Iron-based mixed phosphate cathode material, preparation method thereof, and application in sodium batteries

Abstract

This invention relates to the field of battery materials technology, specifically to an iron-based mixed phosphate cathode material, its preparation method, and its application in sodium-ion batteries. The iron-based mixed phosphate cathode material comprises: polyacrylic acid and sodium, iron, phosphorus, and carbon sources in stoichiometric ratios. The mass ratio of polyacrylic acid to the carbon source is 1% to 30%, and the carbon content in the iron-based mixed phosphate cathode material is 0.5% to 10%. Polyacrylic acid, as a functional additive, combines with the carbon source to form a carbon coating layer of the iron-based mixed phosphate cathode material. Utilizing the aggregation effect and cross-linking template formation of polyacrylic acid, an iron-based mixed phosphate cathode material with high tap density, uniform carbon coating layer, and good conductivity can be obtained, effectively improving the energy density, rate performance, and cycle life of the corresponding sodium-ion battery.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, specifically to an iron-based mixed phosphate cathode material, its preparation method, and its application in sodium batteries. Background Technology

[0002] Among the many battery materials, sodium-ion batteries have received widespread attention due to their advantages such as low development cost, high safety, and wide temperature range adaptability.

[0003] Sodium-ion battery cathode materials are mainly classified into three categories: layered oxides, Prussian blue compounds, and polyanionic materials. Layered oxide cathode materials typically have high specific capacity but poor cycle stability, and often contain elements such as cobalt and nickel, leading to high material costs. Prussian blue compound cathode materials have high energy density, but are difficult to dehydrate and contain cyanide, posing a significant environmental hazard. Polyanionic cathode materials, on the other hand, possess advantages such as high structural stability, long cycle life, and good thermal stability, making them a promising candidate for development in sodium-ion batteries. Among polyanionic cathode materials, iron-based mixed phosphate cathode material Na4Fe3(PO4)2P2O7 (hereinafter referred to as NFPP) has attracted considerable attention due to its pollution-free and low-cost characteristics. However, NFPP also suffers from poor conductivity and low tap density, which limits its practical application.

[0004] Therefore, providing a novel iron-based mixed phosphate cathode material and its preparation method, as well as its application in sodium batteries, has become an urgent technical problem to be solved in this field. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide an iron-based mixed phosphate cathode material, its preparation method, and its application in sodium batteries, which can effectively solve the problems of poor conductivity and low tap density of iron-based mixed phosphate cathode materials.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] The present invention provides an iron-based mixed phosphate cathode material, comprising: polyacrylic acid and sodium source, iron source, phosphorus source and carbon source in stoichiometric ratio, wherein the mass ratio of polyacrylic acid to carbon source is 1% to 30% and the carbon content in the iron-based mixed phosphate cathode material is 0.5% to 10%.

[0008] According to one embodiment of the present invention, the sodium source includes one or more of sodium carbonate, sodium chloride, sodium nitrate, sodium acetate, sodium hydroxide, disodium hydrogen phosphate, and sodium dihydrogen phosphate; the iron source includes one or more of ferric oxalate, ferric phosphate, and ferric nitrate; the phosphorus source includes one or more of phosphoric acid, disodium hydrogen phosphate, and sodium dihydrogen phosphate; and the carbon source includes one or more of tartaric acid, citric acid, malic acid, glucose, acetic acid, and tannic acid.

[0009] Another aspect of the present invention provides a method for preparing an iron-based mixed phosphate cathode material, comprising the following steps:

[0010] S1: Polyacrylic acid and sodium, iron, phosphorus and carbon sources in stoichiometric ratios are added to deionized water and stirred until uniformly mixed to obtain a spray slurry; wherein the mass ratio of the polyacrylic acid to the carbon source is 1%~30%;

[0011] S2: The spray slurry is spray-dried to obtain a precursor material;

[0012] S3: The precursor material is sintered in an argon-hydrogen atmosphere to obtain the iron-based mixed phosphate cathode material.

[0013] According to one embodiment of the present invention, in step S1, the amount of deionized water added is used to control the solid content of the spray slurry to be between 5% and 50%.

