Ionic conductivity and thermal stability of Li2O–Li2S–P2S5 oxysulfide glass

Abstract

Sulfide-based solid electrolytes exhibit high ionic conductivities of more than 10−4 S cm−1 at room temperature. However, sulfide electrolytes have a major disadvantage because they are hygroscopic. To reduce costs and develop practical all-solid-state batteries, it is imperative to enhance their chemical stability in air. In this study, Li2O is added to 75Li2S·25P2S5 glasses to improve their chemical stability to moisture in air, and (75 − x)Li2S·25P2S5·xLi2O samples are synthesized. The effects of the addition of Li2O on the electrical properties and thermal behavior of 75Li2S·25P2S5 glasses are examined. With an increase in the Li2O content, the crystallization temperature of 75Li2S·25P2S5 glasses shifts to the high-temperature side, and the glass phase stabilizes over a wide temperature range. Specifically, the substitution of Li2O with Li2S enhances the thermal stability of glasses. This is attributed to the change in the precipitated crystalline phase, β-Li3PS4 (x = 0, 5) → thio-LISICON III Li3.2P0.96S4 (x = 10, 15). In addition, the (75 − x)Li2S·25P2S5·xLi2O glass ceramics for x = 15 exhibit higher conductivity than pristine glasses because of the thio-LISICON III Li3.2P0.96S4 crystalline phase. Thus, the addition of more than 10 mol% of Li2O ensures a good balance between thermal stability, atmospheric stability, and conductivity.