Method for preparing carbon nanotube-carbon nanofiber composite, and carbon nanotube-carbon nanofiber composite prepared thereby

Abstract

The present invention relates to a method for preparing a carbon nanotube-carbon nanofiber composite, the method comprising: a first step of preparing a spinning solution containing an alkali metal precursor and a carbon-containing polymer; a second step of electrospinning the spinning solution to manufacture carbon-containing polymer nanofibers having the alkali metal precursor bound to the surface thereof; a third step of heat-treating the carbon-containing polymer nanofibers to manufacture carbon nanofibers having the alkali metal precursor bound to the surface thereof; and a fourth step of supplying a carbon source to the carbon nanofibers while heat-treating same to manufacture a carbon nanotube–carbon nanofiber composite having carbon nanotubes bound to the surface thereof, the fourth step including a carbon nanotube seed formation step, a first carbon nanotube growth step, a nanocatalyst activation step, and a second carbon nanotube growth step, whereby a carbon nanotube-carbon nanofiber composite is produced which has 80 to 120 carbon nanotubes per 1 μm2 of carbon nanofibers at the surface of the carbon nanotube-carbon nanofiber composite.

Description

Method for producing a carbon nanotube-carbon nanofiber composite and a carbon nanotube-carbon nanofiber composite produced thereby

[0001] The present invention relates to a method for producing a carbon nanotube-carbon nanofiber composite and a carbon nanotube-carbon nanofiber composite produced thereby.

[0002] Carbon nanofibers refer to fibrous carbon materials with a diameter of 1 ㎛ or less and a carbon element mass content of more than 90%. Carbon atoms are sp, sp 2 , sp 3 It has high mechanical properties, excellent thermal / electrical conductivity, chemical stability, etc. due to the mixed combination of sp, sp. Based on these properties, it has been recently used as an energy storage and conversion material such as an electrode material for secondary batteries and supercapacitors, and a catalyst support for fuel cells. However, the physical contact between nanofibers is not sufficient, so the contact resistance is high, and sp, sp 2 , sp 3 Due to the mixed combination of bonds, it has the disadvantage of having lower electrical conductivity than carbon nanotubes.

[0003] Carbon nanotubes are made up of six carbon atoms sp 2 This refers to carbon materials that form tubes by bonding, with tube diameters ranging from several nanometers to several tens of nanometers. While the tube diameters are nanometers in size, their lengths range from a few micrometers to several millimeters, resulting in a very high aspect ratio. Furthermore, due to its outstanding mechanical, electrical, and thermal properties, it is being used as a substitute for existing materials in the field of nanocomposite manufacturing.

[0004] For these reasons, composite materials formed by forming carbon nanotubes on carbon nanofibers can combine the advantages of both, and can extend excellent electrical and mechanical properties not only in the longitudinal direction of the carbon nanofibers but also in the direction perpendicular thereto, making them ideal two-dimensional fiber-reinforced components. Furthermore, since such composite materials can utilize the relatively large surface area of ​​carbon nanotubes, the bonding area can be substantially expanded, and since functional groups can be introduced to the carbon nanotubes, it is expected that the compatibility of the fiber-reinforced material with the polymer can be improved, and the mechanical strength is also expected to be outstanding, so it is expected to make a great contribution to industrial fields that require high-performance composite materials. Based on these properties, its use as a conductive material for secondary batteries has recently been greatly expanded.

[0005] However, carbon nanotubes exist as nanoscale powders, and their superior properties are difficult to fully utilize due to cohesion caused by van der Waals forces between the tubes. Therefore, unless they are stably dispersed in a solvent, their utility is significantly reduced. Furthermore, for composite materials made of carbon nanotubes and carbon nanofibers to function properly, the bonding between the two must be strong, which is often not the case.

[0006] Composite materials of carbon nanotubes and carbon nanofibers to date have presented solutions to the problem of dispersing carbon nanotubes within polymers, but the bonding strength between carbon nanotubes and carbon nanofibers is weak, and since the carbon nanotubes are not aligned, the mechanical strength when manufactured into composite materials is low.

[0007] In particular, since most non-alkali metal catalysts such as iron (Fe), nickel (Ni), cobalt (Co), and palladium (Pd) are used, numerous non-alkali metal catalyst particles remain as impurities in carbon nanotubes after they are synthesized into composite materials. To remove these non-alkali metal catalyst particles, high-concentration acid treatment is required. However, since the non-alkali metal catalyst particles are not completely removed with a single acid treatment, it is cumbersome to have to perform the process repeatedly multiple times. In addition, there is a problem that environmental costs are added because washing water is required for each acid treatment, and this makes it difficult to mass-produce composite materials of carbon nanotubes and carbon nanofibers.

[0008] Through research, the present inventors have developed a new method for manufacturing a composite material capable of growing carbon nanotubes on carbon nanofibers, which is a method for growing carbon nanotubes on carbon nanofibers, as described in Korean Patent No. 10-2224146, breaking away from the existing composite materials of carbon nanotubes and carbon nanofibers. In the case of the above-mentioned conventional technology, in order to grow carbon nanotubes on carbon nanofibers, a quartz tube is placed centrally inside an electric heater, the temperature inside the quartz tube is maintained at 700°C, and ethanol is heated to produce ethanol vapor, which is supplied into the quartz tube at 50 sccm for 15 minutes through nitrogen gas bubbling. However, in the case of the above conventional technology, the diameter of the carbon nanofiber is 250 to 2000 nm, the diameter of the carbon nanotube is 50 to 70 nm, and the length of the carbon nanotube is 200 to 500 nm, so the thickness (diameter) of the carbon nanofiber is not uniform, and the density and length of the growing carbon nanotube are also varied, and there was difficulty in controlling the length and detailed density of the carbon nanotube.

[0009] Accordingly, the inventors of the present invention have developed a method for manufacturing a carbon nanotube-carbon nanofiber composite in which the length and density of the carbon nanotubes are controlled and a uniform structure is formed by specifying the steps for manufacturing a carbon nanotube-carbon nanofiber composite and controlling the conditions, and have completed the present invention.

[0010] The present invention was invented to solve the above problems, and provides a method for manufacturing a carbon nanotube-carbon nanofiber composite.

[0011] In addition, a carbon nanotube-carbon nanofiber composite manufactured by the above manufacturing method is provided.

[0012] In order to solve the above technical problem, the present invention,

[0013] Step 1: preparing a radiation solution containing an alkali metal precursor and a carbon-containing polymer;

[0014] A second step of producing carbon-containing polymer nanofibers by electrospinning the above-mentioned spinning solution;

[0015] A third step of manufacturing carbon nanofibers having the alkali metal precursor bonded to the surface by heat-treating the carbon-containing polymer nanofibers; and

[0016] A fourth step of manufacturing a carbon nanotube-carbon nanofiber composite having carbon nanotubes bonded to the surface by heat-treating the carbon nanofibers while supplying a carbon source; including,

[0017] The fourth step above is,

[0018] A carbon nanotube seed formation step in which the carbon source is supplied to the carbon nanofibers while heating in an inert gas atmosphere so that the alkali metal precursor is activated into an alkali metal nanocatalyst, and the carbon source is bound to the surface of the carbon nanofibers by the nanocatalyst, thereby forming a carbon nanotube seed for growing into a carbon nanotube;

[0019] A first carbon nanotube growth step of growing carbon nanotubes on the surface of the carbon nanotube fiber by re-supplying the carbon source to the carbon nanofiber while heating it in an inert gas atmosphere after the carbon nanotube seed formation step;

[0020] A nanocatalyst activation step in which hydrogen gas is supplied after the first carbon nanotube growth step and the nanocatalyst is further activated through reduction of the alkali metal nanocatalyst according to the supply of the hydrogen gas; and

[0021] Including a second carbon nanotube growth step in which the carbon source is re-supplied in an inert gas atmosphere after the nanocatalyst activation step to additionally grow carbon nanotubes;

[0022] 1 μm of the carbon nanofiber on the surface of the carbon nanotube-carbon nanofiber composite 2 A method for producing a carbon nanotube-carbon nanofiber composite is provided, characterized in that the carbon nanotube-carbon nanofiber composite is produced having 80 to 120 carbon nanotubes.

[0023] In addition, in order to solve the above technical problem, the present invention,

[0024] Step 1: preparing a radiation solution containing an alkali metal precursor and a carbon-containing polymer;

[0025] A second step of producing carbon-containing polymer nanofibers having the alkali metal precursor bonded to the surface by electrospinning the above-mentioned spinning solution;

[0026] A third step of manufacturing carbon nanofibers having the alkali metal precursor bonded to the surface by heat-treating the carbon-containing polymer nanofibers; and

[0027] A fourth step of manufacturing a branched fine carbon nanofiber-carbon nanofiber composite in which branched fine carbon nanofibers are bonded to the surface by heat-treating the carbon nanofibers while supplying a carbon source; including,

[0028] The fourth step above is,

[0029] A branched fine carbon nanofiber seed formation step in which the carbon source is supplied to the carbon nanofibers while heating in an inert gas atmosphere so that the alkali metal precursor is activated as an alkali metal nanocatalyst, and the carbon source is bound to the surface of the carbon nanofibers by the nanocatalyst, thereby forming a branched fine carbon nanofiber seed for growing into branched fine carbon nanofibers;

[0030] A first branched fine carbon nanofiber growth step in which the carbon source is re-supplied to the carbon nanofiber while heating in an inert gas atmosphere after the branched fine carbon nanofiber seed formation step to grow branched fine carbon nanofibers on the surface of the carbon nanofiber;

[0031] A nanocatalyst activation step in which hydrogen gas is supplied after the first branched fine carbon nanofiber growth step, and the nanocatalyst is further activated through reduction of the alkali metal nanocatalyst according to the hydrogen gas supply; and

[0032] Including a second branched fine carbon nanofiber growth step in which the carbon source is re-supplied in an inert gas atmosphere after the nanocatalyst activation step to additionally grow branched fine carbon nanofibers;

[0033] On the surface of the above branched fine carbon nanofiber-carbon nanofiber composite, the carbon nanofiber is 1 μm in diameter. 2 A method for producing a branched fine carbon nanofiber-carbon nanofiber composite is provided, characterized in that the branched fine carbon nanofiber-carbon nanofiber composite is produced by producing 80 to 120 branched fine carbon nanofibers.