[0014] According to one embodiment of the present invention, in step S1, after stirring until the mixture is uniform, the method further includes: adding oxalic acid and stirring at a stirring rate of 200-500 r / min for 1-15 h to control the pH of the spray slurry between 1 and 5; in step S2, the nozzle temperature of the spray drying is 100-300°C, and the peristaltic pump rate is controlled between 10% and 50%; in step S3, the sintering temperature is 400-600°C, and the sintering time is 1-15 h.

[0015] Another aspect of the present invention provides a method for preparing an iron-based mixed phosphate cathode material, comprising the following steps:

[0016] S1: Polyacrylic acid and sodium, iron, phosphorus and carbon sources in stoichiometric ratios are added to a ball mill jar, ethanol is added and wet ball milling is performed in an inert atmosphere to obtain a precursor material; wherein the mass ratio of the polyacrylic acid to the carbon source is 1%~30%;

[0017] S2: Preheat the precursor material in an argon-hydrogen atmosphere, remove it and grind it until it is uniformly mixed to obtain powder;

[0018] S3: The powder is sintered in an argon-hydrogen atmosphere to obtain the iron-based mixed phosphate cathode material.

[0019] According to one embodiment of the present invention, in step S1, the amount of ethanol added is controlled to maintain the solid content of the precursor material between 60% and 70%; the ball mill rotation speed of the wet ball mill is 200-500 r / min, and the ball milling time is 1-15 h, wherein the ball mill pauses for 1-5 min every 1-10 min of ball milling; in step S2, the preheating temperature is 200-400℃, and the preheating time is 1-15 h; in step S3, the sintering temperature is 400-600℃, and the sintering time is 1-15 h.

[0020] In another aspect, the present invention provides a positive electrode, comprising the aforementioned iron-based mixed phosphate positive electrode material or a positive electrode material prepared by the aforementioned method of preparing iron-based mixed phosphate positive electrode material or a positive electrode material prepared by the aforementioned method of preparing iron-based mixed phosphate positive electrode material.

[0021] Another aspect of the present invention provides a sodium-ion battery, comprising the aforementioned iron-based mixed phosphate cathode material, or a cathode material prepared by the aforementioned method of preparing iron-based mixed phosphate cathode material, or the aforementioned cathode.

[0022] Another aspect of the present invention provides an electrical device including the aforementioned sodium-ion battery.

[0023] Beneficial effects: The iron-based mixed phosphate cathode material of the present invention includes: polyacrylic acid and sodium source, iron source, phosphorus source and carbon source in stoichiometric ratio, wherein the mass ratio of polyacrylic acid to carbon source is 1% to 30% and the carbon content in the iron-based mixed phosphate cathode material is 0.5% to 10%; polyacrylic acid, as a functional additive, combines with carbon source to form carbon coating layer of iron-based mixed phosphate cathode material. By utilizing the aggregation effect of polyacrylic acid and the role of forming cross-linking template, iron-based mixed phosphate cathode material with high tap density, uniform carbon coating layer and good conductivity can be obtained. Attached Figure Description

[0024] Figure 1 This is a schematic flowchart of a method for preparing an iron-based mixed phosphate cathode material according to an embodiment of the present invention.

[0025] Figure 2 This is a schematic flowchart of a method for preparing an iron-based mixed phosphate cathode material according to another embodiment of the present invention.

[0026] Figure 3These are scanning electron microscope (SEM) images of the iron-based mixed phosphate cathode materials prepared in Example 1 and Comparative Example 1 of the present invention; wherein, (a) is a scanning electron microscope image of Comparative Example 1 and (b) is a scanning electron microscope image of Example 1.

[0027] Figure 4 This is an X-ray diffraction pattern of the iron-based mixed phosphate cathode material prepared in Example 1 of the present invention.

[0028] Figure 5 The iron-based mixed phosphate cathode material prepared in Example 1 of this invention was tested in a sodium-ion coin cell at 0.1C (10 mA). The charge-discharge curves for the first 3 cycles of the next cycle.

[0029] Figure 6 Rate performance of the iron-based mixed phosphate cathode material prepared in Example 1 of this invention in a sodium-ion coin cell.

