Sodium ion battery positive electrode, preparation method thereof and sodium ion battery

Abstract

This invention discloses a sodium-ion battery cathode, its preparation method, and a sodium-ion battery. The preparation process includes precursor synthesis, gradient drying, high-temperature sintering, and finished product preparation. The precursor synthesis is carried out under inert gas protection. A first mixed solution containing an iron source, a phosphorus source, and a sodium source is co-precipitated with a second mixed solution containing a structure-directing agent at 45-75°C. The structure-directing agent is a compound of a polyhydroxycarboxylic acid derivative and a nitrogen-containing heterocyclic compound. Gradient drying includes initial drying at 100-120°C and 40%-60% relative humidity, followed by deep drying at 60-80°C and 5%-10% relative humidity. The dried precursor is mixed with a carbon source and sintered at 550-750°C to obtain a sodium iron phosphate-based cathode material matrix. The matrix is ​​then formulated with conductive additives and binders to form an electrode sheet. The prepared cathode material has a uniform phosphorus distribution, a stable electrode structure, high capacity, long cycle life, and good rate performance, and the overall preparation conditions are mild.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery manufacturing technology, specifically to a sodium-ion battery cathode, its preparation method, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries have attracted much attention in the field of large-scale energy storage due to their abundant resources and low cost. Sodium iron phosphate (NaFePO4) cathode materials have become a research hotspot due to their structural stability, environmental friendliness, and high theoretical specific capacity. Currently, sodium iron phosphate materials are mostly prepared by co-precipitation method. However, this method has a common technical problem in the industry: during the drying process of the precursor, the rapid evaporation of moisture generates large capillary forces, resulting in uneven release of internal stress and a high likelihood of generating a large number of microcracks. At the same time, the uneven distribution of phosphorus, iron, and sodium elements, the high spatial entropy of elemental distribution, and the poor uniformity of grain orientation ultimately lead to a severe decline in the cycle life of sodium-ion batteries, limiting the industrial application of high-performance sodium iron phosphate materials.

[0003] Chinese patent application CN119079969A discloses a co-precipitation method for preparing sodium iron phosphate cathode material. The method involves first mixing a phosphorus source with a first solvent and adjusting the pH to 4-8. Then, sodium and iron source solutions are simultaneously added while maintaining a consistent pH. The co-precipitation reaction is then carried out at 95-160℃ under a protective atmosphere of 0.1-0.4 MPa for 4-10 hours. After separation, washing, and drying, the sodium iron phosphate cathode material is obtained. The technical problems this method aims to solve are simplifying the preparation process, reducing energy consumption and equipment requirements, and improving material purity and some electrochemical performance of the battery. However, it still cannot solve the problems of microcracks, uneven elemental distribution, high spatial entropy, and poor grain orientation consistency caused by drying the sodium iron phosphate precursor, making it difficult to improve the long-cycle stability of sodium-ion batteries.

[0004] Therefore, there is an urgent need to develop a new sodium iron phosphate cathode preparation technology to solve the technical problems of easy cracking, uneven element distribution, and chaotic grain orientation of sodium iron phosphate precursors in the existing technology, improve the structural integrity and electrochemical performance of materials, and meet the needs of sodium-ion battery industrial application. Summary of the Invention

[0005] To address the aforementioned technical problems, this application aims to provide a method for preparing a sodium-ion battery cathode, comprising the following steps: (1) Precursor synthesis: Iron source, phosphorus source and sodium source are dissolved in deionized water to form a first mixed solution, and the structure directing agent is dissolved in deionized water to form a second mixed solution. The structure directing agent is composed of polyhydroxycarboxylic acid derivative and nitrogen-containing heterocyclic compound in a mass ratio of 1:1.5 to 1.5:1. Under the protection of inert gas, the first mixed solution and the second mixed solution are simultaneously added dropwise to the container. The reaction temperature is controlled at 45-75℃, the pH value is 6.5-8.5, the stirring rate is 300-600rpm, and the reaction time is 4-8h to obtain the precursor slurry. (2) Gradient drying: The precursor slurry obtained in step (1) is dried. The drying process is divided into two stages: the first stage maintains a temperature of 100-120℃ and a relative humidity of 40%-60% for 0.5-1.5h; the second stage lowers the temperature to 60-80℃ at a rate of 2-3℃ / min, and the relative humidity is reduced to 5%-10% by gas replacement. The drying time is continued for 20-40min at 60-80℃ and a relative humidity of 5%-10% to obtain the dried precursor powder. (3) High temperature sintering: The dry precursor powder obtained in step (2) is mixed evenly with the carbon source coating agent, and heated to 550-750℃ at a heating rate of 2-5℃ / min under an inert atmosphere. The temperature is held for sintering for 6-12h, and after natural cooling, it is crushed and sieved to obtain the composite sodium iron phosphate cathode material matrix. (4) Preparation of finished product: The composite sodium iron phosphate cathode material matrix obtained in step (3) is mixed with conductive additives and binders in proportion, and then an organic solvent is added to disperse it evenly. After subsequent processing, it is made into a cathode sheet.

