A method for heterogeneous interface-assisted synthesis of flake iron phosphate

Abstract

The present invention provides a method for synthesizing flaky iron phosphate using a heterogeneous interface-assisted method, comprising: (1) preparing a phosphorus source and an iron source into raw materials A and B in a ratio of P:Fe=1:1 to 1.5:1, wherein the solvents used for the two raw materials are immiscible; (2) using a heterogeneous interface-assisted synthesis device, dropwise adding raw material B to raw material A mixed with an oxidant to prepare a reaction liquid slurry; (3) after the reaction liquid slurry is allowed to stand and separate, washing and slurrying the lower layer slurry, and aging at high temperature to obtain dihydrated iron phosphate; (4) calcining at high temperature to obtain anhydrous iron phosphate. Specifically, the method is a method for synthesizing a flaky iron phosphate precursor using a two-phase interface reaction-assisted method, which constructs a two-dimensional iron phosphate morphology to a certain extent and improves the quality of the iron phosphate. In addition, the solvents used in the method can be effectively recovered, and the method has good economic value.

Description

Technical Field

[0001] The invention relates to a method for synthesizing flaky ferric phosphate by assisting heterogeneous interface, in particular to a preparation method for synthesizing a flaky ferric phosphate precursor by assisting two-phase interface reaction. Background Art

[0002] In recent years, lithium iron phosphate (LiFePO4) batteries have become the most widely installed battery due to their safety, stable voltage platform, and excellent rate performance. LiFePO4 materials have also become a hot topic in research on cathode materials for many lithium-ion batteries. However, LiFePO4 currently suffers from issues such as electronic conductivity being lower than the lithium ion diffusion rate and low packing density, which seriously hinder its large-scale application and development. Currently, the conventional solution to addressing low electronic conductivity is to nanoparticle LiFePO4. Smaller particle size shortens the ion diffusion path, increases ion migration rate, and improves the electrochemical performance of the material. However, nanoparticle size significantly reduces the packing density of the material, necessitating the development of other technologies to improve the electronic conductivity without compromising the packing density. Two-dimensional nanomaterials, with their unique properties, can enhance electronic conductivity, thereby supporting ultrafast free electron transport and improving electrochemical performance. Furthermore, the agglomeration and staggered stacking structures of two-dimensional nanomaterials can effectively fill the voids in the material, significantly increasing the volumetric density.

[0003] Iron phosphate is one of the primary precursors for the solid-phase preparation of lithium iron phosphate. Its structure significantly influences its morphology and size. Using two-dimensional (2D) flake iron phosphate as a precursor for lithium iron phosphate synthesis effectively reduces particle size, promotes ion migration and diffusion, and enhances its electrochemical performance. The stacking of two-dimensional nanomaterials helps increase the material's compaction density. Therefore, 2D flake iron phosphate precursors can be used to prepare high-energy-density and high-compaction lithium iron phosphate. Summary of the Invention

[0004] In order to develop high-density, high-energy-density lithium iron phosphate materials, the present invention provides a method for heterogeneous interface-assisted synthesis of flake iron phosphate, which is used to prepare a two-dimensional flake iron phosphate precursor that meets the requirements of the above-mentioned type of lithium iron phosphate materials.

[0005] In order to achieve the above-mentioned object of the invention, the present invention provides a method for heterogeneous interface-assisted synthesis of flake iron phosphate, the preparation method of which comprises the following steps:

[0006] S1, adding an oxidant to a phosphorus source solution and then dropwise adding an iron source solution to carry out a heterogeneous interfacial reaction. After the reaction is completed, aging is performed to obtain a reaction solution slurry;

[0007] S2, the reaction liquid slurry in S1 is allowed to stand for stratification, the lower layer of the reaction liquid slurry is washed, concentrated phosphoric acid is added to adjust the solution pH to 1.0-1.5, high-speed stirring is performed to mix uniformly, and high-temperature aging is performed to obtain iron phosphate slurry;

[0008] S3. Filter and wash the iron phosphate slurry in S2, and then spray-dry and calcine to obtain anhydrous iron phosphate.

[0009] In step S1, the phosphorus source is one or more of soluble diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and disodium hydrogen phosphate; the iron source is one or more of ferrous phenylacetate, ferrous benzoate, or ferrous stearate; and the organic solvent is one or more of benzene, toluene, or xylene. The heterogeneous interface is the interface between the aqueous phase and the organic phase, and the organic solvent is an organic solution that is insoluble in water and has a lower density than water, which facilitates subsequent layering. Based on the principle of like dissolves like, the iron source is an organic iron salt that is readily soluble in the organic solvent.

