A method for preparing composite carbon modified sodium vanadium phosphate positive electrode material

Abstract

The invention discloses a method for preparing a composite carbon modified sodium vanadium phosphate positive electrode material, and belongs to the technical field of sodium ion batteries. In view of the problems of cumbersome process, long cycle, high energy consumption, etc. in the process of preparing modified sodium vanadium phosphate by the existing sol-gel method, the invention provides a sol-gel method coupled with a glycine combustion method, in which sodium nitrate is used as both a sodium source and an oxidant to promote the combustion of glycine, thereby shortening the reaction time. The size of the sodium vanadium phosphate particles, the thickness of the surface carbon coating layer, the number of gaps between the particles, and the specific surface area of ​​the material can be further optimized and regulated by adding appropriate amounts of glycine and sodium nitrate in the system, so that the prepared sodium vanadium phosphate positive electrode material has better performance. At the same time, the introduction of the glycine combustion method can change the precursor before calcination from viscosity to brittleness, significantly improving its processing performance. At the same time, assisted by the incorporation of a small amount of fluorine and nitrogen co-doped carbon nanotubes, excellent sodium storage performance is finally obtained.

Description

Technical Field

[0001] The invention belongs to the technical field of sodium ion batteries, and in particular relates to a method for preparing a composite carbon-modified sodium vanadium phosphate positive electrode material. Background Art

[0002] At present, electrochemical storage for large-scale energy storage applications is an important part of the new power system and has received extensive attention and high attention in the industry. Sodium-ion batteries are considered to have great application prospects in the field of large-scale energy storage due to their abundant sodium resources, low cost, high energy density and service life, and similar working principles to lithium-ion batteries.

[0003] As the key to the industrial application of sodium ion batteries, electrode materials are the focus of basic research and large-scale preparation technology development in the field of sodium ion batteries. In terms of positive electrode materials, sodium vanadium phosphate materials with NASICON structure in polyanionic material systems have three-dimensional ion diffusion channels, fast sodium ion diffusion rate, good thermal stability, and have obvious advantages over other types of positive electrode materials in terms of power density, cycle life, safety, and high and low temperature service performance. However, sodium vanadium phosphate materials have the problem of poor conductivity, which seriously affects the excellent cycle stability and rate performance of this type of positive electrode material. At present, researchers mostly use carbon coating to improve this problem, and the sol-gel method is a more commonly used carbon coating method in this material system. For example, the invention patent with the authorization announcement number CN 113735092B adds ethylene glycol to the sol-gel system with oxalic acid as the complexing agent, reacts at a high temperature of 170-200°C for 10-16 hours, and then undergoes a two-step calcination process to obtain a carbon-coated sodium vanadium phosphate composite material. The invention patent with the authorization announcement number CN 104733731B prepares a carbon-coated sodium vanadium phosphate composite material by adding glucose to the sol-gel system, hydrothermal glycosylation reaction and high-temperature calcination. The above preparation methods can improve the performance of sodium vanadium phosphate to varying degrees, but the preparation process is relatively cumbersome, with a long cycle and high energy consumption, and there are many problems faced in large-scale expansion. Summary of the invention

[0004] In view of the problems of complicated process, long cycle and high energy consumption in the existing sol-gel method for preparing modified sodium vanadium phosphate, the present invention provides a preparation method that couples the sol-gel method with the glycine combustion method to shorten the reaction time and improve the efficiency. At the same time, a small amount of fluorine-nitrogen co-doped carbon nanotubes are added to finally obtain excellent sodium storage performance.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for preparing a composite carbon-modified sodium vanadium phosphate positive electrode material comprises the following steps:

[0007] Step 1, weighing raw materials with a molar ratio of ammonium metavanadate: sodium nitrate: diammonium phosphate: glycine: citric acid = 2: 3~5: 3: 10: 4~8, first adding the ammonium metavanadate: sodium nitrate: diammonium phosphate in the raw materials to deionized water in a molar ratio and fully stirring until dissolved, then adding citric acid and glycine in the raw materials and continuing to stir to completely dissolve the raw materials to form a uniform aqueous solution, then adding fluorine and nitrogen co-doped carbon nanotubes to the aqueous solution, and ultrasonically dispersing to obtain a uniform suspension;

[0008] Step 2: The suspension is stirred at a constant temperature of 65-85°C for reaction; then transferred to a forced air oven and baked at 90-120°C for 1-3 hours to obtain a fluffy dry gel.

