High-purity iron-based polyanionic compound, preparation and use thereof

Abstract

The application relates to a sodium ion battery electrode material, in particular to a preparation method of a high-purity phosphate or fluorophosphate polyanion compound and application of the high-purity phosphate or fluorophosphate polyanion compound in a sodium ion battery electrode material; the iron-based phosphate polyanion compound has one or two or more of structures shown in formula I, formula II, formula III, formula IV or V, VI: Na4Fe3(PO4)2P2O7, Na3Fe2(PO4)1P2O7, NaFePO4, Na2FeP2O7, Na2FePO4F, Na5Fe2(PO4)2F3; the prepared phosphate or fluorophosphate polyanion compound is high in purity and has good rate performance and excellent cycle stability.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of sodium ion batteries, and relates to a preparation method of a sodium ion battery electrode material, in particular to a preparation method of a phosphate or fluorophosphate polyanion compound and application of the phosphate or fluorophosphate polyanion compound in a sodium ion battery electrode material. BACKGROUND

[0002] In recent years, with the proposal of the "double carbon" policy and the goal of building a new energy-based new power system, as the key to regulating new energy volatility, indirectness and achieving grid stability, the energy storage industry has always maintained high attention, and alkali metal (Li, Na, K) ion batteries are an important part of portable large-scale fixed energy storage. Among them, lithium ion batteries (LIBs) are widely used in electric vehicles and electronic devices due to their high energy density, high electrode potential, stable cycle performance and environmental friendliness, but in recent years, the limitation of lithium sources has led to high prices of lithium ion battery cathode materials, and sodium ion batteries have also attracted more and more attention because they have similar working principles to lithium ion batteries and low-cost and abundant sodium sources, so it is particularly important to develop sodium ion ion battery cathode materials with excellent performance.

[0003] Polyanion materials such as sodium iron phosphate, sodium pyrophosphate, sodium fluorophosphate, and sodium phosphate pyrophosphate have become the preferred positive electrode materials of alkali metal ion batteries due to their structural stability, safety and other advantages. In the production process of phosphate and fluorophosphate polyanion materials, the precursors are mixed by ball milling or sand milling, and then dried to obtain the precursors before high-temperature calcination to form phases. However, in the phase formation reaction, the chemical reaction mechanism and thermal effect between different types of precursors are not the same, which greatly affects the heat treatment conditions and reaction uniformity of the phase formation process. When the mass of the precursor increases, the above-mentioned thermal effect on the reaction and product uniformity is intensified, which further affects the product purity and has a great impact on the rate performance and cycle stability of the product. SUMMARY

[0004] To solve the above problems, the application provides a method for accurately regulating the heat treatment process of product phase formation according to the thermal effect difference between different precursors, which effectively reduces the influence of heat accumulation in the precursor phase formation process on the uniformity of the overall heat treatment process through the design of a specific reaction container. The introduction of the above technical means can still prepare phosphate and fluorophosphate polyanion materials with high purity, high specific capacity, high rate performance and high cycle stability even when the mass of the precursor increases, which greatly promotes the development and application of such materials in the energy storage field.

[0005] The iron-based phosphate or fluorophosphate polyanion compound has a purity of 99 wt% or higher.

[0006] The positive electrode material contains iron-based phosphate;

[0007] The content of the iron-based phosphate or fluorophosphate polyanionic compound in the positive electrode material is (60-98) wt%;

[0008] The positive electrode material further contains a conductive agent and a binder, and 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%;

[0009] The conductive agent is at least one or two or more of carbon black, conductive graphite, carbon fiber, carbon nanotube, and graphene.

[0010] The binder is at least one or two or more of polyvinylidene fluoride (PVDF) 5130, HSV900, and kynar 761A.

[0011] The prepared phosphate or fluorophosphate polyanionic compound has high purity and good rate performance and excellent cycle stability.

[0012] Beneficial effects

[0013] To solve the above problems, the present application provides a method for accurately regulating the heat treatment process of product phase formation according to the thermal effect difference between different precursors, which effectively reduces the influence of heat accumulation during precursor phase formation on the uniformity of the overall heat treatment process through the design of a specific reaction container. The introduction of the above technical means can prepare phosphate or fluorophosphate polyanionic materials with high purity, high specific capacity, high rate performance, and high cycle stability, greatly promoting the development and application of such materials in the energy storage field. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 The figure shows the geometry of the reaction bed in the claim; in the figure: ① inlet pipe / flange; ② top barrier layer of the reaction bed; ③ reaction bed; ④ peripheral screen barrier layer; ⑤ bottom support layer; ⑥ shortest distance from the peripheral barrier layer of the reaction bed to the wall surface of the inlet flange; ⑦ shortest distance from the peripheral barrier layer of the reaction bed to the wall surface of the outlet flange; ⑧ outlet pipe / flange; ⑨ shortest distance from the reaction bed to the upper and lower inner walls of the reaction tube; ⑩ shortest distance from the reaction bed to the inner wall of the reaction tube; Distance between reaction beds;

[0015] Figure 2 The figure shows the XRD diffraction spectrum of material 1# in Example 1;

[0016] Figure 3XRD diffraction pattern of material A# in Comparative Example 1. DETAILED DESCRIPTION

[0017] Example 1

[0018] Preparation of sodium iron pyrophosphate phosphate Na4Fe3(PO4)2P2O7 material 1

[0019] Step 1) 301.6 g of iron phosphate FePO4, 180 g of ferrous oxalate dihydrate FeC2O4.2H2O, 284 g of disodium hydrogen phosphate, and 174 g of starch were weighed and added to 940 g of deionized water to form a precursor solution with a solid content of 50 wt%, and then transferred to a polytetrafluoroethylene tank mill with a volume of 5 L, and the ball-to-material ratio (mass ratio) was about 2:1 (1.2 kg of 6 cm diameter agate balls and 0.6 kg of 4 cm diameter agate balls were added); the tank mill was transferred to a tank mill machine for tank milling, and the tank milling speed was set to 40 r / min for 48 h;

