A sodium ferric pyrophosphate phosphate positive electrode material, a preparation method and application thereof

Abstract

The application discloses a kind of sodium pyrophosphate iron phosphate sodium positive electrode materials and preparation method and application, it is related to sodium ion battery positive electrode material technical field, first by electrochemical synthesis method preparation doped modified sodium ferrate, subsequently it is as precursor with phosphorus source, iron source is mixed according to specific proportion, high purity, high performance sodium pyrophosphate iron phosphate positive electrode material of phosphoric acid is prepared by two-step sintering process.The method utilizes the high reactivity of sodium ferrate and self-doping effect, realizes the uniform distribution of element at atomic level, significantly improves the purity, structural stability and electrochemical performance of final product.The obtained sodium pyrophosphate iron phosphate material shows high reversible specific capacity, excellent cycle stability and high rate performance in sodium ion battery, suitable for large-scale sodium ion battery production.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery cathode materials technology, specifically to a sodium iron pyrophosphate cathode material, its preparation method, and its application. Background Technology

[0002] With the continuous rise in global lithium resource prices and the increasing prominence of uneven distribution, sodium-ion batteries, as an effective alternative or supplement to lithium-ion batteries, are an important candidate for next-generation large-scale energy storage technology. Among the many sodium-ion battery cathode materials, polyanionic compounds are considered the most promising class of electrode materials due to their excellent structural stability, safety, and suitable voltage platform. Sodium iron pyrophosphate (NFPP), in particular, exhibits great application potential due to its abundant raw materials, low price, environmental friendliness, and good thermal stability.

[0003] In existing technologies, NFPP is mainly prepared using traditional iron sources such as ferrous oxalate and ferric phosphate via solid-state synthesis. However, traditional solid-state synthesis methods for preparing sodium iron pyrophosphate typically require high reaction temperatures and long reaction times, which can easily lead to excessive grain growth, uneven crystallinity, and potential iron reduction, thus affecting the electrochemical performance of the material, particularly rate performance and cycle stability. Furthermore, the products synthesized via solid-state methods often contain incomplete reactions, component segregation, and impurity phases, resulting in actual capacities far lower than theoretical values.

[0004] Chinese Patent Publication No. CN120172384A, published on June 20, 2025, discloses a method for preparing a composite sodium iron pyrophosphate material. The method involves uniformly dispersing sodium, iron, phosphorus, carbon, and surfactant in a solvent, milling the resulting slurry, spray drying, and sintering to obtain the composite sodium iron pyrophosphate material. While this preparation method can improve the slurry agglomeration problem to some extent, it cannot prevent the formation of the electrochemically inactive NaFePO4 impurity phase.

[0005] Chinese patent CN120221637A, published on June 27, 2025, discloses a high-capacity interface-modified sodium iron pyrophosphate composite cathode material constructed using a gas-liquid mixture source plasma, its preparation method, and its application. Sulfur hexafluoride is used as the gas source, and plasma technology is used to modify sodium iron pyrophosphate by synergistic doping with fluorine and sulfur elements and to coat the surface with a carbon layer. Although the material exhibits excellent sodium storage cycle stability and rate performance, the equipment requirements are high, the process control is complex, and it is not conducive to large-scale production.

[0006] Most existing technologies directly modify NFPP synthesis by doping or coating, neglecting the structural design of the precursor material. By carefully designing the composition and structure of the precursor, the crystal structure, elemental distribution, and electrochemical performance of the final product can be optimized from the source. Summary of the Invention

[0007] This invention addresses the problems in the prior art by providing a sodium iron pyrophosphate cathode material, its preparation method, and its application. First, a sodium ferrate precursor is synthesized, and then its high reactivity is used as the iron and sodium source for synthesizing sodium iron pyrophosphate. High-performance cathode material is obtained by optimizing the sintering process.

[0008] To achieve the above-mentioned objectives, the technical solution of the present invention is as follows: A method for preparing a sodium iron pyrophosphate cathode material includes the following steps: Step 1, Precursor Preparation: Using an iron plate as the anode, electrolysis is carried out in a concentrated sodium hydroxide solution under low temperature and high current density conditions, while simultaneously introducing dopant element salts to prepare a doped sodium ferrate precursor. Step 2, Mixing and ball milling: The sodium ferrate precursor, phosphorus source, supplementary iron source and carbon source obtained in Step 1 are mixed according to the preset molar ratio and subjected to high-energy ball milling to make the mixture reach the preset particle size. Step 3, Two-step sintering: The ball-milled mixture obtained in Step 2 is first pre-sintered under an inert or reducing atmosphere, then sintered at high temperature, and finally cooled naturally to obtain the final sodium iron pyrophosphate cathode material.

