Date: 2023-06-08 hits: 1579
Lithium iron phosphate battery is a lithium ion battery that uses lithium iron phosphate (LiFePO4) as the positive electrode material and carbon as the negative electrode material. The rated voltage of the single cell is 3.2V, and the charging cut-off voltage is 3.6V~3.65V.
During the charging process, part of the lithium ions in the lithium iron phosphate come out, are transferred to the negative electrode through the electrolyte, and are embedded in the carbon material of the negative electrode; at the same time, electrons are released from the positive electrode and reach the negative electrode from the external circuit to maintain the balance of the chemical reaction. During the discharge process, lithium ions come out of the negative electrode and reach the positive electrode through the electrolyte. At the same time, the negative electrode releases electrons and reaches the positive electrode from the external circuit to provide energy for the outside world. [1]
Lithium iron phosphate battery has the advantages of high working voltage, high energy density, long cycle life, good safety performance, low self-discharge rate, and no memory effect.
Summary
In the crystal structure of LiFePO4, oxygen atoms are arranged in a hexagonal close-packed arrangement. The PO43-tetrahedron and FeO6 octahedron constitute the space skeleton of the crystal, Li and Fe occupy the octahedral voids, and P occupies the tetrahedral voids, in which Fe occupies the octahedron-shared corner positions, and Li occupies the octahedron-shared edge positions. The FeO6 octahedra are connected to each other on the bc plane of the crystal, and the LiO6 octahedron structures in the b-axis direction are connected to each other to form a chain structure. One FeO6 octahedron shares edges with two LiO6 octahedrons and one PO43-tetrahedron.
Due to the discontinuity of the FeO6 co-edge octahedral network, electronic conduction cannot be formed; at the same time, the PO43-tetrahedron restricts the volume change of the lattice, which affects the deintercalation and electron diffusion of Li+, resulting in the electronic conductivity and ion diffusion of LiFePO4
cathode materials. Extremely inefficient.
The theoretical specific capacity of LiFePO4 battery is high (about 170mAh/g), and the discharge platform is 3.4V. Li+ goes back and forth between the positive and negative poles to achieve charge and discharge. When charging, an oxidation reaction occurs. Li+ moves out of the positive electrode and is inserted into the negative electrode through the electrolyte. Iron changes from Fe2+ to Fe3+, and an oxidation reaction occurs.
On the left side of the lithium iron phosphate battery is the positive electrode made of olivine structure LiFePO4 material, which is connected to the positive electrode of the battery by aluminum foil. On the right is the negative electrode of the battery composed of carbon (graphite), which is connected to the negative electrode of the battery by copper foil. In the middle is a polymer separator, which separates the positive electrode from the negative electrode. Lithium ions can pass through the separator but electrons cannot pass through the separator. The inside of the battery is filled with electrolyte, and the battery is hermetically sealed by a metal case.
The charge and discharge reaction of the lithium iron phosphate battery is carried out between the two phases of LiFePO4 and FePO4. During the charging process, LiFePO4 gradually breaks away from lithium ions to form FePO4, and during the discharging process, lithium ions intercalate into FePO4 to form LiFePO4.
When the battery is charging, lithium ions migrate from the lithium iron phosphate crystal to the surface of the crystal, enter the electrolyte under the action of the electric field force, then pass through the diaphragm, migrate to the surface of the graphite crystal through the electrolyte, and then embed in the graphite lattice.
At the same time, electrons flow to the aluminum foil collector of the positive electrode through the conductor, flow to the copper foil collector of the negative electrode of the battery through the tab, the positive pole of the battery, the external circuit, the pole of the negative pole, and the tab of the negative pole, and then flow to the graphite negative electrode through the conductor , so that the negative charge balance. After lithium ions are deintercalated from lithium iron phosphate, lithium iron phosphate is converted into iron phosphate.
When the battery is discharged, lithium ions are deintercalated from the graphite crystal, enter the electrolyte, then pass through the diaphragm, migrate to the surface of the lithium iron phosphate crystal through the electrolyte, and then re-intercalate into the lattice of lithium iron phosphate.
At the same time, electrons flow through the conductor to the copper foil collector of the negative electrode, through the tab, the negative pole of the battery, the external circuit, the positive pole, and the positive tab to the aluminum foil collector of the positive pole of the battery, and then through the conductor to the iron phosphate The lithium positive electrode balances the charge of the positive electrode. After lithium ions are intercalated into the iron phosphate crystal, the iron phosphate is converted into lithium iron phosphate.
According to reports, the energy density of square aluminum-shell lithium iron phosphate batteries mass-produced in 2018 is about 160Wh/kg. In 2019, some excellent battery manufacturers can probably achieve the level of 175-180Wh/kg. The chip technology and capacity can be made larger, or 185Wh/kg can be achieved.
The electrochemical performance of the cathode material of lithium iron phosphate battery is relatively stable, which determines that it has a stable charging and discharging platform. It is still very safe under special conditions such as filling, extrusion, and acupuncture.
The 1C cycle life of lithium iron phosphate batteries generally reaches 2,000 times, or even more than 3,500 times, while the energy storage market requires more than 4,000-5,000 times, ensuring a service life of 8-10 years, which is higher than the cycle of more than 1,000 times for ternary batteries Life, while the cycle life of long-life lead-acid batteries is about 300 times.
