Preparation method of carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material

Abstract

A method for preparing a carbon-coated Na₃V₂(PO₄)₂F₃ sodium-ion battery positive electrode material is disclosed. This method addresses the poor rate performance of existing Na₃V₂(PO₄)₂F₃ cathode materials prepared by the sol-gel method. The method involves weighing a sodium salt, a vanadium source, a phosphate, a fluoride salt, and a carbon source, treating them using a sol-gel method at 90-95°C, then cooling them to 75-80°C to produce a wet gel. The wet gel is then dried, pre-sintered, and then sintered at high temperature to produce the carbon-coated Na₃V₂(PO₄)₂F₃ sodium-ion battery positive electrode material. The material exhibits an initial capacity of 124.46 mA / g, a capacity retention rate of 77.24% after 1000 cycles, and a capacity of 103.58 mA / g at a rate of 30C. This material can be used in the field of sodium-ion batteries.

Description

Technical Field

[0001] The invention relates to a method for preparing a positive electrode material for a sodium ion battery, and belongs to the technical field of material synthesis. Background Art

[0002] Sodium-ion batteries (SIBs) have attracted widespread attention due to their abundant sodium resources and electrochemical mechanisms similar to those of current mainstream lithium-ion batteries (LIBs). Among the positive electrode materials of sodium-ion batteries, Na3V2(PO4)2F3, as a compound with a NASICON structure, has attracted much attention due to its high conductivity and stable three-dimensional structure. However, this type of material still has some key problems that restrict its practical application: 1) The discharge capacity is too low, and the discharge capacity at a small rate is only about 110mAh / g, which is less competitive; 2) The rate performance is poor. Due to the low intrinsic electronic conductivity of the material, the capacity retention rate is low during high-rate charge and discharge. 3) Poor cycle stability: During long-term cycling, the material structure is unstable and the capacity decays severely. In the existing technology, the sol-gel method is an effective method for preparing Na3V2(PO4)2F3 positive electrode materials, but there is a bottleneck with limited rate performance improvement. Summary of the Invention

[0003] The present invention aims to solve the technical problem of poor rate performance of Na3V2(PO4)2F3 positive electrode materials prepared by the existing sol-gel method, and provides a method for preparing a carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material. By performing variable temperature control in the sol-gel method, the present invention effectively improves the internal conductivity of the material, significantly improving the material's first-cycle coulombic efficiency, cycle life, and rate performance, especially the rate performance under rapid charge and discharge conditions. At the same time, the method of the present invention is simple and cost-effective, and the new positive electrode material prepared can meet the application requirements of the next generation of high-energy-density sodium ion batteries.

[0004] The preparation method of the carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material of the present invention is carried out according to the following steps:

[0005] 1. Weigh sodium salt, trivalent vanadium source, phosphate and fluoride salt according to the molar ratio of Na, V, P and F being 3:2:2:3, and then weigh the carbon source according to the molar ratio of C atoms in the carbon source to V atoms in the vanadium source being (0.75-0.85):1;

[0006] Alternatively, sodium salt, pentavalent vanadium source, phosphate and fluoride salt are weighed according to the molar ratio of Na, V, P and F being 3:2:2:3, and then the carbon source is weighed according to the molar ratio of C atoms in the carbon source to V atoms in the vanadium source being (0.75-0.85):1; and then the reducing agent is weighed, wherein the amount of the reducing agent added is just the amount of the pentavalent vanadium source V 5+ Completely restored to V 3+ the amount;

[0007] 2. Dissolve the carbon source or the carbon source and the reducing agent in water, adjust the pH value to 3.5-5.0, add the vanadium source to obtain a mixed solution; heat the mixed solution to 90-95° C., stir for 20-40 minutes, add the sodium salt, phosphate and fluoride salt, and continue stirring for 20-40 minutes;

[0008] 3. Lower the temperature of the mixed solution to 75-80°C and continue stirring until it becomes a wet gel;

[0009] 4. vacuum drying the wet gel at a temperature of 75-80°C for 10-12 hours to obtain a dry gel;

[0010] 5. Grind the dry gel into powder, place it in a high-temperature furnace, heat it to 200-400°C in an inert atmosphere, pre-sinter it for 4-5 hours, and then heat it to 500-700°C and sinter it at high temperature for 7-9 hours to obtain carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material.

[0011] Furthermore, the sodium salt described in step 1 is one or a combination of sodium metavanadate, sodium chloride, sodium fluoride, sodium hydroxide, and sodium acetate.

