A Li2S cathode has a high theoretical capacity (1166 mA h g−1); however, it is inherently an insulator. To remedy this defect, composite sulfides can be prepared with transition-metal sulfides having better conductivity than Li2S. Among them, a Li8FeS5(Li2S/FeS = 4/1, mol/mol) cathode theoretically delivers 888 mA h g−1. This cathode, however, exhibits poor cycling performance in 1.0 M LiPF6/ethylene carbonate (EC)-dimethyl carbonate (DMC) (EC/DMC = 1/1, v/v) electrolyte; meanwhile, concentrated (3.5 M) electrolyte improves the performance. Herein, we elucidate the degradation mechanisms through analyses of cycled electrolytes and cathode surfaces. We identify organic polysulfides (R-Sn-R) (not lithium polysulfides (Li2Sn)) after charging and organic sulfides (R-S-R) after discharging in the 1.0 M electrolyte. Meanwhile, neither are detected in the 3.5 M electrolyte; instead, salt decomposition products (mainly LiF) are detected on the cathode surface. Therefore, electrolyte modifications can further improve the cyclability of sulfur-based secondary batteries including lithium-sulfur battery.
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