Iron-based polyanionic compounds, preparation and use thereof

Abstract

The application relates to 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; 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 uniform in granularity, complete in carbon coating, high in purity and good in electrochemical performance, especially in rate performance.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion batteries, and relates to sodium-ion battery electrode materials, particularly to the preparation method of phosphate or fluorophosphate polyanionic compounds and their application in sodium-ion battery electrode materials. Background Technology

[0002] In recent years, with the introduction of the "dual-carbon" policy and the goal of building a new power system based on new energy sources, the energy storage industry has maintained a high level of attention as a key factor in regulating the volatility and indirectness of new energy sources and achieving grid connection 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 the high price of lithium-ion battery cathode materials. Sodium-ion batteries, on the other hand, have attracted increasing attention because they have a similar working principle to lithium-ion batteries, and sodium sources are inexpensive and abundant. Therefore, developing 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 pyrophosphate are preferred cathode materials for alkali metal ion batteries due to their advantages such as structural stability and high safety. In the production process of phosphate and fluorophosphate polyanionic materials, the precursors are mixed by ball milling or sand milling, then dried and calcined at high temperature to form a phase. However, the presence of insoluble substances in the precursors leads to obvious stratification of the slurry, resulting in poor mixing of raw materials and failure to achieve uniform and sufficient contact between the raw materials, leading to poor uniformity of the dried precursors. At the same time, phosphate and fluorophosphate polyanionic materials have poor conductivity. Therefore, a carbon source needs to be added during the solid-state high-temperature sintering process. After the carbon source decomposes, a carbon layer is formed on the surface of the material to improve its conductivity. However, some carbon sources, such as sucrose and glucose, are small molecule carbon sources that undergo polymerization or cracking and repolymerization reactions during high-temperature processes, causing problems such as material caking, severe particle agglomeration, and uneven carbon coating after high-temperature calcination. The existence of the above problems results in low purity and poor rate performance of the prepared polyanionic compounds. Summary of the Invention

[0004] To address the aforementioned issues, this invention provides a preparation method that uses starch and its modified forms (carboxymethyl starch, sodium carboxymethyl starch) as both a carbon source and a reducing agent. Nano-starch (nano-carboxymethyl starch, sodium carboxymethyl starch) with a size of 100nm-500nm is obtained through sand milling or ball milling. This nano-starch exhibits a highly stable suspension in aqueous solution and exposes a large specific surface area, allowing finely granulated insoluble raw materials to be adsorbed onto its surface. This results in the insoluble raw materials being suspended (homogeneously dispersed) in the slurry and uniformly coated by other water-soluble materials, improving the uniformity and dispersibility of the raw material mixture. Simultaneously, starch reduces material agglomeration during heat treatment, decreasing the formation of particle clusters and improving the uniformity and integrity of the carbon coating. These multiple effects result in uniform phosphate and fluorophosphate polyanionic compounds with complete carbon coating, high purity, and excellent electrochemical performance, especially rate performance.

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

[0006] The iron-based phosphate polyanionic compound has one or more of the structures shown in 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 may have the structure shown in formula V and / or VI:

[0012] Na₂FePO₄ F Formula V;

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

[0014] The preparation methods of the polyanionic compounds shown in Formula I, Formula II, Formula III, Formula IV, V or VI include:

[0015] Step 1: Disperse the insoluble iron source in water, and transfer the above dispersion into a sand mill and / or a ball mill. After sand milling and / or ball milling, an insoluble iron source dispersion a with a particle size of 1-2 μm is obtained; the ratio of the mass of the insoluble iron source to the mass of the dispersion a (total mass of insoluble iron source and deionized water) in the insoluble iron source dispersion a is 30wt% to 50wt%.

[0016] The insoluble iron source is selected from one or more of the following: iron powder, ferric oxide, ferric oxide, ferrous oxide, ferric oxalate, ferrous oxalate, ferric phosphate, ferric pyrophosphate, ferrous citrate, etc.

[0017] Step 2: Disperse the polymer compound in the above-mentioned insoluble iron source dispersion a, and continue sand milling and / or ball milling to obtain a dispersion b containing polymer compound with a particle size of 100nm-500nm and insoluble iron source with a particle size of 1-2μm; the mass ratio x of polymer compound to insoluble iron source in dispersion b is 0.65≤x≤1.05;

[0018] The polymeric compound is selected from starch and / or modified starch; preferably, the polymeric compound is selected from at least one or more of starch, carboxymethyl starch, and sodium carboxymethyl starch; the polymeric compound also serves as a carbon source and a reducing agent.

[0019] Step 3: Weigh out sodium source and phosphorus source, or sodium source, phosphorus source and soluble iron source, or sodium source, phosphorus source and fluorine source, or sodium source, phosphorus source, fluorine source and soluble iron source according to the stoichiometric ratio of the polyanionic compound to be prepared; add them to the above dispersion b, add or not add water, and then mix evenly to obtain slurry c;

[0020] The solid content (the ratio of the total mass of compounds other than water in slurry c to the mass of slurry c) is 45 wt% to 55 wt%.

[0021] The iron source in the slurry c includes an insoluble iron source, or an insoluble iron source and a soluble iron source; the ratio of the amount of iron ions in the insoluble iron source to the amount of total iron ions in the slurry c is y, 50%. <y≤100%;

[0022] The insoluble iron source and the polymer compound are both derived from dispersion b;

[0023] The soluble iron source is selected from one or more of the following: ferrous nitrate, ferrous nitrate, ferrous sulfate, ferrous sulfate, ferrous chloride, ferrous chloride, ferrous acetate, ferrous ammonium sulfate, ferric citrate, ferric ammonium citrate, sodium ferric citrate succinate, etc.

[0024] Step 4: Transfer the above slurry c into a spray dryer for spray granulation, and obtain precursor powder d after drying;

[0025] Step 5: Heat-treat the above precursor powder d in a specific atmosphere to obtain the polyanionic compound.

[0026] In step (1) or (2):

[0027] A sanding medium needs to be added for the sanding. In step (1), the sanding rotation speed is 3000 to 5000 r / min, and the sanding time is 0.5 to 5 h; in step (2), the sanding rotation speed is 1000 to 3000 r / min, and the sanding time is 0.5 to 5 h

[0028] The sanding medium is one or more selected from the group consisting of natural sand beads, glass beads, steel beads, zirconia beads, zirconium silicate beads and agate beads; the ball-to-material ratio (mass ratio of balls to solid materials) is (1 to 5); the diameter of the sanding medium is 2 to 12 cm, and is preferably a combination of beads with diameters of 10 cm, 8 cm and 4 cm in a mass ratio of 2:1:2;

[0029] A ball milling medium needs to be added for the ball milling. In step (1), the ball milling rotation speed is 600 to 800 r / min, and the ball milling treatment time is 3 to 8 h; in step (2), the ball milling rotation speed is 200 to 400 r / min, and the ball milling treatment time is 2 to 6 h;

[0030] The ball milling medium is one or more selected from the group consisting of zirconia beads, agate beads and zirconium silicate beads; the ball-to-material ratio (mass ratio of balls to solid materials) is (1.5 to 7); the diameter of the ball milling medium is 2 to 12 cm, and is preferably a combination of beads with diameters of 10 cm, 8 cm and 4 cm in a mass ratio of 1:2:2;

[0031] In step (3): mechanical stirring can be selected as the mixing mode, the rotation speed is 200-500 r / min, and the stirring time is 10 min-30 min.

[0032] The spray drying conditions are as follows: a feed flow rate of 1 ml / min to 70 ml / min, an inlet air temperature of 90°C to 210°C, an outlet air temperature of 50°C to 150°C, and a compressed air pressure of 0.01 KPa to 1 KPa; the slurry is pumped into an atomizer of a spray dryer by a peristaltic pump, and the powder is collected;

[0033] Preferably, the feed flow rate is controlled to 20 ml / min to 60 ml / min, the inlet air temperature is controlled to 120°C to 195°C, the outlet air temperature is 80°C to 110°C, and the compressed air pressure is 0.03 KPa to 0.9 KPa.

[0034] The specific atmosphere is selected from at least one of inert atmospheres including argon, helium and nitrogen, or a reducing inert atmosphere containing H2; the heat treatment comprises a sequential treatment process at a first temperature T1 and a treatment process at a second temperature T2; the temperature T1 satisfies 200 ≤ T1 ≤ 400°C, and the treatment time is 0.5 to 6 h; the temperature T2 satisfies 400°C < T2 ≤ 650°C, and the treatment time is 3 to 20 h; the difference between T2 and T1 is greater than or equal to 50°C.

[0035] Preferably, the phosphate polyanionic compound has the structure shown in Formula I, and in slurry c, 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 element, iron element and phosphorus element, respectively.

[0036] Preferably, the phosphate polyanionic compound has the structure shown in Formula II, and in slurry c, 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 element, iron element and phosphorus element, respectively.

[0037] Preferably, the polyanionic compound has the structure shown in Formula III, and in slurry c, 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 element, iron element and phosphorus element, respectively.

[0038] Preferably, the polyanionic compound has the structure shown in Formula IV, and in slurry c, 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 element, iron element and phosphorus element, respectively.

