Iron-based polyanionic compounds and their preparation and application

Abstract

The present invention relates to sodium ion battery electrode materials, in particular to a preparation method of a phosphate or fluorophosphate polyanionic compound and application thereof in sodium ion battery electrode materials; the iron-based phosphate polyanionic compound has one or more structures shown in Formula I, Formula II, Formula III, Formula IV or Formula V, VI: Na4Fe3(PO4)2P2O7, Na3Fe2(PO4)1P2O7, NaFePO4, Na2FeP2O7, Na2FePO4F, Na5Fe2(PO4)2F3; the prepared phosphate or fluorophosphate polyanionic compound particles have small particle size and uniform distribution, and the product has high purity and good electrochemical performance and rate performance.

Description

Technical Field

[0001] The present invention belongs to the field of sodium ion batteries and relates to sodium ion battery electrode materials, in particular to a preparation method of phosphate or fluorophosphate polyanionic compounds and application of the same in sodium ion battery electrode materials. Background Art

[0002] In recent years, with the introduction of the "dual carbon" policy and the goal of building a new power system dominated by renewable energy, the energy storage industry has maintained a high level of attention as a key to regulating the volatility and indirectness of renewable energy and achieving grid stability. Alkali metal (Li, Na, K) ion batteries are an important component of portable large-scale stationary energy storage. Among them, lithium-ion batteries (LIBs) are widely used in electric vehicles and electronic devices due to their significant advantages such as high energy density, high electrode potential, stable cycle performance, and environmental friendliness. However, in recent years, the limitation of lithium sources has led to high prices for lithium-ion battery cathode materials. Sodium-ion batteries have also attracted increasing attention due to their similar operating principles to lithium-ion batteries and the low cost and abundant resources of sodium sources. Therefore, the development of high-performance sodium-ion battery cathode materials is particularly important.

[0003] Polyanionic materials such as sodium iron phosphate, sodium iron pyrophosphate, sodium iron fluorophosphate, and sodium iron phosphate pyrophosphate are preferred cathode materials for alkali metal ion batteries due to their stable structure and high safety. In the production process for these materials, soluble raw materials are often mixed uniformly, dried, and then calcined at high temperatures to form a phase. However, the isotropic shape of the precursor in the homogeneous solution leads to severe agglomeration during the sintering process, resulting in uneven sintering of the material inside and outside. This problem leads to the presence of impurities in the product and extremely poor rate performance. Summary of the Invention

[0004] In order to solve the above problems, the present invention provides a method for introducing acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber, graphene, etc. into the precursor powder obtained after drying, thereby effectively reducing the serious problem of agglomeration of polyanionic materials during calcination, and acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber, and graphene can be effectively removed through specific treatment, so that the prepared material has high purity and good rate performance.

[0005] A method for preparing an iron-based phosphate or fluorophosphate polyanionic compound, characterized in that:

[0006] The iron-based phosphate polyanionic compound has one or more structures represented by Formula I, Formula II, Formula III or Formula IV:

[0007] Na4Fe3(PO4)2P2O7 Formula Ⅰ;

[0008] Na3Fe2(PO4)1P2O7 formula Ⅱ;

[0009] NaFePO4 formula III;

[0010] Na2FeP2O7 formula IV;

[0011] Alternatively, the iron-based fluorophosphate polyanionic compound has a structure shown in Formula V and / or VI:

[0012] Na2FePO4 F formula V;

[0013] Na5Fe2(PO4)2F3 formula VI;

[0014] The preparation method of the polyanionic compound shown in Formula I, Formula II, Formula III, Formula IV, V or VI comprises:

[0015] Step 1), fully dissolving a reducing agent, a soluble iron source, and a pH adjuster in water;

[0016] Step 2) adding a sodium source, a phosphorus source, and a pH adjuster, and optionally adding a fluorine source, to the aqueous solution obtained in step 1 above to form a uniformly mixed precursor solution;

[0017] The amount of the pH adjuster added should ensure that the pH of the aqueous solution in steps 1) and 2) is 2≤pH≤4 (preferably, the amount of the pH adjuster added in step 2 should ensure that the pH of the aqueous solution is the same as that in step 1);

[0018] Step 3), the precursor solution is passed into a drying device for spray drying to obtain a precursor powder;

[0019] Step 4) The precursor powder obtained in step 3) is mixed evenly with the carbon material, and heat-treated in an inert atmosphere or an inert atmosphere containing H2, to obtain a final product of a carbon-coated iron-based phosphate-based polyanion cathode material;

[0020] The carbon material is one or more of acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber, and graphene, and the mass ratio of the carbon material to the precursor powder obtained in step 3) is 0.1% to 5%; more preferably 0.3% to 3%, and more preferably 0.5% to 1.5%;

[0021] Step 5) The product obtained in step 4) is pulverized by air flow to remove the carbon material added in step 4) to obtain an iron-based phosphate polyanionic compound with a particle size range of 0.2 μm-5 μm, preferably 1.5 μm-3.5 μm.

[0022] The iron ion concentration in the aqueous solution in step 1) is (0.001-0.95)n, preferably (0.10-0.90)n, where n is the iron ion concentration when the soluble iron salt in the solution reaches saturation in the clear aqueous solution at 60° C. to 80° C.;

[0023] The iron of the soluble iron source is ferric ion and / or ferrous ion in the solution;

[0024] The stoichiometric ratio of the reducing agent to the ferric iron element required to reduce ferric iron to ferrous iron is set to a (the stoichiometric ratio refers to the ratio of 1 mol Fe 3+ Reduced to 1 mol Fe 2+ The molar amount of the reducing agent exactly (or exactly) required is defined as a);

[0025] When the iron source contains trivalent iron ions, the molar amount of the reducing agent added corresponding to the trivalent iron ions in the iron source is (1.01-1.2)*a*Fe 3+ The molar amount is preferably (1.09 to 1.11)*a*Fe 3+ The molar amount of

[0026] When the iron source contains divalent iron ions, the molar amount of reducing agent added corresponding to the divalent iron ions in the iron source is (0.01~0.2)*a*Fe 2+ The molar amount is preferably (0.09 to 0.1)*a*Fe 2+ The molar amount of

[0027] The molar amount of the reducing agent added to the water is the sum of the molar amounts of the reducing agent added corresponding to the trivalent and divalent iron ions in the above iron source;

[0028] The temperature of the aqueous solution is 60°C to 80°C.

[0029] 1) The iron source is one or more soluble iron salts selected from ferric citrate, ferric nitrate, ferric sulfate, ferric chloride, ferric acetate, ferrous ammonium sulfate, ferrous citrate, ferrous nitrate, ferrous sulfate, ferrous chloride, ferrous acetate, and ferrous pyrophosphate;

[0030] 2) The reducing agent is at least one or more of citric acid, oxalic acid, ammonium citrate, ascorbic acid, formaldehyde, acetaldehyde, lactic acid, and malic acid;

[0031] 3) The pH adjuster is at least one or more of citric acid, oxalic acid, ammonia water (ammonia content 25 wt% to 28 wt%), ammonium oxalate, ammonium hydrogen oxalate, ammonium lactate, ammonium citrate, ammonium hydrogen citrate, ammonium malate, ammonium hydrogen malate, ammonium carbonate, and ammonium bicarbonate;

[0032] 4) The sodium source is at least one or more of sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium acetate, sodium phosphate, sodium pyrophosphate, sodium hydrogen pyrophosphate, sodium hydrogen phosphate, sodium dihydrogen phosphate, sodium citrate, sodium oxalate, sodium nitrate, sodium tartrate, sodium ascorbate, sodium gluconate, sodium glutamate, and sodium formate;

[0033] 5) The phosphorus source is at least one or two or more of sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, triammonium phosphate, pyrophosphoric acid, sodium pyrophosphate, sodium dihydrogen pyrophosphate, and ammonium dihydrogen pyrophosphate;

[0034] 6) The fluorine source is selected from at least one or more of sodium fluoride, ammonium fluoride, polyvinylidene fluoride, polyperfluoroethylene, and hydrofluoric acid.

[0035] Category 1: When the reducing agent is one or both of ascorbic acid and oxalic acid, a is 0.5;

[0036] Category II: When the reducing agent is one or more of citric acid, ammonium citrate, formaldehyde, acetaldehyde, lactic acid, and malic acid, a is 1;

[0037] When the reducing agent is a combination of one or more reducing agents of the first type and one or more reducing agents of the second type, the molar amount of the reducing agent added to the water is determined based on the molar ratio a of the reducing agent to the ferric ion, the molar ratio b of the reducing agent to the divalent iron ion, and the molar amounts of the divalent iron ion and the trivalent iron ion in the iron source.

