Starting from October each year, the temperature gradually decreases, which is a challenge for the low-temperature performance of Lithium Iron Phosphate (LFP) battery packs. As we all know, the high-temperature performance of lithium-ion batteries is excellent, with a peak temperature of 350~500℃ and the ability to release 100% capacity even at high temperatures (60℃).
Lithium Iron Phosphate (LFP) is the positive electrode material of lithium-ion batteries. The P-0 bond in the LFP crystal is stable and difficult to break, so even in high temperatures or overcharging, it will not collapse and generate heat or form strong oxidizing substances like lithium cobalt oxide. Therefore, LFP batteries have good safety, but their low-temperature performance and consistency may be slightly worse than ternary batteries.
As a high-tech product with many chemical raw materials and complex processes, LFP battery packs have high requirements for temperature, humidity, dust, and other production environments. If not controlled properly, the quality of the battery will fluctuate.
Poor conductivity and slow lithium-ion diffusion rate
During high-rate charge and discharge, the actual specific capacity is lower, which is a difficult problem that restricts the development of the LFP industry. This is a major problem that has prevented LFP from being widely used.
The positive electrode of LFP has poor electronic conductivity and is prone to polarization, which reduces the capacity performance. The negative electrode is mainly affected by low-temperature charging, which affects safety issues. The electrolyte may increase in viscosity and lithium-ion migration impedance at low temperatures.
To fundamentally solve the problem of low-temperature performance of Lithium Iron Phosphate battery packs, we need to improve them from four aspects: positive electrode, negative electrode, electrolyte, and binder.
Currently, nano-sizing is the norm. The particle size, resistance, and planar axis length of the positive electrode will affect the low-temperature characteristics of the entire battery. Different processes also have different effects on the positive electrode. The battery made of 100 to 200 nanometer-sized Lithium Iron Phosphate has better low-temperature discharge characteristics, with a discharge of 94% at -20 degrees Celsius. Nanosizing the particle size shortens the migration path and improves the low-temperature discharge performance because Lithium Iron Phosphate discharge is mainly related to the positive electrode.
Considering the charging characteristics, the negative electrode mainly affects the low-temperature charging of Lithium batteries, including the size of the particle and the spacing between the negative electrodes. Three different types of artificial graphite were selected as negative electrodes to study the effect of different interlayer spacing and particle size on low-temperature characteristics. Among the three materials, the particle graphite with larger interlayer spacing has smaller intrinsic impedance and ion migration impedance.
Lithium battery packs do not have a significant problem with low-temperature discharge in winter, mainly low-temperature charging. Regarding the transverse flow ratio, 1C or 0.5C transverse flow ratios are crucial, and it takes a long time to reach the constant voltage. By improving the comparison of three different graphites, we found that one of them improved the constant current ratio of charging at -20 degrees Celsius from 40% to over 70%, with an increase in interlayer spacing and a decrease in particle size.
At -20℃ and -30℃, the electrolyte freezes, viscosity increases, and performance deteriorates. Electrolytes include solvents, lithium salts, and additives. The solvent has an impact ranging from over 70% to over 90% on the low-temperature effect of the Lithium Iron Phosphate battery pack, with an impact of more than ten points. Secondly, different lithium salts have a certain influence on the low-temperature charging and discharging characteristics. By fixing the solvent system and lithium salt base, low-temperature additives can increase the discharge capacity from 85% to 90%. In other words, in the entire electrolyte system, solvents, lithium salts, and additives all have a certain impact on the low-temperature characteristics of our power battery, which also applies to other material systems.
Regarding the adhesive:
Under the conditions of charging and discharging at 20°C, after about 70 to 80 cycles, the entire pole piece exhibits adhesive failure with two types of dot-shaped adhesives, while this problem does not exist when using linear adhesives. After improving the entire system from the positive and negative electrodes, electrolyte, and adhesive, the single cell of the lithium iron phosphate battery has achieved relatively good results. One is the charging characteristic, where the constant current ratio of 0.5C charging at temperatures of -20°C, -30°C, and -40°C can reach 62.9%, and at -20°C temperature, the discharge can reach 94%. These are some characteristics of rate and cycle.