How to improve the low temperature performance of lithium iron phosphate battery?

Recently, at the sub-forum of the 7th Global New Energy Vehicle Conference (GNEV7), the Deputy General Manager of Waterma, Mumin Rao, delivered a speech on the theme of “Research on the Factors Affecting the Low Temperature Performance of Lithium Iron Phosphate power batteries”. The content is organized as follows:

First of all, Mumin Rao introduced that Waterma was founded in 2002 and is headquartered in Shenzhen. At present, there are more than 12,000 employees, and it is also the first batch of battery companies to enter the power battery catalogue of the Ministry of Industry and Information Technology. Among them, more than 50,000 vehicles supporting the Wattma battery are running on the road.

At present, most of the power batteries in the electric bus field use lithium iron phosphate. Mumin Rao said that lithium iron phosphate has several advantages: the cycle life and the ratio are better, but the low temperature is a little worse than other battery systems. Secondly, the current efficiency of electric vehicles at different temperatures is still quite different. Electric vehicles that can run 160 kilometers at normal temperature may only run 60-80 kilometers at -20 degrees, and the efficiency is reduced significantly. Then there is the problem of low temperature charging in winter, including difficult charging and safety accidents.

For lithium iron phosphate battery, Watmar has made a detailed study on the factors affecting its low temperature characteristics: one is the influence of the positive electrode, the lithium iron phosphate positive electrode itself has relatively poor electronic conductivity, and it is easy to generate polarization and reduce the capacity. The second negative electrode, the negative electrode is mainly low temperature charging, because it will affect the safety problem; the third is the electrolyte, which may increase the viscosity at low temperature, and the lithium ion migration impedance will increase; This is the binder, and now this has a relatively large impact on the low temperature performance of the battery.

The whole idea of Watmar is to improve the low temperature performance of lithium iron phosphate battery from the positive electrode, the negative electrode, the electro-hydraulic and the binder.

In terms of the positive electrode, it is now nano-sized, and its particle size, electrical resistance, and axial length of the AB plane affect the low temperature characteristics of the entire battery. lithium iron phosphate was prepared by three processes. From the conditions of our whole preparation, different lithium iron phosphate processes were nano-coated and coated. From the axial length of the AB surface, the increase of the AB-axis length makes lithium The ion migration channel will become larger, which will help improve the performance of the battery rate. Different processes have different effects on the positive electrode. The low-temperature discharge characteristics of the battery made of lithium iron phosphate with a particle diameter of 100 to 200 nanometers are better, and 94% can be released at -20 degrees, that is, the nanometerization of the particle diameter shortens the migration path. The performance of the low temperature discharge is also improved because the lithium iron phosphate discharge is mainly related to the positive electrode.

Considering the charging characteristics from the negative electrode, Mumin Rao believes that low temperature charging of lithium battery is mainly affected by negative electrode, including particle size and spacing change. Three kinds of artificial graphite are selected as negative electrode to study the influence of different layer spacing and particle size on low temperature characteristics. From the perspective of the three materials, the granular graphite with a large interlayer spacing has a smaller bulk impedance and ion mobility resistance in terms of impedance.

In terms of charging, Mumin Rao believes that the problem of discharge at low temperatures in winter is not large, mainly low-temperature charging. Because the cross-flow ratio of 1C or 0.5C is very critical in terms of cross-flow ratio, it takes a very long time to reach constant pressure. By improving the comparison of three different graphites, it is found that one of them has a relatively large 2.6-degree charge constant current ratio. Improvements, from 40% to more than 70%, the increase in layer spacing, and the reduction in particle size.

In the electrolyte, the electrolyte freezes at -20 degrees and -30 degrees, the viscosity increases, and the formation performance deteriorates. The electrolyte is from three aspects: solvent, lithium salt, and additive. Mumin Rao said, "By experiment, we found that the effect of solvent on the low temperature of lithium iron phosphate battery is from more than 70% to more than 90%, and there are more than a dozen points. Secondly, different lithium salts have certain characteristics of low temperature charge and discharge impact. We have fixed the solvent system and lithium salt. The low temperature additive can increase the discharge capacity from 85% to 90%. That is to say, the solvent, lithium salt and additives are all for our power battery in the whole electrolyte system. Low temperature properties have a certain effect, including other material systems."

In terms of binders, Mumin Rao said that there are three kinds, two kinds of spots, one kind of linear. In the case of -20 degrees of charge and discharge, after the two kinds of dots are about 70 to 80 cycles, the entire pole piece has the status of binder failure, and the linear adhesive does not have this problem. In the whole system, after improving from the positive electrode, the negative electrode, the electrolyte to the binder, we have done a good job in the lithium iron phosphate battery cell, one is the charging characteristics, -20, -30, -40 At a temperature of 0.5C, the constant current ratio can reach 62.9%, and the discharge at -20 degrees can release 94%, which is some characteristics of the rate and cycle.

Generally speaking, the normal operation of new energy vehicles in the north, under low temperature conditions, not only the battery cells can solve the charging problem, but also through the pack and the BMS whole cycle battery, and the innovation of the guarantee mode, add up to fully guarantee the normal operation of new energy vehicles in the north.

The page contains the contents of the machine translation.