Cation-Dependent Electrochemistry of Polysulfides in Lithium and Magnesium Electrolyte Solutions

In Li/S and Mg/S batteries, the charge and discharge of the sulfur cathode proceeds through a cascade of bivalent S-x(2-) and radical S-y(.-) polysulfide intermediates. The presence of Li+ or Mg2+ cations in the electrolyte determines the type of intermediates and the overpotentials of their formation in a different manner. Based on systematic cyclic voltammetry (CV) and UV/vis investigations, this work reveals how the mutual interplay of the different cations, the electrolyte solvent, and the polysulfide anions is reflected in the electrochemical behavior of Li2S8/LiTFSI and MgS8/MgTESI2 solutions with dimethyl sulfoxide, dimethylformamide, acetonitrile, dimethoxyethane, tetraethylene glycol dimethyl ether, or tetrahydrofuran as solvent. It was observed that the disproportionation reactions of the polysulfides are generally more pronounced and especially the S-3(.-) radical is less stabilized in Mg2+ than in Li+ containing solutions. In contrast to their Li counterparts, the formation of S-4(2-) polysulfides during the reduction of sulfur is not observed in glyme-based Mg polysulfide solutions. Quantum chemical predictions of stability and disproportionation of the Mg/polysulfide/solvent clusters complemented the CV and UV/vis investigations.