The Li2S–Cu composite electrode materials were prepared by mechanical milling and applied to all-solid-state lithium cells using the Li2S–P2S5 glass–ceramic electrolyte. The addition of Cu and the mechanical activation improved the electrochemical performance of Li2S in all-solid-state cells. The In/Li2S–Cu cells were charged and then discharged at room temperature, suggesting that Li2S was utilized as a lithium source. The cells exhibited high discharge capacity of about 490mAhg−1 at the 1st cycle. The SEM and EDX analyses suggested that the amorphous Li x CuS domain was partially formed by milling, and the domain played an important role in the improvement of capacity. The electrochemical reaction mechanism of the Li2S–Cu composites was discussed on the basis of the mechanism of the S–Cu composite electrode.