Interface reinforced preparation of homogeneous lithium iron phosphate material and application thereof

Abstract

The application discloses a kind of interface reinforced preparation homogeneous lithium iron phosphate material and its application, belong to lithium iron phosphate positive electrode material technical field.The lithium iron phosphate material includes lithium iron phosphate matrix and carbon coating layer;Carbon content C wt 1wt%~2.5wt%, the carbon content curve of the lithium iron phosphate material measured by high-frequency combustion infrared sulfur carbon detector is 3.5s~6.5s;The half-peak width W of unit carbon content in the carbon content curve in the lithium iron phosphate material is 140s~600s;The amount of lithium iron phosphate oxidized after the lithium iron phosphate material is burned by high-frequency combustion infrared sulfur carbon detector m accounts for 30wt%~75wt% of the total mass of material.The specific surface area of the above-mentioned lithium iron phosphate material carbon coating layer is larger and the structure is uniform, the electron transport capacity between lithium iron phosphate material particles is stronger, and the conductivity is good.The battery further prepared therefrom can have better conductivity and rate performance, etc.

Description

Technical Field

[0001] This invention relates to the field of lithium iron phosphate cathode material technology, and more specifically, to a homogeneous lithium iron phosphate material prepared by interface strengthening and its application. Background Technology

[0002] Lithium iron phosphate (LiFePO4), as a cathode material for lithium-ion batteries, has become one of the core materials in the fields of power batteries and energy storage due to its advantages such as high safety, long cycle life, environmental friendliness, and low cost. However, its inherent low electronic conductivity and slow lithium-ion diffusion rate limit the improvement of 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, the electronic conductivity of lithium iron phosphate materials can be significantly improved, the agglomeration of lithium iron phosphate particles can be suppressed, and its structural stability can be enhanced.

[0003] However, current carbon coating technology struggles to achieve both a high specific surface area carbon coating layer and good electrochemical performance in carbon-coated lithium iron phosphate cathode materials.

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

[0005] The purpose of this 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-mentioned technical problems.

[0006] This invention can be implemented as follows: 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 materials wt The carbon content of homogeneous lithium iron phosphate materials ranges from 1.0 wt% to 2.5 wt%, and the half-width (W) of the carbon content curve measured by a high-frequency combustion infrared sulfur-carbon detector is 3.5 s to 6.5 s. The half-width (W) of the carbon content curve per unit carbon content in the homogeneous lithium iron phosphate material is... The value of W ranges from 140s to 600s; After 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%~75wt% of the total mass of carbon-coated lithium iron phosphate cathode material.

[0007] In an optional implementation, the relative standard deviation of the carbon coating structure of the homogeneous lithium iron phosphate material ×100%, where R ranges from 0.1 to 3.0; n represents the number of local locations taken during the Raman test of the homogeneous lithium iron phosphate material to be tested. X i= ; I G and I D These represent the peak intensities of the ordered carbon G bond and the disordered carbon D bond in the Raman spectrum at the i-th local position, respectively. Measured for all local locations X i The average value.

[0008] In an optional embodiment, the homogeneous lithium iron phosphate material further includes at least one of the following features: Feature 1: C wt It ranges from 1.0 wt% to 1.5 wt%. 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 a method for preparing a homogeneous lithium iron phosphate material with interface reinforcement as described in any of the foregoing 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 lithium iron phosphate matrix includes: hydrothermal reaction of lithium source, A source, phosphorus source, iron source and E source according to the general formula of lithium iron phosphate matrix.

[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: The source of 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, it includes at least one of ferrous sulfate, ferrous chloride, and ferrous acetate; Feature 9: The source of E is a water-soluble salt of element E; Feature 10: In 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 hydrothermal reaction temperature is 140~180℃; 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: The lithium iron phosphate matrix is ​​added to the composite carbon source solution and homogenized by ultrasonication to obtain the 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 to 4h.

[0014] Feature 17: Drying is carried out using a fluidized bed drying method; optionally, the inlet air temperature is 180℃~220℃.

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

[0016] In an optional embodiment, the preparation of the composite carbon source solution includes mixing a hydrophobic carbon source, an amphiphilic block copolymer, and a nonpolar 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 includes at least one of polycaprolactone-polyethylene glycol, polystyrene-polyethylene oxide, and polyethylene glycol-polypropylene glycol-polyethylene glycol; Feature 21: Non-polar organic solvents include 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 nonpolar organic solvent is 60℃~80℃; Feature 24: The mixing time of the hydrophobic carbon source, the amphiphilic block copolymer and the nonpolar organic solvent is 1h~2h.

[0018] Thirdly, the present invention provides a battery containing homogeneous lithium iron phosphate material according to any of the foregoing embodiments.

[0019] The beneficial effects of this invention include: The present invention provides a solution that satisfies the above-mentioned C wt A homogeneous lithium iron phosphate material with a content of 1wt%~2.5wt%, w of 3.5s~6.5s, W of 140s~600s, and m of 30wt%~75wt% has a carbon coating layer with a large specific surface area. This carbon coating layer is uniformly applied to the surface of the lithium iron phosphate cathode material, resulting in good conductivity. Batteries further prepared from the above homogeneous lithium iron phosphate material can exhibit both excellent conductivity and rate performance. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[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 the Raman spectrum of the carbon-coated lithium iron phosphate cathode material prepared in Example 1 of the present invention. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0023] The following is a detailed description of the homogeneous lithium iron phosphate material prepared by interface strengthening provided by the present invention and its applications.

