Design Principles and Engineering Strategies for Stabilizing Ni-Rich Layered Oxides in Lithium-Ion Batteries

Abstract

Nickel-rich layered oxides such as LiNi_xMn_yCo_zO₂ (NMC), LiNi_xCo_yAl_zO₂ (NCA), and LiNi_xMn_yCo_zAl_{(1–x–y–z)}O₂ (NMCA), where x ≥ 0.6, have emerged as key cathode materials in lithium-ion batteries due to their high operating voltage and superior energy density. These materials, characterized by low cobalt content, offer a promising path toward sustainable and cost-effective energy storage solutions. However, their electrochemical performance remains below theoretical expectations, primarily due to challenges related to structural instability, limited thermal safety, and suboptimal cycle life. Intensive research efforts have been devoted to addressing these issues, resulting in substantial performance improvements and enabling the development of next-generation lithium-ion batteries with higher nickel content and reduced cobalt dependency. In this review, we present recent advances in material design and engineering strategies to overcome the problems limiting their electrochemical performance (cation mixing, phase stability, oxygen release, microcracks during cycling). These strategies include synthesis methods to optimize the morphology (size of the particles, core–shell and gradient structures), surface modifications of the Ni-rich particles, and doping. A detailed comparison between these strategies and the synergetic effects of their combination is presented. We also highlight the synergistic role of compatible lithium salts and electrolytes in achieving state-of-the-art nickel-rich lithium-ion batteries.

1. Introduction

Lithium-ion batteries (LIBs) have been a major advance in the energy storage, and in the development of many devices, from portable phones and laptops to electric vehicles (EVs). For the latter application, LIBs fall short of meeting the demand for ever-increasing EV mileage, requiring LIBs with ever-increasing energy density. This energy density is determined by the cathode active material. The more redox potential of this material vs. Li⁺/Li, the more energy density; thus, a lot of attention has been devoted to the study of cathode materials with a redox potential > 4 V.

The initial commercial success of LiNi_1/3Mn_1/3Co_1/3O₂ (NMC333) has been the motivation for continuous research on NMC-based cathodes. In these materials, Ni is the active element, as it can cycle between Ni²⁺/³⁺ and Ni³⁺/⁴⁺ redox couples. Mn and Co stabilize the layered structure. Therefore, research has been focused on ways to increase the Ni concentration at the expense of Mn and Co without compromising the layered structure needed to the electrochemical performance. In this context, Ni-rich LiNi_1–x–yCo_xM_yO₂ (NMC, 1 − x − y ≥ 0.5, M = Mn or Al) are a promising choice for high-capacity and low-cost cathode materials [1,2]. They are already utilized in many electric vehicles [3], and many efforts are being made to improve their performance [4,5,6,7,8]. Another family of Ni-rich cathode elements is LiNi_xCo_yAl_zO₂ with x ≥ 0.8 (NCA). NCA and NMC have related structures, very similar electrochemical behavior, and show similar performance. Both cells of the same size 21700 with NMC and NCA cathodes are available at a capacity of 5 Ah. The current commercial high-Ni cathode materials are LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) and LiNi_0.8Co_0.15Al_0.05O₂, i.e., materials with Ni concentration x = 0.8. However, compared to NCA, NMC811 possesses the advantages of lower capital and production cost (simple co-precipitation method without extra heat treatment process), higher energy density, and better thermal stability; thus, it has been the subject of particular attention [9,10].

Super Ni-rich cathode materials with higher Ni contents (x ≥ 0.85) are currently under study [11]. Higher Ni concentrations, however, mean aggravated electrode–electrolyte interphase deterioration [12], microcrack formation [13], and surface rock-salt phase (NiO) formation, which is already a problem when x = 0.8 [14,15].

The increase in Ni concentration did increase the capacity, but the correlated decrease in the stability of the structure associated with the decrease in Co and Mn concentrations resulted in a large increase in the capacity fading and growth of impedance [16,17]. In general, NMC-based cathodes with more Mn content tend to possess better cycling stability [18,19], whereas NMC-based cathodes with more Ni content are incapable of reaching high cutoff voltages due to the lack of Mn⁴⁺ as a structure stabilizer [20].

The investigation of the thermal runaways of 18650 lithium-ion batteries shows that the safety issues of Ni-rich NMC and NCA are similar [21]. More precisely, NCA cells react faster than NMC cells above 1 °C min⁻¹ and, thus, reach thermal runaway a little earlier, but react slower than NMC cells below 1 °C min⁻¹ [22]. The high redox potential of Ni-rich cathode materials is also the cause for their poor thermal and structural stability. Ten years ago, Huggins already warned the community that the partial pressure of oxygen at equilibrium increases with voltage, which accelerates exothermic reactions and causes instability with oxygen release [23]. Therefore, the utilization of Ni-rich cathode elements is possible only if the material is modified to avoid oxygen release and improve safety, following processes that have been the subject of intensive research since then.

Many efforts have been made to find remedies to this drawback, which were reviewed [18,24,25,26,27,28], including cationic doping and coating of the particles [29,30,31], gradient layers and core–shell structuration [32], and electrolyte additives [33]. The studies have revealed that parasitic reactions [16], cation mixing (leading to restructured surface regions) [34], oxygen release to maintain the valence equilibrium upon surface transitioning from the layered to the spinel and finally the rock-salt structure of the surface layer [34,35,36,37,38,39,40], active material dissolution [41], the evolution of surface layers, and surface chemistries during cycling [29,42] are the primary factors responsible for cathode degradation. Higher energy density with Ni-rich cathodes issues from the higher operating voltage of the lithium battery. The gaseous oxygen released from the cathode will migrate to the anode side, where it will undertake exothermic reactions, eventually enhanced by thermal abuse, resulting in thermal runaway [43]. The main cause of thermal runaway is the chemical interactions between the cathode and both the anode and the liquid electrolyte [44]; thus, it is crucial to protect the surface of Ni-rich cathode elements, and/or utilize solid electrolytes. In addition, intergranular cracking can weaken connections between primary particles [18,45,46], and accelerate transition-metal (TM) ion dissolution and migration [47]. Moreover, the surface chemistry and bulk microstructure engage in a complex interplay closely linked to the electrochemical properties [48]. Over the years, different reviews have reported the advances in the research on Ni-rich batteries. They include a discussion on the mitigation strategies and trade-offs, such as the selection of dopants or coating materials [49], and the design [50]. An analysis of surface modification techniques, analysis of the manufacturing process, and cost of the surface modification methods has been published few years ago [51].

In the present work, attention is focused on the progress that had been made recently to obtain batteries with Li-rich cathode materials with improved energy density and rate capability, without compromising safety, at the laboratory scale. For a report and discussion to bridge the technological gap between basic research on materials and industrial-scale fabrication, while controlling the cost-effectiveness in the process, we guide the reader to a recent review on the advances in multi-scale design and fabrications processes [52].

2. Goals and Issues

2.1. Why Decrease the Co Concentration?

The first motivation to reduce the use of cobalt is its scarcity, and thus its high cost [53]. The Democratic Republic of the Congo (DRC) accounts for 69% of global cobalt production. Russia ranks number two, but its production is ten times smaller than that of DRC [54,55], and the DRC has the world’s largest reserves of cobalt ore. Refined cobalt products are centralized mainly in China, accounting for 67% of the world’s refined battery-grade cobalt sulfate (CoSO₄) capacity in 2020. Other than China, only Finland has significant refining capacity for cobalt materials, accounting for only 10% of total supply in 2020 [56]. This concentration in a single country, both in production (DRC) and in refining (China), poses a political problem of energy independence to other countries. The United States has taken steps to increase domestic cobalt supplies with the announcement in 2022 of a USD 3 billion investment toward increasing domestic supplies of refined battery metals [57,58].

Another motivation to decrease the consumption of cobalt comes from its toxicity. Impacts of cobalt-bearing LIB chemistries on the environment are well-documented [59,60,61,62,63,64] and are greater than cobalt-free chemistries [65]. The results show that the refining process is the major contributor to global warming potential, freshwater ecotoxicity, and carcinogenic and noncarcinogenic emissions. This stems from the diesel fuel used in refining processes that also contributes to ozone depletion [53]. Moreover, mining contributes significant levels of particulate matter formation.

Due to the high cost and supply difficulties of cobalt, efforts have been made to reduce the concentration of cobalt in Ni-rich cathode materials [66,67]. Nevertheless, cobalt plays a critical role to ensure thermodynamic stability, so the continued use of cobalt in nickel-based EV batteries is predicted [68,69], even though continuous progress has been made to decrease its concentration [70,71,72,73,74]. Liu et al. demonstrated that the chemical and structural stability of the deep delithiated NMC cathodes are significantly dominated by Co rather than the widely reported Mn [75]. Operando synchrotron X-ray characterization coupled with in situ mass spectrometry reveals that the Co⁴⁺ reduces prior to the reduction of Ni⁴⁺, which postpones the oxygen release and thermal failure. The results of these authors highlight the difficulty to guarantee the safe operation of Ni-rich/Co-free layered oxide cathode materials, even though Mn⁴⁺ can enhance the thermal/crystal stability in NMCs [76]. The role of Mn in the crystal stability, however, is controversial. Song et al. have recently revealed that the real origin of the stability of Co-free Ni-rich cathodes is Li/Ni intermixing rather than the traditional Mn stabilization mechanism [77].

The Ni-rich materials crystallize in the R$\overline{3}$m structure. In this structure, Li/Ni intermixing, a problem we will discuss in the next section, is favored by the strong 180° interplane super-exchange interaction [70,78,79,80,81]. The introduction of the non-magnetic ion Co³⁺ in the TM layer reduces this super-exchange interaction, so that Co can efficiently reduce the Li/Ni exchange [82]. For example, Co is still needed in NCA, where it not only reduces the Li/Ni mixing, but also increases the energy density; Co-free LiNi_0.95Al_0.05O₂ has an improved rate capability, but a smaller cycling ability [83], the reason for the doping with Al [84]. LiNiO₂ suffers from instability issues that plague the material. Some Co-free ultrahigh-Ni NMC cathodes including LiNi_0.90Mn_0.10O₂ [85], LiNi_0.90Mn_0.05Al_0.05O₂ [86], and LiNi_0.96Mg_0.02Mn_0.02O₂ [87], exhibit an improved cycle ability compared to the current commercial cathodes, but a lower capacity because the cutoff voltage is lowered. Due to the dissolution tendency of Mn, efforts are being made to obtain not only Co-free, but also Mn-free ultrahigh-Ni NMC cathodes, namely LiNi_xFe_yAl_zO₂, [88] LiNi_0.96Mg_0.02Ti_0.02O₂ [89,90], LiNi_0.95Al_0.05O₂, and LiNi_0.95Mg_0.05O₂ [91]. All of them, however, suffer from a capacity penalty. On the other hand, Cui et al. have synthesized a LiNi_0.93Al_0.05Ti_0.01Mg_0.01O₂ (NATM) cathode that delivered a capacity of 221 mAh g⁻¹ with a capacity retention of 82% after 800 deep cycles in pouch full cells [92]. This cathode exhibits superior electrochemical performance and improved thermal stability with respect to the Co-containing cathode LiNi_0.94Co_0.06O₂ and the NMC LiNi_0.90Mn_0.05Co_0.05O₂ cathode. Noteworthy, despite the absence of Co, this NATM shows a negligible degree of Ni/Li mixing.

The strategies that have been implemented to overcome these difficulties with Co-free cathode materials, and open questions that remain to be addressed were reviewed by Bianchini et al. [93], and more recently by Ge et al. [71].

2.2. Problems Limiting the Electrochemical Performance

2.2.1. Cation Mixing

The radii of Ni²⁺ (0.69 Å) and Li⁺ (0.76 Å) are similar, so that Li⁺ and Ni²⁺ can exchange positions in a delithiated state through Ni²⁺ diffusion to the octahedral sites of Li⁺. This antisite defect can lower spacing between atomic layers in the lattice of the NMC-based cathodes, hinder Li⁺ movement, and reduce the amount of active Ni and Li. This antisite defect and its negative effect on the electrochemical performance have been extensively studied a long time ago for NMC333, and the problem is the same in the Ni-rich NMCs. In this later case, Cui et al. have shown that this defect is predominantly observed at the surface of the particles [94], where it creates a Li⁺ diffusion bottleneck [95]. As a result, the rate capability is reduced. The antisite defect also presumably plays a role in the irreversibility in the first cycle, which is related to the irreversibility of the Ni charge state [96] and irreversible O3/H1-3 phase transition [97]. In Ni-rich NMC, Mn⁴⁺ can partially be replaced by Ni, which increases the valence of Ni and therefore lowers the possibility of Ni atoms migrating to 3a sites [17]. Nevertheless, Ni²⁺ migration still occurs. The origin and the migration of Ni is well-known and well-documented [98]. The gradual growth of Li vacancies with increasing state of charge leads to easier migration. Moreover, in the high charge voltage, oxygen vacancies are generated, favoring a large Ni migration. Therefore, the tendency of Li/Ni mixing is more severe and frequent with increasing charge state and operating voltage. Using operando neutron diffraction measurements, Wu et al. showed that Li−O−Li configurations resulting from the Li−Ni antisite defect can increase anionic redox activity and promote Ni migrations, particularly at high voltages [99]. Nevertheless, the degradation of electrochemical performance of Ni-rich cathode materials under overcharging conditions primarily originates from the collapse of the cathode material, the precipitation of oxygen, which mainly occurs on the surface of the primary particles, and the formation of voids in the grain boundary region [100].

2.2.2. Phase Stability

Cation mixing can also induce a transition to disordered spinel structure (Fd3m) when LIBs are charged to high voltages (4.8 V) [101,102,103,104]. Spinel phases can also be formed on charged cathodes stored at 90 °C for a week [105]. Investigations on NMC revealed that long term cycling can result in phase changes to an Fm3m structure (cubic rock-salt phase) consisting of Ni²⁺, Mn²⁺, and Co²⁺ [102,106], an effect also observed with NCA cathodes [107]. This phase transition is accompanied with a release of O₂ into the cell, which can react with electrolytes to generate CO₂ and increase the interfacial resistance of the cell [108]. NMC811 cathodes pretreated with ramping to 4.5 V limited the growth of the rock-salt layer in subsequent cycles by isolating the surface of NMC particles [109].

2.2.3. Oxygen Release

Oxygen release is a significant cause of degradation of the Ni-rich cathodes [110,111]. The mechanisms of the oxygen release of the Ni-rich materials are well-documented [112]. This release essentially results from the change of the TM-O bond from ionic to more covalent character induced by Li⁺ extraction, due to the removal of electrons to maintain charge neutrality. In particular, this effect has been clearly evidenced in NCA [113]. This oxygen release results in a destabilization of the layered structure, which is aggravated in highly delithiated states, and increases with Ni concentration [114]. Therefore, in practice, the voltage of the batteries with pristine NMC811 is limited at 4.2 V [34]. Moreover, the oxygen release results in a thermal instability and safety concern [115]. In addition, the thermal stability is impacted by chemical reactions with the electrolyte. This is illustrated in Figure 1 for the case of NMC84 (LiNi_0.84Co_0.11Mn_0.05O₂) charged at 4.3 V in the commonly used carbonate electrolyte with LiPF₆ salt (LB301).

The onset temperature of thermal decomposition is already reduced by the carbonates, in particular dimethyl carbonate (DMC), and further reduced by the introduction of LiPF₆. Furthermore, the outward migration of oxygen is accompanied with an outward migration of Ni. This process of Ni and O co-migration along (003) planes was observed, for example, in NMC811 [116]. This continuous advance of the surface degradation layer toward the particle interior results in an increase of the thickness of the surface inactive rock-salt phase layer, leading to a rapid degradation of the electrochemical properties [15]. In addition, during the delithiation process, the outward diffusion of Li⁺ generates a concentration gradient and vacancies in the layered structure [117]. The smaller variation of the lattice in the Li-rich (vacancy-poor) domains along the c-axis direction than in Li-poor (vacancy-rich) domains results in an anisotropic lattice contraction that may induce microcrack formation at grain boundaries [9,118].

2.2.4. Modifications During Cycling

NMC oxide possesses a R$\overline{3}$m structure (rhombohedral symmetry) with a Li-layer on the 3a site, an NMC layer on the 3b site and an oxygen layer on the 6c site [16]. X-ray measurements and infrared spectroscopy reveal the deformation of the lattice during cycling [16,119,120]. The c-axis parameter expands during charging, due to the repulsive force generated from MO₆ slabs, which are positively charged, whereas the a-parameter is reduced because the radii of the transition metal ions are reduced by the loss of electrons [121] and the increase of their valences [122]. The a-axis of the Ni-rich cathode material remains constant at potentials exceeding 4.3 V, and the steep drop of the c-axis occurs at potentials exceeding 4.2 V, becoming steeper as the Ni content increases [16,120,123,124]. This drop is due to the fact that the repulsion between O²⁻ layers decreases with more covalent M–O bonding at higher delithiated states [125,126]. Two processes are involved in the delithiation of Ni-rich NMC-based cathodes. Until a certain degree of delithiation, Li can exit through oxygen dumbbell hopping, while at higher degree of delithiation, Li can exit through tetrahedral site hopping [127,128].

During cycling, Ni²⁺/Ni⁴⁺ mainly reacts in the low-voltage region and Co³⁺/Co⁴⁺ mainly reacts in the high-voltage region. As Li⁺ is removed from the Li slab during charging, the Ni-rich cathode undergoes the successive phase transitions H1-M-H2–H3, due to change in the repulsive force in O-Li-O caused by local structural distortion [129]. The H2–H3 phase transition is the most problematic for the electrochemical performance, because it is accompanied with an evolution of oxygen in the lattice and large volume changes. These changes can cause microcracks in the cathode active material, an additional exposure of the cathode active material surface to the electrolyte [130]. Both effects result in reduced cycle ability [131], and rate capability [132] (see Figure 2). This problem is particularly important in Ni-rich cathode materials as the onset potential corresponding to the H2–H3 phase transition decreases when the Ni concentration decreases, from 4.6 V vs. Li⁺/Li in NMC111, to 4.2 V in NMC811. While we have explained in a previous section (Section 2.1) the current efforts to reduce the cobalt concentration, it should also be mentioned that cobalt has also adverse effects. The addition of Co to Ni-rich layered structure can cause a more serious H2–H3 phase transition when charging and discharging between 2.8 and 4.5 V [72,133]. Moreover, the structural instability of the Ni-rich cathode materials is strongly correlated with the cell gas release in a charged state. That is why, even though adequate strategy (coating, doping) reviewed hereunder is efficient to reduce the instability, the practical voltage for charging is limited to 4.5 V [134], inasmuch as at such a high potential, the formation of Ni/Li antisite is unavoidable. The main attention on the Ni-rich cathodes these recent years has thus been focused on the study of these degradation mechanics and the solutions to avoid them [135,136,137,138]. Otherwise, operando X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicated that particle fracture from increased structural distortions at >4.3 V was a contributor to capacity fade [139].

2.2.5. Microcracks

The capacity degradation of Ni-rich NMC is not only due to surface chemical instability, but also to mechanical destruction in the form of microcracks [15,140,141,142,143]. The H2 → H3 transition is not the only source of microcracks. As a matter of fact, different sources of cracking may affect Ni-rich cathode materials: intragranular cracking may occur under the effect of cation mixing [15], while the H2–H3 phase transition is mainly responsible for intergranular cracking. In addition, the antisite defect can also be formed during cycling, and cause a structural transition from rom the layered phase to the spinel-like phase [144], so that both the rate capability and the cycle ability are reduced. In addition, the cracking exposes fresh surfaces to the electrolyte, and thus reduces the efficacy of protective surface. It is thus crucial to avoid the cracking of the particles, which has been the subject of intensive research.

The cracking process is initiated by the anisotropic volume change in primary particles and lattice strain, which provokes the mechanical destruction of secondary particles [145,146,147]. Differences in the lattice volume of domains with different Li⁺ concentrations inside a primary particle induce the localization of stress between the domains, which is likely to create intraparticle microcracks along (003) planes [148,149,150,151]. These effects explain that microcracks are generated by the inhomogeneous distribution of Li, so that their formation depends on the state of charge of the cathode [152]. Note that we consider here electrochemical crack formation that occurs mainly at high state-of-charge (SOC) levels. Another mechanism of chemical crack formation has been recently evidenced at low SOC levels, driven by chemical corrosion [153], but this phenomenon is observed at high temperature (~60 °C). The inhomogeneity of the lithium distribution during cycling has been observed in NMCs by operando optical microscopy [154]. The evolution of the stress associated with the anisotropic Li distribution has been studied as a function of the state of charge in different works [155,156,157]. Combined with lattice strain, the anisotropic volume change results in both intergranular and intragranular cracking [158,159,160,161,162]. When the Ni concentration is ≤0.6, only one phase transition H1 ↔ M phase is observed upon cycling [34,163]. Unfortunately, the cracking effects are more important in Ni-rich NMC at Ni concentration x ≥ 0.8, since the structural phase transitions are more severe [164]. During the first charge–discharge cycle, a series of phase transition processes increase interlayer spacing, including H1–M and M–H2 transitions, followed by a shrinking of the interlayer spacing during the H2–H3 phase transition process, which is most damageable to the integrity of the particles [129,164,165,166]. The local stress distribution associated with the anisotropic lattice contraction responsible for the structural instability caused by the H2–H3 transition in the deeply charged state can be overcome by the synthesis of single-crystal Ni-rich cathode elements [167,168]. Consequently, monocrystalline NMCs demonstrate a significantly better cycling ability than polycrystalline NMCs [169]. Even the morphology of the precursors matters. Liang et al. demonstrated that the spheroidization of the precursor can enhance uniformity in structural evolution during solid-phase lithiation [170] (see Figure 3). The resulting LiNi_0.92Co_0.04Mn_0.04O₂ cathode material delivered a capacity of 234.2 mAh g⁻¹, in the range of 2.7–4.3 V, with capacity retention of 89.3% after 200 cycles at 0.5 C. Ni-rich cathode materials with different morphology structures obtained by adjusting the ratio of solvent and water in the solvothermal process [4]. This work illustrates the impact of different morphologies on the electrochemical properties. Coating is also a pertinent strategy, because the surface of Ni-rich materials free from electrolyte attack undergo limited structural degradation [171]. Morphologies of the primary particles of Co-containing Li[Ni_xCo_y(Al or Mn)_1–x–y]O₂ cathodes can be engineered to optimize the electrochemical properties [172,173]. For example, ultrafine-grained structures in the Li[Ni_0.95Co_0.04Mo_0.01]O₂ cathode provided paths deterring the propagation of intergranular cracks, allowing for high-capacity retention and fast-charging capability [173].

Wang et al. introduced a coherent perovskite phase into the layered structure functioning as a ‘rivet’, which significantly mitigates the pernicious structural evolutions by a pinning effect. The resulting enhanced mechanochemical stability was attributed to the presence of significant energy barriers for phase transitions within the rivet phase, similar to a pinning effect [174]. The gas release pattern and stress distribution of NCM particles can be governed by the microstructure engineering [175]. In particular, the strain between the grains in the NMC811 secondary particle can be dissipated by radially aligned grains near the particle surface. However, the mismatched strain between disoriented primary grains in the interior of the particle leads to the development of microcracks in the central region of the NMC811 secondary particle [147]. The electrolyte penetrates in the interior of the particles via these microcracks, resulting in the formation of NiO rock-salt phase that reduces the Li⁺ interfacial resistance [176]. On the other hand, the electrolyte infiltration shortens the Li⁺ diffusion distance [177]. As a result of the balance between these two effects, the Li⁺ diffusion kinetics has been found accelerated in polycrystalline cathodes with respect to single-crystal cathodes without grain boundaries [178]. In addition, typical structural defects of primary particles such as twin boundaries [179], dislocations [180], or even Li⁺/Ni²⁺ anti-site defects [181] serve as preferred sites for the nucleation and propagation of intragranular microcracks. In single-crystallized particles, the tensile stress triggers gliding along the (003) plane. This effect has been observed by through aberration-corrected scanning transmission electron microscopy (STEM) and analyzes by theoretical calculations [182]. These planar gliding processes corresponds to the in-plane migration of Ni ions, the barrier energy of which is reduced in presence of oxygen vacancies [183]. Since these processes are not entirely reversible, they also lead to microcracking along the (003) plane in single-crystallized particles. However, Sun et al. showed that the destructive volume change and H3 phase formation presented in polycrystals are effectively suppressed in single-crystals when cycling at a high cut-off voltage of 4.6 V [184]. This feature clarifies the origin of high-voltage stability in single-crystal cathode, but also the improved cycle ability with respect to polycrystals.

3. Synthesis Methods

A review on the synthesis process has been recently published [185]. Different synthesis routes give good results, including hydrothermal process [186,187,188], sol–gel process [189,190], and solid-sate process [190,191]. The sol–gel method offers some advantages. In particular, Zheng et al. successfully synthesized single-crystal Ni-rich cathode materials via a two-step solid-state method [192], but the preparation and refinement of raw ingredients required is still difficult. That is why the co-precipitated method is widely used for the current commercial synthesis of NMC Ni-rich materials by stirring and mixing the stoichiometric ratio of transition metal salt solutions to form homogeneous precursors [193]. Among various synthetic methods for NMC materials mentioned above, the co-precipitation method is the recognized as one of the most effective processes for preparing Ni-rich cathode materials with the best electrochemical performance [194,195]. This method, however, is difficult to use for NCA because Al³⁺ ions have a propensity to form double hydroxides in solution. Therefore, alternative solutions have been adopted for NCA, namely the hydrothermal method, which is complicated and not cheap, or the solid-state method, which has been reviewed by Wang et al. [196]. However, a recent comparison of the electrochemical properties of LiNi_0.8Mn_0.15Al_0.05O₂ prepared by co-precipitation and sol–gel methods highlights the superior crystallinity, particle uniformity, and compositional accuracy of coprecipitation, making it more effective for enhancing electrochemical stability and efficiency, despite the presence of Al [197]. The common and commercialized method is to prepare precursors via co-precipitation first, and then subject it to high temperature calcination with lithium salt together. Efforts have been made to optimize the synthesis parameters [198], including the calcination temperature [199,200], lithium source type [201,202], and calcination atmosphere [203].

The influence of the precursors on the electrochemical performances of Ni-rich layered oxides has been discussed by several workers. Cui et al. [94] investigated the effect of cationic uniformity in the precursors on Li/Ni mixing of Ni-rich layered cathodes. The cationic uniformity in precursors is regulated by the sol–gel and solvothermal method. All cations in the precursors obtained by the sol–gel method mix uniformly at a molecular scale, while the solvothermal method with a post-addition of Li salts causes a heterogeneous mixture between Li and transition-metal ions. Therefore, advanced Ni-rich layered cathodes should simultaneously possess low Li/Ni mixing and reversible H2–H3 transition.

The conventional precursor for the synthesis of NMC materials is Ni_1–x–yCo_xM_y(OH)₂ particles, prepared by co-precipitation in a mechanically stirred tank reactor, under conditions that lead to a homogeneous elemental distribution [204,205,206,207,208]. This is considered as the most viable option for large-scale production [209,210,211,212]. The effects of the β-(Ni_0.6Mn_0.2Co_0.2)(OH)₂ content in hydroxide precursor on the structure and electrochemical performance of NCM622 cathode materials were investigated by Xu et al. [210]. The cathode material sintered from the mixed phase of α-(Ni_0.6Mn_0.2Co_0.2)(OH)₂ and β-(Ni_0.6Mn_0.2Co_0.2)(OH)₂ shows the highest discharge capacity of 192 mAh g⁻¹ and best rate performance.

The microstructures of Ni_1–x–yCo_xM_y(OH)₂ particles are crucial to obtain Ni-rich cathodes with the best electrochemical properties, and many works have been devoted to optimize them, reviewed in [195]. These microstructures depend on various factors, including the pH value [213,214,215], ammonia concentration [216,217] (see Figure 4), reaction temperature [218], reaction time [219], and stirring rate [220]. Depending on these different parameters, different morphologies of the precursor particles are obtained, including microspheres [160,221,222], rods [223,224,225], and hollow spheres [189]. Among them, the spherical Ni_1–x–yCo_xM_y(OH)₂ particles form spherical LiNi_1–x–yCo_xM_yO₂ after lithiation, a shape that form a more tightly packed structure during battery manufacturing. As a result, the volume energy density is increased, and the Li⁺ ion migration is facilitated [226,227].

These examples illustrate that the morphology of the particles is important. In particular, the role of ammonia concentration in determining the particle shape and size of Ni-rich cathode materials during co-precipitation has been recently deciphered by Zhang et al. Moreover, they showed that ammonia concentration affects the thickness-to-length ratio of the precursor’s primary particles, which in turn influences the morphology of the LiNi_0.95Al_0.05O₂ cathode materials during lithiation [228]. Lower NH₄OH/metal ratios produce more porous precursors, which enhance Li-source diffusion and promote uniform sintering [229]. Pristine NMC811 nano-cubic microstructure is successfully synthesized via a rapid precipitation assisted with hydrothermal treatment, which maintains a high capacity (188.2 mAh·g⁻¹) with a retention of 87.2% at 1 C after 200 cycles, a remarkable result that shows how important the synthesis of nano-sized and single crystallized particles is [230]. We will return to morphology aspects later in this review, when they are monitored by doping, for example.

The lithiation process consists in the calcination of Ni_1–x–yCo_xM_y(OH)₂ with a lithium source (Li₂CO₃ or LiOH) at high temperature and in an oxygen atmosphere [95,231,232,233,234]. New synthesis processes have recently emerged, such as the synthesis of Ni-rich gradient material by Taylor–Couette cylindrical reactor [235], and the synthesis of single-crystallized samples by microfluidic co-precipitation [236]. The lithium salt used as a precursor is usually Li₂CO₃ or LiOH·H₂O. Li₂CO₃ has lower corrosion, and it is suitable for NCM523 and NCM622. Unlike these, preparation of NMC811 should be calcined under the oxygen atmosphere and lower temperature, so the more reactive LiOH·H₂O is the best option. Oxygen partial pressure during the calcination process is also an important parameter [50,237].

The sintering temperature is a crucial parameter. A too-small sintering temperature fails to provide enough energy for the decomposition of the precursors. The high sintering temperature is needed to obtain a good crystallinity, which is essential to the electrochemical performance of the cathode material. However, the high sintering temperature has two negative effects. First, as Li is volatile, some Li evaporates. This problem can be solved by the addition of excess Li during the high-temperature calcination process, which must be adjusted to obtain a stoichiometric final product [238,239]. The consequence is an excess of lithium at the surface. However, a dry cobalt hydroxide coating effective removes the residual lithium, as shown by Kim et al. in the particular case of NMC91 [240]. The second drawback is more challenging; this high temperature makes almost inevitable the cation mixing between Ni and Li [241], which is detrimental to the electrochemical performance. In practice, above 780 °C, the cationic mixed degree increases [242]. Therefore, 750 °C is the best calcination temperature for NMC811 [243]. However, the synthesis temperature of single-crystal structures is usually about 100 °C higher than that of polycrystals [244], implying more severe Li/Ni cationic disorder in single-crystal structures than in polycrystalline structures [245,246]. Post-process (i.e., heat-treatment after washing) is commonly used, as it improves the cycle life of Ni-rich materials [247].

