Ionic Conduction and Solution Structure in LiPF6 and LiBF4 Propylene Carbonate Electrolytes
2018
Hwang, Sunwook | Kim, Dong-Hui | Shin, Jeong Hee | Jang, Jae Eun | Ahn, Kyoung Ho | Lee, Chulhaeng | Lee, Hochun
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.
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