Here, we propose thiol-based electrolyte additives (biphenyl-4,4’-dithiol (BPD)) to enhance the capacity retention of lithium-sulfur batteries. In-situ Raman and UV-Vis spectroscopy are used to investigate the effect of the additive on the sulfur reduction mechanism. Raman spectroscopy shows that long chain polysulfides (S₈^2-) are 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- are observed with continued discharge at ~2.0 V vs Li/Li⁺ in the second reduction process. In the Kinetic study, rate constants obtained for the appearance and disappearance polysulfide species show that short chain polysulfides are directly formed from S₈ decomposition. The polysulfide oxidation and reduction is quasi-reversible. With the BPD additive, an additional reduction process is observed at ~2.1 V vs Li/Li⁺. This reduction is associated with the formation of BPD-polysulfide complexes. The BPD-polysulfide complexes form at ~2.1 V followed by the formation of short chain polysulfides upon further discharge. The reduction of these complexes is reversible during the charge process. The BPD additive inhibits the formation rate of short chain polysulfides, suggesting that there is a strong interaction between BPD and short chain polysulfides. In-situ UV-Vis spectroscopy shows that the polysulfide solubility decreases in the presence of the BPD additive and forms thiol-polysulfide complexes during the cycling. The (-)ESI-MS shows the formation of BPD-S and BPD-S₂ and BPD-S₃, suggesting that BPD interacts with short chain polysulfides. The decomposition of the thiol-based additive was found during the battery cycling and resulted in a dense and smooth SEI film on the Li anode.

References:

(1) Zhang, S.S. et al J. Power Sources 2013, 231, 153-162.

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

(3) Cuisinier, M. et al., J. Phys. Chem. Lett. 2013, 4, 3227.

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