Sodium ferric sulfate composite positive electrode material, preparation method thereof and sodium ion battery

Abstract

The invention discloses a sodium ferric sulfate composite positive electrode material, a preparation method thereof and a sodium ion battery. The preparation method of the sodium ferric sulfate composite positive electrode material comprises the following steps: carrying out ball milling on a sodium ferric sulfate precursor, a carbon source, a dispersing agent and metal acetate to obtain a mixture; and sintering the mixture in an inert atmosphere to obtain the sodium ferric sulfate composite positive electrode material. According to the preparation method disclosed by the invention, a carbon source and metal acetate are adopted as a common coating agent, and after sintering is performed in an inert atmosphere, sodium ferric sulfate co-coated with a sintered product of carbon and metal acetate is obtained; through co-coating, the conductivity of the sodium ferric sulfate material can be improved, the polarization resistance is reduced, the obstruction of sodium ions in the transmission process is reduced, and the charge-discharge rate of the battery under high rate is improved. Meanwhile, co-coating of the sintered product of the carbon and the metal acetate enables the sodium ferric sulfate positive electrode material to have higher electrochemical activity, the capacity of a sodium ion battery can be better improved, and the energy density of the battery is further improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of sodium ion batteries, and in particular relates to a sodium iron sulfate composite positive electrode material and a preparation method thereof, and a sodium ion battery. Background Art

[0002] Sodium iron sulfate has a high working voltage platform, high theoretical discharge capacity, small structural volume change during the charge and discharge process, low preparation cost, readily available raw materials, and is non-toxic and pollution-free. It also has a three-dimensional ion diffusion channel and is considered to be an ideal positive electrode material for large-scale energy storage in sodium ion batteries. However, as a polyanion material, sodium iron sulfate has a low conductivity, which seriously affects the charge and discharge performance of the battery at high rates. Therefore, improving the electronic conductivity and kinetic performance of sodium iron sulfate materials has become a research focus.

[0003] In order to improve the conductivity and kinetics of sodium iron sulfate materials and increase the capacity and cycle stability of batteries, sodium iron sulfate is generally modified by nano-sizing, doping, carbon coating, etc. Although the above modifications can improve its conductivity and kinetics, the improvement of the charge and discharge performance of the sodium ion battery assembled by it at high rates is very limited. Therefore, how to further improve the comprehensive performance of sodium iron sulfate materials and improve the charge and discharge performance of sodium ion batteries at high rates is an urgent problem to be solved. Summary of the invention

[0004] The purpose of the present invention is to provide a sodium iron sulfate composite positive electrode material with good conductivity and a preparation method thereof, and a sodium ion battery.

[0005] In the first aspect, the present invention provides a method for preparing a sodium iron sulfate composite positive electrode material, using the following technical scheme: A method for preparing a sodium iron sulfate composite positive electrode material comprises the following steps: The sodium iron sulfate precursor, carbon source, dispersant and metal acetate are ball-milled to obtain a mixture; the mixture is sintered under an inert atmosphere to obtain a sodium iron sulfate composite positive electrode material.

[0006] Preferably, the method for preparing the sodium iron sulfate precursor comprises the following steps: Sodium sulfate, ferrous sulfate and / or ferrous sulfate heptahydrate and an antioxidant are dissolved in water to obtain a slurry; the slurry is spray-dried to obtain a sodium ferric sulfate precursor.

[0007] Preferably, the chemical formula of the sodium ferric sulfate precursor is Na 2x Fe y (SO 4 ) x+y, wherein: 0<x≤5, 0<y≤5; sodium sulfate, ferrous sulfate and / or ferrous sulfate heptahydrate are added according to the stoichiometric ratio in the chemical formula of the sodium ferric sulfate precursor.

[0008] Preferably, the Fe in the ferrous sulfate and / or ferrous sulfate heptahydrate is 2+ The molar ratio to antioxidant is 10:(1~2).

[0009] Preferably, the spray drying temperature is 150-200°C.

