Method for preparing sodium vanadium fluorophosphate as positive electrode material for sodium-ion batteries on basis of spray drying

Abstract

A method for preparing sodium vanadium fluorophosphate as a positive electrode material for sodium-ion batteries on the basis of spray drying, which method aims to solve the problems of poor electronic conductivity and fluorine loss in sodium vanadium fluorophosphate as a positive electrode material for sodium-ion batteries. The method comprises dissolving a carbon source, a phosphorus source, a sodium source and a vanadium source in water, and then drying same in a spray dryer to obtain a precursor powder; and then uniformly mixing the precursor powder with a fluorine source, and calcining the resulting mixture for 3-6 h to obtain sodium vanadium fluorophosphate, namely, a positive electrode material for sodium ion batteries. The material is made of micron spherical particles having a core-shell structure, the particle size is about 8 μm, the shell thickness is about 200 nm, the initial specific discharge capacity is 110.60 mAh / g at a rate of 1 C, and the specific discharge capacities are 97.90 mAh / g and 74.70 mAh / g at rates of 10 C and 30 C, respectively. The present invention can be used in the field of sodium-ion batteries.

Description

Preparation method of sodium-ion battery cathode material sodium vanadium fluorophosphate based on spray drying TECHNICAL FIELD

[0001] The present application relates to the field of electrode material synthesis and preparation, and particularly relates to a preparation method of sodium-ion battery cathode material sodium vanadium fluorophosphate based on spray drying. BACKGROUND

[0002] Lithium-ion batteries have been widely used in electric vehicles, computers, communications, consumer electronics and other fields, and have become a mature energy storage device. However, the reserves of lithium resources in the earth's crust are relatively scarce, and it is 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 market for lithium-ion batteries will inevitably lead to problems such as depletion of lithium resources and rising prices. Therefore, it is urgent to develop a new type of long-life secondary battery with more abundant resources and lower cost. Similar to lithium-ion batteries, sodium-ion batteries are also "rocking chair" batteries. In comparison, sodium resources are widely distributed and abundant in reserves, and there is almost no depletion problem. With the advantages of low cost and high safety, sodium-ion batteries have gradually become a hot spot in the energy storage field.

[0003] The cathode material is an important component of sodium-ion batteries, and plays a decisive role in the electrochemical performance of the energy density, power density and cycle life of the entire battery system. Among the existing sodium-ion battery cathode materials, sodium vanadium fluorophosphate (Na3V2(PO4)2F3) is a NASICON-type polyanion compound. It has good cycle stability due to its open and stable three-dimensional framework and unobstructed sodium ion diffusion channel. 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 (507Wh / kg) is close to that of commercial lithium iron phosphate cathode material (580Wh / kg). Therefore, sodium vanadium fluorophosphate is considered to be a strong competitor for the next generation of commercial sodium-ion battery cathode materials. However, there are still many problems to be solved in sodium-ion battery cathode material sodium vanadium fluorophosphate. On the one hand, due to the insulating nature of phosphate and fluoride ions in the framework, the conductivity of sodium vanadium fluorophosphate is poor, and its rate performance is also limited to a certain extent. On the other hand, in the preparation process of sodium vanadium fluorophosphate, the inevitable loss of fluorine will lead to the generation of impurity phase sodium vanadium phosphate, which not only damages the cycle stability of the material, but also reduces its average working voltage. At present, some research and reports have tried to solve the above problems through strategies such as nanocrystallization, carbon coating and fluorine regulation. However, the production of sodium-ion battery cathode material sodium vanadium fluorophosphate in most of the literature is relatively small, and it is difficult to reproduce. The reproducibility and consistency are poor, and it is difficult to realize large-scale production while ensuring the performance of the material.

[0004] SUMMARY

[0005] The present application is to solve the problems of poor electronic conductivity and fluorine loss of sodium-ion battery cathode material sodium vanadium fluorophosphate, and provides a preparation method of sodium-ion battery cathode material sodium vanadium fluorophosphate based on spray drying.

