Method for preparing iron-based polyanionic compound by liquid phase method and application of iron-based polyanionic compound

Abstract

The invention discloses a method for preparing an iron-based polyanionic compound by using a liquid phase method and application of the iron-based polyanionic compound. The method comprises the following steps: weighing iron phosphate, a sodium source, a phosphorus source and a carbon source according to a stoichiometric ratio, mixing with deionized water, and performing coarse grinding to obtain a first mixed solution; conveying the solution into a tank body of a nanoscale sand mill, adding a surfactant, and continuously sanding to obtain a precursor solution; adding a dehydrating agent, uniformly mixing, standing, removing supernate, and drying; and crushing the dried material, and sintering in an inert atmosphere to obtain the product. According to the preparation method, FePO4 is taken as a growth template, Na4Fe3 (PO4) 2 (P2O7) spherical powder is synthesized, the surfactant is added, so that the production time can be shortened, the particle size of the product is controlled to be 0.1-0.4 mu m, and the powder is more uniform after being dried and crushed after being dehydrated by the dehydrating agent. And when being applied to the positive electrode of the sodium-ion battery, the composite material has very high specific capacity and excellent rate capability.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparing electrode materials for sodium-ion batteries, and particularly relates to a method for preparing iron-based polyanionic compounds by a liquid-phase method and applications thereof. Background Art

[0002] With the development of industrialization and the growth of the world's population, the global demand for energy has been continuously increasing. So far, the energy supply still mainly relies on non-renewable fossil fuels (such as coal, oil, natural gas, etc.), and the resulting energy shortage and environmental pollution problems have become increasingly serious. Therefore, it is necessary to develop new energy sources to reduce our dependence on fossil fuels. New energy sources mainly based on solar energy and wind energy have the characteristics of rich resources, renewable, and low environmental pollution. However, these renewable energy sources are difficult to directly output as stable and continuous energy. Therefore, there is an urgent need to study high-efficiency and low-cost large-scale energy storage technologies to ensure that new energy sources can be efficiently applied to people's daily lives. Among all energy storage technologies, secondary batteries represented by lithium-ion batteries are considered to be an excellent choice for energy conversion and storage. With the continuous increase in people's demand for lithium-ion batteries, it has directly led to increasingly fierce resource competition and a sharp rise in the cost of lithium resources. To meet the market demand and the needs of social sustainable development, sodium-ion batteries have received extensive attention and research due to their rich sodium resources, low cost, and similar working principles to lithium-ion batteries.

[0003] Currently, the development of sodium-ion batteries is restricted by the cathode materials. The cathode materials being studied include oxides, Prussian blue compounds, and polyanionic compounds. Compared with the other two types, in terms of resource abundance, overall material cost, material electrochemical performance, and environmental sustainability, polyanionic cathode materials are a relatively good choice. Among them, the iron-based polyanionic compound Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) has received extensive attention due to its good structural stability and environmental friendliness. To improve the electrical performance of Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) researchers have adopted different synthesis methods to obtain Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7)Micro-nano powder, but these methods are prone to agglomeration during the process, difficult to control, with high process costs, and the decomposition products seriously pollute the environment. Patent document (CN 113060713 A) provides a method for preparing Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) by wet homogeneous sanding combined with spray drying. This method has high requirements for iron source, phosphorus source, sodium source, carbon source and organic solvents in industrial production. Patent document (CN 113060714 B) provides a method for preparing Na 4 from the FePO 4 liquid phase to prepare Na 3 (PO 4 ) 2 (P 2 O 7 ) method. This method has high requirements for equipment space, and particles are prone to agglomeration during the preparation process and are difficult to control. Therefore, it is urgent to seek suitable raw materials, preparation methods, process conditions and equipment to develop a simple, environmentally friendly and easy-to-industrialize method for preparing Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ). Summary of the Invention

[0004] In view of some problems existing in the preparation process of current Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) materials, the present invention provides a preparation method for Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) that is simple, environmentally friendly and easy to industrialize. This method uses FePO4 for liquid phase sanding and then combines dehydration drying and sintering to prepare high-purity phase Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) materials.

