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.