Modified sodium vanadium fluorophosphate composite material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of sodium ion battery positive electrode materials, and particularly relates to a modified sodium vanadium fluorophosphate composite material as well as a preparation method and application thereof. The modified sodium vanadium fluorophosphate composite material is a tin-doped sodium vanadium fluorophosphate / graphene composite material Na3V2-xSnx (PO4) 2F3 / GR, wherein x is more than or equal to 0.05 and less than or equal to 0.1; wherein the tin ions fully enter sodium vanadium fluorophosphate crystal lattices, the graphene is uniformly distributed around sodium vanadium fluorophosphate particles, and the sodium vanadium fluorophosphate particles are connected through the conductive bridge graphene to form a three-dimensional conductive network structure. According to the modified sodium vanadium fluorophosphate composite material, the sodium vanadium fluorophosphate Na3V2 (PO4) 2F3 is modified jointly by doping tin ions and compounding graphene, so that the sodium vanadium fluorophosphate is effectively modified, and the electrochemical performance of the sodium vanadium fluorophosphate is improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sodium ion battery cathode materials, and particularly relates to a modified sodium vanadium fluorophosphate composite material and a preparation method and application thereof. BACKGROUND

[0002] At present, sodium ion batteries have broad application prospects in large-scale energy storage, low-speed electric vehicles and other fields due to their advantages of abundant raw material reserves, low cost, safety and environmental protection. The cathode materials of sodium ion batteries are mainly divided into three categories: transition metal oxides, polyanion compounds and Prussian blue compounds. Among them, the polyanion compounds have high working potential, large reversible capacity, and good rate performance and cycle stability, and are extremely promising sodium ion battery cathode materials.

[0003] As one of the polyanion compounds, sodium vanadium fluorophosphate has a unique NASICON structure, and its working voltage is as high as about 3.8V, the theoretical energy density is 507Wh / kg, and it has the advantages of high theoretical specific capacity (128mAh / g), etc., and has become a research hotspot of sodium ion battery cathode materials in recent years. Compared with sodium vanadate (Na3V2(PO4)3), sodium vanadium fluorophosphate uses 3 F - substitute 1 PO4 3- Because the fluorine ion has strong electronegativity, its working potential is significantly improved, thereby improving the energy density of the sodium ion battery.

[0004] However, sodium vanadium fluorophosphate also has some deficiencies, such as low electronic conductivity and slow ion diffusion speed, which seriously limits its further application. At present, researchers have improved it by optimizing the synthesis process, coating, ion doping and structure design, etc. to improve its electrochemical performance, and have made significant progress. However, the existing modification methods in the prior art have the following disadvantages: the modification by optimizing the synthesis process, such as low-temperature synthesis, has the problem of high cost; the carbon coating of the material has the problems of uncontrollable uniformity and thickness in actual production; and the complex structure design has the problem of low repeatability in large-scale production.

[0005] The prior art has the following problems by modifying sodium vanadium fluorophosphate through ion doping: 1. If aluminum-doped sodium vanadium fluorophosphate is selected, Na vacancies or V valence state needs to be increased to balance the charge, which will destroy the stability of the framework of sodium vanadium fluorophosphate, thus leading to capacity attenuation. 2. If calcium-doped sodium vanadium fluorophosphate is used, the radius of calcium ions is too large compared with the radius of vanadium ions, the doping amount is limited, and excessive doping may cause lattice distortion, leading to poor long cycle performance. 3. If bismuth ion-doped sodium vanadium fluorophosphate is used, the particles are seriously agglomerated, the size distribution of the crystal grains is uneven, the ion transmission is affected, and a low reversible capacity is caused. 4. If chromium-doped sodium vanadium fluorophosphate is used, it is easy to cause excessive growth of the crystal, the particle size is uneven, and the sodium ion diffusion path is affected. 5. If cobalt-doped sodium vanadium fluorophosphate is used, there are problems of high price and resource scarcity, which is not conducive to large-scale application.

