Electrode and secondary battery including the same

Abstract

The present invention relates to an electrode and a secondary battery including the electrode. The electrode includes an electrode active material layer, and the electrode active material layer includes an electrode active material and a conductive agent. The conductive agent includes: graphene; a carbon nanotube structure in which 2 to 5,000 single-wall carbon nanotube units are bonded to each other; and carbon black, wherein the carbon nanotube structure is included in the electrode active material layer in an amount of 0.01 wt% to 0.5 wt%.

Description

Technical Field

[0001] Cross - Reference to Related Applications

[0002] This application claims the benefit of priority of Korean Patent Application No. 10-2019-0123261, filed on Oct. 4, 2019, the entire disclosure of which is incorporated herein by reference. Technical Field

[0004] The present invention relates to an electrode and a secondary battery including the electrode, the electrode including an electrode active material layer including an electrode active material and a conductive agent, the conductive agent including: graphene; a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded to each other; and carbon black, wherein the carbon nanotube structure is included in the electrode active material layer in an amount of 0.01 wt% to 0.5 wt%. Background Art

[0005] With the recent technological development and increased demand for mobile devices, the demand for batteries as an energy source has increased significantly, and thus various studies on batteries capable of meeting various demands have been carried out. In particular, as a power source for such devices, lithium secondary batteries having excellent life characteristics, cycle characteristics, and high energy density have been actively studied.

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

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

[0008] Single-walled carbon nanotubes are one type of linear conductive agent, and due to their thin and elongated shape, they improve the conductivity in the electrode active material layer. Therefore, generally, single-walled carbon nanotubes are fully dispersed to prepare a dispersion including single-walled carbon nanotube units, where the single-walled carbon nanotube units exist in a single-chain form, and then an electrode paste is prepared through this dispersion, and an electrode active material layer is prepared through this electrode paste. Thus, single-walled carbon nanotubes exist in the electrode active material layer as units (single chains). However, when the charge and discharge of the battery are repeated, the surface of the single-walled carbon nanotube units is damaged or the single-walled carbon nanotube units are broken. Therefore, there is a limitation in maintaining the conductive network in the electrode active material layer. As a result, the conductive network is blocked or reduced, and this reduces the life characteristics of the battery.

[0009] For this reason, there is a method of using multi-walled carbon nanotubes to ensure conductivity in the case where the surface of the carbon nanotubes is damaged. However, due to the structure formed by growing with nodes, multi-walled carbon nanotubes are cut into too short lengths during the preparation of the dispersion, so there are limitations in improving the conductivity of the electrode.

[0010] Meanwhile, planar conductive agents such as graphene also have excellent conductivity, but there are limitations. It is difficult to produce single-layer graphene with a relatively thin thickness, and if graphene with a relatively thick thickness is used, the battery efficiency decreases. In addition, in the case of planar conductive agents, there is a limitation that due to broad planar contact, the mobility of the electrolyte in the battery is restricted. Moreover, graphene mainly exists in the form of covering the surface of the electrode active material, so it is not suitable for forming a long conductive network between the electrode active materials.

[0011] In addition, when using linear conductive agents or planar conductive agents, there is a limitation that the electrode active material is easily damaged during the process of winding the electrode.

[0012] Therefore, a method is needed to suppress the damage of the electrode active material, reduce the resistance of the electrode, and thus improve the life characteristics of the battery. Summary of the Invention

[0013] Technical Problem

[0014] One aspect of the present invention provides an electrode that can suppress the damage of the electrode active material and reduce the resistance of the electrode, thereby improving the life characteristics of the battery.

[0015] Another aspect of the present invention provides a secondary battery including the electrode

[0016] Technical Solution

[0017] According to one aspect of the present invention, there is provided an electrode including an electrode active material layer, the electrode active material layer including an electrode active material and a conductive agent, the conductive agent including: graphene; a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded to each other; and carbon black, wherein the carbon nanotube structure is included in the electrode active material layer in an amount of 0.01 wt% to 0.5 wt%.

