Analysis for performance degradation mechanisms of the retired LiFePO₄/graphite power cells

In this work, the performance degradation mechanism of the retired power cells is investigated after 400 and 800 cycles at 25 degrees C and 45 degrees C to determine whether they satisfy the secondary utilization requirement under the simulated energy storage systems. The results confirm that the capacity of retired cells show a linear relationship with cycle number. When the cycle numbers are more than 2400 times (at 25 degrees C), the capacity of the retired cells is decreased to less than 80% of initial capacity, which means the retired cells have the value of secondary utilization. In contrast to stable capacity delivering of LiFePO4, graphite anode shows obvious capacity fading. Further, the lithium loss at the LiFePO4 cathode accounts for more than 70% of the retired cells capacity loss, and exceeding 85% of the consumed lithium-ion can be found at the graphite anode, leading to the formation of SEI film consisting of Li2CO3 and LiF inorganic salts.