Modified carbon-coated sodium manganese ferric phosphate pyrophosphate / sodium ferric phosphate pyrophosphate positive electrode material as well as preparation method and application of modified carbon-coated sodium manganese ferric phosphate pyrophosphate / sodium ferric phosphate pyrophosphate positive electrode material

Abstract

The invention discloses a modified carbon-coated sodium manganese ferric phosphate pyrophosphate / sodium ferric phosphate pyrophosphate positive electrode material as well as a preparation method and application thereof, and relates to the technical field of new energy materials. The preparation method comprises the following steps: preparing MnFe-MOF from a manganese source, a first iron source and a first organic ligand under a hydrothermal reaction; a second iron source, the MnFe-MOF and a second organic ligand are subjected to a hydrothermal reaction, and MnFe-MOF / Fe-MOF is obtained; uniformly mixing with a sodium source and a phosphorus source, and sintering to obtain a positive electrode material; and carrying out high-temperature gas-phase etching treatment to obtain the fluorine-doped carbon-coated modified sodium manganese ferric phosphate pyrophosphate / sodium ferric phosphate pyrophosphate positive electrode material. The material disclosed by the invention has a coating modified structure, so that an interface side reaction caused by direct contact between sodium ferromanganese phosphate and an electrolyte can be effectively avoided, a manganese dissolution phenomenon is reduced, and the structural stability of the material is improved; and meanwhile, the material has high conductivity, excellent structural stability, excellent long cycle life and excellent rate capability.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy materials, and more specifically, to a modified carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate cathode material, a preparation method, and applications thereof. Background Art

[0002] The electrochemical energy storage mechanism of sodium-ion batteries is similar to that of lithium-ion batteries. Due to their high energy density, excellent low-temperature performance, large reserves of sodium in the earth's crust and low cost, they have become a new type of energy storage battery that is expected to replace lithium-ion batteries.

[0003] Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, NFPP) is a mixed iron-based phosphate. It has more energy-storing sodium ions per unit mole of positive electrode material than sodium iron phosphate, and thus can achieve higher charge and discharge capacity. However, due to its own low voltage platform, the energy density of the material has reached a bottleneck. On the other hand, although sodium manganese pyrophosphate (Na4Mn3(PO4)2P2O7, NMPP) has a high voltage platform, its intrinsic conductivity is poor and manganese dissolution is aggravated, which hinders its commercialization process. Similarly, sodium manganese pyrophosphate (Na4Mn x Fe 3-x (PO4)2P2O7, NMFPP, 0<x<3), by introducing iron atoms to replace manganese atoms in sodium manganese pyrophosphate, forming a solid solution phase of sodium manganese ferrophosphate. This can effectively combine the high voltage platform of sodium manganese pyrophosphate with the good conductivity and excellent stability of sodium ferrophosphate, effectively improving the energy density of sodium ion batteries and becoming a new generation of sodium ion battery positive electrode material. However, a higher Mn content in sodium manganese ferrophosphate will inevitably lead to manganese dissolution-induced Jahn-Teller effect in the positive electrode material during sodium storage, reducing the intrinsic conductivity and material structural stability of sodium manganese ferrophosphate.

[0004] Chinese patent CN118572106A proposes a positive electrode composite material with a double coating layer, which is composed of a sodium manganese ferric pyrophosphate / carbon composite core, and an inorganic coating layer and an organic coating layer sequentially arranged on the surface of the sodium manganese ferric pyrophosphate / carbon composite core from the inside to the outside. The presence of the inorganic coating layer can suppress the distortion of the Jan-Taylor structure and the dissolution of Mn; at the same time, the organic coating layer is added to improve the electronic conductivity of the electrode material, thereby effectively improving the cycle and rate performance of the positive electrode material. However, the carbon coating strategy of adding an external carbon source will result in uneven distribution and serious carbon floating, which affects the quality control of the material; on the other hand, the charge transfer interface resistance between the inorganic coating layer and the sodium manganese ferric pyrophosphate / carbon composite core is large, and the use of an organic coating layer cannot fundamentally improve the electronic structure of the material, and the improvement of conductivity is extremely limited. There is an urgent need to develop a sodium manganese ferric pyrophosphate positive electrode material with high intrinsic conductivity, high material structure stability, and low manganese dissolution. Summary of the Invention

