Negative electrode, secondary battery including the negative electrode, and method for manufacturing the negative electrode

Abstract

The present invention relates to a negative electrode, a secondary battery including the negative electrode, and a method of manufacturing the negative electrode. The negative electrode includes a negative electrode active material layer, where the negative electrode active material layer includes a negative electrode active material and a conductive agent. The negative electrode active material includes a silicon-based active material, and the silicon-based active material includes SiO x (0 ≤ x < 2). The conductive agent includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bound side by side, and the carbon nanotube structure is included in the negative electrode active material layer in an amount of 0.01 wt% to 1.0 wt%.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims the priority of Korean Patent Application No. 10-2019-0094003, filed on August 1, 2019, the disclosure of which is incorporated herein by reference. Technical Field

[0004] The present invention relates to a negative electrode, a secondary battery including the negative electrode, and a method of manufacturing the negative electrode, where the negative electrode includes a negative electrode active material layer, and the negative electrode active material layer includes a negative electrode active material and a conductive agent, where the negative electrode active material includes a silicon-based active material, the silicon-based active material includes SiO x (0 ≤ x < 2), the conductive agent includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side, and the carbon nanotube structure may be included in the negative electrode active material layer in an amount of 0.01 wt% to 1.0 wt%. Background Art

[0005] With the recent increase in the development of technologies and demands for mobile devices, the demand for batteries as an energy source has increased significantly, and various studies have been conducted on batteries that can meet various demands. In particular, as a power source for these devices, lithium secondary batteries having excellent life and cycle characteristics and high energy density have been actively studied.

[0006] A lithium secondary battery refers to a battery including a non-aqueous electrolyte containing lithium ions in an electrode assembly, the electrode assembly including: a positive electrode including a positive electrode active material capable of inserting / extracting lithium ions; a negative electrode including a negative electrode active material capable of inserting / extracting lithium ions; and a microporous separator disposed between the positive electrode and the negative electrode.

[0007] Since the conductivity of the negative electrode may not be ensured only by the negative electrode active material, the resistance of the battery may be too high. Therefore, the negative electrode usually further includes a conductive agent. Generally, point-type conductive agents such as carbon black are mainly used, and linear conductive agents such as carbon nanotubes and carbon nanofibers are also used to increase the battery capacity by further improving the conductivity.

[0008] Single-walled carbon nanotubes are one of the linear conductive agents, and the conductivity in the negative electrode active material layer is increased due to their thin and elongated shape. Therefore, generally, after preparing a negative electrode slurry by using a dispersion in which single-walled carbon nanotubes are completely dispersed, the negative electrode active material layer is prepared by using the negative electrode slurry.

[0009] However, when the battery is repeatedly charged and discharged, as the volume of the negative electrode active material repeatedly expands and contracts, the single-walled carbon nanotubes break, and thus, it may be difficult to maintain the conductive network in the negative electrode active material layer. In particular, in the case where a silicon-based active material is used as the negative electrode active material to increase the battery capacity, since the volume of the silicon-based active material expands excessively due to the charging and discharging of the battery, the breaking phenomenon of the single-walled carbon nanotubes is more severe. As a result, the conductive network is blocked or reduced, which reduces the life characteristics of the battery.

[0010] Therefore, there is a need for a new negative electrode that can increase the capacity and life of the battery when a silicon-based active material is used. Summary of the Invention

[0011] Technical problem

[0012] One aspect of the present invention provides a negative electrode that can improve the capacity and life characteristics of a battery.

[0013] Another aspect of the present invention provides a secondary battery including the negative electrode.

[0014] Still another aspect of the present invention provides a method for preparing the negative electrode.

[0015] Technical solution

[0016] According to one aspect of the present invention, there is provided a negative electrode including a negative electrode active material layer, wherein the negative electrode active material layer includes a negative electrode active material and a conductive agent, wherein the negative electrode active material includes a silicon-based active material, the silicon-based active material including SiO x (0 ≤ x < 2), the conductive agent includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are arranged side by side, and the carbon nanotube structure is included in the negative electrode active material layer in an amount of 0.01 wt% to 1.0 wt%.

[0017] According to another aspect of the present invention, there is provided a method for preparing a negative electrode, the method including the steps of: preparing a conductive agent dispersion (S1); and forming a negative electrode paste including the conductive agent dispersion and a negative electrode active material (S2), wherein the preparation of the conductive agent dispersion (S1) includes: preparing a mixed solution including a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes (S1-1); and dispersing the bundled single-walled carbon nanotubes by applying a shear force to the mixed solution using a homogenizer to form a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are arranged side by side (S1-2), wherein the negative electrode active material includes a silicon-based active material, the silicon-based active material including SiO x(0 ≤ x < 2), and the carbon nanotube structure is included in the negative electrode active material layer in an amount of 0.01 wt% to 1.0 wt%.

[0018] According to another aspect of the present invention, there is provided a secondary battery including the negative electrode.

[0019] Advantageous effects

[0020] Since the negative electrode according to the present invention is prepared by using a conductive agent dispersion in which bundled single-walled carbon nanotubes are appropriately dispersed by a homogenizer under specific conditions, carbon nanotube structures in the form of ropes (long fiber form) are connected to each other to form a network structure in the negative electrode. Therefore, since the carbon nanotube structure is not damaged even when the volume change of the silicon-based active material is too large, the conduction path in the negative electrode active material layer can be maintained. Therefore, when the negative electrode of the present invention is used, due to the use of the carbon-based nanotube structure, the effects of improving the battery capacity due to the use of the silicon-based active material and improving the battery life characteristics due to the carbon nanotube structure can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 is a scanning electron microscope (SEM) image (A, B, and C) of the negative electrode of the present invention;

[0022] Figure 2 is an SEM image (A) of the negative electrode of Example 1 and an SEM image (B) of Comparative Example 2;

[0023] Figure 3 is an SEM image (A) of the negative electrode of Example 1 and an SEM image (B) of Example 3; and

[0024] Figure 4 is an SEM image of the negative electrode of Comparative Example 1. DETAILED DESCRIPTION

[0025] It will be understood that the terms or words used in this specification and the claims should not be construed as having the meanings defined in a common dictionary, and it will be further understood that based on the principle that the inventor can appropriately define the meanings of the terms or words to best explain the present invention, these terms or words should be construed as having meanings consistent with the technical concept of the present invention and the context of the related art.

[0026] The terms used herein are for the purpose of describing specific exemplary embodiments only and are not intended to limit the present invention. In the specification, unless otherwise indicated, the singular forms of the terms may include the plural forms.

[0027] It will be further understood that when used in this specification, the terms "comprising", "including" or "having" specify the presence of the stated features, numbers, steps, elements or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, elements or combinations thereof.

[0028] In this specification, unless otherwise expressly indicated, the expression "%" means % by weight.

