A method for preparing high-rate lithium iron phosphate using in-situ carbon composites and its application

Abstract

This invention relates to the field of lithium-ion battery cathode material technology, specifically to a method for preparing and applying high-rate lithium iron phosphate with in-situ carbon composite. The method includes: Step 1: Using a primary carbon source dispersion as the base liquid, adding an antioxidant, and then adding ferrous sulfate and a phosphorus source solution at the same dropping rate; heating and adding alkaline solution to adjust the pH to 4-6; followed by filtration, washing, and vacuum drying to obtain in-situ carbon composite octahydrate ferrous phosphate; Step 2: Using this as a precursor, adding lithium phosphate, a secondary carbon source, a dispersant, an elemental dopant, and an organic solvent in proportion; dispersing and grinding, and then spray drying to obtain a spray-dried material; Step 3: Dividing the spray-dried material into three equal parts, sintering at different temperatures under a nitrogen atmosphere, and then mixing, pulverizing, and demagnetizing the products to obtain the high-rate lithium iron phosphate cathode material with in-situ carbon composite. This invention significantly improves the rate performance, energy density, and process stability of lithium iron phosphate through in-situ carbon composite, a highly active divalent iron source, and a multi-temperature sintering and particle size distribution strategy.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, specifically to a method for preparing high-rate lithium iron phosphate using in-situ carbon composite and its application. Background Technology

[0002] In the field of lithium-ion battery cathode materials, olivine-structured lithium iron phosphate (LiFePO4) is considered one of the most promising cathode materials for power batteries and large-scale energy storage batteries due to its outstanding advantages such as excellent safety performance, long cycle life (>2000 cycles), environmental friendliness, abundant iron source reserves, and relatively low cost, occupying a key position in the new energy industry. However, the intrinsic physicochemical properties of lithium iron phosphate also bring two major challenges to its practical application: extremely low electronic conductivity (10⁻⁶ ppm). -9 -10 -10 S / cm) and ion diffusion rate (approximately 10) -14 These two bottlenecks together result in poor rate performance of the material; that is, the effective capacity decays rapidly during high-speed charging and discharging, making it difficult to meet the high-current discharge requirements of electric vehicles for rapid start-up, acceleration, and regenerative braking energy recovery. This also limits its application in energy storage frequency regulation scenarios requiring rapid charging and discharging. Furthermore, improving the energy density of lithium iron phosphate materials has become a key research direction for both academia and industry. Numerous patented technologies have focused on the impact of sintering regimes on the crystallinity, particle morphology, and carbon coating quality of lithium iron phosphate. Traditional processes typically employ single-temperature sintering. In the current industry trend of pursuing high compaction and high rate performance, developing new sintering processes and improving particle size distribution are crucial for upgrading and iterating lithium iron phosphate.

[0003] In existing technologies, iron phosphate is the most commonly used precursor for synthesizing lithium iron phosphate. However, the large particle size and compact internal structure of iron phosphate severely hinder ion diffusion during the sintering process, particularly in the carbothermic reduction stage (Fe...). 3+ →Fe 2+ Due to the extended mass transfer path and increased diffusion resistance within the microcrystals, the trivalent iron source needs to undergo Fe... 3+ →Fe 2+ The phase transition of iron phosphate precursors requires higher sintering temperatures and a greater carbon source mass to synthesize lithium iron phosphate, which can easily lead to over-reduction and the formation of magnetic foreign matter (Fe2P, Fe2O3, etc.). For example, CN120423517B discloses a multi-stage high-pressure lithium iron phosphate preparation method. This patent is the applicant's prior application. In this technical solution, the iron and phosphorus sources are iron phosphate, thus requiring the formation of Fe2P precursors. 3+ →Fe 2+ The carbothermic reduction phase transition has problems such as high sintering temperature and easy formation of magnetic foreign matter.

