Aging behavior of LiFePO4-based battery cells at stack level: A Second-Life cycling study

This study investigates capacity degradation in 100 Ah lithium iron phosphate (LiFePO₄, LFP) battery cells during First-Life and Second-Life use. In a laboratory-based case study, cells achieved up to 9600 cycles, with a maximum loss of 42 Ah over 7 years of Second-Life use and a total loss of 56 Ah over >14 years of total cell age. These results highlight the long-term viability of LFP cells for battery energy storage systems. A power law model based on the first 10 data points effectively described capacity trajectories and identified early signs of accelerated degradation. One cell deviated from the predicted progression at cycle #8000, corresponding to a State-of-Health (SoH) of 48 %. Crossing the ±3 standard deviation threshold in the capacity residuals marked the End-of-Second-Life criterion. Incremental Capacity Analysis indicated additional aging mechanisms, such as Loss of Active Material and increased faradaic polarization. Cells with lower initial capacity degraded faster at 70 % SoH, suggesting a History effect overlapping with temperature-dependent aging. Only cells with high initial capacity at 70 % SoH exhibited a linear Arrhenius trend, while cells with low initial capacity degraded slightly faster at 10 °C than at 30 °C, suggesting different aging mechanisms. Within the stack, temperatures recorded in a 10 °C climate chamber reached 22 °C, challenging the reliability of setpoints. Spatial temperature variations suggested non-uniform degradation patterns. These findings underscore the importance of considering capacity history, thermal gradients, and early diagnostic indicators when assessing Second-Life battery performance.