Nitrogen-doped double-carbon-coated three-dimensional conductive composite sodium ferric phosphate positive electrode material, preparation method thereof and application of positive electrode material in sodium ion battery

Abstract

The invention discloses a nitrogen-doped double-carbon-coated three-dimensional conductive composite sodium ferric phosphate positive electrode material, and belongs to the technical field of sodium ion batteries. The material is mainly composed of composite sodium iron phosphate active substance particles, nitrogen-doped double-carbon coating layers coating the surfaces of the active substance particles, and CNTs distributed among the active substance particles and forming a three-dimensional conductive network, and has a multi-dimensional conductive and protection structure, so that the electron transmission efficiency and the interface ion mobility can be greatly improved, and the performance of the material is improved. The overgrowth and aggregation of the active particles in the sintering and circulating process can be effectively inhibited, and a buffer space is provided for the volume change in the sodion deintercalation process, so that high capacity, long cycle life and excellent rate capability are synchronously realized. The invention also discloses a preparation method of the material, and the method has good controllability and industrialization potential. Meanwhile, the invention also discloses an application of the material in a sodium ion battery.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, and specifically relates to a nitrogen-doped double-carbon coated three-dimensional conductive composite sodium iron phosphate cathode material, its preparation method, and its application in sodium-ion batteries. Background Technology

[0002] To address the growing demand for sustainable energy in human society, developing low-cost, high-performance, large-scale energy storage technologies has become a key research topic. Against this backdrop, sodium-ion batteries, due to their similar electrochemical mechanisms to lithium-ion batteries, as well as their advantages of abundant resources, high cost-effectiveness, and strong global availability, are considered one of the most promising energy storage systems. As a core component of sodium-ion batteries, the cathode material directly determines the battery's overall electrochemical performance and economic viability.

[0003] Currently, widely studied cathode materials for sodium-ion batteries mainly include Prussian blue analogues, layered metal oxides, and polyanionic compounds. Among these, Prussian blue analogues often suffer from poor cycle stability due to crystal water and structural defects; layered oxides generally exhibit limitations in structural and air stability. In contrast, polyanionic compounds possess an open framework structure formed by the interconnection of anionic groups and transition metal-oxygen polyhedra, exhibiting good structural stability. Iron-based polyanionic compounds, such as composite sodium iron phosphate materials, have attracted considerable attention due to their low cost, structural stability, and high safety, demonstrating suitable operating voltages and high theoretical capacities. However, these materials are limited by low intrinsic electronic conductivity, sluggish sodium-ion diffusion kinetics, and the unavoidable formation of impurity phases (electrochemically inert m-NaFePO4) during synthesis, severely restricting their specific capacity and rate performance, hindering practical applications. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a nitrogen-doped, double-carbon-coated, three-dimensionally conductive composite sodium iron phosphate cathode material, its preparation method, and its application in sodium-ion batteries.

[0005] The technical solution is as follows: First, this invention discloses a nitrogen-doped double-carbon coated three-dimensional conductive composite sodium iron phosphate cathode material. The material is mainly composed of composite sodium iron phosphate active material particles, a nitrogen-doped double-carbon coating layer on the surface of the composite sodium iron phosphate active material particles, and carbon nanotubes (CNTs) distributed between the composite sodium iron phosphate active material particles to form a three-dimensional conductive network.

[0006] This invention modifies the surface of active particles by using a nitrogen-doped carbon double layer. Building upon this, CNTs are further introduced, interspersed between the active particles to form a three-dimensional conductive network that runs the entire structure, successfully creating a multi-dimensional conductive and protective structure with a "point-surface-volume" synergy. This structure not only significantly improves electron transport efficiency and interfacial ion mobility but also effectively suppresses excessive growth and aggregation of active particles during sintering and cycling, and provides a buffer space for volume changes during sodium ion insertion / extraction, thereby simultaneously achieving high capacity, long cycle life, and excellent rate performance.

