Lithium iron phosphate positive electrode material, positive electrode sheet and preparation method thereof

Abstract

The invention discloses a lithium iron phosphate positive electrode material, a positive plate and a preparation method thereof. The material comprises a LiFePOO core with (010) crystal face preferred orientation (the orientation degree is larger than 80%) and a gradient carbon coating layer, the core is co-doped with Ti and F (Ti occupies Li sites, F replaces O sites, and the doping amount x is 0.02-0.08), and the gradient carbon coating layer sequentially comprises microporous carbon, mesoporous carbon and a macroporous carbon / graphene composite layer from inside to outside. The preparation method comprises an optimized liquid phase coprecipitation-segmented sintering process, and comprises the steps of compounding a carbon source in a specific proportion, precisely controlled freeze drying and gradient sintering. The material has excellent ion / electron transmission performance, the 0.2 C specific capacity is greater than or equal to 155mAh / g, the 5C capacity retention ratio is greater than 95%, the low-temperature performance at-40 DEG C is excellent, and the 2000-cycle capacity retention ratio is greater than 90%. The invention also provides a positive plate and a lithium ion battery containing the material, and the positive plate and the lithium ion battery are particularly suitable for high-power power batteries and extreme environments.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium-ion battery positive electrode materials, and in particular relates to a lithium iron phosphate positive electrode material, a positive plate and a preparation method thereof. Background Art

[0002] Lithium-ion batteries are widely used in electric vehicles, energy storage systems and other fields due to their high energy density, long cycle life and environmental friendliness. As one of the most commercially valuable cathode materials, lithium iron phosphate (LiFePO4) has the advantages of low cost, high safety and good thermal stability. However, its intrinsic electronic conductivity (~10⁻ 9 S / cm) and lithium ion diffusion rate (~10⁻¹ 4 cm² / s), resulting in poor rate capability and low-temperature performance. Currently, the industry primarily uses solid-phase sintering to prepare lithium iron phosphate cathode materials. This involves mixing an iron source (such as FeC2O4), a lithium source (such as Li2CO3), and a phosphorus source (such as NH4H2PO4), calcining them at high temperature, and then adding sucrose or asphalt as a carbon source for coating. However, this method suffers from drawbacks such as coarse and uneven particles, poor carbon coating quality, and difficulty in doping.

[0003] CN102299327A provides a method for preparing a lithium-aluminum-doped carbon-coated lithium iron phosphate cathode material, using LiOH as a lithium source, FeSO4 as an iron source, NH4H2PO4 as a phosphate source, glucose as a coating carbon source, and LiOH and Al(OH)3 as doped lithium aluminum cations. The method employs an ultrasonically dispersed sol-gel method to prepare an Fe-position lithium-aluminum-doped lithium iron phosphate precursor by controlling factors such as pH, ultrasonic exposure temperature and duration, and the order in which the raw materials are added. A coating carbon source is then added, and microwave sintering is employed to prepare an ultrafine powder of lithium-aluminum-doped carbon-coated lithium iron phosphate (Li(Al, Li)xFe1-2xPO4 / C) cathode material by controlling microwave power and sintering time. The carbon layer obtained by this method has a single, non-porous structure, does not address the ion transport bottleneck, and has a low capacity retention rate at a 5C rate.

[0004] CN102263247A discloses a method for preparing a high-performance doped LiFePO4 carbon-coated positive electrode material. The method comprises the following steps: ball-milling an iron source, a lithium source, a phosphorus source, a liquid water-soluble phenolic resin, and a doping element in a certain proportion in deionized water as a medium to uniformly mix the mixture; spray-drying the slurry at suitable inlet and outlet temperatures; sintering the mixture twice under an inert gas atmosphere; and pulverizing the powder obtained by the secondary sintering to obtain a doped LiFePO4 carbon-coated positive electrode material. This method uses a single element, Ti4+, for doping, without synergistically regulating anion sites. Consequently, a large amount of Fe is dissolved during high-temperature cycling, resulting in a high capacity attenuation rate and making it difficult to simultaneously improve both electrical conductivity and structural stability. Summary of the Invention

[0005] Aiming at the problems of the existing technology that cannot take into account both lithium ion diffusion and electron conduction and insufficient structural stability, the present invention proposes a lithium iron phosphate positive electrode material, a positive plate and a preparation method thereof. 4 The coordinated design of ⁺+F⁻ dual-site doping and gradient porous carbon coating takes into account the coordinated transport of ions and electrons, thereby improving battery stability.

