Preparation method of lithium iron phosphate

Abstract

The invention discloses a lithium iron phosphate preparation method, which comprises: (1) adding cellulose to a lithium hydroxide aqueous solution, and uniformly mixing to obtain a cellulose mixed solution; (2) adding urea into the cellulose mixed solution, and uniformly mixing until the urea is completely dissolved to obtain a clear solution; (3) uniformly mixing an iron source, a titanium source and the settled solution to obtain mixed slurry; and (4) grinding the mixed slurry, adding a coagulator, uniformly mixing, drying and calcining to obtain the lithium iron phosphate. Cellulose is used for replacing a traditional carbon source, lithium hydroxide is used for dissolving cellulose and serves as a lithium source at the same time, a completely-dissolved cellulose aqueous solution and iron phosphate are fully ground, lithium iron phosphate grows in a cellulose three-dimensional network in situ, a continuous conductive network wrapping lithium iron phosphate particles is constructed, and the lithium iron phosphate is prepared. And a coagulator is added to'freeze 'the conductive network, so that the conductive network is kept uniform and stable in the subsequent drying process, and the electrochemical performance of the product is improved.

Description

Technical Field

[0001] This invention relates to a method for preparing lithium iron phosphate, and more particularly to a green and environmentally friendly method for preparing lithium iron phosphate. Background Technology

[0002] The EU Battery Regulation (EU) 2023 / 1542 provides a core policy lever for the green transformation of the battery industry chain by setting a minimum proportion of recycled materials used. This mandatory regulation directly drives the demand for alternative renewable raw materials from battery manufacturers. Cellulose, as the most abundant natural polymer compound, has significant advantages such as wide availability, low cost, and biodegradability. In the field of battery materials, cellulose can serve as a green carbon source, structural support, or binder, thereby reducing the carbon footprint of batteries from the source and improving resource recycling. However, due to the high bond energy of the repeating glucose units linked by β-1,4 glycosidic bonds in the cellulose molecule, it is difficult to break, making it difficult to dissolve in water and common organic solvents, which limits the use of cellulose.

[0003] CN103022425A discloses a method for preparing lithium iron phosphate material and a lithium-ion battery. Specifically, it discloses a method of dissolving cellulose in an aqueous lithium hydroxide solution at a low temperature of -5°C to -20°C using a co-solvent such as urea, then preparing a slurry with iron phosphate, followed by sintering to obtain the lithium iron phosphate material. However, this method cannot control the coating of cellulose on lithium iron phosphate, resulting in uneven coating during drying, which in turn affects the performance of the subsequently sintered lithium iron phosphate material. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a method for preparing lithium iron phosphate with environmentally friendly raw material sources and high capacity.

[0005] Technical solution: The preparation method of lithium iron phosphate of the present invention includes the following steps: (1) adding cellulose to lithium hydroxide aqueous solution and mixing evenly to obtain cellulose mixture; (2) adding urea to the aforementioned cellulose mixture and mixing evenly to completely dissolve it to obtain a clear solution; (3) mixing iron source, titanium source and the aforementioned clear solution evenly to obtain a mixed slurry; (4) grinding the aforementioned mixed slurry and adding coagulant, mixing evenly and then drying and calcining to obtain lithium iron phosphate.

[0006] In step (1), the cellulose includes at least one of sugarcane bagasse pulp and straw residue. The cellulose molecular structure has a three-dimensional grid structure and is carbonized during calcination to form a porous carbon skeleton that coats the surface of lithium iron phosphate.

[0007] Preferably, in step (1), the cellulose is a mixture of bagasse pulp and straw residue.

[0008] In step (1), the lithium hydroxide aqueous solution is pre-cooled before adding cellulose. The pre-cooling temperature is ≤8℃. Low temperature is beneficial for lithium hydroxide to break down the long chain of cellulose into short chains.

[0009] Preferably, in step (1), the pre-cooling temperature is 0~8℃.

