Enhancing High-Temperature Cycling Stability and Rate Capability of LiNi_0.71Co_0.09Mn_0.2O₂ Cathodes via Al₂O₃/LiBO₂ Double Coatings

Abstract

Layered nickel-rich cathodes are regarded as promising cathode materials for lithium-ion batteries (LIBs) due to their higher electrochemical capacities and lower cost. However, the development and commercial application of nickel-rich cathodes are severely hindered by significant capacity fading under a high charge cut-off voltage (4.5 V), which arises from interfacial instability and bulk structural degradation during charge–discharge processes. In this study, a two-step double-coating strategy was innovatively adopted to successfully synthesize Al₂O₃/LiBO₂ co-coated LiNi_0.71Co_0.09Mn_0.2O₂ cathode material (denoted as NCM-Al/B). X-ray photoelectron spectroscopy (XPS) verified that Al existed stably in the form of Al³⁺, and B formed B-O-M covalent bonds with transition metals (Ni/Co/Mn), constructing a dual-element synergistic interface. This interface significantly reduced the surface Ni³⁺ content and enhanced the structural stability by suppressing the H₂→H₃ phase transition. The NCM-Al/B material exhibits excellent electrochemical performance: it maintains a remarkable cycling stability with a capacity retention of 91.6% after 100 cycles at 1 C and 25 °C and delivers a discharge capacity of 156.6 mAh·g⁻¹ with a capacity retention of 75.4% after 100 cycles at a high rate of 1 C. This work establishes a chemically driven double-coating strategy and provides a new paradigm for optimizing the performance of high-nickel cathode materials.

1. Introduction

As the consumption of finite fossil fuels continues to rise, the demand for alternative renewable energy is growing at an unprecedented rate. Nevertheless, renewable energy sources are inherently diffuse and intermittent, which has driven substantial interest in energy storage systems that act as a critical bridge between conventional and renewable energy, with lithium-ion batteries (LIBs) emerging as a particularly prominent area of focus [1,2]. Currently, LiNi_0.5Co_0.2Mn_0.3O₂ (NCM-523) and LiNi_0.6Co_0.2Mn_0.2O₂ (NCM-622) represent advanced cathode candidates toward high-energy-density LIBs [3]. However, the high cost and limited energy density of current LIBs restrict their broader application in grid-scale energy storage and electric vehicles (EVs). Consequently, it is critical to pursue next-generation LIBs that exhibit higher gravimetric and volumetric energy density, improved environmental compatibility, and enhanced safety and stability, thereby meeting the stringent requirements for power sources in EVs, energy storage systems, and portable electronic devices [4,5].

Given that cathode materials play a decisive role in determining the overall performance of LIBs, extensive research efforts have been devoted to designing high-performance cathodes featuring high capacity, superior rate capability, and long cycle life [6,7,8]. In this respect, high Ni content LiNi_xCo_yMn_zO₂ (x ≥ 0.6) materials, particularly LiNi_0.7Co_0.1Mn_0.2O₂, have attracted wide attention in new energy power market recently for higher practical reversible capacity (~200 mAh∙g⁻¹) and lower cost [9,10]. However, the ionic radius of Ni²⁺ (0.69 Å) is very close to that of Li⁺ (0.76 Å), which causes Ni²⁺ to irreversibly migrate into the Li layer (i.e., cation mixing) during Li-ion extraction and insertion. Such Li⁺/Ni²⁺ ion exchange can induce the structural transformation of LiNi_0.7Co_0.1Mn_0.2O₂ from the layered phase (R-3m) to the spinel-like phase (Fd-3m) and rock-salt phase (Fm-3m), accompanied by a certain degree of local structural collapse [11,12]. Notably, the degree of Ni/Li cation mixing can be significantly mitigated by the synthesis of highly ordered Ni-rich precursor materials and the excessive addition of lithium [13]. Nevertheless, the residual lithium located on the active material surface can react with moisture to generate by-products such as LiOH and Li₂CO₃. During Li-ion intercalation and deintercalation, the formation of electrochemically inactive rock-salt NiO phase and Li₂CO₃/LiOH layers give rise to high cathode resistance and rapid capacity degradation [13]. To effectively mitigate the key issues arising from Li impurities and crystal structural phase transitions, the bifunctional surface coating strategy has demonstrated significant potential in enhancing the electrochemical performance of layered Ni-rich oxides.

