A method for preparing iron phosphate, lithium iron phosphate and its preparation method and application

Abstract

This application provides a method for preparing iron phosphate, lithium iron phosphate, and their preparation methods and applications, belonging to the field of lithium battery technology. The method for preparing iron phosphate is as follows: S1. Under a protective atmosphere, a mixed solution of iron and phosphorus sources undergoes a primary crystallization reaction to form crystal nuclei; S2. The temperature is raised to 70-85℃, and the pH is adjusted and maintained at 2.8-3.2 to carry out a secondary crystallization reaction; S3. The pH is adjusted to 3.5-4.0, and a tertiary crystallization and ripening reaction is carried out at 80-90℃; the iron phosphate has a particle size D50 of 1-1.5 μm, an iron-to-phosphorus molar ratio Fe / P of 0.98-1.02, and a tap density ≥1.1 g / cm³. 3 This application optimizes the preparation process of lithium iron phosphate, combining specific carbon source combinations and staged calcination steps to effectively improve the electrochemical performance of lithium iron phosphate, enabling it to exhibit excellent overall performance in terms of specific capacity and cycle stability.

Description

Technical Field

[0001] This application relates to the field of lithium battery technology, and in particular to a method for preparing iron phosphate, lithium iron phosphate, the preparation method thereon, and their applications. Background Technology

[0002] Currently, the synthesis methods for lithium iron phosphate (LFP) materials are mainly divided into solid-phase and liquid-phase methods. The solid-phase method primarily utilizes iron salts, lithium salts, and phosphates, sintering them at high temperatures to synthesize LFP. The liquid-phase method involves dissolving soluble iron salts, lithium salts, and phosphates in a solvent, utilizing ionic reactions to produce LFP or its precursors, and then sintering at high temperatures to produce the final product. The solid-phase method is simpler, the raw materials are easier to handle, and the yield is higher, but the morphology of the raw materials is difficult to control, resulting in lower tap and compaction densities in the product. Some new synthesis methods, such as microwave synthesis and ultrasonic co-precipitation, can be categorized under solid-phase synthesis. The liquid-phase method, on the other hand, requires pretreatment in a reaction vessel, as well as drying and filtration processes, making the process more complex. However, the product generally has better sphericity, higher tap density, and excellent capacity and high-rate performance.

[0003] CN103715452A discloses a low-temperature lithium iron phosphate lithium-ion power battery, which uses nano-sized lithium iron phosphate coated with a discontinuous graphene structure as the positive electrode active material. The median particle size of the nano-sized lithium iron phosphate is 5-10 nm, and the graphene is 3-8 layers of multilayer graphene. The coating area accounts for 40%-70% of the total surface area of ​​the lithium iron phosphate material. The positive electrode sheet prepared by this method has a low compaction density, resulting in a low energy density.

[0004] The above-mentioned scheme has the problem of low electrochemical performance. Therefore, it is essential to develop a lithium iron phosphate cathode material that can achieve high electrochemical performance. Summary of the Invention

[0005] This application is made in view of the above-mentioned problems, and its purpose is to provide a method for preparing iron phosphate, lithium iron phosphate, and the preparation method and application thereof.

[0006] Specifically, the first aspect of this application provides a method for preparing iron phosphate, comprising the following steps:

[0007] S1. Under a protective atmosphere, a mixed solution of iron and phosphorus sources and an oxidant are added in parallel to a reactor containing a bottom liquid, and a primary crystallization reaction is carried out at 40-55℃ and pH 1.5-2.0 to form crystal nuclei;

[0008] S2. Heat the system from step S1 to 70-85℃, adjust and maintain the pH at 2.8-3.2, and carry out a secondary crystallization reaction to allow crystal growth;

[0009] S3. Adjust the pH of the system from step S2 to 3.5-4.0, and carry out a three-stage crystallization and ripening reaction at 80-90℃. After the reaction, the system is separated into solid and liquid, washed, and dried to obtain ferric phosphate.

[0010] In the primary crystallization reaction and / or secondary crystallization reaction, a crystal face modifier is added, and the crystal face modifier is at least one selected from citric acid, tartaric acid, malic acid, ascorbic acid and their salts.

