A preparation method of a simple carbon-coated anion-doped sodium iron sulfate as a positive electrode material of a sodium ion battery

Abstract

This invention discloses a simple method for preparing carbon-coated anion-doped sodium iron sulfate as a cathode material for sodium-ion batteries. A yellowish-brown NaFe₂PO₄(SO₄)₂ precursor powder is prepared using a simple sol-gel method, then calcined in an inert atmosphere to obtain a light green NaFe₂PO₄(SO₄)₂ material. This material is then ball-milled with conductive carbon black to obtain carbon-coated anion-doped sodium iron sulfate. The NaFe₂PO₄(SO₄)₂@C nanocomposite material prepared by this method exhibits superior sodium storage performance, high conductivity, and structural stability, making it suitable as a cathode material for high-performance sodium-ion batteries. The reaction principle is: Fe₂(SO₄)₃ + NaH₂PO₄ = NaFe₂PO₄(SO₄)₂ + H₂SO₄↑. This invention provides a simple, environmentally friendly, and low-cost preparation method that is easy to mass-produce.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium-ion battery cathode materials, specifically relating to a simple method for preparing carbon-coated anion-doped sodium iron sulfate as a sodium-ion battery cathode material. Background Technology

[0002] In recent decades, with the development of large-scale industrialization, traditional fossil fuels and other non-renewable energy sources such as coal, oil, and natural gas have been rapidly consumed, leading to several major problems including resource depletion, environmental degradation, the greenhouse effect, acid rain, and smog pollution. Environmentally friendly and renewable energy sources urgently need to be developed. However, traditional renewable energy systems, such as solar, wind, geothermal, and tidal power, are limited by geographical location and the intermittent and unstable nature of voltage output, making them ineffective in solving current energy problems. It is worth mentioning that based on Li... + Na + K + Mg 2+ Zn 2+ Batteries with alkali metal ions, as a simple and efficient electrochemical energy storage system, can achieve stable and efficient voltage output. Among them, sodium reserves (2.75 wt%) are much higher than lithium resources (0.002 wt%), and sodium-ion batteries with similar electrochemical performance to lithium-ion batteries have become the most competitive candidate for large-scale electrochemical energy storage systems.

[0003] Polyanionic materials, i.e., Na x M y (XO4) n (M = transition metal; X = S, P, Si, As, Mo, W) has a series of tetrahedral anionic units (XO4). n- Other derivatives (X) m O 3m+1 ) n- It can be classified into sulfates, phosphates, pyrophosphates, silicates, mixed phosphates, etc. When another electronegative atom X is introduced to form a MOX bond, the strength of the MO covalent bond weakens, and the bond with Na... + The energy difference between the / Na energy levels increases, thus generating a high voltage. Therefore, sulfates, with their high electronegativity, have attracted significant attention among various polyanionic materials. Compared to most other reported cathodes, most contain SO42-. 2- Due to the high electronegativity of S, multi-anionic cathodes with anionic groups can provide relatively high M. n+ / M (n-1)+ Redox voltage. Furthermore, the three-dimensional framework of the polyanion facilitates the provision of stable channels for ion / electron transport and buffers Na+. +The volume change caused by insertion / extraction may benefit the safe operation of sodium-ion batteries. Sulfates can be synthesized through various sustainable and inexpensive methods such as low-temperature solid-state methods, solvothermal methods, and ball milling, which can significantly reduce energy consumption and costs, thus promoting their commercialization. The development of high-potential electrode materials shows that the strong inductive effect of SO4 units gives them excellent electrochemical performance.

[0004] Iron-based sulfates, as a member of the sulfate cathode material system for sodium-ion batteries, possess high operating voltage and capacity, and are inexpensive and safe. However, iron-based sulfates are sensitive to water / oxygen, have poor conductivity, and are prone to decomposition and SO2 release at high temperatures. This limits their synthesis, storage, and application, hindering their commercialization. Some reports suggest addressing these issues through methods such as carbon coating and nanofabrication.

