In recent years, the lithium iron phosphate battery is widely used in the fields of electric vehicles and energy storage because of its high energy density, long cycle life and safety^[1], but the existing battery technology was not enough to meet the requirements of electric vehicles^[2]. So it is of great importance to research performances of battery.

In this paper, the influence of different depth of discharge (DOD) on the cycle life of the battery was investigated. The specific research process is as follows, three kinds of LiFePO₄ batteries of the same type were charged and discharged at three different discharge depths (30% DOD, 50% DOD and 100% DOD) under constant conditions of 40℃and 1C (1.3A), and the discharge capacity decay curve and decay rate curve were measured after a certain number of cycles.

Discharge capacity decay curve and decay rate curve measured under different discharge depth are shown in Figure 1. As can be seen from the left graphic, the discharge capacity of the battery will have a slight increase in the early, this is because the battery anode material has not been fully activated during the initial cycle, as the cycle progresses, the electrolyte gradually penetrates into the interior of the electrode material, and the lithium ions smoothly migrate to the inside of the electrode material and undergo reversible deintercalation reaction, resulting in an increase in the capacity of the battery. In addition, At the beginning of the cycle, the depth of discharge has little effect on the capacity of the three groups of batteries. When the cycle continues, the discharge capacity of the LiFePO₄ battery gradually decreased, the attenuation of battery capacity by the depth of discharge is more and more obvious. The right capacity fading rate curve shows that battery capacity decay rate remained the same at the beginning of the cycle. At this time, the influence of the battery capacity by depth of discharge is almost independent. After the initial cycle, the deeper the depth of discharge, the faster the cell capacity decays, and there is a significant the positive correlation between the depth of discharge and the decay rate of battery capacity. Due to the different depth of discharge, the internal structure of the electrode material will occur to different degrees of deterioration.

Fig. 1 the discharge capacity decay curve and decay rate curve under different discharge depth.

It can be seen from the above studies that the effect of the battery cycle life by depth of discharge is various in different cycle stages. In the early cycle, LiFePO₄ battery capacity at different depth of discharge changes in the same law, indicating that the depth of discharge has no effect on the battery life in the early cycle. But as the cycle continues, the greater depth of discharge, the faster decay of battery capacity, the battery cycle life decline faster.

Acknowledgements

This work is financially supported by the National High Technology Research and Development Program of China (863Program, no.2015BAG01B01).

References

[1] Forgez C, Vinh Do D, Friedrich G, et al. Thermal modeling of a cylindrical LiFePO4/graphite lithium-ion battery[J]. Journal of Power Sources, 2010,195(9):2961-2968.

[2] Ritchie A, Howard W. Recent developments and likely advances in lithium-ion batteries[J]. Journal of Power Sources, 2006,162(2):809-812.