Composite lithium iron phosphate positive electrode material and preparation method and application thereof

Abstract

The application provides a composite lithium iron phosphate positive electrode material and a preparation method and application thereof. The composite lithium iron phosphate positive electrode material comprises hollow spherical lithium iron phosphate and flaky lithium iron phosphate. The hollow spherical lithium iron phosphate comprises an internal cavity and a spherical lithium iron phosphate shell. The flaky lithium iron phosphate is arranged in the internal cavity of the hollow spherical lithium iron phosphate. The surface of the flaky lithium iron phosphate is provided with a nitrogen-doped carbon coating layer. The composite lithium iron phosphate positive electrode material has a unique structure of hollow lithium iron phosphate coating flaky lithium iron phosphate. The sphericity of the lithium iron phosphate positive electrode material is high. The composite structure can not only improve the specific surface area and tap density of the material, but also shorten the transmission path of lithium ions and improve the rate performance of the positive electrode material.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery materials, and relates to a composite lithium iron phosphate positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] The lithium iron phosphate electrode material is mainly used in various lithium ion batteries. American and Japanese scholars published lithium iron phosphate (LiFePO4) with an olivine structure, so that the material has been highly valued and caused extensive research and rapid development. Compared with traditional lithium ion secondary battery positive electrode materials, LiFePO4 has more extensive raw materials, lower price and no environmental pollution.

[0003] However, due to the crystal structure of lithium iron phosphate, LiFePO4 has a low electronic conductivity (10 -9 ~ 10 - 8 S / cm); at the same time, the unique structural characteristics also hinder the diffusion movement of Li + to a certain extent, so that Li + diffuses along the one-dimensional channel of the b-axis direction, resulting in a very low ion diffusion rate (10 -17 ~ 10 -14 cm 2 / s) of LiFePO4 material.

[0004] CN115602833A discloses a method for synthesizing lithium iron phosphate hollow spheres by a solid phase method and a high-performance lithium battery assembled by using the material synthesized by the method. The spherical lithium iron phosphate material is prepared by selecting suitable phosphorus source, lithium source, iron source and carbon source, combining different mixing methods and high-temperature calcination process.

[0005] CN114725374A discloses a lithium iron phosphate material, a preparation method thereof and a battery. The lithium iron phosphate material includes solid large-particle lithium iron phosphate and hollow small-particle lithium iron phosphate distributed between the solid large-particle lithium iron phosphate, the particle size of the solid large-particle lithium iron phosphate is greater than that of the hollow small-particle lithium iron phosphate, and the hollow small-particle lithium iron phosphate is a hollow structure formed by surrounding a plurality of primary particles of lithium iron phosphate.

[0006] The above-mentioned scheme prepares hollow spherical lithium iron phosphate or mixes hollow spherical lithium iron phosphate with solid spherical lithium iron phosphate, which can improve tap density and specific surface area, but has poor rate performance. SUMMARY

[0007] The application aims to provide a composite lithium iron phosphate positive electrode material, a preparation method and application thereof.

[0008] To achieve the above-mentioned purpose, the application adopts the following technical scheme.

[0009] In a first aspect, the application provides a composite lithium iron phosphate positive electrode material, which comprises hollow spherical lithium iron phosphate and flaky lithium iron phosphate.

[0010] The composite lithium iron phosphate positive electrode material has a spherical hollow structure and contains flaky lithium iron phosphate in the hollow sphere. The spherical structure of the lithium iron phosphate is beneficial to improve the tap density of the material. The hollow sphere shell structure and the flaky lithium iron phosphate inside the hollow sphere shell are both beneficial to improve the specific surface area of the material, shorten the transmission path of lithium ions, and improve the rate performance of the positive electrode material. The accumulation of the flaky nanomaterial is beneficial to alleviate the loss of space of the spherical shell-shaped lithium iron phosphate material. In addition, the surface of the flaky nanometer lithium iron phosphate is coated with a dopamine layer, which becomes nitrogen-doped carbon after high-temperature calcination, solving the problem of difficult uniform carbon material coverage of the internal material, improving the electronic conductivity, and improving the rate performance.

[0011] Preferably, the median particle size D50 of the composite lithium iron phosphate positive electrode material is 3.6-4.8 μm, for example, 3.6 μm, 3.8 μm, 4 μm, 4.5 μm or 4.8 μm, etc.

[0012] Preferably, the thickness of the spherical lithium iron phosphate shell is 250-400 nm, for example, 250 nm, 280 nm, 300 nm, 350 nm or 400 nm, etc.

[0013] Preferably, the thickness of the nitrogen-doped carbon coating layer is 10-40 nm, for example, 10 nm, 12 nm, 20 nm, 30 nm or 40 nm, etc.

[0014] Preferably, the surface of the composite lithium iron phosphate positive electrode material is provided with a carbon coating layer.

[0015] Preferably, the thickness of the carbon coating layer is 10-25 nm, for example, 10 nm, 12 nm, 15 nm, 20 nm or 25 nm, etc.

[0016] In a second aspect, the present application provides a preparation method of the composite lithium iron phosphate cathode material according to the first aspect, the preparation method comprising the following steps:

[0017] (1) mixing flaky iron phosphate, a buffer solution and dopamine hydrochloride to obtain a mixed solution, and performing one-step stirring reaction to obtain dopamine-coated flaky iron phosphate, mixing the dopamine-coated flaky iron phosphate, alkyl mercaptan, vinyltrimethoxysilane, a photoinitiator and a solvent, and performing ultraviolet irradiation treatment to obtain hydrophobic iron phosphate;

[0018] (2) mixing an iron source, a phosphoric acid source and a surfactant with water to obtain an aqueous phase solution, mixing the hydrophobic iron phosphate with an oil phase solvent to obtain an oil phase solution, mixing the aqueous phase solution and the oil phase solution, adjusting pH, and performing two-step stirring reaction to obtain the composite iron phosphate;

[0019] (3) mixing the composite iron phosphate, a lithium source and a dispersant, performing grinding and drying, and then performing calcination to obtain the composite lithium iron phosphate cathode material.