[0029] In this specification, the expression "specific surface area" is measured by the Brunauer - Emmett - Teller (BET) method. Specifically, the specific surface area can be calculated from the nitrogen adsorption amount at liquid nitrogen temperature (77K) using BELSORP - mini II manufactured by BEL Japan, Inc.

[0030] The expression "average particle diameter (D 50 )" in this specification can be defined as the particle diameter at which the cumulative volume in the particle size distribution curve is 50%. The average particle diameter (D 50 ) can be measured, for example, by using the laser diffraction method. The laser diffraction method can generally measure particle diameters in the range from the submicron level to several millimeters and can obtain results with high repeatability and high resolution.

[0031] Hereinafter, the present invention will be described in detail.

[0032] Negative electrode

[0033] The negative electrode according to the present invention includes a negative electrode active material layer, wherein the negative electrode active material layer includes a negative electrode active material and a conductive agent, wherein the negative electrode active material includes a silicon - based active material, the silicon - based active material includes SiO x (0 ≤ x < 2), the conductive agent includes a carbon nanotube structure in which 2 to 5,000 single - wall carbon nanotube units are bonded side by side, and the carbon nanotube structure can be included in the negative electrode active material layer in an amount of 0.01 wt% to 1.0 wt%.

[0034] The negative electrode may include a negative electrode active material layer and may more specifically include a current collector and a negative electrode active material layer provided on the current collector. However, the negative electrode does not exclude the so - called "free - standing negative electrode" in which the negative electrode is composed only of the negative electrode active material layer and does not include a current collector.

[0035] The current collector is not particularly limited as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, carbon-treated, aluminum or stainless steel surface-treated with nickel, titanium, silver or the like can be used as the current collector. Specifically, transition metals that can well adsorb carbon (such as copper and nickel) can be used as the current collector.

[0036] The negative electrode active material layer can be provided on one surface or both surfaces of the current collector. Of course, for a free-standing negative electrode, the negative electrode active material layer itself can also be a negative electrode without a current collector.

[0037] The negative electrode active material layer may include a negative electrode active material and a conductive agent.

[0038] The negative electrode active material may include a silicon-based active material.

[0039] The silicon-based active material may include SiO x (0 ≤ x < 2). Therefore, the capacity of the battery can be increased.

[0040] SiO x (0 ≤ x < 2) may have an average particle size (D 50 ) of 0.1 μm to 20 μm, specifically 1 μm to 10 μm. When the average particle size satisfies the above range, the side reaction between SiO x (0 ≤ x < 2) and the electrolyte solution can be suppressed, the reduction of the initial efficiency can be prevented by controlling the lithium silicate formation reaction from SiO x (0 ≤ x < 2), and the initial capacity of the battery can be maximized.

[0041] The silicon-based active material may further include a carbon coating. The carbon coating can be provided on SiO x (0 ≤ x < 2). The carbon coating improves the conductivity of SiO x (0 ≤ x < 2) and plays a role in suppressing the excessive volume expansion of SiO x (0 ≤ x < 2).

[0042] The carbon coating may include at least one of amorphous carbon and crystalline carbon.

[0043] Crystalline carbon can further improve the conductivity of the negative electrode active material. Crystalline carbon may include at least one selected from the group consisting of fullerenes, carbon nanotubes, and graphene.

[0044] Amorphous carbon can suppress the expansion of natural graphite by appropriately maintaining the strength of the coating. Amorphous carbon can be a carbide of at least one selected from the group consisting of tar, pitch, and other organic materials, or can be a carbon-based material formed by using hydrocarbons as a chemical vapor deposition source.

[0045] Carbides of other organic materials may be carbides of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose or ketohexose, and carbides of organic materials selected from combinations of the above.

[0046] The hydrocarbon may be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or a substituted or unsubstituted aromatic hydrocarbon. The aliphatic or alicyclic hydrocarbon of the substituted or unsubstituted aliphatic or alicyclic hydrocarbon may include methane, ethane, ethylene, acetylene, propane, butane, butene, pentane, isobutane or hexane. The substituted or unsubstituted aromatic hydrocarbon may include benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene or phenanthrene.

[0047] The silicon-based active material may have an average particle size (D 50 ) of 0.1 μm to 20 μm, specifically 1 μm to 10 μm. When the average particle size satisfies the above range, side reactions between SiO x (0 ≤ x < 2) and the electrolyte solution can be suppressed, and the reduction of the initial efficiency can be prevented by controlling the lithium silicate formation reaction from SiO x (0 ≤ x < 2), and the initial capacity of the battery can be maximized.

[0048] The negative electrode active material may further include a carbon-based active material. The carbon-based active material may include at least one selected from the group consisting of artificial graphite, natural graphite and graphitized mesophase carbon microspheres. Specifically, the carbon-based active material may be artificial graphite in view of the fact that artificial graphite can effectively control the volume expansion of the negative electrode while maintaining the electrical network with the carbon nanotube structure to be described later, but the carbon-based active material is not limited thereto.

[0049] The weight ratio of the silicon-based active material to the carbon-based active material may be in the range of 0.5:99.5 to 20:80, specifically in the range of 1:99 to 10:90. When the above range is satisfied, the capacity of the battery can be increased while suppressing the excessive volume expansion of the silicon-carbon composite particles.

[0050] The negative electrode active material may be included in the negative electrode active material layer in an amount of 90% by weight to 99% by weight, specifically 95% by weight to 99% by weight. When the above range is satisfied, the energy density of the negative electrode can be highly maintained, and the conductivity and adhesion of the negative electrode can be improved.

[0051] The conductive agent may include a carbon nanotube structure.

[0052] The carbon nanotube structure may include a plurality of single-walled carbon nanotube units. Specifically, the carbon nanotube structure may be a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side with each other, and more specifically, the carbon nanotube structure may be a carbon nanotube structure in which 2 to 4,500 single-walled carbon nanotube units are bonded with each other. Even more specifically, considering the dispersibility of the carbon nanotube structure and the durability of the electrode, it is most desirable that the carbon nanotube structure is a carbon nanotube structure in which 2 to 50 single-walled carbon nanotube units are bonded with each other.

[0053] In the carbon nanotube structure, the single-walled carbon nanotube units may be arranged side by side and bonded (wherein the long axes of the units are bonded parallel to each other to have a flexible cylindrical structure) to form the carbon nanotube structure. The carbon nanotube structures may be connected to each other to exhibit a network structure in the electrode.

