The main factors affecting the low-temperature characteristics of lithium iron phosphate battery packs are as follows:
1. Production environment: Lithium iron phosphate battery pack is a high-tech product with numerous chemical raw materials and complex processes. Its production environment has high requirements for temperature, humidity, dust, etc. If not controlled properly, battery quality will fluctuate.
2. Poor conductivity and slow diffusion rate of lithium ions. When charging and discharging at high rates, the actual specific capacity is low, which is a difficult problem that restricts the development of the lithium iron phosphate industry. The reason why lithium iron phosphate has not been widely applied so late is a major issue.
3. The impact of materials is that the electronic conductivity of lithium iron phosphate cathode itself is relatively poor, and it is also more prone to polarization, which reduces the capacity utilization; The negative electrode is mainly used for low-temperature charging, as it will affect safety issues; The viscosity of the electrolyte may increase at low temperatures, and the migration impedance of lithium ions may also increase; The fourth one is the adhesive, which has a significant impact on the low-temperature performance of the battery.
Methods to improve the low-temperature performance of lithium iron phosphate battery packs:
Mainly improving the low-temperature performance of lithium iron phosphate battery packs from four aspects: positive electrode, negative electrode, electrolyte, and binder.
1. In terms of positive electrodes, they are now all nano sized, and their particle size, electrical resistance, and AB plane axis length will affect the low-temperature characteristics of the entire battery. Different processes have different effects on the positive electrode. The low-temperature discharge characteristics of batteries made of 100 to 200 nanometer sized lithium iron phosphate are relatively good, with a release rate of 94% at -20 degrees Celsius. This means that the nanoparticle size shortens the migration path and improves the low-temperature discharge performance, as lithium iron phosphate discharge is mainly related to the positive electrode.
2. Considering the charging characteristics from the negative electrode perspective, the low-temperature charging of lithium batteries is mainly influenced by the negative electrode, including particle size and changes in the spacing between the negative electrodes. Three different types of artificial graphite were selected as the negative electrode to study the effects of different interlayer spacing and particle size on the low-temperature characteristics. From the perspective of the three materials, for granular graphite with large interlayer spacing, the bulk impedance and ion migration impedance are relatively small in terms of impedance.
3. In terms of charging, lithium battery packs do not have any discharge problems at low temperatures in winter, mainly low-temperature charging. Because in terms of cross current ratio, a cross current ratio of 1C or 0.5C is crucial, and it takes a very long time to reach constant voltage. By improving the comparison of three different types of graphite, it was found that one of them had a significant improvement in the constant current ratio during -20 ° C charging, increasing from 40% to over 70%, with an increase in interlayer spacing and a decrease in particle size.
4. The electrolyte freezes at -20 ° C and -30 ° C, causing an increase in viscosity and a deterioration in performance. Electrolytes come from three aspects: solvents, lithium salts, and additives. The effect of solvents on the low temperature of lithium iron phosphate battery packs ranges from over 70% to over 90%, with more than ten points of influence; Secondly, different lithium salts have a certain impact on the characteristics of low-temperature charging and discharging. On the basis of fixing the solvent system and lithium salt, low-temperature additives can increase the discharge capacity from 85% to 90%. That is to say, in the entire electrolyte system, solvents, lithium salts, and additives have a certain impact on the low-temperature characteristics of our power battery, including other material systems
5. In terms of binders, after approximately 70 to 80 cycles of charging and discharging at 20 degrees, the entire electrode exhibits a situation of adhesive failure, while using linear binders does not have this problem. In the entire system, after the improvement of the positive electrode, negative electrode, electrolyte, and binder, the single unit of lithium iron phosphate battery has achieved good results. One is the charging characteristics, with a constant current ratio of 62.9% at 0.5C charging at -20, -30, and -40 degrees Celsius, and a discharge rate of 94% at -20 degrees Celsius. These are some characteristics of rate and cycling.