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.