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The choice of negative electrode for lithium ion battery-graphite

2019-10-14 21:30

Description

A li-ion battery refers to a secondary battery system in which two lithium intercalation compounds capable of reversibly intercalating and deintercalating, li-ion are used as a positive electrode and a negative electrode, respectively. When charging, lithium ions are deintercalated from the positive electrode, and are inserted into the negative electrode through the electrolyte and the separator; on the contrary, lithium ions are deintercalated from the negative electrode, and are inserted into the positive electrode through the electrolyte and the separator.

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The negative electrode of the lithium ion battery is made by mixing a negative electrode active material, a binder and an additive to form a paste adhesive, which is evenly coated on both sides of the copper foil, and dried and rolled.


Graphite has become the mainstream commercial lithium ion battery anode material due to its high electronic conductivity, large lithium ion diffusion coefficient, small volume change of layered structure before and after lithium insertion, high lithium insertion capacity and low lithium insertion potential.


Graphite has a six-membered ring carbon network layered structure, and the carbon and carbon are SP2 hybrid, and the layers are connected by molecular force.

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There are two different crystal structures in graphite: hexahedral graphite (2H) and rhombohedral graphite (3R).


The 2H phase has ABABA characteristic packing, and the 3R phase packing structure is ABCABC. The two phases can be transformed with each other. The 2H phase is thermodynamically stable and is more abundant in graphite, accounting for about four-fifths of the total. In the anode material of lithium ion batteries, natural graphite and artificial graphite have always used the largest negative electrode material, but artificial Because graphite requires high temperature treatment in the production process, its production cost is greatly increased and the environment is adversely affected. Compared with artificial graphite, natural graphite has many advantages, its low cost, high degree of crystallization, purification, crushing and grading. The technology is mature, the charging and discharging voltage platform is low, and the theoretical specific capacity is high. These have laid a good foundation for its application in the lithium ion battery industry.


Natural graphite is divided into amorphous graphite (earth graphite or microcrystalline graphite) and flake graphite. The theoretical capacity is 372 mAh/g. Amorphous graphite has a low purity and a graphite interplanar spacing (d002) of 0.336 nm.


Mainly 2H crystal plane sorting structure, that is, the graphite layer is arranged in the order of ABAB..., the orientation between the single crystallites is anisotropic, but after processing, the microcrystalline particles have a certain interaction with each other to form a block or particle. Shaped particles have isotropic properties.And the formed massive particles are easily pulverized into particles having a better shape.


In the process of lithium ion intercalation and deintercalation, the volume change is small and the structure is relatively stable, but the reversible specific capacity is only 260 mAh/g, and the irreversible specific capacity is above 100 mAh/g.


The flake graphite has a high degree of crystallinity, and the sheet structure is unitized and has a remarkable anisotropy.


This structure determines the large change in volume of graphite during lithium intercalation and deintercalation, resulting in destruction of the graphite layer structure, large irreversible capacity loss and severe deterioration of cycle performance.


Graphite has good conductivity, high degree of crystallization, and a good layered structure, which is very suitable for repeated insertion-deintercalation of lithium ions. During the charging and discharging process, the graphite layer spacing changes, which tends to cause the graphite layer to peel off and pulverize. Lithium ions and organic solvent molecules are also embedded in the graphite layer and the organic solvent is decomposed, thereby affecting the cycle performance of the battery.


Therefore, when used, researchers tend to focus on the modification of natural graphite, improve its own structural defects, and improve battery performance.


By graphite modification, such as oxidation on the surface of graphite, coating of polymer pyrocarbon, forming a composite graphite with a core-shell structure, the charge and discharge performance of graphite can be improved, and the specific capacity can be improved.