Negative electrode and secondary battery including the same

Abstract

The present invention relates to a negative electrode, comprising: a negative electrode current collector; a first negative electrode active material layer provided on the negative electrode current collector; and a second negative electrode active material layer provided on the first negative electrode active material layer, wherein the second negative electrode active material layer comprises a second negative electrode active material and a second conductive material; and the second negative electrode active material comprises a silicon-based active material and a carbon-based active material, wherein the silicon-based active material comprises SiO x (0≤x<2); and the second conductive material comprises: a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are joined side by side; and a particulate conductive material, wherein in the second negative electrode active material layer, the weight ratio of the carbon nanotube structure to the particulate conductive material is 12.7:87.3 to 0.5:99.5.

Description

TECHNICAL FIELD

[0001] Cross Reference to Related Applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2020-0138049, filed October 23, 2020, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. TECHNICAL FIELD

[0004] The present application relates to a secondary battery including a negative current collector, a first negative active material layer disposed on the negative current collector, and a second negative active material layer disposed on the first negative active material layer, wherein the second negative active material layer includes a second negative active material and a second conductive material, and the second negative active material includes a silicon-based active material and a carbon-based active material, wherein the silicon-based active material includes SiO x (0≤x<2), wherein the second conductive material includes a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are joined side by side and a particulate conductive material, wherein a weight ratio of the carbon nanotube structure to the particulate conductive material is 12.7:87.3 to 0.5:99.5 in the second negative active material layer. BACKGROUND

[0005] With the development of technology and an increasing demand for mobile devices in recent years, there is a rapid increase in demand for secondary batteries as energy sources. Accordingly, various studies have been conducted on batteries that can satisfy various needs. In particular, as a power source for such devices, lithium secondary batteries having high energy density and excellent lifespan and cycle characteristics are being actively studied.

[0006] A lithium secondary battery refers to a battery including a non-aqueous electrolyte containing lithium ions in a battery assembly including a cathode including a cathode active material capable of intercalating / deintercalating lithium ions, an anode including an anode active material capable of intercalating / deintercalating lithium ions, and a microporous separator interposed between the cathode and the anode.

[0007] Meanwhile, since a separate anode active material cannot ensure the conductivity of the anode, there is a problem in that the resistance of the battery is excessively high, and thus, in general, the anode additionally includes a conductive material. Typically, a particulate conductive material such as carbon black is mainly used, and in order to further improve the conductivity, thereby improving the capacity of the battery, a linear conductive material such as a carbon nanotube and a carbon nanofiber is also used.

[0008] A single-walled carbon nanotube is one example of a linear conductive material, which improves the conductivity in the anode active material layer due to its elongated shape. Thus, in general, an anode slurry is prepared by a dispersion liquid obtained by completely dispersing a single-walled carbon nanotube, and then an anode active material layer is prepared by the anode slurry.

[0009] However, when the battery is repeatedly charged and discharged, the single-walled carbon nanotubes are disconnected due to repeated volume expansion / contraction of the negative active material, and thus there is a problem in that it is 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 in order to increase the capacity of the battery, the volume of the silicon-based active material is excessively increased due to charging and discharging of the battery, and thus the phenomenon of the single-walled carbon nanotubes being disconnected occurs more severely. Accordingly, the conductive network is blocked or reduced, which degrades the life characteristics of the battery. In addition, the single-walled carbon nanotubes exist around the surface of the silicon-based active material, and thus cannot smoothly perform the role of electrically connecting adjacent negative active materials to each other.

[0010] In addition, unlike the particulate conductive material, when the single-walled carbon nanotubes are mixed with the negative active material or the like, the edges of the disconnected single-walled carbon nanotubes are exposed, and thus there is a problem in that the side reaction with the electrolyte solution is increased because the reactivity of the single-walled carbon nanotubes is increased compared to the particulate conductive material.

[0011] Meanwhile, when the carbon nanotubes are used as the conductive material, a carbon nanotube dispersion solution having a low solid content should be used in order to uniformly arrange the carbon nanotubes in the negative active material layer. However, when the carbon nanotubes having a low solid content are used, the phenomenon (migration) in which the binder and the conductive material having a relatively low density compared to the negative active material easily move to the upper portion of the negative active material layer (in a direction away from the current collector) occurs when the negative electrode is dried, and thus there is a problem in that the adhesion and the conductivity of the negative electrode are significantly degraded.

[0012] Accordingly, the present application introduces a negative electrode that can connect a conductive network even when the volume of the negative active material is greatly changed, and can minimize problems caused by migration of the binder. SUMMARY

[0013] TECHNICAL PROBLEM

[0014] One aspect of the present application provides a negative electrode that can improve the input / output characteristics and the life characteristics of a battery by reducing problems caused by the migration phenomenon of the binder while smoothly maintaining a conductive network.

[0015] Another aspect of the present application provides a secondary battery including the negative electrode.

[0016] TECHNICAL SOLUTION

[0017] According to an aspect of the present application, there is provided a negative electrode, comprising: 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; wherein the second negative electrode active material layer comprises a second negative electrode active material and a second conductive material; and the second negative electrode active material comprises a silicon-based active material and a carbon-based active material; wherein the silicon-based active material comprises SiO x (0≤x<2); and the second conductive material comprises: a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are connected side by side; and a particulate conductive material; wherein a weight ratio of the carbon nanotube structure to the particulate conductive material in the second negative electrode active material layer is 12.7:87.3 to 0.5:99.5.

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

[0019] Advantageous Effects

[0020] In the negative electrode according to the present application, the second negative electrode active material layer comprises a carbon nanotube structure in a rope shape in which a plurality of single-walled carbon nanotube units are connected side by side, so that the second negative electrode active material layer can be connected even though the volume change of SiO X (0≤x<2) is large, the carbon nanotube structure can connect the second negative electrode active material, thereby improving the life characteristics of the battery. In addition, the second negative electrode active material layer comprises the carbon nanotube structure and the particulate conductive material in a proper weight ratio, so that the conductive network of the second negative electrode active material layer can be more effectively formed. In addition, the negative electrode comprises the first negative electrode active material layer and the second negative electrode active material layer which are sequentially disposed using respective slurries, so that the migration phenomenon of the binder and the conductive material described above can be minimized. In addition, the second negative electrode active material layer comprises the carbon nanotube structure, so that the adhesion of the first negative electrode active material layer and the second negative electrode active material layer can be enhanced. Accordingly, the input / output characteristics and the life characteristics of the battery can be improved. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 is an SEM photograph of a cross-section of a negative electrode according to an embodiment of the present application.

[0022] Figure 2 is an SEM photograph of a negative electrode according to Example 1.

[0023] Figure 3 is an SEM photograph of a negative electrode according to Comparative Example 1.

[0024] Figure 4 is an adhesion evaluation diagram of the negative electrodes of Example 1, Comparative Example 1, and Comparative Example 2.

[0025] Figure 5 is a photograph showing the result of the adhesion test of the negative electrode of Example 1.

[0026] Figure 6 is a photograph showing the result of the adhesion test of the negative electrode of Comparative Example 1.

[0027] Figure 7 is a graph of the resistance evaluation of the negative electrode of Example 1 and Comparative Example 1.

[0028] Figure 8 is a graph of the battery resistance evaluation of the battery using the negative electrode of Example 1, the negative electrode of Comparative Example 1, and the negative electrode of Comparative Example 2, respectively.

