A method for preparing covalently CF2-dominant coal tar pitch-based carbon nanosheets and its application

Abstract

This invention discloses a method for preparing covalent CF2-dominated coal tar pitch-based carbon nanosheets and their applications. The preparation method includes the following steps: coal tar pitch and sodium chloride are ground uniformly to obtain an initial mixture; carbonization is carried out under argon or nitrogen protection to obtain a carbonized mixture; after cooling to room temperature, the mixture is washed and dried to obtain a dried mixture; the dried mixture is then placed in an ozone generator for oxidation to obtain an oxidized mixture; polyvinylidene fluoride is weighed as a fluorine source and placed in the upper heating zone of a dual-temperature tube furnace; the oxidized mixture is placed in the lower heating zone of the dual-temperature tube furnace under inert gas protection, and the reaction yields covalent CF2-dominated coal tar pitch-based carbon nanosheets. The fluorine-doped carbon nanosheets obtained by this preparation method, as a modification material for sodium metal battery separators, can not only effectively inhibit the growth of sodium metal dendrites but also reduce side reactions between sodium metal and the electrolyte. It has advantages such as abundant raw material sources, simple preparation process, low cost, and excellent performance.

Description

Technical Field

[0001] This invention relates to the field of carbon material preparation technology, and in particular to a method for preparing covalent CF2-dominant coal tar pitch-based carbon nanosheets and their applications. Background Technology

[0002] Carbon materials have attracted considerable attention due to their low cost, abundant resources, and high electronic conductivity. One effective method to maximize the application of carbon materials in electrodes is heteroatom doping, represented by fluorine atoms. Fluorine atoms possess the highest electronegativity, and their lone pairs of electrons can modulate the intrinsic reactivity of carbon materials by reconstructing the carbon π-conjugated system, showing great potential for application in energy storage systems. Furthermore, fluorine lacks d-electron orbitals, exhibiting high oxidation stability after gaining an electron. Therefore, when fluorine-doped carbon materials are used in electrolytes, binders, membrane functional layers, or electrode materials, they can significantly improve the electrochemical performance of batteries. Over the past few decades, fluorine-doped carbon materials have been extensively researched and developed in the field of rechargeable batteries.

[0003] Invention patent CN 113644284 B describes a method of mixing conductive carbon black, polyvinylidene fluoride (PVDF), and N-methylpyrrolidone (N-Methylpyrrolidone), and then carbonizing the precipitate at 1000°C under an inert atmosphere to obtain a fluorinated carbon material with a fluorine doping level reaching up to 5 mol%. Patent CN118183683 A describes a method of separating brick-red particles from a mixed solution of resorcinol, fluorophenol, hexamethylenetetramine, and the block copolymer Pluronic F 127 by centrifugation, followed by carbonization at 800°C under a nitrogen atmosphere for 2 hours to obtain a mesoporous fluorine-doped carbon material with a fluorine doping level of 1.2–3.1 wt%. While these fluorine doping methods have achieved the preparation of conventional fluorine-doped carbon materials, they suffer from problems such as low fluorine doping levels and difficulty in controlling specific fluorine species, hindering further improvements in the quality of advanced carbon materials for high-performance electrochemical energy storage and impeding their commercial application. Summary of the Invention

[0004] To address the aforementioned problems of low doping content and difficulty in controlling specific fluorine species in fluorine-doped carbon materials, the present invention aims to provide a method for preparing covalent CF2-dominated coal tar pitch-based carbon nanosheets and their application. The novel fluorine-doped carbon nanosheets prepared can be used as a modification material for sodium metal battery separators, which can not only effectively inhibit the growth of sodium metal dendrites, but also reduce the side reactions between sodium metal and electrolyte. It has the advantages of abundant raw material sources, simple preparation process, low cost, and excellent performance.

[0005] A method for preparing covalently CF2-dominant coal tar pitch-based carbon nanosheets includes the following steps:

[0006] S1 carbonization treatment: Coal tar pitch and sodium chloride are ground evenly to obtain an initial mixture. The initial mixture is then carbonized under argon or nitrogen protection to obtain a carbonized mixture.

