A porous in-situ doped iron phosphate material and a preparation method and application thereof

Abstract

This invention relates to the field of sodium-ion battery cathode material technology, specifically to a porous in-situ doped iron phosphate composite sodium iron phosphate cathode material, its preparation method, and its application. The method includes: synthesizing a porous in-situ doped iron phosphate precursor using a liquid-phase co-precipitation method in the presence of a dispersant and a high-entropy dopant element; mixing and grinding the precursor with a sodium source, a phosphorus source, and a carbon source, spray drying, and then sintering at high temperature in an inert atmosphere to obtain a material with the general chemical formula Na₄Fe₂O₃. 3‑x M x A composite sodium iron phosphate cathode material of (PO4)2P2O7 / C. This invention shortens the solid-phase diffusion distance of sodium ions and suppresses the formation of impurity phases by constructing a small-sized porous precursor, and achieves atomic-level in-situ uniform doping of high-entropy elements in the precursor stage. The resulting material has high specific capacity, excellent rate performance and cycle stability, and the process is compatible with existing lithium iron phosphate production lines, making it easy to scale up production.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery cathode material technology, specifically to a porous in-situ doped iron phosphate composite sodium iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] With the deepening implementation of the "dual-carbon" strategy, the electrochemical energy storage market is developing rapidly. Sodium-ion batteries, due to their abundant sodium resources, low cost, and excellent low-temperature performance, have shown broad application prospects in large-scale energy storage and low-speed electric vehicles. Among them, iron-based mixed polyanionic compounds Na4Fe3(PO4)2P2O7 (NFPP) have become one of the sodium battery cathode materials with the greatest industrialization potential due to their advantages such as low raw material cost, good structural stability, high theoretical specific capacity (approximately 129 mAh / g), and environmental friendliness. Its industrial synthesis method is similar to that of lithium iron phosphate (LFP), both using the "sand milling-spray drying-high temperature sintering" route. Therefore, it can be produced using existing lithium iron phosphate production lines, showing good industrialization prospects.

[0003] However, the industrial application of NFPP cathode materials currently faces a fundamental contradiction: most of the iron phosphate precursors used in NFPP synthesis are products developed for lithium iron phosphate systems. The physical property requirements and process adaptation logic of these two systems differ significantly, leading to several technical problems, primarily in the following three aspects: First, in lithium iron phosphate (LFP) synthesis, after milling, the D50 particle size of the solid particles in the slurry of the iron phosphate precursor needs to be controlled at 0.4-1.0 μm to meet the requirements of subsequent solid-phase reaction for lithium-ion diffusion distance. However, the radius of sodium ions (approximately 1.02 Å) is larger than that of lithium ions (approximately 0.76 Å), resulting in slower solid-phase diffusion kinetics. The NFPP precursor needs to be milled to 0.2-0.4 μm or even finer to ensure sufficient sodium ion reaction and reduce impurity phase formation. Second, iron phosphate precursors prepared by traditional precipitation or solid-phase methods are typically dense, blocky, or large-sized particles with a specific surface area generally below 15 m². 2 / g, which not only leads to low grinding efficiency and high energy consumption, but also requires sodium ions to cross a micrometer-level diffusion distance to reach the particle core during high-temperature solid-state reactions. Long-distance diffusion easily causes incomplete internal reactions, leaving residual FePO4 phase, iron pyrophosphate (Fe2P2O7) impurity phase or local sodium-rich phase; thirdly, traditional carbon coating has limited improvement on conductivity and little effect on shortening ion diffusion distance; although metal ion doping can control the lattice structure and electronic state density, existing doping is mostly introduced in the mixing sintering or post-processing stage, which limits the solid-state diffusion rate and makes it difficult to achieve atomic-level uniform distribution of doped elements, easily resulting in element segregation or even impurity phases.

[0004] Therefore, from an industrialization perspective, developing a systematic technical solution that can construct porous, small-sized precursors specifically designed for NFPP systems from the source, and simultaneously achieve atomic-level in-situ doping of high-entropy elements in the liquid-phase precursor stage, fundamentally solving the core pain points of industrialization such as long solid-phase diffusion paths of sodium ions, low grinding efficiency, and poor doping uniformity, is a key issue that urgently needs to be addressed in the current field. Summary of the Invention

[0005] To address the technical problems of long solid-phase diffusion path of sodium ions in iron phosphate precursors, low grinding efficiency, and poor doping uniformity in the synthesis of NFPP in existing technologies, this invention provides a method and application for preparing composite sodium iron phosphate by in-situ doping with porous iron phosphate. By introducing a composite dispersant to create pores and in-situ co-doping with high-entropy elements during the precursor precipitation stage, a small-sized porous precursor is obtained, which significantly improves grinding efficiency, shortens the solid-phase diffusion distance of sodium ions, and achieves atomic-level uniform doping. The process is also compatible with existing lithium iron phosphate production lines.

