Negative electrode and secondary battery including the same

Abstract

The present invention provides a negative electrode current collector, a first negative electrode active material layer disposed on the negative electrode current collector, and a second negative electrode active material layer disposed on the first negative electrode active material layer, the second negative electrode active material layer including a second negative electrode active material and a second conductive material, the second negative electrode active material including a silicon-based active material, and the silicon-based active material including SiO X (0≦X<2), and the second conductive material comprises a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded in a line; and the carbon nanotube structure is contained in the second negative electrode active material layer in an amount of 0.01 wt % to 1.0 wt %, and a secondary battery including the same.

Description

Technical Field

[0001] [Cross - reference to Related Applications] This application claims the benefit of priority based on Korean Patent Application No. 10 - 2020 - 0048940, filed on April 22, 2020, and all the contents disclosed in the literature of the Korean patent application are incorporated herein by reference in their entirety.

[0002] The present invention includes a negative electrode current collector, a first negative electrode active material layer disposed on the negative electrode current collector, and a second negative electrode active material layer disposed on the first negative electrode active material layer. The second negative electrode active material layer includes a second negative electrode active material and a second conductive material. The second negative electrode active material includes a silicon - based active material, and the silicon - based active material includes SiO X (0 ≦ X < 2), and the second conductive material includes a carbon nanotube structure in which 2 to 5,000 single - wall carbon nanotube units are arranged and bonded together. The carbon nanotube structure is included in the second negative electrode active material layer at 0.01 wt% to 1.0 wt%. The present invention relates to a negative electrode and a secondary battery including the same.

Background Art

[0003] Recently, with the development of technology and the increasing demand for mobile devices, the demand for batteries as an energy source has increased rapidly, and thus various studies on batteries that can meet various requirements have been conducted. In particular, as a power source for such devices, research on lithium secondary batteries having high energy density while having excellent life and cycle characteristics has been actively carried out.

[0004] A lithium secondary battery means a battery including a positive electrode containing a positive electrode active material capable of inserting / desorbing lithium ions, a negative electrode containing a negative electrode active material capable of inserting / desorbing lithium ions, and a non - aqueous electrolyte containing lithium ions in a positive electrode assembly in which a fine porous separator is interposed between the positive electrode and the negative electrode.

[0005] On the one hand, only the negative electrode active material cannot ensure the conductivity of the negative electrode, so there is a problem that the resistance of the battery is too high. Usually, the negative electrode further includes a conductive material. Conventionally, a dot-like conductive material such as carbon black is mainly used, and in order to further improve the conductivity and improve the capacity of the battery, a linear conductive material such as carbon nanotubes and carbon nanofibers is also used.

[0006] Single-walled carbon nanotubes are one of the linear conductive materials, and their elongated shape improves the conductivity in the negative electrode active material layer. Thus, conventionally, after manufacturing a negative electrode slurry through a dispersion liquid in which the single-walled carbon nanotubes are completely dispersed, a negative electrode active material layer is manufactured through the negative electrode slurry.

[0007] However, when the battery is repeatedly charged and discharged, the negative electrode active material repeatedly expands and contracts in volume, so that the single-walled carbon nanotubes are cut, and there is a problem that it is difficult to maintain the conductive network in the negative electrode active material layer. In particular, when a silicon-based active material is used as the negative electrode active material to improve the capacity of the battery, the volume of the silicon-based active material expands too much due to battery charging and discharging, and the cutting phenomenon of the single-walled carbon nanotubes occurs more severely. As a result, the conductive network is blocked or reduced, which deteriorates the life characteristics of the battery. In addition, since the single-walled carbon nanotubes exist surrounding the surface of the silicon-based active material, they cannot smoothly play a role of electrically connecting adjacent negative electrode active materials to each other.

[0008] On the one hand, when using carbon nanotubes as a conductive material, in order to uniformly arrange the carbon nanotubes within the negative electrode active material layer, a carbon nanotube dispersion with a low solid content must be used. However, when using carbon nanotubes with a low solid content, a migration phenomenon occurs where the binder and the conductive material, which are relatively low in density compared to the negative electrode active material during drying of the negative electrode, easily move to the upper layer portion of the negative electrode active material layer (in the direction away from the current collector), resulting in a significant reduction in the negative electrode adhesion and electrical conductivity.

[0009] Therefore, the present invention introduces a negative electrode that can connect a conductive network even with a large volume change of the negative electrode active material and can minimize the problems caused by the inclination phenomenon of the binder.

Summary of the Invention

Problems to be Solved by the Invention

[0010] One problem to be solved by the present invention is to provide a negative electrode that can improve the input / output characteristics and life characteristics by minimizing the problems caused by the inclination phenomenon of the binder while smoothly maintaining the conductive network.

[0011] Another problem to be solved by the present invention is to provide a secondary battery including the negative electrode.

Means for Solving the Problems

[0012] According to an embodiment of the present invention, it includes a negative electrode current collector, a first negative electrode active material layer disposed on the negative electrode current collector, and a second negative electrode active material layer disposed on the first negative electrode active material layer. The second negative electrode active material layer includes a second negative electrode active material and a second conductive material. The second negative electrode active material includes a silicon-based active material, and the silicon-based active material is SiO XIt includes (0≤X<2), and the second conductive material includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are arranged and bonded in parallel; the carbon nanotube structure provides a negative electrode contained in the second negative electrode active material layer at 0.01 wt% to 1.0 wt%.

[0013] According to another embodiment of the present invention, a secondary battery including the negative electrode is provided.

Effect of the Invention

[0014] The negative electrode according to the present invention includes a long rope-shaped carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are arranged and bonded in parallel in the second negative electrode active material layer during rolling. Therefore, the carbon nanotube structure can connect the second negative electrode active material even to a large volume change of SiO X (0≤X<2) and hold them strongly together, and SiO X (0≤X<2) can be prevented from being damaged (for example, cracked). Further, since the negative electrode has a first negative electrode active material layer and a second negative electrode active material layer sequentially arranged as respective slurries, the inclination phenomenon of the binder and the conductive material described above can be minimized. Furthermore, since the second negative electrode active material layer includes a carbon nanotube structure, the adhesive force between the first negative electrode active material layer and the second negative electrode active material layer can be strengthened. Thereby, the input / output characteristics and the life characteristics of the battery can be improved.

Brief Description of the Drawings

[0015]

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Mode for Carrying Out the Invention

[0016] The terms and words used in this specification and the claims should not be construed as being limited to their ordinary and dictionary meanings. In accordance with the principle that the inventor himself / herself can appropriately define the concept of the terms in order to explain the invention in the best way, they must be construed in a meaning and concept conforming to the technical idea of the present invention.

[0017] The terms used in this specification are merely used to explain exemplary embodiments and are not intended to limit the present invention. Singular expressions may include plural expressions unless the context clearly indicates a different meaning.

[0018] In this specification, terms such as "comprising," "including," or "having" are intended to specify the presence of implemented features, numbers, steps, components, or combinations thereof, and should be understood not to preclude in advance the presence or addition possibility of one or more other features, numbers, steps, components, or combinations thereof.

[0019] In this specification, "% " means weight % unless otherwise explicitly indicated.

[0020] In this specification, the "specific surface area" is measured by the BET method, and specifically, it can be calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using BEL Japan's BELSORP-mini II.

[0021] In this specification, the average particle diameter (D 50 ) may be defined as the particle diameter corresponding to 50% of the volume cumulative amount in the particle size distribution curve of the particles. The said average particle diameter (D50 ) may be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes ranging from the submicron region to about several millimeters, and results with high reproducibility and high resolution can be obtained.

