CN115133010B - 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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CN115133010B
CN115133010B CN202211050006.0A CN202211050006A CN115133010B CN 115133010 B CN115133010 B CN 115133010B CN 202211050006 A CN202211050006 A CN 202211050006A CN 115133010 B CN115133010 B CN 115133010B
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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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Abstract

The invention discloses a nitrogen-doped carbon-modified lithium iron phosphate positive electrode material, which is prepared by the following method: 1) Preparing boron doped carbon nanotubes: 2) Preparing a composite nitrogen doped functionalized carbon nano tube: 3) 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 relates to a carbon nano tube LiFePO modified by functionalization 4 The material is coated, so that the defect that the carbon nano tube is difficult to disperse is overcome, the function of improving the conductivity and the mechanical property can be fully exerted 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.

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 anode material.
Background
Lithium ion batteries are widely used in electronic devices in daily life because of their excellent energy density, good rate capability and cycle life. 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 is seriously affected, and the further application is limited. At present, ion doping, coating and other technologies are generally adopted to improve the electrochemical performance of the lithium iron phosphate material. For example, the preparation of a composite lithium iron phosphate anode material disclosed in patent CN109004207B and the preparation of a composite lithium iron phosphate anode material disclosed in patent CN105428617BA method for preparing internal and external conductive carbon modified lithium iron phosphate, and the like.
The carbon coating production is simple and convenient, and good improvement effect can be obtained, so that the carbon coating becomes the main stream direction of the current market. The carbon nano tube has a unique hollow structure, good electric conductivity and mechanical property, and has been applied to coating modification of lithium iron phosphate materials; for example, a method for preparing a lithium iron phosphate/carbon nanotube composite material disclosed in patent CN101734927a, a lithium iron phosphate/carbon nanotube nanocomposite material for a positive electrode material of a lithium battery disclosed in CN201710326942.2, a preparation method thereof, and the like. However, CNTs have large van der Waals forces between themselves due to the entanglement caused by large length-diameter ratio and high-energy surface caused by large specific surface area, and are easy to highly aggregate or intertwine, so that the CNTs have the defect of difficult dispersion; in addition, the single carbon nanotube coating has limited improvement on the conductivity, stability and the like of the lithium iron phosphate material, and a reliable technology for solving the defects is lacking 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 anode material aiming at the defects in the prior art.
In order to solve the technical problems, the invention adopts the following technical scheme: the nitrogen-doped carbon-modified lithium iron phosphate positive electrode material is prepared by the following method:
1) Preparing boron doped carbon nanotubes:
1-1) use of B and B 2 O 3 Covering CNTs on the boron source, and heating for reaction 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 for reaction 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 a composite nitrogen doped functionalized carbon nano tube:
2-1) adding the B-CNTs obtained in the step 1) into hot deionized water, and carrying out ultrasonic treatment to obtain B-CNTs dispersion liquid;
2-2) adding the diethyl triamine penta sodium iron acetate 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 for reaction under heating, filtering after the reaction is finished, sequentially cleaning the solid product with deionized water and ethanol, and drying to obtain the functionalized carbon nanotube;
3) The functionalized carbon nano tube pair LiFePO prepared by the step 2) is adopted 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 boron doped carbon nanotubes:
1-1) use of B and B 2 O 3 Covering CNTs on a boron source serving as the boron source, heating to 1050-1400 ℃ under the protection of argon, and preserving heat for 3-8h;
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 at 35-75 ℃ and 300-900rpm for reaction for 2-6h, filtering after the reaction is finished, washing the solid product with deionized water, and drying for 6-24h to obtain the boron-doped carbon nano tube: B-CNTs;
2) Preparing composite doped functionalized carbon nanotubes:
2-1) adding the B-CNTs obtained in the step 1) into deionized water at 60-85 ℃, and carrying out ultrasonic treatment for 5-30min to obtain B-CNTs dispersion liquid;
2-2) adding the diethyl triamine penta sodium iron acetate into deionized water at 60-85 ℃ and stirring for 5-10min;
2-3) adding the B-CNTs dispersion liquid into the product obtained in the step 2-2), adding DCC, stirring at 45-75 ℃ for reaction for 1-5 hours, filtering after the reaction is finished, sequentially cleaning the solid product with deionized water and ethanol, and drying at 50-70 ℃ for 12-36 hours to obtain the functionalized carbon nano tube;
3) The functionalized carbon nano tube pair LiFePO prepared by the step 2) is adopted 4 Material is subjected toAnd modifying to obtain the nitrogen-doped carbon modified lithium iron phosphate anode material.
Preferably, the alkali solution is sodium hydroxide solution or potassium hydroxide solution with the concentration of 2-5 mol/L.
