Full interpretation of LiFePO4 battery – structure and applications

LiFePO4 battery structure

LiFePO4 battery uses LiFePO4 with olivine structure as the cathode of the battery, and is connected to the cathode of the battery by aluminum foil. In the middle is a polymer separator, which separates the cathode from the anode, but lithium ions can pass through but electrons cannot. The anode of the battery is composed of carbon (graphite), and is connected to the anode of the battery by copper foil.

When the LiFePO4 battery is charged, the lithium ions in the cathode migrate to the anode through the polymer separator; during the discharge process, the lithium ions in the anode migrate to the cathode through the separator. Lithium-ion batteries are named after lithium ions migrate back and forth during charging and discharging.

Main materials of LiFePO4 battery

• Lithium iron phosphate cathode material

The LiFePO4 battery cathode material system can be divided into natural lithium iron phosphate ore and synthetic lithium iron phosphate materials. Among them, natural lithium iron phosphate ore contains Mn impurities, and is easy to weather, and its electrochemical performance is poor, so it is generally not directly used as a lithium iron phosphate cathode material.

The artificially synthesized lithium iron phosphate cathode material can effectively improve the poor conductivity of lithium iron phosphate and the slow diffusion of lithium ions through the synthesis process, so it has good electrochemical activity.

• Graphite anode material

The layered structure of graphite is suitable for the deintercalation of lithium ions, and has high requirements on the electrolyte. During the first charge and discharge process, the solvent will be co-embedded between the graphite layers, causing volume expansion, which can directly lead to the collapse of the graphite layer and deteriorate the cycle performance of the electrode.

Therefore, it is necessary to modify graphite to increase the compatibility between graphite and electrolyte, improve its reversible specific capacity and cycle performance, and form a stable SEI film. The current graphite anodes are mainly divided into natural graphite anodes and artificial graphite anodes. The main modification methods for graphite anodes are surface oxidation, surface coating and doping.

• Separator

Lithium-ion battery separators can be classified in many ways, such as according to the base material, structure and morphology, and usage. At present, the separators of commercial lithium-ion batteries are polyolefin-based materials, such as PP (polypropylene), PE (polyethylene), and composite separators PP/PE/PP.

Commercial LiFePO4 batteries mostly use ceramic-coated wet-process PE membranes (ceramic diaphragms). The surface is coated with a layer of nano-scale alumina material, which is tightly bonded to the substrate after special processing, which significantly improves the high temperature resistance and safety performance of lithium-ion batteries.

• Electrolyte

The electrolyte of LiFePO4 battery is mainly composed of solvent, lithium salt and electrolyte additives. Among them, the solvent is mainly a carbonate solvent, mainly including EC, DMC, DEC and PC, etc., which can ensure the formation of an effective negative passivation film, high ion conductivity and electrochemical stability. The lithium salt mainly uses lithium hexafluorophosphate (LiPF6), and the main additive used is vinylene carbonate (VC), and the additive used in the electrolyte is less than that of the ternary material.

LiFePO4 battery has also developed a series of high-temperature and low-temperature electrolytes according to different application scenarios. By changing lithium salts and additives, the high-temperature performance of LiFePO4 battery can be improved.

Advantages and disadvantages of development and application of LiFePO4 battery

Advantages of LiFePO4 battery

• High safety performance: At high temperature or overcharge, it will not collapse and generate heat or form strong oxidizing substances like lithium cobalt oxide. The decomposition temperature of lithium iron phosphate is about 600 ° C, so it has good safety.

• Long life: Lithium iron phosphate power battery, the cycle life can reach more than 2000 times, and the standard charging can reach 2000 times. Lead-acid batteries of the same quality can be used for a maximum of 1 to 1.5 years, while LiFePO4 batteries are used under the same conditions, and their theoretical life will reach 7 to 8 years, with higher reliability.

• Large capacity: It has a larger capacity than ordinary batteries (lead-acid, etc.). The energy density of lead-acid batteries is about 40WH/kg, and the energy density of mainstream LiFePO4 batteries on the market is above 90WH/kg, which can provide longer battery life.

