A method for preparing functionalized sodium vanadium fluorophosphate by carbothermal reduction and fluorine source compensation

Abstract

This invention discloses a method for preparing functionalized sodium vanadium fluorophosphate through carbothermal reduction and fluorine source compensation, belonging to the field of sodium-ion battery electrode material preparation technology. The method uses a carbon-containing precursor as the starting material, and completes material crystallization through a carbothermal reduction reaction under an inert atmosphere via high-temperature heat treatment. Simultaneously, the chemical state of fluorine is directionally controlled by introducing an additional fluorine source. In the presence of carbon components, the fluorine content and crystal microstructure of sodium vanadium fluorophosphate are adjusted according to the amount of fluorine source compensation, thereby achieving functional design of electrochemical performance. This invention solves the problem of meeting the differentiated performance requirements of sodium vanadium fluorophosphate cathode materials for different application scenarios. Without adding an additional fluorine source, the material exhibits excellent fast-charging capability; with the addition of an appropriate amount of fluorine source, it exhibits high reversible specific capacity. Material functionality can be customized through a single process variable, providing an efficient and feasible technical path for the scenario-based design of sodium-ion battery cathode materials.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery electrode material preparation technology, and relates to a method for preparing functionalized sodium vanadium fluorophosphate through carbothermic reduction and fluorine source compensation. Background Technology

[0002] Energy is the foundation of human society's production and life. With continuous technological advancements, the demand for and application of energy in human society are rapidly increasing and becoming more specialized. Sodium-ion batteries, due to their potential advantages in the abundance of sodium resources, cost-effectiveness, and environmental friendliness, are considered a strong candidate to supplement or even partially replace traditional lithium-ion batteries. In recent years, they have been widely researched and applied in large-scale energy storage (such as grid peak shaving and renewable energy support), and are gradually expanding into scenarios such as consumer electronics and new energy vehicles that demand higher fast-charging performance. Different application scenarios place differentiated demands on battery cathode materials: large-scale energy storage systems focus more on high specific capacity and long-term cycle stability, while power tools or electric vehicles urgently require excellent rate charge and discharge capabilities. Therefore, developing sodium-ion battery cathode materials that can be adapted to different application scenarios has significant application value.

[0003] Sodium vanadium fluorophosphate (chemical formula Na3V2(PO4)2F3) is a sodium-ion battery cathode material with a NASICON structure, exhibiting high operating voltage, good thermal stability, and three-dimensional ion transport channels, showing application potential in the energy storage field. However, its low intrinsic electronic conductivity leads to poor rate performance, severely limiting its use in high-power applications. Therefore, to meet the differentiated performance requirements of sodium vanadium fluorophosphate cathode materials in various applications, it is urgent to develop a simple and directionally applicable method for synthesizing functionalized sodium vanadium fluorophosphate materials with specific electrochemical properties. Summary of the Invention

[0004] The purpose of this invention is to provide a method for synthesizing sodium vanadium fluorophosphate cathode materials suitable for different application scenarios on demand. This method achieves functional design of the electrochemical performance of sodium vanadium fluorophosphate by synergistically regulating the fluorine chemical behavior during the carbothermal reduction process and employing an exogenous fluorine introduction strategy, thereby meeting the application requirements of high-rate fast charging and high-specific-capacity energy storage.

[0005] The technical solution of this invention: A method for preparing functionalized sodium vanadium fluorophosphate via carbothermic reduction and fluorine source compensation includes the following steps: (1) Carbon-containing sodium vanadium fluorophosphate precursor was obtained by drying and low-temperature pre-calcination; (2) The carbon-containing sodium vanadium fluorophosphate precursor is mixed with an additional fluorine source to obtain the material to be treated; (3) Place the material to be treated in an inert atmosphere and perform high-temperature heat treatment at 600℃~850℃ for 2~12 hours to allow it to undergo carbothermic reduction reaction and complete crystallization, thereby obtaining functionalized sodium vanadium fluorophosphate as a positive electrode material.

