Negative electrode and secondary battery including same

Abstract

To provide a negative electrode capable of improving input / output characteristics and lifetime characteristics by minimizing the problems caused by a tilt phenomenon of a binder while maintaining a conductive network smooth.SOLUTION: The present invention relates to a negative electrode including a negative electrode collector, a first negative electrode active material layer disposed on the negative electrode collector, and a second negative electrode active material layer disposed on the first negative electrode active material layer, wherein the second negative electrode active material layer includes a second negative electrode active material and a second conductive agent, wherein the second negative electrode active material includes a silicon-based active material, the silicon-based active material includes SiOX (0≤x<2), the second 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 is included in an amount of 0.01 wt.% to 1.0 wt.% in the second negative electrode active material layer, and a secondary battery including the same.SELECTED DRAWING: Figure 1

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 contents disclosed in the documents of this Korean patent application are incorporated herein by reference.

[0002] The present invention relates to 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, the silicon-based active material being SiO X (0≦X<2), 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. [Background technology]

[0003] Recently, with the technological development and increasing demand for mobile devices, the demand for batteries as an energy source is rapidly increasing, and various researches are being conducted on batteries that can meet various demands. In particular, researches on lithium secondary batteries that have high energy density as well as excellent life and cycle characteristics as a power source for such devices are being actively conducted.

[0004] The lithium secondary battery refers to a battery including a positive electrode assembly having a positive electrode active material capable of inserting / extracting lithium ions, a negative electrode having a negative electrode active material capable of inserting / extracting lithium ions, and a microporous separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte containing lithium ions.

[0005] On the other hand, the negative electrode active material alone cannot ensure the conductivity of the negative electrode, and therefore the resistance of the battery is too high, so that the negative electrode generally further contains a conductive material. Conventionally, a dot-like conductive material such as carbon black has been mainly used, and linear conductive materials such as carbon nanotubes and carbon nanofibers have also been used to further improve the conductivity and thus the capacity of the battery.

[0006] Single-walled carbon nanotubes are one of the linear conductive materials and improve the conductivity in the negative electrode active material layer due to their elongated shape. Therefore, in the past, a negative electrode slurry was prepared using a dispersion liquid in which the single-walled carbon nanotubes were completely dispersed, and then a negative electrode active material layer was prepared using the negative electrode slurry.

[0007] However, when the battery is repeatedly charged and discharged, the negative active material repeatedly expands and contracts in volume, causing the single-walled carbon nanotubes to break, making it difficult to maintain the conductive network in the negative active material layer. In particular, when a silicon-based active material is used as the negative active material to improve the capacity of the battery, the volume of the silicon-based active material expands too much during battery charging and discharging, causing the single-walled carbon nanotubes to break more severely. This causes the conductive network to be cut off or reduced, which reduces the life characteristics of the battery. In addition, the single-walled carbon nanotubes are present by wrapping the surface of the silicon-based active material, and therefore cannot smoothly perform the role of conductively connecting adjacent negative active materials to each other.

[0008] Meanwhile, when carbon nanotubes are used as a conductive material, a carbon nanotube dispersion with a low solid content must be used in order to uniformly distribute the carbon nanotubes in the negative electrode active material layer. However, when carbon nanotubes with a low solid content are used, a migration phenomenon occurs in which the binder, which has a relatively low density compared to the negative electrode active material, and the conductive material easily migrate to the upper layer of the negative electrode active material layer (in the direction away from the current collector) during drying of the negative electrode, resulting in a significant decrease in the adhesive strength and electrical conductivity of the negative electrode.

[0009] Therefore, the present invention provides an anode that can connect a conductive network even when the anode active material has a large volume change, thereby minimizing problems caused by the binder tilt phenomenon. Summary of the Invention [Problem to be solved by the invention]

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

[0011] Another object of the present invention is to provide a secondary battery including the negative electrode. [Means for solving the problem]

[0012] According to one embodiment of the present invention, a negative electrode active material layer 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 X(0≦X<2), 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 included in the second negative electrode active material layer in an amount of 0.01 wt % to 1.0 wt %.

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

[0014] In the negative electrode according to the present invention, the second negative electrode active material layer includes a long rope-shaped carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are aligned and bonded together during rolling, and the carbon nanotube structure is a SiO X Even if the volume of the second negative electrode active material changes significantly (0≦X<2), the second negative electrode active material can be strongly held together by bonding the second negative electrode active material. X (0≦X<2) can suppress damage (e.g., cracks). In addition, since the negative electrode has a first negative electrode active material layer and a second negative electrode active material layer that are sequentially arranged as respective slurries, the aforementioned phenomenon of binder and conductive material tilt can be minimized. Furthermore, since the second negative electrode active material layer includes a carbon nanotube structure, the adhesive strength between the first negative electrode active material layer and the second negative electrode active material layer can be strengthened. As a result, the input / output characteristics and life characteristics of the battery can be improved. [Brief description of the drawings]

[0015] [Figure 1] 4 is a photograph showing an analysis of the binder distribution in the negative electrode of Comparative Example 1 and the negative electrode of Example 1. [Diagram 2] 4 is a SEM photograph of a second negative electrode active material layer of the negative electrode of Example 1. [Diagram 3] 1 is a SEM photograph of a second negative electrode active material layer of the negative electrode of Example 2. [Figure 4] 13 is an SEM photograph of a second negative electrode active material layer of the negative electrode of Comparative Example 3. [Diagram 5] 4 is a SEM photograph of a second negative electrode active material layer of the negative electrode of Example 1. [Figure 6] 13 is an SEM photograph of a second negative electrode active material layer of the negative electrode of Comparative Example 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] The terms and words used in this specification and the claims should not be interpreted limited to their ordinary and dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, based on the principle that the inventor himself / herself can appropriately define the concept of the term in order to explain the invention in the best possible way.