[0030] Figure 7 Cyclic stability of the iron-based mixed phosphate cathode material prepared in Example 1 of this invention at 20C in a sodium-ion coin cell. Detailed Implementation

[0031] The technical solution of the present invention will be illustrated below through specific embodiments. All raw materials and reagents used in the present invention are commercially available.

[0032] This invention discloses an iron-based mixed phosphate cathode material, comprising: polyacrylic acid and sodium, iron, phosphorus, and carbon sources in stoichiometric ratios. The mass ratio of polyacrylic acid to carbon source is 1% to 30%, and the carbon content in the iron-based mixed phosphate cathode material is 0.5% to 10%. Preferably, the mass ratio of polyacrylic acid to carbon source is 5% to 20%.

[0033] In some feasible embodiments, the sodium source includes one or more of sodium carbonate, sodium chloride, sodium nitrate, sodium acetate, sodium hydroxide, disodium hydrogen phosphate, and sodium dihydrogen phosphate. Preferably, the sodium source is sodium hydroxide or sodium carbonate.

[0034] In some feasible implementations, the iron source includes one or more of ferric oxalate, ferric phosphate, and ferric nitrate. Preferably, the iron source is ferric phosphate or ferric nitrate.

[0035] In some feasible implementations, the phosphorus source includes one or more of phosphoric acid, disodium hydrogen phosphate, and sodium dihydrogen phosphate. Preferably, disodium hydrogen phosphate is selected as the phosphorus source.

[0036] In some feasible implementations, the carbon source includes one or more of tartaric acid, citric acid, malic acid, glucose, acetic acid, and tannic acid. Preferably, the carbon source is tartaric acid, citric acid, or glucose.

[0037] The iron-based mixed phosphate cathode material of this invention uses polyacrylic acid as a functional additive, and combines it with a low-cost carbon source to form a carbon coating layer of the iron-based mixed phosphate cathode material. By utilizing the aggregation effect of polyacrylic acid and its role in forming a cross-linking template, an iron-based mixed phosphate cathode material with high tap density, uniform carbon coating layer and good conductivity can be obtained.

[0038] Related carbon coating technologies are prone to structural collapse during the sintering process of precursor materials, leading to problems such as irregular particle morphology and uneven coating (e.g.) Figure 3 (As shown in a), this reduces the tap density and conductivity of the material, ultimately lowering its energy density and affecting its practical applications. The following presents two preparation methods that can significantly improve the tap density and conductivity of iron-based mixed phosphate cathode materials.

[0039] The first method is the spray drying synthesis route; please refer to [link / reference]. Figure 1 The present invention discloses a method for preparing an iron-based mixed phosphate cathode material, comprising the following steps:

[0040] S1: Add polyacrylic acid and sodium, iron, phosphorus and carbon sources in stoichiometric ratio to deionized water and stir until uniformly mixed to obtain spray slurry; wherein, the mass ratio of polyacrylic acid to carbon source is 1%~30%.

[0041] In step S1, the amount of deionized water added is used to control the solid content of the spray slurry to be between 5% and 50%. The stirring time is 1 to 15 hours. Preferably, the mass ratio of polyacrylic acid to carbon source is 5% to 20%. The stirring time is 10 hours.

[0042] Furthermore, after the step of stirring until uniformly mixed, the method further includes: adding oxalic acid as a chelating agent and pH control agent to the mixed solution and stirring at a stirring rate of 200-500 r / min for 1-15 h to control the pH of the spray slurry between 1 and 5. Preferably, the stirring rate is 400 r / min and the stirring time is 12 h to control the pH of the spray slurry between 1 and 3.

[0043] S2: Spray dry the spray slurry to obtain the precursor material.

[0044] In step S2, the nozzle temperature for spray drying is 100~300℃, and the peristaltic pump rate is controlled between 10%~50%. Preferably, the nozzle temperature for spray drying is 250℃, and the peristaltic pump rate is controlled between 10%~20%. During the spray drying process, polyacrylic acid, as a functional additive, utilizes its carboxyl functional groups and polymer chain properties to exert a binding and pelletizing effect, promoting the aggregation of precursor particles and forming highly spherical particles.

