With the rapid expansion of the electric vehicle (EV) market, the demand for batteries with high energy density and excellent stability is growing rapidly (Li M. et al., 2018; Roy et al., 2024). Among these, rechargeable lithium-ion batteries (LIBs) are garnering significant attention and extensive research. In particular, cathode materials, which play the most critical role in battery performance, are the focus of intense investigation (Nitta et al., 2015; Liu et al., 2016). Recent years have seen advancements in the development of diverse cathode materials tailored to meet specific demands. Layered structure cathodes, such as LiNi_xCo_yMn_1-x-yO₂ (NCM), and LiNi_xCo_yAl_1-x-yO₂ (NCA), are common in the research topics due to their high energy density and capacity (Hong et al., 2024; Mitra and Sudakar, 2024; Sim et al., 2024). As the demand for batteries with both higher energy density and improved safety rises, particular attention is being paid to the development of Ni-rich cathode materials with nickel content exceeding 80% (Sim et al., 2024; Ho et al., 2025a). Due to their high capacity and cost efficiency, Ni-rich cathodes are emerging as promising candidates for high energy density batteries (Sun et al., 2021; Ho et al., 2025b). Nonetheless, unlike traditional cathodes such as LiCoO₂ (LCO) and LiNi_1/3Co_1/3Mn_1/3O₂ (NCM333), Ni-rich cathodes suffer from significant surface instability due to the high reactivity of Ni³⁺ (Mun et al., 2014; Kasnatscheew et al., 2017). During cycling, these materials experience degradation, leading to cation mixing, where Li⁺ and Ni²⁺ exchange positions (Cui et al., 2021). This issue occurs because of the similar ionic radii of Li⁺ (0.76 Å) and Ni²⁺ (0.69 Å), and magnetic frustration (Wang W. et al., 2024). Severe cation mixing compromises structural stability, resulting in critical challenges such as reduced battery cycle ability, electrolyte decomposition, and interface instability (Cui et al., 2021; Wang W. et al., 2024).

To address these challenges, researchers are employing strategies such as elemental doping and coating (Sun et al., 2021; Qu et al., 2022). Doping is one of the most effective methods for suppressing the cation mixing failure. Metallic dopants stabilize the structure during synthesis, improving cycle performance (Sun et al., 2021). The choice of dopant and its concentration significantly influence performance, as excessive doping adversely affects both capacity and cycle ability. Recent advancements include strategies like dual doping or high entropy doping with multiple elements to enhance performance (Park et al., 2022; Ma et al., 2025). Coating creates a physical barrier that prevents direct contact between the electrolyte and the cathode materials (Qu et al., 2022). This approach suppresses side reactions with the electrolyte, mitigating the formation of a thick solid electrolyte interphase (SEI) layer and reducing electrolyte consumption, which are common causes of capacity fading (Yan et al., 2022). However, overly thick coating layers negatively affect ionic and charge transfer, ultimately reducing performance. Coating methods are generally categorized into wet and dry techniques. The conventional wet coating process for Ni-rich cathode materials, despite using excellent coating materials, can lead to performance degradation due to the reduction of Ni³⁺ to Ni²⁺ when exposed to wet solvents during the process (Kalluri et al., 2017). In addition, exposure to moisture causes H⁺ from H₂O to exchange with Li⁺ in the layered structure, leading to Li⁺ loss during synthesis (Hartmann et al., 2021), which in turn results in the formation of electrochemically inactive phases (e.g., NiO-like phase) (Kasnatscheew et al., 2017; Hong et al., 2022; Jeong et al., 2022). These inactive phases exacerbate structural instability and creating fundamental issues that severely impact cycle life and safety (Wang F. et al., 2024). Thus, the surface passivation has not been independently investigated by unavoidable possible surface failure.

Dry coating with shear stress avoids the use of solvents, minimizing issues arising from wet coating processes (Ho et al., 2025a). In addition, it is environmentally friendly as it does not require the use of solvents, eliminating the need for additional drying. Furthermore, it offers the advantages of forming a durable and robust coating layer while enabling the production of a uniform coating layer (Herzog et al., 2021). Dry coating methods have merit with reduction of costs and environmental pollution by eliminating the need for solvents utilized in wet processes, offering a promising advantage.

In this study, we evaluate the effects of both dry coating and water-based (D.I. water) wet coating methods on the performance of Ni-rich cathode materials. The dry coating method utilizes high energy generated from blade rotation to coat Al₂O₃ particles onto the active material through collision. Al₂O₃ is selected for its advantages and economic efficiency as a coating material (Han et al., 2017).

