Iron(III) phosphate material, and preparation method therefor and use thereof

Abstract

The present disclosure provides an iron(III) phosphate material, and a preparation method therefor and a use thereof. The preparation method comprises the following steps: (1) mixing 1,3,6,8-tetra(4-carboxyphenyl)pyrene with water, and adjusting the pH to induce reaction to obtain an organic framework solution; (2) mixing an iron salt with the organic framework solution, stirring to obtain a mixed solution, and mixing the mixed solution with a phosphoric acid source for reaction to obtain an iron(III) phosphate hydrate; and (3) aging the iron(III) phosphate hydrate, and performing sintering to obtain the iron(III) phosphate material. Flaky iron(III) phosphate is prepared in the present disclosure, and controllable morphology and in-situ carbon coating of lithium iron phosphate particles are achieved while improving the electronic conductivity and the ion migration capability of lithium iron phosphate.

Description

A kind of iron phosphate material and its preparation method and application Technical Field

[0001] The present disclosure belongs to the technical field of battery materials and relates to an iron phosphate material and a preparation method and application thereof. Background Art

[0002] In 1996, Japan's NTT first proposed Li with olivine structure. x MPO4 is a lithium-ion battery positive electrode material, where M is a complex of cobalt and iron. The United States and Japan have conducted a lot of basic research and have successively discovered lithium battery active materials with olivine structures, which has attracted the attention of many researchers and made this type of material a research focus. Compared with other lithium-ion battery positive electrode active materials, lithium iron phosphate has significant advantages: a specific capacity of approximately 170mAh / g and high initial charge and discharge efficiency; a flat voltage platform during charge and discharge, good cycle life, iron is a non-strategic metal, has abundant reserves, and is inexpensive; it is environmentally friendly and pollution-free. These advantages make it widely favored in the fields of electric vehicles, off-grid solar energy storage power stations, and power grid peak shaving and valley filling. It is considered to be the new generation of lithium-ion batteries with the greatest potential to replace traditional petrochemical energy products.

[0003] However, there are still some problems that need to be solved. The low electronic conductivity and ion migration ability of LiFePO4 affect its large-scale application in high-power and low-temperature environments.

[0004] CN110294466A discloses a method for preparing nano-flaky iron phosphate, which comprises slowly adding iron black to phosphoric acid with a concentration of 1.5 to 3 mol / L, followed by stirring and heating for reaction. After the reaction is completed, the nano-flaky iron phosphate is obtained through the steps of filtration, washing, drying and dehydration.

[0005] CN102079516A discloses a method for preparing flaky iron phosphate crystals: A solution A prepared with a phosphorus source, an iron source, an oxidant, and additive 1, and a solution B prepared with a mixture of sodium hydroxide, ammonia, or both, and additive 2 are introduced into a reactor equipped with a stirring and heating system. The temperature is controlled within the range of 50-80°C, and the pH is controlled within the range of 1-5. Pure anhydrous iron phosphate is thus obtained.

[0006] The flaky iron phosphate obtained by the above scheme has an uneven morphology and poor stability. It breaks and collapses during the subsequent preparation of lithium iron phosphate, affecting the performance of the obtained lithium iron phosphate.

[0007] Summary of the Invention

[0008] 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.

[0009] The purpose of the present disclosure is to provide an iron phosphate material, a preparation method and application thereof. The present disclosure prepares a flaky iron phosphate, which improves the electronic conductivity and ion migration ability of lithium iron phosphate while achieving controllable morphology of lithium iron phosphate particles and in-situ carbon coating.

[0010] To achieve this purpose, the present disclosure adopts the following technical solutions:

[0011] In a first aspect, the present disclosure provides a method for preparing an iron phosphate material, the preparation method comprising the following steps:

[0012] (1) mixing 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene with water, adjusting the pH to induce a reaction, and obtaining an organic skeleton solution;

[0013] (2) mixing the iron salt with the organic framework solution, stirring to obtain a mixed solution, and mixing the mixed solution with a phosphoric acid source to react to obtain hydrated iron phosphate;

[0014] (3) After aging the hydrated iron phosphate, sintering is performed to obtain the iron phosphate material.

