A single crystal sodium vanadium fluorophosphate / carbon positive electrode material and its preparation method and application

Abstract

The present invention discloses a single-crystal sodium vanadyl fluorophosphate / carbon cathode material, its preparation method, and application. A vanadium source and a carbon source are heated and reduced in a solvent, followed by the addition of a sodium source, a phosphorus source, and a fluorine source, followed by stirring and mixing. The mixture is then hydrothermally heated at 120-200°C for 1-50 hours. The resulting sodium vanadyl fluorophosphate is annealed at 350-800°C for 0.1-50 hours under an inert atmosphere to obtain an amorphous carbon-coated sodium vanadyl fluorophosphate cathode material. A carbon source is used as a reducing agent, with the carbon source content being 105%-130% of the amount required to reduce pentavalent or tetravalent vanadium to trivalent vanadium. The excess carbon source is used for in-situ carbon coating. This material improves the poor intrinsic electronic conductivity and structural stability of the sodium vanadyl fluorophosphate through in-situ carbon coating and single crystal synthesis, effectively improving the material's capacity, rate capability, and cycle performance.

Description

Technical Field

[0001] The present invention relates to the field of sodium ion battery positive electrode materials, and in particular to a single crystal sodium vanadium fluorophosphate / carbon positive electrode material and a preparation method and application thereof. Background Art

[0002] Since their commercialization in the 1990s, lithium-ion batteries have been widely used in consumer electronics, electric vehicles, and large-scale energy storage devices. This has increased lithium resource consumption and significantly driven up the price of lithium-ion batteries. The development of a low-cost, high-performance, and long-life energy storage battery is urgently needed. Because sodium (-2.71V) is close to the standard redox potential of lithium (-3.04V), sodium-ion batteries also have a high operating voltage. Furthermore, the abundant sodium reserves in the Earth's crust, reaching 2.83%, far exceeding the concentration of lithium (0.0065%), contribute to their low cost. Furthermore, sodium-ion batteries have a high desolvation capacity in electrolytes, making them promising future applications in cost-effective low-speed electric vehicles, power tools, and energy storage. Furthermore, aluminum foil can be used as the current collector in sodium-ion batteries, replacing the copper foil used as the negative electrode current collector in lithium-ion batteries. This not only significantly reduces the price of sodium-ion batteries, but also reduces the weight of the current collector and addresses the overdischarge issue. As the most expensive material in the battery and the key material that determines the battery's operating voltage, high-performance positive electrode sodium storage materials are the focus of current research.

[0003] Sodium vanadyl fluorophosphate has a stable three-dimensional open framework structure, which allows sodium ions to be reversibly embedded and deintercalated in three directions with little structural change, and has a high theoretical capacity (130 mAh g -1 ) and a high average operating voltage (~3.8V), making it a promising cathode material for sodium-ion batteries. However, sodium vanadyl fluorophosphate's inherently low electronic conductivity and the brittle nature of polycrystalline materials severely hamper its electrochemical performance and application. Summary of the Invention

[0004] In response to the problems existing in the prior art, the present invention proposes a single-crystal sodium vanadium fluorophosphate / carbon positive electrode material and its preparation method and application. This material improves the poor intrinsic electronic conductivity of sodium vanadium fluorophosphate and the problem of easy breakage and shedding of polycrystalline materials during cycling through in-situ carbon coating and single crystal synthesis, effectively improving the material's capacity, rate and cycle performance.

[0005] The present invention is achieved through the following technical solutions:

[0006] A method for preparing a single-crystal sodium vanadyl fluorophosphate / carbon positive electrode material comprises the following steps:

[0007] 1) obtaining a sodium source, a vanadium source, a fluorine source, and a phosphorus source, and a carbon source according to an element ratio of Na:V:P:F=3:2:2:x, wherein 1<x<3;

[0008] 2) Add the vanadium source and carbon source in step 1) to the solvent, heat and stir until the solution turns light green, add the sodium source, fluorine source and phosphorus source to the above solution and continue stirring to mix evenly to obtain a precursor solution; the carbon source is used as a reducing agent to reduce the V 5+ Restore to V 3+ , the carbon source content is 105% to 130% of the amount required to reduce pentavalent or tetravalent vanadium to trivalent vanadium;

[0009] 3) transferring the precursor solution obtained in step 2) to a hydrothermal reactor, heating it to 120-200° C., maintaining the temperature for 1-50 hours, and then naturally cooling it to room temperature to obtain sodium vanadium fluorophosphate single crystal powder (Na3V2O2(PO4)2F);

[0010] 4) The sodium vanadyl fluorophosphate single crystal powder obtained in step 3) is heated to 350°C to 800°C at a heating rate of 2°C / min to 10°C / min under an inert atmosphere, kept at this temperature for 0.1h to 10h, and then naturally cooled to room temperature to obtain a single crystal sodium vanadyl fluorophosphate / carbon positive electrode material.

