A preparation method of sodium vanadium fluorophosphate as a cathode material for sodium-ion batteries based on spray drying

Abstract

A preparation method of sodium vanadium fluorophosphate as a cathode material for sodium-ion batteries based on spray drying. The present invention relates to a preparation method of a cathode material for sodium-ion batteries, and it aims to solve the problems of poor electronic conductivity and fluorine loss of sodium vanadium fluorophosphate as a cathode material for sodium-ion batteries. The method is as follows: dissolve a carbon source, a phosphorus source, a sodium source, and a vanadium source in water and then perform drying treatment in a spray dryer to obtain a precursor powder; then mix the precursor powder with a fluorine source evenly and calcine for 3 to 6 hours to obtain sodium vanadium fluorophosphate as a cathode material for sodium-ion batteries. It is micron-sized spherical particles with a core-shell structure, the particle size is about 8 μm, and the thickness of the spherical shell is about 200 nm. At a 1C rate, its initial discharge specific capacity is 110.60 mAh / g, and at 10C and 30C rates, its discharge specific capacities are 97.90 mAh / g and 74.70 mAh / g respectively. It can be used in the field of sodium-ion batteries.

Description

Technical Field

[0001] The present invention relates to the field of synthesis and preparation of electrode materials, and particularly to a preparation method of sodium vanadium fluorophosphate as a cathode material for sodium-ion batteries based on spray drying. Background Art

[0002] Lithium-ion batteries have been widely used in fields such as electric vehicles, computers, communications, and consumer electronics, and have become a mature energy storage device. However, the reserves of lithium resources in the earth's crust are relatively scarce, making it difficult to meet the growing demand for lithium-ion batteries. In addition, the geographical distribution of lithium resources is extremely uneven, and the development is difficult and costly. The rapidly growing lithium-ion battery market is bound to cause problems such as depletion of lithium resources and rising prices. Therefore, it is urgent to develop a new type of long-life secondary energy storage battery with richer resources and lower costs. Similar to lithium-ion batteries, sodium-ion batteries are also "rocking chair" batteries. In contrast, sodium resources are widely distributed and have rich reserves, and there is almost no depletion problem. With advantages such as low cost and high safety, sodium-ion batteries have gradually become a hot spot in the energy storage field.

[0003] As an important part of sodium-ion batteries, the cathode material plays a decisive role in the electrochemical performance of the entire battery system, such as energy density, power density, and cycle life. Among the existing cathode materials for sodium-ion batteries, sodium vanadium fluorophosphate (Na3V2(PO4)2F3), as a NASICON-type polyanion compound, has good cycle stability due to its open and stable three-dimensional framework and smooth sodium-ion diffusion channels. In addition, due to the high electronegativity of fluorine, sodium vanadium fluorophosphate has a higher average working voltage than sodium vanadium phosphate, about 3.8V (vs. Na / Na + ), and its energy density (507 Wh / kg) is already close to that of commercial lithium iron phosphate cathode materials (580 Wh / kg). Therefore, sodium vanadium fluorophosphate is considered a strong competitor for the next-generation commercial cathode materials for sodium-ion batteries. However, there are still various problems to be solved for sodium vanadium fluorophosphate as a cathode material for sodium-ion batteries. On the one hand, due to the insulation of phosphate groups and fluoride ions in the framework, sodium vanadium fluorophosphate has poor conductivity, and its rate performance is also limited to a certain extent. On the other hand, during the preparation process of sodium vanadium fluorophosphate, inevitable fluorine loss will lead to the formation of impurity sodium vanadium phosphate, which not only damages the cycle stability of the material but also reduces its average working voltage. Currently, some studies and reports have tried to solve the above problems through strategies such as nanosizing, carbon coating, and fluorine regulation. However, the preparation methods of sodium vanadium fluorophosphate as a cathode material for sodium-ion batteries in most literatures have low yields and are difficult to reproduce, with poor reproducibility and consistency, and it is difficult to achieve large-scale production while ensuring the material performance. Summary of the Invention

[0004] The present invention aims to solve the problems of poor electronic conductivity and fluorine loss of sodium vanadium fluorophosphate as the cathode material of sodium-ion batteries, and provides a preparation method of sodium vanadium fluorophosphate as the cathode material of sodium-ion batteries based on spray drying. The method of the present invention has a simple process and is convenient for large-scale production.

