Composite sodium ferric sulfate positive electrode material, sodium ion battery and preparation method

Abstract

The invention relates to the technical field of sodium ion battery positive electrode materials, in particular to a composite sodium ferric sulfate positive electrode material, a sodium ion battery and a preparation method. The preparation method comprises the following steps: preparing a group IIIA metal doped composite sodium ferric sulfate precursor; mixing with a solution containing a sodium source, an iron source and a phosphorus source; and performing staged heat treatment under a protective atmosphere to form a pyrophosphate ferric phosphate sodium coating layer. The particle size D50 of the composite sodium ferric sulfate positive electrode material is 3-5 [mu] m, and the composite sodium ferric sulfate positive electrode material can inhibit iron dissolution and enhance structural stability through bulk phase doping and surface coating cooperation, can prolong the cycle life of a battery and reduce gas production, and is suitable for a high-performance sodium ion battery.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery cathode material technology, specifically to a composite sodium iron sulfate cathode material, a sodium-ion battery, and a preparation method thereof. Background Technology

[0002] Sodium-ion battery technology, proposed in the 1980s, has seen slow development. However, in recent years, with increasing demand for renewable energy storage and concerns about lithium supply and cost, sodium-ion batteries have regained attention. Due to their low cost, high safety, and excellent high and low temperature performance, sodium-ion batteries have become a powerful complement to lithium-ion batteries, leading to continuous market demand growth and making them a strong energy storage technology following lithium-ion batteries. They have been applied in numerous fields. Currently, the most researched cathode materials for sodium-ion batteries include transition metal layered oxides, Prussian blue analogs, and polyanionic compounds. Among these, polyanionic materials have advantages such as good cycle stability and excellent thermal stability compared to other types of materials. Polyanionic cathode materials not only have a high potential plateau but also a relatively stable structure, ensuring that complex phase transitions do not occur during cycling or voltage decay. Na₂Fe₂(SO₄)₃ is particularly prominent, possessing a high voltage plateau of 3.8V and relying solely on Fe. 2+ / Fe 3+ As a redox center, and Na + Ionic portions occupy Fe 2+ The lattice positions of the ions give the material excellent sodium ion transport channels; in Na + During the ion insertion / extraction process, the volume change of the material is only 1.6%, which gives Na2Fe2(SO4)3 a longer cycle life. Compared with traditional cathode materials such as nickel-cobalt-manganese oxides, sodium iron sulfate has a relatively simple synthesis process, lower production costs, and better environmental friendliness, making it suitable for large-scale production.

[0003] However, despite its many advantages, sodium ferric sulfate also has significant drawbacks. It is extremely sensitive to humidity and oxidizing atmospheres, readily absorbing moisture and deteriorating, and is readily oxidized to Fe. 3+ This type of iron-based sulfate, under high voltage, has a charging voltage close to 4.2-4.5V vs Na. + / Na, because of Na + When all the iron is extracted, the crystal structure deforms, which can easily lead to structural collapse and Fe... 3+ Leaching causes a loss of the capacity of the active material. The leached Fe... 3+Iron can also migrate to the negative electrode and be reduced to form elemental iron, clogging the hard carbon pores of the negative electrode and catalyzing the decomposition of the electrolyte. This leads to increased gas production and higher battery impedance, resulting in a significant decrease in battery performance and severe capacity decay. Therefore, there is an urgent need for effective methods to alleviate the instability of iron-based sulfates in air and the reduced cycle life caused by iron dissolution at the positive electrode under high voltage.

[0004] While conventional techniques like carbon coating can improve the conductivity of sodium ferric sulfate, their effects on inhibiting transition metal dissolution and stabilizing the bulk structure are limited, and single-doping methods are insufficient to improve interfacial stability. Therefore, a comprehensive solution that can simultaneously address both bulk and interfacial issues is needed. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention aims to provide a composite sodium iron sulfate cathode material, a sodium-ion battery, and a preparation method that reduces iron leaching through a combination of bulk doping and surface coating. This composite sodium iron sulfate cathode material exhibits high capacity, long cycle life, and excellent rate performance.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a method for preparing a composite sodium ferric sulfate cathode material, comprising the following steps: Preparation of Group IIIA metal-doped composite sodium ferric sulfate precursor; The composite sodium ferric sulfate precursor is mixed with a solution containing sodium, iron, and phosphorus sources to form a homogeneous mixture; Under a protective atmosphere, the mixture is heat-treated to react the sodium, iron, and phosphorus sources on the surface of the composite sodium ferric sulfate precursor to form a sodium ferric phosphate pyrophosphate (NFPP) coating layer.

