Lithium Iron Phosphate Battery: What is This?

Ⅰ. Definition and development of LiFePO4 Battery

1. Definition

Lithium iron phosphate(LiFePO4) battery is a kind of lithium ion battery which uses Lithium iron phosphate as cathode material. Lithium ion batteries with carbon as the negative electrode material have a single rated voltage of 3.2V and a charging cut-off voltage of 3.6V-3.65V.

During the charging process, some lithium ions in Lithium iron phosphate are removed, transferred to the negative electrode through the electrolyte, and embedded in the carbon material of the negative electrode. At the same time, the positive electrode releases electrons, which reach the negative electrode from the external circuit, maintaining the balance of chemical reactions.

During the discharge process, lithium ions detach from the negative electrode and reach the positive electrode through the electrolyte. At the same time, the negative electrode releases electrons, which reach the positive electrode from the external circuit and provide energy to the outside world.

2. Development

(1) 1997: birth period

Professor John B. Goodenough developed Lithium iron phosphate battery.

(2) 2009-2016: development period

State subsidies have promoted the development of new energy vehicles. In the early days, China preferred commercial vehicles, and Lithium iron phosphate battery developed with the advantages of safety and cycle life.

(3) 2017-2019: cooling period

In 2017, China began to provide differentiated subsidies based on energy density. High energy density ternary materials have become the mainstream.

(4) 2019 to present: period of rapid development

From the end of 2019 to 2020, China will launch CTP and blade technologies, which will increase the upper limit of Lithium iron phosphate's volume energy density. The track in the energy storage field is popular, and Lithium iron phosphate has begun to develop at a high speed.

Ⅱ. Advantages and disadvantages of LiFePO4 Battery

1. Advantages

(1) Eco-friendly

LiFePO4 battery is generally considered to be free of any heavy metals and rare metals, non-toxic and pollution-free. The reason why lithium batteries are favored by the industry is mainly due to environmental considerations. As an important branch of lithium battery, Lithium iron phosphate has obvious environmental performance.

(2) Good safety performance

Lithium iron phosphate has good safety performance. It will not collapse and generate heat or form strong oxidizing substances like lithium cobalt oxide during high temperature or overcharging. The decomposition temperature of Lithium iron phosphate is about 600 ℃, so it has good safety.

According to the investigation, in the actual operation of needle puncture or short circuit experiments, we found that a small portion of the samples had combustion phenomenon, but there was no explosion event. In overcharging experiments, the use of high voltage charging that greatly exceeds its own discharge voltage several times results in an explosion phenomenon. In fact, after overcharge reaction, the safety of Lithium iron phosphate has been greatly improved compared with other lithium batteries.

(3) Long lifespan

We all know that long-lived lead-acid batteries can only have a cycling life of about 300 cycles, with a maximum of 500 cycles. The cycle life of Lithium iron phosphate battery can reach more than 2000 times. The life of lead-acid battery of the same quality is generally about 1-2 years, while the theoretical life of Lithium iron phosphate battery will reach 7-10 years when used under the same conditions.

(4) Good high-temperature performance

It is understood that the peak value of Lithium iron phosphate electric heating can reach 350 ℃ -500 ℃, while the peak value of some lithium batteries is only about 200 ℃. Therefore, Lithium iron phosphate has certain advantages in high temperature performance.

(5) Large capacity without memory effect

The LiFePO4 battery has a larger capacity than ordinary batteries (lead acid, etc.). The energy density of the lead acid battery is about 40WH/kg. The energy density of the mainstream Lithium iron phosphate battery in the market is more than 90WH/kg, which can provide a longer life.

When the battery works under the condition that it is always fully charged and not fully discharged, its capacity will be rapidly lower than the Nameplate capacity. This phenomenon is called memory effect. For example, Ni MH and Nickel–cadmium battery have memory, but LiFePO4 battery does not, and its battery capacity will not decrease. No matter what state the battery is in, it can be charged and used at any time, without the need to discharge it before charging. 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.

2. Disadvantages

(1) High manufacturing cost

The preparation cost of Lithium iron phosphate battery materials and the manufacturing cost of the battery are high, and the finished product rate of the battery is low, and the consistency is poor. Although the nanocrystallization and carbon coating of Lithium iron phosphate have improved the electrochemical performance of the materials, they have also brought 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 very rich and the cost is low, the cost of the prepared Lithium iron phosphate product is not low. Even if the initial research and development costs are removed, the process cost of this material combined with the higher cost of preparing batteries will result in a higher cost per unit of energy storage capacity.

