A flaky flower-like single-crystal sodium vanadium oxyfluorophosphate / carbon composite material, a preparation method thereof and a sodium ion battery

Abstract

The application provides a flaky flower-like single-crystal sodium vanadium oxyfluorophosphate / carbon composite material, a preparation method thereof and a sodium ion battery. The flaky flower-like single-crystal sodium vanadium oxyfluorophosphate material is prepared through a hydrothermal reaction. A surfactant is used to control the crystal face growth direction in the material synthesis process, so that the sodium vanadium oxyfluorophosphate grows to form a flaky flower-like single-crystal structure, and then the ion transmission distance is shortened, the ion kinetic characteristics of the material can be effectively improved, and on the basis of ensuring the structural stability of the material, that is, the cycle performance, the rate characteristics of the material are effectively improved. Through carbon coating, the electronic conductivity of the surface of the material particles is improved, and the capacity rate performance of the material is effectively improved.

Description

Technical Field

[0001] This invention relates to the field of cathode materials for sodium-ion batteries, specifically to a layered, flower-like single-crystal sodium fluorophosphate oxyvanadium / carbon composite material, its preparation method, and a sodium-ion battery. Background Technology

[0002] In recent years, electrochemical energy storage technology, due to its high power output, has become a key focus of market development in grid frequency regulation. Sodium-ion batteries, with their abundant raw material reserves, low price, good safety, rapid charge / discharge capabilities, and excellent low-temperature performance, have attracted widespread market attention and are currently being applied in two-wheeled electric vehicles, low-speed electric vehicles, home energy storage, and commercial energy storage. Compared to lithium-ion batteries, they have greater advantages in grid frequency regulation applications. Therefore, improving the high-rate and long-cycle performance of sodium-ion batteries—the highest-cost component and the key factor determining the battery's operating voltage—is crucial for their rapid application in grid frequency regulation.

[0003] Polycrystalline and monocrystalline materials have different electrochemical properties. Polycrystalline materials are composed of many randomly arranged primary grains. During charging and discharging, the volume changes of grains in different directions cause stress and strain between the grains. The secondary grains gradually break down, eventually leading to material deactivation and a significant decrease in material capacity. Monocrystalline materials, on the other hand, have a complete crystal structure, making the transfer of electrons and ions easier, resulting in better battery performance. However, if the monocrystalline particles are too large, the electrons and ions will have to travel too long in the crystal, and the excessive polarization will lead to a significant reduction in battery efficiency, capacity, and rate performance. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention proposes a layered flower-like single-crystal sodium fluorophosphate oxyvanadium / carbon composite material, its preparation method, and a sodium-ion battery. This invention can improve the problems of poor intrinsic electronic conductivity of sodium fluorophosphate oxyvanadium / carbon material, long ion transport distance of large single-crystal particles, and easy breakage and shedding of polycrystalline materials during cycling.

[0005] This invention is achieved through the following technical solution:

[0006] This invention discloses a method for preparing a layered, flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material, comprising the following steps:

[0007] 1) Sodium source, vanadium source, fluorine source and phosphorus source are taken according to the atomic ratio Na:V:P:F=3:2:2:x and dissolved in water. Surfactant is added and the mixture is prepared to obtain precursor solution; where 1<x<3;

[0008] 2) The precursor solution was subjected to a hydrothermal reaction at 100℃~200℃ to obtain layered flower-like single-crystal sodium fluorophosphate powder.

[0009] 3) The lamellar flower-shaped monocrystalline sodium fluorophosphate oxyvanadium powder and carbon source are wet-mixed and dried. The resulting mixture is calcined at 350℃~800℃ under an inert atmosphere to obtain a lamellar flower-shaped monocrystalline sodium fluorophosphate oxyvanadium / carbon composite material.

[0010] Preferably, in step 1), the amount of surfactant added is 0.1% to 50% of the theoretical yield of the layered flower-shaped monocrystalline sodium fluorophosphate powder.