[0006] The technical solution of this invention is as follows: In a first aspect, the present invention provides a method for preparing a porous in-situ doped iron phosphate composite sodium iron phosphate cathode material, comprising the following steps: (1) Using an aqueous solution containing a dispersant and a high-entropy doping element source as the base liquid, add the ferrous iron source and phosphorus source in parallel to react, and add hydrogen peroxide dropwise for oxidation; control the pH of the reaction system to be 2.0 to 5.0, the temperature to be 40 to 80℃, the reaction time to be 0.5 to 4 hours, and the solid content to be ≥15%; after the reaction, the filter cake is obtained by solid-liquid separation and washing; heat treat the filter cake at 500 to 650℃ for 2 to 6 hours to obtain a porous in-situ high-entropy doped anhydrous iron phosphate precursor; (2) The porous in-situ high-entropy doped anhydrous iron phosphate precursor is mixed, dispersed and ground with sodium source, phosphorus source, carbon source and pure water to obtain a slurry, and then spray-dried to obtain spray-dried material. (3) The spray-dried material is sintered at high temperature under an inert atmosphere to obtain a composite sodium iron phosphate cathode material.

[0007] In the above technical solution, a porous anhydrous iron phosphate precursor is prepared by introducing a composite dispersant during the precipitation stage and utilizing its steric hindrance and micellar template effect. This porous structure allows the sodium source to rapidly permeate along the pores during high-temperature sintering, thus converting Na into sodium. + The solid-phase diffusion distance with FePO4 is shortened from the micrometer level to the nanometer level, effectively suppressing the formation of impurity phases and ensuring the material's high specific capacity and cycling stability. Simultaneously, at least three high-entropy doping elements are introduced in the initial stage of liquid-phase synthesis. The dopant ions co-precipitate with iron ions and directly embed into the crystal lattice framework, achieving atomically uniform distribution and avoiding elemental segregation that is prone to occur in traditional solid-phase doping.

[0008] Furthermore, in step (1), the specific surface area of ​​the porous in-situ high-entropy doped anhydrous iron phosphate precursor is ≥25m². 2 / g. This invention controls the specific surface area of ​​the precursor to ≥25m². 2 / g, giving it abundant pore channels, allowing the sodium source to quickly penetrate into the particle's interior along the pore network, thus converting Na into Na + The solid-phase diffusion distance with FePO4 is drastically shortened from the micrometer level to the nanometer level, effectively suppressing the formation of impurity phases such as FePO4 and Fe2P2O7, ensuring high phase purity and electrochemical performance of the material. Furthermore, the loose, porous structure significantly reduces the internal binding force of the particles, greatly improving the efficiency of wet milling compared to commercially available dense iron phosphate (BET is typically below 15 μm). 2 / g), significantly reducing grinding energy consumption and working time.

[0009] Furthermore, in step (1), the ferrous iron source and the phosphorus source are added in parallel at a Fe / P molar ratio of 1:1 to 1:1.05.

[0010] Furthermore, in step (1), the ferrous iron source is any one of ferrous sulfate, ferrous chloride, and ferrous nitrate; and the phosphorus source is any one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, and ammonium phosphate.

[0011] Further, in step (1), the dispersant is at least two of polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, hexadecyltrimethylammonium bromide, amine salt, and quaternary ammonium salt; the amount of dispersant added is 0.5% to 15% of the iron source mass; preferably, the amount of dispersant added is 2% to 8% of the iron source mass. By using at least two dispersants in combination, the synergistic effect of their steric hindrance and micellar template effect can be utilized to effectively control the nucleation and growth process of precursor particles, forming a porous structure composed of loosely packed nanoscale primary particles. If the amount of dispersant is too small (<0.5%), the pore-forming effect is not obvious, the precursor tends to be dense, the specific surface area is low, and it is difficult to achieve rapid solid-phase diffusion of sodium ions in the subsequent process; if the amount is too large (>15%), it is easy to cause the dispersant to decompose violently during heat treatment, resulting in pore collapse or excessive carbonization, which reduces the structural stability of the precursor and increases the raw material cost and subsequent process burden.

