Vanadium sodium fluorophosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention provides a sodium vanadium fluorophosphate positive electrode material and a preparation method and application thereof, and belongs to the technical field of sodium ion battery positive electrode materials. The preparation method comprises the following steps: mixing sodium salt, a vanadium source, a phosphorus source, villiaumite and a carbon source in water, obtaining xerogel by adopting a sol-gel method, calcining to obtain a carbon-coated sodium vanadium fluorophosphate positive electrode material, dispersing the carbon-coated sodium vanadium fluorophosphate positive electrode material in water to form a suspension, adding polyvinyl alcohol and manganese salt, uniformly stirring, heating to evaporate a solvent, and annealing in inert gas to obtain the carbon-coated sodium vanadium fluorophosphate positive electrode material. The carbon and manganese dioxide synergistically co-coated sodium vanadium fluorophosphate positive electrode material is obtained. A sol-gel method and a suspension coating technology are combined to construct the carbon and manganese dioxide dual-coated NVPF composite material, so that the problems of slow sodium ion diffusion kinetics, low electronic conductivity, interface side reaction and the like when the NVPF is used as a sodium ion battery positive electrode material are solved.

Description

Technical Field

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

[0002] Sodium-ion batteries (SIBs) are considered to be one of the most promising alternatives to lithium-ion batteries (LIBs) due to their abundant sodium resources, low cost, and similar chemical properties to those of lithium-ion batteries (LIBs). However, the radius of sodium ions is 1.02Å, while that of lithium ions is 0.76Å. The larger radius and mass of sodium ions cause them to easily cause structural collapse and volume changes when they are extracted and embedded in electrode materials, thus limiting their practical applications. Among them, the polyanionic compound sodium vanadium fluorophosphate (Na3V2(PO4)2F3, abbreviated as NVPF) has an open NASICON three-dimensional structure and a high theoretical capacity (128mAh·g -1 ) and working voltage (3.95V), and is considered an ideal cathode material for SIBs. However, the inherent defects of NVPF include low electronic conductivity (<10 -9 s·cm -1 ), resulting in unsatisfactory rate performance and long cycle performance, and its slow Na + Diffusion kinetics (diffusion coefficients are typically less than 10 -14 cm 2 ·s -1 ), which limits its performance at high current density.

[0003] At present, the performance optimization research of NVPF cathode materials mainly focuses on modification strategies such as carbon-based / metal oxide coating and nanostructure regulation. In terms of conductivity optimization, although the carbon coating layer constructed by sol-gel method or solid phase method can significantly improve the electronic conductivity of the material, the single carbon coating often has the problem of uneven distribution of the surface carbon layer, which leads to the limitation of the structural stability and thermodynamic properties of the material. In terms of ion transport kinetics optimization, the nano-sizing strategy can effectively shorten the Na + However, the inherent high surface energy of nanoparticles can easily lead to particle agglomeration, which also causes the tap density of electrode materials to decrease, which is not conducive to practical applications. In terms of improving interface stability, metal oxides (such as Al2O3, TiO 2-x Coatings (e.g., RuO2) can effectively suppress side reactions at the electrode / electrolyte interface. However, the high cost of precious metal coating materials like RuO2 limits their industrial application prospects. Therefore, developing multifunctional composite coating systems that combine cost advantages with structural stability remains an important research direction for improving the comprehensive electrochemical performance of NVPF cathode materials.

[0004] In order to improve the comprehensive electrochemical performance of NVPF positive electrode materials, existing technologies use two major strategies: carbon coating and metal oxide coating to improve their electronic conductivity and interface stability. However, in terms of carbon coating, the use of organic carbon sources such as glucose, sucrose, and tannic acid can significantly improve the electronic conductivity and rate performance of the material. To further optimize the performance, some studies have adopted a dual carbon coating strategy, such as reducing graphene oxide (rGO) and nanocellulose to collaboratively construct a three-dimensional conductive network, which effectively improves the ion / electron transmission efficiency. However, traditional carbon coating still faces problems such as uneven distribution of the carbon layer, structural phase transition or lattice distortion caused by high-temperature carbonization, and amorphous carbon (sp 3 Too high a ratio of hybridization will reduce conductivity. The root cause of the problems in existing carbon coating or metal oxide coating technology lies mainly in the intrinsic properties of the materials and the physical and chemical mechanisms during the preparation process. The inhomogeneity of the carbon coating layer is due to the phase separation phenomenon and mass transfer differences during the pyrolysis of the organic precursor. The difference in thermodynamic stability of different carbon source components during high-temperature carbonization leads to non-uniform shrinkage. At the same time, the mismatch in thermal expansion coefficients between NVPF and the carbon layer produces significant interfacial stress during sintering, causing VO octahedral lattice distortion and sp 3 The proportion of hybrid carbon increases. In terms of metal oxide coating, Al2O3, TiO 2-x Coating layers such as RuO2 can effectively inhibit the side reactions at the electrode / electrolyte interface. The capacity retention rate of RuO2-coated NVPF is as high as 88% after 500 cycles at 5C rate. The limitation of metal oxide coating is mainly limited by the electronic structure characteristics. The excellent performance of RuO2 comes from its 4d 4 The electronic configuration provides catalytic active sites, but the high cost of precious metals restricts their practicality; while Al2O3, TiO 2-x The insulating properties of transition metal oxides such as NH4O4 and NH4O4 lead to the obstruction of electron transfer, and the interface energy level offset between NH4O4 and NVPF forms a charge transfer barrier. In addition, the difficulty in controlling the thickness of the coating layer involves the balance between surface energy regulation and diffusion dynamics. When the coating layer is too thin, the surface energy is not enough to completely cover the active sites, while when it is too thick, it will significantly increase the Na + Diffusion activation energy. These intrinsic limitations make it difficult for existing coating strategies to simultaneously meet the multiple requirements of conductivity, stability, and economy. To this end, the present invention proposes a carbon and manganese dioxide synergistically co-coated sodium vanadium fluorophosphate positive electrode material and a preparation method thereof. Summary of the Invention

