A high-compaction sodium iron pyrophosphate positive electrode material and a preparation method thereof

Abstract

The application relates to the technical field of batteries and discloses a high-compaction sodium iron pyrophosphate positive electrode material and a preparation method thereof, the positive electrode material being expressed by a chemical formula Na4Fe 2.9 V x Ti y Zr z (PO4)2P2O7, wherein 0.001<=x<=0.01, 0.002<=y<=0.01 and 0.001<=z<=0.01, and the preparation method is as follows: sodium source, iron source, phosphorus source, carbon source, vanadium source, titanium source and zirconium source are weighed and uniformly mixed, first sand milling treatment is carried out to obtain first sand milling slurry, spray drying is carried out, first sintering treatment is carried out to obtain pre-sintered material; proper carbon source is added into the pre-sintered material to carry out second sand milling treatment to obtain second grinding material; the second grinding material is subjected to spray drying, second sintering treatment is carried out to obtain the positive electrode material. The high-compaction sodium iron pyrophosphate positive electrode material provided by the application has high-compaction density, excellent rate performance and cycle performance, and can be used for preparing a sodium ion battery with high-compaction density, high capacity, excellent rate performance and cycle performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery cathode material preparation technology, specifically to a high-pressure solid pyrophosphate sodium iron phosphate cathode material and its preparation method. Background Technology

[0002] Sodium iron phosphate pyrophosphate is one of the three main materials for sodium-ion cathodes, with a theoretical specific capacity of 120 mAh / g. It boasts advantages such as high operating voltage, high safety, long cycle life, and good low-temperature performance, making it suitable for large-scale energy storage applications. However, sodium iron phosphate pyrophosphate materials have consistently suffered from low compaction density; currently, the powder compaction density of industrially produced sodium iron phosphate pyrophosphate materials is generally 1.9~2.0 g / cm³. 3 For sodium-ion batteries, the higher the powder compaction density of the cathode material, the higher the volumetric energy density of the battery can be, and the stronger its product competitiveness.

[0003] Patent application CN119637395A discloses a production line and method for high-pressure sodium-ion battery cathode materials, including a primary sintering unit and a secondary sintering unit. Through equipment layout and process integration, it provides a production line and method capable of continuous large-scale production of high-pressure sodium-ion battery cathode materials. On the one hand, it can improve yield and increase production capacity; on the other hand, it can simultaneously produce two or more different particle sizes and achieve particle size distribution to obtain high-pressure sodium-ion battery cathode materials.

[0004] However, there are still multiple technical routes to choose from for high-pressure solid pyrophosphate sodium iron phosphate cathode materials, and the process stability needs to be improved. Therefore, large-scale production and widespread application of high-pressure solid pyrophosphate sodium iron phosphate cathode materials cannot yet be realized. Summary of the Invention

[0005] To address the shortcomings of existing technologies, one of the objectives of this invention is to provide a high-pressure solid pyrophosphate sodium iron phosphate cathode material and its preparation method.

[0006] The objective of this invention can be achieved through the following technical solutions:

[0007] A high-pressure solid pyrophosphate sodium iron phosphate cathode material, the chemical formula of which is: Na₄Fe 2.9 V x Ti y Zr z (PO4)2P2O7, where 0.001≤x≤0.01, 0.002≤y≤0.01, and 0.001≤z≤0.01.

[0008] The preparation method of the high-pressure solid pyrophosphate sodium iron phosphate cathode material includes the following steps:

[0009] Step (1): Add sodium source, iron source, phosphorus source, carbon source, vanadium source, titanium source and zirconium source to solvent water and disperse evenly to obtain slurry;

[0010] Step (2): Place the mixed slurry in a sand mill and perform a sand milling process to obtain the first grinding material;

[0011] Step (3): Transfer the first grinding material into a spray dryer and spray granulate it to obtain precursor powder;

[0012] Step (4): The precursor powder is subjected to a first sintering treatment to obtain the pre-burned material;

[0013] Step (5): Add a carbon source to the pre-burned material and perform secondary sand milling to obtain the second grinding material;

[0014] Step (6): After spray drying the second abrasive, a second sintering process is performed to obtain the positive electrode material.