[0007] S2 Drying treatment: The carbonized mixture obtained in step S1 is cooled to room temperature, then washed and dried to obtain a dried mixture;

[0008] S3 Oxidation treatment: The dried mixture from step S2 is placed in an ozone generator for oxidation treatment to obtain an oxidized mixture;

[0009] S4 Fluorination treatment: Polyvinylidene fluoride is weighed as the fluorine source and placed in the upper heating zone of a dual-temperature tube furnace. At the same time, the oxidized mixture obtained in step S3 is placed in the lower heating zone of the dual-temperature tube furnace and protected by an inert gas. After the reaction, covalent CF2-dominated coal tar pitch-based carbon nanosheets are obtained.

[0010] Furthermore, in step S1, the softening point of the coal tar pitch is 100-320℃; the mass ratio of coal tar pitch to sodium chloride is 1:10-1:30.

[0011] Further, in step S1, the carbonization process involves transferring the initial mixture into a quartz boat, placing it in a tube furnace, and heating it to 600-1400°C at a heating rate of 2-10°C / min for carbonization, with a carbonization time of 0.5-3 hours.

[0012] Furthermore, in step S2, the drying process is carried out at 80-120°C for 8-12 hours.

[0013] Furthermore, in step S3, the oxidation treatment time is 15-45 minutes, and the ozone generator power is 200-400W.

[0014] Furthermore, in step S4, the inert gas is argon or nitrogen, the upper heating zone is heated to 400-600℃ at a heating rate of 3℃ / min to decompose the fluorine source, and the lower heating zone is heated to 200-400℃ at a heating rate of 2℃ / min to fluorinate the carbon material.

[0015] Further, in step S4, the mass ratio of the added polyvinylidene fluoride to the dried mixture is 1:4-20.

[0016] Furthermore, the upper heating zone and the lower heating zone maintain the same temperature for the same time, controlled within 0.5-2 hours.

[0017] A second objective of this invention is to provide the application of the method in sodium metal anode protection materials.

[0018] Furthermore, the obtained covalent CF2-dominated coal tar pitch-based carbon nanosheets and PVDF binder were dispersed in a N-methylpyrrolidone solution at a mass ratio of 8:2 or 9:1. Then, the nanosheets were placed on a polypropylene membrane by vacuum filtration and dried to form a functional membrane with a carbon coating on the surface. The membrane was then cut into discs for later use. Sodium vanadium phosphate was used as the counter electrode and NS-001 was used as the electrolyte to assemble a battery.

[0019] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0020] The beneficial effects of this invention are as follows:

[0021] 1. The preparation method provided by this invention is simple and efficient. The introduced template agent sodium chloride can be removed by washing with water. It is green, environmentally friendly, and recyclable, reducing production costs. Coal tar pitch is an abundant source of raw materials and is easy to scale up for production.

[0022] 2. In this invention, oxygen-containing functional groups are introduced by carbonization followed by oxidation, and then heteroatom doping is performed by fluorination; wherein the fluorine-containing compound and the carbon oxide material are heated in sections under an inert atmosphere for controllable fluorine doping of the carbon material;

[0023] 3. This invention has the advantages of simple process and large-scale production capability. Fluorine doping can optimize the surface chemical composition, conductivity, and active sites of carbon materials by controlling the microscopic electronic structure, thus having wide applicability.

[0024] 4. The covalent CF2-dominant coal tar pitch-based carbon nanosheets prepared in this invention, as a membrane modification material for sodium metal batteries, can not only inhibit the growth of sodium metal dendrites, but also reduce the side reactions between sodium metal and electrolyte, and have the characteristics of long cycle life and excellent rate performance.

[0025] The accompanying drawings and brief descriptions thereof.

[0026] Figure 1 This is a scanning electron microscope image of the covalent CF2-dominant coal tar pitch-based carbon nanosheets prepared in Example 1 of this invention;

[0027] Figure 2 This is a transmission electron microscope (TEM) image of covalent CF2-dominant coal tar pitch-based carbon nanosheets prepared in Example 1 of this invention;

[0028] Figure 3 These are the X-ray diffraction (XRD) spectra of the coal tar pitch-based carbon nanosheets prepared in Examples 1, 1, 2, and 3 of this invention.

[0029] Figure 4 This is the total X-ray photoelectron spectroscopy (XPS) spectrum of the coal tar pitch-based carbon nanosheets prepared in Examples 1, 1, 2, and 3 of this invention.