[0012] Furthermore, in step (2), the sodium source is one or more of sodium carbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium pyrophosphate; the carbon source is one or more of glucose, sucrose, citric acid, and soluble starch, wherein the amount of carbon source added is based on the carbon content in the final product being 1 to 8 wt%.

[0013] Furthermore, in step (2), the particle size D50 of the slurry obtained after grinding is 0.2 to 0.4 μm.

[0014] Furthermore, in step (2), the inlet air temperature of the spray dryer is 200-240°C and the outlet air temperature is 75-110°C; the moisture content of the spray dryer is less than 5 wt%.

[0015] Furthermore, in step (3), the high-temperature solid-state sintering process is as follows: first, the temperature is raised to 300-400℃ at 2-5℃ / min for pre-firing for 1-3 hours, so that the carbon source and residual dispersant undergo preliminary pyrolysis to form an amorphous carbon coating layer; then, the temperature is raised to 500-650℃ at 2-5℃ / min and held for 6-12 hours. The segmented sintering process is beneficial for the carbon source and residual dispersant to fully pyrolyze at a lower temperature, forming a uniform amorphous carbon coating layer; subsequently, crystal phase reconstruction is carried out at a higher temperature to ensure the full growth of NFPP crystals. In addition, a suitable heating rate (2-5℃ / min) can avoid particle cracking or uneven reaction due to excessively rapid heating, while ensuring production efficiency.

[0016] Secondly, this invention provides a composite sodium iron phosphate cathode material, the chemical formula of which is: Na₄Fe₂O₃. 3-x M x (PO4)2P2O7 / C, where M is at least five doping elements selected from Ni, Cu, Mn, Al, Co, Mg, Zn, and Ti, and 0.03≤x≤0.3.

[0017] Furthermore, the dopant elements are added in the form of soluble salts (such as nitrates, sulfates, or acetates); the total molar amount of the dopant elements accounts for 1% to 10% of the molar amount of iron in the composite sodium iron phosphate cathode material; preferably, the total molar amount of the dopant elements accounts for 2% to 6% of the molar amount of iron in the composite sodium iron phosphate cathode material.

[0018] Thirdly, this invention provides an application of a composite sodium iron phosphate cathode material.

[0019] Furthermore, the composite sodium iron phosphate, as the positive electrode active material, is mixed with a conductive agent and a binder to form a slurry, which is then coated onto the current collector to prepare a sodium-ion battery positive electrode sheet, and further assembled into a sodium-ion battery.

[0020] The beneficial effects of this invention are as follows: 1. This invention introduces a composite dispersant during the precipitation stage, utilizing its steric hindrance and micellar template effect to prepare an anhydrous iron phosphate precursor (BET≥25nm) rich in mesoporous / macroporous structures, composed of loosely packed nanoscale primary particles (50-200nm). 2 This porous structure ( / g) plays a crucial role in solid-state sintering. The sodium source can rapidly permeate throughout the particle's interior along the abundant pore channels, allowing Na... +The solid-phase diffusion distance with FePO4 is drastically shortened from the micrometer level to the nanometer level (equivalent to the radius of a primary particle), effectively suppressing the formation of impurity phases and improving the material's cycle stability. Furthermore, the loose and porous structure of the anhydrous iron phosphate precursor, with its weak internal particle bonding, makes it easily broken and dispersed during wet milling. Compared to commercially available dense anhydrous iron phosphate, milling efficiency can be increased by 30%–50%, significantly reducing production energy consumption and equipment wear costs.

[0021] 2. This invention introduces multiple high-entropy doping elements during the precursor synthesis stage, enabling the dopant ions to interact with F. 3+ Co-precipitation, directly embedded in the FePO4 precursor lattice framework, compared to traditional post-doping or solid-phase ball-milling doping, this in-situ doping method avoids elemental segregation caused by slow solid-phase diffusion, achieving atomically uniform distribution. Furthermore, this invention achieves a synergistic effect of "bulk high-entropy doping" and "morphological porosity" in the NFPP system. High-entropy doping improves the intrinsic conductivity and structural stability of the material at the lattice level, while the porous structure shortens the ion diffusion distance and increases the electrochemical active area at the microscopic morphology level. The synergistic effect of these two processes allows the material to maintain high capacity output at high rates, and effectively suppresses structural degradation during long cycling.