[0011] The iron phosphate has a particle size D50 of 1-1.5 μm, an iron-to-phosphorus molar ratio (Fe / P) of 0.98-1.02, and a tap density ≥1.1 g / cm³. 3 .

[0012] Furthermore, the amount of the crystal plane modifier added is 0.2%-2.0% of the mass of iron element in the iron source.

[0013] A second aspect of this application provides a method for preparing lithium iron phosphate, comprising the following steps:

[0014] (1) The iron phosphate is mixed with lithium source, carbon precursor, dispersant and solvent, and then ground to obtain the first slurry;

[0015] (2) Add nano-conductive carbon material to the first slurry, disperse it evenly to obtain a composite slurry, and spray dry it after homogenization to obtain a composite powder;

[0016] (3) The composite powder is calcined in stages under a protective reducing atmosphere, and the lithium iron phosphate is obtained after post-calcination treatment.

[0017] Further, in step (1), the lithium source is lithium hydroxide, and the molar ratio of lithium to iron in iron phosphate is 1.03-1.08:1;

[0018] And / or, the carbon precursor is at least one of polyethylene glycol, polyvinyl alcohol, and sucrose, and its addition amount is 3%-8% of the mass of iron phosphate;

[0019] The dispersant is ammonium polyacrylate, and its addition amount is 0.1%-0.5% of the mass of ferric phosphate.

[0020] Further, in step (2), the nano-conductive carbon material is a mixture of mercapto-modified carbon fiber and conductive carbon black, and the mass ratio of mercapto-modified carbon fiber to conductive carbon black is 1-3:1.

[0021] Furthermore, in step (2), the inlet temperature of the spray dryer is 180-220°C, the outlet temperature is 80-100°C, and the process is carried out under an inert atmosphere.

[0022] Further, in step (3), the segmented calcination includes:

[0023] First stage: Increase the temperature to 400-500℃ at a rate of 2-5℃ / min and hold for 1-3 hours;

[0024] Second stage: Continue to raise the temperature to 650-750℃ at a rate of 1-3℃ / min, and hold for 3-6 hours;

[0025] The protective reducing atmosphere is a mixture of nitrogen and hydrogen, wherein the volume percentage of hydrogen is 2-10%.

[0026] A third aspect of this application provides a lithium iron phosphate, prepared by the aforementioned method for preparing lithium iron phosphate.

[0027] The fourth aspect of this application provides an application of lithium iron phosphate, wherein the lithium iron phosphate is used in the positive electrode of a lithium-ion battery.

[0028] The present invention has the following beneficial effects:

[0029] The high-tap-density spherical iron phosphate provided by this invention has a particle size (D50) precisely controlled between 1 and 1.5 μm, an iron-to-phosphorus molar ratio (Fe / P) stable within a near-stoichiometric range of 0.98-1.02, and a tap density reaching a high level of ≥1.1 g / cm³. This specific particle size distribution and high tap density provide a good structural basis for the final product when iron phosphate is used as a precursor in the preparation of lithium iron phosphate, which is beneficial for improving the packing density of lithium iron phosphate. By adding crystal facet modifiers (such as citric acid, tartaric acid, etc.) in the primary and / or secondary crystallization reactions, the crystal growth direction and surface morphology of iron phosphate can be effectively controlled, further optimizing its physicochemical properties and laying a solid foundation for subsequent uniform mixing and reaction with lithium sources, etc.

[0030] The method for preparing lithium iron phosphate of this invention involves thoroughly grinding and mixing iron phosphate with a lithium source, a carbon precursor, and a dispersant in a liquid phase environment. This process not only achieves a preliminary uniform distribution of lithium on the surface and near-surface of the iron phosphate particles, avoiding the component segregation problem that easily occurs in traditional solid-phase mixing, but also, with the help of the dispersant, disperses each component at the molecular or nanoscale, creating favorable conditions for subsequent uniform carbon coating and full lithiation reaction. The introduction of the carbon precursor lays the groundwork for subsequent carbon network construction, while the amount of dispersant ammonium polyacrylate added is controlled at 0.1%-0.5% of the iron phosphate mass, which effectively prevents particle agglomeration without introducing excessive impurities that would affect the performance of the final product.