[0015] Preferably, in step (1), the iron source is lamellar iron phosphate, the lamellar iron phosphate has a lamellar thickness of <200nm, and the lamellar iron phosphate has a particle size D50 of <15μm.

[0016] The pores between the layers ensure more thorough contact between the dopant elements and the iron phosphate grains during sintering. At the same time, the thinner layer thickness facilitates the diffusion of dopant ions during high-temperature sintering, thus ensuring the doping effect.

[0017] Preferably, in step (1), the sodium source is one or more of sodium carbonate, sodium oxalate, sodium pyrophosphate, sodium dihydrogen pyrophosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate;

[0018] Preferably, the phosphorus source is one or more of sodium pyrophosphate, sodium dihydrogen pyrophosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate;

[0019] Preferably, the carbon source is one or more of glucose, sucrose, citric acid, ascorbic acid, and polyethylene glycol;

[0020] Preferably, the vanadium source is one or more selected from vanadium carbonate, vanadium pentoxide, and ammonium metavanadate;

[0021] Preferably, the titanium source is one or more of titanium dioxide, tetrabutyl titanate, and tetraisopropyl titanate;

[0022] Preferably, the zirconium source is one or more of sodium zirconate, zirconium dioxide, and zirconium hydroxide.

[0023] Preferably, in step (1), the molar ratio of sodium, iron, and phosphorus is 4:2.9:4, and the carbon source accounts for 6%-10% of the total weight of the iron source;

[0024] Preferably, the molar doping amounts of V, Ti, and Zr are: 0.001≤x≤0.01, 0.002≤y≤0.01, and 0.001≤z≤0.01.

[0025] Preferably, in step (2), the specific steps of the first sand milling process are as follows: the mixed slurry is placed in a sand mill, and zirconia beads with a diameter of 0.3-0.5 mm are used to sand mill at a speed of 800 rpm for 4 hours, and the particle size distribution of the first grinding material is controlled to be D50=8μm and D90=15μm.

[0026] Preferably, in step (3), the specific steps of spray granulation are as follows: the first grinding material is transferred into a spray dryer and granulated under the conditions of an air inlet temperature of 200°C and an atomization pressure of 3 bar to obtain a spherical precursor powder with a smooth surface and a particle size of 10-20 μm.

[0027] Preferably, in step (4), the sintering regime for the first sintering is as follows: the temperature is increased from room temperature to 300°C at a heating rate of 2°C / min, held at N2 atmosphere for 3 hours, then increased to 500°C, and held at 5wt% H2 / N2 reducing atmosphere for 12 hours.

[0028] Preferably, in step (5), the carbon source accounts for 4%-8% of the total weight of the pre-burned material; the sintering regime of the secondary sintering is: heating from room temperature to 530℃ at a heating rate of 2℃ / min, and holding at that temperature for 10h in a N2 atmosphere.

[0029] The second objective of this invention is to provide a sodium-ion battery, comprising a positive electrode material, an electrolyte, a negative electrode material, and a separator, wherein the positive electrode material is the aforementioned high-pressure solid pyrophosphate sodium iron phosphate positive electrode material.

[0030] Compared with the prior art, the advantages of the present invention are as follows:

[0031] (1) The doping scheme adopted in this invention can improve the compaction density and electrochemical activity of sodium iron pyrophosphate: the doping of titanium can improve the compaction density of the material. At the same time, after the introduction of vanadium and zirconium, the doping of vanadium and zirconium atoms can form a three-dimensional diffusion channel of sodium ions in the structure of sodium iron pyrophosphate, thereby improving the capacity and rate performance.