[0029] Figure 9 is a graph of the battery resistance evaluation of the battery using the negative electrode of Example 1 and the negative electrode of Comparative Example 3, respectively. DETAILED DESCRIPTION

[0030] It will be understood that the words or terminology used herein in the specification and claims is not intended to limit the application to specific embodiments having the meaning defined in common dictionaries. It will be further understood that the terms or words should be interpreted as having a meaning consistent with the principles of the present application and the background of the technology in which the application is related, based on the meaning of the terms or words appropriately defined by the inventors to best explain the principles of the present application.

[0031] The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0032] It will be further understood that the terms "comprises" and / or "comprising," or "includes" and / or "including" when used in this specification, specify the presence of stated features, numbers, steps, elements, or combinations thereof, but do not preclude the presence or addition of one or more other features, numbers, steps, elements, or combinations thereof.

[0033] In the present specification, "%" means "% by weight" unless otherwise specified.

[0034] In the present specification, "specific surface area" is measured by the BET method, specifically, it can be calculated from the amount of nitrogen adsorption at a liquid nitrogen temperature (77 K) using Belsorp-mini II of BEL Japan, Ltd.

[0035] In the present specification, the average particle diameter "D 50 " can be defined as the particle diameter corresponding to the point at which the volume accumulates to 50% in the particle diameter distribution curve of the particles. The average particle diameter D 50It can be measured by, for example, a laser diffraction method. The laser diffraction method is generally capable of measuring particle diameters from a submicron region to several millimeters, and thus can obtain results with high reproducibility and high resolution.

[0036] In the present application, a single-walled carbon nanotube unit refers to a tubular unit having a single wall composed of carbon atoms, and a multi-walled carbon nanotube unit refers to a tubular unit having multiple layers of walls composed of carbon atoms in one tube.

[0037] Hereinafter, the present application will be described in detail.

[0038] Negative electrode

[0039] The negative electrode according to the present application includes: a negative electrode current collector; a first negative electrode active material layer provided on the negative electrode current collector; and a second negative electrode active material layer provided 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 material; and the second negative electrode active material includes a silicon-based active material and a carbon-based active material, wherein the silicon-based active material includes SiO x (0≤x<2); and the second conductive material includes: a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are combined side by side; and a particulate conductive material, wherein a weight ratio of the carbon nanotube structure to the particulate conductive material in the second negative electrode active material layer is 12.7:87.3 to 0.5:99.5.

[0040] The negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, as the negative electrode current collector, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel subjected to surface treatment with one of carbon, nickel, titanium, silver, or the like can be used. Specifically, transition metals such as copper and nickel, which adsorb carbon well, can be used as the negative electrode current collector.

[0041] The negative electrode can include a negative electrode active material layer. The negative electrode active material layer can be provided on one surface or both surfaces of the negative electrode current collector.

[0042] The negative electrode active material layer can include a first negative electrode active material layer and a second negative electrode active material layer. The first negative electrode active material layer can be provided on the negative electrode current collector, specifically, can be in contact with the negative electrode current collector. The second negative electrode active material layer can be provided on the first negative electrode active material layer, and the first negative electrode active material layer can be provided between the second negative electrode active material layer and the negative electrode current collector.

[0043] Generally, when carbon nanotubes are used as a conductive material, a carbon nanotube dispersion solution having a low solid content should be used to uniformly arrange the carbon nanotubes in the negative active material layer. However, when a carbon nanotube having a low solid content is used, a phenomenon (migration) in which a relatively low-density binder and conductive material compared to the negative active material is easily moved toward an upper portion of the negative active material layer (a portion far from the negative current collector and close to the surface) occurs when the negative electrode slurry is dried, thereby there is a problem in which the adhesion of the negative electrode and the conductivity are significantly deteriorated. However, the negative electrode of the present application includes a first negative active material layer and a second negative active material layer sequentially disposed using respective slurries, thereby the migration phenomenon of the binder and the conductive material described above can be minimized. Accordingly, the input / output characteristics and the life characteristics of the battery can be improved.

[0044] (1) First negative active material layer

[0045] The first negative active material layer can include a first negative active material.

[0046] The first negative active material can be a negative active material commonly used in the art, and the kind thereof is not particularly limited.

[0047] Specifically, the first negative active material can include a carbon-based active material, and as a particle of the carbon-based active material, one or more selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fiber, and graphitized mesocarbon microbeads can be used. Particularly, when artificial graphite is used, the rate characteristics can be improved.

[0048] The first negative active material can be included in the first negative active material layer in an amount of 70% by weight to 99.5% by weight, preferably 80% by weight to 99% by weight. When the content of the first negative active material satisfies the above range, the energy density of the negative electrode can be improved, the adhesion of the negative electrode can be improved, and the conductivity of the negative electrode can be improved.

[0049] The first negative active material can not include a silicon-based active material. Specifically, the first negative active material can consist only of a carbon-based negative active material. Accordingly, since the adhesion of the negative current collector and the first negative active material can be weakened due to the volume expansion of the silicon-based active material, the life characteristics of the battery can be improved.

[0050] The first negative active material layer can further include a first conductive material.

[0051] The first conductive material can include at least one selected from the group consisting of a carbon nanotube structure, a multi-walled carbon nanotube unit, graphene, and carbon black. The carbon nanotube structure will be described in detail later.

[0052] The first conductive material can be included in the first negative electrode active material layer in an amount of 0.01 to 2.0% by weight, specifically 0.01 to 1.5% by weight, more specifically 0.05 to 1.0% by weight. When the above range is satisfied, only a small amount of the first conductive material needs to be applied, the adhesion and conductivity of the negative electrode can be greatly improved, and a battery having excellent input / output characteristics and life characteristics of the battery can be obtained.

[0053] The thickness of the first negative electrode active material layer can be 1 to 100 μm, specifically 5 to 90 μm, more specifically 10 to 80 μm. When the above range is satisfied, the migration phenomenon of the above-mentioned conductive material and binder can be minimized. Thus, the adhesion and conductivity of the negative electrode can be greatly improved, and the input / output characteristics and life characteristics of the battery can be improved.

[0054] (2) Second negative electrode active material layer

[0055] The second negative electrode active material layer can include a second negative electrode active material and a second conductive material.

[0056] The second negative electrode active material can include a silicon-based active material and a carbon-based active material.

[0057] The silicon-based active material can include SiO x (0≤x<2). The SiO x (0≤x<2) can be specifically 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, not the first negative electrode active material layer, includes SiO x (0≤x<2), the durability of the negative electrode can be improved, and the electrolyte solution impregnation performance can be improved. More specifically, at the interface of the negative electrode current collector, which has the weakest binding force in the negative electrode, and the negative electrode active material layer, there is a problem that the negative electrode active material easily detaches from the negative electrode due to the contraction and expansion of the negative electrode active material during charging and discharging of the battery, and when SiO x (0≤x<2) is located close to the negative electrode current collector, the detachment phenomenon is accelerated. Thus, the durability of the negative electrode deteriorates, and the capacity and life characteristics of the battery deteriorate.

[0058] Meanwhile, during roll-pressing in the negative electrode manufacturing process, the density near the surface of the negative electrode excessively increases, and thus the electrolyte solution impregnation performance deteriorates. When SiO x (0≤x<2) is located close to the surface of the negative electrode, the volume of SiO x (0≤x<2) expands during initial charging of the battery, the density of the negative electrode can be reduced to an appropriate level, and the electrolyte solution impregnation performance can be improved.