Extensive efforts have been devoted to alleviating the structural degradation of Ni-rich cathode materials, including element doping [14,15], surface modification [16,17], and structural regulation [18]. Among these strategies, thin-film surface coatings (e.g., AlF₃, MgO, ZnO, Al₂O₃, H₃BO₃) have been widely employed to reduce the reactivity between the electrode surface and the electrolyte, thereby prolonging the cycling life of cathode materials [17,19]. Typically, such coatings primarily function as protective layers, which can effectively isolate the cathode material from the electrolyte and thus mitigate side reactions during battery cycling. Unfortunately, if the bond energy of the chemical bonds formed between the coating and the host material is relatively low, it will lead to the peeling of the coating layer [17]. Moreover, single modification strategies fail to simultaneously optimize both bulk and surface properties [20,21].

Therefore, designing a bifunctional coating with a stable coating/substrate interface presents a promising route to achieve long-term cycling stability. The bifunctional surface coating strategy has shown great potential in improving the electrochemical performance of nickel-rich layered oxides [22,23,24]. Specifically, the outer layer of the bifunctional coating can resist HF corrosion, while the inner layer can trap lithium impurities on the surface of nickel-rich materials [24]. Currently, Boron-based compounds, for instance, are regarded as effective surface modifiers. Studies have indicated that boric acids can react with residual Li₂CO₃ at elevated temperatures to form a lithium borate layer [25]. The strong B–O bond helps enhance thermal stability, suppress oxygen release, and improve safety [26]. Additionally, boron can diffuse into tetrahedral sites of the layered structure at high temperatures, further stabilizing the bulk material [27]. Likewise, alumina coatings have been extensively studied for their ability to shield the cathode surface from electrolyte decomposition [28,29]. Zeng et al. [30] proposed a demand-tailored surface coating strategy, which integrates the respective advantages of LiBO₂ and LiAlO₂ by constructing a multifunctional LiBO₂/LiAlO₂ composite layer on single-crystal LiNi_0.5Co_0.2Mn_0.3O₂ (NCM) particles. The resulting co-coated NCM material exhibits significantly enhanced rate capability and cycling stability at both 25 °C and 55 °C compared to the bare NCM. Notably, it maintains a reversible specific capacity of 123.8 mAh g⁻¹ after 500 cycles at 1 C. Ho et al. [31] applied a wet-chemical method to deposit a uniform lithium-alumina-boron oxide layer on LiNi_0.88Co_0.06Mn_0.06O₂, demonstrating improved surface homogeneity and conductivity. Nevertheless, studies on co-coating with Al and B remain limited.

In this study, a LiNi_0.71Co_0.09Mn_0.2O₂ cathode material co-coated with Al₂O₃ and LiBO₂ (denoted as NCM-Al/B) was synthesized via a facile wet-coating method, where Al₂O₃ and LiBO₂ served as the bifunctional coating layers. Our investigation reveals that Al and B elements coexist on the material surface: Al stably exists in the form of Al³⁺, while B forms covalent bonds with transition metals (Ni/Co/Mn) via B-O-M bonds. The high electronegativity of B-O-M bonds reduces the content of surface Ni³⁺, thereby leading to a higher proportion of Ni²⁺ in NCM-Al/B than that in NCM-Al. Al³⁺ and B-O-M bonds synergistically stabilize the crystal structure. On one hand, the dual-element synergy optimizes the interfacial impedance of the material, thus enhancing its rate capability; on the other hand, it suppresses the side reactions at the cathode-electrolyte interface, thereby improving the cycling performance of the material. Benefiting from the synergistic effects of this multi-functional hierarchical coating, we anticipate that the modified LiNi_0.71Co_0.09Mn_0.2O₂ cathode will exhibit excellent electrochemical performance.

