Mitigation of Accelerated Degradation of Li-ion BatteriesvFor High Rate Discharge Application 2020-01-0452
The U.S. Army has been pursuing vehicle electrification to achieve enhanced combat effectiveness. The benefits include new capabilities that require high power pulse duty cycles. However as the vehicle platform size decreases, the Energy Storage System (ESS) pulse power discharge rates ( > 40 C rate) can be significantly greater than commercial Hybrid Electric Vehicle ESS, resulting in significantly lower ESS lifetimes. Results of high power pulse duty cycles on lithium iron phosphate cell lifetime performance show a dramatic loss. For a 2 s and 3 s pulse duration tests, the observed degradation is 22 % and 32 % respectively. Although these cells were thermally managed in a convective chamber at 10℃, the 2 s pulse showed a 31℃ temperature rise and the 3 s pulse, a 48℃ temperature rise. The decreased lifetime is as a result of chemical degradation due to the increased temperature, and potentially, mechanical degradation due to pulse induced diffusion stresses. Both of these mechanisms lead to lithium loss and eventual capacity loss. To mitigate the thermally induced degradation, due to pulsing, the use of different thermal management systems is discussed and evaluated based on platform constraints and system objectives.