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Lithium iron phosphate battery refers to lithium ion battery with lithium iron phosphate as cathode material. The cathode materials of lithium-ion batteries mainly include lithium cobaltate, lithium manganate, lithium nickel oxide, ternary materials, lithium iron phosphate, etc.

The cathode of lithium-ion battery is made of lithium iron phosphate. Its safety performance and cycle life have great advantages. These are also the most important technical indicators of power battery. 1C charging and discharging cycle life can reach 2000 times, puncture does not explode, and it is not easy to burn and explode when overcharged. It is easier to use lithium iron phosphate cathode material in series to make large-capacity lithium-ion batteries.

Lithium iron phosphate battery refers to lithium ion battery with lithium iron phosphate as cathode material. The cathode materials of lithium-ion batteries mainly include lithium cobaltate, lithium manganate, lithium nickel oxide, ternary materials, lithium iron phosphate, etc. Lithium cobaltate is the cathode material used by most lithium-ion batteries at present. From the perspective of material principle, lithium iron phosphate is also a process of embedding and disembedding, which is completely the same as lithium cobaltate and lithium manganate.

1 Introduction

Lithium iron phosphate battery is a lithium-ion secondary battery. One of its main uses is for power battery, which has great advantages over NI-MH and Ni-Cd batteries.

Lithium iron phosphate battery has a high charge-discharge efficiency, and the charge-discharge efficiency can reach more than 90% in the case of multiple discharge, while that of lead-acid battery is about 80%.

2 Eight advantages

Improvement of safety performance

The P-O bond in lithium iron phosphate crystal is stable and difficult to decompose. Even at high temperature or overcharge, it will not collapse and heat or form strong oxidizing substances like lithium cobalt, so it has good safety. It is reported that in the actual operation, a small number of samples were found to have burning phenomenon in the puncture or short circuit test, but no explosion occurred. In the overcharge test, the high voltage charge which is much more than several times of its own discharge voltage was used, and the explosion phenomenon was still found. Nevertheless, its overcharge safety has been greatly improved compared with that of ordinary liquid electrolyte lithium cobalt battery.

Life improvement

Lithium iron phosphate battery refers to lithium ion battery with lithium iron phosphate as cathode material.

The cycle life of long-life lead-acid battery is about 300 times, and the maximum is 500 times. The cycle life of lithium iron phosphate power battery is more than 2000 times, and the standard charging (5-hour rate) can reach 2000 times. The lead-acid battery of the same quality is "new half a year, old half a year, and maintenance half a year", which can last for 1 to 1.5 years at most, while the lithium iron phosphate battery will have a theoretical life of 7 to 8 years under the same conditions. In comprehensive consideration, the cost performance is more than 4 times that of lead-acid battery in theory. High current discharge can quickly charge and discharge high current 2C. Under the special charger, the battery can be fully charged within 40 minutes after charging at 1.5C, and the starting current can reach 2C, while the lead-acid battery has no such performance.

High temperature performance

The electrothermal peak value of lithium iron phosphate can reach 350 ℃ - 500 ℃, while that of lithium manganate and lithium cobaltate is only about 200 ℃. The operating temperature range is wide (- 20C -+75C), and the electric peak value of lithium iron phosphate with high temperature resistance can reach 350 ℃ - 500 ℃, while that of lithium manganate and lithium cobaltate is only about 200 ℃.

high-capacity

It has larger capacity than ordinary batteries (lead acid, etc.). The monomer capacity is 5AH-1000AH.

No memory effect

Rechargeable batteries often work under the condition of being fully charged, and the capacity will rapidly fall below the rated capacity. This phenomenon is called memory effect. For example, NiMH and NiCd batteries have memory, but lithium iron phosphate batteries have no such phenomenon. No matter what state the battery is in, it can be used as soon as it is charged, without having to be discharged before charging.