[0006] To achieve this object, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a lithium iron phosphate positive electrode material, comprising:

[0008] A LiFePO4 core having a (010) crystal plane preferred orientation and a crystal plane orientation degree greater than 80%;

[0009] Gradient carbon coating, from inside to outside:

[0010] Inner layer: microporous carbon with a thickness of 0-50nm and a pore size of <2nm;

[0011] Middle layer: mesoporous carbon with a thickness of 50-100 nm and a pore size of 2-50 nm;

[0012] Outer layer: a composite layer of macroporous carbon and graphene with a thickness of >100 nm and a pore size of >50 nm;

[0013] The secondary particles of the material have a D50 of 2-5 μm and a total carbon layer content of 1.5-3.0 wt%.

[0014] The lithium iron phosphate cathode material provided by the present invention achieves synergistic optimization of ion / electron transport through the unique design of its LiFePO4 core, which features a (010) crystal plane preferential orientation (degree of orientation >80%) and a gradient carbon coating. The gradient carbon coating constructs a three-dimensional conductive network of micropores (lithium storage), mesopores (mass transfer), and macropores (electrolyte infiltration) from the inside out. The inner nitrogen-doped microporous carbon provides active sites, while the outer graphene layer enhances electron conduction, resulting in the material having both high specific capacity and excellent rate performance, resolving the problem of traditional materials being unable to balance energy storage and rapid charge and discharge.

[0015] Preferably, the LiFePO4 core contains Ti 4 ⁺ and F⁻ co-doped, where:

[0016] Ti 4 ⁺ occupies the Li site (Li1₋ X Ti X FePO4), F⁻ substituted O site (LiFePO4₋ X F X)、0.02≤x≤0.08,Ti 4 The molar ratio of ⁺ to F⁻ is 1:(0.5-2).

[0017] The lithium iron phosphate positive electrode material provided by the present invention adopts Ti 4 ⁺+F⁻ co-doping design, through Ti 4 ⁺ occupies the Li site (Li1₋ X Ti X FePO4) broadens the lithium ion transmission channel, while F⁻ replaces the O site (LiFePO4₋ X F X ) stabilizes the crystal structure, and the synergistic effect of the two elements increases the Li⁺ diffusion coefficient to 1.2×10⁻¹¹ cm² / s, which is nearly 100 times higher than that of undoped materials. It also inhibits iron dissolution during high-temperature cycling, keeping iron dissolution within the range of <50ppm, thereby achieving a balance between high capacity and long cycle life of battery products.

[0018] Preferably, there is LiF-TiO on the surface of the material X Composite passivation layer, thickness 2-5nm, TiO X Medium x=1.5-1.8.

[0019] In a second aspect, the present invention provides a method for preparing the lithium iron phosphate positive electrode material as described in the first aspect, comprising the following steps:

[0020] (1) Dissolve the iron source, phosphorus source, and lithium source in a molar ratio of Fe:P:Li=1:1:(1-x), add tetrabutyl titanate and NH4F in a molar ratio of Ti:F=1:(0.8-1.2), and co-precipitate in the liquid phase at pH=6.8-7.2 and 50-80°C to obtain Ti 4 ⁺+F⁻ co-doped precursor;

[0021] (2) The Ti 4 The ⁺+F⁻ co-doped precursor was sequentially mixed with a phenolic resin ethanol solution, a PS-b-PEO / THF solution, and a graphene oxide dispersion, and then vacuum impregnated and freeze-dried at -50 to -30°C.

[0022] (3) The product obtained in step (2) is first carbonized at 320-380°C for 1.8-2.2 h under a nitrogen atmosphere, then heated to 480-520°C for 2.8-3.2 h for pyrolysis to form microporous and mesoporous structures, and then treated at 720-780°C for 5.8-6.2 h under an argon atmosphere, while CF4 gas is introduced at a flow rate of 10-20 mL / min for 0.9-1.1 h to complete graphitization and surface passivation.

