Composite negative electrode material structure and lithium secondary battery negative electrode active material comprising same

Abstract

An embodiment of the present invention provides a silicon-graphene-carbon nanotube composite which includes an outer pitch coating layer and of which a cross section including the center includes: a graphene layer; a carbon nanotube layer disposed on the graphene layer and containing a plurality of carbon nanotubes; and a nano silicon particle layer disposed on the graphene layer and containing a plurality of nano silicon particles attached to the carbon nanotubes. A negative electrode active material obtained through the production of the silicon-graphene-carbon nanotube composite can be used to achieve an optimal charge / discharge capacity and initial efficiency in a lithium secondary battery and improve cycle characteristics.

Description

Composite cathode material structure and cathode active material for lithium secondary battery including the same

[0001] The present invention relates to a composite negative electrode material structure and a negative electrode active material for a lithium secondary battery including the same, and more particularly, to a silicon-graphene-carbon nanotube composite as a composite negative electrode material and a negative electrode active material for a lithium secondary battery including the same.

[0002]

[0003] As markets for portable electronic devices and electric vehicles expand, research into rechargeable secondary batteries is actively underway. Among the various types of secondary batteries, lithium secondary batteries consist of a cathode formed with a positive active material layer, an anode formed with a negative active material layer, and a separator that electrically insulates the two electrodes. To maintain uniform ionic conductivity within the electrode active material layer, conductive particles must be well dispersed within the electrode active material slurry.

[0004] However, when using a conventional dispersion method, it is difficult to uniformly disperse the conductive particles in the slurry due to differences in particle sizes and specific gravity of the electrode active material particles and the conductive particles, which reduces electrochemical charge / discharge characteristics and also reduces stability and long-term reliability.

[0005] Our own research has revealed that even if silicon particles are well dispersed on graphite, there is some non-uniformity, and the particle shape is difficult to spheroidize because the silicon particles exist on the expanded state of expanded graphite, making it difficult to apply directly as an anode active material.

[0006] Accordingly, it was necessary to improve uniformity through dispersion. However, in the case of silicon-graphene composites dispersed by the existing dispersion method, the silicon particles and graphene have a structure in which they are randomly overlapped, making it difficult to control the thickness and particle size. In addition, some agglomeration of silicon particles occurs, which leads to a decrease in charge / discharge characteristics and initial efficiency when evaluated electrochemically, and poor long-term reliability. In addition, there is a problem of silicon breaking or detachment due to expansion and contraction during charge / discharge of silicon.

[0007]

[0008] The technical problem to be achieved by the present invention is to provide a structure of a silicon-graphene-carbon nanotube composite capable of manufacturing an optimal negative electrode active material for improving the charge / discharge capacity and initial efficiency of a lithium secondary battery and implementing cycle characteristics.

[0009] The technical problems to be solved by the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention belongs from the description below.

[0010]

[0011] In order to achieve the above technical task, one embodiment of the present invention provides a silicon-graphene-carbon nanotube composite including an outer pitch coating layer, and a cross-section including a center, including: a graphene layer; a carbon nanotube layer positioned on the graphene layer and including a plurality of carbon nanotubes; and a nano silicon particle layer positioned on the graphene layer and including a plurality of nano silicon particles bonded to the carbon nanotubes.

[0012] In an embodiment of the present invention, the thickness of the graphene layer may be characterized as being 1 nm to 300 nm.

[0013] In an embodiment of the present invention, the length of the carbon nanotube is 0.1 to 30 um and the specific surface area is 100 to 500 m 2 / g and bulk density is 0.05~0.2 g / cm 3 And the purity is 95% or higher, and it can be characterized as being single wall or multi wall.

[0014] In an embodiment of the present invention, the thickness of the pitch coating layer may be characterized as being 0.05 to 1 μm.

[0015] In an embodiment of the present invention, the content of the nano silicon may be characterized as being greater than 0 and less than or equal to 85 wt%.

