Lithium iron phosphate cathode material, cathode sheet and its preparation method

Abstract

The application discloses a lithium iron phosphate positive electrode material, a positive electrode sheet and a preparation method thereof. The material comprises a LiFePO4 core with (010) crystal face preferred orientation (orientation degree > 80%) and a gradient carbon coating layer, wherein the core is co-doped with Ti4+ and F- (Ti4+ occupies Li sites, F- substitutes O sites, and the doping amount x = 0.02-0.08), and the gradient carbon coating layer comprises, from inside to outside, microporous carbon, mesoporous carbon and macroporous carbon / graphene composite layers. The material is prepared by an optimized liquid-phase co-precipitation-subsection sintering process, comprising specific proportion carbon source compounding, precise control freezing drying and gradient sintering steps. The material has excellent ion / electron transmission performance, a 0.2C specific capacity of greater than or equal to 155 mAh / g, a 5C capacity retention rate of greater than 95%, excellent low-temperature performance at-40 DEG C, and a 2000-cycle capacity retention rate of greater than 90%. The application further provides a positive electrode sheet and a lithium ion battery comprising the material, and the material is particularly suitable for high-power power batteries and extreme environment applications.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery cathode material technology, and particularly relates to a lithium iron phosphate cathode material, cathode sheet and its preparation method. Background Technology

[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. Lithium iron phosphate (LiFePO4), as one of the most commercially valuable cathode materials, has advantages such as low cost, high safety, and good thermal stability; however, its intrinsic electronic conductivity (~10⁻⁻⁴) is limited. 9 S / cm) and lithium-ion diffusion rate (~10⁻¹) 4 The low speed (cm² / s) results in poor rate performance and low-temperature performance. Currently, the main industrial method for preparing lithium iron phosphate cathode materials is solid-state sintering, which involves mixing an iron source (such as FeC₂O₄), a lithium source (such as Li₂CO₃), and a phosphorus source (such as NH₄H₂PO₄), followed by high-temperature calcination, and then adding sucrose or pitch as a carbon source for coating. However, this method suffers from drawbacks such as coarse and uneven particle size, poor carbon coating quality, and difficulty in doping.

[0003] CN102299327A discloses a method for preparing lithium-aluminum doped carbon-coated lithium iron phosphate cathode material. The method uses LiOH as the lithium source, FeSO4 as the iron source, NH4H2PO4 to introduce phosphate ions, glucose as the coating carbon source, and LiOH and Al(OH)3 to introduce doped lithium-aluminum ions. An ultrasonic dispersion sol-gel method is employed, controlling factors such as pH value, ultrasonic treatment temperature and time, and the order of raw material addition to prepare a Fe-site lithium-aluminum doped lithium iron phosphate precursor. Then, a coating carbon source is added, and a microwave sintering method is used, controlling the microwave power and sintering time to prepare ultrafine powder of lithium-aluminum doped carbon-coated lithium iron phosphate (Li(Al,Li)xFe1-2xPO4 / C) cathode material. However, the carbon layer obtained by this method has a single, non-porous structure, failing to address the ion transport bottleneck, resulting in low capacity retention at 5C rate.

[0004] CN102263247A discloses a method for preparing a high-performance doped LiFePO4 carbon-coated cathode material. The method involves ball milling and mixing an iron source, lithium source, phosphorus source, liquid water-soluble phenolic resin, and dopant elements in a certain proportion in deionized water. The slurry is then spray-dried at appropriate inlet and outlet temperatures and sintered twice under an inert gas atmosphere. The powder obtained from the second sintering is then pulverized to finally obtain the doped LiFePO4 carbon-coated cathode material. However, this method uses Ti4+ as a single element for doping, without synergistic regulation of anion sites. During high-temperature cycling, the amount of Fe dissolved is large, resulting in a high capacity decay rate, making it difficult to simultaneously improve conductivity and structural stability. Summary of the Invention

[0005] To address the problems of existing technologies, such as the inability to simultaneously achieve lithium-ion diffusion and electron conduction, and insufficient structural stability, this invention proposes a lithium iron phosphate cathode material, cathode sheet, and its preparation method, utilizing Ti... 4 The synergistic design of ⁺+F⁻ dual-site doping and gradient porous carbon coating takes into account the coordinated transport of ions and electrons, thus improving battery stability.

