Modified iron(III) phosphate, preparation method therefor and use thereof

Abstract

A method for preparing modified iron(III) phosphate, comprising the following steps: (1) preparing NH2-MIL-53(Al) nanoparticles, mixing the nanoparticles with a solvent, adding an acid solution, and heating and stirring the mixture to obtain hollow NH2-MIL-53(Al) powder; (2) using the hollow NH2-MIL-53(Al) powder to prepare a dispersion, mixing the dispersion with an iron salt solution, stirring and then drying the mixture, mixing the resulting powder with a phosphorus source solution, controlling the pH, and carrying out a reaction; and (3) carrying out sintering treatment on a material resulting from aging to obtain the modified iron(III) phosphate. Provided are iron(III) phosphate prepared according to the method and a positive electrode material. In the method, a hollow metal organic framework nanomaterial is used as a template to provide a micro reaction area for the synthesis of iron(III) phosphate, so that the microscopic morphology of the prepared iron(III) phosphate is restrained and controlled to prepare nanoscale iron(III) phosphate having a polyhedral structure.

Description

A modified ferric phosphate and its preparation method and application Technical Field

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

[0002] As the global energy crisis intensifies, new energy batteries have rapidly developed as a clean and efficient means of energy storage and utilization. Lithium-ion batteries, with their advantages such as high energy density, long cycle life, and lightweight construction, are widely used in applications ranging from electric vehicles to smartphones. Currently, among the cathode materials used in lithium batteries, olivine-type lithium iron phosphate (LiFePO4) has attracted considerable attention due to its low cost, environmental friendliness, excellent cycle performance, and high safety. However, defects inherent in the LiFePO4 structure, such as poor conductivity and slow lithium ion diffusion, have limited its further practical application. To address this issue, LiFePO4 is often modified through carbon coating, doping, and particle size reduction to enhance its electrical properties. Iron phosphate (FePO4) is a key precursor for the preparation of lithium iron phosphate. Parameters such as its purity, specific surface area, micromorphology, and particle size distribution directly influence the electrochemical performance of the cathode material. Therefore, the preparation of iron phosphate with excellent structure and performance is crucial for the synthesis of high-quality lithium iron phosphate.

[0003] In the existing technology for preparing lithium iron phosphate, the ferrophosphate process route is to use iron phosphate as a precursor, evenly mix it with a lithium source and a carbon source, and then synthesize the LiFePO4 / C positive electrode material through processes such as grinding, drying, and high-temperature sintering. The preparation of the precursor iron phosphate mostly adopts the liquid phase co-precipitation method, but the following problems may arise during the precipitation process, such as the particle growth is difficult to control, the obtained product particle size may be too large, and the specific surface area may be too small, which is not conducive to the rapid deintercalation of lithium ions, and thus affects the electrochemical activity of the positive electrode material. In recent years, the industrial production of lithium iron phosphate has become more and more stringent in terms of particle size control, because a large number of studies have shown that lithium iron phosphate with uniform morphology or smaller particles has better electrochemical performance. Therefore, lithium iron phosphate nanomaterials are becoming mainstream.

[0004] Summary of the Invention

[0005] 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.

[0006] The purpose of the present disclosure is to provide a modified iron phosphate and its preparation method and application. The present disclosure uses the hollow metal organic framework NH2-MIL-53(Al) nanomaterial after acid etching as a template to provide a small reaction area for the synthesis of iron phosphate, constrains and controls the micromorphology of the prepared iron phosphate, and prepares a nanoscale iron phosphate with a polyhedral structure.

[0007] To achieve this purpose, the present disclosure adopts the following technical solutions:

[0008] In a first aspect, the present disclosure provides a method for preparing modified ferric phosphate, the preparation method comprising the following steps:

[0009] (1) mixing an aluminum salt and sodium dodecylbenzenesulfonate with a first solvent to obtain a first solution, mixing 2-aminoterephthalic acid with a second solvent to obtain a second solution, mixing the first solution and the second solution, heating and reacting to obtain NH2-MIL-53(Al) nanoparticles, mixing the NH2-MIL-53(Al) nanoparticles with a third solvent, adding an acid solution, heating and stirring, and obtaining hollow NH2-MIL-53(Al) powder;

[0010] (2) preparing the hollow NH2-MIL-53(Al) powder into an NH2-MIL-53(Al) dispersion, mixing the dispersion with an iron salt solution, stirring, and drying, mixing the obtained powder with a phosphorus source solution, controlling the pH, and reacting;

[0011] (3) Sintering the material obtained after aging to obtain the modified iron phosphate.

