Preparation method and application of aluminum fluoride coated sodium vanadyl fluorophosphate positive electrode material

Abstract

The invention belongs to the technical field of batteries, and discloses a preparation method and application of an aluminum fluoride coated sodium vanadyl fluorophosphate positive electrode material. The aluminum fluoride coated sodium vanadyl fluorophosphate positive electrode material has a core-shell structure, the core is sodium vanadyl fluorophosphate NVOPF, and the shell is aluminum fluoride. According to the invention, the morphology of the substrate material is maintained by regulating and controlling the coating conditions. The regulated aluminum fluoride coated sodium vanadyl fluorophosphate positive electrode material has outstanding high temperature resistance, has good specific discharge capacity under high rate, and is of great significance to development of sodium ion battery positive electrode materials with high rate performance and high safety.

Description

Technical Field

[0001] The invention belongs to the technical field of electrochemical energy storage secondary battery application, and in particular relates to a preparation method and application of an aluminum fluoride-coated sodium vanadium oxyfluorophosphate positive electrode material. Background Art

[0002] Fossil fuels have always been the main source to meet global energy needs. However, with the depletion of traditional fossil fuels and the resulting environmental degradation, it is imperative to develop sustainable and renewable energy storage and conversion technologies. Among them, sodium-ion battery technology is gradually regarded as one of the competitive electrochemical energy storage technologies in the field of large-scale energy storage due to its abundant resources, low cost, high safety, and environmental friendliness. In fact, energy storage devices need to meet the requirements of low cost, excellent electrochemical performance, high safety, and good environmental adaptability during application, which is a huge challenge for the energy storage field. High temperature working conditions increase the risk of thermal runaway of batteries. Thermal runaway is an uncontrollable heating process that may eventually lead to fire or explosion. Sodium-ion batteries are more sensitive to changes in ambient temperature. At the same time, the material structure is easily damaged in a high temperature environment, resulting in poor stability and affecting the normal operation of energy storage equipment. Therefore, it is urgent to develop an electrode material with excellent thermal stability to meet the needs of practical applications.

[0003] Polyanion materials have shown great potential as high-temperature electrode materials due to their unique framework structure and good thermal stability. Among them, the 3D tunnel structured sodium-rich Na3V2(PO4)O2F (NVOPF) material can reversibly extract and deintercalate two sodium ions, and the energy density is expected to reach 486 Wh kg ‒1 , compared with commercial lithium-ion battery cathode material LiFePO4 (528 Wh kg ‒1 ) is equivalent. However, the intrinsic conductivity of NVOPF is low, resulting in large polarization and low electrochemical reversibility. Especially in high temperature environments, the destruction of the electrode-electrolyte interface layer will cause the electrode to come into direct contact with the electrolyte, which will inevitably cause electrode / electrolyte side reactions. These side reactions caused by electrolyte corrosion will lead to the dissolution of transition metal vanadium and reduce the cycle stability, greatly limiting the practical application of this material in high temperature environments. In recent years, people have optimized and modified the conductivity of sodium vanadium fluorophosphate materials through carbon coating, intracrystalline doping and other technologies, but few researchers have optimized its high temperature performance. In this regard, achieving the safe use of sodium-ion batteries at high temperatures and promoting their practical application process is a technical problem that needs to be solved urgently.

[0004] The present invention realizes the construction of a uniform and dense nano-aluminum fluoride artificial coating on the NVOPF interface by regulating the pH value of the coating solution, and is applied to sodium ion batteries at high temperatures, which can effectively prevent electrolyte corrosion and side reactions, increase the ion / electron transmission rate, and greatly improve its high temperature resistance, which provides a feasible way to improve the safety of sodium ion batteries. Summary of the invention

[0005] In order to overcome the problem of unstable electrode-electrolyte interface under high temperature conditions in the prior art, the present invention provides a method for preparing an aluminum fluoride-coated sodium vanadium oxyfluorophosphate positive electrode material. By adjusting the coating conditions, the method can achieve complete coating of aluminum fluoride on the surface of the NVOPF material, and the formed aluminum fluoride coating has good uniformity. When applied to sodium ion batteries, excellent high-temperature rate performance can be obtained.

