Highly Conductive Li‐Excess Oxide to Facilitate Durable Interfaces in All‐Solid‐State Batteries

All-solid-state batteries (ASSBs) have garnered significant attention as advanced energy storage systems due to their superior energy density and intrinsic safety, enabled by the incorporation of solid electrolytes instead of conventional liquid electrolytes in lithium-ion batteries (LIBs). However, the commercialization of ASSBs faces significant challenges, particularly due to the interfacial side reactions between Ni-rich cathode materials and sulfide solid electrolytes, which degrade reversibility and cycling stability. To address this limitation, a surface coating strategy is developed for Ni-rich cathode materials. Through the structural engineering of LiAl₅O₈ via optimized lithium concentrations and elemental substitution, its ionic conductivity is significantly enhanced while maintaining a wide electrochemical stability window. The tailored Li₁₊₃ _xAl_5- _x _- _yB_yO₈ is evaluated as a promising surface layer for both LIBs and ASSBs. In practice, ASSBs incorporating Li₁₊₃ _xAl_5- _x _- _yB_yO₈-coated LiNi_0.87Co_0.10Mn_0.03O₂ cathodes into Li₆PS₅Cl solid electrolytes demonstrated enhanced reversibility and stable cycling performance due to improved interfacial stability and the suppression of side reactions. These findings provide a practical pathway for developing high-performance ASSBs for use in electric vehicles.