Hidden Trigger for H₂S Generation by CO₂–Induced Breakdown of Hydrated Sulfide Solid Electrolyte

One of the primary challenges with sulfide solid electrolytes is the generation of toxic H2S gas from moisture-induced hydrolysis. Recently, the influence of atmospheric gas species on the practical fabrication of these electrolytes has been extensively discussed. This study investigates H2S generation from Li4SnS4, a sulfide solid electrolyte known for its high moisture durability, under CO2 exposure. Li4SnS4 was first exposed to humidified Ar gas (dew point 0 °C, ∼6000 ppm of H2O), then to dry CO2 gas (dew point <−70 °C, <3 ppm of H2O). Interestingly, despite the negligible moisture content in dry CO2, significant amounts of H2S were released upon exposure to CO2 after moisture exposure. Furthermore, the amount of H2S generated was proportional to the CO2 concentration. To elucidate the chemical state of the solid electrolyte after the exposure sequence, X-ray diffraction and X-ray photoelectron spectroscopy were performed. These analyses revealed that CO2 induces H2S generation by destabilizing the thermodynamically stable hydrated crystal structure Li4SnS4·4H2O, leading to its structural disruption. This finding suggests that CO2 is an important factor affecting the stability of sulfide solid electrolytes that are stabilized by hydration.