Lithium iron phosphate positive electrode material and preparation method and application thereof

Abstract

The invention discloses a lithium iron phosphate positive electrode material as well as a preparation method and application thereof, and relates to the technical field of lithium ion batteries. The preparation method comprises the following steps: firstly preparing a C-Ni-MOF material, heating the C-Ni-MOF material to reach a molten state, adding a primary lithium iron phosphate material which is not coated with carbon into the C-Ni-MOF in the molten state, and filling gaps of crystal layers of lithium iron phosphate with the C-Ni-MOF under the action force to obtain the lithium iron phosphate LFP-coated C-Ni-MOF which is not coated with carbon, and carrying out high-temperature sintering on the LFP-coated C-Ni-MOF and a carbon source, and carrying out carbon layer coating to obtain the fully-coated LFP-coated C-Ni The LFP-coated C-Ni-MOF obtained by the melting method has high powder pressure, can obtain a battery cell with higher energy density, can improve the liquid absorption rate of a battery positive pole piece to an electrolyte and reduce the internal resistance, and is further beneficial to improving the electronic conductivity and the lithium ion migration rate and improving the rate capability and the cycle performance of the battery cell.

Description

Technical Field

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

[0002] Lithium iron phosphate (LiFePO4) is a cathode material with an olivine-type crystal structure. It exhibits excellent cycle stability and safety performance, making it one of the most widely used cathode materials in commercially available lithium-ion batteries. However, in the LiFePO4 crystal structure, the FeO6 octahedra are connected by shared corners of the bc faces, failing to form a continuous FeO6 octahedral network. Electron conduction can only occur through the Fe-O-Fe pathway, resulting in a relatively low electronic conductivity (approximately 10⁻⁶). -9 S / cm).

[0003] Furthermore, the spatial framework structure formed by the shared edges of FeO6 octahedra and PO4 tetrahedra restricts the variation of lattice volume, thus affecting the Li... + The extraction and insertion during charging and discharging processes lead to the Li + The diffusion rate is low (approximately 10). -16 ~10 -14 cm 2 The low electronic conductivity and lithium-ion migration speed of lithium iron phosphate materials typically prevent them from achieving ideal rate capability and low-temperature performance, which significantly hinders their further commercialization.

[0004] Therefore, there is an urgent need to improve the preparation process of lithium iron phosphate in order to increase the lithium-ion migration rate and improve the rate performance and cycle performance of the battery cells.

[0005] In view of this, the present invention is proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a lithium iron phosphate cathode material, its preparation method, and its application, which aims to significantly improve the rate performance and cycle performance of battery cells.

[0007] This invention is implemented as follows: In a first aspect, the present invention provides a method for preparing a lithium iron phosphate cathode material, comprising: C-Ni-MOF materials were obtained by solvothermal reaction of nickel salt and organic ligand followed by high-temperature carbonization. C-Ni-MOF material is heated to a molten state and then mixed with primary lithium iron phosphate material to obtain modified lithium iron phosphate material; The modified lithium iron phosphate material and carbon source are sintered.

[0008] In an optional embodiment, the preparation process of the modified lithium iron phosphate material includes: calcining the C-Ni-MOF material at 150℃-400℃ for 1h-4h, mixing it with the primary lithium iron phosphate material, and stirring at 150℃-400℃ for 2h-8h.

[0009] In an optional embodiment, the mass ratio of C-Ni-MOF material to primary lithium iron phosphate material is (102-110):100; And / or, the calcination atmosphere of the C-Ni-MOF material is nitrogen or hydrogen.

[0010] In an optional embodiment, the preparation process of C-Ni-MOF material includes: dissolving divalent nickel salt and trimesic acid in a first solvent at a molar ratio of (1.03-1.08):1 to obtain a mixed solution; and using the mixed solution to perform a solvothermal reaction and high-temperature carbonization in sequence; the first solvent is selected from at least one of ethanol, water, dimethylamide and N,N-dimethylformamide; And / or, control the reaction temperature of the solvothermal reaction to be 100℃-200℃ and the reaction time to be 10h-20h; And / or, high-temperature carbonization is carried out in an inert atmosphere, with a high-temperature carbonization temperature of 500℃-800℃ and a high-temperature carbonization time of 3h-15h.