[0032] (2) The secondary sand milling process adopted in this invention, by precisely controlling the sand milling parameters, combines small-diameter grinding beads with low speed to achieve directional cleavage and crushing of lamellar iron phosphate, retaining the lamellar structure while controlling the particle size, ensuring that anisotropic stacked spherical cores are formed during spray drying, and the tap density is increased by 15% compared with traditional particles; the spherical precursor can reduce grain boundary defects in subsequent sintering, and the smooth surface reduces the interfacial resistance during carbon coating.

[0033] (3) The secondary sintering process used in this invention can achieve uniform coating of carbon source and effectively improve the conductivity of the material. First, the sheet precursor is pre-coated with carbon source, which can improve the effect of the first carbon coating. After the first sintering, the second coating is carried out by secondary sand milling, thereby improving the uniformity of carbon coating of the overall structure, overcoming the defects caused by morphological differences, and improving the uniformity of carbon coating of the material as a whole, thereby obtaining better conductivity.

[0034] (4) The chemical ratio of sodium, iron and phosphorus in the raw materials used in this invention is easy to control through ingredient mixing, and the raw materials are simple and readily available; the process used in this invention is simple, reliable, fast, and easy to mass-produce and widely apply.

[0035] (5) XRD results show that the sodium iron pyrophosphate prepared by this method has a typical orthorhombic crystal system and space group Pn21a. In the voltage range of 2-4 V, the discharge specific capacity at 0.1 C can reach as high as 102 mAh / g. After 50 charge-discharge cycles at 1 C rate, the discharge specific capacity still remains at 90 mAh / g, and the coulombic efficiency is stable at 99.9%. Its excellent cycle stability makes it suitable for large-scale energy storage. Attached Figure Description

[0036] Figure 1 This is a scanning electron microscope image of the lamellar iron phosphate used in this invention.

[0037] Figure 2 This is a scanning electron microscope image of sodium iron pyrophosphate prepared in Example 1 of the present invention.

[0038] Figure 3 The image shows the XRD pattern of sodium iron pyrophosphate prepared in Example 1 of this invention.

[0039] Figure 4 This is a rate charge-discharge curve of the sodium-ion battery prepared in Example 1 of the present invention.

[0040] Figure 5 This is a cycle curve of the sodium-ion battery prepared in Example 1 of the present invention at a 1C rate. Detailed Implementation

[0041] The following detailed description, in conjunction with specific embodiments, illustrates a high-pressure solid pyrophosphate sodium iron phosphate cathode material and its preparation method. These embodiments are for comparative and illustrative purposes only, and the present invention is not limited to these embodiments. Example 1

[0042] A method for preparing a high-pressure solid pyrophosphate sodium iron phosphate cathode material includes the following steps:

[0043] (1) Weigh 1.45 mol of sodium oxalate, 2.9 mol of ferric phosphate, 1.1 mol of sodium dihydrogen phosphate, 7.5% of the total weight of ferric phosphate glucose, 0.0015 mol of ammonium metavanadate, 0.003 mol of tetrabutyl titanate and 0.0015 mol of zirconium carbonate and add them to deionized water in sequence for dispersion.

[0044] (2) Place the mixed slurry in a sand mill and perform a sand milling process, that is, use 0.5 mm diameter zirconia beads and sand mill at 800 rpm for 4 hours to control the particle size distribution of the first grinding material D50=8μm, D90=15μm to obtain the first grinding material. Transfer the first grinding material to a spray dryer and granulate it under the conditions of inlet air temperature 200℃ and atomization pressure 3 bar to obtain precursor powder.

[0045] (3) The precursor powder is subjected to a sintering process, that is, the temperature is raised from room temperature to 300℃ at a sintering heating rate of 2℃ / min and held for 3h, and then raised to 500℃ and held for 12h to obtain the pre-sintered material.

[0046] (4) Add 8% of the total weight of polyethylene glycol to the pre-fired material for secondary sand milling to obtain the second grinding material; transfer the second grinding material into a spray dryer for spray granulation.

