Phenomena that limit the capacity of the positive lead acid battery plates - II. Electrochemical impedance Spectroscopy and mechanism of discharge of the plate

This paper is the second part of an investigation whose purpose is to establish the mechanism of the processes that proceed during discharge and cycling of the lead dioxide plate of the lead acid battery (LAB). On the basis of this mechanism some electrical parameters that can show the state-of-charge (SOC) and state-of-health (SOH) have to be distinguished. In the first part of this study the shape of the charge potential transient was examined. In this part the electrochemical impedance spectra of a model lead dioxide electrode was investigated in charged and discharged states as well as the changes in impedance spectra during cycling of the electrode with high and low charge currents. It was established that during cycling the values of charge-transfer resistance (R-et), ion diffusion resistance (R-diff) in the reaction layer, resistance of the grid/PAM interface (R-i), and ohmic resistance of the electrode (R-Omega) change smoothly at first with cycling and after a critical number of cycles have been reached, they start to increase fast. This shows that the electrode is close to the end of its life. The above parameters can be used for the determination of SOH of the positive LAB plate. The discharge processes take place on active centers (ACs) in positive active material (PAM) and interface. A model of the structure of ACs as well as a scheme of the elementary reactions proceeding at discharge in the ACs is proposed. During cycling when the number of ACs in PAM decreases and this one in the interface increases, the capacity of the electrode decreases. On the basis of the changes in the impedance spectra at discharge and charge the changes in equivalent electrical circuit are discussed. (C) 2002 The Electrochemical Society.