Positive electrode active material and preparation method thereof, positive electrode plate and sodium ion secondary battery

Abstract

The invention provides a positive active material and a preparation method thereof, a positive pole piece and a sodium ion secondary battery, and particularly relates to the technical field of sodium ion secondary batteries. The general formula of the positive electrode active material is Na < 4 > Fe < 3-x-y > M < y > (PO4) < 2 > P < 2 > O < 7 > (at) RaOb; wherein the value range of x is 0.03 to 0.27, and the value range of y is 0 to 0.27 and is not 0; m is at least one of transition metal elements; raOb is a metal oxide. According to the positive electrode active material provided by the invention, transition metal elements are introduced for doping, so that the band gap width of the material is effectively reduced, and the ionic conductivity of the material is remarkably improved; and meanwhile, a metal oxide surface coating technology is adopted, so that the energy density and the cycling stability of the material are greatly improved. Through the synergistic effect of transition metal element doping and metal oxide coating, the positive electrode active material shows excellent electrochemical performance.

Description

Technical Field

[0001] The present invention relates to the technical field of sodium ion secondary batteries, and in particular to a positive electrode active material and a preparation method thereof, a positive electrode plate and a sodium ion secondary battery. Background Art

[0002] In recent years, lithium-ion batteries have become an important power source for energy storage due to their high energy density and long cycle life. However, lithium resource scarcity, high cost, and safety issues have limited their large-scale application. In contrast, sodium, a highly abundant and inexpensive element, belongs to the same main group as lithium and shares a similar energy storage mechanism. Therefore, sodium-ion batteries offer broad application prospects in large-scale energy storage.

[0003] In sodium-ion batteries, the specific capacity of cathode materials is typically lower than that of anode materials, a key factor restricting the commercial development of high-energy-density sodium-ion batteries. Therefore, the development of sodium-storage cathode materials with high capacity, high voltage, and high reversibility is crucial. Currently, common sodium-ion battery cathode materials include layered oxides, polyanions, Prussian blue, and organic materials. Among them, polyanion cathode materials have attracted considerable attention due to their unique advantages: a three-dimensional open framework structure, excellent structural stability provided by XO (X = P, S, Si, or B) covalent bonds, minimal volume change during sodium ion insertion and extraction, low raw material cost, and a simple preparation process. However, these materials also suffer from disadvantages such as low electronic conductivity and insufficient electrochemical activity.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] One of the objectives of the present invention is to provide a positive electrode active material, aiming to solve at least one of the above-mentioned technical problems in the prior art.

[0006] A second object of the present invention is to provide a method for preparing a positive electrode active material.

[0007] A third object of the present invention is to provide a positive electrode plate.

[0008] A fourth object of the present invention is to provide a sodium ion secondary battery.

[0009] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are adopted: The first aspect of the present invention provides a positive electrode active material, the general formula of the positive electrode active material is Na4Fe 3-x-y M y (PO4)2P2O7@R a O b; wherein, x ranges from 0.03 to 0.27, y ranges from 0 to 0.27 and is not 0; M is at least one transition metal element; R a O b Includes at least one of Al2O3, SiO2, TiO2, ZrO2, Y2O3, MgO, and ZnO.

[0010] Furthermore, M includes at least one of Mn, Ni, Co, Cu, Ti, Zn, Zr, Cr, Nb, and Sc.

[0011] Furthermore, the positive electrode active material has a NASICON structure.

[0012] The second aspect of the present invention provides a method for preparing the positive electrode active material, comprising the following steps: A. mixing a sodium source, a phosphorus source, an iron source, an M source, and a carbon source in stoichiometric proportions, and dispersing the mixture with a solvent to prepare a precursor solution; then drying the precursor solution to remove the solvent to obtain a precursor; B. heat-treating the precursor in an inert reducing atmosphere to obtain Na4Fe 3-x-y M y (PO4)2P2O7; C, R a O b Coated on the Na4Fe 3-x-y M y (PO4)2P2O7 to obtain the positive electrode active material.

[0013] Furthermore, the sodium source includes at least one of sodium dihydrogen phosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium nitrate, sodium sulfate, sodium bisulfate and sodium citrate, preferably sodium dihydrogen phosphate.

[0014] Preferably, the phosphorus source includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, disodium hydrogen phosphate and sodium dihydrogen phosphate, preferably sodium dihydrogen phosphate.

[0015] Preferably, the iron source includes at least one of ferrous acetate, ferric nitrate, ferrous oxalate and ferrous sulfate, preferably ferric nitrate.

[0016] Preferably, the M source includes at least one of oxidative M, acetic M, sulfuric M, chlorinated M, nitric M and dihydrogen phosphate M, preferably acetic M.

[0017] Preferably, the carbon source comprises at least one of glucose, starch, anhydrous citric acid, ascorbic acid, polydopamine, polyvinyl alcohol and polyacrylonitrile, preferably anhydrous citric acid.

[0018] Preferably, the solvent includes at least one of deionized water, methanol, ethanol and ethylene glycol, preferably deionized water.

[0019] Preferably, the concentration of the carbon source in the precursor solution is 1-10 wt %.

