A metal oxide gradient-coated sodium fluorophosphate material, its preparation method and application

Abstract

This invention relates to the field of cathode materials for sodium-ion batteries, and more particularly to a metal oxide gradient-coated sodium fluorinated iron phosphate material, its preparation method, and its applications; the chemical formula is Na₂FePO₄F@Zn. x Al (4‑2x) / 3 O2; This invention utilizes ALD atomic deposition technology to achieve an increasing x-value for Zn on the surface of sodium iron fluoride phosphate cathode material from the inside out by successively adjusting the ratio of two metal precursors. x Al (4‑2x) / 3 O2 gradient coating; the simultaneous introduction of a certain proportion of the two metal precursor mixture is conducive to the formation of a mixed metal oxide deposition layer with tight interfacial bonding and smoother transition, making full use of the advantages of Al2O3's high mechanical strength and corrosion resistance, and ZnO's high electrical conductivity; gradient coating can give full play to the advantages of the two metal oxides, maximizing the performance of the cathode material; this material does not contain precious metals, has low production cost, good safety performance, and a narrow discharge voltage range, reducing battery management costs and making it suitable for large-scale grid energy storage.

Description

Technical Field

[0001] This invention relates to the field of cathode materials for sodium-ion batteries, and more particularly to a sodium fluorinated iron phosphate material with metal oxide gradient coating, its preparation method, and its application. Background Technology

[0002] With rapid industrial development, traditional fossil fuels are becoming increasingly depleted, and various new renewable energy sources are being widely applied. However, the generation of electricity using new energy sources such as wind and solar power is not regular in terms of time and space. Rechargeable batteries have the ability to convert electrical energy into chemical energy for storage. Among them, lithium-ion batteries have developed well and have mature technology, making them an ideal choice for energy storage power stations. Various new materials have emerged in lithium-ion battery cathode materials. For example, Chinese patent document CN111883748A discloses a method for coating an oxide film on the surface of lithium-ion battery cathode powder material; Chinese patent document CN102244231A proposes to use atomic layer deposition technology to deposit surface modification substances on the surface of cathode active materials. However, the material surface prepared by this patent is a single substance deposition, which has certain performance and application limitations.

[0003] As lithium ore prices gradually rise, high storage costs and resource scarcity hinder the application of lithium-ion batteries in large-scale energy storage power stations. Sodium-ion batteries, as an emerging rechargeable battery, while having lower energy density, are not a critical issue for large-scale energy storage power stations. Furthermore, sodium-ion batteries are abundant and inexpensive, and their key technologies are similar to those of lithium-ion batteries, providing a solid technological foundation. In addition, sodium-ion batteries can achieve wide-temperature discharge and rapid charge-discharge in more complex operating environments, making them a primary choice for large-scale energy storage power stations. However, current sodium-ion battery cathode materials still face numerous challenges, including low energy density and poor cycle performance. Compared to lithium-ion battery cathode materials, especially the common lithium iron phosphate, processing sodium fluorinated iron phosphate (SRF) with low conductivity and poor structural stability to obtain sodium-ion battery cathode materials is more difficult. The preparation methods for lithium-ion battery cathode materials cannot be directly applied to sodium-ion batteries; direct application can easily lead to volume changes and cracking during charge-discharge processes. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a metal oxide gradient-coated sodium fluorophosphate material, its preparation method, and its application, which can simultaneously improve the structural stability and mass transfer efficiency during material cycling and reduce the interfacial resistance between the material and the electrolyte.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] The first aspect of this invention is to provide a sodium fluorinated phosphate material with a metal oxide gradient coating, having the chemical formula Na2FePO4F@Zn. x Al (4-2x) / 3 O2; where 0.3≤x≤1.4.

[0007] A second aspect of the present invention is to provide a method for preparing the above-mentioned metal oxide gradient-coated sodium fluorophosphate material, the steps of which include:

[0008] S1. Anhydrous sodium acetate, sodium fluoride, ammonium dihydrogen phosphate and iron source are mixed, and a dispersant is added for thorough ball milling. The mixture is then dried and pre-sintered in sequence to obtain sodium fluorinated iron phosphate powder.

[0009] S2. Place the sodium ferric fluoride phosphate powder in the ALD reaction chamber, evacuate the vacuum, and heat it to 100-120°C.

