Transport Properties of Electrolyte Solution Comprising LiPF₆, Ethylene Carbonate, and Propylene Carbonate

Abstract

In this study, we experimentally measure the viscosity, η, and ionic conductivity, σ, of 1 mol kg⁻¹ LiPF₆ dissolved in a binary solvent of ethylene carbonate (EC) and propylene carbonate (PC) by varying the EC content from 0 to 60 vol%. Replacing EC with PC does not significantly influence on the mechanism of flow and ionic conduction. The state of solvent solvating to Li⁺ is analyzed using Raman spectra and ¹³C-NMR; the quantitative analysis suggests that the preference of EC in the Li⁺ solvation shell is almost similar to that of PC. The diffusion coefficient, D, of each species, Li⁺, PF₆⁻, EC, and PC, is determined by pulse-gradient spin-echo NMR. In the electrolyte system dealt in this study, the molar conductivity is dominated by only the D of the charge carriers. The hydrodynamic Stokes radius of all constituent species is expressed in terms of D and η using the Stokes–Einstein relation. As for coordinated-Li⁺, a comparison between the Stokes radius and the van der Waals radius or the radius considering the free volume suggests that Li⁺ diffuses while dragging not only a coordination shell that can be spectroscopically detected but also an outer shell that binds loosely to Li⁺.