[0006] The technical scheme of the present invention is as follows: a preparation method of sodium vanadyl fluorophosphate composite microspheres coated with aluminum fluoride, which has a core-shell structure, wherein the core is sodium vanadyl fluorophosphate NVOPF and the shell is aluminum fluoride.

[0007] A method for preparing an aluminum fluoride-coated sodium vanadium oxyfluorophosphate positive electrode material comprises the following preparation steps: (1) adding a vanadium source to a hydrogen peroxide solution, stirring, then adding a fluorine source, a sodium source, a phosphorus source, and carbon tubes to form a mixed solution, and hydrothermally heating to obtain sodium vanadyl fluorophosphate microspheres; (2) dispersing the microspheres described in step (1) in a mixture of a fluorine source and an aluminum source, adjusting the pH of the mixture, and then stirring to dryness; (3) The dried sample obtained in step (2) is subjected to high temperature calcination to obtain sodium vanadyl fluorophosphate microspheres coated with aluminum fluoride.

[0008] The sodium vanadyl fluorophosphate is a product obtained by mixing a solution formed by a vanadium source, a phosphorus source, a sodium source and a fluorine source and then subjecting the solution to a hydrothermal reaction.

[0009] The vanadium source is at least one of vanadium acetylacetonate, ammonium metavanadate, vanadium pentoxide, vanadium trichloride, and vanadyl sulfate, preferably ammonium metavanadate; the fluorine source is at least one of sodium fluoride, potassium fluoride, ammonium fluoride, and magnesium fluoride, preferably sodium fluoride; the sodium source is at least one of sodium fluoride, sodium carbonate, sodium nitrate, sodium hydroxide, sodium bicarbonate, sodium acetate, sodium dihydrogen phosphate, and sodium phosphate, preferably sodium dihydrogen phosphate; the phosphorus source is at least one of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium phosphate and ammonium phosphate, preferably sodium dihydrogen phosphate.

[0010] The concentration of the hydrogen peroxide is 20% to 40% (dispersed in water solvent), preferably 30%, and the added amount is 1 to 5 mL, preferably 2 mL.

[0011] The specifications of the graphitized multi-walled carbon nanotubes are: particle size 5 to 20 nm, length 30 to 60 um, and the added amount is 5 to 15% of the mass of sodium vanadium fluorophosphate, preferably 10%.

[0012] The hydrothermal time in step (1) is 12 to 36 h, preferably 24 h, and the temperature is 140 to 220 °C, preferably 180 °C.

[0013] The aluminum source in step (2) is at least one of aluminum isopropoxide, aluminum hydroxide, aluminum chloride, aluminum nitrate nonahydrate, and aluminum oxide, preferably aluminum nitrate nonahydrate. The temperature of constant temperature heating and stirring is 60 to 90°C, preferably 80°C, the time is 3 to 7 h, preferably 5 h, and the pH is adjusted to 5 to 8, more preferably 6-7.

[0014] In some preferred cases, the pH in step (2) is adjusted to 7.

[0015] The content of aluminum fluoride in the primary coating material in step (2) is 0.5-2%.

[0016] In some preferred cases, the content of aluminum fluoride in step (2) is 1%.

[0017] The high temperature calcination system in step (3) is 2 to 10 °C / min, preferably 2 °C / min, and the calcination is carried out at 400 to 500 °C for 4 to 8 h in an argon atmosphere.

[0018] In some embodiments, the high temperature calcination system in step (3) is 5°C / min, and calcination is performed at 400°C for 5 h.

[0019] In some embodiments of the present invention, the drying is performed by vacuum drying at 80°C.

