Study on Modification of Graphite Anode Materials
Natural graphite has the basic advantages of low cost, high degree of crystallization, mature technology of purification, pulverization and classification, low charge-discharge voltage platform and high theoretical specific capacity. However, the structural defects of natural graphite lead to the first inefficiency and poor cycle. Therefore, it is imperative to develop a modified natural graphite method.
1. Surface oxidation
Surface oxidation is mainly the production of acidic groups (such as -OH, -COOH, etc.) at the irregular electrode interface (saw tooth position and rocking chair position).These groups can prevent the co-intercalation of solvent molecules and improve the wettability between the electrodes/electrolytes and reduce the interface impedance.When lithium is first inserted, it is converted into a lithium carboxylate salt and a surface-Oli group to form a stable SEI film.In addition, oxidation can remove some defect structures in the graphite, and the resulting nano-scale micropores make extra lithium storage space and increase lithium storage capacity.
Surface oxidation generally includes both gas phase oxidation and liquid phase oxidation.Gas phase oxidation mainly uses air, O2, O3, CO2, C2H2 and other gases as oxidants to react with graphite at a gas-solid interface to reduce the active point on the graphite surface and reduce the first irreversible capacity loss.At the same time, more micropores and nanopores are generated, which increases the storage space of lithium ions and is beneficial to increase the reversible capacity and improve the performance of the negative electrode.Some research experts have used ordinary natural graphite to oxidize at 500 °C using air as an oxidant.The stability of the modified graphite structure is improved, and the number of nano-scale micropores and channels is increased while de-defecting the structure.In addition, the oxide layer formed during oxidation is tightly bound to graphite to form a dense passivation film, which prevents the solvation reaction of the electrolyte on the graphite and improves the reversible capacity of the graphite.
The liquid phase oxidation method is realized by a liquid-solid phase reaction using a strong oxidizing agent solution such as barium sulfate, sulfuric acid, nitric acid or hydrogen peroxide.Other researchers used surface oxidation of natural graphite with a saturated solution of sulfuric acid and ammonium persulfate to increase the reversible capacity of graphite to 349 mAhg-1, and the first coulombic efficiency was improved.
2. Surface coating
Surface coating modification of graphite anode materials mainly includes carbon coating, metal or non-metal and its oxide coating and polymer coating.The surface coating is used to improve the reversible specific capacity of the electrode, the first coulombic efficiency, the improvement of cycle performance and the high current charge and discharge performance.The starting point of surface coating modification of graphite materials mainly has the following two points:
（1）Through the surface coating, the specific surface area of the graphite is reduced, and the lithium consumed by the formation of the SEI film is reduced, thereby improving the first coulombic efficiency of the material;
（2）By surface coating, the active point of the graphite surface is reduced, the surface properties are uniform, the co-embedding of the solvent is avoided, and the irreversible loss is reduced.
3. Amorphous carbon coating
A layer of amorphous carbon is coated on the graphite to form a "core-shell" C/C composite, which allows the amorphous carbon to contact the solvent, avoiding direct contact between the solvent and the graphite, and preventing co-intercalation of solvent molecules. The layered peeling phenomenon of graphite expands the selection range of the electrolyte.Wang Guoping people made natural flake graphite into spherical graphite, and coated with a layer of nano-graphitized carbon material on the surface to make modified spherical graphite with core-shell structure.The tap density of the modified spherical graphite is obviously improved, and the reversible capacity is increased to 365 mAh·g-1.At the same time, the first Coulomb efficiency and cycle stability have also been significantly improved.
Natural graphite has the basic advantages of low cost, high degree of crystallization, mature technology of purification, pulverization and classification, low charge-discharge voltage platform and high theoretical specific capacity. However, the structural defects of natural graphite lead to the first inefficiency and poor cycle. Therefore, it is imperative to develop a modified natural graphite method.
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