CN114789996B - High-dispersity carbon nano tube, preparation method thereof and secondary battery - Google Patents

High-dispersity carbon nano tube, preparation method thereof and secondary battery Download PDF

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CN114789996B
CN114789996B CN202111385468.3A CN202111385468A CN114789996B CN 114789996 B CN114789996 B CN 114789996B CN 202111385468 A CN202111385468 A CN 202111385468A CN 114789996 B CN114789996 B CN 114789996B
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carbon nanotubes
slurry
walled carbon
mass
outer diameter
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CN114789996A (en
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王建兴
曹礼洪
方波
谢冬冬
肖敏
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Guangdong Yina Technology Co ltd
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    • 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
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • 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
    • H01M4/00—Electrodes
    • 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
    • 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
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • 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 present invention relates to a highly dispersible carbon nanotube satisfying the following conditions (1), (2) and (3): (1) Comprises single-wall carbon nanotubes and multi-wall carbon nanotubes, wherein the mass ratio of the single-wall carbon nanotubes to the multi-wall carbon nanotubes is 1:30-50; (2) The average outer diameter of the multi-walled carbon nanotubes is 40nm or less, and the standard deviation of the outer diameter of the multi-walled carbon nanotubes is 4nm or less; (3) the aspect ratio of the multi-walled carbon nanotube is not less than 2000. The invention also relates to a method for preparing the carbon nano tube with high dispersibility. The invention also relates to a secondary battery, which comprises a positive pole piece and a negative pole piece, wherein both the positive pole piece and the negative pole piece contain the carbon nano tube with high dispersibility.

Description

High-dispersity carbon nano tube, preparation method thereof and secondary battery
Technical Field
The invention relates to a high-dispersity carbon nano tube and a preparation method thereof. More particularly, the present invention relates to a highly dispersible carbon nanotube and a secondary battery using the same.
Background
Carbon nanotubes are cylindrical carbon materials having an outer diameter of several nanometers to several tens of nanometers. The carbon nanotubes have high electrical conductivity and mechanical strength. Accordingly, carbon nanotubes are used as functional materials in a wide range of fields including electronic engineering and energy engineering. Examples of the functional material are a fuel cell, an electrode, an electromagnetic wave shielding material, a conductive resin, a member for a field emission display (Field Emission Display, FED), an absorbing material for various gases typified by hydrogen, and the like.
As a development example of the functional material, carbon nanotubes may be used as a conductive agent for secondary batteries such as lithium ion batteries, and the conventional technical means of using carbon nanotubes as a conductive agent are as follows: the carbon nano tube is prepared into conductive slurry which is uniformly dispersed, and then the conductive slurry, the positive/negative electrode active material and the binder are mixed to prepare coating slurry which is coated on the positive/negative electrode current collector.
However, the existing conductive paste containing carbon nanotubes has the technical problems that:
1. the solvent of the conductive paste commonly uses N-methyl pyrrolidone (NMP) organic solvent, which has high cost and is not friendly to the environment;
2. if deionized water is used as the solvent of the conductive paste, the dispersibility and stability of the carbon nanotubes in the conductive paste are poor.
Disclosure of Invention
Aiming at the technical problems in the prior art, the invention provides the carbon nano tube with high dispersibility, which can be directly mixed with positive/negative electrode active substances and binders to prepare coating slurry without preparing conductive slurry, and can be uniformly dispersed and stably stored in organic solvents and deionized water.
In order to achieve the above purpose, the invention adopts the following technical scheme: a highly dispersible carbon nanotube characterized by satisfying the following conditions (1), (2) and (3):
(1) Comprises single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs), wherein the mass ratio of the single-wall carbon nanotubes to the multi-wall carbon nanotubes is 1:30-50;
(2) The average outer diameter of the multiwall carbon nanotubes is 40nm or less, and the standard deviation of the outer diameter of the multiwall carbon nanotubes is 4nm or less;
(3) The aspect ratio of the multi-wall carbon nano-tube is not less than 2000.
