CN115133010A - Nitrogen-doped carbon modified lithium iron phosphate positive electrode material - Google Patents

Nitrogen-doped carbon modified lithium iron phosphate positive electrode material Download PDF

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CN115133010A
CN115133010A CN202211050006.0A CN202211050006A CN115133010A CN 115133010 A CN115133010 A CN 115133010A CN 202211050006 A CN202211050006 A CN 202211050006A CN 115133010 A CN115133010 A CN 115133010A
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deionized water
nitrogen
iron phosphate
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CN115133010B (en
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何磊
武志强
阮劲进
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Fuyang Longneng Technology Co ltd
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Longneng Technology Nantong Co ltd
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
    • H01M4/366—Composites as layered products
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B25/00—Phosphorus; Compounds thereof
    • C01B25/16—Oxyacids of phosphorus; Salts thereof
    • C01B25/26—Phosphates
    • C01B25/45—Phosphates containing plural metal, or metal and ammonium
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/158—Carbon nanotubes
    • C01B32/168—After-treatment
    • H—ELECTRICITY
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    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
    • H01M4/5825—Oxygenated metallic salts or polyanionic structures, e.g. borates, phosphates, silicates, olivines
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    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
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    • H—ELECTRICITY
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    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
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    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/624—Electric conductive fillers
    • H01M4/625—Carbon or graphite
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/028—Positive electrodes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Abstract

The invention discloses a nitrogen-doped carbon modified lithium iron phosphate cathode material which is prepared by the following method: 1) preparing the boron-doped carbon nanotube: 2) preparing the composite nitrogen-doped functionalized carbon nanotube: 3) adopting functionalized carbon nano tube to LiFePO 4 And modifying the material to obtain the nitrogen-doped carbon modified lithium iron phosphate anode material. The invention discloses a functional modified carbon nano tube LiFePO 4 The material is coated, so that the defect that the carbon nano tube is difficult to disperse is overcomeSo that the function of improving the conductivity and the mechanical property can be fully exerted in a lithium iron phosphate anode material system, and the enhancement effect of the carbon nano tube on the conductivity can be further improved by doping B, N and introducing iron ions and sodium ions.

Description

Nitrogen-doped carbon modified lithium iron phosphate positive electrode material
Technical Field
The invention relates to the field of battery materials, in particular to a nitrogen-doped carbon modified lithium iron phosphate positive electrode material.
Background
Lithium ion batteries are widely used in electronic devices in daily life due to their excellent energy density, good rate performance and cycle life. The LiFePO4 is one of the most common lithium ion battery anode materials at present, has a stable olivine structure, can reversibly intercalate and deintercalate lithium ions, has the advantages of high energy density, stable performance, high safety, environmental friendliness and the like, and is a lithium ion battery anode material with great potential.
But the electron conductivity and Li of pure lithium iron phosphate material + The diffusion coefficient is poor, the electrochemical performance of the composite material is seriously influenced, and the further application of the composite material is limited. At present, the electrochemical performance of the lithium iron phosphate material is generally improved by adopting the technologies of ion doping, cladding and the like. For example, patent CN109004207B discloses a preparation method of a composite lithium iron phosphate cathode material, and CN105428617B discloses a method for preparing internal and external conductive carbon modified lithium iron phosphate.
The carbon coating is simple and convenient to produce, and can obtain a good improvement effect, so that the carbon coating becomes the mainstream direction of the current market. The carbon nano tube has a unique hollow structure, good conductivity and mechanical properties, and is applied to coating modification of a lithium iron phosphate material; for example, patent CN101734927A discloses a method for preparing a lithium iron phosphate/carbon nanotube composite material, CN201710326942.2 discloses a lithium iron phosphate/carbon nanotube composite material for a lithium battery anode material, and a preparation method thereof. However, CNTs have defects that the CNTs are difficult to disperse because of winding caused by a large length-diameter ratio and a high-energy surface caused by a large specific surface area, and larger van der Waals acting force exists among the CNTs, so that the CNTs are easy to highly aggregate or mutually twine; in addition, the improvement of the conductivity, stability and the like of the lithium iron phosphate material by the single carbon nanotube coating is still limited, and a reliable technology for solving the defects is lacked in the prior art.
Disclosure of Invention
The invention aims to solve the technical problem of providing a nitrogen-doped carbon modified lithium iron phosphate positive electrode material aiming at the defects in the prior art.
In order to solve the technical problems, the technical scheme adopted by the invention is as follows: a nitrogen-doped carbon-modified lithium iron phosphate positive electrode material is prepared by the following method:
1) preparing the boron-doped carbon nanotube:
1-1) with B and B 2 O 3 Covering CNTs on the boron source serving as the mixture of the carbon dioxide and the carbon dioxide, and heating and reacting under the protection of argon;
1-2) adding the product obtained in the step 1-1) into an alkali solution, stirring, transferring the obtained mixture into a reaction kettle, stirring and reacting under heating, filtering after the reaction is finished, washing the solid product with deionized water, and drying to obtain the boron-doped carbon nanotube: B-CNTs;
2) preparing the composite nitrogen-doped functionalized carbon nanotube:
2-1) adding the B-CNTs obtained in the step 1) into hot deionized water, and performing ultrasonic treatment to obtain a B-CNTs dispersion liquid;
2-2) adding the diethyltriamine pentaacetic acid iron sodium into the hot deionized water, and stirring;
2-3) adding the B-CNTs dispersion liquid obtained in the step 2-1) into the product obtained in the step 2-2), adding DCC, stirring and reacting under heating, filtering after the reaction is finished, and sequentially cleaning and drying the solid product by using deionized water and ethanol to obtain a functionalized carbon nanotube;
3) adopting the functionalized carbon nano tube pair LiFePO prepared in the step 2) 4 And modifying the material to obtain the nitrogen-doped carbon modified lithium iron phosphate anode material.
