Lithium polysulfide conformer analysis in ether-based solvents for Li-S batteries

Abstract

A fundamental understanding of polysulfide conformers in common (ether-based) solvents used in prototype lithium-sulfur batteries is mandatory to push the technology towards practical applications. In this work, molecular dynamics simulations were used to perform a detailed conformational analysis and dynamics ascertainment of lithium hexasulfide Li2S6 as a representative of soluble long-chain lithium polysulfides. This novel conformational dynamics study based on 106 structures per studied system provides important insights into open questions such as how many Li+ coordinate polysulfides are correlated to lithium transport and polysulfide clustering. The simulations predict that monoanionic LiS6− in its closed conformation is the predominant structure in 1,2-dioxolane and dimethoxyethane equimolar mixtures. If a second Li+ approaches, that leads to the opening of the polysulfide chain and the formation of neutral Li2S6 (open conformer). By changing the electrolyte composition, the relative distribution of closed and open conformers varies, although closed LiS6− remains the most abundant conformer compared to the open one. There are three major findings from this work: the first is that there is a greater abundance of the closed conformer, in which the closed/open aspect ratio depends on electrolyte composition; the second is that the polysulfide charge distribution resulting from the nearby coordinating lithium cations was determined; and the third is that the number and type of solvent molecules coordinating the lithium polysulfide were determined. These findings, which have been missing up to now to the best of our knowledge, provide crucial information for the design of strategies to suppress the unwanted polysulfide shuttle effect, such as engineered separators and cathode protective layers.