Preparation method of size-controllable sodium vanadium fluorophosphate-carbon composite positive electrode material

Abstract

The application belongs to the technical field of sodium ion battery electrode material preparation, and discloses a preparation method of a size-controllable sodium vanadium fluorophosphate-carbon composite positive electrode material, and the preparation steps comprise the following: firstly, an ionic liquid and carbon material mixed dispersion liquid is obtained through ultrasonic, then a sodium source, a vanadium source, a phosphorus source, a fluorine source and citric acid are added into the dispersion liquid, and after constant temperature stirring, a hydrothermal reaction is carried out, then solid-liquid separation, washing, drying and calcination are carried out, and the sodium vanadium fluorophosphate-carbon composite positive electrode material is obtained. + The application effectively solves the problem of large size of sodium vanadium fluorophosphate in the crystal growth process, and obtains small-sized sodium vanadium fluorophosphate, and the small particle size can shorten the Na + diffusion path and improve the performance of the material in the battery cycle process.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of batteries and relates to a preparation method of a size-controllable sodium vanadium fluorophosphate-carbon composite positive electrode material. BACKGROUND

[0002] With the rapid development of the global economy, the consumption of fossil energy dominated by oil is increasing day by day. Meanwhile, the large-scale use of fossil energy has caused serious environmental problems (such as global warming, air pollution, rising sea level, etc.) and energy crisis problems. Therefore, it is imperative to vigorously develop clean and renewable energy dominated by solar energy and wind energy. However, these energy sources are discontinuous, unstable and extremely susceptible to climate, geographical conditions and other conditions, and cannot guarantee large-scale, continuous supply and use. Moreover, if such intermittent and unstable electric energy is directly integrated into the power grid, it will further threaten the safe operation of the power grid. Therefore, the development of an efficient energy storage system is crucial. Among numerous energy storage systems, lithium ion batteries are widely used due to their long service life, small self-discharge loss, high energy conversion efficiency, high energy density and high power density. However, the uneven distribution of lithium resources (70% in South America) and the increasing price of lithium resources greatly limit the application of lithium ion batteries in large-scale energy storage systems.

[0003] Compared with lithium resources, sodium resources are uniformly distributed and low in cost, and have the same principle as lithium ion batteries. Therefore, sodium ion batteries are expected to be widely used in large-scale energy storage systems. However, due to the molar mass (23 g mol -1 ) of sodium ions being about 3.33 times that of lithium ions (6.9 g mol -1 ), and the radius of sodium ions is about 1.36 times that of lithium ions , sodium ion batteries have problems of low electrochemical performance, large material volume change and poor cycle stability. Therefore, the development of a positive electrode material with high sodium storage performance and high cycle stability is the key to the practical application of sodium ion batteries.

[0004] Among the positive electrode materials of sodium ion batteries, sodium vanadium fluorophosphate exhibits high theoretical specific capacity (128 mAh g -1 ) and high working voltage (>4.0 V) during the charging and discharging process, and can obtain high energy density. However, large particle size of sodium vanadium fluorophosphate will increase the Na + migration path, which seriously limits the electrochemical performance, thereby hindering its large-scale application. SUMMARY

[0005] In view of the problems in the prior art, the application provides a preparation method of a size-controllable sodium vanadium fluorophosphate-carbon composite positive electrode material, solves the problem of large size of sodium vanadium fluorophosphate in the crystal growth process, obtains sodium vanadium fluorophosphate with small size, shortens the Na + diffusion path, and improves the performance of the material in the battery cycle process.

[0006] The technical scheme of the application is as follows:

[0007] A preparation method of a size-controllable sodium vanadium fluorophosphate-carbon composite positive electrode material, comprising the following steps:

[0008] (1) uniformly grinding or ultrasonically mixing ionic liquid ILs and a carbon material dispersion solution to obtain an ILs-carbon material dispersion solution; wherein the concentration of the carbon material dispersion solution is 2-10 mg·ml -1 , and the mass ratio of the ionic liquid ILs to the carbon material is 1:1-4:1;