[0005] The present invention proposes a sodium vanadium fluorophosphate cathode material, its preparation method and application. By combining the sol-gel method with the suspension coating technology, a NVPF composite material (NVPF / C@MnO2) doubly coated with carbon (C) and manganese dioxide (MnO2) is constructed to solve the problems of sluggish sodium ion diffusion kinetics, low electronic conductivity and interfacial side reactions when NVPF is used as a cathode material for sodium ion batteries.

[0006] To achieve the above objectives, the present invention provides the following technical solutions:

[0007] One of the technical solutions of the present invention:

[0008] A method for preparing a sodium vanadium fluorophosphate positive electrode material comprises the following steps:

[0009] Sodium salt, vanadium source, phosphorus source, fluoride salt and carbon source are mixed in water and xerogel is obtained by sol-gel method;

[0010] The dry gel is calcined in an inert gas, and then ground after cooling to obtain a carbon-coated sodium vanadium fluorophosphate positive electrode material (NVPF / C powder);

[0011] The carbon-coated sodium vanadium fluorophosphate cathode material is dispersed in water to form a suspension, polyvinyl alcohol (PVA, as a dispersant) and manganese salt are added and stirred evenly, and then the solvent is evaporated by heating. The obtained solid powder is annealed in an inert gas to obtain a sodium vanadium fluorophosphate cathode material synergistically co-coated with carbon and manganese dioxide (NVPF / C@MnO2).

[0012] Beneficial effects: The present invention utilizes the sol-gel method and suspension coating technology to construct a multi-level functionalized coating system, thereby achieving synergistic optimization of the electrochemical properties of the material. On the one hand, the conductive network of the carbon material is utilized to improve the electron transfer efficiency. On the other hand, the thermal stability and chemical stability of manganese dioxide (MnO2) are utilized to compensate for the defects of single carbon coating, including inhibiting the side reactions at the electrode / electrolyte interface, improving the structural stability of the material, and especially improving the high-rate performance and low-temperature cycle performance. This double-coating design, through the complementary advantages of MnO2 and the carbon layer, not only solves the problem of decreased tap density caused by incomplete carbon coating of nanoparticles, but also avoids the shortcomings of poor heat resistance of pure carbon materials. At the same time, the passivation effect of MnO2 on the electrode surface reduces the decomposition of the electrolyte. Finally, on the basis of maintaining the high operating voltage characteristics of NVPF, a stable interface structure with fast ion / electron dual channels is established, providing new ideas for the development of high-rate performance and long cycle life sodium-ion battery positive electrode materials.

[0013] Furthermore, the molar ratio of the sodium salt, the vanadium source, the phosphorus source and the fluoride salt is (3-5): (1-4): (0.5-2): (3-5).

[0014] Furthermore, based on the C atoms in the carbon source and the V atoms in the vanadium source, the molar ratio of C atoms to V atoms is (4-6):1.

[0015] Furthermore, the vanadium source is vanadium pentoxide (V2O5) or ammonium metavanadate (NH4VO3).

[0016] Furthermore, the phosphorus source is ammonium dihydrogen phosphate (NH4H2PO4) or diammonium hydrogen phosphate ((NH4)2HPO4).

[0017] Furthermore, the carbon source is citric acid (C6H8O7), glucose (C6H 12 O6) or sucrose (C 12 H 22 O 11 ).

[0018] Furthermore, the calcination treatment is to sinter the dry gel at 300-400° C. for 4-8 hours in a nitrogen or argon atmosphere, and then continue to sinter at 650-750° C. for 6-10 hours in a nitrogen or argon atmosphere after heating.

[0019] Beneficial effects: In the preparation method of the present invention, the carbon source can be decomposed during the calcination process to form a uniform and continuous amorphous carbon layer, which significantly improves the electron conductivity. In addition, the carbon source inhibits the excessive growth of NVPF grains during the sol-gel process, forming particles with smaller particle size and uniform distribution, which increases the specific surface area and shortens the Na + The diffusion path prevents particle agglomeration during high-temperature calcination, thereby optimizing the structural stability of the material. In addition, green preparation can be achieved through the sol-gel method, without the need for complex equipment, and there is no harmful residue after decomposition, which fully meets the requirements of large-scale production.