To minimize Ni²⁺ migration, researchers also reported that mitigation methods can be used to modify cathodes [243]. Luo et al. have recently reported an ion-exchange method to prepare Ni-rich layered oxide materials, consisting in the choice of sodium-based layered oxides as precursors, for ion exchange with Li⁺ ion in lithium molten salts [36]. The reaction temperature in this process is reduced to 300 °C. However, additional heat treatment at 700 °C was essential for obtaining well-ordered layered structure. Nevertheless, Ni-rich cathodes synthesized by sodium-to-lithium-ion exchange are devoid of Ni∙_Li antisite defects [248]. In a different approach, Wang et al. reported that the Li/Ni disorder can be facilely tuned through post-synthesis annealing [249]. Recently, Mo et al. adopted a certain amount of H₂O₂ to treat the precursor of spherical Ni_0.8Co_0.1Mn_0.1(OH)₂. the H₂O₂ pre-oxidation reduced Li⁺/Ni²⁺ mixing to below 0.34%, extending the capacity retention of NMC811 to 98.5% over 100 cycles [250]. However, this does not exempt us from using the usual means to reinforce the structural and thermal stability of these materials, such as doping or coating.

These results illustrate the major influence of the precursors on the electrochemical properties. The shape, size distribution, and primary/secondary particle structure of the hydroxide precursors directly translate to the final cathode material, strongly affecting Li-ion diffusion and electrode kinetics. Co-precipitation parameters (pH, temperature, stirring, ammonia feed rate) are crucial—altering them changes nucleation and growth, which in turn impacts particle uniformity and tap density. Solid vs. polycrystalline vs. single-crystal morphologies in precursors yields cathodes with starkly different cycling stability and crack resistance. The precursor’s crystallinity and defect landscape steer the final oxide’s structural integrity. Poor crystallinity can promote lithium–nickel mixing, which undermines capacity and cycle life. As we shall see in the next section, incorporating dopants or surface modifiers during precursor formation—like tungsten (W), Al, or rare-earth ions—can significantly enhance cycle life, mitigate microcracking, and stabilize surface layers.

3.1. Single Crystals Versus Polycrystals

Many works demonstrated that polycrystals have a significantly weaker structural stability than single crystallized Ni-rich particles [178,192,251]. This explains that many efforts have been made to synthesize single-crystallized Ni-rich particles, reviewed in [252]. In particular, Liu et al. reported that heavily cycled commercial-grade pouch cells with NMC811 cathodes showed very little microcracking when single particles are employed [253]. Through comparative experiments, Kong et al. found that single-crystal NMC811 particles have better storage performance than their polycrystalline counterparts [254]. They also demonstrated that the concentration of impurities generated on single-crystal materials is also significantly less, and the electrochemical performance can be better maintained. Lüther et al. studied the electrochemical properties of single- and poly-crystallized NMC811 synthesized with comparable bulk and surface characteristics from identical self-synthesized precursors. This study also demonstrated the superiority of single-crystallized particles [255]. Qian et al. showed that single-crystal NMC622 particles exhibited remarkable cycling stability, and retained ~94% of the initial capacity after 300 cycles at 1 C rate (from 161 to 151 mAh g⁻¹), significantly higher than that of the polycrystalline NMC622 under the same condition (~52%, from 155 to 82 mAh g⁻¹). This result was attributed to the limitation of the Li-ion transport across neighboring primary grains in poly-crystallized particles, as these grains are not perfectly aligned, while there is no such misalignment effect in single-crystal particles [256]. Fan et al. also reported that single-crystallization of LiNi_0.83Co_0.11Mn_0.06O₂ particles mitigate undesired side interactions and significantly prevent the generation of intergranular cracks, to achieve a capacity retention of 84.8% at 45 °C after 600 cycles at a current rate of 1 C [257]. For the same reason, in full cells between 3.00 V and 4.35 V, LiNi_0.83Co_0.10Mn_0.07O₂ delivers a reversible capacity of 167.0 mAh g⁻¹ with capacity retention of 84.8% after 400 cycles at 0.5 C/25 °C, far larger than 107.7 mAh g⁻¹ and 54.1% of conventional polycrystalline LiNi_0.83Co_0.10Mn_0.07O₂ [258]. Still, these results can be improved by appropriate interfacial modification. Examples with single-crystal NMC811 are the in situ planting of an electrolyte film-forming additive on a single-crystal NMC811 surface by consuming the residual lithium to develop a functional Al(Li)BOB nano-layer [259], and surface spinel-phase modification [260]. On the other hand, Ge et al. reported sluggish kinetics in single-crystals governed by ionic transport at low state of charge, which, however, can be alleviated through synergistic interaction with polycrystals integrated into a same electrode [261].

Therefore, the single crystallization is an important path to avoid cracking and ensure that high-nickel cathode materials have both high energy density and long cycling stability [262,263,264]. This is clearly evidenced, for instance, in the work of Zhao et al. [265] on NMC622. Within operando X-ray imaging, spectroscopic technologies, and diffraction methods, Zhang et al. observed that phase transitions from homogeneity to heterogeneity during cycling induce particle crack formations at the surface, and play a dominant role in the structural robustness of single crystals. They also demonstrated that using a lithium source to replenish lattice sites during re-calcination under oxygen flow and high-temperatures (denoted as t-NCM) can induce homogeneous phase distribution, enhancing the capacity retention [266]. In addition, Hu et al. found that at high potentials, the amount of oxygen gas released in single-crystalline Ni-rich cathode materials is significantly lower than in polycrystalline cathode materials [267]. With NMC cathodes, single crystallization avoids cracking during cycling, but only when the Ni content does not exceed 90% [150]. Also, the study of cobalt-free polycrystalline (PC) and single-crystalline (SC) LiNi_0.9Mn_0.1O₂ showed that the PC material has lower Li⁺/Ni²⁺ mixing and a faster Li ion diffusion rate. As a consequence, intergranular cracking, surface pulverization, disorder phase transition, and interfacial side reactions were suppressed in the PC material but not in the SC material [167]. These two last results show that a Ni content greater than 90% is a bottleneck in improving the energy density and cycling performance of high-nickel single-crystal materials [78,268]. On the other hand, the study of single-crystallized and poly-crystallized LiNi_0.76Mn_0.14Co_0.1O₂ cathode materials evidenced that the gas evolution of single crystals is much lower than that of polycrystals even at high voltage potentials [267]. Therefore, the single crystallization of Ni-rich cathode materials is also important to solve the safety issues of these batteries. Single-crystal NMC811 demonstrated 98% capacity retention with no cracking after 1100 cycles [253]. Huang et al. [269] prepared single-crystal Li(Ni_0.9Co_0.05Mn_0.05)O₂, in which an extra pulse high-temperature sintering at 1040 °C for 1 min is added in the traditional calcination process at 750 °C, which increased the tap density by 1/3 to 2.76 g cm⁻³ and suppressed the microcracks, improving both the cycling performance and thermal stability. High-temperature sintering is the most effective synthesis strategy for single-crystal materials, but a sintering temperature exceeding 830 °C can lead to undesirable outcomes, such as Li/Ni mixing and surface reconstruction of the cathode particles. The work of Huang et al. shows that this drawback can be avoided by reducing to the pulse-level the time during which higher sintering temperature is applied. In addition, high tap densities are obtained with small particles, more prone to failure during the cycling process. The work of Huang et al. also demonstrates a way to balance the tap density of the material with the selective failure of smaller particles, without the need of incorporating a mixture of polycrystalline and monocrystalline particles proposed by Li et al. [270]. In addition, the orientation of the crystals matters. For example, CeO₂ with well-oriented (011) facets have a shorter linear Li⁺ migration path and higher electron conductivity, so that they perform better when coated on NMC811 than the (124) and (311) orientations [271]. Single-crystalline-based electrodes show significantly improved cycling ability because the smaller size of the particles result in a more uniform current distribution in the cathode, whereas in the case of polycrystalline electrodes, inert dead zones are caused by large microcracks [272]. The single-crystal architecture can effectively retard the layered to spinel and rock-salt phase transition and mitigate the crack formation [273]. The comparison of polycrystalline and single-crystal cathodes has been reviewed [274,275,276], and demonstrates that the single-crystallization is one of the most prospective routes for the modification of Ni-rich layered cathodes, in particular when the synthesis parameters are chosen to obtained particles with optimum micro-size [277]. The main drawback is the lower redox kinetics of the single-crystal cathodes. The Ni³⁺ oxidizes to Ni⁴⁺ all over the course of delithiation in polycrystals, but only at the end of delithiation in single crystals [261]. The sluggish redox kinetics at low state of charge leads to an abrupt reduction in Li⁺ diffusivity, responsible for responsible for the low Coulombic efficiency and poor rate capability of single-crystal cathodes [9,150]. On the other hand, the single-crystal cathodes are more resistant to mechanical damage during calendar aging [278]. Since the oxygen release occurs mainly near the surface, the small surface area of single crystals significantly limits the activation of oxygen vacancies, which not only improves the electrochemical properties, but also enhances thermal stability. This has been evidenced in NMC811 [279,280]. Single crystals give bigger particles than polycrystals, which tends to give cathodes delivering a capacity slightly smaller than their polycrystalline counterparts, but this drawback can be easily overcome by doping [281].

3.2. Synthesis of Single Crystals

Single crystals have a clear advantage to improve the electrochemical properties, but their synthesis is complex and expensive [282]. Recently, the difficulty has been circumvented by using a facile ball-milling method to transform pristine micron-sized polycrystalline NCM particles into submicron pseudo-single-crystal particles [283]. The co-precipitation method is widely used to prepare the precursors for both the polycrystals and the single crystals. In addition, the spray-drying method is of interest for the synthesis of the precursors for single crystals, because the porous precursor caused by solvent volatilization is conducive to the fusion and growth of grains, allowing for the synthesis of single crystals the single crystals at a lower operating temperature (e.g., 750 °C) [284]. In addition, pores inside Ni-rich cathode materials can effectively reduce stress accumulation caused by anisotropic lattice expansion and shrinkage, and decrease the generation of microcracks [285]. Indeed, the synergetic effects of porosity and radial structure significantly improve the electrochemical performance of Ni-rich cathode materials [286]. Precursors with larger specific surface area exhibit smaller pore size, ensuring the better wettability and enhanced capillarity for homogeneous lithiation reaction [287]. The generation of meso- and macropores can be alleviated by controlling the homogeneity of the synthetic reaction [288]. At present, the sintering processes for single-crystal cathode materials include continuous high-temperature sintering, pulse high-temperature sintering, and molten-salt sintering [289,290,291] (see Figure 5). For high-temperature synthesis, higher calcinating temperature is required, bringing aggravated particle agglomeration and Li/Ni mixing. On the other hand, the low synthesis temperature used in the synthesis of polycrystals is damageable to the mechanical properties. As a consequence, the particles on the upper layer of the electrode may be damaged by the pressure applied during the manufacturing [292]. This destruction of the polycrystalline particles during the calendaring process in the manufacturing of the cathode was also observed by Raman spectroscopy measurements [278]. As an example of the performance achieved with the high-temperature synthesis, single-crystal LiNi_0.83Co_0.10Mn_0.07O₂ in full cells between 3.00 and 4.35 V delivers reversible capacity of 167 mAh g⁻¹ and capacity retention of 84.8% after 400 cycles at 0.5 C [258]. With a low-temperature synthesis (sintered at 750 °C with 50% Li-excess), NMC811 exhibits a discharge capacity of 178.1 mAh g⁻¹ with the capacity retention of 95.1% after 100 cycles at 1 C [293]. As for molten-salt synthesis, modifications on the conventional tunnel furnace to prevent an explosive spillover of hot liquid, additional salts as assisted solvent, and water-washing process to remove the salt residues are explored. However, the water-washing process itself may degrade the electrochemical properties, so additional treatments such as heating and coating are undertaken [294,295]. However, Hu et al. have shown that washing NCA with ammonium dihydrogen phosphate (NH₄H₂PO₄) solution simultaneously enhances electrochemical performance and air stability, due to an in situ generated Li₃PO₄ coating layer [296]. Therefore, a lot of attention is devoted to the fabrication of single-crystallized Li-rich cathode elements [290]. The molten-salt-assisted method using low-melting-point carbonates was used to synthesize single-crystalline NMC622 delivering 174 mAh g⁻¹ at 0.1 C, 3.0–4.3 V, with capacity retention of 87.5% after 300 cycles at 1 C [297]. In addition, this molten-salt-assisted method exhibits its effectiveness in directly regenerating NMC622 from the spent material. Single-crystal NMC811 prepared by molten-salt process still delivered a capacity of 80.7 mAh g⁻¹ after 300 cycles at the high rate of 5 C, which corresponds to a capacity retention of 62.0% [298]. The infusion of molten salts to protect grain boundaries of LiNi_0.5Mn_1.5O₄ increase the capacity of this cathode element from 89.5 to 133 mAh g⁻¹ at 2 C, 50 °C [299]. Wu et al. have recently reported the important relationship between the precursors, lithium salts, sintering atmospheres, sintering temperature, and sintering procedure to be used to obtain such materials [300], including LiNiO₂ [301]. Insight on calcination and sintering of these materials can be found in [301,302]. The aspects of precursor orientational growth, crystal morphology optimization, and composite structure design can be found in [8].

4. Coating

To improve the thermal stability and structural stability, surface modifications of the Ni-rich materials is a predominant strategy [303]. Surface coating is a common process to avoid the presence of residual alkalis, such as LiOH and Li₂CO₃ at the surface of the particles. These residual alkalis are one of the dominant reasons for the phase change and electrochemical degradation [304]. In addition, the coating also protects them against side-reactions with the electrolyte. It is difficult to compare the results obtained with different coating materials, because the efficiency of the coating depends not only on its chemical formula, but also on the synthesis method. Han et al. demonstrated this effect by comparing two different synthesis approaches, wet chemical (WC) coating and atomic layer deposition (ALD), for alumina coatings on LiNi_0.5Mn_0.3Co_0.2O₂ [305]. They showed that the ALD method leads to a better initial quality of the coating layers and is less influenced by the post-coating annealing process, while for WC method the high-temperature treatment is highly needed. Cheng et al. utilized ALD coating and annealing protocol to demonstrate the individual and coupling effects of surface coating and grain boundary engineering on cycling stability of NMC811. This engineering allowed them to achieve superior cycle stability even upon high voltage cycling (91% retention after 200 cycles at 2.7–4.7 V at C/3 rate) [306]. Moreover, the electrochemical performance of the materials depends on the synthesis process and synthesis parameters described above, and also on the morphology. That is why we prefer, when possible, to evaluate the performance by comparing the results reported for coated and uncoated samples in the same work, since the coating is the only parameter that makes a different in this case.

4.1. Li-Free Surface Layers

Carbon coating is commonly used for all kinds of active particles for cathodes of lithium batteries, and it has been also applied to Ni-rich cathode materials with success [307,308,309]. Different forms of conductive carbon have been utilized, including composites such as graphene nanosheets with NMC811 [310], carbon nanotubes with NMC532 [311,312], and interwoven carbon fibers with NMC622 [313]. Density functional theory calculations (DFT) play a crucial role in screening dopants (e.g., Al, Mg, Zr) that can stabilize the Ni-rich structure and reduce cation disordering. It also models interactions with protective coatings such as LiNbO₃ and Al₂O₃, helping design surface modifications that enhance stability and minimize electrolyte decomposition. Ning et al. prepared uniform reduced graphene oxide (rGO)-wrapped NMC811 with the help of a semi-conductive perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) layer, ensuring the adhesion of graphene on the particle surface [314]. Poursalehi et al. employed the scalable electrophoretic deposition (EPD) procedure to make instantly a binder-free Li-ion battery cathode, consisting of LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811), sulfonated reduced graphene oxide (SRGO) sheets, and carbon black (CB) particles [315]. The EPD process caused a squeezing force, which not only wrapped the SRGO sheets around the NMC811 nanoparticles, but also inserted the CB particles into the composite. The volume expansion of the active particles was mitigated during cycling due to wrapping graphene sheets, and the restacking of graphene sheets was inhibited by sulfonated groups and CB spacers. At the current density of 0.2 C, the half-cell with this electrode and 1M LiPF₆ in ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1, v/v) electrolyte delivered discharge capacities of 170.3 and 143.7 mAh g⁻¹ at the 1st and 200th cycle, respectively. The rate capability was also enhanced (42.3 mAh g⁻¹ at 10 C). The method is also scalable, because it takes only 10 min of ultrasonication and EPD when mass-produced SRGO and commercial NMC811 and CB particles are used as the starting materials. Graphitic carbon nitride (g-C₃N₄)-coating can also improve the electrochemical properties of NMC811, provided that the thickness of the coat is thin enough (20 nm) to avoid the hindering of the lithium diffusion [316]. Park et al. used a dispersion of electrochemically exfoliated graphite nanosheets that are functionalized with an amphiphilic surfactant acting as glue for graphite coating on the surface of Ni-rich oxides [317]. As a result, the cathode (99 wt.% NCA; electrode density (ρ) ~4.3 g cm⁻³) exhibited a 38% increase of the areal capacity and 34% increase of the volumetric capacity at current rate of 0.2 C with respect to the bare commercial NCA cathodes. We shall return on the coating devoted to the protection of the particles against side reactions with the electrolyte in a section devoted to the formation of the cathode–electrolyte interface.

Li-free coatings have been extensively investigated, including metal oxides such as SiO₂ [318,319], ZrO₂ [320,321,322], TiO₂ [323,324,325], CeO₂ [326], and Al₂O₃. In particular, Al₂O₃ coating of LiNi_0.6Co_0.2Mn_0.2O₂ improves coulombic efficiency, reduces impedance, and improves capacity retention [327]. Even better electrochemical properties for this material were found with uniform C-Al₂O₃ coating obtained by the pyrolysis of molecular layer deposited alucone in argon, due to the synergistic effect between the amorphous Al₂O₃ and conductive carbon composite coating [328]. For Ni-richer NMC811, the positive effect of Al₂O₃ coating is evidenced by the increase of the capacity retention from 500 cycles to 1100 cycles at 1 C rate [329]. In a comparison between LiAlO₂ and Al₂O₃ coatings of NMC622, Liu et al. showed that LiAlO₂ coating is superior to Al₂O₃ and significantly improves the capacity retention and rate capability [330]. Note, however, that the efficiency of the coating depends on the properties of the coat, so that direct comparison may be misleading. In particular, microporous Al₂O₃ coating [331] and ultrathin Al₂O₃ coating [332] gives remarkable electrochemical properties. Therefore, Al₂O₃ coating is still widely used today, inasmuch as it is now possible to control not only the uniformity but also the thickness of the Al₂O₃ coating, allowing for its optimization [333]. Other metal oxide coatings include ZnO [334], MgO [335], Mg-Al layered double oxide [336], and Ta₂O₅ [337]. A Ta₂O₅ protective layer on the surface of NCA with Ta⁵⁺ entering the lattice gifts the cathode had a capacity retention of 94.46% at 1 C after 200 cycles (60.97% for the pristine NCA) [338]. B₂O₃ coating treatment is an effective approach to reinforce the air storage stability and cyclic stability of Ni-rich cathode materials. Even exposing to ambient air for 8 weeks, B₂O₃-coated NMC811 cathode still delivers a high initial capacity of 161 mAh g⁻¹ and a satisfactory capacity retention of 75.6% after 200 cycles at 0.5 C [339]. B₂O₃-coated LiNi_0.87Co_0.10Al_0.03O₂ with 2.0 wt.% B₂O₃ delivered a capacity of 184 mAh g⁻¹ with a capacity retention of 86% after 100 cycles at 0.2 C [340].

Among metal oxides, Al₂O₃ coating has been most investigated, because of its efficiency. In particular, it introduces new energy levels that prevent the reduction of Ni ions by O²⁻ [341,342,343] (see Figure 6). It can also suppress the Li-Ni cation mixing. It also maintains stabler cathode–electrolyte interface [344]. Wang et al. reported a surface-targeted healing (TH) strategy through atomic layer deposition of Al₂O₃ [345]. The modified NMC811 cathodes exhibited 78.6% capacity retention after 400 cycles at 1 C and 55 °C compared to 70.6% after 200 cycles for the pristine material). Cheng et al. coated polycrystals of LiNi_0.83Mn_0.05Co_0.12O₂ with Al₂O₃ by ALD and a subsequent annealing method according to which the sample was placed at a high temperature of 750 °C and annealed for 2 h. With this process, the primary grains remain intact, and the Al is enriched at the grain boundaries only. After 300 cycles at 4.3 V and at rate C/3, this cathode maintained 143.8 mAh g⁻¹ with 84.7% capacity retention, compared to 118.6 mAh g⁻¹ in case of gradual heating annealing [346]. This improvement was attributed to the rapid heating process, which minimizes Li/O loss and prevents the formation of a disordered phase. It is worth noting that a rapid heating process (by microwave technics in this case) has recently been used to synthesize a Ni-rich Co-free LiNi_0.9Fe_0.05Al_0.05O₂ with also improved electrochemical properties with respect to gradual heating annealing [347].

The crystallization of amorphous TiO₂ applied on NMC811 surfaces can result in Li₂TiO₃-coated NMC811, which can further inhibit phase transformation from H2 to H3 [348]. The capacity retention of Li₂TiO₃-coated NMC811 was raised at 90% (174.6 mAh g⁻¹) after 200 cycles at 1 C with the voltage 2.70–4.55 V, owing to the additional gradient Ti-doping below the surface layer [349]. Sulfated ZrO₂ coatings can incorporate sulfate groups forming –SO³⁻– to stabilize the interface of NMC particles with the electrolyte [350]. The synergetic effect of the ZrO₂ coating in combination with a zirconium doping enhanced the cycling stability of NMC811 [351]. Defect-rich strontium titanate (SrTiO_3–x) coating of NCA cathode exhibited a capacity of 170 mAh g⁻¹ after 200 cycles under 1 C with capacity retention of 81.1% [352]. Al₂(WO₃)₄ is a negative thermal expansion (NTE) material, having the unique properties of abnormal thermal expansion, and has been utilized to significantly suppress heat release and deformation of NMC811 [353]. When cycled at 1 C, NMC811 material modified with 7 wt.% Al₂(WO₃)₄ retained 162.0 and 171.5 mAh g⁻¹ after 100 cycles at 25 and 60 °C, respectively, about 13.8 and 25.4% higher than those of the pristine NMC811. Not only did Al₂(WO₄)₃ reduce the side reactions between the cathode and electrolyte, but it also enhanced the thermal stability of the pristine NMC811.

A drawback of these oxide-based materials utilized to coat the Ni-rich cathode materials their small ionic conductivity. It is thus desirable to investigate more conductive coatings [354,355]. In particular, MoO₃ is attractive, because its ionic conductivity is orders of magnitude larger, compared to Al₂O₃ or MgO. MoO₃-coated NMCs demonstrated the superior capacity retention of 79.8% under the high 5 C [356]. Benefitting from a high ionic and electronic conductivity of NiFe₂O₄, a NiFe₂O₄-coated NMC622 cathode achieved a capacity retention of 81.72% after 100 cycles (at 60 °C@1 C) and an excellent rate capability (109.86 vs. 49.52 mAh·g⁻¹ for the pristine material at 10 C) [357]. This coating was recently extended to NMC811 [358] (see Figure 7). The NiFe₂O₄-coated NMC811 exhibited a superior rate capability of 112.7 mA h g⁻¹ at 10 C, and high-temperature stability of 130 mA h g⁻¹ after 100 cycles at 60 °C. Li₂SiO₃ is also employed, as it is a fast ionic conductor [359]. TiNb₂O₇ coating was found beneficial to the electrochemical performance of LiNi_0.88Co_0.06Mn_0.06O₂, by significantly improving the capacity retention of modified material after 200 cycles at 1 C from 59.8% of the pristine material to 87.2% [360]. A 0.3% Nb₂O₅-coated NMC811 cathode exhibits superior rate capability (146.4 mAh g⁻¹@400 mA g⁻¹) and remarkable rate cyclic stability (188.5 mA h g⁻¹ after 100 cycles with capacity retention of 94.8% [361].

Coatings of TiO_x (TO) and Li_xTi_yO_z (LTO) deposited over NMC622 proved to be efficient to protect significantly the particles and suppress the crack formation during cycling [362]. The surficial Ti⁴⁺ doping induces the reduction of Ni³⁺ to Ni²⁺ at surface region, and the latter tends to migrate into Li-slabs, preforming a 5 nm-thick cation-mixing layer. This less-ordered layer makes the H2–H3 transition reversible [129]. This surficial Ti⁴⁺ substitution in LiNi_0.9Co_0.1O₂ cathode material for the optimized Ti-concentration of 5 mol% enhanced the capacity retention from 69.7% to 97.9%. Coating with metal phosphates is also of interest because they are thermodynamically stable at the operation window of Ni-rich cathode materials [363]. Moreover, the strong covalent bonds between PO₄³⁻ and the metal ions hinder the reactions of the Ni-rich material with the electrolyte. Examples include coating of NCA with Ni₃(PO₄)₂ [364] and NMC811 with Co₃(PO₄)₂ [365], FePO₄ [366,367], AlPO₄ [368], LaPO₄ [369], Li₃PO₄ [370,371,372,373], and MnPO₄ [374]. LiNi_0.83Co_0.12Mn_0.05O₂ was coated with CeP₂O₇ through a polyethylene glycol (PEG)-assisted water deposition method and obtained a cathode exhibiting a retention rate of 92.38% after 100 cycles under the conditions of 2 C and 4.3 V upper cut-off voltage [375]. In a study of the efficiency of 16 metal phosphate coatings on LiNi_0.91Co_0.06Mn_0.03O₂, Min et al. determined that Mn-, Co-, and Fe-phosphate materials improve the electrochemical properties; only TiPO₄ failed [376]. Peng et al. constructed LiNiO₂/Na_1–xNi_1–yPO₄ surface hybrid coating layer and Na bulk doping of NMC811 simultaneously [377]. DFT calculations showed that the improvement of the electrochemical properties was attributable to the removal of surface oxygen vacancy and residual lithium. Zeng et al. coated NMC811 with dihexadecyl phosphate (DHP). The outer long-chain alkyl layer inhibited the adverse chemical reaction with H₂O/CO₂ in ambient air and improved the structural stability, while the phosphate-based inner coating accelerates Li⁺ transport [378].

Huang et al. used alkyl phosphoric material with hydrophobic groups to coat LiNi_0.83Mn_0.11Co_0.06O₂ (N83) [379]. This coat effectively suppressed the surface residual lithium due to H⁺/Li⁺ ion exchange and enhanced the chemical stability of the cathode–air interface. In the case of LaPO₄, the LiNi_0.87Co_0.09Al_0.04O₂ sample modified with the optimum 2 wt.% LaPO₄ exhibited capacity retention rates of 96.0% and 85.1% after 100 cycles at 25 and 60 °C, respectively, against 87.1% and 74.2%, respectively, for the uncoated material. NMC622 was coated with Mn₃(PO₄)₂ [380]. In the case of LaPO₄, the LiNi_0.87Co_0.09Al_0.04O₂ sample modified with the optimum 2 wt.% LaPO₄ exhibited capacity retention rates of 96.0% and 85.1% after 100 cycles at 25 and 60 °C, respectively, against 87.1% and 74.2%, respectively for the uncoated material.

4.2. Li-Based Surface Layers

These coatings with metal oxides or phosphides are efficient to reduce the side reactions with the electrolyte. However, as they do not contain Li in their composition, they increase the charge transfer resistance. To remedy this problem, the Ni-rich particles were also coated with Li-containing layers, such as NMC333 [381], Li₂ZrO₃ [382,383,384], LiAlO₂ [385], Li₂SiO₃ [386], Li₂MnO₃ [387,388], LiNbO₃ [389,390,391,392], Li₂ZrO₃ [105], LiAlF₄ [393], Li₂SO₄ [394], LiBO [395], Li_0.5La₂Al_0.5O₄ [396], Li_xW_yO_z [397] including Li_xWO₃ [398], Li₂MoO₄ [399], and LiTaO₃ [81,400]. Sun et al. reported on a 2.5 nm-thick Li₂ZrO₃ layer coated onto LiNi_0.90Co_0.04Mn_0.03Al_0.03O₂ and demonstrated that the coating layer lowers activation energy and polarization potential, thereby facilitating the Li⁺ motion. At 1 C, the capacity retention rate of the cathode material coated with 1 wt.% of the layer after 100 cycles was 90.2%, while it was only 74.6% for the bare material [383]. The comparison of coated NMCs shows the following trend for coatings to increase in rate performance: TiO₂ < Al₂O₃ < Li₄Zr₃O₈ < LiAlO₂ < Li₄Ti₅O₁₂ < ZrO₂ [401]. The difference in the ranking between Li₄Zr₃O₈ and ZrO₂ comes from the reduced porosity of the Li₄Zr₃O₈ coating, and high porosity is required to obtain a high-rate capability. On the other hand, a high coverage of the surface enhances long-term cycling stability and decrease the crack generation, which is best obtained with Al-containing coats. That is why long-term cycling stability with high capacity has been obtained for the NMC coated with LiAlO₂ [401]. Tang et al. reported an effective etching-induced coating strategy to shield LiNi_0.8Co_0.1Mn_0.1O₂ electrode materials by LiAlO₂ [402]. The 2.2 wt.% LiAlO₂-coated NMC811 cathode delivers a high-rate capacity of 135.2 mAh g⁻¹ at 10 C and capacity retention of 85.8% after 200 cycles at 0.5 C. With an optimized LiAlO₂ coating amount of 3 wt.%, the capacity retention of Ni_0.9Co_0.07Mn_0.01Al_0.02O₂ increased from 70.7% to 88.3% after 100 cycles at 1 C. It also delivered a capacity of 162 mAh g⁻¹ at 5 C, while that of an uncoated sample is only 144 mAh g⁻¹ [385]. This result illustrates the efficiency of LiAlO₂ coating to improve the structural stability of Ni-rich cathode elements. Nevertheless, this coat is not sufficient to protect the electrode materials form HF etching, in contrast with fluoride coatings [403]. To circumvent this drawback, Wang et al. constructed a hybrid LiAlO₂/LiF-AlF₃ coating layer, combining the advantages of LiAlO₂ and fluorinated LiF-AlF₃. The hybrid layer modified NMC611 exhibited a discharge capacity of 166.8 mAh g⁻¹ with capacity retention of 74.5% at 5 C after 300 cycles [404]. Zhang et al. benchmarked Al₂O₃, ZrO₂, and LBO (Li₂O-2B₂O₃) coatings on NMC811, and found that LBO is the best to improve charge–discharge specific capacity, Coulomb efficiency, water absorption stability, cycle characteristics, and resistance stability of the NMC811 cathode material [405]. Jamil et al. coated NCA with Li₂O-BPO₄ and obtained a cathode exhibiting a capacity retention of 92.3% at 1 C in 2.7–4.3 V and 76.2% at high voltage in 2.7–4.7 V [406]. Wu et al. coated LiNi_0.82Co_0.15Mn_0.03O₂ (Ni82) with the fast-ion conductor Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO). This modified cathode delivered a discharge capacity from 168.3 to 121.2 mAh g⁻¹ with the retention rate of 72% after 80 charge/discharge cycle at the very high voltage of 4.7 V [407].

Lithium boron oxide (LiBO) coating of NCA increases the capacity retention to 93.59% after 100 cycles at 2 C [408]. Dot-like Nb₁₂WO₃₃ plus uniform LiBO coating of Li(Ni_0.90Co_0.06Mn_0.04)_0.995Al_0.005O₂ Nb₁₂WO₃₃ provided a rapid Li transmission channel, improving the rate capability with respect to the L-B-O coating alone, with a discharge capacity of 179.1 mAh g⁻¹ at 5 C [409]. The LiBO-coated LiNi_0.90Co_0.06Mn_0.04O₂ material showed a discharge capacity of 222.0 mAh g⁻¹ with 88.1% of capacity retention after 100 cycles at 1 C [395]. Qiao et al. synthesized Li₂SiO₃-coated NMC811 (NCM@LSO) by the precoating and solid-state lithiation methods. The capacity retention reached 97% after 500 cycles at 0.5 C, while that of bare NCM was limited to 79% after 450 cycles [386]. Xiao et al. synthesized LiNi_0.8Co_0.15Al_0.05O₂ using CoAl-layered double hydroxide (CoAl-LDH) nanosheet-coated Ni(OH)₂ as the precursor, improving significantly the properties of NCA, compared with NCA synthesized from nickel–cobalt–aluminum hydroxide and Al(OH)₃-coated nickel–cobalt hydroxide precursors. The improvement was attributed to the formation of a Li_1–x(Co_0.75Al_0.25)_1+xO₂ mesophase as the buffer layer, reducing the Li⁺/Ni²⁺ cation mixing. In addition, the presence of Co on the surface promoted the diffusion of Al during the lithiation–calcination process, thus avoiding the formation of undesired Al-related phases. This cathode delivered a discharge capacity of 194.5 mAh·g⁻¹ at 0.1 C and a capacity retention of 90.9% after 100 cycles at 0.2 C [410].