[0011] Furthermore, in step 1), the vanadium source is one or more of ammonium metavanadate, vanadium pentoxide and vanadyl sulfate; the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate and ammonium phosphate; the carbon source is formic acid (HCOOH), gallic acid (C7H6O5·H2O), oxalic acid (H2C2O4·2H2O), tartaric acid (C4H6O6), malic acid (C4H6O5), citric acid (C6H8O7·H2O) and EDTA (C 10 H 16 N2O8) or more.

[0012] Furthermore, the solvent used in the wet hydrothermal treatment in step 3) is one or both of ethanol and water.

[0013] Furthermore, in step 2), the first stirring speed is 100-800 r / min, the stirring temperature is 40-100° C., and the stirring time is 0.1-6 h.

[0014] Furthermore, the sodium source in step 2) is one or more of sodium fluoride, sodium chloride, sodium carbonate, sodium hydroxide, sodium sulfate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate and sodium nitrate; and the fluorine source is one or more of sodium fluoride, ammonium fluoride, potassium fluoride and magnesium fluoride.

[0015] Furthermore, in step 4), the inert atmosphere is one or both of argon and nitrogen.

[0016] A single crystal sodium vanadyl fluorophosphate / carbon positive electrode material, wherein the positive electrode material is a structure of amorphous carbon-coated sodium vanadyl fluorophosphate single crystal.

[0017] Compared with the prior art, the present invention has the following beneficial technical effects:

[0018] The present invention adopts a strategy of simultaneously improving the electronic conductivity and structural stability of the material, and prepares a single-crystal sodium vanadium oxyfluorophosphate / carbon composite material by hydrothermal synthesis of single crystals and in-situ carbon coating combined with high-temperature annealing. 1) The single-crystal sodium vanadium oxyfluorophosphate / carbon composite material of the present invention has few defects, uniform particle size, excellent capacity rate performance, and ultra-long cycle life. The single-crystal sodium vanadium oxyfluorophosphate / carbon composite material is obtained by controlling the fluorine content. The fluorine content has an important influence on the yield of the single-crystal material. The volume change of the single-crystal material during charging and discharging is small, and no breakage or falling off occurs, which can effectively improve the structural stability of the material. 2) The carbon source of the present invention is used as a reducing agent to convert V 5+ Restore to V 3+ The reducing agent content is 105% to 130% of the amount required to reduce pentavalent or tetravalent vanadium to trivalent vanadium, and the excess carbon source is used for in-situ carbon coating. Through in-situ carbon coating, the electronic conductivity of the surface of the material particles can be greatly improved; 3) The preparation method of the single crystal sodium vanadium oxyfluorophosphate / carbon composite material of the present invention is simple in process, superior in performance, and easy to achieve large-scale production, which is of great significance to the commercialization of sodium ion batteries.

[0019] The single-crystal sodium vanadium oxyfluorophosphate / carbon composite material prepared by the present invention is consistent with the tetragonal crystal structure of sodium vanadium oxyfluorophosphate through X-ray diffraction testing. At the same time, the electrochemical performance test results show that single-crystal materials with different morphologies have excellent cycle stability and significantly enhanced capacity rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a microscopic SEM image of the sample of Example 1 of the present invention;

[0021] Figure 2 This is a microscopic SEM image of a comparative example sample of the present invention;

[0022] Figure 3 The XRD pattern of the sample of Example 1 of the present invention;

[0023] Figure 4 The electrochemical performance curves of Examples 1 and 2 of the present invention and the comparative example samples are shown. DETAILED DESCRIPTION

[0024] The present invention will be further described in detail below with reference to specific embodiments, which are intended to explain the present invention rather than to limit it.

