Charge Transport in [Li(tetraglyme)][bis(trifluoromethane) sulfonimide] Solvate Ionic Liquids: Insight from Molecular Dynamics Simulations.

Abstract

Molecular dynamics simulations using fully atomistic polarizable force field have been performed on solvate ionic liquids (SILs), comprised of tetraglyme (G4) solvent molecules, Li+ cations, and bis(trifluoromethane) sulfonimide (TFSI) anions, [Li(G4)][TFSI]. The SILs with equimolar salt:G4 composition were investigated in the 303-373 K temperature range, whereas several systems with lower salt concentrations were investigated at 373 K. The simulations using polarizable force field demonstrate very good consistency of structural and dynamic properties with experimental data. The ability to accurately sample the ion transport mechanisms is particularly encouraging, taking into account that previous simulations employing nonpolarizable models had challenges in sampling dynamics in these systems. Here, we correlate Li+ ion local environment and glyme conformations with dynamic characteristics, such as residence time of species around Li+, self-diffusion coefficients, transference number, and conductivity. The analysis of contributions to Li+ mobility due to changing its local environment (i.e., moving from one glyme/anion to another) and from translational motion of Li+ with its' coordination environment showed significant dominance of the latter. The contributions of cross-ion dynamic correlations to the total conductivity have been quantified, showing strongly positive contribution from the cation-anion anticorrelation. Despite the high degree of Li-TFSI dissociation and positive contribution of the cation-anion anticorrelated motion to conductivity, the Li+ transference numbers for equimolar SILs are very low under the anion blocking conditions.