In situ raman spectroscopy of sulfur speciation in lithium-sulfur batteries

In situ Raman spectroscopy and cyclic voltammetry were used to investigate the mechanism of sulfur reduction in lithium-sulfur battery slurry cathodes with 1 M lithium bis(trifluoromethane sulfonyl)imide (LiTFSI) and tetraethylene glycol dimethyl ether (TEGDME)/1,3-dioxolane (DIOX) (1/1, v/v). Raman spectroscopy shows that long-chain polysulfides (S₈^2-) were formed via S₈ ring opening in the first reduction process at ∼2.4 V vs Li/Li⁺ and short-chain polysulfides such as S₄^2-, S₄^-, S₃^•-, and S₂O₄^2- were observed with continued discharge at ∼2.3 V vs Li/Li⁺ in the second reduction process. Elemental sulfur can be reformed in the end of the charge process. Rate constants obtained for the appearance and disappearance polysulfide species shows that short-chain polysulfides are directly formed from S₈ decomposition. The rate constants for S₈ reappearance and polysulfide disappearance on charge were likewise similar. The formation of polysulfide mixtures at partial discharge was found to be quite stable. The CS₂ additive was found to inhibit the sulfur reduction mechanism allowing the formation of long-chain polysulfides during discharge only and stabilizing the S₈^2- product.