[0012] The hollow NH2-MIL-53(Al) nanomaterial is used as a template. The prepared iron phosphate precursor retains the polyhedral morphology of the template and has a small particle size. In the synthesis process of iron phosphate, the hollow NH2-MIL-53(Al) shell template can be used as a micro-reaction container to constrain the morphology of the internal synthetic material. 3+ PO4 3- Under suitable reaction conditions, the two ions can be gradually transformed into FePO4 materials with uniform morphology inside the NH2-MIL-53(Al) shell, effectively controlling the growth of particles.

[0013] In one embodiment, the aluminum salt in step (1) comprises aluminum nitrate.

[0014] In one embodiment, the first solvent comprises water.

[0015] In one embodiment, the mixing and stirring time of the first solution is 3 to 5 hours, for example, 3 hours, 3.5 hours, 4 hours, 4.5 hours or 5 hours.

[0016] In one embodiment, the molar concentration of the aluminum salt in the first solution is 0.05 to 0.1 mol / L, for example, 0.05 mol / L, 0.06 mol / L, 0.07 mol / L, 0.08 mol / L or 0.1 mol / L.

[0017] In one embodiment, the mass concentration of sodium dodecylbenzenesulfonate in the first solution is 10-15%, for example, 10%, 11%, 12%, 14% or 15%.

[0018] In one embodiment, the second solvent comprises DMF.

[0019] In one embodiment, the molar concentration of 2-aminoterephthalic acid in the second solution is 0.025 to 0.05 mol / L, for example, 0.025 mol / L, 0.028 mol / L, 0.03 mol / L, 0.04 mol / L or 0.05 mol / L.

[0020] In one embodiment, the volume ratio of the first solution to the second solution is 1:(1.5-2.5), for example: 1:1.5, 1:1.8, 1:2, 1:2.2 or 1:2.5, etc.

[0021] In one embodiment, the temperature of the heating reaction in step (1) is 150-180°C, for example, 150°C, 155°C, 160°C, 170°C or 180°C.

[0022] In one embodiment, the heating reaction time is 24 to 48 hours, for example, 24 hours, 28 hours, 32 hours, 40 hours or 48 hours.

[0023] In one embodiment, the heating reaction is followed by cooling, washing and drying.

[0024] In one embodiment, the third solvent in step (1) comprises anhydrous ethanol.

[0025] In one embodiment, the acid solution includes any one of phytic acid, tannic acid, or lauric acid, or a combination of at least two thereof.

[0026] In one embodiment, the concentration of the acid solution is 50-100 g / L, for example, 50 g / L, 60 g / L, 80 g / L, 90 g / L or 100 g / L.

[0027] In one embodiment, the mass ratio of the NH2-MIL-53(Al) nanoparticles to the solute in the acid solution is 1:(1.5-2.5), for example: 1:1.5, 1:1.8, 1:2, 1:2.2 or 1:2.5, etc.

[0028] In one embodiment, the temperature of the heating and stirring in step (1) is 90-110°C, for example, 90°C, 95°C, 100°C, 105°C or 110°C.

[0029] In one embodiment, the heating and stirring time is 8 to 12 hours, for example, 8 hours, 9 hours, 10 hours, 11 hours or 12 hours.

[0030] In one embodiment, the heating and stirring are followed by centrifugation, washing and drying.

[0031] In one embodiment, the solvent of the NH2-MIL-53(Al) dispersion in step (2) includes water.

[0032] In one embodiment, the solute of the iron salt solution includes ferric nitrate and / or ferric chloride.

[0033] In one embodiment, the concentration of the iron salt solution is 0.1 to 0.3 mol / L, for example, 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L or 0.3 mol / L.