2. Materials and Methods

2.1. Material Synthesis

The Ni_0.71Co_0.09Mn_0.2(OH)₂ precursors were synthesized via a co-precipitation method, during which the concentration of oxygen atmosphere in the reactor was carefully controlled. Firstly, NiSO₄·7H₂O, CoSO₄·6H₂O, and MnSO₄·H₂O (Jinchuan Group Co., Ltd., Jinchang, China) were dissolved in deionized water to formulate a transition metal sulfate solution with a Ni²⁺:Co²⁺:Mn²⁺ molar ratio of 7.1:0.9:2.0. This 2 mol·L⁻¹ solution was gradually fed into a continuous stirred-tank reactor (CSTR) under a N₂ atmosphere. Meanwhile, 4 mol·L⁻¹ NaOH solution (as the precipitant, Fujian Dongnan Electrochemical Co., Ltd., Fuzhou, China) and a suitable amount of NH₃·H₂O (as the chelating agent, Wuhu Sanshun New Energy Material Co., Ltd., Wuhu, China) were independently introduced into the CSTR. Afterwards, the as-prepared hydroxide precursors and LiOH·H₂O (Jiangxi Ganfeng Lithium Group Co., Ltd., Xinyu, China) were homogenously blended at a molar ratio of 1:1.05 by a Pulverisette 7 planetary micro-miller (Fritsch GmbH, Idar-Oberstein, Germany). The mixture was subjected to pre-calcination at 480 °C for 6 h and further calcined at 940 °C for 18 h in a muffle furnace under flowing oxygen, affording the final LiNi_0.71Co_0.09Mn₀.₂O₂ (NCM) products. For the preparation of Al₂O₃-coated NCM (labeled as NCM-Al), the obtained NCM powders were thoroughly mixed and ground with alumina powder (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) at a certain proportion, followed by calcination at 450 °C for 4 h in a muffle furnace. Subsequently, the Al₂O₃-LiBO₂ co-coated NCM (marked as NCM-Al/B) was fabricated by mixing and grinding NCM-Al with a desired amount of boric acid (Xilong Scientific Co., Ltd., Shantou, China) in the same manner as described above, followed by calcination at the same temperature (450 °C for 4 h). For comparison purposes, LiBO₂-coated NCM samples (NCM-B) were synthesized using the same procedure, except that a required amount of boric acid was added directly during the mixing process instead of alumina powder. The schematic diagram is illustrated in Figure 1.

Figure 1. Schematic diagram of the synthesis of pristine and coated NCM.

2.2. Materials Characterization

The phase compositions of the as-prepared samples were characterized by X-ray diffraction (XRD, XRD-6000, Shimadzu Corporation, Kyoto, Japan) with Cu Kα radiation. Surface oxidation states of Ni, Mn, Co, Al and B were probed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo Fisher Scientific Inc., Waltham, MA, USA), in which the binding energies were referenced to the C 1s peak at 284.8 eV for calibration. The surface morphology and microstructure were observed by field-emission scanning electron microscopy (FESEM, SU8700, Hitachi, Ltd., Tokyo, Japan) equipped with an energy-dispersive X-ray spectrometer (EDS, Model Ultim Max 170, Oxford Instruments plc, Abingdon, UK). In addition, the elemental distribution across the surface coating layer of the material was characterized by electron probe microanalysis (EPMA, Model JXA-8530F, JEOL Ltd., Tokyo, Japan) after pretreatment with cross-section polishing (CP, ion milling, IM4000Plus, Hitachi, Ltd., Tokyo, Japan). The specific surface area and pore volume of the samples were calculated from N₂ adsorption–desorption isotherms measured at 77 K using a Micromeritics ASAP 2010 instrument (Micromeritics Instrument Corporation, Norcross, GA, USA), based on the BET and BJH models, respectively.

The concentrations of target metal ions in the samples were quantified using an inductively coupled plasma optical emission spectrometer (ICP-OES, ICAP7200, Thermo Fisher Scientific Inc., Waltham, MA, USA). Prior to instrumental analysis, all sample solutions were diluted to the appropriate linear detection range of the spectrometer to ensure the accuracy and reliability of the analytical results. The mass fraction (x, %) of the analyte in the original sample was calculated according to the following formula:

$$x\left(\%\right)={\displaystyle \frac{cV}{m}}\times 100\%$$

where x is the mass fraction of the target analyte in the sample (%); c is the concentration of the analyte in the solution (g/L); m is the mass of the original sample (g); and V is the total constant volume of the test solution (L).

2.3. Electrochemical Measurements

Electrochemical properties were investigated using 2032-type coin cells (Hefei Kejing Material Technology Co., Ltd., Hefei, China) assembled in an inert atmosphere. The working electrodes were prepared by blending 80 wt% active material, 10 wt% acetylene black (as the conductive agent, Denka Company Limited, Tokyo, Japan), and 10 wt% polyvinylidene difluoride (PVDF, as the binder, Solvay S.A., Brussels, Belgium) in N-methyl-2-pyrrolidone (NMP, Nanjing Changxin Chemical Co., Ltd., Nanjing, Jiangsu, China). The slurry was uniformly cast onto Al foil (Jiangsu Dingsheng New Materials Co., Ltd., Zhenjiang, China), vacuum-dried at 80 °C for 12 h, and subsequently punched into 13 mm-diameter electrodes. Lithium metal foil was used as the counter electrode, and 1 M LiPF₆ dissolved in ethylene carbonate/dimethyl carbonate (EC/DEC, v/v = 1:1, Guangzhou Tinci Materials Technology Co., Ltd., Guangzhou, China) was employed as the electrolyte. Galvanostatic charge–discharge measurements were conducted on a LAND CT-2001A testing system (Wuhan LAND Electronic Co., Ltd., Wuhan, China) between 3.0 and 4.5 V at current densities of 0.2–1 C. Cyclic voltammetry (CV) was performed on a CHI 760E electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd., Shanghai, China) at 0.1 mV s⁻¹ within 3.0–4.5 V (vs. Li/Li⁺). Electrochemical impedance spectroscopy (EIS) was recorded on the same CHI 760E workstation in the frequency range of 10⁻²–10⁵ Hz with an AC amplitude of 10 mV.

