Preparation method of high-compaction-density sodium iron phosphate positive electrode material

Abstract

The application relates to the technical field of battery materials, and discloses a preparation method of a high-compaction-density sodium iron phosphate pyrophosphate positive electrode material, the core of the method being to adopt a specific-synthesis modified gemini quaternary ammonium salt grinding aid and combine a two-stage gradient sand grinding process. The grinding aid has a double-layer hydrophobic structure (short-chain fluorinated aryl and long-chain alkyl) and a double-sulfonic acid / gemini quaternary ammonium salt structure, can be strongly adsorbed on the surface of negatively charged iron phosphate particles through electrostatic action, can provide excellent dispersion effect, and can form a dense coating layer on the surface of the particles. Part of the iron phosphate, the grinding aid and water are superfine ground, the remaining iron phosphate is normally sand ground, a mixed sodium source and a phosphorus source are added, spray drying, sintering and airflow crushing are carried out, and finally, the finished product with a compaction density of 2.10-2.20 g / cm3 is obtained. The application effectively solves the problems of low compaction density and poor grinding efficiency of the sodium iron phosphate pyrophosphate, realizes in-situ heteroatom doping, and is suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of battery materials technology, specifically to a method for preparing a high-density sodium iron pyrophosphate cathode material. Background Technology

[0002] Sodium iron pyrophosphate (NFPP), as a cathode material for sodium-ion batteries, has attracted much attention due to its abundant raw materials, high safety, and environmentally friendly properties. However, its industrial application faces severe challenges: First, NFPP materials prepared by existing processes generally suffer from low compaction density (usually below 2.0 g / cm³), resulting in insufficient volumetric energy density of the battery. The root cause lies in the uniform particle size distribution of the material, which creates numerous voids during stacking. Second, although traditional sand milling techniques attempt to refine the precursor iron phosphate particles, the high hardness and toughness of this material mean that a single sand milling process takes more than 10 hours and still cannot stably obtain submicron-sized fine powder (D). 50 For particles smaller than 0.3 μm, energy consumption is high and particle size distribution is difficult to control precisely. Furthermore, some technologies use a two-stage sintering process to optimize particle size distribution, but this process requires additional secondary processing steps, which increases production costs and may introduce impurities. In addition, NFPP has low intrinsic electronic conductivity, and conventional carbon coating methods have limited improvement effects. Moreover, there is a lack of effective grinding aids that are highly compatible with the surface properties of the material during the grinding process, which restricts the full realization of the material's electrochemical performance.

[0003] Driven by the demand for high compaction density, high specific capacity, high initial efficiency, and low-cost preparation in the energy storage and low-cost electric vehicle markets, developing an innovative process that can synergistically solve the challenges of particle size control, milling efficiency, interfacial conductivity, and cost control has become a key breakthrough for promoting the large-scale commercial application of NFPP. Existing grinding aids often emphasize their acidity and dispersibility, but their structures are common, their functions are limited, and their interactions with the iron phosphate surface are weak and non-specific, resulting in unsatisfactory dispersion, coating uniformity, and subsequent doping effects. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention aims to provide a method for preparing NFPP cathode materials that simultaneously achieves high compaction density, high electrochemical performance, and low cost. This invention utilizes a two-stage gradient milling process in synergy with a functionalized geminal quaternary ammonium salt grinding aid with a specific structure. This grinding aid not only significantly improves milling efficiency and dispersion stability through its unique structure, but also enables the in-situ formation of a uniform and dense heteroatom-doped carbon coating layer during sintering, thereby overcoming the aforementioned industrialization challenges.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a high-density sodium iron pyrophosphate cathode material, the method comprising the following steps: Step (1), Ultrafine grinding: Add some iron phosphate raw material and modified gemini quaternary ammonium salt grinding aid to deionized water and perform ultrafine grinding for 2-4 hours to form D 50 A primary slurry with a particle size of 0.1-0.3 μm; Step (2), gradation grinding: Add the remaining ferric phosphate raw material to the primary slurry and perform ordinary grinding for 1-2 hours until the particle size D is reached. 50 The particle size is 0.3-0.5μm, forming a slurry with an ideal gradation of "fine particles filling the voids between coarse particles"; Step (3), mixing: Add soluble sodium source, phosphorus source and carbon source to the graded slurry, stir for 30 min to form a homogeneous mixed slurry with a solid content of 30%-40%; Step (4), Drying and sintering: The mixed slurry is spray-dried with an inlet air temperature of 200-280℃ and an outlet air temperature of 80-120℃. The resulting precursor powder is heated to 500-600℃ at 3℃ / min under a nitrogen atmosphere and sintered for 8-12 hours. Step (5), Crushing: The sintered powder is processed by an air jet mill to control the particle size D of the finished product. 10 The thickness is 0.1-0.3 μm, D 50 Less than 5μm, D 99 With a thickness of less than 15 μm, a high-density sodium iron pyrophosphate cathode material was obtained.

