Aluminum-doped sodium iron phosphate cathode material and preparation method thereof

Abstract

The application discloses an aluminum-doped sodium iron phosphate-sulfate positive electrode material and a preparation method thereof, and belongs to the technical field of sodium ion battery materials. 3.5 Fe 2‑3x Al 2x (SO4)3(PO4) 0.5 / C, wherein 0.03<=x<=0.2; through aluminum ion doping combined with carbon coating modification, a preparation process of dry mixing-coarse grinding-spray drying-pre-sintering-ultrafine grinding-spray drying-sintering is used, the Fe-Fe bond length in the sodium iron phosphate-sulfate crystal is effectively increased, the coulomb repulsion is reduced, and iron ions are prevented from escaping from original sites in the charging and discharging process. The prepared positive electrode material is high in purity, complete in crystal phase, stable in structure, excellent in ion transmission capacity and electronic conductivity, and greatly improved in discharge specific capacity, rate performance and cycle stability; and the preparation process is simple and controllable, and can be widely applied to the preparation of sodium ion batteries, and is especially suitable for large-scale energy storage fields.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery materials technology, and in particular to an aluminum-doped sodium iron sulfate phosphate cathode material and its preparation method. Background Technology

[0002] Energy storage systems (ESS) are the core support for new power systems and the large-scale application of renewable energy. Electrochemical energy storage technology has become an important development direction in the energy storage field due to its advantages such as fast response and flexible configuration. Lithium-ion batteries have been widely used in power batteries, consumer electronics, and large-scale energy storage. However, due to the limited reserves and uneven distribution of lithium resources, the cost and supply pressure of lithium salts are becoming increasingly prominent, making it difficult to meet the continued expansion demand of the future large-scale energy storage market.

[0003] Sodium and lithium both belong to the alkali metal element family and possess similar electrochemical properties. Furthermore, sodium resources are abundant, widely distributed, and inexpensive, making sodium-ion batteries a promising new rechargeable battery technology with significant potential for large-scale commercialization. Among the polyanionic cathode materials for sodium-ion batteries, sodium iron sulfate phosphate (Na₂SO₄) is a common choice. 3.5 Fe2(SO4)3(PO4) 0.5 NFSP has significant advantages such as high theoretical capacity, high operating voltage (3.8V) and low volume expansion (<4%), making it a preferred cathode material for sodium-ion batteries.

[0004] However, NFSP cathode materials suffer from technical drawbacks such as low intrinsic conductivity and poor cycle performance, especially the capacity decay during charge and discharge, which limits their industrial application. Current technologies often employ strategies such as carbon coating, introducing crystal defects, controlling iron defects, crystal form regulation, and interface modification to modify NFSP materials. However, these modification strategies still have significant room for optimization and struggle to simultaneously achieve optimal capacity, rate performance, and cycle performance. Therefore, further research is needed on the structural design and performance improvement of sodium iron phosphate sulfate-based materials. Summary of the Invention

[0005] The purpose of this invention is to provide an aluminum-doped sodium ferric sulfate phosphate cathode material and its preparation method. By combining aluminum ion doping with a specific preparation process, the structure and electrochemical performance of the cathode material are improved.

[0006] To achieve the above objectives, the present invention provides an aluminum-doped sodium iron sulfate phosphate cathode material, wherein the cathode material is a carbon-coated spherical nanostructure with a chemical composition of Na. 3.5 Fe 2-3x Al 2x (SO4)3(PO4) 0.5 / C, where 0.03≤x≤0.2.

[0007] This invention also provides a method for preparing the above-mentioned aluminum-doped sodium ferric sulfate phosphate cathode material, comprising the following steps: S1. Weigh out sodium source compound, aluminum source compound, iron source compound, phosphorus source compound, sulfur source compound and carbon source compound, add solvent and mix evenly to obtain a mixed slurry; S2. The mixed slurry is subjected to wet coarse grinding until the particle size is 200-600nm to obtain coarse grinding slurry. The coarse grinding slurry is then spray-dried to obtain dried precursor powder. S3. Place the dried precursor powder in an inert gas or an argon-hydrogen mixture and pre-calcine it at 300-450℃ to obtain the pre-calcined product. S4. The pre-calcined product is subjected to wet ultrafine grinding until the particle size is 20-300nm to obtain an ultrafine grinding slurry. The ultrafine grinding slurry is then spray-dried to obtain a secondary dried powder. S5. The secondary dried powder is placed in an inert gas or an argon-hydrogen mixture and sintered at 450-600℃ to obtain aluminum-doped sodium iron sulfate phosphate cathode material.

