Views: 770 Author: Site Editor Publish Time: 2024-03-25 Origin: Site
working principle
Lithium iron phosphate battery refers to a lithium-ion battery using lithium iron phosphate as the positive electrode material. The cathode materials of lithium-ion batteries mainly include lithium cobalt oxide, lithium manganate, lithium nickel oxide, ternary materials, lithium iron phosphate, etc. Among them, lithium cobalt oxide is the cathode material currently used in most lithium-ion batteries.
significance
In the metal trading market, cobalt (Co) is the most expensive and has a small storage capacity. Nickel (Ni) and manganese (Mn) are relatively cheap, while iron (Fe) has a large storage capacity. The price of cathode materials is also in line with the price trends of these metals. Therefore, lithium-ion batteries made of LiFePO4 cathode material should be quite cheap. Another feature of it is that it is environmentally friendly and pollution-free.
The requirements for a rechargeable battery are: high capacity, high output voltage, good charge and discharge cycle performance, stable output voltage, capable of large current charge and discharge, electrochemical stability, and safety during use (not due to overcharge, overdischarge, or short circuit) etc. may cause combustion or explosion due to improper operation), wide operating temperature range, non-toxic or less toxic, and no pollution to the environment. Lithium iron phosphate batteries using LiFePO4 as the positive electrode are good in these performance requirements, especially in terms of large discharge rate discharge (5~10C discharge), stable discharge voltage, safety (no burning, no explosion), and lifespan (number of cycles). ), no pollution to the environment, it is the best, and it is currently the best high-current output power battery.
Structure and working principle
LiFePO4 battery serves as the positive electrode of the battery. It is connected to the positive electrode of the battery by aluminum foil. There is a polymer separator in the middle, which separates the positive electrode from the negative electrode. However, lithium ions Li can pass but electrons e- cannot pass. The right side is composed of carbon (graphite) The negative terminal of the battery is connected to the negative terminal of the battery by copper foil. Between the upper and lower ends of the battery is the electrolyte of the battery, and the battery is sealed by a metal casing.
When the LiFePO4 battery is charged, the lithium ions Li in the positive electrode migrate to the negative electrode through the polymer separator; during the discharge process, the lithium ions Li in the negative electrode migrate to the positive electrode through the separator. Lithium-ion batteries are named because lithium ions migrate back and forth during charging and discharging.
Main performance
The nominal voltage of LiFePO4 battery is 3.2V, the termination voltage is 3.6V, and the termination voltage is 2.0V. Due to the different quality and process of the positive and negative electrode materials and electrolyte materials used by various manufacturers, there will be some differences in their performance. For example, the same model (standard battery in the same package) has a large difference in battery capacity (10% to 20%).
What should be noted here is that lithium iron phosphate power batteries produced by different factories will have some differences in various performance parameters; in addition, some battery properties are not included, such as battery internal resistance, self-discharge rate, charge and discharge temperature, etc.
The capacity of lithium iron phosphate power batteries varies greatly and can be divided into three categories: small ones ranging from a few tenths to several milliamp hours, medium-sized ones tens of milliamp hours, and large ones hundreds of milliamp hours. There are also some differences in the same parameters of different types of batteries.
Over-discharge to zero voltage test:
The lithium iron phosphate power battery using STL18650 (1100mAh) has been discharged to zero voltage test. Test conditions: Fully charge the 1100mAh STL18650 battery with a 0.5C charge rate, and then discharge it with a 1.0C discharge rate until the battery voltage is 0C. 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 a 1.0C rate. Finally, compare the differences between the two zero-voltage storage periods.
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, the battery has 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 restored to 100%.
This test shows that even if the lithium iron phosphate battery is over-discharged (even to 0V) and stored for a certain 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.
1. Improvement of safety performance
The P-O bond in the lithium iron phosphate crystal is stable and difficult to decompose. Even at high temperatures or overcharge, it will not collapse and generate heat or form strong oxidizing substances like lithium cobalt oxide, so it has good safety. Some reports pointed out that in actual operations, a small number of samples were found to be burning during acupuncture or short-circuit experiments, but no explosion occurred. In overcharge experiments, high-voltage charging that was many times higher than the discharge voltage was used, and it was found that there were still explosion phenomenon. Even so, its overcharge safety has been greatly improved compared to ordinary liquid electrolyte lithium cobalt oxide batteries.
2. Improvement of life span
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 cycle life of lithium iron phosphate power batteries reaches more than 2,000 times, and can reach 2,000 times with standard charging (5 hours rate). Lead-acid batteries of the same quality last "half a year for new ones, half a year for old ones, and another half year for maintenance", which is 1 to 1.5 years at most. However, when used under the same conditions, the theoretical life of lithium iron phosphate batteries will reach 7 to 8 years. 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 at a high current of 2C. Under a special charger, the battery can be fully charged within 40 minutes at 1.5C, and the starting current can reach 2C. Lead-acid batteries do not have this performance.
3. Good high temperature performance
The electric heating peak of lithium iron phosphate can reach 350℃-500℃, while the peak value of lithium manganate and lithium cobalt oxide is only about 200℃. The operating temperature range is wide (-20C--75C), and the lithium iron phosphate electric heating peak can reach 350℃-500℃, while the lithium manganate and lithium cobalt oxide are only around 200℃.
4. Large capacity
When the battery is constantly working under the condition of being fully charged and not fully discharged, the capacity will quickly fall below the rated capacity value. This phenomenon is called the memory effect. Nickel metal hydride and nickel cadmium batteries have memory, but lithium iron phosphate batteries do not have this phenomenon. No matter what state the battery is in, it can be charged and used at any time, and there is no need to discharge it first and then charge it.
6. Light weight
The volume of a lithium iron phosphate battery with the same specification and capacity is 2/3 that of a lead-acid battery, and its weight is 1/3 that of a lead-acid battery.
7. Environmental protection
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, compliant with European RoHS regulations, and an absolute green battery certificate. Therefore, the reason why lithium batteries are 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 national support and encouragement development project. With China's accession to the WTO, the export volume of China's electric bicycles will increase rapidly, and electric bicycles entering Europe and the United States are required to be equipped with pollution-free batteries.
However, some experts said that environmental pollution caused by lead-acid batteries mainly occurs in the company's non-standard production process and recycling processing links. In the same way, lithium batteries are good for the new energy industry, but they cannot avoid the problem of heavy metal pollution. Lead, arsenic, cadmium, mercury, chromium, etc. may be released into dust and water during the processing of metal materials. The battery itself is a chemical substance, so it may produce two types of pollution: one is pollution from process effluents in the production project; the other is pollution from batteries after they are scrapped.
Lithium iron phosphate batteries also have their shortcomings: for example, poor low-temperature performance, low tap density of the cathode material, and lithium iron phosphate batteries of the same capacity are larger than lithium-ion batteries such as lithium cobalt oxide, so they do not have advantages in micro batteries. When used in power batteries, lithium iron phosphate batteries, like other batteries, need to face battery consistency issues.
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