[0007] In the above scheme, a two-step core process of group IIIA metal-doped precursor + surface coating with sodium iron phosphate pyrophosphate (NFPP) achieves a synergistic effect of bulk structure stability and interface protection: both by alleviating the structural stress of sodium ion intercalation / deintercalation through bulk doping and reducing Fe... 2+ Oxidation, along with the inhibition of iron ion dissolution and diffusion through a surface coating layer, fundamentally solves the problems of sodium iron sulfate's easy water absorption and deterioration, as well as its structural collapse under high voltage. The resulting material possesses high capacity, long cycle life, and excellent rate performance, providing a reliable preparation path for high-performance sodium-ion battery cathode materials.

[0008] As a preferred embodiment of the present invention, the Group IIIA metal includes at least one of gallium, indium, and thallium. Gallium or indium is preferred, and gallium is more preferred. The Group IIIA metal salt may be selected from, but is not limited to, gallium nitrate, gallium chloride, gallium bromide, gallium hydroxide, or gallium acetate, preferably gallium nitrate or gallium acetate, and most preferably gallium acetate.

[0009] As a preferred technical solution of the present invention, the preparation process of the composite sodium ferric sulfate precursor includes: uniformly dispersing sodium source, iron source, sulfur source and group IIIA metal salt in water in a preset ratio to obtain a mixed solution; heating and spray drying the mixed solution to obtain dried particles; and heating the dried particles to a predetermined temperature and holding them at that temperature for a period of time in an inert atmosphere to obtain the composite sodium ferric sulfate precursor.

[0010] As a preferred embodiment of the present invention, the preparation of the composite sodium ferric sulfate precursor has at least one of the following features a1)-a9): a1) The molar ratio of the sodium source, iron source and sulfur source is 6-2x:x:3, where x = 1.5-2.0; a2) The mass ratio of the Group IIIA metal salt to the total mass of the sodium source, iron source and sulfur source is 0.1-0.5:10; a3) The median particle size D of the composite sodium ferric sulfate precursor 50 It is 4-6 μm; a4) The temperature of the heated spray drying is 150-250℃, and the spray rate is 150-250m. 3 / h; a5) The inert atmosphere includes at least one of argon, nitrogen, and helium; a6) The heating rate is 5-15℃ / min, the predetermined temperature is 350-500℃, and the holding time is 10-40h; a7) The sodium source includes any one of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium nitrate, sodium pyrophosphate, sodium citrate, sodium oxalate, and sodium acetate; a8) The iron source includes at least one of iron oxide, ferric nitrate, ferric phosphate, ferric oxide, ferrous carbonate, ferrous oxalate, ferrous acetate, and ferrous citrate; a9) The sulfur source includes at least one of ferrous sulfate, sodium sulfate, ammonium sulfate, and sodium bisulfate.

[0011] In the above scheme, the precursor is prepared by solution dispersion + heated spray drying + inert atmosphere sintering: spray drying can avoid ion segregation and ensure uniform distribution of sodium, iron, sulfur sources and Group IIIA metal salts; inert atmosphere sintering can prevent oxidation of raw materials (e.g., avoiding Fe oxidation). 2+ Oxidized to Fe3+ This ensures complete precursor crystallization and uniform doping, laying the foundation for the uniformity and effectiveness of subsequent surface coating and reducing material performance fluctuations caused by precursor inhomogeneity.

[0012] As a preferred embodiment of the present invention, the mass ratio of the composite sodium ferric sulfate precursor to the total mass of the sodium source, iron source and phosphorus source is 10:1-3, wherein the molar ratio of the sodium source, iron source and phosphorus source is (3-5):(2-4):(3-5).

[0013] In the above scheme, by limiting the total mass ratio of the precursor to sodium, iron, and phosphorus sources (10:1-3) and the molar ratio of coating materials (3-5:2-4:3-5), the thickness of the NFPP coating layer can be precisely controlled: avoiding a coating layer that is too thin (insufficient protection, easy leaching of iron) or too thick (hindering sodium ion transport and reducing rate performance), while ensuring that the coating materials react fully, the formed coating layer is tightly bonded to the precursor, and the material's cycle stability is improved.

[0014] As a preferred embodiment of the present invention, the sodium source includes any one of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium nitrate, sodium pyrophosphate, sodium citrate, sodium oxalate, and sodium acetate; the iron source includes at least one of iron oxide, ferric nitrate, ferric phosphate, ferric tetroxide, ferrous carbonate, ferrous oxalate, ferrous acetate, and ferrous citrate; and the phosphorus source includes any one of sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, and sodium trihydrogen pyrophosphate.

