Investigation of the low frequency Warburg impedance of Li-ion cells by frequency domain measurements

In this work we investigate the Warburg impedance of Li-ion cells at very low frequencies down to 0.1 mHz. Three different Li-ion cells are used, two 18650-type and one pouch cell of 80 Ah. The measurements are conducted with electrochemical impedance spectroscopy (EIS). The open circuit voltage is measured separately to enable a correction due to voltage hysteresis effects. It is found that the Warburg impedance does not only dependent on the state of charge (SoC) but also on the SoC setting current, the rest time after SoC setting, the sinusoidal excitation amplitude and the charge and discharge history of the cell. It is assumed that the reason for these dependencies are the hysteresis of the open circuit voltage as well as the hysteresis of the cell thickness.