[0007] Preferably, the active ingredient in the composite sodium iron phosphate is Na. 3.4 Fe 2.4 (PO4) 1.4 P2O7.

[0008] Secondly, this invention discloses the preparation method of the above-mentioned composite sodium iron phosphate cathode material, and the preparation process is as follows: A mixed iron source containing a first iron source and a second iron source is dispersed in a solvent, a dispersant and an antioxidant are added, and after stirring evenly, the mixture is ground to a preset particle size to obtain a slurry with a solid content of 40%. Sodium source, phosphorus source, carbon source, nitrogen source and sodium supplement are mixed with deionized water and stirred until completely dissolved to obtain a clear solution with a solute mass fraction of 40%. The slurry and the clarified solution were mixed evenly, and then the CNT slurry was added and continuously dispersed until the CNTs were evenly dispersed to obtain the precursor slurry. The precursor slurry was dried to obtain precursor powder. The precursor powder was subjected to segmented sintering to obtain a composite sodium iron phosphate cathode material. The mixed iron source, sodium source, phosphorus source, carbon source, and nitrogen source were all weighed according to the stoichiometric molar ratio.

[0009] This invention constructs a nitrogen-doped double-carbon coating layer derived from an antioxidant and a carbon source, and introduces CNT bridging to form a three-dimensional conductive network, creating a multi-dimensional conductive and protective structure with "point-surface-volume" synergy. It employs a mixed iron source and a precisely controlled segmented sintering process: the first stage involves preliminary pyrolysis of the carbon and nitrogen sources to form an initial coating layer and induce preliminary crystallization of the precursor; the second stage completes crystal growth, achieves nitrogen doping, and constructs a conductive carbon-based coating layer, thereby suppressing electrochemically inert impurities such as m-NaFePO4, obtaining a high-purity active phase, and improving reversible specific capacity; simultaneously, a sodium supplement is introduced to release additional Na during the first charge-discharge cycle. + It compensates for irreversible capacity loss and sodium loss during sintering. The raw materials are widely available, with transparent cost control. Combined with processes such as spray drying, the process exhibits good repeatability and high product consistency, making it suitable for the large-scale preparation of high-performance sodium-ion battery cathode materials.

[0010] Preferably, the nitrogen source is one or more of urea, biuret, melamine, cyanuric acid, polydopamine, triethanolamine, and methylamine hydrochloride; more preferably, the nitrogen source is one or more of urea, biuret, melamine, cyanuric acid, and polydopamine.

[0011] Preferably, the first iron source and the second iron source are one or more of nano-ferric phosphate, ferrous oxalate dihydrate, ferrous carbonate, ferric oxide, and ferrous oxide, and the first iron source and the second iron source are different from each other; more preferably, the first iron source is nano-ferric phosphate and the second iron source is ferrous oxalate dihydrate.

[0012] Preferably, the dispersant is one or more of polyvinylpyrrolidone (PVP), polyacrylic acid (PAA), polyvinyl alcohol [(Poly(Vinyl Alcohol), PVA], and polyethylene glycol [Poly(Ethylene Glycol) PEG]; more preferably, the dispersant is PVP; Preferably, the antioxidant is ascorbic acid.

[0013] Preferably, the dispersant is 1% of the total mass of the mixed iron source; the antioxidant is 0.4% of the total mass of the mixed iron source.

[0014] Preferably, the solvent is a water / ethanol mixture, wherein the mass ratio of water to ethanol is (3~7):(7~3), and more preferably, the mass ratio of water to ethanol is 1:1.

[0015] Preferably, the preset granularity is D. 50 ≤250nm, D 90 ≤550nm, meaning the median particle size of the powder in the slurry is ≤250nm, and less than 90% of the particles are ≤550nm, indicating that the overall particle size is relatively fine and the distribution is concentrated.

[0016] Preferably, the sodium source is one or more of trisodium phosphate, sodium carbonate, sodium dihydrogen phosphate dihydrate, and disodium hydrogen phosphate dihydrate; more preferably, the sodium source is disodium hydrogen phosphate dihydrate.

