Sodium ferric sulfate positive electrode material as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of positive electrode materials, and particularly relates to a sodium ferric sulfate positive electrode material and a preparation method and application thereof. The preparation method comprises the following steps: (1) preparing a cobalt-doped pre-product; and (2) mixing the cobalt-doped pre-product with a zinc source, and sintering to prepare the sodium ferric sulfate positive electrode material. The sodium ferric sulfate positive electrode material prepared by the invention is high in specific discharge capacity, and good in rate capability and capacity retention rate.

Description

Technical Field

[0001] The invention belongs to the technical field of positive electrode materials, and specifically relates to a sodium iron sulfate positive electrode material and a preparation method and application thereof. Background Art

[0002] As the most promising sustainable energy source, lithium-ion batteries have the advantages of high safety, high energy density, and long service life, and are widely used in various portable mobile devices, electric vehicles, and energy storage. However, due to the high cost and extremely uneven distribution of lithium resources, sodium-ion batteries have attracted new attention due to their advantages such as wide distribution of sodium resources and low cost.

[0003] Indicators such as the cycle stability, capacity, and power density of the positive electrode material determine the performance of the battery. The positive electrode materials in sodium-ion batteries mainly include layered oxides, polyanionic compounds, and Prussian blue. Among them, polyanionic compounds have attracted much attention due to their open skeleton structure, low energy of ion migration pathways, and adjustable voltage range. Polyanionic sodium iron sulfate positive electrode materials have the advantages of low cost and higher operating voltage, but they also have disadvantages such as poor conductivity, easy water absorption, and easy oxidation failure, which results in the material's performance not being able to be brought into play. Summary of the invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the defects of the polyanionic sodium ferric sulfate positive electrode material in the prior art, such as poor conductivity, easy water absorption, easy reaction with air to cause oxidation failure, resulting in a low cycle life of the positive electrode material, thereby providing a sodium ferric sulfate positive electrode material and a preparation method and application thereof.

[0005] To this end, the present invention provides the following technical solutions.

[0006] The present invention provides a method for preparing a sodium iron sulfate positive electrode material, comprising the following steps:

[0007] (1) preparing a cobalt-doped pre-product;

[0008] (2) Mixing the cobalt-doped pre-product and a zinc source, and sintering them to obtain a sodium iron sulfate positive electrode material.

[0009] In an optional embodiment, the zinc source includes at least one of zinc oxide, zinc sulfate, zinc nitrate, zinc chloride, zinc carbonate, zinc acetate, zinc propionate and basic zinc carbonate;

[0010] In an optional embodiment, based on the mass of the cobalt-doped pre-product, the added amount of the zinc source is 500-20000 ppm.

[0011] In an alternative embodiment, the preparation steps of the cobalt-doped pre-product include: mixing a sodium source, an iron source, a cobalt source, an antioxidant, an organic carbon source, an inorganic carbon source, and a dispersant, stirring, and spray drying.

[0012] In an alternative embodiment, the inlet air temperature of the spray drying is 140 - 280 °C;

[0013] In an alternative embodiment, the outlet air temperature of the spray drying is 70 - 140 °C;

[0014] In an alternative embodiment, the ratio of the molar amount of sodium element in the sodium source to the sum of the molar amounts of iron element in the iron source and cobalt element in the cobalt source is 2:(1 - 2);

[0015] In an alternative embodiment, the molar ratio of iron element in the iron source to cobalt element in the cobalt source is (4 - 100):1;

[0016] In an alternative embodiment, based on the sum of the masses of the sodium source and the iron source, the mass of the antioxidant is 0.04 - 6 wt%;

[0017] In an alternative embodiment, based on the sum of the masses of the sodium source and the iron source, the mass of the organic carbon source is 0.01 - 12 wt%;

[0018] In an alternative embodiment, based on the sum of the masses of the sodium source and the iron source, the mass of the inorganic carbon source is 0.05 - 6 wt%;

[0019] In an alternative embodiment, based on the sum of the masses of the sodium source and the iron source, the mass of the dispersant is 0.05 - 3 wt%.

