Degradation mechanisms of lithium sulfide (Li2S) composite cathode in carbonate electrolyte and improvement by increasing electrolyte concentration

Abstract

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–S_n–R) (not lithium polysulfides (Li₂S_n)) 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.