[0011] Preferably, in step 1), the surfactant is one or more of polyvinylpyrrolidone, sodium carboxymethyl cellulose, fatty acid glycerides, fatty acid sorbitan, polysorbate, sodium carboxymethyl cellulose, sodium dodecylbenzene sulfonate, sodium hexadecylbenzene sulfonate, hexadecyltrimethylammonium bromide, sodium dodecyl alcohol sulfonate, and dodecylphenoxy polyoxyethylene ether.

[0012] Preferably, in step 1), the vanadium source is one or more of vanadium tetroxide, vanadium tetrahydroxide, vanadium oxalate, vanadium oxysulfate, vanadium oxalate, and vanadium acetylacetonate; 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 sodium source 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.

[0013] Preferably, in step 2), the hydrothermal reaction time is 1h to 20h.

[0014] Preferably, in step 3), the carbon source is one or more of sucrose, ascorbic acid, glucose, citric acid, fructose, and starch.

[0015] Preferably, in step 3), the amount of carbon source added is 0.1% to 20% of the mass of the layered flower-shaped single-crystal sodium fluorophosphate powder.

[0016] Preferably, in step 3), the calcination time is 1 min to 10 h.

[0017] The present invention provides a layered flower-like monocrystalline sodium fluorophosphate oxyvanadium / carbon composite material obtained by the preparation method described above. The composite material has a core-shell structure, with the core being layered flower-like monocrystalline sodium fluorophosphate oxyvanadium and the outer shell being amorphous carbon.

[0018] The present invention provides a sodium-ion battery comprising, as described above, a layered flower-like monocrystalline sodium vanadium fluorophosphate / carbon composite material as the positive electrode material.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] This invention employs a strategy to shorten ion transport distance by synthesizing layered flower-like single-crystal sodium fluorophosphate (SNP) using a hydrothermal method, combined with in-situ carbon coating via high-temperature annealing, to prepare a layered flower-like single-crystal SNP / carbon composite material. First, this invention prepares the layered flower-like single-crystal SNP material through a hydrothermal reaction. This material exhibits minimal volume change during charge and discharge and avoids the breakage and shedding phenomena observed in polycrystalline materials during cycling, effectively improving the material's structural stability. Second, this invention uses surfactants to control the crystal growth direction during synthesis, enabling the SNP to grow into a layered flower-like single-crystal structure, thereby shortening the ion transport distance and effectively improving the material's ion dynamics characteristics. While maintaining structural stability (i.e., cycling performance), this invention effectively improves the material's rate performance. Third, this invention uses carbon coating to increase the electronic conductivity of the material particle surface, effectively improving the material's rate performance. The preparation method of the layered flower-like single-crystal sodium fluorophosphate oxyvanadium / carbon composite material of the present invention is simple in process, has excellent performance, and is easy to achieve large-scale production, which is of great significance to the commercialization of sodium-ion batteries.

[0021] The layered flower-like single crystals obtained by this invention can reach the micrometer level, with a thickness of only within 50 nm in the 002 crystal plane direction. This can improve the problems of poor intrinsic electronic conductivity of sodium vanadium fluorophosphate, insufficient ionic conductivity caused by the long ion transport distance of large single crystal particles, and rapid capacity decay caused by secondary particle breakage due to volume changes during charge and discharge of polycrystalline materials, thereby improving the capacity, rate capability, and cycle performance of the material. Furthermore, the flower-like structure can effectively increase the contact area between the material and the electrolyte, increase the reactive sites of the material, and reduce the electrochemical impedance of the electrode surface, thus improving the power density and cycle life of the battery. The layered flower-like single crystal sodium vanadium fluorophosphate / carbon composite material prepared by this invention conforms to the tetragonal phase crystal structure of sodium vanadium fluorophosphate as determined by X-ray diffraction, exhibiting few defects in the single crystal structure. Electrochemical performance test results show that this invention effectively improves the rate capability and cycle performance of the material, promoting the rapid application of sodium-ion batteries using this material as the positive electrode in the frequency regulation side of the power grid. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a microscopic SEM image of the sample from Example 1 of the present invention;

[0024] Figure 2This is a microscopic SEM image of the sample from Comparative Example 1 of this invention.