[0006] As a preferred embodiment, in step (1), the amount of modified gemini quaternary ammonium salt grinding aid added is 0.5%-2.0% of the mass of the iron phosphate portion.

[0007] As a preferred embodiment, in step (2), the mass ratio of ultrafine ground ferric phosphate to ferric phosphate subjected to ordinary sand milling is 1:1-5.

[0008] As a preferred embodiment, in step (3), the molar ratio of iron in the iron phosphate raw material to sodium in the sodium source is 0.72-0.75:1.

[0009] As a preferred embodiment, the preparation method of the modified gemini quaternary ammonium salt grinding aid in step (1) includes the following steps: S1. Long-chain alkylamines (tetradecylamine, hexadecylamine, octadecylamine) are added to isopropanol and stirred to dissolve. 3,5-bis(trifluoromethyl)styrene is then added to the solution and the mixture is stirred at 65-70°C for 3-5 hours. After the reaction is complete, the mixture is rotary evaporated and recrystallized from ethyl acetate to obtain intermediate 1. The molar ratio of the long-chain alkylamine to 3,5-bis(trifluoromethyl)styrene is 1:1-1.2. Intermediate 1 is obtained by Michael addition of the amino group in the long-chain alkylamine to the alkenyl group in the 3,5-bis(trifluoromethyl)styrene.

[0010] S2. Add intermediate 1 to isopropanol, then add 1,3-propanesulfonate lactone, and stir at 65-70℃ for 4-5 hours. After the reaction is complete, rotary evaporate and recrystallize from ethyl acetate to obtain intermediate 2. The molar ratio of intermediate 1 to 1,3-propanesulfonate lactone is 1:1-1.2. In this reaction, intermediate 1 and 1,3-propanesulfonate lactone undergo a ring-opening reaction to obtain intermediate 2.

[0011] S3. Under a nitrogen atmosphere, intermediates 2 and 1,4-dibromobutane are added to isopropanol, stirred and dispersed, and refluxed for 12-14 hours to undergo quaternization reaction. After the reaction is completed, the mixture is distilled under reduced pressure and dried to obtain modified geminal quaternary ammonium salt grinding aid. The molar ratio of intermediates 2 and 1,4-dibromobutane is 2.2-2.5:1.

[0012] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Technological innovation and breakthrough in compaction density: "Two-stage gradient sand milling" process, first preparing ultrafine particles (D 50 =0.1-0.3μm) as the filler phase, and then the remaining raw materials are added and ground to a coarser particle size (D). 50 Using 0.3-0.5μm particles as the framework phase, coarse and fine graded ferric phosphate slurry is precisely and controllably prepared. By filling the gaps between coarse particles with ultrafine particles, the problem of stacking of single particle size distribution is fundamentally solved, and the compaction density of NFPP is significantly increased to more than 2.10 g / cm³.