[0015] The present invention utilizes the property of 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene that a hydrogen organic skeleton grows under acidic conditions. After induction, iron salt is added due to the adsorption of iron ions by -COOH. After the addition of phosphate, iron phosphate crystals are induced to grow in a sheet-like manner. After a heating reaction, a hydrated iron phosphate precipitate is obtained. During the aging process, the high temperature causes the hydrogen bonds in the hydrogen organic skeleton to be destroyed and the deprotonation of the carboxylic acid to be induced. Under low temperature conditions, the protons will cause the hydrogen bonds to re-associate and the crystals to be reconstructed, which can increase the looseness of the iron phosphate material to a certain extent.

[0016] In one embodiment, the 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene in step (1) is prepared by the following method:

[0017] Potassium hydroxide and a solvent are mixed to obtain a mixed solution, 4,4',4",4"'-(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid tetramethyl ester is mixed with the mixed solution and subjected to reflux reaction, and the organic solvent is removed under vacuum conditions to obtain the 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene.

[0018] The structural formula of 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene prepared in the present invention is as follows:

[0019] After the hydrogen organic skeleton of 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene disclosed in the present invention is calcined at high temperature to form lithium iron phosphate, a carbon structure is formed inside the skeleton, thereby increasing the electronic conductivity.

[0020] In one embodiment, the mass ratio of potassium hydroxide to tetramethyl 4,4',4",4"'-(pyrene-1,3,6,8-tetrayl)tetrabenzoate is (0.8-1.2):1, for example: 0.8:1, 0.9:1, 1:1, 1.1:1 or 1.2:1.

[0021] In one embodiment, the solvent comprises tetrahydrofuran, 1,4-dioxane and water.

[0022] In one embodiment, the volume ratio of tetrahydrofuran, 1,4-dioxane and water is (4-6):(1-3):2, for example: 4:1:2, 5:2:2, 5:1:2, 6:3:2 or 6:2:2, etc.

[0023] In one embodiment, the mass volume ratio of the tetramethyl 4,4',4",4"'-(pyrene-1,3,6,8-tetrayl)tetrabenzoate to the mixed solution is 1:(80-120) g / mL, for example: 1:80 g / mL, 1:90 g / mL, 1:100 g / mL, 1:110 g / mL or 1:120 g / mL.

[0024] In one embodiment, the temperature of the reflux reaction is 80-90°C, for example, 80°C, 82°C, 85°C, 88°C or 90°C.

[0025] In one embodiment, the reflux reaction time is 6 to 24 hours, for example, 6 hours, 12 hours, 15 hours, 20 hours or 24 hours.

[0026] In one embodiment, the pH in step (1) is 1.5 to 2, for example, 1.5, 1.6, 1.8, 1.9 or 2.

[0027] In one embodiment, the induction reaction time is 10 to 30 minutes, for example, 10 minutes, 15 minutes, 20 minutes, 25 minutes or 30 minutes.

[0028] In one embodiment, the iron salt in step (2) includes any one of ferric nitrate, ferric chloride or ferric sulfate, or a combination of at least two thereof.

[0029] In one embodiment, the molar ratio of the iron salt to 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene in the organic skeleton solution is 1:(0.05-0.2), for example: 1:0.05, 1:0.08, 1:0.1, 1:0.15 or 1:0.2.

[0030] In one embodiment, the stirring speed is 300-1000 rpm, for example, 300 rpm, 500 rpm, 600 rpm, 800 rpm or 1000 rpm.

[0031] In one embodiment, the phosphoric acid source in step (2) includes any one of ammonium hydrogen phosphate, ammonium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate or sodium dihydrogen phosphate, or a combination of at least two thereof.

[0032] In one embodiment, the molar ratio of phosphorus in the phosphoric acid source to iron in the mixed solution is (1-1.5):1, for example: 1:1, 1.2:1, 1.3:1, 1.4:1 or 1.5:1.

[0033] In one embodiment, the reaction temperature in step (2) is 60-100°C, for example, 60°C, 70°C, 80°C, 90°C or 100°C.

[0034] In one embodiment, the reaction time is 2 to 6 hours, for example, 2 hours, 3 hours, 4 hours, 5 hours or 6 hours.

[0035] During the precipitation process of the heating reaction, iron phosphate crystals grow into sheets under the induction of the hydrogen-organic framework, but the structural part of the hydrogen-organic framework coated in the iron phosphate breaks and collapses due to the influence of high temperature. Therefore, during the aging process at room temperature, phosphoric acid can re-associate the broken hydrogen bonds. As a result, in the process of hydrogen bond re-association, the hydrogen-organic framework recrystallizes, resulting in a certain degree of looseness inside the iron phosphate.

