Lithium iron phosphate battery working principle and significance

Lithium iron phosphate battery refers to a lithium-ion battery using lithium iron phosphate as a positive electrode material. The cathode materials of lithium-ion batteries mainly include lithium cobalt, lithium manganese, lithium nickel, ternary material, lithium iron phosphate, and so on. Lithium cobaltate is the anode material used in most lithium-ion batteries.


In the metal market, cobalt (Co) is the most expensive and is not abundant, nickel (Ni) and manganese (Mn) is cheaper, and iron (Fe) is more abundant. The price of the cathode material is also in line with the price of these metals. Lithium-ion batteries made from LiFePO4 should therefore be cheap. Another characteristic of it is environmental protection and pollution-free.


As a rechargeable battery requirements are: high capacity, high output voltage, good charge and discharge cycle performance, output voltage stability, can large current charge and discharge, electrochemical stability performance, safety in use (not due to overcharging, over-discharge and short circuit and other improper operation caused by combustion or explosion), wide operating temperature range, non-toxic or less toxic, no pollution to the environment. LiFePO4 a positive lithium iron phosphate battery in these performance requirements are good, especially in large discharge rate discharge (5 ~ 10C discharge), discharge voltage stable, safety (no combustion, no explosion), life (cycle number), no pollution to the environment, it is the best, is the best large current output power battery.


Structure and working principle. LiFePO4, as the positive terminal of the battery, is connected by aluminum foil to the positive terminal of the battery. In the middle is a polymer diaphragm, which separates the positive terminal from the negative terminal, but lithium-ion Li can pass through while electron e- cannot. On the right is the negative terminal of the battery composed of carbon (graphite), which is connected to the negative terminal of the battery by copper foil.


Between the upper and lower ends of the battery is the battery’s electrolyte, which is enclosed in a metal shell. When a LiFePO4 battery is charged, lithium ions in the positive electrode migrate to the negative electrode through the polymer diaphragm; During the discharge process, lithium-ion Li in the negative electrode migrates through the diaphragm to the positive electrode. Lithium-ion batteries are named because lithium ions migrate back and forth as they are charged and discharged.


Main performance. The nominal voltage of the LiFePO4 battery is 3.2V, the termination charge voltage is 3.6V, and the termination discharge voltage is 2.0V. Due to the different quality and processes of positive and negative electrode materials and electrolyte materials used by various manufacturers, there will be some differences in their performance. For example, the same type of battery (the same package as the standard battery), its battery capacity is very different (10% ~ 20%).


It should be noted that there are some differences in the performance parameters of lithium iron phosphate power batteries produced by different factories; In addition, there are some battery performance is not included, such as battery internal resistance, self-discharge rate, charge, and discharge temperature.


Lithium iron phosphate power batteries vary widely in capacity and can be divided into three categories: small ones with a few tenths to a few milliamps, medium ones with tens of milliamps, and large ones with hundreds of milliamps. There are also some differences in the same type of parameters for different types of batteries.


Over-discharge to zero voltage test: The use of STL18650(1100mAh) lithium iron phosphate power battery has been discharged to zero voltage test.


Test conditions. Charge the 1100mAh STL18650 battery with a 0.5C charge rate, then discharge it with a 1.0C discharge rate until the battery voltage is 0C. The 0V batteries were then divided into two groups: one for 7 days and the other for 30 days. After storage expires, it is charged with 0.5C and discharged with 1.0C. Finally, the difference between the two zero voltage storage periods is compared.


The result of the test is that after 7 days of zero voltage storage, the battery has no leakage, good performance and 100% capacity; After 30 days of storage, no leakage, good performance, and capacity of 98%; After 30 days of storage, the battery is charged and discharged three times, and the capacity is restored to 100%. This test shows that the lithium iron phosphate battery does not leak and damage even if it has been discharged (even to 0V) and stored for a certain time. This is a feature that other types of lithium-ion batteries do not have.