[0006] For example, CN113517426B discloses a kind of sodium vanadium fluorophosphate / reduced graphene oxide composite material and its preparation method and application. The preparation method is according to the following steps: vanadium acetylacetone is dissolved in anhydrous ethanol to obtain a vanadium acetylacetone solution; sodium dihydrogen phosphate and sodium fluoride are dissolved in deionized water to obtain a mixed solution; the vanadium acetylacetone solution and the mixed solution are mixed to obtain a premix; the graphene oxide solution is added to the premix, and then microwave hydrothermal reaction is carried out, and the product is obtained by continuous stirring reaction; the obtained product is washed and dried to obtain a reaction precursor, and the precursor is placed in a tube furnace and subjected to heat treatment in an inert gas atmosphere to obtain a sodium vanadium fluorophosphate / reduced graphene oxide composite material. From the perspective of the electrochemical performance of the material, the patent uses the method of directly pouring the solution into the premix for mixing when the graphene oxide is compounded, which is only a simple stirring and mixing, and cannot ensure that the graphene is uniformly and completely dispersed around the particles, so it is easy to cause uneven loading of the graphene, thereby causing poor battery electrochemical performance.

[0007] Although sodium vanadium fluorophosphate faces the above technical problems of low intrinsic electronic conductivity, relatively slow ion diffusion kinetics, etc., the advantages of sodium vanadium fluorophosphate lie in its high working voltage, high theoretical energy density and stable structure, so how to overcome these difficulties and further improve its performance has become a technical problem to be solved. SUMMARY

[0008] The purpose of the present application is to provide a modified sodium vanadium fluorophosphate composite material and its preparation method and application to solve the above-mentioned problems. The modified sodium vanadium fluorophosphate composite material modifies sodium vanadium fluorophosphate Na3V2(PO4)2F3 by doping tin ions and compounding graphene at the same time, realizes effective modification of sodium vanadium fluorophosphate, and improves its electrochemical performance.

[0009] The technical scheme of the present application is as follows:

[0010] A modified sodium vanadium fluorophosphate composite material is a tin-doped sodium vanadium fluorophosphate / graphene composite material Na3V2(PO4)2F3 / GR, 0.05≤x≤0.1 2- x Sn x The tin ions enter the sodium vanadium fluorophosphate lattice, and the graphene is uniformly distributed around the sodium vanadium fluorophosphate particles, and the graphene serves as a conductive bridge to connect the sodium vanadium fluorophosphate particles to form a three-dimensional conductive network structure.

[0011] It can be seen that, in the modified sodium vanadium fluorophosphate composite material, the tin ions are doped and substituted for vanadium ions, the radius of the tin ions is slightly larger than that of the vanadium ions, which can effectively expand the cell volume without causing excessive lattice expansion, and can provide a wider diffusion channel for sodium ions, and single ion doping is more controllable than multi-element doping in the prior art, which can effectively improve the electrical conductivity of the composite material as a positive electrode material of a sodium ion battery and has good electrochemical performance.

[0012] In addition, the graphene serves as a conductive bridge between the sodium vanadium fluorophosphate particles, forms a good contact and composite structure with the sodium vanadium fluorophosphate, and promotes the diffusion of sodium ions, thereby achieving the common modification of the sodium vanadium fluorophosphate together with the tin ion doping.

[0013] In order to effectively control the generation of impurities in the composite material and improve the electrochemical performance of the composite material as a positive electrode material, under the premise that the electrical conductivity of the modified sodium vanadium fluorophosphate composite material as a positive electrode material is not reduced, a sol-gel method for preparing the above-mentioned composite material is innovatively proposed by using the following raw material ratio, sol-gel process parameters and subsequent treatment condition optimization, that is, the sodium vanadium fluorophosphate Na3V2(PO4)2F3 is commonly modified by doping different amounts of tin ions and compounding graphene.

[0014] The specific scheme is as follows:

[0015] The preparation method of the above-mentioned modified sodium vanadium fluorophosphate composite material comprises the following steps:

[0016] (1) Preparation of gel precursor:

[0017] First, the raw materials ammonium metavanadate, ammonium dihydrogen phosphate, sodium fluoride and tin source are added to the solvent deionized water, mixed uniformly to obtain a raw material mixed solution; wherein the tin source is tin chloride or tin acetate.