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

[0019] Advantageous Effects

[0020] The electrode according to the present invention includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded to each other, and thus can smoothly maintain a conductive network. Therefore, the resistance of the electrode can be kept at a low level, and the life characteristics of the battery can be improved. In addition, since the carbon nanotube structure exists in the form of a long string in the electrode, even if the battery is continuously charged and discharged, a decrease in conductivity due to damage to the carbon nanotube structure can be suppressed, and a long conductive network can be formed. In addition, since the electrode includes graphene, the resistance of the electrode can be further reduced due to the excellent conductivity of graphene. In addition, since graphene helps to form a short conductive network by covering the surface of the electrode active material, a conductive network can be uniformly formed over the entire electrode due to the combined use of graphene and the carbon nanotube structure. In addition, since graphene exists while covering at least a part of the carbon nanotube structure, a tight conductive network can be maintained even during repeated charge and discharge of the battery. Therefore, the life characteristics of the battery can be improved. In addition, since the electrode of the present invention includes carbon black, damage to the electrode active material during winding of the electrode can be suppressed due to the low hardness of the carbon black. Therefore, the life performance of the battery can be improved. Description of the Drawings

[0021] Figure 1 are SEM photographs of a commonly used multi-walled carbon nanotube unit (A) and the carbon nanotube structures (B and C) used in the examples of the present invention.

[0022] Figure 2 are TEM photographs of the carbon nanotube structure (A) used in the examples of the present invention and a completely dispersed single-walled carbon nanotube unit (B) commonly used.

[0023] Figure 3 is an SEM photograph of the electrode in the examples of the present invention. Detailed Description

[0024] The terms or words used in the specification and the claims should not be construed as being limited to the ordinary meaning or dictionary meaning, but should be construed as meanings and concepts consistent with the technical spirit based on the principle that the inventor can appropriately define the concept of the terms in order to interpret the present invention in the best manner.

[0025] The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting of the present invention. Terms in the singular may include plural forms unless the context clearly indicates otherwise.

[0026] It will be understood that when used in this specification, the terms "comprises", "comprising" or "having" 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.

[0027] In this specification, unless otherwise specifically stated, the expression "%" means % by weight.

[0028] In this specification, the expression "specific surface area" is measured by the BET method, wherein, in particular, the specific surface area can be calculated from the nitrogen adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mini II manufactured by Bell Japan.

[0029] In this specification, the average particle size (D 50 ) can be defined as the particle size at which the cumulative volume in the particle size distribution curve is 50%. For example, the average particle size (D) can be measured by using a laser diffraction method. 50 Laser diffraction is generally capable of measuring particle sizes ranging from submicron to several millimeters, and can obtain results with high repeatability and high resolution.

[0030] In the present invention, 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 walls composed of carbon atoms in one tube.

[0031] The term graphene herein refers to a carbonaceous structure in the form of a thin film, which has a form in which one graphite plane or a plurality of graphite planes are stacked and has flexibility.

[0032] Hereinafter, the present invention will be described in detail.

[0033] Electrode

[0034] The electrode according to the present invention includes an electrode active material layer, the electrode active material layer includes an electrode active material and a conductive agent, the conductive agent includes: graphene; a carbon nanotube structure in which 2 to 5,000 single-wall carbon nanotube units are bonded to each other; and carbon black, wherein the carbon nanotube structure is included in the electrode active material layer in an amount of 0.01 wt% to 0.5 wt%.

[0035] The electrode may include an electrode active material layer. The electrode may further include a current collector, and in this case, the electrode active material layer may be disposed on one surface or both surfaces of the current collector.

[0036] There is no particular limitation on the current collector as long as the material of the current collector has conductivity and does not cause adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, their alloys, these materials surface-treated with one of carbon, nickel, titanium, silver, etc., or calcined carbon, etc. may be used.

[0037] The current collector generally may have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the surface of the current collector to improve the adhesion of the electrode active material. In addition, the electrode current collector may be used in various shapes such as a film, sheet, foil, net, porous body, foam body, non-woven fabric body, etc.

