1 Introduction

Lithium iron phosphate battery, the full name of lithium iron phosphate lithium ion battery, refers to the lithium ion battery with lithium iron phosphate as the positive electrode material. Here we talk about the battery naming rules in the industry. At this stage, we usually use the positive electrode material to name the battery. The negative electrode is generally made of graphite as a negative electrode, such as a ternary battery, which refers to NCM or NCA used as a positive electrode material, lithium cobalt oxide battery, which is used as a positive electrode material for lithium cobaltate, and similarly, lithium iron phosphate refers to Lithium iron phosphate material for the positive electrode.



The positive electrode of lithium-ion battery is lithium iron phosphate material, which has great advantages in safety performance and cycle life. These are one of the most important technical indicators of power battery. 1C charging and discharging cycle life can be achieved 2000 times, the puncture does not explode, it is not easy to burn and explode when overcharged. Lithium iron phosphate cathode materials make large-capacity lithium-ion batteries easier to use in series. In principle, lithium iron phosphate is also an embedding and deintercalation process. This principle is identical to lithium cobaltate and lithium manganate.



Compared with lead-acid batteries, he has a good life span. Lead-acid batteries are generally 1-1.5 years old. Compared with nickel-hydrogen batteries, he has a higher working voltage. Compared with nickel-cadmium batteries, he has a better environment. Friendly, this is also an important reason for lithium iron phosphate to overcome these batteries, set off a battery of lithium storm, although at this stage, the popularity of ternary batteries, but because of the safety of the ternary battery is difficult to control at high energy density, lithium iron phosphate The battery is still in large-scale application. However, domestic battery companies are developing large-scale lithium iron phosphate batteries, using high-capacity lithium iron phosphate batteries (100AH, or even 200AH batteries).

2 advantages

The main advantages of lithium iron phosphate battery:

High security performance

The PO bond in the lithium iron phosphate crystal is stable and difficult to decompose. Even at high temperature or overcharge, the structure does not collapse like a lithium cobalt oxide, or forms a strong oxidizing substance. The decomposition temperature of lithium iron phosphate is about 600 ° C. Have good security. Although there have been burns and explosions in the case of overcharging, the overcharge safety has been greatly improved compared with the ordinary liquid electrolyte lithium cobalt oxide battery and ternary battery.

long life

The cycle life of lead-acid batteries is about 300 times, up to about 500 times, while the lithium iron phosphate power battery has a cycle life of more than 2,000 times, and can be used up to 2000 times with standard charging (0.2C, 5 hours). The same quality lead-acid battery is "new half year, old half year, tinkering and half a year", up to 1~1.5 years, while lithium iron phosphate battery is used under the same conditions, the theoretical life will reach 7~8 years . Considering comprehensively, the performance price ratio is theoretically more than four times that of lead-acid batteries. High-current discharge can be quickly charged and discharged with high current 2C. Under the special charger, the battery can be fully charged within 1.5 minutes of 1.5C charging, and the starting current can reach 2C, but the lead-acid battery has no such performance.

Good temperature performance

The peak temperature of lithium iron phosphate can reach 350 ° C -500 ° C while lithium manganate and lithium cobaltate are only around 200 ° C. Wide operating temperature range (-20C--+75C), with high temperature resistance, lithium iron phosphate electric heating peak can reach 350 °C-500 °C, while lithium manganate and lithium cobalt oxide only at 200 °C.

large capacity

It has a larger capacity than ordinary batteries (lead acid, etc.). It is known by the capacity density of the battery that the energy density of the lead-acid battery is about 40 WH/kg. The mainstream lithium iron phosphate battery on the market has an energy density of 90 WH/ More than kg.

No memory effect

Rechargeable batteries work under conditions that are often not fully discharged, and the capacity will quickly fall below the rated capacity. This phenomenon is called the memory effect. Lithium-hydrogen-based and nickel-cadmium batteries have memory, but lithium iron phosphate batteries do not have this phenomenon (lithium-ion batteries generally have no memory effect). No matter what state the battery is in, it can be used with charging, without having to discharge and recharge.

Light weight

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

Environmentally friendly

The battery is generally considered to be free of any heavy metals and rare metals (Ni-MH batteries require rare metals), non-toxic (SGS certified), non-polluting, in line with European RoHS regulations, is a green battery. One of the important reasons why lithium batteries are favored by the industry is environmental considerations.

But please face it a bit. Lithium batteries are good for the new energy industry, but it can't 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 there may be two kinds of pollution: one is the process waste pollution in the production process; the other is the battery pollution after the scrap.

