A positive electrode material, a preparation method thereof and a sodium ion battery

Abstract

The application discloses a positive electrode material and a preparation method and application thereof, and the preparation method comprises the following steps: mixing iron phosphate, a carbon source, a sodium source and water to prepare a slurry; the mixed iron phosphate contains phosphate and pyrophosphate or phosphate and dihydrogen phosphate; the carbon source contains citric acid; the prepared slurry is grinded to obtain a fine slurry; the fine slurry is sprayed and dried to obtain a solid precursor; and the solid precursor is sintered to obtain the positive electrode material. By controlling the reaction parameters, the mixed iron phosphate containing pyrophosphate or dihydrogen phosphate is synthesized, the amount of citric acid in the subsequent process is reduced, the pore generation rate is reduced, the material compaction density is improved, the method is simple and reliable, and the application prospect is good. The prepared positive electrode material has high specific capacity and high compaction density, and is suitable for preparing a positive electrode of a sodium ion battery.

Description

A cathode material, its preparation method, and a sodium-ion battery Technical Field

[0001] This invention relates to a sodium-ion battery cathode material and its preparation method, and more particularly to an iron-based polyanion cathode material, its preparation method, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries, as a potential alternative to lithium-ion batteries in energy storage and low-speed power batteries, have advantages in terms of cost and safety. The cathode material is one of the core materials of sodium-ion batteries, fundamentally determining their electrochemical performance, including energy density, safety, and cycle retention.

[0003] Iron-based polyanionic cathode materials are theoretically cheaper, which can further leverage the cost advantage of sodium-ion batteries. Olivine-type sodium iron phosphate has a similar structure to lithium iron phosphate and can be used as a cathode in sodium-ion batteries, but it cannot be directly synthesized chemically. It requires ion exchange of lithium iron phosphate, a complex process with unsatisfactory performance.

[0004] In the preparation of iron-based polyanionic cathode materials, existing technologies often involve simply mixing sodium, iron, phosphorus, and carbon source compounds and then sintering them to obtain iron-based pyrophosphate materials, which usually have a large number of pores. This is because the preparation method usually requires citric acid as a reactant to convert phosphate groups in the mixture into pyrophosphate groups. During this process, some carboxyl groups will decarboxylate and be converted into carbon dioxide, generating pores and reducing the compaction density of the material.

[0005] To address the issue of low compaction density caused by citric acid, Chinese patent CN116936770A discloses a method for preparing sodium-ion battery cathode materials. This method utilizes a phosphorus and iron source composite (ferric pyrophosphate and ferrous pyrophosphate) that undergoes a solid-phase reaction without decomposition, reducing gas generation during solid-phase sintering. Furthermore, the interstitial spaces between the iron or ferrous pyrophosphate solid particles provide a channel for venting during the solid-phase reaction of the sodium and carbon sources, thus eliminating residual pores in the finished sodium-ion battery cathode material. This results in a material with better crystallinity, superior electrochemical performance, and higher compaction and energy densities. Essentially, this method uses the phosphorus and iron source composite as reactants to construct a specific reaction system, thereby eliminating citric acid as a reactant and reducing gas generation. However, this method relies on a specific reaction system based on iron pyrophosphate and ferrous pyrophosphate. While eliminating citric acid can improve compaction density when the reaction system is changed, it also significantly reduces the performance of the cathode material, making it counterproductive. Summary of the Invention

[0006] Objective of this invention: The objective of this invention is to provide a method for preparing a cathode material, solving the problem of how to prepare a cathode material with high compaction density and excellent performance. Another objective of this invention is to propose a cathode material that balances compaction density and performance. A third objective of this invention is to propose a sodium-ion battery containing the above-mentioned cathode material.

[0007] Technical solution: The present invention provides a method for preparing a positive electrode material, comprising the following steps:

[0008] (1) Mixed ferric phosphate is prepared into a slurry with a carbon source, a sodium source and water; the mixed ferric phosphate is a mixed ferric phosphate containing phosphate and pyrophosphate or a mixed ferric phosphate containing phosphate and dihydrogen phosphate, and the carbon source contains citric acid.

