Modified lithium iron phosphate material, preparation method thereof, positive electrode sheet and secondary battery

Abstract

The application provides a modified lithium iron phosphate material, a preparation method thereof, a positive electrode sheet and a secondary battery. The preparation method comprises the following steps: grinding and mixing a lithium source, an iron source, a phosphorus source, a niobium source, a fluorine-doped zeolite imidazolate framework material nanosheet, a carbon source and a dispersing agent to obtain a raw material system; drying the raw material system to obtain a precursor material; and performing sintering treatment on the precursor material in an inert atmosphere to form the modified lithium iron phosphate material. The modified lithium iron phosphate material comprises niobium-doped lithium iron phosphate particles and a fluorine-doped zeolite imidazolate framework material layer arranged on at least part of the outer surface of the lithium iron phosphate particles. When the lithium iron phosphate material is applied to the secondary battery as a positive electrode active material, the rate performance of the secondary battery can be improved and the capacity of the secondary battery can be increased.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery materials, and particularly relates to a modified lithium iron phosphate material, a preparation method thereof, a positive electrode sheet and a secondary battery. BACKGROUND

[0002] In recent years, due to the increasingly severe energy and environmental problems, vigorously developing new energy has become the consensus of the whole society. With the rapid development of power batteries, secondary batteries with chargeable and dischargeable characteristics are widely used in electronic devices such as mobile phones, computers and automobiles due to their advantages of fast and efficient energy conversion and storage, high energy density, long service life and small self-discharge.

[0003] Secondary batteries usually include a positive electrode sheet containing a positive electrode active material, and the performance of the positive electrode active material has a significant impact on the performance of the secondary battery. Existing positive electrode active materials mainly include lithium cobaltate, lithium nickelate, lithium manganate, lithium iron manganese phosphate, lithium iron phosphate and ternary materials. Lithium iron phosphate material is widely used due to its advantages of structural stability and long cycle life.

[0004] However, the lithium iron phosphate material still has the problems of low capacity and poor rate performance, which limits its application in the field of power batteries. SUMMARY

[0005] The application provides a modified lithium iron phosphate material, a preparation method thereof, a positive electrode sheet and a secondary battery, which can improve the capacity and rate performance.

[0006] In a first aspect, the application provides a preparation method of a modified lithium iron phosphate material, which includes: grinding and mixing a lithium source, an iron source, a phosphorus source, a niobium source, a fluorine-doped zeolitic imidazolate framework material nanosheet, a carbon source and a dispersing agent to obtain a raw material system; drying the raw material system to obtain a precursor material; and performing sintering treatment on the precursor material under an inert atmosphere to form the modified lithium iron phosphate material, wherein the modified lithium iron phosphate material includes niobium-doped lithium iron phosphate particles and a layer of fluorine-doped zeolitic imidazolate framework material arranged on at least part of the outer surface of the lithium iron phosphate particles.

[0007] According to the first aspect of the application, the preparation process of the fluorine-doped zeolitic imidazolate framework material nanosheet includes: stirring and mixing a metal salt solution mixed with a fluorine source and an imidazole ligand solution, and obtaining a precipitate after reaction; and performing calcination on the precipitate under an inert atmosphere after drying to obtain the fluorine-doped zeolitic imidazolate framework material nanosheet.

[0008] According to the embodiment of the first aspect of the present application, in the step of mixing the metal salt solution mixed with the fluorine source and the imidazole ligand solution by stirring, and obtaining the precipitate product after reaction,

[0009] The metal salt comprises a zinc salt; and / or, the fluorine source comprises at least one of sodium fluoride, potassium fluoride, sodium hydrogen fluoride and potassium hydrogen fluoride; and / or, the imidazole ligand comprises at least one of 2-methylimidazole, N-methylimidazole and 1-methylimidazole; and / or, the molar ratio of the metal salt, the fluorine source and the imidazole ligand is (0.6-12) : (0.6-12) : (0.2-10).

[0010] According to the embodiment of the first aspect of the present application, the preparation process of the fluorine-doped zeolitic imidazolate framework nanosheet further satisfies at least one of the following conditions:

[0011] (1) the stirring speed is 300 r / min-700 r / min, and the stirring time is 1 h-10 h;

[0012] (2) the precipitate product is obtained by centrifugal treatment after stirring and mixing, wherein the centrifugal speed is 4000 r / min-9000 r / min, and the centrifugal time is 5 min-20 min.

[0013] (3) the drying temperature is 50℃-100℃;

[0014] (4) the calcination temperature is 700℃-1000℃, and the calcination time is 1 h-6 h.

[0015] According to the embodiment of the first aspect of the present application, in the step of mixing the lithium source, the iron source, the phosphorus source, the niobium source, the fluorine-doped zeolitic imidazolate framework nanosheet, the carbon source and the dispersant by grinding, and obtaining the raw material system,

[0016] The molar ratio of lithium element in the lithium source, iron element in the iron source, phosphorus element in the phosphorus source, and niobium element in the niobium source is 1: (0.1-0.9): (0.1-0.9): (0.8-1.2); and / or, the molar amount of niobium element in the niobium source is 0.2%-1.5% of the molar amount of iron element in the iron source; and / or, the mass percentage content of the fluorine-doped zeolitic imidazolate framework nanosheet is 10%-25% based on the total mass of the raw material system; and / or, the mass percentage content of the dispersant is 0.2%-2% based on the total mass of the raw material system; and / or, the mass percentage content of the carbon source is 5%-15% based on the total mass of the raw material system.

[0017] According to the embodiment of the first aspect of the present application, in the step of grinding and mixing the lithium source, the iron source, the phosphorus source, the niobium source, the fluorine-doped zeolitic imidazolate framework nanosheet, the carbon source and the dispersing agent, the niobium source includes at least one of niobium nitrate, niobium hydroxide, niobium acetate, niobium carbonate, niobium phosphate, di-niobium pentoxide and niobium chloride; and / or, the dispersing agent includes at least one of PEG2000, PEG4000, PEG6000, oleic acid, PVP, graphene and carbon black; and / or, the carbon source includes at least one of citric acid, glucose, sucrose, starch, xylitol, amino acid and malic acid.

[0018] According to the embodiment of the first aspect of the present application, the step of sintering the precursor material under an inert atmosphere to form the modified lithium iron phosphate material includes:

[0019] In the first stage, the temperature is increased from room temperature to 170-320℃ at a rate of 1-15℃ / min, and maintained at this temperature for 2-5h; in the second stage, the temperature is increased to 440-550℃ at a rate of 1-15℃ / min, and maintained at this temperature for 2-6h; in the third stage, the temperature is increased to 650-800℃ at a rate of 1-10℃ / min, and maintained at this temperature for 2-7h.