[0024] The relatively low conductivity of lithium iron phosphate (LFP) cathode materials leads to slow electron and lithium-ion transport during charging and discharging, thus affecting the battery's charge / discharge rate. A common approach to mitigate this is to incorporate a carbon source during LFP cathode material preparation, forming a carbon coating layer on the surface. Carbon coating creates a conductive network, promoting electron and lithium-ion transport. However, conventional carbon-coated LFP materials are prone to carbon buildup, resulting in less than ideal coating performance. Therefore, improving the quality of carbon coating to optimize its effect and ultimately improve the material's conductivity is essential.

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

[0026] The carbon content (C) in this homogeneous lithium iron phosphate material is... wt The concentration is 1.0wt% to 2.5wt%, such as 1.0wt%, 1.002wt%, 1.355wt%, 1.369wt%, 1.387wt%, 1.393wt%, 1.425wt%, 1.454wt%, 1.5wt%, 2.0wt%, or 2.5wt%, etc., or other values ​​within the range of 1.0wt% to 2.5wt%. In some optional embodiments, C wt The carbon content is 1.0wt%~1.5wt%. wt It can be directly measured by a high-frequency combustion infrared sulfur and carbon detector. Among them, the carbon content (C) wt The conductivity of carbon-coated lithium iron phosphate cathode materials with a carbon content of 1.0wt% to 1.5wt% is significantly improved. However, the improvement in conductivity becomes somewhat limited beyond this range.

[0027] The half-maximum width (WHM) of the carbon content curve of homogeneous lithium iron phosphate material measured by a high-frequency combustion infrared sulfur-carbon detector is 3.5s to 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, or other values ​​within the range of 3.5s to 6.5s. In some optional embodiments, WHM is 4.8s to 6.0s. Homogeneous lithium iron phosphate material with WHM of 4.8s to 6.0s exhibits better carbon coating quality, resulting in better conductivity of the carbon-coated lithium iron phosphate cathode material.

[0028] The half-width at half maximum (WHM) of a unit carbon content in the carbon content curve of homogeneous lithium iron phosphate materials The value of W ranges from 140s to 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, or other values ​​within the range of 140s to 600s.

[0029] In the carbon content curve measured by a high-frequency combustion infrared sulfur-carbon detector, under normal conditions and with a constant temperature, the carbon content curve presents a downward-opening parabolic shape with only one peak. The peak width of the carbon content curve reflects the time required for complete carbon combustion. In this scheme, the half-peak width w is taken as the time required to describe carbon combustion, and the half-peak width w is used in conjunction with the carbon content C. wt The ratio of W to σ is used to characterize the half-width (W) of the carbon content per unit of carbon in the carbon content curve. A smaller W corresponds to a smaller W value, meaning a shorter combustion time per unit of carbon content. This indicates a larger contact area between the carbon coating and oxygen during combustion, i.e., a larger specific surface area of ​​the carbon coating. The specific surface area of ​​the carbon coating can reflect the degree of coverage to a certain extent. A larger specific surface area indicates more uniform coating, resulting in a denser and more continuous conductive network. This enhances the electron transport capacity between lithium iron phosphate particles, leading to more uniform overall conductivity of the lithium iron phosphate cathode material.

[0030] After homogeneous lithium iron phosphate material is burned using a high-frequency combustion infrared sulfur-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 cathode material. Examples of oxidized values ​​include 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%, 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 cathode material.

[0031] Ideally, the carbon coating layer should be dense and uniform to effectively isolate oxygen from direct contact with the lithium iron phosphate cathode material and improve its conductivity. A high-frequency combustion infrared sulfur-carbon detector is used to burn the lithium iron phosphate cathode material at high temperature for a short time (e.g., 30 seconds). If the carbon coating layer on the surface of the lithium iron phosphate cathode material has a good coating effect (e.g., complete and uniform coating), the oxidation degree of the carbon-coated lithium iron phosphate cathode material is small after a short period of high-temperature combustion; conversely, the oxidation degree is large. Therefore, the amount of oxidation, *m*, can reflect the degree of carbon coating layer coverage on the surface of the lithium iron phosphate cathode material to a certain extent. In this invention, *m* represents 38wt%~75wt% of the total mass of the homogeneous lithium iron phosphate material, indicating a good carbon coating effect.

[0032] In some alternative implementations, the relative standard deviation of the carbon coating structure of the homogeneous lithium iron phosphate material ×100%, where 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, or other values ​​within the range of 0.1 to 3.0. In some optional embodiments, R is 0.5 to 2.8. Homogeneous lithium iron phosphate materials with R of 0.5 to 2.8 exhibit a more uniform carbon coating structure, resulting in more uniform conductivity at various locations in the carbon-coated lithium iron phosphate cathode material.

[0033] in, n represents the number of local locations taken during the Raman test of the homogeneous lithium iron phosphate material to be tested. X i = ; I G and I DThese represent the peak intensities of the ordered carbon G bond and the disordered carbon D bond in the Raman spectrum at the i-th local position, respectively. Measured for all local locations X i The average value.

[0034] In some alternative implementations, It can be 0.45 to 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 to 1.65.

