What electrochemical properties does graphitized carbon fiber have?

main content:

• ①Graphitized vapor-grown carbon fiber
• ②Vapour grown carbon fiber

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 carbon fiber.

Carbon fiber (CF) has the characteristics of high strength, high modulus, high electrical conductivity, thermal conductivity, low density, and corrosion resistance. It is an important industrial material. There are many types of carbon fiber. According to their sources, they can be divided into rayon carbon fiber (Rayon-CF), polypropylene refined carbon fiber (PAN-CF), mesophase pitch cabon fiber (MPPCF) vapor-grown carbon fiber ( Vapor grown carbon fiber, VGCF). Among these carbon fibers, the most researched lithium ion battery anode materials are vapor grown carbon fiber and mesophase pitch-based carbon fiber. The following mainly introduces these two carbon fiber anode materials.

①Graphitized vapor-grown carbon fiber
Vapor-grown carbon fiber is grown on catalyst particles. It uses ultrafine particles such as transition metals as crystal nuclei and low-carbon hydrocarbons (methane, benzene, etc.) as raw materials. It is directly generated by cracking in hydrogen atmosphere and high temperature. Shaped carbon fiber. The typical VGCF has a diameter of 5~1000nm and a length of 5~100nm. The cross-sectional structure is in the shape of "tree rings", as shown in Figure 1. The inner and outer layers of vapor-grown carbon fiber are different in structure: the inner layer is a highly oriented carbon layer, also known as a basic carbon fiber layer, which is formed close to the metal nucleus; the surrounding is a thermally decomposed carbon layer, which is based on the basic carbon fiber A graphitizable carbon layer formed by the principle of CVD (Chemical Vapor Deposition) around the layer. Vapor-grown carbon fiber belongs to a chaotic layer structure. After graphitization high temperature treatment, the carbon layer structure is similar to graphite single crystal. The degree of graphitization is higher than that of other carbon fibers, the layer plane is parallel to the fiber axis, and the layer spacing is the same as that of graphite, which is 0.335nm.

The form of VGCF is closely related to the reaction temperature, the type of feed gas, the composition of the gas phase, the type of metal catalyst, and the strength of the interaction between the metal and the carrier. The experiment can observe the shape of hollow filament, hollow tree, twist and spiral. The cross-sectional structure of VGCF is shown in Figure 1.

Figure 1 Schematic diagram of the cross-sectional structure of VGCF

Vapor-grown carbon fiber has the characteristics of ultra-high strength, high modulus and good crystal orientation, and is considered to be an ultra-high and short-formed carbon fiber. After graphitization, the physical properties of vapor-grown carbon fibers will also change, as shown in Table 1.

Table 1 Physical and chemical properties of VGCF^①

Graphitized vapor-grown carbon fiber is a kind of graphitized fiber material with tubular hollow structure. As the negative electrode material of lithium ion battery, it has a discharge specific capacity of 320mA·h/g or more and a first charge and discharge efficiency of 93%. It is comparable to other carbon or graphite Compared with similar anode materials, the use of gas-deposited graphite fiber as the anode has more excellent high-current discharge performance, low-temperature discharge performance and longer cycle life. However, due to its complicated preparation process and high material cost, it is used in lithium-ion batteries. A large number of applications are restricted. Someone systematically studied the effect of loading a carbon layer on the surface of carbon fiber and metal on the performance of the material, and found that by loading a conductive carbon layer on the surface of carbon fiber, the cycle performance and electrochemical reaction rate of the material can be improved, and the initial irreversible capacity is also suppressed; At the same time, it is also found that the degree of change in material properties is related to the type of load material used.

Graphitized carbon fiber with a radial structure as a negative electrode material is beneficial to the diffusion of lithium ions. The diffusion coefficient of lithium in the graphitized carbon fiber has three peaks, and its corresponding potential is close to the charge and discharge platform potential, which is similar to the change in the diffusion coefficient of lithium ions in natural graphite, as shown in Figure 2.

Figure 2 The relationship between the logarithm of the diffusion coefficient of graphitized carbon fiber and the potential

Graphitized mesophase pitch-based carbon fiber mesophase pitch-based carbon fiber (mesophase pitch-based carbon fiber, MPCF) is based on pitch (complex mixture of fused-ring aromatic hydrocarbons, divided into coal series and oil series according to the source) as the raw material, using The principle of liquid phase carbonization is to process the pitch to obtain the mesophase, which is then obtained by spinning and further carbonization. High-performance graphitized MCF can be obtained by high-temperature treatment of MPCF at 2800~3000℃.