[0010] Preferably, the carbon source is one or more of acetylene black, Super P, Ketjen black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and conductive graphite; the mass of the carbon source is 1.0-5.0wt% of the mass of the sodium ferric sulfate precursor.

[0011] Preferably, the dispersant is polyethylene glycol-4000 (PEG-4000), polyethylene glycol-6000 (PEG-6000), sodium acrylate (C 3 H 3 NaO 2 ), polyvinyl pyrrolidone or more; the mass of the dispersant is 0.1~1.0 wt% of the mass of the sodium ferric sulfate precursor.

[0012] Preferably, the metal acetate is one or more of nickel acetate, cobalt acetate, copper acetate, zinc acetate, manganese acetate, aluminum acetate, titanium acetate, magnesium acetate, and calcium acetate; the mass of the metal acetate is 0.5-2.0wt% of the mass of the sodium iron sulfate precursor.

[0013] More preferably, the metal acetate is at least three of nickel acetate, cobalt acetate, copper acetate, zinc acetate, manganese acetate, aluminum acetate, titanium acetate, magnesium acetate, and calcium acetate.

[0014] Preferably, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere; the sintering temperature is 350-400° C., and the sintering time is 0.5-1.5 h.

[0015] In a second aspect, the present invention provides a sodium iron sulfate composite positive electrode material prepared by the aforementioned preparation method.

[0016] In a third aspect, the present invention provides a sodium ion battery comprising the aforementioned sodium iron sulfate composite positive electrode material.

[0017] The above one or more technical solutions of the present invention can achieve at least one of the following beneficial effects: (1) The preparation method of the present invention uses a carbon source and a metal acetate as a co-coating agent, and after sintering under an inert atmosphere, a sodium ferric sulfate co-coated with the sintering product of carbon and metal acetate is obtained; the co-coating can improve the conductivity of the sodium ferric sulfate material, reduce polarization resistance, reduce the obstruction of sodium ions in the transmission process, and improve the high-rate charge and discharge rate of the battery. At the same time, the co-coating of the sintering product of carbon and metal acetate can make the sodium ferric sulfate positive electrode material have higher electrochemical activity, which can better improve the capacity of the sodium ion battery, thereby improving the energy density of the battery.

[0018] (2) The sodium iron sulfate precursor prepared by the spray drying method of the present invention has the advantages of narrow particle size distribution and good fluidity, which greatly improves the quality and performance of the product, and can reduce energy consumption and be environmentally friendly, and has broad application prospects.

[0019] (3) During the ball milling process of the preparation method of the present invention, adding a solid dispersant can improve the dispersibility of the material and reduce the agglomeration of the powder, thereby obtaining a more uniformly mixed material; the dispersant can shorten the ball milling time, improve production efficiency, and reduce energy consumption during the ball milling process, thereby reducing production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is the SEM image of the sodium iron sulfate composite positive electrode material prepared in Comparative Example 1.

[0021] Figure 2 This is the SEM image of the sodium iron sulfate composite positive electrode material prepared in Example 5.

[0022] Figure 3 This is a graph showing the first charge and discharge capacity at 0.1C of the sodium iron sulfate composite positive electrode material prepared in Example 5.

[0023] Figure 4 This is a cycle performance diagram of a battery assembled with the sodium iron sulfate composite positive electrode material prepared in Example 5 after 300 charge and discharge cycles at a rate of 10 C. DETAILED DESCRIPTION

[0024] Unless otherwise defined, all technical terms used hereinafter have the same meaning as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention. Unless otherwise specifically stated, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or can be prepared by existing methods.

[0025] As mentioned above, a method for preparing a sodium iron sulfate composite positive electrode material comprises the following steps: The sodium iron sulfate precursor, carbon source, dispersant and metal acetate are ball-milled to obtain a mixture; the mixture is sintered under an inert atmosphere to obtain a sodium iron sulfate composite positive electrode material.

