Porous iron phosphate and preparation method and application thereof

Abstract

The present disclosure provides a kind of porous iron phosphate and its preparation method and application, the preparation method includes the following steps: (1) carbon source, chloride salt and iron source are mixed with solvent, add ammonia water, freeze-drying treatment is carried out after freezing, and dry gel material is obtained;(2) after the dry gel material is carbonized, Fe2O3-C precursor with 3D network structure is obtained;(3) the Fe2O3-C precursor, urea and phosphoric acid are mixed with solvent, after heating reaction, the porous iron phosphate is obtained by sintering treatment.The method of the present disclosure makes the iron phosphate for three-dimensional porous structure, the specific surface area of lithium iron phosphate anode material prepared by using the structure as precursor is increased, lithium ion diffusion path is shortened, speed is accelerated, so that the electrochemical performance of lithium ion battery is improved.

Description

TECHNICAL FIELD

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

[0002] Lithium iron phosphate (LiFePO4) material with olivine structure is considered to be the most potential positive electrode material for lithium ion batteries at present due to its high theoretical capacity, low cost, environmental friendliness and high safety. However, the inherent defects of its structure result in low electronic conductivity and low lithium ion diffusion rate of lithium iron phosphate, which makes its rate performance and low-temperature performance not ideal, and further limits its practical application in power batteries.

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

[0004] CN110104624A discloses a preparation method of porous iron phosphate, which utilizes the expansion of foaming microspheres during heating to prepare porous iron phosphate with 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 etches iron phosphate under heating conditions by using a fluidization device to cooperate with the pulse introduction of gaseous hydrofluoric acid, which can make the hydrofluoric acid and the fluidized iron phosphate fully contact, realize the uniformity of etching and the controllability of etching thickness, and further improve the etching efficiency and the performance of the prepared lithium iron phosphate by adding titanium dioxide.

[0006] The above-mentioned method for preparing porous iron phosphate has high cost and the prepared iron phosphate has poor structural stability. Therefore, it is of great significance to develop a porous structure iron phosphate precursor with low cost, which is beneficial to lithium ion diffusion and interfacial electrolyte penetration, for obtaining a lithium iron phosphate positive electrode material with excellent performance. SUMMARY

[0007] The following is a summary of the subject matter of the detailed description. This summary is not intended to limit the scope of the claims.

[0008] The present disclosure aims to provide a porous iron phosphate and a preparation method and application thereof. The method disclosed in the present disclosure prepares iron phosphate with a three-dimensional porous structure, which increases the specific surface area of lithium iron phosphate positive electrode material prepared by using the iron phosphate as a precursor, shortens the lithium ion diffusion path and accelerates the rate, and thus improves the electrochemical performance of lithium ion batteries.

[0009] To achieve the above object, the present disclosure adopts the following technical solutions:

[0010] In a first aspect, the present disclosure provides a preparation method of porous iron phosphate, comprising the following steps:

[0011] (1) mixing a carbon source, a chloride salt and an iron source with a solvent, adding ammonia water, freezing and then performing freeze-drying treatment to obtain a dry gel material;

[0012] (2) performing carbonization treatment on the dry gel material to obtain a Fe2O3-C precursor with a 3D network structure;

[0013] (3) mixing the Fe2O3-C precursor, urea and phosphoric acid with a solvent, performing heating reaction and then performing sintering treatment to obtain the porous iron phosphate.

[0014] The present disclosure utilizes the recrystallization property of the 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 the crystallization of the iron phosphate by using a porous precursor material, so that the difficult-to-control crystalline morphology is designed in advance, avoiding the deficiencies such as hard agglomeration in the current production process of iron phosphate; the synthesis method is simple and easy to operate, environmentally friendly, and has good industrialization prospects.

[0015] In one embodiment, the carbon source in step (1) includes any one or a combination of at least two of glucose, sucrose or starch.

[0016] In one embodiment, the chloride salt includes sodium chloride and / or potassium chloride.

[0017] In one embodiment, the iron source includes iron nitrate nonahydrate and / or iron chloride hexahydrate.

[0018] In one embodiment, the solvent includes water.

[0019] In one embodiment, the mass concentration of the ammonia water is 20% to 30%, for example, 20%, 22%, 25%, 28% or 30%, etc.

[0020] 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.

[0021] In one embodiment, the mass ratio of the chloride salt to the iron source is (8-10):1.

[0022] In one embodiment, the freezing temperature in step (1) is -15 to -25℃, for example, -15℃, -18℃, -20℃, -22℃, or -25℃, etc.