[0035] The carbon coating layer of lithium iron phosphate (LFP) cathode materials is typically composed of both ordered and disordered carbon. These two phases work together to enhance the electrochemical performance of the LFP cathode material. Disordered carbon, due to its disordered structure and abundant porosity, promotes rapid lithium-ion diffusion and enhances electrolyte wettability; while ordered carbon, with its highly ordered layered structure, provides efficient electron transport channels, significantly reducing interfacial resistance. This combination of ordered and disordered carbon achieves an optimized balance between conductivity and ion diffusion kinetics in the LFP cathode material, thus benefiting the rate performance and cycle stability of downstream battery products. Furthermore, by controlling the manufacturing process, the two carbon phases can be uniformly distributed on the surface of the LFP cathode material, avoiding localized conductivity or uneven ion pathways. This uniform coating structure not only ensures the active material fully participates in the electrochemical reaction but also effectively suppresses particle agglomeration and electrode polarization, ultimately resulting in higher capacity retention and superior long-cycle performance. Conversely, if the carbon coating structure is uneven, it will lead to uneven overall conductivity of the lithium iron phosphate cathode material, which will hinder local electron transport, reduce the overall conductivity, and affect the battery capacity.

[0036] This invention characterizes the carbon coating layer by using the ratio of the Raman coefficients of ordered carbon to disordered carbon. Raman spectroscopy is performed on n local locations within the carbon-coated lithium iron phosphate cathode material sample to be tested, yielding the results for different local locations. I D and I G The ratio of the two values ​​is used to characterize the uniformity of the carbon coating structure on the surface of the homogeneous lithium iron phosphate material by calculating the relative standard deviation R. The smaller the R value, the more uniform the carbon coating structure, which means that the conductivity of the carbon-coated lithium iron phosphate cathode material is more uniform and the electrochemical performance is better.

[0037] Continuing on the above, the homogeneous lithium iron phosphate material provided by the present invention, which satisfies the above-mentioned W range, m as a percentage of the total mass 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 cathode 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 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. Furthermore, it is not excluded that A and E can be other elements.

[0039] Accordingly, the present invention also provides a method for preparing the interface enhancement 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 this invention, the lithium iron phosphate matrix can be prepared using some conventional methods.

[0041] In some alternative embodiments, the general formula of the lithium iron phosphate matrix is ​​Li as described above. 1-x A x Fe 1-y E y Taking PO4 as an example, the preparation of this lithium iron phosphate matrix may include: hydrothermal reaction of lithium source, A source, phosphorus source, iron source and E source according to the general formula of lithium iron phosphate matrix.

[0042] The lithium source may, by example, include lithium hydroxide. Source A 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 phosphates. The iron source is a water-soluble ferrous salt, such as at least one of ferrous sulfate, ferrous chloride, and ferrous acetate. The E source is a water-soluble salt of element E, such as a hydrochloride, sulfate, or nitrate of element E.

[0043] In some alternative implementations, the 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 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, 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 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 the 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 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 alternative 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, which may include LiOH or ammonia.

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

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

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

[0050] It should be noted that existing carbon coating processes often use traditional carbon sources (such as glucose and sucrose) to form the carbon coating layer through high-temperature pyrolysis. This method typically suffers from a low specific surface area of ​​the carbon coating layer, resulting in uneven distribution and an incomplete interface with the active material, making it difficult to form an efficient three-dimensional conductive network. This limitation obstructs charge transport paths, and the electron migration rate between particles cannot meet the demands of high power density. Especially in thick electrode or fast charge / discharge scenarios, this can significantly increase the internal resistance of the lithium iron phosphate cathode material, affecting the overall battery performance. Furthermore, the low specific surface area of ​​the carbon coating layer leads to insufficient wettability of the electrolyte, further restricting the diffusion kinetics of lithium ions at the solid-liquid interface.

[0051] While some technologies have attempted to improve coating effects by introducing highly conductive carbon-based materials such as graphene and carbon nanotubes, these technologies are often difficult to scale up due to complex processes, high costs, or weak interfacial bonding between the carbon layer and the substrate. In particular, increasing the thickness of the carbon coating to improve conductivity often leads to a decrease in the tap density and volumetric energy density of the lithium iron phosphate cathode material. Therefore, a balance needs to be struck between constructing a high specific surface area carbon coating and ensuring the overall electrochemical performance of the carbon-coated lithium iron phosphate cathode material. Furthermore, conventional high-temperature carbonization processes can easily lead to excessive graphitization of the carbon coating, reducing surface functional groups and weakening compatibility with the electrolyte. Simultaneously, it may trigger heterogeneous reduction of iron and phosphorus elements on the surface of the lithium iron phosphate cathode material particles, disrupting the stability of the material's crystal structure.

[0052] Based on this, the present invention provides a novel 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 alternative embodiments, the preparation of the above-mentioned composite carbon source solution may include mixing a hydrophobic carbon source, an amphiphilic block copolymer, and a nonpolar 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 nonpolar 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 more than 15% of the mass of the lithium iron phosphate matrix, it will lead to excessive aggregation of micelles, affecting the uniformity of coating.

[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 more than 10% of the mass of the hydrophobic carbon source, the excess micelles will form empty shell structures, affecting the performance of carbon-coated lithium iron phosphate cathode materials.

[0059] The amount of nonpolar organic solvent can be excessive to completely dissolve the hydrophobic carbon source and the amphiphilic block copolymer. Alternatively, the amount of nonpolar organic solvent can be moderate, as long as it is sufficient to completely dissolve the hydrophobic carbon source and the amphiphilic block copolymer.