[0017] Preferably, the phosphorus source is selected from one or more of sodium dihydrogen phosphate dihydrate, disodium hydrogen phosphate dihydrate, phosphoric acid, and ammonium dihydrogen phosphate; more preferably, the phosphorus source is selected from disodium hydrogen phosphate dihydrate.

[0018] Preferably, the carbon source is one or more of glucose monohydrate, starch, citric acid, and sucrose; more preferably, the carbon source is glucose monohydrate.

[0019] Preferably, sodium acetate is used as the sodium supplement.

[0020] Preferably, the CNT slurry is prepared by dispersing 4% CNTs and 1% polyvinylpyrrolidone (NMP) of the total mass of the mixed iron source in N-methyl pyrrolidone (NMP).

[0021] Preferably, the drying process is spray drying, which involves using a centrifugal or two-fluid spray dryer. The inlet air temperature is 180~240℃, and the outlet air temperature is controlled and maintained within the set range to ensure that the material is fully dried without damaging the equipment. More preferably, the outlet air temperature is 80~110℃.

[0022] Preferably, the segmented sintering process is as follows: First stage: Increase the temperature from room temperature to 280-350℃ at a rate of 1-5℃ / min, and hold for 2-5 hours; Second stage: Continue to increase the temperature at 2~8℃ / min to 480~600℃, and keep it at that temperature for 8~16 hours.

[0023] More preferably, First stage: Increase the temperature from room temperature to 300-330℃ at a rate of 2-4℃ / min, and keep it at that temperature for 5 hours; Second stage: Continue to increase the temperature at 3~6℃ / min to 500~520℃, and keep it at that temperature for 7~10 hours.

[0024] Finally, this invention discloses the application of the above-mentioned composite sodium iron phosphate cathode material in sodium-ion batteries.

[0025] The beneficial effects of this invention are as follows: (1) Multi-strategy synergistic improvement of comprehensive performance: Through the design of composite sodium iron phosphate material composition, nitrogen doping double carbon layer surface coating, construction of CNT three-dimensional conductive network and synergistic application of sodium supplementation technology, a complete material optimization scheme was formed, which improved the electrochemical performance of the material from bulk structure to interface characteristics.

[0026] (2) Excellent conductivity and fast ion transport: By constructing an efficient three-dimensional electronic conduction pathway in the active material using nitrogen-doped double carbon coating and CNTs, the electronic conductivity of the material is significantly improved. At the same time, the uniform carbon coating helps to suppress excessive grain growth and shorten the solid-state diffusion path of sodium ions, thereby effectively improving the rate performance of the material.

[0027] (3) High phase purity and improved reversible capacity: The use of mixed iron sources and precisely controlled segmented sintering process effectively suppresses the formation of electrochemically inert impurities (such as m-NaFePO4), ensuring that the product is a high-purity electrochemically active phase, thereby achieving a higher reversible specific capacity. The introduction of sodium supplement further compensates for sodium loss during sintering and irreversible capacity loss in the first cycle, improving the initial coulombic efficiency and actual usable capacity of the material.

[0028] (4) Enhanced structural stability and cycle performance: The carbon and nitrogen layers coated on the material surface can act as a buffer framework to mitigate the volume changes caused by sodium ions during insertion and extraction, which helps maintain the structural integrity of the electrode material. Nitrogen doping further enhances the interfacial bonding between the carbon layer and the active material, contributing to the improvement of the material's long cycle life.

[0029] (5) The preparation process has good controllability and industrialization potential: the raw materials used in the preparation process are widely available and have low cost. Combined with spray drying and other processes, the process has good repeatability and high product consistency, making it suitable for the large-scale preparation of high-performance sodium-ion battery cathode materials. Attached Figure Description

[0030] Figure 1 The X-ray diffraction (XRD) patterns of the materials prepared in Example 1 and Comparative Example 1 are shown. Figure 2 The image shows a scanning electron microscope (SEM) image of the material prepared in Example 1. Figure 3 The 0.1C first-cycle charge-discharge curves of the materials prepared in Example 1 and Comparative Examples 1-2 are shown. Figure 4 The results of rate performance tests on the materials prepared in Example 1 and Comparative Example 1 are shown. Figure 5 The curve of the long-cycle stability of the material prepared in Example 1 at a high rate of 20°C is shown. Detailed Implementation

[0031] 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.

