Research progress and Prospect of graphitization Technology

Research progress and Prospect of graphitization Technology

Graphite with its stable structure and unique properties, has a wide range of applications in various fields: for example, it is widely used in energy storage equipment because of its high reversible capacity, good conductivity, good charge-discharge potential distribution and relatively low cost; When petroleum coke was graphitized at 2600℃, the graphitization degree was up to 78.8%. Juxing Graphite Electrodes can effectively reduce the cost of steelmaking. When it was used as lithium ion battery anode material, the initial charge-discharge capacity reached 326.1 mA·h·g-1 and the coulomb efficiency reached 78.8%, showing excellent electrochemical properties; Because of its high temperature resistance and high strength, graphite is often used in the smelting industry to make the lining of graphite crucible and metallurgical furnace. The most important thing is that graphite has excellent conductivity, so it is used as the basic material of electrode materials and composites. This makes it possible for its application to achieve high value utilization in high-tech field.

As an effective means to realize high added value and efficient utilization of carbon resources in the preparation of carbon graphite materials, graphitization technology has been widely and deeply studied by scholars at home and abroad in recent years. Researchers have put forward graphitization technology methods such as catalytic graphitization, chemical vapor deposition, microwave heating, high temperature and high pressure and molten salt electrolysis, which promote the continuous development of graphitization technology.  

This paper summarizes the current graphitization methods of artificial graphite, analyzes the advantages and disadvantages of the corresponding graphitization methods, and forecasts the development trend of graphite preparation methods in the future.

1. Catalytic graphite method  

Catalytic graphitization is a graphitization process of carbon assisted by transition metals and their compounds. It is a process from amorphous carbon to dissolved carbon, and finally form crystalline carbon graphite. The principle is as follows: firstly, the carbon is dissolved in the catalyst (metal or alloy). When the solubility of disorderly arranged carbon in the catalyst reaches saturation, it presents an oversaturated state for graphite. At this time, part of the dissolved carbon tends to change to the crystalline state of low-energy graphite and deposit, so as to obtain graphite. Catalytic graphitization is a method to save energy and shorten production cycle in the production of electric carbon product graphite.

The catalytic graphite method can also obtain graphite materials with different pore size distribution and high graphitization degree by changing the conditions to meet the needs of materials in different fields. Researchers have conducted a series of studies on this method, Elena Rodriguez et al. investigated the effect of boron doping in carbon foam on graphitization at 2400~2800℃. The results show that the graphitization process of foamed carbon depends on boron concentration and source. Under the catalytic action of boron, lightweight graphite-like foam can be obtained, and a higher reversible lithium storage capacity can be achieved by using boron oxide-based carbon foam, with a value of about 310mA·h·g-1.

Zhai et al. also prepared porous graphite by ZnCl2 chemical activation and Fe and Ni as catalysts. However, the disadvantage is that in the graphite obtained by this method, a large number of coated catalysts are difficult to completely remove, and the operation is relatively complicated, which brings difficulties to the actual industrial production.

2. Chemical Vapor Deposition (CVD)

Chemical vapor deposition (CVD) usually refers to the method of synthesizing nano materials by thermally decomposing hydrocarbons to react on the substrate surface. It is also the most widely used method in the semiconductor industry, and it is often used to deposit a variety of materials, such as insulating materials and metal alloy materials.  

From a theoretical point of view, chemical vapor deposition process is a process in which two or more gaseous substances form new materials and deposit on the substrate through chemical reaction in the reaction chamber. It is an important material preparation technology, and many researchers also use it in the preparation of graphite.  

For example, Atchudan et al. prepared graphitized carbon nanosheets by atmospheric pressure chemical vapor deposition using Ti-MCM-41 molecular sieve as catalytic template and acetylene as carbon precursor. Cheng et al., graphite was prepared on Ni substrate by inductively coupled plasma enhanced chemical vapor deposition at low temperature. This method can achieve the preparation of high-purity graphite, but the low production efficiency makes it difficult to achieve scalable and low-cost production, and the compatibility of the corresponding manufacturing process and operating environment is poor.

3. Microwave Heating Method    

Microwave heating method is the use of microwave energy to heat solid carbon, under the action of some metal catalyst, at a low temperature (less than 1500 ℃), to achieve solid carbon graphitization, so as to obtain crystalline graphite method. Microwave heating is a way of heating objects by absorbing microwave and converting microwave into heat energy, which is significantly different from the traditional heating method. The traditional heating method is mainly through convection, conduction and radiation to heat the object, the heat is always transferred from the surface of the object to the inside of the object, so it will cause the phenomenon of uneven heating of the object.  

Microwave heating is the internal friction heat generated by the reciprocating motion of dipole molecules in the object to raise the temperature of the object, which does not require any heat transfer process, can make objects inside and outside the heating at the same time. The heating speed is fast and uniform. The transmitted energy will not be dissipated by the medium around the target material, ensuring the maximum utilization of energy. Therefore, many researchers choose to use it for graphitization of carbonaceous materials.  