[0031] After obtaining the first slurry, this method further introduces a nano-conductive carbon material composed of thiol-modified carbon fibers and conductive carbon black in a specific mass ratio (1-3:1). Thiol-modified carbon fibers possess excellent one-dimensional conductive pathway construction capabilities, while conductive carbon black fills the gaps between the thiol-modified carbon fibers. The two work synergistically to form a three-dimensional interconnected, highly efficient conductive network during subsequent calcination. After the nano-conductive carbon material is added to the first slurry and dispersed evenly, it undergoes homogenization to further refine the particles and improve mixing uniformity, followed by spray drying. The spray drying process is carried out under an inert atmosphere, with the inlet temperature controlled at 180-220℃ and the outlet temperature at 80-100℃. These process parameters enable rapid solvent removal, resulting in a composite powder with uniform composition and suitable particle size distribution, while simultaneously preventing oxidation of the material during the drying process.

[0032] For the calcination of composite powders, this application employs a precisely controlled segmented calcination process. The first stage involves heating to 400-500℃ at a rate of 2-5℃ / min and holding for 1-3 hours. The main purpose of this stage is to remove residual solvents and dispersants from the powder and to promote the initial pyrolysis and carbonization of the carbon precursor, forming a preliminary carbon coating layer, while preparing for subsequent high-temperature crystallization. The second stage involves heating to 650-750℃ at a slower rate of 1-3℃ / min and holding for 3-6 hours. This stage is crucial for the formation and growth of the lithium iron phosphate crystal phase. The slow heating rate promotes uniform crystal growth and reduces defects, while the appropriate high-temperature holding ensures the formation of Li... + Sufficient diffusion and intercalation into the iron phosphate lattice lead to the formation of a structurally complete and highly crystalline lithium iron phosphate phase. The entire calcination process is carried out in a protective reducing atmosphere of a mixture of nitrogen and hydrogen (hydrogen volume percentage 2-10%). The presence of hydrogen helps prevent Fe from... 2+ Oxidized to Fe 3+ This ensures the stoichiometry and electrochemical activity of lithium iron phosphate. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of this drawing or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this drawing. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0034] Figure 1 SEM image of ferric phosphate in Preparation Example 1;

[0035] Figure 2 SEM image of ferric phosphate in Preparation Example 2;

[0036] Figure 3 Here is a SEM image of lithium iron phosphate from Example 1;

[0037] Figure 4 Here is a SEM image of lithium iron phosphate in Example 2;

[0038] Figure 5 Here is a SEM image of lithium iron phosphate in Example 3;

[0039] Figure 6 Here is a SEM image of lithium iron phosphate in Example 4;

[0040] Figure 7 This is a SEM image of lithium iron phosphate in Example 5.

[0041] The purpose, features, and advantages of this accompanying drawing will be further explained in conjunction with the embodiments and with reference to the accompanying drawing. Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of this application clearer, the following description and illustration are provided in conjunction with embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application. All other embodiments obtained by those skilled in the art based on the embodiments provided in this application without inventive effort are within the scope of protection of this application.

[0043] Obviously, the following description is merely some examples or embodiments of this application. Those skilled in the art can apply this application to other similar scenarios without any inventive effort. Furthermore, it is understood that although the effort involved in such development may be complex and lengthy, for those skilled in the art related to the content disclosed in this application, any changes to design, manufacturing, or production based on the technical content disclosed in this application are merely conventional technical means and should not be construed as insufficient disclosure of the content of this application.

[0044] An embodiment of the first aspect of this application provides a method for preparing ferric phosphate, comprising the following steps:

[0045] S1. Under a protective atmosphere, a mixed solution of iron and phosphorus sources and an oxidant are added in parallel to a reactor containing a bottom liquid, and a primary crystallization reaction is carried out at 40-55℃ and pH 1.5-2.0 to form crystal nuclei;

[0046] S2. Heat the system from step S1 to 70-85℃, adjust and maintain the pH at 2.8-3.2, and carry out a secondary crystallization reaction to allow crystal growth;

[0047] S3. Adjust the pH of the system from step S2 to 3.5-4.0, and carry out a tertiary crystallization and ripening reaction at 80-90℃. After the reaction, the mixture is separated into solid and liquid phases, washed, and dried to obtain ferric phosphate. A crystal facet modifier is added during the primary and / or secondary crystallization reactions. The crystal facet modifier is at least one selected from citric acid, tartaric acid, malic acid, ascorbic acid, and their salts. The amount of crystal facet modifier added is 0.2%-2.0% of the mass of iron in the iron source.