Preferably, B in the boron source 2 O 3 The mass ratio of (2) 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, stirring and dissolving to obtain solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
3-3) mixing solution A with solution B and then adding H 3 PO 4 Adjusting the pH of the 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 functionalized carbon nano tube dispersion liquid;
3-5) adding the functionalized carbon nano tube dispersion liquid prepared in the step 2) into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment;
3-6) placing the mixture obtained in the step 3-5) in microwaves, heating for 20-60min, cooling after the reaction is finished, filtering, cleaning a solid product, vacuum drying, cooling and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material.
Preferably, the step 3) specifically includes:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, stirring and dissolving to obtain solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
3-3) mixing solution A with solution B and then adding H 3 PO 4 Adjusting the pH 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 functionalized carbon nano tube dispersion liquid;
3-5) adding the functionalized carbon nano tube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 5-30min;
3-6) placing the mixture obtained in the step 3-5) in microwaves, heating for 20-60min at 150-220 ℃, cooling after the reaction is finished, filtering, sequentially cleaning a solid product by deionized water and ethanol, vacuum drying for 2-8h at 95-140 ℃, cooling, and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate positive electrode 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 solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
3-3) adding the functionalized carbon nano tube prepared in the step 2) into deionized water, and performing ultrasonic treatment to obtain functionalized carbon nano tube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing and stirring the solutions, and regulating the pH of the reaction system to 5-7 to obtain a precursor solution;
3-5) adding the functionalized carbon nano tube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out 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, vacuum drying, cooling and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode 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 solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolvingObtaining a solution B;
3-3) adding the functionalized carbon nano tube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 10-45min to obtain functionalized carbon nano tube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing and stirring the solutions, and regulating the pH of the reaction system to 5-6 to obtain a precursor solution;
3-5) adding the functionalized carbon nano tube dispersion liquid prepared in the step 2) into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 5-30min;
3-6) transferring the mixed solution obtained in the step 3-5) into a reaction kettle, and reacting for 0.5-3h at 260-450 ℃ and 15-45 MPa; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, vacuum drying at 95-140 ℃ for 2-8 hours, cooling, and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material.
Preferably, in the step 3-5), the addition amount of the functionalized carbon nanotubes in the functionalized carbon nanotube dispersion is LiFePO in the precursor solution 4 2.5% -10% of the mass of the composition.
Preferably, in the step 3-5), the addition amount of the functionalized carbon nanotubes in the functionalized carbon nanotube dispersion is LiFePO in the precursor solution 4 4% -6.5% of the mass of the composition.
The beneficial effects of the invention are as follows:
the preparation method comprises the steps of firstly preparing a boron-doped carbon nanotube by doping B into the carbon nanotube, and then carrying out N, na and Fe composite doping modification on the boron-doped carbon nanotube by using diethyl triamine pentaacetic acid sodium iron to obtain a B, N-doped functionalized carbon nanotube with uniformly loaded Na and Fe on the surface; finally, the LiFePO is prepared by the functionalized carbon nano tube 4 Carrying out in-situ coating modification on the material to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material;
according to the invention, by doping B in the carbon nano tube, the charge transfer amount between the carbon nano tube and Li in the nitrogen-doped carbon-modified lithium iron phosphate anode material can be increased, and the conductivity between the carbon nano tube and Li can be 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 from an N atom, so that the electron concentration of CNTs is increased, and the conductivity of CNTs can be improved; on the other hand, the doping of N can also obviously promote the hydrophilicity of CNTs, improve the dispersibility of CNTs, and overcome the defect that the conventional CNTs are easy to agglomerate;
in the invention, 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 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, so that 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 between the carbon nano tube and the battery active material, and can improve the dispersibility of the carbon nano tube in an electric positive electrode material system.
The invention relates to a carbon nano tube LiFePO modified by functionalization 4 The material is coated, so that the defect that the carbon nano tube is difficult to disperse is overcome, the function of improving the conductivity and the mechanical property can be fully exerted 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.
Drawings
Fig. 1 is a charge-discharge cycle curve of a battery assembled from lithium iron phosphate cathode materials prepared in examples and comparative examples according to the present invention.
Detailed Description
The present invention is described in further detail below with reference to examples to enable those skilled in the art to practice the same by referring 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 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 were conducted under conventional conditions or conditions recommended by the manufacturer, without specifying the specific conditions. The reagents or apparatus used were conventional products commercially available without the manufacturer's attention.