[0005] The Na 4 Fe 3 (PO4 ) 2 (P 2 O 7 ) preparation method, specifically including the following steps:

[0006] (1) Weigh iron phosphate, sodium source, phosphorus source, carbon source, surfactant, dehydrating agent and deionized water according to the stoichiometric ratio and curing ratio respectively;

[0007] (2) Mix the iron phosphate, sodium source, phosphorus source, carbon source and deionized water, then place them in a coarse sand mill, add sand grinding medium, and grind coarsely for 1 - 2 h to obtain a first mixed solution;

[0008] (3) Transport the first mixed solution through a pipeline into a nano - level sand mill tank, add the surfactant and stir, continue to grind for 1 - 4 h, and control the particle size between 0.2 - 1.0 μm to obtain a precursor solution;

[0009] (4) Add the dehydrating agent to the precursor solution, mix evenly, then transfer it to a static drying tank and let it stand for 1 - 6 h. Open the valve of the static drying tank, drain the supernatant, and then dry to obtain a dried material;

[0010] (5) After crushing the dried material, sinter it under an inert atmosphere to obtain the Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) powder.

[0011] Preferably, in the step (1), the sodium source includes at least one of disodium hydrogen phosphate, sodium carbonate, sodium oxalate, sodium bicarbonate, sodium citrate, trisodium citrate dihydrate, trisodium citrate pentahydrate, sodium gluconate;

[0012] Preferably, in the step (1), the phosphorus source is one or more of phosphoric acid and phosphates; the phosphates include one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate.

[0013] Preferably, in the step (1), the carbon source includes at least one of citric acid, glucose, sucrose, starch, sodium citrate, trisodium citrate dihydrate, trisodium citrate pentahydrate, sodium gluconate.

[0014] Preferably, in the step (1), the surfactant includes an anionic surfactant, a non-ionic surfactant, and an amphoteric ion surfactant; the anionic surfactant includes at least one of sodium alcohol ether sulfate (AES), alkylbenzene sulfonate (LAS), sodium alcohol sulfate (AS), olefin sulfonate (AOS), sulfosuccinate, and phosphate ester; the non-ionic surfactant includes at least one of fatty alcohol polyoxyethylene ether, fatty amine polyoxyethylene ether, fatty acid polyoxyethylene ester, alkylphenol polyoxyethylene ether, polyol ester, and alkanolamide; the amphoteric ion surfactant includes at least one of betaines, imidazolines, amine oxides, and amino acids.

[0015] Preferably, in the step (1), the dehydrating agent includes at least one of cationic polyacrylamide, ester quaternary ammonium salt, dialkyl quaternary ammonium salt, and modified dialkyl quaternary ammonium salt.

[0016] Preferably, in the step (1), the curing ratio is 20%-50%. In the present invention, the curing ratio refers to the percentage of the required feeding amount in the total mass.

[0017] Preferably, in the step (2), the sand milling method of the sand mill includes one of a disk type, a rod pin type, and a turbine type.

[0018] Preferably, in the step (2), the sand milling medium includes one or more of natural sandstone, glass beads, steel beads, zirconia beads, zirconium silicate beads, and agate beads.

[0019] Preferably, in the step (2), the particle size D50 of the particles in the first mixed solution is 1-2 μm.

[0020] Preferably, in the step (4), the static drying tank is a dual-purpose device for static precipitation drainage and drying.

[0021] Preferably, in the step (5), the sintering conditions are: the sintering temperature is 450-550 °C, the time is 8-15 h, and the heating rate of the sintering is 2-5 °C / min.

[0022] Preferably, in the step (5), the inert atmosphere includes one of argon, nitrogen, argon-hydrogen mixture gas, and nitrogen-hydrogen mixture gas.

[0023] Preferably, in the step (5), the ventilation rate of the inert atmosphere is 0.1-0.3 L / h.

[0024] Preferably, in the step (5), the crushing is carried out in a crusher.

[0025] Na prepared by the above method 4 Fe 3(PO 4 ) 2 (P 2 O 7 ) materials also fall within the scope of protection of the present invention.

[0026] Preferably, the Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) material is spherical particles with a particle size of 100 - 400 nm.

[0027] The present invention also protects the application of the above-mentioned Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) material.

[0028] The application is the application of the Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) material as a positive electrode material for a sodium-ion battery in the preparation of a sodium-ion battery.

[0029] The present invention also protects a positive electrode plate for a sodium-ion battery.

[0030] The positive electrode plate for a sodium-ion battery includes a current collector and a positive electrode material disposed on the current collector, and the positive electrode material includes the Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) material prepared above in the present invention.

[0031] Exemplarily, the current collector is, for example, carbon-coated aluminum foil.

[0032] A conductive agent and a binder are also disposed on the current collector. Exemplarily, the mass ratio of the positive electrode material:conductive agent:binder is 100:(2 - 10):(2 - 10).

