How does the solvent composition influence the transport properties of electrolyte solutions? LiPF₆ and LiFSA in EC...

In this study, we experimentally measured the viscosity, η, and ionic conductivity, σ, of the electrolyte solutions of 1 mol kg-1 of LiPF6 or LiFSA dissolved in the binary mixture solvent of EC and DMC in a temperature range of 288 ≤ T/K ≤ 328 by varying the EC content from 0 to 60 vol%, which translates into the molar fraction of EC of 0 ≤ xEC ≤ 0.7. The diffusion coefficient, D, of each species, Li+, PF6-, FSA-, EC and DMC, was determined by pulse gradient spin-echo NMR. The state of molecules around Li+ was examined using the Raman spectra of the solvents and anions; the quantitative analysis suggests that EC is about twice as much preferred as DMC in the solvation shell at low xEC, while the EC-preference decreases with an increase in xEC. The classical Stokes-Einstein relation still quantitatively holds when evaluating the hydrodynamic radius, rSt, of transporting entities from D and η, in that (i) rSt,EC and rSt,DMC without the solute do not significantly differ from those in the solution; (ii) rSt,Li roughly coincides with the size estimated from the solvation number determined by Raman spectroscopy, which implies that rSt,Li reflects the solvation shell size; and (iii) rSt,anion is close to the static size, suggesting that anions are little solvated. The increase in xEC results in a decrease in rSt for all species, among which anions are most influenced, which is consistent with the view that the highly Li+-solvating EC, with its better dielectric shielding effect than DMC, liberates the anions from Li+, whereby enhancing the anion transfer that positively contributes to the ionic conductivity until the viscosity prevails at high xEC.