[0060] The mixing temperature of the above-mentioned hydrophobic carbon source, amphiphilic block copolymer and nonpolar organic solvent can be 60℃~80℃, such as 60℃, 65℃, 70℃, 75℃ or 80℃, or other values ​​within the range of 60℃~80℃.

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

[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 homogenizing it using an ultrasonic homogenizer to ensure that the carbon source is uniformly condensed on the surface of the lithium iron phosphate matrix. The solid-liquid ratio of the lithium iron phosphate matrix to the composite carbon source solution can be from 1 g:0.8 mL to 1 g:1.5 mL. The ultrasonication time can be 2 h to 4 h, such as 2 h, 3 h, or 4 h, or other values ​​within the range of 2 h to 4 h.

[0063] Continuing from the above, this invention employs a hydrophobic carbon source and an amphiphilic block copolymer to jointly prepare a carbon-coated lithium iron phosphate cathode material. The amphiphilic block copolymer combines both hydrophilic and hydrophobic properties. By controlling the reaction conditions, the hydrophobic carbon source with the added amphiphilic block copolymer is directionally and uniformly adsorbed on the surface of the lithium iron phosphate matrix. Specifically, under homogeneous ultrasonic treatment, the hydrophilic end of the amphiphilic block copolymer connects to the lithium iron phosphate matrix, while the hydrophobic end combines with the hydrophobic carbon source, acting as a directional anchoring agent. This results in a uniform distribution of the amphiphilic block copolymer and the hydrophobic carbon source on the lithium iron phosphate matrix. Following the carbonization process described below, a coating layer with high coverage and uniform coverage is formed on the surface of the lithium iron phosphate matrix.

[0064] In some optional embodiments, a fluidized bed drying method can be used to dry the composite. The inlet air temperature can be 180℃~220℃, such as 180℃, 190℃, 200℃, 210℃, or 220℃, or other values ​​within the range of 180℃~220℃.

[0065] In some alternative embodiments, carbonization can be carried out in a tube furnace, specifically including: low-temperature carbonization for 2h to 3h (e.g., 2h, 2.5h, or 3h) at 400°C to 500°C (e.g., 400°C, 450°C, or 500°C), followed by high-temperature carbonization for 0.5h to 1h (e.g., 0.5h, 0.8h, or 1h) at 700°C to 900°C (e.g., 700°C, 750°C, 800°C, 850°C, or 900°C), and then natural cooling.

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

[0067] In some alternative embodiments, 4% to 6% hydrogen gas is introduced during the high-temperature carbonization process by volume. Introducing hydrogen gas neutralizes the oxygen-containing gases 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 hydrophobic carbon sources. During pyrolysis, these sources release 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 lithium iron phosphate substrate, or result in insufficient carbon coating density, low graphitization, and poor conductivity. By adding hydrogen during the high-temperature carbonization process, hydrogen can react with some of the oxygen-containing gases produced by pyrolysis, more effectively removing oxygen and promoting the deoxygenation reaction. Continuing from the above, this invention prepares a carbon-free lithium iron phosphate matrix via a hydrothermal method for subsequent carbon coating. In the preparation of the homogeneous lithium iron phosphate material, a hydrophobic carbon source and an amphiphilic block copolymer are dissolved in a non-polar organic solvent (such as tetrahydrofuran) to obtain a composite carbon source solution. The lithium iron phosphate matrix is ​​then added to the composite carbon source solution and dispersed by homogeneous ultrasonic stirring. The hydrophilic blocks in the amphiphilic block copolymer aggregate to form a polar core (reverse micelles), while the hydrophobic blocks dissolve in the organic solvent to form a micelle shell. The surface of the lithium iron phosphate matrix typically contains hydroxyl groups or uncoordinated phosphate groups (hydrophilic), making it difficult to disperse in non-polar organic solvents. The polar core formed by the hydrophilic blocks adsorbs the lithium iron phosphate matrix through hydrogen bonding, electrostatic interactions, or coordination, encapsulating it within the polar core and preventing aggregation. The polar core formed by the hydrophilic blocks encapsulates the lithium iron phosphate matrix, while the hydrophobic blocks extend into the nonpolar solvent, directly improving its dispersibility and acting as a surfactant to some extent. Furthermore, the nonpolar surface of the hydrophobic carbon source allows it to bind tightly with the hydrophobic blocks in the amphiphilic block copolymer via van der Waals forces, dispersing in the nonpolar organic solvent. The hydrophobic carbon source embeds itself in 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 binds the hydrophobic carbon source (shell or dispersed phase) through reverse micelles, forming a "core-shell" or "embedded" structure, ultimately resulting in a composite structure of "lithium iron phosphate matrix @ micelle core + carbon source @ micelle shell". Using a fluidized bed for drying, with the inlet air temperature controlled at 180℃~220℃, is sufficient to evaporate the nonpolar organic solvent tetrahydrofuran. Subsequent stepwise carbonization treatment can pyrolyze and carbonize the amphiphilic block copolymer, increasing the proportion of ordered carbon, further optimizing the structure of the carbon coating layer, and finally forming a homogeneous lithium iron phosphate material with a large coverage and uniform carbon coating layer structure.

[0069] The method provided by this invention can precisely control the microstructure of the carbon coating layer and significantly improve its specific surface area, thereby optimizing the interfacial charge transport efficiency and enhancing the connectivity of the conductive network. Specifically, the method provided by this invention can obtain C wt It is a homogeneous lithium iron phosphate material with a content of 1wt%~2.5wt%, w of 3.5s~6.5s, W of 140s~600s, and m of 30wt%~75wt%.