[0036] In one embodiment, the treating agent for the aging treatment in step (3) includes phosphoric acid.

[0037] In one embodiment, the aging time is 4 to 15 hours, for example, 4 hours, 6 hours, 8 hours, 10 hours or 15 hours, etc., and can be optionally 8 to 15 hours.

[0038] In one embodiment, the aging treatment temperature is 20-30°C, for example, 20°C, 22°C, 25°C, 28°C or 30°C.

[0039] In one embodiment, the aging is followed by filtering, washing and drying.

[0040] During the aging process disclosed herein, the hydrogen organic skeleton destroyed during the high-temperature precipitation process is re-associated and the crystals are partially reconstructed, thereby causing the interior of the iron phosphate to be loose to a certain extent.

[0041] In one embodiment, the sintering temperature in step (3) is 400-800°C, for example, 400°C, 500°C, 600°C, 700°C or 800°C.

[0042] In one embodiment, the sintering time is 3 to 7 hours, for example, 3 hours, 4 hours, 5 hours, 6 hours or 7 hours.

[0043] In a second aspect, the present disclosure provides an iron phosphate material, which is prepared by the method described in the first aspect.

[0044] The iron phosphate material prepared by the method disclosed herein is flaky iron phosphate, which can shorten the transmission path of lithium ions. During the preparation process, the hydrogen organic skeleton of 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene recrystallizes during the aging process, causing the interior of the iron phosphate to loosen, resulting in an internally loose iron phosphate.

[0045] In a third aspect, the present disclosure provides a lithium iron phosphate, which is prepared by mixing and sintering the iron phosphate material as described in the second aspect with a lithium source.

[0046] Compared with the prior art, the present disclosure has the following beneficial effects:

[0047] (1) The present invention uses 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene having a hydrogen organic skeleton, and after inducing its growth, iron salt is added. Due to the adsorption of iron ions by -COOH, phosphate is added, and the iron phosphate crystals are induced to grow in a sheet-like manner. During the aging process, the high temperature causes the hydrogen bonds in the hydrogen organic skeleton to be destroyed and the deprotonation of the carboxylic acid is induced. Under low temperature conditions, the protons will cause the hydrogen bonds to re-associate and the crystals to reconstruct, which can increase the looseness of the iron phosphate material, thereby producing lithium iron phosphate with high electronic conductivity and ion migration ability.

[0048] (2) The 0.2C discharge specific capacity of the battery made of the iron phosphate material prepared by the method described in the present disclosure can reach more than 159mAh / g, the 1C discharge specific capacity can reach more than 149mAh / g, the 3C discharge specific capacity can reach more than 138mAh / g, and the 10C discharge specific capacity can reach more than 129mAh / g. The 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene prepared under different conditions has different effects in the iron phosphate precipitation process.

[0049] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0051] FIG1 is a SEM image of the iron phosphate material prepared in Example 1 of the present disclosure. DETAILED DESCRIPTION

[0052] 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.

[0053] Example 1

[0054] This embodiment provides an iron phosphate material, and the preparation method of the iron phosphate material is as follows:

[0055] (1) Tetrahydrofuran, 1,4-dioxane and water are mixed in a volume ratio of 5:2:2 to obtain a solvent, potassium hydroxide is added (the mass ratio of potassium hydroxide to 4,4',4",4"'-(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid tetramethyl ester is 1:1) to obtain a mixed solution, and then 4,4',4",4"'-(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid tetramethyl ester is mixed with the mixed solution in a mass-to-volume ratio of 1:100, refluxed at 85° C. for 12 h, the organic solvent is removed under vacuum, an aqueous solution is added and stirred, the pH is adjusted to 1.5 with hydrochloric acid, and the hydrogen organic skeleton is induced to grow for 15 min to obtain an organic skeleton solution;

[0056] (2) Add ferric chloride solution while stirring at 400 rpm. The molar ratio of ferric salt to 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene is 1:0.1. Then add ammonium hydrogen phosphate solution to the above solution, gradually raise the temperature to 70°C, and react for 4 hours, ensuring that the molar ratio of phosphorus to iron is 1:1, to obtain hydrated ferric phosphate.

[0057] (3) The hydrated iron phosphate is added to an aging kettle, and phosphoric acid is added to age the mixture for 10 hours at room temperature. After filtering, washing, and drying, the mixture is calcined at 600° C. for 5 hours in a nitrogen atmosphere to obtain the iron phosphate material.

