LR41 battery.Complete battery negative electrode technical information

　　Complete battery negative electrode technical information

　　one. Graphitized carbon anode material There are many types of graphitized carbon materials depending on the raw materials. Typical ones are graphitized mesophase microbeads, natural graphite and graphitized carbon fiber. In general, it has the following characteristics: 1) The insertion of lithium is positioned below 0.25V (relative to Li+/Li potential) 2) Formation of first-order compounds 3) The maximum reversible capacity is 372mAh/g, which corresponds to LiC6 first-order compound 1 . Natural graphite Graphite crystal has a layered structure of hexagonal carbon network. The carbon atoms in the same carbon layer are arranged in an equilateral hexagonal shape, and the layers are bonded by intermolecular forces, that is, van der Waals forces. X-ray diffraction shows that the interlayer spacing is d002 Generally around 0.335nm, the solid density is 2.2g/cm3 or higher. There are two types of natural graphite: flake graphite and earthy graphite (microcrystalline graphite). The former contains more than 99% carbon after beneficiation and purification, while the latter contains more impurities and is difficult to purify. Therefore, flake graphite is often used as the raw material for negative electrodes in the LIB industry. Flake graphite has a very good layered lithium storage structure. The theoretical lithium insertion capacity can reach 372mAh/g and is cheap and easy to obtain. When used as a LIB negative electrode material, it has outstanding characteristics such as low discharge potential (0.1VvsLi/Li+) and smooth discharge potential curve. advantage. The disadvantage is that due to the weak intermolecular force between graphite layers, that is, van der Waals force, charging the cSEI film not only consumes a large amount of lithium ions and increases the first irreversible capacity loss, but also due to the insertion and extraction of solvated lithium ions. It will cause the volume expansion and contraction of the graphite particles, causing the electrical network between the particles to be partially interrupted, so the cycle life is very poor. Modification of flake graphite (natural graphite coating) can greatly improve its reversible capacity and cycle life. Kuribayashi et al. used phenolic resin to form low-temperature pyrolytic carbon-coated graphite with graphite as the core and phenolic resin pyrolytic carbon as the coating layer. The coating layer improves the interfacial properties of graphite materials to a great extent. Graphite coated with low-temperature pyrolytic carbon not only has a low-potential charge and discharge platform, but also uses low-temperature pyrolytic carbon with good compatibility with the electrolyte to prevent the co-intercalation of solvent molecules and lithium ions, preventing the core graphite material from intercalating lithium. The delamination during the process reduces the irreversible capacity loss during the first charge and discharge process and extends the cycle life of the electrode. Eineli, Menachem and others found that after proper oxidation of graphite, the irreversible capacity decreases and the reversible capacity increases. The reasons for oxidation-modified graphite are as follows: 1) The oxidation generates a small amount of acidic groups such as carboxyl groups on the electrode interface, especially on the irregular interface. During the first charging process, acidic groups are converted into acidic lithium salts and interfacial O-Li groups, which is conducive to the formation of a stable SEI film chemically bonded with the graphite body, thereby preventing the co-intercalation of solvent molecules and lithium ions; 2) Interface generation The active groups are beneficial to increasing the wettability between the electrode/electrolyte; 3) Oxidation causes micropores to be formed on the graphite surface to accommodate more lithium ions. In addition, too many oxidized groups on the surface of the carbon material will lead to an irreversible increase in the electrode's first capacity. The reduction of systems such as Fe powder/HCl, Zn/H2SO4/acetic acid or SnCl1,2/HCl can be used to improve the electrode interface.

　　2. Mesophase carbon microbeads (MCMB) are obtained by processing coal tar pitch to obtain mesophase beads. It is then purified by solvent extraction and other methods, followed by heat treatment to obtain a turbulent layer structure. Among the commercialized carbon materials, graphitized mesophase carbon microbeads are considered to be the most powerful carbon materials. Compared with other carbon materials, MVMB has a diameter between 5-40 μm, a spherical lamellar structure and a smooth surface. The spherical structure is conducive to close packing, so that high-density electrodes can be prepared; MCMB has a smooth surface and low specific energy density. The 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 the lithium force couples to be embedded and detached in all directions of the ball, solving the problem of graphite due to various Anisotropy causes problems such as excessive swelling and collapse of graphite sheets and the inability to charge and discharge with high current. 1) The preparation of mesophase carbon microbeads is guided by the liquid phase carbonization theory. In the process of liquid phase carbonization, from a chemical point of view, thermal decomposition and thermal polycondensation reactions are continuously going on in the liquid phase reactant system. From a physical perspective, the isotropic liquid phase in the reactant system gradually turns into anisotropic mesophase beads, and as the degree of anisotropy of the mesophase gradually increases, the mesophase beads are generated and melted. And, grow up and disintegrate to form a carbon structure. Preparation methods mainly include solvent separation method, emulsification method, centrifugal separation method, supercritical fluid separation and other methods. 2) Physical and chemical properties of mesophase carbon microbeads MCMB prepared through solvent separation has a density of about 1.47g/cm3. Compared with heat-treated mesophase asphalt raw materials, MCMB has relatively low carbon and hydrogen content, and its hydrogen concentration is approximately 60% of that of heat-treated mesophase asphalt raw materials. MCMB is composed of aromatic hydrocarbons with a number average molecular weight of 400-3000 and a weight average molecular weight of about 2500. The composition depends on the type of raw materials and preparation conditions. For mesophase carbon microbeads, d002 decreases as the heat treatment temperature increases. At 2800 degrees, its value is 0.339nm. 3) Electrochemical properties of graphitized mesophase carbon microbeads. Carbon materials with different degrees of graphitization can be produced by heating at different temperatures. When the degree of graphitization is high (average d002 is less than 0.344nm), the reversible capacity begins to increase as the degree of graphitization increases. The reversible capacity changes in the range of 282-325mAh/g depending on the degree of graphitization.