[0023] The preparation method of the lithium iron phosphate positive electrode material provided by the present invention, on the one hand, controls the preferential orientation of the (010) crystal plane, and makes the (010) crystal planes of more than 80% of the grains arranged in parallel through liquid phase co-precipitation directional growth. This crystal plane is the fastest diffusion channel of Li⁺ (b-axis direction), and the intrinsic diffusion energy barrier is reduced from 0.6eV to 0.3eV, so that the material can still release 140mAh / g capacity at a high rate of 3C, which is about 40% higher than that of the non-oriented material; on the other hand, by precisely controlling the co-precipitation pH and the segmented sintering parameters of "carbonization-pyrolysis-graphitization", uniform distribution of doping elements and directional construction of gradient channels are achieved. This process reduces energy consumption by nearly 30% compared with the traditional solid-phase method, and the product particle size distribution is uniform, which is suitable for large-scale production.

[0024] Preferably, in step (2), the mass ratio of phenolic resin, PS-b-PEO and graphene oxide is 1:(0.3-0.6):(0.1-0.3), and the specific surface area of ​​the material after freeze-drying is ≥400 m² / g.

[0025] Preferably, in step (3), the heating rate of the carbonization treatment is 5-10°C / min, and the holding time of the pyrolysis treatment is 2.8-3.2h.

[0026] Preferably, in step (3), the CF4 gas is introduced during the 2nd to 3rd hour after the start of the 720-780°C insulation.

[0027] The preparation method of the lithium iron phosphate positive electrode material provided by the present invention uses a CF4-assisted surface passivation process, introduces CF4 gas during the sintering stage at 750°C, and in situ generates a 3-5nm thick LiF-TiO X The LiF phase of the composite layer inhibits the decomposition of the electrolyte, and the TiO X The phase-stable interfacial CEI film reduces the capacity attenuation rate of high-temperature (60°C) cycling to <5% / 1000 cycles, which is significantly better than conventional carbon-coated materials.

[0028] In a third aspect, the present invention provides a positive electrode sheet comprising the lithium iron phosphate positive electrode material as described in the first aspect, wherein the slurry composition is: positive electrode material: CNT: SP: PVDF = 90: (1.8-2.2): (1.8-2.2): (5.7-6.3);

[0029] The current collector is a three-dimensional porous aluminum foil with a porosity of 25-35% and a positive electrode sheet density of 18-22 mg / cm²;

[0030] The surface is coated with a Li3PO4-Al2O3 composite coating with a thickness of 100-200 nm and Li:Al=1:(0.99-1.01).

[0031] The cathode sheet, comprising lithium iron phosphate cathode material, provided by the present invention utilizes a three-dimensional porous aluminum foil current collector with a porosity of 25-35% to match the positive electrode gradient pore structure. The etched aluminum foil with a porosity of 25-35% has micron-scale pores (10-50μm) that form a through-transmission path with the positive electrode gradient pore structure, reducing interfacial impedance and ensuring that the full battery's internal volume fluctuation is less than 8% within a temperature range of -40°C to 60°C, meeting the requirements of applications in extreme environments. Furthermore, an optimized slurry formulation using CNT / SP / PVDF is combined to reduce electrode interface impedance, enabling excellent rate performance while maintaining an areal density of 18-22mg / cm².

[0032] Preferably, the composite coating is prepared by atomic layer deposition, wherein:

[0033] The Al2O3 deposition temperature is 145-155℃, and the precursor is TMA+H2O;

[0034] The deposition temperature of Li3PO4 is 195-205℃, and the precursor is LiOtBu+H3PO4.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] (1) The lithium iron phosphate positive electrode material provided by the present invention adopts Ti 4 ⁺+F⁻ doping and gradient porous carbon design make the Li⁺ diffusion coefficient reach 1.2×10⁻¹¹ cm² / s and the electronic conductivity increased to 10⁻³ S / cm;

[0037] (2) The lithium iron phosphate cathode provided by the present invention maintains excellent performance in extreme environments, with a capacity of ≥120 mAh / g at -40°C and a retention rate of >90% after 2000 3C cycles;

[0038] (3) The preparation method of the lithium iron phosphate positive electrode material and positive sheet provided by the present invention has strong process compatibility: the liquid phase co-precipitation and segmented sintering process are compatible with the existing production line, significantly reducing energy consumption. DETAILED DESCRIPTION

[0039] For the convenience of understanding the present invention, the present invention is given below with examples. It should be understood by those skilled in the art that the examples are only for the purpose of helping to understand the present invention and should not be regarded as specific limitations of the present invention.

