With the rapid development of China's electric vehicle industry, the market for power batteries is expanding rapidly. At present, the material systems used in power batteries mainly include lithium iron phosphate and ternary materials. At present, ternary material batteries have not yet entered the national financial subsidy catalog Under the circumstances, the lithium iron phosphate battery can be said to be in the limelight.
For electric vehicles, the service life of the battery is a key factor affecting its use cost, while the service life of lithium-ion batteries depends on many factors, such as the selected material system, use environment, working mode, etc. will affect the use of lithium-ion batteries Life expectancy has a significant impact.

Lithium iron phosphate material has a stable olivine structure, so its structural stability is significantly better than that of layered structure materials, such as LiCoO2 and ternary materials. However, the use environment and working mode will have a significant impact on the life of lithium iron phosphate materials. For example, electric vehicles often face working modes such as instantaneous high-current discharge. Generally speaking, high-current discharge will significantly reduce the life of lithium-ion batteries. Therefore, we need to conduct further research on the mechanism of battery degradation caused by high-current pulse discharge.

Recently, Derek N. Wong of the University of Texas at Arlington conducted targeted research on the impact of high-current pulse discharge on the performance of lithium iron phosphate. Derek N. Wong studied the effect of 40A pulsed current on battery performance using lithium iron phosphate 26650 batteries to simulate the real working scenario of lithium-ion batteries used in electric vehicles. The study found that high-rate pulse discharge caused a sharp increase in the internal resistance of lithium iron phosphate batteries, and a large amount of LiF produced by the decomposition of LiPF6 was found on the surface of the negative electrode, which seriously affected the diffusion dynamics of the interface. The main reason for the capacity decline of lithium iron phosphate batteries. Derek N. Wong studied the degradation mechanism of lithium iron phosphate batteries under both continuous and pulsed usage up to 15C, and performed 1C cycles every 20 times to measure their capacity. The study found that when performing 15C pulse charge and discharge, the battery cannot be charged at 15C after a maximum of 40 cycles, but it can still be charged and discharged at 1C, and its 1C capacity decay rate is 6%/20 cycles. The battery that has been charged and discharged continuously at 15C can still be charged and discharged at a rate of 15C continuously after 60 times, but its 1C capacity decline rate is significantly higher than that of the pulse mode, reaching 14%/20 cycles.

The main cause of battery failure in high-current pulse mode is the increase of charge exchange resistance and polarization, which makes the battery voltage rise rapidly to the 4.1V limit voltage at a rate of 15C, resulting in the failure of the battery to complete charging. The AC impedance analysis shows that with the battery pulse discharge, the battery's charge exchange resistance and SEI film resistance continue to increase, and the charge exchange resistance is mainly related to the size of the contact interface between the electrode active material and the electrolyte, and the growth of the SEI film leads to both The increased resistance of the SEI membrane also increases the charge exchange resistance. The active material of the positive electrode was analyzed by means of XAS, and it was found that the active material of the positive electrode did not change significantly, indicating that the loss of the active material of the positive electrode was not the main factor causing the capacity decline of the pulse discharge battery, while the XPS study on the SEI film of the negative electrode found a A special phenomenon, for Li2CO3, a common component of SEI, the battery content of 15C pulse work is about 3%, while the battery content of 15C continuous discharge is about 5%, and there is little difference between the two. As for the LiF component, the battery content of 15C pulse work is about 23%, while the battery content of 15C continuous discharge is about 5.3%, which is the biggest difference between the SEI composition of the pulse work battery and the continuous discharge battery, because LiF The diffusion inhibition effect on Li+ is stronger, which explains why the 15C charging acceptance of lithium iron phosphate battery is rapidly decreased due to the pulse working mode. In the high-current pulse working mode of lithium iron phosphate batteries, it is easier to decompose the lithium salt LiPF6 in the electrolyte into LiF. The presence of LiF makes the ion diffusion resistance and charge exchange resistance of the battery rapidly increase, making the battery , the polarization voltage of the battery rises rapidly, exceeding the limit voltage of the battery, so that the battery cannot be fully charged.
This research reveals the real mechanism of failure of lithium iron phosphate batteries in pulse working mode, points out the research direction for improving the pulse working ability of lithium iron phosphate materials, and has important guiding significance for the production of lithium iron phosphate power batteries.