Method for preparing iron phosphate dihydrate of nanoparticles

Abstract

The invention discloses a method for preparing iron phosphate dihydrate of nanoparticles, which comprises the following steps of: diluting an iron phosphate complex solution by using water, reducing the concentration to 10% or lower of the original concentration, and carrying out solid-liquid separation to obtain the iron phosphate dihydrate nanoparticles. Nanoscale irregular iron phosphate dihydrate particles obtained by the preparation method provided by the invention are further subjected to high-temperature dehydration to form iron phosphate nanoparticles with crystal configuration. According to the method disclosed by the invention, the crystal aging process in the existing iron phosphate dihydrate preparation process is reduced, and the energy consumption is greatly reduced. Besides, nano-particle iron phosphate is mixed with large-particle-size iron phosphate particles, so that gaps among the particles can be reduced in a high compaction process, and the preparation of high-compaction lithium iron phosphate is realized. The preparation method disclosed by the invention is simple and controllable in reaction condition and can realize industrial application.

Description

Technical Field

[0001] This invention belongs to the field of inorganic materials and lithium battery material preparation technology, specifically relating to a method for preparing iron phosphate, a precursor material for lithium-ion batteries. Background Technology

[0002] The development of lithium-ion battery materials is rapid, especially lithium iron phosphate (LFP) and ternary materials. Based on the requirement to improve the energy density of LFP batteries, increasing the compaction density of LFP can be one way to improve the energy density of LFP batteries. Iron phosphate, as a raw material for preparing LFP, has a direct impact on the compaction density of LFP. Preparing high-purity, high-compaction iron phosphate can ultimately improve the energy density of LFP battery packs.

[0003] The main methods for preparing high-pressure iron phosphate products are as follows:

[0004] CN115215313A generates an amorphous ferrous phosphate octahydrate reaction precursor in situ in a first mixture system by controlling the synthesis conditions. Then, the dissolution rate of the amorphous ferrous phosphate octahydrate reaction precursor is controlled by controlling the pH of the second slurry, thereby regulating the concentration of phosphate and ferrous ions in the reaction system. Finally, the supersaturation of ferric and phosphate ions in the reaction system is precisely controlled by adding oxidizing substances.

[0005] CN116534820B provides a method for preparing high-pressure ferric phosphate from industrial monoammonium phosphate and ferrous sulfate. The method adopts a reaction route of oxidation followed by metathesis. First, hydrogen peroxide is used to completely oxidize the refined ferrous sulfate solution. Then, monoammonium phosphate is slowly added. The initial iron concentration of the reaction solution is high, which allows the small amount of added monoammonium phosphate to react rapidly with the high concentration of iron ions to form small particles of ferric phosphate. In subsequent reactions, these small particles of ferric phosphate aggregate and grow, resulting in a complete reaction.

[0006] CN111533103A discloses a method for preparing high-pressure lithium iron phosphate. The method first prepares ferric hydroxide colloid, then reacts it with ammonium bicarbonate and phosphoric acid to obtain filter residue. The washed filter residue is added to a sucrose solution, and the resulting slurry is spray-dried. The spray-dried material is calcined to obtain high-pressure lithium iron phosphate. Then, high-pressure lithium iron phosphate is added to an organic carbon source, lithium carbonate, a dispersant, graphite, and water, and the mixture is mixed and ground until the particle size is 150-250 nm to obtain a ground slurry. This slurry is then calcined twice to obtain high-pressure lithium iron phosphate.

[0007] CN111377426B discloses a method for preparing anhydrous iron phosphate nanoparticles. The resulting material is a micron-sized secondary structure formed by the aggregation of nanoparticles. The iron phosphate material is not a primary nanoparticle.