[0020] Furthermore, the annealing treatment is sintering at 300-500° C. for 2-6 hours in a nitrogen or argon atmosphere.

[0021] Furthermore, the coating amount of manganese dioxide in the sodium vanadium fluorophosphate positive electrode material is 1-5 wt.% (the coating amount refers to the mass proportion of the manganese dioxide layer in the sodium vanadium fluorophosphate positive electrode material being 1-5%).

[0022] The second technical solution of the present invention:

[0023] A sodium vanadium fluorophosphate positive electrode material prepared by the above preparation method.

[0024] The third technical solution of the present invention:

[0025] An application of the above-mentioned sodium vanadium fluorophosphate positive electrode material in sodium ion batteries.

[0026] Furthermore, the above-mentioned carbon and manganese dioxide composite-coated sodium vanadium fluorophosphate positive electrode material is stirred with acetylene black and polyvinylidene fluoride (PVDF) to form a slurry, coated on aluminum foil, and then dried, punched and pressed to produce a carbon and manganese dioxide composite-coated sodium vanadium fluorophosphate positive electrode material electrode sheet, which is used as the positive electrode sheet of the sodium ion battery.

[0027] The carbon and manganese dioxide synergistically co-coated sodium vanadium fluorophosphate cathode material provided by the present invention is successfully constructed by the sol-gel method and suspension coating technology. Compared with the existing technology, the present invention has the following advantages and technical effects:

[0028] (1) The present invention adopts the sol-gel method and suspension coating technology, which is controllable and easy to scale up, avoiding the high energy consumption problem of the traditional high-temperature solid-phase method. Among them, MnO2 is a cheap and environmentally friendly transition metal oxide, which reduces the material cost.

[0029] (2) The present invention uses a carbon layer and an appropriate amount of MnO2 to form a uniform interface layer, wherein the carbon layer provides an electron conduction path and the MnO2 layer optimizes the Na + Diffusion interface, effectively inhibiting Na + Dissolution and side reactions can significantly improve the electronic conductivity and structural stability of the material.

[0030] (3) The carbon and manganese dioxide-coated sodium vanadium fluorophosphate cathode material of the present invention has excellent capacity and rate performance, showing 128.92 mAh g at 0.1C. -1 The high reversible specific capacity is significantly better than the discharge capacity of the uncoated manganese dioxide sample (101.86 mAh·g -1 ). BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The accompanying drawings, which constitute part of the present invention, are provided to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are provided to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0032] Figure 1 The XRD patterns of the sodium vanadium fluorophosphate cathode materials prepared in Comparative Example 1 and Examples 1-5 are shown;

[0033] Figure 2 This is the first charge and discharge curve of Comparative Example 1 at a rate of 0.1C;

[0034] Figure 3 This is the first charge and discharge curve of Example 1 at a rate of 0.1C;

[0035] Figure 4 This is the first charge and discharge curve of Example 2 at a rate of 0.1C;

[0036] Figure 5 This is the first charge and discharge curve of Example 3 at a rate of 0.1C;

[0037] Figure 6 This is the first charge and discharge curve of Example 4 at a rate of 0.1C;

[0038] Figure 7 This is the first charge and discharge curve of Example 5 at a rate of 0.1C;

[0039] Figure 8 The capacity retention rate diagram of Comparative Example 1 and Examples 1-5 after 100 cycles at 1C rate. DETAILED DESCRIPTION

[0040] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0041] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0042] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0043] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

[0044] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0045] The present invention provides a method for preparing a sodium vanadium fluorophosphate cathode material, comprising the following steps:

[0046] Sodium salt, vanadium source, phosphorus source, fluoride salt and carbon source are mixed in water and xerogel is obtained by sol-gel method;

[0047] The dry gel was calcined in an inert gas, cooled, and then ground to obtain a carbon-coated sodium vanadium fluorophosphate cathode material (NVPF / C powder).

[0048] The carbon-coated sodium vanadium fluorophosphate cathode material is dispersed in water to form a suspension, polyvinyl alcohol (PVA, as a dispersant) and manganese salt are added and stirred evenly, then the solvent is heated to evaporate, and the resulting solid powder is annealed in an inert gas to obtain a sodium vanadium fluorophosphate cathode material synergistically co-coated with carbon and manganese dioxide (NVPF / C@MnO2).

[0049] In a preferred embodiment of the present invention, the molar ratio of the sodium salt, the vanadium source, the phosphorus source and the fluoride salt is (3-5): (1-4): (0.5-2): (3-5), and most preferably, the molar ratio of the sodium salt, the vanadium source, the phosphorus source and the fluoride salt is 3.3:2:2:3.3.

[0050] In a preferred embodiment of the present invention, the molar ratio of C atoms to V atoms is (4-6):1, based on the C atoms in the carbon source and the V atoms in the vanadium source. Most preferably, the molar ratio of C atoms to V atoms is 4.8:1.

[0051] In a preferred embodiment of the present invention, the vanadium source is vanadium pentoxide (V2O5) or ammonium metavanadate (NH4VO3), and most preferably, the vanadium source is NH4VO3.