[0022] 3. The process route adopted in this invention is highly compatible with the existing lithium iron phosphate (LFP) production process. The equipment is highly versatile and does not require large-scale modification of the production line. The LFP production line can be directly used to convert or expand the production of NFPP cathode materials, making it easy to achieve large-scale production at the thousand-ton or even ten-thousand-ton level. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a SEM image of the porous in-situ doped iron phosphate prepared in Example 1.

[0025] Figure 2 This is a SEM image of commercially available dense anhydrous ferric phosphate, Comparative Example 4.

[0026] Figure 3 This is a SEM image of the composite sodium iron phosphate cathode material prepared in Example 1.

[0027] Figure 4 The image shows the XRD pattern and component percentage of the composite sodium iron phosphate cathode material prepared in Example 1.

[0028] Figure 5 The image shows the XRD pattern and component percentage of the composite sodium iron phosphate cathode material prepared in Comparative Example 4.

[0029] Figure 6 The first charge-discharge curve of the composite sodium iron phosphate cathode material prepared in Example 1 at 0.1C is shown.

[0030] Figure 7 This is the first charge-discharge curve of the composite sodium iron phosphate cathode material prepared in Comparative Example 4 at 0.1C. Detailed Implementation

[0031] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0032] Example 1 (1) Preparation of porous in-situ high-entropy doped anhydrous iron phosphate precursor Preparation of the base solution: Add 5L of deionized water to the reactor, then add 60g of polyethylene glycol (PEG-2000, dispersant A) and 20g of polyvinylpyrrolidone (PVP-K30, dispersant B) sequentially, and stir to dissolve. Subsequently, add 14.62g of nickel nitrate, 12.08g of manganese sulfate, 15.15g of zinc nitrate, 27.37g of aluminum sulfate, and 12.34g of titanium trichloride (the total molar amount of the five doping elements accounts for 4% of the molar amount of iron in the finished product).

[0033] Prepare the iron salt solution: Dissolve 2780g of ferrous sulfate heptahydrate (FeSO4·7H2O) in 5L of deionized water.

[0034] Prepare the phosphate / oxidant solution: Dissolve 1150g of ammonium dihydrogen phosphate (NH4H2PO4) in 5L of deionized water and mix in 600mL of 30% hydrogen peroxide.

[0035] A liquid-phase co-precipitation method was used. Iron salt and phosphate salt solutions were added dropwise to the bottom liquid of the reactor in a parallel stream at 50°C with vigorous stirring. The pH of the reaction slurry was controlled to 3.0 using ammonia water. After the addition was complete, the mixture was kept at the same temperature and stirred for another 2 hours. The solid content of the reaction slurry was 18%. The reaction solution was filtered, and the filter cake was washed until no SO4 was found. 2- The filter cake was placed in a muffle furnace and heat-treated at 550°C for 4 hours in an air atmosphere. After natural cooling, it was crushed and sieved to obtain a porous in-situ high-entropy doped anhydrous iron phosphate precursor (FePO4).

[0036] The BET specific surface area of ​​the precursor obtained in this embodiment is 30m². 2 / g, the SEM image of the obtained product is as follows Figure 1 As shown in the figure, the particles are loosely packed together from tiny nano-sized primary particles, and are rich in porous structures.

[0037] (2) Slurry preparation and spray drying According to the target product Na4Fe 3-x M x (PO4)2P2O7 stoichiometric proportions of materials: 1500g of the precursor obtained in step (1), 530g of sodium carbonate (Na2CO3), 300g of sodium dihydrogen phosphate (NaH2PO4), and 150g of glucose were added to 2.5L of pure water and ground in a sand mill for 1.5 hours. The particle size D50 of the slurry was measured to be 0.28μm. After grinding, the slurry was transferred to a spray dryer for granulation. The inlet air temperature was 220℃ and the outlet air temperature was 95℃, resulting in a gray-black precursor powder.

[0038] (3) High-temperature solid-state sintering The spray-dried material was placed in an atmosphere furnace and pre-calcined at 300°C for 2 hours under a high-purity nitrogen atmosphere at a rate of 2.5°C / min, followed by a holding time at 550°C for 10 hours at a rate of 2.5°C / min. After cooling to room temperature in the furnace, it was passed through a 300-mesh sieve to obtain a composite sodium iron phosphate cathode material, labeled NFPP-1. ICP and carbon-sulfur analysis showed that the chemical formula of the product was approximately Na₄Fe₂O₃. 2.88 Ni 0.04 Mn 0.04 Zn 0.04 (PO4)2P2O7 / C, with a carbon content of 3.2wt%.