Wherein, when the average outer diameter of the multi-wall carbon nano tube is set as X and the standard deviation of the outer diameter of the multi-wall carbon nano tube is set as sigma, X+/-2 sigma satisfies that the diameter is 16nm or less and X+/-2 sigma or less than 28nm.
The method for manufacturing the carbon nano tube with high dispersibility comprises the following steps:
step one, single-wall carbon nanotubes, multi-wall carbon nanotubes, a dispersing agent and a surfactant are put into deionized water to be mixed to form premixed slurry;
step two, adopting a high-pressure homogenizer or a sand mill to process the premixed slurry to prepare slurry, wherein the viscosity of the slurry is controlled below 5000 cp;
step three, the slurry is subjected to spray drying through spray drying equipment to prepare powder, wherein the inlet temperature is not less than 210 ℃, and the outlet temperature is not less than 130 ℃;
and step four, placing the powder into a tube furnace and calcining at high temperature in an inert gas (such as nitrogen and argon) environment to prepare the carbon nano tube with high dispersibility, wherein the calcining temperature is 300-1000 ℃. The calcination time is 2-8h.
Wherein, in the premix slurry, the total mass of the single-wall carbon nano tube and the multi-wall carbon nano tube is 1-10% of the mass of the premix slurry, the mass of the dispersing agent is not more than 5% of the mass of the premix slurry, and the mass of the surfactant is not more than 1% of the mass of the premix slurry. The dispersing agent is one or more of polyvinylpyrrolidone, polyethylene glycol (with molecular weight of more than 600) and polyvinyl alcohol. The dispersant is completely discharged in the form of gas during the high-temperature calcination in the fourth step.
The surfactant is an alkyl trimethyl ammonium salt type cationic surfactant, and preferably, the surfactant is one or two of hexadecyl trimethyl ammonium bromide (chloride) and octadecyl trimethyl ammonium chloride (bromide). Wherein, the secondary agglomeration of the powder produced in the subsequent step three can be effectively prevented by adding the surfactant to the premix slurry, and the surfactant is completely discharged in the form of gas during the high-temperature calcination in the step four.
The invention also provides a secondary battery (such as a lithium ion secondary battery and a sodium ion secondary battery) which comprises a positive pole piece and a negative pole piece, wherein both the positive pole piece and the negative pole piece contain the carbon nano tube with high dispersibility.
In one embodiment, a battery paste composed of positive/negative electrode active materials, a binder and high-dispersity carbon nanotubes is coated on a current collector to prepare a positive/negative electrode plate. Wherein, the mass fraction of the positive electrode/negative electrode active material, the binder and the high-dispersivity carbon nano tube is respectively 90-98%, 1-5% and 1-5%.
According to a specific embodiment of the present invention, the adhesive is a water-based adhesive or an oil-based adhesive.
The water-based binder includes carboxymethyl cellulose and styrene-butadiene rubber or modified styrene-butadiene rubber. The water-based binder is capable of overcoming volume changes caused by expansion and contraction of the positive/negative electrode active material.
It is understood that the method of applying the mixed slurry to positive/negative electrode tabs and assembling the desired full battery is well known in the art.
Compared with the prior art, the technical scheme of the invention has at least the following beneficial effects:
1. the high-dispersity carbon nano tube provided by the invention can be directly mixed with the anode/cathode active material and the binder to prepare battery slurry in a stirring mode without preparing conductive slurry, and can be uniformly dispersed in the battery slurry;
2. the high-dispersity carbon nano tube provided by the invention has excellent conductivity, so that the addition amount of the high-dispersity carbon nano tube in the battery slurry can be 1/3-1/4 of the addition amount of the conductive carbon black in the battery slurry, namely, the addition amount of the positive electrode/negative electrode active material in the battery slurry can be improved, and the capacity of the secondary battery is improved under the condition of maintaining excellent conductivity.
3. The battery slurry coated on the positive electrode current collector in the secondary battery is oil-based slurry, and the battery slurry coated on the negative electrode current collector in the secondary battery is water-based slurry, and the high-dispersity carbon nano tube provided by the invention has better dispersibility in water-based solvents (such as deionized water) and oil-based solvents (such as N-methyl pyrrolidone), so that the high-dispersity carbon nano tube provided by the invention can be commonly used as a conductive agent on the positive electrode plate and the negative electrode plate of the battery.
