High-compaction-density ferric sodium pyrophosphate positive electrode material, preparation method thereof and positive electrode plate

Abstract

The invention discloses a high-compaction-density ferric sodium pyrophosphate positive electrode material and a preparation method thereof, and belongs to the technical field of sodium ion batteries. The method comprises a double-precursor slurry-separate grinding and mixing process and a sodium supplementing-alcohol secondary sanding-presintering-crushing process, and crystal form transformation is inhibited in the whole process, so that the compaction density, the structural stability and the electrochemical performance of the material are remarkably improved, and high-power industrial production is realized. Meanwhile, the invention further discloses a positive pole piece which comprises a positive current collector, the positive current collector is coated with the phosphoric acid ferric pyrophosphate sodium positive electrode material prepared by the method, the compaction density of the positive pole piece can reach 2.27 g / cm < 3 > to the minimum, the cycle performance is remarkably improved, and the positive pole piece has industrialization feasibility of a large-scale energy storage system.

Description

Technical Field

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

[0002] Sodium ferric pyrophosphate (NFPP) has attracted much attention due to its high theoretical capacity, suitable voltage plateau, and excellent cycling stability, showing promising application prospects in large-scale energy storage systems. However, the compaction density of this material remains relatively low, becoming a key factor restricting its industrialization and market competitiveness. The electrode compaction density of existing industrialized NFPP products is generally around 2.0 g / cm³. -3 Low compaction density directly leads to low volumetric energy density of the electrode, further highlighting the disadvantage of sodium-ion batteries in terms of volumetric energy density. Therefore, significantly improving the compaction density of NFPP materials is of great significance for enhancing their overall performance and industrial application potential. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides a high-density sodium iron pyrophosphate cathode material, its preparation method, and the cathode sheet.

[0004] The technical solution is as follows: First, the present invention provides a method for preparing a high-density sodium iron pyrophosphate cathode material, comprising the following steps: S1. Preparation of the first precursor slurry and the second precursor slurry: Weigh out the first iron source, sodium source, phosphorus source and carbon source according to the stoichiometric ratio, and add a dispersant to obtain mixture one. Then add ultrapure water to mixture one, disperse at high speed, and continue wet grinding to obtain the first precursor slurry. The mass ratio of mixture one to ultrapure water is 3:10 to 1:2. Weigh the second iron source, sodium source, phosphorus source and carbon source according to the stoichiometric ratio, and add the dispersant to obtain mixture two. Then add ultrapure water to mixture two, disperse at high speed, and continue wet grinding to obtain the second precursor slurry. The mass ratio of mixture two to ultrapure water is 3:10 to 1:2. The first iron source and the second iron source are not the same; S2. The first precursor slurry and the second precursor slurry are mixed at a mass ratio of 1:1 to 6:1 and then dried to obtain a dry powder. S3. Mix dry powder one with sodium supplement to obtain mixture three. Add anhydrous ethanol to mixture three, grind and dry to obtain dry powder two. The mass of sodium supplement is 1% to 10% of the mass of dry powder one. The mass ratio of mixture three to anhydrous ethanol is 1:5 to 2:5. S4. The dried powder II is pre-sintered, followed by air jet milling to obtain pre-sintered powder; S5. The pre-sintered powder is then sintered to obtain a high-density sodium iron pyrophosphate cathode material.

[0005] This invention centers on a synergistic process of "dual precursor slurry - separate grinding and subsequent mixing" and "sodium supplementation - alcohol-based secondary sand milling - pre-calcination - pulverization," suppressing crystal transformation throughout the entire process. Specifically, the "dual precursor slurry - separate grinding and subsequent mixing" process precisely controls the stoichiometric ratio of Na, Fe, and P, avoiding uneven grinding and component segregation caused by different iron sources, resulting in a more concentrated slurry particle size distribution and ensuring the uniformity of the dried precursor powder, thereby forming a structurally stable NFPP phase. The sodium supplementation process in the "sodium supplementation - alcohol-based secondary sand milling - pre-calcination - pulverization" process precisely compensates for sodium loss during sintering, ensuring the integrity of the stoichiometric ratio and preventing structural instability. The alcohol-based secondary sand milling process ensures good dispersion and rapid volatilization of the slurry, preventing powder agglomeration. The pre-calcination process eliminates porosity increases caused by volatile organic compounds. The pulverization process achieves highly uniform particle size, significantly improving the material's compaction density, structural stability, and electrochemical performance, enabling high-power industrial production.