[0011] In an optional embodiment, the preparation process of the carbon source includes: mixing an inorganic carbon source, an organic carbon source, and a second solvent, and adjusting the pH value to 8-10; wherein the mass ratio of the inorganic carbon source, the organic carbon source, and the second solvent is 1:(1-1.5):(50-100); the inorganic carbon source is selected from at least one of graphite, graphene, fullerene, and carbon nanotubes; the organic carbon source is selected from at least one of glucose, sucrose, lignin, urea, glycine, and citric acid; and the second solvent is selected from at least one of ethanol, acetone, isopropanol, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide. And / or, the modified lithium iron phosphate material and the mixed carbon source are mixed at a mass ratio of 100:(5-8) and kept at 700℃-800℃ for 8h-12h; the mixed carbon source includes inorganic carbon source and organic carbon source. And / or, the sintering process with the carbon source is carried out under an inert atmosphere, with the heating rate controlled at 2℃ / min-8℃ / min; And / or, after sintering is complete, crush the material and then pass it through a 100-500 mesh sieve.

[0012] In an optional embodiment, the preparation process of primary lithium iron phosphate material includes: mixing phosphorus source, iron source and lithium source, dispersing them in a third solvent to obtain mixed raw material, spray drying the mixed raw material to obtain spray material, and reacting the spray material at 500℃-760℃ for 10h-15h.

[0013] In an optional embodiment, during the spray drying process, the inlet air temperature is controlled at 200℃-250℃ and the outlet air temperature is controlled at 85℃-95℃. And / or, by adjusting the amounts of phosphorus source, iron source and lithium source, the molar ratio of lithium, iron and phosphorus is controlled to be (1.01-1.05):1:(1.01-1.05). And / or, the phosphorus source is selected from at least one of ammonium dihydrogen phosphate and phosphoric acid; And / or, the iron source is selected from at least one of ferric oxide, iron(II,III) oxide, ferric nitrate, ferrous nitrate, ferric chloride, ferrous chloride, ferric citrate, ferrous sulfate, and ferric hydroxide; And / or, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide and lithium oxide; And / or, the process of preparing the mixed raw materials includes: ball milling and mixing a phosphorus source, an iron source and a lithium source, and then dispersing them in a third solvent to adjust the pH to 8-10, wherein the third solvent is selected from at least one of water and ethanol.

[0014] Secondly, the present invention provides a lithium iron phosphate cathode material, which is prepared by any of the preparation methods described in the foregoing embodiments.

[0015] Thirdly, the present invention provides a positive electrode sheet comprising the lithium iron phosphate positive electrode material of the aforementioned embodiments.

[0016] Fourthly, the present invention provides a lithium battery including the positive electrode sheet of the aforementioned embodiments.

[0017] This invention offers the following advantages: First, C-Ni-MOF material is prepared. By heating the C-Ni-MOF material to a molten state, uncoated primary lithium iron phosphate material is added to the molten C-Ni-MOF. Due to the interaction forces, the C-Ni-MOF is attracted into the interlayer spaces of the lithium iron phosphate crystals, resulting in uncoated lithium iron phosphate LFP@C-Ni-MOF. Then, after high-temperature sintering with a carbon source, a carbon layer is applied to obtain a fully coated LFP@C-Ni-MOF. The LFP@C-Ni-MOF obtained by the molten method exhibits high powder pressure, enabling the production of cells with higher energy density. It also improves the electrolyte absorption rate of the battery's positive electrode, reduces internal resistance, and consequently enhances electronic conductivity and lithium-ion migration rate, thereby improving the rate performance and cycle performance of the battery cell. Attached Figure Description

[0018] 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.

[0019] Figure 1 The XRD pattern of A-LFP@C-NiMOF prepared in Example 1; Figure 2 Comparison diagram of powder-electrode compaction of lithium iron phosphate cathode materials prepared in Example 1 and the comparative example; Figure 3 The graphs show the single-cell cycle performance test results of the lithium iron phosphate cathode materials prepared in Example 1 and the comparative example. Detailed Implementation

[0020] 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.

[0021] This invention provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows: S1. Preparation of primary lithium iron phosphate materials Primary lithium iron phosphate materials do not have a carbon coating layer and can be prepared in-house or obtained commercially.

[0022] In some embodiments, the preparation process of primary lithium iron phosphate material includes: mixing a phosphorus source, an iron source, and a lithium source, dispersing them in a solvent to obtain a mixed raw material, spray drying the mixed raw material to obtain a spray powder, and reacting the spray powder at 500℃-760℃ for 10h-15h. Spray drying is beneficial for obtaining primary lithium iron phosphate material with more uniform particle size, resulting in more uniform powder particle size, better particle sphericity, and higher powder tap density.