[0006] In step (1), the carbon-containing sodium vanadium fluorophosphate precursor can be prepared by hydrothermal method, sol-gel method, mechanical ball milling or other conventional methods. The present invention does not limit its preparation method.

[0007] In step (1), the sodium source includes at least one of sodium fluoride, sodium acetate, sodium carbonate, sodium dihydrogen phosphate, sodium oxalate, sodium nitrate, sodium sulfate, and sodium hydroxide.

[0008] In step (1), the vanadium source includes at least one of ammonium metavanadate, vanadium pentoxide, vanadium oxysulfate, vanadium trioxide, vanadium dioxide, vanadium acetylacetonate, vanadium oxyoxalate, vanadium chloride, and vanadium nitrate. The molar ratio between the vanadium source and the sodium source is 2:3.

[0009] In step (1), the phosphorus source includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trimethyl phosphate, triethyl phosphate, phosphorous acid, and sodium hypophosphite. The molar ratio between the phosphorus source and the sodium source is 2:3.

[0010] In step (1), the basic fluorine source includes at least one of sodium fluoride, ammonium fluoride, potassium fluoride, lithium fluoride, calcium fluoride, magnesium fluoride, vanadium trifluoride, iron trifluoride, and aluminum fluoride. The molar ratio between the basic fluorine source and the sodium source is 1:1.

[0011] In step (1), the reducing agent includes at least one of oxalic acid, citric acid, ascorbic acid, tartaric acid, glucose, sucrose, hydrazine hydrate, ethylene glycol, glycerol, phenol, hydroquinone, sodium ascorbate, formic acid, acetic acid, and lactic acid, used to control the valence state of vanadium during the preparation or heat treatment of the carbon-containing sodium vanadium fluorophosphate precursor. The molar ratio between the reducing agent and the vanadium source is 1:1 to 4:1.

[0012] In step (1), the carbon source includes an organic carbon source or a conductive carbon material. The organic carbon source is at least one of glucose, sucrose, citric acid, ascorbic acid, phenolic resin, polyvinyl alcohol, starch, polyacrylonitrile, polyvinylidene fluoride, asphalt, and pitch. The conductive carbon material is at least one of carbon nanotubes, graphene, carbon black, Ketjen black, acetylene black, expanded graphite, and graphite flakes. The mass ratio of the carbon source to the vanadium source is 3:1 to 15:1.

[0013] In step (1), the low-temperature pre-calcination temperature is 100℃~400℃ and the time is 1~8 hours.

[0014] In step (2), the additional fluorine source is at least one of ammonium fluoride, sodium fluoride, potassium fluoride, lithium fluoride, calcium fluoride, magnesium fluoride, and vanadium trifluoride, or it may be an organic fluorine compound that can release fluoride ions under a high-temperature inert atmosphere. The molar ratio of the additional fluorine source to the basic fluorine source is 1:4 to 1:20.

[0015] In step (3), the inert atmosphere is helium, nitrogen or argon.

[0016] In step (3), the high-temperature heat treatment is carried out in a tube furnace or a muffle furnace.

[0017] When no additional fluorine source is introduced, the capacity of the obtained sodium vanadium fluorophosphate cathode material at 30C rate is not less than 50% of the reversible specific capacity at 0.2C rate; when an additional fluorine source is introduced, the reversible specific capacity of the obtained sodium vanadium fluorophosphate cathode material at 0.2C rate is not less than 110 mAh / g.