[0023] In one embodiment, the freezing time is 8 to 15 h, for example, 8 h, 9 h, 10 h, 12 h, or 15 h, etc.

[0024] In one embodiment, the freeze-drying temperature is -50 to -60℃, for example, -50℃, -52℃, -55℃, -58℃, or -60℃, etc.

[0025] In one embodiment, the freeze-drying time is 24 to 36 h, for example, 24 h, 28 h, 30 h, 32 h, or 36 h, etc.

[0026] In one embodiment, the grinding treatment is performed before the carbonization treatment in step (2).

[0027] In one embodiment, the carbonization temperature is 550 to 650℃, for example, 550℃, 580℃, 600℃, 620℃, or 650℃, etc.

[0028] In one embodiment, the carbonization time is 2 to 3 h, for example, 2 h, 2.2 h, 2.5 h, 2.8 h, or 3 h, etc.

[0029] In one embodiment, the washing treatment is performed after the carbonization treatment.

[0030] In one embodiment, the washing agent of the washing treatment includes deionized water.

[0031] In the step of preparing the 3D Fe2O3-C precursor material, the carbon source forms ultra-thin carbon nanosheets after pyrolysis at high temperature, and after washing with deionized water to remove the chloride salt template, a three-dimensional carbon skeleton network is formed. The presence of the carbon network can increase the electronic conductivity of the electrode material. In addition, the three-dimensional porous structure is also beneficial to alleviate the volume change of the electrode material during the lithium extraction / insertion process, and enhances the structural stability.

[0032] 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 the carbon source on the surface of the salt template after pyrolysis at high temperature. The Fe2O3 in the precursor material is loaded on the surface of the three-dimensional porous carbon skeleton.

[0033] In one embodiment, the molar ratio of urea to iron in the Fe2O3-C precursor in step (3) is (5-10):1, for example, 5:1, 6:1, 7:1, 8:1, 9:1, or 10:1, etc.

[0034] In one embodiment, the concentration of the phosphoric acid is 0.05-0.15 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, etc.

[0035] In one embodiment, the molar ratio of iron element in the Fe2O3-C precursor to 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, etc.

[0036] In one embodiment, the temperature of the heating reaction in step (3) is 120-150℃, for example, 120℃, 125℃, 130℃, 140℃ or 150℃, etc.

[0037] In one embodiment, the time of the heating reaction is 12-24h, for example, 12h, 16h, 18h, 20h or 24h, etc.

[0038] In one embodiment, the heating reaction is followed by centrifugation, washing and drying treatment.

[0039] In one embodiment, the temperature of the sintering treatment in step (3) is 500-750℃, for example, 500℃, 520℃, 550℃, 600℃ or 750℃, etc.

[0040] In one embodiment, the time of the sintering treatment is 4-10h, for example, 4h, 5h, 6h, 8h or 10h, etc.

[0041] In a second aspect, the present disclosure provides a porous iron phosphate prepared by the method of the first aspect.

[0042] The iron phosphate prepared by the present disclosure has a three-dimensional porous structure. Compared with irregular solid iron phosphate materials, the material of this structure has a larger specific surface area and abundant mass transfer channels, which is conducive to the full contact of the electrode material with 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 the lithium iron phosphate battery.

[0043] In a third aspect, the present disclosure provides a lithium iron phosphate positive electrode material prepared by mixing and sintering the porous iron phosphate of the second aspect with a lithium source and a carbon source.

[0044] Compared with the prior art, the present disclosure has the following beneficial effects:

[0045] (1) The present disclosure uses soluble chloride salt as a template agent, impregnates and mixes it with iron salt and carbon source, and obtains a three-dimensional interconnected network structure of iron phosphate material through the method of cold drying-calcination-removing template-hydrothermal method; based on the space limitation effect of salt template, the ultra-thin carbon nanosheet formed after calcination of the carbon source will induce the in-situ self-assembly of iron phosphate nanoparticles in its two-dimensional plane, effectively hinder the aggregation of the particles, and maintain the porous framework structure of the iron phosphate product, which is conducive to promoting the rapid diffusion and transfer of lithium ions and electrolyte.

[0046] (2) The 0.1C discharge specific capacity of the lithium iron phosphate battery prepared by the method of the present disclosure can reach 155.5mAh / g or more, and the highest discharge specific capacity can reach 163.5mAh / g; the 1C discharge specific capacity can reach 148.7mAh / g or more, and the highest discharge specific capacity can reach 153.6mAh / g; the first charge-discharge efficiency can reach 98.75% or more, and the electrochemical performance is relatively good.