[0009] Step 3, grinding the dried gel through a 200-mesh sieve to obtain a powder with uniform particle size, and then pretreating the powder at 350-450°C at a heating rate of 1-3°C / min under argon atmosphere for 2-4 hours, and then heat treating at 750-850°C at a heating rate of 5-8°C / min for 3-6 hours to obtain a composite carbon-modified sodium vanadium phosphate positive electrode material.

[0010] Preferably, the raw materials are weighed in a molar ratio of ammonium metavanadate: sodium nitrate: diammonium phosphate: glycine: citric acid = 2: 3~5: 3: 10: 4~8.

[0011] Furthermore, the amount of fluorine-nitrogen co-doped carbon nanotubes used is controlled to be 0.2-0.6 of the molar number of ammonium metavanadate. Preferably, the amount of fluorine-nitrogen co-doped carbon nanotubes used is controlled to be 0.3-0.5 of the molar number of ammonium metavanadate.

[0012] Furthermore, the concentration of the aqueous solution of ammonium metavanadate is controlled at 0.3-0.4 mol / L.

[0013] Furthermore, in the step 1, the ultrasonic dispersion time after adding the fluorine-nitrogen co-doped carbon nanotubes to the aqueous solution is 20 to 40 minutes.

[0014] Further, in step 2, the suspension is stirred at a constant temperature of 65-85° C., and the uniformly dispersed suspension is stirred at a constant temperature in a water bath at 65-85° C. for 2-5 hours. Preferably, the uniformly dispersed suspension is stirred at a constant temperature in a water bath at 70-80° C. for 2.5-4 hours.

[0015] Preferably, the temperature rising mechanism of the powder preheating treatment in step 3 is to increase the temperature to 370-430°C at a heating rate of 1.5-2.5°C / min, and the constant temperature time is 2.5-3.5 hours; the temperature rising mechanism of the heat treatment is to increase the temperature to 770-830°C at a heating rate of 6-7°C / min, and the constant temperature time is 4-5 hours.

[0016] Furthermore, the preparation method of fluorine-nitrogen co-doped carbon nanotubes is as follows: adding carbon nanotube raw materials to trimethylamine trihydrofluoric acid aqueous solution and stirring evenly; then, transferring the suspension to a hydrothermal kettle for hydrothermal reaction, filtering and drying the filtrate to obtain the final product of fluorine-nitrogen co-doped carbon nanotubes.

[0017] Furthermore, the purity of the selected carbon nanotube raw material is above 95%; preferably, the purity of the carbon nanotube raw material is above 96%.

[0018] Furthermore, the concentration of the trimethylamine trihydrofluoric acid aqueous solution used is 25% to 35%; preferably, the concentration of the trimethylamine trihydrofluoric acid aqueous solution is 27% to 33%.

[0019] Furthermore, the solid content of the carbon nanotubes in the solution is controlled at 1-5 mg / mL. Preferably, the solid content of the carbon nanotubes in the solution is controlled at 2-4 mg / mL.

[0020] Furthermore, the reaction temperature of the hydrothermal reaction is 200-220° C., and the reaction time is 2-4 hours. Preferably, the hydrothermal reaction temperature is 205-215° C., and the reaction time is 2.5-3.5 hours.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] The present invention aims at the problems of long reaction time and high energy consumption of a single sol-gel method, and introduces glycine and sodium nitrate into the system to achieve coupling of the sol-gel method and the glycine combustion method. Sodium nitrate serves as both a sodium source and an oxidant to promote the combustion of glycine, thereby shortening the reaction time. By adding appropriate amounts of glycine and sodium nitrate into the system, the size of the sodium vanadium phosphate particles, the thickness of the surface carbon coating layer, the number of gaps between the particles and the specific surface area of ​​the material can be further optimized and regulated, thereby making the prepared sodium vanadium phosphate positive electrode material have more excellent performance. At the same time, the introduction of the glycine combustion method can change the precursor before calcination from viscosity to brittleness, significantly improving its processing performance.