[0038] During step 2), a carbon source may or may not be added to the aqueous solution. Since the reducing agent and the pH adjuster (except ammonia) will introduce a carbon source after addition, the amount of the carbon source in step (b) should ensure that the carbon content in the synthesized final product is 1 wt% to 20 wt%, preferably 4 wt% to 10 wt%;

[0039] The carbon source is at least one or more of oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid, malic acid, sucrose, glucose, soluble starch, and liquid polyethylene glycol;

[0040] The drying process in step 3) is spray drying;

[0041] The spray drying feed flow rate is 40 ml / min to 70 ml / min, the inlet air temperature is 90°C to 190°C, preferably 120°C to 175°C, the exhaust air temperature is 70°C to 180°C, preferably 110°C to 165°C, and the compressed gas (at least one or more of air, argon, and nitrogen, or a mixture of hydrogen (0.1 vol% to 20 vol%) and argon, or a mixture of hydrogen (0.1 vol% to 20 vol%) and nitrogen) pressure is 0.01 KPa to 1 KPa, preferably 0.2 KPa to 0.85 KPa;

[0042] The air flow milling in step 5) requires a fluidized bed air flow mill (with a turbine classifier), a circulating tubular air flow mill (with an inertial classifier), or a jet air flow mill (with a classifying wheel belt), the compressed air pressure is 2 to 12 MPa, and the air flow milling time is 1 h to 5 h.

[0043] The mixing method in step 4) is one or more of sand milling, ball milling and pot milling.

[0044] The sand milling process requires the addition of sand milling media, with a sand milling speed of 3000-5000 r / min and a sand milling time of 0.5-5 h. The sand milling media is one or more of natural sand beads, glass beads, steel beads, zirconium oxide beads, zirconium silicate beads, and agate beads. The ball-to-material ratio (mass ratio of balls to solid materials) is (1-5). The diameter of the sand milling media is 2-12 cm, and is preferably composed of 10 cm, 8 cm, and 4 cm diameters in a mass ratio of 2:1:2.

[0045] The ball milling process requires the addition of ball milling media, with a rotation speed of 600-800 r / min and a ball milling treatment time of 2-6 hours. The ball milling media are one or more of zirconia beads, agate beads, and zirconium silicate beads. The ball-to-material ratio (mass ratio of balls to solid material) is 1.5-7. The diameter of the ball milling media is 2-12 cm, preferably a combination of 10 cm, 8 cm, and 4 cm in diameter at a mass ratio of 1:2:2.

[0046] The pot mill needs to be added with pot mill media, the rotation speed of the pot mill is 40-50 r / min, and the pot mill treatment time is 15-50 hours; the pot mill media is one or more of zirconium oxide beads, agate beads, zirconium silicate beads, and graphite balls; the ball-to-material ratio (mass ratio of balls to solid material) is (1.5-3); the diameter of the pot mill media is 2-12 cm, preferably a combination of 10 cm and 8 cm diameters at a mass ratio of 3:1;

[0047] In step 4), the heat treatment atmosphere is selected from at least one of inert atmospheres including argon, helium, and nitrogen, or an inert atmosphere containing H2 with reducing properties, and the molar content of H2 in the heat treatment atmosphere is 0 - 50%; the heat treatment includes a process of treating at a first temperature T1 and a process of treating at a second temperature T2 in sequence; the temperature T1 is 200 ≤ T1 ≤ 400 °C, and the treatment time is 0.5 - 6 h; the temperature T2 is 400 °C < T1 ≤ 650 °C, and the treatment time is 3 - 20 h; the difference between T2 and T1 is greater than or equal to 50 °C.

[0048] Preferably, the phosphate polyanionic compound has the structure shown in Formula I. In slurry c, the molar ratio of the sodium source, iron source, and phosphorus source is 4:3:4, and the molar ratios of the sodium source, iron source, and phosphorus source are based on the molar amounts of sodium element, iron element, and phosphorus element respectively;

[0049] Preferably, the phosphate polyanionic compound has the structure shown in Formula II. In slurry c, the molar ratio of the sodium source, iron source, and phosphorus source is 3:2:3, and the molar ratios of the sodium source, iron source, and phosphorus source are based on the molar amounts of sodium element, iron element, and phosphorus element respectively;

[0050] Preferably, the polyanionic compound has the structure shown in Formula III. In slurry c, the molar ratio of the sodium source, iron source, and phosphorus source is 1:1:1, and the molar ratios of the sodium source, iron source, and phosphorus source are based on the molar amounts of sodium element, iron element, and phosphorus element respectively;

[0051] Preferably, the polyanionic compound has the structure shown in Formula IV. In slurry c, the molar ratio of the sodium source, iron source, and phosphorus source is 2:1:2, and the molar ratios of the sodium source, iron source, and phosphorus source are based on the molar amounts of sodium element, iron element, and phosphorus element respectively;

[0052] Preferably, the polyanionic compound has the structure shown in Formula V. In slurry c, the molar ratio of the sodium source, iron source, phosphorus source, and fluorine source is 2:1:1:1, and the molar ratios of the sodium source, iron source, phosphorus source, and fluorine source are based on the molar amounts of sodium element, iron element, phosphorus element, and fluorine element respectively;

[0053] Preferably, the polyanionic compound has the structure shown in VI. In slurry c, the molar ratio of the sodium source, iron source, phosphorus source, and fluorine source is 5:2:2:3, and the molar ratios of the sodium source, iron source, phosphorus source, and fluorine source are based on the molar amounts of sodium element, iron element, phosphorus element, and fluorine element respectively.

[0054] The iron-based phosphate or fluorophosphate polyanionic compound prepared by the preparation method.

[0055] The iron-based phosphate or fluorophosphate polyanionic compound prepared by the preparation method or the use of the iron-based phosphate or fluorophosphate polyanionic compound in sodium ion batteries is characterized in that the iron-based phosphate or fluorophosphate polyanionic compound is used as an active substance for a positive electrode material of a sodium ion battery.

[0056] A positive electrode material for a sodium ion battery, characterized in that the positive electrode material comprises an iron-based phosphate or fluorophosphate polyanionic compound or an iron-based phosphate or fluorophosphate polyanionic compound prepared by the preparation method;

[0057] In the positive electrode material, the content of the iron-based phosphate or fluorophosphate polyanionic compound is (60-98) wt%;

[0058] The positive electrode material further contains a conductive agent and a binder, wherein the mass ratio of the iron-based phosphate or fluorophosphate polyanionic compound, the conductive agent, and the binder is (60-98) wt%: (1-39) wt%: (1-39) wt%;

[0059] The conductive agent is at least one or more of carbon black, conductive graphite, carbon fiber, carbon nanotube, and graphene;

[0060] The binder is polyvinylidene fluoride: at least one or more of PVDF5130, HSV900, and kynar761A.

[0061] The significance of introducing acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber and graphene into the precursor is that their rich specific surface area can provide sufficient nucleation sites for the crystallization reaction of the precursor, effectively avoiding the extremely serious problem of agglomeration caused by the simultaneous crystallization reactions of too many isotropic precursors, and the acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber and graphene present in the product after heat treatment can be effectively removed by air flow crushing due to the density difference with the product. After heat treatment, a mixture of phosphate or fluorophosphate positive electrode material and the above-mentioned carbon material is formed. After entering the air flow crushing equipment, the above-mentioned carbon material has low density and enters the cyclone separator first under the action of air flow or centrifugal force before the above-mentioned positive electrode material and is separated first. The positive electrode material has high density and must be crushed multiple times and takes a long time before it can be obtained from the grading device and enter the cyclone separator and be collected. The existence of the above-mentioned separation time difference effectively realizes the removal of carbon material in the above-mentioned positive electrode material. The above method can effectively avoid the problem of agglomeration during the crystallization of the precursor and can separate and remove it from the product. At the same time, it can make the precursor sintered evenly inside and outside, and the product has high purity and excellent rate performance.

[0062] Beneficial effects

[0063] In order to solve the above problems, the present invention provides a method for introducing acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber, graphene, etc. into the precursor powder obtained after drying, thereby effectively reducing the serious problem of agglomeration of polyanionic materials during calcination, and acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber, and graphene can be effectively removed through specific treatment, so that the prepared material has high purity and good rate performance. The significance of introducing acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber and graphene into the precursor is that their rich specific surface area can provide sufficient nucleation sites for the crystallization reaction of the precursor, effectively avoiding the extremely serious problem of agglomeration caused by the simultaneous crystallization reactions of too many isotropic precursors, and the acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber and graphene present in the product after heat treatment can be effectively removed by air flow crushing due to the density difference with the product. After heat treatment, a mixture of phosphate or fluorophosphate positive electrode material and the above-mentioned carbon material is formed. After entering the air flow crushing equipment, the above-mentioned carbon material has low density and enters the cyclone separator first under the action of air flow or centrifugal force before the above-mentioned positive electrode material and is separated first. The positive electrode material has high density and must be crushed multiple times and takes a long time before it can be obtained from the grading device and enter the cyclone separator and be collected. The existence of the above-mentioned separation time difference effectively realizes the removal of carbon material in the above-mentioned positive electrode material. The above method can effectively avoid the problem of agglomeration during the crystallization of the precursor and can separate and remove it from the product. At the same time, it can make the precursor sintered evenly inside and outside, and the product has high purity and excellent rate performance.

[0064] The prepared phosphate and fluorophosphate polyanionic compound particles have small particle size and uniform distribution, high product purity and good electrochemical performance and rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0065] Figure 1 The XRD diffraction pattern of material 1# in Example 1 is shown in FIG.

