Iron phosphate, and preparation method therefor and use thereof

Abstract

Iron phosphate, and a preparation method therefor and a use thereof. The preparation method comprises the following steps: (1) mixing ammonium ferricyanide and a phosphorus source to obtain an iron-phosphorus mixed solution; (2) carrying out spray pyrolysis on the iron-phosphorus mixed solution obtained in step (1) to obtain solid particles; and (3) crushing the solid particles obtained in step (2) to obtain iron phosphate. Iron phosphate has high purity, tap density, and specific surface area. The preparation method can avoid use of a pH regulator and a surfactant, and reduces generation of a by-product and a cleaning step, thereby greatly simplifying the production process, and having high industrial application value.

Description

Ferric phosphate and its preparation method and use Technical Field

[0001] The present disclosure relates to the technical field of lithium-ion batteries, and in particular to iron phosphate, a preparation method thereof, and uses thereof. Background Art

[0002] In the energy storage and new energy sectors, lithium iron phosphate batteries (LiFePO4) have become one of the most promising lithium-ion batteries due to their excellent rate performance, long cycle life, safety, and environmental friendliness. Iron phosphate, as a precursor to lithium iron phosphate (LiFePO4), the cathode material for lithium-ion batteries, has attracted extensive attention and research.

[0003] In existing industrial production, the preparation process of iron phosphate mainly adopts purified ferrous sulfate or titanium dioxide byproduct ferrous phosphate, adds ammonia, ammonium dihydrogen phosphate or diammonium hydrogen phosphate, etc., adopts one-step aging method to carry out crystallization control, and finally obtains iron phosphate product. However, the above process easily leads to insufficient oxidation precipitation, and the problems such as the primary particles of iron phosphate are too large and the specific surface area is too small are not conducive to the subsequent production and processing of lithium iron phosphate. For example, CN111115606A discloses a preparation method for preparing ultrafine spherical iron phosphate by combining liquid phase precipitation with spray drying. The preparation method uses trivalent iron salt solution and phosphoric acid as raw materials, and adopts ammonia and phosphoric acid to adjust the pH value of the reaction. Although the above preparation method can obtain ultrafine spherical iron phosphate product, the use of trivalent iron salt such as iron sulfate leads to the production of byproduct ammonium sulfate, and a large amount of sulfate radicals are easily adsorbed on the surface of iron phosphate, which is not easy to wash, thereby causing the sulfur content to be too high and affecting the performance of lithium iron phosphate.

[0004] Therefore, it is of great significance to provide an iron phosphate having good purity, specific surface area and tap density and a preparation method thereof.

[0005] Summary of the Invention

[0006] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0007] In response to the above problems, the purpose of the present disclosure is to provide an iron phosphate and a preparation method and use thereof. Compared with the prior art, the iron phosphate provided by the present disclosure has higher purity, tap density and specific surface area. The preparation method avoids the use of pH regulators and surfactants, reduces the generation of by-products and cleaning steps, greatly simplifies the production process, and has high industrial application value.

[0008] To achieve this purpose, the present disclosure adopts the following technical solutions:

[0009] In a first aspect, the present disclosure provides a method for preparing ferric phosphate, the preparation method comprising the following steps:

[0010] (1) mixing ammonium ferrocyanide and a phosphorus source to obtain an iron-phosphorus mixed solution;

[0011] (2) spray pyrolysis the iron-phosphorus mixed solution obtained in step (1) to obtain solid particles;

[0012] (3) crushing the solid particles obtained in step (2) to obtain iron phosphate.

[0013] The iron-phosphorus mixed solution prepared by the present invention does not need to use acid or alkali to adjust the pH value and is not easy to agglomerate. Furthermore, the present invention adopts a spray pyrolysis process to control the dispersed morphology of the product, reduce the occurrence of agglomeration, and make the particle size of the obtained iron phosphate particles smaller, easy to grind, with high specific surface area and tap density, and regular morphology. In addition, the ammonium ferricyanide and phosphorus source in the present invention only generate hydrogen cyanide gas and ammonia gas under the conditions of spray pyrolysis, without the generation of other by-products, so the purity of the obtained iron phosphate is high, avoiding the steps of multiple cleanings, simplifying the process flow, and being conducive to industrial application.