[0020] Step 2) The precursor solution after tank milling was transferred to a beaker and then sprayed into a spray drying device (the inlet temperature was 170°C, the outlet temperature was 100°C, the feed flow rate was 50 ml / min, and the compressed air pressure was 0.8 KPa) to obtain a precursor powder with a particle size of 25 μm;

[0021] Step 3) The precursor powder was then made into a reaction bed layer, which was a flat bed layer structure including a bottom support layer and a top screen layer, and an intermediate precursor material clamping zone was formed between the bottom support layer and the top screen layer. An annular screen barrier layer was provided around the intermediate precursor material clamping zone, and the upper and lower ends of the screen barrier layer were fixedly connected to the top screen layer and the bottom support layer, respectively. The intermediate precursor material clamping zone was filled with the precursor powder. The bottom support layer was a flat plate with a thickness of 0.6 cm, and the top screen layer and the peripheral screen barrier layer were both screens with a thickness of 0.25 cm and a mesh size of 1000. The thickness of the intermediate precursor material clamping zone in the bed (i.e., the distance between the bottom support layer and the top screen layer) was 1.5 cm.

[0022] Step 4) the reaction bed is placed in the tube reaction tube, the reaction bed is 2, the interval is 0.5cm, placed in the reaction tube from top to bottom parallel interval, the bottom support layer of the reaction bed is parallel to the axis of the reaction tube (or through the geometric center line of the two opening ends of the reaction tube), the shortest distance from any point on the bottom support layer, the top screen layer, the four around screen barrier layer of the reaction bed to any point on the inner wall surface of the tube furnace is 3cm; the reaction tube is placed at the center position of the reaction furnace, the two opening ends of the reaction tube are connected with the gas inlet pipeline and the gas outlet pipeline of the reaction furnace through flanges, the gas inlet direction is parallel to the axis of the reaction tube, the shortest distance from the four around screen barrier layer of the reaction bed to any point on the inner wall surface of the flange of the tube furnace is 10cm;

[0023] Step 5) heat treatment of the precursor powder in the high-temperature atmosphere tube furnace under the flow of argon, the flow of argon should ensure that the temperature difference when flowing out of the reaction bed and flowing into the reaction bed is less than or equal to 8℃, the temperature of the gas flowing into the reaction bed is -4℃ of the inner wall temperature of the tube furnace during heat treatment to the inner wall temperature of the tube furnace during heat treatment, the flow direction is parallel to the upper surface of the reaction bed from the inlet side to the outlet side;

[0024] Step 6), the heat treatment temperature T1 is 300℃, the heat treatment time is 3h, and the heating rate is 5℃ / min; the heat treatment temperature T2 is 500℃, the heat treatment time is 8h, and the heating rate is 2℃ / min; the sample after heat treatment is a fluffy powder, and the obtained material is marked as sample 1#, and the parameters are shown in Table 1;

[0025] Step 7), the prepared sample 1# is tested by XRD and TG, and it can be seen from the XRD( Figure 1 ) that the synthesized material is pure phase sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity is as high as 99.99%, the conversion rate of iron atoms from the raw material iron phosphate FePO4, ferrous oxalate dihydrate FeC2O4.2H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 is 99.99%, and it can be seen from the TG test that the carbon content of the material is 4.5wt%;

[0026] Example 2

[0027] Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 2#

[0028] The method of example 1 is used to prepare Na4Fe3(PO4)2P2O7 material, the process and conditions are the same as those of example 1, and the difference from example 1 is that

[0029] Step 3) the thickness of the bottom support layer is 0.1cm;

[0030] Step 6) The sample after heat treatment is fluffy powder, the obtained material is recorded as sample 2#, the parameters are shown in Table 1;

[0031] Step 7) The prepared sample 2# is subjected to XRD test, it can be seen from the XRD that the synthesized material is a pure phase sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity is as high as 99.6%, the conversion rate of iron source iron phosphate FePO4 and ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to iron atoms in the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 is 99.6%, and it can be seen from the TG test that the carbon content of the material is 4.3wt%;

[0032] Example 3

[0033] Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 3#

[0034] The sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material is prepared by the method of Example 1, and the process and conditions are the same as those of Example 1, and the difference from Example 1 is that

[0035] Step 3) The thickness of the bottom support layer is 1 cm;

[0036] Step 6) The sample after heat treatment is fluffy powder, the obtained material is recorded as sample 3#, the parameters are shown in Table 1;

[0037] Step 7) The prepared sample 3# is subjected to XRD test, it can be seen from the XRD that the synthesized material is a pure phase sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity is as high as 99.2%, the conversion rate of iron source iron phosphate FePO4 and ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to iron atoms in the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 is 99.2%, and it can be seen from the TG test that the carbon content of the material is 4.3wt%;

[0038] Example 4

[0039] Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 4#

[0040] The sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material is prepared by the method of Example 1, and the process and conditions are the same as those of Example 1, and the difference from Example 1 is that

[0041] Step 3) The thickness of the top screen layer and the four surrounding screen barrier layers is 0.1 cm;

[0042] Step 6) The sample after heat treatment is fluffy powder, the obtained material is recorded as sample 4#, the parameters are shown in Table 1;

[0043] Step 7) XRD test was performed on the prepared sample 4#. From the XRD, 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 99.7%, and the conversion rate of iron source ferric phosphate FePO4 and ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to the iron atom in the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 99.7%; from the TG test, it can be seen that the carbon content of the material is 4.6wt%;

[0044] Example 5

[0045] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material 5#

[0046] The method of Example 1 was used to prepare Na4Fe3(PO4)2P2O7 material, and the process and conditions were the same as those of Example 1, except that

[0047] Step 3) The thickness of the top screen layer and the four surrounding screen blocking layers was 0.5 cm; the sample after heat treatment was fluffy powder, and the obtained material was recorded as sample 5#, and the parameters are shown in Table 1;

[0048] Step 6) The sample after heat treatment was fluffy powder, and the obtained material was recorded as sample 5#, and the parameters are shown in Table 1;

