The important role of graphene in lithium power batteries

The performance of graphene is undoubted, but is it necessarily suitable for lithium-ion batteries? The "graphene battery" currently claimed on the market is an inaccurate concept. Accurately speaking, it is basically to add a little graphene to the material to improve the performance of the lithium battery. It can be called a graphene-based lithium-power battery. It is not excluded that graphene is used as negative electrode material in the laboratory to make lithium power batteries or supercapacitors, but the requirements are relatively high. In summary, graphene can be used as a seasoning for lithium power batteries, but it is not suitable as the main material.

1. Structure and properties of graphene

Physical structure: Graphene, a monoatomic layer flat film composed of carbon atoms, has a thickness of only 0.34 nm, and the thickness of a single layer is equivalent to one-fifth of the diameter of the hair. It is the thinnest and hardest nanomaterial known in the world. It has good light transmission and can be folded. Because there is only one layer of atoms, the motion of electrons is confined to a plane, and graphene also has new electrical properties. The graphene has a specific surface area of about 2630 m 2 /g and a thermal conductivity of 5000 W/m; k.

Electrical properties: Graphene has unique carrier properties and a massless Dirac fermion property. Its electron mobility can reach 2×105cm2/Vt; s, about 140 times of electron mobility in silicon, 20 times of gallium arsenide, high-temperature stability, electrical conductivity up to 108Ω/m, and surface resistance are about 31Ω. /sq (310 Ω / m2), lower than copper or silver, is the best conductive material at room temperature.

In addition, the semi-integer quantum Hall effect of electron carriers and hole carriers in graphene can be observed by changing the chemical potential by an electric field, and Novoselov et al. observed the quantum Hall of graphene at room temperature. effect.

2. The role of graphene in lithium power batteries

It is precise because graphene has the above nano-size effect, great specific surface area, good electrical conductivity and excellent mechanical properties. Graphene has been widely studied by scientists all over the world and has produced "graphene lithium battery". In this concept, what role does graphene participate in in lithium power batteries?

1) Anode material

Graphene has the potential to replace graphite as a new generation of lithium-power batteries anode materials due to its unique two-dimensional structure, excellent electron transport capability and large specific surface area. The lithium storage mechanism of graphene is similar to that of other carbon materials. When charging, lithium ions are separated from the positive electrode and formed into Li2C6 through the electrolyte intercalated carbon material layer. When discharging, lithium ions are extracted and returned to the positive electrode. Due to the special two-dimensional structure of graphene, when the interlayer spacing is greater than 0.7 nm, both sides of graphene can store lithium ions, and lithium can be stored due to the presence of wrinkles in graphene, so the theoretical capacity may be graphite. Twice, above 744mAh/g.

In addition, graphene is mostly micro-nano size, much smaller than bulk graphite, which makes the diffusion path of Li ions shorter. The layer spacing of graphene is usually much larger than that of graphite, which also provides more channels for lithium ion transport. Therefore, compared with graphite, the use of graphene as a negative electrode is more conducive to improving battery performance. Since the concept of graphene battery was put forward, many academic research results show that the reversible capacity of a graphene lithium battery can reach more than 500mAh/g, and has excellent rate performance. The lithium-ion negative electrode prepared under laboratory conditions is mostly prepared by a CVD method, hydrazine reduction, vacuum suction filtration, freeze-drying method, etc., or is in the form of flakes or hollow spheres, which are different.

2) Graphene as a conductive agent

The primary function of the conductive agent is to increase the electronic conductivity. Since the electrolyte is ion-conducting and the electrons are not conductive, the conductive agent promotes the rapid passage of electrons through the living material to the current collector. In addition, the conductive agent can also improve the processability of the pole piece, promote the infiltration of the electrolyte to the pole piece, reduce the electrical resistivity, and thereby improve the service life of the lithium battery.

