NCM811–Sulfide Electrolyte Interfacial Degradation Mechanisms and Regulation Strategies in All‐Solid‐State Lithium Battery

(NCM811) cathode and sulfide solid-state electrolyte (SSE) represents a transformative technology, offering enhanced safety and high energy density through the complete elimination of flammable liquid electrolyte and enabling the lithium metal anode. However, its commercialization is fundamentally limited by complex instabilities at the NCM811/sulfide SSE interface, which trigger coupled mechanical, chemical, and electrochemical degradation. The solid/solid interface creates complex dynamic feedback loops: mechanical stress from anisotropic volume changes accelerates interfacial chemical reactions; chemical degradation progressively alters electrochemical behavior; and continuous electrochemical cycling induces further mechanical instability. This multiscale coupling manifests as progressive contact loss, microcracks, detrimental space charge layer, and impedance growth, which collectively compromise performance under demanding conditions. This review establishes a coherent mechanistic framework to understand these highly interdependent degradation pathways, and systematically evaluates various stabilization strategies, including targeted surface modification, strategic bulk engineering, and innovative synergistic design approaches that specifically address the inherently coupled interface instability. Despite progress, intrinsic material incompatibilities persist, necessitating breakthroughs in materials design, interface engineering, characterization, and manufacturing. This work provides fundamental mechanistic insights into solid-state electrochemistry and practical guidance for developing commercially viable ASSLB.