[0020] Another technical concept of the present invention is to use the aluminum fluoride-coated sodium vanadyl fluorophosphate composite microspheres obtained by the above-mentioned preparation technology as the positive electrode material of sodium ion batteries. After the basic characterization of the composite microsphere material, the electrochemical performance test is carried out on it. The specific method is as follows: The obtained aluminum fluoride-coated sodium vanadyl fluorophosphate microspheres were used as the positive electrode material, aluminum foil as the positive electrode current collector, metallic sodium as the negative electrode, glass fiber as the separator, 1 MNaClO4 dissolved in propylene carbonate (PC) and 5% fluoroethylene carbonate (FEC) solvent as the electrolyte, and assembled into a sodium ion battery in a glove box filled with high-purity argon gas for electrochemical performance testing. The high-temperature performance of the aluminum fluoride-coated sodium vanadyl fluorophosphate is better than that of the uncoated sample, and this high-temperature performance is better than most other polyanionic vanadium-based phosphate materials.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention achieves the regulation of the uniformity and effectiveness of the aluminum fluoride coating layer by changing the pH value and coating amount of the coating solution. The prepared aluminum fluoride-coated sodium vanadium oxyfluorophosphate material can well maintain its intrinsic morphology, and the external aluminum fluoride coating layer is uniform and dense at the nanoscale, which can effectively promote ion transport, reduce side reactions during charging and discharging, and exhibit excellent high-temperature performance.

[0022] (2) The present invention provides a new design idea for the application of sodium vanadium fluorophosphate materials at high temperatures, and can also be extended to other positive electrode materials for sodium ion batteries, and can be widely used in the battery field. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a scanning electron microscope image of the aluminum fluoride coated sodium vanadium oxyfluorophosphate material obtained when the pH of the coating solution is adjusted to 1 in Example 1 of the present invention.

[0024] Figure 2 This is a scanning electron microscope image of the aluminum fluoride-coated sodium vanadium oxyfluorophosphate material obtained when the pH of the coating solution is adjusted to 11 in Example 2 of the present invention.

[0025] Figure 3 This is a scanning electron microscope image of the aluminum fluoride-coated sodium vanadium oxyfluorophosphate material obtained when the pH of the coating solution is adjusted to 7 in Example 3 of the present invention.

[0026] Figure 4 This is a transmission electron microscope image of the aluminum fluoride-coated sodium vanadium oxyfluorophosphate material obtained when the pH of the coating solution is adjusted to 7 in Example 3 of the present invention.

[0027] Figure 5 1 is the XRD diagram of the aluminum fluoride-coated sodium vanadium oxyfluorophosphate composite microspheres of Examples 1, 2, and 3 of the present invention.

[0028] Figure 6 This is the high temperature rate performance of the sodium vanadium oxyfluorophosphate composite microsphere material coated with aluminum fluoride obtained in Examples 1, 2, and 3 of the present invention at 55°C.

[0029] Figure 7 This is the high temperature and long cycle performance of the sodium vanadyl fluorophosphate composite microsphere material coated with aluminum fluoride obtained in Examples 1, 2, and 3 of the present invention at a current density of 10°C.

[0030] Figure 8 This is a rate performance diagram of the aluminum fluoride-coated sodium vanadium oxyfluorophosphate composite microsphere material obtained in Examples 3, 4, and 5 of the present invention. DETAILED DESCRIPTION

[0031] In order to better understand the present invention, the content of the present invention is further explained in conjunction with the following examples, but the content of the present invention is not limited to the following examples. The applicant believes that the aluminum fluoride-coated sodium vanadium oxyfluorophosphate composite microspheres prepared by other methods can also achieve the following technical solutions of the present invention.

[0032] Example 1 (the pH of the coating solution is regulated to 1, and the coating amount is controlled to 1%) A method for preparing aluminum fluoride-coated sodium vanadyl fluorophosphate composite microspheres comprises the following steps: (1) First, 467.92 mg of ammonium metavanadate and 2 mL of hydrogen peroxide were added to 10 mL of deionized water and stirred vigorously for 1 hour to obtain an orange transparent solution, which was recorded as solution A. At the same time, 80 mg of carbon nanotubes were added to 10 mL of deionized water and ultrasonicated in an ice-water bath for 1 hour to make them uniformly dispersed to obtain solution B. Then, solutions A and B were fully mixed and stirred for 30 minutes. Secondly, 483.936 mg of sodium dihydrogen phosphate dihydrate and 83.98 mg of sodium fluoride were poured into the above solution in turn and stirred at room temperature for 30 minutes to make them completely dissolved. Then 60 mL of polyethylene glycol 400 was added and stirred for 1 hour. The above solution was transferred to a 100 mL reactor and heated in an oven at 180 °C for 24 hours. After that, it was taken out and cooled naturally to room temperature. It was washed three times by centrifugation with deionized water and alcohol and dried in vacuum at 80 °C for 24 hours.

