Iron-based polyanionic sodium ion battery positive electrode material and preparation method thereof

Abstract

The invention relates to the technical field of secondary batteries, in particular to an iron-based polyanionic sodium ion battery positive electrode material and a preparation method thereof. According to the preparation method disclosed by the invention, firstly, a simple and feasible process of wet ball milling, sanding, high-temperature spray drying and introduction of protective gas containing reducing gas for reduction calcination is creatively adopted, and secondly, a water-soluble carbon compound is promoted to be embedded into a three-dimensional network of a conductive agent more uniformly by regulating and controlling temperature and stirring in a water bath; and on the other hand, an iron defect strategy is adopted to inhibit the generation of an inactive NaFePO4 impure phase, so that the electrochemical performance of the positive electrode material is effectively improved, and the positive electrode material has a good application prospect. And finally, the conductive network modified carbon coated iron-based polyanionic positive electrode material is prepared, the electrochemical performance of a battery is improved, and the conductive network modified carbon coated iron-based polyanionic positive electrode material has a relatively good practical application value.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and specifically to an iron-based polyanion sodium-ion battery cathode material and its preparation method. Background Technology

[0002] With industrialization and global population growth, the demand for energy is constantly increasing. Currently, the world's energy supply still relies heavily on non-renewable fossil fuels (coal, oil, natural gas, etc.), leading to energy shortages and environmental pollution. Therefore, it is necessary to develop new energy sources to reduce our dependence on fossil fuels. Clean new energy sources such as solar, wind, and tidal power are emerging, but the indirect and cyclical nature of these renewable energy sources in terms of time and space makes it difficult to directly output them as stable and continuous energy. Therefore, there is an urgent need to research efficient, low-cost, large-scale energy storage technologies.

[0003] Among all energy storage technologies, lithium-ion batteries, as a representative of rechargeable batteries, possess advantages such as high capacity, high rate performance, high cycle performance, and environmental friendliness and safety. They are considered an excellent choice for energy conversion and storage and have been widely used in portable electronic devices and electric vehicles. However, with the increasing demand for lithium batteries, coupled with the limited global lithium resources and their relatively concentrated distribution, resource competition has intensified, leading to a surge in lithium resource costs and limiting further development. Compared to lithium-ion rechargeable batteries, sodium-ion batteries, due to their abundant sodium resources, low cost, and similar working principle to lithium-ion batteries, are considered a candidate energy storage battery with great application prospects in large-scale energy storage, thus attracting increasing attention from researchers.

[0004] Currently, commonly used sodium-ion battery cathode materials mainly fall into three categories: layered oxide cathode materials, polyanionic cathode materials, and Prussian cathode materials. Polyanionic cathode materials have attracted considerable attention due to their structural stability, good thermal stability, and small volume change during cycling. Iron-based hybrid polyanionic sodium-ion battery cathode materials structurally contain both PO4 and P2O7 units, combining the advantages of phosphate and pyrophosphate cathodes, thus exhibiting high stability and a good theoretical capacity (129 mAh g⁻¹). -1 Sodium-ion batteries offer advantages such as low cost, environmental friendliness, and ease of synthesis. However, the synthesis of mixed phosphate cathode materials is accompanied by the generation of a small amount of inactive sodium phosphate-iron ore-type NaFePO4 impurities. Since NaFePO4 lacks effective sodium ion diffusion channels, this reduces the capacity of the active material to some extent, limiting the development of high-energy, long-life, and low-cost sodium-ion batteries. Furthermore, the inherently low electronic conductivity and poor ionic conductivity of this material limit its applications and cannot adequately meet its energy storage requirements.

