Modified ferric sodium pyrophosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention provides a modified ferric sodium phosphate pyrophosphate positive electrode material and a preparation method and application thereof.According to the preparation method, mature sanding, spray drying and calcining processes are adopted, and the ferric sodium phosphate pyrophosphate / carbon positive electrode material with a porous spherical structure and high phase purity is prepared by optimizing the combination of raw materials; and the performance breakthrough of high specific capacity, super-large rate capability and excellent super-large rate cycle performance is realized. The provided preparation method is simple and convenient in technological process, high in controllability and suitable for industrialization, the selected raw materials are wide in source and low in cost, the energy consumption in the technological process is relatively low, and the comprehensive production cost has remarkable advantages; the synthesized ferric sodium pyrophosphate / carbon positive electrode material obtains excellent dynamic performance, electrochemical performance and structural stability at an ultra-large rate at 20 DEG C based on special morphology characteristics and relatively high phase purity.

Description

Technical Field

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

[0002] Sodium-ion batteries, with their significant advantages such as abundant resources, low cost, and environmental friendliness, are widely regarded as an important alternative technology to lithium-ion batteries in the field of large-scale energy storage. As a key component of sodium-ion batteries, the performance of the cathode material directly determines the core indicators of the battery, including energy density, cycle life, and safety. Among numerous cathode material systems, the hybrid polyanionic compound sodium iron pyrophosphate has attracted much attention due to its unique advantages: its crystal framework structure is stable, effectively suppressing structural collapse during charging and discharging; it possesses a high theoretical specific capacity (129 mAh / g); and its operating voltage platform is moderate (~3 V vs. Na⁺ / Na).

[0003] Currently, mainstream preparation methods (such as traditional solid-state synthesis) generally suffer from deficiencies in the particle morphology and structure of sodium iron pyrophosphate cathode materials. The resulting products are typically large-sized (micrometer-scale) and densely structured blocky or agglomerated particles. This large-size, low-porosity microstructure has a dual negative impact: firstly, it significantly reduces the contact area between the material and the electrolyte, resulting in poor electrolyte wettability; secondly, it forces sodium ions to undergo a longer diffusion path within the solid phase. The combined effect of these factors leads to slow ion diffusion kinetics, further limiting the bulk sodium ion diffusion rate, ultimately resulting in a significant deterioration in the material's high-rate charge-discharge performance. Summary of the Invention

[0004] The present invention aims to solve the above-mentioned problems of the prior art, and its purpose is to provide a modified sodium iron pyrophosphate cathode material, its preparation method and application.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: The first aspect provides a method for preparing modified sodium iron pyrophosphate cathode material, including: Iron source, phosphorus source, sodium source, dispersing reducing agent, carbon source and water are mixed to prepare a suspension; The suspension was milled to obtain a slurry, and the slurry was spray-dried to obtain a precursor. The precursor was sintered under a protective atmosphere to obtain a modified sodium iron pyrophosphate / carbon composite cathode material.

[0006] Furthermore, the iron source is one or more of ferrous oxalate, ferric phosphate, ferric nitrate, ferric oxide, ferric pyrophosphate, ferrous pyrophosphate, ferric sulfate, and ferrous sulfate.

[0007] Furthermore, the iron source is a mixed iron source of ferric phosphate and ferrous oxalate; the molar ratio of ferric phosphate to ferrous oxalate in the mixed iron source is 1:(0.3~3), preferably 1:(0.5~3).

[0008] Furthermore, the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, sodium pyrophosphate, sodium oxalate, and sodium acetate.

[0009] Furthermore, the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, iron phosphate, iron pyrophosphate, and sodium dihydrogen phosphate.

[0010] Further, the dispersing and reducing agent is one or more of polyethylene glycol, polyvinyl alcohol, polyvinyl butyral, hydroxymethyl cellulose, polyacrylamide, polyoxyethylene, and polyacrylic acid; the amount of the dispersing and reducing agent added is 5% to 20% of the total mass of the iron source, phosphorus source, and sodium source.

[0011] Further, the carbon source is one or more of glucose, ascorbic acid, cyclodextrin, polyvinylpyrrolidone, citric acid, sucrose, polyacrylonitrile, and polyethyleneimine, preferably polyvinylpyrrolidone; the amount of carbon source added is 5% to 10% of the total mass of iron source, phosphorus source and sodium source.