[0022] In the present invention, a single-walled carbon nanotube unit means a tube-shaped unit having one wall made of carbon atoms, and a multi-walled carbon nanotube unit means a tube-shaped unit having multiple walls made of carbon atoms in one tube.

[0023] Hereinafter, the present invention will be specifically described.

[0024] Negative electrode The negative electrode according to the present invention includes a negative electrode current collector, a first negative electrode active material layer disposed on the negative electrode current collector, and a second negative electrode active material layer disposed on the first negative electrode active material layer. The second negative electrode active material layer includes a second negative electrode active material and a second conductive material. The second negative electrode active material includes a silicon-based active material, and the silicon-based active material includes SiO X (0 ≦ X < 2), and the second conductive material includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are arranged and bonded; the carbon nanotube structure may be contained in the second negative electrode active material layer at 0.01 wt% to 1.0 wt%.

[0025] The negative electrode current collector may be any material that has conductivity without inducing a chemical change in the battery, and is not particularly limited. For example, as the negative electrode current collector, copper, stainless steel, aluminum, nickel, titanium, fired carbon, or a material obtained by surface treatment of the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. may be used. Specifically, a transition metal that adsorbs carbon well, such as copper or nickel, may be used as the negative electrode current collector.

[0026] The negative electrode may include a negative electrode active material layer. The negative electrode active material layer may be disposed on one or both surfaces of the negative electrode current collector.

[0027] The negative electrode active material layer may include a first negative electrode active material layer and a second negative electrode active material layer. The first negative electrode active material layer may be disposed on the negative electrode current collector, specifically, it may be in contact with the negative electrode current collector. The second negative electrode active material layer may be disposed on the first negative electrode active material layer, and the first negative electrode active material layer may be disposed between the second negative electrode active material layer and the negative electrode current collector.

[0028] Generally, when using carbon nanotubes as a conductive material, in order to uniformly arrange the carbon nanotubes in the negative electrode active material layer, a carbon nanotube dispersion with a low solid content must be used. However, when using a carbon nanotube dispersion with a low solid content, during the drying of the negative electrode slurry, a phenomenon (tilting phenomenon, migration) occurs in which the binder and the conductive material, which are relatively low in density compared to the negative electrode active material, easily tilt to the upper layer part of the negative electrode active material layer (far from the negative electrode current collector and close to the surface), resulting in a problem that the negative electrode adhesion and electrical conductivity are significantly reduced. On the other hand, since the negative electrode of the present invention has a first negative electrode active material layer and a second negative electrode active material layer sequentially arranged as respective slurries, the tilting phenomenon of the aforementioned binder and conductive material can be minimized. Thereby, the input / output characteristics and life characteristics of the battery can be improved.

[0029] (1) The first negative electrode active material layer The first negative electrode active material layer may include a first negative electrode active material.

[0030] The first negative electrode active material may be a negative electrode active material generally used in the art, and its type is not particularly limited.

[0031] Specifically, the first negative electrode active material may contain at least one of a carbon-based active material and a silicon-based active material. The carbon-based active material particles may be one or more selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fiber, and graphitized mesocarbon microbeads. In particular, when using artificial graphite, the rate performance can be improved. The silicon-based active material is SiO X (0 ≦ X < 2), Si-C composite, and Si-Y alloy (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a transition metal, a Group 13 element, a Group 14 element, a rare earth element, and combinations thereof), and one or more selected from the group may be used. In particular, when using SiO X (0 ≦ X < 2), the high capacity of the battery can be derived. More specifically, the first negative electrode active material may be a carbon-based active material.

[0032] The first negative electrode active material may be contained in the first negative electrode active material layer at 70% to 99.5% by weight, preferably 80% to 99% by weight. When the content of the first negative electrode active material satisfies the above range, the energy density of the negative electrode can be improved, the negative electrode adhesion can be increased, and the electrical conductivity in the negative electrode can be improved.

[0033] The first negative electrode active material layer may further contain a first conductive material.

[0034] The first conductive material may contain at least one selected from the group consisting of a carbon nanotube structure, a multi-wall carbon nanotube unit, graphene, and carbon black. The carbon nanotube structure will be described in detail below.

[0035] The first conductive material may be contained in the first negative electrode active material layer at 0.01% to 2.0% by weight, specifically 0.01% to 1.5% by weight, and more specifically 0.05% to 1.0% by weight. When the above range is satisfied, even with the application of a small content of the first conductive material, the adhesion and electrical conductivity of the negative electrode can be significantly improved, and a battery with excellent input / output characteristics and life characteristics of the battery can be achieved.

[0036] The thickness of the first negative electrode active material layer may be 1 μm to 100 μm, specifically, 5 μm to 90 μm, and more specifically, 10 μm to 80 μm. When the above range is satisfied, the inclination phenomenon of the above-described conductive material and binder can be minimized. Thereby, the adhesive force and electrical conductivity of the negative electrode can be significantly improved, and the input / output characteristics and life characteristics of the battery can be improved.

[0037] (2) Second negative electrode active material layer The second negative electrode active material layer may include a second negative electrode active material and a second conductive material.

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

[0039] The silicon-based active material may include SiO X (0 ≦ X < 2). The SiO X (0 ≦ X < 2) may specifically be SiO. Since the second negative electrode active material includes SiO X (0 ≦ X < 2), the capacity of the battery can be improved. In particular, since the second negative electrode active material layer rather than the first negative electrode active material layer includes SiO X (0 ≦ X < 2), the durability of the negative electrode is improved, and the impregnation property of the electrolytic solution can be improved. More specifically, at the interface between the negative electrode current collector, where the bonding force in the negative electrode is the weakest, and the negative electrode active material layer, during charging and discharging of the battery, there is a problem that the negative electrode active material is easily detached from the negative electrode due to the shrinkage and expansion of the negative electrode active material. When SiO X (0 ≦ X < 2) is located closer to the negative electrode current collector, the detachment phenomenon is accelerated. Thereby, the durability of the negative electrode is reduced, and the capacity and life characteristics of the battery are deteriorated.

[0040] On the other hand, during rolling in the negative electrode manufacturing process, the density near the surface of the negative electrode becomes very high, and thereby, the impregnation property of the electrolytic solution is reduced. However, when SiO X (0 ≦ X < 2) is located near the surface of the negative electrode, during the initial charging of the battery, SiO X(0 ≦ X < 2), the density of the negative electrode can be reduced to an appropriate level by volume expansion, and the impregnation property of the electrolytic solution can be improved.

[0041] The silicon-based active material is SiO X It may further include a carbon coating layer formed on (0 ≦ X < 2). The carbon coating layer may be disposed on the SiO X (0 ≦ X < 2). The carbon coating layer may be disposed on the SiO X (0 ≦ X < 2) to improve the conductivity of the SiO X (0 ≦ X < 2) and play a role in suppressing excessive volume expansion of the SiO

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

[0043] The crystalline carbon can further improve the conductivity of the negative electrode active material. The crystalline carbon may include at least one selected from the group consisting of fullerene, carbon nanotube, and graphene.

[0044] The amorphous carbon can appropriately maintain the strength of the coating layer and suppress the expansion of the natural graphite. The amorphous carbon may be at least one carbide selected from the group consisting of tar, pitch, and other organic substances, or a carbon-based substance formed using a hydrocarbon as a source in chemical vapor deposition method.

[0045] The carbide of the other organic substances may be a carbide of an organic substance selected from the group consisting of carbide of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose, or ketohexose, and combinations thereof.

[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 be methane, ethane, ethylene, acetylene, propane, butane, butene, pentene, isobutene, or hexene, etc. The aromatic hydrocarbon of 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, etc.