[0005] The preparation method of sodium vanadium fluorophosphate as the cathode material of sodium-ion batteries based on spray drying according to the present invention is carried out according to the following steps:

[0006] Step 1: Add a carbon source, a phosphorus source, a sodium source, and a vanadium source to deionized water in sequence, and stir under heating conditions to obtain a clear solution; the purpose of this step is to fully mix and dissolve the added various raw materials and ensure the occurrence of vanadium source reduction and complexation reactions;

[0007] Step 2: Transfer the clear solution obtained in Step 1 to a spray dryer for drying treatment to obtain a precursor powder; this step is a secondary granulation process and removes the moisture in the solution;

[0008] Step 3: After uniformly mixing the precursor powder obtained in Step 2 with a fluorine source in a mass ratio of 10:(1.5 - 3), place it in a tubular furnace, where the fluorine source is polytetrafluoroethylene (PTFE) and / or polyvinylidene fluoride (PVDF), and under the protection of flowing inert gas, heat it at a heating rate of 5 - 10 °C / min to 450 - 700 °C and calcine for 3 - 6 h to obtain sodium vanadium fluorophosphate as the cathode material of sodium-ion batteries, and its chemical formula is Na3V2(PO4)2F3.

[0009] Furthermore, the carbon source in Step 1 is oxalic acid and / or citric acid.

[0010] Furthermore, the phosphorus source in Step 1 is one or a mixture of several of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

[0011] Furthermore, the sodium source in Step 1 is one or a mixture of several of sodium carbonate, sodium acetate, and sodium nitrate.

[0012] Furthermore, the vanadium source in Step 1 is one or a mixture of several of vanadium pentoxide, ammonium metavanadate, and sodium metavanadate.

[0013] Furthermore, the molar ratio of the carbon source, phosphorus source, sodium source, and vanadium source in Step 1 is (1.5 - 4.5):(2.5 - 5.5):(4.5 - 7.5):(2.5 - 5.5), where the carbon source is calculated as carbon.

[0014] Furthermore, the molar amount of the vanadium source in Step 1 and the volume ratio of deionized water is (2.5 - 5.5) mmol:(40 - 60 mL).

[0015] Furthermore, the heating temperature in Step 1 is 55 - 85°C, and the time is 50 - 70 min. The purpose of this heating step is: on the one hand, to accelerate the mixing and dissolution of each reaction raw material, and on the other hand, to promote the vanadium source complexation reaction and reduction reaction.

[0016] Furthermore, in Step 2, the spray drying working conditions are: the outlet air temperature is 110 - 130°C, and the peristaltic speed is 500 - 1500 mL / h. This step is a secondary granulation and rapid drying process. Its purpose is to rapidly remove moisture while constructing a hollow spherical sodium vanadium fluorophosphate cathode material with a micro-nano structure.

[0017] Furthermore, in Step 3, the mass ratio of the precursor powder to the fluorine source is 10:(1 - 3).

[0018] Furthermore, in Step 3, the mixing method of the precursor and the fluorine source is preferably manual grinding with a pestle or ball milling at a rotation speed of 300 revolutions per minute. The purpose is to improve the uniformity of the reaction raw material mixing, thereby improving the purity of the sintered product and shortening the reaction time.

[0019] Furthermore, in Step 3, the inert gas is argon, nitrogen, or a hydrogen-argon mixture.

[0020] The beneficial effects of the present invention compared with the prior art are as follows:

[0021] By using the combined technology of spray drying method and high-temperature solid-phase method, the present invention successfully prepares the sodium vanadium fluorophosphate cathode material for sodium-ion batteries. Utilizing the convenient and effective secondary granulation and rapid drying of spray drying, and using polytetrafluoroethylene (PTFE) and / or polyvinylidene fluoride (PVDF) as the organic fluorine source, taking advantage of their characteristics of being difficult to hydrolyze in aqueous solution, to a certain extent, reduce the loss of fluorine elements during the preparation of the sodium vanadium fluorophosphate cathode material, and conduct carbon coating on the material. During the calcination process, the organic fluorine source can also act as a carbon source to conduct carbon coating on the material and improve the electronic conductivity of the material. The average powder resistivity of the sodium vanadium fluorophosphate prepared by the method of the present invention is 51.97 Ω·cm, and the average powder conductivity is 1.92×10 -2 S / cm. Benefiting from the excellent conductivity, the sodium vanadium fluorophosphate cathode material for sodium-ion batteries prepared by the present invention has satisfactory rate performance. At a 1C rate, its initial discharge specific capacity is 110.60 mAh / g. At 10C and 30C rates, its discharge specific capacities are 97.90 mAh / g and 74.70 mAh / g respectively, showing more excellent rate performance.

