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Why lithium batteries like to use graphite as a negative electrode material?

2019-07-24 08:37

Description

Graphite is an allotrope of carbon, which is closely related.Graphite is the most stable form of carbon.Diamond is a metastable allotrope of carbon. Although its hardness is much higher than that of graphite, it is the hardest substance in nature, but its stability is lower than that of graphite.


The word "graphite" comes from the Greek word "graphein".The material is resistant to high temperatures, corrosion, good electrical conductivity, thermal conductivity and stable chemical properties, and is lighter than aluminum.In addition to being used as a negative electrode material for lithium-ion batteries, high-quality graphite can be used in various fields such as fuel cells, solar cells, semiconductors, light-emitting diodes, and nuclear reactors.


In general, graphite has the advantages of high electronic conductivity, small volume change of layered structure before and after lithium insertion, high lithium insertion capacity and low lithium insertion potential, and has become the mainstream commercial lithium ion battery anode material.

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How to get graphite

There are two ways to obtain graphite: one is natural ore, and the other is the synthesis of coal tar.


The graphite material used in lithium ion batteries is generally prepared by blending 55% synthetic graphite with 45% low purity natural graphite.Manufacturers once favored synthetic graphite because synthetic graphite has better homogeneity and purity than natural graphite.It is different now, and the application of modern chemical purification methods results in a 99.9% pure natural graphite obtained by heat treatment.In contrast, synthetic graphite has a purity of 99%, making the former more popular.Compared to synthetic graphite, the purified natural flake graphite has higher crystallinity and exhibits better electrical conductivity and thermal conductivity.In addition, natural graphite is expected to reduce the production cost of lithium-ion batteries while achieving equivalent or even superior battery performance.


It can be expected that synthetic graphite will eventually be replaced, and in the future it will be cheaper and more environmentally friendly natural graphite, while graphene synthesized from natural graphite will be more powerful.


Lithium intercalation mechanism of graphite

Graphite has good conductivity, high degree of crystallization and good layered structure. It is very suitable for repeated insertion and deintercalation of lithium ions. It is the most widely used and most mature anode material.

After lithium ions are intercalated between the graphite layers, a lithium-incorporated LixC6 (0≤x≤1) is formed, and the theoretical capacity is up to 372 mAh/g (x=1). The reaction formula is: xLi++6C+xe-→LixC6.Lithium ion intercalation causes the deposition between the graphite layer and the layer from ABAB to AAAA.


Modification of graphite


Since the graphite layer spacing (d≤0.34 nm) is smaller than the interplanar layer spacing (0.37 nm) of the graphite lithium intercalation compound LixC6, the graphite layer spacing changes during charging and discharging.

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It is easy to cause the graphite layer to peel off and pulverize, and 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.


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.


Other anode material

At present, graphite is the mainstream commercial lithium battery anode material, its theoretical gram capacity is 372mAh / g, the graphite anode material with better performance on the market has reached 360mAh / g, and the gram capacity gradually reaches the limit value.


Although graphite as a negative electrode material has disadvantages such as low gram capacity and cyclicity deviation, in view of the high cost performance of graphite negative electrode materials, it is not immediately replaced by new materials, for the following reasons:

1. The new anode material technology is immature, and it takes a long time to improve performance;

2. The price of the new anode material is higher, and the price advantage of the graphite anode is obvious;

3. The anode material needs to be used together with the cathode material, the electrolyte, etc., and the specific capacity of the cathode material is generally low.