Thermal Stability of Sulfide Solid Electrolyte with Lithium Metal

Abstract All solid–state battery (ASSB) is widely recognized as one of the most promising high‐energy‐density systems/technologies. However, thermal safety issues induced by highly reactive materials still exist for solid electrolytes (SEs). Insights on thermal behaviors at elevated temperatures and the underlying mechanism for thermal stability of SE‐based systems are still missing. Herein, thermal stability performance of typical sulfide SEs is systematically investigated with metal Li, whose order of interfacial thermal stability is concluded to be Li 6 PS 5 Cl > Li 3 PS 4 > Li 9.54 Si 1.74 P 1.44 S 11.7 Cl 0.3 > Li 4 SnS 4 > Li 7 P 3 S 11 after a comprehensive evaluation. Interestingly, Li 4 SnS 4 , which achieves good air stability, has poor thermal stability with Li metal. This is possibly caused by Li─Sn alloy products generated during thermal decomposition, and their great thermodynamic driving force towards SE for accelerated thermal runaway. Moreover, electrolytes with poor material‐level thermal stability (e.g., Li 7 P 3 S 11 ) may form a dense passivation layer by self‐decomposition with Li metal to retard thermal runaway. Conclusively, the material structure affects the thermodynamic stability of the system, but the reaction products (interphase) affects the kinetic process of the thermal reaction within a certain temperature range. Therefore, thermal stability with both metallic lithium and decomposition products is a necessary condition for interfacial thermal stability of sulfide SEs.