In-Situ Raman Spectroscopy of Sulfur Speciation in Lithium-Sulfur Batteries

Abstract

In this talk we report on our use of in-situ Raman spectroscopy and cyclic voltammetry to investigate the mechanism of sulfur reduction in lithium-sulfur battery slurry cathodes. 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. (1-4) 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, suggesting that polysulfide oxidation and reduction is quasi-reversible. The formation of polysulfide mixtures at partial discharge was found to be quite stable. We also examined the effect of CS2 addition on the sulfur reduction mechanism. Electrochemical and Raman results show that CS2 stabilizes the S8 2-product and inhibits further discharge of the cell