As a preferable technical scheme, the nitrogen-doped carbon-modified lithium iron phosphate anode material in each embodiment of the invention is prepared by the following method:
1) Preparing boron doped carbon nanotubes:
1-1) use of B and B 2 O 3 Covering CNTs on a boron source serving as the boron source, heating to 1050-1400 ℃ under the protection of argon, and preserving heat for 3-8h;
in a preferred embodiment, B is B in the boron source 2 O 3 The mass ratio of (2) is 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 at 35-75 ℃ and 300-900rpm for reaction for 2-6h, filtering after the reaction is finished, washing the solid product with deionized water, and drying for 6-24h to obtain the boron-doped carbon nano tube: B-CNTs;
in a preferred embodiment, the alkaline solution is a sodium hydroxide solution or potassium hydroxide solution having a concentration of 2-5 mol/L.
2) Preparing a composite nitrogen doped functionalized carbon nano tube:
2-1) adding the B-CNTs obtained in the step 1) into deionized water at 60-85 ℃, and carrying out ultrasonic treatment for 5-30min to obtain B-CNTs dispersion liquid;
2-2) adding the diethyl triamine penta sodium iron acetate into deionized water at 60-85 ℃ and stirring for 5-10min;
2-3) adding the B-CNTs dispersion liquid into the product obtained in the step 2-2), adding DCC, stirring at 45-75 ℃ for reaction for 1-5h, filtering after the reaction is finished, sequentially cleaning the solid product with deionized water and ethanol, and drying at 50-70 ℃ for 12-36h to obtain the functionalized carbon nano tube.
3) The functionalized carbon nano tube pair LiFePO prepared by the step 2) is adopted 4 And carrying out in-situ coating modification on the material to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material.
In some preferred embodiments, step 3) is performed using microwave in LiFePO 4 The method comprises the following specific steps of:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, stirring and dissolving to obtain solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
3-3) mixing solution A with solution B and then adding H 3 PO 4 Adjusting the pH 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 functionalized carbon nano tube dispersion liquid;
3-5) adding the functionalized carbon nano tube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 5-30min;
3-6) placing the mixture obtained in the step 3-6) in microwaves, heating for 20-60min at 150-220 ℃, cooling after the reaction is finished, filtering, sequentially cleaning a solid product by deionized water and ethanol, vacuum drying for 2-8h at 95-140 ℃, cooling, and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material.
In further preferred embodiments, step 3) is carried out using a hydrothermal process on LiFePO 4 The method comprises the following specific steps of:
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 solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
3-3) adding the functionalized carbon nano tube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 10-45min to obtain functionalized carbon nano tube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing and stirring the solutions, and regulating the pH of the reaction system to 5-6 to obtain a precursor solution;
3-5) adding the functionalized carbon nano tube dispersion liquid prepared in the step 2) into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 5-30min;
3-6) transferring the mixed solution obtained in the step 3-5) into a reaction kettle, and reacting for 0.5-3h at 260-450 ℃ and 15-45 MPa; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, vacuum drying at 95-140 ℃ for 2-8 hours, cooling, and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material.
In the preferred embodiment, in the two processes of in-situ coating the functionalized carbon nanotubes, in the step 3-5), the addition amount of the functionalized carbon nanotubes in the functionalized carbon nanotube dispersion is LiFePO in the precursor solution 4 And more preferably from 2.5% to 10% by mass, and still more preferably from 4% to 6.5% by mass.
The following is a description of the general principles of the present invention to facilitate an understanding thereof.
First, the general scheme of the invention is as follows: preparing a boron-doped carbon nanotube by doping a conventional carbon nanotube with B, and then carrying out N, na and Fe composite doping on the boron-doped carbon nanotube by using diethylenetriamine pentaacetic acid iron sodium salt to obtain a B, N-doped functionalized carbon nanotube with uniformly loaded Na and Fe on the surface; finally, the LiFePO is prepared by the functionalized carbon nano tube 4 And carrying out in-situ coating modification on the material to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material.
(1) In the invention, B and B are adopted when boron doping is carried out 2 O 3 As a boron source, can reduce energy consumption and increase the doping ratio of B.
B 2 O 3 The boron atoms and the carbon atoms can form B-C bonds 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, and 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, partial oxygen atoms react with CNTs to generate gas, so that the energy consumption is high; when B is used as the boron source, the B is not completely gasified due to the protection of argon, so that the doping proportion of B is reduced easily, and B are used in the invention 2 O 3 As a boron source, can reduce energy consumption and increase the doping ratio of B.
According to the invention, by doping B in the carbon nanotube, the charge transfer amount between the carbon nanotube and Li in the nitrogen-doped carbon-modified lithium iron phosphate anode material can be improved, and the conductivity between the carbon nanotube and Li is improved ([ 1] generation sharp peak. Theory research on the influence of doping on the conductivity of the carbon nanotube [ D ]. North China university of technology, 2017 ].