[0006] This application utilizes a composite of polyhydroxycarboxylic acid derivatives and nitrogen-containing heterocyclic compounds as a structure directing agent to construct a gradient hydrogen bond network, thereby addressing the drying cracking problem of phosphate precursors. By coupling the introduction of the structure directing agent with the gradient drying process, the entropy of phosphorus distribution in the crystal lattice is significantly reduced, while internal stress is fully released, significantly improving the precursor's crack resistance during drying. This results in a composite sodium iron phosphate cathode material with excellent crystal structure integrity and electrochemical performance. Through specific component selection and process parameter configuration, not only is capacity loss caused by traditional doping methods effectively avoided, but the structural integrity of the material after long cycling is also improved. This synergistic effect cannot be achieved by a single improvement measure. A key aspect of this application is that the structure directing agent is composed of a composite of polyhydroxycarboxylic acid derivatives and nitrogen-containing heterocyclic compounds at a mass ratio of 1:1.5 to 1.5:1. This ratio range is crucial for forming a stable gradient hydrogen bond network. If the ratio exceeds this range, the hydrogen bond network density will be insufficient or excessively strong, neither of which can effectively buffer the capillary forces during the drying process.

[0007] Preferably, the mass ratio of iron source, phosphorus source, sodium source, and deionized water in step (1) is (40-60):(35-55):(10-20):(250-450). This ratio can accurately match the stoichiometric ratio of NaFePO4 crystals, avoiding the formation of impurity phases due to excessive or insufficient raw materials, while ensuring a moderate solution concentration. This avoids uneven reaction due to excessive concentration or reduced synthesis efficiency due to excessively low concentration, laying the foundation for the formation of a uniform precursor slurry.

[0008] Preferably, the mass ratio of structure-directing agent to deionized water in step (1) is (2-8):(50-150). This concentration range allows the structure-directing agent to be fully adsorbed onto the surface of the crystal grains and to build a dense gradient hydrogen bond network in the reaction system, without causing a sharp increase in solution viscosity, hindering mass transfer, or introducing too much carbon residue due to excessive concentration. An appropriate amount of directing agent can effectively chelate metal ions and slow down the precipitation rate, thereby achieving a "locked-in" uniform distribution of phosphorus in the crystal lattice and significantly reducing the spatial distribution entropy value.

[0009] Preferably, in step (1), the iron source is selected from at least one of ferrous sulfate heptahydrate or ferrous chloride tetrahydrate; the phosphorus source is selected from at least one of ammonium dihydrogen phosphate or diammonium hydrogen phosphate; and the sodium source is selected from at least one of sodium hydroxide, sodium carbonate, or sodium acetate. The selected iron, phosphorus, and sodium sources are all water-soluble raw materials, and the ions are evenly dispersed after dissolution, which can complete the co-precipitation reaction under mild conditions, avoiding the problem of incomplete reaction caused by poorly soluble raw materials.

[0010] Preferably, the polyhydroxycarboxylic acid derivative is selected from at least one of citric acid, tartaric acid, or gluconic acid; the nitrogen-containing heterocyclic compound is selected from imidazole or melamine. The polyhydroxycarboxylic acid derivative provides abundant hydroxyl groups to form hydrogen bonds, and the nitrogen-containing heterocyclic compound can coordinate with metal ions through nitrogen atoms. The combination of the two can form a stable gradient hydrogen bond network. This network structure can reversibly shrink with the evaporation of water during the drying process, greatly buffering capillary forces and fundamentally inhibiting the generation of microcracks.

[0011] Preferably, in step (3), the mass ratio of the dried precursor powder to the carbon source coating agent is (90-150):(3-15). This ratio can form a carbon coating layer of appropriate thickness, which will not fail to improve the conductivity of the material due to insufficient carbon content, nor will it reduce the specific capacity of the battery due to excessive carbon content crowding out the space of the active material. At the same time, the uniform carbon layer can inhibit grain growth and improve the structural stability of the material.

[0012] Preferably, the carbon source coating agent is selected from at least one of glucose, sucrose, or polyvinyl alcohol. These types of carbon sources readily pyrolyze during high-temperature sintering to form a soft and highly conductive amorphous carbon layer; compared to hard carbon precursors, they can more uniformly wet the precursor surface, preventing hard agglomeration between particles during high-temperature sintering, thereby preserving the nanoscale advantages of the material, shortening the sodium ion diffusion path, and improving the electronic conductivity of the material.

[0013] Preferably, in step (4), the mass ratio of the composite sodium iron phosphate cathode material matrix, conductive additive, binder, and organic solvent is (85-95):(4-6):(3-5):(80-130). This ratio maximizes the retention of the active material proportion, ensuring battery capacity output; an appropriate amount of conductive additive can construct an efficient conductive network; and an appropriate amount of organic solvent facilitates slurry dispersion and subsequent coating processing. This ratio also ensures that the electrode film formed after curing has sufficient porosity and bonding strength, which is beneficial for electrolyte wetting and can effectively resist the volume expansion stress during charging and discharging, thereby improving the cycle life of the electrode.