[0034] The manufacturing method of the present invention by means of solving the above problem can easily grow carbon nanotubes from the surface of carbon nanofibers by simply manufacturing carbon nanofibers having an alkali metal precursor bonded to the surface through electrospinning, carbonizing the carbon nanofibers, and then heat-treating them while supplying a carbon source, thereby enabling mass production of carbon nanotube-carbon nanofiber composites. Furthermore, by specifying the steps for manufacturing a carbon nanotube-carbon nanofiber composite and controlling the conditions, there is an effect of manufacturing a carbon nanotube-carbon nanofiber composite in which the length and density of the carbon nanotubes are controlled and a uniform structure is formed.

[0035] In addition, since the carbon nanotube-carbon nanofiber composite manufactured according to the manufacturing method of the present invention is manufactured using an alkali metal-based catalyst rather than a transition metal-based catalyst, the catalyst particles dissolve in water and can be easily removed, so that after the synthesis of the metal-free carbon nanotube-carbon nanofiber composite is completed, there is no need to go through a cleaning process such as acid treatment, thereby reducing environmental costs. In addition, since carbon nanotubes are grown from carbon nanofibers using a nanocatalyst, the carbon nanofibers and carbon nanotubes have high bonding strength, so that the carbon nanofibers and carbon nanotubes do not separate, resulting in excellent durability.

[0036] Figure 1 illustrates a carbon nanofiber manufacturing step according to one embodiment of the present invention.

[0037] Figure 2 is a graph showing the conditions of the carbon nanofiber stabilization and carbonization step according to one embodiment of the present invention.

[0038] Figure 3 is an SEM photograph of carbon nanofibers manufactured according to one embodiment of the present invention, showing the carbon-containing polymer concentration.

[0039] Figure 4 is an SEM photograph of a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention at different catalyst precursor concentrations.

[0040] Figure 5 is an SEM photograph of a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention, according to carbon source.

[0041] FIG. 6 is an SEM photograph of a carbon nanofiber-carbon nanotube composite manufactured by varying the distance between the gas inlet and the carbon nanotube fiber according to one embodiment of the present invention.

[0042] FIG. 7 is an SEM photograph of a carbon nanofiber-carbon nanotube composite manufactured by varying gas injection times according to one embodiment of the present invention.

[0043] FIG. 8 is an SEM photograph of a carbon nanofiber-carbon nanotube composite manufactured by varying gas flow rates according to one embodiment of the present invention.

[0044] FIG. 9 is a photograph of the shape and microstructure of a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0045] Figure 10 is a nitrogen adsorption / desorption isotherm curve of a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0046] Figure 11 is a pore distribution curve of a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0047] Figure 12 shows the results of ToF-SIMS analysis of a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0048] Figure 13 shows the XPS analysis results of a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0049] Figure 14 is a carbon bond peak of a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0050] FIG. 15 is a Raman spectrum of a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0051] FIG. 16 is a first cycle charge / discharge curve of a lithium ion battery negative electrode material including a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0052] FIG. 17 shows the results of an ultra-high-speed performance evaluation of a lithium ion battery negative electrode material including a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0053] FIG. 18 is a graph of discharge capacity at a charge / discharge rate of 2 C of a lithium ion battery negative electrode material including a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0054] The present invention is susceptible to various modifications and takes various forms, and thus, embodiments are described in detail herein. However, this is not intended to limit the present invention to a specific disclosed form, and it should be understood that all modifications, equivalents, and alternatives fall within the spirit and technical scope of the present invention.

[0055] Throughout the specification, whenever a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0056] Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms defined in commonly used dictionaries should be interpreted as having a meaning consistent with their meaning in the context of the relevant technology, and will not be interpreted in an idealized or overly formal sense unless explicitly defined herein.

[0057] The terminology used herein is merely used to describe specific embodiments and is not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise.

[0058]

[0059] According to one aspect of the present invention, a method for producing a carbon nanotube-carbon nanofiber composite is provided, comprising: a first step of preparing a spinning solution containing an alkali metal precursor and a carbon-containing polymer; a second step of electrospinning the spinning solution to produce carbon-containing polymer nanofibers having the alkali metal precursor bound to a surface; a third step of heat-treating the carbon-containing polymer nanofibers to produce carbon nanofibers having the alkali metal precursor bound to a surface; and a fourth step of supplying a carbon source to the carbon nanofibers and heat-treating them to produce a carbon nanotube-carbon nanofiber composite having carbon nanotubes bound to a surface.

[0060] According to one aspect of the present invention, there is provided a method for producing a carbon nanofiber, comprising: a first step of preparing a spinning solution containing an alkali metal precursor and a carbon-containing polymer; a second step of electrospinning the spinning solution to produce carbon-containing polymer nanofibers; a third step of heat-treating the carbon-containing polymer nanofibers to produce carbon nanofibers having the alkali metal precursor bound to the surface; and a fourth step of heat-treating the carbon nanofibers while supplying a carbon source thereto to produce a carbon nanotube-carbon nanofiber composite having carbon nanotubes bound to the surface thereof; wherein the fourth step comprises a carbon nanotube seed forming step of supplying the carbon source to the carbon nanofibers while heating them in an inert gas atmosphere so that the alkali metal precursor is activated into an alkali metal nanocatalyst, and the carbon source is bound to the surface of the carbon nanofibers by the nanocatalyst, thereby forming carbon nanotube seeds for growing into carbon nanotubes. A first carbon nanotube growth step in which the carbon source is re-supplied to the carbon nanofibers while heating in an inert gas atmosphere after the carbon nanotube seed formation step to grow carbon nanotubes on the surface of the carbon nanotube fibers; a nanocatalyst activation step in which hydrogen gas is supplied after the first carbon nanotube growth step and the nanocatalyst is further activated through reduction of the alkali metal nanocatalyst according to the supply of the hydrogen gas; and a second carbon nanotube growth step in which the carbon source is re-supplied in an inert gas atmosphere after the nanocatalyst activation step to further grow carbon nanotubes; Including, on the surface of the carbon nanotube-carbon nanofiber composite, the carbon nanofibers are 1 μm in diameter. 2 A method for producing a carbon nanotube-carbon nanofiber composite is provided, characterized in that the carbon nanotube-carbon nanofiber composite is produced having 80 to 120 carbon nanotubes.

[0061] The present invention produced nanofibers using electrospinning. Furthermore, during the carbon nanofiber production process, alkaline metal catalysts were incorporated into the fibers, and carbon nanotubes were grown from these catalysts using heat treatment utilizing a carbon source. The nanofibers thus produced were carbonized through heat treatment, and carbon nanotubes were grown from the carbon nanofibers using thermochemical vapor deposition.

[0062] These carbon nanofiber-carbon nanotube composites have a 3D network structure based on a 1D (dimensional) structure, which allows ions and electrons to travel short distances, resulting in excellent ionic and electrical conductivity. Therefore, when applied as a secondary battery electrode material, high energy density and improved output characteristics are expected, and when utilized as a catalyst support for fuel cells, high output characteristics as well as high durability based on an excellent crystalline internal structure are expected.

[0063] In addition, since transition metal catalysts such as Fe, Ni, and Co, which were used in the growth of conventional carbon nanotubes, were not used, it is expected that environmental costs can also be significantly reduced as there is no need for a cleaning process such as high-concentration acid treatment to remove them.

[0064] Furthermore, the steps for manufacturing a carbon nanotube-carbon nanofiber composite with carbon nanotubes bonded to the surface were subdivided to produce a carbon nanotube-carbon nanofiber composite in which carbon nanotubes are uniformly grown and formed at a high density. The specific manufacturing method is described below.

[0065] First, a radiation solution containing an alkali metal precursor and a carbon-containing polymer is prepared (S1).

[0066] Previously, catalysts based on transition metals, i.e., non-alkali metals of Group 8, 9, and 10, such as iron (Fe), cobalt (Co), and nickel (Ni), were mainly used to grow carbon nanotubes on carbon nanofibers. However, when the non-alkali metal catalyst is formed into a nanoparticle form and the synthesis and growth of carbon nanotubes are completed, additional processes such as acid treatment are required to remove the nanoparticles remaining in the metallic state of the non-alkali metal catalyst. In addition, since acid treatment requires washing water, there has been a burden of increased environmental costs.

[0067] Accordingly, in the present invention, an alkali metal precursor based on a Group 1 element excluding hydrogen is dissolved in a solvent to prepare an alkali metal precursor solution, whereby the alkali metal is activated as a nanocatalyst to grow carbon nanotubes from the surface of carbon nanofibers, and the nanocatalyst is easily dissolved and removed in water without having to go through a separate process such as acid treatment to remove the nanocatalyst, thereby enabling the synthesis of a high-purity carbon nanotube-carbon nanofiber composite.

[0068] The alkali metal precursor is selected from the group consisting of a lithium precursor (Li precursor), a sodium precursor (Na precursor), a potassium precursor (K precursor), and mixtures thereof. That is, the alkali metal precursor may be composed of one or more alkali metal salts, alkali metal organic compounds, or alkali metal inorganic compounds selected from the group consisting of lithium (Li), sodium (Na), and potassium (K).

[0069] For example, in the case of a lithium precursor, which is a lithium-containing compound, the compound is selected from the group consisting of lithium benzoate, lithium chloride (LiCl), and mixtures thereof; in the case of a sodium precursor, which is a sodium-containing compound, the compound is selected from the group consisting of sodium benzoate, sodium chloride (NaCl), sodium bicarbonate (NaHCO3), and mixtures thereof; and in the case of a potassium precursor, which is a potassium-containing compound, the compound is selected from the group consisting of potassium benzoate, potassium chloride, potassium hydroxide, and mixtures thereof.

[0070] The solvent is composed of a polar solvent or a non-polar solvent. A polar solvent selected from the group consisting of water, dimethylformamide (DMF), lower alcohols having 1 to 5 carbon atoms, and mixtures thereof, or a non-polar solvent selected from the group consisting of xylene, benzene, toluene, and mixtures thereof can be selected and used.

[0071] An alkali metal precursor solution in which an alkali metal precursor is mixed in a solvent is prepared in the following two ways.

[0072] First, there is a method of preparing an alkali metal precursor solution by dissolving the alkali metal precursor in a polar solvent such as dimethylformamide.