[0012] Furthermore, the pentavalent vanadium source in step one is one or a combination of two of ammonium metavanadate, sodium metavanadate, and vanadium pentoxide; the trivalent vanadium source in step one is one or a combination of two of vanadium acetate, vanadium fluoride, vanadium phosphate, and vanadium trioxide.

[0013] Furthermore, the phosphate in step 1 is one or a combination of ammonium dihydrogen phosphate, phosphoric acid and sodium phosphate.

[0014] Furthermore, the fluoride salt in step 1 is one or a combination of ammonium fluoride, sodium fluoride and vanadium fluoride.

[0015] Furthermore, the carbon source in step 1 is one or a combination of glucose, sucrose, ethylene glycol, polyethylene glycol (PEG), citric acid, and ascorbic acid;

[0016] Furthermore, the reducing agent in step 1 is one or a combination of two of ethylene glycol, citric acid, glucose, oxalic acid, ammonium carbamate, urea, ascorbic acid, and sodium borohydride.

[0017] Furthermore, in step 2, the mass ratio of solute to water is 1:(15-30). The concentration of the solution affects the reaction time. If the concentration is too high, the V element will be partially oxidized, and if the concentration is too low, the reaction will be incomplete and impurities will be generated.

[0018] Furthermore, the material used to adjust the pH value in step 2 is one or a combination of ammonia and sodium hydroxide;

[0019] Furthermore, in step 2, the pH value is adjusted to 4.2-4.6.

[0020] Furthermore, the inert atmosphere in step 5 is nitrogen or argon. Inert gas can avoid oxidation and improve the uniformity of carbon coating.

[0021] The preparation method of the carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material of the present invention greatly improves the high rate performance and cycle performance of the material by adjusting the sol-gel temperature. Uniform carbon coating can provide a channel for the rapid transmission of sodium ions, thereby improving the rate performance of the material. Since the sol-gel method is carried out in an open dissolution system, there is more oxygen in the synthesis environment, and the high temperature is used for driving, the V element in the material is very easy to oxidize. In the preparation process of the material, ensuring that the trivalent V is not oxidized is the key to ensuring that the material capacity is fully released, because for the sodium vanadium fluorophosphate positive electrode material, its capacity is determined by V. 3+ / V 4+If there is incomplete reduction of V elements during the reaction process, it will affect the capacity release, resulting in an initial capacity of the material of only about 110mAh / g. The performance of using a single temperature has its own advantages. Most traditional sol-gel temperatures use 80°C, but the performance at 80°C is average. The low-rate performance at 95°C is excellent, but as the rate increases, the rate of decay is much faster than that of materials prepared at relatively low sol temperatures. This is because high-temperature sol-gel synthesis affects the quality of carbon coating and the particle size of the material, resulting in reduced conductivity. Therefore, under high current conditions, the rate performance decays faster, so the high-rate performance is not as good as that of materials prepared at 85°C. However, the lower the temperature, the better the rate performance, because the temperature will affect the time it takes for the sol-gel to complete. If the temperature is too low and the synthesis time is extended, the performance will decrease. Taking these elements into consideration, the idea of ​​variable temperature operation is proposed. A higher temperature in the early stage can control the oxidation degree of the V element, and a lower temperature (75-80°C) in the later stage can improve the charge transfer kinetics, enhance the capacity and cycle performance of the material at a high rate, and form a uniform and good carbon coating layer while synergistically controlling the material particle size, which can solve the shortcoming of low ionic conductivity of the sodium vanadium fluorophosphate material itself. Under the synergistic optimization of the two temperatures, the rate performance and cycle stability of the NVPF material are significantly improved, especially solving the problems of slow ion migration rate and structural instability under high current charge and discharge, thereby improving the comprehensive performance of the battery. The carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material prepared by the present invention has a first-week capacity of 124.46mA / g (1C rate), a capacity retention rate of 77.24% after 1000 cycles, and a capacity of 103.58mA / g at a 30C rate.