[0014] Preferably, the organic solvent in step (4) is N-methylpyrrolidone. N-methylpyrrolidone has excellent solubility for positive electrode materials, conductive additives, and binders, and its high boiling point and low vapor pressure are beneficial for the slurry to form a smooth and dense coating during the coating process. In addition, the chemical inertness of N-methylpyrrolidone can prevent it from undergoing side reactions with the positive electrode material, thus ensuring the electrochemical stability of the electrode interface.

[0015] Preferably, in step (3), before heating to 550-750℃, the material is pre-fired at 200-350℃ for 1-3 hours to remove volatile organic compounds. Pre-firing provides a gentle pyrolysis path for the structure-directing agent and residual organic compounds, avoiding the rapid decomposition of organic compounds during high-temperature sintering and the resulting gas impact on the material structure, preventing grain breakage and excessive pores, while also allowing the crystals to be initially regularized.

[0016] Preferably, in step (4), the subsequent processing includes coating, drying, rolling and slitting processes.

[0017] Another objective of this application is to provide a sodium-ion battery cathode prepared using the above-described preparation method.

[0018] Another objective of this application is to provide a sodium-ion battery, including a negative electrode, a separator, an electrolyte, and the above-mentioned sodium-ion battery positive electrode, wherein the negative electrode is a hard carbon negative electrode, and the electrolyte is a polyanionic electrolyte system.

[0019] The beneficial effects are as follows: This application uses a specific ratio of polyhydroxycarboxylic acid derivatives and nitrogen-containing heterocyclic compounds to construct a gradient hydrogen bond network on the crystal nucleus surface. This network undergoes reversible structural rearrangement during moisture removal, using a molecular spring effect to buffer capillary forces and internal stresses. Combined with a two-stage gradient drying process, this minimizes the formation of microcracks in the precursor during the drying stage. The resulting positive electrode exhibits a significantly reduced crack area ratio, a significantly decreased electrode detachment rate after cycling, and a significantly improved electrode adhesion. Its structural stability is far superior to products obtained through conventional co-precipitation methods. Through the chelating and guiding effects of organic-inorganic hybrid ligands, combined with low-temperature co-precipitation conditions, a uniform arrangement of phosphorus, iron, and sodium elements is achieved within the NaFePO4 lattice. This effectively locks the spatial distribution of phosphorus, maintaining its spatial distribution entropy at an extremely low level, reducing elemental segregation and impurity phase formation, ensuring a regular crystal structure, and improving grain orientation consistency, thus providing a structural basis for excellent electrochemical performance. The prepared composite sodium iron phosphate cathode material exhibits high initial discharge capacity, significantly improved capacity retention after 1000 long cycles, and excellent 5C high-rate discharge performance. Without sacrificing capacity, it achieves simultaneous breakthroughs in long-cycle stability and rate performance, thus solving the common industry problems of rapid cycle decay and poor rate performance of existing sodium iron phosphate cathode materials. Attached Figure Description

[0020] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a SEM image of the surface morphology of the positive electrode sheet of the sodium-ion battery prepared in Example 1 of this application. Detailed Implementation

[0021] The present invention will now be described in further detail with reference to specific embodiments, so that those skilled in the art can more clearly understand the present invention.

[0022] The following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. All other embodiments obtained by those skilled in the art based on the specific embodiments of the invention without inventive effort are within the protection scope of the invention. The technical solutions of the various embodiments can be combined with each other, but only on the basis that those skilled in the art can implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the protection scope claimed by the invention.

[0023] In the embodiments of this invention, unless otherwise specified, all raw material components are commercially available products well-known to those skilled in the art; in the embodiments of this invention, unless specifically indicated, the technical means used are all conventional means well-known to those skilled in the art. Where specific experimental steps or conditions are not specified in the embodiments, they can be performed according to the conventional experimental steps or conditions described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all conventional reagent products that can be obtained commercially.