[0005] Another effective method to improve the conductivity of iron-based sulfates is element substitution. Element substitution also changes the structure of sulfates, thereby affecting their electrochemical performance. (For example, the presence of (XO4)) u- The framework structure of polyanionic (X = S, P, Si) structures has sufficiently large interstitial spaces to accommodate Na within a wide solid solution range. + Reversible insertion of ions. Most phosphates have good thermal stability and are easy to store. Introducing phosphate into sodium ferric sulfate can significantly improve its thermal stability and electronic conductivity, promote sodium ion transport and improve its electron migration kinetics, thereby effectively enhancing the electrochemical energy storage performance of the material. Summary of the Invention

[0006] The technical problem solved by this invention is to provide a simple method for preparing carbon-coated anion-doped sodium iron sulfate, which is simple in process, mild in reaction conditions, low in cost and high in reaction efficiency. The material prepared by this method has superior sodium storage performance and can be used as a positive electrode material for high-performance sodium-ion batteries.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: a simple method for preparing carbon-coated anion-doped sodium iron sulfate as a positive electrode material for sodium-ion batteries, characterized by the following specific steps:

[0008] Step S1: Preparation of anion-doped sodium ferric sulfate precursor

[0009] Ferric sulfate and anhydrous sodium dihydrogen phosphate were added to deionized water in sequence. After they were mixed evenly, they were heated and stirred into an orange-yellow gel-like solution. The solution was then placed in a vacuum oven to dry. After it was completely dried, it was taken out and ground into a light yellow precursor powder.

[0010] Step S2: Preparation of anion-doped sodium ferric sulfate cathode material

[0011] The precursor powder obtained in step S1 was subjected to an Ar atmosphere at 1-5℃ for min. -1 The temperature is increased to 400-600℃ and heat-treated for 12-24 hours to obtain an anion-doped sodium ferric sulfate material; preferably, the temperature is increased to 1℃ / min. -1 The temperature was increased to 500℃ and heat-treated for 18 hours.

[0012] Step S3: Preparation of carbon-coated anion-doped sodium ferric sulfate nanomaterials

[0013] The anion-doped sodium ferric sulfate powder obtained in step S2 and the carbon-based coating material were added to an agate ball milling jar in a certain proportion, and agate ball milling beads were added for ball milling to obtain carbon-coated anion-doped sodium ferric sulfate material.

[0014] Further, in the above technical solution, the specific process of step S1 is as follows: ferric sulfate is weighed and added to deionized water, and solution A is obtained by stirring. Then, anhydrous sodium dihydrogen phosphate is added to solution A, and solution B is obtained by stirring. After the mixture is evenly mixed, the heating plate is turned on and the heating temperature is set to 120-150℃. Under magnetic stirring, the water is evaporated to an orange-yellow gel state. Heating is stopped, and the mixture is placed in a vacuum oven at 80-120℃ for vacuum drying overnight. The next day, the crystals are taken out and ground into a powder precursor. The mass ratio of ferric sulfate to anhydrous sodium dihydrogen phosphate is 3.995-3.998:1.480-1.485.

[0015] Furthermore, in the above technical solution, in step S1, 3.995-3.998g of ferric sulfate is weighed and added to 100ml of deionized water, and the solution is obtained by magnetic stirring at 400rpm for 5min. Then, 1.480-1.485g of anhydrous sodium dihydrogen phosphate is added to solution A, and the solution is obtained by magnetic stirring at 400rpm for 20min.

[0016] Furthermore, in the above technical solution, the specific process of step S3 is as follows: weigh NaFe2PO4(SO4)2 and carbon-based coating material into an agate ball mill jar, add 10g of agate grinding beads, rotate at 300-500rpm, and mill for 4 hours. The mass ratio of NaFe2PO4(SO4)2 to carbon-based coating material is 0.8-4:0.2.

[0017] Furthermore, in the above technical solution, the carbon-based encapsulation material includes Super P, acetylene black, Ketjen black, carbon nanotubes, and reduced graphene oxide.

