Homogeneous lithium iron phosphate material prepared by interface strengthening and application of homogeneous lithium iron phosphate material

Abstract

The invention discloses a homogeneous lithium iron phosphate material prepared by interface strengthening and application thereof, and belongs to the technical field of lithium iron phosphate positive electrode materials. The lithium iron phosphate material comprises a lithium iron phosphate matrix and a carbon coating layer, the carbon content Cwt is 1-2.5 wt%, and the half-peak width w of a carbon content curve of the lithium iron phosphate material measured by a high-frequency combustion infrared sulfur-carbon detector is 3.5-6.5 s; the half-peak width of unit carbon content in the lithium iron phosphate material in a carbon content curve is # imgabs0 #, W is 140-600s; after the lithium iron phosphate material is combusted by a high-frequency combustion infrared sulfur and carbon detector, the oxidized amount m of the lithium iron phosphate accounts for 30-75wt% of the total mass of the material. According to the lithium iron phosphate material, the carbon coating layer is large in specific surface area and uniform in structure, the electron transmission capacity between particles of the lithium iron phosphate material is high, and the conductivity is good. And the battery further prepared from the composite material has relatively good conductivity, rate capability and the like.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium iron phosphate positive electrode materials, and in particular to a homogeneous lithium iron phosphate material prepared by interface strengthening and applications thereof. Background Art

[0002] Lithium iron phosphate (LiFePO4), a cathode material for lithium-ion batteries, has become a core material in the power battery and energy storage fields due to its advantages such as high safety, long cycle life, environmental friendliness, and low cost. However, its low intrinsic electronic conductivity and slow lithium ion diffusion rate limit its high-rate performance and energy density. To overcome this problem, carbon coating technology has been widely adopted. By constructing a conductive network on the surface of lithium iron phosphate particles, it can significantly improve the electronic conductivity of lithium iron phosphate materials, inhibit lithium iron phosphate particle agglomeration, and enhance their structural stability.

[0003] However, current carbon coating technology makes it difficult to obtain a carbon coating layer with a high specific surface area while ensuring that the carbon-coated lithium iron phosphate positive electrode material has better electrochemical properties.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The object of the present invention is to provide a homogeneous lithium iron phosphate material prepared by interface strengthening and its application, so as to solve or improve the above technical problems.

[0006] The present invention can be implemented like this: In a first aspect, the present invention provides a homogeneous lithium iron phosphate material, comprising a lithium iron phosphate matrix and a carbon coating layer coated on the lithium iron phosphate matrix; Carbon content C in homogeneous lithium iron phosphate material wt The half-peak width w of the carbon content curve of the homogeneous lithium iron phosphate material measured by the high-frequency combustion infrared sulfur-carbon detector is 3.5s~6.5s; the half-peak width of the carbon content curve of the unit carbon content in the homogeneous lithium iron phosphate material is 1.0wt%~2.5wt%. , the value range of W is 140s~600s; After the homogeneous lithium iron phosphate material is burned by the high-frequency combustion infrared sulfur-carbon detector, the amount of lithium iron phosphate oxidized m accounts for 30wt%~75wt% of the total mass of the carbon-coated lithium iron phosphate positive electrode material.

[0007] In an optional embodiment, the relative standard deviation of the carbon coating structure of the homogeneous lithium iron phosphate material is ×100%, the value range of R is 0.1~3.0; among them, ; n is the number of local positions taken when the homogeneous lithium iron phosphate material to be tested is subjected to Raman testing; X i= ; I G and I D represent the peak intensity of ordered carbon G bond and disordered carbon D bond in Raman spectrum at the i-th local position respectively; Measured for all local positions X i The average value of .

[0008] In an optional embodiment, the homogeneous lithium iron phosphate material further includes at least one of the following features: Feature 1: C wt 1.0wt%~1.5wt%; Feature 2: w is 4.8s~6.0s; Feature 3: R is 0.5~2.8; Feature 4: The general formula of lithium iron phosphate matrix is Li 1-x A x Fe 1-y E y PO4; wherein A includes at least one of Na and Mg; E includes at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Zn and Y; 0≤x≤0.1, 0≤y≤0.1.

[0009] In a second aspect, the present invention provides an interface strengthening preparation method for a homogeneous lithium iron phosphate material according to any one of the aforementioned embodiments, comprising the following steps: preparing a carbon coating layer on the surface of a lithium iron phosphate substrate.

[0010] In an optional embodiment, the general formula of the lithium iron phosphate matrix is Li 1-x A x Fe 1-y E y PO4; wherein A includes at least one of Na and Mg; E includes at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Zn and Y; 0≤x≤0.1, 0≤y≤0.1; The preparation of the lithium iron phosphate matrix comprises: according to the general formula of the lithium iron phosphate matrix, subjecting a lithium source, an A source, a phosphorus source, an iron source and an E source to a hydrothermal reaction.

[0011] In an optional embodiment, the preparation of the lithium iron phosphate matrix includes at least one of the following features: Feature 5: The lithium source includes lithium hydroxide; Feature 6: Source A is a water-soluble salt of element A; Feature 7: The phosphorus source includes at least one of phosphoric acid and phosphate; Feature 8: The iron source is a water-soluble ferrous salt; optionally including at least one of ferrous sulfate, ferrous chloride and ferrous acetate; Feature 9: The E source is a water-soluble salt of element E; Feature 10: In terms of molar ratio, (Li in the lithium source + A in the A source): (Fe in the iron source + E in the E source): P in the phosphorus source = (1.02-1.05): (0.95-0.98): 1.0; Feature 11: The molar ratio of Li in the lithium source to A in the A source is (0.9-1):(0-0.1); Feature 12: The molar ratio of Fe in the iron source to E in the E source is (0.9-1):(0-0.1); Feature 13: The pH value of the hydrothermal reaction is 6.0-8.0; Feature 14: The temperature of the hydrothermal reaction is 140-180°C; Feature 15: The hydrothermal reaction time is 2h~8h.

[0012] In an optional embodiment, the preparation of the carbon coating layer includes: mixing a lithium iron phosphate matrix with a composite carbon source solution to obtain a composite; and drying and carbonizing the composite.

[0013] In an optional embodiment, the preparation of the carbon coating layer includes at least one of the following features: Feature 16: Adding the lithium iron phosphate matrix to the composite carbon source solution, homogenizing and ultrasonicating to obtain a composite; optionally, the solid-liquid ratio of the lithium iron phosphate matrix to the composite carbon source solution is 1g:0.8mL to 1g:1.5mL; optionally, the ultrasonication time is 2h~4h.

[0014] Feature 17: Drying is performed using a fluidized bed drying method; optionally, the inlet air temperature is 180°C to 220°C.

[0015] Feature 18: Carbonization includes: in a protective atmosphere, first performing low-temperature carbonization at 400°C~500°C for 2h~3h, and then performing high-temperature carbonization at 700°C~900°C for 0.5h~1h; optionally, heating to 400°C~500°C at a heating rate of 1.5°C / min~2.5°C / min; optionally, heating to 700°C~900°C at a heating rate of 9.5°C / min~10.5°C / min; optionally, 4%~6% hydrogen is introduced during the high-temperature carbonization process, measured in volume percentage.

[0016] In an optional embodiment, the preparation of the composite carbon source solution comprises: mixing a hydrophobic carbon source, an amphiphilic block copolymer and a non-polar organic solvent.

[0017] Optionally, the preparation of the composite carbon source solution includes at least one of the following features: Feature 19: The hydrophobic carbon source includes at least one of polyvinyl chloride and phenolic resin; Feature 20: The amphiphilic block copolymer comprises at least one of polycaprolactone-polyethylene glycol, polystyrene-polyethylene oxide, and polyethylene glycol-polypropylene glycol-polyethylene glycol; Feature 21: The non-polar organic solvent includes tetrahydrofuran; Feature 22: The total mass of the hydrophobic carbon source and the amphiphilic block copolymer is 8% to 15% of the mass of the lithium iron phosphate matrix; wherein the mass of the amphiphilic block copolymer is 6% to 10% of the mass of the hydrophobic carbon source; Feature 23: The mixing temperature of the hydrophobic carbon source, the amphiphilic block copolymer, and the non-polar organic solvent is 60°C to 80°C; Feature 24: The mixing time of the hydrophobic carbon source, the amphiphilic block copolymer and the non-polar organic solvent is 1 hour to 2 hours.