Lithium phosphates have also been utilized as coating materials, including LiFePO₄ to take advantage of the remarkable stability of this cathode element [411,412], Li₃PO₄ [413,414,415,416] (see Figure 8), LiH₂PO₄ [417], LiZr₂(PO₄)₃ [418], and Li_1.3Al_0.3Ti_1.7(PO₄)₃ [419]. Infusing Li₃PO₄ (LPO) in the grain boundaries of secondary particles to coat NMC particles is very promising, because it reinforces the microstructure by shielding off liquid electrolytes within secondary particles [413]. Due to the synergetic effects of Mg-doping and LiFePO₄ coating NMC811, Zhang et al. obtained a cathode that delivered an initial capacity of 181.5 mAh g⁻¹, with 90.8% capacity retention after 100 cycles at 0.2 C, demonstrating the increase of the structural stability due to the coating. The rate capability was also significantly increased, with a capacity still at 120.7 mAh g⁻¹ at 5 C [412]. A still higher capacity at 5 C was obtained with LiH₂PO₄-coated NCA (147.8 mAh g⁻¹). At 0.5 C, this LiH₂PO₄-coated NCA delivered a capacity of 180.2 mAh g⁻¹, decreasing to 172.6 mAh g⁻¹ after 100 cycles, which corresponds to a capacity retention of 95.8% [417].

Coating fluorides on Ni-rich cathode materials can reduce charge transfer resistance and thus improve rate capability, but also increase the cycle life. Directly coating LiF on the surface of Li_1.2Ni_0.2Mn_0.6O₂ stabilized the cathode over 280 cycles and maintained a capacity of 110 mAh g⁻¹ at 1 C [420]. Metal fluoride materials have better resistance to HF attack than metal oxides, and can thus be utilized to coat Ni-rich cathodes. In particular, AlF₃ is known for its beneficial effects in Li-ion batteries [421]. In NMC-based cells, however, AlF₃ coating has been used mainly in NMC333 [422]. On the other hand, NMC811 was coated with PrF₃ [422]. This cathode exhibited a capacity retention of 86.3% after 100 cycles at 1 C.

The primary purpose of the coating is to prevent cathode surfaces containing highly reactive Ni⁴⁺ from contacting the electrolyte and therefore to reduce side reactions. But the coating can also avoid excessive Li on cathode surfaces from causing gas evolution due to its possible transformation into lithium carbonate and hydroxide as well as further side reactions with electrolytes [423]. In particular, metal oxides can allow for excessive Li on the surface of NMC particles to transform into Li_xM_yO_z coatings at certain temperatures. V₂O₅ coating is a good example. Ti-doped V₂O₅ (Ti_0.05V_1.96O₅) on NMC811 effectively enhanced both the rate capability and the cycling stability [424]. DFT calculations demonstrated that the introduction of Ti into V₂O₅ introduced excessive free electrons, while reducing the energy barrier for Li⁺ diffusion. Li et al. proposed a hybrid coating strategy incorporating lithium ions conductor Li_xAlO₂ with superconductor Li_xTi₂O₄ to resolve the issue of excessive Li [425]. With this coating, NMC811 exhibited an initial discharge capacity of 227 mAh g⁻¹ at 4.4 V cutoff voltage with Coulombic efficiency of 87.3%, and reversible capacity of 200 mAh g⁻¹ with 98% capacity retention after 100 cycles at a current density of 0.5 C. Let us recall that the investigation of oxygen release in NMC cathodes has evidenced that stable cycling of NMC is possible up to 4.5 V only for Ni compositions ≤ 0.6, and is possible only up to 4.0 V for pristine NMC811, because of the phase transition H2 → H3 [34]. Therefore, the result of Li et al. suggests that the coating utilized in this work prevents or at least softens this transition.

Recently, Qu et al. developed a lithium boron carbon oxide (Li_xB_yC_1–yO_z) coating using the sol–gel method to recover the air-exposed NMC811 from degradation [426]. The authors demonstrated that this coating is a promising approach to recovering the degraded surface structure and electrochemical performance of NMC cathodes.

4.3. Organic Layers

Organic compounds have also been considered as promising coating materials. Conductive polymers can also be utilized successfully. In particular, Hwang et al. [427] synthesized a lithium-containing “BTJ-L” hybrid oligomer obtained through polymerization of bismaleimide by using a bismaleimide with a polyether monoamine, thiocyanic acid (TCA), and LiOH. NMC811 coated with this polymer demonstrated an improved capacity retention (86.1% vs. 72.9%) after 100 cycles at 1 C, attributed to the excellent wettability toward the electrolyte and the extra Li⁺ ions contributed by the hybrid BTJ-L oligomer additive. Coating with polypyrrole (PPy), another conductive polymer, is efficient to stabilize the Ni-rich particles [428] (see Figure 9). Dry-coating PPy on NCA nanofibers increased the capacity retention from 73.7% to 83.1% after 50 cycles at 0.5 C [429]. Another polymer, polymethyl methacrylate (PMMA) was also successfully coated on NMC8111 [430]. The electron transfer from the interfacial Ni²⁺ to the ester group of PMMA anchors the Ni cations and reduced interfacial side reactions. The NMC811 with the PMMA nanolayer the electrode exhibited a discharge capacity of 181.1 mAh g⁻¹ at 1 C rate, with capacity retention of 91.2% after 100 cycles. Gan et al. used polyvinylpyrrolidone (PVP) as an inductive agent to controllably coat a uniform conductive polyaniline (PANI) layer on NMC811 (NMC811@PANI–PVP) [431]. PANI-coated LiNi_0.9Co_0.085Mn_0.015O₂ also exhibited improved electrochemical properties compared to the pristine material [432]. He et al. coated LiNi_0.95Mn_0.05O₂ (NM95) with polyaniline–polyethylene glycol (PANI-PEG) to obtain a cathode maintaining capacity retention of 94.7% (1 C, 100th cycle) and 79.0% (5 C, 200th cycle) under cut-off voltage of 4.3 V [433]. Huang et al. adopted an effective Mo/PANI co-modification strategy on a LiNi_0.96Co_0.02Mn_0.02O₂ cathode material [434]. This cathode delivered a capacity of 209.3 mAh g⁻¹ with an upper cut-off voltage of 4.4 V at 1 C, and its capacity retention reached 95.7% after 100 cycles. Benefiting of the high conductivity of PANI, the rate capability was also significantly improved, with a capacity of 167.1 mAh g⁻¹ at 10 C. Not only does PANI have a good electronic conductivity, but it also prohibits direct contact of the electrode with the electrolyte, so that the NMC811@PANI–PVP cathode exhibited good cycle ability (88.7% after 100 cycles at 200 mA g⁻¹) and rate performance (152 mAh g⁻¹ at 1000 mA g⁻¹). Polyacrylonitrile (PAN) was employed by Huang et al. as a proof-of-concept organic coating with better electro-negativity on NMC81, because The C≡N functional groups in the PAN coating can adsorb O^α− (α < 2) and provide it with electrons for its reduction to stable O²⁻, thus inhibiting oxygen release. This modified NMC811 displayed a high-capacity retention of 86.3% after 200 cycles at 1 C, 75.1% after 350 cycles at 5 C, and 83.1% after 100 cycles at 1 C and 55 °C [435]. Li et al. coated NMC622 with a cross-linked epoxy natural rubber (CENR). The viscoelastic properties of CENR alleviated mechanical stresses during cycling. Moreover, the strong adhesion to the surface of Ni-rich cathodes suppressed detrimental interfacial reactions [436]. This coated NMC622 cathode material retained 95.9% of its capacity after 300 cycles at 0.5 C and 82.8% after 500 cycles at 1 C. LiNi_0.83Co_0.11Mn_0.06O₂ was coated with 7 nm-thick polysiloxane via in situ hydrolysis-polycondensation of tetraethyl orthosilicate. The corresponding cathode exhibited a capacity retention of 81.8% after 150 cycles at 1 C [437]. Lin et al. Coated NMC811 with poly (pyrrole-co-citral nitrile) (PPC). The discharge capacity of NMC811@PPC after 200 cycles at 1 C in the voltage range 3.0–4.5 V was 173.6 mAh g⁻¹, which is even higher than the initial discharge capacity (102.66%). This little rebound was ascribed to the opening and broadening of the lithium ion channels [438].

Yoon and Shin utilized a cyclopropenium cationic covalent organic framework (C-COF) as a coating material of NCM with 94% Ni content. In the C-COF skeleton, cation moieties attract PF₆⁻ anions in the Li salts of the electrolyte, suppressing the solvation between Li⁺ and PF₆⁻. This coating increased the diffusion coefficient of the lithium by a factor 3, and increased the capacity retention from 60.17 to 71.31% after 200 cycles at 1 C (see Figure 10) [439]. A summary of the effect of different coatings on the cycle ability of Ni-rich cathodes is reported in

Table 1. Effects of coating on cycling ability of Ni-rich cathode elements. Capacity densities are expressed in mAh g⁻¹. Upper cut-off in cycles is 4.5 V for materials, followed by the symbol *. Otherwise, cut-off is 4.3 V. DHP = dihexadecyl phosphate; AP = alkyl phosphoric material (two hydrophobic coats intended to protect the Ni-rich element against moisture). BTJ-L = hybrid oligomer, obtained through polymerization of bismaleimide with a polyether monoamine (i.e., Jeffamine-M1000, JA), trithiocyanuric acid (TCA), and LiOH. CENR = cross-linked epoxy natural rubber.

Cathode Element	Coating	Cycles @C Rate	Pristine		Coated		Ref.
			Capacity (mAh g−1)	Retention (%)	Capacity (mAh g−1)	Retention (%)
NMC811	g-C3N4	225@1 C	170	76.4	170	82.3	[316]
Sb-NCMA93	LiF	1000@1 C 4000@1 C	195	89.6	195	93.7 79.2	[318]
Zr-modified [Ni0.8Co0.1Mn0.1]CO3 *	Zr-rich	100@2 C	198	85.8	195.2	92.2	[320]
NMC622 *	ZrO2	100@0.5 C	190	52.4	187.6	82.5	[321]
LiNi0.85Co0.10Mn0.05O2	ZrO2	200@0.5 C	165	67.9	170	77.6	[322]
NMC532	ZrO2	-	155	61.6	160	77.4	[382]
NMC622 *	TiO2	50@1 C	175.1	78.1	178	88.7	[323]
LiNi0.9Co0.08Al0.02O2	TiO2	100@2 C	178	63.3	181	77.0	[324]
LiNi0.83Co0.11Mn0.06O2 *	CeO2	200@1 C	190	64.2	185	77.0	[326]
NMC811	Al2O3	200@1 C	180.7	82.7	170	95.42	[333]
NMC811 (at 50 °C)	ZnO	100@1 C	175	51.72	182	77.47	[334]
NMC811	Mg-Al oxides	100@0.5 C	200	65.3	176	83.3	[336]
NMC811	B2O3	200@0.5 C	147	58.4	121.7	75.6	[339]
LiNi0.87Co0.10Al0.03O2	B2O3	100@0.2 C	190	45	184	86	[340]
LiNi0.70Co0.15Mn0.15O2	Al2O3	130@C/2	152	60	155	90	[343]
NMC811	Al2O3	400@1 C	800 (mAh)	failed	780 (mAh)	78.6	[345]
LiNi0.83Mn0.05Co0.12O2	Al2O3	300@C/3	193	61.5	170	84.7%	[346]
NMC811	Ti-based	200@1 C	189	44	189	90.0	[349]
LiNi0.8Co0.15Al0.05O2	SrTiO3–x	200@1 C	205	72.1	210	81.1	[352]
NMC811	Al2(WO3)4	100@1 C	190.0	73.0	186.5	86.8	[353]
NMC811	MoO3	100 100	182.5	85.8	186.1 160	92.5 79.8	[356]
NMC622	NiFe2O4	200@C/2	152	67.8	145	81.25	[357]
NMC811	NiFe2O4	200@1 C	162	52.4	162	80.57	[358]
NMC622	Li2SiO3	100@1 C	199	76.4	196	85.5	[359]
NMC811	Li2SiO3	450	170	79	166	98.0	[386]
Li1.06(Ni0.88Co0.06Mn0.06)0.94O2	TiNb2O7	200@1 C	187	59.8	184	87.9	[360]
NMC811	Co3(PO4)2	100@1 C	145	81.3	165	89.5	[365]
NMC811 *	FePO4	400@C/5	195	63	197	86.0	[367]
NMC811 *	Li3PO4	100@1 C	196	75.4	192	88.4	[370]
LiNi0.83Co0.12Mn0.05O2	CeP2O7	100@1 C	179.2	82.8	176.6	92.3	[375]
NMC811	DHP	300@2 C	172	40.7	188	64	[378]
LiNi0.83Mn0.11Co0.06O2	AP	300	190	57.2	190	92.6	[379]
NMC622	Li3PO4	200@1 C	189.3	89.6	189.8	80.9	[380]
NMC622	LiZr2(PO4)3	100@C/5	125.6	65.9	136.2	86.2	[383]
LiNi0.90Co0.04Mn0.03Al0.03O2	Li2ZrO3	100@1 C	178.4	74.6	181.7	90.2	[384]
LiNi0.9Co0.07Mn0.01Al0.02O2	LiAlO2	100@1 C	190	70.7	190	88.3	[385]
NMC811	Li2MnO3	500@1 C	177	63.1	177	80.4	[387]
NMC532	LiNbO3	100@1 C	174.6	73	190	92	[390]
NMC811	LiNbO3	500@1 C	186	19	191	70	[391]
LiNi0.90Co0.05Mn0.05O2 *	LiNbO3	200@1 C	199.45	63.0	193.27	75.6	[392]
LiNi0.85Co0.10Mn0.05O2	Li2SO4	200@1 C	190	73.3	189	81.5	[394]
NMC811 (4.6 V)	LixWyOz	100@1 C	183	80	183	85	[397]
NMC811	LixWO3	100@C/2	180	48	182	93	[398]
NMC811	Li2MoO4	100@1 C	177.7	64.6	177.7	90.5	[399]
LiNi0.82Co0.12Mn0.06O2	LiTaO3	100@C/2	175	55.2	180	92.3	[400]
LiNi0.7Mn0.15Co0.15O2	LiAlO2	100@C/2	170	85	180	88	[401]
LiNi0.6Co0.2Mn0.2O2	LiAlO2/LiF-AlF3	300@5 C	160	4.6	166.8	74.5	[404]
LiNi0.87Co0.1Al0.03O2	Li2O-BPO4	100@1 C	200	75.7	200	92.3	[406]
LiNi0.8Co0.15Al0.05O2	Li-B-O	100@2 C	170.7	72	159.7	94.9	[408]
Li(Ni0.90Co0.06Mn0.04)0.995 Al0.005O2 (at 60 °C)	Nb12WO33 (dots) + Li-B-O	100@3C	217	76.91	227	84.7	[409]
LiNi0.8Co0.15Al0.05O2 *	LiFePO4	100@C/2	204	82.5	210	95	[411]
LiNi0.76Mn0.14Co0.10O2 *	Li3PO4	200@C/3	170	79	180	91.6	[413]
NMC811	Li3PO4	200@C/2	200	59.3	200	84.6	[414]
LiNi0.91Co0.06Mn0.03O2	Li3PO4	60@1 C	192.3	60	180	81.8	[415]
NMC811	Li3PO4	100@C/2	125	60	162	77	[416]
LiNi0.8Co0.15Al0.05O2	LiH2PO4	200@2 C	160.1	73.1	166.4	90.1	[417]
LiNi0.83Co0.06Mn0.06Al0.05O2	LATP	300@1 C	163.1	83.3	189.6	91.5	[419]
NMC811	PrF3	100@1 C	197.4	73.8	186	86.3	[422]
NMC811	Ti0.05V1.96O5	100@1 C	165.3	66.8	170.4	78.7	[424]
NMC811	BTJ-L	100@1 C	171	72.9	173	86.1	[427]
NMC811	PPy	100@1 C	175	84.5	175.3	90.7	[428]
NMC811	PMMA	200@1 C	177	80.1	181.1	91.2	[430]
NMC811	PAN	200@1 C	190	57.5	210	86.3	[435]
NMC811	PANI	100@1 C	198	80.1	200	88.7	[431]
LiNi0.95Mn0.05O2	PANI-PEG	100@1 C	203.7	33.3	219.4	94.7	[433]
NMC622	CENR	500@1 C	150	40.9	150	82.8	[436]
LiNi0.83Co0.11Mn0.06O2	polysiloxane	150@1 C	170	75.8	180	85.4	[437]
LiNi0.948Co0.046Mn0.006O2	COF	200	210	60.1	210	71.3	[439]

.

4.4. Dual Coating

Dual-coating can also be performed to obtain a synergetic effect. For example, Zhao et al. coated NMC with Al₂O₃ and AlPO₄ [440], using a dry coating strategy based on the interaction between NMC811 and Al(OH)₃ nanoparticles to obtain a dual LiAlO₂ and chemical-inert Al₂O₃ coating. They obtained the optimal electrochemical performance when the coating strikes a balance of ion-conducting LiAlO₂ and chemical-inert Al₂O₃ phases [441]. Bai et al. constructed a uniform LiAlO₂+Li₃PO₄ protective layer with gradient Al doping (LAP modification) of single-crystalline LiNi_0.90Co_0.05Mn_0.04Al_0.01O₂. This cathode exhibited a capacity retention of 74.4% at a high voltage of 4.5 V after 200 cycles at 1 C, while the capacity retention without this modification was only 63% after 100 cycles [442]. Similarly, Liu et al. combined La₂Li_0.5Al_0.5O₄ with gradient Al doping of NMC811 and obtained a cathode exhibiting a capacity retention of 90.9%, after 200 cycles at 1 C [443]. Mohsen et al. improved the structural stability and the rate capability of NCA by coating the particles with graphene and Y₂O₃ [444]. Poly (dimethyl diallyl ammonium chloride)/graphene oxide (PDDA/GO) dual-layer coating on the surface of single-crystal NMC811 exhibited a 77% capacity retention after 500 cycles at the current density of 50 mA g⁻¹, against 28% for pristine NMC811 [445].

Liu et al. reported LiNbO₃+Li₃BO₃ dual-coating of single-crystal Li[Ni_0.92Co_0.06Mn_0.02]O₂) (SC-Ni0.92). The all-solid-state battery with this cathode Li₆PS₅Cl solid electrolyte and Li-In anode exhibited a capacity retention rate of 88.4% for 100 cycles at a current density of 1.5 mA cm⁻² (1 C) and a discharge capacity of 150.1 mAh g⁻¹ at a high current density of 7.49 mA cm⁻² (5 C) [446]. This performance was attributed to the melting of Li₃BO₃ during hot sintering, which diffused into the crystalline LiNbO₃ coating to form a dense coating. Yang et al. reported LiF+Li₃BO₃ dual-coating of NMC811. This cathode achieved a capacity of 205.4 mAh·g⁻¹ within a voltage window of 2.8–4.5 V at 1 C. with a capacity retention of 70.6% after 500 cycles [447].

Yang et al. prepared Li_0.5La₂Al_0.5O₄ (LLAO)-coated LiNi_0.82Co_0.14Al_0.04O₂ [396]. In situ coating LLAO is formed from the reaction among La₂O₃, Li₂O, and Al₂O₃ at 800 °C. This reaction, however, will consume some Al atoms in the superficial phase of secondary particles. Therefore, stoichiometric Mn atoms are doped into the Al-deficient layer to form the LiNi_0.82Co_0.14Al_0.04–xMn_xO₂ (LNCAM), which becomes a transition layer suppressing the lattice mismatch on the interface. As a result, the prepared material includes three layers which are the amorphous Li_0.5La₂Al_0.5O₄ in situ coating layer, the LNCAM transition layer, and the pristine LiNi_0.82Co_0.14Al_0.04O₂ (LNCA) phase from the surface to the inside, respectively. This cathode exhibited a capacity retention of 96.2% after 100 cycles in the voltage range of 3.0–4.4 V at 1 C (13% higher than that of the pristine material), and 80.8% at 1 C after 300 cycles. This performance is due to the synergetic effect of the Mn-doping, which suppressed internal strain, and improved the structural stability of NCA, while the coating effectively restrained cracking of the secondary particles. In addition, LLAO is a good ionic conductor, which enhanced the lithium mobility.

5. Doping

Doping is another strategy to minimize side reactions on Ni-rich NMC particle surfaces. Cation doping is of interest if the ionic radius of the dopant is large, increasing distances between atomic layers in the lattice structure for easier lithiation/delithiation. This expansion not only increases the lithium diffusion, but it also improves the structural stability. Moreover, the higher ionic radii enable dopants to reside on cathode particles, preserving the particle surface and refining particle morphology to suppress crack formation [448]. Doping with metal elements M are employed to expand pathways for Li⁺ diffusion, but also to oxidize Ni²⁺ to Ni³⁺ when doped into the material owing to its strong oxidation performance, thus reducing Li⁺/Ni²⁺ mixing. In addition, the strong M–O bonds can maintain the layered structure at high delithiation state.

5.1. Experimental Results

In situ operando analyses now give a comprehensive understanding of elemental doping of Ni-rich materials [449,450]. Fe³⁺ and Sn⁴⁺ with ionic radii larger than substituted Co³⁺ expand Ni-rich NMC crystal lattices horizontally [451], while Ca²⁺ (with a radius larger than Ni²⁺) can expand lattices horizontally [452]. An extensive review on the results published on 46 dopant elements in NMCs through the years has been published by Ko et al. [453]. Hereunder, we focus attention on the best results obtained in recent years. Such doping elements include Mo⁶⁺ [454,455,456,457], W⁶⁺ [134,454,455,456,457,458,459,460,461,462,463], Te⁶⁺ [464], Ta⁵⁺ [465,466,467,468,469,470,471,472] Nb⁵⁺ [473,474,475,476,477], Ce⁴⁺ [478], Zr⁴⁺ [479,480,481,482], Ti⁴⁺ [483,484,485,486,487,488,489,490], Ge⁴⁺ [491], Hf⁴⁺ [492], Sn⁴⁺ [493], Ce⁴⁺ [494], Gd³⁺ [495], and Al³⁺ [2,86]. Al-doping alleviates anisotropic lattice changes and volume changes during cycling. It also maintains a wider LiO₆ inter-slab thickness without collapse at highly charged states, allowing Li-ions to be deintercalated/intercalated reversibly [496]. As a result, Al-doped LiNi_0.7Co_0.1Mn_0.2O₂ at a high cut-off voltage (4.4 V) delivered a capacity of 144.69 mAh g⁻¹, with a capacity retention of 80.26% after 90 cycles at 1 C [497]. Due to the Al-doping, LiNi_0.92Co_0.03Mn_0.03Al _0.02O₂ demonstrates a capacity retention of 92% after 100 cycles at 0.5 C rate, against 69% with undoped LiNi_0.94Co_0.03Mn_0.03O₂ [498]. An important parameter is the concentration of Al, 2% in this case, while the optimized concentration is 3% after Kim et al. [499]. They reported that Li[Ni_0.894Co_0.041Mn_0.034Al_0.031]O₂ cathode demonstrates excellent long-term cycling stability, retaining >90% of its initial capacity after 500 cycles, while the Li[Ni_0.91Co_0.04Mn_0.04Al_0.01]O₂ cathode loses 38.3% of its initial capacity after 500 cycles [499]. Positive effects of Al-doping have also been observed with cobalt-free cathodes, such as LiNi_0.90Mn_0.06Al_0.04O₂ [500], LiNi_0.8Mn_0.16Al_0.04O₂ [501], LiNiO₂ [502], Al-doped NCM [503], and LiNi_xMn_1–xO₂ [504], although the electrochemical performance remains below that of Co-containing Ni-rich cathodes, even with single crystallized particles [505]. Other doping elements of Ni-rich elements include Gd³⁺ [505], Cr³⁺ [506,507], Sc³⁺ [508], Y³⁺ dopant of both NMC [509] and NCA [510], La³⁺ [511], Mg²⁺ [191,512,513,514,515,516,517], Ca²⁺ [518], Sr²⁺ [519,520], Zn²⁺ [13,521], Ag⁺ [522], and Na⁺ [523,524,525].

All the transition metal doping elements we have mentioned form stronger bonds with oxygen and reduced energy loss, promote lithium migration, and reduce the Ni/Li mixing of NMCs. Nevertheless, among V, Nb, and Zr doping, V-doping is the most advantageous to improve the electrochemical properties of NMC811 by reducing significantly the lithium divacancy barrier [526]. This may not be true, however, for NCA cathodes, because this effect in NMC comes from the suppression of the lattice distortion, due to the radius of V⁵⁺ being close to the radius of Mn⁴⁺. Actually, 0.3 wt.% Nb-doped LiNi_0.94Co_0.02Al_0.04O₂ delivers 172 mAh g⁻¹ after 300 charge–discharge (voltage window: 2.8–4.3 V) cycles at 2 C (see Figure 11) [527]. Recently, Li et al. investigated the relationship between the diffusion depth and distribution behavior of foreign elements within the cathode material and the composition of the Li-rich cathode material. They determined the diffusion coefficients between Zr⁴⁺ and transition metal elements TMⁿ⁺ (TM = Ni, Co, Mn; n = 3, 4) [528]. As a result, these coefficients follow the trend: Zr⁴⁺/Mn⁴⁺ > Zr⁴⁺/Mn³⁺ > Zr⁴⁺/Co³⁺ > Zr⁴⁺/Ni³⁺, the Zr⁴⁺/Mn⁴⁺ diffusion couple having the smallest diffusion activation energy of only 0.43 eV. In particular, this work explains the reason for the formation of a Li₂ZrO₃ secondary phase coating layer on the surface of Ni-rich cathode material particles.

Al-doped LNO (LiNi_1–xAl_xO₂) with x = 0.02 exhibits a very good cycle ability, with a capacity of 145 mAh g⁻¹ after 400 cycles at 0.5 C [529]. The 1 mol% Cr-doped NMC622 exhibits a capacity of 100.6 mAh g⁻¹ after 200 cycles at 10 C, corresponding to a retention of 93.1%, in the potential range of 2.7–4.3 V. Extending the potential range to 2.7–4.5 V, the cathode still delivered 166.8 mAh g⁻¹ and keeps 75.0% of the initial value at the 200th cycle at 3 C and 50 °C [507]. At 5 C, the Cu-doped NCM622 exhibits a high discharge capacity of 140.8 mAh g⁻¹ with an increased capacity retention of 77% as compared to that of the bare NCM622 (62%) after 350 cycles [530]. At 1 C, the capacity retention of Ta-doped LiNi_0.88Co_0.09Al_0.03O₂ is 93% (171 mAhg⁻¹) after 100 cycles at 1 C, compared to 68.6% for the pristine material [465]. Park et al. conducted the study of the effects of Co³⁺, Al³⁺, Ti⁴⁺, Nb⁵⁺, Ta⁵⁺, W⁶⁺, and Mo⁶⁺ doping on the properties of LiNi_0.9Mn_0.1O₂ [456] and concluded that Mo-doping give the best results.

Non-metallic dopant elements include B³⁺ [531,532,533,534]. The introduction of high-bond-energy B-O covalent bonds can inhibit the change of 2p orbitals of oxygen atoms and the formation of particle microcracks during cycling. Sun et al. [531] studied the effect of B doping on the microstructure of NCA. The study found that B-doping can reduce the surface energy of the (003) crystal plane, promote the preferential growth of the (003) crystal plane, and change the microstructure. LiNi_0.878Co_0.097Al_0.015B_0.01O₂ cathode material can maintain 83% of its capacity retention after 1000 cycles at a discharge depth of 100%, while the normal undoped LiNi_0.885Co_0.1Al_0.015O₂ showed a capacity retention rate of 49%. LiNi_0.83Co_0.05Mn_0.12O₂ doped with 0.6 mol% B showed enhanced rate capability and excellent cycling stability. The retention after 500 cycles in the soft-pack full cell can reach 91.35%, and the capacity decay per cycle was only 0.0173% [533].

Among Li-site dopants (Na, K, Rb, Ca), Na gives the best results. The substitution of Na⁺ for Li⁺ at 3a sites enlarges the distance between layers, benefitting from the larger radius of Na⁺. Additionally, Na doping suppresses the cation mixing [535] and weakens Li–O bonds, thus enhancing the potential capacity. A Na-doped NCM622 cathode shows an initial discharge specific capacity of 175 mAh g⁻¹ and a high-capacity retention of 90.8% after 100 cycles at 0.2 C, compared to an initial capacity of 160 mAh g⁻¹ and retention of 67.5% for the pristine material [525]. Na⁺-doped Li_0.98Na_0.02Ni_0.6Co_0.05Mn_0.35O₂ exhibited a higher capacity retention rate (93.3% vs. 83.2%) after 100 cycles and a superior rate capacity (121 mAh g⁻¹ vs. 93 mAh g⁻¹) at 3 C current density compared to the pristine NMC under 4.5 V voltage [536]. On a general basis, Li-site doping ions with suitable ionic radii are beneficial to improve Li⁺ diffusion in the positive electrode materials, and thus improve the electrochemical properties. Nevertheless, the dopant must have an ionic radius that is not too big; otherwise, it can generate blocking effects and lattice distortion that are damageable to the cycle life. For example, among alkali metal elements, Na-doped NMC811 has significantly higher capacity and capacity retention than Rb- and K-doped NMC811 [537]. For the same reason, Na-doping has also positive effects in Li-rich, Mn-rich NMC [538].

The bore is a O-dopant. It raises the capacity retention of LiNi_0.95Co_0.04Al_0.01O₂ to 88% after 100 cycles at 0.5 C [539]. Anion doping is mostly conducted using halogens, such as F⁻ [540,541,542,543]. Other halogens doping elements include Br and Cl, which increased the capacity retention of NMC811 by 10%, due to the low-melting eutectic salt and the increase of the electrochemically active surface area [544].

Positive effects of Sn-doping have been demonstrated both in NMC622 [545,546] and more recently in NMC811 [547,548]. In particular, the capacity for the optimized 2% Sn-doping of NMC811 remains at 183.9 mAh g⁻¹ after 270 cycles at 0.5 C, corresponding to 93.7% of the initial discharge capacity. Sn-doped Li[Ni_0.90Co_0.05Mn_0.05]O₂ (Li[Ni_0.897Co_0.05Mn_0.05Sn_0.003]O₂) delivers a discharge capacity of 224.3 mAh g⁻¹ and exhibits a capacity retention of 92.9% after 100 cycles at 4.3 V and 82.9% at 4.4 V [451]. Like doping with other elements, Sn-doping stabilizes the structure, but in addition, it modifies the structure, as Sn-doped material has radially oriented thin primary particles, which is not the case for the undoped material, and this radial orientation is known to increase the electrochemical properties [204,451].