Kim et al. realized graphitization of activated carbon in a short time assisted by catalyst through microwave heating, and used it as anode material of lithium ion battery, which can provide fast rate performance and high charge-discharge specific capacity. 

Lei et al. prepared porous graphitized carbon materials using corn starch as raw materials through microwave assisted catalytic graphitization and chemical activation.

The results showed that corn starch could be partially graphitized under microwave assistance, and the product showed large surface area and good capacitance characteristics. As a new carbon graphitization method, microwave heating can realize the graphitization of solid carbon, but the crystallinity of graphite is not high, and it will also involve the problem of catalyst purification and removal in the subsequent treatment process.

4. High Temperature & High Pressure Method  

High temperature and high pressure method is a method to realize graphitization transformation of amorphous carbon under high temperature and pressure. It uses heat to transform unstable carbon atoms from chaotic structure to graphite crystal structure. This method is a relatively traditional graphitization method, which is also used in the current industrial production of graphite materials. Many researchers have carried out research on this method.

Xing Baolin et al. used bituminous coal as a precursor to prepare synthetic graphite materials by preliminary carbonization and further high temperature graphitization at 2000-2 800 ℃. The results show that the microstructure of synthesized graphite is strongly dependent on graphitization temperature. The synthetic graphite was graphitized at 2800℃, with well-ordered lamellar structure, high degree of graphitization, large surface area and well-developed mesopods, which provided a good way for electrochemical intercalation and deintercalation of lithium ions in carbon matrix.

Ignacio et al. prepared graphite from Spanish anthracite with two different properties by heat treatment in the temperature range of 2400~2 800 ℃, and tested its electrochemical performance as an anode in lithium ion batteries. The XRD (Lc, La, D002) and Raman (ID/IG) crystal parameters of the two kinds of graphite materials prepared from anthracite have a good linear correlation. In this way, the graphite can be produced in batches by Acheson furnace at high temperature and the graphite produced has a better crystallinity. However, this graphitization method needs to be carried out at a temperature of more than 2000 ℃, which has high energy consumption, and the waste generated in the graphitization process will cause air pollution.

More importantly, this method is selective for raw materials. It is only applicable to carbonaceous materials with layered carbon atoms in basic units such as petroleum coke and asphalt coke; However, such as resin carbon, glass carbon and other hard carbon, its structure is porous layer structure, even at 2000 ℃ can not be graphitized. Therefore, the development of high temperature and high pressure method is limited.

5. Molten Salt Electrolysis Method    

Molten salt electrolysis is a new electrochemical graphitization method. Amorphous carbon materials can be graphitized by electrochemical reforming in electrolyte salt at a relatively mild temperature. This method is mainly driven by electrochemistry to remove oxygen and some impurities in the carbon material, and rearrange the carbon atoms, so as to convert amorphous carbon into graphite.  

Compared with the above methods, molten salt electrolysis has significant advantages, such as it can adjust the morphology of the product by adjusting the electrolytic voltage, electrolytic time and electrolytic temperature, to meet the needs of different applications; Graphitization of carbonaceous materials can be achieved at a relatively low temperature (usually less than 1000 ℃）； Compared with other graphitization methods, it has wide adaptability to raw materials; Most importantly, it can realize green production by using inert electrode as anode. Since the electrolyte salt used is easily soluble in water, the electrolytic product can be washed away with water to obtain high purity graphite.

In view of this production method, many researchers have carried out experimental studies, such as Peng et al., by polarizing amorphous carbon cathode at molten CaCl2 at about 827℃, it is transformed into porous graphite containing petal-like nanosheets. This nanostructured graphite can rapidly and reversibly insert/de-embed anions, providing excellent cathode materials for batteries.

Zhu et al. converted the purified inferior coal into graphite with high crystallinity by making amorphous carbon reforming of the cathode through electrochemical drive in the molten salt system of CaCl2 at 950℃. When used as anode material for lithium ion battery, it shows excellent electrochemical properties. Graphitization in this way is still in its infancy and the mechanism of graphitization needs further investigation. In the near future, it will also show incomparable advantages in graphite production.

6.  Conclusion & Prospect

Large scale, low-cost, green and environment-friendly graphitization technology is a long-term goal, and it is also of great significance to the development of graphite industry. With the development of advanced manufacturing technology, the new graphitization method will overcome the shortcomings of the traditional graphitization method, and is expected to realize the low cost, low energy consumption and high efficiency of graphitization treatment. Among many graphitization methods, molten salt electrolysis has a good application prospect because of its advantages of simple process, adjustable morphology, green environmental protection, high purity of product and low energy consumption (5.5kW·h per kilogram of graphite). Of course, it is also necessary to further understand the graphitization reaction process from the molecular level and deeply analyze the kinetic law of atoms driven by electrochemistry, which is of far-reaching significance to the development of graphitization technology of carbon materials, view more graphite progress information contact us.