Porous iron phosphate, preparation method therefor, and use thereof

Abstract

A porous iron phosphate, a preparation method therefor, and the use thereof. The preparation method comprises the following steps: (1) mixing a carbon source, a chloride salt and an iron source with a solvent, adding aqueous ammonia, freezing same, and performing freeze-drying treatment to obtain a xerogel material; (2) carbonizing the xerogel material to obtain a Fe2O3-C precursor having a 3D network structure; and (3) mixing the Fe2O3-C precursor, urea and phosphoric acid with a solvent, heating and reacting same, and then carrying out sintering treatment to obtain the porous iron phosphate. The iron phosphate prepared by the method has a three-dimensional porous structure; and because of said structure, a lithium iron phosphate positive electrode material prepared by means of using the iron phosphate as a precursor exhibits an increased specific surface area, which shortens the diffusion path of lithium ions and accelerates the diffusion, thereby improving the electrochemical properties of lithium ion batteries.

Description

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

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

[0002] Olivine-structured lithium iron phosphate (LiFePO4) is considered the most promising cathode material for lithium-ion batteries due to its high theoretical capacity, low cost, environmental friendliness, and high safety. However, inherent structural defects result in low electronic conductivity and lithium-ion diffusion rate in LiFePO4, resulting in unsatisfactory rate performance and low-temperature performance, further limiting its practical application in power batteries.

[0003] In the ferrophosphide synthesis process, iron phosphate is an important precursor raw material for the preparation of lithium iron phosphate, and lithium iron phosphate crystals can be grown directly on the basis of iron phosphate crystals. Therefore, the structural morphology and particle size of iron phosphate determine the physicochemical properties of lithium iron phosphate and have an important influence on its electrochemical performance.

[0004] CN110104624A discloses a method for preparing porous ferric phosphate, which utilizes the expansion of foamed microspheres during heating to prepare porous ferric phosphate with a high specific surface area.

[0005] CN115465846 discloses a porous iron phosphate and a method for preparing low-temperature lithium iron phosphate using the same. The method uses a fluidizing device with pulsed hydrofluoric acid in the form of gaseous phase to etch the iron phosphate under heating conditions. This etching method ensures full contact between the hydrofluoric acid and the fluidized iron phosphate, achieving uniform etching and controllable etching thickness. The addition of titanium dioxide further improves etching efficiency and the performance of the prepared lithium iron phosphate.

[0006] The method for preparing porous iron phosphate described in the above scheme is costly and the obtained iron phosphate has poor structural stability. Therefore, developing a low-cost porous iron phosphate precursor that is conducive to lithium ion diffusion and interfacial electrolyte penetration is of great significance for obtaining lithium iron phosphate positive electrode materials with excellent performance.

[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 a porous iron phosphate and its preparation method and application. The method described in the present disclosure produces a three-dimensional porous structure of the iron phosphate. This structure increases the specific surface area of ​​the lithium iron phosphate positive electrode material prepared using this as a precursor, shortens the lithium ion diffusion path, and accelerates the rate, thereby improving the electrochemical performance of the lithium-ion battery.

[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 porous ferric phosphate, the preparation method comprising the following steps:

[0012] (1) mixing a carbon source, a chloride salt, and an iron source with a solvent, adding ammonia water, freezing, and then freeze-drying to obtain a dry gel material;

[0013] (2) carbonizing the dry gel material to obtain a Fe2O3-C precursor having a 3D network structure;

[0014] (3) The Fe2O3-C precursor, urea and phosphoric acid are mixed with a solvent, heated for reaction and then sintered to obtain the porous iron phosphate.

[0015] The present invention utilizes the recrystallization property of chloride salt and uses it as a self-assembly template to first prepare a three-dimensional porous substrate loaded with Fe2O3 particles, and then mixes the substrate material with phosphoric acid to prepare a porous iron phosphate material by a high-temperature hydrothermal method; the synthesis method provides a spatial template for iron phosphate crystallization by using a porous precursor material, allowing the difficult-to-control crystal morphology to be designed before the start, avoiding the shortcomings of the current iron phosphate production process such as hard agglomeration; the synthesis method is simple and easy to implement, environmentally friendly, and has good industrial prospects.