3. Results

Figure 2 presents the XRD patterns of the pristine NCM, NCM-Al, NCM-B, and NCM-Al/B samples. After the coating treatment, the NCM particles maintained their original crystal structure (R3m), suggesting that the coating process did not induce significant structural alterations or the formation of additional impurities. All diffraction peaks can be well indexed to an α-NaFeO₂-type hexagonal structure with the R-3m space group. It is clearly observed that the (003) diffraction peaks of NCM-B and NCM-Al/B exhibited a slight shift, whereas no obvious shift was detected for that of NCM-Al. This angular deviation can be attributed to the occupation of tetrahedral interstices in the oxygen framework of the transition metal (TM) and Li layers by B ions with a smaller ionic radius, which leads to the expansion of the crystal unit cell [32]. The obvious splitting of the (006)/(102) and (108)/(110) peaks for all samples confirms the presence of a well-developed layered structure.

Figure 2. XRD patterns of pristine and coated NCM. The dotted line indicates the (003) peak of NCM materials.

In addition, Rietveld refinement of the XRD patterns was performed to investigate the material structure in detail. An interesting phenomenon was observed from the analysis of the unit cell parameters and the cation mixing degree between NCM and the coating materials. When Al³⁺ or B³⁺ ions (with smaller ionic radii) were introduced individually, the unit cell volume decreased slightly, while the cation mixing degree was not significantly improved. As summarized in Table 1, the Ni/Li disordering ratios of pristine NCM, NCM-Al, and NCM-B are 6.46%, 6.85%, and 6.23%, respectively. In contrast, when Al³⁺ and B³⁺ were co-introduced to modify NCM, the unit cell volume of NCM-Al/B increased, and the Ni²⁺/Li⁺ disordering degree was significantly reduced, with its Ni/Li disordering ratio being only 1.97%—much lower than those of pristine NCM, NCM-Al, and NCM-B. This improvement may be attributed to the formation of Al-B bonds and the site occupation where Al ions reside in Li sites and B³⁺ in TM sites, which attracts Li ions and thus effectively reduces Ni/Li cation disorder [33].

Table 1. Rietveld analysis results of the NCM, NCM-Al, NCM-B and NCM-Al/B.

Samples	a (Å)	b (Å)	c (Å)	Volume (Å3)	c/a	Ni/Li (%)
NCM	2.869576	2.869576	14.196152	101.236	4.9471	6.46
NCM-Al	2.869015	2.869015	14.194242	101.183	4.9474	6.85
NCM-B	2.869263	2.869263	14.19583	101.212	4.9476	6.23
NCM-Al/B	2.869672	2.869672	14.198059	101.257	4.9476	1.97

ICP-OES was used to determine the component contents and proportions of NCM, and the mass fractions of Ni, Co, and Mn were determined to be 43.68%, 5.45%, and 11.36%, respectively, thereby obtaining a molar ratio of 0.71:0.09:0.20.

Figure 3 displays the comparative SEM images of the pristine NCM, NCM-Al, NCM-B, and NCM-Al/B samples, which clearly reveal the discrepancies in morphology and microstructure between the uncoated and coated particles. It can be observed that the Al-coated samples present a dispersed coating state in the form of scattered dots and small flakes, while the surfaces of the B-coated samples are covered with a thin film; the surfaces of the Al-B co-coated samples are also uniformly coated with a thin film, indicating that the coating structure of the target product is successfully controlled.

Figure 3. SEM images of (a) NCM, (b) NCM-Al, (c) NCM-B, and (d) NCM-Al/B. EDS spectra of Al, B, Ni, Co, and Mn elements are presented in (a-1–a-5), (b-1–b-5), (c-1–c-5), and (d-1–d-5), respectively. The EPMA elemental mapping images of the NCM-Al/B material (e): Ni, Al, Mn, Co, and B elements correspond to images (e-1), (e-2), (e-3), (e-4), and (e-5), respectively.