[0018] Furthermore, in the above technical solution, the anion-doped sodium ferric sulfate material is specifically a NaFe2PO4(SO4)2@C nanocomposite material, in which amorphous C accounts for approximately 15%-20% of the mass percentage of the NaFe2PO4(SO4)2@C nanocomposite material.

[0019] This invention provides the application of a simple carbon-coated anion-doped sodium iron sulfate material prepared by the above method as a high-performance cathode material for sodium-ion batteries.

[0020] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0021] 1. This invention employs a sol-gel method, which allows for rapid dispersion of raw materials, forming gel blocks with excellent molecular-level uniformity. This enables uniform doping at the molecular level and facilitates easy chemical reactions. The preparation method is simple, low-cost, environmentally friendly, and produces high yields, making it suitable for mass production and collection. The ferric sulfate and sodium dihydrogen phosphate used in the preparation process can be uniformly dissolved in an aqueous solution. The reaction equation is: Fe2(SO4)3 + NaH2PO4 = NaFe2PO4(SO4)2 + H2SO4↑. The sulfuric acid vapor generated in the reaction evaporates along with water vapor as the oil bath temperature increases. Unlike traditional methods for synthesizing sodium ferric sulfate, the iron source (ferric sulfate) used in this synthesis is inexpensive and readily available, and the reaction does not produce the greenhouse gas CO2, thus contributing to environmental protection.

[0022] 2. The structural advantage of this invention is that, through low-temperature synthesis of NaFe2PO4(SO4)2 material, its main framework still maintains the hexagonal NASICON (the compound with the highest electrical conductivity in solid solutions) structure, providing support for Na + A fast-conducting framework structure may have two Na+ atoms. + / The capacity of the formulation unit, the crystal has a three-dimensional sodium ion diffusion channel formed by P / SO4 tetrahedra and FeO6 octahedra connected by shared O atoms at common points.

[0023] 3. The present invention prepares a yellowish-brown NaFe2PO4(SO4)2 colloidal precursor by sol-gel method, which is then dried, ground, and calcined in an inert atmosphere to obtain a light green NaFe2PO4(SO4)2 nanomaterial with excellent conductivity. By introducing phosphate into sodium ferric sulfate, the poor thermal stability and sensitivity to moisture in the air of sulfate are improved. On the other hand, the P atoms partially replace the SO4 tetrahedra to form P / SO4 tetrahedra, which expands the sodium ion transport channels, reduces the sodium ion diffusion distance, and improves the inherent low electronic conductivity of sulfate.

[0024] 4. The NaFe2PO4(SO4)2@C nanocomposite material prepared in this invention is produced by mechanically ball milling the prepared NaFe2PO4(SO4)2 with carbon sources such as super p. Through multiple contacts with conductive carbon black, electrons are conducted into the active material. As the ball milling time increases, the coating of the active material can be completely covered, the particle size of the material gradually decreases, and its capacity and kinetic properties are significantly enhanced. Its one-dimensional conductive structure can greatly shorten the electron movement path and form a cross-linked conductive network.

[0025] 5. This invention prepares NaFe2PO4(SO4)2@C cathode material using a combination of sol-gel and ball milling methods. First, sodium dihydrogen sulfate is added to the mixed ferric sulfate solution in small, frequent additions to control the dropping rate and ensure a more complete reaction. Second, the grinding process following the evaporation of the precursor ensures more uniform mixing and increases the relative surface area of ​​the solid material, thus better stabilizing the crystal structure during calcination. Finally, simple carbon coating is achieved through ball milling, saving reaction time and effectively coating the material with carbon, improving its conductivity and electrochemical performance.

