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

다국어 초록 (Multilingual Abstract)

<P>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 LiPF<SUB>6</SUB> and LiBF<SUB>4</SUB> using Raman spectroscopy, dielectric relaxation spectroscopy (DRS), and pulsed-field gradient NMR (PFG-NMR) spectroscopy. In both LiPF<SUB>6</SUB>-PC and LiBF<SUB>4</SUB>-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 LiPF<SUB>6</SUB>-PC, whereas the contact ion pair (CIP), CD, and agglomerate (AGG) prevail in LiBF<SUB>4</SUB>-PC. Ionic conduction in 0.1-1.5 M LiPF<SUB>6</SUB>-PC and LiBF<SUB>4</SUB>-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 LiBF<SUB>4</SUB>-PC may contribute to ionic conduction via a Grotthuss-type mechanism.</P>
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