[0033] (2) Dissolve 2.65 mg of ammonium fluoride and 8.93 mg of aluminum nitrate nonahydrate in 20 mL of deionized water, respectively. Then, add 200 mg of sodium vanadium fluorophosphate powder to the aluminum nitrate nonahydrate aqueous solution, then slowly drop the ammonium fluoride solution under constant stirring, and add hydrofluoric acid to adjust the pH value of the solution to 1. After continuous stirring at 80 °C for 5 hours, transfer it to a vacuum drying oven, and calcine the obtained powder at 400 °C in argon for 5 hours.

[0034] The scanning electron microscopy (SEM) results of the aluminum fluoride-coated sodium vanadyl fluorophosphate prepared in this example are as follows: Figure 1As shown, since the pH of the solution was adjusted to be too low during the coating process, the solution was strongly acidic and had a strong corrosive effect on the substrate material sodium vanadium fluorophosphate, resulting in the inability to maintain its original morphology, and the emergence of larger holes and more serious cracks, which would hinder the effective formation of a uniform aluminum fluoride coating layer.

[0035] Example 2 (the pH of the coating solution is regulated to 11, and the coating amount is controlled to 1%) The pH of the coating solution in step (2) of Example 1 was changed and adjusted to 11 with aqueous ammonia. The other steps were the same as those of Example 1.

[0036] Figure 2 This is the scanning electron microscope image of this example. Figure 2 It can be observed that due to the high pH value of the coating solution, the solution is strongly alkaline, the material is seriously accumulated, the base material sodium vanadyl fluorophosphate becomes sticky, and the formed aluminum fluoride coating layer is uneven, which hinders the application of the surface coating layer in the circulation process.

[0037] Example 3 (the pH of the coating solution is regulated to 7 and the coating amount is controlled to 1%) The pH of the coating solution in step (2) of Example 1 was changed and adjusted to 7 with aqueous ammonia. The other steps were the same as those of Example 1. The obtained sample was recorded as AF-1.

[0038] The scanning electron microscopy results of the aluminum fluoride-coated sodium vanadyl fluorophosphate prepared in this example are as follows: Figure 3 As shown in the figure, the pH of the coating solution in this case is neutral, and the substrate material is not affected during the drying and secondary high-temperature calcination process, and its morphology can be well maintained. Figure 4 The transmission electron microscopy (TEM) and high-resolution electron microscopy (HRTEM) shown in the figure show that under this coating condition, a uniform and dense aluminum fluoride coating layer with a thickness of ~2.97 nm can be formed on the surface of the sodium vanadium fluorophosphate material. Figure 5 The XRD spectrum analysis shows that the presence of the aluminum fluoride coating layer does not affect the purity of the sodium vanadium fluorophosphate base material, and there is no impurity peak. However, compared with Examples 1 and 2, due to the different pH values ​​of the coating solution, the morphology of the base material changes, and the crystallinity of the material changes. When pH=7, the peak intensity is the largest and the crystallinity is the highest.

[0039] The aluminum fluoride-coated sodium vanadyl fluorophosphate composite microspheres prepared by the present invention are used as the positive electrode active material of the sodium ion battery, and the assembly steps of the sodium ion battery are the same as the conventional assembly method. The aluminum fluoride-coated sodium vanadyl fluorophosphate microspheres are used as the positive electrode material, aluminum foil is used as the positive electrode current collector, metallic sodium is used as the negative electrode, glass fiber is used as the separator, 1 M NaClO4 is dissolved in propylene carbonate (PC) and 5% fluoroethylene carbonate (FEC) solvent as the electrolyte, and the sodium ion battery is assembled in a glove box filled with high-purity argon gas, and then the electrochemical performance test is carried out at 55°C.