[0020] Furthermore, in step A, the drying method includes spray drying, forced air drying, vacuum drying or freeze drying, preferably spray drying.

[0021] Preferably, in step B, the inert reducing atmosphere is a combination of hydrogen and nitrogen or a combination of hydrogen and argon.

[0022] Preferably, in step B, the volume proportion of the hydrogen in the inert reducing atmosphere is 3-7%.

[0023] Preferably, the heat treatment includes a pre-firing process and a calcining process performed in sequence.

[0024] Preferably, the pre-sintering process is: heating to 250-350° C. at a heating rate of 1-5° C. / min and keeping the temperature for 3-10 hours.

[0025] Preferably, the calcination process is: heating to 500-600° C. at a heating rate of 1-5° C. / min, keeping the temperature for 5-10 hours, and cooling with the furnace.

[0026] Furthermore, in step C, the coating process is as follows: 3-x-y M y (PO4)2P2O7 and the R a O b Mix well and dry.

[0027] Preferably, the mixing method includes ball milling, rotary evaporation, mechanical stirring or atomic layer deposition.

[0028] Preferably, the drying method includes forced air drying, vacuum drying or freeze drying, preferably vacuum drying.

[0029] A third aspect of the present invention provides a positive electrode plate, comprising an aluminum foil and a positive electrode active material layer coated on the aluminum foil; the positive electrode active material layer comprises a conductive agent, a binder and the positive electrode active material.

[0030] Furthermore, the positive electrode active material layer includes, by weight, 8 to 12 parts of a conductive agent, 8 to 12 parts of a binder, and 80 parts of a positive electrode active material.

[0031] Preferably, the conductive agent includes acetylene black.

[0032] A fourth aspect of the present invention provides a sodium ion secondary battery comprising the aforementioned positive electrode plate.

[0033] Compared with the prior art, the present invention has at least the following beneficial effects: The positive electrode active material provided by the present invention effectively reduces the band gap width of the material and significantly improves its ionic conductivity by introducing transition metal elements into the sodium ferric phosphate pyrophosphate matrix. At the same time, the metal oxide surface coating technology is used to suppress side reactions at the electrode / electrolyte interface, thereby significantly improving the energy density and cycle stability of the material. The synergistic effect of transition metal element doping and metal oxide coating enables the positive electrode active material to exhibit excellent electrochemical performance. Test results show that the material has a specific capacity of up to 115 mAh / g and a specific capacity of 2.0×10 -3 The conductivity of the battery is 2.5 S / cm, and it can still maintain a capacity retention rate of 91.8% after 5000 cycles, fully demonstrating its excellent structural stability and electrochemical reversibility.

[0034] The method for preparing positive electrode active materials provided by the present invention features a short process flow, simple operation, and large single-batch processing capacity, significantly improving production efficiency and reducing production costs. Furthermore, the method has moderate equipment requirements, requiring no complex precision instruments, and can achieve stable production using conventional industrial equipment. This method exhibits good process adaptability and scalability, providing reliable technical support for the industrial application of positive electrode materials for sodium-ion batteries.

[0035] The positive electrode plate provided by the present invention, in view of the advantages of the above-mentioned positive electrode active material, enables the prepared positive electrode plate to have higher energy density, better rate performance, longer cycle life and higher safety.

[0036] The sodium ion secondary battery provided by the present invention, in view of the advantages of the above-mentioned positive electrode sheet, enables the prepared sodium ion secondary battery to be better applied in the field of large-scale energy storage, broadens the application fields of sodium ion secondary batteries, and promotes the development of downstream industries. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0038] Figure 1 To characterize the XRD spectrum obtained in Example 1; Figure 2 This is the SEM image obtained by characterizing Example 2; Figure 3 This is the cycle performance diagram obtained for Test Example 2; Figure 4 This is the rate performance diagram obtained in Test Example 3. DETAILED DESCRIPTION

[0039] In order to make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0040] Hereinafter, the terms "including", "having" and their cognates, which may be used in various embodiments of the present invention, are intended only to indicate specific features, numbers, steps, operations, elements, components or combinations of the foregoing items, and should not be understood as first excluding the existence of one or more other features, numbers, steps, operations, elements, components or combinations of the foregoing items or the possibility of adding one or more features, numbers, steps, operations, elements, components or combinations of the foregoing items.

[0041] The first aspect of the present invention provides a positive electrode active material, the general formula of the positive electrode active material is Na4Fe 3-x-y M y (PO4)2P2O7@R a O b ; wherein, x ranges from 0.03 to 0.27, y ranges from 0 to 0.27 and is not 0; M is at least one transition metal element; R a O b Includes at least one of Al2O3, SiO2, TiO2, ZrO2, Y2O3, MgO, and ZnO.

