Physicochemical Properties and Structure of Lithium Tetrasulfide Solutions in a LiClO₄/Sulfolane Electrolyte

Lithium tetrasulfide is the final product of sulfur reduction at the high-voltage plateau of the discharge curve of lithium-sulfur batteries (LSBs). When dissolved in an electrolyte system, lithium tetrasulfide binds solvent molecules into the solvate shells of lithium cations. Therefore, the properties of lithium tetrasulfide solutions in electrolyte systems significantly affect the performance of LSBs. This study examines the effect of the concentration of the supporting salt (lithium perchlorate in sulfolane) on the properties (density, ionic conductivity, and self-diffusion coefficients) and structure of lithium tetrasulfide solutions by using molecular dynamics simulation. The range of lithium tetrasulfide concentrations in lithium perchlorate solutions in sulfolane (SL) is limited by a Li₂S₄/SL molar ratio of 1/10 to 1/1. It was found that in solutions of lithium tetrasulfide in sulfolane increasing the concentration of Li₂S₄ salt decreases the solvation number of the lithium cation with respect to sulfolane and increases that of the tetrasulfide anion. When the Li₂S₄ concentration reaches 1 M, the solvation numbers of the lithium cation with respect to SL and S₄^2- become equal and lithium tetrasulfide exists predominantly in a molecular form and/or as neutral clusters.