Measuring the salt activity coefficient in lithium-battery electrolytes

Abstract

Activity coefficients were measured in concentration-cell and melting-point-depression experiments. Results from concentration-cell experiments are presented for solutions of lithium hexafluorophosphate (LiPF6) in propylene carbonate as well as in a 1:1 by weight solution of ethylene carbonate (EC) and ethyl methyl carbonate. A constant transference number, t(+)(0), of 0.38 was estimated for these solutions with a thermodynamic factor increasing between 1 and 4 for the concentration range up to 1.75 M. Measurements of the activity coefficient are also presented from melting-point-depression experiments in solutions of LiPF6 in EC. Differential scanning calorimetry is used to measure melting points for solutions of different concentrations. This technique is shown to be useful for measuring activity coefficients in single-solvent electrolytes; however, data are necessary for the relative partial molal enthalpy of these solutions for accurate results. The thermodynamic factor of LiPF6 solutions in EC varies between ca. 1.1 and 3.17 for the concentration range 0.0625 to 1 M (above this concentration there appears to be a eutectic point). It is shown that the solutions of LiPF6 investigated are not ideal but that an assumption of ideality for these solutions may overestimate the specific energy of a lithium-ion cell by only 0.6%. (C) 2008 The Electrochemical Society