Accelerated Degradation of Li-Ion Batteries for High Rate Discharge Applications

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) to support system requirements can be significantly greater than commercial ESS. Results are reported of high power pulse duty cycles on lithium iron phosphate cells that show a dramatic loss in lifetime performance. 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°C, the 2 s pulse showed a 31°C temperature rise and the 3 s pulse, a 48°C temperature rise. The decreased lifetime is attributed to increased lithium loss due to the increased temperature during pulse discharging. To mitigate the thermally induced degradation, the use of different thermal management systems and alternative cell design is discussed and recommended.