[0005] The purpose of the present invention is to overcome the defects and shortcomings of existing sodium manganese iron pyrophosphate positive electrode materials and provide a preparation method of modified carbon-coated sodium manganese iron pyrophosphate / sodium iron pyrophosphate positive electrode material. The surface is coated with fluorine-doped modified carbon, which can effectively avoid the interfacial side reactions caused by direct contact between the sodium manganese iron pyrophosphate and the electrolyte, reduce manganese dissolution, and improve the material structure stability; at the same time, the interlayer structure is optimized, so that the sodium manganese iron pyrophosphate exhibits high conductivity, excellent structural stability, outstanding long cycle life and rate performance.

[0006] Another object of the present invention is to provide a modified carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material.

[0007] Another object of the present invention is to provide an application of the modified carbon-coated sodium manganese iron pyrophosphate / sodium iron pyrophosphate positive electrode material in the preparation of a sodium ion battery.

[0008] The above-mentioned purpose of the present invention is achieved through the following technical solutions:

[0009] The present invention provides a method for preparing a modified carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material, comprising the following steps:

[0010] S1, under an inert atmosphere, dissolving a manganese source and a first iron source in a solvent, adding a first organic ligand and mixing them uniformly, and obtaining MnFe-MOF after a first hydrothermal reaction;

[0011] S2, under an inert atmosphere, dissolving a second iron source in a solvent and then mixing and dispersing the mixture with the MnFe-MOF, adding a second organic ligand and performing a second hydrothermal reaction to in situ form an Fe-MOF coating layer on the surface of the MnFe-MOF to obtain MnFe-MOF / Fe-MOF;

[0012] S3, after uniformly mixing the MnFe-MOF / Fe-MOF with a sodium source and a phosphorus source, grinding, drying and sintering in sequence to obtain a carbon-coated sodium manganese iron pyrophosphate / sodium iron pyrophosphate positive electrode material;

[0013] S4, using a fluorine source as an etching gas, subjecting the carbon-coated sodium manganese iron phosphate pyrophosphate / sodium iron phosphate pyrophosphate cathode material to a high-temperature vapor phase etching treatment to obtain a fluorine-doped carbon-coated modified sodium manganese iron phosphate pyrophosphate / sodium iron phosphate pyrophosphate cathode material.

[0014] The modified carbon-coated sodium manganese iron phosphate pyrophosphate / sodium iron phosphate pyrophosphate positive electrode material of the present invention comprises the following steps: synthesizing MnFe-MOF by a hydrothermal method, and then in situ synthesizing Fe-MOF on the surface of MnFe-MOF to form MnFe-MOF / Fe-MOF with MnFe-MOF as a core and Fe-MOF as an outer shell; using the MnFe-MOF as a manganese source, an iron source and a carbon source, and mixing and sintering the MnFe-MOF / Fe-MOF with a sodium source and a phosphorus source to obtain a carbon-coated sodium manganese iron phosphate pyrophosphate / sodium iron phosphate pyrophosphate positive electrode material. Due to the sodium iron phosphate pyrophosphate coating derived from the Fe-MOF shell, the interfacial side reactions caused by direct contact between sodium manganese iron pyrophosphate and the electrolyte are effectively avoided, thereby avoiding the manganese dissolution phenomenon that deteriorates the material and destroys the material structure, and improving the material's cycle stability; after the positive electrode material is treated with high-temperature vapor phase etching of a fluorine source, the carbon coating layer is etched and doped with fluorine elements, and the doped atoms in the material are evenly distributed. Due to the electronegativity of fluorine atoms, the electronic structure of the material is effectively optimized after the introduction of doped fluorine, thereby improving the material's conductivity and electrochemical properties, which is helpful to regulate and achieve sodium ion battery positive electrode materials with excellent rate performance and cycle stability.

[0015] In some embodiments, in step S1, the molar ratio of the manganese source, the first iron source and the first organic ligand is (3-4.5):(0.8-2):(1.5-6), preferably (3.5-4):(1-1.5):(1.5-3), and preferably (3.5-4):(1-1.5):2.