[0009] (2) adding a vanadium source and a reducing agent into the ILs-carbon material dispersion solution, uniformly mixing the vanadium source and the reducing agent under stirring at 50-100 DEG C and 300-1000 r / min for 20-80 min to obtain a mixed solution, then adding a fluorine source, a sodium source and a phosphorus source into the mixed solution, uniformly mixing the fluorine source, the sodium source and the phosphorus source under stirring at 50-100 DEG C and 300-1000 r / min for 20-80 min to obtain a uniformly dispersed precursor solution; wherein the molar ratio of the sodium source, the vanadium source, the phosphorus source, the fluorine source and the reducing agent is 3:1:2:3:4.5, and the mass ratio of the vanadium source to the carbon material is 2:1-8:1;

[0010] (3) transferring the precursor solution obtained in step 2 into a reaction kettle, and reacting at a temperature of 50-200 DEG C for 4-24 h; after the reaction is completed, washing and vacuum drying are performed to obtain a solid product;

[0011] (4) calcining the solid product obtained in step 3 under inert atmosphere protection, raising the temperature to 300 DEG C-800 DEG C at a temperature raising rate of 5-10 / min and maintaining for 2-10 h, then raising the temperature to 300 DEG C-800 DEG C at a temperature raising rate of 5-10 / min and maintaining for 2-10 h, and cooling to room temperature to obtain the sodium vanadium fluorophosphate-carbon composite positive electrode material.

[0012] In step (1), the ionic liquid is N-butyl-N-methyl piperidinium bromide ([PP 14one or more combinations of 1 -butyl-3-methylimidazolium bromide ([BMIM]Br), 1 -butyl-3- methylimidazolium dihydrogen phosphate ([BMIM]H2PO4), N-butylpyridinium bis(trifluoromethanesulfonyl)imide ([BPy][NTf2]), 1 -butyl-3-methylimidazolium hydrogen sulfate ([BMIM]HSO4), 1 -pentyl-3-methylimidazolium bromide ([BMIM]Br), 1 -butyl-3-methylimidazolium chloride ([BMIM]Cl), 1 -octyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([OMIM][NTf2]), 1 -butyl-2,3-dimethylimidazolium tetrafluoroborate ([BMMIM]BF4), 1 -butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4), 1 -butyl-3-methylimidazolium acetate ([BMIM]Ac), 1 -hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([HMIM][NTf2]), N-hexylpyridinium bis(trifluoromethanesulfonyl)imide ([HPy][NTf2]), 1 -ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]BF4).

[0013] The carbon material in step (1) is one or more combinations of carbon fiber, mesocarbon microbead, glassy carbon, hard carbon, porous activated carbon graphene, graphene oxide, single-walled carbon nanotube, multi-walled carbon nanotube, carboxylated multi-walled carbon nanotube, hydroxylated multi-walled carbon nanotube.

[0014] The reducing agent in step (2) includes any one or a combination of at least two of citric acid, citric acid monohydrate, oxalic acid, glucose, sucrose, ascorbic acid.

[0015] The phosphorus source in step (2) includes any one or a combination of at least two of sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, pyrophosphoric acid, sodium pyrophosphate, and sodium dihydrogen pyrophosphate.

[0016] The vanadium source in step (2) includes any one or a combination of at least two of vanadium powder, vanadium pentoxide, vanadium trioxide, metavanadic acid, sodium metavanadate, sodium orthovanadate, vanadyl sulfate, oxovanadium acetate, vanadium tetrachloride, and vanadyl trichloride.

[0017] The fluorine source in step (2) includes any one or a combination of at least two of sodium fluoride, ammonium fluoride, hydrofluoric acid, and trifluoroacetic acid.

[0018] The sodium source in step (2) includes any one or a combination of at least two of sodium fluoride, sodium acetate, sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate.

[0019] The washing in step (3) is performed by centrifugation or vacuum filtration using one or a combination of two or more of water, ethanol, methanol, isobutanol, ethylene glycol, acetone, tetrahydrofuran, dimethyl sulfoxide, propylene carbonate, ethylene carbonate, and N-methylpyrrolidone as the solvent.

[0020] The inert atmosphere in step (4) is an N2 atmosphere or an Ar atmosphere.

[0021] Compared with the prior art, the present application has the following beneficial effects:

[0022] (1) The present application introduces ILs to prepare sodium vanadium fluorophosphate with a smaller particle size, reduces the diffusion path of Na+ during charging and discharging, and improves the electrochemical performance of the material. +

[0023] (2) The present application in-situ composites GO with NVPF, and the three-dimensional framework structure formed thereby provides more channels for sodium ion diffusion and electron conduction.