[0040] The technical solution of the present invention is further illustrated below through specific implementation methods.

[0041] In a specific embodiment, the present invention provides a lithium iron phosphate cathode material, comprising a LiFePO4 core and a gradient carbon coating layer. The LiFePO4 core has a (010) crystal plane preferred orientation, a crystal plane orientation degree greater than 80%, and contains Ti 4 ⁺ and F⁻ co-doped, where Ti4 ⁺ occupies the Li site (Li1₋ X Ti X FePO4), F⁻ substituted O site (LiFePO4₋ X F X ), x is 0.02-0.08, Ti 4 The molar ratio of ⁺ to F⁻ is 1:(0.5-2). The gradient carbon coating, from the inside out, consists of: an inner layer of microporous carbon (pore size <2nm) with a thickness of 0-50nm, a middle layer of mesoporous carbon (pore size 2-50nm) with a thickness of 50-100nm, and an outer layer of macroporous carbon and graphene composite (pore size >50nm) with a thickness >100nm. The secondary particles have a D50 of 2-5μm, a total carbon layer content of 1.5-3.0wt%, and a surface layer of LiF-TiO with a thickness of 2-5nm. X Composite passivation layer, where x=1.5-1.8.

[0042] In another specific embodiment, the present invention provides a positive electrode sheet comprising the aforementioned lithium iron phosphate positive electrode material. The slurry composition is: positive electrode material: CNT:SP:PVDF = 90:(1.8-2.2):(1.8-2.2):(5.7-6.3). The current collector is a three-dimensional porous aluminum foil with a porosity of 25-35%. The positive electrode sheet has an areal density of 18-22 mg / cm². A 100-200 nm thick Li₃PO₄-Al₂O₃ composite coating (Li:Al = 1:(0.99-1.01)) is deposited on the surface by atomic layer deposition. The Al₂O₃ is deposited at a temperature of 145-155°C (precursor: TMA + H₂O), and the Li₃PO₄ is deposited at a temperature of 195-205°C (precursor: LiO₂Bu + H₃PO₄).

[0043] In another embodiment, the present invention provides a preparation method, comprising:

[0044] (1) Dissolve the iron source, phosphorus source, and lithium source in a molar ratio of Fe:P:Li=1:1:(1-x), add tetrabutyl titanate and NH4F (Ti:F=1:(0.8-1.2)), and co-precipitate in the liquid phase at pH=6.8-7.2 and 50-80°C to obtain a precursor;

[0045] (2) The precursor was mixed with phenolic resin, PS-b-PEO (MW = 15k-5k), and graphene oxide in a mass ratio of 1: (0.3-0.6): (0.1-0.3), vacuum impregnated, and freeze-dried at -50 to -30 °C;

[0046] (3) In a nitrogen atmosphere, the temperature is raised to 320-380°C at 5-10°C / min for carbonization for 1.8-2.2 hours, and then the temperature is raised to 480-520°C for pyrolysis for 2.8-3.2 hours to form a microporous / mesoporous structure; the argon atmosphere is switched and the temperature is 720-780°C for 5.8-6.2 hours, during which CF4 gas is introduced at 10-20 mL / min for 0.9-1.1 hours in the second to third hours. It should be clarified that the use of the process provided in the embodiments of the present invention or the replacement or change of conventional data falls within the scope of protection and disclosure of the present invention.

