Research Progress of Ternary Cathode Materials: Failure Mechanism and Heat Treatment for Repair and Regeneration

Abstract

With the large-scale application of lithium-ion batteries in the field of new energy, many retired lithium batteries not only cause environmental pollution problems but also lead to serious waste of resources. Repairing failed lithium batteries and regenerating new materials has become a crucial path to break through this dilemma. Based on the research on the failure mechanism of ternary cathode materials, this paper systematically combs through the multiple factors leading to their failure, extensively summarizes the influence of heat treatment process parameters on the performance of recycled materials, and explores the synergistic effect between heat treatment technology and other processes. Studies have shown that the failure of ternary cathode materials is mainly attributed to factors such as cation mixing disorder, the generation of microcracks, phase structure transformation, and the accumulation of by-products. Among them, cation mixing disorder damages the crystal structure of the material, microcracks accelerate the pulverization of the active substance, phase structure transformation leads to lattice distortion, and the generation of by-products will hinder ion transport. The revelation of these failure mechanisms lays a theoretical foundation for the efficient recycling of waste materials. In terms of recycling technology, this paper focuses on the application of heat treatment technology. On the one hand, through synergy with element doping and surface coating technologies, heat treatment can effectively improve the crystal structure and surface properties of the material. On the other hand, when combined with processes such as the molten salt method, coprecipitation method, and hydrothermal method, heat treatment can further optimize the microstructure and electrochemical properties of the material. Specifically, heat treatment plays multiple key roles in the recycling process of ternary cathode materials: repairing crystal structure defects, enhancing the electrochemical performance of the material, removing impurities, and promoting the uniform distribution of elements. It is a core link to achieving the efficient reuse of waste ternary cathode materials.

1. Introduction

As time goes by and energy sources are being consumed and depleted, finding alternative sources of energy is one of the solutions to the existing energy problems [1]. Lithium-ion batteries (LIBs) are one of the most essential types of new energy battery technology, with a broad range and large number of applications [2]. Among them, ternary lithium-ion batteries (NCM/NCA) have established significant market dominance in power applications due to their superior energy density (250–300 Wh/kg) and cost-performance advantages [3]. Ternary lithium batteries are named after the composition of the cathode material. There are two types of materials, LiNi_xMn_yCo_1-x-yO₂ (NCM) and LiNi_xAl_yCo_1-x-yO₂ (NCA) [4], in which metallic elements (Ni, Co, Mn, and Al) each serve specialized functions in maintaining the structural integrity of the crystal lattice and influencing their electrochemical function [5]. Only NCM is discussed here, which is further divided into LiNi_1/3Mn_1/3Co_1/3O₂ (NCM111), LiNi_0.5Mn_0.3Co_0.2O₂ (NCM532), LiNi_0.6Mn_0.2Co_0.2O₂ (NCM622), LiNi_0.8Mn_0.1Co_0.1O₂ (NCM811), etc. Different types of ternary cathode materials have different characteristics [6]. Due to the many advantages of ternary batteries and the pursuit of high-capacity batteries, the sales of ternary lithium batteries continue to rise. However, the life of lithium batteries is always limited, following an extended period of sustained cycling, as well as the internal structure of the battery changing and the generation of by-products and other reasons [7]. If spent batteries are not appropriately managed within the context of sustainable economic development, this will inevitably lead to both ecological contamination and the waste of valuable materials [8]. If waste batteries are casually discarded in the environment, with the ingress of rainwater, the leakage of electrolytes will occur and react with the binder, generating a large amount of fluorine-containing waste liquid and HF gas [9]. On the one hand, heavy metals such as Ni, Co, and Mn will enter the human body through the cycle, especially cobalt elements, which will enter water bodies and soil, affecting the ecosystem. The current collectors of both positive and negative electrodes are indigestible substances, which will cause environmental pollution. On the other hand, the reserves of these elements are limited, and the disposal of negative carbon electrode graphite will lead to a waste of resources. To solve the above problems, first, the remaining power in waste batteries should be fully utilized. Some automobile companies have already adopted AC/DC inverters to transfer the remaining power in the batteries back to the power grid [10]. Finally, the materials of all parts in waste batteries should be classified for recycling. In response to the problem of harmful substances leaking due to the reaction of the electrolyte solution, freezing technology is adopted to inactivate the batteries by freezing the electrolyte [11]. Various technologies are used to leach and precipitate valuable metal elements, and the current collectors of the positive and negative electrodes are recycled through high-temperature smelting.

Currently, the treatment methods for used batteries include step-by-step utilization, recycling by traditional methods, recovery of valuable metal elements, and direct recycling [12]. Step-by-step utilization refers to the application of used power batteries in different fields in stages according to their remaining capacity and characteristics, intending to maximize the utilization of battery capacity [13]. For example, retired high-capacity batteries are used in energy storage devices such as electric vehicles, which consume less energy. Traditional recovery methods include hydrometallurgy [14] and pyrometallurgy [15]. If recycling is carried out according to traditional methods, the economic benefits are not satisfactory, and if the cost of recycling is too great, then the significance of recycling will be greatly reduced. Traditional hydrometallurgy is a series of pretreatments of waste batteries; the cathode material will be removed, and leaching, precipitation, and many other steps to recover the cathode material of the various components will be carried out [16]. The advantage is that the treatment process is more mature, and the recovery efficiency is high, but the process is complex, involving a long cycle, and it has low economic efficiency. It uses many additional chemical reagents, resulting in a secondary waste of resources, in addition to the production of many harmful substances. The pyrometallurgy involves melting large quantities of crushed battery material at high temperatures. The advantage is that it is easy to operate and can handle a wide range of samples, but the process generates numerous harmful gases and results in a large waste of energy [17].

Compared with other common recycling methods, the preparation process of direct recycling is shorter. It can make the most of the remaining lithium in waste materials and is applicable to a variety of cathode materials. As shown in Figure 1, the direct recycling of waste cathode materials skips the materials consumed in the purification step and multiple dissolution and precipitation steps, greatly shortening the recycling process [18]. Therefore, the direct recycling of waste battery materials is the best choice from both economic and environmental perspectives, and it is also the optimal approach for dealing with waste batteries [19]. Waste ternary cathode materials contain valuable metals, such as Li, Ni, Co, Mn, etc., which are crucial components in recycled power batteries [20]. However, the Earth's reserves are limited, and extraction costs are high, so the extraction of critical minerals from spent batteries can alleviate the resource shortages and have a high economically sustainable value [21]. Direct recycling methods include solid-phase regeneration [22], the hydrothermal method [23], the molten salt method [24], co-precipitation [25], the sol-gel method [26], etc. These methods all involve the step of solid-phase heat treatment. The solid-phase heat treatment is carried out in an air or oxygen atmosphere, and the ideal materials are obtained by utilizing the diffusion and oxidation processes of thermal molecules. Solid-phase and molten-salt methods have lower temperatures and shorter cycle times than pyrometallurgy, the operation process is simple, and the regenerated materials can be restored to near-original levels. The solid-phase regeneration restores the layer structure of the ternary material by solid-state high-temperature reactions after supplementing the lithium source without introducing other impurities, thus restoring the channels for lithium ions to move between the positive and negative electrodes [27]. Chen et al. [28] described a process for calcining the precipitated metal oxalate precursor NCM523 in two stages. In the first stage, a specific proportion of lithium carbonate was added, and the mixture was calcined at 500 °C for 4 h. In the second stage, an optimal temperature of 850 °C was selected, along with a calcination time of 12 h, to produce the regenerated ternary material. Under 1C charge conditions in a half battery, this material exhibited an initial discharge-specific capacity of 147 mAh g⁻¹, and it maintained a capacity retention rate of 86% after 400 cycles. This approach minimizes cation mixing in the material and restores the phase structure by effectively substituting the missing lithium ions within the material. The molten salt method involves introducing reduced fusion temperature salt to the degraded cathode material and liquefying the salt under moderate thermal conditions to establish an optimized liquid-like medium for Li⁺ carriers. The lithium source reaches the interior of the material more quickly and efficiently, both lowering the sintering temperature and restoring the material to its ideal state [29]. All other repair and regeneration methods involve the step of heat treatment, and heat treatment, whether as a stand-alone method for treating failed cathode materials or as a post-treatment step for other methods, is integral to the process of repairing and regenerating materials. However, because our research on the failure mechanism and regeneration method of ternary materials is not perfect enough, the direct regeneration technique is still in the research period and cannot be utilized industrially.

To solve the above problems, we must understand the reasons for the failure of NCM widely and repair and regenerate them to address their failure mechanisms to accurately restore the material structure. The reasons for material failure are: (1) Phase structure changes in NCM often lead to the degradation of electrochemical capability, resulting from transformations from layered to rock salt or spinel phases [31]. (2) The mixing of cations [32]. The ionic radii of lithium and divalent nickel ions are close, and lithium ions are slightly larger than divalent nickel ions, so Ni²⁺ tends to occupy the position of Li⁺, resulting in lithium/nickel mixing. (3) The depletion of lithium ions through the generation of multiple chemical by-products on the Solid Electrolyte Interphase (SEI) membrane of the negative electrode [33]. (4) Dissolution of transition metals in electrolytes, etc. [34]. The reasons for the reduction in battery capacity are not limited to those mentioned above but are also related to the way the battery is assembled, the manufacturer's production process, and other factors.