[0025] Figure 3 This is a microscopic SEM image of the sample from Comparative Example 2 of this invention;

[0026] Figure 4 The XRD patterns of samples from Examples 1-4 and Comparative Example 2 of this invention are shown below.

[0027] Figure 5 These are the rate performance curves of the samples from Example 1, Comparative Example 1, and Comparative Example 2 of the present invention. Detailed Implementation

[0028] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0029] It should be noted that the process equipment or apparatus not specifically mentioned in the following embodiments are all conventional equipment or apparatus in the art.

[0030] It should be noted that the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products, or apparatuses. Furthermore, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not intended to limit the order of the method steps or define the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0031] The preparation method of the above-mentioned lamellar flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material of the present invention adopts a hydrothermal combined with high-temperature annealing method, specifically including the following steps:

[0032] 1) Select sodium, vanadium, fluorine, and phosphorus sources according to the atomic ratio Na:V:P:F = 3:2:2:x, and select surfactants accordingly; where 1 < x < 3.

[0033] 2) Add the sodium source, vanadium source, fluorine source and phosphorus source from step 1) to deionized water and stir until the solution is clear. Then add the surfactant to the above solution and continue stirring to mix evenly to obtain the precursor solution.

[0034] 3) Transfer the precursor solution obtained in step 2) to a hydrothermal reactor, heat it to 100℃~200℃, keep it at the temperature for 1h~20h and then cool it naturally to room temperature to obtain layered flower-like single crystal sodium vanadium fluorophosphate powder (Na3V2O2(PO4)2F).

[0035] 4) The obtained lamellar flower-shaped monocrystalline sodium fluorophosphate powder and carbon source are wet-mixed and dried. The resulting mixture is heated to 350℃~800℃ at a heating rate of 2℃ / min~10℃ / min under an inert atmosphere, held at the temperature for 1min~10h, and then naturally cooled to room temperature to obtain the lamellar flower-shaped monocrystalline sodium fluorophosphate / carbon composite material.

[0036] The vanadium source mentioned in step 1) of this invention is a tetravalent vanadium compound, specifically one or more of vanadium tetroxide (VO2), vanadium tetrahydroxide (V(OH)4), vanadium oxalate (V(C2O4)2), vanadium oxysulfate (VOSO4), vanadium oxalate (VOC2O4), and vanadium acetylacetonate (VO(AcAc)2); 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 surfactant is one or more of polyvinylpyrrolidone (PVP), sodium carboxymethyl cellulose, fatty acid glycerides, fatty acid sorbitan, polysorbate, sodium carboxymethyl cellulose, sodium dodecylbenzene sulfonate, sodium hexadecylbenzene sulfonate, hexadecyltrimethylammonium bromide, sodium dodecyl alcohol sulfonate, and dodecylphenoxy polyoxyethylene ether. The sodium source 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.

[0037] In step 3) of this invention, the lamellar flower-like single-crystal sodium fluorophosphate powder and the carbon source are wet-mixed and dried. The method used is one of ball milling, magnetic stirring and high-speed mixing. The carbon source is one or more of sucrose, glucose, citric acid, fructose and starch.

[0038] The surfactant in this invention is used as a material to control the growth of specific crystal faces, and its amount is 0.1% to 10% of the theoretical yield of flaky monocrystalline sodium fluorophosphate. The carbon source in this invention is mainly used as carbon coating, and its amount is 0.1% to 50% of the mass of flaky monocrystalline sodium fluorophosphate powder.

[0039] The inert atmosphere mentioned in step 4) of this invention is one or both of argon and nitrogen.