[0073] It is preferable to dissolve the alkali metal precursor in the range of 0.02 to 0.3 mol per 1 liter of the solvent. In this case, the amount of the alkali metal precursor used is not particularly limited, but if it is mixed in an amount less than 0.02 mol per 1 liter of the solvent, the alkali metal precursor is not activated or functionalized as an alkali metal nanocatalyst when heat treatment is performed, so not only does it take a long time until carbon nanotubes can grow from the surface of the carbon nanofiber, but there are also cases where the supplied carbon source does not grow into carbon nanotubes, which limits its application in the field of energy applications. In particular, if the alkali metal precursor is added in too small an amount, the reaction rate cannot be increased, which is not preferable in terms of production efficiency.

[0074] Secondly, a method of preparing an alkali metal precursor solution by solvating the alkali metal cation by forming a complex by allowing the alkali metal cation of the alkali metal precursor to be coordinated to the cavity of the crown ether through the addition of a crown ether.

[0075] Crown ether (x-Crown ether-y; x is the total number of atoms in the ring, y is the number of oxygen atoms) is an oligomer of ethylene oxide with repeating ethyleneoxy (-CH2CH2O-) units. When an alkali metal cation in an alkali metal precursor solution is inserted into the cavity at the center of the crown ether, a stable structure is formed with the alkali metal cation, thereby allowing the alkali metal cation to be solvated and dissolved, thereby increasing the solubility of the alkali metal precursor as a solute, especially in a nonpolar solvent. In other words, the crown ether is a metal ion, that is, Li + , Na + , K +Because it forms a stable complex with alkali metal cations such as benzene, xylene, and toluene, it can easily solvate alkali metal precursors that are insoluble in nonpolar solvents composed of hydrocarbons such as benzene, xylene, and toluene.

[0076] When an alkali metal precursor is dissolved in a solvent through a crown ether, it is converted into a transparent alkali metal precursor solution. In order to dissolve the maximum amount of alkali metal precursor, it is recommended to control the ratio with the crown ether (for example, the weight ratio of the alkali metal precursor and the crown ether may be 1:0.1 to 100). The solubility of the alkali metal precursor solution can also be controlled by controlling the amount of the crown ether. However, the amount in which the alkali metal precursor and the crown ether can be mixed is not limited. In the case of the crown ether, it can be selected and used from the group consisting of 12-Crown-4, 15-Crown-5, 18-Crown-6, and mixtures thereof.

[0077] In particular, solvents such as water or dimethylformamide presented in the first method are polar and the alkali metal precursor dissolves well in them, but since the alkali metal precursor does not dissolve in nonpolar solvents (e.g., xylene) that are different from polar solvents, crown ether plays an important role in solvating the solute and increasing the solubility.

[0078] A spinning solution is prepared by dissolving a carbon-containing polymer in the alkali metal precursor solution. It is preferable to prepare a spinning solution capable of electrospinning by adding 1 to 15 wt% of a carbon-containing polymer to 85 to 99 wt% of the alkali metal precursor solution and dissolving it while stirring.

[0079] In particular, if the carbon-containing polymer is less than 1 wt%, it is difficult to form carbon nanofibers with a uniform shape even if the spinning solution is electrospun. If it exceeds 15 wt%, the alkali metal precursor solution is contained relatively little, so the amount of nano-catalyst that can be activated later is relatively reduced, which also reduces the amount of carbon nanotubes that can be grown from the surface of the carbon nanofibers. In other words, if the alkali metal precursor solution is less than 85 wt%, the amount of nano-catalyst that can be activated is small, which means that the amount of carbon nanotubes that can be grown is also small, and if the alkali metal precursor solution exceeds 99 wt%, there is a disadvantage that there is not enough space in the carbon nanofibers for the nano-catalyst to form, making it difficult for the carbon nanotubes to grow stably. In order to stably grow carbon nanotubes on the carbon nanofibers after complete molding into carbon nanofibers, it is most desirable to contain the carbon-containing polymer at 9 wt%, taking into account the volatilization of the solvent in the alkali metal precursor solution.

[0080] The carbon-containing polymer may be considered a precursor of carbon nanofibers, and may be selected from the group consisting of polyacrylonitrile (PAN), polyvinyl acetate (PVA), polyvinyl pyrrolidone (PVP), polycarbonate (PC), polyvinyl chloride (PVC), cellulose, cellulose acetate, and mixtures thereof. In the present invention, polyacrylonitrile is used, but any carbon-containing polymer that can be formed into carbon nanofibers is not particularly limited.

[0081] Next, the above-mentioned spinning solution is electrospun to produce carbon-containing polymer nanofibers (S2).

[0082] The above-mentioned manufactured spinning solution is electrospun to produce carbon-containing polymer nanofibers having an alkali metal precursor bound to the surface. In order to perform electrospinning, first, a (+) or (-) high voltage terminal is connected to the nozzle, and when a sufficiently high voltage is applied while connecting the conductor to ground, an electromagnetic field is formed between the nozzle and the conductor, and the spinning solution inside the nozzle is affected. When the electromagnetic force is greater than the surface tension and viscosity of the spinning solution, a Taylor cone is formed and stretched at the end to produce a nano-sized composite fiber, which is a composite nanofiber. However, the 'composite nanofiber' and 'nano-sized composite fiber' mentioned in the present invention mean 'carbon-containing polymer nanofibers having an alkali metal precursor bound to the surface.'

[0083] In order to produce nano-sized composite fibers through electrospinning, it is desirable to satisfy the following conditions: molecular weight of the carbon-containing polymer, properties of the spinning solution, voltage, distance between the nozzle and the conductor, fluid amount and concentration of the carbon-containing polymer, parameters, movement of the nozzle, size of the conductor, and size of the nozzle.

[0084] The molecular weight conditions of the carbon-containing polymer are as follows. That is, if the molecular weight (Mw) of the carbon-containing polymer is less than 45,000 or more than 1,000,000, it is difficult to uniformly form composite nanofibers, so it is preferable to make it within the range of 45,000 to 1,000,000.

[0085] With respect to the viscosity of the spinning solution, if it is less than 1 Pa·s, the viscosity is too low and the spinning solution breaks before forming nano-sized composite fibers during the electrospinning process, resulting in droplets rather than composite nanofibers. In addition, if it exceeds 1,000 Pa·s, the viscosity becomes too high and more electromagnetic force is required to elute from the nozzle, which may cause an overcurrent and burn out the experimental equipment. Therefore, it is preferable to have a viscosity in the range of 1 to 1,000 Pa·s. With respect to the conductivity of the spinning solution, if it exceeds 53 μs / cm, it is not suitable for forming carbon nanofibers. Therefore, it is preferable that the spinning solution have a conductivity of 53 μs / cm or less. Regarding the surface tension of the spinning solution, if it exceeds 450 dyn / cm, the electromagnetic force becomes smaller than the surface tension of the spinning solution, so Taylor cone formation does not occur, making it difficult to form a composite nanofiber shape. Therefore, it is preferable that the surface tension of the spinning solution be 450 dyn / cm or less.

[0086] In the case of voltage conditions, if a voltage of 30 kV or less is applied for electrospinning of the spinning solution, an electromagnetic field is formed between the nozzle and the conductor, so there is no need to apply a voltage exceeding 30 kV.

[0087] The distance between the nozzle and the conductor, i.e. the distance between the nozzle containing the spinning solution and the conductor, is maintained to be 30 cm or less to form nano-sized composite fibers. If the distance between the nozzle and the conductor exceeds 30 cm, the distance between the nozzle and the conductor is too far, resulting in a small electromagnetic force, making it difficult to uniformly form the nano-sized composite fibers. In addition, there is the disadvantage that droplets, rather than nanofibers, may be visible. Preferably, the distance is less than 10 cm.

[0088] In the case of the fluid amount of carbon-containing polymer, it should be less than 25 ml / min so that the spinning solution is formed into a Taylor cone and stretched well into nanofibers. However, if it exceeds 25 ml / min, the fluid amount is too large and the stretching amount is small, so the probability of manufacturing non-uniform nanofibers increases, which can also increase the defect rate.

[0089] For the carbon-containing polymer concentration, it is desirable to be 7 to 11 wt% for manufacturing nano-sized fibers.

[0090] The parameters are related to basic environmental aspects such as temperature, humidity, and airflow. If the temperature is below 35℃, the humidity is below 60%, and the airflow is below 1, then carbon-containing polymer nanofibers with alkali metal precursors bonded to the surface can be manufactured through electrospinning.

[0091] The conditions for nozzle movement and conductor size are as follows. First, the spinning solution can be stably stretched into a conductor through electrospinning only when the nozzle movement is less than 0.1 mm / min, and the composite nanofibers electrospun from the nozzle can be stably captured on the conductor even if the conductor size is 10㎠ or larger. However, if the conductor size is less than 10㎠, the area is too small, which has the disadvantage of not securing enough space to capture the composite nanofibers.

[0092] The nozzle size conditions are as follows. If the nozzle size is less than 0.01 mm or greater than 1.7 mm, it does not help in forming carbon nanofibers with an alkali metal precursor bonded to the surface. Therefore, the nozzle size is preferably in the range of 0.01 to 1.7 mm.

[0093] Before manufacturing carbon nanofibers through carbonization, carbon-containing polymer nanofibers having an alkali metal precursor bonded to their surface are preheated at a heating rate of 8 to 12°C / min in the air to 100 to 300°C for 20 minutes to 1 hour to stabilize the carbon-containing polymer. If the preheat treatment is performed at a temperature lower than 100°C, it is difficult to stabilize the carbon-containing polymer nanofibers, and if the preheat treatment is performed at a temperature higher than 300°C, the temperature may be unnecessarily high, which may cause deterioration of the shape or properties of the nanofibers.

[0094] The heating rate can be 8 to 12°C / min, with 10°C / min being most preferred. Furthermore, if the preheat treatment is performed for less than 20 minutes, it is difficult to stabilize the carbon-containing polymer of the carbon-containing polymer nanofibers, similar to the temperature conditions. Furthermore, if the preheat treatment is performed for more than 1 hour, the results are not as outstanding as when performed for less time. In particular, preheat treatment should be performed in an oxygen atmosphere to ensure a smooth oxygen supply, which has the advantage of allowing rapid formation of carbon-containing polymer nanofibers.

[0095] The diameter of the carbon nanofibers of the present invention manufactured accordingly may be 150 to 200 nm, but is not limited thereto.

[0096] Next, the carbon-containing polymer nanofibers are heat-treated to produce carbon nanofibers having the alkali metal precursor bonded to the surface (S3).