[0039] Preferably, the polyanionic compound has the structure shown in Formula V, and in slurry c, the molar ratio of sodium source, iron source, phosphorus source and fluorine source is 2:1:1:1, and 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.

[0040] Preferably, the polyanionic compound has the structure shown in VI, and in slurry c, the molar ratio of sodium source, iron source, phosphorus source and fluorine source is 5:2:2:3, and 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.

[0041] 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.

[0042] 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.

[0043] The fluorine source is selected from at least one or more of sodium fluoride, ammonium fluoride, polyvinylidene fluoride, perfluorinated polyvinyl fluoride, and hydrofluoric acid.

[0044] The preparation method yields iron-based phosphate or fluorophosphate polyanionic compounds.

[0045] The iron-based phosphate or fluorophosphate polyanionic compounds have a particle size of 1.5 μm-6 μm and a carbon content of 1%-10%.

[0046] The iron-based phosphate or fluorophosphate polyanionic compounds are used as active materials in sodium-ion battery cathode materials.

[0047] In the cathode material, the content of the iron-based phosphate or fluorophosphate polyanionic compound is (60-98) wt%.

[0048] The positive electrode material also 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%.

[0049] The conductive agent is at least one or more of carbon black, conductive graphite, carbon fiber, carbon nanotubes, and graphene.

[0050] The adhesive is at least one or more of polyvinylidene fluoride: PVDF5130, HSV900, and kynar761A.

[0051] Beneficial effects

[0052] To address the aforementioned issues, this invention provides a preparation method that uses starch and its modified forms (carboxymethyl starch, sodium carboxymethyl starch) as both a carbon source and a reducing agent. Nano-starch (nano-carboxymethyl starch, sodium carboxymethyl starch) with a size of 100nm-500nm is obtained through sand milling or ball milling. This nano-starch exhibits a highly stable suspension in aqueous solution and exposes a large specific surface area, allowing finely granulated insoluble raw materials to be adsorbed onto its surface. This results in the insoluble raw materials being suspended (homogeneously dispersed) in the slurry and uniformly coated by other water-soluble materials, improving the uniformity and dispersibility of the raw material mixture. Simultaneously, starch reduces material agglomeration during heat treatment, decreasing the formation of particle clusters and improving the uniformity and integrity of the carbon coating. These multiple effects result in uniform phosphate and fluorophosphate polyanionic compounds with complete carbon coating, high purity, and excellent electrochemical performance, especially rate performance. Attached Figure Description

[0053] Figure 1 This is the XRD diffraction pattern of material 1# in Example 1.

[0054] Figure 2 This is an SEM image of material 1# in Example 1.

[0055] Figure 3This is a TEM image of material 1# in Example 1.

[0056] Figure 4 This is the TG curve of material 1# in Example 1. Detailed Implementation

[0057] Example 1 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material #1

[0058] Step 1) Disperse 30.16g of insoluble iron source ferric phosphate (FePO4) in 45.24g of deionized water, and transfer the dispersion to a 150mL agate ball mill jar. Add 18g of agate balls with a diameter of 10cm, 36g of agate balls with a diameter of 8cm, and 36g of agate balls with a diameter of 4cm according to a ball-to-material ratio (mass ratio) of approximately 3:1. Transfer the ball mill jar to a planetary ball mill for ball milling. Set the ball milling speed to 700r / min and the time to 4h to obtain an insoluble iron source dispersion a with a particle size of 1.5-1.6μm. The ratio i1 of the mass of insoluble iron source to the total mass of insoluble iron source and deionized water in insoluble iron source dispersion a is 40wt%.

[0059] Step 2) Add 26.39g of starch to the agate ball mill jar containing the insoluble iron source dispersion a, and transfer the ball mill jar to a planetary ball mill for ball milling. Set the ball milling speed to 300r / min and the time to 4h to obtain a polymer compound containing a particle size of 300-350nm and an insoluble iron source dispersion b. The mass ratio x of the polymer compound to the insoluble iron source in dispersion b is 0.875. The insoluble iron source particles are uniformly attached to the polymer compound particles.

[0060] Step 3) Weigh 28.392g of sodium source / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b and mix evenly to obtain slurry c. Then add 80.102g of deionized water to adjust the solid content of slurry c to 50wt% (the ratio of the total mass of compounds other than water in slurry c to the mass of slurry c). The ratio of the amount of iron ions in the insoluble iron source to the amount of total iron ions in the iron source is y = 66.7%. Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 40h (no stratification phenomenon occurred).

[0061] Step 4) The slurry c is fed into a spray drying device for spray drying (inlet temperature 170℃, outlet temperature 100℃, feed flow rate 50ml / min, compressed air pressure 0.8KPa) to obtain precursor powder d;

[0062] Step 5) The precursor powder d is then transferred into a high-temperature tube under argon atmosphere for heat treatment h1. 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 550℃, the heat treatment time is 8h, and the heating rate is 2℃ / min. The heat-treated sample is a fluffy powder, and the obtained material is called sample 1#. The parameters are shown in Table 1.

[0063] Step 6) The prepared sample 1# was characterized by XRD, SEM, TEM and its carbon content (TG) was tested. The XRD values ​​were obtained from the XRD... Figure 1 It can be seen that the synthesized material is pure-phase sodium iron pyrophosphate Na4Fe3(PO4)2P2O7, with a purity of up to 99.99%. The conversion rate of iron atoms from the insoluble iron source ferric phosphate FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 is 99.99%. From SEM (… Figure 2 It can be seen that the particle size is uniform, with a particle size of 2μm-3μm, as shown by TEM. Figure 3 It can be seen from TG that the outer surface of the material is coated with carbon. Figure 4 The data shows that the carbon content of the material is 5 wt%.

[0064] Example 2 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material #2

[0065] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0066] Step 1) Add 30.16g of insoluble iron source ferric phosphate FePO4 to 70.37g of deionized water and disperse. Set the ball mill speed to 700r / min and the time to 3h to obtain an insoluble iron source dispersion a with a particle size of 1.5-1.7μm. The ratio of the mass of insoluble iron source to the total mass of insoluble iron source and deionized water in insoluble iron source dispersion a is 30wt%.

[0067] Step 2) Obtain a dispersion b containing a polymer compound with a particle size of 300-350 nm and an insoluble iron source; the insoluble iron source particles are uniformly attached to the polymer compound particles;

[0068] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b, and mix well to obtain slurry c. Then add 54.972g of deionized water to adjust the solid content of slurry c to 50wt% (the mass ratio of the total mass of compounds other than water in slurry c to the mass of slurry c). Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 35h (no stratification phenomenon occurred).

[0069] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 2#. The parameters are shown in Table 1.

[0070] Step 6) The prepared sample 2# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD analysis showed that the synthesized material was pure-phase sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) with a purity of 99.89%. The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 99.89%. SEM showed uniform particle size (2.5μm-3.5μm). TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 5wt%.

[0071] Example 3: Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material #3

[0072] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0073] Step 1) Add 30.16g of insoluble iron source ferric phosphate FePO4 to 30.16g of deionized water and disperse. Set the ball mill speed to 700r / min and the time to 5h to obtain an insoluble iron source dispersion a with a particle size of 1.4-1.5μm. The ratio of the mass of insoluble iron source to the total mass of insoluble iron source and deionized water in insoluble iron source dispersion a is 50wt%.

[0074] Step 2) Obtain a dispersion b containing a polymer compound with a particle size of 300-350 nm and an insoluble iron source; the insoluble iron source particles are uniformly attached to the polymer compound particles;

[0075] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b, and mix well to obtain slurry c. Then add 95.182g of deionized water to adjust the solid content of slurry c to 50wt% (the mass ratio of the total mass of compounds other than water in slurry c to the mass of slurry c). Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 30h (no stratification phenomenon occurred).

[0076] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 3#. The parameters are shown in Table 1.

[0077] Step 6) The prepared sample #3 was characterized by XRD, SEM, TEM, and carbon content (TG) analysis. XRD analysis showed that the synthesized material was pure-phase sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) with a purity of 99.80%. The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 99.80%. SEM analysis showed uniform particle size (3μm-4μm). TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 5wt%.

[0078] Example 4: Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material #4

[0079] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0080] Step 1) Set the ball milling speed to 800 r / min and the time to 8 h to obtain an insoluble iron source dispersion a with a particle size of 1-1.1 μm;

[0081] Step 2) Add 26.39g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1-1.1μm, and transfer the ball mill jar to a planetary ball mill for ball milling to obtain a polymer compound containing a particle size of 300-350nm and an insoluble iron source dispersion b; the insoluble iron source particles are uniformly attached to the polymer compound particles;

[0082] Step 3) Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 25 hours (no stratification occurred);

[0083] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 4#. The parameters are shown in Table 1.

[0084] Step 6) The prepared sample #4 was characterized by XRD, SEM, TEM, and carbon content (TG) analysis. XRD analysis showed that the synthesized material was pure-phase sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) with a purity of 99.85%. The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 99.85%. SEM analysis showed uniform particle size (1.5μm-2.5μm). TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 5wt%.