[0033] Exemplarily, the conductive agent is, for example, SP, and the binder is, for example, polyvinylidene fluoride PVDF.

[0034] The present invention also protects a sodium-ion battery.

[0035] The sodium-ion battery includes a positive electrode plate, a negative electrode plate, and a separator disposed between the positive electrode plate and the negative electrode plate; wherein, the positive electrode plate is the positive electrode plate for a sodium-ion battery as described above in the present invention.

[0036] Exemplarily, the negative electrode plate is a sodium metal sheet.

[0037] Exemplarily, the electrolyte includes one or both of EC and DMC; for example, an electrolyte composed of EC and DMC in a mass ratio of 1:1. Among them, EC is ethylene carbonate, and DMC is dimethyl carbonate.

[0038] Exemplarily, the separator is made of glass fiber, for example.

[0039] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0040] (1) The method provided by the present invention uses FePO 4 as a growth template to synthesize Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) spherical powder. Adding a surfactant can shorten the production time and control the particle size of the product within 0.1 - 0.4 μm. After dehydration using a dehydrating agent and then drying and crushing, the Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) powder is more uniform.

[0041] (2) The present invention adopts a method for preparing a high-purity phase material of Na 4 Fe 4 (PO 3 ) 4 (P 2 (P 2 O 7 ) through liquid-phase sanding, combined with rapid dehydration drying and sintering. During the preparation process, the reaction is mild, no large amount of decomposition products are generated, the material utilization rate is relatively high, the requirements for equipment are simple, there is no pollution, the cost is low and it is easy to obtain, which is suitable for large-scale industrial production.

[0042] (3) The Na 4 Fe 3 (PO 4 ) 2 (P 2 O7 ) The powder has a uniform particle size distribution and exhibits very high specific capacity and excellent rate performance when applied to the positive electrode of sodium-ion batteries. The battery performance test results show that Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) has a specific capacity of 106 mAh / g; after 50 charge-discharge cycles, the specific capacity remains at about 99%, and it can be used as an electrode material in sodium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention, and those of ordinary skill in the art can obtain other drawings based on these drawings without creative efforts.

[0044] Figure 1 Scanning electron microscope image of the positive electrode active material Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) prepared in Example 1 of the present invention.

[0045] Figure 2 XRD patterns of the positive electrode active material Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) prepared in Examples 1, 2, and 3 of the present invention.

[0046] Figure 3 Charge-discharge curves of the sodium-ion batteries prepared in Examples 1, 2, 3, and 4 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0047] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in combination with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0048] Unless otherwise specified, the methods described in the following examples are all conventional methods. Unless otherwise specified, the raw materials can all be obtained from public commercial channels.

[0049] Example 1

[0050] A positive electrode active material with the chemical formula Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ), and the preparation method of this positive electrode active material is as follows:

[0051] 1) Using FePO 4 , Na 2 CO 3 , Na 2 HPO 4 , sodium citrate dihydrate as raw materials, sodium fatty alcohol polyether sulfate as surfactant, cationic polyacrylamide as dehydrating agent, and deionized water as solvent. Among them, Na 2 HPO 4 is both a sodium source and a phosphorus source, FePO 4 is an iron source, Na 2 CO 3 is a supplementary sodium source, and sodium citrate dihydrate is a carbon source; Mix 452.46 g of FePO 4 , 105.99 g of Na 2 CO 3 , 141.96 g of Na 2 HPO 4 , 82.35 g of sodium citrate dihydrate, and 2236.5 g of deionized water, then place them in a coarse sand mill for coarse grinding for 2 h to obtain a first mixed solution (particle size D50 is 1.67 μm).

[0052] 2) Transport the first mixed solution through a pipeline to the tank of a nanoscale sand mill, add 6.7095 g of sodium fatty alcohol polyether sulfate and stir, then continue sand grinding at 1200 rpm for 1 h (control the particle size at 0.42 μm) to obtain a precursor solution.

[0053] 3) Add 13.419 g of cationic polyacrylamide to the precursor solution, mix evenly, then transfer it to a static drying tank and let it stand for 3 h. Open the valve of the static drying tank, drain the supernatant, and then dry it to obtain a dried material;

[0054] 4) Then crush the dried material using a crusher, and place it in an inert atmosphere (high-purity N 2 , the ventilation rate is 0.2 L / h) for sintering, and calcine it at 510 °C (the heating rate is 2 °C / min) for 13 h to obtain Na 4 Fe 3(PO 4 ) 2 (P 2 O 7 ) is used as the cathode material for sodium-ion batteries.

