Investigation of calendar ageing of lithium-ion battery through physical models with ex-situ validation

The estimation of the state of health of a lithium-ion battery is a topic of interest with the spread of battery electric vehicles. According to the desired long lifetime, calendar ageing is a matter of concern due to the known deterioration effect of mid-high environmental temperatures, even during parking periods. In this work, the combined use of an optimised sequence of tests and physical models is applied to investigate degradation due to calendar ageing on a commercial NMC + LMO|Graphite cell type. A calendar ageing campaign on 12 samples is carried out with periodic interruptions for characterisation. This method, already applied to a low-temperature charging campaign, proves its suitability in identifying ageing mechanisms and reproducing the performance of aged cells. SEI layer growth with significant consumption of lithium and electrolyte material is the dominant phenomenon, but mass-transport limitations arise from the positive electrode, too. Physical model parameter identification is challenged with verification measurements like tests on coin cell configuration and microscopies, resulting in compatibility. This work further verifies the suitability of such a methodology for the degradation of lithium-ion batteries.