[0085] Example 5: Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material #5

[0086] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0087] Step 1) Set the ball milling speed to 600 r / min and the time to 3 h to obtain an insoluble iron source dispersion a with a particle size of 1.8-2 μm;

[0088] Step 2) Add 26.39g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1.8-2μm to obtain a polymer compound containing a particle size of 300-350nm and an insoluble iron source dispersion b; the insoluble iron source particles are uniformly attached to the polymer compound particles.

[0089] Step 3) Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 20 hours (no stratification occurred);

[0090] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 5#. The parameters are shown in Table 1.

[0091] Step 6) The prepared sample #5 was characterized by XRD, SEM, TEM, and carbon content (TG) analysis. XRD analysis showed that the synthesized material was pure-phase sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) with a purity of 99.83%. The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 99.83%. SEM analysis showed uniform particle size (3.5 μm-4.5 μm). TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 5 wt%.

[0092] Example 6 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 6#

[0093] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0094] Step 2) Set the ball milling speed to 400 r / min and the time to 6 h to obtain a dispersion b containing a polymer compound with a particle size of 100-160 nm and an insoluble iron source; the insoluble iron source particles are uniformly attached to the polymer compound particles.

[0095] Step 3) Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 30 hours (no stratification occurred);

[0096] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 6#, and the parameters are shown in Table 1.

[0097] Step 6) The prepared sample 6# was characterized by XRD, SEM, TEM, and carbon content (TG) analysis. XRD analysis showed that the synthesized material was pure-phase sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) with a purity of 99.79%. The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 99.79%. SEM analysis showed uniform particle size (1.5 μm-3 μm). TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 5 wt%.

[0098] Example 7 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material #7

[0099] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0100] Step 2) Set the ball milling speed to 200 r / min and the time to 2 h to obtain a dispersion b containing a polymer compound with a particle size of 450-500 nm and an insoluble iron source; the insoluble iron source particles are uniformly attached to the polymer compound particles.

[0101] Step 3) Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 25 hours (no stratification occurred);

[0102] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 7#. The parameters are shown in Table 1.

[0103] Step 6) The prepared sample 7# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD analysis showed that the synthesized material was pure-phase sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) with a purity of 99.05%. The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 99.05%. SEM analysis showed uniform particle size (2μm-5μm). TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 5wt%.

[0104] Example 8 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material #8

[0105] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0106] Step 2) Add 19.60g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1.5-1.6μm, set the ball milling speed to 300r / min, and the time to 3h; to obtain a polymer compound containing a particle size of 320-370nm and an insoluble iron source dispersion b; the mass ratio x of the polymer compound to the insoluble iron source in dispersion b is 0.65; the insoluble iron source particles are uniformly attached to the polymer compound particles;

[0107] Step 3) Weigh 28.392g of sodium source / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b and mix evenly to obtain slurry c. Then add 73.312g of deionized water to adjust the solid content of slurry c to 50wt% (the mass ratio of the total mass of compounds other than water in slurry c to the mass of slurry c). Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 20h (no stratification phenomenon occurred). The obtained material is recorded as sample 8#, and the parameters are shown in Table 1.

[0108] Step 6) The prepared sample #8 was characterized by XRD, SEM, TEM, and carbon content (TG) analysis. XRD analysis showed that the synthesized material was pure-phase sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) with a purity of 99.35%. The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 99.35%. SEM analysis showed uniform particle size (3μm-6μm). TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 1 wt%.

[0109] Example 9 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material #9

[0110] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0111] Step 2) Add 31.67g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1.5-1.6μm, set the ball milling speed to 300r / min, and the time to 5h; to obtain a polymer compound containing a particle size of 260-310nm and an insoluble iron source dispersion b; the mass ratio x of the polymer compound to the insoluble iron source in dispersion b is 1.05; the insoluble iron source particles are uniformly attached to the polymer compound particles;

[0112] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b and mix evenly to obtain slurry c. Then add 85.38g of deionized water to adjust the solid content of slurry c to 50wt%. Take out a portion of slurry c and let it stand. It can be observed that slurry c has no sedimentation (no stratification) after 25h.

[0113] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 9#, and the parameters are shown in Table 1.

[0114] Step 6) The prepared sample 9# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD analysis showed that the synthesized material was pure-phase sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) with a purity of 99.45%. The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 99.45%. SEM analysis showed uniform particle size (2μm-5μm). TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 7wt%.

[0115] Example 10 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 10#

[0116] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0117] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source - ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b, and mix well to obtain slurry c. Then add 107.96g of deionized water to adjust the solid content of slurry c to 45wt%. Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 38 hours (no stratification phenomenon occurred).

[0118] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 10#, and the parameters are shown in Table 1.

[0119] Step 6) The prepared sample 10# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD analysis showed that the synthesized material was pure-phase sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) with a purity of 99.55%. The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 99.55%. SEM showed uniform particle size (3μm-4μm). TEM analysis revealed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 5wt%.

[0120] Example 11 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 11#

[0121] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0122] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b and mix well to obtain slurry c. Then add 57.31g of deionized water to adjust the solid content of slurry c to 55wt%. Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 35h (no stratification phenomenon occurred).

[0123] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 11#, and the parameters are shown in Table 1.

[0124] Step 6) The prepared sample 11# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD analysis showed that the synthesized material was pure-phase sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) with a purity of 99.35%. The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 99.35%. SEM showed uniform particle size (2μm-5μm). TEM analysis revealed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 5wt%.

[0125] Example 12 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 12#

[0126] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0127] Step 1) Disperse 23.08g of insoluble iron source iron phosphate FePO4 in 34.6g of deionized water, transfer the above ball mill jar to a planetary ball mill for ball milling, set the ball milling speed to 700r / min, and the time to 4h;

[0128] Step 2) Add 20.17g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1.5-1.6μm, and transfer the ball mill jar to a planetary ball mill for ball milling. Set the ball milling speed to 300r / min and the time to 4h; to obtain a polymer compound containing a particle size of 300-350nm and an insoluble iron source dispersion b; the insoluble iron source particles are uniformly attached to the polymer compound particles;

[0129] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 59.388g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b, and mix well to obtain slurry c. Then add 96.43g of deionized water to adjust the solid content of slurry c to 50wt%. The ratio y of the amount of iron ions in the insoluble iron source to the amount of total iron ions in the iron source is 51%. Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle (no stratification) within 40h.

[0130] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 12#, and the parameters are shown in Table 1.

[0131] Step 6) The prepared sample 12# was characterized by XRD, SEM, TEM, and its carbon content (TG) was tested. XRD showed that the synthesized material was pure-phase sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a purity of 99.35%. The conversion rate of iron atoms from the insoluble iron source ferric phosphate FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 99.35%. SEM... Figure 2 It can be seen that the particle size is uniform, with a particle size of 1.5μm-2.5μm, as shown in the TEM ( ). Figure 3 The TG (chemical chromatography) data shows that the material's outer surface is coated with carbon, and the carbon content is 2 wt%.

[0132] Example 13 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 13#

[0133] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0134] Step 1) Disperse 45.24g of insoluble iron source ferric phosphate FePO4 in 67.87g of deionized water, transfer the above ball mill jar to a planetary ball mill for ball milling, set the ball milling speed to 700r / min, and the time to 4h;

[0135] Step 2) Add 39.59g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1.5-1.6μm, and transfer the ball mill jar to a planetary ball mill for ball milling. Set the ball milling speed to 300r / min and the time to 4h; to obtain a polymer compound containing a particle size of 300-350nm and an insoluble iron source dispersion b; the insoluble iron source particles are uniformly attached to the polymer compound particles;

[0136] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate (Na2HPO4) and add it to the above dispersion b. Mix well to obtain slurry c. Then add 45.322g of deionized water to adjust the solid content of slurry c to 50wt%. The ratio y of the amount of iron ions in the insoluble iron source to the amount of total iron ions in the iron source is 100%. Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle (no stratification occurred) within 35 hours.

[0137] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 13#, and the parameters are shown in Table 1.

[0138] Step 6) The prepared sample 13# was characterized by XRD, SEM, TEM, and its carbon content (TG) was tested. XRD analysis showed that the synthesized material was pure-phase sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a purity of 99.25%. The conversion rate of iron atoms from the insoluble iron source ferric phosphate FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 99.25%. SEM analysis showed... Figure 2 It can be seen that the particle size is uniform, with a particle size of 3μm-5μm, as shown by TEM ( Figure 3 The TG (chemical chromatography) data shows that the material's outer surface is coated with carbon, and the carbon content is 7.5 wt%.

[0139] Example 14 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 14#

[0140] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0141] Step 1) Disperse 53.967g of insoluble iron source ferrous oxalate dihydrate FeC2O4.H2O in 80.95g of deionized water;

[0142] Set the rotation speed to 700 r / min and the time to 3.5 h;

[0143] Step 2) Add 47.22g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1.5-1.6μm; to obtain a polymer compound containing a particle size of 320-350nm and an insoluble iron source dispersion b; the insoluble iron source particles are uniformly attached to the polymer compound particles;

[0144] Step 3) Weigh 28.392g of sodium source / phosphorus source - disodium hydrogen phosphate Na2HPO4 and add it to the above dispersion b. Mix well to obtain slurry c. Then add 48.63g of deionized water to adjust the solid content of slurry c to 50wt%. The ratio y of the amount of iron ions in the insoluble iron source to the amount of total iron ions in the iron source is 100%. Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 45h (no stratification phenomenon occurred).