[0052] In a preferred embodiment of the present invention, the phosphorus source is ammonium dihydrogen phosphate (NH4H2PO4) or diammonium hydrogen phosphate ((NH4)2HPO4), and most preferably, the phosphorus source is NH4H2PO4.

[0053] In a preferred embodiment of the present invention, the carbon source is citric acid (C6H8O7), glucose (C6H 12 O6) or sucrose (C 12 H 22 O 11 ), most preferably, the carbon source is citric acid (C6H8O7).

[0054] In a preferred embodiment of the present invention, the sodium salt and the fluoride salt are added in the form of NaF, but are not limited to such compounds. For example, the sodium salt can be sodium carbonate, sodium hydroxide, sodium nitrate, etc., and the fluoride salt can be ammonium fluoride, ammonium hexafluorophosphate, etc.

[0055] In a preferred embodiment of the present invention, the calcination treatment is to sinter the dry gel at 300-400° C. for 4-8 hours in a nitrogen or argon atmosphere, and then continue to sinter at 650-750° C. for 6-10 hours in a nitrogen or argon atmosphere after heating.

[0056] In a preferred embodiment of the present invention, the annealing treatment is sintering at 300-500° C. for 2-6 hours in a nitrogen or argon atmosphere.

[0057] In a preferred embodiment of the present invention, the coating amount of manganese dioxide in the sodium vanadium fluorophosphate positive electrode material is 1-5 wt.%, preferably 1 wt.%.

[0058] Exemplarily, a method for preparing a sodium vanadium fluorophosphate positive electrode material in an embodiment of the present invention specifically includes the following steps:

[0059] The vanadium source was dissolved in deionized water and stirred at 80°C. The carbon source was dissolved in deionized water and added to the vanadium source solution. The mixture was heated and stirred continuously until the solution turned from yellow to dark blue. Then, the sodium salt and phosphorus source were added and stirred until the solution turned blue. The mixture was transferred to a vacuum oven at 100°C and dried for 12 hours to obtain a xerogel.

[0060] The dry gel was calcined in two steps under an argon atmosphere: first, sintered at 300-400°C for 4-8 h, then heated to 650-750°C for 6-10 h (heating rate 5°C / min), and then ground to obtain NVPF / C powder after cooling.

[0061] NVPF / C powder was dispersed in deionized water, and 0.5 wt.% polyvinyl alcohol (PVA) solution was added. After ultrasonic treatment for 20 min, magnetic stirring was performed at room temperature (25 ± 3 °C, the same below) for 3 h to obtain NVPF / C suspension.

[0062] Manganese salt was dissolved in deionized water, then slowly added dropwise to the NVPF / C suspension and stirred for 3 h. After stirring at 60 °C for 1 h, the temperature was raised to 80 °C and the solvent was evaporated to obtain a black slurry.

[0063] The black slurry was dried in an oven at 120°C for 12 hours, ground and sintered at 300-500°C for 2-6 hours in an argon atmosphere, and then ground after cooling to obtain a sodium vanadium fluorophosphate positive electrode material synergistically coated with carbon and manganese dioxide (NVPF / C@MnO2).

[0064] In the preparation process of the sodium vanadium fluorophosphate positive electrode material synergistically co-coated with carbon and manganese dioxide in the embodiment of the present invention, first, the vanadium source is dissolved in deionized water and stirred at 80°C to form a vanadium ion solution; after the carbon source is added, the carbon source molecules and the vanadium ions are evenly mixed through coordination or reduction reactions, and the solution changes from yellow (the characteristic color of pentavalent vanadium ions) to dark blue, indicating that a stable sol system (vanadium-carbon composite sol) has been formed; then, sodium salt and phosphorus source are added, and the reactants are further dispersed at the molecular level in the sol, and a blue gel (a mixed gel containing vanadium, sodium, phosphorus, and carbon) is formed after stirring; and the xerogel is obtained by vacuum drying at 100°C, at which time the carbon source is evenly embedded in the gel network. The xerogel is calcined in two steps under argon (inert atmosphere): sintering at 300-400°C for 4-8 hours to remove organic impurities from the gel and initially carbonize the carbon source; and sintering at 650-750°C for 6-10 hours (at a temperature ramp of 5°C / min) to promote the formation of the NVPF crystal structure and completely carbonize the carbon source, forming a uniform carbon layer (C layer) on the surface of the NVPF particles to produce NVPF / C powder. During this process, the sol-gel method ensures molecular-level homogeneous mixing of the carbon source and the NVPF precursor. The resulting carbon layer tightly and evenly coats the NVPF particle surface, preventing carbon agglomeration and paving the way for subsequent electron conduction.