[0008] Judging from the aforementioned patents or patent applications, preparing iron phosphate with nanoparticles or micron aggregates with nano-primary particles is one of the important routes for preparing high-pressure lithium iron phosphate. However, so far, there is a lack of a simple and low-cost industrial application method to obtain nanoscale iron phosphate particles. Summary of the Invention

[0009] The purpose of this application is to provide a method for preparing irregular nanoparticles of iron phosphate dihydrate. The inventors of this application believe that lithium iron phosphate particles prepared using nano-sized iron phosphate as seed crystals have high compaction density, or that mixing nanoparticles with large-particle iron phosphate materials can also improve the compaction density of lithium iron phosphate materials.

[0010] The present invention provides a method for preparing iron phosphate dihydrate nanoparticles, the method comprising: diluting an iron phosphate complex solution with water to reduce the concentration to 10% or less of the original concentration, preferably to 10%-0.5% of the original concentration, and performing solid-liquid separation to obtain iron phosphate dihydrate nanoparticles.

[0011] Ferric phosphate dihydrate nanoparticles are dehydrated at high temperature to obtain ferric phosphate nanoparticles. The high-temperature dehydration temperature is 400-1000℃, preferably 500-800℃.

[0012] In the iron phosphate complex solution, the phosphorus-to-iron ratio is 2-10, preferably 3.75-6; the concentration of phosphorus in the complex solution is greater than 2 mol / L, preferably 2.5 mol / L-14.6 mol / L. The phosphorus (P) includes all phosphorus elements in the solution, including the iron phosphate complex, phosphoric acid, and phosphate ions. For ease of calculation, the amount of phosphoric acid feedstock added in the initial reaction to form the iron phosphate complex can be used for calculation.

[0013] The dilution method includes adding the complexing solution to water, or adding water to the complexing solution. Both methods yield nanoparticles. The inventors observed that once the dilution concentration (10% or less of the original concentration) is reached, a large amount of precipitation occurs immediately, indicating that the ferric phosphate dihydrate nanoparticles have been formed.

[0014] Preferably, the volume ratio of water to complexing solution is (12-120):1, and more preferably, the volume ratio of water to complexing solution is (16-50):1.

[0015] The dilution can achieve the desired effect at temperatures ranging from 0°C to 90°C, but is preferably carried out between 20°C and 85°C. Excessively high temperatures are uneconomical and pose operational risks, while excessively low temperatures can cause separation difficulties.

[0016] The diluted solution will produce a precipitate, which can be separated into iron phosphate nanoparticles through solid-liquid separation. The solid-liquid separation is performed using conventional methods, such as filtration, centrifugation, and settling. Considering the small size of the nanoparticles, centrifugation is preferred to obtain solid iron phosphate dihydrate nanoparticles.

[0017] The separated mother liquor can be recycled to partially replace the water used for dilution. The principle for replacing part of the dilution water with mother liquor is to ensure that the concentration of the complex after dilution reaches 10% or less of the original concentration. For example, the mother liquor can be mixed with water at volume ratios of 2:1, 1:1, 1:2, 1:3, 1:4, 1:10, 1:20, etc., and used to replace the dilution water. Recycling the mother liquor can reduce the pressure on wastewater treatment.

[0018] This invention further provides a method for preparing iron phosphate nanoparticles, the method comprising:

[0019] S1. Preparation of ferric phosphate complex solution;

[0020] S2. Dilute the ferric phosphate complex solution with water to 10% or less of the original concentration, and separate the solid and liquid to obtain solid ferric phosphate dihydrate nanoparticles.

[0021] S3, iron phosphate dihydrate nanoparticles are dehydrated at high temperature to obtain iron phosphate nanoparticles.

[0022] In the iron phosphate complex solution, the phosphorus-to-iron ratio is 2-10, preferably 3.75-6; the concentration of phosphorus in the complex solution is greater than 2 mol / L, preferably 2.5 mol / L-14.6 mol / L. The phosphorus (P) includes all phosphorus elements in the solution, including the iron phosphate complex, phosphoric acid, and phosphate ions.