[0005] CN 110326136A discloses a novel high-potential multilayer carbon-coated polyanionic sodium-ion battery cathode material and its preparation method. A polyanionic cathode material with a three-dimensional carbon-coated network is prepared by combining high-energy milling and freeze-drying. The resulting product has uniform particle size, uniform carbon layer, and excellent electrochemical performance. CN 116960308A discloses a carbon-coated modified polyanionic cathode material, its preparation method, and its application. The preparation method introduces a complex containing bacterial cellulose, sodium carboxymethyl cellulose, and chitosan during the precursor mixing stage. After further mixing, spray drying, and calcination, the polyanionic compound in the prepared product is coated with a three-dimensional nitrogen-doped carbon network structure, significantly improving the product's charge-discharge performance and rate capability. Summary of the Invention

[0006] This invention aims to provide an iron-based polyanionic sodium-ion battery cathode material and its preparation method, the specific scheme of which is as follows:

[0007] A type of iron-based polyanionic cathode material, wherein the cathode material is an iron-deficient Na4Fe modified with conductive network and carbon coating. 3-x (PO4)P2O7@yC sodium-ion battery cathode material.

[0008] A method for preparing an iron-based polyanionic cathode material includes the following steps:

[0009] (1) Sodium salt, iron salt and phosphate salt are added to the solvent in sequence, and the mixture is obtained by wet stirring and ball milling.

[0010] (2) The liquid obtained in step (1) is slowly added to the sand mill and refined and mixed by high-energy sand milling to obtain a uniform liquid A;

[0011] (3) The carbon-containing compound, dispersant and conductive agent are added slowly in sequence to the deionized water solvent and ultrasonically stirred to finally obtain water-based slurry B;

[0012] (4) The aqueous slurry B from step (3) is slowly added to the liquid A obtained in step (2), and while heating and stirring in a water bath, it is sprayed and granulated by a peristaltic pump through a high-temperature centrifugal spray dryer to finally obtain modified iron-based polyanionic positive electrode phosphate precursor powder.

[0013] (5) The phosphate precursor powder obtained in step (4) is sintered in a tube furnace under a hydrogen-nitrogen mixed protective atmosphere. After sintering, it is cooled in the furnace to obtain Na4Fe. 3-x (PO4)P2O7@yC;

[0014] (6) Na4Fe from step (5) 3-x(PO4)P2O7@yC was sieved through a 100-mesh sieve and subjected to air jet milling to obtain an iron-based polyanionic cathode material coated with conductive network modified carbon.

[0015] (7) The general formula of the modified iron-deficient polyanionic cathode material with conductive network modified carbon coating obtained in step (6) is Na4Fe 3-x (PO4)P2O7@yC; where the residual carbon content y is 1%-5%, and the relative iron deficiency 3-x ranges from 0.03 to 0.09 (appropriate iron deficiency can effectively suppress the formation of inactive NaFePO4 impurity phase in the material, thereby effectively improving the electrochemical performance of the cathode material).

[0016] The sodium salt is one or more of sodium pyrophosphate and sodium dihydrogen phosphate; the iron salt is one or more of ferric nitrate, ferrous oxalate, and ferric phosphate; the phosphate salt is mainly one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the carbon-containing compound is at least one of glucose, sucrose, citric acid, and soluble starch; the conductive agent is one or more of graphene, single-walled carbon nanotubes, and multi-walled carbon nanotubes; and the dispersant is one or more of polyethylene glycol (PEG) Mw10000 and polyvinylpyrrolidone (PVP).

[0017] In step (1), the wet ball milling has a ball-to-material ratio of 5:1, a rotation speed of 350-500 rpm, and a milling time of 60-120 min; in step (2), the slurry refined by sand milling has a solid content of 30-40%, a sand mill rotation speed of 1500-2500 rpm, and a milling time of 3-5 h.

[0018] Step (3) The conductive agent accounts for 0.1%-5% of the total mass of the raw materials, the dispersant accounts for 0.001-0.05% of the mass, the ultrasonic machine frequency is 40KHz, the power is 500W, the ultrasonic time is 6-8h, the water bath stirring speed is 500-800rpm, and the stirring time is 18-24h.

[0019] The water bath temperature in step (4) is 60-80℃, and the water bath stirring speed is 500-800rpm.

[0020] The high-temperature centrifugal spray dryer described in step (4) has a spray granulation feed rate of 1.2 to 2.0 L / h, a fan frequency of 30 to 50 Hz, an inlet air temperature of 200 to 220 ℃, and an outlet air temperature of 100 to 110 ℃.