[0040] like Figure 6 As shown in the figure, the aluminum fluoride-coated sodium vanadyl fluorophosphate composite microspheres prepared in Example 3 as the positive electrode material of the sodium ion battery have better rate performance than those in Examples 1 and 2, and their initial capacity can reach 126.8 mAh g at 0.2 C. ‒1 After being tested at different current densities of 0.2 C, 0.5 C, 1 C, 2 C, 5 C, 10 C, and 20 C, and then returning to 0.2 C, its capacity remained at 126.4 mAh g ‒1 , the capacity attenuation rate is only 0.3%, which can be ignored. Figure 7 It shows that at a current density of 10 C, the first cycle discharge capacity of the composite material obtained in Example 3 can reach 92.4 mAh g ‒1 After 500 cycles, the capacity remains at 84 mAh g ‒1 , the capacity retention rate is 90.9%, with excellent high-temperature long-cycle performance. The results show that by regulating the pH of the coating solution, a uniform and dense nano-aluminum fluoride coating layer can be formed on the surface of the substrate material. Under high-temperature test conditions, the coating can be used as an artificial CEI layer to effectively promote ion transmission, prevent excessive expansion and damage of the positive electrode material, reduce electrode-electrolyte interface side reactions, and greatly improve the high-temperature resistance of the material.

[0041] Example 4 (the pH of the coating solution is regulated to 7, and the coating amount is controlled to 0.5%) The amounts of ammonium fluoride and aluminum nitrate nonahydrate in step (2) of Example 3 were changed, and 1.325 mg of ammonium fluoride and 4.465 mg of aluminum nitrate nonahydrate were dissolved in 20 mL of deionized water, respectively. The other conditions were the same as those in the steps of Example 3, and the obtained sample was recorded as AF-0.5.

[0042] Example 5 (the pH of the coating solution is regulated to 7, and the coating amount is controlled to 2%) The amounts of ammonium fluoride and aluminum nitrate nonahydrate in step (2) of Example 3 were changed, and 5.3 mg of ammonium fluoride and 17.86 mg of aluminum nitrate nonahydrate were dissolved in 20 mL of deionized water, respectively. The other conditions were the same as those in the steps of Example 3, and the obtained sample was recorded as AF-2.

[0043] like Figure 8 As shown, the aluminum fluoride-coated sodium vanadyl fluorophosphate composite microspheres prepared in this embodiment are used as the positive electrode material of the sodium ion battery. Compared with Example 3 and Example 4, the results show that its rate performance is the worst. When the coating amount is controlled at 1%, the rate performance of the obtained sample is the best. When the coating amount is too low, the sample cannot be completely covered, and when the coating amount is too high, the diffusion rate of sodium ions will be reduced.

[0044] In summary, the present invention regulates the pH and coating amount of the coating solution to form an effective aluminum fluoride coating layer, and the prepared aluminum fluoride-coated sodium vanadium oxyfluorophosphate material is tested under high temperature conditions of 55°C and exhibits excellent electrochemical properties. This technology provides a new idea for the application of sodium vanadium oxyfluorophosphate positive electrode materials under high temperature conditions in sodium ion batteries, and improves the actual use safety of sodium ion batteries.

[0045] The above-mentioned embodiments are only preferred technical solutions of the present invention and should not be regarded as limitations of the present invention. The embodiments and features in the embodiments of the present application can be arbitrarily combined with each other without conflict. The protection scope of the present invention shall be the technical solutions recorded in the claims, including the equivalent replacement solutions of the technical features in the technical solutions recorded in the claims. That is, equivalent replacement improvements within this scope are also within the protection scope of the present invention.

