An impedance model based on a transmission line circuit and a frequency dispersion Warburg component for the study of EIS in Li-ion batteries

In this study, an impedance model represented as an equivalent electrical circuit (EEC) and comprised of a transmission line circuit and a frequency dispersion Warburg component is developed for the study of the electrochemical impedance spectroscopy (EIS) of Li-ion batteries. The model considers the impedance response of a porous cathode electrode comprised of solid particles surrounded by solution. Theory of diffusion of charge carriers in the solid and solution phases of the cathode composite electrode has been considered in the impedance model. The simulated impedance response predicted by the impedance model is compared with EIS measurements carried out in a LiPo battery pack. In addition, a theoretical comparison between the simulated EIS response from the developed model with the simulated impedance response of an EEC representing a Li-ion battery and reported in the literature is carried out as well. The effect of increasing and decreasing the cathodic diffusion distance of lithium ions across the solution and solid phases on the Nyquist impedance complex-plot is simulated. The impedance model is able to decouple diffusion mechanisms in the solid and solution phases of the cathode electrode represented at different frequencies of the Nyquist complex-plot. The results show that the impedance model can provide an insight into the different electrochemical mechanisms of the Li-ion battery represented in the Nyquist complex impedance plot at different frequencies. The model could assist the understanding of the different phenomenological processes in the electrode during the decrease of state of charge of Li-ion batteries using EIS.