Electrochemical redox of Li2S–CaS and –CaX2 (X = Cl, Br, and I) cathode materials for all-solid-state lithium-sulfur batteries

In this study, we have evaluated the cell performances of Li2S–CaS and –CaX2 (X = Cl, Br, and I) cathode materials prepared by high-energy ball milling in all-solid-state lithium-sulfur batteries. The cell using 85Li2S·15CaS (mol%) cathode exhibited the highest capacity of 819 mAh g−1 after 15 cycles among the prepared (100−x)Li2S–xCaS cathodes (x = 0, 5, 10, and 15). The electrochemical tests showed that the electrochemical redox reactions of the Li2S–CaS cathodes involved a large contribution of the degradation products from solid electrolytes. Additionally, the cells using the Li2S-CaX2 (X = Cl, Br, and I) showed a higher utilization than Li2S cathode. Cyclic voltammetry at different scan rates demonstrated that the addition of CaX2 (X = Cl, Br, and I) increased the Li-ion diffusion coefficient of cathode materials, which was the highest value for the 90Li2S·10CaI2 cathode material among the fabricated cathodes. In the electrochemical redox of the 90Li2S·10CaI2 cathode, an additional oxidation reaction appeared at a slightly higher reaction voltage than the oxidation voltage of Li2S. These electrochemical properties allow to excellent capacity, performance stability, and reversibility of the all-solid-state lithium-sulfur batteries using the 90Li2S·10CaI2 cathode material.