Ageing in Commercial Li-ion Batteries: Lifetime Testing and Modelling for Electrified Vehicle Applications

Sammanfattning: In this thesis, ageing in a commercial pouch cell for vehicle application is investigated through lifetime testing and modelling. The lifetime tests investigate the impact of temperature, current, depth of discharge (DOD) and state of charge (SOC). Results from lifetime tests are used in the development of an empirical ageing model, to study the ageing as a result of different user cases in a vehicle application. The results showed that the battery lifetime in a vehicle application could be prolonged, without interfering with the driving itself, by better planning of the charging. Lifetime test results and ageing analysis of the tested cells are used as guidance for the development of a physics-based ageing model. The model includes capacity loss and resistance increase due to resistive film formation and loss of active material. The lifetime test results showed that for the studied cell, staying below 40% SOC level will improve the lifetime considerably. This was seen in both the calendar ageing tests and the cycling ageing tests. The lifetime tests performed in small SOC intervals at different SOC levels showed that the ageing is separated into two groups, with more pronounced ageing at high SOC and less ageing at low SOC.   The main ageing contribution is the growth of Solid Electrolyte Interphase (SEI), which is also the case seen in the model when using parameters for the SEI growth extracted from calendar ageing data. The more pronounced ageing at higher SOC levels can partly be explained by the SEI growth that is increased at higher SOC level and partly from the contribution of manganese dissolution. This was confirmed by Post Mortem (PM) analysis and successfully captured by the model.