Preferably, the nitrogen-doped carbon-modified lithium iron phosphate positive electrode material is prepared by the following method:
1) preparing the boron-doped carbon nanotube:
1-1) with B and B 2 O 3 Covering CNTs on the boron source, heating to 1050-;
1-2) adding the product obtained in the step 1-1) into excessive alkali solution, stirring for 5-30min, transferring the obtained mixture into a reaction kettle, stirring and reacting for 2-6h at 35-75 ℃ and at 300-900rpm, filtering after the reaction is finished, washing the solid product with deionized water, and drying for 6-24h to obtain the boron-doped carbon nanotube: B-CNTs;
2) preparing a composite doped functionalized carbon nanotube:
2-1) adding the B-CNTs obtained in the step 1) into deionized water at the temperature of 60-85 ℃, and performing ultrasonic treatment for 5-30min to obtain a B-CNTs dispersion liquid;
2-2) adding sodium iron diethyltriamine pentaacetate into deionized water at the temperature of 60-85 ℃, and stirring for 5-10 min;
2-3) adding the B-CNTs dispersion liquid into the product obtained in the step 2-2), adding DCC, stirring and reacting for 1-5h at 45-75 ℃, filtering after the reaction is finished, sequentially cleaning the solid product with deionized water and ethanol, and drying for 12-36h at 50-70 ℃ to obtain the functionalized carbon nanotube;
3) adopting the functionalized carbon nano tube pair LiFePO prepared in the step 2) 4 And modifying the material to obtain the nitrogen-doped carbon modified lithium iron phosphate anode material.
Preferably, the alkali solution is a sodium hydroxide solution or a potassium hydroxide solution with a concentration of 2 to 5 mol/L.
Preferably, B is B in the boron source 2 O 3 The mass ratio of (A) to (B) is 1:0.3-1: 5. .
Preferably, the step 3) specifically includes:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, and stirring for dissolving to obtain a solution A;
3-2) reacting LiOH. H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
3-3) mixing solution A with solution B, and then adding H 3 PO 4 Adjusting the pH value of a reaction system to 5-7, adding ethanol, and stirring to obtain a precursor solution;
3-4) adding the functionalized carbon nano tube prepared in the step 2) into deionized water, and performing ultrasonic treatment to obtain a functionalized carbon nano tube dispersion liquid;
3-5) adding the functional carbon nano tube dispersion liquid prepared in the step 2) into the precursor liquid under continuous stirring, stirring and performing ultrasonic treatment;
3-6) placing the mixture obtained in the step 3-5) in a microwave, heating for 20-60min, cooling after the reaction is finished, filtering, cleaning a solid product, drying in vacuum, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
Preferably, the step 3) specifically includes:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, and stirring for dissolving to obtain a solution A;
3-2) reacting LiOH. H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
3-3) mixing solution A with solution B, and then adding H 3 PO 4 Adjusting the pH value of the reaction system to 5-6, adding ethanol, and stirring for 5-15min to obtain a precursor solution;
3-4) adding the functionalized carbon nano tube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 10-45min to obtain a functionalized carbon nano tube dispersion liquid;
3-5) adding the functionalized carbon nanotube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 5-30 min;
3-6) placing the mixture obtained in the step 3-5) in microwave, heating at the temperature of 150-220 ℃ for 20-60min, cooling after the reaction is finished, filtering, sequentially cleaning the solid product with deionized water and ethanol, vacuum-drying at the temperature of 95-140 ℃ for 2-8h, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
Preferably, the step 3) specifically includes:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing ethanol, adding ascorbic acid and lauric acid, and stirring for dissolving to obtain a solution A;
3-2) reacting LiOH & H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
3-3) adding the functionalized carbon nano tube prepared in the step 2) into deionized water, and performing ultrasonic treatment to obtain a functionalized carbon nano tube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing the solutions, stirring, and adjusting the pH value of a reaction system to 5-7 to obtain a precursor solution;
3-5) adding the functionalized carbon nanotube dispersion liquid into the precursor liquid under continuous stirring, stirring and performing ultrasonic treatment;
3-6) transferring the mixed solution obtained in the step 3-5) into a reaction kettle, and reacting under heating and pressurizing; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, drying in vacuum, cooling and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
Preferably, the step 3) specifically includes:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing ethanol, adding ascorbic acid and lauric acid, and stirring for dissolving to obtain a solution A;
3-2) reacting LiOH. H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
3-3) adding the functionalized carbon nanotube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 10-45min to obtain a functionalized carbon nanotube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing the solutions, stirring, and adjusting the pH value of a reaction system to 5-6 to obtain a precursor solution;
3-5) adding the functional carbon nano tube dispersion liquid prepared in the step 2) into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 5-30 min;
3-6) transferring the mixed solution obtained in the step 3-5) to a reaction kettle, and reacting for 0.5-3h at the temperature of 260-450 ℃ and under the pressure of 15-45 MPa; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, drying for 2-8h at 95-140 ℃ in vacuum, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
Preferably, in the step 3-5), the addition amount of the functionalized carbon nanotubes in the functionalized carbon nanotube dispersion liquid is LiFePO in the precursor liquid 4 2.5-10% of the mass of (A).
Preferably, in the step 3-5), the amount of the functionalized carbon nanotubes added in the functionalized carbon nanotube dispersion liquid is LiFePO in the precursor liquid 4 4 to 6.5 percent of the mass of (A).
The invention has the beneficial effects that:
b is doped into a carbon nano tube to prepare a boron-doped carbon nano tube, and then N, Na and Fe are compositely doped and modified into the boron-doped carbon nano tube through diethyltriamine pentaacetic acid iron sodium salt to obtain a functional carbon nano tube which is doped with B, N and uniformly loaded with Na and Fe on the surface; finally, LiFePO is subjected to functionalization by the carbon nano tube 4 Carrying out in-situ coating modification on the material to prepare the nitrogen-doped carbon modified lithium iron phosphate anode material;
in the invention, the carbon nano tube is doped with the B, so that the charge transfer amount between the carbon nano tube and the Li in the nitrogen-doped carbon modified lithium iron phosphate anode material can be increased, and the conductivity between the carbon nano tube and the Li is improved;
in the invention, nitrogen is doped in the carbon nano tube, on one hand, N and C are combined to form a C-N bond, and simultaneously, an electron is lost by an N atom, so that the electron concentration of CNTs is increased, and the conductivity of the CNTs can be improved; on the other hand, the doping of N can also obviously improve the hydrophilicity of CNTs, improve the dispersibility of the CNTs and overcome the defect that the conventional CNTs are easy to agglomerate;
in the invention, iron ions are doped in the carbon nano tube, so that the Fermi level can be reduced, the energy band structure of the carbon nano tube is changed, the contact potential barrier between the carbon nano tube and the electrode is reduced, and the conductivity of the carbon nano tube is improved; furthermore, the iron ions in the invention can be uniformly loaded on the surface of the three-dimensional network structure of the carbon nano tube, the state density near the Fermi level can be increased, a stable conductive network is constructed, and the conductivity can be improved;
in the invention, sodium ions are doped in the carbon nano tube, so that the surface active reaction sites of the carbon nano tube can be increased, a stable conductive network can be constructed in the battery active material, and the conductivity of the carbon nano tube can be further improved; and the introduction of iron ions and sodium ions can enhance the surface contact of the carbon nano tube and the battery active material, and can improve the dispersibility of the carbon nano tube in an electric anode material system.