[0065] NVPF / C powder was then dispersed in deionized water. A PVA solution was added and ultrasonicated for 20 minutes, followed by magnetic stirring at room temperature for 3 hours. The PVA, acting as a dispersant and binder, interacted with the NVPF / C surface through its hydroxyl groups, inhibiting particle agglomeration and forming a stable NVPF / C suspension (uniformly dispersed particles). Upon addition of the manganese salt solution, manganese ions were uniformly deposited on the NVPF / C particle surface through electrostatic adsorption or hydrogen bonding with the PVA. After stirring at 60°C for 1 hour, the solvent was evaporated at 80°C, allowing the manganese salt precursor to adhere evenly to the particle surface, forming a black slurry. The slurry was dried at 120°C and sintered at 300-500°C under argon for 2-6 hours. The manganese salt decomposed, forming a uniform MnO2 layer on the NVPF / C surface, ultimately yielding the NVPF / C@MnO2 composite cathode material (a double-coated structure with a carbon layer on the inside and a MnO2 layer on the outside). During this process, the suspension coating technology ensures the uniform coverage of the MnO2 layer on the surface of the carbon coating layer through the dispersion effect of PVA and the uniform deposition of manganese ions, avoiding local agglomeration.

[0066] The embodiment of the present invention further provides a positive electrode material of sodium vanadium fluorophosphate synergistically co-coated with carbon and manganese dioxide prepared by the above preparation method.

[0067] The carbon (such as amorphous carbon) in the sodium vanadium fluorophosphate positive electrode material synergistically coated with carbon and manganese dioxide in the embodiment of the present invention has good electronic conductivity. The uniform carbon layer formed by the sol-gel method directly connects the NVPF particles to construct an "electronic conduction network" throughout the entire material, reducing the transmission resistance of electrons between particles. The close contact between the carbon layer and the NVPF particles (derived from the molecular-level mixing of the sol-gel method) avoids the interface barrier of electron transmission and significantly improves the electronic conductivity of the overall material. The MnO2 layer can optimize the Na + Diffusion interface, the MnO2 layer has a layered or tunnel structure, and its lattice gap is suitable for Na + Rapid diffusion, can be used as Na + transmission buffer layer, reducing Na + The diffusion resistance on the NVPF surface is reduced due to Na + The structural stress caused by insertion / extraction, on the other hand, the MnO2 layer can inhibit the Na + Dissolution and side reactions: During the charge and discharge process of NVPF, vanadium ions are easily dissolved due to electrolyte corrosion, leading to structural collapse; the MnO2 layer acts as a physical barrier to isolate the direct contact between NVPF and the electrolyte, inhibiting the dissolution of vanadium ions; at the same time, MnO2 has high chemical stability, which can reduce the decomposition side reactions of electrolytes (such as carbonates) under high voltage and protect the crystal structure of NVPF. The carbon layer and the MnO2 layer form an "electron-ion dual pathway" synergistically: the carbon layer ensures rapid electron conduction, solving the problem of low intrinsic conductivity of NVPF; the MnO2 layer optimizes the Na + The uniform coating of both (without local defects) ensures that the transmission efficiency of electrons and ions is simultaneously improved during the charge and discharge cycle, and the structure is not easily damaged by stress or side reactions, ultimately achieving a significant improvement in electronic conductivity and structural stability.

[0068] The technical solution of the present invention is further illustrated by the following examples.

[0069] Example 1

[0070] A method for preparing a sodium vanadium fluorophosphate positive electrode material specifically comprises the following steps:

[0071] 5 mmol of NH4VO3 was weighed and dissolved in 50 mL of deionized water. The mixture was stirred at 80°C to obtain an NH4VO3 solution. 4 mmol of C6H8O7 was dissolved in 20 mL of deionized water and added to the NH4VO3 solution (the molar ratio of C atoms to V atoms was 4.8:1). The mixture was heated and stirred at 80°C until the solution changed from yellow to dark blue. 8.25 mmol of NaF and 5 mmol of NH4H2PO4 were then added (to avoid the loss of NaF during the calcination process, the molar excess of NaF was 10%. The molar ratio of sodium salt, vanadium source, phosphorus source and fluoride salt was 3.3:2:2:3.3). The mixture was stirred until the solution turned blue. The mixture was then transferred to a vacuum oven at 100°C and dried for 12 h to obtain a xerogel.

[0072] The dry gel was calcined in two steps under argon atmosphere: first sintered at 350 °C for 4 h, then heated to 675 °C for 8 h (heating rate 5 °C / min), and then ground to obtain NVPF / C powder.

[0073] 2 g of NVPF / C powder was dispersed in 50 mL of deionized water, and 2 mL of 0.5 wt.% PVA solution was added. After ultrasonication for 20 min, magnetic stirring was performed at room temperature for 3 h to obtain an NVPF / C suspension.

[0074] 0.058 g of Mn(NO3)2·4H2O was dissolved in 30 mL of deionized water, and then added dropwise to the NVPF / C suspension at a rate of 0.1 mL / s and stirred for 3 h. After stirring at 60 °C for 1 h, the temperature was raised to 80 °C and the solvent was evaporated to obtain a black slurry.

[0075] The black slurry was dried in an oven at 120°C for 12 hours, ground, and sintered at 400°C for 4 hours under an argon atmosphere. After cooling, the slurry was ground to obtain a sodium vanadium fluorophosphate cathode material synergistically coated with carbon and manganese dioxide.

[0076] The manganese dioxide loading amount in the sodium vanadium fluorophosphate positive electrode material prepared in this embodiment is 1 wt.%.