[0047] Other aspects can be apparent after reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF DRAWINGS

[0048] The accompanying drawings are used to provide a further understanding of the technical solutions of the present disclosure, and constitute a part of the specification, and are used together with the embodiments of the present application to explain the technical solutions of the present disclosure, and do not constitute a limitation on the technical solutions of the present disclosure.

[0049] Figure 1 is the SEM image of the porous iron phosphate prepared by the embodiment 1 of the present disclosure.

[0050] Figure 2 is a schematic diagram of the preparation process of the porous iron phosphate according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0051] The technical solutions of the present disclosure will be further described by specific embodiments. 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 a specific limitation on the present disclosure.

[0052] Embodiment 1

[0053] The present embodiment provides a porous iron phosphate, and a schematic diagram of the preparation process of the porous iron phosphate is shown in Figure 2 The preparation method of the porous iron phosphate is as follows:

[0054] (1) 1 g of glucose, 5.4 g of sodium chloride and 2 mmol of iron chloride hexahydrate (mass ratio of chloride salt to iron source is 10:1) were dissolved in 80 mL of deionized water, and ultrasonic treatment was performed for 20 min. 1.5 mL of 25% ammonia water was added dropwise into the mixture, and stirring was performed for 15 min. Then, the mixture was frozen in a refrigerator at -20°C for 12 h, and then freeze-dried under vacuum at -55°C for 24 h to obtain a dry gel product, and the product was ground into a fine powder;

[0055] (2) The obtained powder was transferred into a tube furnace, and carbonization was performed at 550°C for 3 h under a nitrogen atmosphere at a temperature increasing rate of 5°C / min. After cooling to room temperature, the product powder was washed with deionized water for multiple times to remove the chloride salt template, to obtain a 3D Fe2O3-C precursor product, and then the product was dried in a vacuum oven at 100°C for 12 h;

[0056] (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 uniform stirring, 20 mL of a 0.1 mol / L phosphoric acid solution (Fe:P = 1:1) was added into the mixture, and stirring was performed at room temperature for 20 min. Then, the mixture was transferred into 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 with deionized water and anhydrous ethanol for multiple times, and then dried in a vacuum oven at 100°C for 12 h to obtain a 3D FePO4·2H2O material. Then, the prepared FePO4·2H2O was placed in a muffle furnace, and heated to 600°C at a temperature increasing rate of 8°C / min, and kept at 600°C for 8 h to remove the crystallization water, to obtain a 3D porous anhydrous FePO4 material (the porous iron phosphate).

[0057] The SEM image of the porous iron phosphate is shown in Figure 1 As can be seen from Figure 1 It can be seen that the iron phosphate material prepared in the present disclosure has a 3D porous structure.

[0058] Example 2

[0059] The present example provides a porous iron phosphate, and a schematic diagram of the preparation process of the porous iron phosphate is shown in Figure 2 The preparation method of the porous iron phosphate is as follows:

[0060] (1) 1.6 g glucose, 6.5 g sodium chloride and 3 mmol iron chloride hexahydrate (mass ratio of chloride salt to iron source is 8:1) were dissolved in 80 mL deionized water, and ultrasonic treatment was performed for 20 min. 2 mL of 25% ammonia water was added dropwise into the mixture, and stirring was performed for 15 min. The mixture was frozen in a refrigerator at -15°C for 15 h, and then freeze-dried at -60°C under vacuum for 24 h to obtain a dry gel product. The product was ground into a fine powder;

[0061] (2) The powder was transferred into a tube furnace, and carbonization was performed at 550°C for 3 h under a nitrogen atmosphere at a temperature increasing rate of 5°C / min. After cooling to room temperature, the product powder was washed with deionized water for multiple times to remove the chloride salt template, to obtain a 3D Fe2O3-C precursor product. The product was then dried in a vacuum oven at 100°C for 12 h;

[0062] (3) The Fe2O3-C precursor product was ultrasonically dispersed in 15 mL deionized water, and then 1.45 g of urea (urea:Fe = 8:1) was added. After uniform stirring, 30 mL of 0.1 mol / L phosphoric acid solution (Fe:P = 1:1) was added into the mixture, and stirring was performed at room temperature for 20 min. The mixture was then transferred into 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 with deionized water and anhydrous ethanol for multiple times. The product was then dried in a vacuum oven at 100°C for 12 h to obtain a 3D FePO4·2H2O material. Subsequently, the prepared FePO4·2H2O was placed in a muffle furnace, and heated at a temperature increasing rate of 8°C / min to 650°C for 8 h to remove the crystallization water, to obtain a 3D porous anhydrous FePO4 material (the porous iron phosphate).

