Method for manufacturing silicon composite comprising nitrogen-doped carbon nanotubes and reduced graphene oxide, silicon composite manufactured thereby, and anode active material comprising same

Abstract

The present invention relates to a method for manufacturing a silicon composite including nitrogen-doped carbon nanotubes and reduced graphene oxide, a silicon composite manufactured thereby, and an anode active material including same, wherein the manufacture is performed by the method comprising the steps of: preparing a graphene oxide dispersion solution and a pre-treated carbon nanotube dispersion solution separately; preparing a mixed solution by mixing silicon-based particles in a solvent and adding the graphene oxide dispersion solution and the pre-treated carbon nanotube dispersion solution thereto; preparing composite particles in which the pre-treated nanotubes are combined with the graphene oxide by surrounding and coating the silicon-based particles with the graphene oxide while spray-drying the mixed solution so that the solvent evaporates; and preparing a silicon composite in which the nitrogen-doped carbon nanotubes are combined with the reduced graphene oxide (rGO) by reducing the composite particles with a nitrogen-containing reducing agent. By the method for manufacturing a silicon composite of the present invention, a silicon composite is manufactured by combining nitrogen-doped carbon nanotubes with reduced graphene oxide (rGO) through chemical reduction at a relatively low temperature in a solution phase. The silicon composite manufactured accordingly may increase the rate of diffusion of ions because single-walled carbon nanotubes doped with nitrogen and having excellent conductivity are complexed with graphene, and can be used as an anode active material having excellent charging / discharging speeds, capacity retention rates, and electrode stability.

Description

Method for manufacturing a silicon composite comprising nitrogen-doped carbon nanotubes and reduced graphene oxide, a silicon composite manufactured thereby, and a negative electrode active material comprising the same

[0001] The present invention relates to a method for producing a silicon composite including nitrogen-doped carbon nanotubes and reduced graphene oxide, a silicon composite produced thereby, and a negative electrode active material including the same.

[0002] With the recent surge in demand for miniaturized and lightweight electronic devices and ultra-large-scale power storage systems, global interest in new energy sources is growing. Research and development is particularly focused on secondary batteries, which are environmentally friendly, offer high energy density, and enable rapid charging and discharging.

[0003] For example, lithium secondary batteries are increasingly important not only as power sources for portable electronic devices, but also as power sources for hybrid vehicles, plug-in hybrid vehicles, electric vehicles, and non-IT applications such as industrial tools and robots. Furthermore, with the widespread adoption of renewable energy sources like solar and wind power, their use as large-capacity energy storage devices is expected to expand to improve power quality due to the intermittent nature of these sources.

[0004] The performance of lithium secondary batteries is crucial to their application, and there is a continuously increasing demand for the development of lithium secondary batteries with improved storage capacity (charge capacity per unit weight or unit volume of active material), output (discharge rate per unit weight or unit volume of active material), stability, and lifespan (the number of repeatable charge / discharge cycles while maintaining storage capacity) characteristics compared to existing lithium secondary batteries. These characteristics are primarily determined by the properties of the anode active material used as the anode material.

[0005] Graphite is currently used as an anode material for lithium secondary batteries, but its maximum theoretical storage capacity is approximately 372 mAh / g. Recently, this theoretical capacity has been almost fully realized in commercial lithium secondary batteries. Therefore, in order to increase the energy storage capacity of lithium secondary batteries, an anode material with a higher storage capacity than graphite must be developed.

[0006] Among the candidates for negative electrode materials for lithium secondary batteries that can replace graphite, metals that electrochemically react with lithium to form alloys (Si, Sn, As, Ge, Bi, Al, In, Pb, and Ga) are attracting attention as high-capacity materials.

[0007] Among them, silicon is an abundant resource with a high theoretical capacity (~4,200 mAh / g) and a low charge potential (~0.2 V vs Li / Li+), so much research is being conducted to develop it as a high-capacity anode material for next-generation lithium secondary batteries.

[0008] However, due to the large volume change (>300%) that occurs when lithium ions are charged and discharged, polarization and cracks occur in the negative electrode, and the silicon surface created at this time reacts with the electrolyte to continuously create a new solid electrolyte interface (SEI), which increases internal resistance, resulting in low efficiency and a decrease in capacity with repeated cycling, which shortens the lifespan. In addition, silicon has very low electrical conductivity, so a method of manufacturing it in the form of a composite with a conductive material is required.