The review on ternary materials focuses on the types of direct regeneration technologies for waste materials, modification pathways for nickel-rich ternary materials, technological methods for the recovery of valuable metals, pretreatment technologies, surface degradation mechanisms of ternary materials, and environmental issues in recycling. There are also the following issues rarely mentioned, such as the reasons for the failure of ternary cathode materials in different environments, how to solve the Cathode Electrolyte Interphase (CEI) film [35] and SEI film [36] in ternary materials, the negative impacts brought by both of them as well as their formation mechanisms and impacts on the materials, and how to efficiently solve the problem of the regeneration of waste materials in the era of artificial intelligence. This paper imports causes of failure in ternary cathode materials and the utilization of heat treatment methods in the repair and regeneration of NCM, in which the causes of microcrack production and the research progress of heat treatment as a separate regeneration method (solid-phase sintering) are described in detail. Meanwhile, it serves as an important application of hydrothermal precipitation and molten-salt sintering for the regeneration of waste ternary cathode materials as a post-treatment step. As shown in Figure 2, we summarize the reasons for the failure of ternary materials and the accompanying structural changes, and we summarize the wide application of heat treatment methods during regeneration using ternary cathode materials. Thermal treatment can be directly applied to repair and regenerate ternary cathode materials with a low degree of failure. At the same time, it can also be combined with other regeneration methods and act synergistically on ternary cathode materials with a high degree of failure, such as the molten salt method, hydrothermal method, coprecipitation method, etc. It summarizes the repair mechanism and advantages and disadvantages of the heat treatment method, puts forward the current challenges requiring resolution urgently and the future research direction of waste NCM batteries, and elaborates on the application prospects of the heat treatment method in other regeneration methods.

2. Failure Mechanism of Ternary Cathode Materials

NCM have emerged as leading candidates for Li-ion battery chemistry due to their inherent strengths. Figure 3a,b shows the schematic structure of the ternary material and the Li⁺ transport pathway. The NCM material belongs to a α-NaFeO₂ layered structure with Ni, Mn, Co atoms and oxygen atoms, forming MO₆ octahedra with a O3-type structure and a space group of R-3 m [37]. Lithium ions reside in the MO6 octahedral layer and move between layers, utilizing different transport pathways depending on their phase structures [38]. The causes of failure of ternary cathode materials are due to multiple factors, and, in most conditions, it is the combination of multiple factors that leads to material inversion. Some researchers have explored the three main reasons for the invalidation of nickel-rich and low-cobalt NCM, which are cation mixing, microcrack generation, and residual lithium compound generation, and have examined these issues in detail (Figure 3c) [39]. The failure mechanism of ternary materials determines the choice of regeneration methods, so a comprehensive exploration of the causes of their failure is a critical step. For example, methods for repairing ternary cathode materials include removing impurities generated at the interface, replenishing irreversibly lost lithium ions, repairing internal defects and cracks in the structure, reducing the mixing of cations, and eliminating residual stresses [12,40]. A more precise grasp of the internal alterations of the material allows for a more oriented choice of restorative regeneration.

2.1. Li/Ni Mixture

When a battery undergoes long-term cycling, reacts with the electrolyte, and is under other such conditions, it will consume a substantial quantity of lithium ions, thus forming lithium vacancies. At this time, Ni²⁺ migrates to the previously formed vacancies through various pathways under the action of various external forces, causing changes in the structure of the material [41]. There are several ways to solve this problem, the most common of which is to repair the oxygen lattice deficiency on the lamellar surface and to reduce the oxygen release energy to oxidize Ni²⁺ to Ni³⁺, raising Li⁺/Ni²⁺. The crystallographic integrity of layered oxides is conventionally confirmed when the intensity ratio between (003) and (104) diffraction peaks exceeds 1.2, as determined by X-ray diffraction analysis; it has a low cationic mixing degree and an intact lamellar structure [42]. Cheng et al. [43] conducted an observation of the surface microstructure of the obtained NCM622 cathode material after repeated charge-discharge cycles. It was found that a large number of lithium defects and impurity phases were formed in the material. By calculating the peak intensity ratio of I₍₀₀₃₎/I₍₁₀₄₎ to be 1.18, this indicates a severe mixing of the cation. Meanwhile, the content of Ni²⁺ calculated from the peak area in the subsequent Ni 2p spectrum is 66.9%, which is much higher than the content of divalent nickel in the regenerated sample. The above analysis shows that the structure of the recycled waste cathode material has been severely damaged. At the same time, the formation of lithium vacancies will also lead to the occurrence of phase transitions in the material. As shown in Figure 4a, the thermal breakdown of the NCM cathode material during the charging and discharging process of ternary batteries will first cause the transfer of transition metals, leading to nickel ions occupying the lithium vacancies first and the subsequent migration of other metals, leading to phase transitions [44]. In addition to conducting XRD tests on the failed cathode materials and analyzing the peak intensity ratio of I₍₀₀₃₎/I₍₁₀₄₎ to assess the degree of cation mixing in the materials, ICP tests can also be utilized to directly detect the contents of lithium and transition metals to predict the degree of the failure of the materials. Researchers have detected the contents of lithium and transition metals in the failed NCM523 using ICP and found that their contents were severely insufficient. Moreover, through experiments, they doped the highly electronegative PO₄³⁻ polyanions into NCM523 to form strong bonds with Ni, which reduced the degree of cation mixing in the material. The degraded NCM523 material during cycling was repaired. The mechanism of doping repair is shown in Figure 4b [45].

The level of cation mixing is not only affected by the absence of lattice oxygen, but is also related to cobalt content and manganese content. Because the reduction potentials of cobalt and oxygen overlap, cobalt promotes the activity of lattice oxygen, which leads to the release of oxygen. In the transition metal oxide lattice, the strong superexchange pathways formed between Ni²⁺ and Mn⁴⁺ ions across adjacent layers enhance oxygen lattice integrity. This makes controlled cation mixing essential for achieving better thermal stability in high-nickel-content active materials [46]. By coating the surface of the ternary cathode material with a protective layer that does not change the structure of the material, it is possible to prevent the dissolution of the transition metal into the electrolyte, thereby reducing the mixing of cations. Han et al. [47] reported a novel method for the regeneration of single-crystal NCM811, which realizes the regeneration of the material through a curable PMMA (purchased from HX-R Ltd. Co., Chengdu, China) surface layer. As shown in the SEM and TEM images in Figure 4c–e, the materials prepared by adding PMMA and PVDF (KF1100 purchased from Kureha Corporation, Tokyo, Japan) with different mass fractions to SC-NCM have organic layers with different thicknesses. More importantly, it was found that the peak intensity ratio of I₍₀₀₃₎/I₍₁₀₄₎ for SC-NCM/PMMA is 2.25, which is much larger than 1.52 for SC-NCM. This indicates that the PMMA layer changes the valence state of surface nickel, increases the ratio of Ni³⁺ within the NCM lattice, and reduces the degree of cation mixing [47].

Furthermore, the diffusion paths and velocities of Li⁺ are different in different mixing states of Li/Ni. In the absence of cation mixing, the spread path of Li⁺ is from the neighboring tetrahedral positions to the new empty octahedral positions when the intermediate tetrahedron and the four octahedrons are adjacent to each other. However, in the case of Li and Ni mixing, the lithium ions of the tetrahedral lithium are repelled by the cations of the transition metal layer and the divalent nickel ions, and the rate of diffusion to the new octahedra becomes particularly slow, so that the cation mixing seriously affects the spread kinetics of Li⁺ in the NCM material [48].

Figure 4. (a) Phase transition and possible transition metal cation migration paths during thermal decomposition of charged-state NMC cathode material (“A” represents the original position, and “B” represents the octahedral position). Reprinted with permission from ref. [44] 2014, American Chemical Society. (b) Schematic representation of the process by which failed NCM523 material is repaired. Reprinted with permission from ref. [45] 2021, Elsevier. (c–e) SEM and TEM images of different samples. Reprinted with permission from ref. [47] 2020, American Chemical Society. (f) Diagram of the decomposition path of a Ni-rich cathode. Reprinted from ref. [49].

Figure 4. (a) Phase transition and possible transition metal cation migration paths during thermal decomposition of charged-state NMC cathode material (“A” represents the original position, and “B” represents the octahedral position). Reprinted with permission from ref. [44] 2014, American Chemical Society. (b) Schematic representation of the process by which failed NCM523 material is repaired. Reprinted with permission from ref. [45] 2021, Elsevier. (c–e) SEM and TEM images of different samples. Reprinted with permission from ref. [47] 2020, American Chemical Society. (f) Diagram of the decomposition path of a Ni-rich cathode. Reprinted from ref. [49].