[0016] In some embodiments, in step S1, the reaction conditions of the first hydrothermal reaction are: reaction temperature 100-120° C., reaction time 5-10 h.

[0017] In some embodiments, in step S2, the molar ratio of the MnFe-MOF, the second iron source, and the second organic ligand is (1-10):(0.5-2):(0.5-2), preferably (1-10):1:(0.8-2).

[0018] In some embodiments, in step S2, the reaction conditions of the second hydrothermal reaction are: reaction temperature 100-120° C., reaction time 5-10 h.

[0019] In some embodiments, the first iron source and the second iron source are independently selected from one or more of ferrous chloride, ferric chloride, ferrous sulfate, ferric nitrate, ferric citrate, ferric acetate, ammonium ferric sulfate, or ferrous oxalate, preferably ferrous chloride.

[0020] In some embodiments, the first organic ligand and the second organic ligand are carboxylic acid ligands or amine ligands; preferably, the first organic ligand and the second organic ligand are independently selected from 1,2-benzenedicarboxylic acid, 1,3-benzenedicarboxylic acid, 1,4-benzenedicarboxylic acid, 1,2,4-benzenetricarboxylic acid, 1,3,5-benzenetricarboxylic acid, 1,2,4,5-benzenetetracarboxylic acid, 4-hydroxybenzoic acid, 2-aminoterephthalic acid, 4-aminobenzoic acid, 2,5-dihydroxybenzoic acid, The present invention further comprises one or more of 1,3,5-dihydroxybenzoic acid, 2-hydroxyterephthalic acid, 2,3-dihydroxybenzoic acid, 2,6-dihydroxybenzoic acid, 2,3-dihydroxyterephthalic acid, 2,5-dihydroxyterephthalic acid, p-hydroxydibenzoic acid, 5-nitroisophthalic acid, nitroterephthalic acid, fumaric acid, citric acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, 2-aminoterephthalic acid, 4-aminobenzoic acid, dicyandiamide, melamine or urea. More preferably, 1,3,5-benzoic acid is used.

[0021] In some embodiments, the manganese source is selected from one or more of manganous chloride, manganous sulfate, manganous nitrate, manganous carbonate, manganous acetate, or manganous oxalate, preferably manganous chloride.

[0022] In some embodiments, the sodium source is selected from one or more of sodium carbonate, sodium bicarbonate, sodium phosphate, sodium nitrate, sodium hydroxide, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium citrate, sodium pyrophosphate, sodium chloride or sodium oxalate, preferably sodium carbonate.

[0023] In some embodiments, the phosphorus source is selected from one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, sodium pyrophosphate, ammonium hypophosphite, ammonium polyphosphate, ammonium hexafluorophosphate, sodium phosphite, sodium metaphosphate, sodium tripolyphosphate, or sodium hexafluorophosphate, preferably sodium dihydrogen phosphate.

[0024] In some embodiments, in step S3, the stoichiometric ratio of (Mn+Fe):Na:P elements between the MnFe-MOF / Fe-MOF and the sodium source and the phosphorus source is 3:(4-4.4):(4-4.4).

[0025] In some embodiments, in step S3, the sintering temperature is 550-700° C., and the sintering time is 6-14 hours.

[0026] In some embodiments, in step S4, in the high-temperature vapor phase etching treatment, the mass ratio of the fluorine source and the carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material is (2-8):1, preferably, the mass ratio is (4-7):1.

[0027] In some embodiments, during the high-temperature vapor phase etching process in step S4, the carrier gas flow rate is 0.1-0.3 L / min; preferably, the carrier gas flow rate is 0.18-0.22 L / min. Optionally, the carrier gas is an inert gas, including but not limited to nitrogen and / or argon.

[0028] In some embodiments, in step S4, the fluorine source is ammonium fluoride, and the high-temperature vapor phase etching process is performed at a temperature of 350-480°C for 1-4 hours. Ammonium fluoride is thermally decomposed into HF at the gas inlet, which is transported with the carrier gas to the cathode material for etching and doping with fluoride ions.

[0029] The present invention protects a modified carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material, which is prepared by the preparation method of the modified carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material.