[0020] In an alternative embodiment, the sodium source includes sodium sulfate;

[0021] Preferably, the sodium sulfate is anhydrous sodium sulfate;

[0022] In an alternative embodiment, the iron source includes ferrous sulfate;

[0023] Preferably, the ferrous sulfate is ferrous sulfate heptahydrate;

[0024] In an alternative embodiment, the cobalt source includes at least one of cobalt sulfate, cobalt oxalate, cobalt phosphate, cobalt carbonate, cobalt chloride, cobalt silicate, and cobalt nitrate;

[0025] In an alternative embodiment, the antioxidant includes at least one of ascorbic acid, citric acid, phytic acid, uric acid, oxalic acid, formic acid, acetic acid, and benzoic acid;

[0026] In an alternative embodiment, the organic carbon source includes at least one of glucose, sucrose, starch, fructose, and stearic acid;

[0027] In an alternative embodiment, the inorganic carbon source includes at least one of acetylene black, graphene, carbon nanotubes, carbon fibers, activated carbon, and conductive carbon black;

[0028] In an alternative embodiment, the dispersant includes at least one of sodium dodecyl sulfate, polyethylene glycol, sodium alkylbenzene sulfonate, sodium alkyl sulfonate, sodium stearate, and polyvinylpyrrolidone.

[0029] In an alternative embodiment, in the preparation of the cobalt-doped pre-product, an antifoaming agent is further added during the mixing;

[0030] Preferably, the antifoaming agent includes at least one of hydroxyethyl cellulose, fatty alcohols, fatty acid esters, soybean oil, and corn oil;

[0031] Preferably, based on the total mass of the sodium source, iron source, cobalt source, antioxidant, organic carbon source, inorganic carbon source, and dispersant, the addition amount of the antifoaming agent is 1 - 100 ppm.

[0032] In an alternative embodiment, a solvent is further added during the stirring, and a spray liquid is obtained after stirring, and the solid content of the spray liquid is 15 - 40%;

[0033] In an alternative embodiment, the solvent includes water;

[0034] In an alternative embodiment, the rotation speed of the stirring is 100 - 400 r / min;

[0035] In an alternative embodiment, the stirring time is 2 - 3 h.

[0036] In an alternative embodiment, the sintering is carried out under a protective atmosphere;

[0037] Preferably, the protective atmosphere includes at least one of nitrogen and argon;

[0038] Preferably, the sintering includes a first sintering and a second sintering;

[0039] Preferably, the temperature of the first sintering is 100 - 300 °C;

[0040] Preferably, the time of the first sintering is 0.1 - 12 h;

[0041] Preferably, the heating rate of the first sintering is 2 - 5 °C / min;

[0042] Preferably, the temperature of the second sintering is 300 - 400 °C;

[0043] Preferably, the time of the second sintering is 0.1 - 24 h;

[0044] Preferably, the heating rate of the second sintering is 2 - 5 °C / min.

[0045] The present invention also provides a sodium iron sulfate cathode material prepared by the above preparation method.

[0046] In common polyanionic sodium iron sulfate cathode materials, the sulfate is extremely easy to absorb water, which makes the discharge specific capacity, rate performance, capacity retention rate, etc. of the cathode material deteriorate; in the present invention, a cobalt-doped cathode material with a carbon composite structure is used as the core, and a zinc coating layer is coated on the outside of the core; the carbon composite structure and the zinc coating layer have a certain isolation effect on moisture, and water molecules are not easy to contact with the cathode material, making the conductivity and cycle life of the prepared cathode material better.

[0047] The present invention also provides an application of the sodium iron sulfate cathode material prepared by the above preparation method in a sodium-ion battery.

[0048] The technical solution of the present invention has the following advantages:

[0049] 1. The preparation method of the sodium iron sulfate cathode material provided by the present invention includes the following steps: (1) preparing a cobalt-doped pre-product; (2) mixing the cobalt-doped pre-product and a zinc source, and sintering to obtain the sodium iron sulfate cathode material. The sodium iron sulfate cathode material prepared by the present invention has a high discharge specific capacity, good rate performance, and good capacity retention rate. Specifically, cobalt doping introduces more electron conduction channels into the cathode material, improving the conductivity and electron conduction performance of the sodium iron sulfate cathode material; at least one of iron atoms, oxygen atoms, sodium atoms, or sulfur atoms can form chemical bonds with cobalt atoms, enhancing the structural stability of the cathode material, inhibiting the volume expansion of the cathode material during charge and discharge, making the cycle performance of the cathode material better, and extending the battery cycle life.