[0009] (2) The prepared slurry is ground to obtain a fine slurry;

[0010] (3) The fine slurry is spray-dried to obtain a solid precursor;

[0011] (4) The cathode material is obtained by sintering the solid precursor.

[0012] This invention does not simply replace citric acid with a carbon source that produces less gas to solve the problem of low compaction density. Instead, it introduces pyrophosphate or dihydrogen phosphate into the system and mixes them at the molecular level to prepare mixed iron phosphate. This reduces the amount of citric acid used, thereby directly reducing carbon dioxide production and increasing compaction density. It does not increase compaction density by constructing exhaust channels. To maintain the performance of the cathode material, this invention retains citric acid in the reaction raw materials.

[0013] Preferably, the mixed ferric phosphate is prepared by the following method:

[0014] (11) Dissolve an iron source containing ferrous ions, a phosphorus source, and a complexing agent in water to obtain a base solution, and adjust the pH of the base solution to acidic; the phosphorus source includes one of dihydrogen phosphate or pyrophosphate; the complexing agent includes at least one of ammonium sulfate, glucose, and citric acid;

[0015] (12) After heating the base liquid, add hydrogen peroxide aqueous solution dropwise to produce a precipitate and obtain the reaction solution;

[0016] (13) Heat the reaction solution to 80-98℃ and keep it warm for 2-5 hours. Take the solid to obtain mixed iron phosphate.

[0017] Preferably, in step (11), the ferrous ion-containing iron source includes one or more of ferrous sulfate, ferrous nitrate, ferrous chloride, ferrous citrate, ferrous oxalate, and ferrous oxide; the dihydrogen phosphate includes one or more of sodium dihydrogen phosphate, potassium dihydrogen phosphate, and ammonium dihydrogen phosphate; the pyrophosphate includes one or more of sodium pyrophosphate, potassium pyrophosphate, sodium acid pyrophosphate, and potassium acid pyrophosphate; the pH of the bottom solution is adjusted to 0.9-1.7 by an inorganic acid. At high temperatures, some of the pyrophosphate decomposes to produce dihydrogen phosphate, which further reacts to form phosphate. Therefore, the resulting mixed ferric phosphate is either a mixed ferric phosphate containing phosphate and pyrophosphate or a mixed ferric phosphate containing phosphate and dihydrogen phosphate.

[0018] Preferably, the pH is between 0.95 and 1.5. The inorganic acid can be selected from at least one of phosphoric acid, sulfuric acid, and nitric acid. A strongly acidic reaction environment can reduce the hydroxide content in the base solution. Excessive hydroxide will lead to excessive iron in the reaction system. This excess iron will not combine with pyrophosphate or phosphate, resulting in an imbalance of elemental ratios, which in turn affects the performance of the cathode material.

[0019] Preferably, in step (11), the concentration of ferrous ions in the base solution is in the range of 0.1-0.5 mol / L, the concentration of phosphorus is in the range of 0.2-0.7 mol / L, and the concentration of complexing agent is in the range of 0.01-0.05 mol / L.

[0020] Preferably, in step (12), after the bottom liquid is heated to 50-70°C, an aqueous solution of hydrogen peroxide is added dropwise; the molar ratio of hydrogen peroxide to ferrous ions in the bottom liquid is 1:1-3.

[0021] Preferably, in step (1), the carbon source further includes at least one of glucose, sucrose, polyethylene glycol, soluble starch, maltose, cyclodextrin, carbon nanotubes, acetylene black, and graphene; the sodium source includes at least one of sodium carbonate, sodium acetate, sodium nitrate, sodium sulfate, sodium chloride, and sodium oxalate.

[0022] Preferably, in step (1), the molar ratio of sodium in the sodium source to phosphorus in the mixed iron phosphate is 0.95-1.05:1; and the molar ratio of carbon source to phosphorus in the mixed iron phosphate is 0.18-0.25:1.

[0023] Preferably, in step (4), the solid precursor is sintered at 400-600°C for 8-15 hours.

[0024] The present invention uses the above-described preparation method to prepare a positive electrode material.

