Electrolytic characteristics of ethylene carbonate-diglyme-based electrolytes for lithium batteries

Dependencies of viscosity (eta) and conductivity (kappa) on temperature and solvent composition are examined for solutions of LiPF6 and of LiClO4 in e thylene carbonate (EC):diglyme (DG) binary mixtures to identify organic ele ctrolyte systems with the highest conductivity as possible at room temperat ure. The energies of activation for viscous flow and for conductivity, dete rmined as functions of the composition of the mixed solvent, show these two transport processes to be strongly related. Conductivities of electrolyte solutions prepared with LiPF6 and LiClO4 are comparable, but lower activati on energy is required for LiClO4. A flat maximum of conductivity at about 5 0% (v/v) in EC is observed when the composition of the mixed solvent is var ied. A high value, independent of the considered temperature, has been foun d for the product kappa eta, meaning that free ions are the predominant ion ic species in these media. As a consequence, the macroscopic viscosity of e lectrolytes under study mainly governs their conductivity. Cyclic voltammet ry, performed on negative (LixC) and positive (LixNiO2) intercalation elect rodes in the optimized electrolyte (LiClO4 or LiPF6 (1 M)-50:50 EC:DG), giv es evidence for the formation of passive layers during the first cycle and reversible electrochemical intercalation during the following cycles. DSC r esults show that this electrolyte system is a potential candidate for Li se condary batteries or electrochromic devices working at ambient or higher te mperatures as it is safe and stable at least to 200 degrees C. (C) 2000 Els evier Science Ltd. All rights reserved.