[0054] Conventional electrodes including carbon nanotubes are generally prepared by dispersing bundle type or entangled type carbon nanotubes (in the form of single-walled carbon nanotube units or multi-walled carbon nanotube units attached or entangled with each other) in a dispersion medium to prepare a conductive agent dispersion, and then using the conductive agent dispersion. In this case, the carbon nanotubes are completely dispersed in a typical conductive agent dispersion, and thus exist as a conductive agent dispersion in which single-chain form carbon nanotube units are dispersed. In this case, in the conventional conductive agent dispersion, the carbon nanotube units are easily cut by an excessive dispersion process, such that the length of the carbon nanotube units is less than the initial length. In addition, the carbon nanotube units are also likely to be cut during the roll pressing process of the negative electrode, and there is another limitation in which, during battery operation, the carbon nanotube units (especially single-walled carbon nanotube units) are cut due to the excessive volume change of the silicon-based active material. Therefore, the conductivity of the negative electrode may be reduced, thereby deteriorating the life characteristics of the battery. In addition, for multi-walled carbon nanotube units, due to the mechanism of node growth (not a smooth linear shape, but having nodes due to defects generated during the growth process), the structural defects are very high. Therefore, during the dispersion process, the multi-walled carbon nanotube units are more easily cut, and the cut short multi-walled carbon nanotube units may aggregate with each other via the π-π stacking of the carbon of the unit. Therefore, it is difficult to disperse the multi-walled carbon nanotube units more uniformly and exist in the electrode paste.

[0055] Alternatively, regarding the carbon nanotube structure included in the negative electrode of the present invention, since it is in the form of a rope in which 2 to 5,000 single-walled carbon nanotube units are joined side by side with each other and relatively maintain a high crystallinity without structural defects, even if the volume change of the silicon-carbon composite particles is too large, it can well maintain its length without being cut, and thus the conductivity of the electrode can be maintained. In addition, due to the high conductivity of the single-walled carbon nanotube units with high crystallinity, the conductivity of the electrode increases, and thus the input characteristics, output characteristics, and life characteristics of the battery can be significantly improved. Furthermore, since the carbon nanotube structures can be connected to each other to have a network structure in the electrode, the occurrence of cracks can be prevented by suppressing the excessive volume change of the silicon-based active material (see Figure 1 A of Figure 1 ), and at the same time, a strong conductive network can be ensured. In addition, even if cracks occur in the silicon-based active material, since the carbon nanotube structure connects the silicon-based active material while passing through the cracks, the conductive network can be maintained (see Figure 1 B of

[0056] ). In addition, since the carbon nanotube structure is not easily broken and can maintain its long shape, the conductive network can be enhanced in the entire negative electrode active material layer (see Figure 1 C of

[0057] ). In addition, the peeling of the silicon-based active material can be suppressed to significantly improve the electrode adhesion.

[0058] In the carbon nanotube structure, the average diameter of the single-walled carbon nanotube units can be 0.5 nm to 10 nm, specifically 1 nm to 9 nm. When the average diameter is satisfied, even if a very small amount of conductive agent is used, it has the effect of maximizing the conductivity in the electrode. When observing the prepared electrode through a transmission electron microscope (TEM), the average diameter corresponds to the average of the diameters of the first 100 single-walled carbon nanotubes with large diameters and the diameters of the last 100 single-walled carbon nanotubes with small diameters. 2 / g to 1,000 m 2 / g, specifically 600 m 2 / g to 800 m 2 / g. When the above ranges are satisfied, since the specific surface area is large, the conduction path in the electrode can be smoothly ensured. Therefore, even when a very small amount of conductive agent is used, it has the effect of maximizing the conductivity in the electrode. The specific surface area of the single-walled carbon nanotube unit can be calculated from the nitrogen adsorption amount at the liquid nitrogen temperature (77K) using BELSORP-mini II of BEL Japan, Inc.

[0059] The average diameter of the carbon nanotube structure can be 2 nm to 200 nm, specifically 5 nm to 150 nm, and more specifically 5 nm to 50 nm. When the above ranges are satisfied, since it is effective for forming a conductive network structure and is beneficial for the connection between active material particles, excellent conductivity can be achieved. When the prepared electrode is observed by scanning electron microscope (SEM), the average length corresponds to the average of the lengths of the top 100 carbon nanotube structures with large lengths and the bottom 100 carbon nanotube structures with small lengths.

[0060] The average length of the carbon nanotube structure can be 1 μm to 100 μm, specifically 5 μm to 50 μm. When the above ranges are satisfied, since it is effective for forming a conductive network structure and is beneficial for the connection between active material particles, excellent conductivity can be achieved. When the prepared electrode is observed by SEM, the average length corresponds to the average of the lengths of the top 100 carbon nanotube structures with long lengths and the bottom 100 carbon nanotube structures with short lengths.

[0061] The carbon nanotube structure can be included in the negative electrode active material layer in an amount of 0.01 wt% to 1.0 wt%, particularly 0.01 wt% to 0.5 wt%, and more particularly 0.08 wt% to 0.3 wt%. When the above ranges are satisfied, since the conduction path of the negative electrode can be ensured, the life characteristics of the battery can be improved while maintaining a low level of electrode resistance. During the preparation of the conductive agent dispersion, in the case where the bundled carbon nanotubes are completely dispersed (as a general dispersion method, dispersion is carried out to separate the single chains of the carbon nanotube units from each other as much as possible), no carbon nanotube structure is formed, or even if a carbon nanotube structure is inadvertently formed, the carbon nanotube structure is only formed in an extremely small amount (for example, 0.0005 wt%). That is, the above amount range may never be achieved by conventional methods.

[0062] Regarding the prior art in which the negative electrode includes multi-walled carbon nanotube units, a large amount (e.g., greater than 0.5 wt%) of multi-walled carbon nanotube units must be used to compensate for the low conductivity of the multi-walled carbon nanotube units. Additionally, in the case of preparing the negative electrode by using a conductive agent dispersion in which single-walled carbon nanotube units are completely dispersed, since the single-walled carbon nanotube units may be cut, the single-walled carbon nanotube units cannot be used in a small amount.

[0063] In contrast, the carbon nanotube structure included in the negative electrode of the present invention has a form in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side with each other. Therefore, even if the volume change of the silicon-based active material is too large, the carbon nanotube structure will not be cut and can well maintain its length. Therefore, the conductivity of the negative electrode can be maintained, and due to the high conductivity of the single-walled carbon nanotube units, the conductivity of the negative electrode can be smoothly ensured. Therefore, even if the amount of the carbon nanotube structure in the negative electrode is very low, the capacity and life characteristics of the battery can be excellent.

[0064] In some cases, the surface of the single-walled carbon nanotube units can be treated by an oxidation treatment or a nitridation treatment to improve the affinity with the dispersant.

[0065] The negative electrode active material layer may further include carboxymethyl cellulose (CMC). Carboxymethyl cellulose can be a material that is included in the conductive agent dispersion required for preparing the negative electrode paste from the beginning and, in addition, can be additionally added to enhance the role of the binder during the preparation of the negative electrode paste.