The scheme of the present invention will be explained below with reference to examples. It will be appreciated by those skilled in the art that the following examples are for illustrative purposes only and are not to be construed as limiting the invention. Unless otherwise indicated, the reagents, software and instrumentation involved in the examples below are all conventional commercial products or open source.
Drawings
The invention is further illustrated by the accompanying drawings, which are not to be construed as limiting the invention in any way.
Fig. 1 is an image obtained by observing the carbon nanotubes provided with high dispersibility in example 4 using a Scanning Electron Microscope (SEM) at a magnification of 40K.
Fig. 2 is an image obtained by observing the CNT dispersion provided in comparative example 1 at a magnification of 40K using a Scanning Electron Microscope (SEM).
Fig. 3 is an image obtained by observing the carbon nanotubes of example 4 providing high dispersibility at a magnification of 80K using a Scanning Electron Microscope (SEM).
Fig. 4 is an image obtained by observing the CNT dispersion provided in comparative example 1 at 80K magnification using a Scanning Electron Microscope (SEM).
Fig. 5 is a graph showing the evaluation results of the rate performance of lithium ion secondary batteries prepared using the conductive agents provided in examples 1 to 4 and comparative examples 1 to 2.
Fig. 6 is a graph showing measurement results of resistance characteristics of lithium ion secondary batteries prepared using the conductive agents provided in examples 1 to 4 and comparative examples 1 to 2.
Detailed Description
It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
[ highly dispersible carbon nanotubes ]
The highly dispersible carbon nanotubes provided by the present invention are described in detail below. The high-dispersibility carbon nanotubes include single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs).
The carbon nanotubes have a shape in which planar graphite is wound into a cylindrical shape. The single-walled carbon nanotubes have a structure in which a layer of graphite is wound. The multiwall carbon nanotube has a structure in which two or more layers of graphite are wound. In addition, the sidewall of the multiwall carbon nanotube may not have a graphite structure. For example, carbon nanotubes having side walls with an amorphous structure may also be used as the multiwall carbon nanotubes.
The shape of the highly dispersible carbon nanotubes provided by the present invention is not limited. Examples of the shape include various shapes including a needle shape, a cylindrical tubular shape, a fishbone shape (fishbone or cup layered), a sheet, and a coil shape. In the present embodiment, the shape of the highly dispersible carbon nanotube is preferably needle-like or cylindrical tube-like. The highly dispersed carbon nanotubes may be in a single shape or in a combination of two or more shapes.
The highly dispersible carbon nanotubes provided by the present invention may be in the form of, for example: graphite whiskers, filamentous carbon, graphite fibers, ultrafine carbon tubes, carbon fibrils, carbon microtubes, and carbon nanofibers, but are not limited thereto. The highly dispersible carbon nanotubes provided by the present invention may have a single form of these or a form in which two or more kinds of these are combined.
The highly dispersible carbon nanotube provided by the present invention is characterized by satisfying the following conditions (1), (2) and (3):
(1) Comprises single-wall carbon nanotubes and multi-wall carbon nanotubes, wherein the mass ratio of the single-wall carbon nanotubes to the multi-wall carbon nanotubes is 1:30-50;
(2) The average outer diameter of the multiwall carbon nanotubes is 40nm or less, and the standard deviation of the outer diameter of the multiwall carbon nanotubes is 4nm or less;
(3) The aspect ratio of the multi-wall carbon nano-tube is not less than 2000.
The inventors have unexpectedly found that when the above conditions are satisfied, the carbon nanotubes exhibit excellent high dispersibility and electrical conductivity, and can exhibit excellent dispersibility in both aqueous slurries and oil-based slurries.
When the average outer diameter of the multiwall carbon nanotubes is X and the standard deviation of the outer diameter of the multiwall carbon nanotubes is σ, X.+ -. 2σ satisfies 16 nm. Ltoreq.X.+ -. 2σ. Ltoreq.28 nm.
