Elucidation of Liquid Structures and Transport Properties of Highly Concentrated LiN(SO₂F)₂/Ethylene Carbonate Electrolytes

Abstract

The Li+ conduction mechanism in highly concentrated ethylene carbonate (EC) solution of lithium bis(fluorosulfonyl)amide (LiFSA) was studied based on the measurements of thermal properties, density, viscosity, ionic conductivity, self-diffusion coefficients, Raman spectra, and proton nuclear magnetic resonance relaxation time. The compositions of EC and LiFSA were varied from dilute to concentrated states ([EC]/[LiFSA] = 9.2–0.80). For compositions of [EC]/[LiFSA] ≤ 1.5, the free solvents decrease, and contact ion pairs and aggregates become dominant, causing a significant change in the transport properties. Faster diffusion of Li+ compared with solvent (Li+ hopping conduction) was observed when the molar composition of [EC]/[LiFSA] ≤ 1.0 as in the case of sulfolane.

Molecular Level Origin of Ion Dynamics in Highly Concentrated Electrolytes

“…However, these transport phenomena correspond to averaged dynamics on a relatively large scale, and as such, their correlation with microscopic behaviors on a molecular level (e.g., solvation/coordination structures, relaxation of the solvent structure, and the lifetimes of ion–ion/ion–solvent complexes) are obscured. Although several groups have investigated the above relationships by means of dielectric relaxation spectroscopy, − quasi-elastic neutron scattering, two-dimensional infrared spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy based on the Bloembergen–Purcell–Pound theory, , it remains a challenge to obtain more direct information, particularly on the microscopic scale. In addition, current studies based on the use of molecular dynamics (MD) simulations endeavor to clarify the relationship between the molecular level behavior and the transport phenomena; − however, it is still a challenging issue being examined to date.…”