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How to Modify Natural Graphite

How to Modify Natural Graphite

The modification of natural graphite is mainly carried out from two aspects: first, the surface modification is carried out to reduce the formation of excessive SEI film by changing the surface structure and properties of natural graphite, thus reducing the loss of irreversible capacity; Second, to modify the structure, by changing the structure and shape of natural graphite, to improve the capacity of charge and discharge.

1. Carbon Coating

In 1994, Kuribayashit and Yamashita first proposed a new core-shell model coating method in the 7th International Conference on Lithium Batteries, which can greatly improve the electrical properties of composite graphite. This method is mainly based on graphite material "core", on the surface coated a kind of amorphous carbon material, coating methods have spirit deposition method, organic solvent pyrolysis method, the mixed FenSuiFa, such as the cladding material of amorphous carbon precursor consists of phenolic resin, epoxy resin, urea, asphalt, coal tar, vinyl, etc., through high temperature carbonization step by step for after the amorphous carbon.

Some experts and scholars coated asphalt on the surface of natural graphite, adding 5%, carbonization at 400℃ for 3h, high-temperature treatment at 850℃ for 2h, the reversible capacity of the product can reach 362mAh/g, and the first charge and discharge efficiency is 92%. The main mechanism of the good electrical performance of composite coating is that on the one hand, the amorphous carbon shell is not active to organic solvents. Its chaotic layer structure makes it difficult for small organic solvent molecules to co-insert into the lamellar. On the other hand, the layer spacing of amorphous carbon is larger than that of graphite, and the diffusion performance of lithium ion is better, which greatly improves the rate performance.

2. Surface Oxidation

Oxidation treatment methods are mainly gas phase oxidation method and liquid phase oxidation method. Gas phase oxidation method mainly uses air, oxygen, ozone gas as oxidant, through the gas phase, solid phase interface reaction to complete the oxidation process. However, due to the gas phase oxidation can only occur in the gas-solid interface, it is difficult to guarantee the uniformity of oxidation, is not conducive to commercial use, the liquid phase oxidation method using cerium sulfate, nitric acid and hydrogen peroxide as oxidant, oxidizing more evenly, so commercial use of liquid phase oxidation method, general with experts and scholars using iron phosphate potassium as oxidant, the graphite oxidation treatment, The treated graphite surface contains microporous structure, forming a layer of dense oxide film, compared with untreated natural graphite, reversible capacity and cycling performance are greatly improved. Main mechanism for on the one hand, the surface oxidation treatment can increase the graphite material surface nanoscale pore quantity, increase the lithium storage space, increase the reversible capacity, on the other hand, surface oxidation can eliminate some higher position on the graphite surface, is advantageous to the irreversible capacity is reduced, and also will make the graphite surface with even the reductive decomposition of electrolyte and inhibit further decomposition of the electrolyte, Improve battery cycle performance.

3. Surface Reduction

Graphite surface there is a certain amount of oxygen adsorption of organic functional groups and some impurities, decomposition of graphite for the first time in the process of charging and discharging the solvent and the formation of the SEI film will cause negative effect, lead to irreversible capacity loss increase, Japanese experts using diethyl ether to restore the surface of graphite, oxygen-containing organic functional groups on the surface of the graphite fundamental disappear, The SEI film solvent decomposition platform with graphite electrode at 0.5V also disappears, because the amount of solvent reduction decomposition required for SEI film is greatly reduced, which further indicates that the SEI film generated is thinner and denser.

4. Mechanical Activation

The main purpose of mechanical activation is to reduce the size of graphite particles and increase the number of end faces in the graphite material. The decrease of particle size and the increase of end faces can provide more places for lithium ions to embed and release, and also facilitate the increase of the embedding and release rate, so as to improve the reversible capacity and rate performance of graphite materials.

Then mechanical grinding of graphite can improve the reversible capacity and rate performance of graphite, but after grinding, the specific surface of graphite will increase, so that the consumption of lithium ions to form the SEI film will increase, and the reversible capacity will be reduced, so in practice It is difficult to achieve in production. On the other hand, different grinding methods will also have a certain impact on graphite. For example, the irreversible capacity of natural graphite that has been ball-milled for a long time can reach 580mAh/g, but the cycle performance is average.

5.Doping

As we all know, doping is to introduce some metal elements or non-metal elements into the natural graphite material, and then change the microstructure and electronic state of the graphite material, and then improve its electrochemical performance. At present, the most widely used doping non-metallic elements are phosphorus, nitrogen, boron, silicon, etc., and the main metal elements are potassium, magnesium, aluminum, copper, nickel, cobalt, etc. For example, studies have shown that when boron is introduced into natural graphite as a doping element, the capacity of the composite can reach 315mA/g at the addition level of 3.8% boron.