[0038] The electrode active material layer may include an electrode active material and a conductive agent.

[0039] The electrode active material may be a positive electrode active material or a negative electrode active material commonly used in the art, and there is no particular limitation on its type.

[0040] For example, a lithium oxide containing at least one metal such as cobalt, manganese, nickel, or aluminum may be used as the positive electrode active material. More particularly, the lithium oxide may include: lithium manganese-based oxides (such as LiMnO2, LiMn2O, etc.); lithium cobalt-based oxides (such as LiCoO2, etc.); lithium nickel-based oxides (such as LiNiO2, etc.); lithium nickel manganese-based oxides (such as LiNi 1-Y1 Mn Y1 O2 (where 0 < Y1 < 1), LiNi Z1 Mn 2-Z1 O4 (where 0 < Z1 < 2), etc.); lithium nickel cobalt-based oxides (such as LiNi 1-Y2 Co Y2 O2 (where 0 < Y2 < 1), etc.); lithium manganese cobalt-based oxides (such as LiCo 1-Y3 Mn Y3 O2 (where 0 < Y3 < 1), LiMn 2-Z2 Co z2O4 (where 0 < Z2 < 2, etc.); lithium nickel cobalt manganese-based oxides (such as Li(Ni P1 Co Q1 Mn R1 )O2 (where 0 < P1 < 1, 0 < Q1 < 1, 0 < R1 < 1, and P1 + Q1 + R1 = 1) or Li(Ni P2 Co Q2 Mn R2 )O4 (where 0 < P2 < 2, 0 < Q2 < 2, 0 < R2 < 2, and P2 + Q2 + R2 = 2), etc.); or lithium nickel cobalt manganese-other metal (M) oxides (such as Li(Ni P3 Co Q3 Mn R3 M 1 S )O2 (where M 1 is selected from the group consisting of aluminum (Al), copper (Cu), iron (Fe), vanadium (V), chromium (Cr), titanium (Ti), zirconium (Zr), zinc (Zn), tantalum (Ta), niobium (Nb), magnesium (Mg), boron (B), tungsten (W), and molybdenum (Mo), and P3, Q3, R3, and S are the atomic fractions of each independent element, where 0 < P3 < 1, 0 < Q3 < 1, 0 < R3 < 1, 0 < S < 1, and P3 + Q3 + R3 + S = 1), etc.), and may include any one of them or a mixture of two or more.

[0041] Meanwhile, the negative electrode active material may include: for example, carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metal compounds that can alloy with lithium such as silicon (Si), aluminum (Al), tin (Sn), lead (Pb), zinc (Zn), bismuth (Bi), indium (In), magnesium (Mg), gallium (Ga), cadmium (Cd), Si alloy, Sn alloy, or Al alloy; metal oxides that may or may not be doped with lithium such as SiO v (0 < v < 2), SnO2, vanadium oxides, and lithium vanadium oxides; or composites including metal compounds and carbonaceous materials such as Si-C composites or Sn-C composites, and any one of them or a mixture of two or more may be used. In addition, a thin film of metallic lithium can be used as the negative electrode active material. In addition, both low-crystalline carbon and high-crystalline carbon can be used as carbonaceous materials.

[0042] The electrode active material may be included in an amount of 70 wt% to 99.5 wt%, preferably 80 wt% to 99 wt%, based on the total weight of the electrode active material layer. When the content of the electrode active material satisfies the above range, excellent energy density, electrode adhesion, and conductivity can be achieved.

[0043] The conductive agent may include graphene, carbon nanotube structures, and carbon black.

[0044] (1) Graphene

[0045] Graphene is mainly disposed on the surface of the electrode active material to help form a conductive network between adjacent electrode active materials.

[0046] Graphene may include a structure in which 1 to 1,000 graphite planes are stacked. In particular, the stacked graphite planes may be 1 to 500, and more particularly 1 to 100. When the above ranges are satisfied, the characteristics of high-quality graphene with flexibility due to its thin thickness can be utilized, and graphene is easily and uniformly dispersed in the electrode active material layer. In addition, even when a low content of graphene is used, the conductivity of the electrode active material layer can be significantly improved.