[0145] Step 4) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 14#, and the parameters are shown in Table 1.

[0146] Step 6) The prepared sample 14# was characterized by XRD, SEM, TEM, and carbon content (TG) analysis. XRD analysis showed that the synthesized material was pure-phase sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) with a purity of 99.05%. The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 99.05%. SEM analysis showed uniform particle size, ranging from 1.5 μm to 3.5 μm. TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 10 wt%.

[0147] Example 15 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 15#

[0148] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0149] Step 2) Add 19.60g of carboxymethyl starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1.5-1.6μm, to obtain a polymer compound containing a particle size of 250-300nm and an insoluble iron source dispersion b; set the ball milling speed to 300r / min and the time to 3h; the mass ratio x of the polymer compound to the insoluble iron source in dispersion b is 0.65; the insoluble iron source particles are uniformly attached to the polymer compound particles;

[0150] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source - ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b, and mix well to obtain slurry c. Then add 73.312g of deionized water to adjust the solid content of slurry c to 50wt%. Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 15 hours (no stratification occurred).

[0151] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 15#, and the parameters are shown in Table 1.

[0152] Step 6) The prepared sample 15# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a purity of up to 99.55%. The conversion rate of iron atoms from the insoluble iron source iron phosphate FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3.9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 99.55%. SEM showed that the particle size was uniform, with a particle size of 4μm-6μm. TEM showed that the outer surface of the material was coated with carbon. TG showed that the carbon content of the material was 1.5wt%.

[0153] Example 16 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 16#

[0154] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0155] Step 2) Add 24.13g of carboxymethyl starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1.5-1.6μm. Set the rotation speed to 300r / min and the time to 3.5h to obtain a polymer compound containing a particle size of 260-300nm and an insoluble iron source dispersion b. The mass ratio x of the polymer compound to the insoluble iron source in dispersion b is 0.8. The insoluble iron source particles are uniformly attached to the polymer compound particles.

[0156] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source - ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b, and mix well to obtain slurry c. Then add 77.84g of deionized water to adjust the solid content of slurry c to 50wt%. Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 38 hours (no stratification phenomenon occurred).

[0157] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 16#, and the parameters are shown in Table 1.

[0158] Step 6) The prepared sample 16# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a purity of up to 99.93%. The conversion rate of iron atoms from the insoluble iron source iron phosphate FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3.9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 99.93%. SEM showed that the particle size was uniform, with a particle size of 1.5μm-2.5μm. TEM showed that there was carbon coating on the outer surface of the material. TG showed that the carbon content of the material was 4.5wt%.

[0159] Example 17 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 17#

[0160] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0161] Step 2) Add 31.67g of carboxymethyl starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1.5-1.6μm. Set the rotation speed to 300r / min and the time to 4h to obtain a polymer compound containing a particle size of 300-350nm and an insoluble iron source dispersion b. The mass ratio of the polymer compound to the insoluble iron source in dispersion b is x = 1.05. The insoluble iron source particles are uniformly attached to the polymer compound particles.

[0162] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source - ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b, and mix well to obtain slurry c. Then add 85.38g of deionized water to adjust the solid content of slurry c to 50wt%. Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 25h (no stratification phenomenon occurred).

[0163] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 17#, and the parameters are shown in Table 1.

[0164] Step 6) The prepared sample 17# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a purity of up to 99.87%. The conversion rate of iron atoms from the insoluble iron source iron phosphate FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3.9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 99.87%. SEM showed that the particle size was uniform, with a particle size of 2μm-4.5μm. TEM showed that there was carbon coating on the outer surface of the material. TG showed that the carbon content of the material was 6.5wt%.

[0165] Example 18 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 18#

[0166] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0167] Step 2) Add 19.60g of sodium carboxymethyl starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1.5-1.6μm. Set the rotation speed to 300r / min and the time to 2.5h to obtain a polymer compound containing a particle size of 350-400nm and an insoluble iron source dispersion b. The mass ratio of the polymer compound to the insoluble iron source in dispersion b is x = 0.65. The insoluble iron source particles are uniformly attached to the polymer compound particles.

[0168] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source - ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b, and mix well to obtain slurry c. Then add 73.312g of deionized water to adjust the solid content of slurry c to 50wt%. Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 15h (no stratification phenomenon occurred).

[0169] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 18#, and the parameters are shown in Table 1.

[0170] Step 6) The prepared sample 18# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a purity of up to 99.85%. The conversion rate of iron atoms from the insoluble iron source iron phosphate FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3.9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 99.85%. SEM showed that the particle size was uniform, with a particle size of 4.5μm-6μm. TEM showed that there was carbon coating on the outer surface of the material. TG showed that the carbon content of the material was 2.1wt%.

[0171] Example 19 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 19#

[0172] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0173] Step 2) Add 27.144g of sodium carboxymethyl starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1.5-1.6μm. Set the rotation speed to 300r / min and the time to 3h to obtain a polymer compound containing a particle size of 350-400nm and an insoluble iron source dispersion b. The mass ratio of the polymer compound to the insoluble iron source in dispersion b is 0.9. The insoluble iron source particles are uniformly attached to the polymer compound particles.

[0174] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source - ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b, and mix well to obtain slurry c. Then add 80.856g of deionized water to adjust the solid content of slurry c to 50wt%. Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 36 hours (no stratification phenomenon occurred).

[0175] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 16#, and the parameters are shown in Table 1.

[0176] Step 6) The prepared sample 19# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a purity of up to 99.92%. The conversion rate of iron atoms from the insoluble iron source iron phosphate FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3.9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 99.92%. SEM showed that the particle size was uniform, with a particle size of 2.5μm-3.5μm. TEM showed that there was carbon coating on the outer surface of the material. TG showed that the carbon content of the material was 5.5wt%.

[0177] Example 20 Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material 20#

[0178] Na4Fe3(PO4)2P2O7 material was prepared using the method described in Example 1, with the same process and conditions as in Example 1, except that:

[0179] Step 2) Add 31.67g of carboxymethyl starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a with a particle size of 1.5-1.6μm. Set the rotation speed to 300r / min and the time to 3.5h to obtain a polymer compound containing a particle size of 340-380nm and an insoluble iron source dispersion b. The mass ratio x of the polymer compound to the insoluble iron source in dispersion b is 1.05. The insoluble iron source particles are uniformly attached to the polymer compound particles.

[0180] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source - ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b, and mix well to obtain slurry c. Then add 85.382g of deionized water to adjust the solid content of slurry c to 50wt%. Take out a portion of slurry c and let it stand. It can be observed that slurry c did not settle within 20h (no stratification phenomenon occurred).

[0181] Step 5) The heat-treated sample is a fluffy powder. The obtained material is designated as sample 20#, and the parameters are shown in Table 1.

[0182] Step 6) The prepared sample 20# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material was pure phase sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a purity of up to 99.79%. The conversion rate of iron atoms from the insoluble iron source iron phosphate FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3.9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 99.79%. SEM showed that the particle size was uniform, with a particle size of 2μm-5μm. TEM showed that there was carbon coating on the outer surface of the material. TG showed that the carbon content of the material was 7.7wt%.

[0183] Test Example 1

[0184] Sample 1# prepared in Example 1 was used as the positive electrode active material for sodium-ion batteries. It was mixed uniformly with conductive agent acetylene black and binder polyvinylidene fluoride at a mass ratio of 8:1:1. An appropriate amount of solvent N-methylpyrrolidone was added and mixed thoroughly to form a paste. This paste was then applied to an aluminum current collector, dried, and cut into 14mm diameter discs. The areal density of the active material was 1.1–3.2 mg / cm³. 2 (Here it is 2.1 mg / cm) 2 This is used as the positive electrode of the battery. The negative electrode is a sodium metal sheet, the electrolyte is 1M NaClO4 / EC / DEC (EC / DEC V / V = 1:1), and the separator is a glass fiber membrane. The assembled battery was subjected to charge-discharge tests at a voltage range of 1.8-3.6V, and the discharge specific capacity was tested under conditions of 0.2C / 1.0C / 5.0C / 10C / 50C. The discharge specific capacity under 0.2C / 50C conditions is recorded in Table 1.

[0185] Test Example 2-20

[0186] Samples 2#-20# were tested according to the test method in Test Example 1, and the discharge specific capacity under 0.2C / 50C conditions was recorded in Table 1.

