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

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

References:

(1) Barchasz, C. et al., Anal. Chem. 2012, 84, 3973.

(2) Cuisinier, M. at al., J. Phys. Chem. Lett. 2013, 4, 3227.

(3) Yeon, J.-T. et al., J. Electrochem. Soc. 2012, 159, A130.

(4) Hagen, M. et al., J. Electrochem. Soc. 2013, 160, A1205.