Ionic Conduction and Speciation in LiPF<sub>6</sub> and LiBF<sub>4</sub> Dimethyl Sulfoxide Electrolytes: Comparison with Propylene Carbonate Electrolytes
Bonhyeop Koo, Hyejin Lee, Hyejin Lee, Sunwook Hwang, Hochun Lee, Hochun Lee
Abstract
Innovative Li-ion battery technology requires a fundamental understanding of the intricate ion–ion and ion–solvent interactions that govern the ion transport properties of electrolyte solutions. This study investigates the ion conduction and solution structure of 0.1–3.0 M LiPF 6 and LiBF 4 dimethyl sulfoxide (DMSO) solutions using Raman, dielectric relaxation, and pulsed-field gradient nuclear magnetic resonance spectroscopies for a comparison with a previous work on propylene carbonate (PC) solutions. Notably, LiBF 4 –DMSO displays higher ion conductivity than LiPF 6 –DMSO, whereas LiBF 4 –PC exhibits lower conductivity than LiPF 6 –PC. This different conductivity trend is rationalized by the interplay between solution viscosity and the degree of salt dissociation. In DMSO solution, viscosity governs the ion conductivity as salts readily dissociate into free ions and solvent-shared ion pairs owing to the high donor number of DMSO solvent. In contrast, the number of charge carriers determines the conductivity in PC solution, where charge-neutral contact ion pairs are dominant due to the low donicity of PC. Moreover, the conductivity of the DMSO solutions obeys the Nernst–Einstein (NE) theory, whereas that of LiBF 4 –PC deviates from the NE theory. This study underscores the crucial role of the microscopic solution structure in determining the ionic conduction of electrolyte solutions.