[0020] In the present application, dopamine is first oxidized to benzoquinone, intramolecular cyclization reaction occurs, then 5,6-dihydroxyindole is generated through intramolecular rearrangement reaction, and then self-polymerization occurs to form polydopamine aggregates. The aggregates perform Brownian motion in the solution, and a polydopamine coating layer with high-strength irreversible covalent bonds is formed on the nanophosphate iron. The polydopamine layer is rich in hydroxyl groups, and hydrolysis of the vinyltrimethoxysilane forms silanol, which condenses with the hydroxyl groups in the polydopamine. Under the action of the photoinitiator, the alkyl mercaptan undergoes click reaction with the vinyltrimethoxysilane to form a super-hydrophobic phosphate iron material.

[0021] The hydrophobicity of the hydrophobically modified phosphate iron is utilized to dissolve the phosphate iron in the oil phase solution, and then the phosphoric acid source and the iron source are added in the aqueous phase solution. After preparing an oil-in-water emulsion and adjusting the pH value of the solution, the phosphate iron is grown on the oil-water interface by using the physical adsorption and hydrogen bonding of the surfactant to the iron ions, a phosphate iron shell layer is formed, and phosphate iron nanosheets are stored in the oil phase droplets. Finally, the composite phosphate iron material is formed, and the composite lithium iron phosphate cathode material according to the present application is obtained by mixing the composite phosphate iron material with a lithium source and sintering.

[0022] Preferably, the length and width of the flaky iron phosphate in step (1) are independently 200-400 nm, for example, 200 nm, 250 nm, 300 nm, 350 nm or 400 nm, and the like. The values within this range are also applicable, in addition to the listed values.

[0023] Preferably, the buffer solution in step (1) comprises a Tris-HCl buffer solution.

[0024] Preferably, the pH of the buffer solution in step (1) is 8.2-8.8, for example 8.2, 8.3, 8.5, 8.6 or 8.8, and the like, not only limited to the listed values, other values not listed in the range of values are also applicable.

[0025] Preferably, the mass concentration of the sheet-shaped iron phosphate in the mixed solution in step (1) is 3.5-8 g / L, for example 3.5 g / L, 4 g / L, 5 g / L, 6 g / L or 8 g / L, and the like, not only limited to the listed values, other values not listed in the range of values are also applicable.

[0026] Preferably, the mass concentration of dopamine hydrochloride in the mixed solution in step (1) is 0.38-0.95 g / L, for example 0.38 g / L, 0.4 g / L, 0.5 g / L, 0.8 g / L or 0.95 g / L, and the like, not only limited to the listed values, other values not listed in the range of values are also applicable.

[0027] Preferably, the one-step stirring reaction in step (1) is carried out for 15-24 h, for example 15 h, 18 h, 20 h, 22 h or 24 h, and the like, not only limited to the listed values, other values not listed in the range of values are also applicable.

[0028] Preferably, after the one-step stirring reaction in step (1), centrifugation and drying are carried out.

[0029] Preferably, the centrifugation is carried out at a speed of 8000-12000 rpm, for example 8000 rpm, 9000 rpm, 10000 rpm, 11000 rpm or 12000 rpm, and the like, not only limited to the listed values, other values not listed in the range of values are also applicable.

[0030] Preferably, the centrifugation is carried out for 8-15 min, for example 8 min, 9 min, 10 min, 12 min or 15 min, and the like, not only limited to the listed values, other values not listed in the range of values are also applicable.

[0031] Preferably, the solvent in step (1) comprises ethyl acetate.

[0032] Preferably, the mass-volume ratio of the dopamine-coated sheet-shaped iron phosphate to the solvent in step (1) is 20-35 g:100 mL, for example 20 g:100 mL, 25 g:100 mL, 28 g:100 mL, 30 g:100 mL or 35 g:100 mL, and the like, not only limited to the listed values, other values not listed in the range of values are also applicable.

[0033] Preferably, the mass ratio of dopamine to vinyltrimethoxysilane in the dopamine-coated iron phosphate platelets of step (1) is (3.5-4.5): 1, such as 3.5: 1, 3.8: 1, 4: 1, 4.2: 1, or 4.5: 1, etc., not limited to the listed values, other values not listed within the range are also applicable.

[0034] Preferably, the alkyl mercaptan of step (1) includes any one or a combination of at least two of dodecyl mercaptan, tetradecyl mercaptan, hexadecyl mercaptan, or octadecyl mercaptan.

[0035] Preferably, the mass ratio of alkyl mercaptan to vinyltrimethoxysilane in step (1) is 1:(1.8-2.2), such as 1:1.8, 1:1.9, 1:2, 1:2.1, or 1:2.2, etc., not limited to the listed values, other values not listed within the range are also applicable.

[0036] Preferably, the photoinitiator of step (1) includes 2,2-dimethoxy-2-phenylacetophenone.

[0037] Preferably, the mass ratio of photoinitiator to vinyltrimethoxysilane in step (1) is 5:(5.7-6.2), such as 5:5.7, 5:5.8, 5:5.9, 5:6, or 5:6.2, etc., not limited to the listed values, other values not listed within the range are also applicable.

[0038] Preferably, the intensity of the ultraviolet irradiation treatment of step (1) is 230-270 W, such as 230 W, 240 W, 250 W, 260 W, or 270 W, etc., not limited to the listed values, other values not listed within the range are also applicable.

[0039] Preferably, the wavelength of the ultraviolet irradiation treatment of step (1) is 350-380 nm, such as 350 nm, 355 nm, 360 nm, 370 nm, or 380 nm, etc., not limited to the listed values, other values not listed within the range are also applicable.

[0040] Preferably, the time of the ultraviolet irradiation treatment of step (1) is 45-60 min, such as 45 min, 48 min, 50 min, 55 min, or 60 min, etc., not limited to the listed values, other values not listed within the range are also applicable.

[0041] Preferably, the ultraviolet irradiation treatment of step (1) is followed by centrifugation, washing, and drying.

[0042] Preferably, the iron source of step (2) comprises any one or a combination of at least two of ferric sulfate, poly ferric sulfate, ferric chloride or ferric nitrate, typical but non-limiting combinations include a combination of ferric sulfate and poly ferric sulfate, a combination of ferric sulfate and ferric chloride, or a combination of ferric chloride and ferric nitrate, etc.