[0022] (2) The present invention uses spray drying method to prepare sodium vanadium fluorophosphate, a micron-sized spherical cathode material for sodium-ion batteries with a unique core-shell structure, which combines the advantages of short ion diffusion path of nanoparticles and high tap density of microparticles. The sodium vanadium fluorophosphate cathode material prepared by the present invention has a particle size of about 8 μm, a spherical shell thickness of about 200 nm, and a uniform microscopic morphology. In addition, the test results show that at a pressure of 30 MPa, the density of the sodium vanadium fluorophosphate prepared by the present invention is 4.545 g / cm 3 , and a higher tap density corresponds to a higher volumetric energy density, meeting the requirements of commercial applications. Therefore, while ensuring the electrochemical performance, the sodium vanadium fluorophosphate cathode material prepared by the present invention also improves the tap density to a certain extent, which indirectly enhances the economic benefits of this material and can be commercially applied.

[0023] (3) The raw materials required for the preparation method in the present invention are widely available and have low costs. At the same time, the synthesis process is simple, the product reproducibility is high, and it can be mass-produced at one time to meet the actual application requirements of sodium-ion batteries and can be used in the field of sodium-ion batteries. Description of the Drawings

[0024] Figure 1 Scanning electron microscope image of sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries prepared in Example 1;

[0025] Figure 2 X-ray diffraction patterns of sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries prepared in Example 1 and Comparative Example 1;

[0026] Figure 3 Transmission electron microscope images of sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries prepared in Example 1 and Comparative Example 1;

[0027] Figure 4 Tap density-pressure relationship diagram of sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries prepared in Example 1 and Comparative Example 1;

[0028] Figure 5 Cycling performance diagram of sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries prepared in Example 1 and Comparative Example 1 at 1C rate;

[0029] Figure 6 Cycling performance diagram of sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries prepared in Example 1 and Comparative Example 1 at 10C rate;

[0030] Figure 7 Constant current charge-discharge curve diagram of sodium vanadium fluorophosphate, a cathode material for sodium-ion batteries prepared in Comparative Example 2 at 1C rate;

[0031] Figure 8Rate performance graphs of sodium vanadium fluorophosphate, the cathode material for sodium-ion batteries, prepared in Example 1, Comparative Example 1, Comparative Example 3, and Comparative Example 4;

[0032] Figure 9 Cycling performance graph of sodium vanadium fluorophosphate, the cathode material for sodium-ion batteries, prepared in Example 1, at a high temperature of 60°C;

[0033] Figure 10 Cycling performance graph of sodium vanadium fluorophosphate, the cathode material for sodium-ion batteries, prepared in Example 1, at a low temperature of -20°C. Detailed implementation manners

[0034] The beneficial effects of the present invention are verified by the following examples.

[0035] Example 1: The preparation method of sodium vanadium fluorophosphate, the cathode material for sodium-ion batteries, based on spray drying, is carried out according to the following steps:

[0036] I. Element composition measurement is carried out according to the synthesis of 20 mmol of sodium vanadium fluorophosphate:

[0037] Accurately weigh 5.4 g of anhydrous oxalic acid and 4.6 g of ammonium dihydrogen phosphate, dissolve them in 200 mL of deionized water in sequence, and keep heating at 80°C, stirring and dissolving until a clear solution is obtained;

[0038] Weigh 3.18 g of anhydrous sodium carbonate and slowly add it to the above-mentioned clear solution until it is completely dissolved. During this period, keep heating at 80°C and stirring. A large number of bubbles can be observed;

[0039] Add 3.64 g of vanadium pentoxide to the above-mentioned clear solution, keep heating and stirring at 80°C for 60 min. During this period, it can be observed that the yellow suspension gradually turns into a yellow-green clear solution and finally into a dark blue clear solution;

[0040] II. Transfer the above-mentioned dark blue clear solution to a spray dryer through a peristaltic pump. Set the outlet air temperature to 120°C, the peristaltic pump feed rate to 1000 mL / h, the needle setting to 5.0, and the air speed setting to 40.0. Carry out spray drying. After removing the moisture in the solution, a blue precursor powder is obtained;

[0041] III. Use a pestle to uniformly mix the precursor powder obtained in step II with PTFE powder at a mass ratio of 10:1.8, and then transfer the mixed powder to a tube furnace. Under the protection of a flowing argon gas atmosphere, heat it to 600°C at a heating rate of 5°C / min and calcine for 4 h. After natural cooling, sodium vanadium fluorophosphate Na3V2(PO4)2F3, the cathode material for sodium-ion batteries, is obtained.

