Lithium iron phosphate battery refers to a lithium-ion battery that uses lithium iron phosphate as the positive electrode material. The positive electrode materials of lithium-ion batteries mainly include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, ternary materials, lithium iron phosphate, etc. Among them, lithium cobalt oxide is the positive electrode material used by most lithium-ion batteries at present. From the principle of materials, lithium iron phosphate is also an embedding and de-embedding process, and this principle is exactly the same as that of lithium cobalt oxide and lithium manganese oxide.
1. Introduction
Lithium iron phosphate batteries are lithium-ion secondary batteries. One of their main uses is in power batteries. They have great advantages over NI-MH and Ni-Cd batteries.
Lithium iron phosphate batteries have a high charging and discharging efficiency, which can reach over 90% under rate discharge conditions, while lead-acid batteries are about 80%.
2. Eight advantages
Improved safety performance
The PO bonds in lithium iron phosphate crystals are stable and difficult to decompose. Even at high temperatures or when overcharged, they will not collapse and generate heat or form strong oxidizing substances like lithium cobalt oxide, so they have good safety. It has been reported that in actual operations, a small number of samples were found to burn during needle puncture or short-circuit tests, but no explosion occurred. In overcharge tests, high voltages that were several times higher than their own discharge voltage were used for charging, and explosions were still found. Despite this, its overcharge safety has been greatly improved compared to ordinary liquid electrolyte lithium cobalt oxide batteries.
Improved life expectancy
Lithium iron phosphate battery refers to a lithium-ion battery that uses 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 is more than 2000 times, and can reach 2000 times with standard charging (5-hour rate). Lead-acid batteries of the same quality are "new for half a year, old for half a year, and maintenance for another half a year", which is at most 1 to 1.5 years, while lithium iron phosphate batteries can reach 7 to 8 years in theory under the same conditions. Taking all factors into consideration, the performance-price ratio is theoretically more than 4 times that of lead-acid batteries. High-current discharge can be fast charged and discharged with high current 2C. Under a special charger, the battery can be fully charged within 40 minutes with 1.5C charging, and the starting current can reach 2C, which lead-acid batteries do not have.
Good high temperature performance
The peak value of lithium iron phosphate electric heating can reach 350℃-500℃, while lithium manganese oxide and lithium cobalt oxide are only around 200℃. Wide operating temperature range (-20C--+75C), high temperature resistance The peak value of lithium iron phosphate electric heating can reach 350℃-500℃, while lithium manganese oxide and lithium cobalt oxide are only around 200℃.
Large capacity
It has a larger capacity than ordinary batteries (lead acid, etc.). The single cell capacity is 5AH-1000AH.
No memory effect
If a rechargeable battery is often fully charged but not fully discharged, its capacity will quickly drop below the rated capacity. This phenomenon is called the memory effect. NiMH and NiCd batteries have memory, but LiFePO4 batteries do not. No matter what state the battery is in, it can be charged and used at any time without having to fully discharge before charging.
Lightweight
The volume of a lithium iron phosphate battery of the same specification and capacity is 2/3 of that of a lead-acid battery, and its weight is 1/3 of that of a lead-acid battery.
Environmental friendly
The battery is generally considered to be free of any heavy metals and rare metals (NiMH batteries require rare metals), non-toxic (SGS certified), pollution-free, and in compliance with European RoHS regulations, and is an absolute green battery. Therefore, the reason why lithium batteries are favored by the industry is mainly due to environmental considerations. Therefore, the battery is included in the "863" national high-tech development plan during the "15th Five-Year Plan" period and has become a project that the country focuses on supporting and encouraging development. With China's entry into the WTO, the export volume of Chinese 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 the environmental pollution caused by lead-acid batteries mainly occurs in the non-standard production process and recycling links of enterprises. Similarly, lithium batteries are good in the new energy industry, but they cannot avoid 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 it may cause two kinds of pollution: one is the process waste pollution in the production process; the other is the battery pollution after scrapping.