The invention discloses a functional modified carbon nano tube LiFePO 4 The material is coated, so that the defect that the carbon nano tube is difficult to disperse is overcome, and the material can fully play a role in improving the conductivity in a lithium iron phosphate positive electrode material systemAnd mechanical property, and the enhancement effect of the carbon nano tube on the conductivity can be further improved by doping B, N and introducing iron ions and sodium ions.
Drawings
Fig. 1 is a charge-discharge cycle curve of a battery assembled by lithium iron phosphate cathode materials prepared in examples and comparative examples of the present invention.
Detailed Description
The present invention is further described in detail below with reference to examples so that those skilled in the art can practice the invention with reference to the description.
It will be understood that terms such as "having," "including," and "comprising," as used herein, do not preclude the presence or addition of one or more other elements or groups thereof.
The test methods used in the following examples are all conventional methods unless otherwise specified. The material reagents and the like used in the following examples are commercially available unless otherwise specified. The following examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.
As a preferred technical scheme, the nitrogen-doped carbon-modified lithium iron phosphate positive electrode material in each embodiment of the present invention is prepared by the following method:
1) preparing the boron-doped carbon nanotube:
1-1) with B and B 2 O 3 Covering CNTs on the boron source, heating to 1050-;
in a preferred embodiment, the boron source is B: B 2 O 3 The mass ratio of (1: 0.3) - (1: 5);
1-2) adding the product obtained in the step 1-1) into excessive alkali solution, stirring for 5-30min, transferring the obtained mixture into a reaction kettle, stirring and reacting for 2-6h at 35-75 ℃ and at 300-900rpm, filtering after the reaction is finished, washing the solid product with deionized water, and drying for 6-24h to obtain the boron-doped carbon nanotube: B-CNTs;
in a preferred embodiment, the alkali solution is a sodium hydroxide solution or a potassium hydroxide solution having a concentration of 2 to 5 mol/L.
2) Preparing the composite nitrogen-doped functionalized carbon nanotube:
2-1) adding the B-CNTs obtained in the step 1) into deionized water at the temperature of 60-85 ℃, and performing ultrasonic treatment for 5-30min to obtain a B-CNTs dispersion liquid;
2-2) adding sodium iron diethyltriamine pentaacetate into deionized water at the temperature of 60-85 ℃, and stirring for 5-10 min;
2-3) adding the B-CNTs dispersion liquid into the product obtained in the step 2-2), adding DCC, stirring and reacting for 1-5h at 45-75 ℃, filtering after the reaction is finished, sequentially cleaning the solid product with deionized water and ethanol, and drying for 12-36h at 50-70 ℃ to obtain the functionalized carbon nanotube.
3) Adopting the functionalized carbon nano tube pair LiFePO prepared in the step 2) 4 The material is subjected to in-situ coating modification to prepare the nitrogen-doped carbon modified lithium iron phosphate anode material.
In some preferred embodiments, the microwave method is used for LiFePO in the step 3) 4 Coating the functionalized carbon nano tube in situ, which comprises the following specific steps:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, and stirring to dissolve to obtain a solution A;
3-2) reacting LiOH. H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
3-3) mixing solution A with solution B, and then adding H 3 PO 4 Adjusting the pH value of the reaction system to 5-6, adding ethanol, and stirring for 5-15min to obtain a precursor solution;
3-4) adding the functionalized carbon nanotube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 10-45min to obtain a functionalized carbon nanotube dispersion liquid;
3-5) adding the functionalized carbon nanotube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 5-30 min;
3-6) placing the mixture obtained in the step 3-6) in a microwave, heating at the temperature of 150-220 ℃ for 20-60min, cooling after the reaction is finished, filtering, sequentially cleaning the solid product with deionized water and ethanol, vacuum-drying at the temperature of 95-140 ℃ for 2-8h, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
In other preferred embodiments, the hydrothermal method is used for LiFePO in step 3) 4 Coating the functionalized carbon nano tube in situ, which comprises the following specific steps:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing ethanol, adding ascorbic acid and lauric acid, and stirring for dissolving to obtain a solution A;
3-2) reacting LiOH. H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
3-3) adding the functionalized carbon nanotube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 10-45min to obtain a functionalized carbon nanotube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing the solutions, stirring, and adjusting the pH value of a reaction system to 5-6 to obtain a precursor solution;
3-5) adding the functional carbon nano tube dispersion liquid prepared in the step 2) into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 5-30 min;
3-6) transferring the mixed solution obtained in the step 3-5) to a reaction kettle, and reacting for 0.5-3h at the temperature of 260-450 ℃ and under the pressure of 15-45 MPa; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, drying for 2-8h at 95-140 ℃ in vacuum, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
In a preferred embodiment, in the two processes for in-situ coating of the functionalized carbon nanotubes, in step 3-5), the addition amount of the functionalized carbon nanotubes in the functionalized carbon nanotube dispersion liquid is LiFePO in the precursor liquid 4 2.5 to 10% by mass, more preferably 4 to 6.5% by mass of (A).
The following description is made for the purpose of illustrating the general principles of the present invention and for the purpose of promoting an understanding of the invention.
Firstly, the general scheme of the invention is as follows: firstly, doping B into a conventional carbon nano tube to prepare a boron-doped carbon nano tube, and then carrying out N, Na and Fe composite doping on the boron-doped carbon nano tube by using diethyltriamine pentaacetic acid iron sodium salt to obtain doped B, N, wherein the surface of the doped B, N is uniformly loaded with NaFe functionalized carbon nanotubes; finally, LiFePO is subjected to functionalization of the carbon nano tube 4 The material is subjected to in-situ coating modification to prepare the nitrogen-doped carbon modified lithium iron phosphate cathode material.
(1) In the present invention, B and B are used for boron doping 2 O 3 The mixture of (3) is used as a boron source, so that the energy consumption can be reduced, and the doping proportion of B can be improved.
