Lithium ion Battery Aging: Battery Lifetime Testing and Physics-based Modeling for Electric Vehicle Applications

Sammanfattning: Electrification of vehicles is one solution in reducing emissions from the transport sector, and in particular to limit tail-pipe emissions in cities. In order to make the electrification successful and cost effective, there is a high demand for longer drive range and longer battery lifetime. By improving the understanding for the underlying aging mechanisms of vehicle lithium ion batteries, the utilization can be improved and thus cost effective. This thesis contributes with an extensive test matrix for lifetime testing, including calendar and cycling aging, on large commercial cells for automotive applications. The test matrix is constructed to investigate the effect of temperature, C-rate, SOC level, test procedure and DOD. A physics-based model including SEI formation, film resistance increase and loss of electrode volume fraction as a function of number of cycles is constructed. Aging parameters were determined through parameterizing the model using calibration experiments. Lifetime cycling data show that the expected temperature dependence can not be seen for 10% DOD for SOC levels below 50% SOC, it is first for SOC levels higher than 60% that this is observed. Also SOC levels higher than 30% is accelerating the aging, while SOC levels below this show very low aging effects, however, when using cycling in the whole SOC interval, the classical decreased lifetime with higher temperatures were obtained. For cells tested in different DODs, but with the same cut-off voltage, longer lifetime was observed for the small DOD, although the initial aging was similar up to 1200 FCE. After that, the aging decelerated for the small DODs while it accelerated for the large DODs.