[0097] After the formation of nanofibers that stabilize the carbon-containing polymer, the carbon is further heat-treated at a heating rate of 3 to 7°C / min to 800 to 1,200°C for 30 minutes to 1 hour and 30 minutes in an inert gas atmosphere to carbonize, followed by a natural cooling process to complete the production of carbon nanofibers. Carbonization here refers to a heat treatment process that increases the carbon / hydrogen ratio of the carbon-containing polymer that forms the carbon-containing polymer nanofibers, and refers to a process that converts the carbon-containing component into carbon.

[0098] Regarding the heating rate for carbonization, the reason why the heating rate is relatively slow at 3 to 7°C / min compared to the preliminary heat treatment to stabilize the carbon-containing polymer nanofibers is to ensure that there are no problems in forming carbon nanofibers during the carbonization process as the temperature increases, while also preventing deterioration of the physical properties of the carbon nanofibers. For stable production of carbon nanofibers, it is most desirable to perform the process at 5°C / min.

[0099] Regarding the heat treatment time for carbonization, if it's less than 30 minutes, the intended carbonization effect will be minimal. If it exceeds 1 hour and 30 minutes, the process becomes inefficient due to the excessively long time. From a process efficiency perspective, 60 minutes of heat treatment is the most desirable method for carbonization.

[0100] In the present invention, the inert gas atmosphere may be, for example, a gas such as helium, nitrogen, argon, or carbon dioxide. In other words, the carbon-containing polymer of the carbon-containing polymer nanofibers can be carbonized and converted into carbon nanofibers through heat treatment under an inert atmosphere.

[0101] Finally, a carbon nanotube-carbon nanofiber composite is manufactured by supplying a carbon source to the carbon nanofiber and heat-treating the carbon nanofiber, thereby bonding carbon nanotubes to the surface (S4).

[0102] First of all, in the past, for the growth of carbon nanotubes on carbon nanofibers, the carbon nanofibers were coated with a non-alkali metal catalyst such as iron (Fe), and when carbon dioxide and other carbon-containing gases were passed through them, carbon atoms began to dissolve in the iron particles, ultimately forming vertical tubes of carbon atoms around the carbon nanofibers. However, in this case, there was a drawback that the iron particles remained inside the carbon nanotubes, so acid treatment had to be repeated several times to remove the iron particles.

[0103] To improve this, a method was developed in which the alkali metal of the alkali metal precursor is activated as a nanocatalyst, and the carbon source is crystallized into carbon nanotubes through this nanocatalyst to grow them from the surface of carbon nanofibers. However, the problem occurred that the thickness (diameter) of the carbon nanofibers was not uniform, so there was a large deviation in the density and length of the growing carbon nanotubes, and a structurally non-uniform shape appeared. In other words, there was difficulty in controlling the length and detailed density of the carbon nanotubes.

[0104] Accordingly, the present invention is characterized in that the steps for manufacturing the carbon nanotube-carbon nanofiber composite are specified and the conditions are controlled to manufacture a carbon nanotube-carbon nanofiber composite in which the length and density of the carbon nanotube are controlled and a uniform structure is formed.

[0105] Specifically, the step (S4) of manufacturing the carbon nanotube-carbon nanofiber composite includes: a carbon nanotube seed forming step (S4-1) of supplying the carbon source to the carbon nanofiber while heating it in an inert gas atmosphere so that the alkali metal precursor is activated as an alkali metal nanocatalyst, and the carbon source is bound to the surface of the carbon nanofiber by the nanocatalyst, thereby forming a carbon nanotube seed for growing into a carbon nanotube; a first carbon nanotube growth step (S4-2) of re-supplying the carbon source to the carbon nanofiber while heating it in an inert gas atmosphere after the carbon nanotube seed forming step so that carbon nanotubes grow on the surface of the carbon nanotube fiber; a nanocatalyst activation step (S4-3) of supplying hydrogen gas after the first carbon nanotube growth step and further activating the nanocatalyst through reduction of the alkali metal nanocatalyst according to the supply of the hydrogen gas; And a second carbon nanotube growth step (S4-4) in which the carbon source is re-supplied in an inert gas atmosphere after the nanocatalyst activation step to additionally grow carbon nanotubes.

[0106] First, carbon nanotube seeds are formed (S4-1).

[0107] The nanocatalyst of the present invention can activate an alkali metal precursor into an alkali metal nanocatalyst simply by heat treatment in the presence of a carbon source, without the need to separately prepare a catalyst in particle form using a Group 1 element other than hydrogen, particularly sodium. The nanocatalyst can form carbon nanotube seeds that enable only pure carbon nanotubes to grow from the surface of carbon nanofibers.

[0108] As an example, carbon source vapor can be supplied into the quartz tube through argon gas bubbling to activate seeds on the surface of carbon nanotube fibers.

[0109] Although the nanocatalyst is an alkali metal, the alkali metal of the alkali metal precursor applied in the present invention, particularly sodium, is soluble in water, so there is no need to use a separate acid to remove it, and thus a metal-free carbon nanotube-carbon nanofiber composite can be synthesized. In other words, even if some of the nanocatalyst remains in the carbon nanotube, it can be removed by dissolving it in normal water rather than acid treatment due to the high reactivity of the alkali metal cation. In addition, it is characterized by being able to be removed simply by vaporization or evaporation depending on the heat treatment temperature.

[0110] After forming carbon nanotube seeds on the surface of carbon nanofibers, a heated carbon source is re-supplied in an inert gas atmosphere to primarily grow carbon nanotubes on the surface of the carbon nanotube fibers (S4-2).

[0111] By growing carbon nanotubes from carbon nanofibers using a nanocatalyst, high bonding strength is created between the carbon nanofibers and carbon nanotubes, preventing separation of the carbon nanofibers and carbon nanotubes. Therefore, no separate means for bonding the carbon nanofibers and carbon nanotubes are required. As the carbon source is crystallized into carbon nanotubes by the nanocatalyst, they can grow from the surface of the carbon nanofibers.

[0112] Specifically, in order to grow carbon nanotubes on carbon nanofibers, a quartz tube is placed centrally inside an electric heater, and the temperature inside the quartz tube is maintained at 600 to 700°C, and the temperature is increased at 10°C per minute in a hydrogen atmosphere.

[0113] As a carbon source capable of growing and synthesizing carbon nanotubes from the surface of carbon nanofibers, at least one of a liquid-type carbon source, a gas-type carbon source, and a solid-type carbon source is selected and used. The liquid-type carbon source is selected from the group consisting of ethanol (C2H6O), benzene (C6H6), xylene, toluene (C7H8), and mixtures thereof. The gas-type carbon source is selected from the group consisting of methane (CH4), propylene (C3H6), propane (C3H4), propane (C3H8), butane (C4H 10 ), butylene (C4H8), butadiene (C4H6), ethylene (C2H2) and mixtures thereof. As a solid-phase carbon source, camphor (C) which is one of the monoterpene ketones 10 H 160 ) can be used.

[0114] Inert gases can be used, such as helium, nitrogen, argon, and carbon dioxide.

[0115] Preferably, the carbon source can be supplied by vaporizing a liquid carbon source, and in one embodiment, ethanol can be heated to 150°C and the generated ethanol vapor can be supplied into the quartz tube at 1000 to 2400 sccm through argon gas bubbling.

[0116] Next, hydrogen gas is supplied, and the nanocatalyst is further activated through reduction of the alkali metal nanocatalyst according to the supply of the hydrogen gas (S4-3).

[0117] The present invention has a technical feature in that it further includes a nanocatalyst activation step by supplying hydrogen gas during the carbon nanotube growth step, thereby uniformly growing carbon nanotubes and increasing the density of carbon nanotubes on the surface of carbon nanofibers.

[0118] The hydrogen gas supplied in the middle can assist in the reduction of additional alkali metal nanocatalysts, further enhancing their activation and increasing the purity of carbon nanotubes. This can enhance the activation of the limited alkali metal nanocatalysts and increase the concentration of alkali metal precursors in the process from 0.02 mol to 0.3 mol per liter of solvent.

[0119] In one embodiment, hydrogen gas can be supplied at 200 to 500 sccm for at least 30 minutes.

[0120] After this, the carbon source is re-supplied in an inert gas atmosphere to further grow carbon nanotubes (S4-4).

[0121] S4-4 can proceed under the same conditions as S4-2, and carbon nanotubes can be grown from the nanocatalyst additionally activated in S4-3. Accordingly, the carbon nanofibers having a diameter of 1 μm can be grown on the surface of the carbon nanotube-carbon nanofiber composite of the present invention. 2 It is characterized by the number of carbon nanotubes in the above-mentioned sugars being 80 to 120.

[0122] Previously, even when carbon nanofibers and carbon nanotubes were combined, the density of the carbon nanotubes was low. In this case, the carbon nanofibers were 1 μm. 2 The number of carbon nanotubes in the present invention was only 10 to 30. On the other hand, when carbon nanotubes were grown under controlled conditions according to the present invention, the carbon nanofibers were 1 μm in size. 2 The above carbon nanotubes have excellent technological characteristics in that they are 3 to 10 times denser than conventional ones, with 80 to 120 carbon nanotubes per tube.

[0123] In addition, in the carbon nanotube-carbon nanofiber composite, the length of the carbon nanotube is 30 to 120 nm, and preferably 50 to 100 nm. In the case of the prior art, the diameter of the carbon nanofiber is 250 to 2000 nm, the diameter of the carbon nanotube is 50 to 70 nm, and the length of the carbon nanotube is 200 to 500 nm, so there was a problem that the thickness (diameter) of the carbon nanofiber was not uniform and the deviation in the length of the growing carbon nanotube was also large, but when the carbon nanotube is grown under controlled conditions according to the present invention, it is characterized in that the length is formed uniformly.

[0124] Beyond the above-mentioned density and length ranges, not only will electrical conductivity decrease due to insufficient electrical conduction paths, but the composite may also face limitations in various applications due to insufficient carbon nanotube growth. For example, when evaluating performance as a lithium-ion battery anode, performance outside the above ranges did not significantly differ from that of single carbon nanofibers.

[0125] The present invention can manufacture a metal-free carbon nanotube-carbon nanofiber composite using an alkali metal-based nanocatalyst, and since the nanocatalyst can be vaporized and removed, it is not attached or bound to the carbon nanotubes grown on the carbon nanofibers, so there is no need to perform a post-treatment process such as an additional heat treatment or acid treatment to remove the nanocatalyst. In addition, even if some of the nanocatalyst remains in the carbon nanotubes, due to the high reactivity of alkali metal ions to water, it can be removed by dissolving it in normal water rather than acid treatment, so the advantage in the process is maintained.