[0006] The preparation method of sodium-ion battery cathode material sodium vanadium fluorophosphate based on spray drying of the present application is carried out according to the following steps:

[0007] Step one: carbon source, phosphorus source, sodium source and vanadium source are sequentially added to deionized water in order, and stirred under heating conditions to obtain a clear solution; the purpose of this step is to fully mix and dissolve the added raw materials, and to ensure the occurrence of vanadium source reduction and complexation reaction;

[0008] Step two: the clear solution obtained in step one is transferred to a spray dryer for drying treatment to obtain a precursor powder; this step is a secondary granulation process and removes water from the solution;

[0009] Step three: the precursor powder obtained in step two is mixed with fluorine source at a mass ratio of 10:(1.5-3) and then placed in a tube furnace, wherein the fluorine source is polytetrafluoroethylene (PTFE) and / or polyvinylidene fluoride (PVDF), and the temperature is raised to 450-700℃ at a heating rate of 5-10℃ / min under the protection of flowing inert gas for calcination for 3-6h to obtain sodium-ion battery cathode material sodium vanadium fluorophosphate, with a chemical formula of Na3V2(PO4)2F3.

[0010] Further, the carbon source in step one is oxalic acid and / or citric acid.

[0011] Further, the phosphorus source in step one is one or a mixture of several of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate and disodium hydrogen phosphate.

[0012] Further, the sodium source in step one is one or a mixture of several of sodium carbonate, sodium acetate and sodium nitrate.

[0013] Further, the vanadium source in step one is one or a mixture of several of vanadium pentoxide, ammonium metavanadate and sodium metavanadate.

[0014] Further, the molar ratio of the carbon source, phosphorus source, sodium source and vanadium source in step one is (1.5-4.5):(2.5-5.5):(4.5-7.5):(2.5-5.5), wherein the carbon source is calculated based on carbon.

[0015] Further, the ratio of the amount of substance of the vanadium source to the volume of deionized water in step one is (2.5-5.5) mmol:(40-60 mL).

[0016] Further, the heating temperature in step one is 55-85℃, and the time is 50-70 min. The purpose of heating in this step is to accelerate the mixing and dissolution of the reaction materials, and to promote the complexation and reduction reactions of the vanadium source.

[0017] Further, in step two, the spray drying working conditions are as follows: the air outlet temperature is 110-130℃, and the peristaltic speed is 500-1500 mL / h. This step is a secondary granulation and rapid drying process, and the purpose is to quickly remove water while constructing the hollow spherical sodium vanadium fluorophosphate positive electrode material with micro-nano structure.

[0018] Further, in step three, the mass ratio of the precursor powder to the fluorine source is 10:(1-3).

[0019] Further, in step three, the mixing method of the precursor and the fluorine source is preferably manual grinding with a pestle or ball milling at a speed of 300 rpm, which aims to improve the uniformity of the reaction materials, thereby improving the purity of the product after sintering and shortening the reaction time.

[0020] Further, in step three, the inert gas is argon, nitrogen, or hydrogen-argon mixed gas.

[0021] The beneficial effects of the present application relative to the prior art are as follows:

[0022] The present application successfully prepares the sodium-ion battery positive electrode material sodium vanadium fluorophosphate by combining the spray drying method with the high-temperature solid-phase method. The secondary granulation and rapid drying by spray drying are convenient and effective, and polytetrafluoroethylene (PTFE) and / or polyvinylidene fluoride (PVDF) are used as organic fluorine sources, which are difficult to hydrolyze in aqueous solution, thereby reducing the loss of fluorine elements to some extent during the preparation of the sodium vanadium fluorophosphate positive electrode material. The material is coated with carbon, and the organic fluorine source can also act as a carbon source during the calcination process, thereby improving the electronic conductivity of the material. The average powder resistivity of the sodium vanadium fluorophosphate prepared by the method of the present application is 51.97 Ω·cm, and the average powder conductivity is 1.92×10 -2 S / cm. Benefiting from the excellent conductivity, the sodium-ion battery positive electrode material sodium vanadium fluorophosphate prepared by the present application has satisfactory rate performance. The initial discharge specific capacity is 110.60 mAh / g at 1C rate, and the discharge specific capacities are 97.90 mAh / g and 74.70 mAh / g at 10C and 30C rates, respectively, which has more excellent rate performance.

