Concerted Influence of H 2 O and CO 2 : Moisture Exposure of Sulfide Solid Electrolyte Li 4 SnS 4 .

Although moisture-induced deterioration mechanisms in sulfide solid electrolytes to enhance atmospheric stability have been investigated, the additional impact of CO 2 exposure remains unclear. This study investigated the generation of H 2 S from Li 4 SnS 4 under H 2 O and CO 2 exposure. Li 4 SnS 4 was exposed to Ar gas at a dew point of 0 °C with and without 500 ppm of CO 2 , and its ion conductive properties were evaluated. Although the lithium-ion conductivity of Li 4 SnS 4 decreased regardless of the presence of CO 2 , the amount of H 2 S generated with CO 2 was five times higher. To elucidate the underlying mechanism, X-ray diffraction and Raman spectroscopy were used. Without CO 2 , hydrate Li 4 SnS 4 ·4H 2 O formation markedly increased, whereas, with CO 2 , it increased a little. The difference revealed distinct deterioration mechanisms leading to a decrease in lithium-ion conductivity: without CO 2 , adsorbed H 2 O and Li 4 SnS 4 ·4H 2 O contributed to the decrease, while with CO 2 , a weak acid dissociation reaction could reduce the thermodynamic stability of the moisture-exposed Li 4 SnS 4 surface including Li 4 SnS 4 ·4H 2 O and adsorbed H 2 O, promoting H 2 S release and carbonate formation. This was supported by the recovery of lithium-ion conductivity after vacuum heating. The concerted influence of H 2 O and CO 2 provides valuable insights into the fundamental deterioration mechanisms in sulfide solid electrolytes that could be applied in battery manufacturing processes.