[0012] Furthermore, the particle size D50 of the slurry is ≤300nm, preferably 100~300nm.

[0013] Furthermore, the particle size of the precursor is 1~40μm.

[0014] Furthermore, the milling speed is 2000~3500 r / min, and the milling time is 60~300 min.

[0015] Furthermore, the spray drying is a two-fluid spray or a centrifugal spray; the inlet temperature during spray drying is 180~300℃, and the outlet temperature is 90~150℃.

[0016] Furthermore, the sintering temperature is 450~600℃; the sintering holding time is 8~15h; the sintering heating rate is 1~5℃ / min; and the protective atmosphere is one or more of argon atmosphere, nitrogen atmosphere, and helium atmosphere.

[0017] Secondly, a modified sodium iron pyrophosphate cathode material is provided, which is prepared using the preparation method described in the first aspect.

[0018] Furthermore, the modified sodium iron pyrophosphate cathode material has a porous spherical morphology.

[0019] Furthermore, the purity of the modified sodium iron pyrophosphate cathode material is not less than 95%.

[0020] Furthermore, the modified sodium iron pyrophosphate cathode material has the following properties: the first discharge specific capacity of the assembled button cell is ≥105mAh / g at 0.1C; the discharge specific capacity at 20C rate is ≥90mAh / g; and the capacity retention rate is ≥85% after 5000 cycles at 20C.

[0021] Thirdly, a sodium-ion battery is provided, comprising the modified sodium iron pyrophosphate cathode material described in the second aspect.

[0022] Compared with the prior art, one or more of the above technical solutions can achieve at least one of the following beneficial effects: The provided preparation method employs mature sand milling, spray drying, and calcination processes. By optimizing the combination of raw materials, a sodium iron pyrophosphate / carbon cathode material with a porous spherical structure and high phase purity is prepared, achieving a performance breakthrough in high specific capacity, ultra-high rate performance, and excellent ultra-high rate cycling performance.

[0023] The synthesized sodium iron pyrophosphate / carbon cathode material features a unique porous spherical structure and a continuous carbon coating layer on the surface and through which sodium ions diffuse within the solid phase. This significantly shortens the diffusion path of sodium ions and substantially improves the overall electronic conductivity of the material, endowing it with excellent electrical and ionic conductivity. This effectively improves the material's kinetic performance at ultra-high rates (20C). The unique porous structure of the sodium iron pyrophosphate / carbon cathode material effectively buffers volume changes during charge and discharge. Combined with the protective effect of the carbon coating layer, the material exhibits excellent long-cycle stability, especially at ultra-high rates.

[0024] The preparation method provided is simple, highly controllable, and suitable for industrialization. The selected raw materials are widely available and inexpensive, and the energy consumption of the process is relatively low, resulting in a significant advantage in overall production cost. Attached Figure Description

[0025] 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, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a flowchart illustrating the preparation process of sodium iron pyrophosphate / carbon cathode material in some embodiments of the present invention.

[0027] Figure 2SEM images of the cathode materials prepared in Examples 1-4 and Examples 7 and 8.

[0028] Figure 3 The images show the XRD patterns of the cathode materials prepared in Examples 1-9.

[0029] Figure 4 The image shows the refined XRD pattern of the cathode material prepared in Example 1.

[0030] Figure 5 The first-cycle charge-discharge curves of the electrodes obtained from the positive electrode materials prepared in Examples 1, 7, and 8 are shown.

[0031] Figure 6 The graph shows the rate performance of the electrodes obtained from the positive electrode materials prepared in Examples 1, 7, and 8.

[0032] Figure 7 The graph shows the 20C cycle performance of the electrode obtained from the positive electrode material prepared in Example 1. Detailed Implementation

[0033] Existing technologies suffer from the toxicity of residual organic solvents and inefficient mixing processes, making it difficult to balance high material performance with industrial feasibility. Through extensive research, the applicant combined aqueous nano-sand milling dispersion with spray granulation technology, and optimized the raw materials, to prepare a sodium iron pyrophosphate / carbon cathode material with a porous spherical structure and high phase purity. This resulted in a performance breakthrough with high specific capacity, high rate performance, and excellent high-rate cycling performance. Based on this, the present invention was completed.

