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Effect of large current pulse discharge on lithium iron phosphate battery

- Jan 19, 2019 -

With the rapid development of China's electric vehicle industry, the market of power battery is expanding rapidly. Currently, the material system of power battery mainly includes lithium iron phosphate and ternary material system. At present, when the ternary material battery has not entered the list of state financial subsidies, lithium iron phosphate material battery is gaining momentum.

For electric vehicles, the battery life is a key factor affecting the service cost, while the service life of lithium ion battery depends on a variety of factors, such as the selected material system, service environment, working mode and so on, which will have a significant impact on the service life of lithium ion battery.

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Lithium iron phosphate has a stable olivine structure, so its structural stability is significantly better than that of layered structural materials, such as LiCoO2 and ternary materials. But using the environment and working mode of lithium iron phosphate material life have a significant impact, such as electric cars can work patterns such as faces the instantaneous large-current discharge, generally large current discharge will significantly reduce the service life of lithium ion battery, so we need to large current pulse discharge battery failure of further study on the mechanism of reduction.

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Derek n. Wong of the university of Texas at Arlington recently conducted a study on the effect of large current pulses on the performance of lithium iron phosphate. Derek n. Wong used a lithium iron phosphate 26650 battery to study the effect of 40A pulse current on the performance of the battery in order to simulate the real working scenario of lithium ion batteries used in electric vehicles. It was found that the high-rate pulse discharge caused a sharp increase in the internal resistance of the lithium iron phosphate battery, and a large amount of LiF generated by LiPF6 decomposition was found on the surface of the negative electrode, which seriously affected the diffusion kinetic characteristics of the interface. This is the main reason for the capacity decline of lithium iron phosphate battery caused by high-current pulse discharge. Derek n. Wong studied the degradation mechanism of lithium iron phosphate batteries using continuous and pulsed currents up to 15C, with a 1C cycle every 20 times to measure their capacity. It was found that when the 15C pulse charging and discharging was carried out, the battery could not be charged for 15C after 40 cycles at most, but still could be charged and discharged for 1C, and its 1C capacity degradation rate was 6/20 cycles. For batteries with continuous 15C charge and discharge, after 60 times, they can still have continuous 15C rate charge and discharge, but the capacity decline rate of 1C is significantly higher than that of pulse mode, reaching 14%/20 cycles.

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Large current pulse work mode is the main reason of the failure battery charge exchange impedance and the increase of the polarization, makes the battery voltage increases rapidly under the ratio of 15 c to 4.1 V voltage, which can lead to the battery cannot complete charge ac impedance analysis showed that the pulse discharge battery, the battery charge exchange impedance and the impedance of the SEI film continues to increase, and the charge exchange impedance and electrode active material and electrolyte related to the size of the contact interface, the SEI film both lead to the growth of the SEI film impedance increases, has increased the charge exchange impedanceBy means of XAS analysis the positive active material, found that the positive active material did not significantly change, showed that the positive active material loss is not the main factor of pulse discharge battery capacity decline down, and in view of the negative of the SEI film XPS studies have found a special phenomenon, for a common component of SEI Li2CO3, 15 c battery pulse work content is about 3%, but the content of 15 c continuous continuous discharge of battery is about 5%, the difference is not bigFor the composition of LiF, 15 c pulse batteries work content is about 23%, but the content of 15 c continuous discharge of battery is about 5.3% or so, this is pulsed SEI components with continuous discharge battery is the biggest difference, due to the proliferation inhibition of LiF on Li + more strengthened, this explains why the pulse work mode caused the lithium iron phosphate battery charging 15 c accept ability fast downLithium iron phosphate batteries work in large current pulse mode, it is easier to make the electrolyte of lithium for LiF LiPF6 decomposition, the existence of LiF make the ion diffusion impedance of batteries and charge exchange impedance increases rapidly, make the battery in large current charge, the polarization of the battery voltage rise rapidly, beyond the limits of the battery voltage, makes the battery charging was unable to complete the study reveals that the pulse working mode of lithium iron phosphate battery failure mechanism of the real, in order to improve the lithium iron phosphate material pulse pointed out the research direction of working ability, production of lithium iron phosphate power battery has important guiding significance