[0049] Step 7) XRD test was performed on the prepared sample 5#. From the XRD, 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 99.1%, and the conversion rate of iron source ferric phosphate FePO4 and ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to the iron atom in the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 99.1%; from the TG test, it can be seen that the carbon content of the material is 4.9wt%;

[0050] Example 6

[0051] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material 6#

[0052] The method of Example 1 was used to prepare Na4Fe3(PO4)2P2O7 material, and the process and conditions were the same as those of Example 1, except that

[0053] Step 3) The thickness of the middle precursor material clamping area in the bed layer (i.e. the distance between the bottom support layer and the top screen layer) was 1 cm;

[0054] Step 6) The sample after heat treatment was fluffy powder, and the obtained material was recorded as sample 6#, and the parameters are shown in Table 1;

[0055] Step 7) XRD test was performed on the prepared sample 6#, from which 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 99.9%, and the conversion rate of iron source ferric phosphate FePO4 and ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to iron atoms in the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 99.9%; from the TG test, it can be seen that the carbon content of the material is 4wt%;

[0056] Example 7

[0057] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material 7#

[0058] The method of Example 1 was used to prepare Na4Fe3(PO4)2P2O7 material, and the process and conditions were the same as in Example 1, except that

[0059] Step 3) The thickness of the clamping zone of the precursor material in the bed (i.e. the distance between the bottom support layer and the top screen layer) was 4cm;

[0060] Step 6) The sample after heat treatment was a fluffy powder, and the obtained material was recorded as sample 7#, the parameters of which are shown in Table 1;

[0061] Step 7) XRD test was performed on the prepared sample 7#, from which 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%, and the conversion rate of iron source ferric phosphate FePO4 and ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to iron atoms in the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 99%, from the TG test, it can be seen that the carbon content of the material is 4.1wt%;

[0062] Example 8

[0063] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material 8#

[0064] The method of Example 1 was used to prepare Na4Fe3(PO4)2P2O7 material, and the process and conditions were the same as in Example 1, except that

[0065] Step 4) The shortest distance from any point on the bottom support layer, top screen layer and peripheral screen barrier of the reaction bed to any point on the inner wall surface of the tube furnace tube was 0.1cm;

[0066] Step 6) The obtained material was recorded as sample 8#, the parameters of which are shown in Table 1;

[0067] Step 7) XRD test was performed on the prepared sample 8#. From the XRD, 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 99.1%, and the conversion rate of iron source ferric phosphate FePO4 and ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to the iron atom in the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 99.1%; from the TG test, it can be seen that the carbon content of the material is 5wt%;

[0068] Example 9

[0069] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material 9#

[0070] The method of Example 1 was used to prepare Na4Fe3(PO4)2P2O7 material, and the process and conditions were the same as in Example 1, except that

[0071] Step 4) The shortest distance from the screen barrier around the reaction bed to any point on the inner wall surface of the flange at both ends of the reaction tube is 1 cm;

[0072] Step 6) The obtained material is recorded as sample 9#, and the parameters are shown in Table 1;

[0073] Step 7) XRD test was performed on the prepared sample 9#. From the XRD, 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 99.2%, and the conversion rate of iron source ferric phosphate FePO4 and ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to the iron atom in the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 99.2%; from the TG test, it can be seen that the carbon content of the material is 4.6wt%

[0074] Example 10

[0075] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material 10#

[0076] The method of Example 1 was used to prepare Na4Fe3(PO4)2P2O7 material, and the process and conditions were the same as in Example 1, except that

[0077] Step 5) The argon gas flow condition should ensure that the temperature difference between the temperature when flowing out of the reaction bed and the temperature when flowing into the reaction bed is less than or equal to 20℃;

[0078] Step 6) The obtained material is recorded as sample 10#, and the parameters are shown in Table 1;

[0079] Step 7) XRD test was performed on the prepared sample 10#, from which it can be seen from the XRD that the synthesized material is pure phase sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material, with a purity of up to 99.1%, and the conversion rate of iron source ferric phosphate FePO4 and ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to iron atoms in the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 99.1%, and from the TG test it can be seen that the carbon content of the material is 4.7wt%.

[0080] Example 11

[0081] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material 11

[0082] The method of Example 1 was used to prepare Na4Fe3(PO4)2P2O7 material, with the same process and conditions as Example 1, except that

[0083] Step 5) The argon gas flow condition should ensure that the temperature of the gas flowing into the reaction bed is -10°C to the inner wall temperature of the tube furnace during heat treatment;

[0084] Step 6) The obtained material is recorded as sample 11#, and the parameters are shown in Table 1;

[0085] Step 7) XRD test was performed on the prepared sample 11#, from which it can be seen from the XRD that the synthesized material is pure phase sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material, with a purity of up to 99.3%, and the conversion rate of iron source ferric phosphate FePO4 and ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to iron atoms in the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 99.3%, and from the TG test it can be seen that the carbon content of the material is 4.5wt%

[0086] Test Example 1

[0087] The sample 1# prepared in Example 1 was used as the positive active material of a sodium ion battery, mixed uniformly with a conductive agent acetylene black, a binder polyvinylidene fluoride in a mass ratio of 8:1:1, and an appropriate amount of solvent N-methyl pyrrolidone was added and mixed uniformly to form a paste, which was coated on an aluminum current collector, dried, and cut into a circular piece with a diameter of 14mm, and the active material surface density was 1.1-3.2mg / cm 2 (here 2.1mg / cm 2), and the electrolyte was selected as 1 M NaClO4 / EC / DEC (V / V of EC / DEC = 1:1), and the separator was selected as a glass fiber membrane. The assembled battery was subjected to charge and discharge test, the voltage range was 1.5-4.0 V, the discharge specific capacity under the conditions of 0.2 C / 1.0 C / 5.0 C / 10 C / 50 C was tested, and the discharge specific capacity under the conditions of 0.2 C / 50 C was recorded in Table 1. At the same time, the cycle retention rate of the battery after 1000 cycles at the rate of 1.0 C and the voltage range of 1.5-4.0 V was also recorded in Table 1.