2.2. Formation of Microcracks in Materials

Microcracks generated on the surface of NCM are another important cause of their failure. The cracks can be caused by many reasons, such as the phase transition of the material, lattice distortion, oxygen loss, or other mechanical stresses, which can lead to the formation of microcracks [50]. Different types of ternary materials produce cracks for different reasons, as shown in Figure 4f for the microcrack generation process of ternary cathode materials with different nickel contents [49]. When the Ni content is lower than 0.8, microcracking occurs from the inside of the substances, and no visible cracks develop on the surface of the material. However, when this value is exceeded, the cracks produced are more pronounced and can be observed over the entire surface of the material. This is because of the penetration of the electrolyte due to cracks and the formation of NiO impurities, which further leads to an increase in crack growth. This shows that the production of microcracks is a key reason for the failure of NCM, so a deeper understanding of the reasons for the generation of microcracks is essential.

2.2.1. Microcracks Caused by Phase Transitions

After a long time of cycling, the layered structure of ternary materials will change to rock salt phase or spinel phase through point defects and line dislocations, which is not favorable for lithium ion passage and reduces the transmission rate of lithium ions between positive and negative electrodes [51]. Structural phase transitions lead to volume changes in the material and the accumulation of mechanical stresses along the crystal interface, eventually leading to crack formation [52]. For nickel-rich cathodes, several phase transitions occur, changing the initial laminar structure (H1) into a monoclinic phase (M) and then progressing to two additional hexagonal phases (H2 and H3) [49]. Researchers have studied the structural variations of nickel-rich NCM cathodes in solid-state batteries using in situ microscopy, and the results of the study indicate that when the lithium metal is depleted, the reversed-phase boundary extends into the laminar structure, where the lithium metal and other metal ions are mixed in the laminar structure, which results in the conversion of the twin boundaries into rock salt phase [53]. Because the weak electrochemical activity of the generated rock salt phase is not ionically conductive, it leads to a remarkable reduction in capacity caused by a slowing down of the kinetic rate. Mu et al. [54] investigated the effect of oxygen release on cracks and phase transitions in the material, and the crack breeding simulated by in situ STEM (JEOL 2100 S/TEM, JEOL Ltd., Tokyo, Japan) probing and finite element analysis showed that oxygen release can cause phase transitions, and the accumulation of such phase transitions can cause cracks. As shown in Figure 5a, the change in the length of inter-crystal cracks produced with time-varied thermal activation. In addition, as shown in Figure 5b, other researchers have suggested that the generation of rock salt and spinel phase transitions is related to the cycling rate, with the former being formed at low cycling rates and the latter at high cycling rates [55]. The development of the rock salt phase occurs through a dual-phase procedure spanning extended cycling intervals. Initially, a primary surface reconstruction layer approximately 2 nm thick forms, succeeded by directional expansion into the material's bulk along a specific crystallographic plane [56].

Effective elemental doping can inhibit the occurrence of phase transitions and address the issues caused by phase transitions in materials. U.-H. et al. [57] used tungsten doping to stabilize the rock-salt phase formed on the surface of Li[Ni_xCo_yMn_1-x-y]O₂ (NCM) and LiNiO₂ (LNO) materials. When observing the TEM (JEOL 2100F and aberration-corrected FEI 300 kV Titan, JEOL Ltd., Tokyo, Japan) images of the cycled materials, many intergranular and intragranular cracks were found. However, in the TEM image of the 1 mol% tungsten-doped LNO material after cycling, there was no obvious surface damage in the [001] rock-salt region, and the lattice structure remained intact. The restoration of the layered architecture becomes distinctly evident during the direct regeneration process of ternary compounds, making the re-establishment of the phase configuration a salient characteristic in the direct rejuvenation of cathode materials [58]. An in-depth understanding of the variation of phase structure in ternary cathode materials is particularly important for the design of cathode materials with stable electrochemical performance.

Figure 5. (a) Variation of crystal crack length with time. Reprinted with permission from ref. [54] 2018, American Chemical Society. (b) Phase transitions occurring at different cycling rates. Reprinted with permission from ref. [55] 2018, American Chemical Society. (c) Volume variations of secondary particles were monitored in real-time using optical microscopy during the cycling of NCM111 and NCM811 cathodes paired with lithium metal anodes. Reprinted with permission from ref. [59] 2017, American Chemical Society. (d) Capacity of the ternary material as observed in terms of its crystal structure decay mechanism. Reprinted with permission from ref. [59] 2017, American Chemical Society. (e) Dislocation modeling of NCM333. Reprinted from ref. [60].

Figure 5. (a) Variation of crystal crack length with time. Reprinted with permission from ref. [54] 2018, American Chemical Society. (b) Phase transitions occurring at different cycling rates. Reprinted with permission from ref. [55] 2018, American Chemical Society. (c) Volume variations of secondary particles were monitored in real-time using optical microscopy during the cycling of NCM111 and NCM811 cathodes paired with lithium metal anodes. Reprinted with permission from ref. [59] 2017, American Chemical Society. (d) Capacity of the ternary material as observed in terms of its crystal structure decay mechanism. Reprinted with permission from ref. [59] 2017, American Chemical Society. (e) Dislocation modeling of NCM333. Reprinted from ref. [60].

2.2.2. Microcracks Due to Lattice Distortion

Lattice distortion causes defects in crystals and also causes a phase transition in the material that results in a loss of capacity [61]. There are various factors affecting the generation of defects in crystals, one of which is the generation of dislocations between crystals, which are usually formed when batteries are charged and discharged at high voltages. Mao et al. [62] carried out an in-depth study of the causes of capacity changes in NCM622 due to high-voltage charging, and the results show that capacity degradation and cycling stability are highly related to microcracks. After multiple cycles through different voltage windows (3~4.4 V, 3~4.6 V, and 3~4.9 V) and then observations using various test techniques, an attempt was made to propose a solution to this problem. Similarly, other researchers have not only studied the cathode capacity decay mechanism of low-Ni NCM111 but have also explored high-Ni NCM811, comparing the similarities and differences in degradation of the two materials in terms of crystallography and microstructure. As shown in Figure 5c, the results indicate that both materials exhibit volume changes to varying degrees. However, the microcracks in NCM811 are more pronounced. It is believed that the aging of the material is related to the rupture of lattice bonds caused by the mechanical stress generated at the crystal interfaces. Moreover, from Figure 5d, the changes of the crystal along the c-axis and a-axis of the ternary layered structure during charging and discharging processes can be observed [59]. In addition, Yan et al. [60] observed the nucleation and growth of intracrystalline cracks in NCM cathode materials using an advanced scanning transmission microscope and found that dislocation-generated cracks are the key to intracrystalline crack formation. In Figure 5e, it is shown that after 100 cycles at a high cutoff voltage of 4.7 V, NCM333 shows a high density of dislocations. Cracks can disrupt the connections between grains, which can lead to poorer conductivity of the material, exposing the surface of the new material to the electrolyte and causing the development of new corrosion, side reactions, and other problems. One study observed the changes in different ternary batteries during charging and discharging; as the lithium layer is gradually consumed, the charge-based repulsion forces among octahedral transition metal clusters intensify, and the initial c-lattice parameter increases, but the c-lattice parameter of all types of ternary batteries decreases rapidly when the voltage exceeds 4 V [63]. Variation of the lattice parameter over a wide range is adverse to the electrochemical performance of the material because both shrinkage and expansion lead to changes in the mechanical stresses in the material. Ryu et al. [64] investigated the capacity decay mechanism of Ni-rich ternary materials, and they found that with the rise in the number of cycles, the stability of the material deteriorates due to the presence of lattice distortion near the charging end, which leads to the sudden onset of anisotropic contraction, and proposed that, to solve this problem, the morphology of the particles must be controlled to eliminate the original lattice changes. In a previous study, researchers reported the observation of unexpected nucleation processes and the growth of cracks within the lattice in cathode materials, and the researchers predicted that cracks in crystals start at grain boundaries or the surface of the crystal and that dislocations cause cracks to form at different locations [60]. So, the formation of lattice cracks is one of the important mechanisms of material degradation, which has long been verified. During the charging of layered materials, the precipitation of Li⁺ causes the lattice to expand along the c-direction but contract in the a-direction, and this change in stress leads to a drastic change in the crystal structure.

2.2.3. Microcracking Due to Oxygen Release

Oxygen release introduces localized stresses and triggers the formation of along-crystal cracks, ultimately leading to microcracks in the material as a result of particle segregation [65]. The liberation of oxygen atoms results in the accumulation of vacancy defects in the oxygen sublattice as well as sequential phase transformations. As shown in Figure 6a, the heating XRD images with the NCM and the process of generating phase transitions after migration with lithium ions attribute the oxygen release to the instability in the highly delithiated and highly valent Ni⁴⁺ [66]. By exploring the battery cycling performance of Li/NCM811 at high voltage (4.5 V and 4.7 V), it was found that the oxygen release was due to the reaction of oxygen through oxygen precipitation reaction, the reaction between the active oxygen precipitation intermediates and the electrolyte solvent, and the oxygen release caused the secondary particles to crack and the degradation of the battery performance [67].