[0018] In a third aspect, the present invention provides a battery comprising the homogeneous lithium iron phosphate material according to any one of the aforementioned embodiments.

[0019] The beneficial effects of the present invention include: The present invention provides the above-mentioned C wt A homogeneous lithium iron phosphate material having a mass fraction of 1 wt% to 2.5 wt%, a mass fraction of 3.5s to 6.5s, a mass fraction of 140s to 600s, and an mass fraction of 30 wt% to 75 wt% comprises a carbon coating layer having a large specific surface area. The carbon coating layer is uniformly coated on the surface of the lithium iron phosphate cathode material, resulting in the carbon-coated lithium iron phosphate cathode material having good electrical conductivity. A battery prepared from the homogeneous lithium iron phosphate material can have both good electrical conductivity and rate capability. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without paying any creative work.

[0021] Figure 1 This is a graph showing the carbon content of the carbon-coated lithium iron phosphate cathode material prepared in Example 1 of the present invention; Figure 2 This is a Raman spectrum of the carbon-coated lithium iron phosphate positive electrode material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0022] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer are used. Where the manufacturer of the reagents or instruments is not specified, all are conventional products that can be purchased commercially.

[0023] The homogeneous lithium iron phosphate material prepared by interface strengthening provided by the present invention and its application are described in detail below.

[0024] The relatively low electrical conductivity of lithium iron phosphate cathode materials results in a slow transmission rate of electrons and lithium ions during the charge and discharge process, thus affecting the charge and discharge rate of the battery. This defect is usually improved by adding a carbon source during the preparation of the lithium iron phosphate cathode material to form a carbon coating layer on the surface of the lithium iron phosphate cathode material. Carbon coating can form a conductive network to promote the transmission of electrons and lithium ions. However, conventional carbon-coated lithium iron phosphate materials are prone to carbon accumulation, and the coating effect is not ideal. Therefore, it is necessary to improve the quality of the carbon coating to optimize the coating effect and thus optimize the conductive properties of the material.

[0025] The present invention provides a homogeneous lithium iron phosphate material, which comprises a lithium iron phosphate matrix and a carbon coating layer coated on the lithium iron phosphate matrix.

[0026] The carbon content C in the homogeneous lithium iron phosphate material wt 1.0 wt% to 2.5 wt%, such as 1.0 wt%, 1.002 wt%, 1.355 wt%, 1.369 wt%, 1.387 wt%, 1.393 wt%, 1.425 wt%, 1.454 wt%, 1.5 wt%, 2.0 wt% or 2.5 wt%, and may be other values within the range of 1.0 wt% to 2.5 wt%. In some optional embodiments, C wt The carbon content C wt It can be directly measured by high-frequency combustion infrared sulfur and carbon detector. Among them, the carbon content C wt The conductivity of the carbon-coated lithium iron phosphate positive electrode material with a concentration of 1.0wt% to 1.5wt% is significantly improved. Beyond the above range, the improvement in conductivity has certain limitations.

[0027] The half-peak width w of the carbon content curve of the homogeneous lithium iron phosphate material measured by a high-frequency combustion infrared sulfur-carbon detector is 3.5s~6.5s, such as 3.5s, 4.0s, 4.5s, 4.830s, 4.916s, 4.936s, 5.0s, 5.038s, 5.129s, 5.206s, 5.5s, 5.983s, 6.0s or 6.5s, etc., and can also be other values within the range of 3.5s~6.5s. In some optional embodiments, w is 4.8s~6.0s. The homogeneous lithium iron phosphate material with w of 4.8s~6.0s has better carbon coating quality, and the carbon-coated lithium iron phosphate positive electrode material has better conductivity.

[0028] The half-peak width of the carbon content curve per unit carbon content in homogeneous lithium iron phosphate material The value range of W is 140s~600s, such as 140s, 160s, 200s, 250s, 300s, 350s, 332.2s, 359.1s, 364.3s, 369.8s, 373.7s, 400s, 419.8s, 450s, 500s, 502.7s, 550s or 600s, and can also be other values in the range of 140s~600s.

[0029] In the carbon content curve measured by the high-frequency combustion infrared sulfur and carbon detector, under normal circumstances, the temperature is constant and the carbon content curve presents a parabola with only one peak opening downward. The peak width of the carbon content curve reflects the time it takes for the carbon to be completely burned. In this scheme, the half-peak width w is used to describe the time it takes for the carbon to be burned. The half-peak width w is used to express the carbon content C. wt The ratio of w to carbon content is used to characterize the half-peak width W of the unit carbon content in the carbon content curve. The smaller w, the smaller the corresponding W, that is, the shorter the time it takes for the unit carbon content to burn, indicating that the carbon coating has a large contact area with oxygen during the combustion process, that is, the specific surface area of the carbon coating is large. The specific surface area of the carbon coating can reflect the coverage of the carbon coating to a certain extent. The larger the specific surface area of the carbon coating, the more uniform its coating is, and the denser and continuous conductive network it can form, which is beneficial to enhancing the electron transfer ability between lithium iron phosphate particles and making the overall conductive performance of the lithium iron phosphate positive electrode material more uniform.

[0030] After the homogeneous lithium iron phosphate material is burned by a high-frequency combustion infrared sulfur and carbon detector, the amount of lithium iron phosphate oxidized, m, accounts for 30wt% to 75wt% of the total mass of the carbon-coated lithium iron phosphate positive electrode material, such as 30wt%, 35wt%, 38.79wt%, 40wt%, 45wt%, 45.02wt%, 48.94wt%, 49.87wt%, 50wt%, 53.12wt%, 55wt%, 60wt%, 65wt%, 66.78wt%, 70wt%, 74.94wt% or 75wt%, etc., and can also be other values within the range of 30wt% to 75wt%. In some optional embodiments, m accounts for 38wt% to 75wt% of the total mass of the carbon-coated lithium iron phosphate positive electrode material.

[0031] Generally, an ideal carbon coating should be dense and uniform to effectively isolate oxygen from direct contact with the lithium iron phosphate cathode material and effectively improve the conductivity of the lithium iron phosphate cathode material. The lithium iron phosphate cathode material is burned at high temperature for a short period of time (for example, 30 seconds) using a high-frequency combustion infrared sulfur-carbon detector. If the carbon coating on the surface of the lithium iron phosphate cathode material is well coated (e.g., completely and uniformly coated), the carbon-coated lithium iron phosphate cathode material will exhibit a low degree of oxidation after the short, high-temperature combustion. Conversely, the degree of oxidation will be high. Therefore, the amount of oxidation, m, can, to a certain extent, reflect the degree of coating of the carbon coating on the surface of the lithium iron phosphate cathode material. In the present invention, a carbon-coated lithium iron phosphate cathode material in which m accounts for 38 wt% to 75 wt% of the total mass of the homogeneous lithium iron phosphate material exhibits a good carbon coating effect.

[0032] In some optional embodiments, the relative standard deviation of the carbon coating structure of the homogeneous lithium iron phosphate material is ×100%, R ranges from 0.1 to 3.0, such as 0.1, 0.5, 0.694, 1.0, 1.001, 1.003, 1.263, 1.5, 1.641, 2.0, 2.5, 2.540, 2.792, or 3.0, and may also be other values within the range of 0.1 to 3.0. In some optional embodiments, R is 0.5 to 2.8. The carbon coating layer structure in the homogeneous lithium iron phosphate material with R of 0.5 to 2.8 is more uniform, making the conductivity of each position of the carbon-coated lithium iron phosphate positive electrode material more uniform.