[0018] The vanadium source ammonium metavanadate, the phosphorus source ammonium dihydrogen phosphate, sodium fluoride and the tin source are analytically pure. The raw materials for preparing the tin-doped sodium vanadium fluorophosphate are designed and matched according to the molar ratio, and the molar ratio of the vanadium source to the phosphorus source is 1:1.

[0019] The sodium fluoride is calculated according to the stoichiometric ratio, and 10% excess sodium fluoride is added to prevent volatilization of the sodium source and the fluorine source during preparation.

[0020] The tin source is designed according to the doping amount of tin ions of 0.05-0.1 mol.

[0021] Then, citric acid is added to the obtained raw material mixed solution as a complexing agent, and stirring is continued for 1-2 h, so that the metal ions in the solution are fully complexed with the citric acid to form a stable sol system; wherein the molar ratio of citric acid to ammonium metavanadate is 3:(1.9-2).

[0022] Unlike the molar ratio of 1:1 of citric acid to vanadium source commonly used in the preparation of sodium vanadium phosphate fluoride material by the sol-gel method in the prior art, the molar ratio of citric acid to vanadium source in this step is controlled to be 3:(1.9-2), so that the citric acid not only serves as a complexing agent, but also enhances the synergistic coordination between tin ions and vanadium ions, inhibits the oxidation of vanadium, and improves the uniformity of the precursor.

[0023] Meanwhile, the proportion of citric acid will affect the change of the crystal structure. If the proportion of citric acid is too low, the complexing is not complete, and a heterogeneous precursor will be formed, resulting in impure crystal forms. If the proportion of citric acid is too high, residual citric acid will be present, and the residual citric acid will carbonize during subsequent pre-sintering, resulting in the generation of impurity phases.

[0024] The obtained sol is heated in a water bath at 60-100℃, and continuous stirring is performed, so that it is gradually dried to form a gel.

[0025] Finally, the obtained gel is dried at 80-120℃ for 12-24 h to remove the water therein, and a dried gel precursor is obtained.

[0026] (2) Pre-sintering treatment:

[0027] The dried gel precursor obtained in step (1) is placed in a muffle furnace, and is heated to 300-450℃ at a heating rate of 5-10℃ / min under an air atmosphere, and is kept at this temperature for 3-5 h to perform pre-sintering, so that the gel precursor undergoes a preliminary redox reaction and a crystal structure transformation, and an intermediate product with a certain crystal form is formed.

[0028] After the pre-sintered product is naturally cooled to room temperature, it is taken out and ground into a powder, and the obtained pre-sintered powder is obtained.

[0029] The pre-sintering can remove the water in the gel precursor, and the low-temperature sintering can gently remove the water and volatile impurities (NH3, CO2) in the precursor, avoiding the generation of cracks during high-temperature sintering. Compared with direct high-temperature sintering, pre-sintering is more energy-saving and controllable.

[0030] (3) Graphene composite treatment:

[0031] The pre-sintered powder obtained in step (2) is mixed with graphene in a set ratio, and ball milling is performed in an ethanol (absolute ethanol, analytical pure) medium using a ball mill at a speed of 300 r / min-500 r / min for 4-10 h, so that the graphene and the pre-sintered powder are uniformly dispersed and combined.

[0032] The graphene is combined with the pre-sintered tin-doped sodium vanadium fluorophosphate particles by ball milling, which can ensure uniform combination of the graphene and improve the electrochemical performance of the composite material as a positive electrode material for sodium ion batteries.

[0033] The mixture after ball milling is vacuum dried at 80℃-120℃ for 12-24 h to remove solvents such as ethanol, and a composite of graphene and pre-sintered powder is obtained.