[0088] Test Example 2-11

[0089] The sample 2#-11# was tested according to the test method of Test Example 1, and the discharge specific capacity under the conditions of 0.2 C / 50 C and the cycle retention rate of the battery after 1000 cycles at the rate of 1.0 C and the voltage range of 1.5-4.0 V were recorded in Table 1.

[0090] Comparative Example 1 Preparation of sodium iron pyrophosphate phosphate material A Na4Fe3(PO4)2P2O7

[0091] Step 1) 301.6 g of iron phosphate FePO4, 180 g of ferrous oxalate dihydrate FeC2O4.2H2O, 284 g of disodium hydrogen phosphate, and 174 g of starch were weighed and added to a precursor solution in 940 g of deionized water, and the solid content in the precursor solution was 50 wt%, and then the precursor solution was transferred to a polytetrafluoroethylene tank mill with a volume of 5 L, and the ball-to-material ratio (mass ratio) was about 2:1 (1.2 kg of 6 cm diameter agate balls and 0.6 kg of 4 cm diameter agate balls were added); the tank mill was transferred to a tank mill machine, and the ball milling speed was set to 40 r / min, and the time was 48 h;

[0092] Step 2) The precursor solution after tank milling was transferred to a beaker, and then sprayed into a spray drying device (the inlet temperature was 170°C, the outlet temperature was 100°C, the feeding flow rate was 50 ml / min, and the compressed air pressure was 0.8 KPa), to obtain a precursor powder, and the particle size of the precursor powder was 25 μm;

[0093] Step 3) The precursor powder is then made into a reaction bed layer, which is a flat bed layer structure, including a bottom support layer and a top screen layer, forming an intermediate precursor material clamping area between the bottom support layer and the top screen layer, and an annular screen barrier layer is provided around the intermediate precursor material clamping area, the upper and lower ends of the screen barrier layer are respectively fixed with the top screen layer and the bottom support layer; the intermediate precursor material clamping area is filled with precursor powder; the bottom support layer is a flat plate with a thickness of 0.05 cm, the bottom support layer, the top screen layer and the four screen barrier layers are all screens, the thickness of the top screen layer and the four screen barrier layers is 0.25 cm, the screen mesh is 1000, and the thickness of the intermediate precursor material clamping area in the bed layer (i.e. the distance between the bottom support layer and the top screen layer) is 1.5 cm;

[0094] Step 4) The reaction bed layer is placed in a tubular reaction tube, the reaction bed layer is 2, the distance is 0.5 cm, and the reaction bed layer is placed in the reaction tube in parallel and at intervals from top to bottom, the bottom support layer of the reaction bed layer is parallel to the axis of the reaction tube (or the geometric center line passing through the two opening ends of the reaction tube), the shortest distance from any point on the bottom support layer, the top screen layer and the four screen barrier layers of the reaction bed layer to any point on the inner wall surface of the tubular furnace tube is 3 cm; the reaction tube is placed at the center position of the reaction furnace, the two opening ends of the reaction tube are connected with the gas inlet pipeline and the gas outlet pipeline of the reaction furnace through flanges, the gas inlet direction is parallel to the axis of the reaction tube, and the shortest distance from the four screen barrier layers of the reaction bed layer to any point on the inner wall surface of the flange of the tubular furnace is 10 cm;

[0095] Step 5) The precursor powder is heat treated in a high-temperature atmosphere tubular furnace under an argon gas flow atmosphere, the argon gas flow condition should ensure that the temperature difference when flowing out of the reaction bed layer and when flowing into the reaction bed layer is less than or equal to 8℃, the temperature of the gas flowing into the reaction bed layer is -4℃ of the inner wall temperature of the tubular furnace during heat treatment to the inner wall temperature of the tubular furnace during heat treatment, and the flow direction is parallel to the upper surface of the reaction bed layer from the inlet side to the outlet side;

[0096] Step 6) The T1 heat treatment temperature is 300℃, the heat treatment time is 3h, and the heating rate is 5℃ / min; the T2 heat treatment temperature is 500℃, the heat treatment time is 8h, and the heating rate is 2℃ / min; the sample after heat treatment is a fluffy powder, and the obtained material is marked as sample A#, and the parameters are shown in Table 1;

[0097] Step 7) The prepared sample A# is tested by XRD and TG, and the XRD pattern of the sample A# is shown in Fig. 1, and the TG curve of the sample A# is shown in Fig. 2; Figure 2It can be seen from the XRD that the synthesized material is mainly sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity is only 92%, the conversion rate of iron source ferric phosphate FePO4, ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to the iron atom of the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 92%, there are 3.5% of sodium iron phosphate NaFePO4 and 4.5% of sodium iron pyrophosphate Na2FeP2O7 impurities, and the carbon content is about 4wt%; Comparative Example 2

[0098] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material B

[0099] The sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1, and the process and conditions were the same as those of Comparative Example 1, and the difference from Comparative Example 1 was that

[0100] Step 3) The thickness of the bottom support layer was 1.5 cm;

[0101] Step 6) The sample after heat treatment was a fluffy powder, and the obtained material was recorded as sample B#, and the parameters were shown in Table 2;

[0102] Step 7) The prepared sample B# was subjected to XRD test, and it can be seen from the XRD that the synthesized material is mainly sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity is only 91%, the conversion rate of iron source ferric phosphate FePO4, ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to the iron atom of the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 91%, there are 4% of sodium iron phosphate NaFePO4 and 5% of sodium iron pyrophosphate Na2FeP2O7 impurities, and the carbon content is about 4.1wt%;

[0103] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material C

[0104] The sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1, and the process and conditions were the same as those of Comparative Example 1, and the difference from Comparative Example 1 was that

[0105] Step 3) The thickness of the bottom support layer was 0.6 cm, and the thickness of the top screen layer and the four surrounding screen barrier layers was 0.05 cm;

[0106] Step 6) The sample after heat treatment was a fluffy powder, and the obtained material was recorded as sample C#, and the parameters were shown in Table 2;