[0034] In one embodiment, the mass concentration ratio of NH2-MIL-53(Al) and iron ions after mixing is (0.8-1.2):1, for example: 0.8:1, 0.9:1, 1:1, 1.1:1 or 1.2:1, etc.

[0035] In one embodiment, the stirring time is 3 to 5 hours, for example, 3 hours, 3.5 hours, 4 hours, 4.5 hours or 5 hours.

[0036] In one embodiment, the solute of the phosphorus source solution in step (2) includes any one of ammonium dihydrogen phosphate, ammonium hydrogen phosphate or sodium hydrogen phosphate, or a combination of at least two thereof.

[0037] In one embodiment, the concentration of phosphorus in the phosphorus source solution is 0.1 to 0.3 mol / L, for example, 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L or 0.3 mol / L.

[0038] In one embodiment, the molar ratio of phosphorus in the phosphorus source solution to iron in the iron salt solution is 1:(0.97-1.05), for example: 1:0.97, 1:0.99, 1:1, 1:1.02 or 1:1.05.

[0039] In one embodiment, the method of controlling pH comprises adding an alkaline solution.

[0040] In one embodiment, the alkaline solution comprises aqueous ammonia and / or sodium hydroxide solution.

[0041] In one embodiment, the pH is 1.5 to 2.2, for example, 1.5, 1.6, 1.8, 2 or 2.2.

[0042] In one embodiment, heating and stirring are performed during the reaction.

[0043] In one embodiment, the heating and stirring temperature is 70-90°C, for example, 70°C, 75°C, 80°C, 85°C or 90°C.

[0044] In one embodiment, the heating and stirring time is 4 to 10 hours, for example, 4 hours, 5 hours, 7 hours, 8 hours or 10 hours.

[0045] In one embodiment, the heating and stirring are followed by aging.

[0046] In one embodiment, the aging time is 1 to 2 hours, for example, 1 hour, 1.2 hours, 1.5 hours, 1.8 hours or 2 hours.

[0047] In one embodiment, the temperature of the sintering treatment in step (3) is 500-750°C, for example, 500°C, 520°C, 550°C, 600°C or 750°C.

[0048] In one embodiment, the sintering treatment time is 4 to 10 hours, for example, 4 hours, 5 hours, 7 hours, 8 hours or 10 hours.

[0049] In one embodiment, the sintering process comprises a nitrogen atmosphere.

[0050] In the sintering process disclosed in the present invention, the nitrogen-containing organic ligands of the NH2-MIL-53(Al) template will undergo carbonization under high temperature conditions under a nitrogen atmosphere, forming a thin layer of nitrogen-doped carbon coated on the surface of the FePO4 material. Nitrogen doping can improve the electrochemical activity of the carbon material. Compared with using only a carbon layer coating, the electronic conductivity of the material coated with the nitrogen-doped carbon layer can be further enhanced.

[0051] In one embodiment, the sintering process is followed by a washing process.

[0052] In one embodiment, the washing treatment includes acetic acid washing and water washing.

[0053] In a second aspect, the present disclosure provides a modified ferric phosphate, which is prepared by the method described in the first aspect.

[0054] The modified iron phosphate particles prepared in the present invention have a smaller particle size and a larger specific surface area. The size of the lithium iron phosphate prepared therefrom is also smaller, which is beneficial to increase the contact area between the electrode material and the electrolyte, shorten the diffusion path of lithium ions, and increase the diffusion rate of lithium ions, thereby enhancing the electrochemical activity of the battery material.

[0055] In a third aspect, the present disclosure provides a lithium iron phosphate positive electrode material, which is prepared by mixing and sintering the modified iron phosphate as described in the second aspect with a lithium source.