[0058] The SEM image of the iron phosphate material is shown in FIG1 .

[0059] Example 2

[0060] This embodiment provides an iron phosphate material, and the preparation method of the iron phosphate material is as follows:

[0061] (1) Tetrahydrofuran, 1,4-dioxane and water are mixed in a volume ratio of 4:1:2 to obtain a solvent, potassium hydroxide is added (the mass ratio of potassium hydroxide to 4,4',4",4"'-(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid tetramethyl ester is 1:1) to obtain a mixed solution, and then 4,4',4",4"'-(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid tetramethyl ester is mixed with the mixed solution in a mass-to-volume ratio of 1:80, refluxed at 80° C. for 24 h, the organic solvent is removed under vacuum, an aqueous solution is added and stirred, the pH is adjusted to 1.8 with hydrochloric acid, and the hydrogen organic skeleton is induced to grow for 15 min to obtain an organic skeleton solution;

[0062] (2) Add ferric chloride solution while stirring at 300 rpm. The molar ratio of ferric salt to 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene is 1:0.05. Then add ammonium hydrogen phosphate solution to the above solution, gradually raise the temperature to 60°C, and react for 6 hours, ensuring that the molar ratio of phosphorus to iron is 1:1, to obtain hydrated ferric phosphate.

[0063] (3) The hydrated iron phosphate is added to an aging kettle, and phosphoric acid is added and aged for 8 hours at 20°C. After filtering, washing, and drying, the hydrated iron phosphate is calcined at 400°C for 7 hours in a nitrogen atmosphere to obtain the iron phosphate material.

[0064] Example 3

[0065] This embodiment provides an iron phosphate material, and the preparation method of the iron phosphate material is as follows:

[0066] (1) Tetrahydrofuran, 1,4-dioxane and water were mixed in a volume ratio of 6:3:2 to obtain a solvent, potassium hydroxide was added (the mass ratio of potassium hydroxide to 4,4',4",4"'-(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid tetramethyl ester was 1:1) to obtain a mixed solution, and then 4,4',4",4"'-(pyrene-1,3,6,8-tetrayl)tetrabenzoic acid tetramethyl ester was mixed with the mixed solution in a mass-to-volume ratio of 1:120, and the mixture was refluxed at 90° C. for 6 h. The organic solvent was removed under vacuum, an aqueous solution was added and stirred, the pH was adjusted to 2 with hydrochloric acid, and the hydrogen organic skeleton was induced to grow for 25 min to obtain an organic skeleton solution;

[0067] (2) Add ferric chloride solution while stirring at 1000 rpm. The molar ratio of ferric salt to 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene is 1:0.2. Then add ammonium hydrogen phosphate solution to the above solution, gradually raise the temperature to 100°C, and react for 2 hours, ensuring that the molar ratio of phosphorus to iron is 1:1, to obtain hydrated ferric phosphate.

[0068] (3) The hydrated iron phosphate is added to an aging kettle, and phosphoric acid is added and aged for 15 hours at 30°C. After filtering, washing, and drying, the hydrated iron phosphate is calcined at 800°C for 3 hours in a nitrogen atmosphere to obtain the iron phosphate material.

[0069] Example 4

[0070] The only difference between this embodiment and embodiment 1 is that the growth time of the induced hydrogen organic framework is 30 minutes, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0071] Example 5

[0072] The only difference between this embodiment and embodiment 1 is that the growth time of the induced hydrogen organic framework is 10 minutes, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0073] Example 6

[0074] The only difference between this embodiment and embodiment 1 is that the molar ratio of the iron salt to 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene in the organic skeleton solution is 1:0.01, and other conditions and parameters are exactly the same as those in embodiment 1.

[0075] Example 7

[0076] The only difference between this embodiment and embodiment 1 is that the molar ratio of the iron salt to 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene in the organic skeleton solution is 1:0.3, and other conditions and parameters are exactly the same as those in embodiment 1.

[0077] Example 8

[0078] The only difference between this embodiment and embodiment 1 is that the aging time is 4 hours, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0079] Example 9

[0080] The only difference between this embodiment and embodiment 1 is that the aging temperature is 70° C., and other conditions and parameters are exactly the same as those in embodiment 1.

