Ionic Conduction and Solution Structure in LiPF6 and LiBF4 Propylene Carbonate Electrolytes

Expanding the performance limit of current Li-ion batteries requires ion–ion and ion–solvent interaction, which governs the ion transport behavior of the electrolytes, to be fully understood as a matter of crucial importance. We herein examine the ionic speciation and conduction behavior of propylene carbonate (PC) electrolytes of 0.1–3.0 M LiPF6 and LiBF4 using Raman spectroscopy, dielectric relaxation spectroscopy (DRS), and pulsed-field gradient NMR (PFG-NMR) spectroscopy. In both LiPF6–PC and LiBF4–PC, free ions and a solvent-shared ion pair (SIP) are dominant species at dilute salt concentrations (<0.8 M), and SIP becomes dominant at intermediate concentrations (0.8–1.5 M). At higher concentrations (1.5–3.0 M), the solvent-shared dimer (SSD) and contact dimer (CD) are dominant in LiPF6–PC, whereas the contact ion pair (CIP), CD, and agglomerate (AGG) prevail in LiBF4–PC. Ionic conduction in 0.1–1.5 M LiPF6–PC and LiBF4–PC is governed by the migration of free ions and SIP. Notably, above 1.5 M of the two PC electrolytes, SSD participates in ionic conduction via the migration mode as well. Furthermore, it is suggested that the large number of CIPs present in LiBF4–PC may contribute to ionic conduction via a Grotthuss-type mechanism.