Fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of sodium ion batteries, and particularly relates to a fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material as well as a preparation method and application thereof. In order to solve the problems of poor conductivity, low gram volume and the like of a positive electrode material, sodium dihydrogen phosphate, vanadium pentoxide, L-alanine, gelatin, amine fluoride and tetrabutyl titanate are added into an ethanol / water mixed solvent according to a certain proportion, the mixture is transferred into a ball mill for reaction and dried, and uniform powder is obtained. And then, performing high-temperature solid-phase sintering on the powder in rapid Joule thermal equipment to obtain the fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material. The method has the advantages of simplicity in operation, easiness in amplification, energy conservation, consumption reduction and the like. Meanwhile, the prepared powder is fluorine-nitrogen co-doped carbon uniformly-coated sodium vanadium phosphate single crystal, and is high in gram volume and excellent in rate capability and cycle performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium-ion batteries, and particularly relates to a fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material, a preparation method thereof, and an application thereof. Background Art

[0002] Sodium-ion batteries have broad application prospects in the field of large-scale energy storage due to advantages such as rich sodium resources and long service life. The cathode material is the key to determining the performance of sodium-ion batteries. Compared with other cathode materials with commercial prospects, such as layered oxides and Prussian blue, polyanion-type compounds have received extensive attention due to their low environmental sensitivity, low cost, excellent cycle stability, and rate performance. However, polyanion-type cathode materials generally have problems such as poor conductivity and low specific capacity, which hinder their commercialization process.

[0003] Carbon coating and anion and cation doping for the structure of sodium vanadium phosphate are the two most effective strategies for modifying sodium vanadium phosphate materials. The organic combination of the carbon coating layer and sodium vanadium phosphate can significantly improve the charge transfer ability and structural stability of the material, and improve the reversible capacity and cycle stability of sodium vanadium phosphate materials. At the same time, doping elements such as magnesium, iron, and titanium at the vanadium site, and fluorine doping at the anion site can all improve the specific capacity, cycle, and rate performance of sodium vanadium phosphate materials to varying degrees. For example, the invention patent CN 115849328B uses isopropyl titanate as the titanium source to develop a class of carbon-coated sodium vanadium phosphate materials with good rate performance and excellent cycle life by controlling the doping ratio of titanium at the vanadium site, but its specific capacity in the range of 2.0 - 3.9V is low (less than 100mAh / g), which somewhat limits its practicality. In addition, existing research shows that compared with polycrystalline cathode materials, single-crystalline cathode materials have better structural stability and cycle stability during the sodiation and desodiation processes of sodium. Summary of the Invention

[0004] To solve the problems and deficiencies existing in the existing sodium vanadium phosphate materials, the present invention provides a fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material, a preparation method thereof, and an application thereof. This material is modified in multiple aspects such as the carbon coating layer structure, element doping, and crystal structure control.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] The present invention provides a preparation method of a fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material, comprising the following steps:

[0007] Step 1, dissolve sodium dihydrogen phosphate, vanadium pentoxide, L-alanine, ammonium fluoride, tetrabutyl titanate, and gelatin in a mixed solvent prepared from ethanol and water, and stir.

[0008] Step 2: Ball-mill the mixture obtained in Step 1, and then perform baking to obtain a sodium vanadium phosphate precursor.

[0009] Step 3: Grind the sodium vanadium phosphate precursor obtained in Step 2, and then sinter it by using a rapid Joule heating method under an inert atmosphere to obtain a fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material.

[0010] Furthermore, in Step 1, the molar ratio of sodium dihydrogen phosphate, vanadium pentoxide, L-alanine, ammonium fluoride, and tetrabutyl titanate is 6: 1.8 - 1.9: 3 - 5: 0.01 - 0.05: 0.2 - 0.4, and the mass ratio of gelatin to sodium dihydrogen phosphate is 0.02 - 0.1: 1.

[0011] Furthermore, in Step 1, the volume ratio of ethanol to water is 1: 0.5 - 1.

[0012] Furthermore, in Step 2, the rotation speed of the ball-milling is 300 - 450 revolutions per minute, and the time is 2 - 10 hours.

[0013] Furthermore, in Step 2, the baking temperature is 80 - 120 °C, and the time is 5 - 24 hours.

[0014] Furthermore, in Step 3, grind until it completely passes through a 60 - 100 mesh sieve.

[0015] Furthermore, in Step 3, the sintering temperature is 950 - 1050 °C, and the time is 30 - 120 seconds.

[0016] The present invention also provides a fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material prepared by the above preparation method, which is a single crystal material, and its crystal structure contains a small amount of titanium doping and fluorine doping.