[0018] The beneficial effects of this invention are as follows: This invention provides a method for preparing functionalized sodium vanadium fluorophosphate through carbothermal reduction and fluorine source compensation. The method uses a carbon-containing sodium vanadium fluorophosphate precursor as the starting material, and completes material crystallization through high-temperature heat treatment under an inert atmosphere. Simultaneously, the chemical state of fluorine is directionally controlled by introducing an additional fluorine source. During the high-temperature heat treatment, due to the presence of the carbon component, sodium vanadium fluorophosphate undergoes a carbothermal reduction reaction. Whether or not the fluorine source is compensated directly affects the fluorine content and crystal microstructure of the final product, thereby achieving on-demand switching between high rate performance and high specific capacity. The material obtained without adding an additional fluorine source exhibits excellent fast-charging capability due to the formation of a specific fluorine defect microstructure, while the material obtained with the addition of an additional fluorine source exhibits higher reversible capacity due to structural integrity. This method is simple, requiring only conventional high-temperature equipment, and allows for material function customization through a single process variable, providing an efficient and feasible technical path for the scenario-based design of sodium-ion battery cathode materials. Attached Figure Description

[0019] Figure 1 The images show XRD comparisons of the materials prepared in Examples 1-3 and Comparative Example 1, where (a) is the overall XRD spectrum and (b) is a magnified view of (a) at 31-34 degrees.

[0020] Figure 2 The graph shows a comparison of the rate performance of the materials prepared in Examples 1-3 and Comparative Example 1.

[0021] Figure 3 SEM image of the material prepared in Example 1.

[0022] Figure 4 The constant current charge-discharge curve of the material prepared in Example 1 is shown.

[0023] Figure 5 The constant current charge-discharge curve of the material prepared in Example 2 is shown.

[0024] Figure 6 The constant current charge-discharge curve of the material prepared in Example 3 is shown.

[0025] Figure 7 The constant current charge-discharge curve of the material prepared for Comparative Example 1. Detailed Implementation

[0026] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0027] Example 1: (1) Add 10 mmol ammonium metavanadate, 15 mmol oxalic acid, 15 mmol sodium fluoride and 10 mmol ammonium dihydrogen phosphate to acetone, and ball mill in a ball mill jar at 400 rpm for 5 hours to obtain a uniform slurry; (2) Dry the slurry at 80℃ for 12 hours and grind it into powder; (3) The above powder was placed in a tube furnace and pre-calcined at 350°C for 4 hours under an argon atmosphere to obtain sodium vanadium fluorophosphate precursor; (4) The obtained sodium vanadium fluorophosphate precursor was mixed with 25% of its mass of sucrose and an additional 3.75 mmol of ammonium fluoride (equivalent to 25% of the basic fluorine source addition) and ball-milled again at 400 rpm for 4 hours. After drying at 80°C for 12 hours, it was ground into powder to obtain sodium vanadium fluorophosphate precursor containing carbon. (5) The sodium vanadium fluorophosphate precursor containing carbon was heated to 600°C at a heating rate of 5°C / min under a nitrogen atmosphere, calcined for 8 hours, and then cooled to room temperature in the furnace to obtain a high specific capacity sodium vanadium fluorophosphate cathode material.

[0028] The electrochemical performance of the material prepared in Example 1 was tested, and the reversible specific capacity at 0.2C was 123 mAh / g, and the discharge capacity at 30C was 53 mAh / g (equivalent to 43.1% of the reversible specific capacity at 0.2C rate).

[0029] Example 2: (1) Add 10 mmol ammonium metavanadate, 15 mmol oxalic acid, 15 mmol sodium fluoride and 10 mmol ammonium dihydrogen phosphate to acetone, and ball mill in a ball mill jar at 400 rpm for 5 hours to obtain a uniform slurry; (2) Dry the slurry at 80℃ for 12 hours and grind it into powder; (3) The above powder was placed in a tube furnace and pre-calcined at 350°C for 4 hours under an argon atmosphere to obtain sodium vanadium fluorophosphate precursor; (4) The obtained sodium vanadium fluorophosphate precursor was mixed with 25% of its mass of sucrose and an additional 2.25 mmol of ammonium fluoride (equivalent to 15% of the basic fluorine source addition) and ball-milled again at 400 rpm for 4 hours. After drying at 80°C for 12 hours, it was ground into powder to obtain sodium vanadium fluorophosphate precursor containing carbon. (5) The sodium vanadium fluorophosphate precursor containing carbon was heated to 600°C at a heating rate of 5°C / min under a nitrogen atmosphere, calcined for 8 hours, and then cooled to room temperature in the furnace to obtain a sodium vanadium fluorophosphate cathode material with moderate specific capacity and rate performance.