[0033] in, ; n is the number of local positions taken when the homogeneous lithium iron phosphate material to be tested is subjected to Raman testing; X i = ; I G and I Drepresent the peak intensity of ordered carbon G bond and disordered carbon D bond in Raman spectrum at the i-th local position respectively; Measured for all local positions X i The average value of .

[0034] In some optional embodiments, It can be 0.45~1.65, such as 0.45, 0.5, 0.8, 1.0, 1.2, 1.5 or 1.65, or other values within the range of 0.45~1.65.

[0035] The carbon coating of lithium iron phosphate cathode materials is typically composed of a combination of ordered and disordered carbon. These two carbon phases work together to enhance the electrochemical performance of lithium iron phosphate cathode materials. The disordered carbon, with its disordered structure and abundant porosity, promotes rapid lithium ion diffusion and enhances electrolyte wettability. The ordered carbon, with its highly ordered layered structure, provides efficient electron transport pathways, significantly reducing interfacial resistance. This combination of ordered and disordered carbon ensures an optimal balance between conductivity and ion diffusion kinetics in lithium iron phosphate cathode materials, benefiting the rate performance and cycling stability of downstream battery products. Furthermore, by manipulating the production process, the two carbon phases form a uniformly distributed composite structure on the surface of the lithium iron phosphate cathode material, avoiding localized inhomogeneities in conductivity or ion pathways. This uniform coating not only ensures full participation of the active material in the electrochemical reaction but also effectively inhibits particle agglomeration and electrode polarization, ultimately resulting in higher capacity retention and superior long-term cycling performance. On the contrary, if the structure of the carbon coating layer is uneven, the overall conductivity of the lithium iron phosphate positive electrode material will be uneven, which will hinder the local electron transmission of the lithium iron phosphate positive electrode material, reduce the overall conductivity, and affect the capacity of the battery.

[0036] The present invention characterizes the coating condition of the carbon coating layer by using the ratio of the Raman coefficients of ordered carbon and disordered carbon. Raman tests are performed at n local positions in the carbon-coated lithium iron phosphate cathode material sample to be tested, and the Raman coefficients of different local positions are obtained. I D and I G The ratio of the carbon coating layer to the carbon coating layer is finally calculated by calculating the relative standard deviation R to characterize the uniformity of the carbon coating layer structure on the surface of the homogeneous lithium iron phosphate material. When the R value is smaller, the carbon coating layer structure is more uniform, that is, the conductive properties of the carbon-coated lithium iron phosphate positive electrode material are more uniform and the electrochemical performance is better.

[0037] As mentioned above, the homogeneous lithium iron phosphate material provided by the present invention satisfies the above-mentioned W range, m accounts for the total mass range of the homogeneous lithium iron phosphate material, and R range, has a carbon coating layer with a large specific surface area, and the carbon coating layer is uniformly coated on the surface of the lithium iron phosphate positive electrode material, thereby ensuring that the homogeneous lithium iron phosphate material has good conductivity.

[0038] In some optional embodiments, the general formula of the lithium iron phosphate matrix provided by the present invention can be Li 1-x A x Fe 1- y E y PO4; wherein A comprises at least one of Na and Mg; and E comprises at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Zn, and Y; and 0≤x≤0.1, 0≤y≤0.1. Furthermore, it is not excluded that A and E may be other elements.

[0039] Correspondingly, the present invention also provides an interface strengthening preparation method of the above-mentioned homogeneous lithium iron phosphate material, which may include the following steps: preparing a carbon coating layer on the surface of the lithium iron phosphate substrate.

[0040] In the present invention, the lithium iron phosphate matrix can be prepared by some conventional methods.

[0041] In some optional embodiments, the general formula of the lithium iron phosphate matrix is the above-mentioned Li 1-x A x Fe 1-y E y Taking PO4 as an example, the preparation of the lithium iron phosphate matrix may include: according to the general formula of the lithium iron phosphate matrix, hydrothermally reacting a lithium source, an A source, a phosphorus source, an iron source and an E source.

[0042] The lithium source may illustratively include lithium hydroxide. The A source may be a water-soluble salt of element A, such as a hydrochloride, sulfate, or nitrate of element A. The phosphorus source may include at least one of phosphoric acid and a phosphate. The iron source may be a water-soluble ferrous salt, such as ferrous sulfate, ferrous chloride, and ferrous acetate. The E source may be a water-soluble salt of element E, such as a hydrochloride, sulfate, or nitrate of element E.

[0043] In some optional embodiments, the molar ratio of (Li in the lithium source + A in the A source): (Fe in the iron source + E in the E source): P in the phosphorus source can be (1.02~1.05): (0.95~0.98): 1.0, such as 1.02:0.95:1.0, 1.03:0.96:1.0, 1.04:0.97:1.0 or 1.05:0.98:1.0, etc., or other values within the range of (1.02~1.05): (0.95~0.98): 1.0.

[0044] The molar ratio of Li in the lithium source to A in the A source can be (0.9-1):(0-0.1), such as 0.9:0.1, 0.92:0.08, 0.95:0.05, 0.98:0.02, or 1:0, or can be other values within the range of (0.9-1):(0-0.1). When the molar ratio of Li in the lithium source to A in the A source is 1:0, it indicates that the lithium iron phosphate matrix does not contain element A.

[0045] The molar ratio of Fe in the iron source to E in the E source can be (0.9-1):(0-0.1), such as 0.9:0.1, 0.92:0.08, 0.95:0.05, 0.98:0.02, or 1:0, or can be other values within the range of (0.9-1):(0-0.1). When the molar ratio of Fe in the iron source to E in the E source is 1:0, it indicates that the lithium iron phosphate matrix does not contain E element.

[0046] In some optional embodiments, the pH value of the hydrothermal reaction can be 6.0 to 8.0, such as 6.0, 6.5, 7.0, 7.5, or 8.0, or other values within the range of 6.0 to 8.0. Specifically, the pH value can be adjusted by adding a pH adjuster, wherein the pH adjuster can include LiOH or ammonia.

[0047] In some optional embodiments, the temperature of the hydrothermal reaction can be 140~180℃, such as 140℃, 145℃, 150℃, 155℃, 160℃, 165℃, 170℃, 175℃ or 180℃, etc., or other values within the range of 140~180℃.

[0048] In some optional embodiments, the hydrothermal reaction time may be 2 h to 8 h, such as 2 h, 3 h, 4 h, 5 h, 6 h, 7 h or 8 h, or other values within the range of 2 h to 8 h.

[0049] In some specific embodiments, the lithium iron phosphate matrix can be prepared according to the following process: the lithium source and the source A are dissolved in water according to the molar ratio of the above elements to prepare a solution, wherein the total concentration of Li and A in the solution is 3.0 mol / L to 4.5 mol / L. The above solution is added to a 1000 mL autoclave, and then the phosphorus source is added. After the air in the dead volume of the autoclave is purged with helium, the autoclave is sealed, stirred, and the temperature is heated from room temperature to 40°C to 50°C. Subsequently, the feed valve and the exhaust valve are opened, and the solution of the iron source and the source B is uniformly added over 10 minutes to 15 minutes. After stirring for 25 minutes to 35 minutes, a pH adjuster is added to adjust the pH to 6.0 to 8.0. Stirring is continued, and then the autoclave is sealed again and reacted at 140 to 180°C for 2 hours to 8 hours. After the reaction is completed, the autoclave is quickly cooled to 80°C by cooling water, the discharge valve is opened, the product is filtered and washed to obtain a filter cake and a mother liquor, and the filter cake is vacuum dried at 60°C to 120°C for 10h to 15h to obtain a lithium iron phosphate matrix.

[0050] It should be noted that in existing carbon coating processes, traditional carbon sources (such as glucose, sucrose, etc.) are often used to form a carbon coating layer through high-temperature cracking. This type of method usually has the problem of low specific surface area of the carbon coating layer, resulting in uneven distribution of the carbon coating layer, incomplete contact interface with the active material, and difficulty in forming an efficient three-dimensional conductive network. This limitation blocks the charge transfer path, and the migration rate of electrons between particles cannot meet the high power density requirements. Especially in thick electrode or fast charge and discharge scenarios, it is easy to cause a significant increase in the internal resistance of the lithium iron phosphate positive electrode material, affecting the overall performance of the battery. In addition, the carbon coating layer with a low specific surface area is not wettable to the electrolyte, further limiting the diffusion dynamics of lithium ions at the solid-liquid interface.

