Aging Behavior Beyond SOH 80: An Experimental Aging Study on Commercial Lithium–Ion Batteries with Different Cathode Materials: Capacity Loss, Resistance Change and Impedance Modeling

New insights into lithium–ion battery aging behavior beyond a state of health of 80%, as well as for three different aging diagnostics and modeling methods, are obtained through this study conducted on four different cell types. Commercial cylindrical cells of type 21,700 are subjected to calendar aging and cyclic aging with different parameters in a long-term study. The impact of the aging parameters on the four different cathode materials assessed (lithium–nickel–manganese cobalt oxide (NMC), lithium–nickel–cobalt–aluminum oxide high energy (NCA-HE), lithium–nickel–cobalt–aluminum oxide high power, and lithium–iron phosphate oxide (LFP)) is examined with a portfolio of diagnostic methods: capacity test, Ohmic resistance test and the core of this study, impedance analysis, together with sophisticated equivalent circuit modeling (ECM). It was found that the NMC cell degraded fastest under all aging conditions, the most durable was in most cases the NCA-HE cell. Only for one cyclic aging procedure, the LFP cells performs the best. The diagnostics shows that quantitative analysis of Nyquist plots is not sufficient for aging tracking and that some aging effects can only be detected by pulse discharging test. Moreover, capacity and mean Ohmic resistance deduced from electrochemical impedance spectroscopy test allow extrapolation to forecast further aging under diverse aging conditions.