B 2 O 3 The boron atoms are decomposed into boron atoms and oxygen atoms at high temperature, the boron atoms can form a B-C bond with carbon atoms to realize B doping, and B-CNTs are obtained, wherein the main reaction equation is as follows:
B 2 O 3 →2[B]+3[O]；
CNTs+[O]=CO↑；
[B]+CNTs=B-CNTs；
b is gasified at high temperature to generate boron atoms, the boron atoms can form B-C bonds with carbon atoms, and the main reaction equation is as follows:
B→[B]；
[B]+CNTs=B-CNTs；
B 2 O 3 when the boron source is used, gas can be generated due to the reaction of partial oxygen atoms and CNTs, so that the energy consumption is high; when B is used as a boron source, the doping proportion of B is easily reduced because B is not completely gasified due to the protection of argon, so B and B are used in the invention 2 O 3 The mixture of (3) is used as a boron source, so that the energy consumption can be reduced, and the doping proportion of B can be improved.
In the invention, the carbon nano tube is doped with B, so that the charge transfer amount between the carbon nano tube and Li in the nitrogen-doped carbon modified lithium iron phosphate cathode material can be increased, and the conductivity between the carbon nano tube and Li is increased (1 generation of sharp peak. theoretical research on the influence of doping on the conductivity of the carbon nano tube [ D ]. North China university of science and engineering, 2017.).
(2) In step 1-2), the invention adds excessive alkali solution to remove unreacted B and B 2 O 3 On the other hand, excessive alkali solution can also act on the carbon nano tube to introduce abundant hydroxyl groups to the surface of the carbon nano tube, so that hydroxylation of the carbon nano tube is realized.
In the invention, diethyltriamine pentaacetic acid iron sodium is used as a functional reagent for modifying B-CNTs, and the chemical structural formula of the diethyltriamine pentaacetic acid iron sodium is shown as the following formula I:
Figure DEST_PATH_IMAGE001
formula I
It can be seen that the iron sodium diethyltriamine pentaacetate has rich carboxyl functional groups, iron ions and sodium ions are complexed thereon, and it contains N element.
According to the invention, the diethylenetriamine pentaacetic acid iron sodium is uniformly and firmly grafted to the surface of B-CNTs through the condensation reaction of a carboxyl functional group on the diethylenetriamine pentaacetic acid iron sodium and a hydroxyl functional group of the B-CNTs, so that the N doping of the B-CNTs is realized on one hand, and a large amount of uniformly loaded iron ions and sodium ions can be introduced into the surface of the B-CNTs on the other hand.
The carbon nano tube is doped with nitrogen, on one hand, N and C are combined to form a C-N bond, and meanwhile, N atoms lose one electron, so that the electron concentration of the CNTs is increased, and the conductivity of the CNTs can be improved (1 Zhang Asia east. Experimental research on the conductivity of the carbon nano tube by doping transition metals [ D ]. North China university of science); on the other hand, the doping of N can also obviously improve the hydrophilicity of CNTs, improve the dispersibility of the CNTs, and overcome the defect that the conventional CNTs (the conventional CNTs are easy to highly aggregate or intertwine with each other due to large length-diameter ratio caused winding and high-energy surface caused by large specific surface area, and larger van der Waals acting force exists among the CNTs, so that the CNTs are difficult to disperse) are easy to aggregate.
The iron ions are doped in the carbon nano tube, so that the Fermi level can be reduced, the energy band structure of the carbon nano tube is changed, the contact potential barrier between the carbon nano tube and an electrode is reduced, and the conductivity of the carbon nano tube is improved; further, the iron ions in the invention can be uniformly loaded on the surface of the three-dimensional network structure of the carbon nano tube, the state density near the Fermi level can be increased, a stable conductive network is constructed, and the conductivity is improved.
The sodium ions are doped in the carbon nano tube, so that the surface active reaction sites of the carbon nano tube can be increased, a stable conductive network can be constructed in the battery active material, and the conductivity of the carbon nano tube can be further improved.
The introduction of iron ions and sodium ions can enhance the surface contact of the carbon nano tube and the battery active material, and can improve the dispersibility of the carbon nano tube in an electric anode material system.
(3) In the invention, the functionalized carbon nano tube is used for LiFePO 4 The material is coated in situ, a three-dimensional conductive network can be formed, and the electron conductivity and the cycle stability of the lithium iron phosphate anode material are enhanced; and when the carbon nano tube is coated in situ, the carbonization of a secondary carbon source (citric acid or ascorbic acid and lauric acid) can form a CB (carbon black) coating layer to be coated on the LiFePO 4 The particle surface can inhibit the agglomeration or enlargement of the particles and enhance the conductivity of the particles; the invention enables LiFePO to be realized through the combined action of the functionalized carbon nano tube and the CB 4 The material has better conductivity, and the multiplying power and the cycle performance of the lithium iron phosphate can be improved.
The invention utilizes the functionalized modified carbon nano tube to LiFePO 4 The material is coated, the defect that the carbon nano tube is difficult to disperse is overcome, so that the material can fully play a role in improving the conductivity and the mechanical property in a lithium iron phosphate positive electrode material system, and meanwhile, the enhancement effect of the carbon nano tube on the conductivity can be further improved by doping B, N and introducing iron ions and sodium ions.
Typical but non-limiting examples of the invention are as follows:
example 1
A nitrogen-doped carbon-modified lithium iron phosphate positive electrode material is prepared by the following method:
1) preparing the boron-doped carbon nanotube:
1-1) with B and B 2 O 3 The mixture is used as a boron source, CNTs (a commercial multi-wall carbon nano tube with the length of 0.5-2 microns and the diameter of 30-50nm, which is purchased from Jiangsu Xiancheng nano material science and technology Co., Ltd.) are covered on the excessive boron source, the mixture is heated to 1250 ℃ under the protection of argon, the heating rate is 15 ℃/min, and the heat preservation is carried out for 5 hours; wherein B is B 2 O 3 In mass ratio ofIs 1: 2;
1-2) adding the product obtained in the step 1-1) into an excessive sodium hydroxide solution with the concentration of 4mol/L, stirring for 20min, transferring the obtained mixture into a reaction kettle, stirring and reacting at 55 ℃ and 600rpm for 5h, filtering after the reaction is finished, washing the solid product with deionized water, and drying for 12h to obtain the boron-doped carbon nanotube: B-CNTs;
2) preparing the composite nitrogen-doped functionalized carbon nanotube:
2-1) adding the B-CNTs obtained in the step 1) into hot deionized water at 70 ℃, and performing ultrasonic treatment for 15min to obtain a B-CNTs dispersion liquid;
2-2) adding sodium iron diethyltriamine pentaacetate into hot deionized water at the temperature of 70 ℃, and stirring for 7 min;
2-3) adding the B-CNTs dispersion liquid into the product obtained in the step 2-2), adding DCC (1, 3-dicyclohexylcarbodiimide), stirring and reacting for 3h at 60 ℃, filtering after the reaction is finished, sequentially cleaning the solid product with deionized water and ethanol, and drying for 24h at 55 ℃ to obtain the functionalized carbon nanotube; wherein the mass ratio of the diethyltriamine pentaacetic acid iron sodium to the B-CNTs is 6: 100.