[0032] 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.

[0033] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0034] I. Examples and Comparative Examples 1. Example 1 S1. Disperse 2 mol of nano-ferric phosphate and 0.4 mol of ferrous oxalate dihydrate in a water / ethanol mixture (mass ratio 1:1). Add 1% ascorbic acid and 0.4% PVP (by mass of the total iron source mixture). Stir until homogeneous and then mill to D. 50 ≤250nm, D 90 ≤550nm, resulting in a slurry with a solid content of 40%; S2. Dissolve 1.7 mol disodium hydrogen phosphate dihydrate, 0.1 mol sodium acetate, 0.3 mol glucose monohydrate and 0.1 mol urea in deionized water to obtain a clear solution with a solute mass fraction of 40%. S3. First, mix the slurry obtained in S1 and the clarified solution obtained in S2 evenly. Then, add 5% of the total mass of the mixed iron source (nano-iron phosphate + ferrous oxalate dihydrate) CNT slurry and disperse at high speed at 60℃ for 40 min to obtain a precursor slurry with uniform composition and good CNT dispersion. S4. Spray dry the precursor slurry obtained in S3. The spray drying is carried out using a centrifugal spray dryer with an inlet air temperature of 230℃ and an outlet air temperature of 85~110℃ to obtain precursor powder. S5. Under a nitrogen atmosphere, the precursor powder obtained in S4 was subjected to segmented sintering: the temperature was increased from room temperature to 300℃ at a rate of 3℃ / min and held for 5 hours to allow the organic components to completely volatilize; the temperature was then increased to 500℃ at a rate of 4℃ / min and held for 8 hours, followed by natural cooling to room temperature to obtain Na. 3.4 Fe 2.4 (PO4) 1.4 P2O7 material.

[0035] The CNT slurry is prepared by dispersing arrayed carbon nanotubes (4% by mass of the mixed iron source) and polyvinylpyrrolidone (1% by mass of the mixed iron source) in NMP.

[0036] 2. Example 2 S1. Same as in Example 1, a slurry with a solid content of 40% was obtained; S2. Dissolve 1.7 mol disodium hydrogen phosphate dihydrate, 0.1 mol sodium acetate, 0.3 mol glucose monohydrate and 0.1 mol biuret in deionized water to obtain a clear solution with a solute mass fraction of 40%; S3. Same as in Example 1, a precursor slurry with uniform composition and good CNT dispersion is obtained; S4. Spray dry the precursor slurry obtained in S3. The spray drying is carried out using a centrifugal spray dryer with an inlet air temperature of 180℃ and an outlet air temperature of 85~110℃ to obtain precursor powder. S5. The precursor powder obtained in S4 was subjected to segmented sintering under a nitrogen atmosphere: the temperature was increased from room temperature to 330℃ at a rate of 3℃ / min and held for 5 hours to allow the organic components to completely volatilize; the temperature was then increased to 500℃ at a rate of 3℃ / min and held for 8 hours, followed by natural cooling to room temperature to obtain Na. 3.4 Fe 2.4 (PO4) 1.4 P2O7 material.