[0026] The method of the present invention can make the carbon source and the metal acetate evenly distributed on the body of the sodium ferric sulfate precursor by ball milling and mixing the sodium ferric sulfate precursor, and obtain the sodium ferric sulfate composite positive electrode material co-coated with two coating agents of the carbon and metal acetate sintering products by sintering. The combination of the two coating agents can improve the conductivity of the material, reduce the polarization resistance, reduce the obstruction of sodium ions in the transmission process, and improve the charge and discharge rate of the battery at a high rate. At the same time, the co-coating of the sintering products of carbon and metal acetate can make the sodium ferric sulfate positive electrode material have higher electrochemical activity, can better improve the capacity of the sodium ion battery, and then improve the energy density of the battery.

[0027] In the preparation method of the present invention, a solid dispersant is added for ball milling, which can improve the dispersibility of the material and reduce the agglomeration of powders, thereby obtaining a more uniformly mixed material; the dispersant can shorten the ball milling time, improve production efficiency, reduce energy consumption during the ball milling process, and thus reduce production costs.

[0028] Preferably, the method for preparing the sodium iron sulfate precursor comprises the following steps: Sodium sulfate, ferrous sulfate and / or ferrous sulfate heptahydrate and an antioxidant are dissolved in water to obtain a slurry; the slurry is spray-dried to obtain a sodium ferric sulfate precursor.

[0029] The sodium iron sulfate precursor prepared by the spray drying method in the present invention has the advantages of narrow particle size distribution and good fluidity, greatly improving the quality and performance of the product, and can reduce energy consumption, and the process is environmentally friendly, and has broad application prospects.

[0030] Preferably, the chemical formula of the sodium ferric sulfate precursor is Na 2x Fe y (SO 4 ) x+y , wherein: 0<x≤5, such as 1, 2, 3, 4, 5, etc.; 0<y≤5, y can be 1, 2, 3, 4, 5, etc. Sodium sulfate, ferrous sulfate and / or ferrous sulfate heptahydrate are added according to the stoichiometric ratio in the chemical formula of the sodium ferric sulfate precursor.

[0031] Preferably, the Fe in the ferrous sulfate and / or ferrous sulfate heptahydrate is 2+ The molar ratio to the antioxidant is 10:(1-2), for example, 10:1, 10:1.3, 10:1.5, 10:1.7, 10:2, etc.

[0032] Preferably, the spray drying temperature is 150-200°C, for example, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, etc.

[0033] Preferably, the carbon source is one or more of acetylene black, Super P, Ketjen black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and conductive graphite. Typical but non-limiting combinations include a combination of acetylene black and Super P, a combination of Super P and Ketjen black, a combination of Ketjen black and single-walled carbon nanotubes, a combination of multi-walled carbon nanotubes and conductive graphite, and the like.

[0034] Preferably, the mass of the carbon source is 1.0-5.0 wt % of the mass of the sodium ferric sulfate precursor, for example, 1.0 wt %, 2.0 wt %, 3.0 wt %, 4.0 wt %, 5.0 wt %, etc.

[0035] Preferably, the dispersant is polyethylene glycol-4000 (PEG-4000), polyethylene glycol-6000 (PEG-6000), sodium acrylate (C 3 H 3 NaO 2 ), one or more of polyvinyl pyrrolidone, typical but non-limiting combinations include a combination of polyethylene glycol-4000 and sodium acrylate, a combination of polyethylene glycol-6000 and sodium acrylate, a combination of polyvinyl pyrrolidone and sodium acrylate, etc.

[0036] Preferably, the mass of the dispersant is 0.1-1.0 wt% of the mass of the sodium ferric sulfate precursor, for example, 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1.0wt%, etc.

[0037] Preferably, the metal acetate is one or more of nickel acetate, cobalt acetate, copper acetate, zinc acetate, manganese acetate, aluminum acetate, titanium acetate, magnesium acetate, and calcium acetate; typical but non-limiting combinations include a combination of nickel acetate and cobalt acetate, a combination of cobalt acetate and copper acetate, a combination of copper acetate and zinc acetate, a combination of zinc acetate and manganese acetate, a combination of magnesium acetate and calcium acetate, and the like.