[0050] The sodium iron sulfate cathode material prepared by the present invention uses a cobalt-doped cathode material with a carbon composite structure as the core, and a zinc coating layer is coated on the outside. The carbon composite structure and the zinc coating layer have the following advantages: (1) It can effectively prevent the cobalt-doped cathode material from directly contacting the electrolyte, delaying the water absorption and oxidation problems of the cathode material, reducing the occurrence of side reactions, enhancing the chemical stability of the cathode material, and thus increasing the cycle life of the cathode material; (2) It improves the electron and ion transport rates, reduces the interfacial resistance, improves the rate performance of the cathode material, and enables the battery to maintain good performance during high-current charge and discharge. Detailed Embodiments

[0051] The following embodiments are provided to better understand the present invention further. It is not limited to the best mode described, and does not limit the content and protection scope of the present invention. Any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with those of other prior arts falls within the protection scope of the present invention.

[0052] For those not specifying specific experimental steps or conditions in the embodiments, the operations or conditions of the conventional experimental steps described in the literature in this field can be followed. For the reagents or instruments not specifying the manufacturer, they are all conventional reagent products that can be obtained through commercial purchase.

[0053] Example 1

[0054] This example provides a preparation method for a sodium iron sulfate cathode material, including the following steps:

[0055] (1) Mix Na 2 SO 4 , FeSO 4 ·7H 2 O, and CoSO 4 ·7H 2 O. Based on the sum of the masses of Na 2 SO 4 and FeSO 4 ·7H 2 O, add 2 wt% ascorbic acid, 5 wt% glucose, 4 wt% acetylene black, 1 wt% sodium dodecyl sulfate, and hydroxymethyl cellulose. Among them, the molar ratio of the sodium element in Na 2 SO 4 to the sum of the molar amounts of the iron element in FeSO 4 ·7H 2 O and the cobalt element in CoSO 4 ·7H 2 O is 2:1.2; the molar ratio of the iron element in FeSO 4 ·7H 2 O to the cobalt element in CoSO 4 ·7H 2 O is 99:1; based on the sum of the masses of Na 2 SO 4 , FeSO 4 ·7H 2 , CoSO 4 ·7H 2 O, ascorbic acid, glucose, acetylene black, and sodium dodecyl sulfate, the addition amount of the hydroxymethyl cellulose is 50 ppm.

[0056] (2) Add deionized water and the mixture from step (1) to the reactor, stir at 200 r / min for 2 h to obtain a spray liquid with a solid content of 25%; spray-dry to obtain a cobalt-doped pre-product. The inlet air temperature for the spray drying is 200 °C, and the outlet air temperature for the spray drying is 100 °C.

[0057] (3) Crush and screen the cobalt-doped pre-product, based on the mass of the cobalt-doped pre-product, add 5000 ppm of zinc oxide and mix evenly. Under a nitrogen atmosphere, heat in a box furnace from room temperature to 200 °C at a rate of 5 °C / min for the first sintering for 6 h, and then heat from 200 °C to 370 °C at a rate of 5 °C / min for the second sintering for 12 h to obtain the sodium iron sulfate cathode material.

[0058] Example 2

[0059] This example provides a method for preparing a sodium iron sulfate cathode material, which includes the following steps:

[0060] (1) Mix Na 2 SO 4 , FeSO 4 ·7H 2 O and cobalt oxalate. Based on the sum of the masses of Na 2 SO 4 and FeSO 4 ·7H 2 O, add 6 wt% citric acid, 12 wt% sucrose, 6 wt% graphene, 3 wt% polyethylene glycol and hydroxymethylcellulose. Among them, the molar amount of sodium element in Na 2 SO 4 is in a ratio of 1:1 to the sum of the molar amounts of iron element in FeSO 4 ·7H 2 O and cobalt element in cobalt oxalate; the molar ratio of iron element in FeSO 4 ·7H 2 O to cobalt element in cobalt oxalate is 50:1; based on the sum of the masses of Na 2 SO 4 , FeSO 4 ·7H 2 O, cobalt oxalate, citric acid, sucrose, graphene, and polyethylene glycol, the addition amount of hydroxymethylcellulose is 100 ppm.