[0034] Some embodiments of the present invention provide a method for preparing a modified sodium iron pyrophosphate cathode material, comprising: Iron source, phosphorus source, sodium source, dispersing reducing agent, carbon source and water are mixed to prepare a suspension; The suspension was milled to obtain a slurry, and the slurry was spray-dried to obtain a precursor. The precursor was sintered under a protective atmosphere to obtain a modified sodium iron pyrophosphate / carbon composite cathode material.

[0035] In some embodiments, the iron source is one or more of ferrous oxalate, ferric phosphate, ferric nitrate, ferric oxide, ferric pyrophosphate, ferrous pyrophosphate, ferric sulfate, and ferrous sulfate.

[0036] In some preferred embodiments, the iron source is a mixed iron source of ferric phosphate and ferrous oxalate; the molar ratio of ferric phosphate to ferrous oxalate in the mixed iron source is 1:(0.3~3), preferably 1:(0.5~3), for example 1:0.5, 1:0.6, 1:0.8, 1:1, 1:1.2, 1:1.5, 1:1.8, 1:2, 1:2.2, 1:2.5, 1:2.8, 1:3, etc.

[0037] In some embodiments, the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, sodium pyrophosphate, sodium oxalate, and sodium acetate.

[0038] In some embodiments, the phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, ferric phosphate, ferric pyrophosphate, and sodium dihydrogen phosphate.

[0039] In some preferred embodiments, the dispersing reducing agent is one or more of polyethylene glycol, polyvinyl alcohol, polyvinyl butyral, hydroxymethyl cellulose, polyacrylamide, polyoxyethylene, and polyacrylic acid, and is more preferably polyethylene glycol.

[0040] Excessive use of the dispersing and reducing agent will negatively impact the electrochemical performance of the sodium iron pyrophosphate cathode material, such as reducing its capacity. Conversely, insufficient use will result in poor dispersion and consequently, deteriorate the overall material performance. In some preferred embodiments, the amount of the dispersing and reducing agent added is 5% to 20% of the total mass of the iron, phosphorus, and sodium sources, for example, 5%, 8%, 10%, 12%, 15%, 18%, 20%, etc.

[0041] In some embodiments, the carbon source is one or more of glucose, ascorbic acid, cyclodextrin, polyvinylpyrrolidone, citric acid, sucrose, polyacrylonitrile, and polyethyleneimine, with polyvinylpyrrolidone being the preferred carbon source.

[0042] Excessive carbon source can lead to a decrease in electrochemical performance, such as capacity, while insufficient carbon source will not adequately improve conductivity, resulting in a decrease in electrochemical performance. In some preferred embodiments, the amount of carbon source added is 5% to 10% of the total mass of iron, phosphorus, and sodium sources, for example, 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0043] In some embodiments, the solid-liquid ratio of the slurry is 20-40%.

[0044] In some preferred embodiments, the particle size D50 of the slurry is ≤300nm, preferably 100~300nm, such as 100nm, 150nm, 200nm, 250nm, 300nm, etc.

[0045] In some preferred embodiments, the particle size of the precursor is 1~40μm, such as 1μm, 3μm, 5μm, 8μm, 10μm, 12μm, 15μm, 18μm, 20μm, 22μm, 25μm, 28μm, 30μm, 32μm, 35μm, 38μm, 40μm, etc.

[0046] In some preferred embodiments, the rotational speed of the sand mill is 2000~3500 r / min, for example 2000 r / min, 2500 r / min, 3000 r / min, 3500 r / min; the sand milling time is 60~300 min, for example 60 min, 80 min, 100 min, 150 min, 200 min, 250 min, 300 min, etc.

[0047] In some preferred embodiments, the spray drying is a two-fluid spray or a centrifugal spray; the inlet temperature during spray drying is 180~300℃, for example 180℃, 200℃, 220℃, 250℃, 280℃, 300℃, etc., and the outlet temperature is 90~150℃, for example 90℃, 120℃, 150℃, etc.

[0048] In some preferred embodiments, the sintering temperature is 450~600℃, for example 450℃, 500℃, 550℃, 600℃, etc.; the sintering holding time is 8~15h, for example 8h, 10h, 12h, 15h, etc.; the sintering heating rate is 1~5℃ / min, for example 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, etc.; the protective atmosphere is one or more of argon atmosphere, nitrogen atmosphere, and helium atmosphere.

