Stage-Dominated Thermal Runaway in Sulfide ASSBs: Decoupled Electrochemical Ignition and Chemical Cascades

Sulfide all-solid-state batteries (ASSBs) are poised to revolutionize energy storage with unparalleled energy densities and reduced flammability risks, yet emerging studies reveal a critical safety paradox where cathode-electrolyte interactions induce thermal runaway at unexpectedly low temperatures, challenging the assumption of inherent thermal stability. While current research prioritizes electrochemical performance, the thermochemical degradation dynamics between oxide cathode and thiophosphate electrolyte remain poorly understood, with even conflicting mechanistic interpretations. Herein, leveraging multiscale calorimetry and in-situ gaseous analytical techniques, it is noted that the metastable interphase between nickel-rich cathodes and thiophosphate electrolytes, formed through electrochemically preconditioned but persistently overlooked, serve as primary triggers for exothermic cascades, a phenomenon starkly distinct from liquid electrolyte counterparts. Thermal degradation in composite cathodes evolves through dual mechanistic phases: First, delithiated cathode materials react with electrochemical precondition generated sulfur-rich species (-S-S-, -P-S-P-, and Li ₃ PS ₄ ), driving rapid heat accumulation below 160 ℃ by interphase-dominated chemistry, accompanied by gaseous emissions (SO ₂ , CO ₂ , O ₂ ); subsequently, sulfur-oxygen interdiffusion in bulk phases accelerates solid-solid exothermic reactions, driving thermal propagation and eventual runaway. This dual-stage mechanism generalizes across other sulfide systems. Crucially, it is demonstrated that the electrochemically formed interface through Ge-S bond engineering effectively suppresses thermal cascades, where the as-designed Li ₄ GeS ₄ -modified interface system achieves unprecedented thermal safety without compromising electrochemical performance. Our study establishes a new paradigm for thermal runaway causality by prioritizing interfacial thermodynamics over bulk material compatibility as the primary determinant of thermal safety—a framework fundamentally divergent from conventional liquid electrolyte batteries.