[0066] Figure 2 This is the SEM image of material 1# in Example 1;

[0067] Figure 3 TEM image of material 1# in Example 1;

[0068] Figure 4 This is the TG curve of material 1# in Example 1. DETAILED DESCRIPTION

[0069] Example 1

[0070] The mass ratio of carbon material to precursor powder is 1%

[0071] 1) Add 2L of deionized water to a beaker, heat to 70°C, and add 3.3mol of citric acid monohydrate (C6H 10 O8 (addition can achieve Fe 3+ to Fe 2+ Complete reduction and inhibition of the reduction product Fe 2+ The reducing agent used for reoxidation), 3 mol of ferric nitrate nine hydrate Fe(NO3)3.9H2O, and at the same time, an appropriate amount of ammonia water (ammonia content 26wt%) was added dropwise to adjust the pH of the aqueous solution to 3, and the above raw materials were stirred until fully dissolved;

[0072] 2) 4 mol of sodium dihydrogen phosphate dihydrate NaH2PO4.2H2O (sodium source / phosphorus source) was then added to the aqueous solution, and an appropriate amount of ammonia water (ammonia content 26 wt%) was simultaneously added dropwise to maintain a constant pH value (i.e., the same pH value as in step 1) above), and stirring was continued until the raw materials were completely dissolved to form a precursor solution;

[0073] 3) The precursor solution was passed into a spray drying apparatus protected by an argon atmosphere for spray drying (inlet temperature of 170°C, outlet temperature of 130°C, feed flow rate of 50 ml / min, compressed argon pressure of 0.8 kPa) to obtain 1.2 kg of precursor powder;

[0074] 4) Then, 1.2 kg of the precursor powder and 12 g of carbon black were added to a jar mill, and 909 g of graphite balls with a diameter of 10 cm and 303 g of graphite balls with a diameter of 6 cm were added. The jar mill speed was 40-50 r / min, and the jar mill treatment time was 24 h. The jar mill was then transferred to a high-temperature rotary kiln protected by an argon atmosphere for heat treatment. The heat treatment temperature for h1 and T1 was 300 ° C, the heat treatment time was 3 h, and the heating rate was 5 ° C / min; the heat treatment temperature for T2 was 550 ° C, the heat treatment time was 8 h, and the heating rate was 2 ° C / min. The sample after heat treatment was a fluffy powder. XRD analysis of the above powder showed that the product was 99% sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material and 1% carbon black material, and no other impurity phases were present.

[0075] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with a classifying wheel frequency set to 50 Hz and a high-pressure air pressure set to 7 MPa for 2 h. The obtained material was recorded as sample 1#;

[0076] 6) XRD, SEM, TEM characterization and carbon content (TG) test were performed on the prepared sample 1#. Figure 1) It can be seen that the synthesized material is pure phase sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material with a purity of up to 99.99%. There is no diffraction peak of carbon black, that is, 1% of the carbon black material in step 4) can be effectively removed by the air flow crushing treatment in step 5). From SEM ( Figure 2 ) It can be seen that the particle size is uniform and small, with a particle size of 1μm-2μm. From TEM ( Figure 3 ) can be seen from the material surface that there is an amorphous carbon layer covering the outer surface, with a thickness of about 5 to 10 nm. TG( Figure 4 ) shows that the amorphous carbon content of the material surface coating is 5wt%.

[0077] Example 2

[0078] The mass ratio of carbon material to precursor powder is 0.3%

[0079] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Example 1, and the process and conditions were the same as those of Example 1, except that:

[0080] 4) The mass of the carbon black is 3.6 g. The sample after heat treatment is a fluffy powder. XRD analysis of the powder shows that the product is 99.7% sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material and 0.3% carbon black material, with no other impurity phases present;

[0081] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with a classifying wheel frequency set to 50 Hz and a high-pressure air pressure set to 7 MPa for 2 h. The obtained material was recorded as sample 2#;

[0082] 6) The prepared sample 2# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material with a purity of up to 99.95%. There was no diffraction peak of carbon black, that is, the 0.3% carbon black material in step 4) can be effectively removed by the air flow milling treatment in step 5). SEM showed that the particle size was uniform and small, with a particle size of 2μm-3μm. TEM showed that there was an amorphous carbon layer coating the outer surface of the material with a thickness of about 3 to 8nm. TG showed that the amorphous carbon content coated on the outer surface of the material was 4.9wt%.

[0083] Example 3

[0084] The mass ratio of carbon material to precursor powder is 3%

[0085] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Example 1, and the process and conditions were the same as those of Example 1, except that:

[0086] 4) The mass of the carbon black is 36 g, and the sample after heat treatment is a fluffy powder. XRD analysis of the powder shows that the product is 97% sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material and 3% carbon black material, with no other impurity phases present;

[0087] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with a classifying wheel frequency set to 50 Hz and a high-pressure air pressure set to 7 MPa for 2 h. The obtained material was recorded as sample 3#;

[0088] 6) The prepared sample 3# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material with a purity of up to 99.97%. There was no diffraction peak of carbon black, that is, 3% of the carbon black material in step 4) can be effectively removed by the air flow milling treatment in step 5). SEM showed that the particle size was uniform and small, with a particle size of 0.5μm-1.5μm. TEM showed that there was an amorphous carbon layer coating the outer surface of the material with a thickness of about 4 to 8nm. TG showed that the amorphous carbon content coated on the outer surface of the material was 4.8wt%.

[0089] Example 4

[0090] The mass ratio of carbon material to precursor powder is 0.1%

[0091] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Example 1, and the process and conditions were the same as those of Example 1, except that:

[0092] 4) The mass of the carbon black is 1.2 g. The sample after heat treatment is a fluffy powder. XRD analysis of the powder shows that the product is 99.9% sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material and 0.1% carbon black material, with no other impurity phases present;

[0093] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with a classifying wheel frequency set to 50 Hz and a high-pressure air pressure set to 7 MPa for 2 h. The obtained material was recorded as sample 4#;

[0094] 6) The prepared sample 4# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material with a purity of up to 99.94%. There was no diffraction peak of carbon black, that is, the 0.1% carbon black material in step 4) can be effectively removed by the air flow milling treatment in step 5). SEM showed that the particle size was uniform and small, with a particle size of 2.5μm-4.5μm. TEM showed that there was an amorphous carbon layer coating the outer surface of the material with a thickness of about 3 to 9nm. TG showed that the amorphous carbon content coated on the outer surface of the material was 4.85wt%.

[0095] Example 5

[0096] The mass ratio of carbon material to precursor powder is 5%

[0097] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Example 1, and the process and conditions were the same as those of Example 1, except that:

[0098] 4) The mass of the carbon black is 60 g, and the sample after heat treatment is a fluffy powder. XRD analysis of the powder shows that the product is 95% sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material and 5% carbon black material, with no other impurity phases present;

[0099] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with a classifying wheel frequency set to 50 Hz and a high-pressure air pressure set to 7 MPa for 2 h. The obtained material was recorded as sample 5#;

[0100] 6) The prepared sample 5# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material with a purity of up to 99.99%. There was no diffraction peak of carbon black, that is, 5% of the carbon black material in step 4) can be effectively removed by the air flow milling treatment in step 5). SEM showed that the particle size was uniform and small, with a particle size of 0.2μm-1.5μm. TEM showed that there was an amorphous carbon layer coating the outer surface of the material with a thickness of about 5 to 9nm. TG showed that the amorphous carbon content coated on the outer surface of the material was 4.95wt%.

[0101] Example 6

[0102] The high-pressure air pressure of the jet mill is set to 2 MPa

[0103] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Example 1, and the process and conditions were the same as those of Example 1, except that:

[0104] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with the high-pressure air pressure set to 2 MPa. The obtained material was recorded as sample 6#;

[0105] 6) The prepared sample 6# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material with a purity of up to 99.94%. There was no diffraction peak of carbon black, that is, 1% of the carbon black material in step 4) can be effectively removed by the air flow milling treatment in step 5). SEM showed that the particle size was uniform and small, with a particle size of 3μm-5μm. TEM showed that there was an amorphous carbon layer coating the outer surface of the material with a thickness of about 5 to 8nm. TG showed that the amorphous carbon content coated on the outer surface of the material was 5.01wt%.

[0106] Example 7

[0107] The high-pressure air pressure of the jet mill is set to 12 MPa

[0108] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Example 1, and the process and conditions were the same as those of Example 1, except that:

[0109] 5) The powder obtained above was pulverized using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with the high-pressure air pressure set to 12 MPa. The obtained material was recorded as sample 7#;

[0110] 6) The prepared sample 7# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material with a purity of up to 99.98%. There was no diffraction peak of carbon black, that is, 1% of the carbon black material in step 4) can be effectively removed by the air flow milling treatment in step 5). SEM showed that the particle size was uniform and small, with a particle size of 0.5μm-1μm. TEM showed that there was an amorphous carbon layer coating the outer surface of the material with a thickness of about 3 to 7nm. TG showed that the amorphous carbon content coated on the outer surface of the material was 4.8wt%.