[0004] Furthermore, the most common method to improve the conductivity of lithium iron phosphate (LFP) is to add organic carbon sources (such as glucose or sucrose) to the raw materials and sinter them at high temperature under an inert atmosphere, causing the carbon source to pyrolyze and form an amorphous carbon coating layer on the surface of the LFP particles. However, this method often involves physical mixing with the carbon source after the reactant precursor has formed, resulting in an inherent defect of uneven carbon distribution. For example, CN / 121225563A discloses a high-pressure, high-rate LFP synthesized from ferrous phosphate and its preparation method, but the physical mixing of the carbon source leads to uneven coating. It is evident that traditional physical mixing methods cannot ensure uniform contact between the carbon source and the reactant precursor at the microscale, resulting in incomplete and uneven carbon layer coating after sintering. Some active materials fail to effectively connect with the conductive network, thus limiting the overall conductivity improvement and failing to fully realize the potential of the precursor route. Summary of the Invention

[0005] To address the technical problems in existing lithium iron phosphate (LFP) preparation methods that utilize iron phosphate precursors, such as high sintering temperatures, large carbon source quantities, easy formation of magnetic foreign matter, uneven physical mixing and coating of carbon sources, and poor particle size distribution in single-temperature sintering processes, this invention provides a method for preparing high-rate LFP using in-situ carbon composite technology and its application. By introducing a primary carbon source during the synthesis stage of the octahydrate ferrous phosphate precursor, in-situ carbon composite is achieved. Combined with optimized spray granulation and multi-temperature-range sintering processes, this method solves the problems of uneven carbon distribution, low precursor reactivity, and poor particle size distribution found in existing technologies, thereby preparing LFP cathode materials with excellent rate performance.

[0006] The technical solution of this invention is as follows: In a first aspect, the present invention provides a method for preparing high-rate lithium iron phosphate using in-situ carbon composites, comprising the following steps: Step 1: Prepare ferrous sulfate solution, phosphorus source solution and primary carbon source dispersion respectively; using the primary carbon source dispersion as the reaction base liquid, first add antioxidant to the reaction system, then add ferrous sulfate solution and phosphorus source solution to the reaction system at the same dropping rate, heat to the preset temperature, add alkali solution to the reaction system to adjust the pH value to 4-6, stir and keep the reaction at the temperature for a certain time, filter, wash and vacuum dry to obtain in-situ carbon-composite octahydrate ferrous phosphate; Step 2: Using in-situ carbon-composite ferrous phosphate octahydrate as a precursor, lithium phosphate, secondary carbon source, dispersant, metal dopant and organic solvent are added in a certain proportion for dispersion to obtain a dispersed slurry; after grinding the dispersed slurry to a certain particle size, it is spray-dried to obtain a spray-dried material. Step 3: Divide the spray-dried material into three equal parts and sinter them at different temperatures under a nitrogen atmosphere. After sintering, mix and crush the three sintered products, and after a demagnetization process, obtain an in-situ carbon composite high-rate lithium iron phosphate cathode material.

[0007] This invention uses a primary carbon source dispersion as the reaction substrate. By simultaneously adding ferrous sulfate and phosphorus source solutions, combined with antioxidant protection and precise pH control, the primary carbon source can be uniformly adsorbed and coated on the crystal surface and gaps during the growth of ferrous phosphate crystals, forming an in-situ "crystal-carbon layer" composite structure, thus solving the problem of uneven carbon layer coating from the source.

[0008] Furthermore, in step one, the phosphorus source is any one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate; the primary carbon source is any one of graphene, carbon nanotubes, and acetylene black; and the antioxidant is any one of ascorbic acid and tea polyphenols.

[0009] The primary carbon source uses nanomaterials such as graphene and carbon nanotubes, which have a large specific surface area and excellent conductivity. They can also rapidly disperse and recombine in situ during the growth of ferrous phosphate crystals. Compared to traditional post-coated carbon sources, they exhibit stronger bonding with the crystal matrix and a more continuous conductive network. The antioxidants used are ascorbic acid or tea polyphenols, both of which have strong reducing properties and can effectively inhibit Fe in the reaction system. 2+ Oxidized to Fe 3+ .