[0048] Particle size D50 is 0.5-2.0 μm, iron / phosphorus molar ratio (Fe / P) is 0.98-1.02, and tap density is ≥1.1 g / cm³. 3 .

[0049] The iron source in step S1 is a 1.0 mol / L FeSO4 solution. Its preparation method is as follows: dissolve FeSO4·7H2O in deionized water, add citric acid (1.0% of the mass of FeSO4·7H2O), and stir until completely dissolved. The phosphorus source is H3PO4.

[0050] FeSO4 solution and H3PO4 solution were mixed at a volume ratio of 1:1 and stirred for 30 minutes to obtain a mixed solution of iron source and phosphorus source.

[0051] Add 20 L of deionized water as the base solution to a 50 L reactor, start stirring (200-300 rpm), purge with nitrogen for protection, and heat to 40-55 °C. Adjust the pH of the base solution to 1.5-2.5 with dilute sulfuric acid. Using two constant flow pumps, add a mixed solution of iron and phosphorus sources and 30% hydrogen peroxide dropwise to the reactor at rates of 200 mL / min and 40 mL / min, respectively. After the addition is complete, mature the solution at 50 °C and pH 2.0 for 30 minutes to obtain a suspension containing a large number of fine crystal nuclei.

[0052] In step S2, the temperature of the reaction system is increased from 50℃ to 75℃ at a heating rate of 2℃ / min. At the same time as heating, 10% ammonia water is added dropwise to adjust and maintain the pH at 2.8-3.2. The heating and pH adjustment are completed simultaneously within 30 minutes, and the reaction is carried out at 70-85℃ for 3 hours.

[0053] Step S3: While maintaining stirring, raise the temperature of the reaction system to 85°C, slowly add ammonia water dropwise, and raise the pH value of the reaction system to 3.5 within 30 minutes. Under the conditions of 85°C and pH=3.5, mature for 2 hours. After maturity, stop heating and allow it to cool naturally to below 40°C.

[0054] The reaction slurry was subjected to solid-liquid separation using a filter press. The filter cake was washed with hot deionized water at 70°C. The wet filter cake was placed in a vacuum drying oven and dried for 10 hours at 100°C and -0.095 MPa. The slurry was then passed through a 400-mesh sieve to obtain ferric phosphate.

[0055] An embodiment of the second aspect of this application provides a method for preparing lithium iron phosphate, comprising the following steps:

[0056] (1) The iron phosphate and lithium source compound are mixed in a solvent and subjected to hydrothermal or solvothermal reaction at 100-130°C to obtain a composite slurry;

[0057] (2) Add a carbon source to the composite slurry, homogenize it, and then spray dry it to obtain composite powder;

[0058] The carbon source includes a water-soluble carbon precursor and a conductive carbon material;

[0059] (3) The composite powder is calcined in stages under a protective reducing atmosphere, and the lithium iron phosphate is obtained after post-calcination treatment.

[0060] In step (1), the lithium source is 0.485 kg LiOH·H2O dissolved in 2 L of deionized water and stirred until a clear lithium salt solution is obtained.

[0061] Ferric phosphate, carbon precursor polyethylene glycol (PEG-400), and dispersant ammonium polyacrylate were added to a lithium salt solution and initially stirred to form a slurry. The slurry was then transferred to a sand mill and ground using 0.3 mm zirconia beads at 2000 rpm for 4 hours until the slurry particle size D100 < 1 μm and the viscosity was approximately 300-500 mPa·s.

[0062] In step (2), the mixture of mercapto-modified carbon fiber and conductive carbon black (Super P) is slowly added to the ground slurry while running a high-speed shear disperser (8000 rpm), and shear dispersion is continued for 1 hour. Drying is performed using a centrifugal spray drying tower with an inlet temperature of 180-220℃ and an outlet temperature of 80-100℃, under a nitrogen atmosphere. The composite powder is collected from the bottom of the drying tower and the cyclone separator.