[0023] Specifically, the high-temperature reaction temperature of the sprayed material can be 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, 760℃, etc.; the high-temperature reaction time can be 10h, 11h, 12h, 13h, 14h, 15h, etc. The high-temperature reaction can be carried out under a nitrogen atmosphere, but is not limited to this.

[0024] In some embodiments, the molar ratio of lithium, iron, and phosphorus is controlled to be (1.01-1.05):1:(1.01-1.05) by adjusting the amounts of phosphorus source, iron source, and lithium source. For example, it can be 1.01:1:1.01, 1.02:1:1.02, 1.03:1:1.03, 1.04:1:1.04, 1.05:1:1.05, etc. The phosphorus source is selected from at least one of ammonium dihydrogen phosphate and phosphoric acid, and can be any one or more of the above. The iron source is selected from at least one of ferric oxide, magnetite, ferric nitrate, ferrous nitrate, ferric chloride, ferrous chloride, ferric citrate, ferrous sulfate, and ferric hydroxide, and can be any one or more of the above. The lithium source is selected from at least one of lithium carbonate, lithium hydroxide, and lithium oxide, and can be any one or more of the above, with a purity >99%.

[0025] Furthermore, the process of preparing the mixed raw materials includes: ball milling and mixing the phosphorus source, iron source, and lithium source to ensure uniform mixing of the raw materials, thereby achieving fine particle size and uniform composition; then dispersing them in a solvent and adjusting the pH value to 8-10 (e.g., 8.0, 8.5, 9.0, 9.5, 10.0, etc.) to form a homogeneous solution, thus obtaining the first mixture. The solvent is selected from at least one of water and ethanol, and can be any one or more of the above. The reagent used to adjust the pH value can be ammonia water or sodium hydroxide solution, but is not limited to these.

[0026] In some embodiments, during the spray drying process, the inlet air temperature is controlled at 200℃-250℃, such as 200℃, 210℃, 220℃, 230℃, 240℃, 250℃, etc.; the outlet air temperature is controlled at 85℃-95℃, such as 85, 88, 90, 93, 95, etc.

[0027] S2. Preparation of C-Ni-MOF materials Primary Ni-MOFs are obtained by solvothermal reaction of nickel salts and organic ligands. Then, the primary Ni-MOFs are carbonized at high temperature in an inert atmosphere (such as argon) to obtain C-Ni-MOF materials.

[0028] In some embodiments, the nickel salt can be a divalent nickel salt, and the organic ligand can be trimesic acid, but is not limited thereto. The divalent nickel salt and trimesic acid are dissolved in a solvent at a molar ratio of (1.03-1.08):1 to obtain a clear mixed solution. After stirring at a constant temperature, the mixed solution is transferred to a hydrothermal reactor for a solvothermal reaction to obtain a primary Ni-MOF. The primary Ni-MOF is then subjected to high-temperature carbonization in an inert atmosphere (such as argon) to obtain a C-Ni-MOF material. The solvent used is selected from at least one of ethanol, water, dimethylamide, and N,N-dimethylformamide, and any one or more of these solvents can be used.

[0029] Specifically, the molar ratio of divalent nickel salt to trimesic acid can be 1.03:1, 1.04:1, 1.05:1, 1.06:1, 1.07:1, 1.08:1, etc. The type of divalent nickel salt is not limited; it can be nickel chloride tetrahydrate, nickel chloride hexahydrate, etc.

[0030] In some embodiments, the reaction temperature of the solvothermal reaction is controlled to be 100℃-200℃, such as 100℃, 130℃, 150℃, 180℃, 200℃, etc.; the reaction time is 10h-20h, such as 10h, 13h, 15h, 18h, 20h, etc.

[0031] In some embodiments, the high-temperature carbonization temperature is 500℃-800℃, such as 500℃, 550℃, 600℃, 650℃, 700℃, 750℃, 800℃, etc.; the high-temperature carbonization time is 3h-15h, such as 3h, 5h, 8h, 10h, 13h, 15h, etc.

[0032] S3. Preparation of modified lithium iron phosphate materials The C-Ni-MOF material is heated to a molten state and then mixed with primary lithium iron phosphate material. Due to the force, the C-Ni-MOF material can fill the interlayer gaps of lithium iron phosphate crystals to obtain carbon-free lithium iron phosphate LFP@C-Ni-MOF, which is a modified lithium iron phosphate material.