[0022] The carbon-coated Na₃V₂(PO₄)₂F₃ sodium-ion battery cathode material prepared by the method of the present invention is a novel cathode material with excellent performance, simple processing, and controllable cost. It can meet the current application requirements of high-rate sodium-ion batteries. This optimization method is simple, easy to implement, and low-cost, and can be used in high-energy-density sodium-ion batteries for low-speed vehicles, grid energy storage, and emergency backup power supplies. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 XRD spectra of the carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material prepared in Example 1 and the materials prepared in Comparative Examples 1 to 4;

[0024] Figure 2 This is an SEM image of the carbon-coated Na3V2(PO4)2F3 sodium ion battery cathode material prepared in Example 1;

[0025] Figure 3Comparison of the first-week charge-discharge curves of the carbon-coated Na3V2(PO4)2F3 sodium-ion battery positive electrode material prepared in Example 1 and the materials prepared in Comparative Examples 1 to 4;

[0026] Figure 4 Comparison of charge and discharge curves of the carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material prepared in Example 1 and the materials prepared in Comparative Examples 1 to 4 at a rate of 30C;

[0027] Figure 5 A comparison chart of the rate performance of the carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material prepared in Example 1 and the materials prepared in Comparative Examples 1 to 4;

[0028] Figure 6 This is a performance comparison chart of the carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material prepared in Example 1 and the materials prepared in Comparative Examples 1 to 4 after 1000 cycles at a 5C rate. DETAILED DESCRIPTION

[0029] The beneficial effects of the present invention are demonstrated with the following examples.

[0030] Example 1: The preparation method of the carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material of this embodiment is carried out according to the following steps:

[0031] 1. Weigh 0.252 g of sodium fluoride, 0.364 g of vanadium pentoxide, and 0.46 g of ammonium dihydrogen phosphate, where the molar ratio of Na, V, P, and F is 3:2:2:3. Then, use 0.6147 g of citric acid as a carbon source and reducing agent, and the molar ratio of citric acid to V atoms is 0.8:1.

[0032] 2. Dissolve citric acid in water, add ammonia water to adjust the pH to 4.4, and then add vanadium pentoxide to obtain a mixed solution; raise the temperature of the mixed solution to 90°C and stir for 30 minutes, then add sodium fluoride and ammonium dihydrogen phosphate, and continue stirring for 30 minutes;

[0033] 3. Lower the temperature of the mixed solution to 80°C and continue stirring for 60 minutes to obtain a wet gel;

[0034] 4. Dry the wet gel in vacuum at 80°C for 12 h to obtain a dry gel;

[0035] 5. Grind the dry gel into powder, place it in a high-temperature furnace, heat it to 300°C in an inert atmosphere, pre-sinter it for 4 hours, and then heat it to 600°C and sinter it at high temperature for 8 hours to obtain carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material.

[0036] Comparative Example 1: The difference between this comparative example and Example 1 is that the temperature in step 2 and step 3 is 80°C, and the other steps and parameters are the same as those in Example 1.

[0037] Comparative Example 2: This comparative example differs from Example 1 in that the temperature in step 2 and step 3 is both 85°C, and the other steps and parameters are the same as those in Example 1.

[0038] Comparative Example 3: This comparative example differs from Example 1 in that the temperature in step 2 and step 3 is both 90° C., and the other steps and parameters are the same as those in Example 1.

[0039] Comparative Example 4: This comparative example differs from Example 1 in that the temperature in step 2 and step 3 is both 95°C, and the other steps and parameters are the same as those in Example 1.

[0040] Example 1 is prepared by adopting a variable temperature strategy, while Comparative Examples 1 to 4 are materials prepared by sol-gel at a conventional single temperature. The materials prepared in Example 1 and Comparative Examples 1 to 4 were subjected to X-ray diffraction (XRD) tests, and the obtained XRD patterns are as follows: Figure 1 As shown. Figure 1 As can be seen, the main diffraction peak positions of the sample prepared using the variable temperature sol-gel strategy in Example 1 are essentially identical to those of the sample prepared using a conventional single-temperature method, indicating that the variable temperature strategy did not significantly alter the material's crystal structure. However, the peak intensity of the sample prepared using the variable temperature strategy is slightly enhanced, the peak shape is sharper, and the peak position is slightly shifted, closer to the standard pattern, indicating structural optimization.

[0041] Figure 2 This is a scanning electron microscope photo of the carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material prepared in Example 1. Figure 2 It can be seen that the variable temperature strategy is used to prepare a nano-scale uniform block structure. The small particle size ensures that the material has a large specific surface area and can fully contact with the electrolyte.