An irreversible reaction occurs as follows [68]

Li₂O–2e⁻→ ½O₂+2Li⁺

(1)

LiOH–2e⁻→½O₂+Li⁺+H⁺

(2)

Li₂CO₃–2e⁻→½O₂+CO₂+2Li⁺

(3)

As shown in Figure 6b, the vibrational modes of lattice oxygen are observed by Raman spectroscopy, and an irreversible transition within the NCA is seen to begin at the inflection point x = 0.2, and this transition is associated with lattice oxidation, which corresponds to the release of oxygen observed at the particle surface [69]. Oxygen release can also cause the battery to produce gas during cycling, leading to battery inflation, and the battery may explode and other dangerous accidents may occur. Researchers investigated the causes of CO₂ production in the batteries through isotopic labeling and in situ gas and other analyses, and, in addition to being produced due to the decomposition of lithium carbonate at high voltages, its main contributor was the dissolved NCM lattice oxygen in the electrolyte [70]. In addition, researchers have found that under high-rate charging and discharging conditions, the depth of the redox reaction on the surface of particles is higher than that in the particle bulk, and the inhomogeneity of the particle reaction will exacerbate the pulverization of particles. From the TEM images of NCM at 10C, it can also be seen that the degree of degradation of the surface structure is much more severe than that at 0.2C [71]. The discharge rate is proportional to the capacity decay of the battery and the increase in the internal temperature of the battery. One of the reasons is that, at a high discharge rate, the rate of lithium extraction from the battery materials will increase, making the active materials of the battery unstable. The second reason is that high-rate discharging causes a thermal effect inside the battery, which will lead to an increase in the aging rate of the battery [72].

Figure 6. (a) XRD patterns of delithiated NCM materials and their corresponding crystal structures after heating at 200, 350, and 500 °C. Reprinted with permission from ref. [66] (b) 2022, Elsevier. Constant current discharge/charge potential curves and corresponding -dx/|dE| NCA during two cycles (3.0–4.3 V vs. Li⁺/Li) and a third overcharging cycle (3.0–4.8 V vs. Li⁺/Li) along with the operation performed for the CO₂ and O₂ release analysis. Reprinted with permission from ref. [69] 2018, American Chemical Society.

Figure 6. (a) XRD patterns of delithiated NCM materials and their corresponding crystal structures after heating at 200, 350, and 500 °C. Reprinted with permission from ref. [66] (b) 2022, Elsevier. Constant current discharge/charge potential curves and corresponding -dx/|dE| NCA during two cycles (3.0–4.3 V vs. Li⁺/Li) and a third overcharging cycle (3.0–4.8 V vs. Li⁺/Li) along with the operation performed for the CO₂ and O₂ release analysis. Reprinted with permission from ref. [69] 2018, American Chemical Society.

2.3. Interaction Between the Electrolyte and the Active Component in the Material

During the prolonged cycling of the battery, the electrolyte undergoes reactions with the active substances in the cathode material. The resulting byproducts deposit on the SEI film, leading to an increase in thickness and hindering the transport of lithium ions [73]. The SEI film is weakened, resulting in degradation of the battery. Organic solvents in the electrolyte decompose, and the decomposition products react with the components in the SEI membrane, leading to a degradation of the membrane and an increase in resistance [74]. Using numerical simulations, some researchers have found that when cracks form in the material, the electrolyte infiltrates deeper into the material, along with the cracks, and the degree of deeper penetration affects the rate of the diffusion of ions, which also leads to an imbalance in the fitness between the electrons in the secondary particles and ionic conductivity [75]. The multiscale architecture of secondary particles, particularly their internal grain boundaries, constitutes a critical factor contributing to intense reactions occurring at the electrode-electrolyte interface. Notably, reducing the particle size of the material represents an effective strategy by which to mitigate such interfacial challenges. Fan et al. [76] comprehensively investigated ternary materials with primary particle diameters of 3–6 μm in order to ameliorate this dilemma and found that micrometer-sized NCM particles can effectively mitigate the interaction between the cathode and electrolyte and also avoid the creation of cracks between the grains. At a high cutoff voltage, the substances in the ternary cathode material are more likely to react, and especially the electrolyte will decompose, in which the organic solvents and lithium salts are more likely to react, leading to safety issues, such as battery flatulence [77]. At this point, the SEI membrane may be damaged or reorganized, causing an improvement in the resistance of the interface and a decrease in the performance of the cell. Nickel ions are also prone to migration and recombination at high voltages, when side reactions between the surface of the cathode and the electrolyte increase [78]. In addition, under the conditions of low temperature and deep discharge, the deterioration of the electrolyte, separator, and current collector in the battery will be aggravated, leading to a rupture of the SEI film, thus causing the degradation of the lithium-ion battery [79]. Zhu et al. [80] studied the failure mechanism of lithium batteries at different temperatures ranging from −10 °C to 45 °C and at different depths of discharge. The results show that when the temperature is −10 °C and the depth of discharge reaches 105%, the capacity retention rate of the battery drops sharply, and, at the same time, the conductivity decreases and the loss of lithium ions increases. It can be concluded from the SEM (JSM-6340F, JEOL Ltd., Tokyo, Japan) images that the electrolyte decomposes at low temperatures, and the electrolyte salt decomposes when the depth of discharge is 110%. Researchers found that after NCM cycles at −10 °C, the metallic lithium deposited on the surface of the negative electrode participates in the gas evolution reaction, producing hydrogen, carbon monoxide, and carbon dioxide. At the same time, it was found that, at low temperatures, the formation of “dead lithium” on the negative electrode and the dissolution of transition metals in the positive electrode material are the causes of battery failure [81].

In summary, the failure of NCM is majorly caused by the following reasons. First, the cations in the material cause mixed rows. Factors such as lattice distortion and increased oxygen release energy in the material, and the content of nickel elements increases the level of cation mixing. In recycled NCM, methods to decrease the mixing of cations include doping effective elements, synthesizing material particles of appropriate size, and coating an effective surface layer on the surface of the new material. Second, the phase structure of the material is changed, and microcracks are generated. Phase transformation, lattice distortion, and oxygen loss of the material can lead to the generation of microcracks, and the phase transformation can change the layered structure of the ternary cathode material into spinel phase or salt rock phase [50]. External stress on the material during chronic cycling can lead to the formation of lattice cracks. Both changes are responsible for making the ternary cathode material less capable and less safe. Solving this problem requires improving the heat stabilization of the material and controlling the morphology of the microparticle to reduce the lattice stress changes. Third, the electrolyte and the active material in the material react. During electrochemical cycling, both the electrolyte and active material participate in oxidation-reduction processes driven by the electrochemical gradient. The reaction process will cause the battery's performance degradation, thermal runaway, and other safety issues. Therefore, the composition, concentration, purity, and other parameters of the electrolyte need to be strictly controlled to ensure the safety and stability of the battery. In addition, under the conditions of high voltage, low temperature, and overcharging and discharging, ternary lithium batteries will affect the structure of materials and the diffusion rate of lithium ions, accelerating the degradation of batteries to varying degrees.

3. Progress in the Application of the Heat Treatment Method in Repairing Regenerated Ternary Cathode Materials

The heat treatment method is widely used in the repair and regeneration of ternary cathode materials. Firstly, by utilizing heat treatment technology, based on the difference in the melting points between the ternary cathode material and the aluminum foil, the separation of the cathode active material from the current collector (aluminum foil) can be achieved. The binder and PVDF will decompose at around 450 °C because they will further damage the structure of the material during the process of heat treatment for regenerating the material at high temperatures [82]. At the same time, the conductive carbon, binder, and residual electrolyte can be removed to varying degrees. The heat treatment technology is mainly divided into low-temperature annealing, high-temperature sintering, and some emerging technologies, such as microwave-assisted heat treatment, plasma heat treatment, and multi-step heat treatment. Through different heat treatment technologies, the layered cathode material can be replenished with the lithium element lost during the cycling process, thus quickly realizing the regeneration of the ternary cathode material. In addition, the heat treatment technology can also be combined with other technologies such as element doping and surface coating to further enhance the performance of the regenerated material. The process parameters during heat treatment will also affect the performance of the regenerated material, so it is necessary to explore these process parameters.