[0025] The present invention further involves uniformly mixing the above-mentioned positive electrode material, conductive agent, and binder in a specific ratio, coating the mixture onto aluminum foil, drying it at 120°C for 6 hours, and then cutting the dried electrode sheet into circular pieces. A coin cell is then assembled using the positive electrode disc, separator, sodium metal negative electrode sheet, and sodium-ion battery electrolyte to obtain a sodium-ion battery, wherein the positive electrode of the sodium-ion battery contains the above-mentioned positive electrode material.

[0026] Beneficial Effects: Compared with existing technologies, this invention has the following significant advantages: By controlling reaction parameters, this invention synthesizes mixed iron phosphate containing pyrophosphate or dihydrogen phosphate, reducing the amount of citric acid used in subsequent steps, lowering the porosity, and thus improving the material's compaction density. The method is simple and reliable, and has good application prospects. The cathode material obtained by this invention possesses both high specific capacity and high compaction density, making it suitable for preparing cathodes for sodium-ion batteries. Attached Figure Description

[0027] Figure 1 is a SEM image of the cathode material prepared in Example 1;

[0028] Figure 2 is a SEM image of the cathode material prepared in Example 2;

[0029] Figure 3 is a SEM image of the cathode material prepared in Example 3;

[0030] Figure 4 shows the SEM image of the cathode material prepared in Comparative Example 1;

[0031] Figure 5 shows the SEM image of the cathode material prepared in Comparative Example 2. Detailed Implementation

[0032] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0033] Example 1: A method for preparing a sodium-ion battery cathode material is as follows:

[0034] (1) Dissolve ferrous sulfate, sodium pyrophosphate and glucose in water to prepare a mixed bottom solution, wherein the concentration of ferrous ions is 0.25 mol / L, the molar concentration of phosphorus is 0.4 mol / L and the concentration of glucose is 0.025 mol / L. Then add concentrated sulfuric acid dropwise to the mixed solution to adjust the pH value to 1.2.

[0035] (2) Raise the temperature of the base liquid to 65°C, add hydrogen peroxide solution dropwise while stirring, and a precipitate is produced to obtain the reaction solution. The amount of hydrogen peroxide is equivalent to 0.5 times the amount of ferrous ions in the base liquid.

[0036] (3) Heat the reaction solution to 95°C and keep it at that temperature for 2.5 hours to allow it to react fully. Then filter the solid, wash and dry it to obtain a mixed iron phosphate solid.

[0037] (4) Add the mixed iron phosphate, sucrose, citric acid and sodium carbonate to water and mix evenly. The molar ratio of phosphorus, sucrose, citric acid and sodium in the mixed iron phosphate is 1:0.1:0.1:0.98. During the sintering process, most of the carbon will volatilize in the form of carbon dioxide or carbon monoxide.

[0038] (5) The prepared slurry is milled to obtain a fine slurry, and the particle size of the solid particles in the fine slurry is less than 50 μm;

[0039] (6) The fine slurry is spray-dried to obtain a solid precursor;

[0040] (7) The precursor was sintered at 480℃ for 12h to obtain the cathode material product, as shown in Figure 1.

[0041] Example 2: The rest is the same as in Example 1, except that the pH value is adjusted to 0.9 in step (1). The positive electrode material prepared in this example is shown in Figure 2.

[0042] Example 3: The rest is the same as in Example 1, except that sodium pyrophosphate in step (1) is replaced with ammonium dihydrogen phosphate. The cathode material prepared in this example is shown in Figure 3.

[0043] Example 4: A method for preparing a sodium-ion battery cathode material is as follows:

[0044] (1) Ferrous nitrate, potassium pyrophosphate, and a complexing agent were dissolved in water to obtain a base solution. Phosphoric acid was added dropwise to adjust the pH of the base solution to 0.95. The concentration of ferrous ions in the base solution was 0.1 mol / L, the concentration of phosphorus was 0.2 mol / L, and the concentration of the complexing agent was 0.01 mol / L. In this embodiment, the complexing agent was ammonium sulfate.