[0047] The average particle size (D 50 ) of the silicon-based active material may be 0.1 μm to 20 μm, specifically, it may be 1 μm to 10 μm. When the above range is satisfied, the side reaction between the SiO X (0 ≦ X < 2) and the electrolyte can be suppressed, the lithium silicate formation reaction from the SiO X (0 ≦ X < 2) can be controlled, the decrease in the initial efficiency can be prevented, and the initial capacity of the battery can be maximized.

[0048] The second negative electrode active material may further contain 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 mesocarbon microbeads. Specifically, together with the carbon nanotube structure described later, in terms of effectively controlling the volume expansion of the negative electrode while maintaining the electrical network, the carbon-based active material is preferably artificial graphite, but is not limited thereto.

[0049] The weight ratio of the silicon-based active material to the carbon-based active material may be 0.5:99.5 to 20:80, specifically, it may be 1:99 to 10:90. When the above range is satisfied, the excessive volume expansion of the second negative electrode active material can be suppressed, and the capacity of the battery can be improved.

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

[0051] The second conductive material may further 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 arranged side by side and bonded to each other. More specifically, considering the durability and conductive network of the second negative electrode active material layer, the carbon nanotube structure may be a carbon nanotube structure in which 2 to 4,500, preferably 2 to 4,000, more preferably 2 to 200 single-walled carbon nanotube units are bonded to each other. Considering the improvement of the dispersibility of the carbon nanotube structure and the durability of the negative electrode, the carbon nanotube structure may be one in which 2 to 50 single-walled carbon nanotube units are arranged side by side and bonded to each other.

[0053] In the carbon nanotube structure, the single-walled carbon nanotube units may be arranged side by side and bonded (the long axes of the units are bonded parallel to each other to form a flexible bundle-shaped cylindrical structure) to form the carbon nanotube structure. In the second negative electrode active material layer, the carbon nanotube structures may be connected to each other to show a network structure.

[0054] Conventional electrodes containing carbon nanotubes are generally manufactured by dispersing bundle-type or entangled-type carbon nanotubes (single-wall carbon nanotube units or multi-wall carbon nanotube units attached to or intertwined with each other) in a dispersion medium to produce a conductive material dispersion liquid, and then using the conductive material dispersion liquid. At this time, in the conventional conductive material dispersion liquid, the carbon nanotubes are completely dispersed and exist as a conductive material dispersion liquid in which single carbon nanotube units are dispersed. The conventional conductive material dispersion liquid causes the carbon nanotube units to be easily cut due to an excessive dispersion process, resulting in a shorter length compared to the initial state. Also, during the rolling process of the negative electrode, the carbon nanotube units may be easily cut, and additional problems may occur where the carbon nanotube units (especially single-wall carbon nanotube units) are cut according to the excessive volume change of the silicon-based active material during battery operation. This leads to a problem of reduced conductivity of the negative electrode and reduced battery life characteristics. Furthermore, in the case of multi-wall carbon nanotube units, the structure has high defects due to the node growth mechanism (nodes exist due to defects generated during the growth process rather than smooth linearity). Therefore, during the dispersion process, the multi-wall carbon nanotube units are more easily cut, and the cut multi-wall carbon nanotube units are likely to aggregate with each other due to the π-π stacking of the carbon in the units. As a result, it is difficult to exist more uniformly dispersed in the negative electrode slurry.

[0055] In contrast, in the case of the carbon nanotube structure included in the second negative electrode active material layer of the present invention, 2 to 5,000 single-wall carbon nanotube units that maintain high crystallinity with relatively few structural defects are arranged side by side and combined with each other to have a rope-like shape. Therefore, the length can be smoothly maintained without being cut even by the volume change of the second negative electrode active material, and the conductivity of the negative electrode can be maintained even during the continuous charge and discharge process of the battery. Further, due to the high electrical conductivity of the single-wall carbon nanotube units having high crystallinity, the conductivity of the negative electrode can be increased, the resistance of the negative electrode can be reduced, and the input / output characteristics and life characteristics of the battery can be significantly improved. Also, within the second negative electrode active material layer that directly receives pressure during rolling, the carbon nanotube structures can be connected to each other to have a network structure, so damage (for example, cracking phenomena such as cracks) to the second negative electrode active material can be suppressed. Also, even if cracks occur in the second negative electrode active material, the carbon nanotube structures connect the second negative electrode active material across the cracks, so a conductive network can be maintained. Furthermore, since the carbon nanotube structures can maintain a long shape without being easily cut, the conductive network can be strengthened throughout the second negative electrode active material layer. Also, the desorption of the second negative electrode active material is suppressed, and the negative electrode adhesion can be significantly improved.

[0056] Also, since the carbon nanotube structure is included in the second negative electrode active material layer, the adhesion between the first negative electrode active material layer and the second negative electrode active material layer can be significantly improved. Due to the long rope-like shape formed by the horizontal bonding of the single-wall carbon nanotube units inside the carbon nanotube structure, the second negative electrode active materials can be well connected to each other by van der Waals forces, and the negative electrode can be firmly constructed. Furthermore, the surfaces of the carbon nanotube structure and the carbon-based active material of the first negative electrode active material layer can be more tightly bonded by π-π bonding (stacking) occurring between the same type of carbon, so the adhesion between the first negative electrode active material layer and the second negative electrode active material layer can be further strengthened.

[0057] In the carbon nanotube structure, the average diameter of the single-wall carbon nanotube unit may be 0.5 nm to 10 nm, specifically, it may be 1 nm to 9 nm. When the average diameter is satisfied, even a very small amount of the content of the conductive material can maximize the conductivity in the negative electrode. The average diameter corresponds to the average value of the 100 single-wall carbon nanotube units with the largest diameters and the 100 single-wall carbon nanotube units with the smallest diameters when the manufactured negative electrode is observed through TEM.

[0058] In the carbon nanotube structure, the average length of the single-wall carbon nanotube unit may be 1 μm to 100 μm, specifically, it may be 5 μm to 50 μm. When the average length is satisfied, a long conductive path for the conductive connection between the second negative electrode active materials can be formed, and a unique network structure can be formed. Therefore, even a very small amount of the content of the conductive material can maximize the conductivity in the negative electrode. The average length corresponds to the average value of the 100 single-wall carbon nanotube units with the largest lengths and the 100 single-wall carbon nanotube units with the smallest lengths when the manufactured negative electrode is observed through TEM.

[0059] The specific surface area of the single-wall carbon nanotube unit is 500 m 2 / g to 1,000 m 2 / g, specifically, it may be 600 m 2 / g to 800 m 2 / g. When the above range is satisfied, a wide specific surface area can ensure a smooth conductive path in the negative electrode. Therefore, even a very small amount of the content of the conductive material can maximize the conductivity in the negative electrode. The specific surface area of the single-wall carbon nanotube unit can be specifically calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mini II of BEL Japan.

[0060] The average diameter of the carbon nanotube structure may be 2 nm to 500 nm, specifically, it may be 5 nm to 200 nm, and more specifically, it may be 5 nm to 50 nm. When the above range is satisfied, it is effective for the formation of a conductive network, advantageous for the connection between the second negative electrode active materials, and can embody excellent electrical conductivity. The average length corresponds to the average value of the diameters of the top 100 carbon nanotube structures with larger diameters and the bottom 100 carbon nanotube structures when the manufactured negative electrode is observed through SEM.

[0061] The average length of the carbon nanotube structure may be 1 μm to 500 μm, specifically, it may be 1 μm to 100 μm, and more specifically, it may be 2 μm to 50 μm. When the above range is satisfied, it is effective for the formation of a conductive network, advantageous for the connection between the second negative electrode active materials, and can embody excellent electrical conductivity. The average length corresponds to the average value of the lengths of the top 100 carbon nanotube structures with larger lengths and the bottom 100 carbon nanotube structures when the manufactured negative electrode is observed through SEM.