[0126] Furthermore, the method is characterized by specifying the steps for manufacturing a carbon nanotube-carbon nanofiber composite and controlling the conditions to manufacture a carbon nanotube-carbon nanofiber composite in which the length and density of the carbon nanotubes are controlled and a uniform structure is formed.

[0127]

[0128] In addition, according to one aspect of the present invention, a method for producing a branched fine carbon nanofiber-carbon nanofiber composite is provided, including: a first step of preparing a spinning solution containing an alkali metal precursor and a carbon-containing polymer; a second step of electrospinning the spinning solution to produce carbon-containing polymer nanofibers having the alkali metal precursor bound to the surface; a third step of heat-treating the carbon-containing polymer nanofibers to produce carbon nanofibers having the alkali metal precursor bound to the surface; and a fourth step of supplying a carbon source to the carbon nanofibers and heat-treating them to produce a branched fine carbon nanofiber-carbon nanofiber composite having branched fine carbon nanofibers bound to the surface.

[0129] In addition to a carbon nanotube-carbon nanofiber composite, the present invention can provide a method for manufacturing a branched fine carbon nanofiber-carbon nanofiber composite. The branched fine carbon nanofiber-carbon nanofiber composite has a structure in which branched fine carbon nanofibers, rather than carbon nanotubes, grow from the surface of carbon nanofibers. The branched structure means a structure in which fine fibers grow in multiple directions when a carbon source is supplied to the surface of carbon nanofibers formed through an electrospinning and carbonization process and heat treatment is performed.

[0130] Structures formed in this manner offer higher reactivity, a larger surface area, improved electrical conductivity, and ion diffusion characteristics, making them ideal for energy storage and conversion devices. For example, when used as a cathode material for lithium-ion batteries, they can simultaneously achieve high energy density and excellent rate characteristics. They can also be utilized as composite materials for high-performance electrodes, such as supercapacitors and fuel cell electrodes.

[0131]

[0132] In addition, according to one aspect of the present invention, there is provided a method for producing a carbon nanofiber, comprising: a first step of preparing a spinning solution containing an alkali metal precursor and a carbon-containing polymer; a second step of electrospinning the spinning solution to produce carbon-containing polymer nanofibers having the alkali metal precursor bound to the surface; a third step of heat-treating the carbon-containing polymer nanofibers to produce carbon nanofibers having the alkali metal precursor bound to the surface; and a fourth step of heat-treating the carbon nanofibers while supplying a carbon source to the carbon nanofibers to produce a branched fine carbon nanofiber-carbon nanofiber composite having branched fine carbon nanofibers bound to the surface; wherein the fourth step comprises: a branched fine carbon nanofiber seed forming step of supplying the carbon source to the carbon nanofibers while heating in an inert gas atmosphere so that the alkali metal precursor is activated as an alkali metal nanocatalyst, and the carbon source is bound to the surface of the carbon nanofibers by the nanocatalyst, thereby forming branched fine carbon nanofiber seeds for growing into branched fine carbon nanofibers; A first branched fine carbon nanofiber growth step in which the carbon source is re-supplied to the carbon nanofibers while heating in an inert gas atmosphere after the branched fine carbon nanofiber seed formation step to grow branched fine carbon nanofibers on the surface of the carbon nanofibers; a nanocatalyst activation step in which hydrogen gas is supplied after the first branched fine carbon nanofiber growth step and the nanocatalyst is further activated through reduction of the alkali metal nanocatalyst according to the supply of the hydrogen gas; and a second branched fine carbon nanofiber growth step in which the carbon source is re-supplied in an inert gas atmosphere after the nanocatalyst activation step to further grow branched fine carbon nanofibers; Including, on the surface of the branched fine carbon nanofiber-carbon nanofiber composite, the carbon nanofibers are 1 μm in diameter. 2A method for producing a branched fine carbon nanofiber-carbon nanofiber composite is provided, characterized in that the branched fine carbon nanofiber-carbon nanofiber composite is produced by producing 80 to 120 branched fine carbon nanofibers.

[0133] According to one embodiment of the present invention, a carbon-containing polymer nanofiber containing an alkali metal precursor is converted into carbon nanofibers through electrospinning and heat treatment, and branched fine carbon nanofibers are grown from the surface of the carbon nanofibers, thereby producing a branched fine carbon nanofiber-carbon nanofiber composite.

[0134] The above manufacturing method comprises four steps.

[0135] In the first step, a spinning solution containing alkali metal precursors such as lithium, sodium, and potassium and a carbon-containing polymer is prepared. Polar solvents such as water, dimethylformamide (DMF), and lower alcohols containing 1 to 5 carbon atoms, or nonpolar organic solvents such as xylene, benzene, and toluene, can be used. If necessary, crown ethers can be added to improve the solubility of the alkali metal precursor.

[0136] In the second step, the above-mentioned spinning solution is electrospun to form carbon-containing polymer nanofibers containing an alkali metal precursor. During electrospinning, viscosity, conductivity, surface tension, voltage, and spinning conditions must be appropriately controlled. The viscosity of the spinning solution can be set to 1 to 1,000 Pa·s, the conductivity to 53 μs / cm or less, and the surface tension to 450 dyn / cm or less.

[0137] In the third step, the electrospun nanofibers are initially stabilized, and then carbonization heat treatment is performed in an inert gas atmosphere to produce carbon nanofibers with an alkali metal precursor bonded to the surface. The heat treatment temperature is preferably 800 to 1,200°C, and the heat treatment time is preferably 30 to 90 minutes.

[0138] In the fourth step, a carbon source is supplied to the carbon nanofibers and heat-treated to induce the formation of branched, fine carbon nanofibers on the surface. This step is further subdivided into the following substeps.

[0139] 1) Branched fine carbon nanofiber seed formation stage

[0140] By heating and supplying a carbon source in an inert gas atmosphere, an alkali metal precursor present on the surface of carbon nanofibers is activated into an alkali metal nanocatalyst, and the carbon source is adsorbed and bound based on the nanocatalyst to form a growth nucleus (seed) of branched fine carbon nanofibers.

[0141] 2) First branched fine carbon nanofiber growth stage

[0142] After seed formation, the carbon source is heated and re-supplied in an inert gas atmosphere, and a fibrous carbon structure with a branched structure, i.e., branched micro-carbon nanofibers, grows on the surface of the carbon nanofibers.

[0143] 3) Nanocatalyst activation step

[0144] By supplying hydrogen gas, the reduction and reactivation of alkali metal nanocatalysts are induced. This enhances catalytic activity and enables the growth of more uniform, high-density, branched, fine carbon nanofibers in subsequent steps.

[0145] 4) Second branched fine carbon nanofiber growth stage

[0146] A carbon source is then supplied again to further grow branched, fine carbon nanofibers. This creates a composite structure with high density and cohesive strength.

[0147] The composite manufactured through the above four-step process has a structure in which 80 to 120 branched fine carbon nanofibers are formed per 1 μm² of carbon nanofiber, and provides excellent performance in terms of electrical conductivity, surface area, and mechanical stability.

[0148] In particular, the present invention does not use transition metal-based catalysts such as iron (Fe) and nickel (Ni), which are commonly used in existing technologies, but uses alkali metal-based nanocatalysts that are easily soluble in water, so that post-treatment processes such as acid treatment are unnecessary, and environmental pollution and production costs can be significantly reduced.

[0149] These branched fine carbon nanofiber-carbon nanofiber composites can be utilized as electrode materials for high-performance energy storage and conversion devices such as lithium-ion batteries, fuel cells, and supercapacitors, and can be effectively applied to various fields such as functional filters, electronic devices, and composite material reinforcement due to their porous structure and high reactivity.

[0150] Hereinafter, embodiments of the present invention will be described in more detail. However, the following embodiments are provided merely to aid understanding of the present invention, and the scope of the present invention is not limited thereby.

[0151]

[0152] <Example>

[0153] In this example, various experiments were conducted to determine variables such as catalyst amount, spinning solution concentration, carbon source, gas injection distance, heat treatment time, and gas flow rate while producing nanofibers through electrospinning and using chemical vapor deposition to synthesize carbon nanotubes on the produced carbon nanofibers.

[0154]

[0155] Manufacturing carbon nanofibers using electrospinning

[0156] To manufacture carbon nanofibers, polyacrylonitrile (PAN, Polyacrylonitrile, Sigma-Aldrich) was used as a carbon precursor, and a product with a molecular weight of 150,000 g / mol was used to maintain an appropriate viscosity. Figure 1 illustrates a carbon nanofiber manufacturing step according to one embodiment of the present invention.

[0157] In addition, an alkali metal was used as a catalyst for growing carbon nanotubes from carbon nanofibers. Generally, carbon nanotubes are synthesized using transition metal catalysts. However, these transition metal catalysts act as impurities later, causing various side reactions, which leads to many problems, and therefore have the disadvantage of having to be removed through acid washing, etc. On the other hand, alkali metals are easily soluble in water, so unlike transition metals, a purification process through acid treatment is not necessary, and they have the advantage of being sufficiently removed through low-temperature heat treatment. Therefore, alkali metals were used as catalysts. The catalyst used was lithium benzoate (C7H5LiO2, Sigma-Aldrich), which was added to a molar concentration of 0.3 mol / L in dimethylformamide (DMF, N,N-Dimethylformamide, Sigma-Aldrich) and stirred in a stirrer for more than 1 hour. After that, 9 wt% polymer PAN powder was added and stirred in a stirrer again to complete the spinning solution.

[0158] Furthermore, to produce carbon nanofibers with a smooth surface and no bead formation, the concentration of the spinning solution was varied and confirmed. PAN was dissolved in DMF, a carbon-containing polymer solvent, at concentrations of 7 wt%, 9 wt%, and 11 wt%, respectively, and electrospun. Furthermore, lithium benzoate, a lithium precursor, was dissolved at concentrations of 0.03 mol / L and 0.3 mol / L to confirm the amounts required for high yield growth of carbon nanotubes.

[0159] The prepared spinning solution was fabricated into nanofibers using an electrospinning system (Nano NC). The distance between the nozzle and the collecting plate was 20 cm, and a voltage of 12 kV was applied between them to obtain nanofibers.

[0160] The stabilization and carbonization process of the nanofibers after radiation was carried out in a quartz tube with a diameter of 50 mm and a length of 1000 mm, and was carried out using an infrared heat treatment furnace. Fig. 2 is a graph showing the conditions of the carbon nanofiber stabilization and carbonization step according to an embodiment of the present invention. Referring to Fig. 2, the stabilization process of the nanofibers was first carried out at 200 ° C. for 40 minutes in an air atmosphere, and then the nanofibers were carbonized by heating at 1000 ° C. for 1 hour at a heating rate of 5 ° C. / min in a nitrogen atmosphere. Thereafter, the nanofibers were cooled to room temperature in a nitrogen atmosphere.

