Why is graphite cathode materials appropriate for lithium battery?

APR 16, 2019   Pageview:63

Lithium ion battery refers to a secondary battery system which two different lithium compounds can reversibly embedded and dis-embedded as positive and negative poles of lithium ion battery. During charging, lithium ion gets off from the positive pole and embedded in the cathode through the electrolyte and the diaphragm; Discharge process is different. Lithium ion takes off from the cathode and embedded in the positive pole through the electrolyte and the diaphragm.

Lithium ion battery cathode is made from the cathode active material, adhesives and additives mixed paste adhesive evenly on both sides of copper foil, after drying, rolling.

The advantage of the graphite as the cathode material

Graphite is carbon allotropes, both are closely related. Graphite is the most stable form of carbon element.(diamond is a kind of metastable allotropes of carbon, although its hardness is much higher than that of graphite, is the hardest substance in nature, but its stability is lower than that of graphite.)

The term graphite from the Greek "graphein", the material is high temperature resistant, corrosion resistance, good electrical conductivity, thermal conductivity and stable chemical properties, and is lighter than aluminum. In addition to the anode materials for lithium ion batteries, high quality graphite can also be used for fuel cells, solar cells, semiconductor, light-emitting diodes (leds), as well as different domains such as nuclear reactors.

In general, the graphite with high electrical conductivity, the layered structure in small volume change, before and after the intercalated-li capacity high and embedded lithium electricity low bit advantages, has become the mainstream of commercial lithium-ion battery cathode materials.

The access of the graphite

For graphite in one of two ways: one is the natural ore, the second is the synthesis of coal tar.And lithium ion battery using graphite material is usually made of 55% graphite and 45% of the preparation of low purity of natural graphite to reconcile.

Manufacturers once popular synthetic graphite, because homogeneity and purity of synthetic graphite is superior to natural graphite. Now is not the same, application of modern chemical purification methods make the heat treatment can obtain 99.9% purity of natural graphite. Synthesis, by contrast, the purity of graphite was 99%, and the former is more popular.

Compared with the synthetic graphite, purified natural flake graphite has higher crystallinity, show better electrical conductivity and thermal conductivity. In addition, natural graphite is expected to reduce the production cost of lithium ion battery, at the same time, still can obtain the same or even more excellent battery performance.

Can be expected is a synthetic graphite will be replaced, the future belongs to cheaper and more environmentally friendly, natural graphite and graphene synthesized from natural graphite, more will do.

The intercalated-li mechanism of graphite

Graphite good electrical conductivity, high degree of crystallization, has good layer structure, very suitable for lithium ion repeatedly embedded - embedded, is currently the most widely used, the most mature technology of cathode materials. Lithium ion embedded after graphite layers, form the intercalated-li combined LixC6 (0 x 1 or less or less), theoretical capacity is 372 mAh/g (x = 1), reactive as: xLi++ 6 c + xe - - > LixC6

Lithium ion embedded between the graphite layer and layer accumulation mode changed from ABAB to AAAA, as shown in the figure below.

Modification of graphite

Due to graphite layer spacing (d 0.34 nm or less) less than the crystal layer spacing of graphite LixC6 intercalated-li compounds (0.37 nm), which in the process of charging and discharging, graphite layer spacing change, easy to cause flake graphite layer, pulverization, will happen lithium ions and organic solvent molecules common embedded graphite layer and decomposition of organic solvent, which affect cell cycle performance.

By graphite modification, such as in carbon, graphite surface oxidation, coating polymer pyrolysis to form the compound graphite with core - shell structure, can improve the charge and discharge performance of graphite and improve the specific capacity.

Other anode materials

At present, the graphite anode materials is the mainstream of commercial li-ion battery, its theoretical capacity of 372 mAh/g, on the market with better performance of graphite anode materials have been able to reach 360 mAh/g, g capacity gradually tends to limit value. Although graphite as the cathode material has a low capacity of g, such as circularity deviation disadvantage, but with high cost performance of graphite anode materials class, it will not immediately replaced by new materials, the specific reasons are as follows:

New cathode materials technology is not mature, also need a long time for performance improvement;

The price of new anode materials is higher, the graphite anode price advantage is obvious;

Anode materials need to match with the anode materials, electrolyte and use, and at present the anode material of specific capacity is generally low.

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