Investigation of degradation mechanisms in lithium-ion batteries by incremental open-circuit-voltage characterization and impedance spectra

Non-invasive investigation methods of lithium-ion batteries (LIBs) are essential for online diagnosis and identification of degradation mechanisms and thus enable optimization of the operating conditions and cell materials. In this study, nine large format pouch cells with a nickel-manganese-cobalt-oxide (NMC) cathode were cyclic aged with varying average state of charge (SOC) and charging current. The loss of lithium inventory (LLI) and of active material (LAM) of the anode were identified by evaluating the electrochemical impedance spectroscopy (EIS) and the open circuit voltage (OCV). By analyzing the distribution of relaxation times (DRT), the relevant electrochemical processes were identified and their polarization and time constant determined. The parameters of some processes changed unexpectedly during aging, which challenged the assignment of the processes. The analysis of the high-resolution pulse data obtained by the incremental OCV measurement revealed changes in the balancing of the electrodes due to LLI as the reason. Additionally, LLI was indicated as the main aging mechanism and LAM of the cathode was quantified. Finally, it was found that the pulse resistances in certain SOC ranges correlate strongly with the capacity as long as LLI limits the capacity. It was shown that high-resolution pulse data should be considered when evaluating EIS. Furthermore, the pulse resistances provide complementary information about the aging processes and can allow a quick estimation of the capacity. An incremental OCV measurement combined with EIS as non-invasive techniques has therefore proven to be advantageous.