[0020] In the second aspect, the embodiment of the present application provides a modified lithium iron phosphate material, which includes niobium-doped lithium iron phosphate particles and a layer of fluorine-doped zeolitic imidazolate framework material coated on at least part of the surface of the lithium iron phosphate particles; optionally, the molar amount of niobium in the niobium-doped lithium iron phosphate particles is 0.2%-1.5% of the molar amount of iron; and the mass percentage of the layer of fluorine-doped zeolitic imidazolate framework material is 5wt%-10wt% based on the total mass of the modified lithium iron phosphate material.

[0021] In the third aspect, the embodiment of the present application provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode active layer arranged on at least one side of the positive electrode current collector, and the positive electrode active layer includes the lithium iron phosphate material prepared by the preparation method of any one of the embodiments of the first aspect of the present application or the modified lithium iron phosphate material of any one of the embodiments of the second aspect of the present application.

[0022] In the fourth aspect, the present application provides a secondary battery, which includes the positive electrode sheet of any one of the embodiments of the third aspect of the present application.

[0023] According to the preparation method of the embodiment of the present application, the Li-O bond length in the lithium iron phosphate and the length of the b-axis direction in the crystal structure thereof are regulated by niobium doping, and appropriate niobium doping can weaken the Li-O bond energy in the lithium iron phosphate, lengthen the bond length, make the Li +It is easier to deintercalate from the crystal structure of lithium iron phosphate, in addition, appropriate niobium doping can also make the length of lithium iron phosphate crystal along the b-axis direction shortened (Li + diffusion channel), thereby improving the diffusion rate of Li + , and further improving the charge and discharge specific capacity and rate performance of lithium iron phosphate. At the same time, by doping zeolite imidazolate framework material nanosheets with fluorine, on the one hand, the lithium iron phosphate particles can be anchored in the reaction system to enhance the structural stability, and it can also form a continuous conductive network between the lithium iron phosphate particles to enhance the electronic conductivity of lithium iron phosphate, thereby reducing the internal resistance of the modified lithium iron phosphate material. The conductive network can effectively inhibit the growth of lithium iron phosphate crystal grains and prevent the agglomeration of lithium iron phosphate particles during the calcination process. On the other hand, fluorine doping can increase the carbon defects in the zeolite imidazolate framework material nanosheets, reduce the electron cloud density around the carbon atoms, and further improve the electronic conductivity of the zeolite imidazolate framework material. In addition, the introduction of fluorine can effectively inhibit the growth of the (010) crystal plane in lithium iron phosphate, reduce the length of the b-axis in the modified lithium iron phosphate material, shorten the diffusion path of Li + , and further improve the diffusion rate of Li + , thereby improving the charge and discharge specific capacity and rate performance of the modified lithium iron phosphate material, and further improving the electrochemical performance of the modified lithium iron phosphate material. The present application embodiment improves the electrochemical performance of the modified lithium iron phosphate material by synergistic effect of niobium doping and fluorine-doped zeolite imidazolate framework material nanosheets. The preparation method of the present application embodiment has simple preparation process, high repeatability and low cost, and is suitable for large-scale industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments of the present application. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creating laborious work.

[0025] Figure 1 XRD patterns of the positive electrode active materials prepared in the embodiments and comparative examples of the present application;

[0026] Figure 2 Scanning electron microscope (SEM) images of the positive electrode active materials in the embodiments and comparative examples of the present application; wherein, the a, b figures are scanning electron microscope images of the positive electrode active material provided by comparative example 2; the c, d figures are scanning electron microscope images of the positive electrode active material provided by example 1; the e, f figures are scanning electron microscope images of the positive electrode active material provided by example 2; the g, h figures are scanning electron microscope images of the positive electrode active material provided by example 3;

[0027] Figure 3A charge-discharge curve of the positive electrode active material provided for Example 2 is shown. DETAILED DESCRIPTION

[0028] Hereinafter, embodiments of the lithium iron phosphate material and the preparation method thereof, the positive electrode sheet and the secondary battery according to the present application will be described in detail with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed descriptions are omitted. For example, there will be cases where detailed descriptions of matters known well, repeated descriptions of substantially identical structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0029] Method for preparing modified lithium iron phosphate material

[0030] In a first aspect, the embodiments of the present application provide a preparation method of a modified lithium iron phosphate material.

[0031] As shown in Figure 1 , the preparation method comprises:

[0032] In step S100, a lithium source, an iron source, a phosphorus source, a niobium source, a fluorine-doped zeolitic imidazolate framework material nanosheet, a carbon source and a dispersing agent are ground and mixed to obtain a raw material system;

[0033] In step S200, the raw material system is dried to obtain a precursor material;

[0034] In step S300, the precursor material is sintered under an inert atmosphere to form a modified lithium iron phosphate material, wherein the modified lithium iron phosphate material comprises niobium-doped lithium iron phosphate particles and a layer of fluorine-doped zeolitic imidazolate framework material arranged on at least part of the outer surface of the lithium iron phosphate particles.

[0035] According to the preparation method of the embodiments of the present application, the Li-O bond length in the lithium iron phosphate and the length of the b-axis direction in the crystal structure thereof are regulated by niobium doping. Appropriate niobium doping can weaken the Li-O bond energy in the lithium iron phosphate, lengthen the bond length, make Li + more easily deintercalate from the lithium iron phosphate crystal structure, in addition, appropriate niobium doping can also shorten the length of the b-axis direction of the lithium iron phosphate crystal (diffusion channel of Li + ), thereby improving the Li +the diffusion rate of Li+, thereby improving the specific capacity and rate performance of the modified lithium iron phosphate material. The present application improves the electrochemical performance of the modified lithium iron phosphate material through the synergistic effect of niobium-doped and fluorine-doped zeolitic imidazolate framework nanosheets, and the preparation method of the present application has simple preparation process, high repeatability, low cost, and is suitable for large-scale industrial production. + the diffusion rate of Li+, thereby improving the specific capacity and rate performance of the modified lithium iron phosphate material. The present application improves the electrochemical performance of the modified lithium iron phosphate material through the synergistic effect of niobium-doped and fluorine-doped zeolitic imidazolate framework nanosheets, and the preparation method of the present application has simple preparation process, high repeatability, low cost, and is suitable for large-scale industrial production. + the diffusion rate of Li+, thereby improving the specific capacity and rate performance of the modified lithium iron phosphate material. The present application improves the electrochemical performance of the modified lithium iron phosphate material through the synergistic effect of niobium-doped and fluorine-doped zeolitic imidazolate framework nanosheets, and the preparation method of the present application has simple preparation process, high repeatability, low cost, and is suitable for large-scale industrial production. The present application improves the electrochemical performance of the modified lithium iron phosphate material through the synergistic effect of niobium-doped and fluorine-doped zeolitic imidazolate framework nanosheets, and the preparation method of the present application has simple preparation process, high repeatability, low cost, and is suitable for large-scale industrial production.

