Hard-Soft Gradient-Engineered Oxychloride Coating on Ni-Rich Cathodes for All-Solid-State Lithium Batteries

All-solid-state lithium batteries (ASSLBs) employing Ni-rich layered oxide cathodes (NRLOs) and sulfide solid-state electrolytes (SSEs) hold great promise for high energy density and enhanced safety, yet they suffer from severe interfacial instability. This study precisely constructs a composite gradient coating on the surface of NRLOs by using atomic layer deposition. The oxygen-rich inner layer, which is induced by the reaction between TiCl₄ and predeposited Li₂O/LiOH, functions as an electrochemically inert barrier that enhances structural integrity and suppresses transition-metal dissolution, whereas the chlorine-rich outer layer acts as a compliant interfacial region with favorable compatibility toward sulfide SSE, which accommodates volume changes during cycling and mitigates mechanical stress at the cathode/sulfide SSE interface. Additionally, the well-structured LiCl nanocrystals formed within the coating layer improve both the cycling stability and Li⁺ transport kinetics. The optimized NCM95 cathode demonstrates exceptional cycling stability (96.9% capacity retention after 200 cycles) and rate capability (103 mAh g^-1 at 2 C) at room temperature. This work underscores the importance of designing multifunctional coatings and provides a scalable approach to stabilizing the cathode/electrolyte interface for high-energy ASSLBs.