What is Lithium Iron Phosphate Battery?

The lithium iron phosphate battery is a lithium ion battery using lithium iron phosphate (LiFePO4) as the positive electrode material and carbon as the negative electrode material.

During the charging process, some of the lithium ions in the lithium iron phosphate are extracted, transferred to the negative electrode through the electrolyte, and embedded in the negative electrode carbon material; at the same time, electrons are released from the positive electrode and reach the negative electrode from the external circuit to maintain the balance of the chemical reaction. During the discharge process, lithium ions come out from the negative electrode and reach the positive electrode through the electrolyte. At the same time, the negative electrode releases electrons and reaches the positive electrode from the external circuit to provide energy for the outside world.

Chinese name: 磷酸铁锂电池

Foreign name: Lithium iron phosphate Battery

Abbreviation: LIFEPO4

Positive electrode: lithium iron phosphate

Negative Electrode: Carbon (Graphite)

Rated voltage: 3.2V charging

Cut-off voltage: 3.6V~3.65V

Advantages: high working voltage, high energy density, long cycle life, good safety performance, low self-discharge rate, no memory effect

What is Lithium Iron Phosphate Battery
What is Lithium Iron Phosphate Battery?

Introduction to lithium iron phosphate battery

In the crystal structure of LiFePO4, oxygen atoms are arranged in a hexagonal close-packed arrangement.

The PO43-tetrahedra and FeO6 octahedra constitute the spatial skeleton of the crystal, Li and Fe occupy the octahedral voids, while P occupies the tetrahedral voids, in which Fe occupies the corner-sharing positions of the octahedra and Li occupies the edge-sharing positions of the octahedra. FeO6 octahedra are connected to each other on the bc plane of the crystal, and LiO6 octahedral structures in the b-axis direction are connected to each other into a chain-like structure. 1 FeO6 octahedron shares edges with 2 LiO6 octahedra and 1 PO43-tetrahedron.

Due to the discontinuity of the FeO6 edge-sharing octahedral network, electronic conduction cannot be formed; at the same time, the PO43-tetrahedron limits the volume change of the lattice, which affects the deintercalation and electron diffusion of Li+, resulting in the electronic conductivity and ion diffusion of LiFePO4 cathode material. Very inefficient.

The theoretical specific capacity of LiFePO4 battery is high (about 170mAh/g), and the discharge platform is 3.4V. Li+ is de-intercalated back and forth between the positive and negative electrodes to realize charge and discharge. During charging, an oxidation reaction occurs, Li+ migrates from the positive electrode, and is embedded in the negative electrode through the electrolyte. The iron changes from Fe2+ to Fe3+, and an oxidation reaction occurs.

Structural characteristics of lithium iron phosphate battery

The left side of the lithium iron phosphate battery is a positive electrode composed of an olivine structure LiFePO4 material, which is connected to the positive electrode of the battery by an aluminum foil. 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 a copper foil. In the middle is a polymer separator, which separates the positive and negative electrodes, through which lithium ions can pass but electrons cannot. The interior of the battery is filled with electrolyte, and the battery is hermetically sealed by a metal casing.

The charge-discharge reaction of lithium iron phosphate battery is carried out between the two phases of LiFePO4 and FePO4. During the charging process, LiFePO4 is gradually separated from the lithium ions to form FePO4, and during the discharge process, the lithium ions are intercalated into FePO4 to form LiFePO4.

The charging and discharging principle of lithium iron phosphate battery

When the battery is charged, lithium ions migrate from the lithium iron phosphate crystal to the crystal surface, enter the electrolyte under the action of the electric field force, then pass through the separator, and then migrate to the surface of the graphite crystal through the electrolyte, and then embed in the graphite lattice.

At the same time, the electrons flow to the aluminum foil collector of the positive electrode through the conductor, flow to the copper foil collector of the negative electrode of the battery through the tab, the positive pole of the battery, the external circuit, the negative pole and the negative pole, and then flow to the graphite negative pole through the conductor. , so that the charge of the negative electrode reaches a balance. After the lithium ions are deintercalated from the lithium iron phosphate, the lithium iron phosphate is converted into iron phosphate.