[0055] The above-prepared cathode material powder is made into a pole piece. SP is used as the conductive agent with a mass fraction of 10%; polyvinylidene fluoride PVDF is used as the binder with a mass fraction of 10%, and the mass is accurate to one-thousandth. The pole piece uses carbon-coated aluminum foil as the current collector. Carbon composite sodium iron pyrophosphate (80%), SP (10%), polyvinylidene fluoride (10%) and N-methylpyrrolidone are stirred to make a slurry, and the slurry is evenly coated on the carbon-coated aluminum foil and dried in an oven at 120°C. Strictly control the mixing and coating process. The coating thickness of the measured pole piece should be uniform, and it is cut into a positive pole piece with a diameter of ф12mm and a thickness of 14.5μm.

[0056] In an inert gas glove box where the water and oxygen content are both less than or equal to 0.0005%, a sodium metal sheet is used as the negative electrode material, glass fiber is used as the separator, and 1mol / L NaClO 4 / EC+DEC (mass ratio 1:1) is used as the electrolyte. They are assembled into a test battery. After the battery is sealed, it is tested with a sodium-ion battery electrochemical performance tester.

[0057] Figure 1 The scanning electron microscope image of the positive electrode active material Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) prepared in this example can be seen that the powder is spherical in morphology, the product particle size is controllable, and it is in the range of 200-400nm.

[0058] Figure 2 The XRD pattern of the positive electrode active material Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) prepared in this example. It can be seen from the figure that the characteristic peaks of Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) are obvious, the phase is relatively pure, and there is no impurity phase.

[0059] The charge-discharge curve of the sodium-ion battery obtained in this example is as shown in Figure 3 (voltage range 1.5-4.0V). FromFigure 3 It can be seen that the specific capacity of the button-type half-cell assembled with the material prepared in this example as the positive electrode material reaches 101.6 mAh / g at 0.2C.

[0060] The sodium-ion battery prepared in this example was subjected to 50 charge-discharge cycle performance tests, and the specific capacity retention rate was 99.1% at 0.2C.

[0061] Example 2

[0062] A positive electrode active material with the chemical formula Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ). The preparation method of the positive electrode active material is as follows: 1) Using FePO 4 , NaHCO 3 , Na 2 HPO 4 , citric acid as raw materials, sodium fatty alcohol polyether sulfate as a surfactant, polyacrylamide as a dehydrating agent, and deionized water as a solvent. Among them, Na 2 HPO 4 is both a sodium source and a phosphorus source, FePO 4 is an iron source, NaHCO 3 is a supplementary sodium source, and citric acid is a carbon source; 452.46 g of FePO 4 , 168.02 g

[0063] NaHCO 3 , 141.96 g of Na 2 HPO 4 , 58.84 g of citric acid, and 2737.6 g of deionized water were mixed and then coarsely ground in a coarse sand mill for 2.5 h to obtain a first mixed solution (particle size D50 is 1.73 μm).

[0064] 2) The first mixed solution was transported through a pipeline to the tank of a nanoscale sand mill, 8.2128 g of sodium fatty alcohol polyether sulfate was added and stirred, and sand milling was continued at 1500 rpm for 1 h (the particle size was controlled at 0.46 μm) to obtain a precursor solution.

[0065] 3) 16.4256 g of cationic polyacrylamide was added to the precursor solution, and after mixing evenly, it was transferred to a static drying tank and left standing for 3 h. The valve of the static drying tank was opened, and after the supernatant was removed, drying was carried out to obtain a dried material;

[0066] 4) Then the dried material was crushed using a crusher and placed in an inert atmosphere (high-purity N 2, sinter at a ventilation rate of 0.2 L / h, calcine at 520 °C (heating rate: 2 °C / min) for 10 h to obtain Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) as the cathode material for sodium-ion batteries. The particle size of the product is 200 - 400 nm.

[0067] Prepare the cathode electrode and sodium-ion battery according to the method in Example 1 with the above-prepared cathode material and conduct electrochemical performance tests.

[0068] From Figure 3 it can be seen that the specific capacity of the coin-type half-cell assembled with the material prepared in this example as the cathode material reaches 100.9 mAh / g at 0.2C.

[0069] Conduct 50 charge-discharge cycle performance tests on the sodium-ion battery prepared in this example, and the specific capacity retention rate is 98.7% at 0.2C.