[0010] Further, in step (1), the amount of cellulose added is 15-25% of the mass of the iron source; in step (2), the amount of urea added is 1-1.5 times the mass of cellulose.

[0011] Further, in step (1), the mixing is stirring, and the stirring time is 0.5~2 h.

[0012] In step (1), the mass concentration of the lithium hydroxide aqueous solution is 6-14%, which is used to break down the long chain of cellulose into short chains and participate in the generation of lithium iron phosphate as a lithium source precursor.

[0013] In step (2), the urea is an aqueous solution of urea with a mass concentration of 20-40%, which is used to help dissolve cellulose in an aqueous solution of lithium hydroxide.

[0014] Furthermore, in step (1), the temperature of the urea aqueous solution is 5~15℃.

[0015] Further, in step (2), the mixing is stirring, the stirring temperature is 15~40℃, and the stirring time is 1~3h.

[0016] In step (3), the iron source is iron phosphate, and its addition amount is 1:1 to 1.05:1 of the total molar ratio of Li:Fe in the system. The titanium source is titanium oxide, and its addition amount is 0.1 to 1% of the mass of the iron source.

[0017] In step (4), the coagulant includes at least one of phosphoric acid, ammonium sulfate, and ammonium phosphate. The amount of coagulant added is 0.1-5% of the mass of the iron source, which is used to cause the cellulose disintegrated into short chains to precipitate in situ on the surface of the iron phosphate particles, so that it remains uniform and stable during the subsequent drying process.

[0018] Furthermore, the phosphoric acid concentration is 85%.

[0019] Preferably, in step (4), the coagulant is ammonium sulfate or a mixture of ammonium sulfate and phosphoric acid. Ammonium sulfate can disrupt the coordination of lithium ions and hydroxyl groups on cellulose, thereby better promoting the precipitation of cellulose.

[0020] In step (4), the mixing temperature is 40~60℃.

[0021] In step (4), the drying is centrifugal spray drying, the inlet air temperature of the centrifugal spray is 180~230℃, the outlet air temperature is 90~105℃, and the spray particle size D50 is controlled to be 20~50 μm.

[0022] Furthermore, in step (4), the grinding temperature is 10~35℃, and the particle size of the slurry after grinding is 0.3~0.5μm. Grinding allows the cellulose to be fully coated on the surface of the iron phosphate.

[0023] Furthermore, in step (4), the calcination temperature is 650~850℃, the calcination time is 10~24 h, and the calcination is carried out under inert gas protection.

[0024] Invention Principle: This invention utilizes a lithium hydroxide aqueous solution dissolved at low temperature with urea as a solubilizer to dissolve cellulose. The small-molecule lithium hydroxide not only breaks down the long chains of cellulose into shorter chains but also penetrates the three-dimensional network structure of cellulose, dispersing uniformly on the cellulose skeleton. Simultaneously, lithium hydroxide also serves as a lithium source precursor, allowing it to combine with iron phosphate during sintering to grow in situ into lithium iron phosphate. Subsequently, the resulting clear and homogeneous cellulose-lithium hydroxide aqueous solution is wet-milled with iron phosphate to ensure uniform mixing of the components, effectively dispersing and fixing the iron phosphate particles within the three-dimensional cellulose network. Then, a coagulant is added to initiate the reassembly and precipitation of short-chain cellulose, "freezing" the previously formed three-dimensional network structure, encapsulating the iron phosphate particles, and ensuring the uniformity and stability of the structure during subsequent centrifugal drying. Finally, high-temperature calcination carbonizes the cellulose to form a porous carbon skeleton, coating the lithium iron phosphate particles and forming a continuous conductive network, thereby improving the electrochemical performance of the lithium iron phosphate product.