[0033] In some embodiments, the preparation process of modified lithium iron phosphate material includes: calcining C-Ni-MOF material in a nitrogen or hydrogen atmosphere at 150°C-400°C for 1-4 hours, then mixing it with primary lithium iron phosphate material, and stirring at 150°C-400°C for 2-8 hours, so that the molten state of C-Ni-MOF penetrates into the interior of lithium iron phosphate particles to form a core-shell structured composite lithium iron phosphate material.

[0034] Specifically, the calcination temperature can be 150℃, 200℃, 250℃, 300℃, 350℃, 400℃, etc., and the calcination time can be 1h, 2h, 3h, 4h, etc. When mixing with primary lithium iron phosphate materials, the stirring temperature can be controlled at 150℃, 200℃, 250℃, 300℃, 350℃, 400℃, etc., and the stirring time can be 2h, 3h, 4h, 5h, 6h, 7h, 8h, etc.

[0035] In some embodiments, the mass ratio of C-Ni-MOF material to primary lithium iron phosphate material is (102-110):100, such as 102:100, 103:100, 104:100, 105:100, 106:100, 107:100, 108:100, 109:100, 110:100, etc. By adjusting the mass ratio of C-Ni-MOF material to primary lithium iron phosphate material, the introduced nickel content is made more suitable, which is beneficial to further improve electronic conductivity and lithium-ion migration rate, thereby improving the rate performance and cycle performance of the battery cell.

[0036] S4, carbon coating Modified lithium iron phosphate material and carbon source were sintered at high temperature to obtain fully coated LFP@C-Ni-MOF.

[0037] In some embodiments, the preparation process of the carbon source includes: mixing an inorganic carbon source, an organic carbon source, and a solvent to obtain a mixed solution, and adjusting the pH of the mixed solution to 8-10, such as 8.0, 8.5, 9.0, etc. The carbon source is added in solution form, which allows the carbon source to be uniformly dispersed in the precursor during the preparation process, forming a more uniform carbon layer coating, while optimizing the particle morphology (e.g., reducing the particle size to the submicron level and improving sphericity), thereby improving electron transport efficiency.

[0038] Further, the mass ratio of the inorganic carbon source, organic carbon source, and solvent is 1:(1-1.5):(50-100), such as 1:1.0:50, 1:1.1:60, 1:1.2:70, 1:1.3:80, 1:1.4:90, 1:1.5:100, etc. The inorganic carbon source is selected from at least one of graphite, graphene, fullerene, and carbon nanotubes, and can be any one or more of the above; the organic carbon source is selected from at least one of glucose, sucrose, lignin, urea, glycine, and citric acid, and can be any one or more of the above; the solvent is selected from at least one of ethanol, acetone, isopropanol, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide, and can be any one or more of the above.

[0039] Further, the modified lithium iron phosphate material and the mixed carbon source are mixed at a mass ratio of 100:(5-8), the mixed carbon source including inorganic carbon source and organic carbon source, and held at 800℃-1000℃ for 8h-12h, and then naturally cooled to room temperature in an inert atmosphere in the furnace to obtain a carbon-coated, fully coated LFP@C-Ni-MOF. Specifically, the mass ratio of modified lithium iron phosphate material to carbon source can be 100:5, 100:6, 100:7, 100:8, etc., the high-temperature sintering temperature can be 800℃, 850℃, 900℃, 950℃, 1000℃, etc., and the holding time can be 8h, 9h, 10h, 11h, 12h, etc. The sintering process with the carbon source is carried out in an inert atmosphere, and the heating rate is controlled at 2℃ / min-8℃ / min, such as 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, etc.

[0040] S5, Crushing and Screening After sintering, the material is crushed and then passed through a 100-500 mesh sieve to obtain the cathode material lithium iron phosphate.

[0041] In some embodiments, the sintered material is crushed at a speed of 300 rpm to 1000 rpm for 0.5 h to 2 h to refine the particles, and then sieved.

[0042] This invention also provides a lithium iron phosphate cathode material, which is prepared by the preparation method provided in this invention. It has good electronic conductivity and lithium-ion migration rate, which is beneficial to improving the rate performance and cycle performance of the battery cell.

[0043] This invention also provides a positive electrode sheet, comprising the aforementioned lithium iron phosphate positive electrode material. Using the lithium iron phosphate positive electrode material as the positive electrode active material, the positive electrode sheet can be prepared using existing positive electrode sheet preparation methods. Through multi-layer coating of lithium iron phosphate with C-Ni-MOF, the powder compaction of lithium iron phosphate particles is greatly improved, increasing the compaction density of the lithium iron phosphate electrode sheet, which is beneficial to improving electrochemical performance.