[0056] Compared with the prior art, the present disclosure has the following beneficial effects:

[0057] (1) The template NH2-MIL-53(Al) used in the present disclosure has the high specific surface area characteristic of MOF materials, so its own adsorption capacity is relatively good. After the material is etched into a hollow structure, its specific surface area will increase, the number of active adsorption sites will increase, and its adsorption capacity will be enhanced accordingly. The present disclosure first places the hollow NH2-MIL-53(Al) material after acid etching in an iron salt solution to adsorb Fe 3+ Then the powder was collected and vacuum dried and then placed in a phosphate solution. At this time, Fe 3+ The NH2-MIL-53(Al) material will continue to adsorb PO4 3- , at a specific pH value, iron phosphate materials with uniform MOF morphology were in situ synthesized, which could be evenly dispersed and had small particle size.

[0058] (2) The modified iron phosphate particles prepared in the present invention are fine powders. Compared with the iron phosphate agglomerates prepared in most existing technologies, the iron phosphate precursor prepared in the present invention does not require a crushing step in the subsequent synthesis of lithium iron phosphate, which simplifies the process of preparing iron phosphate positive electrode materials.

[0059] (3) The modified iron phosphate prepared in the present invention has good performance indicators and regular morphology. The lithium iron phosphate positive electrode material prepared therefrom also exhibits excellent electrochemical performance after being assembled into a lithium battery. The first charge and discharge capacity of LiFePO4 / C at room temperature can reach up to 158.6 mAh / g; the first charge and discharge efficiency can reach 99.2%; and the discharge capacity retention rate can still reach 99.5% after 300 cycles at 1C at 45°C.

[0060] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] 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.

[0062] FIG1 is a schematic diagram of a process for preparing modified ferric phosphate according to an embodiment of the present disclosure.

[0063] FIG2 is a SEM image of the modified ferric phosphate prepared in Example 1 of the present disclosure. DETAILED DESCRIPTION

[0064] 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.

[0065] Example 1

[0066] This embodiment provides a modified iron phosphate. The schematic diagram of the preparation process of the modified iron phosphate is shown in FIG1 . The modified iron phosphate is prepared by the following method:

[0067] (1) 2 mmol of Al(NO₃)₃·9H₂O was dissolved in 20 mL of deionized water and ultrasonically treated for 10 min. 10 wt% of sodium dodecylbenzenesulfonate was then added and stirred to obtain solution A. 2 mmol of 2-aminoterephthalic acid (NH₂-H₂BDC) was dissolved in 40 mL of dimethylformamide (DMF) and ultrasonically treated to obtain solution B. The two solutions were mixed and transferred to a 100 mL autoclave, which was then heated in an oven at 150°C for 48 h. After cooling to room temperature, the NH₂-MIL-53(Al) product was collected by centrifugation and washed three times with DMF and then with anhydrous ethanol. Subsequently, the sample was vacuum-dried at 120°C for 12 h to obtain NH2-MIL-53(Al) nanoparticles. 0.5 g of NH2-MIL-53(Al) nanoparticles was dispersed in 20 mL of anhydrous ethanol and ultrasonically treated to obtain dispersion A. 13 mL of phytic acid solution (80 g / L) was dropwise added to dispersion A and stirred uniformly at room temperature. The mixture was then placed in a 50 mL autoclave and heated in a 90°C oven for 8 h. After the reaction, the mixture was washed with anhydrous ethanol and deionized water, and dried in a vacuum oven at 120°C for 12 h to obtain hollow NH2-MIL-53(Al) powder.

[0068] (2) 0.5 g of hollow NH2-MIL-53(Al) powder was ultrasonically dispersed in 20 mL of deionized water, then poured into 100 mL of 0.1 mol / L ferric chloride solution and stirred at room temperature for 3 h (the mass ratio of NH2-MIL-53(Al) to Fe was 0.89:1). The product was collected by centrifugation and dried in an oven at 100 °C for 12 h. Subsequently, the dried powder was dispersed in 100 mL of 0.1 mol / L ammonium hydrogen phosphate solution, and 25% ammonia water was slowly added dropwise under stirring to control the pH value of the solution to 1.8. Subsequently, the mixture was heated at 70 °C, stirred continuously and kept warm for 6 h (speed of 400 rpm), and then allowed to stand for 1 h. The precipitate was collected and washed several times with deionized water, and then dried in a vacuum oven at 100 °C for 12 h to obtain a crude modified iron phosphate product.