[0107] The sample after heat treatment was a fluffy powder, and the obtained material was recorded as sample C#, and the parameters were shown in Table 2;

[0108] Step 7) XRD test was performed on the prepared sample C#, from which it can be seen from the XRD that the synthesized material is mainly sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity is only 96%, the conversion rate of iron source ferric phosphate FePO4, ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to the iron atom of the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 96%, there are also 2% of sodium iron phosphate NaFePO4 and 2% of sodium iron pyrophosphate Na2FeP2O7 impurities, and the carbon content is about 4.3wt%;

[0109] Comparative Example 4

[0110] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material D

[0111] The sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1, and the process and conditions were the same as those of Comparative Example 1, and the difference from Comparative Example 1 was that

[0112] Step 3) the thickness of the bottom support layer was 0.6 cm, and the thickness of the top screen layer and the surrounding screen barrier layer was 0.8 cm;

[0113] Step 6) the sample after heat treatment was a fluffy powder, and the obtained material was recorded as sample D#, and the parameters are shown in Table 2;

[0114] Step 7) XRD test was performed on the prepared sample D#, from which it can be seen from the XRD that the synthesized material is mainly sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity is only 93%, the conversion rate of iron source ferric phosphate FePO4, ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to the iron atom of the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 93%, there are also 3% of sodium iron phosphate NaFePO4 and 4% of sodium iron pyrophosphate Na2FeP2O7 impurities, and the carbon content is about 4.2wt%;

[0115] Comparative Example 5

[0116] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material E

[0117] The sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1, and the process and conditions were the same as those of Comparative Example 1, and the difference from Comparative Example 1 was that

[0118] Step 3) the thickness of the bottom support layer was 0.6 cm, and the thickness of the top screen layer and the surrounding screen barrier layer was 0.8 cm;

[0119] Step 6) The sample after heat treatment is fluffy powder, the obtained material is recorded as sample E#, and the parameters are shown in Table 2;

[0120] Step 7) The prepared sample E# is subjected to XRD test, and it can be seen from the XRD that the synthesized material is mainly sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity is only 88%, the conversion rate of iron source iron phosphate FePO4, ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to iron atom in the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 88%, there are still 7% of sodium iron phosphate NaFePO4 and 5% of sodium iron pyrophosphate Na2FeP2O7 impurities, and the carbon content is about 4.6wt%;

[0121] Comparative Example 6

[0122] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material F#

[0123] The sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material is prepared by the method of Comparative Example 1, and the process and conditions are the same as those of Comparative Example 1, and the difference from Comparative Example 1 is that

[0124] Step 3) The thickness of the bottom support layer is 0.6 cm, and the thickness of the clamping area of the precursor material in the middle of the bed (i.e. the distance between the bottom support layer and the top screen layer) is 5 cm;

[0125] Step 6) The sample after heat treatment is fluffy powder, the obtained material is recorded as sample F#, and the parameters are shown in Table 2;

[0126] Step 7) The prepared sample F# is subjected to XRD test, and it can be seen from the XRD that the synthesized material is mainly sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity is only 83%, the conversion rate of iron source iron phosphate FePO4, ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to iron atom in the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 83%, there are still 9% of sodium iron phosphate NaFePO4 and 8% of sodium iron pyrophosphate Na2FeP2O7 impurities, and the carbon content is about 4.7wt%;

[0127] Comparative Example 7

[0128] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material G#

[0129] The sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material is prepared by the method of Comparative Example 1, and the process and conditions are the same as those of Comparative Example 1, and the difference from Comparative Example 1 is that

[0130] Step 3) The thickness of the bottom support layer is 0.6 cm;

[0131] Step 4) the shortest distance from any point on the bottom support layer, the top screen layer, the four perimeter screen barrier layers of the reaction bed to any point on the inner wall surface of the reaction furnace tube is 0.05 cm;

[0132] Step 6) the sample after heat treatment is fluffy powder, and the obtained material is recorded as sample G#, and the parameters are shown in Table 2;

[0133] Step 7) the prepared sample G# is subjected to XRD test, and it can be seen from the XRD that the synthesized material is mainly sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity is only 82%, the conversion rate of iron source iron phosphate FePO4, ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to iron atoms in the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 82%, there are also 10% of sodium iron phosphate NaFePO4 and 8% of sodium iron pyrophosphate Na2FeP2O7 impurities, and the carbon content is about 4.8 wt%;

[0134] Comparative Example 8

[0135] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material H#

[0136] The sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material is prepared by the method of Comparative Example 1, and the process and conditions are the same as those of Comparative Example 1, and the difference from Comparative Example 1 is that

[0137] Step 3) the thickness of the bottom support layer is 0.6 cm;

[0138] Step 4) the shortest distance from any point on the four perimeter screen barrier layers of the reaction bed to any point on the inner wall surface of the two end flanges of the reaction tube is 0.5 cm;

[0139] Step 6) the sample after heat treatment is fluffy powder, and the obtained material is recorded as sample H#, and the parameters are shown in Table 2;

[0140] Step 7) the prepared sample H# is subjected to XRD test, and it can be seen from the XRD that the synthesized material is mainly sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity is only 85%, the conversion rate of iron source iron phosphate FePO4, ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to iron atoms in the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 is 85%, there are also 8% of sodium iron phosphate NaFePO4 and 7% of sodium iron pyrophosphate Na2FeP2O7 impurities, and the carbon content is about 4.4 wt%;

[0141] Comparative Example 9

[0142] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material I

[0143] The sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1, the process and conditions being the same as in Comparative Example 1, except that

[0144] Step 3) the thickness of the bottom support layer was 0.6 cm;

[0145] Step 5) the temperature difference between the temperature when the gas flowed out of the reaction bed and the temperature when the gas flowed into the reaction bed should be 25°C under the condition of argon gas flow;

[0146] Step 6) the sample after heat treatment was a fluffy powder, and the obtained material was recorded as sample I#, the parameters being shown in Table 2;