[0070] Example 3

[0071] A cathode active material with the chemical formula Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ), and the preparation method of this cathode active material is as follows:

[0072] 1) Using FePO 4 , Na 2 CO 3 , Na 2 HPO 4 , and sodium gluconate as raw materials, fatty alcohol polyoxyethylene ether as the surfactant, polyacrylamide as the dehydrating agent, and deionized water as the solvent. Among them, Na 2 HPO 4 is both the sodium source and the phosphorus source, FePO 4 is the iron source, Na 2 CO 3 is the supplementary sodium source, and sodium gluconate is the carbon source; mix 452.46 g of FePO 4 , 105.99 g of Na 2 CO 3 , 141.96 g of Na 2 HPO 4 , 61.08 g of sodium gluconate, and 2003.92 g of deionized water, and then place them in a coarse sand mill for coarse grinding for 2 h to obtain the first mixed solution (particle size D50 is 1.54 μm).

[0073] 2) The first mixed solution is transported through a pipeline into the nano-scale sand mill tank, and 6.0118 g of sodium lauryl ether sulfate is added and stirred. Then, it is continuously sanded at 1500 rpm for 1 h (the particle size is controlled at 0.40 μm) to obtain a precursor solution.

[0074] 3) Add 12.0235 g of cationic polyacrylamide to the precursor solution. After mixing evenly, transfer it to a static drying tank and let it stand for 3 h. Then, open the valve of the static drying tank, drain the supernatant, and perform drying to obtain a dried material.

[0075] 4) Then, the dried material is crushed using a crusher and placed in an inert atmosphere (high-purity N 2 , with a gas flow rate of 0.2 L / h) for sintering, and calcined at a temperature of 520 °C (heating rate: 2 °C / min) for 10 h to obtain Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) as the cathode material for sodium-ion batteries.

[0076] The cathode electrode sheet and sodium-ion battery are prepared from the above-prepared cathode material according to the method in Example 1, and the electrochemical performance is tested.

[0077] From Figure 3 it can be seen that the specific capacity of the button-type half-cell assembled with the material prepared in this example as the cathode material reaches 98.2 mAh / g at 0.2C.

[0078] The sodium-ion battery prepared in this example is subjected to 50 charge-discharge cycle performance tests, and the specific capacity retention rate at 0.2C is 97.6%.

[0079] Example 4

[0080] A cathode active material with the chemical formula of Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) is prepared as follows:

[0081] 1) Using FePO 4 , Na 2 C 2 O 4 , H 3 PO 4 , glucose as raw materials, fatty alcohol polyoxyethylene ether as a surfactant, polyacrylamide as a dehydrating agent, and deionized water as a solvent. Among them, Na2 C 2 O 4 is used as the sodium source, H 3 PO 4 is used as the phosphorus source, FePO 4 is used as the iron source, and glucose is used as the carbon source; 420.8 g of FePO 4 , 249.8 g of Na 2 C 2 O 4 , 128.9 g of H 3 PO 4 , 36.33 g of glucose, and 1253.8 g of deionized water are mixed and then coarsely ground in a coarse sand mill for 2 h to obtain a first mixed solution (with a particle size D50 of 1.63 μm).

[0082] 2) The first mixed solution is transported through a pipeline into a nano-scale sand mill tank, 5.937 g of fatty alcohol polyoxyethylene sulfate is added and stirred, and sand milling is continued at 1500 rpm for 1 h (the particle size is controlled at 0.40 μm) to obtain a precursor solution.

[0083] 3) 11.48 g of cationic polyacrylamide is added to the precursor solution, and after mixing evenly, it is transferred to a static drying tank and left standing for 3 h. The valve of the static drying tank is opened, and after the supernatant is removed, drying is carried out to obtain a dried material;

[0084] 4) Then the dried material is crushed using a crusher and then sintered in an inert atmosphere (high-purity N 2 , with a gas flow rate of 0.2 L / h), and calcined at a temperature of 520 °C (with a heating rate of 2 °C / min) for 10 h, that is, Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) is obtained as the positive electrode material for a sodium-ion battery.

[0085] The positive electrode sheet and the sodium-ion battery are prepared from the above-prepared positive electrode material according to the method in Example 1, and the electrochemical performance is tested.

[0086] From Figure 3 it can be seen that the specific capacity of the button-type half-cell assembled with the material prepared in this example as the positive electrode material reaches 106.9 mAh / g at 0.2C.