[0047] The average length of graphene can be 1 μm to 100 μm, particularly 0.1 μm to 10 μm, and more particularly 0.1 μm to 5 μm. When the above ranges are satisfied, graphene can be present while appropriately covering the electrode active material. In addition, graphene covers at least a part of the carbon nanotube structure, so that the separation of the carbon nanotube structure from the electrode active material can be further suppressed. Therefore, the conductivity between the electrode active materials and the electrode active material can be improved. In the electrode observed by SEM or TEM, the average length corresponds to the average of the lengths of the first 100 graphene with longer lengths and the lengths of the last 100 graphene with shorter lengths. In this article, the expression "the length of graphene" means the longest length when assuming that there is a line from one point to another point in one graphene.

[0048] The average thickness of graphene can be 0.3 nm to 300 nm, particularly 1 nm to 100 nm, and more particularly 1 nm to 50 nm. When the above ranges are satisfied, the characteristics of high-quality graphene with flexibility due to its thin thickness can be utilized to better cover the electrode active material, and graphene is easily and uniformly dispersed in the electrode active material layer. In addition, even when a low content of graphene is used, the conductivity of the electrode active material layer can be significantly improved. In the electrode observed by SEM or TEM, the average thickness corresponds to the average of the thicknesses of the first 100 graphene with thicker thicknesses and the thicknesses of the last 100 graphene with thinner thicknesses.

[0049] The BET specific surface area of graphene can be 100 m 2 / g to 500 m 2 / g, particularly 100 m 2 / g to 400 m 2 / g, and more particularly 100 m 2 / g to 300 m 2 / g. When within the above range, the dispersibility of graphene in the electrode paste is good, and even with a small amount of graphene, the conductivity of the electrode active material layer can be significantly improved. The BET specific surface area can be measured by the nitrogen adsorption BET method.

[0050] Graphene can be included in the electrode active material layer in an amount of 0.01 wt% to 1.0 wt%, particularly 0.05 wt% to 0.5 wt%, and more particularly 0.1 wt% to 0.5 wt%. When within the above range, applying only a small amount of graphene can significantly improve the adhesion and conductivity of the electrode, and can improve the input / output characteristics and life characteristics of the battery.

[0051] (2) Carbon nanotube structure

[0052] The carbon nanotube structure can include a plurality of single-walled carbon nanotube units. In particular, the carbon nanotube structure can be a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side to each other. More particularly, considering the durability of the electrode and the conductive network, most preferably the carbon nanotube structure is a carbon nanotube structure in which 2 to 4,500, preferably 50 to 4,000, and more preferably 1,000 to 4,000 single-walled carbon nanotube units are bonded to each other.

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

[0054] Conventional electrodes including carbon nanotubes are generally manufactured in the following manner: A conductive agent dispersion is prepared by dispersing bundle type or entangled type carbon nanotubes (in the form where single-walled carbon nanotube units or multi-walled carbon nanotube units are attached to or entangled with each other) in a dispersion medium, and then the conductive agent dispersion is used. In this case, the carbon nanotubes are completely dispersed in the conventional conductive agent dispersion and exist in the form of a conductive agent dispersion in which carbon nanotube units are dispersed in a single-chain form. In the conventional conductive agent dispersion, the carbon nanotube units are easily cut by an excessive dispersion process, so that the length of the carbon nanotube units is shorter than the initial length. In addition, the carbon nanotube units may be easily cut during the roll pressing process of the electrode, and another limitation is that due to excessive volume changes of the electrode active material during battery operation, the carbon nanotube units (especially single-walled carbon nanotube units) are cut or their surfaces are damaged. Therefore, due to the deterioration of the conductivity of the electrode, there is a limitation in that the input / output characteristics and life characteristics of the battery deteriorate. In addition, regarding multi-walled carbon nanotube units, there are high structural defects due to the mechanism of node growth (not a smooth linear shape, but there are nodes due to defects generated during the growth process). Therefore, during the dispersion process, multi-walled carbon nanotube units are more easily cut (see (A) of Figure 1 ), and the short-cut multi-walled carbon nanotube units are likely to aggregate with each other via π-π stacking based on the carbon surface bonding structure (sp2) of the units. Therefore, it is difficult to disperse multi-walled carbon nanotube units more uniformly and have them present in the electrode paste.