[0039] Example 2 This embodiment is the same as the steps in Embodiment 1, the main difference being that: in step (1), the doping elements are adjusted to titanium, magnesium, aluminum, copper and nickel, and the specific amounts added are: titanium sulfate 24.00g, magnesium nitrate 14.83g, aluminum nitrate 21.30g, copper sulfate 15.96g, nickel sulfate 15.48g (the total molar amount of the five doping elements accounts for 5% of the molar amount of iron in the finished product). The dispersant is adjusted to hexadecyltrimethylammonium bromide (CTAB) 20g and polyvinyl alcohol (PVA) 40g; in step (3), the sintering temperature is to pre-fire at 3℃ / min to 300℃ for 2 hours, and then at 3℃ / min to 600℃ for 8 hours.

[0040] Example 3 This embodiment is the same as the steps in embodiment 1, the main difference being: in step (1), the amount of dispersant used is reduced (the total amount added is 2.5% of the iron source mass), that is, PEG 20g and PVP 10g; in step (2), the D50 of the slurry after grinding is controlled at 0.35μm; in step (3), the sintering temperature is to pre-fire at 300℃ for 3 hours by heating at 5℃ / min, and then hold at 500℃ for 12 hours by heating at 5℃ / min.

[0041] Comparative Example 1 (No Dispersant Control) No dispersant was added during the precursor synthesis process, and all other conditions were the same as in Example 1. The resulting anhydrous ferric phosphate precursor was a dense mass with a BET specific surface area of ​​11 m². 2 / g. Under the same grinding conditions, sand milling took 3.5 hours to achieve a slurry D50 of 0.45μm.

[0042] Comparative Example 2 (Undoped Control) No dopant elements were added, and all other conditions were the same as in Example 1.

[0043] Comparative Example 3 (Post-doping control) The dopant element is not added during the precursor synthesis stage, but is added together with the sodium source and carbon source in the form of oxides or salts during the wet grinding in step two. The other conditions are the same as in Example 1.

[0044] Comparative Example 4 (Dense anhydrous iron phosphate precursor control) Commercially available dense anhydrous ferric phosphate (BET≈5.5m) was used. 2 / g, primary particle size >1μm) replaced the self-made porous precursor of Example 1, and was doped, ground, and sintered according to the same formulation as in Example 1. The results showed that grinding to the same fineness took 4 hours; XRD pattern ( Figure 5 The presence of distinct heterophase peaks of sodium pyrophosphate and sodium iron phosphate leads to a significant decrease in reversible capacity, resulting in a reduced 0.1C discharge specific capacity ( Figure 7 It has a capacity of only 95mAh / g.

[0045] Test Example 1 Scanning electron microscopy test Scanning electron microscopy (SEM) tests were performed on the porous in-situ doped iron phosphate prepared in Example 1 and the commercially available dense anhydrous iron phosphate in Comparative Example 4. The results are as follows: Figure 1 , Figure 2 As shown in the figure, the precursor of Example 1 has abundant porous structure and primary particle size is less than 100 nm; while the commercially available iron phosphate precursor has no obvious porous structure between particles, and the primary particle size is nearly 1 micrometer, with large particles. It can be seen that the precursor prepared in Example 1 of this invention contains abundant small-sized porous structure, which can significantly shorten the solid-phase diffusion distance of sodium ions and effectively suppress the formation of impurity phases.

[0046] The composite sodium iron phosphate cathode material prepared in Example 1 was subjected to SEM testing, and the results are as follows: Figure 3 As shown in the figure, the secondary agglomerates exhibit a typical spray microsphere morphology, with a size of 5-20 micrometers. The accumulation of particles of different sizes can effectively increase the packing density of the material. In addition, the enlarged image on the right shows that the primary particles are small, only tens of nanometers in size, and have abundant pore structures between them, which is conducive to electrolyte wetting and effectively shortens the transport distance of sodium ions.

[0047] Test Example 2 XRD phase analysis The composite sodium iron phosphate cathode material prepared in Example 1 was subjected to XRD testing, and the results are as follows: Figure 4 As shown, its diffraction peak positions are consistent with the characteristic diffraction peaks of standard card PDF#98-022-0348 Na4Fe3(PO4)2P2O7, and no other impurity peaks were observed, indicating that a composite sodium iron phosphate cathode material with high phase purity was successfully synthesized.