[0008] Preferably, in step S1, the aluminum source compound is one or more of the following: aluminum oxide, aluminum hydroxide, aluminum nitrate, aluminum chloride, aluminum sulfate, aluminum phosphate, aluminum isopropoxide, aluminum acetylacetone, aluminum stearate, aluminum fluoride, aluminum dihydrogen phosphate, aluminum acetate, aluminum ethoxide, aluminum trifluoroacetate, and aluminum methacrylate.

[0009] Preferably, in step S1, the sodium source compound is one or more of the following: sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium nitrate, sodium fluoride, sodium fluorophosphate, sodium trifluoroacetate, sodium oxalate, sodium acetate, sodium persulfate, sodium hydroxide, sodium formate, sodium citrate, sodium pyrophosphate, sodium dihydrogen pyrophosphate, and sodium alginate. The phosphorus source compound is one or more of the following: sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, sodium dihydrogen pyrophosphate, phosphoric acid, ammonium dihydrogen phosphate, and triammonium phosphate; The iron source compound is one or more of the following: reduced iron powder, iron(II,III) oxide, iron oxide, ferrous oxide, ferric nitrate, ferric chloride, ferric sulfate, ferric phosphate, ferric oxalate, ferrous oxalate, ferric acetate, ferric citrate, and ferric fluoride. The sulfur source compound is one or more of the following: sodium sulfide, sodium sulfide nonahydrate, sodium hydrosulfide, sodium sulfate, sodium sulfite, sodium thiosulfate, ammonium sulfide, ammonium thiocyanate, ammonium sulfate, thiourea, dithioglycolamide, cysteine, dimethyl sulfoxide, sodium persulfate, potassium sulfide, ferrous sulfide, and sodium metabisulfite. The carbon source compound is one or more of glucose, sucrose, citric acid, malic acid, polyaniline, polyethylene glycol, adipic acid, phenolic resin, polypyrrole, ascorbic acid, chitosan, and soluble starch.

[0010] Preferably, in step S1, the solvent is a mixture of water and ethanol, wherein the molar ratio of water to ethanol is 1:1.

[0011] Preferably, both the wet coarse grinding in step S2 and the wet ultrafine grinding in step S4 are performed using a sand mill, with a grinding speed of 2000 r / min; wherein the grinding time for wet coarse grinding is 3 hours and the grinding time for wet ultrafine grinding is 2 hours.

[0012] Preferably, the grinding particle size of the wet coarse grinding in step S2 is 300 nm; and the grinding particle size of the wet ultrafine grinding in step S4 is 120 nm.

[0013] Preferably, the spray drying in steps S2 and S4 adopts one of centrifugal spray, two-fluid spray, and four-fluid spray; the inlet temperature of the spray dryer is 200-350℃ and the outlet temperature is 60-120℃.

[0014] Preferably, the pre-firing temperature in step S3 is 350°C; and the sintering temperature in step S5 is 550°C.

[0015] Preferably, the inert gas in steps S3 and S5 is nitrogen or argon; the volume content of hydrogen in the argon-hydrogen mixture is 5%.

[0016] Therefore, the aluminum-doped sodium ferric sulfate phosphate cathode material and its preparation method provided by the present invention have the following beneficial effects: (1) Suppressing iron ion deposition and improving cycle stability: This invention effectively increases the Fe-Fe bond length by doping aluminum ions with larger ionic radii into the NFSP crystal structure, thereby reducing the Coulomb repulsion inside the crystal. This suppresses the phenomenon that iron ions decompose from their original sites and occupy sodium sites during high-voltage charging of NFSP materials, avoids the capacity decay problem caused by the inability to fully insert during the migration back during discharge, and ensures that sodium ions can be smoothly and completely inserted during discharge, thus improving the cycle stability of the battery.

[0017] (2) Improve electronic conductivity and ion transport capability: On the one hand, the present invention optimizes the crystal structure of the material by aluminum doping, and on the other hand, forms an effective conductive network by carbon coating. At the same time, the multi-step grinding-spray drying-sintering process is adopted to make the material particles small and uniform with high crystallinity, which synergistically improves the electronic conductivity and sodium ion diffusion rate of the material, thereby improving the discharge specific capacity and rate performance of the cathode material.

