What role does graphene play in lithium batteries

Graphene's performance is undoubtedly excellent, but is it suitable for lithium ion batteries? Currently, the "graphene battery" claimed in the market is an inaccurate concept. To be accurate, it is basically to add a little graphene to the material to improve some performance of lithium battery, which can be called graphene-based lithium ion battery. It is not excluded that graphene is used as a negative electrode material to make lithium batteries or super capacitors in the laboratory, but the requirements are relatively high. In summary, graphene can be used as a condiment for lithium batteries, but it is not suitable as the main material.

1. Structure and properties of graphene

Physical structure: graphene is a flat film with a single atomic layer composed of carbon atoms. Its thickness is only 0.34 nanometers. The thickness of a single layer is equal to 1/150,000th of the diameter of a human hair. It is the thinnest and hardest nanometer material known in the world, with good light transmission and folding ability. Because there is only one layer of atoms, the electrons are confined to a single plane, and graphene has a whole new set of electrical properties. The specific surface area of graphene was about 2630m2/g, and the thermal conductivity was 5000W/m&dot. K.

Electrical properties: graphene has unique carrier properties and massless Dirac fermion properties. Its electronic mobility can reach 2 x 105cm2/ v. S, which is about 140 times that of the electron mobility in silicon, gallium arsenide 20 times, high temperature stability, electrical conductivity of up to 108 Ω/m, the surface resistance is about 31 Ω/sq (310 Ω/m2), lower than that of copper or silver, is the best material conductivity at room temperature.

In addition, the half-integer quantum hall effect of electron carriers and hole carriers in graphene can be observed by changing the chemical potential through the action of electric field, while Novoselov et al. observed this quantum hall effect of graphene at room temperature.

2 graphene in lithium batteries

It is because of graphene with nanometer size effect of the above, has a great specific surface area, good electrical conductivity and excellent mechanical properties, such as graphene widely research by scientists all over the world, and created a "graphene lithium battery" concept, graphene is what roles involved in the lithium battery in?

Negative material

Due to its unique two-dimensional structure, excellent electronic transmission capacity and large specific surface area, graphene has great potential to replace graphite as a new generation of cathode material for lithium ion batteries. The lithium storage mechanism of graphene is similar to that of other carbon materials. Lithium ions are separated from the positive pole during charging and formed by electrolyte embedded in the interlayer of carbon material to form Li2C6. Lithium ions are separated and returned to the positive pole during discharge. Due to the special two-dimensional structure of graphene, when the sheet spacing is greater than 0.7nm, both sides of graphene can store lithium ions. At the same time, due to the existence of pleated graphene, lithium can also be stored. Therefore, the theoretical capacity of graphene may be twice that of graphite, higher than 744mAh/g.

In addition, graphene is mostly of micro-nano size, which is much smaller than bulk graphite, making the diffusion path of Li ions shorter, and the spacing between graphene layers is usually much larger than that of graphite, which also provides more channels for the transmission of lithium ions. Therefore, compared with graphite, using graphene as the negative electrode is more conducive to improving the battery performance. Since the concept of graphene battery was proposed, many academic studies have shown that the reversible capacity of graphene lithium battery can reach over 500mAh/g, and it has excellent multiplier performance. The lithium electronegative electrode prepared under laboratory conditions mostly adopts CVD method, hydrazine hydrate reduction, vacuum filtration and freeze-drying method to prepare graphene, which is either flaky or hollow spherical.

2. Graphene ACTS as a conductive agent

The primary role of the conductive agent is to improve the electronic conductivity, because the electrolyte is ionic conductivity, and electrons are unable to conduct, so the conductive agent is to promote the rapid passage of electrons through the living material to the collector fluid. In addition, the conductive agent can also improve the processing of the electrode sheet, promote the infiltration of electrolyte on the electrode sheet, reduce the resistivity, so as to improve the service life of lithium battery.