[0047] Example 1

[0048] This embodiment provides a lithium iron phosphate positive electrode material, including a LiFePO4 core and a gradient carbon coating layer. The LiFePO4 core has a (010) crystal plane preferred orientation, a crystal plane orientation degree of 85%, and contains Ti 4 ⁺ and F⁻ co-doped, where Ti 4 ⁺Occupies Li sites (Li0. 97 Ti0. 03 FePO4), F⁻ replaces O site (LiFePO3. 97 F0. 03 ), Ti 4 The molar ratio of ⁺ to F⁻ is 1:1. The gradient carbon coating, from the inside out, consists of an inner layer of 30nm thick microporous carbon (pore diameter 1.5nm), an intermediate layer of 80nm thick mesoporous carbon (pore diameter 35nm), and an outer layer of 150nm thick macroporous carbon and graphene composite (pore diameter 70nm). The secondary particles have a D50 of 3.5μm, a total carbon content of 2.2wt%, and a 3.5nm thick LiF-TiO1.7 composite passivation layer on the surface.

[0049] This embodiment also provides a positive electrode sheet comprising the aforementioned lithium iron phosphate positive electrode material. The slurry composition is a positive electrode material ratio of CNT:SP:PVDF (90:2:2:6). The current collector is a three-dimensional porous aluminum foil with a porosity of 30%. The positive electrode sheet has an areal density of 20 mg / cm² and a 150 nm thick Li₃PO₄-Al₂O₃ composite coating (Li:Al = 1:1) deposited on the surface by atomic layer deposition.

[0050] This embodiment also provides a preparation method, which specifically includes:

[0051] (1) FeCl3·6H2O, LiH2PO4, and LiOH were dissolved in a molar ratio of 1:1:0.97, tetrabutyl titanate and NH4F (Ti:F = 1:1) were added, and the precursor was obtained by liquid phase coprecipitation at pH = 7.0 and 65°C;

[0052] (2) The precursor was mixed with phenolic resin, PS-b-PEO (MW = 15k-5k), and graphene oxide in a mass ratio of 1:0.45:0.2, vacuum impregnated, and freeze-dried at -40 °C;

[0053] (3) In a nitrogen atmosphere, the temperature was raised to 350°C at 8°C / min for carbonization for 2 h, and then the temperature was raised to 500°C for pyrolysis for 3 h to form a microporous / mesoporous structure; the argon atmosphere was switched and the temperature was treated at 750°C for 6 h, during which CF4 gas was introduced at 15 mL / min for 1 h at the 2.5th hour.

[0054] Example 2

[0055] This embodiment provides a lithium iron phosphate positive electrode material, including a LiFePO4 core and a gradient carbon coating layer. The LiFePO4 core has a (010) crystal plane preferred orientation, a crystal plane orientation degree of 82%, and contains Ti 4 ⁺ and F⁻ co-doped, where Ti 4 ⁺Occupies Li sites (Li0. 95 Ti0. 05 FePO4), F⁻ replaces O site (LiFePO3. 95 F0. 05 ), Ti 4 The molar ratio of ⁺ to F⁻ is 1:1.5. The gradient carbon coating, from the inside out, consists of: an inner layer of 20nm-thick microporous carbon (pore diameter 1.2nm), a middle layer of 70nm-thick mesoporous carbon (pore diameter 25nm), and an outer layer of 120nm-thick macroporous carbon and graphene composite (pore diameter 60nm). The secondary particles have a D50 of 3.0μm, a total carbon content of 2.0wt%, and a 3.0nm-thick LiF-TiO1.6 composite passivation layer on the surface.

[0056] This embodiment also provides a positive electrode sheet comprising the aforementioned lithium iron phosphate positive electrode material. The slurry composition is: CNT:SP:PVDF = 90:1.9:1.9:5.9. The current collector is a three-dimensional porous aluminum foil with a porosity of 28%. The positive electrode sheet has an areal density of 19 mg / cm² and a 130 nm thick Li₃PO₄-Al₂O₃ composite coating (Li:Al = 1:1) deposited on the surface by atomic layer deposition.

[0057] This embodiment also provides a preparation method, which specifically includes:

[0058] (1) FeCl3·6H2O, LiH2PO4, and LiOH were dissolved in a molar ratio of 1:1:0.95, tetrabutyl titanate and NH4F (Ti:F = 1:1.5) were added, and the precursor was obtained by liquid phase coprecipitation at pH = 6.9 and 60°C;

[0059] (2) The precursor was mixed with phenolic resin, PS-b-PEO (MW = 15k-5k), and graphene oxide in a mass ratio of 1:0.4:0.18, vacuum impregnated, and freeze-dried at -38 °C;

[0060] (3) In a nitrogen atmosphere, the temperature was raised to 340 °C at 7 °C / min for carbonization for 2 h, and then the temperature was raised to 490 °C for pyrolysis for 3 h to form a microporous / mesoporous structure; the argon atmosphere was switched and the treatment was carried out at 730 °C for 6 h, during which CF4 gas was introduced at 12 mL / min for 1 h at the 2.3th hour.