[0024] (3) The sodium vanadium fluorophosphate carbon composite positive electrode material prepared by the method of the present application has a high reversible capacity and exhibits good cycle stability, and has a broad application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 XRD pattern of the I-NVPF@rGO composite material prepared in Example 1.

[0026] Figure 2 SEM pattern of the I-NVPF@rGO composite material prepared in Example 1.

[0027] Figure 3 The first three cycle charge-discharge curve of the I-NVPF@rGO composite material prepared in Example 1 at 0.1C.

[0028] Figure 4 Long cycle performance graph of the I-NVPF@rGO composite material prepared in Example 1 at 10C and 1000 cycles.

[0029] Figure 5 Rate performance graph of the I-NVPF@rGO composite material prepared in Example 1.

[0030] Figure 6 XRD pattern of the H-NVPF@rGO composite material prepared in Comparative Example 2.

[0031] Figure 7 SEM pattern of the H-NVPF@rGO composite material prepared in Comparative Example 2.

[0032] Figure 8 ​The rate performance graph of the H-NVPF@rGO composite material prepared for Comparative Example 2. DETAILED DESCRIPTION

[0033] The application will be further described in detail by specific examples and drawings, which are an explanation of the application rather than a limitation.

[0034] Example 1

[0035] (1) 38.64 μL [EMIM]BF4 and 5 mL graphene oxide (GO) aqueous dispersion solution were added to 7.5 mL deionized water, and dispersed uniformly under ultrasonic for 1 h to obtain a [EMIM]BF4-GO dispersion solution.

[0036] (2) 271.00 mg V2O5, 864.59 mg C6H8O7 were added to the [EMIM]BF4-GO dispersion solution, and mixed uniformly under stirring, and then 188.96 mg NaF, 345.09 mg NH4H2PO4 and 2.46 mg CH3COONa were added to the mixed solution, and stirred and mixed uniformly at 70°C and 300 r / min for 80 min to obtain a precursor solution in which the raw materials were dispersed uniformly.

[0037] (3) The precursor solution was transferred to a reaction kettle, the reaction temperature was 110°C, and the reaction time was 9 h; after the reaction was completed, the solid product was obtained after washing and vacuum drying.

[0038] (4) The obtained solid product was calcined under the protection of an inert atmosphere, the temperature was increased to 350°C at a rate of 5°C / min and maintained for 4 h, and then the temperature was increased to 650°C at a rate of 5°C / min and maintained for 8 h, and after cooling to room temperature, the I-NVPF@rGO composite material was obtained.

[0039] Example 2

[0040] (1) 38.64 μL [EMIM]BF4 and 5 mL graphene oxide (GO) aqueous dispersion solution were added to 7.5 mL deionized water, and dispersed uniformly under ultrasonic for 1 h to obtain a [EMIM]BF4-GO dispersion solution.

[0041] (2) 271.00 mg V2O5, 864.59 mg C6H8O7 were added into the [EMIM]BF4-GO dispersion solution, mixed uniformly under stirring, and a mixed solution was obtained by stirring at 70°C and 300 r / min for 40 min; then, 188.96 mg NaF, 345.09 mg NH4H2PO4 and 2.46 mg CH3COONa were added into the mixed solution, and the precursor solution with uniformly dispersed raw materials was obtained by stirring and mixing uniformly at 70°C and 300 r / min for 80 min.

[0042] (3) The precursor solution was transferred into a reaction kettle, the reaction temperature was 80°C, and the reaction time was 9 h; after the reaction was completed, the solid product was obtained by washing and vacuum drying.

[0043] (4) The obtained solid product was calcined under the protection of an inert atmosphere, the temperature was increased to 350°C at a rate of 5°C / min and maintained for 4 h, and then the temperature was increased to 650°C at a rate of 5°C / min and maintained for 8 h, and the composite material was obtained after cooling to room temperature.

[0044] Example 3

[0045] (1) 38.64 μL [EMIM]BF4 and 5 mL graphene oxide (GO) aqueous dispersion solution were added into 7.5 mL deionized water, and the [EMIM]BF4-GO dispersion solution was obtained by dispersing uniformly under ultrasonic for 1 h.

[0046] (2) 271.00 mg V2O5, 864.59 mg C6H8O7 were added into the [EMIM]BF4-GO dispersion solution, mixed uniformly under stirring, and a mixed solution was obtained by stirring at 70°C and 300 r / min for 40 min; then, 188.96 mg NaF, 345.09 mg NH4H2PO4 and 2.46 mg CH3COONa were added into the mixed solution, and the precursor solution with uniformly dispersed raw materials was obtained by stirring and mixing uniformly at 70°C and 300 r / min for 80 min.