[0006] To achieve this objective, the present invention adopts the following technical solution:

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

[0008] The LiFePO4 core has a (010) preferred orientation and a crystal orientation degree > 80%.

[0009] The gradient carbon coating, from the inside out, consists of:

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

[0011] Intermediate 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 >100nm and a pore size of >50nm;

[0013] The material has a secondary particle size (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 this invention achieves synergistic optimization of ion / electron transport through a unique design of the LiFePO4 core with preferred orientation of the (010) crystal plane (orientation degree > 80%) and a gradient carbon coating layer. The gradient carbon coating layer constructs a three-dimensional conductive network from the inside out, consisting of micropores (lithium storage), mesopores (mass transfer), and macropores (electrolyte wetting). The inner nitrogen-doped microporous carbon provides active sites, while the outer graphene enhances electron conduction, enabling the material to simultaneously possess high specific capacity and excellent rate performance, solving the problem that traditional materials cannot simultaneously achieve energy storage and rapid charge / discharge.

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

[0016] Ti 4 ⁺ Occupies the Li site (Li1₋xTixFePO4), F⁻ Substitutes for the O site (LiFePO4₋xFx), 0.02≤x≤0.08, Ti 4 The molar ratio of F⁺ to F⁻ is 1:(0.5-2).

[0017] The lithium iron phosphate cathode material provided by this invention uses Ti 4 ⁺+F⁻ co-doping design, through Ti 4 The presence of Li⁺ at Li sites (Li1₋xTixFePO4) broadens the lithium-ion transport channels, while F⁻ substituting for O sites (LiFePO4₋xFx) stabilizes the crystal structure. The synergistic effect of the two elements increases the diffusion coefficient of Li⁺ to 1.2×10⁻¹¹ cm² / s, which is nearly 100 times higher than that of undoped materials. It also suppresses iron dissolution during high-temperature cycling, keeping iron dissolution within the range of <50ppm, thus achieving a balance between high capacity and long cycle life in battery products.

[0018] Preferably, the material surface has a LiF-TiOx composite passivation layer with a thickness of 2-5 nm, and x=1.5-1.8 in TiOx.

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

[0020] (1) Iron, phosphorus, and lithium sources were dissolved in a molar ratio of Fe:P:Li = 1:1:(1-x), and tetrabutyl titanate and NH4F in a molar ratio of Ti:F = 1:(0.8-1.2) were added. Co-precipitation was carried out in the liquid phase at pH 6.8-7.2 and 50-80℃ to obtain Ti 4 ⁺+F⁻ co-doped precursor;

[0021] (2) The Ti 4 The ⁺+F⁻ co-doped precursor was sequentially mixed with phenolic resin ethanol solution, PS-b-PEO / THF solution and 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℃ for 1.8-2.2h under a nitrogen atmosphere, and then heated to 480-520℃ for 2.8-3.2h for pyrolysis to form microporous and mesoporous structures. Then, it is treated at 720-780℃ for 5.8-6.2h under an argon atmosphere, while CF4 gas is introduced at a flow rate of 10-20mL / min for 0.9-1.1h to complete graphitization and surface passivation.

