Ion Solvation and Transport in Tetraglyme-based Electrolytes

The molecular understanding of cation transport is central to the rational design of electrolyte systems for Li-ion batteries. The transference number, t⁺₀, is a key transport parameter that reflects the fraction of electric current contributed by the working cation relative to a reference velocity, which is usually taken to be that of the solvent. We present the molecular dynamics (MD) simulation study of a model liquid electrolyte consisting of tetraglyme and bis(trifluoromethanesulfonyl)imide (LiTFSI). Under the Onsager transport frameworks, the measured t⁺₀ decreases with increasing amount of salt for concentration below 2 mol/kg. This behavior of t⁺₀ is in quantitative agreement with that measured by electrophoretic NMR (eNMR). At these low salt concentrations, the cations are constantly solvated by solvent with either one or two tetraglyme molecules, where the two-chain motif dominates. At a particular electrolyte concentration, 1.8 mol/kg, t⁺₀ approaches zero as most of the solvent molecules are in the two-chain motif, corresponding to identical values of average field-induced velocities of cations and solvents in eNMR. At concentrations above 4 mol/kg, where some cations are not solvated by solvent, we observe negative values of t⁺₀. The clustering between ions is shown to affect the behavior of t⁺₀ at these high salt concentrations.