[0077] Example 2

[0078] A method for preparing a sodium vanadium fluorophosphate positive electrode material specifically comprises the following steps:

[0079] 5 mmol of NH4VO3 was weighed and dissolved in 50 mL of deionized water. The mixture was stirred at 80°C to obtain an NH4VO3 solution. 4 mmol of C6H8O7 was dissolved in 20 mL of deionized water and added to the NH4VO3 solution (the molar ratio of C atoms to V atoms was 4.8:1). The mixture was heated and stirred at 80°C until the solution changed from yellow to dark blue. 8.25 mmol of NaF and 5 mmol of NH4H2PO4 were then added (to avoid the loss of NaF during the drying process, the molar excess of NaF was 10%. The molar ratio of sodium salt, vanadium source, phosphorus source and fluoride salt was 3.3:2:2:3.3). The mixture was stirred until the solution turned blue. The mixture was then transferred to a vacuum oven at 100°C and dried for 12 h to obtain a xerogel.

[0080] The dry gel was calcined in two steps under argon atmosphere: first sintered at 350 °C for 4 h, then heated to 675 °C for 8 h (heating rate 5 °C / min), and then ground to obtain NVPF / C powder.

[0081] 2 g of NVPF / C powder was dispersed in 50 mL of deionized water, and 2 mL of 0.5 wt.% PVA solution was added. After ultrasonication for 20 min, magnetic stirring was performed at room temperature for 3 h to obtain an NVPF / C suspension.

[0082] 0.174 g of Mn(NO3)2·4H2O was dissolved in 30 mL of deionized water, and then added dropwise to the NVPF / C suspension at a rate of 0.1 mL / s and stirred for 3 h. After stirring at 60 °C for 1 h, the temperature was raised to 80 °C and the solvent was evaporated to obtain a black slurry.

[0083] The black slurry was dried in an oven at 120°C for 12 hours, ground, and sintered at 400°C for 4 hours under an argon atmosphere. After cooling, the slurry was ground to obtain a sodium vanadium fluorophosphate cathode material synergistically coated with carbon and manganese dioxide.

[0084] The manganese dioxide loading in the sodium vanadium fluorophosphate positive electrode material prepared in this embodiment is 3 wt.%.

[0085] Example 3

[0086] A method for preparing a sodium vanadium fluorophosphate positive electrode material is the same as that in Example 1, except that the carbon and manganese dioxide are synergistically co-coated, and the manganese dioxide loading in the sodium vanadium fluorophosphate positive electrode material is 5 wt.%. The method specifically comprises the following steps:

[0087] 5 mmol of NH4VO3 was dissolved in 50 mL of deionized water and stirred at 80°C to obtain an NH4VO3 solution. 4 mmol of C6H8O7 was dissolved in 20 mL of deionized water and added to the NH4VO3 solution (the molar ratio of carbon atoms to vanadium atoms was 4.8:1). The solution was heated and stirred at 80°C until the color of the solution changed from yellow to dark blue. 8.25 mmol of NaF and 5 mmol of NH4H2PO4 were then added (to avoid the loss of NaF during the drying process, the molar excess of NaF was 10%; the molar ratio of sodium salt, vanadium source, phosphorus source, and fluoride salt was 3.3:2:2:3.3). The solution was stirred until it turned blue and then transferred to a vacuum oven at 100°C for 12 h to obtain a xerogel.

[0088] The dry gel was calcined in two steps under argon atmosphere: first sintered at 350 °C for 4 h, then heated to 675 °C for 8 h (heating rate 5 °C / min), and then ground to obtain NVPF / C powder.

[0089] 2 g of NVPF / C powder was dispersed in 50 mL of deionized water, and 2 mL of 0.5 wt.% PVA solution was added. After ultrasonication for 20 min, magnetic stirring was performed at room temperature for 3 h to obtain an NVPF / C suspension.

[0090] 0.29 g of Mn(NO3)2·4H2O was dissolved in 30 mL of deionized water, and then added dropwise to the NVPF / C suspension at a rate of 0.1 mL / s and stirred for 3 h. After stirring at 60 °C for 1 h, the temperature was raised to 80 °C and the solvent was evaporated to obtain a black slurry.

[0091] The black slurry was dried in an oven at 120°C for 12 hours, ground, and sintered at 400°C for 4 hours under an argon atmosphere. After cooling, the slurry was ground to obtain a sodium vanadium fluorophosphate cathode material synergistically coated with carbon and manganese dioxide.

[0092] The manganese dioxide loading in the sodium vanadium fluorophosphate positive electrode material prepared in this embodiment is 5 wt.%.

[0093] Example 4

[0094] A method for preparing a sodium vanadium fluorophosphate positive electrode material specifically comprises the following steps:

[0095] Weigh 2.5mmol V2O5 and dissolve it in 50mL deionized water. Stir it at 80℃ to obtain V2O5 solution. 12O6 was dissolved in 20 mL of deionized water and added to a V2O5 solution (the molar ratio of C atoms to V atoms was 6:1 at this time). The solution was heated and stirred at 80°C until the solution changed from yellow to dark blue. Then, 9 mmol of NaF and 4 mmol of (NH4)2HPO4 were added (to avoid the loss of NaF during the drying process, the molar excess of NaF was 20%, and the molar ratio of sodium salt, vanadium source, phosphorus source and fluoride salt was 3.6:1:1.6:3.6 at this time). The solution was stirred until it turned blue and then transferred to a vacuum oven at 100°C for 12 h to obtain a xerogel.

