Range anxiety is one of the most significant barriers to mainstream adoption of electric vehicles (EVs). An effective solution to range anxiety is increasing energy density of EV batteries combined with fast charging. Energy density of EV batteries has advanced significantly over the past few years, from 150 Wh/kg in 2012 to ~220 Wh/kg of state-of-the-art cells, and is targeting >300 Wh/kg by 2020. A practical approach to higher energy density of Li-ion cells is to increase areal loading of electrode active materials. Two critical challenges, however, arise. One is that cycle life of Li-ion cells drops significantly with increasing loading amount. The other is that the ability of fast charging is drastically limited with increasing electrode loading. Gallagher et al. (1) tested a sets of graphite/NMC622 cells with different areal loadings, and found that cells with higher areal loadings have much poorer cycle life. Furthermore, though the cells were cycled at room temperature with only 1C charge rate, a significant amount of lithium deposit was observed in cells with higher anode loading, indicating serious lithium plating in these high energy cells. These results differ greatly from those in the literature based on high power cells. Aging of high power cells is typically believed to be dominated by growth of solid-electrolyte interphase (SEI), and lithium plating is expected to occur only at harsh charge conditions (e.g. low temperature, high charge rate). To date, research on aging behavior of high energy Li-ion cells is still rather limited (2).
In this talk, we will give a comprehensive discussion on the aging characteristics of high energy Li-ion batteries for next generation EVs, with special focus on fast charging conditions @2-3C rate. An aging model will be introduced which incorporates both SEI growth and lithium plating. As such, cell aging associated with SEI growth and with lithium plating can be distinguished. The model is applied to predict the cycle life of Li-ion cells at various operating conditions. Specially, we will focus on the effects of a) areal loading density, b) operating temperature, and c) charge rate on the cycle life of Li-ion cells.
1. K. G. Gallagher, S. E. Trask, C. Bauer, T. Woehrle, S. F. Lux, M. Tschech, P. Lamp, B. J. Polzin, S. Ha, B. Long, Q. Wu, W. Lu, D. W. Dees and A. N. Jansen, J. Electrochem. Soc., 163, A138 (2016).
2. Y. Leng, S. Ge, D. Marple, X.G. Yang, C. Bauer, P. Lamp and C.Y. Wang, J. Electrochem. Soc., 164, A1037 (2017).