SIMULTANEOUS MEASUREMENTS AND MODELING OF THE ELECTROCHEMICAL IMPEDANCE AND THE CYCLIC VOLTAMMETRIC CHARACTERISTICS OF GRAPHITE-ELECTRODES DOPED WITH LITHIUM

Slow scan rate cyclic voltammetry (CV) and highly resolved (with respe ct to potential) electrochemical impedance spectroscopy (EIS) have bee n applied for lithiated graphite electrodes of different thicknesses. The impedance spectra have been successfully modeled for the whole ran ge of intercalation potentials, using a combination of a Voigt-type eq uivalent circuit analog and the Frumkin and Melik-Gaykazyan (FMG) mode l. The Voigt-type analog, which is a series combination of R parallel to C circuits, models the Li ion migration through the surface films c overing the graphite particles. The FMG model combines a finite-length Warburg element, which reflects solid state Li diffusion in the graph ite particles in series with capacitance that reflects the bulk capaci ty of the graphite particles (doped with intercalated lithium). The hi ghly anisotropic nature of the graphite particles predetermines differ ent extensive properties, including their charge transfer resistance a nd the parameters of the finite-length Warburg. The application of sma ll-amplitude EIS and slow scan rate CV to very thin graphite electrode s (micronic and submicronic thicknesses) enabled us to obtain a good s eparation of the various processes which take place along the intercal ation reaction path (e.g., Li+ migration through the passivating surfa ce films, solid stale diffusion of Li ion in graphite, electron migrat ion across the boundaries of the graphite particles partly covered by the passivating films, interfacial charge transfer, accumulation-consu mption of Li into graphite, and phase transition). The application of an electroanalytical model based on a Frumkin-type adsorption isotherm complicated with a slow charge transfer for the voltammetric behavior of these electrodes at slow scan rates is discussed.