[0010] In step S1, the oxidant is a mixture of one or more of H2O2, O2, and air. The dropwise addition time of raw material B is controlled to be 30 minutes, the stirring speed is 600-1200 rpm, and after the dropwise addition is completed, the stirring speed is 300-400 rpm, and the reaction time is 50-60 minutes.

[0011] In step S2, the mass ratio of filter cake, deionized water and concentrated phosphoric acid is 10-15:20-25:1-1.6, the stirring speed is 400-600 rpm, and the reaction is carried out at 90-95 °C for 70-80 min. -1 The product is oxidized by potassium permanganate solution, extracted and purified by deionized water, and recycled for the preparation of raw material B.

[0012] The heating rate in step S3 is 5-10°C / min, the sintering temperature is 550-650°C, and the sintering time is controlled within 3-8 hours.

[0013] The present invention achieves the following benefits by synthesizing two-dimensional ferric phosphate with the assistance of heterogeneous interfaces:

[0014] 1) The present invention uses a heterogeneous interface-assisted synthesis device to provide an interface reaction zone between the organic phase and the aqueous phase. This method uses high-speed shear force stirring to construct a multi-site oil-water reaction interface, and reacts at the interface between the aqueous phase and the organic phase to generate iron phosphate crystal nuclei, while inducing Fe 3+ and PO4 3- Directed growth along the surface of the crystal nucleus results in the formation of two-dimensional flake iron phosphate. The material synthesized by this method has a regular morphology, small iron phosphate particle size (D50 is 3-4 μm), and a narrow thickness (50-100 nm), making it suitable for the synthesis of high-density lithium iron phosphate.

[0015] 2) Using an organic iron salt as the iron source improves the solubility of the raw material based on the principle of like dissolves like. Furthermore, the organic solvent easily separates from the aqueous phase and can be recycled in the preparation of raw materials after reaction and extraction, demonstrating excellent practical and economic value. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 These are SEM images of ferric phosphate dihydrate and anhydrous ferric phosphate prepared in the present invention.

[0017] Figure 2 The XRD patterns of ferric phosphate dihydrate and anhydrous ferric phosphate prepared in the present invention are shown. DETAILED DESCRIPTION

[0018] To avoid repetition, the present invention is further described below with reference to the accompanying drawings and specific embodiments, which does not limit the scope of protection thereof.

[0019] The two-dimensional iron phosphate crystal nucleus of the present invention is formed by contacting an aqueous phosphate solution and an organic iron salt solution. The two-phase reaction interface provides a nucleation site for the two-dimensional iron phosphate crystal nucleus. The oil-water reaction interface is constructed by high-pressure dripping and high-speed shear stirring, which promotes the formation and reaction of multiple sites and finally induces Fe 3+ and PO4 3- Oriented growth occurs along the surface of the crystal nucleus, resulting in a two-dimensional sheet of iron phosphate.

[0020] The phosphorus source is an inorganic salt ion compound soluble in water.

[0021] The organic solvent is a common solvent that has a density lower than that of water and is immiscible with water.

[0022] The iron source is an organic iron salt ion compound soluble in the organic solvent.

[0023] The method for preparing flaky iron phosphate provided by the present invention comprises the following steps:

[0024] S1. Weigh the phosphorus source and iron source in a ratio of P:Fe=1:1 and dissolve them in water and organic solvent respectively to prepare 1 mol L -1 Raw materials A and B;

[0025] S2, adding an oxidant to raw material A to obtain a mixed bottom liquid, then adding the mixed bottom liquid and raw material B to the reaction chamber and the feeding chamber of the heterogeneous interface reaction device respectively, starting stirring, controlling the discharge valve of the feeding chamber to dropwise add raw material B into the reaction chamber, controlling the dropwise addition flow rate, adjusting the stirring speed after the dropwise addition is completed, and aging to obtain a reaction liquid slurry;

[0026] S3. After standing and stratifying, the lower layer of reaction liquid slurry is taken out, filtered and washed to obtain a filter cake, deionized water and 85% phosphoric acid are added to the filter cake and mixed evenly, the temperature is raised to react, and the reaction is continued after the reaction liquid slurry turns white. After cooling, the material is discharged to obtain dihydrated iron phosphate slurry.

[0027] S4. Filter and wash the ferric phosphate dihydrate slurry in S3, dry it, and then calcine it to obtain anhydrous ferric phosphate.

