A method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for sodium ion batteries

Abstract

The present invention provides a method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for sodium ion batteries. The method increases the solubility by heating and stirring, allowing the material to be uniformly dispersed in the solution in an ionic state. Simultaneously, the interaction between a small molecule A-type thiol-containing carbon source and a high molecular weight B-type carbon source is utilized to prevent the carbon source from crystallizing. Furthermore, a stable and evenly distributed carbon-coated structure is formed after carbonization, significantly improving the electronic conductivity of the sodium vanadium phosphate material. The carbon-coated sodium vanadium phosphate material prepared by this method exhibits excellent electrochemical performance as a sodium ion battery positive electrode, with an initial discharge capacity of >110 mAh / g at a 1 C rate, a discharge capacity of >105 mAh / g at a 5 C rate, and a capacity retention rate of >99.2% after 1000 cycles at a 5 C rate. This achieves a dual improvement in both electronic conductivity and electrochemical performance of the carbon-coated sodium vanadium phosphate positive electrode material.

Description

Technical Field

[0001] The present invention specifically relates to a method for preparing a carbon-coated sodium vanadium phosphate cathode material for sodium ion batteries, belonging to the field of new energy battery material technology, which is related to sodium ion battery cathodes. Background Art

[0002] As one of the key materials in sodium ion battery system, cathode material largely determines the performance index, cycle life and manufacturing cost of the battery. + The positive electrode material of the Super Ionic Conductor (NASICON) structure sodium vanadium phosphate (Na3V2(PO4)3, NVP) belongs to the hexagonal crystal system and has the dual advantages of high rate performance and cycle performance. It has two unique sodium ion migration channels (Na1 site and Na2 site). The sodium ion reversible deintercalation at the Na2 site is located at the 3.3V voltage platform, corresponding to V 3+ / V 4+The redox reaction can provide a theoretical capacity of about 117 mAh / g, but the extremely low electronic conductivity also seriously limits the performance of its capacity. At present, the commonly used preparation methods of sodium vanadium phosphate include high-temperature solid-phase method, sol-gel method, electrostatic spinning method and hydrothermal method. Among them, the high-temperature solid-phase method is simple and easy to industrialize. In the work of CN115954456A, a high-temperature solid-phase method is adopted to ball-mill a vanadium source (V2O5, VO2 or NH4VO3), a sodium source (Na2CO3, CH3COONa or NaCl), a phosphorus source (NH4H2PO4, (NH4)2HPO4 or H3PO4) and a carbon source (citric acid, oxalic acid or gluconic acid) in a certain proportion and then perform segmented high-temperature calcination. However, the obtained sodium vanadium phosphate product is easy to agglomerate, resulting in a large particle size and uneven distribution. The sol-gel method is easy to control the stoichiometric ratio, and the product particles are uniform and have a controllable morphology. A soluble sodium source, a vanadium source, and a phosphorus source are condensed and hydrolyzed under the action of a complexing agent to form a sol. The gel formed after drying is heat-treated to obtain a sodium vanadium phosphate product. In the work of CN115872383A, the sol-gel method is coupled with the glycine combustion method. Sodium nitrate is used as a sodium source and an oxidant to promote the combustion of glycine, thereby shortening the reaction time. However, the sol-gel method is complex and has a long processing cycle, making it difficult to industrialize. In the work of CN115275140, ammonium metavanadate, oxalic acid, and sodium dihydrogen phosphate are used as raw materials, and boric acid and polyvinyl pyrrolidone are used as dopants and binders, respectively. Nanofibrous boron-doped sodium vanadium phosphate is successfully prepared by electrospinning technology and high-temperature annealing process. The material has a large specific surface area and good conductivity, which is conducive to improving electrochemical performance. However, the preparation cost of this method is high, the process is difficult, and the work efficiency is low, making it difficult to achieve large-scale industrial application. The hydrothermal method is a common liquid-phase chemical method with mild reaction conditions and easy operation. The prepared products have small particle size and uniform distribution. However, the reaction is carried out in a sealed reactor, and the crystal growth process cannot be observed and controlled. At the same time, it is highly dependent on equipment and has high costs. Industrial production has major safety risks.

[0003] Therefore, there is an urgent need for a simple and effective preparation method that can, on the one hand, generate large quantities of pure phase sodium vanadium phosphate materials in situ to meet the needs of industrial applications, and on the other hand, improve the inherent defects of the material (low electronic conductivity) to exert high capacity and high stability characteristics. Summary of the Invention