[0013] (2) Innovation in the molecular structure and multifunctional synergy of the grinding aid: The core of this invention lies in the design and synthesis of a multifunctional grinding aid with a unique "gemini structure" and "double hydrophobic structure". Its effect is far more than the simple superposition of various functional groups, but produces a significant synergistic effect, specifically: The bilayer hydrophobic structure achieves spatial stability and strong anchoring: One end of the grinding aid molecule of this invention is a short-chain 3,5-bis(trifluoromethyl)phenyl structure, which has extremely strong hydrophobicity and steric hindrance effect; the other end is a long-chain alkyl (C14-C18), which also provides excellent hydrophobic segments. This "bilayer hydrophobic structure" can be strongly inserted into and anchored on the surface of iron phosphate particles, and prevent particle agglomeration through strong steric hindrance effect, so that ultrafine particles can be stably dispersed. The dispersion effect is far superior to that of traditional single-chain structure grinding aids, thus achieving the effect of spatial stability.

[0014] The twin structure of the grinding aid achieves dense coating and multi-mechanism adsorption: The final product is a bis-quaternary ammonium salt and bis-sulfonate structure. This "twin structure" gives it two positively charged quaternary ammonium salt centers and two negatively charged sulfonate centers, resulting in multiple adsorption mechanisms. Mechanism 1: Electrostatic adsorption: The quaternary ammonium salt cation head is strongly adsorbed onto the negatively charged iron phosphate particle surface through electrostatic attraction (environmental pH (7) > IEP (~4)). Mechanism 2: Coordination bonding: The oxygen atom in the sulfonate group (-SO3⁻) has a lone pair of electrons, which can act as an electron donor to coordinate with the iron ions (Fe³⁺) on the iron phosphate surface, forming a strong Fe-OS coordination bond. This chemical bonding greatly enhances the adsorption strength and stability of the grinding aid on the particle surface, achieving a strong anchoring effect. Furthermore, through multiple adsorption mechanisms, a dense organic monomolecular film is formed on the iron phosphate surface, laying the foundation for its subsequent functions.

[0015] (3) Synergistic enhancement mechanism of in-situ pyrolysis and multi-element doped carbon layer: The above-mentioned dense organic monolayer undergoes in-situ pyrolysis during sintering, transforming into a uniform heteroatom (N, S, F) doped carbon layer, which significantly improves the material properties from both physical and chemical aspects, specifically: N and S co-doping optimizes the electronic structure of carbon layers: N and S atoms derived from quaternary ammonium salts and sulfonic acid groups are doped into the amorphous carbon matrix. The co-doping effect of N atoms (providing n-type doping) and S atoms (expanding the carbon layer spacing) can introduce a large number of defects and active sites near the Fermi level of the carbon layer, significantly changing its electron cloud distribution, greatly improving the intrinsic electronic conductivity of the carbon coating layer, and promoting the rapid transport of electrons during charging and discharging.

[0016] F doping constructs a stable interface layer: During pyrolysis, F derived from trifluoromethyl forms a thermodynamically stable metal fluoride (such as NaF) interface layer or CF bonds. This interface layer effectively isolates the electrode material from direct contact with the electrolyte, greatly suppressing irreversible interfacial side reactions and the dissolution of transition metal ions during the first charge and discharge process, thereby significantly improving the initial coulombic efficiency. Simultaneously, the introduction of F further optimizes the interfacial ion transport kinetics.

[0017] N / S / F doping produces a synergistic effect: N / S doping mainly improves electronic conductivity, while F doping mainly enhances interface stability. The three work together to solve the core problems of low electronic conductivity and poor initial performance of NFPP materials.

[0018] (4) Precise dispersion mechanism based on scientific principles: Ferric phosphate has a very low isoelectric point (IEP) (pH ~4). In the neutral aqueous medium (pH≈7) used in this invention, the particle surface carries a net negative charge. The active ingredient of the grinding aid of this invention—a cationic quaternary ammonium salt—can be precisely and firmly adsorbed onto the particle surface through the aforementioned strong electrostatic attraction and coordination bonding. This design based on the intrinsic properties of the material makes its dispersion efficiency far higher than that of traditional nonionic or anionic dispersants.

[0019] (5) Process and cost optimization: This method can achieve particle size distribution, carbon coating and multi-element doping in one sintering step, eliminating the complex steps of traditional secondary sintering, secondary grinding and external doping source, significantly reducing production costs and the risk of impurity introduction, and greatly shortening the total sand grinding time, which is suitable for large-scale production. Attached Figure Description

[0020] Figure 1 This is the reaction route for modified gemini quaternary ammonium salt grinding aids.