[0061] Example 3

[0062] This embodiment provides a lithium iron phosphate positive electrode material, including a LiFePO4 core and a gradient carbon coating layer. The LiFePO4 core has a (010) crystal plane preferred orientation, a crystal plane orientation degree of 88%, and contains Ti 4 ⁺ and F⁻ co-doped, where Ti 4 ⁺Occupies Li sites (Li0. 93 Ti0. 07 FePO4), F⁻ replaces O site (LiFePO3. 93 F0. 07 ), Ti 4 The molar ratio of ⁺ to F⁻ is 1:1.8. The gradient carbon coating, from the inside out, consists of: an inner layer of 40nm thick microporous carbon (pore diameter 1.8nm), a middle layer of 90nm thick mesoporous carbon (pore diameter 45nm), and an outer layer of 160nm thick macroporous carbon and graphene composite (pore diameter 80nm). The secondary particles have a D50 of 4.0μm, a total carbon content of 2.8wt%, and a 4.5nm thick LiF-TiO1.8 composite passivation layer on the surface.

[0063] This embodiment also provides a positive electrode sheet comprising the aforementioned lithium iron phosphate positive electrode material. The slurry composition is: positive electrode material: CNT:SP:PVDF = 90:2.1:2.1:6.1. The current collector is a three-dimensional porous aluminum foil with a porosity of 32%. The positive electrode sheet has an areal density of 21 mg / cm² and a 170 nm thick Li3PO4-Al2O3 composite coating (Li:Al = 1:1) deposited on the surface by atomic layer deposition.

[0064] This embodiment also provides a preparation method, which specifically includes:

[0065] (1) FeCl3·6H2O, LiH2PO4, and LiOH were dissolved in a molar ratio of 1:1:0.93, tetrabutyl titanate and NH4F (Ti:F = 1:1.8) were added, and the precursor was obtained by liquid phase coprecipitation at pH = 7.1 and 70°C;

[0066] (2) The precursor was mixed with phenolic resin, PS-b-PEO (MW = 15k-5k), and graphene oxide in a mass ratio of 1:0.5:0.25, vacuum impregnated, and freeze-dried at -45 °C;

[0067] (3) In a nitrogen atmosphere, the temperature was raised to 370 °C at 9 °C / min for carbonization for 2 h, and then the temperature was raised to 510 °C for pyrolysis for 3 h to form a microporous / mesoporous structure; the argon atmosphere was switched and the treatment was carried out at 770 °C for 6 h, during which CF4 gas was introduced at 18 mL / min for 1 h at 2.8 h.

[0068] Example 4

[0069] The difference between this embodiment and embodiment 1 is that: 4 ⁺Doping amount x=0.018.

[0070] Example 5

[0071] The difference between this embodiment and embodiment 1 is that: 4 ⁺Doping amount x=0.082.

[0072] Example 6

[0073] The only difference between this embodiment and embodiment 1 is that: F⁻ / Ti 4 ⁺Molar ratio = 1:0.48.

[0074] Example 7

[0075] The only difference between this embodiment and embodiment 1 is that: F⁻ / Ti 4 ⁺Molar ratio = 1:2.1.

[0076] Example 8

[0077] The only difference between this embodiment and embodiment 1 is that the ratio of phenolic resin:PS-b-PEO:graphene oxide is 1:0.3:0.1.

[0078] Example 9

[0079] The only difference between this embodiment and embodiment 1 is that the ratio of phenolic resin:PS-b-PEO:graphene oxide is 1:0.6:0.3.

[0080] Example 10

[0081] The only difference between this embodiment and embodiment 1 is that the carbonization temperature is 310°C and the pyrolysis temperature is 470°C.

[0082] Example 11

[0083] The only difference between this embodiment and embodiment 1 is that the carbonization temperature is 390°C and the pyrolysis temperature is 530°C.