[0014] In one embodiment, the ammonium ferrocyanide in step (1) is mixed in the form of an ammonium ferrocyanide solution.

[0015] In one embodiment, the concentration of iron ions in the ammonium ferricyanide solution is 100-200 g / L, for example, 100 g / L, 110 g / L, 120 g / L, 130 g / L, 140 g / L, 150 g / L, 160 g / L, 170 g / L, 180 g / L, 190 g / L or 200 g / L, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0016] In one embodiment, the phosphorus source includes any one of phosphoric acid, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate, or a combination of at least two thereof.

[0017] In this disclosure, phosphoric acid, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate reacts with ammonium ferricyanide in a spray pyrolysis process to produce ferric phosphate, hydrogen cyanide, and ammonia gas without producing other byproducts, thereby improving the purity of the ferric phosphate product. Using diammonium hydrogen phosphate as an example, the main reaction is as follows: (NH₄)₂HPO₄ + (NH₄)₃[Fe(CN)₆] → FePO₄ + 6HCN + 5NH₃.

[0018] In one embodiment, the molar ratio of iron to phosphorus in the iron-phosphorus mixed solution of step (1) is (0.98-1.02):1, for example, it can be 0.98:1, 0.99:1, 1.00:1, 1.01:1 or 1.02:1, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0019] In one embodiment, the carrier gas used in the spray pyrolysis in step (2) includes air.

[0020] In one embodiment, the flow rate of the carrier gas is 20-30 L / min, for example, it can be 20 L / min, 22 L / min, 24 L / min, 26 L / min, 28 L / min or 30 L / min, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0021] In one embodiment, the flow rate of the iron-phosphorus mixture in the spray pyrolysis is 1.5-1.8 mL / min, for example, 1.5 mL / min, 1.6 mL / min, 1.7 mL / min or 1.8 mL / min, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0022] In one embodiment, the average particle size of the atomized droplets of the iron-phosphorus mixture in the spray pyrolysis is 5-15 μm, for example, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm or 15 μm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0023] In the present disclosure, the particle size of the obtained solid particles can be further adjusted by selectively controlling the flow rate of the iron-phosphorus mixed solution and the particle size of the atomized droplets.

[0024] In one embodiment, the spray pyrolysis temperature is 450-650°C, for example, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 520°C, 550°C, 580°C, 600°C or 650°C, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0025] In the present disclosure, the temperature of the spray pyrolysis is selectively controlled to promote the formation of ferric phosphate and the volatilization of substances other than ferric phosphate, thereby reducing the content of impurities in the ferric phosphate and obtaining high-purity ferric phosphate.

[0026] In one embodiment, the average particle size of the solid particles in step (2) is 3-10 μm, for example, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0027] In one embodiment, the smoke after the spray pyrolysis in step (2) is subjected to gas-solid separation to obtain solid powder and tail gas; the tail gas is absorbed by an absorbent to obtain an absorption liquid, the absorption liquid is mixed with an iron salt to obtain regenerated ammonium ferrocyanide, and the regenerated ammonium ferrocyanide is reused in step (1) to prepare an iron-phosphorus mixed solution; the solid powder is mixed with the solid particles in step (3) and then crushed.

[0028] In the present disclosure, there is no particular limitation on the type of the iron salt, which may be any trivalent iron salt commonly used in the art, for example, any one of ferric sulfate, ferric chloride, or ferric nitrate, or a combination of at least two thereof.

[0029] In one embodiment, the absorbent comprises an ammonium carbonate solution.