[0063] The method for preparing the mercapto-modified carbon fiber is as follows: the carbon fiber is refluxed with a 65% nitric acid solution at 80°C for 4 hours, washed with deionized water until neutral and dried; then the treated carbon fiber is added to an ethanol solution containing 3-mercaptopropyltrimethoxysilane (the mass ratio of 3-mercaptopropyltrimethoxysilane to carbon fiber is 1:10), and stirred at 60°C for 6 hours. After the reaction is completed, the mixture is filtered, washed with ethanol, and vacuum dried to obtain the mercapto-modified carbon fiber.

[0064] The thiol (-SH) functional groups on the surface of thiol-modified carbon fibers can interact with metal ions (such as Fe) on the surface of lithium iron phosphate particles. 2+ Li + This forms a strong chemical adsorption effect, which anchors the mercapto-modified carbon fiber more firmly on the surface and between lithium iron phosphate particles. This not only enhances the interfacial bonding force between the conductive network and the active material, effectively inhibiting the shedding of the conductive network and the aggregation of active particles during cycling, but also facilitates the rapid transfer of electrons at the interface and reduces interfacial impedance.

[0065] In step (3), the composite powder obtained by spray drying is evenly spread in an alumina crucible; the crucible is then pushed into a container filled with nitrogen. 2 / The sintering process is carried out in a tubular sintering furnace containing a H2 (95 / 5) mixture; the staged calcination includes:

[0066] First stage: Increase the temperature to 400-500℃ at a rate of 2-5℃ / min and hold for 1-3 hours;

[0067] Second stage: Continue to heat to 650-750℃ at a rate of 1-3℃ / min, and hold for 3-6 hours.

[0068] The segmented calcination process allows for precise control of the temperature and heating rate at different reaction stages. In the first stage, the temperature is maintained at 400-500℃ for 1-3 hours, primarily achieving the initial decomposition and shaping of the carbon precursor, as well as the removal of some organic impurities, creating a stable environment for subsequent lithium-ion intercalation and crystal growth. In the second stage, the temperature is further increased to 650-750℃ and maintained for 3-6 hours, promoting the full intercalation of lithium ions into the iron phosphate lattice to complete the solid-phase reaction. Simultaneously, the carbon precursor is completely carbonized, forming a uniform and highly conductive carbon coating layer and conductive network on the surface and between the lithium iron phosphate particles. The entire calcination process is conducted under a protective reducing atmosphere provided by a mixture of nitrogen and hydrogen (hydrogen volume percentage 2-10%), ensuring that iron remains stably in its +2 valence state throughout the reaction, effectively avoiding the adverse effects of high-valence iron formation on the material's electrochemical performance. After calcination, the product is naturally cooled to room temperature, followed by post-processing steps such as pulverization and sieving to obtain high-performance lithium iron phosphate material.

[0069] An embodiment of the third aspect of this application provides a lithium iron phosphate, prepared by the aforementioned method for preparing lithium iron phosphate.

[0070] An embodiment of the fourth aspect of this application provides an application of lithium iron phosphate, wherein the lithium iron phosphate is used in the positive electrode of a lithium-ion battery.

[0071] Example

[0072] The following examples describe the disclosure of this invention in more detail. These examples are merely illustrative, as various modifications and variations will be apparent to those skilled in the art within the scope of this disclosure. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on weight. Unless otherwise stated, all reagents used in the examples are available commercially or synthesized using conventional methods and are ready for use without further processing. Unless otherwise stated, all instruments used in the examples are available commercially.

[0073] Preparation Example 1

[0074] A method for preparing ferric phosphate includes the following steps:

[0075] S1. Dissolve 2.78 kg FeSO4·7H2O and 27.8 g citric acid in 8 L of deoxygenated water to obtain an iron source solution; dilute 1.15 kg 85% H3PO4 to 10 L to obtain a phosphorus source solution; mix the iron source and phosphorus source; add 20 L of deionized water to a 50 L reactor, heat to 50 °C, and adjust the pH to 1.5; add the mixture of liquid iron source and phosphorus source and 30% H2O2 dropwise at rates of 300 mL / min and 50 mL / min respectively, controlling the pH at 1.8 and the temperature at 50 °C, for 1 hour;

[0076] S2. Increase the temperature to 80℃ at 1℃ / min, and simultaneously add 10% ammonia water to raise the pH to 3.0. Maintain the reaction at constant temperature and pH for 3 hours.