[0021] Figure 2 This is a scanning electron microscope image of the grinding particles prepared in Example 1 of the present invention.

[0022] Figure 3 The image shows the XRD pattern of sodium iron pyrophosphate prepared in Example 1 of this invention.

[0023] Figure 4 This is a 0.1C charge-discharge curve of the sodium-ion battery prepared in Example 1 of the present invention. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] It should be noted that the raw materials used in the following embodiments are all commercially available.

[0026] The performance testing of the coin cell is carried out according to the following steps: The prepared positive electrode active material was mixed with a conductive agent and a binder in a mass ratio of 90:5:5, and N-methylpyrrolidone was added as a dispersant. After thorough mixing, a stable positive electrode slurry was prepared. This slurry was uniformly coated onto the surface of a carbon-coated aluminum foil current collector, dried, and pressed into a sheet to serve as the positive electrode. A sodium metal sheet was used as the counter electrode, and a glass fiber separator was selected. The electrolyte was a sodium-ion battery-specific electrolyte with sodium hexafluorophosphate as the solute. After battery assembly, a constant current charge-discharge test was conducted at a rate of 0.1C within a voltage range of 2.0–4.0 V (vs. Na⁺ / Na) to evaluate its electrochemical performance.

[0027] Preparation of modified gemini quaternary ammonium salt grinding aid: Preparation Example 1 S1. In a 500 mL three-necked flask equipped with a condenser and a stirrer, add 0.25 mol tetradecylamine and 200 mL isopropanol, stir to dissolve, and then slowly add 0.3 mol 3,5-bis(trifluoromethyl)styrene. Heat to 68 °C and stir for 4 hours. After the reaction is complete, remove most of the solvent by rotary evaporation and recrystallize with ethyl acetate to obtain intermediate 1-A.

[0028] S2. Add 0.25 mol of intermediate 1-A to 200 mL of isopropanol, stir to dissolve, add 0.28 mol of 1,3-propanesulfonate lactone, and react at 68 °C for 4.5 hours. After the reaction is complete, rotary evaporate and recrystallize from ethyl acetate to obtain intermediate 2-A.

[0029] S3. Under nitrogen protection, 0.22 mol of intermediate 2-A and 0.1 mol of 1,4-dibromobutane were added to 250 mL of isopropanol, stirred and heated to reflux for 13 hours. After the reaction was completed, the solvent was removed by vacuum distillation, and the product was dried in a vacuum drying oven at 60 °C for 24 hours to obtain grinding aid A.

[0030] Preparation Example 2 The difference between this preparation example and preparation example 1 is that tetradecylamine in S1 is replaced with hexadecylamine, while the remaining steps are the same as in preparation example 1. The product obtained is denoted as grinding aid B.

[0031] Preparation Example 3 The difference between this preparation example and preparation example 1 is that tetradecylamine in S1 is replaced with octadecylamine, and the remaining steps are the same as in preparation example 1. The product obtained is denoted as grinding aid C.

[0032] Examples and comparative examples of NFPP cathode material preparation: Example 1

[0033] The preparation method of the positive electrode material in this embodiment includes the following steps: (1) Ultrafine grinding: Weigh 1 mol of ferric phosphate, deionized water and grinding aid B (0.5% of the total mass of ferric phosphate) and mix them. Use zirconia beads with a diameter of 0.3 mm and grind at 1200 rpm for 3 hours to obtain D. 50 =0.14μm primary slurry.

[0034] (2) Grading and grinding: Add 5 mol of ferric phosphate to the above primary slurry and continue grinding for 1.5 hours under the same grinding conditions to obtain D. 50 =0.38μm graded slurry.

[0035] (3) Mixing: Add 2 mol sodium dihydrogen phosphate, 3 mol sodium carbonate and 0.4 mol glucose to the graded slurry, stir for 30 min to form a homogeneous mixed slurry with a solid content of 30%.