[0016] In one embodiment, the carbon source in step (1) comprises any one of glucose, sucrose or starch, or a combination of at least two of them.

[0017] In one embodiment, the chloride salt comprises sodium chloride and / or potassium chloride.

[0018] In one embodiment, the iron source comprises ferric nitrate nonahydrate and / or ferric chloride hexahydrate.

[0019] In one embodiment, the solvent comprises water.

[0020] In one embodiment, the mass concentration of the ammonia water is 20% to 30%, for example, 20%, 22%, 25%, 28% or 30%.

[0021] In one embodiment, the mass ratio of the chloride salt to the carbon source in step (1) is (4-6):1, for example: 4:1, 4.5:1, 5:1, 5.5:1 or 6:1, etc.

[0022] In one embodiment, the mass ratio of the chloride salt to the iron source is (8-10):1.

[0023] In one embodiment, the freezing temperature in step (1) is -15 to -25°C, for example, -15°C, -18°C, -20°C, -22°C or -25°C.

[0024] In one embodiment, the freezing time is 8 to 15 hours, for example, 8 hours, 9 hours, 10 hours, 12 hours or 15 hours.

[0025] In one embodiment, the freeze-drying temperature is -50 to -60°C, for example, -50°C, -52°C, -55°C, -58°C or -60°C.

[0026] In one embodiment, the freeze-drying treatment time is 24 to 36 hours, for example, 24 hours, 28 hours, 30 hours, 32 hours or 36 hours.

[0027] In one embodiment, the carbonization treatment in step (2) is preceded by a grinding treatment.

[0028] In one embodiment, the temperature of the carbonization treatment is 550-650°C, for example, 550°C, 580°C, 600°C, 620°C or 650°C.

[0029] In one embodiment, the carbonization treatment time is 2 to 3 hours, for example, 2 hours, 2.2 hours, 2.5 hours, 2.8 hours or 3 hours.

[0030] In one embodiment, the carbonization treatment is followed by a washing treatment.

[0031] In one embodiment, the detergent of the washing treatment comprises deionized water.

[0032] In the preparation of the 3D Fe2O3-C precursor material disclosed herein, the carbon source undergoes high-temperature pyrolysis to form ultrathin carbon nanosheets. After deionized water washing to remove the chloride template, an interwoven three-dimensional carbon skeleton network is formed. The presence of this carbon network increases the electronic conductivity of the electrode material. Furthermore, this three-dimensional porous structure helps mitigate volume changes in the electrode material during lithium removal and insertion, enhancing its structural stability.

[0033] The Fe2O3-C precursor material includes a three-dimensional porous carbon skeleton, which is a three-dimensional carbon network skeleton formed by self-assembly of a carbon source on a salt template surface after high-temperature pyrolysis, and the Fe2O3 in the precursor material is loaded on the surface of the three-dimensional porous carbon skeleton.

[0034] In one embodiment, the molar ratio of the urea in step (3) to the iron element in the Fe2O3-C precursor is (5-10):1, for example: 5:1, 6:1, 7:1, 8:1, 9:1 or 10:1.

[0035] In one embodiment, the concentration of the phosphoric acid is 0.05 to 0.15 mol / L, for example, 0.05 mol / L, 0.08 mol / L, 0.1 mol / L, 0.12 mol / L or 0.15 mol / L.

[0036] In one embodiment, the molar ratio of the iron element in the Fe2O3-C precursor to the phosphorus element in the phosphoric acid is (0.98-1.06):1, for example: 0.98:1, 0.99:1, 1:1, 1.02:1 or 1.06:1.

[0037] In one embodiment, the temperature of the heating reaction in step (3) is 120-150°C, for example, 120°C, 125°C, 130°C, 140°C or 150°C.