[0017] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. A singular expression may include a plural expression unless a clearly different meaning is present in the context.

[0018] In this specification, terms such as "comprise", "comprise", or "have" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the presence or additional possibility of one or more other features, numbers, steps, components, or combinations thereof.

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

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

[0021] In this specification, the average particle size (D 50 The average particle size (D) may be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve.50 ) may be measured, for example, by using a laser diffraction method. The laser diffraction method generally allows measurement of particle sizes from the submicron range to about several mm, and provides results with high reproducibility and high resolution.

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

[0023] The present invention will be specifically described below.

[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 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 is 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 bonded in a line, and the carbon nanotube structure may be included in the second negative electrode active material layer in an amount of 0.01 wt % to 1.0 wt %.

[0025] The negative electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, the negative electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. Specifically, a transition metal that easily adsorbs carbon, 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 disposed on one or both sides 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, 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] In general, when carbon nanotubes are used as a conductive material, a carbon nanotube dispersion with a low solid content must be used to uniformly distribute the carbon nanotubes in the negative electrode active material layer. However, when a carbon nanotube dispersion with a low solid content is used, the binder and conductive material, which have a relatively low density compared to the negative electrode active material, tend to migrate toward the upper layer of the negative electrode active material layer (the side farther from the negative electrode current collector and closer to the surface) during drying of the negative electrode slurry, resulting in a significant decrease in negative electrode adhesiveness and electrical conductivity. Meanwhile, the negative electrode of the present invention has a first negative electrode active material layer and a second negative electrode active material layer sequentially disposed as respective slurries, thereby minimizing the above-mentioned binder and conductive material migration phenomenon. As a result, the input / output characteristics and life characteristics of the battery can be improved.

[0029] (1) 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 commonly used in the art, and the type thereof is not particularly limited.

[0031] Specifically, the first negative electrode active material may include at least one of a carbon-based active material and a silicon-based active material, and the carbon-based active material particles may be at least one selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fiber, and graphitized mesocarbon microbeads. In particular, when artificial graphite is used, the rate characteristics can be improved. The silicon-based active material is SiO X (0≦X<2), Si-C composites, and Si-Y alloys (wherein Y is an element selected from the group consisting of alkali metals, alkaline earth metals, transition metals, Group 13 elements, Group 14 elements, rare earth elements, and combinations thereof), and in particular, SiO X When (0≦X<2) is used, a high capacity of the battery can be obtained. More specifically, the first negative electrode active material may be a carbon-based active material.

[0032] The first negative electrode active material may be included in the first negative electrode active material layer in an amount of 70 wt% to 99.5 wt%, preferably 80 wt% to 99 wt%. When the content of the first negative electrode active material satisfies this range, the energy density of the negative electrode may be improved, the negative electrode adhesive strength may be increased, and the electrical conductivity in the negative electrode may be improved.

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

[0034] The first conductive material may include 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 included in the first negative electrode active material layer in an amount of 0.01 wt % to 2.0 wt %, specifically 0.01 wt % to 1.5 wt %, more specifically 0.05 wt % to 1.0 wt %. If this range is satisfied, the adhesive strength and electrical conductivity of the negative electrode can be significantly improved even with only a small content of the first conductive material, and a battery with excellent input / output characteristics and life characteristics 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, more specifically 10 μm to 80 μm. If the thickness is within the above range, the aforementioned gradient of the conductive material and binder can be minimized. As a result, the adhesive strength and electrical conductivity of the negative electrode are 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 is SiO X (0≦X<2). X (0≦X<2) may specifically be SiO. The second negative electrode active material is SiO X (0≦X<2), the capacity of the battery can be improved. In particular, the second negative electrode active material layer is made of SiO X Since the negative electrode contains a SiO 2 (0≦X<2), the durability of the negative electrode is improved and the impregnation of the electrolyte is improved. More specifically, at the interface between the negative electrode current collector, which has the weakest binding strength in the negative electrode, and the negative electrode active material layer, the negative electrode active material easily detaches from the negative electrode due to contraction and expansion of the negative electrode active material during charging and discharging of the battery. X When (0≦X<2) is located, the desorption phenomenon is accelerated, which reduces the durability of the negative electrode and deteriorates the capacity and life characteristics of the battery.

[0040] On the other hand, when the negative electrode is rolled during the manufacturing process, the density of the negative electrode near the surface becomes very high, which reduces the impregnation of the electrolyte. X When (0≦X<2) is reached, SiO XDue to the volume expansion (0≦X<2), the density of the negative electrode is reduced to an appropriate level, improving the impregnation of the electrolyte.

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

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

[0043] The crystalline carbon may 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 may appropriately maintain the strength of the coating layer and suppress the expansion of the natural graphite. The amorphous carbon may be a carbon-based material formed by using at least one carbide selected from the group consisting of tar, pitch, and other organic substances, or a hydrocarbon as a source of a chemical vapor deposition process.

[0045] The carbonized organic matter may be a carbonized organic matter selected from carbonized 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 be benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, or phenanthrene, etc.

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

[0048] The second 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 mesocarbon microbeads. Specifically, the carbon-based active material is preferably, but is not limited to, artificial graphite, since it can effectively control the volume expansion of the negative electrode while maintaining an electrical network together with a carbon nanotube structure described later.