[0028] The method for preparing lithium iron phosphate provided by the present invention comprises the following steps:

[0029] S5, mixing the iron phosphate obtained in steps S1 to S4 with a lithium source and a carbon source, and freeze-drying the mixture to obtain a lithium iron phosphate precursor;

[0030] S6. Sintering the lithium iron phosphate precursor under an inert atmosphere to obtain lithium iron phosphate.

[0031] Example 1

[0032] A1: Weigh ammonium dihydrogen phosphate and ferrous phenylacetate in a ratio of P:Fe = 1:1, dissolve them in water and benzene respectively to prepare 1 mol L -1 Raw materials A and B;

[0033] A2: Add 400 g of 25% hydrogen peroxide to 2 L of raw material A to obtain a mixed bottom solution. Place the mixed bottom solution in a heterogeneous interfacial reactor (a heterogeneous interfacial reactor known in the art, such as a liquid phase reactor). Stir at 700 rpm. Then, add 2 L of raw material B dropwise to the mixed bottom solution at a rate of 150 mL min. -1 , a heterogeneous interfacial reaction was carried out, the pressure was controlled at 0.1 MPa, the reaction temperature was 43 ° C during the reaction process, the stirring speed was adjusted to 400 rpm after the dropwise addition was completed, and the reaction liquid slurry was obtained after aging for 60 min;

[0034] A3: The reaction liquid slurry obtained in A2 was allowed to stand for 30 minutes. The lower layer of reaction liquid slurry was taken out from the discharge bin, filtered and washed to obtain a filter cake. 5 L of deionized water and 150 g of food-grade 85 phosphoric acid were added to the filter cake and mixed evenly. The temperature was raised to 95 ° C, and the stirring speed was controlled at 500 rpm. After the reaction liquid slurry turned white, it was kept warm for 2 hours. After cooling, the material was discharged to obtain dihydrated iron phosphate slurry. In order to achieve effective utilization, the upper layer organic solvent in the present invention was 0.5 mol·L -1 Oxidize with potassium permanganate for 30 minutes, add water and mix evenly, extract the upper organic solution and reuse it in the preparation of raw material B;

[0035] A4: Filter and wash the ferric phosphate dihydrate slurry in A3 with pure water until the conductivity of the washing water is less than 0.3 mS·cm-1 , the product was dried at 95 °C, and finally the dihydrated ferric phosphate was calcined at 600 °C to obtain anhydrous ferric phosphate.

[0036] A5: Add the iron phosphate prepared in steps A1 to A4 to a stirring tank, and add lithium carbonate, graphite black, and glucose, wherein the molar ratio of FePO4:Li2CO3 is 1:0.6, and graphite black and glucose are added at 5% and 10% by weight of the iron phosphate, respectively; use deionized water as the dispersion medium, stir for 24 hours, and then freeze at -15°C for 24 hours, and then dry in a freeze dryer to obtain a lithium iron phosphate precursor;

[0037] A6. calcining the lithium iron phosphate precursor at 800° C. for 12 h under a nitrogen atmosphere and naturally cooling it to room temperature to obtain lithium iron phosphate.

[0038] Example 2-3

[0039] In Examples 2-3, the iron source in Step A1 was modified, while all other conditions remained the same as in Example 1. In Example 2, ferrous stearate was used as the iron source; in Example 3, ferrous benzoate was used as the iron source. Under the same conditions (as in Example 1), the reaction mixture was reacted to obtain a reaction slurry, which was then subjected to subsequent steps to obtain anhydrous ferric phosphate and lithium iron phosphate.

[0040] Examples 4-5

[0041] In Examples 4-5, the type of dispersant for raw material B in step A1 was changed, while all other conditions remained the same as in Example 1. In Example 4, toluene was used as the dispersant for raw material B, while in Example 5, xylene was used as the dispersant for raw material B. Under the same conditions (as in Example 1), a reaction liquid slurry was obtained, and anhydrous ferric phosphate and lithium iron phosphate were obtained through subsequent steps.

[0042] Examples 6-7

[0043] In Examples 6-7, the ratio of P to Fe in step A1 was changed, while the other conditions remained the same as in Example 1. In Example 8, the ratio of P to Fe was 1.5:1, and in Example 9, the ratio of P to Fe was 0.8:1. Under the same conditions (as in Example 1), the reaction mixture was reacted to obtain a reaction slurry, and anhydrous ferric phosphate and lithium iron phosphate were obtained in subsequent steps.