[0010] Furthermore, in step two, the secondary carbon source is any one or more of glucose, sucrose, and starch; the dispersant is any one or more of PEG2000, PEG6000, and PVA2000; the metal dopant is any one or more of titanium dioxide, niobium pentoxide, vanadium pentoxide, ammonium metavanadate, and magnesium acetate; and the organic solvent is methanol or ethanol.

[0011] The secondary carbon source uses sugars such as glucose and sucrose, which are easily carbonized during subsequent sintering, forming a thin and uniform carbon film on the particle surface. This forms a dual carbon network of "in-situ composite carbon + surface-coated carbon" with the primary carbon source, further improving the material's conductivity. Simultaneously, the residual pores after carbonization of the sugar source provide channels for lithium-ion migration, optimizing rate performance. The dispersant used is PEG series or PVA2000, which has good surface activity, adsorbing onto the surface of each component particle, reducing interparticle forces, preventing particle agglomeration, and ensuring uniform slurry dispersion. Furthermore, the dispersant is almost completely decomposed and volatilized during sintering, leaving no impurities, and the resulting pores facilitate electrolyte wetting. The metal dopant uses various metal oxides or salts, whose metal ions can partially replace Fe in the lithium iron phosphate lattice. 2+ Or Li +By adjusting the lattice constant, the lithium-ion diffusion channels are increased, and the lithium-ion migration resistance is reduced. Simultaneously, doping ions stabilize the lattice structure, suppressing particle volume expansion and structural collapse during cycling. Furthermore, methanol or ethanol is chosen as the organic solvent, as its moderate polarity effectively disperses the components, and its low boiling point allows it to easily volatilize during spray drying, rapidly forming dry spherical particles and preventing particle adhesion.

[0012] Furthermore, in step two, the molar ratio of ferrous phosphate octahydrate, lithium phosphate, and metal dopant is iron:lithium:metal in the metal dopant = 1:(1-1.05):(0.006-0.06); the mass ratio of ferrous phosphate octahydrate, secondary carbon source, and dispersant is ferrous phosphate octahydrate:secondary carbon source:dispersant = 1:(0.01-0.15):(0.01-0.15).

[0013] Furthermore, in step two, the solid content of the dispersed slurry is 30-45%. Controlling the solid content to 30-45% is crucial; too low a content results in excessive solvent evaporation during spray drying, increasing energy consumption and causing the particles to be too loose with too many pores, affecting compaction density. Too high a content increases slurry viscosity, reduces fluidity, hinders uniform spraying, and easily leads to the formation of large or agglomerated particles.

[0014] Furthermore, in step two, the slurry particle size is ground to a D50 of 0.35-0.55μm.

[0015] Furthermore, in step two, the inlet air temperature of the spray dryer is 200-240℃, the outlet air temperature of the spray dryer is 75-110℃, nitrogen is used as the gas source, and the moisture content of the material obtained by spray drying is less than 5wt%.

[0016] Furthermore, in step three, the sintering temperature of material one is 760-820℃, the sintering temperature of material two is 1-5℃ higher than that of material one, and the sintering temperature of material three is 1-5℃ lower than that of material one. The sintering time for all three materials is the same, 6-12 hours. By dividing the spray-dried material into three equal parts for gradient temperature sintering, and by controlling different sintering temperatures, the three parts of the material form lithium iron phosphate particles with different grain sizes. The sintering temperature of material one at 760-820℃ ensures that the carbothermic reduction reaction proceeds fully, forming a complete and structurally stable lithium iron phosphate lattice. Material two is sintered at a temperature 1-5℃ higher, resulting in particles with slightly larger grain sizes. Material three is sintered at a temperature 1-5℃ lower, resulting in particles with slightly smaller grain sizes. Smaller particles have a larger specific surface area and more active sites, which can improve the rate performance and charge / discharge capacity of the material. Step three of this invention achieves a multi-level gradation effect between particles based on different sintering temperatures. Small particles fill the spaces between large particles, which effectively improves the electrochemical performance of the material while increasing its bulk density and compaction density, thus achieving a significant increase in energy density.

