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

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.