[0004] Based on the above-mentioned related technical means, the present invention increases the solubility of vanadium source, sodium source, phosphorus source and carbon source by heating and high-speed stirring, and realizes mixed carbon coating of pure phase sodium vanadium phosphate material by small molecule type A thiol-containing carbon source and high molecular type B carbon source after high temperature calcination, wherein the high molecular type B carbon source can enhance the viscosity and adsorption of the precursor solution, effectively preventing the crystallization of type A thiol-containing carbon source, in addition, the large-sized carbon layer formed after high-temperature calcination phase transition can well coat the in-situ generated sodium vanadium phosphate material, while promoting the small molecule type A thiol-containing carbon source to form a large-sized carbon layer. Small-particle sulfur-doped carbon (sulfur and carbon produce a synergistic effect through dp orbital coupling to adjust the Fermi level and improve electronic conductivity) particles formed by carbonization of the carbon source extend the core of the sodium vanadium phosphate material through internal pores, forming a uniform carbon coating structure inside and outside, thereby improving the electronic conductivity of the sodium vanadium phosphate material and thus having excellent battery performance. The first discharge capacity is greater than 110mAh / g at a 1C rate, the discharge capacity is greater than 105mAh / g at a 5C rate, and the capacity retention rate is greater than 99.2% after 1000 cycles at a 5C rate.

[0005] The preparation method of sodium vanadium phosphate composite sodium ion battery positive electrode material provided by the present invention is innovative on the basis of the traditional high-temperature solid-phase method. The solubility is improved by heating and stirring to achieve ionic mixing in the solvent. At the same time, the high electronic conductivity of the material is achieved by uniformly carbonizing the sodium vanadium phosphate material inside and outside with a small molecule A-type thiol-containing carbon source and a high molecular B-type carbon source after high-temperature carbonization. The overall operation is simple, the process is simple, the dependence on experimental equipment is low, and large-scale commercial production is easy to achieve. It provides a new idea for preparing pure and highly conductive sodium vanadium phosphate materials and other lithium / sodium electrode materials. The prepared sodium vanadium phosphate composite material has high specific capacity and long cycle life when applied to the field of sodium ion battery positive electrode materials, which is conducive to promoting the development of the sodium electricity industry and even the new energy sector.

[0006] In response to the above problems, the purpose of this patent is to provide a method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for sodium ion batteries, which specifically includes the following steps:

[0007] (1) Preparation of sodium vanadium phosphate composite precursor dispersion: Weigh a certain amount of sodium source, vanadium source, phosphorus source and carbon source, and completely disperse them in a solvent by heating and stirring to obtain a uniform sodium vanadium phosphate composite precursor dispersion;

[0008] (2) Preparation of sodium vanadium phosphate composite solid precursor: The sodium vanadium phosphate composite precursor dispersion is used to fully and evenly mix the sodium source, vanadium source, phosphoric acid and carbon source solid materials, and the solvent is evaporated by drying to obtain the sodium vanadium phosphate composite solid precursor;

[0009] (3) Preparation of carbon-coated sodium vanadium phosphate positive electrode material: Weigh a certain amount of the sodium vanadium phosphate composite solid precursor and place it in corundum, put it into a tubular furnace for calcination, heat it under a protective atmosphere, wait for it to naturally cool to room temperature, then take it out, crush it with a powder grinder and mix it thoroughly, then pour the crushed powdered solid into a sieve and sieve it to obtain a high-performance carbon-coated sodium vanadium phosphate positive electrode material with uniform particle size distribution.

[0010] The introduction of ionic forms in the process of the present invention is conducive to better achieving uniform compounding of precursor materials, improving the bonding strength between the raw material components under high temperature, enhancing the structural stability of the material, and is conducive to generating high-purity and high-crystallinity sodium vanadium phosphate materials, thereby improving the specific capacity and cycle performance of carbon-coated sodium vanadium phosphate positive electrode materials.

[0011] Preferably, the sodium source in step (1) comprises one or more of sodium acetate, sodium carbonate, disodium tetraacetate, sodium nitrate, sodium sulfate, sodium chloride, sodium bicarbonate, sodium phosphate, disodium hydrogen phosphate and sodium dihydrogen phosphate.

[0012] Preferably, the vanadium source in step (1) includes one or more of ammonium metavanadate, vanadium pentoxide, vanadium oxide, vanadium trioxide, vanadium phosphate monohydrate, vanadium sulfate, vanadyl sulfate, sodium orthovanadate and vanadyl oxalate.

[0013] Preferably, the phosphorus source in step (1) comprises one or more of sodium hypophosphite, disodium hydrogen phosphate, sodium dihydrogen phosphate, trisodium phosphate, phosphoric acid, ammonium dihydrogen phosphate and diammonium hydrogen phosphate;

[0014] Preferably, the carbon source described in step (1) includes a small molecule type A thiol-containing carbon source and a polymer type B carbon source, and the molar ratio of the small molecule type A thiol-containing carbon source to the polymer type B carbon source is 1:1-3:1, wherein the small molecule type A thiol-containing carbon source includes one or more of 2-mercaptobutyric acid, 3-mercaptobutyric acid, 3-mercaptopropionic acid, 2-mercaptoethylamine, 2-mercaptopyridine, 2-mercaptopyrazine and 1,5-dimercaptonaphthalene; the polymer type B carbon source includes one or more of polyvinyl alcohol, polyethylene oxide, polyacrylic acid, polyvinyl alcohol, polyisobutylene, polyglycine, polyaniline, polyacrylonitrile and polypyrrole.

