Extreme fast charging (XFC) capability is important for wider adoption of battery electric vehicles (1). However, barriers to XFC still exist, including rapid heat generation and potential overheating of Li-ion batteries. Both life and safety of Li-ion batteries could be jeopardized by overheating (2-4). Moreover, temperature distributions in individual Li-ion cells would be non-uniform during XFC (5) due to low thermal conductivity of Li-ion cell materials (6, 7). The non-uniform temperature distribution could then lead to non-uniform degradation and underestimate of overheating by monitoring cell surface temperature. Indeed, a recent numerical modeling study (5) predicts overheating and non-uniform temperature distributions in Li-ion cells during XFC.

In this presentation we will report characterization of Li-ion cell behaviors during XFC through in situ measurement of temperature distributions. Figure 1 shows representative results of a 2Ah Li-ion cell during 1C, 3C, 5C and 7C constant current charging followed by 4.2V constant voltage charging. It can be seen that all temperatures increase much more rapidly during 7C charging than during 1C charging. It can be also seen that the difference between internal core temperature and external surface temperature during 7C charging is much larger than that during 1C charging. The results suggest overheating and obvious non-uniform temperature distributions during XFC even for a small Li-ion cell with capacity of 2 Ah and thickness of only 4 mm.

More results, including heat generation rate, voltage behaviors, effects of cooling, and degradation will be presented and discussed.

Figure 1. Rise of local temperatures in a 2 Ah Li-ion cell during 1C, 3C, 5C and 7C constant current charging followed by 4.2V constant voltage charging with forced convection air cooling