[0023] (2) The sodium-ion battery cathode material sodium vanadium fluorophosphate with unique core-shell structure is prepared by the spray drying method, which has the advantages of short ion diffusion path of nanoparticles and high compaction density of microparticles. The sodium vanadium fluorophosphate cathode material prepared by the application has a particle size of about 8 microns, a shell thickness of about 200 nanometers, and uniform micro-morphology. In addition, test results show that the density of the sodium vanadium fluorophosphate prepared by the application is 4.545 g / cm 3 at a pressure of 30 MPa, which corresponds to a higher compaction density and a higher volume specific energy, meeting the needs of commercial applications. Therefore, the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared by the application not only ensures the electrochemical performance, but also improves the compaction density to a certain extent, which indirectly improves the economic benefits of the material and can be applied commercially.

[0024] (3) The raw materials required by the preparation method in the application have a wide source and low cost, and the synthesis process is simple, the product has high reproducibility, and can be produced in large quantities at one time, meeting the needs of the practical application of sodium-ion batteries and can be used in the field of sodium-ion batteries. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 is a scanning electron microscope picture of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1;

[0026] Figure 2 is an X-ray diffraction spectrum of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 and Comparative Example 1;

[0027] Figure 3 is a transmission electron microscope picture of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 and Comparative Example 1;

[0028] Figure 4 is a compaction density-pressure relationship diagram of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 and Comparative Example 1;

[0029] Figure 5 is a cycle performance diagram of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 and Comparative Example 1 at a 1C rate;

[0030] Figure 6 is a cycle performance diagram of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 and Comparative Example 1 at a 10C rate;

[0031] Figure 7 is a constant current charge-discharge curve diagram of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Comparative Example 2 at a 1C rate;

[0032] Figure 8 is a rate performance diagram of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1, Comparative Example 1, Comparative Example 3 and Comparative Example 4;

[0033] Figure 9 is a graph of the cycle performance of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 at a high temperature of 60°C;

[0034] Figure 10 is a graph of the cycle performance of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 at a low temperature of -20°C. DETAILED DESCRIPTION

[0035] The beneficial effects of the present application are verified by the following examples.

[0036] Example 1: The preparation method of the spray-drying-based sodium-ion battery cathode material sodium vanadium fluorophosphate in this example is carried out in the following steps:

[0037] I. The elemental composition is measured according to the synthesis of 20 mmol of sodium vanadium fluorophosphate:

[0038] Accurately weigh 5.4 g of anhydrous oxalic acid and 4.6 g of ammonium dihydrogen phosphate, and sequentially dissolve them in 200 mL of deionized water while maintaining 80°C heating and stirring until a clear solution is obtained;

[0039] Weigh 3.18 g of anhydrous sodium carbonate and slowly add it to the above clear solution until it is completely dissolved, maintaining 80°C heating and stirring in the process, and a large amount of bubbles can be observed;

[0040] Add 3.64 g of vanadium pentoxide to the above clear solution, maintain 80°C heating and stirring for 60 min, and observe that the yellow suspension gradually changes to a yellow-green clear solution and finally to a dark blue clear solution;

[0041] II. The above dark blue clear solution is transferred to a spray dryer through a peristaltic pump, the outlet temperature is set to 120°C, the peristaltic pump inlet speed is 1000 mL / h, the needle is set to 5.0, and the air speed is set to 40.0, and spray drying is carried out, after removing the water in the solution, a blue precursor powder is obtained;

[0042] III. The precursor powder obtained in step II is uniformly mixed with PTFE powder in a mass ratio of 10:1.8 using a pestle, and then the mixed powder is transferred to a tube furnace, which is heated to 600°C at a rate of 5°C / min under a flowing argon atmosphere, and calcined for 4 h, and after natural cooling, the sodium-ion battery cathode material sodium vanadium fluorophosphate Na3V2(PO4)2F3 is obtained.