[0047] (5) The second abrasive after spray granulation is subjected to secondary sintering treatment, that is, the temperature is raised from room temperature to 530℃ at a sintering heating rate of 2℃ / min and held for 10h, and then held for 10h in N2 atmosphere to obtain the positive electrode material. Example 2

[0048] A method for preparing a high-pressure solid pyrophosphate sodium iron phosphate cathode material includes the following steps:

[0049] (1) Weigh 1.45 mol of sodium oxalate, 2.9 mol of ferric phosphate, 1.1 mol of sodium dihydrogen phosphate, 7.5% of the total weight of ferric phosphate, 0.001 mol of ammonium metavanadate, 0.002 mol of tetrabutyl titanate, and 0.001 mol of zirconium carbonate and add them to deionized water in sequence for dispersion.

[0050] (2) Place the mixed slurry in a sand mill and perform a sand milling process, that is, use 0.3 mm diameter zirconia beads and sand mill at 800 rpm for 4 hours to control the particle size distribution of the first grinding material D50=8μm, D90=15μm to obtain the first grinding material. Transfer the first grinding material to a spray dryer and granulate it under the conditions of inlet air temperature 200℃ and atomization pressure 3 bar to obtain precursor powder.

[0051] (3) The precursor powder is subjected to a sintering process, that is, the temperature is raised from room temperature to 300℃ at a sintering heating rate of 2℃ / min and held for 3h, and then raised to 500℃ and held for 12h to obtain the pre-sintered material.

[0052] (4) Add 8% of the total weight of polyethylene glycol to the pre-fired material for secondary sand milling to obtain the second grinding material; transfer the second grinding material into a spray dryer for spray granulation.

[0053] (5) The second abrasive after spray granulation is subjected to secondary sintering treatment, that is, the temperature is raised from room temperature to 530℃ at a sintering heating rate of 2℃ / min and held for 10h, and then held for 10h in N2 atmosphere to obtain the positive electrode material. Example 3

[0054] A method for preparing a high-pressure solid pyrophosphate sodium iron phosphate cathode material includes the following steps:

[0055] (1) Weigh 1.45 mol of sodium oxalate, 2.9 mol of ferric phosphate, 1.1 mol of sodium dihydrogen phosphate, 7.5% of the total weight of ferric phosphate, 0.002 mol of ammonium metavanadate, 0.004 mol of tetrabutyl titanate, and 0.002 mol of zirconium carbonate and add them to deionized water in sequence for dispersion.

[0056] (2) Place the mixed slurry in a sand mill and perform a sand milling process, that is, use 0.5 mm diameter zirconia beads and sand mill at 800 rpm for 4 hours to control the particle size distribution of the first grinding material D50=8μm, D90=15μm to obtain the first grinding material. Transfer the first grinding material to a spray dryer and granulate it under the conditions of inlet air temperature 200℃ and atomization pressure 3 bar to obtain precursor powder.

[0057] (3) The precursor powder is subjected to a sintering process, that is, the temperature is raised from room temperature to 300℃ at a sintering heating rate of 2℃ / min and held for 3h, and then raised to 500℃ and held for 12h to obtain the pre-sintered material.

[0058] (4) Add 8% of the total weight of polyethylene glycol to the pre-fired material for secondary sand milling to obtain the second grinding material; transfer the second grinding material into a spray dryer for spray granulation.

[0059] (5) The second abrasive after spray granulation is subjected to secondary sintering treatment, that is, the temperature is raised from room temperature to 530℃ at a sintering heating rate of 2℃ / min and held for 10h, and then held for 10h in N2 atmosphere to obtain the positive electrode material. Example 4

[0060] A method for preparing a high-pressure solid pyrophosphate sodium iron phosphate cathode material includes the following steps:

[0061] (1) Weigh 1.45 mol of sodium oxalate, 2.9 mol of ferric phosphate, 1.1 mol of sodium dihydrogen phosphate, 10% of the total weight of ferric phosphate glucose, 0.0015 mol of ammonium metavanadate, 0.003 mol of tetrabutyl titanate and 0.0015 mol of zirconium carbonate and add them to deionized water in sequence for dispersion.