[0006] Because of its stable crystal structure and suitable reactivity, ferric phosphate can be used as the primary iron source to improve raw material utilization and the formation of the main phase of the material. On the other hand, the particle characteristics of ferric phosphate, combined with a special sand milling process, can maximize the particle grinding effect and refine the particles. This not only increases the contact area in subsequent reactions and significantly improves the raw material conversion efficiency, but also lays a key foundation for the uniformity of the mixed slurry composition.

[0007] Preferably, the first iron source is iron orthophosphate.

[0008] Preferably, in the preparation of the first precursor slurry, the mass of the dispersant is 1 to 5% of the total mass of the first iron source, sodium source, phosphorus source and carbon source; more preferably, the mass of the dispersant is 1.5% of the total mass of the first iron source, sodium source, phosphorus source and carbon source.

[0009] Preferably, in the preparation of the second precursor slurry, the mass of the dispersant is 1 to 5% of the total mass of the second iron source, sodium source, phosphorus source and carbon source; more preferably, the mass of the dispersant is 1.5% of the total mass of the first iron source, sodium source, phosphorus source and carbon source.

[0010] Preferably, in the preparation of the first precursor slurry, the mass ratio of mixture one to ultrapure water is 2:5; in the preparation of the second precursor slurry, the mass ratio of mixture two to ultrapure water is 2:5.

[0011] Preferably, the D50 particle size of the first precursor slurry and the second precursor slurry is ≤550nm.

[0012] Preferably, in S2, the mass ratio of the first precursor slurry to the second precursor slurry is 1:1 to 4:1; more preferably, the mass ratio of the first precursor slurry to the second precursor slurry is 3:1.

[0013] Preferably, in S2, the drying includes the following steps: the first precursor slurry and the second precursor slurry are put into a mixing tank and mixed at 1000~1500r / min at room temperature for 30~60min, and then sent to a spray dryer for spray drying under the conditions of inlet air temperature of 210~260℃ and outlet air temperature of 96~110℃.

[0014] Preferably, in S3, the mass of the sodium supplement is 1% to 5% of the mass of the dried powder; more preferably, the mass ratio of the sodium supplement to the dried powder is 1:45.

[0015] Preferably, in S3, the mass ratio of mixture three to anhydrous ethanol is 2:5.

[0016] Preferably, in S3, the drying is carried out by spray drying, with an inlet air temperature of 160-180℃ and an outlet air temperature of 60-80℃.

[0017] Preferably, in step S4, the step of pre-sintering the dry powder II is as follows: the dry powder II is heated to 280-380°C at a rate of 2-5°C / min under an inert atmosphere and held at that temperature for 3-8 hours, and then naturally cooled to room temperature to obtain pre-sintered particles; more preferably, the inert atmosphere is nitrogen, argon, or a hydrogen-argon mixture containing 5%-10% hydrogen by volume.

[0018] Preferably, the step of final sintering the pre-sintered powder to obtain high-density sodium iron pyrophosphate cathode material is as follows: the pre-sintered powder is heated to 480-575°C in an inert atmosphere at a rate of 2-5°C / min and held for 8-16 hours, and then naturally cooled to room temperature to obtain high-density sodium iron pyrophosphate cathode material.

[0019] Preferably, the second iron source is one or more of ferrous oxalate, ferrous carbonate, ferrous oxide, ferrous acetate, ferrous sulfate, ferric oxide, ferric tetroxide, ferric nitrate, ferric hydroxide, iron powder, or iron concentrate; more preferably, the second iron source is ferrous oxalate.

[0020] Preferably, the sodium source is one or more of sodium carbonate, sodium bicarbonate, sodium nitrate, sodium acetate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium oxalate, trisodium phosphate, sodium pyrophosphate, and sodium citrate; more preferably, the sodium source is sodium carbonate.

[0021] Preferably, the carbon source is one or more of soluble starch, glucose, sucrose, citric acid, and ascorbic acid; more preferably, the carbon source is one or two of glucose and ascorbic acid.

[0022] Preferably, the phosphorus source is one or more of phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, trisodium phosphate, and sodium pyrophosphate; more preferably, the phosphorus source is disodium hydrogen phosphate or sodium dihydrogen phosphate.

[0023] Preferably, the dispersant is one or more of polyvinylpyrrolidone, polyethylene glycol, and polyacrylic acid; more preferably, the dispersant is polyvinylpyrrolidone; particularly preferably, the dispersant is polyvinylpyrrolidone (K90).