[0037] 3. Example 3 S1. Same as in Example 1, a slurry with a solid content of 40% was obtained; S2. Dissolve 1.7 mol disodium hydrogen phosphate dihydrate, 0.1 mol sodium acetate, 0.3 mol glucose monohydrate and 0.1 mol melamine in deionized water to obtain a clear solution with a solute mass fraction of 40%; S3. Same as in Example 1, a precursor slurry with uniform composition and good CNT dispersion is obtained; S4. Spray dry the precursor slurry obtained in S3. The spray drying is carried out using a centrifugal spray dryer with an inlet air temperature of 240℃ and an outlet air temperature of 85~110℃ to obtain precursor powder. S5. Under a nitrogen atmosphere, the precursor powder obtained in S4 was subjected to segmented sintering: the temperature was increased from room temperature to 330℃ at a rate of 2℃ / min and held for 5 hours to allow the organic components to completely volatilize; the temperature was then increased to 520℃ at a rate of 5℃ / min and held for 7 hours, followed by natural cooling to room temperature to obtain Na. 3.4 Fe 2.4 (PO4) 1.4 P2O7 material.

[0038] 4. Example 4 S1. Same as in Example 1, a slurry with a solid content of 40% was obtained; S2. Dissolve 1.7 mol disodium hydrogen phosphate dihydrate, 0.1 mol sodium acetate, 0.3 mol glucose monohydrate and 0.1 mol cyanuric acid in deionized water to obtain a clear solution with a solute mass fraction of 40%. S3. Same as in Example 1, a precursor slurry with uniform composition and good CNT dispersion is obtained; S4. Same as in Example 1, to obtain precursor powder; S5. Under a nitrogen atmosphere, the precursor powder obtained in S4 was subjected to segmented sintering: the temperature was increased from room temperature to 330℃ at a rate of 3℃ / min and held for 5 hours to allow the organic components to completely volatilize; the temperature was then increased to 520℃ at a rate of 5℃ / min and held for 7 hours, followed by natural cooling to room temperature to obtain Na. 3.4 Fe 2.4 (PO4) 1.4 P2O7 material.

[0039] 5. Example 5 S1. Same as in Example 1, a slurry with a solid content of 40% was obtained; S2. Dissolve 1.7 mol disodium hydrogen phosphate dihydrate, 0.1 mol sodium acetate, 0.3 mol glucose monohydrate and 0.1 mol polydopamine in deionized water to obtain a clear solution with a solute mass fraction of 40%. S3. Same as in Example 1, a precursor slurry with uniform composition and good CNT dispersion is obtained; S4. Same as in Example 1, to obtain precursor powder; S5. Under a nitrogen atmosphere, the precursor powder obtained in S4 was subjected to segmented sintering: the temperature was increased from room temperature to 300℃ at a rate of 4℃ / min and held for 5 hours to allow the organic components to completely volatilize; the temperature was then increased to 550℃ at a rate of 6℃ / min and held for 10 hours, followed by natural cooling to room temperature to obtain Na. 3.4 Fe 2.4 (PO4) 1.4 P2O7 material.

[0040] 6. Comparative Example 1 S1. Same as in Example 1, a slurry with a solid content of 40% was obtained; S2. Dissolve 1.7 mol disodium hydrogen phosphate dihydrate, 0.1 mol sodium acetate and 0.3 mol glucose monohydrate in deionized water to obtain a clear solution with a solute mass fraction of 40%. S3. Same as in Example 1, a precursor slurry with uniform composition and good CNT dispersion is obtained; S4. Same as in Example 1, to obtain precursor powder; S5. Same as in Example 1, obtain Na 3.4 Fe 2.4 (PO4) 1.4 P2O7 material.

[0041] 7. Comparative Example 2 S1. Same as in Example 1, a clear solution with a solute mass fraction of 40% was obtained; S2. Dissolve 1.7 mol of disodium hydrogen phosphate dihydrate, 0.3 mol of glucose monohydrate and 0.1 mol of urea in deionized water to obtain a clear solution with a solid content of 40%. S3. Same as in Example 1, a precursor slurry with uniform composition and good CNT dispersion is obtained; S4. Same as in Example 1, to obtain precursor powder; S5. Same as in Example 1, obtain Na 3.4 Fe 2.4 (PO4) 1.4 P2O7 material.