Light weight

The volume of lithium iron phosphate battery with the same specification and capacity is 2/3 of that of lead-acid battery, and the weight is 1/3 of that of lead-acid battery.

environment protection

The battery is generally considered to be free of any heavy metals and rare metals (NiMH battery requires rare metals), non-toxic (SGS certification passed), non-polluting, compliant with European RoHS regulations, and an absolute green environmental protection battery certificate. Therefore, the reason why lithium battery is favored by the industry is mainly due to environmental protection. 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 China's entry into the WTO, the export volume of China's electric bicycles will increase rapidly, and the electric bicycles entering Europe and the United States have been required to be equipped with pollution-free batteries.

However, some experts said that the environmental pollution caused by lead-acid batteries mainly occurred in the nonstandard production process and recycling process of enterprises. Similarly, lithium battery belongs to the new energy industry, but it can not avoid the problem of heavy metal pollution. Lead, arsenic, cadmium, mercury, chromium, etc. may be released into dust and water during metal material processing. The battery itself is a chemical substance, so it may produce two kinds of pollution: one is the pollution of process waste in the production project; The second is battery pollution after scrapping.

The lithium iron phosphate battery also has its disadvantages: for example, the low temperature performance, the low compaction density of the cathode material, and the volume of the lithium iron phosphate battery with the same capacity is larger than that of the lithium ion battery such as lithium cobaltate, so it has no advantages in terms of miniature batteries. When used for power batteries, lithium iron phosphate batteries, like other batteries, need to face the problem of battery consistency.

Comparison of power battery

At present, the most promising cathode materials for power lithium-ion batteries are modified lithium manganate (LiMn2O4), lithium iron phosphate (LiFePO4) and lithium nickel cobalt manganate (Li (Ni, Co, Mn) O2). Due to the lack of cobalt resources and the high proportion of nickel and cobalt and the large price fluctuation, the nickel-cobalt lithium manganate ternary material is generally believed to be difficult to become the mainstream of power lithium-ion batteries for electric vehicles, but it can be mixed with spinel lithium manganate in a certain range.

Industry application

Carbon-coated aluminum foil brings technological innovation and industrial upgrading to lithium battery industry; Improve the performance of lithium products and improve the discharge rate.

With the increasing requirements of domestic battery manufacturers on battery performance, it is generally recognized that new energy battery materials are conductive materials, conductive coating aluminum foil, and copper foil.

Its advantages are that when processing battery materials, it often has high rate charge and discharge performance, large specific capacity, but poor cycle stability, serious attenuation and other reasons, so it has to make a choice.

This is a magic coating that will improve the performance of the battery and bring it into a new era.

The conductive coating is composed of well-dispersed nano-conductive graphite coated particles. It can provide excellent static conductivity and is a protective energy absorption layer. It can also provide good covering and protection performance. The coating is water-based and solvent-based and can be applied to aluminum sheet, copper sheet, stainless steel, aluminum and titanium bipolar plates.

Carbon coating can improve the performance of lithium battery as follows:

1. Reduce the internal resistance of the battery and inhibit the dynamic internal resistance increase during the charge-discharge cycle;

2. Significantly improve the consistency of battery pack and reduce the cost of battery pack;

3. Improve the adhesion between the active material and the fluid collector, and reduce the manufacturing cost of the electrode;

4. Reduce polarization, improve magnification performance and reduce thermal effect;

5. Prevent the electrolyte from corroding the collector;

6. Comprehensive factors can further extend the service life of the battery;

7. Coating thickness: 1~3 for conventional single surface μ m。

In recent years, Japan and South Korea have mainly developed power lithium-ion batteries with modified lithium manganate and nickel-cobalt lithium manganate as cathode materials, such as Panasonic EV Energy Co., Ltd., Hitachi, Sony, New Kobe Electric, NEC, Sanyo Electric, Samsung and LG, which are jointly established by Toyota and Panasonic.