[0017] Furthermore, in step three, the particle size D50 of the high-rate lithium iron phosphate composite with in-situ carbon is 1.2 ± 0.3 μm.

[0018] Secondly, the present invention provides an application of in-situ carbon-composite high-rate lithium iron phosphate, wherein the in-situ carbon-composite high-rate lithium iron phosphate is used in lithium-ion batteries.

[0019] The beneficial effects of this invention are as follows: (1) In this invention, ferrous phosphate octahydrate is selected as the precursor. Compared with ferric phosphate, sintering is not required in the precursor preparation process, which can save sintering costs, reduce one process, improve production efficiency, and significantly reduce production costs. It also avoids the Fe required by traditional trivalent iron sources in the carbothermic reduction process. 3+ →Fe 2+ Phase transition. On the one hand, this significantly reduces the sintering temperature and the amount of carbon source required, suppresses magnetic foreign matter generated by excessive reduction, and improves the chemical purity and batch consistency of the product; on the other hand, the higher reactivity of the precursor promotes the complete growth of LiFePO4 crystals during sintering, which is beneficial to obtaining cathode materials with high crystallinity and few crystal defects, thereby improving the specific capacity and cycle stability of the material.

[0020] (2) This invention introduces a primary conductive carbon source (such as graphene, carbon nanotubes, etc.) during the synthesis stage of ferrous phosphate octahydrate precursor and adopts a reaction method of co-precipitation and using a dispersion as the base liquid, thereby achieving in-situ, nanoscale uniform composite of the carbon source during the crystallization process of ferrous phosphate. This structure effectively avoids the problems of incomplete coating and uneven conductive layer thickness caused by traditional physical mixing of carbon sources, and constructs a stable three-dimensional conductive network inside the material, significantly enhancing the electron transport efficiency between particles and laying a structural foundation for high-rate performance.

[0021] (3) The present invention adopts a multi-temperature sintering strategy, which makes the final product contain particles with different particle sizes and sintering degrees. After mixing and airflow pulverization, efficient particle gradation is achieved, filling the voids under a single particle size distribution, significantly improving the compaction density of the electrode sheet, and thus improving the volumetric energy density of the battery. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 These are SEM images of materials 1, 2, and 3 prepared at different sintering temperatures in Example 1 after being pulverized.

[0024] Figure 2 This is a SEM image of the lithium iron phosphate cathode material prepared in Example 1.

[0025] Figure 3 This is the XRD pattern of the lithium iron phosphate cathode material prepared in Example 1.

[0026] Figure 4 This is the XRD pattern of the in-situ carbon-composite octahydrate ferrous phosphate precursor prepared in Example 1.

[0027] Figure 5 The first charge-discharge curve of the lithium iron phosphate cathode material prepared in Example 1 at 0.1C is shown. Detailed Implementation

[0028] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0029] Example 1 A method for preparing high-rate lithium iron phosphate using in-situ carbon composites includes the following steps: Step 1: Prepare ferrous sulfate and ammonium dihydrogen phosphate solutions according to the stoichiometric ratio Fe:P=3:2. Disperse graphene in pure water, with the graphene mass being 0.5% of the ferrous sulfate mass. Using the graphene dispersion as a base, first add ascorbic acid at 0.4% of the ferrous sulfate mass. Add the ferrous sulfate solution and phosphorus source solution to the reaction system at the same dropping rate, control the reaction temperature at 40℃, and adjust the pH of the solution to 5.0 by adding ammonia dropwise. After stirring and keeping warm for 2 hours, filter, wash, and vacuum dry to obtain in-situ carbon-composite octahydrate ferrous phosphate.

