Dynamic Heterogeneity of Solvent Motion and Ion Transport in Concentrated Electrolytes.

Abstract

Molecular-level understanding of the cation transference number t+0, an important property that characterizes the transport of working cations, is critical to the bottom-up design of battery electrolytes. We quantify t+0 in a model tetraglyme-based electrolyte using molecular dynamics simulation and the Onsager approach. t+0 exhibits a concentration dependence in three distinct regimes: dilute, intermediate, and concentrated. The cluster approximation uncovers dominant correlations and dynamic heterogeneity in each regime. In the dilute regime, t+0 decreases sharply as increasing numbers of solvent molecules become coordinated with Li+. The crossover to the intermediate regime, t+0 ≈ 0, occurs when all solvent molecules become coordinated, and a plateau is obtained because anions enter the Li+ solvation shell, resulting in ion pairs that do not contribute to t+0. Transference in concentrated electrolytes is dominated by the presence of cations in a variety of negatively charged and solvent-excluded clusters, resulting in t+0 < 0.