[0045] (2) After the bottom liquid is heated to 50°C, hydrogen peroxide aqueous solution is added dropwise. The molar ratio of hydrogen peroxide to ferrous ions in the bottom liquid is 1:1, and a precipitate is formed to obtain the reaction solution.

[0046] (3) Heat the reaction solution to 80°C and keep it warm for 5 hours. Take the solid to obtain mixed iron phosphate.

[0047] (4) Mix ferric phosphate with citric acid, glucose, sodium acetate and water to make a slurry, wherein the molar ratio of phosphorus, glucose and sodium in the mixed ferric phosphate is 1:0.2:0.98;

[0048] (5) The prepared slurry is ground to obtain a fine slurry, and the particle size of the solid particles in the fine slurry is less than 50 μm; the fine slurry is spray-dried to obtain a solid precursor.

[0049] (6) The cathode material is obtained by sintering the solid precursor at 400℃ for 15h.

[0050] Example 5: A method for preparing a sodium-ion battery cathode material is as follows:

[0051] (1) Ferrous chloride, sodium pyrophosphate, and a complexing agent were dissolved in water to obtain a base solution. Concentrated nitric acid was added dropwise to adjust the pH of the base solution to 1.5. The concentration of ferrous ions in the base solution was 0.5 mol / L, the concentration of phosphorus was 0.7 mol / L, and the concentration of the complexing agent was 0.01 mol / L. In this embodiment, the complexing agent was citric acid.

[0052] (2) After the bottom liquid is heated to 70°C, hydrogen peroxide aqueous solution is added dropwise. The molar ratio of hydrogen peroxide to ferrous ions in the bottom liquid is 1:3, and a precipitate is formed to obtain the reaction solution.

[0053] (3) Heat the reaction solution to 98°C and keep it warm for 2 hours. Take the solid to obtain mixed iron phosphate.

[0054] (4) Mix ferric phosphate with citric acid, polyethylene glycol, sodium nitrate and water to form a slurry, wherein the molar ratio of phosphorus, citric acid, polyethylene glycol and sodium in the mixed ferric phosphate is 1:0.1:0.1:0.98;

[0055] (5) The prepared slurry is ground to obtain a fine slurry, and the particle size of the solid particles in the fine slurry is less than 50 μm;

[0056] The fine slurry was spray-dried to obtain a solid precursor;

[0057] (6) The cathode material is obtained by sintering the solid precursor at 600℃ for 8 hours.

[0058] Example 6: A method for preparing a sodium-ion battery cathode material is as follows:

[0059] (1) Ferrous citrate, potassium acid pyrophosphate, and a complexing agent were dissolved in water to obtain a base solution. Concentrated sulfuric acid was added dropwise to adjust the pH of the base solution to 1.0. The concentration of ferrous ions in the base solution was 0.3 mol / L, the concentration of phosphorus was 0.5 mol / L, and the concentration of the complexing agent was 0.03 mol / L. In this embodiment, the complexing agent was ammonium sulfate.

[0060] (2) After the bottom liquid is heated to 60°C, hydrogen peroxide aqueous solution is added dropwise. The molar ratio of hydrogen peroxide to ferrous ions in the bottom liquid is 1:1.5, and a precipitate is formed to obtain the reaction solution.

[0061] (3) Heat the reaction solution to 90°C and keep it warm for 3 hours. Take the solid to obtain mixed iron phosphate.

[0062] (4) Mix ferric phosphate with citric acid, soluble starch, sodium sulfate and water to make a slurry, wherein the molar ratio of phosphorus, citric acid, soluble starch and sodium in the mixed ferric phosphate is 1:0.1:0.1:0.98;

[0063] (5) The prepared slurry is ground to obtain a fine slurry, and the particle size of the solid particles in the fine slurry is less than 50 μm; the fine slurry is spray-dried to obtain a solid precursor.

[0064] (6) The cathode material is obtained by sintering the solid precursor at 500℃ for 11h.

[0065] Example 7: A method for preparing a sodium-ion battery cathode material is as follows:

[0066] (1) Ferrous oxalate, sodium dihydrogen phosphate, and a complexing agent were dissolved in water to obtain a base solution. Concentrated nitric acid was added dropwise to adjust the pH of the base solution to 1.3. The concentration of ferrous ions in the base solution was 0.2 mol / L, the concentration of phosphorus was 0.6 mol / L, and the concentration of the complexing agent was 0.05 mol / L. In this embodiment, the complexing agent was glucose.

