Non-invasive investigation of predominant processes in the impedance spectra of high energy lithium-ion batteries with Nickel-Cobalt-Aluminum cathodes

Analysis and prediction of the state of health (SOH) of batteries in the electromobility sector is of supreme importance. In this regard, non-invasive techniques present unique opportunities. Electrochemical impedance spectroscopy (EIS) allows detailed investigations of predominant processes at both electrodes. However, a proper understanding of the impedance spectra is crucial, especially for the assignment of the major phenomena to their corresponding electrodes. In this work, impedance spectra of a high energy lithium-ion battery (Panasonic NCR18650B: NCA vs. graphite) are investigated. Half-cells and commercial cells are measured by means of EIS and analyzed using the distribution of relaxation times (DRT) and an equivalent circuit model (ECM). Additionally, differential voltage analyses (DVA) are performed, activation energies are calculated and resistances at two different SOH are compared to each other. It is concluded that the predominant processes are the charge transfers of the two electrodes. While at low SOC the NCA's charge transfer dominates, at high SOC the graphite's charge transfer dominates. Impedance spectra analysis at different SOH with the aim of investigating the aging processes of both electrodes separately proved that the electrodes show severe degradations at a SOH of about C/C-0 = 70%.