WO3-Mediated Conversion of Surface Residual Lithium into a Li2WO4 Protective Layer for Enhanced Air Stability and Electrochemical Performance of Single-Crystal High-Nickel NCM Cathodes

High-nickel single-crystal NCM cathodes are essential for high-energy-density power batteries in the electric vehicles (EVs), but they suffer from severe surface residual lithium compounds (RLCs) that degrade electrochemical performance. Conventional modification methods can only remove RLCs once and fail to prevent their re-formation upon air exposure. Here, we report a WO3 treatment strategy that simultaneously eliminates surface LCs and constructs an in situ Li2WO4 protective layer on single-crystal NCM613 and NCM8866 cathodes. An optimal WO3 addition of 0.5 wt % achieves the best balance between RLCs removal and electrochemical performance restoration. The Li2WO4 layer acts as a fast-ion conductor to facilitate Li+ transport and as a chemically stable barrier to block moisture and CO2, thereby suppressing RLCs regeneration during air storage. After 6 days of humid air exposure, the total RLCs content of the WO3-treated 8-NCM increases by only 18%, compared with 57.6% for the pristine material. The optimized [email protected] delivers an initial discharge capacity of 207 mAh g-1 (vs 137 mAh g-1 for pristine) with significantly improved cycling stability and rate capability. Galvanostatic intermittent titration technique (GITT) confirms enhanced Li+ diffusion kinetics. Moreover, the Li2WO4 layer passivates the cathode surface, mitigating electrolyte side reactions and microcrack formation during long-term cycling. This work provides a simple, scalable, and effective surface modification approach to overcome the critical bottleneck of RLCs re-formation, which is particularly relevant for long-life, high-safety power batteries in EVs applications.