[0047] (3) The precursor solution was transferred into a reaction kettle, the reaction temperature was 140°C, and the reaction time was 9 h; after the reaction was completed, the solid product was obtained by washing and vacuum drying.

[0048] (4) The obtained solid product was calcined under the protection of an inert atmosphere, the temperature was increased to 350°C at a rate of 5°C / min and maintained for 4 h, and then the temperature was increased to 650°C at a rate of 5°C / min and maintained for 8 h, and the composite material was obtained after cooling to room temperature.

[0049] Example 4

[0050] (1) 38.64 μL [EMIM]BF4 and 5 mL graphene oxide (GO) aqueous dispersion solution were added into 7.5 mL deionized water, and dispersed uniformly under ultrasonic for 1 h to obtain a [EMIM]BF4-GO dispersion solution.

[0051] (2) 271.00 mg V2O5 and 864.59 mg C6H8O7 were added into the [EMIM]BF4-GO dispersion solution, and mixed uniformly under stirring, and a mixed solution was obtained by stirring at 70 °C and 300 r / min for 40 min; then, 188.96 mg NaF, 345.09 mg NH4H2PO4 and 2.46 mg CH3COONa were added into the mixed solution, and mixed uniformly by stirring at 70 °C and 300 r / min for 80 min to obtain a precursor solution in which raw materials were uniformly dispersed.

[0052] (3) The precursor solution was transferred into a reaction kettle, the reaction temperature was 170 °C, and the reaction time was 9 h; after the reaction was completed, the solid product was obtained by washing and vacuum drying.

[0053] (4) The obtained solid product was calcined under the protection of inert atmosphere, the temperature was increased to 350 °C at a rate of 5 °C / min and kept for 4 h, and then the temperature was increased to 650 °C at a rate of 5 °C / min and kept for 8 h, and the composite material was obtained after cooling to room temperature.

[0054] Example 5

[0055] (1) 38.64 μL [EMIM]BF4 and 5 mL graphene oxide (GO) aqueous dispersion solution were added into 7.5 mL deionized water, and dispersed uniformly under ultrasonic for 1 h to obtain a [EMIM]BF4-GO dispersion solution.

[0056] (2) 271.00 mg V2O5 and 864.59 mg C6H8O7 were added into the [EMIM]BF4-GO dispersion solution, and mixed uniformly under stirring, and a mixed solution was obtained by stirring at 70 °C and 300 r / min for 40 min; then, 188.96 mg NaF, 345.09 mg NH4H2PO4 and 2.46 mg CH3COONa were added into the mixed solution, and mixed uniformly by stirring at 70 °C and 300 r / min for 80 min to obtain a precursor solution in which raw materials were uniformly dispersed.

[0057] (3) The precursor solution was transferred into a reaction kettle, the reaction temperature was 200 °C, and the reaction time was 9 h; after the reaction was completed, the solid product was obtained by washing and vacuum drying.

[0058] (4) The obtained solid product is calcined under the protection of inert atmosphere, with a temperature increasing rate of 5°C / min to 350°C and kept for 4h, and then with a temperature increasing rate of 5°C / min to 650°C and kept for 8h, and after cooling to room temperature, a composite material is obtained.

[0059] From Examples 1-5, lower hydrothermal temperature cannot prepare NVPF, and higher hydrothermal temperature accelerates the nucleation rate of NVPF crystal, resulting in the increase of the particle size of prepared NVPF, so the hydrothermal temperature is most suitable at 110°C.

[0060] Example 6

[0061] (1) 38.64 μL of [EMIM]BF4 is added to 5 mL of graphene oxide (GO) aqueous dispersion solution in 7.5 mL of deionized water, and dispersed uniformly under ultrasonic for 1 h to obtain a [EMIM]BF4-GO dispersion solution.

[0062] (2) 271.00 mg of V2O5, 864.59 mg of C6H8O7 are added to the [EMIM]BF4-GO dispersion solution, and mixed uniformly under stirring, and then 188.96 mg of NaF, 345.09 mg of NH4H2PO4 and 2.46 mg of CH3COONa are added to the mixed solution, and stirred and mixed uniformly under the conditions of 70°C and 300 r / min for 80 min to obtain a precursor solution with uniformly dispersed raw materials.