[0077] S3. Maintain the temperature at 85℃, add ammonia water to adjust the pH to 3.5, and age under these conditions for 2 hours; cool, filter under pressure, wash with hot water, and vacuum dry at 90℃ for 12 hours.

[0078] Results: Ferric phosphate D50 = 1.18 μm, tap density 1.12 g / cm³ 3 Fe / P = 1.002. Specific surface area of ​​iron phosphate: 10 m² 2 / g, with a dense surface and no well-developed mesopores.

[0079] SEM image of ferric phosphate is shown below Figure 1 .

[0080] Preparation Example 2

[0081] A method for preparing ferric phosphate includes the following steps:

[0082] S1. Dissolve 2.78 kg FeSO4·7H2O and 30.8 g tartaric acid in 8 L of deoxygenated water to obtain an iron source solution; dilute 1.15 kg 85% H3PO4 to 10 L to obtain a phosphorus source solution; mix the iron source and phosphorus source; add 20 L of deionized water to a 50 L reactor, heat to 50 °C, and adjust the pH to 1.5; add the mixture of liquid iron source and phosphorus source and 30% H2O2 dropwise at rates of 300 mL / min and 50 mL / min respectively, controlling the pH at 1.8 and the temperature at 50 °C, for 1 hour;

[0083] S2. Increase the temperature to 75℃ at 1℃ / min, and simultaneously add 10% ammonia water to raise the pH to 3.0. Maintain the reaction at constant temperature and pH for 2 hours.

[0084] S3. Maintain the temperature at 78℃, add 150g urea (final concentration 0.5 mol / L) and 20g P123, add ammonia water dropwise to raise the pH to 3.6, raise the temperature to 80℃, and age for 4 hours; cool, filter under pressure, wash with hot water, and vacuum dry at 100℃ for 10 hours.

[0085] Results: Ferric phosphate D50 = 1.09 μm, tap density 1.15 g / cm³ 3 Fe / P = 1.001.

[0086] SEM image of ferric phosphate is shown below Figure 2 .

[0087] Example 1

[0088] A method for preparing lithium iron phosphate includes the following steps:

[0089] (1) Mix 0.95 kg of iron phosphate, 0.485 kg of LiOH·H2O, 0.12 kg of polyethylene glycol, and 0.075 kg of ammonium polyacrylate with 2 L of water and grind for 4 hours until D100 < 1 μm to obtain a composite slurry;

[0090] (2) Add a mixture of 0.03 kg mercapto-modified carbon fiber and 0.02 kg conductive carbon black (SuperP) to the composite slurry, treat the mixed slurry again at 8000 rpm for 1 hour using a high-speed shear disperser, and dry it in a centrifugal spray drying tower with an inlet temperature of 200°C and an outlet temperature of 90°C under nitrogen atmosphere protection to obtain composite powder;

[0091] The preparation method of the mercapto-modified carbon fiber is as follows: Weigh 100g of carbon fiber, add 500mL of 65% nitric acid solution, reflux at 80℃ for 4 hours, cool to room temperature, filter, and then wash repeatedly with deionized water until the washing solution is neutral. Place the washed carbon fiber in a vacuum drying oven and dry at 80℃ for 12 hours to obtain oxidized carbon fiber. Then, add the dried oxidized carbon fiber to 500mL of ethanol solution containing 10g of 3-mercaptopropyltrimethoxysilane, stir and react for 6 hours in an oil bath under nitrogen protection at 60℃. After the reaction is completed, filter, and wash the obtained solid three times with a mixture of 100mL ethanol and 0.05g ascorbic acid. The last wash is with 100mL of pure ethanol. Finally, place the washed solid in a vacuum drying oven and vacuum dry at 60℃ under nitrogen atmosphere for 8 hours to obtain mercapto-modified carbon fiber.