[0049] Some embodiments provide a modified sodium iron pyrophosphate cathode material prepared by the above preparation method.

[0050] In some preferred embodiments, the modified sodium iron pyrophosphate cathode material has a porous spherical morphology.

[0051] In some preferred embodiments, the purity of the modified sodium iron pyrophosphate cathode material is not less than 95%, for example, not less than 95%, not less than 96%, not less than 97%, not less than 98%, not less than 99%, etc.

[0052] In some preferred embodiments, the modified sodium iron pyrophosphate cathode material has the following properties: the assembled button cell has an initial discharge specific capacity of ≥105 mAh / g at 0.1C, for example, not less than 106 mAh / g, not less than 107 mAh / g, not less than 108 mAh / g, not less than 109 mAh / g, not less than 110 mAh / g, not less than 111 mAh / g, not less than 112 mAh / g, not less than 113 mAh / g, not less than 114 mAh / g, not less than 115 mAh / g, etc.; a discharge specific capacity of ≥90 mAh / g at 20C rate, for example, not less than 91 mAh / g, not less than 92 mAh / g, not less than 93 mAh / g, not less than 94 mAh / g, not less than 95 mAh / g, etc.; and a capacity retention rate of ≥85% after 5000 cycles at 20C, for example, not less than 85%, not less than 86%, not less than 87%, not less than 88%, not less than 89%, not less than 90%, etc.

[0053] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0054] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0055] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0056] Example 1 This embodiment provides a method for preparing a porous spherical sodium iron pyrophosphate cathode material, and the process flow diagram is shown below. Figure 1 As shown, it includes the following steps: (1) Weigh 13.415g of ferric phosphate, 16g of ferrous oxalate dihydrate, 19.2836g of sodium dihydrogen phosphate, and 4.676g of sodium carbonate. Simultaneously add 10% (by weight of raw materials) of polyethylene glycol as a dispersant and reducing agent, and 5% (by weight of raw materials) of polyvinylpyrrolidone as a carbon source. Disperse the mixture in deionized water and stir to obtain a suspension. Mill the suspension until the particle size D50 of the solid particles in the suspension reaches 0.30μm. (2) The slurry obtained in step (1) is dried in a spray drying tower. A two-fluid dryer is selected for the spray dryer. The gas pressure is 0.30 MPa. The spray drying inlet temperature is set to 220℃ and the outlet temperature is controlled at 125℃ to obtain the spray-dried material. (3) The spray-dried powder was heated to 520°C at a heating rate of 5°C / min under nitrogen protection and heat-treated for 10h to obtain sodium iron pyrophosphate / carbon composite cathode material.

[0057] Example 2 This embodiment provides a method for preparing a porous spherical sodium iron pyrophosphate cathode material, including the following steps: (1) Weigh 20.122g of ferric phosphate, 8g of ferrous oxalate dihydrate, 13.9478g of sodium dihydrogen phosphate, and 7.0138g of sodium carbonate. Simultaneously add 10% (by weight of raw materials) of polyethylene glycol as a dispersant and reducing agent, and 5% (by weight of raw materials) of polyvinylpyrrolidone as a carbon source. Disperse the mixture in deionized water and stir to obtain a suspension. Mill the suspension until the particle size D50 of the solid particles in the suspension reaches 0.30μm. (2) The slurry obtained in step (1) is dried in a spray drying tower. A two-fluid dryer is selected for the spray dryer. The gas pressure is 0.30 MPa. The spray drying inlet temperature is set to 220℃ and the outlet temperature is controlled at 125℃ to obtain the spray-dried material. (3) The spray-dried powder was heated to 520°C at a heating rate of 5°C / min under nitrogen protection and heat-treated for 10h to obtain sodium iron pyrophosphate / carbon composite cathode material.