[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, 1:99 to 10:90. When this range is satisfied, 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 included in the second 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 second negative electrode active material layer is included in the second negative electrode active material layer in an amount of 90% by weight to 99% by weight, the energy density of the negative electrode may be kept high, and the electrical conductivity and adhesive strength of the negative electrode may 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 aligned and bonded to each other. More specifically, in consideration of 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 aligned and bonded to each other. In consideration of improving the dispersibility of the carbon nanotube structure and the durability of the negative electrode, the carbon nanotube structure may be a carbon nanotube structure in which 2 to 50 single-walled carbon nanotube units are aligned and bonded to each other.

[0053] In the carbon nanotube structure, the single-walled carbon nanotube units may be arranged and bonded together (long axes of the units are bonded together in parallel to each other to form a flexible bundled 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 form a network structure.

[0054] A conventional electrode including carbon nanotubes is generally prepared by dispersing bundle-type or entangled-type carbon nanotubes (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 material dispersion, and then using the conductive material dispersion. In this case, the carbon nanotubes are completely dispersed in the conventional conductive material dispersion, and the conductive material dispersion is prepared by dispersing single-stranded carbon nanotube units. In the conventional conductive material dispersion, the carbon nanotube units are easily cut due to an excessive dispersion process, and have a short length compared to the initial length. In addition, the carbon nanotube units may be easily cut during the rolling process of the negative electrode, and there is an additional problem that the carbon nanotube units (especially single-walled carbon nanotube units) are cut due to an excessive volume change of the silicon-based active material during the operation of the battery. This causes a problem of a decrease in the conductivity of the negative electrode and a decrease in the life characteristics of the battery. Furthermore, in the case of multi-walled carbon nanotube units, structural defects are high due to the mechanism of node growth (nodes exist due to defects that occur during the growth process rather than smooth lines). Therefore, during the dispersion process, the multi-walled carbon nanotube units are more easily cut, and the short cut multi-walled carbon nanotube units are more likely to aggregate with each other due to π-π interaction (π-π stacking) between the carbons of the units. As a result, they are less likely to be uniformly dispersed in the negative electrode slurry.

[0055] In contrast, the carbon nanotube structure included in the second negative electrode active material layer of the present invention has a rope shape in which 2 to 5,000 single-walled carbon nanotube units that maintain high crystallinity without structural defects are arranged side by side and bonded to each other, and therefore can smoothly maintain its length without being cut even when the volume of the second negative electrode active material changes, and therefore can maintain the conductivity of the negative electrode even during continuous charging and discharging of the battery. In addition, due to the high electrical conductivity of the single-walled carbon nanotube units with 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. In addition, in the second negative electrode active material layer that is directly subjected to pressure during rolling, the carbon nanotube structures can be connected to each other to form a network structure, and therefore damage to the second negative electrode active material (e.g., cracking phenomenon, etc.) can be suppressed. In addition, even if a crack occurs in the second negative electrode active material, the carbon nanotube structure connects the second negative electrode active material across the crack, so that the conductive network can be maintained. Furthermore, since the carbon nanotube structure is not easily broken and can maintain a long shape, the conductive network can be strengthened throughout the second negative electrode active material layer, and the detachment of the second negative electrode active material is suppressed, thereby significantly improving the negative electrode adhesive strength.

[0056] In addition, since the carbon nanotube structure is included in the second negative electrode active material layer, the adhesive strength between the first negative electrode active material layer and the second negative electrode active material layer can be significantly improved. The carbon nanotube structure has a long rope shape formed by horizontally bonding single-walled carbon nanotube units therein, and therefore the second negative electrode active material can be well connected to each other by van der Waals forces, thereby forming a strong negative electrode. Furthermore, the carbon nanotube structure and the surface of the carbon-based active material of the first negative electrode active material layer can be bonded more tightly by π-π bonding (stacking) occurring between the same kind of carbon, so that the adhesive strength 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-walled carbon nanotube units may be 0.5 nm to 10 nm, specifically, 1 nm to 9 nm. If the average diameter is satisfied, the conductivity in the negative electrode can be maximized even with a very small amount of conductive material. The average diameter corresponds to the average value of the top 100 single-walled carbon nanotube units with the largest diameter and the bottom 100 single-walled carbon nanotube units with the smallest diameter when the manufactured negative electrode is observed through a TEM.

[0058] In the carbon nanotube structure, the average length of the single-walled carbon nanotube units may be 1 μm to 100 μm, specifically, 5 μm to 50 μm. If the average length is satisfied, a long conductive path for conductive connection between the second negative electrode active materials may be formed, and a unique network structure may be formed, so that the conductivity in the negative electrode can be maximized even with a very small amount of conductive material. The average length corresponds to an average value of the top 100 single-walled carbon nanotube units and the bottom 100 single-walled carbon nanotube units when the manufactured negative electrode is observed through a TEM.

[0059] The specific surface area of ​​the single-walled carbon nanotube unit is 500 m 2 / g~1,000m 2 / g, specifically, 600m 2 / g~800m 2 / g. If this range is satisfied, a conductive path in the negative electrode can be smoothly secured due to the large specific surface area, so that the conductivity in the negative electrode can be maximized even with a very small amount of conductive material. Specifically, the specific surface area of ​​the single-walled carbon nanotube unit can be calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mini II from BEL Japan.