[0043] Preferably, the phosphate source of step (2) comprises any one or a combination of at least two of H3PO4, (NH4)2HPO4, NH4H2PO4, (NH4)3PO4, NaH2PO4 or Na2HPO4, typical but non-limiting combinations include a combination of H3PO4 and (NH4)2HPO4, a combination of (NH4)2HPO4 and NH4H2PO4, or a combination of NaH2PO4 and Na2HPO4, etc.

[0044] Preferably, the surfactant of step (2) comprises any one or a combination of at least two of dodecyl benzene sulfonic acid, polyoxyethylene sorbitan monooleate, fatty alcohol polyoxyethylene ether, polyoxyethylene castor oil, polyoxyethylene oleyl ether or polyoxyethylene lauryl ether, typical but non-limiting combinations include a combination of dodecyl benzene sulfonic acid and polyoxyethylene sorbitan monooleate, a combination of fatty alcohol polyoxyethylene ether and polyoxyethylene castor oil, or a combination of polyoxyethylene oleyl ether and polyoxyethylene lauryl ether, etc.

[0045] Preferably, the molar concentration of iron in the aqueous solution of step (2) is 0.5-2 mol / L, for example: 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.5 mol / L or 2 mol / L, etc., not only limited to the listed values, other values not listed within the range of values are also applicable.

[0046] Preferably, the molar concentration of phosphate in the aqueous solution of step (2) is 0.5-2 mol / L, for example: 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.5 mol / L or 2 mol / L, etc., not only limited to the listed values, other values not listed within the range of values are also applicable.

[0047] Preferably, the mass concentration of surfactant in the aqueous solution of step (2) is 1%-3%, for example: 1%, 1.5%, 2%, 2.5% or 3%, etc., not only limited to the listed values, other values not listed within the range of values are also applicable.

[0048] Preferably, the pH of the aqueous solution of step (2) is 1-1.5, for example: 1, 1.1, 1.2, 1.4 or 1.5, etc., not only limited to the listed values, other values not listed within the range of values are also applicable.

[0049] Preferably, the oil phase solvent of step (2) comprises any one or a combination of at least two of edible oil, motor oil, synthetic mineral oil or liquid paraffin, typical but non-limiting combinations include a combination of edible oil and motor oil, a combination of motor oil and synthetic mineral oil or a combination of edible oil and liquid paraffin, etc.

[0050] Preferably, the mass-volume ratio of the hydrophobic ferric phosphate and the oil phase solvent of step (2) is 5-20 g: 100 mL, for example, 5 g: 100 mL, 8 g: 100 mL, 10 g: 100 mL, 15 g: 100 mL or 20 g: 100 mL, etc., not only limited to the listed values, other values not listed within the range of values are also applicable.

[0051] Preferably, the volume ratio of the aqueous phase solution and the oil phase solution of step (2) is (1-4): 1, for example, 1: 1, 1.5: 1, 2: 1, 3: 1 or 4: 1, etc., not only limited to the listed values, other values not listed within the range of values are also applicable.

[0052] Preferably, the pH adjusting agent of step (2) comprises ammonia water.

[0053] Preferably, the pH of step (2) is 1.5-2.2, for example, 1.5, 1.8, 2, 2.1 or 2.2, etc., not only limited to the listed values, other values not listed within the range of values are also applicable.

[0054] Preferably, the speed of the two-step stirring reaction of step (2) is 400-600 rpm, for example, 400 rpm, 450 rpm, 500 rpm, 550 rpm or 600 rpm, etc., not only limited to the listed values, other values not listed within the range of values are also applicable.

[0055] Preferably, the time of the two-step stirring reaction of step (2) is 8-12 h, for example, 8 h, 9 h, 10 h, 11 h or 12 h, etc., not only limited to the listed values, other values not listed within the range of values are also applicable.

[0056] Preferably, after the two-step stirring reaction of step (2), a filtering, washing and drying process is performed.

[0057] Preferably, the lithium source of step (3) comprises lithium hydroxide and / or lithium carbonate.

[0058] Preferably, the dispersant of step (3) comprises ethanol.

[0059] Preferably, the mixing of step (3) further adds a carbon source.

[0060] Preferably, the carbon source comprises any one or a combination of at least two of glucose, sucrose, stearic acid, cyclodextrin, citric acid, aniline, cellulose acetate or polyvinylpyrrolidone, and typical but non-limiting combinations include a combination of glucose and sucrose, a combination of citric acid and cyclodextrin, or a combination of cellulose acetate and polyvinylpyrrolidone, etc.

[0061] Preferably, the mass of the carbon source is 4% to 8%, for example 4%, 5%, 6%, 7% or 8%, etc., of the total mass of the composite iron phosphate lithium source, and the range of values is not limited to the listed values, and other values not listed in the range of values are also applicable.

[0062] Preferably, the grinding time of step (3) is 2 to 6 hours, for example 2 hours, 3 hours, 4 hours, 5 hours or 6 hours, etc., and the range of values is not limited to the listed values, and other values not listed in the range of values are also applicable.

[0063] Preferably, the drying temperature of step (3) is 60 to 80°C, for example 60°C, 65°C, 70°C, 75°C or 80°C, etc., and the range of values is not limited to the listed values, and other values not listed in the range of values are also applicable.

[0064] Preferably, the atmosphere of the calcination of step (3) comprises any one or a combination of at least two of nitrogen, helium or argon.

[0065] Preferably, the calcination temperature of step (3) is 600 to 850°C, for example 600°C, 650°C, 700°C, 800°C or 850°C, etc., and the range of values is not limited to the listed values, and other values not listed in the range of values are also applicable.

[0066] Preferably, the calcination time of step (3) is 6 to 15 hours, for example 6 hours, 8 hours, 10 hours, 12 hours or 15 hours, etc., and the range of values is not limited to the listed values, and other values not listed in the range of values are also applicable.

[0067] In a third aspect, the present application provides a lithium ion battery comprising the composite lithium iron phosphate positive electrode material according to the first aspect.