[0024] Raw material source: Ferrous sulfate heptahydrate, purchased from Shandong Xinhe New Materials Co., Ltd. Ferrous chloride tetrahydrate was purchased from Shandong Shouhua Chemical Co., Ltd. Ammonium dihydrogen phosphate, CAS No. 7722-76-1, was purchased from Jiangsu Kelunduo Food Ingredients Co., Ltd. Diammonium hydrogen phosphate, CAS No. 7783-28-0, purchased from Shandong Yueyang New Materials Co., Ltd. Sodium acetate, CAS No. 127-09-3, was purchased from Gongyi Hongyuan Environmental Protection Technology Co., Ltd. Citric acid, CAS No. 5949-29-1, purchased from Shandong Xinmingyang New Materials Co., Ltd. Tartaric acid, CAS number 87-69-4, was purchased from Henan Wanshan New Material Technology Co., Ltd. Gluconic acid, CAS No. 526-95-4, was purchased from Nantong Runfeng Petrochemical Co., Ltd. Imidazole, CAS No. 288-32-4, purchased from Hubei Langbowan Biopharmaceutical Co., Ltd. Melamine, CAS No. 108-78-1, was purchased from Shandong Shengrui Chemical Technology Co., Ltd. Polyvinyl alcohol, CAS No. 9002-89-5, was purchased from Hunan Jinyu Fine Chemical Co., Ltd. N-methylpyrrolidone (NMP), CAS No. 872-50-4, was purchased from Jinan Chengyijia Chemical Technology Co., Ltd. Conductive carbon black, CAS No. 1333-86-4, purchased from Tianjin Youmeng Chemical Technology Co., Ltd. Polyvinylidene fluoride (PVDF), brand and model: Arkema HSV900 from France, purchased from Shanghai Youcheng International Trade Co., Ltd. All other raw materials are commercially available.

[0025] Example 1 This embodiment provides a sodium-ion battery positive electrode and a battery.

[0026] A method for preparing a sodium-ion battery cathode includes the following steps: S1 Preparation of the first mixed solution: Dissolve 40 parts by mass of iron source, 35 parts by mass of phosphorus source, and 10 parts by mass of sodium source in 450 parts by mass of deionized water, and stir until completely dissolved to form the first mixed solution. In this step, the iron source is ferrous chloride tetrahydrate; the phosphorus source is diammonium hydrogen phosphate; and the sodium source is sodium carbonate.

[0027] S2 Preparation of the second mixed solution: Dissolve 2 parts of the structure-directing agent in 50 parts of deionized water and stir until completely dissolved to form the second mixed solution. The structure-directing agent is composed of a polyhydroxycarboxylic acid derivative and a nitrogen-containing heterocyclic compound in a 1:1 mass ratio. The polyhydroxycarboxylic acid derivative is tartaric acid; the nitrogen-containing heterocyclic compound is imidazole.

[0028] S3 coprecipitation reaction: In a reactor protected by inert gas (nitrogen), the first and second mixed solutions were added. The reaction temperature was controlled at 45±2℃, the pH value was maintained between 6.5 and 8.5, the stirring rate was 300 rpm, and the reaction time was 4 h to obtain the precursor slurry. During this process, the hydroxyl groups of tartaric acid and the nitrogen atoms of imidazole synergistically formed a gradient hydrogen bond network on the surface of the generated sodium iron phosphate crystal nuclei.

[0029] S4 Gradient Drying: The precursor slurry obtained in step S3 is conveyed to a spray drying tower for drying. The drying process is divided into two stages: First stage: Set the internal temperature of the spray drying tower to 100±2℃, and maintain the relative humidity of the tower environment at 40±2% by injecting water vapor into the air inlet of the drying tower. The drying time is 1.5h, so that the hydrogen bond network on the surface of the precursor particles undergoes reversible rearrangement, buffering the capillary force generated by water evaporation. Second stage: The temperature of the spray drying tower is set to decrease to 60±2℃ at a rate of 2℃ / min. At the same time, a dry inert gas (nitrogen) is introduced into the sealed container at a constant wind speed using a dehumidification device to replace the humid air in the spray drying tower, so that the relative humidity decreases to 5±2%. The drying time is continued for 40 minutes at 60±2℃ and 5±2% relative humidity to complete the deep drying and obtain the dried precursor powder. S5 High-Temperature Sintering: 90 parts by weight of the above-mentioned dried precursor powder and 3 parts by weight of carbon source coating agent were mixed evenly and placed in a tube furnace. Under an inert atmosphere, the temperature was raised to 550±2℃ at a heating rate of 2℃ / min, and sintered at this temperature for 12 hours. After natural cooling, the mixture was pulverized through a 400-mesh sieve to obtain the composite sodium iron phosphate cathode material matrix. The carbon source coating agent was glucose.

[0030] S6 Preparation of Positive Electrode Slurry: Mix 85 parts by weight of the composite sodium iron phosphate positive electrode material matrix with 4 parts by weight of the conductive additive and 3 parts by weight of the binder, then add 80 parts by weight of N-methylpyrrolidone and disperse evenly to prepare the positive electrode slurry. The conductive additive is specifically conductive carbon black, and the binder is specifically polyvinylidene fluoride.

[0031] S7 Coating and Drying: A transfer coating machine is used to coat the positive electrode slurry onto a 12μm aluminum foil current collector, forming a wet film with a thickness of 180μm. The wet film is then dried with hot air in the coating machine's oven at a relative humidity of 50±2%, a temperature of 90±2℃, an air velocity of 0.45m / s, and a drying time of 8 minutes, forming the positive electrode active material layer. After rolling, the sodium-ion battery positive electrode sheet is obtained. Figure 1 As shown, Figure 1 This is a SEM image of the positive electrode of the sodium-ion battery prepared in Example 1 of this application.

