Molecular modeling of electrolyte and polysulfide ions for lithium-sulfur batteries

The operation of a lithium-sulfur (Li-S) battery involves the transport of Li+ ions and soluble sulfides mostly in the form of solvated ions. Key challenges in the development of Li-S battery technology are the diffusion of Li+ in micropores filled with sulfur and eliminating the “shuttling” of polysulfides. Ion dimensions in solvated and desolvated forms are key parameters determining the diffusion coefficient and the rate of transport of such ions, while constrictivity effects due to the effect of pore size compared to ion size control both transport and filling of the pores. We present molecular simulations to determine the solvation parameters of electrolyte ions and sulfides S22−, S42−, S62−, and S82− in two different electrolyte systems: LiTFSI in DOL/DME and LiPF6 in EC/DMC. The calculated parameters include the coordination number and the geometrically optimized model and dimensions, using the van der Waals surface approach, of the solvated and desolvated ions. The desolvation energy of the electrolyte ions is also calculated. Such data is useful for the modeling and design of the pore sizes of cathode host materials to be able to accommodate the different sulfides while minimizing their “shuttling” between cathode and anode.