[0058] Example 3 This embodiment provides a method for preparing a porous spherical sodium iron pyrophosphate cathode material, including the following steps: (1) Weigh 6.707 g of ferric phosphate, 24 g of ferrous oxalate dihydrate, 24.6191 g of sodium dihydrogen phosphate, and 2.3378 g of sodium carbonate. Simultaneously add 10% (by weight of raw materials) of polyethylene glycol as a dispersant and reducing agent, and 5% (by weight of raw materials) of polyvinylpyrrolidone as a carbon source. Disperse these components in deionized water and mix thoroughly to obtain a suspension. Mill the suspension until the particle size D50 of the solid particles in the suspension reaches 0.30 μm. (2) The slurry obtained in step (1) is dried in a spray drying tower. A two-fluid dryer is selected for the spray dryer. The gas pressure is 0.30 MPa. The spray drying inlet temperature is set to 220℃ and the outlet temperature is controlled at 125℃ to obtain the spray-dried material. (3) The spray-dried powder was heated to 520°C at a heating rate of 5°C / min under nitrogen protection and heat-treated for 10h to obtain sodium iron pyrophosphate / carbon composite cathode material.

[0059] Example 4 This embodiment provides a method for preparing a porous spherical sodium iron pyrophosphate cathode material, including the following steps: (1) Weigh 8.9445g of ferric phosphate, 21.3372g of ferrous oxalate dihydrate, 22.8447g of sodium dihydrogen phosphate, and 3.1177g of sodium carbonate. Simultaneously add 10% (by weight of raw materials) of polyethylene glycol as a dispersant and reducing agent, and 5% (by weight of raw materials) of polyvinylpyrrolidone as a carbon source. Disperse the mixture in deionized water and stir to obtain a suspension. Mill the suspension until the particle size D50 of the solid particles in the suspension reaches 0.30μm. (2) The slurry obtained in step (1) is dried in a spray drying tower. A two-fluid dryer is selected for the spray dryer. The gas pressure is 0.30 MPa. The spray drying inlet temperature is set to 220℃ and the outlet temperature is controlled at 125℃ to obtain the spray-dried material. (3) The spray-dried powder was heated to 520°C at a heating rate of 5°C / min under nitrogen protection and heat-treated for 10h to obtain sodium iron pyrophosphate / carbon composite cathode material.

[0060] Example 5 This embodiment provides a method for preparing a porous spherical sodium iron pyrophosphate cathode material, including the following steps: (1) Weigh 13.415g of ferric phosphate, 16g of ferrous oxalate dihydrate, 19.2836g of sodium dihydrogen phosphate, and 4.676g of sodium carbonate. Simultaneously, add 10% (by weight of raw materials) of polyvinyl alcohol to replace polyethylene glycol as a dispersant and reducing agent, and add 5% (by weight of raw materials) of polyvinylpyrrolidone as a carbon source. Disperse the mixture in deionized water and stir to obtain a suspension. Mill the above suspension until the particle size D50 of the solid particles in the suspension is 0.30μm. (2) The slurry obtained in step (1) is dried in a spray drying tower. A two-fluid dryer is selected for the spray dryer. The gas pressure is 0.30 MPa. The spray drying inlet temperature is set to 220℃ and the outlet temperature is controlled at 125℃ to obtain the spray-dried material. (3) The spray-dried powder was heated to 520°C at a heating rate of 5°C / min under nitrogen protection and heat-treated for 10h to obtain sodium iron pyrophosphate / carbon composite cathode material.

[0061] Example 6 This embodiment provides a method for preparing a porous spherical sodium iron pyrophosphate cathode material, including the following steps: (1) Weigh 13.415g of ferric phosphate, 16g of ferrous oxalate dihydrate, 19.2836g of sodium dihydrogen phosphate, and 4.676g of sodium carbonate. Simultaneously, add 10% by weight of polyacrylic acid to replace polyethylene glycol as a dispersant and reducing agent, and add 5% by weight of polyvinylpyrrolidone as a carbon source. Disperse the mixture in deionized water and stir to obtain a suspension. Mill the suspension until the particle size D50 of the solid particles in the suspension is 0.30μm. (2) The slurry obtained in step (1) is dried in a spray drying tower. A two-fluid dryer is selected for the spray dryer. The gas pressure is 0.30 MPa. The spray drying inlet temperature is set to 220℃ and the outlet temperature is controlled at 125℃ to obtain the spray-dried material. (3) The spray-dried powder was heated to 520°C at a heating rate of 5°C / min under nitrogen protection and heat-treated for 10h to obtain sodium iron pyrophosphate / carbon composite cathode material.