[0015] As a preferred embodiment of the present invention, the mixing process is carried out using a sand mill with a rotation speed of 500-3000 rpm and a milling time of 0.5-2 hours. The particle size D of the material after milling is... 50 It is 2-4 μm.

[0016] In the above scheme, by using a milling speed of 500-3000 rpm and a sand milling time of 0.5-2h, the particle size of the mixture is controlled at D50=2-4μm, which can ensure that the precursor and the coating material are in full contact and dispersed evenly, avoid uneven coating thickness caused by local aggregation of coating material, ensure that each precursor particle is covered by the NFPP layer, and eliminate the risk of iron leaching from uncoated areas.

[0017] As a preferred embodiment of the present invention, the protective atmosphere includes one or more of argon, nitrogen, and helium, and hydrogen is added to the protective atmosphere to form a reducing atmosphere, with a hydrogen content of 5-10%; the heat treatment adopts segmented sintering, first heating from room temperature to 250-400℃ at a heating rate of 1.5℃ / min and holding for 10-12h, then heating to 500-800℃ at a heating rate of 2.5℃ / min and holding for 15-24h.

[0018] In the above scheme, a protective atmosphere containing 5-10% hydrogen is used: hydrogen can form a reducing environment, preventing Fe from being absorbed during heat treatment. 2+ Oxidation is avoided, while preventing the formation of oxide impurities on the material surface; segmented sintering (low temperature holding for 10-12 hours + high temperature holding for 15-24 hours): the low temperature stage allows the coating material to be uniformly adsorbed on the precursor surface, and the high temperature stage ensures that the NFPP layer is fully crystallized and dense, avoiding cracking or peeling of the coating layer caused by one-step sintering, further improving the protective performance of the coating layer and reducing battery gas production.

[0019] Secondly, this invention provides a composite sodium ferric sulfate cathode material, which is prepared by the preparation method described above, and its particle size D 50 It is 3-5μm.

[0020] Thirdly, the present invention provides a sodium-ion battery, comprising a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode comprises the composite sodium iron sulfate positive electrode material as described above.

[0021] Compared with existing technologies, this invention consists of two core stages: precursor bulk doping and surface coating layer formation. These two stages, through precise control of process parameters, achieve a dual effect of lattice stabilization and interface protection, fundamentally suppressing iron dissolution and structural damage. Specific effects are analyzed below: Group IIIA (IUPAC Group 13) metals, preferably Ga and In, with their trivalent cations (Ga 3+ and In 3+ ), with a radius slightly larger than Fe 3+ Ga / In doping slightly expands the lattice parameters, alleviating structural stress during sodium ion insertion / extraction. Simultaneously, because the Ga / In-O bond energy is stronger than the Fe-O bond energy, doping enhances the chemical stability of the lattice framework, suppressing structural collapse or phase transitions during cycling. In electrochemical reactions, Ga / In doping helps stabilize Fe... 3+ / Fe 2+ Redox potential, reducing Fe 2+ Oxidized to Fe 3+ This reduces the amount of iron leached out.

[0022] Applying sodium iron phosphate pyrophosphate (Na4Fe2(P2O7)2) as a coating layer to the surface of sodium iron sulfate particles can effectively inhibit iron dissolution through the following mechanisms: (1) Sodium iron phosphate pyrophosphate has certain ionic conductivity and structural toughness, which can alleviate the stress caused by the volume change of the material during charging and discharging, and reduce the generation of microcracks. Microcracks are an important channel for iron ion dissolution, and reducing cracks helps to maintain the integrity of the material; (2) Pyrophosphate (P2O7)2 4- (3) Pyrophosphate is a strong coordinating anion that can react with Fe. It has strong thermal and chemical stability and is not easily hydrolyzed or reacted with acid. Compared with sulfate, pyrophosphate is less likely to produce an acidic environment, thus avoiding a local pH decrease and reducing the dissolution kinetics of iron. 2+ / Fe 3+ To form stable coordination structures (such as [Fe(P2O7)]). 2- The coating "locks" iron ions in the coating layer. Even if slight dissolution occurs at the interface, the iron ions trapped by the coating layer are difficult to diffuse further, thus inhibiting their migration. Detailed Implementation

[0023] To enable those skilled in the art to better understand the technical solutions of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples. However, these should not be construed as limiting the present invention and are merely examples.

