Temperature dependence studies of a.c. impedance of lithium-ion cells

The a.c. impedance behaviour of a commercial sealed Li-ion cell is studied in the temperature range -10 to 40 degreesC at various state-of-charge (SOC) values. The data comprise an inductive part in the frequency region of 100 kHz-100 Hz, and two capacitive parts in the frequency region of 100 Hz-10 mHz. The data are analysed using an equivalent circuit and a nonlinear least square fitting procedure, and the impedance parameters are evaluated. The inductance is found to be independent of temperature and SOC of the cell. The resistance corresponding to the high frequency semicircle of the Nyquist impedance plot, which is attributed to the surface film on the electrodes, shows a weak dependence on the SOC of the cell. On the other hand, it exhibits Arrhenius dependence on temperature. A value of 0.4 +/- 0.05 eV is obtained for activation energy of charge-transport in the surface film. The resistance corresponding to the low frequency semicircle is due to the electron-transfer reactions, and exhibits a strong dependence on the SOC and also temperature. The data are analysed using the Butler-Volmer kinetic equation for activation controlled charge-transfer reaction, and the kinetic parameters, namely, the apparent exchange current, transfer coefficient and activation energy of the cell reaction are evaluated.