When the battery is discharged, lithium ions are deintercalated from the graphite crystal, enter the electrolyte, and then pass through the separator, migrate to the surface of the lithium iron phosphate crystal through the electrolyte, and then re-insert into the lattice of the lithium iron phosphate.

At the same time, the electrons flow to the copper foil collector of the negative electrode through the conductor, and flow to the aluminum foil collector of the positive electrode of the battery through the tab, the negative pole of the battery, the external circuit, the positive pole and the positive pole, and then flow to the iron phosphate through the conductor. The lithium positive electrode balances the charge of the positive electrode. After the lithium ions are intercalated into the iron phosphate crystal, the iron phosphate is converted into lithium iron phosphate.

Features of LiFePO4 battery

higher energy density

According to reports, the energy density of the square aluminum shell lithium iron phosphate battery mass-produced in 2018 is about 160Wh/kg. In 2019, some excellent battery manufacturers can probably achieve the level of 175-180Wh/kg. The chip technology and capacity are made larger, or 185Wh/kg can be achieved.

good safety performance

The electrochemical performance of the cathode material of lithium iron phosphate battery is relatively stable, which determines that it has a stable charging and discharging platform. Therefore, the structure of the battery will not change during the charging and discharging process, and it will not burn and explode. It is still very safe under special conditions such as charging, squeezing, and acupuncture.

long cycle life

The 1C cycle life of lithium iron phosphate batteries generally reaches 2,000 times, or even more than 3,500 times, while the energy storage market requires more than 4,000-5,000 times, ensuring a service life of 8-10 years, which is higher than 1,000 cycles of ternary batteries. The cycle life of long-life lead-acid batteries is about 300 times.

Synthesis of LiFePO4

The synthesis process of lithium iron phosphate has been basically perfected, and it is mainly divided into solid phase method and liquid phase method. Among them, the high-temperature solid-phase reaction method is the most commonly used, and some researchers combine the microwave synthesis method in the solid-phase method and the hydrothermal synthesis method in the liquid-phase method—the microwave hydrothermal method.

In addition, the synthesis methods of lithium iron phosphate also include bionic method, cooling drying method, emulsification drying method, pulsed laser deposition method, etc. By choosing different methods, synthesizing products with small particle size and good dispersion performance can effectively shorten the diffusion path of Li+ , the contact area between the two phases increases, and the diffusion rate of Li+ increases.

Industrial application of lithium iron phosphate battery

Application of new energy vehicle industry

China’s “Energy-saving and New Energy Vehicle Industry Development Plan” proposes that “the overall goal of new energy vehicle development is: by 2020, the cumulative production and sales of new energy vehicles will reach 5 million units, and the scale of energy-saving and new energy vehicle industry will be at the forefront of the world.” . Lithium iron phosphate batteries are widely used in passenger cars, passenger cars, logistics vehicles, low-speed electric vehicles, etc. due to their advantages of good safety and low cost. Influenced by policy, ternary batteries occupy a dominant position with the advantage of energy density, but lithium iron phosphate batteries still occupy irreplaceable advantages in passenger cars, logistics vehicles and other fields. In the field of passenger cars, lithium iron phosphate batteries accounted for about 76%, 81%, 78% of the 5th, 6th, and 7th batches of the “Catalogue of Recommended Models for the Promotion and Application of New Energy Vehicles” (hereinafter referred to as the “Catalogue”) in 2018. %, still maintaining the mainstream. In the field of special vehicles, lithium iron phosphate batteries accounted for about 30%, 32%, and 40% of the 5th, 6th, and 7th batches of the “Catalogue” in 2018, respectively, and the proportion of applications has gradually increased.

Yang Yusheng, an academician of the Chinese Academy of Engineering, believes that the use of lithium iron phosphate batteries in the extended-range electric vehicle market can not only improve the safety of vehicles, but also support the marketization of extended-range electric vehicles, eliminating the mileage, safety, price, and cost of pure electric vehicles. Anxiety about charging, subsequent battery issues, etc. During the period from 2007 to 2013, many car companies have launched projects of extended-range pure electric vehicles.