[0034] In the prior art, graphene is usually directly poured into the solution to be combined and stirred, which can easily cause oxidation or structural changes of the graphene in a strong acid environment. In order to solve the problems of uneven mixing of graphene with the solution used for combination and easy destruction of the graphene structure in the prior art, the present application fully considers the uniform combination of tin-doped sodium vanadium fluorophosphate particles and graphene during the design process, and innovatively proposes to use ball milling to mix graphene and tin-doped sodium vanadium fluorophosphate particles, which realizes high uniformity of the two and makes the graphene structure complete. At the same time, wet ball milling is used for ball milling at a low speed (300-500 r / min), which can realize more uniform combination without destroying the graphene structure. Ethanol is used as a dispersant in the ball milling process, and the dispersant can be completely removed by vacuum drying, which is conducive to better contact between graphene and particles during calcination.

[0035] (4) Calcination treatment:

[0036] The composite obtained in step (3) is placed in a tube furnace, heated to 600℃-750℃ at a heating rate of 5℃ / min-10℃ / min under an argon atmosphere, and held for 6-10 h for calcination.

[0037] The calcined product is naturally cooled to room temperature and then ground into powder to obtain the modified sodium vanadium fluorophosphate composite material.

[0038] In the present application, the amount of graphene added in step (3) of the preparation method of the modified sodium vanadium fluorophosphate composite material is 1%-5% of the mass of sodium vanadium fluorophosphate.

[0039] Preferably, the amount of graphene added in step (1) is 2% of the mass of sodium vanadium fluorophosphate.

[0040] In the present application, the preparation method of the modified sodium vanadium fluorophosphate composite material, in step (1), the preparation of the raw material mixed solution is stirring and mixing at a speed of 400 r / min-800 r / min for 2-4 h at 25-45 DEG C.

[0041] The modified sodium vanadium fluorophosphate composite material is used as a positive material in a sodium ion battery.

[0042] When the modified sodium vanadium fluorophosphate composite material is used as a positive material of a sodium ion battery, the discharge specific capacity at 1C rate is 120-130 mAh / g after electrochemical performance test, the capacity retention rate is 90%-98% after 160 cycles, and the discharge specific capacity at 10C rate is maintained at 100-110 mAh / g. Compared with pure sodium vanadium fluorophosphate without doping and without graphene composite, the electrochemical performance is significantly improved.

[0043] The modified sodium vanadium fluorophosphate composite material of the present application has the following beneficial effects: in the modified sodium vanadium fluorophosphate composite material, tin ions are doped to replace vanadium sites, Sn 4+ The radius (0.69) is slightly larger than V 3+ (0.64), the substitution makes the VO6 octahedron slightly expand, which can effectively expand the cell volume without causing excessive lattice expansion, provides a wider diffusion channel for sodium ions, accelerates the diffusion of sodium ions, and the single ion doping is more controllable than the multi-element doping in the prior art, which can effectively improve the electrical conductivity of the composite material as a positive material of a sodium ion battery and has good electrochemical performance.

[0044] Meanwhile, the graphene is compounded, the graphene plays a conductive bridge role between the sodium vanadium fluorophosphate particles, forms a good contact and composite structure with the sodium vanadium fluorophosphate, forms a three-dimensional conductive channel, and the bulk phase and interface electrical conductivity of the sodium vanadium fluorophosphate are simultaneously improved, and the kinetic bottleneck of the sodium battery positive material is broken through. BRIEF DESCRIPTION OF DRAWINGS

[0045] Figure 1 The X-ray diffraction pattern of the modified sodium vanadium fluorophosphate composite material prepared in Example 2 of the present application.

[0046] Figure 2 The EDS-mapping diagram of the modified sodium vanadium fluorophosphate composite material prepared in Example 2 of the present application.

[0047] Figure 3 The scanning electron microscope diagram of the modified sodium vanadium fluorophosphate composite material prepared in Example 2 of the present application.

[0048] Figure 4The modified sodium vanadium fluorophosphate composite material prepared in Example 2 of the present application and the unmodified Na3V2(PO4)2F3 material in Comparative Example 1 are used as positive electrode materials for sodium ion batteries, and a comparison chart of 160 cycles at 0.1C is shown below.

