Electrochemical impedance spectroscopy (EIS) is employed to investigate the electrochemical properties of LiCoO2 cathode. At initial state, the Nyquist plots consist of one compressed semicircle in high and intermediate frequency ranges that reflect charge transfer resistance and a straight line inclined at a constant angle to the real axis in lower frequency range that reflects the diffusion properties of lithium ion in solid materials. When the cell was charged for several hours, the Nyquist plots consist of two compressed semicircles alike in appearance of a half ellipse at high and intermediate frequency ranges and an anomalous line at lower frequency range, and the shape of the second semicircle is a half ellipse in appearance almost. The information of electrolyte resistance, surface film resistance and charge transfer resistance have been obtained by modeling with two different equivalent circuits according with the Nyquist plots with different shape. It can be speculated that both the passivating film on the surface of LiCoO2 cathode and the channels for lithium ion transfer in LiCoO2 cathode are mostly formed during the first charge/discharge process. Both diffusion coefficient of lithium ion in LiCoO2 cathode and exchange current density are in accord with the analysis of the Nyquist plots of electrochemical impedance spectra, indicating the rationality of the speculation
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