The outer diameter and the average outer diameter of the multiwall carbon nanotubes were determined as follows. First, photographing is performed while observing the multiwall carbon nanotubes by using a transmission electron microscope. Next, in the observation photograph, 300 arbitrary multiwall carbon nanotubes were selected, and the outer diameters of the multiwall carbon nanotubes were measured. Next, the average outer diameter (nm) of the multiwall carbon nanotubes was calculated as the number average of the outer diameters.
From the viewpoint of ease of dispersion, the aspect ratio of the multiwall carbon nanotubes of the present embodiment is preferably 2000 to 5000 μm, more preferably 3000 to 4000.
In the present embodiment, the single-walled carbon nanotube and the multi-walled carbon nanotube may be both surface-treated carbon nanotubes. In addition, the single-walled carbon nanotube and the multi-walled carbon nanotube may be carbon nanotube derivatives to which functional groups typified by carboxyl groups are added. Further, a multiwall carbon nanotube containing an organic compound, a metal atom, or a fullerene can be used.
[ method for producing multiwall carbon nanotubes ]
The manufacturing methods of single-walled carbon nanotubes and multi-walled carbon nanotubes are substantially the same. In the present embodiment, the method for producing the multiwall carbon nanotubes is not particularly limited, and carbon nanotubes produced by any method may be used. Multiwall carbon nanotubes can be generally produced using laser ablation, arc discharge, thermal chemical vapor deposition (ChemicalVapor Deposition, CVD), plasma CVD, and combustion. Further, for example, a multiwall carbon nanotube can be produced by contact-reacting a carbon source with a catalyst at 500 to 1000 ℃ in an atmosphere having an oxygen concentration of 1% by volume or less.
[ method for producing highly-dispersible carbon nanotubes ]
The method for manufacturing the carbon nano tube with high dispersibility comprises the following steps:
step one, single-wall carbon nanotubes, multi-wall carbon nanotubes, a dispersing agent and a surfactant are put into deionized water to be mixed to form premixed slurry;
step two, adopting a high-pressure homogenizer or a sand mill to process the premixed slurry to prepare slurry, wherein the viscosity of the slurry is controlled below 5000 cp;
step three, the slurry is subjected to spray drying through spray drying equipment to prepare powder, wherein the inlet temperature is not less than 210 ℃, and the outlet temperature is not less than 130 ℃;
and step four, placing the powder into a tube furnace and calcining at high temperature in an inert gas (such as nitrogen and argon) environment to prepare the carbon nano tube with high dispersibility, wherein the calcining temperature is 300-1000 ℃. The calcination time is 2-8h.
Depending on the properties required for the dispersion of the single-walled carbon nanotubes and the multi-walled carbon nanotubes, a dispersant and a surfactant of a suitable type can be suitably used in a suitable amount. Surfactants are largely classified as anionic, cationic, nonionic, and amphoteric. The dispersing agent is one or more of polyvinylpyrrolidone, polyethylene glycol (with molecular weight of above 600) and polyvinyl alcohol. The dispersant is completely discharged in the form of gas during the high-temperature calcination in the fourth step. The surfactant is an alkyl trimethyl ammonium salt type cationic surfactant, and preferably, the surfactant is one or two of hexadecyl trimethyl ammonium bromide (chloride) and octadecyl trimethyl ammonium chloride (bromide).
The additive may be appropriately formulated in the premix slurry as required within a range that does not hinder the object of the present embodiment. Examples of the additive include: wetting penetrant, antioxidant, crosslinking agent, preservative, viscosity regulator, pH regulator, leveling agent and defoamer, but are not limited thereto.
[ electrode for Secondary Battery ]
The electrode for a secondary battery may be a positive electrode or a negative electrode, and particularly may be a positive electrode. In addition, the electrode may be manufactured by a general method, except for including the above-described highly dispersed carbon nanotubes.
Specifically, when the electrode is a positive electrode, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector.
The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity. Further, the positive electrode current collector may generally have a thickness of 3 to 500 μm, and may have fine irregularities at the surface thereof to increase the adhesion of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous material, a foam, a non-woven fabric, and the like.