[0023] The method for preparing lithium iron phosphate cathode material provided by the present invention, on the one hand, controls the preferred orientation of the (010) crystal plane, and through liquid-phase co-precipitation directional growth, makes the (010) crystal plane of >80% of the grains parallel. This crystal plane is the fastest diffusion channel of Li⁺ (b-axis direction), which reduces the intrinsic diffusion barrier from 0.6eV to 0.3eV, so that the material can still release 140mAh / g capacity at 3C high rate, which is about 40% higher than that of 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 doped elements and directional construction of gradient channels are achieved. This process reduces energy consumption by nearly 30% compared with the traditional solid-state method, and the product has a uniform particle size distribution, 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 ≥400m² / g.

[0025] Preferably, in step (3), the heating rate of the carbonization treatment is 5-10℃ / 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 second to third hour after the start of the 720-780℃ heat preservation.

[0027] The method for preparing lithium iron phosphate cathode material provided by this invention utilizes a CF4-assisted surface passivation process. During the sintering stage at 750°C, CF4 gas is introduced to generate a 3-5 nm thick LiF-TiOx composite layer in situ. The LiF phase inhibits electrolyte decomposition, and the TiOx phase stabilizes the CEI film at the interface, reducing the capacity decay rate during high-temperature (60°C) cycling to <5% / 1000 cycles, which is significantly better than conventional carbon-coated materials.

[0028] Thirdly, 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 surface density of 18-22 mg / cm².

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

[0031] The cathode sheet containing lithium iron phosphate cathode material provided by this invention adopts a three-dimensional porous aluminum foil current collector with a porosity of 25-35% to match the cathode gradient pore structure. The etched aluminum foil with a porosity of 25-35% forms a through transport path with the micron-level channels (10-50μm) and the cathode gradient pore structure, reducing the interface impedance and enabling the full cell to have a volume fluctuation of <8% in the temperature range of -40℃ to 60℃, meeting the requirements of extreme environment applications. At the same time, combined with the CNT / SP / PVDF optimized slurry formulation, the electrode interface impedance is reduced, and the areal density can reach 18-22mg / cm² while still maintaining excellent rate performance.

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

[0033] 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 beneficial effects of the present invention are as follows:

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

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

[0038] (3) The preparation method of lithium iron phosphate cathode material and cathode sheet provided by the present invention has strong process compatibility: the liquid phase co-precipitation and segmented sintering process are adapted to existing production lines, significantly reducing energy consumption. Detailed Implementation

[0039] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0040] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0041] In one 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 preferred (010) crystal plane orientation with an orientation degree >80%, and contains Ti. 4 Ti is co-doped with F and F, where Ti4 ⁺ Occupies the Li site (Li1₋xTixFePO4), F⁻ Substitutes for the O site (LiFePO4₋xFx), x is 0.02-0.08, Ti 4 The molar ratio of F⁺ to F⁻ is 1:(0.5-2). The gradient carbon coating layers, from the inside out, are as follows: 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 layer (pore size >50nm) with a thickness >100nm. The secondary particle size D50 of the material is 2-5μm, the total carbon content is 1.5-3.0wt%, and a 2-5nm thick LiF-TiOx composite passivation layer exists on the surface, where x=1.5-1.8.

[0042] In another specific embodiment, the present invention provides a positive electrode sheet comprising the above-mentioned 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 surface density is 18-22 mg / cm², and a 100-200 nm thick Li3PO4-Al2O3 composite coating (Li:Al=1:(0.99-1.01)) is prepared on the surface by atomic layer deposition, wherein the Al2O3 deposition temperature is 145-155℃ (precursor TMA+H2O), and the Li3PO4 deposition temperature is 195-205℃ (precursor LiOtBu+H3PO4).

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

[0044] (1) Dissolve iron, phosphorus and lithium sources 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℃ to obtain the precursor;

[0045] (2) The precursor is 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 then freeze-dried at -50 to -30°C;

[0046] (3) Carbonize at 320-380℃ for 1.8-2.2h under a nitrogen atmosphere by heating at 5-10℃ / min, then pyrolyze at 480-520℃ for 2.8-3.2h to form a microporous / mesoporous structure; switch to an argon atmosphere and treat at 720-780℃ for 5.8-6.2h, during which CF4 gas is introduced at 10-20mL / min for 0.9-1.1h in the 2nd-3rd h. It should be noted that any substitution or variation of conventional data using the process provided in the embodiments of the present invention falls within the protection and disclosure scope of the present invention.