[0062] (2) Place the mixed slurry in a sand mill and perform a sand milling process, that is, use 0.3 mm diameter zirconia beads and sand mill at 800 rpm for 4 hours to control the particle size distribution of the first grinding material D50=8μm, D90=15μm to obtain the first grinding material. Transfer the first grinding material to a spray dryer and granulate it under the conditions of inlet air temperature 200℃ and atomization pressure 3 bar to obtain precursor powder.

[0063] (3) The precursor powder is subjected to a sintering process, that is, the temperature is raised from room temperature to 300℃ at a sintering heating rate of 2℃ / min and held for 3h, and then raised to 500℃ and held for 12h to obtain the pre-sintered material.

[0064] (4) Add 4% of the total weight of polyethylene glycol to the pre-fired material for secondary sand milling to obtain the second grinding material; transfer the second grinding material into a spray dryer for spray granulation.

[0065] (5) The second abrasive after spray granulation is subjected to secondary sintering treatment, that is, the temperature is raised from room temperature to 530℃ at a sintering heating rate of 2℃ / min and held for 10h, and then held for 10h in N2 atmosphere to obtain the positive electrode material. Example 5

[0066] A method for preparing a high-pressure solid pyrophosphate sodium iron phosphate cathode material includes the following steps:

[0067] (1) Weigh 1.45 mol of sodium oxalate, 2.9 mol of ferric phosphate, 1.1 mol of sodium dihydrogen phosphate, 6% of the total weight of ferric phosphate glucose, 0.0015 mol of ammonium metavanadate, 0.003 mol of tetrabutyl titanate and 0.0015 mol of zirconium carbonate and add them to deionized water in sequence for dispersion.

[0068] (2) Place the mixed slurry in a sand mill and perform a sand milling process, that is, use 0.5 mm diameter zirconia beads and sand mill at 800 rpm for 4 hours to control the particle size distribution of the first grinding material D50=8μm, D90=15μm to obtain the first grinding material. Transfer the first grinding material to a spray dryer and granulate it under the conditions of inlet air temperature 200℃ and atomization pressure 3 bar to obtain precursor powder.

[0069] (3) The precursor powder is subjected to a sintering process, that is, the temperature is raised from room temperature to 300℃ at a sintering heating rate of 2℃ / min and held for 3h, and then raised to 500℃ and held for 12h to obtain the pre-sintered material.

[0070] (4) Add 8% of the total weight of polyethylene glycol to the pre-fired material for secondary sand milling to obtain the second grinding material; transfer the second grinding material into a spray dryer for spray granulation.

[0071] (5) The second abrasive after spray granulation is subjected to secondary sintering treatment, that is, the temperature is raised from room temperature to 530℃ at a sintering heating rate of 2℃ / min and held for 10h, and then held for 10h in N2 atmosphere to obtain the positive electrode material.

[0072] Comparative Example 1

[0073] A method for preparing a high-pressure solid pyrophosphate sodium iron phosphate cathode material includes the following steps:

[0074] (1) Weigh 1.45 mol of sodium oxalate, 2.9 mol of ferric phosphate, 1.1 mol of sodium dihydrogen phosphate, and 7.5% of glucose by weight of ferric phosphate and add them to deionized water to disperse.

[0075] (2) Place the mixed slurry in a sand mill and perform a sand milling process, that is, use 0.5 mm diameter zirconia beads and sand mill at 800 rpm for 4 hours to control the particle size distribution of the first grinding material D50=8μm, D90=15μm to obtain the first grinding material. Transfer the first grinding material to a spray dryer and granulate it under the conditions of inlet air temperature 200℃ and atomization pressure 3 bar to obtain precursor powder.

[0076] (3) The precursor powder is subjected to a sintering process, that is, the temperature is raised from room temperature to 300℃ at a sintering heating rate of 2℃ / min and held for 3h, and then raised to 500℃ and held for 12h to obtain the pre-sintered material.

[0077] (4) Add 8% of the total weight of polyethylene glycol to the pre-fired material for secondary sand milling to obtain the second grinding material; transfer the second grinding material into a spray dryer for spray granulation.