[0063] (3) The precursor solution is transferred to a reaction kettle, the reaction temperature is 110°C, and the reaction time is 5h; after the reaction is completed, the solid product is obtained after washing and vacuum drying.

[0064] (4) The obtained solid product is calcined under the protection of inert atmosphere, with a temperature increasing rate of 5°C / min to 350°C and kept for 4h, and then with a temperature increasing rate of 5°C / min to 650°C and kept for 8h, and after cooling to room temperature, a composite material is obtained.

[0065] Example 7

[0066] (1) 38.64 μL of [EMIM]BF4 is added to 5 mL of graphene oxide (GO) aqueous dispersion solution in 7.5 mL of deionized water, and dispersed uniformly under ultrasonic for 1 h to obtain a [EMIM]BF4-GO dispersion solution.

[0067] (2) 271.00 mg V2O5, 864.59 mg C6H8O7 were added into the [EMIM]BF4-GO dispersion solution, mixed uniformly under stirring, and a mixed solution was obtained by stirring at 70°C and 300 r / min for 40 min; then, 188.96 mg NaF, 345.09 mg NH4H2PO4 and 2.46 mg CH3COONa were added into the mixed solution, and the precursor solution with uniformly dispersed raw materials was obtained by stirring and mixing uniformly at 70°C and 300 r / min for 80 min.

[0068] (3) The precursor solution was transferred into a reaction kettle, the reaction temperature was 110°C, and the reaction time was 7 h; after the reaction was completed, the solid product was obtained by washing and vacuum drying.

[0069] (4) The obtained solid product was calcined under the protection of an inert atmosphere, the temperature was increased to 350°C at a rate of 5°C / min and maintained for 4 h, and then the temperature was increased to 650°C at a rate of 5°C / min and maintained for 8 h, and the composite material was obtained after cooling to room temperature.

[0070] Example 8

[0071] (1) 38.64 μL [EMIM]BF4 and 5 mL graphene oxide (GO) aqueous dispersion solution were added into 7.5 mL deionized water, and the [EMIM]BF4-GO dispersion solution was obtained by dispersing uniformly under ultrasonic for 1 h.

[0072] (2) 271.00 mg V2O5, 864.59 mg C6H8O7 were added into the [EMIM]BF4-GO dispersion solution, mixed uniformly under stirring, and a mixed solution was obtained by stirring at 70°C and 300 r / min for 40 min; then, 188.96 mg NaF, 345.09 mg NH4H2PO4 and 2.46 mg CH3COONa were added into the mixed solution, and the precursor solution with uniformly dispersed raw materials was obtained by stirring and mixing uniformly at 70°C and 300 r / min for 80 min.

[0073] (3) The precursor solution was transferred into a reaction kettle, the reaction temperature was 110°C, and the reaction time was 7 h; after the reaction was completed, the solid product was obtained by washing and vacuum drying.

[0074] (4) The obtained solid product was calcined under the protection of an inert atmosphere, the temperature was increased to 350°C at a rate of 5°C / min and maintained for 4 h, and then the temperature was increased to 650°C at a rate of 5°C / min and maintained for 8 h, and the composite material was obtained after cooling to room temperature.

[0075] It can be obtained by Examples 1 and 6-8 that a shorter hydrothermal time can not convert part of the raw materials into NVPF, and a longer hydrothermal time can cause the further growth of NVPF crystals, so the hydrothermal time of 9h is the most suitable.

[0076] Example 9 is carried out on the basis of Example 4 by replacing [EMIM]BF4 with [BMIM]H2PO4.

[0077] Example 10 is carried out on the basis of Example 4 by replacing [EMIM]BF4 with [BMIM]BF4.

[0078] Example 11 is carried out on the basis of Example 4 by replacing GO with carboxylated multi-walled carbon nanotubes.

[0079] Example 12 is carried out on the basis of Example 4 by replacing citric acid with citric acid monohydrate.

[0080] Example 13 is carried out on the basis of Example 1 by replacing the second step of calcination at a temperature increasing rate of 5℃ / min to 650℃ and keeping for 8h with a temperature increasing rate of 5℃ / min to 600℃ and keeping for 8h.

[0081] Comparative Example 1

[0082] On the basis of Example 1, without adding graphene oxide, Comparative Material I-NVPF is prepared.