[0009] To this end, in 'Method for manufacturing a crumpled silicon-carbon nanotube-graphene composite, a composite manufactured thereby, and a secondary battery including the composite (Registration No.: 10-1813893)', a method for manufacturing a crumpled silicon-carbon nanotube-graphene composite by manufacturing a mixture including silicon, carbon nanotubes, graphene oxide, and a solvent, spray-drying the mixture, and then heat-treating the mixture is presented.

[0010] However, since reduction of carbon nanotubes and graphene oxide is achieved only by heat treatment at a temperature of 500 to 1,000°C in an atmosphere of one or more gases selected from the group consisting of argon, helium, and nitrogen, there is a problem in that a high temperature of at least 500°C or higher is required and a separate inert gas atmosphere must be provided.

[0011] Accordingly, the inventors of the present invention formed a stable solid electrolyte interphase (SEI) layer by forming a layer of lithiophilic carbon nanotubes having a high affinity for lithium ions on the surface of silicon. At this time, by using nitrogen-doped carbon nanotubes manufactured in a solution under relatively low temperature conditions, not only electrical conductivity but also ion diffusion is improved, and a method for manufacturing a silicon composite including nitrogen-doped carbon nanotubes and reduced graphene oxide, which can be used as an anode active material for secondary batteries, and a silicon composite manufactured thereby were developed, and the present invention was completed.

[0012] The present invention was invented to solve the above problems, and the technical task of the present invention is to provide a method for manufacturing a silicon composite including nitrogen-doped carbon nanotubes and reduced graphene oxide, a silicon composite manufactured thereby, and a negative electrode active material including the same.

[0013] In order to solve the above technical problem, the present invention,

[0014] A step of preparing a graphene oxide dispersion solution and a pretreated carbon nanotube dispersion solution, respectively;

[0015] A step of preparing a mixed solution by mixing silicon particles in a solvent and adding the above graphene oxide dispersion solution and the above pretreated carbon nanotube dispersion solution;

[0016] A step of spray-drying the above mixed solution so that the solvent evaporates, coating the graphene oxide to surround the silicon-based particles, and manufacturing a composite particle in which the pretreated carbon nanotubes are bonded to the graphene oxide; and

[0017] A step of reducing the above composite particles with a reducing agent containing nitrogen to produce a silicon composite in which nitrogen-doped carbon nanotubes and reduced graphene oxide (rGO) are combined; including,

[0018] The above pretreated carbon nanotubes are,

[0019] Characterized in that the content of oxygen-containing functional groups is at least 10 wt%,

[0020] A method for manufacturing a silicon composite including nitrogen-doped carbon nanotubes and reduced graphene oxide is provided.

[0021] In addition, in order to solve the above other technical problems, a silicone composite manufactured according to the manufacturing method of the present invention is provided.

[0022] In addition, to solve the other technical problems mentioned above, a negative electrode active material including a silicon composite of the present invention is provided.

[0023] The method for manufacturing a silicon composite of the present invention by means of solving the above problem has the effect of manufacturing a carbon nanotube doped with nitrogen through chemical reduction at a relatively low temperature in a solution phase, and manufacturing a silicon composite by combining it with reduced graphene oxide (rGO).

[0024] In addition, the silicon composite of the present invention has the effect of increasing the diffusion speed of ions by complexing single-walled carbon nanotubes with reduced graphene oxide and nitrogen doped through chemical reduction and having excellent conductivity.

[0025] In addition, the negative electrode active material including the silicon composite of the present invention has a high ion diffusion rate and charge / discharge rate, and excellent capacity retention rate and electrode stability, including nitrogen-doped carbon nanotubes and reduced graphene oxide.

[0026] FIG. 1 is an X-ray photoelectron spectroscopy (XPS) graph of a pretreated oxidized single-walled carbon nanotube according to an embodiment of the present invention.

[0027] FIG. 2 is an N1s XPS graph of silicon particles, silicon composite SEM and TEM images, and pretreated carbon nanotubes manufactured according to an embodiment of the present invention.