3.1. Regeneration of Failed Ternary Cathode Materials by Heat Treatment Technology

Solid-phase sintering is the heat treatment to obtain regenerated material under the condition that an additional lithium source is added to the failed material [83]. As shown in Figure 7a, Yang et al. [28] first performed acid leaching on the obtained NCM waste materials. Then, a precipitant was added to make the metal ions in the leaching solution polymerize and precipitate. Finally, lithium carbonate was added for lithium supplementation and sintering. A two-stage sintering process was adopted. The low-to-medium temperature sintering allowed the lithium salt to melt completely, and the high-temperature sintering accelerated the diffusion of lithium ions. Combining high-temperature sintering with other physical processing methods can transform polycrystalline materials into single-crystalline materials. Single-crystalline materials have advantages such as higher crystallinity, thermal stability, and mechanical strength, and thus exhibit more excellent electrochemical performance. Nam et al. [84] pressed the prepared solid-phase precursor powder into spherical particles with a diameter of 13 mm under a pressure of 60 Mpa and then upgraded the polycrystalline materials to single-crystalline materials through decomposition sintering. Mechanical processing increases the contact area between solid particles and improves the mass transfer rate, which is conducive to the growth of crystal grains during the sintering process. During the cycling process, the residual lithium compounds formed on the surface of the layered cathode material not only led to lithium loss but also reduced the electronic conductivity. By directly using solid-phase sintering to convert the residual lithium compounds on the surface of the waste materials into a lithium source that can be replenished into the interior of the materials, the failed materials can be repaired and regenerated without adding an additional lithium source. Chi et al. [85] studied the electrochemical properties of regenerated materials from the lithium-containing residues of spent single-crystalline NCM811 at different sintering temperatures (800 °C, 850 °C, and 900 °C) and summarized the optimal sintering temperature. The study shows that at 800 °C, the diffusion of Li in the crystal is insufficient, while at a higher temperature of 900 °C, the diffusion distance of Li along the particles will increase. Both situations will result in the electrochemical properties of the regenerated materials being poorer compared to those at 850 °C.

Before the repair and regeneration process, other components in the positive electrode material except for the ternary cathode material, such as the PVDF binder, the electrolyte, and the conductive carbon, need to be removed. In order to completely remove the acetylene black in the cathode material, Zhou et al. [86] heated the recycled material for 5 h to obtain pure SNCM material. Subsequently, lithium acetate was added to the SNCM material and sintered to obtain the regenerated material. The research results show that the heat-treated ternary positive electrode waste is beneficial for the subsequent sintering and regeneration. As shown in Figure 7b,c, the cell performance is optimized and comparable to commercial materials compared to S-NCM. SEM images display that the impurities on the particle surface are removed, and cracks and broken particles produced by cycling of the material disappear. The heat treatment technology can be applied not only to cathode materials but also to anode materials. Mancini et al. [87] used the heat treatment technology to recycle and regenerate cathode materials and anode graphite. An additional lithium source is added to the cathode material to supplement the missing lithium, and then the cathode material is regenerated by the common solid-state sintering method. At the same time, the binder in the graphite is removed at the first temperature of 600 °C, and the purity of the graphite is improved at a high temperature of 2300 °C. Figure 7d shows the SEM images and TG curves of the graphite anode before and after heat treatment. It can be seen from the figure that the impurities on the surface of the treated material are completely removed, and the decrease in the mass of the material before 600 °C is due to the removal of impurities. In addition, trace amounts of sintering aids can be used to regulate the particle size of the target product. By adjusting the ionic mobility and surface energy, they can promote the growth of crystals into larger particle sizes. The Vegard slope can be used to demonstrate that sintering aids can regulate the growth of crystals at lower temperatures [88].

3.2. Synergistic Effects of Heat Treatment Technology, Element Doping, and Surface Coating

The mere inclusion of a lithium source does not enable the ternary cathode material to attain optimal electrochemical performance at elevated voltages. The preparation of a coated layer on the surface of the material and the internal doping with elements that can reduce the structural disorder are effective modification methods, while the cladding and doping can be realized only by heat treatment operations. The preparation of the coating for the cathode material can effectively prevent the active substances in the cathode material from reacting with the electrolyte to produce by-products, thereby avoiding the generation of gas in the battery and the consumption of lithium ions [52]. A researcher altered single-crystal NCM523 using a three-in-one modification technique involving sodium doping and a dual coating of alumina and lithium borate and sintered the precursor powder with sodium carbonate and lithium carbonate under the atmosphere of air after mixing them in a certain ratio. The coating layer did not alter the structural composition of the ternary cathode material, and the modified material can achieve a maximum voltage of 4.6 V while sustaining a specific capacity of 158.9 mAh g⁻¹ over 200 cycles at a rate of 1 C. The regeneratively coated N-NCM@AB exhibits an ordered layer structure [89]. Combining element doping with heat treatment can successfully dope elements into the interior of the material. This not only improves the capacity of the layered material but also represents a relatively easy-to-implement technical route. Researchers mixed SrO and TiO₂ powders with various oxidation precursors of the ternary cathode material, carried out ball milling, and then sintered at different temperatures to obtain Sr/Ti co-doped samples. The electrochemical performance shows that the half-cell can provide a high reversible capacity of 191.1 mAh g⁻¹ at 0.1C [90]. Enhancing the surface condition of ternary cathode materials is a necessity for improving battery performance. Conventional coating techniques by wet or atomic layer deposition suffer from being time-consuming, highly complex, possibly triggering surface lithium deficiency and phase remodeling, and, most importantly, serious impediments to the performance of mass-produced batteries.

Wu et al. [91] introduced a dry coating technique utilizing a high-energy mixer with Al(OH)₃ nanoparticles as the coating material, and the preparation process is illustrated in Figure 8a. The principle of reconstructing the protective layer on the surface of nickel-rich cathodes via dry coating primarily involves utilizing the LiAlO₂ phase produced by the reaction between residual lithium and aluminum hydroxide on the cathode's surface, alongside the transformation of unreacted lithium hydroxide into the Al₂O₃ phase. These two phases coalesce to create a mixed layer that envelops the cathode surface, establishing a stabilizing layer. Figure 8b shows the XRD patterns for different Al doping amounts, and all the diffraction peaks show a good layer structure, demonstrating that the surface covering does not influence the crystal structure of the material. A coating that efficiently envelops the material's surface diminishes charge transfer resistance, separates the electrolyte from direct contact with the cathode material, and concurrently preserves the integrity of the lithium-ion diffusion pathway. Similarly, a layer of superionic conductor LiAlO₂ is coated on the surface of the failed NCM523. Here, not only high-temperature annealing and surface coating are combined, but also mechanical activation is involved. The mechanical ball milling breaks the secondary particles, thus increasing the surface area of the material, which is beneficial for the coating on the material surface during the sintering process [92].

3.3. Influence of Heat Treatment Process Parameters on Regenerated Materials

Ball milling the precursor material can effectively improve the efficiency of the heat treatment [94]. Tang et al. [95] first used air jet milling and ball milling to uniformly reduce the particle size of the raw materials and then carried out lithium supplementation and sintering. From the SEM image of the regenerated material, it was observed that the secondary particles were completely broken, and the primary particles were uniformly dispersed. And, from the TEM image, lattice fringes of about 0.47 nm were formed, which corresponded to the (003) crystal plane, indicating that lithium ions successfully diffused into the structure of the material during the sintering process. In addition to being able to control the particle size of the pretreated material before heat treatment, during the heat treatment, the selection of atmosphere, temperature, different lithium sources, and time will all affect the morphology and characteristics of the material. Through in situ XRD exploration of the different regeneration temperatures of the sintered and regenerated NCM523 with the addition of lithium carbonate, as the temperature increased, all diffraction peaks became sharper and shifted to the left. At the same time, it was found that when the temperature exceeded 950 °C, the peak intensity ratio of I₍₀₀₃₎/I₍₁₀₄₎ was greater than 1.2 and decreased as the temperature continued to rise, indicating that a relatively high temperature is not suitable for the repair of the ternary layered structure [96].

With the continuous upgrading of the power system, the pursuit of batteries with high energy density has become increasingly intense. High-nickel ternary materials can achieve this goal, and many researchers have used failed low-nickel ternary batteries to convert them into high-nickel materials through certain means. Gao et al. [93] obtained a single-crystal NCM811 material with a uniform distribution of nickel elements by optimizing the sintering temperature and time. As shown in Figure 8c, it is a schematic diagram of the process of directly upgrading polycrystalline D-NCM111 to NCM811. To ensure that the added nickel source can completely enter D-NCM111, different sintering times of 10 h, 12 h, and 15 h were explored at 900 °C. The optimal sintering time of 15 h was determined by observing the growth trend of the particle size under different sintering times. As shown in Figure 8d, its electrochemical performance shows that it is 198 mAh g⁻¹ at C/10 and 173 mAh g⁻¹ at 1C, while maintaining good cycle stability. Thermal treatment technology is one of the most common methods for directly repairing and regenerating ternary cathode materials. It has the advantages of simple operation, mature technology, and variability. However, when it is applied on a large scale in industrial applications, there are the following difficulties. Firstly, the application of thermal treatment technology requires precise control of temperature and atmosphere. If the temperature is too high, the structure of the material may be further damaged, while if the temperature is too low, the repair effect cannot be achieved. Moreover, the temperature fluctuation usually needs to be controlled within a few degrees Celsius, which requires high precision of the heating equipment. In industrial production, the scale of the equipment is usually enlarged, which requires continuous energy consumption and regular maintenance of the equipment, indirectly increasing the production cost. Secondly, for the materials undergoing thermal treatment, certain pretreatment steps are required to remove impurities and improve the purity, which will reduce the production efficiency.

