Quantitative Analysis of Time-Domain Supported Electrochemical Impedance Spectroscopy Data of Li-Ion Batteries: Reliable Activation Energy Determination at Low Frequencies

For an accurate characterization of transport and mobility processes in batteries using electrochemical impedance spectroscopy (EIS), a large frequency range of up to ten decades must be covered. It is experimentally demonstrated, for the first time, that the phase of the impedance measurements converges in the sub-millihertz range, which permits a reliable quantification of diffusion kinetics. To avoid a considerable change of the state of charge (SOC) of the battery and to mitigate the very long measurement times caused by standard EIS, a combination of EIS and time domain measurements, the time-domain supported electrochemical impedance spectroscopy (TD-EIS), is employed. To ensure an utmost comparability and reproducibility of the results with minimum influence of the cell fabrication, the method is demonstrated using three equivalent, industrially manufactured lithium-ion pouch cells at varying temperatures. The obtained impedance data were fitted by an electrical equivalent circuit battery model for an accurate estimate of charge transfer resistance and, in particular, also solid-state diffusion rate. Both processes follow an Arrhenius law, allowing the determination of activation energies with small variance. The obtained results are within the range of literature values measured for similar systems. The relevance of very low frequency impedance data for accurate fitting of mobility parameters in batteries is discussed.