[0062] Example 7 This embodiment provides a method for preparing a porous spherical sodium iron pyrophosphate cathode material, including the following steps: (1) Weigh 26.83 g of ferric phosphate, 8.6124 g of sodium dihydrogen phosphate, and 9.3519 g of sodium carbonate. Simultaneously add 10% polyethylene glycol by weight of the raw materials as a dispersant and reducing agent, and 5% polyvinylpyrrolidone by weight of the raw materials as a carbon source. Disperse the mixture in deionized water and stir to obtain a suspension. Mill the above suspension until the particle size D50 of the solid particles in the suspension is 0.30 μm. (2) The slurry obtained in step (1) is dried in a spray drying tower. A two-fluid dryer is selected for the spray dryer. The gas pressure is 0.30 MPa. The spray drying inlet temperature is set to 220℃ and the outlet temperature is controlled at 125℃ to obtain the spray-dried material. (3) The spray-dried powder was heated to 520°C at a heating rate of 5°C / min under nitrogen protection and heat-treated for 10h to obtain sodium iron pyrophosphate / carbon composite cathode material.

[0063] Example 8 This embodiment provides a method for preparing a porous spherical sodium iron pyrophosphate cathode material, including the following steps: (1) Weigh 32g of ferrous oxalate dihydrate and 29.9548g of sodium dihydrogen phosphate. Simultaneously add 10% polyethylene glycol by weight of the raw materials as a dispersant and reducing agent, and 5% polyvinylpyrrolidone by weight of the raw materials as a carbon source. Disperse the mixture in deionized water and stir to obtain a suspension. Mill the above suspension until the particle size D50 of the solid particles in the suspension is 0.30μm. (2) The slurry obtained in step (1) is dried in a spray drying tower. A two-fluid dryer is selected for the spray dryer. The gas pressure is 0.30 MPa. The spray drying inlet temperature is set to 220℃ and the outlet temperature is controlled at 125℃ to obtain the spray-dried material. (3) The spray-dried powder was heated to 520°C at a heating rate of 5°C / min under nitrogen protection and heat-treated for 10h to obtain sodium iron pyrophosphate / carbon composite cathode material.

[0064] Example 9 This embodiment provides a method for preparing a porous spherical sodium iron pyrophosphate cathode material, including the following steps: (1) Weigh 13.415g of ferric phosphate, 16g of ferrous oxalate dihydrate, 19.2836g of sodium dihydrogen phosphate, and 4.676g of sodium carbonate. Simultaneously add 10% (by weight of raw materials) of polyethylene glycol as a dispersant and reducing agent, and 5% (by weight of raw materials) of glucose to replace polyvinylpyrrolidone as a carbon source. Disperse the mixture in deionized water and stir to obtain a suspension. Mill the suspension until the particle size D50 of the solid particles in the suspension reaches 0.30μm. (2) The slurry obtained in step (1) is dried in a spray drying tower. A two-fluid dryer is selected for the spray dryer. The gas pressure is 0.30 MPa. The spray drying inlet temperature is set to 220℃ and the outlet temperature is controlled at 125℃ to obtain the spray-dried material. (3) The spray-dried powder was heated to 520°C at a heating rate of 5°C / min under nitrogen protection and heat-treated for 10h to obtain sodium iron pyrophosphate / carbon composite cathode material.

[0065] The sodium iron pyrophosphate cathode materials prepared in the examples and comparative examples were characterized as follows: Figure 2SEM images of sodium ferric pyrophosphate prepared in Examples 1-4 and Examples 7-8 are shown. It can be observed that the material in Example 1 consists of secondary spherical particles assembled from primary nanoparticles, exhibiting uniform micropores (pore size 50-100 nm) and high sphericity on the surface. The material in Example 2, synthesized with a high proportion of ferric phosphate, shows decreased sphericity and a reduced number of surface pores. The structure in Example 3 exhibits collapse, with some particles unable to maintain their spherical shape, leading to closure of internal pores. The material in Example 7, using ferric phosphate as the iron source, has low particle sphericity, with some spherical particles collapsing inwards and only a few pores on the surface. The material in Example 8, synthesized with ferrous oxalate as the iron source, has a blocky structure with no obvious pores on the surface. Example 4 has a similar microstructure to Example 1, demonstrating that under the same conditions (carbon source, dispersant, preparation process, etc.), a certain proportion of composite iron source (ferric phosphate + ferrous oxalate) synergistically guides the formation of porous spherical structures. Furthermore, the added carbon source can stabilize the pore structure to a certain extent, and the dispersant and carbon source also affect the micropore structure and conductivity.