[0036] [Step S100]

[0037] Step S100 provides each raw material and mixes each raw material for preliminary reaction; illustratively, a lithium source, an iron source, a phosphorus source, and an organic solvent are added to the organic solvent according to a preset stoichiometric ratio, and after stirring and uniform mixing, a raw material system is formed.

[0038] In some embodiments, the lithium source includes at least one of an inorganic lithium salt and an organic lithium salt. The above-mentioned lithium source is easy to obtain and easy to react with other components.

[0039] Illustratively, the inorganic lithium salt includes at least one of lithium carbonate, lithium hydroxide, lithium nitrate, lithium silicate, lithium sulfate, lithium phosphate, lithium orthosilicate, lithium permanganate, lithium metaphosphate, lithium fluoride, lithium bromide, lithium oxide, lithium nitride, and lithium sulfide.

[0040] Illustratively, the organic lithium salt includes at least one of lithium oxalate, lithium formate, lithium octanoate, lithium citrate, lithium salicylate, lithium trifluoroacetate, lithium acetoacetate, lithium difluorophosphate, lithium hexafluorophosphate, lithium benzoate, lithium pyruvate, lithium acetate, lithium methoxide, and lithium ethoxide.

[0041] When the lithium source contains both lithium and phosphorus elements, the material can act as both a lithium source and a phosphorus source.

[0042] In some embodiments, the iron source includes at least one of an inorganic iron salt and an organic iron salt. The above-mentioned iron source is readily available and easy to react with other components.

[0043] Illustratively, the inorganic iron salt includes at least one of ferric nitrate, ferric phosphate, ferric pyrophosphate, ferrous nitrate, ferrous sulfide, ferrous sulfate, ferrous phosphate, ferrous iodide, ferrous fluoride, ferrous bromide, and ferrous chloride.

[0044] Illustratively, the organic iron salt includes at least one of ferric citrate, ferric oxalate, ferrous oxalate, ferrous acetylacetonate, ferrous gluconate, and ferrous acetate.

[0045] When the iron source contains both iron element and phosphorus element, the material serves as both iron source and phosphorus source.

[0046] In some embodiments, the phosphorus source includes at least one of a phosphorus-containing sodium salt, a phosphorus-containing ammonium salt, and a phosphorus-containing potassium salt. The above-mentioned phosphorus source is readily available and easy to react with other components.

[0047] Illustratively, the phosphorus-containing sodium salt includes at least one of sodium pyrophosphate, sodium phosphite, sodium metaphosphate, sodium tripolyphosphate, and sodium hexafluorophosphate.

[0048] Illustratively, the phosphorus-containing ammonium salt includes at least one of ammonium hypophosphite, ammonium polyphosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium hexafluorophosphate, and ammonium phosphate.

[0049] Illustratively, the phosphorus-containing potassium salt includes at least one of potassium hypophosphite, potassium pyrophosphate, potassium phosphate tribasic, potassium phosphite, potassium metaphosphate, and potassium hexafluorophosphate.

[0050] In some embodiments, the niobium source includes at least one of niobium nitrate, niobium hydroxide, niobium acetate, niobium carbonate, niobium phosphate, niobium pentoxide, and niobium chloride. The above-mentioned niobium source is readily available and easy to react with other components.

[0051] By niobium doping, the niobium element replaces part of the iron element, which can control the Li-O bond length in lithium iron phosphate and the length of the b-axis direction in the crystal structure of lithium iron phosphate. Appropriate niobium doping can weaken the Li-O bond energy in lithium iron phosphate and lengthen the bond length, making Li + more easily deintercalated from the crystal structure of lithium iron phosphate. In addition, appropriate niobium doping can also shorten the length of the b-axis direction of the lithium iron phosphate crystal (the diffusion channel of Li + ), thereby improving the diffusion rate of Li + and thus improving the specific capacity and rate performance of lithium iron phosphate.

[0052] In some embodiments, the molar amount of niobium element in the niobium source is 0.2%-1.5% of the molar amount of iron element in the iron source.

[0053] Exemplarily, the molar amount of niobium element in the niobium source is 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or a range between any two of the above values of the molar amount of iron element in the iron source.

[0054] In some embodiments, the dispersant includes at least one of PEG2000, PEG4000, PEG6000, oleic acid, PVP, graphene and carbon black.

[0055] In some embodiments, the mass percentage of the dispersant is 0.2%-2% based on the total mass of the raw material system.

[0056] Exemplarily, the mass percentage of the dispersant is 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0% or a range between any two of the above values based on the total mass of the raw material system.

[0057] In some embodiments, the mass percentage of the fluorine-doped zeolitic imidazolate framework nanosheet is 10%-25% based on the total mass of the raw material system.

[0058] Exemplarily, the mass percentage of the fluorine-doped zeolitic imidazolate framework nanosheet is 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25% or a range between any two of the above values based on the total mass of the raw material system.

[0059] In some embodiments, the particle size of the raw material system is 1 μm or below. Exemplarily, the particle size of the raw material system is 0.01 μm, 0.02 μm, 0.04 μm, 0.06 μm, 0.08 μm, 0.1 μm, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm or a range between any two of the above values.

[0060] In some embodiments, the carbon source includes at least one of citric acid, glucose, sucrose, starch, xylitol, amino acid and malic acid. Carbon improves the electronic conductivity of lithium iron phosphate and can also act as a reducing agent to avoid oxidation of ferrous ions in a high-temperature environment.

[0061] In some embodiments, the mass percentage of the carbon source is 5%-15% based on the total mass of the raw material system.

[0062] Illustratively, the mass percentage of the carbon source is 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, or a range between any two of the above values.

[0063] In some embodiments, in the step of grinding and mixing the lithium source, the iron source, the phosphorus source, the niobium source, the fluorine-doped zeolitic imidazolate framework nanosheet, the carbon source, and the dispersant to obtain the raw material system, the molar ratio of lithium in the lithium source, iron in the iron source, and phosphorus in the phosphorus source is (0.8-1.2):(0.8-1.2):(0.8-1.2). When the molar ratio of the lithium source, the iron source, and the phosphorus source is in the above range, the desired lithium iron phosphate primary crystal system can be prepared, and the stability of the content of each element in the system is ensured.