[0023] The present invention also adds a small amount of fluorine-nitrogen co-doped carbon nanotubes into the system to further improve the cycle stability and rate performance of the sodium vanadium phosphate material. Compared with conventional acid-treated (carboxylated) carbon nanotubes, fluorine-nitrogen co-doped carbon nanotubes have better wettability and dispersibility in aqueous solution and better stability after dispersion. Therefore, a better dispersion effect and uniformity of the carbon nanotubes in the final sodium vanadium phosphate product are guaranteed. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1This is the X-ray diffraction pattern of the product of Example 1 of the present invention, indicating that the purity of the sodium vanadium phosphate component in the prepared material is very high.

[0025] Figure 2 This is a scanning electron microscope photo of the product of Example 1 of the present invention. It can be seen that the particle size of the prepared sodium vanadium phosphate powder is about 5 microns. At the same time, it can be found that each individual micron particle is formed by the accumulation of many nanoparticles, and there are abundant accumulation pores between the nanoparticles.

[0026] Figure 3 This is the cycle performance of the product of Example 1 of the present invention. It can be seen that the material has excellent sodium storage cycle stability. The capacity retention rate after 1000 charge and discharge cycles is 81.2%.

[0027] Figure 4 The rate performance of the product of Example 1 of the present invention. The test results show that the rate performance of the material is excellent. The rate capacity retention rate at 10C current density can still reach more than 80%. DETAILED DESCRIPTION

[0028] Example 1

[0029] In the first step, the raw materials are weighed according to the molar ratio of ammonium metavanadate: sodium nitrate: diammonium phosphate: glycine: citric acid = 2:3:3:10:4. First, add ammonium metavanadate: sodium nitrate: diammonium phosphate in a molar ratio to deionized water and stir thoroughly until dissolved. Among them, the concentration of the aqueous solution of ammonium metavanadate is controlled at 0.4 mol / L. After that, add the pre-weighed citric acid and glycine according to the above ratio and continue to stir to completely dissolve the raw materials to form a uniform aqueous solution. Finally, fluorine-nitrogen co-doped carbon nanotubes are added to the solution, and ultrasonic dispersion is performed for 40 minutes to obtain a uniform suspension. The amount of fluorine-nitrogen co-doped carbon nanotubes used is controlled to 0.2 of the molar number of ammonium metavanadate.

[0030] In the second step, the uniformly dispersed suspension was placed in a water bath at 65°C and stirred. After reacting for 5 hours, it was transferred to a forced air oven and baked at 90°C for 3 hours to obtain a fluffy dry gel.

[0031] In the third step, the prepared dry gel was ground through a 200-mesh sieve to obtain a powder with a relatively uniform particle size. After that, the powder was pretreated at 350°C at a heating rate of 3°C / min for 4 hours under the protection of an argon atmosphere, and then heat-treated at 850°C at a heating rate of 5°C / min for 3 hours to obtain a composite carbon-modified sodium vanadium phosphate positive electrode material.

[0032] The preparation process of fluorine-nitrogen co-doped carbon nanotubes is as follows:

[0033] Add the carbon nanotube raw material with a purity of 95% to the trimethylamine trihydrofluoric acid aqueous solution with a mass concentration of 25% and stir evenly. The solid content of carbon nanotubes in the solution is controlled at 5 mg / mL. After that, transfer the suspension to a hydrothermal kettle and react at 200°C for 4 hours. After filtration, dry the filtrate to obtain the final product.