[0187] Comparative Example 1: Preparation of Sodium Iron Pyrophosphate Na4Fe3(PO4)2P2O7 Material A#

[0188] Step 1) Disperse 30.16g of insoluble iron source ferric phosphate (FePO4) in 45.24g of deionized water, and transfer the dispersion to a 150mL agate ball mill jar. Add agate balls of 10cm diameter, 36g of 8cm diameter, and 36g of 4cm diameter according to a ball-to-material ratio (mass ratio) of approximately 3:1. Transfer the mill jar to a planetary ball mill for ball milling at 700r / min for 4h to obtain an insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. The ratio of the mass of insoluble iron source to the total mass of insoluble iron source and deionized water in dispersion a1 is i. 11 It is 40 wt%;

[0189] Step 2) 26.39 g of soluble polyethylene glycol (weight average molecular weight: 300-400) was added to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a diameter of 1.5-1.6 μm to disperse it, thereby obtaining dispersion b1 containing soluble polyethylene glycol and insoluble iron source; the mass ratio x1 of soluble polyethylene glycol to insoluble iron source ferric phosphate in dispersion b1 was 0.875;

[0190] Step 3) Weigh 28.392g of sodium source / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b1 and mix evenly to obtain slurry c1. Then add 80.102g of deionized water to adjust the solid content of slurry c to 50wt%. The iron source in slurry c1 is insoluble iron source ferric phosphate and soluble iron source ferric nitrate nonahydrate. The ratio of the amount of iron ions in the insoluble iron source to the amount of total iron ions in the iron source is y1, which is 66.7wt%. Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 2 hours (obvious stratification occurs).

[0191] Step 4) The slurry c1 is fed into a spray drying device for spray drying (inlet temperature 170℃, outlet temperature 100℃, feed flow rate 50ml / min, compressed air pressure 0.8KPa) to obtain precursor powder d1;

[0192] Step 5) The precursor powder d1 was then transferred into a high-temperature tube under an argon atmosphere for heat treatment h1'. The heat treatment temperature T1' was 300℃, the heat treatment time was 3h, and the heating rate was 5℃ / min; the heat treatment temperature T2' was 550℃, the heat treatment time was 8h, and the heating rate was 2℃ / min. The heat-treated sample was a fluffy powder, and the obtained material was denoted as sample A#. The parameters are shown in Table 1.

[0193] Step 6) The prepared sample A# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD analysis showed the presence of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) phase with a purity of 93%, along with 7 wt% of other impurities, mainly sodium iron phosphate (NaFePO4) (3.5 wt%) and sodium phosphate (Na3PO4) (2.5 wt%). The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) in the raw material to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 93%. SEM analysis showed a particle size of 3 μm-4 μm. TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 5 wt%.

[0194] Comparative Example 2: Preparation of Sodium Iron Pyrophosphate Na4Fe3(PO4)2P2O7 Material B#

[0195] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0196] Step 2) Add 26.39g of citric acid to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a diameter of 1.5-1.6μm and disperse to obtain dispersion b1 containing citric acid and insoluble iron source; the mass ratio of citric acid to insoluble iron source ferric phosphate in dispersion b1 is x1, which is 0.875.

[0197] Step 3) Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 1.5 hours (obvious stratification occurs).

[0198] Step 5) After heat treatment, the sample showed agglomeration. The obtained material is denoted as sample B#, and the parameters are shown in Table 1.

[0199] Step 6) The prepared sample B# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD analysis showed the presence of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) phase with a purity of 92%, along with 8 wt% of other impurities, mainly sodium iron phosphate (NaFePO4) (4 wt%) and sodium phosphate (Na3PO4) (3 wt%). The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 92%. SEM showed a particle size of 15 μm-20 μm. TEM showed carbon coating on the outer surface of the material. TG analysis indicated a carbon content of 4.5 wt%.

[0200] Comparative Example 3: Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material C#

[0201] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0202] Step 2) Add 26.39g of lauric acid to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. Transfer the ball mill jar to a planetary ball mill for ball milling. Set the ball milling speed to 300r / min and the time to 2h to obtain lauric acid and insoluble iron source dispersion b1 with a particle size of 250-300nm. The mass ratio x of lauric acid to insoluble iron source in dispersion b1 is 0.875.

[0203] Step 3) Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 20 hours (obvious stratification occurs).

[0204] Step 5) After heat treatment, the sample showed agglomeration. The obtained material is denoted as sample C#, and the parameters are shown in Table 1.

[0205] Step 6) The prepared sample C# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD analysis showed the presence of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) phase with a purity of 98.5%, along with 1.5 wt% of other impurities (too low to be classified). The conversion rate of iron atoms from the insoluble iron source FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 98.5%. SEM showed a particle size of 10 μm-20 μm. TEM showed carbon coating on the outer surface of the material. TG analysis indicated a carbon content of 5 wt%. Comparative Example 4: Preparation of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) material D#

[0206] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0207] Step 1) Disperse 30.16g of insoluble iron source ferric phosphate (FePO4) in 120.64g of deionized water. Set the ball mill speed to 700r / min and the time to 2h. The ratio of the mass of insoluble iron source to the total mass of insoluble iron source and deionized water in the insoluble iron source dispersion a1 is i. 11 It is 20 wt%;

[0208] Step 2) Add 26.39g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm, and transfer the ball mill jar to a planetary ball mill for ball milling. Set the ball milling speed to 300r / min and the time to 2h to obtain starch containing a particle size of 300-350nm and insoluble iron source dispersion b1; the mass ratio x of starch to insoluble iron source in dispersion b1 is 0.875;

[0209] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source - ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b1 and mix well to obtain slurry c1. Then add 4.702g of deionized water to adjust the solid content of slurry c to 50wt% (the mass ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles after 10 hours (obvious stratification occurs).

[0210] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample D#, and the parameters are shown in Table 1.

[0211] Step 6) The prepared sample D# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD analysis showed the presence of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) phase with a purity of 94%, along with 6 wt% of other impurities, mainly sodium iron phosphate (NaFePO4) (3.5 wt%) and sodium phosphate (Na3PO4) (2.5 wt%). The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) in the raw materials to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 94%. SEM analysis showed a particle size of 6 μm-8 μm. TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 5 wt%.

[0212] Comparative Example 5: Preparation of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) material E#

[0213] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0214] Step 1) Disperse 30.16g of insoluble iron source ferric phosphate (FePO4) in 20.11g of deionized water. Set the ball milling speed to 700r / min and the time to 6h to obtain an insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. The ratio of the mass of insoluble iron source in dispersion a1 to the total mass of insoluble iron source and deionized water is i. 11 It is 60 wt%;

[0215] Step 2) Add 26.39g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm, and transfer the ball mill jar to a planetary ball mill for ball milling. Set the ball milling speed to 300r / min and the time to 5h to obtain starch insoluble iron source dispersion b1 with a particle size of 300-350nm; the mass ratio x of starch to insoluble iron source in dispersion b1 is 0.875;

[0216] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4 and 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, add them to the above dispersion b1 and mix well to obtain slurry c1. Then add 105.232g of deionized water to adjust the solid content of slurry c1 to 50wt% (the mass ratio of the total mass of compounds other than water in slurry c to the mass of slurry c). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 8 hours (obvious stratification occurs).

[0217] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample E#, and the parameters are shown in Table 1.

[0218] Step 6) The prepared sample E# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD analysis showed the presence of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) phase with a purity of 93%, along with 7 wt% of other impurities, mainly sodium iron phosphate (NaFePO4) (3.5 wt%) and sodium phosphate (Na3PO4) (2.5 wt%). The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 93%. SEM showed a particle size of 6.5 μm-7.5 μm. TEM showed carbon coating on the outer surface of the material. TG analysis indicated a carbon content of 5 wt%.

[0219] Comparative Example 6: Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material F#

[0220] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0221] Step 1) Set the ball milling speed to 800 r / min and the time to 9 h to obtain an insoluble iron source dispersion a1 with a particle size of 0.4-0.6 μm;

[0222] Step 2) Add 26.39g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 0.4-0.6μm. Transfer the ball mill jar to a planetary ball mill and set the ball milling speed to 300r / min for 4h to obtain starch containing a particle size of 300-350nm and insoluble iron source dispersion b1. The mass ratio x1 of starch to insoluble iron source ferric phosphate in dispersion b1 is 0.875.

[0223] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4, 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, and the above iron source dispersion b1. Then add 80.102g of deionized water to adjust the solid content of slurry c1 to 50wt% (the mass ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 12 hours (obvious stratification occurs).

[0224] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample F#, and the parameters are shown in Table 1.

[0225] Step 6) The prepared sample F# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD analysis showed the presence of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) phase with a purity of 93%, along with 7 wt% of other impurities, mainly sodium iron phosphate (NaFePO4) (3.5 wt%) and sodium phosphate (Na3PO4) (2.5 wt%). The conversion rate of iron atoms from the insoluble iron source (FePO4) and the soluble iron source (Fe(NO3)3·9H2O) in the raw material to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 93%. SEM analysis showed a particle size of 2 μm-7 μm. TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 5 wt%.

[0226] Comparative Example 7: Preparation of sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 material G#

[0227] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0228] Step 1) Set the ball milling speed to 500 r / min and the time to 3 h to obtain an insoluble iron source dispersion a1 with a particle size of 2.5-3.0 μm;

[0229] Step 2) Add 26.39g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 2.5-3.0μm. Transfer the ball mill jar to a planetary ball mill and set the ball milling speed to 300r / min for 4h to obtain starch containing a particle size of 300-350nm and insoluble iron source dispersion b1. The mass ratio x1 of starch to insoluble iron source ferric phosphate in dispersion b1 is 0.875.