[0063] Example 3

[0064] This example provides a porous iron phosphate. A preparation flowchart of the porous iron phosphate is shown in Figure 2 The preparation method of the porous iron phosphate is as follows:

[0065] (1) 1.6 g glucose, 6.5 g sodium chloride and 3 mmol iron chloride hexahydrate (mass ratio of chloride salt to iron source is 8:1) were dissolved in 80 mL deionized water, and ultrasonic treatment was performed for 20 min. 2 mL of 25% ammonia water was added dropwise into the mixture, and stirring was performed for 15 min. The mixture was frozen in a refrigerator at -15°C for 15 h, and then freeze-dried at -60°C under vacuum for 24 h to obtain a dry gel product. The product was ground into a fine powder;

[0066] (2) The obtained powder was transferred into a tube furnace and carbonized at 650°C for 2h under nitrogen atmosphere at a heating rate of 5°C / min. After cooling to room temperature, the product powder was washed with deionized water for several times to remove the chloride salt template, i.e. to obtain the 3D Fe2O3-C precursor product, which was then dried in a vacuum oven at 100°C for 12h;

[0067] (3) The Fe2O3-C precursor product was ultrasonically dispersed in 15mL deionized water, and then 1.45g urea (urea:Fe = 8:1) was added. After stirring uniformly, 30mL of 0.1mol / L phosphoric acid solution (Fe:P = 1:1) was added to the above mixture, which was stirred at room temperature for 20min. Subsequently, the mixture was transferred into a 100mL polytetrafluoroethylene-lined stainless steel autoclave, which was placed in an oven at 140°C for 24h. After cooling to room temperature, the product was collected by centrifugation and washed with deionized water and anhydrous ethanol for several times, and then dried in a vacuum oven at 100°C for 12h to obtain the 3D FePO4·2H2O material. Subsequently, the prepared FePO4·2H2O was placed in a muffle furnace and heated to 750°C at a heating rate of 8°C / min, and kept for 4h to remove the crystallization water, thereby obtaining the 3D porous anhydrous FePO4 material (the porous iron phosphate).

[0068] Example 4

[0069] The difference between this example and Example 1 is only that the mass ratio of sodium chloride to iron source is 6:1, and other conditions and parameters are completely the same as those in Example 1.

[0070] Example 5

[0071] The difference between this example and Example 1 is only that the mass ratio of sodium chloride to iron source is 12:1, and other conditions and parameters are completely the same as those in Example 1.

[0072] Example 6

[0073] The difference between this example and Example 1 is only that the stainless steel autoclave was placed in an oven at 120°C for 12h, and other conditions and parameters are completely the same as those in Example 1.

[0074] Example 7

[0075] The difference between this example and Example 1 is only that the stainless steel autoclave was placed in an oven at 140°C for 24h, and other conditions and parameters are completely the same as those in Example 1.

[0076] Comparative Example 1

[0077] The difference between this comparative example and Example 3 is only that no sodium chloride was added, and other conditions and parameters are completely the same as those in Example 1.

[0078] Comparative Example 2

[0079] The comparative example is distinguished from Example 3 only in that the 3D Fe2O3-C precursor material is not prepared by steps (1) and (2), and step (3) directly uses ferric chloride for hydrothermal reaction, and other conditions and parameters are completely the same as Example 1.

[0080] Performance test:

[0081] The iron phosphate prepared in the examples and comparative examples is dispersed in anhydrous ethanol according to the metering ratio of 1:1.03:0.09 of lithium source, iron source and carbon source, ball milled for 2h to mix uniformly at a speed of 3000rpm, and then spray dried to obtain a precursor powder. Then, the precursor powder is heated to 400℃ at a heating rate of 5℃ / min under a nitrogen atmosphere for 1.5h, and then calcined at a high temperature of 700℃ for 8h to obtain a LiFePO4 / C positive electrode material. The lithium iron phosphate positive electrode material is matched into a button cell for lithium ion battery electrochemical performance test (controlling the charge and discharge voltage between 2.5-4.5V), and the test results are shown in Table 1:

[0082] Table 1

[0083]

[0084] As can be seen from Table 1, according to Examples 1-7, the 0.1C discharge specific capacity of the lithium iron phosphate battery prepared from the porous iron phosphate prepared by the method of the present disclosure can reach 155.5mAh / g or more, the 1C discharge specific capacity can reach 148.7mAh / g or more, and the first charge and discharge efficiency can reach 98.75% or more.