[0055] Alternatively, regarding the carbon nanotube structure included in the electrode of the present invention, since the carbon nanotube structure is in the form of a rope, in which 2 to 5,000 single-walled carbon nanotube units with relatively high crystallinity and no structural defects are arranged side by side and bonded to each other (see (B) and (C) of Figure 1 and Figure 2(A)), even when the volume of the electrode active material changes, the lengths of these single-walled carbon nanotube units can be well maintained without being cut. Therefore, even during the continuous charge / discharge process of the battery, the conductivity of the electrode can be maintained. In addition, due to the high conductivity of the single-walled carbon nanotube units with high crystallinity, the conductivity of the electrode increases, so the input / output characteristics and life characteristics of the battery can be significantly improved. In addition, since the carbon nanotube structures can be connected to each other to have a network structure in the electrode, the generation of cracks can be prevented by suppressing excessive changes in the volume of the electrode active material, and a strong conductive network can be ensured at the same time. In addition, even if cracks are generated in the electrode active material, since the carbon nanotube structures can connect the electrode active material while passing through the cracks, the conductive network can be maintained. In addition, since the carbon nanotube structures are not easily damaged and can maintain their long shapes, the conductive network can be enhanced throughout the electrode active material layer. In addition, the peeling of the electrode active material can be suppressed, thus significantly improving the electrode adhesion.

[0056] In particular, from the perspective of being used in combination with graphene, graphene is mainly disposed on the surface of the electrode active material to help ensure the conductivity of short lengths, and the carbon nanotube structure can help ensure the conductivity of long lengths through its long length and network structure. In addition, since graphene exists when at least a part of the carbon nanotube structure is covered, a tight conductive network can be maintained even during repeated charge and discharge of the battery. Therefore, when graphene and the carbon nanotube structure are used in combination, since a tight and uniform conductive network can be formed on the entire electrode active material layer even when the total amount of the conductive agent is reduced, the input / output characteristics and life characteristics of the battery can be significantly improved.

[0057] In the carbon nanotube structure, the average diameter of the single-walled carbon nanotube units can be 0.5 nm to 5 nm, especially 1 nm to 5 nm. When this average diameter is satisfied, even with a very small amount of the conductive agent, there is an effect of maximizing the conductivity of the electrode. When observing the fabricated electrode by TEM, the average diameter corresponds to the average of the diameters of the first 100 single-walled carbon nanotube units with larger diameters and the last 100 single-walled carbon nanotube units with smaller diameters.

[0058] In a carbon nanotube structure, the average length of single-walled carbon nanotube units can be from 1 μm to 100 μm, particularly from 5 μm to 50 μm. When this average length is satisfied, since a long conduction path for forming a conductive connection between electrode active material particles can be formed and a unique network structure can be formed, even with a very small amount of conductive agent, there is an effect of maximizing the conductivity of the electrode. When observing the fabricated electrode by TEM, the average length corresponds to the average of the lengths of the first 100 single-walled carbon nanotube units with longer lengths and the last 100 single-walled carbon nanotube units with shorter lengths.

[0059] The specific surface area of single-walled carbon nanotube units can be from 500 m 2 / g to 1,000 m 2 / g, particularly from 600 m 2 / g to 800 m 2 / g. When the above range is satisfied, since the conduction path in the electrode can be smoothly ensured by the large specific surface area, even with a very small amount of conductive agent, there is an effect of maximizing the conductivity of the electrode. The specific surface area of single-walled carbon nanotube units can be calculated from the nitrogen adsorption amount at liquid nitrogen temperature (77 K) using BELSORP-mini II of Bell Japan Co., Ltd.