[0017] The present invention also provides an application of the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material, which is used as a positive electrode material for a sodium ion battery.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] 1. In the present invention, by selecting two complexing agents with a large difference in molecular weight, namely L-alanine and gelatin, the coordination environment of vanadium ions and the system viscosity are flexibly regulated, and they also serve as reducing agents and carbon sources. By finely adjusting the ratio of the two complexing agents, the particle size of sodium vanadium phosphate and the thickness of the surface carbon coating layer are synergistically regulated. At the same time, ammonium fluoride can promote the formation of a fluorine and nitrogen co-doped carbon coating layer, which has higher conductivity and carbonate electrolyte wettability. In addition, the appropriate addition of ammonium fluoride can also incorporate a small amount of fluorine atoms into the anion sites of the sodium vanadium phosphate crystal structure, effectively improving the stability of the sodium ion diffusion channel and accelerating the sodium ion diffusion kinetics.

[0020] 2. Introducing an appropriate amount of titanium atoms at the vanadium site of sodium vanadium phosphate can induce the generation of sodium vacancies in the crystal structure, increase the sodium ion diffusion path and improve the material conductivity. At the same time, it can also shorten the bond length, reduce the unit cell volume, and further improve the crystal structure stability.

[0021] 3. The rapid Joule heating method can complete the sintering reaction in an extremely short time, greatly reducing the energy consumption and improving the sintering process efficiency. At the same time, compared with the conventional sintering process, the higher sintering temperature of the Joule heating sintering process and the improvement of the material conductivity caused by appropriate titanium doping synergistically induce the formation of the single crystal structure of sodium vanadium phosphate, which further improves the structure stability of the material during the sodiation and desodiation process of sodium, and reduces the occurrence of side reactions. Brief Description of the Drawings

[0022] Figure 1 The X-ray diffraction pattern of the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material prepared in Example 1. It can be found that the purity of the material is relatively high, and the relatively wide diffraction peaks can prove that the grain size of the material is small.

[0023] Figure 2 The typical scanning electron microscope photograph of the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material prepared in Example 1. It can be seen that the average particle size of the material is about 1 micron.

[0024] Figure 3 The high-resolution transmission electron microscope photograph of the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material prepared in Example 1. It can be found that the crystal form of the material is relatively good.

[0025] Figure 4 The selected area electron diffraction pattern of the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material prepared in Example 1. It can be seen that the material exhibits typical single crystal structure characteristics.

[0026] Figure 5 The charge-discharge curve of the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material prepared in Example 1. It can be seen that the material exhibits typical high voltage plateau characteristics of sodium vanadium phosphate materials, and the irreversible capacity in the first cycle is small.

[0027] Figure 6 The rate performance graph of the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material prepared in Example 1 proves that the rate performance of the material is excellent.

[0028] Figure 7 The cyclic performance test results of the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material prepared in Example 1. It can be seen that the cyclic stability of the material is excellent.

[0029] Figure 8 The selected area electron diffraction pattern of the sodium vanadium phosphate material prepared in Comparative Example 1. It can be found that the material exhibits typical polycrystalline structure characteristics. Detailed implementation mode

[0030] In order to further elaborate the technical solution of the present invention, the present invention will be further described below through embodiments.

[0031] Example 1

[0032] First, after accurately weighing sodium dihydrogen phosphate, vanadium pentoxide, L-alanine, ammonium fluoride, and tetrabutyl titanate according to a molar ratio of 6:1.8:5:0.02:0.4, gelatin was weighed according to a mass ratio of gelatin to sodium dihydrogen phosphate of 0.04:1. Subsequently, the above raw materials were added to a mixed solvent prepared by mixing ethanol and water at a volume ratio of 1:1 and stirred at 50 °C for 2 hours. Then, the obtained mixed solution was transferred to a ball milling tank and reacted at a rotation speed of 400 revolutions per minute for 5 hours and then taken out. Then, it was baked in a forced air oven at 100 °C for 12 hours to obtain a sodium vanadium phosphate precursor. Finally, after grinding the sodium vanadium phosphate precursor until it completely passed through an 80-mesh sieve, it was placed in a rapid Joule heating device and reacted for 60 seconds at a sintering temperature of 1000 °C under an inert atmosphere to obtain a fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate powder material.