Furthermore, EDS analysis demonstrates that the Ni, Co, and Mn elements are uniformly distributed in the bulk of the as-prepared pristine material. For the Al-coated sample, Al is dispersed on the surface in the form of small flakes; B element exhibits relatively uniform large-area flake-like coating; while the Al-B co-coated sample combines both coating morphologies, achieving an overall homogeneous coating layer. Figure 3e further characterizes the Al-B co-coated sample via EPMA. The diffusion of Al and B elements inside the crystal lattice is not obvious, albeit a small quantity of B atoms penetrates into the crystal bulk. This phenomenon can be ascribed to the relatively low coating temperature required for B element [34]. The B coating process adopted in this study enables partial diffusion of B into the material matrix. Nevertheless, owing to the low atomic weight of B, it is difficult to observe B element clearly via EPMA and EDS, resulting in relatively low definition and failure to display more detailed information. Even so, the observed results in this work have provided certain confirmation for evaluating the coating state of the target samples.

XPS characterization was employed to analyze the surface chemical states of the main elements in the pristine and coated NCM samples, with the corresponding XPS spectra depicted in Figure 4. In the uncoated NCM, the average valence state of Ni is approximately +2.71, and Ni mainly exists in a mixed state of Ni³⁺ and Ni²⁺ with a slightly higher content of Ni³⁺ than Ni²⁺. Accordingly, the Ni³⁺ peak of the bare NCM matrix is more intense, which conforms to the intrinsic surface chemical state of the material after synthesis without contacting the electrolyte. In contrast, the Ni³⁺ peak of the coated samples is weakened while the Ni²⁺ peak is intensified, which is mainly ascribed to the chemical interaction between the coating layer and the material surface, leading to the variation in surface chemical states. Specifically for Al coating, Al³⁺ reacts with surface Ni³⁺ to form Al-O-Ni bonds, and such bonding increases the electron cloud density of Ni, triggering the partial “reduction” of surface Ni³⁺ to Ni²⁺. As for B coating, B³⁺ interacts with surface Ni³⁺ to generate B-O-Ni bonds, exerting a similar effect to that of Al coating. Then the Al-B co-coating exhibited a general trend of reducing both the Li⁺/Ni²⁺ disordering degree and discharge loss compared with the other samples. Owing to the simultaneous Al and B coating, a small amount of B is observed to penetrate into the crystal interior from the EPMA results. This may be ascribed to the fact that the total amount of dopant elements involved in surface reactions increases after Al/B co-coating, which is more conducive to their infiltration into the lattices of the transition regions of partial grains, resulting in the distribution of coating elements not merely on the material surface. The aforementioned improvement in cation disorder can be partly attributed to the synergistic doping substitution of Al and B ions [13]. A small amount of Al³⁺ or B³⁺ ions can enter the TM layer and occupy Ni sites, resulting in the partial reduction of Ni³⁺ to Ni²⁺ [35]. In addition, further analysis of the existing forms of coating elements on the surfaces of NCM-Al, NCM-B, and NCM-Al/B samples indicates that Al ions exist in the form of Al³⁺, whereas B forms B-O-M chemical bond (where M represents transition metals). Figure 4e,f demonstrate that after Al/B co-coating, the chemical state of boron on the surface of NCM materials is similar to that in the single B-coated sample. However, the synergistic effect between Al and B exerts a significant influence on the chemical state of Ni in the surface layer, with an intensified characteristic peak of Ni²⁺ and a decreased proportion of the Ni³⁺ peak. This further verifies that the introduction of B in the co-coating system promotes the more sufficient formation of Al-O-Ni bonds between Al³⁺ and surface Ni³⁺, and such chemical bonding leads to the conversion of partial surface Ni³⁺ into Ni²⁺.

Figure 4. XPS spectra of Ni 2p for (a) NCM, (b) NCM-Al, (c) NCM-B, and (d) NCM-Al/B, Al 2p for (e) NCM-Al and NCM-Al/B, and B 1s for (f) NCM-Al and NCM-Al/B.

