The chemical properties of lithium iron phosphate (LiFePO4) are very stable, especially its high-temperature stability is very good. Even at very high temperatures, it cannot decompose and release oxygen, so the safety performance of lithium iron phosphate batteries is very good and it is not easy to occur combustion, explosion, and other dangers. With reasonable structural design, its safety is further improved, and therefore the battery does not ignite or explode in situations such as impact, puncture, and short circuits. lithium iron phosphate batteries were charged to full capacity and were quickly pierced with 8mm diameter steel nails while recording changes in the battery's voltage and temperature.
In the initial stage of the steel nail piercing, due to internal short circuiting, the battery voltage dropped rapidly, and a certain amount of heat was released, causing the battery's temperature to rise. However, since the vacuum inside the battery significantly decreased after being pierced, causing the short-circuited contact part to deform and have poor contact, no more heat was released. Therefore, the voltage tended to stabilize, and the battery temperature only rose slightly.
The weight energy density is an important indicator of battery performance. When fully charged, a 20Ah LiFePO4 battery was discharged to 2.0V at a rate of 0.3C, and the discharge curve was integrated to obtain the energy output of the battery. After integration calculation, a 20Ah LiFePO4 battery released 70.7Wh of energy with a weight of 580g. Therefore, the weight energy density of lithium iron phosphate batteries can be calculated as 121.90Wh/kg.
Since there are significant geographical differences in the use of electric vehicles, there are low-temperature weather conditions in the winter in the north, and lower temperatures will inevitably have a certain impact on the battery performance. Therefore, in order to understand the low-temperature discharge performance of lithium iron phosphate batteries, a test was conducted where 20Ah LiFePO4 power batteries were stored at -20℃, -10℃, 0℃, 25℃, and 55℃ for 20 hours, and then discharged at 0.3C rate in this low-temperature environment (where the discharge capacity at room temperature at 0.3C is 100%).
lithium iron phosphate batteries can only discharge about 55% of their capacity at room temperature under -20℃, so it may have an adverse effect on electric vehicles in use. However, it is obvious that as the temperature decreases, the discharge capacity of a single battery decreases significantly, but for electric vehicles, hundreds of batteries are usually combined, and when the battery is working, it will release a certain amount of heat, hence the battery's temperature will inevitably rise. Therefore, the low-temperature discharge problem is not very serious for the actual application of battery packs. During the test, since the single battery had a larger exposed surface area, the temperature was basically the same as the environment throughout the entire testing process, so the discharge capacity was greatly affected.
At higher temperatures, lithium iron phosphate batteries are less affected, and at 55℃, the discharge capacity is only slightly increased compared to 25℃.
The above research shows that lithium iron phosphate batteries have long cycle life, high safety, and energy density. Moreover, lithium iron phosphate batteries do not use toxic heavy metal elements such as lead, cadmium, mercury, hexavalent chromium, and other toxic heavy metal elements in the entire production process, and phosphate iron lithium batteries are also more environmentally friendly since the battery packaging materials do not contain polybrominated biphenyls and polybrominated diphenyl ethers. Therefore, lithium iron phosphate batteries will have broader applications in the field of electric vehicles and large-scale chemical energy storage.