Modification of Cathode Surface for Sulfide Electrolyte‐Based All‐Solid‐State Batteries Using Sulfurized LiNbO ₃ Coating

All‐solid‐state batteries (ASSBs) with sulfide electrolytes are promising for next‐generation battery systems owing to their superior safety and favorable electrochemical properties. However, interfacial instability between the oxide cathode and sulfide electrolyte induces undesirable side reactions, degrading cell performance. This study develops a sulfurized LiNbO ₃ coating to stabilize this interface. While conventional LiNbO ₃ coatings reduce interfacial side reactions, their limited compatibility with sulfide electrolytes, due to Li‐ion chemical potential differences, hinders ion transport. The sulfurized LiNbO ₃ coating improves compatibility, acting as a buffer that reduces the Li‐ion potential gradient and enhances interfacial conductivity. The coating effectively suppresses side reactions, lowering cathode degradation and interfacial resistance. A simple and cost‐effective sulfur treatment process is used, where sulfur sublimation at 300 °C forms a sulfurized outer layer on the coating. Electrochemical evaluations of the coating reveal significant capacity, rate capability, and cyclic performance improvements over conventional LiNbO ₃ coatings. These findings underscore sulfur treatment as an effective method for stabilizing interfaces and enabling smooth Li‐ion transport, highlighting the advantages of the sulfurized LiNbO ₃ coating method. Overall, sulfurized LiNbO ₃ coatings offer scalable solutions to interfacial challenges in sulfide‐based ASSBs, thereby promoting improved performance and commercialization of solid‐state battery systems.