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

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

[0062] The carbon nanotube structure may be included in the second negative electrode active material layer at 0.01 wt % to 1.0 wt %, specifically at 0.01 wt % to 0.5 wt %, more specifically at 0.01 wt % to 0.2 wt %. When the range is satisfied, the conductive path of the second negative electrode active material layer is secured, and the negative electrode resistance is maintained at a low level, while the life characteristics of the battery can be improved. When the bundle-type carbon nanotubes are completely dispersed (dispersed so that the carbon nanotube units are separated from each other as much as possible in a general dispersion method) during the preparation of the conductive material dispersion liquid, the carbon nanotube structure is not generated, or even if it is generated unintentionally, it is generated in a very small amount (for example, 0.0005 wt %). That is, the range of the content is never achievable by a general method. The carbon nanotube structure has a configuration in which 2 to 5,000 single-walled carbon nanotube units are arranged and bonded to each other, so that the carbon nanotube structure is not broken even when the volume of the second negative electrode active material changes, and the length can be smoothly maintained. Therefore, the conductive network of the second negative electrode active material layer can be maintained, and the conductivity of the second negative electrode active material layer can be smoothly secured due to the high conductivity of the carbon nanotube structure. As a result, even if the content of the carbon nanotube structure in the second negative electrode active material layer is low, the input / output characteristics and life characteristics of the battery may be excellent.

[0063] Meanwhile, in some cases, the single-walled carbon nanotube unit may be surface-treated by oxidation or nitridation to improve 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 units, and graphene. In this case, the one-dimensional linear conductive network may become a two-dimensional or higher conductive network by combining the material with the carbon nanotube structure, thereby improving the conductivity of the second negative electrode active material layer and improving the input / output characteristics and life characteristics of the battery.

[0065] The thickness of the second negative electrode active material layer may be 1 μm to 100 μm, specifically 5 μm to 90 μm, more specifically 10 μm to 80 μm. If the thickness is within the above range, the conductive material and binder tilt phenomenon described above can be minimized. As a result, the adhesive strength of the negative electrode (adhesive strength between the negative electrode active material layer and the current collector), the adhesive strength between the first negative electrode active material layer and the second negative electrode active material layer, and the electrical conductivity of the negative electrode can be 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 ratio does not satisfy the above range, the effect of suppressing the migration of the conductive material and binder described above decreases, and the effect of improving the diffusion resistance by improving the porosity of the second negative electrode active material layer decreases. If the thickness of the first negative electrode active material layer is too thin outside the above range, the effect of suppressing the migration of the conductive material and binder described above decreases, and the effects of improving the negative electrode adhesive strength and the interface resistance decrease.

[0067] An interface exists between the first and second negative electrode active material layers. This can be confirmed by examining a cross section of a manufactured negative electrode. In contrast, if the negative electrode active material layer is formed as a single layer structure (a single coating of one negative electrode slurry is performed) rather than 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 include 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 is used to ensure adhesion between negative electrode active materials or between a negative electrode active material and a current collector, and may be a general binder used in the art, and the type of the binder is not particularly limited. Examples of the binder include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof, and any of these may be used alone or in combination.

[0069] The binder may be included in the first negative electrode active material layer (or the second negative electrode active material layer) in an amount of 10 wt % or less, preferably 0.1 wt % to 5 wt %. When the binder content satisfies this range, an increase in negative electrode resistance can be minimized and excellent negative electrode adhesion can be achieved.

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

[0071] The method for manufacturing a 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 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; 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 is 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 bonded in a line, and 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 above-mentioned embodiment.

[0072] (1) Preparing a first negative electrode slurry and a second negative electrode slurry The first negative electrode slurry may be prepared by the same method as a typical negative electrode slurry, for example, by preparing a mixture including a first negative electrode active material (same as the first negative electrode active material of the above embodiment), a first conductive material (same as the first conductive material of the above embodiment), and a solvent (which may further include a binder), and then stirring the mixture to prepare the first negative electrode slurry.

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

[0074] Examples of the solvent include water, dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP) and other amide-based polar organic solvents; 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, and octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, and hexylene glycol; and various solvents such as glycerin, trimethylolpropane, pentaerythritol, and sorbitol. Examples of the solvent include, but are not limited to, hydric alcohols, 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, and tetraethylene glycol monobutyl ether, ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, and cyclopentanone, and esters such as ethyl acetate, γ-butyrolactone, and ε-propiolactone, and any one or a mixture of two or more of these may be used. The solvent may be the same as or different from the dispersion medium used in the conductive material dispersion, and may be preferably water.

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

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

[0077] The preparation of the carbon nanotube structure dispersion may include the steps of: preparing a mixture containing a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes (a combination or aggregate of single-walled carbon nanotube units) (S1-1); and applying a shear force to the mixture to disperse the bundled single-walled carbon nanotubes, thereby forming a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are combined in a line (S1-2).

[0078] In the step S1-1, the mixture may be prepared by adding the bundled single-walled carbon nanotubes and a dispersant to a dispersion medium. The bundled single-walled carbon nanotubes are the above-mentioned single-walled carbon nanotube units that are combined to form a bundle, and typically 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 is 500 m 2 / g~1,200m 2 / g, specifically, 500m 2 / g~1,000m 2 When this range is satisfied, the large specific surface area can smoothly secure a conductive path in the second negative electrode active material layer, so that the conductivity in the second negative electrode active material layer can be maximized even with a very small amount of conductive material. In addition, in order to strengthen the adhesive strength between the first negative electrode active material layer and the second negative electrode active material layer, 2 / g~800m 2 It is preferred that the molecular weight is / g.

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

[0081] Examples of the dispersion medium include water, amide polar organic solvents such as 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, and octanol; glycols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, and hexylene glycol; and various solvents such as glycerin, trimethylolpropane, pentaerythritol, and sorbitol. Examples of the dispersing medium include, but are not limited to, hydric alcohols, 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, and tetraethylene glycol monobutyl ether, ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, and cyclopentanone, and esters such as ethyl acetate, γ-butyrolactone, and ε-propiolactone, and any one or a mixture of two or more of these may be used. More specifically, the dispersing medium may be the same as or different from the solvent for preparing the negative electrode slurry, and may be preferably 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, and specifically may be carboxymethyl cellulose, polyvinylidene fluoride, polyvinylpyrrolidone, or hydrogenated nitrile butadiene rubber.