[0034] The sodium ion battery performance evaluation of the composite carbon modified sodium vanadium phosphate positive electrode material is carried out using a conventional 2025 type button half-cell. Specifically, nitrogen methyl pyrrolidone is used as a solvent, and the positive electrode material, PVDF and Super P are fully mixed in a ball mill at a mass ratio of 9:0.5:0.5, and then evenly coated on aluminum foil using a coater, punched and accurately weighed. The electrode sheet is transferred to an argon-protected glove box, and the battery is assembled with a metal sodium sheet as a reference / counter electrode, 0.8 mol / L NaClO4 / EC:DMC:EMC (1:1:1) as an electrolyte, and a glass fiber membrane as a diaphragm. After aging for 12 hours, the battery test is carried out. The test results show that the first discharge capacity of the positive electrode material at a current density of 0.5 C is 102 mAh / g, the first coulomb efficiency is 91.0%, the capacity retention rate after 1000 cycles is 81.2%, and the rate capacity retention rate at a current density of 10 C is 82.5%.

[0035] Example 2

[0036] In the first step, the raw materials are weighed according to the molar ratio of ammonium metavanadate: sodium nitrate: diammonium phosphate: glycine: citric acid = 2:4:3:10:5. First, add ammonium metavanadate: sodium nitrate: diammonium phosphate in a molar ratio into deionized water and stir thoroughly until dissolved. Among them, the concentration of the aqueous solution of ammonium metavanadate is controlled at 0.35 mol / L. After that, add the pre-weighed citric acid and glycine according to the above ratio and continue stirring to completely dissolve the raw materials to form a uniform aqueous solution. Finally, add fluorine-nitrogen co-doped carbon nanotubes to the solution, and ultrasonically disperse for 30 minutes to obtain a uniform suspension. The amount of fluorine-nitrogen co-doped carbon nanotubes used is controlled to 0.4 of the molar number of ammonium metavanadate.

[0037] In the second step, the uniformly dispersed suspension was placed in a water bath at 85°C and stirred. After reacting for 2 hours, it was transferred to a forced air oven and baked at 120°C for 1 hour to obtain a fluffy dry gel.

[0038] In the third step, the prepared dry gel was ground through a 200-mesh sieve to obtain a powder with a relatively uniform particle size. After that, the powder was pretreated at 400°C at a heating rate of 2°C / min for 3 hours under the protection of an argon atmosphere, and then heat-treated at 750°C at a heating rate of 6°C / min for 4 hours to obtain a composite carbon-modified sodium vanadium phosphate positive electrode material.

[0039] The preparation process of fluorine-nitrogen co-doped carbon nanotubes is as follows:

[0040] Add the carbon nanotube raw material with a purity of 95% to the trimethylamine trihydrofluoric acid aqueous solution with a mass concentration of 30% and stir evenly. The solid content of carbon nanotubes in the solution is controlled at 1 mg / mL. After that, transfer the suspension to a hydrothermal kettle and react at 220°C for 2 hours. After filtration, dry the filtrate to obtain the final product.

[0041] The sodium ion battery test results show that the first discharge capacity of the positive electrode material at a current density of 0.5 C is 106 mAh / g, the first coulombic efficiency is 93.7%, the capacity retention rate after 1000 cycles is 84.5%, and the rate capacity retention rate at a current density of 10 C is 83.3%.

[0042] Example 3

[0043] In the first step, the raw materials are weighed according to the molar ratio of ammonium metavanadate: sodium nitrate: diammonium phosphate: glycine: citric acid = 2:5:3:10:8. First, add ammonium metavanadate: sodium nitrate: diammonium phosphate in molar ratio to deionized water and stir thoroughly until dissolved. Among them, the concentration of the aqueous solution of ammonium metavanadate is controlled at 0.3 mol / L. After that, add the pre-weighed citric acid and glycine according to the above ratio and continue to stir to completely dissolve the raw materials to form a uniform aqueous solution. Finally, fluorine-nitrogen co-doped carbon nanotubes are added to the solution, and ultrasonic dispersion is performed for 20 minutes to obtain a uniform suspension. The amount of fluorine-nitrogen co-doped carbon nanotubes used is controlled to 0.6 of the molar number of ammonium metavanadate.

[0044] In the second step, the uniformly dispersed suspension was placed in a water bath at 75°C and stirred. After reacting for 3 hours, it was transferred to a forced air oven and baked at 110°C for 2 hours to obtain a fluffy dry gel.