Other material comparison

At present, the most promising cathode materials for power-type lithium-ion batteries are mainly modified lithium manganate (LiMn2O4), lithium iron phosphate (LiFePO4) and lithium nickel cobalt manganese oxide (Li(Ni, Co, Mn)O2) ternary material. Nickel-cobalt-manganate ternary materials are generally considered to be difficult to become the mainstream of power-type lithium-ion batteries for electric vehicles due to the lack of cobalt resources and high nickel and cobalt, and the price fluctuations, but can be related to spinel manganate. Lithium is mixed in a certain range.



3 disadvantages

Lithium iron phosphate batteries also have their disadvantages: for example, poor low temperature performance, low tap density of the positive electrode material, and a lithium iron phosphate battery having a capacity of more than lithium cobalt oxide, and thus have no advantage in terms of a micro battery. When used in a power battery, a lithium iron phosphate battery, like other batteries, needs to face battery consistency problems.

The threat of elemental iron

During the sintering process in the preparation of lithium iron phosphate, iron oxide is likely to be reduced to elemental iron under a high temperature reducing atmosphere. Elemental iron can cause micro-short circuit of the battery, which is the most taboo substance in the battery. This is also the main reason why Japan has not used this material as a positive electrode material for a lithium-ion battery.

Performance defect

Lithium iron phosphate has some performance defects, such as low tap density and compaction density, resulting in lower 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. The test results of the lithium iron phosphate type lithium ion battery indicate that the lithium iron phosphate battery cannot drive the electric vehicle at a low temperature (below 0 ° C). Although some manufacturers claim that the lithium iron phosphate battery has a good capacity retention rate at low temperatures, it is in the case where the discharge current is small and the discharge cutoff voltage is very low, in which case the device cannot be started at all.

High manufacturing costs

The preparation cost of the material and the manufacturing cost of the battery are high, the battery yield is low, and the consistency is poor. The nanocrystallization and carbon coating of lithium iron phosphate, while improving the electrochemical performance of the material, also brings other problems such as a decrease in energy density, an increase in synthesis cost, poor electrode processing performance, and environmentally demanding problems. 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 previous research and development cost is removed, the process cost of the material is higher. The cost of preparing the battery will make the cost of the final unit of stored energy higher.

Poor consistency

Poor product consistency. Whether it is from material preparation or manufacturing. It is difficult to ensure product consistency, and the voltage platform of lithium iron phosphate is narrower, which increases the observability of the battery.



Intellectual property issue

It is a pity to tell you that the basic patent for lithium iron phosphate is owned by the University of Texas, and the carbon coated patent is applied by Canadians. These two basic patents cannot be circumvented. If the cost of the patent is calculated, the cost of the product will be further increased.

4 structure and principle

The internal junction of the LiFePO4 battery is an olivine-structured LiFePO4 as the positive electrode of the battery. The aluminum foil is connected to the positive electrode of the battery. The middle is a polymer separator. It separates the positive electrode from the negative electrode, but the lithium ion Li+ can pass and the electron e- cannot pass. On the right is a battery negative electrode composed of carbon (graphite), which is connected by a copper foil to the negative electrode of the battery. Between the upper and lower ends of the battery is the electrolyte of the battery, and the battery is hermetically sealed by a metal casing.



When the LiFePO4 battery is charged, the lithium ion Li+ in the positive electrode migrates toward the negative electrode through the polymer separator; during the discharge, the lithium ion Li+ in the negative electrode migrates toward the positive electrode through the separator. Lithium-ion batteries are named after the lithium ions migrate back and forth during charging and discharging. In general, the nominal voltage of the LiFePO4 battery is 3.2V, the termination charging voltage is 3.6V, and the termination discharge voltage is 2.0V.

Lithium iron phosphate power batteries have large differences in capacity and can be divided into three categories: small fractions to a few milliamperes, medium tens of milliampere-hours, and large hundreds of milliampere-hours. There are some differences in the same parameters for different types of batteries. It is used in 18650 cylindrical batteries and also in square batteries.

5 applications

Since the lithium iron phosphate power battery has the above characteristics and produces batteries of various capacities, it has been widely used. Its main application areas are:

Large electric vehicles: buses, electric cars, scenic tour buses and hybrid vehicles;

Light electric vehicles: electric bicycles, golf carts, small flat battery cars, forklifts, cleaning cars, electric wheelchairs, etc.;

Power tools: electric drills, chainsaws, lawn mowers, 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 miner's lamps (safest);

Replace the 3V disposable lithium battery in the camera and the 9V nickel-cadmium or nickel-hydrogen rechargeable battery (the same size);

Small medical equipment and portable instruments.