[0069] (3) The modified ferric phosphate crude product was placed in a muffle furnace and heated to 650°C at a rate of 5°C / min in a nitrogen atmosphere for 8 hours to obtain a nitrogen-doped carbon-coated anhydrous FePO4 crude product. Subsequently, the crude product was washed with acetic acid for 3 hours and deionized water for 1 hour to remove the residual impurity aluminum oxide in the product. After drying in a vacuum oven, the modified ferric phosphate was obtained.

[0070] The SEM image of the prepared modified iron phosphate is shown in FIG2 .

[0071] Example 2

[0072] This embodiment provides a modified iron phosphate. The schematic diagram of the preparation process of the modified iron phosphate is shown in FIG1 . The modified iron phosphate is prepared by the following method:

[0073] (1) 1 mmoL Al(NO₃)₃·9H₂O was dissolved in 20 mL of deionized water and ultrasonically treated for 10 min. 10 wt% sodium dodecylbenzenesulfonate was then added and stirred to obtain solution A. 1 mmoL NH₂-H₂BDC was dissolved in 40 mL of DMF and ultrasonically treated to obtain solution B. The two solutions were mixed and transferred to a 100 mL autoclave, which was then heated in an oven at 170°C for 36 h. After cooling to room temperature, the NH₂-MIL-53(Al) product was collected by centrifugation and washed three times with DMF and then with anhydrous ethanol. Subsequently, the sample was vacuum-dried at 120°C for 12 hours to obtain NH2-MIL-53(Al) nanoparticles. 0.8 g of NH2-MIL-53(Al) nanoparticles was dispersed in 30 mL of anhydrous ethanol and ultrasonically treated to obtain dispersion A. 15 mL of tannic acid solution (80 g / L) was dropwise added to dispersion A and stirred evenly at room temperature. The mixture was then placed in a 100 mL high-pressure reactor and heated in an oven at 90°C for 12 hours. After the reaction, the mixture was washed with anhydrous ethanol and deionized water, and dried in a vacuum oven at 120°C for 12 hours to obtain hollow NH2-MIL-53(Al) powder.

[0074] (3) 0.8 g of hollow NH2-MIL-53(Al) powder was ultrasonically dispersed in 20 mL of deionized water, and then poured into 100 mL of 0.15 mol / L ferric chloride solution and stirred at room temperature for 3 h (the mass ratio of NH2-MIL-53(Al) to Fe was 0.95:1). The product was collected by centrifugation and dried in an oven at 100 °C for 12 h. Subsequently, the dried powder was dispersed in 100 mL of 0.15 mol / L ammonium hydrogen phosphate solution and poured into the above mixture. 25% sodium hydroxide solution was slowly added dropwise under stirring to control the pH value of the solution to 1.5. Subsequently, the mixture was heated at 70 °C, stirred continuously and kept warm for 10 h (speed of 400 rpm), and then allowed to stand for 1 h. The precipitate was collected and washed several times with deionized water, and then dried in a vacuum oven at 100 °C for 12 h to obtain a crude modified iron phosphate product.

[0075] (3) The modified ferric phosphate crude product prepared above is then placed in a muffle furnace and heated to 500°C at a rate of 5°C / min in a nitrogen atmosphere and maintained at this temperature for 10 hours to obtain a crude anhydrous FePO4 product coated with a nitrogen-doped carbon layer. Subsequently, the crude product is washed with acetic acid for 3 hours and deionized water for 1 hour to remove the residual impurity aluminum oxide in the product. After drying in a vacuum oven, the modified ferric phosphate is obtained.