[0009] Preferably, in step one, the current density of the electrolysis is 0.5~5 A / cm², the electrolysis temperature is -10℃~15℃, and the concentration of the concentrated sodium hydroxide solution is 10~20 mol / L.

[0010] Preferably, in step one, the general chemical formula of the doped sodium ferrate precursor is Na₂Fe. 1-x M x O4, wherein M is at least one of Mn, Ni, Co, Mg, Ti, and Al, and the value of x is in the range of 0.01 ≤ x ≤ 0.1; the dopant salt is one or more soluble salts of the corresponding dopant element.

[0011] Preferably, in step two, the phosphorus source is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, or ammonium phosphate; the supplementary iron source is one or more of ferrous oxalate, ferric phosphate, or ferric oxide; and the carbon source is one or more of glucose, sucrose, polyethylene glycol, polyvinyl alcohol, β-cyclodextrin, chitosan, and sodium alginate.

[0012] Preferably, in step two, the preset molar ratio is Na:(Fe+M):P = 4:3:4, wherein the molar amounts of Na and Fe are provided by the sodium ferrate precursor and the supplementary iron source.

[0013] Preferably, in step two, the high-energy ball milling process conditions are: rotation speed 500-1000 r / min, grinding time 2-10 h; and the preset particle size is 300-500 nm.

[0014] Preferably, in step three, the protection under an inert or reducing atmosphere is: argon, nitrogen, or a mixture of the gas and hydrogen, wherein the volume content of hydrogen does not exceed 5%.

[0015] Preferably, in step three, the temperature is increased to 300-500°C at a heating rate of 1-5°C / min and held for 2-6 hours for pre-sintering; then the temperature is increased to 600-800°C at a heating rate of 2-8°C / min and held for 5-20 hours for crystallization treatment.

[0016] A sodium iron pyrophosphate cathode material is prepared by the above method; the material has a carbon content of 1-2 wt% and a specific surface area of ​​20-60 m². 2 / g, compacted density higher than 2.0 g / cm³ 3 .

[0017] Application of the above-mentioned sodium iron pyrophosphate cathode material in the cathode of sodium-ion batteries.

[0018] The beneficial effects of this invention are: I. This invention provides a method for preparing sodium iron pyrophosphate cathode material. The doping with sodium ferrate enhances its reactivity, making it more readily react with other raw materials during sintering, thus lowering the reaction temperature, shortening the reaction time, and preventing the formation of impurity phases. This method effectively inhibits the formation of the electrochemically inert sodium iron phosphate phase and improves the phase purity of the product.

[0019] II. The present invention provides a method for preparing sodium iron pyrophosphate cathode material. By introducing doping elements in the precursor stage, atomic-level uniform doping is achieved, which effectively improves the conductivity and ion diffusion rate of the material. The resulting sodium iron pyrophosphate cathode material has high reversible specific capacity and excellent cycle stability.

[0020] Third, the method for preparing sodium iron pyrophosphate cathode material provided by this invention is more environmentally friendly and has lower energy consumption than traditional methods, meeting the requirements of green manufacturing. Detailed Implementation

[0021] The present invention will be further described in detail below with reference to embodiments, but the implementation of the present invention is not limited thereto.

[0022] Example 1 A 14 mol / L NaOH solution was prepared in an electrolytic cell and maintained at 5°C. A pure iron plate was used as the anode, a nickel plate as the cathode, and the electrode spacing was 1.0 cm. Manganese nitrate was added to the electrolyte to make the Mn / Fe molar ratio 0.03:0.97. Electrolysis was performed at a current density of 2.0 A / cm². After electrolysis, the precipitate was collected, washed three times with anhydrous ethanol, and dried in a vacuum drying oven at 60°C for 12 hours to obtain the Mn-doped sodium ferrate precursor.

[0023] The Mn-doped sodium ferrate precursor prepared above was mixed with ammonium dihydrogen phosphate and ferrous oxalate in a Na:(Fe+Mn):P molar ratio of 4:3:4. Glucose was added as a carbon source (to make the carbon content of the final product 1.5 wt%). The mixture was then placed in a high-energy ball mill jar with zirconia grinding beads at a mass ratio of 1:15 and ball-milled at 600 r / min for 5 hours to obtain a uniform powder with a particle size of approximately 400 nm.