Lithium iron phosphate batteries also have their disadvantages: for example, poor low-temperature performance, low tap density of the positive electrode material, and the volume of lithium iron phosphate batteries of the same capacity is larger than lithium-ion batteries such as lithium cobalt oxide, so they do not have an advantage in micro batteries. When used in power batteries, lithium iron phosphate batteries, like other batteries, need to face the problem of battery consistency.
Comparison of power batteries
At present, the most promising cathode materials for power lithium-ion batteries are modified lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4) and nickel cobalt manganese oxide (Li(Ni,Co,Mn)O2) ternary materials. Due to the lack of cobalt resources, the high cost of nickel and cobalt, and the large price fluctuations, it is generally believed that nickel cobalt manganese oxide ternary materials are unlikely to become the mainstream of power lithium-ion batteries for electric vehicles, but they can be mixed with spinel lithium manganese oxide within a certain range.
Industry Applications
Carbon-coated aluminum foil brings technological innovation and industrial upgrading to the lithium battery industry;
Improve the performance of lithium battery products and improve the discharge rate.
As domestic battery manufacturers have increasingly higher requirements for battery performance, new energy battery materials are generally recognized in China: conductive materials, conductive coated aluminum foil, and copper foil.
Its advantages are: when processing battery materials, it often has good high-rate charge and discharge performance and large specific capacity, but due to poor cycle stability and severe attenuation, it has to be given up.
This is a magical coating that will take battery performance into a new era.
Conductive coating is composed of dispersed nano-conductive graphite coated particles. It can provide excellent static conductivity and is a protective energy absorption layer. It also provides good covering and protection performance. The coating is water-based and solvent-based and can be applied to aluminum, copper, stainless steel, aluminum and titanium bipolar plates.
Carbon coating improves the performance of lithium batteries in the following ways:
1. Reduce the internal resistance of the battery and suppress the increase of dynamic internal resistance during the charge and discharge cycle;
2. Significantly improve the consistency of battery packs and reduce battery pack costs;
3. Improve the bonding adhesion between active materials and current collectors and reduce the manufacturing cost of pole pieces;
4. Reduce polarization, improve rate performance, and reduce thermal effects;
5. Prevent the electrolyte from corroding the current collector;
6. Comprehensive factors to extend battery life;
7. Coating thickness: conventional single-sided thickness 1 ~ 3μm.
In recent years, Japan and South Korea have mainly developed power lithium-ion batteries with modified lithium manganese oxide and nickel-cobalt lithium manganese oxide ternary materials as positive electrode materials, such as Panasonic EV Energy Co., Ltd., a joint venture between Toyota and Panasonic, Hitachi, Sony, Shin Kobe Electric, NEC, Sanyo Electric, Samsung and LG. The United States mainly develops power lithium-ion batteries with lithium iron phosphate as positive electrode materials, such as A123 Systems and Valence, but major American automakers choose manganese-based positive electrode material system power lithium-ion batteries in their PHEV and EV, and it is said that A123 is considering entering the field of lithium manganese oxide materials, while European countries such as Germany mainly adopt the way of cooperating with battery companies in other countries to develop electric vehicles, such as Daimler Benz and France Saft Alliance, German Volkswagen and Japan Sanyo Agreement Cooperation. At present, Volkswagen of Germany and Renault of France are also developing and producing power lithium-ion batteries with the support of their own governments.
3. Disadvantages
Whether a material has application development potential, in addition to paying attention to its advantages, the more critical thing is whether the material has fundamental defects.