[0076] Example 3

[0077] This embodiment provides a modified iron phosphate. The schematic diagram of the preparation process of the modified iron phosphate is shown in FIG1 . The modified iron phosphate is prepared by the following method:

[0078] (1) 1.5 mmol Al(NO₃)₃·9H₂O was dissolved in 20 mL deionized water and sonicated for 10 min. 12 wt% sodium dodecylbenzenesulfonate was then added and stirred to obtain solution A. 1.5 mmol NH₂-H₂BDC was dissolved in 40 mL DMF and sonicated to obtain solution B. The two solutions were mixed and transferred to a 100 mL autoclave, which was then heated in an oven at 180°C for 24 h. After cooling to room temperature, the NH₂-MIL-53(Al) product was collected by centrifugation and washed three times with DMF and then with anhydrous ethanol. Subsequently, the sample was vacuum-dried at 120°C for 12 h to obtain NH2-MIL-53(Al) nanoparticles. 0.8 g of NH2-MIL-53(Al) nanoparticles was dispersed in 30 mL of anhydrous ethanol and ultrasonically treated to obtain dispersion A. 25 mL of lauric acid solution (80 g / L) was dropwise added to dispersion A and stirred uniformly at room temperature. The mixture was then placed in a 100 mL high-pressure reactor and heated in an oven at 110°C for 8 h. After the reaction, the mixture was washed with anhydrous ethanol and deionized water, and dried in a vacuum oven at 120°C for 12 h to obtain hollow NH2-MIL-53(Al) powder.

[0079] (3) 0.8 g of hollow NH2-MIL-53(Al) powder was ultrasonically dispersed in 20 mL of deionized water, and then poured into 50 mL of 0.3 mol / L ferric chloride solution and stirred at room temperature for 3 h (the mass ratio of NH2-MIL-53(Al) to Fe was 0.95:1). The product was collected by centrifugation and dried in an oven at 100°C for 12 h. Subsequently, the dried powder was dispersed in 50 mL of 0.3 mol / L ammonium hydrogen phosphate solution and poured into the above mixture. 20% sodium hydroxide solution was slowly added dropwise under stirring to control the pH value of the solution to 2.2. Subsequently, the mixture was heated at 90°C, stirred continuously and kept warm for 4 h (speed of 400 rpm), and then allowed to stand for 1 h. The precipitate was collected and washed several times with deionized water, and then dried in a vacuum oven at 100°C for 12 h to obtain a crude modified iron phosphate product.

[0080] (3) The modified ferric phosphate crude product prepared above was placed in a muffle furnace and heated to 750°C at a rate of 5°C / min in a nitrogen atmosphere and maintained at that temperature for 4 hours to obtain a crude anhydrous FePO4 product coated with a nitrogen-doped carbon layer. Subsequently, the crude product was washed with acetic acid for 3 hours and deionized water for 1 hour to remove the residual impurity aluminum oxide in the product. After drying in a vacuum oven, the modified ferric phosphate was obtained.

[0081] Example 4

[0082] The only difference between this embodiment and embodiment 1 is that the volume of the phytic acid solution is 6.25 mL (the mass ratio of NH2-MIL-53(Al) nanoparticles to the solute in the acid solution is 1:1), and the other conditions and parameters are exactly the same as those in embodiment 1.

[0083] Example 5

[0084] The only difference between this embodiment and embodiment 1 is that the volume of the phytic acid solution is 18.75 mL (the mass ratio of NH2-MIL-53(Al) nanoparticles to the solute in the acid solution is 1:3), and the other conditions and parameters are exactly the same as those in embodiment 1.

[0085] Example 6

[0086] The only difference between this embodiment and Example 1 is that the mass of NH2-MIL-53(Al) powder is 0.28 g (the mass concentration ratio of NH2-MIL-53(Al) to iron ions is 0.5:1), and the other conditions and parameters are exactly the same as those in Example 1.

[0087] Example 7

[0088] The only difference between this embodiment and Example 1 is that the mass of NH2-MIL-53(Al) powder is 0.84 g (the mass concentration ratio of NH2-MIL-53(Al) to iron ions is 1.5:1), and the other conditions and parameters are exactly the same as those in Example 1.

[0089] Comparative Example 1

[0090] The only difference between this comparative example and Example 2 is that NH2-MIL-53(Al) is not added, and the other conditions and parameters are exactly the same as those in Example 1.

[0091] Comparative Example 2

[0092] The only difference between this comparative example and Example 2 is that the hollow NH2-MIL-53(Al) is directly placed in the mixed precursor solution of iron salt and phosphate. Other conditions and parameters are exactly the same as those in Example 1.