At present, the commonly used conductive agents are SP, acetylene black, etc. The conventional carbon black is spherical, and it is more easily mixed with each other when mixed with the active material, but the contact form is point contact, which limits the exertion of the conductive effect and increases the addition of the conductive agent. the amount. On the other hand, graphene is a sheet-like structure, and the contact with the active material is point-to-surface contact, which can maximize the function of the conductive agent and the like, and reduce the amount of the conductive agent, thereby making it possible to use the active material more and increase the capacity of the lithium battery. However, the sheet form of graphene is also its drawback. The sheet-like graphene is more difficult to disperse in the solvent and is more likely to be agglomerated together, but it is necessary to increase the amount of graphene. At the same time, its sheet structure is not conducive to the diffusion of lithium ions, resulting in the increased internal resistance of the cell and accelerated battery failure.

In theory, the ultra-fast conductivity of graphene can improve the rate performance of the battery, but the fact is that the monolithic layer of graphene hinders the diffusion of lithium ions, especially in the case of large rate charge and discharge, the internal polarization of the battery is increased, and the battery is discharged. The capacity is reduced. Related studies have shown that partial replacement of conductive carbon black with graphene can reduce the amount of conductive agent and increase the energy density of the battery to a certain extent under low-rate discharge conditions.

3. Graphene lithium battery industrialization process?

Successful examples of graphene anode materials and graphene conductive agents prepared under laboratory and research and development conditions provide a solid scientific research foundation for the commercialization of graphene lithium-ion batteries in industrial production, so what is the reality of graphene lithium power batteries? What?

The first product was the product “Ene King” launched by Dongxu Optoelectronics in 2016. On July 8, 2016, Dongxu Optoelectronics held a press conference for graphene-based lithium-ion batteries at Diaoyutai, and launched the world's first graphene-based lithium-ion battery product, “Alkenwang”.

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

The third product is the “domestic graphene battery” made by Dongxu Optoelectronics and Best by the media. The fact is that graphene is used on the diaphragm...

Of course, there are a variety of graphene lithium battery patents on the market, but they are only in the patent stage. Including Samsung, Panasonic, LG, etc. all have patent applications related to graphene. At present, there is no enterprise on the market for mass production of graphene-based lithium-ion batteries.

4. Summary

The performance of graphene is undoubted, but is it necessarily suitable for lithium-ion batteries? In the small series, graphene is not suitable for lithium-ion batteries, consider the following:

1.Cost problem

The cost of using graphene as a conductive agent is much higher than that of ordinary carbon black. The cost of a lithium battery is a key control factor, and the cost of raw materials is not reduced. Even if the electrical properties of graphene are good, one ton of hundreds of thousands of cost-consuming battery manufacturers Also can't afford it.

2. Process problems

The process problem caused by the sheet-like structure of graphene is mainly the preparation of the electrode paste. The electrode paste is required to have good fluidity, dispersibility, and suitable viscosity. The dispersion of flake graphene in the electrode paste is a very difficult problem, especially in the case where the electrode paste cannot add a dispersant to aid dispersion. The surface area of graphene is large, which has a great influence on the sedimentation stability of the slurry and cannot guarantee the consistency of each batch and affect the battery performance.

3. Other factors

The sheet-like structure of graphene inhibits the diffusion of lithium ions, which tends to cause serious polarization of the battery, resulting in a decrease in battery capacity. The rich functional group on the surface of graphene is a small wound on the surface of graphene. Adding too much will not only reduce the energy density of the battery but also increase the liquid absorption of the electrolyte. On the other hand, it will increase the side reaction with the electrolyte and affect the cycle. There may even be security issues.

The "graphene battery" currently claimed on the market is an inaccurate concept. Accurately speaking, basically adding a little graphene to the material to improve the performance of the lithium battery can be called a graphene-based lithium-power battery. It is not excluded that graphene is used as negative electrode material in the laboratory to make lithium power batteries or supercapacitors, but the requirements are relatively high. In summary, graphene can be used as a seasoning for lithium power batteries, but it is not suitable as the main material.

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