The United States mainly develops power-type lithium-ion batteries with lithium iron phosphate as the cathode material, such as A123 Systems and Valence, but the major automobile manufacturers in the United States choose manganese based cathode material system power lithium-ion batteries in their PHEVs and EVs, and it is said that the United States A123 company is considering entering the field of lithium manganate materials, while Germany and other European countries mainly adopt the way of cooperation with battery companies in other countries to develop electric vehicles, For example, Daimler-Benz and the Saft alliance of France, Volkswagen of Germany and Sanyo of Japan, etc. At present, Volkswagen in Germany and Renault in France are also developing and producing power lithium-ion batteries with the support of their governments.

shortcoming

In addition to focusing on its advantages, whether a material has potential for application and development is more critical than whether it has fundamental defects.

At present, lithium iron phosphate is widely selected as the cathode material of power lithium-ion batteries in China. Market analysts from governments, 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.

The main reasons are as follows: First, due to the influence of the research and development direction in the United States, Valence and A123 companies in the United States first used lithium iron phosphate as the cathode material of lithium-ion batteries. Secondly, lithium manganate material with good high temperature cycling and storage performance for power lithium-ion batteries has not been prepared in China. However, lithium iron phosphate also has fundamental defects that cannot be ignored, which can be summarized as follows:

1. During the sintering process during the preparation of lithium iron phosphate, iron oxide may be reduced to elemental iron under high temperature reducing atmosphere. Simple iron can cause micro short circuit of battery, which is the most taboo substance in battery. This is also the main reason why Japan has not used this material as the cathode material of power lithium-ion batteries;

2. Lithium iron phosphate has some performance defects, such as low compaction density and compaction density, resulting in low energy density of lithium ion battery. The low temperature performance is poor, even if it is nano and carbon coated, this problem cannot be solved. When Dr. Don Hillebrand, director of the Energy Storage System Center of Argonne National Laboratory of the United States, talked about the low-temperature performance of lithium iron phosphate batteries, he used the term "terrible" to describe it. Their test results of lithium iron phosphate lithium-ion batteries showed that lithium iron phosphate batteries could not drive electric vehicles at low temperatures (below 0 ℃). Although some manufacturers claim that the capacity retention rate of lithium iron phosphate battery is good at low temperature, it is under the condition of low discharge current and low discharge cut-off voltage. In this case, the device cannot be started at all.

3. 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 nanometer and carbon coating of lithium iron phosphate improve the electrochemical performance of the material, it also brings other problems, such as the reduction of energy density, the improvement of synthesis cost, poor electrode processing performance and harsh environmental requirements. Although the chemical elements Li, Fe and P in lithium iron phosphate are 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 the material plus the high cost of battery preparation will make the final unit energy storage cost higher.

4. 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 of lithium iron phosphate is a complex multiphase reaction, including solid phosphate, iron oxide and lithium salt, 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 FXMITTERMAIER&SOEHNEOHG (DE) on June 25, 1993, and the application results were published on August 19, the same year. The basic patent of lithium iron phosphate is owned by the University of Texas, while the carbon coating patent is applied by Canadians. These two basic patents cannot be bypassed. If the royalty is included in the cost, the product cost will be further increased.

In addition, from the experience of research and development 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 leading in some basic research, there is no large lithium-ion battery manufacturer so far. Therefore, it is more reasonable for Japan to choose lithium manganate as the cathode material of power lithium-ion battery. Even in the United States, half of the manufacturers use lithium iron phosphate and lithium manganate as cathode materials for power lithium-ion batteries, and the federal government also supports the research and development of these two systems.

In view of the above problems of lithium iron phosphate, it is difficult to be widely used as the cathode material of power lithium-ion batteries in new energy vehicles and other fields. If it can solve the problem of poor high temperature cycling and storage performance of lithium manganate, with its advantages of low cost and high rate performance, it will have great potential in the application of power lithium-ion batteries.