[0051] Although some technologies have been tried to improve the coating effect by introducing highly conductive carbon-based materials such as graphene and carbon nanotubes, these technologies are often difficult to apply on a large scale due to complex processes, high costs, or weak interfacial bonding between the carbon layer and the substrate. In particular, when the thickness of the carbon coating layer is increased to improve conductivity, it is often accompanied by a decrease in the tap density of the lithium iron phosphate positive electrode material and a loss in volumetric energy density. Therefore, it is necessary to find a balance between constructing a carbon coating layer with a high specific surface area and ensuring the overall electrochemical performance of the carbon-coated lithium iron phosphate positive electrode material. In addition, the conventional high-temperature carbonization process can easily lead to an excessive degree of graphitization of the carbon coating layer, a reduction in surface functional groups, and a weakening of compatibility with the electrolyte. At the same time, it may trigger heterogeneous reduction of iron and phosphorus elements on the surface of the lithium iron phosphate positive electrode material particles, destroying the stability of the material's crystal structure.

[0052] Based on this, the present invention provides a new method for preparing a carbon coating layer, which may include: mixing a lithium iron phosphate matrix with a composite carbon source solution to obtain a composite; and drying and carbonizing the composite.

[0053] In some optional embodiments, the preparation of the composite carbon source solution may include: mixing a hydrophobic carbon source, an amphiphilic block copolymer and a non-polar organic solvent.

[0054] The hydrophobic carbon source may include at least one of polyvinyl chloride and phenolic resin. The amphiphilic block copolymer may include at least one of polycaprolactone-polyethylene glycol, polystyrene-polyethylene oxide, and polyethylene glycol-polypropylene glycol-polyethylene glycol. The non-polar organic solvent may include tetrahydrofuran.

[0055] The total mass of the hydrophobic carbon source and the amphiphilic block copolymer can be 8% to 15% of the mass of the lithium iron phosphate matrix, such as 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%, or other values within the range of 8% to 15%.

[0056] If the total mass of the hydrophobic carbon source and the amphiphilic block copolymer is less than 8% of the mass of the lithium iron phosphate matrix, it is not conducive to the uniform coating of the hydrophobic carbon source on the surface of the lithium iron phosphate matrix, resulting in poor coating quality; if the total mass of the hydrophobic carbon source and the amphiphilic block copolymer is higher than 15% of the mass of the lithium iron phosphate matrix, it will cause excessive aggregation of micelles, affecting the coating uniformity.

[0057] The mass of the amphiphilic block copolymer can be 6% to 10% of the mass of the hydrophobic carbon source, such as 6%, 7%, 8%, 9% or 10%, or other values within the range of 6% to 10%.

[0058] If the mass of the amphiphilic block copolymer is less than 6% of the mass of the hydrophobic carbon source, it is not conducive to the effective encapsulation of the hydrophobic carbon source molecules by the hydrophobic segments in the amphiphilic block copolymer; if the mass of the amphiphilic block copolymer is higher than 10% of the mass of the hydrophobic carbon source, the excess micelles will form a hollow shell structure, affecting the performance of the carbon-coated lithium iron phosphate positive electrode material.

[0059] The amount of the non-polar organic solvent may be excessive to completely dissolve the hydrophobic carbon source and the amphiphilic block copolymer. In addition, the amount of the non-polar organic solvent may not be excessive as long as it can completely dissolve the hydrophobic carbon source and the amphiphilic block copolymer.

[0060] The mixing temperature of the hydrophobic carbon source, the amphiphilic block copolymer and the non-polar organic solvent can be 60°C to 80°C, such as 60°C, 65°C, 70°C, 75°C or 80°C, or other values within the range of 60°C to 80°C.

[0061] The mixing time of the hydrophobic carbon source, the amphiphilic block copolymer and the non-polar organic solvent can be 1 h to 2 h, such as 1 h, 1.5 h or 2 h, or other values within the range of 1 h to 2 h.

[0062] In some optional embodiments, the preparation of the above-mentioned composite may include: adding a lithium iron phosphate matrix to a composite carbon source solution, and performing homogenization and sonication using an ultrasonic homogenizer to uniformly condense the carbon source on the surface of the lithium iron phosphate matrix. The solid-to-liquid ratio of the lithium iron phosphate matrix to the composite carbon source solution may be 1g:0.8mL to 1g:1.5mL. The sonication time may be 2h to 4h, such as 2h, 3h, or 4h, or other values within the range of 2h to 4h.

[0063] As above, the present invention adopts a hydrophobic carbon source and an amphiphilic block copolymer to prepare a carbon-coated lithium iron phosphate positive electrode material, wherein the amphiphilic block copolymer takes into account both hydrophilic and hydrophobic properties at the same time, and combines the control of the reaction conditions to make the hydrophobic carbon source added with the amphiphilic block copolymer be uniformly adsorbed on the surface of the lithium iron phosphate matrix. Specifically, under the action of homogeneous ultrasound, the hydrophilic end of the amphiphilic block copolymer is connected to the lithium iron phosphate matrix, and the hydrophobic end of the amphiphilic block copolymer is combined with the hydrophobic carbon source to play a role of directional anchoring, so that the amphiphilic block copolymer and the hydrophobic carbon source are evenly distributed on the lithium iron phosphate matrix. After the following carbonization process, a coating with large coverage and uniform coating is formed on the surface of the lithium iron phosphate matrix.

[0064] In some optional embodiments, the composite may be dried using a fluidized bed drying method, wherein the inlet air temperature may be 180°C to 220°C, such as 180°C, 190°C, 200°C, 210°C, or 220°C, or other values within the range of 180°C to 220°C.

[0065] In some optional embodiments, carbonization can be carried out in a tubular furnace, and specifically may include: in a protective atmosphere (such as an argon atmosphere), first performing low-temperature carbonization at 400°C to 500°C (such as 400°C, 450°C or 500°C, etc.) for 2h to 3h (such as 2h, 2.5h or 3h, etc.), and then performing high-temperature carbonization at 700°C to 900°C (such as 700°C, 750°C, 800°C, 850°C or 900°C, etc.) for 0.5h to 1h (such as 0.5h, 0.8h or 1h, etc.), and then naturally cooling.

[0066] In some optional embodiments, the temperature is increased to 400°C~500°C at a heating rate of 1.5°C / min~2.5°C / min (such as 1.5°C / min, 2°C / min or 2.5°C / min, etc.), and the temperature is increased to 700°C~900°C at a heating rate of 9.5°C / min~10.5°C / min (such as 9.5°C / min, 10°C / min or 10.5°C / min, etc.).

[0067] In some optional embodiments, 4% to 6% by volume of hydrogen is introduced during the high-temperature carbonization process. The introduction of hydrogen can neutralize the oxygen-containing gas generated during the high-temperature pyrolysis of the hydrophobic carbon source.

