As a high-performance secondary green battery, lithium-ion batteries have
high voltage, high energy density (including volume energy, mass specific
energy), low self-discharge rate, wide operating temperature range, long cycle
life, environmental protection, It has the advantages of no memory effect and
the ability to charge and discharge with high current. The improvement of
lithium-ion battery performance is largely determined by the improvement of
electrode material properties, especially cathode materials. Currently, the most
widely studied cathode materials include LiCoO2, LiNiO2, and LiMn2O4. However,
due to the toxicity of cobalt and limited resources, the difficulty in preparing
lithium nickel oxide, and the poor cycle performance and high temperature
performance of lithium manganate, etc., these factors have restricted their
application and development. Therefore, the development of new high-energy and
cheap cathode materials is crucial to the development of lithium-ion
batteries.
Lithium iron phosphate (LiFepO4) has an olivine structure, which is
slightly twisted hexagonal close-packing. Its space group is pmnb type. The
crystal structure is shown in Figure 2.1
LiFepO4 is composed of FeO6 octahedron and pO4 tetrahedron to form a space
skeleton. . In the crystal lattice, one FeO6 octahedron 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 hexagonally packed oxygen atoms, lithium ions can only be
deintercalated on a two-dimensional plane, and therefore have 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 of the material, resulting in
the material having high thermal stability.
Wang et al. made a detailed analysis of the electrochemical properties of
LiFepO4. Figure 2.2 is the cyclic load voltammogram of LiFepO4. Two peaks are
formed in the C-V diagram. During the anode scan, Li+ is detached from the
LixFepO4 structure and forms oxidation at 3.52V. peak; when scanning at 4.0~3.0,
Li+ is embedded into the LixFepO4 structure, correspondingly forming a reduction
peak at 3.32V; the redox peak in the C-V curve indicates that a reversible
lithium ion intercalation and extraction reaction occurs on the LiFepO4
electrode.
Properties 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 lithium-ion battery cathode material; it does
not contain any heavy metal elements that are harmful to the human body;
3) Long life
Under 100% DOD conditions, it can be charged and discharged more than 2,000
times; (Reason: The lithium iron phosphate lattice has good stability, and the
insertion and extraction of lithium ions has little effect on the lattice, so it
has good reversibility. Existing shortcomings The reason is that the electrode
has poor ionic conductivity and is not suitable for large current charging and
discharging, which hinders its application. Solution: coat the electrode surface
with conductive materials and dope to modify the electrode.)
The service life of lithium iron phosphate batteries is closely related to
its operating temperature. If the operating temperature is too low or too high,
it will cause great hidden dangers in the charging, discharging and use
processes. Especially when used in electric vehicles in northern China, lithium
iron phosphate batteries cannot supply normal power or the power supply is too
low in autumn and winter, and the working environment temperature needs to be
adjusted to maintain its performance. At present, domestic solutions to the
constant temperature working environment of lithium iron phosphate batteries
need to consider space constraints. The more common solution is to use airgel
felt as the insulation layer.
4) Charging performance
Lithium batteries with lithium iron phosphate cathode material can be
charged at a high rate and can be fully charged in as little as 1 hour.
Lithium iron phosphate production process flow
1. Drying and removing water from iron phosphate
(1) Drying process in the drying room: Fill the stainless steel sagger with
the raw material ferric phosphate and place it in the drying room, and adjust
the drying room temperature to 220±
20℃, 6-10 hours drying. The material is discharged and transferred to the
next process to the rotary kiln for sintering.
(2) Rotary furnace sintering process: After the rotary furnace is heated up
and the nitrogen gas is supplied to meet the requirements, the material (from
the drying room in the previous process) is fed.
Material), adjust the temperature to 540±20℃, and sintering for 8-12
hours.
2. Grinding machine mixing process
During normal production, the two grinders are put into operation at the
same time. The specific feeding and operation of the two equipment are the same
(one can be run independently during debugging). The procedures are 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, mix and grind for 1-3 hours. Stop the machine and transfer
the discharged material to the disperser. Take samples to measure particle
size.
(3) Cleaning: Weigh 100Kg of pure water, clean the grinder 3-5 times, and
transfer all the washing liquid to the disperser.
3. Dispersing machine material dispersing process
(1) Transfer about 500Kg of materials (including materials for cleaning the
grinder) mixed by two grinders in 2.2 (or mixed twice by one grinder) into the
disperser, then add 100Kg of pure water, adjust the mixing speed, and fully Stir
and disperse for 1-2 hours, then 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
feeding spray drying to obtain dry materials.
(2) The solid content can be adjusted to 15%~30% according to the spray
particle size.
5. Hydraulic press material briquetting and charging: Adjust the pressure
of the hydraulic press to 150 tons and 175 tons respectively, load the
spray-dried material into the mold, maintain the pressure for a certain period
of time, and compact it into blocks. Put the sagger into the push plate stove.
At the same time, several groups of bulk samples were put in for comparison with
the materials pressed into blocks.
6. For sintering in the push plate furnace, first raise the temperature,
pass nitrogen, and reach the atmosphere requirement of less than 100 ppm. Push
the sagger into the push plate furnace and set the heating section to 300-550°C
for 4-6 hours; the constant temperature section to 750°C for 8-10 hours; cool
down. The process lasts for 6-8 hours and the materials are discharged.
7. Roller ultra-fine grinding
Input the burned materials in the push plate furnace into the ultra-fine
grinding mill, adjust the rotation speed, perform roller grinding and then send
it to the ultra-fine grinding mill for grinding. Samples are taken from each
batch to test for particle size.
8. Screening and packaging
The ground materials are screened and packaged. Available in 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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