[0042] Comparative Example 1: The difference between this comparative example and Example 1 is that the mass ratio of the precursor powder to the PTFE powder in Step 3 is 10:1, and the others are the same as in Example 1, obtaining the sodium-ion battery cathode material sodium fluorophosphate vanadate.

[0043] Comparative Example 2: The difference between this comparative example and Example 1 is that the mass ratio of the precursor powder to the PTFE powder in Step 3 is 10:5, and the others are the same as in Example 1, obtaining the sodium-ion battery cathode material sodium fluorophosphate vanadate.

[0044] Comparative Example 3: The difference between this comparative example and Example 1 is that the calcination time in Step 3 is 2 h, and the others are the same as in Example 1, obtaining the sodium-ion battery cathode material sodium fluorophosphate vanadate.

[0045] Comparative Example 4: The difference between this comparative example and Example 1 is that the calcination time in Step 3 is 10 h, and the others are the same as in Example 1, obtaining the sodium-ion battery cathode material sodium fluorophosphate vanadate.

[0046] Figure 1 Figure Figure 1 is the scanning electron microscope image of the sodium-ion battery cathode material sodium fluorophosphate vanadate prepared in Example 1. It can be

[0047] Figure 2 observed that the micro-morphology of this sample is a hollow spherical shell with a particle size of about 8 μm and a spherical shell thickness of 170 - 200 nm, which conforms to the basic characteristics of spray drying. The nano-scale spherical shell provides a shorter sodium-ion transmission channel, while the micron-scale secondary particles are convenient for increasing the loading of the active material. Figure 2 Figure

[0048] Figure 3 is the X-ray diffraction pattern of the sodium-ion battery cathode material sodium fluorophosphate vanadate prepared in Example 1 and Comparative Example 1. It can be

[0049] Figure 4 Figure 2 seen that the two groups of samples in Example 1 and Comparative Example 1 can both be indexed to the sodium fluorophosphate vanadate Na3V2(PO4)2F3 structure. By comparing the two, it can be found that the sodium-ion battery cathode material sodium fluorophosphate vanadate obtained in Example 1 shows a higher purity. In its XRD diffraction pattern, almost no diffraction peaks corresponding to the impurity phase sodium vanadate phosphate can be observed. In contrast, in Comparative Example 1, due to the less amount of PTFE used, it is not enough to make up for the fluorine loss during the material synthesis process, so more impurity phase peaks of sodium vanadate phosphate Na3V2(PO4)3 can be significantly observed.

[0048] Figure 3 Figure

[0049] Figure 4 is the transmission electron microscope image of the sodium-ion battery cathode material sodium fluorophosphate vanadate prepared in Example 1 and Comparative Example 1. In Example 1, only the lattice fringes corresponding to sodium fluorophosphate vanadate can be observed, while in Comparative Example 1, in addition to the lattice fringes of sodium fluorophosphate vanadate, the lattice fringes corresponding to sodium vanadate phosphate can also be observed, which once again proves that Example 1 contains less impurity phase.

[0049] Figure 4The graph of the relationship between the tap density and pressure of the sodium vanadium fluorophosphate cathode material prepared in Example 1 and Comparative Example 1 shows that Figure 4 the tap density of the sample in Example 1 at different pressures is significantly higher than that of the sample in Comparative Example 1, indicating that the sodium vanadium fluorophosphate cathode material prepared in Example 1 better meets the economic requirements of practical applications.

[0050] Using the sodium vanadium fluorophosphate cathode materials obtained in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4 as the active materials, Super P as the conductive agent, and PVDF as the binder, they were mixed into a paste at a mass ratio of 7:2:1 to obtain the positive electrode slurry. The slurry was coated on the aluminum foil, dried and cut into positive electrode sheets with a diameter of 14 mm, and finally half-cells were assembled and charged and discharged within a voltage range of 2.5 - 4.5 V (vs. Na + / Na).