[0024] Preferably, sodium acetate is used as the sodium supplement.

[0025] Preferably, the chemical formula of the high-density sodium iron pyrophosphate cathode material is Na. 3.6 Fe 2.6 (PO4) 1.6 P2O7.

[0026] Second, the present invention provides a high-density sodium iron pyrophosphate cathode material, which is obtained by the above preparation method.

[0027] Third, the present invention provides a positive electrode sheet, including a positive current collector, on which a high-density sodium iron pyrophosphate positive electrode material obtained by the above preparation method is coated.

[0028] The beneficial effects of this invention are as follows: (1) The “dual precursor slurry-separate grinding and subsequent mixing” process avoids the problem of uneven grinding and component segregation caused by the difference in characteristics of different iron sources from the root, ensuring that the stoichiometric ratio of Na, Fe and P elements in the mixed slurry is accurately matched to the target composition of phosphate-sodium iron pyrophosphate; at the same time, the separate grinding and subsequent mixing mode can make the particle size distribution of the mixed slurry more concentrated, providing core support for the subsequent spray drying to prepare uniform dry precursor powder and the final sintering to form a stable phosphate-sodium iron pyrophosphate phase structure, effectively optimizing the structural stability of the material.

[0029] (2) The compaction density and performance uniformity are significantly improved by the “sodium supplementation-alcohol-based secondary sand milling-pre-calcination-pulverization” process. The pre-calcination process avoids the increase in product porosity caused by organic volatiles during the subsequent sintering process, thus clearing the way for the improvement of compaction density. In the pulverization and alcohol-based sand milling process, ethanol media is easier to ensure the dispersibility of slurry than water, and it can evaporate quickly in the subsequent spray drying, reducing the agglomeration of precursor powder. Combined with the precise control of intermediate particle size by airflow pulverization after pre-calcination, the high uniformity of material particle size can be achieved. In the sodium supplementation process, the loss of sodium element in the precursor during the drying, pre-calcination and subsequent formal sintering processes can be precisely compensated, avoiding structural instability caused by sodium defects and ensuring the integrity of the product stoichiometry.

[0030] (3) The compaction density of the positive electrode sheet made from the material obtained by the method of the present invention can be as low as 2.27 g / cm³. 3 Furthermore, its cycle performance is significantly improved, demonstrating the industrialization feasibility of large-scale energy storage systems. Attached Figure Description

[0031] Figure 1 The X-ray diffraction (XRD) patterns of the materials obtained in Example 1 and Comparative Example 1 are shown. Figure 2 The image shows a scanning electron microscope (SEM) image of the material obtained in Example 1. Figure 3 SEM images of the electrode sheets prepared from the material obtained in Example 1; Figure 4 SEM images of the electrode fabricated from the material obtained in Comparative Example 3; Figure 5 The first charge-discharge curves of the materials obtained in Example 1 and Comparative Examples 1-2 are shown. Detailed Implementation

[0032] The specific embodiments listed in this invention are merely examples, and the invention is not limited to the specific embodiments described below. For those skilled in the art, any equivalent modifications and substitutions to the embodiments described below are also within the scope of this invention. Therefore, all equivalent transformations and modifications made without departing from the spirit and scope of this invention should be covered within its scope. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. All reagents or instruments whose manufacturers are not specified are commercially available conventional products. To better illustrate this invention, numerous specific details are provided in the following detailed embodiments. Those skilled in the art should understand that this invention can be practiced even without certain specific details. In other embodiments, methods, means, equipment, and steps well known to those skilled in the art are not described in detail in order to highlight the main points of this invention.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art. Unless otherwise specified, all units used in this specification are International Standard Units (SI), and all numerical values ​​and ranges appearing in this invention should be understood to include systematic errors unavoidable in industrial production.

[0034] Polyvinylpyrrolidone (K90) indicates that its K value is approximately 90, corresponding to an average molecular weight Mv of approximately 1,000,000 to 1,500,000. It is used as a thickener, dispersant, or film-forming aid for high viscosity grades.

