Graphite materials are one of the most studied carbon anode materials for lithium-ion batteries, and they are also the most important anode materials currently used in commercial production of lithium-ion batteries. Artificial graphite is a kind of carbon that is easily graphitized (such as pitch and coke). It is made by high temperature graphitization treatment at 1900~2800℃ in a protective atmosphere, such as N2. Common artificial graphite includes graphitized mesophase carbon microbeads.
①Basic concept of mesophase
Generally, a substance exhibits optical anisotropy when it exists as a crystal, and it exhibits an optical isotropy when it exists as a liquid; when the temperature is higher than the melting point, the substance changes from a solid to a liquid, and when the temperature is lower than the crystallization point, the substance changes from a liquid to a solid. This is not true for a class of substances. They transform from an optically anisotropic crystal to an optically isotropic liquid (or inverse process) in the intermediate stage, which will present an optically anisotropic turbid liquid state. From the perspective of phase science, this turbid liquid is certainly not a solid phase, but it has optical anisotropy, so it cannot be regarded as a liquid phase, so it is called a mesophase; from a crystallographic point of view, it is a liquid It also has optical anisotropy and is also called liquid crystal.
②Classification of mesophase
There are many types of mesophase compounds. According to the different formation structures, they can be divided into three types: smectic type—close to crystals and have a certain crystal lattice; nematic type—the compound molecules inside the crystal grains are aligned, but the molecules of the compound It is not single, and the center of gravity is disordered in structure; cholesteric-composed of cholesteric compounds. The mesophases obtained by liquid phase carbonization of asphalt and heavy oil are nematic, and their molecular structure cannot be described by a single model, but they have some common features.
③Molecular structure characteristics of mesophase
The molecule itself has an anisotropic structure, that is, the shape of the molecule is rod-shaped or flat; the molecule contains more than two aromatic rings, and the electrons in the molecule can flow in a larger range.
④Formation of mesophase
At normal temperature, the molecules of the liquid crystal compound are combined and aligned by van der Waals force, showing the optical anisotropy of the crystal. At higher temperatures (liquid phase temperature), the kinetic energy of molecular motion is greater than the binding energy of intermolecular forces, and the molecules are randomly oriented, showing the optical isotropy of the liquid. Within a certain temperature range (intermediate phase temperature), the kinetic energy of molecular motion and the binding energy of intermolecular force corresponding to this temperature are not much different. At this time, the intermolecular force can no longer maintain the orderly arrangement of the molecules, but it can also make several The molecules are aligned in a sub-assembly, so the whole system is in a liquid state and has optical anisotropy.
The mesophase carbon microspheres are prepared through a liquid phase carbonization process. The reaction temperature of the liquid phase carbonization reaction is usually below 500-550°C, and the reactant system is in a liquid state. The liquid phase carbonization process, from a chemical point of view, is the continuous thermal decomposition and thermal polycondensation reaction in the liquid phase reaction system (the gas phase carbonization process is led by free radical reactions at high temperatures), which is accompanied by hydrogen transfer; From a scientific point of view, the isotropic liquid phase in the reactant system gradually becomes anisotropic mesophase spheres, and as the degree of anisotropy of the mesophase gradually increases, the mesophase spheres are formed, merged, and grow. Great disintegration and formation of carbon structure. The process of preparing mesophase carbon microspheres is to control the liquid phase carbonization reaction of the reaction system, so that the number and size of the mesophase microspheres produced meet the requirements or reach the optimization process.
According to the theory of liquid phase carbonization, the difficulty of liquid phase carbonization of various hydrocarbons is, in order from difficult to easy, alkanes, alkenes, aromatics and polycyclic aromatic hydrocarbons. Therefore, the raw materials for preparing mesophase carbon microspheres are mostly hydrocarbons containing heavy components of polycyclic aromatic hydrocarbons. The raw materials for liquid phase carbonization include coal-based pitch, heavy oil, and petroleum-based heavy oil. As the raw materials for preparing mesophase carbon microbeads, most of them are also selected from these raw materials, such as medium temperature coal pitch, coal tar, catalytic cracking residue or a combination thereof. The different components of the raw materials (such as the content of pyridine insolubles PI and the content of quinoline insolubles QI), additional substances (such as carbon black, coke powder, graphite powder, organometallic compounds and mesophase carbon microspheres, etc.) and substances at the reaction temperature System viscosity has varying degrees of influence on the formation, growth, fusion and structure of small mesospheres.