[0038] Preferably, the mass of the metal acetate is 0.5-2.0 wt % of the mass of the sodium iron sulfate precursor, for example, 0.5 wt %, 1.0 wt %, 1.5 wt %, 2.0 wt %, etc.

[0039] Further preferably, the metal acetate is at least three of nickel acetate, cobalt acetate, copper acetate, zinc acetate, manganese acetate, aluminum acetate, titanium acetate, magnesium acetate, and calcium acetate. Typical but non-limiting combinations include: a combination of nickel acetate, cobalt acetate, and copper acetate, a combination of titanium acetate, magnesium acetate and calcium acetate, a combination of copper acetate, zinc acetate and manganese acetate, a combination of aluminum acetate, titanium acetate, magnesium acetate and calcium acetate, a combination of copper acetate, aluminum acetate, titanium acetate, magnesium acetate and calcium acetate, etc.

[0040] When multiple metal acetates are used as coating agents in the present invention, since the coating layer contains multiple elements, the coating layer has a highly disordered high entropy (HE) surface structure, which can further improve the thermodynamic and electrochemical properties of the material, improve the surface properties of the material, and enhance its structural stability.

[0041] Preferably, the inert atmosphere is a nitrogen atmosphere or an argon atmosphere.

[0042] Preferably, the sintering temperature is 350-400°C, for example, 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, etc.

[0043] Preferably, the sintering time is 0.5-1.5 h, for example, 0.5 h, 1.0 h, 1.5 h, etc.

[0044] In a second aspect, the present invention provides a sodium iron sulfate composite positive electrode material prepared by the aforementioned preparation method.

[0045] In a third aspect, the present invention provides a sodium ion battery comprising the aforementioned sodium iron sulfate composite positive electrode material.

[0046] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively and meticulously below in conjunction with the accompanying drawings and preferred embodiments of the present invention, but the protection scope of the present invention is not limited to the following specific embodiments.

[0047] Example 1 The preparation method of the sodium iron sulfate composite positive electrode material in this embodiment is as follows: (1) Put Na 2 SO 4 、FeSO 4 . 7H 2 O was dissolved in deionized water at a molar ratio of Na:Fe of 3:2, followed by the addition of ascorbic acid (the molar amount of ascorbic acid is FeSO 4 . 7H 2 O molar amount), and after being fully dissolved, a mixed slurry is obtained; the mixed slurry is spray-dried at 180 ° C to obtain a sodium iron sulfate precursor.

[0048] (2) Sodium iron sulfate precursor was placed in a stainless steel ball mill, and then Ketjen black was added as a conductive coating agent (the amount added was 5wt% of the mass of the sodium iron sulfate precursor), calcium acetate was added as a metal coating agent (the amount added was 1wt% of the mass of the sodium iron sulfate precursor), and sodium acrylate was added as a dispersant (the amount added was 0.5wt% of the mass of the sodium iron sulfate precursor), and then ball milled for 6 hours under an argon atmosphere, wherein: the process parameters of ball milling were: ball-to-material ratio was 10:1; the ball milling speed was 400rpm; after ball milling, a mixture was obtained. The mixture was heated to 380°C at a heating rate of 2°C / min under an argon atmosphere, and kept at this temperature for 8 hours. After natural cooling, it was crushed and sieved to obtain a sodium iron sulfate composite positive electrode material.

