(Laboratory of New Carbon Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084)
China is rich in natural graphite resources, and it is one of the effective ways to upgrade China's graphite industry to modify natural graphite and apply it to high-energy lithium-ion batteries. High purity microcrystalline graphite is molded and coated with carbon film. The first cycle efficiency increased to 89.9%, and the cycle stability was obviously improved. The results show that microcrystalline graphite coated on the surface is an excellent composite anode material for lithium ion secondary batteries. Using H2SO4-GIC graphite intercalation composite technology to pre-expand flake graphite, sub-micron-nano voids are formed in graphite particles, which improves the discharge capacity, fast charge-discharge ability and cycle life of graphite products, and is especially suitable for the development requirements of high-energy lithium-ion batteries [1~1].
Natural graphite; Surface coating; Pre-expansion; Negative electrode material; Lithium ion battery.
Introduction to the first author: Shen Wanci, a professor in the Department of Materials Science and Engineering of Tsinghua University, has been engaged in the research and development of graphite and new carbon materials for a long time. E-mail :shenwc@mail.tsinghua.edu.cn.
I. Introduction
China graphite products can be divided into flake graphite and microcrystalline graphite. Flake graphite refers to graphite with grain size greater than 1μm and developed lamellar structure, but with low grade of raw ore and carbon content generally below 10%. Microcrystalline graphite, also known as amorphous graphite, aphanitic graphite and earthy graphite, has a crystal mass of less than 65,438+0 microns, which is characterized by small grains condensed into polycrystals. The raw ore has a high grade and the carbon content is generally above 50%. The carbon content of Lutang Mine in Chenzhou is above 80%.
Microcrystalline graphite, as the anode material of lithium ion battery, has high lithium intercalation capacity and cycle stability, and is rich in resources and low in price. Modification of natural microcrystalline graphite for high-energy lithium-ion batteries is one of the effective ways to promote China's graphite industry. Similarly, flake graphite can also be used as the negative electrode material of lithium ion battery, but the expansion and contraction of graphite in the process of electricity storage must be solved, otherwise it will directly affect the service life of the battery.
Second, the molding of microcrystalline graphite
Microcrystalline graphite particles are composed of many grains with disordered orientation, which are easily crushed during the spheroidization of microcrystalline graphite, and most of the particles are crushed into fine particles below 10 μ m, which are unfavorable to the negative electrode performance of graphite. Natural graphite for lithium ion batteries requires small specific surface area, high tap density and uniform particles to improve its negative electrode performance, which requires narrow particle size distribution, smooth surface and high sphericity. Natural graphite must be deeply processed by powder to meet the requirements of lithium ion batteries. However, ordinary mechanical crushing is difficult to meet these requirements. In this paper, microcrystalline graphite purified by chemical method (with purity of C≥99.5%) was used as raw material, and the forming effect of microcrystalline graphite in stirring mill system was studied. Table 1 shows the carbon content and particle size of microcrystalline graphite used in this study.
Table 1 microcrystalline graphite used in the test
Stirring mill is a SX-8 small-scale stirring ball mill produced by Wuxi Xinda Powder Machinery Co., Ltd. The volume of the mixing barrel is 8L, and the standard handling capacity is 3L.
Plastic processing of (1) natural microcrystalline graphite
It is formed by wet stirring and grinding: spherical zirconia grinding ball with a diameter of 3 mm; The slurry concentration is 20%; The ratio of ball to material is 20∶ 1 (mass ratio); The filling rate is1/2; Ammonium polyacrylate (or sodium hexametaphosphate) was added as grinding aid, and the proportion was 0.3% (relative to the mass of graphite). Different technical parameters were used in the experiment, as shown in Table 2.
Table 2 Experimental conditions and parameters of spheroidizing treatment of natural microcrystalline graphite
Table 3 Specific surface area and particle size of microcrystalline graphite before and after molding
(2) Plastic experimental results
As can be seen from Table 3, the specific surface area of microcrystalline graphite has decreased after grinding, because the shape of microcrystalline graphite particles after stirring and grinding is closer to spherical shape, and the specific surface area of spherical particles is smaller under the same conditions. At the same time, the particle size of graphite particles decreased after stirring mill molding, which indicated that stirring mill had a certain crushing effect in the molding process.