(2) The invention is based on the step 1-2) of adding an excess of an alkaline solution, on the one hand, for removing unreacted B and B 2 O 3 On the other hand, excessive alkali solution can also act on the carbon nanotubes, so that abundant hydroxyl groups are introduced to the surfaces of the carbon nanotubes, and hydroxylation of the carbon nanotubes is realized.
In the invention, diethyl triamine sodium iron pentaacetate is used as a functional reagent for modifying B-CNTs, and the chemical structural formula of the diethyl triamine sodium iron pentaacetate is shown as the following formula I:
Figure DEST_PATH_IMAGE001
i
It can be seen that the sodium iron diethylenetriamine pentaacetate has a rich carboxyl function, on which iron ions and sodium ions are complexed, and which contains N element.
According to the invention, the ferric sodium diethylenetriamine pentaacetate is uniformly and firmly grafted on the surface of the B-CNTs through the condensation reaction of the carboxyl functional group on the ferric sodium diethylenetriamine pentaacetate and the hydroxyl functional group of the B-CNTs, so that on one hand, N doping of the B-CNTs is realized, and on the other hand, a large amount of iron ions and sodium ions which can be uniformly loaded can be simultaneously introduced on the surface of the B-CNTs.
On one hand, N and C are combined to form a C-N bond, and meanwhile, an N atom loses one electron, so that the electron concentration of CNTs is increased, and the conductivity of CNTs can be improved ([ 1] Zhang Yadong. Experimental research on improving the conductivity of carbon nanotubes by doping transition metals [ D ]. North China university of science); on the other hand, the doping of N can also obviously promote the hydrophilicity of CNTs, improve the dispersibility of CNTs, and overcome the defects that conventional CNTs (the conventional CNTs have larger van der Waals acting force among CNTs and are easy to gather or intertwine and difficult to disperse because of the winding caused by large length-diameter ratio and the high-energy surface caused by large specific surface area) are easy to agglomerate.
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 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, so that 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 are 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 between the carbon nano tube and the battery active material, and can improve the dispersibility of the carbon nano tube in an electric positive electrode material system.
(3) In the invention, liFePO is prepared by functionalized carbon nano tube 4 The material is coated in situ, so that a three-dimensional space conductive network can be formed, and the electron conduction capacity and the cycling stability of the lithium iron phosphate anode material are enhanced; and the carbon nano tube is coated in situ and simultaneously, the secondary process is carried outCarbonization of carbon sources (citric acid or ascorbic acid and lauric acid) can form a CB (carbon black) coating layer to coat LiFePO 4 Particle surface can inhibit particle agglomeration or enlargement, and enhance particle conductivity; the invention leads LiFePO to be realized by the combined action of the functionalized carbon nano tube and the CB 4 The material has better conductivity and can improve the multiplying power and the cycle performance of lithium iron phosphate.
The invention uses the functionalized modified carbon nano tube to carry out LiFePO 4 The material is coated, so that the defect that the carbon nano tube is difficult to disperse is overcome, the function of improving the conductivity and the mechanical property can be fully exerted 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
The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material is prepared by the following method:
1) Preparing boron doped carbon nanotubes:
1-1) use of B and B 2 O 3 Covering CNTs (commercial multiwall carbon nanotubes with the length of 0.5-2 microns and the diameter of 30-50nm, which are purchased from Jiangsu Xianfeng nano materials science and technology Co., ltd.) on excessive boron source, heating to 1250 ℃ under the protection of argon, heating at a heating rate of 15 ℃/min, and preserving heat for 5h; wherein B is B 2 O 3 The mass ratio of (2) 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 at 55 ℃ and 600rpm for reaction 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 nano tube: B-CNTs;
2) Preparing a composite nitrogen doped functionalized carbon nano tube:
2-1) adding the B-CNTs obtained in the step 1) into hot deionized water at 70 ℃, and carrying out ultrasonic treatment for 15min to obtain B-CNTs dispersion liquid;
2-2) adding the diethyl triamine penta acetic acid sodium iron into the hot deionized water at 70 ℃ and stirring for 7min;
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 a solid product with deionized water and ethanol, and drying at 55 ℃ for 24 hours to obtain the functionalized carbon nanotube; wherein the mass ratio of the diethylenetriamine pentaacetic acid iron sodium to the B-CNTs is 6:100.