• More friendly to the environment: the battery is generally considered to be free of any heavy metals and rare metals, non-toxic (SGS certified), non-polluting, and a green battery. One reason why lithium batteries have been given great expectations is that they are more environmentally friendly.

• High stability: LiFePO4 battery will not be overcharged or overdischarged, safer and more stable. LiFePO4 battery also has better resistance to humidity and high pressure.

Disadvantages of LiFePO4 battery

In contrast, the tap density of the positive electrode material is small, and the volume of the LiFePO4 battery with the same capacity is larger than that of lithium-ion batteries such as lithium cobalt oxide, so it does not have advantages in micro-batteries. In terms of power batteries, LiFePO4 batteries, like other batteries, need to face the problem of battery consistency.

• The threat of elemental iron: During the sintering process during the preparation of lithium iron phosphate, iron oxide may be reduced to elemental iron in a high-temperature reducing atmosphere. The most taboo substance in the battery is simple iron, which can cause a micro-short circuit in the battery.

• High manufacturing cost: The manufacturing cost of LiFePO4 battery and the preparation cost of materials are high, and the battery yield is low and the consistency is poor. Although the nanonization and carbon coating of lithium iron phosphate improve the electrochemical performance of the material, it also brings other problems, such as the reduction of energy density, the increase of synthesis cost, poor electrode processing performance and harsh environmental requirements.

• Poor performance in low temperature environment: The test results of lithium iron phosphate lithium ion battery show that LiFePO4 battery cannot drive electric vehicles at low temperature. Although some manufacturers claim that the capacity retention rate of LiFePO4 battery is not bad at low temperature, but that is when the discharge current is small and the discharge cut-off voltage is very low. In this case, the device cannot start to work at all.

Application fields of LiFePO4 battery

• Applications in the new energy vehicle industry

LiFePO4 battery is widely used in passenger cars, buses, logistics vehicles, low-speed electric vehicles, motorcycle battery pack, etc. due to its advantages in safety and low cost. Although, in the current field of new energy passenger vehicles, due to the impact of new energy vehicle subsidy policies and the advantages of energy density, ternary batteries once occupied a dominant position, but LiFePO4 batteries still occupy an irreplaceable advantage in the fields of passenger cars and logistics vehicles.

• Application in starting power supply

The start-up LiFePO4 battery not only has the characteristics of a power lithium battery, but also has an instantaneous high power output capability. Replace traditional lead-acid batteries with power-type lithium batteries with energy less than one degree of electricity, and replace traditional starter motors and generators with BSG motors.

This not only has the function of idling start and stop, but also has the functions of engine stop coasting, coasting and braking energy recovery, acceleration assist and electric cruise function.

• Application of LiFePO4 battery in 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. It is suitable for large-scale electric energy storage, and has good application prospects in the fields of safe grid connection of renewable energy power stations, power grid peak regulation, distributed power stations, UPS power supplies, and emergency power systems.

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 LiFePO4 battery.

On the one hand, due to its ultra-long life, safe use, large capacity, 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 supporting LiFePO4 battery has become the mainstream choice in the market.

According to reports, LiFePO4 battery has been tried to be used in electric buses, electric trucks, user-side and grid-side frequency regulation. Compared with lead-acid batteries, LiFePO4 batteries have the advantages of long cycle life, safety and stability, environmental protection, and low self-discharge rate. With the continuous maturity of integrated technology and the continuous reduction of costs, LiFePO4 batteries will be widely used in UPS batteries.

• Applications in other fields

LiFePO4 battery is also widely used in many fields closely related to life because of its good cycle life, safety, and low temperature performance.

Conclusion

There is no doubt that LiFePO4 battery will open up more space in more diversified market applications. Cost and safety will continue to be the first factors that battery suppliers need to consider in various battery application scenarios, and the advantages of LiFePO4 battery are gradually emerging.

In addition, in the emerging application market, including scenarios such as electric forklifts, electric mopeds, data center backup power, elevator backup power, and medical device power supplies, LiFePO4 battery will bring certain opportunities and space. It is foreseeable that the application range of LiFePO4 battery will expand rapidly in the future, and its position in the power battery market will be strengthened.