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 spectrosc opy. Impedance spectra at various potentials were fitted with a modifi ed Randles equivalent circuit including a pseudocapacitance to express the observed finite diffusional behavior. The variations of electroch emical parameters with electrode potential, such as the charge-transfe r resistance, the pseudocapacitance, the Warburg prefactor, and, final ly, the chemical diffusion coefficient of lithium ion within graphite, were evaluated and discussed. It was shown that the charge-transfer r eaction takes place on the whole surface of graphite particles, wherea s 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 t he intercalation.