[0025] The preparation method of a single crystal sodium vanadyl fluorophosphate / carbon positive electrode material of the present invention is prepared by hydrothermal combined with high temperature annealing through the following steps:

[0026] 1) taking a sodium source, a vanadium source, a phosphorus source, and a fluorine source according to Na:V:P:F=3:2:2:x, and taking a carbon source;

[0027] 2) Add the vanadium source and carbon source from step 1) to a beaker of solvent to form a solution, and heat and stir the solution until the solution turns light green. Add the sodium source, fluorine source, and phosphorus source to the solution and continue stirring for 0.5 h to obtain a uniformly mixed precursor solution.

[0028] 3) transferring the precursor solution in step 2) to a hydrothermal reactor, heating it to 120-200° C., maintaining the temperature for 1-50 hours, and then naturally cooling it to room temperature to obtain sodium vanadium fluorophosphate single crystal powder (Na3V2O2(PO4)2F);

[0029] 4) heating the sodium vanadyl fluorophosphate single crystal powder in step 3) to 350° C. to 800° C. at a heating rate of 2° C. / min to 10° C. / min under an inert atmosphere, keeping the temperature constant for 0.1 h to 10 h, and then naturally cooling to room temperature to obtain an amorphous carbon-coated sodium vanadyl fluorophosphate single crystal (sodium vanadyl fluorophosphate / carbon) positive electrode material;

[0030] In step 2), a carbon source is used as a reducing agent to convert V 5+ or V 4+ Restore to V 3+ The reducing agent is in excess to reduce pentavalent or tetravalent vanadium to 10% to 30% of the required amount of trivalent vanadium, and the excess carbon source is used for in-situ carbon coating. Specific embodiments

[0032] Example 1

[0033] (1) Weigh 0.004 mol of ammonium metavanadate, 0.005 mol of oxalic acid dihydrate, 0.006 mol of sodium fluoride, and 0.004 mol of ammonium dihydrogen phosphate;

[0034] (2) adding the ammonium metavanadate and oxalic acid dihydrate weighed in step (1) to a beaker containing 80 ml of deionized water, stirring for half an hour in a water bath at 80° C. with a magnetic stirring rotor speed of 400 r / min until a light green solution is formed, then adding the weighed sodium fluoride and ammonium dihydrogen phosphate to the solution and continuing to stir for 0.5 h to form a precursor solution;

[0035] (3) transferring the precursor solution in step (2) to a hydrothermal reactor with ethanol as the solvent, heating it to 180° C., keeping the temperature constant for 10 hours, and then naturally cooling it to room temperature to obtain sodium vanadium fluorophosphate single crystal powder (Na3V2O2(PO4)2F);

[0036] (4) heating the sodium vanadyl fluorophosphate single crystal powder in step 3) to 600° C. at a heating rate of 4° C. / min under an inert atmosphere, keeping the temperature constant for 2 h, and then naturally cooling to room temperature to obtain an amorphous carbon-coated sodium vanadyl fluorophosphate single crystal (sodium vanadyl fluorophosphate / carbon) positive electrode material;

[0037] Example 2

[0038] (1) Weigh 0.004 mol of ammonium metavanadate, 0.005 mol of citric acid, 0.006 mol of sodium fluoride, and 0.004 mol of phosphoric acid;

[0039] (2) adding the ammonium metavanadate and citric acid weighed in step (1) to a beaker containing 80 ml of deionized water, stirring for half an hour in a water bath at 70° C. with a magnetic stirring rotor speed of 500 r / min until a light green solution is formed, then adding the weighed sodium fluoride and ammonium dihydrogen phosphate to the solution and continuing to stir for 0.5 h to form a precursor solution;

[0040] (3) transferring the precursor solution in step (2) to a hydrothermal reactor with ethanol and water as solvent, heating to 180° C., keeping the temperature constant for 8 hours, and then naturally cooling to room temperature to obtain sodium vanadium fluorophosphate single crystal powder (Na3V2O2(PO4)2F);

[0041] (4) heating the sodium vanadyl fluorophosphate single crystal powder in step 3) to 350° C. at a heating rate of 4° C. / min under a nitrogen atmosphere, keeping the temperature constant for 1 hour, and then naturally cooling to room temperature to obtain an amorphous carbon-coated sodium vanadyl fluorophosphate single crystal (sodium vanadyl fluorophosphate / carbon) positive electrode material;

[0042] Example 3

[0043] (1) Weigh 0.002 mol of vanadium pentoxide, 0.005 mol of malic acid, 0.002 mol of ammonium fluoride, and 0.004 mol of sodium dihydrogen phosphate;