[0062] The carbon nanotube structure may be contained in the second negative electrode active material layer at 0.01% by weight to 1.0% by weight, specifically, it may be contained at 0.01% by weight to 0.5% by weight, and more specifically, it may be contained at 0.01% by weight to 0.2% by weight. When the above range is satisfied, a conductive path of the second negative electrode active material layer is ensured, and while maintaining a low level of negative electrode resistance, the life characteristics of the battery can be improved. When manufacturing the conductive material dispersion liquid, when the bundled carbon nanotubes are completely dispersed (a general dispersion method, and maximally dispersed so that single carbon nanotube units are separated from each other), the carbon nanotube structure may not be generated, or even if it is generated unintentionally, it will be generated in a very small amount (for example, 0.0005% by weight). That is, the above content range cannot be achieved by general methods. Since the carbon nanotube structure has a form in which 2 to 5,000 single-walled carbon nanotube units are arranged side by side and bonded to each other, the carbon nanotube structure can smoothly maintain its length without being cut even due to the volume change of the second negative electrode active material. Therefore, the conductive network of the second negative electrode active material layer can be maintained, and due to the high conductivity of the carbon nanotube structure, the conductivity of the second negative electrode active material layer can be smoothly ensured. As a result, even when the content of the carbon nanotube structure in the second negative electrode active material layer is at a low level, there is a possibility of excellent input / output characteristics and life characteristics of the battery.

[0063] On the other hand, in some cases, the single-walled carbon nanotube unit may be surface-treated by oxidation treatment or nitridation treatment or the like in order to improve the affinity with the dispersant.

[0064] The second conductive material may further include at least one selected from the group consisting of fullerene, carbon black, carbon nanotube unit, and graphene. In this case, by the composite application of the substance and the carbon nanotube structure, a one-dimensional linear conductive network can become a two-dimensional or higher-dimensional conductive network, so that the conductivity of the second negative electrode active material layer is improved, and the input / output characteristics and life characteristics of the battery can be improved.

[0065] The thickness of the second negative electrode active material layer may be 1 μm to 100 μm, specifically, 5 μm to 90 μm, and more specifically, 10 μm to 80 μm. When the above range is satisfied, the above-described conductive material and binder inclination phenomenon can be minimized. Thereby, the adhesion of the negative electrode (the adhesion between the negative electrode active material layer and the current collector), the adhesion between the first negative electrode active material layer and the second negative electrode active material layer, and the electrical conductivity of the negative electrode are significantly improved, and the input / output characteristics and life characteristics of the battery can be improved.

[0066] The thickness of the second negative electrode active material layer is preferably equal to or greater than the thickness of the first negative electrode active material layer. The ratio of the thickness of the first negative electrode active material layer to the thickness of the second negative electrode active material layer may be 10:90 to 50:50, specifically, 20:80 to 50:50, and more specifically, 25:75 to 50:50. If the above range is not satisfied, the effect of suppressing the above-described conductive material and binder inclination phenomenon (migration) is reduced, and the effect of improving the diffusion resistance by improving the porosity of the second negative electrode active material layer is reduced. Even when the thickness of the first negative electrode active material layer is outside the above range and is very thin, the effect of suppressing the above-described conductive material and binder inclination phenomenon is reduced, and the effects of improving the negative electrode adhesion and the interfacial resistance are diminished.

[0067] An interface exists between the first negative electrode active material layer and the second negative electrode active material layer. This can be confirmed by the cross-section of the manufactured negative electrode. On the other hand, if the negative electrode active material layer is formed in a single-layer structure (only one application is performed through one negative electrode slurry) instead of a multi-layer structure, the interface is not observed.

[0068] The first negative electrode active material layer and the second negative electrode active material layer may each further contain a binder, and the binder of the first negative electrode active material layer and the binder of the second negative electrode active material layer may be the same or different. The binder may be a general binder used in the art to ensure the adhesive force between the negative electrode active materials or between the negative electrode active material and the current collector, and its type is not particularly limited. Examples of the binder include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof. One of these alone or a mixture of two or more thereof may be used.

[0069] The binder may be contained in the first negative electrode active material layer (or the second negative electrode active material layer) at 10% by weight or less, preferably 0.1% to 5% by weight. When the content of the binder satisfies the above range, excellent negative electrode adhesion can be realized while minimizing the increase in negative electrode resistance.

[0070] Method for manufacturing a negative electrode Next, the method for manufacturing the negative electrode of the present invention will be described.

[0071] The method for manufacturing the negative electrode of the present invention includes the steps of preparing a first negative electrode slurry and a second negative electrode slurry; forming a first negative electrode active material layer on a negative electrode current collector through the first negative electrode slurry; and forming a second negative electrode active material layer on the first negative electrode active material layer through the second negative electrode slurry. The second negative electrode slurry includes a second negative electrode active material and a second conductive material. The second negative electrode active material includes a silicon-based active material, and the silicon-based active material includes SiO X (where 0 ≤ X < 2), and the second conductive material includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are arranged and bonded together. The carbon nanotube structure may be included in the second negative electrode active material layer at 0.01 wt% to 1.0 wt%. The first negative electrode active material layer, the second negative electrode active material layer, the second negative electrode active material, the second conductive material, and the carbon nanotube structure are the same as those in the foregoing embodiments.

[0072] (1) Step of preparing a first negative electrode slurry and a second negative electrode slurry The first negative electrode slurry may be the same as the normal method for manufacturing a negative electrode slurry. For example, after preparing a mixture containing a first negative electrode active material (the same as the first negative electrode active material in the foregoing embodiments), a first conductive material (the same as the first conductive material in the foregoing embodiments), and a solvent (a binder may further be included), the mixture is stirred to prepare the first negative electrode slurry.

[0073] However, when the first negative electrode slurry contains a carbon nanotube structure, a carbon nanotube structure dispersion liquid described later must be prepared.

[0074] Examples of the solvent include amide-based polar organic solvents such as water, dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexylene glycol; polyhydric alcohols such as glycerin, trimethylolpropane, pentaerythritol, or sorbitol; 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 propyl ketone, or cyclopentanone; esters such as ethyl acetate, γ-butyrolactone, and ε-propiolactone. Any one or a mixture of two or more of these may be used, but is not limited thereto. The solvent may be the same as or different from the dispersion medium used in the conductive material dispersion liquid, and preferably may be water.

[0075] The second negative electrode slurry may be prepared by preparing a mixture containing a second negative electrode active material, a carbon nanotube structure body dispersion liquid, and a solvent, and then stirring the mixture.

[0076] The carbon nanotube structure dispersion may be prepared as follows.

[0077] The preparation of the carbon nanotube structure dispersion may include the steps of: preparing a mixed solution containing a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes (aggregates or aggregates of single-walled carbon nanotube units) (S1-1); and applying a shearing force to the mixed solution to disperse the bundled single-walled carbon nanotubes and form a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are arranged and bonded (S1-2).

[0078] In the step S1-1, the mixed solution may be prepared by adding the bundled single-walled carbon nanotubes and the dispersant to the dispersion medium. The bundled single-walled carbon nanotubes are those in which the aforementioned single-walled carbon nanotube units are bonded and exist in a bundle shape, and usually contain 2 or more, substantially 500 or more, for example, 5,000 or more single-walled carbon nanotube units.