[0032] S8 Sodium-ion Battery Assembly: The sodium-ion battery positive electrode, negative electrode (specifically a hard carbon negative electrode), and separator (specifically a glass fiber membrane) prepared above are stacked and assembled into a soft-pack cell in an argon glove box (water content < 0.1 ppm, oxygen content < 0.1 ppm) in the order of positive electrode-separator-negative electrode, with an electrode alignment error < 0.5 mm. Electrolyte (specifically a polyanionic electrolyte, model TC-ENA25C, purchased from Guangzhou Tinci Advanced Materials Co., Ltd.) is injected into the cell at a rate of 1.2 g / Ah. The injected cell is then left to stand at 45±2℃ for 24 h to allow the electrolyte to fully wet the electrode and separator. Then, it is charged to 1.5V at 0.05C at 45±2℃ and left to stand for 30 min. The cell is then cooled to 10±2℃ and left to stand for 36 h to allow the electrolyte to fully penetrate into the electrode micropores at low temperature. Then, the cell was charged to 2.2V at 0.05C at -10±2℃ and allowed to stand for 30 minutes; then it was charged to 2.5V at a constant current of 0.1C and allowed to stand for 30 minutes; the cell was then heated to 25±2℃ and vacuum degassed and packaged, with a cell thickness error of <0.1mm after packaging. It was then aged at 25±2℃ for 24 hours.

[0033] Example 2 The difference between this embodiment and Embodiment 1 is that: S1 Preparation of the first mixed solution: Dissolve 50 parts by mass of iron source, 45 parts by mass of phosphorus source, and 15 parts by mass of sodium source in 350 parts by mass of deionized water, and stir until completely dissolved to form the first mixed solution. In this step, the iron source is ferrous sulfate heptahydrate; the phosphorus source is ammonium dihydrogen phosphate; and the sodium source is sodium hydroxide.

[0034] S2 Preparation of the second mixed solution: Dissolve 5 parts of the structure-directing agent in 100 parts of deionized water and stir until completely dissolved to form the second mixed solution. The structure-directing agent is composed of a polyhydroxycarboxylic acid derivative and a nitrogen-containing heterocyclic compound in a mass ratio of 1:1.5. The polyhydroxycarboxylic acid derivative is citric acid; the nitrogen-containing heterocyclic compound is imidazole.

[0035] S3 coprecipitation reaction: In a reactor filled with inert gas (nitrogen) protection, the first mixed solution and the second mixed solution are added to the reactor. The reaction temperature is controlled at 60±2℃, the pH value is maintained between 6.5 and 8.5, the stirring rate is 450rpm, and the reaction time is 6h to obtain the precursor slurry.

[0036] S4 Gradient Drying: The precursor slurry obtained in step S3 is conveyed to a spray drying tower for drying. The drying process is divided into two stages: First stage: Set the internal temperature of the spray drying tower to 110±2℃, and maintain the relative humidity of the environment inside the tower at 50±2% by injecting water vapor into the air inlet of the drying tower, with a drying time of 1 hour; Second stage: The temperature of the spray drying tower is set to decrease to 70±2℃ at a rate of 3℃ / min. At the same time, a dry inert gas (nitrogen) is introduced into the sealed container at a constant wind speed using a dehumidification device to replace the humid air in the spray drying tower, so that the relative humidity decreases to 8±2%. The drying time is continued for 30 minutes at 70±2℃ and 8±2% relative humidity to complete the deep drying and obtain the dried precursor powder. S5 High-Temperature Sintering: 120 parts by weight of the above-mentioned dried precursor powder and 9 parts by weight of carbon source coating agent were mixed evenly and placed in a tube furnace. Under an inert atmosphere, the temperature was raised to 650±2℃ at a heating rate of 3℃ / min, and sintered at this temperature for 9 hours. After natural cooling, the mixture was pulverized and passed through a 400-mesh sieve to obtain the composite sodium iron phosphate cathode material matrix. The carbon source coating agent was sucrose.

[0037] S6 Preparation of positive electrode slurry: According to the mass ratio, 90 parts of composite sodium iron phosphate positive electrode material matrix, 5 parts of conductive additive, and 4 parts of binder are mixed in proportion, and 110 parts of N-methylpyrrolidone are added and dispersed evenly to prepare positive electrode slurry.

[0038] Example 3 The difference between this embodiment and Embodiment 1 is that: S1 Preparation of the first mixed solution: Dissolve 60 parts by mass of iron source, 55 parts by mass of phosphorus source, and 20 parts by mass of sodium source in 250 parts by mass of deionized water, and stir until completely dissolved to form the first mixed solution. In this step, the iron source is composed of ferrous sulfate heptahydrate and ferrous chloride tetrahydrate in a mass ratio of 1:1; the phosphorus source is composed of ammonium dihydrogen phosphate and diammonium hydrogen phosphate in a mass ratio of 1:1; and the sodium source is sodium acetate.