[0092] (3) Spread the composite powder obtained by spray drying evenly in an alumina crucible; push the crucible into a container filled with nitrogen. 2 / In a tubular sintering furnace containing H2 (95 / 5) mixed gas, the composite powder is subjected to segmented calcination. The segmented calcination includes: a first stage: heating to 450°C at a rate of 3°C / min and holding for 2 hours; a second stage: continuing to heat to 700°C at a rate of 2°C / min and holding for 5 hours, followed by post-calcination treatment to obtain lithium iron phosphate.

[0093] SEM image (see) Figure 3 .

[0094] Example 2

[0095] This embodiment is basically the same as that of embodiment 1, except that in step (1), 0.12 kg of polyethylene glycol is replaced with 0.11 kg of polyvinyl alcohol.

[0096] SEM image (see) Figure 4 .

[0097] Example 3

[0098] This embodiment is basically the same as embodiment 1, except that in step (2), a mixture of 0.025 kg of mercapto-modified carbon fiber and 0.015 kg of conductive carbon black is added to the composite slurry.

[0099] SEM image (see) Figure 5 .

[0100] Example 4

[0101] This embodiment is basically the same as that of embodiment 1, except that the calcination temperature in the second stage of step (3) is 680℃.

[0102] SEM image (see) Figure 6 .

[0103] Example 5

[0104] This embodiment is basically the same as that of Example 1, except that step (1) uses 100g of iron phosphate as in Preparation Example 2.

[0105] SEM image (see) Figure 7 .

[0106] Comparative Example 1

[0107] 0.95 kg of ferric phosphate, 0.49 kg of Li2CO3 (Li / Fe=1.05), 0.12 kg of sucrose, and carbon black were dry-mixed in a high-speed mixer at 6000 rpm for 2 hours, and then calcined in argon at 750°C for 10 hours.

[0108] Comparative Example 2

[0109] This comparative example is basically the same as Example 1, except that grinding is not performed in step (1).

[0110] Comparative Example 3

[0111] This comparative example is basically the same as Example 1, except that in step (3), the composite powder is heated to 700°C at 5°C / min and kept at that temperature for 6 hours, and then cooled.

[0112] Comparative Example 4

[0113] This comparative example is basically the same as Example 1, except that step (2) does not add mercapto-modified carbon fiber and conductive carbon black.

[0114] Experimental Case

[0115] The lithium iron phosphates prepared in Examples 1-5 and Comparative Examples 1-4 were used to prepare lithium batteries. CR2032 coin cells were assembled using a positive electrode (active material: acetylene black: PVDF = 92:4:4), a lithium negative electrode, and a 1M LiPF6 in EC / DMC electrolyte. The electrical performance test results are shown in Table 1.

[0116] Table 1 Performance Test Results

[0117]

[0118] As shown in Table 1, the lithium iron phosphate materials prepared in the embodiments of this application exhibit significant advantages in terms of electrical performance. This indicates that by optimizing the preparation process of iron phosphate, combining specific carbon source combinations, and implementing steps such as staged calcination, this application effectively improves the electrochemical performance of lithium iron phosphate, resulting in excellent overall performance in terms of specific capacity and cycle stability.

[0119] Comparative Example 1 uses a traditional solid-state method to prepare lithium iron phosphate. This method does not involve a spray drying step, the carbon source is only sucrose, and the calcination process is a single-stage high-temperature long-term calcination, which results in large and unevenly distributed product particles. The thickness of the carbon coating layer is difficult to control precisely, and the lithium-ion intercalation efficiency is low. As a result, the initial discharge specific capacity and high-rate performance of the material are inferior to those of the embodiments in this application.

[0120] Comparative Example 2 was not ground, resulting in a larger slurry particle size. This significantly reduced the mixing uniformity and contact area of ​​iron phosphate and lithium source in the liquid phase, leading to poor lithium ion intercalation in the subsequent hydrothermal reaction. Furthermore, the dispersion of carbon source and active material was also affected, ultimately resulting in a decrease in the conductivity and structural integrity of lithium iron phosphate. The 1C discharge specific capacity and cycle stability were both lower than those of Example 1.

