To improve the utilization of Li2S active material, an essential increase of Li+ ion conductivity for Li2S itself was investigated by modifying the crystal structure of Li2S. Li2S-LiI solid solutions were prepared by mechanical milling and their ionic conductivities were remarkably increased. All-solid-state cells with Li2S-LiI composite electrodes exhibited the highest utilization of Li2S with high reversibility and cyclability in all-solid-state cells with Li2S reported so far.