The electrochemical performances of pristine NCM, NCM-Al, NCM-B, and NCM-Al/B were evaluated under a series of harsh conditions, including a high cut-off voltage (3.0–4.5 V) and an elevated temperature (45 °C). Figure 5a presents the charge–discharge curves of the bare and coated NCM samples for the first two cycles at a current rate of 0.1 C within 3.0–4.5 V. Compared with the first cycle, the charging plateau of the second cycle exhibited a slight upshift accompanied by a slight capacity decrease. During the charging process, a cathode electrolyte interphase (CEI) layer was formed on the surface of the electrode via the reaction between the electrode and electrolyte, which hindered Li⁺ migration and alleviated polarization to a certain extent. In addition, the Coulombic efficiencies of pristine NCM, NCM-Al, NCM-B, and NCM-Al/B at the end of the first cycle were 84.08%, 85.57%, 88.86%, and 87.03%, respectively. This result indicates that the surface coating effectively improved the structural stability of the materials and enhanced their electrochemical performances compared with the uncoated NCM. As clearly shown in Figure 5b, the discharge capacity of the coated samples in the second cycle was slightly higher than that in the first cycle, whereas this behavior was not observed in the pristine NCM. This phenomenon may be attributed to the partial blocking of Li⁺ intercalation by coating ions during the initial coating process; after activation, the coated NCM materials can exhibit better electrochemical cycling performance than that uncoated [36]. Figure 5b also shows the rate performances of the different electrode materials. As the current rate increased, the discharge capacity of all electrodes gradually decreased, and the difference in discharge capacity between the pristine and coated NCM samples expanded significantly. It is worth noting that the performances of NCM-Al and NCM-Al/B, especially NCM-Al/B, showed a distinct improvement over that of the pristine NCM at high current rates, demonstrating that Al coating and Al/B co-coating have noticeable merits in improving the rate performance of NCM cathode materials. In contrast, the rate capability of NCM-B was not satisfactory, although it exhibited good performance at low current rates.

Figure 5. Electrochemical performance of NCM, NCM-Al, NCM-B and NCM-Al/B as cathodes for LIBs: (a) charge–discharge curves for 1st, 2nd cycle at 0.1 C; (b) rate capability; (c) continued cycling performance at 1 C; (d) median potential curve of continued cycling at 1 C; (e) median potential curve at various C-rates.

Currently, the EV and power tool places increasingly stringent requirements on cathode materials with exceptional high-rate performance during charge–discharge cycling under diverse extreme operating environments. To validate the favorable role of oxide coating for improving the long-cycle stability of NCM, long-term cycling measurements were carried out at 1 C under elevated operating temperature. Figure 5c shows the cycling performances of the various coated and uncoated electrodes over different charge–discharge cycles. Within the first 50 cycles, the electrochemical performance gap between NCM-B and pristine NCM was narrow; however, as the cycle number increased to 100, this gap gradually widened, indicating that the B-coated sample had higher capacity retention and better cycling performance than the uncoated NCM. During the long-cycle test, the Al coating on the NCM surface effectively improved its structural stability at high temperatures and increased its capacity retention to a certain extent. Notably, for the NCM-Al/B cathode system, Al and B exhibited a synergistic effect in promoting capacity increase and improving capacity retention rate. The electrochemical performances of the pristine NCM, NCM-Al, NCM-B, and NCM-Al/B samples measured at 45 °C are illustrated in Figure 5c. After 100 cycles, the NCM-Al/B sample delivers the highest capacity retention of 74.7%, followed by NCM-Al (68.1%), NCM-B (66.2%), and pristine NCM (64.3%). Correspondingly, the NCM-Al/B cathode maintains the highest absolute discharge capacity throughout the cycling test, along with the most stable and highest Coulombic efficiency among all tested samples.

To clarify the specific improvement mechanism of Al/B co-coating on NCM materials, additional electrochemical tests were conducted on pristine NCM, NCM-Al, NCM-B, and NCM-Al/B samples at room temperature (25 °C). Table 2 presents the cycling curves of these samples at 25 °C. It can be observed that after activation at 0.1 C, the specific discharge capacities of the samples at a constant current density of 1 C for the first cycle were 185.8 mAh·g⁻¹ (pristine NCM), 186.7 mAh·g⁻¹ (NCM-Al), 193.1 mAh·g⁻¹ (NCM-B), and 193.5 mAh·g⁻¹ (NCM-Al/B), with corresponding Coulombic efficiencies of 92%, 90.7%, 92.16%, and 92.25%, respectively. These results confirm that B coating can significantly enhance the material’s capacity and optimize the host crystal structure. The long-term electrochemical performance of these materials at a high current density (1 C) was also investigated. It is found that Al and/or B coating can improve the cycling performance of NCM, particularly for NCM-B and NCM-Al/B. More interestingly, the difference in electrochemical performance between NCM-Al and pristine NCM was relatively small at 25 °C; a similar trend was also observed between NCM-Al/B and NCM-B. In contrast, the electrochemical performances of the four materials measured at 45 °C exhibited a significant gradient difference, consistent with the results presented in Figure 5c.

Table 2. Electrochemical performance of the NCM, NCM-Al, NCM-B and NCM-Al/B electrodes at different cycles (1 C rate, 185 mA∙g⁻¹).