[0147] Step 7) the prepared sample I# was subjected to XRD test, and it could be seen from the XRD that the synthesized material was mainly sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity being only 86%, the conversion rate of iron atoms from the raw material iron phosphate FePO4, ferrous oxalate dihydrate FeC2O4.2H2O to the product sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 being 86%, there being 9% of sodium iron phosphate NaFePO4 and 5% of sodium iron pyrophosphate Na2FeP2O7 impurities, and the carbon content being about 5.1 wt%;

[0148] Comparative Example 10

[0149] Preparation of sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material J

[0150] The sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 material was prepared by the method of Comparative Example 1, the process and conditions being the same as in Comparative Example 1, except that

[0151] Step 3) the thickness of the bottom support layer was 0.6 cm;

[0152] Step 5) the temperature when the gas flowed into the reaction bed was the inner wall surface temperature of the tube furnace during heat treatment minus 15°C to the inner wall surface temperature of the tube furnace during heat treatment;

[0153] Step 6) the sample after heat treatment was a fluffy powder, and the obtained material was recorded as sample J#, the parameters being shown in Table 2;

[0154] Step 7) XRD test was performed on the prepared sample I#, from which it can be seen from the XRD that the synthesized material is mainly sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material, the purity is only 85%, the conversion rate of iron source iron phosphate FePO4, ferrous oxalate dihydrate FeC2O4.2H2O in the raw material to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 iron atom is 84%, there are also 9% of sodium iron phosphate NaFePO4 and 8% of sodium iron pyrophosphate Na2FeP2O7 impurities, and the carbon content is about 3.9wt%;

[0155] Comparative Test Example 1

[0156] The sample A# prepared in Comparative Example 1 was used as a positive active material for a sodium ion battery, mixed with a conductive agent acetylene black, a binder polyvinylidene fluoride at a mass ratio of 8:1:1, and an appropriate amount of a solvent N-methyl pyrrolidone was added and mixed uniformly to form a paste, which was coated on an aluminum current collector, dried, and cut into a circular sheet with a diameter of 14 mm, and the active material area density was 1.1-3.2 mg / cm 2 (here 2.1 mg / cm 2 ), which was used as a positive electrode of the battery. The negative electrode was selected as a metal sodium sheet, the electrolyte was selected as 1M NaClO4 / EC / DEC (V / V of EC / DEC = 1:1), and the separator was selected as a glass fiber membrane. The assembled battery was tested for charge and discharge, the voltage range was 1.5-4.0V, the discharge specific capacity under the conditions of 0.2C / 1.0C / 5.0C / 10C / 50C was tested, and the discharge specific capacity under the conditions of 0.2C / 50C was recorded in Table 2. At the same time, the cycle retention rate of the battery after 1000 cycles at a rate of 1.0C and a voltage range of 1.5-4.0V was also recorded in Table 2.

[0157] Test Examples 2-10

[0158] The samples B#-J# were tested according to the test method of Comparative Test Example 1, and the discharge specific capacity under the conditions of 0.2C / 50C and the cycle retention rate of the battery after 1000 cycles at a rate of 1.0C and a voltage range of 1.5-4.0V were recorded in Table 2.

[0159] Table 1 Discharge specific capacity test data of example materials

[0160]

[0161] Table 2 Discharge specific capacity test data of comparative materials

[0162]

[0163] The present application is to utilize the difference in thermal effect between different precursors to precisely control the heat treatment process of product phase formation, and to effectively reduce the influence of heat accumulation during precursor phase formation on the uniformity of the overall heat treatment process by designing a specific bed-type reaction vessel. Even when the mass of the precursor increases, high-purity, high-specific capacity, high-rate performance, and high-cycle stability phosphate and fluorophosphate polyanionic materials can still be prepared, greatly promoting the development and application of such materials in the energy storage field (e.g., in Example 1: 125 mAh / g @ 0.2 C; 90 mAh / g @ 50 C; CR = 90% @ 1.0 C, 1000 times, material purity up to 99.99%).

[0164] In the precise control of the reaction bed layer structure of the heat treatment process, the thickness of the bottom support layer (0.1 cm to 1 cm) and the thickness of the top screen layer and the surrounding screen barrier layer (0.1 cm to 0.5 cm) will affect the heat transfer process, and in turn affect the uniformity of the above heat treatment process. The thickness of the precursor material clamping area in the middle of the bed layer (1 cm to 4 cm) will also affect the above heat treatment process, and in turn greatly affect the purity and electrochemical performance of the product (e.g., in Examples 1-7: 113-125 mAh / g @ 0.2 C; 80-90 mAh / g @ 50 C; CR = 80%-90% @ 1.0 C, 1000 times, material purity only 99%-99.99%). However, when the thickness of the bottom support layer (>1 cm), the thickness of the top screen layer and the surrounding screen barrier layer (>0.5 cm), and the thickness of the precursor material clamping area in the middle of the bed layer (>4 cm) are too large, the heat in the reaction bed layer cannot be effectively transferred in the high-temperature tube furnace, resulting in uneven heat and impurities of sodium iron phosphate NaFePO4 and sodium iron pyrophosphate Na2FeP2O7, which seriously affect the electrochemical performance of the material. At the same time, when the thickness of the bottom support layer (<0.1 cm), the thickness of the top screen layer and the surrounding screen barrier layer (<0.1 cm), and the thickness of the precursor material clamping area in the middle of the bed layer (<1 cm) are too small, the precursor material is blown away by the gas flow, causing local stoichiometric imbalance. All the above problems result in different proportions of sodium iron phosphate NaFePO4 and sodium iron pyrophosphate Na2FeP2O7 impurities in the product (e.g., in Comparative Examples 1-6: 93-103 mAh / g @ 0.2 C; 62-70 mAh / g @ 50 C; CR = 68%-75% @ 1.0 C, 1000 times, material purity only 83%-96%).