The structure and electrochemical properties of MPCF are closely related to the heat treatment temperature. As the treatment temperature increases, d₀₀₂ decreases, L_c increases, and capacity increases. As shown in Figure 3. Graphitized MPCF treated at 3000°C, d₀₀₂ is close to highly oriented pyrolytic graphite (HOPG). The unheated MPCF has a turbo stratic disorder structure with small crystal grain size. The adjacent carbon layers are randomly arranged in rotation, and stacked in a more or less parallel arrangement. As the temperature increases, the degree of graphitization increases, and the degree of regular arrangement of the carbon layers also increases. Figure 4 is an SEM image of MPCF treated at 1000°C and 3000°C. The MPCF carbon layers treated at low temperature (1000°C) are arranged randomly, and the MPCF carbon layers treated at high temperature (3000°C) are arranged regularly, and the diameter of the carbon fiber is reduced.

Figure 3 The variation of MPCF grain thickness L_c(002) and interlayer spacing d₀₀₂ with temperature at 650~3000℃

Figure 4 (a, d) 1000, (b, e) 2000, (c, f) SEM images of MPCF processed at 3000°C).

Someone has studied the electrochemical performance of MPCF treated at 2800℃. It is believed that there are two types of lithium storage sites. One is the interlayer position of graphite. Its lithium insertion characteristics are similar to those of natural graphite forming high-order lithium insertion compounds. The inserted lithium is reversible; the other is disorganized carbon, that is, curved monolayer or sp³ carbon, which can insert lithium during the initial charge, but the inserted lithium cannot be released during the discharge.

②Vapour grown carbon fiber
The iron, nickel or cobalt catalyst is made into ultra-fine particles (particles) and then evenly spread on the ceramic substrate and placed in a high-temperature furnace. The hydrocarbon gas (such as benzene vapor) is fed into the furnace at an appropriate concentration and flow rate. Hydrocarbon molecules are pyrolyzed under the action of high temperature (1000~1300℃) and catalyst to get carbon fiber.

The process of growing carbon fiber includes the following five stages: the adsorption of hydrocarbon molecules on the surface of the catalyst particles; the catalytic pyrolysis of the adsorbed hydrocarbons and the precipitation of carbon; the diffusion of carbon in the catalyst particles; the precipitation of carbon on the other side of the catalyst particles, and the growth of fibers ; The catalyst particles lose their activity and the fiber stops growing.

There are many explanations for the above-mentioned process. Among them, a representative one is that after the hydrocarbon molecules contact the metal catalyst particles, they are pyrolyzed at high temperature and undergo a series of gas phase carbonization reactions. The precipitated carbon is dissolved from the side where the hydrocarbon and the metal contact Into the metal; the carbon atoms in the metal particles move from the high concentration side to the low concentration side according to the law of diffusion; the temperature on the side where the gas phase carbonization reaction of the metal particles occurs is higher and the temperature on the other side is lower, when the carbon atoms When the directional movement reaches a certain level, carbon atoms are supersaturated on the low-temperature side, so carbon atoms are continuously precipitated from the low-temperature side, and finally carbon fibers are formed.

The study of the VGCF process found that the activation energy of the carbon filament growth process is close to the activation energy of the carbon diffusion in the corresponding metal (see Table 2), so the diffusion of carbon in the metal particles is considered to be the control step of VGCF.

Table 2 Comparison of activation energy of carbon filament growth and activation energy of carbon diffusion process in metal particles

The smaller the metal particles, the faster the growth rate of VGCF. Metal particles are too large, the growth rate is slow, and it is easy to inactivate. The size of the particles must also be large enough to hold the carbon and complete the carbon structure conversion process. The dynamics of carbon diffusion are not very clear. One view is that the decomposition of hydrocarbons on the metal surface is an exothermic reaction, and the precipitation of carbon from the inside of the metal particles is an endothermic reaction. These two processes cause the temperature gradient of different parts of the metal particles to be diffused. Power, but this view cannot explain that the cracking reaction is an endothermic process, such as methane cracking; the other is that the carbon content of each part of the metal particles is different, and the concentration gradient of carbon provides the driving force.