[0021] Step (5) The sintering process is divided into two stages. The first stage sintering temperature is 300-400℃ and the holding time is 2-5h. The second stage sintering temperature is 500-600℃ and the holding time is 10-14h. The heating rate is 2℃ / min. The first stage calcination is a pre-sintering process to remove certain moisture and impurities. The second stage calcination is a high-temperature solid-phase reaction.

[0022] In step (5), under the hydrogen-argon mixed protective atmosphere, a certain amount of reducing hydrogen is mixed in on the basis of argon as the main protective gas, and the volume content of the reducing gas is greater than or equal to 1%.

[0023] This invention provides a method for preparing a modified iron-deficient mixed polyanionic cathode material with conductive network-modified carbon coating. Firstly, based on the traditional preparation process combining ball milling, sand milling, spray drying, and high-temperature solid-state sintering, an innovative water bath temperature-controlled stirring method is adopted to promote the uniform embedding of water-soluble carbon compounds into the three-dimensional network of the conductive agent, forming a uniform three-dimensional conductive network layer on the material surface. This improves the electronic conductivity of the cathode material, ultimately yielding an iron-deficient mixed phosphate cathode material for sodium-ion batteries with three-dimensional conductive network-modified carbon coating. This method has advantages such as simple operation, low cost, environmental friendliness, and ease of industrial scale-up. The resulting cathode material has uniform particle size, fewer impurities, high electronic conductivity, and excellent electrochemical performance.

[0024] The modified iron-deficient polyanionic cathode material with conductive carbon coating produced by high-temperature solid-state method and processes such as sand milling and spray drying has the following advantages: Firstly, by controlling the temperature and stirring in a water bath, water-soluble carbon compounds are more uniformly embedded into the three-dimensional network of the conductive agent and uniformly coated on the material surface to form a uniform three-dimensional conductive network layer, thereby improving the electronic conductivity of the cathode material. Secondly, the iron defect process is used to suppress the formation of inactive NaFePO4 impurity phase in the cathode material, thereby improving the discharge specific capacity and rate performance. Finally, by setting a low-temperature heat preservation pre-sintering section and mixing an appropriate amount of reducing gas (5% hydrogen and 95% argon) into the protective gas during the sintering process, the uniformity of carbon coating is further improved and the impurity content in the cathode material is reduced, effectively improving the cycle stability of the cathode material. Attached Figure Description

[0025] Figure 1 This is a process flow diagram of the preparation of the modified iron-deficient polyanionic cathode material with conductive modified carbon coating according to the present invention.

[0026] Figure 2 These are scanning electron microscope (SEM) images of the polyanionic cathode materials from Examples 2 and 8.

[0027] Figure 3These are the 1C capacity voltage diagrams for Examples 6 and 8. Detailed Implementation

[0028] The cathode material and its preparation method of the present invention will be further described in detail below through some specific examples.

[0029] Example 1

[0030] This embodiment prepares a carbon-coated modified polyanionic cathode material, and the specific preparation method is as follows:

[0031] The sodium salt is sodium pyrophosphate; the iron salt is ferrous oxalate; the phosphate salt is mainly phosphoric acid; and the carbon-containing compound is glucose. The raw materials, including phosphate, iron, and sodium salts, were weighed in an elemental molar ratio of 2:3:4. Deionized water was used as the solvent, and the solid content was controlled at 30%. The amount of carbon source added was calculated based on a theoretical residual carbon content of 1%. Wet ball milling (ball-to-material ratio of 5:1) was used at 500 rpm for 2 hours. The slurry was then added to a sand mill (with the solid content of the sand mill slurry maintained at 30%), and milled at 2500 rpm for 4 hours. Afterward, high-temperature spray drying was performed. The spray dryer was set with a feed rate of 1.5 L / h, an inlet air temperature of 220℃, an outlet air temperature of 110℃, and a fan frequency of 50 Hz. The precursor powder was obtained after high-temperature spray drying. The precursor powder was sintered in a tube furnace with a mixture of hydrogen (5%) and argon (95%) as the protective gas. The tube furnace sintering was set with two holding sections. The first holding section was at 300℃ for 4 hours and the second holding section was at 550℃ for 12 hours. The heating rate of both sections was 2℃ / min. After natural cooling, the material was passed through a 100-mesh sieve and air jet pulverization to obtain the polyanionic cathode material Na4Fe3(PO4)2P2O7@1%C.