[0035] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0036] I. Examples and Comparative Examples Example 1 S1. Preparation of the first precursor slurry and the second precursor slurry: Ferric phosphate, sodium carbonate, and disodium hydrogen phosphate in a molar ratio of 2.6:1.3:1 were thoroughly mixed with 5% glucose and 1.5% polyvinylpyrrolidone (K90) by mass of the three (ferric phosphate, sodium carbonate, and disodium hydrogen phosphate) to obtain mixture one. Ultrapure water was then added to mixture one, with a mass ratio of mixture one to ultrapure water of 2:5. The mixture was thoroughly dispersed in a high-speed stirring device until no obvious agglomerated particles were found. Then, it was transferred to a sand mill for sand milling to obtain a first precursor slurry with D50≤550nm. Ferrous oxalate and sodium dihydrogen phosphate in a molar ratio of 2.6:3.6 were thoroughly mixed with 4.5% glucose, 0.5% ascorbic acid, and 1.5% polyvinylpyrrolidone (K90) by mass of the two (ferrous oxalate and sodium dihydrogen phosphate) to obtain mixture two. Ultrapure water was then added to mixture two, wherein the mass ratio of mixture two to ultrapure water was 2:5. After being thoroughly dispersed and uniformly dispersed by a high-speed disperser, the mixture was transferred to a sand mill for sand milling to obtain a second precursor slurry with D50≤550nm. S2. The first precursor slurry and the second precursor slurry are added into a mixing tank at a mass ratio of 3:1. After mixing at room temperature and 1500 r / min for 60 min, the mixture is sent to a spray dryer and spray dried under the conditions of inlet air temperature of 250℃ and outlet air temperature of 96~110℃ to obtain dried powder. S3. Mix dry powder one with sodium acetate at a mass ratio of 45:1 to obtain mixture three. Then add anhydrous ethanol to mixture three, wherein the mass ratio of mixture three to anhydrous ethanol is 2:5. After grinding with sand until the particle size D50≤200nm, a mixed slurry is obtained. The mixed slurry is fed into a spray dryer and spray dried under the conditions of inlet air temperature of 200℃ and outlet air temperature of 70~80℃ to obtain dry powder two. S4. The dried powder II is loaded into an alumina crucible and placed in a roller kiln. Under a nitrogen atmosphere, the temperature is raised to 380℃ at 2℃ / min and held for 3.5h. After natural cooling to room temperature, pre-sintered particles are obtained. The pre-sintered particles are then transferred to an air jet mill and pulverized under nitrogen protection to obtain pre-sintered powder. S5. The pre-sintered powder is loaded back into the alumina crucible and placed in a roller kiln. Under a nitrogen atmosphere, the temperature is increased to 500℃ at 3℃ / min and held for 8 hours. Then, it is naturally cooled to room temperature to obtain Na. 3.6 Fe 2.6 (PO4) 1.6 P2O7 cathode material.

[0037] Example 2 S1. Preparation of the first precursor slurry and the second precursor slurry: Same as in Example 1, a first precursor slurry was obtained; Ferric oxide and sodium dihydrogen phosphate in a molar ratio of 1.3:3.6 were thoroughly mixed with 4.5% glucose, 0.5% ascorbic acid, and 1.5% polyvinylpyrrolidone (K90) by mass of the two (ferric oxide and sodium dihydrogen phosphate) to obtain mixture B2. Ultrapure water was then added to mixture B2, wherein the mass ratio of mixture B2 to ultrapure water was 2:5. After being thoroughly dispersed and uniformly dispersed by a high-speed disperser, the mixture was transferred to a sand mill for sand milling to obtain a second precursor slurry with D50≤550nm. S2. Same as in Example 1, to obtain a dry powder; S3. Same as in Example 1, obtain dry powder II; S4. Same as in Example 1, obtain pre-sintered powder; S5. Same as in Example 1, obtain Na 3.6 Fe 2.6 (PO4) 1.6 P2O7 cathode material.

[0038] Example 3 S1. Preparation of the first precursor slurry and the second precursor slurry: Same as in Example 1, a first precursor slurry was obtained. Ferrous oxalate, sodium pyrophosphate, and ammonium dihydrogen phosphate in a molar ratio of 26:9:18 were thoroughly mixed with 4.5% glucose, 0.5% ascorbic acid, and 1.5% polyvinylpyrrolidone (K90) by mass of the three (ferrous oxalate, sodium pyrophosphate, and ammonium dihydrogen phosphate) to obtain mixture B2. Ultrapure water was then added to mixture B3, wherein the mass ratio of mixture B to ultrapure water was 2:5. After being thoroughly dispersed and uniformly dispersed by a high-speed disperser, the mixture was transferred to a sand mill for sand milling to obtain a second precursor slurry with D50≤550nm. S2. Same as in Example 1, to obtain a dry powder; S3. Same as in Example 1, obtain dry powder II; S4. Same as in Example 1, obtain pre-sintered powder. S5. Same as in Example 1, obtain Na 3.6 Fe 2.6 (PO4) 1.6 P2O7 cathode material.