[0024] The ball-milled powder was placed in a tube furnace and pre-sintered at 400°C for 4 hours under an argon atmosphere at a heating rate of 3°C / min. Then, it was heated to 650°C for 12 hours at a heating rate of 5°C / min for crystallization. After natural cooling, Mn-doped sodium iron pyrophosphate phosphate / carbon composite cathode material was obtained.

[0025] Example 2 A 16 mol / L NaOH solution was prepared in an electrolytic cell, and the solution temperature was maintained at 0℃. A pure iron plate was used as the anode, a nickel plate as the cathode, and the electrode spacing was 1.2 cm. Nickel nitrate and magnesium nitrate were added to the electrolyte to make the (Ni+Mg) / Fe molar ratio 0.05:0.95 (where the Ni / Mg molar ratio was 1:1). Electrolysis was performed at a current density of 3.0 A / cm². After electrolysis, the precipitate was collected, washed three times with anhydrous ethanol, and dried in a vacuum drying oven at 60℃ for 12 hours to obtain the Ni-Mg composite-doped sodium ferrate precursor.

[0026] The Ni-Mg composite-doped sodium ferrate precursor prepared above was mixed with diammonium hydrogen phosphate and iron phosphate in a Na:(Fe+Ni+Mg):P molar ratio of 4:3:4. Sucrose and polyethylene glycol (mass ratio 1:1) were added as a composite carbon source (to make the carbon content of the final product 1.8 wt%). The mixture was then placed in a high-energy ball mill jar with zirconia grinding beads at a mass ratio of 1:18 and ball-milled at 700 r / min for 4 hours to obtain a uniform powder with a particle size of approximately 350 nm.

[0027] The ball-milled powder was placed in a tube furnace and pre-sintered at 450°C for 3 hours under a mixed atmosphere of argon and hydrogen (3% hydrogen by volume) at a heating rate of 2°C / min. Then, it was heated to 700°C at a heating rate of 6°C / min and held for 10 hours for crystallization. After natural cooling, Ni-Mg composite doped sodium iron pyrophosphate phosphate / carbon composite cathode material was obtained.

[0028] Example 3 A 12 mol / L NaOH solution was prepared in an electrolytic cell, and the solution temperature was maintained at 10℃. A pure iron plate was used as the anode, a stainless steel plate as the cathode, and the electrode spacing was 1.0 cm. Titanium sulfate was added to the electrolyte to make the Ti / Fe molar ratio 0.06:0.94. Electrolysis was performed at a current density of 1.5 A / cm². After electrolysis, the precipitate was collected, washed three times with anhydrous ethanol, and dried in a vacuum drying oven at 60℃ for 12 hours to obtain the Ti-doped sodium ferrate precursor.

[0029] The Ti-doped sodium ferrate precursor prepared above was mixed with phosphoric acid and iron oxide in a Na:(Fe+Ti):P molar ratio of 4:3:4. β-cyclodextrin was added as a carbon source (to make the carbon content of the final product 1.2 wt%). The mixture was then placed in a high-energy ball mill jar with zirconia grinding beads at a mass ratio of 1:12 and ball-milled at 550 r / min for 6 hours to obtain a uniform powder with a particle size of approximately 450 nm.

[0030] The ball-milled powder was placed in a tube furnace and pre-sintered at 350°C for 5 hours under a nitrogen atmosphere at a heating rate of 4°C / min. Then, it was heated to 600°C for 15 hours at a heating rate of 4°C / min for crystallization. After natural cooling, Ti-doped sodium iron pyrophosphate / carbon composite cathode material was obtained.

[0031] Example 4 An 18 mol / L NaOH solution was prepared in an electrolytic cell, and the solution temperature was maintained at -5℃. A pure iron plate was used as the anode, a nickel plate as the cathode, and the electrode spacing was 1.5 cm. Aluminum sulfate and cobalt nitrate were added to the electrolyte to achieve a (Al+Co) / Fe molar ratio of 0.09:0.91 (where the Al / Co molar ratio was 2:1). Electrolysis was performed at a current density of 4.0 A / cm². After electrolysis, the precipitate was collected, washed three times with anhydrous ethanol, and dried in a vacuum drying oven at 60℃ for 12 hours to obtain the Al-Co composite-doped sodium ferrate precursor.