3) Adopting the functionalized carbon nano tube pair LiFePO prepared in the step 2) 4 The material is subjected to in-situ coating modification to prepare the nitrogen-doped carbon modified lithium iron phosphate anode material, and the method comprises the following specific steps of:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, and stirring to dissolve to obtain a solution A;
3-2) reacting LiOH. H 2 Adding O into deionized water, and stirring to dissolve to obtain a solution B;
3-3) mixing solution A with solution B, and then adding H 3 PO 4 Adjusting the pH value of the reaction system to 6, adding ethanol, and stirring for 8min to obtain a precursor solution; wherein, according to Fe: p: raw materials are added with the molar ratio of Li being 1:1: 1.5;
3-4) adding the functionalized carbon nanotube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 30min to obtain a functionalized carbon nanotube dispersion liquid;
3-5) adding the functional carbon nano tube dispersion liquid into the precursor liquid under continuous stirring, stirring and ultra-treatingSounding for 15 min; wherein the addition amount of the functionalized carbon nanotubes in the functionalized carbon nanotube dispersion liquid is LiFePO in the precursor liquid 4 4% of the mass of (a),
3-6) placing the mixture obtained in the step 3-5) in a microwave, heating for 40min at 190 ℃, cooling after the reaction is finished, filtering, sequentially cleaning solid products with deionized water and ethanol, vacuum-drying for 5h at 115 ℃, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
Example 2
A nitrogen-doped carbon modified lithium iron phosphate cathode material is prepared by the following method:
1) preparing the boron-doped carbon nanotube:
1-1) with B and B 2 O 3 The mixture is used as a boron source, CNTs (a commercial multi-wall carbon nano tube with the length of 0.5-2 microns and the diameter of 30-50nm, which is purchased from Jiangsu Xiancheng nano material science and technology Co., Ltd.) are covered on the excessive boron source, the mixture is heated to 1250 ℃ under the protection of argon, the heating rate is 15 ℃/min, and the heat preservation is carried out for 5 hours; wherein B is B 2 O 3 The mass ratio of (A) to (B) is 1: 2;
1-2) adding the product obtained in the step 1-1) into an excessive sodium hydroxide solution with the concentration of 4mol/L, stirring for 20min, transferring the obtained mixture into a reaction kettle, stirring and reacting at 55 ℃ and 600rpm for 5h, filtering after the reaction is finished, washing the solid product with deionized water, and drying for 12h to obtain the boron-doped carbon nanotube: B-CNTs;
2) preparing the composite nitrogen-doped functionalized carbon nanotube:
2-1) adding the B-CNTs obtained in the step 1) into hot deionized water at 70 ℃, and performing ultrasonic treatment for 15min to obtain a B-CNTs dispersion liquid;
2-2) adding the diethyltriamine pentaacetic acid iron sodium into hot deionized water at the temperature of 70 ℃, and stirring for 7 min;
2-3) adding the B-CNTs dispersion liquid into the product obtained in the step 2-2), adding DCC (1, 3-dicyclohexylcarbodiimide), stirring and reacting for 3h at 60 ℃, filtering after the reaction is finished, sequentially cleaning the solid product with deionized water and ethanol, and drying for 24h at 55 ℃ to obtain the functionalized carbon nanotube; wherein the mass ratio of the diethyltriamine pentaacetic acid iron sodium to the B-CNTs is 6: 100.
3) Adopting the functionalized carbon nano tube pair LiFePO prepared in the step 2) 4 The material is subjected to in-situ coating modification to prepare the nitrogen-doped carbon modified lithium iron phosphate anode material, and the method comprises the following specific steps:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, and stirring to dissolve to obtain a solution A;
3-2) reacting LiOH & H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
3-3) mixing solution A with solution B, and then adding H 3 PO 4 Adjusting the pH value of the reaction system to 6, adding ethanol, and stirring for 8min to obtain a precursor solution; wherein, according to Fe: p: raw materials are added with the molar ratio of Li being 1:1: 1.5;
3-4) adding the functionalized carbon nanotube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 30min to obtain a functionalized carbon nanotube dispersion liquid;
3-5) adding the functionalized carbon nanotube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 15 min; wherein the addition amount of the functionalized carbon nanotubes in the functionalized carbon nanotube dispersion liquid is LiFePO in the precursor liquid 4 5% of the mass of (a),
3-6) placing the mixture obtained in the step 3-5) in a microwave, heating for 40min at 190 ℃, cooling after the reaction is finished, filtering, sequentially cleaning solid products by using deionized water and ethanol, drying for 5h in vacuum at 115 ℃, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
Example 3
A nitrogen-doped carbon modified lithium iron phosphate cathode material is prepared by the following method:
1-1) with B and B 2 O 3 The mixture is used as a boron source, CNTs (a commercially available multi-walled carbon nanotube with the length of 0.5-2 microns and the diameter of 30-50nm, which is purchased from Jiangsu Xiancheng nano material science and technology Co., Ltd.) is covered on the excessive boron source, the mixture is heated to 1250 ℃ under the protection of argon, the heating rate is 15 ℃/min, and the heat preservation is carried out for 5 hours; wherein B is B 2 O 3 The mass ratio of (A) to (B) is 1: 2;
1-2) adding the product obtained in the step 1-1) into an excessive sodium hydroxide solution with the concentration of 4mol/L, stirring for 20min, transferring the obtained mixture into a reaction kettle, stirring and reacting at 55 ℃ and 600rpm for 5h, filtering after the reaction is finished, washing the solid product with deionized water, and drying for 12h to obtain the boron-doped carbon nanotube: B-CNTs;
2) preparing the composite nitrogen-doped functionalized carbon nanotube:
2-1) adding the B-CNTs obtained in the step 1) into hot deionized water at 70 ℃, and performing ultrasonic treatment for 15min to obtain a B-CNTs dispersion liquid;
2-2) adding sodium iron diethyltriamine pentaacetate into hot deionized water at the temperature of 70 ℃, and stirring for 7 min;
2-3) adding the B-CNTs dispersion liquid into the product obtained in the step 2-2), adding DCC (1, 3-dicyclohexylcarbodiimide), stirring and reacting for 3h at 60 ℃, filtering after the reaction is finished, sequentially cleaning the solid product with deionized water and ethanol, and drying for 24h at 55 ℃ to obtain the functionalized carbon nanotube; wherein the mass ratio of the diethyltriamine pentaacetic acid iron sodium to the B-CNTs is 6: 100.