[0042] 8. Comparative Example 3 S1. Same as in Example 1, a slurry with a solid content of 40% was obtained; S2. Same as in Example 1, a clear solution with a solute mass fraction of 40% is obtained; S3. Mix the slurry obtained in S1 with the clarified solution obtained in S2, and disperse at high speed at 60℃ for 40 min to obtain a precursor slurry with uniform composition; S4. Same as in Example 1, to obtain precursor powder; S5. Same as in Example 1, obtain Na 3.4 Fe 2.4 (PO4) 1.4 P2O7 material.

[0043] 9. Comparative Example 4 S1. Disperse 2 mol of nano-ferric phosphate and 0.4 mol of ferrous oxalate dihydrate in an aqueous solution, add 1% ascorbic acid (by mass of the mixed iron source) and 0.4% PVP (by mass of the mixed iron source), stir evenly, and then mill to D. 50 ≤250nm, D 90≤550nm, resulting in a slurry with a solid content of 30%; S2. Same as in Example 1, a clear solution with a solute mass fraction of 30% is obtained; S3. Same as in Example 1, a precursor slurry with uniform composition and good CNT dispersion is obtained; S4. Same as in Example 1, to obtain precursor powder; S5. Same as in Example 1, obtain Na 3.4 Fe 2.4 (PO4) 1.4 P2O7 material.

[0044] II. Performance Testing and Characterization The XRD patterns of the materials prepared in Example 1 and Comparative Example 1 are as follows: Figure 1 As shown, the materials prepared in Example 1 and Comparative Example 1 both exhibit a typical sodium iron pyrophosphate (NFPP) crystal structure, and their diffraction peaks are in complete agreement with the standard card PDF 89-0579. The high consistency of the two spectra and the sharp diffraction peaks indicate that the materials have good crystallinity and high phase purity, which verifies that the synergistic effect of the two iron sources under a specific chemical ratio is conducive to the formation of a regular phase structure.

[0045] SEM images of the materials obtained in Example 1 are as follows: Figure 2 As shown, CNTs are uniformly distributed inside and on the surface of the material through in-situ doping, forming a stable three-dimensional conductive framework, which significantly reduces the contact resistance between particles and greatly improves the electronic conductivity of the material.

[0046] The materials prepared in the embodiments and comparative examples of the present invention were respectively assembled into CR2032 coin cells to test their electrical performance. The specific assembly process is as follows: Electrode preparation: The active material, conductive agent acetylene black, and binder polyvinylidene fluoride were uniformly mixed at a mass ratio of 8:1:1, and then N-methylpyrrolidone solvent was added to prepare a slurry. The resulting slurry was uniformly coated onto an aluminum foil current collector, and after vacuum drying at 120℃ for 12 hours, it was punched into a 14mm disc to obtain the positive electrode. The surface loading of the active material in the positive electrode was controlled at (2.0±0.5) mg / cm².

[0047] Battery assembly: In an argon-filled glove box, using a sodium metal sheet as the counter electrode and reference electrode, and a glass fiber membrane as the separator, a 1 mol / L NaClO4 ethylene carbonate / propylene carbonate solution was used as the base, with 5% fluoroethylene carbonate (FEC) added as a film-forming additive to prepare the final electrolyte for assembling CR2032 coin cells.

[0048] Electrochemical Testing: All electrochemical tests were conducted at a constant temperature of 27°C. Constant current charge-discharge tests were performed within the range of 1.5–4.0V using the Newway battery testing system. Charge-discharge specific capacity was calculated based on the mass of the active material and the generally accepted theoretical capacity (129 mAh / g) of the relevant sodium iron phosphate system. Cycle performance was evaluated using capacity retention, defined as the ratio of the discharge capacity in the Nth cycle to the discharge capacity in the first cycle.

[0049] The 0.1C first-cycle charge-discharge curves of the materials prepared in Example 1 and Comparative Examples 1-2 are as follows: Figure 3 As shown, compared with Comparative Examples 1 and 2, Example 1 exhibits a higher reversible capacity after introducing nitrogen doping and double carbon coating strategies, which fully demonstrates that the introduction of nitrogen doping and double carbon coating can significantly enhance the electrochemical performance of the material.