[0083] (1) 38.64 μL of [EMIM]BF4 is added to 12.5 mL of deionized water, and uniformly dispersed under ultrasonic for 1h.

[0084] (2) 271.00 mg of V2O5, 864.59 mg of C6H8O7 is added to the above solution, and uniformly mixed under stirring, and stirred for 40 min at 70℃ and 300 r / min to obtain a mixed solution; then, 188.96 mg of NaF, 345.09 mg of NH4H2PO4 and 2.46 mg of CH3COONa are added to the mixed solution, and uniformly mixed under stirring for 80 min at 70℃ and 300 r / min to obtain a precursor solution with uniformly dispersed raw materials.

[0085] (3) The precursor solution is transferred to a reaction kettle, and the reaction temperature is 110℃, and the reaction time is 9h; after the reaction is completed, the solid product is obtained after washing and vacuum drying.

[0086] (4) The obtained solid product is calcined under the protection of inert atmosphere, and the temperature is increased to 350℃ at a rate of 5℃ / min and kept for 4h, and then increased to 650℃ at a rate of 5℃ / min and kept for 8h, and cooled to room temperature to obtain I-NVPF.

[0087] Comparative Example 2

[0088] On the basis of Example 1, without adding [EMIM]BF4, a comparative material H-NVPF@rGO composite material was prepared.

[0089] (1) 5 mL of an aqueous graphene oxide (GO) dispersion solution was added to 7.5 mL of deionized water, and uniformly dispersed under ultrasonic for 1 h to obtain a GO dispersion solution.

[0090] (2) 271.00 mg of V2O5, 864.59 mg of C6H8O7 were added to the above solution, and uniformly mixed under stirring, and stirred at 70°C and 300 r / min for 40 min to obtain a mixed solution; then, 188.96 mg of NaF, 345.09 mg of NH4H2PO4 and 2.46 mg of CH3COONa were added to the mixed solution, and uniformly mixed under stirring at 70°C and 300 r / min for 80 min to obtain a precursor solution in which the raw materials were uniformly dispersed.

[0091] (3) The precursor solution was transferred to a reaction kettle, the reaction temperature was 110°C, and the reaction time was 9 h; after the reaction was completed, the solid product was obtained after washing and vacuum drying.

[0092] (4) The obtained solid product was calcined under the protection of an inert atmosphere, the temperature was increased to 350°C at a rate of 5°C / min and maintained for 4 h, and then the temperature was increased to 650°C at a rate of 5°C / min and maintained for 8 h, and after cooling to room temperature, the H-NVPF@rGO composite material was obtained.

[0093] The I-NVPF@rGO composite material obtained from Example 1 and the H-NVPF@rGO composite material obtained from Comparative Example 2 were physically characterized, and Figure 1 and Figure 6 By comparison, it can be seen that the diffraction peak intensity of I-NVPF@rGO and H-NVPF@rGO is lower but the half peak width is wider, so the NVPF particle size in I-NVPF@rGO is smaller, which shows that the introduction of ILs effectively reduces the particle size of NVPF; by Figure 2 and Figure 7 By comparison, it can be seen that the NVPF cubic block particle size in I-NVPF@rGO is 0.3-1 μm, and the NVPF cubic block particle size in H-NVPF@rGO is 5-8 μm, so the NVPF particle size in I-NVPF@rGO is smaller, which is also consistent with the results of XRD. The I-NVPF@rGO composite material obtained from Example 1 and the H-NVPF@rGO composite material obtained from Comparative Example 2 were electrochemically tested, and Figure 3 andFigure 4 It can be obtained that I-NVPF@rGO has good cycle stability; by Figure 5 and Figure 8 It can be obtained that the discharge specific capacity of I-NVPF@rGO at 0.2-20C is higher than that of H-NVPF@rGO, which shows that I-NVPF@rGO synthesized with the assistance of ILs has excellent rate performance and structural stability. This excellent cycle performance and electrochemical reversibility is mainly due to the smaller NVPF cubic block which shortens the diffusion path of Na + / e - , so it has higher discharge specific capacity and coulombic efficiency at large rate, that is, it has excellent electrochemical performance.