[0032] Example 2

[0033] Compared to Example 1, the only difference is that the amount of C source added has been changed, as follows:

[0034] Phosphate, iron, and sodium salts were weighed according to the elemental molar ratio of 2:3:4. Deionized water was used as the solvent, and the solid content was controlled at 30%. The amount of C source added was calculated based on the theoretical residual carbon content of 3%. Finally, carbon-coated polyanionic cathode material Na4Fe3(PO4)2P2O7@3%C was prepared.

[0035] Example 3

[0036] Compared to Example 1, the only difference is that the amount of C source added has been changed, as follows:

[0037] Phosphate, iron, and sodium salts were weighed according to the elemental molar ratio of 2:3:4. Deionized water was used as the solvent, and the solid content was controlled at 30%. The amount of carbon source added was calculated based on the theoretical residual carbon content of 5%. Finally, carbon-coated polyanionic cathode material Na4Fe3(PO4)2P2O7@5%C was prepared.

[0038] Example 4

[0039] Compared to Example 2, the only difference is that an iron defect strategy was introduced on top of the 3% C source, reducing the relative iron content to suppress the formation of inactive NaFePO4 impurity phases. Specifically:

[0040] Phosphate, iron, and sodium salts were weighed according to an elemental molar ratio of 2:2.97:4. Deionized water was used as the solvent, and the solid content was controlled at 30%. The amount of carbon source added was calculated based on a theoretical residual carbon content of 3%. The ball milling, sand milling, spraying, sintering, and air jet milling processes and parameter settings were the same as in Example 1, ultimately yielding a carbon-coated iron-deficient polyanionic cathode material, Na4Fe. 2.97 (PO4)2P2O7@3%C.

[0041] Example 5

[0042] Compared to Example 4, the only difference is that the amount of iron salt added was changed, further reducing the relative iron content, as detailed below:

[0043] Phosphate, iron, and sodium salts were weighed according to an elemental molar ratio of 2:2.94:4. The amount of carbon source added was calculated based on a theoretical residual carbon content of 3%. The ball milling, sand milling, spray milling, sintering, and air jet milling processes and parameter settings were the same as in Example 1, ultimately yielding a carbon-coated iron-deficient polyanionic cathode material, Na4Fe. 2.94 (PO4)2P2O7@3%C.

[0044] Example 6

[0045] Compared to Example 5, the only difference is that the amount of iron salt added was changed, further reducing the relative iron content, as detailed below:

[0046] Phosphate, iron, and sodium salts were weighed according to the elemental molar ratio of 2:2.91:4. The amount of carbon source added was calculated based on a theoretical residual carbon content of 3%. The ball milling, sand milling, spraying, sintering, and air jet milling processes and parameter settings were the same as in Example 1, ultimately yielding a carbon-coated iron-deficient polyanionic cathode material, Na4Fe. 2.91 (PO4)2P2O7@3%C.

[0047] Example 7

[0048] Compared to Example 6, the only difference is that, in addition to changing the amount of iron salt added, a conductive agent is further introduced to improve the electronic conductivity of the material, specifically as follows:

[0049] The sodium salt is sodium pyrophosphate; the phosphate salt is mainly phosphoric acid; the carbon-containing compound is glucose; the conductive agent is single-walled carbon nanotubes; and the dispersant is polyethylene glycol Mw10000 (PEG). First, phosphate, iron, and sodium salts were weighed in an elemental molar ratio of 2:2.91:4. Using deionized water as a solvent, the solid content was controlled at 30%. Wet ball milling was employed at 500 rpm for 2 hours to obtain a uniformly mixed slurry. The slurry was then added to a sand mill at 2500 rpm for 4 hours to obtain a uniform slurry A. A carbon-containing compound (calculated based on a theoretical residual carbon content of 3%), a conductive agent (single-walled carbon nanotube aqueous slurry (SWCNT) added at 0.1% of the total raw material mass), and a dispersant (PEG added at 0.001% of the total raw material mass) were mixed. The mixture was then ultrasonicated at 40 kHz with a power of 500 W for 8 hours, and stirred in a water bath at 500 rpm for 24 hours to obtain a uniformly mixed aqueous slurry B. Aqueous slurry B was slowly added to slurry A, and stirred in a water bath at 60°C with a stirring speed of 800 rpm. While the mixture was being heated and stirred in the water bath, it was also being sprayed and granulated by a peristaltic pump through a high-temperature centrifugal spray dryer to obtain modified iron-based polyanionic positive electrode phosphate precursor powder. The spray dryer was set with the following parameters: feed rate 1.5 L / h, inlet air temperature 220°C, outlet air temperature 110°C, and fan frequency 50 Hz. The precursor powder was obtained after high-temperature spray drying. The precursor powder was sintered in a tube furnace with a mixture of hydrogen (5%) and argon (95%) as the protective gas. The tube furnace sintering was set with two holding sections. The first holding section was at 300℃ for 4 hours and the second holding section was at 550℃ for 12 hours. The heating rate of both sections was 2℃ / min. After natural cooling, the material was passed through a 100-mesh sieve and air jet pulverization to obtain the polyanionic cathode material Na4Fe3(PO4)2P2O7@1%C.

[0050] Example 8

[0051] Compared to Example 7, the only difference is that the amount of conductive agent doped has been changed, while maintaining the original residual carbon content and iron deficiency. Specifically:

[0052] Phosphate, iron, and sodium salts were weighed according to the elemental molar ratio of 2:2.91:4. The amount of carbon source added was calculated based on a theoretical residual carbon content of 3%. Aqueous single-walled carbon nanotube slurry (SWCNT) was added at 0.2% of the total raw material mass, and PEG was added at 0.002% of the total raw material mass. All other process conditions and parameters were the same as in Example 7. The final product was an iron-deficient polyanionic cathode material, Na4Fe, with conductive network modified carbon coating. 2.91 (PO4)2P2O7@3%C@0.2%SWCNT.

[0053] Example 9

[0054] Compared to Example 8, the only difference is that the amount of conductive agent doped has been changed, while maintaining the original residual carbon content and iron deficiency. Specifically:

[0055] Phosphate, iron, and sodium salts were weighed according to the elemental molar ratio of 2:2.91:4. The amount of carbon source added was calculated based on a theoretical residual carbon content of 3%. Aqueous single-walled carbon nanotube slurry (SWCNT) was added at 0.3% of the total raw material mass, and PEG was added at 0.003% of the total raw material mass. All other process conditions and parameters were the same as in Example 7. The final product was an iron-deficient polyanionic cathode material, Na4Fe, with conductive network-modified carbon coating. 2.91 (PO4)2P2O7@3%C@0.3%SWCNT.

[0056] Example 10

[0057] Compared to Example 9, the only difference is that, based on the original residual carbon content and iron deficiency content, the types and doping amounts of the conductive agent and dispersant have been changed. Specifically:

[0058] Phosphate, iron, and sodium salts were weighed according to the elemental molar ratio of 2:2.91:4. The amount of carbon source added was calculated based on a theoretical residual carbon content of 3%. Multi-walled carbon nanotube aqueous slurry (MWCNT) was added at 1.0% of the total raw material mass, and polyvinylpyrrolidone (PVP) was added at 0.01% of the total raw material mass. All other process conditions and parameters were the same as in Example 7. Finally, an iron-deficient polyanionic cathode material, Na4Fe, with conductive network modified carbon coating, was prepared. 2.91 (PO4)2P2O7@3%C@1%MWCNT.

[0059] Example 11

[0060] Compared to Example 10, the only difference is that the amount of conductive agent doped has been changed, while maintaining the original residual carbon content and iron deficiency. Specifically:

[0061] Phosphate, iron, and sodium salts were weighed according to the elemental molar ratio of 2:2.91:4. The amount of carbon source added was calculated based on a theoretical residual carbon content of 3%. Multi-walled carbon nanotube aqueous slurry (MWCNT) was added at 3.0% of the total raw material mass, and polyvinylpyrrolidone (PVP) was added at 0.03% of the total raw material mass. All other process conditions and parameters were the same as in Example 7. Finally, an iron-deficient polyanionic cathode material, Na4Fe, with conductive network modified carbon coating, was prepared. 2.91 (PO4)2P2O7@3%C@3%MWCNT.