[0010] S3. Under an inert atmosphere, a mixed vapor of precursor aluminum source and precursor zinc source is deposited on the surface of the sodium fluorinated iron phosphate powder. Then, an inert gas is introduced to blow away the excess precursor aluminum source and precursor zinc source mixture on the surface of the sodium fluorinated iron phosphate powder. A precursor oxygen source is then introduced to react with the precursor aluminum source and precursor zinc source mixture on the surface of the sodium fluorinated iron phosphate powder in a redox reaction. After that, an inert gas is introduced to blow away the excess precursor oxygen source and by-reaction products on the surface of the sodium fluorinated iron phosphate powder. This completes one ALD deposition.

[0011] S4. Repeat step S3 to perform multiple ALD depositions, and gradually increase the content of precursor zinc source in the mixed vapor of precursor aluminum source and precursor zinc source during each ALD deposition to obtain sodium fluorinated phosphate material with metal oxide gradient coating.

[0012] Preferably, in step S1, the molar ratio of the anhydrous sodium acetate, the sodium fluoride, the ammonium dihydrogen phosphate and the iron source is (1.0-1.5):(1.0-1.3):(1.0-1.5):(1.0-1.5).

[0013] Preferably, in step S1, the iron source includes at least one of ferrous oxalate, ferrous sulfate, and ferrous chloride.

[0014] Preferably, in step S1, the mass ratio of the dispersant to the mixture is 1:(1-3); the dispersant includes at least one of anhydrous ethanol, acetone, isopropanol, and N-methylpyrrolidone.

[0015] Preferably, in step S1, the ball milling speed is 300-500 rpm, the time is 6-8 h, and the mass ratio of the milling balls to the mixture is (8-10):1.

[0016] The drying process is carried out at a temperature of 60–100°C for 8–14 hours.

[0017] The pre-sintering treatment includes: pre-purifying with a protective gas at room temperature for 0.5 to 2 hours, then heating to 350 to 450°C at a rate of 3 to 5°C / min and calcining for 2 to 6 hours; the protective gas includes at least one of argon, hydrogen-argon, nitrogen, and helium.

[0018] Preferably, in step S3, the flow rate of the inert gas during deposition is 50 mL / min, and the duration is 40–60 s; the flow rate of the inert gas during blowing-off is 100 mL / min, and the duration is 80–120 s.

[0019] Preferably, the precursor aluminum source includes at least one of: trimethylaluminum, dimethylethylaluminum, diethylaluminum, diisobutylaluminum, 2-aminopyridine diethylaluminum, 2-tert-butylaminomethylpyridine dimethylaluminum, pyrrole-2-imine dimethylaluminum, and diethylaluminum;

[0020] The precursor zinc source includes at least one of: diethyl zinc, dimethyl zinc, pyridine zinc complexes, pyrimidine zinc complexes, and nitrogen heterocyclic carbene zinc complexes;

[0021] The precursor oxygen source includes at least one of H2O, O2, and O3.

[0022] Preferably, in steps S3 to S4, the number of ALD depositions is 8 to 10, wherein in the first ALD deposition, the content of the precursor zinc source in the mixed steam is 20% to 40%, and in the last ALD deposition, the content of the precursor zinc source in the mixed steam is 60% to 80%.

[0023] A third aspect of the present invention is to provide the application of the above-described metal oxide gradient-coated sodium fluorophosphate material or the metal oxide gradient-coated sodium fluorophosphate material prepared by the above-described preparation method in the cathode material of sodium-ion batteries.

[0024] The present invention adopts the above technical solution and has the following technical effects compared with the prior art:

[0025] (1) The present invention avoids the oxidation of ferrous iron during sintering, which would lead to a significant reduction in the material specific capacity, and improves the material yield by pre-passing inert gas.

[0026] (2) This invention utilizes ALD atomic deposition technology to achieve a Zn content that increases from the inside to the outside of the sodium iron fluoride phosphate cathode material by successively adjusting the ratio of two metal precursors. x Al (4-2x) / 3O2 gradient coating; the simultaneous introduction of a mixture of two metal precursors in a certain proportion facilitates the formation of a mixed metal oxide deposition layer with tight interfacial bonding and smoother transition. This fully utilizes the advantages of Al2O3's high mechanical strength and corrosion resistance, and ZnO's high electrical conductivity. The high Al2O3 content in the inner layer effectively improves the structural stability of the material during cycling, effectively reducing the erosion and mechanical damage to the cathode material from the external environment. The high ZnO content in the outer layer helps improve mass transfer efficiency, reduces the interfacial resistance between the material and the electrolyte, and maximizes the overall performance of the cathode material. Gradient coating fully leverages the advantages of both metal oxides, maximizing the performance of the cathode material. This material does not contain precious metals, has low production costs, good safety performance, and a narrow discharge voltage range, reducing battery management costs and making it suitable for large-scale grid energy storage. Attached Figure Description