Mg²⁺ acts differently. It possesses a similar radius to Li⁺ and will enter the Li-layer rather than the transition metal layer on the surface region of Li-rich NMC particles [549]. The larger valence with respect to Li⁺ is responsible for a repulsion of Mg²⁺ to transition metal ions, allowing for the suppression of cation mixing, and delaying the surface transformation to a spinel structure. As for anionic doping, fluorine is most popular. Not only does it expand both lattice parameters by reducing transition metal ions (mainly Ni and Mn), but it also increases mean charging voltages due to increased bond energy of Li–F as compared with Li–O. In particular, 0.5% Mg-doping of LiNi_0.9Co_0.05Mn_0.05O₂ reduces the capacity loss at 1 C rate from 3.5% to 1.7% after 100 cycles [99].

Kim et al. compared the electrochemical performance of Li[Ni_0.92–xCo_0.04Mn_0.04Al_x]O₂ cathodes for different Al concentrations x. Primary particle elongation and radial alignment increase with increasing x, which dissipates strain resulting from the abrupt contraction of unit cells during the H2 → H3 transition. The best results were obtained for an Al content of 3 mol%. The Li[Ni_0.894Co_0.041Mn_0.034Al_0.031]O₂ cathode demonstrated a capacity retention above 90% after 500 cycles, and an energy density of 740 Wh kg⁻¹ [499]. For NCA, the optimum Al concentration is 5.6%. At C/3 rate, the capacity retention of LiNi_0.85Co_0.094Al_0.056O₂ is almost 100% after 100 cycles, [550].

The cathode obtained by incorporating 2 mol% Zn²⁺ into LiNi_0.94Co_0.06O₂ delivers an initial discharge capacity of 200 mA h g⁻¹ at a C/2 rate, and retains 74% of the initial capacity after 500 cycles in a full cell assembled with a graphite anode (against 62% before Zn doping) [13]. Yi et al. synthesized a LiNiO₂ co-doped with 1 mol% Mg and 2 mol% B [551]. This cathode in pouch full cell configurations with graphite anode exhibited a high initial 1 C discharge capacity of 210 mAh g⁻¹ with a 20% capacity retention improvement over 500 cycles when benchmarked against pristineLiNiO₂. The result was attributed to the stabilization of the CEI reducing TM dissolution.

All the dopants serve as pillars for enhancing the structural stability. In addition, high-valence dopants Nb⁵⁺, Ta⁵⁺, Sb⁵⁺, Mo⁶⁺, and W⁶⁺ modify the microstructure through lowering the energy of the (003) surface, which dramatically dissipates the strain arising from H2 → H3 phase transition [459,552]. The change in morphology, crystal structure, and electrochemical properties associated with such doping is illustrated in Figure 12 for the particular case of liNiO_2. They are thus the subject of increasing interest, and their application as modifiers in Ni-rich cathodes [553]. W-doping showed remarkable results. At 1 C, the 1.0-mol% W-doped Li[Ni_0.90Co_0.05Mn_0.05]O₂ cathode in a pouch-type full cell using a graphite anode retained 89% of its initial capacity after 500 cycles [134]. Kim et al. constructed a W-doped Li[Ni_0.8Co_0.15Mn_0.05]O₂ cathode that showed nearly no drop in specific capacity even after 1000 cycles at full depth of discharge at 1 C [554]. Kim et al. obtained W-doped Li[Ni_0.95Co_0.04Al_0.01]O₂ cathode with columnar grains by introducing WS₂. [460]. The study of W-doped LiNi_0.88Co_0.09Mn_0.03O₂ also demonstrated that the primary particles of the cathode become finer with the increase of W-doping amount [555]. As a result, at a high rate of 10 C, this W-modified cathode exhibited capacity retentions of 94.68% and 89.63% at 25 and 45 °C after 100 cycles, respectively. At full depth of discharge, Li[Ni_0.90Co_0.09Ta_0.01]O₂ exhibits 90% capacity retention after 2000 cycles, when Li[Ni_0.90Co_0.09Al_0.01]O₂ cathode loses ~40% of its initial capacity within 500 cycles [466]. Another example of high-valence doping is provided by Nb-doped Ni-rich cathode that consists of the elongated and radially aligned primary particles with increased oxygen stable (003) planes [556]. In this work, Nb-doped LiNi_0.9Co_0.1O₂ cathodes in a pouch-type full cell exhibited a capacity retention of 91.9% at 1 C after 500 cycles and 80.5% at 5 C after 2000 cycles within 3.0–4.2 V. The Nb-doping is efficient to stabilize the structure, but coating may be desirable to increase the conductivity at the surface of the particles. For example, a dual-modification strategy of Nb doping and Li₄SiO₄ coating combined the two effects to obtain a NCA cathode exhibiting a specific capacity of 199.2 mA h g⁻¹ at 1 C with a capacity retention of 93.3% after 100 cycles due to the doping, but it also delivered a high capacity of 160.0 mAh g⁻¹ at 10 C, much higher than the pristine NCA (138.2 mAh g⁻¹), due to the coating [557]. Note, however, that a simple dual-site Nb-doped NMC811 cathode delivered 202.8 mAh·g⁻¹ with a capacity retention of 81% after 200 cycles at 2 C, and delivered a capacity of 176 mAh·g⁻¹ at 10 C rate [558]. This result demonstrates that Nb doping is capable of effectively stabilizing the structure at a high voltage, suppressing the Li/Ni disordering, and improving the electrochemical performance of the NCM cathode. Zhang et al. demonstrated recently that the introduction of Co at a doping level (1.5%) into LiNi_0.9Mn_0.1O₂ is sufficient to obtain a cathode exhibiting a discharge specific capacity of 167.1 mAh g⁻¹ after 100 cycles at 1 C and 121.2 mAh g⁻¹ after 200 cycles at 5 C, respectively [559].

Machine learning techniques can be applied to efficiently and accurately screen appropriate dopants [560]. Recently, Kim et al. developed a machine-learning-based model for Ni-rich cathodes. They implemented 33 elements as potential dopants, which led to the identification of 101 Co-free compounds demonstrating their superior performance. Among them, the top ten Li_0.85D′_xD″_(0.15–x)O₂ compounds are obtained. Only one of them contains Mn: LiNi_0.85Ti_0.05Mn_0.1O₂, and one of them has only one dopant: LiNi_0.85B_0.15O₂. The other ones are co-doped, with B, Cu, Al, Cr, Ti, and V as D′ and D″ dopants. Their calculated energy density is in the range 1160–1204 mWh g⁻¹, calculated capacity in the range 277–287 mAh g⁻¹, with a calculated volume change during cycling ≤ 2.6% [561]. For comparison with cathodes that are currently investigated, NMC811 has a theoretical capacity of 200 mAh g⁻¹ [562], average voltage 3.5 V when doped with Fe [563], and volume change of 7.8% ± 1.5% [564]; NCA80 (LiNi_0.80Co_xAl_yO₂) has a theoretical capacity of 279 mAh g⁻¹, average voltage 3.9 V at x = 0.15 and y = 0.05 [565], and volume change of 4.2% at x = 0.16 and y = 0.04 [566]. This result illustrates how useful artificial intelligence (AI) is for helping to determine next-generation cathode candidates. A comparative study of the effects of doping on the cycling ability of Ni-rich cathode materials is summarized in

Table 2. Effects of single doping on cycling ability of Ni-rich cathode elements. Capacity densities are expressed in mAh g⁻¹. Asterisk symbol after cathode element indicates that upper voltage cut-off for cycles is 4.5 V. Otherwise, unless specified, cut-off is 4.3 V.

Cathode	Dopant	Cycles at C Rate	Pristine		Doped		Ref.
			Capacity (mAh g−1)	Retention (%)	Capacity (mAh g−1)	Retention (%)
NMC811	Mo	100@C/3	150	78	180	81.6	[454]
LiNi0.84Co0.11Mn0.05O2	Mo	80@C/10 80@C/2	195 191	46.5 -	205 191	- 87.2	[455]
NCMA94	Mo	100@C/2	238.3	87	236.8	93.1	[457]
LiNiO2	W	100@C/2	230	83.7	228	90.3	[458]
NCA95	W	100@C/2	220	81	230	90.3	[460]
LiNi0.83Co0.11Mn0.06O2	W	500@2 C	140	53.5	165	69.9	[461]
NMC811	W	200@1 C	170	81.3	170	92.3	[463]
LiNi0.88Co0.09Mn0.03O2	W	100@1 C	192	62.21	188	93.51	[555]
LiNi0.9Mn0.1O2	W	200@1 C	170	77.1	170	95.3	[463]
LiNi0.88Co0.09Al0.03O2	Ta	100@1 C	208	68	198	93	[465]
NMC622 *	Ta	100@1 C	143.4	80.1	148.1	83.6	[467]
Li[Ni0.91Co0.09]O2	Ti Ta Mo	100@C/2	227–230	78.8	227–230	94 97 94.9	[468]
LiNi0.865Co0.095Al0.04O2	Ta	200@1 C	180	97.87	188	79	[469]
LiNi0.83Co0.11Mn0.06O2	Ta	200@1 C	91.4	50	160.3	97.5	[472]
NMC8111	Nb	300@1 C	184.9	79.8	180.2	96.9	[473]
NMC8111	Nb	100@1 C	181.5	67.6	181.6	94.55	[474]
LiNi0.83Co0.11Mn0.06O2	Nb	200@1 C	181	61.2	181	86.6	[477]
LiNi0.94Co0.02Al0.04O2	Nb	300@2 C	192	67.4	194	88.7	[527]
NMC811 *	Ce	100@5 C	129.6	70.7	161	86.9	[478]
LiNi0.90Co0.07Al0.03O2 *	Zr	100@C/2	205.9	62.02	191.7	84.86	[480]
LiNi0.9Al0.1O2	Zr	100@1 C	105	81.04	177.5	92.45	[481]
LiNi0.83Co0.12Mn0.05O2	Zr	100@5 C	178	86.5	180	97.6	[482]
NMC811	Ti	50@C/2	180	91.1	180	96.7	[484]
NMC811	Ti	100@C/10	116.5	64.1	180.6	96.9	[486]
LiNi0.83Co0.11Mn0.06O2	Ti	100@2 C	168.5	76.3	164	95.4	[488]
LiNi0.88Co0.06Mn0.06O2	Ti	100@1 C	181.5	78.9	193.8	93.6	[489]
LiNi0.90Co0.05Mn0.05O2	Ti	150@1 C	195	72.6	170	94.4	[490]
LiNi0.9Co0.08Al0.02O2	Hf	100@1 C	200	82.0	200	95.3	[492]
NMC622 *	Ce	100@1 C	167.2	63.63	188.7	74.66	[494]
LiNi0.88Co0.09Al0.03O2	Gd	100@1 C	151	77	176	89.3	[495]
NMC90	Sn	100@C/2	230	87.1	230	92.9	[451]
NMC811	Sn	270@C/2	192	61.6	190.3	96.6	[547]
LiNi0.90Co0.04Mn0.03Al0.03O2	Sn	200@1 C	185	67	185	83	[493]
LiNi0.80Co0.15Mn0.05O2	Al	100@C/10	214.1	92.4	208.1	97.2	[496]
NMC811 (4.4 V)	Al	90@1 C	180	70	180	80	[497]
LiNi0.92Co0.03Mn0.03Al0.02O2	Al	100@C/2	210	69	190	92	[498]
Li[Ni0.92Co0.04Mn0.04]O2 (4.2 V)	Al	1000@1 C	193	53.4	186	88.3	[499]
LiNi0.90Mn0.06Al0.04O2 LiNi0.90Mn0.06Co0.04O2	Al	200@1 C 200@1 C	- 170	- 78.3	165 -	82.9% -	[500]
LiNi0.8Mn0.16Al0.04O2	Al	200@1 C	161	69.4	178	82.2	[501]
LiNi0.6Mn0.4O2 *	Al	100@1 C	136.8	70.4	179	87.6	[504]
LiNiO2	Al	600@C/2	187	66.8	175	81.7	[529]
LiNi0.6Co0.2Mn0.2O2	Cr	200@1 C	137.2	84.9	149.8	91.0	[507]
NMC811	Sc	150@1 C	185	77.9	185	81.7	[508]
LiNi0.925Co0.03Mn0.045O2	Y	150@1 C	197.7	65.39	196	82.7	[509]
LiNi0.88Co0.09Al0.03O2	Y	100@1 C	190.8	60.1	192.7	88.9	[510]
LiNi0.9Co0.1O2	La	300@C/2	205	62.2	209	75.2	[511]
LiNi0.88Co0.05Mn0.07O2	Sr	100@1 C	191.4	88.48	189.1	96.61	[520]
LiNi0.83Co0.12Mn0.05O2 *	Mg	200@1 C	201.8	74.0	199.7	87.2	[512]
LiNi0.6Co0.2Mn0.2O2	Mg	100@1 C	162.6	79.33	180	90.02	[549]
LiNi0.9Co0.05Mn0.05O2	Zn	80@C/2	190	69.8	190	91.7	[521]
NMC622	Na	100@C/5	160	67.5	175	90.8	[525]
LiNi0.6Co0.05Mn0.35O2 (4.45 V)	Na	150@C/2	160	80.23	160	84.4	[536]
NMC811	B	100@C/2	178	93	180	97	[532]
LiNi0.83Co0.05Mn0.12O2	B	500@C/2	1.73 Ah	66.95	1.78 Ah	91.35	[533]
NMC811	B	120@C/2	170	53	170	87	[534]
NMC811	F	100@C/2	170	58	175	93	[540]
LiNi0.9Co0.05Mn0.05O2	F	100@2 C	176.7	85.3	181.1	95.5	[541]
LiNi0.8Co0.15Al0.05O2	F	100@2 C	183	77.6	157.8	98.3	[542]
LiNi0.82Mn0.18O2 *	F	100@C/2	170	47	180	70	[543]
NMC811 *	Cl Br	100@C/2 100@C/2	197 197	81.8 81.8	202 197	96.1 95.3	[544]

.

5.2. Theoretical Calculations

Density functional theory calculations provide critical insights into Ni-rich cathode materials at the atomic and electronic levels, aiding in their optimization and performance enhancement. They enable the prediction of phase transitions and structural changes during battery operation. By analyzing lattice parameters and atomic interactions, DFT helps identify factors contributing to phase segregation and degradation, such as the migration of Ni ions into the Li layer. DFT is used to calculate the band structure and density of states (DOS), revealing how Ni concentration affects charge transport. Additionally, Li-ion diffusion barriers can be computed to optimize ionic conductivity and rate capability. These insights help fine-tune material compositions for better conductivity and fast-charging capabilities. By evaluating lithium chemical potential, DFT aids in understanding the stability of Ni-rich cathodes and their decomposition tendencies. This information is vital for designing materials with higher capacity retention and minimal voltage fade. DFT calculations allow for the investigation of oxygen evolution. By understanding these degradation pathways, researchers can develop strategies to mitigate these effects and enhance cathode longevity. DFT calculations determine vacancy formation energies and their impact on charge compensation mechanisms. By analyzing these defects, researchers can fine-tune material stoichiometry to achieve optimal electrochemical properties. That is why DFT calculations are very often included in the publications to support the experimental results and their interpretation. Another example of the contribution of DFT is the calculation on LiNi_0.9Co_0.1O₂ providing insights into the performance-limiting factors of this cathode element, emphasizing the need for a higher concentration of cobalt doping to effectively stabilize the Ni-rich cathode [567].

In the context of doping and coating strategies, DFT plays a crucial role in screening dopants that can stabilize the Ni-rich structure and reduce cation disordering. It also models interactions with protective coatings, helping design surface modifications that enhance stability and minimize electrolyte decomposition. For example, DFT calculations demonstrate that Mg²⁺ doping not only inhibits the H2–H3 phase, but also improves the number of free electrons near the fermi level from 15.66 for the pristine LiNi_0.80Co_0.05Mn_0.15O₂ (FNCM) to 17.30 for Mg-doped FNCM [568]. Bader atomic charge calculation found that Ga-doping brings an additional charge of 0.1864 to the oxygen atom in LiNiO₂ [569]. DFT calculations also gave insight on the effects of oxidized dopants with more electrons near the fermi level, which reduces charge transfer resistance and improves electron conductivity [570,571,572] (see Figure 13). DFT is also useful in determining the chemical stability of the coatings. For example, it shows that the coating with phosphates LiMPO₄ (M = Ni, Co, Mg) has improved stability compared with sulfates, which is confirmed by experiments [573]. The calculation of binding energy between the Li-rich material and the coating layer also gives insight into the structural stability and improvement of the lithium diffusion [574,575,576,577]. Finally, DFT calculations facilitate the exploration and parameter optimization of new materials, and the coupling with machine learning will be of great value in selecting the most appropriate coating materials and dopants for Ni-rich cathodes [578]. A first example of this coupling has been recently reported [579].

5.3. Morphology Aspects

Doping has beneficial effects on the morphology of the particles. For example, we have already mentioned that Nb⁵⁺ doping significantly improves the electrochemical properties of Ni-rich materials. In addition, Nb⁵⁺ doping of LiNi_0.88Co_0.05Mn_0.07O₂ modifies the particle morphology, yielding radially distributed, elongated, rod-like structures. This cathode delivered an initial capacity of 200.3 mAh g⁻¹ at 1 C, within a voltage range of 2.7–4.5 V, with a capacity retention of 92.9% after 100 cycles [580]. This rod-like morphology effectively mitigates the anisotropic volume changes during cycling and thus increases the cycle ability. Li et al. have shown that the one-dimensional morphology of the primary particles provides shorter electron/ion diffusion distances and high tolerance to corresponding stress changes, significantly improving the electrochemical properties [223]. Other dopants preserve the rod-like structure. In particular, excess Al dopant effectively preserves the needle-like morphology of compositionally partitioned precursors during lithiation [581], and segregates at grain boundaries, inhibiting the growth of primary particles [582]. This segregation of high-oxidation-state dopants at grain boundaries is commonly observed because their ionic radii limit their solubility [583]. The amount of dopant also modifies the size and morphology of primary particles. B-dopant is an example. It lowers the surface energy of the (003) planes, thereby producing a strong crystallographic texture in B-doped cathodes [531]. In addition, as the bore dopant concentration increases, the primary particles elongate along the radial direction [584]. All these results show that the orientation of primary particles has been extensively controlled using high-valence dopants during calcination. Concentration gradients also favor the rod-like structure, as will be seen in the section devoted to them. Ta-doping increases the number of small primary particles and increases the degree of heterogeneity of particle orientation. This effectively mitigates the local strain accumulation during cycling due to the random orientation of primary particles [472].

In addition, the role of the hydroxide precursor is also critical in synthesizing radially aligned Ni-rich cathodes. Through stepwise control of ammonia concentration and pH during precipitation, elongated primary particles with appropriate size along [001] are promoted, facilitating the formation of radially aligned particles in the precursor. This approach enabled Wang et al. to synthesize LiNi_0.94Co_0.02Mn_0.04O₂ cathodes with exceptional radially aligned microstructure [585]. This cathode material delivered a capacity of 230 mAh g⁻¹ at 0.05 C and 186 mAh g⁻¹ at 5 C, with capacity retention of 95.6% after 100 cycles at 1 C and 25 °C in half cell and 93.2% after 1000 cycles at 1 C and 30 °C in full cell. This is an illustration of the morphological relationship of cathode materials to their electrochemical performance, which has been reviewed by Meng et al. [586].

Other morphologies are also of interest. Wu et al. successfully synthesized NMC811 cathode material, arranging the hexagonal nanosheets with exposed {104} facets by a co-precipitation process and a high-temperature lithiation reaction. The tightly adhered nanosheets on the (001) surface help to alleviate stress changes caused by the anisotropic structure and inhibit the fragmentation of secondary particles [587]. This cathode exhibited a discharge capacity of 203.8 mAh·g⁻¹ at 0.1 C and stable cycling performance (89.3% capacity retention after 100 cycles at 1 C, 55.3% after 300 cycles at 5 C, and 59.6% after 300 cycles at 10 C).

5.4. Synergetic Effects of Co-Doping and-or Multiple Coating

Synergetic effects can be obtained by combing multiple dopants and/or multiple coatings [588,589].

5.4.1. Dual Doping

For example, as the Mg-doping is beneficial for stabilizing the structure, while Zr-doping improves the kinetics, the electrochemical properties of NMC811 are enhanced by Mg+Zr co-doping with the discharge capacity 199.7 mAh g⁻¹ at cut-off potential of 4.7 V, along with a capacity retention of 74.4% after 100 cycles [590]. W+Mg co-doping improves the electrochemical performance, because of the reduced particle size induced by the W⁶⁺ and increased free electron induced by the Mg²⁺ [591] (see Figure 14). This co-doping of LiNi_0.9Co_0.06Mn_0.04O₂ single-crystal raised the capacity retention to 86.7% after 150 cycles at 2 C current density. K+F co-doping of single-crystalline LiNi_0.85Co_0.05Mn_0.10O₂ (NCM85) endowed this cathode with a very high reversible capacity of 222.3 mAh g⁻¹.

A superior capacity retention of 91.3% was obtained after 500 times at 1 C in pouch-type full cells, and a prediction value of 75.3% is given after cycling for 5000 h [592]. K+Ti-co-doped NMC811 delivered a capacity of 160.42 mAh g⁻¹ and 91.19% capacity retention at 1 C after 200 cycles between 2.75 and 4.35 V due to the synergistic effect of K doping and Li₂TiO₃ coating associated with the introduction of Ti [593].

K doping enlarges the interlayer space to accelerate Li⁺ transport, while Ti doping can strengthen Ni–O bonds to improve the structure stability. The Li₂TiO₃ coating protected the surface of the particles against side reactions with the electrolyte. Al+Fe co-doping into NMC622 combines the polarization effects of Fe with the lower chemical potential attributed to Al [545]. However, the performance was not as good as in NMC doped with Sn, studied in the same work. He et al. synthesized single-crystal LiNi_0.92Co_0.04Mn_0.04O₂ with cation Mo⁶⁺ and anion F⁻ elements co-doped in the near-surface region. For this purpose, nano-MoO₃ was additionally added at the stage of mixing precursor of the NMC synthesis. Then, NMC was powder was blended with NH₄F via ball-milling [594]. Al+Na co-doping combines Al-doping that forms strong Al-O bonds and reduces the Li⁺/Ni²⁺ mixing, and Na-doping that widens the ion diffusion channels. As a result, the full concentration gradient Li_1–yNa_yNi_0.80Co_0.05Mn_0.15–xAl_xO₂ co-doped with 0.1 mol% Al and 1 mol% Na exhibited a capacity retention of 93% after 200 cycles at a rate of 1 C [595]. Mo⁶⁺ and F⁻ constructed an appropriate Li⁺/Ni²⁺ antisite defects structure at the particle surface, boosting the interfacial stability and reducing the lattice mismatch. In addition, this surface modification maintained the low-defect Li⁺ layered channel inside the bulk simultaneously. The corresponding cathode delivered a high capacity of 204 mAh g⁻¹ at 1 C. A full cell constructed with this cathode element exhibited a capacity retention of 91.9% after 400 cycles in 2.8–4.4 V, against 76.9% for the pristine cathode. Na+F co-doping of NMC622 also significantly improved the electrochemical properties [596]. Nb+B co-doping of NMC811 exhibits strong radial orientation and exposes a large number of active (003) lattice plane on the particle surface. As a cathode, this modified NMC811 maintained a capacity retention of 86.71% after 200 cycles at 1 C [597]. Li et al. synthesized a Ce+Gd dopant-concentrated layer single-crystal LiNi_0.83Co_0.07Mn_0.10O_2, including a Gd doped LiCeO₂ (LCGO) shell and Ce/Gd dopant-concentrated layer [598]. The corresponding cathode capacity retention of 90.2% after 100 cycles at 1.0 C. In a different approach, Park and Kim introduced B and Ca in the cathode–electrolyte interface, where the B-functional group decreases the residual Li⁺ species, while Ca suppresses the microcracks [599]. With this CEI, the capacity retention after 100 cycles at 0.5 C was improved from 44.3% to 72.4%. Recently, Usman et al. synthesized V+Zn-co-doped LiNiO₂ and found that LiNi_0.52Zn_0.16V_0.32O₂ has a discharge capacity of 48–246 mAhg⁻¹ with an intercalation voltage of 5.77–3.35 V [600]. The Ti+Al co-substituted LiNi_0.92Co_0.08O₂ cycled in a voltage range of 2.75−4.3 V at a rate of 0.2 C, delivered an initial capacity of 197.7 mAh g⁻¹ with a capacity retention of 82.6% after 100 cycles [601]. Ti+F-co-doped NMC811 delivered a discharge capacity of 202.2 mA h g⁻¹ at 1 C and 45 °C, with a capacity retention of 88.1% after 200 cycles (only 45.2% for the pristine NMC811). This performance was attributed not only to the bulk doping effect, but also the fact that Ti⁴⁺ and F⁻ co-dopants induce the formation of an ultra-thin rock-salt phase on the cathode surface, which provides a protective layer [602]. Such a surface rock-salt nanolayer was also successfully developed on the LiNi_0.9Co_0.1O₂ subsurface via heteroatom anchoring utilizing high-valence element molybdenum modification [603]. With this modification, the cathode exhibited a capacity of 245.4 mAh g⁻¹ at 0.1 C, remarkable rate performance of 169.3 mAh g⁻¹ at 10 C at 4.5 V, and a high capacity retention of 70.5% after 1000 cycles in full cells at a high cut-off voltage of 4.4 V. Fe+F-co-doped Ni-rich, Li-rich cathode elements have improved electrochemical properties, because the Fe dopant increases both the electronic and ionic conduction, while the F dopant can improve the cycling stability remarkably by lowering the O_2p band top to suppress the lattice oxygen escape [604]. In addition, F doping within the oxygen sites with a larger ionic radius increases the distance between the (003) planes, and thus increases the lithium diffusivity. Moreover, the bonding energy between transition metals (TMs) and F (TM-F) is larger than that of TM-O, so that F can fix Ni²⁺ at the Ni site, thereby reducing the cation mixing [543]. In+Sn-co-doped LiNi_0.85Mn_0.09Al_0.06O₂ exhibited a capacity retention of ~100% and ~90% within the voltage range of 2.7–4.5 V at 30 °C and 45 °C, respectively, after 100 cycles at 0.4 C [605]. This result is due not only to the bulk doping, but also to in situ formed coating of LiInO₂ effectively protecting the cathode. Na⁺ inserted into the Li site expands the Li slab, due to its large ionic radius, while Al³⁺ stabilizes the structure through a high Al-O bonding energy. Combing these two advantages, Na+Al-co-doped LiNi_0.88Co_0.08Mn_0.04O₂ cathode material exhibited a capacity retention of 84% after 50 cycles at 1 C [606], higher compared with the pristine material doped with only Al or only Na. Peng et al. synthesized NMC811 modified by Mg+Ti not only in the bulk, but also by the formation of an Mg₂TiO₄ and Mg_0.5–xTi_2–y(PO₄)₃ outer layer with Mg and Ti vacancies [607]. The corresponding cathode cycled in the voltage range 2.7–4.5 V at 1 C rate exhibited a capacity retention of 89.3% after 200 cycles. Zhang et al. evaluated concentration-gradient Mg+Al-co-doped LiNi_0.95Co_0.03Al_0.01Mg_0.01O₂. As a cathode, this material exhibited a high-rate capacity (172.9 mAh g⁻¹ at 10 C rate), enhanced cyclability (95.6%, 100 cycles at 1 C) and less microcrack formation [608]. Ba+Al co-doping of LiNi_0.85Co_0.05Mn_0.10O₂ (NCM85) single-crystalline cathode increased the capacity up to 87.5% after 500 cycles at 1 C in a pouch-type full cell [609]. The strong Zr-O bond facilitates the lithium diffusion, while Ga doping can promote cation order and strengthen the TM-O bond energy, diminishing the oxygen loss. As a result, Zr+Ga-co-doped LiNi_0.94Co_0.03Mn_0.03O₂ exhibited a capacity retention of 91.9% at 0.5 C after 100 cycles, compared to 72.64% for the pristine material [610]. The optimal amount (0.5 mol.%) of each Na⁺ and Y³⁺ dopants increased the capacity retention of NCA to 92.34% of its capacity after 100 charge and discharge cycles at a rate of 0.5 C, compared to 67.11% for the pristine NCA [611]. In this case, the calculated Bader charge of oxygen in pristine and modified materials increased from −1.18 to −1.38 e in the fully lithiated structures, confirming the significant impact of doping on reducing the oxygen release rate.

Co-doping with high-valence dopants is also efficient. Li-ion pouch cells with Ta⁵⁺- and Mo⁶⁺-doped Li[Ni_0.91Co_0.09]O₂ cathodes retain about 81.5% of their initial specific capacity after 3000 cycles at 200 mA g⁻¹ [468]. Y³⁺ and W⁶⁺ co-doping of the single-crystal cobalt-free LiNi_0.9Mn_0.1O₂ cathode exhibited a capacity retention of 82.1% at 1.0 C after 100 cycles (compared to 71.9% for the undoped cathode), and at 3 C the co-doped sample displayed a high specific capacity of 152.0 mAh g⁻¹, while the value for the pristine is only 139.8 mAh g⁻¹ [612]. More recently, Zhang et al. co-doped LiNi_0.9Mn_0.1O₂ with Zr⁴⁺ and Mo⁶⁺ substituting transition metal element and Mg²⁺ replacing Li⁺ to obtain a cathode exhibiting 87.5% capacity retention after 210 cycles at 1 C and 85.5% after 150 cycles at 2 C [613] (see Figure 15).

The high-valence dopant W⁶⁺ and low-valence Na⁺ co-doping is also a good strategy, since the high-valence doping element improves the structural stability by forming strong metal–oxygen binding forces, while the low-valence doping element eliminates high cation mixing. A single-crystal Ni-rich LiNi_0.83Co_0.12Mn_0.05O₂ cathode material with this-co-doping exhibited a capacity retention of 87% after 1000 cycles (vs. graphite/Li⁺) in pouch-type full cells at a high temperature of 55 °C [614].

Other co-doping improving the electrochemical and structural stability of NMC materials includes Na+F [538,615], Na+Al [616], Al+Nb [617,618,619], Ga+B [620], Al+Y [621], Zr+B [622,623], Zr+Mg [590], Zr+Al [624], Zr+La [625], Ti+Ta [626], Mo+F [627], Mo+Ti [628], Ti+Al [601,629], Cu+Ti [630], B+Ti [631], Mg+Ti [632,633], Mg+F [634], Na+Y [611], and Co+Ti [635]. Xiao et al. gave evidence of the important role of Co+Al doping in inhibiting cation disorder to increase the lithium-ion diffusion rate in LiNi_0.84Mn_0.10Co_0.03Al_0.03O₂ [636]. Shi et al. selected Co+La co-coating. The single-crystal NMC811 prepared by Shi et al. with 2 wt.% La₂Li_0.5Co_0.5O₄ coating exhibited a capacity retention of 81.15% after 200 cycles at 1 C [637]. This is a 18% improvement with respect to the pristine NMC811 prepared by Shi et al., but smaller than that of the pristine nano-sized and single crystallized NMC811 particles of Lao et al. [230], which emphasizes the important role of the morphology to optimize the electrochemical properties. Yoon et al. reported a room-temperature synthesis route to achieve a full surface coverage of secondary particles and facile infusion into grain boundaries, and thus offered a complete Co+B ‘coating-plus-infusion’ strategy, by applying cobalt boride metallic glass to NMC811 [638]. The effect of B+Al doping of LiNiO₂ was investigated by Kim et al. [639]. They found that the low solubility of B into the host structure leads to a surface-confined distribution of B, inhibiting the growth of primary particles, whereas the highly soluble Al facilitates primary particle growth. As a consequence of this difference, boron-doping was found the most effective dopant strategy for microstructural engineering of the primary particles. The B-doped LiNiO₂ cathode exhibited a capacity retention of 81% in full cells, after 300 cycles, significantly better compared with Al-doped LiNiO₂. While B accumulates close to the surface, Ti is uniformly distributed in NMCs, so that the formation of strong Ti-O bond effectively alleviates the bulk phase structure degradation. As a result, Ti+B-modified NMC exhibited a capacity retention of 95.4% after 150 cycles between 2.7–4.3 V at 1 C and excellent rate performance of 137.7 mAh g⁻¹ at 5 C [640]. Kuo et al. investigated the effect of B³⁺ and Ru⁵⁺ on the structure and electrochemical performance of NMC811 [641]. They demonstrated that B-doping leads to the formation of a boron oxide-containing surface coating, mainly on the outer surface of secondary particles, thus improving the cycle ability. On the other hand, Ru preferentially occupies incoherent grain boundary sites, resulting in smaller primary particle size. DFT calculations confirm that B-doping forms an enriched surface, and also find that Al-doping results in Al-rich bulk [642].