[0028] Figure 3 is a CV graph of a battery sample according to a manufacturing example of the present invention.

[0029] Figure 4 is a graph showing the capacity retention rate according to the charge / discharge cycle of a battery sample according to a manufacturing example of the present invention.

[0030] Figure 5 is a graph showing the rate characteristics according to the charge / discharge cycle of a battery sample according to a manufacturing example of the present invention.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035]

[0036] According to one aspect of the present invention, a method for producing a silicon composite comprising nitrogen-doped carbon nanotubes and reduced graphene oxide, comprising the steps of: preparing a graphene oxide dispersion solution and a pretreated carbon nanotube dispersion solution, respectively; mixing silicon-based particles in a solvent, and adding the graphene oxide dispersion solution and the pretreated carbon nanotube dispersion solution to prepare a mixed solution; spray-drying the mixed solution such that the solvent evaporates and the graphene oxide coats the silicon-based particles so as to surround them, producing composite particles in which the pretreated carbon nanotubes are bonded to the graphene oxide; and reducing the composite particles with a nitrogen-containing reducing agent to produce a silicon composite in which nitrogen-doped carbon nanotubes and reduced graphene oxide (rGO) are bonded; wherein the pretreated carbon nanotubes are characterized in that the content of oxygen-containing functional groups is at least 10 wt%.

[0037]

[0038] First, a graphene oxide dispersion solution and a pretreated carbon nanotube dispersion solution are prepared, respectively.

[0039] The graphene oxide dispersion solution of the present invention has a structure in which graphene oxide includes an oxygen-containing functional group, such as an epoxy group or a hydroxyl group, on the surface, but no oxygen-containing functional group, such as a lactol group or a carboxyl group, exists on the surface, and graphite and chlorate (ClO3) - ) is mixed in a powder form as an oxidizing agent, and concentrated nitric acid is added to oxidize the graphite to produce graphite oxide, and then the graphite oxide produced in advance is exfoliated under alkaline conditions with a pH of 10 or higher using a basic solution to produce a low-defect graphene oxide dispersion solution.

[0040] More specifically, graphite and a chlorate-based oxidizing agent are mixed in a powdered form, then a small amount of a strong acid such as concentrated nitric acid is added to oxidize at room temperature, resulting in graphite oxide substituted with oxygen-containing functional groups on the surface or edges. Afterwards, the graphite oxide is treated with a basic solution to control the pH to 10 or higher to create an alkaline condition, and shear stress is applied to exfoliate the graphite, thereby removing lactol and carboxyl groups, which have a relatively higher oxidation degree than epoxy and hydroxyl groups, from the surface. Through this, rather than removing lactol and carboxyl groups through a separate treatment process from graphene oxide having epoxy, hydroxy, lactol, and carboxyl groups bonded to the surface and edges, graphene oxide is formed in which lactol and carboxyl groups do not exist on the surface from the beginning.

[0041] That is, while conventional graphene oxide contains a large number of oxygen-containing functional groups with a high degree of oxidation as graphite is oxidized using strong oxidizing agents such as sulfuric acid and potassium permanganate, the graphene oxide of the present invention minimizes the content of lactol groups and carboxyl groups introduced into the graphene layer by the chlorate-based oxidizing agent used in the oxidation of graphite, and mainly introduces hydroxyl groups and epoxy groups to induce hydrogen bonding between materials.

[0042] However, the graphene oxide of the present invention is applicable not only to low-defect graphene oxide that exhibits heat-resistant stability by including an oxygen-containing functional group such as an epoxy group or a hydroxyl group on the surface, but not to high-oxidation oxygen-containing functional groups such as a lactol group or a carboxyl group on the surface, but also to conventionally commercialized graphene oxide.

[0043] The pretreated carbon nanotube dispersion solution of the present invention is prepared by mixing carbon nanotubes and a strong acid, adding an oxidizing agent to prepare a carbon nanotube liquid crystal phase, stirring the carbon nanotube liquid crystal phase at room temperature to cause an oxidation reaction, neutralizing the mixture to prepare carbon nanotubes having oxygen-containing functional groups introduced thereto, and then washing and filtering the mixture.