The addition of other substances in the raw materials has a significant effect on the preparation of mesophase carbon microspheres. The addition of carbon black can promote the formation of the mesophase carbon microbead core and prevent the fusion between microspheres. At the same time, the mesocarbon microspheres obtained by increasing the amount of carbon black show a trend of decreasing diameter, increasing number, and uniform distribution, and at the same time can increase the yield of mesocarbon microspheres. In addition, by controlling the addition amount and optimizing the thermal reaction conditions, the morphology and quantity of the mesophase carbon microspheres can be controlled. The addition of organometallic compounds such as ferrocene and carbonyl iron to the raw materials can also effectively promote the uniform formation of small spheres and prevent their fusion. However, carbon black and organometallic compounds are difficult to graphitize substances, their addition is bound to introduce impurities, and then affect the performance of mesophase carbon microsphere products.
① Preparation of mesophase carbon microspheres
The methods for preparing mesophase carbon microspheres mainly include thermal polycondensation and emulsification, and there are other methods such as amphiphilic carbon.
The preparation of mesophase carbon microspheres by thermal polycondensation involves two steps: heat-treating the fused-ring aromatic compound to polymerize and produce mesophase small spheres (the spheres are rich in the mother liquor of the polycondensation product), and use appropriate methods to reduce the mesophase. The spheres are separated from the mother liquor.
The process of preparing mesophase carbon microspheres by thermal polycondensation method is roughly as follows: put the reaction materials into a reactor of a certain capacity, seal to isolate the air, and then increase to a certain temperature at a certain heating rate under the protection of pure N2 (usually at a certain temperature). 350~450℃), keep it at this temperature for a certain period of time, and then naturally cool to room temperature. In addition, it can also be kept for a period of time under the protection of pure N2 flow at a low temperature (such as 100~300℃), and then perform self-boosting polymerization in a closed state. Stirring is continued during the reaction. After the constant temperature is over, the product (small spheres rich in mesophase) is cooled to room temperature. The technological process is shown as in Figure 2.
Figure 2 The process flow of preparing mesophase carbon microspheres by polymerization
To prepare mesophase carbon microspheres by the emulsification method, first heat the fused ring aromatic compound to obtain the spherical mesophase, and then emulsify the mesophase into mesophase small spheres. After the mesophase small spheres obtained by the thermal polycondensation method or the emulsification method are carbonized and graphitized, the mesophase carbon microsphere material products with special properties can be obtained.
Emulsification process: crush the solid mesophase pitch with a softening point of about 300℃ through a sieve (200 mesh or 325 mesh Taylor sieve) and dissolve it in a certain amount of thermally stable medium (such as silicone oil), and use ultrasonic waves under N2 purge Stir and disperse, heat while stirring (the temperature is 300~400℃). Emulsify to form a suspension. Then it is cooled to room temperature, the mesophase carbon microspheres are separated from the thermally stable medium with a centrifugal separator, and washed with benzene, and the mesophase carbon microspheres are obtained after drying.
②Structure and properties of mesophase carbon microspheres
The H/C atomic ratio of the mesophase carbon microspheres is 0.35~0.5, and the density is 1.4~1.6g/cm3. It is formed by the accumulation of polycyclic condensed aromatic hydrocarbon plane molecules. The structure of mesophase carbon microspheres can be represented by a model similar to a globe (Figure 3). Planar molecules are arranged in a large plane in the sphere. The macromolecular level on the equatorial plane is a plane. The other planes located in the upper and lower hemispheres of the equator are still arranged parallel to each other, but when the planes are close to the surface of the sphere, the planes are curved and curved. It is perpendicular to the surface, which is the result of the mutual stability of the intermolecular force at the macromolecular level and the surface tension of the sphere. Due to the difference of raw materials and reaction conditions, the mesophase spheres show many variations, such as heart spherical shell shape, oblate shape and so on.
Figure 3 SEM images showing an overview of the carbonized mesophase sample (a), a single mesophase sphere (b) and the cross section of mesophase sphere (c), with Brooks–Taylor model inserted (d)
The molecular structure of the "spider web model" mesophase is composed of aromatic rings with a size of 0.6 to 1.3 nm, which are macromolecules connected by biaryl groups or methylene groups, with a relative molecular mass of 400 to 4000. The spherulites have large molecules as the nucleus, and low molecular weight molecules are also condensed by the van der Waals force of the π-π bond, so they also show anisotropy, and the spherulites are slightly thermoplastic due to the presence of low molecules.