[0070] In summary, the homogeneous lithium iron phosphate material prepared by interface enhancement in this application refers to the homogeneous lithium iron phosphate material provided by this invention that can be prepared using the interface enhancement preparation method of this invention. However, those skilled in the art will understand that the homogeneous lithium iron phosphate material of this invention is not limited to the preparation method of this invention; the preparation method of this invention is merely an example. Interface enhancement in this invention refers to the adsorption of hydrophilic blocks in the amphiphilic block copolymer of the composite carbon source onto the surface of the lithium iron phosphate matrix, while the hydrophobic blocks combine with the hydrophobic carbon source. Subsequent carbonization treatment can enhance the interfacial bonding between the lithium iron phosphate matrix and the carbon layer, ultimately resulting in 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 whose positive electrode material includes the above-mentioned homogeneous lithium iron phosphate material.

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

[0073] The present invention also provides a battery comprising the above-described battery cells. This battery can possess superior conductivity, rate performance, and low-temperature performance.

[0074] The present invention also provides an electrical device comprising the aforementioned battery cell and / or battery. As examples, the electrical device may include, but is not limited to, mobile phones, tablets, laptops, electric toys, power tools, electric vehicles, electric cars, ships, spacecraft, etc. Electric toys may include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc., while spacecraft may include airplanes, rockets, space shuttles, and spacecraft, etc.

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

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

[0077] 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. Phosphoric acid was added next, and the autoclave was purged of the dead volume with helium. The autoclave was then sealed and stirred. The temperature was raised from room temperature to 50°C, and then the feed valve and exhaust valve were opened. An aqueous solution of ferrous sulfate was added uniformly over 12 min, with a molar ratio of Li:Fe:P = 1.05:0.98:1.0. After stirring for 30 min, a pH adjuster (lithium hydroxide) was added to adjust the pH to 6.0, and stirring continued. The autoclave was then sealed again, and the reaction was carried out at 160°C for 6 h. 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 no sulfate ions were found, yielding a filter cake and mother liquor. The filter cake was vacuum dried at 100°C for 12 h to obtain the lithium iron phosphate matrix.

[0078] S2: Preparation of composite carbon source solution.

[0079] Weigh 13.8g of hydrophobic carbon source (polyvinyl chloride) and 1.2g of amphiphilic block copolymer (polycaprolactone-polyethylene glycol) and dissolve them in excess tetrahydrofuran nonpolar organic solvent. Heat to 80℃ and stir for 2 hours to form a composite carbon source solution.

[0080] S3: Prepare a carbon coating layer.

[0081] S31: Preparation of the complex.

[0082] Weigh 100g 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 1g:1.5mL (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 stir ultrasonically for 3h to obtain the composite.

[0083] S32: Drying.

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

[0085] S33: Carbonization.

[0086] The dried composite was placed in a tube furnace for stepwise carbonization: under an argon atmosphere, the temperature was increased to 500°C at a heating rate of 2°C / min and held for 3 hours; then 5% hydrogen was introduced, and the temperature was increased to 900°C at a heating rate of 10°C / min under a mixed atmosphere and held for 1 hour, followed by natural cooling to obtain carbon-coated lithium iron phosphate cathode 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: Preparation of lithium iron phosphate matrix LiFePO4.

[0088] Lithium hydroxide was dissolved in water to prepare a 3.0 mol / L lithium hydroxide aqueous solution, which was then added to a 1000 mL autoclave. Phosphoric acid was added next, and the autoclave was purged of the dead volume with helium. The autoclave was then sealed and stirred. The temperature was raised from room temperature to 50°C. The feed and exhaust valves were then opened, and an aqueous solution of ferrous acetate was added uniformly over 12 min. The molar ratio of added substances was Li:Fe:P = 1.02:0.96:1.0. After stirring for 30 min, a pH adjuster (lithium hydroxide) was added to adjust the pH to 6.5. Stirring continued, and the autoclave was sealed again. The reaction was carried out at 140°C for 4 h. 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 no acetate ions were found, yielding a filter cake and mother liquor. The filter cake was vacuum dried at 80°C for 12 h to obtain the lithium iron phosphate matrix.

[0089] S2: Preparation of composite carbon source solution.

[0090] Weigh 9.1g of hydrophobic carbon source (polyvinyl chloride) and 0.9g of amphiphilic block copolymer (polycaprolactone-polyethylene glycol) and dissolve them in excess tetrahydrofuran nonpolar organic solvent. Heat to 60℃ and stir for 2 hours to form a composite carbon source solution.

[0091] S3: Prepare a carbon coating layer.

[0092] S31: Preparation of the complex.

[0093] Weigh 100g 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 1g:1mL (that is, the total mass of the hydrophobic carbon source and the amphiphilic block copolymer is 10% of the mass of the lithium iron phosphate matrix). Use an ultrasonic homogenizer to homogenize and stir ultrasonically for 3h to obtain the composite.

[0094] S32: Drying.

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

[0096] S33: Carbonization.