[0061] (2) Add deionized water and the mixture from step (1) to the reactor, stir at 400 r / min for 3 h to obtain a spray liquid with a solid content of 40%; spray-dry to obtain a cobalt-doped pre-product. The inlet air temperature for the spray drying is 280 °C, and the outlet air temperature for the spray drying is 140 °C.

[0062] (3) Crush and screen the cobalt-doped pre-product. Based on the mass of the cobalt-doped pre-product, add 20,000 ppm of zinc sulfate and mix evenly. Under an argon atmosphere, in a box furnace, heat it to 300 °C at a rate of 2 °C / min for the first sintering for 12 h, and then heat it to 400 °C at a rate of 2 °C / min for the second sintering for 24 h to obtain the sodium iron sulfate cathode material.

[0063] Example 3

[0064] This example provides a method for preparing a sodium iron sulfate cathode material, which includes the following steps:

[0065] (1) Mix Na 2 SO 4 , FeSO 4 ·7H 2 O and cobalt phosphate. Based on the sum of the masses of Na 2 SO 4 and FeSO 4 ·7H 2 O, add 0.04 wt% acetic acid, 0.01 wt% fructose, 0.05 wt% activated carbon, 0.05 wt% polyvinylpyrrolidone and hydroxymethylcellulose. Among them, the molar amount of sodium element in Na 2 SO 4 is in a ratio of 2:1 to the sum of the molar amounts of iron element in FeSO 4 ·7H 2 O and cobalt element in cobalt phosphate; the molar ratio of iron element in FeSO 4 ·7H 2 O to cobalt element in cobalt phosphate is 4:1; based on the sum of the masses of Na 2 SO 4 , FeSO 4 ·7H 2 O, cobalt phosphate, ascorbic acid, glucose, acetylene black, and sodium dodecyl sulfate, the addition amount of the hydroxymethylcellulose is 5 ppm.

[0066] (2) Add deionized water and the mixture in step (1) to the reaction kettle, stir at 100 r / min for 2.5 h to obtain a spray liquid, and the solid content of the spray liquid is 15%; spray dry to obtain a cobalt-doped pre-product. The inlet air temperature of the spray drying is 140 °C, and the outlet air temperature of the spray drying is 70 °C.

[0067] (3) Crush and screen the cobalt-doped pre-product. Based on the mass of the cobalt-doped pre-product, add 500 ppm of zinc acetate and mix evenly. Under a nitrogen atmosphere, in a box furnace, heat it to 100 °C at a rate of 3 °C / min for the first sintering for 1 h, and then heat it to 300 °C at a rate of 3 °C / min for the second sintering for 1 h to obtain the sodium iron sulfate cathode material.

[0068] Comparative Example 1

[0069] This comparative example provides a method for preparing a cathode material, which includes the following steps:

[0070] (1) Mix Na 2 SO 4 and FeSO 4 ·7H 2 O. Based on the total mass of Na 2 SO 4 and FeSO 4 ·7H 2 O, add 2 wt% ascorbic acid and mix; the molar ratio of sodium element in Na 2 SO 4 to iron element in FeSO 4 ·7H 2 O is 2:1.2.

[0071] (2) Add deionized water and the mixture from step (1) to a reaction kettle, stir at 200 r / min for 2 h to obtain a spray liquid, and the solid content of the spray liquid is 25%; spray-dry to obtain a pre-product. The inlet air temperature of the spray drying is 200 °C, and the outlet air temperature of the spray drying is 100 °C.

[0072] (3) Crush and screen the pre-product, and in a nitrogen atmosphere, heat it in a box furnace from room temperature to 200 °C at a rate of 5 °C / min for the first sintering for 6 h, and then heat it from 200 °C to 370 °C at a rate of 5 °C / min for the second sintering for 12 h to obtain the cathode material.

[0073] Comparative Example 2

[0074] This comparative example provides a method for preparing a cathode material, which includes the following steps:

[0075] (1) Mix Na 2 SO 4 and FeSO 4 ·7H 2 O. Based on the total mass of Na 2 SO 4 and FeSO 4 ·7H 2 O, add 2 wt% ascorbic acid, 5 wt% glucose, 4 wt% acetylene black, 1 wt% sodium dodecyl sulfate, and hydroxymethyl cellulose; among them, the molar ratio of sodium element in Na 2 SO 4 to iron element in FeSO 4 ·7H 2 O is 2:1.2; based on Na 2 SO 4 and FeSO 4 ·7H 2Based on the sum of the masses of O, ascorbic acid, glucose, acetylene black, and sodium dodecyl sulfate, the addition amount of the hydroxymethyl cellulose is 50 ppm.