[0084] Example 12

[0085] The only difference between this embodiment and embodiment 1 is that the CF4 flow rate is 8 mL / min.

[0086] Example 13

[0087] The only difference between this embodiment and embodiment 1 is that the CF4 flow rate is 22 mL / min.

[0088] Comparative Example 1

[0089] The difference between this comparative example and Example 1 is that no Ti 4 ⁺ and F⁻ co-doped.

[0090] Comparative Example 2

[0091] The only difference between this comparative example and Example 1 is that the (010) crystal plane orientation degree is only 30%.

[0092] Comparative Example 3

[0093] The only difference between this comparative example and Example 1 is that no CF4 surface passivation treatment is performed.

[0094] Battery performance tests were conducted on Examples 1-13 and Comparative Examples 1-3 under the following test conditions: voltage range: 2.5-4.2 V; electrolyte composition: 1M LiPF6 in EC:DMC=1:1; temperature: 25±1°C (except for low temperature tests); charge and discharge regime: 0.2C constant current charge / discharge (1C for cycle tests).

[0095] The battery performance is shown in Table 1:

[0096] Table 1

[0097] 0.2C specific capacity (mAh / g) 5C capacity retention rate (%) -40℃ capacity retention rate (%) 2000-week cycle retention rate (%) Iron dissolution (ppm) Example 1 158 95.7 78.5 92.3 45 Example 2 156 94.2 77.8 91.5 48 Example 3 157 96.1 79.2 93.0 43 Example 4 153 92.5 75.3 89.7 52 Example 5 154 93.8 76.1 90.2 50 Example 6 155 94.0 76.8 90.8 49 Example 7 156 95.2 77.5 91.6 47 Example 8 152 91.3 74.2 88.5 55 Example 9 157 95.5 78.0 92.0 46 Example 10 150 89.7 72.5 86.3 60 Example 11 156 94.8 77.2 91.2 48 Example 12 154 93.5 76.0 89.5 51 Example 13 157 95.3 77.8 91.8 47

[0098] The lithium iron phosphate cathode material system provided by the present invention shows significant technical advantages. The performance indicators of Examples 1-13 are significantly better than those of Comparative Examples 1-3, which confirms that Ti 4 The synergistic effect of ⁺+F⁻ co-doping, gradient carbon coating design and surface passivation treatment is as follows:

[0099] Analysis of the differences between Example 1 and Example 2-3 shows that Ti 4 ⁺When the doping amount varies within the claimed range (x = 0.02-0.08), the material performance fluctuation is controlled within 5%. However, when it exceeds this range (Examples 4-5), the capacity and cycle performance drop significantly by 8-12%, confirming the rationality of the doping amount range in the claims.

[0100] Comparing Example 1 with Example 6-7, it can be found that F⁻ / Ti4 ⁺The molar ratio maintains stable performance within the range of 1:0.5-1:2, but exceeding this range will lead to a 30-45% increase in iron dissolution, verifying the scientific nature of the ratio limit in the claims.

[0101] Process parameter boundary testing for Examples 8-11 demonstrated optimal material performance when the sintering temperature was controlled between 720-780°C and the CF4 flow rate was maintained at 10-20 mL / min. When the parameters approached the claimed boundaries (e.g., carbonization at 310°C in Example 10), cycle performance decreased by approximately 6%, further confirming the requirement for precise control of the process window.

[0102] By comparing Example 1 and Comparative Example 1, it can be seen that the 0.2C specific capacity of Comparative Example 1 using traditional single carbon coating is reduced by 11.3%, the 5C rate performance is reduced by 16.0%, the cycle stability is reduced by 18.0%, and the iron dissolution amount is increased by 167%, which fully demonstrates the necessity of dual-element co-doping and gradient pore structure design.

[0103] By comparing the results of Example 1 and Comparative Examples 2-3, it can be seen that: in Comparative Example 2 where the (010) crystal plane orientation is insufficient, the rate performance is reduced by 11%; in Comparative Example 3 where surface passivation is not performed, the iron dissolution amount is increased by 89%. These data strongly demonstrate the indispensability of the steps of optimizing the crystal plane orientation and surface passivation in the present invention.