[0097] The dried composite was placed in a tube furnace for stepwise carbonization: under an argon atmosphere, the temperature was increased to 400°C at a heating rate of 2°C / min and held for 2 hours; then 5% hydrogen was introduced, and the temperature was increased to 800°C at a heating rate of 10°C / min under a mixed atmosphere and held for 0.5 hours, followed by natural cooling to obtain carbon-coated lithium iron phosphate cathode 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] 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 phosphoric acid. The autoclave was purged of the dead volume with helium, sealed, and stirred. The temperature was raised from room temperature to 50°C. The feed and exhaust valves were then opened, and an aqueous solution of ferrous acetate was added at a uniform rate over 12 minutes. The amount of iron added was expressed as a molar ratio of (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 continued, and the autoclave was then sealed again. The reaction was carried out at 180°C for 8 hours. After the reaction is complete, the autoclave is rapidly cooled to 80°C with cooling water. The discharge valve is then opened, and the product is filtered and washed until no acetate ions are present, yielding a filter cake and mother liquor. The filter cake is then vacuum dried at 120°C for 12 hours to obtain the lithium iron phosphate matrix.

[0100] S2: Preparation of composite carbon source solution.

[0101] Weigh 7.3g of hydrophobic carbon source (phenolic resin) and 0.7g of amphiphilic block copolymer (polystyrene-polyethylene oxide) and dissolve them in excess tetrahydrofuran nonpolar organic solvent. Heat to 70℃ and stir for 1 hour to form a composite carbon source solution.

[0102] S3: Prepare a carbon coating layer.

[0103] S31: Preparation of the complex.

[0104] Weigh 100g 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 1g:0.8mL (that is, the total mass of the hydrophobic carbon source and the amphiphilic block copolymer is 8% of the mass of the lithium iron phosphate matrix). Use an ultrasonic homogenizer to homogenize and stir ultrasonically for 4h to obtain the composite.

[0105] S32: Drying.

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

[0107] S33: Carbonization.

[0108] The dried composite was placed in a tube furnace for stepwise carbonization: under an argon atmosphere, the temperature was increased to 500°C at a heating rate of 2°C / min and held for 2 hours; then 5% hydrogen was introduced, and the temperature was increased to 700°C at a heating rate of 10°C / min under a mixed atmosphere and held for 0.5 hours, followed by natural cooling to obtain carbon-coated lithium iron phosphate cathode material.

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

[0110] 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. Phosphoric acid was added next, and the autoclave was purged of the dead volume with helium. The autoclave was then sealed and stirred. The temperature was raised from room temperature to 50 °C. The feed and exhaust valves were then opened, and an aqueous solution of ferrous sulfate and manganese carbonate was added uniformly over 12 min (the molar ratio of Fe in the iron source to Mn in the manganese source was 0.987:0.013). The molar ratio of added substances was Li:(Fe+Mn):P = 1.05:0.98:1.0. After stirring for 30 min, a pH adjuster (lithium hydroxide) was added to adjust the pH to 6.0. Stirring continued, and the autoclave was then sealed again. The reaction was carried out at 160 °C for 6 h. After the reaction is complete, the autoclave is rapidly cooled to 80°C with cooling water. The discharge valve is opened, and the product is filtered and washed until no sulfate and carbonate ions are present, resulting in filter cake and mother liquor. The filter cake is then vacuum dried at 100°C for 12 hours to obtain 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 7.52g and the mass of polycaprolactone-polyethylene glycol is 0.48g.

[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 nonpolar organic solvent is 60°C and the mixing time is 1 hour.

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

[0115] Example 8 The difference between this embodiment and Embodiment 1 is that hydrogen gas was not introduced during the high-temperature carbonization process in S33.

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

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

[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, stepwise carbonization is not performed. The carbonization process is carried out directly in an argon atmosphere, with the temperature increased to 900°C at a rate of 2°C / min, held for 3 hours, and then cooled naturally.

[0120] Comparative Example 5 This comparative example provides a carbon-coated lithium iron phosphate cathode material prepared using a conventional carbon coating process, the preparation method of which includes: S1: Weigh 100g of lithium iron phosphate and add it to 2000mL of ethanol / water mixed solvent (volume ratio of ethanol to water is 3:1) to obtain lithium iron phosphate precursor solution. S2: Weigh 10g of glucose and dissolve it in deionized water to form a carbon source solution with a concentration of 10wt%; S3: The carbon source solution is slowly added to the lithium iron phosphate precursor solution, and then placed in a tube furnace. Under an argon atmosphere, the temperature is increased to 900°C at a heating rate of 2°C / min, and held for 5 hours for sintering. After natural cooling, carbon-coated lithium iron phosphate cathode material is obtained.

[0121] Test case The lithium iron phosphate materials provided in Examples 1-8 and Comparative Examples 1-5 were subjected to the following tests: (1) Carbon content test The carbon content (mass percentage) in lithium iron phosphate materials was determined using an HCS-140 high-frequency combustion infrared sulfur and carbon detector from Dekai Instruments Co., Ltd., 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 (WHM) of the carbon content curve was obtained by fitting the carbon content distribution map using Origin software. For example, the WHM of the carbon content curves in Table 1 below were obtained by the following method: baseline subtraction was performed on the carbon content curve using Origin software, followed by Gaussian fitting to obtain the WHM, with a goodness of fit (COD) above 0.9.

[0122] The carbon content curve obtained in Example 1 is shown below. Figure 1 As shown.