[0040] The layered flower-like monocrystalline sodium fluorophosphate oxyvanadium / carbon composite material obtained by the above method has a core-shell structure, with the core being layered flower-like monocrystalline sodium fluorophosphate oxyvanadium and the outer shell being amorphous carbon, i.e., a layered flower-like monocrystalline sodium fluorophosphate oxyvanadium composite material coated with amorphous carbon.

[0041] Example 1

[0042] (1) Weigh 0.004 mol of vanadium acetylacetonate, 0.001 mol of sodium carbonate, 0.004 mol of sodium fluoride, 0.004 mol of ammonium dihydrogen phosphate and 0.00412 g of sodium carboxymethyl cellulose (0.5% of the mass of the lamellar flower-shaped monocrystalline sodium vanadium fluorophosphate powder).

[0043] (2) Add the acetylacetone vanadium, sodium carbonate, sodium fluoride and ammonium dihydrogen phosphate weighed in step (1) to a beaker containing 80 ml of deionized water. Stir for 1 hour at a magnetic stirring rotor speed of 400 r / min until a light green solution is formed. Then add the weighed sodium carboxymethyl cellulose to the solution and continue stirring for 0.5 h to form a precursor solution.

[0044] (3) Transfer the precursor solution in step (2) to a hydrothermal reactor with water as the solvent, heat it to 120°C, keep it at a constant temperature for 10 hours and then cool it to room temperature naturally to obtain layered flower-shaped monocrystalline sodium fluorophosphate (Na3V2O2(PO4)2F). Mix the layered flower-shaped monocrystalline sodium fluorophosphate and 2% by mass of sucrose in a stirrer and then dry it in an oven.

[0045] (4) The mixture obtained in step (3) is heated to 600°C at a heating rate of 4°C / min under an inert atmosphere, held at the temperature for 1 min and then naturally cooled to room temperature to obtain amorphous carbon-coated lamellar flower-like monocrystalline sodium fluorophosphate oxyvanadium / carbon composite material.

[0046] Example 2

[0047] (1) Weigh 0.004 mol of vanadium acetylacetonate, 0.002 mol of sodium carbonate, 0.002 mol of sodium fluoride, 0.004 mol of diammonium hydrogen phosphate and 0.412 g of polyvinylpyrrolidone (accounting for 50% of the mass of the layered flower-shaped monocrystalline sodium vanadium fluorophosphate powder).

[0048] (2) Add the acetylacetonate vanadyl, sodium carbonate, sodium fluoride and diammonium hydrogen phosphate weighed in step (1) to a beaker containing 80 ml of deionized water. Stir for 1 hour at a magnetic stirring rotor speed of 400 r / min until a light green solution is formed. Then add the weighed polyvinylpyrrolidone to the solution and continue stirring for 0.5 h to form a precursor solution.

[0049] (3) Transfer the precursor solution in step (2) to a hydrothermal reactor with water as the solvent, heat it to 100°C, keep it at a constant temperature for 20 hours and then cool it to room temperature naturally to obtain layered flower-shaped monocrystalline sodium fluorophosphate (Na3V2O2(PO4)2F). After ball milling and mixing the layered flower-shaped monocrystalline sodium fluorophosphate and glucose accounting for 0.1% of the mass of the layered flower-shaped monocrystalline sodium fluorophosphate in a ball mill, dry it in an oven.

[0050] (4) The mixture obtained in step (3) is heated to 350°C at a heating rate of 2°C / min under an inert atmosphere, kept at the temperature for 10 hours and then naturally cooled to room temperature to obtain amorphous carbon-coated lamellar flower-like monocrystalline sodium fluorophosphate oxyvanadium / carbon composite material.

[0051] Example 3

[0052] (1) Weigh 0.004 mol of vanadium oxysulfate, 0.003 mol of sodium carbonate, 0.002 mol of ammonium fluoride, 0.004 mol of phosphoric acid and 0.0824 g of sodium hexadecylbenzenesulfonate (accounting for 10% of the mass of the lamellar flower-shaped monocrystalline sodium vanadium oxyfluoride powder).