[0047] Example 1

[0048] This embodiment provides a lithium iron phosphate cathode material, comprising a LiFePO4 core and a gradient carbon coating layer. The LiFePO4 core has a preferred (010) crystal plane orientation with an orientation degree of 85%, and contains Ti. 4 Ti is co-doped with F and F, where Ti 4 ⁺ Occupies the Li site (Li0). 97 Ti0. 03 FePO4), F⁻ substituting the O site (LiFePO3). 97 F0. 03 ), Ti 4 The molar ratio of ⁺ to F⁻ is 1:1. The gradient carbon coating layers, from the inside out, consist of: an inner layer of 30 nm thick microporous carbon (1.5 nm pore size), a middle layer of 80 nm thick mesoporous carbon (35 nm pore size), and an outer layer of 150 nm thick macroporous carbon and graphene composite layer (70 nm pore size). The secondary particle size (D50) of the material is 3.5 μm, the total carbon content is 2.2 wt%, and a 3.5 nm thick LiF-TiO1.7 composite passivation layer exists 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 positive electrode material: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 surface density is 20mg / cm², and a 150nm thick Li3PO4-Al2O3 composite coating (Li:Al=1:1) is prepared on the surface by atomic layer deposition.

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

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

[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 then freeze-dried at -40℃;

[0053] (3) Carbonize at 350℃ for 2 hours under nitrogen atmosphere at 8℃ / min, then pyrolyze at 500℃ for 3 hours to form microporous / mesoporous structure; switch to argon atmosphere and treat at 750℃ for 6 hours, during which CF4 gas is introduced at 15mL / min for 1 hour on the 2.5th hour.

[0054] Example 2

[0055] This embodiment provides a lithium iron phosphate cathode material, comprising a LiFePO4 core and a gradient carbon coating layer. The LiFePO4 core has a preferred (010) crystal plane orientation with an orientation degree of 82%, and contains Ti. 4 Ti is co-doped with F and F, where Ti 4 ⁺ Occupies the Li site (Li0). 95 Ti0. 05 FePO4), F⁻ substituting the O site (LiFePO3). 95 F0. 05 ), Ti 4 The molar ratio of ⁺ to F⁻ is 1:1.5. The gradient carbon coating layers, from the inside out, consist of: an inner layer of 20 nm thick microporous carbon (pore size 1.2 nm), a middle layer of 70 nm thick mesoporous carbon (pore size 25 nm), and an outer layer of 120 nm thick macroporous carbon and graphene composite layer (pore size 60 nm). The secondary particle size D50 of the material is 3.0 μm, the total carbon content is 2.0 wt%, and a 3.0 nm thick LiF-TiO1.6 composite passivation layer exists 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 positive electrode material: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 surface density is 19 mg / cm², and a 130 nm thick Li3PO4-Al2O3 composite coating (Li:Al=1:1) is prepared on the surface by atomic layer deposition.

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

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

[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 then freeze-dried at -38℃;

[0060] (3) Carbonize at 340℃ for 2 hours under nitrogen atmosphere at 7℃ / min, then pyrolyze at 490℃ for 3 hours to form microporous / mesoporous structure; switch to argon atmosphere and treat at 730℃ for 6 hours, during which CF4 gas is introduced at 12mL / min for 1 hour in the 2nd and 3rd hours.