4. Synergistic Effects of Heat Treatment Technology and Other Regeneration Technologies

The combination of low-temperature annealing technology and low-melting-point molten salt, by using the liquid-like environment created by the molten salt, can significantly accelerate the diffusion rate of lithium ions into the interior of the material during the heat treatment process. Thus, a new emerging method for directly repairing and regenerating ternary layered cathode materials has been derived–the molten salt sintering method. In addition, heat treatment technology is mostly used as the last step in the hydrothermal method and co-precipitation method to supplement the missing metal elements and further improve the performance of the materials. The combined use of multiple technologies helps to improve the crystallinity of the materials, remove impurities, and repair the material structure more effectively.

4.1. Combination of Heat Treatment Technology and Molten Salt-Molten Salt Sintering Method

The molten salt regeneration method is a technology to recover and regenerate waste materials. It has the advantages of being environmentally friendly, efficient, and economical [97]. The repair principle is that at high temperatures, molten salt can lower the reaction temperature and promote ion diffusion, thereby accelerating the repair and regeneration process of the cathode material. Generally, to convert polycrystalline materials into single-crystal materials, it usually requires long-time sintering at high temperatures. However, using molten salt as a high-temperature solvent can achieve the single crystallization of materials at relatively low temperatures. Ni et al. [98] used molten salt as a reaction medium and high-temperature solvent to eliminate the grain boundaries of nickel-rich cathode materials. Single-crystal materials can solve the problem that the materials are prone to crack after cycling under high pressure.

The combination of eutectic molten salt and heat treatment is an effective regeneration method. Different molten salt systems and the ratios of molten salts determine different heat treatment temperatures. Shi et al. [99] aimed to use the low-eutectic lithium molten salt combined with thermal annealing to restore the NCM523 material with significant capacity loss under normal pressure. Through the phase diagram analysis of the mixture of lithium nitrate and lithium hydroxide, they found that when the molar ratio of the mixture is 3:2, the melting point is only 176 °C. Therefore, they chose to treat the mixed lithium salts and the failed cathode material at 300 °C. Similarly, taking the recycling and regeneration of NCM523 cathode material as an example, another molten salt system, LiNO₃-KNO₃-KC, was used to treat the degraded cathode material. The heat treatment temperature was determined by the DSC curves of the waste materials and the mixed molten salt. Therefore, the mass ratio of the waste materials was selected as SC: KCl: KNO₃: LiNO₃ = 1: 8: 8: 0.8. The process of molten salt-assisted lithium-ion migration is shown in Figure 9a. Lithium ions in the molten salt smoothly migrate to the positions of vacancies, and the phase structure of the waste materials is repaired. As can be seen from Figure 9b, its electrochemical performance is very close to that of commercial materials, indicating a significant regeneration effect. The XRD pattern in Figure 9c shows that the regenerated NCM523 exhibits a significant improvement in kinetics, and the lattice parameters and structure of the material have been restored [100].

The choice of the system of molten salt has a significant impact on the effect of material regeneration, directly affecting the reaction efficiency during the reaction mechanism, the purity of the product, the minimum temperature required for the reaction, and, moreover, directly determining the efficacy of the regenerated material. Jiang et al. [101] selected the eutectic salt system of LiOH−Li₂CO₃. As can be seen from Figure 10a, to make lithium carbonate and lithium hydroxide reach the eutectic point of 433 °C, the molar ratio of LiOH/Li₂CO₃ is 0.86:0.14 at this time. Figure 10b is a schematic diagram of the process for restoring and regenerating NCM523 through lithium carbonate and lithium hydroxide. The micro-cracks generated in the material after molten salt heat treatment are repaired, and the layered structure is restored. The XRD of the regenerated material shows the α-NaFeO₂ layered structure with the space group R-3 m (Figure 10c), indicating that the damaged structure of the waste material is completely restored, and the degree of cation mixing is also reduced. The liquid environment created by molten salts acts in a similar way to ionic liquids. Cathode materials can be directly regenerated by restoring their composition and lattice-structure defects [102]. The molten salt method can effectively make up for the shortcomings of the high sintering temperature and long processing time of the fire method [103]. Adding some fluxes to the molten salt is beneficial to further reduce the temperature of the molten salt. Ma et al. [104] selected LiI-LiOH salt, which possesses the lowest eutectic point in the binary eutectic lithium salt system, and also added Co₂O₃ and MnO₂ for the complementary element to regenerate the NCM523, and this method is also applicable to the NCM622 material. Again, the spent NCM523 was regenerated by molten salt, and Qin et al. [105] chose a ternary molten salt for regeneration, and it is noteworthy that the regenerated material reaches a high multiplicity of 132 mAh g⁻¹ discharge specific capacity at 5C. They carried out the re-lithiation reaction by using a mixed molten salt of lithium nitrate, lithium hydroxide, and lithium acetate with a molar volume ratio of 9:6:10 mixed with NCM523 heated at 400 °C for 4 h. The XRD plots illustrate that the regenerated material has the same peak morphology as the commercial one, with the 003 crystallographic planes shifted compared to the waste material. It indicates that the lattice structure of the material was successfully restored. Combining the molten salt method with heat treatment technology can efficiently upgrade low-nickel layered cathode materials into nickel-rich materials, thereby enhancing the capacity of the materials. Wang et al. [106] also upgraded the used NCM111 material to nickel-rich NCM622 by the molten salt way with the addition of lithium and nickel. Higher capacity and cycling stability were achieved to maximize the value of the used material. The results show that the obtained NCM622 material is almost the same as the commercial one in terms of peak 003 in XRD, and the peaks 108 and 110 have a narrower peak splitting like the commercial one, as shown in Figure 10d.

It is also possible to utilize the special properties of molten salts to dope and coat the ternary cathode materials, further optimizing the performance of the regenerated materials. The researchers proposed to use lithium hydroxide and lithium carbonate in molten salt for NCM523 regeneration and lithium titanate coating deposition in one step. Both regenerate the ternary material at a lower temperature, while the formed lithium titanate coating can promote charge transfer and inhibit the dissolution of transition metals. The regenerated material shows excellent electrochemical conductivity, especially at a high multiplicity of 10C, which can reach a discharge specific capacity of 125 mAh g⁻¹ [107]. In addition, Xing et al. [108] examined the procedure of regeneration of NCM523 by Al doping using the molten salt method, and it was found that the element Al successfully entered the interior of NCM523, and the process of molten salt treatment formed a lithium-aluminum layer on the material's surface, which repaired the cracks produced by the previous crystals. Following further heat treatment, the nickel ions undergo oxidation, resulting in a progressive increase in the concentration of Ni³⁺ relative to divalent nickel. Therefore, the combination of molten salt and heat treatment technology will further increase the rate of regenerating cathode materials, which is a promising method. However, it is also necessary to consider how to reduce the consumption of lithium salts and recycle them. When this method is applied in industrial applications, it is necessary to consider that the molten salts used should have good thermal stability, low corrosiveness, and be easily accessible. At the same time, the treated cathode materials usually need to be washed with water, which not only generates a large amount of waste liquid containing fluorine and lithium but also leads to a certain loss of lithium elements, increasing the treatment cost. In addition, the molten salt method in the laboratory is usually carried out in a tube furnace. However, in industrial production, it is a huge challenge to design large-scale continuous production reaction equipment that can withstand high temperatures and corrosion.