[0030] Step 2: Add 1000g of in-situ carbon-composite ferrous phosphate octahydrate, 231.5g of lithium phosphate, 25g of sucrose, 80g of PEG6000, and 4g of titanium dioxide to 2L of methanol, and mechanically stir for 30 minutes to mix evenly and obtain a uniformly dispersed slurry. Transfer the slurry to a sand mill and grind it to a D50 of 0.4μm. Then spray dry it, controlling the inlet air temperature at 220℃ and the outlet air temperature at 95℃ to obtain the spray-dried material.

[0031] Step 3: The spray-dried material was divided into three equal portions. Under a nitrogen atmosphere, these three portions were sintered at different temperatures: Material 1 was sintered at 780℃; Material 2 was sintered at a temperature 2℃ higher than Material 1; and Material 3 was sintered at a temperature 2℃ lower than Material 1. The sintering time for all three portions was the same, 10 hours. After sintering, the products sintered at the three different temperatures were mixed, and the particle size D50 was controlled to be 1.2±0.2μm by airflow milling. After sieving and demagnetization, the in-situ carbon composite high-rate lithium iron phosphate cathode material was finally obtained.

[0032] Example 2 A method for preparing high-rate lithium iron phosphate using in-situ carbon composites includes the following steps: Step 1: Prepare ferrous sulfate and ammonium dihydrogen phosphate solutions according to the stoichiometric ratio Fe:P=3:2. Disperse carbon nanotubes in pure water, with the mass of carbon nanotubes being 0.5% of the mass of ferrous sulfate. Using the carbon nanotube dispersion as a base, first add ascorbic acid at 0.4% of the mass of ferrous sulfate. Add the ferrous sulfate solution and phosphorus source solution to the reaction system at the same dropping rate, control the reaction temperature at 40℃, and adjust the pH of the solution to 5.0 by adding ammonia dropwise. After stirring and keeping warm for 2 hours, filter, wash, and vacuum dry to obtain in-situ carbon-composite octahydrate ferrous phosphate.

[0033] Step 2: Add 1000g of in-situ carbon-composite ferrous phosphate octahydrate, 231.5g of lithium phosphate, 25g of glucose, 80g of PEG6000, and 4g of titanium dioxide to 2L of methanol, and mechanically stir for 30 minutes to mix evenly and obtain a uniformly dispersed slurry. Transfer the slurry to a sand mill and grind it to a D50 of 0.4μm, and then spray dry it. Control the inlet air temperature at 220℃ and the outlet air temperature at 95℃ to obtain the spray-dried material.

[0034] Step 3: The spray-dried material was divided into three equal portions. Under a nitrogen atmosphere, these three portions were sintered at different temperatures: Material 1 was sintered at 780℃; Material 2 was sintered at a temperature 2℃ higher than Material 1; and Material 3 was sintered at a temperature 2℃ lower than Material 1. The sintering time for all three portions was the same, 10 hours. After sintering, the products sintered at the three different temperatures were mixed, and the particle size D50 was controlled to be 1.2±0.2μm by airflow milling. After sieving and demagnetization, the in-situ carbon composite high-rate lithium iron phosphate cathode material was finally obtained.

[0035] Example 3 A method for preparing high-rate lithium iron phosphate using in-situ carbon composites includes the following steps: Step 1: Prepare ferrous sulfate and ammonium dihydrogen phosphate solutions according to the stoichiometric ratio Fe:P=3:2. Disperse graphene in pure water, with the graphene mass being 0.5% of the ferrous sulfate mass. Using the graphene dispersion as a base, first add ascorbic acid at 0.4% of the ferrous sulfate mass. Add the ferrous sulfate solution and phosphorus source solution to the reaction system at the same dropping rate, control the reaction temperature at 40℃, and adjust the pH of the solution to 5.0 by adding ammonia dropwise. After stirring and keeping warm for 2 hours, filter, wash, and vacuum dry to obtain in-situ carbon-composite octahydrate ferrous phosphate.