[0042] The positive electrode active material provided by the present invention effectively reduces the band gap width of the material and significantly improves its ionic conductivity by introducing transition metal elements into the sodium ferric phosphate pyrophosphate matrix. At the same time, the metal oxide surface coating technology is used to suppress side reactions at the electrode / electrolyte interface, thereby significantly improving the energy density and cycle stability of the material. The synergistic effect of transition metal element doping and metal oxide coating enables the positive electrode active material to exhibit excellent electrochemical performance. Test results show that the material has a specific capacity of up to 115 mAh / g and a specific capacity of 2.0×10 -3 The conductivity of the battery is 2.5 S / cm, and it can still maintain a capacity retention rate of 91.8% after 5000 cycles, fully demonstrating its excellent structural stability and electrochemical reversibility.

[0043] Typical, but not limiting, Na4Fe 3-x-y M y (PO4)2P2O7@R a O b, x may be, for example, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26 or 0.27, or any value within the range of 0.03 to 0.27. y may be, for example, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, or 0.27, or any value within the range of 0 to 0.27 except 0.

[0044] Furthermore, M includes at least one of Mn, Ni, Co, Cu, Ti, Zn, Zr, Cr, Nb, and Sc.

[0045] Furthermore, the positive electrode active material has a NASICON structure.

[0046] The second aspect of the present invention provides a method for preparing the positive electrode active material, comprising the following steps: A. mixing a sodium source, a phosphorus source, an iron source, an M source, and a carbon source in stoichiometric proportions, and dispersing the mixture with a solvent to prepare a precursor solution; then drying the precursor solution to remove the solvent to obtain a precursor; B. heat-treating the precursor in an inert reducing atmosphere to obtain Na4Fe 3-x-y M y (PO4)2P2O7; C, R a O b Coated on the Na4Fe 3-x-y M y (PO4)2P2O7 to obtain the positive electrode active material.

[0047] The method for preparing positive electrode active materials provided by the present invention features a short process flow, simple operation, and large single-batch processing capacity, significantly improving production efficiency and reducing production costs. Furthermore, the method has moderate equipment requirements, requiring no complex precision instruments, and can achieve stable production using conventional industrial equipment. This method exhibits good process adaptability and scalability, providing reliable technical support for the industrial application of positive electrode materials for sodium-ion batteries.

[0048] Furthermore, the sodium source includes at least one of sodium dihydrogen phosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium nitrate, sodium sulfate, sodium bisulfate and sodium citrate, preferably sodium dihydrogen phosphate.

[0049] Preferably, the phosphorus source includes at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, disodium hydrogen phosphate and sodium dihydrogen phosphate, preferably sodium dihydrogen phosphate.

[0050] Preferably, the iron source includes at least one of ferrous acetate, ferric nitrate, ferrous oxalate and ferrous sulfate, preferably ferric nitrate.

[0051] Preferably, the M source includes at least one of oxidative M, acetic M, sulfuric M, chlorinated M, nitric M and dihydrogen phosphate M, preferably acetic M.

[0052] Preferably, the carbon source comprises at least one of glucose, starch, anhydrous citric acid, ascorbic acid, polydopamine, polyvinyl alcohol and polyacrylonitrile, preferably anhydrous citric acid.

[0053] Preferably, the solvent includes at least one of deionized water, methanol, ethanol and ethylene glycol, preferably deionized water.

[0054] Preferably, the concentration of the carbon source in the precursor solution is 1-10 wt %.

[0055] The purpose of adding a carbon source to the precursor solution is to decompose it to generate carbon during the sintering process, forming a uniform carbon layer on the surface of the material, preventing the material from oxidation, enhancing stability and improving conductivity; the carbon source can act as an organic reducing agent to reduce metal salts to low-valent oxides, promoting the formation of target products; the weak acidity of the carbon source can also adjust the pH value of the reaction system and optimize the synthesis conditions; at the same time, the carbon source can also act as a dispersant adsorbed on the surface of the particles, reducing particle aggregation through electrostatic repulsion or steric hindrance effects, and ensuring product uniformity.

[0056] Typically but not limitatively, the concentration of the carbon source in the precursor solution can be, for example, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt% or 10 wt%, or any value within the range of 1 to 10 wt%.

[0057] Furthermore, in step A, the drying method includes spray drying, forced air drying, vacuum drying or freeze drying, preferably spray drying.

[0058] Preferably, in step B, the inert reducing atmosphere is a combination of hydrogen and nitrogen or a combination of hydrogen and argon.

[0059] Preferably, in step B, the volume proportion of the hydrogen in the inert reducing atmosphere is 3% to 7%. Typically, but not limiting, the volume proportion of the hydrogen in the inert reducing atmosphere may be, for example, 3%, 4%, 5%, 6%, or 7%, or any value within the range of 3% to 7%.

[0060] Preferably, the heat treatment includes a pre-sintering process and a calcining process performed in sequence. It should be noted that after the pre-sintering process is completed, the material is cooled to room temperature in the furnace and then the subsequent calcining process is performed.

[0061] Preferably, the pre-sintering process comprises: heating to 250-350°C at a heating rate of 1-5°C / min and holding the temperature for 3-10 hours. Typically, but not limiting, the heating rate may be, for example, 1°C / min, 2°C / min, 3°C / min, 4°C / min, or 5°C / min, or any value within the range of 1°C / min-5°C / min; the temperature after heating may be, for example, 250°C, 260°C, 270°C, 280°C, 290°C, 300°C, 310°C, 320°C, 330°C, 340°C, or 350°C, or any value within the range of 250-350°C; and the holding time may be, for example, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours, or any value within the range of 3 hours-10 hours.

