Progress and Challenges in Buffer Layers Between Cathode Materials and Sulfide Solid Electrolytes in All‐Solid‐State Batteries

All‐solid‐state batteries (ASSBs), configured with solid electrolytes, have received considerable attention as a future energy solution across diverse sectors of modern society. Unlike conventional liquid electrolytes in particular, sulfide solid electrolytes have various advantages, such as high ionic conductivity (>10−3 S cm−1), good ductile properties, and thermal stability. Despite these advantages, the practical application of sulfide solid electrolytes in ASSBs is still limited due to their interfacial instability with commercial cathode materials. Unfortunately, the spontaneous formation of a space charge layer (SCL) at the interface between the cathode material and the solid electrolyte leads to heightened interfacial resistance, obstructing Li+ transport. To address this issue, proper interfacial engineering is still required to facilitate smooth Li+ migration across the interfaces. In this respect, various functional materials have been under exploration as buffer layers, which are intended to suppress the formation of the SCL at these interfaces. Herein, focus is given on the critical significance of these buffer layers between cathode materials and sulfide solid electrolytes in the development of ASSBs. Considering the present limitations, future research directions for next‐generation ASSBs are discussed. These insights are poised to offer valuable guidance for the strategic design and development of highly reliable ASSBs. The practical use of sulfide solid electrolytes is limited by interfacial instability with cathode materials, hindering Li+ movement. To resolve this issue, various buffer layers are being explored to facilitate Li+ migration and suppress side reactions at interfaces. The importance of the buffer layers in all solid‐state batteries is emphasized and future research directions to improve their performance are discussed.