[0165] In other optimized conditions, the shortest distance between any point on the upper, lower, and peripheral parts of the reaction bed and the inner wall surface of the tube furnace also affects the heat dissipation process and sintering uniformity. When the shortest distance between any point on the bottom support layer, top screen layer, and peripheral screen barrier layer of the reaction bed and any point on the inner wall surface of the tube furnace is > 0.1 cm, and the shortest distance between any point on the peripheral screen barrier layer and any point on the inner wall surface of the flanges at both ends of the reaction tube is > 1 cm (as in Examples 8-9: 114-116 mAh / g @ 0.2 C; 83-84 mAh / g @ 50 C; CR = 81%-83% @ 1.0 C, 1000 times, material purity 99.1%-99.2%), while when the shortest distance between any point on the bottom support layer and top screen layer of the reaction bed and any point on the inner wall surface of the tube furnace is < 0.1 cm, and the shortest distance between the peripheral screen barrier layer of the reaction bed and any point on the inner wall surface of the tube furnace is < 1 cm, the material cannot exhibit excellent electrochemical performance due to the influence of sintering process uniformity (as in Comparative Examples 7-8: 90-96 mAh / g @ 0.2 C; 55-67 mAh / g @ 50 C; CR = 60%-63% @ 1.0 C, 1000 times, material purity only 82%-85%).

[0166] In other optimized conditions, the gas conditions in the high-temperature tube furnace also affect the heat dissipation process of the above-mentioned bed-type reactor, thereby affecting the product performance. The gas flow conditions should ensure that the temperature difference between the gas flowing out of the reaction bed and the gas flowing into the reaction bed is less than or equal to 20°C, and the temperature of the gas flowing into the reaction bed is -10°C to the inner wall surface temperature of the tube furnace during heat treatment, which will not affect the uniformity of the above heat treatment process (as in Examples 10-11: 112-115 mAh / g @ 0.2 C; 83-85 mAh / g @ 50 C; CR = 82%-83% @ 1.0 C, 1000 times, material purity 99.1%-99.3%). When the temperature difference between the gas flowing into and out of the reaction bed is > 20°C, and the difference between the inner wall surface temperature of the tube furnace during heat treatment and the temperature of the gas flowing into the reaction bed is > 10°C, the introduction of gas may affect the uniformity and sustainability of the temperature zone during the heat treatment process, thereby affecting the material performance (as in Comparative Examples 9-10: 96-98 mAh / g @ 0.2 C; 69-70 mAh / g @ 50 C; CR = 63%-64% @ 1.0 C, 1000 times, material purity only 85%-86%).

Claims

1. A method for preparing iron-based phosphate or fluorophosphate polyanionic compounds, characterized in that, The iron-based phosphate polyanionic compound has one or more of the structures shown in 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 may have the structure shown in formula V and / or VI: Na2FePO4F, Formula V; Na5Fe2(PO4)2F3 Formula VI; The preparation methods of the polyanionic compounds shown in Formula I, Formula II, Formula III, Formula IV, V or VI include: Step 1: Mix carbon source, sodium source, iron source and phosphorus source uniformly in solvent to obtain precursor solution, or mix carbon source, sodium source, iron source, phosphorus source and fluorine source uniformly in solvent to obtain precursor solution; after drying the precursor solution, a precursor powder with a particle size of 20μm-30μm is obtained. Step 2: Prepare the precursor powder into a reaction bed; The reaction bed is a flat-plate bed structure, including a bottom support layer and a top screen layer. An intermediate precursor material holding area is formed between the bottom support layer and the top screen layer. An annular screen blocking layer is provided around the intermediate precursor material holding area, with its upper and lower ends fixed to the edges of the top screen layer and the bottom support layer, respectively. Precursor powder is filled into the intermediate precursor material holding area. The bottom support layer is a flat plate, a sieve plate, or a sieve, with a thickness of 0.1cm to 1cm. The top screen layer and the surrounding screen blocking layers are both sieves, with a thickness of 0.1cm to 0.5cm. The sieve plate or sieve mesh size is 800 to 2000. The thickness of the intermediate precursor material holding area, i.e., the distance between the bottom support layer and the top screen layer, is 1cm to 4cm. Step 3: Place the reaction bed inside the reaction tube, with the bottom support layer parallel to the axis of the reaction tube. The reaction tube is positioned at both ends of the center of the reactor. The two open ends of the reaction tube are connected to the inlet and outlet gas lines of the reactor via flanges. The gas inlet direction is parallel to the axis of the reaction tube. The shortest distance from any point on the bottom support layer, top screen layer, or surrounding screen barrier layer of the reaction bed to any point on the inner wall surface of the reaction tube is >0.1cm; the shortest distance from any point on the surrounding screen barrier layer of the reaction bed to any point on the inner wall surface of the flanges at both ends of the reaction tube is >1cm. Step 4: Heat-treat the precursor powder in a high-temperature tubular furnace under a specific flowing atmosphere. The gas flow conditions under the specific atmosphere should ensure that the temperature difference between the gas exiting the reaction bed and the gas entering the reaction bed is less than or equal to 20°C. The temperature of the gas entering the reaction bed is from the inner wall temperature of the tubular furnace during heat treatment to -10°C to the inner wall temperature of the tubular furnace during heat treatment. The gas flow direction is parallel to the upper surface of the reaction bed and points from the inlet side to the outlet side. The temperature in the heat treatment process refers to the temperature at the inner wall of the high-temperature atmosphere tube furnace during heat treatment; 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 ≤ 600 °C, the treatment time is 3 - 20 h; the difference between T2 and T1 is greater than or equal to 50 °C.