(2) Poor product consistency

At present, Lithium iron phosphate material factory in China cannot solve this problem. From the perspective of material preparation, the synthesis reaction of Lithium iron phosphate is a complex multiphase reaction, including solid phosphate, iron oxide and lithium salt, carbon added precursor and reducing gas phase. In this complex reaction process, it is difficult to ensure the consistency of the reaction.

(3) Poor performance in low temperature environments

LiFePO4 battery has some performance defects, such as low tamping density and compaction density, resulting in low energy density of lithium ion battery. The low temperature performance is poor, and even if it is nano sized and carbon coated, it does not solve this problem. When Dr. Don Hillebrand, director of the Energy Storage System Center of the Argonne National Laboratory, talked about the low-temperature performance of the lithium iron phosphate battery, he described it as terrible. Their test results of the Lithium iron phosphate type lithium ion battery showed that the Lithium iron phosphate battery could not drive electric vehicles at low temperatures (below 0 ℃). Although some manufacturers claim that the capacity retention rate of lithium iron phosphate batteries is good at low temperatures, that is when the discharge current is small and the discharge cutoff voltage is very low. In this situation, the device simply cannot start working.

(4) The threat of elemental iron

In the sintering process of preparing Lithium iron phosphate, it is possible for iron oxide to be reduced to elemental iron under high temperature reducing atmosphere. Simple iron can cause micro short circuits in batteries. This is the most taboo substance in batteries and the main reason why Japan has not used this material as the positive electrode material for power lithium-ion batteries.

(5) Intellectual property issues

At present, the basic patent of Lithium iron phosphate is owned by the University of Texas in the United States, while the carbon coating patent has been applied by Canadians. These two fundamental patents cannot be circumvented. If patent royalties are included in the cost, the product cost will further increase.

In addition, based on its experience in research and production of lithium-ion batteries, Japan was the earliest 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 currently no large lithium-ion battery manufacturer in the country. Therefore, Japan's choice of modified lithium manganese oxide as the positive electrode material for power lithium-ion batteries is more reasonable. Even in the United States, half of the manufacturers use Lithium iron phosphate and half of the manufacturers use lithium manganate as cathode materials for power lithium-ion batteries. The federal government also supports.

Ⅲ. Main materials and working principle of LiFePO4 Battery

1. Main materials

There are four main materials for Lithium iron phosphate battery, namely, positive material, negative material, electrolyte and diaphragm.

(1) Positive electrode material

Because the performance of the positive electrode material directly affects the performance of the lithium battery pack, its cost also directly determines the cost of the battery.

The cathode material system of Lithium iron phosphate battery can be divided into natural lithium iron phosphate ore and synthetic Lithium iron phosphate material. Among them, natural phosphate iron lithium ore contains Mn impurities and is prone to weathering, resulting in poor electrochemical performance. Therefore, the natural lithium iron phosphate ore is generally not directly used as Lithium iron phosphate cathode material. The synthetic Lithium iron phosphate cathode material can effectively improve the poor conductivity of Lithium iron phosphate and the slow diffusion of lithium ion through the synthesis process. Therefore, it has good electrochemical activity.

(2) Negative electrode material

At present, the main negative electrode materials are natural graphite and artificial graphite. As one of the four major components of the Lithium iron phosphate battery package, the anode material plays an important role in improving the capacity and cycling performance of the battery. It is at the core of the middle reaches of the lithium battery industry.

The layered structure of graphite is suitable for the de intercalation of lithium ions and has high requirements for electrolytes. During the first charging and discharging process, the solvent will co embed between the graphite layers, causing volume expansion. It can directly cause the collapse of the graphite layer and deteriorate the cycling performance of the electrode. Therefore, we need to increase the compatibility between graphite and electrolyte, improve its reversible specific capacity and cycling performance, in order to form a stable SEI film.

(3) Electrolyte

The electrolyte plays the role of conducting ions between the positive and negative electrodes of the lithium battery, which guarantees the Lithium iron phosphate battery to obtain high voltage, high specific energy and other advantages.

The electrolyte of LiFePO4 battery is mainly composed of solvent, lithium salt and electrolyte additives. The solvent is mainly carbonate solvent, including Ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC) and propylene carbonate (PC). It can ensure the formation of an effective negative electrode passivation film, high ion conductivity, and electrochemical stability. Lithium hexafluorophosphate (LiPF6) is mainly used as the lithium salt, and the main additive is vinyl carbonate (VC). There are relatively few additives used in the electrolyte compared to ternary materials.