[0230] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4, 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, and the above iron source dispersion b1. Then add 80.102g of deionized water to adjust the solid content of slurry c1 to 50wt% (the ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 10 hours (obvious stratification occurs).

[0231] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample G#, and the parameters are shown in Table 1.

[0232] Step 6) The prepared sample G# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 phase with a purity of 93.5%, and 6.5 wt% of other impurities, mainly sodium iron phosphate NaFePO4 (3.5 wt%) and sodium phosphate Na3PO4 (2.5 wt%). The conversion rate of iron atoms from the insoluble iron source FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 93.5%. SEM showed that the particle size was 7μm-12μm. TEM showed that the outer surface of the material was coated with carbon. TG showed that the carbon content of the material was 5 wt%.

[0233] Comparative Example 8: Preparation of Sodium Iron Pyrophosphate Na4Fe3(PO4)2P2O7 Material H#

[0234] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0235] Step 2) Add 26.39g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. Transfer the ball mill jar to a planetary ball mill and set the ball milling speed to 650r / min for 4h to obtain starch containing 70-90nm particle size and soluble iron source dispersion b1. The mass ratio x1 of starch to insoluble iron source ferric phosphate in dispersion b1 is 0.875.

[0236] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4, 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, and the above iron source dispersion b1. Then add 80.102g of deionized water to adjust the solid content of slurry c1 to 50wt% (the ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 8 hours (obvious stratification occurs).

[0237] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample H#, and the parameters are shown in Table 1.

[0238] Step 6) The prepared sample H# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 phase with a purity of 93.5%, and 6.5 wt% of other impurities, mainly sodium iron phosphate NaFePO4 (3.5 wt%) and sodium phosphate Na3PO4 (2.5 wt%). The conversion rate of iron atoms from the insoluble iron source FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 93.5%. SEM showed that the particle size was 3μm-8μm. TEM showed that the outer surface of the material was coated with carbon. TG showed that the carbon content of the material was 5 wt%.

[0239] Comparative Example 9: Preparation of Sodium Iron Pyrophosphate Na4Fe3(PO4)2P2O7 Material I#

[0240] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0241] Step 2) Add 26.39g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. Transfer the ball mill jar to a planetary ball mill and set the ball milling speed to 200r / min for 1.5h to obtain starch containing a particle size of 600-650nm and insoluble iron source dispersion b1; the mass ratio x1 of starch to insoluble iron source ferric phosphate in dispersion b1 is 0.875.

[0242] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4, 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, and the above iron source dispersion b1. Then add 80.102g of deionized water to adjust the solid content of slurry c1 to 50wt% (the ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 6 hours (obvious stratification occurs).

[0243] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample I#, and the parameters are shown in Table 1.

[0244] Step 6) The prepared sample I# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 phase with a purity of 93.5%, and 6.5 wt% of other impurities, mainly sodium iron phosphate NaFePO4 (3.5 wt%) and sodium phosphate Na3PO4 (2.5 wt%). The conversion rate of iron atoms from the insoluble iron source FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 93.5%. SEM showed that the particle size was 8μm-12μm. TEM showed that the outer surface of the material was coated with carbon. TG showed that the carbon content of the material was 5 wt%.

[0245] Comparative Example 10: Preparation of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) material J#

[0246] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0247] Step 2) Add 15.08g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. Transfer the ball mill jar to a planetary ball mill and set the ball milling speed to 300r / min for 2h to obtain starch containing a particle size of 300-350nm and insoluble iron source dispersion b1; the mass ratio x1 of starch to insoluble iron source ferric phosphate in dispersion b1 is 0.5.

[0248] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4, 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, and the above iron source dispersion b1. Then add 68.792g of deionized water to adjust the solid content of slurry c1 to 50wt% (the ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 9 hours (obvious stratification occurs).

[0249] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample J#, and the parameters are shown in Table 1.

[0250] Step 6) The prepared sample J# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 phase with a purity of 93%, and 7wt% other impurities, mainly sodium iron phosphate NaFePO4 (3.5wt%) and sodium phosphate Na3PO4 (2.5wt%). The particle size was 7μm-12μm. TEM showed that the outer surface of the material was coated with carbon. TG showed that the conversion rate of iron atoms from the insoluble iron source FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 93%. SEM showed that the carbon content of the material was 0.5wt%.

[0251] Comparative Example 11: Preparation of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) material K#

[0252] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0253] Step 2) Add 34.684g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. Transfer the ball mill jar to a planetary ball mill and set the ball milling speed to 300r / min for 5.5h to obtain starch containing a particle size of 300-350nm and insoluble iron source dispersion b1; the mass ratio x1 of starch to insoluble iron source ferric phosphate in dispersion b1 is 1.15.

[0254] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4, 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, and the above iron source dispersion b1. Then add 88.396g of deionized water to adjust the solid content of slurry c1 to 50wt% (the ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 12 hours (obvious stratification occurs).

[0255] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample K#, and the parameters are shown in Table 1.

[0256] Step 6) The prepared sample K# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 phase with a purity of 92%, and 8 wt% of other impurities, mainly sodium iron phosphate NaFePO4 (4 wt%) and sodium phosphate Na3PO4 (3 wt%). The conversion rate of iron atoms from the insoluble iron source FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3.9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 92%. SEM showed that the particle size was 6μm-10μm. TEM showed that the outer surface of the material was coated with carbon. TG showed that the carbon content of the material was 12 wt%.

[0257] Comparative Example 12: Preparation of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) material L#

[0258] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0259] Step 2) Add 26.39g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. Transfer the ball mill jar to a planetary ball mill and set the ball milling speed to 300r / min for 4h to obtain starch containing a particle size of 300-350nm and insoluble iron source dispersion b1. The mass ratio x1 of starch to insoluble iron source ferric phosphate in dispersion b1 is 0.875.

[0260] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4, 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, and the above iron source dispersion b1. Then add 187.54g of deionized water to adjust the solid content of slurry c1 to 35wt% (the ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 8 hours (obvious stratification occurs).

[0261] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample L#, and the parameters are shown in Table 1.

[0262] Step 6) The prepared sample L# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 phase with a purity of 92.5%, and 7.5 wt% of other impurities, mainly sodium iron phosphate NaFePO4 (4 wt%) and sodium phosphate Na3PO4 (3 wt%). The conversion rate of iron atoms from the insoluble iron source FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 92.5%. SEM showed that the particle size was 5 μm-8 μm. TEM showed that the outer surface of the material was coated with carbon. TG showed that the carbon content of the material was 5 wt%.

[0263] Comparative Example 13: Preparation of Sodium Iron Pyrophosphate Na4Fe3(PO4)2P2O7 Material M#

[0264] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0265] Step 2) Add 26.39g of starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. Transfer the ball mill jar to a planetary ball mill and set the ball milling speed to 300r / min for 4h to obtain starch containing a particle size of 300-350nm and insoluble iron source dispersion b1. The mass ratio x1 of starch to insoluble iron source ferric phosphate in dispersion b1 is 0.875.

[0266] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4, 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, and the above iron source dispersion b1. Then add 22.25g of deionized water to adjust the solid content of slurry c1 to 65wt% (the ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 6 hours (obvious stratification occurs).

[0267] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample M#, and the parameters are shown in Table 1.

[0268] Step 6) The prepared sample M# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 phase with a purity of 92.5%, and 7.5 wt% of other impurities, mainly sodium iron phosphate NaFePO4 (4 wt%) and sodium phosphate Na3PO4 (3 wt%). The conversion rate of iron atoms from the insoluble iron source FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3.9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 92.5%. SEM showed that the particle size was 6μm-9μm. TEM showed that the outer surface of the material was coated with carbon. TG showed that the carbon content of the material was 5 wt%.

[0269] Comparative Example 14: Preparation of Sodium Iron Pyrophosphate Na4Fe3(PO4)2P2O7 Material N#

[0270] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0271] Step 2) Add 13.572g of carboxymethyl starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. Transfer the ball mill jar to a planetary ball mill and set the ball milling speed to 300r / min for 2.5h to obtain a dispersion b1 containing carboxymethyl starch with a particle size of 250-300nm and an insoluble iron source. The mass ratio x1 of carboxymethyl starch to insoluble iron source ferric phosphate in dispersion b1 is 0.45.

[0272] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4, 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, and the above iron source dispersion b1. Then add 67.284g of deionized water to adjust the solid content of slurry c1 to 50wt% (the ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 8 hours (obvious stratification occurs).

[0273] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample N#, and the parameters are shown in Table 1.

[0274] Step 6) The prepared sample N# was characterized by XRD, SEM, TEM, and carbon content (TG) test. XRD analysis showed the presence of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) phase with a purity of 93.2%, along with 6.8 wt% of other impurities, mainly sodium iron phosphate (NaFePO4) (3.5 wt%) and sodium phosphate (Na3PO4) (2.5 wt%). The conversion rate of iron atoms from the insoluble iron source FePO4 and the soluble iron source ferric nitrate nonahydrate (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 93.2%. SEM showed a particle size of 6.5 μm-11.5 μm. TEM showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 0.8 wt%.