[0062] Preferably, the calcination process is: heating to 500-600° C. at a heating rate of 1-5° C. / min, keeping the temperature for 5-10 hours, and cooling with the furnace.

[0063] Typically but not limitatively, the heating rate may be, for example, 1°C / min, 2°C / min, 3°C / min, 4°C / min or 5°C / min, or any value within the range of 1°C / min to 5°C / min; the temperature after heating may be, for example, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C or 600°C, or any value within the range of 500°C to 600°C; the holding time may be, for example, 5h, 6h, 7h, 8h, 9h or 10h, or any value within the range of 5h to 10h.

[0064] Furthermore, in step C, the coating process is as follows: 3-x-y M y (PO4)2P2O7 and the R a O b Mix well and dry.

[0065] Preferably, the mixing method includes ball milling, rotary evaporation, mechanical stirring or atomic layer deposition.

[0066] Preferably, the drying method includes forced air drying, vacuum drying or freeze drying, preferably vacuum drying.

[0067] A third aspect of the present invention provides a positive electrode plate, comprising an aluminum foil and a positive electrode active material layer coated on the aluminum foil; the positive electrode active material layer comprises a conductive agent, a binder and the positive electrode active material.

[0068] The positive electrode plate provided by the present invention, in view of the advantages of the above-mentioned positive electrode active material, enables the prepared positive electrode plate to have higher energy density, better rate performance, longer cycle life and higher safety.

[0069] Furthermore, the positive electrode active material layer includes, by weight, 8 to 12 parts of a conductive agent, 8 to 12 parts of a binder, and 80 parts of a positive electrode active material.

[0070] Typically but not limiting, the amount of the conductive agent can be, for example, 8 parts, 9 parts, 10 parts, 11 parts or 12 parts, or any value within the range of 8 to 12 parts; the amount of the binder can be, for example, 8 parts, 9 parts, 10 parts, 11 parts or 12 parts, or any value within the range of 8 to 12 parts; the amount of the positive electrode active material is 80 parts.

[0071] Preferably, the conductive agent includes acetylene black.

[0072] A fourth aspect of the present invention provides a sodium ion secondary battery comprising the aforementioned positive electrode plate.

[0073] The sodium ion secondary battery provided by the present invention, in view of the advantages of the above-mentioned positive electrode sheet, enables the prepared sodium ion secondary battery to be better applied in the field of large-scale energy storage, broadens the application fields of sodium ion secondary batteries, and promotes the development of downstream industries.

[0074] The present invention is further illustrated below by specific examples and comparative examples. However, it should be understood that these examples are merely for the purpose of further explanation and should not be construed as limiting the present invention in any form. The raw materials used in the examples and comparative examples of the present invention, unless otherwise specified, were prepared under conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used without manufacturer's indication are all commercially available conventional products.

[0075] Example 1 This embodiment provides a positive electrode active material. 20 mmol of anhydrous citric acid, 14.55 mmol of ferric nitrate nonahydrate, 0.45 mmol of cobalt acetate tetrahydrate, and 20 mmol of sodium dihydrogen phosphate dihydrate are dissolved in 30 mL of deionized water, stirred evenly, and spray-dried to obtain a precursor powder.

[0076] The precursor powder was heated to 300°C in a tube furnace at a heating rate of 2°C / min and kept at this temperature for 6 hours, then heated to 550°C at a heating rate of 2°C / min and kept at this temperature for 8 hours. The whole process was protected by argon-hydrogen mixed gas (hydrogen volume accounted for 5%), and cooled with the furnace to obtain Na4Fe 2.91 Co 0.09 (PO4)2P2O7.

[0077] Dissolve zinc nitrate Zn(NO3)2 in ethanol to obtain zinc salt solution, and then mix the zinc salt solution with Na4Fe 2.91 Co 0.09 (PO4)2P2O7 powders were mixed in a mass ratio of 95:5 and rotary evaporated.

[0078] The temperature of the rotary evaporation was 50-60 ° C, the speed was controlled at 100-200 rpm, and the mixture was ensured to be evenly stirred during the evaporation process. The vacuum degree was 0.08-0.1 MPa to obtain the surface-coated product and obtain the positive electrode active material Na4Fe 2.91 Co 0.09 (PO4)2P2O7@ZnO.

[0079] Example 2 This example provides a positive electrode active material. 20 mmol of anhydrous citric acid, 12.3 mmol of ferric nitrate nonahydrate, 0.6 mmol of manganese acetate tetrahydrate, 0.75 mmol of nickel acetate tetrahydrate, 1.35 mmol of cobalt acetate tetrahydrate, and 20 mmol of sodium dihydrogen phosphate dihydrate are dissolved in 30 mL of deionized water. The mixture is stirred and spray-dried to obtain a precursor powder.

