Thermal Runaway Behavior of Li6PS5Cl Solid Electrolytes for LiNi0.8Co0.1Mn0.1O2 and LiFePO4 in All-Solid-State Batteries

All-solid-state batteries (ASSBs) have received much attention because of their high energy density and safety. However, the safety of argyrodite-type Li6PS5Cl (LPSCl)-based ASSBs is still not assured because their thermal stability has been assessed under selected mild conditions. Herein, we introduce the poor thermal stability of LPSCl with Ni-rich layered oxide cathode materials as the trigger of thermal runaway. The charged composite cathode pellets containing Li1–xNi0.8Co0.1Mn0.1O2 and LPSCl are explosively burned at 150 °C even in Ar. Moreover, the mechanical abuse gives rise to violent burning at room temperature. This is due to vigorous exothermic chemical reactions between delithiated Li1–xNi0.8Co0.1Mn0.1O2 and LPSCl. However, LPSCl with LiFePO4 exhibits excellent thermal stability, such as no violent exothermic reactions even at 350 °C. This is because LPSCl is metastable with delithiated Li1–xFePO4. Moreover, LiFePO4 shows excellent electrochemical performance, such as a high reversible capacity of 141 mAh g–1 and stable capacity retention over 1000 cycles, despite the fact that LiFePO4 is known to be poorly electrochemically active for ASSBs. These findings provide fundamental insights to improve the thermal stability and electrochemical performance of LPSCl-based ASSBs.