Zhang et al. [483] synthesized high-voltage single-crystal cathode material LiNi_0.6Co_0.1Mn_0.3O₂ with in situ doping of Zr⁴⁺ and Ti⁴⁺ by precursor co-precipitation. The synergistic effect of Zr/Ti co-doping on the transition metal (TM) sites in the SC-NCM material not only effectively improved the diffusion migration rate of Li⁺ but also reduced the stress concentration and cation disorder inside particles. The capacity retention rate of its soft-packed full battery was 80.6% after 4000 cycles.

Al+Zr-co-doped single-crystalline LiNi_0.88Co_0.09Mn_0.03O₂ delivered a discharge capacity of 121.2 mAh g⁻¹ at 5.0 C (1000 mA g⁻¹) with a cut-off voltage of 4.6 V after 350 cycles, while the pristine material falls to nearly zero after 200 cycles under the same conditions [643]. The authors attributed the effect of the Al/Zr doping to the reduction of the Li⁺ ion energy barrier from 500 meV in the pristine material to 70–130 meV, and the increase of the Ni atomic transfer barrier from the initial 2.07 eV to 2.37–2.68 eV, indicating that Ni can exist in a stable state. The Al³⁺ and PO₄³⁻ co-doping of NMC811 inhibited the mixing of lithium and nickel, and maintained the good lamellar structure of the cathode material [644]. As a result, the capacity retention rate of the cathode with this modified NMC811 was 14.1% higher than that of the raw material after 100 cycles at 1 C. The rate capability was also increased, with a capacity of 155.8 mAh g⁻¹ at 5 C. In the particular case of NCA, Al+Mn, Mg+Mn, Fe+Al, Ti+MgAl+Ti+Mg co-doping also proved useful to improve the electrochemical properties and structural stability, as reviewed in [71]. A comparative study of the dual-doping effect on Ni-rich cathodes is reported in

Table 3. Effects of dual-doping on cycling ability of Ni-rich cathode elements. Capacity densities are expressed in mAh g⁻¹. Asterisk symbol after cathode element indicates that upper voltage cut-off for cycles is 4.5 V. Otherwise, unless specified, cut-off is 4.3 V.

Cathode Element	Dual Doping	Cycle at C-Rate	Pristine		Doped		Ref.
			Capacity (mAh g−1)	Retention (%)	Capacity (mAh g−1)	Retention (%)
NMC811 (cut-off at 4.8 V)	Mg+Zr	100@C/10	201.8	66	232.2	70.5	[590]
LiNi0.90Co0.06Mn0.04O2	Mg+W	150@2 C	170	64.8	180	86.7	[591]
LiNi0.85Co0.05Mn0.10O2 *	K+F	500@1 C	185	75	190	91.5	[592]
NMC811	K+Ti	200@1 C	182	59.4	175	91.2	[593]
LiNi0.92Co0.04Mn0.04O2 *	Mo+F	200@1 C	200	0	204	87.1	[594]
LiNi0.80Co0.05Mn0.15O2 *	Al+Na	200@1 C	196	70	205	84	[595]
NMC811	B+Nb	200@1 C	175	76.1	175	86.7	[597]
LiNi0.83Co0.07Mn0.10O2	Ce+Gd	100@1 C	192.6	82.5	194.6	90.2	[598]
LiNi0.83Co0.11Mn0.06O2	B+Ca	100@C/2	190	44.3	190	72.4	[599]
LiNi0.92Co0.08O2	Ti+Al	100@C/5	208.7	76.5	197.7	93.8	[601]
LiNi0.85Mn0.09Al0.06O2	In+Sn	100@C/2	160	86.6	162	100	[605]
LiNi0.88Co0.08Mn0.04O2	Na+Al	50@1 C	183.91	78.6	192.49	83.7	[606]
LiNi0.95Co0.03Al0.01Mg0.01O2	Mg+Al	100@1 C	190	60.8	200	95.6	[608]
LiNi0.8Co0.15Al0.05O2	Na+Y	100@C/2	172	67.1	175	92.3	[611]
LiNi0.9Mn0.1O2	W+Y	100@1 C	178	71.9	180	82.1	[612]
LiNi0.9Mn0.1O2	Zr+Mo+Mg	150@2 C	180	63.2	160	85.5	[613]
LiNi0.83Co0.12Mn0.05O2 at 55 °C	Na+W	1000@1 C 500@1 C	185	0 80	197.5	87	[614]
NMC811	Na+Al	80@1 C	175	84.5	175	93	[616]
LiNi0.96Co0.04O2 (cut-off 4.4 V)	Al+Nb	300	200	48.5	200	77.8	[617]
Li[Ni0.92Co0.04Mn0.04]O2	Al+Nb	500@1 C	195	0	183	75.6	[618]
LiNi0.83Co0.12Mn0.05O2	Al+Nb	100@1 C	172	75	172	95.1	[619]
LiNi0.94Co0.06O2	Al+Y	500@1 C	200	54.6	200	80.2	[621]
Li[Ni0.885Co0.100Al0.015]O2	Zr+B	1000@1 C	190	48	178	95	[623]
LiNi0.83Co0.12Mn0.05O2	Zr+Al	300@5 C	150	26.1	157	80.5	[624]
LiNi0·83Co0·11Mn0·06O2 *	Zr+La	200@1 C	200.9	66.2	189.0	86.1	[625]
NMC811	Ti+Ta	250@1 C	155.4	59.8	161.1	87.1	[626]
NMC811	Mo+F	500@1 C	182	67.3	181	87.1	[627]
Ni0.9Co0.05Mn0.05O2	Mo+Ti	100@C/2	96.09	53	98.85	69	[628]
Li(Ni0·83Co0·12Mn0.05)O2 *	Ti+Al	500@1 C	170	35.1	180	71.8	[629]
NMC811	Ti+B	100@1 C	186.8	71.1	173.4	94.7%	[631]
Li[Ni0.90Co0.05Mn0.05]O2	Ti+Mg	100@1 C	157	86.6	158	98.9	[632]
LiNi0.89Co0.11O2	Ti+Mg	200@1 C	195	74.3	190	86.5	[633]
LiNi0.8Co0.15Al0.015O2	Mg+F	200@1 C	175	55.7	175	82.6	[634]
NMC811	Co+Ti	400@1 C	175	89.9	170	97.1	[635]

.

5.4.2. Doping Plus Coating

Doping is often concomitant with surface coating to benefit from the synergistic effect [645]. When cycled at a discharge rate of 3 C for 500 cycles within 2.75–4.3 V, the cell with Ge-doped ((Ni+Mn):Ge = 100:0.5 in molar ratio) LiNi_0.9Mn_0.1O₂ cathode and graphite anode exhibited a discharge capacity retention of 80.5%, whereas that of the cell without Ge is only 52.9% [491]. Note that the improvement does not come only from the Ge⁴⁺-doping, but also from the fact that the doping is accompanied with the coating with a Li₄GeO₄ layer. The doping stabilizes the ordered layered structure and limits the Ni-Li substitution, which increases the rate capability, while the coat performs as a barrier to reduce the parasitic reactions, which increases the cycle life. Park et al. have advanced in microstructure engineering by the formation of F-rich species on the surfaces of internal grains, combined with 2 mol% F⁻ doping inside Li[Ni_0.900Co_0.047Mn_0.048Mo_0.005]O₂ [646]. The full cell with this cathode material and graphite anode delivered a capacity of 177.2 mAh g⁻¹ (90.4% of its initial capacity) after 800 cycles at 1 C and 45 °C. Synergetic effects are obtained by combining doping and coating. We have already mentioned the capacity retention of 93.7% obtained for 2% Sn-doped NMC811, after 270 cycles at 0.5 C. This capacity retention rises to 96.6% when the Sn-doped particles are also Li₂SnO₃-coated [547]. The positive effect of Sn-doping is mainly due to higher structural stability and reduction of the Li/Ni mixing, and lower polarization than those of the bare NMC811. The Li₂SnO₃-coating improves the interfacial stability. The same Li₂SnO₃-coating plus Sn⁴⁺ doping strategy has been recently utilized to modify LiNi_0.8Mn_0.2O₂ [647] (see Figure 16). This Co-free cathode with the optimized amount (2 wt.%) Li₂SnO₃ delivered an initial capacity of 173.20 mA h g⁻¹ and a capacity retention of 90.54% after 300 cycles at 0.2 C, outperforming the results with pristine LiNi_0.8Mn_0.2O₂ cathode.

The same strategy consisting in combining coating and doping was applied, choosing boron instead of the tin element. Lithium tetraborate Li₂B₄O₇ coating and boron doping applied to the Co-less Ni-rich LiNi_0.925Co_0.03Mn_0.045O₂ cathode increased the capacity retention at 0.2 C from 59.65% for the pristine material to 72.86% after 100 cycles at 55 °C. At room temperature, the capacity retention of this B-modified cathode was raised to 76.29% after 150 cycles at 1 C [648]. Li et al. synthesized B-doped and La₄NiLiO₈-coated LiNi_0.825Co_0.115Mn_0.06O₂. This cathode demonstrated a capacity retention of 93.49% after 500 cycles at 1 C. [649]. This outstanding performance is due to the combination of the protecting role played by the La₄NiLiO₈ coat, and the boron doping that not only stabilized the structure by restraining the H2–H3 transition, but also adjusted the orientation of primary particles to a radial alignment, which is obstructive to the arise of microcracks. This result outperforms a previous study of the NMC811 with the same coating but Ti-doped instead of B-doped, which exhibited a capacity of 158.3 mAh g⁻¹ after 200 cycles at 1 C, corresponding to a capacity retention of 90.6% [650]. Kim et al. synthesized a fast ionic conductor Li₃BO₃ coating and B³⁺-doped co-modified LiNi_0.91Co_0.06Mn_0.03O₂ [651]. This significantly improved the rate capability, with a discharge capacity of 88.6 mAh g⁻¹ at 5 C while pristine delivered only 45.8 mAh g⁻¹. It also improved the cycle ability, with the capacity retention of 79.4% as compared to 67.3% for the pristine material after 100 cycles 2 C. The same synergetic effect of Li₃BO₃ coating was used to modify single-crystal Zr-doped LiNi_0.83Co_0.11Mn_0.06O₂ (Zr-NCM83@B) [652]. The Zr-NCM83@B//graphite pouch-type full cell displayed excellent capacity retention of 83.5% over 1400 cycles at 1 C with high voltage range of 2.8–4.4 V.

Cao et al. produced a dual-modified coating of B₂O₃ and LiBO₂ to the high-nickel material LiNi_0.89Co_0.08Mn_0.03O₂ [653]. With this protection, the cathode delivered a high capacity of 180.4mAh g⁻¹ along with an excellent capacity retention of 90% after 100 cycles at 1 C in 2.75–4.35 V (59% for the pristine material). The same dual-modified coating was applied to NMC811 with the same success [654]. Li₃BO₃-coating was also associated with La-doping. When applied to NMC811, the capacity was raised to 230.6 mAh g⁻¹ at 0.5 C under a high cutoff voltage of 4.8 V, with a 73.8% capacity retention after 100 cycles [655].

NASICON is an acronym for sodium (Na) super ionic conductor, which usually refers to a family of solids with the chemical formula Na_1+xZr₂Si_xP_3–xO₁₂, 0 < x < 3. We use this term in the following in a broader sense, for similar compounds where Na, Zr and/or Si are replaced by isovalent elements. Tang et al. constructed a protective NASICON-type Li_1.3Y_0.3Zr_1.7(PO₄)₃ (LYZP) coating with Zr gradient doping on the surface/subsurface of single-crystalline LiNi_0.83Co_0.11Mn_0.06O₂ (SC-NCM83) [656]. The full-cell LYZP-modified SC-NCM83//graphite demonstrated a capacity retention of 83.7% after 500 cycles at 0.5 C, a performance that highlights its potential for commercial applications. Another NASICON-type LiZr₂(PO₄)₃ coating of NCA enhanced the capacity to 182 mAh g⁻¹ at 1 C in 2.7–4.3 V, with the capacity retention of 84.6% after 100 cycles [657].

The Li-free Nb oxide treatment removed surface impurities forming a LiNbO₃/Li₃NbO₄ coating of Ni-rich cathode materials, to reduce the first capacity loss and to improve the rate performance. Nb substitution stabilized the structure, increasing the capacity retention to 93.2% after 250 cycles at C/3 rate in the voltage range 2.8–4.4 V [658]. Xin et al. performed Li-Nb-O coating and Nb substitution on NMC811, enhancing both the rate (158 vs. 135 mAh g⁻¹ at 2 C) and capacity retention (89.6 vs. 81.6% after 60 cycles) performance [659]. Zhang et al. proposed dual-modification strategy, in which Sm was both doped into and coated on NMC811 [660]. With the optimum Sm content (0.5%), this modified NMC811 demonstrated a discharge specific capacity of 184.2 mAh g⁻¹ at 1 C rate, a capacity retention of 94.19% after 100 cycles at 1 C, and a good rate performance (152.2 mAh g⁻¹ at 5 C).

A thermochemical investigation shows that Zr preferentially form secondary phases rather than becoming doped in the NMC crystal; therefore, the equilibrium doping concentration of Zr in the NMC is very low and the material is a mixture of almost undoped NMC with a Li₂ZrO₃ secondary phase [661]. In the case of LiNi_0.96Mn_0.04O₂, the introduction of Zr resulted in the Li₂ZrO₃ coating of the particles [662]. On the other hand, Zheng et al. successively processed NCA carbonate oxalate precursors with (NH₄)₂HPO₄ and ZrOCl₂ solutions to obtain an ultrathin Zr-contained gradient layer at the surface of NCA. Experimental characterization together with DFT calculations revealed that Zr at lithium site strengthens the adjacent (Ni,Co)-O bonds, inhibiting the oxygen evolution and stabilizing the surface. As a result, the cathode delivered a high capacity of 219.7 mA h·g⁻¹, 23% higher than NCA at 0.1 C, and exhibits superior rate capability (128.2 mAh·g⁻¹ capacity at 16 C) and cycling stability (capacity of 110.5 mAh g⁻¹ after 300 cycles at 1 C) [574]. Dai et al. synthesized a highly-antioxidative BaZrO₃ thermal barrier engineered LiNi_0.8Co_0.1Mn_0.1O₂ cathode. Analyses showed that the Zr element was penetrated uniformly into the interior region, so that the material benefited of the rigid Zr–O bonds, which improve the structural stability. On the other hand, the Ba element was only concentrated in the exterior region of secondary particles, impeding the fast heat exchange between electrode and electrolyte [663]. This cathode in a pouch cell delivered a specific energy density of 690 Wh kg⁻¹ at active material level with an excellent capacity retention of 92.5% after 1400 cycles under 1 C at 25 °C. At high temperature of 55 °C, the pouch cell exhibited 88.7% in capacity retention after 1200 cycles. Here, Zr was effectively incorporated in the structure to form Zr-O bonds ensuring stability of lattice O upon cycling. As a result, the cell with this cathode and graphite anode exhibited a superior capacity retention of 92.5% after 1400 cycles at 25 °C, and a resultant 174.1 mAh g⁻¹ with 88.7% in capacity retention after 1200 cycles at 55 °C. In situ Zr-doping achieved a gradient distribution of Zr inside the particles of full-concentration-gradient LiNi_0.80Co_0.05Mn_0.15O₂. The corresponding cathode delivered an initial discharge specific capacity of 181.7 mAh g⁻¹ with 91.9% capacity retention at room temperature and 85.2% at 50 °C, after 200 cycles (1 C, 2.7–4.3 V) [664]. Instead of Zr, yttrium can be utilized [665]. Luo et al. utilized synergetic effects of Y³⁺ doping and Y₂O₃ coating of NMC to prolong the lifetime of Ni-rich Co-less material exposed to air [666]. The same synergetic effect has been observed in ultrahigh-Ni NMC cathodes, such as in situ synthesized Y-doped and yttrium orthophosphate (YPO₄)-modified LiNi_0.88Co_0.09Al_0.03O₂ [510]. The enrichment of these functional elements Ti [92], Zr, or Y at the particle surface is easily obtained in situ due to the slow kinetics of their diffusion.

Ti-doped and TiO₂-coated NMC811 full cells cycled for 200 cycles between 2.8 and 4.4 V at C/3 improves from 86.1% for the pristine NMC811 to 89.4% and 91.5%, respectively [667]. TiO₂ coating improves the cycling stability of NMC811 by minimizing contact between the active material and the electrolyte [668] and reduces the air and moisture sensitivity of NMC622 [669]. Due to the synergistic effects of the Li₃PO₄ coating and Ti-doping, the dual-modified LiNi_0.9Co_0.09Mo_0.01O₂ exhibited a discharge capacity of 218.7 mAh g⁻¹ at 0.1 C (1 C = 220 mA g⁻¹) and a capacity retention of 93.7% at 0.5 C after 100 cycles [670]. Al₂O₃-coated/Al³⁺ and Zr⁴⁺ co-doped NMC811 delivered an initial capacity of 196 mAh g⁻¹ and arrived at a high discharge capacity of 120 mAh g⁻¹ after 200 cycles at 0.5 C, corresponding to a capacity retention of 98%, compared to 70.4% for the pristine material [671].

Recently, Chen et al. modified LiNi_0.84Co_0.11Mn_0.05O₂ with Al³⁺ doped bulk lattice, W⁶⁺ doped subsurface region and hetero-epitaxially grown Li₂WO₄. The modified LiNi_0.84Co_0.11Mn_0.05O₂ showed a capacity retention rate of 88.98% after 200 cycles at 1 C rate in the potential range of 2.7–4.3 V and 82.41% after 200 cycles at 1 C with a high cut-off voltage of 4.5 V at 30 °C [672]. With Zr⁴⁺ doping plus Li₆Zr₂O₇ coating LiNi_0.83Co_0.12Mn_0.05O₂ cycled at 1 C delivered a capacity of 183.7 mAh g⁻¹ with a capacity retention of 94.6% after 200 cycles, compared to 129.9 mAh g⁻¹ and 68.6%, respectively, for the pristine material [479]. W⁶⁺ doping plus WO₃ coating improved the capacity retention of NMC above 90.2% after 200 cycles at 1 C [673]. Li et al. have recently evidenced the fastest inter-diffusivity of W⁶⁺/Mnⁿ⁺ (n = 3 and 4) couple in Ni-rich cathodes. As a result, when Mn proportion is decreased to 10%, Li₆WO₆ coating of grain boundaries and bulk W-doping have been achieved in LiNi_0.9Mn_0.1O₂ and LiNi_0.8Co_0.1Mn_0.1O₂ cathodes, so cyclic stabilities of W-modified LiNi_0.9Mn_0.1O₂ have been prominently improved [463]. Recently, Li(Ni_0.80Mn_0.18Al_0.02)_1–xW_xO₂ with the optimized value x = 0.02 as a cathode delivered an initial discharge specific capacity of 212.9 mAh g⁻¹ at 0.1 C in 2.7–4.5 V and the capacity retention reached 91.91% after 100 cycles at 1 C [674], also emphasizing the effect of tungsten, which promotes Li⁺ migration and suppresses surface side reactions. WO₃-coated and Mg²⁺-doped NMC811 exhibited a capacity retention of 73.5% after 100 cycles at 1 C rate against 53.6% for the pristine material [675].

Xiao et al. coated K-doped LiNi_0.83Co_0.11Mn_0.06O₂ with Li₃PO₄. This cathode delivered a discharge specific capacity of 148.9 mAh·g⁻¹ with a capacity retention of 84.1% after 200 cycles at 1.0 C [676]. PO₄³⁻-doping and Li₃PO₄-coating of dual modification of LiNiO₂ significantly improved the structural stability, and thus the cycle ability of this electrode which exhibited a capacity retention of 81.6% (from 180.8 to 147.6 mAh g⁻¹) after 200 cycles at 0.5 C, when cycled in the range 3–4.3V, and 51.0% capacity retention after 1000 cycles at 1 C [677]. The good cycle ability is due to the coating, while the improved rate capability (a capacity of 130.2 mAh g⁻¹ at 5 C) is attributable to the enlargement in c-axis of the material crystal structure upon the PO₄³⁻ doping. Li₃PO₄ coating was also associated with Nb⁵⁺ doping of LiNi_0.83Co_0.11Mn_0.06O₂ (NCM83) cathode powders [678]. The corresponding half-cell cell delivered an initial discharge specific capacity of 173.5 mAh g⁻¹ at 1.0 C and 25 °C with a capacity retention ratio of 88.8% after 200 cycles. Li₃PO₄ coating was also associated with Mg²⁺ doping to modify LiNi_0.95Mn_0.05O₂ to obtain a cathode with increased rate capability (130.6 mAh·g⁻¹ at 10 C, 1 C = 189.6 mA·h·g⁻¹) [679].

Tang et al. introduced C₆H₉O₆Sb with the precursors before calcination to synthesize LiNi_0.92Co_0.04Mn_0.04O₂ which was not only coated with Li₃Ni₂SbO₆, but also doped with Sb³⁺ which has an ionic radius close to that of Ni²⁺ [680]. This Sb-modified NMC demonstrated a capacity at 5 C equal to 88.5% at 0.1 C and a capacity retention of 82.9% at 5 C after 125 cycles. Combining in situ Na⁺ doping with carbon coating single-crystal NMC811, Sun et al. obtained a cathode delivering a capacity of 140 mAh g⁻¹ after 500 cycles at 5 C, which corresponds of a capacity retention of 83.1% [681]. In the case of Li₃PO₄ coating, the Ni-rich particles were not only coated, but also doped with Mg²⁺ [380,517]. In particular, Li₃PO₄-coated and Mg²⁺-doped single-crystal LiNi_0.925Co_0.03Mn_0.045O₂ cathode exhibited a retention of 89.36% after 100 cycles at 1 C (79.78% with the pristine NMC cathode) [380].

Yu et al. combined in one-step Ta doping and CeO₂ coating of NMC811 [682] (see Figure 17). The strong Ta–O bond helped forming the elongated primary particles for inhibiting microcrack generation, and the CeO₂ coating layer could effectively capture the remaining escaped oxygen. As a result, this cathode retained 90.9% of its initial capacity after 1000 cycles at 2.7–4.5 V and 1 C rate in a pouch-type full cell. LiNi_0.92Co_0.04Mn_0.04O₂ (NCM92) coated with a nano-level CeO₂ buffer layer and simultaneously doped with Ce showed the discharge specific capacity of 141.4 mAh g⁻¹ after 80 cycles at 0.2 C with 79.32% capacity retention at 60 °C [683]. Liu et al. prepared Mg/Ti/Sb-co-doped and CeO₂-coated LiNi_0.9Al_0.1O₂ cathode material. At a high voltage of 2.8–4.5 V and 1 C, the capacity retention of this cathode after 100 cycles reached 90.2% (from 200 mAh g⁻¹ to 181 mAh g⁻¹) [684].

Recently, Li₄TeO₅ surface coating along with bulk Te-gradient doping of single-crystalline Ni-rich LiNi_0.90Co_0.05Mn_0.05O₂ showed improved Li⁺ diffusion kinetics and thermodynamic stability, as well as an improved capacity retention of 95.83% and 82.12% after 200 cycles at the cut-off voltage of 4.3 and 4.5 V [685]. This illustrates the best performance that can be achieved when combining the synergetic effects of coating, doping, and single-crystal morphology. Wang et al. introduced an advanced surface reconstruction strategy combining a LiScF₄ coating, Sc+F surface co-doping, and a cation-mixing layer, which improved the thermal stability, structural integrity, and electrochemical performance of Ni-rich cathodes [686]. A comparison between different results obtained on doped plus coated Ni-rich cathodes is reported in

Table 4. Effects of doping plus coating on the cycling ability of Ni-rich cathode elements. Capacity densities are expressed in mAh g⁻¹. Unless specified, cut-off voltage is 4.3 V.

Cathode Element	Dopant + Coating	Cycles at C-Rate	Pristine		Coated		Ref.
			Capacity (mAh g−1)	Retention (%)	Capacity (mAh g−1)	Retention (%)
NMC811	Mg + LiFePO4	100	183.48	81.7	181.5	90.8	[412]
LiNi0.8Co0.15Al0.05O2	Ta + Ta2O5	200@1 C	168	60.97	180	94.46	[338]
LiNi0.90Co0.05Mn0.05O2	Ta + Li2MnO3	300@1 C	201.9	85.8	204.2	91.6	[388]
LiNi0.82Co0.14Al0.04O2	Mn + Li0.5La2Al0.5O4	100@1 C	181	83.2	182	96.2	[396]
	Zr + WO3/Li2WO4	@C/3	173	85	180	93	[645]
NMC811	Na + LiNiO2 + Na1–xNi1–yPO4	200@1 C	200	59	202	88	[377]
NMC811	Sn + Li2SnO3	280@C/2	192	61.6	190.3	96.6	[547]
LiNi0.8Mn0.2O2	Sn + Li2SnO3	300@C/5	189.70	81.23	191.3	90.54	[647]
LiNi0.925Co0.03Mn0.045O2 At 55 °C	B + Li2B4O7	100	210	59.65	210	72.86	[648]
LiNi0.825Co0.115Mn0.06O2	B + La4NiLiO8	500@1 C	165	73	165	93.49	[649]
LiNi0.91Co0.06Mn0.03O2	B + Li3BO3	100@2 C	170	67.3	175	79.4	[651]
LiNi0.89Co0.08Mn0.03O2	B + B2O3 & LiBO2	100@1 C	195	59	195	90	[653]
LiNi0.83Co0.11Mn0.06O2 (cut-off 4.4 V)	Zr + Li3BO3	1400@1 C	700 Wh/kg	71.4	700 Wh/kg	83.5	[652]
NMC811	B + LiBO2/B2O3	140@1 C	170	91	170	>99	[654]
NMC811 cut-off 4.6 V	La + Li3BO3	300@C/2	176.9	27.0	201.4	63.7	[655]
LiNi0.83Co0.11Mn0.06O2 (full cell)	Zr + Li1.3Y0.3Zr1.7(PO4)3	500@C/2	19.87 mAh	70.6	23.48 mAh	83.7	[656]
NMC811	Sm dopant + coat	100@1 C	184	57.6	184.2	94.2	[660]
LiNi0.96Mn0.04O2	Al+Zr+ Li2ZrO3	100@1 C	187	75	195	81.2	[662]
NMC811	Zr + BaZrO3	1400@1 C	190	92.3	190	92.5	[663]
NMC811 (cut-off 4.4 V)	Ti + TiO2	200@C/3	193	86.1	192	90.2	[667]
NMC811 (cut-off 4.3 V)	Ti + TiO2	1000@1 C	175	24.3	160	63.4	[668]
LiNi0.9Co0.09Mo0.01O2	Ti + Li3PO4	100@1 C	156.1	75.1	189.8	92.1	[670]
NMC811	Zr + Al2O3	100@C/5	180	82.7	186	92	[671]
LiNi0.84Co0.11Mn0.05O2	Al + Li2WO4	200@1 C	172	72.9	178	88.9	[672]
NMC811	W + WO3	200@1 C	170.21	74.5	184.21	90.2	[673]
LiNiO2	PO43− + Li3PO4	1000@1 C	200	23.3	175	51.0	[677]
LiNi0.83Co0.11Mn0.06O2	Nb + Li3PO4	200@1 C	178	59.5	173.5	88.8	[678]
LiNi0.92Co0.04Mn0.04O2	Sb + Li3Ni2SbO6	200@5 C	185.7	39.2	149.4	75.7	[680]
NMC811	Na + carbon	500@5 C	125	41.1	165	83.1	[681]
LiNiO2 (cut-off 4.5 V)	Ta + CeO2	100@1 C	195	85	205	96.8	[682]
NMC92 at 60 °C	Ce + CeO2	80@C/5	91.8	49.25%	141.5	79.32%	[683]
LiNi0.9Al0.1O2 (4.5 V)	Mg/Ti/Sb + CeO2	100@1 C	192	60.1%	200	90.2%	[684]
LiNi0.90Co0.05Mn0.05O2	Te + Li4TeO5	200@1 C	198	82.05%	198	95.83%	[685]

.

5.4.3. Multiple Coatings

Multiple coatings were achieved with PPY. In particular, dual-conductive layers composed of Li₃PO₄ and PPy were utilized to improve the performance of NMC811, due to the conversion of excessive Li into Li₃PO₄ [687]. Another dual-conductive coating utilized to coat NMC811 is PPy-LiAlO₂. With the optimal amount of PPy coating (2 wt.%), the PPy-LiAlO₂ coated NMC811delivered a capacity of 118 mAh g⁻¹ at 10 C. The capacity retention was 90.9% after 100 loops at 1 C [688]. NMC811 was also enwrapped by a functional reduced graphene oxide (RGO)–KH560 polymer composite layer which consists of an inner high-flexibility epoxy-functionalized silane (KH560) layer and an outer RGO layer [689]. Here, the KH560 layer was especially critical in connecting the layer of outer RGO and the inner surface of the active material, while uniform coating with RGO alone is almost impossible, due to the 2D nature of RGO or graphite. The NMC811 cathode with RGO (0.5%)–KH560 (0.5%) coating exhibited a capacity retention of 95.2 after 150 cycles at 1 C.

Recently, Tan et al. designed a single-crystal LiNi_0.9Co_0.05Mn_0.05O₂ in which coherently grown MgO₆ octahedra and BO₄ tetrahedra were incorporated into the lattice, and a stabilizing Mg₃(BO₃)₂ layer was concurrently formed on the particle surface [690]. These modifications reduced planar gliding and delamination cracking, even during prolonged cycling. A pouch-type full cell with this cathode and a graphite anode sustained a specific discharge capacity of 177 mAh g⁻¹ after 500 cycles, with 90% capacity retention.

Cheng et al. coated NCA with a triple composite Li-ion conductor coating layer (Y(PO₃)₃-Li₃PO₄-YPO₄) [691]. The metal phosphite Y(PO₃)₃ was here to optimize the surface residual alkaline lithium salt of NCA to expel H₂O and CO₂ in advance, which effectively reduced the electrolyte’s hydrolysis and gas generation. The electrode modified with 2 mol% Y(PO₃)₃ showed excellent rate performance (156.3 mAh·g⁻¹ at 5 C) with a capacity retention of 88.3% after 200 cycles.