[0096] The dry gel was calcined in two steps under argon atmosphere: first sintered at 300 °C for 8 h, then heated to 650 °C for 10 h (heating rate 5 °C / min), and then ground to obtain NVPF / C powder.

[0097] 2 g of NVPF / C powder was dispersed in 50 mL of deionized water, and 2 mL of 0.5 wt.% PVA solution was added. After ultrasonication for 20 min, magnetic stirring was performed at room temperature for 3 h to obtain an NVPF / C suspension.

[0098] 0.058 g of Mn(NO3)2·4H2O was dissolved in 30 mL of deionized water, and then added dropwise to the NVPF / C suspension at a rate of 0.1 mL / s and stirred for 3 h. After stirring at 60 °C for 1 h, the temperature was raised to 80 °C and the solvent was evaporated to obtain a black slurry.

[0099] The black slurry was dried in an oven at 120°C for 12 hours, ground, and sintered at 500°C for 2 hours in an argon atmosphere. After cooling, the slurry was ground to obtain a sodium vanadium fluorophosphate cathode material synergistically coated with carbon and manganese dioxide.

[0100] The manganese dioxide loading amount in the sodium vanadium fluorophosphate positive electrode material prepared in this embodiment is 1 wt.%.

[0101] Example 5

[0102] A method for preparing a sodium vanadium fluorophosphate positive electrode material specifically comprises the following steps:

[0103] Weigh 2.5mmol V2O5 and dissolve it in 50mL deionized water, stir it at 80℃ to get V2O5 solution, and add 2mmolC 12 H 22 O 11Dissolve in 20 mL of deionized water, add to V2O5 solution (at this time, the molar ratio of C atoms to V atoms is 6:1), maintain heating and stirring at 80°C until the solution changes from yellow to dark blue, then add 9 mmol of NaF and 4 mmol of (NH4)2HPO4 (to avoid the loss of NaF during the drying process, the molar excess of NaF is 20%, and the molar ratio of sodium salt, vanadium source, phosphorus source and fluoride salt is 3.6:1:1.6:3.6), stir until the solution turns blue, transfer to a vacuum oven at 100°C and dry for 12 h to obtain a xerogel;

[0104] The dry gel was calcined in two steps under argon atmosphere: first sintered at 400 °C for 5 h, then heated to 750 °C for 6 h (heating rate 5 °C / min), and then ground to obtain NVPF / C powder.

[0105] 2 g of NVPF / C powder was dispersed in 50 mL of deionized water, and 2 mL of 0.5 wt.% PVA solution was added. After ultrasonication for 20 min, magnetic stirring was performed at room temperature for 3 h to obtain an NVPF / C suspension.

[0106] 0.058 g of Mn(NO3)2·4H2O was dissolved in 30 mL of deionized water, and then added dropwise to the NVPF / C suspension at a rate of 0.1 mL / s and stirred for 3 h. After stirring at 60 °C for 1 h, the temperature was raised to 80 °C and the solvent was evaporated to obtain a black slurry.

[0107] The black slurry was dried in an oven at 120°C for 12 hours, ground, and sintered at 300°C for 6 hours in an argon atmosphere. After cooling, the slurry was ground to obtain a sodium vanadium fluorophosphate cathode material synergistically coated with carbon and manganese dioxide.

[0108] The manganese dioxide loading amount in the sodium vanadium fluorophosphate positive electrode material prepared in this embodiment is 1 wt.%.

[0109] Comparative Example 1

[0110] The same as Example 1, except that the manganese dioxide coating process is omitted, specifically comprising the following steps:

[0111] 5 mmol of NH4VO3 was weighed and dissolved in 50 mL of deionized water. The mixture was stirred at 80°C to obtain an NH4VO3 solution. 4 mmol of C6H8O7 was dissolved in 20 mL of deionized water and added to the NH4VO3 solution. The mixture was heated and stirred at 80°C until the solution changed from yellow to dark blue. 8.25 mmol of NaF and 5 mmol of NH4H2PO4 were then added (to avoid loss of NaF during the drying process, the molar excess of NaF was 10%; the molar ratio of sodium salt, vanadium source, phosphorus source, and fluoride salt was 3.3:2:2:3.3). The mixture was stirred until the solution turned blue. The mixture was then transferred to a vacuum oven at 100°C and dried for 12 h to obtain a xerogel.

[0112] The dry gel was calcined in two steps under argon atmosphere: first sintered at 350 °C for 4 h, then heated to 675 °C for 8 h (heating rate 5 °C / min), and then ground to obtain NVPF / C powder.