[0081] Comparative Example 1

[0082] The only difference between this comparative example and Example 1 is that 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene is not added, and other conditions and parameters are exactly the same as those in Example 1.

[0083] Performance testing:

[0084] The iron phosphate material and Li2CO3 prepared in the embodiment and comparative example were loaded into a ball mill and wet-milled with anhydrous ethanol at a speed of 600 rpm for 5 h; glucose, lithium carbonate and the prepared iron phosphate precursor in a mass ratio of 0.05:1.05:1.0 were calcined at 600°C under a nitrogen atmosphere to obtain a lithium iron phosphate material.

[0085] The prepared lithium iron phosphate positive electrode material was mixed with a cyclohexane solution of acetylene black and polyvinylidene fluoride (PVDF) at room temperature and pressure to form a slurry (according to the positive electrode material: acetylene black: PVDF weight ratio of 75:15:10), and evenly coated on an aluminum foil substrate as the positive electrode of a simulated battery. The negative electrode of the simulated battery used a lithium sheet, and the electrolyte was 1 mol LiPF6 dissolved in a mixed solvent of 1L EC and DMC (volume ratio 1:1). The positive electrode, negative electrode, electrolyte and polypropylene porous diaphragm were assembled into a simulated battery in an argon-protected glove box. The discharge specific capacity of 0.2C, 1C, 3C and 10C was tested, and the test results are shown in Table 1:

[0086] Table 1

[0087] As can be seen from Table 1, it can be obtained from Examples 1-3 that the 0.2C discharge specific capacity of the battery made of the iron phosphate material prepared by the method described in the present disclosure can reach more than 159 mAh / g, the 1C discharge specific capacity can reach more than 149 mAh / g, the 3C discharge specific capacity can reach more than 138 mAh / g, and the 10C discharge specific capacity can reach more than 129 mAh / g. The 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene prepared under different conditions has different effects in the iron phosphate precipitation process.

[0088] By comparing Example 1 with Examples 4-5, it can be seen that during the preparation process of the iron phosphate material described in the present disclosure, the time for inducing the growth of the hydrogen organic skeleton will affect its performance. By controlling the time for inducing the growth of the hydrogen organic skeleton to 15 to 25 minutes, the performance of the iron phosphate material obtained is better. If the growth time of the hydrogen organic skeleton is long, nanoparticles will be formed, and the iron phosphate cannot be well formed into a flaky form. If the growth time of the hydrogen organic skeleton is too short, the skeleton does not grow, and the iron phosphate cannot be well formed into a flaky form.

[0089] By comparing Example 1 with Example 6-7, it can be seen that in the preparation process of the iron phosphate material disclosed in the present invention, the molar ratio of the iron salt to 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene in the organic skeleton solution will affect its performance. The molar ratio of the iron salt to 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene in the organic skeleton solution is controlled at 1:0.05 to 0.2, and the performance of the iron phosphate material obtained is better. If the molar ratio of the iron salt to 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene in the organic skeleton solution is 1:0.05 to 0.2, the performance of the iron phosphate material obtained is better. If the molar ratio of iron salt to 1,3,6,8-tetra(4-carboxyphenyl)pyrene in the organic skeleton solution is too small (the amount of 1,3,6,8-tetra(4-carboxyphenyl)pyrene is too large), the organic skeleton content is too small, the effect is weakened, and the carbon network structure finally formed is not perfect; if the molar ratio of iron salt to 1,3,6,8-tetra(4-carboxyphenyl)pyrene in the organic skeleton solution is too small (the amount of 1,3,6,8-tetra(4-carboxyphenyl)pyrene is too large), the carbon content in the iron phosphate will be too high, affecting the effect of its active substance.

[0090] From the comparison between Example 1 and Example 8, it can be seen that during the preparation process of the iron phosphate material described in the present disclosure, the aging time will affect its performance. If the aging time is too short, the hydrogen organic framework inside the iron phosphate particles will not be able to re-crystallize and will not be loose enough.

[0091] From the comparison between Example 1 and Example 9, it can be seen that during the preparation process of the iron phosphate material described in the present disclosure, the aging temperature will affect its performance. If the aging temperature is too high, the hydrogen organic framework will not re-associate and transform, and the iron phosphate will not be loose enough.

