Influence of temperature dependent aging mechanism on the safety behavior of Li-ion pouch cells

Compared to many other characteristics of Li-ion batteries, the safety behavior is a critical property and should be examined carefully. However, the safety behavior can drastically change dependent on the prior aging conditions such as ambient temperature [1-4]. The temperature dependent aging behavior of commercial pouch cells is evaluated using an Arrhenius plot. Post-Mortem analysis of aged cells from both branches of the V-shaped Arrhenius plot confirmed two dominating aging mechanisms: Li plating and SEI growth for low and high temperature range, respectively.

Heat-wait-seek (HWS) tests under quasi-adiabatic conditions in an accelerated rate calorimeter (ARC) are a well-known method to evaluate the safety behavior of Li-ion batteries and can reveal critical temperatures such as the onset of self-heating (SH). In addition to the temperature measurement, multiple sensors for voltage, resistance, strain, and ultrasonic transmission were placed on the pouch cell for a better understanding of the ongoing processes while the HWS tests. Differential scanning calorimetry and thermal gravimetric analysis was performed for the passive cell components such as electrolyte, separator, and pouch foil for an enhanced understanding of the processes on the way to the thermal runaway (TR).

HWS tests on cells aged at different temperatures (0 °C – 50 °C) until different state-of-health (SOH) reveal the trend that a more pronounced aging increases or decreases the onset of self-heating depending on whether SEI growth or Li plating is the dominating aging mechanism. Especially low temperature aged cells with Li plating show a drastically decreased safety behavior due to violent reactions of the electrolyte with Li. Thereby, the amount of plated Li is more crucial than the exact aging temperature for safety. In contrast, cells aged at higher temperatures with a distinct solid-electrolyte-interface (SEI) layer on the anode show an increased onset of self-heating leading to improved safety due to a more stable SEI. Especially, higher aging temperatures and higher number of cycles, both lead to an increased safety.

Thus, the change in dominating aging mechanism observed by the Arrhenius plot correlates with the change in safety at about 25 °C. As the cell aging mainly happens on the anode side, other critical events such as cell swelling, venting, separator melting, or TR are only minor influenced by aging. These results justify, that new cells are sufficient to estimate the minimum safety over the entire battery life, if Li plating can be excluded.

Acknowledgment

We gratefully acknowledge the German Federal Ministry of Education and Research (BMBF) for the financial support of the projects AnaLiBa (03XP0347C) and MiCha (03XP0317C) within the AQua cluster.

References

[1] M. Feinauer, A.A. Abd_El_Latif, P. Sichler, M. Wohlfahrt_Mehrens, T. Waldmann, Journal of Power Sources 570 (2023) 233046.

[2] T. Waldmann, M. Wohlfahrt-Mehrens, Electrochimica Acta 230 (2017) 454–460.

[3] M. Börner, A. Friesen, M. Grützke, Y.P. Stenzel, G. Brunklaus, J. Haetge, S. Nowak, F.M. Schappacher, M. Winter, Journal of Power Sources 342 (2017) 382–392.

[4] D. Ren, H. Hsu, R. Li, X. Feng, D. Guo, X. Han, L. Lu, X. He, S. Gao, J. Hou, Y. Li, Y. Wang, M. Ouyang, eTransportation 2 (2019) 100034.