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) approximate to 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.