Start the application on the power

In addition to the characteristics of power lithium batteries, the starter lithium iron phosphate battery also has the ability to output high power instantaneously. The traditional lead-acid battery is replaced by a power lithium battery with an energy less than one kilowatt hour, and the traditional starter motor and generator are replaced by a BSG motor. , not only has the function of idling start-stop, but also has the functions of engine shutdown and coasting, coasting and braking energy recovery, acceleration booster and electric cruise.

Applications in the energy storage market

LiFePO4 battery has a series of unique advantages such as high working voltage, high energy density, long cycle life, low self-discharge rate, no memory effect, green environmental protection, etc., and supports stepless expansion, suitable for large-scale electric energy storage, in renewable Energy power stations have good application prospects in the fields of safe grid connection of power generation, power grid peak regulation, distributed power stations, UPS power supplies, and emergency power supply systems.

According to the latest energy storage report recently released by GTM Research, an international market research organization, the application of grid-side energy storage projects in China in 2018 continued to increase the consumption of lithium iron phosphate batteries.

With the rise of the energy storage market, in recent years, some power battery companies have deployed energy storage business to open up new application markets for lithium iron phosphate batteries. On the one hand, due to the characteristics of ultra-long life, safe use, large capacity, and green environmental protection, lithium iron phosphate can be transferred to the field of energy storage, which will extend the value chain and promote the establishment of a new business model. On the other hand, the energy storage system supporting the lithium iron phosphate battery has become the mainstream choice in the market. According to reports, lithium iron phosphate batteries have been tried to be used in electric buses, electric trucks, user-side and grid-side frequency regulation.

1. Safe grid connection of renewable energy power generation such as wind power generation and photovoltaic power generation. The inherent randomness, intermittency and volatility of wind power generation determine that its large-scale development will inevitably have a significant impact on the safe operation of the power system. With the rapid development of the wind power industry, especially most of the wind farms in my country are “large-scale centralized development and long-distance transmission”, the grid-connected power generation of large-scale wind farms poses severe challenges to the operation and control of large power grids.

Photovoltaic power generation is affected by ambient temperature, solar light intensity and weather conditions, and photovoltaic power generation presents the characteristics of random fluctuations. my country presents a development trend of “decentralized development, low-voltage on-site access” and “large-scale development, medium and high voltage access”, which puts forward higher requirements for power grid peak regulation and safe operation of power systems.

Therefore, large-capacity energy storage products have become a key factor in solving the contradiction between the grid and renewable energy generation. The lithium iron phosphate battery energy storage system has the characteristics of fast conversion of working conditions, flexible operation mode, high efficiency, safety and environmental protection, and strong scalability. Local voltage control problem, improve the reliability of renewable energy power generation and improve power quality, so that renewable energy can become a continuous and stable power supply. [2]

With the continuous expansion of capacity and scale, and the continuous maturity of integrated technology, the cost of energy storage systems will be further reduced. After long-term safety and reliability tests, lithium iron phosphate battery energy storage systems are expected to be used in renewable energy such as wind power and photovoltaic power generation. It has been widely used in the safe grid connection of energy generation and the improvement of power quality.

2. Power grid peak regulation. The main means of power grid peak regulation has always been pumped storage power stations. Because the pumped-storage power station needs to build two reservoirs, the upper and lower reservoirs, which are greatly restricted by geographical conditions, it is not easy to construct in the plain area, and the area is large and the maintenance cost is high. The use of lithium iron phosphate battery energy storage system to replace the pumped storage power station, to cope with the peak load of the power grid, not limited by geographical conditions, free site selection, less investment, less land occupation, low maintenance cost, will play an important role in the process of power grid peak regulation .