[0015] Preferably, the molar ratio of the sodium source to the vanadium source in step (1) is 1:1-3:1, the molar ratio of the sodium source to the phosphorus source is 1:3-1:1, and the mass ratio of the sodium vanadium phosphate positive electrode active material precursor (including the sodium source, the vanadium source and the phosphorus source) to the carbon source is 5:1-15:1. In particular, the sodium vanadium phosphate precursor material can also be added with a series of doping elements such as manganese source, boron source, chromium source and cobalt source that can improve the performance of sodium vanadium phosphate.

[0016] Preferably, the solvent in step (1) includes one or more of deionized water, anhydrous ethanol, anhydrous ether and anhydrous methanol, and the mass of the sodium vanadium phosphate positive electrode active material precursor (including a sodium source, a vanadium source and a phosphorus source) and the carbon source accounts for 10%-30% of the total mass of the solution.

[0017] Preferably, in step (1), the temperature of the heated solvent is 60° C.-80° C., the stirring speed is 300-2000 rpm, and the heating and stirring time is 0.5-10 h. The solution needs to be heated and stirred until it is completely clear and free of solid precipitation.

[0018] Preferably, the drying method in step (2) can be one or more of spray drying, blast drying and vacuum drying. Wherein, the spray drying fan is set to 75-95, the temperature is set to 160-250°C, the peristaltic pump is set to 10-50mL / min, and the needle is set to 5-15s. By adjusting the solid content of the solution and the spray drying parameters, the particle size of the outlet particles can be precisely controlled, and the particle size range is 2-10μm. The time required for the spray drying depends on the sample amount; the blast drying temperature is set to 80-180°C, and the drying time is 10-30h; the pressure in the vacuum drying box needs to be less than -30MPa, the drying temperature is set to 80-180°C, and the drying time is 10-30h.

[0019] Preferably, the sodium vanadium phosphate composite solid precursor material obtained in step (2) can be ground into powder by solid phase grinding.

[0020] Preferably, the protective atmosphere in step (3) can be nitrogen, helium, argon, neon or other inert gases, and the air flow rate is 2-50 mL / min.

[0021] Preferably, in step (3), the heating rate is 2-10°C / min, the calcination temperature is 600-1000°C, and the calcination time is 5-20h. In particular, a staged calcination method can be used to fully increase the carbonization time. The heating rate of the first calcination is 2-10°C / min, the first calcination temperature is 300-500°C, and the first calcination time is 2-5h. The heating rate of the second calcination is 2-10°C / min, the second calcination temperature is 600-1000°C, and the calcination time is 5-20h.

[0022] Preferably, the power of the powdering machine in step (3) is 200-2000W, the working time does not exceed 30s, the output particle size should be less than 0.5mm, and the sieve mesh number is 200-350 mesh.

[0023] The present invention uses a battery assembled from the carbon-coated sodium vanadium phosphate positive electrode material obtained by the preparation method.

[0024] The composite cathode material of the present invention is used in a sodium ion battery system with high rate performance and long cycle stability.

[0025] The present invention improves the solubility of raw materials by heating and stirring so that they exist in the solvent in the form of ions to achieve sufficient and uniform mixing. At the same time, a small molecule type A thiol-containing carbon source and a high molecular type B carbon source are calcined and phase-transformed to jointly coat the in-situ generated sodium vanadium phosphate material, significantly improving the ionic / electronic conductivity and electrochemical properties of the sodium vanadium phosphate material. The purpose is to achieve new commercial sodium ion battery applications with high energy density, long cycle life and low manufacturing cost through highly conductive and highly active positive electrode materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is the XRD pattern of the carbon-coated sodium vanadium phosphate positive electrode material prepared in Example 1.

[0027] Figure 2 This is an SEM image of the carbon-coated sodium vanadium phosphate positive electrode material precursor prepared in Example 1.

[0028] Figure 3 This is the SEM image of the carbon-coated sodium vanadium phosphate positive electrode material prepared in Example 1.

[0029] Figure 4 This is the particle size distribution diagram of the carbon-coated sodium vanadium phosphate positive electrode material prepared in Example 1.

[0030] Figure 5 This is a cycling performance diagram of the carbon-coated sodium vanadium phosphate positive electrode material prepared in Example 1 at a 5C rate.

[0031] Figure 6 This is the SEM image of the carbon-coated sodium vanadium phosphate positive electrode material prepared in Example 2.

[0032] Figure 7 This is the SEM image of the cobalt-doped carbon-coated sodium vanadium phosphate positive electrode material prepared in Example 3.