[0043] Comparative Example 1: The mass ratio of the precursor powder to PTFE powder in step III of this comparative example is 10:1, which is different from Example 1, and the others are the same as Example 1, and the sodium-ion battery cathode material sodium vanadium fluorophosphate is obtained.

[0044] Comparative Example 2: The difference between this comparative example and Example 1 is that the mass ratio of precursor powder to PTFE powder in step three is 10:5, and other conditions are the same as Example 1, obtaining sodium-ion battery cathode material sodium vanadium fluorophosphate.

[0045] Comparative Example 3: The difference between this comparative example and Example 1 is that the calcination time in step three is 2h, and other conditions are the same as Example 1, obtaining sodium-ion battery cathode material sodium vanadium fluorophosphate.

[0046] Comparative Example 4: The difference between this comparative example and Example 1 is that the calcination time in step three is 10h, and other conditions are the same as Example 1, obtaining sodium-ion battery cathode material sodium vanadium fluorophosphate.

[0047] Figure 1 is a scanning electron microscope image of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1. From Figure 1, it can be observed that the micro-morphology of the sample is a hollow spherical shell with a particle size of about 8μm, and the shell thickness is 170-200nm, which is consistent with the basic characteristics of spray drying. The nanoscale shell provides a shorter sodium ion transmission channel, and the micrometer-sized secondary particles facilitate the increase of active material loading.

[0048] Figure 2 is an X-ray diffraction pattern of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 and Comparative Example 1. As can be seen from Figure 2, both the samples of Example 1 and Comparative Example 1 can be indexed as sodium vanadium fluorophosphate Na3V2(PO4)2F3 structure. By comparing the two, it can be found that the sodium-ion battery cathode material sodium vanadium fluorophosphate obtained in Example 1 exhibits higher purity. In its XRD diffraction pattern, almost no diffraction peak corresponding to the impurity phase sodium vanadium phosphate Na3V2(PO4)3 can be observed. In contrast, in Comparative Example 1, due to the smaller amount of PTFE, it is not enough to make up for the loss of fluorine during the synthesis of the material, so more sodium vanadium phosphate Na3V2(PO4)3 phase impurity peaks can be observed.

[0049] Figure 3 is a transmission electron microscope image of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 and Comparative Example 1. In Example 1, only the lattice fringes corresponding to sodium vanadium fluorophosphate can be observed, while in Comparative Example 1, in addition to the lattice fringes of sodium vanadium fluorophosphate, the lattice fringes corresponding to sodium vanadium phosphate can also be observed, which again proves that Example 1 contains less impurity phase.

[0050] Figure 4 is a graph of the compaction density-pressure relationship of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 and Comparative Example 1. As can be seen from Figure 4, the compaction density of the sample of Example 1 under different pressures is significantly higher than that of the sample of Comparative Example 1, indicating that the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 meets the economic requirements of actual application.

[0051] The sodium-ion battery cathode material sodium vanadium fluorophosphate obtained in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4 was used as an active material, Super P was used as a conductive agent, and PVDF was used as a binder. The active material, the conductive agent and the binder were mixed in a mass ratio of 7:2:1 and a paste was obtained. The paste was coated on an aluminum foil, dried, and then cut into a cathode sheet with a diameter of 14 mm. Finally, a half battery was assembled and subjected to charge-discharge test in a voltage range of 2.5-4.5 V (vs. Na + / Na).