[0067] (2) After the bottom liquid is heated to 55°C, hydrogen peroxide aqueous solution is added dropwise. The molar ratio of hydrogen peroxide to ferrous ions in the bottom liquid is 1:2.5, and a precipitate is formed to obtain the reaction solution.

[0068] (3) Heat the reaction solution to 95°C and keep it warm for 4 hours. Take the solid to obtain mixed iron phosphate.

[0069] (4) Mix ferric phosphate with citric acid, maltose, sodium chloride and water to make a slurry, wherein the molar ratio of phosphorus, citric acid, maltose and sodium in the mixed ferric phosphate is 1:0.1:0.1:0.98;

[0070] (5) The prepared slurry is ground to obtain a fine slurry, and the particle size of the solid particles in the fine slurry is less than 50 μm; the fine slurry is spray-dried to obtain a solid precursor.

[0071] (6) The cathode material is obtained by sintering the solid precursor at 550℃ for 10h.

[0072] Example 8: A method for preparing a sodium-ion battery cathode material is as follows:

[0073] (1) Ferrous oxide, potassium dihydrogen phosphate, and a complexing agent were dissolved in water to obtain a base solution. Concentrated sulfuric acid was added dropwise to adjust the pH of the base solution to 1.1. The concentration of ferrous ions in the base solution was 0.4 mol / L, the concentration of phosphorus was 0.3 mol / L, and the concentration of the complexing agent was 0.01 mol / L. In this embodiment, the complexing agent was citric acid.

[0074] (2) After the bottom liquid is heated to 65°C, hydrogen peroxide aqueous solution is added dropwise. The molar ratio of hydrogen peroxide to ferrous ions in the bottom liquid is 1:3, and a precipitate is formed to obtain the reaction solution.

[0075] (3) Heat the reaction solution to 98°C and keep it warm for 5 hours. Take the solid to obtain mixed iron phosphate.

[0076] (4) Mixed ferric phosphate with citric acid, cyclodextrin, carbon nanotubes, sodium oxalate and water to form a slurry, wherein the molar ratio of phosphorus, citric acid, cyclodextrin and carbon nanotube mixed carbon source and sodium in the mixed ferric phosphate is 1:0.1:0.12:0.98; the molar ratio of cyclodextrin to carbon nanotubes is 10:1.

[0077] (5) The prepared slurry is ground to obtain a fine slurry, and the particle size of the solid particles in the fine slurry is less than 50 μm; the fine slurry is spray-dried to obtain a solid precursor.

[0078] (6) The cathode material is obtained by sintering the solid precursor at 500℃ for 10h.

[0079] Example 9: A method for preparing a sodium-ion battery cathode material is as follows:

[0080] (1) Ferrous sulfate, ferrous nitrate, sodium pyrophosphate, phosphoric acid, and a complexing agent were dissolved in water to obtain a base solution. Concentrated sulfuric acid was added dropwise to adjust the pH of the base solution to 1.4. The concentration of ferrous ions in the base solution was 0.5 mol / L, the concentration of phosphorus was 0.3 mol / L, and the concentration of the complexing agent was 0.01 mol / L. In this embodiment, the complexing agent was glucose; the molar ratio of ferrous sulfate to ferrous nitrate was 2:1.

[0081] (2) After the bottom liquid is heated to 60°C, hydrogen peroxide aqueous solution is added dropwise. The molar ratio of hydrogen peroxide to ferrous ions in the bottom liquid is 1:2, and a precipitate is formed to obtain the reaction solution.

[0082] (3) Heat the reaction solution to 85°C and keep it warm for 4 hours. Take the solid to obtain mixed iron phosphate.

[0083] (4) Mix ferric phosphate with citric acid, acetylene black, graphene, sodium carbonate and water to form a slurry; wherein the molar ratio of phosphorus, citric acid, acetylene black and graphene mixed carbon source and sodium in the mixed ferric phosphate is 1:0.1:0.15:0.98; the molar ratio of acetylene black to graphene is 1:1.