[0111] Example 8

[0112] The air flow crushing time is set to 1h

[0113] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Example 1, and the process and conditions were the same as those of Example 1, except that:

[0114] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) for 1 hour. The obtained material was recorded as sample 8#;

[0115] 6) The prepared sample 8# was characterized by XRD, SEM, TEM and carbon content (TG) test. From XRD, it can be seen that the synthesized material is a pure phase sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material with a purity of up to 99.93%. There is no diffraction peak of carbon black, that is, 1% of the carbon black material in step 4) can be effectively removed by the air flow crushing treatment in step 5). From SEM, it can be seen that the particle size is uniform and the particle size is small, with a particle size of 3.5μm-5μm. From TEM, it can be seen that there is an amorphous carbon layer coating on the outer surface of the material with a thickness of about 6 to 10nm. TG shows that the amorphous carbon content coated on the outer surface of the material is 5.05wt%.

[0116] Example 9

[0117] The air flow crushing time is set to 5h

[0118] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Example 1, and the process and conditions were the same as those of Example 1, except that:

[0119] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) for 5 hours. The obtained material was recorded as sample 9#;

[0120] 6) The prepared sample 9# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material with a purity of up to 99.96%. There was no diffraction peak of carbon black, that is, 1% of the carbon black material in step 4) can be effectively removed by the air flow milling treatment in step 5). SEM showed that the particle size was uniform and small, with a particle size of 0.5μm-2μm. TEM showed that there was an amorphous carbon layer coating the outer surface of the material with a thickness of about 3 to 8nm. TG showed that the amorphous carbon content coated on the outer surface of the material was 4.75wt%.

[0121] Example 10

[0122] Preparation of Na3Fe2(PO4)1P2O7

[0123] The Na3Fe2(PO4)1P2O7 material was prepared by the method of Example 1, and the process and conditions were the same as those of Example 1, except that:

[0124] 2) Add 4.5 mol of sodium dihydrogen phosphate dihydrate NaH2PO4.2H2O (sodium source / phosphorus source) to the above aqueous solution;

[0125] 4) The T1 heat treatment temperature was 300°C, the heat treatment time was 3 hours, and the heating rate was 5°C / min; the T2 heat treatment temperature was 500°C, the heat treatment time was 6 hours, and the heating rate was 2°C / min. The sample after heat treatment was a fluffy powder. XRD analysis of the above powder showed that the product was 99% sodium iron pyrophosphate Na3Fe2(PO4)1P2O7 material and 1% carbon black material, with no other impurity phases.

[0126] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with a classifying wheel frequency set to 50 Hz and a high-pressure air pressure set to 7.5 MPa for 2.5 h. The obtained material was recorded as sample 10#;

[0127] 6) The prepared sample 10# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron phosphate pyrophosphate Na3Fe2(PO4)1P2O7 material with a purity of up to 99.99%. There was no diffraction peak of carbon black, that is, 1% of the carbon black material in step 4) can be effectively removed by the air flow milling treatment in step 5). SEM showed that the particle size was uniform and small, with a particle size of 1.5μm-2.5μm. TEM showed that there was an amorphous carbon layer coating the outer surface of the material with a thickness of about 5 to 10nm. TG showed that the amorphous carbon content coated on the outer surface of the material was 4.7wt%.

[0128] Example 11

[0129] Preparation of Na2FeP2O7

[0130] The Na2FeP2O7 material was prepared by the method of Example 1, and the process and conditions were the same as those of Example 1, except that:

[0131] 2) Add 6 mol of sodium dihydrogen phosphate dihydrate NaH2PO4.2H2O (sodium source / phosphorus source) to the above aqueous solution;

[0132] 4) The T1 heat treatment temperature was 300°C, the heat treatment time was 3 hours, and the heating rate was 5°C / min; the T2 heat treatment temperature was 500°C, the heat treatment time was 6 hours, and the heating rate was 2°C / min. The sample after heat treatment was a fluffy powder. XRD analysis of the above powder showed that the product was 99% sodium iron pyrophosphate Na2FeP2O7 material and 1% carbon black material, with no other impurity phases.

[0133] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with a classifying wheel frequency set to 50 Hz and a high-pressure air pressure set to 8 MPa for 2 h. The obtained material was recorded as sample 11#;

[0134] 6) The prepared sample 11# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron pyrophosphate Na2FeP2O7 material with a purity of up to 99.97%. There was no diffraction peak of carbon black, that is, 1% of the carbon black material in step 4) can be effectively removed by the air flow milling treatment in step 5). SEM showed that the particle size was uniform and small, with a particle size of 2μm-3μm. TEM showed that there was an amorphous carbon layer covering the outer surface of the material with a thickness of about 5 to 9nm. TG showed that the amorphous carbon content of the outer surface of the material was 5.1wt%.

[0135] Test Example 1

[0136] Sample 1# prepared in Example 1 was used as the positive electrode active material for a sodium-ion battery. It was mixed with acetylene black (a conductive agent) and polyvinylidene fluoride (PVDF) (a binder) in a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (a solvent) was added to form a paste, which was then applied to an aluminum current collector. After drying, the paste was cut into 14 mm diameter discs with an active material surface density of 1.1-3.2 mg / cm² (2.1 mg / cm² in this case). This served as the positive electrode. A sodium metal sheet was used as the negative electrode, the electrolyte was 1M NaClO₄ / EC / DEC (EC / DEC V / V = 1:1), and the separator was a glass fiber membrane. The assembled battery was subjected to charge and discharge tests over a voltage range of 1.8-3.6 V. The discharge specific capacities were measured at 0.2C / 1.0C / 5.0C / 10C / 50C. The discharge specific capacities at 0.2C / 50C are reported in Table 1.

[0137] Test Example 2-11

[0138] Samples 2#-11# were tested according to the test method of Test Example 1, and the discharge specific capacities at 0.2C / 50C were recorded in Table 1.

[0139] Comparative Example 1 (no carbon material added)

[0140] 1) Add 2L of deionized water to a beaker, heat to 70°C, and add 3.3mol of citric acid monohydrate (C6H 10 O8 (addition can achieve Fe 3+ to Fe 2+ Complete reduction and inhibition of the reduction product Fe 2+ The reducing agent used for reoxidation), 3 mol of ferric nitrate nine hydrate Fe(NO3)3.9H2O, and at the same time, an appropriate amount of ammonia water (ammonia content 26wt%) was added dropwise to adjust the pH of the aqueous solution to 3, and the above raw materials were stirred until fully dissolved;

[0141] 2) 4 mol of sodium dihydrogen phosphate dihydrate NaH2PO4.2H2O (sodium source / phosphorus source) was then added to the aqueous solution, and an appropriate amount of ammonia water (ammonia content 26 wt%) was simultaneously added dropwise to maintain a constant pH value (i.e., the same pH value as in step 1) above), and stirring was continued until the raw materials were completely dissolved to form a precursor solution;

[0142] 3) The precursor solution was passed into a spray drying apparatus protected by an argon atmosphere for spray drying (inlet temperature of 170°C, outlet temperature of 130°C, feed flow rate of 50 ml / min, compressed argon pressure of 0.8 kPa) to obtain 1.2 kg of precursor powder;

[0143] 4) 1.2 kg of the precursor powder was then transferred into a high-temperature rotary kiln protected by an argon atmosphere for heat treatment, wherein the heat treatment temperature for h1 and T1 was 300° C., the heat treatment time was 3 h, and the heating rate was 5° C. / min; the heat treatment temperature for T2 was 550° C., the heat treatment time was 8 h, and the heating rate was 2° C. / min. The heat-treated sample formed a block with a certain hardness. A small amount of the block was ground and subjected to XRD analysis, which showed that the product contained 95% sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material and 1% carbon black material, and also included 4% impurity phase, which mainly included 1% unreacted raw materials, 2% sodium iron phosphate NaFePO4, and 1% Na3Fe2(PO4)1P2O7;

[0144] 5) The obtained block was first crushed using a pulverizer, and then subjected to air flow milling using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1). The classifying wheel frequency was set to 50 Hz and the high-pressure air pressure was set to 7 MPa for 2 hours. The obtained material was recorded as sample A#;

[0145] 6) The prepared sample A# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a content of 95.6%, and there was no diffraction peak of carbon black, that is, 1% of the carbon black material in step 4) can be effectively removed by the air flow crushing treatment in step 5), but it also included 4.04% impurity phases, mainly including 1.04% unreacted raw materials, 2% sodium iron phosphate NaFePO4 and 1% Na3Fe2(PO4)1P2O7 impurity phases; SEM showed that the particle size was uniform with a particle size of 5μm-10μm, and TEM showed that there was an amorphous carbon layer coating the outer surface of the material with a thickness of about 5~9nm. TG showed that the amorphous carbon content coated on the outer surface of the material was 3.5~5wt%.