[0066] The weight average molecular weight of carboxymethyl cellulose can be 50,000 g / mol to 150,000 g / mol, specifically 90,000 g / mol to 110,000 g / mol. In the case of satisfying the above range, since carboxymethyl cellulose can easily penetrate between the single-walled carbon nanotube units in the bundle-type carbon nanotubes, a suitable dispersion of the bundle-type carbon nanotubes can be obtained and the phase stability of the conductive agent dispersion can be improved. Therefore, the conductivity of the prepared negative electrode can be significantly improved, and the input / output characteristics and life performance of the battery can be improved.

[0067] The degree of substitution of carboxymethyl cellulose can range from 0.1 to 3, specifically from 0.5 to 2. The degree of substitution refers to the extent to which the hydroxyl groups of carboxymethyl cellulose are substituted by functional group A. The degree of substitution represents the extent to which the hydroxyl groups in the molecular structure of carboxymethyl cellulose are substituted by functional group A, that is, the extent to which the hydroxyl groups are substituted by functional group A. Specifically, if one of the three hydroxyl groups present in a repeating unit is substituted by functional group A, the degree of substitution of the repeating unit is 1, if all three hydroxyl groups are substituted by functional group A, the degree of substitution of the repeating unit is 3, and if all three hydroxyl groups are unsubstituted, the degree of substitution of the repeating unit is 0. In carboxymethyl cellulose, the extent to which the hydroxyl groups are substituted by functional group A represents the average value of the degree of substitution of each repeating unit. In addition, functional group A may include at least one of -CH 2 CO 2 Na and -CH 2 CO 2 H.

[0068] When carboxymethyl cellulose has the above degree of substitution, since functional group A can interact smoothly with single-walled carbon nanotube units, the dispersibility of the carbon nanotube structure can be improved. Therefore, the electrode adhesion can be further improved and the life characteristics of the battery can be improved.

[0069] The negative electrode active material layer may further include a binder. The binder is used to ensure the adhesion between negative electrode active material particles or between the negative electrode active material and the current collector. Among them, binders commonly used in the art can be used, and there is no particular limitation on their type. The binder may include, for example, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. One kind can be used alone or a mixture of two or more of them can be used.

[0070] The binder can be included in an amount of 10% by weight or less, preferably 0.1% by weight to 5% by weight, based on the total weight of the electrode active material layer. When the amount of the binder satisfies the above range, excellent electrode adhesion can be achieved while minimizing the increase in electrode resistance.

[0071] Method for preparing a negative electrode

[0072] Next, a method for preparing a negative electrode of the present invention will be described.

[0073] The method for preparing a negative electrode of the present invention includes the following steps: preparing a conductive agent dispersion (S1); and forming a negative electrode paste including the conductive agent dispersion and a negative electrode active material (S2), wherein the preparation of the conductive agent dispersion (S1) includes: preparing a mixed solution including a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes (S1-1); and dispersing the bundled single-walled carbon nanotubes by applying a shear force to the mixed solution by means of a homogenizer to form a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side (S1-2), wherein the negative electrode active material includes a silicon-based active material, the silicon-based active material includes SiO x (0 ≤ x < 2), and the carbon nanotube structure may be included in the negative electrode active material layer in an amount of 0.01 wt% to 1.0 wt%. The negative electrode of the above embodiment can be prepared by the above method.

[0074] (1) Preparing a conductive agent dispersion (S1)

[0075] The preparation of the conductive agent dispersion (S1) may include: preparing a mixed solution including a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes (S1-1); and dispersing the bundled single-walled carbon nanotubes by applying a shear force to the mixed solution by means of a homogenizer to form a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side (S1-2).

[0076] In step S1-1, the mixed solution can be prepared by adding bundled carbon nanotubes and a dispersant to the dispersion medium. In the bundled carbon nanotubes, the above single-walled carbon nanotube units are bonded together in the form of a bundle, and the bundled carbon nanotubes generally include 2 or more, substantially 500 or more, for example 5,000 or more single-walled carbon nanotube units.

[0077] The specific surface area of the bundled single-walled carbon nanotubes can be 500 m 2 / g to 1,000 m 2 / g, specifically 600 m 2 / g to 800 m 2 / g. When the above range is satisfied, due to the large specific surface area, the conduction path in the electrode can be smoothly ensured, so that even if a very small amount of conductive agent is used, it has the effect of maximizing the conductivity in the electrode.

[0078] The bundled single-walled carbon nanotubes may be included in the mixed solution in an amount of 0.1 wt% to 1.0 wt%, specifically 0.2 wt% to 0.5 wt%. When the above range is satisfied, since the bundled single-walled carbon nanotubes are dispersed at an appropriate level, an appropriate level of carbon nanotube structure can be formed, and the dispersion stability can be improved.

[0079] The dispersion medium may include, for example: water (H 2 O); amide-based polar organic solvents such as dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropanol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol; diols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexanediol; polyols such as glycerol, trimethylolpropane, pentaerythritol, or sorbitol; ethylene glycol ethers such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, or tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl isopropyl ketone, or cyclopentanone; esters such as ethyl acetate, γ-butyrolactone, and ε-propiolactone, and any one of them or a mixture of two or more of them may be used, but the present invention is not limited thereto. More specifically, the dispersion medium may be N-methylpyrrolidone (NMP).

[0080] The dispersant may include at least one selected from the group consisting of hydrogenated nitrile rubber, polyvinylidene fluoride, polystyrene, polyvinylpyrrolidone, polyvinyl alcohol, pyrene butyric acid, pyrene sulfonic acid, tannic acid, pyrene methylamine, sodium dodecyl sulfate, and carboxymethyl cellulose, and specifically may include at least one of carboxymethyl cellulose, polyvinylidene fluoride, polyvinylpyrrolidone, and hydrogenated nitrile rubber. In particular, the dispersant may include carboxymethyl cellulose. Since the carboxymethyl cellulose is the same as the carboxymethyl cellulose described for the negative electrode in the above embodiment, its description will be omitted.

[0081] The weight ratio of the bundled carbon nanotubes to the dispersant in the conductive agent dispersion may be in the range of 1:0.1 to 1:10, specifically in the range of 1:1 to 1:10. When the above range is satisfied, since the bundled single-walled carbon nanotubes are dispersed at an appropriate level, an appropriate level of carbon nanotube structure can be formed, and the dispersion stability can be improved.

[0082] The solid content in the mixed solution can be in the range of 0.1% by weight to 20% by weight, specifically in the range of 1% by weight to 10% by weight. When the above range is satisfied, since the bundled single-walled carbon nanotubes are dispersed at an appropriate level, an appropriate level of carbon nanotube structure can be formed, and the dispersion stability can be improved. In addition, the negative electrode paste can have a viscosity and elasticity suitable for the electrode preparation process, and also helps to increase the solid content of the negative electrode paste.