Lithium iron phosphate is now widely chosen as the positive electrode material for power lithium-ion batteries in China. Market analysts from the government, scientific research institutions, enterprises and even securities companies are optimistic about this material and regard it as the development direction of power lithium-ion batteries. There are two main reasons for this: First, influenced by the research and development direction of the United States, Valence and A123 of the United States first used lithium iron phosphate as the positive electrode material for lithium-ion batteries. Secondly, China has not yet prepared lithium manganese oxide materials with good high-temperature cycle and storage performance for power lithium-ion batteries. However, lithium iron phosphate also has fundamental defects that cannot be ignored. In summary, there are the following points:
1. During the sintering process of lithium iron phosphate preparation, iron oxide may be reduced to elemental iron in a high-temperature reducing atmosphere. Elemental iron can cause micro-short circuits in batteries and is the most taboo substance in batteries. This is also the main reason why Japan has not used this material as a positive electrode material for power lithium-ion batteries.
2. Lithium iron phosphate has some performance defects, such as low tap density and compaction density, which leads to low energy density of lithium-ion batteries. The low-temperature performance is poor, and even nano-sizing and carbon coating cannot 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 used the word "terrible" to describe it. Their test results on lithium iron phosphate lithium-ion batteries showed that lithium iron phosphate batteries cannot drive electric vehicles at low temperatures (below 0°C). Although some manufacturers claim that the capacity retention rate of lithium iron phosphate batteries is not bad at low temperatures, that is when the discharge current is small and the discharge cut-off voltage is very low. In this condition, the equipment cannot start working at all.
3. The material preparation cost and battery manufacturing cost are high, the battery yield is low, and the consistency is poor. Although the nano-sizing and carbon coating of lithium iron phosphate improve the electrochemical properties of the material, it also brings other problems, such as reduced energy density, increased synthesis cost, poor electrode processing performance, and stringent environmental requirements. Although the chemical elements Li, Fe and P in lithium iron phosphate are very abundant and the cost is low, the cost of the prepared lithium iron phosphate product is not low. Even if the initial R&D cost is removed, the process cost of the material plus the higher cost of preparing the battery will make the final unit energy storage cost higher.
4. Poor product consistency. At present, no lithium iron phosphate material factory in China can solve this problem. From the perspective of material preparation, the synthesis reaction of lithium iron phosphate is a complex multiphase reaction, with solid 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.
5. Intellectual property issues. The earliest patent application for lithium iron phosphate was obtained by FX MITTERMAIER & SOEHNE OHG (DE) on June 25, 1993, and the application result was announced on August 19 of the same year. The basic patent of lithium iron phosphate is owned by the University of Texas, and the carbon coating patent is applied for by Canadians. These two basic patents cannot be circumvented. If the patent royalties are calculated in the cost, the product cost will be further increased.
In addition, from the experience of R&D and production of lithium-ion batteries, Japan is the first country to commercialize lithium-ion batteries and has always occupied the high-end lithium-ion battery market. Although the United States is ahead in some basic research, it has not yet had a large lithium-ion battery manufacturer. Therefore, it makes more sense for Japan to choose lithium manganese oxide as the positive electrode material for power lithium-ion batteries. Even in the United States, half of the manufacturers use lithium iron phosphate and lithium manganese oxide as positive electrode materials for power lithium-ion batteries, and the federal government also supports the research and development of these two systems at the same time. In view of the above-mentioned problems of lithium iron phosphate, it is difficult to be widely used as a positive electrode material for power lithium-ion batteries in new energy vehicles and other fields. If the problem of poor high-temperature cycling and storage performance of lithium manganese oxide can be solved, it will have great potential for application in power lithium-ion batteries with its advantages of low cost and high rate performance.
4. Working Principle and Characteristics The full name of lithium iron phosphate battery is lithium iron phosphate lithium-ion battery. This name is too long, so it is abbreviated 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 battery. Some people also call it "lithium iron (LiFe) power battery".
significance
In the metal trading market, cobalt (Co) is the most expensive and has a limited storage capacity. Nickel (Ni) and manganese (Mn) are relatively cheap, while iron (Fe) is the cheapest. The price of positive electrode materials is also consistent with the price of these metals. Therefore, lithium-ion batteries made of LiFePO4 positive electrode materials should be the cheapest. Another feature of it is that it is environmentally friendly.