[0026] 6. Electrochemical test results of the assembled battery show that the NaFe2PO4(SO4)2@C cathode material exhibits good performance at 5 mA g. -1 After activation, at 10 mA g -1 Under certain conditions, the capacity retention rate after 100 cycles is 90%, while the NaFe2PO4(SO4)2 cathode material retains 90% capacity at 5 mA g. -1 After activation, at 10 mA g -1 Under certain conditions, the capacity retention rate after 100 cycles was only 76.9%. Although the NaFe(SO4)2 cathode material prepared in Comparative Example 2 maintained a capacity retention rate of only 76.9% after 5 mAg cycles... -1 After activation, at 10 mA g -1 Under the given conditions, the capacity retention rate after 100 cycles is approximately 90%, but its initial cycle specific capacity is significantly lower than that of the present invention. This indicates that the NaFe2PO4(SO4)2@C cathode material prepared in this invention exhibits superior electrochemical performance compared to the NaFe(SO4)2 cathode material. Attached Figure Description

[0027] Figure 1 X-ray diffraction patterns of the NaFe2PO4(SO4)2@C cathode material prepared in Example 1, the NaFe2PO4(SO4)2 cathode material prepared in Comparative Example 1, and the NaFe(SO4)2 prepared in Comparative Example 2.

[0028] Figure 2The image shows a scanning electron microscope image of the NaFe(SO4)2 material prepared in Comparative Example 2.

[0029] Figure 3 The image shows a scanning electron microscope image of the NaFe2PO4(SO4)2 cathode material obtained in Comparative Example 1.

[0030] Figure 4 This is a scanning electron microscope image of the NaFe2PO4(SO4)2@C cathode material obtained in Example 1;

[0031] Figure 5 The rate performance diagrams show the NaFe2PO4(SO4)2@C cathode material prepared in Example 1, the NaFe2PO4(SO4)2 cathode material prepared in Comparative Example 1, and the NaFe(SO4)2 cathode material prepared in Comparative Example 2 as cathode materials for sodium-ion batteries.

[0032] Figure 6 The diagram shows the cycle performance of the NaFe2PO4(SO4)2@C cathode material prepared in Example 1, the NaFe2PO4(SO4)2 cathode material prepared in Comparative Example 1, and the NaFe(SO4)2 cathode material prepared in Comparative Example 2 as cathode materials for sodium-ion batteries. Detailed Implementation

[0033] The following examples further illustrate the above-described content of the present invention, but it should not be construed as limiting the scope of the subject matter of the present invention to the following examples. All technologies implemented based on the above-described content of the present invention fall within the scope of the present invention.

[0034] Example 1

[0035] Preparation of NaFe2PO4(SO4)2@C nanocomposites

[0036] Dissolve 3.998 g of ferric sulfate in 100 ml of deionized water and stir magnetically at 400 rpm for 5 min. Then add 1.485 g of anhydrous sodium dihydrogen phosphate to the solution and stir magnetically at 400 rpm for 30 min. Once the mixture is homogeneous, turn on the heating plate and heat to 120 °C. Stir magnetically until a yellowish-brown gel is formed. Stop heating and place the mixture in a vacuum oven at 80 °C to dry overnight. The next day, remove the crystals and grind them into a powder precursor. In an Ar atmosphere, heat the powder at 1 °C for 1 min. -1The temperature was increased to 500℃ and heat-treated for 18 hours to obtain an anion-doped sodium iron sulfate material NaFe2PO4(SO4)2. 0.8000g of NaFe2PO4(SO4)2 and 0.2g of super p were weighed into an agate ball mill jar, and 10g of agate grinding beads were added for ball milling to obtain carbon-coated anion-doped sodium iron sulfate material. The mass ratio of the sodium iron sulfate material to the carbon-based coating material was 80:20; the ball-to-material ratio in the ball milling process was 20:1, the rotation speed was 300 rpm, and the milling time was 4 hours.

[0037] Comparative Example 1

[0038] Preparation of NaFe2PO4(SO4)2 cathode material (without carbon coating)

[0039] 3.998 g of ferric sulfate was dissolved in 100 ml of deionized water and magnetically stirred at 400 rpm for 5 min. Then, 1.485 g of anhydrous sodium dihydrogen phosphate was added to the solution, and the mixture was magnetically stirred at 400 rpm for 30 min. After thorough mixing, the heating plate was opened, and the temperature was raised to 120 °C. The mixture was then magnetically stirred until a yellowish-brown gel was formed. Heating was stopped, and the mixture was placed in a vacuum oven at 80 °C and vacuum-dried overnight. The next day, the crystals were removed and ground into a powder precursor. The resulting precursor powder was then subjected to an Ar atmosphere at 1 °C for 1 min. -1 The temperature was increased to 500℃ and heat-treated for 18 hours to obtain an anion-doped sodium ferric sulfate material.

