Interpretation and Modelling of the Electrochemical Impedance of LiFePO₄/Li₄Ti₅O₁₂Batteries

In this work, a comprehensive equivalent circuit model (ECM) was developed for the a commercial 26650 LiFePO4/Li4Ti5O12 cell in which the main processes at the anode and cathode electrodes and their contributions to the full-cell impedance were investigated and quantified separately. To unambiguously separate the anode and cathode processes, EIS spectra from Li4Ti5O12/Li and LiFePO4/Li half cells were modeled at different state of charge (SOCs) and SOC history and the fit results were used to develop the full-cell ECM. The distribution of relaxation times (DRT) analysis was used in all cases to better resolve the frequencies of the processes, and thereby help their accurate identification. The LiFePO4/Li4Ti5O12-cell impedance was found to be significantly dependant on the SOC history, i.e. it changes slightly only at low SOCs during charge, whereas, it changes significantly during discharge. The SOC-dependence of the full-cell impedance was found to be mainly due to the decrease of the LiFePO4 electrode's charge transfer resistance with SOC increase. While Li4Ti5O12 electrode processes did not show a significant change with SOC history, it was revealed that the SOC-histoty dependence of the LiFePO4/Li4Ti5O12 full cell was also mainly due to the variation of charge transfer frequency and resistance of LiFePO4 electrode.