[0161]

[0162] Manufacturing of carbon nanofiber-carbon nanotube composites

[0163] Chemical vapor deposition (CVD) was used to grow carbon nanotubes from carbon nanofibers. An alumina boat was placed centrally within the quartz tube used for the carbonization process, and the carbonized carbon nanofibers were placed on top. Ethanol (C2H6O, Duksan) was used as the carbon precursor required for carbon nanotube growth. Various liquid carbon sources were used to identify the one with the best yield and quality of carbon nanotubes. During the process, the distance between the gas inlet and the carbon nanofibers within the quartz tube was adjusted to find the optimal experimental conditions.

[0164] To grow carbon nanotubes on carbon nanofibers, a quartz tube was placed centrally inside an electric heater, and the temperature inside the quartz tube was maintained at 650°C and increased at 10°C per minute (hydrogen atmosphere). After that, ethanol was heated to 150°C, and the generated ethanol vapor was supplied into the quartz tube through argon gas bubbling at 2,400 sccm for 30 minutes to form seeds on the surface of the carbon nanofibers. After removing the heating tape, ethanol vapor was supplied at 2,400 sccm for 30 minutes through argon gas bubbling. After that, the hydrogen gas line was changed, and hydrogen gas was supplied at 300 sccm for 30 minutes, and then ethanol vapor was supplied again at 2,400 sccm for 30 minutes through argon gas bubbling.

[0165] Specifically, ethanol vapor was supplied at 2400 sccm into the quartz tube through argon gas bubbling at 650°C. For the first 30 minutes, the quartz tube, through which the ethanol vapor was flowing, was heated to 400°C using a heating tape, thereby increasing the activation of the ethanol as it reached the carbon nanofibers inside the quartz tube.

[0166] After approximately 30 minutes of ethanol vapor supply, hydrogen gas was injected at 300 sccm for 30 minutes. The high energy of the heated ethanol and the hydrogen gas supplied in the middle assisted in the additional reduction of the alkali metal nanocatalyst, further promoting the activation of the catalyst and increasing the purity of the carbon nanotubes. This enabled the activation of the limited alkali metal nanocatalyst to be increased and the concentration of the alkali metal precursor to be increased from 0.02 mol to 0.3 mol per liter of solvent.

[0167] At this time, the exact amount of gas was supplied by controlling it with a mass flow controller (MFC). Argon gas was injected into a triangular flask through the MFC, and ethanol heated to 150℃ was bubbled inside the triangular flask and supplied into the quartz tube. At this time, the amount and length of carbon nanotubes were controlled according to the amount and time of argon gas supplied. After the heat treatment was completed, only argon gas was injected to cool the carbon nanotubes to room temperature so that they would not burn.

[0168] Furthermore, the growth rate of carbon nanotubes was confirmed by conducting experiments with heat treatment times of 15, 30, and 60 minutes to grow carbon nanotubes. The gas injected here was argon gas, which was bubbled as a carbon source and then injected into the quartz tube. The same experiment was conducted at argon gas flow rates of 400 sccm, 1,000 sccm, and 2,400 sccm, and the growth trend of each carbon nanotube was confirmed.

[0169]

[0170] lithium-ion battery manufacturing

[0171] The fabricated carbon nanotube-carbon nanofiber composite was punched into 14 mm-diameter circular shapes for cell assembly. Unlike typical lithium-ion batteries, no binder, current collector, or active material was used, and the fabricated sample weighed approximately 2 mg. The lithium-ion battery was assembled into a CR2030 type coin cell (Wellcose Corp.).

[0172] The cell consists of a case, spacer disc, spring, gasket, and cap, and uses lithium metal as the counter electrode. All assembly processes were performed in an argon-filled glove box.

[0173] Polypropylene (Celgard 2400) was used as a separator, and a solvent containing 1 M lithium hexafluorophosphate (LiPF6) dissolved in ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1 was used as an electrolyte.

[0174]

[0175] <Experimental Example>: Analysis of the Carbon Nanofiber-Carbon Nanotube Structure

[0176] (1) The synthesized carbon nanofibers-carbon nanotubes were analyzed for their shape and microstructure using a field emission transmission electron microscope (FE-TEM, JEOL, JEM-2100F). FE-TEM irradiates an electron beam onto a sample, and the electron beam collides with the atoms constituting the interior of the sample, and the electrons that pass through the sample are used to obtain an image of the sample. The sample was prepared by adding a small amount of ethanol to disperse it ultrasonically, placing it on a carbon grid, and drying it. The analysis was performed under an acceleration voltage set to 200 kV.

[0177] (2) The presence and distribution of lithium ions on the surface of carbon nanofibers and carbon nanotubes were confirmed using a time-of-flight secondary ion mass spectrometer (ToF-SIMS). The ToF-SIMS model 5 from ION-ToF GmbH was used. A pulsed primary ion beam was used to ionize the material on the sample surface, and the secondary ions on the ionized sample surface were accelerated at a constant acceleration voltage. The mass spectrum was analyzed by measuring the time it took for the ions to fly from the ion source to the detector.

[0178] (3) The chemical composition of the sample was determined using X-ray photoelectron spectroscopy (XPS). When the sample is exposed to X-rays with a certain energy, photoelectrons are emitted from the sample. By measuring the kinetic energy of these photoelectrons, the binding energy can be determined. The chemical bonding state and the quality of the elements were analyzed by measuring this binding energy, which is an inherent property of atoms. Thermofisher's ESCALAB 250 was used.

[0179] (4) Nitrogen adsorption / desorption isotherms were measured using the BELSORP-max device from MicrotracBEL. The specific surface area of ​​the sample is measured by the volumetric adsorption method. Pore size measurement is also possible, and chemical adsorption of compounds is measured. Pore size measurement of micropores (less than 2 nm), mesopores (2-50 nm), and macropores is possible. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method. Pore size was calculated using the Barrett-Joyner-Halena (BJH) method and the micropore analysis (MP) method.

[0180] (5) The degree of graphite crystallinity according to the carbon nanotube growth density of pure carbon nanofibers and carbon nanofiber-carbon nanotubes was confirmed using a Raman spectrometer (micro Raman spectrometer), and the NRS-5100 equipment from JASCO was used. By detecting the wavelength change and intensity of light scattered by the laser with a CCD detector, the vibrational energy structure of molecules was obtained as a spectrum, and the bonding state, structure, and characteristics between molecules were confirmed.

[0181] (6) To measure the electrical conductivity and resistance of pure carbon nanofibers and carbon nanofiber-carbon nanotubes, a surface resistance measuring device (four-point probe) was used. The 4PX-P200 device from WIT was used. By supplying a constant current and measuring the voltage, the resistance characteristics were analyzed accurately and quickly without destroying the sample.

[0182]

[0183] <Evaluation and Results>

[0184] (1) Setting of electrospinning and chemical vapor deposition conditions

[0185] Carbon-containing polymer concentration

[0186] In order to find conditions under which beads do not form in the fibers when producing nanofibers through electrospinning, polyacrylonitrile, a carbon-containing polymer, was added to the solvent DMF at concentrations of 7 wt%, 9 wt%, and 11 wt%, respectively, and compared.

[0187] Figure 3 is a SEM photograph of carbon nanofibers manufactured according to an embodiment of the present invention according to the concentration of the carbon-containing polymer. Referring to Figure 3, at a concentration of 7 wt%, the viscosity of the spinning solution was low, so the voltage had to be increased during electrospinning, resulting in the formation of beads. Conversely, at a concentration of 11 wt%, the viscosity was high, so spinning did not proceed well and beads were formed. However, at a concentration of 9 wt%, spinning proceeded well at a voltage of 11 kV with an appropriate viscosity, and nanofibers were produced without forming beads.

[0188]

[0189] Catalyst precursor concentration

[0190] To investigate the effect of catalyst precursor concentration on carbon nanotube growth, the amount of lithium benzoate, a catalyst precursor contained in the spinning solution, was varied and compared. Carbon nanofibers were produced by adding 0.03 mol / L and 0.3 mol / L of lithium benzoate to the spinning solution solvent, DMF.

[0191] Figure 4 is a SEM photograph of a carbon nanofiber-carbon nanotube composite manufactured according to an embodiment of the present invention at different catalyst precursor concentrations. Referring to Figure 4, the density of carbon nanotubes according to the catalyst concentration is shown. At 0.03 mol / L, carbon nanotubes can be seen growing sparsely on the carbon nanofibers, whereas at 0.3 mol / L, carbon nanotubes can be seen growing at a high density. This shows that the higher the amount of catalyst, the higher the density of carbon nanotubes.

[0192]

[0193] Carbon source selection

[0194] In order to select a carbon source suitable for carbon nanotube growth, experiments were conducted using various carbon sources. Figure 5 shows SEM images of carbon nanofiber-carbon nanotube composites manufactured according to one embodiment of the present invention, according to different carbon sources. Liquid carbon sources were used as the carbon source, and referring to Figure 5, first, in the experiment using acetone (C3H6O, Duksan), it was confirmed that carbon nanotubes hardly grew from the carbon nanofibers. Next, in the experiment using ethanol (C2H6O, Duksan), it was confirmed that high-density carbon nanotubes grew. This is believed to be because the OH radicals contained in ethanol and propanol activated the carbon fiber surface, allowing the carbon source to easily diffuse and dissolve into the catalyst present on the carbon fiber surface, thereby helping the tube growth.

[0195] To confirm this, additional experiments were conducted using toluene (C7H8, Sigma-Aldrich) and propanol (propyl alcohol, C3H8O Duksan), and it was confirmed that small carbon nanotubes of 10 nm were grown only in propanol. This confirmed that OH radicals affect the growth of carbon nanotubes. As a result, it was confirmed that even for carbon sources containing the same OH functional group, the higher the ratio of hydrocarbons excluding OH groups, the weaker the frequency of occurrence of OH groups, which can affect the growth of carbon nanotubes.

[0196]

[0197] Distance between gas inlet and carbon nanofiber

[0198] In chemical vapor deposition, the distance between the gas inlet and carbon nanofibers located inside a quartz tube was adjusted to observe differences in carbon nanotube growth density. First, heat treatment was performed with the carbon nanofibers and the gas inlet as close as possible, at a distance of 10 cm. Next, the carbon nanofibers were positioned in the middle of the quartz tube, approximately 30 cm from the gas inlet, for the experiment.

