Kinetic Characterization of Single Particles of LiCoO₂ by AC Impedance and Potential Step Methods

This is the first report of impedance technique run on single particle LiCoO₂ electrodes with the aim of clarifying its electronic and ionic transport properties. Measurements were successfully conducted on a LiCoO₂ particle of 15 μm diam resulting in impedance magnitude on the order of MΩ. The impedance spectra exhibited (i) one semicircle in the high frequency region, (ii) Warburg impedance in low frequencies, and finally, (iii) a limiting capacitance in the very low frequencies. The spectra were analyzed using a modified Randles-Ershler circuit, so that the reaction kinetics could be precisely evaluated. The charge transfer resistance decreased as the potential increased, whereas the double layer capacitance was almost invariant with the potential. Thus, the apparent chemical diffusion coefficient (D_app) of lithium ions was determined to be 10¹¹ to 10^-7 cm²/s as function of electrode potential. These results are in agreement with those obtained by potential step chronoamperometry technique.