[0025] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The present invention uses cellulose to replace the traditional carbon source. Lithium hydroxide is used to dissolve cellulose and serve as a lithium source. The completely dissolved cellulose aqueous solution is thoroughly ground with iron phosphate, so that lithium iron phosphate grows in situ in the three-dimensional network of cellulose, constructing a continuous conductive network that encapsulates lithium iron phosphate particles. The conductive network is "frozen" by adding a coagulant, so that it remains uniform and stable during the subsequent drying process, thereby improving the electrochemical performance of lithium iron phosphate products; (2) The temperature and amount of coagulant in the coagulation process are reasonably controlled during the precipitation process of adding coagulant, thereby regulating the degree of cellulose coating on lithium iron phosphate and enhancing the cycle life of lithium iron phosphate batteries; (3) Straw residue and sugarcane bagasse, which are difficult to utilize, are selected as cellulose sources. The raw material sources are environmentally friendly, reducing the carbon footprint of lithium iron phosphate preparation process from the source and improving the circularity of resources. Attached Figure Description

[0026] Figure 1This is a scanning electron microscope (SEM) image of lithium iron phosphate prepared in Example 3 of the present invention. Detailed Implementation

[0027] The technical solution of the present invention will be further described below with reference to the embodiments and comparative examples. Unless otherwise stated, all reagents used are commercially available and used directly without purification.

[0028] Example 1

[0029] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 80 g of sugarcane bagasse slurry to the pre-cooled lithium hydroxide aqueous solution and stir for 1 h to obtain a cellulose mixture;

[0030] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0031] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0032] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 25 g of ammonium sulfate and add it to the ground mixed slurry. Stir at 40°C for 1 hour and then centrifuge spray dry to obtain the lithium iron phosphate precursor. The inlet air temperature of the centrifugal spray is 220°C and the outlet air temperature is 100°C. The spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, then crush and sieve to obtain the lithium iron phosphate product.

[0033] Example 2

[0034] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0035] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0036] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0037] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 25 g of ammonium sulfate and add it to the ground mixed slurry. Stir at 40°C for 1 hour and then centrifuge spray dry to obtain the lithium iron phosphate precursor. The inlet air temperature of the centrifugal spray is 220°C and the outlet air temperature is 100°C. The spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, then crush and sieve to obtain the lithium iron phosphate product.

[0038] Example 3

[0039] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0040] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0041] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0042] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 15 g of ammonium sulfate and 10 g of phosphoric acid (phosphoric acid concentration of 85%), add them to the ground mixed slurry, stir at 40°C for 1 hour, and then centrifuge spray dry to obtain lithium iron phosphate precursor. The inlet air temperature of the centrifuge spray is 220°C, the outlet air temperature is 100°C, and the spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, and then crush and sieve to obtain lithium iron phosphate product.

[0043] Example 4

[0044] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0045] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0046] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0047] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 15 g of ammonium sulfate and 10 g of phosphoric acid (phosphoric acid concentration of 85%), add them to the ground mixed slurry, stir at 60°C for 1 hour, and then centrifuge spray dry to obtain lithium iron phosphate precursor. The inlet air temperature of the centrifuge spray is 220°C, the outlet air temperature is 100°C, and the spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, and then crush and sieve to obtain lithium iron phosphate product.

[0048] Example 5

[0049] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0050] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0051] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0052] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 15 g of ammonium phosphate and 10 g of phosphoric acid (phosphoric acid concentration of 85%), add them to the ground mixed slurry, stir at 40°C for 1 hour, and then centrifuge spray dry to obtain lithium iron phosphate precursor. The inlet air temperature of the centrifuge spray is 220°C, the outlet air temperature is 100°C, and the spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, and then crush and sieve to obtain lithium iron phosphate product.

[0053] Example 6

[0054] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0055] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 40% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0056] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0057] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 25 g of ammonium sulfate and add it to the ground mixed slurry. Stir at 40°C for 1 hour and then centrifuge spray dry to obtain the lithium iron phosphate precursor. The inlet air temperature of the centrifugal spray is 220°C and the outlet air temperature is 100°C. The spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, then crush and sieve to obtain the lithium iron phosphate product.