[0048] Test Example 3 Electrochemical performance testing The composite sodium iron phosphate cathode materials prepared in Examples 1-3 and Comparative Examples 1-3 were respectively operated as follows: the active material, binder PVDF, and conductive additive SP were dispersed in NMP solution according to the mass ratio of 90:5:5. After being mixed evenly, they were prepared into a slurry, coated on aluminum foil, and then vacuum dried, rolled, and cut into sheets to obtain cathode sheets.

[0049] The negative electrode uses a sodium metal sheet, the separator is glass fiber, and the electrolyte is 1 mol / L NaPF6 (EC:DEC=1:1Vol%). The charge / discharge specific capacity of the coin cell was tested within a voltage range of 2.0V-4.2V, and the test results are shown in Table 1. The initial charge / discharge curve of the composite sodium iron phosphate positive electrode material prepared in Example 1 at 0.1C is shown below. Figure 6 As shown.

[0050] Test Example 4 Specific surface area test BET tests were performed on the precursors prepared in Examples 1-3 and Comparative Examples 1-3, and the results are shown in Table 1. A composite dispersant was introduced during the precursor synthesis stage to construct anhydrous iron phosphate precursors with small size and large specific surface area. The specific surface area of ​​the examples was significantly larger than that of the comparative examples, which effectively shortened the time required for the milling stage and promoted more complete subsequent solid-phase reactions.

[0051] Table 1. Comparison of data from Examples 1-3 and Comparative Examples 1-4.

[0052] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for preparing a porous in-situ doped iron phosphate composite sodium iron phosphate cathode material, characterized in that, Includes the following steps: (1) Using an aqueous solution containing a dispersant and a high-entropy doping element source as the base liquid, add the ferrous iron source and phosphorus source in parallel to react, and add hydrogen peroxide dropwise for oxidation; control the pH of the reaction system to be 2.0 to 5.0, the temperature to be 40 to 80℃, the reaction time to be 0.5 to 4 hours, and the solid content to be ≥15%; after the reaction, the filter cake is obtained by solid-liquid separation and washing; heat treat the filter cake at 500 to 650℃ for 2 to 6 hours to obtain a porous in-situ high-entropy doped anhydrous iron phosphate precursor; (2) The porous in-situ high-entropy doped anhydrous iron phosphate precursor is mixed, dispersed and ground with sodium source, phosphorus source, carbon source and pure water to obtain a slurry, and then spray-dried to obtain spray-dried material. (3) The spray-dried material is sintered at high temperature under an inert atmosphere to obtain a composite sodium iron phosphate cathode material.

2. The preparation method according to claim 1, characterized in that, In step (1), the ferrous iron source and the phosphorus source are added in parallel at a Fe / P molar ratio of 1:1 to 1:1.

05.

3. The preparation method according to claim 1, characterized in that, In step (1), the ferrous iron source is any one of ferrous sulfate, ferrous chloride, and ferrous nitrate; the phosphorus source is any one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, and ammonium phosphate.

4. The preparation method according to claim 1, characterized in that, In step (1), the dispersant is at least two of polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, hexadecyltrimethylammonium bromide, amine salt, and quaternary ammonium salt; the amount of the dispersant added is 0.5% to 15% of the mass of the iron source.

5. The preparation method according to claim 1, characterized in that, In step (2), the sodium source is one or more of sodium carbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, and sodium pyrophosphate; the carbon source is one or more of glucose, sucrose, citric acid, and soluble starch, wherein the amount of carbon source added is based on the carbon content in the final product being 1 to 8 wt%.

6. The preparation method according to claim 1, characterized in that, In step (2), the particle size D50 of the slurry obtained after grinding is 0.2 to 0.4 μm.

7. The preparation method according to claim 1, characterized in that, In step (2), the inlet air temperature of the spray dryer is 200-240°C and the outlet air temperature is 75-110°C; the moisture content of the spray dryer is less than 5 wt%.

8. The preparation method according to claim 1, characterized in that, In step (3), the high-temperature solid-state sintering process is as follows: first, the temperature is raised to 300-400℃ at 2-5℃ / min for 1-3 hours for pre-firing, and then the temperature is raised to 500-650℃ at 2-5℃ / min for 6-12 hours for holding.

9. A composite sodium iron phosphate cathode material prepared by the preparation method according to any one of claims 1-8, characterized in that, The chemical formula of the composite sodium iron phosphate cathode material is: Na₄Fe₂O₃ 3-x M x (PO4)2P2O7 / C, where M is at least five doping elements selected from Ni, Cu, Mn, Al, Co, Mg, Zn, and Ti, and 0.03≤x≤0.

3.

10. An application of the composite sodium iron phosphate cathode material as described in claim 9.

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