TGA analysis was conducted to determine the coating temperature for Al(NO₃)₃·9H₂O. Supplementary Figure S1 shows the TGA curve of Al(NO₃)₃·9H₂O under an O₂ atmosphere. As observed in Supplementary Figure S1, no further weight loss occurs beyond 400°C. Therefore, temperatures of 400°C and 700°C were selected for the coating process. This is because Al₂O₃ is formed at 400°C, and at 700°C, it reacts with Li to form LiAlO₂ (Li L. et al., 2018). As shown in Supplementary Figure S2, the results indicate that 700°C demonstrates improved electrochemical performance. Additionally, a comparison of Al₂O₃ coating contents at 0.1, 0.5 and 1wt%, respectively, as seen in Supplementary Figure S3, that 0.5wt% exhibits enhanced electrochemical performance. Therefore, 0.5wt% was selected as the coating content for both dry and wet coating methods, and the process was carried out at 700°C.

XRD pattern analysis was conducted to examine whether the dry and wet Al₂O₃ coatings affected the crystal structure of NCM811. As shown in the XRD patterns in Figure 1A, both bare NCM811 and coated NCM811 correspond to α-NaFeO₂ layered structure (R3m lattice space group, JCPDS card No. 82–1,495) (Yang et al., 2024). The distinct peak splitting of (006)/(102) and (108)/(110) peaks in all samples had indicated that the layered structure of the material had been well-developed (Sim et al., 2020). The ratio I ₍₀₀₃₎/I ₍₁₀₄₎ had served as an indicator of the degree of cation mixing in layered structures, where a higher value had been preferred (Sim et al., 2020). Moreover, it had been reported that, due to the similarity in ionic radii between Li⁺ (0.76 Å) and Ni²⁺ (0.69 Å), these ions had migrated from 3a sites to 3b sites, and a value below 1.2 had indicated severe cation mixing (Weber et al., 2020). The values for bare NCM811, D-NCM811, and W-NCM811 were 1.92, 1.94, and 1.96, respectively, showing similar results, which had suggested that the extent of cation mixing had been comparable across all samples. Additionally, due to the low content of Al₂O₃ after coating, peaks for Al₂O₃ had not been observed in the XRD patterns (Li L. et al., 2018). However, as shown in Figure 1B, the dry coated sample exhibited a shift to the left after coating. This shift had potentially been attributed to the expansion of the interlayer spacing (c-axis direction) that had been caused by changes in the oxygen arrangement following the loss of Li ions at the surface, and this behavior could also be associated with the formation of a small amount of LiAlO₂ during the coating process (Li L. et al., 2018).

The effects of the coating process on the particles were examined using FE-SEM. Figures 2A–C shows the secondary particles of bare NCM811, D-NCM811, and W-NCM811, respectively, while Figures 2D–I present the Ni and Al element EDS mapping for each sample. The images reveal no significant changes in the particle morphology before and after coating. Additionally, as confirmed by the EDS mapping, the uniform distribution of Al demonstrates that the coating layer was uniformly applied. The size of the secondary particles for all samples was approximately 20 μm.

HR-TEM analysis was conducted to confirm the coating layer. Figures 3A–C shows the images of bare NCM811, D-NCM811, and W-NCM811, respectively. The HR-TEM images in Figure 3B, C show that the Al₂O₃ coating layer exhibited similar morphology and thickness, independent of the coating method. HR-TEM analysis in ADF mode was conducted, and the EDS line scan results are shown in Figure 3D, with the corresponding Al peak illustrated in Figure 3E. The Al₂O₃ coating layer analysis revealed that no Al peaks were present in uncoated regions, whereas significant Al peaks were observed in the coating layer regions. The results indicate that the Al₂O₃ coating layer was formed with a thickness of approximately 10 nm.

ICP-OES analysis was conducted, and the results are presented in Table 1 to determine the Al atomic percentage across the active material. The Al atomic percentages for D-NCM811 and W-NCM811 were determined to be 0.77% and 0.78%, respectively, showing nearly identical values. These results are consistent with HR-TEM observations, which also revealed that the thickness and morphology of the coating layers for both samples were comparable.

