Concerted Influence of H₂O and CO₂: Moisture Exposure of Sulfide Solid Electrolyte Li₄SnS₄

Although moisture-induced deterioration mechanisms in sulfide solid electrolytes to enhance atmospheric stability have been investigated, the additional impact of CO₂ exposure remains unclear. This study investigated the generation of H₂S from Li₄SnS₄ under H₂O and CO₂ exposure. Li₄SnS₄ was exposed to Ar gas at a dew point of 0 °C with and without 500 ppm of CO₂, and its ion conductive properties were evaluated. Although the lithium-ion conductivity of Li₄SnS₄ decreased regardless of the presence of CO₂, the amount of H₂S generated with CO₂ was five times higher. To elucidate the underlying mechanism, X-ray diffraction and Raman spectroscopy were used. Without CO₂, hydrate Li₄SnS₄·4H₂O formation markedly increased, whereas, with CO₂, it increased a little. The difference revealed distinct deterioration mechanisms leading to a decrease in lithium-ion conductivity: without CO₂, adsorbed H₂O and Li₄SnS₄·4H₂O contributed to the decrease, while with CO₂, a weak acid dissociation reaction could reduce the thermodynamic stability of the moisture-exposed Li₄SnS₄ surface including Li₄SnS₄·4H₂O and adsorbed H₂O, promoting H₂S release and carbonate formation. This was supported by the recovery of lithium-ion conductivity after vacuum heating. The concerted influence of H₂O and CO₂ provides valuable insights into the fundamental deterioration mechanisms in sulfide solid electrolytes that could be applied in battery manufacturing processes.