Analysis of ac impedance of AgO-Zn cells: effects of state-of-charge, temperature and cycle-life

Abstract

The ac impedance of commercial silver oxide-zinc cells has been studied in the temperature range −20 to 40 °C at various state-of-charge (SOC) values in conjunction with the effect of charge–discharge cycling. The impedance data in the Nyquist form comprise an inductive part in the frequency region 100–10 kHz, and a capacitive semicircle in the frequency region 10 kHz–10 mHz in the SOC range between 0 and 0.4, or two capacitive semicircles at SOC ≥ 0.4. The impedance parameters have been evaluated by analysis of the data using an equivalent circuit and a non-linear least squares fitting procedure. The inductance has been found to be independent of both temperature and SOC of the cell. The resistance values corresponding to the semicircles of the Nyquist impedance plot, which are attributed to the positive electrode, show a strong dependence on the SOC of the cell. The energy of activation for the charge-transfer reaction has been calculated from the temperature dependence studies. There is a decline in the capacity values of the cells on cycling, while the constant phase elements are found to increase with increasing cycle number. The scanning electron micrographs reveal an increase in particle size of the negative electrode on cycling, which causes the performance of AgO-Zn cells to deteriorate gradually.