[0052] FIG. 5 is a cycle performance graph of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 and Comparative Example 1 at a 1C rate; FIG. 6 is a cycle performance graph of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 and Comparative Example 1 at a 10C rate; and FIG. 7 is a constant current charge-discharge curve graph of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Comparative Example 2 at a 1C rate. As can be seen from FIG. 5 and FIG. 6, the half batteries assembled using the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 and Comparative Example 1 as an active material respectively showed a huge difference in performance, and the half battery assembled using the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 as an active material had a higher capacity and a better cycle performance than the half battery assembled using the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Comparative Example 1 as an active material. +The initial discharge specific capacity of the sodium-ion battery cathode material prepared in Example 1 was 110.52 mAh / g at 1C rate, and the discharge specific capacity was 98.35 mAh / g after 500 cycles, with a cycle retention rate of 88.99% in the voltage range of 1.5-4.1 V (vs. Na+ / Na). The discharge specific capacity of the sodium-ion battery cathode material prepared in Example 1 was 90.65 mAh / g after 1000 cycles at 10C rate, with a cycle retention rate of 93.41%, and the cycle performance was excellent. The initial discharge specific capacity of the sodium-ion battery cathode material prepared in Comparative Example 1 was 106.91 mAh / g at 1C rate, and the discharge specific capacity was 79.03 mAh / g after 500 cycles at 1C rate, with a cycle retention rate of 73.92%. The initial discharge specific capacity of the sodium-ion battery cathode material prepared in Comparative Example 1 was 93.18 mAh / g, and the discharge specific capacity was 43.39 mAh / g after 1000 cycles at 10C rate, with a cycle retention rate of 46.56%, and the cycle performance was poor. This was caused by the poor purity of sodium vanadium fluorophosphate in Comparative Example 1. As can be seen from FIG. 7, the initial charge-discharge curve of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Comparative Example 2 at 1C rate had a discharge specific capacity of only 86.98 mAh / g, which was much lower than 110.52 mAh / g of Example 1. This was caused by the excessive introduction of non-active substances. Because the amount of PTFE in Comparative Example 2 was excessive, the low-voltage platform corresponding to sodium vanadium phosphate was not observed in the constant-current charge-discharge curve. However, the excessive PTFE generated substances that were not electrochemically active during the sintering process, which reduced the proportion of the active substance sodium vanadium fluorophosphate, and thus the initial discharge specific capacity at 1C rate was only 86.98 mAh / g. In summary, the amount of PTFE needs to be controlled within a reasonable range. Too little PTFE will result in more impurities, and too much PTFE will reduce the proportion of active substances, thereby damaging the capacity.

[0053] FIG. 8 is a rate performance graph of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1, Comparative Example 1, Comparative Example 3, and Comparative Example 4. As can be seen from FIG. 8, the discharge specific capacities of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 at 1C, 5C, and 10C rates were 110.60 mAh / g, 103.71 mAh / g, and 97.90 mAh / g, respectively. The discharge specific capacities of the material prepared in Comparative Example 1 at 1C, 5C, and 10C rates were 101.76 mAh / g, 82.07 mAh / g, and 68.26 mAh / g, respectively, all of which were lower than those of Example 1.

[0054] It can also be seen from FIG. 8 that the discharge specific capacity of the sodium ion battery positive electrode material prepared in Comparative Example 3 at 1C, 5C and 10C rates is 87.54 mAh / g, 38.58 mAh / g and 13.88 mAh / g, respectively, which is much lower than that of the sodium ion battery positive electrode material prepared in Example 1. The reason is that the calcination in Comparative Example 3 is only 2 h, and the short calcination time leads to incomplete reaction. The discharge specific capacity of the sodium ion battery positive electrode material in Comparative Example 4 at 1C, 5C and 10C rates is 83.15 mAh / g, 49.61 mAh / g and 34.58 mAh / g, respectively, which is also lower than that of the sodium ion battery positive electrode material in Example 1. This is because the calcination time in Comparative Example 4 is 10 h, and the long calcination time causes additional loss of fluorine. The comparison results of Example 1 and Comparative Examples 3 and 4 show that the calcination time also has a great influence on the sodium ion battery positive electrode material sodium vanadium fluorophosphate, and the calcination time also needs to be controlled within a reasonable range. Too short calcination time may lead to incomplete reaction and incomplete material formation, while too long calcination time causes additional loss of fluorine, which is not conducive to the improvement of the electrochemical performance of the sodium ion battery positive electrode material sodium vanadium fluorophosphate.

