Development of Graphene-coated lithium battery materials by Sichuan University

lithium iron phosphate is a commonly used cathode material for lithium ion batteries, and it is favored by power battery manufacturers because of its good thermal stability and safety.

The relevant safety tests show that under the existing technical conditions, only lithium ion batteries using lithium iron phosphate materials can pass all safety tests and do not ignite or explode in acupuncture and compression experiments. This is very important for electric vehicles, electric buses and other areas where battery safety requirements are extremely high.

However, lithium iron phosphate materials also have this innate deficiency. The main reason is that the working voltage is low, only about 3.4 V, and the conductivity is poor. This not only makes the energy density of the material much lower than that of lithium cobalt acid and other materials, but also affects the battery. Fast charging and discharging performance.

In order to increase the working voltage of lithium iron phosphate materials, people try to replace Fe elements in lithium iron phosphate materials with Mn elements, but relevant experiments and calculations have shown that LiMnPO4 has very poor conductivity and electron conductivity is much lower than LiFePO4. As a result, the doubling performance of the material is extremely poor and it is almost impossible to discharge.

As a result, people retreated to the second place and instead studied LiMn1-xFexPO4, a solid solution material of lithium iron phosphate and lithium manganese phosphate. On the one hand, this material inherits the "relatively good" conductivity of LiFePO4 and also inherits the higher operating voltage of LiMnPO4.

In order to improve the conductivity of lithium ferromanganese phosphate materials, people have tried a variety of materials to cover, of which the most successful and most mature is the graphite coating method, but because graphite can not form a continuous conductive network on the surface of the material particles, Therefore, the improvement of graphite on the properties of lithium ferromanganese phosphate material is very limited.

Graphene materials consist of single-layer or low-layer graphite atoms and have good conductivity. They are currently the most conductive materials in known materials. The appearance of graphene gives people an additional choice. The excellent conductivity of graphene, It can significantly improve the electron conductivity of lithium iron phosphate material and improve the doubling performance of the material.

At present, there are two main methods for graphene coating lithium iron phosphate: backward method and forward method. The backward method is to form a layer of graphene layer on the surface of the material particle by mechanical mixing and self-assembly on the surface of the synthesized lithium iron phosphate material particle.

The forward method is to form pyrolysis carbon by pyrolysis of Fe organic matter, form a layer of graphene layer on the surface of the material particle by catalytic carbonization, or synthesize the precursor FePO4 directly in the graphene oxide solution to attach it to the graphene oxide. On the sheet, lithium iron phosphate material is synthesized.

Since there is only one dimensional Li + diffusion channel for olivine materials, we prefer to cover a layer of several hundred nanometers of graphene on the lithium iron phosphate primary particle surface to achieve the simultaneous improvement of the material's electron conductivity and Ionic conductivity. The purpose.

Recently, WeiXi of Sichuan University and others synthesized graphene coated with ferromanganese phosphate lithium through the forward method. They first synthesized the graphene-coated nanometer Li3PO4 material in the graphene oxide solution by co-precipitation, and then reacted the precursor with Mn2 + and Fe2 + in the ethylene glycol solution by solvent heat method to obtain LiMn 0.5 Fe 0.5 PO4 material. The graphene oxide is then reduced to graphene. The material inherits the morphology of the precursor Li3PO3. Its particle diameter is only about 20 nm, which greatly shortens the diffusion distance of Li +. The graphene network structure gives the material good conductivity. performance.

Electrochemical tests found that there are two voltage platforms in the material, 3.4-3 .6 V and 4.0-4 .1 V, respectively, corresponding to Fe2 + / Fe3 + and Mn2 + / Mn3 +, respectively.

The capacity test found that the material can reach 166mAh/g after being carbon coated again. Due to the good conductivity of the material, the material has a good multiplier performance at 0.1 C, 0.2 C, 0.5 C, 1C, 3C, 5C, At the multiple of 10C and 20C, the specific capacity of the material reached 166,156,136,126,115,107,101,90mAh/g, respectively, and the energy density of the material also reached 612Wh/kg, which is higher than that of lithium cobalt acid. With 500 cycles at 1C times, the material has a capacity retention rate of 92 %, which shows excellent recycling performance.

The graphene coated nanometer LiMn 0.5 Fe 0.5 PO4 material synthesized by this method has overcome the problem of poor material conductivity and Li + diffusion difficulties, improved the doubling performance of the material, and improved the energy density of the material. At present, the biggest problem with this method is that the cost of graphene is too high, which increases the cost of the entire material.

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