LiFePO4 battery has also developed a series of high-temperature electrolyte and low-temperature electrolyte according to different application scenarios. By changing the lithium salt and additives, we can improve the high-temperature performance of Lithium iron phosphate battery.

(4) Diaphragm

In the structure of lithium batteries, the separator is one of the key inner components. The performance of the separator determines the interface structure, internal resistance, and other characteristics of the battery, directly affecting the capacity, cycling, and safety performance of the lithium battery pack. The excellent performance of the separator plays an important role in improving the comprehensive performance of the battery.

There are various classification methods for lithium-ion battery separators, such as based on substrate, structural morphology, and usage. Starting from the types of materials, lithium-ion battery separator materials are usually divided into polyolefin separator, inorganic composite separator, non-woven separator, and polymer electrolyte. Due to the advantages of polyolefin materials such as good insulation, low density, high strength mechanical properties, and electrochemical corrosion resistance, the separators for commercial lithium-ion batteries are currently polyolefin based materials, such as PP (polypropylene), PE (polyethylene), and composite separators PP/PE/PP.

Most commercial LiFePO4 battery use wet PE membrane (ceramic membrane) coated with ceramics. We apply a layer of nanoscale alumina material on its surface. After special processing, the ceramic diaphragm is tightly bonded to the substrate, which significantly improves the high-temperature resistance and safety performance of lithium-ion batteries. Ceramic membranes generally require uniform particle size of alumina coated on the surface, which can be well adhered to the membrane without blocking the membrane pores. At the same time, they have special crystal structure requirements for alumina to ensure its compatibility and wettability with the electrolyte. Aluminum oxide coatings have excellent high-temperature resistance and can maintain intact morphology above 180 ℃. At the same time, the aluminum oxide coating can neutralize the free HF in the electrolyte, improve the acid resistance of the battery, and thus improve safety.

2. Working principle

Let's take a look at how batteries are charged and discharged.

LiFePO4 battery charging:

During charging, lithium ions (Li+) are released from the cathode and moved to the anode through the electrolyte. When fully charged, the anode stores more lithium than the cathode.

Discharge of LiFePO4 batteries:

If an electrical load is applied to the battery, the opposite reaction will occur. Lithium ions flow from the anode to the cathode, ultimately storing more lithium than the anode.

Finally, the movement of lithium ions within the battery generates an electron flow between the two electrodes, generating an electric charge outside the battery.

Ⅳ. Industrial application of LiFePO4 Battery

1. Start the application on the power supply

In addition to the characteristics of power lithium battery, the start-up Lithium iron phosphate battery also has the capability of instantaneous high power output. It replaces traditional lead-acid batteries with power type lithium batteries with energy less than one kilowatt hour, and replaces traditional starting motors and generators with BSG motors. It not only has the function of starting and stopping at idle speed, but also has the functions of engine shutdown and coasting, coasting and braking energy recovery, acceleration assistance, and electric cruise control.

2. Application of new energy vehicle industry

LiFePO4 battery is widely used in passenger cars, buses, logistics vehicles, Neighborhood Electric Vehicle, etc. due to its advantages of good safety and low cost. In the current field of new energy passenger vehicles, affected by the national subsidy policy for new energy vehicles, although the ternary battery is dominant by virtue of its energy density, LiFePO4 battery still occupies an irreplaceable advantage in passenger cars, logistics vehicles and other fields. In the field of passenger cars, LiFePO4 battery will remain the mainstream in the promotion and application of new energy vehicles in 2018.

Yang Yusheng, an academician of the CAE Member, believes that the use of LiFePO4 battery in the incremental electric vehicle market can not only improve the safety of vehicles, but also support the marketization of incremental electric vehicles, and eliminate the anxiety of pure electric vehicles about mileage, safety, price, charging, and follow-up battery issues.

3. Application of 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 and environmental protection, and supports stepless expansion. So it is suitable for large-scale energy storage. It has good application prospects in the fields of safe grid connection of renewable energy power stations, grid peak shaving, distributed power stations, UPS power supply, emergency power supply systems, etc.

According to the latest energy storage report recently released by international market research institute GTM Research, the application of grid side energy storage projects in China in 2018 has led to a continuous increase in the use of Lithium iron phosphate battery.

With the rise of the energy storage market, in recent years, some power battery enterprises have laid out energy storage business to open up new application markets for Lithium iron phosphate battery. On the one hand, because of its super long life, safe use, large capacity, green and environmental protection, Lithium iron phosphate can be transferred to the energy storage field, which will extend the value chain and promote the establishment of a new business model. On the other hand, the energy storage system matched with LiFePO4 battery has become the mainstream choice in the market. It is reported that Lithium iron phosphate battery has been tried to be used in electric buses, electric trucks, user side and grid side FM.