[0050] The rate performance test results of the materials prepared in Example 1 and Comparative Example 1 are as follows: Figure 4 As shown, Example 1 exhibits better capacity retention than Comparative Example 1 at different current densities, demonstrating that nitrogen doping can effectively enhance the ion and electron transport dynamics of the material, thereby significantly improving its rate performance.

[0051] The long-cycle stability curve of the material prepared in Example 1 at a high rate of 20°C is shown below. Figure 5 As shown, Example 1 still maintains 88.85% capacity retention after 9500 cycles at a high rate of 20C, indicating that the nitrogen doping strategy combined with the construction of a three-dimensional conductive network significantly enhances the structural stability and cycle life of the material.

[0052] The electrochemical performance test results of Examples 1-5 and Comparative Examples 1-4 are shown in Table 1.

[0053] Table 1. Electrochemical performance test results of Examples 1-5 and Comparative Examples 1-4

[0054] From Table 1, we can obtain: (1) The key role of nitrogen doping in improving electrochemical performance, especially cycle stability: Examples 1-5 (nitrogen-doped) all showed significantly better long-term cycle performance than Comparative Example 1 (without nitrogen doping). Specifically, Examples 1-5 maintained a capacity retention of over 90% (90.14%~92.32%) during high-rate (5C) long-cycle (2000 cycles); while the capacity retention of Comparative Example 1 was only 82.16%. This data strongly demonstrates that nitrogen doping can effectively enhance the conductivity of the carbon coating layer while optimizing the stability of the electrode / electrolyte interface, thereby significantly improving the structural integrity and capacity retention of the material under high-rate long-cycle conditions.

[0055] (2) The key impact of the synergistic effect of sodium replenishment strategy and nitrogen doping on improving first-cycle performance: Comparing Example 1 (with sodium replenishment agent) and Comparative Example 2 (without sodium replenishment agent), it can be seen that the sodium replenishment strategy has a significant effect on improving the first-cycle charge-discharge efficiency and reversible capacity. Specifically, the 0.1C first-cycle discharge capacity of Example 1 is as high as 109.88mAh. g -1 This is significantly higher than the 99.31mAh of Comparative Example 2. g -1 This verifies that the sodium supplement (sodium acetate) provides an additional sodium source, effectively compensating for the irreversible consumption of active sodium caused by the formation of the solid electrolyte interphase (SEI) film; meanwhile, the 0.1C first-cycle discharge capacity of Example 1 (nitrogen-doped) is significantly higher than that of Comparative Example 1 (undoped), which has a capacity of 103.221 mAh. g -1 This further illustrates that nitrogen doping not only improves conductivity, but also provides adsorption sites that promote sodium ion interfacial migration and reaction kinetics. The synergistic effect of these two factors significantly enhances the material's first-cycle coulombic efficiency and reversible specific capacity (0.1C first-cycle discharge capacity).

[0056] (3) The decisive contribution of CNT-constructed three-dimensional conductive network to rate performance: As can be seen from Example 1 (with CNTs added) and Comparative Example 3 (without CNTs added), the lack of a CNT three-dimensional conductive network (Example 3) not only reduces the first-cycle discharge capacity at 0.1C (99.55mAh) g -1 The rate performance is relatively low, and its long-cycle capacity retention (80.75%) is far lower than that of the embodiments of the present invention. This fully demonstrates that the macroscopic conductive framework constructed by CNTs, in synergy with the microscopic conductive network formed by the nitrogen-doped carbon layer, provides an efficient channel for the rapid transport of electrons and ions, which is the core guarantee for the material to obtain excellent rate performance and cycle stability.