[0055] It can also be seen from FIG. 8 that the discharge specific capacity of the sodium ion battery positive electrode material prepared in Example 1 at 10C, 20C and 30C rates is 97.55 mAh / g, 86.09 mAh / g and 74.70 mAh / g, respectively, which has excellent rate performance. In comparison, the discharge specific capacity of Comparative Example 1 is 68.26 mAh / g, 43.39 mAh / g and 21.28 mAh / g, respectively, the discharge specific capacity of Comparative Example 3 is 13.88 mAh / g, 1.42 mAh / g and 1.07 mAh / g, respectively, and the discharge specific capacity of Comparative Example 4 is 34.58 mAh / g, 22.07 mAh / g and 13.88 mAh / g, respectively. The rate performance of Comparative Examples 1, 3 and 4 cannot meet the actual demand, which also indicates that inappropriate PTFE dosage and calcination time during the preparation of sodium vanadium fluorophosphate deteriorate the material performance.

[0056] FIG. 9 is a cycle performance diagram of the sodium ion battery positive electrode material sodium vanadium fluorophosphate prepared in Example 1 at a high temperature environment of 60°C. It can be seen from FIG. 9 that the sodium vanadium fluorophosphate prepared in Example 1 can provide a first discharge specific capacity of 108.90 mAh / g at a 10C rate in a high temperature environment of 60°C, and the discharge specific capacity is 87.25 mAh / g after 500 cycles, with a cycle retention rate of 80.46%.

[0057] Figure 10 is a graph of the cycle performance of the sodium-ion battery cathode material sodium vanadium fluorophosphate prepared in Example 1 at a low temperature environment of -20°C. As can be seen from Figure 10, the sodium vanadium fluorophosphate prepared in Example 1 can provide a first discharge specific capacity of 92.37 mAh / g at a 1C rate in a low temperature environment of -20°C, and the discharge specific capacity is 92.12 mAh / g after 100 cycles, with a cycle retention rate of up to 99.73%. As can be seen from Figures 9 and 10, the sodium vanadium fluorophosphate prepared in Example 1 has good wide-temperature-range performance, is more suitable for extreme environments, and has good application prospects.

[0058] Example 2: The preparation method of the spray-drying-based sodium-ion battery cathode material sodium vanadium fluorophosphate in this example is carried out in the following steps:

[0059] I. Element composition measurement according to the synthesis of 20 mmol of sodium vanadium fluorophosphate:

[0060] Accurately weigh 5.4 g of anhydrous oxalic acid and 4.6 g of ammonium dihydrogen phosphate, and sequentially dissolve them in 200 mL of deionized water while maintaining heating at 80°C, and stirring to obtain a clear solution;

[0061] Weigh 3.18 g of anhydrous sodium carbonate, and slowly add it to the above clear solution until it is completely dissolved, while maintaining heating at 80°C and stirring, and a large amount of bubbles can be observed;

[0062] Add 3.64 g of vanadium pentoxide to the above clear solution, maintain heating at 80°C and stirring for 60 min, and during this time, it can be observed that the yellow suspension gradually changes to a yellow-green clear solution, and finally to a dark blue clear solution;

[0063] II. Transfer the above dark blue clear solution to a spray dryer through a peristaltic pump, set the outlet temperature to 120°C, the peristaltic pump inlet speed to 1000 mL / h, the needle to 5.0, and the air speed to 40.0, and carry out spray drying, and after removing the water in the solution, obtain a blue precursor powder;

[0064] III. Uniformly mix the above precursor powder with PTFE powder in a mass ratio of 10:2 using a pestle. Then transfer the mixed powder to a tube furnace, and under the protection of a flowing argon atmosphere, heat it to 600°C at a heating rate of 5°C / min for 4 h, and after natural cooling, obtain the sodium-ion battery cathode material sodium vanadium fluorophosphate Na3V2(PO4)2F3.