[0060] The average diameter of the carbon nanotube structure can be from 2 nm to 200 nm, particularly from 5 nm to 150 nm, and more particularly from 50 nm to 120 nm. When the above range is satisfied, since it effectively forms a conductive network structure and is beneficial to the connection between active material particles, excellent conductivity can be achieved. When observing the fabricated electrode by SEM, the average diameter corresponds to the average of the diameters of the first 100 carbon nanotube structures with larger diameters and the last 100 carbon nanotube structures with smaller diameters.

[0061] The average length of the carbon nanotube structure can be from 1 μm to 500 μm, particularly from 5 μm to 100 μm, and more particularly from 10 μm to 70 μm. When the above range is satisfied, since it effectively forms a conductive network structure and is beneficial to the connection between active material particles, excellent conductivity can be achieved. When observing the fabricated electrode by SEM, the average length corresponds to the average of the lengths of the first 100 carbon nanotube structures with longer lengths and the last 100 carbon nanotube structures with shorter lengths.

[0062] The carbon nanotube structure may be included in the electrode active material layer in an amount of 0.01% by weight to 0.5% by weight, particularly 0.01% by weight to 0.15% by weight, and more particularly 0.01% by weight to 0.1% by weight. When the above range is satisfied, since the conduction path of the electrode can be ensured, the life characteristics of the battery can be improved while maintaining the electrode resistance at a low level. During the preparation of the conductive agent dispersion, when the bundled carbon nanotubes are completely dispersed (as a general dispersion method, dispersion is performed to separate the single-chain carbon nanotube units from each other as much as possible), the carbon nanotube structure is not formed, or even if the carbon nanotube structure is inadvertently formed, only a very small amount (for example, 0.0005% by weight) of the carbon nanotube structure is formed. That is, it may be impossible to reach the above-mentioned amount range by a general method. Since the carbon nanotube structure has a form in which 2 to 5,000 single-walled carbon nanotube units are arranged side by side and combined with each other, even if the volume of the electrode active material changes, the carbon nanotube structure will not be cut and its length can be well maintained. Therefore, due to the high conductivity of the carbon nanotube structure, the conductivity of the electrode can be maintained and the conductivity of the electrode can be smoothly ensured. Therefore, even if the content of the carbon nanotube structure in the electrode is small, the input / output characteristics and life characteristics of the battery can be excellent.

[0063] In some cases, the surface of the single-walled carbon nanotube units may be treated by an oxidation treatment or a nitridation treatment in order to improve the affinity with the dispersion.

[0064] (3) Carbon black

[0065] Carbon black can be used to reduce the resistance of the electrode and can be used to suppress the damage of the electrode active material when winding the electrode at the same time.

[0066] The average particle diameter (D 50 ) of the carbon black may be 1 nm to 500 nm, particularly 10 nm to 250 nm, and more particularly 20 nm to 200 nm. When the above range is satisfied, when winding the electrode, the damage of the electrode active material can be minimized by the carbon black, and a short conductive network can be easily formed.

[0067] Carbon black may be included in the electrode active material layer in an amount of 0.01% by weight to 1% by weight, particularly 0.05% by weight to 1% by weight, and more particularly 0.05% by weight to 0.5% by weight. When the above range is satisfied, when winding the electrode, the damage of the electrode active material can be minimized by the carbon black, and a short conductive network can be easily formed.

[0068] The weight ratio of graphene, carbon nanotube structure, and carbon black can be from 0.01 to 3:0.01 to 0.5:0.1 to 10, particularly from 0.01 to 2:0.03 to 0.5:0.1 to 5, and more particularly from 0.05 to 1:0.05 to 0.5:0.1 to 1. When the above ranges are satisfied, a tight and uniform conductive network can be formed, and when the electrode is wound, damage to the electrode active material can be effectively suppressed. Therefore, the life characteristics of the battery can be improved.