[0030] Figure 5 These are the O1s XPS spectra of coal tar pitch-based carbon nanosheets prepared in Examples 1, 1, 2, and 3 of this invention.

[0031] Figure 6 These are the F1s XPS spectra of coal tar pitch-based carbon nanosheets prepared in Examples 1, 1, 2, and 3 of this invention.

[0032] Figure 7 This is a full-cell cycling performance diagram of the covalent CF2-dominant coal tar pitch-based carbon nanosheets prepared in Example 1 of this invention as a sodium metal membrane modification material.

[0033] Figure 8 This is a rate performance diagram of a full cell assembled using covalent CF2-dominant coal tar pitch-based carbon nanosheets prepared in Example 1 of this invention as a sodium metal membrane modification material. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] Example 1

[0036] This embodiment discloses a method for preparing covalent CF2-dominant coal tar pitch-based carbon nanosheets, including the following steps:

[0037] S1. Add 1g of high-temperature coal tar pitch with a softening point of 280℃ and 20g of sodium chloride to a mortar and grind them evenly to obtain a uniformly mixed coal tar pitch and sodium chloride mixture. Transfer the prepared mixture to a quartz boat, place it in a tube furnace, and carbonize it to 800℃ at a heating rate of 5℃ / min under nitrogen protection for 2 hours.

[0038] S2. After the carbonized mixture is cooled to room temperature, it is washed with deionized water, filtered three times, and then transferred to a vacuum drying oven and dried at 80°C for 10 hours.

[0039] S3. Weigh 0.5g of the dried mixture and transfer it to the corundum boat. Place it in an ozone generator for oxidation treatment for 30 minutes. The ozone generator has a power of 300W.

[0040] S4. Weigh 6g of PVDF as a fluorine source and place it in the upper heating zone (upper air vent) of a dual-temperature tube furnace. At the same time, place the mixture obtained after oxidation in step S3 in the lower heating zone (lower air vent) of the dual-temperature tube furnace and introduce argon or nitrogen for inert gas protection.

[0041] S5. Based on step S4, fluorination treatment is performed, i.e., the upper heating zone is heated to 500℃ at a heating rate of 3℃ / min to decompose PVDF. Simultaneously, the lower heating zone is heated to 300℃ at a heating rate of 2℃ / min to fluorinate the carbon material. The holding time for both heating zones is the same, controlled at 1 hour. After cooling to room temperature, the material in the lower heating zone is removed to obtain the target material, covalently CF2-dominated coal tar pitch-based carbon nanosheets.

[0042] By scanning electron microscopy (SEM) Figure 1 ) and high-magnification transmission electron microscopy characterization ( Figure 2 As can be seen, the material prepared in Example 1 exhibits a smooth, sheet-like structure with a low carbon microcrystalline structure. The internal carbon layers are randomly arranged, with a small number of long-range ordered nano-carbon clusters (within the dashed box) embedded within them. Figure 3 As shown in the XRD pattern, the carbon interlayer spacing of the material obtained in Example 1 is 0.391 nm. Figure 4 The XPS spectrum shows that the fluorine content of the material obtained in Example 1 reaches 12.89 at.%. Figure 5 As shown, the oxygen-containing functional groups in Example 1 are mainly COO-, COH, and C=O groups; and compared with the ozone-oxidized coal tar pitch-based carbon nanosheets (Comparative Example 2), the COH group content in Example 1 is significantly reduced, and the COC group disappears. The F1s XPS spectrum of Example 1 shows a covalent CF2 (687.8 eV) peak, indicating that the material obtained in Example 1 is mainly composed of covalent CF2, and the covalent CF2 is mainly generated by the substitution of CO groups (COH and COC) by fluorinated free radicals produced during the fluorination process. Figure 6 As shown.

[0043] The fluorinated carbon nanosheets obtained in Example 1 were ultrasonically dispersed with a binder in a N-methylpyrrolidone solution at a mass ratio of 8:2. The dispersion was then transferred to a PP membrane using a vacuum filtration method, dried, and dried to form a functional membrane with a carbon coating. The membrane was then cut into 19mm diameter discs for later use. A CR2032 battery case, sodium vanadium phosphate as the counter electrode, and NS-001 as the electrolyte were used to assemble the battery. Electrochemical performance tests were conducted using a Land CT2001A battery testing system with a charge / discharge voltage range of 0.01-2.5V. The reversible capacity of the covalently CF2-dominated coal tar pitch-based carbon nanosheets at a charge / discharge current density of 2C was measured to be 101 mAh g⁻¹. -1 Its rate performance and cycle performance are as follows: Figure 7 and Figure 8 As shown.