4.2. Combination of Heat Treatment Technology and Co-Precipitation Method

The recovery and regeneration of failed ternary cathode materials by the co-precipitation method generally includes dissolving the valuable metals in the collected cathode materials into a solution with a leaching agent to form a solution containing metal ions. Then, a precipitating agent is added to the leaching solution to make the target metal ions precipitate in the form of hydroxides or salts. After washing and drying, the dried precursor is mixed with a lithium source and heat-treated at a high temperature to obtain the final regenerated product. In fact, the co-precipitated product is generally subjected to heat treatment. Heat treatment can promote the rearrangement of atoms inside the material to form a specific crystal structure. Among them, conditions such as the treatment temperature, atmosphere, and time have an important impact on the structure and properties of the final material. Huang et al. [109] proposed a method for the rapid co-precipitation preparation of ternary materials. What makes it different from other co-precipitation methods is that it uses spray drying to dry the ball-milled and mixed precursors. The preparation flow chart is shown in Figure 11a. This method combines co-precipitation, ball milling, spray drying, and heat treatment. With this method, a highly efficient spherical nanoparticle material was prepared. It has excellent electrochemical properties. Even under a high cut-off voltage (4.5 V) and high temperature (60 °C), the obtained material still has good cycle stability and rate performance. The combination of heat treatment and the co-precipitation method can not only prepare ternary cathode materials but also extract target elements. Hu et al. [110] used the anoxic complexation ammonia precipitation-spontaneous precipitation process to recover manganese. Through the step of reduction roasting, manganese was converted into manganese oxide at 800 °C. Since manganese oxide can achieve good results in ammonia leaching, taking advantage of this, the leaching rate of manganese was increased to over 96%. In addition to using acid as a leaching agent, some researchers have chosen p-toluenesulfonic acid as a deep eutectic solvent (DES) to leach the metal components from waste ternary cathode materials. Then, combined with precipitation and high-temperature solid-phase methods, the ternary composite material NCM523 is regenerated. Under the optimal process conditions, the newly synthesized ternary material exhibits stable cycling performance [111]. The selection of heat treatment temperature has an important impact on further improving the performance of the precipitated product. Chen et al. [112] mainly investigated the influences of different sintering temperatures on the physical and electrochemical behavior of regenerated materials after precipitation of waste materials. The appropriate sintering temperature was found to be of great significance for the regeneration of NCM. The characteristics of the co-precipitated materials sintered at 780 °C, 830 °C, and 880 °C were examined. The EDS spectra clearly indicate a uniform distribution of Ni, Co, Mn, and O elements across the particles, whereas the high-resolution electron microscope image in Figure 11b reveals organized lattice stripes. Figure 11c illustrates the electrochemical characteristics at various sintering temperatures, revealing that optimal results are achieved at a sintering temperature of 830 °C. By combining the advantages of heat treatment and wet co-precipitation technology, a large amount of failed ternary cathode materials can be processed. Firstly, the waste ternary materials can be treated by carbothermal reduction, which is beneficial for the subsequent leaching of valuable metals. Secondly, the heat treatment operation with the addition of a lithium source in the final step is also indispensable [113]. The disadvantage of this method is that the regeneration process produces toxic gases and wastewater emissions, along with the utilization of hazardous chemical reagents detrimental to the environment and human life and health. We need to find more environmentally friendly treatment reagents to reduce the negative effects of using this method.

Using organic acids as leaching agents is an effective way to reduce environmental pollution and improve the utilization rate of reagents. Wang et al. [82] utilized the synergistic effect between malic acid and glucose to leach the valuable metals from the failed NCM111. Then, a precipitant was added to make the valuable metals precipitate simultaneously. Finally, lithium was supplemented, and a two-stage sintering process was carried out to obtain a regenerated material with excellent electrochemical properties. Compared with the commercial material, the crystal structure of the regenerated prepared material showed almost no difference with it, while the I₍₀₀₃₎/I₍₁₀₄₎ value reached 1.24, showing that the regenerated material had good cationic hybridity, as shown in Figure 11d. Heat treatment, as the final operational step, not only eliminated the impurities generated during leaching and precipitation but also perfectly fused the supplemented lithium ions with the original material. Through the TEM (Transmission Electron Microscopy) observation of the ternary cathode material obtained by sintering after precipitation, the structural shape changes from the initial spherical shape to a flaky one after roasting. This indicates that the heat treatment process is conducive to the formation of the layered structure of the ternary material [114]. Based on Le Chatelier's principle, Zhou et al. [115] proposed a green closed-loop recycling method for waste NCM523 materials. They explored the influence of the heat treatment temperature on the electrical conductivity of the materials and selected different temperatures (830 °C, 880 °C, and 930 °C) to treat the precursor materials. Since the narrower the bandgap, the better the electrical conductivity, the electrical conductivity was determined by measuring the bandgap of the materials treated at different temperatures. The bandgap at 880 °C was 2.383 eV, which was narrower than that at other temperatures. Under this condition, the surface of the sintered material was smooth, and the particle size was uniform, being larger than the range between 600 and 900 nm. In order to completely remove the pollutant BPA that appears in the waste residue after acid leaching, Liang et al. [116] studied the influence of the calcination temperature on the removal rate of BPA. It was found that when the calcination temperature was between 300 °C and 400 °C, the removal rate could reach 99.0%.

In addition to studying the influence of different heat treatment temperatures on recycled materials, the addition of a sintering aid can reduce the heat treatment temperature and is beneficial for the conversion of impurity elements into compounds, which is conducive to the extraction of target elements. Zhong et al. [117] proposed the extraction of lithium through sulfur-assisted roasting and the direct recycling of ternary materials using the leaching residue. As shown in Figure 12a, it is a mechanism diagram of the extraction of lithium by sulfur-assisted roasting in argon. Under the combined action of oxygen and sulfur at 500 °C, the lithium in the ternary material is directionally converted into lithium sulfate, and the leaching rate of lithium reaches 98.91%. As shown in Figure 12b, it is an analysis of the phase transformation through the XRD patterns of the products at different temperatures. With the continuous increase of temperature, the leaching efficiency shows a trend of first increasing and then decreasing. The method of heating treatment by acid leaching is more widely used, not only for small-scale experimental studies in the laboratory but also for large-scale industrialized treatment [118]. However, the process of regenerating ternary cathode materials by combining acid leaching, co-precipitation, and heat treatment technologies is long and complex in operation. Moreover, a large variety and quantity of chemical reagents are required, which is likely to cause environmental pollution and resource waste. Especially when this method is applied in industrial applications, cost control, environmental protection, and the acquisition of stable and uniform precursor precipitated compounds need to be considered.

4.3. Combination of Heat Treatment Technology and Hydrothermal Method

The hydrothermal regeneration of cathode materials occurs in a specialized closed reactor, such as an autoclave, utilizing an aqueous solution as the reaction medium. Waste battery cathode materials are combined with an appropriate solvent mixture, and the system is subjected to heating and pressurization to establish a high-temperature, high-pressure reaction environment [120]. After the product is obtained by hydrothermal treatment, it is also necessary to pass the product through thermal treatment to enhance the crystallinity and stability of the precursor. Zhou et al. [119] used a combination of CH₃CH₂OH-LiNO₃ solvent thermal relithiation and thermal annealing steps to directly regenerate the spent NCM523, and this method can accurately replenish lithium to the missing sites in the material. The regenerated material at the optimal hydrothermal temperature of 130 °C and sintering at 850 °C displays a great stable cycling performance with 90.23% capacity retention after 560 cycles at 1 C, as shown in Figure 12c,d. The robust electrostatic repulsion generated by the formation of a salt rock or spinel phase on the cathode material's surface impedes lithium-ion transport within the lithium layer of the transition metal octahedron, thereby diminishing the material's electrochemical properties. Yang et al. [121] successfully repaired the surface crystal defects of lithium-deficient NCM622 material by pre-oxidizing the failed NCM622 material via hydrothermal treatment succeeded by brief annealing of the material. The β-NiOOH derived from pre-oxidation promotes the transformation of the original laminar structure during the subsequent brief annealing process, resulting in a regenerated material that demonstrates a capacity of 153.82 mAh g⁻¹ between 2.8 V and 4.3 V at a rate of 1 C. Researchers utilized the stability at high temperatures of the layered structure of the transition metals in Ni_0.5Co_0.2Mn_0.3(OH)₂ formed during the hydrothermal reaction. During the heat treatment stage, a topological transformation from Ni_0.5Co_0.2Mn_0.3(OH)₂ to the NCM523 cathode material was achieved. This breakthrough has significantly improved the efficiency of lithium supplementation during the regeneration process and has constructed a smooth pathway for the efficient transport of lithium ions, which is of great significance for enhancing the electrochemical performance of the NCM523 material [122].

Hydrothermal relithiation is a key step for replenishing lithium in failed ternary materials [123]. Xu et al. [124] carried out a hydrothermal relithiation reaction on the failed cathode materials by using a mixture of 0.1 M lithium hydroxide and 3.9 M potassium hydroxide. They also explored the influence of different lithium sources, annealing temperatures, and annealing times on further improving the performance of the products after hydrothermal treatment. By analyzing the XPS spectra, it was found that the intensity of the O 1S peak decreased with an increase of the annealing temperature. When the lithium source was lithium carbonate, the optimal annealing temperature was 850 °C. Chan et al. [125] successfully regenerated LiNi_0.15Mn_0.15Co_0.70O₂ by hydrothermal relithiation at 220 °C for 2 h and sintering at 850 °C for 4 h in an air atmosphere. The kinetic analysis determined that the hydrothermal relithiation phase was primarily governed by chemisorption, and the experimental isothermal data adhered to the Langmuir isothermal model, demonstrating that the hydrothermal relithiation reaction was predominantly regulated by monomolecular layer adsorption. During the hydrothermal treatment of waste ternary materials, the raw materials are amalgamated at the atomic level to dissolve insoluble substances, and crystalline growth is induced by regulating the temperature gradient within the autoclave, thereby generating convection to achieve a supersaturated state. This method can control the crystal structure morphology of the recycled material and is widely applicable to the raw materials. However, hydrothermal treatment usually needs to be carried out in a high-temperature and high-pressure reaction kettle. For industrial production, there are huge potential safety hazards. At the same time, the operation of heat treatment increases the steps of the preparation process, further increasing the energy consumption. However, in industrial production, ensuring a uniform distribution of temperature and pressure within a large-scale reaction kettle poses a huge challenge, and the safety of the equipment also represents a significant potential hazard. In addition to combining treatment technologies, the hydrothermal method also requires separation steps and a drying environment, incurring substantial time and economic costs. Moreover, during industrial production, certain waste liquids, exhaust gases, and solid waste are generated, all of which need to be further treated before being discharged.