3) The functionalized carbon nano tube pair LiFePO prepared by the step 2) is adopted 4 The material is subjected to in-situ coating modification to prepare the nitrogen-doped carbon-modified lithium iron phosphate anode material, which comprises the following specific steps:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, stirring and dissolving to obtain solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
3-3) mixing solution A with solution B and then adding H 3 PO 4 Adjusting the pH of the reaction system to 6, adding ethanol, and stirring for 8min to obtain a precursor solution; wherein, according to Fe: p: adding raw materials in a molar ratio of Li of 1:1:1.5;
3-4) adding the functionalized carbon nano tube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 30min to obtain functionalized carbon nano tube dispersion liquid;
3-5) adding the functionalized carbon nano tube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 15min; wherein the addition amount of the functionalized carbon nano tube in the functionalized carbon nano tube dispersion liquid is LiFePO in the precursor liquid 4 4% of the mass of (c) the total weight of the composition,
3-6) placing the mixture obtained in the step 3-5) in microwaves, heating for 40min at 190 ℃, cooling after the reaction is finished, filtering, sequentially cleaning a solid product by deionized water and ethanol, vacuum drying for 5h at 115 ℃, cooling, and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate positive electrode material.
Example 2
The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material is prepared by the following method:
1) Preparing boron doped carbon nanotubes:
1-1) use of B and B 2 O 3 Covering CNTs (commercial multiwall carbon nanotubes with the length of 0.5-2 microns and the diameter of 30-50nm, which are purchased from Jiangsu Xianfeng nano materials science and technology Co., ltd.) on excessive boron source, heating to 1250 ℃ under the protection of argon, heating at a heating rate of 15 ℃/min, and preserving heat for 5h; wherein B is B 2 O 3 The mass ratio of (2) 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 at 55 ℃ and 600rpm for reaction 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 nano tube: B-CNTs;
2) Preparing a composite nitrogen doped functionalized carbon nano tube:
2-1) adding the B-CNTs obtained in the step 1) into hot deionized water at 70 ℃, and carrying out ultrasonic treatment for 15min to obtain B-CNTs dispersion liquid;
2-2) adding the diethyl triamine penta acetic acid sodium iron into the hot deionized water at 70 ℃ and stirring for 7min;
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 a solid product with deionized water and ethanol, and drying at 55 ℃ for 24 hours to obtain the functionalized carbon nanotube; wherein the mass ratio of the diethylenetriamine pentaacetic acid iron sodium to the B-CNTs is 6:100.
3) The functionalized carbon nano tube pair LiFePO prepared by the step 2) is adopted 4 The material is subjected to in-situ coating modification to prepare the nitrogen-doped carbon-modified lithium iron phosphate anode material, which comprises the following specific steps:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, stirring and dissolving to obtain solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtainTo solution B;
3-3) mixing solution A with solution B and then adding H 3 PO 4 Adjusting the pH of the reaction system to 6, adding ethanol, and stirring for 8min to obtain a precursor solution; wherein, according to Fe: p: adding raw materials in a molar ratio of Li of 1:1:1.5;
3-4) adding the functionalized carbon nano tube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 30min to obtain functionalized carbon nano tube dispersion liquid;
3-5) adding the functionalized carbon nano tube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 15min; wherein the addition amount of the functionalized carbon nano tube in the functionalized carbon nano tube dispersion liquid is LiFePO in the precursor liquid 4 5% of the mass of (c) the total weight of the composition,
3-6) placing the mixture obtained in the step 3-5) in microwaves, heating for 40min at 190 ℃, cooling after the reaction is finished, filtering, sequentially cleaning a solid product by deionized water and ethanol, vacuum drying for 5h at 115 ℃, cooling, and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate positive electrode material.
Example 3
The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material is prepared by the following method:
1-1) use of B and B 2 O 3 Covering CNTs (commercial multiwall carbon nanotubes with the length of 0.5-2 microns and the diameter of 30-50nm, which are purchased from Jiangsu Xianfeng nano materials science and technology Co., ltd.) on excessive boron source, heating to 1250 ℃ under the protection of argon, heating at a heating rate of 15 ℃/min, and preserving heat for 5h; wherein B is B 2 O 3 The mass ratio of (2) 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 at 55 ℃ and 600rpm for reaction 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 nano tube: B-CNTs;
2) Preparing a composite nitrogen doped functionalized carbon nano tube:
2-1) adding the B-CNTs obtained in the step 1) into hot deionized water at 70 ℃, and carrying out ultrasonic treatment for 15min to obtain B-CNTs dispersion liquid;
2-2) adding the diethyl triamine penta acetic acid sodium iron into the hot deionized water at 70 ℃ and stirring for 7min;
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 a solid product with deionized water and ethanol, and drying at 55 ℃ for 24 hours to obtain the functionalized carbon nanotube; wherein the mass ratio of the diethylenetriamine pentaacetic acid iron sodium to the B-CNTs is 6:100.