[0044] In the present invention, the carbon nanotube may be at least one of a single-walled carbon nanotube (SWCNT), a double-walled carbon nanotube (DWCNT), and a multi-walled carbon nanotube (MWCNT).

[0045] Among them, single-walled carbon nanotubes have a diameter of only 1 nm, so they cannot exist alone and exhibit the characteristic of agglomeration due to van der Waals attraction. Due to this characteristic, single-walled carbon nanotubes originally have a bundle structure, and in the past, many technologies were developed to dismantle the bundle structure of single-walled carbon nanotubes. In other words, when the bundle of single-walled carbon nanotubes becomes large, dispersion becomes difficult, so from the perspective of dispersion, the technology was developed to make the bundle small to improve dispersion and to minimize the bundle size to maximize electrical conductivity.

[0046] In the case of multi-walled carbon nanotubes, although aggregation may occur due to their large diameter, they exhibit characteristics similar to the entanglement of polymer chains. In the case of double-walled carbon nanotubes, some with small diameters have characteristics similar to single-walled carbon nanotubes, and some with large diameters have characteristics similar to multi-walled carbon nanotubes.

[0047] Carbon nanotubes, with these properties, are attracting attention as conductive materials due to their unique electrical, thermal, and physicochemical properties. However, they suffer from the disadvantage of difficulty in ensuring dispersion stability due to aggregation phenomena such as bundling or entanglement. In particular, the aggregation of carbon nanotubes is difficult to physically handle, hindering the production of carbon nanotube dispersions.

[0048] Therefore, a method for pretreating carbon nanotubes is needed to disperse them in a stable state without causing them to aggregate or precipitate in a solvent.

[0049] Specifically, the pretreated carbon nanotubes of the present invention are manufactured by oxidizing carbon nanotubes at room temperature using a strong acid and an alkali metal-containing oxidizing agent, and manufacturing them so that oxygen-containing functional groups are introduced to the surface so that they can be easily dispersed in a solvent. That is, after oxidizing carbon nanotubes through acid treatment, the pretreated oxidized carbon nanotubes are manufactured by repeatedly washing them with an aqueous solution and removing impurities using a centrifuge. Preferably, single-walled carbon nanotubes are used for manufacturing.

[0050] In the present invention, the strong acid may be at least one selected from the group consisting of fuming nitric acid, sulfuric acid, nitric acid, hydrochloric acid, phosphoric acid, and hydrogen peroxide.

[0051] In the present invention, the alkali metal-containing oxidizing agent may be at least one selected from the group consisting of potassium permanganate (KMnO4), sodium chlorate (NaClO3), sodium perchlorate (NaClO4), potassium chlorate (KClO3), potassium perchlorate (KClO4), and potassium peroxymonosulfate (KHSO5).

[0052] The oxidation of the present invention is performed by adding the strong acid and the alkali metal-containing oxidizing agent to carbon nanotubes and stirring them at room temperature for 30 minutes to a week or leaving them in a kneaded state to oxidize the carbon nanotubes.

[0053] The oxidizing agent is preferably added in an amount of 0.5 to 10 parts by weight based on 1 part by weight of the single-walled carbon nanotube. If the amount of the chlorate-based oxidizing agent is less than 0.5 parts by weight, the single-walled carbon nanotube is not sufficiently oxidized, and if it exceeds 10 parts by weight, the degree of oxidation is excessive, which has the disadvantage of rapidly reducing the electrical conductivity of the single-walled carbon nanotube.

[0054] Stirring or kneading of carbon nanotubes can be selected depending on the amount of strong acid added. However, due to the tendency of single-walled carbon nanotubes to clump together, kneading with the strong acid and leaving it to stand is most preferable. After neutralizing the strong acid with distilled water, repeated washing via filtering or centrifugation is performed, followed by drying to obtain pure oxidized single-walled carbon nanotubes with no strong acid residue remaining on the surface.

[0055] Next, the pretreated carbon nanotubes are dispersed in a solvent to prepare a carbon nanotube dispersion solution. The pretreated carbon nanotubes can be dispersed in one or more solvents including water (H2O), dimethylformamide (DMF), an alcohol-based solvent, and N-methylformamide (NMP), to prepare a carbon nanotube dispersion solution.

