Structural and Transport Properties of Li/S Battery Electrolytes: Role of the Polysulfide Species

Abstract

Lithium--sulfur (Li/S) batteries are regarded as one of the most promising energy storage devices beyond lithium-ion batteries because of their high energy density of 2600 Wh/kg and an affordable cost of sulfur. Meanwhile, some challenges inherent to Li/S batteries remain to be tackled, for instance, the polysulfide (PS) shuttle effect, the irreversible solidification of Li$_2$S, and the volume expansion of the cathode material during discharge. On the molecular level, these issues originate from the structural and solubility behavior of the PS species in bulk and in the electrode confinement. In this study, we use classical molecular dynamics (MD) simulations to develop a working model for PS of different chain lengths in applied electrolyte solutions of lithium bistriflimide (LiTFSI) in 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) mixtures. We investigate conductivities, diffusion coefficients, solvation structures, and clustering behavior and verify our simulation model with experimental measurements available in literature and newly performed by us. Our results show that diffusion coefficients and conductivities are significantly influenced by the chain length of PS. The conductivity contribution of the short chains, like Li$_2$S$_4$, is lower than of longer PS chains, such as Li$_2$S$_6$ or Li$_2$S$_8$, despite the fact that the diffusion coefficient of Li$_2$S$_4$ is higher than for longer PS chains. The low conductivity of Li$_2$S$_4$ can be attributed to its low degree of dissociation and even to a formation of large clusters in the solution. It is also found that an addition of 1 M LiTFSI into PS solutions considerably reduces the clustering behavior. Our simulation model enables future systematic studies in various solvating and confining systems for the rational design of Li/S electrolytes.