Molecular Level Origin of Ion Dynamics in Highly Concentrated Electrolytes

Single-ion conducting liquid electrolytes are key to achieving rapid charge/discharge in Li secondary batteries. The Li⁺ transference (or transport) numbers are the defining properties of such electrolytes and have been discussed in the framework of concentrated solution theories. However, the connection between macroscopic transference and microscopic ion dynamics remains unclear. Molecular dynamics simulations were performed to obtain direct information regarding the microscopic behaviors in highly concentrated electrolytes, and the relationships between these behaviors and the transference number were determined under anion-blocking conditions. Various solvents with different donor numbers (DNs) were used along with a Li salt of the weakly Lewis basic bis(fluorosulfonyl)amide anion for electrolyte preparation. Favorable ordered Li⁺ structuring and a continuous Li⁺ conduction pathway were observed for the fluoroethylene carbonate-based electrolyte due to its low DN. The properties were less pronounced at higher DNs, e.g., for the dimethyl sulfoxide-based electrolyte. The τ_Li-solvent^life/τ_dipole^relax ratio was introduced as a factor for ion dynamics, and the two mechanisms of ion transport were considered an exchange mechanism (τ_Li-solvent^life/τ_dipole^relax < 1) and a vehicle mechanism (translational motion of solvated Li⁺) (τ_Li-solvent^life/τ_dipole^relax ≥ 1). Vehicle-type transport was dominant with high DNs, while exchangeable transport was preferable at lower DNs. These findings should aid the further selection of solvents and Li salts to prepare single-ion conducting electrolytes.