Impedance study on the electrochemical lithium intercalation into natural graphite powder

Electrochemical lithium intercalation into natural graphite powder of different sizes was studied by alternating current impedance spectroscopy. Impedance spectra at various potentials were fitted with a modified Randles equivalent circuit including a pseudocapacitance to express the observed finite diffusional behavior. The variations of electrochemical parameters with electrode potential, such as the charge-transfer resistance, the pseudocapacitance, the Warburg prefactor, and, finally, the chemical diffusion coefficient of lithium ion within graphite, were evaluated and discussed. It was shown that the charge-transfer reaction takes place on the whole surface of graphite particles, whereas lithium ion is intercalated from the edge plane and diffuses to the interior. The kinetics of the charge-transfer reaction was independent of the structure of the host. In contrast, the diffusivity of lithium ion within graphite was strongly dependent on the host structure, and the dependence was explained in terms of differences in in-plane and stacking order of lithium-graphite intercalation compounds formed by the intercalation.