Safety-Critical Influence of Ageing on Mechanical Properties of Lithium-Ion Pouch Cells

While the effect of ageing has been thoroughly analysed, to improve the cycle life of lithium-ion batteries, its impact on safety in case of a mechanical loading is still a new field of research. It has to be found out how mechanical properties, such as the tolerable failure force or deformation, change over the operational lifetime of a battery. To answer this question, mechanical abuse tests were carried out with pouch cells used in recent electric vehicles in a fresh state and after usage over 160.000 km. These tests were complemented with a detailed component level analysis, in order to identify mechanisms that lead to changed cell behaviour. For the analysed aged cells, a significantly different mechanical response was observed in comparison with the respective fresh samples. The tolerable force was severely reduced (up to −27%), accompanied by a notable reduction in the allowable deformation level (up to −15%) prior to failure, making the aged cells clearly more safety critical. Based on the subsequent component tests, the predominant mechanism for this different behaviour was concluded to be particle cracking in the cathode active material. The found results are partly in contrast with the (few) other already published works. It is, however, unclear if this difference is rooted in different cell chemistries or types, or another battery state resulting from varying ageing procedures. This underlies the importance of further investigations in this research field to close the apparent gap of knowledge.