Insight into the effect of current rates on the ageing performance of LiFePO4/graphite cells with the ultra-long cycles

Abstract

Lithium iron phosphate batteries are considered one of the most popular power sources for electric vehicles and storage systems because of their excellent safety and cycle life. However, when degradation occurs, battery performance becomes unreliable. Thus, accurate identification of the battery performance during the long cycle is especially important. In this paper, the performance of LiFePO₄/graphite cells cycled at different rates to 5000 cycles is compared in detail. During the cycle, the voltage, capacity, and reference performance tests are recorded to find the method for cell state assessment under operating conditions. We find that the voltages acquired from the end of the rest period can reflect the cell state of charge to some extent. The capacity differences between operating conditions and the capacity increase during the early cycle stage of rate discharge can all be explained clearly in terms of the changes in their corresponding rest voltages. Moreover, the voltage differences between the end of the rest period and the start of the operation are useful parameters to evaluate the change in ohmic polarization with the cycle. In addition, a non-destructive method is proposed by comparing the voltage and corresponding differential voltage curves of the cell and the two separate electrodes. Based on this method, the ageing states of cells cycled under different conditions are quantitatively evaluated. An increased capacity ratio of negative to positive during the cycle is discovered in cells with better cycle performance, providing theoretical support for cell design with ultra-long cycles.