[0039] Example 4 S1. Preparation of the first precursor slurry and the second precursor slurry: Same as in Example 1, a first precursor slurry was obtained; Same as in Example 1, a second precursor slurry was obtained; S2. The first precursor slurry and the second precursor slurry are added into a mixing tank at a mass ratio of 1:1 and mixed at 1500 r / min at room temperature for 60 min. Then, the mixture is sent to a spray dryer and spray dried under the conditions of inlet air temperature of 250℃ and outlet air temperature of 96~110℃ to obtain dried powder. S3. Same as in Example 1, obtain dry powder II; S4. Same as in Example 1, obtain pre-sintered powder; S5. Same as in Example 1, obtain Na 3.6 Fe 2.6 (PO4) 1.6 P2O7 cathode material.

[0040] Application Example 1 This embodiment provides the use of Na 3.6 Fe 2.6 (PO4) 1.6 The steps for preparing positive electrode sheets from P2O7 positive electrode material.

[0041] S1. Preparation of positive electrode slurry: Na 3.6 Fe 2.6 (PO4) 1.6 P2O7 cathode material: Super P Carbon Black (SP): Carbon Nanotubes (CNTs, 2.8wt% CNTs + 1wt% dispersant dissolved in NMP solvent): PVDF-5130 are mixed in a mass ratio of 96:1.5:0.5:2. The steps are as follows: S11. Na 3.6 Fe 2.6 (PO4) 1.6 P2O7 cathode material, carbon black SP, and PVDF-5130 were added to a stirred tank and stirred and dispersed at 20 / 1800 rpm (where 20 is the revolution speed of the stirring paddle and 1800 is the rotation speed) for 30 minutes. During this period, the tank wall was scraped once at the 15th minute and once at the 30th minute to remove the residue on the tank wall and ensure that all materials are fully dispersed and uniformly mixed to obtain a premix. S12. Add CNT and N-methyl-2-pyrrolidone (NMP) to the premix, control the solid content of the positive electrode slurry to 60%, and continue to stir at 15 / 1800 rpm for 30 minutes. After stirring, scrape the wall again to ensure that all residues on the reactor wall re-enter the slurry system and achieve full dispersion. S13. Stir at 28 / 3000 rpm for 2 hours until the slurry is uniformly mixed and there is no phase separation. When the viscosity of the slurry is stably controlled below 6000 mPa·s, the positive electrode slurry is obtained.

[0042] S2. Coating the positive electrode sheet: The positive electrode slurry obtained in S1 is coated onto the surface of carbon-coated aluminum foil. The process parameters are as follows: By adjusting the height of the coating machine's scraper, the surface density of the positive electrode sheet was stably controlled at 350 g / m². 2 ; After coating, the electrode sheets are placed in an oven to dry, with the oven temperature controlled at 95~112℃. Coating operation parameters: Coating speed 2~10m / min, coating speed ratio (ratio of linear speed of coating roller to transfer roller) is 1.35.

[0043] S3. Press-cut positive electrode sheet: The cold pressing and die-cutting parameters for the positive electrode sheet are as follows: The coated and dried positive electrode sheet is subjected to cold pressing with rollers. The total pressing pressure is controlled at 100~200T (1T≈9.80665kN), and the compaction density of the electrode sheet after cold pressing is controlled at 1.9~2.4g / cm³. 3 ; The cold-pressed electrode sheet is die-cut to obtain a positive electrode sheet that meets the size requirements.

[0044] Comparative Example 1 The difference from Example 1 is that the slurry is processed using a co-grinding sand mill spray drying method, specifically: S1. Ferric orthophosphate, disodium hydrogen phosphate, and ferrous oxalate in a molar ratio of 9:9:4 are mixed with 5% glucose, 0.5% ascorbic acid, and 1.5% dispersant (K90) of the total mass of the three (ferric orthophosphate + disodium hydrogen phosphate + ferrous oxalate) to obtain a mixture. Ultrapure water is then added to the mixture, wherein the mass ratio of the mixture to ultrapure water is 2:5. After being fully dispersed in a high-speed stirring device until there are no obvious agglomerated particles, the mixture is transferred to a sand mill for sand milling to obtain a slurry with D50≤550nm. S2. The slurry is fed into a spray dryer and spray dried under the conditions of an inlet air temperature of 250℃ and an outlet air temperature of 96~110℃ to obtain dried powder. S3. Same as in Example 1, obtain dry powder II; S4. Same as in Example 1, obtain pre-sintered powder; S5. Same as in Example 1, obtain Na 3.6 Fe 2.6 (PO4) 1.6 P2O7 cathode material.

