Modelling the degradation of Li-ion batteries in mobility and stationary applications based on operational stress factors

The electric mobility heavily relies on lithium-ion batteries due to their high energy density. The limited lifetime of these batteries is still a challenge. To estimate and optimize the lifetime of a battery for different use cases, empirical degradation models are utilized. This study presents an empirical lifetime model for NMC and LFP battery chemistries. The model considers both calendar and cyclic capacity fading and total degradation is their cumulative sum. The calendar lifetime model consists of temperature, state of charge (SoC) and time stress factors. Cyclic degradation is considered due to five stress factors. These stress factors are temperature, depth of discharge (DoD), mean state of charge (mSoC), and charging and discharging C-rates. The model is parametrized for both NMC and LFP chemistry using published lifetime testing data. Unlike other studies that are based on the lifetime testing data of a few cells from a single manufacturer, this study parameterized the model with a huge dataset of lifetime experiments collected from the published literature. The collected dataset consists of 413 calendar and 650 cyclic lifetime tests. Due to the wide range of datasets, the model is applicable to estimate the degradation in diverse operational conditions for multiple use cases. The study also discussed and compared the parametrized lifetime functions of all stress factors for both chemistries and highlighted better chemistry under different operational conditions. This analysis revealed that the calendar degradation rate is less sensitive to SoC variation in LFP chemistry compared to NMC. In the case of storage temperature, the NMC chemistry showed lower degradation rate. Cyclic lifetime functions revealed that LFP chemistry is better for deep cyclic operations and under high charging C-rates than NMC. It was also found that the optimal operational temperature of LFP chemistry is lower than NMC chemistry and discharging C-rate degradation is almost identical for both cases. The study includes a case study of an electric long-haul truck in which the life of the battery is estimated for a journey between two Finnish cities.