[0068] The high-temperature carbonization stage is a critical period for the pyrolysis and carbonization of the hydrophobic carbon source. During pyrolysis, the hydrophobic carbon source releases a large amount of oxygen-containing gases (such as H₂O, CO, CO₂, and small organic molecules). In a purely inert atmosphere, these released oxygen-containing gases may form oxygen-containing functional groups on or near the surface of the lithium iron phosphate matrix, or lead to a less dense carbon coating, low graphitization, and poor conductivity. By adding hydrogen during the high-temperature carbonization process, the hydrogen reacts with some of the oxygen-containing gases produced by pyrolysis, more effectively removing oxygen and promoting the deoxygenation reaction. Continuing from the above, the present invention prepares a carbon-free lithium iron phosphate matrix by a hydrothermal method for subsequent carbon coating. In the process of preparing the homogeneous lithium iron phosphate material, a non-polar organic solvent (such as tetrahydrofuran) is used to dissolve the hydrophobic carbon source and the amphiphilic block copolymer to obtain a composite carbon source solution, and then the lithium iron phosphate matrix is added to the composite carbon source solution and dispersed by homogeneous ultrasonic stirring. The hydrophilic blocks in the above-mentioned amphiphilic block copolymer aggregate to form a polar core (reverse micelle), while the hydrophobic blocks dissolve in the organic solvent to form a micelle shell. The surface of the lithium iron phosphate matrix usually has hydroxyl groups or uncoordinated phosphate groups (hydrophilicity), which are difficult to disperse in non-polar organic solvents. The polar core formed by the above-mentioned hydrophilic blocks adsorbs the lithium iron phosphate matrix through hydrogen bonds, electrostatic interactions or coordination interactions, encapsulating it in the polar core to prevent agglomeration. The polar core formed by the hydrophilic blocks encapsulates the lithium iron phosphate matrix, while the hydrophobic blocks extend into the nonpolar solvent, directly enhancing its dispersibility and, to a certain extent, acting as a surfactant. Furthermore, the hydrophobic carbon source has a nonpolar surface, allowing it to tightly bind to the hydrophobic blocks in the amphiphilic block copolymer through van der Waals forces. Dispersed in the nonpolar organic solvent, the hydrophobic carbon source embeds within the micelle shell formed by the hydrophobic blocks or blends with the hydrophobic segments to form a "carbon-polymer" composite phase. The amphiphilic block copolymer simultaneously encapsulates the lithium iron phosphate matrix (core) and the bound hydrophobic carbon source (shell or dispersed phase) through reverse micelles, forming a "core-shell" or "mosaic" structure, ultimately resulting in a composite structure of "lithium iron phosphate matrix@micelle core+carbon source@micelle shell." Drying is performed in a fluidized bed with the inlet air temperature controlled at 180°C to 220°C, sufficient to evaporate the nonpolar organic solvent, tetrahydrofuran. Subsequently, a step-by-step carbonization treatment is used to pyrolyze and carbonize the amphiphilic block copolymer, increase the proportion of ordered carbon, further optimize the structure of the carbon coating layer, and ultimately form a homogeneous lithium iron phosphate material with a large coverage and uniform carbon coating structure.

[0069] The above method provided by the present invention can accurately control the microstructure of the carbon coating layer and significantly increase its specific surface area, thereby optimizing the interfacial charge transfer efficiency and strengthening the connectivity of the conductive network. Specifically, the above method provided by the present invention can obtain C wt The homogeneous lithium iron phosphate material has a mass ratio of 1wt% to 2.5wt%, a mass ratio of 3.5s to 6.5s, a mass ratio of 140s to 600s, and a mass ratio of 30wt% to 75wt%.

[0070] In summary, the homogeneous lithium iron phosphate material prepared by interface strengthening in this application refers to the homogeneous lithium iron phosphate material provided by the present invention, which can be prepared using the interface strengthening preparation method of the present invention. However, those skilled in the art will appreciate that the homogeneous lithium iron phosphate material of the present invention is not limited to the preparation method of the present invention, which is merely an example. Interface strengthening in the present invention refers to the adsorption of the hydrophilic block of the amphiphilic block copolymer in the composite carbon source used in the preparation process on the surface of the lithium iron phosphate particle matrix, while the hydrophobic block is combined with the hydrophobic carbon source. Subsequent carbonization treatment can enhance the interfacial bonding between the lithium iron phosphate matrix and the carbon layer, ultimately obtaining a lithium iron phosphate material with a uniform (i.e., homogeneous) carbon coating structure.

[0071] In addition, the present invention also provides a battery cell, the positive electrode material of which includes the above-mentioned homogeneous lithium iron phosphate material.

[0072] For example, the battery cells can be used in, but are not limited to, electrical devices such as vehicles, ships, or aircraft.

[0073] The present invention also provides a battery comprising the above battery cell, which can have good conductivity, rate performance and low-temperature performance.

[0074] The present invention also provides an electrical device comprising the aforementioned battery cell and / or battery. For example, the electrical device may include, but is not limited to, a mobile phone, a tablet, a laptop computer, an electric toy, an electric tool, a battery-powered vehicle, an electric car, a ship, a spacecraft, and the like. The electric toy may include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys. The spacecraft may include airplanes, rockets, space shuttles, and spacecraft.

[0075] The features and performance of the present invention are further described in detail below with reference to the embodiments.

[0076] Example 1 This embodiment provides a homogeneous lithium iron phosphate material, the preparation method of which includes: S1: preparing lithium iron phosphate matrix LiFePO4.

[0077] A lithium source (lithium hydroxide) was dissolved in water to prepare a 4.0 mol / L aqueous lithium hydroxide solution, which was then added to a 1000 mL autoclave. A phosphorus source (phosphoric acid) was then added. After purging the air from the dead volume of the autoclave with helium, the autoclave was sealed and stirred. The temperature was heated from room temperature to 50°C. The feed and exhaust valves were then opened, and an aqueous iron source (ferrous sulfate) solution was added at a constant rate over 12 minutes. The molar ratio of Li:Fe:P was 1.05:0.98:1.0. After stirring for 30 minutes, a pH adjuster (lithium hydroxide) was added to adjust the pH to 6.0. Stirring was continued, and the autoclave was resealed and reacted at 160°C for 6 hours. After the reaction was complete, the autoclave was rapidly cooled to 80°C with cooling water. The discharge valve was opened, and the product was filtered and washed until sulfate ions were free, resulting in a filter cake and mother liquor. The filter cake was vacuum-dried at 100°C for 12 hours to obtain the lithium iron phosphate matrix.

[0078] S2: preparing a composite carbon source solution.

[0079] 13.8 g of a hydrophobic carbon source (polyvinyl chloride) and 1.2 g of an amphiphilic block copolymer (polycaprolactone-polyethylene glycol) were weighed and dissolved in an excess of tetrahydrofuran, a non-polar organic solvent. The mixture was heated to 80°C and stirred for 2 h to form a composite carbon source solution.

[0080] S3: preparing a carbon coating layer.

[0081] S31: preparing a complex.

[0082] Weigh 100 g of the lithium iron phosphate matrix prepared in S1 above and add it to the composite carbon source solution formed in S2 at a solid-liquid ratio of 1 g:1.5 mL (the total mass of the hydrophobic carbon source and the amphiphilic block copolymer is 15% of the mass of the lithium iron phosphate matrix). Use an ultrasonic homogenizer to homogenize and ultrasonically stir for 3 h to obtain a composite.

[0083] S32: Dry.

[0084] The composite was placed on a fluidized bed dryer for drying, and the air inlet temperature of the fluidized bed dryer was controlled at 200°C.

[0085] S33: Carbonization.

[0086] The dried composite was placed in a tubular furnace for step-by-step carbonization: under an argon atmosphere, the temperature was raised to 500°C at a heating rate of 2°C / min and kept warm for 3 hours; then 5% hydrogen was introduced, and the temperature was raised to 900°C at a heating rate of 10°C / min under a mixed atmosphere, kept warm for 1 hour, and then naturally cooled to obtain a carbon-coated lithium iron phosphate positive electrode material (i.e., homogeneous lithium iron phosphate material).

[0087] Example 2 This embodiment provides a homogeneous lithium iron phosphate material, the preparation method of which includes: S1: preparing lithium iron phosphate matrix LiFePO4.

[0088] A lithium source (lithium hydroxide) was dissolved in water to prepare a 3.0 mol / L aqueous lithium hydroxide solution, which was then added to a 1000 mL autoclave. A phosphorus source (phosphoric acid) was then added. After purging the air from the dead volume of the autoclave with helium, the autoclave was sealed and stirred. The temperature was heated from room temperature to 50°C. The feed and exhaust valves were then opened, and an aqueous iron source (ferrous acetate) solution was added at a constant rate over 12 minutes. The molar ratio of Li:Fe:P was 1.02:0.96:1.0. After stirring for 30 minutes, a pH adjuster (lithium hydroxide) was added to adjust the pH to 6.5. Stirring was continued, and the autoclave was resealed and reacted at 140°C for 4 hours. After the reaction was complete, the autoclave was rapidly cooled to 80°C with cooling water. The discharge valve was opened, and the product was filtered and washed until acetate ions were free, resulting in a filter cake and mother liquor. The filter cake was vacuum-dried at 80°C for 12 hours to obtain the lithium iron phosphate matrix.