[0033] Figure 8 This is the SEM image of the boron-doped carbon-coated sodium vanadium phosphate positive electrode material prepared in Example 4. DETAILED DESCRIPTION

[0034] The specific implementation methods of the present invention will be described in more detail below. Although the specific implementation methods of the present invention are described below, it is worth noting that the present invention can be implemented in various ways within the conditions of the claims and is not limited by the specific implementation methods. In addition, it is worth noting that if the specific technical solutions or means are not indicated in the specific implementation methods, they must be carried out in accordance with the conditions limited by the claims. The drugs and reagents mentioned in the specific implementation methods are all conventional products that can be purchased through market channels.

[0035] Example 1

[0036] Preparation of sodium vanadium phosphate composite precursor dispersion: ammonium metavanadate, anhydrous sodium carbonate, and ammonium dihydrogen phosphate are added in a V:Na:P molar ratio of 2:3:3, and a small molecule A-type thiol-containing carbon source 2-mercaptobutyric acid and a polymer B-type carbon source polyisobutylene are added in a mass ratio of 3:1 to deionized water, wherein the mass ratio of the sodium vanadium phosphate positive electrode active material precursor (including a sodium source, a vanadium source, and a phosphorus source) to the carbon source is 5:1, the solid content of the solution is adjusted to 20%, and magnetic stirring is performed at a speed of 400 rpm. After the solution temperature is heated to 70°C, stirring is continued for 0.5h until the solution completely becomes clear to obtain a uniform precursor dispersion.

[0037] Preparation of sodium vanadium phosphate composite solid precursor: The above-mentioned precursor dispersion was spray dried, and the spray drying fan setting value was adjusted to 95, the temperature was set to 180°C, the peristaltic pump setting value was 20, and the needle setting value was 10s. When the outlet temperature rose to above 80°C, the air pump was turned on. After the water inlet sprayed all the materials, the peristaltic pump was turned off (about 5 minutes). When the inlet temperature dropped below 90°C, the fan was turned off and the green sodium vanadium phosphate composite solid precursor was collected in the collector.

[0038] Preparation of carbon-coated sodium vanadium phosphate positive electrode material: Weigh 2g of the sodium vanadium phosphate composite solid precursor and place it in corundum, put it into a tubular furnace for calcination, heat it under a nitrogen atmosphere, adjust the airflow rate to 10mL / min, first heat it to 500℃ at a rate of 3°C / min, keep it warm for 2h, then heat it to 850℃ at a rate of 5°C / min, keep it warm for 10h, wait for it to naturally cool to room temperature, then take it out, crush it with a powder grinder with a power of 200W, crush it continuously for 20s to ensure thorough mixing, then pour the crushed powdered solid into a 325-mesh sieve and sieve it to make the discharge particle size less than 0.045mm, and finally obtain a high-performance carbon-coated sodium vanadium phosphate positive electrode material with uniform particle size distribution.

[0039] Assembly of sodium ion batteries with high rate performance and long cycle stability: The NVP / C (0.35 g) prepared above was weighed with PVDF and SP in a mass ratio of 7:2:1, and NMP was used as a solvent to prepare a 5 wt.% PVDF solution. The solid content of the slurry was controlled at 22%-25%. It was coated on aluminum foil by a coater and dried with air at 120°C for 1-3 hours. Then, a manual punch was used to cut it into 12 mm diameter discs and used as working electrodes. After vacuum drying at 150°C overnight, the pole piece was weighed. A metal sodium sheet was used as the counter electrode, and a glass fiber diaphragm was used to form a sodium ion transmission channel. 1.0 M NaClO4 inEC:PC=1:1Vol%with 5.0% FEC (NC-004) was used as the experimental electrolyte system to assemble button cells (half-cells) and perform charge and discharge rate tests and stability tests.

[0040] Figure 1 The XRD pattern of the prepared carbon-coated sodium vanadium phosphate cathode material is shown below. The XRD pattern of the carbon-coated sodium vanadium phosphate cathode material corresponds exactly to the NASICON structure sodium vanadium phosphate Na₃V₂(PO₄)₃ standard database card 00-062-0345, and is devoid of any other impurity peaks, demonstrating that the method described in this patent successfully prepared a pure, highly crystalline sodium vanadium phosphate material.

[0041] Figure 2 This is an SEM image of the prepared carbon-coated sodium vanadium phosphate cathode material precursor. After spray drying, the solid precursor exhibits a micron-scale spherical structure with a smooth surface and dense structure.

[0042] Figure 3 This is a SEM image of the prepared carbon-coated sodium vanadium phosphate cathode material. After high-temperature calcination, the sodium vanadium phosphate cathode material maintains the dense microspherical structure of the precursor. At the same time, the surface of the microspheres becomes roughened at high temperatures, making it more conducive to electrolyte infiltration.