MCMB prepared by solvent separation is generally insoluble in quinoline solvents, and MCMB does not melt during heat treatment and maintains its spherical shape. With the increase of heat treatment temperature, the hydrogen content of MCMB decreases. During the period of 500~1000℃, the density of MCMB gradually increases from 1.5g/cm3 to 1.8g/cm3, and the specific surface area has a maximum value at 700℃. MCMB can be graphite The degree of chemical conversion is not as high as that of petroleum coke. The reason may be that the MCMB graphite crystallites are restricted by the shape of the microspheres. Generally speaking, the mesophase spheres are the primary stage of mesophase growth, and the aromatic lamellae are ordered in the liquid phase carbonization stage. The degree of arrangement is lower than that of the precursor fused mesophase pitch such as petroleum coke. MCMB and its heat-treated products are hydrophobic, but due to the very high reactivity of carbon atoms on the periphery of MCMB, they have high activity for various surface modifications.
Figure 4Schematic model of the sintering mechanism of carbon blocks from MCMBs during carbonization
The physical and chemical properties of MCMB vary greatly with the heat treatment temperature. MCMB treated at low temperature is an amorphous soft carbon material. There are many nano-scale micropores in its structure. These micropores can store Lithium makes MCMB have an ultra-high specific capacity. The intercalation capacity of lithium during pyrolysis and carbonization under 700℃ can reach more than 600mA·h/g, but the irreversible capacity is high; with the increase of temperature, these microporous The number of pores is reduced, and the number of micropores is also decreasing, and the amount of lithium storage decreases, but at the same time the degree of graphitization increases. At this time, the microporous lithium storage is the main factor; when the temperature further increases, the number of micropores basically remains stable and graphitization Degree is the main factor that affects MCMB. The increase in temperature increases the degree of graphitization, which also has a higher lithium insertion capacity. The main discussion here is graphitized materials, and MCMB is graphitized at high temperature. When the temperature is above 1500℃, the change of the structural parameters of MCMB with temperature obviously reflects the increase of graphitization degree. When the temperature increases, the lengths La and Lc of the graphite crystallites in the a-axis and c-axis directions continue to increase, and the interplanar spacing d002 decreases with the increase in temperature.
The negative electrode materials of industrialized lithium-ion batteries are all carbon materials, including natural graphite, MCMB, coke, etc. Among these materials, MCMB is considered to be the most promising carbon material, not only because of its specific capacity It can reach 300mA·h/g-1. The more important reason is that, compared with other carbon materials, MCMB has a diameter of 5-40μm, a spherical lamellar structure and a smooth surface, which gives it the following unique advantages: the spherical structure is conducive to close packing, which can produce high density The electrode; MCMB's smooth surface and low specific surface area can reduce the occurrence of side reactions on the electrode surface during the charging process, thereby reducing the Coulomb loss during the first charging process; the spherical lamellar structure allows lithium ions to be in all directions of the ball Insertion and discharge solves the problems of graphite sheet swelling, collapse and incapability of rapid high-current charging and discharging caused by excessively high anisotropy of graphite materials.
③The electrochemical performance of graphitized mesophase carbon microspheres
Graphitized mesophase carbon microspheres refer to carbon materials obtained by graphitizing MCMB through high temperature (above 2000°C) treatment. The main difference between different graphitized MCMBs is the size and number of graphite crystallites; there is also a certain number of micropores in the graphitized MCMB that are basically unchanged, but it has little effect on the capacity. The lithium insertion mechanism of graphitized MCMB is the same as that of natural graphite, lithium is inserted between graphite layers to form GIC, so the degree of graphitization has a great influence on its performance. Generally, the higher the temperature, the greater the degree of graphitization (the d002 peak is also stronger, as shown in Figure 5), and the higher the capacity of MCMB. Figure 6 shows the charge and discharge curves of MCMB under different temperature treatments.
Figure 5 The XRD pattern of MCMB under the same degree
Figure 6 MCMB charging and discharging curves at different processing temperatures
The particle size of MCMB also has a great influence on the first discharge capacity, cycle performance, and high current discharge characteristics of the material. The smaller the average particle size, the shorter the distance between the insertion and extraction of lithium ions in the microspheres. Under the same time and diffusion rate, the extraction of lithium ions is easier, so the specific capacity is larger; but the smaller the particle size, the more The larger the surface area, the larger the area of the SEI membrane formed, which results in greater irreversible capacity loss, thereby reducing the Coulomb efficiency. During the charge and discharge cycle of the electrode, lithium ions are continuously inserted and extracted in MCMB to cause volume changes. MCMB with a larger particle size changes greatly during the charge and discharge process, which leads to damage to the structure of part of the MCMB,cannot participate in the electrode reaction, and the cycle performance deteriorates.