[0059] The silicon-based active material can further include a carbon coating layer formed on SiO x (0≤x<2). The carbon coating layer can be provided on SiO x (0≤x<2). The carbon coating layer serves to improve the electrical conductivity of SiO x (0≤x<2) and suppress excessive volume expansion of SiO x (0≤x<2).

[0060] The carbon coating layer can include at least one of amorphous carbon and crystalline carbon.

[0061] The crystalline carbon can further improve the electrical conductivity of the negative active material. The crystalline carbon can include at least one selected from the group consisting of fluorene, carbon nanotube, and graphene.

[0062] The amorphous carbon can appropriately maintain the strength of the coating layer, thereby suppressing the expansion of the natural graphite. The amorphous carbon can be a carbonized product of at least one organic material selected from the group consisting of tar, pitch, and other organic materials, or a carbon-based material formed by using a hydrocarbon as a chemical vapor deposition source.

[0063] The carbonized product of the other organic material can be a carbonized product of an organic material selected from the group consisting of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldo-hexose or keto-hexose, and combinations thereof.

[0064] The hydrocarbon can be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or a substituted or unsubstituted aromatic hydrocarbon. The aliphatic or alicyclic hydrocarbon in the substituted or unsubstituted aliphatic or alicyclic hydrocarbon can be methane, ethane, ethylene, acetylene, propane, butane, butene, pentene, isobutene, or hexane, etc. The aromatic hydrocarbon in the substituted or unsubstituted aromatic hydrocarbon can be benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene, or phenanthrene, etc.

[0065] The average particle diameter (D 50 ) of the silicon-based active material can be 0.1 μm to 20 μm, specifically 1 μm to 10 μm. When the above range is satisfied, a side reaction between SiO x (0≤x<2) and an electrolyte solution can be suppressed, a reaction of SiO x (0≤x<2) to form lithium silicate can be controlled to prevent a decrease in initial efficiency, and an initial capacity of a battery can be maximized.

[0066] The carbon-based active material can include at least one selected from the group consisting of artificial graphite, natural graphite, and graphitized mesocarbon microbeads. Specifically, in terms of being able to maintain an electrical network together with the carbon nanotube structure to be described later while effectively controlling the volume expansion of the negative electrode, the carbon-based active material is preferably artificial graphite, but is not limited thereto.

[0067] The weight ratio of the silicon-based active material to the carbon-based active material can be 0.5:99.5 to 20:80, specifically 1:99 to 12:88. When the above range is satisfied, excessive volume expansion of the second negative active material can be suppressed, and the capacity of the battery can be improved.

[0068] The second negative active material can be included in the second negative active material layer in an amount of 90% by weight to 99% by weight, specifically 95% by weight to 99% by weight. When the above range is satisfied, the energy density of the negative electrode can be maintained at a high level, and the conductivity of the negative electrode and the adhesion of the negative electrode can be improved.

[0069] The second conductive material can include a carbon nanotube structure and a particulate conductive material.

[0070] The carbon nanotube structure can include a plurality of single-walled carbon nanotube units. Specifically, the carbon nanotube structure can be a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are connected side by side with each other. More specifically, the carbon nanotube structure can 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 connected with each other, in consideration of the durability of the second negative active material layer and the conductive network. In consideration of improving the dispersibility of the carbon nanotube structure and the durability of the negative electrode, the carbon nanotube structure can be formed by 2 to 50 single-walled carbon nanotube units being arranged and combined side by side with each other.

[0071] In the carbon nanotube structure, the single-walled carbon nanotube units can be arranged and combined side by side (cylindrical structure in the form of a bundle in which long axes of the units are combined parallel to each other, thereby having flexibility) to form the carbon nanotube structure. In the second negative active material layer, the carbon nanotube structures can be connected with each other to exhibit a network structure.

[0072] Typical electrodes including carbon nanotubes are generally manufactured by dispersing bundle-type or entangled-type carbon nanotubes (in which single-walled carbon nanotube units or multi-walled carbon nanotube units are attached or entangled with each other) in a dispersion medium to prepare a conductive material dispersion liquid, and then using the conductive material dispersion liquid. At this time, the carbon nanotubes are completely dispersed in the typical conductive material dispersion liquid, and exist as a conductive material dispersion liquid in which single-strand-like carbon nanotube units are dispersed. In the typical conductive material dispersion, the carbon nanotube units are easily cut due to an excessive dispersion process, and thus become shorter than at the beginning. In addition, the carbon nanotube units can also be easily cut during a rolling process of the negative electrode, and there is also a problem that the carbon nanotube units (particularly, single-walled carbon nanotube units) are cut due to an excessive volume change of the silicon electrode active material when the battery is driven. Thus, the conductivity of the negative electrode is reduced, and there is a problem that the life characteristics of the battery are reduced. In addition, the multi-walled carbon nanotube units have a high defect in structure due to a node growth mechanism (rather than a smooth line, there are nodes generated by defects occurring during the growth process). Thus, the multi-walled carbon nanotube units are more easily cut during the dispersion process, and the cut multi-walled carbon nanotube units are easily aggregated with each other by π-π stacking of carbon of the units. Thus, the units are more difficult to be uniformly dispersed and exist in the negative electrode slurry.

[0073] On the other hand, the carbon nanotube structure included in the second negative electrode active material layer of the present application has a long rope shape in which a plurality of single-walled carbon nanotube units maintaining high crystallinity while being relatively free of structural defects are arranged side by side and connected to each other, and thus maintains its length without being cut even if the volume of the second negative electrode active material is changed, thereby maintaining the conductivity of the negative electrode even during continuous charge and discharge of the battery. In addition, since the single-walled carbon nanotube units having high crystallinity have high conductivity, the conductivity of the negative electrode can be improved to reduce the negative electrode resistance, and the input / output characteristics and the life characteristics of the battery can be greatly improved. In addition, since the carbon nanotube structures are connected to each other to have a network structure directly by pressure during a rolling process, damage (e.g., a fracture phenomenon such as a crack) to the second negative electrode active material can be inhibited. In addition, even if a crack exists in the second negative electrode active material, the carbon nanotube structures can connect the second negative electrode active material through the crack, and thus the conductive network can be maintained. In addition, since the carbon nanotube structures can maintain a long shape without being easily broken, the conductive network of the entire second negative electrode active material layer can be enhanced. In addition, the deintercalation of the second negative electrode active material is inhibited, and thus the adhesion of the negative electrode can be greatly improved.

[0074] Further, since the carbon nanotube structure is included in the second negative active material layer, adhesion between the first negative active material layer and the second negative active material layer can be greatly improved. Since the carbon nanotube structure can be well connected to each other by van der Waals force due to the long rope shape formed by horizontally combining the single-walled carbon nanotube units inside the carbon nanotube structure, and can be firmly constituted as a negative electrode. Further, since the surface of the carbon nanotube structure and the carbon-based active material of the first negative active material layer can be more closely combined by π-π bonding occurring between homogenous carbons, adhesion between the first negative active material layer and the second negative active material layer can be further improved.

[0075] In the carbon nanotube structure, the average diameter of the single-walled carbon nanotube units can be 0.1 nm to 10 nm, specifically 1 nm to 9 nm. When the above average diameter is satisfied, even if the least amount of conductive material is used, it has an effect of maximizing the conductivity in the negative electrode. When the manufactured negative electrode is observed by TEM, the average diameter corresponds to the average of the diameters of the first 100 single-walled carbon nanotube units having a larger diameter and the diameters of the last 100 single-walled carbon nanotube units having a smaller diameter.