[0039] S2 Preparation of the second mixed solution: Dissolve 8 parts of the structure-directing agent in 150 parts of deionized water and stir until completely dissolved to form the second mixed solution. The structure-directing agent is composed of a polyhydroxycarboxylic acid derivative and a nitrogen-containing heterocyclic compound in a mass ratio of 1.5:1. The polyhydroxycarboxylic acid derivative is gluconic acid; the nitrogen-containing heterocyclic compound is melamine.

[0040] S3 coprecipitation reaction: In a reactor filled with inert gas (argon) protection, the first mixed solution and the second mixed solution are added to the reactor. The reaction temperature is controlled at 75±2℃, the pH value is maintained between 6.5 and 8.5, the stirring rate is 600rpm, and the reaction time is 8h to obtain the precursor slurry.

[0041] S4 Gradient Drying: The precursor slurry obtained in step S3 is conveyed to a spray drying tower for drying. The drying process is divided into two stages: First stage: Set the internal temperature of the spray drying tower to 120±2℃, and maintain the relative humidity of the environment inside the tower at 60±2% by injecting water vapor into the air inlet of the drying tower, with a drying time of 0.5h; Second stage: The temperature of the spray drying tower is set to decrease to 80±2℃ at a rate of 3℃ / min. At the same time, a dry inert gas (argon) is introduced into the sealed container at a constant wind speed using a dehumidification device to replace the humid air in the spray drying tower, so that the relative humidity decreases to 10%. The drying time is continued for 20 minutes at 80±2℃ and relative humidity of 10±2% to complete the deep drying and obtain the dried precursor powder. S5 High-Temperature Sintering: 150 parts by weight of the above-mentioned dried precursor powder and 15 parts by weight of carbon source coating agent were mixed evenly and placed in a tube furnace. Under an inert atmosphere, the temperature was raised to 750±2℃ at a heating rate of 5℃ / min, and sintered at this temperature for 6 hours. After natural cooling, the mixture was pulverized through a 400-mesh sieve to obtain the composite sodium iron phosphate cathode material matrix. The carbon source coating agent was polyvinyl alcohol.

[0042] S6 Preparation of positive electrode slurry: According to the mass ratio, 95 parts of composite sodium iron phosphate positive electrode material matrix, 6 parts of conductive additive, and 5 parts of binder are mixed in proportion, and 130 parts of N-methylpyrrolidone are added and dispersed evenly to prepare positive electrode slurry.

[0043] Comparative Example 1 The difference between this comparative example and Example 2 is that no structure-directing agent is added at all, and the step of preparing the second mixed solution in S2 is omitted. In S3, only the first mixed solution (iron, phosphorus, and sodium sources) is added dropwise to the reactor, without adding the second mixed solution. All other components, process parameters, and operating steps are consistent with Example 2.

[0044] Comparative Example 2 The difference between this comparative example and Example 2 is that the mass ratio of citric acid to imidazole in the structure-directing agent is adjusted to 1:4. All other components, process parameters, and operating steps remain the same as in Example 2.

[0045] Comparative Example 3 The difference between this comparative example and Example 2 is that only citric acid is added as a structure-directing agent in step S2, and imidazole is not added. The amount of citric acid used is the same as the total amount of structure-directing agent used in Example 2 (5 parts), and it is dissolved in 100 parts of deionized water to form a second mixed solution. All other components, process parameters, and operating steps are consistent with those in Example 2.

[0046] Comparative Example 4 The difference between this comparative example and Example 2 is that only imidazole is added as a structure-directing agent in step S2, and citric acid is not added. The amount of imidazole used is the same as the total amount of structure-directing agent used in Example 2 (5 parts), and it is dissolved in 100 parts of deionized water to form a second mixed solution. The remaining components, process parameters, and operating steps are consistent with those of Example 2.

[0047] Comparative Example 5 The difference between this comparative example and Example 2 is that the relative humidity in step S4, the gradient drying process, is maintained at 8±2% until drying is complete. All other components, process parameters, and operating steps remain the same as in Example 2.

[0048] Comparative Example 6 The difference between this comparative example and Example 2 is as follows: Step S3 coprecipitation reaction: In a reactor filled with inert gas (nitrogen), the first mixed solution and the second mixed solution are added to the reactor, and the reaction temperature is controlled at 95±2℃. The remaining components, process parameters, and operating steps are consistent with those of Example 2.

[0049] Test method: Phosphorus distribution entropy: A surface scan was performed using an energy dispersive spectroscopy (EDS) instrument equipped with a field emission scanning electron microscope (FE-SEM-EDS, S4800, 15.0 kV). The scanned area was 50 μm × 50 μm, with a pixel resolution of 512 × 512. The spatial distribution entropy was calculated using an improved information entropy algorithm: H = - ,in Entropy represents the normalized proportion of the phosphorus element feature X-ray intensity of the i-th pixel to the total phosphorus element intensity of the entire region. A lower entropy value indicates a more uniform distribution.