[0040] Comparative Example 2

[0041] Preparation of NaFe(SO4)2 cathode material (no carbon coating, no anion doping)

[0042] Weigh 4.8219 g of NH4Fe(SO4)2·12H2O and place it in a beaker containing 50 ml of deionized water. Stir magnetically at 300 rpm for 5 minutes until homogeneous. Add 0.8401 g of NaHCO3 slowly while stirring. Stir magnetically at 300 rpm for 30 minutes until homogeneous. Transfer the mixture to an oil bath and stir magnetically at 80°C until the water evaporates to a white gel state. Stop heating and place the mixture in a vacuum oven at 80°C to dry overnight. The next day, remove the crystals and grind them into a powder precursor. In an air atmosphere, heat the resulting precursor powder at 1°C for 1 minute... -1 The temperature was increased to 200℃ and heat-treated for 12 hours at a rate of 1℃ / min. -1 The temperature was increased to 350℃ and heat-treated for 24 hours to obtain NaFe(SO4)2 cathode material.

[0043] Example 2

[0044] Preparation of NaFe2PO4(SO4)2@C nanocomposites

[0045] 3.998 g of ferric sulfate and 1.485 g of anhydrous sodium dihydrogen phosphate were dissolved in 100 ml of deionized water and magnetically stirred at 400 rpm for 30 min until homogeneous. The mixture was then transferred to an oil bath and evaporated at 120 °C with magnetic stirring until a gel state was reached. Heating was stopped, and the mixture was placed in a vacuum oven at 80 °C and dried overnight. The next day, the crystals were removed and ground into a powder precursor. The resulting precursor powder was then subjected to an Ar atmosphere at 1 °C for 1 min. -1 The temperature was increased to 500℃ and heat-treated for 18 hours to obtain an anion-doped sodium ferric sulfate material. The obtained anion-doped sodium ferric sulfate powder and Super P were added to an agate ball mill jar in a specific ratio, and agate grinding balls were added for ball milling to obtain carbon-coated anion-doped sodium ferric sulfate material. The mass ratio of the sodium ferric sulfate material to the carbon-based coating material was 70:30; the ball-to-material ratio in the ball milling process was 20:1, the rotation speed was 300 rpm, and the milling time was 4 hours.

[0046] Example 3

[0047] Preparation of NaFe2PO4(SO4)2@C nanocomposites

[0048] 3.998 g of ferric sulfate and 1.485 g of anhydrous sodium dihydrogen phosphate were dissolved in 100 ml of deionized water. The mixture was magnetically stirred at 400 rpm for 30 min until homogeneous. The solution was then transferred to an oil bath and evaporated at 120 °C with magnetic stirring until a gel state was reached. Heating was stopped, and the mixture was placed in a vacuum oven at 80 °C and dried overnight. The next day, the crystals were removed and ground into a powder precursor. The resulting precursor powder was then subjected to Ar atmosphere at 1 °C for 1 min. -1 The temperature was increased to 500℃ and heat-treated for 18 hours to obtain an anion-doped sodium ferric sulfate material. The obtained anion-doped sodium ferric sulfate powder and Super P were added to an agate ball mill jar in a specific ratio, and agate grinding balls were added for ball milling to obtain carbon-coated anion-doped sodium ferric sulfate material. The mass ratio of the sodium ferric sulfate material to the carbon-based coating material was 60:40; the ball-to-material ratio in the ball milling process was 20:1, the rotation speed was 300 rpm, and the milling time was 4 hours.