[0080] The precursor powder was heated to 350°C in a tube furnace at a heating rate of 4°C / min and kept at that temperature for 4 hours, then heated to 600°C at a heating rate of 4°C / min and kept at that temperature for 6 hours. The whole process was protected by argon-hydrogen mixture (hydrogen volume ratio was 5%) and cooled with the furnace. Finally, the obtained material was coated with TiO2 at a ratio of 95:5 to obtain Na4Fe 2.46 Co 0.27 Ni 0.15 Mn0. 12 (PO4)2P2O7@TiO2.

[0081] Example 3 This embodiment provides a positive electrode active material, which is prepared by dissolving 20 mmol of anhydrous citric acid, 13.8 mmol of ferric nitrate nonahydrate, 0.75 mmol of nickel acetate tetrahydrate, 0.45 mmol of cobalt acetate tetrahydrate, and 20 mmol of sodium dihydrogen phosphate dihydrate in 30 mL of deionized water, stirring the mixture uniformly, and spray drying the mixture to obtain a precursor powder.

[0082] The precursor powder was heated to 250°C in a tube furnace at a heating rate of 2°C / min and kept at this temperature for 8 hours, then heated to 550°C at a heating rate of 2°C / min and kept at this temperature for 10 hours. The whole process was protected by an argon-hydrogen mixture (hydrogen volume ratio was 5%) and cooled in the furnace. The obtained material was mixed with Al2O3 in a ratio of 95:0.5, uniformly mixed by mechanical ball milling for 30 minutes, and then dried to obtain Na4Fe 2.76 Co 0.09 Ni 0.15 (PO4)2P2O7@Al2O3.

[0083] Example 4 This embodiment provides a positive electrode active material, which is prepared by dissolving 20 mmol of anhydrous citric acid, 13.8 mmol of ferric nitrate nonahydrate, 0.75 mmol of nickel acetate tetrahydrate, 0.45 mmol of cobalt acetate tetrahydrate, and 20 mmol of sodium dihydrogen phosphate dihydrate in 30 mL of deionized water, stirring the mixture uniformly, and spray drying the mixture to obtain a precursor powder.

[0084] The precursor powder was heated to 250°C in a tube furnace at a heating rate of 2°C / min and kept at this temperature for 8 hours, then heated to 550°C at a heating rate of 2°C / min and kept at this temperature for 10 hours, with argon-hydrogen mixture (hydrogen volume ratio is 5%) as protection during the whole process. After cooling in the furnace, the obtained material was uniformly mixed with MgO at a ratio of 95:5 by mechanical ball milling for 10 minutes, and then dried to obtain Na4Fe 2.76 Co 0.09 Ni 0.15 (PO4)2P2O7@MgO.

[0085] Example 5 This embodiment provides a positive electrode active material. 20 mmol of anhydrous citric acid, 13.8 mmol of ferric nitrate nonahydrate, 0.75 mmol of chromium acetate dihydrate, and 20 mmol of sodium dihydrogen phosphate dihydrate are dissolved in 30 mL of deionized water, stirred evenly, and spray-dried to obtain a precursor powder.

[0086] The precursor powder was heated to 250°C in a tube furnace at a heating rate of 2°C / min and kept at this temperature for 8 hours, then heated to 550°C at a heating rate of 2°C / min and kept at this temperature for 10 hours. The whole process was protected by an argon-hydrogen mixture (hydrogen volume ratio was 5%) and cooled in the furnace. The obtained material was mixed with Al2O3 in a ratio of 95:0.5, uniformly mixed by mechanical ball milling for 30 minutes, and then dried to obtain Na4Fe 2.85 Cr 0.15 (PO4)2P2O7@Al2O3.

[0087] Example 6 This embodiment provides a positive electrode active material. 20 mmol of anhydrous citric acid, 13.8 mmol of ferric nitrate nonahydrate, 0.75 mmol of zinc acetate dihydrate, and 20 mmol of sodium dihydrogen phosphate dihydrate are dissolved in 30 mL of deionized water, stirred evenly, and spray-dried to obtain a precursor powder.

[0088] The precursor powder was heated to 250°C in a tube furnace at a heating rate of 2°C / min and kept at this temperature for 8 hours, then heated to 550°C at a heating rate of 2°C / min and kept at this temperature for 10 hours. The whole process was protected by an argon-hydrogen mixture (hydrogen volume ratio was 5%) and cooled in the furnace. The obtained material was mixed with TiO2 in a ratio of 95:0.5, uniformly mixed by mechanical ball milling for 30 minutes, and then dried to obtain Na4Fe 2.85 Zn 0.15 (PO4)2P2O7@TiO2.

[0089] Example 7 This embodiment provides a positive electrode active material. 20 mmol of anhydrous citric acid, 13.8 mmol of ferric nitrate nonahydrate, 0.75 mmol of manganese acetate tetrahydrate, and 20 mmol of sodium dihydrogen phosphate dihydrate are dissolved in 30 mL of deionized water, stirred evenly, and spray-dried to obtain a precursor powder.

