Graphite vs Silicon materials

Graphite material is a veteran of the lithium-ion battery industry and has many excellent qualities. However, with the emergence of a group of high-performance negative materials in recent years, it has threatened the status of graphite materials and has performed a drama of love and death. Silicon negative materials as an outstanding representation of new materials and graphite is really love and hate.

Silicon negative electrode material theory has a capacity of 4200mAh/g or more, which is much higher than the graphite negative electrode (372mAh/g) and is a powerful competitor for the next generation of lithium ion battery negative electrode materials. However, there is a natural defect in the Silicon negative electrode. Lithium embedded in Si's cell will cause serious expansion of Si material, which will expand to 300 % in volume, causing positive material expansion and pulverization, resulting in a rapid decrease in capacity. To overcome these disadvantages of Silicon negative poles, Scientists combine the two materials and use graphite to overcome the disadvantages of Silicon negative poles. Although Silicon was originally intended to replace the graphite negative electrode, the last two materials came together. You have me and I have you.

According to the distribution method of Silicon, silicon-carbon composites are mainly divided into cladding, embedding, and molecular contact types. According to the morphology, they are divided into granular and thin film types. According to the number of silicon-carbon types, they are divided into silicon-carbon binary composite and silicon-carbon multicomponent composite.

There are many methods for preparing silicon-carbon composites, such as high-energy ball grinding (both mechanical activation methods, Its main principle is to use mechanical energy to induce chemical reactions or induce changes in the structure, structure and properties of the material), chemical vapor precipitation (both CVD method), sputtering deposition method (this is the main method for preparing membrane materials, using gas discharge. The ions produced, Under the action of the electric field, high-speed bombardment of the target material causes the atoms in the target material to escape and deposit on the substrate to form a thin film), steaming plating method (heating and evaporation of the material, vaporizing the material, and depositing it on the substrate to form a thin film), high temperature cracking solution, etc..

At present, the main method used is the high-temperature cracking method. This method is relatively simple compared to other methods and has a good application prospect. The commonly used method is to disperse the nanoparticles in organic solvents and add the corresponding organic matter. After drying, the reaction cracking reaction occurs at a high temperature to form a Sicarbon composite material. For example, Pengei.G added nanometer Si, hexachlorocyclic triphosphonitrile(HCCP) and 4,4 '-dihydroxydiphenylsulfoxide(BSP) to a mixed solution of tetrahydrofuran and ethanol, and then added triethylamine(TEA) for decentralized cleaning and drying. The high temperature cracking results in a Si-C composite material with a specific capacity of more than 1200 mAh/g and a 40 cycle capacity retention rate of 95.6 %.

High-energy ball grinding is also a research hotspot. Using the mechanical energy produced by high-speed ball grinding, it promotes the chemical reaction of the system and obtains the target product at a lower cost. For example, Chil.Hoon et al. use a high-energy ball grinding method to first mix iron powder, copper powder, and nanosilicon particles together, and then add graphite to grind in this ball to obtain a Fe-Cu/Si/C composite material.

Gas deposition is a commonly used method in the laboratory. Pengei.G et al. used gas deposition to deposit multi-walled carbon nanotubes(MWNT) on the surface of nanometer Si particles, and carbon nanotubes formed a good conductive network. The capacity and cyclic performance of the composite materials are very good. The initial charging ratio can reach 1592 mAh/g or more. After 20 cycles, the specific capacity can still reach 1400 mAh/g.

At present, there are many methods for preparing Si-C composite materials. Some methods (sputtering deposition method, etc.) have a very good recycling performance. However, these methods currently show that the production cost is too high to produce on a large scale. This limits its application in production. At present, the more practical methods are high temperature cracking method and high energy ball grinding method. The product with better performance can be obtained by optimizing the process.

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