The full name of lithium iron phosphate battery is lithium iron phosphate lithium ion battery, this name is too long, referred to as lithium iron phosphate battery. Because its performance is particularly suitable for power applications, the word “power” is added to the name, that is, lithium iron phosphate power lithium battery. Some people also call it “lithium iron (LiFe) power lithium battery” or “LFP battery”.
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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 are mainly lithium cobalt oxide, lithium manganate, lithium nickel oxide, ternary materials, lithium iron phosphate, etc. Among them, lithium cobalt oxide is the cathode material used in the vast majority of lithium-ion batteries.
Structure and working principle
LiFepO4 is used as the positive electrode of the battery, which is connected by aluminum foil and the positive electrode of the battery. The middle is a polymer separator, which separates the positive electrode and the negative electrode, but the lithium ion Li can pass through but the electron e- cannot pass through, and the right side is composed of carbon (graphite). The negative electrode of the battery is connected by the copper foil to the negative electrode of the battery. Between the upper and lower ends of the battery is the electrolyte of the battery, and the battery is hermetically sealed by a metal casing. When LiFepO4 battery is charged, the lithium ion Li in the positive electrode migrates to the negative electrode through the polymer separator; during the discharge process, the lithium ion Li in the negative electrode migrates to the positive electrode through the separator. Lithium-ion batteries are named after lithium ions migrate back and forth during charging and discharging.Structure and working principleLiFepO4 is used as the positive electrode of the battery, which is connected by aluminum foil and the positive electrode of the battery. The middle is a polymer separator, which separates the positive electrode and the negative electrode, but the lithium ion Li can pass through but the electron e- cannot pass through, and the right side is composed of carbon (graphite). The negative electrode of the battery is connected by the copper foil to the negative electrode of the battery. Between the upper and lower ends of the battery is the electrolyte of the battery, and the battery is hermetically sealed by a metal casing. When LiFepO4 battery is charged, the lithium ion Li in the positive electrode migrates to the negative electrode through the polymer separator; during the discharge process, the lithium ion Li in the negative electrode migrates to the positive electrode through the separator. Lithium-ion batteries are named after lithium ions migrate back and forth during charging and discharging.
The nominal voltage of the LiFepO4 battery is 3.2V, the final charge voltage is 3.6V, and the final discharge voltage is 2.0V. Due to the different quality and process of positive and negative electrode materials and electrolyte materials used by various manufacturers, there will be some differences in their performance. For example, the capacity of the battery of the same type (standard battery in the same package) is quite different (10% to 20%).
It should be noted here that lithium iron phosphate power lithium batteries produced by different factories will have some differences in various performance parameters; in addition, some battery performance is not included, such as battery internal resistance, self-discharge rate, charge and discharge temperature, etc. .
The capacity of lithium iron phosphate power lithium batteries is quite different and can be divided into three categories: small tenths to a few milliamp hours, medium tens of milliamp hours, and large hundreds of milliamp hours. There are also some differences in the same parameters of different types of batteries.
Overdischarge to zero voltage test: STL18650 (1100mAh) lithium iron phosphate power lithium battery was used for discharge to zero voltage test.
Test conditions: 1100mAh STL18650 battery is fully charged with a 0.5C charge rate, and then discharged to a battery voltage of 0C with a 1.0C discharge rate. Then divide the batteries placed at 0V into two groups: one group is stored for 7 days, and the other group is stored for 30 days; after the storage expires, it is fully charged with a 0.5C charging rate, and then discharged with 1.0C. Finally, the differences between the two zero-voltage storage periods are compared.
The result of the test is that after 7 days of zero voltage storage, the battery has no leakage, good performance, and the capacity is 100%; after 30 days of storage, there is no leakage, good performance, and the capacity is 98%; after 30 days of storage, the battery is subjected to 3 charge-discharge cycles, The capacity is back to 100%. This test shows that even if the lithium iron phosphate battery is overdischarged (even to 0V) and stored for a period of time, the battery will not leak or be damaged. This is a feature that other types of lithium-ion batteries do not have.
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In the metal trading market, cobalt (Co) is the most expensive, and there is not much storage, nickel (Ni) and manganese (Mn) are cheaper, and iron (Fe) has more storage. The prices of cathode materials are also in line with the prices of these metals. Therefore, lithium-ion batteries made of LiFepO4 cathode materials should be quite cheap. Another feature of it is that it is environmentally friendly and non-polluting.
As a rechargeable battery, the requirements are: high capacity, high output voltage, good charge-discharge cycle performance, stable output voltage, high-current charge-discharge, electrochemical stability, and safety in use (not due to overcharge, overdischarge and short circuit). It can cause combustion or explosion due to improper operation), wide operating temperature range, non-toxic or less toxic, and no pollution to the environment. The lithium iron phosphate battery using LiFepO4 as the positive electrode has good performance requirements, especially in terms of large discharge rate discharge (5 ~ 10C discharge), stable discharge voltage, safety (non-burning, non-exploding), life (cycle) Times), no pollution to the environment, it is the best, and is currently the best high-current output power lithium battery.
Here is the six advantages and three disadvantages of Lithium iron phosphate battery.
- The improvement of safety performance The PO bond in the lithium iron phosphate crystal is stable and difficult to decompose. Even at high temperature or overcharge, it will not collapse and generate heat or form strong oxidizing substances like lithium cobalt oxide, so it has good safety. sex. A report pointed out that in the actual operation, a small number of samples were found to be burning in the acupuncture or short-circuit experiments, but there was no explosion. explosion phenomenon. Even so, its overcharge safety has been greatly improved compared with ordinary liquid electrolyte lithium-ion cobalt oxide batteries.