[0089] S2: preparing a composite carbon source solution.

[0090] 9.1 g of a hydrophobic carbon source (polyvinyl chloride) and 0.9 g of an amphiphilic block copolymer (polycaprolactone-polyethylene glycol) were weighed and dissolved in an excess of tetrahydrofuran, a non-polar organic solvent. The mixture was heated to 60°C and stirred for 2 h to form a composite carbon source solution.

[0091] S3: preparing a carbon coating layer.

[0092] S31: preparing a complex.

[0093] 100 g of the lithium iron phosphate matrix prepared in S1 was weighed and added to the composite carbon source solution formed in S2 at a solid-liquid ratio of 1 g:1 mL (i.e., the total mass of the hydrophobic carbon source and the amphiphilic block copolymer was 10% of the mass of the lithium iron phosphate matrix). The mixture was homogenized and ultrasonically stirred for 3 h using an ultrasonic homogenizer to obtain a composite.

[0094] S32: Dry.

[0095] The composite was placed on a fluidized bed dryer for drying, and the air inlet temperature of the fluidized bed dryer was controlled at 180°C.

[0096] S33: Carbonization.

[0097] The dried composite was placed in a tubular furnace for step-by-step carbonization: under an argon atmosphere, the temperature was raised to 400°C at a heating rate of 2°C / min and kept warm for 2 hours; then 5% hydrogen was introduced, and the temperature was raised to 800°C at a heating rate of 10°C / min under a mixed atmosphere, kept warm for 0.5 hours, and then naturally cooled to obtain a carbon-coated lithium iron phosphate positive electrode material.

[0098] Example 3 This embodiment provides a homogeneous lithium iron phosphate material, the preparation method of which includes: S1: Preparation of lithium iron phosphate matrix Li 0.998 Na 0.002 FePO4.

[0099] A lithium source (lithium hydroxide) and sodium chloride were dissolved in deionized water at a molar ratio of 0.998:0.002 to prepare an aqueous solution with a total metal element concentration of 4.5 mol / L. This solution was then added to a 1000 mL autoclave, followed by a phosphorus source (phosphoric acid). After purging the air from the dead volume of the autoclave with helium, the autoclave was sealed and stirred. The temperature was heated from room temperature to 50°C. The feed and exhaust valves were then opened, and an aqueous solution of an iron source (ferrous acetate) was added at a uniform rate over 12 minutes. The molar ratio of the added substances was (Li+Na):Fe:P=1.03:0.97:1.0. After stirring for 30 minutes, a pH adjuster (lithium hydroxide) was added to adjust the pH to 8.0. Stirring was continued, and the autoclave was sealed again. The reaction was then heated at 180°C for 8 hours. After the reaction is completed, the autoclave is quickly cooled to 80°C by cooling water, the discharge valve is opened, the product is filtered and washed until there is no acetate ion, to obtain a filter cake and a mother liquor, and the filter cake is vacuum dried at 120°C for 12 hours to obtain a lithium iron phosphate matrix.

[0100] S2: preparing a composite carbon source solution.

[0101] 7.3 g of a hydrophobic carbon source (phenolic resin) and 0.7 g of an amphiphilic block copolymer (polystyrene-polyethylene oxide) were weighed and dissolved in an excess of tetrahydrofuran, a non-polar organic solvent. The mixture was heated to 70° C. and stirred for 1 h to form a composite carbon source solution.

[0102] S3: preparing a carbon coating layer.

[0103] S31: preparing a complex.

[0104] 100 g of the lithium iron phosphate matrix prepared in S1 was weighed and added to the composite carbon source solution formed in S2 at a solid-liquid ratio of 1 g:0.8 mL (i.e., the total mass of the hydrophobic carbon source and the amphiphilic block copolymer was 8% of the mass of the lithium iron phosphate matrix). The mixture was homogenized and ultrasonically stirred for 4 h using an ultrasonic homogenizer to obtain a composite.

[0105] S32: Dry.

[0106] The composite was placed on a fluidized bed dryer for drying, and the air inlet temperature of the fluidized bed dryer was controlled at 220°C.

[0107] S33: Carbonization.

[0108] The dried composite was placed in a tubular furnace for step-by-step carbonization: under an argon atmosphere, the temperature was raised to 500°C at a heating rate of 2°C / min and kept warm for 2 hours; then 5% hydrogen was introduced, and the temperature was raised to 700°C at a heating rate of 10°C / min under a mixed atmosphere, kept warm for 0.5 hours, and then naturally cooled to obtain a carbon-coated lithium iron phosphate positive electrode material.

[0109] Example 4 This embodiment provides a homogeneous lithium iron phosphate material, the preparation method of which includes: S1: Preparation of lithium iron phosphate matrix LiFe 0.987 Mn 0.013 PO4.

[0110] A lithium source (lithium hydroxide) was dissolved in water to prepare a 4.0 mol / L lithium hydroxide aqueous solution, which was then added to a 1000 mL autoclave. A phosphorus source (phosphoric acid) was then added. After purging the air from the dead volume of the autoclave with helium, the autoclave was sealed and stirred. The temperature was heated from room temperature to 50°C. The feed and exhaust valves were then opened, and aqueous solutions of an iron source (ferrous sulfate) and a manganese source (manganese carbonate) were added at a uniform rate over 12 minutes (the molar ratio of Fe in the iron source to Mn in the manganese source was 0.987:0.013). The molar ratio of the added substances was Li:(Fe+Mn):P=1.05:0.98:1.0. After stirring for 30 minutes, a pH adjuster (lithium hydroxide) was added to adjust the pH to 6.0. Stirring was continued, and the autoclave was resealed and reacted at 160°C for 6 hours. After the reaction is completed, the autoclave is quickly cooled to 80°C by cooling water, the discharge valve is opened, and the product is filtered and washed until there is no sulfate and carbonate ions to obtain a filter cake and a mother liquor. The filter cake is vacuum dried at 100°C for 12 hours to obtain a lithium iron phosphate matrix.

[0111] S2 to S3 are the same as the corresponding steps in Example 1.

[0112] Example 5 The difference between this embodiment and embodiment 1 is that in S2, the mass of polyvinyl chloride is 7.52 g, and the mass of polycaprolactone-polyethylene glycol is 0.48 g.

[0113] Example 6 The difference between this embodiment and embodiment 1 is that in S2, the mixing temperature of the hydrophobic carbon source, the amphiphilic block copolymer and the non-polar organic solvent is 60° C., and the mixing time is 1 h.

[0114] Example 7 The difference between this embodiment and embodiment 1 is that in S31, the homogenizing ultrasonic stirring time is 2 hours.

[0115] Example 8 The difference between this embodiment and embodiment 1 is that in S33, no hydrogen is introduced during the high-temperature carbonization process.

[0116] Comparative Example 1 The difference between this comparative example and Example 1 is that in S2, the mass of polyvinyl chloride is 13.5 g, and the mass of polycaprolactone-polyethylene glycol is 0.5 g.

[0117] Comparative Example 2 The difference between this comparative example and Example 1 is that in S2, the mass of polyvinyl chloride is 13.5 g, and the mass of polycaprolactone-polyethylene glycol is 5 g.

[0118] Comparative Example 3 The difference between this comparative example and Example 1 is that S2 does not contain the amphiphilic block copolymer (polycaprolactone-polyethylene glycol).

[0119] Comparative Example 4 The difference between this comparative example and Example 1 is that in S33, no step-by-step carbonization is performed, and the carbonization process is directly carried out in an argon atmosphere, with the temperature being raised to 900°C at a heating rate of 2°C / min, kept at that temperature for 3 hours, and then naturally cooled.