[0018] (3) Raw materials are readily available and the process is simple: The raw materials of this invention are widely available and inexpensive. Sodium and aluminum are both common chemical raw materials, avoiding the use of rare metals. This meets the development needs of sodium-ion batteries for low cost and large scale. Furthermore, the process of wet mixing, multi-stage grinding combined with spray drying and two-step sintering is simple and reasonable in process design, and the process parameters are controllable, making it easy to achieve large-scale production.

[0019] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0020] Figure 1 This is a comparison chart of the rate performance of Embodiment 1 and Comparative Example 1 of the present invention; Figure 2 This is a SEM image of Example 1 of the present invention at a magnification of 5 μm. Detailed Implementation

[0021] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention should be considered equivalent substitutions and are included within the protection scope of the present invention. Furthermore, it should be understood that after reading the contents of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims and are all within the protection scope of the present invention.

[0022] In this document, the term "embodiment" means that a specific feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The term "embodiment" appearing in various places throughout the specification does not necessarily refer to the same embodiment, nor does it specifically limit its independence or connection with other embodiments. In principle, in this application, as long as there are no technical contradictions or conflicts, the technical features mentioned in each embodiment can be combined in any way to form corresponding implementable technical solutions.

[0023] Unless otherwise defined, the technical terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the use of related terms herein is merely for the purpose of describing particular embodiments and is not intended to limit this application.

[0024] Unless otherwise specified, the reagents, instruments, and equipment used in this invention are all commonly used by those skilled in the art, and the testing standards all use national or international standards commonly used in the field, without further explanation.

[0025] Example 1 This embodiment provides an NFSP sodium ion cathode material Na 3.5 Fe 1.85 Al 0.1 (SO4)3(PO4) 0.5 The preparation method of / C includes the following steps: S1. Weigh 0.15 mol sodium sulfate, 0.075 mol sodium carbonate, 0.278 mol ferrous sulfate, 0.075 mol sodium dihydrogen phosphate, 0.0075 mol aluminum sulfate, and 0.01 mol glucose, and dissolve them in 150 mL of water / ethanol (the molar ratio of ethanol to water is 1:1). Mix thoroughly to obtain a mixed slurry.

[0026] S2. Transfer the mixed slurry to a sand mill and sand mill at a speed of 2000 r / min. After grinding for 3 hours, a coarsely ground slurry with a particle size of 300 nm is obtained. The coarsely ground slurry is then centrifugally spray-dried at an inlet temperature of 280℃ and an outlet temperature of 90℃ to obtain a dried precursor powder.

[0027] S3. Place the dried precursor powder in a tube furnace with a hydrogen-argon mixed gas atmosphere (hydrogen volume content 5%) and pre-calcine it at 350°C for 5 hours to obtain the pre-calcined product.

[0028] S4. Take out the pre-calcined product and transfer it to a sand mill. Sand mill at a speed of 2000 r / min for 2 hours to obtain an ultrafine slurry with a particle size of 120 nm. Centrifugal spray drying is used on the ultrafine slurry with an inlet temperature of 280℃ and an outlet temperature of 90℃ to obtain a secondary dried powder.

[0029] S5. The secondary dried powder was placed in a tube furnace with a hydrogen-argon mixed gas atmosphere (hydrogen volume content 5%) and sintered at 550℃ for 8 hours. The resulting solid product was the aluminum-carbon coated aluminum-iron-sodium phosphate sulfate composite material with a high specific surface area. The specific surface area of ​​the prepared composite material was 12 m². 2 / g.

[0030] The composite material prepared in this embodiment was mixed with SP and PVDF in a ratio of 8:1:1 and stirred evenly. After coating and drying for 3 hours, it was taken out and rolled. Positive electrode sheets with a diameter of 8 mm were punched out using a punching machine. After weighing, they were assembled in a glove box. The assembly was carried out in the following order: negative electrode shell, gasket, sodium sheet, separator, electrolyte, positive electrode sheet, gasket, and positive electrode shell. Finally, the battery was sealed using a sealing machine to complete the battery assembly. The assembled coin cells were subjected to standard electrochemical performance tests.