[0068] Compared with the prior art, the present application has the following beneficial effects:

[0069] (1) The composite lithium iron phosphate positive electrode material has a unique structure of hollow lithium iron phosphate coated on flaky lithium iron phosphate, the sphericity of the lithium iron phosphate positive electrode material is high, and the composite structure not only increases the specific surface area and tap density of the material, but also shortens the transmission path of lithium ions and improves the rate performance of the positive electrode material.

[0070] (2) The specific surface area of the composite lithium iron phosphate positive electrode material prepared by the method can reach 20.21 m 2 / g or above, and the tap density can reach 1.11 g / cm 3 / g or above, the 1C discharge specific capacity can reach 153 mAh / g or above, the 2C discharge specific capacity can reach 144 mAh / g or above, the 5C discharge specific capacity can reach 140 mAh / g or above, and the 10C discharge specific capacity can reach 129 mAh / g or above. BRIEF DESCRIPTION OF DRAWINGS

[0071] Figure 1 Figure 1 is an SEM image of the composite lithium iron phosphate positive electrode material prepared in Example 1. DETAILED DESCRIPTION

[0072] The technical solutions of the present application will be further described through specific embodiments. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations of the present application.

[0073] Example 1

[0074] The present embodiment provides a composite lithium iron phosphate positive electrode material, which is prepared by the following method:

[0075] (1) Tris-hydroxymethyl aminomethane is added to deionized water, the concentration of the Tris-hydroxymethyl aminomethane is 0.012 mol / L, and a Tris-HCl buffer solution is obtained after adjusting the pH to 8.3 with hydrochloric acid. Then, flaky nanometer iron phosphate with a length and width of 200-400 nm is added to the Tris-HCl buffer solution, the mass concentration of the flaky nanometer iron phosphate is 5 g / L, and the flaky nanometer iron phosphate is uniformly dispersed after being treated by ultrasonic for 8 min. Then, hydrochloric acid dopamine is added to make the concentration of the hydrochloric acid dopamine 0.65 g / L, and the hydrochloric acid dopamine is further treated by ultrasonic dispersion for 8 min. After stirring for 20 h, the mixture is treated by centrifugation at a speed of 10,000 rpm for 10 min to obtain dopamine-coated flaky iron phosphate. Then, the dopamine-coated flaky iron phosphate is added into ethyl acetate, the mass-volume ratio of the dopamine-coated flaky iron phosphate and the ethyl acetate is 25 g:100 mL, and then octadecyl mercaptan, vinyl trimethoxysilane and 2,2-dimethoxy-2-phenyl phenylacetone are added to the solution, the mass ratio of the vinyl trimethoxysilane and the dopamine in the dopamine-coated flaky iron phosphate is 1:4, the mass ratio of the octadecyl mercaptan and the vinyl trimethoxysilane is 1:2, and the mass ratio of the 2,2-dimethoxy-2-phenyl phenylacetone and the vinyl trimethoxysilane is 5:6. After stirring in a sealed environment and irradiation with ultraviolet light with a wavelength of 365 nm and a power of 250 W for 50 min, the mixture is treated by centrifugation, washing and drying to obtain hydrophobic iron phosphate;

[0076] (2) mixing ferric chloride salt, sodium dihydrogen phosphate and polyoxyethylene sorbitan monooleate with deionized water to obtain an aqueous phase solution, the concentrations of ferric chloride and sodium dihydrogen phosphate in the aqueous phase solution are both 1 mol / L, the mass fraction of polyoxyethylene sorbitan monooleate is 2 wt%, hydrochloric acid is used to adjust the pH to 1.2, then the above-mentioned hydrophobic ferric phosphate is placed in a liquid paraffin solution, the mass-volume ratio of hydrophobic ferric phosphate to liquid paraffin is 10 g:100 mL, after being uniformly mixed, the solution is used as an oil phase solution, two phases are mixed according to the oil-water volume ratio of 1:2.4, a water-in-oil emulsion is obtained by processing at a speed of 5500 rpm for 5 min, ammonia water is added to adjust the pH to 1.7, and stirring is carried out at a speed of 500 rpm, after reaction for 10 h, filtration, washing and drying are carried out to obtain a composite ferric phosphate, and a SEM image of the composite ferric phosphate is as shown in Figure 1 The composite ferric phosphate prepared in the application is spherical, in order to more intuitively understand the structure of the composite ferric phosphate, the composite ferric phosphate material is excessively broken, and a SEM image as shown in Figure 1 It can be seen from Figure 1 that the inside is nanosheet-shaped ferric phosphate, a ferric phosphate shell, and the ferric phosphate nanosheet is coated with dopamine, and a carbon layer is formed after calcination to prevent the nanosheet from agglomeration;

[0077] (3) mixing the above-mentioned composite ferric phosphate, lithium hydroxide, glucose and ethanol to obtain a mixed material, the molar ratio of lithium to iron in the lithium hydroxide and the ferric phosphate is 1:1, the addition amount of glucose is 6% of the total mass of the composite ferric phosphate and lithium hydroxide, the mixed material is placed in a grinder for grinding and stirring for 5 h, and then is placed in a drying box at 80 DEG C for drying; the dried mixed material is placed in a tube furnace, and is kept at 850 DEG C for 9 h under a nitrogen atmosphere, and finally the composite lithium ferric phosphate positive electrode material is obtained.

[0078] The median particle size D50 of the composite lithium ferric phosphate positive electrode material is 3.9 pm, the thickness of the hollow lithium ferric phosphate shell in the composite lithium ferric phosphate positive electrode material is 310 nm, the thickness of the nitrogen-doped coating layer on the surface of the sheet-shaped lithium ferric phosphate is 15 nm, and the thickness of the carbon coating layer on the surface of the composite lithium ferric phosphate positive electrode material is 15 nm.

[0079] Example 2

[0080] The embodiment provides a composite lithium ferric phosphate positive electrode material, and the composite lithium ferric phosphate positive electrode material is prepared by the following method.