[0044] (2) adding the vanadium pentoxide and malic acid weighed in step (1) to a beaker containing 80 ml of deionized water, stirring for half an hour in a water bath at 80° C. with a magnetic stirring rotor speed of 500 r / min until a light green solution is formed, then adding weighed ammonium fluoride and sodium dihydrogen phosphate to the solution and continuing to stir for 0.5 h to form a precursor solution;

[0045] (3) transferring the precursor solution in step (2) to a hydrothermal reactor with water as the solvent, heating it to 180° C., keeping the temperature constant for 12 hours, and then naturally cooling it to room temperature to obtain sodium vanadium fluorophosphate single crystal powder (Na3V2O2(PO4)2F);

[0046] (4) heating the sodium vanadyl fluorophosphate single crystal powder in step 3) to 800° C. at a heating rate of 6° C. / min under an argon atmosphere, keeping the temperature constant for 0.1 h, and then naturally cooling to room temperature to obtain an amorphous carbon-coated sodium vanadyl fluorophosphate single crystal (sodium vanadyl fluorophosphate / carbon) positive electrode material;

[0047] Example 4

[0048] (1) Weigh 0.005 mol of vanadyl sulfate, 0.0065 mol of oxalic acid dihydrate, 0.0075 mol of sodium fluoride, and 0.005 mol of potassium dihydrogen phosphate;

[0049] (2) adding the vanadyl sulfate and oxalic acid dihydrate weighed in step (1) to a beaker containing 80 ml of deionized water, stirring for half an hour in a water bath at 90° C. with a magnetic stirring rotor speed of 400 r / min until a light green solution is formed, then adding weighed ammonium fluoride and sodium dihydrogen phosphate to the solution and continuing to stir for 0.5 h to form a precursor solution;

[0050] (3) transferring the precursor solution in step (2) to a hydrothermal reactor, heating it to 180° C., keeping the temperature constant for 15 h, and then naturally cooling it to room temperature to obtain sodium vanadium fluorophosphate single crystal powder (Na3V2O2(PO4)2F);

[0051] (4) heating the sodium vanadyl fluorophosphate single crystal powder in step 3) to 600° C. at a heating rate of 10° C. / min under an inert atmosphere, keeping the temperature constant for 10 h, and then naturally cooling to room temperature to obtain an amorphous carbon-coated sodium vanadyl fluorophosphate single crystal (sodium vanadyl fluorophosphate / carbon) positive electrode material;

[0052] Comparative Example

[0053] Sodium vanadium fluorophosphate polycrystalline (Na3V2O2(PO4)2F) material was prepared by sol-gel-high temperature sintering method, and the following steps were performed:

[0054] (1) Weigh 0.02 mol of ammonium metavanadate, 0.025 mol of oxalic acid dihydrate, 0.01 mol of sodium fluoride, 0.01 mol of sodium carbonate, and 0.02 mol of ammonium dihydrogen phosphate;

[0055] (2) dissolving the ammonium metavanadate and oxalic acid dihydrate weighed in step (1) into 100 ml of deionized water, stirring for 6 hours in a water bath at 80° C. with a magnetic stirring rotor speed of 500 r / min until a light green solution is formed, and then dissolving the weighed sodium fluoride and ammonium dihydrogen phosphate in the solution to form a precursor solution;

[0056] (3) The precursor solution obtained in step (2) was continuously stirred and heated to evaporate the solvent, and finally a sol-gel Na3V2O2(PO4)2F precursor 1 was obtained. The precursor 1 was dried by forced air at 80°C for 12 hours and then ground for 20 minutes to obtain a uniform powdered Na3V2O2(PO4)2F precursor 2;

[0057] (4) The powdered precursor 2 in step (3) was transferred to a tubular furnace and annealed under Ar atmosphere protection, heated to 600°C at a heating rate of 4°C / min, kept constant at this temperature for 8 hours, and then naturally cooled to room temperature to obtain the sodium vanadium fluorophosphate polycrystalline (sodium vanadium fluorophosphate / carbon) positive electrode material of the control group.

[0058] A series of physical properties were characterized for the sodium vanadyl fluorophosphate single crystal (sodium vanadyl fluorophosphate / carbon) cathode material prepared in Example 1, and the results are shown in the figure. Figure 1 This is an SEM image of sodium vanadium fluorophosphate single crystal material, which shows that the prepared material is a relatively regular micron-sized cube. Figure 2 This is an SEM image of a sodium vanadyl fluorophosphate polycrystalline material as a control example, illustrating that irregular polycrystalline particles are prepared by the sol-gel method. Figure 3 This is the XRD pattern of the sodium vanadium oxyfluorophosphate single crystal material prepared in Example 1. The X-ray diffraction test results show that the prepared single crystal material has good crystallinity.