[0036] (4) Drying and sintering: The slurry is spray-dried with an inlet air temperature of 250°C and an outlet air temperature of 100°C to obtain precursor powder. The precursor powder is then sintered in a nitrogen atmosphere at a temperature of 3°C / min to 550°C for 10 hours.

[0037] (5) Crushing: The sintered material is crushed by an air jet mill to obtain sodium iron pyrophosphate cathode material.

[0038] Test results: compaction density is 2.19 g / cm³; 0.1C discharge capacity is 110.8 mAh / g, and initial efficiency is 94.1%. Example 2

[0039] The steps for preparing sodium iron pyrophosphate cathode material in this embodiment are roughly the same as those in Example 1, except that grinding aid A is used instead of grinding aid B.

[0040] After ultrafine grinding, the first slurry D 50 =0.16μm; Graded slurry D 50 =0.42μm.

[0041] Test results: compaction density is 2.16 g / cm³; 0.1C discharge capacity is 109.2 mAh / g, with an initial efficiency of 93.9%. Example 3

[0042] The steps for preparing sodium iron pyrophosphate cathode material in this embodiment are roughly the same as those in Example 1, except that grinding aid C is used instead of grinding aid B.

[0043] After ultrafine grinding, the first slurry D 50 =0.19μm; Graded slurry D 50 =0.45μm.

[0044] Test results: compaction density is 2.14 g / cm³; 0.1C discharge capacity is 108.6 mAh / g, and initial efficiency is 91.9%.

[0045] Comparative Example 1 Weigh out 6 mol of ferric phosphate and deionized water (adjust the total slurry solid content to 30%) and mix them. Then, using 0.3 mm diameter zirconia beads, mill the mixture at 1200 rpm for 2 hours to obtain D. 50 The slurry with a particle size of 0.78 μm was then added with 2 mol sodium dihydrogen phosphate, 3 mol sodium carbonate and 0.4 mol glucose. The stirring, drying, sintering and pulverizing steps were exactly the same as in Example 1.

[0046] Test results: compaction density is only 1.95 g / cm³; 0.1C discharge capacity is 102.9 mAh / g, with an initial efficiency of 83.5%.

[0047] Comparative Example 2 The steps for preparing sodium iron pyrophosphate cathode material in this comparative example are roughly the same as those in Example 1, except that cetyltrimethylammonium bromide is used instead of grinding aid B.

[0048] Test results: After ultrafine grinding, the first slurry D 50 =0.28μm; Graded slurry D 50 =0.50μm.

[0049] The compacted density of the finished product is 2.06 g / cm³; the 0.1C discharge capacity is 105.2 mAh / g, and the first-time efficiency is 84.5%.

[0050] Comparative Example 3 The steps for preparing sodium iron pyrophosphate cathode material in this comparative example are roughly the same as those in Example 1, except that sodium dodecylbenzenesulfonate is used instead of grinding aid B.

[0051] Test results: The ultrafine grinding effect is poor, and the slurry D is produced in one step. 50 =0.52μm; Graded slurry D 50 =0.68μm.

[0052] The compacted density of the finished product is 2.03 g / cm³; the discharge capacity at 0.1C is 105.4 mAh / g, and the first-time efficiency is 89.2%.

[0053] By comparing the above examples with the comparative examples, the following conclusions can be clearly drawn: This invention significantly improves performance: all samples using the twin-structure grinding aids described in this invention (Examples 1-3) showed significantly better grinding efficiency, final compaction density, specific capacity, and first-efficiency than all comparative examples. This indicates that the synergistic effect of N, S, and F elements and the twin molecular structure are crucial for improving the overall performance of NFPP. Influence of grinding aid chain length: Within the scope of this invention, the hexadecylamine (C16)-based grinding aid (Example 1) exhibited the best overall performance, indicating that the alkyl chain length needs to be optimized to obtain the best steric hindrance and dispersion effect. Limited effect of single elements: Although comparative examples 2 (containing only nitrogen) and 3 (containing only sulfur) may have a parameter close to that of this invention (such as the first-efficiency of comparative example 3), they could not simultaneously achieve high levels in the four key indicators of grinding efficiency, compaction density, specific capacity, and first-efficiency, demonstrating the necessity of multifunctional synergy. Mechanism verification: The compaction density and specific capacity of comparative example 2 were higher than those of comparative examples 1 and 3, confirming that cationic grinding aids have an inherent advantage in adsorption and grinding of negatively charged iron phosphate. The twin structure of this invention represents a further leap forward, achieving a comprehensive breakthrough in performance.