[0044] This application utilizes a novel fluorine-doped carbon material prepared using a carbonization-oxidation-post-fluorination strategy based on the traditional salt template method. First, asphalt and sodium chloride template are ground uniformly, and carbon nanosheets are generated through template replication during carbonization. Second, oxygen-containing functional groups (COH) are introduced during ozone oxidation to increase the content of specific oxygen species in the carbon material. Finally, fluorine-containing free radicals generated by the thermal decomposition of PVDF substitute the oxygen-containing functional groups on the carbon material, thereby promoting the effective doping of specific fluorine species. XPS analysis shows (…). Figure 4 The fluorine doping level is as high as 12.89 at.%, and the peak fitting results of the F element show that the proportion of covalent CF2 reaches >99%. Figure 6 This indicates that the effective preparation of covalent CF2-dominated coal tar pitch-based carbon nanosheets has been achieved.

[0045] Example 2

[0046] S1. Add 1g of high-temperature coal tar pitch with a softening point of 103℃ and 10g of sodium chloride to a mortar and grind them evenly to obtain a uniformly mixed coal tar pitch and sodium chloride mixture. Transfer the prepared mixture to a quartz boat, place it in a tube furnace, and carbonize it to 600℃ at a heating rate of 2℃ / min under nitrogen protection for 0.5h.

[0047] S2. After the carbonized mixture is cooled to room temperature, it is washed with deionized water, filtered three times, and then transferred to a vacuum drying oven and dried at 100°C for 8 hours.

[0048] S3. Weigh 0.5g of the dried mixture and transfer it to the corundum boat. Place it in an ozone generator for oxidation treatment for 15 minutes. The ozone generator has a power of 200W.

[0049] S4. Weigh 2g of PVDF as a fluorine source and place it in the upper heating zone (upper air vent) of the dual-temperature tube furnace. At the same time, place the mixture obtained after oxidation in step 3 in the lower heating zone (lower air vent) of the dual-temperature tube furnace and introduce argon or nitrogen for inert gas protection.

[0050] S5. Based on step 4, fluorination treatment is performed. Specifically, the upper heating zone is heated to 400℃ at a rate of 3℃ / min to decompose PVDF. Simultaneously, the lower heating zone is heated to 200℃ at a rate of 2℃ / min to fluorinate the carbon material. The holding time for both heating zones is the same, controlled at 0.5h. After cooling to room temperature, the material from the lower heating zone is removed, yielding the target material, covalently CF2-dominated coal tar pitch-based carbon nanosheets.

[0051] The carbon interlayer spacing of the material obtained in Example 2 was 0.397 nm, and the fluorine content was only 2.29 at.%. The fluorinated carbon nanosheets obtained in Example 2 were ultrasonically dispersed with a binder at a mass ratio of 9:1 in a N-methylpyrrolidone solution, and then drawn onto a PP separator using a vacuum filtration method. After drying, a functional separator with a carbon coating was formed and cut into 19 mm diameter discs for later use. A CR2032 battery case, sodium vanadium phosphate as the counter electrode, and NS-001 as the electrolyte were used to assemble the battery. Electrochemical performance tests were performed on the battery using a Land CT2001A battery testing system with a charge / discharge voltage range of 0.01-2.5 V. The reversible capacity of the covalently CF2-dominated coal tar pitch-based carbon nanosheets was measured to be 95 mAh g⁻¹ under a 2C charge / discharge current density. -1 .

[0052] Example 3

[0053] S1. Add 1g of high-temperature coal tar pitch with a softening point of 320℃ and 30g of sodium chloride to a mortar and grind until uniform to obtain a homogeneous mixture of coal tar pitch and sodium chloride. Transfer the prepared mixture to a quartz boat, place it in a tube furnace, and carbonize it to 1400℃ at a heating rate of 10℃ / min under nitrogen protection for 3 hours.

[0054] S2. After carbonization, the mixture is cooled to room temperature and taken out. It is then washed with deionized water, filtered three times, and then transferred to a vacuum drying oven. It is dried at 120°C for 12 hours and then taken out.