[0049] Figure 5 The modified sodium vanadium fluorophosphate composite materials prepared in Examples 1-3 of the present application and the unmodified Na3V2(PO4)2F3 material in Comparative Example 1 are used as positive electrode materials for sodium ion batteries, and a comparison chart of rate at 10C is shown below.

[0050] Figure 6 The modified sodium vanadium fluorophosphate composite materials prepared in Examples 1-3 of the present application and the unmodified Na3V2(PO4)2F3 material in Comparative Example 1 are used as positive electrode materials for sodium ion batteries, and a comparison chart of first charge-discharge at 0.1C is shown below. DETAILED DESCRIPTION

[0051] The technical solutions of the present application will be described in detail below with reference to the accompanying drawings.

[0052] Example 1

[0053] The modified sodium vanadium fluorophosphate composite material is a tin-doped sodium vanadium fluorophosphate / graphene composite material Na3V 1.95 Sn 0.05 (PO4)2F3 / GR.

[0054] The preparation method of the modified sodium vanadium fluorophosphate composite material includes the following specific steps:

[0055] (1) Preparation of gel precursor:

[0056] First, 1.95 mol of NH4VO3, 0.05 mol of SnCl4·5H2O, 3.3 mol of NaF, and 2 mol of NH4H2PO4 are dissolved in 15 mL of deionized water, and stirred at 500 r / min at 30°C for 2 h to fully dissolve and mix the materials to form a uniform raw material mixed solution.

[0057] Then, 3 mol of citric acid is added dropwise to the obtained raw material mixed solution, and stirring is continued to form a stable sol system.

[0058] The obtained sol is heated in a water bath at 80°C, and continuously stirred for 2 h, and dried to form a gel.

[0059] Finally, the obtained gel is dried at 80°C for 12 h to obtain a dried gel precursor.

[0060] (2) Pre-sintering treatment:

[0061] The dried gel precursor obtained in step (1) is placed in a muffle furnace, and pre-sintered at 350℃ under air atmosphere at a temperature increasing rate of 5℃ / min, and the temperature is kept for 3h.

[0062] After the pre-sintered product is naturally cooled to room temperature, it is taken out and ground to obtain a pre-sintered powder;

[0063] (3) Composite treatment with graphene:

[0064] The pre-sintered powder obtained in step (2) is mixed with 15mL of graphene suspension with a concentration of 2mg / mL, and mixed ball-milling is performed in an ethanol medium using a ball mill at a rotation speed of 300r / min for 6h.

[0065] The ball-milled mixture is vacuum dried at 80℃ for 12h to obtain a pre-sintered powder and graphene composite.

[0066] (4) Calcination treatment:

[0067] The composite obtained in step (3) is placed in a tube furnace, and calcined at 650℃ under argon atmosphere at a temperature increasing rate of 5℃ / min, and the temperature is kept for 7h.

[0068] After the calcined product is naturally cooled to room temperature, it is ground again into a powder to obtain the modified sodium vanadium fluorophosphate composite material Na3V 1.95 Sn 0.05 (PO4)2F3 / GR.

[0069] The prepared composite material has a discharge specific capacity of 113mAh / g at 1C rate as a positive electrode material of a sodium ion battery, and the capacity retention rate is 85.4% after 160 cycles at 1C, and the discharge specific capacity is 87mAh / g at 10C rate.

[0070] Example 2

[0071] The modified sodium vanadium fluorophosphate composite material is a tin-doped sodium vanadium fluorophosphate / graphene composite material Na3V 1.93 Sn 0.07 (PO4)2F3 / GR.

[0072] The preparation method of the modified sodium vanadium fluorophosphate composite material includes the following specific steps:

[0073] (1) Preparation of gel precursor:

[0074] First, 1.93 mol of NH4VO3, 0.07 mol of SnCl4·5H2O, 3.3 mol of NaF, and 2 mol of NH4H2PO4 were dissolved in 15 mL of deionized water, mixed uniformly, and a raw material mixed solution was obtained.

[0075] Then, 3 mol of citric acid was added dropwise to the obtained raw material mixed solution, and stirring was continued to form a stable sol system.