[0084] (5) The prepared slurry is ground to obtain a fine slurry, and the particle size of the solid particles in the fine slurry is less than 50 μm; the fine slurry is spray-dried to obtain a solid precursor.

[0085] (6) The cathode material is obtained by sintering the solid precursor at 500℃ for 8 hours.

[0086] Example 10: A method for preparing a sodium-ion battery cathode material is as follows:

[0087] (1) Ferrous chloride, ferrous oxide, sodium dihydrogen phosphate, and a complexing agent were dissolved in water to obtain a base solution. Concentrated nitric acid was added dropwise to adjust the pH of the base solution to 1.2. The concentration of ferrous ions in the base solution was 0.4 mol / L, the concentration of phosphorus was 0.4 mol / L, and the concentration of the complexing agent was 0.04 mol / L. In this embodiment, the complexing agent was ammonium sulfate; the molar ratio of ferrous chloride to ferrous oxide was 1:3.

[0088] (2) After the bottom liquid is heated to 70°C, hydrogen peroxide aqueous solution is added dropwise. The molar ratio of hydrogen peroxide to ferrous ions in the bottom liquid is 1:2, and a precipitate is formed to obtain the reaction solution.

[0089] (3) Heat the reaction solution to 95°C and keep it at that temperature for 3.5 hours. Take the solid to obtain mixed iron phosphate.

[0090] (4) Mix ferric phosphate with citric acid, glucose, sucrose, sodium nitrate and water to form a slurry; wherein the molar ratio of phosphorus, citric acid, glucose and sucrose mixed carbon source and sodium in the mixed ferric phosphate is 1:0.1:0.08:0.98; and the molar ratio of glucose to sucrose is 1:2.

[0091] (5) The prepared slurry is ground to obtain a fine slurry, and the particle size of the solid particles in the fine slurry is less than 50 μm; the fine slurry is spray-dried to obtain a solid precursor.

[0092] (6) The cathode material is obtained by sintering the solid precursor at 400℃ for 8 hours.

[0093] Comparative Example 1: The cathode material was prepared according to the following method:

[0094] (1) Mix commercially available ferric phosphate, trisodium phosphate, sodium carbonate and citric acid in water at a molar ratio of 1:0.33:0.17:0.35 to form a slurry;

[0095] (2) The prepared slurry is milled to obtain a fine slurry;

[0096] (3) The fine slurry is spray-dried to obtain a solid precursor;

[0097] (4) The precursor was sintered at 480℃ for 12h to obtain the cathode material product, as shown in Figure 4.

[0098] Comparative Example 2: Everything else was the same as in Example 1, except that citric acid was replaced with an equal amount of sucrose. The microstructure of the obtained cathode material is shown in Figure 5.

[0099] Testing of cathode material products:

[0100] The positive electrode material, conductive agent, and binder were mixed evenly in a 95:5:5 ratio and coated onto aluminum foil. The mixture was then dried at 120°C for 6 hours. The dried electrode was cut into circular pieces. A coin cell was assembled using the positive electrode discs, separator, sodium metal negative electrode, and sodium-ion battery electrolyte. The battery was charged and discharged within a 2.0-4.0V voltage window, cycling twice at 0.1C, and then cycling more than 30 times at 1C. (1C = 120mA / g)

[0101] The compaction density test was conducted using the UTM7305 compaction density tester from Shenzhen Sansi Test Instrument Co., Ltd., under a pressure of 3T.

[0102] The contents of Na, Fe, and P were tested using ICP, and the contents of C were tested using an infrared carbon-sulfur analyzer.

[0103] The test results are shown in Tables 1 and 2:

[0104] Table 1. Test results of the main element content of cathode materials prepared by different methods

[0105]

[0106]

[0107] As can be seen from Table 1, the proportions of elements in each case are similar.

[0108] Table 2 shows the performance indicators of various materials in the case studies.