3) Adopting the functionalized carbon nano tube pair LiFePO prepared in the step 2) 4 The material is subjected to in-situ coating modification to prepare the nitrogen-doped carbon modified lithium iron phosphate anode material, and the method comprises the following specific steps:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing ethanol, adding ascorbic acid and lauric acid, and stirring for dissolving to obtain a solution A;
3-2) reacting LiOH. H 2 Adding O into deionized water, and stirring to dissolve to obtain a solution B;
3-3) adding the functionalized carbon nanotube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 30min to obtain a functionalized carbon nanotube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing the solutions, stirring, and adjusting the pH value of a reaction system to 5-6 to obtain a precursor solution; wherein, according to the weight ratio of Fe: p: raw materials are added with the molar ratio of Li being 1:1: 1.5;
3-5) adding the functionalized carbon nanotube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 15 min; wherein the functionalized carbonThe addition amount of the functionalized carbon nanotube in the nanotube dispersion liquid is LiFePO in the precursor liquid 4 4% of the mass of (c);
3-6) transferring the mixed solution obtained in the step 3-5) into a reaction kettle, and reacting for 2 hours at the temperature of 420 ℃ and under the pressure of 25 MPa; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, drying for 5 hours in vacuum at the temperature of 115 ℃, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
Example 4
A nitrogen-doped carbon modified lithium iron phosphate cathode material is prepared by the following method:
1-1) with B and B 2 O 3 The mixture is used as a boron source, CNTs (a commercially available multi-walled carbon nanotube with the length of 0.5-2 microns and the diameter of 30-50nm, which is purchased from Jiangsu Xiancheng nano material science and technology Co., Ltd.) is covered on the excessive boron source, the mixture is heated to 1250 ℃ under the protection of argon, the heating rate is 15 ℃/min, and the heat preservation is carried out for 5 hours; wherein B is B 2 O 3 The mass ratio of (A) to (B) is 1: 2;
1-2) adding the product obtained in the step 1-1) into an excessive sodium hydroxide solution with the concentration of 4mol/L, stirring for 20min, transferring the obtained mixture into a reaction kettle, stirring and reacting at 55 ℃ and 600rpm for 5h, filtering after the reaction is finished, washing the solid product with deionized water, and drying for 12h to obtain the boron-doped carbon nanotube: B-CNTs;
2) preparing the composite nitrogen-doped functionalized carbon nanotube:
2-1) adding the B-CNTs obtained in the step 1) into hot deionized water at 70 ℃, and performing ultrasonic treatment for 15min to obtain a B-CNTs dispersion liquid;
2-2) adding sodium iron diethyltriamine pentaacetate into hot deionized water at the temperature of 70 ℃, and stirring for 7 min;
2-3) adding the B-CNTs dispersion liquid into the product obtained in the step 2-2), adding DCC (1, 3-dicyclohexylcarbodiimide), stirring at 60 ℃ for reaction for 3 hours, filtering after the reaction is finished, sequentially cleaning the solid product by deionized water and ethanol, and drying at 55 ℃ for 24 hours to obtain the functionalized carbon nanotube; wherein the mass ratio of the diethyltriamine pentaacetic acid iron sodium to the B-CNTs is 6: 100.
3) Adopting the functions prepared in the step 2)Carbon nanotube pair LiFePO 4 The material is subjected to in-situ coating modification to prepare the nitrogen-doped carbon modified lithium iron phosphate anode material, and the method comprises the following specific steps:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing ethanol, adding ascorbic acid and lauric acid, and stirring for dissolving to obtain a solution A;
3-2) reacting LiOH. H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
3-3) adding the functionalized carbon nanotube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 30min to obtain a functionalized carbon nanotube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing the solutions, stirring, and adjusting the pH value of a reaction system to 5-6 to obtain a precursor solution; wherein, according to Fe: p: raw materials are added with the molar ratio of Li being 1:1: 1.5;
3-5) adding the functionalized carbon nanotube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 15 min; wherein the addition amount of the functional carbon nanotubes in the functional carbon nanotube dispersion liquid is LiFePO in the precursor liquid 4 5% of the mass of (c);
3-6) transferring the mixed solution obtained in the step 3-5) into a reaction kettle, and reacting for 2 hours at the temperature of 420 ℃ and under the pressure of 25 MPa; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, drying for 5 hours in vacuum at the temperature of 115 ℃, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
The following comparative examples are provided to further illustrate the present invention.
Comparative example 1
A lithium iron phosphate cathode material is prepared by the following method:
1-1) FeSO 4 ·7H 2 Adding O into deionized water containing ethanol, adding ascorbic acid and lauric acid, and stirring for dissolving to obtain a solution A;
1-2) reacting LiOH. H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
1-3) mixing solution A, solution B and solution H 3 PO 4 Mixing the solutions, stirring, and adjusting pH of the reaction system to 5-6 to obtainTo a precursor solution; wherein, according to the weight ratio of Fe: p: raw materials are added with the molar ratio of Li being 1:1: 1.5;
1-4) placing the precursor solution obtained in the step 1-3) in a microwave, heating for 40min at 190 ℃, cooling after the reaction is finished, filtering, sequentially cleaning solid products with deionized water and ethanol, drying for 5h in vacuum at 115 ℃, cooling, and grinding to obtain the lithium iron phosphate cathode material.