[0038] In one embodiment, the heating reaction time is 12 to 24 hours, for example, 12 hours, 16 hours, 18 hours, 20 hours or 24 hours.

[0039] In one embodiment, the heating reaction is followed by centrifugation, washing and drying.

[0040] 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.

[0041] In one embodiment, the sintering treatment time is 4 to 10 hours, for example, 4 hours, 5 hours, 6 hours, 8 hours or 10 hours.

[0042] In a second aspect, the present disclosure provides a porous iron phosphate, which is prepared by the method described in the first aspect.

[0043] The iron phosphate prepared in the present invention has a three-dimensional porous structure. Compared with irregular solid iron phosphate materials, the material with this structure has a larger specific surface area and abundant mass transfer channels, which is conducive to the full contact between the electrode material and the electrolyte, shortens the diffusion path of lithium ions, enhances the transmission and migration of lithium ions and electrons, and promotes the lithiation / delithiation process, thereby improving the rate performance of lithium iron phosphate batteries.

[0044] In a third aspect, the present disclosure provides a lithium iron phosphate positive electrode material, which is prepared by mixing and sintering the porous iron phosphate as described in the second aspect with a lithium source and a carbon source.

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

[0046] (1) The present invention adopts a soluble chloride salt as a template, impregnates and mixes it with an iron salt and a carbon source, and obtains an iron phosphate material with a three-dimensional interconnected network structure through a cold drying-calcination-template removal-hydrothermal method; based on the spatial confinement effect of the salt template, the ultrathin carbon nanosheets formed after the carbon source is calcined will induce the in situ self-assembly of iron phosphate nanoparticles in its two-dimensional plane, effectively hindering the aggregation of particles and maintaining the porous framework structure of the iron phosphate product, which is conducive to promoting the rapid diffusion and transfer of lithium ions and electrolytes.

[0047] (2) The porous iron phosphate prepared by the method disclosed in the present invention can produce lithium iron phosphate batteries with a 0.1C discharge capacity of more than 155.5 mAh / g, and a maximum discharge capacity of 163.5 mAh / g; a 1C discharge capacity of more than 148.7 mAh / g, and a maximum discharge capacity of 153.6 mAh / g; an initial charge and discharge efficiency of more than 98.75%, and excellent electrochemical performance.

[0048] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] 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.

[0050] FIG1 is a SEM image of the porous iron phosphate prepared in Example 1 of the present disclosure.

[0051] FIG2 is a schematic diagram of the preparation process of porous iron phosphate according to an embodiment 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 a porous iron phosphate. The schematic diagram of the preparation process of the porous iron phosphate is shown in FIG2 . The preparation method of the porous iron phosphate is as follows:

[0055] (1) 1 g of glucose, 5.4 g of sodium chloride, and 2 mmol of ferric chloride hexahydrate (the mass ratio of chloride to iron source is 10:1) were dissolved in 80 mL of deionized water and ultrasonically treated for 20 min. 1.5 mL of 25% ammonia water was added dropwise to the mixture and stirred for 15 min. The mixture was placed in a refrigerator at -20°C for 12 h and then freeze-dried at -55°C under vacuum for 24 h to obtain a xerogel product, which was then ground into a fine powder.

[0056] (2) The obtained powder was transferred to a tube furnace and heated to 550°C at a heating rate of 5°C / min under a nitrogen atmosphere for carbonization for 3 h. After cooling to room temperature, the product powder was washed with deionized water several times to remove the chloride template to obtain the 3D Fe2O3-C precursor product. The product was then placed in a vacuum oven at 100°C for 12 h;