[0049] Figure 1 XRD was used to characterize the NaFe2PO4(SO4)2@C nanocomposite material obtained in Example 1, the NaFe2PO4(SO4)2 cathode material obtained in Comparative Example 1, and the NaFe(SO4)2 cathode material obtained in Comparative Example 2. Figure 1It can be seen that the XRD peak positions of NaFe2PO4(SO4)2@C and NaFe2PO4(SO4)2 are consistent, and the peak intensity of NaFe2PO4(SO4)2@C decreases compared to NaFe2PO4(SO4)2, proving that the carbon-based material is well coated on the surface of NaFe2PO4(SO4)2, with fewer impurity peaks and a purer crystal phase. Both the synthesized NaFe2PO4(SO4)2@C and NaFe2PO4(SO4)2 cathode materials show good agreement with the standard card (PDF#85-2265) of the NaTi2(PO4)3 phase. The NaFe(SO4)2 cathode material synthesized in Comparative Example 2 also shows good agreement with the standard card (PDF#27-0718) of the NaFe(SO4)2 phase. Figure 1 It can be seen that the position of the XRD peak of NaFe2PO4(SO4)2 has changed compared with NaFe(SO4)2, but the position of the main peak remains the same, indicating that the crystal structure of the anion-doped composite material is consistent with that of NaFe(SO4)2.

[0050] The NaFe2PO4(SO4)2@C cathode material obtained in Example 1 was characterized by SEM, such as... Figure 4 As shown in the figure, the NaFe2PO4(SO4)2@C cathode material prepared in Example 1 has a smoother and flatter surface compared with the NaFe2PO4(SO4)2 cathode material obtained in Comparative Example 1, and its particle size is smaller. This proves that the carbon material is well coated on the surface to form a conductive layer, which lays a good foundation for the subsequent improvement of electrochemical performance.

[0051] Application examples

[0052] The NaFe2PO4(SO4)2 prepared in Comparative Example 1, the NaFe(SO4)2 prepared in Comparative Example 2, and the NaFe2PO4(SO4)2@C cathode materials prepared in Example 1 were ground and mixed with conductive carbon (SuperP) and binder (PVDF) in a mass ratio of 7:2:1. A certain amount of N-methylpyrrolidone (NMP) was then added dropwise to form a slurry, which was uniformly coated onto an aluminum foil current collector to obtain the working electrode. Metallic sodium was used as the counter electrode, and a glass fiber microporous filter membrane (GF / A) was used as the separator. 1 mol L -1 NaClO4 was used as the electrolyte, and batteries were assembled in a glove box. After being left to stand in air for 10 hours, the assembled batteries were subjected to charge-discharge tests on a Newway charge-discharge tester, with the test range being 2-4.4V. At 0.005A g... -1 0.01A g -1 0.02A g -1 0.03A g -1 0.05A g-1 0.08Ag -1 0.1A g -1 The rate performance of the assembled battery was tested at a current density of 0.01 A g. -1 The cycle performance of the assembled battery was tested under the current density conditions.

[0053] from Figure 5 As can be seen from the data, the NaFe2PO4(SO4)2@C cathode material prepared in Example 1 exhibits good performance at 0.005A g. -1 At current density, the reversible specific capacity reached 97 mAh g for the first time. -1 The NaFe2PO4(SO4)2 cathode material prepared in Comparative Example 1 was tested at 0.005 A g. -1 At current density, the initial reversible specific capacity is only 60 mAh g. -1 The NaFe(SO4)2 cathode material prepared in Comparative Example 2 was tested at 0.005 A g. -1 At a current density of only 15 mAh g, the initial reversible specific capacity is only 15 mAh g. -1 .from Figure 6 As can be seen from the data, the NaFe2PO4(SO4)2@C cathode material prepared in Example 1 has a compatibility with 0.005A g. -1 Activation was performed at 0.01 A g. -1 Under the given conditions, the capacity retention rate after 100 cycles was 89%, while the NaFe2PO4(SO4)2 cathode material prepared in Comparative Example 1 exhibited a capacity retention rate of 0.005 A g. -1 After activation, at 0.01 A g -1 Under certain conditions, the capacity retention rate after 100 cycles was only 84%, while the NaFe(SO4)2 cathode material prepared in Comparative Example 2 had a capacity retention rate of only 84% after 0.005 A g. -1 Activation at 0.01 Ag -1 Although the capacity retention rate after 100 cycles under the given conditions was 90%, its initial specific capacity was too low. This indicates that the NaFe2PO4(SO4)2@C cathode material prepared in this example exhibits better rate performance and cycle stability compared to the NaFe2PO4(SO4)2 cathode materials prepared in Comparative Example 1 and Comparative Example 2 when used as cathode materials for sodium-ion batteries.