Currently, the commonly used conductive agents include SP and acetylene black, etc. The traditional carbon black is spherical, which is more likely to be mixed evenly with the living substance. However, the contact form is spot contact, which limits the play of the conductive role and increases the amount of the conductive agent. However, graphene is a sheet structure with point-to-surface contact with the active material, which can maximize the role of the conductive agent and reduce the amount of conductive agent, so as to use more active material and improve the capacity of lithium battery. However, graphene in sheets is also a drawback. Sheets of graphene are more difficult to disperse in solvents and more likely to agglomerate together. Instead, we need to increase the amount of graphene we use. At the same time, its lamellar structure is not conducive to the diffusion of lithium ions, resulting in increased internal resistance of the cell and accelerated battery failure.

In theory, the ultra-fast conductivity of graphene can improve the multiplier performance of the battery, but the fact is that the result of graphene single layer hinders the diffusion of lithium ions, especially in the case of charging and discharging at a high rate, the polarization inside the battery increases and the discharge capacity of the battery decreases. Relevant studies have shown that partial substitution of graphene for conductive carbon black can reduce the amount of conductive agent and improve the energy density of the battery to a certain extent under the condition of low rate discharge.

3. Industrialization process of graphene lithium battery

Successful examples of graphene anode materials and graphene conductive agents prepared under laboratory and r&d conditions have provided a solid scientific research foundation for the productization of graphene lithium ion batteries in industrial production. So what is the situation of graphene lithium ion batteries in reality?

The first product is the "king of ene", launched by dong xu in 2016. On July 8, 2016, east light held a graphene-based lithium ion battery product launch in diaoyutai, and launched the world's first graphene-based lithium ion battery product -- "king of ene".

The second product is the industry's first high-temperature and long-life graphene-assisted lithium battery launched by huawei in December 2016. Graphene-assisted high-temperature lithium ion battery technology breakthroughs mainly come from three aspects: adding special additives in the electrolyte to remove trace water and avoid high-temperature decomposition of the electrolyte; The positive electrode of the battery is made of modified large single crystal ternary material to improve the thermal stability of the material. At the same time, the use of new materials graphene, lithium ion batteries and the environment can achieve efficient heat dissipation.

The third product is the "domestic graphene battery" made by Dongxu photoelectric and best, as mentioned by the media. The fact is that graphene is used on the diaphragm...

Of course, there are a variety of patents on graphene lithium batteries on the market, but only at the patent stage. Samsung, Panasonic, LG and others all have graphene related patent applications. Currently, there is no mass production of graphene-based lithium ion batteries in the market.

4. To summarize

Graphene's performance is undoubtedly excellent, but is it suitable for lithium ion batteries? In brief, graphene is not suitable for lithium ion batteries. Here are some considerations:

1. Cost

The cost of using graphene as a conductive agent is much higher than that of ordinary carbon black. The cost of lithium battery is a key control factor, which does not reduce the cost of raw materials. Even if the electrical properties of graphene are good, the cost of hundreds of thousands of tons of batteries cannot be used by the battery manufacturers.

2. Process problems

The main technological problem caused by the flake structure of graphene is the preparation of electrode slurry. The electrode slurry should have good fluidity, dispersibility and suitable viscosity. The dispersion of sheet graphene in electrode slurry is a difficult problem, especially as the slurry cannot be dispersed by adding dispersants. The surface area of graphene is very large, which has a great influence on the settling stability of slurry, and the consistency of each batch cannot be guaranteed, which affects the battery performance.

3. Other factors

The flake structure of graphene inhibits the diffusion of lithium ions, which is likely to cause severe battery polarization and lower battery capacity. The rich functional groups on the graphene surface are small wounds on the graphene surface. Excessive addition will not only reduce the energy density of the battery, but also increase the amount of liquid absorbed by the electrolyte. On the other hand, it will increase the side reactions with the electrolyte, which will affect the circularity and may even cause safety problems.

Currently, the "graphene battery" claimed in the market is an inaccurate concept. To be accurate, it is basically to add a little graphene to the material to improve the performance of some lithium batteries. It can be called graphene-based lithium ion battery. It is not excluded that graphene is used as a negative electrode material to make lithium batteries or supercapacitors in the laboratory, but the requirements are relatively high. In summary, graphene can be used as a condiment for lithium batteries, but it is not suitable as the main material.

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