Comparison of Sulfur Cathode Reactions between a Concentrated Liquid Electrolyte System and a Solid-State Electrolyte System by Soft X-Ray Absorption Spectroscopy

Sulfur is one of the promising next-generation cathode materials because of its low cost and high theoretical gravimetric capacity. However, the reaction mechanism of the sulfur cathode is largely influenced by the electrolyte and the intermediate sulfur species during the first discharge process has not been quantitatively explored in different electrolytes. In this study, we elucidated the reaction mechanism of sulfide cathodes by using three different electrolyte systems, viz., a conventional liquid electrolyte [LiPF6/ethylene carbonate (EC)/ethylene-methyl carbonate (EMC)], a concentrated liquid electrolyte [lithium bis(trifluorosulfonyl)amide (LiTFSA)/tetraglyme (G4):1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE)], and a solid-state electrolyte (Li3PS4). Soft X-ray absorption spectroscopy was used to examine the reaction mechanism of the sulfur cathode in the liquid and solid-state electrolytes during the first discharge process. In the conventional electrolyte, the sulfur cathode was reduced to long-chain polysulfide (S62–) during the first discharge process, and the polysulfide subsequently dissolved into the electrolyte. In the concentrated electrolyte, the sulfur cathode was reduced to midchain polysulfide (S42–) at the initial stage of the first discharge process and then reduced to short-chain polysulfide (S22–) and Li2S, followed by the formation of long-chain polysulfide (S62–). In the solid-state electrolyte, the sulfur cathode was reduced to long-chain polysulfide (S62–) at the initial stage of the first discharge process and was gradually reduced to mid-chain polysulfide (S42–), short-chain polysulfide (S22–), and Li2S. The differences in these reaction pathways govern electrochemical properties such as the difference in discharge voltage.