Do lithium iron phosphate batteries really need to be activated

The full name of lithium iron phosphate battery is lithium iron phosphate lithium ion battery, which is too long, called lithium iron phosphate battery for short. Since its performance is particularly suitable for power applications, the word "power" was added to the name, that is, lithium iron phosphate power battery. It is also called "lithium iron (LiFe) power battery".

The working principle of

Lithium iron phosphate battery refers to the lithium ion battery with lithium iron phosphate as the positive electrode material. The cathode materials of lithium ion battery mainly include lithium cobalt acid, lithium manganese acid, lithium nickel acid, ternary materials, lithium iron phosphate, etc. lithium cobalt oxide is the anode material used in most lithium ion batteries.

meaning

In the metals market, cobalt (Co) is the most expensive and has less storage, nickel (Ni) and manganese (Mn) are cheaper and iron (Fe) has more. The prices of anode materials are also in line with those of these metals. Therefore, lithium ion batteries made of LiFePO4 anode materials should be relatively cheap. Another characteristic of it is no pollution to the environment.

As the requirements of the rechargeable batteries are: high capacity, high output voltage, good charge and discharge cycle performance, stable output voltage, large current charge and discharge, electrochemical stability, use of safety (did not charge, discharge and short circuit caused by improper operation, such as the combustion or explosion), wide working temperature range, non-toxic or less poison, no pollution to the environment. Adopting LiFePO4 as anode of lithium iron phosphate batteries are good on these performance requirements, particularly in large discharge rate discharge (5 ~ 10 c discharge), stable discharge voltage, security, not burning, not an explosion, and life (cycles), on the environment pollution-free, it is the best, is currently the best large current output power battery.

Structure and working principle

The LiFePO4, as the positive electrode of the battery, is connected with the positive electrode by aluminum foil. In the middle is a polymer membrane, which separates the positive electrode from the negative electrode. However, lithium ion Li can pass through while electron e- cannot. Between the upper and lower ends of the battery is the electrolyte of the battery, which is sealed by a metal shell.

When the LiFePO4 battery is charged, lithium Li in the positive electrode migrates to the negative electrode through the polymer membrane. During the discharge process, lithium Li in the negative electrode migrates to the positive electrode through the membrane. Lithium-ion batteries are named for the movement of lithium ions back and forth between charge and discharge.

The main performance

The nominal voltage of LiFePO4 battery is 3.2v, the termination charging voltage is 3.6v, and the termination discharge voltage is 2.0v. Due to the different quality and process of anode and cathode materials and electrolyte materials used by various manufacturers, there will be some differences in their performance. For example, the battery capacity of the same type (standard battery in the same package) varies greatly (10% ~ 20%).

It should be noted here that the performance parameters of lithium iron phosphate power batteries produced by different factories will have some differences. In addition, some battery properties are not included, such as battery internal resistance, self-discharge rate, charging and discharging temperature.

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

Over discharge to zero voltage test:

The STL18650(1100mAh) lithium iron phosphate battery was used for the test from discharge to zero voltage. Test conditions: 1100mAh STL18650 battery was fully charged with 0.5c charging rate, and then discharged to 0C battery voltage with 1.0c discharging rate. Then put the 0V battery into two groups: one group for 7 days, another group for 30 days; When the storage expires, fill it with 0.5c charging rate and discharge it with 1.0c. Finally, the differences between the two ZVS storage periods were 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, no leakage, good performance, 98% capacity; After 30 days of storage, the battery was charged and discharged for 3 more cycles, and the capacity was restored to 100%.

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

Advantages of lithium iron phosphate batteries

1. Improvement of safety performance

The p-o bond in the crystal of lithium iron phosphate is stable and difficult to decompose. Even at high temperature or over charge, it will not generate heat or form strong oxidizing substances like lithium cobalt oxide, so it has good safety. Some reports pointed out that in the actual operation of acupuncture or short circuit experiment, a small part of samples were found to have combustion phenomenon, but there was no explosion event. However, in the overcharge experiment, a high voltage charge that was several times higher than its own discharge voltage was used, and explosion phenomenon was still found. Nevertheless, its overcharge safety has been greatly improved compared with ordinary liquid electrolyte lithium cobalt battery.

2. Improvement of life expectancy

Lithium iron phosphate battery refers to lithium ion battery with lithium iron phosphate as the positive electrode material.

Long life lead-acid battery cycle life of 300 times or so, the highest is 500 times, and lithium iron phosphate power battery cycle life of more than 2000 times, the standard charge (5 hour rate) use, can reach 2000 times. Lead-acid battery of the same quality is "new half year, old half year, maintenance and maintenance half year", the most is 1~1.5 years time, and lithium iron phosphate battery in the same condition, the theoretical life will reach 7~8 years. Overall, the performance price is more than four times that of the theoretical lead-acid battery. Large current discharge can be large current 2C quick charge and discharge, in the special charger, 1.5C charge can be full of 40 minutes, starting current up to 2C, while the lead-acid battery does not have this performance.

3, high temperature performance is good

Lithium iron phosphate can reach 350 ℃ to 500 ℃ and the electric peak and cobalt acid lithium manganese acid lithium only at about 200 ℃. Wide range of operating temperature (20 c - 75 - c), have a high temperature resistant properties of lithium iron phosphate can reach 350 ℃ to 500 ℃ heating peak lithium cobalt and lithium manganese acid and acid only at about 200 ℃.