[0079] The specific surface area of the bundled single-walled carbon nanotubes may be 500 m 2 / g to 1,200 m 2 / g, specifically, it may be 500 m 2 / g to 1,000 m 2 / g. When the above range is satisfied, a wide specific surface area can smoothly ensure the conductive path in the second negative electrode active material layer, so that even with a very small content of the conductive material, the conductivity in the second negative electrode active material layer can be maximized. In addition, in order to strengthen the adhesion between the first negative electrode active material layer and the second negative electrode active material layer, it is preferably 500 m 2 / g to 800 m 2 / g.

[0080] The bundled single-walled carbon nanotubes may be contained in the mixed solution at 0.1% by weight to 1.0% by weight, specifically, may be contained at 0.2% by weight to 0.5% by weight. When the above range is satisfied, the bundled single-walled carbon nanotubes can be dispersed at an appropriate level, an appropriate level of carbon nanotube structure can be formed, and the dispersion stability can be improved.

[0081] Examples of the dispersion medium include amide-based polar organic solvents such as water, dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropyl alcohol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexylene glycol; polyhydric alcohols such as glycerin, trimethylolpropane, pentaerythritol, or sorbitol; 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 propyl ketone, or cyclopentanone; esters such as ethyl acetate, γ-butyrolactone, and ε-propiolactone. Any one or a mixture of two or more of these may be used, but is not limited thereto. More specifically, the dispersion medium may be the same as or different from the solvent for producing the negative electrode slurry, and preferably may be water.

[0082] The dispersant may include at least one of hydrogenated nitrile butadiene rubber, polyvinylidene fluoride, polystyrene, polyvinylpyrrolidone, polyvinyl alcohol, pyrene butyric acid, pyrene sulfonic acid, tannic acid, pyrene methylamine, sodium dodecyl sulfate, and carboxymethyl cellulose. Specifically, it may be carboxymethyl cellulose, polyvinylidene fluoride, polyvinylpyrrolidone, or hydrogenated nitrile butadiene rubber.

[0083] In the carbon nanotube structure dispersion liquid, the weight ratio of the bundled carbon nanotubes to the dispersant may be 1:0.1 to 1:10. Specifically, it may be 1:1 to 1:10. When the above range is satisfied, the bundled single-walled carbon nanotubes can be dispersed at an appropriate level, a carbon nanotube structure at an appropriate level can be formed, and the dispersion stability can be improved.

[0084] The solid content in the mixed liquid may be 0.1 wt% to 20 wt%. Specifically, it may be 1 wt% to 10 wt%. When the above range is satisfied, the bundled single-walled carbon nanotubes can be dispersed at an appropriate level, a carbon nanotube structure at an appropriate level can be formed, and the dispersion stability can be improved. In addition, the second negative electrode slurry (the slurry for manufacturing the second negative electrode active material layer) can have a viscosity and elasticity suitable for the formation of the second negative electrode active material layer, which also contributes to increasing the solid content of the second negative electrode slurry.

[0085] In the S1-2 stage, the step of dispersing the bundled carbon nanotubes in the mixed solution may be performed using a mixing device such as a homogenizer, bead mill, ball mill, basket mill, attrition mill, universal stirrer, clear mixer, spike mill, TK mixer, or sonification device. Among them, the bead mill method is preferable in that it can precisely control the diameter of the carbon nanotube structure, achieve a uniform distribution of the carbon nanotube structure, and has advantages in terms of cost.

[0086] The bead mill method may be as follows. The mixed solution can be put into a container containing beads or the like, and the container can be rotated to disperse the bundled single-walled carbon nanotubes.

[0087] Here, the conditions under which the bead mill method is performed are as follows.

[0088] The average particle diameter of the beads may be 0.5 mm to 1.5 mm, specifically, it may be 0.5 mm to 1.0 mm. When the above range is satisfied, the carbon nanotube structure during the dispersion process is not cut, the diameter can be appropriately controlled, and a dispersion solution with a uniform composition can be produced.

[0089] The rotation speed of the container may be 500 RPM to 10,000 RPM, specifically, it may be 2,000 RPM to 6,000 RPM. When the above range is satisfied, the carbon nanotube structure during the dispersion process is not cut, the diameter can be appropriately controlled, and a dispersion solution with a uniform composition can be produced.

[0090] The time for performing the bead mill may be from 0.5 hours to 2 hours, specifically, it may be from 0.5 hours to 1.5 hours, and more specifically, it may be from 0.8 hours to 1 hour. When the above range is satisfied, the carbon nanotube structure during the dispersion process can be appropriately controlled in terms of diameter without being cut, and a dispersion solution with a uniform composition can be produced. The bead mill execution time means the total time to which the bead mill is applied. For example, if the bead mill is performed several times, it means the total time over those several times.

[0091] The bead mill conditions are for dispersing the bundled single-walled carbon nanotubes to an appropriate level. Specifically, this excludes the case where the bundled single-walled carbon nanotubes are completely dispersed into a single single-walled carbon nanotube. That is, the bead mill conditions are for forming a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are arranged and bonded to each other in the produced carbon nanotube structure dispersion liquid with the bundled single-walled carbon nanotubes being dispersed to an appropriate level. This can be achieved only when conditions such as the composition of the mixed liquid and the conditions of the dispersion process (for example, the bead mill process) are strictly adjusted.

[0092] A carbon nanotube structure dispersion liquid can be formed by the above process.

[0093] The negative electrode slurries (the first negative electrode slurry and the second negative electrode slurry) may further contain a binder as required. Here, as the binder, the binder of the above-described embodiment may be used.

[0094] (2) Forming a first negative electrode active material layer on the negative electrode current collector via the first negative electrode slurry and forming a second negative electrode active material layer on the first negative electrode active material layer via the second negative electrode slurry Next, a first negative electrode active material layer is formed through the first negative electrode slurry manufactured as described above. Specifically, the first negative electrode active material layer may be formed by a method of applying the first negative electrode slurry onto a negative electrode current collector and then drying it, or by a method of applying the first negative electrode slurry onto a separate support, peeling the film obtained from the support, and laminating it onto the negative electrode current collector. If necessary, after forming the first negative electrode active material layer by the method as described above, a rolling step may be further performed. At this time, drying and rolling may be performed under appropriate conditions in consideration of the physical properties of the negative electrode to be finally manufactured, and are not particularly limited.

[0095] Thereafter, a second negative electrode active material layer is formed through the second negative electrode slurry manufactured as described above. Specifically, the second negative electrode active material layer may be formed by a method of applying the second negative electrode slurry onto the first negative electrode active material layer and then drying it, or by a method of applying the second negative electrode slurry onto a separate support, peeling the film obtained from the support, and laminating it onto the first negative electrode active material layer. If necessary, after forming the second negative electrode active material layer by the method as described above, a rolling step may be further performed. At this time, drying and rolling may be performed under appropriate conditions in consideration of the physical properties of the negative electrode to be finally manufactured, and are not particularly limited.

[0096] Secondary battery Next, a secondary battery according to another embodiment of the present invention will be described.

[0097] The secondary battery according to another embodiment of the present invention may include the negative electrode of the above-described embodiment.

[0098] Specifically, the secondary battery may include the negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and the negative electrode is the same as the negative electrode of the above-described embodiment. Since the negative electrode has been described above, a specific description thereof will be omitted.

[0099] 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 containing the positive electrode active material.

[0100] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity without inducing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or those obtained by surface treatment with carbon, nickel, titanium, silver, etc. on the surface of aluminum or stainless steel may be used. Further, the positive electrode current collector may usually have a thickness of 3 μm to 500 μm, and the adhesion of the positive electrode active material can also be enhanced by forming fine irregularities on the surface of the current collector. For example, it may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabric bodies, etc.