[0064] Illustratively, the molar ratio of lithium in the lithium source, iron in the iron source, and phosphorus in the phosphorus source is (0.8-1.2):(0.8-1.2):(0.8-1.2), (0.9-1.2):(0.8-1.2):(0.8-1.2), (1.0-1.2):(0.8-1.2):(0.8-1.2), (0.8-1.2):(0.9-1.2):(0.8-1.2), (0.8-1.2):(1.0-1.2):(0.8-1.2), (0.8-1.2):(0.8-1.2):(0.9-1.2), (0.8-1.2):(0.8-1.2):(1.0-1.2), or (0.8-1.2):(0.8-1.2):(0.9-1.1).

[0065] [Step S200]

[0066] In step S200, the raw material system is dried to remove the liquid dispersant, so that the solid substances in the raw material system are separated to obtain the precursor material, which is prepared for subsequent calcination.

[0067] In some embodiments, the temperature of the drying treatment is 60°C-90°C. Illustratively, the temperature of the drying treatment can be 60°C, 61°C, 62°C, 63°C, 64°C, 65°C, 66°C, 67°C, 68°C, 69°C, 70°C, 71°C, 72°C, 73°C, 74°C, 75°C, 76°C, 77°C, 78°C, 79°C, 80°C, 81°C, 82°C, 83°C, 84°C, 85°C, 86°C, 87°C, 88°C, 89°C, 90°C, or a range between any two of the above values.

[0068] In some embodiments, the drying process is performed for 1 h to 24 h. For example, the drying process can be performed for 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, 12 h, 13 h, 14 h, 15 h, 16 h, 17 h, 18 h, 19 h, 20 h, 21 h, 22 h, 23 h, 24 h, or a range defined by any two of the above values.

[0069] [Step S300]

[0070] Step S300 calcines the precursor material under an inert atmosphere to form a modified lithium iron phosphate material, wherein the modified lithium iron phosphate material comprises niobium-doped lithium iron phosphate particles and a layer of fluorine-doped zeolitic imidazolate framework material disposed on at least a portion of the outer surface of the lithium iron phosphate particles.

[0071] The precursor material can form fine-grained particles after calcination; the fluorine-doped zeolitic imidazolate framework material nanosheet can form a layered material after the calcination process, and the layered material can also improve the stability and electrochemical stability of the material.

[0072] In some embodiments, the inert atmosphere can be an argon (Ar) or nitrogen (N2) atmosphere.

[0073] In some embodiments, the calcination process comprises: a first stage, in which the temperature is increased from room temperature to 170-320°C at a rate of 1-15°C / min, and maintained at this temperature for 2-5 h; a second stage, in which the temperature is increased to 440-550°C at a rate of 1-15°C / min, and maintained at this temperature for 2-6 h; and a third stage, in which the temperature is increased to 650-800°C at a rate of 1-10°C / min, and maintained at this temperature for 2-7 h.

[0074] The first stage increases the temperature to 170-320°C and maintains it at this temperature for 2-5 h to remove the crystal water in the precursor material. For example, the first stage increases the temperature to 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, 290°C, 300°C, 310°C, 320°C, or a range defined by any two of the above values; the temperature is maintained at the above value for 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, 5 h, or a range defined by any two of the above values; and the temperature is increased at a rate of 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, 11°C / min, 12°C / min, 13°C / min, 14°C / min, 15°C / min, or a range defined by any two of the above values.

[0075] The second stage is heated to 440-550°C and kept at this temperature for 2-6 hours, so that the carbon source is decomposed and carbonized to form a carbon coating layer on the surface of the lithium iron phosphate and the lithium iron phosphate crystals are initially formed. Illustratively, the second stage is heated to 440°C, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, or a range defined by any two of the above values; the holding time at the above temperature is 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, or a range defined by any two of the above values; the heating rate is 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, 11°C / min, 12°C / min, 13°C / min, 14°C / min, 15°C / min, or a range defined by any two of the above values.

[0076] The third stage is heated to 650-800°C and kept at this temperature for 2-7 hours, so that the modified lithium iron phosphate crystals can grow sufficiently. Illustratively, the third stage is heated to 650°C, 660°C, 670°C, 680°C, 690°C, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, 760°C, 770°C, 780°C, 790°C, 800°C, or a range defined by any two of the above values; the holding time at the above temperature is 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, or a range defined by any two of the above values; the heating rate is 1°C / min, 2°C / min, 3°C / min, 4°C / min, 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, 10°C / min, or a range defined by any two of the above values.

[0077] By using the above calcination conditions, the rate and degree of crystal growth in the modified lithium iron phosphate material can be adjusted, thereby facilitating the obtaining of a modified lithium iron phosphate material with excellent electrochemical performance.

[0078] [Step S110]

[0079] In some embodiments, in step S100, the preparation method of the embodiments of the present application can further include step S110 for preparing fluorine-doped zeolitic imidazolate framework nanosheets, and step S110 can include:

[0080] The metal salt solution mixed with the fluorine source and the imidazole ligand solution are stirred and mixed, and the precipitate is obtained after the reaction.

[0081] After drying the precipitated product, calcination is performed under an inert atmosphere to obtain fluorine-doped zeolitic imidazolate framework nanosheets.

[0082] In some embodiments, the metal salt comprises a zinc salt; illustratively, the zinc salt is zinc nitrate.

[0083] In some embodiments, the fluorine source comprises at least one of sodium fluoride, potassium fluoride, sodium bifluoride, and potassium bifluoride.

[0084] In some embodiments, the imidazole ligand comprises at least one of 2-methylimidazole, N-methylimidazole, and 1-methylimidazole.

[0085] In some embodiments, the molar ratio of the metal salt, the fluorine source, and the imidazole ligand is (0.6-12):(0.6-12):(0.2-10).

[0086] Illustratively, the molar ratio of the metal salt, the fluorine source, and the imidazole ligand can be (0.6-12):(0.6-12):(0.2-10), (1-12):(0.6-12):(0.2-10), (2-12):(0.6-12):(3-10), (0.6-12):(0.6-12):(0.2-10), (5-12):(0.6-12):(0.2-10), (7-12):(0.6-12):(0.2-10), (8-12):(0.6-12):(0.2-10), (10-12):(0.6-12):(0.2-10), (5-7):(0.6-12):(0.2-10), (6-7):(0.6-12):(0.2-10), (0.6-12):(3-12):(0.2-10), (0.6-12):(3-8):(0.2-10), (0.6-12):(0.6-12):(1-10), (0.6-12):(0.6-12):(1-5), (0.6-12):(0.6-12):(6-9).

