Deterioration process of argyrodite solid electrolytes during exposure to humidity-controlled air

Sulfide-based solid electrolytes (SEs) are among the most promising candidates for realizing all-solid-state lithium-ion batteries due to their high ionic conductivity. However, their low atmospheric stability is a major drawback. When exposed to air, their conductivity precipitously decreases, along with a toxic H2S gas generation, mainly from the reaction between the SEs and water molecules in the air. Understanding the sulfide-based SEs deterioration mechanism under air exposure is necessary to develop them with high air stability. This study investigated the degradation process of sulfide-based SEs primarily by transmission electron microscope (TEM) observation. As a promising SE for all-solid-state sulfide-based batteries, we focused on the argyrodite exhibiting high ionic conductivity. Besides the reaction with H2O and CO2 in air, TEM observations revealed that argyrodite decomposed with decreased crystallinity, followed by the decomposed components generation. The deterioration process and the main factor of the decreased ionic conductivity are discussed based on the reaction products caused by air exposure.