[0031] Test results show that at 1C rate, the discharge specific capacity reaches 131mAh / g after 500 cycles, with a capacity retention rate of 91.3%.

[0032] Example 2 This embodiment provides an NFSP sodium ion cathode material Na 3.5 Fe 1.7 Al 0.2 (SO4)3(PO4) 0.5 The preparation method of / C includes the following steps: S1. Weigh 0.15 mol sodium sulfate, 0.075 mol sodium carbonate, 0.256 mol ferrous sulfate, 0.075 mol sodium dihydrogen phosphate, 0.015 mol aluminum sulfate, and 0.01 mol glucose, and dissolve them in 150 mL of water / ethanol (the molar ratio of ethanol to water is 1:1). Mix thoroughly to obtain a mixed slurry.

[0033] S2. Transfer the mixed slurry to a sand mill and sand mill at a speed of 2000 r / min. After grinding for 3 hours, a coarsely ground slurry with a particle size of 300 nm is obtained. The coarsely ground slurry is then centrifugally spray-dried at an inlet temperature of 280℃ and an outlet temperature of 90℃ to obtain a dried precursor powder.

[0034] S3. Place the dried precursor powder in a tube furnace with a hydrogen-argon mixed gas atmosphere (hydrogen volume content 5%) and pre-calcine it at 350°C for 5 hours to obtain the pre-calcined product.

[0035] S4. Take out the pre-calcined product and transfer it to a sand mill. Sand mill at a speed of 2000 r / min for 2 hours to obtain an ultrafine slurry with a particle size of 120 nm. Centrifugal spray drying is used on the ultrafine slurry with an inlet temperature of 280℃ and an outlet temperature of 90℃ to obtain a secondary dried powder.

[0036] S5. The secondary dried powder is placed in a tube furnace with a hydrogen-argon mixed gas atmosphere (hydrogen volume content 5%) and sintered at 525℃ for 8 hours. The resulting solid product is a high specific surface area aluminum-carbon coated aluminum phosphate-iron sodium sulfate composite material.

[0037] The composite material prepared in this embodiment was mixed with SP and PVDF in a ratio of 8:1:1 and stirred evenly. After coating and drying for 3 hours, it was taken out and rolled. Positive electrode sheets with a diameter of 8 mm were punched out using a punching machine. After weighing, they were assembled in a glove box. The assembly was carried out in the following order: negative electrode shell, gasket, sodium sheet, separator, electrolyte, positive electrode sheet, gasket, and positive electrode shell. Finally, the battery was sealed using a sealing machine to complete the battery assembly. The assembled coin cells were subjected to standard electrochemical performance tests.

[0038] Test results show that at 1C rate, the discharge specific capacity reaches 125mAh / g after 500 cycles, with a capacity retention rate of 88.5%.

[0039] Example 3 This embodiment provides an NFSP sodium ion cathode material Na 3.5 Fe 1.91 Al 0.06 (SO4)3(PO4) 0.5 The preparation method of / C includes the following steps: S1. Weigh 0.15 mol sodium sulfate, 0.075 mol sodium carbonate, 0.287 mol ferrous sulfate, 0.075 mol sodium dihydrogen phosphate, 0.0045 mol aluminum sulfate, and 0.01 mol glucose, and dissolve them in 150 mL of water / ethanol (the molar ratio of ethanol to water is 1:1). Mix thoroughly to obtain a mixed slurry.

[0040] S2. Transfer the mixed slurry to a sand mill and sand mill at a speed of 2000 r / min. After grinding for 3 hours, a coarsely ground slurry with a particle size of 300 nm is obtained. The coarsely ground slurry is then centrifugally spray-dried at an inlet temperature of 280℃ and an outlet temperature of 90℃ to obtain a dried precursor powder.

[0041] S3. Place the dried precursor powder in a tube furnace with a hydrogen-argon mixed gas atmosphere (hydrogen volume content 5%) and pre-calcine it at 350°C for 5 hours to obtain the pre-calcined product.

[0042] S4. Take out the pre-calcined product and transfer it to a sand mill. Sand mill at a speed of 2000 r / min for 2 hours to obtain an ultrafine slurry with a particle size of 120 nm. Centrifugal spray drying is used on the ultrafine slurry with an inlet temperature of 280℃ and an outlet temperature of 90℃ to obtain a secondary dried powder.