[0069] Graphene, carbon nanotube structure, and carbon black can form the same structure as the electrode active material in the Figure 3 electrode. In particular, the carbon nanotube structure has a long length, connects the electrode active material particles, and connects with each other to form a network structure. Graphene covers a part of the nanotube structure, so the conductive connection through the carbon nanotube structure is enhanced, and the entire conductive network is tightly formed. Carbon black is disposed on the electrode active material, graphene, and carbon nanotube structure, and plays a buffering role in preventing damage to the electrode active material when the electrode is wound.

[0070] The electrode active material layer may further include a binder. The binder is used to ensure the adhesion between the electrode active material particles or the adhesion between the electrode active material and the current collector. Among them, common binders used in the art can be used, and there is no particular limitation on its type. The binder may include, for example: polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene polymer (EPDM), sulfonated EPDM, carboxymethyl cellulose (CMC), styrene - butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of them can be used alone or a mixture of two or more of them can be used.

[0071] The binder may be included in an amount of 10% by weight or less, for example, 0.1% to 5% by weight, based on the total weight of the electrode active material layer. When the content of the binder satisfies the above range, excellent electrode adhesion can be achieved while minimizing the increase in the resistance of the electrode.

[0072] Method for Manufacturing an Electrode

[0073] Next, a method for manufacturing the electrode of the present invention will be described.

[0074] The method for preparing an electrode according to the present invention may include the following steps: preparing a graphene dispersion, a carbon black dispersion, and a carbon nanotube structure dispersion (S1), and forming an electrode paste including the graphene dispersion, the carbon black dispersion, the carbon nanotube structure dispersion, and an electrode active material (S2). Since the graphene, carbon black, and carbon nanotube structure are the same as those in the above-described embodiment, the detailed description thereof will be omitted.

[0075] (1) Step (S1) of preparing a graphene dispersion, a carbon black dispersion, and a carbon nanotube structure dispersion

[0076] 1) Preparation of graphene dispersion

[0077] After preparing a mixed solution including the graphene, a dispersion medium, and a dispersant of the above-described embodiment, a graphene dispersion may be prepared by methods such as a homogenizer, a bead mill, a ball mill, a basket mill, a grinder, a universal stirrer, a transparent mixer, a pin mill, a TK mixer, and ultrasonic dispersion. The dispersion medium and the dispersant may be the same as those used in the preparation of the carbon nanotube structure dispersion to be described below, and thus will be described below.

[0078] 2) Preparation of carbon black dispersion

[0079] After preparing a mixed solution including the carbon black, a dispersion medium, and a dispersant of the above-described embodiment, a carbon black dispersion may be prepared by methods such as a homogenizer, a bead mill, a ball mill, a basket mill, a grinder, a universal stirrer, a transparent mixer, a pin mill, a TK mixer, and ultrasonic dispersion. The dispersion medium and the dispersant may be the same as those used in the preparation of the carbon nanotube structure dispersion to be described below, and thus will be described below.

[0080] 3) Preparation of carbon nanotube structure dispersion [[ID=D19]]

[0081] The preparation of the carbon nanotube structure dispersion may include the following steps: preparing a mixed solution including a dispersion medium, a dispersant, and bundled single-walled carbon nanotubes (bonded bodies or aggregates of single-walled carbon nanotubes) (S1-1); and dispersing the bundled single-walled carbon nanotubes by applying a shear force to the mixed solution to form a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side (S1-2).

[0082] In step S1-1, the mixed solution may be prepared by adding the bundled single-walled carbon nanotubes and the dispersant to the dispersion medium. In the bundled single-walled carbon nanotubes, the above-described single-walled carbon nanotube units are bonded to exist in the form of a bundle, and the bundled single-walled carbon nanotubes generally include 2 or more, substantially 500 or more, for example, 5,000 or more single-walled carbon nanotube units.

[0083] The specific surface area of the bundled single-walled carbon nanotubes can be 500 m 2 / g to 1,000 m 2 / g, particularly 600 m 2 / g to 800 m 2 / g. When the above range is satisfied, since the conduction path in the electrode can be smoothly ensured by the large specific surface area, the conductivity of the electrode can reach the maximum even with a very small amount of conductive agent.