[0087] By adopting the above steps, the amount of fluorine doping in the fluorine-doped zeolite imidazolate framework nanosheet crystals can be adjusted. Fluorine doping has two effects. On the one hand, fluorine atoms can replace carbon atoms in the material, form carbon defects, and change the electron cloud density around the carbon atoms, thereby enhancing the electronic conductivity of the nanosheet material. If the amount of fluorine doping is too small, not enough carbon defects can be formed, which is not conducive to the enhancement of the electronic conductivity of the nanosheet material. If the amount of fluorine doping is too high, more defects will be formed, which will reduce the structural stability of the material and is not conducive to the cyclic stability of the material. On the other hand, fluorine atoms have strong adsorption and can be adsorbed on some crystal planes of the lithium iron phosphate crystal to inhibit the growth of the crystal plane. For example, adsorption on the (010) crystal plane can inhibit the growth of the crystal along the (010) direction, so that the transport path of Li + becomes shorter, thereby enhancing the transport rate of Li + and the electrochemical performance of the battery. If the amount of F doping is too low, the growth of the (010) crystal plane cannot be inhibited, which will eventually lead to a slow transport rate of Li + and a decrease in the electrochemical performance of the battery, such as the specific charge capacity, rate, cycle, and low-temperature performance. In addition, if the amount of F doping is too high, the growth of the crystal will be inhibited, which will result in smaller lithium manganese iron particles. Although this will enhance the rate and specific charge capacity of lithium iron phosphate to some extent, it will cause many problems, such as low compaction of the material, resulting in low energy density of the material, large specific surface area, high porosity, easy water absorption, poor processing performance, and many particle gaps, which will increase the ohmic resistance of the material, etc.

[0088] In some embodiments, the stirring speed is 300 r / min to 700 r / min, and the stirring time is 1 h to 10 h. For example, the stirring speed is 300 r / min, 350 r / min, 400 r / min, 450 r / min, 500 r / min, 550 r / min, 600 r / min, 650 r / min, 700 r / min, or a range formed by any two of the above values. The stirring time is 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, or a range formed by any two of the above values.

[0089] In some embodiments, the precipitated product is subjected to drying, and then calcination under inert atmosphere to obtain the fluorine-doped zeolitic imidazolate framework nanosheets. In some embodiments, the temperature for drying is 50-100℃.

[0090] In some embodiments, the precipitated product is subjected to drying, and then calcination under inert atmosphere to obtain the fluorine-doped zeolitic imidazolate framework nanosheets. In some embodiments, the temperature for drying is 50-100℃.

[0091] In some embodiments, the temperature for drying can be 50℃, 52℃, 54℃, 56℃, 58℃, 60℃, 62℃, 64℃, 66℃, 68℃, 70℃, 72℃, 74℃, 76℃, 78℃, 80℃, 82℃, 84℃, 86℃, 88℃, 90℃, 92℃, 94℃, 96℃, 98℃, 100℃, or a range defined by any two of the aforementioned values.

[0092] In some embodiments, the precipitated product is subjected to drying, and then calcination under inert atmosphere to obtain the fluorine-doped zeolitic imidazolate framework nanosheets. In some embodiments, the temperature for calcination is 700-1000℃, and the time for calcination is 1-6h.

[0093] In some embodiments, the temperature for calcination can be 700℃, 720℃, 740℃, 760℃, 780℃, 800℃, 820℃, 840℃, 860℃, 880℃, 900℃, 920℃, 940℃, 960℃, 980℃, 1000℃, or a range defined by any two of the aforementioned values.

[0094] In some embodiments, the time for calcination can be 1h, 2h, 3h, 4h, 5h, 6h, or a range defined by any two of the aforementioned values.

[0095] By using the above calcination conditions, the rate and extent of crystal growth in the fluorine-doped zeolitic imidazolate framework nanosheets can be adjusted.

[0096] Modified lithium iron phosphate material

[0097] In a second aspect, the present application also provides a modified lithium iron phosphate material, comprising niobium-doped lithium iron phosphate particles and a layer of fluorine-doped ZIF material coated on at least part of the surface of the lithium iron phosphate particles.

[0098] The Li-O bond length in the lithium iron phosphate and the length of the b-axis direction in the crystal structure of the lithium iron phosphate are regulated by niobium doping. Appropriate niobium doping can weaken the Li-O bond energy in the lithium iron phosphate, lengthen the bond length, make the Li + more easily deintercalate from the crystal structure of the lithium iron phosphate, in addition, appropriate niobium doping can also shorten the length of the b-axis direction of the lithium iron phosphate crystal (the diffusion path of Li + ), thereby improving the diffusion rate of Li + , thereby improving the specific capacity and rate performance of the lithium iron phosphate.

[0099] Meanwhile, by fluorine-doping zeolitic imidazolate framework material nanosheets, on the one hand, the lithium iron phosphate particles can be anchored in the reaction system to enhance the structural stability thereof, and the fluorine-doped zeolitic imidazolate framework material nanosheets can form a continuous conductive network among the lithium iron phosphate particles, thereby enhancing the electronic conductivity of the lithium iron phosphate, reducing the internal resistance of the modified lithium iron phosphate material, and the conductive network can effectively inhibit the growth of lithium iron phosphate crystal grains and prevent the agglomeration of lithium iron phosphate particles during calcination, on the other hand, fluorine doping can increase the carbon defects in the zeolitic imidazolate framework material nanosheets, reduce the electron cloud density around the carbon atoms, and thereby improve the electronic conductivity of the zeolitic imidazolate framework material, in addition, the introduction of fluorine can effectively inhibit the growth of the (010) crystal plane in the lithium iron phosphate, reduce the length of the b-axis in the modified lithium iron phosphate material, shorten the diffusion path of Li + , and thereby improve the diffusion rate of Li + , thereby improving the specific capacity and rate performance of the modified lithium iron phosphate material, and further improving the electrochemical performance of the modified lithium iron phosphate material.

[0100] The electrochemical performance of the modified lithium iron phosphate material is improved by the synergistic effect of niobium doping and fluorine-doped zeolitic imidazolate framework material nanosheets.

[0101] In some embodiments, the molar amount of niobium in the niobium-doped lithium iron phosphate particles is 0.2%-1.5% of the molar amount of iron.

[0102] For example, the molar amount of niobium in the niobium source is 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, or a range formed by any two of the above values, of the molar amount of iron in the iron source.

[0103] In some embodiments, the mass percentage of the fluorine-doped zeolitic imidazolate framework material layer is 5wt%-10wt% based on the total mass of the modified lithium iron phosphate material.

[0104] For example, the mass percentage of the fluorine-doped zeolitic imidazolate framework material layer is 5%, 6%, 7%, 8%, 9%, 10% or a range between any two of the above values based on the total mass of the modified lithium iron phosphate material.

[0105] Positive electrode sheet

[0106] In a third aspect, the embodiments of the present application provide a positive electrode sheet. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer arranged on at least one side of the positive electrode current collector. The positive electrode active layer comprises a positive electrode active material, and the positive electrode active material comprises the modified lithium iron phosphate material according to any one of the embodiments of the second aspect of the present application.