[0043] Figure 4 The particle size distribution of the prepared carbon-coated sodium vanadium phosphate cathode material is shown in Figure 2. The particle size distribution of the carbon-coated sodium vanadium phosphate cathode material is in the range of 0.4-40 μm, with a D10 particle size of 2.610 μm, a D50 particle size of 7.721 μm, and a D90 particle size of 16.817 μm.

[0044] Figure 5 The figure shows the cycling performance of the prepared carbon-coated sodium vanadium phosphate cathode material at a 5C rate. During 1000 cycles, the coulombic efficiency of the prepared carbon-coated sodium vanadium phosphate cathode material remained basically at 100%, while the capacity retention rate was as high as 99.99%, with almost no attenuation.

[0045] Example 2

[0046] Preparation of sodium vanadium phosphate composite precursor dispersion: vanadyl sulfate, sodium dihydrogen phosphate and carbon nanotubes are added to anhydrous ethanol at a V:Na:P molar ratio of 2:3:3, and the small molecule A-type thiol-containing carbon source 1,5-dimercaptonaphthalene and the polymer B-type carbon source polyvinyl alcohol are added at a mass ratio of 3:1. The mass ratio of the sodium vanadium phosphate positive electrode active material precursor (including sodium source, vanadium source and phosphorus source) to the carbon source is 15:1. The solid content of the solution is adjusted to 10%, and magnetic stirring is performed at a speed of 2000 rpm. After the solution temperature is heated to 60°C, stirring is continued for 2 hours until the solution is completely converted into a completely mixed suspension state, and all substances except carbon nanotubes are dissolved to obtain a uniform precursor dispersion.

[0047] Preparation of sodium vanadium phosphate composite solid precursor: The precursor dispersion is subjected to blast drying at a blast drying temperature of 120° C. for 20 h until it is completely dried, and solid-phase grinding is performed to obtain sodium vanadium phosphate composite solid precursor powder.

[0048] Preparation of carbon-coated sodium vanadium phosphate positive electrode material: Weigh 3g of the sodium vanadium phosphate composite solid precursor and place it in corundum, put it into a tubular furnace for calcination, heat it under an argon atmosphere, adjust the airflow rate to 20mL / min, and heat it to 700℃ at a heating rate of 5℃ / min. After keeping it warm for 20h, wait for it to naturally cool to room temperature and then take it out. Use a powder grinder with a power of 2000W to crush it. Crushing it continuously for 10s to ensure thorough mixing. Then pour the crushed powdered solid into a 350-mesh sieve and sieve it so that the discharge particle size is less than 0.045mm. Finally, a high-performance carbon-coated sodium vanadium phosphate positive electrode material with uniform particle size distribution is obtained.

[0049] Assembly of sodium ion batteries with high rate performance and long cycle stability: The NVP / C (0.35 g) prepared above was weighed with PVDF and SP in a mass ratio of 7:2:1, and NMP was used as a solvent to prepare a 5 wt.% PVDF solution. The solid content of the slurry was controlled at 22%-25%. It was coated on aluminum foil by a coater and dried with air at 120°C for 1-3 hours. Then, a manual punch was used to cut it into 12 mm diameter discs and used as working electrodes. After vacuum drying at 150°C overnight, the pole piece was weighed. A metal sodium sheet was used as the counter electrode, and a glass fiber diaphragm was used to form a sodium ion transmission channel. 1.0 M NaClO4 inEC:PC=1:1Vol%with 5.0% FEC (NC-004) was used as the experimental electrolyte system to assemble button cells (half-cells) and perform charge and discharge rate tests and stability tests.

[0050] Figure 6 The SEM image of the prepared carbon-coated sodium vanadium phosphate cathode material shows a fluffy and porous structure.

[0051] Example 3

[0052] Preparation of cobalt-doped sodium vanadium phosphate composite precursor dispersion: vanadium trioxide, sodium chloride, cobalt acetate tetrahydrate and phosphoric acid are added in a V:Co:Na:P mass ratio of 1.8:0.3:3:3, small molecule A-type thiol-containing carbon source 2-mercaptoethylamine and polymer B-type carbon source polyglycine are added to anhydrous ethanol in a mass ratio of 2:1, wherein the mass ratio of sodium vanadium phosphate positive electrode active material precursor (including sodium source, vanadium source and phosphorus source) to carbon source is 10:1, added to anhydrous ether, and the solid content of the solution is adjusted to 15%. Magnetic stirring is performed at a speed of 1000 rpm. After the solution temperature is heated to 80°C, stirring is continued for 5 hours until the solution is completely transformed into a completely mixed suspension state, and all substances except vanadium pentoxide are dissolved to obtain a uniform precursor dispersion.

[0053] Preparation of cobalt-doped sodium vanadium phosphate composite solid precursor: The above-mentioned precursor dispersion is vacuum dried, the pressure in the vacuum drying box is less than -30 MPa, the vacuum drying temperature is set to 180°C, and the drying time is 10 hours until it is completely dried. After solid phase grinding, cobalt-doped sodium vanadium phosphate composite solid precursor powder is obtained.

