Comparison of activation energies of laminated lithium-ion secondary cell using LiCoO2 and LiMn2O4 as cathode material by AC impedance method

Abstract

In this study, laminated lithium-ion cells were prepared using LiCoO2 or LiMn2O4 for the cathode and graphite carbon for the anode. The electrochemical impedance spectroscopy (EIS) measurements of these cells at elevated temperature (25 °C−60 °C) were carried out to determine the activation energy of the charge/discharge, which is determined from the electrochemical parameters based on an equivalent circuit. Two semicircles were observed at almost all temperatures and the state of charge (SOC) range. For the high frequency semicircle, the activation energy behavior is similar between the LiCoO2-based cell and LiMn2O4-based cell, the highest value was observed at 25% SOC, and the lowest value was observed at 0 and 100% SOCs. On the other hand, for the low frequency semicircle, the behavior of the LiCoO2-based cell is different from that of the LiMn2O4-based cell. The highest value was observed at 50% SOC for the LiCoO2-based cell, and the value was almost the same over the entire SOC range for the LiMn2O4-based cell. At a low frequency, the activation energy of the LiCoO2-based cell was higher than that of the LiMn2O4-based cell over the entire SOC range. Therefore, because cells used for EIS measurements consisted of the same materials except for the cathode material, it is strongly suggested that the semicircle in the high frequency region is mostly caused by the anode reaction and the semicircle in the low frequency region is predominantly caused by the cathode reaction.