[0032] The Al-Co composite-doped sodium ferrate precursor prepared above was mixed with ammonium phosphate and ferrous oxalate in a Na:(Fe+Al+Co):P molar ratio of 4:3:4. Chitosan and citric acid (mass ratio 2:1) were added as a composite carbon source (to make the carbon content of the final product 2.0 wt%). The mixture was then placed in a high-energy ball mill jar with zirconia grinding beads at a mass ratio of 1:20 and ball-milled at 800 r / min for 3 hours to obtain a uniform powder with a particle size of approximately 300 nm.

[0033] Step 3: Two-step sintering process The ball-milled powder was placed in a tube furnace and pre-sintered at 500°C for 2 hours under an argon atmosphere. Then, it was heated to 750°C for 8 hours under a heating rate of 7°C. After natural cooling, Al-Co composite-doped sodium iron pyrophosphate phosphate / carbon composite cathode material was obtained.

[0034] Example 5 A 20 mol / L NaOH solution was prepared in an electrolytic cell, and the solution temperature was maintained at 15℃. A pure iron plate was used as the anode, and a stainless steel plate as the cathode, with an electrode spacing of 1.0 cm. Manganese nitrate, nickel nitrate, and titanium sulfate were added to the electrolyte to achieve a (Mn+Ni+Ti) / Fe molar ratio of 0.1:0.9 (where the Mn:Ni:Ti molar ratio was 2:1:1). Electrolysis was performed at a current density of 0.8 A / cm². After electrolysis, the precipitate was collected, washed three times with anhydrous ethanol, and dried in a vacuum drying oven at 60℃ for 12 hours to obtain the Mn-Ni-Ti multi-component doped sodium ferrate precursor.

[0035] The Mn-Ni-Ti multi-component doped sodium ferrate precursor prepared above was mixed with ammonium dihydrogen phosphate, phosphoric acid, and iron phosphate in a Na:Fe+Mn+Ni+Ti:P molar ratio of 4:3:4. Glucose and polyvinyl alcohol (mass ratio 3:1) were added as a composite carbon source (to make the carbon content of the final product 1.6 wt%). The mixture was then placed in a high-energy ball mill jar with zirconia grinding beads at a mass ratio of 1:16 and ball-milled at 650 r / min for 5 hours to obtain a uniform powder with a particle size of approximately 400 nm.

[0036] The ball-milled powder was placed in a tube furnace and pre-sintered at 400°C for 4 hours under a nitrogen and hydrogen mixed atmosphere (hydrogen volume content 5%) at a heating rate of 3°C / min. Then, it was heated to 680°C at a heating rate of 5°C / min and held for 12 hours for crystallization. After natural cooling, a Mn-Ni-Ti multi-component doped sodium iron pyrophosphate / carbon composite cathode material was obtained.

[0037] Example 6 A 10 mol / L NaOH solution was prepared in an electrolytic cell, and the solution temperature was maintained at -10℃. A pure iron plate was used as the anode, a stainless steel plate as the cathode, and the electrode spacing was 1.0 cm. Manganese nitrate, nickel nitrate, and titanium sulfate were added to the electrolyte to make the (Mn+Ni+Ti) / Fe molar ratio 0.01:0.9 (where the Mn:Ni:Ti molar ratio was 2:1:1). Electrolysis was performed at a current density of 0.5 A / cm². After electrolysis, the precipitate was collected, washed three times with anhydrous ethanol, and dried in a vacuum drying oven at 60℃ for 12 hours to obtain the Mn-Ni-Ti multi-component doped sodium ferrate precursor.

[0038] The Mn-Ni-Ti multi-component doped sodium ferrate precursor prepared above was mixed with ammonium dihydrogen phosphate and phosphoric acid, and iron phosphate and iron oxide in a Na:Fe+Mn+Ni+Ti:P molar ratio of 4:3:4. Chitosan and sodium alginate (mass ratio 3:1) were added as a composite carbon source (to make the carbon content of the final product 1.6 wt%). The mixture was then placed in a high-energy ball mill jar with zirconia grinding beads at a mass ratio of 1:16 and ball-milled at 1000 r / min for 2 hours to obtain a uniform powder with a particle size of approximately 500 nm.