[0030] In one embodiment, the mass concentration of the ammonium carbonate solution is 10-40%, for example, it can be 10%, 12%, 14%, 16%, 18%, 20%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38% or 40%, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0031] In the present disclosure, tail gas is absorbed using an ammonium carbonate solution to obtain regenerated ammonium ferricyanide, thereby enabling material recycling. Taking ferric sulfate as an example, the absorption and regeneration reactions for ammonium ferricyanide are as follows: (NH₄)₂CO₃+2HCN=2NH₄CN+H₂O+CO₂; Fe₂(SO₄)₃+12NH₄CN=2(NH₄)₃[Fe(CN)₆]+3(NH₄)₂SO₄.

[0032] In one embodiment, the crushing pressure in step (3) is 0.2-0.6 MPa, for example, 0.2 MPa, 0.3 MPa, 0.4 MPa, 0.5 MPa or 0.6 MPa, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0033] As an optional technical solution of the first aspect of the present disclosure, the preparation method comprises the following steps:

[0034] (1) mixing an ammonium ferricyanide solution having an iron ion concentration of 100-200 g / L and a phosphorus source to obtain an iron-phosphorus mixed solution, wherein the molar ratio of iron to phosphorus in the iron-phosphorus mixed solution is (0.98-1.02):1, and the phosphorus source comprises any one of phosphoric acid, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate, or a combination of at least two thereof;

[0035] (2) spray pyrolysis of the iron-phosphorus mixed solution obtained in step (1) at a flow rate of 1.5-1.8 mL / min, using air with a flow rate of 20-30 L / min as a carrier gas, at a temperature of 450-650° C., wherein the average particle size of the atomized droplets of the iron-phosphorus mixed solution in the spray pyrolysis is 5-15 μm, to obtain solid particles, wherein the average particle size of the solid particles is 3-10 μm;

[0036] The smoke after the spray pyrolysis is subjected to gas-solid separation to obtain solid powder and tail gas, the tail gas is absorbed by an ammonium carbonate solution with a mass concentration of 10-40% to obtain an absorption liquid, the absorption liquid is mixed with an iron salt to obtain regenerated ammonium ferrocyanide, and the regenerated ammonium ferrocyanide is reused in step (1) to prepare an iron-phosphorus mixed solution;

[0037] (3) mixing the solid particles and solid powder obtained in step (2), and crushing them at 0.2-0.6 MPa to obtain iron phosphate.

[0038] In a second aspect, the present disclosure provides an iron phosphate, which is obtained by the preparation method of the iron phosphate described in the first aspect of the present disclosure.

[0039] The iron phosphate provided by the present disclosure has high purity, high tap density and high specific surface area, and has a regular morphology, which is beneficial to improving the electrical performance of lithium-ion batteries.

[0040] In one embodiment, the purity of the ferric phosphate is 99.96-99.99%, for example, 99.96%, 99.97%, 99.98% or 99.99%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0041] In one embodiment, the tap density of the iron phosphate is 1.22-1.25 g / cm 3 , for example, it can be 1.22 g / cm 3 , 1.23g / cm 3 , 1.24g / cm 3 or 1.25g / cm 3 , but not limited to the listed values, other unlisted values ​​within the numerical range are also applicable.

[0042] In one embodiment, the specific surface area of ​​the iron phosphate is 9.96-1.52 m 2 / g, for example, it can be 9.96m 2 / g, 9.98m 2 / g, 1.00m 2 / g, 1.1m 2 / g, 1.2m 2 / g, 1.3m 2 / g, 1.4m 2 / g, 1.5m 2 / g or 1.52m 2 / g, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0043] In a third aspect, the present disclosure provides a use of the iron phosphate as described in the second aspect of the present disclosure, wherein the iron phosphate is used to prepare a lithium iron phosphate positive electrode material.

[0044] The iron phosphate provided by the present disclosure is used to prepare lithium iron phosphate positive electrode materials, which can increase the gram capacity of the lithium iron phosphate material and thus improve the electrochemical performance.