[0107] Secondary battery

[0108] In a fourth aspect, the embodiments of the present application also provide a secondary battery. The secondary battery according to the embodiments of the present application comprises the positive electrode sheet according to any one of the embodiments of the third aspect of the present application. The lithium iron phosphate particles of the modified lithium iron phosphate material in the positive electrode sheet according to the embodiments of the present application have a small grain size, the transport path of lithium ions is short, and the surface of the lithium iron phosphate particles is coated with a carbon coating layer, which can improve the overall conductivity of the positive electrode active material, thereby effectively improving the electrochemical performance of the secondary battery, especially the rate capability and low-temperature performance.

[0109] In some embodiments, the secondary battery can further comprise a negative electrode sheet. The negative electrode sheet can be a negative electrode sheet commonly used in the art, for example, the negative electrode sheet comprises a negative electrode current collector and a negative electrode active layer. The negative electrode current collector can be a copper foil, and the negative electrode active layer can be graphite or the like.

[0110] In some embodiments, the secondary battery can further comprise a separator. The separator can be an organic polymer film layer, for example, a polypropylene film layer or a polyethylene film layer, or the like.

[0111] In some embodiments, the secondary battery can further comprise an electrolyte. The electrolyte can be an electrolyte commonly used in the art, for example, lithium hexafluorophosphate is used as a lithium salt.

[0112] The present application will be described in detail below with reference to specific embodiments.

[0113] Embodiment 1

[0114] A preparation method of a modified lithium iron phosphate positive electrode active material, the specific steps are as follows:

[0115] (1) Add 0.03 mol of sodium fluoride into an aqueous solution containing 0.3 mol of zinc nitrate (1 L), then add an aqueous solution containing 0.06 mol of 2-methylimidazole (50 mL) dropwise into the above solution, and stir at room temperature at a speed of 500 r / min, then centrifuge the above solution at a speed of 6000 r / min in a centrifuge to obtain a ZIF precursor, dry the separated ZIF precursor at 60°C, then place the dried ZIF precursor into a tube furnace and calcine at 900°C for 3 h, and then naturally cool to obtain fluorine-doped zeolitic imidazolate framework nanosheets;

[0116] (2) Add 1.02 mol of lithium hydroxide, 1 mol of iron nitrate nonahydrate, 1 mol of ammonium dihydrogen phosphate, 0.002 mol of niobium nitrate, 0.01 mol of PVP, 0.1 mol of glucose, and 10 g of fluorine-doped zeolitic imidazolate framework nanosheets into a sand mill, and grind until the sand mill particle size in the slurry is below 1 μm;

[0117] (3) Spray dry the above slurry to obtain a solid precursor material, then calcine the precursor material in a nitrogen atmosphere, first heat it from room temperature to 250°C at a rate of 10°C / min, and keep it at this temperature for 3 h, then heat it to 520°C at a rate of 5°C / min, and keep it at this temperature for 4 h, and finally heat it to 750°C at a rate of 5°C / min, and keep it at this temperature for 6 h, and then naturally cool to obtain a Nb-doped LFP / F-doped ZIF positive electrode active material.

[0118] Example 2

[0119] A method for preparing a modified lithium iron phosphate positive electrode active material, which is similar to the preparation method of Example 1, except that 0.008 mol of niobium nitrate is added in step (2).

[0120] Example 3

[0121] A method for preparing a modified lithium iron phosphate positive electrode active material, which is similar to the preparation method of Example 1, except that 0.015 mol of niobium nitrate is added in step (2).

[0122] Example 4

[0123] A method for preparing a modified lithium iron phosphate positive electrode active material, which is similar to the preparation method of Example 2, except that 0.08 mol of sodium fluoride is added in step (1).

[0124] Comparative Example 1

[0125] A preparation method of a positive electrode active material, except that the following steps are different, other steps are consistent with those in Example 1: 0.0075 mol of dihydrogen diamine is added in step (1) to prepare nitrogen-doped zeolitic imidazolate framework nanosheet material; 10 g of nitrogen-doped zeolitic imidazolate framework nanosheet material is added in step (2); and a niobium-doped lithium iron phosphate composite nitrogen-doped zeolitic imidazolate framework material layer is prepared in step (3).

[0126] Comparative Example 2

[0127] A preparation method of a positive electrode active material, except that the following steps are different, other steps are consistent with those in Example 1: 0.004 mol of V2O5 is added in step (2); and a vanadium-doped lithium iron phosphate composite fluorine-doped zeolitic imidazolate framework material layer is prepared in step (3).

[0128] Preparation of lithium ion batteries of examples and comparative examples

[0129] 1. Preparation of positive electrode sheet

[0130] (1) Preparation of positive electrode slurry: positive electrode active material, superconducting carbon black (SP) and binder polyvinylidene fluoride (PVDF) are added into a maroon ball mill tank according to a mass ratio of 90:5:5, and a solvent N-methyl pyrrolidone (NMP) is added for ball milling to obtain a positive electrode slurry; and the lithium iron phosphate materials prepared in the above examples and comparative examples are used as positive electrode active materials of lithium ion batteries.

[0131] (2) Coating of positive electrode slurry: the scale of the doctor blade of the coating machine is adjusted, and the ball-milled positive electrode slurry is uniformly coated on an aluminum foil; the coated electrode sheet is placed in a vacuum drying oven for baking, cold pressing and punching to obtain a positive electrode sheet.

[0132] 2. Preparation of lithium ion battery

[0133] In a glove box, the lithium ion batteries are assembled in the order of negative electrode shell, spring, steel sheet, lithium sheet, isolation film, positive electrode sheet and positive electrode shell, and 10 μL of electrolyte (lithium salt including 1 mol / L lithium hexafluorophosphate LiPF6, and organic solvent including ethylene carbonate EC and dimethyl carbonate DMC (volume ratio 1:1)) is injected in the process, and then a sealing machine is used to seal the button cell, and lithium ion batteries are obtained.

[0134] Performance test

[0135] 1. Cell parameter characterization of positive electrode active material

[0136] The positive electrode active materials prepared in the examples and comparative examples are subjected to X-ray diffraction test, Figure 1The XRD patterns of the positive electrode active materials prepared in the examples and comparative examples are shown. The cell parameters of the positive electrode active materials can be characterized by refining the XRD patterns.

[0137] 2. Performance Characterization of Lithium-ion Batteries

[0138] The electrochemical performance of coin cells was tested using the LAND electrochemical testing system, with a discharge cutoff voltage of 2.0V and a charge cutoff voltage of 4.3V. The electrochemical performance of coin cells prepared with the positive electrode active materials of the examples and comparative examples was tested at 0.1C and 1C rates.