[0042] The materials prepared in Example 1 and Comparative Examples 1 to 4 were mixed with a conductive agent (super P) and a binder (PVDF) in a mass ratio of 7:2:1, and an appropriate amount of N-methylpyrrolidone (NMP) solvent was added. The mixture was stirred for 4 hours to prepare a slurry, which was then coated on aluminum foil to a thickness of 200 μm. The slurry was dried in a vacuum oven at 80°C for 12 hours, cut into discs with a diameter of 10 mm, and compacted to a thickness of about 60 μm to prepare electrodes. Pure metallic sodium sheet was used as the counter electrode, glass fiber (GF-D) was used as the separator, and 1 mol·L -1An organic solution of NaClO4 in dimethyl carbonate (DMC), ethylene carbonate (EC) (volume ratio 1:1), and 5% fluoroethylene carbonate (FEC) was used as the electrolyte. CR2025 button cells and 2032 button cells were assembled in a glove box filled with high-purity Ar with water and oxygen contents less than 1 ppm for testing. The test conditions are as follows:

[0043] (1) First week charge and discharge test: 1C (1C = 128.3 mA / g);

[0044] (2) Cyclic performance test: 5C, voltage range 2.0-4.5V;

[0045] (3) Rate performance test: 1C, 5C, 10C, 12C, 20C, 30C.

[0046] Figure 3 This figure compares the first-week charge-discharge curves of the carbon-coated Na₃V₂(PO₄)₂F₃ sodium-ion battery cathode material prepared in Example 1 and the materials prepared in Comparative Examples 1 to 4. As can be seen from the figure, the sample prepared using the variable temperature strategy in Example 1 has higher charge and discharge capacities than the sample prepared using a single temperature. This reflects that the material has a good structure that allows for more complete embedding and de-embedding of sodium ions, and the platform in the high-voltage discharge range is longer, thereby increasing the material's capacity.

[0047] Figure 4 This is a comparison of the charge and discharge curves of the carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material prepared in Example 1 and the materials prepared in Comparative Examples 1 to 4 at a rate of 30C. As can be seen from the figure, under the high-rate charge and discharge mode of 30C, the material prepared by the variable temperature strategy in Example 1 better maintains the platform in the high-voltage range, while the material prepared by a single temperature has a certain degree of attenuation in the high-voltage section. At the same time, the performance loss of the material in the low-voltage discharge platform discharge range is less, so the overall capacity is higher than that of the material prepared at a single temperature. This shows that the sample prepared by the variable temperature strategy has excellent electronic conductivity and can greatly ensure the material performance under high-rate rapid charge and discharge conditions.

[0048] Figure 5This is a rate performance comparison chart of the carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material prepared in Example 1 and the materials prepared in Comparative Examples 1 to 4. As can be seen from the figure, under the condition of single temperature preparation, the material prepared at high temperature has a higher relative capacity, but suffers from a larger loss at a high rate, while the sample prepared at a relatively low temperature of 85°C performs better under high rate conditions. After adopting the variable temperature strategy in Example 1, the overall capacity is increased by 3-5 mA / g compared with the single temperature. It not only combines all the advantages of the material prepared at a single temperature, but also can release a larger capacity on its basis. At a rate of 1C, the capacity is as high as 124.46 mA / g, which is close to the theoretical capacity value of the material (128.8 mA / g).

[0049] Figure 6 This figure compares the performance of the carbon-coated Na₃V₂(PO₄)₂F₃ sodium-ion battery cathode material prepared in Example 1 and the materials prepared in Comparative Examples 1 to 4 after 1000 cycles at a 5C rate. As can be seen from the figure, the material prepared in Example 1 using the variable temperature strategy not only has a higher discharge capacity, but also has the highest capacity retention rate during the cycle. This indicates that the variable temperature strategy not only regulates the electronic conductivity of the material, but also optimizes the bulk structure of the material, making the material structure more stable and having better cycle performance.

[0050] The electrical performance data of the carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material prepared in Example 1 and the materials prepared in Comparative Examples 1 to 4 are shown in Table 1.

[0051] Table 1 Electrical properties of materials prepared in Example 1 and Comparative Examples 1 to 4

[0052]

[0053] from Figures 3 to 6 As can be seen from Table 1, the carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material prepared by sol-gel using a variable temperature strategy in Example 1 has a higher first-week capacity, better cycle stability and rate performance than the materials prepared by sol-gel at a conventional single temperature in Comparative Examples 1 to 4. By taking simple temperature control measures, not only the capacity of the material is improved, but also the structural degradation of the material during the cycle is effectively suppressed. In Example 1, the sol-gel was first performed at 90°C for 1 hour and then the temperature was adjusted to 80°C until it became a wet gel. The material performance reached the best, with a first-week capacity of 124.46 mA / g (1C rate), a capacity retention rate of 77.24% after 1000 cycles, and a capacity of 103.58 mA / g at a rate of 30C. Compared with the sample with the best rate performance at 85°C for sol-gel and the sample with the best performance at low rate at 95°C for sol-gel, the improved material has improved performance in all aspects.

