The structure of graphite In graphite, the graphite layer is conductive and the carbon atoms on the sp2-hybrid orbitals are layered along the hexagonal plane. In addition, the delocalized π electrons have van der Waals forces between the graphite layers. Since π electrons can move freely between graphite layers, graphite has good electronic conductivity. π electrons have weak van der Waals forces, but the graphite layer has anisotropy and strong covalent bonds. Lithium ions can be inserted and extracted between graphite layers.
Graphite is usually arranged in the order of ABAB along the c axis, forming a hexagonal structure, and at the same time it is stacked in the order of ABCABC to form a rhombohedral structure.
Since the base surface of the graphite crystal is perpendicular to the c-axis and the end surface is parallel to the c-axis, it has anisotropy. The anisotropy of graphite affects the electrochemical reaction on the negative electrode of the secondary battery. The base surface of the stone is not active for electrochemical reactions, while the end surface of the graphite exhibits considerable activity. Therefore, the electrochemical properties of the rock mass depends on the ratio of the base surface to the end surface. In addition, the high activity of the graphite end faces accelerates the formation of surface functional groups containing oxygen atoms. Because of this, artificial graphite with high-end surface (synthesized into thermal decomposition) can be obtained by heating pitch coke at a temperature higher than 2500°C.
Carbon that can be petrified under heat treatment is called graphitizable carbon. At high temperatures, the arrangement of atoms facilitates the formation of a layered structure, so carbon is easier to graphitize to form graphite, which is also called soft carbon. Carbon that cannot be graphitized when the temperature is higher than 2500°C is called hard carbon or hard carbon. When the temperature is relatively low (below 1000°C), a small amount of graphite planes in the graphitizable carbon are stacked in parallel, but they show a disordered structure along the c-axis. As the temperature increases, the graphite surface will increase and orderly pile up. For soft carbon, when the temperature is higher than 2000°, the disordered layer structure will be significantly reduced. At a temperature of about 3000°, heat treatment of soft carbon will produce graphite with a regular structure, as shown in Figure 1.
In order to promote carbonization, the carbon precursor should contain high-density polycyclic aromatic compounds that are easily converted to graphite planes. Moreover, adjacent graphite planes should be properly aligned. The connection between the carbon layers in the graphitizable carbon is very weak, allowing rearrangement to form a graphite structure.
Generally speaking, the graphitization process includes two processes, the expansion and accumulation of the graphite layer in the three-dimensional space of the graphite plane. During the graphitization process, graphite experienced an increase in density, an increase in grain size (La and Lc), and a decrease in (002) crystal plane spacing. Here La is the crystal grain size parallel to the base plane, Lc is the crystal grain size perpendicular to the base plane, and La and Lc are the crystal grain sizes on the a-axis and c-axis, respectively. The crystal structure of the graphitized product is determined by a variety of factors, such as the crystal grain size. Interplanar spacing d002 and graphitization temperature. By studying these factors, we can predict whether a specific carbon material can be used as a negative electrode material in a lithium secondary battery. La and Lc can be obtained through XRD analysis and the formula t=kλ/βcos(θ). Here, in the formula, t is the layer height of the La or Lc accumulation layer, k is the shape factor of the crystal grains, the k of La and Lc are 0.9 and 1.84, respectively, λ is the wavelength of X-ray, β is the width of the diffraction peak, and θ Is the angle of incidence.