[0093] Performance testing:

[0094] Lithium carbonate, modified iron phosphate, and glucose were dispersed in anhydrous ethanol at a molar ratio of 1:1.03:0.07 for the lithium source, iron source, and carbon source. The mixture was ball-milled for 3 hours until uniformly mixed at a speed of 4000 rpm, and then spray-dried to obtain a precursor powder. Subsequently, the precursor powder was heated to 400°C under a nitrogen atmosphere at a heating rate of 5°C / min for 1.5 hours, and then heated to 700°C for 8 hours to obtain a LiFePO4 / C positive electrode material. The obtained lithium iron phosphate positive electrode material was assembled into a button cell for lithium-ion battery electrochemical performance testing. The test results are shown in Table 1:

[0095] Table 1

[0096] As can be seen from Table 1, from Examples 1-7, the first charge and discharge specific capacity of the battery made of the modified iron phosphate disclosed in the present invention can reach more than 150.7 mAh / g, the first charge and discharge efficiency can reach more than 96.5%, and the discharge capacity retention rate after 300 cycles at 1C can reach more than 97.3%.

[0097] By comparing Example 1 and Examples 4-5, it can be seen that in the preparation process of the modified iron phosphate described in the present invention, the mass ratio of NH2-MIL-53 (Al) nanoparticles and the solute in the acid solution affects its performance. The mass ratio of NH2-MIL-53 (Al) nanoparticles and the solute in the acid solution is controlled at 1: (1.5~2.5), and the performance of the modified iron phosphate obtained is better. If the amount of acid is too large, it will cause certain damage to the structure of NH2-MIL-53 (Al), which may cause the product iron phosphate to agglomerate; if the amount of acid is too small, the hollow volume inside the etched NH2-MIL-53 (Al) is small, resulting in a smaller specific surface area and a reduction in active adsorption sites, which in turn affects the synthesis of the product iron phosphate.

[0098] By comparing Example 1 with Examples 6-7, it can be seen that in the preparation process of the modified ferric phosphate disclosed in the present invention, the mass concentration ratio of NH2-MIL-53 (Al) and iron ions affects its performance. When the mass concentration ratio of NH2-MIL-53 (Al) and iron ions is controlled at (0.8-1.2):1, the performance of the modified ferric phosphate obtained is better. If the addition amount of NH2-MIL-53 (Al) is too large or too small, it will affect its performance. 3+ and PO4 3- The adsorption of ferric phosphate affects the purity of the product.

[0099] By comparing Example 1 and Comparative Example 1, it can be seen that compared with the large-sized irregular iron phosphate agglomerated particles prepared directly by the co-precipitation method in Comparative Example 1, the iron phosphate particles prepared by the hollow template NH2-MIL-53(Al) in the present disclosure are smaller in size and more evenly dispersed, and the electrochemical performance of the lithium battery made of its positive electrode material is also better.

[0100] From the comparison between Example 1 and Comparative Example 2, it can be seen that if the hollow NH2-MIL-53(Al) template is directly placed in a mixed precursor solution of iron salt and phosphate, some Fe 3+ PO4 3- A large amount of iron phosphate with irregular morphology and easy agglomeration may be generated, which will reduce the electrochemical performance of the lithium iron phosphate positive electrode material prepared therefrom.

Claims

1. A preparation method of modified iron phosphate, comprising the following steps: (1) Mix an aluminum salt, sodium dodecylbenzenesulfonate with a first solvent to obtain a first solution, mix 2-aminoterephthalic acid with a second solvent to obtain a second solution, mix the first solution and the second solution, and heat and react to obtain NH 2 -MIL-53(Al) nanoparticles, mix the NH 2 -MIL-53(Al) nanoparticles with a third solvent, add an acid solution, heat and stir to obtain hollow NH 2 -MIL-53(Al) powder; (2) Make hollow NH 2 -MIL-53(Al) powder into an NH 2 -MIL-53(Al) dispersion, mix the dispersion with an iron salt solution, stir and then dry, mix the obtained powder with a phosphorus source solution, control the pH, and carry out the reaction; (3) Sintering the material obtained after aging to obtain the modified iron phosphate.