[0076] (2) Add deionized water and the mixture from step (1) to the reaction kettle, stir at 200 r / min for 2 h to obtain a spray liquid, and the solid content of the spray liquid is 25%; obtain a pre-product by spray drying, the inlet air temperature of the spray drying is 200 °C, and the outlet air temperature of the spray drying is 100 °C.

[0077] (3) Crush and screen the pre-product, based on the mass of the pre-product, add 5000 ppm of zinc oxide and mix evenly. Under a nitrogen atmosphere, heat the box furnace to 200 °C at a rate of 5 °C / min for the first sintering for 6 h, and then heat to 370 °C at a rate of 5 °C / min for the second sintering for 12 h to obtain the cathode material.

[0078] Comparative Example 3

[0079] This comparative example provides a method for preparing a cathode material, including the following steps:

[0080] (1) Mix Na 2 SO 4 , FeSO 4 ·7H 2 O and CoSO 4 ·7H 2 O, based on the sum of the masses of Na 2 SO 4 and FeSO 4 ·7H 2 O, add 2 wt% ascorbic acid; the molar amount of sodium element in Na 2 SO 4 and the molar sum of iron element in FeSO 4 ·7H 2 O and cobalt element in CoSO 4 ·7H 2 O have a ratio of 2:1.2; the molar ratio of iron element in FeSO 4 ·7H 2 O and cobalt element in CoSO 4 ·7H 2 O is 99:1.

[0081] (2) Add deionized water and the mixture from step (1) to the reaction kettle, stir at 200 r / min for 2 h to obtain a spray liquid, and the solid content of the spray liquid is 25%; obtain a pre-product by spray drying, the inlet air temperature of the spray drying is 200 °C, and the outlet air temperature of the spray drying is 100 °C.

[0082] (3) Pulverize and sieve the pre-product. Under a nitrogen atmosphere, heat it in a box furnace from room temperature to 200 °C at a rate of 5 °C / min for the first sintering for 6 h, and then heat it from 200 °C to 370 °C at a rate of 5 °C / min for the second sintering for 12 h to obtain the positive electrode material.

[0083] Test Example

[0084] Mix the positive electrode materials prepared in the above Examples 1-3 and Comparative Examples 1-3 with acetylene black and PVDF uniformly at a mass ratio of 80:10:10, add 1-methyl-2-pyrrolidone and ball mill for 1 h to make a slurry, uniformly coat it on an aluminum sheet, dry it, and press it into a positive electrode plate. Use a sodium metal sheet as the negative electrode and a 1 mol / L 4 PC solution of NaClO as the electrolyte to assemble a 2032 coin cell. Perform performance tests using a Land test system:

[0085] (1) Test method for the initial discharge specific capacity at 0.1C: At room temperature, within a cut-off voltage of 2.0 - 4.5 V, charge once at 0.1C and discharge once at 0.1C; the results are shown in Table 1;

[0086] (2) Test method for rate performance: At room temperature, within a cut-off voltage of 2.0 - 4.5 V, charge once at 1C and discharge once at 1C; calculate according to the following formula, and the results are shown in Table 1;

[0087] Rate performance = 1C discharge capacity / 0.1C discharge capacity;

[0088] (3) Capacity retention rate: At room temperature, within a cut-off voltage of 2.0 - 4.5 V, charge once at 1C and discharge once at 1C, and cycle 50 times; the results are shown in Table 1.

[0089] Table 1

[0090]

[0091] The initial discharge specific capacity at 0.1C, rate performance, and capacity retention rate of the sodium iron sulfate positive electrode material prepared by the present invention are excellent. By comparing with Comparative Examples 1-3, it can be seen that the carbon composite structure, doped cobalt element, and coated zinc coating layer of the sodium iron sulfate positive electrode material provided by the present invention have good synergistic effects, significantly improving the discharge specific capacity, rate performance, and capacity retention rate of the positive electrode material.