[0024] Unless otherwise specified, the test methods or experimental methods described in the following examples are conventional methods; unless otherwise specified, the reagents and materials are obtained from conventional commercial sources or prepared by conventional methods.

[0025] Example 1 This embodiment provides a method for preparing a composite sodium ferric sulfate cathode material, including the following steps: (1) Sodium carbonate, ferric nitrate, ammonium sulfate and gallium acetate were uniformly dispersed in water to obtain a mixed solution; the mixed solution was spray-dried at 200°C with a spraying rate of 250 m / s. 3 / h, dry particles are obtained; in a nitrogen atmosphere, the dry particles are heated to 400℃ at a heating rate of 10℃ / min and held at that temperature for 35h to obtain a gallium-doped sodium ferric sulfate composite sodium ferric sulfate precursor material with a particle size of 4.5μm, wherein the molar ratio of sodium carbonate, ferric nitrate, and ammonium sulfate is 2:2:3, and the mass ratio of (sodium carbonate + ferric nitrate + ammonium sulfate) to gallium acetate is 10:0.3.

[0026] (2) The composite sodium ferric sulfate precursor, sodium carbonate, ferric nitrate, and ammonium dihydrogen phosphate were mixed and milled for 2 hours at a mill speed of 1200 rpm. The particle size of the mixed powder after milling was 3.2 μm. The molar ratio of sodium carbonate, ferric nitrate, and ammonium dihydrogen phosphate was 4:3:4. The mass ratio of the composite sodium ferric sulfate precursor to (sodium carbonate + ferric nitrate + ammonium dihydrogen phosphate) was 10:1.5.

[0027] (3) The milled powder was placed in a tube furnace for sintering to obtain NFPP-coated metal-doped composite sodium ferric sulfate material with a particle size D50 of 3.8 μm. The sintering process was carried out in two stages: the temperature was increased from room temperature to 350℃ at a rate of 1.5℃ / min and held for 12 h, and then increased to 700℃ at a rate of 2.5℃ / min and held for 20 h. The protective reducing atmosphere was a nitrogen-hydrogen mixture with a hydrogen content of 8%.

[0028] Example 2 This embodiment is basically the same as embodiment 1, except that the mass ratio of (sodium carbonate + ferric nitrate + ammonium sulfate) to gallium acetate in step (1) is 10:0.2.

[0029] Example 3 This embodiment is basically the same as embodiment 1, except that the mass ratio of (sodium carbonate + ferric nitrate + ammonium sulfate) to gallium acetate in step (1) is 10:0.4.

[0030] Example 4 This embodiment is basically the same as that of embodiment 1, except that the mass ratio of the composite sodium ferric sulfate precursor (sodium carbonate + ferric nitrate + ammonium dihydrogen phosphate) in step (2) is 10:1.0.

[0031] Example 5 This embodiment is basically the same as that of embodiment 1, except that the mass ratio of the composite sodium ferric sulfate precursor (sodium carbonate + ferric nitrate + ammonium dihydrogen phosphate) in step (2) is 10:2.0.

[0032] Comparative Example 1 This embodiment provides a method for preparing a sodium-ion battery cathode material, including: Sodium carbonate, ferric nitrate, ammonium sulfate, and gallium acetate were uniformly dispersed in water to obtain a mixed solution. The mixed solution was then spray-dried at 200°C at a spray rate of 250 m / s. 3 / h, dry particles are obtained; in a nitrogen atmosphere, the dry particles are heated to 400℃ at a heating rate of 10℃ / min and held at that temperature for 35h to obtain a gallium-doped sodium ferric sulfate composite sodium ferric sulfate precursor material with a particle size of 4.5μm, wherein the molar ratio of sodium carbonate, ferric nitrate, and ammonium sulfate is 2:2:3, and the mass ratio of (sodium carbonate + ferric nitrate + ammonium sulfate) to gallium acetate is 10:0.3.

[0033] Comparative Example 2 This embodiment provides a method for preparing a sodium-ion battery cathode material, including: Sodium carbonate, ferric nitrate, and ammonium sulfate were uniformly dispersed in water to obtain a mixed solution. The mixed solution was then spray-dried at 200°C with a spray rate of 250 m / s. 3 / h, dry particles are obtained; in a nitrogen atmosphere, the dry particles are heated to 400℃ at a heating rate of 10℃ / min and held at that temperature for 35h to obtain sodium ferric sulfate material with a particle size of 4.2μm, wherein the molar ratio of sodium carbonate, ferric nitrate and ammonium sulfate is 2:2:3.