[0066] Figure 3 The XRD patterns of sodium iron pyrophosphate prepared in Examples 1-9 are shown in Table 1. The horizontal axis represents the 2θ diffraction angle of X-rays, and the vertical axis represents the diffraction intensity. The phase purity of the refined XRD patterns is shown in Table 1. The refined XRD pattern of sodium iron pyrophosphate prepared in Example 1 is shown in Table 1. Figure 4 As shown in Table 1, compared with other examples, Example 1 has almost no impurity phase (main phase content ≥99%). Furthermore, comparing Examples 1, 5, and 6 shows that the type of dispersant significantly affects the purity of sodium iron pyrophosphate. Comparing Examples 1 and 9 shows that the carbon source also significantly affects the purity of sodium iron pyrophosphate. Comparing Examples 1-4 and Examples 7-8 shows that the type of iron source also has a certain impact on the purity of sodium iron pyrophosphate.

[0067] The positive electrode materials obtained in Examples 1-9 were mixed and homogenized at a mass ratio of active material:SP:PVDF of 7:2:1, and then coated onto aluminum foil. The membrane was then dried in a 120°C forced-air drying oven for 4 hours. The electrode membrane was punched into a 12mm diameter disc using a punching machine. Sodium metal was used as the counter electrode, and the electrolyte was a mixture of 1 mol / L NaClO4, EC + PC (1:1 vol%) + 5% FEC, where EC is ethylene carbonate, PC is polycarbonate, FEC is fluoroethylene carbonate additive, and the separator was glass fiber. The CR2025 type button cell was assembled in a glove box. The above button cells were subjected to constant current charge-discharge tests within the range of 1.5~4V.

[0068] Figure 5 , Figure 6The figures show the first-cycle charge-discharge curves and rate performance graphs of button batteries assembled using the cathode materials obtained in Examples 1, 7, and 8, within a voltage range of 1.5–4V at 0.1C (1C = 129 mAh / g), respectively. The electrode obtained using the cathode material in Example 1 exhibits a 0.1C charging capacity of 117.2 mAh / g and a discharge capacity of 116.4 mAh / g, demonstrating excellent capacity performance. Furthermore, Example 1 consistently demonstrates excellent capacity performance across rates from 0.5 to 30C. Notably, at a 20C charge-discharge rate, Example 1 still maintains a specific capacity of 99.6 mAh / g. A comparison of Example 1 with Examples 7 and 8 shows that the iron source significantly affects the discharge specific capacity and rate performance of the cathode material.

[0069] Figure 7 The graph shows the cycling performance of the button cell assembled with the cathode material obtained in Example 1 at a rate of 20C. Within a voltage range of 1.5 to 4V, Example 1 exhibits a capacity retention of 91.67% after 5000 cycles at 20C, demonstrating excellent cycle stability.

[0070] The discharge specific capacity and initial coulombic efficiency of the coin cells assembled with the cathode materials prepared in Examples 1-9 at 0.1C, as well as the discharge specific capacity and cycle performance at 20C, are shown in Table 1.

[0071] Table 1 As shown in Table 1, compared with Examples 5-6 and 9, the coin cells assembled with the cathode material obtained in Example 1 have higher discharge specific capacity and initial coulombic efficiency, significantly enhanced high-rate performance, and significantly increased capacity retention at high rates. Analysis suggests this may be related to the purity of the product. It can also be seen that the sodium iron pyrophosphate prepared using PEG as a dispersant and PVP as a carbon source has the highest purity, and the best discharge specific capacity, ultra-high rate performance, and ultra-high rate cycle performance. This indicates that the combination of PEG and PVP has a synergistic effect, significantly improving phase purity and providing better control over the pore structure and morphology of the material. Compared with Examples 2-4 and 7-8, the coin cells assembled with the cathode material obtained in Example 1 have higher discharge specific capacity, significantly enhanced high-rate performance, and significantly increased capacity retention at high rates. Analysis suggests this may be related to the morphology of the cathode material; porous, spherical sodium iron pyrophosphate with a certain pore structure performs better.