[0139] Test results

[0140] Table 1. Cell parameters of the positive electrode active materials provided in Examples 1-3 and Comparative Examples 1-2;

[0141]

[0142] Table 2 Electrochemical data of the positive electrode active materials provided in the comparative examples and embodiments.

[0143]

[0144] Table 3. Resistivity data of the positive electrode active materials provided in the comparative examples and embodiments.

[0145] Performance index Resistivity (Ω·cm) Comparative example 1 7683.2 Comparative example 2 15362.7 Example 2 38.4

[0146] The X-ray diffraction patterns of the positive electrode active materials prepared in the examples and comparative examples are as follows: Figure 1 As shown, from Figure 1 It can be seen that the diffraction peaks of the materials prepared in the comparative examples and embodiments correspond to the diffraction peaks of the lithium iron phosphate standard card, and there are no other impurity peaks in the XRD patterns of the materials prepared in each comparative example and embodiment, which confirms that the materials prepared in the comparative examples and embodiments are all pure phase lithium iron phosphate. In addition, for the embodiments, as the Nb doping amount increases, the characteristic peaks of the embodiments shift towards the direction with larger diffraction angles, which confirms that the Nb element has been successfully incorporated into the crystal structure of lithium iron phosphate. Furthermore, according to the Bragg equation, Nb doping leads to a decrease in the lattice spacing of lithium iron phosphate. Refining the XRD patterns yields refined data (see Table 1). For the embodiments, as the Nb doping amount increases, the axial length of lithium iron phosphate along the b-axis and its cell volume both show a trend of first decreasing and then increasing, while the Li-O bond length in lithium iron phosphate shows a trend of first increasing and then decreasing. When the Nb doping amount is 0.008 mol, the axial length of LFP along the b-axis is the shortest, which makes Li + The diffusion path is shortened, and the Li-O bond in lithium iron phosphate has the longest bond length, making the Li-O bond more prone to breakage.+ more easily detached from the unit cell of lithium iron phosphate. In summary, when the doping amount of Nb is 0.008 mol, the diffusion rate of Li + in lithium iron phosphate should be the fastest.

[0147] Comparative Example 1 has no significant change in the bond length of Li-O in the crystal, but the length along the b-axis is significantly longer than that of Example 2, which may be because part of the F element in the ZIF material in Example 2 is specifically adsorbed on the (010) crystal face of the material, thereby inhibiting the growth of the (010) crystal face in the lithium iron phosphate crystal. Since the F element has a smaller radius than the N element (which makes it more easily deintercalated from the ZIF material than the N element), and a stronger electronegativity (which makes it more easily adsorbed on the LFP surface than the N element), ultimately leading to the growth of the LFP crystal along the b-axis direction being inhibited in Example 2. In addition, since F has a stronger electronegativity than N, when F is doped into the ZIF material, it has a stronger attraction to the electron cloud density of the surrounding carbon atoms, which makes more positively charged carriers-holes on the carbon atom sites. The generation of these holes can accelerate the transfer of charges in the LFP material, reducing the resistivity of the lithium iron phosphate material. Example 2 has a larger unit cell volume of the prepared lithium iron phosphate than Comparative Example 2, because the ionic radius of V is smaller than that of Nb. In addition, the Li-O bond length of vanadium-doped lithium iron phosphate is shorter than that of niobium-doped lithium iron phosphate, and the b-axis length is longer than that of niobium-doped lithium iron phosphate, which will result in a slower diffusion rate of Li + in vanadium-doped lithium iron phosphate.

[0148] Example 3 changes the doping amount of niobium element, which is within an appropriate range, making the Li-O bond energy in lithium iron phosphate weaker and the bond length longer, making Li + more easily deintercalated from the lithium iron phosphate crystal structure, in addition, it can also make the length of the lithium iron phosphate crystal along the b-axis direction (the diffusion channel of Li + ) shorter, thereby improving the diffusion rate of Li + , thereby improving the charge and discharge specific capacity and rate performance of lithium iron phosphate. Example 4 changes the doping amount of fluorine element in the ZIF material, which is within an appropriate range, forming an appropriate amount of carbon defects and changing the electron cloud density around the carbon atoms, enhancing the electronic conductivity of the nanosheet material, and making the transport path of Li + shorter, thereby facilitating the enhancement of the transport rate of Li + , enhancing the electrochemical performance of the battery.

[0149] The scanning electron microscope images of the lithium iron phosphate positive electrode active materials in the examples and comparative examples are shown in Figure 2 , and Figure 2It can be seen from the figure that when vanadium is doped into lithium iron phosphate, the lithium iron phosphate particles are in block shape, irregular morphology, and the particle size distribution is very uneven. The particles of niobium-doped lithium iron phosphate are elliptical or spherical, and as the amount of Nb doping increases, the agglomeration of lithium iron phosphate particles is significantly improved. When the Nb doping amount is 0.008 mol, the agglomeration of lithium iron phosphate particles is not obvious, but as the amount of Nb doping continues to increase, the agglomeration of lithium iron phosphate particles becomes very serious, which shows that appropriate amount of Nb doping can effectively inhibit the agglomeration of LFP particles.

[0150] The charge-discharge curves of the niobium-doped lithium iron phosphate positive active material provided in Example 2 at 0.1C and 1C rates are shown in FIG. 2. Figure 3 It can be seen from the figure that the charge-discharge specific capacity of the lithium iron phosphate prepared in Example 2 at 0.1C and 1C rates, and the coulombic efficiency thereof can be calculated accordingly. Similarly, we can calculate the charge-discharge specific capacity and coulombic efficiency of the LFP positive active materials described in other examples and comparative examples as shown in Table 2. It can be seen from Table 2 that compared with Comparative Example 1, the first charge-discharge specific capacity and coulombic efficiency of the niobium-doped lithium iron phosphate prepared in Example 2 at 0.1C and 1C rates are significantly improved, which may be related to the shorter b-axis and Li-O bond length of the lithium iron phosphate prepared in Example 2, and the smaller resistivity of the LFP prepared in Example 2 (see Table 3), which makes the diffusion rate of Li + faster and the electronic conductivity higher in Example 2, thereby making the charge-discharge specific capacity and coulombic efficiency higher, which proves that the introduction of F element in ZIF material is more beneficial to the improvement of the electrochemical performance of lithium iron phosphate than the introduction of N element.