[0044] This invention also provides a lithium battery, including the aforementioned positive electrode sheet, and may further include a negative electrode sheet, an electrolyte, a separator, etc. Improvements to the positive electrode active material are beneficial for enhancing the rate performance and cycle performance of the lithium battery.

[0045] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0046] Example 1 This embodiment provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows: (1) Preparation of primary lithium iron phosphate materials Lithium source (lithium carbonate), iron source (ferrous oxalate), and phosphorus source (ammonium dihydrogen phosphate) were weighed according to a stoichiometric ratio (i.e., molar ratio) of lithium:iron:phosphorus source = 1.03:1:1.03. The raw materials were uniformly mixed by ball milling to ensure fine particle size and uniform composition. The raw materials were dispersed in water (with the solid content controlled at 30%), and the pH was adjusted to 8 to form a homogeneous solution, thus obtaining the first mixture. The first mixture was spray-dried at an inlet air temperature of 230°C and an outlet air temperature of 91°C. The resulting sprayed material was reacted at 650°C under a nitrogen atmosphere for 12 hours to obtain primary lithium iron phosphate material.

[0047] (2) Preparation of C-Ni-MOF materials Divalent nickel salt (nickel chloride tetrahydrate) and trimesic acid were dissolved in a mixed solvent at a molar ratio of 1.05:1 to form a clear solution. The mixed solvent was prepared by mixing anhydrous ethanol, deionized water, and dimethylamide in a volume ratio of 1:1.05:100. The concentration of the divalent nickel salt in the clear solution was 1.2 mol / L. After stirring at 200°C for 30 min, the mixed solution was transferred to a hydrothermal reactor for reaction at 150°C for 15 h to obtain primary Ni-MOF. The primary Ni-MOF was then subjected to high-temperature carbonization (650°C) for 3 h in an inert atmosphere (argon) to obtain C-Ni-MOF.

[0048] (3) Preparation of modified lithium iron phosphate materials C-Ni-MOF material was calcined at 230℃ for 2 hours under a nitrogen atmosphere, then mixed with primary lithium iron phosphate material and stirred at 180℃ for 4 hours. The molten C-Ni-MOF material permeated into the lithium iron phosphate particles to form a core-shell structured modified lithium iron phosphate material. The mass ratio of C-Ni-MOF material to primary lithium iron phosphate material was 105:100.

[0049] (4) Carbon coating The modified lithium iron phosphate material from step (3) was mixed with the mixed carbon source at a mass ratio of 94:6 (i.e., 100:6.4) and mixed evenly at a rotation speed of 200 rpm. The temperature was increased to 740℃ at a rate of 5℃ / min and held for 10h. Then, the temperature was allowed to cool naturally to room temperature in an argon furnace to obtain a fully coated LFP@C-Ni-MOF.

[0050] The preparation process of the carbon source includes: mixing an inorganic carbon source, an organic carbon source, and a solvent to obtain a mixture, and adjusting the pH of the mixture to 8.5. The mass ratio of the inorganic carbon source, the organic carbon source, and the solvent is 1:1.2:50. The inorganic carbon source is graphene, the organic carbon source is glucose, and the solvent is ethanol.

[0051] (5) Crushing and screening The above sintered material was crushed at 600 rpm for 1 hour and sieved through a 300-mesh sieve to obtain the cathode material lithium iron phosphate A-LFP@C-Ni-MOF.

[0052] The XRD pattern of the prepared A-LFP@C-Ni-MOF is shown below. Figure 1 As shown.

[0053] Example 2 This embodiment provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows: (1) Preparation of primary lithium iron phosphate materials Lithium source (lithium hydroxide), iron source (ferric nitrate), and phosphorus source (phosphoric acid) were weighed according to a stoichiometric ratio (i.e., molar ratio) of lithium:iron:phosphorus source = 1.01:1:1.01. The raw materials were uniformly mixed by ball milling to ensure fine particle size and uniform composition. The raw materials were dispersed in the solvent ethanol (with the solid content controlled at 30%), and the pH was adjusted to 8 to form a homogeneous solution, thus obtaining the first mixture. The first mixture was spray-dried at an inlet air temperature of 200℃ and an outlet air temperature of 85℃. The resulting sprayed material was reacted at 500℃ under a nitrogen atmosphere for 15 hours to obtain primary lithium iron phosphate material.