XPS surface analysis was used to determine the Ni²⁺ ratios, as shown in Figures 4A–C. The Ni²⁺ ratios for bare NCM811, D-NCM811, and W-NCM811 were measured to be 19.3, 18.2, and 33.7%, respectively. The Ni²⁺ ratio in D-NCM811, prepared via the dry coating method, showed minimal change, whereas the ratio in W-NCM811 significantly increased following the wet coating method. This result demonstrates that wet processing conditions, even without coating (Wet-W/O coating), lead to a substantial increase in the NiO-like phase on the NCM811 surface, as shown in Figure 4D (Zhang, 2020). The electrochemically inactive NiO-like phase formed on the NCM811 surface acts as a resistive layer, hindering Li-ion diffusion (Ivanishchev et al., 2023). The formation of this resistive NiO-like phase likely caused the increased resistance components observed during the earlier cycle ability evaluation. This was further exacerbated by the resulting cation mixing (Cui et al., 2021). Figure 4E presents the Al 2p spectra. Analysis of NCM811 after both dry and wet coating processes confirmed the formation of LiAlO₂ in the dry coated sample (Liu et al., 2018). This formation is attributed to the additional reaction between Li residues distributed on the surface of NCM811 and the coated Al₂O₃. The observed result aligns well with the peak shifting identified in the XRD analysis. According to reported studies, LiAlO₂ is known to enhance electrochemical conductivity (Liu et al., 2018). These findings suggest that the dry coating process contributes to improving the electrochemical performance of Ni-rich cathode materials.

Electrochemical analyses were conducted to evaluate the effects of different coating methods. Figures 5A–C illustrates the galvanostatic charge-discharge voltage profiles for bare NCM811, D-NCM811, and W-NCM811 at a 0.1C for the 1st cycle and at a 0.5C from the 2nd to the 150th cycles. The initial charge and discharge capacities at a 0.1C were 229.0, 227.9, and 224.6 mAh g^-1 and 197.0, 197.4, and 194.6 mAh g^-1 for bare NCM811, D-NCM811, and W-NCM811, respectively. The corresponding initial Coulombic efficiencies were 86.0%, 86.6%, and 86.5%. These results suggest that both dry and wet coating methods mitigate initial electrolyte side reactions. Figure 5D shows the cycling performance at 0.5C from the 2nd to the 150th cycles. The retention rates for bare NCM811, D-NCM811, and W-NCM811 were 78.1%, 84.3%, and 77.4%, respectively, while the corresponding average Coulombic efficiencies were 98.9%, 99.3%, and 99.0%. Furthermore, as observed in the discharge voltage curves in Figures 5A–C, the overpotentials increased with the number of cycles in the order of bare NCM811, D-NCM811, and W-NCM811. These results confirm, consistent with previous findings, that D-NCM811 effectively suppressed both initial electrolyte side reactions and surface degradation during cycling. In contrast, W-NCM811 exhibited lower overpotential increases compared to bare NCM811. The instability of W-NCM811, caused by the formation of a NiO-like phase, leads to a degradation in cycle characteristics.

The rate performance evaluation was conducted by two sequences. In the first stage, as shown in Figure 6A, each C-rate from 0.1 to 20 C was tested for 3 cycles. In the second stage, after 150 cycles at a 0.5 C-rate, the rate performance was re-evaluated by testing each C-rate from 0.1 to 10 C for three cycles, as illustrated in Figure 6B. This method reflects the operation rate of LIBs in the application under repeating cycling. The first-stage rate performance evaluation showed that the performance decreased in the order of bare NCM811 > D-NCM811 > W-NCM811. This result indicates that, during the first-stage rate performance test, the interfacial resistance caused by the Al₂O₃ and LiAlO₂ coating layer was greater than the resistance caused by side reactions with the electrolyte. Therefore, bare NCM811 exhibited better rate characteristics compared to D-NCM811. For W-NCM811, the NiO-like phase formed during the coating process introduced additional resistance, leading to greater capacity loss compared to both bare NCM811 and D-NCM811. To evaluate the rate performance after substantial cell degradation, further assessments were conducted following the 150th cycle, as shown in Figure 6B. This stage was designed to investigate the impact of the thick SEI layer and cation mixing on battery performance. Unlike the first stage, D-NCM811 demonstrated the highest discharge capacity under high-rate conditions after prolonged cycling, particularly at 5C. This shows that the interfacial resistance initially increased due to the coating layer formed by Al₂O₃ and LiAlO₂ on the surface of NCM811. However, after 150 cycles, the stabilized CEI layer and the formation of Al₂O₃ and LiAlO₂ demonstrate higher performance. Furthermore, the discharge capacities of bare NCM811 and W-NCM811 were similar. These differences were most noticeable in Figure 6C. This suggests that their reduced discharge capacities under high-rate cycling from distinct degradation mechanisms: electrolyte side reactions dominated in bare NCM811, whereas NiO-like phase formation and cation mixing were predominant in W-NCM811 (Zhang, 2020). In contrast, D-NCM811 effectively suppressed reactions with the electrolyte, achieving higher discharge capacities even under high-rate conditions. These findings indicate that the dry coating method applied to D-NCM811 was more effective in mitigating degradation phenomena such as resistance increase from side reactions and cation mixing. These issues were prevalent in bare NCM811 and W-NCM811, respectively. As a result, D-NCM811 exhibited superior rate performance after prolonged cycling degradation.