[0078] (5) The second abrasive after spray granulation is subjected to secondary sintering treatment, that is, the temperature is raised from room temperature to 530℃ at a sintering heating rate of 2℃ / min and held for 10h, and then held for 10h in N2 atmosphere to obtain the positive electrode material.

[0079] Comparative Example 2

[0080] A method for preparing a high-pressure solid pyrophosphate sodium iron phosphate cathode material includes the following steps:

[0081] (1) Weigh 1.45 mol of sodium oxalate, 2.9 mol of ferric phosphate, 1.1 mol of sodium dihydrogen phosphate, 7.5% of the total weight of ferric phosphate, glucose, 0.01 mol of ammonium metavanadate, 0.01 mol of tetrabutyl titanate and 0.01 mol of zirconium carbonate and add them to deionized water in sequence for dispersion.

[0082] (2) Place the mixed slurry in a sand mill and perform a sand milling process, that is, use 0.5 mm diameter zirconia beads and sand mill at 800 rpm for 4 hours to control the particle size distribution of the first grinding material D50=8μm, D90=15μm to obtain the first grinding material. Transfer the first grinding material to a spray dryer and granulate it under the conditions of inlet air temperature 200℃ and atomization pressure 3 bar to obtain precursor powder.

[0083] (3) The precursor powder is subjected to a sintering process, that is, the temperature is raised from room temperature to 300℃ at a sintering heating rate of 2℃ / min and held for 3h, and then raised to 500℃ and held for 12h to obtain the pre-sintered material.

[0084] (4) Add 8% of the total weight of polyethylene glycol to the pre-fired material for secondary sand milling to obtain the second grinding material; transfer the second grinding material into a spray dryer for spray granulation.

[0085] (5) The second abrasive after spray granulation is subjected to secondary sintering treatment, that is, the temperature is raised from room temperature to 530℃ at a sintering heating rate of 2℃ / min and held for 10h, and then held for 10h in N2 atmosphere to obtain the positive electrode material.

[0086] Comparative Example 3

[0087] A method for preparing a high-pressure solid pyrophosphate sodium iron phosphate cathode material includes the following steps:

[0088] (1) Weigh 1.45 mol of sodium oxalate, 2.9 mol of ferric phosphate, 1.1 mol of sodium dihydrogen phosphate, 7.5% of glucose (total weight of ferric phosphate), 8% of polyethylene glycol (total weight of ferric phosphate), 0.01 mol of ammonium metavanadate, 0.01 mol of tetrabutyl titanate, and 0.01 mol of zirconium carbonate and add them to deionized water in sequence for dispersion.

[0089] (2) The mixed slurry is placed in a sand mill for sand milling, that is, zirconia beads with a diameter of 0.5 mm are used to sand mill at 800 rpm for 4 hours. The particle size distribution of the first grinding material is controlled to be D50=8μm and D90=15μm to obtain the first grinding material. The first grinding material is then transferred to a spray dryer and granulated under the conditions of inlet air temperature of 200℃ and atomization pressure of 3 bar to obtain precursor powder.

[0090] (3) The precursor powder is sintered, that is, the temperature is raised from room temperature to 300℃ at a sintering heating rate of 2℃ / min and held for 3h, and then raised to 500℃ and held for 12h to obtain the cathode material.

[0091] test:

[0092] Sodium iron pyrophosphate prepared in Examples 1-5 and Comparative Examples 1-3 was used as the positive electrode active material for sodium-ion batteries. It was mixed with conductive agent SP and binder PVDF at a mass ratio of 90:5:5, with aluminum foil as the current collector, forming the positive electrode. A metallic sodium sheet was used as the negative electrode for sodium-ion batteries, glass fiber was used as the separator, and a 1 mol / L NaPF6 EC / DMC (volume ratio 1:1) + 5% FEC solution was used as the electrolyte. The batteries were assembled in a glove box under an argon atmosphere with an oxygen partial pressure of less than 0.01 PPM, a moisture pressure of less than 0.01 PPM, and constant current / constant voltage charge-discharge mode. Charge-discharge tests were conducted using a voltage range of 2-4 V and a current density of 0.1 C-0.2 C-0.5 C-1 C.