[0030] The electrochemical performance of the material prepared in Example 2 was tested, and the reversible specific capacity at 0.2C was 112 mAh / g, and the discharge capacity at 30C was 63 mAh / g (equivalent to 56.2% of the reversible specific capacity at 0.2C rate).

[0031] Example 3: (1) Add 10 mmol ammonium metavanadate, 15 mmol oxalic acid, 15 mmol sodium fluoride and 10 mmol ammonium dihydrogen phosphate to acetone, and ball mill in a ball mill jar at 400 rpm for 5 hours to obtain a uniform slurry; (2) Dry the slurry at 80℃ for 12 hours and grind it into powder; (3) The above powder was placed in a tube furnace and pre-calcined at 350°C for 4 hours under an argon atmosphere to obtain sodium vanadium fluorophosphate precursor; (4) The obtained sodium vanadium fluorophosphate precursor was mixed with 25% of its mass of sucrose and an additional 0.75 mmol of ammonium fluoride (equivalent to 5% of the basic fluorine source addition) and ball-milled again at 400 rpm for 4 hours. After drying at 80°C for 12 hours, it was ground into powder to obtain sodium vanadium fluorophosphate precursor containing carbon. (5) The sodium vanadium fluorophosphate precursor containing carbon was heated to 600°C at a heating rate of 5°C / min under a nitrogen atmosphere, calcined for 8 hours, and then cooled to room temperature in the furnace to obtain sodium vanadium fluorophosphate cathode material with high rate performance.

[0032] The electrochemical performance of the material prepared in Example 3 was tested, and the reversible specific capacity at 0.2C was 106 mAh / g, and the discharge capacity at 30C was 73 mAh / g (equivalent to 68.9% of the reversible specific capacity at 0.2C rate).

[0033] Comparative Example 1: (1) Add 10 mmol ammonium metavanadate, 15 mmol oxalic acid, 15 mmol sodium fluoride and 10 mmol ammonium dihydrogen phosphate to acetone, and ball mill at 400 rpm for 5 hours in a ball mill jar to obtain a uniform slurry; (2) Dry the slurry at 80℃ for 12 hours and grind it into powder; (3) The above powder was placed in a tube furnace and pre-calcined at 350°C for 4 hours under an argon atmosphere to obtain sodium vanadium fluorophosphate precursor; (4) The obtained sodium vanadium fluorophosphate precursor was mixed with 15% of its mass of sucrose and an additional 6.0 mmol of ammonium fluoride (equivalent to 40% of the basic fluorine source addition) and ball-milled again at 400 rpm for 4 hours. After drying at 80°C for 12 hours, it was ground into powder to obtain sodium vanadium fluorophosphate precursor. (5) The sodium fluorophosphate precursor containing carbon fluoride is heated to 600°C at a heating rate of 5°C / min under a nitrogen atmosphere, calcined for 8 hours, and then cooled to room temperature in the furnace to obtain sodium fluorophosphate material with excessive fluorine doping.

[0034] Electrochemical test results: The reversible specific capacity at 0.2C is 83 mAh / g, but the discharge capacity at 30C is only 29 mAh / g (equivalent to 34.9% of the reversible specific capacity at 0.2C rate), indicating that excessive fluorine has damaged the kinetic stability of the material and is not conducive to practical applications.

[0035] The above results show that, in the same carbothermic reduction system, as the amount of additional fluorine source added increases from 5% to 25% of the basic fluorine source, the specific capacity of the material first increases and then tends to saturate, while the rate performance continuously decreases. Excessive (greater than 40%) fluorine source addition impairs the kinetic stability of the material. By precisely controlling the amount of additional fluorine source added (preferably 5%–25%), the switching between high specific capacity and high rate performance of sodium vanadium fluorophosphate can be achieved, meeting the needs of different application scenarios. This fully verifies the technical effectiveness of the present invention.