[0085] As can be seen from the comparison of Example 1 and Examples 4-5, in the preparation process of the porous iron phosphate of the present disclosure, the amount of added chloride salt will affect the structure and thus the performance. If the amount of added chloride salt is too large, the carbon sheets may be stacked after the removal of the salt template; if the amount of added chloride salt is too small, the pore structure of the product may not be constructed enough, which will all cause the electrical performance of the electrode material to decrease.

[0086] As can be seen from the comparison of Example 1 and Examples 6-7, in the preparation process of the porous iron phosphate of the present disclosure, the hydrothermal reaction temperature and its duration will affect the structure and thus the performance. If the reaction time is too short, it may affect the phase transition of the product particles and affect the crystallinity; if the reaction temperature is too high, the product particles are prone to agglomeration, and the particle size may be large, resulting in a decrease in the performance of the electrode material.

[0087] From the comparison of Example 1 and Comparative Examples 1-2, it can be seen that the present disclosure utilizes the recrystallization of the 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 structure of iron phosphate material by a high-temperature hydrothermal method. Compared with 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 has a three-dimensional porous structure, which increases the contact area of the lithium iron phosphate positive electrode material and the electrolyte and shortens the diffusion path of lithium ions, thus improving the electrochemical performance of the lithium ion battery.

Claims

1. A method for preparing porous iron phosphate, characterized in that, The preparation method includes the following steps: (1) Mix carbon source, chloride salt and iron source with solvent, add ammonia water, freeze and freeze dry to obtain dry gel material; (2) After carbonization treatment of the dry gel material, a Fe2O3-C precursor with a 3D network structure is obtained; (3) The Fe2O3-C precursor, urea and phosphoric acid are mixed with a solvent, heated and reacted, and then sintered to obtain the porous iron phosphate; The carbon source in step (1) includes any one or a combination of at least two of glucose, sucrose, or starch; The iron source includes ferric nitrate nonahydrate and / or ferric chloride hexahydrate.

2. The preparation method according to claim 1, characterized in that, The chloride salts include sodium chloride and / or potassium chloride.

3. The preparation method according to claim 1, characterized in that, The solvent includes water.

4. The preparation method according to claim 1, characterized in that, The mass concentration of the ammonia water is 20%~30%.

5. The preparation method according to claim 1, characterized in that, The mass ratio of chloride salt and carbon source in step (1) is (4~6):

1.

6. The preparation method according to claim 1, characterized in that, The mass ratio of the chloride salt to the iron source is (8~10):

1.

7. The preparation method according to claim 1, characterized in that, The freezing temperature in step (1) is -15~-25℃.

8. The preparation method according to claim 1, characterized in that, The freezing time is 8-15 hours.

9. The preparation method according to claim 1, characterized in that, The freeze-drying process is carried out at a temperature of -50 to -60°C.

10. The preparation method according to claim 1, characterized in that, The freeze-drying process takes 24 to 36 hours.

11. The preparation method according to claim 1, characterized in that, Before the carbonization process described in step (2), the material is ground.

12. The preparation method according to claim 1, characterized in that, The carbonization process is carried out at a temperature of 550~650℃.

13. The preparation method according to claim 1, characterized in that, The carbonization process takes 2-3 hours.

14. The preparation method according to claim 1, characterized in that, The carbonization process is followed by a washing process.

15. The preparation method according to claim 14, characterized in that, The detergent used in the washing process includes deionized water.

16. The preparation method according to claim 1, characterized in that, The molar ratio of urea to iron in the Fe2O3-C precursor in step (3) is (5~10):

1.

17. The preparation method according to claim 1, characterized in that, The concentration of phosphoric acid is 0.05~0.15 mol / L.

18. The preparation method according to claim 1, characterized in that, The molar ratio of iron to phosphorus in the Fe2O3-C precursor is (0.98~1.06):

1.

19. The preparation method according to claim 1, characterized in that, The heating reaction in step (3) is carried out at a temperature of 120~150℃.

20. The preparation method according to claim 1, characterized in that, The heating reaction takes 12 to 24 hours.

21. The preparation method according to claim 1, characterized in that, The heating reaction is followed by centrifugation, washing, and drying.

22. The preparation method according to claim 1, characterized in that, The sintering temperature in step (3) is 500~750℃.

23. The preparation method according to claim 1, characterized in that, The sintering process takes 4 to 10 hours.

24. A porous iron phosphate prepared by the method according to any one of claims 1-23.

25. A lithium iron phosphate cathode material prepared by sintering porous iron phosphate as described in claim 24 with a lithium source and a carbon source.

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