All-solid-state batteries with Li2O-Li2S-P2S5 glass electrolytes synthesized by two-step mechanical milling

Sulfide solid electrolytes, which show high ion conductivity, are anticipated for use as electrolyte materials for all-solid-state batteries. One drawback of sulfide solid electrolytes is their low chemical stability in air. They are hydrolyzed by moisture and generate H2S gas. Substituting oxygen atoms for sulfur atoms in sulfide solid electrolytes is effective for suppression of H2S gas generation in air. Especially, the xLi2O·(75-x)Li2S·25P2S5 (mol%) glasses hardly generated H2S gas in air. However, substituting oxygen atoms for sulfur atoms caused a decrease in conductivity. The x = 7 glass showed high chemical stability in air and maintained high conductivity of 2.5 × 10−4 S cm−1 at room temperature. Performance of cells using the 7Li2O·68Li2S·25P2S5 and the 75Li2S·25P2S5 glasses as solid electrolytes were compared. All-solid-state C/LiCoO2 cell using the 7Li2O·68Li2S·25P2S5 glass produced performance as good as that obtained using the 75Li2S·25P2S5 glass. Capacity retention and change of interfacial resistance of the former cell were superior to those of the latter cell after storage at 4.0 V and 60 °C. The diffusion of oxygen element into the 7Li2O·68Li2S·25P2S5 glass was less than that into the 75Li2S·25P2S5 glass after storage at the voltage of 4.0 V at 60 °C. Improvement of the stability of sulfide solid electrolytes to moisture was related to cell performance as well as an increase in conductivity.