[0045] Comparative Example 2 The difference from Example 1 is that S4, namely the alcohol-based milling and sodium supplementation process, is omitted. Specifically: S1. Preparation of the first precursor slurry and the second precursor slurry: Same as in Example 1, a first precursor slurry was obtained; Same as in Example 1, a second precursor slurry was obtained; S2. Same as in Example 1, obtain dry powder; S3. Omit this step (do not perform alcohol-based milling and add sodium supplement); S4. The dried powder is loaded into an alumina crucible and placed in a roller kiln. Under a nitrogen atmosphere, the temperature is raised to 380℃ at 2℃ / min and held for 3.5h. The powder is then naturally cooled to room temperature to obtain pre-sintered particles. The pre-sintered particles are then transferred to an air jet mill and pulverized under nitrogen protection to obtain pre-sintered powder. S5. The pre-sintered powder is loaded back into the alumina crucible and placed in a roller kiln. Under a nitrogen atmosphere, the temperature is increased to 500℃ at 3℃ / min and held for 8 hours. Then, it is naturally cooled to room temperature to obtain Na. 3.6 Fe 2.6 (PO4) 1.6 P2O7 cathode material.

[0046] Comparative Example 3 The difference from Example 1 is that the airflow pulverization process in S4 is omitted. Instead, the pre-sintered powder is cooled to room temperature and then sintered directly according to the process flow in S5. Specifically: S1. Preparation of the first precursor slurry and the second precursor slurry: Same as in Example 1, a first precursor slurry was obtained; Same as in Example 1, a second precursor slurry was obtained; S2. Same as in Example 1, to obtain a dry powder; S3. Same as in Example 1, obtain dry powder II; S4. The dried powder II is loaded into an alumina crucible, placed in a roller kiln, heated to 380℃ at 2℃ / min under a nitrogen atmosphere and held for 3.5h, and then naturally cooled to room temperature to obtain pre-sintered particles. S5. The pre-sintered particles were loaded back into the alumina crucible and placed in a roller kiln. Under a nitrogen atmosphere, the temperature was increased to 500℃ at 3℃ / min and held for 8 hours. The mixture was then allowed to cool naturally to room temperature to obtain Na. 3.6 Fe 2.6 (PO4) 1.6 P2O7 cathode material.

[0047] Comparative Example 4 The difference from Example 1 is that only the mass ratio of the first precursor slurry and the second precursor slurry in S2 is changed from 3:1 to 1:3, specifically: S1. Preparation of the first precursor slurry and the second precursor slurry: Same as in Example 1, a first precursor slurry was obtained; Same as in Example 1, a second precursor slurry was obtained; S2. The first precursor slurry and the second precursor slurry are added into a mixing tank at a mass ratio of 1:3. After mixing at room temperature and 1500 r / min for 60 min, the mixture is sent to a spray dryer and spray dried under the conditions of inlet air temperature of 250℃ and outlet air temperature of 96~110℃ to obtain dried powder. S3. Same as in Example 1, obtain dry powder II; S4. Same as in Example 1, obtain pre-sintered powder; S5. Same as in Example 1, obtain Na 3.6 Fe2.6 (PO4) 1.6 P2O7 cathode material.

[0048] Comparative Example 5 The difference from Example 1 is that only the formulation with ferric phosphate as the main iron source is replaced, specifically: S1. Preparation of the first precursor slurry and the second precursor slurry: Ferrous oxalate and sodium dihydrogen phosphate in a molar ratio of 2.6:3.6 were thoroughly mixed with 4.5% glucose, 0.5% ascorbic acid, and 1.5% dispersant, accounting for 4.5% of the total mass of the two (ferrous oxalate and sodium dihydrogen phosphate), to obtain mixture one. Ultrapure water was then added to mixture one, wherein the mass ratio of mixture one to ultrapure water was 2:5. After being thoroughly dispersed in a high-speed stirring device until there were no obvious agglomerated particles, the mixture was transferred to a sand mill for sand milling to obtain a first precursor slurry with D50≤550nm. Ferric oxide and sodium dihydrogen phosphate in a molar ratio of 1.3:3.6 were thoroughly mixed with 4.5% glucose, 0.5% ascorbic acid, and 1.5% dispersant, accounting for 4.5% of the total mass of the two (ferric oxide and sodium dihydrogen phosphate), to obtain mixture two. Then, ultrapure water was added to mixture two, wherein the mass ratio of mixture two to ultrapure water was 2:5. After being fully dispersed in a high-speed stirring device until there were no obvious agglomerated particles, it was transferred to a sand mill for sand milling to obtain a second precursor slurry with D50≤550nm. S2. Same as in Example 1, to obtain a dry powder; S3. Same as in Example 1, obtain dry powder II; S4. Same as in Example 1, obtain pre-sintered powder; S5. Same as in Example 1, obtain Na 3.6 Fe 2.6 (PO4) 1.6 P2O7 cathode material.