[0058] Example 7

[0059] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 6% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0060] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0061] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0062] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 25 g of ammonium sulfate and add it to the ground mixed slurry. Stir at 40°C for 1 hour and then centrifuge spray dry to obtain the lithium iron phosphate precursor. The inlet air temperature of the centrifugal spray is 220°C and the outlet air temperature is 100°C. The spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, then crush and sieve to obtain the lithium iron phosphate product.

[0063] Example 8

[0064] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 14% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0065] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0066] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0067] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 25 g of ammonium sulfate and add it to the ground mixed slurry. Stir at 40°C for 1 hour and then centrifuge spray dry to obtain the lithium iron phosphate precursor. The inlet air temperature of the centrifugal spray is 220°C and the outlet air temperature is 100°C. The spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, then crush and sieve to obtain the lithium iron phosphate product.

[0068] Example 9

[0069] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0070] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0071] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0072] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 0.5 g of ammonium sulfate and add it to the ground mixed slurry. Stir at 40°C for 1 hour and then centrifuge spray dry to obtain the lithium iron phosphate precursor. The inlet air temperature of the centrifugal spray is 220°C and the outlet air temperature is 100°C. The spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, then crush and sieve to obtain the lithium iron phosphate product.

[0073] Example 10

[0074] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 100 g of straw residue to the pre-cooled lithium hydroxide aqueous solution and stir for 1 h to obtain a cellulose mixture;

[0075] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0076] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0077] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 25 g of ammonium sulfate and add it to the ground mixed slurry. Stir at 40°C for 1 hour and then centrifuge spray dry to obtain the lithium iron phosphate precursor. The inlet air temperature of the centrifugal spray is 220°C and the outlet air temperature is 100°C. The spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, then crush and sieve to obtain the lithium iron phosphate product.

[0078] Example 11

[0079] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0080] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0081] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0082] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 25 g of phosphoric acid (85% phosphoric acid concentration), add it to the ground mixed slurry, stir at 40°C for 1 hour, and then centrifuge spray dry to obtain lithium iron phosphate precursor. The inlet air temperature of the centrifugal spray is 220°C, the outlet air temperature is 100°C, and the spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, and then crush and sieve to obtain lithium iron phosphate product.

[0083] Comparative Example 1

[0084] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0085] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0086] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0087] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 15 g of ammonium sulfate and 10 g of phosphoric acid (phosphoric acid concentration of 85%), add them to the ground mixed slurry, stir at 20°C for 1 hour, and then centrifuge spray dry to obtain lithium iron phosphate precursor. The inlet air temperature of the centrifugal spray is 220°C, the outlet air temperature is 100°C, and the spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, and then crush and sieve to obtain lithium iron phosphate product.

[0088] Comparative Example 2

[0089] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0090] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0091] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0092] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 25 g of sulfuric acid and add it to the ground mixed slurry. Stir at 40°C for 1 hour and then centrifuge spray dry to obtain the lithium iron phosphate precursor. The inlet air temperature of the centrifugal spray is 220°C and the outlet air temperature is 100°C. The spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, then crush and sieve to obtain the lithium iron phosphate product.

[0093] Comparative Example 3

[0094] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0095] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0096] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0097] (4) Grind the aforementioned mixed slurry to a particle size of 0.35 μm. Weigh 15 g of ammonium sulfate and 10 g of phosphoric acid (phosphoric acid concentration of 85%), add them to the ground mixed slurry, stir at 70°C for 1 hour, and then centrifuge spray dry to obtain lithium iron phosphate precursor. The inlet air temperature of the centrifugal spray is 220°C, the outlet air temperature is 100°C, and the spray particle size D50 is controlled to be 20~50 μm. After drying, calcine the lithium iron phosphate precursor at 830°C under N2 protection for 10 h, and then crush and sieve to obtain lithium iron phosphate product.