[0027] Figure 1 Zn prepared in Example 1 of this invention x Al (4-2x) / 3 SEM images of sodium ferric fluorophosphate coated with O2 gradient;

[0028] Figure 2 These are comparative images of the discharge curves of sodium fluorinated iron phosphate prepared in Example 1, Comparative Example 1, and Comparative Example 2 of this invention. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0031] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the scope of the invention.

[0032] Example 1

[0033] This embodiment provides a method for preparing sodium fluorinated phosphate material with metal oxide gradient coating, the steps of which include:

[0034] S1. Anhydrous sodium acetate, sodium fluoride, ammonium dihydrogen phosphate and ferrous oxalate (the molar ratio of anhydrous sodium acetate, sodium fluoride, ammonium dihydrogen phosphate and ferrous oxalate is 1.0:1.0:1.0:1.0) are mixed, a dispersant is added and the mixture is fully ball-milled and mixed, and then dried and pre-sintered in sequence to obtain sodium fluorophosphate powder.

[0035] The ball milling mixing speed is 500 rpm, the time is 6 hours, and the mass ratio of the milling balls to the mixture is 8:1.

[0036] The drying process is carried out at a temperature of 60°C for 14 hours.

[0037] The pre-sintering treatment includes: pre-purifying with argon gas at room temperature for 1 hour, then heating to 350°C at a rate of 3°C / min and calcining for 6 hours;

[0038] The mass ratio of the dispersant to the mixture is 1:1; the dispersant is a mixture of anhydrous ethanol and acetone with a volume ratio of 1:5.

[0039] S2. Place the sodium ferric fluoride phosphate powder in the ALD reaction chamber, evacuate the vacuum, and heat it to 100°C.

[0040] S3. Under an inert atmosphere (flow rate 50 mL / min, duration 60 s), a mixed vapor of trimethylaluminum and diethylzinc is deposited on the surface of the sodium fluorinated iron phosphate powder. Then, an inert gas is introduced (flow rate 100 mL / min, duration 120 s) to remove excess trimethylaluminum and diethylzinc mixture from the surface of the sodium fluorinated iron phosphate powder. H2O is then introduced to react with the trimethylaluminum and diethylzinc mixture on the surface of the sodium fluorinated iron phosphate powder in a redox reaction. Afterwards, an inert gas is introduced (flow rate 100 mL / min, duration 100 s) to remove excess H2O and by-products from the surface of the sodium fluorinated iron phosphate powder; thus completing one ALD deposition.

[0041] S4. Repeat step S3 for a total of 9 ALD depositions, and increase the content of diethylzinc in the mixed steam of trimethylaluminum and diethylzinc in each ALD deposition (20% diethylzinc in the mixed steam for the first ALD deposition; 60% diethylzinc in the mixed steam for the last ALD deposition) to obtain sodium fluorinated phosphate material with metal oxide gradient coating.

[0042] Example 2

[0043] This embodiment provides another method for preparing sodium fluorophosphate material with metal oxide gradient coating, the steps of which include:

[0044] S1. Anhydrous sodium acetate, sodium fluoride, ammonium dihydrogen phosphate and ferrous chloride (the molar ratio of anhydrous sodium acetate, sodium fluoride, ammonium dihydrogen phosphate and ferrous chloride is 1.0:1.1:1.3:1.0) are mixed, a dispersant is added and the mixture is thoroughly ball-milled, and then dried and pre-sintered in sequence to obtain sodium fluorophosphate powder.

[0045] The ball milling mixing speed is 300 rpm, the time is 8 hours, and the mass ratio of the milling balls to the mixture is 9:1.

[0046] The drying process is carried out at a temperature of 80°C for 12 hours.

[0047] The pre-sintering treatment includes: pre-purifying with hydrogen and argon gas at room temperature for 1.5 hours, then heating to 400°C at a rate of 4°C / min and calcining for 4 hours;

[0048] The mass ratio of the dispersant to the mixture is 1:2; the dispersant is a mixed solution of acetone and isopropanol with a volume ratio of 1:1.

[0049] S2. Place the sodium ferric fluorophosphate powder in the ALD reaction chamber, evacuate the vacuum, and heat it to 120°C.