[0062] Example 12

[0063] Compared to Example 11, the only difference is that the doping amount of the conductive agent has been changed, while maintaining the original residual carbon content and iron deficiency content. Specifically:

[0064] Phosphate, iron, and sodium salts were weighed according to the elemental molar ratio of 2:2.91:4. The amount of carbon source added was calculated based on a theoretical residual carbon content of 3%. Multi-walled carbon nanotube aqueous slurry (MWCNT) was added at 5.0% of the total raw material mass, and polyvinylpyrrolidone (PVP) was added at 0.05% of the total raw material mass. All other process conditions and parameters were the same as in Example 7. Finally, an iron-deficient polyanionic cathode material, Na4Fe, with conductive network modified carbon coating, was prepared. 2.91 (PO4)2P2O7@3%C@5%MWCNT.

[0065] Comparative Example 1

[0066] Compared with Example 1, this comparative example does not use C source in the raw materials, and the other process conditions and parameters are the same as in Example 1. The final polyanionic cathode material Na4Fe3(PO4)2P2O7 is prepared.

[0067] Preparation of coin cells in the examples and comparative examples: The prepared sodium-ion battery positive electrode material was mixed with conductive agent acetylene black and binder polyvinylidene fluoride (PVDF) at a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (NMP) solution was then added, and the mixture was thoroughly ground to obtain a battery positive electrode slurry. This positive electrode slurry was coated onto a current collector aluminum foil, vacuum dried, rolled, and then stamped into a positive electrode sheet. A sodium metal sheet was used as the negative electrode, the electrolyte ratio was 1 mol / L NaClO4 EC:DEC (1:1 vol%), and glass fiber filter paper was used as the separator. Coin cells were assembled in a glove box under an argon atmosphere.

[0068] Table 1 Comparison of parameters and electrochemical performance of each example and comparative example.

[0069]

[0070] Results and Analysis

[0071] According to the results in Table 1, the relative iron deficiency in this invention is 3%, the relative Fe content is 2.91%, and the final product, Na4Fe, is a conductive network modified carbon-coated iron-deficient polyanionic cathode material with a doping content of 0.2% SWCNT and 3% residual carbon. 2.91 The (PO4)2P2O7@3%C@0.2%SWCNT assembled battery exhibited the best electrochemical performance. The first-cycle discharge specific capacity at 0.1C current density was 110.5 mAh g. -1， The initial discharge efficiency is 99.2%; and the discharge specific capacity is 100.2 mAh g at a high current density of 1C. -1 .

[0072] This invention innovatively employs a simple and feasible process: wet ball milling, sand milling, high-temperature spray drying, and reduction calcination with a protective gas containing reducing gases. Secondly, by controlling the temperature and stirring in a water bath, water-soluble carbon compounds are more uniformly embedded into the three-dimensional network of the conductive agent, forming a uniform three-dimensional conductive network layer on the material surface, thus improving the electronic conductivity of the cathode material. Furthermore, an iron defect strategy is used to suppress the formation of inactive NaFePO4 impurity phases, thereby effectively improving the electrochemical performance of the cathode material. Ultimately, a conductive network-modified carbon-coated iron-based polyanionic cathode material is prepared, improving the electrochemical performance of the battery and possessing significant practical application value.

[0073] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. An iron-based polyanionic cathode material, characterized in that: The cathode material is an iron-deficient Na4Fe2O3 modified with conductive network and carbon coating. 3-x (PO4)P2O7@yC sodium-ion battery cathode material.