[0036] Step 2: Add 1000g of in-situ carbon-composite ferrous phosphate octahydrate, 233g of lithium phosphate, 25g of sucrose, 80g of PEG6000, and 5g of titanium dioxide to 2L of methanol, and mechanically stir for 30 minutes to mix evenly and obtain a uniformly dispersed slurry. Transfer the slurry to a sand mill and grind it to a D50 of 0.45μm. Then spray dry it, controlling the inlet air temperature at 220℃ and the outlet air temperature at 95℃ to obtain the spray-dried material.

[0037] Step 3: The spray-dried material was divided into three equal portions. Under a nitrogen atmosphere, these three portions were sintered at different temperatures: Material 1 was sintered at 800℃; Material 2 was sintered at a temperature 2℃ higher than Material 1; and Material 3 was sintered at a temperature 2℃ lower than Material 1. The sintering time for all three portions was the same, 10 hours. After sintering, the products sintered at the three different temperatures were mixed, and the particle size D50 was controlled to be 1.2±0.2μm by airflow milling. After sieving and demagnetization, the in-situ carbon composite high-rate lithium iron phosphate cathode material was finally obtained.

[0038] Comparative Example 1 The only difference between Comparative Example 1 and Example 1 is step 3: the sprayed material is sintered at the same temperature, both at 780°C for 10 hours, without multi-temperature sintering gradation.

[0039] Comparative Example 2 The difference between Comparative Example 2 and Example 1: Step 1 is omitted, and ferric phosphate, a trivalent iron source, is directly used as the precursor.

[0040] 1000g of ferric phosphate, 253.2g of lithium carbonate, 57g of sucrose, 90g of PEG6000, and 7.0g of titanium dioxide were added to 2L of pure water and mechanically stirred for 30 minutes to obtain a uniformly dispersed slurry. The dispersed slurry was ground to a D50 of 0.4μm and then spray-dried, with the inlet air temperature controlled at 220℃ and the outlet air temperature at 95℃. Sintering was carried out under a nitrogen atmosphere at a heating rate of 3℃ / min at 780℃ for 10 hours. The sintered material was then pulverized by an air jet mill, with the particle size D50 controlled at 1.2±0.2μm.

[0041] Comparative Example 3 The only difference between Comparative Example 3 and Example 1 is in step one: in the synthesis of ferrous phosphate octahydrate, no primary carbon source is added, and pure phase ferrous phosphate octahydrate is directly synthesized.

[0042] Test Example 1 Compacted density test The compaction density of the lithium iron phosphate cathode materials prepared in Examples 1-3 and Comparative Examples 1-4 was tested using a compaction density meter at a pressure of 3T. The test results are shown in Table 1.

[0043] Test Example 2 Scanning electron microscopy test Materials 1 (sintered at 780℃), 2 (sintered at 782℃), and 3 (sintered at 778℃) prepared in Example 1 were separately pulverized. The scanning electron microscopy (SEM) results and primary particle size distribution are shown below. Figure 1 As shown. By Figure 1It is known that the difference in sintering temperature directly leads to a significant difference in the primary particle size of the materials; the higher the sintering temperature, the larger the particle size. The average primary particle sizes of materials one, two, and three are 302 nm, 452 nm, and 175 nm, respectively. Therefore, the innovative sintering process of this application utilizes the difference in material size for particle gradation, which can achieve efficient packing between particles of different sizes, effectively filling the structural voids under a single particle size distribution, thereby significantly improving the compaction density of the electrode sheet and ultimately increasing the volumetric energy density of the battery.

[0044] The lithium iron phosphate cathode material prepared in Example 1 was subjected to scanning electron microscopy (SEM) testing. The SEM images are shown below. Figure 2 As shown. By Figure 2 It is evident that the prepared lithium iron phosphate samples exhibit near-spherical particle shapes; the particles of varying sizes are densely packed, and the synergistic effect of particle size distribution and near-spherical primary particles significantly enhances the material's packing density. Furthermore, layered graphene is observed embedded between lithium iron phosphate particles, providing rapid channels for ion and electron transport between particles, which will significantly enhance the electrochemical performance of lithium iron phosphate.