[0090] The precursor powder was heated to 250°C in a tube furnace at a heating rate of 2°C / min and kept at this temperature for 8 hours, then heated to 550°C at a heating rate of 2°C / min and kept at this temperature for 10 hours. The whole process was protected by an argon-hydrogen mixture (hydrogen volume ratio was 5%) and cooled in the furnace. The obtained material was mixed with Al2O3 in a ratio of 95:0.5, uniformly mixed by mechanical ball milling for 30 minutes, and then dried to obtain Na4Fe 2.85 Mn 0.15 (PO4)2P2O7@Al2O3.

[0091] Example 8 This embodiment provides a positive electrode active material. 20 mmol of anhydrous citric acid, 13.8 mmol of ferric nitrate nonahydrate, 0.75 mmol of copper acetate monohydrate, and 20 mmol of sodium dihydrogen phosphate dihydrate are dissolved in 30 mL of deionized water, stirred evenly, and spray-dried to obtain a precursor powder.

[0092] The precursor powder was heated to 250°C in a tube furnace at a heating rate of 2°C / min and kept at this temperature for 8 hours, then heated to 550°C at a heating rate of 2°C / min and kept at this temperature for 10 hours. The whole process was protected by an argon-hydrogen mixture (hydrogen volume ratio was 5%) and cooled in the furnace. The obtained material was mixed with Al2O3 in a ratio of 95:0.5, uniformly mixed by mechanical ball milling for 30 minutes, and then dried to obtain Na4Fe2.85 Cu 0.15 (PO4)2P2O7@Al2O3.

[0093] Example 9 This embodiment provides a positive electrode active material. 20 mmol of anhydrous citric acid, 13.8 mmol of ferric nitrate nonahydrate, 0.75 mmol of titanium dioxide, and 20 mmol of sodium dihydrogen phosphate dihydrate are dissolved in 30 mL of deionized water, stirred evenly, and spray-dried to obtain a precursor powder.

[0094] The precursor powder was heated to 250°C in a tube furnace at a heating rate of 2°C / min and kept at this temperature for 8 hours, then heated to 550°C at a heating rate of 2°C / min and kept at this temperature for 10 hours. The whole process was protected by an argon-hydrogen mixture (hydrogen volume ratio was 5%) and cooled in the furnace. The obtained material was mixed with Al2O3 in a ratio of 95:0.5, uniformly mixed by mechanical ball milling for 30 minutes, and then dried to obtain Na4Fe 2.85 Ti 0.15 (PO4)2P2O7@Al2O3.

[0095] Comparative Example 1 This comparative example provides a positive electrode active material. Unlike Example 3, nickel acetate tetrahydrate and cobalt acetate tetrahydrate are not added to the raw materials, and the amount of ferric nitrate nonahydrate is changed to 15 mmol; finally, the Al2O3 coating process is not performed, and Na4Fe3(PO4)2P2O7 material is obtained.

[0096] Comparative Example 2 This comparative example provides a positive electrode active material. Unlike Example 3, nickel acetate tetrahydrate and cobalt acetate tetrahydrate are not added to the raw materials, and the amount of ferric nitrate nonahydrate is changed to 15 mmol; finally, an Al2O3 coating process is performed to obtain Na4Fe3(PO4)2P2O7@Al2O3 material.

[0097] Comparative Example 3 This comparative example provides a positive electrode active material. The difference from Example 3 is that the final Al2O3 coating process is eliminated to obtain Na4Fe 2.76 Co 0.09 Ni 0.15 (PO4)2P2O7 material.

[0098] Comparative Example 4 This comparative example provides a positive electrode active material. Different from Example 3, 15 mmol of ferric nitrate nonahydrate is selected as acetate and dissolved in 30 mL of deionized water. No element doping or metal oxide coating is adopted. The material is stirred evenly and spray-dried to obtain a precursor powder.

[0099] Comparative Example 5 This comparative example provides a positive electrode active material. 20 mmol of anhydrous citric acid, 13.8 mmol of ferric nitrate nonahydrate, 0.75 mmol of nickel acetate tetrahydrate, 0.45 mmol of cobalt acetate tetrahydrate, and 20 mmol of sodium dihydrogen phosphate dihydrate are dissolved in 30 mL of deionized water. The mixture is stirred and spray-dried to obtain a precursor powder.

[0100] The precursor powder was heated to 250°C in a tube furnace at a heating rate of 2°C / min and kept at this temperature for 8 hours, and then heated to 550°C at a heating rate of 2°C / min and kept at this temperature for 10 hours. The whole process was protected by argon-hydrogen mixed gas (hydrogen volume accounted for 5%), with a heating rate of 2°C / min and cooled in the furnace to obtain Na4Fe 2.76 Co 0.09 Ni 0.15 (PO4)2P2O7.

[0101] Characterization Example 1 The Na4Fe obtained in Example 3 2.76 Co 0.09 Ni 0.15 (PO4)2P2O7@Al2O3 was subjected to X-ray diffraction test. The experimental conditions were as follows: copper target (λ = 0.1518nm), 2θ angle range of 5-65°. The XRD pattern is shown in Figure 2. Figure 1 shown.