Claims

1. A method for preparing aluminum fluoride-coated sodium vanadyl fluorophosphate composite microspheres, characterized in that: The method comprises the following preparation steps: (1) adding a vanadium source to a hydrogen peroxide solution, stirring, then adding a fluorine source, a sodium source, a phosphorus source, and carbon tubes to form a mixed solution, and hydrothermally heating to obtain sodium vanadyl fluorophosphate microspheres; (2) dispersing the microspheres described in step (1) in a mixed solution of a fluorine source and an aluminum source, adjusting the pH of the mixed solution, and then stirring for coating; (3) The dried sample obtained in step (2) is subjected to high temperature calcination to obtain aluminum fluoride-coated sodium vanadyl fluorophosphate composite microspheres.

2. The method for preparing the aluminum fluoride-coated sodium vanadyl fluorophosphate composite microspheres according to claim 1, characterized in that: The vanadium source in step (1) is one or more of vanadium acetylacetonate, ammonium metavanadate, vanadium pentoxide, vanadium trichloride, and vanadyl sulfate; the fluorine source is one or more of sodium fluoride, potassium fluoride, ammonium fluoride, and magnesium fluoride; the sodium source is one or more of sodium fluoride, sodium carbonate, sodium nitrate, sodium hydroxide, sodium bicarbonate, sodium acetate, sodium dihydrogen phosphate, and sodium phosphate; the phosphorus source is one or more of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, potassium dihydrogen phosphate, dipotassium hydrogen phosphate, potassium phosphate, and ammonium phosphate.

3. The method for preparing the composite microspheres of sodium vanadyl fluorophosphate coated with aluminum fluoride according to claim 1, characterized in that: The mass concentration of hydrogen peroxide in step (1) is 20% to 40%; the carbon tubes are graphitized multi-walled carbon nanotubes, and the added amount is 5 to 15% of the mass of sodium vanadium fluorophosphate.

4. The method for preparing the aluminum fluoride-coated sodium vanadyl fluorophosphate composite microspheres according to claim 1, characterized in that: The hydrothermal time in step (1) is 12 to 36 hours, and the temperature is 140 to 220°C.

5. The method for preparing the composite microspheres of sodium vanadyl fluorophosphate coated with aluminum fluoride according to claim 1, characterized in that: The coating process of step (2) comprises: adding the sodium vanadium fluorophosphate material prepared in step (1) into a solution containing an aluminum source to prepare a dispersion, heating and stirring the dispersion at a constant temperature and slowly adding a fluorine source, adjusting the pH of the solution with hydrogen fluoride and / or ammonia water, and stirring to obtain a primary coating material.

6. The method for preparing the aluminum fluoride-coated sodium vanadyl fluorophosphate composite microspheres according to claim 1, characterized in that: The aluminum source in step (2) is selected from one or more of aluminum isopropoxide, aluminum hydroxide, aluminum chloride, aluminum nitrate nonahydrate, and aluminum oxide; the fluorine source is one or more of sodium fluoride, potassium fluoride, ammonium fluoride, and magnesium fluoride; the temperature of constant temperature heating and stirring is 60 to 90°C, the time is 3 to 7 hours, and the pH is adjusted to 5 to 8.

7. The method for preparing the aluminum fluoride-coated sodium vanadyl fluorophosphate composite microspheres according to claim 1, characterized in that: The content of aluminum fluoride in the primary coating material in step (2) is 0.5-2% of the mass of sodium vanadyl fluorophosphate.

8. The method for preparing the aluminum fluoride-coated sodium vanadyl fluorophosphate composite microspheres according to claim 1, characterized in that: The high-temperature calcination in step (3) is carried out in an argon atmosphere, the heating rate is 2 to 10 °C / min, preferably 2 °C / min, the calcination temperature is 400 to 500 °C, and the calcination time is 4 to 8 h.

9. A sodium ion battery positive electrode material, characterized in that: The positive electrode material is a composite microsphere of sodium vanadyl fluorophosphate coated with aluminum fluoride prepared by the preparation method according to any one of claims 1 to 8.

10. A battery, characterized in that: Including the sodium ion battery positive electrode material according to claim 9.

Images