Salt and Solvent Control of Elemental Sulfur (Cyclo-S8) and Lithium-Ion Diffusion in Liquid Li–S Battery Electrolytes Revealed by DFTB Molecular Dynamics

Abstract

Abstract In recent years, lithium–sulfur (Li–S) batteries have become attractive owing to their high energy density and higher charge-storage capacity than Li-ion batteries. Despite this, practical applications of Li–S batteries remain limited by several challenges that still need to be resolved, including Li-dendrite formation, which leads to short-circuiting. In the present study, divide-and-conquer density-functional tight-binding molecular dynamics (DC-DFTB-MD) simulations examine neutral cyclo-S8, while lithium polysulfides and polysulfide anions are not included. Increasing LiOTf concentration generally reduces the self-diffusion coefficients because denser, more highly coordinated ionic environments restrict translational motion. The Li+ ion preferentially coordinates with electronegative oxygen atoms of the solvent and triflate anion, whereas nonpolar, neutral S8 interacts mainly through induced-polarization and dispersion forces. These competing coordination environments explain the weak direct Li+–S8 association and the coupled decline of ionic and molecular mobility with salt loading.