[0081] (1) Tris-HCl buffer solution is prepared by adding trihydroxymethyl aminomethane into deionized water, the concentration of trihydroxymethyl aminomethane is 0.012 mol / L, and the pH is adjusted to 8.5 by hydrochloric acid, then the flaky nanometer iron phosphate with length and width of 200-400 nm is added into the Tris-HCl buffer solution, the mass concentration of the flaky nanometer iron phosphate is 3.5 g / L, and the flaky nanometer iron phosphate is uniformly dispersed by ultrasonic treatment for 8 min, then hydrochloric acid dopamine is added to make the concentration of 0.38 g / L, and the dopamine-coated flaky iron phosphate is obtained by continuing ultrasonic dispersion treatment for 8 min, stirring reaction for 15 h, and centrifugal treatment at a rotating speed of 8000 rpm for 15 min, then the dopamine-coated flaky iron phosphate is put into ethyl acetate, the mass-volume ratio of the dopamine-coated flaky iron phosphate and the ethyl acetate is 20 g:100 mL, then octadecyl mercaptan, vinyl trimethoxysilane and 2,2-dimethoxy-2-phenyl phenylacetone are added into the solution, the mass ratio of the vinyl trimethoxysilane to dopamine in the dopamine-coated flaky iron phosphate is 1:3.5, the mass ratio of the octadecyl mercaptan to the vinyl trimethoxysilane is 1:1.8, and the mass ratio of the 2,2-dimethoxy-2-phenyl phenylacetone to the vinyl trimethoxysilane is 5:5.7, and the mixture is stirred in a sealed environment and irradiated by ultraviolet light with 230 W and λ=350 nm for 60 min, then the hydrophobic iron phosphate is obtained by centrifugation, washing and drying;

[0082] (2) The water phase solution is prepared by mixing ferric chloride, sodium dihydrogen phosphate and polyoxyethylene sorbitan monooleate with deionized water, the concentration of ferric chloride and sodium dihydrogen phosphate in the water phase solution is 0.5 mol / L, and the mass fraction of polyoxyethylene sorbitan monooleate is 1 wt%, then the pH is adjusted to 1.5 by hydrochloric acid, then the above-mentioned hydrophobic iron phosphate is put into liquid paraffin solution, the mass-volume ratio of the hydrophobic iron phosphate and the liquid paraffin is 5 g:100 mL, the mixture is uniformly mixed as an oil phase solution, then the oil-water volume ratio is 3:7, the two phases are mixed, and the water-in-oil emulsion is obtained by treatment at a rotating speed of 5500 rpm for 5 min, then the pH is adjusted to 1.8 by adding ammonia water, the mixture is stirred at a rotating speed of 400 rpm, and the composite iron phosphate is obtained by filtration, washing and drying after reaction for 10 h;

[0083] (3) The composite iron phosphate is mixed with lithium hydroxide, glucose and ethanol to obtain a mixed material, the molar ratio of lithium in the lithium hydroxide to iron in the composite iron phosphate is 1:1, and the addition amount of glucose is 4% of the total mass of the composite iron phosphate and lithium hydroxide, then the mixed material is put into a grinder for grinding and stirring for 2 h, and then the mixed material is dried in a 60 ℃ drying box, then the dried mixed material is put into a tube furnace, the composite lithium iron phosphate positive electrode material is finally obtained by heat preservation at 750 ℃ for 8 h in a nitrogen atmosphere.

[0084] The median particle size D50 of the composite lithium iron phosphate positive electrode material is 4.6 μm, the thickness of the hollow lithium iron phosphate shell in the composite lithium iron phosphate positive electrode material is 280 nm, the thickness of the nitrogen-doped coating layer on the surface of the sheet-shaped lithium iron phosphate is 11 nm, and the thickness of the carbon coating layer on the surface of the composite lithium iron phosphate positive electrode material is 12 nm.

[0085] Example 3

[0086] The present embodiment provides a composite lithium iron phosphate positive electrode material, which is prepared by the following method:

[0087] (1) Tris-hydroxymethyl aminomethane is added to deionized water, the concentration of the Tris-hydroxymethyl aminomethane is 0.012 mol / L, and a Tris-HCl buffer solution is obtained after adjusting the pH to 8.8 with hydrochloric acid. Sheet-shaped nanometer lithium iron phosphate with a length and width of 200-400 nm is added to the Tris-HCl buffer solution, and the mass concentration of the sheet-shaped nanometer lithium iron phosphate is 8 g / L. After being uniformly dispersed by ultrasonic treatment for 8 min, hydrochloric acid dopamine is added to make the concentration of the hydrochloric acid dopamine 0.95 g / L. After being uniformly dispersed by ultrasonic treatment for another 8 min, the reaction is stirred for 24 h, and then centrifugal treatment is performed at a speed of 12000 rpm for 10 min to obtain dopamine-coated sheet-shaped lithium iron phosphate. The dopamine-coated sheet-shaped lithium iron phosphate is added into ethyl acetate, and the mass-volume ratio of the dopamine-coated sheet-shaped lithium iron phosphate and the ethyl acetate is 35 g:100 mL. After being uniformly stirred, octadecyl mercaptan, vinyltrimethoxysilane, and 2,2-dimethoxy-2-phenylphenylethanone are added to the solution. The mass ratio of the vinyltrimethoxysilane to the dopamine in the dopamine-coated sheet-shaped lithium iron phosphate is 1:4.5, the mass ratio of the octadecyl mercaptan to the vinyltrimethoxysilane is 1:2.2, and the mass ratio of the 2,2-dimethoxy-2-phenylphenylethanone to the vinyltrimethoxysilane is 5:6.2. After being stirred in a sealed environment and irradiated with ultraviolet light with a wavelength of 380 nm and a power of 270 W for 45 min, the hydrophobic lithium iron phosphate is obtained by centrifugation, washing, and drying.