[0121] Comparative Example 3 did not use a segmented calcination process, but instead directly raised the temperature to 700℃ at a relatively fast heating rate and held it at that temperature. This resulted in incomplete decomposition of the carbon precursor in the first stage and the occurrence of local overheating and carbonization. The resulting carbon layer structure was uneven. At the same time, the rapid insertion of lithium ions at high temperature may lead to an increase in lattice defects. The cycling performance of the material dropped significantly, and the capacity retention rate was only 68.6% after 1000 cycles.

[0122] Comparative Example 4 did not add mercapto-modified carbon fiber and conductive carbon black. It relied solely on the carbon layer formed by polyethylene glycol carbonization, which made it difficult to construct an effective conductive network. The electronic conduction path of the material was limited, and the high-rate discharge performance was significantly weakened. The 1C discharge specific capacity was reduced by about 9 mAh / g compared with Example 1, and the capacity decay rate was accelerated during cycling.

[0123] These comparative results fully demonstrate the synergistic effect of key steps in this application, such as optimization of the iron phosphate preparation process, specific carbon source combinations, and segmented calcination, on improving the electrochemical performance of lithium iron phosphate.

[0124] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A method for preparing lithium iron phosphate, characterized in that, Includes the following steps: (1) Preparation of ferric phosphate, including the following steps: S1. Under a protective atmosphere, a mixed solution of iron and phosphorus sources and an oxidant are added in parallel to a reactor containing a bottom liquid, and a primary crystallization reaction is carried out at 40-55℃ and pH 1.5-2.0 to form crystal nuclei; S2. Heat the system from step S1 to 70-85℃, adjust and maintain the pH at 2.8-3.2, and carry out a secondary crystallization reaction to allow crystal growth; S3. Adjust the pH of the system from step S2 to 3.5-4.0, and carry out a three-stage crystallization and ripening reaction at 80-90℃. After the reaction, the system is separated into solid and liquid, washed, and dried to obtain ferric phosphate. In the primary crystallization reaction and / or secondary crystallization reaction, a crystal face modifier is added, and the crystal face modifier is at least one selected from citric acid, tartaric acid, malic acid, ascorbic acid and its salts. The iron phosphate has a particle size D50 of 1-1.5 μm, an iron-to-phosphorus molar ratio (Fe / P) of 0.98-1.02, and a tap density ≥1.1 g / cm³. 3 ; The iron phosphate was mixed with a lithium source, a carbon precursor, and a dispersant, and then ground to obtain a first slurry. (2) Add nano-conductive carbon material to the first slurry, disperse it evenly to obtain a composite slurry, and spray dry it after homogenization to obtain a composite powder; The nano-conductive carbon material is a mixture of mercapto-modified carbon fiber and conductive carbon black, wherein the mass ratio of mercapto-modified carbon fiber to conductive carbon black is 1-3:

1. (3) The composite powder is calcined in stages under a protective reducing atmosphere, and the lithium iron phosphate is obtained after post-calcination treatment.

2. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (1), the amount of the crystal plane modifier added is 0.2%-2.0% of the mass of iron element in the iron source.

3. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (1), the lithium source is lithium hydroxide, and the molar ratio of lithium to iron in iron phosphate is 1.03-1.08:

1. And / or, the carbon precursor is at least one of polyethylene glycol, polyvinyl alcohol, and sucrose, and its addition amount is 3%-8% of the mass of iron phosphate; And / or, the dispersant is ammonium polyacrylate, and its addition amount is 0.1%-0.5% of the mass of ferric phosphate.

4. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (2), the inlet temperature of the spray dryer is 180-220℃ and the outlet temperature is 80-100℃, and the process is carried out under an inert atmosphere.

5. The method for preparing lithium iron phosphate according to claim 1, characterized in that, In step (3), the segmented calcination includes: First stage: Increase the temperature to 400-500℃ at a rate of 2-5℃ / min and hold for 1-3 hours; Second stage: Continue to raise the temperature to 650-750℃ at a rate of 1-3℃ / min, and hold for 3-6 hours; The protective reducing atmosphere is a mixture of nitrogen and hydrogen, wherein the volume percentage of hydrogen is 2-10%.

6. A lithium iron phosphate, characterized in that, It is prepared by the method of lithium iron phosphate according to any one of claims 1-5.

7. An application of lithium iron phosphate as described in claim 6, characterized in that, The lithium iron phosphate is used in the positive electrode of lithium-ion batteries.

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