[0076] In the carbon nanotube structure, the average length of the single-walled carbon nanotube units can be 1 μm to 100 μm, specifically 5 μm to 50 μm. When the above average length is satisfied, a longer conductive path for conductive connection between the second negative active material can be formed, and a unique network structure can be formed, so even if the least amount of conductive material is used, it has an effect of maximizing the conductivity in the negative electrode. When the manufactured negative electrode is observed by TEM, the average length corresponds to the average of the lengths of the first 100 single-walled carbon nanotube units having a longer length and the lengths of the last 100 single-walled carbon nanotube units having a shorter length.

[0077] The specific surface area of the single-walled carbon nanotube units can be 500 m 2 / g to 1,000 m 2 / g, specifically 600 m 2 / g to 800 m 2 / g. When the above range is satisfied, since the specific surface area is large, a conductive path can be smoothly secured in the negative electrode, so even if the least amount of conductive material is used, it has an effect of maximizing the conductivity in the negative electrode. The specific surface area of the single-walled carbon nanotube units can be calculated from the nitrogen adsorption amount at liquid nitrogen temperature (77 K) using Belsorp-mino II of BEL Japan Co., Ltd.

[0078] The average diameter of the carbon nanotube structure can 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, an electrically conductive network structure is effectively formed, and connection between the second negative active materials is facilitated, so that excellent electrical conductivity can be achieved. When the manufactured negative electrode is observed by TEM, the average diameter corresponds to the average of the diameters of the first 100 carbon nanotube structures having a larger diameter and the last 100 carbon nanotube structures having a smaller diameter.

[0079] The average length of the carbon nanotube structure can be 3 μm to 15 μm, specifically 4 μm to 13 μm, and more specifically 5 μm to 10 μm. When the above range is satisfied, an electrically conductive network structure is effectively formed, and connection between the second negative active materials is facilitated, so that excellent electrical conductivity can be achieved. When the manufactured negative electrode is observed by TEM, the average length corresponds to the average of the lengths of the first 100 carbon nanotube units having a longer length and the last 100 carbon nanotube units having a shorter length.

[0080] The carbon nanotube structure can be included in the second negative active material layer in an amount of 0.005 wt% to 0.07 wt%, specifically 0.005 wt% to 0.05 wt%, and more specifically 0.01 wt% to 0.03 wt%. When the above range is satisfied, the electrically conductive path of the second negative active material layer can be secured, so that the life characteristics of the battery can be improved while maintaining a low level of negative electrode resistance. When the electrically conductive material dispersion liquid is prepared, in the case where the bundled carbon nanotube is completely dispersed (a single-walled carbon nanotube unit is dispersed to be as much as possible separated from each other by a commonly used dispersion method), the carbon nanotube structure is not generated, or even if it is accidentally generated, only a very small amount (for example, 0.0005 wt%) is generated. That is, it is not possible to achieve the above content range by a common method. The carbon nanotube structure has a form in which 2 to 5,000 single-walled carbon nanotube units are arranged side by side and combined with each other, so that the carbon nanotube structure can smoothly maintain its length without being cut, despite the volume change of the second negative active material. Accordingly, the electrically conductive network of the second negative active material layer can be maintained, and the electrical conductivity of the second negative electrode can be smoothly secured due to the high electrical conductivity of the carbon nanotube unit. Therefore, even when the content of the carbon nanotube structure in the second negative active material layer is low, the input / output characteristics and the life characteristics of the battery can be excellent.

[0081] Meanwhile, in some cases, the single-walled carbon nanotube unit can be surface-treated by an oxidation treatment or a nitration treatment to improve the affinity with the dispersant.

[0082] The particulate conductive material serves as a pivot for the conductive network in the second negative active material layer. Specifically, when the particulate conductive material is included in the second negative active material layer together with the carbon nanotube structure, the particulate conductive material serves as a pivot and the carbon nanotube structure forms a longer conductive path, so that the conductivity of the second negative active material layer can be effectively improved. Accordingly, the conductive network in the vertical and horizontal directions in the second negative active material layer is improved, the resistance of the second negative active material layer is reduced, and thus the rapid charging performance of the battery can be improved.

[0083] The particulate conductive material can include carbon black, and specifically, the particulate conductive material can be carbon black. Carbon black has high dispersibility and conductivity, and thus is advantageous for use in combination with the carbon nanotube structure. The carbon black can be at least one selected from the group consisting of acetylene black, ketjen black, channel black, and furnace black, but is not limited thereto.

[0084] The particulate conductive material can have an average particle diameter (D 50 ) of 0.1 μm to 100 μm, and specifically 0.1 μm to 5 μm. When the above range is satisfied, the energy density of the battery can be improved.

[0085] In the second negative active material layer, the weight ratio of the carbon nanotube structure to the particulate conductive material can be 12.7:87.3 to 0.5:99.5, and specifically 9:91 to 0.5:99.5, and more specifically 5:95 to 0.5:99.5. When the content of the particulate conductive material exceeds the above range (12.7:87.3 to 0.5:99.5) and is less than the above range, it is difficult to impart sufficient conductivity to the carbon-based active material of the second negative active material layer, and the conductive network is easily broken due to excessive volume expansion of the silicon-based active material, thereby causing a problem in that the long-term durability and rapid charging performance of the negative electrode are degraded. In contrast, when the content of the particulate conductive material exceeds the above range (12.7:87.3 to 0.5:99.5) and is greater than the above range, the conductive network of the second negative active material layer is not sufficiently formed, and the viscosity of the negative electrode is excessively increased, so that the processability of the mixing and coating of the negative electrode slurry is deteriorated.

[0086] Meanwhile, more specifically, with reference to the weight ratio of 9:91 to 0.5:99.5 or 5:95 to 0.5:99.5, it can be seen that the content of the carbon nanotube structure is extremely low. Since the carbon nanotube structure is used instead of using a general multi-walled carbon nanotube or a single-walled carbon nanotube unit present in a single strand form, an effective conductive network can be formed even with a small amount of the carbon nanotube structure.

[0087] The thickness of the second negative active material layer can be 1 to 100 µm, specifically 5 to 90 µm, and more specifically 10 to 80 µm. When the above range is satisfied, the migration phenomenon of the above-described conductive material and binder can be minimized. Accordingly, the adhesion of the negative electrode (adhesion between the negative active material layer and the current collector), the adhesion between the first negative active material layer and the second negative active material layer, the conductivity of the negative electrode are greatly improved, and the input / output characteristics and the life characteristics of the battery can be improved.

[0088] Preferably, the thickness of the second negative active material layer is greater than or equal to the thickness of the first negative active material layer. The ratio of the thickness of the first negative active material layer to the thickness of the second negative active material layer can be 1:1 to 1:2, specifically 1:1 to 1:1.5. When the above range is satisfied, the effect of suppressing the migration phenomenon of the above-described conductive material and binder is reduced, and the effect of increasing the diffusion resistance by increasing the porosity of the second negative active material layer is reduced. When the thickness of the first negative active material layer exceeds the above range and is too thin, the effect of suppressing the migration phenomenon of the above-described conductive material and binder is also reduced, and thus the effect of improving the adhesion of the negative electrode and the effect of increasing the interfacial resistance are insignificant.

[0089] An interface exists between the first negative active material and the second negative active material layer. This can be confirmed by the cross-section of the manufactured negative electrode. In contrast, if the negative active material layer is formed in a single layer structure (one-time coating by only one negative electrode slurry) rather than a multi-layer structure, the interface is not observed.