[0092] Obviously, the above examples are only for clear illustration and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation manners here. And the obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for preparing a sodium iron sulfate positive electrode material, characterized in that: The steps include: (1) preparing a cobalt-doped pre-product; (2) Mixing the cobalt-doped pre-product and a zinc source, and sintering them to obtain a sodium iron sulfate positive electrode material.

2. The preparation method according to claim 1, characterized in that: The zinc source comprises at least one of zinc oxide, zinc sulfate, zinc nitrate, zinc chloride, zinc carbonate, zinc acetate, zinc propionate and basic zinc carbonate; and / or, Based on the mass of the cobalt-doped pre-product, the added amount of the zinc source is 500-20000 ppm.

3. The preparation method according to claim 1 or 2, characterized in that: The preparation steps of the cobalt-doped pre-product include: mixing a sodium source, an iron source, a cobalt source, an antioxidant, an organic carbon source, an inorganic carbon source and a dispersant, stirring, and spray drying.

4. The preparation method according to claim 3, characterized in that: The ratio of the molar amount of the sodium element in the sodium source to the sum of the molar amounts of the iron element in the iron source and the cobalt element in the cobalt source is 2:(1-2); and / or, The molar ratio of the iron element in the iron source to the cobalt element in the cobalt source is (4-100):1; and / or, Based on the sum of the mass of the sodium source and the iron source, the mass of the antioxidant is 0.04-6wt%; and / or, Based on the sum of the mass of the sodium source and the iron source, the mass of the organic carbon source is 0.01-12wt%; and / or, Based on the sum of the mass of the sodium source and the iron source, the mass of the inorganic carbon source is 0.05-6wt%; and / or, Based on the sum of the masses of the sodium source and the iron source, the mass of the dispersant is 0.05-3wt%.

5. The preparation method according to claim 3 or 4, characterized in that: The sodium source comprises sodium sulfate; and / or, The iron source comprises ferrous sulfate; and / or, The cobalt source comprises at least one of cobalt sulfate, cobalt oxalate, cobalt phosphate, cobalt carbonate, cobalt chloride, cobalt silicate and cobalt nitrate; and / or, The antioxidant comprises at least one of ascorbic acid, citric acid, phytic acid, uric acid, oxalic acid, formic acid, acetic acid and benzoic acid; and / or, The organic carbon source comprises at least one of glucose, sucrose, starch, fructose and stearic acid; and / or, The inorganic carbon source comprises at least one of acetylene black, graphene, carbon nanotubes, carbon fibers, activated carbon and conductive carbon black; and / or, The dispersant includes at least one of sodium lauryl sulfate, polyethylene glycol, sodium alkylbenzene sulfonate, sodium alkyl sulfonate, sodium stearate and polyvinyl pyrrolidone.

6. The preparation method according to claim 3, characterized in that: The mixing also includes adding a defoaming agent; Preferably, the defoaming agent comprises at least one of hydroxymethyl cellulose, fatty alcohol, fatty acid ester, soybean oil and corn oil; Preferably, based on the sum of the mass of the sodium source, the iron source, the cobalt source, the antioxidant, the organic carbon source, the inorganic carbon source and the dispersant, the added amount of the defoaming agent is 1-100 ppm.

7. The preparation method according to any one of claims 3 to 6, characterized in that: The stirring further comprises adding a solvent, and after stirring, a spray liquid is obtained, and the solid content of the spray liquid is 15-40%; and / or, The stirring speed is 100-400 r / min; and / or, The stirring time is 2-3h; and / or, The inlet air temperature of the spray drying is 140-280°C; and / or, The outlet air temperature of the spray drying is 70-140°C.

8. The preparation method according to any one of claims 1 to 7, characterized in that: The sintering is carried out under a protective atmosphere; Preferably, the protective atmosphere comprises at least one of nitrogen and argon; Preferably, the sintering includes a first sintering and a second sintering; Preferably, the temperature of the first sintering is 100-300°C; Preferably, the first sintering time is 0.1-12h; Preferably, the temperature of the second sintering is 300-400°C; Preferably, the second sintering time is 0.1-24h.

9. The sodium iron sulfate positive electrode material prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the sodium iron sulfate positive electrode material prepared by the preparation method according to any one of claims 1 to 8 in sodium ion batteries.

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