Simultaneous Measurements and Modeling of the Electrochemical Impedance and the Cyclic Voltammetric Characteristics of Graphite Electrodes Doped with Lithium

Abstract

Slow scan rate cyclic voltammetry (CV) and highly resolved (with respect to potential) electrochemical impedance spectroscopy (EIS) have been applied for lithiated graphite electrodes of different thicknesses. The impedance spectra have been successfully modeled for the whole range of intercalation potentials, using a combination of a Voigt-type equivalent circuit analog and the Frumkin and Melik-Gaykazyan (FMG) model. The Voigt-type analog, which is a series combination of R||C circuits, models the Li ion migration through the surface films covering the graphite particles. The FMG model combines a finite-length Warburg element, which reflects solid state Li diffusion in the graphite particles in series with capacitance that reflects the bulk capacity of the graphite particles (doped with intercalated lithium). The highly anisotropic nature of the graphite particles predetermines different extensive properties, including their charge transfer resistance and the parameters of the finite-length Warburg. The application of small-amplitude EIS and slow scan rate CV to very thin graphite electrodes (micronic and submicronic thicknesses) enabled us to obtain a good separation of the various processes which take place along the intercalation reaction path (e.g., Li+ migration through the passivating surface films, solid state diffusion of Li ion in graphite, electron migration across the boundaries of the graphite particles partly covered by the passivating films, interfacial charge transfer, accumulation−consumption 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.