[0030] The chemical formula of the sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate of the present invention is Na4Mn x Fe 3-x (PO4)2P2O7 / Na4Fe3(PO4)2P2O7, wherein 0<x<3. Preferably, 2≤x≤2.5.

[0031] The present invention protects the use of the modified carbon-coated sodium manganese iron pyrophosphate / sodium iron pyrophosphate positive electrode material in the preparation of sodium ion batteries.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The present invention provides a method for preparing a modified carbon-coated sodium manganese ferrous phosphate pyrophosphate / sodium ferrous phosphate pyrophosphate cathode material. Fe-MOF is synthesized in situ on the surface of a MnFe-MOF to form a MnFe-MOF / Fe-MOF with a MnFe-MOF core and a Fe-MOF shell. The MnFe-MOF / Fe-MOF core is then used as a manganese source, an iron source, and a carbon source. The core is then mixed with a sodium source and a phosphorus source and sintered to obtain a carbon-coated sodium manganese ferrous phosphate pyrophosphate / sodium ferrous phosphate pyrophosphate cathode material. The sodium ferrous phosphate pyrophosphate coating derived from the Fe-MOF shell effectively avoids interfacial side reactions caused by direct contact between the sodium manganese ferrous phosphate pyrophosphate and the electrolyte, thereby preventing manganese dissolution that deteriorates the material and damages the material structure, thereby improving the material's cyclic stability.

[0034] Furthermore, after the positive electrode material is treated with high-temperature vapor phase etching of a fluorine source, the electronic structure of the material is effectively optimized after the introduction of doped fluorine due to the electronegativity of fluorine atoms, thereby improving the conductivity and electrochemical properties of the material, which helps to regulate and achieve sodium ion battery positive electrode materials with excellent rate performance and cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is an SEM image of a fluorine-doped carbon-coated modified sodium manganese iron pyrophosphate / sodium iron pyrophosphate positive electrode material of Example 1 of the present invention. DETAILED DESCRIPTION

[0036] The present invention will be further described below with reference to specific embodiments, but the examples do not limit the present invention in any form.

[0037] Example 1

[0038] A fluorine-doped carbon-coated modified sodium manganese iron pyrophosphate / sodium iron pyrophosphate (Na4Mn 2.4 Fe 0.6 The preparation method of (PO4)2P2O7 / Na4Fe3(PO4)2P2O7) positive electrode material comprises the following steps:

[0039] In step S1, 2 mol of 1,3,5-benzoic acid was dispersed in 20 L of N,N-dimethylacetamide to form solution A. In a nitrogen-filled reactor, 4 mol of manganous chloride and 1 mol of ferrous chloride were dissolved in 10 L of deionized water to form solution B. Solution A was then slowly pumped into the reactor containing solution B. After complete addition, the mixture was stirred for 30 minutes to form a uniform solution system, and then hydrothermally reacted at 100°C for 6 hours. After cooling to room temperature, the mixture was washed with N,N-dimethylacetamide, deionized water, and anhydrous ethanol, respectively, and dried in a vacuum oven at 60°C for 12 hours to produce MnFe-MOF.

[0040] In step S2, 1 mol of 1,3,5-benzoic acid was dispersed in 10 L of N,N-dimethylacetamide to form solution C. In a nitrogen-filled reactor, 1 mol of ferrous chloride was dissolved in 30 L of deionized water to form solution B'. The MnFe-MOF was then added to the reactor containing solution B' and stirred for 30 minutes to form a homogeneous system D. Subsequently, solution C was slowly pumped into the reactor containing system D, stirred for 30 minutes, and then hydrothermally reacted at 100°C for 8 hours. After cooling to room temperature, the mixture was washed with N,N-dimethylacetamide, deionized water, and anhydrous ethanol, respectively, and dried in a vacuum oven at 60°C for 12 hours to obtain MnFe-MOF / Fe-MOF.

[0041] S3, according to the quantitative relationship of the stoichiometric ratio of Na: (Mn + Fe): P elements of 4.05:3:4.05, the corresponding sodium carbonate, MnFe-MOF / Fe-MOF, and sodium dihydrogen phosphate are respectively weighed and dispersed in ultrapure water, and a slurry with a solid content of 40% is prepared for wet grinding and spray drying to obtain sodium manganese iron pyrophosphate / sodium iron pyrophosphate precursor.