[0045] Test Example 3 XRD phase analysis XRD phase analysis was performed on the lithium iron phosphate cathode material prepared in Example 1, and the results are as follows: Figure 3 As shown. By Figure 3 It can be seen that the prepared lithium iron phosphate sample corresponds to the LiFePO4 standard card PDF#83-2092, indicating that the lithium iron phosphate material with olivine structure was successfully prepared, and the sample has high purity with no other impurity peaks found.

[0046] XRD phase analysis was performed on the in-situ carbon-composite octahydrate ferrous phosphate prepared in Example 1, and the results are as follows: Figure 4 As shown. By Figure 4 It can be seen that the prepared in-situ carbon-reinforced octahydrate ferrous phosphate corresponds to Fe3(PO4)2 The 8H2O standard card PDF#83-2453 indicates that a pure phase sample was successfully prepared. Due to the low carbon content, no carbon diffraction peaks were found.

[0047] Test Example 4 Electrochemical performance testing The lithium iron phosphate cathode materials prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to the following operations: The active material, binder PVDF, and conductive additive SP were dispersed in NMP solution at a mass ratio of 95:2.5:2.5. After thorough mixing, a slurry was prepared, coated onto aluminum foil, vacuum dried, rolled, and cut into sheets to obtain the cathode sheet. The mass of the active material in the cathode sheet was approximately 12 mg. The anode was a lithium metal sheet, the separator was a polypropylene porous membrane, and the electrolyte was 1 mol / L LiPF6 / EC+DEC+DMC (where the volume ratio of EC:DEC:DMC = 1:1:1). The charge / discharge specific capacity of the coin cell was tested in the voltage range of 2.0V-3.75V, and the test results are shown in Table 1.

[0048] The first charge-discharge curve of the lithium iron phosphate cathode material prepared in Example 1 at 0.1C is shown below. Figure 5 As shown.

[0049] Table 1 shows a comparison of data from various embodiments and comparative examples of the present invention: Table 1. Compaction density and electrochemical performance test results of Examples 1-3 and Comparative Examples 1-3

[0050] The compaction density and electrochemical performance of Examples 1-3 of the present invention are significantly better than those of Comparative Examples 1-3.

[0051] Comparative Example 1, which did not employ multi-temperature sintering gradation (but instead used single-temperature sintering), had a more significant impact on the compaction density and 5C high-rate discharge capacity. Its compaction density was 2.49 g / cc, lower than the 2.62 g / cc of Example 1; its 1C and 5C discharge capacities were 143.7 mAh / g and 122.7 mAh / g, respectively, a decrease of 8.5 mAh / g and 8.9 mAh / g compared to Example 1, highlighting the core role of multi-temperature sintering in particle size distribution and high-rate performance.

[0052] Comparative Example 2 did not use a highly active divalent iron source (instead using the traditional trivalent iron source, iron phosphate), thus failing to achieve in-situ carbon recombination, which significantly affected the compaction density and 5C high-rate discharge capacity results. Its compaction density was only 2.48 g / cc, 0.14 g / cc lower than Example 1; its 5C discharge capacity was 113.0 mAh / g, 18.6 mAh / g lower than Example 1, and its first-efficiency rating also dropped to 97.2%, significantly inferior to the levels of the examples.

[0053] Comparative Example 3, which did not add a primary carbon source and eliminated in-situ carbon recombination, showed inferior results in terms of compaction density, high-rate discharge capacity, and first-time efficiency compared to the examples. Its 5C discharge capacity was only 119.1 mAh / g, 12.5 mAh / g lower than Example 1, demonstrating that in-situ carbon recombination during the precursor synthesis stage is crucial for constructing a uniform conductive network and improving electrochemical performance.