[0033] The fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material was uniformly mixed with PVDF and Super P at a mass ratio of 85:10:5, coated on aluminum foil, and vacuum dried to make an electrode. A CR2032 type button half-cell was assembled in a glove box. The electrolyte system was a 0.8 mol / L sodium perchlorate / ethylene carbonate / dimethyl carbonate solution. The battery test results at room temperature showed that the first-cycle discharge specific capacity of this material was 112 mAh / g at a current density of 0.1C, and the first Coulombic efficiency was 93.8%.

[0034] Example 2

[0035] First, after accurately weighing sodium dihydrogen phosphate, vanadium pentoxide, L-alanine, ammonium fluoride, and tetrabutyl titanate according to a molar ratio of 6:1.9:4:0.04:0.2, gelatin was weighed according to a mass ratio of gelatin to sodium dihydrogen phosphate of 0.06:1. Subsequently, the above raw materials were added to a mixed solvent prepared by mixing ethanol and water at a volume ratio of 1:0.75 and stirred at 60 °C for 1.5 hours. The obtained mixed solution was transferred to a ball milling tank and reacted at a rotation speed of 450 revolutions per minute for 2 hours and then taken out. Then, it was baked in a forced air oven at 120 °C for 5 hours to obtain a sodium vanadium phosphate precursor. Finally, after grinding the sodium vanadium phosphate precursor until it completely passed through a 60-mesh sieve, it was placed in a rapid Joule heating device and reacted for 30 seconds at a sintering temperature of 1050 °C under an inert atmosphere to obtain a fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate powder material.

[0036] The test results of the half-cell show that the first-cycle discharge specific capacity of the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material is 108 mAh / g at a current density of 0.1C, and the first Coulombic efficiency is 93.4%.

[0037] Example 3

[0038] First, accurately weigh sodium dihydrogen phosphate, vanadium pentoxide, L-alanine, ammonium fluoride, and tetrabutyl titanate according to the molar ratio of 6:1.83:3:0.05:0.34, and then weigh gelatin according to the mass ratio of gelatin to sodium dihydrogen phosphate of 0.02:1. Subsequently, add the above raw materials to a mixed solvent prepared by mixing ethanol and water at a volume ratio of 1:0.5 and stir at 40°C for 3 hours. Transfer the obtained mixed solution into a ball milling tank, react at a rotation speed of 350 rpm for 8 hours, and then take it out. Then bake in a forced-air oven at 80°C for 24 hours to obtain the sodium vanadium phosphate precursor. Finally, grind the sodium vanadium phosphate precursor until it completely passes through a 70-mesh sieve, place it in a rapid Joule heating device, and react for 120 seconds at a sintering temperature of 950°C under an inert atmosphere to obtain the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate powder material.

[0039] The test results of the half-cell show that the first-cycle discharge specific capacity of the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material is 102 mAh / g at a current density of 0.1C, and the first Coulombic efficiency is 92.5%.

[0040] Example 4

[0041] First, accurately weigh sodium dihydrogen phosphate, vanadium pentoxide, L-alanine, ammonium fluoride, and tetrabutyl titanate according to the molar ratio of 6:1.87:4.5:0.03:0.26, and then weigh gelatin according to the mass ratio of gelatin to sodium dihydrogen phosphate of 0.1:1. Subsequently, add the above raw materials to a mixed solvent prepared by mixing ethanol and water at a volume ratio of 1:0.75 and stir at 80°C for 1 hour. Transfer the obtained mixed solution into a ball milling tank, react at a rotation speed of 300 rpm for 10 hours, and then take it out. Then bake in a forced-air oven at 90°C for 16 hours to obtain the sodium vanadium phosphate precursor. Finally, grind the sodium vanadium phosphate precursor until it completely passes through a 100-mesh sieve, place it in a rapid Joule heating device, and react for 90 seconds at a sintering temperature of 1000°C under an inert atmosphere to obtain the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate powder material.

[0042] The test results of the half-cell show that the first-cycle discharge specific capacity of the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material is 101 mAh / g at a current density of 0.1C, and the first Coulombic efficiency is 91.7%.

[0043] Example 5

[0044] First, accurately weigh sodium dihydrogen phosphate, vanadium pentoxide, L-alanine, ammonium fluoride, and tetrabutyl titanate according to the molar ratio of 6:1.9:3.5:0.01:0.2, and then weigh gelatin according to the mass ratio of gelatin to sodium dihydrogen phosphate of 0.08:1. Subsequently, add the above raw materials to a mixed solvent prepared by mixing ethanol and water at a volume ratio of 1:0.6 and stir at 70 °C for 1.5 hours. Transfer the obtained mixed solution into a ball milling tank, react at a rotation speed of 380 revolutions per minute for 6 hours, and then take it out. Then bake in a forced-air oven at 110 °C for 9 hours to obtain a sodium vanadium phosphate precursor. Finally, grind the sodium vanadium phosphate precursor until it completely passes through a 90-mesh sieve, place it in a rapid Joule heating device, and react at a sintering temperature of 980 °C for 100 seconds under an inert atmosphere to obtain a fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate powder material.