[0042] The precursor was then heated in a nitrogen atmosphere box furnace at a rate of 5°C / min to 600°C and held at that temperature for 12 hours. After cooling to room temperature, the precursor was jet-milled and sieved to produce the carbon-coated sodium manganese ferrous pyrophosphate / sodium ferric pyrophosphate cathode material.

[0043] S4, weigh 1g of the above-mentioned carbon-coated sodium manganese iron phosphate pyrophosphate / sodium iron phosphate pyrophosphate cathode material into a sagger and place it near the outlet end of a nitrogen atmosphere tube furnace. Use 5g of ammonium fluoride as a fluorine source, place ammonium fluoride near the inlet end of a nitrogen atmosphere tube furnace (inner diameter 75mm, length 1000mm), and heat it to 450℃ at a heating rate of 10℃ / min and keep it at this temperature for 3h for etching and doping treatment. After naturally cooling to room temperature, a fluorine-doped carbon-coated modified sodium manganese iron phosphate pyrophosphate / sodium iron phosphate pyrophosphate cathode material is obtained.

[0044] Example 2

[0045] A fluorine-doped carbon-coated modified sodium manganese iron pyrophosphate / sodium iron pyrophosphate (Na4Mn 2.1 Fe 0.9 The preparation method of (PO4)2P2O7 / Na4Fe3(PO4)2P2O7) positive electrode material comprises the following steps:

[0046] In step S1, 2 mol of 1,3,5-benzoic acid was dispersed in 20 L of N,N-dimethylacetamide to form solution A. In a nitrogen-filled reactor, 3.5 mol of manganous chloride and 1.5 mol of ferrous chloride were dissolved in 10 L of deionized water to form solution B. Solution A was then slowly pumped into the reactor containing solution B. After complete addition, the mixture was stirred for 30 minutes to form a uniform solution system, and then hydrothermally reacted at 100°C for 6 hours. After cooling to room temperature, the mixture was washed with N,N-dimethylacetamide, deionized water, and anhydrous ethanol, respectively, and dried in a vacuum oven at 60°C for 12 hours to produce MnFe-MOF.

[0047] In step S2, 1 mol of 1,3,5-benzoic acid was dispersed in 10 L of N,N-dimethylacetamide to form solution C. In a nitrogen-filled reactor, 1 mol of ferrous chloride was dissolved in 30 L of deionized water to form solution B'. The MnFe-MOF was then added to the reactor containing solution B' and stirred for 30 minutes to form a homogeneous system D. Subsequently, solution C was slowly pumped into the reactor containing system D, stirred for 30 minutes, and then hydrothermally reacted at 100°C for 8 hours. After cooling to room temperature, the mixture was washed with N,N-dimethylacetamide, deionized water, and anhydrous ethanol, respectively, and dried in a vacuum oven at 60°C for 12 hours to obtain MnFe-MOF / Fe-MOF.

[0048] S3, according to the quantitative relationship of the stoichiometric ratio of Na: (Mn + Fe): P elements of 4.07:3:4.04, the corresponding sodium carbonate, MnFe-MOF / Fe-MOF, and sodium dihydrogen phosphate are respectively weighed and dispersed in ultrapure water, and a slurry with a solid content of 38% is prepared for wet grinding and spray drying to obtain sodium manganese iron pyrophosphate / sodium iron pyrophosphate precursor.

[0049] The precursor was then placed in a nitrogen atmosphere box furnace with an air flow rate of 0.2 L / min and heated to 620°C at a rate of 5°C / min, where it was held constant for 10 hours. After cooling to room temperature, the carbon-coated sodium manganese iron pyrophosphate / sodium iron pyrophosphate cathode material was prepared after airflow milling and sieving.