[0072] As shown in Table 1, the method provided by this invention prepares porous spherical materials through a composite iron source synergistic mechanism. The phase purity is significantly improved through the strong dispersing effect of polyethylene glycol. Furthermore, the conductivity of the material is improved by in-situ coating of nitrogen-doped carbon with polyvinylpyrrolidone. Finally, a sodium iron pyrophosphate cathode material with high capacity, good rate performance, and high cycle stability is prepared.

[0073] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing modified sodium iron pyrophosphate cathode material, characterized in that, include: Iron source, phosphorus source, sodium source, dispersing reducing agent, carbon source and water are mixed to prepare a suspension; The suspension was milled to obtain a slurry, which was then spray-dried to obtain a precursor. The precursor was sintered under a protective atmosphere to obtain a modified sodium iron pyrophosphate / carbon composite cathode material.

2. The preparation method of the modified sodium iron pyrophosphate cathode material as described in claim 1, characterized in that, The iron source is one or more of ferrous oxalate, ferric phosphate, ferric nitrate, ferric oxide, ferric pyrophosphate, ferrous pyrophosphate, ferric sulfate, and ferrous sulfate. The sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, sodium pyrophosphate, sodium oxalate, and sodium acetate. The phosphorus source is one or more of phosphoric acid, ammonium dihydrogen phosphate, ammonium monohydrogen phosphate, ferric phosphate, ferric pyrophosphate, and sodium dihydrogen phosphate.

3. The preparation method of the modified sodium iron pyrophosphate cathode material as described in claim 1, characterized in that, The dispersing and reducing agent is one or more selected from polyethylene glycol, polyvinyl alcohol, polyvinyl butyral, hydroxymethyl cellulose, polyacrylamide, polyoxyethylene, and polyacrylic acid; the amount of the dispersing and reducing agent added is 5% to 20% of the total mass of the iron source, phosphorus source, and sodium source; The carbon source is one or more selected from glucose, ascorbic acid, cyclodextrin, polyvinylpyrrolidone, citric acid, sucrose, polyacrylonitrile, and polyethyleneimine, preferably polyvinylpyrrolidone; the amount of carbon source added is 5% to 10% of the total mass of the iron, phosphorus, and sodium sources. The iron source is a mixed iron source of ferric phosphate and ferrous oxalate; the molar ratio of ferric phosphate to ferrous oxalate in the mixed iron source is 1:(0.3~3), preferably 1:0.5~3.

4. The preparation method of the modified sodium iron pyrophosphate cathode material as described in claim 1, characterized in that, The particle size of the slurry is D50≤300nm, preferably 100~300nm; The precursor has a particle size of 1~40μm.

5. The method for preparing the modified sodium iron pyrophosphate cathode material as described in claim 1, characterized in that, The milling speed is 2000~3500 r / min, and the milling time is 60~300 min.

6. The method for preparing the modified sodium iron pyrophosphate cathode material as described in claim 1, characterized in that, The spray drying is a two-fluid spray or a centrifugal spray; the inlet temperature during spray drying is 180~300℃, and the outlet temperature is 90~150℃.

7. The method for preparing the modified sodium iron pyrophosphate cathode material as described in claim 1, characterized in that, The sintering temperature is 450~600℃; the sintering holding time is 8~15h; the sintering heating rate is 1~5℃ / min; and the protective atmosphere is one or more of argon, nitrogen, and helium atmospheres.

8. A modified sodium iron pyrophosphate cathode material, characterized in that, It is prepared by the preparation method described in any one of claims 1 to 7.

9. The modified sodium iron pyrophosphate cathode material as described in claim 8, characterized in that, The modified sodium iron pyrophosphate cathode material has a porous spherical morphology. The purity of the modified sodium iron pyrophosphate cathode material is not less than 95%; The modified sodium iron pyrophosphate cathode material has the following properties: the first discharge specific capacity of the assembled button cell is ≥105mAh / g at 0.1C; the discharge specific capacity at 20C rate is ≥90mAh / g; and the capacity retention rate is ≥85% after 5000 cycles at 20C.

10. A sodium-ion battery, characterized in that, Including the modified sodium iron pyrophosphate cathode material as described in claim 8 or 9.

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