[0045] The test results of the half-cell show that the first-cycle discharge specific capacity of the fluorine and nitrogen co-doped carbon-coated sodium vanadium phosphate material is 105 mAh / g at a current density of 0.1C, and the first Coulombic efficiency is 92.3%.

[0046] Comparative Example 1

[0047] First, accurately weigh sodium dihydrogen phosphate, vanadium pentoxide, L-alanine, ammonium fluoride, and tetrabutyl titanate according to the molar ratio of 6:1.8:5:0.02:0.4, and then weigh gelatin according to the mass ratio of gelatin to sodium dihydrogen phosphate of 0.04:1. Subsequently, add the above raw materials to a mixed solvent prepared by mixing ethanol and water at a volume ratio of 1:1 and stir at 50 °C for 2 hours. Then, transfer the obtained mixed solution into a ball milling tank, react at a rotation speed of 400 revolutions per minute for 5 hours, and then take it out. Then bake in a forced-air oven at 100 °C for 12 hours to obtain a sodium vanadium phosphate precursor. Finally, grind the sodium vanadium phosphate precursor until it completely passes through an 80-mesh sieve, place it in a tubular furnace protected by an argon atmosphere, heat at a heating rate of 5 °C per minute, keep it at a constant temperature of 350 °C for 3 hours, and then sinter at 800 °C for 5 hours to obtain a carbon-coated sodium vanadium phosphate powder material.

[0048] The test results of the half-cell show that the first-cycle gravimetric capacity of the hard carbon cathode material is 92 mAh / g, and the first Coulombic efficiency is 88.2%.

[0049] The above shows and describes the main features and advantages of the present invention. For those skilled in the art, it is obvious that the present invention is not limited to the details of the above exemplary embodiments, and can be implemented in other specific forms without departing from the spirit or basic characteristics of the present invention. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting. The scope of the present invention is defined by the appended claims rather than the above description. Therefore, all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present invention.

[0050] In addition, it should be understood that although this specification is described according to embodiments, not every embodiment only contains an independent technical solution. This narrative way of the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing a fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material, characterized in that: The following steps are involved: Step 1, dissolving sodium dihydrogen phosphate, vanadium pentoxide, L-alanine, ammonium fluoride, tetrabutyl titanate and gelatin in a mixed solvent prepared from ethanol and water, and stirring; Step 2, ball-milling the mixed solution obtained in step 1, and then baking to obtain a sodium vanadium phosphate precursor; Step 3: Grind the sodium vanadium phosphate precursor obtained in step 2, and then sinter it in an inert atmosphere by using a rapid Joule heating method to obtain a fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material.

2. The method for preparing a fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material according to claim 1, characterized in that: In the step 1, the molar ratio of sodium dihydrogen phosphate, vanadium pentoxide, L-alanine, ammonium fluoride and tetrabutyl titanate is 6:1.8-1.9:3-5:0.01-0.05:0.2-0.4, and the mass ratio of gelatin to sodium dihydrogen phosphate is 0.02-0.1:

1.

3. The method for preparing a fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material according to claim 1, characterized in that: The volume ratio of ethanol to water in step 1 is 1:0.5-1.

4. The method for preparing a fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material according to claim 1, characterized in that: The stirring temperature in step 1 is 40 to 80° C. and the stirring time is 1 to 3 hours.

5. The method for preparing a fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material according to claim 1, characterized in that: The ball milling speed in step 2 is 300 to 450 rpm, and the time is 2 to 10 hours.

6. The method for preparing a fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material according to claim 1, characterized in that: The baking temperature in step 2 is 80-120° C. and the baking time is 5-24 hours.

7. The method for preparing a fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material according to claim 1, characterized in that: In step 3, the powder is ground until it completely passes through a 60-100 mesh sieve.

8. The method for preparing a fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material according to claim 1, characterized in that: The sintering temperature in step 3 is 950-1050° C. and the sintering time is 30-120 seconds.

9. The fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material is a single crystal material, and contains a small amount of titanium doping and fluorine doping in the crystal structure.

10. The use of the fluorine-nitrogen co-doped carbon-coated sodium vanadium phosphate material according to claim 9, characterized in that: Cathode materials for sodium-ion batteries.

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