Table 1. The efficiency improvement of regenerative ternary cathode materials through heat treatment as a single method or post-treatment step.

Waste Material	Regeneration Method	Failed Material Capacity/Regenerative Capacity	Magnification	Increase Efficiency	Reference
NCM523	Acid leaching + calcination	29 mAh g−1/147 mAh g−1	1C	83%	Ref [28]
NCM111	Mechanical activation + calcination	75 mAh g−1/165 mAh g−1	0.2C	54.5%	Ref [27]
NCM	Solid sintering	175 mAh g−1/191.1 mAhg −1	0.1C	8.4%	Ref [90]
NCM111	Leaching + precipitation + sintering	−/134.3 mAh g−1	0.5C	-	Ref [82]
NCM111	Solid sintering	−/173 mAh g−1	1C	-	Ref [93]
NCM111	Molten salt sintering	120 mAh g−1/150 mAh g−1	1C	20%	Ref [106]
NCM111	Direct calcination	80 mAh g−1/129.1 mAh g−1	0.5C	37%	Ref [85]
NCM111	Hot lithium of ionic liquid	100 mAh g−1/145 mAh g−1	1C	31%	Ref [126]
NCM811	Sol-gel method	−/180 mAh g−1	1C	-	Ref [26]
NCM811	Coprecipitation + sintering	−/190 mAh g−1	1C	-	Ref [76]
NCM622	Peroxidation sintering	90 mAh g−1/153.82 mAh g−1	1C	41%	Ref [121]
NCM811	Molten salt sintering	−/187.2 mAh g−1	1C	-	Ref [29]
NCM523	Solid sintering	−/157.7 mAh g−1	0.5C	-	Ref [127]
NCM811	Pre-oxidation +sintering	−/180 mAh g−1	1C	-	Ref [128]
NCM523	Molten salt process	120 mAh g−1/150.6 mAh g−1	1C	-	Ref [107]
NCM523	Molten salt process	100 mAh g−1/160 mAh g−1	0.5C	37.5%	Ref [105]
NCM523	Molten salt process	−/159 mAh g−1	1C	-	Ref [99]
NCM523	Molten salt process	120 mAh g−1/146.3mAh g−1	1C		Ref [101]
NCM523	Molten salt process	−/152.5 mAh g−1	1C	-	Ref [102]
NCM523	Molten salt method for Al doping	45 mAh g−1/158.6 mAh g−1	1C		Ref [129]
NCM523	Molten salt process	−/160 mAh g−1	0.2C	-	Ref [100]
NCM523	Molten salt process	20 mAh g−1/150 mAh g−1	1C	71.5%	Ref [104]
NCM523	Hydrothermal method + molten salt sintering	60 mAh g−1/140 mAh g−1	1C	57%	Ref [23]
NCM523	Coprecipitation + calcination	−/173.9 mAh g−1	0.2C	-	Ref [130]
NCM622	Leaching + precipitation + sintering	−/181 mAh g−1	0.1C	-	Ref [131]
NCM111	Leaching + precipitation + sintering	−/164.9 mAh g−1	0.2C	-	Ref [25]
NCM1.51.57	Hydrothermal method + sintering	99 mAh g−1/150.7 mAh g−1	C/3	34%	Ref [125]
NCM111	Hydrothermal method + short annealing time	130 mAh g−1/150.4 mAh g−1	C/3	13.3%	Ref [124]
NCM622	Hydrothermal method + sintering	87 mAh g−1/153.82 mAh g−1	1C	43%	Ref [121]
NCM111	Leaching + sintering	−/200 mAh g−1	0.5C	-	Ref [82]
NCM111	Leaching + precipitation + sintering	−/160 mAh g−1	0.5C	-	Ref [114]
NCM523	Leaching + roasting	−/156.9 mAh g−1	0.5C	-	Ref [115]
NCM	Leaching + roasting	85 mAh g−1/144.3 mAh g−1	1C	40.9%	Ref [117]
NCM622	Leaching + roasting	−/152.2 mAh g−1	1C	-	Ref [118]

lists the improvement rates of the capacity of NCM materials regenerated by various methods mentioned above.

5. Conclusions and Outlook

This paper delineates the factors contributing to the failure of NCM and reviews the advancements in heat treatment for the recycling and regeneration of ternary cathode materials while addressing the issues that must be resolved by regeneration methods, informed by an understanding of the materials' failure mechanisms. The electrochemical performance of NCM deteriorates during chronic cycling due to the generation of microcracks, the mixing and discharging of cations, and the production of by-products. Here we respectively detail how the materials become degraded internally, which is of great significance in utilizing waste materials for recycling. Ternary materials, being a crucial component in lithium-ion batteries, hold a significant role in the power market and energy storage sector. With the explosive growth in the quantity of LIBs, there is an urgent need to solve the disposal problem of used batteries. The heat treatment technology plays an important role in the recycling of waste ternary cathode materials. Under the condition of supplementing the required metal elements, the heat treatment technology can be directly used to recycle the materials. We have also explored how the synergistic effect of heat treatment technology, element doping, and surface coating can improve the performance of recycled materials. Analyze the influence of the selection of process parameters during heat treatment on the electrochemical performance of recycled materials. In addition, the technologies combining heat treatment technology with eutectic molten salts, coprecipitation, and hydrothermal methods are also relatively mature. These technologies can not only give play to the advantages of wet chemical methods but also enhance the effect of heat treatment. As the requirements for ternary cathode materials in commercialization are gradually increasing, it is necessary to continuously improve aspects such as the cut-off voltage, cycle life, and capacity. While enabling the reuse of failed materials, it is also essential to find more suitable approaches for industrial production. There are several suggestions for recycling ternary cathode materials:

(1) Material regeneration for failure mechanisms.

The active materials of ternary batteries will gradually become obsolete during long-term cycling, and the materials will undergo phase structure transformation, mixing and arranging of cations, generation of cracks between lattices, side reactions, and other problems. The alteration of the layered phase structure of the ternary material results in the disruption or obstruction of the lithium-ion transport channel, hence diminishing the efficiency of lithium-ion transport. The mixing of cations damages the original distribution state of the elements, which in turn destroys the laminar construction of the material, and the material deteriorates. Lattice cracks inevitably occur under harsh conditions such as high pressure and high temperature, and the electrolyte also reacts with the valuable metals in the material over a long period of time, depleting the valuable elements and generating gases that lead to potentially hazardous production of batteries. Researchers should accurately restore the structure of the material to realize the reuse of the active material for its failure mechanism, but there are still many unknown influences that have not been explored in depth, such as the production of SEI film and the impact it brings.

(2) Shorten the preparation process when the heat treatment technology is combined with other methods to improve production efficiency and save energy.

Combining heat treatment with the molten salt method, coprecipitation method, and hydrothermal method is an effective way to recycle failed materials. However, a large amount of salt is required in the process of molten salt recycling, which is then removed after washing. This operation will lead to a large amount of waste of molten salt, and the appropriate ratio of molten salt to waste materials, treatment temperature, and the system used are experimental factors that are relatively difficult to determine. In addition, the coprecipitation and hydrothermal methods originally require the use of additional chemical reagents, and at the same time, waste needs to be removed. Combining heat treatment with them will undoubtedly increase the production cost, which is unfavorable for industrial production. We still need to find efficient, low-pollution, and economical chemical additives to reduce the production cost and consider how to shorten the preparation process.

(3) Optimize the process parameters of the combination of heat treatment and other methods to achieve maximum energy efficiency when combined with other methods.

As a post-treatment step for the molten salt method, coprecipitation method, and hydrothermal method, heat treatment plays roles such as homogenizing material particles, removing impurities generated during the treatment process, and improving the crystallinity of the materials. However, at the same time, it also increases the length of the recycling process, making the entire process cycle more cumbersome and more energy-consuming. Therefore, we need to continue to explore the best way to apply its advantages in the post-treatment stage of other recycling methods and try our best to reduce energy consumption, shorten the process, and improve efficiency.

This research examines the causes of failure in ternary cathode materials and the impact of heat treatment on their repair and regeneration. The inadequacy of ternary cathode materials mostly arises from cation mixing and discharging, the occurrence of phase transitions, and the formation of microcracks. Heat treatment during the repair of ternary cathode materials can rejuvenate the material's crystal structure, facilitate phase transitions, and diminish cation mixing and exclusion, so enhancing the electrochemical performance of the degraded material and enabling the recycling of waste material. However, the heat treatment technology requires further optimization of process parameters to improve the efficiency and quality of recycling. Given the rising commercialization demands for ternary cathode materials, future research must focus on the advancement of more efficient and eco-friendly regeneration techniques to satisfy industrial production requirements while minimizing resource waste and environmental pollution.