[0054] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for preparing high-rate lithium iron phosphate using in-situ carbon composites, characterized in that, Includes the following steps: Step 1: Prepare ferrous sulfate solution, phosphorus source solution and primary carbon source dispersion respectively; using the primary carbon source dispersion as the reaction base liquid, first add antioxidant to the reaction system, then add ferrous sulfate solution and phosphorus source solution to the reaction system at the same dropping rate, heat to the preset temperature, add alkali solution to the reaction system to adjust the pH value to 4-6, stir and keep the reaction at the temperature, filter, wash and vacuum dry to obtain in-situ carbon-composite octahydrate ferrous phosphate; Step 2: Using in-situ carbon-composite ferrous phosphate octahydrate as a precursor, lithium phosphate, secondary carbon source, dispersant, metal dopant and organic solvent are added and dispersed to obtain a dispersed slurry; after grinding the dispersed slurry to a certain particle size, it is spray-dried to obtain a spray-dried material. Step 3: Divide the spray-dried material into three equal parts and sinter them at different temperatures under a nitrogen atmosphere. After sintering, mix and crush the three sintered products, and after demagnetization, obtain an in-situ carbon composite high-rate lithium iron phosphate.

2. The method for preparing high-rate lithium iron phosphate with in-situ carbon composite as described in claim 1, characterized in that, In step one, the phosphorus source is any one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and ammonium phosphate; the primary carbon source is any one of graphene, carbon nanotubes, and acetylene black; and the antioxidant is any one of ascorbic acid and tea polyphenols.

3. The method for preparing high-rate lithium iron phosphate with in-situ carbon composite as described in claim 1, characterized in that, In step two, the secondary carbon source is any one or more of glucose, sucrose, and starch; the dispersant is any one or more of PEG2000, PEG6000, and PVA2000; the metal dopant is any one or more of titanium dioxide, niobium pentoxide, vanadium pentoxide, ammonium metavanadate, and magnesium acetate; and the organic solvent is methanol or ethanol.

4. The method for preparing high-rate lithium iron phosphate with in-situ carbon composite as described in claim 1, characterized in that, In step two, the molar ratio of ferrous phosphate octahydrate, lithium phosphate, and metal dopant is ferrous element: lithium element: metal element in metal dopant = 1:(1-1.05):(0.006-0.06); the mass ratio of ferrous phosphate octahydrate, secondary carbon source, and dispersant is ferrous phosphate octahydrate: secondary carbon source: dispersant = 1:(0.01-0.15):(0.01-0.15).

5. The method for preparing high-rate lithium iron phosphate with in-situ carbon composite as described in claim 1, characterized in that, In step two, the solid content of the dispersed slurry is 30-45%.

6. The method for preparing high-rate lithium iron phosphate with in-situ carbon composite as described in claim 1, characterized in that, In step two, the slurry particle size is ground to a D50 of 0.35-0.55μm.

7. The method for preparing high-rate lithium iron phosphate with in-situ carbon composite as described in claim 1, characterized in that, In step two, the inlet air temperature of spray drying is 200-240℃, the outlet air temperature of spray drying is 75-110℃, nitrogen is used as the gas source, and the moisture content of the material obtained by spray drying is less than 5wt%.

8. The method for preparing high-rate lithium iron phosphate with in-situ carbon composite as described in claim 1, characterized in that, In step three, the sintering temperature of material one is 760-820℃, the sintering temperature of material two is 1-5℃ higher than that of material one, and the sintering temperature of material three is 1-5℃ lower than that of material one. The sintering time for the three materials is the same, which is 6-12h.

9. The method for preparing high-rate lithium iron phosphate with in-situ carbon composite as described in claim 1, characterized in that, In step three, the particle size D50 of the high-rate lithium iron phosphate composite with in-situ carbon is 1.2 ± 0.3 μm.

10. An application of high-rate lithium iron phosphate with in-situ carbon composite, characterized in that, The in-situ carbon-composite high-rate lithium iron phosphate is prepared by the in-situ carbon-composite high-rate lithium iron phosphate preparation method according to any one of claims 1-9, and the in-situ carbon-composite high-rate lithium iron phosphate is used in lithium-ion batteries.

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