[0146] Comparative Example 2 (outside the lower limit of carbon material mass ratio)

[0147] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1. The process and conditions were the same as those of Comparative Example 1. The difference from Comparative Example 1 was that:

[0148] 4) Then, 1.2 kg of precursor powder and 0.6 g of carbon black were added to a jar mill, and 909 g of graphite balls with a diameter of 10 cm and 303 g of graphite balls with a diameter of 6 cm were added. The rotation speed of the jar mill was 40-50 r / min, and the jar mill treatment time was 24 h. Then, the jar mill was transferred to a high-temperature rotary kiln protected by an argon atmosphere for heat treatment. h1, T1 heat treatment temperature was 300 ° C, heat treatment time was 3 h, and heating rate was 5 ° C / min; T2 heat treatment temperature was 550 ° C, heat treatment time was 8 h , the heating rate is 2 ° C / min, and the sample after heat treatment is a coexistence of small blocks with a certain hardness and fluffy powder. A small amount of the above blocks and powder are ground and then XRD detection is performed, which shows that the product is 96.5% of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 and 1% of carbon black material, and also includes 2.5% impurity phase (mainly including 1% unreacted raw materials, 0.5% of sodium iron phosphate NaFePO4 and 1% of Na3Fe2(PO4)1P2O7 impurity phase;

[0149] 5) The small blocks of a certain hardness obtained above were first crushed using a pulverizer and then mixed with the fluffy powder. The mixture was then jet milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1). The classifying wheel frequency was set to 50 Hz and the high-pressure air pressure was set to 7 MPa for 2 hours. The resulting material was recorded as Sample B#.

[0150] 6) The prepared sample B# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a content of 97.5%. There was no diffraction peak of carbon black, that is, 1% of the carbon black material in step 4) can be effectively removed by the air flow milling treatment in step 5). However, it also contained 2.53% impurity phases, mainly including 1% unreacted raw materials, 0.5% sodium iron phosphate NaFePO4 and 1.03% Na3Fe2(PO4)1P2O7 impurity phases. SEM showed that the particle size was uniform with a particle size of 950nm-5μm. TEM showed that the outer surface of the material was coated with an amorphous carbon layer with a thickness of about 5-10nm. TG showed that the amorphous carbon content of the outer surface of the material was 4-5wt%.

[0151] Comparative Example 3 (outside the upper limit of carbon material mass ratio)

[0152] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1. The process and conditions were the same as those of Comparative Example 1. The difference from Comparative Example 1 was that:

[0153] 4) 1.2 kg of precursor powder and 66 g of carbon black were then added to a jar mill, along with 909 g of 10 cm diameter graphite balls and 303 g of 6 cm diameter graphite balls. The mill was rotated at 40-50 rpm for 24 h. The product was then heat treated in a high-temperature rotary kiln protected by an argon atmosphere. The heat treatment temperature for h1, T1, was 300 ° C. The heat treatment time was 3 h and the heating rate was 5 ° C. / min. The heat treatment temperature for T2 was 550 ° C. The heat treatment time is 8 hours, the heating rate is 2°C / min, and the sample after heat treatment is a fluffy powder; a small amount of the above powder is taken for XRD detection, which shows that the product is 93% of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 and 5.5% of carbon black material, and also includes 1.5% impurity phase (mainly including 1% unreacted raw materials, 0.5% of sodium iron phosphate NaFePO4 and 0.5% of Na3Fe2(PO4)1P2O7 impurity phase;

[0154] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with a classifying wheel frequency set to 50 Hz and a high-pressure air pressure set to 7 MPa for 2 h. The obtained material was recorded as sample C#;

[0155] 6) The prepared sample C# was characterized by XRD, SEM, and TEM, and its carbon content (TG) was tested. XRD showed that the synthesized material contained sodium ferric pyrophosphate Na4Fe3(PO4)2P2O7 at a content of 98.5%. A diffraction peak of carbon black was present at a content of 0.5%, indicating that the 5.5% carbon black material in step 4) could not be effectively removed by the airflow milling treatment in step 5). The material also contained 1% impurity phase (mainly including 0.5% unreacted raw materials, 0.3% sodium ferric phosphate NaFePO4, and 0.2% Na3Fe2(PO4)1P2O7 impurity phase. SEM showed that the particles were uniform in size, with a particle size of 150nm-400nm. TEM showed that an amorphous carbon layer was coated on the outer surface of the material, with a thickness of approximately 4-8nm. TG showed that the amorphous carbon content of the outer surface of the material was 4.5-5wt%. Comparative Example 4 (Adding a Dispersant to the Liquid Phase) (Carbon Material)

[0156] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1. The process and conditions were the same as those of Comparative Example 1. The difference from Comparative Example 1 was that:

[0157] 2) Add 36g of carbon black to the precursor solution and mix well;

[0158] 4) After heat treatment, the sample contained fluffy powder, as well as blocks of a certain hardness and fluffy powder. A small amount of the blocks and powder was ground and then subjected to XRD analysis, which showed that the product contained 97% sodium ferric pyrophosphate Na4Fe3(PO4)2P2O7 and 1% carbon black material, and also contained 2% impurity phases (mainly including 0.5% unreacted raw materials, 1% sodium ferric phosphate NaFePO4, and 0.5% Na3Fe2(PO4)1P2O7 impurity phases).

[0159] 5) The small blocks of a certain hardness obtained above were first crushed using a pulverizer and then mixed with the fluffy powder. The mixture was then jet milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1). The classifying wheel frequency was set to 50 Hz and the high-pressure air pressure was set to 7 MPa for 2 hours. The resulting material was recorded as sample D#.

[0160] 6) The prepared sample D# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a content of 98%, and there was no diffraction peak of carbon black, that is, 1% of the carbon black material in step 4) can be effectively removed by the air flow milling treatment in step 5), but it also contained 2.02% impurity phases, mainly including 1% unreacted raw materials, 0.52% of sodium iron phosphate NaFePO4 and 0.5% of Na3Fe2(PO4)1P2O7 impurity phases. SEM showed that the particle size was uniform with a particle size of 1μm-10μm. TEM showed that the outer surface of the material was coated with an amorphous carbon layer with a thickness of about 4-10nm. TG showed that the amorphous carbon content of the outer surface of the material was 2-6wt%.

[0161] Comparative Example 5 (adding dispersant to liquid phase) (surfactant)

[0162] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1. The process and conditions were the same as those of Comparative Example 1. The difference from Comparative Example 1 was that:

[0163] 2) Add 36 g of polyvinyl pyrrolidone (M=40000) to the precursor solution and mix well;

[0164] 4) After heat treatment, the sample contained fluffy powder, most of which were blocks of a certain hardness. A small amount of the blocks was ground and then subjected to XRD analysis, which showed that the product contained 94% sodium ferric pyrophosphate Na4Fe3(PO4)2P2O7 and 1% carbon black material, and also contained 5% impurity phases, mainly including 2% unreacted raw materials, 2% sodium ferric phosphate NaFePO4, and 1% Na3Fe2(PO4)1P2O7 impurity phases.

[0165] 5) The blocks obtained above with a certain hardness were first crushed using a pulverizer and then mixed with the fluffy powder. The mixture was then jet milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1). The classifying wheel frequency was set to 50 Hz and the high-pressure air pressure was set to 7 MPa for 2 hours. The resulting material was recorded as Sample E#.

[0166] 6) The prepared sample E# was characterized by XRD, SEM, TEM, and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a content of 94.5%, and there was no diffraction peak of carbon black, that is, 1% of the carbon black material in step 4) can be effectively removed by the air flow milling treatment in step 5), but it also contained 5.05% impurity phases, mainly including 2.05% unreacted raw materials, 2% sodium iron phosphate NaFePO4, and 1% Na3Fe2(PO4)1P2O7 impurity phases. SEM showed that the particle size was uniform with a particle size of 5μm-15μm. TEM showed that the outer surface of the material was coated with an amorphous carbon layer with a thickness of about 4-8nm. TG showed that the amorphous carbon content of the outer surface of the material was 1.5-7wt%;

[0167] Comparative Example 6 (no airflow pulverization treatment, direct grinding to obtain products with similar particle sizes)

[0168] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1. The process and conditions were the same as those of Comparative Example 1. The difference from Comparative Example 1 was that:

[0169] 4) Then, 1.2 kg of precursor powder and 36 g of carbon black were added to a jar mill, and 909 g of graphite balls with a diameter of 10 cm and 303 g of graphite balls with a diameter of 6 cm were added. The rotation speed of the jar mill was 40-50 r / min, and the jar mill treatment time was 24 h. Then, the jar mill was transferred to a high-temperature rotary kiln protected by an argon atmosphere for heat treatment. h1, T1 heat treatment temperature was 300 ° C, heat treatment time was 3 h, and heating rate was 5 ° C / min; T2 heat treatment temperature was 550 ° C, heat treatment time was 8 h, heating rate was 2 ° C / min, and the sample after heat treatment was a fluffy powder. A small amount of the above powder was taken and ground for XRD detection, which showed that the product was 99% sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 and 1% carbon black material, without other impurity phases;

[0170] 5) Grind the heat-treated sample and record the resulting material as sample F#;

[0171] 6) The prepared sample F# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a content of 99%, and there was a diffraction peak of carbon black with a content of 1%, that is, 1% of the carbon black material in step 4) was not effectively removed by the air flow milling treatment in step 5). SEM showed that the particle size was uniform with a particle size of 2μm-3μm. TEM showed that there was an amorphous carbon layer covering the outer surface of the material with a thickness of about 4 to 7nm. TG showed that the amorphous carbon content of the outer surface of the material was 6wt%.