[0098] Comparative Example 4

[0099] (1) Weigh 127.5 g of lithium hydroxide, dissolve it in water to prepare a 10% lithium hydroxide aqueous solution, and pre-cool it to 5°C. Add 40 g of sugarcane bagasse pulp and 50 g of straw residue to the pre-cooled lithium hydroxide aqueous solution, and stir for 1 h to obtain a cellulose mixture;

[0100] (2) Prepare 100 g of urea aqueous solution with a mass fraction of 20% and pre-cool it to 5°C. Add the urea aqueous solution to the aforementioned cellulose mixture and stir at room temperature for 2 h to completely dissolve it, obtaining a clear and transparent solution;

[0101] (3) Weigh 500 g of anhydrous ferric phosphate (Fe:Li=1:1.02) and 3 g of titanium oxide respectively and add them to the aforementioned clear solution. Stir for 30 min to mix them evenly to obtain a mixed slurry.

[0102] (4) The aforementioned mixed slurry was ground to a particle size of 0.35 μm. After grinding, it was centrifugally spray-dried to obtain the lithium iron phosphate precursor. The inlet air temperature of the centrifugal spray was 220℃, the outlet air temperature was 100℃, and the spray particle size D50 was controlled to be 20~50 μm. After drying, the lithium iron phosphate precursor was calcined at 830℃ under N2 protection for 10 h, and then crushed and sieved to obtain the lithium iron phosphate product.

[0103] Electrochemical performance tests were conducted on the lithium iron phosphate prepared in Examples 1-10 and Comparative Examples 1-4. The compaction density was tested using a compaction density meter (3 tons pressure), the carbon content was tested using a carbon-sulfur analyzer, and the carbon layer resistance was tested using a four-probe powder resistance meter. The lithium iron phosphate prepared in each example and comparative example was mixed with PVDF (polyvinylidene fluoride) and SP (carbon black) in a ratio of 90:5:5 to form electrode sheets, which were then assembled into button batteries. The 0.5C discharge capacity and 1C / 100-cycle retention rate were tested, and the results are shown in Table 1.

[0104] Table 1. Electrochemical performance test results of the examples and comparative examples. Serial Number <![CDATA[Compaction density (g / cm 3 )]]> 0.5C discharge capacity (mAh / g) 1C / 100-week cycle retention rate (%) Carbon content (%) Carbon layer resistance (Ω·cm) Example 1 2.66 138 94 1.34 9.6 Example 2 2.64 143 95.5 1.44 5.8 Example 3 2.64 146 97 1.47 4.5 Example 4 2.61 143 96 1.47 6 Example 5 2.61 138 93 1.42 8 Example 6 2.63 140 95 1.37 9 Example 7 2.62 143 95 1.45 6 Example 8 2.66 142 95 1.41 7 Example 9 2.64 137 94 1.33 10 Example 10 2.65 140 95 1.5 7.8 Example 11 2.66 137 94 1.40 9 Comparative Example 1 2.61 140 95 1.42 6.5 Comparative Example 2 2.64 128 92 1.3 15 Comparative Example 3 2.66 139 94.5 1.39 8 Comparative Example 4 2.66 135 93 1.33 12

[0105] As shown in Table 1, Examples 1-10 all exhibited excellent electrochemical performance, and as shown in the attached figures, the lithium iron phosphate particles prepared by this invention were uniform in size. Comparing Examples 1, 2, and 10, it is evident that when a mixture of straw residue and bagasse slurry is used as the carbon source, the electrochemical performance of the lithium iron phosphate prepared using this mixture is superior to that using either straw residue or bagasse slurry alone. This is because lithium hydroxide solution and urea solution have different effects on the dissolution of fibers and the breaking of hydrogen bonds in straw residue and bagasse slurry, resulting in the formation of short-chain cellulose with different molecular weights. During the preparation of lithium iron phosphate, short-chain cellulose with different molecular weights produces better crystallinity and a layered effect on the surface of lithium iron phosphate, thereby giving the lithium iron phosphate material better electrochemical performance.