[0090] Each of the first negative active material layer and the second negative active material layer can further include a binder, and the binder of the first negative active material layer and the binder of the second negative active material layer can be the same or different. The binder is to secure the adhesion between the negative active materials or between the negative active material and the current collector. Any binder commonly used in the art can be used, and the type thereof is not particularly limited. The binder can be, for example, a vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated-EPDM, carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, etc., and any one or a mixture of two or more thereof can be used.

[0091] The binder can be included in the first negative electrode active material layer (or the second negative electrode active material layer) in an amount of 10% by weight or less, preferably 0.1 to 5% by weight. When the content of the binder satisfies the above range, excellent adhesion of the negative electrode can be achieved while minimizing the increase in the resistance of the negative electrode.

[0092] Method for manufacturing negative electrode

[0093] Next, a method of manufacturing a negative electrode according to the present application will be described.

[0094] The method of manufacturing a negative electrode according to the present application includes: 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 by the first negative electrode slurry, and forming a second negative electrode active material layer on the first negative electrode active material layer by the second negative electrode slurry, wherein 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 a carbon-based active material, wherein the silicon-based active material includes SiO x (0≤x<2), and the second conductive material includes a carbon nanotube structure in which a plurality of single-walled carbon nanotube units are joined side by side and a particulate conductive material, wherein in the second negative electrode active material layer, the weight ratio of the carbon nanotube structure to the particulate conductive material can be 12.7:87.3 to 0.5:99.5. The first negative electrode active material layer, the second negative electrode active material layer, the second negative electrode active material, the second conductive material, the carbon nanotube structure, and the particulate conductive material are the same as those in the above-described embodiments.

[0095] (1) preparing a first negative electrode slurry and a second negative electrode slurry

[0096] The method of preparing the first negative electrode slurry can be the same as the commonly used method of preparing a negative electrode slurry. For example, the first negative electrode slurry is prepared by preparing a mixture including a first negative electrode active material (the same as the first negative electrode active material in the above-described embodiments), a first conductive material (the same as the first conductive material in the above-described embodiments), and a solvent (which can further include a binder), and then stirring the mixture.

[0097] However, when the first negative electrode slurry contains the carbon nanotube structure, a carbon nanotube structure dispersion liquid, which will be described later, should be prepared.

[0098] Solvents can be, for example: 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 (isopropanol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, and octanol; and glycols, such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-propanediol, etc. Butanediol, 1,5-pentanediol, and hexanediol; polyols such as glycerol, trimethylolpropane, pentaerythritol, and 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, 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. Any one of these and mixtures of two or more of them may be used, but the solvent is not limited thereto. The solvent may be the same as or different from the dispersion medium used in the conductive material dispersion, and is preferably water.

[0099] The second negative electrode slurry can be prepared by preparing a mixture comprising a second negative electrode active material, a carbon nanotube structure dispersion, a particulate conductive material and a solvent, and then stirring the mixture.

[0100] Carbon nanotube structured dispersions can be prepared as follows.

[0101] The preparation of a carbon nanotube structure dispersion may include: step S1-1, preparing a mixed solution comprising a dispersion medium, a dispersant and bundled single-walled carbon nanotubes (a combination or aggregate of single-walled carbon nanotube units); and step S1-2, applying a shear force to the mixed solution to disperse the bundled single-walled carbon nanotubes, thereby forming a carbon nanotube structure in which multiple single-walled carbon nanotube units are combined side by side.

[0102] In step S1-1, a mixed solution can be prepared by introducing bundled single-walled carbon nanotubes and a dispersant into a dispersion medium. Bundled single-walled carbon nanotubes are carbon nanotubes in which the above-mentioned single-walled carbon nanotube units are combined together and exist in a bundle, typically including more than 2, substantially more than 500, for example more than 5000 single-walled carbon nanotube units.

[0103] The specific surface area of ​​bundled single-walled carbon nanotubes can reach 500 m². 2 / g to 1,200m 2 / g, specifically 550m 2 / g to 1,200m 2 / g. When the above range is satisfied, since the specific surface area is large, the electrically conductive path can be smoothly ensured in the second negative electrode active material layer, thereby having an effect of maximizing the electrical conductivity in the second negative electrode active material layer even though the minimum amount of the electrically conductive material is used. In addition, in order to enhance the adhesion between the first negative electrode active material layer and the second negative electrode active material layer, the specific surface area of the bundle-type single-walled carbon nanotube can be preferably 600 m 2 / g to 1,200 m 2 / g.

[0104] The bundle-type single-walled carbon nanotube can be included in the mixed solution in an amount of 0.1% by weight to 1.0% by weight, specifically 0.2% by weight to 0.5% by weight. When the above range is satisfied, the bundle-type single-walled carbon nanotube is dispersed to a suitable level, thereby a carbon nanotube structure of a suitable level can be formed, and the dispersion stability can be improved.

[0105] The dispersion medium can be, for example, an amide-based polar organic solvent such as water, dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); an alcohol such as methanol, ethanol, 1-propanol, 2-propanol (isopropanol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, and octanol; a glycol such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, and hexanediol; a polyol such as glycerol, trimethylolpropane, pentaerythritol, and sorbitol; a glycol ether 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; a ketone such as acetone, methyl ethyl ketone, methyl propyl ketone, and cyclopentanone; and an ester such as ethyl acetate, γ-butyrolactone, and ε-propylolactone. Any one of them and a mixture of two or more of them can be used, but the dispersion medium is not limited thereto. More specifically, the dispersion medium can be the same as or different from the solvent for preparing the negative electrode slurry, and can be preferably water.

[0106] The dispersant can include at least one of hydrogenated nitrile rubber, polyvinylidene fluoride, polystyrene, polyvinylpyrrolidone, polyvinyl alcohol, pyrene butyric acid, pyrene sulfonic acid, tannic acid, pyrene methylamine, sodium dodecyl sulfate, and carboxymethyl cellulose, and can be specifically carboxymethyl cellulose, polyvinylidene fluoride, polyvinylpyrrolidone, or hydrogenated nitrile rubber.

[0107] In the carbon nanotube structure dispersion liquid, the weight ratio of the bundle-type carbon nanotube to the dispersant can be 1:0.1 to 1:10, specifically 1:1 to 1:10. When the above range is satisfied, the bundle-type single-walled carbon nanotube is dispersed to an appropriate level, so that a carbon nanotube structure can be formed to an appropriate level, and dispersion stability can be improved.

[0108] The solid content in the mixed solution can be 0.1% by weight to 20% by weight, specifically 1% by weight to 10% by weight. When the above range is satisfied, the bundle-type single-walled carbon nanotube is dispersed to an appropriate level, so that a carbon nanotube structure can be formed to an appropriate level, and dispersion stability can be improved. In addition, when the above range is satisfied, the second negative electrode slurry (a slurry for preparing a second negative electrode active material layer) can have a viscosity and elasticity suitable for forming a second negative electrode active material layer, and the solid content of the second negative electrode slurry can be increased.

[0109] In step S1-2, the process of dispersing the bundle-type carbon nanotube in the mixed solution can be performed using a mixing device such as a homogenizer, an in-line mixer, a bead mill, a ball mill, a basket mill, a colloid mill, a general stirrer, a dissolver, a pin mill, a TK mixer, or a sonification device.

[0110] Specifically, step S1-2 can perform primary dispersion of the mixed solution by the in-line mixer, and then perform secondary dispersion of the mixed solution, which has been subjected to the primary dispersion, by the homogenizer.