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

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

[0085] In the step S1-2, the process of dispersing the bundle-type carbon nanotubes in the mixture may be performed using a mixer such as a homogenizer, a bead mill, a ball mill, a basket mill, an attrition mill, a universal mixer, a clear mixer, a spike mill, a TK mixer, or an ultrasonic dispersion (sonification) device, etc. Among them, the bead mill method is preferred because it can precisely control the diameter of the carbon nanotube structure, can achieve uniform distribution of the carbon nanotube structure, and has an advantage in terms of cost.

[0086] The bead mill method may be as follows: The mixture is placed in a container containing beads, and the container is rotated to disperse the bundled single-walled carbon nanotubes.

[0087] Here, the conditions for carrying out the bead mill method are as follows.

[0088] The average particle size of the beads may be 0.5 mm to 1.5 mm, specifically, 0.5 mm to 1.0 mm. When the above range is satisfied, the carbon nanotube structure is not cut during the dispersion process, 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, 2,000 RPM to 6,000 RPM. When the rotation speed is within the above range, the carbon nanotube structures are not cut during the dispersion process, the diameter size can be appropriately controlled, and a dispersion solution with a uniform composition can be produced.

[0090] The time for which the bead mill is performed may be 0.5 to 2 hours, specifically 0.5 to 1.5 hours, and more specifically 0.8 to 1 hour. If the above range is satisfied, the carbon nanotube structures are not cut during the dispersion process, and the diameter size can be appropriately controlled, so that a dispersion solution with a uniform composition can be produced. The bead milling time means the total time for which the bead mill is applied, and for example, if the bead mill is performed several times, it means the total time over the several times.

[0091] The bead mill conditions are for dispersing the bundled single-walled carbon nanotubes to an appropriate level, specifically, excluding the case where the bundled single-walled carbon nanotubes are completely dispersed into one single-walled carbon nanotube. That is, the bead mill conditions are for dispersing the bundled single-walled carbon nanotubes to an appropriate level and forming a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are aligned and bonded to each other in the carbon nanotube structure dispersion liquid produced. This can only be achieved when the composition of the mixture liquid, the conditions of the dispersion process (e.g., bead mill process), etc. are strictly controlled.

[0092] Through the above process, a carbon nanotube structure dispersion liquid can be formed.

[0093] The negative electrode slurries (first and second negative electrode slurries) may further include a binder, if necessary. Here, the binder may be the same as that of the above-described embodiment.

[0094] (2) forming a first negative electrode active material layer on a negative electrode current collector using the first negative electrode slurry and forming a second negative electrode active material layer on the first negative electrode active material layer using the second negative electrode slurry. Next, a first negative electrode active material layer is formed using the first negative electrode slurry prepared as described above. Specifically, the first negative electrode active material layer may be formed by applying the first negative electrode slurry on a negative electrode current collector and drying it, or by applying the first negative electrode slurry on a separate support and peeling it off from the support to obtain a film, which is then laminated on the negative electrode current collector. If necessary, after forming the first negative electrode active material layer by the above method, a rolling process may be further performed. In this case, the 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 using the second negative electrode slurry prepared as above. Specifically, the second negative electrode active material layer may be formed by applying the second negative electrode slurry on the first negative electrode active material layer and drying it, or by applying the second negative electrode slurry on a separate support and peeling it off from the support to obtain a film, which is then laminated on the first negative electrode active material layer. If necessary, after forming the second negative electrode active material layer by the above method, a rolling process may be further performed. In this case, the 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] A 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-mentioned embodiment. Since the negative electrode has been described above, detailed 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 including the positive electrode active material.

[0100] In the positive electrode, the positive electrode current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery, and may be, for example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. The positive electrode current collector may generally have a thickness of 3 μm to 500 μm, and the adhesive force of the positive electrode active material can be increased by forming fine irregularities on the surface of the current collector. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0101] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; a lithium iron oxide such as LiFe3O4; 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; chemical formula LiNi 1-c2 M c2 O2 (wherein 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); 2-c3 M c3 Examples of the lithium manganese composite oxide include, but are not limited to, lithium manganese composite oxides represented by the formula Li2Mn3MO8 (wherein 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 Li2Mn3MO8 (wherein M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); and LiMn2O4 in which part of the Li in the chemical formula is replaced with an alkaline earth metal ion. 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 in addition to the positive electrode active material described above.

[0103] In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and can be used without any particular limitation as long as it has electronic conductivity without causing chemical changes in the battery that is constructed. Specific examples include 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 black, and carbon fibers; 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; and conductive polymers such as polyphenylene derivatives, and the like, and one or more of these may be used alone or in mixture.

[0104] In addition, the positive electrode binder serves to improve adhesion between particles of the positive electrode active material and adhesion between the positive electrode active material and the positive electrode current collector.Specific examples of the positive electrode binder 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, and various copolymers thereof, and one or more of these may be used alone or in combination.

[0105] The separator separates the negative electrode and the positive electrode and provides a passage for lithium ions to move. Any separator that is generally used in secondary batteries may be used without any particular limitation. In particular, a separator that has low resistance to ion movement of the electrolyte and has excellent humidification ability of the electrolyte solution is preferred. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene 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. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting glass fiber, polyethylene terephthalate fiber, etc. may be used. In addition, a coated separator containing a ceramic component or a polymer material may be used to ensure heat resistance or mechanical strength, and may be used in a single layer or multilayer structure.