[0023] The preparation method of the ferric phosphate complex solution adopts conventional methods, such as reacting iron powder with phosphoric acid, and then using an oxidizing agent to oxidize ferrous ions to ferric ions to form a ferric phosphate complex solution; or reacting ferric oxide with phosphoric acid to obtain a ferric phosphate complex solution; or reacting ferric oxide with phosphoric acid, adding an oxidizing agent to oxidize ferrous ions to ferric ions to form a ferric phosphate complex solution. The preparation method described in Chinese Patent CN116924365A can be referenced.

[0024] The inventors of this application have discovered that by adjusting the feed ratio of ferric phosphate complex solution to water and the temperature during dilution, higher yields of nanoparticle ferric phosphate dihydrate can be achieved.

[0025] In step S2, the volume ratio of water to complexing solution is controlled to be greater than 12, and the preferred volume ratio is 16 to 50.

[0026] In step S2, the system temperature is controlled to be between 0°C and 90°C during dilution, preferably between 20°C and 85°C, and more preferably between 20°C and 60°C.

[0027] The temperature for high-temperature dehydration in step S3 is 400-1000℃, preferably 500-800℃.

[0028] The iron phosphate nanoparticles obtained after solid-liquid separation using the aforementioned method are amorphous nanoparticles of iron phosphate dihydrate. Iron phosphate dihydrate (or iron phosphate dihydrate) is conventionally considered to be iron phosphate. These amorphous iron phosphate dihydrate nanoparticles undergo high-temperature dehydration to transform into crystalline iron phosphate nanoparticles, thus avoiding the prolonged aging and crystallization process required in other methods.

[0029] Terminology Explanation:

[0030] Ferric phosphate complex: Ferric phosphate-phosphoric acid complex formed by ferric phosphate and phosphoric acid, existing in solution form. In this invention, "ferric phosphate complex," "ferric phosphate complex solution," "ferric phosphate complex liquid," and "complex liquid" have the same meaning and can be used interchangeably.

[0031] Phosphorus-to-iron ratio: In this invention, the phosphorus-to-iron ratio (P:Fe) refers to the molar ratio (molar ratio) of phosphorus to iron in a system (e.g., a solution of ferric phosphate complex). Phosphorus refers to all phosphorus elements in the system, and iron refers to all iron elements in the system. For example, in the solution or product obtained by reacting 3 mol of phosphoric acid with 1 mol of ferric hydroxide, the phosphorus-to-iron ratio is 3:1, while the phosphorus-to-iron ratio of 6 mol of phosphoric acid with 1 mol of iron(III) oxide is 2:1. In this invention, the phosphorus-to-iron ratio is described as 3:1 or 3, which have the same meaning, both indicating that phosphorus:iron = 3:1.

[0032] Iron ions: The iron ions described in this invention include free iron ions, and free iron ions include ferrous ions (Fe2+). 2+ ), trivalent iron ions (Fe 3+ ).

[0033] Room temperature: refers to indoor temperature. In this invention, room temperature is the temperature that does not require heating, which is usually 15-25℃.

[0034] Dilution concentration: refers to the concentration of the solute after dilution relative to the original solution. Although the original solute may change during the dilution process, the degree of dilution is still calculated based on the original solute concentration. For example, the iron phosphate complex in this application may decompose during the dilution process; however, the degree of dilution is calculated based on the original concentration of the iron phosphate complex, iron ions, or phosphate ions.

[0035] min: indicates minutes.

[0036] H or h: indicates hours.

[0037] Beneficial effects

[0038] This invention provides, for the first time, a method for preparing nano-sized iron phosphate dihydrate materials. The method yields irregularly shaped nanoparticles of iron phosphate dihydrate, which can be used to improve the compaction density of lithium iron phosphate. The method obtains nano-sized, irregular iron phosphate dihydrate particles, which are then further dehydrated at high temperatures to form crystalline iron phosphate nanoparticles. This method reduces the crystal aging process in current iron phosphate dihydrate preparation processes, significantly lowering energy consumption. Furthermore, mixing the nanoparticles of iron phosphate with larger-diameter iron phosphate particles reduces the gaps between particles during high-pressure compaction, enabling the preparation of high-pressure compacted lithium iron phosphate. The preparation method described in this application has simple and controllable reaction conditions, allowing for industrial-scale application. Attached Figure Description

[0039] Figure 1 The image shown is an SEM image of the product from Example 1.