(1) Peak shaving of the power grid. Pumped-storage hydroelectricity has always been the main means of peak shaving of power grid. Because Pumped-storage hydroelectricity is greatly restricted by geographical conditions, it is not easy to build in plain areas, and it occupies a large area and has high maintenance costs. We use LiFePO4 battery energy storage system instead of Pumped-storage hydroelectricity to deal with peak load of power grid, which is not limited by geographical conditions. It has the advantages of free site selection, less investment, less land occupation, and low maintenance costs, and will play an important role in the peak shaving process of the power grid.

(2) 4 UPS power supply. The continuous and rapid development of the Chinese economy has led to the diversification of user demand for UPS power, resulting in a sustained demand for UPS power from more industries and enterprises.

Compared with lead-acid battery, LiFePO4 battery has the advantages of long cycle life, safety and stability, environmental protection, low self discharge rate, etc. With the continuous maturity of integrated technology and the continuous reduction of cost, Lithium iron phosphate battery will be widely used in UPS battery.

(3) Safe grid connection of renewable energy generation such as wind power and photovoltaic power. 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. Most wind farms in China belong to "large-scale centralized development and long-distance transmission". With the rapid development of the wind power industry, the grid connected generation of large-scale wind farms poses a serious challenge to the operation and control of the large power grid.

Photovoltaic power generation is influenced by environmental temperature, sunlight intensity, and weather conditions, and exhibits a random fluctuation characteristic. China is showing a development trend of "decentralized development, low voltage local access" and "large-scale development, medium to high voltage access", which puts forward higher requirements for peak shaving of the power grid and safe operation of the power system.

Therefore, high-capacity energy storage products have become a key factor in solving the contradiction between the power grid and renewable energy generation. The LiFePO4 battery energy storage system has the characteristics of fast condition conversion, flexible operation mode, high efficiency, safety, environmental protection and strong scalability. The implementation of engineering applications in the national wind and solar energy storage and transmission demonstration project will effectively improve equipment efficiency, solve local voltage control problems, and improve the reliability and quality of renewable energy generation, making renewable energy a continuous and stable power supply.

With the continuous expansion of capacity and scale, the continuous maturity of integration technology, the cost of the energy storage system will be further reduced. After long-term testing of safety and reliability, the Lithium iron phosphate battery energy storage system is expected to be widely used in the safe grid connection of wind power generation, photovoltaic power generation and other renewable energy power generation and in improving power quality.

(4) Distributed power plants. The inherent defects of large power grids are difficult to ensure the quality, efficiency, safety and reliability requirements of power supply. For important units and enterprises, we often need dual or even multiple power sources as backup and protection. The Lithium iron phosphate battery energy storage system can reduce power failure caused by power grid failure and various accidents. It plays an important role in ensuring safe and reliable power supply for hospitals, banks, Command and control centers, data processing centers, chemical material industries and precision manufacturing industries.

4. Applications in other fields

LiFePO4 battery is also widely used in the military field because of its good cycle life, safety, low temperature performance and other advantages. On October 10, 2018, a battery enterprise in Shandong made a strong appearance at the first Military-civil fusion scientific and technological innovation achievement exhibition in Qingdao, demonstrating military products including -45 ℃ military ultra-low temperature batteries.

Ⅴ. Can a lithium battery last 20 years?

However, most lithium-ion batteries will survive much longer than the minimum, in the region of 10–15 years. These batteries may last up to 3 times as long as cheaper lead-acid batteries, which only last five to seven years.

Ⅵ. How do you maintain a LiFePO4 battery?

To be able to keep the internal chemistry of LiFePO4 working stably, it is recommended to set the charge to 90% SOC to stop charging and to stop discharging at 10% SOC. You can set this range easily with BMS, charger or inverter. Also, it is recommended to perform a full charge every 6 months.

Ⅶ. Can LiFePO4 batteries be used to start a car?

You can nearly draw all the energy saved in a LiFePO4 cell whereas from a lead-acid cell it is around 30-35%. Furthermore you can start your car more times repeatable because the voltage stays consistent over the whole decharging process, whereas lead-acid is dropping rapidly.

Ⅷ. LiFePO4 vs lithium ion battery：which is better?

In most ways, LiFePO4 batteries are better than comparable lithium-ion batteries. Lithium iron phosphate batteries are less prone to combustion and thermal runaway, making them safer for home use. Plus, a longer cycle life means the LiFePO4 batteries will outlast lithium-ion for up to five times longer.