[0050] S3. Under an inert atmosphere (flow rate 50 mL / min, duration 40 s), a mixed vapor of dimethylethylaluminum and dimethylzinc is deposited on the surface of the sodium fluorinated iron phosphate powder. Then, an inert gas is introduced (flow rate 100 mL / min, duration 100 s) to remove excess dimethylethylaluminum and dimethylzinc mixture from the surface of the sodium fluorinated iron phosphate powder. O2 is then introduced to react with the dimethylethylaluminum and dimethylzinc mixture on the surface of the sodium fluorinated iron phosphate powder in a redox reaction. Afterward, an inert gas is introduced (flow rate 100 mL / min, duration 80 s) to remove excess O2 and by-reaction products from the surface of the sodium fluorinated iron phosphate powder. This completes one ALD deposition.

[0051] S4. Repeat step S3 for a total of 8 ALD depositions, and increase the content of dimethyl zinc in the mixed vapor of dimethyl ethyl aluminum and dimethyl zinc in each ALD deposition (40% dimethyl zinc in the mixed vapor for the first ALD deposition; 80% dimethyl zinc in the mixed vapor for the last ALD deposition) to obtain sodium fluorinated phosphate material with metal oxide gradient coating.

[0052] Example 3

[0053] This embodiment provides another method for preparing sodium fluorophosphate material with metal oxide gradient coating, the steps of which include:

[0054] S1. Anhydrous sodium acetate, sodium fluoride, ammonium dihydrogen phosphate, and an iron source (the molar ratio of the anhydrous sodium acetate, sodium fluoride, ammonium dihydrogen phosphate, and the iron source is 1.5:1.3:1.2:1.2; the iron source is a mixture of ferrous oxalate and ferrous chloride with a molar ratio of 1:1) are mixed, a dispersant is added, and the mixture is thoroughly ball-milled and mixed. The mixture is then dried and pre-sintered sequentially to obtain sodium fluorophosphate powder.

[0055] The ball milling mixing speed is 400 rpm, the time is 7 h, and the mass ratio of the milling balls to the mixture is 10:1.

[0056] The drying process is carried out at a temperature of 100°C for 10 hours.

[0057] The pre-sintering treatment includes: pre-purging nitrogen at room temperature for 2 hours, then heating to 450°C at a rate of 5°C / min and calcining for 3 hours;

[0058] The mass ratio of the dispersant to the mixture is 1:3; the dispersant is a mixed solution of anhydrous ethanol and N-methylpyrrolidone with a volume ratio of 1:6.

[0059] S2. Place the sodium ferric fluoride phosphate powder in the ALD reaction chamber, evacuate the vacuum, and heat it to 110°C.

[0060] S3. Under an inert atmosphere (flow rate 50 mL / min, duration 60 s), a mixed vapor of diisobutylaluminum and diethylzinc is deposited on the surface of the sodium fluorinated iron phosphate powder. Then, an inert gas is introduced (flow rate 100 mL / min, duration 80 s) to remove excess diisobutylaluminum and diethylzinc mixture from the surface of the sodium fluorinated iron phosphate powder. H2O is then introduced to react with the diisobutylaluminum and diethylzinc mixture on the surface of the sodium fluorinated iron phosphate powder in a redox reaction. Afterward, an inert gas is introduced (flow rate 100 mL / min, duration 120 s) to remove excess H2O and by-products from the surface of the sodium fluorinated iron phosphate powder. This completes one ALD deposition.

[0061] S4. Repeat step S3 for a total of 10 ALD depositions, and increase the content of diethylzinc in the mixed steam of diisobutylaluminum and diethylzinc in each ALD deposition (30% diethylzinc in the mixed steam for the first ALD deposition; 70% diethylzinc in the mixed steam for the last ALD deposition) to obtain sodium fluorinated phosphate material with metal oxide gradient coating.

[0062] Comparative Example 1

[0063] This comparative example provides another method for preparing sodium fluorinated iron phosphate material with metal oxide gradient coating. Steps S1-S2 are the same as in Example 1. In step S3, only trimethylaluminum is introduced, and only one ALD deposition is performed without gradient coating.

[0064] Comparative Example 2

[0065] This comparative example provides another method for preparing sodium fluorinated phosphate material with metal oxide gradient coating. Steps S1-S2 are the same as in Example 1. In step S3, only diethylzinc is introduced, and only one ALD deposition is performed without gradient coating.

