Crystallinity Tailoring of TiNb ₂ O ₇ Coating Enables Enhanced Interfacial Stability and Electrochemical Performance of NCM811 Cathode for Sulfide‐Based All‐Solid‐State Lithium Batteries

ABSTRACT Elemental interdiffusion and interfacial side reactions between sulfide solid‐state electrolytes (SSEs) and nickel‐rich layered oxide cathodes inevitably form high‐impedance, ion‐insulating phases at cathode/electrolyte (C/E) interface, which severely degrades the electrochemical performance of all‐solid‐state lithium batteries (ASSLBs). Herein, we propose a crystallinity‐tailored TiNb 2 O 7 (TNO) coating strategy for LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) cathodes and the long‐term interfacial stability of NCM811/SSE interfaces. The ion‐conductive yet electron‐insulating TNO coating not only guarantees rapid Li + migration across the C/E interface but also effectively suppresses the oxidative decomposition of SSEs at high voltage. More importantly, compared with the high‐crystallinity TNO coating, the low‐crystallinity counterpart exhibits superior structural adaptability, which can well accommodate the volume changes of NCM811 during Li + insertion/extraction. This unique structural advantage endows the TNO coating with durable protective capability for NCM811 cathodes, especially under high‐current‐density cycling conditions. Benefiting from the above synergistic effects, the NCM811 cathode modified with low‐crystallinity TNO coating synthesized at 500°C (NCM811@T500) delivers a high specific discharge capacity of 150.9 mAh g − 1 after 100 cycles at 0.069 mA cm − 2 and 25°C. This work provides a novel crystallinity‐regulation strategy for cathode surface modification, and offers valuable insights into the rational design of high‐performance protective coatings for nickel‐rich cathodes in sulfide‐based ASSLBs.