[0151] In addition, compared with Examples 1-4, the charge-discharge specific capacity and efficiency of the lithium iron phosphate prepared in Comparative Example 2 are greatly reduced, because vanadium-doped lithium iron phosphate P has shorter Li-O bond, longer b-axis length and higher resistivity than niobium-doped lithium iron phosphate (see Table 3). For niobium-doped lithium iron phosphate, as the amount of Nb doping increases, the charge-discharge specific capacity and coulombic efficiency of lithium iron phosphate generally show a trend of first increasing and then decreasing, because the doped Nb can replace the Fe site in lithium iron phosphate, which causes the Li-O bond in lithium iron phosphate to be lengthened. On the one hand, this will widen the transmission channel of Li + , accelerate the diffusion of Li + , on the other hand, it can also reduce the binding energy of Li-O bond, making the Li-O bond more prone to break, thereby increasing the diffusion rate of Li + . Therefore, as the amount of Nb doping increases, the Li-O bond in lithium iron phosphate is more prone to break, but when the amount of Nb doping is too much, the lattice structure of lithium iron phosphate will be distorted, causing the b-axis length in lithium iron phosphate to be longer, which prolongs the transmission channel of Li+ the diffusion path of Li+, reduces the diffusion rate of Li+, and thus finally leads to the decrease of the specific charge capacity and efficiency of lithium iron phosphate. + the diffusion path of Li+, reduces the diffusion rate of Li+, and thus finally leads to the decrease of the specific charge capacity and efficiency of lithium iron phosphate.

[0152] Although the illustrative embodiments have been shown and described, it should be understood by those skilled in the art that the above-described embodiments are not to be construed as limiting, and that changes, alternatives and modifications can be suggested as being apparent to one skilled in the art without departing from the spirit, principles and scope of the application.

Claims

1. A method for preparing a modified lithium iron phosphate material, characterized in that, include: The raw material system is obtained by grinding and mixing lithium source, iron source, phosphorus source, niobium source, fluorine-doped zeolite imidazole ester framework structure material nanosheets, carbon source and dispersant; The raw material system is dried to obtain the precursor material; The precursor material is sintered in an inert atmosphere to form a modified lithium iron phosphate material, wherein the modified lithium iron phosphate material includes niobium-doped lithium iron phosphate particles and a fluorine-doped zeolite imidazole ester framework structure material layer disposed on at least part of the outer surface of the lithium iron phosphate particles.

2. The preparation method according to claim 1, characterized in that, The preparation process of the fluorine-doped zeolite imidazolium ester framework structured nanosheets includes: A metal salt solution containing a fluorine source and an imidazole ligand solution were stirred and mixed, and a precipitate was obtained after the reaction. After drying the precipitated product, it was calcined under an inert atmosphere to obtain fluorine-doped zeolite imidazole ester framework structured nanosheets.

3. The preparation method according to claim 2, characterized in that, In the step of stirring and mixing a metal salt solution containing a fluorine source and an imidazole ligand solution to obtain a precipitate after reaction, The metal salt includes zinc salts; and / or, The fluorine source includes at least one of sodium fluoride, potassium fluoride, sodium hydrogen fluoride, and potassium hydrogen fluoride; and / or, The imidazole ligand comprises at least one of 2-methylimidazolium, N-methylimidazolium, and 1-methylimidazolium; and / or, The molar ratio of the metal salt, the fluorine source, and the imidazole ligand is (0.6-12):(0.6-12):(0.2-10).

4. The preparation method according to claim 2 or 3, characterized in that, The preparation process of the fluorine-doped zeolite imidazolium ester framework structure nanosheets also satisfies at least one of the following conditions: (1) The stirring speed is 300 r / min - 700 r / min, and the stirring time is 1h - 10h; (2) The precipitate is obtained by centrifugation after stirring and mixing, wherein the centrifugation speed is 4000 r / min-9000 r / min and the centrifugation time is 5 min-20 min; (3) The drying temperature is 50°C. o C-100 o C; (4) The calcination temperature is 700°C. o C-1000 o C, the calcination time is 1h-6h.

5. The preparation method according to any one of claims 1-3, characterized in that, In the step of grinding and mixing lithium source, iron source, phosphorus source, niobium source, fluorine-doped zeolite imidazole ester framework structure material nanosheets, carbon source and dispersant to obtain the raw material system, The molar ratio of lithium in the lithium source, iron in the iron source, and phosphorus in the phosphorus source is 1:(0.1-0.9):(0.1-0.9):(0.8-1.2); and / or, The molar amount of niobium in the niobium source is 0.2%-1.5% of the molar amount of iron in the iron source; and / or, Based on the total mass of the raw material system, the mass percentage of the fluorine-doped zeolite imidazole ester framework structured material nanosheets is 10%-25%; and / or, Based on the total mass of the raw material system, the mass percentage of the dispersant is 0.2%-2%; and / or, Based on the total mass of the raw material system, the mass percentage of the carbon source is 5%-15%.

6. The preparation method according to any one of claims 1-3, characterized in that, In the step of grinding and mixing lithium source, iron source, phosphorus source, niobium source, fluorine-doped zeolite imidazole ester framework structure material nanosheets, carbon source and dispersant to obtain the raw material system, The niobium source includes at least one selected from niobium nitrate, niobium hydroxide, niobium acetate, niobium carbonate, niobium phosphate, niobium pentoxide, and niobium chloride; and / or, The dispersant includes at least one selected from PEG2000, PEG4000, PEG6000, oleic acid, PVP, graphene, and carbon black; and / or, The carbon source includes at least one of citric acid, glucose, sucrose, starch, xylitol, amino acids, and malic acid.

7. The preparation method according to any one of claims 1-3, characterized in that, The step of sintering the precursor material in an inert atmosphere to form the modified lithium iron phosphate material includes: Phase 1, 1-15 o The temperature was increased from room temperature to 170 °C / min at a rate of C / min. o C-320 o C, and maintain at this temperature for 2-5 hours; The second phase, from 1 to 15 o The temperature was raised to 440 °C / min at a rate of C / min. o C-550 o C, and maintain at this temperature for 2-6 hours; The third stage, with 1-10 o The temperature was raised to 650 °C / min at a rate of C / min. o C-800 o C, and keep at this temperature for 2-7 hours.

8. A modified lithium iron phosphate material, characterized in that, It includes niobium-doped lithium iron phosphate particles and a fluorine-doped zeolite imidazole ester framework material layer covering at least a portion of the surface of the lithium iron phosphate particles.

9. The modified lithium iron phosphate material according to claim 8, characterized in that, The molar amount of niobium in the niobium-doped lithium iron phosphate particles is 0.2%-1.5% of the molar amount of iron.

10. The modified lithium iron phosphate material according to claim 8, characterized in that, Based on the total mass of the modified lithium iron phosphate material, the mass percentage of the fluorine-doped zeolite imidazole ester framework structure material layer is 5wt%-10wt%.

11. A positive electrode plate, characterized in that, It includes a positive current collector and a positive active layer disposed on at least one side of the positive current collector, wherein the positive active layer includes the modified lithium iron phosphate material as described in any one of claims 8-10.

12. A secondary battery, characterized in that, Including the positive electrode sheet as described in claim 11.

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