[0049] II. Performance Testing and Characterization Figure 1 The XRD patterns of the materials obtained in Example 1 and Comparative Example 1 show that both spectra exhibit only characteristic peaks of the NFPP phase, with no other impurities or unreacted precursors detected. This indicates that both preparation methods can successfully obtain high-purity NFPP structures under the same stoichiometric ratios and synthesis conditions. All diffraction peaks in the figures are sharp and narrow, indicating good crystallinity of the samples. The peak shape of the spectrum in Example 1 is particularly sharp, and the peak positions perfectly match those of the standard card, indicating that the atomic arrangement within the crystal lattice of this sample exhibits long-range order, verifying its excellent crystallinity.

[0050] Figure 2The SEM image of the material obtained in Example 1 shows that by optimizing the particle size through the pulverization process, the particle size of the material becomes more concentrated; after pre-calcination, the secondary particle size is smaller and the distribution is more uniform, which meets the requirements of high compaction density; the material can form a denser film structure during rolling, shortening the electron transport distance and improving electrochemical performance.

[0051] Figure 3 and Figure 4 SEM images of the electrode sheets prepared from the materials obtained in Example 1 and Comparative Example 3 are shown. It can be seen that the electrode sheet prepared from the material obtained in Example 1 has a smooth surface with few pores and a reasonable particle size distribution. Small particles fully fill the gaps between large particles, thus significantly improving density. In contrast, the electrode sheet prepared from the material obtained in Comparative Example 3 exhibits more grooves and a large number of unfilled areas. Due to the overall large size of the particles, the pressure they withstand during rolling exceeds their limits, causing particle deformation and edge cracking. Further increasing the compaction density may lead to strip breakage and edge curling, which is detrimental to subsequent processing. Furthermore, the insufficient and uneven distribution of small particles fails to effectively fill the gaps between large particles, resulting in a lower compaction density.

[0052] Electrochemical tests were performed on the materials obtained in Examples 1-4 and Comparative Examples 1-5, according to Na... 3.6 Fe 2.6 (PO4) 1.6 A mixture of P2O7, carbon black (SP), and CNT in a mass ratio of 8:1:1 was used. Etched aluminum foil was used as the current collector, sodium foil as the counter electrode, and a glass fiber diaphragm was employed. The electrolyte was 1 mol / L NaClO4 EC:PC + 5% FEC, indicating that 1 mol / L sodium perchlorate (NaClO4) was used as the conductive salt, and the solvent consisted of a 1:1 volume ratio of ethylene carbonate (EC) and propylene carbonate (PC), with an additional 5 wt% fluoroethylene carbonate (FEC) added as a film-forming additive. Assembly was performed in a glove box with a water and oxygen content of less than 0.01 PPM, and the test voltage range was 1.5–4 V.

[0053] Example 1, the first charge-discharge curves of the materials obtained in Comparative Examples 1 and 2 are as follows: Figure 5As shown, the voltage plateaus and shapes of the three curves are basically the same, indicating that their crystal structures are identical. The material prepared by the graded sand milling-sodium supplementation process (Example 1) exhibits the highest charge-discharge specific capacity, with a charge specific capacity of 117.13 mAh / g and a discharge specific capacity of 107.64 mAh / g. The material prepared by single sand milling (Comparative Example 1) has slightly lower capacities, at 112.27 mAh / g (charge specific capacity) and 102.28 mAh / g (discharge specific capacity), respectively. The material obtained without alcohol-based sand milling and the addition of sodium supplement (sodium acetate) (Comparative Example 2) has the worst performance, with only 108.13 mAh / g (charge specific capacity) and 98.51 mAh / g (discharge specific capacity). This further confirms that the dual precursor slurry "separate grinding and subsequent mixing" process provided by the present invention achieves the uniformity of raw materials and precise control of phase structure, fundamentally avoiding uneven grinding and component segregation caused by differences in the characteristics of different iron sources; at the same time, the synergistic process of pre-calcination-alcohol-based secondary sand milling-sodium supplementation significantly improves electrochemical performance, laying the foundation for achieving high energy density.

