Electrochemical impedance spectroscopy (EIS) for the initial Li-ion insertion and extraction in Li4Ti5O12 (LTO) anode at different potentials were studied. At the intermediate stage of Li-ion insertion process, as the frequency decreased, the EIS spectra exhibited three semicircles and a slightly inclined line, which correspond to the Schottky contact reflecting the electronic properties of the material, the charge transfer step and solid state diffusion process respectively. At the intermediate stage of Li-ion extraction process, it was observed that the low-frequency semicircle turned into two joint semicircles. This interesting and significant phenomenon was attributed to gas generation in LTO cells. Kinetic parameters obtained from fitting the experimental impedance spectra in the first charge–discharge cycle on LTO side have been analyzed in detail. A new model was proposed to interpret the mechanism of Li-ion insertion/extraction reaction along with side reactions (gas generation) in the LTO electrode.