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 Li 2 S. In this study, for improving the utilization and understanding the conversion reaction of Li 2 S/S in the all‐solid‐state cells, Li 2 S‐based solid solutions composed of Li 2 S 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 80Li 2 S·20LiI (mol%) reaches more than 1100 mAh g −1 at 0.5 C, corresponding to 95% of theoretical capacity. The reversible full utilization of Li 2 S is first demonstrated in this work. The significantly improved utilization of Li 2 S is originated by the increase of electrochemical reaction sites rather than the ionic conductivities of Li 2 S‐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 Li 2 S cells with not only solid electrolytes but also organic liquid electrolytes.