[0172] Comparative Example 7 (air flow crushing conditions outside the protection range, lower pressure limit 1 MPa)

[0173] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1. The process and conditions were the same as those of Comparative Example 1. The difference from Comparative Example 1 was that:

[0174] Subsequently, 1.2 kg of precursor powder and 36 g of carbon black were added to a jar mill, and 909 g of graphite balls with a diameter of 10 cm and 303 g of graphite balls with a diameter of 6 cm were added. The rotation speed of the jar mill was 40-50 r / min, and the jar mill treatment time was 24 h. The jar mill was then transferred to a high-temperature rotary kiln protected by an argon atmosphere for heat treatment, h1, T1 heat treatment temperature was 300 ° C, heat treatment time was 3 h, and heating rate was 5 ° C / min; T2 heat treatment temperature was 550 ° C, heat treatment time was 8 h, and heating rate was 2 ° C / min. The sample after heat treatment was a fluffy powder. A small amount of the above powder was taken for XRD detection, which showed that the product was 99% sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 and 1% carbon black material, without other impurity phases.

[0175] 5) The powder obtained above was subjected to air flow milling using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1). The classifying wheel frequency was set to 50 Hz, the high-pressure air pressure was set to 1 MPa, and the time was 2 h. The obtained material was recorded as sample G#

[0176] 6) The prepared sample G# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD showed that the synthesized material contained sodium ferric pyrophosphate Na4Fe3(PO4)2P2O7 at a content of 99.7%, and a diffraction peak of carbon black at a content of 0.2%, i.e., the 1% carbon black material in step 4) was not effectively removed by the air flow milling treatment in step 5). SEM showed that the particles were uniform in size, with a particle size of 5 μm-20 μm. TEM showed that an amorphous carbon layer was coated on the outer surface of the material, with a thickness of approximately 4 to 10 nm. TG showed that the amorphous carbon content of the outer surface of the material was 6 wt%;

[0177] Comparative Example 8 (air flow crushing conditions outside the protection range, upper pressure limit)

[0178] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1. The process and conditions were the same as those of Comparative Example 1. The difference from Comparative Example 1 was that:

[0179] 4) Then, 1.2 kg of precursor powder and 36 g of carbon black were added to a jar mill, and 909 g of graphite balls with a diameter of 10 cm and 303 g of graphite balls with a diameter of 6 cm were added. The rotation speed of the jar mill was 40-50 r / min, and the jar mill treatment time was 24 h. The jar mill was then transferred to a high-temperature rotary kiln protected by an argon atmosphere for heat treatment. h1, T1 heat treatment temperature was 300 ° C, heat treatment time was 3 h, and heating rate was 5 ° C / min; T2 heat treatment temperature was 550 ° C, heat treatment time was 8 h, and heating rate was 2 ° C / min. The sample after heat treatment was a fluffy powder. A small amount of the above powder was taken for XRD detection, indicating that the product was 99% sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 and 1% carbon black material, without other impurity phases.

[0180] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with a classifying wheel frequency set to 50 Hz and a high-pressure air pressure set to 13 MPa for 2 h. The obtained material was recorded as sample H#;

[0181] 6) The prepared sample H# was characterized by XRD, SEM, and TEM, and its carbon content (TG) was tested. XRD showed that the synthesized material contained sodium ferric phosphate pyrophosphate Na4Fe3(PO4)2P2O7 at a content of 99.95%, and no carbon black diffraction peak was present. That is, the 1% carbon black material in step 4) was effectively removed by the airflow milling treatment in step 5). SEM showed that the particles were uniform in size, with a particle size of 0.2 μm to 1 μm. TEM showed that an amorphous carbon layer was present on the outer surface of the material, with a thickness of approximately 4 to 8 nm. TG showed that the amorphous carbon content of the outer surface of the material was 4.6 wt%;

[0182] Comparative Example 9 (air flow crushing conditions outside the protection range, time lower limit)

[0183] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1. The process and conditions were the same as those of Comparative Example 1. The difference from Comparative Example 1 was that:

[0184] 4) Then, 1.2 kg of precursor powder and 36 g of carbon black were added to a jar mill, and 909 g of graphite balls with a diameter of 10 cm and 303 g of graphite balls with a diameter of 6 cm were added. The rotation speed of the jar mill was 40-50 r / min, and the jar mill treatment time was 24 h. The jar mill was then transferred to a high-temperature rotary kiln protected by an argon atmosphere for heat treatment. h1, T1 heat treatment temperature was 300 ° C, heat treatment time was 3 h, and heating rate was 5 ° C / min; T2 heat treatment temperature was 550 ° C, heat treatment time was 8 h, and heating rate was 2 ° C / min. The sample after heat treatment was a fluffy powder. A small amount of the above powder was taken for XRD detection, indicating that the product was 99% sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 and 1% carbon black material, without other impurity phases.

[0185] 5) The powder obtained above was milled using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with a classifying wheel frequency set to 50 Hz and a high-pressure air pressure set to 7 MPa for 0.5 h. The obtained material was recorded as Sample 1#;

[0186] 6) The prepared sample I# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD showed that the synthesized material contained sodium ferric pyrophosphate Na4Fe3(PO4)2P2O7 at a content of 99.5%. A diffraction peak of carbon black was present, with a content of 0.5%, i.e., the 1% carbon black material in step 4) was not effectively removed by the airflow milling treatment in step 5). SEM showed that the particles were uniform in size, with a particle size of 10 μm-20 μm. TEM showed that an amorphous carbon layer was present on the outer surface of the material, with a thickness of approximately 5 to 8 nm. TG showed that the amorphous carbon content of the outer surface of the material was 5.2 wt%;

[0187] Comparative Example 10 (air flow crushing conditions outside the protection range, time upper limit)

[0188] The Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1. The process and conditions were the same as those of Comparative Example 1. The difference from Comparative Example 1 was that:

[0189] 4) Then, 1.2 kg of the precursor powder and 36 g of carbon black were added to a jar mill, and 909 g of graphite balls with a diameter of 10 cm and 303 g of graphite balls with a diameter of 6 cm were added. The jar mill was rotated at 40 to 50 r / min and the jar mill treatment time was 24 h. The jar mill was then transferred to a high-temperature rotary kiln protected by an argon atmosphere for heat treatment. h1, T1 heat treatment temperature was 300°C, heat treatment time was 3 h, and heating rate was 5°C / min; T2 heat treatment temperature was 550°C, heat treatment time was 8 h, and heating rate was 2°C / min. The sample after heat treatment was a fluffy powder. A small amount of the above powder was taken for XRD detection, which showed that the product was 99% sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 and 1% carbon black material, without other impurity phases.

[0190] 5) The powder obtained above was subjected to air flow milling using a fluidized bed jet mill with a classifying wheel (manufacturer: Weifang Zhengyuan; model: LHL / Y-1) with the classifying wheel frequency set to 50 Hz and the high-pressure air pressure set to 7 MPa for 5.5 hours. The obtained material was recorded as sample J#;

[0191] 6) The prepared sample J# was characterized by XRD, SEM, and TEM, and its carbon content (TG) was measured. XRD revealed that the synthesized material contained 99.93% sodium ferric pyrophosphate (Na4Fe3(PO4)2P2O7), and no carbon black diffraction peaks were present. This indicated that the 1% carbon black material in step 4) was effectively removed by the airflow milling treatment in step 5). SEM revealed that the particles were uniform in size, ranging from 0.5 μm to 1 μm. TEM revealed that an amorphous carbon layer was present on the outer surface of the material, with a thickness of approximately 5 to 10 nm. TG revealed that the amorphous carbon content on the outer surface of the material was 4.5 wt%.

[0192] Comparative test example 1

[0193] Sample A#, prepared in Comparative Example 1, was used as the positive electrode active material for a sodium-ion battery. It was mixed with acetylene black (a conductive agent) and polyvinylidene fluoride (PVDF) (a binder) in a mass ratio of 8:1:1. An appropriate amount of N-methylpyrrolidone (a solvent) was added to form a paste, which was then applied to an aluminum current collector. After drying, the paste was cut into 14 mm diameter discs with an active material surface density of 1.1-3.2 mg / cm² (2.1 mg / cm² in this case). This was used as the positive electrode. A sodium metal sheet was used as the negative electrode, the electrolyte was 1M NaClO₄ / EC / DEC (EC / DEC V / V = 1:1), and the separator was a glass fiber membrane. The assembled battery was subjected to charge and discharge tests over a voltage range of 1.8-3.6 V. The discharge specific capacities were measured at 0.2C / 1.0C / 5.0C / 10C / 50C. The discharge specific capacities at 0.2C / 50C are reported in Table 2.

[0194] Comparative Test Example 2-10

[0195] Samples B#-J# were tested according to the test method of Comparative Test Example 1, and the discharge specific capacity under 0.2C / 50C conditions was recorded in Table 2.

