State-of-Health Estimation of Li-ion Batteries: Cycle Life Test Methods

Abstract: Despite a rapid development, cost, performance and durability of the energy storage system are still a hindrance for a wide commercialisation of heavy-duty hybrid electric vehicles (HEV). The purpose of the work presented in this thesis is to investigate how different load cycle properties affect the cycle life and ageing processes of Li-ion cells developed for use in HEVs. The cycle life of commercial LiFePO4/graphite Li-ion cells was tested using a range of operating conditions and battery load cycles based on conditions relevant to heavy-duty HEVs. Established methods for cell performance evaluation have been combined with half-cell measurements and analysis methods such as incremental capacity analysis, differential voltage analysis and impedance spectroscopy to characterise the cell ageing in terms of capacity fade, power fade and impedance rise. Furthermore, a simplified cell fade model is used to distinguish between different likely ageing mechanisms. Loss of cyclable lithium is found to be the main contribution to ageing during the first phase of cycling, followed by an accelerated loss of active anode material towards the end of the battery cycle life. The longest lifetime is observed for cells cycled with low peak currents and a narrow SOC range. In addition, high charge current is found to affect the cycle life profoundly. On the contrary, a moderate temperature increase did not result in a shorter cycle life. Despite similarities in average current and SOC range, the load cycle properties are found to have a significant effect on the ageing characteristics, indicating that a more detailed evaluation of load cycle properties is needed to enable a cycle life estimation model.