[0054] Preparation of cobalt-doped carbon-coated sodium vanadium phosphate positive electrode material: Weigh 5 g of the sodium vanadium phosphate composite solid precursor and place it in corundum, put it into a tubular furnace for calcination, heat it under a helium atmosphere, adjust the airflow rate to 40 mL / min, and raise the temperature to 300°C at a rate of 6°C / min. After keeping it warm for 5 hours, raise it to 1000°C at 10°C / min and keep it warm for 5 hours. Use a powder grinder with a power of 1000 W for crushing, and crush it continuously for 5 seconds to ensure thorough mixing. Then pour the crushed powdered solid into a 200-mesh sieve and sieve it to make the output particle size less than 0.074 mm, and finally obtain a high-performance cobalt-doped carbon-coated sodium vanadium phosphate positive electrode material with uniform particle size distribution.

[0055] Assembly of sodium ion batteries with high rate performance and long cycle stability: The NVP / C (0.35 g) prepared above was weighed with PVDF and SP in a mass ratio of 7:2:1, and NMP was used as a solvent to prepare a 5 wt.% PVDF solution. The solid content of the slurry was controlled at 22%-25%. It was coated on aluminum foil by a coater and dried with air at 120°C for 1-3 hours. Then, a manual punch was used to cut it into 12 mm diameter discs and used as working electrodes. After vacuum drying at 150°C overnight, the pole piece was weighed. A metal sodium sheet was used as the counter electrode, and a glass fiber diaphragm was used to form a sodium ion transmission channel. 1.0 M NaClO4 inEC:PC=1:1Vol%with 5.0% FEC (NC-004) was used as the experimental electrolyte system to assemble button cells (half-cells) and perform charge and discharge rate tests and stability tests.

[0056] Figure 7 The SEM image of the prepared cobalt-doped carbon-coated sodium vanadium phosphate cathode material is shown. The carbon-coated sodium vanadium phosphate cathode material exhibits a uniformly distributed block structure.

[0057] Example 4

[0058] Preparation of boron-doped sodium vanadium phosphate composite precursor dispersion: vanadium pentoxide, sodium borohydride, disodium hydrogen phosphate and fructose are added to anhydrous ethanol in a mass ratio of V:Na:P:B of 2:3:2.8:0.2, and a small molecule type A thiol-containing carbon source 2-mercaptopyridine and a polymer type B carbon source polyaniline are added to anhydrous ethanol in a mass ratio of 3:1. Among them, the mass ratio of the sodium vanadium phosphate positive electrode active material precursor (including sodium source, vanadium source and phosphorus source) to the carbon source is 5:1. They are added to anhydrous ether, and the solid content of the solution is adjusted to 20%. The solution is magnetically stirred at a speed of 500 rpm. After the solution temperature is heated to 80°C, stirring is continued for 2 hours until the solution becomes completely clear to obtain a uniform precursor dispersion.

[0059] Preparation of boron-doped sodium vanadium phosphate composite solid precursor: The above-mentioned precursor dispersion was spray-dried, and the spray drying fan setting value was adjusted to 90, the temperature was set to 230°C, the peristaltic pump setting value was 30, and the needle setting value was 6s. When the outlet temperature rose to above 80°C, the air pump was turned on. After the water inlet sprayed all the materials, the peristaltic pump was turned off (about 5 minutes), and the fan was turned off when the inlet temperature dropped below 90°C. The boron-doped sodium vanadium phosphate composite solid precursor was collected in the collector.

[0060] Preparation of boron-doped carbon-coated sodium vanadium phosphate positive electrode material: Weigh 3 g of the sodium vanadium phosphate composite solid precursor and place it in corundum, put it into a tubular furnace for calcination, heat it under a helium atmosphere, adjust the airflow rate to 15 mL / min, and increase the temperature to 500°C at a rate of 2°C / min. After keeping it warm for 3 hours, increase the temperature to 900°C at a rate of 5°C / min and keep it warm for 20 hours. Use a powder grinder with a power of 500 W to crush it. Crush it continuously for 25 seconds to ensure thorough mixing. Then pour the crushed powdered solid into a 300-mesh sieve and sieve it so that the output particle size is less than 0.048 mm. Finally, a high-performance boron-doped carbon-coated sodium vanadium phosphate positive electrode material with uniform particle size distribution is obtained.

[0061] Assembly of sodium ion batteries with high rate performance and long cycle stability: The NVP / C (0.35 g) prepared above was weighed with PVDF and SP in a mass ratio of 7:2:1, and NMP was used as a solvent to prepare a 5 wt.% PVDF solution. The solid content of the slurry was controlled at 22%-25%. It was coated on aluminum foil by a coater and dried with air at 120°C for 1-3 hours. Then, a manual punch was used to cut it into 12 mm diameter discs and used as working electrodes. After vacuum drying at 150°C overnight, the pole piece was weighed. A metal sodium sheet was used as the counter electrode, and a glass fiber diaphragm was used to form a sodium ion transmission channel. 1.0 M NaClO4 inEC:PC=1:1Vol%with 5.0% FEC (NC-004) was used as the experimental electrolyte system to assemble button cells (half-cells) and perform charge and discharge rate tests and stability tests.