[0120] Comparative Example 5 This comparative example provides a carbon-coated lithium iron phosphate positive electrode material prepared using a conventional carbon coating process, and the preparation method thereof includes: S1: Weigh 100 g of lithium iron phosphate and add it to 2000 mL of ethanol / water mixed solvent (the volume ratio of ethanol to water is 3:1) to obtain a lithium iron phosphate precursor solution; S2: Weigh 10 g of glucose and dissolve it in deionized water to form a carbon source solution with a concentration of 10 wt%; S3: Slowly add the carbon source solution to the lithium iron phosphate precursor solution, then place it in a tube furnace, and heat it to 900°C at a heating rate of 2°C / min under an argon atmosphere. Keep it warm for 5 hours for sintering, and then cool it naturally to obtain a carbon-coated lithium iron phosphate positive electrode material.

[0121] Test example The lithium iron phosphate materials provided in Examples 1 to 8 and Comparative Examples 1 to 5 were subjected to the following tests: (1) Carbon content test The carbon content (mass percentage) in lithium iron phosphate materials was determined using a Dekai Instruments HCS-140 high-frequency combustion infrared sulfur and carbon detector in accordance with the YS / T 1028.4-2015 standard for chemical analysis of lithium iron phosphate. During the test, 0.1 g of lithium iron phosphate material was weighed, flux was added, and the test conditions were 18 MHz and 2.2 kW. The half-peak width of the carbon content curve was obtained by fitting the carbon content distribution plot using Origin software. For example, the half-peak width of the carbon content curve in Table 1 below was obtained by performing a baseline subtraction on the carbon content curve using Origin software, followed by a Gaussian fit to obtain the half-peak width, with a goodness-of-fit (COD) of 0.9 or higher.

[0122] Among them, the carbon content curve obtained in Example 1 is as follows Figure 1 shown.

[0123] (2) Test of the oxidation amount of lithium iron phosphate Lithium iron phosphate material is burned in a high-frequency combustion infrared sulfur and carbon detector with a certain amount of flux added, and the burning time is 30s. When measuring the amount of lithium iron phosphate oxidized in the lithium iron phosphate material, a blank sample is first set up, that is, the blank sample is only added with flux, and the weight is weighed after the same burning time. Therefore, the amount of lithium iron phosphate oxidized m = [m (after burning) + C wt +S wt ]-m(before burning)-m'. Among them, m(after burning) is the mass of lithium iron phosphate material after burning, m(before burning) is the mass of lithium iron phosphate material before burning, S wt The sulfur content in lithium iron phosphate material is directly measured by a high-frequency combustion infrared sulfur-carbon detector.

[0124] (3) Raman test The Raman spectrometer of Japan Horiba iHR550 was used to perform Raman testing on the lithium iron phosphate material to be tested, with a scanning range of 50cm -1 ~4000cm -1 , the excitation wavelength is 532nm, the power reaching the sample surface is 0.45mW, according to ×100% to calculate the relative standard deviation R of the carbon coating structure; where, ; n is the number of local positions taken when the lithium iron phosphate material to be tested is subjected to Raman testing (n=5); X i = ; I G and I D represent the peak intensity of ordered carbon G bond and disordered carbon D bond in Raman spectrum at the i-th local position respectively; Measured for all local positions X i The average value of .

[0125] The Raman spectrum obtained in Example 1 is as follows: Figure 2 shown.

[0126] (4) Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) testing The microstructure and chemical elements of lithium iron phosphate materials were tested using a G220 transmission electron microscope from FEI Company of the United States combined with energy dispersive spectroscopy (EDS).

[0127] (5) Local impedance test (LEIS) The impedance of multiple local locations of lithium iron phosphate material was tested using a Princeton VersaSCAN LEIS micro-area scanning electrochemical workstation. Lithium iron phosphate material, acetylene black and polyvinylidene fluoride were uniformly mixed in a mass ratio of 90:5:5 to form an electrode material as a working electrode, and a metal lithium sheet was used as a counter electrode. Ten local locations were marked on the test surface of the working electrode, with each local location point spaced 0.1 mm apart, and the frequency range was 10 -2 Hz~10 5 Hz, scanning speed is 0.1mV / s~0.5mV / s.

[0128] Based on local impedance testing and circuit simulations, the high-frequency charge transfer resistance (Rct) values of multiple local locations on the lithium iron phosphate material were obtained. Impedance data was recorded during the test, and software tools were used to analyze and fit the test data to extract the charge transfer resistance (Rct), a parameter related to the charge transfer process.

[0129] (6) Electrode resistivity At room temperature (25°C), the resistivity (Ω·cm) of the lithium iron phosphate positive electrode sheet was tested using a two-probe test method, and the current intensity was set to 2A.

[0130] (7) Battery performance test The lithium iron phosphate material obtained above was assembled into a button-type lithium-ion battery for electrochemical performance testing. The specific steps were as follows: The lithium iron phosphate material, acetylene black, and polyvinylidene fluoride were uniformly mixed in N-methylpyrrolidone at a mass ratio of 90:5:5. The mixture was then coated onto aluminum foil and dried in a vacuum drying oven. After drying, the cathode was pressed into a 12 mm diameter cathode sheet using a tablet press in an argon glove box. The anode was a metallic lithium sheet, the electrolyte was 1 mol / L LiPF6-EC:DMC (volume ratio 1:1), and a polypropylene porous membrane was used as the separator. The electrochemical performance was tested at a voltage of 2.0 V to 3.9 V, a temperature of 25°C, and a current density of 1C = 170 mAh / g. The capacity retention was calculated using the following formula.

[0131] Capacity retention rate = ×100%.

[0132] Use an IT5100 battery internal resistance meter to test the internal resistance of the button cell battery. Touch the positive and negative leads of the meter to the two ends of the cylindrical battery. After the voltage stabilizes, read the value on the meter to obtain the internal resistance of the battery. Calculate the internal resistance growth rate using the following formula.

[0133] Internal resistance growth rate = )×100%.

[0134] The results of the above tests are shown in Tables 1 and 2.

[0135] Table 1 Test results

[0136] Table 2 Test results

[0137] Combined with Table 1 and Table 2, the lithium iron phosphate materials provided by the embodiments of the present invention all meet the requirements of C wt The values of 1.0wt% to 2.5wt%, w of 3.5s to 6.5s, m of 30wt% to 75wt% of the total mass of the lithium iron phosphate material, W of 140s to 600s, and R of 0.1 to 3.0 indicate that the lithium iron phosphate material provided by the embodiment of the present invention has a high-quality carbon coating layer. The carbon coating layer not only has a large coating specific surface area but also has a uniform carbon layer structure. The corresponding carbon-coated lithium iron phosphate positive electrode material has a uniformly distributed conductive network, resulting in overall better charge transfer resistance (Rct) and electrode sheet resistivity than the comparative example. The lithium ion battery prepared in the embodiment also has a lower internal resistance growth rate and excellent cycle capacity retention. Furthermore, the charge transfer resistance (Rct) and electrode sheet resistivity of the lithium iron phosphate material provided by the embodiment of the present invention are smaller, and the corresponding lithium ion battery has better performance such as capacity retention and cycle stability.

[0138] The results in Table 1 show that adjusting the reaction parameters of each step in the preparation process can regulate the carbon coating of the lithium iron phosphate material, thereby adjusting W and R. Specifically, in step S2, the difference in the content of the hydrophobic carbon source and the amphiphilic block copolymer will affect the coverage of the hydrophobic carbon source on the surface of the lithium iron phosphate substrate; on the other hand, by incorporating a certain amount of amphiphilic block copolymer into the hydrophobic carbon source, the uniformity of the carbon coating can be adjusted.

[0139] Combining Example 1 and Comparative Examples 1 to 3, it can be seen that whether or not an amphiphilic block copolymer is added, as well as whether the amount of amphiphilic block copolymer added is too much or too little, will affect the uniformity of the carbon coating structure. For example, the amphiphilic block copolymer can, to a certain extent, "fix" the position of the hydrophobic carbon source on the lithium iron phosphate matrix. If the amphiphilic block copolymer is not included or the amount of the amphiphilic block copolymer added is small, the "fixing" effect is poor; if the amount of amphiphilic block copolymer added is too much, the excess micelles will form a hollow shell structure, affecting the performance of the carbon-coated lithium iron phosphate cathode material.