[0056] The pretreated carbon nanotubes of the present invention are characterized by having an oxygen-containing functional group content of at least 10 wt%. More preferably, the content may be at least 20 wt%.

[0057] In the present invention, the oxygen-containing functional group is characterized in that it is at least one selected from the group consisting of a hydroxy group (-OH), a carboxy group (-COOH), and a carbonyl group (-C(=O)-).

[0058] Next, silicon particles are mixed in a solvent, and the above graphene oxide dispersion solution and the above pretreated carbon nanotube dispersion solution are added to prepare a mixed solution.

[0059] In the present invention, the silicon-based particles are silicon-based metal particles, and can be applied in various ways as long as they are silicon-based metal particles of nano or micro size. That is, any one or more silicon-based particles of silicon, silicon alloy, and silicon oxide can be used. The metal elements included in the silicon alloy can be at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti, Ge, P, As, Sb, Bi, S, Se, Te, and Po.

[0060] In the present invention, any solvent that can disperse the silicon particles may be used without limitation. Water is preferred.

[0061] After mixing silicon particles and a solvent, the above graphene oxide dispersion solution and the pretreated carbon nanotube dispersion solution are mixed, and a mixed solution is prepared without a separate dispersant.

[0062] In the above 100 wt% of the mixed solution, graphene oxide may be less than 0.3 wt%, and single-walled carbon nanotubes may be less than 0.2 wt%. When graphene oxide is less than 0.3 wt% and single-walled carbon nanotubes are less than 0.2 wt%, graphene forms a complete coating on the outside of the silicon-based particles, and single-walled carbon nanotubes can exist between the graphene oxide and the silicon-based particles and on the graphene oxide.

[0063] Through spray drying, the solvent in the mixed solution is evaporated together with water, and a composite layer consisting of a composite structure of graphene oxide and single-walled carbon nanotubes is coated on a single silicon-based particle or multiple silicon-based particles, thereby forming a composite particle.

[0064] At this time, since the mixed solution is in a state where graphene oxide and single-walled carbon nanotubes are uniformly mixed with each other, graphene oxide and single-walled carbon nanotubes can be coated on silicon particles in a mixed form.

[0065] That is, when the solvent evaporates during spray drying, relatively large graphene oxide forms composite particles by wrapping around small silicon particles and single-walled carbon nanotubes through hydrogen bonding.

[0066] However, since this step is before heat treatment or chemical reduction, single-walled carbon nanotubes refer to oxidized single-walled carbon nanotubes before reduction.

[0067] Finally, the above composite particles are reduced with a reducing agent containing nitrogen to produce a silicon composite in which nitrogen-doped carbon nanotubes and reduced graphene oxide (rGO) are combined.

[0068] In the present invention, reduction can be performed by chemical treatment using a reducing agent containing nitrogen. Preferably, a reducing agent containing hydrazine can be used.

[0069] The above hydrazine reducing agent treatment can be performed by dispersing the composite particles in a solvent such as DMF, adding a hydrazine solution, and treating at a temperature of 50 to 150°C for 30 minutes to 12 hours. Alternatively, the composite particles can be treated in a sealed container containing a hydrazine solution.

[0070] By performing reduction through the above chemical treatment, not only reduced graphene oxide is formed, but also nitrogen-doped carbon nanotubes are formed. Through this, a silicon composite in which the nitrogen-doped carbon nanotubes and reduced graphene oxide are combined can be manufactured.

[0071] The above nitrogen-doped carbon nanotube is characterized in that the nitrogen content is at least 1 wt%. If the nitrogen content is less than 1 wt%, the property of having sufficient affinity for lithium ions (lithiophilicity) is weak, and thus the effect of forming a stable SEI (solid electrolyte interface) layer is not observed, which is undesirable.

[0072] The above nitrogen-doped carbon nanotube is characterized in that the nitrogen content is at least 1 wt%. If the nitrogen content is less than 1 wt%, the property of having sufficient affinity for lithium ions (lithiophilicity) is weak, and thus the effect of forming a stable SEI (solid electrolyte interface) layer is not observed, which is undesirable. More preferably, it may be at least 4 wt%.