3. Distributed power station.

Due to the defects of the large power grid itself, it is difficult to guarantee the quality, efficiency, safety and reliability requirements of power supply. For important units and enterprises, dual power supplies or even multiple power supplies are often required as backup and protection. The lithium iron phosphate battery energy storage system can reduce or avoid power outages caused by power grid failures and various unexpected events, and ensure safe and reliable power supply in hospitals, banks, command and control centers, data processing centers, chemical material industries and precision manufacturing industries. Play an important role.

4UPS power supply. The continuous and rapid development of China’s economy has led to the decentralization of UPS power supply users’ needs, which has caused more industries and more enterprises to have continuous demand for UPS power supply.

Compared with lead-acid batteries, lithium iron phosphate batteries have the advantages of long cycle life, safety and stability, green environmental protection, and low self-discharge rate. will be widely used.

Applications in other fields

Lithium iron phosphate batteries are also widely used in the military field due to their good cycle life, safety, low temperature performance and other advantages. On October 10, 2018, a battery company in Shandong made a strong appearance at the first Qingdao Military-Civilian Integration Technology Innovation Achievement Exhibition, and exhibited military products including -45℃ military ultra-low temperature batteries.

Lithium iron phosphate battery energy storage system

LiFePO4 battery has a series of unique advantages such as high working voltage, high energy density, long cycle life, green environmental protection, etc., and supports stepless expansion, and can be used for large-scale electrical energy storage after forming an energy storage system. The lithium iron phosphate battery energy storage system consists of a lithium iron phosphate battery pack, a battery management system (BMS), a converter device (rectifier, inverter), a central monitoring system, and a transformer.

In the charging stage, the intermittent power supply or the power grid charges the energy storage system, and the alternating current is rectified into direct current through the rectifier to charge the energy storage battery module and store energy; in the discharging stage, the energy storage system discharges to the power grid or the load, and the energy storage battery module The DC power of the inverter is converted into AC power through the inverter, and the inverter output is controlled by the central monitoring system, which can provide stable power output to the grid or load.

Echelon utilization of lithium iron phosphate battery

Generally speaking, the retired lithium iron phosphate battery of electric vehicles still has nearly 80% of the capacity remaining, and there is still 20% of the capacity from the lower limit of 60% completely scrapped capacity, which can be used in occasions with lower power requirements than automobiles, such as low-speed electric vehicles, Communication base stations, etc., to realize the cascade utilization of waste batteries. Lithium iron phosphate batteries retired from automobiles still have high utilization value. The cascade utilization process of power battery is as follows: enterprise recycling retired battery – dismantling – testing and grading – sorting by capacity – battery module reorganization. At the level of battery preparation, the residual energy density of the waste lithium iron phosphate battery can reach 60~90Wh/kg, and the recycling life can reach 400~1000 times. With the improvement of the battery preparation level, the recycling life may be further improved. Compared with lead-acid batteries with a cycle life of 45Wh/kg and a cycle life of about 500 times, waste lithium iron phosphate batteries still have performance advantages. Moreover, the cost of waste lithium iron phosphate batteries is low, only 4000~10000 yuan/t, which is very economical.

Recycling characteristics of lithium iron phosphate batteries

Rapid growth and large scrap

Since the development of the electric vehicle industry, China has become the world’s largest consumer market for lithium iron phosphate. Especially from 2012 to 2013, the growth rate was nearly 200%. In 2013, the sales volume of lithium iron phosphate in China was about 5797t, accounting for more than 50% of global sales.

In 2014, 75% of lithium iron phosphate cathode materials were sold to China. The theoretical life of lithium iron phosphate batteries is 7 to 8 years (calculated in 7 years). It is expected that about 9400t of lithium iron phosphate will be scrapped by 2021. If the huge amount of waste is not treated, it will bring not only environmental pollution, but also energy waste and economic loss.

