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.

The kinetics of the stage transformation of lithium-graphite intercalation compounds from dilute stage 1 to stage 4 were studied using potential-step chronoamperometry and alternating current impedance spectroscopy. Highly oriented pyrolytic graphite was used as a host material. The current-transient curve showed a current hump, suggesting that the stage transformation was initiated by the nucleation and growth of stage 4. The phase-boundary movement was discussed quantitatively using a simple geometric model. The phase boundary progressed in proportion to time during the initial stage. The rate constant was inversely proportional to the product of the interfacial resistance and the geometric edge-plane area, indicating that the phase-boundary movement was determined by the rate of the reaction at the graphite/electrolyte interface. In the following stage, the phase boundary advanced in proportion to the square root of time. The parabolic rate constant obtained experimentally was in satisfactory agreement with that calculated using Wagner's classical model which describes the diffusion within two phases separated by a phase boundary. These results indicated that the phase-boundary movement was initially determined by the rate of the interfacial electrochemical reaction and was controlled thereafter by a diffusion process.

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In order to elucidate surface reactions on graphite negative electrodes of secondary lithium ion batteries, topographical changes of the basal plane of a highly oriented pyrolytic graphite (HOPG) in 1 M LiClO4/ethylene carbonate−diethyl carbonate (1:1 by volume) were observed under polarization by electrochemical scanning tunneling microscopy. A step edge on the basal plane of HOPG was treated as a model of the edge plane of HOPG. When the sample potential was stepped to 1.1 V vs Li/Li+, two kinds of hill-like structure of ca. 10 Å height appeared on the HOPG surface. The first hill was formed far apart from a step edge and was almost unchanged with time. The second hill was formed in the vicinity of the step and was spread out with time. The formation of the second hill caused the exfoliation of graphite layers. The observed height of the hills was comparable to the values of the increment of the interlayer spacing for ternary graphite intercalation compounds of alkali metal with solvent molecules prepared by a solution method. It was considered that the intercalation of solvated lithium ion is responsible for the formation of the hills and that this process corresponds to the initial stage of the solvent decomposition and subsequent film formation processes.