[0049] Comparative Example 1 The preparation method of the sodium iron sulfate positive electrode material in this comparative example is as follows: Will Na 2 SO 4 、FeSO 4 . 7H 2 O was dissolved in deionized water at a molar ratio of Na:Fe of 3:2, followed by the addition of ascorbic acid (the molar amount of ascorbic acid is FeSO 4 . 7H 2 O molar amount), after fully dissolving, a mixed slurry is obtained; the mixed slurry is spray-dried at 180°C to obtain a sodium iron sulfate precursor. The sodium iron sulfate precursor is heated to 380°C at a heating rate of 2°C / min under an argon atmosphere, and is kept at this temperature for 8 hours. After natural cooling, it is crushed and sieved to obtain a sodium iron sulfate positive electrode material.

[0050] The SEM image of the sodium iron sulfate positive electrode material prepared in this comparative example is as follows Figure 1 As shown, it is a spherical structure.

[0051] Comparative Example 2 The process is basically the same as Example 1, except that calcium acetate is not added in step (2).

[0052] Comparative Example 3 The method is basically the same as Example 1, except that: the sodium iron sulfate composite positive electrode material does not contain carbon; Ketjen black is not added in step (2) of the corresponding preparation method; and the sodium iron sulfate composite positive electrode material is prepared accordingly.

[0053] Example 2 The method is basically the same as Example 1, except that in step (2), calcium acetate and magnesium acetate are used as metal coating agents, and the addition amount of calcium acetate and magnesium acetate is 0.5wt% of the sodium ferric sulfate precursor. The corresponding sodium ferric sulfate composite positive electrode material is prepared.

[0054] Example 3 The same as Example 1, except that in step (2), calcium acetate, magnesium acetate and aluminum acetate are used as metal coating agents, and the addition amount of calcium acetate, magnesium acetate and aluminum acetate is 0.33wt% of the sodium iron sulfate precursor. The corresponding structure of the prepared sodium iron sulfate composite positive electrode material is Na 3 Fe 2 (SO 4 ) 3.5 @C / M, M is the sintering product of calcium acetate, magnesium acetate, aluminum acetate and zinc acetate.

[0055] Example 4 The same as Example 1, except that in step (2), calcium acetate, magnesium acetate, aluminum acetate and zinc acetate are used as metal coating agents, and the addition amount of calcium acetate, magnesium acetate, aluminum acetate and zinc acetate is 0.25wt% of the sodium iron sulfate precursor. The structural formula of the corresponding sodium iron sulfate composite positive electrode material prepared is Na 3 Fe 2 (SO 4 ) 3.5 @C / M, M is the sintering product of calcium acetate, magnesium acetate, aluminum acetate and zinc acetate.

[0056] Example 5 The method is basically the same as Example 1, except that in step (2), calcium acetate, magnesium acetate, aluminum acetate, zinc acetate and titanium acetate are added as metal coating agents; the added amounts of calcium acetate, magnesium acetate, aluminum acetate, zinc acetate and titanium acetate are all 0.2 wt% of the mass of the sodium ferric sulfate precursor.

[0057] The SEM image of the sodium iron sulfate composite positive electrode material prepared in this example is as follows Figure 2 As shown, its microscopic surface is a highly disordered coating.

[0058] Example 6 The preparation method of the sodium iron sulfate composite positive electrode material in this embodiment is as follows: (1) Put Na 2 SO 4 、FeSO 4 . 7H 2 O was dissolved in deionized water at a molar ratio of Na:Fe of 6:5, followed by the addition of citric acid (the molar amount of citric acid is FeSO 4 . 7H 2 O molar amount), and after being fully dissolved, a mixed slurry is obtained; the mixed slurry is spray-dried at 200° C. to obtain a sodium iron sulfate precursor.

[0059] (2) Sodium iron sulfate precursor was placed in a stainless steel ball mill, and then carbon nanotubes (CTS600) were added as a conductive coating agent (the amount added was 3wt% of the mass of the sodium iron sulfate precursor), nickel acetate, cobalt acetate, zinc acetate, manganese acetate and magnesium acetate as metal coating agents (the amount of nickel acetate, cobalt acetate, zinc acetate, manganese acetate and magnesium acetate added was 0.4wt% of the mass of the sodium iron sulfate precursor) and polyethylene glycol-4000 was used as a dispersant (the amount added was 1.0wt% of the mass of the sodium iron sulfate precursor), and then ball milled for 5h under argon atmosphere, wherein: the process parameters of ball milling were: ball-to-material ratio was 15:1; ball milling speed was 500rpm; after ball milling, a mixture was obtained. The mixture was heated to 400℃ under argon atmosphere at a heating rate of 4℃ / min, and kept at this temperature for 4h. After natural cooling, it was crushed and sieved to obtain a sodium iron sulfate composite positive electrode material.