[0054] (2) Preparation of C-Ni-MOF materials Divalent nickel salt (nickel chloride tetrahydrate) and trimesic acid were dissolved in a mixed solvent at a molar ratio of 1.03:1 to form a clear solution. The mixed solvent was prepared by mixing anhydrous ethanol, deionized water, and dimethylamide in a volume ratio of 1:1.05:100. The concentration of the divalent nickel salt in the clear solution was 1.2 mol / L. After stirring at 200°C for 30 min, the mixed solution was transferred to a hydrothermal reactor for reaction at 100°C for 20 h to obtain primary Ni-MOF. The primary Ni-MOF was then subjected to high-temperature carbonization (500°C) for 15 h in an inert atmosphere (argon) to obtain C-Ni-MOF.

[0055] (3) Preparation of modified lithium iron phosphate materials After calcining C-Ni-MOF material at 150°C for 4 hours under a nitrogen atmosphere, it was mixed with primary lithium iron phosphate material and stirred at 150°C for 8 hours. The molten state of C-Ni-MOF penetrated into the interior of the lithium iron phosphate particles to form a core-shell structured modified lithium iron phosphate material. The mass ratio of C-Ni-MOF material to primary lithium iron phosphate material was 102:100.

[0056] (4) Carbon coating The modified lithium iron phosphate material and the mixed carbon source in step (3) were mixed at a mass ratio of 100:5 and mixed evenly at a rotation speed of 200 rpm. The temperature was increased to 500℃ at a rate of 2℃ / min and held for 12h. Then, the temperature was naturally cooled to room temperature in an argon furnace to obtain a carbon-coated LFP@C-Ni-MOF with full coating.

[0057] The preparation process of the carbon source includes: mixing an inorganic carbon source, an organic carbon source, and a solvent to obtain a mixture, and adjusting the pH of the mixture to 8. The mass ratio of the inorganic carbon source, the organic carbon source, and the solvent is 1:1:50. The inorganic carbon source is graphene, the organic carbon source is glucose, and the solvent is ethanol.

[0058] (5) Crushing and screening The above sintered material was crushed at 600 rpm for 1 hour and sieved through a 300-mesh sieve to obtain the cathode material lithium iron phosphate A-LFP@C-Ni-MOF.

[0059] Example 3 This embodiment provides a method for preparing lithium iron phosphate cathode material, the steps of which are as follows: (1) Preparation of primary lithium iron phosphate materials Lithium source (lithium carbonate), iron source (ferrous oxalate), and phosphorus source (ammonium dihydrogen phosphate) were weighed according to a stoichiometric ratio (i.e., molar ratio) of lithium:iron:phosphorus source = 1.05:1:1.05. The raw materials were uniformly mixed by ball milling to ensure fine particle size and uniform composition. The raw materials were dispersed in water (with the solid content controlled at 30%), and the pH was adjusted to 10 to form a homogeneous solution, thus obtaining the first mixture. The first mixture was spray-dried at an inlet air temperature of 250°C and an outlet air temperature of 95°C. The resulting sprayed material was reacted at 760°C under a nitrogen atmosphere for 10 hours to obtain primary lithium iron phosphate material.

[0060] (2) Preparation of C-Ni-MOF materials Divalent nickel salt (nickel chloride tetrahydrate) and trimesic acid were dissolved in a mixed solvent at a molar ratio of 1.08:1 to form a clear solution. The mixed solvent was prepared by mixing anhydrous ethanol, deionized water, and dimethylamide in a volume ratio of 1:1.05:100. The concentration of the divalent nickel salt in the clear solution was 1.2 mol / L. After stirring at 200°C for 30 min, the mixed solution was transferred to a hydrothermal reactor for reaction at 200°C for 10 h to obtain primary Ni-MOF. The primary Ni-MOF was then subjected to high-temperature carbonization (800°C) for 3 h in an inert atmosphere (argon) to obtain C-Ni-MOF.

[0061] (3) Preparation of modified lithium iron phosphate materials C-Ni-MOF material was calcined at 400℃ for 1 hour under a nitrogen atmosphere, then mixed with primary lithium iron phosphate material and stirred at 400℃ for 2 hours. The molten C-Ni-MOF material permeated into the lithium iron phosphate particles to form a core-shell structured modified lithium iron phosphate material. The mass ratio of C-Ni-MOF material to primary lithium iron phosphate material was 110:100.

