Ionic Liquids in Quasi-Solid-State Li–S Batteries with Sulfide-Based Solid Electrolytes: A Density Functional Theory and Ab Initio Molecular Dynamics Study

Lithium–sulfur (Li–S) batteries are promising next-generation energy storage systems due to their high theoretical capacity and abundance of sulfur. However, practical applications are hindered by the shuttle effect of lithium polysulfides (LiPSs) and lithium dendrite growth in liquid electrolytes. This study employs density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations to investigate the interfacial behavior between ionic liquids (ILs) and LiPSs on a Li3PS4 solid electrolyte surface. Two imidazolium-based cations ([C4mim]+ and [C2mim]+) and two anions ([BF4]− and [TFSI]−) were selected to construct 24 coadsorption models. Through comprehensive analysis using electrostatic potential (ESP), atoms in molecules (AIM), independent gradient model (IGM), electron density difference (EDD), and AIMD simulations, we reveal that anion type and sulfur chain length critically influence interfacial charge transfer and bonding. [TFSI]− promotes oxidation in long-chain LiPSs, while [BF4]− favors electron accumulation in short-chain species. Strong interactions, such as Li···F/O bonds, dominate between anions and LiPSs, whereas cations exhibit weaker van der Waals interactions. The coadsorption structure enhances Li+ desorption and transport, and the Li3PS4 substrate effectively stabilizes sulfur species. These insights provide theoretical guidance for designing efficient quasi-solid Li–S batteries with enhanced interfacial stability and electrochemical performance.