[0060] Example 7 The preparation method of the sodium iron sulfate composite positive electrode material in this embodiment is as follows: (1) Put Na 2 SO 4 、FeSO 4 . 7H 2 O was dissolved in deionized water at a molar ratio of Na:Fe of 8:5, followed by the addition of oxalic acid (the molar amount of oxalic acid is FeSO 4 . 7H 2 The mixed slurry was spray-dried at 150° C. to obtain a sodium iron sulfate precursor.

[0061] (2) Sodium iron sulfate precursor was placed in a stainless steel ball mill, and then carbon nanotubes (CTS720) were added as conductive coating agents (the amount added was 8wt% of the mass of the sodium iron sulfate precursor), nickel acetate, cobalt acetate and zinc acetate as metal coating agents (the amount of nickel acetate, cobalt acetate and zinc acetate added was 0.2wt% of the mass of the sodium iron sulfate precursor) and polyvinyl pyrrolidone as a dispersant (the amount added was 0.2wt% of the mass of the sodium iron sulfate precursor), and then ball milled for 6h under argon atmosphere, wherein: the process parameters of ball milling were: ball-to-material ratio was 20:1; ball milling speed was 300rpm; after ball milling, a mixture was obtained. The mixture was heated to 350℃ under argon atmosphere at a heating rate of 3℃ / min, and kept at this temperature for 12h. After natural cooling, it was crushed and sieved to obtain a sodium iron sulfate composite positive electrode material.

[0062] The positive electrode materials prepared in Examples 1 to 7 and Comparative Examples 1 to 2 were assembled into button cells. The specific method was as follows: the positive electrode material, acetylene black and PVDF were weighed and mixed in a mass ratio of 8:1:1, N-methylpyrrolidone solution (NMP) was added for homogenization, and the slurry was evenly coated on an aluminum foil, dried in a vacuum drying oven at 80° C. for 6 h, and the positive electrode sheet was obtained by rolling and cutting (with a diameter of 14 mm). A metal sodium sheet was used as a counter electrode, and 1 mol / L NaClO 4 The electrolyte is an electrolyte whose solvent is a combination of ethylene carbonate, propylene carbonate and fluoroethylene carbonate (the volume ratio of ethylene carbonate and propylene carbonate is 1:1; fluoroethylene carbonate accounts for 5% of the total volume of the solvent). Glass fiber is used as a separator, and button half cells are assembled in a glove box filled with argon to obtain a sodium ion battery.

[0063] The sodium ion battery was aged for 12 h, then activated at 25°C and 2.0-4.3 V charge and discharge voltage at a rate of 0.1 C for 3 cycles, and then cycled at a rate of 10 C for 300 cycles. The test data can be seen in Table 1, and the test performance diagram in Example 5 can be seen in Figure 3 and Figure 4 .

