The effect of slow interfacial kinetics on the chronoamperometric response of composite lithiated graphite electrodes and on the calculation of the chemical diffusion coefficient of Li ions in graphite.
This paper deals with a study of the shape of the chronoamperometric response (current, I, vs time, t) and, eventually, the mechanism of Li-ions insertion and deinsertion to/from composite graphite electrodes obtained by a small-amplitude (incremental) technique, such as potentiostatic intermittent titration (PITT). The dependences of log I, the Cottrell parameter It(1/2), and the differential parameter d log I/d log t on the process duration (vs log t) were carefully examined both for single- and two-phase coexistence domains. log I vs log t curves for single-phase domains were characterized by a single monotonic curve with a gradually increasing slope. In contrast, the same curves for two-phase domains consist of two sequential downward concave lines. Both types of response were explained by using the cell-impedance-control model. To separate the contributions of solid-state diffusion, Ohmic drops, and slow interfacial charge-transfer kinetics to the chronoamperometric response, the data were presented in the form of the inverse Cottrell parameter, (It(1/2))(-1) vs t(-1/2), from which the chemical diffusion coefficient (D) could be obtained. Refined values of D for Li insertion into graphite obtained herein agree very well with values of the component diffusion coefficient obtained from quasielastic neutron scattering for Li insertion into HOPG, reported in the literature.