[0140] Combining Example 1 and Comparative Example 4, it can be seen that in step S33, the stepwise carbonization (low-temperature, slow carbonization in the early stage, followed by high-temperature, short carbonization in the later stage) can effectively convert the carbon source into disordered and ordered carbon, achieving a uniform carbon layer structure while maintaining uniform distribution of the carbon source. Specifically, Comparative Example 4, which uses a process of directly increasing the temperature at a constant rate, produces a carbon-coated lithium iron phosphate cathode material with poor performance.

[0141] Combining Example 1 and Comparative Example 5, it can be seen that: compared with the conventional carbon coating process, the process of combining the addition of amphiphilic block polymers and step-by-step carbonization in Example 1 can achieve the best carbon coating effect of lithium iron phosphate, which can not only meet the requirements of a large specific surface area of the carbon coating layer, but also meet the requirements of a uniform structure of the carbon coating layer, thereby better improving the conductive properties of the lithium iron phosphate positive electrode material.

[0142] In summary, the homogeneous lithium iron phosphate material provided by the present invention has a large specific surface area and a uniform carbon coating. It also exhibits strong electron transport between lithium iron phosphate particles and excellent electrical conductivity. Batteries prepared from this lithium iron phosphate material can exhibit both excellent electrical conductivity and rate capability.

[0143] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A homogeneous lithium iron phosphate material, characterized in that: It includes a lithium iron phosphate matrix and a carbon coating layer coated on the lithium iron phosphate matrix; The carbon content C in the homogeneous lithium iron phosphate material wt The carbon content of the homogeneous lithium iron phosphate material is 1.0wt%~2.5wt%, and the half-peak width w of the carbon content curve measured by the high-frequency combustion infrared sulfur-carbon detector is 3.5s~6.5s; the half-peak width of the carbon content curve of the unit carbon content in the homogeneous lithium iron phosphate material is 1.0wt%~2.5wt%. , the value range of W is 140s~600s; After the homogeneous lithium iron phosphate material is burned by the high-frequency combustion infrared sulfur-carbon detector, the amount m of oxidized lithium iron phosphate accounts for 30wt% to 75wt% of the total mass of the homogeneous lithium iron phosphate material.

2. The homogeneous lithium iron phosphate material according to claim 1, characterized in that Relative standard deviation of the carbon coating structure of the homogeneous lithium iron phosphate material ×100%, the value range of R is 0.1~3.0; among them, ; n is the number of local positions taken when the homogeneous lithium iron phosphate material to be tested is subjected to Raman testing; X i = ; I G and I D represent the peak intensity of ordered carbon G bond and disordered carbon D bond in Raman spectrum at the i-th local position respectively; Measured for all local positions X i The average value of .

3. The homogeneous lithium iron phosphate material according to claim 1 or 2, characterized in that: The homogeneous lithium iron phosphate material further includes at least one of the following features: Feature 1: C wt 1.0wt%~1.5wt%; Feature 2: w is 4.8s~6.0s; Feature 3: R is 0.5~2.8; Feature 4: The general formula of the lithium iron phosphate matrix is Li 1-x A x Fe 1-y E y PO4; wherein A includes at least one of Na and Mg; E includes at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Zn and Y; 0≤x≤0.1, 0≤y≤0.

1.

4. A method for preparing a homogeneous lithium iron phosphate material by interface strengthening according to any one of claims 1 to 3, characterized in that: The following steps are involved: A carbon coating layer is prepared on the surface of a lithium iron phosphate substrate.

5. The interface strengthening preparation method according to claim 4, characterized in that: The general formula of the lithium iron phosphate matrix is Li 1-x A x Fe 1-y E y PO4; wherein A includes at least one of Na and Mg; E includes at least one of Al, Ni, Co, Mn, Ti, La, Ce, Cr, Mo, Ca, Zn and Y; 0≤x≤0.1, 0≤y≤0.1; The preparation of the lithium iron phosphate matrix comprises: according to the general formula of the lithium iron phosphate matrix, subjecting a lithium source, an A source, a phosphorus source, an iron source and an E source to a hydrothermal reaction.

6. The interface strengthening preparation method according to claim 5, characterized in that: The preparation of the lithium iron phosphate matrix includes at least one of the following features: Feature 5: The lithium source includes lithium hydroxide; Feature 6: The source A is a water-soluble salt of element A; Feature 7: The phosphorus source includes at least one of phosphoric acid and phosphate; Feature 8: The iron source is a water-soluble ferrous salt; Feature 9: The E source is a water-soluble salt of element E; feature 10: In terms of molar ratio, (Li in lithium source + A in A source): (Fe in iron source + E in E source): P in phosphorus source = (1.02-1.05): (0.95-0.98): 1.0; Feature 11: The molar ratio of Li in the lithium source to A in the A source is (0.9-1):(0-0.1); Feature 12: The molar ratio of Fe in the iron source to E in the E source is (0.9-1):(0-0.1); feature 13: The pH value of the hydrothermal reaction is 6.0~8.0; Feature 14: The temperature of the hydrothermal reaction is 140-180°C; Feature 15: The hydrothermal reaction time is 2h~8h.

7. The interface strengthening preparation method according to claim 4, characterized in that: The preparation of the carbon coating layer comprises: mixing the lithium iron phosphate matrix with a composite carbon source solution to obtain a composite; and drying and carbonizing the composite.

8. The interface strengthening preparation method according to claim 7, characterized in that: The preparation of the carbon coating layer includes at least one of the following features: Feature 16: adding the lithium iron phosphate matrix to the composite carbon source solution, homogenizing and ultrasonicating to obtain a composite; preferably, the solid-liquid ratio of the lithium iron phosphate matrix to the composite carbon source solution is 1g:0.8mL to 1g:1.5mL; preferably, the ultrasonication time is 2h~4h; feature 17: Drying is performed using a fluidized bed drying method; preferably, the inlet air temperature is 180°C to 220°C; Feature 18: Carbonization includes: in a protective atmosphere, first performing low-temperature carbonization at 400°C~500°C for 2h~3h, and then performing high-temperature carbonization at 700°C~900°C for 0.5h~1h; preferably, heating to 400°C~500°C at a heating rate of 1.5°C / min~2.5°C / min; preferably, heating to 700°C~900°C at a heating rate of 9.5°C / min~10.5°C / min; preferably, 4%~6% of hydrogen is introduced during the high-temperature carbonization process, in terms of volume percentage.

9. The interface strengthening preparation method according to claim 7, characterized in that: The preparation of the composite carbon source solution comprises: mixing a hydrophobic carbon source, an amphiphilic block copolymer and a non-polar organic solvent; Preferably, the preparation of the composite carbon source solution includes at least one of the following features: Feature 19: The hydrophobic carbon source comprises at least one of polyvinyl chloride and phenolic resin; Feature 20: The amphiphilic block copolymer comprises at least one of polycaprolactone-polyethylene glycol, polystyrene-polyethylene oxide, and polyethylene glycol-polypropylene glycol-polyethylene glycol; Feature 21: The non-polar organic solvent comprises tetrahydrofuran; Feature 22: The total mass of the hydrophobic carbon source and the amphiphilic block copolymer is 8% to 15% of the mass of the lithium iron phosphate matrix; wherein the mass of the amphiphilic block copolymer is 6% to 10% of the mass of the hydrophobic carbon source; Feature 23: The mixing temperature of the hydrophobic carbon source, the amphiphilic block copolymer and the non-polar organic solvent is 60° C. to 80° C. Feature 24: The mixing time of the hydrophobic carbon source, the amphiphilic block copolymer and the non-polar organic solvent is 1 h to 2 h.

10. A battery, characterized in that: The battery contains the homogeneous lithium iron phosphate material according to any one of claims 1 to 3.

Images