[0016]

[0017] In order to achieve the above technical problem, another embodiment of the present invention provides a method for producing a silicon-graphene-carbon nanotube composite, comprising: preparing a silicon-graphite-carbon nanotube fusion precursor; treating the silicon-graphite-carbon nanotube fusion precursor with plasma; dispersing the plasma-treated silicon-graphite-carbon nanotube fusion precursor by a wet grinding process to form an exfoliated silicon-graphene-carbon nanotube composite; coating the exfoliated silicon-graphene-carbon nanotube composite with a binder; and granulating the binder-coated exfoliated silicon-graphene-carbon nanotube composite to produce a granulated silicon-graphene-carbon nanotube composite.

[0018] In an embodiment of the present invention, the binder coating may be characterized by coating a pitch liquid.

[0019] In an embodiment of the present invention, the solvent used in the wet grinding process may be characterized by having a Hansen Solubility Parameter of 17 to 23 MPa ½ or 45 to 50 MPa ½.

[0020] In an embodiment of the present invention, the solvent used in the wet grinding process may be characterized by having a boiling point of 60 to 160°C.

[0021]

[0022] In order to achieve the above technical task, another embodiment of the present invention provides an anode active material for a lithium secondary battery, including the silicon-graphene-carbon nanotube composite.

[0023]

[0024] According to an embodiment of the present invention, by manufacturing a silicon-graphene-carbon nanotube composite, it is possible to achieve optimal charge / discharge capacity and initial efficiency of a lithium secondary battery using a negative electrode active material, and to improve cycle characteristics.

[0025] The effects of the present invention are not limited to the effects described above, and should be understood to include all effects that can be inferred from the detailed description of the present invention or the composition of the invention described in the claims.

[0026]

[0027] Figure 1 is a schematic diagram of a cross-section of a silicon-graphene-carbon nanotube composite.

[0028] Figure 2 is a schematic diagram according to the composition of (a) silicon-carbon nanotube, (b) silicon-graphene, and (c) silicon-graphene-carbon nanotube fusion precursors.

[0029] Figure 3 is a schematic diagram of a silicon-graphene-carbon nanotube composite.

[0030] Figure 4 is an FE-SEM image of a cross-section of a silicon-graphene-carbon nanotube composite.

[0031] Figure 5 is a graph showing the electrochemical evaluation of (a) a comparative example and (b) an exemplary example.

[0032] Figure 6 is a cycle retention graph of a comparative example and an embodiment.

[0033]

[0034] Hereinafter, the present invention will be described with reference to the attached drawings. However, the present invention can be implemented in various different forms and is therefore not limited to the embodiments described herein. In the drawings, irrelevant parts have been omitted for clarity of description, and similar parts have been designated with similar reference numerals throughout the specification.

[0035] Throughout the specification, when a part is said to be "connected (connected, contacted, or coupled)" to another part, this includes not only cases where it is "directly connected," but also cases where it is "indirectly connected" with another part in between. Furthermore, when a part is said to "include" a component, this does not exclude other components, but rather implies that it may include other components, unless otherwise specifically stated.

[0036] The terminology used herein is merely used to describe specific embodiments and is not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly indicates otherwise. In this specification, it should be understood that the terms "comprises" or "has" indicate the presence of a feature, number, step, operation, component, part, or combination thereof described in the specification, but do not exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0037] Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings.

[0038]

[0039] The terms used in this specification are defined as follows.

[0040] “Composite cathode material” refers to a silicon-graphene-carbon nanotube composite.

[0041]

[0042] Figure 1 is a schematic diagram of a cross-section of a silicon-graphene-carbon nanotube composite.

[0043] Figure 2 is a schematic diagram according to the composition of (a) silicon-carbon nanotube, (b) silicon-graphene, and (c) silicon-graphene-carbon nanotube fusion precursors.

[0044] Figure 3 is a schematic diagram of a silicon-graphene-carbon nanotube composite.

[0045] A silicon-graphene-carbon nanotube composite according to an embodiment of the present invention is described with reference to FIGS. 1 to 3.

[0046] A silicon-graphene-carbon nanotube composite according to one embodiment of the present invention may include an outer pitch coating layer, and a cross-section including a center may include: a graphene layer; a carbon nanotube layer positioned on the graphene layer and including a plurality of carbon nanotubes; and a nano silicon particle layer positioned on the graphene layer and including a plurality of nano silicon particles bonded to the carbon nanotubes.