(III) Electrochemical performance
The prepared graphite was evenly mixed with polyvinylidene fluoride (PVDF) (10% by mass), and then it was dissolved with dimethyl pyrrolidone (NMP) to make paste and coated on copper foil. After drying and rolling, the film with a thickness of about 65438 000 μ m was obtained. A membrane with a diameter of 65438±02mm was used as the experimental electrode. After vacuum drying at 65438 050℃ for 24 h, the electrode diaphragm was assembled into an experimental button cell (model 2025) in an argon glove box. The electrolyte is1mol/l-lipf6/EC-dec (1:1) (Merck), and the diaphragm is Celgard#2500. The electrochemical performance of lithium sheet was tested by constant current charge and discharge method. The discharge speed ranges from 0. 1C to 1C, and the discharge cut-off voltage is 0V and the charging cut-off voltage is 3V. The battery test system is blue electricity CT 2001a.
After stirring and grinding, the first lithium intercalation capacity and reversible capacity of microcrystalline graphite increased from 370 Ma h/g and 284 Ma h/g to 386 Ma h/g and 308 Ma h/g, respectively, and the first efficiency increased to 78.2%. It can be seen that the reversible capacity of microcrystalline graphite is not high, which is slightly lower than the average of 320 Ma h/g of flake graphite. However, microcrystalline graphite has anisotropic structural characteristics and shows good cycle performance during repeated charge and discharge, so microcrystalline graphite will have more advantages as a lithium ion secondary battery, and the key is to improve the first cycle efficiency.
Thirdly, the surface coating of microcrystalline graphite.
From the mechanism point of view, surface modification is mainly to reduce the active sites on the graphite surface, reduce the coulomb consumption of SEI formation, optimize the performance of SEI film, and thus reduce the irreversible capacity loss. At the same time, a carbon film is formed on the graphite surface in advance, which is beneficial to prevent the decomposition of electrolyte on the graphite surface and improve the stability of the graphite cathode. However, the density of surface carbon film directly affects the modification effect. The dense and uniform carbon film can effectively block the insertion of solvated ions, and at the same time, some nano-scale holes can be generated during carbonization, which provides more channels for the insertion of lithium ions.
Surface coating technology of (1) microcrystalline graphite
The preparation process of coated graphite adopts impregnation method, that is, spherical flake graphite and phenolic resin are evenly mixed according to a certain proportion, and ethanol solvent is added to adjust the viscosity to obtain slurry meeting the requirements of dispersion process. After stirring, filtering, drying and other processes, the surface of graphite particles is coated with a layer of phenolic resin, which is still a dispersed ellipsoid or spherical particle after coating. After high temperature carbonization, flake graphite coated with resin carbon was prepared.
Phenolic resin for coating is liquid linear phenolic resin, model 9 17 (Beijing Furunda Resin Factory), with solid content of 62.4%. Thermogravimetric analysis (TGA STA 409C) was performed after the ethanol solvent was removed. The experiment shows that at 1000℃, the weight loss rate of the resin is 6 1%, and the pyrolysis carbon content is 39%. Graphite for coating is a natural microcrystalline graphite which is formed by stirring and grinding and spheroidized by PCS system.
Table 4 Comparison of Cyclic Properties of Microcrystalline Graphite with Different Coating Amount
Figure 1 Cyclic capacity curve of microcrystalline graphite with different coating contents
(2) Experimental results and discussion of surface coating.
Table 4 lists the comparison of cycle performance under different coating amounts. It can be seen that the first cycle efficiency and cycle stability of microcrystalline graphite are improved after it is coated with resin and carbonized at 1000℃.
It can be seen from Figure 1 that surface coating is an effective method to modify the electrochemical performance of microcrystalline graphite, which can not only improve the initial efficiency, but also show better cycle performance, indicating that surface coating microcrystalline graphite is a good composite negative electrode material for lithium ion secondary batteries.
Fig. 2 Cycle performance after cycle treatment
4. Flake graphite is used as anode material of lithium ion battery.
When the project team studied the use of natural flake graphite as anode material, it was found that the charge and discharge capacity of natural graphite was higher than that of artificial mesophase carbon microspheres (MCMB). The capacity of MCMB is about 300 mAh, while the capacity of flake graphite is about 340 mAh ... But considering the cycle performance, flake graphite cathode is poor, and the capacity loss is great after repeated charging and discharging. The main reason is that graphite crystal expands and contracts about 10% during charging and discharging, and the multiple expansion and contraction of flake graphite concentrated in one direction damages the negative electrode film, resulting in the decline of performance. In order to solve this problem, the principle of graphite intercalation compound (GICs) is put forward, which forms micro-nano voids in graphite particles and prefabricates lattice expansion and contraction spaces, thus improving cycle performance. The key of this technology lies in the slow and orderly deintercalation, so that the escape of intercalation gas only causes micro-nano holes in graphite, and does not cause obvious volume expansion. Usually, H2SO4-GIC, MClx-GICs or other receptor GICs are used to deintercalate 100 ~ 300℃ at low temperature, and then the graphite is treated. The anode material prepared in this way not only has high flake graphite capacity, but also has good cycle performance (Figure 2). At present, the product performance has been tested on the battery.