[0050] S4, weighing 1g of the above-mentioned carbon-coated sodium manganese iron phosphate pyrophosphate / sodium iron phosphate pyrophosphate cathode material into a sagger and placing it near the outlet end of a nitrogen atmosphere tube furnace, using 5g of ammonium fluoride as a fluorine source, placing ammonium fluoride near the inlet end of the nitrogen atmosphere tube furnace with an inlet flow rate of 0.2L / min, and heating the temperature to 450°C at a heating rate of 10°C / min and maintaining the temperature for 4 hours for etching and doping treatment. After naturally cooling to room temperature, a fluorine-doped carbon-coated modified sodium manganese iron phosphate pyrophosphate / sodium iron phosphate pyrophosphate cathode material is obtained.

[0051] Example 3

[0052] A method for preparing a fluorine-doped carbon-coated modified sodium manganese iron pyrophosphate / sodium iron pyrophosphate positive electrode material, which differs from Example 1 in that it comprises the following steps:

[0053] Steps S1 to S3 of this embodiment are the same as those of embodiment 1;

[0054] S4. Weigh 1g of the carbon-coated sodium manganese iron phosphate pyrophosphate / sodium iron phosphate pyrophosphate cathode material into a sagger and place it in a nitrogen atmosphere tube furnace near the gas outlet. 7g of ammonium fluoride is used as a fluorine source. The ammonium fluoride is placed in a nitrogen atmosphere tube furnace near the gas inlet with an inlet flow rate of 0.2L / min. The temperature is raised to 450°C at a heating rate of 10°C / min and maintained at this temperature for 3 hours for etching and doping treatment. After naturally cooling to room temperature, a fluorine-doped carbon-coated modified sodium manganese iron phosphate pyrophosphate / sodium iron phosphate pyrophosphate cathode material is obtained.

[0055] Comparative Example 1

[0056] A method for preparing a sodium manganese iron pyrophosphate / sodium iron pyrophosphate positive electrode material, wherein the material of this comparative example is not subjected to fluorine doping treatment; specifically, the method comprises the following steps:

[0057] In step S1, 2 mol of 1,3,5-benzoic acid was dispersed in 20 L of N,N-dimethylacetamide to form solution A. In a nitrogen-filled reactor, 4 mol of manganous chloride and 1 mol of ferrous chloride were dissolved in 10 L of deionized water to form solution B. Solution A was then slowly pumped into the reactor containing solution B. After complete addition, the mixture was stirred for 30 minutes to form a uniform solution system, and then hydrothermally reacted at 100°C for 6 hours. After cooling to room temperature, the mixture was washed with N,N-dimethylacetamide, deionized water, and anhydrous ethanol, respectively, and dried in a vacuum oven at 60°C for 12 hours to produce MnFe-MOF.

[0058] In step S2, 1 mol of 1,3,5-benzoic acid was dispersed in 10 L of N,N-dimethylacetamide to form solution C. In a nitrogen-filled reactor, 1 mol of ferrous chloride was dissolved in 30 L of deionized water to form solution B'. The MnFe-MOF was then added to the reactor containing solution B' and stirred for 30 minutes to form a homogeneous system D. Subsequently, solution C was slowly pumped into the reactor containing system D, stirred for 30 minutes, and then hydrothermally reacted at 100°C for 8 hours. After cooling to room temperature, the mixture was washed with N,N-dimethylacetamide, deionized water, and anhydrous ethanol, respectively, and dried in a vacuum oven at 60°C for 12 hours to obtain MnFe-MOF / Fe-MOF.

[0059] S3, according to the quantitative relationship of the stoichiometric ratio of Na: (Mn + Fe): P elements of 4.05:3:4.05, the corresponding sodium carbonate, MnFe-MOF / Fe-MOF, and sodium dihydrogen phosphate are respectively weighed and dispersed in ultrapure water, and a slurry with a solid content of 40% is prepared for wet grinding and spray drying to obtain sodium manganese iron pyrophosphate / sodium iron pyrophosphate precursor.

[0060] The precursor was then heated in a nitrogen atmosphere box furnace at a rate of 5°C / min to 600°C and held at that temperature for 12 hours. After cooling to room temperature, the precursor was jet-milled and sieved to produce the carbon-coated sodium manganese ferrous pyrophosphate / sodium ferric pyrophosphate cathode material.