3) The functionalized carbon nano tube pair LiFePO prepared by the step 2) is adopted 4 The material is subjected to in-situ coating modification to prepare the nitrogen-doped carbon-modified lithium iron phosphate anode material, 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 solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
3-3) adding the functionalized carbon nano tube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 30min to obtain functionalized carbon nano tube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing and stirring the solutions, and regulating the pH of the reaction system to 5-6 to obtain a precursor solution; wherein, according to Fe: p: adding raw materials in a molar ratio of Li of 1:1:1.5;
3-5) adding the functionalized carbon nano tube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 15min; wherein the addition amount of the functionalized carbon nano tube in the functionalized carbon nano tube dispersion liquid is LiFePO in the precursor liquid 4 4% by mass of (3);
3-6) transferring the mixed solution obtained in the step 3-5) into a reaction kettle, and reacting for 2 hours at 420 ℃ and 25 MPa; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, vacuum drying at 115 ℃ for 5 hours, cooling, and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material.
Example 4
The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material is prepared by the following method:
1-1) use of B and B 2 O 3 Covering CNTs (commercial multiwall carbon nanotubes with the length of 0.5-2 microns and the diameter of 30-50nm, which are purchased from Jiangsu Xianfeng nano materials science and technology Co., ltd.) on excessive boron source, heating to 1250 ℃ under the protection of argon, heating at a heating rate of 15 ℃/min, and preserving heat for 5h; wherein B is B 2 O 3 The mass ratio of (2) 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 at 55 ℃ and 600rpm for reaction 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 nano tube: B-CNTs;
2) Preparing a composite nitrogen doped functionalized carbon nano tube:
2-1) adding the B-CNTs obtained in the step 1) into hot deionized water at 70 ℃, and carrying out ultrasonic treatment for 15min to obtain B-CNTs dispersion liquid;
2-2) adding the diethyl triamine penta acetic acid sodium iron into the hot deionized water at 70 ℃ and stirring for 7min;
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 a solid product with deionized water and ethanol, and drying at 55 ℃ for 24 hours to obtain the functionalized carbon nanotube; wherein the mass ratio of the diethylenetriamine pentaacetic acid iron sodium to the B-CNTs is 6:100.
3) The functionalized carbon nano tube pair LiFePO prepared by the step 2) is adopted 4 The material is subjected to in-situ coating modification to prepare the nitrogen-doped carbon-modified lithium iron phosphate anode material, 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 solution A;
3-2) LiOH.H 2 O addition and dissociationStirring and dissolving the mixture in the child water 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 for 30min to obtain functionalized carbon nano tube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing and stirring the solutions, and regulating the pH of the reaction system to 5-6 to obtain a precursor solution; wherein, according to Fe: p: adding raw materials in a molar ratio of Li of 1:1:1.5;
3-5) adding the functionalized carbon nano tube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 15min; wherein the addition amount of the functionalized carbon nano tube in the functionalized carbon nano tube dispersion liquid is LiFePO in the precursor liquid 4 5% by mass of (2);
3-6) transferring the mixed solution obtained in the step 3-5) into a reaction kettle, and reacting for 2 hours at 420 ℃ and 25 MPa; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, vacuum drying at 115 ℃ for 5 hours, cooling, and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material.
Comparative examples are provided below to further illustrate the invention.
Comparative example 1
A lithium iron phosphate positive electrode material is prepared by the following method:
1-1) FeSO is carried out 4 ·7H 2 Adding O into deionized water containing ethanol, adding ascorbic acid and lauric acid, and stirring for dissolving to obtain solution A;
1-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
1-3) mixing solution A, solution B and H 3 PO 4 Mixing and stirring the solutions, and regulating the pH of the reaction system to 5-6 to obtain a precursor solution; wherein, according to Fe: p: adding raw materials in a molar ratio of Li of 1:1:1.5;
1-4) placing the precursor liquid obtained in the step 1-3) in microwaves, heating at 190 ℃ for 40min, cooling after the reaction is finished, filtering, sequentially cleaning a solid product by deionized water and ethanol, vacuum drying at 115 ℃ for 5h, cooling, and grinding to obtain the lithium iron phosphate anode material.
Comparative example 2
A lithium iron phosphate positive electrode material is prepared by the following method:
1) CNTs are adopted for LiFePO 4 The material is subjected to in-situ coating modification to prepare the lithium iron phosphate anode material, which comprises the following specific steps:
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 solution A;
1-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
1-3) adding CNTs (commercial multiwall carbon nanotubes with the length of 0.5-2 microns and the diameter of 30-50nm, purchased from Jiangsu Xianfeng nanomaterial technologies Co., ltd.) into deionized water, and performing ultrasonic treatment for 30min to obtain a carbon nanotube dispersion;
1-4) mixing solution A, solution B and H 3 PO 4 Mixing and stirring the solutions, and regulating the pH of the reaction system to 5-6 to obtain a precursor solution; wherein, according to Fe: p: adding raw materials in a molar ratio of Li of 1:1:1.5;
1-5) adding the carbon nano tube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 15min; wherein the addition amount of the carbon nano tube in the carbon nano tube dispersion liquid is LiFePO in the precursor liquid 4 5% by mass of (2);
1-6) placing the mixture obtained in the step 1-5) in microwaves, heating for 40min at 190 ℃, cooling after the reaction is finished, filtering, sequentially cleaning a solid product by deionized water and ethanol, vacuum drying for 5h at 115 ℃, cooling, and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate positive electrode material.