[0076] The obtained sol was heated in a water bath at 80°C, and stirring was continued for 2 h, and drying was performed to form a gel.

[0077] Finally, the obtained gel was dried at 80°C for 12 h to obtain a dried gel precursor.

[0078] (2) Pre-sintering treatment:

[0079] The dried gel precursor obtained in step (1) was placed in a muffle furnace, and pre-sintering was performed at a temperature increasing rate of 5°C / min to 350°C under an air atmosphere, and the temperature was maintained for 3 h.

[0080] After the pre-sintered product was naturally cooled to room temperature, it was taken out and ground to obtain a pre-sintered powder;

[0081] (3) Graphene complexing treatment:

[0082] The pre-sintered powder obtained in step (2) was mixed with 15 mL of a graphene suspension with a concentration of 2 mg / mL, and ball milling was performed in an ethanol medium using a ball mill at a rotation speed of 300 r / min for 6 h.

[0083] The ball-milled mixture was vacuum dried at 80°C for 12 h to obtain a complex of the pre-sintered powder and graphene.

[0084] (4) Calcination treatment:

[0085] The complex obtained in step (3) was placed in a tube furnace, and calcination was performed at a temperature increasing rate of 5°C / min to 650°C under an argon atmosphere, and the temperature was maintained for 7 h.

[0086] After the calcined product was naturally cooled to room temperature, it was ground again into a powder to obtain the modified sodium vanadium fluorophosphate composite material Na3V 1.93 Sn 0.07 (PO4)2F3 / GR.

[0087] The prepared composite material was used as a positive electrode material for a sodium ion battery, and the discharge specific capacity was as high as 123 mAh / g at a 1C rate, the capacity retention rate was 96.6% after 160 cycles at 1C, and the discharge specific capacity was 106 mAh / g at a 10C rate.

[0088] Example 3

[0089] The modified sodium vanadium fluorophosphate composite material is a tin-doped sodium vanadium fluorophosphate / graphene composite material Na3V 1.9 Sn 0.1 (PO4)2F3 / GR.

[0090] The preparation method of the modified sodium vanadium fluorophosphate composite material includes the following specific steps:

[0091] (1) Preparation of gel precursor:

[0092] First, 1.9 mol of NH4VO3, 0.1 mol of SnCl4·5H2O, 3.3 mol of NaF, and 2 mol of NH4H2PO4 are dissolved in 15 mL of deionized water, and uniformly mixed to obtain a raw material mixed solution.

[0093] Then, 3 mol of citric acid is added dropwise to the obtained raw material mixed solution, and stirring is continued to form a stable sol system.

[0094] The obtained sol is heated in a water bath at 80°C, and continuously stirred for 2 h, and dried to form a gel.

[0095] Finally, the obtained gel is dried at 80°C for 12 h to obtain a dry gel precursor.

[0096] (2) Pre-sintering treatment:

[0097] The dry gel precursor obtained in step (1) is placed in a muffle furnace, and heated to 350°C at a heating rate of 5°C / min in an air atmosphere, and heat treated for 3 h to perform pre-sintering.

[0098] After the pre-sintered product is naturally cooled to room temperature, it is taken out and ground to obtain a pre-sintered powder;

[0099] (3) Graphene composite treatment:

[0100] The pre-sintered powder obtained in step (2) is mixed with 15 mL of graphene suspension with a concentration of 2 mg / mL, and mixed and ball milled in an ethanol medium using a ball mill at a rotation speed of 300 r / min for 6 h.

[0101] The ball-milled mixture is vacuum dried at 80°C for 12 h to obtain a composite of the pre-sintered powder and graphene.

[0102] (4) Calcination treatment:

[0103] The composite obtained in step (3) was placed in a tube furnace, and calcined at 650℃ for 7h under argon atmosphere at a heating rate of 5℃ / min.

[0104] After the calcined product was naturally cooled to room temperature, it was ground into powder again to obtain the modified sodium vanadium fluorophosphate composite material Na3V 1.9 Sn 0.1 (PO4)2F3 / GR.