[0047] The thickness of the graphene layer can range from 1 nm to 300 nm. When the thickness exceeds 300 nm, some nano-Si particles aggregate and grow coarsely rather than evenly dispersed, which increases swelling during the final composite electrochemical evaluation, negatively affecting long-term reliability during the electrochemical evaluation.

[0048] Carbon nanotubes have a length of 0.1 to 30 μm and a specific surface area of ​​100 to 500 m 2 / g and bulk density is 0.05~0.2 g / cm 3And the purity is 95% or higher, and it can be single wall or multi wall. When the length of the carbon nanotube is less than 0.1 um, entanglements between nano Si and adjacent nano Si may not occur, so the detachment of nano Si cannot be prevented during the electrochemical evaluation of the final complex, and it is also difficult to form a conductive network between graphene and graphene layers, so an appropriate range of sizes is required for the carbon nanotube. When using carbon nanotubes larger than 30 um, the effect is sharply reduced because the self-entanglement becomes larger rather than the optimal entanglement, so a length of 30 um or less is appropriate. In the case of single wall and multi wall, single wall has superior performance, but multi wall carbon nanotubes with the inherent high conductivity of carbon nanotubes can also be applied.

[0049] The specific surface area of ​​expanded graphite is 10 to 100 m 2 / g, whereas in the case of carbon nanotubes, it is 100-500 m, which is higher than that of expanded graphite. 2 / g has a high specific surface area value, so that there are many condensation sites for Si, and it is possible to condense up to 85 wt% of Si into nano Si.

[0050] The thickness of the pitch coating layer can be 0.05 to 1 μm.

[0051] The content of nano silicon can be greater than 0 and less than 85 wt%. When Si is mixed in at a level exceeding 85 wt%, rather than condensing into nano silicon due to excess Si, there is a high possibility that nano Si growth will occur in the fusion precursor, thereby forming μm-scale Si.

[0052]

[0053] Hereinafter, a method for manufacturing a silicon-graphene-carbon nanotube composite according to another embodiment of the present invention will be described.

[0054] A method for manufacturing a silicon-graphene-carbon nanotube composite according to one embodiment of the present invention may include the steps of: preparing a silicon-graphite-carbon nanotube fusion precursor; plasma treating the silicon-graphite-carbon nanotube fusion precursor; dispersing the plasma-treated silicon-graphite-carbon nanotube fusion precursor through a wet grinding process to form an exfoliated silicon-graphene-carbon nanotube composite; binder-coating the exfoliated silicon-graphene-carbon nanotube composite; and granulating the binder-coated exfoliated silicon-graphene-carbon nanotube composite to manufacture a granulated silicon-graphene-carbon nanotube composite.

[0055] The first step is to prepare a silicon-graphite-carbon nanotube fusion precursor. The silicon can be amorphous Si of the μm scale or submicron-scale silicon sludge produced as a byproduct during the manufacture of solar cell ingots. The graphite can be expanded graphite intercalated with a strong acid. The Si-expanded graphite mixture can be prepared by mixing raw materials of Si powder of 0 to 85 wt%, expanded graphite of 99.9 to 15 wt%, and carbon nanotube of 20 to 0 wt% in a mixer. The silicon content is 0 to 85 wt%, and the silicon content can be applied solely as a fusion precursor, a mixture of a fusion precursor and silicon, or a composite of expanded graphite, carbon nanotubes, and silicon. Here, the specific surface area of ​​the expanded graphite is 10 to 100 m 2 / g, but in the case of carbon nanotubes, it is 100-500 m higher than expanded graphite. 2 / g has a high specific surface area value, so that there are many Si condensation sites, and Si can be condensed into nano-Si up to 85 wt%. When Si is mixed at a level higher than 85 wt%, rather than condensing into nano-silicon due to excessive Si, there is a high possibility that many nano-Si growths will occur in the fusion precursor, forming μm-scale Si.

[0056] Here, the carbon nanotubes used in the production of fusion precursors are single-walled or multi-walled, and their main properties include a bulk density of 0.05 to 0.2 g / cm. 3 , and the specific surface area is 100~500m 2 / g and materials with a purity of 95% or higher were used.

