The Fundamental Understanding of Lithium Polysulfides in Ether-Based Electrolyte for Lithium–Sulfur Batteries

The liquid electrolyte in a lithium–sulfur battery is important for the dissolution–deposition reaction through the solubility of polysulfides. To get insight into the sulfur chemistry, fundamental understanding of the dissolved polysulfides should be explored. In this work, the dissolved forms of Li₂S₆, as prototypical polysulfides, in 1,2-dimethoxyethane (DME) were studied by ab initio calculations. The calculated first-order and second-order dissociation constants are 3.52 and 17.61, respectively, for Li₂S₆ dissociated into LiS₆^– and S₆^2–. Simultaneously, Li₂S₆ can also dissociate into LiS₃^• radicals with dissociation constant of 7.80. The experimental ultraviolt–visible spectrum of stoichiometric 2 mM Li₂S₆ in DME was interpreted by the individual contributions of various polysulfide species using time-dependent density functional theory calculations. The concentrations of Li₂S₆ and the dissociated LiS₆^–, S₆^2–, and LiS₃^• were further calculated for different electrolyte-to-sulfur ratios. It is found that neutral Li₂S₆ is dominant in DME, while polysulfide anions and radicals are rare.