First, the crystal structure of graphite
Graphite is a substance composed of a single carbon element, and the crystal structure belongs to the hexagonal crystal system, with a hexagonal layered structure. The carbon atoms at the level combine with σ bonds formed by sp2 hybrid orbitals and delocalized π bonds formed by Pz orbitals to form a solid hexagonal grid-like plane with a carbon-carbon atom spacing of 1.42Å and a strong bond energy (345KJ/mol) between carbon atoms, while the plane of carbon atoms is combined with a weak van der Waals force (bond energy of 16.7KJ/mol) and a plane spacing of 3.354Å.
The composition of natural graphite and synthetic graphite may vary depending on the source and synthesis method/condition, but both mainly consist of 2H phases with 0-30% 3R phase content. Due to Bragg reflection, the 2H/3R phase composition can be determined by powder diffraction (neutrons or X-rays) and quantified by structural modeling. It is worth noting that the layered nature of graphite can produce a high degree of preference orientation, which affects the relative peak intensity in the obtained diffraction pattern. In addition, polycrystalline graphite may exhibit a large number of microstructural defects (stacking faults, point defects, dislocations, etc.), which can widen the profile of the diffraction peak.
Second, the characteristics of artificial graphite and natural graphite
Artificial graphite:
Artificial graphite is similar to polycrystalline in crystallography. There are many types of artificial graphite, and the production process is very different. In a broad sense, all graphite materials obtained by carbonization of organic matter and then graphitization after high temperature treatment can be collectively referred to as artificial graphite, such as carbon (graphite) fiber, pyrolytic carbon (graphite), foam graphite, etc.
Artificial graphite in a narrow sense usually refers to the bulk solid materials prepared by batching, kneading, molding, carbonization (industrially known as roasting) and graphitization with carbon raw materials (petroleum coke, asphalt coke, etc.) with low impurity content as aggregate, coal pitch, etc. as binder, and through batching, kneading, molding, carbonization (industrially known as roasting) and graphitization.
Natural graphite:
Natural graphite is transformed by carbon-rich organic matter under the long-term action of high temperature and high pressure geological environment, which is a gift from nature. The process characteristics of natural graphite are mainly determined by its crystalline form. There are many types of natural graphite, according to different crystal forms, the industry divides natural graphite into three categories: dense crystalline graphite, flake graphite and cryptocrystalline graphite. The mainland mainly has two categories: flake graphite and cryptocrystalline graphite.
Dense crystalline graphite is also called bulk graphite. Such graphite crystals are obvious and visible to the naked eye. The particle diameter is greater than 0.1 mm. The crystals are arranged in a disorderly manner and have a dense block-like structure. The grade is very high, the general carbon content is 60% ~ 65%, sometimes up to 80% ~ 98%, but its plasticity and smoothness are not as good as flake graphite.
Natural flake graphite belongs to the pegmatite in crystallography, which is a single crystal, named for its scaly crystals, with large scales and fine scales. The lubricity and plasticity of this graphite are superior to other types of graphite, so its industrial value is the largest. Although the grade of flake graphite ore is not high, the carbon content is generally between 3% ~ 25%, but it is one of the best floating ores in nature, and high-grade graphite concentrate can be obtained after multiple grinding and selection.
Cryptocrystalline graphite, also known as amorphous graphite or earthy graphite, has been called microcrystalline graphite in recent years. The crystal diameter of this graphite is generally less than 1 micron, and the crystal shape can only be seen under the electron microscope, which can be regarded as a collection of graphite crystals.
Natural microcrystalline graphite is usually converted from coal in a high temperature and high pressure geological environment, so natural microcrystalline graphite is usually associated with coal, and the transition region from anthracite to natural microcrystalline graphite can often be seen in the natural microcrystalline graphite ore body. This type of graphite is characterized by an earthy surface, lack of luster, lower lubricity than flake graphite, and poor selectivity. However, the grade is high, the carbon content is generally 60%~80%, and a few are as high as 90%.
Third, the difference and connection between artificial graphite and natural graphite
Preparation method and performance comparison
Natural graphite has low cost, high specific capacity and high compaction density, and artificial graphite has advantages in long cycle, high temperature and high magnification.
Cost: Natural graphite generally has no graphitization process, and the cost is lower. From the perspective of the production process of the two, artificial graphite needs to be graphitized process, and high-end products need to be carbonized, and the ratio of electricity to energy consumption and carbon emissions is large, so the cost of artificial production is higher than that of natural graphite.
Energy density: natural graphite is more advantageous, but artificial is gradually approaching. The core indicators are gram capacity (natural at 355-370mAh/g, artificial in 280-365mAh/g), compaction density (natural in 1.6-1.8g/cc, artificial in 1.4-1.7g/cc), first charge and discharge efficiency (natural in more than 95%, artificial in more than 92%).
Other properties: natural graphite has a regular layered structure, lithium ions are embedded very slowly, and due to the high anisotropy of the material, it is easy to lead to insufficient contact between the active material and the current collector, resulting in poor natural graphite rate performance, but it is also using various modification processes to improve.
Crystals and tissue structure
The crystal development of natural graphite is relatively perfect, and the graphitization degree of natural flake graphite is usually above 98%, while the graphitization degree of natural microcrystalline graphite is usually below 93%. The degree of crystal development of artificial graphite depends on the raw material and heat treatment temperature, in general, the higher the heat treatment temperature, the higher the degree of graphitization. At present, the degree of graphitization of artificial graphite produced by industry is usually less than 90%.
Natural flake graphite is a single crystal with a relatively simple microstructure, only crystallographic defects (point defects, dislocations, laminar faults, etc.), and macroscopic structural characteristics of anisotropy. The grains of natural microcrystalline graphite are small, and there are holes between the grains arranged chaotically, and there are holes after impurity removal, which shows the structural characteristics of isotropy on a macroscopic basis. Artificial graphite can be regarded as a multi-phase material, including the graphite phase converted by carbonaceous particles such as petroleum coke or asphalt coke, the graphite phase converted by the coal pitch binder coated around the particles, the pores formed by particle accumulation or coal pitch binder after heat treatment.
Physical properties and physical morphology
Natural graphite usually exists in powder form and can be used alone, but is usually used in combination with other materials. There are many forms of artificial graphite, both powdery, fibrous and blocky, while narrow artificial graphite is usually blocky and needs to be processed into a certain shape when used.
Natural graphite and artificial graphite have both commonalities and performance differences. For example, natural graphite and artificial graphite are good conductors of heat and electricity, but for graphite powders of the same purity and particle size, natural flake graphite has the best heat transfer and electrical conductivity, followed by natural microcrystalline graphite, and artificial graphite is the lowest. Graphite has better lubricity and certain plasticity, natural flake graphite crystal development is more perfect, friction coefficient is small, lubricity is the best, plasticity is the highest, and dense crystalline graphite and cryptocrystalline graphite are second, artificial graphite is poor.