[0045] S3: The precursor material is sintered in an argon-hydrogen atmosphere to obtain an iron-based mixed phosphate cathode material.

[0046] In step S3, the argon-hydrogen atmosphere comprises 95% argon and 5% hydrogen, the sintering temperature is 400~600℃, and the sintering time is 1~15h. Preferably, the sintering temperature is 520℃ and the sintering time is 8h.

[0047] During the sintering process, in the low-temperature range (200 ~ 400 ºC) of sintering heating, the polyacrylic acid molecular chains dehydrate to form cross-linked polymeric anhydrides, which interact with the carboxyl groups of the carbon source to form a three-dimensional network structure, acting as a rigid template to maintain the integrity of the spherical structure. In the high-temperature carbonization / sintering stage (400 ~ 600 ºC), the polyacrylic acid template thermally decomposes and escapes, and the carbon source pyrolyzes into a conductive carbon layer that uniformly coats the particle surface. Thanks to the support of the rigid template, the spherical particles undergo isotropic shrinkage, retaining high sphericity to the maximum extent, significantly improving the tap density of the material and the uniformity of the carbon coating layer.

[0048] The preparation method of the iron-based mixed phosphate cathode material in this invention introduces polyacrylic acid as a multifunctional additive, combines it with a low-cost carbon source as a carbon precursor, and uses spray drying granulation to form a material coating layer and a spherical precursor with uniform material, robust structure, and excellent conductivity. This improves the conductivity of the iron-based mixed phosphate cathode material while ensuring that the material retains a micron-level spherical morphology after sintering (e.g., ...). Figure 3 (As shown in b), it effectively improves the tap density, carbon coating uniformity, rate performance, and cycle stability of iron-based mixed phosphate cathode materials.

[0049] Furthermore, the raw materials used in the preparation method of this iron-based mixed phosphate cathode material are abundant, inexpensive, and environmentally friendly. In addition, this preparation method not only yields iron-based mixed phosphate cathode materials with uniform composition and coating thickness, but also facilitates large-scale industrial production.

[0050] The second method is a solid-state reaction synthesis route; please refer to [link / reference]. Figure 2 The present invention discloses a method for preparing an iron-based mixed phosphate cathode material, comprising the following steps:

[0051] S1: Polyacrylic acid and sodium, iron, phosphorus and carbon sources in stoichiometric ratios are added to a ball mill jar, ethanol is added and wet ball milling is performed in an inert atmosphere to obtain the precursor material; wherein, the mass ratio of polyacrylic acid to carbon source is 1%~30%.

[0052] In step S1, the amount of ethanol added is such that the solid content of the precursor material is controlled between 60% and 70%. The ball mill speed for wet ball milling is 200-500 r / min, and the milling time is 1-15 h, wherein the ball mill rests for 1-5 min every 1-10 min of milling. Preferably, the mass ratio of polyacrylic acid to carbon source is 5%-20%. In another step, the ball mill speed for wet ball milling is 400 r / min, and the milling time is 10 h, wherein the ball mill rests for 1 min every 5 min of milling.

[0053] S2: Preheat the precursor material in an argon-hydrogen atmosphere, remove it and grind it until it is evenly mixed to obtain powder.

[0054] In step S2, the argon-hydrogen atmosphere comprises 95% argon and 5% hydrogen. A tube furnace is selected as the preheating equipment, with a preheating temperature of 100-400°C and a preheating time of 1-15 hours. Preferably, the preheating temperature is 300°C and the preheating time is 6 hours. Specifically, the precursor material is placed in the tube furnace, and the furnace is evacuated and circulated three times to remove air. An argon-hydrogen mixture is then introduced, and the furnace is preheated at 300°C for 6 hours. The mixture is then removed and ground until homogeneous to obtain powder.

[0055] S3: The powder is sintered in an argon-hydrogen atmosphere to obtain an iron-based mixed phosphate cathode material.