Comparative example 3
A lithium iron phosphate positive electrode material is prepared by the following method:
1) Preparing boron doped carbon nanotubes:
1-1) use of B and B 2 O 3 Is prepared by coating CNTs (commercially available multi-walled carbon nanotubes with a length of 0.5-2 μm and a diameter of 30-50nm, available from Jiangsu first Feng Na) on excessive boron source Rice materials science and technology Co., ltd.) under the protection of argon gas, heating to 1250 ℃, heating at 15 ℃/min, and preserving heat for 5 hours; wherein B is B 2 O 3 The mass ratio of (2) 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 at 55 ℃ and 600rpm for reaction 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 nano tube: B-CNTs;
2) LiFePO is prepared by adopting B-CNTs 4 The material is subjected to in-situ coating modification to prepare the lithium iron phosphate anode material, which 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 solution A;
2-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain 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 and stirring the solutions, and regulating the pH of the reaction system to 5-6 to obtain a precursor solution; wherein, according to Fe: p: adding raw materials in a molar ratio of Li of 1:1:1.5;
2-5) adding the carbon nano tube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 15min; wherein the addition amount of the B-CNTs in the carbon nano tube dispersion liquid is LiFePO in the precursor liquid 4 5% by mass of (2);
2-6) transferring the mixed solution obtained in the step 1-5) into a reaction kettle, and reacting for 2 hours at 420 ℃ and 25 MPa; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, vacuum drying at 115 ℃ for 5 hours, cooling, and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material.
The lithium iron phosphate positive electrode materials prepared in examples 1-4 and comparative example 1-are assembled into button cells, and when the button cells are charged and discharged, the first discharge capacity of examples 1-4 can be achieved at 1C multiplying power: 151.9mAh/g, 154.6mAh/g, 158.3mAh/g and 162.1mAh/g, and according to FIG. 1, after 100 charge and discharge cycles, the discharge capacity is 151.5mAh/g, 154.2mAh/g, 154.4mAh/g and 161.9mAh/g in sequence, and the capacity retention rate is 99.7%, 99.9% and 99.9% in sequence, so that the discharge capacity is basically unchanged, indicating that the lithium iron phosphate positive electrode material has good stability, and the lithium iron phosphate positive electrode material prepared by the invention has excellent electrochemical performance. The first discharge capacities of comparative examples 1 to 3 were: after 100 charge and discharge cycles, the discharge capacity is 108.3mAh/g, 122.8mAh/g and 128.3mAh/g, and the capacity retention rate is 89.4%, 91.4% and 91.9% in sequence. The comparative example 1 is not coated with carbon nanotubes, and the first discharge capacity and the charge-discharge cycle stability are remarkably reduced; the CNTs and the B-CNTs are respectively adopted for coating in the comparative example 2 and the comparative example 3, so that the first discharge capacity and the charge-discharge cycle stability of the comparative example are obviously improved, but the difference is still obvious compared with the example 2, and the improvement effect of the carbon nano tube on conductivity is greatly improved mainly due to the fact that the functionalized modified carbon nano tube is adopted for coating in the example 2 and the doping B, N and the iron ions and the sodium ions are introduced.
Although embodiments of the present invention have been disclosed above, it is not limited to the use of the description and embodiments, it is well suited to various fields of use for the invention, and further modifications may be readily apparent to those skilled in the art, and accordingly, the invention is not limited to the particular details without departing from the general concepts defined in the claims and the equivalents thereof.