[0055] S3. Weigh 0.5g of the dried mixture and transfer it to the corundum boat. Place it in an ozone generator for oxidation treatment for 45 minutes. The ozone generator has a power of 400W.

[0056] S4. Weigh 10g of PVDF as a fluorine source and place it in the upper heating zone (upper air vent) of the dual-temperature tube furnace. At the same time, place the mixture obtained after oxidation in step 3 in the lower heating zone (lower air vent) of the dual-temperature tube furnace and introduce argon or nitrogen for inert gas protection.

[0057] S5. Based on step 4, fluorination treatment is performed: the upper heating zone is heated to 600℃ at a rate of 3℃ / min to fluorinate the PVDF. Simultaneously, the lower heating zone is heated to 400℃ at a rate of 2℃ / min to fluorinate the carbon material. The holding time for both heating zones is the same, controlled at 2 hours. After cooling to room temperature, the material in the lower heating zone is removed, yielding the target material, covalently CF2-dominated coal tar pitch-based carbon nanosheets.

[0058] The carbon interlayer spacing of the material obtained in Example 3 was 0.378 nm, and the fluorine content was 7.89 at.%. The fluorinated carbon nanosheets obtained in Example 3 were ultrasonically dispersed with a binder at a mass ratio of 8:2 in a N-methylpyrrolidone solution, and then drawn onto a PP separator using a vacuum filtration method. After drying, a functional separator with a carbon coating was formed and cut into 19 mm diameter discs for later use. A CR2032 battery case, sodium vanadium phosphate as the counter electrode, and NS-001 as the electrolyte were used to assemble the battery. Electrochemical performance tests were performed on the battery using a Land CT2001A battery testing system with a charge / discharge voltage range of 0.01-2.5 V. The reversible capacity of the covalently CF2-dominated coal tar pitch-based carbon nanosheets was measured to be 98 mAh g⁻¹ under a 2C charge / discharge current density. -1 .

[0059] Comparative Example 1

[0060] S1. Add 1g of high-temperature coal tar pitch with a softening point of 280℃ and 20g of sodium chloride to a mortar and grind them evenly to obtain a uniformly mixed coal tar pitch and sodium chloride mixture. Transfer the prepared mixture to a quartz boat, place it in a tube furnace, and carbonize it to 800℃ at a heating rate of 5℃ / min under nitrogen protection for 2 hours.

[0061] S2. After the mixture obtained by carbonization is cooled to room temperature, it is taken out, washed with deionized water, filtered three times, and then transferred to a vacuum drying oven. After drying at 80°C for 10 hours, the target material coal tar pitch-based carbon nanosheets are obtained.

[0062] like Figure 3 As shown in the XRD pattern, the carbon interlayer spacing of the material obtained in Comparative Example 1 is 0.387 nm. Figure 5 As shown, the oxygen-containing functional groups in Comparative Example 1 are mainly COO-, COC, COH, and C=O groups. The carbon nanosheets obtained in Comparative Example 1 were ultrasonically dispersed with a binder at a mass ratio of 8:2 in a N-methylpyrrolidone solution, and then drawn onto a PP separator using a vacuum filtration method. After drying, a functional separator with a carbon coating was prepared and cut into 19mm diameter discs for later use. A CR2032 battery case, sodium vanadium phosphate as the counter electrode, and NS-001 as the electrolyte were used to assemble the battery. The electrochemical performance of the battery was tested using a Land CT2001A battery testing system with a charge / discharge voltage range of 0.01-2.5V. The reversible capacity of the coal tar pitch-based carbon nanosheets under a 2C charge / discharge current density was measured to be 93 mAh g⁻¹. -1 .

[0063] Compared to Example 1, Comparative Example 1 only underwent carbonization treatment. The full cell assembled using the coal tar pitch-based carbon nanosheets prepared therefrom as the sodium metal membrane modification material showed a significant decrease in reversible capacity under the same test conditions.

[0064] Comparative Example 2

[0065] Step 1: Add 1g of high-temperature coal tar pitch with a softening point of 280℃ and 20g of sodium chloride to a mortar and grind until homogeneous to obtain a uniform mixture of coal tar pitch and sodium chloride. Transfer the mixture to a quartz boat, place it in a tube furnace, and carbonize it at 800℃ under nitrogen protection at a heating rate of 5℃ / min for 2 hours.