[0113] Performance Testing

[0114] The XRD patterns of the sodium vanadium fluorophosphate cathode materials prepared in Comparative Example 1 and Examples 1-5 are shown in FIG. Figure 1 ,Depend on Figure 1 It can be seen that the characteristic diffraction peaks of the six samples in Comparative Example 1 and Examples 1-5 fully match the standard diffraction patterns of NVPF, with no additional stray scattering peaks present. This result indicates that neither the residual carbon introduced by citric acid nor the manganese dioxide coating, nor the subsequent secondary heat treatment, caused changes in the NVPF crystal structure.

[0115] The sodium vanadium fluorophosphate cathode materials prepared in Examples 1-5 and Comparative Example 1 were used as the positive electrode materials for sodium-ion batteries. The cathode material, acetylene black, and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 8:1:1, and N-methylpyrrolidone (NMP) was added to form a slurry. The slurry was coated on aluminum foil and dried in a vacuum oven at 120°C for 10 hours. CR2032 cells were assembled in an argon glove box using sodium metal as the negative electrode, Whatman GF / D as the separator, and 1M NaClO4 / PC (containing 5 vol.% FEC) as the electrolyte. The assembled cells were allowed to rest for 12 hours and then placed in an electrochemical channel to test their constant current charge and discharge (GCD) performance. Testing included material specific capacity, cycle performance, and rate capability.

[0116] Figure 2-7 The first charge and discharge curves of Comparative Example 1 and Examples 1-5 at a rate of 0.1C are shown respectively. Figure 8 The figure shows the capacity retention rate of Comparative Example 1 and Examples 1-5 after 100 cycles at 1C rate. Figure 2-7 It can be seen that the first discharge specific capacities of the five electrode materials in Examples 1-5 after co-coating with MnO2 and carbon at 0.1C are 128.92, 120.08, 121.11, 111.27, and 110.16 mAh·g, respectively. -1 , which is higher than the discharge capacity of the uncoated comparative example 1 (101.86 mAh·g -1 The electrode materials of Comparative Example 1 and Examples 1-5 were activated for three cycles at a rate of 0.1C and then subjected to 100 charge-discharge tests at 1C. The initial reversible discharge specific capacities of the six electrode materials were 90.30, 125.27, 119.29, 119.48, 108.42, and 107.11 mAh·g, respectively. -1, indicating that the introduction of MnO2 improves the capacity of NVPF. After 100 cycles, the capacity retention rates of the six electrode materials are 82.8%, 95.9%, 93.3%, 87.9%, 85.2% and 89.0%, respectively, proving that the cycle stability of the material is further improved after MnO2 coating.

[0117] The above are only preferred specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or replacements that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention should be covered by the scope of protection of the present invention.

Claims

1. A method for preparing a sodium vanadium fluorophosphate positive electrode material, characterized in that: The following steps are involved: Sodium salt, vanadium source, phosphorus source, fluoride salt and carbon source are mixed in water and xerogel is obtained by sol-gel method; The dry gel is calcined in an inert gas, and then ground after cooling to obtain a carbon-coated sodium vanadium fluorophosphate positive electrode material; The carbon-coated sodium vanadium fluorophosphate cathode material is dispersed in water to form a suspension, polyvinyl alcohol and manganese salt are added and stirred evenly, and then the solvent is evaporated by heating. The obtained solid powder is annealed in an inert gas to obtain a sodium vanadium fluorophosphate cathode material synergistically co-coated with carbon and manganese dioxide.

2. The method for preparing the sodium vanadium fluorophosphate cathode material according to claim 1, wherein: The molar ratio of the sodium salt, the vanadium source, the phosphorus source and the fluoride salt is (3-5): (1-4): (0.5-2): (3-5).

3. The method for preparing the sodium vanadium fluorophosphate cathode material according to claim 1, wherein: Calculated based on the C atoms in the carbon source and the V atoms in the vanadium source, the molar ratio of C atoms to V atoms is (4-6):

1.

4. The method for preparing the sodium vanadium fluorophosphate cathode material according to claim 1, wherein: The vanadium source is vanadium pentoxide or ammonium metavanadate; and / or, The phosphorus source is ammonium dihydrogen phosphate or diammonium hydrogen phosphate; and / or, The carbon source is citric acid, glucose or sucrose.

5. The method for preparing the sodium vanadium fluorophosphate cathode material according to claim 1, wherein: The calcination treatment is to sinter the dry gel at 300-400° C. for 4-8 hours in a nitrogen or argon atmosphere, and then continue to sinter at 650-750° C. for 6-10 hours in a nitrogen or argon atmosphere after heating.

6. The method for preparing the sodium vanadium fluorophosphate cathode material according to claim 1, characterized in that: The annealing treatment is sintering at 300-500° C. for 2-6 hours in a nitrogen or argon atmosphere.

7. The method for preparing the sodium vanadium fluorophosphate positive electrode material according to claim 1, characterized in that: The carbon and manganese dioxide are synergistically coated with the sodium vanadium fluorophosphate positive electrode material, and the coating amount of manganese dioxide is 1-5 wt.%.

8. A sodium vanadium fluorophosphate positive electrode material prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the sodium vanadium fluorophosphate cathode material according to claim 8 in a sodium ion battery.

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