[0199] FIG. 6 is an SEM photograph of a carbon nanofiber-carbon nanotube composite manufactured by varying the distance between the gas inlet and the carbon nanotube fiber according to one embodiment of the present invention. Referring to FIG. 6, the shorter the distance between the gas inlet and the carbon nanofiber, the higher the density of the grown carbon nanotube tends to be. In other words, it was confirmed that the closer the distance between the argon gas inlet and the carbon nanofiber, the higher the density and longer the length of the carbon nanotube.

[0200]

[0201] Argon gas injection time and flow rate

[0202] This study aimed to determine the extent of carbon nanotube growth according to argon gas injection time. The injection time was 15, 30, and 60 minutes to determine trends. Argon gas was injected through ethanol, the carbon source.

[0203] Figure 7 is a SEM image of a carbon nanofiber-carbon nanotube composite manufactured by varying gas injection times according to one embodiment of the present invention. Referring to Figure 7, it can be seen that the length and density of the carbon nanotubes increase as the reaction time increases. It can be confirmed that the carbon nanotubes hardly grow at all with a reaction time of 15 minutes, indicating that 30 minutes are required for carbon nanotube growth.

[0204] Additionally, the flow rate of argon gas introduced through ethanol, a carbon source, was controlled to confirm the trend. As the argon gas flow rate increased, the amount of ethanol injected increased, with injections of 400 sccm, 1,000 sccm, and 2,400 sccm, respectively.

[0205] FIG. 8 is an SEM photograph of a carbon nanofiber-carbon nanotube composite manufactured by varying gas flow rates according to an embodiment of the present invention. Referring to FIG. 8, carbon nanotubes hardly grew at a flow rate of 400 sccm, and a high growth rate was observed at 1,000 sccm. However, when a flow rate of 2,400 sccm was introduced, carbon with a length of 200 nm or more grew in an amorphous form. This shows that excessive introduction of argon gas results in a longer carbon nanotube length but lowers its quality, and that carbon nanotubes grow to an appropriate length at 1,000 sccm.

[0206] That is, it was confirmed that the growth length of carbon nanotubes tended to increase as the argon gas flow rate and time increased.

[0207]

[0208] (2) Analysis of carbon nanofiber-carbon nanotube microstructure and catalyst

[0209] Surface morphology and microstructure

[0210] FIG. 9 is a photograph of the shape and microstructure of a carbon nanofiber-carbon nanotube composite manufactured according to an embodiment of the present invention. Referring to FIG. 9, the SEM analysis results confirmed that carbon nanotubes with a thickness of about 10 to 20 nm were grown from carbon nanofibers with a thickness of about 150 to 200 nm. In addition, HR-TEM analysis was performed to confirm the microcrystalline structure of the carbon nanofibers and carbon nanotubes, and the results confirmed that the carbon nanotubes synthesized according to an embodiment of the present invention grew to a length of about 50 to 100 nm.

[0211] Next, the specific surface area and pore size of the carbon nanofiber-carbon nanotube composite were evaluated. Fig. 10 is a nitrogen adsorption / desorption isotherm of a carbon nanofiber-carbon nanotube composite manufactured according to an embodiment of the present invention. Examining the adsorption / desorption isotherm of Fig. 10, it can be seen that as the density of the carbon nanotubes increases, the adsorption amount at low pressures of a relative pressure of 0.1 or less decreases. The adsorption amount at low pressure is due to the adsorption amount in micropores with a size of 2 nm or less. This graph trend is believed to be because the proportion of micropores on the surface of the carbon nanofibers gradually disappears, which is due to the growth of carbon nanotubes in these micropores. This can also be confirmed from the pore distribution curve of Fig. 11. That is, it can be seen that the contribution of micropores with a size of 2 nm or less to the total adsorption amount decreases as the carbon nanotubes grow.

[0212] Furthermore, as carbon nanotubes grow, differences in the adsorption / desorption curves occur, resulting in a hysteresis region. This hysteresis phenomenon indicates the presence of mesopores ranging in size from 2 nm to 50 nm. It is thought that as carbon nanotubes grow, the spaces between the tubes create these mesopores.

[0213] Meanwhile, the specific surface area, micropore volume, and average pore size of each material are shown in Table 1.

[0214] SamplesS BET (m 2 / g)V mic (cm 3 / g)D ap (nm)Carbon nanofiber979.70.44112.2438Carbon nanofiber-carbon nanotube low density739.40.32682.3934Carbon nanofiber-carbon nanotube high density448.00.16272.8967Carbon nanofiber-carbon nanotube ultra high density851.740.442.7948

[0215] Referring to Table 1, as the density of carbon nanotubes increased, the micropore volume and average pore size decreased, and the specific surface area also decreased accordingly. In other words, it can be seen that micropores were created in the carbon nanofibers by solvent evaporation during the electrospinning process to produce nanofibers, and carbon nanotubes grew from the micropores. However, when the density of carbon nanotubes finally reached its highest, the specific surface area increased again. This indicates that as the carbon nanotubes grew, the pores were blocked, reducing the specific surface area, and then the specific surface area increased again due to the further growth of the carbon nanotubes.

[0216] In the present invention, the volume range of micropores is 0 to 0.5 cm 3 / g, and the specific surface area is 300 to 900 m 2 / g is preferable. If the micropore volume is outside the above range, it means that the carbon nanotubes did not grow well from the carbon nanofibers, which can be confirmed by the decrease in the density of the carbon nanotubes.

[0217]

[0218] Chemical composition

[0219] ToF-SIMS analysis was performed to confirm the distribution and presence of lithium, which was used as a growth catalyst for carbon nanotubes. Fig. 12 shows the results of ToF-SIMS analysis of a carbon nanofiber-carbon nanotube composite manufactured according to an embodiment of the present invention. Fig. 12(a) shows the nanofibers before carbonization, and (b) shows the carbon nanofibers after carbonization. It was confirmed that lithium was present in both and was evenly distributed throughout. This indicates that Li was used as a catalyst in the synthesis of carbon nanotubes.

[0220] Next, XPS analysis was performed to confirm the presence and activation of the lithium catalyst in the carbon nanofiber-carbon nanotube composite. Fig. 13 shows the XPS analysis results of the carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention. Referring to Fig. 13, lithium spectra were found in both the carbon nanofibers and the carbon nanofiber-carbon nanotubes, and the spectra shown in Fig. 13 mainly correspond to lithium metal. It can be seen that the peak intensity becomes stronger as the carbon nanotubes grow. This shows that the lithium metal catalyst is activated as the heat treatment time and the supply of ethanol, which is a carbon source, increase during the carbon nanotube growth.

[0221] Figure 14 is a carbon bond peak of a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention. Referring to Figure 14, it can be observed that the peak intensity of the C-C bond (285.2 eV) decreases as the carbon nanotubes grow. Compared to pure carbon nanofibers, the C-C peak of the sample with a high carbon nanotube density decreased by 43.7%. sp 3 This peak, indicating bonding, is a peak that mainly appears in carbon nanofibers. This indicates that the peak decreases as the carbon nanotubes grow.

[0222] That is, it was confirmed that carbon nanotubes were grown in alkaline metals.

[0223]

[0224] Electrical conductivity

[0225] The electrical conductivity and surface resistance of carbon nanofibers and carbon nanofiber-carbon nanotubes were measured using a surface resistance meter, and the results are shown in Table 2. The applied current for the measurement was 100 mA.

[0226] SamplesResistance (Ohm)Sheet resistance (Ohm / square)Conductivity(S cm -1 )Carbon nanofiber11.948.32.07Carbon nanofiber-carbon nanotube low density11.345.92.17Carbon nanofiber-carbon nanotube high density4.920.24.95Carbon nanofiber-carbon nanotube ultra high density2.711.98.40

[0227] Referring to Table 2, it can be seen that the electrical conductivity of the composite material in which carbon nanotubes are grown on carbon nanofiber-carbon nanotube is higher than that of pure carbon nanofibers, and it was confirmed that the higher the density of carbon nanotubes, the higher the electrical conductivity. This is because the growth of carbon nanotubes in the one-dimensional matrix of carbon nanofibers forms a network, which increases the total contact area and provides a short travel distance for ions and electrons. In other words, it was confirmed that the electrical conductivity increases as carbon nanotubes grow from pure carbon nanofibers.

[0228]

[0229] Structural analysis

[0230] Raman spectroscopy (micro Raman spectrometer) was performed to determine the degree of crystallinity and graphitization of carbon nanofibers and carbon nanofiber-carbon nanotubes. The Raman spectrum of carbon shows two peaks corresponding to the D band and the G band, and is generally located at 1350 and 1580 cm -1 is located in

[0231] Figure 15 is a Raman spectrum of a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention. The Raman spectra were measured using Raman spectroscopy for carbon nanofibers (a), carbon nanofiber-carbon nanotubes with a low carbon nanotube density (b), and carbon nanofiber-carbon nanotubes with a high carbon nanotube density (c).

[0232] Referring to Figure 15, D-band and G-band peaks were observed in all three materials, confirming that the graphite crystallinity increased with the growth of carbon nanotubes. The D-band is mainly due to the disorder and imperfection of carbon crystals in general, such as nano-graphene layers, defects, voids within the CNT walls, and heteroatoms. The G-band is related to the graphitic carbon structure. I D / I G The band ratio is a key factor in evaluating the disorder and defects in the structure of carbon materials, and I D / I G A higher ratio indicates a higher degree of disorder and more defects. For single-walled carbon nanotubes (SWCNTs), I D / I G The ratio is low. The higher the density of carbon nanotubes, the higher the carbon nanofiber-carbon nanotube ratio. D / I G The ratio is small and pure carbon nanofibers are the largest. This is because the overall degree of crystallization increases as the density of carbon nanotubes increases.

[0233]

[0234] Electrochemical performance

[0235] FIG. 16 is a first cycle charge / discharge curve of a lithium ion battery negative electrode material including a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0236] Referring to Figure 16, in relation to the density of carbon nanotubes, the carbon nanofibers 1 μm on the surface of the carbon nanotube-carbon nanofiber composite 2When the number of carbon nanotubes in the above-mentioned composite is 80 to 120, the initial Coulombic efficiency reaches approximately 60.6%, which is approximately 5% better than the initial Coulombic efficiency of carbon nanofibers (approximately 55.5%). This suggests that the process of manufacturing the carbon nanofiber-carbon nanotube composite promoted the activation of lithium, resulting in an increase in the initial efficiency.

