An AC Impedance Study of the Degradation of Porous Nickel Battery Electrodes

AC impedance spectra of porous nickel battery electrodes were recorded periodically during charge/discharge cycling in concentrated solution at various temperatures. A transmission line model (TLM) was adopted to represent the impedance of the porous electrodes, and various model parameters were adjusted in a curve fitting routine to reproduce the experimental impedances. Degradation processes were deduced from changes in model parameters with electrode cycling time. In developing the TLM, impedance spectra of planar (nonporous) electrodes were used to represent the pore wall and backing plate interfacial impedances. These data were measured over a range of potentials and temperatures, and an equivalent circuit model was adopted to represent the planar electrode data. Cyclic voltammetry was used to study the characteristics of the oxygen evolution reaction on planar nickel electrodes during charging, since oxygen evolution can affect battery electrode charging efficiency and ultimately electrode cycle life if the overpotential for oxygen evolution is sufficiently low. Transmission line modeling results suggest that porous rolled and bonded nickel electrodes undergo restructuring during charge/discharge cycling prior to failure. The average pore length and the number of active pores decreases during cycling, while the average solid phase resistivity increases. The average solution phase resistivity remains relatively constant during cycling, and the total porous electrode impedance is relatively insensitive to the solution/backing plate interfacial impedance.