6. Core–Shell and Gradient Structures

6.1. Core–Shell Structures

The design of core–shell structures is a strategy commonly used to improve the electrochemical properties of Ni-rich cathode materials [692,693,694,695,696,697]. In such structures, the core is a Ni-rich material, and the shell is a Mn-rich component, which is structurally stronger and has thus the ability to protect the inner part from contact with the electrolyte. For example, Li et al. [692] prepared a core–shell structure with the inner core composed of NMC811, while the outer layer named Li–Mn–O contains Mn⁴⁺, with higher concentration than that of the core. The corresponding cathode delivered the same capacity (118 mAh g⁻¹) as that of pristine NMC811 at the rate of 10 C in the voltage range of 3.0–4.3 V, but the capacity decay was limited to 18.4% after 200 cycles compared to 27% decay with the pristine material. Cathode materials with the overall composition of NMC811 in which the nickel–rich composition (LiNi_0.9Mn_0.05Co_0.05O₂) is the core and the manganese–rich composition (LiNi_0.33Mn_0.33Co_0.33O₂) is the shell improved capacity retention of 76.6% compared to the commercial homogeneous NMC811 [695]. Bae et al. synthesized LiNi_0.8Co_0.1Mn_0.1Al_xO₂ with an inner porous layer. This structure was obtained by the intermittent addition of Al salt during hydroxide precursor co-precipitation, exhibiting the spherical morphology with the structure of core (active material particle) and shell (the inner porous layer. This modified cathode material demonstrated a capacity retention of 98.2% after 100 cycles at 1 C and a cut-off voltage of 4.5 V [698]. In addition, this synthesis is scalable to commercial production due to its simple and facile procedure. Shi et al. redesigned Li[Ni_0.7Co_0.1Mn_0.2]O₂ into a core–shell structure with the core of Li[Ni_0.8Co_0.1Mn_0.1]O₂ delivering a high capacity and the shell of Li[Ni_0.45Co_0.1Mn_0.45]O₂ providing structurally stability [699]. The core–shell electrodes displayed enhanced cycling stability and thermal stability compared with the non-core–shell electrodes. Xia et al. proposed core–shell-structured Ni-rich cathodes composed of LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) shell and LiNi_0.9Co_0.05Mn_0.05O₂ (NCM9055) core. In addition, the NCM622 shell had radially oriented primary particles to reduce the surface side reaction with electrolyte and alleviate internal stress. The 40 wt.% NCM622 coating NCM9055 cathode exhibited a discharge capacity of 160.5 mAh g⁻¹ with a capacity retention of 85.0% after 200 cycles at 0.5 C [700]. Ryu et al. synthesized a hybrid structure in which Li[Ni_0.92Co_0.04Mn_0.03Al_0.01]O₂ in the interior of the particle is encapsulated by Li[Ni_0.845Co_0.067Mn_0.078Al_0.01]O₂ that acts as a buffer against microstrain [693]. This hybrid cathode structure suppresses the formation of microcracks by creating a non-uniform spatial distribution of the microstrain within the cathode particle. As a result, this hybrid cathode material retained 84.7% of its initial capacity after 1500 cycles at 1 C, which is a 50% increase with respect to the retention obtained with the pristine Li[Ni_0.9Co_0.045Mn_0.045Al_0.01] bulk material. The presence of Li_xW_yO_z secondary phases in the grain boundaries eliminated the interdiffusion of Mn and Al from the shell to the core in these Ni-rich core–shell materials. Park et al. [696] constructed a core–shell structure with the core consisting mainly of Li[Ni_0.957Mn_0.043]O₂, while in the 3 µm-thick shell. the Ni concentration decreases to 83% and the Mn concentration increases to 17% at the secondary particle surface, resulting in the surface composition of Li[Ni_0.834Mn_0.166]O₂. Therefore, the cathode material had the composition of Co-free Li[Ni_0.9Mn_0.1]O₂. This cathode retains 78.5% of its initial capacity after 2000 cycles at 1 C charge and 0.8 C discharge rates, and retains 79.5% capacity after 1000 cycles under fast-charging conditions (3 C charge and 1 C discharge), demonstrating the effective dissipation of internal strain and chemical protection provided by the Mn-rich shell. This recent result outperforms the electrochemical properties of prior Co-free cathodes [86,91], and proves that both cycle life and rate capability can be reached even with Co-free Ni-rich cathodes. So far, such a performance was obtained only with presence of cobalt, because Co³⁺ alleviates magnetic frustration during cycling, thus reducing the disordered cation mixing of Li⁺ and Ni²⁺ [701], and increases electronic conductivity through CoO₆ octahedra with delocalized electronic states. The success of the cathode in [696] also comes from the fact that Mn-based shells are less prone to interdiffusion, compared to Mg- and Al-based shells, while interdiffusion has been identified as a major challenge to overcome for the preservation of core–shell structure. Han et al. proposed a new approach that fully utilizes Co by forming a Co nanoshell on a Ni-rich [Ni_0.90Co_0.05Mn_0.05](OH)₂ precursor via a single coprecipitation process [702]. The Co nanoshell acts as a sintering inhibitor during calcination, thereby generating columnar structures that are not observed in the baseline Li[Ni_0.90Co_0.05Mn_0.05]O₂ cathode material. The Co nanoshell markedly improved the Li⁺ kinetics and cycling stabilities of Ni-rich cathode materials.

Xue et al. introduced a proton-rich (ammonium bicarbonate) shell on LiNi_0.92Co_0.06Mn_0.02O₂ single crystals [703]. Once released during the first charge, the proton is immediately captured by LiPF₆, in situ electrochemically conversing to LiF and Li₃PO₄ sub-nano particle dense maskant. This surface modification enables 95% capacity retention after 100 cycles at a high voltage of 4.5 V in the half cell, and 83% capacity retention after 800 cycles in the full cell with graphite anode.

Tungsten oxide (WO₃) coating on the surface of precursors leads to the formation of Li_xW_yO_z secondary phases during the heat treatment with LiOH·H₂O. These Li_xW_yO_z phases infuse into the grain boundaries and prevent interdiffusion between the core and shell phases [704]. This process was proposed by Rathore et al. who constructed a cobalt-free core–shell cathode material composed of inner core Ni(OH)₂ and outer shell Ni_xM_y(OH)₂ [697], enhancing the mechanical strength and electrochemical performance. This behavior is probably not unique to W, and it is expected that elements like Mo, Zr and Nb, which are also found in the grain boundaries, can be used to prevent the interdiffusion between shell and core, so that Co-free Ni-rich cathodes are very promising for the next generation of lithium batteries. Recently, Liu et al. used synergetic effects of Li₂WO₄ coating and W⁶ doping to obtain a modified single-crystal LiNi_0.9Co_0.05Mn_0.05O₂ (Ni90) cathode exhibiting 89.8% capacity retention after 300 cycles at 0.5 C [705]. Chu et al. coated single-crystal LiNi_0.92Co_0.04Mn_0.04O₂ with a layer composed of Li–B–O species and Li₂WO₄. The cycling stability of such a cathode at a high cut-off voltage of 4.5 V was improved from 71.8% to 87.1% at 1 C after 100 cycles [706]. Du et al. introduced W into LiNi_0.9Co_0.05Mn_0.05O₂ and discovered that, during high-temperature solid-state reactions, a Li₄NiWO₆ defect phase is formed. This phase, combined with the amorphous Li_xW_yO_z coating layer improved the electrochemical properties of this cathode, which exhibited a capacity retention of 97.31% (200.5 mAh g⁻¹ to 195.1 mAh g⁻¹) after 200 cycles in the voltage range 3–4.3 V at 0.5 C and 30 °C [707].

The hollow multi-shell structure (HoMS) allows Ni-rich cathode materials to have sufficient pore space to buffer the structural instability caused by volume expansion in the cyclic process. In particular, Ding et al. chose micro- and nanostructured Co₃O₄ as a template of the HoMS because the annealed Co₃O₄ template has a unique HoMS layer structure, to prepare a NMC622cathode material that demonstrated a capacity of 148.5 mAh g⁻¹ after 100 cycles at 0.5 C, corresponding to a capacity retention of 90.4% [708].

6.2. Gradient Layers

Concentration gradient cathodes are a good strategy to concentrate high Ni-concentrations in the core region, and smaller Ni concentrations near the surface to improve the structural stability. The concentration gradient design originates from a simple core–shell structure in which the core is Ni-rich NMC while the shell has other NMC compositions (lower Ni content and higher Mn content) to stabilize the layer structure at the surface. Concentration gradients are an alternative to core–shell structure to confine the Ni-rich part of the particles in the core. Actually, the two procedures are related when some interdiffusion occurs between the shell and the core. However, concentration gradient concept has an advantage, as it can effectively solve the problem of the reduced Li diffusion caused by the contraction of the crystal cell volume between the high-nickel core and the gradient shell [582,709,710,711]. Moreover, the core–shell design is responsible for voids generated during cycling between the shell and the core during long-term cycling and mismatches of different lattice parameters. Gradient layers avoid this problem, and have been used constantly for a decade to synthesize Ni-rich cathode materials [712].

The improved electrochemical properties of a gradient material with a high-nickel NMC811 as the core and a low-nickel LiNi_0.46Co_0.23Mn_0.31O₂ as the shell illustrate the benefit of the reduction of surface microcracks, and the rod-shaped morphology of the primary particles with radial growth orientation [713]. As a result, the capacity retention of 70% after 1000 cycles at a high temperature of 55 °C. The same morphology and same benefit were experienced with a material of average composition LiNi_0.75Co_0.10Mn_0.15O₂ with gradual composition with the inner core composed of high-nickel low-Mn LiNi_0.86Co_0.10Mn_0.04O₂, and the outer layer composed of low-nickel high-Mn LiNi_0.70Co_0.10Mn_0.10O₂ [709]. More recently, Wu et al. synthesized a Ni concentration gradient LiNi_0.8Co_0.15Al_0.05O₂ (NCG-NCA) cathode material using the characteristic reaction of Ni²⁺ and dimethylglyoxime [710]. Owing to the high nickel content in the core, and high cobalt content on the surface stabilizing the surface of the particles, the capacity retention was increased at 75% after 200 cycles at a current density of 10 C (1 C = 160 mA g⁻¹) under a high cut-off voltage of 4.5 V, compared with 50% for the homogeneous NCA material. Park et al. synthesized a core–shell with concentration gradient Li[Ni_0.90Co_0.045Mn_0.045Al_0.01]O₂ (CSG-NCMA90) consisting of fine, elongated primary particles that are radially aligned from the center of a spherical secondary particle [582]. This microstructure effectively suppressed the formation of cracks in the charged state, in agreement with the general trend that a spherical secondary cathode particle shape with uniform morphology can reduce the production of microcracks by effectively dispersing structural strain during cycling [714]. Moreover, these elongated particles largely featured (001) facets on their lateral sides, and were tolerant of electrolyte attack, thus suppressing surface degradation, enabling the CSG-NCMA90 cathode to retain 90.7% of its initial capacity after 1000 cycles at 100% depth of discharge (in the voltage range 3.0–4.2 V) at 1 C rate. The oriented growth of primary grains along (001) and parallel stacking of nanosheets can be achieved by optimizing the stable adsorption structures of metal-ammonia complexes on the precursor crystal facets under aqueous ammonia conditions [715,716]. Ryu et al. constructed a hybrid-structured Li[Ni_0.9Co_0.045Mn_0.045Al_0.01] (HS-NCMA90) cathode, in which Li[Ni_0.92Co_0.04Mn_0.03Al_0.01]O₂ forms the interior of the cathode particle enclosed in a buffer layer of Li[Ni_0.845Co_0.067Mn_0.078Al_0.01]O₂. This structure develops radially aligned primary particles, enabling the primary particles to contract uniformly, thereby effectively suppressing the propagation of microcracks toward the outer surface [693]. The pouch cell with this cathode and a graphite anode retained retaining 90.0% of its initial capacity after 1000 cycles at 1 C in the voltage range 3.0–4.2 V. Another example of the effectiveness of the nanorod configuration is provided by Liu et al. who designed a unique bayberry-like Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ cathode material, composed of a spherical core and the shell self-assembled by radially oriented nanorods [717] (see Figure 18). The radial texturing of the nanorods in shell layer formed a natural protective interface constituted by thermodynamically stable (003) planes, so this cathode delivered remarkable high-rate long-term cycling stability with capacity retentions of 91.2% after 500 cycles at 1 C and of 81.3% after 1000 cycles at 5 C. Guo et al. constructed a concentration-gradient LiNi_0.753Mn_0.247O₂ cathode that consists of the elongated and radially aligned primary particles through Mo+Ti+Mg doping [718]. With the synergetic effect of gradient concentrations and multi-doping, this Co-free Ni-rich cathode exhibited a capacity retention of 81.1% after 1000 cycles in a pouch full cell (commercial graphite anode) at 1 C within 2.7–4.5 V.

These results illustrate that concentration gradient layers can reshape primary particles into a rod-like or even needle-shaped structure, which can promote Li⁺ movement by minimizing diffusion pathways [719]. Such cathode materials are usually prepared by calcining hydroxide precursors with Li salts at an optimum temperature, high enough to obtain a good crystallization, but small enough to keep the rod-like morphology of the precursors. Jeyakumar et al. synthesized a double concentration-gradient Li[Ni_0.90Co_0.04Mn_0.03Al_0.03]O₂ with a Ni-rich-core, an Mn-rich shell, and Al on top surface [235]. This cathode exhibited a capacity retention of 91.5% after 100 cycles and 70.3% after 500 cycles at 1 C (against 83.4% and 47.6% for the material with uniform composition). While the concentration gradient concerns usually the concentration of nickel, Liu et al. synthesized a 0.5(LiNi_0.8Mn_0.2O₂)_bulk0.5(LiNi_0.8Co_0.2O₂)_surface sample (denoted as NM-NC82), where the Ni concentration is constant (x = 0.8) with a composition design where the central component is Ni/Mn/Co (8:2:0, molar ratio), then gradually changes to Ni/Mn/Co (8:0:2, molar ratio) toward the surface of the particle [133]. Benefiting from lower stiffness in Co-enriched surfaces, this cathode material showed negligible morphology damage, due to an improved stability of the H2–H3 phase transition. Maximizing the advantages of coating, doping, and concentration gradient, Park et al. constructed a core–shell structure in which the shell with Ni fraction of ≈84% uniformly surrounded the core part of Ta-doped NMC particles of composition Li[Ni_0.935Co_0.040Mn_0.020Ta_0.005]O₂ with Ni fraction of ≈97% [720]. When cycled in pouch-type full cells using graphite anodes in the voltage range of 3.0–4.2 V, this cathode demonstrated a capacity retention of 92.6% after 1000 cycles when charged at 2 C (discharge current 1 C).

Wu et al. identified recently the effectiveness of using such an anionic surfactant (β-styryl phosphonic acid) to assist the preparation of Ni_0.8Co_0.1Mn_0.1(OH)₂ precursors in the coprecipitation [721]. With this configuration, the NMC811 exhibited capacity retention rates could reach 93% (164.5 mAh g⁻¹) after 100 cycles and 84.3% (149.0 mAh g⁻¹) after 300 cycles. Shang et al. synthesized NMC811 with dual Ni gradients on both primary and secondary particles by applying Ni-MOF-74 during the coprecipitation of precursors [711]. This cathode delivered a capacity of 203.5 mAh g⁻¹ at 0.1 C. At C/3, and the capacity retention was 88.5% after 300 cycles with a coulombic efficiency larger than 99.86%. At 1 C, the capacity retention was 84.1% after 500 cycles.

Gradient NMC cathode materials with high stability and excellent performance can be synthesized by capitalizing on the unique reactivity of the NMC precursor with ammonia and KMnO₄ [722]. According to this method, surficial Ni is dissolved from the NMC precursor through ammoniacal leaching and subsequently MnO₂ is deposited on the surface by reducing KMnO₄. This process allows for precise adjustment with reduced surficial Ni and enriched Mn. A 1.2 Ah pouch cell configurated with this modified NMC cathode and graphite anode demonstrates a lifespan of over 500 cycles with only 8% capacity loss.

NMC811 with a surface gradient layer and lithium phosphorus oxynitride (LIPON) coating showed improved capacity retention [723]. The formation of highly electronegative PO₄³⁻ polyanions in the layered material after P-doping enables the expansion of the layer spacing and stabilization of the lattice structure. Liu et al. synthesized P-doped NMC811 by a co-precipitation process, to obtain a cathode that exhibited a capacity retention of 83.53% after 100 cycles at 1 C [724]. Choi et al. coated 1 wt.% lithium and cobalt acetate precursor on the inner and outer surfaces of commercial polycrystalline NMC811, which improved electrochemical properties in cells with liquid electrolytes. On the other hand, when the coating was mainly on the outer surface of the material, the performance was improved in cells with solid-state electrolytes, as the contact between the electrolyte and the active material is only at the surface of the polycrystalline NMC811 [725].

Gradient layers can be extended into full concentration gradient (FCG) layers or multiple gradient layers with different slopes, to maximize the Ni content at the core as compared with single-sloped gradient layers. For instance, full-concentration-gradient Li[Ni_0.78Co_0.10Mn_0.12]O₂ obtained with the optimized calcination temperature of 790 °C exhibited remarkable cycling stability, retaining 86.3% of its initial capacity after 4000 cycles, and superior rate capability [726]. Wu et al. studied a series of full concentration gradient-tailored Li-rich layered oxides, and found that the gradient-tailored materials with medium-slope show the best electrochemical performance (capacity retention of 88.4% within 200 cycles at 200 mA g⁻¹) [727]. Zhang et al. combined the full concentration strategy of Ni-rich material (overall composition: LiNi_0.75Co_0.15Mn_0.1O₂) with Ti pillar and Li₂ZrO₃ (LZO) coating modification [728]. The corresponding cathode exhibited 92.3% capacity retention at 1 C, and delivered a capacity of 137.9 mAh g⁻¹ at 10 C rate. Radially oriented rod-shaped primary particles in a cathode, resulting from the concentration grading (or compositional partitioning) of the cathode, also showed markedly improved electrochemical and thermal properties. [581,693,729]. In particular, a cathode with full-concentration-gradient Li[Ni_0.78Co_0.10Mn_0.12]O₂ active element retained 86.3% of its initial discharge capacity after 4000 cycles at 1 C in a full cell [726]. NMC811 with radially orientated primary particles resulting from Mn composition grading delivered ≈180.1 mAh g⁻¹ at 1 C and retained 96.2% of its initial discharge capacity after 100 cycles [730]. This is also the capacity retention obtained by designing a concentration gradient core–shell structure of Li(Ni_0.794Co_0.11Mn_0.096)O₂ consisting of a nickel-rich core with a Ni:Co:Mn mole ratio of 9:0.5:0.5, two interlayers with increasing Mn content gradient, and a high-manganese thin shell [731]. Ni_0.95Mn_0.025Co_0.025O₂ with a Ni/Co gradient and radially oriented particles demonstrated greatly improved cycling stability (capacity fade per cycle of 0.04 ± 0.01% over 300 cycles at 1 C) [732]. Guo et al. reported the design and synthesis of a full concentration gradient LiNi_0.75Mn_0.20Co_0.05O₂ cathode with a Mn-rich Ni-poor surface, which has been realized by in situ forming a PO₄³⁻ gradient distribution after Li_0.1B_0.967PO₄ coating [733]. This cathode delivered a capacity of 212.6 mAh g⁻¹ at 2.7–4.5 V with an energy density of >800 Wh kg⁻¹_cathode. In commercial-grade full cells with commercial graphite anodes, a superior cycle life of 80.5% capacity retention is achieved at 1 C within 2.7–4.5 V after 1700 cycles. This performance is comparable to the state-of-the-art cathodes with Ni content of 90% at 2.7–4.3 V, but is safer as the exothermic peak is at higher temperature (261.4 °C). The concentration gradient design is thus particularly effective in the fabrication of high-density lithium batteries.

Gradient structure refers usually to a Ni concentration gradient, but concentration-gradient doping element is also a good strategy to improve the electrochemical properties [734,735]. Mu et al. developed a gradient Zr element doping, the orientations of which were achieved via coprecipitation in a positive or negative correlation between the concentrations of Zr and Ni. They found that the material behaves better when the Zr content is abundant in the core. The gradient Zr-doped LiNi_0.65Co_0.20Mn_0.15O₂ delivered a higher capacity retention of 85.6% after 300 cycles at 1 C (70.7%, 121.4 mAh g⁻¹ for the pristine sample) and an outstanding rate performance of 122.5 mAh g⁻¹ at 20 C [736]. Zhang et al. improved the performance of LiNiO₂ via concentration-gradient yttrium modification with LiYO₂-Y₂O₃ coating layer [737]. Other doping elements with gradient concentrations also led to better electrochemical properties. In particular, concentration-gradient niobium-doping strategy was applied to modify single-crystal LiNi_0.83Co_0.12Mn_0.05O₂, with Nb concentration decreasing linearly from the surface to the core of the particle [738].

7. Cells with Ni-Rich Cathodes

The general progress on the engineering of electrolytes for lithium batteries is out of the scope of the present work and have been reviewed elsewhere (for a recent review, see [739]). We only report in the following recent results obtained concerning batteries based on Ni-rich cathodes with Ni content > 0.5 of interest in the present work. The reason why we specify this point comes from the fact that electrolytes working for Ni concentrations ≤ 0.5 do not necessarily work at higher concentrations. For instance, for various additives, which can greatly enhance the performance of Li-ion batteries with LiNi_xCo_yMn_1–x–yO₂ (x < 0.6) cathodes, there is no obvious improvement for Ni-richer cathode materials [740].

7.1. Solid Electrolytes

All-solid-state lithium batteries (ASSLBs) with Ni-rich cathodes are the most promising solution to improve the safety and thermal stability limited by the conventional organic liquid electrolytes. In particular, sulfide-based solid electrolytes have been considered [741]. However, the interfacial resistance caused by poorly conductive side products, the contact loss, and the space charge layer effect have limited the application of Ni-rich oxide cathodes in sulfide-based ASSBs [742]. Again, coating has also been considered as a solution to overcome these difficulties. Payandeh et al. studied the effect of Li₂CO₃/LiNbO₃ coating on the cycling performance of NCM-851005 (85% Ni content) in pellet-stack solid-state-battery with Li₆PS₅Cl as solid electrolyte and Li₄Ti₅O₁₂ anode and found that 80% of the initial specific discharge capacity is retained after 200 cycles (~160 mAh·g⁻¹, ~1.7 mAh·cm⁻²) at C/5 and 45 °C [743]. Walther et al. showed that the Li₂CO₃/LiNbO₃ coating suppresses the interfacial reaction at the cathode active material/Li₆PS₅Cl interface, in particular, the formation of oxygenated phosphorous and sulfur compounds [744]. An ASSB with LiNbO₃-coated NMC811, an electrolyte consisted of a polar poly(vinylidene fluoride–co–trifluoroethylene) (P(VDF–TrFE)) framework and Li₆PS₅Cl via an electrospinning–permeation–heating–pressing way Li₆PS₅Cl electrolyte, and Li-In anode showed 71% capacity retention after 2000 cycles at current density of 0.1 mA cm⁻² [745]. Li₃PO₄ coating also fulfill the purpose, because this phosphate contains the same anion (O²⁻) and cation (P⁵⁺) species as those present in the cathode and sulfide electrolyte, respectively, preventing the exchange of S²⁻ and O²⁻ in Li₆PS₅Cl and NMC [746]. Atomic layer deposition of LiAl(PO₃)₄ onto LiNi_0.88Co_0.09Mn_0.03O₂ (NCM88) accelerated the charge transfer and mitigated the interfacial deterioration. The ASSB with this modified cathode, Li₆PS₅Cl electrolyte, and Li-In anode, with a high loading of 20.1 mg cm⁻² exhibited a reversible capacity of 4.25 mAh/cm² and a capacity retention of 98.3% after 440 cycles at 0.5 C [747] (see Figure 19). Lin et al. constructed a LiNi_0.83Co_0.12Mn_0.05O₂ (NCM83125) cathode modified by Zr+F co-doping, plus cyclized polyacrylonitrile (cPAN) coating. This polymer was chosen because the pyridine-based structure with delocalized π bonds via thermal cyclization of PAN units endows cPAN with satisfactory electronic conductivity. ASSLBs utilizing this modified NCM83125 as a cathode, Li₆PS₅Cl electrolyte, and Li-In anode, exhibited a higher initial capacity (175 mAh g⁻¹) and Coulombic efficiency (82.7%), a capacity retention of 95% after 300 cycles at a rate of 0.2 C, and an enhanced rate performance (109 mAh g⁻¹ at 3 C) [748]. Via atomic layer deposition of Li₃PO₄ and subsequent in situ formation of a gradient Li₃P_1+xO₄S_4x coating, Liang et al. obtained a conformal covering for NMC811 particles [371]. Tested in combination with an indium metal negative electrode and a Li₁₀GeP₂S₁₂ solid electrolyte, this modified NMC811 enabled the delivery of a specific discharge capacity of 128 mAh g⁻¹ after almost 250 cycles at 0.178 mA cm⁻² and 25 °C. using advanced molecular layer deposition (MLD) technique, Su et al. coated single-crystal NMC811 with an elastic and sticky alucone/glycol layer [749]. This coat enabled NMC to be in close contact with the sulfide electrolyte, so that the ASSB with a mass loading of 20.4 mg cm⁻² at 60 °C exhibited the capacity retention of 80.0% after 1000 cycles at 1 C.

To overcome the limitation of the performance due to the low conductivity of metal oxides used as coating layers, Kim et al. coated Li[Ni_0.88Co_0.11Al_0.01]O₂ with Li₃YCl₆, the conductivity of which is 0.40 mS cm⁻¹. The solid-state battery with this Li₃YCl₆-coated NCA, Li₆PS₅Cl as solid electrolyte, and Li-In anode exhibited a capacity of 134 mA h g⁻¹ at 2 C (vs. 53 with the pristine NCA) [750]. Li_6.25La₃Zr₂Al_0.25O₁₂ -coated Zr doped NMC811 exhibited a capacity retention of 70.1% after 1000 cycles at 1 C in a full ASSB cell with Li-In anode and Li₆PS₅Cl electrolyte, demonstrating that this coating layer enhanced the stability between the Ni-rich cathode material and the sulfide electrolyte [751]. Wang et al. proposed a new process, where LiNi_0.88Co_0.09Mn_0.03O₂ (NCM88) cathode is sulfidized via a time-/cost-efficient strategy in a mixed N₂ and CS₂ gas atmosphere, improving the interfacial compatibility with Li₆PS₅Cl. The corresponding ASSB with Li₆PS₅Cl electrolyte and Li₄Ti₅O₁₂ anode delivered a capacity of 200.7 mAh g⁻¹, 87% capacity retention after 500 cycles, and satisfactory rate capability (158.3 mAh g⁻¹ at 1 C) [752]. Kitsche et al. prepared HfO₂-coated LiNi_0.85Co_0.10Mn_0.05O₂ (NCM-851005) via atomic layer deposition (ALD) of tetrakis(ethylmethylamido)hafnium(IV) with O₃ as the oxidant [753]. The test in a cell with this modified cathode, Li₆PS₅Cl electrolyte and Li₄Ti₅O₁₂ anode exhibited a capacity retention of 82% after 60 cycles at 0.5 C and 45 °C. A drawback with the use of thiophosphate-based electrolytes is their limited stability toward the oxidizing potentials of Ni-rich cathodes, which limits the cycle ability of the ASSBs. Therefore, efforts are made to choose different solid electrolytes, such as chloride-based SEs, principally Li₃InCl₆. However, Li₃InCl₆ is unstable when in contact with the Li anode, so that Gao et al. introduced a layer of Li₃InCl₆ adjacent to the LiNi_0.83Mn_0.06Co_0.11O₂ cathode, and a Li₆PS₅Cl layer placed between this and the Li-In alloy acting as the anode [754]. Therefore, Li₆PS₅Cl could not be entirely suppressed. Nevertheless, the cell made of NMC, solid electrolyte, and carbon nanofiber mass ratios of 65, 30, and 5 wt.%, respectively, showed significantly improved electrochemical properties. When cycled at 1 mA cm⁻² and 80 °C under 2 MPa stack pressure at a cutoff voltage of 4.2 V, the cell delivered a capacity of 118 mA h g⁻¹_total, with capacity retention of 94% ± 1.2% after 50 cycles. Note the stack pressure was needed to obtain this result by limiting the volume change of the cathode material to approximately 2.5% during cycling.

The coating of the particles proved useful to suppress the cracking of the particles and protect them from side reactions with the electrolyte. However, the process increases the cost of fabrication of the cathodes. Recently, solid-state electrolytes (SSE) were employed to prevent the cracking of the Ni-rich particles without the need of coating, provided that intimate contact between the cathode material and the solid electrolyte is maintained. For this purpose, sulfide solid electrolytes should be avoided with unprotected Ni-rich materials, because they decompose at a cathode voltage as low as 2.6 V [755]. Note, however, that Kim et al. constructed an all-solid-state battery utilizing the argyrodite-type sulfide electrolyte, Li₆PS₅Cl (LPSCl), as the solid electrolyte, with an NMC811 cathode material coated with lithium lanthanum titanate (LLTO). The authors employed a rapid technique known as flash-light sintering (FLS) to fabricate a uniform and pure perovskite protective layer without inducing cation mixing in NMC [756]. The Li-In/LPSCl/modified NMC811 cell cycled in the voltage range 2.5–4.25 V exhibited an initial capacity at 1 C of approximatively 100 mAh g⁻¹ lower than that of bare NMC811 (167.96 mAh g⁻¹), and a better cycle ability, with a capacity retention of 95%, but over 40 cycles only. Zhao et al. coated quasi single-crystalline LiNi_0.83Co_0.12Mn_0.05O₂ (SC83) particles to protect the cathode material from Li₆PS₅Cl solid electrolyte. The all-solid-state cell exhibited a capacity retention of 88% after 1000 cycles at the 1 C rate, compared to only 71% for the uncoated counterpart [757] (see Figure 20).

On the other hand, the employment of >4 V-class cathode materials is enabled without any protective coating with lithium metal halide solid-state electrolytes, due to their high electrochemical oxidation stability [758,759,760,761]. An all-solid-state Li battery with LiNi_0.85Co_0.1Mn_0.05O₂ cathode and Li₂In_xSc_0.666–xCl₄ (0 ≤ x ≤ 0.666) electrolyte exhibits a long life of >3000 cycles with 80% capacity retention at room temperature. High cathode loadings are also demonstrated in ASSBs with stable capacity retention of >4 mAh cm⁻² (~190 mAh g⁻¹) [762]. A remarkable performance was achieved in an all-solid-state cell composed of a single crystallized uncoated NMC811 as a cathode, a Li₃YCl₆ electrolyte, and a Li–In alloy anode: this ASSB delivered a discharge capacity of 170 mAh g⁻¹ at C/5 and an impressive capacity retention of nearly 90% after 1000 cycles [763]. Liu et al. reported the development of free-standing-type composite cathodes containing NCM (Ni stoichiometries ≥ 0.83) active materials and Li₃InCl₆ (LIC) solid electrolyte. The strong interface was realized via a novel in situ recrystallization process of LIC. The resulting free-standing composite cathode containing LiNi_0.88Co_0.06Mn_0.05Al_0.01O₂ (NCM88) and LIC sustained 1500 charge–discharge cycles under a low operating stacking pressure ~2 MPa [764].

7.2. Liquid Electrolytes

With liquid electrolytes, the residual Li species present on the surfaces of Ni-rich particles promote the decomposition of electrolytes to produce corrosive HF. This enhances the dissolution of the TM ions in the electrolyte [765], and it reduces the Li⁺ diffusion coefficient [766]. The migration of these TM ions to the anode side hinders the insertion of lithium inside the anode, which implies a loss of capacity and the deterioration of the battery [767]. Note that there is a difference between NMC and NCA materials concerning the dissolution of TM ions. The disproportionation of Mn ions (Mn³⁺ → Mn⁴⁺ + Mn²⁺) may lead to more severe dissolution of Mn than Ni and Co ions [768]. This is consistent with the fact that the capacity loss of batteries is closely related to the concentration of Mn ions in the cathode. Actually, a Ni-rich pristine NCA cathode exhibits much less dissolution of TM (where TM = Ni) than a Ni-rich pristine NMC (where TM = Ni + Mn), due to the stabilized effect of Al³⁺ [769].