[0092] By comparing Example 1 and Comparative Example 1, it can be seen that the present disclosure utilizes the property of 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene that the hydrogen organic skeleton grows under acidic conditions. After induction, the iron salt is added due to the adsorption of iron ions by -COOH, and then the phosphate is added, so that the iron phosphate crystals are induced to grow in a sheet-like shape, and then after a heating reaction, a hydrated iron phosphate precipitate is obtained; during the aging process, the high temperature causes the hydrogen bonds in the hydrogen organic skeleton to be destroyed and the deprotonation of the carboxylic acid is induced. Under low temperature conditions, the protons will cause the hydrogen bonds to re-associate and the crystals to reconstruct, which can increase the looseness of the iron phosphate material to a certain extent.

Claims

1. A preparation method of iron phosphate material, comprising the following steps: (1) Mix 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene with water, adjust the pH to carry out an induction reaction to obtain an organic framework solution; (2) Mix an iron salt with the organic framework solution, stir to obtain a mixed solution, mix the mixed solution with a phosphorus source, and carry out a reaction to obtain hydrated iron phosphate; (3) After aging the hydrated iron phosphate, sinter it to obtain the iron phosphate material.

2. The preparation method according to claim 1, wherein, the 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene in step (1) is prepared by the following method: Mix potassium hydroxide with a solvent to obtain a mixed solution, take tetramethyl 4,4',4”,4”'-(pyrene-1,3,6,8-tetrayl)tetrabenzoate and mix it with the mixed solution for reflux reaction, and remove the organic solvent under vacuum conditions to obtain the 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene.

3. The preparation method according to claim 2, wherein, the mass ratio of the potassium hydroxide to tetramethyl 4,4',4”,4”'-(pyrene-1,3,6,8-tetrayl)tetrabenzoate is (0.8 - 1.2):

1.

4. The preparation method according to claim 2 or 3, wherein, the solvent includes tetrahydrofuran, 1,4-dioxane and water.

5. The preparation method according to claim 4, wherein, the volume ratio of the tetrahydrofuran, 1,4-dioxane and water is (4 - 6):(1 - 3):

2.

6. The preparation method according to any one of claims 2 - 5, wherein, the mass-volume ratio of the tetramethyl 4,4',4”,4”'-(pyrene-1,3,6,8-tetrayl)tetrabenzoate to the mixed solution is 1:(80 - 120) g / mL.

7. The preparation method according to any one of claims 2 - 6, wherein, the temperature of the reflux reaction is 80 - 90 °C; Optionally, the time of the reflux reaction is 6 - 24 h.

8. The preparation method according to any one of claims 1 - 7, wherein, the pH in step (1) is 1.5 - 2; Optionally, the time of the induction reaction is 10 - 30 min.

9. The preparation method according to any one of claims 1 - 8, wherein, the time of the induction reaction is 15 - 25 min.

10. The preparation method according to any one of claims 1 - 9, wherein, the iron salt in step (2) includes any one or a combination of at least two of iron nitrate, iron chloride or iron sulfate; Optionally, the molar ratio of the iron salt to 1,3,6,8-tetrakis(4-carboxyphenyl)pyrene in the organic framework solution is 1:(0.05 - 0.2); Optionally, the stirring speed is 300 - 1000 rpm.

11. The preparation method according to any one of claims 1 - 10, wherein, the phosphorus source in step (2) includes any one or a combination of at least two of ammonium hydrogen phosphate, ammonium dihydrogen phosphate, sodium phosphate, disodium hydrogen phosphate or sodium dihydrogen phosphate; Optionally, the molar ratio of the phosphorus element in the phosphorus source to the iron element in the mixed solution is (1 - 1.5):

1.

12. The preparation method according to any one of claims 1-11, wherein, the temperature of the reaction in step (2) is 60-100 °C; optionally, the reaction time is 2-6 h.

13. The preparation method according to any one of claims 1-12, wherein, the treatment agent for the aging treatment in step (3) includes phosphoric acid; optionally, the temperature of the aging treatment is 20-30 °C; optionally, the aging time is 4-15 h; optionally, filtration, washing and drying treatments are carried out after aging.

14. The preparation method according to any one of claims 1-13, wherein, the aging time of the aging treatment in step (3) is 8-15 h.

15. The preparation method according to any one of claims 1-14, wherein, the sintering temperature in step (3) is 400-800 °C; optionally, the sintering time is 3-7 h.

16. An iron phosphate material prepared by the method according to any one of claims 1-15.

17. Lithium iron phosphate prepared by mixing and sintering the iron phosphate material according to claim 16 with a lithium source.

Images