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

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

[0108] Examples of the non-aqueous organic solvent that can be used 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, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0109] In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, are highly viscous organic solvents having a high dielectric constant and dissociating lithium salts well, and therefore can be preferably used. When such cyclic carbonates are mixed with linear carbonates having a low viscosity and a low dielectric constant, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte having high electrical conductivity can be prepared, and therefore can be more preferably used.

[0110] The metal salt may be a lithium salt, which is a substance that is easily dissolved in the non-aqueous electrolyte. For example, the anion of the lithium salt may be 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 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, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing a decrease in battery capacity, and improving the discharge capacity of the battery.

[0112] According to yet another embodiment of the present invention, there is provided a battery module including the secondary battery as a unit cell, and a battery pack including the same. The battery module and the battery pack include the secondary battery having high capacity, high-rate characteristics, and cycle characteristics, and may be used as a power source for a medium- to large-sized device selected from the group consisting of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system.

[0113] The present invention will now be described in more detail with reference to 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 carboxymethylcellulose (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. The mixture was stirred using a beads mill to disperse the carbon black in the solvent to prepare a carbon black dispersion. At this time, the particle size of the beads was 1 mm, the rotation speed of the stirring vessel containing the beads was 3,000 RPM, and the stirring was performed 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 A bundle-type carbon nanotube (specific surface area: 650 m2) consisting of single-walled carbon nanotube units with an average diameter of 1.5 nm and an average length of 5 μm or more. 2 A mixture was produced by mixing 0.4 parts by weight of cellulose acetate (0.25 g / g) and 0.6 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) with 99.0 parts by weight of water as a dispersion medium so that the solid content was 1.0% by weight.

[0117] The mixture was stirred using a bead mill to disperse the bundled single-walled carbon nanotubes in the solvent to prepare a carbon nanotube structure dispersion. The beads had a particle size of 1 mm, the rotation speed of the stirring vessel containing the beads was 3,000 RPM, and the stirring was performed for 60 minutes. The carbon nanotube structure dispersion contained carbon nanotube structures in which 2 to 5,000 single-walled carbon nanotube units were bound together in a line.

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

[0119] Production Example 3: Production of carbon nanotube structure dispersion A carbon nanotube structure dispersion 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 (substitution degree: 1.0) in Production Example 2. In the dispersion, the carbon nanotube structure was 0.4% by weight, and the carboxymethyl cellulose was 0.6% by weight.

[0120] Preparation Example 4: Preparation of single-walled carbon nanotube unit dispersion A bundle-type carbon nanotube (specific surface area: 650 m2) consisting of single-walled carbon nanotube units with an average diameter of 1.5 nm and an average length of 5 μm or more. 2 A mixture was prepared by mixing 0.2 parts by weight of cellulose acetate (0.2 parts by weight / g) and 1.2 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) with 98.6 parts by weight of water as a dispersion medium to give a solids content of 1.4% by weight.

[0121] The mixture was stirred in a bead mill to disperse the bundled single-walled carbon nanotubes in the solvent, thereby preparing 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 was performed for a total of 4 cycles (with 60 minutes of natural cooling between each cycle) under the above conditions, with 60 minutes of stirring being one cycle. Thus, a single-walled carbon nanotube unit dispersion was prepared. In the dispersion, the bundled single-walled carbon nanotubes were completely dispersed, and only single-walled carbon nanotube units were present as individual units, and the above-mentioned carbon nanotube structure was not detected. In addition, the single-walled carbon nanotube unit was 0.2 wt % and the carboxymethyl cellulose was 1.2 wt % in the single-walled carbon nanotube unit dispersion.

[0122] Preparation Example 5: Preparation of multi-walled carbon nanotube unit dispersion The bundled carbon nanotubes (specific surface area: 185 m) are composed of multi-walled carbon nanotube units with an average diameter of 10 nm and an average length of 1 μm. 2 A mixture was prepared by mixing 4.0 parts by weight of cellulose acetate (100,000 g / g) and 0.6 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) with 95.4 parts by weight of water as a dispersion medium to give a solids content of 4.6% by weight.

[0123] The mixture was dispersed in a spike mill filled with 80% beads having a size of 0.65 mm, and discharged at a discharge rate of 2 kg / min. This process was repeated twice to completely disperse the bundled multi-walled carbon nanotubes, producing a multi-walled carbon nanotube unit dispersion. In the dispersion, the multi-walled carbon nanotube unit (average diameter: 10 nm) was 4.0 wt % and the carboxymethyl cellulose was 0.6 wt %.

[0124] Examples and Comparative Examples Example 1: Preparation of negative electrode (1) Formation of the first negative electrode active material layer Carbon black dispersion of Production Example 1, average particle size (D 50 A first negative electrode slurry was prepared by mixing artificial graphite having a particle size of 21 μm, binders of styrene butadiene rubber (SBR) and carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) with water. The negative electrode slurry was applied to a negative electrode current collector (copper (Cu) metal thin film) having 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 in an amount of 94.80 wt %, the SBR was contained in an amount of 3.5 wt %, the CMC was contained in an amount of 1.2 wt %, and the carbon black was contained in an amount of 0.50 wt %.

[0126] (2) Formation of second negative electrode active material layer Carbon nanotube structure dispersion liquid of Production Example 2, negative electrode active material (average particle size (D 50 ) of 21 μm: Average particle size (D 50 A second negative electrode slurry was prepared by mixing SiO (weight ratio = 94:6) having a particle size of 6.6 μm, SBR and carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) as binders with water. The second negative electrode slurry was applied onto the first negative electrode active material layer, dried at 130° C., and rolled to form a second negative electrode active material layer (thickness: 50 μm).