[0083] In step S1-2, the mixed solution can be stirred by a homogenizer, and in this process, the bundled single-walled carbon nanotubes can be dispersed to form a carbon nanotube structure. In the carbon nanotube structure, 2 to 5,000 single-walled carbon nanotube units are bonded side by side, and the carbon nanotube structure is the same as the carbon nanotube structure described for the negative electrode in the above embodiment.

[0084] The homogenizer can include a first nozzle and a second nozzle. When pressure is applied to the mixed solution, the mixed solution passes through the first nozzle and the second nozzle in sequence. Since the diameter of the second nozzle is smaller than that of the first nozzle, the mixed solution is subjected to a shear force when passing through the nozzles, and in this case, the bundled single-walled carbon nanotubes are dispersed.

[0085] The diameter of the first nozzle can be in the range of 100 mm to 500 mm, specifically 150 mm to 300 mm, and more specifically 150 mm to 250 mm. The diameter of the second nozzle can be in the range of 100 μm to 1,000 μm, specifically 200 μm to 800 μm, and more specifically 200 μm to 650 μm. In addition, the pressure can be in the range of 500 Bar to 1,800 Bar, specifically 500 Bar to 1,600 Bar, and more specifically 800 Bar to 1,600 Bar. If the pressure is 1800 Bar or higher, since the bundled single-walled carbon nanotubes are completely dispersed, it may not be possible to smoothly form a carbon nanotube structure.

[0086] In step S1, different from the conventional method of completely dispersing bundled single-walled carbon nanotubes, the bundled single-walled carbon nanotubes are not completely dispersed, but by appropriately combining conditions, such as the conditions of using a homogenizer (nozzle size, pressure, etc.), the physical properties of the bundled single-walled carbon nanotubes used, and the dispersant used, they are dispersed at an appropriate level. Therefore, in the formed conductive agent dispersion, there is no or almost no single-walled carbon nanotube unit existing independently in a single-chain form, and most of the bundled single-walled carbon nanotubes can exist as the above carbon nanotube structure.

[0087] (2) Forming a negative electrode paste (S2) including a conductive agent dispersion and a negative electrode active material

[0088] When preparing the conductive agent dispersion through the above process, the negative electrode active material is mixed with the conductive agent dispersion to form a negative electrode paste. In this case, the above negative electrode active material can be used as the negative electrode active material.

[0089] In addition, if necessary, the negative electrode paste may further include a binder and a solvent. In this case, the binder of the above embodiment can be used as the binder. The solvent may include, for example, amide-based polar organic solvents such as dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropanol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol; diols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexanediol; polyols such as glycerol, trimethylolpropane, pentaerythritol, or sorbitol; ethylene glycol ethers such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, or tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl isopropyl ketone, or cyclopentanone; esters such as ethyl acetate, γ-butyrolactone, and ε-propiolactone. Any one of them or a mixture of two or more of them can be used, but the present invention is not limited thereto. The solvent may be the same as or different from the dispersion medium used in the pre-dispersion, and N-methylpyrrolidone (NMP) is preferably used.

[0090] The negative electrode active material includes a silicon-based active material, and the silicon-based active material may include SiO x (0 ≤ x < 2). The negative electrode active material is the same as the negative electrode active material described for the negative electrode in the above embodiment.

[0091] The negative electrode active material may be included in an amount of 70% to 99.5% by weight, preferably 80% to 90% by weight, based on the total solid content in the negative electrode paste. When the amount of the negative electrode active material satisfies the above range, excellent energy density, electrode adhesion, and conductivity can be achieved.

[0092] In addition, in the case of further including a binder, the binder may be included in an amount of 10% or less, specifically 0.1% to 5% by weight, based on the total solid content in the negative electrode paste.

[0093] The solid content in the negative electrode paste can be in the range of 40% by weight to 80% by weight, specifically in the range of 40% by weight to 60% by weight. When the above range is satisfied, during the drying period after the negative electrode paste is coated, migration of the conductive agent and the binder due to evaporation of the solvent can be suppressed, and a negative electrode having excellent electrode adhesiveness and conductivity can be prepared. In addition, a high-quality electrode with less deformation during rolling can be prepared.

[0094] The carbon nanotube structure can be included in an amount of 0.01% by weight to 1.0% by weight, specifically 0.01% by weight to 0.5% by weight, in the solid content of the negative electrode paste. When the above range is satisfied, since the conduction path of the electrode can be ensured, the life characteristics of the battery can be improved while maintaining the electrode resistance at a low level.

[0095] Next, a negative electrode active material layer is formed by drying the negative electrode paste prepared as described above. Specifically, the negative electrode active material layer can be formed by coating the electrode current collector with the negative electrode paste and then drying the coated current collector, or the negative electrode active material layer can be formed by casting the negative electrode paste on a separate support and then laminating the film separated from the support onto the negative electrode current collector. If necessary, after the negative electrode active material layer is formed by the above method, a rolling process can be further performed. In this case, considering the physical properties of the finally obtained electrode, drying and rolling can be performed under appropriate conditions, and there is no particular limitation. The carbon nanotube structure can be included in the negative electrode active material layer in an amount of 0.01% by weight to 1.0% by weight.

[0096] <Secondary battery>

[0097] Next, a secondary battery according to another embodiment of the present invention will be described.

[0098] The secondary battery according to another embodiment of the present invention may include the negative electrode of the above embodiment.

[0099] Specifically, the secondary battery may include a negative electrode, a positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, wherein the negative electrode is the same as the negative electrode of the above embodiment. Since the negative electrode has been described above, its detailed description will be omitted.

[0100] The positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including a positive electrode active material.

[0101] In the positive electrode, there is no particular limitation on the positive electrode current collector as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, or silver can be used. In addition, the positive electrode current collector generally can have a thickness of 3 μm to 500 μm and can have a surface with fine roughness to improve the adhesion to the positive electrode active material. The positive electrode current collector can be used in various shapes such as films, sheets, foils, meshes, porous bodies, foams, non-woven fabric bodies, etc.

[0102] The positive electrode active material can be a commonly used positive electrode active material. Specifically, the positive electrode active material can include layered compounds such as lithium cobalt oxide (LiCoO 2 ) or lithium nickel oxide (LiNiO 2 ) or compounds substituted by one or more transition metals; lithium iron oxide such as LiFe 3 O 4 ; lithium manganese oxide such as Li 1+c1 Mn 2-c1 O 4 (0 ≤ c1 ≤ 0.33), LiMnO 3 , LiMn 2 O 3 , and LiMnO 2 ; lithium copper oxide (Li 2 CuO 2 ); vanadium oxides such as LiV 3 O 8 , V 2 O 5 , and Cu 2 V 2 O 7 ; Ni-site type lithium nickel oxide represented by the chemical formula LiNi 1-c2 M c2 O 2 (where M is at least one selected from the group consisting of cobalt (Co), manganese (Mn), aluminum (Al), copper (Cu), iron (Fe), magnesium (Mg), boron (B), and gallium (Ga), and c2 satisfies 0.01 ≤ c2 ≤ 0.3); LiMn represented by the chemical formula 2-c3 M c3 O 2 (where M is at least one selected from the group consisting of Co, Ni, Fe, chromium (Cr), zinc (Zn), and tantalum (Ta), and c3 satisfies 0.01 ≤ c3 ≤ 0.1) or Li 2 Mn 3 MO 8A lithium manganese composite oxide represented by (wherein M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); and having Li partially substituted by an alkaline earth metal ion in LiMn 2 O 4 , but the positive electrode active material is not limited thereto. The positive electrode may be metallic lithium.