[0088] (2) mixing ferric chloride salt, sodium dihydrogen phosphate and polyoxyethylene sorbitan monooleate with deionized water to obtain an aqueous solution, the concentration of ferric chloride and sodium dihydrogen phosphate in the aqueous solution is 2 mol / L, the mass fraction of polyoxyethylene sorbitan monooleate is 3 wt%, and hydrochloric acid is used to adjust the pH to 1, then the above hydrophobic ferric phosphate is placed in a liquid paraffin solution, the mass-volume ratio of hydrophobic ferric phosphate to liquid paraffin is 20 g:100 mL, after uniform mixing, the solution is used as an oil phase solution, two phases are mixed according to the volume ratio of oil to water is 3:7, a water-in-oil emulsion is obtained by processing at a speed of 5500 rpm for 5 min, ammonia water is added to adjust the pH to 2.2, and stirring is carried out at a speed of 600 rpm, after reaction for 8 h, filtration, washing and drying are carried out to obtain a composite ferric phosphate;

[0089] (3) mixing the above composite ferric phosphate with lithium hydroxide, glucose and ethanol to obtain a mixed material, the molar ratio of lithium to iron in lithium hydroxide and ferric phosphate is 1:1, and the addition amount of glucose is 8% of the total mass of the composite ferric phosphate and lithium hydroxide, the mixed material is placed in a grinder for grinding and stirring for 6 h, and then is placed in a 70°C drying box for drying; the dried mixed material is placed in a tube furnace, and is kept at 600°C for 15 h under a nitrogen atmosphere, and finally the composite lithium ferric phosphate positive electrode material is obtained.

[0090] The median particle size D50 of the composite lithium ferric phosphate positive electrode material is 4.4 μm, the thickness of the hollow lithium ferric phosphate shell in the composite lithium ferric phosphate positive electrode material is 440 nm, the thickness of the nitrogen-doped coating layer on the surface of the sheet-shaped lithium ferric phosphate is 20 nm, and the thickness of the carbon coating layer on the surface of the composite lithium ferric phosphate positive electrode material is 24 nm.

[0091] Example 4

[0092] The difference between this example and Example 1 is that the mass concentration of dopamine hydrochloride in the mixed solution in step (1) is 0.35 g / L (correspondingly, the amount of vinyltrimethoxysilane, octadecanethiol and 2,2-dimethoxy-2-phenylacetophenone is reduced), and other conditions and parameters are completely the same as in Example 1.

[0093] Example 5

[0094] The difference between this example and Example 1 is that the mass concentration of dopamine hydrochloride in the mixed solution in step (1) is 1 g / L (correspondingly, the amount of vinyltrimethoxysilane, octadecanethiol and 2,2-dimethoxy-2-phenylacetophenone is increased), and other conditions and parameters are completely the same as in Example 1.

[0095] Example 6

[0096] The embodiment differs from example 1 only in that the time of the ultraviolet irradiation treatment in step (1) is 30 min, and other conditions and parameters are completely the same as in the example.

[0097] Example 7

[0098] The embodiment differs from example 1 only in that the time of the ultraviolet irradiation treatment in step (1) is 70 min, and other conditions and parameters are completely the same as in the example.

[0099] Example 8

[0100] The embodiment differs from example 1 only in that the volume ratio of the aqueous solution and the oil phase solution in step (2) is 3:2, and other conditions and parameters are completely the same as in the example.

[0101] Example 9

[0102] The embodiment differs from example 1 only in that the volume ratio of the aqueous solution and the oil phase solution in step (2) is 4:1, and other conditions and parameters are completely the same as in the example.

[0103] Comparative Example 1

[0104] The comparative example differs from example 1 only in that step (1) is not performed, i.e. the nano-ferric phosphate is not subjected to hydrophobic treatment, and other conditions and parameters are completely the same as in example 1.

[0105] Comparative Example 2

[0106] The comparative example only uses hollow ferric phosphate to directly prepare a lithium ferric phosphate positive electrode material.

[0107] Comparative Example 3

[0108] The comparative example only uses flaky ferric phosphate to directly prepare a lithium ferric phosphate positive electrode material.

[0109] Performance test:

[0110] Tap density test:

[0111] The mass of the dry measuring cylinder m1 is weighed, a certain amount of solid sample is added into the measuring cylinder (about 5 mL), the mouth of the measuring cylinder is blocked, and the measuring cylinder is vertically vibrated until the volume of the sample in the cylinder no longer decreases. The volume V (mL) of the sample is recorded. The mass m2 (g) of the measuring cylinder + sample is weighed; the tap density p (g / cm3) of the sample to be tested is obtained by the formula p = (m2-m1) / V. 3 ) is obtained.

[0112] Specific surface area test:

[0113] The specific surface area of the lithium ferric phosphate powder material in the example is determined by using a specific surface area tester, and the results are shown in the following table.

[0114] Battery assembly:

[0115] A homogeneous slurry was prepared by mixing the positive electrode material, acetylene black and PVDF in a mass ratio of 75:15:10, and was uniformly coated on an aluminum foil substrate as the positive electrode of the simulation battery. Lithium sheet was used as the negative electrode of the simulation battery, a polypropylene porous membrane was used as the separator, and an electrolyte was prepared by dissolving 1 mol of LiPF6 in 1 L of a mixed solvent of EC and DMC (volume ratio 1:1). The positive electrode, negative electrode, electrolyte and separator were assembled into a battery in an argon-protected glove box.

[0116] Rate and cycle test procedure of the simulation battery:

[0117] First, constant current charging to 4.2V, then discharging to 2.0V at a rate current, the discharged capacity is the discharge capacity at this rate, and then discharging to 2.0V at a constant current after discharging. Then the next rate test is performed. The test results are shown in Table 1:

[0118] Table 1

[0119]

[0120]

[0121] As can be seen from Table 1, according to Examples 1-3, the specific surface area of the composite lithium iron phosphate positive electrode material prepared by the method of the present application can reach 20.21 m 2 / g or more, the tap density can reach 1.11 g / cm 3 The above, the specific capacity of the battery at 0.2C discharge can reach 160 mAh / g or more, the specific capacity at 1C discharge can reach 153 mAh / g or more, the specific capacity at 2C discharge can reach 144 mAh / g or more, the specific capacity at 5C discharge can reach 140 mAh / g or more, and the specific capacity at 10C discharge can reach 129 mAh / g or more.