[0053] (2) Add the vanadium sulfate, sodium carbonate, ammonium fluoride and phosphoric acid weighed in step (1) to a beaker containing 80 ml of deionized water. Stir for 1 hour at a magnetic stirring rotor speed of 400 r / min until a light green solution is formed. Then add the weighed sodium hexadecylbenzenesulfonate to the solution and continue stirring for 0.5 h to form a precursor solution.

[0054] (3) Transfer the precursor solution in step (2) to a hydrothermal reactor with water as the solvent, heat it to 200°C, keep it at a constant temperature for 20 hours and then cool it to room temperature naturally to obtain layered flower-shaped monocrystalline sodium fluorophosphate (Na3V2O2(PO4)2F). Mix the layered flower-shaped monocrystalline sodium fluorophosphate and fructose accounting for 20% of the mass of the layered flower-shaped monocrystalline sodium fluorophosphate in a high-speed mixer and then dry it in an oven.

[0055] (4) The mixture obtained in step (3) is heated to 800°C at a heating rate of 2°C / min under an inert atmosphere, kept at the temperature for 1 hour and then naturally cooled to room temperature to obtain amorphous carbon-coated lamellar flower-like monocrystalline sodium fluorophosphate oxyvanadium / carbon composite material.

[0056] Example 4

[0057] (1) Weigh 0.004 mol of vanadium oxalate, 0.001 mol of sodium sulfate, 0.002 mol of potassium fluoride, 0.004 mol of sodium dihydrogen phosphate and 0.0412 g of hexadecyltrimethylammonium bromide (5% of the mass of the layered flower-shaped single crystal sodium fluorophosphate powder).

[0058] (2) Add the vanadium oxalate, sodium sulfate, potassium fluoride and sodium dihydrogen phosphate weighed in step (1) to a beaker containing 80 ml of deionized water. Stir for 1 hour at a magnetic stirring rotor speed of 400 r / min until a light green solution is formed. Then add the weighed hexadecyltrimethylammonium bromide to the solution and continue stirring for 0.5 h to form a precursor solution.

[0059] (3) Transfer the precursor solution in step (2) to a hydrothermal reactor with water as the solvent, heat it to 120°C, keep it at a constant temperature for 12 hours and then cool it to room temperature naturally to obtain layered flower-shaped monocrystalline sodium fluorophosphate (Na3V2O2(PO4)2F). After ball milling the layered flower-shaped monocrystalline sodium fluorophosphate and starch accounting for 2% of the mass of the layered flower-shaped monocrystalline sodium fluorophosphate in a ball mill, dry them in an oven.

[0060] (4) The mixture obtained in step (3) is heated to 600°C at a heating rate of 2°C / min under an inert atmosphere, kept at the temperature for 2 hours and then naturally cooled to room temperature to obtain amorphous carbon-coated lamellar flower-like monocrystalline sodium fluorophosphate oxyvanadium / carbon composite material.

[0061] Comparative Example 1

[0062] Bulk single-crystal sodium vanadium fluorophosphate (Na3V2O2(PO4)2F) material was prepared by hydrothermal method, and the following steps were performed:

[0063] (1) Weigh out 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;

[0064] (2) Add the ammonium metavanadate and oxalic acid dihydrate weighed in step (1) to a beaker containing 80 ml of deionized water. Stir for half an hour under the conditions of magnetic stirring rotor speed of 400 r / min and 60℃ water bath until a light green solution is formed. Then add the weighed sodium fluoride and ammonium dihydrogen phosphate to the solution and continue stirring for 0.5 h to form a precursor solution.