The positive electrode active material layer formed on the positive electrode current collector may further include a positive electrode active material, a highly dispersed carbon nanotube, and optionally a binder. The positive electrode active material may be a compound capable of reversibly intercalating and deintercalating lithium ions (lithiated intercalation compound), particularly a composite metal oxide of lithium and a metal such as cobalt, manganese, nickel, or a combination thereof.
The content of the positive electrode active material may be 90 to 98% wt with respect to the total weight of the positive electrode active material layer based on the solid content. When the content of the positive electrode active material is less than 90% by weight, the capacity may be reduced, and when the content of the positive electrode active material is more than 98% by weight, the adhesiveness and conductivity with the positive electrode current collector may be reduced due to the relative reduction in the content of the binder and the highly dispersible carbon nanotubes.
In addition, since the highly dispersible carbon nanotubes can exhibit excellent dispersibility in both aqueous and oily slurries, the binder may be an aqueous binder or an oily binder.
The positive electrode may be manufactured by a conventional method of manufacturing a positive electrode, except for using the above-described highly dispersible carbon nanotubes. Specifically, the positive electrode can be manufactured by the following method: a composition for forming a positive electrode active material layer (prepared by dispersing or dissolving a positive electrode active material, a binder, and highly dispersed carbon nanotubes in a solvent) is coated on a positive electrode current collector, followed by drying and rolling; or the composition for forming the positive electrode active material layer is cast onto a different support, and then a film peeled from the support is laminated on the positive electrode current collector. In this case, the solvent may be a solvent commonly used in the art without particular limitation.
In addition, when the electrode is an anode, the anode includes an anode current collector and an anode active material layer disposed on the anode current collector.
Such a negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. Further, the thickness of the negative electrode current collector may be generally 3 to 500 μm, and may have fine irregularities on the surface thereof to increase the adhesiveness of the negative electrode active material, like the positive electrode current collector. For example, the negative electrode current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous material, a foam, a nonwoven fabric, and the like.
In addition, the anode active material layer may include an anode active material, highly dispersed carbon nanotubes, and an optional binder.
The anode active material may be a compound capable of reversibly intercalating and deintercalating lithium ions. As a specific example, the anode active material may be any one or a mixture of two or more of the following: carbon materials such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, and the like; a metal compound capable of forming an alloy with lithium, such as Si, al, sn, pb, zn, bi, in, mg, ga, cd, si alloy, sn alloy, al alloy, or the like; metal oxides capable of doping and dedoping lithium, e.g. SiO x (0<x<2)、SnO 2 Vanadium oxide or lithium vanadium oxide; and a composite material including the metal compound and the carbon material, such as a Si-C composite material or a Sn-C composite material. In addition, the anode active material may be a lithium metal thin film. As the carbon material, low crystalline carbon, high crystalline carbon, and the like can be used. Representative examples of the low crystalline carbon may be soft carbon and hard carbon, and representative examples of the high crystalline carbon may be high temperature calcined carbon such as amorphous, plate-like, flake-like, spherical or fibrous natural graphite or artificial graphite, coagulated graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesophase carbon microspheres, mesophase pitch, petroleum or coal tar pitch-derived coke, or the like.
In addition, the binder may be the same as those described above for the positive electrode.
The negative electrode may be manufactured by: a composition for forming a negative electrode (prepared by dispersing or dissolving a negative electrode active material, a highly dispersible carbon nanotube, a binder in a solvent) is coated on a negative electrode current collector, followed by drying; or the composition for forming the anode is cast onto a different support, and then a film peeled from the support is laminated on the anode current collector. In this case, the solvent may be the same as the above-described solvent for the positive electrode.
An electrochemical device comprising the above electrode. The electrochemical device may be a battery, a capacitor, or the like, and may be a lithium secondary battery.
The lithium secondary battery includes, in particular, a positive electrode, a negative electrode disposed at a position facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.