[0122] As can be seen from the comparison of Example 1 and Examples 4-5, the amount of dopamine hydrochloride added during the preparation of the composite lithium iron phosphate positive electrode material of the present application will affect its performance. If the mass concentration of dopamine hydrochloride in the mixed solution of step (1) is controlled to be 0.38-0.95 g / L, the performance of the composite lithium iron phosphate positive electrode material prepared is better. If the mass concentration of dopamine hydrochloride in the mixed solution is too large, the dopamine layer will increase, increasing the transmission distance of lithium ions, increasing the cost, and increasing the carbon content, ultimately reducing the theoretical capacity and the rate performance. If the mass concentration of dopamine hydrochloride in the mixed solution is too small, the hydrophobic modification of the iron phosphate will not be complete, resulting in the overflow of part of the nanosheet-shaped iron phosphate, destroying the stability of the emulsion, affecting the inside of the shell, and reducing the specific surface area and the rate performance.

[0123] From the comparison of Example 1 and Examples 6-7, it can be seen that the intensity of the ultraviolet irradiation treatment in the preparation process of the composite lithium iron phosphate positive electrode material of the present application affects its performance. If the intensity of the ultraviolet irradiation treatment is controlled at 230-270 W, the performance of the composite lithium iron phosphate positive electrode material prepared is better. If the time of the ultraviolet irradiation treatment is too long, the performance of the material does not become better. If the time of the ultraviolet irradiation treatment is too short, the hydrophobic modification of the nano-phosphorous acid iron is not sufficient, which affects the emulsion microspheres and finally affects the electrochemical performance of the material.

[0124] From the comparison of Example 1 and Examples 8-9, it can be seen that the volume ratio of the aqueous solution and the oil phase solution in step (2) in the preparation process of the composite lithium iron phosphate positive electrode material of the present application affects its performance. If the volume ratio of the aqueous solution and the oil phase solution is controlled at 1-4:1, the performance of the composite lithium iron phosphate positive electrode material prepared is better. If the aqueous solution accounts for too large a proportion, the amount of the oil phase becomes less, which reduces the yield of the emulsion microspheres, increases the shell thickness, and finally reduces the rate performance and the specific surface area. If the aqueous solution accounts for too small a proportion, the stability of the emulsion spheres is reduced, which affects the sphericity, and finally affects the tap density and the rate performance of the material.

[0125] From the comparison of Example 1 and Comparative Example 1, it can be seen that if the nano-phosphorous acid iron is not subjected to hydrophobic treatment, only hollow phosphorous acid iron can be formed, and part of the phosphorous acid iron is accumulated, which reduces the specific surface area and the tap density. From the comparison of Example 1 and Comparative Examples 2-3, it can be seen that if only hollow phosphorous acid iron or flaky phosphorous acid iron is used, the tap density is low. The composite lithium iron phosphate positive electrode material of the present application has a spherical hollow structure and contains flaky lithium iron phosphate in the hollow sphere, which is beneficial to improve the tap density of the material. The hollow sphere shell structure and the flaky lithium iron phosphate in the hollow sphere shell are both beneficial to improve the specific surface area of the material, shorten the transmission path of lithium ions, and improve the rate performance of the positive electrode material. The accumulation of the flaky nano-material is beneficial to alleviate the loss of space of the spherical shell-shaped lithium iron phosphate material. In addition, the surface of the flaky nano-lithium iron phosphate is coated with a dopamine layer, which is carbonized at high temperature to form nitrogen-doped carbon, which solves the problem that the internal material is difficult to be uniformly covered with carbon material, improves the electronic conductivity, and improves the rate performance.

[0126] The applicant declares that the above description is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto. It should be understood by those skilled in the art that any changes or replacements within the technical scope disclosed by the present application can be easily thought of by any person skilled in the art, and all fall within the protection scope and disclosure scope of the present application.

Claims

1. A composite lithium iron phosphate cathode material, characterized in that, The composite lithium iron phosphate positive electrode material comprises hollow spherical lithium iron phosphate and flaky lithium iron phosphate, wherein the hollow spherical lithium iron phosphate comprises an internal cavity and a spherical lithium iron phosphate shell, the flaky lithium iron phosphate is arranged in the internal cavity of the hollow spherical lithium iron phosphate, and the surface of the flaky lithium iron phosphate is provided with a nitrogen-doped carbon coating layer.

2. The composite lithium iron phosphate cathode material of claim 1, wherein, The composite lithium iron phosphate positive electrode material has a median particle size D50 of 3.6-4.8 μm.

3. The composite lithium iron phosphate cathode material of claim 1, wherein, The thickness of the spherical lithium iron phosphate shell is 250-400 nm.

4. The composite lithium iron phosphate cathode material of claim 1, wherein, The thickness of the nitrogen-doped carbon coating layer is 10-30 nm.

5. The composite lithium iron phosphate cathode material of claim 1, wherein, The composite lithium iron phosphate positive electrode material is provided with a carbon coating layer on the surface.

6. The composite lithium iron phosphate cathode material of claim 5, wherein, The thickness of the carbon coating layer is 12-25 nm.

7. A method for preparing the composite lithium iron phosphate cathode material according to any one of claims 1-6, characterized in that, The preparation method comprises the following steps: (1) mixing flaky lithium iron phosphate, a buffer solution and dopamine hydrochloride to obtain a mixed solution, and performing one-step stirring reaction to obtain dopamine-coated flaky lithium iron phosphate, mixing the dopamine-coated flaky lithium iron phosphate, alkyl mercaptan, vinyltrimethoxysilane, a photoinitiator and a solvent, and performing ultraviolet irradiation treatment to obtain hydrophobic lithium iron phosphate; (2) mixing an iron source, a phosphorus source, a surfactant and water to obtain an aqueous phase solution, mixing the hydrophobic lithium iron phosphate and an oil phase solvent to obtain an oil phase solution, mixing the aqueous phase solution and the oil phase solution, adjusting pH, and performing two-step stirring reaction to obtain composite lithium iron phosphate; (3) mixing the composite lithium iron phosphate, a lithium source and a dispersant, performing grinding and drying, and then performing calcination to obtain the composite lithium iron phosphate positive electrode material.

8. The production method according to claim 7, wherein In step (1), the length and width of the flaky lithium iron phosphate are independently 200-400 nm.

9. The production method according to claim 7, wherein In step (1), the buffer solution comprises a Tris-HCl buffer solution.

