Li2S‐Based Solid Solutions as Positive Electrodes with Full Utilization and Superlong Cycle Life in All‐Solid‐State Li/S Batteries

All‐solid‐state Li/S batteries have received a lot of attention in the view point of high safety and long cycle life. However, the all‐solid‐state Li/S cells have suffered from the low utilization of Li2S. In this study, for improving the utilization and understanding the conversion reaction of Li2S/S in the all‐solid‐state cells, Li2S‐based solid solutions composed of Li2S and one of lithium halides of LiCl, LiBr, and LiI are applied as active materials to the all‐solid‐state cells. Capacity of the cells with 80Li2S·20LiI (mol%) reaches more than 1100 mAh g−1 at 0.5 C, corresponding to 95% of theoretical capacity. The reversible full utilization of Li2S is first demonstrated in this work. The significantly improved utilization of Li2S is originated by the increase of electrochemical reaction sites rather than the ionic conductivities of Li2S‐based solid solutions. Furthermore, the cell retains the capacity of 980 mAh g−1 at 2 C for 2000 cycles without capacity degradation. These are the best cycling performances in the reported Li2S cells with not only solid electrolytes but also organic liquid electrolytes.