[0051] Figure 5 The graph shows the cycle performance of the sodium vanadium fluorophosphate cathode material prepared in Example 1 and Comparative Example 1 at a 1C rate; Figure 6 The graph shows the cycle performance of the sodium vanadium fluorophosphate cathode material prepared in Example 1 and Comparative Example 1 at a 10C rate; Figure 7 The graph shows the constant current charge-discharge curve of the sodium vanadium fluorophosphate cathode material prepared in Comparative Example 2 at a 1C rate. It can be seen from Figure 5 and Figure 6 that the half-cells assembled with the sodium vanadium fluorophosphate cathode materials prepared in Example 1 and Comparative Example 1 respectively show great differences in performance. Within the voltage range of 2.5 - 4.5 V (vs. Na + / Na), at a 1C rate, the initial discharge specific capacity of Example 1 is 110.52 mAh / g, and after 500 cycles, the discharge specific capacity is 98.35 mAh / g, and the cycle retention rate is 88.99%. At a 10C rate, the discharge specific capacity after 1000 cycles is 90.65 mAh / g, and the cycle retention rate is 93.41%, with excellent cycle performance. For the sodium vanadium fluorophosphate cathode material prepared in Comparative Example 1, at a 1C rate, the initial discharge specific capacity is 106.91 mAh / g, and after 500 cycles at a 1C rate, the discharge specific capacity is 79.03 mAh / g, and the cycle retention rate is 73.92%. At a 10C rate, the initial discharge specific capacity is 93.18 mAh / g, and the discharge specific capacity after 1000 cycles is 43.39 mAh / g, and the cycle retention rate is 46.56%, with poor cycle performance. This is due to the poor purity of the sodium vanadium fluorophosphate in Comparative Example 1. It can be seen from Figure 7It can be seen that for the first charge-discharge curve of the sodium-ion battery cathode material sodium fluorophosphate vanadate prepared in Comparative Example 2 at a current rate of 1C, its discharge specific capacity is only 86.98 mAh / g, far lower than 110.52 mAh / g of Example 1. This is caused by the excessive introduction of inactive substances. Due to the excessive amount of PTFE in Comparative Example 2, in its constant current charge-discharge curve, the low-voltage plateau corresponding to sodium vanadate phosphate can no longer be observed. However, the excess PTFE will generate electrochemically inactive substances during the sintering process, resulting in a decrease in the proportion of the active substance sodium fluorophosphate vanadate. Therefore, its first discharge specific capacity at a current rate of 1C is only 86.98 mAh / g. In summary, the amount of PTFE needs to be controlled within a reasonable range. Too little amount will lead to the generation of more impurity phases, while too much amount will reduce the proportion of active substances, thereby damaging the capacity.

[0052] Figure 8 Figure showing the rate performance of the sodium-ion battery cathode material sodium fluorophosphate vanadate prepared in Example 1, Comparative Example 1, Comparative Example 3, and Comparative Example 4. From Figure 8 It can be seen that for the sodium-ion battery cathode material sodium fluorophosphate vanadate of Example 1, the discharge specific capacities at 1C, 5C, and 10C current rates are 110.60 mAh / g, 103.71 mAh / g, and 97.90 mAh / g respectively. The discharge specific capacities of the material of Comparative Example 1 at 1C, 5C, and 10C current rates are 101.76 mAh / g, 82.07 mAh / g, and 68.26 mAh / g respectively, all lower than those of Example 1.

[0053] From Figure 8 It can also be seen that for the sodium-ion battery cathode material prepared in Comparative Example 3, the discharge specific capacities at 1C, 5C, and 10C current rates are 87.54 mAh / g, 38.58 mAh / g, and 13.88 mAh / g respectively, far lower than those of the sodium-ion battery cathode material prepared in Example 1. The reason is that the calcination in Comparative Example 3 is only 2 h, and the calcination time is too short, resulting in incomplete reaction. The discharge specific capacities of the sodium-ion battery cathode material of Comparative Example 4 at 1C, 5C, and 10C current rates are 83.15 mAh / g, 49.61 mAh / g, and 34.58 mAh / g respectively, also lower than those of the sodium-ion battery cathode material of Example 1. This is because the calcination time of Comparative Example 4 is 10 h, and too long calcination time will cause additional fluorine loss. The comparison results between Example 1 and Comparative Example 3 and Comparative Example 4 show that the calcination time also has a great influence on the sodium-ion battery cathode material sodium fluorophosphate vanadate, and the calcination time also needs to be controlled within a reasonable range. Too short time may lead to insufficient reaction and incomplete material forming, while too long calcination time will cause additional fluorine loss, both of which are not conducive to the improvement of the electrochemical performance of the sodium-ion battery cathode material sodium fluorophosphate vanadate.