[0073] According to the above-described manufacturing method, the present invention spray-dries a mixed solution containing graphene oxide, silicon-based particles, pretreated oxidized single-walled carbon nanotubes, and a solvent to wrap graphene oxide around the outside of silicon-based particles, manufacture composite particles in which a portion of the oxidized single-walled carbon nanotubes is embedded on the graphene oxide, and then chemically reduces the composite particles manufactured in this manner to nitrogen-dope them, thereby manufacturing a silicon composite containing nitrogen-doped carbon nanotubes and reduced graphene oxide.

[0074]

[0075] According to another aspect of the present invention, a silicon composite comprising nitrogen-doped carbon nanotubes and reduced graphene oxide is provided, characterized in that it is manufactured according to the above manufacturing method.

[0076] The silicon composite of the present invention has a technical feature in that it can increase the diffusion speed of ions by complexing single-walled carbon nanotubes with graphene and nitrogen doping through chemical reduction, and the oxygen-containing functional group content of the carbon nanotubes is 10 wt% or more, so that it can be dispersed in an aqueous solution without a dispersant.

[0077] In addition, according to another aspect of the present invention, a negative electrode active material of a secondary battery is provided, including the silicon composite of the present invention.

[0078] The negative electrode active material including the silicon composite of the present invention has the characteristics of high ion diffusion rate and charge / discharge rate, and excellent capacity retention rate and electrode stability, including nitrogen-doped carbon nanotubes and reduced graphene oxide.

[0079]

[0080] 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.

[0081]

[0082] <Example>

[0083] <Example 1>: Nitrogen-doped single-walled carbon nanotube-reduced graphene oxide-silicon composite

[0084] (1) Preparation of graphene oxide dispersion solution

[0085] First, a graphene oxide dispersion solution for composite with Si-based anode particles was prepared as follows. 10 g of graphite and 75 g of sodium chlorate (NaClO3) were mixed in a powder state, and a small amount of 50 mL of fuming nitric acid was added, followed by an oxidation process at room temperature for 1 to 3 hours. Afterwards, 1 L of distilled water was added to neutralize, and 1 L of 1 M hydrochloric acid solution was added. Centrifugation at 1,000 rpm was repeated twice to produce purified graphite oxide. Afterwards, the pH was controlled to 10 or higher with a KOH solution, and graphene oxide was exfoliated using a homogenizer at 10,000 rpm for 30 minutes under alkaline conditions. Subsequently, distilled water was added, and centrifugation was performed to remove the alkaline solution, thereby producing a graphene oxide dispersion solution.

[0086]

[0087] (2) Preparation of pretreated carbon nanotube dispersion solution

[0088] A mixture was prepared by mixing 10 g of single-walled carbon nanotubes with 1,000 mL of fuming nitric acid, and slowly adding 70 g of sodium chlorate (NaClO3). The mixture was stirred at room temperature for approximately 3 hours to initiate an oxidation reaction. Afterwards, the carbon nanotubes were neutralized using distilled water, hydrochloric acid, and hydrogen peroxide, and then washed, filtered, and dried to remove the acid and oxidizing agent, thereby obtaining oxidized carbon nanotubes.

[0089] Figure 1 shows the results of X-ray photoelectron spectroscopy (XPS) of oxidized single-walled carbon nanotubes pretreated according to an embodiment of the present invention. In the XPS survey scan (Figure 1(a)), the C / O element ratio was confirmed to be approximately 3.75, indicating that the content of oxygen atoms was more than 20%. Figure 1(b) shows the results of C1s XPS analysis showing the chemical structure of carbon atoms. CO bonds and C=O, COOH bonds from numerous hydroxyl groups were confirmed.

[0090]

[0091] (3) Preparation of nitrogen-doped single-walled carbon nanotube-reduced graphene oxide-silicon composite

[0092] A 1 wt% silicon alloy was mixed in water, and the resulting graphene oxide and oxidized single-walled carbon nanotube dispersion solution was mixed to produce a mixed dispersion having 1 wt% silicon alloy, 0.08 wt% graphene oxide, and 0.04 wt% single-walled carbon nanotube without a separate dispersant. The mixed dispersion thus produced was used in a spray dryer to produce silicon alloy composite particles uniformly coated with graphene oxide and oxidized single-walled carbon nanotubes. The produced particles were chemically treated with hydrazine to produce silicon alloy composite particles in which reduced graphene oxide and single-walled carbon nanotubes were combined.

