Do you know the Seven advantages and five disadvantages of lithium iron phosphate batteries

The full name of lithium iron phosphate battery is lithium iron phosphate battery. The name is too long. Lithium iron phosphate battery is abbreviated as lithium iron phosphate battery. Since its performance is particularly suitable for power applications, the word "power" is added to the name, namely lithium iron phosphate power battery. Others call it "lithium iron (LiFe) power battery".

Working principle

Lithium iron phosphate battery refers to lithium ion battery using lithium iron phosphate as cathode material. lithium cobalt oxide, lithium manganate, lithium nickel oxide, ternary materials and lithium iron phosphate are the main cathode materials for lithium ion batteries. Among them, lithium cobalt oxide is the cathode material used in most lithium-ion batteries.

Significance

In the metal market, cobalt (Co) is the most expensive and has less storage capacity. Nickel (Ni) and manganese (Mn) are cheaper, while iron (Fe) has more storage capacity. The prices of cathode materials are also in line with those of these metals. Therefore, lithium-ion batteries made of LiFePO4 cathode materials should be very cheap. Another characteristic is that it is environmentally friendly and pollution-free.

The requirements for rechargeable batteries are: high capacity, high output voltage, good charge-discharge cycle performance, stable output voltage, high current charge-discharge, electrochemical stability, safety in use (not to cause combustion or explosion due to improper operation of overcharge, overdischarge and short circuit), wide operating temperature range, non-toxic or less toxic, and no pollution to the environment. Lithium iron phosphate battery with LiFePO4 as cathode has good performance requirements, especially in high discharge rate discharge (5-10C discharge), stable discharge voltage, safety (non-combustion, non-explosion), life (cycle number) and no pollution to the environment. It is the best high current output power battery at present.

Structure and working principle

LiFePO4 serves as the positive pole of the battery. It is connected with the positive pole of the battery by aluminium foil and polymer membrane in the middle. It separates the positive pole from the negative pole. However, Li ion can pass through the negative pole of the battery, but electronic e-can not. On the right side, the negative pole of the battery is composed of carbon (graphite), and copper foil is connected with the negative pole of the battery. Between the upper and lower ends of the battery is the electrolyte of the battery, which is sealed and encapsulated by a metal shell.

When LiFePO4 battery is charged, lithium ion Li in positive electrode migrates to negative electrode through polymer diaphragm, and lithium ion Li in negative electrode migrates to positive electrode through diaphragm during discharge. Lithium-ion batteries are named for the back-and-forth migration of lithium ions during charging and discharging.

main performance

The nominal voltage of LiFePO4 battery is 3.2V, the termination charge voltage is 3.6V and the termination discharge voltage is 2.0V. Because the quality and technology of positive and negative electrode materials and electrolyte materials used by various manufacturers are different, their performances will be different. For example, the capacity of the same type of battery (standard battery with the same package) is quite different (10%-20%).

It is pointed out here that there are some differences in performance parameters of lithium iron phosphate power batteries produced in different factories. In addition, some battery performances are not included, such as battery internal resistance, self-discharge rate, charging and discharging temperature, etc.

The capacity of lithium iron phosphate power batteries is quite different, which can be divided into three categories: small zero to several milliampere hours, medium tens of milliampere hours and large hundreds of milliampere hours. The same parameters of different types of batteries are also different.

Overdischarge to Zero Voltage Test:

The lithium iron phosphate power battery STL18650 (1100mAh) was used to test the discharge to zero voltage. Test conditions: 1 100 mAh STL18650 battery was filled with 0.5C charge rate, and then discharged to 0 C battery voltage with 1.0C discharge rate. The batteries are divided into two groups: one group is stored for 7 days, the other group is stored for 30 days; after storage expires, the batteries are filled with 0.5C charge rate, and then discharged with 1.0C. Finally, the differences between the two zero-voltage storage periods are compared.

The test results show that after 7 days of zero voltage storage, the battery has no leakage, good performance and 100% capacity; after 30 days storage, the battery has no leakage, good performance and 98% capacity; after 30 days storage, the battery has three charge and discharge cycles, and the capacity restores to 100%.

This test shows that even if the lithium iron phosphate battery is overdischarged (even up to 0V) and stored for a certain period of time, the battery will not leak or damage. This is a characteristic that other types of lithium-ion batteries do not possess.

advantage

1. 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 like lithium cobalt oxide or form strong oxidizing substances, so it has good safety. It has been reported that a small number of samples were found to be burning in the needle-punching or short-circuit experiments, but no explosion occurred. In the over-charging experiments, the explosion was still found when the high voltage charge was used, which was several times larger than the discharge voltage itself. Nevertheless, its overcharge safety has been greatly improved compared with ordinary liquid electrolyte lithium cobalt oxide batteries.

2. Improvement of life span

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

The cycle life of long-life lead-acid batteries is about 300 times, and the maximum is 500 times. The cycle life of lithium iron phosphate power batteries is more than 2000 times, and the standard charging rate (5-hour rate) can reach 2000 times. The lead-acid batteries with the same quality are "new half year, old half year, maintenance half year", and the maximum time is 1-1.5 years. The theoretical life of lithium iron phosphate batteries under the same conditions will reach 7-8 years. Considering comprehensively, the ratio of performance to price is more than four times that of lead-acid battery in theory. High current discharge can charge and discharge 2C rapidly. Under the special charger, the battery can be filled within 40 minutes after 1.5C charging. The starting current can reach 2C, but the lead-acid battery has no such performance.