[0040] Figure 2 The image shown is an SEM image of the product from Example 2.

[0041] Figure 3 The image shown is the SEM image of the product of Example 3.

[0042] Figure 4 The image shown is the SEM image of the product of Example 4.

[0043] Figure 5 The image shown is an SEM image of the product from Example 5.

[0044] Figure 6 The image shown is an SEM image of the product from Example 7. Detailed Implementation

[0045] The present invention will be further described below with reference to embodiments. It should be noted that the embodiments are not intended to limit the scope of protection of the present invention, and those skilled in the art will understand that any improvements and variations made based on the present invention are within the scope of protection of the present invention.

[0046] The particle size measurement method is as follows: Particle size can be marked using a scanning electron microscope. Calibration is performed according to the magnification and scale. The test results are then verified using a Malvern laser particle size analyzer. The Malvern laser particle size analysis method is as follows: A small amount of sample (0.003g-0.1g each time) is taken with a sample spoon and added to the testing instrument in three portions. The change in the system shading rate of the testing instrument is observed. Data is recorded when the shading rate reaches between 9% and 10%.

[0047] The common reagents used in the following examples are all commercially available.

[0048] Example 1: Dilution at room temperature

[0049] Measure 31 mL of H3PO4 solution (wt = 85%) and 19 mL of H2O to form a mixed solution. Accurately weigh 7.9844 g of Fe2O3 powder. Mix the Fe2O3 powder with the phosphoric acid aqueous solution to form a mixed system. Transfer the mixed system to an oil bath and keep it at 90℃ for 3 h. Filter to obtain ferric phosphate complex solution.

[0050] At room temperature (20℃), different volumes of complexing solution (1 mL, 2 mL, 3 mL, 4 mL, 5 mL, 6 mL, 7 mL) were added dropwise to 100 mL of water. After stirring for 30 minutes, the mixture was centrifuged, washed, and dried to obtain amorphous nano-sized iron phosphate dihydrate powder. When 6 mL of complexing solution was added, the precipitate showed significant dissolution; when 7 mL was added, the precipitate completely dissolved, and no further precipitate was formed even after the reaction ended. Therefore, from the perspective of efficiency in obtaining iron phosphate dihydrate, at room temperature, the volume ratio of water to complexing solution should be greater than 16, with a better ratio of 33. Figure 1 The image shows the SEM pattern of the product obtained by adding 1 mL of complexing solution to 100 mL of water. As can be seen from the image, the morphology of the product is nanoparticles. Figure 1 The average particle size of the nanoparticles is approximately 106.4 nm.

[0051] Table 1 Experimental Results of Example 1-X

[0052]

[0053] Example 2: Dilution at 60°C

[0054] Measure 31 mL of H3PO4 solution (wt = 85%) and 19 mL of H2O to form a mixed solution. Accurately weigh 7.9844 g of Fe2O3 powder. Mix the Fe2O3 powder with the phosphoric acid aqueous solution to form a mixed system. Transfer the mixed system to an oil bath and keep it at 90℃ for 3 h. Filter to obtain ferric phosphate complex solution.

[0055] Take 100 mL of water and keep it at 60°C in an oil bath. Add different volumes of complexing solution (1 mL, 2 mL, 3 mL, 4 mL, 5 mL, 6 mL) dropwise to the 100 mL of water at a constant temperature of 60°C. Stir for 30 minutes, then centrifuge, wash, and dry. Figure 2 The image shows the morphology of the product prepared by incubating 100 mL of water and 2 mL of complexing solution at 60 °C for 30 min. The average particle size was measured to be approximately 390 nm.