[0106] Comparing Examples 2 and 6 and 8, it can be seen that although lithium hydroxide is the key component for dissolving cellulose and urea can act as a co-solvent to promote the dissolution of cellulose in the lithium hydroxide aqueous solution, when too much of it is added, it leads to the excessive dissolution of cellulose into small molecules. The carbon layer formed during the subsequent calcination process is mostly amorphous carbon with poor three-dimensional structure, which ultimately results in an increase in powder resistance.

[0107] Comparing Examples 2 and 9 with Comparative Example 4, it can be seen that the addition of a coagulant is key to the excellent electrochemical performance of the lithium iron phosphate product prepared in this invention, and the electrochemical performance of the lithium iron phosphate product is significantly improved with increasing coagulant content. Without a coagulant, the three-dimensional network structure formed by the precursor in the slurry stage cannot be maintained during subsequent centrifugal drying, resulting in inhomogeneity and compromised stability due to centrifugation. Insufficient coagulant dosage prevents complete precipitation of short-chain cellulose in the slurry, making it difficult to uniformly coat the surface of the iron phosphate particles.

[0108] Further comparison of Examples 2, 3, 5, 11 and Comparative Example 2 shows that the type of coagulant also has a significant impact on the electrochemical performance of lithium iron phosphate products. When ammonium sulfate or a mixture of ammonium sulfate and phosphoric acid is used as a coagulant, ammonium sulfate can act as a crystal nucleus to initiate the re-precipitation process of cellulose and disrupt the coordination between lithium ions and the hydroxyl groups on cellulose, further promoting cellulose precipitation. When ammonium sulfate and phosphoric acid are mixed, phosphoric acid can act as a precursor phosphorus source, promoting the subsequent growth of lithium iron phosphate. Therefore, ammonium sulfate or a mixture of ammonium sulfate and phosphoric acid is preferred as a coagulant.

[0109] Comparing Examples 3-4 with Comparative Examples 1 and 3, it can be seen that the temperature of the coagulation process after adding the coagulant also determines the electrochemical performance of the lithium iron phosphate product. When the temperature is too low, the cellulose coagulation is insufficient, failing to adequately coat the surface of the iron phosphate particles and failing to remain stable during centrifugal drying. Conversely, when the coagulation temperature is too high, excessive cellulose precipitation results in the final lithium iron phosphate product containing too much carbon on its surface. This carbon cannot form a three-dimensional network structure on the lithium iron phosphate surface, thereby reducing the lithium ion migration rate and causing a decline in its electrochemical performance.

Claims

1. A method for preparing lithium iron phosphate, characterized in that, Includes the following steps: (1) Add cellulose to the lithium hydroxide aqueous solution and mix evenly to obtain a cellulose mixture; (2) Add urea to the aforementioned cellulose mixture and mix evenly to completely dissolve it to obtain a clear solution; (3) Mix the iron source, titanium source and the aforementioned clear solution evenly to obtain a mixed slurry; (4) Grind the aforementioned mixed slurry and add a coagulant, mix evenly and then dry and calcine to obtain lithium iron phosphate.

2. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (1), the cellulose includes at least one of sugarcane bagasse pulp and straw residue.

3. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (1), the lithium hydroxide aqueous solution is pre-cooled before adding cellulose, and the pre-cooling temperature is ≤8℃.

4. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (1), the mass concentration of the lithium hydroxide aqueous solution is 6-14%.

5. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (2), the urea is an aqueous solution of urea with a mass concentration of 20-40%.

6. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (3), the iron source is iron phosphate and the titanium source is titanium oxide.

7. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (4), the coagulant includes at least one of phosphoric acid, ammonium sulfate, and ammonium phosphate, and the amount of coagulant added is 0.1 to 5% of the mass of the iron source.

8. The method for preparing lithium iron phosphate according to claim 7, characterized in that, In step (4), the coagulant is ammonium sulfate or a mixture of ammonium sulfate and phosphoric acid.

9. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (4), the mixing temperature is 40~60℃.

10. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (4), the drying is centrifugal spray drying.

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