Detailed production process of lithium iron phosphate

Introduction to Lithium Iron Phosphate

Lithium ion batteries, as a high-performance secondary green battery, 

have advantages such as high voltage, high energy density (including volume 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 high currents. 

The improvement of lithium-ion battery performance is largely determined by the improvement of electrode material properties, 

especially positive electrode materials. 

The most widely studied positive electrode materials currently include LiCoO2, LiNiO2, and LiMn2O4. 

However, due to the toxicity and limited resources of cobalt, difficulties in preparing lithium nickelate, 

and poor cycling and high-temperature performance of lithium manganese oxide, 

their application and development are constrained. 

Therefore, developing new high-energy and inexpensive positive electrode materials is crucial for the development of lithium-ion batteries.

In 1997, Padhi et al. reported that lithium iron phosphate (LiFePO4) with an olivine structure can reversibly intercalate and deintercalate lithium, 

and has the characteristics of high specific capacity, good cycling performance, stable electrochemical performance, and low cost. 

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 field of electrochemistry at home and abroad. 

In recent years, with the increasing application of lithium batteries, there has been more and more research on LiFePO4.

Structure of lithium iron phosphate

Lithium iron phosphate (LiFePO4) has an olivine structure, which is a slightly twisted hexagonal dense packing. 

Its space group is Pmnb type, and its crystal structure is shown in Figure 2.1

LiFePO4 consists of a spatial skeleton consisting of FeO6 octahedra and PO4 tetrahedra. 

P occupies tetrahedral positions, while Fe and Li fill the octahedral voids,

with Fe occupying octahedral positions at common angles and Li occupying octahedral positions at common edges. 

One FeO6 octahedron shares a common edge with two FeO6 octahedra and one PO4 tetrahedron, 

while the PO4 tetrahedron shares a common edge with one FeO6 octahedron and two LiO6 octahedra. 

Due to the close arrangement of nearly hexagonal oxygen atoms, lithium ions can only be deintercalated 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 its 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 loading voltammetry of LiFePO4, with two peaks formed in the C-V diagram. 

During anodic scanning, Li+dissociates from the LixFePO4 structure and forms an oxidation peak at 3.52V; 

When Li+is embedded into the LixFePO4 structure during the 4.0-3.0 scan, a reduction peak is formed at 3.32V; 

The oxidation-reduction peak in the C-V curve indicates a reversible lithium ion insertion and extraction reaction occurring on the LiFePO4 electrode.

Performance of lithium iron phosphate

1) High energy density

Its theoretical specific capacity is 170 mAh/g, and the actual specific capacity of the product can exceed 140 mAh/g (0.2 ° C, 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 be charged and discharged 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 the poor ion conductivity of the electrode, which is not suitable for high current charging and discharging, 

and is hindered in application. Solution: Coating the electrode surface with conductive materials and doping for electrode modification.)

The service life of lithium iron phosphate batteries is closely related to their operating temperature. 

If the operating temperature is too low or too high, it can cause significant adverse hazards during the charging, discharging, 

and usage processes. Especially used in electric vehicles in northern China, in autumn and winter, 

lithium iron phosphate batteries cannot supply power normally or the power supply is too low, 

so 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 with lithium iron phosphate cathode materials can be charged at high rates and can be fully charged in as little as 1 hour.

Production process of lithium iron phosphate

1. Drying iron phosphate to remove water

(1) Drying process in the drying room: Fill the stainless steel container 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 material to the next process and sinter it in the rotary furnace.

(2) Rotary furnace sintering process: After the rotary furnace is heated and nitrogen gas is introduced to meet the requirements, 

the material (from the drying room of the previous process) is fed

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 devices are the same (one can also run 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 discharged material to the disperser. 

Sampling and particle size measurement.

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

3. Material dispersion process of disperser machine

(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. 

Load it into a bowl and transfer it to the pusher furnace.

 At the same time, several sets of bulk samples were placed and compared with the compressed block shaped materials.

6. First, heat up the plate pushing furnace and introduce nitrogen gas to achieve an atmosphere of below 100ppm.

 Then, push the crucible into the plate pushing furnace at a temperature range of 300-550 ℃ for 4-6 hours; 

Constant temperature range of 750 ℃ for 8-10 hours; The cooling stage takes 6-8 hours and the material is discharged.

7. Roller press ultrafine grinding

Input the materials burned in the push plate furnace into the ultrafine mill, adjust the speed, perform roller pressing grinding, 

and then send them to the ultrafine mill for grinding. 

Each batch is sampled and tested for particle size.

8. Screening and packaging

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

9. Inspection and storage

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