[0034] Comparative Example 3 Comparative Example 3 is basically the same as Example 1, except that in step 3, only one heating stage is used, directly heating to 700°C at a heating rate of 2.5°C / min and holding for 32 hours.

[0035] Soft-pack battery manufacturing: Sodium-ion soft-pack batteries include positive electrode, negative electrode, electrolyte and separator.

[0036] (1) The positive electrode, conductive agent, and binder are mixed evenly in a specific ratio with N-methylpyrrolidone as solvent to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on aluminum foil by a coating machine, and then dried in an oven to obtain a coated electrode sheet, which is then rolled and die-cut to obtain a positive electrode sheet.

[0037] (2) Hard carbon, binder SBR (styrene-butadiene latex), thickener CMC (sodium carboxymethyl cellulose), and conductive agent are mixed evenly with deionized water as solvent to obtain a negative electrode slurry. The negative electrode slurry is evenly coated on aluminum foil by a coating machine, and then dried in an oven to obtain a coated electrode sheet. The negative electrode sheet is obtained by rolling and die cutting.

[0038] (3) The positive electrode, negative electrode and separator are stacked to obtain the core package, and then the electrode tabs are welded and aluminum-plastic film is used to encapsulate the cell to be injected with liquid.

[0039] (4) The cells are baked, injected with electrolyte, formed and tested for capacity to finally obtain sodium iron sulfate soft pack batteries.

[0040] High-temperature storage gas generation performance: After storing the battery cell at 60℃ for 0 and 30 days, record the thickness H1 of the battery cell before storage and the thickness H2 of the battery cell after storage. Record the gas expansion rate of the battery cell as (H2-H1) / H1*100%.

[0041] Iron leaching test: After discharging the soft-pack battery to 2.5V after 500 cycles, disassemble it and remove all electrolyte, negative electrode, and separator. Mix the clear solution obtained by digesting the negative electrode and separator with aqua regia and filtering it with the removed electrolyte. Measure the total iron concentration (Fe) using an ICP-OES instrument. 2+ and Fe 3+ The total iron dissolution amount is obtained by summing the concentrations of the iron. The iron dissolution amount is then converted into a relative mass fraction (μg / g) of the positive electrode active material, where the mass of the positive electrode active material is the mass of the active material on all the positive electrode plates of the soft pack before cycling.

[0042] Cyclic performance: The battery was tested at 25°C on an electrochemical workstation battery testing system. The test charge / discharge current density was 1C / 1C, and the charge / discharge voltage window was 2.5V-3.95V, as shown in Table 1.

[0043] Table 1 Based on the data in Table 1 regarding iron dissolution after 500 cycles, the number of cycles at 80% SOH, and the gas production expansion rate, and through Examples 1-5 and Comparative Examples 1-2, it can be seen that the present invention reduces iron dissolution from sodium ferric sulfate by combining gallium metal bulk doping with NFPP surface coating, thereby improving battery cycle life and reducing battery gas production. Referring to Examples 1-3, as the gallium metal doping amount increases, the iron dissolution amount and gas production expansion rate show a trend of first decreasing and then increasing, while the cycle life first increases and then decreases. This indicates that the gallium metal doping amount needs to be strictly controlled; too little content has no significant improvement effect, while too much content will increase excessive defects and structural changes, resulting in increased iron dissolution and gas production. Similarly, referring to Examples 1 and Examples 4-5, it is shown that the NFPP coating amount within a preferred range has superior performance.

[0044] A comparison of the performance data of Comparative Examples 1-3 and Example 1 shows that: Comparative Example 1, which only performed gallium doping without NFPP coating, had an iron dissolution rate more than three times that of Example 1 after 500 cycles, a cycle life of less than half that of Example 1 at 80% SOH, and a gas expansion rate more than three times that of Example 1. Therefore, bulk doping alone cannot effectively solve the problems of iron dissolution and interface stability; it needs to be combined with NFPP surface coating to achieve significant improvement. Comparative Example 2, without any doping or coating, had the worst iron dissolution rate, cycle life, and gas expansion rate among all the schemes, with performance far lower than Example 1 and Comparative Example 1. Therefore, bulk doping can fundamentally improve the structural stability of the material and reduce iron dissolution, which is an important prerequisite for improving battery performance. Without it, material defects and oxidation risks increase significantly. Comparative Example 3 uses a one-stage sintering process. The iron dissolution rate and gas production expansion rate are higher than those of Example 1, but the cycle life is lower than that of Example 1. This indicates that segmented sintering (low-temperature adsorption + high-temperature crystallization) can make the coating layer more dense and uniform, reduce the risk of cracking and falling off, and is more conducive to reducing gas production and iron dissolution and improving material stability compared with one-stage sintering.