10. The production method according to claim 7, wherein In step (1), the pH of the buffer solution is 8.2-8.

8.

11. The production method according to claim 7, wherein In step (1), the mass concentration of the flaky lithium iron phosphate in the mixed solution is 3.5-8 g / L.

12. The production method according to claim 7, wherein In step (1), the mass concentration of dopamine hydrochloride in the mixed solution is 0.38-0.95 g / L.

13. The production method according to claim 7, wherein In step (1), the one-step stirring reaction is performed for 15-24 h.

14. The production method according to claim 7, wherein In step (1), centrifugation and drying treatment are performed after the one-step stirring reaction.

15. The production method according to claim 14, wherein The centrifugation is performed at a speed of 8000-12000 rpm.

16. The production method according to claim 14, wherein The centrifugation is performed for 8-15 min.

17. The production method according to claim 7, wherein In step (1), the solvent comprises ethyl acetate.

18. The production method according to claim 7, wherein In step (1), the mass-volume ratio of the dopamine-coated flaky lithium iron phosphate to the solvent is 20-35 g:100 mL.

19. The production method according to claim 7, wherein In step (1), the mass ratio of dopamine to vinyltrimethoxysilane in the dopamine-coated flaky lithium iron phosphate is (3.5-4.5):

1.

20. The production method according to claim 7, wherein In step (1), the alkyl mercaptan comprises any one or a combination of at least two of dodecyl mercaptan, tetradecyl mercaptan, hexadecyl mercaptan or octadecyl mercaptan.

21. The production method according to claim 7, wherein In step (1), the mass ratio of the alkyl mercaptan to vinyltrimethoxysilane is 1:(1.8-2.2).

22. The production method according to claim 7, wherein In step (1), the photoinitiator comprises 2,2-dimethoxy-2-phenylacetophenone.

23. The production method according to claim 7, wherein In step (1), the mass ratio of the photoinitiator to vinyltrimethoxysilane is 5:(5.7-6.2).

24. The production method according to claim 7, wherein In step (1), the intensity of the ultraviolet irradiation treatment is 230-270 W.

25. The production method according to claim 7, wherein The wavelength of the ultraviolet irradiation treatment in step (1) is 350-380 nm.

26. The production method according to claim 7, wherein The time of the ultraviolet irradiation treatment in step (1) is 45-60 min.

27. The production method according to claim 7, wherein The ultraviolet irradiation treatment in step (1) is followed by centrifugation, washing, and drying.

28. The production method according to claim 7, wherein The iron source in step (2) includes any one or a combination of at least two of ferric sulfate, polyferric sulfate, ferric chloride, or ferric nitrate.

29. The production method according to claim 7, wherein The phosphoric acid source in step (2) includes any one or a combination of at least two of H3PO4, (NH4)2HPO4, NH4H2PO4, (NH4)3PO4, NaH2PO4, or Na2HPO4.

30. The production method according to claim 7, wherein The surfactant in step (2) includes any one or a combination of at least two of dodecylbenzenesulfonic acid, polyoxyethylene sorbitan monooleate, fatty alcohol polyoxyethylene ether, polyoxyethylene castor oil, polyoxyethylene oleyl ether, or polyoxyethylene lauryl ether.

31. The production method according to claim 7, wherein The molar concentration of iron in the aqueous solution in step (2) is 0.5-2 mol / L.

32. The production method according to claim 7, wherein The molar concentration of phosphate in the aqueous solution in step (2) is 0.5-2 mol / L.

33. The production method according to claim 7, wherein The mass concentration of the surfactant in the aqueous solution in step (2) is 1%-3%.

34. The production method according to claim 7, wherein The pH of the aqueous solution in step (2) is 1-1.

5.

35. The preparation method according to claim 7, characterized in that, The oil phase solvent in step (2) includes any one or a combination of at least two of edible oil, machine oil, synthetic mineral oil, or liquid paraffin.

36. The production method according to claim 7, wherein The mass-volume ratio of the hydrophobic ferric phosphate to the oil phase solvent in step (2) is 5-20 g:100 mL.

37. The production method according to claim 7, wherein The volume ratio of the aqueous solution to the oil phase solution in step (2) is (1-4):

1.

38. The production method according to claim 7, wherein The pH adjusting agent in step (2) includes ammonia water.

39. The production method according to claim 7, wherein The pH in step (2) is 1.5-2.

2.

40. The method of claim 7, wherein the compound is ###00021### 40A. The speed of the two-step stirring reaction in step (2) is 400-600 rpm.

41. The production method according to claim 7, wherein The time of the two-step stirring reaction in step (2) is 8-12 h.

42. The method of claim 7, wherein the compound is ###0010### 7 The two-step stirring reaction in step (2) is followed by filtration, washing, and drying.

43. The production method according to claim 7, wherein The lithium source in step (3) includes lithium hydroxide and / or lithium carbonate.

44. The production method according to claim 7, wherein The dispersant in step (3) includes ethanol.

45. The preparation method according to claim 7, characterized in that, The mixing in step (3) further includes a carbon source.

46. The method of claim 45, wherein the step of preparing is performed by a method comprising: The carbon source includes any one or a combination of at least two of glucose, sucrose, stearic acid, cyclodextrin, citric acid, aniline, cellulose acetate, or polyvinylpyrrolidone.

47. The preparation method according to claim 45, characterized in that, The mass of the carbon source is 4%-8% of the total mass of the composite ferric lithium phosphate and the lithium source.

48. The production method according to claim 7, wherein The time of the grinding in step (3) is 2-6 h.

49. The production method according to claim 7, wherein The temperature of the drying in step (3) is 60-80℃.

50. The method of claim 7, wherein the compound is ###0010### The atmosphere of the calcination in step (3) includes any one or a combination of at least two of nitrogen, helium, or argon.

51. The production method according to claim 7, wherein The temperature of the calcination in step (3) is 600-850℃.

52. The method of claim 7, wherein the compound is ###0010### The time of the calcination in step (3) is 6-15 h.

53. A lithium-ion battery, characterized by, The lithium ion battery includes the composite ferric lithium phosphate cathode material according to any one of claims 1-6.

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