The U.S. Army has been pursuing vehicle electrification to achieve increased combat effectiveness and new capabilities, potentially requiring high power pulse duty cycles. However as Energy Storage System (ESS) pulse power discharge rates (> 40 C rate) increase, there is a significantly lower ESS lifetimes. Results of high power pulse duty cycles on lithium iron phosphate cell lifetime performance show a dramatic loss. For 2s and 3s, 120 A pulse tests, the observed degradation after 80 hours cycling is 22 % and 32 % respectively, significantly higher than previously reported values at lower discharge rates, but similar temperatures. A 7 year calendar aged cell was also tested with a 2s pulse and showed severe degradation (53% loss after 40 hours cycling). The decreased lifetime of the high pulse duty cycling aged cells is a result of the increased strain / heating at high currents, and subsequent SEI fracture, and thermally accelerated SEI formation. This mechanism leads to lithium consumption at the anode, and eventual capacity loss. To mitigate the pulsing induced thermal degradation, the use of different thermal management systems based on immersion cooling is proposed based on simulation results.