[0103] The positive electrode active material layer may include a positive electrode conductive agent, a positive electrode binder, and the above positive electrode active material.

[0104] In this case, the positive electrode conductive agent is used to provide conductivity to the electrode. Any conductive agent can be used without particular limitation as long as it has electronic conductivity and does not cause adverse chemical changes in the battery. Specific examples of the positive electrode conductive agent may be: graphite, such as natural graphite and artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal carbon black, and carbon fiber; metal powders, such as copper powder, nickel powder, aluminum powder, and silver powder, or metal fibers; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxides; or conductive polymers, such as derivatives of polyphenylene. One kind can be used alone or a mixture of two or more of them can be used.

[0105] Furthermore, the positive electrode binder is used to improve the binding between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples of the positive electrode binder may be: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. One kind can be used alone or a mixture of two or more of them can be used.

[0106] The separator separates the negative electrode and the positive electrode and provides a transmission path for lithium ions. Any separator can be used as the separator without particular limitation as long as it is commonly used in secondary batteries. In particular, a separator having a high moisture retention capacity for the electrolyte and a low resistance to the transmission of electrolyte ions can be used. Specifically, a porous polymer membrane can be used, for example, a porous polymer membrane made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure having two or more layers thereof can be used. In addition, a typical porous non-woven fabric can be used, for example, a non-woven fabric formed of high melting point glass fiber or polyethylene terephthalate fiber. In addition, a coated separator including a ceramic component or a polymer component can be used to ensure heat resistance or mechanical strength, and a separator having a single-layer or multi-layer structure can be selectively used.

[0107] The electrolyte may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten-type inorganic electrolyte that can be used in the preparation of a lithium secondary battery, but the present invention is not limited thereto.

[0108] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0109] Examples of the non-aqueous organic solvent may be aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetra-hydroxyl franc, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0110] In particular, cyclic carbonates (ethylene carbonate and propylene carbonate) in the carbonate-based organic solvents dissociate the lithium salt in the electrolyte solution well due to their high dielectric constant as high-viscosity organic solvents, so cyclic carbonates can be preferably used. Since an electrolyte solution having high conductivity can be prepared when the cyclic carbonate is mixed with a linear carbonate such as dimethyl carbonate and diethyl carbonate having low viscosity and low dielectric constant in an appropriate ratio, the cyclic carbonate can be more preferably used.

[0111] A lithium salt can be used as the metal salt, and the lithium salt is a material that is easily soluble in a non-aqueous electrolyte solution. Among them, for example, as the anion of the lithium salt, any one selected from the group consisting of the following can be used: F - , Cl - , I - , NO 3 - , N(CN) 2 - , BF 4 - , ClO 4 - , PF 6 - , (CF 3 ) 2 PF 4 - , (CF 3 ) 3 PF 3 - , (CF 3 ) 4 PF 2 - , (CF 3 ) 5 PF - , (CF 3 ) 6 , P - , CF 3 , SO 3 - , CF 3 , CF 2 , SO 3 - , (CF 3 , SO 2 ) 2 , N - , (FSO 2 ) 2 , N - , CF 3 , CF 2 , (CF 3 ) 2 , CO - , (CF 3 , SO 2 ) 2 , CH - , (SF 5 ) 3 , C - , (CF 3 , SO 2 ) 3 , C - , CF 3 , (CF2 ) 7 SO 3 - CF 3 CO 2 - , CH 3 CO 2 - 、SCN - and (CF 3 CF 2 SO 2 ) 2 N - .

[0112] In order to improve the life characteristics of the battery, prevent the reduction of the battery capacity, and improve the discharge capacity of the battery, in addition to the above-mentioned electrolyte components, at least one additive may be further included in the electrolyte, for example, a halogenated alkylene carbonate compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum chloride.

[0113] According to another embodiment of the present invention, a battery module including the secondary battery as a unit cell and a battery pack including the battery module are provided. Since the battery module and the battery pack include secondary batteries having high capacity, high rate performance, and high cycle characteristics, the battery module and the battery pack can be used as a power source for medium and large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0114] Hereinafter, the present invention will be described in more detail according to specific examples.

[0115] Preparation Example 1: Preparation of Conductive Agent Dispersion

[0116] 0.4 parts by weight of bundled carbon nanotubes (specific surface area 650 m 2(g) and 0.6 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) were mixed in 99.0 parts by weight of N-methylpyrrolidone (NMP) as a dispersion medium to prepare a mixed solution such that the solid content was 1.0% by weight. After adding the mixed solution to a homogenizer, a pressure of 1,000 Bar was applied to the mixed solution so that the mixed solution passed successively through a first nozzle with a diameter of 200 mm and a second nozzle with a diameter of 500 μm.

[0117] The conductive agent dispersion prepared in this way includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side with each other. In the conductive agent dispersion, the amount of the carbon nanotube structure is 0.4% by weight, and the amount of carboxymethyl cellulose is 0.6% by weight.

[0118] Preparation Example 2: Preparation of Conductive Agent Dispersion

[0119] A conductive agent dispersion was prepared in the same manner as in Preparation Example 1, except that: in Preparation Example 1, the weight average molecular weight of carboxymethyl cellulose was changed to 400,000 g / mol (degree of substitution: 1.0), and in the homogenizer, a pressure of 500 Bar was applied to the mixed solution so that the mixed solution passed successively through a first nozzle with a diameter of 300 mm and a second nozzle with a diameter of 800 μm. In the conductive agent dispersion, the amount of the carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side with each other is 0.4% by weight, and the amount of carboxymethyl cellulose is 0.6% by weight.

[0120] Preparation Example 3: Preparation of Conductive Agent Dispersion

[0121] A conductive agent dispersion was prepared in the same manner as in Preparation Example 1, except that: in Preparation Example 1, the weight average molecular weight of carboxymethyl cellulose was changed to 100,000 g / mol (degree of substitution: 1.0), and in the homogenizer, a pressure of 2,000 Bar was applied to the mixed solution so that the mixed solution passed successively through a first nozzle with a diameter of 200 mm and a second nozzle with a diameter of 500 μm. In the conductive agent dispersion, the amount of single-walled carbon nanotube units is 0.2% by weight, and the amount of carboxymethyl cellulose is 1.2% by weight. The carbon nanotube structure shown in Preparation Examples 1 and 2 was not detected.

