Materials‐Property Classification of Interlayers for Stabilizing Lithium Metal Interfaces in Sulfide‐Based All‐Solid‐State Batteries

ABSTRACT Sulfide‐based all‐solid‐state lithium metal batteries (ASSLMBs) promise high energy density by using lithium metal anodes, but their practical implementation is limited by severe interfacial instability. At the lithium metal/sulfide electrolyte interface, coupled chemical, electrochemical, and mechanical degradation accelerates impedance growth and cell failure. Interlayer strategies are promising for stabilizing sulfide‐based all‐solid‐state batteries because they can mitigate interfacial degradation, improve contact stability, and regulate ion/electron transport at critical electrode–electrolyte interfaces. However, their mechanisms have largely been interpreted based on phenomenological observations and empirical performance improvements, rather than through intrinsic material property–mechanism relationships, highlighting the need for a material property‐driven design framework. Here, we categorize interlayer strategies into metal, polymer, ceramic, and composite systems and reinterpret their functions from an electro‐chemo‐mechanical perspective. We discuss how stiffness, ductility, fracture toughness, electronic conductivity, electrochemical stability, chemical stability, lithiophilicity, and Li‐ion transport govern interfacial behavior during assembly, plating, stripping, and long‐term cycling. By correlating representative interlayer systems with their electro‐chemo‐mechanical properties, this perspective clarifies the relationship between material characteristics and interfacial stabilization behavior in sulfide‐based ASSLMBs. This property‐driven framework is expected to provide practical criteria for screening unexplored interlayer materials and guiding the rational design of stable lithium metal/sulfide electrolyte interfaces.