[0061] Comparative Example 2

[0062] A method for preparing a carbon-coated sodium manganese iron phosphate pyrophosphate positive electrode material, wherein the surface of the positive electrode material is not coated with Fe-MOF modification and fluorine doping treatment, comprises the following steps:

[0063] In step S1, 2 mol of 1,3,5-benzoic acid was dispersed in 20 L of N,N-dimethylacetamide to form solution A. In a nitrogen-filled reactor, 4 mol of manganous chloride and 1 mol of ferrous chloride were dissolved in 10 L of deionized water to form solution B. Solution A was then slowly pumped into the reactor containing solution B. After complete addition, the mixture was stirred for 30 minutes to form a uniform solution system, and the reaction was continued at 100°C for 6 hours. After cooling to room temperature, the mixture was washed with N,N-dimethylacetamide, deionized water, and anhydrous ethanol, respectively, and dried in a vacuum oven at 60°C for 12 hours to produce MnFe-MOF.

[0064] S2, according to the quantitative relationship of Na: (Mn + Fe): P elements of 4.05:3:4.05, the corresponding sodium carbonate, MnFe-MOF, and sodium dihydrogen phosphate were weighed and dispersed in ultrapure water, and a slurry with a solid content of 40% was prepared for wet grinding and spray drying to obtain a sodium manganese iron phosphate pyrophosphate precursor. The sodium manganese iron phosphate pyrophosphate precursor was then placed in a nitrogen atmosphere box furnace and heated to 600°C at a heating rate of 5°C / min and kept at this temperature for 12 hours. After naturally cooling to room temperature, the carbon-coated sodium manganese iron phosphate pyrophosphate positive electrode material was obtained after air flow milling and sieving.

[0065] Performance Testing

[0066] 1. SEM characterization

[0067] The positive electrode material obtained in Example 1 was examined by scanning electron microscopy (SEM). Figure 1 shown.

[0068] The results show that the microstructure of the cathode material obtained in Example 1 is composed of nanoscale primary particles, the primary particles have a spherical morphology, and the particle size distribution is uniform.

[0069] 2. Physical and chemical index test

[0070] The positive electrode material samples prepared in the above embodiments and comparative examples were subjected to physical and chemical index tests, and the results are shown in Table 1.

[0071] Powder resistance test method: Use an automatic powder resistivity tester to apply an excitation current to the battery powder samples prepared in the examples and comparative examples, measure the voltage, and obtain the surface resistivity and resistance data of the materials.

[0072] Electrochemical testing method: The battery powder samples prepared in the examples or comparative examples were used as the positive electrode active material. The active material: conductive agent carbon black: binder polyvinylidene fluoride were weighed and dispersed in N-methylpyrrolidone dispersant at a mass ratio of 92:4:4 to form a uniformly dispersed positive electrode slurry. The slurry was then coated on aluminum foil and dried in a vacuum oven at 120°C for 12 hours. The slurry was then punched and weighed to produce circular electrode sheets. 2032-type sodium ion button cells were assembled in an inert atmosphere glove box (O₂ ≤ 0.01 ppm, H₂O ≤ 0.01 ppm) using a sodium metal sheet as the negative electrode, the prepared electrode sheet as the positive electrode, glass fiber as the separator, and a mixed solution of 1 mol / L NaClO₄ dissolved in dimethyl carbonate and ethylene carbonate (dimethyl carbonate:ethylene carbonate at a ratio of 1:1 vol%) as the electrolyte. The cells were left to stand for 12 hours before electrochemical performance testing at room temperature. The test voltage range is set to 1.7~4.3V, and the charge and discharge test is performed at a current rate of 0.2C / 1C.

[0073] Manganese dissolution test method: The inductively coupled plasma test method was used to detect the amount of Mn dissolved in the electrolyte after the batteries assembled with the battery powder samples prepared in the examples and comparative examples were cycled 100 times at a current density of 1C.

[0074] Table 1 Physical and chemical index data

[0075]

[0076]

[0077] The results show that the powder resistance of the fluorine-doped carbon-coated modified sodium manganese iron pyrophosphate / sodium iron pyrophosphate positive electrode material of the present invention is below 18.32Ω·cm, the 0.2C first charge specific capacity is above 118.9mAh / g, the 0.2C first discharge efficiency is above 98.7%, the 1C 100 cycle capacity retention rate is above 96.9%, and the manganese dissolution is lower than 42ppm.