2. The preparation method according to claim 1, wherein the aluminum salt in step (1) includes aluminum nitrate.

3. The preparation method according to claim 1 or 2, wherein the first solvent includes water.

4. The preparation method according to any one of claims 1-3, wherein the mixing and stirring time of the first solution is 3-5 h.

5. The preparation method according to any one of claims 1-4, wherein the molar concentration of the aluminum salt in the first solution is 0.05-0.1 mol / L.

6. The preparation method according to any one of claims 1-5, wherein the mass concentration of sodium dodecylbenzenesulfonate in the first solution is 10-15%.

7. The preparation method according to any one of claims 1-6, wherein the second solvent includes DMF.

8. The preparation method according to any one of claims 1-7, wherein the molar concentration of 2-aminoterephthalic acid in the second solution is 0.025-0.05 mol / L.

9. The preparation method according to any one of claims 1-8, wherein the volume ratio of the first solution to the second solution is 1:(1.5-2.5).

10. The preparation method according to any one of claims 1-9, wherein the temperature of the heating reaction in step (1) is 150-180 °C; Optionally, the time of the heating reaction is 24-48 h; Optionally, cooling, washing and drying treatments are carried out after the heating reaction.

11. The preparation method according to any one of claims 1-10, wherein the third solvent in step (1) includes absolute ethanol.

12. The preparation method according to any one of claims 1-11, wherein the acid solution includes any one or a combination of at least two of phytic acid, tannic acid or lauric acid.

13. The preparation method according to any one of claims 1-12, wherein the concentration of the acid solution is 50-100 g / L.

14. The preparation method according to any one of claims 1-13, wherein The NH 2 -MIL-53(Al) nanoparticles and the solute in the acid solution have a mass ratio of 1:(1.5 - 2.5).

15. The preparation method according to any one of claims 1-14, wherein the temperature of the heating and stirring in step (1) is 90-110 °C; Optionally, the time of the heating and stirring is 8-12 h; Optionally, centrifugation, washing and drying treatments are carried out after the heating and stirring.

16. The preparation method according to any one of claims 1-15, wherein The NH described in step (2) 2 -MIL-53(Al) dispersion has a solvent including water; Optionally, the solute of the iron salt solution includes iron nitrate and / or iron chloride; Optionally, the concentration of the iron salt solution is 0.1-0.3 mol / L; Optionally, the mass concentration ratio of the NH 2 -MIL-53(Al) to iron ions after mixing is (0.8 to 1.2):1; Optionally, the stirring time is 3-5 h.

17. The preparation method according to any one of claims 1-16, wherein the solute of the phosphorus source solution in step (2) includes any one or a combination of at least two of ammonium dihydrogen phosphate, ammonium hydrogen phosphate or sodium hydrogen phosphate; Optionally, the concentration of phosphorus element in the phosphorus source solution is 0.1-0.3 mol / L; Optionally, the molar ratio of phosphorus element in the phosphorus source solution to iron element in the iron salt solution is 1: (0.97 - 1.05); Optionally, the method for controlling pH includes adding an alkaline solution; Optionally, the alkaline solution includes ammonia water and / or sodium hydroxide solution; Optionally, the pH is 1.5 - 2.2; Optionally, heating and stirring are carried out during the reaction; Optionally, the temperature of the heating and stirring is 70 - 90 °C; Optionally, the time of the heating and stirring is 4 - 10 h; Optionally, aging is carried out after the heating and stirring; Optionally, the time of the aging is 1 - 2 h.

18. The preparation method according to any one of claims 1 - 17, wherein, the temperature of the sintering treatment in step (3) is 500 - 750 °C; Optionally, the time of the sintering treatment is 4 - 10 h; Optionally, the atmosphere of the sintering treatment includes a nitrogen atmosphere; Optionally, washing treatment is carried out after the sintering treatment; Optionally, the washing treatment includes acetic acid washing and water washing.

19. A modified iron phosphate prepared by the method according to any one of claims 1 - 18.

20. A lithium iron phosphate cathode material prepared by mixing and sintering the modified iron phosphate according to claim 19 with a lithium source.

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