Li-ion batteries degrade during their usage (cyclic aging) and during storage (calendar aging). The reasons for degradation on the cell level are aging mechanisms on the material and electrode level which are accessible by Post-Mortem analysis. However, even after extensive usage of batteries and a reduction of the capacity to 70-80 % of the initial capacity, batteries are still useful. The sustainability and resource independence can be improved strongly by re-using batteries from a 1st life application (e.g. battery electric vehicle) in 2nd life applications (e.g. home storage system) before recycling. High safety levels are mandatory for re-using batteries in 2nd life applications. However, safety tests are usually only performed on new cells.
In our extensive study with commercial 18650 cells (graphite | NCA) we evaluate the safety behavior of aged cells with differently known aging mechanisms with respect to new cells. Aging was carried out at 0°C and at 45°C to provoke the aging mechanisms of lithium plating and other side reactions without lithium plating, respectively. The new and aged cells were subjected to Post-Mortem analysis by electrochemical analysis and physico-chemical methods (SEM, EDX, GD-OES) to be sure about the aging mechanisms. Additionally, another type of commercial 18650 cells were analyzed to show the difference in the main aging mechanism of the anode in case of Si-graphite anodes.
In literature, many groups focus on accelerating rate calorimetry (ARC) for safety analysis. In our study, we employed electrical (overcurrent, overcharge, overdischarge), mechanical (nail penetration) and thermal (ARC coupled with online mass-spectrometry) abuse tests with the commercial cells with NCA cathodes. Our approach not only allows the comparison of different safety test triggers, but also the impact of different aging mechanisms on safety.
Most interestingly, our results clearly show that the aging mechanism of lithium plating leads to increased hazard levels, whereas aged cells without lithium plating often show a safer behavior compared with new cells. Furthermore, lithium plating often leads to strongly increased aging rates. Therefore, the critical aging mechanism of lithium plating must be avoided in 1st life applications for long life time and for safe use in 2nd life applications. Finally, we show workflows for detecting and avoiding lithium plating and finding appropriate operating conditions [2] and methods based on sensors and electrochemical methods for early detection of safety critical events. Our results are a valuable contribution for safely re-using Li-ion batteries in 2nd life applications which have the potential to increase sustainability, reduce costs, and reduce the dependence on critical raw materials.

References
[1] M. Wohlfahrt-Mehrens, T. Waldmann, Aging Mechanisms and Lifetime Predictions. In: Garche, J. (eds.) Encyclopedia of Electrochemical Power Sources, 2nd Edition. vol. 4, pp. 412-425. UK: Elsevier. https://dx.doi.org/10.1016/B978-0-323-96022-9.00307-8
[2] T. Waldmann, C. Hogrefe, M. Flügel, I. Pivarníková, C. Weisenberger, E. Delz, M. Bolsinger, L. Boveleth, N. Paul, M. Kasper, M. Feinauer, R. Schäfer, K. Bischof, T. Danner, V. Knoblauch, P. Müller-Buschbaum, R. Gilles, A. Latz, M. Hölzle and M. Wohlfahrt-Mehrens, J. Electrochem. Soc., 171(7), 70526 (2024). https://doi.org/10.1149/1945-7111/ad5ef8

Acknowledgment
Funding of the projects CIRCULUS (03ETE035F) by the German Federal Ministry of Economic Affairs and Climate Action (BMWK) and AnaLiBa (03XP0347C) within the AQua-Cluster by the German Federal Ministry of Education and Research (BMBF) and are gratefully acknowledged.