[0078] Compared with Example 1, the material of Comparative Example 1 was not subjected to fluorine doping post-treatment, and the manganese dissolution phenomenon was serious, the conductivity of the material was poor, and its electrochemical performance was affected.

[0079] The surface of the sodium manganese ferric pyrophosphate in Comparative Example 2 was not coated with sodium ferric pyrophosphate, resulting in significant manganese dissolution in the material, poor material stability, low conductivity, and decreased battery capacity.

[0080] The above embodiments of the present invention are merely examples for the purpose of clearly illustrating the present invention and are not intended to limit the embodiments of the present invention. Those skilled in the art will appreciate that other variations or modifications may be made based on the above description. It is not necessary and impossible to enumerate all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention are intended to be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a modified carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material, characterized in that: The following steps are involved: S1, dissolving a manganese source and a first iron source in a solvent, adding a first organic ligand and mixing them uniformly, and performing a first hydrothermal reaction to obtain MnFe-MOF; S2, dissolving a second iron source in a solvent and mixing and dispersing the mixture with the MnFe-MOF, adding a second organic ligand and performing a second hydrothermal reaction to in situ form an Fe-MOF coating layer on the surface of the MnFe-MOF to obtain MnFe-MOF / Fe-MOF; S3, after uniformly mixing the MnFe-MOF / Fe-MOF with a sodium source and a phosphorus source, grinding, drying and sintering in sequence to obtain a carbon-coated sodium manganese iron pyrophosphate / sodium iron pyrophosphate positive electrode material; S4, using a fluorine source as an etching gas, subjecting the carbon-coated sodium manganese iron phosphate pyrophosphate / sodium iron phosphate pyrophosphate cathode material to a high-temperature vapor phase etching treatment to obtain a fluorine-doped carbon-coated modified sodium manganese iron phosphate pyrophosphate / sodium iron phosphate pyrophosphate cathode material.

2. The method for preparing the modified carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material according to claim 1, characterized in that: In step S1, the molar ratio of the manganese source, the first iron source and the first organic ligand is (3-4.5): (0.8-2): (1.5-6).

3. The method for preparing the modified carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material according to claim 1, characterized in that: In step S2, the molar ratio of the second iron source to the second organic ligand is 1:(0.8-2).

4. The method for preparing the modified carbon-coated sodium manganese iron pyrophosphate / sodium iron pyrophosphate positive electrode material according to claim 1, characterized in that: The reaction conditions of the first hydrothermal reaction and the second hydrothermal reaction are independently selected from: reaction temperature 100-120° C., reaction time 5-10 h.

5. The method for preparing the modified carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material according to claim 1, characterized in that: In step S3, the stoichiometric ratio of (Mn+Fe):Na:P among the MnFe-MOF / Fe-MOF, the sodium source and the phosphorus source is 3:(4-4.4):(4-4.4).

6. The method for preparing the modified carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material according to claim 1, characterized in that: In step S3, the sintering temperature is 550-700° C., and the sintering time is 6-14 hours.

7. The method for preparing the modified carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material according to claim 1, characterized in that: In step S4, during the high-temperature vapor phase etching process, the mass ratio of the fluorine source to the carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material is (2-8):

1.

8. The method for preparing the modified carbon-coated sodium manganese ferric pyrophosphate / sodium ferric pyrophosphate positive electrode material according to claim 1, characterized in that: In step S4, the fluorine source is ammonium fluoride, the temperature of the high-temperature vapor phase etching treatment is 350-480° C., and the treatment time is 1-4 hours.

9. A modified carbon-coated sodium manganese iron pyrophosphate / sodium iron pyrophosphate positive electrode material, characterized in that: The modified carbon-coated sodium manganese iron pyrophosphate / sodium iron pyrophosphate cathode material is prepared by the preparation method of any one of claims 1 to 8.

10. Use of the modified carbon-coated sodium manganese iron pyrophosphate / sodium iron pyrophosphate positive electrode material according to claim 9 in the preparation of a sodium ion battery.

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