[0045] Compared with the prior art, the present disclosure has the following beneficial effects:

[0046] (1) The preparation method of ferric phosphate provided by the present invention uses ammonium ferrocyanide as the iron source and adopts a spray pyrolysis process, which not only can obtain ferric phosphate with uniform morphology, high specific surface area and high tap density, but also does not require the use of pH regulators and surfactants, avoids the introduction of acid ions and impurity cations, and does not produce any by-products, thereby effectively improving the purity of the ferric phosphate.

[0047] (2) The preparation method provided by the present disclosure can increase the purity of iron phosphate to above 99.90%, and the tap density reaches 1.05 g / cm 3 Above, the specific surface area reaches 9.23m 2 / g or more; under optimal conditions, the purity of iron phosphate can be increased to more than 99.96%, and the tap density can reach 1.22g / cm 3 Above, the specific surface area reaches 9.96m 2 / g or more; the iron phosphate provided by the present disclosure is used to prepare lithium iron phosphate positive electrode materials, which can make the compaction density of lithium iron phosphate reach 2.40g / cm 3 Above, the 1C gram capacity reaches above 154.6mAh / g; under optimal conditions, the compaction density of lithium iron phosphate can reach 2.55g / cm 3 Above, the 1C gram capacity reaches above 160.5mAh / g.

[0048] (3) The preparation method provided by the present disclosure does not require multiple washing processes, simplifies the process flow, and can achieve the recycling of materials.

[0049] (4) The iron phosphate provided by the present disclosure is used to prepare lithium iron phosphate positive electrode materials, which can increase the gram capacity of lithium iron phosphate and thus improve the electrochemical performance.

[0050] Still other aspects will become apparent upon reading and understanding the detailed description. DETAILED DESCRIPTION

[0051] The technical solution of the present disclosure is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present disclosure and should not be regarded as specific limitations of the present disclosure.

[0052] Example 1

[0053] This embodiment provides a method for preparing iron phosphate, which comprises the following steps:

[0054] (1) mixing an ammonium ferricyanide solution with a concentration of 150 g / L of iron ions and diammonium hydrogen phosphate to obtain an iron-phosphorus mixed solution, wherein the molar ratio of iron to phosphorus in the iron-phosphorus mixed solution is 1:1;

[0055] (2) spray pyrolysis of the iron-phosphorus mixed solution obtained in step (1) at a flow rate of 1.7 mL / min, using air with a flow rate of 25 L / min as a carrier gas, at a temperature of 550° C., wherein the average particle size of the atomized droplets of the iron-phosphorus mixed solution in the spray pyrolysis is 10 μm, to obtain solid particles, wherein the average particle size of the solid particles is 6 μm;

[0056] The smoke after the spray pyrolysis is subjected to gas-solid separation to obtain solid powder and tail gas, the tail gas is absorbed by an ammonium carbonate solution with a mass concentration of 20% to obtain an absorption liquid, the absorption liquid and ferric sulfate are mixed in a stoichiometric ratio to obtain regenerated ammonium ferrocyanide, and the regenerated ammonium ferrocyanide is reused in step (1) to prepare an iron-phosphorus mixed solution;

[0057] (3) mixing the solid particles and solid powder obtained in step (2), and crushing them at 0.4 MPa to obtain iron phosphate.

[0058] Example 2

[0059] This embodiment provides a method for preparing iron phosphate, which comprises the following steps:

[0060] (1) mixing an iron ion solution having a concentration of 100 g / L ammonium ferricyanide and phosphoric acid (the mass concentration of the phosphoric acid is 80%) to obtain an iron-phosphorus mixed solution, wherein the molar ratio of iron to phosphorus in the iron-phosphorus mixed solution is 0.98:1;

[0061] (2) spray pyrolysis of the iron-phosphorus mixed solution obtained in step (1) at a flow rate of 1.8 mL / min, using air with a flow rate of 20 L / min as a carrier gas, at a temperature of 650° C., wherein the average particle size of the atomized droplets of the iron-phosphorus mixed solution in the spray pyrolysis is 5 μm, to obtain solid particles, wherein the average particle size of the solid particles is 3 μm;