[0054] The above embodiments describe the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are only illustrative of the principles of the present invention. Various changes and modifications can be made to the present invention without departing from the scope of the principles of the present invention, and all such changes and modifications fall within the protection scope of the present invention.

Claims

1. A method for preparing a simple carbon-coated anion-doped sodium iron sulfate material as a positive electrode material for sodium-ion batteries, characterized in that The specific steps are as follows: Step S1: Preparation of anion-doped sodium ferric sulfate precursor Ferric sulfate and anhydrous sodium dihydrogen phosphate were added to deionized water in sequence. After they were mixed evenly, they were heated and stirred into an orange-yellow gel-like solution. The solution was then placed in a vacuum oven to dry. After it was completely dried, it was taken out and ground into a light yellow precursor powder. Step S2: Preparation of anion-doped sodium ferric sulfate cathode material The precursor powder obtained in step S1 was subjected to an Ar atmosphere at 1-5℃ for min. -1 The temperature was increased to 400-600℃ and heat-treated for 12-24 hours to obtain an anion-doped sodium ferric sulfate material. Step S3: Preparation of carbon-coated anion-doped sodium ferric sulfate nanomaterials The anion-doped sodium ferric sulfate powder obtained in step S2 and the carbon-based coating material were added to an agate ball milling jar in a certain proportion, and agate ball milling beads were added for ball milling to obtain carbon-coated anion-doped sodium ferric sulfate material. The specific process of step S1 is as follows: Ferrous sulfate is weighed and added to deionized water, and solution A is obtained by stirring. Then, anhydrous sodium dihydrogen phosphate is added to solution A, and solution B is obtained by stirring. After the mixture is evenly mixed, the heating plate is turned on and the heating temperature is set to 120-150℃. Under magnetic stirring, the water is evaporated to an orange-yellow gel state. Heating is stopped, and the mixture is placed in a vacuum oven at 80-120℃ and vacuum dried overnight. The next day, the crystals are taken out and ground into a powder precursor. The mass ratio of ferrous sulfate to anhydrous sodium dihydrogen phosphate is 3.995-3.998:1.480-1.

485. The carbon-coated anion-doped sodium iron sulfate material is specifically a NaFe2PO4(SO4)2@C nanocomposite material, in which amorphous C accounts for 15%-20% of the mass percentage of the NaFe2PO4(SO4)2@C nanocomposite material.

2. The process for the preparation of a simple carbon-coated anion-doped sodium iron sulfate material as a positive electrode material for sodium-ion batteries according to claim 1, characterized in that The specific process of step S3 is as follows: weigh the anion-doped sodium ferric sulfate material and the carbon-based coating material into an agate ball milling jar, add 10g of agate ball milling beads, rotate at 300-500rpm, and mill for 4h. The mass ratio of the anion-doped sodium ferric sulfate material to the carbon-based coating material is 0.8-4:0.

2.

3. The process for the preparation of simple carbon-coated anion-doped sodium iron sulfate material as a positive electrode material for sodium-ion batteries according to claim 1, characterized in that Carbon-based coating materials include Super P, acetylene black, Ketjen black, carbon nanotubes, and reduced graphene oxide.

4. The application of the simple carbon-coated anion-doped sodium iron sulfate material prepared by the method according to any one of claims 1-3 as a cathode material for sodium-ion batteries.

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