2. A method for preparing the iron-based polyanionic cathode material as described in claim 1, characterized in that, Includes the following steps: (1) Sodium salt, iron salt and phosphate salt are added to the solvent in sequence, and the mixture is obtained by wet stirring and ball milling; (2) The liquid obtained in step (1) is slowly added to the sand mill and refined and mixed by high-energy sand milling to obtain a uniform liquid A; (3) The carbon-containing compound, dispersant and conductive agent are added slowly to the deionized water solvent in sequence and ultrasonically stirred to finally obtain water-based slurry B; (4) The aqueous slurry B from step (3) is slowly added to the liquid A obtained in step (2), and while heating and stirring in a water bath, it is sprayed and granulated by a peristaltic pump through a high-temperature centrifugal spray dryer to finally obtain modified iron-based polyanionic positive electrode phosphate precursor powder. (5) The phosphate precursor powder obtained in step (4) is sintered in a tube furnace under a hydrogen-nitrogen mixed protective atmosphere. After sintering, it is cooled in the furnace to obtain Na4Fe. 3-x (PO4)P2O7@yC; (6) Take the Na4Fe from step (5) 3-x (PO4)P2O7@yC was sieved through a 100-mesh sieve and subjected to air jet milling to obtain an iron-based polyanionic cathode material coated with conductive network modified carbon. (7) The general formula of the modified iron-deficient polyanionic cathode material with conductive network modified carbon coating obtained in step (6) is Na4Fe 3-x (PO4)P2O7@yC; where the residual carbon content y is 1% - 5%, and the relative iron deficiency 3-x ranges from 0.03 to 0.

09.

3. The method for preparing an iron-based polyanionic sodium-ion battery cathode material as described in claim 2, characterized in that: The sodium salt is one or more of sodium pyrophosphate and sodium dihydrogen phosphate; the iron salt is one or more of ferric nitrate, ferrous oxalate, and ferric phosphate; the phosphate salt is mainly one or more of phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; the carbon-containing compound is at least one of glucose, sucrose, citric acid, and soluble starch; the conductive agent is one or more of graphene, single-walled carbon nanotubes, and multi-walled carbon nanotubes; and the dispersant is one or more of polyethylene glycol (PEG) Mw10000 and polyvinylpyrrolidone (PVP).

4. The method for preparing an iron-based polyanionic sodium-ion battery cathode material as described in claim 2, characterized in that: In step (1), the wet ball milling has a ball-to-material ratio of 5:1, a rotation speed of 350-500 rpm, and a milling time of 60-120 min; in step (2), the slurry refined by sand milling has a solid content of 30-40%, a sand mill rotation speed of 1500-2500 rpm, and a milling time of 3-5 h.

5. The method for preparing an iron-based polyanionic sodium-ion battery cathode material as described in claim 2, characterized in that: Step (3) The conductive agent accounts for 0.1%-5% of the total mass of the raw materials, the dispersant accounts for 0.001-0.05% of the mass, the ultrasonic machine frequency is 40KHz, the power is 500W, the ultrasonic time is 6-8h, the water bath stirring speed is 500-800rpm, and the stirring time is 18-24h.

6. The method for preparing an iron-based polyanionic sodium-ion battery cathode material as described in claim 2, characterized in that: The water bath temperature in step (4) is 60-80℃, and the water bath stirring speed is 500-800rpm.

7. The method for preparing an iron-based polyanionic sodium-ion battery cathode material as described in claim 2, characterized in that: The high-temperature centrifugal spray dryer described in step (4) has a spray granulation feed rate of 1.2 ~ 2.0 L / h; a fan frequency of 30 ~ 50 Hz; an inlet air temperature of 200 ~ 220 ℃; and an outlet air temperature of 100 ~ 110 ℃.

8. The method for preparing an iron-based polyanionic sodium-ion battery cathode material as described in claim 2, characterized in that: Step (5) The sintering process is divided into two stages. The first stage sintering temperature is 300-400℃ and the holding time is 2-5h. The second stage sintering temperature is 500-600℃ and the holding time is 10-14h. The heating rate is 2℃ / min. The first stage calcination is a pre-sintering process to remove certain moisture and impurities. The second stage calcination is a high-temperature solid-phase reaction.

9. The method for preparing an iron-based polyanionic sodium-ion battery cathode material as described in claim 2, characterized in that: In step (5), under the hydrogen-argon mixed protective atmosphere, a certain amount of reducing hydrogen is mixed in on the basis of argon inert gas as the main protective gas, and the volume content of reducing gas is greater than or equal to 1%.

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