[0093] The sodium iron phosphate pyrophosphate prepared in Examples 1-5 and Comparative Examples 1-3 were used as test samples for the compaction density tester. The test was conducted using a PCD2000 compaction density tester from Yuaneng Technology under a test pressure of 3T.

[0094] Table 1: Performance Test Data of Each Embodiment and Comparative Example

[0095]

[0096] Table 1 shows the test results of the eight groups of experiments. The compaction density test results show that the material prepared by doping 1500 ppm vanadium, 3000 ppm titanium, and 1500 ppm zirconium per mol of sodium iron pyrophosphate has a higher compaction density. In Example 2, the doping ratio of each element was reduced, and in Example 3, the doping ratio of each element was increased; the compaction density of the prepared materials was lower than that of Example 1. In Comparative Example 1, no element was doped, and in Comparative Example 2, an excessive amount of element was doped; the compaction density of the prepared materials was significantly lower than that of Example 1. This indicates that the element doping ratio in Example 1 is the most reasonable. In Example 4, the glucose ratio in the primary abrasive was increased, and in Example 5, the glucose sugar ratio in the primary abrasive was decreased; the compaction density of the prepared materials was slightly lower than that of Example 1. This indicates that the carbon source pre-coating effect in Example 1 is the most reasonable. In Comparative Example 3, no secondary sintering process was used; the compaction density of the prepared materials was significantly lower than that of Example 1. This indicates that the secondary sintering process used in Example 1 is more reasonable.

[0097] Figure 1 This is a scanning electron microscope image of the layered iron phosphate used in this invention. As can be seen from the image, the iron phosphate exhibits a typical layered lamellar structure, with a lamellar thickness of <200 nm; the particle size D50 of the layered iron phosphate is <15 μm.

[0098] Figure 2 The image shows a scanning electron microscope (SEM) image of sodium iron pyrophosphate prepared in Example 1 of this invention. As can be seen from the image, the particles are spherical, with a particle size of 2-20 μm. The particle surfaces are relatively smooth, and the carbon coating on the particle surfaces is well-done.

[0099] Figure 3 The image shows the XRD pattern of the NFPP prepared in Example 1 of this invention. The crystal structure of the material was analyzed by X-ray diffraction. The results showed that diffraction peaks of the NFPP composite phase existed at 9.8º, 15.8º, 16.7º, 32.1º, and 33.6º, corresponding to the (200), (011), (210), (022), and (222) crystal planes of the material, respectively. The XRD results confirm that the material synthesized using this formulation and process is a sodium iron phosphate pyrophosphate crystal phase.

[0100] Figure 4 This is a rate charge-discharge curve of the sodium-ion battery prepared in Example 1 of the present invention. The sample can provide discharge specific capacities of 106.50 mAh / g, 105.01 mAh / g, 102.94 mAh / g, and 101.54 mAh / g at rates of 0.1C, 0.2C, 0.5C, and 1C, respectively, and the 1C capacity retention rate is 95.34%.

[0101] Figure 5 This is a cycle curve of the sodium-ion battery prepared in Example 1 of this invention at a 1C rate. After 50 charge-discharge cycles at a 1C rate, the discharge specific capacity still remains at 100.64 mAh / g, and the coulombic efficiency is stable at 99.9%, indicating that the material has excellent cycle stability.

[0102] This invention illustrates a high-pressure solid pyrophosphate sodium iron phosphate cathode material and its preparation method through the above embodiments. However, this invention is not limited to the above embodiments, meaning that this invention does not necessarily depend on the above embodiments for implementation. Those skilled in the art should understand that any improvements to this invention, equivalent substitutions of raw materials, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection and disclosure scope of this invention.

