(Tsinghua University Department of Materials Science and Engineering, New Carbon Materials Laboratory, Beijing 100084)
The layers of carbon atoms in graphite are combined by van der Waals force, which is easy to be opened by external force to insert other molecules and atoms, thus forming graphite intercalation compounds (GICs). By controlling the oxidation/intercalation process of GICs modification, the research group invented high-quality and low-sulfur expandable graphite, with the expansion volume greater than 160 mL/g and the residual sulfur content less than 800× 10-6. MClx-GICs(M is a transition metal) micropowder is invented as electromagnetic wave absorption shielding material, which completely shields infrared light and laser for more than 15 min. By controlling the intercalation/delamination process, high temperature expanded graphite was prepared as an oil absorption material, and the adsorption capacity of heavy oil exceeded 80 g/g, and the sewage purification effect was far better than that of activated carbon. Low-temperature expanded micro-expanded graphite was invented as anode material of lithium ion battery, with reversible capacity of 370 Ma h/g and good cycle performance [1 ~ 20].
Graphite intercalation compound; Expanded graphite; Process control.
Brief introduction of the first author: Kang Feiyu, male, doctor of engineering, professor, mainly engaged in deep processing technology of natural graphite and research on porous carbon materials. E-mail: fykang @ qing-hua.edu.cn.
I. Introduction
Natural flake graphite has excellent physical and chemical properties and has broad application prospects in various high-tech and industrial fields. However, natural flake graphite is flaky powder, and its shape, structure and properties are difficult to meet the requirements of different scientific and technological fields. In this study, flake graphite was modified into functional graphite by graphite intercalation technology, and the oxidation/intercalation and intercalation/deintercalation processes were controlled to obtain high-quality expandable graphite materials, porous graphite materials, flexible graphite bipolar plate materials, lithium ion battery cathode materials, electromagnetic wave absorbing materials and so on.
Graphite is a typical layered structure, which is composed of carbon atoms superimposed in a hexagonal network structure. On the network plane, carbon atoms are strongly combined with metal large π bonds through valence bonds, and the atomic spacing is only 0. 142nm, while carbon atoms are weakly combined through van der Waals force, and the interlayer spacing is 0.335nm. This structure determines that different atoms, molecules and ions can be inserted into graphite layers to form various types. The most widely used GICs is acceptor GICs, that is, the insert accepts electrons from the carbon atom layer. GICs is a non-stoichiometric compound, and the carbon layer and intercalation retain their respective structures, so it can be considered as a nano-scale composite material. Due to the exchange of electrons between layers, GICs has many special physical and chemical properties, such as high conductivity, catalysis and selective adsorption. Therefore, GICs treatment can provide many possibilities for graphite modification. This paper introduces the preparation of high quality expandable graphite and electromagnetic wave absorbing (stealth) materials by controlling oxidation/intercalation process with GICs technology. Porous graphite and anode materials for lithium ion batteries were prepared by controlling the intercalation/deintercalation process of porous graphite.
Second, graphite intercalation composite modification technology
(A) H2O2-H2SO4*** embedding technology: synthesis of low-sulfur expandable graphite.
The formation of acceptor GICs is an oxidation-embedding process. First of all, [O] (and other oxidizing substances) react with π electrons in the graphite layer to oxidize, which increases the interlayer spacing and makes the intercalation agent enter the graphite layer to realize intercalation. Oxidation process is the control link of receptor GICs formation. When the intercalation agent itself is not oxidized enough, the intercalation reaction is very slow, or even impossible. At this time, in order to ensure the formation of GICs, it is necessary to rely on external chemical oxidant or electrochemical anodic oxidation to realize intercalation reaction.
At present, the most widely used GICs material in industry is expandable graphite, which is the main raw material for preparing flexible graphite and porous graphite. Expandable graphite is a product obtained by gasifying the intercalating agent of GICs at high temperature and rapid heating, which causes huge internal pressure in the graphite GICs, which makes the graphite particle layer expand and expand several tens to hundreds of times in the direction of C axis. Most GICs are expandable, but considering comprehensively, H2SO4-GICs intercalated sulfuric acid is the most economical as expandable graphite, so it is also called acidified graphite in engineering.