Highly delithiated (highly charged) states generate large amounts of Ni⁴⁺ that can react with electrolytes. This results in a thickening of the cathode–electrolyte interface (CEI), reducing the number of available Li⁺ and generating a resistive surface layer [18]. The high Ni⁴⁺ concentration also leads to a decay in the electrochemical reaction activity [9]. It will also result in a decrease of the thermal stability and thus an increase of the risk of thermal runaway of the battery [92,770]. Above 4.2 V, the evolution of CO₂ gas and the generation of a heat flow from NMC cathodes [16] give evidence of such a reaction of the particles with the electrolyte. In addition, remaining at a highly delithiated state can cause significant electrolyte oxidation resulting in other side reactions [14]. To remedy this drawback, additives are employed in the electrolytes. In particular, a mixture with 2 wt.% prop-1-ene-1,3-sultone (PES) + 2 wt.% tris-(trimethyl-silyl) phosphite (TTSPi) + 2 wt.% 1,3,2-dioxathiolane-2,2-dioxide (DTD) in the control electrolyte (1 M LiPF₆ ethylene carbonate (EC): ethyl methyl carbonate (EMC) 3:7 w:w electrolyte), named PES211, can be used in Ni-rich cells to suppress the gas evolution [771]. In addition, rock-salt surface layers known to be formed with the use of VC are not formed with PES211. In practice, however, the Ni-rich cells should not be fully charged, which, however, implies a loss of energy density available with these batteries [772]. In addition, additives added to the electrolyte must help in the formation of the cathode–electrolyte interphase (CEI), but also of the solid–electrolyte interphase (SEI) on the anode side. In particular, in a full cell graphite/Ni-rich cathode, the dissolved Ni migrate to the graphite side and the strong correlation between Ni²⁺ and Li⁺ in the SEI is the main reason for the increase of SEI resistivity [773]. Many results are reported for full cells with graphite anode. However, other anodes can be chosen, in particular calcium tetrafluoroborate (Ca(BF₄)₂) as additive for the rational design of interfacial layers on both Li metal anode and Ni-rich cathode [774]. In this case, Li-Ca alloy suppressed the growth of Li dendrite by reducing the nucleation polarization. The preferential oxidation of BF₄⁻ in Ca(BF₄)₂ results in the formation of B-based CEI. For batteries with Ni-rich cathodes and Si-based anodes, we guide the reader to a recent review devoted to them [775]. In addition, we would like to mention a result too recent to be included in this review, dating from 2023. Huang et al. introduced tetravinylsilane (TVSi) as a multifunctional electrolyte additive into a commercial electrolyte to engineer tailored interphases simultaneously on Si anode and LiNi_0.92Mn_0.05Co_0.03O₂ cathode [776]. This full cell demonstrated a capacity retention of 83% after 200 cycles at 1 C. In addition, Zhang et al. proposed 3-Fluoro-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)picolinonitrile (FTDP), which decomposed to form B-contained and cyano (CN) group-rich CEI, as well as LiF-, Li₃N-rich solid SEI, simultaneously, resulting in the integrity and stability of electrodes [777]. The 1.6 Ah NMC811/SiO_x pouch cell with 0.2 wt.% of this additive delivered a high-capacity retention of 84.0% after 300 cycles at the current of 1.0 A.

7.2.1. Additives

The surface reactivity of the layered cathodes is related to the presence of surface-active oxygen resulting from TM-O bond covalency, which increases with increasing Ni content [778]. The consequence is a dehydrogenation of the ethylene carbonate (EC), which has been observed on NMC811 electrodes at a voltage of 3.8 V vs. Li⁺/Li [779]. This oxidation of the carbonate solvent is considered as the main route for the electrolyte decomposition [780,781]. This oxidative decomposition of the electrolyte generates a CEI that must be homogeneous to protect the particles from further degradation [782], and thin enough to avoid a thick resistive CEI that would reduce the rate capability [783]. To construct such a CEI, an electrolyte additive is a good strategy [784,785]. The most popular additive for this purpose is fluoroethylene carbonate (FEC) [786,787,788]. The reason for the use of fluorinated carbonates is their lower HOMO and LUMO energies compared to their nonfluorinated counterparts, due to the strong electronegativity of fluorine. Therefore, they have higher oxidation potentials and higher reduction potentials, so that they can be used in high-voltage cells. In addition, they are known to generate a SEI protecting the anode.

Organic additives can be sacrificed at high voltages (4.3–4.5 V) because their HOMOs are higher than the HOMOs of organic solvents, so that they are able to prevent the oxidative decomposition of the solvents [789]. The electro-polymerization of electrolyte additives can also provide electrons to counteract the decomposition of solvents [790]. Other solvents can be used to allow for the formation of more even CEIs, reducing contact between electrode surfaces and electrolytes [791].

Replacing Mn and Co with Ni in layered LiNi_xMn_yCo_1–x–yO₂ (NMC) positive electrodes promotes the dehydrogenation of carbonate-based electrolytes on the oxide surface, which generates protic species to decompose LiPF₆ in the electrolyte. The stability of the surface of NMC can be enhanced by decreasing free-solvent activity in the electrolyte through controlling salt concentration and salt dissociation. In particular, NMC811 with capacities greater than 150 mAh g⁻¹ (77% retention) after 500 cycles could be achieved by simply increasing the LiPF₆ concentration [792]. Otherwise, protection of the Ni-rich particles is needed to avoid thermal runaway due to the reaction of NMC with the LiPF₆ salt of the electrolyte. Forero-Saboya et al. used lithium hydridoaluminate as electrolyte additive, which acts as HF scavenger. Moreover, its oxidation results in the formation of an Al-rich protective layer on the cathode. With this additive in the electrolyte, the LiNi_0.86Mn_0.07Co_0.07O₂ exhibited a capacity retention of 66% at C/3 after 200 cycles with a graphite anode, raising at 87% with Li₄Ti₅O₁₂ anode [793]. Hu et al. proposed the dual additives of tetrabutyl titanate (TBT) and lithium difluoroxalate borate (LiDFOB) to form a stable Ti-, B-, and F-rich CEI [794]. The LiDFOB additive is functionalized by fluorine (F)-rich functional groups, which leads to the formation of the LiF-based SEI layers via reductive decomposition. In addition, TBT stabilized the electrolyte by removing H₂O/HF. The Li∣NMC811 cell with the dual additives exhibited a capacity retention of 86% after 200 cycles at 1 C and a high cut-off voltage of 4.5 V. This is also the capacity retention obtained with N,O-bis(trimethylsilyl)acetamide additive [795]. Li₃(CB₁₁H₁₂)₂(CB₉H₁₀) hydroborate electrolyte was successfully integrated in a NMC811 cell. With the Li-In anode, the cell had a discharge capacity at C/10 of ~145 mAh g⁻¹ at room temperature. It retained 98% of the initial discharge capacity after 100 cycles at C/5 and 70% after 1000 cycles at C/2 [796]. Capacity retentions of 97% after 100 cycles at C/5 and 75% after 350 cycles at C/2 are also achieved with a graphite anode without any excess Li. The energy density per cathode composite weight of 460 Wh kg⁻¹ is on par with the best solid-state batteries.

Hu et al. tuned the spatial linking structure of dipentacyclic anhydride (DPA) to obtain a new class of DPA-based additives to obtain robust CEI and SEI layers [797]. Among them, the best results were obtained with 1,2,3,4-cyclobutane-tetracarboxylic dianhydride (CBDA). The CBDA with a quaternary ring spatial linking structure changes Li⁺ coordination, which decreases the content of organic species in the CEI. In addition, CBDA makes more PF₆⁻ and FEC to transfer to both of the electrodes’ surfaces for decomposition owing to their stronger binding energies. As a result, a prototypical 216 Wh kg⁻¹ (1.2 Ah) NMC811//graphite pouch cell with this additive in the electrolyte exhibited an ultrahigh capacity retention 94% over 800 cycles at a current density of 1 C. Moreover, prolonged stable cycling for 900 cycles (93% capacity retention at 1 C) in an NMC811//graphite pouch cell was enabled by CBDA, outperforming the best cycling performance of NMC811//Li-metal batteries incorporating monocyclic anhydride additives (83% after 200 cycles at 1 C) [798]. Jiang et al. employed glutaric anhydride (GA) as a multifunctional electrolyte additive to in situ construct a CEI, which significantly enhanced the cycling performance of Li/NMC811 cell [799]. Therefore, anhydride additives can be used with Ni-rich based batteries, although the functions and mechanisms of these additives are not well understood and need further investigations.

Qin et al. reported a Janus-faced CEI film constructed by succinonitrile (SN) and cyclohexylbenzene (CHB) additives [800]. SN is adsorbed on electrode particles, while CHB is electrochemically polymerized generating an outer layer subsequently. The Li/LiNi_0.8Co_0.1Mn_0.1O₂ cell with SN+CHB electrolyte presented excellent long-cycle performance with a capacity retention ~72.3% in 600 cycles at the rate of 0.5/1.0 C between 2.8–4.7 V.

VC is known to generate a poly(carbonate)-based SEI layer, but it usually hinders Li⁺ migration at the Ni-rich NCM interface [801,802]. A co-additive is thus recommended to compensate for the inferior properties of the VC additive. Kim et al. proposed the choice of LiDFOB as a co-additive of VC for Ni-rich NCM and SiO_x electrode materials [803]. The LiNi_0.83Co_0.11Mn_0.06O₂ (NCM83)//SiO_x cell with this modifies electrolyte exhibited a capacity retention of 72.7% after 100 cycles at 1 C. Dai et al. selected LiNO₃ and LiBF₄ as dual additives. With an electrolyte consisting in 1.0 M LiPF₆ in SL/FEC/EMC with 0.5 wt.% LiBF₄–LiNO₃, NMC811-graphite cell retains 90.4% of the initial capacity after 200 cycles at 2 C (~6 mAh cm⁻²), owing to the formation of a uniform and LiF-inorganic (including B and S)-rich CEI and a dense, thin Li₃N–LiF-rich SEI [804].

Excessive Li is usually added in the preparation of NMC-based cathodes and tends to accumulate on the surface of particles and react with electrolytes to convert to lithium carbonate and hydroxide [423,805]. LiPF₆ can react with excessive alkaline Li on the surfaces of cathode particles to produce chemically stable Li₃PO₄ and LiF, and significantly suppress parasitic reactions caused by excessive Li [806]. Phelan et al. showed that higher concentration of LiPF₆ leads to formation of more Li_xPO_yF_z-/LiF-rich CEI that protects Ni-rich cathode materials [807]. Qiao et al. introduced Li₂O as a preloaded sacrificial agent on a NMC811 cathode to provide an additional Li source to offset the irreversible loss of Li, and employed fluorinated ether additive to neutralize O₂⁻ species released through Li₂O oxidation [808]. This led to the construction of a LiF-based layer at the CEI. The pouch cell constructed with this modified NMC811 cathode has a gravimetric energy density of 320 Wh kg⁻¹, maintaining 80% capacity after 300 cycles. On the other hand, reactions between LiPF₆ and trace amounts of water generate HF. To prevent such reactions, 3-IPTS can be used as an electrolyte additive: it can consume the trace amounts of water in electrolytes [789]. Zhang et al. demonstrated an effective approach to inhibit the dissolution of TM ions by using medium concentrated lithium bis(fluorosulfonyl)imide (LiFSI)-based electrolytes with an optimized fluoroethylene carbonate (FEC) content. In addition, this electrolyte creates the formation of a LiF-rich layer on the LiNi_0.90Mn_0.05Co_0.05O₂ cathode surface [809]. Triallyl cyanurate (TAC) additive forms robust CEI via oxidation decomposition and inhibits LiPF₆ hydrolysis. It also modifies SEI on the anode through reduction decomposition. As a result, TAC substantially enhances the electrochemical performance of Ah-level pouch cells with various Ni-rich cathodes, achieving long cycle life at both room temperature and 45 °C, alongside improved high-temperature storage capabilities, with both graphite and graphite-SiO_x anodes [810] (see Figure 21). Zhuang et al. proposed bis(di-tert-butyl)-4-dimethylaminophenylphosphine (Bis-4TMPA) as electrolyte additive [811]. The phosphine group in Bis-4TMPA reacted with the oxygen species and formed a stable CEI, while the nitrogen-containing group reacted with HF to reduce the corrosion on the electrode. The LiNi_0.8Co_0.1Mn_0.1O₂ with the addition of 0.5 wt.% Bis-4TMPA exhibited a capacity retention rate of 76.3% after 400 cycles (1 C = 200 mAh g⁻¹), against 69.1% for the pristine NMC811. Zou et al. utilized 4-fluoro-N, N-dimethylbenzenesulfonamide (FBSN) as an additive to refrain the dehydrogenation of EC [812]. This result is due to the formation of a CEI composed of LiF and benzene ring skeleton along with conductive S-rich and N-rich lithium species. Moreover, this CEI can be effectively self-repaired in time when damaged. As a result, the LiNi_0.9Co_0.05Mn_0.05O₂ (NCM90)//Li cells with 2% FBSN-containing electrolyte delivers a capacity retention of 90.2% after 200 cycles at 1 C, and 83.1% after 200 cycles even at higher voltage of 4.5 V With 2,4,6-triphenyl boroxine (TPBX) additive, the graphite/NMC811 maintained 84% of the initial discharged capacity after 100 cycles at 0.5 C (69% for the cells using the baseline electrolyte [813]. Zhang et al. proposed a diazacyclo type additive, 2-fluoropyrazine (2-FP). When adding 0.2% 2-FP to the electrolyte, the capacity retention of single-crystal LiNi_0.90Co_0.05Mn_0.05O₂ (NCM90) cathode was promoted from 72.3% to 82.1% after 200 cycles at 1 C [814]. This improvement is attributed to the formation of a CEI film through ring-opening electrochemical polymerization of 2-FP upon the NCM90 electrode particles; This film suppressed of the decomposition of LiPF₆ and protected the cathode from side reactions with the electrolyte. Yang et al. fabricated a robust CEI through in situ polymerization of ethylene carbonate induced by aluminum isopropoxide [815]. This increased the capacity retention of the NMC811//Li cell at 1 C rate after the 3rd cycle from 80.8% to 97.8% with a highly reversible capacity of 176 mAh g⁻¹ after 200 cycles at 1 C.

Sulfone and sulfonate additives are used to stabilize both CEI of Ni-rich cathodes and the SEI at the anode side [816]. Kim et al. constructed a sulfonate-based CEI layer that selectively protect the surface of the Ni-rich NCMA cathode materials and control parasitic reactions. They selected 3-(N,N-dimethylpalmitylammonio)propane sulfonate (PAPS), which features amphiphilic characteristics due to its combination of hydrophilic (sulfonate) and hydrophobic (carbon) functional group. Above the melting point of PAPS (242 °C), the liquefied PAPS evenly attached to the Ni-rich NCMA material, and a further increase in temperature decomposed the PAPS precursor to form the sulfonate-based CEI layer [817]. The capacity retention of the PAPS-modified NCMA cathode after 100 cycles at 0.5 C improved at both 25 °C (92.7%) and at 45 °C (86.4%) compared with that of the pristine NCMA cathode (58.5% and 28.8%, respectively). Enhanced performance of the NMC811//graphite cell was achieved with 2-thiophene sulfonamide (2-TS) additive [818]. After 500 cycles in the voltage range of 2.80–4.20 V, the cell with 1% 2-TS lost 8.40% of the initial discharge capacity (23.59% without the additive). Li et al. proposed p-toluene sulfonylmethyl isocyanide (TOSMIC) additive. The high reduction activity of TOSMIC additive is mainly contributed by its sulfone group, while the high oxidation activity is primarily caused by its phenyl group. The graphite/LiNi_0.8Co_0.15Al_0.05O₂ cell with 3 wt.% TOSMIC exhibited a capacity retention of 67% after 100 cycles at 0.5 C [819]. The 3,3-diethylene di-sulfite (DES)-functionalized electrolyte regulates both the graphite anode and the NMC811 cathode, increasing the capacity retention of the cell to 82.53% after 150 cycles at 1 C when the cut-off voltage is raised to 4.5 V [820]. A noticeable result was obtained with 4-(allyloxy)phenyl fluorosulfate (APFS) additive. On one hand, the radical copolymerization of vinylene carbonate (VC) with APFS via electrochemical initiation creates a spatially deformable polymeric SEI on the SiG-C (30 wt.% graphite + 70 wt.% SiC composite) anode. On the other hand, it generates a thermally stable CEI containing S-O and S-F species on the NMC cathode material. The SiG-C/LiNi_0.8Co_0.1Mn_0.1O₂ full cell with the combined use of VC with APFS exhibited a capacity retention of 72.5% with a capacity of 143.5 mAh g⁻¹ after 300 cycles at 45 °C [821]. The crucial role of VC polymerization in enhancing CEI stability and mitigating surface reconstruction on NMC cathodes has been explored recently [822]. Liu et al. proposed bis(4-fluorophenyl) sulfone (BFS) as an additive, which improved the capacity retention of NMC811 from 75.19% to 83.04% after 100 cycles at the rate of 1 C [823]. This additive led to a stable CEI, and can be oxidized at state of charge into two sulfonates, which prevent the carbonate electrolyte from decomposition. Another sulfone-based additive, phenyl trans-styryl sulfone (PTSS) increases the capacity retention of NMC811//graphite full cell from 48% to 63% after 100 cycles at 1 C [824]. This is attributed to the formation of the CEI, but in addition, the PTSS induced-SEI film enhances the interphasial stability of graphite anode. A sulfonate-based CEI was obtained by using ammonium sulfate [825], increasing the capacity retention of NMC from 52.2% to 75.6% after 100 cycles at 0.5 C at 45 °C. Joung and Yim proposed N,N,N,N-tetraethylsulfamide to improve surface stability for Ni-rich oxides cathode [826]. This sulfone and amine group-based additive effectively constructed an effective CEI layer on NMC811 cathodes, inhibiting electrolyte decomposition. In addition, it lowered the fluoride concentration in the cell. Ahn et al. proposed allyl phenyl sulfone additive [827]. A sulfonate ester additive, 1,3-propane sultone, is effective to stabilize the CEI of NMC, and the SEI on the lithium anode by constructing a LiF/Li₂SO₃-rich interphase layer [828]. Like in the other sulfone-based additives already cited, the sulfone functional group effectively suppresses electrolyte decomposition during cycling. The allyl functional group promotes additional crosslinking reaction during CEI formation, which renders the CEI more durable. In addition, it scavenged fluoride (F⁻) species in the cell with NMC811. As a result, the capacity retention at a relatively high temperature of 45 °C where the standard electrolyte decomposition is accelerated, the capacity retention of the Li//NMC811 cell at 0.5 C raises from 64.3% with the pristine electrolyte, to 78.9% with 0.25 wt.% additive. A cell with SiO_x/C anode, NCM90 (LiNi_0.9Co_0.05Mn_0.05O₂) cathode, in electrolyte with allyl phenyl sulfone additive still delivered a capacity of 777 mAh g⁻¹ after 200 cycles at 0.5 C and 30 °C, corresponding to 79.3% capacity retention (57.8% capacity retention without the additive) [829]. Trimethylsilyl 2-(fluorosulfonyl) difluoroacetate (TMSFS) additive in the electrolyte enhanced the capacity retention of LiNi_0.9Co_0.05Mn_0.05O₂ (NCM90) from 73% to 85.1% at 1 C up to 200 cycles [830], compared to 78% obtained with bis(vinylsulphonyl)methane instead of TMSFS [831]. Another scavenger of HF is 1-(trimethylsilyloxy)cyclohexene (TMSCH) with functional groups (C=C, Si-O) electrolyte additive, which in addition regulars both the CEI and the SEI with lithium metal anode [832]. The retention of the Li/NMC811 batteries at 1 C are enhanced from 68.2% to 84.9% at 30 °C and 64.2% to 85.8% at 45 °C. with this additive. Choi et al. chose a sultone additive: 1,4-butane sultone (BS), because its HOMO is similar with that of the carbonate-based solvents, and can thus form a stable CEI. The Li[Ni_0.75Mn_0.25]O₂‖graphite full cell using the carbonate-based electrolyte with 1 wt.% BS additive exhibited a capacity retention of ~80% after 200 cycles at 200 mA g⁻¹ (~68% in absence of BS) [833].

Phosphorous containing additives can also participate in the formation of stable and robust CEI, and scavenge HF. Jo et al. introduced monobasic sodium phosphate (NaH₂PO₄) additive, which improved the thermal stability of NMC811 and enhanced the electrochemical performance of graphite//NMC811 cell at 60 °C [834]. On the other hand, lithium bis(trimethylsilyl) phosphate (LiTMSP) was incorporated as an electrolyte additive in graphite/NMC811 for improving the low-temperature performance (at −10 °C). Zhao et al. utilized carbonate-based electrolytes containing vinylene carbonate (VC) additive to stabilize graphite and phosphate compounds like triallyl phosphate (TPPC2) and tripropargyl phosphate (TPPC3) additives to stabilize the Ni-rich cathode. The artificial graphite/NMC811 with this modified electrolyte increased the capacity retention of NMC811/AG cells to 90.5% after 500 cycles at 1.0 C and 55 °C [835]. A better compromise between voltage drop, gas emission, and thermal stability in artificial graphite/NMC811 can be obtained with lithium difluoro(dioxalato)phosphate (LiDFDOP) additive, singly and in combination with co-additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) [836].

Allyltrimethylsilane additive has the same functions owing to C=C, Si-C groups, enhancing the capacity retention of NMC811 to from 46.4% to 82.9% after 200 cycles at 1 C [837]. We have already mentioned the improvement of the electrochemical performance of Ni-rich cathodes by coating the particles with siloxane [437]. 1,3,5-Trimethyl-1,3,5-tris(3,3,3-trifluoropropyl) cyclotrisiloxane (D₃F) as an electrolyte additive is also effective. 5 wt.% of this additive enhances the capacity retention of the graphite/NMC811cell from 38 to 76% after 300 cycles at 1 C [838]. Among siloxane additives, propargyloxytrimethylsilane (PMSL), allyloxytrimethylsilane (AMSL), and trimethylmethoxysilane (TMSL) have been tested by Chen et al. They concluded that the highest cycle ability at 60 °C is obtained with PMSL, the artificial graphite//NMC811 cell with this additive exhibiting a capacity retention of 85% after 300 cycles at 0.5 C in the voltage range 3.0–4.3 V [839]. Ren et al. introduced cyano-groups into a siloxane to synthesize a novel cyano-siloxane additive, namely 2,2,7,7-tetramethyl-3,6-dioxa-2,7-disilaoctane-4,4,5,5-tetracarbonitrile (TDSTCN). 0.5 wt.% TDSTCN additive enables ultrahigh nickel LiNi_0.9Co_0.05Mn_0.05O₂//graphite (NCM90//Gr) full cells with significantly increased cycle life, especially at an elevated temperature of 50 °C and a high charging cut-off voltage of 4.5 V [840]. Zhang et al. introduced lithium hexamethyldisilazide (LiHMDS) with a low oxidation potential, which improve the cycling stability of Li//NMC811 batteries at high-stress conditions such as high voltage (4.5 V) and high temperature (60 °C) [841].

Fluorine-containing electrolyte additives have excellent kinetic reactivity, which can preferentially generate stable SEI films and uniform CEI [842]. Lithium difluoro(oxalato)borate (LiDFOB) additive is a film-forming additive protecting the surface of Ni-rich cathodes [843]. The same additive has been used with a fluorinated electrolyte composed of 1 M LiPF₆ as a lithium salt in fluoroethylene carbonate (FEC)/ethyl methyl carbonate (EMC)/2,2,2-trifluoroethyl acetate (TFA) (1:3:1 by volume) [844]. The NMC811 cathode with this electrolyte and 0.2 M LiDFOB additive exhibited superior discharge capability (247.2 mAh g⁻¹), cycling stability (81.4% retention at 200 cycles, 0.5 C) and rate performance (154.5 mAh g⁻¹ at 5 C) at 4.6 V compared to commercial electrolytes (66.2% retention at 200 cycles, 108.0 mAh g⁻¹ at 5 C) in Li//NMC811 half-cells. Ahn et al. proposed 2,4-difluorobiphenyl (FBP) as a fluorine-based additive to form a stable CEI, where the biphenyl provides an overcharge protection [845]. This additive successfully enhanced the high-voltage cycling stability of the Ni-rich NCM83 cathode. Chen et al. utilized LiDFOB and tris(trimethylsilyl)phosphate (TMSP) as dual additives, where TMSP was added to scavenge HF [846]. Owing to the synergistic effect of LiDFOB and TMSP, the Li/LiNi_0.8Co_0.1Mn_0.1O₂ half cells exhibited the capacity retention 76.3% after 500 cycles at a super high voltage of 4.7 V and 1 C rate. 2-aminoethyldiphenyl borate (AEDB) significantly improves long-term cycling of a graphite//LiNi_0.85Co_0.1Mn_0.05O₂ full cell, with a capacity retention of 88% after 100 cycles with cutoff voltage set to 4.35 V, by forming a stable CEI [847]. LiNi_0.8Mn_0.1Co_0.1O₂, or NMC811 is particularly attractive, as it can achieve a notably high specific capacity (200–220 mAh g⁻¹ at a relatively high average discharge voltage of ~3.8 V vs. Li⁺/Li), stable cycling with an ultrahigh cut-off voltage of 4.8 V can be realized by using an appropriate amount of lithium difluorophosphate (LiPO₂F₂₎ in a common commercial electrolyte. The Li//LiNi_0.76Mn_0.14Co_0.10O₂ cell retains 97% of the initial capacity (235 mAh g⁻¹) after 200 cycles [848]. The decomposition products (Li₃PO₄ and LiF) of LiPO₂F₂ generated a protective SEI. This result is even better than the performance of NMC811 in presence of sulfonamide-based electrolyte [849].

Isocyanate additives also improve the electrochemical performance of Ni-rich cathodes [789]. Oh et al. proposed icosafluoro-15-crown-5-ether (IF-CE) as an additive [850]. Each molecule of this additive contains 20 fluorine atoms, which participate at the formation of SEI on the anode, and of CEI. A pouch-type full cell with 5 wt.% SiO-graphite anode and LiNi_0.88Co_0.08Mn_0.04O₂ cathode, and IF-CE + FEC additives in the electrolyte, demonstrates an initial capacity of 194 mAh g⁻¹ at 1 C, with capacity retention of 78% after 300 cycles. Peng et al. selected fluorine- and nitrogen-containing methyl-2-nitro-4-(trifluoromethyl)benzoate (MNTB) regulated with FEC to generate a robust CEI film while capturing HF [851] (see Figure 22 and Figure 23). The NMC811//Li batteries with 15% MNTB-containing electrolyte exhibited a specific capacity of 150.12 mA h g⁻¹ with a capacity retention of 81.10% after 300 cycles with 0.5 C charging and 1 C discharging in the voltage range of 3.0–4.3 V. Lei et al. chose pentafluorophenyl trifluoroacetate (PFTF) to adapt the NMC811cell by forming LiF-rich CEI/SEI films and eliminate HF, extending the cycling stability of the symmetrical Li cell is expanded to 500 h [852]. Cyclopentyl isocyanate (CPI) with a single isocyanate (−NCO) functional group as a bifunctional electrolyte additive was used to improve the interfacial stability of Ni-rich cathode LiNi_0.83Co_0.12Mn_0.05O₂ [853]. Tan et al. utilized anhydride additives to improve the electrochemical performance of LiNi_0.5Mn_1.5O₄ cathode [854]. Han et al. used 4-fluorophenyl isocyanate (4-FI) as an additive to improve the low-temperature performance of the NMC811//SiO_x@G battery due to the optimization of F in the SEI membrane components. The capacity retention of the NMC811/SiOx@G pouch cell increased from 92.5% (without additive) to 94.2% (with 1% 4-FI) after 200 cycles at 0.5 C. At −20 °C, the cyclic stability of the NMC811//SiO_x@G pouch cell increased from 83.2% (without additive) to 88.6% (with 1% 4-FI) after 100 cycles at 0.33 C [855]. 4-fluorobenzene isocyanate (4-FBC) electrolyte additive is also efficient to form a CEI film on Ni-rich cathodes. Tested in the NMC532//SiO@Graphite pouch full cells with electrolytes containing a mass fraction of 1% 4-FBC additives demonstrate improved capacity retention of 81.3% after 200 cycles at 1 C (39.1% without additive) [856]. 1,2,4-1H-Triazole (HTZ) was added to the electrolyte to improve the interfacial stability of LiNi_0.9Co_0.05Mn_0.05O₂, resulting in an increase of both rate capability and cycle ability up to 60 °C [857]. Owing to its −N=C=O group, isocyanoethyl methacrylate additive improves the performance of NMC811//graphite, increasing the capacity retention from 62.8% to 81.9% after 150 cycles at 1 C in the voltage range 2.75–4.4 V [858].

Other HF scavengers include silane derivatives [859]. In particular, with 3% silane-based electrolyte additive, vinyltrimethylsilane (VTMS) in the LiPF₆-based electrolyte, the graphite//NMC811 cell maintained a discharge capacity of 131.9 mAh g⁻¹ after 500 cycles in the voltage range of 2.8–4.3 V at 1 C [860]. Cheng et al. reported that a dual additive containing lithium bis(oxalate)borate and dopamine produces a uniform inorganic/polymer CEI that eliminates undesirable interfacial side reactions [861]. Guan et al. synthesized niobium-doped strontium titanate (Nb-STO) via a facile solvothermal method and used it as a surface modification layer onto the NMC811 cathode. [862]. Not only did Nb-STO prevent the dissolution of the transition metals, it also alleviated the internal resistance growth from uncontrollable CEI evolution. With this protection, the capacity of NMC811 was ~190 mAh g⁻¹ after 100 cycles under 1 C with capacity retention over 84% in the voltage range of 3.0–4.5 V (61% with pristine NMC811). Zhang et al. proposed an electrolyte additive, cis-1,2,3,6-tetrahydrophthalic anhydride (THPA), to enhance the interfacial stability of the Ni-rich LiNi_0.90Co_0.05Mn_0.05O₂ (NCM90) cathode [863]. This additive not only hinders the oxidation of carbonate solvents, but also generates a conductive protective CEI film. The capacity retention of NCM90 with an electrolyte containing 2.0% THPA reached 93.2% after 120 cycles at 1 C (1 C = 180 mA g⁻¹) at 30 °C, against 80.0% in absence of this additive. Zou et al. proposed a diboron electrolyte additive 5,5,5′,5′-tetramethyl-2,2′-bi-1,3,2-dioxaborinane (TBDB) to construct a stable LiNi_0.9Co_0.05Mn_0.05O₂ (NCM90)/electrolyte interface [864]. With 0.3% TBDB-containing electrolyte, the NCM90 cathode shows a capacity retention of 84.3% after 200 cycles at 1 C at a charge cutoff of 4.4 V (70.1% with the baseline electrolyte). Zeng et al. employed Tolylene-2,4-diisocyanate (TDI) additive containing lone-pair electrons. With this additive in the electrolyte, the graphite//NMC811 cell exhibited a capacity retention of 94% at 1 C 1000 cycles at 25 °C (78% in absence of this additive) [865]. The authors attributed this significant increase of the cycle ability to the isocyanate group playing a crucial role in inhibiting the generation of H₂O and HF, which leads to the formation of a thin and dense CEI. Lim et al. synthesized tetrakis(methacryloyloxyethyl)pyrophosphate additive to form a stable CEI, which increased the capacity retention of graphite/NMC811 cell to 82.2% after 300 cycles at a 1.0 C rate [866].