[0087] The sodium-ion battery prepared in this example is subjected to 50 charge-discharge cycle performance tests, and the specific capacity retention rate is 99.2% at 0.2C.

Claims

1. A kind of Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) preparation method, comprising the following steps: (1) Weigh ferric phosphate, sodium source, phosphorus source, carbon source, surfactant, dehydrating agent and deionized water according to the stoichiometric ratio and curing ratio respectively; (2) Mix the ferric phosphate, sodium source, phosphorus source, carbon source and deionized water, then place them in a coarse sand mill, add sand grinding media, and grind coarsely for 1 - 2 h to obtain a first mixed solution; (3) Transfer the first mixed solution to the nano - level sand mill tank through a pipeline, add the surfactant and stir, continue to grind for 1 - 4 h, and control the particle size between 0.2 - 1.0 μm to obtain a precursor solution; (4) Add the dehydrating agent to the precursor solution, mix evenly, then transfer it to a static drying tank and let it stand for 1 - 6 h. Open the valve of the static drying tank, drain the supernatant, and then dry to obtain a dried material; (5) After crushing the dried material, sinter it under an inert atmosphere to obtain the Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) powder.

2. The preparation method according to claim 1, wherein: In the step (1), the sodium source includes at least one of disodium hydrogen phosphate, sodium carbonate, sodium oxalate, sodium bicarbonate, sodium citrate, trisodium citrate dihydrate, trisodium citrate pentahydrate, sodium gluconate; Or, in the step (1), the phosphorus source is one or more of phosphoric acid and phosphates; the phosphates include one or more of sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate; Or, in the step (1), the carbon source includes at least one of citric acid, glucose, sucrose, starch, sodium citrate, trisodium citrate dihydrate, trisodium citrate pentahydrate, sodium gluconate; Or, in the step (1), the surfactant includes anionic surfactant, non - ionic surfactant, zwitterionic surfactant; the anionic surfactant includes at least one of sodium lauryl ether sulfate, alkylbenzene sulfonate, sodium lauryl sulfate, olefin sulfonate, sulfosuccinate, phosphate ester; the non - ionic surfactant includes at least one of fatty alcohol polyoxyethylene ether, fatty amine polyoxyethylene ether, fatty acid polyoxyethylene ester, alkylphenol polyoxyethylene ether, polyol ester, alkanolamide; the zwitterionic surfactant includes at least one of betaines, imidazolines, amine oxides, amino acids; Or, in the step (1), the dehydrating agent includes at least one of cationic polyacrylamide, ester - based quaternary ammonium salt, dialkyl quaternary ammonium salt, modified dialkyl quaternary ammonium salt.

3. The preparation method according to claim 1, wherein: In the step (1), the curing ratio is 20% - 50%.

4. The preparation method according to claim 1, wherein: In the step (2), the sand grinding method of the sand mill includes one of disk type, rod - pin type, and turbine type; Or, in the step (2), the sand grinding media includes one or more of natural sand, glass beads, steel beads, zirconia beads, zirconium silicate beads, agate beads; Or, in the step (2), the particle size D50 of the particles in the first mixed solution is 1 - 2 μm.

5. The preparation method according to claim 1, wherein: In the step (4), the static drying tank is a dual - purpose device for static precipitation, drainage and drying.

6. The preparation method according to claim 1, wherein: In the step (5), the sintering conditions are as follows: the sintering temperature is 450 to 550 °C, the time is 8 to 15 h, and the heating rate of the sintering is 2 - 5 °C / min; Alternatively, in the step (5), the inert atmosphere includes one of argon, nitrogen, argon-hydrogen mixture gas, and nitrogen-hydrogen mixture gas; Alternatively, in the step (5), the ventilation rate of the inert atmosphere is 0.1 - 0.3 L / h.

7. The Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) powder prepared by the method according to any one of claims 1 - 6.

8. The Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) powder, It is characterized in that: The Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) powder is spherical particles with a particle size of 100 - 400 nm.

9. A positive electrode sheet for a sodium-ion battery, comprising a current collector and a positive electrode material disposed on the current collector, The feature lies in: The positive electrode material includes Na as described in claim 7 or 8 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) powder 10. A sodium-ion battery, comprising a positive electrode sheet, a negative electrode sheet, and a separator disposed between the positive electrode sheet and the negative electrode sheet; It is characterized in that: The positive electrode sheet is the positive electrode sheet described in claim 9.

Images