[0062] Figure 8 The SEM image of the prepared boron-doped carbon-coated sodium vanadium phosphate cathode material is shown. The carbon-coated sodium vanadium phosphate cathode material exhibits a microsphere / block structure as a whole.

[0063] Example 5

[0064] The implementation steps are the same as those in Example 1, except that the sodium source is disodium tetraacetate. In step 5.1, a sodium vanadium phosphate composite precursor dispersion is obtained. The other steps are the same as those in Example 1 to obtain a carbon-coated sodium vanadium phosphate positive electrode material.

[0065] Example 6

[0066] The implementation steps are the same as those in Example 1, except that the phosphorus source is trisodium phosphate. In step 6.1, a sodium vanadium phosphate composite precursor dispersion is obtained. The other steps are the same as those in Example 1 to obtain a carbon-coated sodium vanadium phosphate positive electrode material.

[0067] Example 7

[0068] The implementation steps are the same as those in Example 1, except that the small molecule A-type thiol-containing carbon source is 3-mercaptopropionic acid. In step 7.1, a sodium vanadium phosphate composite precursor dispersion is obtained. The other steps are the same as those in Example 1 to obtain a carbon-coated sodium vanadium phosphate positive electrode material.

[0069] Example 8

[0070] The implementation steps are the same as those in Example 1, except that the polymer B carbon source is polyacrylonitrile to obtain a sodium vanadium phosphate composite solid precursor. The other steps are the same as those in Example 1 to obtain a carbon-coated sodium vanadium phosphate positive electrode material.

[0071] Example 9

[0072] The implementation steps are the same as those in Example 1, except that the spray drying temperature in step 9.2 is set to 200° C. The other steps are the same as those in Example 1 to obtain a carbon-coated sodium vanadium phosphate positive electrode material.

[0073] Example 10

[0074] The implementation steps are the same as those in Example 1, except that the secondary calcination time in step 10.3 is adjusted to 20 h. The other steps are the same as those in Example 1 to obtain a carbon-coated sodium vanadium phosphate positive electrode material.

[0075] Comparative Example 1

[0076] The implementation steps are the same as those in Example 1, except that the small molecule A-type thiol-containing carbon source 2-mercaptobutyric acid is not added. Other steps are the same as those in Example 1 to obtain a carbon-coated sodium vanadium phosphate positive electrode material.

[0077] Comparative Example 2

[0078] The implementation steps are the same as those in Example 4, except that the polymer B carbon source polyisobutylene is not added. The other steps are the same as those in Example 1, to obtain a carbon-coated sodium vanadium phosphate positive electrode material.

[0079] Table 1: Main physicochemical parameters and sodium ion battery performance of Examples 1-10 and Comparative Examples 1-2:

[0080]

[0081] According to the contents of Table 1, it can be seen that according to the present invention, a pure phase and high crystallinity carbon-coated sodium vanadium phosphate positive electrode material with good performance for sodium ion batteries can be successfully prepared, including multiple types of metal / non-metal doped carbon-coated sodium vanadium phosphate positive electrode materials.

[0082] According to the contents of Table 1, it can be seen that the D50 particle size of the carbon-coated sodium vanadium phosphate positive electrode material is in the range of 4-10μm. Precise control of the experimental parameters during the preparation process can adjust the particle size of the carbon-coated sodium vanadium phosphate positive electrode material and the battery performance. The introduction of small molecule type A thiol-containing carbon source or polymer type B carbon source is beneficial to improving the electronic conductivity, specific capacity and cycle stability of sodium vanadium phosphate.

[0083] It should be understood that the above embodiments are merely illustrative of the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent variations or modifications made in accordance with the spirit and substance of the present invention are intended to be encompassed within the scope of protection of the present invention.

