Lithium-ion battery anode material PK: graphite vs silicon material
Graphite materials are the veterans of the lithium-ion battery industry and have many excellent qualities. However, with the emergence of a number of high-performance anode materials in recent years, it has threatened the status of graphite materials and performed a drama of falling in love and killing each other. As an outstanding representative of new materials, silicon anode material is really unreasonable with graphite.
The theoretical specific capacity of silicon anode materials is above 4200mAh/g, which is much higher than that of graphite anodes (372mAh/g). It is a strong competitor for the next generation of lithium-ion battery anode materials. However, the silicon negative electrode has natural defects. The insertion of lithium into the Si unit cell will cause serious expansion of the Si material, with a volume expansion of 300%, causing the positive electrode material to expand and pulverize, resulting in a rapid decrease in capacity. In order to overcome the For these shortcomings, scientists combined the two materials and used graphite to overcome the shortcomings of silicon anode. Although silicon was originally intended to replace graphite anodes, the last two materials came together. You have me in you, and you in me.
According to the distribution of silicon, silicon-carbon composite is mainly divided into coating type, embedded type and molecular contact type. According to the form, it is divided into particle type and film type. According to the type of silicon-carbon, it is divided into silicon-carbon binary composite and silicon-carbon Multiple compound.
There are many methods for preparing silicon-carbon composites, such as high-energy ball milling (both mechanical activation method, the main principle is to use mechanical energy to induce chemical reactions or to induce changes in material structure, structure and performance), chemical vapor deposition (both CVD method) ), sputtering deposition method (this is the main method of preparing film materials, using ions generated by gas discharge, under the action of an electric field, bombard the target at high speed, so that atoms in the target escape and deposit on the substrate to form a thin film), Evaporation method (heating and evaporating the material, so that the material is vaporized/sublimated, and deposited on the substrate to form a thin film), high temperature cracking method, etc.
The main method currently used is the high-temperature cracking method. Compared with other methods, this method has a relatively simple process and has a good application prospect. A common method is to disperse nano silicon particles in an organic solvent, add corresponding organics, and then react and crack at high temperature after drying to generate Si-carbon composite materials. For example, Pengfei.G et al. added nano-Si, hexachlorocyclotriphosphazene (HCCP) and 4,4’-dihydroxydiphenylsulfone (BSP) to a mixed solution of tetrahydrofuran and ethanol, and then added triethylamine (TEA ) After dispersing, cleaning and drying, the Si-C composite material is obtained by pyrolysis at high temperature. The specific capacity exceeds 1200mAh/g and the capacity retention rate reaches 95.6% after 40 cycles.
High-energy ball milling is also a current research hotspot. The mechanical energy generated by high-speed ball milling is used to promote the chemical reaction of the system and obtain the target product at a lower cost. For example, Chil. Hoon et al. used high-energy ball milling to first mix iron powder, copper powder, and nano-silicon particles together, and then add graphite to the ball mill to obtain Fe-Cu/Si/C multi-element composite materials.
Vapor deposition method is a commonly used method in the laboratory. Pengfei.G et al. used vapor deposition method to deposit multi-walled carbon nanotubes (MWNT) on the surface of nano-Si particles. The carbon nanotubes formed a good conductive network and the capacity of the composite material And the cycle performance is very good, the first charging specific capacity can reach more than 1592mAh/g, after 20 cycles, the specific capacity can still reach 1400mAh/g.
At present, there are many methods for preparing Si-C composite materials. The thin-film Si electrodes prepared by some methods (sputtering deposition method, etc.) have very good cycle performance, but these methods are currently too expensive to produce on a large scale. , Which limits its application in production. At present, the more practical methods are high-temperature cracking method and high-energy ball milling method. Products with better performance can be obtained by optimizing the process.
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