[0045] In the third step, the prepared dry gel was ground through a 200-mesh sieve to obtain a powder with a relatively uniform particle size. After that, the powder was pretreated at 450°C at a heating rate of 1°C / min for 2 hours under argon atmosphere protection, and then heat-treated at 800°C at a heating rate of 8°C / min for 6 hours to obtain a composite carbon-modified sodium vanadium phosphate positive electrode material.

[0046] Add the carbon nanotube raw material with a purity of >98% to a 35% mass concentration of trimethylamine trihydrofluoric acid aqueous solution and stir evenly. The solid content of carbon nanotubes in the solution is controlled at 3 mg / mL. Then, transfer the suspension to a hydrothermal kettle and react at 210°C for 3 hours. After filtration, dry the filtrate to obtain the final product.

[0047] The sodium ion battery test results show that the first discharge capacity of the positive electrode material at a current density of 0.5 C is 110 mAh / g, the first coulombic efficiency is 95.5%, the capacity retention rate after 1000 cycles is 88.4%, and the rate capacity retention rate at a current density of 10 C is 85.1%.

[0048] Example 4

[0049] In the first step, the raw materials are weighed according to the molar ratio of ammonium metavanadate: sodium nitrate: diammonium phosphate: glycine: citric acid = 2:5:3:10:8. First, add ammonium metavanadate: sodium nitrate: diammonium phosphate in molar ratio into deionized water and stir thoroughly until dissolved. Among them, the concentration of the aqueous solution of ammonium metavanadate is controlled at 0.32 mol / L. After that, add the pre-weighed citric acid and glycine according to the above ratio and continue stirring to completely dissolve the raw materials to form a uniform aqueous solution. Finally, add fluorine-nitrogen co-doped carbon nanotubes to the solution and ultrasonically disperse for 25 minutes to obtain a uniform suspension. The amount of fluorine-nitrogen co-doped carbon nanotubes used is controlled to 0.5 of the molar number of ammonium metavanadate.

[0050] In the second step, the uniformly dispersed suspension was placed in a water bath at 80° C. and stirred. After reacting for 2.5 hours, the suspension was transferred to a forced air oven and baked at 100° C. for 2.5 hours to obtain a fluffy dry gel.

[0051] In the third step, the prepared dry gel was ground through a 200-mesh sieve to obtain a powder with a relatively uniform particle size. After that, the powder was pretreated at 430°C at a heating rate of 1.5°C / min for 2.5 hours under argon atmosphere protection, and then heat-treated at 780°C at a heating rate of 7°C / min for 5 hours to obtain a composite carbon-modified sodium vanadium phosphate positive electrode material.

[0052] Add the carbon nanotube raw material with a purity of >96% to a trimethylamine trihydrofluoric acid aqueous solution with a mass concentration of 28% and stir evenly. The solid content of carbon nanotubes in the solution is controlled at 2 mg / mL. After that, transfer the suspension to a hydrothermal kettle and react at 215°C for 2.5 hours. After filtration, dry the filtrate to obtain the final product.

[0053] The sodium ion battery test results show that the first discharge capacity of the positive electrode material at a current density of 0.5 C is 112 mAh / g, the first coulombic efficiency is 94.6%, the capacity retention rate after 1000 cycles is 89.9%, and the rate capacity retention rate at a current density of 10 C is 86.7%.

[0054] Example 5

[0055] In the first step, the raw materials are weighed according to the molar ratio of ammonium metavanadate: sodium nitrate: diammonium phosphate: glycine: citric acid = 2:5:3:10:4. First, add ammonium metavanadate: sodium nitrate: diammonium phosphate in molar ratio into deionized water and stir thoroughly until dissolved. Among them, the concentration of the aqueous solution of ammonium metavanadate is controlled at 0.38 mol / L. After that, add the pre-weighed citric acid and glycine according to the above ratio and continue stirring to completely dissolve the raw materials to form a uniform aqueous solution. Finally, add fluorine-nitrogen co-doped carbon nanotubes to the solution and ultrasonically disperse for 35 minutes to obtain a uniform suspension. The amount of fluorine-nitrogen co-doped carbon nanotubes used is controlled to 0.3 of the molar number of ammonium metavanadate.

