Fluorinated graphite is an advanced carbon-graphite functional material formed by the reaction of carbon and fluorine at high temperature. Its excellent performance and unique quality make it a wonderful thing in new functional materials.
First, the structural characteristics of fluorinated graphite
Graphite fluoride is a compound in which a fluorine atom is covalently bonded to a carbon atom constituting a network plane.
The chemical structure of fluorinated graphite can be expressed by (CFx)n, x is an indefinite value, and its range is generally 0. The specific process is a process of breaking the carbon layer π bond (sp2 hybridization) and combining with carbon atoms to form a CF bond. It is one of the most effective methods for modifying and controlling the physical and chemical properties of carbon materials, which can significantly improve the surface polarity, electrical conductivity, adsorption capacity and capacitive properties of carbon materials.
It currently has two stable compounds, one is polymonofluorocarbon, the chemical formula is (CF)n; the other is polymonofluorodicarbon, and the chemical formula is (C2F)n. Since fluorine has a small atomic radius and high electronegativity, its compounds exhibit unique properties.
Second, the nature of graphite fluoride
C/F atomic ratio
The performance of fluorinated graphite varies with the ratio of C/F atoms in the molecular structure. The C/F ratio ≤ 1 is called a high fluorinated graphite fluoride; the C/F ratio > 1 is called a low fluorinated graphite fluoride.
Highly fluorinated graphite has excellent thermal stability, is an electrical and thermal insulator, is not corroded by strong acids and alkalis, and has lubricating properties superior to MoS2 and flake graphite. The low fluorinated graphite is grayish black and has poor thermal stability and is generally not used as a lubricant. But it is an ideal material for making high-energy anhydrous battery cathodes.
A little weight loss above 500 ° C is a compound with poor thermal stability.
It is the lowest friction coefficient of greases and solid lubricants (molybdenum disulfide, graphite, etc.). The friction coefficient is only 0.1~0.13 at 27~344°C.
Surface energy: The surface energy of fluorinated graphite is extremely low.
Corrosion resistant; reduction reaction with hydrogen to form hydrogen fluoride; reaction with alkali metal and alkali metal halide under high temperature/heating melting to form alkali metal fluoride and amorphous carbon.
Fluorinated graphite is the active material of the battery. Using lithium as the anode and fluorinated graphite as the cathode to form the non-aqueous battery, the energy density is about 5 times that of the water-based manganese battery, and the voltage is 2 times; the storage performance is good, no gas is generated, the self-discharge electrode is small; the working voltage is stable, The temperature effect is small and it is used as usual in the range of -20~70 °C.
Third, the synthesis process of fluorinated graphite
High temperature synthesis
This is the most common method used in the synthesis of fluorinated graphite, allowing graphite and fluorine gas to react directly at a certain temperature.
The synthesis method using fluorine gas as raw material can produce corresponding fluorinated graphite by selecting different reaction temperatures; however, due to the toxic gas contained in the raw material, the reaction process may explode or leak gas, which has a great safety hazard. And the reaction process is accompanied by a large number of by-products, resulting in a relatively low yield. In recent years, some researchers have used fluorine-containing substances as raw materials for fluorine in order to optimize the synthetic environment and reduce safety hazards.
Low temperature synthesis
The low-temperature synthesis method, also known as the indirect reaction method, refers to a process in which graphite is first reacted with a pentahalide to form a graphite intercalation compound at a temperature of 250 to 500 ° C, and then reacted with fluorine gas at room temperature to form a graphite fluoride.
The method has the advantages of relatively low reaction temperature and large control range, and greatly reduces the possibility of explosion; and the number of by-products is small, the yield of fluorinated graphite is high, and the recovered raw materials can be recycled; however, due to the reaction process Fluorine gas, therefore, it is necessary to pay attention to the problems of sealing, safety, etc., and the requirements for production equipment are high.
Catalytic synthesis means that when a metal fluoride such as LiF is present, the reaction of graphite with fluorine gas can be carried out smoothly at a temperature lower than 300 °C. Among them, metal fluoride mainly plays a catalytic role, and the presence of metal fluoride can improve certain properties of fluorinated graphite, such as electrical conductivity.
In the middle of the two poles of the battery, graphite and hydrofluoric acid are electrolyzed due to the influence of the electric field, and an electrochemical reaction can generate graphite fluoride, and the progress of the electrolysis process can be controlled by the concentration of the reaction liquid, the reaction temperature, and the amount of the electrolyte.
Fourth, the application of fluorinated graphite
Used as waterproof enamel material
Fluorinated graphite is one of the most hydrophobic materials due to its strong covalent nature and the low polarization of C-F. The low surface of fluorinated graphite enables it to be used on water and oil resistant materials to reduce or control the wetting of solid surfaces by various liquids.
Used as a release agent
The low surface of fluorinated graphite enables it to be used for molding various materials, and can also be used as a mold release agent for die casting, plywood forming, powder molding, sintering, plastics, etc., and as an abrasive. For grinding in optics.
Used as a solid lubricant
Graphite fluoride has unparalleled high lubricity and chemical stability of graphite and MoS2. It can be used as a solid lubricant or modified as a lubricating additive. It is widely used in harsh conditions such as high temperature, high speed and high load, and is resistant to acid and alkali corrosion, and is used in the field of energy saving and antifriction lubrication.
Used as a positive or positive electrode additive for batteries
Fluorinated graphite replaces fluorine as a positive electrode active material, and combines with lithium metal and an organic solvent in which LiBF4 or LiClO4 is dissolved to form an electrode of a lithium battery, which can increase the output voltage by a factor of two compared with an ordinary battery, and has a wide temperature range and a long storage period. Form a high output, high energy, high density battery.
Application in nuclear reactors
Fluorinated graphite is mainly used as a moderator, a reflective material and a coating material in a nuclear reactor. The utility model has the advantages that the gas projection rate is small, the antifouling performance is good, the thermal neutron absorption section is small, the thermal neutron loss in the reactor is small, and the thermal efficiency of the reaction is high.