[0123] (2) Test of the amount of lithium iron phosphate oxidized Lithium iron phosphate (LFP) material was burned in a high-frequency combustion infrared sulfur-carbon detector with a certain amount of flux added, and the combustion time was 30 seconds. When measuring the amount of LFP oxidation (m) in the LFP material, a blank sample was first set up, i.e., this blank sample only had flux added. After burning for the same duration, it was weighed (m'). Therefore, the amount of LFP oxidation (m) = [m(after combustion) + C] wt +S wt ]-m(before combustion)-m'. Where, m(after combustion) is the mass of lithium iron phosphate material after combustion, m(before combustion) is the mass of lithium iron phosphate material before combustion, and S wt The sulfur content in lithium iron phosphate materials is directly measured using a high-frequency combustion infrared sulfur and carbon detector.

[0124] (3) Raman test Raman spectroscopy of lithium iron phosphate material was performed using a Horiba iHR550 Raman spectrometer (Japan), with a scanning range of 50 cm⁻¹. -1 ~4000cm -1 The excitation wavelength was 532 nm, and the power reaching the sample surface was 0.45 mW. According to... Calculate the relative standard deviation R of the carbon coating structure using ×100%; where... n represents the number of local locations taken during the Raman test of the lithium iron phosphate material to be tested (n=5). X i = ; I G and I D These represent the peak intensities of the ordered carbon G bond and the disordered carbon D bond in the Raman spectrum at the i-th local position, respectively. Measured for all local locations X i The average value.

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

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

[0127] (5) Local Impedance Testing (LEIS) Impedance at multiple local locations of lithium iron phosphate (LFP) material was measured using a Princeton VersaSCAN LEIS microarea scanning electrochemical workstation. An electrode material was prepared by uniformly mixing LFP, acetylene black, and polyvinylidene fluoride (PVDF) in a 90:5:5 mass ratio to serve as the working electrode, with a lithium metal sheet as the counter electrode. Ten local location points were marked on the test surface of the working electrode, spaced 0.1 mm apart, with a frequency range of 10 Hz. -2 Hz~10 5 Hz, with a scan rate of 0.1mV / s to 0.5mV / s.

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

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

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

[0131] Capacity retention rate = ×100%.

[0132] The internal resistance of the assembled button cells was tested using an IT5100 battery internal resistance meter. The positive and negative probes of the internal resistance meter were connected to both ends of the cylindrical battery, and the internal resistance value was obtained by reading the value on the instrument after the voltage stabilized. The internal resistance growth rate was then calculated 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] Referring to Tables 1 and 2, the lithium iron phosphate materials provided in the embodiments of the present invention all simultaneously satisfy C wt With a composition of 1.0wt%~2.5wt%, w of 3.5s~6.5s, m of 30wt%~75wt% of the total mass of the lithium iron phosphate material, W of 140s~600s, and R of 0.1~3.0, the lithium iron phosphate material provided in this embodiment of the invention possesses a high-quality carbon coating layer. This carbon coating layer not only has a large specific surface area but also a uniform carbon layer structure. Consequently, the carbon-coated lithium iron phosphate cathode material has a uniformly distributed conductive network, resulting in a better overall charge transfer impedance (Rct) and electrode resistivity compared to the comparative example. Furthermore, the lithium-ion battery prepared according to the embodiment also exhibits a lower internal resistance growth rate and excellent cycle capacity retention. Moreover, the lithium iron phosphate material provided in this embodiment of the invention has lower charge transfer impedance (Rct) and electrode resistivity, and the corresponding lithium-ion battery exhibits superior capacity retention and cycle stability.

[0138] The results in Table 1 show that adjusting the reaction parameters in each step of the preparation process can regulate the carbon coating layer of the lithium iron phosphate material, thereby controlling W and R. Specifically, in step S2, the different contents of the hydrophobic carbon source and the amphiphilic block copolymer affect the effect of the hydrophobic carbon source covering the lithium iron phosphate matrix surface; on the other hand, by incorporating a certain amount of amphiphilic block copolymer into the hydrophobic carbon source, the uniformity of the carbon coating layer can be controlled.

[0139] As can be seen from Example 1 and Comparative Examples 1-3, the presence or absence of amphiphilic block copolymers, and the amount of amphiphilic block copolymers added, both affect the uniformity of the carbon coating structure. For example, amphiphilic block copolymers can, to a certain extent, "fix" the position of the hydrophobic carbon source on the lithium iron phosphate matrix. If no amphiphilic block copolymer is added or the amount of amphiphilic block copolymer added is small, the "fixation" effect is poor. If too much amphiphilic block copolymer is added, the excess micelles will form empty shell structures, 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, by performing stepwise carbonization (low-temperature slow carbonization in the early stage and high-temperature brief carbonization in the later stage), the carbon source can be better converted into disordered carbon and ordered carbon, achieving the goal of uniform carbon layer structure under the premise of uniform carbon source distribution. Specifically, Comparative Example 4 uses a process of directly heating at a certain rate, and the carbon-coated lithium iron phosphate cathode material prepared by it has poor performance.

[0141] As can be seen from Example 1 and Comparative Example 5, compared with the conventional carbon coating process, the process of adding amphiphilic block polymer and stepwise carbonization in Example 1 can achieve the best carbon coating effect for lithium iron phosphate. It can satisfy both the requirement that the carbon coating layer has a large specific surface area and the requirement that the carbon coating layer has a uniform structure, thereby better improving the conductivity of lithium iron phosphate cathode material.

[0142] In summary, the homogeneous lithium iron phosphate material provided by this invention has a large specific surface area and a uniform carbon coating layer. It also exhibits strong electron transport capabilities between lithium iron phosphate particles and good electrical conductivity. Batteries further prepared from the above-mentioned lithium iron phosphate material can possess both excellent conductivity and rate performance.

