Intermolecular Interactions in Lactone-Based Electrolytes

Abstract

The viscosities (η), conductivities (σ), and enthalpies of dissolution of (LiTF) and (LiTFSI) are measured in γ-butyrolactone (BL), γ-valerolactone (VL), and mixtures of ethylene carbonate (EC) with tetrahydrofuran (THF) and diglyme (DG): EC:THF (20:80, in moles) and EC: DG (45:55, in moles). From the variations of η with the salt concentration, the B and D coefficients in the extended Jones-Dole equation are determined at different temperatures. The concentration dependence of the conductivity is analyzed on the basis of a model involving (i) a chemical equilibrium between free ions and ion pairs, (ii) the use of the Walden product to correct molar conductivities for viscosity variations, and (iii) the calculation of ion activity coefficients by the cube root law derived from the quasi-lattice model. According to this model, the limiting molar conductivities and ion pair dissociation constants are inferred. The solvation enthalpies of the salts are deduced from the heat of dissolution of LiTF and LiTFSI in BL, VL, and the EC:DG mixture. The formation of solvent-separated ion pairs in lactones and contact ion pairs in the EC mixture is deduced from the analysis of the experimental data as the result of a competition between ion-ion and ion-solvent interactions. The dissociation coefficient of ion pairs determined by Raman spectroscopy agrees well with those deduced from conductivity measurements. From the variation of the area of the band at 676 cm−1 with salt concentration, a coordination number of 4 molecules is found for the solvation of the by BL molecules. © 2002 The Electrochemical Society. All rights reserved.