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

Molecular-level understanding of the cation transference number , an important property that characterizes the transport of working cations, is critical to the bottom-up design of battery electrolytes. We quantify in a model tetraglyme-based electrolyte using molecular dynamics simulation and the Onsager approach. 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, decreases sharply as increasing numbers of solvent molecules become coordinated with Li. The crossover to the intermediate regime, ≈ 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 . Transference in concentrated electrolytes is dominated by the presence of cations in a variety of negatively charged and solvent-excluded clusters, resulting in < 0.