[0043] S5. The secondary dried powder is placed in a tube furnace with a hydrogen-argon mixed gas atmosphere (hydrogen volume content 5%) and sintered at 525℃ for 8 hours. The resulting solid product is a high specific surface area aluminum-carbon coated aluminum phosphate-iron sodium sulfate composite material.

[0044] The composite material prepared in this embodiment was mixed with SP and PVDF in a ratio of 8:1:1 and stirred evenly. After coating and drying for 3 hours, it was taken out and rolled. Positive electrode sheets with a diameter of 8 mm were punched out using a punching machine. After weighing, they were assembled in a glove box. The assembly was carried out in the following order: negative electrode shell, gasket, sodium sheet, separator, electrolyte, positive electrode sheet, gasket, and positive electrode shell. Finally, the battery was sealed using a sealing machine to complete the battery assembly. The assembled coin cells were subjected to standard electrochemical performance tests.

[0045] Test results show that at 1C rate, the discharge specific capacity reaches 122mAh / g after 500 cycles, with a capacity retention rate of 85.6%.

[0046] Example 4 This embodiment provides an NFSP sodium ion cathode material Na 3.5 Fe 1.4 Al 0.4 (SO4)3(PO4) 0.5 The preparation method of / C includes the following steps: S1. Weigh 0.15 mol sodium sulfate, 0.075 mol sodium carbonate, 0.21 mol ferrous sulfate, 0.075 mol sodium dihydrogen phosphate, 0.03 mol aluminum sulfate, and 0.01 mol glucose, and dissolve them in a water / ethanol solution (the molar ratio of ethanol to water is 1:1). Mix thoroughly to obtain a mixed slurry.

[0047] S2. Transfer the mixed slurry to a sand mill and sand mill at a speed of 2000 r / min. After grinding for 3 hours, a coarsely ground slurry with a particle size of 300 nm is obtained. The coarsely ground slurry is then centrifugally spray-dried at an inlet temperature of 280℃ and an outlet temperature of 90℃ to obtain a dried precursor powder.

[0048] S3. Place the dried precursor powder in a tube furnace with a hydrogen-argon mixed gas atmosphere (hydrogen volume content 5%) and pre-calcine it at 350°C for 5 hours to obtain the pre-calcined product.

[0049] S4. Take out the pre-calcined product and transfer it to a sand mill. Sand mill at a speed of 2000 r / min for 2 hours to obtain an ultrafine slurry with a particle size of 120 nm. Centrifugal spray drying is used on the ultrafine slurry with an inlet temperature of 280℃ and an outlet temperature of 90℃ to obtain a secondary dried powder.

[0050] S5. The secondary dried powder is placed in a tube furnace with a hydrogen-argon mixed gas atmosphere (hydrogen volume content 5%) and sintered at 525℃ for 8 hours. The resulting solid product is a high specific surface area aluminum-carbon coated aluminum phosphate-iron sodium sulfate composite material.

[0051] The composite material prepared in this embodiment was mixed with SP and PVDF in a ratio of 8:1:1 and stirred evenly. After coating and drying for 3 hours, it was taken out and rolled. Positive electrode sheets with a diameter of 8 mm were punched out using a punching machine. After weighing, they were assembled in a glove box. The assembly was carried out in the following order: negative electrode shell, gasket, sodium sheet, separator, electrolyte, positive electrode sheet, gasket, and positive electrode shell. Finally, the battery was sealed using a sealing machine to complete the battery assembly. The assembled coin cells were subjected to standard electrochemical performance tests.

[0052] Test results show that at 1C rate, the discharge specific capacity reaches 120mAh / g after 500 cycles, with a capacity retention rate of 85.1%.

[0053] Comparative Example 1 This comparative example provides an NFSP sodium ion cathode material Na3.5 Fe2(SO4)3(PO4) 0.5 The preparation method of / C includes the following steps: S1. Weigh 0.15 mol sodium sulfate, 0.075 mol sodium carbonate, 0.3 mol ferrous sulfate, 0.075 mol sodium dihydrogen phosphate, and 0.01 mol glucose, and dissolve them in 150 mL of water / ethanol (the molar ratio of ethanol to water is 1:1). Mix thoroughly to obtain a mixed slurry.

[0054] S2. Transfer the mixed slurry to a sand mill and sand mill at a speed of 2000 r / min. After grinding for 3 hours, a coarsely ground slurry with a particle size of 300 nm is obtained. The coarsely ground slurry is then centrifugally spray-dried at an inlet temperature of 280℃ and an outlet temperature of 90℃ to obtain a dried precursor powder.