[0275] Comparative Example 15: Preparation of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) material O#

[0276] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0277] Step 2) Add 33.176g of carboxymethyl starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. Transfer the ball mill jar to a planetary ball mill and set the ball milling speed to 300r / min for 4.5h to obtain carboxymethyl starch containing particle size of 250-300nm and insoluble iron source dispersion b1; the mass ratio x1 of carboxymethyl starch to insoluble iron source ferric phosphate in dispersion b1 is 1.1.

[0278] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4, 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, and the above iron source dispersion b1. Then add 86.888g of deionized water to adjust the solid content of slurry c1 to 50wt% (the ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 10 hours (obvious stratification occurs).

[0279] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample O#, and the parameters are shown in Table 1.

[0280] Step 6) The prepared sample O# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 phase with a purity of 91.5%, and 8.5 wt% of other impurities, mainly sodium iron phosphate NaFePO4 (4.5 wt%) and sodium phosphate Na3PO4 (3 wt%). The conversion rate of iron atoms from the insoluble iron source FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3.9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 91.5%. SEM showed that the particle size was 6μm-10.5μm. TEM showed that the outer surface of the material was coated with carbon. TG showed that the carbon content of the material was 10.5 wt%.

[0281] Comparative Example 16: Preparation of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) material P#

[0282] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0283] Step 2) Add 18.096g of sodium carboxymethyl starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. Transfer the ball mill jar to a planetary ball mill and set the ball milling speed to 300r / min for 2h to obtain sodium carboxymethyl starch with a particle size of 350-400nm and insoluble iron source dispersion b1. The mass ratio x1 of sodium carboxymethyl starch to insoluble iron source ferric phosphate in dispersion b1 is 0.6.

[0284] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4, 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, and the above iron source dispersion b1. Then add 71.808g of deionized water to adjust the solid content of slurry c1 to 50wt% (the ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 10 hours (obvious stratification occurs).

[0285] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample P#, and the parameters are shown in Table 1.

[0286] Step 6) The prepared sample P# was characterized by XRD, SEM, TEM, and carbon content (TG) testing. XRD analysis showed the presence of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) phase with a purity of 93.2%, along with 6.8 wt% of other impurities, mainly sodium iron phosphate (NaFePO4) (3.5 wt%) and sodium phosphate (Na3PO4) (2.5 wt%). The conversion rate of iron atoms from the insoluble iron source FePO4 and the soluble iron source ferric nitrate nonahydrate (Fe(NO3)3·9H2O) to the product sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) was 93.2%. SEM analysis showed a particle size of 7.5 μm-13 μm. TEM analysis showed carbon coating on the outer surface of the material. TG analysis showed a carbon content of 1.7 wt%.

[0287] Comparative Example 17: Preparation of sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) material Q#

[0288] Na4Fe3(PO4)2P2O7 material was prepared using the same method as Comparative Example 1, with the same process and conditions, except that:

[0289] Step 2) Add 36.192g of sodium carboxymethyl starch to the agate ball mill jar containing the above-mentioned insoluble iron source dispersion a1 with a particle size of 1.5-1.6μm. Transfer the ball mill jar to a planetary ball mill and set the ball milling speed to 300r / min for 4h to obtain sodium carboxymethyl starch containing a particle size of 350-400nm and insoluble iron source dispersion b1. The mass ratio x1 of starch to insoluble iron source ferric phosphate in dispersion b1 is 1.2.

[0290] Step 3) Weigh 28.392g of sodium / phosphorus source - disodium hydrogen phosphate Na2HPO4, 40.4g of soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O, and the above iron source dispersion b1. Then add 89.904g of deionized water to adjust the solid content of slurry c1 to 50wt% (the ratio of the total mass of compounds other than water in slurry c1 to the mass of slurry c1). Take out a portion of slurry c1 and let it stand. It can be observed that slurry c1 settles within 13 hours (obvious stratification occurs).

[0291] Step 5) The heat-treated sample is a fluffy powder. The obtained material is denoted as sample Q#, and the parameters are shown in Table 1.

[0292] Step 6) The prepared sample Q# was characterized by XRD, SEM, TEM and carbon content (TG) test. XRD showed that the synthesized material contained sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 phase with a purity of 91.5%, and 8.5 wt% of other impurities, mainly sodium iron phosphate NaFePO4 (4.5 wt%) and sodium phosphate Na3PO4 (3 wt%). The conversion rate of iron atoms from the insoluble iron source FePO4 and the soluble iron source ferric nitrate nonahydrate Fe(NO3)3·9H2O to the product sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 was 91.5%. SEM showed that the particle size was 7μm-11μm. TEM showed that the outer surface of the material was coated with carbon. TG showed that the carbon content of the material was 13 wt%.

[0293] Comparative Test Example 1

[0294] Sample A#, prepared in Comparative Example 1, was used as the positive electrode active material for sodium-ion batteries. It was mixed uniformly with conductive agent acetylene black and binder polyvinylidene fluoride at a mass ratio of 8:1:1. An appropriate amount of solvent N-methylpyrrolidone was added and mixed thoroughly to form a paste. This paste was then coated onto an aluminum current collector, dried, and cut into 14mm diameter discs. The areal density of the active material was 1.1–3.2 mg / cm³. 2 (Here it is 2.1 mg / cm) 2 This material was used as the positive electrode of the battery. A sodium metal sheet was used as the negative electrode. The electrolyte was 1M NaClO4 / EC / DEC (EC / DEC V / V = 1:1). A glass fiber membrane was used as the separator. The assembled battery was subjected to charge-discharge tests at voltages ranging from 1.8 to 3.6V, and the discharge specific capacity was tested under conditions of 0.2C / 1.0C / 5.0C / 10C / 50C. The discharge specific capacity under 0.2C / 50C conditions is recorded in Table 2.

[0295] Comparative test case 2-17

[0296] Samples B#-Q# 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.

[0297] Table 1. Discharge specific capacity test data of materials in the examples.

[0298]

[0299] Table 2. Discharge specific capacity test data of comparative materials

[0300]

[0301]

[0302] This invention patent utilizes the adsorption effect of starch (carboxymethyl starch, sodium carboxymethyl starch) with specific nanoparticle sizes on insoluble iron sources within a specific particle size range to improve the uniform dispersion of precursor raw materials in slurry. It is more suitable for raw material systems where insoluble iron sources dominate, effectively improving the uniformity of the precursor solution in such systems, ensuring uniform dispersion of insoluble iron sources and other soluble raw materials, and increasing the purity of the product (e.g., Examples 1-20: 0.2C: 116-126 mAh.g). -1 50C: 80-90mAh.g -1 Starch (carboxymethyl starch, sodium carboxymethyl starch) can reduce the caking phenomenon during high-temperature calcination of materials, resulting in a loose powdery state after calcination, thus improving the uniformity and rate performance of the material. While polyethylene glycol, as a carbon source, can also reduce caking during high-temperature calcination, it cannot evenly disperse the precursor raw materials, leading to uneven dispersion, impurities in the product, and reduced initial and rate performance. Lauric acid, as a carbon source, can improve the uniformity of precursor dispersion, but severe caking during high-temperature calcination results in poor uniformity and reduced rate performance. Citric acid, as a carbon source, not only causes poor uniform dispersion of precursor raw materials but also severe caking during high-temperature calcination, resulting in impurities in the prepared material and reduced initial and rate performance (e.g., Comparative Examples 1-3: 0.2C: 105-115mAh g). -1 50C: 65-75mAh.g -1 );

[0303] Under other optimal conditions, the mass ratio y (0.65–1.05) of starch to insoluble iron source in dispersion b and the particle size of starch (100 nm–500 nm) will affect the stability of the adsorption between starch and insoluble iron source in dispersion b. When the mass ratio of starch to insoluble iron source y < 0.65, or when the starch particle size is too large (> 500 nm), the contact area is too small to provide sufficient adsorption sites for the insoluble iron source, resulting in sedimentation due to the inability to maintain a suspended state. When the mass ratio of starch to insoluble iron source y > 1.05, the starch molecules cannot maintain a suspended state in the solution due to excessively strong intermolecular adsorption, resulting in sedimentation. When the starch particle size is too small (< 100 nm), the light weight cannot effectively support the insoluble iron source, preventing it from maintaining a suspended state and causing sedimentation. All of these factors lead to insufficient contact between the insoluble iron source and other soluble raw materials, resulting in impurities in the product and a decrease in the initial and rate performance of the material. (Examples 6-9: 0.2C: 118-120 mAh g) -1 50C: 82-87 mAh.g -1Comparative Examples 8-11: 0.2C: 102-105mAh.g -1 50C: 70-74mAh.g -1 )