Claims

1. A method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for a sodium ion battery, characterized in that: The following steps are involved: (1) Preparation of sodium vanadium phosphate composite precursor dispersion: Weigh a certain amount of sodium source, vanadium source, phosphorus source and carbon source, and completely disperse them in a solvent by heating and stirring to obtain a uniform sodium vanadium phosphate composite precursor dispersion; the carbon source includes a small molecule type A thiol-containing carbon source and a high molecular type B carbon source, wherein the small molecule type A thiol-containing carbon source includes one or more of 2-mercaptobutyric acid, 3-mercaptobutyric acid, 3-mercaptopropionic acid, 2-mercaptoethylamine, 2-mercaptopyridine, 2-mercaptopyrazine and 1,5-dimercaptonaphthalene; the high molecular type B carbon source includes one or more of polyvinyl alcohol, polyethylene oxide, polyacrylic acid, polyisobutylene, polyglycine, polyaniline, polyacrylonitrile and polypyrrole; (2) Preparation of sodium vanadium phosphate composite solid precursor: The sodium vanadium phosphate composite precursor dispersion is used to fully and evenly mix the sodium source, vanadium source, phosphoric acid and carbon source solid materials, and the solvent is evaporated by drying to obtain the sodium vanadium phosphate composite solid precursor; (3) Preparation of carbon-coated sodium vanadium phosphate positive electrode material: Weigh a certain amount of the sodium vanadium phosphate composite solid precursor and place it in corundum, put it into a tubular furnace for calcination, heat it under a protective atmosphere, wait for it to naturally cool to room temperature, then take it out, crush it with a powder grinder and mix it thoroughly, then pour the crushed powdered solid into a sieve and sieve it to obtain a high-performance carbon-coated sodium vanadium phosphate positive electrode material with uniform particle size distribution.

2. The method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for a sodium ion battery according to claim 1, characterized in that: The sodium source in step (1) includes one or more of sodium acetate, sodium carbonate, disodium tetraacetate, sodium nitrate, sodium sulfate, sodium chloride, sodium bicarbonate, sodium phosphate, disodium hydrogen phosphate and sodium dihydrogen phosphate; The vanadium source includes one or more of ammonium metavanadate, vanadium pentoxide, vanadium trioxide, vanadium phosphate monohydrate, vanadium sulfate, vanadyl sulfate, sodium orthovanadate and vanadyl oxalate; The phosphorus source includes one or more of sodium hypophosphite, disodium hydrogen phosphate, sodium dihydrogen phosphate, trisodium phosphate, phosphoric acid, ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

3. The method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for a sodium ion battery according to claim 1, characterized in that: In step (1), the molar ratio of the small molecule type A thiol-containing carbon source to the high molecule type B carbon source is 1:1-3:

1.

4. The method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for a sodium ion battery according to claim 1, characterized in that: The molar ratio of the sodium source to the vanadium source in step (1) is 1:1-3:1, the molar ratio of the sodium source to the phosphorus source is 1:3-1:1, and the mass ratio of the sodium vanadium phosphate positive electrode active material precursor to the carbon source is 5:1-15:

1.

5. The method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for a sodium ion battery according to claim 1, characterized in that: The solvent in step (1) includes one or more of deionized water, anhydrous ethanol, anhydrous ether and anhydrous methanol, and the mass of the sodium vanadium phosphate positive electrode active material precursor and the carbon source accounts for 10%-30% of the total mass of the solution.

6. The method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for a sodium ion battery according to claim 1, characterized in that: In step (1), the temperature of the heated solvent is 60°C-80°C, the stirring speed is 300-2000 rpm, and the heating and stirring time is 1-10 h; heating and stirring are required until the solution is completely clear and there is no solid precipitation.

7. The method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for a sodium ion battery according to claim 1, characterized in that: The drying method in step (2) can be one or more of spray drying, blast drying and vacuum drying; wherein, the spray drying fan is set to 75-95, the temperature is set to 160-250 ℃, the peristaltic pump is set to 10-50 mL / min, the needle is set to 5-15 s, and the particle size of the outlet particles can be precisely controlled by adjusting the solid content of the solution and the spray drying parameters, with a particle size range of 2-10 μm. The time required for the spray drying depends on the sample amount; the blast drying temperature is set to 80-180 ℃, and the drying time is 10-30 h; the pressure in the vacuum drying box needs to be less than -30 MPa, the drying temperature is set to 80-180 ℃, and the drying time is 10-30 h; the obtained sodium vanadium phosphate composite solid precursor material is ground into powder by solid phase grinding.

8. The method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for a sodium ion battery according to claim 1, characterized in that: In step (3), the protective atmosphere can be nitrogen, helium, argon, neon or other inert gases, with a gas flow rate of 2-50 mL / min; the heating rate is 2-10 °C / min, the calcination temperature is 600-1000 °C, and the calcination time is 5-20 h.

9. The method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for a sodium ion battery according to claim 1, characterized in that: The carbonization time is fully increased by adopting the staged calcination method. The heating rate of the first calcination is 2-10 ℃ / min, the first calcination temperature is 300-500 ℃, the first calcination time is 2-5 h, the heating rate of the second calcination is 2-10 ℃ / min, the second calcination temperature is 600-1000 ℃, and the calcination time is 5-20 h.

10. The method for preparing a carbon-coated sodium vanadium phosphate positive electrode material for sodium ion batteries according to claim 1, characterized in that: In step (3), the power of the powder pulverizer is 200-2000 W, the working time does not exceed 30 seconds, the output particle size is less than 0.5 mm, and the sieve mesh size is 200-350 mesh.

11. A sodium vanadium phosphate composite material prepared according to the method according to any one of claims 1 to 10.

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