[0055] S3. Place the dried precursor powder in a tube furnace with a hydrogen-argon mixed gas atmosphere (hydrogen volume content 5%) and pre-calcine it at 350°C for 5 hours to obtain the pre-calcined product.

[0056] S4. Take out the pre-calcined product and transfer it to a sand mill. Sand mill at a speed of 2000 r / min for 2 hours to obtain an ultrafine slurry with a particle size of 120 nm. Centrifugal spray drying is used on the ultrafine slurry with an inlet temperature of 280℃ and an outlet temperature of 90℃ to obtain a secondary dried powder.

[0057] S5. The secondary dried powder is placed in a tube furnace with a hydrogen-argon mixed gas atmosphere (hydrogen volume content 5%) and sintered directly at 525℃ for 8 hours. The resulting solid product is the sodium iron sulfate phosphate composite material without aluminum carbon coating.

[0058] The composite material prepared in this comparative example was mixed with SP and PVDF in a ratio of 8:1:1 and stirred evenly. After coating and drying for 3 hours, it was removed and rolled. Positive electrode sheets with a diameter of 8 mm were punched out using a punching machine. After weighing, the electrodes were assembled in a glove box, placing them in the following order: negative electrode shell, gasket, sodium sheet, separator, electrolyte, positive electrode sheet, gasket, and positive electrode shell. Finally, the batteries were sealed using a sealing machine to complete the assembly. Standard electrochemical performance tests were performed on the assembled coin cells.

[0059] Test results show that at 1C rate, the discharge specific capacity reaches 117mAh / g after 500 cycles, with a capacity retention rate of 83.9%.

[0060] To verify the effect of aluminum doping on the crystal structure of sodium ferric sulfate phosphate, the products obtained in Example 1 and Comparative Example 1 were subjected to Rieteld X-ray diffraction refinement, and the crystallographic parameters are shown in Tables 1 and 2.

[0061] Table 1: Crystallographic parameters of Example 1 obtained by Rietveld refinement

[0062] Table 2: Crystallographic parameters of Comparative Example 1 obtained through Rietveld refinement

[0063] As shown in Tables 1 and 2, the cell parameters of the aluminum-doped material in Example 1 increased compared to the undoped material, indicating that aluminum was successfully doped into the lattice to widen its volume. This facilitates the extraction and insertion of sodium ions during charging and discharging, effectively reducing the migration energy barrier.

[0064] To visually compare the effect of aluminum doping on the rate performance of sodium ferric sulfate phosphate cathode material, this invention tested the performance of Example 1 and Comparative Example 1 at different rate ranges. The results are as follows: Figure 1 As shown. From Figure 1 It can be seen that the discharge specific capacity of Example 1 at high rate is significantly better than that of Comparative Example 1, indicating that it can still achieve the extraction and insertion of sodium ions in the material during a shorter charge and discharge time, thereby providing capacity.

[0065] The SEM image of Example 1 is as follows: Figure 2 As shown, by Figure 2 As can be seen, Example 1 successfully synthesized a spherical particle cathode material with a large specific surface area, which allows for a large contact area between the material and the electrolyte to fully react. In addition, the small radius of the spherical particles results in the shortest transport path for sodium ions between the particles, which also confirms the excellent discharge specific capacity of Example 1 at high rates.

[0066] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. An aluminum-doped sodium ferric sulfate phosphate cathode material, characterized in that, The cathode material is a carbon-coated spherical nanostructure with the chemical composition Na. 3.5 Fe 2-3x Al 2x (SO4)3(PO4) 0.5 / C, where 0.03≤x≤0.

2.

2. The method for preparing an aluminum-doped sodium ferric sulfate phosphate cathode material as described in claim 1, characterized in that, Includes the following steps: S1. Weigh out sodium source compound, aluminum source compound, iron source compound, phosphorus source compound, sulfur source compound and carbon source compound, add solvent and mix evenly to obtain a mixed slurry; S2. The mixed slurry is subjected to wet coarse grinding until the particle size is 200-600nm to obtain coarse grinding slurry. The coarse grinding slurry is then spray-dried to obtain dried precursor powder. S3. Place the dried precursor powder in an inert gas or an argon-hydrogen mixture and pre-calcine it at 300-450℃ to obtain the pre-calcined product. S4. The pre-calcined product is subjected to wet ultrafine grinding until the particle size is 20-300nm to obtain an ultrafine grinding slurry. The ultrafine grinding slurry is then spray-dried to obtain a secondary dried powder. S5. The secondary dried powder is placed in an inert gas or an argon-hydrogen mixture and sintered at 450-600℃ to obtain aluminum-doped sodium iron sulfate phosphate cathode material.

