Determination of electrochemical impedance of lithium-ion battery from charge curve by wavelet transformation

Lithium-ion batteries (LIBs) are being used as power sources for use in electric vehicles and aircraft, and so on. Early detection technique of battery deterioration is required with the spread adoption of electric vehicles and aircraft. Electrochemical impedance spectroscopy is a powerful method to evaluate the internal state of LIB. We recently have proposed a method to determine the impedance spectra of LIB by wavelet transformation (WT) of the external input and the output signals. In the present paper, the impedance spectra were determined by performing WT on the charge curves of the LIB without an external input signal. A capacitive semicircle and Warburg impedance appeared in the impedance spectrum determined by the WT, which is roughly consistent with the impedance spectrum measured by a frequency response analyzer (FRA). In addition, the new impedance determination method was applied to the evaluation of deteriorated batteries. The diameter of the semicircle increased with fading the battery capacity. Curve fitting was applied to the spectra determined by the WT. The sum of the resistance related to electrode structure, R_c, and the charge transfer resistance, R_ct, increased as the capacity loss increased. Moreover, the parameter values were in good agreement with those calculated from the impedance measured using FRA, indicating that this new impedance determination method is useful for evaluation of deteriorated batteries. Furthermore, we emphasize that this method can diagnose the battery deterioration simply by charging the battery.