[0101] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material is a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; the chemical formula Li 1+c1 Mn 2-c1 O4 (0 ≦ c1 ≦ 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; the chemical formula LiNi 1-c2 M c2 O2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01 ≦ c2 ≦ 0.3) of Ni-site type lithium nickel oxide; the chemical formula LiMn 2-c3 M c3 O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 0.01 ≦ c3 ≦ 0.1) or lithium manganese composite oxide represented by Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn). Examples include LiMn2O4 in which a part of the Li in the chemical formula is substituted with an alkaline earth metal ion, but it is not limited to these. The positive electrode may be Li-metal.

[0102] The positive electrode active material layer may include a positive electrode conductive material and a positive electrode binder together with the above-described positive electrode active material.

[0103] At this time, the positive electrode conductive material is used to impart conductivity to the electrode, and in the battery to be configured, it can be used without particular limitation as long as it has electronic conductivity without causing a chemical change. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based substances such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. Among these, one alone or a mixture of two or more may be used.

[0104] In addition, the positive electrode binder plays a role in improving the adhesion between positive electrode active material particles and the adhesive force between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof. Among these, one alone or a mixture of two or more may be used.

[0105] The separator membrane separates the negative electrode and the positive electrode and provides a migration path for lithium ions. Usually, any separator membrane that can be used in a secondary battery can be used without particular limitation. In particular, a separator membrane that has low resistance to the ion migration of the electrolyte and excellent moisture retention ability of the electrolyte solution is preferable. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as a polyethylene homopolymer, a polypropylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof may be used. Also, a normal porous nonwoven fabric, for example, a nonwoven fabric made of high-melting glass fibers, polyethylene terephthalate fibers, etc. may be used. Further, a coated separator membrane containing a ceramic component or a polymer substance may be used to ensure heat resistance or mechanical strength, and optionally, it may be used in a single-layer or multilayer structure.

[0106] Examples of the electrolyte include, but are not limited to, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, and a molten inorganic electrolyte that can be used in the manufacture of a lithium secondary battery.

[0107] Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0108] Examples of the non-aqueous organic solvent include aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate ester, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl propionate, ethyl propionate, etc. may be used.

[0109] In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates in the carbonate-based organic solvent, are organic solvents with high viscosity, high dielectric constant, and good dissociation of lithium salts, so they can be preferably used. When such cyclic carbonates are mixed with low-viscosity and low-dielectric-constant linear carbonates such as dimethyl carbonate and diethyl carbonate in an appropriate ratio and used, an electrolyte having high electrical conductivity can be prepared, so they can be more preferably used.

[0110] The metal salt may be a lithium salt. The lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte. For example, as the anion of the lithium salt, F - , Cl - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4-, PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 -, (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - One or more selected from the group consisting of may be used.

[0111] In addition to the constituent components of the electrolyte, the electrolyte may further contain one or more additives such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride for the purpose of improving the life characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery.

[0112] According to still another embodiment of the present invention, there are provided a battery module including the secondary battery as a unit cell and a battery pack including the same. Since the battery module and the battery pack include the secondary battery having high capacity, high-speed characteristics, and cycle characteristics, they may be used as a power source for medium and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0113] Hereinafter, the present invention will be described in more detail with specific examples.

[0114] Production Example 1: Production of Carbon Black Dispersion 0.4 parts by weight of carbon black (Imerys, 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 with 99.0 parts by weight of water as a dispersion medium to prepare a mixed solution with a solid content of 1.0% by weight. The mixed solution was stirred by a beads-mill method to disperse the carbon black in the solvent and produce a carbon black dispersion. At this time, the particle size of the beads was 1 mm, the rotational speed of the stirring vessel containing the beads was 3,000 RPM, and the stirring was carried out for 60 minutes.

[0115] In the carbon black dispersion, the carbon black was 0.4% by weight and the carboxymethyl cellulose was 0.6% by weight.

[0116] Production Example 2: Production of Carbon Nanotube Structure Dispersion 0.4 parts by weight of a bundle-type carbon nanotube composed of single-wall carbon nanotube units with an average diameter of 1.5 nm and an average length of 5 μm or more (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 with 99.0 parts by weight of water as a dispersion medium to prepare a mixture with a solid content of 1.0% by weight.

[0117] The mixture was stirred by a bead-mill method to disperse the bundle-type single-wall carbon nanotubes in the solvent and produce a carbon nanotube structure dispersion. At this time, the particle size of the beads was 1 mm, the rotational speed of the stirring vessel containing the beads was 3,000 RPM, and the stirring was carried out for 60 minutes. The carbon nanotube structure dispersion contained a carbon nanotube structure in which 2 to 5,000 single-wall carbon nanotube units were arranged and bonded.

[0118] In the carbon nanotube structure dispersion liquid, the carbon nanotube structure was 0.4% by weight and the carboxymethyl cellulose was 0.6% by weight.

[0119] Production Example 3: Production of Carbon Nanotube Structure Dispersion Liquid A carbon nanotube structure dispersion liquid was produced in the same manner as in Production Example 2, except that the weight average molecular weight of carboxymethyl cellulose was changed to 400,000 g / mol (degree of substitution: 1.0) in Production Example 2. In the dispersion liquid, the carbon nanotube structure was 0.4% by weight and the carboxymethyl cellulose was 0.6% by weight.

[0120] Production Example 4: Production of Single-Walled Carbon Nanotube Unit Dispersion Liquid 0.2 part by weight of a bundle-type carbon nanotube (specific surface area: 650 m 2 / g) composed of single-walled carbon nanotube units with an average diameter of 1.5 nm and an average length of 5 μm or more and 1.2 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) were mixed with 98.6 parts by weight of water as a dispersion medium to produce a mixture so that the solid content became 1.4% by weight.

[0121] The mixture was stirred by a bead-mill method to disperse bundled single-walled carbon nanotubes in a solvent, thereby producing a single-walled carbon nanotube unit dispersion. At this time, the particle size of the beads was 1 mm, and the rotation speed of the stirring vessel containing the beads was 3,000 RPM. Stirring for 60 minutes under the above conditions was defined as one cycle, and stirring was performed for a total of 4 cycles (natural cooling was performed for 60 minutes between each cycle), thereby producing a single-walled carbon nanotube unit dispersion. In the dispersion, the bundled single-walled carbon nanotubes were completely dispersed, and the single-walled carbon nanotube units existed only in single units, and the above-described carbon nanotube structure was not detected. Also, in the single-walled carbon nanotube unit dispersion, the single-walled carbon nanotube units were 0.2% by weight, and the carboxymethyl cellulose was 1.2% by weight.

[0122] Production Example 5: Production of a multi-walled carbon nanotube unit dispersion A bundled carbon nanotube (specific surface area: 185 m 2 / g) consisting of multi-walled carbon nanotube units with an average diameter of 10 nm and an average length of 1 μm, 4.0 parts by weight, and carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0), 0.6 parts by weight, were mixed with 95.4 parts by weight of water as a dispersion medium to prepare a mixed solution with a solid content of 4.6% by weight.

[0123] The above mixed solution was put into a spike mill filled with 80% beads having a size of 0.65 mm and dispersed, and discharged at a discharge rate of 2 kg / min. Such a process was performed twice to completely disperse the bundled multi-walled carbon nanotubes, thereby producing a multi-walled carbon nanotube unit dispersion. In the dispersion, the multi-walled carbon nanotube units (average diameter: 10 nm) were 4.0% by weight, and the carboxymethyl cellulose was 0.6% by weight.

