Self-discharge of graphite electrodes at elevated temperatures studied by CV and electrochemical impedance spectroscopy

We propose a new method for the kinetic treatment of self-discharge of intercalation electrodes based on a combination of cyclic voltammetry (CV) and chronopotentiometry under net zero current conditions. This was applied to composite graphite electrodes in 1 M LiPF6/ethylene carbonate + dimethyl carbonate (1:1) + 5% dimethyl polycarbonate at four different temperatures from 25 to 80degreesC, and was quantitatively proved for small degrees of self-discharge. The degree of self-discharge and the self-discharge current, I-sd, were shown to substantially increase with the increase in temperature. Self-discharge experiments, which continued for 50 h, were complemented by detailed impedance characterizations of the gradually discharged electrodes, which show an increase in the diameter of the high-frequency semicircles (HFS) with open-circuit time at different temperatures. A comparison between Arrhenius plots for the HFS, the chemical diffusion coefficient, and the self-discharge current may create a basis for a new method of "electrochemical temperature spectroscopy'' for identification of the nature of the rate-determining step of self-discharge for different temperature ranges. Our results are discussed in light of studies by others of self-discharge phenomena and mechanisms (Li-graphite electrodes). (C) 2004 The Electrochemical Society.