[0056] Table 2 Experimental Results of Example 2-X

[0057]

[0058] Example 3: Dilution at 70°C

[0059] 31 mL of H3PO4 and 19 mL of H2O were measured to form a phosphoric acid aqueous solution. 7.9844 g of Fe2O3 powder was accurately weighed and added to the phosphoric acid aqueous solution. The mixture was heated to 90 °C and kept at this temperature for 3 hours. The resulting complex was obtained by filtration. 100 mL of water was measured and kept at 70 °C in an oil bath. Different volumes of the complex (1 mL, 3 mL, 6 mL) were added dropwise to the 100 mL water at a constant temperature of 70 °C. After stirring for 30 minutes, the mixture was centrifuged, washed, and dried. The morphology of the mixture after adding 6 mL of the complex and incubating at 70 °C for 30 minutes is shown below. Figure 3 As shown in the figure, the average particle size was measured to be approximately 370 nm.

[0060] Table 3 Experimental Results of Example 3-X

[0061]

[0062] Example 4: Dilution at 60℃ + Increasing the concentration of ferric phosphate complex

[0063] 42 mL of H3PO4 and 8 mL of H2O were measured to form a phosphoric acid aqueous solution. 10.646 g of Fe2O3 powder was accurately weighed and added to the phosphoric acid aqueous solution. The mixture was heated to 90 °C and maintained at this temperature for 3 hours. The resulting complex solution was obtained by filtration. 100 mL of water was measured and kept at 60 °C in an oil bath. Different volumes of the complex solution (1 mL, 2 mL, 3 mL, 6 mL) were added dropwise to the 100 mL water at a constant temperature of 60 °C. After stirring for 30 minutes, the mixture was centrifuged, washed, and dried. The morphology of 3 mL of the complex solution added to 100 mL of H2O is shown in the figure. Figure 4 As shown in the figure, the average particle size was measured to be approximately 450 nm.

[0064] Table 4 Experimental Results of Example 4-X

[0065]

[0066] Example 5: Insulation at 80℃

[0067] Measure 31 mL of H3PO4 solution (wt = 85%) and 19 mL of H2O to form a mixed solution. Accurately weigh 7.9844 g of Fe2O3 powder. Mix the Fe2O3 powder with the phosphoric acid aqueous solution to form a mixed system. Transfer the mixed system to an oil bath and keep it at 90℃ for 3 h. Filter to obtain ferric phosphate complex solution.

[0068] Measure 100 mL of water and incubate it in an oil bath at 80°C. Add different volumes of complexing solution (6 mL, 7 mL, 8 mL, 9 mL, 10 mL, 11 mL) dropwise to the 100 mL water at a constant temperature of 80°C. After stirring for 30 minutes, centrifuge, wash, and dry. The 7 mL complexing solution was characterized by SEM as follows: Figure 5 As shown in the figure, the particle size was measured to be 410 nm. When the volume of the complexing solution added was greater than 9 mL, the resulting white precipitate dissolved and, after incubation at 80 °C for 30 min, the morphology was irregular polyhedral micron-sized particles. When the volume of the complexing solution added was 11 mL, no precipitate formed after incubation at 80 °C for 30 min.

[0069] Table 5 Experimental Results of Example 5-X

[0070]

[0071] Example 6: Dilution at 0°C

[0072] Measure 31 mL of H3PO4 solution (wt = 85%) and 19 mL of H2O to form a mixed solution. Accurately weigh 7.9844 g of Fe2O3 powder. Mix the Fe2O3 powder with the phosphoric acid aqueous solution to form a mixed system. Transfer the mixed system to an oil bath and keep it at 90℃ for 3 h. Filter to obtain ferric phosphate complex solution.

[0073] Measure 100 mL of H2O into a three-necked flask, transfer the flask to an ice-water mixture, and measure the system temperature at 0 °C after 30 min. Accurately measure 1 mL of the complexing solution, add it to the three-necked flask, stir for 30 min, and centrifuge to obtain a colorless gel. Transfer the gel to a 90 °C oven, and after 12 h, the gel turns into a white powder, which is nanoparticles.

[0074] The experimental results show that when the complexing solution is poured into water for dilution, a precipitate forms. However, during centrifugation and washing, the precipitate directly turns into a gel. Drying the gel product yields a white powder, which is nanoparticles.