Electrodes	Temperature (°C)	1st Specific Discharge Capacity (mAh·g−1)	100th Specific Discharge Capacity (mAh·g−1)	Capacity Retention Rate (%)
NCM	25	185.8	161.6	86.98%
	45	199	128.9	64.77%
NCM-Al	25	186.7	161.7	86.61%
	45	199.9	133.2	66.63%
NCM-/B	25	193.1	177.4	91.87%
	45	205.6	141.2	68.68%
NCM-Al/B	25	193.5	176.9	91.42%
	45	207.7	156.6	75.40%

Based on these comparative experiments, the following conclusions can be drawn: (I) Al coating can improve the cycling performance of NCM materials to a certain extent and endow them with enhanced rate performance; (II) B coating is more advantageous in enhancing the cycling performance of the materials but has little ability to improve rate performance, especially at high current rates; (III) Al/B co-coating exhibits outstanding effectiveness in improving both the cycling stability and rate capability of NCM materials at elevated temperatures.

To further elucidate the underlying mechanism of capacity decay, the potential changes during the charge–discharge processes of the materials were monitored, as shown in Figure 5d,e. It is noteworthy that the capacity decay trend exhibits a positive correlation with voltage attenuation, meaning that materials with a relatively gentle voltage decay trend possess better capacity retention. The NCM-Al, NCM-B, and NCM-Al/B electrode materials can effectively mitigate potential decay over long-term cycling, which is associated in part with the decreased residual lithium on the surface of NCM electrode materials via the coating treatment [36,37]. Among these samples, the NCM-Al/B exhibits the most prominent improvement, and this enhancement is attributed to the interaction between Al and B atoms, which can effectively slow down capacity decay. Further analysis of the potential decay rate reveals that the NCM-Al/B has the smallest potential attenuation, confirming that both Al/B co-coating and Al single coating exert a significant effect on improving rate capability, whereas B single coating has no obvious effect on enhancing capacity retention.

To gain deeper insight into the structural evolution of the electrodes upon repeated charge–discharge cycles and its correlation with capacity degradation, differential capacity (dQ/dV) curves were calculated from the corresponding charge–discharge profiles. Figure 6 presents the dQ/dV curves of pristine NCM, NCM-B, and NCM-Al/B samples under different charge–discharge cycles at 45 °C. It can be clearly observed that all samples (NCM, NCM-B, and NCM-Al/B) undergo three stages of phase transition during the charging process: from the hexagonal phase (H₁) to the monoclinic phase (M), and subsequently to another hexagonal phase (H₂)—a common characteristic of high-Ni ternary cathode materials. However, the subsequent H₂→H₃ phase transition causes a drastic contraction of the unit cell along the c-axis, which imposes mechanical strain on the cathode and subsequently deteriorates its rechargeability. For pristine NCM and NCM-Al cathodes, the intensity of the H₂→H₃ oxidation peak exhibits a dramatic decline when the cycle number increases to 10; in contrast, the NCM-B and NCM-Al/B cathodes show a sharp drop in the H₂→H₃ oxidation peak intensity at the 30th and 50th cycles, respectively. This observation reflects the inferior reversibility of the H₂→H₃ phase transition upon cycling, which is ascribed to the serious structural degradation triggered by the mechanical strain originating from this detrimental phase transition [19].

Figure 6. Differential curves during long cycling of (a) NCM, (b) NCM-Al, (c) NCM-B and (d) NCM-Al/B at 45 °C.

Such structural collapse severely restricts the movement of the material’s redox potential, leading to severe polarization and a decline in capacity retention. Once the structure is damaged, the internal electrolyte is exposed, thereby exacerbating capacity decay [24,38]. In contrast, the Al/B co-coating can effectively mitigate the subsequent contraction and expansion of the material during charge–discharge processes, thereby delaying capacity decay. Furthermore, as shown in Figure 6, polarization becomes progressively worse with cycling, causing a shift in the redox potential. At 45 °C within the voltage window of 3.0–4.5 V, the reduction potential deviation between the 1st and 100th cycles for NCM-Al/B is 0.311 V, which is considerably smaller than those of pristine NCM (0.540 V), NCM-Al (0.588 V), and NCM-B (0.497 V). The superior cycling and rate performances of NCM-Al/B are attributed to the reduction in internal cation mixing by the relatively small amount of co-coating, which mitigates external side reactions and stabilizes the layered structure during charge–discharge processes. Last but not least, NCM-Al/B can still deliver excellent performance under high-temperature conditions. This may be attributed to the fact that high temperature facilitates the migration of a portion of Al and B ions into the bulk material, where these ions effectively stabilize the crystal structure and suppress structural collapse [39].

