Keisuke Shigenobu, Seiji Tsuzuki, Frederik Philippi, Taku Sudoh, Yosuke Ugata, Kaoru Dokko, Masayoshi Watanabe, Kazuhide Ueno, and Wataru Shinoda
J. Phys. Chem. B, 127, 10422-10433(2023).
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 behaviours in highly concentrated electrolytes, and the relationships between these behaviours and the transference number were determined under anion-blocking conditions. Various solvents with different donor numbers were used along with an Li salt of the weakly Lewis basic bis(fluorosulfonyl)amide anion for electrolyte preparation. Favourable ordered Li+ structuring and a continuous Li+ conduction pathway were observed for the fluoroethylene carbonate-based electrolyte due to its low donor number. The properties were less pronounced at higher donor numbers, e.g., for the dimethyl sulfoxide-based electrolyte. The τ(Li-solvent)life/τdipolerelax ratio was introduced as a factor for ion dynamics, and two mechanisms of ion transport were considered – an exchange mechanism (τ(Li-solvent)life/τdipolerelax < 1) and a vehicle mechanism (translational motion of solvated Li+) (τ(Li-solvent)life/τdipolerelax≥ 1). Vehicle-type transport was dominant with high donor numbers, while exchangeable transport was preferable at lower donor numbers. These findings should aid the further selection of solvents and Li salts to prepare single-ion conducting electrolytes.