[0093] Fig. 2(a) is a SEM image of a silicon alloy particle, and Figs. 2(b) and (c) show SEM and TEM images, respectively, of a nitrogen-doped single-walled carbon nanotube-reduced graphene-silicon alloy composite manufactured according to an embodiment of the present invention. It shows that graphene is mainly present on the outer surface of the particle and carbon nanotubes are mainly present between the silicon alloy and the graphene. Fig. 2(d) shows an N1s XPS graph of a reduced single-walled carbon nanotube. It was confirmed that pyrrolic N, Pyrazole, pyridinic-N, and graphitic-N were introduced into the single-walled carbon nanotube by reduction of hydrazine.

[0094]

[0095] <Comparative Example>: Preparation of a single-walled carbon nanotube-reduced graphene oxide-silicon composite

[0096] 1 wt% of silicon alloy, 0.08 wt% of graphene oxide, and 0.04 wt% of nitrogen-free oxidized single-walled carbon nanotubes were mixed without a separate dispersant. The prepared mixed dispersion was spray-dried to prepare a silicon composite uniformly coated with graphene oxide and nitrogen-free oxidized single-walled carbon nanotubes. The prepared composite was reduced by heat treatment at 300°C in an inert gas atmosphere to prepare a composite including nitrogen-free carbon nanotubes.

[0097]

[0098] <Manufacturing Example>

[0099] In order to confirm the characteristics of a secondary battery using a negative electrode active material including a silicon composite according to an embodiment and comparative example of the present invention, a battery sample was manufactured.

[0100]

[0101] <Manufacturing Example 1>: Sample A

[0102] A silicone composite manufactured according to the above example: polyacrylic acid (water-soluble binder) was mixed in an aqueous solution at a weight ratio of 95:5 without adding a separate conductive agent to manufacture a slurry, which was then coated on a copper current collector to form a cathode layer, and dried at 100°C to remove moisture. Using the electrode manufactured in this way, a CR2032 standard coin cell was manufactured inside an argon atmosphere glove box.

[0103] Lithium metal foil was used as the counter electrode, and a polyolefin separator was interposed between the two electrodes. 1 M LiPF6 was used as the electrolyte, and a mixed solution of ethylene carbonate, dimethyl carbonate, and fluoroethylene carbonate was used as the solvent. This was designated as Sample A.

[0104]

[0105] <Manufacturing Example 2>: Sample B

[0106] A slurry was prepared in the same manner as in Manufacturing Example 1, except that a silicon composite manufactured according to the comparative example was used, and then a battery sample was fabricated. This will be referred to as Sample B.

[0107]

[0108] <Manufacturing Example 3>: C sample

[0109] A slurry was prepared by mixing silicon alloy alone, silicon alloy: conductive material (carbon black): polyacrylic acid (water-soluble binder) in an aqueous solution at a weight ratio of 87:5:5, and a battery sample was prepared by preparing the slurry in the same manner as in Manufacturing Example 1. This will be referred to as the C sample.

[0110]

[0111] <Evaluation and Results>

[0112] Cyclic voltammetry (CV) and capacity retention versus charge / discharge cycles were measured using coin cells of samples A, B, and C, respectively.

[0113] Fig. 3 is a CV graph of a battery sample according to a manufacturing example of the present invention. Referring to Fig. 3, compared to the case of the C sample where only a silicon alloy was applied, in the case of the A and B composites where single-walled carbon nanotubes and reduced graphene were introduced, it was confirmed that lithiation and delithiation occurred due to rapid electron transfer and lithium ion diffusion even without the use of an additional conductive material due to excellent electrical conductivity. In particular, in the case of the A sample, nitrogen-doped single-walled carbon nanotubes with lithiophilicity were introduced onto the surface of the silicon alloy, showing faster lithiation and delithiation than the B sample where nitrogen was not doped.