[0105] The prepared composite material was used as a positive electrode material of a sodium ion battery, and had a discharge specific capacity of 105mAh / g at a 1C rate, a capacity retention rate of 73% after 160 cycles at 1C, and a discharge specific capacity of 81mAh / g at a 10C rate.

[0106] Example 4

[0107] The difference between this example and Example 2 is that 0.07mol of tin acetate was added in step (1) of this example.

[0108] The prepared composite material was used as a positive electrode material of a sodium ion battery, and had a discharge specific capacity of 122mAh / g at a 1C rate, a capacity retention rate of 96% after 160 cycles at 1C, and a discharge specific capacity of 104mAh / g at a 10C rate.

[0109] Comparative Example 1

[0110] 2mol of NH4VO3, 3.3mol of NaF, and 2mol of NH4H2PO4 were dissolved in 15mL of deionized water, and 3mol of citric acid was added dropwise. The sol was heated in a water bath at 80℃ and continuously stirred for two hours until a gel was formed.

[0111] The obtained gel was dried at 80℃ for 12 hours, and then placed in a muffle furnace and pre-sintered at 350℃ for 3h under air atmosphere at a heating rate of 5℃ / min.

[0112] Then the obtained powder was mixed with 15mL of a graphene suspension with a concentration of 2mg / mL in a ball mill, and ball-milled at a speed of 300r / min for 6 hours. The mixture after ball-milling was vacuum dried at 80℃ for 12 hours.

[0113] The composite was placed in a tube furnace and calcined at 650℃ for 7h under argon atmosphere at a heating rate of 5℃ / min, and finally obtained Na3V2(PO4)2F3 powder.

[0114] The prepared Na3V2(PO4)2F3 material is used as a positive electrode material of a sodium ion battery, and has a discharge specific capacity of 91 mAh / g at a 1C rate, a capacity retention rate of 53.2% after 160 cycles at 1C, and a discharge specific capacity of only 60 mAh / g at a 10C rate.

[0115] Comparative Example 2

[0116] Different from Example 3, the modified sodium vanadium fluorophosphate composite material is Na3V 1.98 Sn 0.02 (PO4)2F3 / GR. That is, the tin doping amount of the present comparative example is 0.02 mol.

[0117] The adding amounts of the tin source and the vanadium source in the preparation method are adaptively adjusted according to the molar ratio, and the other conditions are the same as those in Example 3.

[0118] The prepared composite material is used as a positive electrode material of a sodium ion battery, and has a discharge specific capacity of 96 mAh / g at a 1C rate, a capacity retention rate of 65% after 160 cycles at 1C, and a discharge specific capacity of 72 mAh / g at a 10C rate.

[0119] Comparative Example 3

[0120] Different from Example 3, the modified sodium vanadium fluorophosphate composite material is Na3V 1.85 Sn 0.15 (PO4)2F3 / GR. That is, the tin doping amount of the present comparative example is 0.15 mol.

[0121] The adding amounts of the tin source and the vanadium source in the preparation method are adaptively adjusted according to the molar ratio, and the other conditions are the same as those in Example 3.

[0122] The prepared composite material is used as a positive electrode material of a sodium ion battery, and has a discharge specific capacity of 99 mAh / g at a 1C rate, a capacity retention rate of 68% after 160 cycles at 1C, and a discharge specific capacity of 78 mAh / g at a 10C rate.

[0123] Comparative Example 4

[0124] Different from Example 3, the tin source of the modified sodium vanadium fluorophosphate composite material is tin oxide (SnO2).

[0125] The prepared composite material is used as a positive electrode material of a sodium ion battery, and has a discharge specific capacity of 92 mAh / g at a 1C rate, a capacity retention rate of 58% after 160 cycles at 1C, and a discharge specific capacity of 66 mAh / g at a 10C rate.

[0126] Comparative Example 5

[0127] The difference between the example 3 is that in the preparation method of the modified sodium vanadium fluorophosphate composite material, 1.9 mol of citric acid is added dropwise in the obtained raw material mixed solution in step (1), that is, the molar ratio of citric acid to vanadium source is controlled to 1:1.