[0057] The following is a step of plasma treating the above silicon-graphite-carbon nanotube fusion precursor.

[0058] The step of plasma treating the above silicon-graphite-carbon nanotube fusion precursor is performed by performing DC plasma treatment with N2 purge at 20 kW to cause vaporization of Si, and at the same time, as the expanded graphite expands, nano-sized Si is condensed between the expanded graphite layers and on the surface of the carbon nanotube bundle, thereby finally obtaining a fusion precursor in which plasma-treated silicon, graphite, and carbon nanotubes are combined.

[0059]

[0060] The following is a step of forming a silicon-graphene-carbon nanotube composite in an exfoliated form by dispersing the above plasma-treated silicon-graphite-carbon nanotube fusion precursor through a wet grinding process.

[0061] The above grinding process may be characterized as a wet grinding process. When dispersing a silicon-graphite fusion precursor using a conventional dry grinding process, the exfoliation state of the graphite (plate graphite / graphene) is not uniform, making it difficult to control the thickness and particle size, and agglomeration of silicon particles may occur, which may result in a decrease in charge / discharge characteristics and initial efficiency and poor long-term reliability when evaluated electrochemically.

[0062] The solvent used in the above wet grinding process may be a single component among aqueous and organic solvents or a combination of solvents that can be mixed. Alcohols including water, ethanol, isopropanol, N-butanol amyl alcohol, cyclohexanol, etc.; ketones such as acetone, MEK, MIBK, cyclohexanone; esters including ethyl acetate, isopropanol acetate, N-butyl acetate, amyl acetate, etc.; hydrocarbons including mineral spirit, heptane, cyclohexane, toluene, xylene, etc.; glycoether acetates including butyl cellosolve, ethyl cellosolve, acetate, butyl carbitol, butyl carbitol acetate, etc.; 1.1.1-trichloroethane (1.1.1-TCE), TCE, Solvents such as halogenated hydrocarbons including EDC (1,2-dichloroethane), furan solvents including tetrahydrofuran, and lactam solvents including NMP (N-Methyl-2-pyrrolidone) can be used.

[0063] The optimal solvent conditions are boiling point between 60 and 160°C, and the Hansen Solubility Parameter (MPa) of the solvent. ½ ) is preferably between 17 and 23 or between 45 and 50. More preferably, it can be between 18 and 22 or between 47 and 48.

[0064] Solvents that satisfy this include Acetaldoxime, Acetic acid, Acetic anhydride, Acetonecyanhydrin, N-Acetyl caprolactam, Acetylacetone, Acetylbromide, Allyl acetate, Arryl acetoacetate, Allyl Alcohol, Amyl acetate, Benzene, N-benzyl pyrrolidone, 4-bromo-1-butene, 1-butanethiol, 2-buanol, 1-butene, Carbon tetrachloride, Chloro acetaldehyde, Cyclohexanone, Cyclohexanol, 2-chloro allyl alcohol, 4-chloro-1,2-butadiene, 1-chloro-2-butene, Ethanol, Isoamyl acetate, Methyl ethyl ketone, Isoamyl alcohol, Xylene, Tetrahydrofuran, Toluene, and Water. It is preferable to use these solvents alone or in combination.

[0065] The Hansen Solubility Parameter is a unique value for each solvent developed by Charles M. Hansen in 1966. It can be composed of three parameters: a dispersion component, a hydrogen bonding component, and a terminal polar component. The Total Solubility Parameter can be calculated using the following equation.

[0066]

[0067] HANSEN SOLUBILITY PARAMETERS (HSP)

[0068] d 2 = dD 2 + dP 2 + dH 2

[0069]

[0070] d = square root of cohesive energy density

[0071] δD: Energy derived from dispersion force

[0072] δP: Energy derived from polarity

[0073] δH: Energy derived from hydrogen bonding

[0074]

[0075] The above wet grinding process may be characterized by chemically activating a fusion precursor obtained from plasma.

