New insight into studies of the cycling performance of Li-graphite electrodes: A combination of cyclic voltammetry, electrochemical impedance, and differential self-discharge measurements

Based on a combination of cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy techniques, a new method of self-discharge characterization of lithiated graphite electrodes as a function of cycle number has been elaborated. Different domains were identified in the plot of the self-discharge current, I_sd vs. cycle number. During the beginning of cycling, the electrode's impedance and I_sd, both decrease as cycling proceeds. This is due to reorganization and reformation of the surface films on the electrode surface. After several tens of consecutive cycles, I_sd start to increase. In parallel, the impedance of the deintercalated graphite electrode increases as well. Both I_sd and the electrode's impedance reach steady value upon prolonged cycling. This relates to the semiconducting properties of the surface films formed on the electrode surface which change upon cycling toward an increasingly better electronic conductivity of the surface films (their passivity decreases). This, in turn, creates favorable conditions for electroadsorption of Li ions on the surface films of the long-term cycled electrode reflected as an increase in R _ct. A porous electrode model was adopted in order to qualitatively describe the evolution of the impedance spectra of the fully lithiated graphite electrodes with cycle number, at room temperature.