Polysulfide Speciation in Li–S Battery Electrolyte via In-Operando Optical Imaging and Ex-Situ UV-vis Spectra Analysis

Abstract

Lithium sulfur (Li-S) batteries have received significant attention as an energy storage system with excellent prospects for emerging applications due to their high energy density and low-cost. However, there are fundamental challenges impeding the commercialization of Li-S batteries. Notorious among those challenges is the “polysulfide shuttle” consisting of the dissolution into the electrolyte solvent and subsequent crossover to the anode of long-chain lithium polysulfides. Sparingly-solvating electrolytes have been exploited as an approach to reduce the dissolution of polysulfides and thereby the shuttle effect. Using an optical in operando lithium-sulfur cell and ex situ UV-vis spectroscopy, we elucidate the speciation of polysulfides in fully and sparingly solvating electrolytes for Li-S batteries. Extensive literature meta-analysis reveals that the most unambiguous effect of sparingly-solvating electrolytes is in improving the coulombic efficiency of sulfur-cells. Experimental optical imaging and UV-vis characterization elucidate a shift towards shorter-chain polysulfides in electrolytes with increasing lithium-salt concentration (more sparingly solvating). The shift to shorter-chain polysulfides corresponds to a reduction of polysulfide species participating in shuttling which corroborate the increased coulombic efficiency in sparingly-solvating electrolytes.