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.