[0066] Detection Examples

[0067] 1. Morphological test

[0068] The sodium fluorophosphate material with metal oxide gradient coating prepared by the method described in Example 1 was subjected to gold sputtering treatment, and the surface morphology of the sample was characterized using a ZEISS GEMINI 500 scanning electron microscope (Germany). The results are as follows: Figure 1 As shown, the material prepared by the method described in Example 1 is a spherical particle with a diameter of 2 to 3 μm.

[0069] 2. Battery cycle performance test

[0070] To test the electrochemical performance of the materials, half-cells were assembled using sodium fluorophosphate materials prepared by the methods described in Examples 1-3 and Comparative Examples 1-2 as positive electrodes for electrochemical performance testing. First, 175 mg of sodium fluorophosphate material and 50 mg of SuperP were weighed and ground in an agate mortar for approximately 20 minutes until thoroughly mixed. The mixture was then transferred to a paste bottle, and 500 mg of a pre-prepared 5 wt% PVDF NMP solution was added, followed by approximately 800 μL of NMP solution. The mixture was magnetically stirred for approximately 12 hours until a slightly viscous paste-like liquid was formed. Subsequently, a 120 μm spatula was used to evenly coat the paste onto an aluminum foil, and the foil was vacuum-dried at 120 °C for 12 hours. The resulting positive electrode sheet was then formed into a 12 mm diameter positive electrode sheet using a die-cutting machine. Using this electrode as the positive electrode, sodium sheet as the negative electrode, glass fiber as the separator, and 1.0M NaClO4 / EC-PC (volume ratio 1:1, 5.0 Vol% FEC) as the electrolyte, a CR2032 coin cell was assembled in an argon-filled glove box.

[0071] The button cell battery was subjected to constant current charge-discharge at 0.2C (1C = 120 mAh / g), with a voltage range of 2.0–4.0V. Its charge-discharge curve is shown below. Figure 2 As shown in the figure; the charging and discharging results are shown in Table 1;

[0072] Table 1

[0073]

[0074] The sodium iron fluoride phosphate material obtained in Comparative Example 1 has only a single Al2O3 coating layer on its surface. This can reduce the structural damage caused by side reactions between the material and the electrolyte to some extent, but it does not significantly improve the conductivity of the material. During the battery charge-discharge test, it exhibits significant polarization, resulting in a loss of some capacity. As shown in Table 1, compared with gradient coating, the sodium iron fluoride phosphate material with a single Al2O3 coating has a lower initial discharge specific capacity and a relatively lower initial coulombic efficiency. The modification effect of the coating layer on the sodium iron fluoride phosphate material is not as good as that of Zn. x Al (4-2x) / 3 O2 gradient coating layer.

[0075] The sodium iron fluoride phosphate material obtained in Comparative Example 2 has only a single ZnO coating layer on its surface, which can improve the conductivity of the material to a certain extent and reduce the capacity loss caused by polarization. However, due to the relatively large molecular weight of ZnO, it has a certain impact on the specific capacity of the material, and ZnO has poor mechanical properties, which may lead to partial detachment during coating or cycling, thus failing to exert its modification effect normally. As shown in Table 1, compared with gradient coating, the sodium iron fluoride phosphate material with a single ZnO coating has a lower initial discharge specific capacity and a relatively lower initial coulombic efficiency. The modification effect of the coating layer on the sodium iron fluoride phosphate material is not as good as that of ZnO. x Al (4-2x) / 3 O2 gradient coating layer.

[0076] This invention utilizes ALD atomic deposition technology and, by successively adjusting the ratio of two metal precursors, achieves an increase in the x-value of Zn on the surface of sodium iron fluoride phosphate cathode material from the inside out. x Al (4-2x) / 3 O2 gradient coating, along with the introduction of a certain proportion of a mixture of two metal precursors, facilitates the formation of a mixed metal oxide deposition layer with tight interfacial bonding and smoother transition. This fully utilizes the advantages of Al2O3's high mechanical strength and corrosion resistance, as well as ZnO's high electrical conductivity. The high Al2O3 content in the inner layer effectively improves the structural stability of the material during cycling, while the high ZnO content in the outer layer helps improve mass transfer efficiency, reduce the interfacial resistance between the material and the electrolyte, and maximize the overall performance of the cathode material.

[0077] The above description is merely a preferred embodiment of the present invention and does not limit the implementation and protection scope of the present invention. Those skilled in the art should realize that any equivalent substitutions and obvious changes made based on the description and illustrations of the present invention should be included within the protection scope of the present invention.

