Flammability of Sulfide Solid-State Electrolytes β-Li₃PS₄ and Li₆PS₅Cl— Volatilization and Autoignition of Sulfur Vapor—New Insight into All-Solid-State Battery Thermal Runaway

In this study we show that sulfide solid-state electrolytes, β-Li₃PS₄ (LPS) and Li₆PS₅Cl (LPSCl), are flammable solids. Both solid-state electrolytes release sulfur vapor in a dry, oxidizing environment at elevated temperature < 300 °C. Sulfur vapor is a highly flammable gas, which then auto-ignites to produce a flame. This behavior was related to all-solid-state battery (ASSB) thermal runaway (TR) by conducting several thermal experiments at a heating rate of 40 °C min^-1. First, 100% state-of-charge high-Ni NCM cathode composites composed of either LPS or LPSCl underwent TR at 217.2 °C and 221.7 °C, respectively. Second, 100% state-of-charge high-Ni NCM evolved oxygen at 197.7 °C. Third, LPS and LPSCl evolved sulfur in pure oxygen at only 220.7 and 203.3 °C, respectively. These data suggest that solid-solid and solid-gas reaction mechanisms likely initiate ASSB TR and may generate sulfur vapor in addition to SO₂. The subsequent gas-gas reaction of sulfur vapor with additional oxygen may then sustain and aggravate ASSB TR. To improve all-solid-state battery safety, current work focuses on eliminating the oxygen source by changing the cathode active material. The conclusion of this study suggests that ASSB safety can also be realized by the development of solid-state electrolytes with less susceptibility to sulfur volatilization.