Effect of soluble polysulfides on the viscosity and ionic conductivity of electrolytes for magnesium-sulfur batteries

Abstract

In the present work, the influence of polysulfide species and their concentration on the viscosity and ionic conductivity of the electrolyte for magnesium-sulfur batteries was investigated. Polysulfide solutions were synthesized with different Mg:S composition (MgS4, MgS6, MgS8) from elemental sulfur and elemental magnesium in mono-, di-, tetraglyme, dioxolane and tetrahydrofuran. X-ray photoelectron spectroscopy, elemental analysis and UV/VIS spectroscopy were used to detect the formation of magnesium polysulfides in mono- and diglyme. Regardless of the starting composition Mg:S and the solvent used, polysulfides with the molecular formula MgSx (x = 6-8) were formed. With dioxolane and tetrahydrofuran no magnesium polysulfides could be synthesized. Viscosity measurements showed an increased viscosity with increasing molality of the polysulfide solutions (0.1 < 0.5 < 1.0 < 5.0 < 10.0) mmol ∙ kg-1. A significant influence of the solvent on the viscosity could be observed. By using mono- and diglyme, an increased conductivity was observed with increasing molality of the polysulfide solutions. Solutions with tetrahydrofuran and dioxolane as solvent showed only less or no conductivity. In the case of the polysulfide solutions analyzed, the influence of the solvent on the viscosity and conductivity is greater than the influence of polysulfides.