Solvent-Dictated Lithium Sulfur Redox Reactions: An Operando UV-vis Spectroscopic Study.

Fundamental understanding of solvent's influence on Li-S redox reactions is required for rational design of electrolyte for Li-S batteries. Here we employ operando UV-vis spectroscopy to reveal that Li-S redox reactions in high-donor-number solvents, for example, dimethyl sulfoxide (DMSO), undergo multiple electrochemical and chemical reactions involving S8(2-), S6(2-), S4(2-), and S3(•-), where S3(•-) is the most stable and dominant reaction intermediate. In low-donor-number solvents, for example, 1,3-dioxolane:1,2-dimethoxyethane, the dominant reaction intermediate, is found to be S4(2-). The stability of these main polysulfide intermediates determines the reaction rates of the disproportionation/dissociation/recombination of polysulfides and thereby affects the reaction rates of the Li-S batteries. As an example, we show that dimethylformamide, a high-donor-number solvent, which exhibits stronger stabilization of S3(•-) compared with DMSO, significantly reduces Li-S cell polarization compared with DMSO. Our study reveals solvent-dependent Li-S reaction pathways and highlights the role of polysulfide stability in the efficiency of Li-S batteries.