The oxidative decomposition of nitrile leads to the formation of a stable CEI responsible for the high-voltage stability of nitrile-containing electrolytes. Li et al. utilized lithium difluoro(oxalato)borate (LiDFOB) as the lithium salt and adiponitrile (C₆H₈N₂) as the additive. The LiNi_0.5Mn_1.5O₄//Li half-cell with this electrolyte delivered a capacity of 100.9 mAh g⁻¹ at 0.5 C, with a capacity retention of 81.2% after 50 cycles, significantly better than the result obtained in absence of the adiponitrile additive [867]. The additive adiponitrile complexed with Ni⁴⁺ ions on the surface of the cathode electrode inhibited the dissolution of Ni⁴⁺ and contributed to the formation of a CEI protective and conductive passivation film. The authors also calculated that the Ni⁴⁺ ion with a strong cathode potential and the cyanide group with a strong anode potential participated in charge transfer to stabilize the cathode material. In addition, adiponitrile also stabilizes the lithium anode, so that using 1 wt.% adiponitrile in 0.8 m LiTFSI + 0.2 M LiDFOB + 0.05 M LiPF₆ dissolved in EMC/FEC = 3:1 electrolyte, the Li/full concentration gradient Li[Ni_0.73Co_0.10Mn_0.15Al_0.02]O₂ battery achieves a cycle retention of 75% over 830 cycles under high-capacity loading of 1.8 mAh cm⁻² and fast charge–discharge time of 2 h [868]. Another nitrile additive, 1,3,6-hexanetricarbonitrile was used to raise the capacity retention of LiNi_0.83Co_0.07Mn_0.1O₂ to 81.42% at 1 C after 300 cycles at a cutoff voltage of 4.5 V [869]. Note, however, that nitrile has a poor reductive stability. Therefore, it should not be used with lithium anode, unless another additive is added to form a SEI protecting the lithium or graphite anode.

Qiu et al. designed 1-methyl-1-cyanopropylpyrrolidine bisfluoromethanesulfonimide salt (PYR_1(4CN)TFSI) to reduce the interface impedance on the NMC811 cathode side and enhance the compatibility between the electrolytes and the lithium metal anode. With the addition of 0.5 wt.% PYR_1(4CN)TFSI in the electrolyte, the Li//LiNi_0.8Co_0.1Mn_0.1O₂ cells delivered 107 mAh g⁻¹ after 200 cycles at 1 C [870]. The result is attributed to the combination of TFSI⁻ with pyrrole, which promotes the migration of Li⁺, while the cyano group can coordinate with oxygen atom electron pairs on TFSI⁻ to participate in the SEI film on the lithium anode. Park et al. utilized 5-methyl-4-((trifluoromethoxy)methyl)-1,3-dioxol-2-one and 5-methyl-4-((trimethylsilyloxy)methyl)-1,3-dioxol-2-one to provide spatial flexibility to the vinylene carbonate-derived SEI via polymeric propagation with the vinyl group of vinylene carbonate. This electrolyte was compatible with Si-C anode, and with high-voltage cathode, so the Si-C/NMC cell exhibited 81.5% capacity retention after 400 cycles at 1 C and fast-charging capability (1.9% capacity fading after 100 cycles at 3 C) [871]. Zhuang et al. introduced lithium tetrafluoro (1,2-dihydroxyethane-1,1,2,2-tetracarbonitrile) phosphate (LiTFTCP) additive, adding the effects of cyano-groups (−CN) into lithium fluorinated phosphate. NMC811//SiO_x-graphite full cells (cathode loading 17.2 mg cm⁻², 3.5 mAh cm⁻²) using 0.5 wt.% LiTFTCP additive deliver a capacity retention of 90.24% (172.8 mAh g⁻¹/191.5 mAh g⁻¹) after 600 cycles at 1 C, 30 °C, in the voltage range 2.7–4.25 V, with an average Coulombic efficiency (ACE) of 99.91% [872]. This remarkable result is due to cyano-enriched CEI layer and the stable LiF-enriched SEI layer.

Song et al. reported a systematic study of electrolyte additives in single-crystal and bimodal artificial graphite/NMC811, including vinylene carbonate (VC), 1,3,2-dioxathiolane-2,2-dioxide (ethylene sulfate or DTD), fluoroethylene carbonate (FEC), lithium difluorophosphate (LFO), and lithium bis(oxalate) borate (LiBOB) [873]. They concluded that 2VC + 1DTD cells possess the best or close to the best performance on long-term cycling at 40 °C and 55 °C, but their high charge transfer resistance (R_ct) leads to relatively poor performance in 20 °C 1 C long-term cycling. 2FEC + 1LFO cells are also the top performer on formation gas and R_ct and possess lower R_ct and ∆R_ct than 2VC + 1DTD cells in cycling and storage tests, but perform slightly worse in capacity retention during cycling at 40 and 55 °C.

Innovative electrolyte solvent technologies are also a good strategy. We have already mentioned fluorinated solvents [146,833]. We can also cite fluoroacetonitrile (FAN) solvent enabling 4.5-V graphite//NMC811 pouch cells (1.2 Ah, 2.85 mAh cm⁻²) to achieve high reversibility (0.62 Ah) when the cells are charged and discharged even at −65 °C [874], fluorinated cyclic ether allowing Li (50 µm)//NMC811 (4 mAh cm⁻²) cells to maintain 80% capacity after 568 and 218 cycles at room temperature and 60 °C, respectively [875], and 1,2-difluorobenzene enabling highly stable cycling of Li//NMC811 cells (4.4 V) at C/3 charge and 1 C discharge (1 C = 2 mA cm⁻²) for 500 cycles with a capacity retention of 93% [876]. By dissolving fluorinated electrolytes into highly fluorinated non-polar solvents, the LiNi_0.8Co_0.15Al_0.05O₂//Li battery can still deliver ~50% of its room-temperature capacity at −85 °C [877]. With an all-fluorinated electrolyte containing fluoroethylene carbonate (FEC), and ethyl (2,2,2-trifluoroethyl) carbonate (ETFEC), the Li//NMC811 cell showed a capacity retention of 72.3% with an average CE of 99.8% at high voltage (up to 4.6 V) after 225 cycles at 1 C [878]. β-fluorinated sulfone-based electrolytes enable the very stable long-term cycling of graphite//LiNi_0.6Co_0.2Mn_0.2O₂ full cells [879].

10% sulfone plus 0.02 M Lithium difluoro(oxalato)borate (LiDFOB) enabling ~400 Wh kg⁻¹ NMC811//Li cells at 4.7 V to demonstrate an 82% capacity retention after 200 cycles [880]. Chen et al. chose ethyl butyrate (EB) as a solvent to be fluorinated because of its low melting point. In particular, ethyl heptafluorobutyrate (EHFB) enabled 4.5 V graphite//NMC811 pouch cells (1 Ah) to perform stably over 200 cycles at −10 °C with only 2% capacity loss [881]. Xiao et al. enhanced the solubility of LiNO₃ in ester-based electrolytes by employing triethyl phosphate (TEP) as a co-solvent. As a result, NO₃⁻ is preferentially adsorbed on the electrode surface to derive a CEI layer with the homogeneous oxynitridation, while NO₃⁻ dominated weakly dissociated solvation clusters (NSC) suppress the dendrite formation on the lithium metal anode [882]. With this design, the Li//NMC811 full cell achieves 80% capacity retention after 300 cycles of 0.5 C. Another solubilizer, tris (2,2,2-trifluoroethyl) borate (TTFEB), utilizes its electron-deficient B atom to snatch electron-rich NO₃⁻ anion of insoluble LiNO₃ and thus forms a unique TTFEB-LiNO₃ solvation structure in carbonate electrolytes [883]. The full cell, featuring a thin Li anode (50 µm) and a high-loading NMC811 cathode (4.04 mAh cm⁻²) in the dual-additive electrolyte (FEC+TTFEB), demonstrated a capacity retention of 81.5% after 140 cycles at 1 C.

7.2.2. Electrolyte Formulations

An et al. designed a fire-resistant liquid electrolyte formulation consisting of propylene carbonate and 2,2,2-trifluoroethyl group-containing linear ester solvents paired with 1 mol L⁻¹ LiPF₆ salt and FEC additive. The replacement of linear carbonate with fluorinated linear ester yielded a fire-resistant and outperforming electrolyte under harsh condition. The fire-resistant electrolyte-incorporated industrial 730 mAh graphite//NMC811 Li-ion pouch battery achieved 82% retention after 400 cycles under 4.3 V charge voltage, 45 °C and 1 C [884]. Huang et al. designed an electrolyte consisting in1 M LiPF₆ in FEC/acetonitrile (FEC/AN, 7/3 by vol.). Combining the FEC-dominated solvation structure and the AN-rich environment, the designed electrolyte triples the capacity of the 1 Ah graphite//NMC811 pouch cells at 8 C in comparison with the commercial electrolyte [885].

Other electrolytes have been formulated to improve the performance of batteries with Ni-rich cathodes. In particular, fluorinated linear carboxylate esters with low freezing point and low viscosity as co-solvents enhance the performance of such batteries at subzero temperatures [886,887], while perfluorinated electrolytes improve their thermal stability [888,889], and EC-free electrolytes are studied to enhance the high-voltage cycle life and safety [890,891,892,893,894].

Another class of electrolytes is obtained with substitution of LiPF₆ salt by other salts (LiTFSI, LiFSI, and LiDFOB). In particular, dual-salt LiDFOB-LiBF₄ in a sulfolane-based electrolyte with fluorobenzene additive to restrain the decomposition of sulfonate endowed graphite//NMC811cell with capacity retentions of 83% after 500 cycles at 25 °C and 82% after 400 cycles at 60 °C [895]. Even better results were reported with 1.6 m LiFSI and (2-cyanoethyl)triethoxysilane (TEOSCN), which enabled long-term cycling mesocarbon microbead (MCMB)//NMC811 for 1000 cycles with an ultrahigh-capacity retention of 91% at 25 °C and for 500 cycles with a retention of 81% at 60 °C [896]. Xu et al. chose LiTFSI as the salt, and selected solvents with low donor number (DN < 10) and high dielectric constant to minimize Li⁺ desolvation energy, and thus the interfacial resistance, while still dissociating LiTFSI. These properties were fulfilled with an electrolyte consisting in 1 M LiTFSI is dissolved into methyl difluoroacetate (MDFA), methyl 2,2-difluoro-2 (fluorosulfonyl)acetate (MDFSA), and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (TTE). In addition, the fluorination endowed the electrolyte with non-flammability characteristics [897]. NMC811//graphite pouch cells with this lean electrolyte (2.5 grams per ampere hour) achieve stable cycling with an average Coulombic efficiency of more than 99.9 per cent at −30 °C. Su et al. developed a dual-anion regulated electrolyte using LiTFSI and lithium difluoro(bisoxalato)phosphate (LiDFBOP) as anion regulators. The incorporation of TFSI⁻ in the solvation sheath reduces the desolvation energy of Li⁺, and DFBOP⁻ promotes the formation of the CEI. As a result, Li//LiNi_0.83Co_0.11Mn_0.06O₂ pouch cells exhibited 84.6% capacity retention at 0.1 C charge and 0.5 C discharge rates [898].

Chen et al. also utilized LiFSI as the primary salt and TTE as the dilutant. Moreover, trimethyl phosphate (TMP) provided the basis for the introduction of lithium nitrate (LiNO₃) as the secondary lithium salt. LiNO₃ regulated the solvation structure and successfully prepared the phosphate ester-based localized high-concentration electrolyte with an apparent Li⁺ concentration of 1.15 M. Meanwhile, NO₃⁻ generated nitride-dominated electrode/electrolyte interfaces on the lithium anode and nickel-rich cathode surfaces. As a result, the Li//NMC811 battery with this electrolyte maintained 81.7% capacity retention after 800 cycles at 4.3 V, 1 C, and 93.3% capacity retention after 200 cycles at a high voltage of 4.5 V [899]. On the other hand, Yang et al. kept a carbonate electrolyte, and used double salts synergistic effect (LiPF₆+LiFSI) to construct a super concentrated (DSSC) electrolyte of 1 M LiPF₆ + 8 M LiFSI in EC:DMC (LiPF₆: LiFSI = 1:8, EC:DMC = 1:1 by weight) The LiNi_0.90Co_0.05Mn_0.05O₂ (NCM9055)//Li cells with this electrolyte exhibited the capacity retention of 93.04% and reversible capacity of 173.8 mAh g⁻¹ after 100 cycles at 1 C rate. This result was attributed to the formation of a stable and uniform 2–3 nm rock-salt phase protection layer on the surface of NCM9055, induced by the electrolyte [900]. Zhang et al. proposed an electrolyte consisting of 1.9 m LiFSI in a mixture of 2,2,2-trifluoroethyl trifluoromethanesulfonate (TTMS) and 2,2,2-trifluoroethyl methanesulfonate (TM) (1:2 by volume). These two solvents integrate the properties of sulfonates and fluorination forming a Li-rich CEI and low impedance SEI layer containing S species on graphite, so that this electrolyte endows 4.6 V NMC811//graphite (1 Ah) with a high-capacity retention of 83% after 1000 cycles [901]. Zhang et al. fabricated an advanced ether-based localized high-concentration electrolyte (LHCE) consisting of LiFSI, 1,2-dimethoxyethane (DME) and TTE with a molar ratio of 1:1.2:3 [902]. The LiNi_0.94Co_0.06O₂ (NC94) in this advanced ether-based LHCE exhibited capacity retentions of 81.4% after 500 cycles at 25 °C and 91.6% after 100 cycles at 60 °C in the voltage range of 2.8–4.4 V in Li//NC94 cells at 1 C cycling rate.

Note, however, that the LiFSI corrodes aluminum of the current collector at voltage ≥ 4.3 V [903], which is thus the limit of the upper potential range to be used, even if NMC is coated. Moreover, LiFSI and LiTFSI salts corrode the Ni-rich cathode materials [904], because the nickel ions induce the breakage of C-F bond of these fluor-lithium salts driven by voltage. To avoid these drawbacks, Qiao et al. designed a non-corrosive sulfonimide salt lithium((difluoromethanesulfonyl) (trifluoromethanesulfonyl)imide (LiDFTFSI)) [905]. The improvement of the electrochemical properties by the replacement of LiPF₆ or LiTFSI by this salt demonstrates the decisive role of the salt anion. Another demonstration is the outstanding improvement of the rate capability of Li//NMC811 pouch cell with an asymmetric Li salt, lithium 1,1,1-trifuoro-N-[2-[2-(2-methoxyethoxy)ethoxy)]ethyl] methanesulfonamide (LiFEA) [906]. This pouch cell with LiFEA maintained 81% capacity after 100 cycles under fast-cycling conditions (charging: 1.46 mA cm⁻², discharging: 3.66 mA cm⁻²). A highly fluorinated (8-CF₃) aluminum-centered lithium salt of lithium perfluoropinacolatoaluminate (LiFPA) facilitates the formation of a passivating CEI layer (see Figure 24) [907]. A Li//NMC622 cell in a high loading of 3.5 mAh cm⁻² with LiFPA-EC/DMC electrolyte can perform 100 stable cycles delivered a capacity of 3.43 mAh cm⁻² after 100 cycles at 0.2 C, corresponding to a capacity retention rate of 97.7%. Wu et al. utilized a dual-anion ionic liquid, 0.8Pyr₁₄FSI-0.2LiTFSI. This salt allows for stable interfaces at both Li-metal and Ni-rich electrodes. With this specific electrolyte, the LiNi_0.88Co_0.09Mn_0.03O₂ (NCM88)//Li cell achieved an initial specific capacity of 214 mAh g⁻¹ and outstanding capacity retention of 88% over 1000 cycles at 0.3C, and an average Coulombic efficiency of 99.94% [908].

Since Li-derived impurities at the surface of the particles degrade the electrochemical performance, attempts have been made to remove them. Exposition of the Ni-rich layered materials in air results in the presence of LiOH and Li₂CO₃ impurities at the surface, which can react with the electrolyte to generate CO₂. Rinsing with water is a way to remove them [909]. Unfortunately, the water molecules induce the extraction of Li⁺ from the surface due to Li⁺/H⁺ exchange [910], triggering surface degradation and increasing the CEI [294]. We have mentioned earlier that the best strategy to remove the Li-derived impurities is coating. In addition, coating of NMC avoids the dehydrogenation of EC solvent, observed at voltage as low as 3.8 V on the surface of pristine NMC811 [779]. Wang et al. replaced the highly flammable organic solvents EC in conventional electrolytes with dimethyl methyl phosphonate (DMMP) and 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropylether (HFE) to obtain an electrolyte having a wide potential window beyond 5.0 V, and forming an effective CEI. The Li//NMC622 battery with this electrolyte showed no capacity loss over 100 cycles, and an average CE of 99.6% at a high cutoff voltage of 4.5 V [911].

The interaction between the electrolytes and the Ni-rich materials is not yet well understood. A step has been made, however, by a recent work of Wang et al., who proposed a sp²-induction mechanism to address coordination deficiency through the coupling of interfacial orbitals between molecules and the cathode surface [912]. Among the sp²-hybrid high-fluorinated olefins in this work, (perfluorobutyl)ethylene (PFBE) is identified as an optimal inducing molecule. The Li//LiNi_0.8Mn_0.1Co_0.1O₂ (30 μm Li, high load 3.7 mAh cm⁻² NMC811) cell with PFBE electrolyte achieving 80% capacity retention over 320 cycles (voltage range 2.8–4.4 V, charge/discharge 0.2/0.3 C), compared to 175 cycles with a PFBE-absent electrolyte. The electrolyte was composed of LiFSI salt and methoxytrimethylsilane (MOTMS) solvent, chosen because PFBE cannot dissolve salt (molar ratio LiFSI:MOTMS 1.2:2).

7.3. Recycling Ni-Rich Batteries

Lithium, cobalt, and nickel resources are limited [913,914]. To overcome this criticality and the energy costs and environmental hazards associated with the extraction of these metals, large-scale recycling of Ni-rich-based batteries is mandatory [915,916]. At present, only cobalt is recycled at a low price, taking into account the high cost of cobalt. Advancements in recycling technologies to mitigate environmental impact and recover valuable materials is thus desirable [917,918]. The industrial recycling processes of positive electrode materials comprise pyrometallurgy and hydrometallurgy. They are energy-intensive due to their reliance on high temperatures, and potentially damaging due to toxic chemicals. In hydrometallurgical processes, acidic leaching in the presence of a reducing agent is commonly used, including the use of H₂O₂ as a reducing agent for Co³⁺ [919]. To limit the extensive amount of chemicals used in hydrometallurgy, an alternative approach has emerged consisting of solid-state reactions to convert oxide-based materials into salts that are readily soluble in aqueous media. This approach, hereafter denoted salt conversion, has already been demonstrated for different salts including sulfates [920,921,922]. The fundamental understanding of the reactivity of layered compounds with these media has been investigated in this paper. The authors established the conditions to convert layered lithiated transition metal compounds into sulfate-based products targeting the stabilization of the metal into the langbeinite structure K₂M₂(SO₄)₃. This conversion enables subsequent metal recovery using only water as medium, which therefore prevents the classical use of highly acidic solutions usually used in hydrometallurgy to destabilize the oxidic compounds. To illustrate the possibility of recovering metals using salt conversion, the authors highlighted LiCoO₂ as an example. However, the langbeinite-type structure can accommodate different cations such as M = Mg²⁺, Ca²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺, and Cd²⁺ [923] Therefore, the extension of the salt conversion to NMC and NCA compounds supports the versatility of this method for different types of electrode chemistry of LIBs, opening practical outcomes to the vital question of battery recycling. Moreover, Wang et al. published a synthesis method of new cathodes, using as raw materials NMCs obtained from spent lithium-ion batteries recycling system [924]. Hamitouche et al. proposed a method to chemically convert lithiated transition metal oxides into readily water-soluble sulfate products [925]. This method comprises a temperature-driven solid-state reaction between the electrode material and potassium hydrogenosulfate molten salt yielding K₂M₂(SO₄)₃ and potassium/lithium sulfates.

Recently, Ji et al. [926] proposed a greener and more economic effective pathway, centered on “repair” to regenerate spent Ni-rich cathode materials, following a process already proposed to regenerate LiFePO₄ cathode by using a multifunctional organic lithium salt [927]. Ji et al. introduced a closed-loop upcycling strategy that leverages the detritus of the cathode and anode current collectors produced during dismantling process, involving two dopants (Al and Cu) in a ternary eutectic molten salt system (NaCl-KCl-LiOH). The two dopants work synergistically, as Al³⁺ tends to fill TM layer vacancies, strengthening bonding and reducing antisite defects, while Cu²⁺ tends to fill Li layer vacancies. The process was applied to highly degraded polycrystalline LiNi_0.83Co_0.12Mn_0.05O₂ cathode. The upcycled material achieved a first-cycle discharge capacity of over 215 mAh g⁻¹ and maintained 93.3% capacity retention after 100 cycles at 1 C, and a fast-charging capability of 85 mAh g⁻¹ at 15 C. The 1.2 Ah pouch cell using this upcycled cathode material retained 91.1% capacity retention after 200 cycles.

It is crucial to address the non-uniform lithium distribution among the repeatedly reacted cathode particles. The solution can be achieved by chemical relithiation by using a lithiation agent in the regeneration process [915,916,927,928,929,930,931]. The problem is that these reagents can cause overlithiation and structural damage. In particular, the introduction of redox mediators (RMs), such as quinone-based and organic electron-donating materials avoid this problem and has proven more effective in the restoration of NMC cathode materials [932,933]. Recently, Kim et al. achieved 100% efficiency of chemical relithiation of NMC622 assisted by the redox mediator 3,5-di-tert-butyl-o-benzoquinone (DTBQ) [934]. This result offers a facile and sustainable solution for Ni-rich battery recycling.

Zheng et al. proposed a different process. They investigated selective leaching of lithium from spent NCM using advanced oxidant ammonium persulfate (NH₄)₂S₂O₈, resulting in the recovery of lithium carbonate Li₂CO₃. Then, the recovered Li₂CO₃ is combined with the selective leaching residue to synthesize fully-regenerated NCM via high-temperature solid-phase synthesis [935]

7.4. Safety Issues

Ni-rich lithium batteries suffer from severe thermal runaway under abuse conditions [936,937,938]. The release of oxygen by the cathode material is part of the problem, and cross-talking effects between cathode and anode play a critical role. The cross-talk of cathode-released oxygen to fully lithiated graphite anode [939,940], as well as the reaction between oxygen species and the electrolyte [941], can trigger the thermal runaway. It has also been suggested that the recombination of atomic hydrogen accumulated in graphite anode plays an important role [942]. LiH induced exothermic reactions at the anode side and H₂ generation and migration to the cathode side are also at the origin of thermal runaway [943,944,945,946]. The conclusion of these different works is that the thermal-induced cross-talking effects between the cathode and anode triggers the thermal runaway of the Ni-rich lithium batteries. This also implies that the stability of the CEI and SEI is more essential than that of the electrolyte components [947].

8. Conclusions

Progress in recent years on Ni-rich cathode elements have been made along different issues. Structural and chemical stability have been enhanced. Single-crystallized materials show better mechanical integrity compared to polycrystalline materials, reducing cracking and degradation. Efforts in research should then focus on this morphology. Introducing doping elements can stabilize the crystal structure, mitigate cation mixing, and improve thermal stability, and surface coating can suppress side reactions with electrolytes and reduce microcracking.

Progress has also been achieved to mitigate oxygen loss and electrolyte decomposition. High-voltage stable electrolytes with additives help reduce electrolyte decomposition at high voltages. Replacing liquid electrolytes with ceramic solid electrolytes can enhance stability and lifespan. Designing gradient Ni concentration across cathode particles can minimize oxygen release and improve cycle life.

Mechanical and microstructural improvements include crack-resistant design preventing the performance loss. Combining Ni-rich cathodes with other materials (e.g., Li-rich layered oxides) can balance energy density and stability.

Improved manufactory and scalability results from lower cobalt content, which reduces cobalt dependence, cuts costs, and improves sustainability.

An optimized sintering process has been achieved, due to advanced synthesis techniques like spray drying and atomic layer deposition, which can refine particle morphology and enhance material uniformity.

However, additional research is needed, and the future outlook depends on following research along the following lines:

• The interaction between the electrolytes and the Ni-rich materials and the related CEI needs further investigation. Newly-developed experimental set-ups, such as cryogenic transmission electron microscopy revealing atomic-resolution CEI structures, should help for this purpose. An example is the dynamic evolution of over cycling and its impact on the performance of NMC811, revealed by the ultrafine images of CEI obtained by this process in presence of FEC-based electrolytes [948].
• Safety issues with high-voltage cathode materials would plead in favor of the use of solid electrolytes. As shown in this review, some attempts are promising, but the research should continue in the next years. The space charge layer between the cathode and a ceramic solid electrolyte may result in high polarization and capacity degradation. Another difficulty comes from the deterioration of the contact between the cathode and solid electrolyte upon cycling, due to the rigid ceramic nature of the solid electrolyte and the change in the lattice parameters of the cathode material, in particular at high charge. The columnar shape of the Ni-rich particles or actually any elongated form will help [949], pointing to the need for further investigation on the correlations between morphology, synthesis, and electrochemical performance Elongated polymer electrolytes have already made possible the commercialization of LiFePO₄-based batteries, due to the low voltage (3.5 V) of the cells. With higher voltage cells, the same polymers cannot be used, because of their poor antioxidative ability. Further investigations on polymer electrolytes with wide electrochemical window are thus needed, and under studies [950]. Further research is needed to pair Ni-rich cathodes with lithium metal or silicon anodes. The reaction of the electrolytes with the Ni-rich cathode and the anode materials is not well understood, and further studies on the additives able to generate both a SEI and a CEI protecting the electrodes against side reactions with an electrolyte stable at high voltage are still needed.
• The application of AI to the research on Li-rich batteries is needed and most promising. It is possible today to predict material degradation pathways according to operating environments, which will help to optimize the design and composition of electrode materials for enhanced performance and durability of the Ni-rich cathode materials. Machine learning (ML) can accelerate the design of new Ni-rich cathode chemistries, and can also be used to build next-generation battery architectures. Many trials have been made to dope Ni-rich cathode materials with different single and multiple doping elements. ML models can predict dopants that improve stability without expensive trial-and-error experiments. In particular, deep learning models trained on large materials databases (e.g., Materials Project) predict stable Ni-rich structures with minimal degradation. ML will also help designing single-crystal vs. polycrystalline Ni-rich cathodes to reduce cracking. Computer vision models will be used to analyze SEM/TEM images to detect defects and suggest improvements in synthesis processes.

Ni-rich lithium-based cathode materials, particularly those in the layered LiNi_xCo_yMn_zO₂ (NCM) and LiNi_xCo_yAl_zO₂ (NCA) families, have emerged as frontrunners in the pursuit of high-energy-density lithium-ion batteries. Their increased nickel content directly contributes to elevated specific capacities, making them attractive for applications ranging from electric vehicles to grid storage. Over the past two decades, the field has experienced remarkable advancements in material synthesis, structural stabilization, and electrochemical performance optimization. This review has comprehensively explored the multifaceted aspects of Ni-rich LIBs, spanning from fundamental crystal chemistry and degradation mechanisms to cutting-edge approaches in coating, doping, and electrolyte engineering. Despite these advancements, the commercialization and long-term viability of Ni-rich cathodes are still hindered by persistent challenges. Chief among these are structural degradations during cycling, irreversible phase transitions, surface instability, gas evolution (notably O₂ and CO₂), transition metal dissolution, and thermal runaway risks at high states of charge. These degradation pathways are exacerbated by the high reactivity of Ni⁴⁺ and the increased lattice strain that arises during deep lithiation/delithiation cycles. Addressing these issues requires a deep understanding of bulk–surface–interface interactions under both normal and abusive conditions.

Innovations in material engineering, such as gradient compositions, core–shell morphologies, single-crystal structures, and atomic-scale doping, have demonstrated notable progress in mitigating degradation while improving thermal stability and capacity retention. Furthermore, the strategic development of advanced electrolytes, including localized high-concentration electrolytes (LHCEs), fluorinated solvents, and hybrid solid–liquid systems, has opened new frontiers in interfacial stability. Coupled with surface coatings based on metal oxides, phosphates, or ion-conductive polymers, these approaches form a synergistic framework to extend cycle life and enhance safety.

At the device level, progress in electrode architecture, electrode–electrolyte interface design, and formation protocols have collectively contributed to more robust, scalable battery systems. However, translating these laboratory-scale improvements into commercial viability necessitates further breakthroughs in synthesis cost reduction, scalable coating technologies, and recyclability strategies.

The field also stands to benefit from deeper integration of advanced characterization techniques—such as operando X-ray diffraction (XRD), transmission electron microscopy (TEM), neutron scattering, and synchrotron-based spectroscopy—alongside computational modeling and machine learning. These tools are critical for real-time insight into degradation pathways and for predictive materials discovery. Data-driven optimization and digital twin modeling may accelerate the design of next-generation Ni-rich cathode systems that are both high-performing and durable.

Looking forward, Ni-rich cathode chemistries will likely remain central to LIB technology for the foreseeable future, especially as efforts to reduce or eliminate cobalt become more urgent for ethical and economic reasons. Sustainable sourcing of nickel, closed-loop recycling systems, and lifecycle assessments will be crucial components of this ecosystem. Simultaneously, a holistic approach that incorporates safety engineering, environmental considerations, and total cost of ownership must guide the development and deployment of these advanced materials.

To our view, a breakdown of the best, most effective strategies-ranked by impact and practicality are as follows:

• Surface coating to prevent electrolyte attack, suppress transition metal (TM) dissolution, and stabilize the cathode–electrolyte interface. The benefits are the reduction of surface reactivity, minimization of HF attack, and improvement of the cycling stability. To fulfill this goal, however, the coating must be uniform, thin (<10 nm), and ionically conductive to avoid impeding Li⁺ transport.
• Doping, to enhance structural stability, suppress phase transitions, and reduce oxygen release. The cation doping stabilizes the layered structure and suppresses Li/Ni disorder, while anion doping, in particular F⁻ (fluorination), strengthens TM–O bonds and suppresses oxygen evolution. The benefits are improved structural integrity, better capacity retention, and reduced gas generation.
• Single-crystal morphology minimizes grain boundaries and microcracks, which are common initiation sites for degradation. The benefits are improved mechanical integrity, longer cycle life, and better thermal stability. However, there are drawbacks: this morphology is more difficult and costly to synthesize. It may require higher sintering temperatures.
• Gradient composition or core–shell design combines a Ni-rich core for capacity with a stable outer shell (lower-Ni or doped) to buffer against side reactions. These architectures enhance thermal and chemical stability and delay impedance rise.
• Optimized electrolyte formulations reduce parasitic reactions, stabilize CEI (cathode–electrolyte interface), and suppress gas generation.
• Particle size and morphology engineering is useful to reduce mechanical stress and optimize Li⁺ diffusion. Spherical secondary particles with controlled porosity or dense, crack-resistant structures are preferred, resulting in improved structural stability and rate capability.
• Thermal and pressure management during cycling, to reduce gas generation and mechanical damage. The strategy for this purpose is to use stack pressure or thermal management in cell design.
• Advanced synthesis techniques, to achieve controlled doping, precise morphology, and homogeneous element distribution.

The efforts in the next years should then be focused on the integrated design combining the above approaches, such as the synthesis of a Ni-rich cathode with gradient composition, surface coating (e.g., LiNbO₃), W-doping, and used with a fluorinated LHCE, which will significantly outperform a conventional NMC811 cell in both energy density and cycle life, even at high voltages (>4.3 V).

In conclusion, while Ni-rich lithium-ion batteries have achieved a high level of maturity, their full potential will only be realized through continued interdisciplinary collaboration across materials science, electrochemistry, manufacturing, and systems engineering (see Table S1). The challenges are complex, but the rewards—in terms of decarbonizing transportation, stabilizing renewable grids, and powering the next generation of electronic devices—are immense. With sustained innovation and strategic investment, Ni-rich cathode technologies are well poised to drive the next wave of energy storage breakthroughs.