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

[0128] Example 2: Preparation of negative electrode A negative electrode was manufactured in the same manner as in Example 1, except that the carbon nanotube structure dispersion liquid of Preparation Example 3 was used instead of the carbon nanotube structure dispersion liquid of Preparation Example 2 when forming the second negative electrode active material layer in Example 1.

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

[0130] Example 4: Preparation of negative electrode A negative electrode was manufactured in the same manner as in Example 1, except that the carbon nanotube structure dispersion of Preparation Example 2 and the multi-walled carbon nanotube unit dispersion of Preparation Example 5 were used instead of the carbon black dispersion of Preparation Example 1 when forming the first negative electrode slurry in Example 1. In the manufactured first negative electrode active material layer, the weight ratio of the carbon nanotube structure to the multi-walled carbon nanotubes was 10:90. In the first negative electrode active material layer, the artificial graphite was included at 94.80 wt%, the SBR was included at 3.5 wt%, the CMC was included at 1.2 wt%, the carbon nanotube structure was included at 0.05 wt%, and the multi-walled carbon nanotube units were included at 0.45 wt%.

[0131] Example 5: Preparation of negative electrode A negative electrode was manufactured in the same manner as in Example 1, except that the carbon nanotube structure dispersion liquid of Preparation Example 2 was used instead of the carbon black dispersion liquid of Preparation Example 1 when forming the first negative electrode slurry in Example 1.

[0132] The first negative electrode active material layer contained 95.25 wt % of the artificial graphite, 3.5 wt % of the SBR, 1.2 wt % of the CMC, and 0.05 wt % of the carbon nanotube structure.

[0133] Comparative Example 1: Production of negative electrode Carbon nanotube structure dispersion liquid of Production Example 2, negative electrode active material (average particle size (D 50) of 21 μm: Average particle size (D 50 A negative electrode slurry was prepared by mixing SiO (weight ratio = 94:6) with a particle size of 6.6 μm, SBR and carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) as binders with water. 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 negative electrode active material layer (thickness: 100 μm).

[0134] In the negative electrode active material layer, the negative electrode active material was contained in a total amount of 95.75 wt %, the SBR was contained in a total amount of 3.0 wt %, the CMC was contained in a total amount of 1.2 wt %, and the carbon nanotube structure was contained in a total amount of 0.05 wt %.

[0135] Comparative Example 2: Production of negative electrode A negative electrode was prepared in the same manner as in Example 1, except that the single-walled carbon nanotube unit dispersion liquid of Preparation Example 4 was used instead of the carbon nanotube structure of Preparation 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 96.25% by weight (average particle size (D 50 ) of 21 μm: Average particle size (D 50 The SiO weight ratio is 94:6) with a particle size of 6.6 μm, the SBR is 2.5 wt %, the CMC is 1.2 wt %, and the single-walled carbon nanotube unit is 0.05 wt %.

[0137] Comparative Example 3: Production of negative electrode A negative electrode was prepared in the same manner as in Example 1, except that the multi-walled carbon nanotube unit dispersion liquid of Preparation Example 5 was used instead of the carbon nanotube structure of Preparation 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 95.80 wt % (average particle size (D 50 ) of 21 μm: Average particle size (D 50The SiO weight ratio is 94:6) with an average particle size of 6.6 μm, the SBR is 2.5 wt %, the CMC is 1.2 wt %, and the multi-wall carbon nanotube unit is 0.5 wt %.

[0139] [Table 1]

[0140] In the examples 1, 3 to 5 and the comparative example 1, the carbon nanotube structures had an average diameter of 10 nm and an average length of 8.2 μm. In the example 2, the carbon nanotube structures had an average diameter of 100 nm and an average length of 15.6 μm. In the comparative example 2, the single-walled carbon nanotube units had an average diameter of 1.6 nm and an average length of 1.8 μm.

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

[0142] The average diameter and the average length correspond to the average values ​​of the top 100 carbon nanotube structures (or multi-wall carbon nanotube units or single-wall carbon nanotube units) having the largest diameter (or length) and the bottom 100 carbon nanotube structures (or multi-wall carbon nanotube units or single-wall carbon nanotube units) having the largest diameter (or length) when the manufactured negative electrode is observed through a TEM.

[0143] Experimental example 1: Observation of the negative electrode (1) Binder distribution FIG. 1 is a photograph showing the analysis of the binder distribution of the negative electrode of Comparative Example 1 and the negative electrode of Example 1. The analysis was performed by an osmium oxide (OsO4) staining method. Specifically, the negative electrode was dyed by exposing it to an osmium oxide fume atmosphere for 3 days in a closed special chamber prepared in a glove box, and the atmosphere was replaced with an Ar atmosphere. After 1 day, the cross section of the negative electrode cut by ion milling was analyzed by image mapping using SEM-EDX.

[0144] FIG. 1 shows analytical photographs of Comparative Example 1 (left), in which a single layer of anode active material layer is formed, and Example 1 (right), in which two layers of first and second anode active material layers are formed. From the photograph on the right, it can be seen that in Example 1, the first and second anode active material layers have different compositions, and the binder distribution ratios for the first and second anode active material layers are also different. In particular, when the first and second anode active material layers are formed as in Example 1, it can be seen that SiO is located only in the second anode active material layer, and the binder amount in the first anode active material layer is greater. It can also be seen that the binder distribution changes in the thickness direction of the anode active material layer.

