Electrochemical impedance spectroscopy on lead-acid cells during aging

Kwiecien, Monika; Sauer, Dirk Uwe (Thesis advisor); Kowal, Julia (Thesis advisor)

Aachen : ISEA (2019)
Book, Dissertation / PhD Thesis

In: Aachener Beiträge des ISEA 132
Page(s)/Article-Nr.: 1 Online-Ressource (xii, 138 Seiten) : Illustrationen, Diagramme

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2019

Abstract

The diagnostics of the aging of lead-acid batteries is an important topic for applications in the automotive and stationary sector. Lead-acid batteries, especially as starter batteries, are still by far the most widely produced battery technology. The impedance of the battery is an easy parameter to determine, but in the past no clear dependencies between impedance change and ageing were found. This was due to a lack of control of the boundary conditions and failure to consider the dependencies of the impedance on state-of-charge, current rate and temperature. Furthermore, often only single impedance values were considered. In this thesis, electrochemical impedance spectroscopy (EIS) on lead-acid batteries was applied with regard to ageing estimation and the determination of parameters of electrochemical processes. Test cells were constructed with two positive and one negative electrode in order to obtain the state of the electrodes as homogeneous as possible. At the test cells two different ageing processes relevant for the lead-acid battery were forced; on the one hand the corrosion of the positive electrode and on the other hand the accumulation of the discharge product (lead sulphate) at the surface of the negative electrode (sulphation). During these tests, EIS measurements with superimposed charge and discharge currents were regularly carried out together with capacity tests. At the end of the tests the amount of corroded material and lead sulphate was determined. Due to the sulphation, characteristic changes in the spectrum of the negative electrode could be observed, while the spectrum of the positive electrode hardly changed. The changes could be related to the loss of capacity and the amount of accumulated lead sulphate in the electrodes. In contrast, no changes in the spectra were observed in the corrosion tests. In addition, it was simulatively investigated whether the parameter determination of electrochemical processes from EIS measurements is possible for porous electrodes. With an electrical equivalent circuit diagram consisting of several branches with locally different impedances to represent the current distribution on the electrodes, the total impedance for different excitations was calculated. Thus, the possibilities to calculate the active surface area of the electrode with the help of EIS were analyzed. The determined deviations of the surface area determination depend strongly on the variance of the local impedances. An exact determination of electrochemical parameters from EIS measurements on porous electrodes is not possible in the case of lead-acid batteries due to the pronounced current dependency of the charge-transfer process. Since a changing variance of the local impedance with ageing is to be expected, changes in the spectrum can not necessarily be attributed to changes in electrochemical processes. Qualitative statements about the ageing state of the battery can, however, be made with the aid of impedance, if the impedance is measured at several frequencies and under comparable conditions. In addition, characteristic changes of the impedance for the investigated ageing processes were found, which can be considered in diagnostic algorithms in order to detect not only the ageing state but also the dominant ageing process.