Claims

1. A method for preparing a size-controllable sodium vanadium fluorophosphate-carbon composite positive electrode material, characterized in that, The method comprises the following steps: (1) grinding or ultrasonic mixing the ionic liquid ILs and the carbon material dispersion liquid uniformly to obtain an ILs-carbon material dispersion solution; wherein the concentration of the carbon material dispersion solution is 2-10 mg·ml -1 , the mass ratio of the ionic liquid ILs to the carbon material is 1:1-4:1; (2) adding a vanadium source and a reducing agent into the IL-carbon material dispersion solution, mixing uniformly under stirring at 50-100℃ and 300-1000r / min for 20-80min to obtain a mixed solution, then adding a fluorine source, a sodium source and a phosphorus source into the mixed solution, mixing uniformly under stirring at 50-100℃ and 300-1000r / min for 20-80min to obtain a uniformly dispersed precursor solution, wherein the sodium source, the vanadium source, the phosphorus source, the fluorine source and the reducing agent are in a molar ratio of 3:1:2:3:4.5, and the mass ratio of the vanadium source to the carbon material is 2:1-8:1; (3) transferring the precursor solution obtained in step 2 into a reaction kettle, and reacting at a temperature of 50-200℃ for 4-24h, then washing and vacuum drying to obtain a solid product; (4) calcining the solid product obtained in step 3 under inert atmosphere, raising the temperature to 300-800℃ at a rate of 5-10 / min and maintaining for 2-10h, then raising the temperature to 300-800℃ at a rate of 5-10 / min and maintaining for 2-10h, and cooling to room temperature to obtain a sodium vanadium fluorophosphate-carbon composite positive electrode material.

2. The production method according to claim 1, characterized by, The ion liquid in step (1) is one or a combination of more than two of N-butyl-N-methyl piperidinium bromide, 1-butyl-3-methyl imidazole dihydrogen phosphate, N-butyl pyridine bis(trifluoromethanesulfonyl) imide, 1-butyl-3-methyl imidazole hydrogen sulfate, 1-pentyl-3-methyl imidazole bromide, 1-butyl-3-methyl imidazole chloride, 1-octyl-3-methyl imidazole bis(trifluoromethanesulfonyl) imide, 1-butyl-2,3-dimethyl imidazole tetrafluoroborate, 1-butyl-3-methyl imidazole tetrafluoroborate, 1-butyl-3-methyl imidazole acetate, 1-hexyl-3-methyl imidazole bis(trifluoromethanesulfonyl) imide, N-hexyl pyridine bis(trifluoromethanesulfonyl) imide, 1-ethyl-3-methyl imidazole tetrafluoroborate.

3. The preparation method according to claim 1, characterized in that, The carbon material in step (1) is one or a combination of more than two of carbon fiber, mesocarbon microbead, glassy carbon, hard carbon, porous activated carbon graphene, graphene oxide, single-walled carbon nanotube, multi-walled carbon nanotube, carboxylated multi-walled carbon nanotube and hydroxylated multi-walled carbon nanotube.

4. The method of claim 1, wherein, The reducing agent in step (2) includes any one or a combination of at least two of citric acid, citric acid monohydrate, oxalic acid, glucose, sucrose and ascorbic acid.

5. The preparation method according to claim 1, characterized in that, The phosphorus source in step (2) includes any one or a combination of at least two of sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, sodium pyrophosphate, sodium dihydrogen pyrophosphate.

6. The method of claim 1, wherein, The vanadium source in step (2) includes any one or a combination of at least two of vanadium powder, vanadium pentoxide, vanadium trioxide, metavanadic acid, sodium metavanadate, sodium orthovanadate, vanadyl sulfate, oxovanadic acid, vanadium tetrachloride and vanadyl trichloride.

7. The preparation method according to claim 1, characterized in that, The fluorine source in step (2) includes any one or a combination of at least two of sodium fluoride, ammonium fluoride, hydrofluoric acid and trifluoroacetic acid.

8. The method of claim 1, wherein, The sodium source in step (2) includes any one of sodium fluoride, sodium acetate, sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, or a combination of at least two thereof.

9. The method of claim 1, wherein, The washing in step (3) is performed by centrifugation or vacuum filtration using one or a combination of more than two of water, ethanol, methanol, isobutanol, ethylene glycol, acetone, tetrahydrofuran, dimethyl sulfoxide, propylene carbonate, ethylene carbonate, and N-methylpyrrolidone as a solvent.

10. The method of claim 1, wherein, The inert atmosphere in step (4) is N2atmosphere or Ar atmosphere.

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