[0059] The single crystal sodium vanadium fluorophosphate / carbon composite material prepared in Example 1-3 is mixed with the conductive agent super P / carbon nanotubes and the binder CMC in a mass ratio of 7:2:1 or 8:1:1 and dissolved in ultrapure water. The slurry is scraped onto aluminum foil and dried and rolled. The cut pieces are used as the positive electrode, the metal sodium sheet, hard carbon or sodium titanium phosphate are used as the negative electrode, the glass fiber membrane is used as the diaphragm, the solute is 1M NaPF6 or NaClO4, the solvent is a mixture of solvents such as EC (ethylene carbonate), PC (propylene carbonate) and DEC (diethyl carbonate), the additive is FEC with a mass fraction of 1% to 5% as the electrolyte, the aluminum foil is used as the current collector, and the batteries are assembled into CR2025 / CR2016 button-type sodium ion batteries for electrochemical performance testing. The results are as follows: Figure 4As shown, samples of single-crystalline sodium vanadium fluorophosphate / carbon composite materials synthesized using polycrystalline materials prepared by the sol-gel method and oxalic acid and citric acid as chelating agents were subjected to constant current charge and discharge tests at a current density of 1C. After single crystallization and in-situ carbon coating, the specific capacity and cycle stability of the material were significantly improved. After 200 cycles, the capacity of the comparative example and Examples 1 and 2 remained at 83.82mAh / g, 98.40mAh / g, and 112.78mAh / g, respectively, indicating that the synthesized single-crystalline sodium vanadium fluorophosphate / carbon composite material effectively improves the electrochemical performance of the material.

[0060] Example 5

[0061] (1) Weigh 0.004 mol of vanadyl sulfate, 0.0065 mol of formic acid, 0.006 mol of sodium chloride, 0.004 mol of phosphoric acid, and 0.004 mol of potassium fluoride;

[0062] (2) adding the vanadyl sulfate and oxalic acid dihydrate weighed in step (1) to a beaker containing 80 ml of deionized water, stirring for 0.1 hour in a water bath at 100° C. with a magnetic stirring rotor speed of 100 r / min until a light green solution is formed, then adding weighed sodium chloride, potassium fluoride and phosphoric acid to the solution and continuing to stir for 0.5 h to form a precursor solution;

[0063] (3) transferring the precursor solution in step (2) to a hydrothermal reactor, heating it to 120° C., keeping the temperature constant for 50 hours, and then naturally cooling it to room temperature to obtain sodium vanadium fluorophosphate single crystal powder (Na3V2O2(PO4)2F);

[0064] (4) heating the sodium vanadyl fluorophosphate single crystal powder in step 3) to 800° C. at a heating rate of 2° C. / min under an inert atmosphere, keeping the temperature constant for 0.1 h, and then naturally cooling to room temperature to obtain an amorphous carbon-coated sodium vanadyl fluorophosphate single crystal (sodium vanadyl fluorophosphate / carbon) positive electrode material;

[0065] Example 6

[0066] (1) Weigh 0.002 mol of vanadium pentoxide, 0.002 mol of vanadyl sulfate, 0.0065 mol of gallic acid, 0.006 mol of sodium carbonate, 0.004 mol of diammonium hydrogen phosphate, and 0.003 mol of magnesium fluoride;

[0067] (2) adding the vanadyl sulfate and oxalic acid dihydrate weighed in step (1) to a beaker containing 80 ml of deionized water, stirring for 3 hours in a water bath at 40° C. with a magnetic stirring rotor speed of 800 r / min until a light green solution is formed, then adding weighed sodium carbonate, magnesium fluoride and diammonium hydrogen phosphate to the solution and continuing to stir for 0.5 h to form a precursor solution;

[0068] (3) transferring the precursor solution in step (2) to a hydrothermal reactor, heating it to 200° C., keeping the temperature constant for 1 hour, and then naturally cooling it to room temperature to obtain sodium vanadium fluorophosphate single crystal powder (Na3V2O2(PO4)2F);