[ manufacturing of Secondary Battery ]
In this example, lithium nickel manganese cobalt oxide as a positive electrode active material, a conductive agent, and PVDF as a binder were mixed at 95:2.5:2.5 in N-methylpyrrolidone (NMP) as a solvent to prepare a composition for forming a positive electrode (viscosity: 5000 mPas). The composition was coated on an aluminum current collector, and then dried and rolled at 130 deg.c, thereby manufacturing a positive electrode.
In addition, natural graphite as a negative electrode active material, a conductive agent, and an aqueous binder were mixed in deionized water at a weight ratio of 85:10:5 to prepare a composition for forming a negative electrode, and the composition was coated on a copper current collector to manufacture a negative electrode.
A porous polyethylene separator was interposed between the thus-manufactured positive electrode and negative electrode to manufacture an electrode assembly, the electrode assembly was placed in a case, and then an electrolyte solution was injected into the inside of the case, thereby manufacturing a lithium ion secondary battery. In this case, by adding 1.0M lithium hexafluorophosphate (LiPF 6 ) The electrolyte solution was prepared by dissolving in an organic solvent consisting of ethylene carbonate/dimethyl carbonate/methylethyl carbonate.
Examples
The present invention will be described in more detail with reference to the following examples. The present invention is not limited to the following examples unless the gist thereof is exceeded. In the examples, "%" means "% by mass" unless otherwise specified. In addition, the "carbon nanotube" may be simply referred to as "CNT".
Comparative example 1
The present embodiment provides an existing CNT dispersion, and the preparation process of the CNT dispersion includes:
measuring 0.2g of carbon nano tube and 0.2g of polyvinylpyrrolidone as a resin type dispersing agent in a 450mL SM sample bottle;
step two, 200mL of isopropyl alcohol was added, and dispersion treatment was performed at an amplitude of 50% for 5 minutes using an ultrasonic homogenizer under a low temperature environment, to prepare a CNT dispersion.
Comparative example 2
The prior ultra-dense super P conductive carbon black.
Example 1
1g of single-walled carbon nanotubes, 30g of multi-walled carbon nanotubes, 2g of polyvinylpyrrolidone, 1g of cetyltrimethylammonium bromide and 495g of deionized water are weighed, and then the components are placed in a beaker to be uniformly mixed to form premixed slurry. The premix slurry was then poured into a high pressure homogenizer cup and homogeneously dispersed at a pressure of 800bar to form a slurry with a particle size D50<20 μm. And (3) spray drying the slurry, wherein the feeding temperature is 210 ℃, and the discharging temperature is 130 ℃, so that black powder is obtained. Placing the powder in a tube furnace, introducing nitrogen, controlling the heating speed of the tube furnace to be 10 ℃/min, heating to 500 ℃, preserving heat for 6 hours, cooling to normal temperature, stopping introducing inert gas, taking out black powder, and finally obtaining the carbon nano tube with high dispersibility.
Example 2
1g of single-walled carbon nanotubes, 50g of multi-walled carbon nanotubes, 1g of polyvinylpyrrolidone, 3g of octadecyl trimethyl ammonium chloride and 479g of deionized water are weighed, and then the components are placed in a beaker to be uniformly mixed to form premixed slurry. The premix slurry was then poured into a high pressure homogenizer cup and homogeneously dispersed at a pressure of 800bar to form a slurry with a particle size D50<20 μm. And (3) spray drying the slurry, wherein the feeding temperature is 210 ℃, and the discharging temperature is 130 ℃, so that black powder is obtained. Placing the powder in a tube furnace, introducing nitrogen, controlling the heating speed of the tube furnace to be 10 ℃/min, heating to 600 ℃, preserving heat for 8 hours, cooling to normal temperature, stopping introducing inert gas, taking out black powder, and finally obtaining the carbon nano tube with high dispersibility.
Example 3
1g of single-walled carbon nanotubes, 40g of multi-walled carbon nanotubes, 4g of polyvinyl alcohol, 3g of cetyltrimethylammonium chloride and 473g of deionized water are weighed, and then the components are placed in a beaker and uniformly mixed to form a premixed slurry. The premix slurry was then poured into a high pressure homogenizer cup and homogeneously dispersed at a pressure of 800bar to form a slurry with a particle size D50<20 μm. And (3) spray drying the slurry, wherein the feeding temperature is 210 ℃, and the discharging temperature is 130 ℃, so that black powder is obtained. Placing the powder in a tube furnace, introducing nitrogen, controlling the heating speed of the tube furnace to be 10 ℃/min, heating to 500 ℃, preserving heat for 7 hours, cooling to normal temperature, stopping introducing inert gas, taking out black powder, and finally obtaining the carbon nano tube with high dispersibility.