[0056] In the second step, the uniformly dispersed suspension was placed in a water bath at 70° C. and stirred. After reacting for 4 hours, the suspension was transferred to a forced air oven and baked at 110° C. for 1.5 hours to obtain a fluffy dry gel.

[0057] In the third step, the prepared dry gel was ground through a 200-mesh sieve to obtain a powder with a relatively uniform particle size. After that, the powder was pretreated at 370°C at a heating rate of 2.5°C / min for 2.5 hours under argon atmosphere protection, and then heat-treated at 830°C at a heating rate of 5°C / min for 4 hours to obtain a composite carbon-modified sodium vanadium phosphate positive electrode material.

[0058] Add the carbon nanotube raw material with a purity of >97% to a 27% mass concentration of trimethylamine trihydrofluoric acid aqueous solution and stir evenly. The solid content of carbon nanotubes in the solution is controlled at 2 mg / mL. After that, transfer the suspension to a hydrothermal kettle and react at 205°C for 3.5 hours. After filtration, dry the filtrate to obtain the final product.

[0059] The sodium ion battery test results show that the first discharge capacity of the positive electrode material at a current density of 0.5 C is 104 mAh / g, the first coulombic efficiency is 97.0%, the capacity retention rate after 1000 cycles is 82.9%, and the rate capacity retention rate at a current density of 10 C is 84.4%.

[0060] Example 6

[0061] In the first step, the raw materials are weighed according to the molar ratio of ammonium metavanadate: sodium nitrate: diammonium phosphate: glycine: citric acid = 2:3:3:10:8. First, add ammonium metavanadate: sodium nitrate: diammonium phosphate in molar ratio to deionized water and stir thoroughly until dissolved. Among them, the concentration of the aqueous solution of ammonium metavanadate is controlled at 0.3 mol / L. After that, add the pre-weighed citric acid and glycine according to the above ratio and continue to stir to completely dissolve the raw materials to form a uniform aqueous solution. Finally, fluorine-nitrogen co-doped carbon nanotubes are added to the solution, and ultrasonic dispersion is performed for 20 minutes to obtain a uniform suspension. The amount of fluorine-nitrogen co-doped carbon nanotubes used is controlled to 0.3 of the molar number of ammonium metavanadate.

[0062] In the second step, the uniformly dispersed suspension was placed in a water bath at 65°C and stirred. After reacting for 3 hours, it was transferred to a forced air oven and baked at 100°C for 2 hours to obtain a fluffy dry gel.

[0063] In the third step, the prepared dry gel was ground through a 200-mesh sieve to obtain a powder with a relatively uniform particle size. After that, the powder was pretreated at 450°C at a heating rate of 2.5°C / min under argon atmosphere for 2 hours, and then heat-treated at 830°C at a heating rate of 7°C / min for 6 hours to obtain a composite carbon-modified sodium vanadium phosphate positive electrode material.

[0064] Add the carbon nanotube raw material with a purity of >98% to a 35% mass concentration of trimethylamine trihydrofluoric acid aqueous solution and stir evenly. The solid content of carbon nanotubes in the solution is controlled at 4 mg / mL. Then, transfer the suspension to a hydrothermal kettle and react at 210°C for 3 hours. After filtration, dry the filtrate to obtain the final product.

[0065] The sodium ion battery test results show that the first discharge capacity of the positive electrode material at a current density of 0.5 C is 109 mAh / g, the first coulombic efficiency is 95.9%, the capacity retention rate after 1000 cycles is 90.6%, and the rate capacity retention rate at a current density of 10 C is 87.1%.

[0066] The contents not described in detail in the specification of the present invention belong to the prior art known to the professional and technical personnel in the field. Although the illustrative specific embodiments of the present invention are described above to facilitate the understanding of the present invention by the technical personnel in the field, it should be clear that the present invention is not limited to the scope of the specific embodiments. For the ordinary technical personnel in the field, as long as various changes are within the spirit and scope of the present invention defined and determined by the attached claims, these changes are obvious, and all inventions and creations using the concept of the present invention are protected.

