In this study, we have prepared Li2S-LiI solid solutions by mechanical milling . The starting materials of crystalline Li2S and LiI were milled using a planetary ball mill apparatus. X-ray diffraction (XRD) for the (100-x)Li2S∙xLiI (mol%, 0 ≤ x ≤ 20) milled samples indicated that XRD peaks attributable to crystalline LiI disappeared and those attributable to Li2S shifted to the lower angle side with an increase in the LiI content. The lattice constant of Li2S increased with an increase in the LiI content, suggesting that solid solutions in the system Li2S-LiI were prepared by mechanical milling. The solid solution at the composition of x = 20 showed the highest Li+ ion conductivity of 2.2×10-6 S cm-1, which is two orders of magnitude higher than that of Li2S. An all-solid-state cell (Li-In/Li3PS4/Li2S-LiI) operated as a rechargeable battery at room temperature. The cell with the x=20 solid solution as an active material exhibited a stable reversible capacity of 930 mAh per gram of Li2S (80% utilization) for 50 cycles at 0.07C. Further conductivity enhancement was achieved in the ternary system Li2S-LiI-LiBr, and the highest conductivity was over 10-5 S cm-1at the composition with 40 mol% lithium halides. The concept of increasing ionic conductivity of sulfur-based active materials is effective in increasing their utilization and reversible capacity for Li/S cells.
This research was financially supported by ALCA-SPRING project.
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