The requirements for rechargeable batteries are: high capacity, high output voltage, good charge and discharge cycle performance, stable output voltage, high current charge and discharge, electrochemical stability, safety in use (will not cause combustion or explosion due to improper operation such as overcharging, overdischarging and short circuit), wide operating temperature range, non-toxic or less toxic, and no pollution to the environment. Lithium iron phosphate batteries using LiFePO4 as positive electrodes are good in these performance requirements, especially in high discharge rate discharge (5-10C discharge), stable discharge voltage, safety (no combustion, no explosion), life (number of cycles), and no pollution to the environment. It is the best, and is currently the best high-current output power battery.
Structure and working principle
The internal structure of LiFePO4 battery is that the positive electrode of the battery is made of olivine structure LiFePO4, which is connected to the positive electrode of the battery by aluminum foil. In the middle is a polymer separator, which separates the positive electrode from the negative electrode, but lithium ions Li+ can pass through while electrons e- cannot. On the right is the negative electrode of the battery composed of carbon (graphite), which is connected to the negative electrode 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 shell.
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 end-of-charge voltage is 3.6V, and the end-of-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 model (standard battery of the same package) varies greatly (10% to 20%).
It should be noted here that the performance parameters of lithium iron phosphate power batteries produced by different factories will be somewhat different; in addition, some battery performance is not included, such as battery internal resistance, self-discharge rate, charging and discharging temperature, etc.
The capacity of lithium iron phosphate power batteries varies greatly and can be divided into three categories: small ones with a few tenths to a few mAh, medium ones with tens of mAh, and large ones with hundreds of mAh. There are also some differences in the same parameters of different types of batteries. The most widely used small standard cylindrical packaged lithium iron phosphate power battery has the following parameters: diameter 18mm, height 650mm (model 18650).
Over discharge to zero voltage test
The STL18650 (1100mAh) lithium iron phosphate power battery was used for a discharge to zero voltage test. Test conditions: The 1100mAh STL18650 battery was fully charged at a 0.5C charge rate, and then discharged at a 1.0C discharge rate until the battery voltage was 0C. The batteries that were discharged to 0V were then divided into two groups: one group was stored for 7 days, and the other group was stored for 30 days; after the storage period expired, they were fully charged at a 0.5C charge rate, and then discharged at 1.0C. Finally, the differences between the two zero voltage storage periods were compared.
The test results showed that after seven days of zero-voltage storage, the battery had no leakage, good performance, and a capacity of 100%; after 30 days of storage, there was no leakage, good performance, and a capacity of 98%; after 30 days of storage, the battery was subjected to three more charge and discharge cycles, and the capacity returned to 100%.
This test shows that even if the 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.
Characteristics of lithium iron phosphate battery
Through the above introduction, LiFePO4 batteries can be summarized into the following characteristics.
High-efficiency output: standard discharge is 2-5C, continuous high-current discharge can reach 10C, and instantaneous pulse discharge (10S) can reach 20C;
Good performance at high temperature: when the external temperature is 65℃, the internal temperature is as high as 95℃, and the temperature can reach 160℃ when the battery is discharged. The battery structure is safe and intact;
Even if the battery is damaged internally or externally, it will not burn or explode, and is the safest;
Excellent cycle life, after 500 cycles, its discharge capacity is still greater than 95%;
There is no damage even if it is over-discharged to zero volts;
Fast charging;
low cost;
No pollution to the environment.
Application of lithium iron phosphate power battery
Because lithium iron phosphate power batteries have the above characteristics and various batteries with different capacities are produced, they are quickly widely used. Its main application areas are:
Large electric vehicles: buses, electric cars, tourist attractions buses and hybrid vehicles, etc.;
Light electric vehicles: electric bicycles, golf carts, small flat battery vehicles, forklifts, cleaning vehicles, electric wheelchairs, etc.;
Power tools: electric drill, electric saw, lawn mower, etc.;
Remote control cars, boats, airplanes and other toys;
Energy storage equipment for solar and wind power generation;
UPS and emergency lights, warning lights and mining lamps (the safest);
Replaces 3V disposable lithium batteries and 9V NiCd or NiMH rechargeable batteries in cameras (exactly the same size);
Small medical instruments and portable equipment, etc.