Significant harm

LiPF6, organic carbonate, copper and other chemical substances contained in lithium iron phosphate batteries are listed in the national hazardous waste list. LiPF6 is highly corrosive and easily decomposes to produce HF in contact with water; organic solvents and their decomposition and hydrolysis products will cause serious pollution to the atmosphere, water, soil, and harm the ecosystem; heavy metals such as copper accumulate in the environment, and eventually Humans are harmed through the biological chain; once phosphorus enters lakes and other water bodies, it is very easy to cause eutrophication of water bodies. It can be seen that if the discarded lithium iron phosphate batteries are not recycled, it will cause great harm to the environment and human health.

Recycling technology is immature

The existing data show that the recycling of waste lithium iron phosphate batteries is divided into two types: one is to recover metals, and the other is to regenerate lithium iron phosphate cathode materials.

(1) Wet recovery of lithium and iron

This type of process is mainly to recover lithium. Because lithium iron phosphate does not contain precious metals, the recovery process of lithium cobaltate is modified. Firstly, the lithium iron phosphate battery is disassembled to obtain a positive electrode material, which is crushed and sieved to obtain powder; then an alkaline solution is added to the powder to dissolve aluminum and aluminum oxides, and filtered to obtain a filter residue containing lithium, iron, etc.; the filter residue is used The mixed solution of sulfuric acid and hydrogen peroxide (reducing agent) is leached to obtain leaching solution; adding alkali to precipitate ferric hydroxide, and filtering to obtain filtrate; burning ferric hydroxide to obtain ferric oxide; finally adjusting the pH value of leaching solution (5.0 ~ 8.0), filtering The filtrate is obtained from the leaching solution, and solid sodium carbonate is added to concentrate and crystallize to obtain lithium carbonate.

(2) Regenerated lithium iron phosphate

The single recovery of a certain element makes the economic benefit of the recovery of lithium iron phosphate without precious metals relatively low. Therefore, the solid-phase regeneration of lithium iron phosphate is mainly used to treat waste lithium iron phosphate batteries. This process has high recovery benefits and high comprehensive utilization rate of resources.

Firstly, the lithium iron phosphate battery is disassembled to obtain the positive electrode material, which is crushed and sieved to obtain powder; after that, the residual graphite and binder are removed by heat treatment, and then the alkaline solution is added to the powder to dissolve aluminum and aluminum oxides; Filter residue containing lithium, iron, etc., analyze the molar ratio of iron, lithium and phosphorus in the filter residue, add iron source, lithium source and phosphorus source, adjust the molar ratio of iron, lithium and phosphorus to 1:1:1; add carbon source, After ball milling, the new lithium iron phosphate cathode material is obtained by calcining in an inert atmosphere.

Incomplete recycling system

The national “863” plan, “973” plan and “Eleventh Five-Year” high-tech industry development plan all classify lithium iron phosphate batteries as key support areas, but the battery production technical requirements are relatively strict, resulting in high battery prices. On electric motorcycles and a small number of cars. Therefore, vehicle power batteries have not yet been scrapped in large quantities, and a systematic and professional vehicle power battery recycling system has not yet been established. The existing recycling system has certain problems, and the recycling efficiency is low.

This problem is mainly caused by the following aspects:

(1) Less recyclable amount

A large number of used batteries are scattered in the hands of the people, but the people have no place to put them in, so they are disposed of together with domestic waste, so that the waste batteries recovered from individuals are almost zero, and most of the recycled batteries are produced in the production process of production enterprises Scrap or old materials in stock, the number of large power batteries recovered is even less.

(2) The recycling system is not perfect

A system dedicated to recycling batteries has not yet been established in China, and it is mainly the extensive collection of small workshops. my country is a major producer and consumer of lithium-ion batteries, but due to its large population, the per capita battery ownership is relatively small. For a long time, recycling companies did not recycle individual lithium-ion batteries that had no recycling value.

(3) High barriers to entry

If an enterprise wants to recycle and dispose of used batteries, it must apply for a hazardous waste business license in accordance with the “Environmental Protection Law of the People’s Republic of China” and the “Administrative Measures for Hazardous Waste Experience Permits”. On the contrary, there are a large number of small-scale and low-tech companies, which cause the problem that batteries cannot be collected in a centralized manner.