Example 4
1g of single-walled carbon nanotube, 35g of multi-walled carbon nanotube, 1g of polyethylene glycol, 3g of octadecyl trimethyl ammonium bromide and 491g of deionized water are weighed, and then the components are placed in a beaker and uniformly mixed to form premixed slurry. The premix slurry was then poured into a high pressure homogenizer cup and homogeneously dispersed at a pressure of 800bar to form a slurry with a particle size D50<20 μm. And (3) spray drying the slurry, wherein the feeding temperature is 210 ℃, and the discharging temperature is 130 ℃, so that black powder is obtained. Placing the powder in a tube furnace, introducing nitrogen, controlling the heating speed of the tube furnace to be 10 ℃/min, heating to 600 ℃, preserving heat for 7 hours, cooling to normal temperature, stopping introducing inert gas, taking out black powder, and finally obtaining the carbon nano tube with high dispersibility.
The carbon nanotubes provided with high dispersibility in example 4 and the CNT dispersion provided in comparative example 1 were observed at a magnification of 40K and a magnification of 80K, respectively, using a Scanning Electron Microscope (SEM), and the results are shown in fig. 1 to 4, respectively. As can be seen from SEM images of different magnifications, the highly dispersed carbon nanotubes are consistent with the dispersion state of the CNT dispersion at different magnifications. Therefore, the high-dispersity carbon nano tube provided by the invention can be directly mixed with the anode/cathode active material and the binder to prepare the battery paste in a stirring mode without preparing the conductive paste, and the high-dispersity carbon nano tube can be uniformly dispersed in the battery paste.
According to the above-described manufacturing of the secondary battery, a lithium ion secondary battery was manufactured by using examples 1 to 4 and comparative examples 1 to 2 as the conductive agent, respectively. And the following tests were performed:
1. rate performance evaluation
The lithium ion secondary batteries prepared by using the conductive agents provided in examples 1-4, comparative examples 1-2 were charged to a voltage of 4.25V at a Constant Current (CC) of 0.1C at 25 ℃, and thereafter, the charging in the first cycle was performed by charging the coin-type batteries to a current of 0.05mA at a Constant Voltage (CV) of 4.25V. After the lithium ion secondary battery was left to stand for 20 minutes, the lithium ion secondary battery was discharged to a voltage of 3.0V at a constant current of 0.1C to measure the discharge capacity at the first cycle. Thereafter, the rate performance was evaluated by changing the discharge condition to 2C. The results are shown in fig. 5.
According to experimental results, the lithium ion secondary batteries prepared by using the highly dispersed carbon nanotubes provided in examples 1 to 4 as a conductive agent showed more excellent rate performance.
The CNT dispersion provided in comparative example 1 formed a conductive layer on the entire surface of the positive/negative electrode active material layer. Therefore, since lithium ions in the electrolyte cannot easily move to the electrode, the lithium ion secondary battery prepared by using the CNT dispersion provided in comparative example 1 as a conductive agent has a reduced rate performance compared to the lithium ion secondary batteries prepared by using the highly dispersed carbon nanotubes provided in examples 1 to 4 as a conductive agent.
In addition, the lithium ion secondary battery prepared by using the super high super P conductive carbon black provided in comparative example 2 as a conductive agent, has an increased resistance of the active material due to poor dispersibility of the super high super P conductive carbon black in the positive/negative electrode active material layers.
The lithium ion secondary battery prepared by using the highly dispersible carbon nanotubes provided in examples 1 to 4 as a conductive agent, the highly dispersible carbon nanotubes were uniformly dispersed in the positive/negative electrode active material layer, thereby forming a conductive layer having a network structure, so that high rate performance could be achieved by suppressing an increase in mass transfer resistance in the conductive layer.
