Thermal Runaway in Sulfide‐Based All‐Solid‐State Batteries: Risk Landscape, Diagnostic Gaps, and Strategic Directions

Abstract All‐solid‐state batteries (ASSBs) with sulfide solid electrolytes (SSEs) are widely considered safer alternatives to conventional lithium‐ion batteries (LIBs), owing to their nonflammable nature and promise for high performance and scalable manufacturing. However, emerging evidence reveals that ASSBs remain susceptible to thermal runaway (TR), primarily driven by interfacial instabilities at the cathode–SSE interfaces, alongside contributions from the Li metal–SSE interface. In this review, TR phenomena is benchmarked in sulfide‐based ASSBs against those in LIBs, highlighting the roles of gas evolution, oxygen release, and exothermic interfacial reactions—particularly between SSEs and delithiated layered oxide cathodes. Based on heat release calculations, safety metrics are reframed, underscoring their unique advantage of ASSBs in tolerating internal short circuits rather than exhibiting lower heat generation. It is further assessed how experimental parameters—including delithiation methods, the differential scanning calorimetry configuration (open vs. closed systems), heating rate, and postmortem analysis protocols—influence the interpretation of thermal behavior. These findings emphasize the need for standardized diagnostic protocols to enable consistent and fair safety evaluations. Lastly, mitigation strategies are discussed, including thermally stable SSEs, composite electrode design, and flame‐retardant integration. It is concluded that thermal safety in ASSBs must be proactively engineered through coordinated advances in materials, interfaces, and system‐level validation.