Figure 7 exhibits the results of the electrochemical impedance spectroscopy (EIS) measurements for pristine NCM, NCM-Al, NCM-B and NCM-Al/B after the 1st cycle. EIS was employed to investigate the interfacial reaction kinetics of the samples. All Nyquist plots consist of two typical features: a depressed semicircle in the high–medium frequency region and a sloped linear line in the low-frequency region. The acquired EIS spectra were simulated using the equivalent circuit illustrated in Figure 7b, which comprises the electrolyte resistance (R_e), cathode-electrolyte interface (CEI) resistance (R_CEI), capacitance related to the CEI (C_CEI), charge-transfer resistance (R_ct), non-ideal capacitance of the surface layer (C_dl) serving as an approximate substitute for the electric double-layer capacitance, and Warburg impedance associated with lithium-ion diffusion (W). All data of the fitting resistance are listed in Table 3. The CEI film resistance (R_CEI) values of the four samples are 12.9 Ω (NCM), 11.8 Ω (NCM-Al), 7.86 Ω (NCM-B) and 7.79 Ω (NCM-Al/B), respectively, and their corresponding charge-transfer resistance (R_ct) values are 19.1 Ω, 8.25 Ω, 10.1 Ω, 9.62 Ω. The smaller R_s values verify that the coating layers effectively lower the electrode/electrolyte interfacial resistance, which enhances the electrochemical performance of the samples. The Li⁺ diffusion coefficient (D_Li⁺) was calculated according to the following equation:

$$D={\displaystyle \frac{R^2{T}^2}{2A^2\mathrm{n}4F^4{C}^2{\sigma }^2}}$$

$$Z_W=R_D+R_L+\sigma {\omega }^{\raisebox{1ex}{$-1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}$$

where A denotes the surface area of the electrode (cm²), n represents the number of electrons transferred during the reaction, F is the Faraday constant, C is the concentration of Li⁺ (mol∙cm⁻³), σ denotes the Warburg factor, R is the gas constant, T is the absolute temperature, Z_W is the Warburg impedance, ω is the angular frequency in the low frequency region (0.1–0.01 Hz), and R_D and R_L denote the diffusive resistance and liquid resistance, respectively.

Figure 7. (a) Nyquist plots of NCM, NCM-Al, NCM-B and NCM-Al/B after the 1st cycle; (b) the equivalent circuit of the simulated EIS spectra.

Table 3. Electrochemical impedance parameters of the NCM, NCM-Al, NCM-B and NCM-Al/B electrodes after the 1st cycle.

Samples	RCEI (Ω)	Rct (Ω)	DLi (cm2·s−1)
NCM	12.9	19.1	1.93 × 10−10
NCM-Al	11.8	8.25	5.63 × 10−10
NCM-B	7.86	10.1	4.86 × 10−10
NCM-Al/B	7.79	9.62	6.08 × 10−10

As listed in Table 3, the D_Li+ of NCM, NCM-Al, NCM-B and NCM-Al/B are 1.93 × 10⁻¹⁰, 5.63 × 10⁻¹⁰, 4.86 × 10⁻¹⁰ and 6.08 × 10⁻¹⁰ cm²∙s⁻¹, respectively. This result reveals that the co-modified materials exhibit corresponding higher D_Li+ than pristine one before and after cycling. The primary cause is that the residual lithium-rich passivation layer on the surface of the original material is converted into a conductive layer, reducing thickness of CEI passivation film.

4. Conclusions

In summary, we developed a facile two-step coating approach to synthesize Al₂O₃/LiBO₂ hybrid nanolayer co-coated LiNi_0.71Co_0.09Mn_0.2O₂ cathode materials. XPS analysis confirms that Al exists as Al³⁺ and B forms B-O-M bonds with transition metals (Ni/Co/Mn), creating a dual-element synergistic interface. This interface significantly reduces surface Ni³⁺ content (NCM-Al/B shows a higher Ni²⁺ proportion compared to NCM-Al), enhances crystal structure stability (evidenced by minimal H₂→H₃ phase transition in XRD), and mitigates cathode-electrolyte interfacial side reactions. As a consequence, the Al-B co-coated LiNi_0.71Co_0.09Mn_0.2O₂ cathode exhibits excellent cycling stability and outstanding rate performance: it retains 75.4% of its capacity (156.6 mAh·g⁻¹) at a high current rate of 1 C after 100 cycles at 45 °C, and 91.6% of its capacity (176.9 mAh·g⁻¹) at 1 C after 100 cycles at 25 °C. EIS analysis further reveals that the dual-coating layer reduces charge-transfer resistance and improves the Li⁺ diffusion coefficient of NCM materials, confirming the synergistic mechanism. Notably, the dual-modification strategy proposed in this work also serves as an effective approach for improving the electrochemical performance of other cathode materials.