[0057] (3) The Fe2O3-C precursor product was ultrasonically dispersed in 15 mL of deionized water, and then 0.96 g of urea (urea:Fe=8:1) was added. After stirring evenly, 20 mL of 0.1 mol / L phosphoric acid solution (Fe:P=1:1) was added to the above mixture and stirred at room temperature for 20 min. The mixture was then transferred to a 100 mL polytetrafluoroethylene-lined stainless steel autoclave and heated in an oven at 120°C for 24 h. After cooling to room temperature, the product was collected by centrifugation and washed several times with deionized water and anhydrous ethanol, and then dried in a vacuum oven at 100°C for 12 h to obtain a 3D FePO4·2H2O material. Subsequently, the FePO4·2H2O prepared above was placed in a muffle furnace and heated to 600°C at a heating rate of 8°C / min for 8 h to remove the crystallization water, thereby obtaining a 3D porous anhydrous FePO4 material (the porous iron phosphate).

[0058] The SEM image of the porous iron phosphate is shown in FIG1 . As can be seen from FIG1 , the iron phosphate material prepared in the present disclosure has a 3D porous structure.

[0059] Example 2

[0060] This embodiment provides a porous iron phosphate. The schematic diagram of the preparation process of the porous iron phosphate is shown in FIG2 . The preparation method of the porous iron phosphate is as follows:

[0061] (1) 1.6 g of glucose, 6.5 g of sodium chloride, and 3 mmol of ferric chloride hexahydrate (the mass ratio of chloride salt to iron source is 8:1) were dissolved in 80 mL of deionized water and ultrasonically treated for 20 min. 2 mL of 25% ammonia water was added dropwise to the mixture and stirred for 15 min. The mixture was then placed in a refrigerator at -15°C for 15 h and then freeze-dried at -60°C under vacuum for 24 h to obtain a xerogel product, which was then ground into a fine powder.

[0062] (2) The obtained powder was transferred to a tube furnace and heated to 550°C at a heating rate of 5°C / min under a nitrogen atmosphere for carbonization for 3 h. After cooling to room temperature, the product powder was washed with deionized water several times to remove the chloride template to obtain the 3D Fe2O3-C precursor product. The product was then placed in a vacuum oven at 100°C for 12 h;

[0063] (3) The Fe2O3-C precursor product was ultrasonically dispersed in 15 mL of deionized water, and then 1.45 g of urea (urea:Fe=8:1) was added. After stirring evenly, 30 mL of 0.1 mol / L phosphoric acid solution (Fe:P=1:1) was added to the mixture and stirred at room temperature for 20 min. The mixture was then transferred to a 100 mL polytetrafluoroethylene-lined stainless steel autoclave and heated in an oven at 150°C for 12 h. After cooling to room temperature, the product was collected by centrifugation and washed several times with deionized water and anhydrous ethanol, and then dried in a vacuum oven at 100°C for 12 h to obtain a 3D FePO4·2H2O material. Subsequently, the FePO4·2H2O prepared above was placed in a muffle furnace and heated to 650°C at a heating rate of 8°C / min for 8 h to remove the crystallization water, thereby obtaining a 3D porous anhydrous FePO4 material (the porous iron phosphate).

[0064] Example 3

[0065] This embodiment provides a porous iron phosphate. The schematic diagram of the preparation process of the porous iron phosphate is shown in FIG2 . The preparation method of the porous iron phosphate is as follows:

[0066] (1) 1.6 g of glucose, 7.2 g of sodium chloride, and 3 mmol of ferric chloride hexahydrate (the mass ratio of chloride salt to iron source is 9:1) were dissolved in 80 mL of deionized water and ultrasonically treated for 20 min. 2 mL of 25% ammonia water was added dropwise to the mixture and stirred for 15 min. The mixture was placed in a refrigerator at -25°C for 8 h and then freeze-dried at -50°C under vacuum for 36 h to obtain a xerogel product, which was then ground into a fine powder.