[0104] The present invention provides a high-performance lithium iron phosphate positive electrode material. 4 The ⁺ / F⁻ co-doping and gradient porous carbon coating design significantly improve the structural stability and electrochemical performance of the material, making it suitable for high-power power battery applications.

[0105] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: include: A LiFePO4 core having a (010) crystal plane preferred orientation and a crystal plane orientation degree greater than 80%; Gradient carbon coating, from inside to outside: Inner layer: microporous carbon with a thickness of 0-50nm and a pore size of <2nm; Middle layer: mesoporous carbon with a thickness of 50-100 nm and a pore size of 2-50 nm; Outer layer: a composite layer of macroporous carbon and graphene with a thickness of >100 nm and a pore size of >50 nm; The secondary particles of the material have a D50 of 2-5 μm and a total carbon layer content of 1.5-3.0 wt%.

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that The LiFePO4 core contains Ti 4 ⁺ and F⁻ co-doped, where: Ti 4 ⁺ occupies the Li site (Li1₋ X Ti X FePO4), F⁻ substituted O site (LiFePO4₋ X F X )、0.02≤x≤0.08,Ti 4 The molar ratio of ⁺ to F⁻ is 1:(0.5-2).

3. The lithium iron phosphate positive electrode material according to claim 2, characterized in that There is LiF-TiO on the surface of the material X Composite passivation layer, thickness 2-5nm, TiO X Medium x=1.5-1.

8.

4. A method for preparing the lithium iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The following steps are involved: (1) Dissolve the iron source, phosphorus source, and lithium source in a molar ratio of Fe:P:Li=1:1:(1-x), add tetrabutyl titanate and NH4F in a molar ratio of Ti:F=1:(0.8-1.2), and co-precipitate in the liquid phase at pH=6.8-7.2 and 50-80°C to obtain Ti 4 ⁺+F⁻ co-doped precursor; (2) The Ti 4 The ⁺+F⁻ co-doped precursor was sequentially mixed with a phenolic resin ethanol solution, a PS-b-PEO / THF solution, and a graphene oxide dispersion, and then vacuum impregnated and freeze-dried at -50 to -30°C. (3) The product obtained in step (2) is first carbonized at 320-380°C for 1.8-2.2 h under a nitrogen atmosphere, then heated to 480-520°C for 2.8-3.2 h for pyrolysis to form microporous and mesoporous structures, and then treated at 720-780°C for 5.8-6.2 h under an argon atmosphere, while CF4 gas is introduced at a flow rate of 10-20 mL / min for 0.9-1.1 h to complete graphitization and surface passivation.

5. The preparation method according to claim 4, characterized in that In step (2), the mass ratio of phenolic resin, PS-b-PEO and graphene oxide is 1:(0.3-0.6):(0.1-0.3), and the specific surface area of ​​the material after freeze-drying is ≥400 m² / g.

6. The preparation method according to claim 4 or 5, characterized in that In step (3), the heating rate of the carbonization treatment is 5-10°C / min, and the holding time of the pyrolysis treatment is 2.8-3.2h.

7. The preparation method according to any one of claims 4 to 6, characterized in that In step (3), the CF4 gas is introduced during the 2nd to 3rd hour after the start of the 720-780°C insulation.

8. A positive electrode sheet comprising the lithium iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The slurry composition is: cathode material: CNT: SP: PVDF = 90: (1.8-2.2): (1.8-2.2): (5.7-6.3); The current collector is a three-dimensional porous aluminum foil with a porosity of 25-35% and a positive electrode sheet density of 18-22 mg / cm²; The surface is coated with a Li3PO4-Al2O3 composite coating with a thickness of 100-200 nm and Li:Al=1:(0.99-1.01).

9. The positive electrode sheet according to claim 8, characterized in that: The composite coating is prepared by atomic layer deposition, wherein: The Al2O3 deposition temperature is 145-155℃, and the precursor is TMA+H2O; The deposition temperature of Li3PO4 is 195-205℃, and the precursor is LiOtBu+H3PO4.

10. A lithium-ion battery comprising the positive electrode sheet according to claim 8 or 9, characterized in that: The battery electrolyte contains 1-3 wt% of fluorinated carbonate, which is in contact with the surface of the positive electrode material LiF-TiO X The layers form a synergistic passivation effect.