[0064] Table 1 From Table 1 and Figure 3~4 It can be seen from the data that the sodium ion battery assembled with the uncoated sodium iron sulfate positive electrode material in Comparative Example 1 has basically no cycle performance at a 10C rate; the sodium ion battery assembled with the sodium iron sulfate composite positive electrode material in Comparative Example 3 has basically no cycle performance at a 10C rate, indicating that when the sintering product of calcium acetate in an inert atmosphere is used alone to coat sodium iron sulfate, it has basically no effect on the improvement of the performance of the positive electrode material. The cycle performance of the sodium battery assembled with the sodium iron sulfate composite positive electrode material at a 10C rate in Comparative Example 2 is significantly improved compared with Comparative Example 1, indicating that carbon coating can significantly improve the electrochemical properties of sodium iron sulfate. The battery assembled with the sodium iron sulfate composite positive electrode material in Example 1 has further improved the specific capacity and cycle performance compared with Comparative Example 2. In Examples 2 to 5, the types of acetates are successively increased, and the specific capacity and cycle performance of the sodium iron sulfate composite positive electrode material assembled into a sodium ion battery will increase slightly with the increase in the type. The process parameters in Examples 6 and 7 are different, and the performance of the sodium ion battery assembled with the sodium iron sulfate composite positive electrode material prepared therefrom will fluctuate within a small range.

[0065] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or additions made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a sodium iron sulfate composite positive electrode material, characterized in that: The following steps are involved: The sodium iron sulfate precursor, carbon source, dispersant and metal acetate are ball-milled to obtain a mixture; the mixture is sintered under an inert atmosphere to obtain a sodium iron sulfate composite positive electrode material.

2. The method for preparing the sodium iron sulfate composite positive electrode material according to claim 1, characterized in that: The preparation method of the sodium iron sulfate precursor comprises the following steps: Sodium sulfate, ferrous sulfate and / or ferrous sulfate heptahydrate and an antioxidant are dissolved in water to obtain a slurry; the slurry is spray-dried to obtain a sodium ferric sulfate precursor.

3. The method for preparing the sodium iron sulfate composite positive electrode material according to claim 2, characterized in that: The chemical formula of the sodium iron sulfate precursor is Na 2x Fe y (SO4) x+y , wherein: 0<x≤5, 0<y≤5; sodium sulfate, ferrous sulfate and / or ferrous sulfate heptahydrate are added according to the stoichiometric ratio in the chemical formula of the sodium ferric sulfate precursor; The Fe in the ferrous sulfate and / or ferrous sulfate heptahydrate 2+ The molar ratio to antioxidant is 10:(1~2).

4. The method for preparing the sodium iron sulfate composite positive electrode material according to claim 2 or 3, characterized in that: The spray drying temperature is 150~200℃.

5. The method for preparing the sodium iron sulfate composite positive electrode material according to claim 1, characterized in that: The carbon source is one or more of acetylene black, Super P, Ketjen black, single-walled carbon nanotubes, multi-walled carbon nanotubes, and conductive graphite; the mass of the carbon source is 1.0-5.0wt% of the mass of the sodium iron sulfate precursor; The dispersant is one or more of polyethylene glycol-4000, polyethylene glycol-6000, sodium acrylate, and polyvinyl pyrrolidone; the mass of the dispersant is 0.1-1.0 wt% of the mass of the sodium ferric sulfate precursor.

6. The method for preparing the sodium iron sulfate composite positive electrode material according to claim 1, characterized in that: The metal acetate is one or more of nickel acetate, cobalt acetate, copper acetate, zinc acetate, manganese acetate, aluminum acetate, titanium acetate, magnesium acetate, and calcium acetate; the mass of the metal acetate is 0.5-2.0wt% of the mass of the sodium iron sulfate precursor.

7. The method for preparing the sodium iron sulfate composite positive electrode material according to claim 6, characterized in that: The metal acetate is at least three of nickel acetate, cobalt acetate, copper acetate, zinc acetate, manganese acetate, aluminum acetate, titanium acetate, magnesium acetate and calcium acetate.

8. The method for preparing the sodium iron sulfate composite positive electrode material according to claim 1, characterized in that: The inert atmosphere is a nitrogen atmosphere or an argon atmosphere; the sintering temperature is 350~400°C, and the sintering time is 0.5~1.5h.

9. A sodium iron sulfate composite positive electrode material, characterized in that: The method is prepared by any one of claims 1 to 8.

10. A sodium ion battery, characterized in that: Including the sodium iron sulfate composite positive electrode material as described in claim 9.

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