[0067] (2) The obtained powder was transferred to a tube furnace and heated to 650°C at a heating rate of 5°C / min under a nitrogen atmosphere for carbonization for 2 h. After cooling to room temperature, the product powder was washed with deionized water several times to remove the chloride template to obtain the 3D Fe2O3-C precursor product. The product was then placed in a vacuum oven at 100°C for 12 h;

[0068] (3) The Fe2O3-C precursor product was ultrasonically dispersed in 15 mL of deionized water, and then 1.45 g of urea (urea:Fe=8:1) was added. After stirring evenly, 30 mL of 0.1 mol / L phosphoric acid solution (Fe:P=1:1) was added to the mixture and stirred at room temperature for 20 min. The mixture was then transferred to a 100 mL polytetrafluoroethylene-lined stainless steel autoclave and heated in an oven at 140°C for 24 h. After cooling to room temperature, the product was collected by centrifugation and washed several times with deionized water and anhydrous ethanol, and then dried in a vacuum oven at 100°C for 12 h to obtain a 3D FePO4·2H2O material. Subsequently, the FePO4·2H2O prepared above was placed in a muffle furnace and heated to 750°C at a heating rate of 8°C / min for 4 h to remove the crystallization water, thereby obtaining a 3D porous anhydrous FePO4 material (the porous iron phosphate).

[0069] Example 4

[0070] The only difference between this embodiment and embodiment 1 is that the mass ratio of sodium chloride to iron source is 6:1, and 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 mass ratio of sodium chloride to iron source is 12:1, and 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 stainless steel autoclave is placed in an oven at 120° C. and heated for 12 h. 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 stainless steel autoclave is placed in an oven at 140° C. and heated for 24 h. Other conditions and parameters are exactly the same as those in embodiment 1.

[0077] Comparative Example 1

[0078] The only difference between this comparative example and Example 3 is that no sodium chloride is added, and other conditions and parameters are exactly the same as those in Example 1.

[0079] Comparative Example 2

[0080] The only difference between this comparative example and Example 3 is that steps (1) and (2) are not performed to prepare the 3D Fe2O3-C precursor material, and ferric chloride is directly used for the hydrothermal reaction in step (3). Other conditions and parameters are exactly the same as those in Example 1.

[0081] Performance testing:

[0082] The examples and comparative examples were used to prepare iron phosphate. Lithium carbonate, iron phosphate and glucose were dispersed in anhydrous ethanol in a stoichiometric ratio of 1:1.03:0.09 among the lithium source, iron source and carbon source. The mixture was ball-milled for 2 hours until uniformly mixed at a rotation speed of 3000 rpm, and then spray-dried to obtain a precursor powder. Subsequently, the precursor powder was heated to 400°C at a heating rate of 5°C / min under a nitrogen atmosphere for 1.5 hours, and then heated to 700°C for high-temperature calcination for 8 hours to obtain LiFePO4 / C positive electrode material. The lithium iron phosphate positive electrode material was prepared into a button cell for lithium-ion battery electrochemical performance testing (controlling the charge and discharge voltage between 2.5-4.5V). The test results are shown in Table 1:

[0083] Table 1

[0084] As can be seen from Table 1, from Examples 1-7, the porous iron phosphate prepared by the method described in the present disclosure can make the lithium iron phosphate battery have a 0.1C discharge specific capacity of more than 155.5 mAh / g, a 1C discharge specific capacity of more than 148.7 mAh / g, and an initial charge and discharge efficiency of more than 98.75%.

[0085] By comparing Example 1 with Examples 4-5, it can be seen that during the preparation process of the porous iron phosphate described in the present disclosure, the amount of chloride salt added will affect its structure and thus its performance. If the amount of chloride salt added is too large, it may cause accumulation of carbon flakes after the salt template is removed; if the amount of chloride salt added is too small, the pore structure of the product will be insufficiently constructed, which will cause the electrical properties of the electrode material to deteriorate.

[0086] By comparing Example 1 with Examples 6-7, it can be seen that in the preparation process of the porous iron phosphate described in the present disclosure, the hydrothermal reaction temperature and duration will affect its structure and thus its performance. If the reaction time is too short, it may affect the phase transition of the product particles and affect their crystallinity; when the reaction temperature is too high, the product particles are prone to agglomeration and the particle size may also be larger, resulting in reduced performance of the electrode material.

