The Li
2O-Li
2S-P
2S
5 glasses, which were prepared by a reaction of the Li
2S-P
2S
5 glass and Li
2O, hardly generated H
2S gas in air. These glasses showed relatively high ion conductivities about 10
−4 S cm
−1 at room temperature. However, these conductivities were lower than those of original sulfide glasses. In this study, an addition of LiI to the Li
2O-Li
2S-P
2S
5 glasses was examined in order to improve ion conductivities. The 30LiI·70(0.07Li
2O·0.68Li
2S·0.25P
2S
5) glass showed high ion conductivity of 1.3 × 10
−3 S cm
−1 at room temperature, and hardly generated H
2S gas in air. On the other hand, the 30LiI·70(0.07Li
2O·0.68Li
2S·0.25P
2S
5) glass was electrochemically stable in the potential range of 0–10 V. Moreover, the all-solid-state C/LiCoO
2 cell using the 30LiI·70(0.07Li
2O·0.68Li
2S·0.25P
2S
5) glass as solid electrolytes worked as lithium secondary batteries at room temperature, and exhibited a good cycle performance. Therefore, the 30LiI·70(0.07Li
2O·0.68Li
2S·0.25P
2S
5) glass was a suitable electrolyte for all-solid-state cells because the glass had high conductivity, wide electrochemical window and high chemical stability to moisture.
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