[0066] Step 2: After the carbonized mixture is cooled to room temperature, it is taken out, washed with deionized water, filtered three times, and then transferred to a vacuum drying oven and dried at 80°C for 10 hours.

[0067] Step 3: Weigh 0.5g of the dried mixture and transfer it to a corundum boat. Place the boat in an ozone generator for oxidation treatment for 30 minutes. The ozone generator has a power of 300W. The target material, coal tar pitch-based carbon nanosheets, is obtained.

[0068] like Figure 3 As shown in the XRD pattern, the carbon interlayer spacing of the material obtained in Comparative Example 2 is 0.392 nm. Figure 4 The XPS spectra show that the oxygen content of the material obtained in Comparative Example 2 is as high as 9.67 at.%. Figure 5 As shown, the oxygen-containing functional groups in Comparative Example 2 are mainly COO-, COC, COH, and C=O groups. Compared with Comparative Example 1, the COH content is significantly higher, indicating that the COH functional group is mainly introduced during the ozone oxidation process. The carbon nanosheets obtained in Comparative Example 2 were ultrasonically dispersed with a binder at a mass ratio of 8:2 in a N-methylpyrrolidone solution, then filtered onto a PP membrane, dried to form a functional membrane with a carbon coating, and cut into 19mm diameter discs for later use. A CR2032 battery case, sodium vanadium phosphate as the counter electrode, and NS-001 as the electrolyte were used to assemble the battery. The electrochemical performance of the battery was tested using a Land CT2001A battery testing system with a charge / discharge voltage range of 0.01-2.5V. The reversible capacity of the coal tar pitch-based carbon nanosheets under a 2C charge / discharge current density was measured to be 91 mAh g⁻¹. -1 .

[0069] Compared with Example 1, Comparative Example 2, without fluorination after the introduction of oxygen-containing functional groups, showed a significant decrease in the first reversible capacity of the full cell assembled using the coal tar pitch-based carbon nanosheets prepared therefrom as the sodium metal membrane modification material under the same test conditions.

[0070] Comparative Example 3

[0071] Step 1: Add 1g of high-temperature coal tar pitch with a softening point of 280℃ and 20g of sodium chloride to a mortar and grind until homogeneous to obtain a uniform mixture of coal tar pitch and sodium chloride. Transfer the mixture to a quartz boat, place it in a tube furnace, and carbonize it at 800℃ under nitrogen protection at a heating rate of 5℃ / min for 2 hours.

[0072] Step 2: After the carbonized mixture is cooled to room temperature, it is taken out, washed with deionized water, filtered three times, and then transferred to a vacuum drying oven and dried at 80°C for 10 hours.

[0073] Step 3: Weigh 6g of PVDF as the fluorine source and place it in the upper heating zone (upper air vent) of the dual-temperature tube furnace. At the same time, place the mixture obtained after drying in step 2 in the lower heating zone (lower air vent) of the dual-temperature tube furnace and introduce argon or nitrogen for inert gas protection.

[0074] Step 4: Based on Step 3, fluorination treatment is performed. Specifically, the upper heating zone is heated to 500℃ at a rate of 3℃ / min to fluorinate the PVDF. Simultaneously, the lower heating zone is heated to 300℃ at a rate of 2℃ / min to fluorinate the carbon material. The holding time for both heating zones is the same, controlled at 1 hour. After cooling to room temperature, the material from the lower heating zone is removed, yielding the target material, fluorine-doped coal tar pitch-based carbon nanosheets.

[0075] like Figure 3 As shown in the XRD pattern, the carbon interlayer spacing of the material obtained in Comparative Example 3 is 0.407 nm. Figure 4 The XPS spectrum shows that the fluorine content of the material obtained in Comparative Example 3 reaches 9.72 at.%. Figure 5 As shown, the oxygen-containing functional groups in Comparative Example 3 are mainly COO- and COH groups. Compared with Comparative Example 1, the COH content is significantly higher, indicating that the fluorination effect of direct fluorination without oxidation is poor. The carbon nanosheets obtained in Comparative Example 3 were ultrasonically dispersed with a binder in a N-methylpyrrolidone solution at a mass ratio of 8:2, and then drawn onto a PP membrane using a vacuum filtration method. After drying, a functional membrane with a carbon coating on the surface was prepared and cut into 19mm diameter discs for later use. A CR2032 battery case, sodium vanadium phosphate as the counter electrode, and NS-001 as the electrolyte were used to assemble the battery. The electrochemical performance of the battery was tested using a Land CT2001A battery testing system with a charge / discharge voltage range of 0.01-2.5V. The reversible capacity of the fluorinated coal tar pitch-based carbon nanosheets under a charge / discharge current density of 2C was measured to be 99 mAhg. -1 .