[0050] Electrode crack area ratio: The coating appearance evaluation method was based on GB / T 24533-2019 "Graphite Anode Materials for Lithium-ion Batteries," with adaptive adjustments made to suit the characteristics of the cathode material. Scanning electron microscopy (SEM) was used to photograph the surface and cross-sectional morphology of the cathode electrode (200x magnification for surface and 500x magnification for cross-section). ImageJ software was used for binarization and image analysis to calculate the crack area ratio. Ten fields of view were randomly selected for each sample, and the average value was taken.

[0051] Electrochemical performance testing: Referring to GB / T 44265-2024 Technical Specifications for Sodium-ion Batteries in Power Storage Stations, the initial discharge capacity was tested at 0.2C at 25℃, and the capacity retention rate was tested after 1000 cycles at 0.5C. Discharge capacities at different rates (0.2C, 0.5C, 1C, 2C, 5C) were also tested. The 5C capacity retention rate was calculated as: 5C discharge capacity / 0.2C discharge capacity * 100%. Three parallel sets of batteries were tested for each sample, and the average value was taken. The relative standard deviation (RSD) was controlled within 5%.

[0052] Electrode adhesion: The adhesion of the electrode was tested using a pressure-sensitive adhesive tape holding power tester. The electrode detachment rate after cycling was calculated as the ratio of the difference in electrode mass before and after cycling to the initial mass of the active material.

[0053] Table 1. Performance test results of samples from each embodiment and comparative example.

[0054] As shown in the table above, Examples 1-3, using different combinations of raw materials and process conditions within the range of process parameters, all exhibited phosphorus distribution entropy values ​​between 3.14 and 3.35 bits, electrode crack area ratio ≤ 2.8%, initial discharge capacity ≥ 142.8 mAh / g, capacity retention rate ≥ 80.7% after 1000 cycles, 5C rate capacity retention rate ≥ 72.2%, electrode adhesion ≥ 9.4 N / mm, and detachment rate ≤ 2.9% after 1000 cycles. The data dispersion among the examples was small and far superior to that of the comparative examples, demonstrating that the preparation method provided by this invention has a good process window and repeatability stability.

[0055] In summary, the phosphorus distribution entropy of the cathode material obtained in Comparative Example 1 (without structure-directing agent) increased to 5.62 bits, the electrode crack area ratio reached 6.2%, the initial discharge capacity was only 108.4 mAh / g, and the capacity retention rate after 1000 cycles decreased to 62.4%. This indicates that without the structure-directing agent, the drying stress of the precursor cannot be mitigated, the elemental distribution is uneven, and the electrochemical performance of the material is severely degraded. The phosphorus distribution entropy values ​​of Comparative Example 3 (only the polyhydroxycarboxylic acid derivative citric acid was added) and Comparative Example 4 (only the nitrogen-containing heterocyclic compound imidazole was added) were 5.39 bits and 4.57 bits, respectively, and the crack area ratios were both higher than 5.9%. Their performance was also significantly lower than that of the examples, indicating that a single component cannot construct a stable gradient hydrogen bond network. Only the polyhydroxycarboxylic acid derivative provides hydroxyl hydrogen bond sites, and the nitrogen-containing heterocyclic compound provides metal ion coordination sites. The combination of the two can synergistically buffer capillary forces and lock the phosphorus distribution.

[0056] In Comparative Example 2, the mass ratio of the polyhydroxycarboxylic acid derivative to the nitrogen-containing heterocyclic compound was 1:4. Its phosphorus distribution entropy value increased to 4.47 bits, the crack area ratio was 5.8%, and the capacity retention rate after 1000 cycles was 72.7%. All indicators were worse than those of Example 2 (entropy value 3.14 bits, crack area ratio 2.1%, cycle retention rate 82.4%). This confirms that when the compounding ratio exceeds the protection range, the hydrogen bond network density is inappropriate and cannot effectively relieve drying stress, thus verifying the rationality of the ratio range.

[0057] Comparative Example 5 omitted gradient humidity control and maintained a relative humidity of 8±2% throughout the process. Its phosphorus element distribution entropy value increased to 4.68 bits, crack area ratio was 5.5%, and capacity retention rate after 1000 cycles was 76.3%. Although it was slightly better than the group without structure-directing agent, it was still significantly weaker than Example 2. This indicates that there is a synergistic effect between gradient drying and structure-directing agent, which together ensures the integrity of the precursor.

[0058] Comparative Example 6 increased the coprecipitation reaction temperature to 95±2℃, exceeding the 45-75℃ limit of this invention. The phosphorus element distribution entropy value was as high as 6.26bit, the crack area ratio was 7.2%, the initial discharge capacity was 110.6mAh / g, and the cycle retention rate was only 70.1%. This indicates that excessively high temperature will destroy the regulatory effect of the structure guiding agent on crystal nucleus growth, leading to element segregation and abnormal grain growth.