[0102] from Figure 1 The XRD pattern of the material shows a highly crystalline, pure phase Pn21a space group crystal structure (PDF standard card number: PDF#890579). No significant impurity diffraction peaks were observed, indicating that doping and coating did not affect the material's crystal structure. This result demonstrates that the preparation method of the present invention can synthesize a phase-pure, highly crystalline polyanionic material.

[0103] Characterization Example 2 The Na4Fe obtained in Example 3 2.76 Co 0.09 Ni 0.15 (PO4)2P2O7@Al2O3 was analyzed by scanning electron microscopy. Figure 2 As shown. Figure 2 As can be seen from the figure, the material as a whole is a spherical structure with a size of 2 to 6 μm and a uniform coating layer on the surface, which shows that the preparation method of the present invention can synthesize polyanionic materials with small size and uniform morphology.

[0104] Test Example 1 The positive electrode active materials obtained in the Examples and Comparative Examples were dissolved in N-methylpyrrolidone at a ratio of 80 parts positive electrode active material, 10 parts acetylene black, and 10 parts binder. The mixture was then coated onto treated aluminum foil and dried under vacuum at 90°C to produce a positive electrode sheet. In an anhydrous and oxygen-free glove box, the prepared positive electrode sheet, sodium sheet, and separator were then assembled with an NC-008 electrolyte (1 mol / L sodium salt of NaClO₄ dissolved in a 1:1 volume ratio of ethylene carbonate (EC) and propylene carbonate (PC), supplemented with 5% fluoroethylene carbonate (FEC)). The resulting button-type cells were then assembled for electrochemical performance testing.

[0105] Cycling performance testing: The battery was charged and discharged at room temperature at a current of 129 mA / g at 1C. The charge and discharge voltage range was 1.8 to 4.2 V. Before the 1C cycle, the battery was activated with a low current of 0.1C for three cycles. The data are shown in Table 1 below.

[0106] Charge and discharge rate performance testing: The batteries were tested at rates of 0.1C, 0.3C, 0.5C, 1C, 2C, and 5C. The 1C current was 129 mA / g, the charge and discharge voltage range was 1.8-4.2 V, and the charge and discharge temperature was room temperature. The data are shown in Table 1 below.

[0107] Ionic conductivity test: Apply AC signals of different frequencies to the battery, measure its impedance response, and obtain the bulk resistance (R b ), and then calculate the ionic conductivity (σ).

[0108] Table 1

[0109] As shown in Table 1, the battery of Example 3 exhibits a discharge capacity exceeding 110 mAh / g at 0.1C. After 300 cycles, the discharge capacity is 105.5 mAh / g, with a capacity retention rate of 93.54%. In contrast, the battery of Comparative Example 1 exhibits a discharge capacity of no more than 110 mAh / g at 0.1C, and after 300 cycles, the capacity retention rate is only 86.85%. These results demonstrate that Ni and Co co-doping can improve the capacity and cycling stability of this polyanion material.

[0110] Comparing the data in the table reveals a synergistic effect between doping and coating. The effects of each alone are limited: Coating alone (Comparative Example 2) achieved a 0.1C specific capacity of 92 mAh / g and a cycle retention rate of 87.07%, outperforming the uncoated material but below the doping and coating system.

[0111] Doping only (Comparative Example 3): 0.1C specific capacity 102 mAh / g, cycle retention rate 86.57%, rate performance (5C specific capacity 81.0 mAh / g) is significantly improved, but the cycle stability is not as good as the double-modified system.

[0112] The synergistic effect is significant: Example 3 (Ni, Co co-doping + Al2O3 coating) performs best in capacity, cycle stability, rate performance and ionic conductivity, proving that the dual mechanism of doping to reduce the band gap width and enhance ionic conduction and coating to suppress interface side reactions is effective.

[0113] It can be seen that through the dual modification of "transition metal doping + metal oxide coating", the material is significantly superior to traditional polyanion positive electrode materials in capacity (increased by 28%), cycle life (retention rate increased by 12%), ionic conductivity (increased by 300%) and rate performance (capacity increased by 34% at 5C current density).

[0114] Test Example 2 The button cell of Example 3 prepared in Test Example 1 was subjected to a charge-discharge cycle test at a rate of 5C. The current at 1C was 129 mA / g, the charge-discharge voltage range was 1.8-4.2 V, and the charge-discharge temperature was room temperature. The obtained data were plotted to obtain Figure 3 .

[0115] from Figure 3 It can be seen that after the positive electrode active material provided in Example 3 is prepared into a battery, it still has a capacity retention rate of 91.8% after 5000 cycles, showing excellent cycle stability.

[0116] Test Example 3 The button cells assembled from Example 3, Comparative Example 1 and Comparative Example 5 were tested for rate performance at different current densities. The images obtained are as follows: Figure 4 The charge and discharge voltage range is 1.8~4.2V.