[0124] Examples and Comparative Examples Example 1: Production of Negative Electrode (1) Formation of First Negative Electrode Active Material Layer The carbon black dispersion of Production Example 1, artificial graphite with an average particle size (D 50 ) of 21 μm, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) were mixed with water to produce a first negative electrode slurry. The negative electrode slurry was applied to a negative electrode current collector (copper (Cu) metal thin film) with a thickness of 20 μm, and then dried at 130°C and rolled to form a first negative electrode active material layer (thickness: 50 μm).

[0125] In the first negative electrode active material layer, the artificial graphite was contained at 94.80% by weight, the SBR was 3.5% by weight, the CMC was 1.2% by weight, and the carbon black was 0.50% by weight.

[0126] (2) Formation of Second Negative Electrode Active Material Layer The carbon nanotube structure dispersion of Production Example 2, negative electrode active material (average particle size (D 50 ) of 21 μm artificial graphite: average particle size (D 50 ) of 6.6 μm SiO weight ratio = 94:6), SBR as a binder, and carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) were mixed with water to produce a second negative electrode slurry. The second negative electrode slurry was applied onto the first negative electrode active material layer, and then dried at 130°C and rolled to form a second negative electrode active material layer (thickness: 50 μm).

[0127] In the second negative electrode active material layer, the negative electrode active material was contained at a total of 96.25% by weight, the SBR was 2.5% by weight, the CMC was 1.2% by weight, and the carbon nanotube structure was 0.05% by weight.

[0128] Example 2: Production of Negative Electrode When forming the second negative electrode active material layer in Example 1, a negative electrode was manufactured in the same manner as in Example 1, except that the carbon nanotube structure body dispersion liquid of Production Example 3 was used instead of the carbon nanotube structure body dispersion liquid of Production Example 2.

[0129] Example 3: Production of Negative Electrode When forming the first negative electrode slurry in Example 1, a negative electrode was manufactured in the same manner as in Example 1, except that the multi-walled carbon nanotube unit body dispersion liquid of Production Example 5 was used instead of the carbon black dispersion liquid of Production Example 1.

[0130] Example 4: Production of Negative Electrode When forming the first negative electrode slurry in Example 1, a negative electrode was manufactured in the same manner as in Example 1, except that the carbon nanotube structure body dispersion liquid of Production Example 2 and the multi-walled carbon nanotube unit body dispersion liquid of Production Example 5 were used instead of the carbon black dispersion liquid of Production Example 1. In the manufactured first negative electrode active material layer, the weight ratio of the carbon nanotube structure body to the multi-walled carbon nanotube was 10:90. In the first negative electrode active material layer, the artificial graphite was 94.80% by weight, the SBR was 3.5% by weight, the CMC was 1.2% by weight, the carbon nanotube structure body was 0.05% by weight, and the multi-walled carbon nanotube unit body was 0.45% by weight.

[0131] Example 5: Production of Negative Electrode When forming the first negative electrode slurry in Example 1, a negative electrode was manufactured in the same manner as in Example 1, except that the carbon nanotube structure body dispersion liquid of Production Example 2 was used instead of the carbon black dispersion liquid of Production Example 1.

[0132] In the first negative electrode active material layer, the artificial graphite is contained at 95.25% by weight, the SBR at 3.5% by weight, the CMC at 1.2% by weight, and the carbon nanotube structure body at 0.05% by weight.

[0133] Comparative Example 1: Production of Negative Electrode The carbon nanotube structure body dispersion liquid of Production Example 2, negative electrode active material (average particle size (D 50) is artificial graphite with an average particle size (D 50 ) is SiO with an average particle size of 6.6 μm (weight ratio = 94:6), SBR as a binder, and carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) were mixed with water to produce a negative electrode slurry. After applying the negative electrode slurry to a negative electrode current collector (copper (Cu) metal thin film) with a thickness of 20 μm, it was dried at 130 °C and rolled to form a negative electrode active material layer (thickness: 100 μm).

[0134] In the negative electrode active material layer, the negative electrode active material is contained at a total of 95.75% by weight, the SBR is 3.0% by weight, the CMC is 1.2% by weight, and the carbon nanotube structure is 0.05% by weight.

[0135] Comparative Example 2: Production of negative electrode A negative electrode was produced in the same manner as in Example 1, except that a single-walled carbon nanotube unit dispersion of Production Example 4 was used instead of the carbon nanotube structure of Production Example 2 when forming the second negative electrode active material layer of Example 1.

[0136] In the second negative electrode active material layer, the negative electrode active material is a total of 96.25% by weight (average particle size (D 50 ) is artificial graphite with an average particle size of 21 μm: average particle size (D 50 ) is SiO with an average particle size of 6.6 μm (weight ratio = 94:6), the SBR is 2.5% by weight, the CMC is 1.2% by weight, and the single-walled carbon nanotube unit is 0.05% by weight.

[0137] Comparative Example 3: Production of negative electrode A negative electrode was produced in the same manner as in Example 1, except that a multi-walled carbon nanotube unit dispersion of Production Example 5 was used instead of the carbon nanotube structure of Production Example 2 when forming the second negative electrode active material layer of Example 1.

[0138] In the second negative electrode active material layer, the negative electrode active material is a total of 95.80% by weight (average particle size (D 50 ) is artificial graphite with an average particle size of 21 μm: average particle size (D 50) is 6.6 μm (SiO weight ratio = 94:6), the SBR is 2.5% by weight, the CMC is 1.2% by weight, and the multi-wall carbon nanotube unit is included at 0.5% by weight.

[0139]

Table 1

[0140] In Examples 1, 3 to 5, and Comparative Example 1, the average diameter of the carbon nanotube structure was 10 nm and the average length was 8.2 μm. In Example 2, the average diameter of the carbon nanotube structure was 100 nm and the average length was 15.6 μm. In Comparative Example 2, the average diameter of the single-wall carbon nanotube unit was 1.6 nm and the average length was 1.8 μm.

[0141] In the negative electrodes of Examples 3 and 4 and Comparative Example 3, the average diameter of the multi-wall carbon nanotube unit was 10.8 nm and the average length was 1.3 μm.

[0142] The average diameter and average length correspond to the average value of the 100 carbon nanotube structures (or multi-wall carbon nanotube units or single-wall carbon nanotube units) with the largest diameter (or length) and the 100 carbon nanotube structures (or multi-wall carbon nanotube units, single-wall carbon nanotube units) when the manufactured negative electrode is observed through TEM.

[0143] Experimental Example 1: Observation of Negative Electrode (1) Binder Distribution Figure 1 is a photograph analyzing the binder distribution of the negative electrode of Comparative Example 1 and the negative electrode of Example 1. The above analysis was performed by the osmium oxide (OsO4) staining method. Specifically, the negative electrode was exposed / left in the Fume atmosphere of osmium oxide in a sealed special chamber prepared in a glove box for 3 days for staining, and then replaced with an Ar atmosphere. One day later, the cross-section of the negative electrode cut by ion milling was analyzed by SEM-EDX for image mapping.

[0144] Figure 1 is an analytical photograph of Comparative Example 1 (left side), where the negative electrode active material layer is formed in a single layer, and Example 1 (right side), where two layers of the first and second negative electrode active material layers are formed. Looking at the right-side photograph, it can be seen that in Example 1, the first and second negative electrode active material layers have different compositions from each other, and it can also be seen that the ratios of the binder distribution for the first and second negative electrode active material layers are also different. In particular, when the first and second negative electrode active material layers are formed respectively as in Example 1, it can be seen that SiO is located only in the second negative electrode active material layer, and it can be seen that the amount of binder in the first negative electrode active material layer is more. Also, it can be seen that the binder distribution changes in the thickness direction of the negative electrode active material layer.