[0196] Table 1 Test data of discharge specific capacity of materials in Example

[0197]

[0198] Table 2 Comparative Example Material Discharge Specific Capacity Test Data

[0199]

[0200] The present invention provides a method of introducing acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber, graphene, etc. into the precursor powder obtained after drying. The rich specific surface area of ​​the above materials provides growth sites for the isotropic homogeneous precursor heat treatment process of the iron-based polyanion material, effectively avoiding the problem of caking during the calcination of the isotropic homogeneous precursor. Moreover, the above substances can be effectively removed by air flow crushing due to the density difference between them and the product, so that the material sintering uniformity is high during the heat treatment process and the material particles are small and evenly distributed. As shown in Example 1, the purity of the sodium iron phosphate pyrophosphate material prepared can reach 99.99%. The XRD diffraction spectrum shows that the added carbon black can be effectively removed. The particle size is 1μm-2μm. The specific capacity of the material under 0.2C conditions is 125mAh.g -1 , the specific capacity of the material under 50C conditions is 90mAh.g -1 ;

[0201] The rich specific surface area of ​​materials such as acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber, and graphene provides growth sites for the isotropic homogeneous precursor heat treatment process of iron-based polyanionic materials. At the same time, their mass ratio with the precursor also has a certain optimization relationship. When the mass ratio of acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber, and graphene is too low (<0.1%), it is impossible to ensure that the precursor forms sufficient contact with them. This insufficient contact causes the precursors that are not covered by them to form lumps during the subsequent heat treatment, resulting in uneven sintering. When the mass ratio of acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber, and graphene is too high (>5%), its rich specific surface area isolates the contact between the precursors to a certain extent. Although they will not form lumps during heat treatment, this insufficient contact between the precursors may not be able to effectively contact during crystal growth, resulting in impurities in the product. Therefore, when the mass ratio of acetylene black to the precursor is too high or too low, it will affect the performance of the product. Through experimental verification and optimization, it can be found that when the mass ratio of the precursor is 0.1% to 5%, the performance of the material is relatively excellent. For example, in Examples 1 to 5, the purity of the prepared sodium iron pyrophosphate material is as high as 99.9%. The XRD diffraction spectrum shows that the added carbon black can be effectively removed. The particle size is 0.2μm-4.5μm, and the specific capacity of the material under 0.2C conditions is 112-125mAh.g -1 The specific capacity of the material under 50C conditions is 70 to 90 mAh.g -1 When the optimal mass ratio of the precursor is 0.1% to 3%, as in Examples 1, 2, and 4, the specific capacity of the material at 0.2C is 117 to 125 mAh.g -1 The specific capacity of the material under 50C conditions is 75 to 90 mAh.g -1When the mass ratio of the carbon black to the precursor is less than 0.1%, as in Comparative Examples 1 to 2, the product exhibits varying degrees of agglomeration after the heat treatment process, affecting the purity and rate performance of the product. The prepared sodium iron pyrophosphate material contains impurities with a purity of 95.6% to 97.5%, but the XRD diffraction spectrum shows that the added carbon black can be effectively removed, with a particle size of 950nm-10μm and a specific capacity of 105 to 106mAh.g at 0.2C. -1 The specific capacity of the material under 50C conditions is 50~55mAh.g -1 When the mass ratio of carbon black to the precursor is greater than 5%, as in Comparative Example 3, although the product does not form agglomerates during heat treatment and the particle size is 150nm-800nm, the insufficient contact between the precursors leads to the appearance of impurities in the product with a purity of 98.5%. However, the XRD diffraction spectrum shows that the added carbon black can be effectively removed. The specific capacity of the material under 0.2C conditions is 110mAh.g -1 The specific capacity of the material under 50C conditions is 65mAh.g -1 However, this type of carbon material cannot achieve the above-mentioned dispersion effect when added to the precursor solution because it is insoluble in water. For example, in Comparative Example 4, the particle size is 1μm to 10μm, the purity is 98%, and the specific capacity of the material under 0.2C conditions is 106mAh.g -1 The specific capacity of the material under 50C conditions is 60mAh.g -1 ;

[0202] However, other surfactant dispersants are directly added to the solution, but they decompose during the high-temperature calcination process or cannot provide a rich specific surface area, which cannot achieve the effect of reducing product agglomeration. For example, in Comparative Example 5, the particle size is 5 μm to 10 μm, the purity is 94.5%, and the specific capacity of the material under 0.2C conditions is 108 mAh.g -1 , the specific capacity of the material under 50C conditions is 50mAh.g -1 If the calcined material is not subjected to airflow pulverization, the carbon material added in step 4) cannot be effectively removed, such as in Comparative Example 6, resulting in the appearance of carbon material impurities in the positive electrode. The material purity is 99%, the particle size is 2μm to 3μm, and the specific capacity of the material under the condition of purity of 0.2C is 106mAh.g -1 The specific capacity of the material under 50C conditions is 60mAh.g -1 When the airflow milling treatment conditions are not suitable, the airflow milling pressure is less than 2MPa or the time is less than 1h, the carbon material in the product cannot be effectively removed. For example, in Comparative Examples 7 and 9, the material purity is 99.5%-99.7%, the particle size is 5μm-20μm, and the specific capacity of the material under 0.2C conditions is 100-101mAh.g -1The specific capacity of the material under 50C conditions is 48-49 mAh.g -1 When the pressure of the airflow milling is greater than 12 MPa or the time is greater than 5.5 h, the structural stability of the product may be affected. For example, in Comparative Examples 8 and 10, the purity of the material is 99.93%-99.95%, the particle size is 0.2 μm-1 μm, and the specific capacity of the material under 0.2C conditions is 110-111 mAh.g -1 The specific capacity of the material under 50C conditions is 75-76 mAh.g -1 .

Claims

1. A method for preparing an iron-based phosphate or iron-based fluorophosphate polyanionic compound, characterized in that: The iron-based phosphate polyanionic compound has one or more structures represented by Formula I, Formula II, Formula III or Formula IV: Na4Fe3(PO4)2P2O7 Formula Ⅰ; Na3Fe2(PO4)1P2O7 formula Ⅱ; NaFePO4 formula III; Na2FeP2O7 formula IV; Alternatively, the iron-based fluorophosphate polyanionic compound has a structure shown in Formula V and / or VI: Na2FePO4F formula V; Na5Fe2(PO4)2F3 formula VI; The preparation method of the polyanionic compound shown in Formula I, Formula II, Formula III, Formula IV, V or VI comprises: Step 1) fully dissolving the reducing agent, soluble iron source, and pH adjuster in water; Step 2), adding a sodium source, a phosphorus source and a pH adjuster, and optionally adding a fluorine source, to the aqueous solution obtained in step 1 above to form a uniformly mixed precursor solution; The amount of the pH adjuster added should ensure that the pH of the aqueous solution in steps 1) and 2) is 2≤pH≤4, respectively. The amount of the pH adjuster added in step 2) should ensure that the pH of the aqueous solution is the same as that in step 1); Step 3), the precursor solution is passed into a drying device for spray drying to obtain a precursor powder; Step 4) uniformly mixing the precursor powder obtained in step 3) with the carbon material, and heat-treating the mixture in an inert atmosphere or an inert atmosphere containing H2, to obtain the final product of the carbon-coated iron-based phosphate-based polyanion cathode material; The carbon material is one or more of acetylene black, carbon black, Ketjen black, graphite, carbon nanotubes, carbon fiber, and graphene, and the mass ratio of the carbon material to the precursor powder obtained in step 3) is 0.1% to 5%; In step 5, the product obtained in step 4) is pulverized by air flow to remove the carbon material added in step 4) to obtain an iron-based phosphate polyanionic compound with a particle size range of 0.2 μm-5 μm.

2. The method according to claim 1, wherein: The mass ratio of the carbon material to the precursor powder obtained in step 3) is 0.3% to 3%, and the particle size of the iron-based phosphate polyanionic compound ranges from 1.5 μm to 3.5 μm.

3. The method according to claim 1, wherein: The mass ratio of the carbon material to the precursor powder obtained in step 3) is 0.5% to 1.5%.

4. The method according to claim 1, wherein: The iron ion concentration in the aqueous solution in step 1) is (0.001-0.95)n, where n is the iron ion concentration when the soluble iron salt in the solution reaches a saturated state in the clear aqueous solution obtained above at 60°C-80°C; The iron of the soluble iron source is ferric ion and / or ferrous ion in the solution; The stoichiometric ratio of the reducing agent to the ferric iron element required to reduce ferric iron to ferrous iron is set to a, where a is defined as the stoichiometric ratio of 1 mol Fe 3+ Reduced to 1 mol Fe 2+ Exactly the molar amount of reducing agent required; When the iron source contains trivalent iron ions, the molar amount of the reducing agent added corresponding to the trivalent iron ions in the iron source is (1.01~1.2)*a*Fe 3+ The molar amount of When the iron source contains divalent iron ions, the molar amount of reducing agent added corresponding to the divalent iron ions in the iron source is (0.01~0.2)*a*Fe 2+ The molar amount of The molar amount of the reducing agent added to the water is the sum of the molar amounts of the reducing agent added corresponding to the trivalent and divalent iron ions in the above iron source; The temperature of the aqueous solution is 60°C to 80°C.