One of the important quality indexes of sulfuric acid intercalated expandable graphite is its residual sulfur content, which is a harmful element and will affect the quality of subsequent products such as flexible graphite. The residual sulfur content is determined by sulfuric acid oxidation-intercalation process and insertion amount. After the common expandable graphite is expanded at 900 ~ 1000℃, the residual sulfur content is1300×10-6 ~ 2000×10-6. The key technology is sulfur reduction. According to GICs theory, firstly, the intercalation of oxidant is used to reduce the insertion of H2SO4, and then a method is designed to reduce the volatilization, that is, the residual intercalated H2SO4, so as to reduce sulfur. In fact, the oxidant itself is also an intercalation agent, which has the same intercalation relationship with H2SO4. The stronger the oxidation, the stronger the intercalation process. The strength of oxidant can be judged by the standard electrode potential of oxidant, as shown in table 1.
Table 1 standard electrode potentials of different oxidants
It can be seen from the table 1 that pure hydrogen peroxide H2O2 is a strong oxidant. The intercalation system of H2O2-H2SO4 can also avoid the secondary pollution of graphite and environment caused by nitrogen oxides and metal ion residues in other oxidant systems.
Therefore, it is necessary to increase the oxidation strength and H2O2 addition. However, the strong exothermic effect of the mixture of H2O2 and H2SO4 partially decomposes H2O2, and it is difficult to reach the addition amount of more than 10%.
Figure 1 shows the relationship between volatile matter (mainly residual H2SO4) and expansion rate and residual sulfur content. The volatilization rate of common expanded graphite is 10% ~ 15%. If it is controlled between 5%- 10%, the residual sulfur can be reduced to below 800× 10-6, and the expansion volume is greater than160 ml/g. The key to reduce the volatile matter is H2O2***, which can reduce the dosage of H2SO4.
Figure 1 Schematic diagram of the relationship between volatile matter of expanded graphite and expansion volume (1) and residual sulfur content (2)
Based on GICs theory, this study designed a temperature-controlled mixing method and device to increase the oxidation intensity by using the relationship between oxidation and embedding, and added excessive hydrogen peroxide into the H2O2-H2SO4 system to prevent the decomposition of H2SO4. The excessive H2O2 and H2SO4 were successfully mixed evenly, so that H2O2 and H2SO4 were * * * embedded, thus preparing qualified expandable graphite with low sulfur and high quality. This technology has not been reported at home and abroad.
In the process of preparing high-quality expandable graphite, electrochemical anodic oxidation was also invented to control the oxidation/insertion process. Electrochemical method is to put graphite on the anode side of electrochemical reaction chamber without oxidant, and promote the embedding reaction of H2SO4 through anodic oxidation. Its advantage is that it reacts when power is on and stops when power is off. The oxidation/intercalation process can be controlled by on-off and reaction voltage, current and current, so as to control the intercalation amount and obtain high-quality expandable graphite. Moreover, electrochemical anodic oxidation can also use intercalation agents such as organic acids that cannot be intercalated by chemical methods to prepare ultra-low sulfur and sulfur-free expandable graphite for nuclear energy. Electrochemical anodic oxidation has been reported abroad, but it has not been industrialized because of the uneven electrochemical reaction. The invention of this study solved the key technology, designed and manufactured an electrochemical reactor with uniform electric field, and realized industrial production (this technology won the third prize of 1993 national invention).
(2) GICs oxidation-intercalation process control technology: synthesis of graphite-based electromagnetic wave absorbing materials.
In this study, mclx-GICs and composite expanded graphite absorbing (stealth) materials were also developed by using the technology of controlling the intercalation process of graphite interlayer oxidation. According to the test results, the mass extinction coefficient of GICs for infrared waves is 4 to 40 times that of ordinary smoke suppressants. The radar wave attenuation of the prepared composite expanded graphite is much greater than that of the conventional jammer. Using the oxidation/intercalation process control technology of this project, the GICs with the best extinction performance was screened, and the preparation of chloride GICs with different grades and mixed chloride GICs with different proportions was realized. The preparation of composite expanded graphite is based on the intercalation/delamination process control technology of the following projects. According to the basic equation, the explosion temperature and time parameters of gunpowder are introduced for dynamic calculation, and the expansion effect is selected with reference to the expandable graphite coupled radar wave. Using these technologies, a broadband photoelectric jamming bomb with excellent shielding effect against infrared, laser and radar waves is designed and manufactured. In the dynamic test of live ammunition launch, the shielding effect of infrared, laser and radar waves is remarkable, and the shielding effect of infrared and laser completely exceeds 15 min (Figure 4). This technology has obtained the invention patent "A preparation method of graphite-based composite material for electromagnetic shielding" (patent number CN02 124 1392).