[0075] Example 7: Dilution at 90°C

[0076] Measure 31 mL of H3PO4 solution (wt = 85%) and 19 mL of H2O to form a mixed solution. Accurately weigh 7.9845 g of Fe2O3 powder. Mix the Fe2O3 powder with the phosphoric acid aqueous solution to form a mixed system. Transfer the mixed system to an oil bath and keep it at 90℃ for 3 h. Filter to obtain ferric phosphate complex solution.

[0077] 100 mL of water was measured and kept at 90 °C in an oil bath. Different volumes of complexing solution (5 mL, 6 mL, 7 mL, 8 mL, 9 mL, 10 mL, 11 mL) were added dropwise to the 100 mL water at a constant temperature of 90 °C. After stirring for 30 minutes, the mixture was centrifuged, washed, and dried. When the volume of complexing solution added exceeded 10 mL, the precipitate dissolved first, and then precipitated again after being kept at 90 °C for 30 minutes. The prepared product was characterized by SEM, and the morphology showed that the product consisted of irregular micron-sized particles. The morphology of the product after adding 9 mL of complexing solution is shown in the figure below. Figure 6 As shown, the product morphology consists of irregular micron-sized particles. These experimental conditions cannot prepare dispersed nanoparticles.

[0078] Table 6 Experimental Results of Example 7-X

[0079]

[0080] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing ferric phosphate dihydrate nanoparticles, characterized in that: The ferric phosphate complex solution was diluted with water to reduce the concentration to 10% or less of the original concentration, preferably to 10%-0.5%. Solid-liquid separation was then performed to obtain ferric phosphate dihydrate nanoparticles. In the iron phosphate complex solution, the phosphorus-to-iron ratio is 2-10, preferably 3.75-6; the concentration of phosphorus in the complex solution is greater than 2 mol / L, preferably 2.5 mol / L-14.6 mol / L.

2. The method as described in claim 1, characterized in that, The dilution method includes: adding the complexing solution to water, or adding water to the complexing solution.

3. The method as described in claim 1 or 2, characterized in that, The volume ratio of water to complexing solution is (12-120):1, and the preferred volume ratio of water to complexing solution is (16-50):

1.

4. The method according to any one of claims 1-3, characterized in that, The dilution is carried out at 0°C to 90°C, preferably at 20-85°C.

5. The method according to any one of claims 1-4, characterized in that, After dilution, a precipitate will form in the solution. Through solid-liquid separation, iron phosphate dihydrate nanoparticles can be obtained.

6. The method as described in claim 5, characterized in that, The mother liquor after solid-liquid separation is recycled and reused, partially replacing the aforementioned dilution water.

7. A method for preparing iron phosphate nanoparticles, the method comprising: S1. Preparation of ferric phosphate complex solution; S2. Dilute the ferric phosphate complex solution with water to 10% or less of the original concentration, and separate the solid and liquid to obtain ferric phosphate dihydrate nanoparticles. S3, iron phosphate dihydrate nanoparticles are dehydrated at high temperature to obtain iron phosphate nanoparticles.

8. The method as described in claim 7, characterized in that, The preparation method of the ferric phosphate complex solution is selected from: reacting iron powder with phosphoric acid, and then using an oxidizing agent to oxidize ferrous ions into ferric ions to form an ferric phosphate complex solution; or reacting ferric oxide with phosphoric acid to obtain an ferric phosphate complex solution; or reacting ferric oxide with phosphoric acid, adding an oxidizing agent to oxidize ferrous ions into ferric ions to form an ferric phosphate complex solution.

9. The method as described in claim 7 or 8, characterized in that, The high-temperature dehydration temperature is 400-1000℃, preferably 500-800℃.

10. The method according to any one of claims 1-9, characterized in that, Amorphous iron phosphate dihydrate nanoparticles are transformed into anhydrous iron phosphate nanoparticles with a crystalline structure after high-temperature dehydration.

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