Ionic Conduction and Speciation in LiPF6 and LiBF4 Dimethyl Sulfoxide Electrolytes: Comparison with Propylene Carbonate Electrolytes

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 LiPF6 and LiBF4 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, LiBF4–DMSO displays higher ion conductivity than LiPF6–DMSO, whereas LiBF4–PC exhibits lower conductivity than LiPF6–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 LiBF4–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.