Fig. 2 attenuation curves of 8 mm radar waves by conventional jammers (a) and (b) and composite expanded graphite (c) and (d)
Fig. 3 Dynamic test curve of photoelectric jamming bomb
(GICs intercalation-delamination process control technology: preparation of expanded graphite and its oil absorption characteristics.
The application of GICs in treating modified graphite: firstly, the GICs obtained is directly used to prepare the above infrared extinction material MCl2-GICs micropowder; The other is pure graphite obtained after GICs deintercalation, that is, graphite is modified by GICs treatment as an intermediate process, and flexible graphite, porous graphite and deintercalated GICs graphite lithium ion battery cathode materials are made by controlling the intercalation-deintercalation process.
The deintercalation of GICs means that the intercalated foreign substances escape from the plane layer of carbon atoms. Usually, in vacuum and atmospheric environment, the intercalation comes out as a gas. Theoretically, the thermodynamic parameters of GICs deintercalation are calculated by first principles and Real program, and the basic equation of equilibrium is calculated by the minimum free energy method consisting of the system free energy equation and the system mass conservation equation. Then, the phase transition and thermal decomposition parameters of GICs in the process of deintercalation are introduced, and the kinetic parameters of deintercalation reaction are calculated by Kissinger-Ozawa calculation method. Fig. 4 is the gas volume generated by some GICs deintercalation obtained from the basic equation. The theoretical calculation is basically consistent with the de-insertion (expansion) experiment.
Gas volume produced by several GICs deintercalation reactions.
According to theoretical analysis and experimental results, graphite materials with different uses were obtained. The control of the insertion-deintercalation process of GICs treatment modification is mainly to control the type, insertion amount, deintercalation temperature and heating rate of the insert. For porous expanded graphite used for liquid phase adsorption and manufacturing flexible graphite, the plug-in with large amount of desorption reaction gas should be selected, and rapid desorption at high temperature should be adopted. For graphite used as the negative electrode of lithium ion battery, an insert containing a small amount of reaction gas should be selected and slowly removed at low temperature.
Fig. 5 Oil absorption capacity of porous graphite
Porous graphite, because of its hydrophobic and oleophilic properties and porous structure, has great adsorption capacity for oils and macromolecular organic compounds, and the adsorption capacity of dispersed porous graphite for heavy oil in water is greater than 80 g/g, which is beyond the reach of other oil-absorbing materials (Figure 5). In the engineering application experiments of oil removal in cooling water pool of Baotou Steel Strip Steel Plant and COD removal in printing and dyeing wastewater of Qinghe Wool Mill, the porous graphite low-density poly-plate prepared by intercalation/delamination control technology in this study has better decontamination effect than activated carbon. Porous graphite has a good development prospect as a water pollution control material. This technology has applied for the invention patent "Preparation of an oil-contaminated adsorbent and its recovery and regeneration method" (application number 2004 1007978.6438+0). At the same time, porous graphite micropowder, as a new conductive additive for the positive electrode of high-energy alkaline battery, has replaced Japanese imported products and has been industrialized at present.
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Study on modification technology of graphite intercalation compound and its application
Kang Feiyu, Zou Lin, Shen Wanci, Zheng Yongping, Gai Guo Sheng, Ren Hui, Gu Jialin
(Laboratory of New Carbon Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084)
Abstract: Because of the weak binding force between carbon layers and van der Waals interaction, graphite with layered structure is easy to form graphite intercalation compounds through intercalation reaction. By controlling the oxidation intercalation process, high-quality expanded graphite with low residual sulfur content and MClx-GICs(M =Fe, Co, Ni, Cu, Zn) powder for electromagnetic wave absorption and shielding materials were invented. The expansion volume of expandable graphite can be more than 65438±060ml/g/g, and the residual sulfur content is less than 800ppm. MClx-GICs powder can completely shield infrared rays and lasers for 65438+ 05 minutes. We also invented high-temperature expanded graphite for heavy oil adsorption and micro-expanded expanded graphite for anode materials of lithium ion batteries by controlling the intercalation/deintercalation process. The adsorption capacity of expanded graphite for heavy oil can reach 80 g/g, and it also shows better performance than commercial activated carbon in sewage treatment. Low-temperature micro-expansion expanded graphite as anode material has high reversible capacity of 370 mAh/g and good cycle performance.
Key words: graphite intercalation compound, expanded graphite, process control.