[0059] In summary, this application addresses the technical challenges of traditional sodium iron phosphate cathode material preparation, such as microcracks during precursor drying, uneven distribution of phosphorus, iron, and sodium elements, high spatial distribution entropy, and poor grain orientation consistency, by employing a structure-directing agent formed by compounding polyhydroxycarboxylic acid derivatives and nitrogen-containing heterocyclic compounds in a specific mass ratio. This is achieved by coupling low-temperature co-precipitation, two-stage gradient drying, and segmented high-temperature sintering processes, thus solving these problems at the precursor molecular assembly mechanism level. Experimental data show that the composite sodium iron phosphate cathode material prepared in this application exhibits significantly reduced phosphorus distribution entropy, a substantial decrease in electrode crack area ratio and post-cycle shedding rate, and a marked improvement in electrode adhesion. Furthermore, it possesses high initial discharge capacity, excellent long-cycle stability, and high-rate discharge performance, effectively enhancing the overall electrochemical performance and structural stability of sodium-ion batteries.

[0060] The above are merely embodiments of the present invention and are not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of the present invention should be included within the scope of the claims of the present invention.

Claims

1. A method for preparing a sodium-ion battery cathode, characterized in that, Includes the following steps: (1) Precursor synthesis: Iron source, phosphorus source and sodium source are dissolved in deionized water to form a first mixed solution, and a structure directing agent is dissolved in deionized water to form a second mixed solution. The structure directing agent is composed of a polyhydroxycarboxylic acid derivative and a nitrogen-containing heterocyclic compound in a mass ratio of 1:1.5 to 1.5:

1. Under the protection of an inert gas, the first mixed solution and the second mixed solution are simultaneously added dropwise to a container. The reaction temperature is controlled at 45-75℃, the pH value is 6.5-8.5, the stirring rate is 300-600rpm, and the reaction time is 4-8h to obtain a precursor slurry. (2) Gradient drying: The precursor slurry obtained in step (1) is dried. The drying process is divided into two stages: the first stage maintains a temperature of 100-120℃ and a relative humidity of 40%-60% for 0.5-1.5h; the second stage lowers the temperature to 60-80℃ at a rate of 2-3℃ / min, while the relative humidity is lowered to 5%-10% through gas replacement. The drying time is continued for 20-40min at 60-80℃ and 5%-10% relative humidity to obtain dried precursor powder. (3) High temperature sintering: The dry precursor powder obtained in step (2) is mixed evenly with the carbon source coating agent, and heated to 550-750℃ at a heating rate of 2-5℃ / min under an inert atmosphere. The temperature is held for sintering for 6-12h, and after natural cooling, it is crushed and sieved to obtain the composite sodium iron phosphate cathode material matrix. (4) Preparation of finished product: The composite sodium iron phosphate cathode material matrix obtained in step (3) is mixed with conductive additives and binders in proportion, and then an organic solvent is added to disperse it evenly. After subsequent processing, it is made into a cathode sheet.

2. The method for preparing a sodium-ion battery cathode according to claim 1, characterized in that, The mass ratio of iron source, phosphorus source, sodium source and deionized water in step (1) is (40-60):(35-55):(10-20):(250-450).

3. The method for preparing a sodium-ion battery cathode according to claim 1, characterized in that, The mass ratio of the structure-directing agent and deionized water in step (1) is (2-8):(50-150).

4. The method for preparing a sodium-ion battery cathode according to claim 1, characterized in that, The iron source in step (1) is selected from at least one of ferrous sulfate heptahydrate or ferrous chloride tetrahydrate; the phosphorus source is selected from at least one of ammonium dihydrogen phosphate or diammonium hydrogen phosphate; and the sodium source is selected from at least one of sodium hydroxide, sodium carbonate or sodium acetate.

5. The method for preparing a sodium-ion battery cathode according to claim 1, characterized in that, The polyhydroxycarboxylic acid derivative in step (1) is selected from at least one of citric acid, tartaric acid or gluconic acid; the nitrogen-containing heterocyclic compound is selected from imidazole or melamine.

6. The method for preparing a sodium-ion battery cathode according to claim 1, characterized in that, The mass ratio of the dried precursor powder to the carbon source coating agent in step (3) is (90-150):(3-15).

7. The method for preparing a sodium-ion battery cathode according to claim 1, characterized in that, The carbon source coating agent is selected from at least one of glucose, sucrose, or polyvinyl alcohol.

8. The method for preparing a sodium-ion battery cathode according to claim 1, characterized in that, The mass ratio of the composite sodium iron phosphate cathode material matrix, conductive additive, binder and organic solvent in step (4) is (85-95):(4-6):(3-5):(80-130).

9. A sodium-ion battery positive electrode, characterized in that, It is prepared by any one of the preparation methods described in claims 1 to 8.

10. A sodium-ion battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The positive electrode is the sodium-ion battery positive electrode according to claim 9, the negative electrode is a hard carbon negative electrode, and the electrolyte is a polyanion electrolyte system.

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