[0122] Preparation Example 4: Preparation of Conductive Agent Dispersion

[0123] 4.0 parts by weight of a bundle-type carbon nanotube composed of multi-walled carbon nanotube units with an average diameter of 10 nm and an average length of 1 μm (specific surface area: 185 m 2 / g) and 0.6 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) were mixed in 99.0 parts by weight of N-methylpyrrolidone (NMP) as a dispersion medium to prepare a mixed solution such that the solid content was 5.2% by weight. After adding the mixed solution to a homogenizer, a pressure of 500 Bar was applied to the mixed solution so that the mixed solution passed successively through a first nozzle with a diameter of 300 mm and a second nozzle with a diameter of 800 μm. In the conductive agent dispersion, the amount of multi-walled carbon nanotube units (average diameter: 10 nm) was 0.4% by weight, and the amount of carboxymethyl cellulose was 0.6% by weight. No carbon nanotube structures shown in Preparation Examples 1 and 2 were detected.

[0124] Examples and comparative examples

[0125] Example 1: Preparation of the negative electrode

[0126] A silicon-based active material and artificial graphite were used as the negative electrode active materials. The silicon-based active material consisted of SiO and a carbon coating provided on the SiO, and the average particle diameter (D 50 ) was 6.6 μm. The average particle diameter (D 50 ) of the artificial graphite was 21 μm. The negative electrode active materials, styrene-butadiene rubber (SBR) as a binder, carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0), and the conductive agent dispersion of Preparation Example 1 were mixed to prepare a negative electrode slurry (solvent: H 2 O). The weight ratio of the negative electrode active material particles, the binder, and the carbon nanotube structure was 96.75:3.2:0.05. The weight ratio of SiO to artificial graphite was 5:95, and the weight ratio of SBR to CMC was 2.0:1.2.

[0127] The negative electrode slurry was coated on a 20-μm-thick copper (Cu) metal film as the negative electrode current collector at a loading amount of 160 mg / 25 cm 2 and dried. In this case, the temperature of the circulating air was 70°C. Subsequently, the negative electrode current collector coated with the slurry and dried was roll-pressed and dried in a vacuum oven at 130°C for 8 hours to prepare a negative electrode including a negative electrode active material layer (see A of Figure 2 and A of Figure 3 ).

[0128] The carbon nanotube structure was included in the negative electrode active material layer in an amount of 0.05% by weight.

[0129] Example 2: Preparation of the negative electrode

[0130] The negative electrode was prepared in the same manner as in Example 1, except that: the negative electrode active material, binder, and carbon nanotube structure were adjusted such that the weight ratio of the negative electrode active material, binder, and carbon nanotube structure in Example 1 was 96.7:3.2:0.1.

[0131] Example 3: Preparation of Negative Electrode

[0132] The negative electrode was prepared in the same manner as in Example 1, except that: in Example 1, the conductive agent dispersion of Preparation Example 2 was used instead of the conductive agent dispersion of Preparation Example 1 (see Figure 3 ) of B).

[0133] Comparative Example 1: Preparation of Negative Electrode

[0134] The negative electrode was prepared in the same manner as in Example 1, except that: in Example 1, the conductive agent dispersion of Preparation Example 3 was used instead of the conductive agent dispersion of Preparation Example 1 (see Figure 4 ).

[0135] Comparative Example 2: Preparation of Negative Electrode

[0136] The negative electrode was prepared in the same manner as in Example 1, except that: the conductive agent dispersion of Preparation Example 4 was used instead of the conductive agent dispersion of Preparation Example 1, and the weight ratio of the negative electrode active material, binder, and multi-walled carbon nanotube unit in Example 1 was 95.8:3.2:1.0 (see Figure 2 ) of B).

[0137] Comparative Example 3: Preparation of Negative Electrode

[0138] (1) Preparation of Carbon Black Dispersion

[0139] 0.4 parts by weight of carbon black (Imerys Graphite&Carbon, Super C65) with an average particle size of 35 nm and 0.6 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) were mixed in 99.0 parts by weight of water as a dispersion medium to prepare a mixed solution such that the solid content was 1.0% by weight. After adding the mixed solution to a homogenizer, a pressure of 500 Bar was applied to the mixed solution so that the mixed solution passed successively through a first nozzle with a diameter of 300 mm and a second nozzle with a diameter of 800 μm. In the conductive agent dispersion, the amount of carbon black was 0.4% by weight, and the amount of carboxymethyl cellulose was 0.6% by weight.

[0140] (2) Preparation of Negative Electrode

[0141] Thereafter, a negative electrode was prepared in the same manner as in Example 1, except that the above conductive agent dispersion was used instead of the conductive agent dispersion of Preparation Example 1, and the weight ratio of the negative electrode active material, binder, and carbon black was 95.8:3.2:1.0.

[0142] Comparative Example 4: Preparation of Negative Electrode

[0143] A negative electrode was prepared in the same manner as in Example 1, except that the conductive agent dispersion of Preparation Example 4 and the carbon black dispersion used in Comparative Example 3 were used, and the weight ratio of the negative electrode active material, binder, multi-walled carbon nanotube unit, and carbon black was 95.8:3.2:0.15:0.85.

[0144] [Table 1]

[0145]

[0146]

[0147] When the prepared negative electrode was observed by transmission electron microscopy (TEM), the average diameter and average length of the carbon nanotube structure and the average diameter of the carbon nanotube unit corresponded to the average value of the top 100 carbon nanotube structures or single-walled carbon nanotube units having a large diameter or length and the bottom 100 carbon nanotube structures or single-walled carbon nanotube units having a small diameter or length.

[0148] Test Example 1: Observation of the negative electrode

[0149] The negative electrode active material layers of the negative electrodes of Example 1, 3, and Comparative Example 1 were observed with a scanning electron microscope.

[0150] Refer to Figure 2 A of, regarding Example 1, it can be understood that carbon nanotube structures in the form of long ropes with flexibility are well formed into a conductive network in the negative electrode. In contrast, referring to Figure 2 B of, multi-walled carbon nanotube units are formed with short lengths, and carbon nanotube structures in the form of long ropes with flexibility as in Example 1 are not observed.

[0151] In the negative electrode of Comparative Example 1, since single-walled carbon nanotube units with a diameter of 1.5 nm exist in a completely dispersed state, carbon nanotube structures are not observed, and since the single-walled carbon nanotube units exist in a single-chain form, it is difficult to accurately observe the single-walled carbon nanotube units with the resolution of a scanning electron microscope (SEM).

[0152] Refer to Figure 3 A of, carbon nanotube structures in the form of ropes with a diameter of about 10 nm in Example 1 were observed, and referring to Figure 3For B, carbon nanotube structures in the form of ropes with a diameter of about 100 nm in Example 3 were observed.