[0076] Titanate coupling agents such as isopropyl di(dioctylphosphite) titanate, tetraoctyl bis(ditridecylphosphite) titanate, and isopropyl triisostearoyl titanate, which can be dispersed organically or inorganically on the surface for activation, silane coupling agents such as vinyltrimethoxysilane, 3-aminopropyltriethoxysilane, and 3-(trimethoxysilyl)propylsuccinic anhydride, fatty acids such as stearic acid, palmitic acid, and oleic acid, and acrylate copolymers, copolymers having pigment-affinity groups, sodium alkylnaphthalenesulfonate, sodium polyacrylate, olefin-sodium maleate copolymers, carboxymethylcellulose, alkylbenzene(naphthalene)sulfonate, fatty acid amide, polyoxyethylene alkylamine, alkylamine (acetate, fatty acid salt), Inorganic Si / Graphite / CNT can be activated to be dispersed in a solvent by adding amine derivatives such as alkyl secondary (tertiary) amines (amides) and alkylimidazolines, emulsifiers such as xanthan gum, sucrose fatty acid esters, glycerin fatty acid esters, propylene glycol fatty acid esters, polyvinylpyridolidone, polyethylene-polypropylene glycol, and polymeric dispersants such as alkylphenols, fatty acids, and higher fatty acid amines.

[0077] The above wet grinding process may be characterized by homogenizing the fusion precursor using equipment that applies shear stress and cavitation to the activated slurry.

[0078]

[0079] The next step is to binder-coat the silicon-graphene-carbon nanotube composite in the above-described exfoliated form.

[0080] Binder coating is a coating process for granulation. The binder is a resin that dissolves in a solvent and acts as a binding agent for coating and granulating the exfoliated particles. Thermoplastic resins, thermosetting resins, pitch, and hydrocarbon resins are used. The thermoplastic binder may be one of acryl, ethyl cellulose, polyester, polysulfone, phenoxy, and polyamide, or a mixture of at least two or more thereof, and the thermosetting binder may be one of amino, epoxy, and phenol, or a mixture of at least two or more thereof. The pitch may be either coal-based or petroleum-based. In this example, coal tar pitch was added at 10 wt% to the composite and mixed using a mixer.

[0081]

[0082] The following is a step of manufacturing a granulated silicon-graphene-carbon nanotube composite by granulating the above-mentioned binder-coated, exfoliated silicon-graphene-carbon nanotube composite.

[0083] Equipment is used to obtain the desired rotational force for spheroidizing the particles coated with the binder. Spheroidization is possible by controlling the working rpm and process time, such as a ball mill, an attrition mill, a paste mixer, an ultra-fine grinder capable of controlling the rotational force, or a mechano fusion that can compact the particles with the rotational force.

[0084]

[0085] Hereinafter, a negative electrode active material for a lithium secondary battery according to another embodiment of the present invention will be described.

[0086] A negative electrode active material for a lithium secondary battery according to one embodiment of the present invention may include the silicon-graphene-carbon nanotube composite.

[0087] The negative electrode active material composed of the silicon-graphene-carbon nanotube composite exhibiting a uniform distribution without pores and agglomeration through dispersion can achieve the optimal charge / discharge capacity and initial efficiency of a lithium secondary battery and implement cycle characteristics.

[0088]

[0089] Hereinafter, embodiments and experimental examples of the present invention will be described in detail.

[0090]

[0091] <Example>

[0092]

[0093] Dispersion is carried out by selecting from among high shear mixer, ball mill, attrition mill, high pressure homogenizer, three roll mill, basket mill, apex mill, paste mixer, planetary mixer, spike mill, and ultrasonic. In the present invention, the first dispersion was carried out by applying a basket mill of 3000 rpm, 2 hr, and 0.4 mm Zirconia ball, and then a 4000 Watt class ultrasonic disperser.

[0094] By applying this technique, graphite and Si were further delaminated. If equipment capable of applying high shear is added, a greater degree of delamination can be achieved.

[0095] Table 1 illustrates the process conditions for the existing comparative examples and manufacturing examples according to the manufacturing process. A pigment-friendly copolymer dispersant was applied for plasma treatment and activation, and coin-half cells were manufactured with different boiling points (60–160°C) and different Hansen solubility parameters.