[0061] Example 3

[0062] This embodiment provides a lithium iron phosphate cathode material, comprising a LiFePO4 core and a gradient carbon coating layer. The LiFePO4 core has a preferred (010) crystal plane orientation with an orientation degree of 88%, and contains Ti. 4 Ti is co-doped with F and F, where Ti 4 ⁺ Occupies the Li site (Li0). 93 Ti0. 07 FePO4), F⁻ substituting the O site (LiFePO3). 93 F0. 07 ), Ti 4 The molar ratio of ⁺ to F⁻ is 1:1.8. The gradient carbon coating layers, from the inside out, consist of: an inner layer of 40 nm thick microporous carbon (pore size 1.8 nm), a middle layer of 90 nm thick mesoporous carbon (pore size 45 nm), and an outer layer of 160 nm thick macroporous carbon and graphene composite layer (pore size 80 nm). The material has a secondary particle size (D50) of 4.0 μm, a total carbon content of 2.8 wt%, and a 4.5 nm 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 surface density is 21 mg / cm², and a 170 nm thick Li3PO4-Al2O3 composite coating (Li:Al=1:1) is prepared on the surface by atomic layer deposition.

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

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

[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 then freeze-dried at -45℃;

[0067] (3) Carbonize at 370℃ for 2 hours under nitrogen atmosphere at 9℃ / min, then pyrolyze at 510℃ for 3 hours to form microporous / mesoporous structure; switch to argon atmosphere and treat at 770℃ for 6 hours, during which CF4 gas is introduced at 18mL / min for 1 hour on the 2.8th hour.

[0068] Example 4

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

[0070] Example 5

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

[0072] Example 6

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

[0074] Example 7

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

[0076] Example 8

[0077] The only difference between this embodiment and Example 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 Example 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℃ and the pyrolysis temperature is 470℃.

[0082] Example 11

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

[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 only difference between this comparative example and Example 1 is that Ti is not performed. 4 ⁺ and F⁻ co-doped.

[0090] Comparative Example 2

[0091] The only difference between this comparative example and Example 1 is that (010) the crystal 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 was tested for Examples 1-13 and Comparative Examples 1-3 under the following conditions: voltage range: 2.5-4.2V; electrolyte composition: 1M LiPF6 in EC:DMC=1:1; temperature: 25±1℃ (except for low temperature test); charge / discharge regime: 0.2C constant current charge / discharge (1C for cycle test).

[0095] Battery performance is shown in Table 1:

[0096] Table 1

[0097] 0.2C specific capacity (mAh / g) 5C Capacity Retention Rate (%) -40℃ capacity retention (%) 2000-week cycle retention rate (%) Iron leaching amount (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 this invention exhibits significant technical advantages. The performance indicators of Examples 1-13 are all significantly better than those of Comparative Examples 1-3, confirming the superior performance of Ti. 4 The synergistic effect of ⁺+F⁻ co-doping, gradient carbon coating design, and surface passivation treatment is manifested in the following ways:

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

[0100] Comparing Example 1 and Examples 6-7, it can be found that F⁻ / Ti4 The molar ratio can maintain stable performance in the range of 1:0.5-1:2, but exceeding this range will lead to an increase of 30-45% in iron leaching, which verifies the scientific validity of the ratio limit in the claims.

[0101] The process parameter boundary tests in Examples 8-11 show that the material performance is optimal when the sintering temperature is controlled at 720-780℃ and the CF4 flow rate is maintained at 10-20 mL / min. When the parameters approach the claim boundaries (such as carbonization at 310℃ in Example 10), the cycle performance decreases by about 6%, further confirming the requirement for precise control of the process window.

[0102] Comparing Example 1 and Comparative Example 1, it can be seen that Comparative Example 1, which uses traditional single carbon coating, has a 11.3% decrease in 0.2C specific capacity, a 16.0% decrease in 5C rate performance, an 18.0% decrease in cycle stability, and a 167% increase in iron dissolution, which fully demonstrates the necessity of dual-element co-doping and gradient pore structure design.

[0103] Comparing the results of Example 1 and Comparative Examples 2-3, it can be seen that: (010) Comparative Example 2, which has insufficient crystal orientation, has a reduced rate performance of 11%; Comparative Example 3, which has not undergone surface passivation, has an increased iron dissolution of 89%. These data strongly demonstrate the indispensability of the optimization of crystal orientation and surface passivation steps in this invention.