[0069] (4) heating the sodium vanadyl fluorophosphate single crystal powder in step 3) to 350° C. at a heating rate of 10° C. / min under an inert atmosphere, keeping the temperature constant for 10 h, and then naturally cooling to room temperature to obtain an amorphous carbon-coated sodium vanadyl fluorophosphate single crystal (sodium vanadyl fluorophosphate / carbon) positive electrode material;

[0070] Example 7

[0071] The difference from the embodiment is that the sodium source is one or more of sodium hydroxide, sodium sulfate, disodium hydrogen phosphate, sodium phosphate and sodium nitrate, the phosphorus source is one or more of disodium hydrogen phosphate, sodium phosphate and ammonium phosphate, and the carbon source is (HCOOH), (C7H6O5·H2O), tartaric acid (C4H6O6), and EDTA (C 10 H 16 N2O8) or more.

[0072] In response to the shortcomings of sodium vanadyl fluorophosphate and the current research status, the present invention adopts a strategy of simultaneously improving the electronic conductivity and structural stability of the material. Through hydrothermal and high-temperature annealing synthesis, in-situ carbon coating and large-scale single crystallization, a single-crystalline sodium vanadyl fluorophosphate / carbon composite material is prepared, which significantly improves the electrochemical performance of sodium vanadyl fluorophosphate and is of great significance to the commercialization of sodium-ion batteries.

[0073] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a single crystal sodium vanadyl fluorophosphate / carbon positive electrode material, characterized in that The following steps are involved: 1) A sodium source, a vanadium source, a fluorine source, and a phosphorus source are respectively obtained according to the element ratio of Na: V: P: F = 3:2:2:x, and a carbon source is obtained, wherein 1<x<3; the carbon source is one or more of formic acid, gallic acid, tartaric acid, malic acid, citric acid, and EDTA; 2) Add the vanadium source and carbon source in step 1) to the solvent, heat and stir until the solution turns light green, add the sodium source, fluorine source and phosphorus source to the above solution and continue stirring to mix evenly to obtain a precursor solution; the carbon source is used as a reducing agent to reduce the V 5+ Restore to V 3+ , the carbon source content is 105% to 130% of the amount required to reduce pentavalent or tetravalent vanadium to trivalent vanadium; 3) Transferring the precursor solution obtained in step 2) to a hydrothermal reactor, heating it to 180-200°C, maintaining the temperature for 1-50 hours, and then naturally cooling it to room temperature to obtain sodium vanadyl fluorophosphate single crystal powder; the solvent used in the hydrothermal reaction is water; 4) The sodium vanadyl fluorophosphate single crystal powder obtained in step 3) is heated to 350°C to 800°C at a heating rate of 2°C / min to 10°C / min under an inert atmosphere, maintained at this temperature for 0.1 h to 10 h, and then naturally cooled to room temperature to obtain a single crystal sodium vanadyl fluorophosphate / carbon positive electrode material.

2. The method for preparing a single crystal sodium vanadyl fluorophosphate / carbon cathode material according to claim 1, wherein: In step 1), the vanadium source is one or more of ammonium metavanadate, vanadium pentoxide and vanadyl sulfate; and the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate and ammonium phosphate.

3. The method for preparing the single crystal sodium vanadyl fluorophosphate / carbon positive electrode material according to claim 1, characterized in that: In step 2), the first stirring speed is 100 to 800 r / min, the stirring temperature is 40 to 100°C, and the stirring time is 0.1 to 6 h.

4. The method for preparing a single crystal sodium vanadyl fluorophosphate / carbon cathode material according to claim 1, characterized in that: The sodium source in step 2) is one or more of sodium fluoride, sodium chloride, sodium carbonate, sodium hydroxide, sodium sulfate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate and sodium nitrate; the fluorine source is one or more of sodium fluoride, ammonium fluoride, potassium fluoride and magnesium fluoride.

5. The method for preparing the single crystal sodium vanadyl fluorophosphate / carbon positive electrode material according to claim 1, characterized in that: In step 4), the inert atmosphere is one or both of argon and nitrogen.

6. A single crystal sodium vanadyl fluorophosphate / carbon positive electrode material prepared according to the preparation method according to any one of claims 1 to 5.

7. The single crystal sodium vanadyl fluorophosphate / carbon cathode material according to claim 6, characterized in that: The positive electrode material is a structure of amorphous carbon-coated sodium vanadium fluorophosphate single crystal.

8. Use of the single crystal sodium vanadyl fluorophosphate / carbon positive electrode material according to claim 6 or 7 in a sodium ion battery.

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