[0054] The method of the present invention, based on the preparation by the sol-gel method, breaks through the temperature setting of the traditional sol-gel method through a temperature control, and improves the ion migration ability while controlling the vanadium element from being oxidized. The capacity, cycle stability and rate performance of the material are significantly improved. The modification method is simple and easy to implement, low in cost and can be applied on a large scale. The sol-gel method using a variable temperature strategy provided by the present invention has the following advantages: (1) simple operation and easy large-scale production; (2) the process does not produce toxic gases and is environmentally friendly; (3) the improvement effect is significant and multiple electrochemical performance indicators can be improved at the same time; (4) the scope of application is wide and can be extended to other types of sodium ion battery positive electrode materials. This simple and effective modification method provides a new idea for the development of high-performance sodium vanadium fluorophosphate positive electrode materials, and is expected to promote the industrial development of polyanion sodium ion batteries.

Claims

1. A method for preparing a carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material, characterized in that The method proceeds as follows:

1. Weigh sodium salt, trivalent vanadium source, phosphate and fluoride salt according to the molar ratio of Na, V, P and F being 3:2:2:3, and then weigh the carbon source according to the molar ratio of C atoms in the carbon source to V atoms in the vanadium source being (0.75-0.85):1; Alternatively, sodium salt, pentavalent vanadium source, phosphate and fluoride salt are weighed according to the molar ratio of Na, V, P and F being 3:2:2:3, and then the carbon source is weighed according to the molar ratio of C atoms in the carbon source to V atoms in the vanadium source being (0.75-0.85):1; and then the reducing agent is weighed, wherein the amount of the reducing agent added is just the amount of the pentavalent vanadium source V 5+ Completely restored to V 3+ the amount; 2. Dissolve the carbon source or the carbon source and the reducing agent in water, adjust the pH value to 3.5-5.0, add the vanadium source to obtain a mixed solution; heat the mixed solution to 90-95° C., stir for 20-40 minutes, add the sodium salt, phosphate and fluoride salt, and continue stirring for 20-40 minutes; 3. Lower the temperature of the mixed solution to 75-80°C and continue stirring until it becomes a wet gel; 4. vacuum drying the wet gel at a temperature of 75-80°C for 10-12 hours to obtain a dry gel; 5. Grind the dry gel into powder, place it in a high-temperature furnace, heat it to 200-400°C in an inert atmosphere, pre-sinter it for 4-5 hours, and then heat it to 500-700°C and sinter it at high temperature for 7-9 hours to obtain carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material.

2. The method for preparing a carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material according to claim 1, characterized in that: The sodium salt described in step 1 is one or a combination of sodium metavanadate, sodium chloride, sodium fluoride, sodium hydroxide, and sodium acetate.

3. The method for preparing a carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material according to claim 1 or 2, characterized in that: The pentavalent vanadium source described in step one is one or a combination of two of ammonium metavanadate, sodium metavanadate, and vanadium pentoxide; the trivalent vanadium source described in step one is one or a combination of two of vanadium acetate, vanadium fluoride, vanadium phosphate, and vanadium trioxide.

4. The method for preparing a carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material according to claim 1 or 2, characterized in that: The phosphate described in step 1 is one or a combination of ammonium dihydrogen phosphate, phosphoric acid and sodium phosphate.

5. The method for preparing a carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material according to claim 1 or 2, characterized in that: The fluoride salt described in step 1 is one or a combination of ammonium fluoride, sodium fluoride and vanadium fluoride.

6. The method for preparing a carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material according to claim 1 or 2, characterized in that: The carbon source in step 1 is one or a combination of glucose, sucrose, ethylene glycol, polyethylene glycol, citric acid, and ascorbic acid.

7. The method for preparing a carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material according to claim 1 or 2, characterized in that: The reducing agent in step 1 is one or a combination of two of ethylene glycol, citric acid, glucose, oxalic acid, ammonium carbamate, urea, ascorbic acid, and sodium borohydride.

8. The method for preparing a carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material according to claim 1 or 2, characterized in that: In step 2, the mass ratio of solute to water is 1:(15-30).

9. The method for preparing a carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material according to claim 1 or 2, characterized in that: The material used to adjust the pH value in step 2 is one or a combination of ammonia water and sodium hydroxide.

10. The method for preparing a carbon-coated Na3V2(PO4)2F3 sodium ion battery positive electrode material according to claim 1 or 2, characterized in that The inert atmosphere in step 5 is nitrogen or argon.

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