Finally, EIS analysis was conducted on cells with D-NCM811 and W-NCM811 to investigate resistance components, focusing on surface resistance (R_film) and charge transfer resistance (R_ct) observed in Nyquist plots after charging the cells to 4.3 V versus Li/Li⁺ (Ivanishchev et al., 2023). Figure 7A presents the EIS results from the 1st cycle, where both the first semicircle resistance (R_film) and the second semicircle resistance (R_ct) for D-NCM811 were observed to be smaller than those for W-NCM811 (Liang et al., 2015). As shown in the results presented in Table 2, the R_film and R_ct values were 5.0 and 37.63 Ω for D-NCM811 and 7.8 and 45.86 Ω for W-NCM811. This result indicates that the higher R_film and R_ct values in W-NCM811 are directly correlated with its lower capacity retention, highlighting the importance of interfacial stability in achieving long-term performance (Strehle et al., 2021). The greater R_film and R_ct values in W-NCM811 after the first charge can be attributed to the NiO-like phase formed on its surface, which contributes significantly to interfacial resistance. Additionally, as shown in Figure 7B, EIS results after the 100th cycle revealed that both R_film and R_ct exhibited more pronounced differences compared to their initial values. According to the results listed in Table 2, the R_film and R_ct values were 8.7 and 130.06 Ω for D-NCM811 and 17.7 Ω and 155.49 for W-NCM811. In D-NCM811, the Al₂O₃ and LiAlO₂ coating layers effectively suppressed side reactions with the electrolyte, thereby preventing a significant increase in surface resistance over time. In contrast, W-NCM811 displayed a notable increase in resistance attributed to the pre-formed NiO-like phase on its surface as well as additional resistance from cation mixing during cycling. The lower R_ct observed in D-NCM811 suggests that the Al₂O₃ and LiAlO₂ coating promotes Li⁺ transport by providing a uniform ion-conductive pathway, whereas the NiO-like phase in W-NCM811 acts as a kinetic barrier, hindering Li⁺ transport (Wang et al., 2023). These observations emphasize the effectiveness of the Al₂O₃ coating in mitigating degradation in D-NCM811, while highlighting the detrimental effects of the NiO-like phase and cation mixing on the resistance and performance of W-NCM811 (Hu et al., 2021).

Ni-rich cathode materials exhibit high theoretical capacities but face challenges such as surface instability and significant capacity fading, primarily due to the high reactivity of Ni. This study investigates and compares the effects of dry and wet coating processes on the structural and electrochemical properties of these materials. The wet coating process uses deionized water, while the dry coating process involves high-speed mixing. XRD analysis reveals no distinct peaks for Al₂O₃ or LiAlO₂ in D-NCM811, suggesting that these phases are formed only in small quantities through a reaction between surface lithium and Al₂O₃ without significantly altering the structure. In contrast, no such reactions or changes are observed in NCM811 or W-NCM811. SEM and TEM analyses confirm that the particle morphology remains largely unchanged, while ICP analysis shows aluminum levels of 77% and 78% for dry and wet coatings, respectively. XPS analysis indicates that the Ni³⁺/Ni²⁺ ratio is lowest in dry coating, while wet coating results in an excessive presence of Ni²⁺ due to the formation of a NiO-like phase. Moreover, Al bonding analysis reveals signatures consistent with Al₂O₃ or LiAlO₂ after dry coating, corroborating the XRD results and confirming the limited formation of these phases. Electrochemical tests reveal that the initial discharge capacities at a 0.1C rate for NCM811, W-NCM811, and D-NCM811 are 197.0, 197.4, and 194.6 mAh g^-1, respectively, with retention rates of 78.1%, 84.3%, and 77.4% after 150 cycles at a 0.5C rate. While the first phase rate performance of D-NCM811 is initially lower than that of bare NCM811, its second phase performance after 150 cycles surpasses others, particularly at 0.5C, due to the Al₂O₃ and LiAlO₂ improving conductivity and providing a stable surface layer over time. In contrast, W-NCM811 displays lower performance caused by structural degradation and instability due to the formation of the NiO-like phase. The EIS results further support these findings, showing smaller interfacial resistance arcs for D-NCM811 compared to W-NCM811 across cycles. These results demonstrate that the dry coating process provides significant advantages over the wet coating process by enhancing the long-term stability and electrochemical performance of Ni-rich cathode materials. However, challenges such as achieving coating uniformity and scalability in large-scale manufacturing remain and should be addressed in future research. Nevertheless, this study underscores the critical importance of surface coating technologies in addressing persistent issues of Ni-rich cathodes and highlights the dry coating process as a promising approach for realizing high-performance and durable batteries.