[0065] (3) Transfer the precursor solution in step (2) to a hydrothermal reactor with water as the solvent, heat it to 200°C, keep it at a constant temperature for 10 hours and then cool it to room temperature naturally to obtain bulk sodium vanadium fluorophosphate single crystal powder (Na3V2O2(PO4)2F).

[0066] (4) The bulk sodium fluorophosphate oxyvanadium single crystal powder in step (3) is heated to 600°C at a heating rate of 4°C / min under an inert atmosphere, held at the temperature for 4 hours and then naturally cooled to room temperature to obtain amorphous carbon-coated bulk sodium fluorophosphate oxyvanadium single crystal (sodium fluorophosphate oxyvanadium / carbon) cathode material.

[0067] Comparative Example 2

[0068] (1) Weigh 0.004 mol of acetylacetone vanadium oxide, 0.001 mol of sodium carbonate, 0.004 mol of sodium fluoride, and 0.004 mol of ammonium dihydrogen phosphate.

[0069] (2) Add the acetylacetonate vanadyl, sodium carbonate, sodium fluoride and ammonium dihydrogen phosphate weighed in step (1) to a beaker containing 80 ml of deionized water, and stir for 1 hour at a magnetic stirring rotor speed of 400 r / min until a light green precursor solution is formed.

[0070] (3) Transfer the precursor solution in step (2) to a hydrothermal reactor with water as the solvent, heat it to 120°C, keep it at a constant temperature for 10 hours and then cool it to room temperature naturally to obtain bulk single crystal sodium fluorophosphate (Na3V2O2(PO4)2F). Mix the bulk single crystal sodium fluorophosphate powder and 2% of the mass of the bulk single crystal sodium fluorophosphate in a stirrer and then dry it in an oven.

[0071] (4) The mixture obtained in step (3) is heated to 600°C at a heating rate of 4°C / min under an inert atmosphere, held at the temperature for 1 min and then naturally cooled to room temperature to obtain amorphous carbon-coated blocky single crystal sodium fluorophosphate vanadium / carbon composite material.

[0072] A series of physical property characteristics were performed on the layered flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material prepared in the examples, and the results are shown in the figure. Figure 1 The image shows a SEM image of the single-crystal sodium vanadium fluorophosphate material prepared in Example 1, illustrating that the prepared material has a relatively regular sheet-like structure. Figure 2 and Figure 3 The images show SEM images of the bulk single-crystal sodium fluorophosphate prepared in Comparative Examples 1 and 2, illustrating that without the addition of a surfactant, a bulk single crystal with fully grown crystal planes is obtained. Figure 4 The XRD comparison diagrams of the single crystal materials prepared in Examples 1-4 and Comparative Example 2 show that the single crystal sodium fluorophosphate prepared by both methods exhibits good crystallinity.

[0073] The sodium vanadium fluorophosphate / carbon composite materials prepared in Examples 1, Comparative Examples 1 and 2 were mixed with conductive agent Super P and binder CMC in an equal mass ratio of 8:1:1 and dissolved in ultrapure water. The slurry was coated onto aluminum foil, dried, rolled, and cut into sheets to serve as the positive electrode. Metallic sodium sheets were used as the negative electrode, glass fiber membranes as the separator, and aluminum foil as the current collector. These were assembled into CR2016 coin-type sodium-ion batteries, and their electrochemical performance was tested. The bulk monocrystalline sodium vanadium fluorophosphate / carbon composite materials of Comparative Examples 1 and 2, and the layered flower-like monocrystalline sodium vanadium fluorophosphate / carbon composite material of Example 1 were subjected to constant current charge-discharge tests at different rates. The results are as follows: Figure 5 As shown. From Figure 5 As can be seen, the specific capacities of the layered flower-like monocrystalline sodium fluorophosphate oxyvanadium / carbon composite material in Example 1 reached 122.6, 115.3, 111.7, 108.5 and 104.9 mAh / g at rates of 1, 2, 5, 10 and 20C, respectively. These are significantly higher than those of Comparative Example 1 (118.8, 112.6, 98.3, 82.5 and 61.2 mAh / g) and Comparative Example 2 (96.8, 92.3, 80.9, 67.6 and 51.2 mAh / g), which fully demonstrates that the layered flower-like monocrystalline sodium fluorophosphate oxyvanadium / carbon composite material of the present invention can exhibit superior rate performance after the ion transport distance is shortened.

