We characterized high-power lithium-ion cells in terms of performance and cycle and calendar life at 45°C. Among other parameters, we measured the C/25 capacity every 4 weeks during the test. Differentiation of the C/25 voltage versus capacity data with respect to capacity (dV/dQ) has been used to elucidate another type of side reaction at the anode. In cycle-life cells, with their higher capacity throughput, the analysis showed that one phase transition (a peak in the profile) was disappearing with time. In contrast, this effect was not seen in calendar-life cells.