3. Good high temperature performance

The peak value of lithium iron phosphate can reach 350 500 while that of lithium manganate and lithium cobalt is only about 200 C. The working temperature range is wide (-20C-75C), and the peak value of lithium iron phosphate with high temperature resistance can reach 350-500 (-20C-75C), while lithium manganate and lithium cobalt oxide are only about 200 (-20C-75C).

4. Large capacity

Charging batteries often work under conditions that are full and unfinished, and their capacity rapidly falls below the rated capacity. This phenomenon is called memory effect. For example, nickel-hydrogen and nickel-cadmium batteries have memory, but lithium iron phosphate batteries do not have this phenomenon. No matter what state the batteries are in, they can be used as they are charged without first putting them out and recharging them.

6. Light weight

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

7, environmental protection

Lithium iron phosphate batteries are generally considered to contain no heavy metals and rare metals (nickel-hydrogen batteries need rare metals), non-toxic (SGS certification), non-polluting, in line with European RoHS regulations, for the absolute green battery certificate. Therefore, the reason why lithium batteries are favored by the industry is mainly environmental considerations. Therefore, the batteries are included in the "863" national high-tech development plan during the "Tenth Five-Year Plan" period, which has become a national key project to support and encourage development. With China's entry into WTO, the export volume of Chinese electric bicycles will increase rapidly, and the electric bicycles entering Europe and America have been required to equip with pollution-free batteries.

But some experts say that the environmental pollution caused by lead-acid batteries mainly occurs in the enterprise's non-standard production process and recycling process. Likewise, lithium batteries are good in the new energy industry, but they can't avoid the problem of heavy metal pollution. Lead, arsenic, cadmium, mercury and chromium may be released into dust and water during metal processing. Battery itself is a kind of chemical substance, so it may produce two kinds of pollution: one is the process excrement pollution in production engineering; the other is the battery pollution after scrapping.

Lithium iron phosphate batteries also have some disadvantages, such as poor performance at low temperature, low compacting density of cathode materials, larger volume of lithium iron phosphate batteries with equal capacity than lithium cobalt oxide batteries, so they do not have advantages in micro-batteries. For power batteries, lithium iron phosphate batteries, like other batteries, need to face the problem of battery consistency.

Lithium iron phosphate battery

shortcoming

Whether a material has the potential of application and development, besides paying attention to its advantages, the more crucial is whether the material has fundamental defects.

At present, lithium iron phosphate is widely used as cathode material for power lithium ion batteries in China. Market analysts from government, research institutes, enterprises and even securities companies all take this material as the development direction of power lithium ion batteries. There are two main reasons for this: First, influenced by the direction of research and development in the United States, Valence and A123 Company in the United States first used lithium iron phosphate as cathode material for lithium ion batteries. Secondly, lithium manganate materials with good high temperature cycling and storage performance for power lithium-ion batteries have not been prepared in China. But lithium iron phosphate also has some fundamental defects which can not be ignored. In summary, there are the following points:

1. In the sintering process of lithium iron phosphate preparation, the possibility of iron oxide being reduced to elementary iron under high temperature reducing atmosphere exists. Iron is the most taboo substance in batteries, which can cause short circuit of batteries. This is also the main reason why Japan has not used this material as cathode material for power lithium-ion batteries.

2. There are some performance defects in lithium iron phosphate, such as low compaction density and compaction density, which results in low energy density of lithium ion batteries. Low temperature performance is poor, even if it is nano-sized and carbon-coated, it does not solve this problem. Dr. Don Hillebrand, Director of the Energy Storage System Center of Argonne National Laboratory, talked about the low-temperature performance of lithium iron phosphate batteries, which he described as terrible. Their test results on lithium iron phosphate batteries showed that lithium iron phosphate batteries could not drive electric vehicles at low temperatures (below 0 C). Although some manufacturers claim that the capacity retention rate of lithium iron phosphate batteries is good at low temperature, it is in the case of low discharge current and low cut-off voltage. In this case, the device simply can not start work.

3. The cost of material preparation is higher than that of battery manufacture, and the yield of battery is lower, and the consistency is poor. Although Nanocrystallization 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 processing performance of the electrodes and stringent environmental requirements. Although the chemical elements Li, Fe and P in lithium iron phosphate are abundant and the cost is low, the cost of lithium iron phosphate products is not low. Even if the cost of previous research and development is removed, the process cost of this material plus the cost of preparing batteries will make the final unit energy storage cost higher.

4. The product consistency is poor. At present, there is no lithium iron phosphate material factory in China to solve this problem. From the point of view of material preparation, the synthesis of lithium iron phosphate is a complex multiphase reaction, including solid phase phosphate, iron oxides and lithium salts, precursors of carbon addition and reductive vapor phase. In this complex reaction process, it is difficult to ensure the consistency of the reaction.

5. Intellectual property rights. At present, the basic patent of lithium iron phosphate is owned by the University of Texas in the United States, while the carbon-coated patent is applied by Canadians. These two basic patents can not be bypassed. If the royalty is calculated in the cost, the cost of the product 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 been occupying the high-end lithium-ion batteries 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 modified lithium manganate as cathode material for power lithium ion batteries. Even in the United States, the manufacturers using lithium iron phosphate and lithium manganate as cathode materials for power lithium-ion batteries account for half of the total. The federal government also supports the development of both systems. In view of the above problems of lithium iron phosphate, it is difficult to be widely used as cathode material for power lithium ion batteries in new energy automobiles and other fields. If lithium manganate can solve the problem of poor high temperature cycle and storage performance, it will have great potential in the application of power lithium-ion batteries with its advantages of low cost and high rate performance.

The page contains the contents of the machine translation.