[0065] Assemble a half-cell using the sodium-ion battery cathode material sodium vanadium fluorophosphate obtained in Example 2 as the active material, and test its electrochemical performance in the voltage range of 2.5-4.5 V (vs. Na +The charge-discharge test is carried out in the voltage interval of (0-3.0) V. The results show that the initial specific discharge capacity is 110.56 mAh / g at 1C rate, and the specific discharge capacity is 90.97 mAh / g after 400 cycles, and the cycle retention rate is 82.28%. The rate test results show that the discharge specific capacities at 1C, 5C and 10C rates are 110.85 mAh / g, 99.06 mAh / g and 88.62 mAh / g, respectively.

[0066] The hollow spherical sodium vanadium fluorophosphate positive electrode material with micro-nano structure and high compact density is prepared by the spray drying method, and the organic compound which is not easy to hydrolyze is used as a fluorine source, so that the fluorine content of the material is ensured, and the electronic conductivity is improved. The prepared sodium vanadium fluorophosphate positive electrode material of sodium ion battery has the advantages of high performance, large batch and reproducibility, and helps to promote the commercial application of sodium ion battery.

Claims

1. A method for preparing a spray-drying-based sodium-ion battery cathode material, sodium vanadium fluorophosphate, characterized in that The method is carried out according to the following steps: Step one: carbon source, phosphorus source, sodium source, vanadium source are added into deionized water in sequence under heating condition, and stirring to obtain a clear solution; Step two: the clear solution obtained in step one is transferred into a spray dryer for drying treatment to obtain a precursor powder; Step three: the precursor powder obtained in step two is mixed with fluorine source in a mass ratio of 10:(1.5-3), and then placed in a tube furnace, wherein the fluorine source is polytetrafluoroethylene and / or polyvinylidene fluoride, and calcined at a temperature of 450-700℃ for 3-6h under the protection of flowing inert gas at a heating rate of 5-10℃ / min to obtain sodium-ion battery positive electrode material sodium vanadium fluorophosphate with a chemical formula of Na3V2(PO4)2F3.

2. The method according to claim 1, wherein the method for preparing a sodium-ion battery cathode material of sodium vanadium fluorophosphate based on spray drying is characterized in that, The carbon source in step one is oxalic acid and / or citric acid.

3. The preparation method of a spray-drying-based sodium-ion battery cathode material sodium vanadium fluorophosphate according to claim 1 or 2, characterized in that, The phosphorus source in step one is a mixture of one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

4. The preparation method of a spray-drying-based sodium-ion battery cathode material sodium vanadium fluorophosphate according to claim 1 or 2, characterized in that, The sodium source in step one is a mixture of one or more of sodium carbonate, sodium acetate, and sodium nitrate.

5. The preparation method of a spray-drying-based sodium-ion battery cathode material sodium vanadium fluorophosphate according to claim 1 or 2, characterized in that, The vanadium source in step one is a mixture of one or more of vanadium pentoxide, ammonium metavanadate, and sodium metavanadate.

6. The preparation method of a spray-drying-based sodium-ion battery cathode material sodium vanadium fluorophosphate 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 one is (1.5-4.5):(2.5-5.5):(4.5-7.5):(2.5-5.5), wherein the carbon source is calculated based on carbon.

7. The preparation method of a spray-drying-based sodium-ion battery cathode material sodium vanadium fluorophosphate according to claim 1 or 2, characterized in that, The ratio of the amount of substance of the vanadium source to the volume of deionized water in step one is (2.5-5.5)mmol:(40-60mL).

8. The preparation method of a spray-drying-based sodium-ion battery cathode material sodium vanadium fluorophosphate according to claim 1 or 2, characterized in that, In step two, the spray drying working conditions are: air outlet temperature of 110-130℃, and peristaltic speed of 500-1500mL / h.

9. The preparation method of a spray-drying-based sodium-ion battery cathode material sodium vanadium fluorophosphate according to claim 1 or 2, characterized in that, The mass ratio of the precursor powder to the fluorine source in step three is 10:(1-3).

10. The method for preparing a spray-drying-based sodium-ion battery cathode material sodium vanadium fluorophosphate according to claim 1 or 2, characterized in that, In step three, the inert gas is argon, nitrogen, or hydrogen-argon mixed gas.

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