[0062] The smoke after the spray pyrolysis is subjected to gas-solid separation to obtain solid powder and tail gas, the tail gas is absorbed by an ammonium carbonate solution with a mass concentration of 10% to obtain an absorption liquid, the absorption liquid and ferric sulfate are mixed in a stoichiometric ratio to obtain regenerated ammonium ferrocyanide, and the regenerated ammonium ferrocyanide is reused in step (1) to prepare an iron-phosphorus mixed solution;

[0063] (3) mixing the solid particles and solid powder obtained in step (2), and crushing them at 0.2 MPa to obtain iron phosphate.

[0064] Example 3

[0065] This embodiment provides a method for preparing iron phosphate, which comprises the following steps:

[0066] (1) mixing an ammonium ferricyanide solution and ammonium dihydrogen phosphate with an iron ion concentration of 200 g / L to obtain an iron-phosphorus mixed solution, wherein the molar ratio of iron to phosphorus in the iron-phosphorus mixed solution is 1.02:1;

[0067] (2) spray pyrolysis of the iron-phosphorus mixed solution obtained in step (1) at a flow rate of 1.5 mL / min, using air with a flow rate of 30 L / min as a carrier gas, at a temperature of 450° C., wherein the average particle size of the atomized droplets of the iron-phosphorus mixed solution in the spray pyrolysis is 15 μm, to obtain solid particles, wherein the average particle size of the solid particles is 10 μm;

[0068] The smoke after the spray pyrolysis is subjected to gas-solid separation to obtain solid powder and tail gas, the tail gas is absorbed by an ammonium carbonate solution with a mass concentration of 40% to obtain an absorption liquid, the absorption liquid and ferric sulfate are mixed in a stoichiometric ratio to obtain regenerated ammonium ferrocyanide, and the regenerated ammonium ferrocyanide is reused in step (1) to prepare an iron-phosphorus mixed solution;

[0069] (3) mixing the solid particles and solid powder obtained in step (2), and crushing them at 0.6 MPa to obtain iron phosphate.

[0070] Example 4

[0071] This embodiment provides a method for preparing ferric phosphate. The only difference between the preparation method and Example 1 is that the temperature of the spray pyrolysis is 400°C.

[0072] Example 5

[0073] This embodiment provides a method for preparing ferric phosphate. The only difference between the preparation method and Example 1 is that the temperature of the spray pyrolysis is 700°C.

[0074] Comparative Example 1

[0075] This comparative example provides a method for preparing ferric phosphate, which differs from that of Example 1 only in that the ammonium ferricyanide solution in step (1) is replaced by a ferric sulfate solution with an equal iron ion concentration.

[0076] The purity of the ferric phosphate obtained in Examples 1-5 and Comparative Example 1 was detected by X-ray fluorescence spectrometer, and the tap density and specific surface area of ​​the ferric phosphate obtained in Examples 1-5 and Comparative Example 1 were obtained by BET method. The results are shown in Table 1.

[0077] The iron phosphate obtained in Examples 1-5 and Comparative Example 1 was used to prepare a lithium iron phosphate positive electrode material. The method was as follows: iron phosphate and lithium carbonate were weighed at a molar ratio of phosphorus to lithium of 1:1, the iron phosphate, lithium carbonate and sucrose (the mass of sucrose accounted for 10% of the mass of iron phosphate) were mixed evenly, and then calcined at 720°C for 6 hours under a nitrogen protective atmosphere, and then cooled to obtain a lithium iron phosphate positive electrode material.