2. Evaluation of resistance characteristics
The resistance characteristics of lithium ion secondary batteries prepared using the conductive agents provided in examples 1 to 4 and comparative examples 1 to 2 were evaluated by the following methods.
Specifically, the resistance was measured by discharging each of the batteries charged and discharged at room temperature (25 ℃) at 2.5C based on 50% SOC for 30 seconds, and then the interface resistance and the mass transfer resistance were measured by potentiostatic Electrochemical Impedance Spectroscopy (EIS), respectively. The results are shown in fig. 6.
According to the experimental results, the interfacial resistance of the lithium ion secondary battery prepared by using the highly dispersed carbon nanotubes provided in examples 1 to 4 as a conductive agent was significantly reduced compared to that of the lithium ion secondary battery prepared by using the conductive agent provided in comparative examples 1 to 2, and thus excellent output was obtained. In particular, the conductive network formed by the highly dispersed carbon nanotubes can significantly reduce interface resistance, and the highly dispersed carbon nanotubes can reduce mass transfer resistance by controlling the porosity of the conductive layer.
The technical features of the above-described embodiments may be arbitrarily combined, and all possible combinations of the technical features in the above-described embodiments are not described for brevity of description, however, as long as there is no contradiction between the combinations of the technical features, they should be considered as the scope of the description.
The above examples illustrate only a few embodiments of the invention, which are described in detail and are not to be construed as limiting the scope of the invention. It should be noted that it will be apparent to those skilled in the art that several variations and modifications can be made without departing from the spirit of the invention, which are all within the scope of the invention. Accordingly, the scope of protection of the present invention is to be determined by the appended claims.

Claims (5)

1. A method for preparing carbon nanotubes is characterized in that,
the carbon nanotubes satisfy the following conditions (1), (2) and (3):
(1) Comprises single-wall carbon nanotubes and multi-wall carbon nanotubes, wherein the mass ratio of the single-wall carbon nanotubes to the multi-wall carbon nanotubes is 1:30-50;
(2) The average outer diameter of the multi-walled carbon nanotubes is 40nm or less, the standard deviation of the outer diameter of the multi-walled carbon nanotubes is 4nm or less, X is the average outer diameter of the multi-walled carbon nanotubes, and X + -2σ satisfies 16nm < X + -2σ < 28nm, where σ is the standard deviation of the outer diameter of the multi-walled carbon nanotubes;
(3) The length-diameter ratio of the multi-wall carbon nano tube is not lower than 2000;
the method comprises the following steps:
step one, putting the single-walled carbon nanotubes, the multi-walled carbon nanotubes, a dispersing agent and a surfactant into deionized water, and mixing to form premixed slurry;
step two, adopting a high-pressure homogenizer or a sand mill to process the premixed slurry to prepare slurry, wherein the viscosity of the slurry is controlled below 5000 cp;
step three, the slurry is subjected to spray drying through spray drying equipment to prepare powder, wherein the inlet temperature is not less than 210 ℃, and the outlet temperature is not less than 130 ℃;
and step four, placing the powder into a tube furnace and calcining at a high temperature in an inert gas environment, wherein the calcining temperature is 300-1000 ℃ and the calcining time is 2-8h.
2. The method of claim 1, wherein the total mass of the single-walled carbon nanotubes and the multi-walled carbon nanotubes is 1-10% of the mass of the premix slurry, the mass of the dispersant is no greater than 5% of the mass of the premix slurry, and the mass of the surfactant is no greater than 1% of the mass of the premix slurry.
3. The method of claim 2, wherein the dispersing agent is one or more of polyvinylpyrrolidone, polyethylene glycol with molecular weight of 600 or more, and polyvinyl alcohol.
4. A method according to claim 3, wherein the surfactant is an alkyl trimethylammonium salt type cationic surfactant.
5. The method of claim 4, wherein the surfactant is one of cetyltrimethylammonium bromide, cetyltrimethylammonium chloride, octadecyltrimethylammonium chloride, and octadecyltrimethylammonium bromide.
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