Claims

1. A metal oxide gradient-coated sodium fluorophosphate material, characterized in that, The chemical formula is Na2FePO4F@Zn x Al （4-2x） / 3 O2; metal oxides form a gradient coating layer on the surface of sodium iron fluoride phosphate material, in which the x value of the sodium iron fluoride phosphate material increases from the inside to the outside; wherein, 0.3≤x≤1.

4.

2. A method for preparing sodium fluorophosphate material with metal oxide gradient coating as described in claim 1, characterized in that the step... include: S1. Anhydrous sodium acetate, sodium fluoride, ammonium dihydrogen phosphate and iron source are mixed, and a dispersant is added for thorough ball milling. The mixture is then dried and pre-sintered in sequence to obtain sodium fluorinated iron phosphate powder. S2. Place the sodium ferric fluorophosphate powder in the ALD reaction chamber, evacuate the vacuum, and heat it to 100~120℃. S3. Under an inert atmosphere, a mixed vapor of precursor aluminum source and precursor zinc source is deposited on the surface of the sodium fluorinated iron phosphate powder. Then, an inert gas is introduced to blow away the excess precursor aluminum source and precursor zinc source mixture on the surface of the sodium fluorinated iron phosphate powder. A precursor oxygen source is then introduced to react with the precursor aluminum source and precursor zinc source mixture on the surface of the sodium fluorinated iron phosphate powder in a redox reaction. After that, an inert gas is introduced to blow away the excess precursor oxygen source and by-reaction products on the surface of the sodium fluorinated iron phosphate powder. This completes one ALD deposition. S4. Repeat step S3 to perform 8-10 ALD depositions, and gradually increase the content of precursor zinc source in the mixed steam of precursor aluminum source and precursor zinc source during each ALD deposition to obtain the metal oxide gradient coated sodium iron fluoride phosphate material; wherein, in the first ALD deposition, the content of the precursor zinc source in the mixed steam is 20%-40%, and in the last ALD deposition, the content of the precursor zinc source in the mixed steam is 60%-80%.

3. The preparation method according to claim 2, characterized in that, In step S1, the molar ratio of the anhydrous sodium acetate, the sodium fluoride, the ammonium dihydrogen phosphate and the iron source is (1.0~1.5):(1.0~1.3):(1.0~1.5):(1.0~1.5).

4. The preparation method according to claim 2, characterized in that, In step S1, the iron source includes at least one of ferrous oxalate, ferrous sulfate, and ferrous chloride.

5. The preparation method according to claim 2, characterized in that, In step S1, the mass ratio of the dispersant to the mixture is 1:(1~3); the dispersant includes at least one of anhydrous ethanol, acetone, isopropanol, and N-methylpyrrolidone.

6. The preparation method according to claim 2, characterized in that, In step S1, the ball milling speed is 300~500 rpm, the time is 6~8 h, and the mass ratio of the ball milling beads to the mixture is (8~10):1; The drying process is carried out at a temperature of 60~100℃ for 8~14 hours. The pre-sintering treatment includes: pre-purifying with a protective gas at room temperature for 0.5 to 2 hours, then heating to 350 to 450°C at a rate of 3 to 5°C / min and calcining for 2 to 6 hours; the protective gas includes at least one of argon, hydrogen argon, nitrogen, and helium.

7. The preparation method according to claim 2, characterized in that, In step S3, the inert gas flow rate during deposition is 50 mL / min, and the duration is 40-60 s; the inert gas flow rate during blowing is 100 mL / min, and the duration is 80-120 s.

8. The preparation method according to claim 2, characterized in that, The precursor aluminum source includes at least one of the following: trimethylaluminum, dimethylethylaluminum, diethylaluminum, diisobutylaluminum, 2-aminopyridine diethylaluminum, 2-tert-butylaminomethylpyridine dimethylaluminum, and pyrrole-2-imine dimethylaluminum; The precursor zinc source includes at least one of: diethyl zinc, dimethyl zinc, pyridine zinc complexes, pyrimidine zinc complexes, and nitrogen heterocyclic carbene zinc complexes; The precursor oxygen source includes at least one of H2O, O2, and O3.

9. The application of a metal oxide gradient-coated sodium fluorophosphate material as described in claim 1 or a metal oxide gradient-coated sodium fluorophosphate material prepared by any one of claims 2-8 in the cathode material of a sodium-ion battery.

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