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Introduction to lithium iron phosphate

As a high-performance secondary green battery, lithium-ion batteries have advantages such as high voltage, high energy density (including volumetric energy and mass specific energy), low self discharge rate, wide temperature range, long cycle life, environmental protection, no memory effect, and the ability to charge and discharge at high currents. The improvement of lithium-ion battery performance largely depends on the improvement of electrode material performance, especially the positive electrode material. At present, the most widely studied positive electrode materials include LiCoO2, LiNiO2, and LiMn2O4. However, due to the toxicity and limited resources of cobalt, the preparation of lithium nickel oxide is difficult, and the poor cycling and high-temperature performance of lithium manganese oxide restrict their application and development. Therefore, the development of new high-energy and inexpensive cathode materials is crucial for the development of lithium-ion batteries.

In 1997, Padhi et al. reported that lithium iron phosphate (LiFepO4) with olivine structure can reversibly intercalate and remove lithium, and has characteristics such as high specific capacity, good cycling performance, stable electrochemical performance, and low price. It is the preferred new generation of green cathode materials, especially as a material for dynamic lithium-ion batteries. The discovery of lithium iron phosphate has attracted the attention of many researchers in the electrochemical field both domestically and internationally. In recent years, with the increasingly widespread application of lithium batteries, research on LiFepO4 has been increasing.

The structure of lithium iron phosphate

Lithium iron phosphate (LiFepO4) has a olivine structure, which is a slightly twisted hexagonal dense packing. Its spatial group is pmn b type, and the crystal structure is shown in Figure 2.1

LiFepO4 consists of a spatial skeleton consisting of FeO6 octahedra and pO4 tetrahedra, with p occupying tetrahedral positions and Fe and Li filling in octahedral voids, where Fe occupies a common octahedral position and Li occupies a common octahedral position. A FeO6 octahedron in the lattice shares edges with two FeO6 octahedrons and one pO4 tetrahedron, while the pO4 tetrahedron shares edges with one FeO6 octahedron and two LiO6 octahedrons. Due to the close arrangement of nearly hexagonal stacked oxygen atoms, lithium ions can only be deintercaled in a two-dimensional plane, resulting in a relatively high theoretical density (3.6g/cm3). In this structure, the voltage of Fe2+/Fe3+relative to metallic lithium is 3.4V, and the theoretical specific capacity of the material is 170mA · h/g. The formation of strong p-O-M covalent bonds in the material greatly stabilizes the crystal structure, resulting in high thermal stability of the material.

Wang et al. conducted a detailed analysis of the electrochemical performance of LiFepO4. Figure 2.2 shows the cyclic load voltammetry of LiFepO4, forming two peaks in the C-V plot. During anodic scanning, Li+detached from the LixFepO4 structure and formed an oxidation peak at 3.52V; When Li+is embedded into the LixFepO4 structure during scanning from 4.0 to 3.0, a corresponding reduction peak is formed at 3.32V; The oxidation-reduction peak in the C-V curve indicates a reversible lithium ion intercalation reaction occurring on the LiFepO4 electrode.

The performance of lithium iron phosphate

1) High energy density

Its theoretical specific capacity is 170mAh/g, and the actual specific capacity of the product can exceed 140mAh/g (0.2C, 25 ° C);

2) Security

It is currently the safest positive electrode material for lithium-ion batteries; Does not contain any harmful heavy metal elements to the human body;

3) Long lifespan

Under 100% DOD conditions, it can charge and discharge more than 2000 times; (Reason: Lithium iron phosphate has good lattice stability, and the insertion and extraction of lithium ions have little effect on the lattice, so it has good reversibility. The disadvantage is that the electrode ion conductivity is poor, which is not suitable for high current charging and discharging, and is hindered in application. Solution: Coating conductive materials on the electrode surface and doping for electrode modification.)

The service life of lithium iron phosphate batteries is closely related to their operating temperature. Using temperatures that are too low or too high can cause significant adverse hazards during their charging, discharging, and usage processes. Especially when used in electric vehicles in northern China, lithium iron phosphate batteries cannot provide normal power or the power supply is too low in autumn and winter. It is necessary to adjust the working environment temperature to maintain their performance. At present, space constraints need to be considered to solve the constant temperature working environment of lithium iron phosphate battery in China. The more common solution is to use aerogel felt as the insulation layer.

4) Charging performance

Lithium batteries made of lithium iron phosphate cathode materials can be charged at high rates and can be fully charged within 1 hour at the fastest.

Production process flow of lithium iron phosphate

1. Drying and dehydration of iron phosphate

(1) Drying process in the drying room: Fill a stainless steel bowl with raw material iron phosphate and place it in the drying room, adjust the temperature of the drying room to 220 ±

Dry at 20 ℃ for 6-10 hours. Transfer the discharge to the next process for sintering in the rotary furnace.

(2) Rotary furnace sintering process: After the rotary furnace is heated up and nitrogen gas is supplied to meet the requirements, the material is fed (from the drying room of the previous process)

Material), adjust the temperature to 540 ± 20 ℃, and sinter for 8-12 hours.

2. Grinding machine mixing process

During normal production, two grinding machines are put into operation simultaneously, and the specific feeding and operation of the two equipment are the same (one can also be operated separately during debugging). The program is as follows:

(1) Lithium carbonate grinding: Weigh 13Kg of lithium carbonate, 12Kg of sucrose, and 50Kg of pure water, mix and grind for 1-2 hours. Pause.

(2) Mixing and grinding: Add 50kg of iron phosphate and 25kg of pure water to the above mixture, and mix and grind for 1-3 hours. Stop the machine and transfer the discharge to the disperser. Sampling to measure particle size.

(3) Cleaning: Weigh 100Kg of pure water and clean the grinder 3-5 times. Transfer all the washing solution to the disperser.

3. Dispersal machine material dispersion process

(1) Transfer about 500Kg of materials (including materials for cleaning the grinder) mixed by two grinders (or mixed twice by one grinder) in 2.2 to the disperser, add 100Kg of pure water, adjust the mixing speed, fully mix and disperse for 1-2 hours, and wait for pumping into the spray drying equipment.

4. Spray drying process

(1) Adjust the inlet temperature of the spray drying equipment to 220 ± 20 ℃, the outlet temperature to 110 ± 10 ℃, and the feeding speed to 80Kg/hr. Then, start the feeding spray drying to obtain the dried materials.

(2) The solid content can be adjusted to 15%~30% according to the particle size of spray.

5. The pressure of the hydraulic press is adjusted to 150 tons and 175 tons respectively, and the spray dried materials are loaded into the mold, kept for a certain time, and compacted into blocks. Put it into a bowl and transfer it into a push plate furnace. At the same time, several sets of bulk samples were placed and compared with the compressed materials.

6. Heat up the sintering process in the push plate furnace first, pass nitrogen gas to achieve the required atmosphere of 100ppm or less. Then, push the bowl into the push plate furnace and follow the heating range of 300-550 ℃ for 4-6 hours; Constant temperature range of 750 ℃ for 8-10 hours; The cooling period lasts for 6-8 hours and the material is discharged.

7. Roller press ultrafine grinding

Feed the materials fired in the pusher furnace into the ultrafine mill, adjust the speed, and perform roller grinding before feeding them into the ultrafine mill for grinding. Sampling and testing particle size for each batch.

8. Screening and packaging

Screen and package the grinding materials. There are two specifications: 5Kg and 25Kg.

9. Inspection and warehousing

Product inspection, labeling and storage. Including: product name, inspector, material batch, date.

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