[0096]

[0097]

[0098]

[0099]

[0100] <Experimental Example 1> Cross-section of silicon-graphene-carbon nanotube composite according to dispersion

[0101]

[0102] Figure 4 is an FE-SEM image of a cross-section of a silicon-graphene-carbon nanotube composite. Referring to Figure 4, it can be confirmed that a silicon particle layer is uniformly dispersed and distributed on the graphene layer and a carbon nanotube layer is wrapped around the silicon in various directions.

[0103]

[0104] <Experimental Example 2> Electrochemical Evaluation

[0105]

[0106] Figure 5 is a graph showing the electrochemical evaluation of (a) a comparative example and (b) an exemplary example. Figure 6 is a cycle retention graph of the comparative example and the exemplary example. Table 2 shows these data.

[0107]

[0108]

[0109]

[0110]

[0111] Through this, it can be confirmed that the initial discharge capacity, initial efficiency, cycle capacity change, and capacity retention rate are improved when the mixed electrode manufactured using the silicon-graphene-carbon nanotube composite is applied as a negative electrode active material.

[0112]

[0113] The foregoing description of the present invention is for illustrative purposes only, and those skilled in the art will readily appreciate that the present invention can be readily modified into other specific forms without altering the technical spirit or essential characteristics of the present invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive. For example, each component described as a single entity may be implemented in a distributed manner, and similarly, components described as distributed may be implemented in a combined manner.

[0114] The scope of the present invention is indicated by the claims described below, and all changes or modifications derived from the meaning and scope of the claims and their equivalent concepts should be interpreted as being included in the scope of the present invention.

Claims

1. Contains an external pitch coating layer, The cross section containing the center is, graphene layer; A carbon nanotube layer positioned on the graphene layer and including a plurality of carbon nanotubes; and A silicon-graphene-carbon nanotube composite comprising a nano silicon particle layer comprising a plurality of nano silicon particles positioned on the graphene layer and bonded to the carbon nanotubes.

2. In paragraph 1, A silicon-graphene-carbon nanotube composite, characterized in that the thickness of the graphene layer is 1 nm to 300 nm.

3. In paragraph 1, The length of the above carbon nanotubes is 0.1 to 30 um and the specific surface area is 100 to 500 m 2 / g and bulk density is 0.05~0.2 g / cm 3 A silicon-graphene-carbon nanotube composite having a purity of 95% or higher and characterized by being single-walled or multi-walled.

4. In paragraph 1, A silicon-graphene-carbon nanotube composite, characterized in that the thickness of the pitch coating layer is 0.05 to 1 μm.

5. In paragraph 1, A silicon-graphene-carbon nanotube composite, characterized in that the silicon content is greater than 0 and less than or equal to 85 wt%.

6. Step of preparing a silicon-graphite-carbon nanotube fusion precursor; A step of plasma treating the above silicon-graphite-carbon nanotube fusion precursor; A step of dispersing the plasma-treated silicon-graphite-carbon nanotube fusion precursor through a wet grinding process to form a silicon-graphene-carbon nanotube composite in an exfoliated form; A step of binder coating the silicon-graphene-carbon nanotube composite in the above-mentioned peeled form; and A method for producing a silicon-graphene-carbon nanotube composite, comprising: a step of granulating the binder-coated, exfoliated silicon-graphene-carbon nanotube composite to produce a granulated silicon-graphene-carbon nanotube composite; 7. In paragraph 6, A method for manufacturing a silicon-graphene-carbon nanotube composite, characterized in that the above binder coating comprises coating a pitch solution.

8. In paragraph 6, A method for producing a silicon-graphene-carbon nanotube composite, wherein the solvent used in the above wet grinding process has a Hansen Solubility Parameter of 17 to 23 MPa ½ or 45 to 50 MPa ½.

9. In paragraph 6, A method for producing a silicon-graphene-carbon nanotube composite, characterized in that the solvent used in the above wet grinding process has a boiling point of 60 to 160°C.

10. A negative electrode active material for a lithium secondary battery, comprising the silicon-graphene-carbon nanotube composite of clause 1.

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