Critical Factors Contributing to the Thermal Runaway of Thiophosphate Solid Electrolytes for All‐Solid‐State Batteries

Although all‐solid‐state batteries are suggested as a means to tackle the safety concerns associated with current Li‐ion batteries, there is presently a lack of comprehensive understanding regarding their thermal safety. In this context, critical factors contributing to the thermal runaway of thiophosphate solid electrolytes with charged Li1‐xNi0.8Co0.1Mn0.1O2 (NCM) under thermal and mechanical abuse conditions are demonstrated, considering parameters such as heating rate under thermal abuse conditions and the hybridization of S atom in structure. In particular, the thermal behavior of various solid electrolytes, including thiophosphates, thioantimonates, and halides, is investigated to clarify critical elements in Li6PS5Cl (LPSCl) contributing to its thermal instability when combined with charged NCM. Various ex situ analyses, along with density functional theory calculations, reveal a correlation between the hybridization of S atoms and the thermal instability of solid electrolytes, suggesting that sulfur acts as a key element triggering the thermal runaway of sulfide‐based solid electrolytes. The safety of thiophosphate‐based ASSBs remains uncertain, as a few recent literatures have reported the thermal runaway behavior of sulfide‐based ASSBs under abusive conditions. This work elucidates the factors influencing the thermal runaway of thiophosphate solid electrolytes, with a specific focus on clarifying the role of heating rates under thermal abuse and identifying critical elements in Li6PS5Cl that trigger ignition.