[0054] In summary, this invention effectively and synergistically solves the industrial problems of low grinding efficiency, insufficient compaction density, and poor electrochemical performance of NFPP materials through specific grinding aid molecular design, synthesis, and application processes.

[0055] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing a high-density sodium iron pyrophosphate cathode material, characterized in that, The preparation method includes the following steps: Step (1), Ultrafine grinding: Add some iron phosphate raw material and modified gemini quaternary ammonium salt grinding aid to deionized water and perform ultrafine grinding for 2-4 hours to form D 50 A primary slurry with a particle size of 0.1-0.3 μm; Step (2), gradation grinding: Add the remaining ferric phosphate raw material to the primary slurry and perform ordinary grinding for 1-2 hours until the particle size D is reached. 50 The particle size is 0.3-0.5μm, forming a slurry with an ideal gradation of "fine particles filling the voids between coarse particles"; Step (3), mixing: Add soluble sodium source, phosphorus source and carbon source to the graded slurry, stir for 30 min to form a homogeneous mixed slurry with a solid content of 30%-40%; Step (4), Drying and sintering: The mixed slurry is spray-dried with an inlet air temperature of 200-280℃ and an outlet air temperature of 80-120℃. The resulting precursor powder is heated to 500-600℃ at 3℃ / min under a nitrogen atmosphere and sintered for 8-12 hours. Step (5), Crushing: The sintered powder is processed by an air jet mill to control the particle size D of the finished product. 10 The thickness is 0.1-0.3 μm, D 50 Less than 5μm, D 99 With a thickness of less than 15 μm, a high-density sodium iron pyrophosphate cathode material was obtained. In step (1), the preparation method of the modified Gemini quaternary ammonium salt grinding aid includes the following steps: S1. Add long-chain alkylamine to isopropanol, stir to dissolve, add 3,5-bis(trifluoromethyl)styrene, stir at 65-70℃ for 3-5 h, after the reaction is completed, rotary evaporate, recrystallize from ethyl acetate to obtain intermediate 1. S2. Add intermediate 1 to isopropanol, then add 1,3-propanesulfonate lactone, and stir the reaction at 65-70℃ for 4-5 hours. After the reaction is complete, evaporate by rotary evaporation and recrystallize from ethyl acetate to obtain intermediate 2. S3. Under a nitrogen atmosphere, intermediates 2 and 1,4-dibromobutane were added to isopropanol, stirred and dispersed, and refluxed for 12-14 hours. After the reaction was completed, the mixture was distilled under reduced pressure and dried to obtain the modified gemini quaternary ammonium salt grinding aid.

2. The method for preparing the high-density sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In step (1), the amount of modified gemini quaternary ammonium salt grinding aid added is 0.5%-2.0% of the mass of the iron phosphate in that part.

3. The method for preparing the high-density sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In step (2), the mass ratio of ultrafine ground ferric phosphate to ferric phosphate that has undergone ordinary sand milling is 1:1-5.

4. The method for preparing the high-density sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In step (3), the molar ratio of iron in the iron phosphate raw material to sodium in the sodium source is 0.72-0.75:

1.

5. The method for preparing the high-density sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In S1, the molar ratio of long-chain alkylamine and 3,5-bis(trifluoromethyl)styrene is 1:1-1.

2.

6. The method for preparing the high-density sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In S1, the long-chain alkylamine is one of tetradecylamine, hexadecylamine, and octadecylamine.

7. The method for preparing the high-density sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In S2, the molar ratio of intermediate 1 and 1,3-propanesulfonate lactone is 1:1-1.

2.

8. The method for preparing the high-density sodium iron pyrophosphate cathode material according to claim 1, characterized in that, In S3, the molar ratio of intermediate 2 to 1,4-dibromobutane is 2.2-2.5:1.

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