3. The method for preparing an aluminum-doped sodium ferric sulfate phosphate cathode material according to claim 2, characterized in that: In step S1, the aluminum source compound is one or more of the following: aluminum oxide, aluminum hydroxide, aluminum nitrate, aluminum chloride, aluminum sulfate, aluminum phosphate, aluminum isopropoxide, aluminum acetylacetone, aluminum stearate, aluminum fluoride, aluminum dihydrogen phosphate, aluminum acetate, aluminum ethoxide, aluminum trifluoroacetate, and aluminum methacrylate.

4. The method for preparing an aluminum-doped sodium ferric sulfate phosphate cathode material according to claim 2, characterized in that: In step S1, the sodium source compound is one or more of the following: sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium nitrate, sodium fluoride, sodium fluorophosphate, sodium trifluoroacetate, sodium oxalate, sodium acetate, sodium persulfate, sodium hydroxide, sodium formate, sodium citrate, sodium pyrophosphate, sodium dihydrogen pyrophosphate, and sodium alginate. The phosphorus source compound is one or more of the following: sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, sodium dihydrogen pyrophosphate, phosphoric acid, ammonium dihydrogen phosphate, and triammonium phosphate; The iron source compound is one or more of the following: reduced iron powder, iron(II,III) oxide, iron oxide, ferrous oxide, ferric nitrate, ferric chloride, ferric sulfate, ferric phosphate, ferric oxalate, ferrous oxalate, ferric acetate, ferric citrate, and ferric fluoride. The sulfur source compound is one or more of the following: sodium sulfide, sodium sulfide nonahydrate, sodium hydrosulfide, sodium sulfate, sodium sulfite, sodium thiosulfate, ammonium sulfide, ammonium thiocyanate, ammonium sulfate, thiourea, dithioglycolamide, cysteine, dimethyl sulfoxide, sodium persulfate, potassium sulfide, ferrous sulfide, and sodium metabisulfite. The carbon source compound is one or more of glucose, sucrose, citric acid, malic acid, polyaniline, polyethylene glycol, adipic acid, phenolic resin, polypyrrole, ascorbic acid, chitosan, and soluble starch.

5. The method for preparing an aluminum-doped sodium ferric sulfate phosphate cathode material according to claim 2, characterized in that: In step S1, the solvent is a mixture of water and ethanol, wherein the molar ratio of water to ethanol is 1:

1.

6. The method for preparing an aluminum-doped sodium ferric sulfate phosphate cathode material according to claim 2, characterized in that: Both the wet coarse grinding in step S2 and the wet ultrafine grinding in step S4 are carried out using a sand mill, with a grinding speed of 2000 r / min. The grinding time for wet coarse grinding is 3 hours, and the grinding time for wet ultrafine grinding is 2 hours.

7. The method for preparing an aluminum-doped sodium ferric sulfate phosphate cathode material according to claim 2, characterized in that: The grinding particle size of wet coarse grinding in step S2 is 300 nm; the grinding particle size of wet ultrafine grinding in step S4 is 120 nm.

8. The method for preparing an aluminum-doped sodium ferric sulfate phosphate cathode material according to claim 2, characterized in that: The spray drying in steps S2 and S4 uses one of centrifugal spray, two-fluid spray, or four-fluid spray; the inlet temperature of the spray dryer is 200-350℃ and the outlet temperature is 60-120℃.

9. The method for preparing an aluminum-doped sodium ferric sulfate phosphate cathode material according to claim 2, characterized in that: The pre-firing temperature in step S3 is 350℃; the sintering temperature in step S5 is 550℃.

10. The method for preparing an aluminum-doped sodium ferric sulfate phosphate cathode material according to claim 2, characterized in that: The inert gas in steps S3 and S5 is nitrogen or argon; the volume content of hydrogen in the argon-hydrogen mixture is 5%.

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