5. The method according to claim 1, wherein: 1) The iron source is one or more soluble iron salts selected from ferric citrate, ferric nitrate, ferric sulfate, ferric chloride, ferric acetate, ferrous ammonium sulfate, ferrous citrate, ferrous nitrate, ferrous sulfate, ferrous chloride, ferrous acetate, and ferrous pyrophosphate; 2) The reducing agent is at least one or more of citric acid, oxalic acid, ammonium citrate, ascorbic acid, formaldehyde, acetaldehyde, lactic acid, and malic acid; 3) The pH adjuster is at least one or more of citric acid, oxalic acid, aqueous ammonia with an ammonia content of 25 wt% to 28 wt%, ammonium oxalate, ammonium hydrogen oxalate, ammonium lactate, ammonium citrate, ammonium hydrogen citrate, ammonium malate, ammonium hydrogen malate, ammonium carbonate, and ammonium bicarbonate; 4) The sodium source is at least one or more of sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium acetate, sodium phosphate, sodium pyrophosphate, sodium hydrogen pyrophosphate, sodium hydrogen phosphate, sodium dihydrogen phosphate, sodium citrate, sodium oxalate, sodium nitrate, sodium tartrate, sodium ascorbate, sodium gluconate, sodium glutamate, and sodium formate; 5) The phosphorus source is at least one or two or more of sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, triammonium phosphate, pyrophosphoric acid, sodium pyrophosphate, sodium dihydrogen pyrophosphate, and ammonium dihydrogen pyrophosphate; 6) The fluorine source is selected from at least one or more of sodium fluoride, ammonium fluoride, polyvinylidene fluoride, polyperfluoroethylene, and hydrofluoric acid.

6. The method according to claim 1, wherein: Category 1: When the reducing agent is one or both of ascorbic acid and oxalic acid, a is 0.5; Category II: When the reducing agent is one or more of citric acid, ammonium citrate, formaldehyde, acetaldehyde, lactic acid, and malic acid, a is 1; When the reducing agent is a combination of one or more reducing agents of the first type and one or more reducing agents of the second type, the molar amount of the reducing agent added to the water is determined based on the molar ratio a of the reducing agent to the ferric ion, the molar ratio b of the reducing agent to the divalent iron ion, and the molar amounts of the divalent iron ion and the trivalent iron ion in the iron source.

7. The method according to claim 1, wherein: In step 2), a carbon source may or may not be added to the aqueous solution. Since the reducing agent and the pH adjuster other than ammonia water will introduce a carbon source after addition, the amount of the carbon source should ensure that the carbon content in the synthesized final product is 1 wt%-20 wt%; The carbon source is at least one or more of oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid, malic acid, sucrose, glucose, soluble starch, and liquid polyethylene glycol; Step 3) The drying process is spray drying; The spray drying feed flow rate is 40 ml / min to 70 ml / min, the air inlet temperature is 90°C to 190°C, the exhaust temperature is 70°C to 180°C, the compressed gas pressure is 0.01 KPa to 1 KPa, and the compressed gas is at least one or two or more of air, argon, and nitrogen, or a mixture of 0.1 vol% to 20 vol% of hydrogen and argon, or a mixture of 0.1 vol% to 20 vol% of hydrogen and nitrogen; The air jet milling described in step 5) uses a fluidized bed air jet mill, a circulating tube air jet mill or an opposed jet air jet mill, the compressed air pressure is 2 - 12 MPa, and the air jet milling time is 1 h - 5 h.

8. The method according to claim 7, wherein: The amount of the carbon source should ensure that the carbon content in the synthesized end product is 4 wt% - 10 wt%; The spray inlet air temperature is 120°C - 175°C, the exhaust air temperature is 110°C - 165°C, and the compressed gas pressure is 0.2 KPa - 0.85 KPa.

9. According to the preparation method described in claim 1, wherein The mixing method described in step 4) is one or more of sand grinding, ball milling, and tank milling, For sand grinding, a sand grinding medium needs to be added, the sand grinding speed is 3000 - 5000 r / min, and the sand grinding time is 0.5 - 5 h; the sand grinding medium is one or more of natural sand beads, glass beads, steel beads, zirconia beads, zirconium silicate beads, and agate beads; the ball-to-material ratio, that is, the mass ratio of balls to solid materials, is 1 - 5; the diameter of the sand grinding medium is 2 - 12 cm; For ball milling, a ball milling medium needs to be added, the rotation speed of the ball milling is 600 - 800 r / min, and the ball milling treatment time is 2 - 6 h; the ball milling medium is one or more of zirconia beads, agate beads, and zirconium silicate beads; the ball-to-material ratio is 1.5 - 7; the diameter of the ball milling medium is 2 - 12 cm; For tank milling, a tank milling medium needs to be added, the rotation speed of the tank milling is 40 - 50 r / min, and the tank milling treatment time is 15 - 50 h; the tank milling medium is one or more of zirconia beads, agate beads, zirconium silicate beads, and graphite balls; the ball-to-material ratio is 1.5 - 3; the diameter of the tank milling medium is 2 - 12 cm; The heat treatment atmosphere described in step 4) is selected from an inert atmosphere including at least one of argon, helium, and nitrogen, or an inert atmosphere containing 0 - 50% by mole of H2 with reducing properties; the heat treatment includes a process of being treated at the first temperature T1 and a process of being treated at the second temperature T2 in sequence; the temperature T1 is 200 ≤ T1 ≤ 400°C, and the treatment time is 0.5 - 6 h; the temperature T2 is 400°C < T1 ≤ 650°C, and the treatment time is 3 - 20 h; the difference between T2 and T1 is greater than or equal to 50°C.

10. According to the preparation method described in claim 9, wherein The sand grinding medium is composed of sand beads with diameters of 10 cm, 8 cm, and 4 cm combined in a mass ratio of 2:1:2; The ball milling medium is composed of balls with diameters of 10 cm, 8 cm, and 4 cm combined in a mass ratio of 1:2:2; The tank milling medium is composed of balls with diameters of 10 cm and 8 cm combined in a mass ratio of 3:

1.

11. According to the preparation method described in claim 1, wherein The phosphate polyanionic compound has the structure shown in formula I. In slurry c, the molar ratio of the sodium source, iron source, and phosphorus source is 4:3:4, and the molar ratios of the sodium source, iron source, and phosphorus source are based on the molar amounts of sodium element, iron element, and phosphorus element respectively. The phosphate polyanionic compound has a structure shown in Formula II. In slurry C, the molar ratio of the sodium source, the iron source, and the phosphorus source is 3:2:3, and the molar ratios of the sodium source, the iron source, and the phosphorus source are calculated based on the molar amounts of the sodium element, the iron element, and the phosphorus element, respectively. The polyanionic compound has a structure shown in Formula III. In slurry c, the molar ratio of the sodium source, the iron source, and the phosphorus source is 1:1:1, and the molar ratios of the sodium source, the iron source, and the phosphorus source are calculated based on the molar amounts of the sodium element, the iron element, and the phosphorus element, respectively. The polyanionic compound has a structure shown in Formula IV. In slurry c, the molar ratio of the sodium source, the iron source, and the phosphorus source is 2:1:2, and the molar ratio of the sodium source, the iron source, and the phosphorus source is based on the molar amount of the sodium element, the iron element, and the phosphorus element, respectively; The polyanionic compound has a structure shown in Formula V. In slurry C, the molar ratio of the sodium source, the iron source, the phosphorus source, and the fluorine source is 2:1:1:1, and the molar ratio of the sodium source, the iron source, the phosphorus source, and the fluorine source is based on the molar amount of the sodium element, the iron element, the phosphorus element, and the fluorine element, respectively; The polyanionic compound has a structure shown as VI. In slurry c, the molar ratio of the sodium source, iron source, phosphorus source, and fluorine source is 5:2:2:3, and the molar ratio of the sodium source, iron source, phosphorus source, and fluorine source is based on the molar amount of the sodium element, iron element, phosphorus element, and fluorine element, respectively.

12. An iron-based phosphate or fluorophosphate polyanionic compound prepared by the preparation method according to any one of claims 1 to 11.

13. Use of the iron-based phosphate or fluorophosphate polyanionic compound according to claim 12 in a sodium ion battery, characterized in that: The iron-based phosphate or fluorophosphate polyanionic compound is used as an active substance for a positive electrode material of a sodium ion battery.

14. A positive electrode material for a sodium ion battery, characterized in that The positive electrode material contains the iron-based phosphate or fluorophosphate polyanionic compound according to claim 12; In the positive electrode material, the content of the iron-based phosphate or fluorophosphate polyanionic compound is 60-98 wt %; The positive electrode material further contains a conductive agent and a binder, wherein the mass ratio of the iron-based phosphate or fluorophosphate polyanionic compound, the conductive agent, and the binder is (60-98) wt%: (1-39) wt%: (1-39) wt%; The conductive agent is at least one or more of carbon black, conductive graphite, carbon fiber, carbon nanotube, and graphene; The binder is polyvinylidene fluoride: at least one or more of PVDF5130, HSV900, and kynar761A.

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