[0104] This invention provides a high-performance lithium iron phosphate cathode material, achieved through the use of Ti 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 battery applications.

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

Claims

1. A lithium iron phosphate cathode material, characterized in that, Comprising: LiFePO4 core, the LiFePO4 core having (010) crystal plane preferred orientation, the degree of crystal plane orientation > 80%; Gradient carbon coating layer, from inside to outside in turn: Inner layer: microporous carbon with thickness of 0-50 nm and pore size < 2 nm; Middle layer: mesoporous carbon with thickness of 50-100 nm and pore size of 2-50 nm; Outer layer: composite layer of macroporous carbon with thickness of > 100 nm and pore size of > 50 nm and graphene; The material secondary particle D50 is 2-5 μm, and the total content of the carbon layer is 1.5-3.0 wt%; Ti is contained in the LiFePO4 core 4+ and F - co-doping, wherein: Ti 4+ occupies Li site Li 1-x Ti x FePO4, F - substitutes O site LiFePO 4-x F x , 0.02≤x≤0.08, Ti 4+ with F - in a molar ratio of 1:(0.5-2); LiF-TiO x Composite passivation layer, thickness 2-5 nm, TiO x x = 1.5-1.

8.

2. A method for preparing the lithium iron phosphate cathode material of claim 1, characterized in that, Comprising the following steps: (1) iron source, phosphorus source, lithium source are dissolved according to the molar ratio of Fe:P:Li=1:1:(1-x), and tetrabutyl titanate and NH4F with the molar ratio of Ti:F=1:(0.8-1.2) are added, and co-precipitation is carried out in liquid phase at pH=6.8-7.2 and 50-80℃, to obtain Ti 4+ +F - co-doped precursor; (2) the Ti 4+ +F - The co-doped precursor was mixed with phenolic resin ethanol solution, PS-b-PEO / THF solution and graphene oxide dispersion solution in sequence, vacuum impregnated, and freeze-dried at -50 to -30°C. (3) The product obtained in step (2) is first subjected to carbonization treatment at 320-380℃ for 1.8-2.2h under a nitrogen atmosphere, then heated to 480-520℃ for 2.8-3.2h pyrolysis treatment to form microporous and mesoporous structures, and then treated at 720-780℃ under an argon atmosphere for 5.8-6.2h, while passing CF4 gas at a flow rate of 10-20mL / min for 0.9-1.1h, completing graphitization and surface passivation, and the CF4 gas is passed during the 2-3h period after 720-780℃ holding.

3. The preparation method according to claim 2, 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 m 2 / g.

4. The production method according to claim 2 or 3, characterized by, In step (3), the heating rate of the carbonization treatment is 5-10℃ / min, and the holding time of the pyrolysis treatment is 2.8-3.2h.

5. A positive electrode sheet comprising the lithium iron phosphate positive electrode material of claim 1, characterized in that: 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: three-dimensional porous aluminum foil with porosity of 25-35%, positive electrode sheet surface density of 18-22 mg / cm 2 ; The positive electrode sheet is coated with a Li3PO4-Al2O3 composite coating on the surface, with a thickness of 100-200nm, Li: Al = 1: (0.99-1.01).

6. The positive electrode sheet according to claim 5, characterized by The composite coating is prepared by atomic layer deposition, wherein: The Al2O3 deposition temperature is 145-155℃, and the precursor is TMA+H2O; The Li3PO4 deposition temperature is 195-205℃, and the precursor is LiOtBu+H3PO4.

7. A lithium ion battery comprising the positive electrode sheet of claim 5 or 6, characterized in that: The battery electrolyte is added with 1-3wt% fluorinated carbonate, and the surface LiF-TiO x layer forms a synergistic passivation effect.