Since the advent of lithium-ion batteries, research and development work around it has continued. In the late 1990s, lithium polymer batteries were developed. After 2002, lithium iron phosphate power batteries were introduced.
The interior of a lithium ion battery is mainly composed of a positive electrode, a negative electrode, an electrolyte, and a separator. The difference in the positive and negative electrodes and electrolyte materials and the difference in process make the battery have different performance and have different names. At present, the cathode material of lithium ion battery on the market is mainly lithium cobalt oxide (LiCoO2), and a few lithium ion batteries using lithium manganese oxide (LiMn2O4) and lithium nickel oxide (LiNiO2) as positive electrode materials, generally the latter two positive electrodes. The lithium ion battery of the material is called "lithium manganese battery" and "lithium nickel battery". The newly developed lithium iron phosphate power battery is a lithium ion battery using lithium iron phosphate (LiFePO4) material as the positive electrode of the battery, and it is a new member of the lithium ion battery family.
LiFePO4 is a positive electrode material for lithium ion batteries. LiFePO4 exists in the form of lithium iron phosphate in nature. It has stable structure, abundant resources, good safety performance and no toxicity. Compared with the traditional lithium ion battery cathode materials LiMn2O4 and LiCoO2, LiFePO4 raw materials are more widely available, cheaper and have no environmental pollution. It is environmentally friendly and has good thermal stability. It is one of the most competitive materials for the next generation of lithium ion battery anodes. Although the LiFePO4 material has many excellent electrochemical properties, it also has problems such as a small diffusion coefficient. Therefore, domestic and foreign scholars have begun to study the improvement of the conductivity of LiFePO4.
Generally, the electrolyte of a lithium ion battery is liquid. Later, a solid-state and gel-type polymer electrolyte was developed. This lithium ion battery is called a lithium polymer battery, and its performance is superior to that of a liquid electrolyte lithium ion battery. The full name of lithium iron phosphate battery should be lithium iron phosphate lithium ion battery, the name is too long, referred to as lithium iron phosphate battery. Because its performance is particularly suitable for power applications, the word "power" is added to the name, namely lithium iron phosphate power battery. Some people call it "LiFe (LiFe) power battery".
The working principle of an electrode in LiFePO4 battery is as follows:
(1) When discharging, the device is a primary battery, and the valence of Fe element is changed from +3 to +2, and the electron is reduced. Therefore, the electrode is a positive electrode, and the electrode reaction formula is FePO4+e-+Li+=LiFePO4.
(2) Electrolyzing 100 mL of a mixed solution containing 0.01 mol of CuSO4 and 0.01 mol of NaCl, and transferring 0.02 mol of e- in the circuit,
Anode: 2Cl--2e -=Cl2↑,
0.01 mol 0.01 mol 0.005 mol
0.01mol 0.01mol 0.0025mol
Therefore, the volume of gas generated on the anode under standard conditions = (0.005 mol + 0.0025 mol) & TImes; 22.4 L / mol = 0.168 L;
(3) Iron is used as anode, carbon is used as cathode electrolysis to saturate brine, anode Fe loses electrons to form ferrous ions, then the anode electrode equation is Fe-2e-=Fe2+, cathode cathode hydrogen ion generates electrons to generate hydrogen, then cathode electrode equation It is: 2H++2e-=H2↑; the formation of Fe2+, H2, and OH- in solution, the total equation of electrolysis is: Fe+2H2O energization/.Fe(OH)2↓+H2↑;
The internal structure of the FePO4 battery is shown in the figure below.
Structure and working principle of LiFePO4 battery
The internal structure of the LiFePO4 battery is shown in Figure 1. On the left is the olivine-structured LiFePO4 as the positive electrode of the battery. The aluminum foil is connected to the positive electrode of the battery. The middle is the polymer separator. It separates the positive electrode from the negative electrode, but the lithium ion Li+ can pass and the electron e- cannot pass. The right side is composed of A battery negative electrode composed of carbon (graphite) is connected to the negative electrode of the battery by a copper foil. Between the upper and lower ends of the battery is the electrolyte of the battery, and the battery is hermetically sealed by a metal casing.
Lithium iron phosphate battery works
Above is an olivine structure of LiFePO4 as the positive electrode of the battery, an aluminum foil is connected to the positive electrode of the battery, and on the left is a polymer diaphragm, which separates the positive electrode from the negative electrode, but lithium ion Li can pass and electron e- cannot pass, and the right side is a battery negative electrode composed of carbon (graphite), which is connected by a copper foil to the negative electrode of the battery. Between the upper and lower ends of the battery is the electrolyte of the battery, and the battery is hermetically sealed by a metal casing.
When the LiFePO4 battery is charged, the lithium ion Li in the positive electrode migrates to the negative electrode through the polymer separator; during the discharge, the lithium ion Li in the negative electrode migrates toward the positive electrode through the separator. Lithium-ion batteries are named after the lithium ions migrate back and forth during charging and discharging.
1. When the battery is charged, Li migrates from the 010 surface of the lithium iron phosphate crystal to the surface of the crystal. Under the action of the electric field force, the electrolyte enters the electrolyte, passes through the separator, and then migrates to the surface of the graphite crystal through the electrolyte, and then embeds the graphite crystal. In the grid. At the same time, the electrons flow through the conductor to the aluminum foil collector of the positive electrode, and flow through the copper foil of the negative electrode through the ear, the battery pole, the external circuit, the negative pole, and the negative electrode, and then flow to the graphite negative through the electrical conductor. The charge of the negative electrode reaches equilibrium. After lithium ions are deintercalated from lithium iron phosphate, lithium iron phosphate is converted into iron phosphate, and its lattice structure changes as shown in Figure-2 above.
2. When the battery is discharged, Li is deintercalated from the graphite crystal, enters the electrolyte, passes through the separator, and then migrates to the surface of the lithium iron phosphate crystal through the electrolyte, and then re-inserted into the crystal lattice of lithium iron phosphate via the 010 surface. . At the same time, the battery flows through the conductor to the copper foil collector of the negative electrode, and flows through the ear, the battery negative column, the external circuit, the positive pole, and the positive electrode to the aluminum foil current collector of the battery positive electrode, and then flows to the iron phosphate through the electric conductor. The lithium positive electrode makes the charge of the positive electrode reach equilibrium.
From the working principle of lithium iron phosphate battery, the charging and discharging process of lithium iron phosphate battery requires the participation of lithium ions and electrons, and the migration speed of lithium ions and the migration speed of electrons should be balanced. This requires that the positive and negative electrodes of the lithium ion battery must be a mixed conductor of ions and electrons, and the ion conductivity and electronic conductivity must be the same. However, it is well known that lithium iron phosphate has poor electrical conductivity. While the conductivity of the graphite negative electrode is better, in order to achieve large-rate discharge, it is still necessary to improve the conductivity of the negative electrode, so that the electronic conductivity of the negative electrode and the ability of lithium ions to be deintercalated from the graphite are balanced.
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