Determination of Mass Transfer Parameters and Ionic Association of LiPF₆: Organic Carbonates Solutions

Abstract

We report herein on the determination of several mass transport properties (diffusion coefficients for cations, anions, and neutral ionic aggregates; cation transference number) and degree of salt association in electrolyte solutions of practical interest (LiPF6 solutions in binary organic carbonate mixtures), through an analysis of pulsed field gradient NMR and specific conductivity data. Results were obtained as a function of LiPF6 salt concentration in a binary ethylene carbonate (EC) - dimethyl carbonate (DMC) mixture at 1:1 volume ratios and 30°C, also from 5°C to 35°C for 1 M LiPF6 dissolved in binary mixtures of EC with DMC and EMC at 3:7, 1:1 and 7:3 volume ratios. Li+ transference numbers have significantly lower values than the transport numbers, ranging from 0.31 to 0.35, and display a correct (decreasing) variation with salt concentration, contrary to the trend observed for transport numbers. A remarkably high degree of ion pairing (from 36% to 67%) was observed for the solutions investigated in the present work.