[0078] Electrical performance test: The lithium iron phosphate positive electrode material prepared from the iron phosphate obtained in Examples 1-5 and Comparative Example 1, a conductive agent (Super P), and a binder (polyvinylidene fluoride, PVDF) were mixed in a mass ratio of 90:5:5, and after mixing, added to the solvent N-methylpyrrolidone (NMP) and stirred to obtain a slurry of the lithium iron phosphate positive electrode material. The slurry was coated on the positive electrode current collector aluminum foil and dried to obtain a positive electrode sheet. Ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) were mixed in a mass ratio of 1:1:1 to prepare an electrolyte with a LiPF6 concentration of 1.0 mol / L. A polyethylene film (PE) was used as an isolating membrane and a metal lithium sheet was used as the negative electrode to assemble a 2025 button battery, and the electrochemical performance was tested. Charge and discharge the button cell at 2-3.75V, 25°C, and 0.1C, recording the capacity of the cell. Divide the measured capacity by the mass of the lithium iron phosphate cathode material to obtain the gram capacity of the lithium iron phosphate cathode material in the lithium-ion battery. The compaction density of the lithium iron phosphate cathode material in Table 1 was measured using a PRCD compaction density meter.

[0079] Table 1

[0080] The following points can be seen from the data in Table 1:

[0081] (1) From the data of Examples 1-5, it can be seen that the preparation method provided by the present disclosure can increase the purity of ferric phosphate to above 99.90%, and the tap density reaches 1.05 g / cm 3 Above, the specific surface area reaches 9.23m 2 / g or more; under optimal conditions, the purity of iron phosphate can be increased to more than 99.96%, and the tap density can reach 1.22g / cm 3Above, the specific surface area reaches 9.96m 2 / g or more; the iron phosphate provided by the present disclosure is used to prepare lithium iron phosphate positive electrode materials, which can make the compaction density of the lithium iron phosphate positive electrode material reach 2.40g / cm 3 Above, the 1C gram capacity reaches above 154.6mAh / g; under optimal conditions, the compaction density of the lithium iron phosphate positive electrode material reaches 2.55g / cm 3 Above, the 1C gram capacity reaches above 160.5mAh / g.

[0082] (2) A comprehensive comparison of the data of Example 1 and Examples 4-5 shows that the only difference between Example 1 and Examples 4-5 is that the temperature of the spray pyrolysis is not within the optional range of the present disclosure. The purity, tap density and specific surface area of ​​the iron phosphate in Example 1 are significantly higher than those in Examples 4-5, and the compacted density and 1C gram capacity of the lithium iron phosphate in Example 1 are significantly higher than those in Examples 4-5. It can be seen that the present disclosure can promote the formation of iron phosphate and the volatilization of other substances in the form of gas by controlling the temperature of the spray pyrolysis, thereby significantly improving the purity, tap density and specific surface area of ​​the iron phosphate, and thereby improving the compacted density and 1C gram capacity of the lithium iron phosphate positive electrode material.

[0083] (3) A comprehensive comparison of the data of Example 1 and Comparative Example 1 shows that the only difference between Comparative Example 1 and Example 1 is that the ammonium ferricyanide solution is replaced by a ferric sulfate solution with an equal iron ion concentration. The purity, tap density and specific surface area of ​​the iron phosphate in Example 1 are significantly better than those in Comparative Example 1, and thus the compacted density and 1C gram capacity of the lithium iron phosphate positive electrode material are also significantly better than those in Comparative Example 1. It can be seen that the present disclosure prepares iron phosphate by spray pyrolysis using ammonium ferricyanide as an iron source, which can avoid the excessively high impurity content of the prepared iron phosphate caused by the use of a sulfur-containing iron source in Comparative Example 1, thereby improving the purity of the iron phosphate, and can prepare iron phosphate with a high specific surface area and a high tap density without the use of a pH regulator and a surfactant, which is beneficial to improving the compacted density and gram capacity of the lithium iron phosphate positive electrode material.

[0084] In summary, the preparation method provided by the present disclosure can make iron phosphate have higher purity, tap density and specific surface area, reduce the production of by-products and cleaning steps, greatly simplify the production process, and has high industrial application value.

Claims

1. A method for preparing ferric phosphate, comprising the following steps: (1) mixing ammonium ferrocyanide and a phosphorus source to obtain an iron-phosphorus mixed solution; (2) spray pyrolysis the iron-phosphorus mixed solution obtained in step (1) to obtain solid particles; (3) crushing the solid particles obtained in step (2) to obtain iron phosphate.