[0237] The difference can also be seen in the first cycle charge capacity. At a current density of 0.1C, the charge capacity of the carbon nanofiber-carbon nanotube composite was approximately 1268.4 mAh / g, which is approximately 49% higher than the charge capacity of the carbon nanofiber alone (approximately 852.1 mAh / g).

[0238] FIG. 17 shows the results of an ultra-high-speed performance evaluation of a lithium ion battery negative electrode material including a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0239] Referring to Fig. 17, in order to confirm the stability of the carbon nanotube-carbon nanofiber composite and the carbon nanofiber, the current density was gradually increased from 1 C to 100 C and then decreased to 1 C again. The carbon nanotube-carbon nanofiber composite showed discharge capacities of approximately 341, 309, 266, 226, 194, 163, 140, 126, 111, and 94 mAh / g at current densities of 1, 2, 5, 10, 20, 30, 40, 50, 70, and 100 C, respectively.

[0240] Meanwhile, conventional carbon nanofibers showed discharge capacities of approximately 278, 223, 159, 107, 34, 5, 0.1, 0.1, 0.1, 0.1, and 0.1 mAh / g. Stability was confirmed by recovering the original discharge capacity when the current density was increased to 100 C, which is a very high current density, and then decreased to 1 C.

[0241] As the charge / discharge rate increases, the carbon nanofibers' fading rate increases even more rapidly than that of the carbon nanotube-carbon nanofiber composite. This suggests that the carbon nanotubes grown on the carbon nanofibers form a network of electrically conductive pathways, facilitating the flow of ions and electrons, thus positively impacting the rate performance.

[0242] FIG. 18 is a graph of discharge capacity at a charge / discharge rate of 2 C of a lithium ion battery negative electrode material including a carbon nanofiber-carbon nanotube composite manufactured according to one embodiment of the present invention.

[0243] Referring to Figure 18, the discharge capacity initially exhibited 260 mAh / g, but continued to increase as cycling progressed. At approximately the 800th cycle, the discharge capacity increased to approximately 407 mA / g, demonstrating excellent life characteristics. Furthermore, the ultra-high-speed performance evaluation confirmed that the capacity was similar to that at a 2 C rate.

[0244]

[0245] Thus, the present invention can easily grow carbon nanotubes from the surface of carbon nanofibers by simply manufacturing carbon nanofibers having an alkali metal precursor bonded to the surface through electrospinning, carbonizing the carbon nanofibers, and then supplying a carbon source and performing heat treatment, thereby enabling mass production of carbon nanotube-carbon nanofiber composites. Furthermore, the present invention is characterized in that it specifies the steps for manufacturing a carbon nanotube-carbon nanofiber composite and controls the conditions to manufacture a carbon nanotube-carbon nanofiber composite in which the length and density of the carbon nanotubes are controlled and a uniform structure is formed.

[0246] In addition, since the carbon nanotube-carbon nanofiber composite of the present invention is manufactured using an alkali metal-based catalyst rather than a transition metal-based catalyst, the catalyst particles dissolve in water and can be easily removed, so that after the synthesis of the metal-free carbon nanotube-carbon nanofiber composite is completed, there is no need to go through a cleaning process such as acid treatment, thereby reducing environmental costs. In addition, the present invention is excellent in that it provides a carbon nanotube-carbon nanofiber composite with excellent durability because carbon nanotubes are grown from carbon nanofibers using a nanocatalyst, and the carbon nanofibers and carbon nanotubes have high bonding strength so that the carbon nanofibers and carbon nanotubes do not separate.

[0247]

[0248] The above description is merely an illustrative description of the technical idea of ​​the present invention, and those skilled in the art will appreciate that various modifications and variations can be made without departing from the essential characteristics of the present invention. Therefore, the embodiments disclosed in the present invention are not intended to limit the technical idea of ​​the present invention, but rather to illustrate it, and the scope of the technical idea of ​​the present invention is not limited by these embodiments. The scope of protection of the present invention should be interpreted by the claims, and all technical ideas within a scope equivalent thereto should be interpreted as being included in the scope of the rights of the present invention.

Claims

1. Step 1: preparing a radiation solution containing an alkali metal precursor and a carbon-containing polymer; A second step of producing carbon-containing polymer nanofibers having the alkali metal precursor bonded to the surface by electrospinning the above-mentioned spinning solution; A third step of manufacturing carbon nanofibers having the alkali metal precursor bonded to the surface by heat-treating the carbon-containing polymer nanofibers; and A fourth step of manufacturing a carbon nanotube-carbon nanofiber composite having carbon nanotubes bonded to the surface by heat-treating the carbon nanofibers while supplying a carbon source; including, The fourth step above is, A carbon nanotube seed formation step in which the carbon source is supplied to the carbon nanofibers while heating in an inert gas atmosphere so that the alkali metal precursor is activated into an alkali metal nanocatalyst, and the carbon source is bound to the surface of the carbon nanofibers by the nanocatalyst, thereby forming a carbon nanotube seed for growing into a carbon nanotube; A first carbon nanotube growth step of growing carbon nanotubes on the surface of the carbon nanotube fiber by re-supplying the carbon source to the carbon nanofiber while heating it in an inert gas atmosphere after the carbon nanotube seed formation step; A nanocatalyst activation step in which hydrogen gas is supplied after the first carbon nanotube growth step and the nanocatalyst is further activated through reduction of the alkali metal nanocatalyst according to the supply of the hydrogen gas; and Including a second carbon nanotube growth step in which the carbon source is re-supplied in an inert gas atmosphere after the nanocatalyst activation step to additionally grow carbon nanotubes; 1 μm of the carbon nanofiber on the surface of the carbon nanotube-carbon nanofiber composite 2 A method for producing a carbon nanotube-carbon nanofiber composite, characterized in that the carbon nanotube-carbon nanofiber composite has 80 to 120 carbon nanotubes.

2. In paragraph 1, In the above 4th step, A method for producing a carbon nanotube-carbon nanofiber composite, characterized in that the carbon source is vaporized from a liquid carbon source, supplied together with the inert gas, and then heat-treated at 600 to 700°C.

3. In paragraph 1, A method for producing a carbon nanotube-carbon nanofiber composite, characterized in that the length of the carbon nanotube in the carbon nanotube-carbon nanofiber composite is 30 to 120 nm.

4. In paragraph 1, The above alkali metal precursor is, A method for producing a carbon nanotube-carbon nanofiber composite, characterized in that the composite is selected from the group consisting of a lithium precursor (Li precursor), a sodium precursor (Na precursor), a potassium precursor (K precursor), and a mixture thereof.

5. In paragraph 1, The above carbon-containing polymer is, A method for producing a carbon nanotube-carbon nanofiber composite, characterized in that the composite is selected from the group consisting of polyacrylonitrile (PAN), polyvinyl acetate (PVA), polyvinyl pyrrolidone (PVP), polycarbonate (PC), polyvinylchloride (PVC), cellulose, cellulose acetate, and mixtures thereof.

6. A carbon nanotube-carbon nanofiber composite characterized by being manufactured according to any one of claims 1 to 5.

7. Step 1: Preparing a radiation solution containing an alkali metal precursor and a carbon-containing polymer; A second step of producing carbon-containing polymer nanofibers having the alkali metal precursor bonded to the surface by electrospinning the above-mentioned spinning solution; A third step of manufacturing carbon nanofibers having the alkali metal precursor bonded to the surface by heat-treating the carbon-containing polymer nanofibers; and A method for producing a carbon nanotube-carbon nanofiber composite, comprising a fourth step of producing a carbon nanotube-carbon nanofiber composite having carbon nanotubes bonded to the surface by heat-treating the carbon nanofibers while supplying a carbon source.

8. Step 1: Preparing a radiation solution containing an alkali metal precursor and a carbon-containing polymer; A second step of producing carbon-containing polymer nanofibers having the alkali metal precursor bonded to the surface by electrospinning the above-mentioned spinning solution; A third step of manufacturing carbon nanofibers having the alkali metal precursor bonded to the surface by heat-treating the carbon-containing polymer nanofibers; and A method for producing a branched fine carbon nanofiber-carbon nanofiber composite, comprising a fourth step of producing a branched fine carbon nanofiber-carbon nanofiber composite in which branched fine carbon nanofibers are bonded to the surface by heat-treating the carbon nanofibers while supplying a carbon source.

9. Step 1: Preparing a radiation solution containing an alkali metal precursor and a carbon-containing polymer; A second step of producing carbon-containing polymer nanofibers having the alkali metal precursor bonded to the surface by electrospinning the above-mentioned spinning solution; A third step of manufacturing carbon nanofibers having the alkali metal precursor bonded to the surface by heat-treating the carbon-containing polymer nanofibers; and A fourth step of manufacturing a branched fine carbon nanofiber-carbon nanofiber composite in which branched fine carbon nanofibers are bonded to the surface by heat-treating the carbon nanofibers while supplying a carbon source; including, The fourth step above is, A branched fine carbon nanofiber seed formation step in which the carbon source is supplied to the carbon nanofibers while heating in an inert gas atmosphere so that the alkali metal precursor is activated as an alkali metal nanocatalyst, and the carbon source is bound to the surface of the carbon nanofibers by the nanocatalyst, thereby forming a branched fine carbon nanofiber seed for growing into branched fine carbon nanofibers; A first branched fine carbon nanofiber growth step in which the carbon source is re-supplied to the carbon nanofiber while heating in an inert gas atmosphere after the branched fine carbon nanofiber seed formation step to grow branched fine carbon nanofibers on the surface of the carbon nanofiber; A nanocatalyst activation step in which hydrogen gas is supplied after the first branched fine carbon nanofiber growth step, and the nanocatalyst is further activated through reduction of the alkali metal nanocatalyst according to the hydrogen gas supply; and Including a second branched fine carbon nanofiber growth step in which the carbon source is re-supplied in an inert gas atmosphere after the nanocatalyst activation step to additionally grow branched fine carbon nanofibers; On the surface of the above branched fine carbon nanofiber-carbon nanofiber composite, the carbon nanofiber is 1 μm in diameter. 2 A method for producing a branched fine carbon nanofiber-carbon nanofiber composite, characterized in that the branched fine carbon nanofiber-carbon nanofiber composite comprises 80 to 120 branched fine carbon nanofibers.

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