2. The preparation method according to claim 1, wherein In step 1, the method of mixing the sodium source, iron source, phosphorus source, and carbon source evenly in the solvent can adopt at least one or two or more of ball milling, pot milling, sand milling, etc.; the solvent is water, ethanol, or a mixed solvent of water and ethanol, and the solid content of the precursor solution is 30wt% - 75wt%: For the sand milling, a sand milling medium needs to be added. In step 1, the sand milling speed is 2000 - 5000 r / min, and the sand milling time is 0.5 - 5 h; The sand milling medium is one or more of natural sand beads, glass beads, steel beads, zirconia beads, zirconium silicate beads, agate beads; the ball-to-material ratio is 1 - 5; the diameter of the sand milling medium is 2 - 12 cm; For the ball milling, a ball milling medium needs to be added. In step 1, the rotation speed of the ball milling is 200 - 800 r / min, and the ball milling treatment time is 3 - 15 h; The ball milling medium is one or more of zirconia beads, agate beads, zirconium silicate beads; the ball-to-material ratio is 1.5 - 7; the diameter of the ball milling medium is 2 - 12 cm; For the pot milling, a pot milling medium needs to be added. In step 1, the rotation speed of the pot milling is 20 - 40 r / min, and the pot milling treatment time is 20 h - 72 h; The pot milling medium is one or more of zirconia beads, agate beads, zirconium silicate beads; the ball-to-material ratio is 1.1 - 3; the diameter of the pot milling medium is 4 - 12 cm; The solvent is water, ethanol, or a mixed solvent of water and ethanol, wherein the mass proportion of water in the total mass of the mixed solvent is (30 - 80)wt%, and the mass proportion of ethanol in the total mass of the mixed solvent is (20 - 70)wt%.

3. The preparation method according to claim 1, wherein In step 1, the drying method is spray drying. Control the feed flow rate to be 20 mL / min - 60 mL / min, control the inlet air temperature to be 120 °C - 195 °C, the outlet air temperature to be 80 °C - 110 °C, and the compressed air pressure to be 0.03 KPa - 0.9 KPa. Pump the precursor solution into the atomizer of the spray dryer through a peristaltic pump, and collect the powder.

4. The preparation method according to claim 1, wherein In step 1, the materials of the bottom support layer, top screen layer, and surrounding screen barrier layer are any one or two or more of alumina, silica, crystalline natural graphite, porous fused alumina, high-purity quartz sand.

5. The preparation method according to claim 1, wherein In step 3, there are more than 2 reaction bed layers, which are placed parallel and spaced from top to bottom in the tube furnace. The upper surface of the reaction bed layer is placed parallel to the horizontal plane, and the distance between adjacent reaction bed layers from top to bottom > 0.5 cm; The tubular furnace described in step 3 is heated by electric heating elements disposed on the outer wall of the tubular furnace and / or disposed inside the wall. The electric heating elements are one or more of the following: electric heating wire, electric heating rod, etc.

6. The preparation method according to claim 1, characterized in that, The specific atmosphere mentioned in step 4 is selected from at least two or more of the following inert atmospheres: argon, helium, and nitrogen, or an inert atmosphere gas containing an H2 volume concentration greater than 0 and less than 50%.

7. The preparation method according to claim 1, characterized in that: The sodium source is selected from at least one or more of sodium hydroxide, sodium carbonate, sodium bicarbonate, sodium acetate, sodium citrate, sodium oxalate, sodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium pyrophosphate, and sodium hydrogen pyrophosphate. The iron source is selected from one or more of the following: iron powder, iron(II) oxide, ferric oxide, ferrous oxide, ferric oxalate, ferrous oxalate (dihydrate), ferric phosphate, ferric pyrophosphate, ferrous citrate, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, ferric chloride, ferrous chloride, ferric acetate, ferrous ammonium sulfate, ferric citrate, ferric ammonium citrate, and sodium ferric citrate succinate. The phosphorus source is selected from at least one or more of the following: diammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, pyrophosphate, sodium pyrophosphate, and sodium hydrogen pyrophosphate. The fluorine source is selected from at least one or more of sodium fluoride, ammonium fluoride, polyvinylidene fluoride, perfluorinated polyvinylidene fluoride, and hydrofluoric acid. The carbon source is selected from at least one or more of starch, carboxymethyl starch, sodium carboxymethyl starch, sodium alginate, citric acid, oxalic acid, ammonium citrate, ascorbic acid, formaldehyde, acetaldehyde, lactic acid, malic acid, glucose, sucrose, maltose, and maltodextrin.

8. The preparation method according to claim 1 or 7, characterized in that, The amount of carbon source added should ensure that the mass content of carbon in the phosphate polyanionic compound is 4wt%~5wt%; The phosphate polyanionic compound has the structure shown in Formula I. In the precursor solution, the molar ratio of sodium source, iron source and phosphorus source is 4:3:4, and the molar ratio of sodium source, iron source and phosphorus source is expressed in terms of the molar amounts of sodium, iron and phosphorus elements, respectively. The phosphate polyanionic compound has the structure shown in Formula II. In the precursor solution, the molar ratio of sodium source, iron source and phosphorus source is 3:2:3, and the molar ratio of sodium source, iron source and phosphorus source is expressed in terms of the molar amounts of sodium, iron and phosphorus elements, respectively. The polyanionic compound has the structure shown in Formula III. In the precursor solution, the molar ratio of sodium source, iron source and phosphorus source is 1:1:1, and the molar ratio of sodium source, iron source and phosphorus source is expressed in terms of the molar amounts of sodium, iron and phosphorus elements, respectively. The polyanionic compound has the structure shown in Formula IV. In the precursor solution, the molar ratio of sodium source, iron source and phosphorus source is 2:1:2, and the molar ratio of sodium source, iron source and phosphorus source is expressed in terms of the molar amounts of sodium, iron and phosphorus elements, respectively. The polyanionic compound has the structure shown in Formula V. In the precursor solution, the molar ratio of sodium source, iron source, phosphorus source and fluorine source is 2:1:1:

1. The molar ratio of sodium source, iron source, phosphorus source and fluorine source is expressed in terms of the molar amounts of sodium, iron, phosphorus and fluorine elements, respectively. The polyanionic compound has the structure shown in VI. In the precursor solution, the molar ratio of sodium source, iron source, phosphorus source and fluorine source is 5:2:2:

3. The molar ratio of sodium source, iron source, phosphorus source and fluorine source is expressed in terms of the molar amounts of sodium, iron, phosphorus and fluorine elements, respectively.

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