[0128] The prepared composite material is used as a positive electrode material of a sodium ion battery, and the discharge specific capacity at 1C rate reaches 100 mAh / g, the capacity retention rate after 160 cycles at 1C is 70%, and the discharge specific capacity at 10C rate reaches 79 mAh / g.

Claims

1. A modified sodium vanadium fluorophosphate composite material, characterized in that, The composite material is a tin-doped sodium vanadium fluorophosphate / graphene composite material Na3V. 2-x Sn x (PO4)2F3 / GR, 0.05≤x≤0.1; In this process, tin ions fully enter the sodium vanadium fluorophosphate lattice, while graphene is uniformly distributed around the sodium vanadium fluorophosphate particles. The individual sodium vanadium fluorophosphate particles are connected by conductive bridges, forming a three-dimensional conductive network structure.

2. A method for preparing the modified sodium vanadium fluorophosphate composite material as described in claim 1, characterized in that, Includes the following steps: (1) Preparation of gel precursor: First, the raw materials ammonium metavanadate, ammonium dihydrogen phosphate, sodium fluoride, and tin source are added to the solvent deionized water and mixed evenly to obtain a raw material mixed solution; wherein, the tin source is tin chloride or tin acetate; Then, citric acid was added to the resulting raw material mixture solution, and stirring was continued for 1-2 hours to form a stable sol system; wherein the molar ratio of citric acid to ammonium metavanadate was 3:(1.9-2); The resulting sol was heated in a water bath at 60℃-100℃ and continuously stirred and dried to form a gel; Finally, the obtained gel was dried at 80℃-120℃ for 12-24h to obtain the dried gel precursor. (2) Pre-sintering treatment: The dried gel precursor obtained in step (1) was placed in a muffle furnace and heated to 300℃-450℃ at a heating rate of 5℃ / min-10℃ / min under an air atmosphere, and held for 3-5 hours for pre-sintering. After the pre-sintered product is naturally cooled to room temperature, it is taken out and ground to obtain pre-sintered powder. (3) Composite treatment with graphene: The pre-sintered powder obtained in step (2) is mixed with graphene in a set ratio and ball-milled in ethanol medium for 4-10 hours at a speed of 300 r / min-500 r / min. The ball-milled mixture was vacuum dried at 80℃-120℃ for 12-24h to obtain a composite of pre-sintered powder and graphene. (4) Calcination treatment: The composite obtained in step (3) was placed in a tube furnace and heated to 600℃-750℃ at a heating rate of 5℃ / min-10℃ / min under an argon atmosphere, and held for 6-10h for calcination. After the calcined product is naturally cooled to room temperature, it is ground into powder again to obtain the modified sodium vanadium fluorophosphate composite material.

3. The preparation method of the modified sodium vanadium fluorophosphate composite material according to claim 2, characterized in that, In step (3), the amount of graphene added is 1%-5% of the mass of sodium vanadium fluorophosphate.

4. The preparation method of the modified sodium vanadium fluorophosphate composite material according to claim 3, characterized in that, In step (3), the amount of graphene added is 2% of the mass of sodium vanadium fluorophosphate.

5. The method for preparing the modified sodium vanadium fluorophosphate composite material according to claim 2, characterized in that, The preparation of the raw material mixture solution in step (1) is carried out by stirring and mixing at a speed of 400r / min-800r / min for 2-4 hours at 25℃-45℃.

6. The application of the modified sodium vanadium fluorophosphate composite material as described in claim 1 or the modified sodium vanadium fluorophosphate composite material prepared by the preparation method according to any one of claims 2-5 in a sodium-ion battery, characterized in that, This composite material is used as the positive electrode material for sodium-ion batteries.

7. The application according to claim 6, characterized in that, When the modified sodium vanadium fluorophosphate composite material is used as the positive electrode material of sodium-ion battery, the discharge specific capacity at 1C rate is 120mAh / g-130mAh / g; after 160 cycles, the capacity retention rate is 90%-98%; and the discharge specific capacity at 10C rate is maintained at 100mAh / g-110mAh / g.

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