Determination of state-of-discharge of zinc-silver oxide button cells. III. In situ impedance measurements of each electrode

Abstract

The ac impedance of each electrode of Zn-Ag20 button cells (30–50 mA h) has been measured over an extended range of frequencies using a Hg/HgO reference electrode located in a small hole drilled through the positive case terminal. The high frequency impedance spectrum of the Ag2O cathode is a straight line with a 22.5° slope typical for diffusion at a porous electrode. The low frequency end exhibits a 45° sloped straight line characteristic of diffusion processes at a planar electrode. The deposition process is fast and hence the change transfer resistance is usually not clearly evident in the complex plane impedance plot. The impedance response of the zinc anode shows a capacitive loop at high frequencies and some inductive effects characteristic of adsorption processes. At low frequency the complex plane impedance plot of the total cell is a straight line of slope close to 45° mainly ascribed to the Ag20 cathode. At high frequencies equally important contributions from the two electrodes are evident. The main change in impedance which results from discharge is the decrease of the characteristic relaxation frequency of the high frequency capacitive loop of the zinc anode. The determination of the state-of-discharge at a frequency higher than 1 Hz is best realized if (i) the characteristic relaxation frequency of the high frequency capacitive loop occurring at the zinc anode decreases with discharge and (ii) the response of the Ag2O cathode is quasi-linear over the entire frequency range. Under these conditions the characteristic relaxation frequency at the zinc anode or some related parameters can be clearly seen on the spectrum measured at the two terminals and used as a state-of-discharge indicator.