- Lifetime improvement Lithium iron phosphate battery refers to a lithium ion battery using lithium iron phosphate as the positive electrode material. The cycle life of long-life lead-acid batteries is about 300 times, and the maximum is 500 times, while the lithium iron phosphate power lithium battery has a cycle life of more than 2,000 times, and can be used for 2,000 times with standard charging (5-hour rate). The lead-acid battery of the same quality is “new half year, old half year, and maintenance and maintenance for half a year”, which is 1 to 1.5 years at most, while lithium iron phosphate battery is used under the same conditions, and the theoretical life will reach 7 to 8 years. 《Related articles: How many times is the lithium battery life Cycle? How Long Is The cycle Life Of Lithium Battery? 》
Comprehensive consideration, the performance-price ratio is theoretically more than 4 times that of lead-acid batteries. High-current discharge can quickly charge and discharge high-current 2C. Under the special charger, the battery can be fully charged within 40 minutes of 1.5C charging, and the starting current can reach 2C, but lead-acid batteries do not have this performance.
- Good high temperature performance, the electric heating peak of lithium iron phosphate can reach 350℃-500℃, while lithium manganate and lithium cobaltate are only around 200℃. Wide operating temperature range (-20C–75C), with high temperature resistance, the electric heating peak of lithium iron phosphate can reach 350℃-500℃, while lithium manganate and lithium cobaltate are only around 200℃.
- Large-capacity’s rechargeable batteries often work under the condition of being fully charged, and the capacity will quickly drop below the rated capacity. This phenomenon is called the memory effect. Like nickel-metal hydride and nickel-cadmium batteries, there is memory, but lithium iron phosphate batteries do not have this phenomenon. No matter what state the battery is in, it can be used at any time without having to discharge it before charging.
- Light weight
The volume of the lithium iron phosphate battery with the same specification and capacity is 2/3 of the volume of the lead-acid battery, and the weight is 1/3 of the lead-acid battery. 《Related articles: the comparison between lithium battery and AGM battery》
- Environmentally friendly
Lithium iron phosphate batteries are generally considered to be free of any heavy metals and rare metals (nickel-metal hydride batteries require rare metals), non-toxic (SGS certified), non-polluting, comply with European RoHS regulations, and are an absolute green battery certificate. Therefore, the reason why the lithium-ion battery is favored by the industry is mainly due to environmental protection considerations. Therefore, the battery has been included in the “863” national high-tech development plan during the “Tenth Five-Year Plan” period and has become a key project supported and encouraged by the state.
With my country’s entry into the WTO, the export volume of my country’s electric bicycles will increase rapidly, and electric bicycles entering Europe and the United States have been required to be equipped with non-polluting batteries. However, some experts said that the environmental pollution caused by lead-acid batteries mainly occurred in the company’s non-standard production process and recycling process.
In the same way, lithium-ion batteries are good in the new energy industry, but they cannot prevent the problem of heavy metal pollution. Lead, arsenic, cadmium, mercury, chromium, etc. in the processing of metal materials may be released into dust and water.
The battery itself is a chemical substance, so there may be two kinds of pollution: one is the process waste pollution in the production project; the other is the battery pollution after scrapping. Lithium iron phosphate batteries also have their shortcomings: for example, low temperature performance is poor, the tap density of positive electrode materials is low, and the volume of lithium iron phosphate batteries of equal capacity is larger than that of lithium ion batteries such as lithium cobalt oxide, so it has no advantages in micro batteries. When used in power lithium batteries, lithium iron phosphate batteries are the same as other batteries, and they have to face the problem of battery consistency.
Lithium iron phosphate has some performance defects, such as low tap density and compaction density, resulting in low energy density of lithium-ion batteries. The low temperature performance is poor, and even nano-encapsulation and carbon coating did not solve this problem. When Dr. Don Hillebrand, director of the Energy Storage System Center of Argonne National Laboratory in the United States, talked about the low temperature performance of lithium iron phosphate batteries, he described it as terrible. It is not possible to drive an electric vehicle at low temperature (below 0°C). Although some manufacturers claim that the lithium iron phosphate battery has a good capacity retention rate at low temperatures, it is in the case of a small discharge current and a low discharge cut-off voltage. In this condition, the device cannot start working at all.
The preparation cost of the material and the manufacturing cost of the battery are high, the yield of the battery is low, and the consistency is poor. Although the nanoscale and carbon coating of lithium iron phosphate improves the electrochemical performance of the material, it also brings other problems, such as the reduction of energy density, the increase of synthesis cost, poor electrode processing performance, and harsh environmental requirements. Although the chemical elements Li, Fe and p in lithium iron phosphate are very rich and the cost is low, the cost of the prepared lithium iron phosphate product is not low. Even if the early research and development costs are removed, the process cost of this material is relatively high. The cost of preparing the battery will make the final unit energy storage cost higher.
Poor product consistency. At present, there is no lithium iron phosphate material factory in China that can solve this problem. From the perspective of material preparation, the synthesis reaction of lithium iron phosphate is a complex heterogeneous reaction, including solid phase phosphate, iron oxide and lithium salt, plus carbon precursor and reducing gas phase. In this complex reaction process, it is difficult to ensure the consistency of the reaction.
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