[0111] The homogenizer can be a high pressure homogenizer (High Pressure Homogenizer) including a primary nozzle and a secondary nozzle. When pressure is applied to the mixed solution, the mixed solution passes through the primary nozzle and the secondary nozzle in sequence. Since the diameter of the secondary nozzle is smaller than that of the primary nozzle, the mixed solution is subjected to shear force when passing through the nozzle, at which time the bundle-type single-walled carbon nanotube is dispersed.

[0112] The diameter of the primary nozzle can be 100 mm to 500 mm, specifically 150 mm to 300 mm, and more specifically 150 nm to 250 mm. The diameter of the secondary nozzle can be 100 μm to 1000 μm, specifically 200 μm to 800 μm, and more specifically 200 μm to 650 μm. In addition, the pressure can be 500 Bar to 1800 Bar, and more specifically 800 Bar to 1600 Bar. When the pressure is greater than 1800 Bar, the bundle-type single-walled carbon nanotube is completely dispersed, so that a carbon nanotube structure can not be smoothly formed.

[0113] Unlike the method of a typical completely dispersed bundle type single-walled carbon nanotube, conditions of the application homogenizer (nozzle size, pressure, etc.), physical properties of the bundle type single-walled carbon nanotube to be used, conditions of the dispersant used, etc. are appropriately combined to disperse the bundle type single-walled carbon nanotube at an appropriate level without complete dispersion. Thus, in the conductive material dispersion liquid formed, the above-described carbon nanotube structure can exist mainly, and there are no or almost no single-walled carbon nanotube units that exist independently in a single strand form.

[0114] If necessary, the negative electrode slurry (the first negative electrode slurry and the second negative electrode slurry) can further include a binder. At this time, as the binder, the binder of the above-described embodiment can be used.

[0115] (2) forming a first negative electrode active material layer on the negative electrode current collector by the first negative electrode slurry, and forming a second negative electrode active material layer on the first negative electrode active material layer by the second negative electrode slurry

[0116] The first negative electrode active material layer and the second negative electrode active material layer can be formed by the following methods, but are not limited thereto.

[0117] As a first method, the first negative electrode slurry can be applied to the negative electrode current collector and dried to form the first negative electrode active material layer, and then the second negative electrode slurry can be applied to the first negative electrode active material layer and dried to form the second negative electrode active material layer. The roll pressing process can be performed immediately after the first negative electrode slurry is dried and immediately after the second negative electrode slurry is dried, or can be performed only immediately after the second negative electrode slurry is dried.

[0118] On the other hand, as a second method, the first negative electrode slurry and the second negative electrode slurry can be simultaneously applied, dried, and then roll pressed so that the first negative electrode slurry is positioned on the negative electrode current collector and the second negative electrode slurry is positioned on the first negative electrode slurry to form the first negative electrode active material layer and the second negative electrode active material layer.

[0119] Secondary battery

[0120] Next, a secondary battery according to another embodiment of the present application will be described.

[0121] The secondary battery according to another embodiment of the present application can include the negative electrode of the above-described embodiment.

[0122] Specifically, the secondary battery can include a negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. The negative electrode is the same as the negative electrode of the above-described embodiment. Since the negative electrode has been described above, a detailed description thereof will be omitted.

[0123] The positive electrode can include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including a positive electrode active material.

[0124] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel treated with one of carbon, nickel, titanium, silver, or the like on the surface can be used. In addition, the thickness of the positive electrode current collector can generally be 3 μm to 500 μm, and a slight concavo-convex can be formed on the surface of the positive electrode current collector to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam, and a nonwoven fabric.

[0125] The positive electrode active material can be a positive electrode active material commonly used in the art. Specifically, the positive electrode active material can be a layered compound such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; a lithium iron oxide such as LiFe3O4; a lithium manganese oxide such as LiMnO4 (0≤c1≤0.33), LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); a vanadium oxide such as LiV3O8, V2O5, and Cu2V2O7; a Ni-site type lithium nickel oxide represented by the formula LiNi 1+c1 Mn 2-c1 O4 (0≤c1≤0.33), LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); a vanadium oxide such as LiV3O8, V2O5, and Cu2V2O7; a Ni-site type lithium nickel oxide represented by the formula LiNi 1-c2 M c2 O2 (where M is any one of Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and 0.01≤c2≤0.3); a lithium manganese composite oxide represented by the formula LiMn 2-c3 M c3 O2 (where M is any one of Co, Ni, Fe, Cr, Zn, or Ta, and 0.01≤c3≤0.1), or the formula Li2Mn3MO8 (where M is any one of Fe, Co, Ni, Cu, or Zn); LiMn2O4 in which part of Li in the formula is substituted with an alkaline earth metal ion; and the like, but is not limited thereto. The positive electrode can be a lithium metal Li-metal.

[0126] The positive electrode active material layer can include a positive electrode conductive material and a positive electrode binder, as well as the positive electrode active material described above.

[0127] At this time, the positive electrode conductive material is used to impart electrical conductivity to the electrode, and any positive electrode conductive material can be used without particular limitation, as long as it has electronic conductivity and does not cause chemical changes in the battery. Specific examples of the positive electrode conductive material can 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 whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxides; or conductive polymers, such as derivatives of polyaniline, polypyrrole, and polyacetylene, any one of them or a mixture of two or more of them can be used.

[0128] Further, the positive electrode binder is used to improve the binding between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples of the positive electrode binder can include: polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, any one of them or a mixture of two or more of them can be used.

[0129] The separator is used to separate the negative electrode and the positive electrode and to provide a movement path for lithium ions. Any separator can be used without particular limitation, as long as it is a separator commonly used in secondary batteries. In particular, a separator having excellent moisture retention of the electrolyte and low resistance to ion movement in the electrolyte is preferred. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminate structure having two or more layers can be used. Further, a typical porous nonwoven fabric, for example, a nonwoven fabric formed of glass fibers having a high melting point, polyethylene terephthalate fibers, or the like can be used. Further, a coated separator including a ceramic component or a polymeric material can be used to ensure heat resistance or mechanical strength, and can be selectively used in a single layer or a multi-layer structure.

[0130] The electrolyte can be an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, a molten-type inorganic electrolyte, etc., which can be used in the preparation of a lithium secondary battery, but is not limited thereto.

[0131] Specifically, the electrolyte can include a non-aqueous organic solvent and a lithium salt.

[0132] As the non-aqueous organic solvent, for example, an aprotic organic solvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxy methane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate can be used.

[0133] In particular, among carbonate-based organic solvents, a cyclic carbonate such as ethylene carbonate and propylene carbonate can be preferably used because it is a high-viscosity organic solvent and has a high dielectric constant to well dissociate a lithium salt. Such a cyclic carbonate can be more preferably used because when it is mixed with a linear carbonate such as dimethyl carbonate and diethyl carbonate having a low viscosity and a low dielectric constant in an appropriate ratio, an electrolyte having a high electric conductivity can be prepared.

[0134] A lithium salt can be used as the metal salt. The lithium salt is a material that is easily dissolved in a non-aqueous electrolyte solution. For example, as the anion of the lithium salt, one or more selected from the group consisting of 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 - .

[0135] In the electrolyte, in order to improve the life characteristics of the battery, suppress the decrease in the capacity of the battery, and increase the discharge capacity of the battery, in addition to the above-mentioned electrolyte components, one or more additives, for example, a halogenated alkylene carbonate compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, a cyclic ether, ethylenediamine, a glyme, triamide hexaphosphoric acid, a nitrobenzene derivative, sulfur, a quinonimine dye, an N-substituted oxazolidinone, an N,N-substituted imidazolidine, an ethylene glycol dialkyl ether, an ammonium salt, a pyrrole, 2-methoxyethanol, or aluminum trichloride, or the like, can be further included.