(4) High recovery cost

A large number of lithium iron phosphate materials are used in the positive electrode of power or energy storage batteries, and the demand is far greater than that of ordinary small batteries. Recycling them has high social value, but the recycling cost is high, and lithium iron phosphate batteries do not contain valuable Metals with low economic value.

(5) Weak awareness of recycling

For a long time, there has been little publicity and education on the recycling of used batteries in my country, resulting in a lack of citizens’ in-depth understanding of the pollution hazards of used batteries, and no awareness of conscious recycling.

Dismantling and recycling of lithium iron phosphate batteries

Batteries that do not have cascade utilization value in retired lithium iron phosphate batteries and batteries after cascade utilization will eventually enter the dismantling and recycling stage. The difference between lithium iron phosphate batteries and ternary material batteries is that they do not contain heavy metals, and the recovery is mainly Li, P, and Fe. The added value of the recovered products is low, and a low-cost recovery route needs to be developed. There are mainly two recycling methods: fire method and wet method.

Fire recovery process

The traditional fire recovery is generally high-temperature incineration of electrode sheets, which burns the carbon and organic matter in the electrode fragments, and the remaining ash that cannot be burned is finally screened to obtain fine powder materials containing metals and metal oxides. The process of this method is simple, but the treatment process is long and the comprehensive recovery rate of valuable metals is low. The improved fire recovery technology removes the organic binder through calcination, separates the lithium iron phosphate powder from the aluminum foil, and obtains the lithium iron phosphate material, and then adds an appropriate amount of raw materials to obtain the required lithium, iron, and phosphorus. Molar ratio, a new lithium iron phosphate was synthesized by a high-temperature solid-phase method. According to cost estimates, the improved pyro-dry recycling of waste lithium iron phosphate batteries can be profitable, but the newly prepared lithium iron phosphate according to this recycling process has many impurities and unstable performance.

Wet recycling process

Wet recovery is mainly to dissolve the metal ions in the lithium iron phosphate battery through acid-base solution, and further use precipitation, adsorption, etc. to extract the dissolved metal ions in the form of oxides and salts. Most of the reaction processes use H2SO4, NaOH and Reagents such as H2O2. The wet recycling process is simple, the equipment requirements are not high, and it is suitable for industrial scale production.

Wet recycling of lithium iron phosphate batteries is mainly based on recycling positive electrodes. When the lithium iron phosphate positive electrode is recovered by the wet process, the aluminum foil current collector must be separated from the positive electrode active material first. One of the methods is to use lye to dissolve the current collector, and the active material does not react with the lye, and the active material can be obtained by filtration. The second method is to dissolve the binder PVDF with an organic solvent, so that the lithium iron phosphate positive electrode material is separated from the aluminum foil, the aluminum foil is reused, the active material can be subjected to subsequent treatment, and the organic solvent can be distilled to realize its recycling. Compared with the two methods, the second method is more environmentally friendly and safe. One of the recovery of lithium iron phosphate in the positive electrode is to generate lithium carbonate. This recycling method has a low cost and is adopted by most lithium iron phosphate recycling enterprises. However, iron phosphate (content 95%), the main component of lithium iron phosphate, has not been recycled, resulting in a waste of resources.

The ideal wet recovery method is to convert waste lithium iron phosphate cathode materials into lithium salts and iron phosphates to achieve full element recovery of Li, Fe, and P. In order to turn lithium ferrous phosphate into lithium salt and iron phosphate, ferrous iron needs to be oxidized to ferric iron, and lithium is leached by acid leaching or alkali leaching. Some scholars use oxidative calcination to separate aluminum flakes and lithium iron phosphate, and then leaching and separating with sulfuric acid to obtain crude iron phosphate, and the solution is decontaminated with sodium carbonate to precipitate into lithium carbonate; the filtrate is evaporated and crystallized to obtain anhydrous sodium sulfate product and sold as a by-product; The crude iron phosphate is further refined to obtain battery-grade iron phosphate, which can be used for the preparation of lithium iron phosphate materials. This process has been relatively mature after years of research.

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