[0087] A comparison of Example 1 and Comparative Examples 1-2 demonstrates that the present disclosure utilizes the recrystallization properties of chloride salts as a self-assembly template to first prepare a three-dimensional porous substrate loaded with Fe2O3 particles. The substrate material is then mixed with phosphoric acid and subjected to a high-temperature hydrothermal method to prepare a porous iron phosphate material. Compared to the solid carbon block / iron phosphate mixture prepared in Comparative Example 1 and the solid, irregular iron phosphate particles prepared in Comparative Example 2, the iron phosphate prepared in the present disclosure exhibits a three-dimensional porous structure. This structure increases the contact area between the lithium iron phosphate cathode material and the electrolyte, shortening the diffusion path of lithium ions and thereby improving the electrochemical performance of the lithium-ion battery.

Claims

1. A preparation method of porous iron phosphate, comprising the following steps: (1) Mix a carbon source, a chloride salt, and an iron source with a solvent, add ammonia water, freeze it, and then perform freeze-drying treatment to obtain a dry gel material; (2) After carbonizing the dry gel material, obtain an Fe2O3-C precursor with a 3D network structure; (3) Mix the Fe2O3-C precursor, urea, and phosphoric acid with a solvent, heat and react, and then perform sintering treatment to obtain the porous iron phosphate.

2. The preparation method according to claim 1, wherein, The carbon source in step (1) includes any one or a combination of at least two of glucose, sucrose, or starch.

3. The preparation method according to claim 1 or 2, wherein The chloride salt includes sodium chloride and / or potassium chloride.

4. The preparation method according to any one of claims 1-3, wherein, The iron source includes iron(III) nitrate nonahydrate and / or iron(III) chloride hexahydrate.

5. The preparation method according to any one of claims 1-4, wherein, The solvent includes water.

6. The preparation method according to any one of claims 1-5, wherein, The mass concentration of the ammonia water is 20% - 30%.

7. The preparation method according to any one of claims 1-6, wherein, The mass ratio of the chloride salt to the carbon source in step (1) is (4 - 6):

1.

8. The preparation method according to any one of claims 1-7, wherein, The mass ratio of the chloride salt to the iron source is (8 - 10):

1.

9. The preparation method according to any one of claims 1-8, wherein, The temperature of the freezing in step (1) is -15 to -25 °C; Optionally, the time of the freezing is 8 - 15 h; Optionally, the temperature of the freeze-drying treatment is -50 to -60 °C; Optionally, the time of the freeze-drying treatment is 24 - 36 h.

10. The preparation method according to any one of claims 1-9, wherein, Perform grinding treatment before the carbonization treatment in step (2); Optionally, the temperature of the carbonization treatment is 550 - 650 °C; Optionally, the time of the carbonization treatment is 2 - 3 h; Optionally, perform washing treatment after the carbonization treatment; Optionally, the detergent for the washing treatment includes deionized water.

11. The preparation method according to any one of claims 1-10, wherein, The molar ratio of urea to iron element in the Fe2O3-C precursor in step (3) is (5 - 10):1; Optionally, the concentration of the phosphoric acid is 0.05 - 0.15 mol / L; Optionally, the molar ratio of iron element in the Fe2O3-C precursor to phosphorus element in the phosphoric acid is (0.98 - 1.06):

1.

12. The preparation method according to any one of claims 1-11, wherein, The temperature of the heating reaction in step (3) is 120 - 150 °C; Optionally, the time of the heating reaction is 12 - 24 h; Optionally, perform centrifugation, washing, and drying treatment after the heating reaction.

13. The preparation method according to any one of claims 1-12, 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.

14. A porous iron phosphate prepared by the method according to any one of claims 1 - 13.

15. A lithium iron phosphate cathode material prepared by mixing and sintering the porous iron phosphate according to claim 14 with a lithium source and a carbon source.

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