[0136] According to still another embodiment of the present application, a battery module including the secondary battery as a unit cell, and a battery pack including the battery module are provided. The battery module and the battery pack include the secondary battery having high capacity, high rate performance, and high cycle characteristics, and thus can be used as a power source for a medium- and 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.

[0137] Hereinafter, the present application will be described in greater detail with reference to specific embodiments.

[0138] Preparation Example 1: Preparation of carbon nanotube structure dispersion liquid

[0139] 0.4 wt% of carbon nanotube structure having a specific surface area of 1160 m 2A bundle type carbon nanotube (specific surface area: 185 m2 / g) composed of multi-walled carbon nanotube units having an average diameter of 10 nm and an average length of 1 μm, 0.6 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) as a dispersant, and 95.4 parts by weight of water as a dispersion medium were mixed to prepare a mixed solution having 4.6% by weight of solid content.

[0140] Preparation Example 2: Preparation of multi-walled carbon nanotube unit dispersion

[0141] A bundle type carbon nanotube (specific surface area: 185 m 2 / g) composed of multi-walled carbon nanotube units having an average diameter of 10 nm and an average length of 1 μm, 0.6 parts by weight of carboxymethyl cellulose (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) as a dispersant, and 95.4 parts by weight of water as a dispersion medium were mixed to prepare a mixed solution having 4.6% by weight of solid content.

[0142] The mixed solution was introduced into a bead mill in which 80% was filled with beads having a size of 0.65 mm to be dispersed, and then discharged at a discharge rate of 2 kg / min. The above procedure was repeated twice to completely disperse the bundle type carbon nanotube, thereby preparing a multi-walled carbon nanotube unit dispersion. In the dispersion, the multi-walled carbon nanotube unit (average diameter: 10 nm) was 4.0% by weight, and the carboxymethyl cellulose was 0.6% by weight.

[0143] Preparation Example 3: Preparation of single-walled carbon nanotube unit dispersion

[0144] A bundle type carbon nanotube (specific surface area: 185 m 2A 1 kg mixture was prepared by mixing 0.45 wt% of single-walled carbon nanotubes (TUBALL, OCSiAl Co., Ltd.), 0.15 wt% of polyvinylpyrrolidone (K15, Zhangzhou Huafu Chemical Co., Ltd.) as a dispersant, and 0.15 wt% of tannic acid (Sigma Aldrich Co., Ltd.) as a dispersant, and water as a solvent. This mixture was then treated for 30 minutes using a Verso (silverson Co., Ltd.) high-shear inline mixer at 10,000 rpm. Subsequently, the mixture was passed sequentially through a 200 mm diameter primary nozzle and a 500 μm diameter secondary nozzle using a PICOMAX device (High Pressure Homogenizer) from Micronox Co., Ltd. at 2000 bar pressure. This process yielded a single-walled carbon nanotube unit dispersion. No carbon nanotube structures were observed in this dispersion.

[0145] Examples and comparative examples

[0146] Example 1: Manufacturing of the negative electrode

[0147] (1) Formation of the first negative electrode slurry

[0148] The carbon nanotube structure dispersion prepared in Example 1, with an average particle size (D) 50 Artificial graphite with a particle size of 21 μm and an average particle size (D) 50 A first negative electrode slurry was prepared by mixing 18 μm natural graphite, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) with water. In the first negative electrode slurry, the weight ratio of artificial graphite, natural graphite, SBR, CMC, and carbon nanotube structures was 56.94:37.96:3.0:1.1:1.0.

[0149] (2) Formation of the second negative electrode slurry

[0150] Average particle size (D) 50 Carbon black with a particle size of less than 5 μm, carbon nanotube structure dispersion prepared in Example 1, and second negative electrode active material (average particle size (D) 50 Artificial graphite with a particle size of 21 μm: average particle size (D) 50) and SBR and carboxymethyl cellulose (CMC) (weight average molecular weight: 100,000 g / mol, degree of substitution: 1.0) as a binder were mixed with water to prepare a second negative electrode slurry. In the second negative electrode slurry, the weight ratio of the second negative electrode active material, SBR, CMC, carbon nanotube structure, and carbon black was 96.255:1.6:1.145:0.03:0.97.

[0151] (3) Formation of the first negative electrode active material layer and the second negative electrode active material layer

[0152] The first negative electrode slurry and the second negative electrode slurry were simultaneously applied to a negative electrode current collector (copper (Cu) metal thin film) having a thickness of 8 μm, the first negative electrode slurry was disposed on the negative electrode current collector, and the second negative electrode slurry was disposed on the first negative electrode slurry. Thereafter, the negative electrode current collector on which the first negative electrode slurry and the second negative electrode slurry were applied was dried at 130°C and roll-pressed to form a second negative electrode active material layer (thickness: 70.4 μm) and a first negative electrode active material layer (thickness: 74 μm).

[0153] Comparative Example 1: Manufacture of a negative electrode

[0154] A negative electrode was manufactured in the same manner as in Example 1, except that, when the second negative electrode active material layer of Example 1 was formed, the multi-walled carbon nanotube unit dispersion liquid of Preparation Example 2 was used instead of the carbon nanotube structure of Preparation Example 1, and in the second negative electrode slurry, the weight ratio of the second negative electrode active material, SBR, CMC, multi-walled carbon nanotube unit, and carbon black was 96.2:1.6:1.2:0.5:0.5.

[0155] Comparative Example 2: Manufacture of a negative electrode

[0156] A negative electrode was manufactured in the same manner as in Example 1, except that, when the second negative electrode active material layer of Example 1 was formed, in the second negative electrode slurry, the weight ratio of the second negative electrode active material, SBR, CMC, carbon nanotube structure, and carbon black was adjusted to 96.296:1.6:1.104:0.00272:0.99728.

[0157] Comparative Example 3: Manufacture of a negative electrode

[0158] A negative electrode was manufactured in the same manner as in Example 1, except that the single-walled carbon nanotube unit dispersion liquid of Preparation Example 3 was used instead of the carbon nanotube structure of Preparation Example 1.

[0159] [Table 1]

[0160]

[0161] Table 1 above shows the average diameter and average length of each of the carbon nanotube structure, multi-walled carbon nanotube, and single-walled carbon nanotube unit present in the second negative electrode active material layer. The average diameter corresponds to the average of the diameters of the first 100 carbon nanotube structures (or multi-walled carbon nanotubes, or single-walled carbon nanotube units) having a larger diameter and the diameters of the last 100 carbon nanotube structures (or multi-walled carbon nanotubes, or single-walled carbon nanotube units) having a smaller diameter. The average length corresponds to the average of the lengths of the first 100 carbon nanotube structures (or multi-walled carbon nanotubes, or single-walled carbon nanotube units) having a longer length and the lengths of the last 100 carbon nanotube structures (or multi-walled carbon nanotubes, or single-walled carbon nanotube units) having a shorter length. The above were confirmed by SEM.

[0162] Test example 1: observation of negative electrode

[0163] (1) Confirmation of the presence of carbon nanotube structures

[0164] Figure 1 and 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 Comparative Example 1.