Summary and prospect of verb (abbreviation of verb)
China's lithium-ion battery industry will still maintain an average annual growth rate of over 30%. In 2005, the annual output of domestic small lithium-ion batteries exceeded 654.38+0 billion, and the annual demand for graphite anode materials was 5000 ~ 654.38+0 million t, and the world demand was about 2× 654.38+0.04 t, but the current supply gap is very large. With the rapid development of electric vehicles, the demand for anode materials for lithium batteries will be more vigorous.
In view of the abundant natural graphite resources, low price and high lithium intercalation capacity, it is one of the effective ways to improve the domestic graphite industry by modifying natural microcrystalline graphite for high-energy lithium-ion batteries. Considering the cost and performance comprehensively, natural graphite has the most development potential in lithium ion battery anode materials, but there are also some problems to be solved urgently, such as irreversible capacity loss in the first cycle and cycle stability. The continuous development of natural graphite modification technology includes spheroidizing treatment, surface resin coating, intercalation/delamination micro-expansion treatment and so on. Graphite products's discharge capacity, fast charge-discharge ability and cycle life are improved, and modified natural graphite will be the first choice for high-energy lithium-ion batteries.
Reference materials and materials
[1] He Ming, Gai Guo Sheng, Shen Wanci, et al. Effect of ball milling process on the structure and properties of natural microcrystalline graphite lithium-ion anode materials. Battery, 2002,32 (4):197-200.
He Ming, Chen Xiangbiao, Kang Feiyu, et al. Preparation and electrochemical properties of resin carbon coated microcrystalline graphite. Battery, 2003,33 (5): 281-284.
Chen Xiangbiao, Liu Xuan, Shen Wanci. Study on lithium intercalation behavior of coated flake graphite. Battery, 2004,34 (6): 394-396
Zhang Jing, Zheng Yongping, Shen Wanci, et al. Study on natural graphite modified by GICs technology as anode material of lithium ion battery. Battery, 2006, 36 (4): 257-259
Shen Wanci, et al. A graphite anode film product for lithium ion battery, and its preparation method and application. Patent number: ZL 97 1 2 1933.8.
Shen Wanci, et al. Preparation method of carbon-coated graphite micropowder. Patent number: ZL 02 1257 15.9.
[7]Andersson A M, Abraham D P, Haasch R, et al. Surface characteristics of high-power lithium ion battery electrodes. Acta Electrochemistry, 2002,149 (10): A1358-1369.
[8] the latest development of broussely m. saft lithium-ion battery. J. Power supply, 1999, 81/82:140-143.
Zhang Wanhong, Min Yue. Research status and development direction of lithium ion power battery and its anode materials. New materials industry, 2006, 9: 54-59.
Zhang Shichao. Technical status and development trend of key materials industry for lithium ion batteries. New materials industry, 2006, 3: 32-36.
[1 1] Dong Jian, Zhou Wei, Liu Xuan, et al. Effect of microcrystalline graphite as anode material on electrochemical performance of secondary lithium-ion battery. Carbon Science and Technology, 1999, (1): 1-6.
Study on natural graphite as anode material of lithium ion battery
Shen Wanci, Li Xinlu, Lin Zou, Kang Feiyu, Zheng Yongping
(Laboratory of New Carbon Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084)
Abstract: China is rich in natural graphite resources. The application of modified natural graphite in lithium-ion batteries will be an effective way to promote China's graphite industry. In the study, high-purity microcrystalline graphite was spheroidized and its surface was covered with a carbon film. The first cycle efficiency was improved to 89.9%, and the cycle stability was significantly improved. Experiments show that carbon-coated microcrystalline graphite is an excellent cathode material for lithium ion batteries. In addition, flake graphite powder with slight peeling was prepared by sulfuric acid -GIC technology. The results show that submicron and nano pores are formed in graphite samples, which improves reversible capacity, rate capacity and cycle life. The product meets the requirements of lithium ion batteries well.
Key words: natural graphite, surface coating, mild peeling, negative electrode material, lithium ion battery.