[0044] Examples 8-10

[0045] In Examples 8-10, the stirring speed during the dropwise addition process in Step A2 was varied, while all other conditions remained the same as in Example 1. The stirring speed in Example 10 was 500 rpm, and in Example 11 it was 1000 rpm. In Example 11, aging was also performed at 700 rpm after the iron source was added. Under the same other conditions (as in Example 1), a reaction solution slurry was obtained, and anhydrous ferric phosphate and lithium iron phosphate were obtained in subsequent steps.

[0046] Comparative Example 1-2

[0047] Comparative Examples 1-2 varied the type of dispersant for raw material B in step A1, while all other conditions remained the same as in Example 1. Ethanol was used as the dispersant for raw material B in Comparative Example 1, while ethylene glycol was used as the dispersant for raw material B in Comparative Example 2. Under the same conditions (as in Example 1), a reaction mixture slurry was obtained, and anhydrous ferric phosphate and lithium iron phosphate were obtained in subsequent steps.

[0048] Comparative Example 3

[0049] Comparative Example 3 varied the iron source and dispersant in step A1, while remaining consistent with Example 1. In step A1, ferrous sulfate crystals were dissolved in deionized water to prepare raw material B. The reaction was conducted under the same conditions (as in Example 1) to obtain a reaction liquid slurry, which was then subjected to subsequent steps to obtain anhydrous ferric phosphate.

[0050] Comparative Example 4

[0051] Comparative Example 4 is a comparative example in which two-dimensional iron phosphate is directly used as the raw material and the same process as in Example 1 is used to prepare two-dimensional lithium iron phosphate. The two-dimensional iron phosphate is a commercially available two-dimensional iron phosphate with a thickness of 50-100 nm. The two-dimensional lithium iron phosphate is prepared under the same conditions as in Example 1.

[0052] Table 1 shows the performance data of lithium iron phosphate of the embodiment and comparative example

[0053]

Claims

1. A method for synthesizing flaky iron phosphate by heterogeneous interface assistance, characterized in that: The steps include: S1. After adding an oxidant to the phosphorus source solution, an iron source solution is added dropwise to carry out a heterogeneous interfacial reaction. After the reaction is completed, the reaction solution is aged to obtain a reaction liquid slurry; the iron source is one or more of ferrous phenylacetate, ferrous benzoate or ferrous stearate, and the phosphorus source and iron source solutions are respectively prepared with water and an organic solvent, wherein the organic solvent is selected from one or more of benzene, toluene or xylene, and the iron source solution is added dropwise at a rate of 130-200 mL / min; the stirring speed during the dropwise addition is 600-1200 rpm; the stirring speed after the dropwise addition is completed is 300-400 rpm, and the reaction time is 50-60 min; S2. The reaction liquid slurry in S1 is allowed to stand for stratification. The lower layer of the reaction liquid slurry is washed to obtain a filter cake. Water and concentrated phosphoric acid are added to the filter cake to adjust the pH of the solution to 1.0-1.

5. The mixture is stirred and dispersed uniformly at 400-600 rpm. The mixture is heated to 90-95°C and reacted for 70-80 minutes. After the reaction liquid slurry turns white, it is kept warm and aged to obtain iron phosphate slurry. S3. Filter and wash the iron phosphate slurry in S2, and then spray-dry and calcine to obtain anhydrous iron phosphate.

2. The method for synthesizing flaky iron phosphate by heterogeneous interface-assisted method according to claim 1, characterized in that: In step S1, the molar ratio of P:Fe in the phosphorus source and the iron source is controlled to be 1:1 to 1.5:

1.

3. The method for synthesizing flaky iron phosphate by heterogeneous interface-assisted method according to claim 1, characterized in that: In step S1, the phosphorus source is selected from one or more of soluble diammonium hydrogen phosphate, ammonium dihydrogen phosphate and disodium hydrogen phosphate.

4. The method for heterogeneous interface-assisted synthesis of flake iron phosphate according to claim 3, characterized in that: The oxidant is selected from H2O2, O2, air or a mixture thereof.

5. The method for synthesizing flaky iron phosphate by heterogeneous interface-assisted method according to claim 1, characterized in that: In step S1, the heterogeneous interface reaction pressure is 0.1-0.2 MPa, and the reaction temperature is 40-55°C.

6. The method for synthesizing flaky iron phosphate by heterogeneous interface-assisted method according to claim 1, characterized in that: During the calcination process in step S3, the heating rate is 5-10°C / min, the sintering temperature is 550-650°C, and the sintering time is controlled within 3-8 hours.

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