[0074] This invention addresses the application requirements of sodium vanadium fluorophosphate in high-power scenarios such as power grid frequency regulation. It employs a strategy that simultaneously improves the material's electronic conductivity and structural stability while shortening the ion transport distance. Through hydrothermal and high-temperature annealing synthesis, in-situ carbon coating and sheet-like single-crystal synthesis are achieved, resulting in a layered flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material. This significantly improves the capacity and rate performance of sodium vanadium fluorophosphate, which is of great significance for promoting the application of sodium-ion batteries in high-power scenarios.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions 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 layered, flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material, characterized in that, It consists of the following steps: 1) Sodium source, vanadium source, fluorine source and phosphorus source are taken according to the atomic ratio Na:V:P:F=3:2:2:x and dissolved in water. Surfactant is added and mixed to obtain precursor solution; wherein, 1<x<3; the vanadium source is one or more of vanadium tetroxide, vanadium tetrahydroxide, vanadium oxalate, vanadium oxysulfate, vanadium oxyoxalate and vanadium acetylacetonate. 2) The precursor solution was subjected to a hydrothermal reaction at 100 ℃~200 ℃ to obtain layered flower-like single crystal sodium fluorophosphate powder. 3) The lamellar flower-shaped monocrystalline sodium fluorophosphate oxyvanadium powder and carbon source are wet-mixed and dried. The resulting mixture is calcined at 350℃~800℃ under an inert atmosphere to obtain a lamellar flower-shaped monocrystalline sodium fluorophosphate oxyvanadium / carbon composite material.

2. The method for preparing the lamellar flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material according to claim 1, characterized in that, In step 1), the amount of surfactant added is 0.1% to 50% of the theoretical yield of the layered flower-shaped monocrystalline sodium fluorophosphate powder.

3. The method for preparing the lamellar flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material according to claim 1, characterized in that, In step 1), the surfactant is one or more of the following: polyvinylpyrrolidone, sodium carboxymethyl cellulose, fatty acid glycerides, fatty acid sorbitan, polysorbate, sodium carboxymethyl cellulose, sodium dodecylbenzene sulfonate, sodium hexadecylbenzene sulfonate, hexadecyltrimethylammonium bromide, sodium dodecyl alcohol sulfonate, and dodecylphenoxy polyoxyethylene ether.

4. The method for preparing the lamellar flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material according to claim 1, characterized in that, In step 1), 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 sodium source 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.

5. The method for preparing the lamellar flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material according to claim 1, characterized in that, In step 2), the hydrothermal reaction time is 1 h to 20 h.

6. The method for preparing the lamellar flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material according to claim 1, characterized in that, In step 3), the carbon source is one or more of sucrose, ascorbic acid, glucose, citric acid, fructose, and starch.

7. The method for preparing the lamellar flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material according to claim 1, characterized in that, In step 3), the amount of carbon source added is 0.1% to 20% of the mass of the layered flower-shaped single crystal sodium fluorophosphate powder.

8. The method for preparing the lamellar flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material according to claim 1, characterized in that, In step 3), the calcination time is 1 min to 10 h.

9. A layered, flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material obtained by the preparation method according to any one of claims 1 to 8, characterized in that, The composite material has a core-shell structure, with the core being a layered, flower-like single-crystal sodium fluorophosphate and the outer shell being amorphous carbon.

10. A sodium-ion battery, characterized in that, Including the layered flower-like single-crystal sodium vanadium fluorophosphate / carbon composite material as described in claim 9, which is used as a positive electrode material.

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