[0054] From Figure 8It can also be seen that at the 10C, 20C, and 30C rates, the discharge specific capacities of the sodium-ion battery cathode material sodium fluorophosphate vanadate prepared in Example 1 are 97.55 mAh / g, 86.09 mAh / g, and 74.70 mAh / g respectively, with relatively excellent rate performance. In contrast, the discharge specific capacities of Comparative Example 1 are 68.26 mAh / g, 43.39 mAh / g, and 21.28 mAh / g respectively, the discharge specific capacities of Comparative Example 3 are 13.88 mAh / g, 1.42 mAh / g, and 1.07 mAh / g respectively, and the discharge specific capacities of Comparative Example 4 are 34.58 mAh / g, 22.07 mAh / g, and 13.88 mAh / g respectively. The rate performances of Comparative Example 1, Comparative Example 3, and Comparative Example 4 cannot meet the actual requirements, which also indicates that during the preparation of sodium fluorophosphate vanadate, inappropriate PTFE dosage and calcination time both deteriorate the material performance.

[0055] Figure 9 Figure for the cycling performance of the sodium-ion battery cathode material sodium fluorophosphate vanadate prepared in Example 1 at a high temperature of 60 °C; from Figure 9 It can be seen that in a high temperature environment of 60 °C, the sodium fluorophosphate vanadate prepared in Example 1 can provide an initial discharge specific capacity of 108.90 mAh / g at a rate of 10C. After 500 cycles, its discharge specific capacity is 87.25 mAh / g, and the cycling retention rate is 80.46%.

[0056] Figure 10 Figure for the cycling performance of the sodium-ion battery cathode material sodium fluorophosphate vanadate prepared in Example 1 at a low temperature of -20 °C. From Figure 10 It can be seen that in a low temperature environment of -20 °C, the sodium fluorophosphate vanadate prepared in Example 1 can provide an initial discharge specific capacity of 92.37 mAh / g at a rate of 1C. After 100 cycles, its discharge specific capacity is 92.12 mAh / g, and the cycling retention rate is as high as 99.73%. According to Figure 9 and Figure 10 It can be seen that the sodium fluorophosphate vanadate prepared in Example 1 has good wide-temperature performance, is more adaptable to extreme environments, and has good application prospects.

[0057] Example 2: The preparation method of the sodium-ion battery cathode material sodium fluorophosphate vanadate based on spray drying in this example is carried out according to the following steps:

[0058] I. According to the elemental composition measurement for synthesizing 20 mmol of sodium fluorophosphate vanadate:

[0059] Accurately weigh 5.4 g of anhydrous oxalic acid and 4.6 g of ammonium dihydrogen phosphate, dissolve them in 200 mL of deionized water in sequence, and keep heating at 80 °C, stirring and dissolving until a clear solution is obtained;

[0060] Weigh 3.18 g of anhydrous sodium carbonate and slowly add it to the above-mentioned clear solution until it is completely dissolved. During this process, heat it at 80 °C and stir. It can be observed that a large number of bubbles are generated.

[0061] Add 3.64 g of vanadium pentoxide to the above-mentioned clear solution, heat and stir at 80 °C for 60 min. During this period, it can be observed that the yellow suspension gradually turns into a yellow-green clear solution and finally into a dark blue clear solution.

[0062] Second, transfer the above-mentioned dark blue clear solution to a spray dryer through a peristaltic pump. Set the outlet air temperature to 120 °C, the peristaltic pump inlet speed to 1000 mL / h, the through needle to 5.0, and the wind speed to 40.0. Carry out spray drying. After removing the moisture in the solution, a blue precursor powder is obtained.

[0063] Third, use a pestle to uniformly mix the above-mentioned precursor powder and PTFE powder in a mass ratio of 10:2. Then transfer the mixed powder to a tube furnace. Under the protection of a flowing argon gas atmosphere, heat it to 600 °C at a heating rate of 5 °C / min and calcine for 4 h. After natural cooling, the sodium-ion battery cathode material sodium fluorophosphate vanadate Na3V2(PO4)2F3 is obtained.