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

[0146] 2 and 3, it can be seen that there is a long rope-like carbon nanotube structure in which a plurality of single-walled carbon nanotube units are arranged and bonded to each other. In particular, in the negative electrode of Example 1 of FIG. 2, since hydrogenated nitrile butadiene rubber having a relatively low weight average molecular weight is used, it can be seen that a carbon nanotube structure having an average diameter of about 10 nm is formed, and in the case of FIG. 3, it can be seen that a carbon nanotube structure having an average diameter of about 100 nm is formed. In both FIG. 2 and FIG. 3, it can be seen that the carbon nanotube structures form a network structure.

[0147] On the other hand, in Fig. 4, only short multi-walled carbon nanotube units were visible, and no carbon nanotube structure was observed, and in Fig. 6, only single-walled carbon nanotube units present as individual units were visible, and no carbon nanotube structure was observed.

[0148] (3) Confirmation of the network using the second conductive material Figure 5 is an SEM photograph of the second negative electrode active material layer of the negative electrode of Example 1. Referring to (A) of Figure 5, it can be seen that the carbon nanotube structure can suppress cracking of SiO. Referring to (B) of Figure 5, it can be seen that the carbon nanotube structure keeps the conductive network from being broken on the cracked SiO. Referring to (C) of Figure 5, it can be seen that a long conductive network is formed due to the long length of the carbon nanotube structure.

[0149] Experimental Example 2: Evaluation of negative electrode adhesive strength The negative electrode adhesive strength (adhesion strength between the negative electrode active material layer and the current collector) was measured under dry conditions. Specifically, double-sided tape was attached to a glass slide, and a negative electrode punched to 20 mm x 180 mm was placed on the tape and adhered by rolling it back and forth 10 times with a 2 kg roller. The force at which the electrode was peeled off from the glass slide was measured by pulling it at 200 mm / min using a UTM (TA Co.) device. The measurement angle between the glass slide and the negative electrode was 90°. The measurement results are shown in Table 2 below.

[0150] Experimental Example 3: Evaluation of Adhesion Strength Between First and Second Negative Electrode Active Materials For Examples 1 to 4 and Comparative Examples 1 to 3, the shear strength (N / mm 2 ) was measured. The measurements were performed using a SAICAS (Surface and Interfacial Characterization Analysis System) device (SAICAS EN-EX, Daipla Wintes Japan). Specifically, a micro-sized diamond blade was used to cut each negative electrode in a tilted state from the surface toward the inside, and the shear strength was measured at the interface between the first negative electrode active material layer and the second negative electrode active material layer by the force applied to the blade, and the results are 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 produced as follows.

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

[0153] The prepared positive electrode slurry was applied to an aluminum metal thin film, which was a positive electrode current collector, having a thickness of 15 μm, and dried. At this time, the temperature of the circulating air was 110° C. Then, the positive electrode slurry 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 positive electrodes prepared above, and the porous polyethylene separators were stacked together, and an electrolyte (ethylene carbonate (EC) / ethyl methyl carbonate (EMC)=1 / 2 (volume ratio) and lithium hexafluorophosphate (LiPF61 mol) was injected into the assembled batteries to prepare lithium secondary batteries.

[0155] 1) Evaluation of discharge capacity by C-Rate The charge C-rate was fixed at 0.2C, and the discharge C-rate was increased from 0.2C to 2.0C. The 0.2C discharge capacity and the 2.0C discharge capacity (%) of each lithium secondary battery were measured and shown in Table 2.

[0156] 2) Capacity retention rate (lifetime characteristics) Each lithium secondary battery was charged and discharged under the following conditions. Each lithium secondary battery was charged / discharged at 0.33 C / 0.33 C in the voltage range of 4.25 V to 2.8 V at 45° C. for a total of 100 cycles. The discharge capacity after 100 cycles (capacity retention rate) was evaluated based on the discharge capacity after 1 cycle being 100%. The results are 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 includes 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. 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 includes a first negative electrode active material and a first conductive material, 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, the second conductive material includes a carbon nanotube structure in which single-walled carbon nanotube units are bonded together; The carbon nanotube structure has an average diameter of 2 nm to 500 nm, The average diameter of the single-walled carbon nanotube units is 0.5 nm to 10 nm. Negative electrode.

2. The negative electrode of claim 1 , wherein the carbon nanotube structures in the second negative electrode active material layer are interconnected to form a network structure.

3. In the carbon nanotube structure, The negative electrode of claim 1 , wherein the single-walled carbon nanotube units are bonded together with their major axes aligned parallel to each other.

4. 2. The negative electrode according to claim 1, wherein the carbon nanotube structure has an average length of 1 μm to 500 μm.

5. 2. The negative electrode according to claim 1, wherein the carbon nanotube structure has an average length of 2 μm to 50 μm.

6. The negative electrode described in claim 1, wherein the first conductive material includes at least one selected from the group consisting of the carbon nanotube structure and a multi-wall carbon nanotube unit body.

7. 2. The negative electrode according to claim 1, wherein the carbon nanotube structure has an average diameter of 5 nm to 200 nm.

8. The negative electrode of claim 1, wherein the silicon-based active material comprises SiOX (0≦X<2).

9. 2. The anode of claim 1, wherein the carbon nanotube structure is a carbon nanotube structure in which 2 to 50 single-walled carbon nanotube units are bonded to each other.

10. The negative electrode of claim 1 , wherein 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 %.

11. The negative electrode according to claim 1 , wherein the first negative electrode active material layer has a thickness of 1 μm to 100 μm.

12. The negative electrode according to claim 1 , wherein the second negative electrode active material layer has a thickness of 1 μm to 100 μm.

13. A secondary battery comprising the negative electrode according to claim 1.

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