[0045] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a composite sodium ferric sulfate cathode material, characterized in that, Includes the following steps: Preparation of Group IIIA metal-doped composite sodium ferric sulfate precursor; The composite sodium ferric sulfate precursor is mixed with a solution containing sodium, iron, and phosphorus sources to form a homogeneous mixture; Under a protective atmosphere, the mixture is heat-treated to cause the sodium, iron, and phosphorus sources to react on the surface of the composite sodium ferric sulfate precursor to form a sodium ferric phosphate pyrophosphate coating layer.

2. The preparation method according to claim 1, characterized in that, The Group IIIA metals include at least one of gallium, indium, and thallium.

3. The preparation method according to claim 1, characterized in that, The preparation process of the composite sodium ferric sulfate precursor includes: uniformly dispersing sodium source, iron source, sulfur source and group IIIA metal salt in water according to a preset ratio to obtain a mixed solution; heating and spray drying the mixed solution to obtain dried particles; and heating the dried particles to a predetermined temperature and holding them at that temperature for a period of time in an inert atmosphere to obtain the composite sodium ferric sulfate precursor.

4. The preparation method according to claim 3, characterized in that, The preparation of the composite sodium ferric sulfate precursor has at least one of the following characteristics a1)-a9): a1) The molar ratio of the sodium source, iron source and sulfur source is 6-2x:x:3, where x = 1.5-2.0; a2) The mass ratio of the Group IIIA metal salt to the total mass of the sodium source, iron source and sulfur source is 0.1-0.5:10; a3) The median particle size D of the composite sodium ferric sulfate precursor 50 It is 4-6 μm; a4) The temperature of the heated spray drying is 150-250℃, and the spray rate is 150-250m. 3 / h; a5) The inert atmosphere includes at least one of argon, nitrogen, and helium; a6) The heating rate is 5-15℃ / min, the predetermined temperature is 350-500℃, and the holding time is 10-40h; a7) The sodium source includes any one of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium nitrate, sodium pyrophosphate, sodium citrate, sodium oxalate, and sodium acetate; a8) The iron source includes at least one of iron oxide, ferric nitrate, ferric phosphate, ferric oxide, ferrous carbonate, ferrous oxalate, ferrous acetate, and ferrous citrate; a9) The sulfur source includes at least one of ferrous sulfate, sodium sulfate, ammonium sulfate, and sodium bisulfate.

5. The preparation method according to claim 1, characterized in that, The mass ratio of the composite sodium ferric sulfate precursor to the total mass of the sodium source, iron source and phosphorus source is 10:1-3, wherein the molar ratio of the sodium source, iron source and phosphorus source is (3-5):(2-4):(3-5).

6. The preparation method according to claim 5, characterized in that, The sodium source includes any one of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium nitrate, sodium pyrophosphate, sodium citrate, sodium oxalate, and sodium acetate; the iron source includes at least one of iron oxide, ferric nitrate, ferric phosphate, ferric tetroxide, ferrous carbonate, ferrous oxalate, ferrous acetate, and ferrous citrate; the phosphorus source includes any one of sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, and sodium trihydrogen pyrophosphate.

7. The preparation method according to claim 1, characterized in that, The mixing process is carried out using a sand mill at a speed of 500-3000 rpm for 0.5-2 hours. The particle size D of the material after sand milling is... 50 It is 2-4 μm.

8. The preparation method according to claim 1, characterized in that, The protective atmosphere includes one or more of argon, nitrogen, and helium. Hydrogen is added to the protective atmosphere to form a reducing atmosphere, and the hydrogen content is 5-10%. The heat treatment adopts segmented sintering. First, the temperature is raised from room temperature to 250-400℃ at a heating rate of 1.5℃ / min and held for 10-12 hours. Then, the temperature is raised to 500-800℃ at a heating rate of 2.5℃ / min and held for 15-24 hours.

9. A composite sodium ferric sulfate cathode material, characterized in that, The composite sodium ferric sulfate cathode material is prepared by the preparation method according to any one of claims 1-8, and its particle size D 50 It is 3-5μm.

10. A sodium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, an electrolyte, and a separator, wherein the positive electrode comprises the composite sodium ferric sulfate positive electrode material as described in claim 9.

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