[0057] (4) Optimization effect of water / ethanol mixed solvent system on precursor quality and final performance: Comparative Example 4, which uses pure water system for sand milling, has the following performance indicators: [0.1C first discharge capacity 96.57mAh] g -1 The long-cycle capacity retention rate of 86.47% was significantly inferior to that of Example 1, which used a water / ethanol mixed solvent. This indicates that the introduction of ethanol improves the dispersibility and rheological properties of the slurry, helps to obtain a more uniform precursor with better particle size, and ultimately achieves superior electrochemical performance.

[0058] In summary, this invention successfully prepared a sodium-ion battery cathode material with high first-cycle reversible capacity, excellent rate performance, and ultra-long cycle life through the synergistic use of nitrogen doping, sodium replenishment strategy, CNT three-dimensional conductive network, and optimized solvent system. The precise comparison of each comparative example and embodiment systematically verifies the unique and indispensable technical effects brought about by each technical feature of this invention.

[0059] 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 nitrogen-doped double-carbon-coated three-dimensional conductive composite sodium iron phosphate cathode material, characterized in that, The positive electrode material is mainly composed of composite sodium iron phosphate active material particles, a nitrogen-doped double-carbon coating layer coated on the surface of the composite sodium iron phosphate active material particles, and carbon nanotubes distributed between the composite sodium iron phosphate active material particles and forming a three-dimensional conductive network.

2. The nitrogen-doped double-carbon-coated three-dimensional conductive composite sodium iron phosphate cathode material of claim 1, wherein, The complex sodium iron phosphate active material is Na 3.4 Fe 2.4 (PO4) 1.4 P2O7.

3. A method for preparing the nitrogen-doped double-carbon-coated three-dimensional conductive composite sodium iron phosphate cathode material according to any one of claims 1-2, characterized in that, The preparation process is as follows: A mixed iron source containing a first iron source and a second iron source is dispersed in a solvent, a dispersant and an antioxidant are added, and after uniform stirring, grinding treatment is performed to a preset particle size to obtain a slurry with a solid content of 40%; A sodium source, a phosphorus source, a carbon source, a nitrogen source, and a sodium supplement agent are mixed with deionized water, and stirred until completely dissolved to obtain a clear solution with a solute mass fraction of 40%; The slurry and the clear solution are mixed uniformly, then CNT slurry is added and continuously dispersed until the CNT is uniformly dispersed to obtain a precursor slurry; The precursor slurry is subjected to drying treatment to obtain a precursor powder; The precursor powder is subjected to a segmented sintering treatment to obtain the composite sodium iron phosphate positive electrode material. The mixed iron source, the sodium source, the phosphorus source, the carbon source, and the nitrogen source are all taken according to the stoichiometric molar ratio.

4. The preparation method of the nitrogen-doped double-carbon-coated three- dimensional conductive composite sodium iron phosphate cathode material according to claim 3, characterized in that, The nitrogen source is one or more of urea, biuret, melamine, cyanuric acid, polydopamine, triethanolamine, and methylamine hydrochloride.

5. The method of claim 3, wherein the method is characterized by: The first iron source and the second iron source are one or more of nano iron phosphate, ferrous oxalate dihydrate, ferrous carbonate, diiron trioxide, and ferrous oxide, and the first iron source and the second iron source are different from each other.

6. The method of claim 3, wherein the method is characterized by: The antioxidant is ascorbic acid.

7. The method of claim 3, wherein the method is characterized by: The carbon source is one or more of glucose monohydrate, starch, citric acid, and sucrose.

8. The method of claim 3, wherein the method is characterized by: The sodium supplement agent is sodium acetate.

9. The method of claim 3, wherein the method is characterized by: The segmented sintering treatment step is as follows: First segment: from room temperature to 280-350℃ at a rate of 1-5℃ / min, and holding for 2-5h; Second segment: continue to heat at a rate of 2-8℃ / min to 480-600℃, and hold for 8-16h.

10. The application of a nitrogen-doped double-carbon-coated three-dimensional conductive composite sodium iron phosphate positive electrode material according to any one of claims 1-2 in a sodium ion battery.

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