[0109]

[0110] As shown in Table 2, Examples 1-3 achieved higher compaction densities while maintaining high specific capacity. Comparative Example 2 had the lowest compaction density. Simply replacing the carbon source citric acid with sucrose increased the compaction density, but severely degraded performance. Figures 1-3 show that the cathode material samples prepared in Examples 1-3 had higher sphericity and no voids, while the cathode material sample prepared in Comparative Example 1 exhibited pitting and surface voids. This is due to the porosity caused by carbon dioxide produced from the decomposition of citric acid, resulting in numerous hollow spheres and a decrease in compaction density.

Claims

1. A method for preparing a positive electrode material, characterized in that, The process includes the following steps: (1) Preparation of mixed ferric phosphate: Iron source containing ferrous ions, phosphorus source and complexing agent are dissolved in water to obtain a base solution. The pH of the base solution is adjusted to acidic. After heating the base solution, hydrogen peroxide aqueous solution is added dropwise to produce a precipitate and obtain a reaction solution. The reaction solution is heated to 80-98℃ and kept at that temperature for 2-5 hours. After solid-liquid separation, the solid is taken to obtain mixed ferric phosphate. The phosphorus source contains one of dihydrogen phosphate or pyrophosphate. The complexing agent contains at least one of ammonium sulfate, glucose and citric acid. (1) Mixed iron phosphate: The mixed iron phosphate is a mixed iron phosphate containing phosphate and pyrophosphate, or a mixed iron phosphate containing phosphate and dihydrogen phosphate; (2) Preparation of slurry: The mixed iron phosphate is mixed with carbon source, sodium source and water to prepare slurry; The carbon source contains citric acid; (3) Grinding treatment: The prepared slurry is ground to obtain fine slurry; (4) Spray drying: The fine slurry is spray dried to obtain solid precursor; (5) Sintering: The solid precursor is sintered to obtain positive electrode material.

2. The method for preparing the cathode material according to claim 1, characterized in that, In step (1), the ferrous ion-containing iron source includes one or more of ferrous sulfate, ferrous nitrate, ferrous chloride, ferrous citrate, ferrous oxalate, and ferrous oxide; the dihydrogen phosphate includes one or more of sodium dihydrogen phosphate, potassium dihydrogen phosphate, and ammonium dihydrogen phosphate; the pyrophosphate includes one or more of sodium pyrophosphate, potassium pyrophosphate, sodium acid pyrophosphate, and potassium acid pyrophosphate; and the pH of the bottom solution is adjusted to 0.9-1.7 by an inorganic acid.

3. The method for preparing the cathode material according to claim 1, characterized in that, In step (1), the concentration of ferrous ions in the substrate ranges from 0.1 to 0.5 mol / L, the concentration of phosphorus element ranges from 0.2 to 0.7 mol / L, and the concentration of complexing agent ranges from 0.01 to 0.05 mol / L.

4. The method for preparing the cathode material according to claim 1, characterized in that, In step (1), the bottom liquid is heated to 50-70°C and then hydrogen peroxide aqueous solution is added dropwise; the molar ratio of hydrogen peroxide to ferrous ions in the bottom liquid is 1:1-3.

5. The method for preparing the positive electrode material according to claim 1, characterized in that, In step (2), the carbon source further includes at least one of glucose, sucrose, polyethylene glycol, soluble starch, maltose, cyclodextrin, carbon nanotubes, acetylene black, and graphene; the sodium source includes at least one of sodium carbonate, sodium acetate, sodium nitrate, sodium sulfate, sodium chloride, and sodium oxalate.

6. The method for preparing the cathode material according to claim 1, characterized in that, In step (2), the molar ratio of sodium in the sodium source to phosphorus in the mixed iron phosphate is 0.95-1.05:1; the molar ratio of carbon source to phosphorus in the mixed iron phosphate is 0.18-0.25:

1.

7. The method for preparing the cathode material according to claim 1, characterized in that, In step (5), the solid precursor is sintered at 400-600°C for 8-15 hours.

8. The cathode material prepared by the preparation method according to any one of claims 1-7.

9. A sodium-ion battery, characterized in that, The positive electrode of the battery comprises the positive electrode material as described in claim 8.

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