Claims

1. A high-pressure solid pyrophosphate sodium iron phosphate cathode material, characterized in that, The chemical formula of the cathode material is: Na₄Fe₂ 2.9 V x Ti y Zr z (PO4)2P2O7, where 0.001≤x≤0.01, 0.002≤y≤0.01, 0.001≤z≤0.01; The method for preparing the cathode material includes the following steps: Step (1): Add sodium source, iron source, phosphorus source, carbon source, vanadium source, titanium source and zirconium source to solvent water and disperse evenly to obtain slurry. The iron source is lamellar iron phosphate, the lamellar iron phosphate has a lamellar thickness of <200nm and a particle size D50 of <15μm. The molar doping amount of V, Ti and Zr is: 0.001≤x≤0.01, 0.002≤y≤0.01, 0.001≤z≤0.

01. Step (2): Place the mixed slurry in a sand mill and perform a sand milling process to obtain the first grinding material; Step (3): Transfer the first grinding material into a spray dryer and spray granulate it to obtain precursor powder; Step (4): The precursor powder is subjected to a first sintering treatment to obtain the pre-burned material; Step (5): Add a carbon source to the pre-burned material and perform secondary sand milling to obtain the second grinding material; Step (6): After spray drying the second abrasive, a second sintering process is performed to obtain the positive electrode material.

2. The high-pressure solid pyrophosphate sodium iron phosphate cathode material according to claim 1, characterized in that, In step (1), the sodium source is one or more of sodium carbonate, sodium oxalate, sodium pyrophosphate, sodium dihydrogen pyrophosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate. The phosphorus source is one or more of sodium pyrophosphate, sodium dihydrogen pyrophosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. The carbon source is one or more of glucose, sucrose, citric acid, ascorbic acid, and polyethylene glycol; The vanadium source is one or more of vanadium carbonate, vanadium pentoxide, and ammonium metavanadate. The titanium source is one or more of titanium dioxide, tetrabutyl titanate, and tetraisopropyl titanate. The zirconium source is one or more of sodium zirconate, zirconium dioxide, and zirconium hydroxide.

3. The high-pressure solid pyrophosphate sodium iron phosphate cathode material according to claim 1, characterized in that, In step (1), the molar ratio of sodium, iron and phosphorus is 4:2.9:4, and the carbon source accounts for 6%-10% of the total weight of the iron source.

4. The high-pressure solid pyrophosphate sodium iron phosphate cathode material according to claim 1, characterized in that, In step (2), the specific steps of the first sand milling process are as follows: the mixed slurry is placed in a sand mill, and zirconia beads with a diameter of 0.3-0.5 mm are used to sand mill at a speed of 800 rpm for 4 hours, and the particle size distribution of the first grinding material is controlled to be D50=8μm and D90=15μm.

5. The high-pressure solid pyrophosphate sodium iron phosphate cathode material according to claim 1, characterized in that, In step (3), the specific steps of spray granulation are as follows: the first grinding material is transferred into a spray dryer and granulated under the conditions of air inlet temperature of 200℃ and atomization pressure of 3bar to obtain spherical precursor powder with smooth surface and particle size of 10-20μm.

6. The high-pressure solid pyrophosphate sodium iron phosphate cathode material according to claim 1, characterized in that, In step (4), the sintering regime for the first sintering is as follows: the temperature is increased from room temperature to 300℃ at a heating rate of 2℃ / min, held at N2 atmosphere for 3h, then increased to 500℃, and held at 5wt% H2 / N2 reducing atmosphere for 12h.

7. The high-pressure solid pyrophosphate sodium iron phosphate cathode material according to claim 1, characterized in that, In step (5), the carbon source accounts for 4%-8% of the total weight of the pre-burned material; the sintering regime of the secondary sintering is: heating from room temperature to 530℃ at a heating rate of 2℃ / min, and holding at that temperature for 10h in N2 atmosphere.

8. A sodium-ion battery, comprising a positive electrode material, an electrolyte, a negative electrode material, and a separator, characterized in that, The cathode material is the high-pressure solid pyrophosphate sodium iron phosphate cathode material as described in claim 1.

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