Impedance analysis of porous electrode structures in batteries and fuel cells

Today technical electrodes in batteries and fuel cells rely on complex multiphase microstructures that facilitate electronic, ionic and, in case of fuel cells, diffusive gas transport to the active reaction sites distributed in the electrode volume. The impedance of such electrodes can be described by the well-established transmission line model (TLM) approach. In a TLM, transport, charge transfer phenomena and capacitive effects are coupled considering microstructural features of the electrode. Its application for impedance data analysis of technical cells is challenging as the TLM impedance extends over a wide frequency range and quite often a strong overlapping with other contributions takes place. In this paper the application of the distribution of relaxation times (DRT) to the analysis of technical electrodes in batteries and fuel cells is elucidated. Different examples how to apply the DRT to analyze impedance spectra of solid oxide-, polymer electrolyte- and lithium ion-cells will be discussed. It will be shown that the TLM is usually represented by multiple peaks in the DRT, which might be strongly affected if contributions of different electrode layers overlap in the spectra. Related error sources and countermeasures are illustrated. Approaches how the DRT can be applied for the analysis of measured spectra and how it is able to support CNLS-fitting are presented.