[0062] (4) Carbon coating The modified lithium iron phosphate material and the mixed carbon source in step (3) were mixed at a mass ratio of 100:8 and mixed evenly at a rotation speed of 200 rpm. The temperature was increased to 800℃ at a rate of 8℃ / min and held for 8 hours. Then, the temperature was allowed to cool naturally to room temperature in an argon furnace to obtain a carbon-coated, fully coated LFP@C-Ni-MOF.

[0063] The preparation process of the carbon source includes: mixing an inorganic carbon source, an organic carbon source, and a solvent to obtain a mixture, and adjusting the pH of the mixture to 10. The mass ratio of the inorganic carbon source, the organic carbon source, and the solvent is 1:1.5:100. The inorganic carbon source is graphene, the organic carbon source is glucose, and the solvent is ethanol.

[0064] (5) Crushing and screening The above sintered material was crushed at 600 rpm for 1 hour and sieved through a 300-mesh sieve to obtain the cathode material lithium iron phosphate A-LFP@C-Ni-MOF.

[0065] Example 4 The only difference from Example 1 is that in step (3), “C-Ni-MOF material is calcined at 230°C for 2 hours in a nitrogen atmosphere” is changed to “C-Ni-MOF material is calcined at 230°C for 1 hour in a nitrogen atmosphere”.

[0066] Example 5 The only difference from Example 1 is that in step (3), “C-Ni-MOF material is calcined at 230°C for 2 hours in a nitrogen atmosphere” is changed to “C-Ni-MOF material is calcined at 250°C for 2 hours in a nitrogen atmosphere”.

[0067] Comparative Example 1 This comparative example provides a method for preparing a lithium iron phosphate cathode material, the steps of which are as follows: (1) Preparation of primary lithium iron phosphate material: the steps are the same as in Example 1.

[0068] (2) The primary lithium iron phosphate material was dispersed in ethanol, ultrasonically treated, and then pyromellitic acid and Ni(NO3)26H2O (molar ratio consistent with Example 1) were added. The mixture was stirred, heated to react, cooled to room temperature, centrifuged to separate the product, washed, and the washed product was vacuum dried to obtain B-LFP@C Ni MOF.

[0069] (3) Carbon coating: The steps are the same as step (4) in Example 1, to obtain fully coated B-LFP@C-Ni-MOF.

[0070] (4) Crushing and screening: The steps are the same as step (5) in Example 1, to obtain the positive electrode material lithium iron phosphate B-LFP@C-Ni-MOF.

[0071] Comparative Example 2 In this comparative example, the primary lithium iron phosphate material obtained in step (1) of Example 1 is carbon coated. The coating step is the same as step (4) of Example 1. After the coating is completed, it is crushed and screened (the step is the same as step (5) of Example 1).

[0072] Comparative Example 3 The only difference from Example 1 is that the primary Ni-MOF is not carbonized at high temperature in step (2).

[0073] Comparative Example 4 The only difference from Example 1 is that “C-Ni-MOF material” is replaced with “C-Zn-MOF material”.

[0074] Experimental Example 1 The performance of the cathode materials prepared in the test examples and comparative examples is shown in Table 1.

[0075] Test methods: (1) Positive electrode sheet of LFP@C-Ni-MOF (experimental group): LiFePO4 (Example A-LFP@C-Ni-MOF or Comparative Example B-LFP@C-Ni-MOF), carbon black and PVDF are mixed in a ball mill jar at a mass ratio of 88:2:5:5. N-methylpyrrolidone is used as the dispersion medium. The mixture is stirred and prepared into a positive electrode slurry. The slurry is coated on the positive electrode current collector and dried and rolled to obtain a positive electrode sheet. (2) Graphite, SBR, CMC and carbon black are mixed in a mass ratio of 96:1:1.5:1.5. Water is used as the dispersion medium. The mixture is stirred and prepared into a negative electrode slurry. The slurry is coated on the negative electrode current collector and dried and rolled to obtain a negative electrode sheet. (3) The above positive electrode sheets and negative electrode sheets are combined to form single sheets. The electrical performance of each group of batteries is compared.

[0076] Table 1. Performance comparison of the cathode materials prepared in the examples and comparative examples.

[0077] A comparison of powder compaction and electrode compaction in Example 1 and Comparative Example 1 Figure 2 As shown, the material in Example 1 has a higher compaction density.

[0078] Comparison of cycling performance between Example 1 and Comparative Example 1 Figure 3As shown, the cathode material prepared in Example 1 has superior cycle performance.

