Accelerated Aging Analysis on Cycle Life of LiFePO4/Graphite Batteries Based on Different Rates

Abstract In this study, different charge/discharge rates (1/3C, 1C, 1.5C, 1.8C, 2.0C and 2.5C) are used to accelerate the aging of commercial LiFePO 4 /graphite cells. The capacity attenuation mechanism is investigated by disassembling the aged full cells and analyzing the morphologies and chemical composition of electrode surfaces, the single electrode potentials, and so forth, when the capacity retention of full cell decays to 95 %, 90 %, 85 %, and 80 %. Through the tests, the capacity fade of the full cell is mainly ascribed to the irreversible loss of active lithium. However, when the test rate is not lower than 2.0C, the dominant mechanism for consumption of active lithium is changed to lithium deposition on the anode during the charging process instead of the generation of SEI film. As a consequence, the capacity decay rate at 2.0C or 2.5C is accelerated to a large degree. The capacity loss with cycling time is fitted by Arrhenius equations. The pre‐exponential factor at a given rate ( A rate ) increases linearly with the rising test rate when it is not more than 1.8C, and this corresponds to the results of disassembly analysis. Therefore, an unknown A rate in a reasonable range can be extrapolated by the data of other A rate values at relatively high rates. In addition, the calculated A rate after 10 % capacity loss can be used to replace the fitted value by the whole aging data. This provides a rapid and practical approach to estimate the lifetime of the full cell.