[0165] Referring to Figure 1 , it can be seen that the second negative electrode active material layer including the silicon-based active material is present. Referring to Figure 2 , it can be seen that carbon nanotube structures in the form of long ropes in which a plurality of single-walled carbon nanotube units are connected side by side and combined with each other, and the silicon-based active material are connected to each other through the carbon nanotube structures are present. On the other hand, in Figure 3 , only multi-walled carbon nanotube units having a short length are observed, and no carbon nanotube structures are observed, and the multi-walled carbon nanotube structures are completely adsorbed on the surface of the artificial graphite, and do not normally function to connect the silicon-based active material to each other.

[0166] Test example 2: evaluation of adhesion of negative electrode

[0167] The negative electrode adhesion of the negative electrodes of Example 1 and Comparative Examples 1 and 2 was evaluated. The negative electrode adhesion was measured under dry conditions. Specifically, a double-sided tape was attached to a glass slide, a negative electrode punched into 20 mm x 180 mm was placed on the glass slide, and then the negative electrode was adhered to the glass slide by rolling with a 2 kg roller back and forth 10 times, and using a UTM (TA Co., Ltd.) device, the negative electrode was pulled at a speed of 200 mm / min to measure the force required to peel the negative electrode from the glass slide. At this time, the measurement angle of the glass slide and the negative electrode was 90°. The measurement results are shown in Figure 4 .

[0168] Referring to Figure 4It can be seen that the adhesion of the negative electrode of Example 1 is higher than that of Comparative Example 1 and Comparative Example 2.

[0169] Test example 3: evaluation of adhesion between first negative electrode active material layer and second negative electrode active material layer

[0170] Figure 5 is the result of the adhesion test of the negative electrode of Example 1, Figure 6 is the result of the adhesion test of the negative electrode of Comparative Example 1.

[0171] Comparing Figure 5 and Figure 6 , in the case of the negative electrode of Example 1, the adhesion between the first negative electrode active material layer and the second negative electrode active material layer is excellent, so that the current collector is peeled off with the first negative electrode active material layer to expose the current collector (see Figure 5 ). On the other hand, in the case of the negative electrode of Comparative Example 1, the adhesion between the first negative electrode active material layer and the second negative electrode active material layer is relatively weak, so that only the second negative electrode active material layer is peeled off (see Figure 6 ).

[0172] Test example 4: evaluation of resistance of negative electrode

[0173] The resistance of the negative electrode of Example 1 and Comparative Example 1 was evaluated and is shown in Figure 5 .

[0174] Specifically, the negative electrode resistance was set as the negative electrode layer resistance and the interface contact resistance, and thus the resistance values of the first negative electrode active material layer and the second negative electrode active material layer were calculated using the potential difference measured between the probes.

[0175] Referring to Figure 7 , it can be seen that the "interface resistance between the negative electrode active material layer and the current collector (interface of Figure 5 ) of Example 1 is lower than that of Comparative Example 1. This can be due to the fact that the silicon-based active material can be smoothly connected through the carbon nanotube structure, and also the migration phenomenon of the negative electrode binder can be inhibited, so that the negative electrode binder can be uniformly distributed.

[0176] Test example 5: evaluation of capacity retention rate (life characteristics)

[0177] A battery was manufactured in the following manner using the negative electrode of Example 1 and each of Comparative Examples 1 and 2.

[0178] Li[Ni 0.6 Mn 0.2 Co 0.2O2 as a positive electrode active material. The positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder were mixed at a weight ratio of 94:4:2 with N-methyl-2-pyrrolidone as a solvent to prepare a positive electrode slurry.

[0179] The prepared positive electrode slurry was applied to an aluminum metal thin film having a thickness of 15 μm as a positive electrode current collector, followed by drying. At this time, the temperature of the circulating air was 110°C. Thereafter, the aluminum metal thin film to which the positive electrode slurry was applied and dried was roll-pressed, and then dried in a vacuum oven at 130°C for 2 hours to form a positive electrode active material layer.

[0180] The negative electrode of each of Example 1 and Comparative Examples 1 and 2, the prepared positive electrode, and a porous polyethylene separator were assembled using a Stacking method, an electrolyte solution (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 1 / 2 (volume ratio) and lithium hexafluorophosphate (LiPF6 1 mole) was injected into the assembled battery to manufacture a lithium secondary battery.

[0181] Evaluation of capacity retention rate (life characteristics)

[0182] Each lithium secondary battery was subjected to charge and discharge under the following conditions.

[0183] Each lithium secondary battery was subjected to a total of 300 cycles, in which 0.33C / 0.33C charging / discharging at 45°C in a voltage range of 4.2V to 2.8V was set as one cycle. Thereafter, the discharge capacity (capacity maintenance rate) was evaluated based on the discharge capacity after one cycle of 100%, and is shown in Figure 9 .

[0184] Referring to Figure 8 It can be seen that Example 1 has improved initial deterioration and excellent long-term life characteristics compared to Comparative Examples 1 and 2.

[0185] Test example 6: evaluation of resistance of battery

[0186] A battery was manufactured in the same manner as in Test Example 5 using the negative electrode of each of Example 1 and Comparative Example 3.

[0187] Thereafter, each battery was charged to SOC 50%, and then discharged under conditions of 2.5C and 10 second pulses. The resistance was calculated by dividing the Δ voltage value of Vi-Vf by the magnitude of the current to evaluate the battery resistance, which is shown in Figure 9 .

[0188] Referring to Figure 9As can be seen, when the negative electrode of Example 1 including the carbon nanotube structure is used, the battery resistance is lower than the battery resistance when the negative electrode of Comparative Example 3 including the single-walled carbon nanotube unit is used.

Claims

1. A negative electrode, comprising: Negative 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, wherein: The second negative electrode active material layer includes a second negative electrode active material and a second conductive material; and The second negative electrode active material includes silicon-based active materials and carbon-based active materials. in: The silicon-based active material includes SiO₂. x where 0 ≤ x < 2; The second conductive material includes: Carbon nanotube structure, in which multiple single-walled carbon nanotube units are bonded together side by side; and A particulate conductive material, wherein in the second negative electrode active material layer, the weight ratio of the carbon nanotube structure to the particulate conductive material is from 12.7:87.3 to 0.5:99.5, and the content of the carbon nanotube structure in the second negative electrode active material layer is from 0.005% to 0.07% by weight.

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

3. The negative electrode according to claim 1, wherein in the carbon nanotube structure, the single-walled carbon nanotube units are combined in a state in which the long axes of the single-walled carbon nanotube units are arranged parallel to each other.

4. The negative electrode according to claim 1, wherein the average length of the carbon nanotube structure is 3 μm to 15 μm.

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

6. The negative electrode according to claim 1, wherein in the carbon nanotube structure, the average diameter of the single-walled carbon nanotube unit is from 0.1 nm to 10 nm.

7. The negative electrode according to claim 1, wherein the carbon nanotube structure is a carbon nanotube structure formed by combining 2 to 50 single-walled carbon nanotube units.

8. The negative electrode according to claim 1, wherein the particulate conductive material comprises carbon black.

9. The negative electrode according to claim 1, wherein the average particle size D of the particulate conductive material is... 50 The range is from 0.1 μm to 100 μm.

10. The negative electrode according to claim 1, wherein the first negative electrode active material layer comprises a first negative electrode active material and a first conductive material, wherein the first conductive material comprises at least one selected from the group consisting of the carbon nanotube structure, multi-walled carbon nanotube units, graphene and carbon black.

11. The negative electrode according to claim 1, wherein the ratio of the thickness of the first negative electrode active material layer to the thickness of the second negative electrode active material layer is 1:1 to 1:

2.

12. A secondary battery comprising the negative electrode as described in claim 1.

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