[0079] 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 method for preparing a lithium iron phosphate cathode material, characterized in that, include: C-Ni-MOF materials were obtained by solvothermal reaction of nickel salt and organic ligand followed by high-temperature carbonization. The C-Ni-MOF material is heated to a molten state and then mixed with primary lithium iron phosphate material to obtain modified lithium iron phosphate material; The modified lithium iron phosphate material and the carbon source are sintered.

2. The preparation method according to claim 1, characterized in that, The preparation process of the modified lithium iron phosphate material includes: calcining the C-Ni-MOF material at 150℃-400℃ for 1h-4h, mixing it with the primary lithium iron phosphate material, and stirring at 150℃-400℃ for 2h-8h.

3. The preparation method according to claim 2, characterized in that, The mass ratio of the C-Ni-MOF material to the primary lithium iron phosphate material is (102-110):100; And / or, the calcination atmosphere of the C-Ni-MOF material is nitrogen or hydrogen.

4. The preparation method according to claim 1, characterized in that, The preparation process of the C-Ni-MOF material includes: dissolving divalent nickel salt and trimesic acid in a first solvent at a molar ratio of (1.03-1.08):1 to obtain a mixed solution; and using the mixed solution to carry out a solvothermal reaction and high-temperature carbonization in sequence; the first solvent is selected from at least one of ethanol, water, dimethylamide and N,N-dimethylformamide. And / or, control the reaction temperature of the solvothermal reaction to be 100℃-200℃ and the reaction time to be 10h-20h; And / or, high-temperature carbonization is carried out in an inert atmosphere, with a high-temperature carbonization temperature of 500℃-800℃ and a high-temperature carbonization time of 3h-15h.

5. The preparation method according to claim 1, characterized in that, The preparation process of the carbon source includes: mixing an inorganic carbon source, an organic carbon source, and a second solvent, and adjusting the pH value to 8-10; wherein the mass ratio of the inorganic carbon source, the organic carbon source, and the second solvent is 1:(1-1.5):(50-100); the inorganic carbon source is selected from at least one of graphite, graphene, fullerene, and carbon nanotubes; the organic carbon source is selected from at least one of glucose, sucrose, lignin, urea, glycine, and citric acid; the second solvent is selected from at least one of ethanol, acetone, isopropanol, tetrahydrofuran, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide. And / or, the modified lithium iron phosphate material and the mixed carbon source are mixed at a mass ratio of 100:(5-8), and kept at 500℃-800℃ for 8h-12h, wherein the mixed carbon source includes inorganic carbon source and organic carbon source. And / or, the sintering process with the carbon source is carried out under an inert atmosphere, and the heating rate is controlled at 2℃ / min-8℃ / min; And / or, after sintering is complete, crush the material and then pass it through a 100-500 mesh sieve.

6. The preparation method according to claim 1, characterized in that, The preparation process of the primary lithium iron phosphate material includes: mixing a phosphorus source, an iron source and a lithium source, dispersing them in a third solvent to obtain a mixed raw material, spray drying the mixed raw material to obtain a spray material, and reacting the spray material at 500℃-760℃ for 10h-15h.

7. The preparation method according to claim 6, characterized in that, During the spray drying process, the inlet air temperature is controlled at 200℃-250℃ and the outlet air temperature is controlled at 85℃-95℃. And / or, by adjusting the amounts of the phosphorus source, the iron source and the lithium source, the molar ratio of lithium, iron and phosphorus is controlled to be (1.01-1.05):1:(1.01-1.05). And / or, the phosphorus source is selected from at least one of ammonium dihydrogen phosphate and phosphoric acid; And / or, the iron source is selected from at least one of ferric oxide, iron(II,III) oxide, ferric nitrate, ferrous nitrate, ferric chloride, ferrous chloride, ferric citrate, ferrous sulfate, and ferric hydroxide; And / or, the lithium source is selected from at least one of lithium carbonate, lithium hydroxide and lithium oxide; And / or, the process of preparing the mixed raw materials includes: ball milling and mixing a phosphorus source, an iron source and a lithium source, and then dispersing them in a third solvent to adjust the pH value to 8-10, wherein the third solvent is selected from at least one of water and ethanol.

8. A lithium iron phosphate cathode material, characterized in that, It is prepared by the preparation method according to any one of claims 1-7.

9. A positive electrode plate, characterized in that, Including the lithium iron phosphate cathode material as described in claim 8.

10. A lithium battery, characterized in that, Includes the positive electrode sheet as described in claim 9.

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