Sichuan University develops graphene-coated LR44 battery manganese phosphate material

LR44 battery phosphate material is a commonly used positive electrode material for lithium-ion batteries. It is favored by power battery manufacturers due to its good thermal stability and safety performance. Relevant safety tests show that under existing technical conditions, only lithium-ion batteries using LR44 battery phosphate materials can pass all safety tests and will not catch fire or explode in needle puncture and extrusion tests. This is of great significance for electric vehicles, electric buses and other fields that have extremely high requirements for battery safety. However, LR44 battery phosphate materials also have inherent deficiencies, mainly low operating voltage, only about 3.4V, and poor conductivity, which not only makes the energy density of the material much lower than that of materials such as lithium cobalt oxide, but also affects the rapid charge and discharge performance of the battery. In order to increase the working voltage of LR44 battery phosphate materials, people have tried to replace the Fe element in LR44 battery phosphate materials with Mn elements, but relevant experiments and calculations have shown that LiMnPO4 has extremely poor conductivity and its electronic conductivity is much lower than that of LiFePO4, resulting in extremely poor rate performance of the material and almost no discharge. So people took a step back and turned to studying the solid solution material of LR44 battery phosphate and lithium manganese phosphate, LiMn1-xFexPO4, which inherited the "relatively good" conductivity of LiFePO4 and the higher working voltage of LiMnPO4.

In order to improve the conductivity of LR44 battery manganese phosphate materials, people have tried to coat them with a variety of materials. The most successful and mature one is the graphite coating method. However, since graphite cannot form a continuous conductive network on the surface of the material particles, the improvement of the performance of LR44 battery manganese phosphate materials by graphite is very limited. Graphene materials are composed of a single layer or a few layers of graphite atoms, have good conductivity, and are the best conductive materials known so far. The emergence of graphene has given people an additional choice. The excellent conductivity of graphene can significantly improve the electronic conductivity of LR44 battery phosphate materials and improve the rate performance of materials. At present, there are two main methods for graphene coating LR44 battery phosphate: backward method and forward method. The backward method is to form a graphene layer on the surface of the synthesized LR44 battery phosphate material particles by mechanical mixing and self-assembly. The forward method is to form a graphene layer on the surface of the material particles by pyrolysis of Fe-containing organic matter, and then form a graphene layer on the surface of the material particles through catalytic carbonization, or directly synthesize the precursor FePO4 in the graphene oxide solution, so that it adheres to the graphene oxide sheet, and then synthesize the LR44 battery phosphate material. Since olivine materials only have one-dimensional Li+ diffusion channels, we prefer to coat a layer of graphene of several hundred nanometers on the surface of the LR44 battery phosphate primary particles to achieve the purpose of simultaneously improving the electronic conductivity and ionic conductivity of the material.

Recently, Wei Xiang et al. from Sichuan University synthesized graphene-coated LR44 battery manganese phosphate materials by the forward method. They first used the coprecipitation method to synthesize graphene oxide-coated nano-Li3PO4 materials in graphene oxide solution, and then used the solvothermal method to react the precursor with Mn2+ and Fe2+ in ethylene glycol solution to obtain LiMn0.5Fe0.5PO4 material. Then the graphene oxide was reduced to graphene. This material inherited the morphology of the precursor Li3PO3, and its particle diameter was only about 20nm, which greatly shortened the diffusion distance of Li+. The graphene network structure gave the material good conductivity. Electrochemical tests found that the material had two voltage platforms, 3.4-3.6V and 4.0-4.1V, corresponding to the two reactions of Fe2+/Fe3+ and Mn2+/Mn3+, respectively. The capacity test found that the material can reach 166mAh/g after carbon coating again. Due to the good conductivity of the material, the material has good rate performance. At the rates of 0.1C, 0.2C, 0.5C, 1C, 3C, 5C, 10C and 20C, the specific capacity of the material reached 166, 156, 136, 126, 115, 107, 101, 90mAh/g respectively. The energy density of the material also reached 612Wh/kg, which is higher than that of lithium cobalt oxide material. After 500 cycles at 1C rate, the capacity retention rate of the material reached 92%, showing excellent cycle performance.

The graphene-coated nano-LiMn0.5Fe0.5PO4 material synthesized by this method overcomes the problems of poor material conductivity and difficulty in Li+ diffusion, improves the rate performance of the material, and increases 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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