[0117] pass Figure 4 It can be seen that the battery assembled in Example 3 has a discharge capacity of 91.7 mAh / g at a current density of 5C. After charge and discharge tests at five different current densities, the discharge capacity still reaches 107 mAh / g when the current density returns to 0.1C, which is superior to the performance of Comparative Example 1. This result shows that Co doping can improve the rate performance of the polyanion material. In addition, combined with Table 1, by comparing the data of Example 3 and Example 4, it can be seen that the introduction of Ni doping on the basis of Co doping, the synergistic effect of the two elements, and the use of metal oxide coating can further improve the rate performance of the polyanion material.

[0118] Finally, it should be noted that the above-described embodiments are only specific implementations of the present invention, which are used to illustrate the technical solutions of the present invention, rather than to limit them. The scope of protection of the present invention is not limited thereto. Although the present invention has been described in detail with reference to the above-described embodiments, those skilled in the art should understand that any person skilled in the art can modify or easily conceive of changes to the technical solutions described in the above-described embodiments within the technical scope disclosed by the present invention, or replace some of the technical features therein with equivalents. Such modifications, changes, or replacements do not deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A positive electrode active material, characterized in that The general formula of the positive electrode active material is Na4Fe 3-x-y M y (PO4)2P2O7@R a O b ; Among them, the value range of x is 0.03~0.27, and the value range of y is 0~0.27 and is not 0; M is at least one transition metal element; R a O b Includes at least one of Al2O3, SiO2, TiO2, ZrO2, Y2O3, MgO, and ZnO.

2. The positive electrode active material according to claim 1, characterized in that M includes at least one of Mn, Ni, Co, Cu, Ti, Zn, Zr, Cr, Nb, and Sc.

3. The positive electrode active material according to claim 1, characterized in that The positive electrode active material has a NASICON structure.

4. A method for preparing the positive electrode active material according to any one of claims 1 to 3, characterized in that: The following steps are involved: A. mixing a sodium source, a phosphorus source, an iron source, an M source, and a carbon source according to stoichiometric amounts, and dispersing the mixture with a solvent to prepare a precursor solution; then drying the precursor solution to remove the solvent to obtain a precursor; B. Heat treating the precursor in an inert reducing atmosphere to obtain Na4Fe 3-x-y M y (PO4)2P2O7; C. R a O b Coated on the Na4Fe 3-x-y M y (PO4)2P2O7 to obtain the positive electrode active material.

5. The preparation method according to claim 4, characterized in that The sodium source comprises at least one of sodium dihydrogen phosphate, sodium carbonate, sodium bicarbonate, sodium acetate, sodium oxalate, sodium nitrate, sodium sulfate, sodium bisulfate and sodium citrate, preferably sodium dihydrogen phosphate; Preferably, the phosphorus source comprises at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, disodium hydrogen phosphate and sodium dihydrogen phosphate, preferably sodium dihydrogen phosphate; Preferably, the iron source comprises at least one of ferrous acetate, ferric nitrate, ferrous oxalate and ferrous sulfate, preferably ferric nitrate; Preferably, the M source includes at least one of oxidative M, acetic M, sulfuric M, chlorinated M, nitric M and dihydrogen phosphate M, preferably acetic M; Preferably, the carbon source comprises at least one of glucose, starch, anhydrous citric acid, ascorbic acid, polydopamine, polyvinyl alcohol and polyacrylonitrile, preferably anhydrous citric acid; Preferably, the solvent comprises at least one of deionized water, methanol, ethanol and ethylene glycol, preferably deionized water; Preferably, the concentration of the carbon source in the precursor solution is 1-10 wt %.

6. The preparation method according to claim 4, characterized in that In step A, the drying method includes spray drying, forced air drying, vacuum drying or freeze drying, preferably spray drying; Preferably, in step B, the inert reducing atmosphere is a combination of hydrogen and nitrogen or a combination of hydrogen and argon; Preferably, in step B, the volume proportion of the hydrogen in the inert reducing atmosphere is 3-7%; Preferably, the heat treatment includes a pre-sintering process and a calcining process performed in sequence; Preferably, the pre-sintering process is: heating to 250-350°C at a heating rate of 1-5°C / min and keeping the temperature for 3-10 hours; Preferably, the calcination process is: heating to 500-600° C. at a heating rate of 1-5° C. / min, keeping the temperature for 5-10 hours, and cooling with the furnace.

7. The preparation method according to claim 4, characterized in that In step C, the coating process is as follows: 3-x-y M y (PO4)2P2O7 and the R a O b Mix well and dry; Preferably, the mixing method includes ball milling, rotary evaporation, mechanical stirring or atomic layer deposition; Preferably, the drying method includes forced air drying, vacuum drying or freeze drying, preferably vacuum drying.

8. A positive electrode plate, characterized in that: The invention comprises an aluminum foil and a positive electrode active material layer coated on the aluminum foil; The positive electrode active material layer includes a conductive agent, a binder, and the positive electrode active material according to any one of claims 1 to 3.

9. The positive electrode sheet according to claim 8, characterized in that: The positive electrode active material layer comprises 8 to 12 parts by weight of a conductive agent, 8 to 12 parts by weight of a binder, and 80 parts by weight of a positive electrode active material; Preferably, the conductive agent includes acetylene black.

10. A sodium ion secondary battery, characterized in that: Including the positive electrode sheet according to claim 8 or 9.

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