[0064] Assemble a half-cell with the sodium-ion battery cathode material sodium fluorophosphate vanadate obtained in Example 2 as the active material, and carry out charge-discharge tests in the voltage range of 2.5 - 4.5 V (vs. Na + / Na). The results show that at a 1C rate, its initial discharge specific capacity is 110.56 mAh / g. After 400 cycles, the discharge specific capacity is 90.97 mAh / g, and the cycle retention rate is 82.28%. The rate test results show that its discharge specific capacities at 1C, 5C, and 10C rates are 110.85 mAh / g, 99.06 mAh / g, and 88.62 mAh / g respectively.

[0065] The present invention prepares a hollow spherical sodium fluorophosphate vanadate cathode material with a micro-nano structure and a high tap density through a spray drying method, and uses an organic compound that is not easily hydrolyzed as a fluorine source. While ensuring the fluorine content of the material, the electronic conductivity is improved. The prepared sodium-ion battery cathode material sodium fluorophosphate vanadate has the advantages of high performance, large batch, and reproducibility, which helps to promote the commercial application of sodium-ion batteries.

Claims

1. A method for preparing sodium vanadium fluorophosphate as a positive electrode material for sodium ion batteries based on spray drying, characterized in that The method proceeds as follows: Step 1: Add a carbon source, a phosphorus source, a sodium source, and a vanadium source into deionized water in order, and stir under heating conditions to obtain a clear solution; Step 2: transferring the clarified solution obtained in step 1 to a spray dryer for drying to obtain a precursor powder; Step 3: The precursor powder obtained in step 2 is mixed evenly with a fluorine source in a mass ratio of 10:(1.5-3), and then placed in a tubular furnace, wherein the fluorine source is polytetrafluoroethylene powder. Under the protection of flowing inert gas, the temperature is increased to 450-700°C at a heating rate of 5-10°C / min and calcined for 3-6h to obtain sodium vanadium fluorophosphate, a positive electrode material for sodium ion batteries, whose chemical formula is Na3V2(PO4)2F3.

2. The method for preparing sodium vanadium fluorophosphate as a positive electrode material for sodium ion batteries based on spray drying according to claim 1, characterized in that: The carbon source described in step 1 is oxalic acid and / or citric acid.

3. A method for preparing sodium vanadium fluorophosphate as a positive electrode material for sodium ion batteries based on spray drying according to claim 1 or 2, characterized in that: The phosphorus source described in step 1 is one or a mixture of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

4. A method for preparing sodium vanadium fluorophosphate as a positive electrode material for sodium ion batteries based on spray drying according to claim 1 or 2, characterized in that: The sodium source described in step 1 is one or a mixture of sodium carbonate, sodium acetate and sodium nitrate.

5. A method for preparing sodium vanadium fluorophosphate as a positive electrode material for sodium ion batteries based on spray drying according to claim 1 or 2, characterized in that: The vanadium source described in step 1 is one or a mixture of vanadium pentoxide, ammonium metavanadate, sodium metavanadate.

6. A method for preparing sodium vanadium fluorophosphate as a positive electrode material for sodium ion batteries based on spray drying according to claim 1 or 2, characterized in that: The molar ratio of the carbon source, phosphorus source, sodium source and vanadium source in step 1 is (1.5-4.5):(2.5-5.5):(4.5-7.5):(2.5-5.5), wherein the carbon source is calculated as carbon.

7. A method for preparing sodium vanadium fluorophosphate as a positive electrode material for sodium ion batteries based on spray drying according to claim 1 or 2, characterized in that: The ratio of the amount of the vanadium source described in step 1 to the volume of deionized water is (2.5-5.5) mmol: (40-60 mL).

8. A method for preparing sodium vanadium fluorophosphate as a positive electrode material for sodium ion batteries based on spray drying according to claim 1 or 2, characterized in that: In step 2, the spray drying working conditions are: the air outlet temperature is 110-130° C., and the creeping speed is 500-1500 mL / h.

9. A method for preparing sodium vanadium fluorophosphate as a positive electrode material for sodium ion batteries based on spray drying according to claim 1 or 2, characterized in that: In step three, the inert gas is argon, nitrogen or a mixture of hydrogen and argon.

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