Charge–discharge performance of all-solid-state Li/S batteries using several solid electrolytes to enhance energy density is investigated at 25 °C. The sulfur content in the positive composite electrode is 50 wt%. A correlation between the P/S ratio in a solid electrolyte and the reactivity of sulfur is observed. The capacity of a positive composite electrode using a Li1.5PS3.3{60Li2S–40P2S5(mol%)} electrolyte is 1096 mAh g−1 under 6.4 mA cm−2 at 25 °C. This value is much higher than the capacity for a Li4.0PS4.5{80Li2S–20P2S5} electrolyte (565 mAh g−1), although the ionic conductivity of Li1.5PS3.3 was lower than Li4.0PS4.5 (2 × 10−5 S cm−1, 5 × 10−4 S cm−1, respectively).