4. Large capacity

A rechargeable battery operating at a constant full capacity can rapidly fall below the rated capacity, a phenomenon called memory effect. For example, nickel-metal hydride and nickel-cadmium batteries have memory, while lithium iron phosphate batteries have no such phenomenon. No matter what state the batteries are in, they can be filled and used at any time, without having to be put out before recharging.

6. Light weight

Lithium iron phosphate batteries of the same size are two-thirds the size and one-third the weight of lead-acid batteries.

7, the 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 environmental protection battery certificate. Therefore, lithium battery is favored by the industry, mainly due to environmental considerations, so the battery was included in the "863" national high-tech development plan during the "tenth five-year plan" period, which has become the national key support and encourage development projects. With China's accession to the WTO, China's exports of electric bicycles will rapidly increase, and the United States and Europe have entered the e-bike requirements with pollution-free batteries.

But some experts said that the environmental pollution caused by lead-acid batteries mainly occurred in the non-standard production process and recycling process. Similarly, lithium batteries belong to the new energy industry is good, but it can not avoid the problem of heavy metal pollution. Metal materials such as lead, arsenic, cadmium, mercury and chromium can be released into dust and water. Battery itself is a chemical substance, so there may be two kinds of pollution: one is the production process waste pollution; Second, the battery pollution after scrapping.

Lithium iron phosphate battery also has its disadvantages: such as low temperature performance is poor, the positive electrode material density is small, the volume of equal capacity lithium iron phosphate battery is larger than lithium ion battery such as lithium cobalt acid, so it does not have an 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.

Disadvantages of lithium iron phosphate batteries

Whether a material has the potential of application development, in addition to focusing on its advantages, more critical is whether the material has fundamental defects.

At present, lithium iron phosphate is widely used as the positive electrode material for dynamic 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 dynamic lithium ion batteries. The reasons for this are as follows: the first is the influence of the research and development direction of the United States. Valence and A123 of the United States were the first to use lithium iron phosphate as the cathode material for lithium ion batteries. Secondly, lithium manganate materials with good high temperature circulation and storage performance for dynamic lithium ion batteries have not been prepared in China. However, lithium iron phosphate also has fundamental defects that cannot be ignored, which can be summarized as follows:

1. In the sintering process of the preparation of lithium iron phosphate, there is a possibility that the iron oxide can be reduced to the elemental iron under the high temperature reducing atmosphere. The elemental iron is the most taboo substance in the battery because it can cause the micro-short circuit of the battery. This is also the main reason why Japan has not used this material as a positive electrode material for dynamic lithium ion batteries.

2. Lithium iron phosphate has some performance defects, such as low density of vibration and compaction, resulting in low energy density of lithium ion battery. The low temperature performance is poor, even if the nano and carbon coating did not solve this problem. Argonne national laboratory, Dr DonHillebrand, director of the center for energy storage system when it comes to the lithium iron phosphate battery performance at low temperature with terrible to describe, their type of lithium iron phosphate lithium ion battery test results show that shows that the lithium iron phosphate battery at low temperature (below 0 ℃) could not make the electric car. Although some manufacturers claim that the capacity retention rate of lithium iron phosphate battery is good at low temperature, that is in the case of low discharge current and low cut-off voltage. In this case, the device won't start at all.

3. The production cost of materials is higher than that of batteries, with low yield and poor consistency. Although the nanometer and carbon coating of lithium iron phosphate improve the electrochemical properties of the materials, they also bring other problems, such as the reduction of energy density, the increase of synthesis cost, poor electrode processing performance and the harsh requirements on the environment. Although the chemical elements Li, Fe and P in lithium iron phosphate are abundant and the cost is low, the cost of the prepared lithium iron phosphate product is not low. Even if the early-stage research and development cost is removed, the material's process cost and the high cost of preparing batteries 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 point of view of material preparation, the synthesis reaction of lithium iron phosphate is a complex multiphase reaction, including solid phase phosphates, iron oxides and lithium salts, plus carbon precursor and reducing gas phase. In this complex reaction process, it is difficult to guarantee the consistency of the reaction.

5. Intellectual property issues. At present, the basic patent of lithium iron phosphate is owned by the university of Texas, while the carbon coating patent is applied by the Canadian. There is no way around these two foundational patents, and if royalties are included in the cost, the cost of the product will rise further.

In addition, from the research and development and production of lithium ion battery experience, Japan is the earliest commercial lithium ion battery countries, and has been occupying the high-end lithium ion battery market. The United States, though leading in some basic research, has so far not had a large lithium ion battery manufacturer. Therefore, it is more reasonable for Japan to choose modified lithium manganate as the positive electrode material for dynamic lithium ion batteries. Even in the United States, manufacturers of lithium iron phosphate and lithium manganate as anode materials for power lithium-ion batteries are evenly split, and the federal government supports the development of both systems. In view of the above problems existing in lithium iron phosphate, it is difficult to be widely used as a positive electrode material for dynamic 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, it will have great potential in the application of dynamic lithium ion batteries due to its advantages of low cost and high magnification performance.

The page contains the contents of the machine translation.