[0076] Compared to Example 1, Comparative Example 3 did not introduce oxygen-containing functional groups through ozone oxidation. The full cell assembled using fluorine-doped coal tar pitch-based carbon nanosheets prepared therein as the sodium metal membrane modification material showed a slight decrease in the first reversible capacity under the same test conditions.

[0077] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for the preparation of covalent CF2-dominant coal-tar pitch-based carbon nanosheets, characterized by, The method comprises the following steps: S1 carbonization treatment: uniformly grinding coal pitch and sodium chloride to obtain an initial mixture, and performing carbonization treatment on the initial mixture under protection of argon or nitrogen to obtain a carbonized mixture; S2 drying treatment: after the carbonized mixture obtained in step S1 is cooled to room temperature, washing and drying treatment are performed to obtain a dried mixture; S3 oxidation treatment: placing the dried mixture in step S2 in an ozone generator to perform oxidation treatment, thereby obtaining an oxidized mixture; S4 fluorination treatment: taking polyvinylidene fluoride as a fluorine source and placing it in an upper heating zone of a double-tube furnace, and simultaneously placing the oxidized mixture obtained in step S3 in a lower heating zone of the double-tube furnace, and then introducing inert gas protection, and after reaction, covalent CF2-dominant coal pitch-based carbon nanosheets are obtained; the inert gas is argon or nitrogen, the upper heating zone is heated to 400-600 ℃ at a heating rate of 3 ℃ / min to decompose the fluorine source, and the lower heating zone is heated to 200-400 ℃ at a heating rate of 2 ℃ / min to fluorinate the carbon material.

2. A process for the preparation of covalent CF2-dominant coal pitch-based carbon nanosheets as claimed in claim 1, wherein, In step S1, the softening point of the coal pitch is 100-320 ℃, and the mass ratio of the coal pitch to sodium chloride is 1:10-1:

30.

3. A process for the preparation of covalent CF2-dominant coal pitch-based carbon nanosheets as claimed in claim 2, wherein, In step S1, the carbonization treatment is performed by placing the initial mixture into a quartz boat and placing it in a tube furnace, and heating to 600-1400 ℃ at a heating rate of 2-10 ℃ / min for carbonization, and the carbonization time is 0.5-3 h.

4. The method for preparing covalently CF2-dominant coal tar pitch-based carbon nanosheets as described in claim 1, characterized in that, In step S2, the drying treatment is performed at 80-120 ℃ for 8-12 h.

5. A process for the preparation of covalent CF2-dominant coal pitch-based carbon nanosheets as claimed in claim 1, wherein, In step S3, the oxidation treatment time is 15-45 min, and the ozone generator power is 200-400 W.

6. A process for the preparation of covalent CF2-dominant coal pitch-based carbon nanosheets as claimed in claim 1, wherein, The upper heating zone and the lower heating zone have the same constant temperature time, which is controlled to be 0.5-2 h.

7. A process for the preparation of covalent CF2-dominant coal pitch-based carbon nanosheets as claimed in claim 6 wherein, In step S4, the mass ratio of the amount of polyvinylidene fluoride added to the dried mixture is 1:4-20.

8. The method according to any one of claims 1 to 7 is used in sodium metal negative electrode protection materials.

9. The use according to claim 8, wherein the compound is ###0002### The obtained covalent CF2-dominant coal pitch-based carbon nanosheets and PVDF binder are dispersed in a solution of nitrogen methyl pyrrolidone at a mass ratio of 8:2 or 9:1, and then placed on a polypropylene separator by using a suction filtration method, and dried to obtain a functional separator with a carbon coating on the surface, and then cut into a round piece for use; Vanadium sodium phosphate is used as a counter electrode, and NS-001 is used as an electrolyte to assemble a battery.

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