Molecular Dynamics Study on Tetraglyme Solutions of Two Lithium Salts with Isomeric Anions: LiTFSI and LiFPFSI

We used classical molecular dynamics simulations with a polarizable force field to compare Li-ion-conducting electrolytes based on tetraglyme as a solvent and two isomeric salts, LiTFSI and LiFPFSI. Analysis of the structural information and dynamics of the systems reveals that, for both salts, very stable Li(tetraglyme)+ solvates form in the electrolyte. For an equimolar salt/solvent composition, solutions exhibit properties of a solvate ionic liquid. In LiFPFSI electrolytes, the Li-anion interactions are slightly weaker than those in LiTFSI solutions, resulting in more stable Li+ solvates. Although the diffusion coefficients of ions are similar for both salts, the ionic conductivities of LiFPFSI electrolytes estimated from the simulations are 40-70% larger than the conductivities of LiTFSI solutions. This enhancement originates from constructive contributions to the conductivity arising from anticorrelated motions of cations and anions, a feature characteristic of ionic liquids. Therefore, the detailed analysis of ion-ion correlations is necessary for a deeper understanding of ion transport in concentrated solutions.