　　3. Pitch-based carbon fiber When pitch-based carbon fiber is used as an anode material, it has a lot to do with pretreatment. Carbon fibers prepared with low viscosity have a high degree of graphitization and large discharge capacity; while carbon fibers prepared with high viscosity have good rapid charge and discharge capabilities, which may be related to Lithium ions are more likely to diffuse in carbon fibers with lower crystallinity; when optimized, the reversible capacity is 315mAh/g, the irreversible capacity is only 10mAh/g, and the first charge and discharge efficiency reaches 97%. For graphitized carbon prepared from coke, although its capacity is lower than that of graphite, its rapid charge and discharge ability is stronger than that of graphite. Compared with graphite, graphitized metastable phase pitch-based carbon fiber has a diffusion coefficient of lithium ions that is one order of magnitude higher, and its charge and discharge behavior under high current is also better than graphite. When graphitized carbon fiber is inserted into lithium, there is first a relatively important process: the formation of a passivation film or electrolyte-electrode interface film. The quality of the interface film has a very obvious impact on its electrochemical performance. Its formation is generally divided into the following three steps: 1) The beginning of film formation above 0.5V; 2) The main film formation process at 0.55-0.5V; 3) The insertion of lithium begins at 0.2-0V. If the membrane is unstable or not dense enough, on the one hand, the electrolyte will continue to decompose, and on the other hand, the solvent will be inserted, leading to the destruction of the carbon structure. The quality of the surface film has a great relationship with the type of carbon material and the composition of the electrolyte.

　　two. Effect of particle size on electrochemical properties of graphite materials 1. Effect of particle size The first irreversible specific capacity loss of graphite electrodes is mainly caused by the formation of SEI film. The smaller the graphite particles, the greater the specific surface area that can be in contact with the electrolyte. The SEI film formed during the first charge and discharge process consumes more charges, and the greater the irreversible capacity loss. The smaller the particles, the greater the problem that needs to be overcome for embedding. The smaller the van der Waals force, the easier it is to embed, and the smaller the particles, the greater the number of channels for lithium ions to embed and detach, which is more conducive to quickly reaching the complete lithium embedding state. Under high-rate scanning conditions, the voltage hysteresis is The smaller it is, the better the large current charging and discharging performance. As the graphite particle size decreases, the irreversible capacity gradually increases; and for the reversible capacity, the reversible capacity also increases with the decrease of the particle size. Therefore, graphite particles that are too large or too small are not conducive to the reversible intercalation and deintercalation of lithium ions. Only appropriate particle sizes can maximize the reversible intercalation and deintercalation of lithium ions. 2. Effect of Particle Size Distribution Generally speaking, the narrower the particle size distribution, the better the performance of the battery. Choosing a reasonable particle size distribution area can not only improve battery charging and discharging efficiency, but also improve battery cycle life.

　　three. Determination of various negative electrode materials: Since there are many types of negative electrode materials, there are not only low-end natural graphite, mid-range artificial graphite, modified natural graphite, but also high-end MCMB (mesophase carbon microspheres). So be especially careful when identifying materials. Different materials have different costs. Prices also vary widely. First, let’s talk about natural graphite. Natural graphite is a material obtained by simply classifying natural graphite after medium-temperature treatment. Judging from its crystal shape: the shape is plate-like, scaly or irregular round. In terms of particle size distribution, the D10 and D50 of natural graphite are both relatively small, and the average particle size is the smallest among all negative electrodes, so it is easier to judge.

　　Modified natural graphite: Since the structure of graphite affects its electrochemical performance, for high-quality graphite, its processing and electrochemical performance can be improved by grading, modifying its morphology, and performing preliminary graphitization treatment. However, in the SEM image, it can be found that the big difference from artificial graphite and MCMB is that the crystal stacking is more serious. Once there are solvent molecules, they can significantly affect the degree of exfoliation of graphite, resulting in poor cycle performance. Therefore, when natural graphite is modified, it should be made into a spherical shape as much as possible to improve cycle performance. Modified natural graphite is mostly in regular spherical shape and has a large particle size distribution as a result of classification. Since it is only a simple morphology modification, its particles are smaller than MCMB and artificial graphite, that is, the D50 is smaller. The average particle size is between 15 and 20µm.

　　Artificial graphite: Artificial graphite is basically spherical or regular cylindrical, with excellent processing performance and excellent electrochemical properties. Because its average particle size is very close to that of modified natural graphite, the two can be distinguished. Judging only from the crystal morphology, its surface is smoother than that of modified graphite crystal, and the graphite layer is not easy to see.

　　Mesophase carbon microspheres: The crystals are smooth and spherical, with a large average particle size and a relatively concentrated particle size distribution. Most particle sizes are >20µm. It can be roughly judged from its particle size distribution diagram and SEM picture.

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