[0145] (2) Confirmation of the presence of the carbon nanotube structure Figure 2 is an SEM photograph of the second negative electrode active material layer of the negative electrode of Example 1, Figure 3 is an SEM photograph of the second negative electrode active material layer of the negative electrode of Example 2, Figure 4 is an SEM photograph of the second negative electrode active material layer of the negative electrode of Comparative Example 3, and Figure 6 is an SEM photograph of the second negative electrode active material layer of the negative electrode of Comparative Example 2.

[0146] Referring to FIGS. 2 and 3, it can be confirmed that there exists a long rope-shaped carbon nanotube structure in which a plurality of single-wall carbon nanotube units are arranged side by side and joined together. In particular, in the negative electrode of Example 1 in FIG. 2, since a hydrogenated nitrile butadiene rubber with a relatively low weight average molecular weight was used, it can be seen that a carbon nanotube structure having an average diameter at the 10 nm level was formed. In the case of FIG. 3, it can be seen that a carbon nanotube structure having an average diameter at the 100 nm level was formed. It can be seen that in both FIGS. 2 and 3, the carbon nanotube structures form a network with each other.

[0147] On the other hand, in FIG. 4, only short multi-wall carbon nanotube units can be seen, and no carbon nanotube structure was observed. Also, in FIG. 6, only single-wall carbon nanotube units existing in single units can be seen, and no carbon nanotube structure was observed.

[0148] (3) Confirmation of the network by the second conductive material FIG. 5 is an SEM photograph of the second negative electrode active material layer of the negative electrode of Example 1. Referring to FIG. 5(A), it can be seen that the cracks in SiO can be suppressed by the carbon nanotube structure. Referring to FIG. 5(B), it can be seen that the carbon nanotube structure maintains the conductive network from being cut on the SiO where cracks have occurred. Referring to FIG. 5(C), it can be seen that due to the long length of the carbon nanotube structure, a long conductive network is formed.

[0149] Experimental Example 2: Evaluation of the negative electrode adhesion The negative electrode adhesion force (the adhesion force between the negative electrode active material layer and the current collector) was measured under dry conditions. Specifically, double-sided tape was attached to a slide glass, and a negative electrode punched into a size of 20 mm × 180 mm was placed thereon and adhered by reciprocating 10 times with a 2 kg roller. Then, using a UTM (TA Corporation) device, the force to peel off from the slide glass was measured while pulling at 200 mm / min. At this time, the measurement angle between the slide glass and the negative electrode was 90°. The measurement results are shown in Table 2 below.

[0150] Experimental Example 3: Evaluation of the Adhesion Force of the First Negative Electrode Active Material and the Second Negative Electrode Active Material For Examples 1 to 4 and Comparative Examples 1 to 3, the shear strength (N / mm 2 ) was measured by the following method. The measurement was performed using a SAICAS (Surface and Interfacial Charaterization Analysis System) device (SAICAS EN-EX, Daipla Wintes Japan). Specifically, while cutting each negative electrode with a blade of micro size made of diamond material with the surface facing inward and tilted, the shear strength was measured by the force applied to the blade at the interface between the first negative electrode active material layer and the second negative electrode active material layer, and this is shown in Table 2.

[0151] Experimental Example 4: Evaluation of Discharge Capacity and Capacity Retention Rate by C-Rate Using the negative electrodes of Examples 1 to 5 and Comparative Examples 1 to 3, batteries were manufactured as follows respectively.

[0152] Li[Ni 0.6 Mn 0.2 Co 0.2 O2 was used as the positive electrode active material. The positive electrode active material, carbon black as the conductive material, and polyvinylidene fluoride (PVdF) as the binder were mixed in a weight ratio of 94:4:2 in a solvent N-methyl-2-pyrrolidone to produce a positive electrode slurry.

[0153] The manufactured positive electrode slurry was applied to and dried on an aluminum metal thin film, which is a positive electrode current collector with a thickness of 15 μm. At this time, the temperature of the circulated air was 110°C. Next, it was rolled and dried in a vacuum oven at 130°C for 2 hours to form a positive electrode active material layer.

[0154] Each of the negative electrodes of Examples 1 to 5 and Comparative Examples 1 to 3, the manufactured positive electrode, and the porous polyethylene separator were assembled using a stacking method, and an electrolytic solution (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1 / 2 (volume ratio), lithium hexafluorophosphate (LiPF6 1 mol)) was injected into the assembled battery to manufacture a lithium secondary battery.

[0155] 1) Evaluation of discharge capacity by C-Rate The charging C-Rate was fixed at 0.2C, and while increasing the discharge C-Rate from 0.2C to 2.0C, the 2.0C discharge capacity (%) with respect to the 0.2C discharge capacity was measured for each lithium secondary battery and then shown in Table 2.

[0156] 2) Evaluation of capacity retention rate (life characteristics) Each lithium secondary battery was charged and discharged under the following conditions. Each lithium secondary battery was charged and discharged at 45°C in a voltage range of 4.25V to 2.8V at 0.33C / 0.33C as one cycle condition, and a total of 100 cycles were carried out. Then, the discharge capacity after 100 cycles (capacity retention rate) was evaluated based on the discharge capacity after 1 cycle being 100% and shown in Table 2.

[0157]

Table 2

[0158] Referring to Table 2, it can be seen that when the second negative electrode active material layer contains a carbon nanotube structure, the input / output characteristics and life characteristics of the battery are improved, and the negative electrode adhesion and the adhesion between the first negative electrode active material layer and the second negative electrode active material layer can be improved.

Claims

1. It includes a negative electrode current collector, a first negative electrode active material layer disposed on the negative electrode current collector, and a second negative electrode active material layer disposed on the first negative electrode active material layer. The first negative electrode active material layer contains a first negative electrode active material. The first negative electrode active material is a carbon-based active material. The second negative electrode active material layer contains a second negative electrode active material and a second conductive material. The second negative electrode active material contains a silicon-based active material. The silicon-based active material contains SiO Ｘ (0 ≤ X < 2), The second conductive material includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are arranged side by side and bonded. The carbon nanotube structure is contained in the second negative electrode active material layer at 0.01 wt% to 1.0 wt%. The first negative electrode active material layer further contains a first conductive material. The first conductive material contains at least one selected from the group consisting of the carbon nanotube structure and multi-walled carbon nanotube units, a negative electrode.

2. In the second negative electrode active material layer, the carbon nanotube structures are connected to each other to show a network structure, the negative electrode according to Claim 1.

3. In the carbon nanotube structure, The single-walled carbon nanotube units are bonded in a state where the long axes of the single-walled carbon nanotube units are arranged parallel to each other, the negative electrode according to Claim 1.

4. The average length of the carbon nanotube structure is 1 μm to 500 μm, the negative electrode according to Claim 1.

5. The average length of the carbon nanotube structure is 2 μm to 50 μm, the negative electrode according to Claim 1.

6. The average diameter of the carbon nanotube structure is 2 nm to 500 nm, and the negative electrode according to claim 1.

7. The average diameter of the carbon nanotube structure is 5 nm to 200 nm, and the negative electrode according to claim 1.

8. Within the carbon nanotube structure, the average diameter of the single-walled carbon nanotube unit is 0.5 nm to 10 nm, and the negative electrode according to claim 1.

9. The carbon nanotube structure is a carbon nanotube structure in which 2 to 50 single-walled carbon nanotube units are bonded to each other, and the negative electrode according to claim 1.

10. The thickness of the first negative electrode active material layer is 1 μm to 100 μm, and the negative electrode according to claim 1.

11. The thickness of the second negative electrode active material layer is 1 μm to 100 μm, and the negative electrode according to claim 1.

12. A secondary battery including the negative electrode according to claim 1.

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