[0056] In step S3, the argon-hydrogen atmosphere comprises 95% argon and 5% hydrogen. A tube furnace is selected as the sintering equipment, with a sintering temperature of 400-600℃ and a sintering time of 1-15 hours. Preferably, the sintering temperature is 520℃ and the sintering time is 8 hours. Specifically, the powder is placed in the tube furnace, and the furnace is evacuated and circulated three times to remove air. Then, an argon-hydrogen mixed gas is introduced, and sintering is performed at 520℃ for 8 hours to obtain the iron-based mixed phosphate cathode material.

[0057] The method for preparing iron-based mixed phosphate cathode material in this invention introduces polyacrylic acid as a multifunctional additive and combines it with a low-cost carbon source as a carbon precursor to form a material coating layer with uniform material, robust structure, and excellent conductivity. This effectively improves the tap density, carbon coating uniformity, rate performance, and cycle stability of the iron-based mixed phosphate cathode material.

[0058] Furthermore, the raw materials used in the preparation method of this iron-based mixed phosphate cathode material are abundant, inexpensive, and environmentally friendly. In addition, this preparation method not only yields iron-based mixed phosphate cathode materials with uniform composition and coating thickness, but also features a simple process that is easily scalable for large-scale industrial production.

[0059] Example 1

[0060] This embodiment discloses a method for preparing an iron-based mixed phosphate cathode material, comprising the following steps: adding polyacrylic acid and FePO4, NaOH, Na2HPO4 and a carbon source in stoichiometric ratio to deionized water, mixing and stirring for 10 hours to obtain a spray slurry; wherein the mass ratio of polyacrylic acid to carbon source is 15%; spray drying the spray slurry, with the nozzle temperature at 250°C and the peristaltic pump controlled between 10% and 20%, to obtain a precursor material after spraying; evacuating and purging the tubular furnace with argon and hydrogen gas three times to completely remove the air, placing the precursor material in the tubular furnace for sintering at 520°C for 8 hours to obtain the iron-based mixed phosphate cathode material.

[0061] This embodiment also discloses a positive electrode, including the iron-based mixed phosphate positive electrode material prepared in this embodiment.

[0062] This embodiment also discloses a sodium-ion battery, including the iron-based mixed phosphate cathode material prepared in this embodiment or the cathode of this embodiment.

[0063] Example 2

[0064] This embodiment discloses a method for preparing an iron-based mixed phosphate cathode material. The only difference between this embodiment and Example 1 is that the mass ratio of polyacrylic acid to carbon source is 20%.

[0065] This embodiment also discloses a positive electrode, including the iron-based mixed phosphate positive electrode material prepared in this embodiment.

[0066] This embodiment also discloses a sodium-ion battery, including the iron-based mixed phosphate cathode material prepared in this embodiment or the cathode of this embodiment.

[0067] Example 3

[0068] This embodiment discloses a method for preparing an iron-based mixed phosphate cathode material. The only difference between this embodiment and Example 1 is that the mass ratio of polyacrylic acid to carbon source is 10%.

[0069] This embodiment also discloses a positive electrode, including the iron-based mixed phosphate positive electrode material prepared in this embodiment.

[0070] This embodiment also discloses a sodium-ion battery, including the iron-based mixed phosphate cathode material prepared in this embodiment or the cathode of this embodiment.

[0071] Example 4

[0072] This embodiment discloses a method for preparing an iron-based mixed phosphate cathode material. The only difference between this embodiment and Example 1 is that the mass ratio of polyacrylic acid to carbon source is 5%.

[0073] This embodiment also discloses a positive electrode, including the iron-based mixed phosphate positive electrode material prepared in this embodiment.

[0074] This embodiment also discloses a sodium-ion battery, including the iron-based mixed phosphate cathode material prepared in this embodiment or the cathode of this embodiment.

[0075] Example 5

[0076] This embodiment discloses a method for preparing an iron-based mixed phosphate cathode material, comprising the following steps: adding polyacrylic acid and Na2CO3, Fe(NO3)3, NaH2PO4, NH4H2PO4 and a carbon source in a stoichiometric ratio into a ball mill jar in a glove box, adding ethanol and performing wet ball milling in an inert atmosphere to obtain a precursor material; wherein, the mass ratio of polyacrylic acid to carbon source is 15%, the ball mill speed is 400 r / min, the running time is 10 h, and there is a 1 min pause every 5 min of grinding; the precursor material is placed in a tube furnace with an argon-hydrogen mixture and preheated at 300 °C for 6 h, taken out and ground evenly, and then placed back into the tube furnace with an argon-hydrogen mixture and sintered at 520 °C for 8 h to obtain the iron-based mixed phosphate cathode material.