[0039] The ball-milled powder was placed in a tube furnace and pre-sintered at 300°C for 6 hours under a nitrogen and hydrogen mixed atmosphere (hydrogen volume content 5%) at a heating rate of 1°C / min. Then, it was heated to 800°C at a heating rate of 8°C / min and held for 5 hours for crystallization. After natural cooling, a Mn-Ni-Ti multi-component doped sodium iron pyrophosphate / carbon composite cathode material was obtained.

[0040] Example 7 A 10 mol / L NaOH solution was prepared in an electrolytic cell, and the solution temperature was maintained at 15℃. A pure iron plate was used as the anode, and a stainless steel plate as the cathode, with an electrode spacing of 1.0 cm. Manganese nitrate, nickel nitrate, and titanium sulfate were added to the electrolyte to achieve a (Mn+Ni+Ti) / Fe molar ratio of 0.1:0.9 (where the Mn:Ni:Ti molar ratio was 2:1:1). Electrolysis was performed at a current density of 5 A / cm². After electrolysis, the precipitate was collected, washed three times with anhydrous ethanol, and dried in a vacuum drying oven at 60℃ for 12 hours to obtain the Mn-Ni-Ti multi-component doped sodium ferrate precursor.

[0041] The Mn-Ni-Ti multi-component doped sodium ferrate precursor prepared above was mixed with ammonium dihydrogen phosphate, phosphoric acid, and iron phosphate in a Na:Fe+Mn+Ni+Ti:P molar ratio of 4:3:4. Glucose and polyvinyl alcohol (mass ratio 3:1) were added as a composite carbon source (to make the carbon content of the final product 1.6 wt%). The mixture was then placed in a high-energy ball mill jar with zirconia grinding beads at a mass ratio of 1:16 and ball-milled at 500 r / min for 10 hours to obtain a uniform powder with a particle size of approximately 300 nm.

[0042] The ball-milled powder was placed in a tube furnace and pre-sintered at 400°C for 4 hours under a nitrogen and hydrogen mixed atmosphere (hydrogen volume content 5%) at a heating rate of 3°C / min. Then, it was heated to 600°C at a heating rate of 2°C / min and held for 20 hours for crystallization. After natural cooling, a Mn-Ni-Ti multi-component doped sodium iron pyrophosphate / carbon composite cathode material was obtained.

[0043] Comparative Example 1 Mn-doped sodium iron pyrophosphate cathode material prepared according to the traditional solid-state method: Sodium carbonate, ferrous oxalate, manganese oxalate, and ammonium dihydrogen phosphate were mixed in a Na:(Fe+Mn):P molar ratio of 4:3:4, with a Mn / Fe molar ratio of 0.03:0.97. Glucose was added as a carbon source (to achieve a final product carbon content of 1.5 wt%). The mixture was then placed in a high-energy ball mill with zirconia grinding beads at a mass ratio of 1:15 and ball-milled at 600 r / min for 5 hours. The milled powder was then placed in a tube furnace and heated directly to 650℃ at a heating rate of 5℃ / min under an argon atmosphere, and held at that temperature for 12 hours for crystallization. After natural cooling, Mn-doped sodium iron pyrophosphate / carbon composite cathode material was obtained.

[0044] Comparative Example 2 This comparative example was not doped, and the remaining steps were basically the same as in Example 1.

[0045] A 14 mol / L NaOH solution was prepared in an electrolytic cell, and the solution temperature was maintained at 5℃. A pure iron plate was used as the anode, a nickel plate as the cathode, and the electrode spacing was 1.0 cm. Electrolysis was performed at a current density of 2.0 A / cm². After electrolysis, the precipitate was collected, washed three times with anhydrous ethanol, and dried in a vacuum drying oven at 60℃ for 12 hours to obtain the undoped sodium ferrate precursor.

[0046] The undoped sodium ferrate precursor prepared above was mixed with ammonium dihydrogen phosphate and ferrous oxalate in a Na:Fe:P molar ratio of 4:3:4, and glucose was added as a carbon source (to make the carbon content of the final product 1.5 wt%). The mixture was then placed in a high-energy ball mill jar with zirconia grinding beads at a mass ratio of 1:15 and ball-milled at 600 r / min for 5 hours to obtain a uniform powder with a particle size of approximately 400 nm.

[0047] The ball-milled powder was placed in a tube furnace and pre-sintered at 400°C for 4 hours under an argon atmosphere at a heating rate of 3°C / min. Then, it was heated to 650°C for 12 hours at a heating rate of 5°C / min for crystallization. After natural cooling, undoped sodium iron pyrophosphate / carbon composite cathode material was obtained.

