Interlayer‐Driven Interfacial Stabilization in Solid Electrolytes for Lithium Batteries: Promises and Challenges

Despite major progress in developing solid electrolytes (SEs) with high ionic conductivity, the performance of all‐solid‐state Li‐metal batteries (ASSLBs) remains dominated by interfacial impedance that develops at the electrode‐electrolyte interface. Sulfide and halide SEs have emerged as leading candidates for high‐energy density ASSLBs owing to their exceptional ionic conductivities, low grain‐boundary resistance, and favorable mechanical deformability. However, their practical implementation is still constrained by severe interfacial instabilities with both Li‐metal anodes and high‐voltage layered oxide cathodes. Interlayer engineering, specifically the incorporation of a functional interlayer between the electrode and SE, has become one of the most effective strategies to mitigate these challenges, enabling suppression of electrolyte decomposition, reduction of space‐charge effects, homogenization of Li flux, and stabilization of interphases under high current densities. In this review, we aim to recapitulate the recent developments made in the interlayer‐engineering approaches that span over a range of sulfide‐ and halide‐based SE systems, which play a central role as fast Li + ‐conducting media in enabling high‐energy‐density ASSLB architectures, and distill unified design principles that connect electrochemical stability, ion‐transport behavior, and mechanical compliance. Finally, we discuss future directions and research opportunities that define key priorities for scalable interlayer engineering, aimed at accelerating the development of next‐generation high‐performance ASSLBs.