Fast Charging of Li-Ion Batteries in Extreme Cold

Fast charging is an enabling technology for electric vehicles (EV). Unfortunately lithium-ion batteries are notoriously incapable of fast charging at subzero temperatures,^1-3 thereby hindering EVs from dependable and convenient operation under all weather conditions. In this work we describe a novel battery structure, called all-climate battery (ACB), which adds a resistor sheet inside the electrochemical cell to modulate the cell’s internal resistance according to temperature. Based on ACB technology, we have built 12Ah pouch cells using graphite anode, NMC cathode and 1M LiPF₆ in EC:EMC (3:7 by wt) + 2% VC and showed that such ACB cells are able to be charged from 20% to 90% state of charge (SOC) in -30^oC environment in ~22 minutes, while keeping the cell voltage never exceeding 4.2V (a necessary condition to avoid Li plating).

Figure 1 displays the experimental results of fast charging a 12Ah ACB cell from -30^oC with a charge protocol of CV (4.2V) and limited by CC (3C or 36A). It is seen that the charge current under 4.2V charge voltage starts only at ~7A; however, it rapidly increases to 36A (3C limit) in around 400 seconds. Simultaneously the cell outer surface temperature increases from the initial temperature of -30^oC to above the freezing point. Thereafter, the ACB cell undergoes 3C constant current charging with the cell voltage dips below 4.2V between 400 and 1100 seconds. During this period, the cell SOC rapidly increases from the initial SOC of 20%, eventually reaching 90% in ~22 min. Figure 2 compares two charge experiments under the same charge protocols of a 12Ah ACB cell and its baseline cell without ACB technology. It is clearly shown that ACB technology has dramatically improved the fast charge capability of Li-ion batteries.

More efforts are ongoing to accelerate the fast charge process from subzero temperatures and to further reduce the charge time from 22 min to within 15 min. These advanced improvements will be elaborated in this presentation. 

References:

1. S. S. Zhang, K. Xu, and T. R. Jow, Electrochem. Commun., 4, 928 (2002).

2. M. C. Smart, J. F. Whitacre, B. V. Ratnakumar, and K. Amine, J. Power Sources, 168, 501 (2007).

3. Y. Ji, Y. Zhang, and C.-Y. Wang, J. Electrochem. Soc., 160, A636 (2013).