[0048] The key parameters and test results of Examples 1 to 7 and Comparative Examples 1 to 2 are shown in Table 1.

[0049] Table 1

[0050] All materials prepared in the examples formed a pure-phase NFPP structure without any impurity phases. Compared with Comparative Examples 1 and 2, the materials prepared in Examples 1-7 have lower specific surface areas and higher compaction densities, which is beneficial for improving the energy density of the electrode. Electrochemical test results show that the doped NFPP cathode materials prepared in the examples of this invention are significantly superior to Comparative Examples 1 and 2 in terms of initial discharge specific capacity, initial coulombic efficiency, and cycle stability.

[0051] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A method for preparing a sodium iron pyrophosphate cathode material, characterized in that, Includes the following steps: Step 1, Precursor Preparation: Using an iron plate as the anode, electrolysis is carried out in a concentrated sodium hydroxide solution under low temperature and high current density conditions, while simultaneously introducing dopant element salts to prepare a doped sodium ferrate precursor. Step 2, Mixing and ball milling: The sodium ferrate precursor, phosphorus source, supplementary iron source and carbon source obtained in Step 1 are mixed according to the preset molar ratio and subjected to high-energy ball milling to make the mixture reach the preset particle size. Step 3, Two-step sintering: The ball-milled mixture obtained in Step 2 is first pre-sintered under an inert or reducing atmosphere, then sintered at high temperature, and finally cooled naturally to obtain the final sodium iron pyrophosphate cathode material.

2. The method for preparing a sodium iron pyrophosphate cathode material according to claim 1, characterized in that: In step one, the current density of the electrolysis is 0.5~5 A / cm², the electrolysis temperature is -10℃~15℃, and the concentration of the concentrated sodium hydroxide solution is 10~20 mol / L.

3. The method for preparing a sodium iron pyrophosphate cathode material according to claim 2, characterized in that: In step one, the general chemical formula of the doped sodium ferrate precursor is Na₂Fe. 1-x M x O4, wherein M is at least one of Mn, Ni, Co, Mg, Ti, and Al, and the value of x is in the range of 0.01 ≤ x ≤ 0.1; the dopant salt is one or more soluble salts of the corresponding dopant element.

4. The method for preparing a sodium iron pyrophosphate cathode material according to claim 3, characterized in that: In step two, the phosphorus source is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, or ammonium phosphate; the supplementary iron source is one or more of ferrous oxalate, ferric phosphate, or ferric oxide; and the carbon source is one or more of glucose, sucrose, polyethylene glycol, polyvinyl alcohol, β-cyclodextrin, chitosan, or sodium alginate.

5. The method for preparing a sodium iron pyrophosphate cathode material according to claim 4, characterized in that: In step two, the preset molar ratio is Na:(Fe+M):P = 4:3:4, wherein the molar amounts of Na and Fe are provided by the sodium ferrate precursor and the supplementary iron source.

6. The method for preparing a sodium iron pyrophosphate cathode material according to claim 5, characterized in that: In step two, the high-energy ball milling process conditions are: rotation speed 500-1000 r / min, grinding time 2-10 h; and the preset particle size is 300-500 nm.

7. The method for preparing a sodium iron pyrophosphate cathode material according to claim 6, characterized in that: In step three, the protection under an inert or reducing atmosphere is: argon, nitrogen, or a mixture of the gas and hydrogen, wherein the volume content of hydrogen does not exceed 5%.

8. The method for preparing a sodium iron pyrophosphate cathode material according to claim 7, characterized in that: In step three, the temperature is increased to 300-500℃ at a heating rate of 1-5℃ / min and held for 2-6 hours for pre-sintering; then the temperature is increased to 600-800℃ at a heating rate of 2-8℃ / min and held for 5-20 hours for crystallization treatment.

9. A sodium iron pyrophosphate cathode material, characterized in that: The material is prepared by the method described in any one of claims 1 to 8, which is a sodium iron pyrophosphate cathode material; the material has a carbon content of 1-2 wt% and a specific surface area of ​​20-60 m². 2 / g, compacted density higher than 2.0 g / cm³ 3 .

10. The application of the sodium iron pyrophosphate cathode material according to claim 9 as a cathode in a sodium-ion battery.

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