6. Working principle and characteristics The full name of lithium iron phosphate battery is lithium iron phosphate battery. This name is too long, so it is called lithium iron phosphate battery for short. Because its performance is particularly suitable for power applications, the word "power", namely lithium iron phosphate power battery, is added to the name. Some people also call it "LiFe power battery".

significance

In the metal trading market, cobalt (Co) is the most expensive and has a small storage capacity. Nickel (Ni) and manganese (Mn) are cheaper, while iron (Fe) is the cheapest. The price of cathode materials is also consistent with the price of these metals. Therefore, the lithium ion battery made of LiFePO4 cathode material should be the cheapest. Another characteristic of it is that it has no pollution to the environment.

As a rechargeable battery, the requirements are: high capacity, high output voltage, good charge-discharge cycle performance, stable output voltage, large current charge-discharge, electrochemical stability, safety in use (no combustion or explosion due to improper operation of overcharge, overcharge and short circuit), wide working 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 high discharge rate discharge (5~10C discharge), stable discharge voltage, safety (non combustion, no explosion), life (cycles), and no pollution to the environment. It is the best and the best high current output power battery at present.

Structure and working principle

The internal junction of LiFePO4 battery is olivine-structured LiFePO4 as the positive pole of the battery, which is connected by aluminum foil and the positive pole of the battery. In the middle is a polymer diaphragm, which separates the positive pole from the negative pole. However, lithium ion Li+can pass through but electronic e - cannot. On the right is the negative pole of the battery composed of carbon (graphite), which is connected by copper foil and the negative pole of the battery. The electrolyte of the battery is between the upper and lower ends of the battery, and the battery is sealed by a metal shell.

When the LiFePO4 battery is charged, the lithium ion Li+in the positive electrode migrates to the negative electrode through the polymer membrane; During the discharge process, lithium ion Li+in the negative electrode migrates to the positive electrode through the diaphragm. Lithium-ion battery is named after the migration of lithium ions during charging and discharging.

main performance

The nominal voltage of LiFePO4 battery is 3.2V, the ending charge voltage is 3.6V, and the ending 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, their performance will be somewhat different. For example, the battery capacity of the same model (standard battery in the same package) is quite different (10%~20%).

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

The capacity of lithium iron phosphate power battery is quite different, which can be divided into three categories: small ones ranging from a few milliampere hours to a few milliampere hours, medium ones ranging from tens of milliampere hours, and large ones ranging from hundreds of milliampere hours. Similar parameters of different types of batteries also have some differences. At present, the small standard cylindrical packaged lithium iron phosphate power battery is widely used. Its parameters and dimensions are: diameter 18 mm, height 650 mm (model 18650).

Overdischarge to zero voltage test

Use STL18650 (1100mAh) lithium iron phosphate power battery for over-discharge to zero voltage test. Test conditions: charge 1100mAh STL18650 battery with 0.5C charge rate, and then discharge it to the battery voltage of 0C with 1.0C discharge rate. Then divide the 0V battery into two groups: one group is stored for 7 days, the other group is stored for 30 days; After the storage expires, it shall be fully charged with 0.5C charging rate, and then discharged with 1.0C. Finally, the difference between the two zero voltage storage periods is compared.

The result of the test is that the battery has no leakage after being stored at zero voltage for 7 days, with good performance and 100% capacity; After storage for 30 days, it has no leakage, good performance, and the capacity is 98%; After 30 days of storage, the battery will be charged and discharged again for 3 times, and the capacity will be restored to 100%.

This test shows that the battery will not leak or damage even if it is over-discharged (even to 0V) and stored for a certain time. This is a characteristic that other lithium ion batteries do not have.

Characteristics of lithium iron phosphate battery

Through the above introduction, LiFePO4 battery can be summarized as follows.

High efficiency output: standard discharge is 2~5C, continuous high current discharge can reach 10C, and instantaneous pulse discharge (10S) can reach 20C;

Performance at high temperature

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