[0054] The performance test data of the materials obtained in Examples 1-4 and Comparative Examples 1-5 are shown in Table 1.

[0055] Table 1 Performance test data of the materials obtained in Examples 1-4 and Comparative Examples 1-5

[0056] Table 1 shows that the synthesis of NFPP with ferric orthophosphate as the main phase material, combined with the synergistic effect of the "dual precursor slurry-separate grinding and mixing" process and the "sodium supplementation-alcohol-based secondary sand milling-pre-sintering-pulverization" process (Examples 1-4), can significantly improve the compaction density of the electrode. The lowest compaction density of the obtained material can reach 2.27 g / cm³. 3 The value was significantly higher than that of the control group 1-5.

[0057] The embodiments described above merely illustrate specific implementation methods of this application, and while the descriptions are detailed and specific, they should not be construed as limiting the scope of protection of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the technical solution of this application, and these modifications and improvements all fall within the scope of protection of this application.

Claims

1. A method for preparing a high-density sodium iron pyrophosphate cathode material, characterized in that, Includes the following steps: S1. Preparation of the first precursor slurry and the second precursor slurry: Weigh out the first iron source, sodium source, phosphorus source and carbon source according to the stoichiometric ratio, and add a dispersant to obtain mixture one. Then add ultrapure water to mixture one, disperse at high speed, and continue wet grinding to obtain the first precursor slurry. The mass ratio of mixture one to ultrapure water is 3:10 to 1:

2. Weigh the second iron source, sodium source, phosphorus source and carbon source according to the stoichiometric ratio, and add a dispersant to obtain mixture two. Then add ultrapure water to mixture two, disperse at high speed, and continue wet grinding to obtain the second precursor slurry. The mass ratio of mixture two to ultrapure water is 3:10 to 1:

2. The first iron source and the second iron source are not the same; S2. The first precursor slurry and the second precursor slurry are mixed at a mass ratio of 1:1 to 6:1 and then dried to obtain a dry powder. S3. The first dry powder is mixed with the sodium supplement to obtain the third mixture. Anhydrous ethanol is then added to the third mixture, which is ground and dried to obtain the second dry powder. The mass of the sodium supplement is 1% to 10% of the mass of the first dry powder. The mass ratio of the third mixture to the anhydrous ethanol is 1:5 to 2:

5. S4. The dried powder II is pre-sintered, followed by air jet milling to obtain pre-sintered powder; S5. The pre-sintered powder is subjected to final sintering to obtain the high-density sodium iron pyrophosphate cathode material.

2. The method for preparing high-density sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The first iron source is iron orthophosphate.

3. The method for preparing high-density sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In the preparation of the first precursor slurry, the mass of the dispersant is 1-5% of the total mass of the first iron source, the sodium source, the phosphorus source and the carbon source; in the preparation of the second precursor slurry, the mass of the dispersant is 1-5% of the total mass of the second iron source, the sodium source, the phosphorus source and the carbon source.

4. The method for preparing high-density sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In S2, the mass ratio of the first precursor slurry to the second precursor slurry is 1:1 to 4:

1.

5. The method for preparing high-density sodium iron pyrophosphate cathode material according to claim 4, characterized in that, The mass ratio of the first precursor slurry to the second precursor slurry is 3:

1.

6. The method for preparing high-density sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In S3, the mass of the sodium supplement is 1% to 5% of the mass of the dried powder.

7. The method for preparing high-density sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In S3, the mass ratio of the mixture to the anhydrous ethanol is 2:

5.

8. The method for preparing high-density sodium iron pyrophosphate cathode material according to any one of claims 1-7, characterized in that, The chemical formula of the high-density sodium ferric pyrophosphate cathode material is Na. 3.6 Fe 2.6 (PO4) 1.6 P2O7.

9. A high-density sodium iron pyrophosphate cathode material, characterized in that, It was obtained using the high-density sodium iron pyrophosphate cathode material preparation method as described in claim 8.

10. A positive electrode sheet, comprising a positive current collector, wherein the positive current collector is coated with a sodium iron pyrophosphate positive electrode material prepared by the high-density sodium iron pyrophosphate positive electrode material preparation method according to any one of claims 1-7.

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