[0304] Similarly, the mass fraction (30wt%–50wt%) and particle size (1μm–2μm) of the insoluble iron source in dispersion a both affect the adsorption strength between the insoluble iron source and starch in dispersion b. When the mass fraction of the insoluble iron source is too low (<20wt%), the amount of insoluble iron source and starch per unit volume in dispersion b is too small, making it difficult to achieve sufficient contact and form a stable adsorption effect, resulting in significant sedimentation of the slurry within 10 hours. When the mass fraction of the insoluble iron source is too high (>60wt%), the amount of insoluble iron source and starch per unit volume in dispersion b is too large, resulting in excessively heavy adsorbed agglomerates that also settle. Both of these effects lead to insufficient contact between the insoluble iron source and other soluble raw materials, resulting in impurities in the product and a decrease in the initial performance and rate performance of the material; (e.g., Examples 1-3: 0.2C: 121-125mAh g). -1 50C: 86-90mAh.g -1 Comparative Examples 4-5: 0.2C: 112-113 mAh.g -1 50C: 72-73mAh.g -1 When the particle size of the insoluble iron source is <1μm, it cannot be adsorbed by starch due to its excessively large specific surface energy; when the particle size of the insoluble iron source is too large (>2μm), it cannot be effectively carried by starch due to the limited contact sites, resulting in sedimentation. This leads to insufficient contact between the insoluble iron source and other soluble raw materials, resulting in impurities in the product and a decrease in the initial and rate performance of the material. (e.g., Examples 4-5: 0.2C: 120-121mAh g) -1 50C: 88-89mAh.g -1 Comparative Examples 6-7: 0.2C: 10⁵-10⁸ mAh.g -1 50C: 71-72mAh.g -1 );

[0305] The solid content of the slurry affects the spray drying effect and the duration of stable suspension. Spray drying efficiency is highest when the solid content of the slurry is between 45 wt% and 55 wt%. (Examples 10-11: 0.2C: 120-121 mAh.g) -1 50C: 86-87mAh.g -1 Comparative Examples 12-13: 0.2C: 100-101 mAh.g -1 50C: 70-72mAh.g -1 )

[0306] Materials with excellent performance can also be prepared by using carboxymethyl starch or sodium carboxymethyl starch. Sodium iron pyrophosphate materials prepared with carboxymethyl starch particle size of 250nm-350nm (sodium carboxymethyl starch particle size of 350nm-450nm), insoluble iron source particle size of 1.5-1.6μm, and a mass ratio (y) of carboxymethyl starch (sodium carboxymethyl starch) to insoluble iron source of 0.65-1.05 exhibit superior performance. (Examples 15-20: 0.2C: 116-124mAh.g) -1 50C: 82-89mAh.g -1 Comparative Examples 14-17: 0.2C: 97-99mAh.g -1 50C: 69-71mAh.g -1 ).

Claims

1. A method for preparing iron-based phosphate polyanionic compounds, characterized in that, The iron-based phosphate polyanionic compound has the structure shown in Formula I: Na4Fe3(PO4)2P2O7 Formula I; The preparation method of the polyanionic compound shown in Formula I includes: Step (1): Disperse the insoluble iron source in water, transfer the dispersion to a sand mill and / or a ball mill, and after sand milling and / or ball milling, obtain an insoluble iron source dispersion a with a particle size of 1-2 μm; the mass of the insoluble iron source in the insoluble iron source dispersion a is 30wt%~50wt% of the mass of the dispersion a. The insoluble iron source is selected from one or more of the following: iron powder, ferric oxide, ferric oxide, ferrous oxide, ferric oxalate, and ferric phosphate. Step (2): The polymer compound is dispersed in the above-mentioned insoluble iron source dispersion a, and then sand milled and / or ball milled to obtain a dispersion b containing a polymer compound with a particle size of 100nm-500nm and an insoluble iron source with a particle size of 1-2μm; the mass ratio x of the polymer compound to the insoluble iron source in the dispersion b is 0.65≤x≤1.05; The polymeric compound is selected from at least one or more of starch, carboxymethyl starch, and sodium carboxymethyl starch; the polymeric compound also serves as a carbon source and a reducing agent. Step (3): Weigh out sodium source and phosphorus source, or sodium source, phosphorus source and soluble iron source, according to the stoichiometric ratio of the polyanionic compound to be prepared; add them to the above dispersion b, add or not add water, and then mix evenly to obtain slurry c; The solid content of the slurry c is 45wt%~55wt%; The iron source in the slurry c includes an insoluble iron source, or an insoluble iron source and a soluble iron source; the ratio of the amount of iron ions in the insoluble iron source to the amount of total iron ions in the iron source of the slurry c is y = 50%. <y≤100%; The insoluble iron source and the polymer compound are both derived from dispersion b; The soluble iron source is selected from one or more of the following: ferrous nitrate, ferrous nitrate, ferrous sulfate, ferrous sulfate, ferrous chloride, ferrous chloride, ferrous acetate, ferrous ammonium sulfate, ferric citrate, ferric ammonium citrate, and sodium ferric citrate succinate. Step (4): The above slurry c is transferred into a spray dryer for spray granulation, and the precursor powder d is obtained after drying. Step (5): The precursor powder d is heat-treated in a specific atmosphere to obtain the polyanionic compound.

2. The preparation method according to claim 1, characterized in that, In step (1) or (2): The sand milling process requires the addition of sand milling media. In step (1), the sand milling speed is 3000-5000 r / min, and the sand milling time is 0.5-5 h. In step (2), the sand milling speed is 1000-3000 r / min, and the sand milling time is 0.5-5 h. The grinding media is one or more of the following: natural sand beads, glass beads, steel beads, zirconium oxide beads, zirconium silicate beads, and agate beads; the ball-to-material ratio is 1-5; and the diameter of the grinding media is 2-12 cm. Ball milling requires adding a ball milling medium, in step (1), the rotation speed of the ball milling is 600 to 800r / min, and the ball milling treatment time is 3 to 8h; in step (2), the rotation speed of the ball milling is 200 to 400r / min, and the ball milling treatment time is 2 to 6h; The ball milling medium is one or more selected from zirconia beads, agate beads and zirconium silicate beads; the ball-to-material ratio is 1.5 to 7; the diameter of the ball milling medium is 2 to 12cm; In step (3): mechanical stirring can be selected as the mixing method, with a rotation speed of 200-500r / min and a time of 10min-30min.

3. The preparation method according to claim 1, characterized in that, The conditions of the spray drying are as follows: a feed flow rate of 1ml / min to 70ml / min, an inlet air temperature of 90°C to 210°C, an outlet air temperature of 50°C to 150°C, and a compressed air pressure of 0.01KPa to 1KPa; the slurry is pumped into an atomizer of a spray dryer by a peristaltic pump, and the powder is collected.

4. The preparation method according to claim 1, characterized in that, The feed flow rate is controlled to 20ml / min to 60ml / min, the inlet air temperature is controlled to 120°C to 195°C, the outlet air temperature is 80°C to 110°C, and the compressed air pressure is 0.03KPa to 0.9KPa.

5. The preparation method according to claim 1, characterized in that, The specific atmosphere is selected from at least one of argon, helium and nitrogen, or a reducing inert atmosphere containing H2; the heat treatment comprises a sequential treatment process at a first temperature T1 and a treatment process at a second temperature T2; the temperature T1 satisfies 200≤T1≤400°C, and the treatment time is 0.5 to 6h; the temperature T2 satisfies 400°C<T2≤650°C, and the treatment time is 3 to 20h; the difference between T2 and T1 is greater than or equal to 50°C.

6. The preparation method according to claim 1, characterized in that, The phosphate polyanionic compound has a structure represented by formula I, in slurry c, the molar ratio of a sodium source, an iron source and a phosphorus source is 4:3:4, wherein the molar ratio of the sodium source, the iron source and the phosphorus source is calculated based on the molar amounts of sodium element, iron element and phosphorus element, respectively.

7. The preparation method according to claim 1, characterized in that, The sodium source is selected from at least one or more than two 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 phosphorus source is selected from at least one or more than two of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, pyrophosphoric acid, sodium pyrophosphate and sodium hydrogen pyrophosphate.

8. An iron-based phosphate polyanionic compound prepared by the preparation method according to any one of claims 1-7.

9. The iron-based phosphate polyanionic compound according to claim 8, characterized in that, The particle size of the iron-based phosphate polyanionic compound is 1.5μm-6μm, and the carbon content is 1% to 10%.

10. The application of the iron-based phosphate polyanionic compound prepared by the preparation method according to any one of claims 1-7, or the iron-based phosphate polyanionic compound according to claim 8 or claim 9, in a sodium-ion battery, characterized in that, The iron-based phosphate polyanionic compound is used as an active material for a positive electrode material of a sodium-ion battery.

11. A positive electrode material for a sodium-ion battery, characterized in that, The positive electrode material contains an iron-based phosphate polyanionic compound prepared by any one of claims 1-7 or an iron-based phosphate polyanionic compound as described in claim 8 or claim 9; In the cathode material, the content of the iron-based phosphate polyanionic compound is 60~98 wt%; The positive electrode material also contains a conductive agent and a binder, wherein the mass ratio of the iron-based phosphate polyanionic compound, the conductive agent, and the binder is 60~98wt%:1~39wt%:1~39wt%; The conductive agent is at least one or more of carbon black, conductive graphite, carbon fiber, carbon nanotubes, and graphene. The adhesive is polyvinylidene fluoride.

12. The positive electrode material of the sodium-ion battery according to claim 11, characterized in that, Polyvinylidene fluoride is selected from at least one or more of PVDF5130, HSV900, and kynar761A.

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