The degradation and ageing of Li-ion batteries will in many cases contribute to reduced thermal stability which potentially affects the safety performance of the batteries. The fact that aging of lithium-ion cells leads to a reduced capacity and cell life, is extensively covered in the literature by several research groups, e.g. Vetter[1]. The safety effects of ageing are far less studied, with only a handful of empirical studies published, e.g. [2-4].

This poster presents ageing data and safety aspects of large commercial Li-ion cells. The cells have been aged and cycled at 5, 25 and 45 °C. Several diagnostic tools have been applied to characterise the ageing mechanisms. These tools include high power pulse characterisation, entropy spectroscopy, incremental capacity analysis and impedance spectroscopy.

The ageing mechanisms are however different at low and high temperatures and this will affect the thermal stability of the aged cells. The thermal stability of cells which have been aged with different ageing mechanisms was characterized with an Accelerated Rate Calorimeter (ARC). It was e.g. observed that a cell cycled at 5 °C for 3000 cycles reaching 70% State-of-Health showed a reduced thermal runaway limit from 240 to 150 °C compared to the uncycled cell. This was also lower than the exotherm onset temperature for the uncycled cell.

References

1. Vetter, J., et al., Ageing mechanisms in lithium-ion batteries. Journal of Power Sources, 2005. 147(1-2): p. 269-281.
2. Fleischhammer, M., et al., Interaction of cyclic ageing at high-rate and low temperatures and safety in lithium-ion batteries. Journal of Power Sources, 2015. 274: p. 432-439.
3. Gilljam, M., et al., 7E. Effect of electrical energy and aging on cell safety, in Safety of Lithium Batteries, J. Garche and K. Brandt, Editors. 2017, Elsevier.
4. Friesen, A., et al., Influence of temperature on the aging behavior of 18650-type lithium ion cells: A comprehensive approach combining electrochemical characterization and post-mortem analysis. Journal of Power Sources, 2017. 342: p. 88-97.