2. The preparation method according to claim 1, wherein The ammonium ferrocyanide of step (1) is mixed in the form of an ammonium ferrocyanide solution; the concentration of iron ions in the ammonium ferrocyanide solution is 100-200 g / L; The phosphorus source includes any one of phosphoric acid, ammonium dihydrogen phosphate or diammonium hydrogen phosphate, or a combination of at least two thereof.

3. The preparation method according to claim 1 or 2, wherein The molar ratio of iron element to phosphorus element in the iron-phosphorus mixed solution of step (1) is (0.98-1.02):

1.

4. The preparation method according to any one of claims 1 to 3, wherein: The carrier gas used in the spray pyrolysis in step (2) includes air; the flow rate of the carrier gas is 20-30L / min.

5. The preparation method according to any one of claims 1 to 4, wherein: The flow rate of the iron-phosphorus mixed solution in the spray pyrolysis is 1.5-1.8 mL / min.

6. The preparation method according to any one of claims 1 to 5, wherein: The average particle size of the atomized droplets of the iron-phosphorus mixed solution in the spray pyrolysis is 5-15 μm.

7. The preparation method according to any one of claims 1 to 6, wherein: The temperature of the spray pyrolysis is 450-650°C.

8. The preparation method according to any one of claims 1 to 7, wherein: The average particle size of the solid particles in step (2) is 3-10 μm.

9. The preparation method according to any one of claims 1 to 8, wherein: The smoke after the spray pyrolysis in step (2) is subjected to gas-solid separation to obtain solid powder and tail gas; the tail gas is absorbed by an absorbent to obtain an absorption liquid, the absorption liquid is mixed with an iron salt to obtain regenerated ammonium ferrocyanide, and the regenerated ammonium ferrocyanide is reused in step (1) to prepare an iron-phosphorus mixed solution; the solid powder is mixed with the solid particles in step (3), and then crushed; The absorbent comprises an ammonium carbonate solution; the mass concentration of the ammonium carbonate solution is 10-40%.

10. The preparation method according to any one of claims 1 to 9, wherein: The crushing pressure in step (3) is 0.2-0.6 MPa.

11. The preparation method according to any one of claims 1 to 10, wherein: The preparation method comprises the following steps: (1) mixing an ammonium ferrocyanide solution having an iron ion concentration of 100-200 g / L and a phosphorus source to obtain an iron-phosphorus mixed solution, wherein the molar ratio of iron element to phosphorus element in the iron-phosphorus mixed solution is (0.98-1.02):1, and the phosphorus source comprises any one of phosphoric acid, diammonium phosphate or diammonium hydrogen phosphate, or a combination of at least two thereof; (2) spray pyrolysis of the iron-phosphorus mixed solution obtained in step (1) at a flow rate of 1.5-1.8 mL / min, using air with a flow rate of 20-30 L / min as a carrier gas, at a temperature of 450-650° C., wherein the average particle size of the atomized droplets of the iron-phosphorus mixed solution in the spray pyrolysis is 5-15 μm, to obtain solid particles, wherein the average particle size of the solid particles is 3-10 μm; The smoke after the spray pyrolysis is subjected to gas-solid separation to obtain solid powder and tail gas, the tail gas is absorbed by an ammonium carbonate solution with a mass concentration of 10-40% to obtain an absorption liquid, the absorption liquid is mixed with an iron salt to obtain regenerated ammonium ferrocyanide, and the regenerated ammonium ferrocyanide is reused in step (1) to prepare an iron-phosphorus mixed solution; (3) Mixing the solid particles and solid powder obtained in step (2), and crushing them at 0.2-0.6 MPa to obtain iron phosphate.

12. Ferric phosphate obtained by the method for preparing ferric phosphate according to any one of claims 1 to 11.

13. Use of the iron phosphate according to claim 12 to prepare lithium iron phosphate positive electrode material.

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