Comparative example 2
A lithium iron phosphate cathode material is prepared by the following method:
1) using CNTs to LiFePO 4 The material is subjected to in-situ coating modification to prepare the lithium iron phosphate anode material, and the method comprises the following specific steps of:
1-1）FeSO 4 ·7H 2 adding O into deionized water containing ethanol, adding ascorbic acid and lauric acid, and stirring for dissolving to obtain a solution A;
1-2) reacting LiOH. H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
1-3) adding CNTs (multi-wall carbon nano tubes sold in the market, the length of which is 0.5-2 microns, the diameter of which is 30-50nm and is purchased from Jiangsu Xiancheng nano material science and technology limited) into deionized water, and carrying out ultrasonic treatment for 30min to obtain a carbon nano tube dispersion liquid;
1-4) mixing solution A, solution B and H 3 PO 4 Mixing the solutions, stirring, and adjusting the pH value of a reaction system to 5-6 to obtain a precursor solution; wherein, according to Fe: p: raw materials are added with the molar ratio of Li being 1:1: 1.5;
1-5) adding the carbon nano tube dispersion liquid into the precursor liquid under the condition of continuous stirring, stirring and carrying out ultrasonic treatment for 15 min; wherein the addition amount of the carbon nanotubes in the carbon nanotube dispersion liquid is LiFePO in the precursor liquid 4 5% of the mass of (c);
1-6) placing the mixture obtained in the step 1-5) in a microwave, heating for 40min at 190 ℃, cooling after the reaction is finished, filtering, sequentially cleaning solid products with deionized water and ethanol, vacuum-drying for 5h at 115 ℃, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
Comparative example 3
A lithium iron phosphate cathode material is prepared by the following method:
1) preparing the boron-doped carbon nanotube:
1-1) with B and B 2 O 3 The mixture is used as a boron source, CNTs (a commercially available multi-walled carbon nanotube with the length of 0.5-2 microns and the diameter of 30-50nm, which is purchased from Jiangsu Xiancheng nano material science and technology Co., Ltd.) is covered on the excessive boron source, the mixture is heated to 1250 ℃ under the protection of argon, the heating rate is 15 ℃/min, and the heat preservation is carried out for 5 hours; wherein B is B 2 O 3 The mass ratio of (A) to (B) is 1: 2;
1-2) adding the product obtained in the step 1-1) into an excessive sodium hydroxide solution with the concentration of 4mol/L, stirring for 20min, transferring the obtained mixture into a reaction kettle, stirring and reacting at 55 ℃ and 600rpm for 5h, filtering after the reaction is finished, washing the solid product with deionized water, and drying for 12h to obtain the boron-doped carbon nanotube: B-CNTs;
2) using B-CNTs to LiFePO 4 The material is subjected to in-situ coating modification to prepare the lithium iron phosphate anode material, and the method comprises the following specific steps:
2-1）FeSO 4 ·7H 2 adding O into deionized water containing ethanol, adding ascorbic acid and lauric acid, and stirring for dissolving to obtain a solution A;
2-2) reacting LiOH. H 2 Adding O into deionized water, and stirring to dissolve to obtain a solution B;
2-3) adding the B-CNTs prepared in the step 1) into deionized water, and performing ultrasonic treatment for 30min to obtain a carbon nano tube dispersion liquid;
2-4) mixing solution A, solution B and H 3 PO 4 Mixing the solutions, stirring, and adjusting the pH value of a reaction system to 5-6 to obtain a precursor solution; wherein, according to Fe: p: adding raw materials with the molar ratio of Li being 1:1: 1.5;
2-5) adding the carbon nano tube dispersion liquid into the precursor liquid under the condition of continuous stirring, stirring and carrying out ultrasonic treatment for 15 min; wherein the addition amount of the B-CNTs in the carbon nano tube dispersion liquid is LiFePO in the precursor liquid 4 5% of the mass of (c);
2-6) transferring the mixed solution obtained in the step 1-5) into a reaction kettle, and reacting for 2 hours at the temperature of 420 ℃ and under the pressure of 25 MPa; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, drying for 5 hours in vacuum at the temperature of 115 ℃, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
The lithium iron phosphate positive electrode materials prepared in the examples 1 to 4 and the comparative example 1 are assembled into a button cell, and when the button cell is charged and discharged, the first discharge capacity of the examples 1 to 4 can reach the following value at the 1C multiplying power: 151.9mAh/g, 154.6mAh/g, 158.3mAh/g and 162.1mAh/g, referring to FIG. 1, after 100 charge-discharge cycles, the discharge capacities are 151.5mAh/g, 154.2mAh/g, 154.4mAh/g and 161.9mAh/g in sequence, the capacity retention rates are 99.7%, 99.9% and 99.9% in sequence, and the discharge capacities are basically unchanged, which shows that the lithium iron phosphate cathode material prepared by the invention has good stability. The first discharge capacities of comparative examples 1 to 3 were, in order: 121.2mAh/g, 134.4mAh/g and 139.6mAh/g, after 100 charge-discharge cycles, the discharge capacities are 108.3mAh/g, 122.8mAh/g and 128.3mAh/g in sequence, and the capacity retention rates are 89.4%, 91.4% and 91.9% in sequence. Comparative example 1 no carbon nanotube was coated, and the first discharge capacity and charge-discharge cycle stability were significantly reduced; compared with the comparative example, the first discharge capacity and the charge-discharge cycle stability are obviously improved, but the difference is still obvious compared with the example 2, mainly due to the fact that the carbon nano tube which is subjected to functional modification is adopted for coating in the example 2, and meanwhile, the enhancement effect of the carbon nano tube on the conductivity is greatly improved by doping B, N and introducing iron ions and sodium ions.
While embodiments of the invention have been disclosed above, it is not limited to the applications listed in the description and the embodiments, which are fully applicable in all kinds of fields of application of the invention, and further modifications may readily be effected by those skilled in the art, so that the invention is not limited to the specific details without departing from the general concept defined by the claims and the scope of equivalents.

Claims (10)

1. The nitrogen-doped carbon-modified lithium iron phosphate cathode material is characterized by being prepared by the following method:
1) preparing the boron-doped carbon nanotube:
1-1) with B and B 2 O 3 Covering CNTs on the boron source serving as the mixture of the carbon dioxide and the carbon dioxide, and heating and reacting under the protection of argon;
1-2) adding the product obtained in the step 1-1) into an alkali solution, stirring, transferring the obtained mixture into a reaction kettle, stirring and reacting under heating, filtering after the reaction is finished, washing the solid product with deionized water, and drying to obtain the boron-doped carbon nanotube: B-CNTs;
2) preparing the composite nitrogen-doped functionalized carbon nanotube:
2-1) adding the B-CNTs obtained in the step 1) into hot deionized water, and performing ultrasonic treatment to obtain a B-CNTs dispersion liquid;
2-2) adding sodium iron diethyltriamine pentaacetate into hot deionized water, and stirring;
2-3) adding the B-CNTs dispersion liquid obtained in the step 2-1) into the product obtained in the step 2-2), adding DCC, stirring and reacting under heating, filtering after the reaction is finished, and washing and drying the solid product by deionized water and ethanol in sequence to obtain a functionalized carbon nano tube;
3) adopting the functionalized carbon nano tube pair LiFePO prepared in the step 2) 4 And modifying the material to obtain the nitrogen-doped carbon modified lithium iron phosphate anode material.
2. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material as claimed in claim 1, which is prepared by the following method:
1) preparing the boron-doped carbon nanotube:
1-1) with B and B 2 O 3 The mixture is used as a boron source, CNTs is covered on the boron source, the mixture is heated to 1050-1400 ℃ under the protection of argon, and the temperature is kept for 3 to 8 hours;
1-2) adding the product obtained in the step 1-1) into excessive alkali solution, stirring for 5-30min, transferring the obtained mixture into a reaction kettle, stirring and reacting for 2-6h at 35-75 ℃ and at 900rpm, filtering after the reaction is finished, washing the solid product with deionized water, and drying for 6-24h to obtain the boron-doped carbon nanotube: B-CNTs;
2) preparing a composite doped functionalized carbon nanotube:
2-1) adding the B-CNTs obtained in the step 1) into deionized water at the temperature of 60-85 ℃, and performing ultrasonic treatment for 5-30min to obtain a B-CNTs dispersion liquid;
2-2) adding sodium iron diethyltriamine pentaacetate into deionized water at the temperature of 60-85 ℃, and stirring for 5-10 min;
2-3) adding the B-CNTs dispersion liquid into the product obtained in the step 2-2), adding DCC, stirring and reacting for 1-5h at 45-75 ℃, filtering after the reaction is finished, sequentially cleaning the solid product with deionized water and ethanol, and drying for 12-36h at 50-70 ℃ to obtain the functionalized carbon nanotube;
3) adopting the functionalized carbon nano tube pair LiFePO prepared in the step 2) 4 And modifying the material to obtain the nitrogen-doped carbon modified lithium iron phosphate anode material.
3. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material of claim 2, wherein the alkali solution is a sodium hydroxide solution or a potassium hydroxide solution with a concentration of 2-5 mol/L.
4. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material as claimed in claim 2, wherein B is B in the boron source 2 O 3 The mass ratio of (A) to (B) is 1:0.3-1: 5.
5. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material as claimed in claim 1, wherein the step 3) specifically comprises:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, and stirring to dissolve to obtain a solution A;
3-2) reacting LiOH. H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
3-3) mixing solution A with solution B, and then adding H 3 PO 4 Adjusting the pH value of a reaction system to 5-7, adding ethanol, and stirring to obtain a precursor solution;
3-4) adding the functionalized carbon nanotube prepared in the step 2) into deionized water, and performing ultrasonic treatment to obtain a functionalized carbon nanotube dispersion liquid;
3-5) adding the functionalized carbon nanotube dispersion liquid into the precursor liquid under continuous stirring, stirring and performing ultrasonic treatment;
3-6) placing the mixture obtained in the step 3-5) in a microwave, heating for 20-60min, cooling after the reaction is finished, filtering, cleaning a solid product, drying in vacuum, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
6. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material as claimed in claim 5, wherein the step 3) specifically comprises:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, and stirring to dissolve to obtain a solution A;
3-2) reacting LiOH & H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
3-3) mixing solution A with solution B, and then adding H 3 PO 4 Adjusting the pH value of the reaction system to 5-6, adding ethanol, and stirring for 5-15min to obtain a precursor solution;
3-4) adding the functionalized carbon nanotube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 10-45min to obtain a functionalized carbon nanotube dispersion liquid;
3-5) adding the functionalized carbon nanotube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 5-30 min;
3-6) placing the mixture obtained in the step 3-5) in a microwave, heating at the temperature of 150-220 ℃ for 20-60min, cooling after the reaction is finished, filtering, sequentially cleaning the solid product with deionized water and ethanol, vacuum-drying at the temperature of 95-140 ℃ for 2-8h, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
7. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material as claimed in claim 1, wherein the step 3) specifically comprises:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing ethanol, and adding ascorbic acid and laurelAcid, stirring and dissolving to obtain a solution A;
3-2) reacting LiOH & H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
3-3) adding the functionalized carbon nanotube prepared in the step 2) into deionized water, and performing ultrasonic treatment to obtain a functionalized carbon nanotube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing the solutions, stirring, and adjusting the pH value of a reaction system to 5-7 to obtain a precursor solution;
3-5) adding the functional carbon nano tube dispersion liquid prepared in the step 2) into the precursor liquid under continuous stirring, stirring and performing ultrasonic treatment;
3-6) transferring the mixed solution obtained in the step 3-5) into a reaction kettle, and reacting under heating and pressurizing; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, drying in vacuum, cooling and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
8. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material as claimed in claim 7, wherein the step 3) specifically comprises:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing ethanol, adding ascorbic acid and lauric acid, and stirring for dissolving to obtain a solution A;
3-2) reacting LiOH & H 2 Adding O into deionized water, and stirring and dissolving to obtain a solution B;
3-3) adding the functionalized carbon nanotube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 10-45min to obtain a functionalized carbon nanotube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing the solutions, stirring, and adjusting the pH value of a reaction system to 5-6 to obtain a precursor solution;
3-5) adding the functional carbon nano tube dispersion liquid prepared in the step 2) into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 5-30 min;
3-6) transferring the mixed solution obtained in the step 3-5) to a reaction kettle, and reacting for 0.5-3h at the temperature of 260-450 ℃ and under the pressure of 15-45 MPa; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, drying for 2-8h at 95-140 ℃ in vacuum, cooling, and grinding to obtain the nitrogen-doped carbon modified lithium iron phosphate cathode material.
9. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material as claimed in claim 6 or 8, wherein in the step 3-5), the addition amount of the functionalized carbon nanotubes in the functionalized carbon nanotube dispersion liquid is LiFePO in the precursor liquid 4 2.5-10% of the mass of (A).
10. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material as claimed in claim 9, wherein in the step 3-5), the addition amount of the functionalized carbon nanotubes in the functionalized carbon nanotube dispersion liquid is LiFePO in the precursor liquid 4 4 to 6.5 percent of the mass of (A).
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