  1. Improvement of safety performance The P-O bond in lithium iron phosphate crystal is stable and difficult to decompose. Even at high temperatures or overcharges, it will not collapse and heat or form strong oxidizing substances like lithium cobalt acid structure, so it has good safety.


It has been reported that a small number of samples were found burning in the actual operation of acupuncture or short-circuit experiments, but no explosion occurred. In the overcharge experiment, high voltage charging was used which was several times higher than the self-discharge voltage, and the explosion was still found. However, its overcharge safety is much improved compared with that of ordinary liquid electrolyte lithium cobalt acid batteries.
 improvement lithium iron phosphate battery refers to lithium iron phosphate as the positive material of lithium-ion batteries. The cycle life of a long-life lead-acid battery is about 300 times, the highest is 500 times, and the cycle life of the lithium iron phosphate battery is more than 2000 times, and the standard charge (5-hour rate) can be used for 2000 times. The same quality of the lead-acid battery is “new half a year, old half a year, maintenance and maintenance of half a year”, the maximum time is 1~1.5 years, and lithium iron phosphate battery under the same conditions, the theoretical life will reach 7~8 years.


Overall consideration, the performance price ratio is more than 4 times of the theoretical lead acid battery. The high current discharge can be charged and discharged quickly with high current 2C. Under the special charger, the battery can be charged at 1.5C within 40 minutes, and the starting current can reach 2C, while the lead-acid battery has no such performance.


  1. high-temperature performance is good lithium iron phosphate peak electric heating up to 350℃-500℃ and lithium manganese acid and lithium cobalt acid only about 200℃. The operating temperature range is wide (-20C– 75C), with high-temperature resistance characteristics of lithium iron phosphate electric peak of 350℃-500℃ and lithium manganese acid and lithium cobalt acid only about 200℃.


  1. When the battery works under the condition of a full ∩ charger, its capacity will be lower than the rated capacity rapidly. This phenomenon is called the memory effect. Like nickel-metal hydride, nickel-cadmium batteries have memory, and lithium iron phosphate batteries do not have this phenomenon, no matter what state the battery is in, can be used with the charge, and do not need to put the first charge.


  1.  Light weight

The volume and weight of a lithium iron phosphate battery with the same capacity are 2/3 of the volume and 1/3 of the weight of a lead-acid battery.


  1. Environmental Protection

Lithium iron phosphate batteries are generally considered to be free of any heavy metals and rare metals (nickel metal hydride batteries need rare metals), non-toxic (SGS certification), pollution-free, in line with European RoHS regulations, for the absolute green battery certificate. Therefore, the reason why lithium battery is favored by the industry is mainly due to the consideration of environmental protection. Therefore, the battery has been included in the “863” national high-tech development plan during the “Tenth Five-Year Plan”, and has become a key project supported and encouraged by the state.


With China’s accession to the WTO, the export volume of Chinese electric bicycles will increase rapidly, and electric bicycles entering Europe and the United States have been required to be equipped with pollution-free batteries. But some experts say that the environmental pollution caused by lead-acid batteries mainly occurs in the non-standard production process and recycling process of enterprises.


Likewise, lithium batteries are a good part of the new energy industry, but they are not immune to the problem of heavy metal pollution. Lead, arsenic, cadmium, mercury, chromium, etc. may be released into dust and water during metal processing.


The battery itself is a chemical, so there are two kinds of pollution possible: one is the process waste pollution in the production engineering; Second, the battery pollution after scrapping. Lithium iron phosphate battery also has its disadvantages: for example, low-temperature performance is poor, the positive material vibration density is small, the volume of lithium iron phosphate battery of the same capacity is larger than lithium cobalt acid lithium-ion battery, so it does not have the advantage in the micro battery. When used in power batteries, lithium iron phosphate batteries, like other batteries, need to face the problem of battery consistency.

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