Claims

1. A method for preparing functionalized sodium vanadium fluorophosphate via carbothermic reduction and fluorine source compensation, characterized in that, The method includes the following steps: (1) Carbon-containing sodium vanadium fluorophosphate precursor was obtained by drying and low-temperature pre-calcination; (2) The carbon-containing sodium vanadium fluorophosphate precursor is mixed with an additional fluorine source to obtain the material to be treated; (3) Place the material to be treated in an inert atmosphere and perform high-temperature heat treatment at 600℃~850℃ for 2~12 hours to allow it to undergo carbothermic reduction reaction and complete crystallization, thereby obtaining functionalized sodium vanadium fluorophosphate as a positive electrode material.

2. The method according to claim 1, characterized in that, The additional fluorine source is at least one of ammonium fluoride, sodium fluoride, potassium fluoride, lithium fluoride, calcium fluoride, magnesium fluoride, and vanadium trifluoride, or an organic fluorine compound that can release fluoride ions under a high-temperature inert atmosphere.

3. The method according to claim 1 or 2, characterized in that, The carbon-containing sodium vanadium fluorophosphate precursor is prepared by ball milling and mixing sodium source, vanadium source, phosphorus source, and basic fluorine source with reducing agent and carbon source in a stoichiometric ratio of 3:2:2:3, followed by drying and optional low-temperature pre-calcination. The molar ratio of reducing agent to vanadium source is 1:1 to 4:1, and the molar ratio between reducing agent and vanadium source is 1:1 to 4:

1. The sodium source includes at least one of sodium fluoride, sodium acetate, sodium carbonate, sodium dihydrogen phosphate, sodium oxalate, sodium nitrate, sodium sulfate, and sodium hydroxide. The vanadium source includes at least one of ammonium metavanadate, vanadium pentoxide, vanadium oxysulfate, vanadium trioxide, vanadium dioxide, vanadium acetylacetone, vanadium oxyoxalate, vanadium chloride, and vanadium nitrate. The phosphorus source includes ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, trimethyl phosphate, and triethyl phosphate. The fluorine source includes at least one of phosphorous acid and sodium hypophosphite; the basic fluorine source includes at least one of sodium fluoride, ammonium fluoride, potassium fluoride, lithium fluoride, calcium fluoride, magnesium fluoride, vanadium trifluoride, iron trifluoride, and aluminum fluoride; the reducing agent includes at least one of oxalic acid, citric acid, ascorbic acid, tartaric acid, glucose, sucrose, hydrazine hydrate, ethylene glycol, glycerol, phenol, hydroquinone, sodium ascorbate, formic acid, acetic acid, and lactic acid; the carbon source is an organic carbon source or a conductive carbon material, wherein the organic carbon source is at least one of glucose, sucrose, citric acid, ascorbic acid, phenolic resin, polyvinyl alcohol, starch, polyacrylonitrile, polyvinylidene fluoride, asphalt, and pitch, and the conductive carbon material is at least one of carbon nanotubes, graphene, carbon black, Ketjen black, acetylene black, expanded graphite, and graphite flakes.

4. The method according to claim 1, characterized in that, The carbon-containing sodium vanadium fluorophosphate precursor is prepared by hydrothermal method, sol-gel method or mechanical ball milling method.

5. The method according to claim 1, characterized in that, The inert atmosphere is helium, nitrogen, or argon, and the high-temperature heat treatment is carried out in a tube furnace or a muffle furnace.

6. The method according to claim 1, characterized in that, The ratio of the additional fluorine source to the basic fluorine source is 1:4 to 1:

20.

7. The method according to claim 1, characterized in that, The mass ratio of the carbon source to the vanadium source is 3:1 to 15:

1.

8. The method according to claim 1, characterized in that, The low-temperature pre-baking temperature is 100℃~400℃, and the time is 1~8 hours.

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