[0077] This embodiment also discloses a positive electrode, including the iron-based mixed phosphate positive electrode material prepared in this embodiment.

[0078] This embodiment also discloses a sodium-ion battery, including the iron-based mixed phosphate cathode material prepared in this embodiment or the cathode of this embodiment.

[0079] Comparative Example 1

[0080] This embodiment discloses a method for preparing an iron-based mixed phosphate cathode material. The only difference between this embodiment and Example 1 is that polyacrylic acid is not added in this embodiment.

[0081] This embodiment also discloses a positive electrode, including the iron-based mixed phosphate positive electrode material prepared in this embodiment.

[0082] This embodiment also discloses a sodium-ion battery, including the iron-based mixed phosphate cathode material prepared in this embodiment or the cathode of this embodiment.

[0083] test

[0084] (1) The iron-based mixed phosphate cathode materials prepared in Example 1 and Comparative Example 1 were subjected to scanning electron microscopy (SEM) tests, and the morphology images are shown below. Figure 3 As shown; where, Figure 3 a is a comparative topographic diagram. Figure 3 b is a morphological diagram of Example 1.

[0085] (2) The iron-based mixed phosphate cathode material prepared in Example 1 was subjected to XRD testing, and the test results are as follows: Figure 4 As shown.

[0086] (3) The tap density of the iron-based mixed phosphate cathode materials prepared in Examples 1-5 and Comparative Example 1 was tested respectively. The test results are shown in Table 1.

[0087] (4) Electrochemical performance tests were performed on the sodium-ion batteries prepared in Examples 1-5 and Comparative Example 1, including:

[0088] The prepared positive electrode sheet was combined with metallic sodium (approximately 1 mm thick) to form a CR2032 button cell in an Ar-filled glove box. The separator was Whatman GF / D glass fiber, the electrolyte was a 1 mol / L NaPF6 solution, and the solvent was EC (ethylene carbonate) / diethyl carbonate (DEC) (v:v = 1:1), containing 5 vol% fluoroethylene carbonate (FEC) additive.

[0089] The obtained CR2032 button cell was subjected to constant current charge-discharge experiments on the Xinwei battery testing system, ranging from 1.7 to 4V. The test results are shown in Table 1; among them, the iron-based mixed phosphate cathode material prepared in Example 1 was tested in a sodium-ion button half-cell at 0.1C (10 mA). The charge / discharge curves for the first three cycles are as follows: Figure 5 As shown; the rate performance of the iron-based mixed phosphate cathode material prepared in Example 1 in a sodium-ion coin cell is as follows. Figure 6 As shown; the cycle stability of the iron-based mixed phosphate cathode material prepared in Example 1 at 20C in a sodium-ion coin half-cell is as follows. Figure 7 As shown.

[0090] Table 1: Test results of tap density and electrochemical performance of Examples 1-5 and Comparative Example 1.

[0091]

[0092] Table 1 shows that in the spray-drying synthesis route, when the polyacrylic acid content is 15%, the resulting iron-based mixed phosphate cathode material exhibits excellent rate performance, cycle performance, and high tap density, achieving synergistic optimization of rate performance (20C capacity retention > 85%) and cycle life (capacity retention close to 99% after 1000 cycles). In the solid-state reaction synthesis route, when the polyacrylic acid content is 15%, the resulting iron-based mixed phosphate cathode material has similar electrochemical performance to that obtained by the spray-drying synthesis route, but the morphology is different, resulting in different tap densities.