Here is an example of using lithium iron phosphate power batteries to replace lead-acid batteries. A 36V/10Ah (360Wh) lead-acid battery weighs 12kg, can travel about 50km on a single charge, can be charged about 100 times, and can be used for about 1 year. If a lithium iron phosphate power battery is used, with the same 360Wh energy (12 10Ah batteries connected in series), it weighs about 4kg, can travel about 80km on a single charge, can be charged up to 1,000 times, and has a service life of 3 to 5 years. Although the price of lithium iron phosphate power batteries is much higher than that of lead-acid batteries, the overall economic effect is still better with lithium iron phosphate power batteries, and they are lighter in use.
5. Battery performance
The performance of lithium-ion power batteries mainly depends on the positive and negative electrode materials. Lithium iron phosphate as a lithium battery material has only appeared in recent years. The domestic development of large-capacity lithium iron phosphate batteries was in July 2005. Its safety performance and cycle life are unmatched by other materials, which are also the most important technical indicators of power batteries. The 1C charge and discharge cycle life is up to 2000 times. A single battery overcharge voltage of 30V does not burn, and puncture does not explode. Lithium iron phosphate positive electrode materials make large-capacity lithium-ion batteries easier to use in series. To meet the needs of frequent charging and discharging of electric vehicles. It has the advantages of non-toxicity, non-pollution, good safety performance, wide source of raw materials, low price, and long life. It is an ideal positive electrode material for the new generation of lithium-ion batteries.
This project belongs to the development of functional energy materials among high-tech projects, and is a key area supported by the national "863" plan, "973" plan and the "Eleventh Five-Year" high-tech industry development plan.
The positive electrode of lithium-ion batteries is made of lithium iron phosphate material, which has great advantages in safety performance and cycle life, which are also one of the most important technical indicators of power batteries. The 1C charge and discharge cycle life can reach 2,000 times, and it will not explode when punctured, and it is not easy to burn or explode when overcharged. Large-capacity lithium-ion batteries made of lithium iron phosphate positive electrode materials are easier to use in parallel and series.
6. Scientific research applications
Lithium iron phosphate battery
Recently, there have been reports of progress in new batteries that are expected to replace traditional lithium batteries, giving us hope that mobile phones and tablets will have longer battery life. Unfortunately, most of them remain in the laboratory research stage, and it is hard to say when or even whether they can be put into large-scale commercial use.
In the white paper on lithium iron phosphate battery technology released by Deboch TEC.GmbH, it is stated that after using composite nanomaterials, the energy density of a single 32650 cell (32mm diameter/65mm length) can be increased to 6000mAh. Compared with the current industry's 32650 cell specification of 5000mAh, the energy density has increased by 1000mAh, or 20%, for the same volume. One cell can charge an iPhone 4S nearly four times.
What is even more exciting is that, when used in a single low-rate charge and discharge environment, this battery can maintain a charge of about 80% after being used for up to 3,000 cycles, while ordinary lithium batteries can only do this after about 500 cycles. Calculated as charging and discharging once every 3 days, it can be used continuously for 24 years, making it a truly long-life battery.
This new battery technology can be widely used in various devices such as portable mobile power supplies, small UPS, laptop batteries, car batteries, etc. In addition, in response to different usage environments, Deboch TEC.GmbH also uses different battery cell colors according to the number of cycle charging: gold for military grade, with a cycle number of 3,000 times; blue for civilian automobiles, with a cycle number of 2,500 times; and green, with a cycle number of 2,000 times, is suitable for small portable mobile devices.
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