[0143] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included 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 covering the lithium iron phosphate matrix; The homogeneous lithium iron phosphate material contains C. wt The carbon content of the homogeneous lithium iron phosphate material is 1.0 wt% to 1.5 wt%, and the half-width (W) of the carbon content curve measured by a high-frequency combustion infrared sulfur-carbon detector is 3.5 s to 6.5 s; the half-width (W) of the carbon content curve per unit carbon content in the homogeneous lithium iron phosphate material is 1.0 wt% to 1.5 wt%. The value of W ranges from 140s to 600s; 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%~75wt% of the total mass of the homogeneous lithium iron phosphate material. The relative standard deviation of the carbon coating structure of the homogeneous lithium iron phosphate material ×100%, where R ranges from 0.1 to 3.0; n represents the number of local locations taken during the Raman test of the homogeneous lithium iron phosphate material to be tested. X i = ; I G and I D These represent the peak intensities of the ordered carbon G bond and the disordered carbon D bond in the Raman spectrum at the i-th local position, respectively. Measured for all local locations X i The average value.

2. The homogeneous lithium iron phosphate material according to claim 1, characterized in that, The homogeneous lithium iron phosphate material also includes at least one of the following characteristics: Feature 1: w is 4.8s~6.0s; Feature 2: R ranges from 0.5 to 2.8; Feature 3: 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.

3. A method for preparing homogeneous lithium iron phosphate material with interface reinforcement as described in claim 1 or 2, characterized in that, Includes the following steps: A carbon coating layer was prepared on the surface of a lithium iron phosphate substrate; The preparation of the carbon coating layer includes: mixing the lithium iron phosphate matrix with a composite carbon source solution to obtain a composite; drying and carbonizing the composite; the preparation of the composite carbon source solution includes: mixing a hydrophobic carbon source, an amphiphilic block copolymer and a nonpolar organic solvent.

4. The interface strengthening preparation method according to claim 3, 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 includes: subjecting a lithium source, an A source, a phosphorus source, an iron source, and an E source to a hydrothermal reaction according to the general formula of the lithium iron phosphate matrix.

5. The interface strengthening preparation method according to claim 4, characterized in that, The preparation of the lithium iron phosphate matrix includes at least one of the following features: Feature 4: The lithium source includes lithium hydroxide; Feature 5: The A source is a water-soluble salt of element A; Feature 6: The phosphorus source includes at least one of phosphoric acid and phosphate; Feature 7: The iron source is a water-soluble ferrous salt; Feature 8: The E source is a water-soluble salt of element E; Feature 9: In 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 10: The molar ratio of Li in the lithium source to A in the A source is (0.9~1):(0~0.1); Feature 11: The molar ratio of Fe in the iron source to E in the E source is (0.9~1):(0~0.1); Feature 12: The pH value of the hydrothermal reaction is 6.0~8.0; Feature 13: The hydrothermal reaction temperature is 140~180℃; Feature 14: The hydrothermal reaction time is 2h~8h.

6. The interface strengthening preparation method according to claim 4, characterized in that, The preparation of the carbon coating layer includes at least one of the following features: Feature 15: The lithium iron phosphate matrix is ​​added to the composite carbon source solution, homogenized and sonicated to obtain the composite; Feature 16: Drying is carried out using a fluidized bed drying method; Feature 17: Carbonization includes: first, low-temperature carbonization at 400℃~500℃ for 2h~3h in a protective atmosphere, and then high-temperature carbonization at 700℃~900℃ for 0.5h~1h.

7. The interface strengthening preparation method according to claim 6, characterized in that, The solid-liquid ratio of the lithium iron phosphate matrix to the composite carbon source solution is from 1g:0.8mL to 1g:1.5mL.

8. The interface strengthening preparation method according to claim 6, characterized in that, The ultrasound session lasted 2 to 4 hours.

9. The interface strengthening preparation method according to claim 6, characterized in that, The air inlet temperature is 180℃~220℃.

10. The interface strengthening preparation method according to claim 6, characterized in that, The temperature is increased to 400℃~500℃ at a heating rate of 1.5℃ / min~2.5℃ / min.

11. The interface strengthening preparation method according to claim 6, characterized in that, The temperature is increased to 700℃~900℃ at a heating rate of 9.5℃ / min~10.5℃ / min.

12. The interface strengthening preparation method according to claim 6, characterized in that, By volume percentage, 4% to 6% of hydrogen gas is introduced during the high-temperature carbonization process.

13. The interface strengthening preparation method according to claim 4, characterized in that, The preparation of the composite carbon source solution includes at least one of the following characteristics: Feature 18: The hydrophobic carbon source includes at least one of polyvinyl chloride and phenolic resin; Feature 19: The amphiphilic block copolymer comprises at least one of polycaprolactone-polyethylene glycol, polystyrene-polyethylene oxide, and polyethylene glycol-polypropylene glycol-polyethylene glycol; Feature 20: The nonpolar organic solvent includes tetrahydrofuran; Feature 21: 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 22: The mixing temperature of the hydrophobic carbon source, the amphiphilic block copolymer and the nonpolar organic solvent is 60℃~80℃; Feature 23: The mixing time of the hydrophobic carbon source, the amphiphilic block copolymer and the nonpolar organic solvent is 1h to 2h.

14. A battery, characterized in that, The battery contains the homogeneous lithium iron phosphate material as described in claim 1 or 2.

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