Claims

1. A method for preparing a composite carbon-modified sodium vanadium phosphate positive electrode material, characterized in that: The following steps are involved: Step 1, weighing raw materials with a molar ratio of ammonium metavanadate: sodium nitrate: diammonium phosphate: glycine: citric acid = 2: 3~5: 3: 10: 4~8, first adding the ammonium metavanadate: sodium nitrate: diammonium phosphate in the raw materials to deionized water in a molar ratio and fully stirring until dissolved, then adding citric acid and glycine in the raw materials and continuing to stir to completely dissolve the raw materials to form a uniform aqueous solution, then adding fluorine and nitrogen co-doped carbon nanotubes to the aqueous solution, and ultrasonically dispersing to obtain a uniform suspension; Step 2, stirring the suspension at a constant temperature of 65-85° C. and reacting; then transferring the suspension to a forced air oven and baking at 90-120° C. for 1-3 hours to obtain a fluffy dry gel; Step 3, grinding the dried gel through a 200-mesh sieve to obtain a powder with uniform particle size, and then pre-treating the powder at 350-450°C at a heating rate of 1-3°C / min for 2-4 hours under argon atmosphere protection, and then heat-treating at 750-850°C at a heating rate of 5-8°C / min for 3-6 hours to obtain a composite carbon-modified sodium vanadium phosphate positive electrode material; The preparation method of fluorine-nitrogen co-doped carbon nanotubes comprises the following steps: adding carbon nanotube raw materials to trimethylamine trihydrofluoric acid aqueous solution and stirring evenly; then, transferring the suspension to a hydrothermal kettle for hydrothermal reaction, filtering and drying the filtrate to obtain the final product, which is fluorine-nitrogen co-doped carbon nanotubes.

2. The method for preparing a composite carbon-modified sodium vanadium phosphate positive electrode material according to claim 1, characterized in that: The raw materials are weighed in a molar ratio of ammonium metavanadate: sodium nitrate: diammonium phosphate: glycine: citric acid = 2: 3.5-4.5: 3: 10: 5-7.

3. The method for preparing a composite carbon-modified sodium vanadium phosphate positive electrode material according to claim 1, characterized in that: The usage amount of fluorine and nitrogen co-doped carbon nanotubes is controlled to be 0.2-0.6 of the molar number of ammonium metavanadate.

4. The method for preparing a composite carbon-modified sodium vanadium phosphate positive electrode material according to claim 1, characterized in that: The concentration of the aqueous solution of ammonium metavanadate is controlled at 0.3~0.4 mol / L.

5. The method for preparing a composite carbon-modified sodium vanadium phosphate positive electrode material according to claim 1, characterized in that: The ultrasonic dispersion time after adding fluorine and nitrogen co-doped carbon nanotubes to the aqueous solution in step 1 is 20 to 40 minutes.

6. The method for preparing a composite carbon-modified sodium vanadium phosphate positive electrode material according to claim 1, characterized in that: In the step 2, the suspension is stirred at a constant temperature of 65-85° C. for reaction. Specifically, the uniform suspension is stirred at a constant temperature in a water bath at 65-85° C. for reaction time of 2-5 hours.

7. The method for preparing a composite carbon-modified sodium vanadium phosphate positive electrode material according to claim 1, characterized in that: The temperature rise mechanism of the powder pretreatment in step 3 is to increase the temperature to 370-430°C at a heating rate of 1.5-2.5°C / min, and the constant temperature time is 2.5-3.5 hours; the temperature rise mechanism of the heat treatment is to increase the temperature to 770-830°C at a heating rate of 6-7°C / min, and the constant temperature time is 4-5 hours.

8. The method for preparing a composite carbon-modified sodium vanadium phosphate positive electrode material according to claim 1, characterized in that: The purity of the selected carbon nanotube raw material is above 95%; the concentration of the trimethylamine trihydrofluoric acid aqueous solution used is 25%-35%; and the solid content of the carbon nanotube in the solution is controlled at 1-5 mg / mL.

9. The method for preparing a composite carbon-modified sodium vanadium phosphate positive electrode material according to claim 1, characterized in that: The reaction temperature of the hydrothermal reaction is 200-220° C., and the reaction time is 2-4 hours.

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