[0153] The carbon nanotube structures of Examples 1 to 3 (the image of Example 2 is not attached) are in the form of ropes and are connected to each other in the negative electrode active material layer to present a network structure.

[0154] Test Example 2: Evaluation of battery life characteristics

[0155] Using the negative electrodes of Examples 1 to 3 and Comparative Examples 1 to 4, batteries were prepared as follows.

[0156] Li[Ni 0.6 Mn 0.2 Co 0.2 O 2 was used as the positive electrode active material. The positive electrode active material, carbon black as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder were mixed in a weight ratio of 94:4:2 in an N-methyl-2-pyrrolidone solvent to prepare a positive electrode slurry.

[0157] The prepared positive electrode slurry was coated on a 15-μm-thick aluminum metal film as a positive electrode current collector and dried. In this case, the temperature of the circulating air was 110°C. Subsequently, the coated and dried positive electrode current collector was roll-pressed and dried in a vacuum oven at 130°C for 2 hours to form a positive electrode active material layer.

[0158] Each of the negative electrodes of Examples 1 to 3 and Comparative Examples 1 to 4, the above-prepared positive electrode, and a porous polyethylene separator were assembled by using a stacking method, and an electrolyte solution (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1 / 2 (volume ratio), lithium hexafluorophosphate (LiPF 6 1 mol)) was injected into the assembled battery to prepare a lithium secondary battery.

[0159] Charging and discharging of each lithium secondary battery were performed under the following conditions.

[0160] Charging conditions: Charge at a constant current of 0.5C to 4.25V, and then charge at 4.2V until the current flows at a rate of 0.1C

[0161] Discharging conditions: Discharge at a current rate of 0.5C to 2.8V

[0162] When the above charging and discharging were set as 1 cycle, 100 cycles were performed at 45°C. Thereafter, the discharge capacity (capacity retention rate) after 100 cycles was evaluated based on 100% of the discharge capacity after one cycle and is shown in Table 2.

[0163] [Table 2]

[0164]

[0165]

[0166] Referring to Table 1, for Examples 1-3 including a carbon nanotube structure, it can be understood that the lifetime characteristics of the battery are superior to those of the comparative examples. The reason is that the presence of the carbon nanotube structure in the negative electrode can suppress the decrease in the capacity retention rate caused by the volume expansion of the silicon-based active material.

[0167] When comparing Example 1 and Example 3, it can be understood that the case where the diameter of the carbon nanotube structure is 10 nm exhibits a higher battery capacity retention rate compared to the case where the diameter of the carbon nanotube structure is 100 nm.

Claims

1. A negative electrode comprising a negative electrode active material layer, wherein the negative electrode active material layer comprises a negative electrode active material and a conductive agent, wherein the negative electrode active material comprises a silicon-based active material and a carbon-based active material, The silicon-based active material includes SiO x , where 0 ≤ x < 2, the carbon-based active material comprises at least one selected from the group consisting of artificial graphite, natural graphite, and graphitized mesophase carbon microspheres, the conductive agent comprises a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are arranged side by side, and the carbon nanotube structure is included in the negative electrode active material layer in an amount of 0.01 wt% to 1.0 wt%, wherein, in the negative electrode, the carbon nanotube structures are connected to each other to present a network structure, wherein the average diameter of the carbon nanotube structure is 5 nm to 50 nm, wherein the average diameter of the single-walled carbon nanotube unit is 0.5 nm to 9 nm.

2. The negative electrode according to claim 1, wherein, in the carbon nanotube structure, the single-walled carbon nanotube units are arranged and combined side by side.

3. The negative electrode according to claim 1, wherein the average length of the single-walled carbon nanotube unit is 1 μm to 100 μm.

4. The negative electrode according to claim 1, wherein the specific surface area of the single-walled carbon nanotube unit is 500 m 2 / g to 1,000 m 2 / g.

5. The negative electrode according to claim 1, wherein the carbon nanotube structure is included in the negative electrode active material layer in an amount of 0.08 wt% to 0.3 wt%.

6. The negative electrode according to claim 1, wherein the average particle size D of the silicon-based active material 50 is from 0.1 μm to 20 μm.

7. The negative electrode according to claim 1, wherein the silicon-based active material further comprises a carbon coating disposed on the SiO x where 0 ≤ x < 2.

8. The negative electrode according to claim 1, wherein the weight ratio of the silicon-based active material to the carbon-based active material is in the range of 0.5:99.5 to 20:

80.

9. The negative electrode according to claim 1, wherein the negative electrode active material layer further comprises carboxymethyl cellulose.

10. The negative electrode according to claim 9, wherein the weight average molecular weight of the carboxymethyl cellulose is 50,000 g / mol to 150,000 g / mol.

11. The negative electrode according to claim 9, wherein the degree of substitution of the carboxymethyl cellulose is in the range of 0.1 to 3.

12. A method for preparing the negative electrode according to any one of claims 1 to 11, the method comprising the following steps: preparing a conductive agent dispersion S1, and forming a negative electrode slurry S2 comprising the conductive agent dispersion and a negative electrode active material, wherein the preparation S1 of the conductive agent dispersion comprises: preparing a mixed solution S1-1 comprising a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes; and dispersing the bundled single-walled carbon nanotubes by applying a shear force to the mixed solution using a homogenizer to form a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are arranged side by side S1-2, wherein the negative electrode active material comprises a silicon-based active material and a carbon-based active material, The silicon-based active material includes SiO x , where 0 ≤ x < 2, the carbon-based active material comprises at least one selected from the group consisting of artificial graphite, natural graphite, and graphitized mesophase carbon microspheres, and the carbon nanotube structure is included in the negative electrode active material layer in an amount of 0.01 wt% to 1.0 wt%, wherein, in the negative electrode, the carbon nanotube structures are connected to each other to present a network structure, wherein the average diameter of the carbon nanotube structure is 5 nm to 50 nm, The average diameter of the single-walled carbon nanotube units is from 0.5 nm to 9 nm.

13. The method according to claim 12, wherein the dispersant comprises carboxymethyl cellulose, and the weight-average molecular weight of the carboxymethyl cellulose is from 50,000 g / mol to 150,000 g / mol.

14. The method according to claim 12, wherein the specific surface area of the bundle-shaped single-walled carbon nanotubes is 500 m 2 / g to 1,000 m 2 / g.

15. The method according to claim 12, wherein the homogenizer has a first nozzle with a diameter of 100 mm to 500 mm and a second nozzle with a diameter of 100 μm to 1,000 μm.

16. The method according to claim 12, wherein the pressure applied to the mixed solution in the homogenizer is in the range of 500 Bar to 1800 Bar.

17. A secondary battery, comprising: the negative electrode of claim 1; a positive electrode; a separator disposed between the negative electrode and the positive electrode; and an electrolyte.

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