[0093] Combination Figure 3 As shown in Table 1 (see Example 1 and Comparative Example 1), the addition of polyacrylic acid in Example 1 resulted in an iron-based mixed phosphate cathode material that maintained a good spherical morphology. Figure 3 As shown in b; Comparative Example 1, without the addition of polyacrylic acid, produced an iron-based mixed phosphate cathode material with irregular morphology, such as... Figure 3 As shown in Figure a, the added polyacrylic acid can prevent the destruction of the material morphology during sintering, avoid structural collapse, and effectively improve the tap density, carbon coating uniformity, rate performance, and cycle stability of the iron-based mixed phosphate cathode material.

[0094] The above are merely embodiments of the present invention and do not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing an iron-based mixed phosphate cathode material, characterized in that, Includes the following steps: S1: Polyacrylic acid and sodium, iron, phosphorus and carbon sources in stoichiometric ratios are added to deionized water and stirred until uniformly mixed to obtain a spray slurry; wherein the mass ratio of the polyacrylic acid to the carbon source is 1%~30%; S2: The spray slurry is spray-dried to obtain a precursor material; S3: The precursor material is sintered in an argon-hydrogen atmosphere to obtain the iron-based mixed phosphate cathode material; In step S1, after stirring until uniformly mixed, the process further includes adding oxalic acid and stirring at a speed of 200-500 r / min for 1-15 h to control the pH of the spray slurry between 1 and 5. In step S2, the nozzle temperature for spray drying is 100-300℃, and the peristaltic pump rate is controlled between 10% and 50%. In step S3, the sintering temperature is 400-600℃, and the sintering time is 1-15 h.

2. The method for preparing the iron-based mixed phosphate cathode material according to claim 1, characterized in that, In step S1, the amount of deionized water added is used to control the solid content of the spray slurry to be between 5% and 50%.

3. A method for preparing an iron-based mixed phosphate cathode material, characterized in that, Includes the following steps: S1: Polyacrylic acid and sodium, iron, phosphorus and carbon sources in stoichiometric ratios are added to a ball mill jar, ethanol is added and wet ball milling is performed in an inert atmosphere to obtain a precursor material; wherein the mass ratio of the polyacrylic acid to the carbon source is 1%~30%; S2: Preheat the precursor material in an argon-hydrogen atmosphere, remove it and grind it until it is uniformly mixed to obtain powder; S3: The powder is sintered in an argon-hydrogen atmosphere to obtain the iron-based mixed phosphate cathode material; In step S1, the amount of ethanol added is controlled to maintain the solid content of the precursor material between 60% and 70%; the ball mill speed in the wet ball milling is 200-500 r / min, and the ball milling time is 1-15 h, wherein the ball mill pauses for 1-5 min every 1-10 min of ball milling; in step S2, the preheating temperature is 200-400℃, and the preheating time is 1-15 h; in step S3, the sintering temperature is 400-600℃, and the sintering time is 1-15 h.

4. An iron-based mixed phosphate cathode material, characterized in that, The iron-based mixed phosphate cathode material is prepared by the preparation method of any one of claims 1 to 2 or by the preparation method of claim 3; the iron-based mixed phosphate cathode material comprises: polyacrylic acid and sodium source, iron source, phosphorus source and carbon source in stoichiometric ratio, wherein the mass ratio of polyacrylic acid to the carbon source is 1% to 30% and the carbon content in the iron-based mixed phosphate cathode material is 0.5% to 10%.

5. The iron-based mixed phosphate cathode material according to claim 4, characterized in that, The sodium source includes one or more of sodium carbonate, sodium chloride, sodium nitrate, sodium acetate, sodium hydroxide, disodium hydrogen phosphate, and sodium dihydrogen phosphate; the iron source includes one or more of ferric oxalate, ferric phosphate, and ferric nitrate; the phosphorus source includes one or more of phosphoric acid, disodium hydrogen phosphate, and sodium dihydrogen phosphate; and the carbon source includes one or more of tartaric acid, citric acid, malic acid, glucose, acetic acid, and tannic acid.

6. A positive electrode, characterized in that, Including the iron-based mixed phosphate cathode material as described in any one of claims 4 to 5.

7. A sodium-ion battery, characterized in that, Includes the positive electrode as described in claim 6.

8. An electrical appliance, characterized in that, Including the sodium-ion battery as described in claim 7.

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