Claims (10)

1. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material is characterized by being prepared by the following steps:
1) Preparing boron doped carbon nanotubes:
1-1) use of B and B 2 O 3 Covering CNTs on the boron source, and heating for reaction 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 for reaction 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 a composite nitrogen doped functionalized carbon nano tube:
2-1) adding the B-CNTs obtained in the step 1) into hot deionized water, and carrying out ultrasonic treatment to obtain B-CNTs dispersion liquid;
2-2) adding the diethyl triamine penta sodium iron acetate 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 for reaction under heating, filtering after the reaction is finished, sequentially cleaning the solid product with deionized water and ethanol, and drying to obtain the functionalized carbon nanotube;
3) The functionalized carbon nano tube pair LiFePO prepared by the step 2) is adopted 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 according to claim 1, which is prepared by the following method:
1) Preparing boron doped carbon nanotubes:
1-1) use of B and B 2 O 3 Covering CNTs on a boron source serving as the boron source, heating to 1050-1400 ℃ under the protection of argon, and preserving heat for 3-8h;
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 at 35-75 ℃ and 300-900rpm for reaction for 2-6h, filtering after the reaction is finished, washing the solid product with deionized water, and drying for 6-24h to obtain the boron-doped carbon nano tube: B-CNTs;
2) Preparing composite doped functionalized carbon nanotubes:
2-1) adding the B-CNTs obtained in the step 1) into deionized water at 60-85 ℃, and carrying out ultrasonic treatment for 5-30min to obtain B-CNTs dispersion liquid;
2-2) adding the diethyl triamine penta sodium iron acetate into deionized water at 60-85 ℃ and stirring for 5-10min;
2-3) adding the B-CNTs dispersion liquid into the product obtained in the step 2-2), adding DCC, stirring at 45-75 ℃ for reaction for 1-5 hours, filtering after the reaction is finished, sequentially cleaning the solid product with deionized water and ethanol, and drying at 50-70 ℃ for 12-36 hours to obtain the functionalized carbon nano tube;
3) The functionalized carbon nano tube pair LiFePO prepared by the step 2) is adopted 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 according to claim 2, wherein the alkali solution is a sodium hydroxide solution or a potassium hydroxide solution having a concentration of 2-5 mol/L.
4. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material according to claim 2, wherein B in the boron source is B 2 O 3 The mass ratio of (2) is 1:0.3-1:5.
5. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material according to claim 1, wherein the step 3) specifically comprises:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, stirring and dissolving to obtain solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
3-3) mixing solution A with solution B and then adding H 3 PO 4 Adjusting the pH of the 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 functionalized carbon nano tube dispersion liquid;
3-5) adding the functionalized carbon nano tube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment;
3-6) placing the mixture obtained in the step 3-5) in microwaves, heating for 20-60min, cooling after the reaction is finished, filtering, cleaning a solid product, vacuum drying, cooling and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material.
6. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material according to claim 5, wherein the step 3) specifically comprises:
3-1）FeSO 4 ·7H 2 adding O into deionized water containing citric acid, stirring and dissolving to obtain solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
3-3) mixing solution A with solution B and then adding H 3 PO 4 Adjusting the pH 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 functionalized carbon nano tube dispersion liquid;
3-5) adding the functionalized carbon nano tube dispersion liquid into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 5-30min;
3-6) placing the mixture obtained in the step 3-5) in microwaves, heating for 20-60min at 150-220 ℃, cooling after the reaction is finished, filtering, sequentially cleaning a solid product by deionized water and ethanol, vacuum drying for 2-8h at 95-140 ℃, cooling, and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate positive electrode material.
7. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material according to claim 1, 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 solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
3-3) adding the functionalized carbon nano tube prepared in the step 2) into deionized water, and performing ultrasonic treatment to obtain functionalized carbon nano tube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing and stirring the solutions, and regulating the pH of the reaction system to 5-7 to obtain a precursor solution;
3-5) adding the functionalized carbon nano tube dispersion liquid prepared in the step 2) into the precursor liquid under continuous stirring, stirring and carrying out 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, vacuum drying, cooling and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material.
8. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material according to 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 solution A;
3-2) LiOH.H 2 Adding O into deionized water, stirring and dissolving to obtain solution B;
3-3) adding the functionalized carbon nano tube prepared in the step 2) into deionized water, and performing ultrasonic treatment for 10-45min to obtain functionalized carbon nano tube dispersion liquid;
3-4) mixing solution A, solution B and H 3 PO 4 Mixing and stirring the solutions, and regulating the pH of the reaction system to 5-6 to obtain a precursor solution;
3-5) adding the functionalized carbon nano tube dispersion liquid prepared in the step 2) into the precursor liquid under continuous stirring, stirring and carrying out ultrasonic treatment for 5-30min;
3-6) transferring the mixed solution obtained in the step 3-5) into a reaction kettle, and reacting for 0.5-3h at 260-450 ℃ and 15-45 MPa; and after the reaction is finished, cooling, centrifuging, filtering, washing a solid product, vacuum drying at 95-140 ℃ for 2-8 hours, cooling, and grinding to obtain the nitrogen-doped carbon-modified lithium iron phosphate anode material.
9. According to claim 6 or 8The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material is characterized in that 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 the composition.
10. The nitrogen-doped carbon-modified lithium iron phosphate positive electrode material according to 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% -6.5% of the mass of the composition.
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