[0114] FIG. 4 is a graph showing the capacity retention rate according to the charge / discharge cycle of a battery sample according to a manufacturing example of the present invention. Referring to FIG. 4, in the case of sample C, to which only a silicon alloy was applied, even though 8 wt% of carbon black was added as a conductive material, the electrical conductivity was low and the physical networking between particles was insufficient, resulting in a capacity retention rate of 30.3% after 100 charge / discharge cycles. On the other hand, sample A, to which a silicon alloy-nitrogen-doped carbon nanotube-graphene composite was applied, showed a significantly improved capacity retention rate of 82.3% after 100 charge / discharge cycles. In addition, looking at the SEM image of the electrode cross-section after the charge / discharge test inserted in FIG. 4, cracks occurred when carbon black was used as a conductive material, but when composited with carbon nanotubes and graphene, the electrode layer was stable without the formation of cracks.

[0115] Figure 5 is a graph showing the rate characteristics according to the charge / discharge cycle of a battery sample according to a manufacturing example of the present invention. Referring to Figure 5, it was confirmed that Sample A, to which a composite including nitrogen-doped single-walled carbon nanotubes was applied, exhibited high rate characteristics even at fast charge / discharge rates.

[0116]

[0117] The present invention relates to a method for producing a silicon composite containing nitrogen-doped carbon nanotubes, a composite produced therefrom, and an anode active material containing the same, wherein a mixed solution containing graphene oxide, silicon particles, pretreated carbon nanotubes, and a solvent is spray-dried to introduce composite particles in which oxidized single-walled carbon nanotubes and graphene oxide are combined on the outside of silicon particles, and nitrogen atoms are introduced onto the surface of carbon nanotubes with a high oxidation degree by chemically reducing the composite particles using a reducing agent containing nitrogen. The present invention is particularly significant in that the nitrogen-doped carbon nanotubes can be produced at a relatively low temperature in a solution, rather than at a high temperature.

[0118] According to the present invention, by forming a lithium-philic carbon nanotube layer having a high affinity for lithium ions on the surface of a silicon alloy, a stable SEI (solid electrolyte interphase) layer is formed, and as can be confirmed from the results of the above-described examples and test examples, it can be confirmed that the present invention is excellent in that it can secure the safety of a silicon-based negative electrode material by improving not only the electrical conductivity but also the lithium ion diffusion of the negative electrode material for a silicon-based secondary battery.

[0119]

[0120] 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. A step of preparing a graphene oxide dispersion solution and a pretreated carbon nanotube dispersion solution, respectively; A step of preparing a mixed solution by mixing silicon particles in a solvent and adding the above graphene oxide dispersion solution and the above pretreated carbon nanotube dispersion solution; A step of spray-drying the above mixed solution so that the solvent evaporates, coating the graphene oxide to surround the silicon-based particles, and manufacturing a composite particle in which the pretreated carbon nanotubes are bonded to the graphene oxide; and A step of reducing the above composite particles with a reducing agent containing nitrogen to produce a silicon composite in which nitrogen-doped carbon nanotubes and reduced graphene oxide (rGO) are combined; including, The above pretreated carbon nanotubes are, Characterized in that the content of oxygen-containing functional groups is at least 10 wt%, A method for manufacturing a silicon composite comprising nitrogen-doped carbon nanotubes and reduced graphene oxide.

2. In paragraph 1, The above nitrogen-doped carbon nanotubes are, Characterized in that the nitrogen content is at least 1 wt%, A method for manufacturing a silicon composite comprising nitrogen-doped carbon nanotubes and reduced graphene oxide.

3. In paragraph 1, The above pretreated carbon nanotubes are, A step of producing a carbon nanotube liquid crystal phase by mixing carbon nanotubes and a strong acid and then adding an oxidizing agent; A step of producing a carbon nanotube having an oxygen-containing functional group introduced therein by stirring the above carbon nanotube liquid crystal phase at room temperature and then neutralizing the mixture to cause an oxidation reaction; and A pretreated carbon nanotube, characterized in that it is manufactured by including a step of washing and filtering the carbon nanotube into which the oxygen-containing functional group is introduced; A method for manufacturing a silicon composite comprising nitrogen-doped carbon nanotubes and reduced graphene oxide.

4. A silicon composite comprising nitrogen-doped carbon nanotubes and reduced graphene oxide, characterized in that it is manufactured according to any one of claims 1 to 3.

5. A negative electrode active material comprising a silicon composite including nitrogen-doped carbon nanotubes and reduced graphene oxide according to Article 4.

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