[0084] The bundled single-walled carbon nanotubes can be included in the mixed solution in an amount of 0.1 wt% to 1.0 wt%, for example, 0.2 wt% to 0.5 wt%. When the above range is satisfied, since the bundled single-walled carbon nanotubes are dispersed at an appropriate level, an appropriate level of carbon nanotube structure can be formed, and the dispersion stability can be improved.

[0085] The dispersion medium can include, for example, polar organic solvents having an amide group, such as dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols, such as methanol, ethanol, 1-propanol, 2-propanol (isopropanol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol; diols, such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexanediol; polyhydric alcohols, such as glycerol, trimethylolpropane, pentaerythritol, or sorbitol; ethylene glycol ethers, such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, or tetraethylene glycol monobutyl ether; ketones, such as acetone, methyl ethyl ketone, methyl propyl ketone, or cyclopentanone; esters, such as ethyl acetate, γ-butyrolactone, and ε-propiolactone, and any one or a mixture of two or more of them can be used, but it is not limited thereto. More particularly, the dispersion medium can be N-methylpyrrolidone (NMP).

[0086] The dispersant can include at least any one of hydrogenated nitrile rubber, polyvinylidene fluoride, and carboxymethyl cellulose, particularly hydrogenated nitrile rubber or polyvinylidene fluoride.

[0087] The weight ratio of the bundled carbon nanotubes to the dispersant in the conductive agent dispersion can be in the range of 1:0.1 to 1:7, particularly 1:1 to 1:6. When the above range is satisfied, since the bundled single-walled carbon nanotubes are dispersed at an appropriate level, an appropriate level of carbon nanotube structure can be formed, and the dispersion stability can be improved.

[0088] The solid content in the mixed solution can be in the range of 0.1 wt% to 20 wt%, particularly 1 wt% to 10 wt%. When the above range is satisfied, since the bundled single-walled carbon nanotubes are dispersed at an appropriate level, an appropriate level of carbon nanotube structure can be formed, and the dispersion stability can be improved. In addition, the electrode paste can have a viscosity and elasticity suitable for the electrode preparation process, and also helps to increase the solid content of the electrode paste.

[0089] In step S1-2, the process of dispersing the bundled carbon nanotubes in the mixed solution can be carried out by using a mixing device such as a homogenizer, bead mill, ball mill, basket mill, grinder, universal stirrer, transparent mixer, pin mill, TK mixer or an ultrasonic (sonification) dispersion device. Among them, the bead mill method is preferred because it can control the diameter size of the carbon nanotube structure, achieve a uniform distribution of the carbon nanotube structure, and has an advantage in cost.

[0090] The bead mill method can be as follows. Add the mixed solution to a container containing beads, and rotate the container, whereby the bundled single-walled carbon nanotubes can be dispersed.

[0091] In this case, the conditions for performing the bead mill method are as follows.

[0092] The average diameter of the beads can be 0.5 mm to 1.5 mm, particularly 0.5 mm to 1.0 mm. When this range is satisfied, during the dispersion process, the carbon nanotube structure will not be damaged and the diameter of the carbon nanotube structure can be appropriately regulated, and a dispersion solution with a uniform composition can be prepared.

[0093] The rotation speed of the container can be 500 RPM to 10,000 RPM, particularly 2,000 RPM to 6,000 RPM. When this range is satisfied, during the dispersion process, the carbon nanotube structure will not be damaged and the diameter of the carbon nanotube structure can be appropriately regulated, and a dispersion solution with a uniform composition can be prepared.

[0094] The time for performing bead milling can be from 0.5 hour to 2 hours, particularly from 0.5 hour to 1.5 hours, and more particularly from 0.8 hour to 1 hour. When within this range, during the dispersion process, the carbon nanotube structure is not damaged and the diameter of the carbon nanotube structure can be appropriately regulated, and a dispersion solution with a uniform composition can be prepared. The execution time of bead milling refers to the total time of using bead milling. For example, if bead milling is performed several times, the execution time refers to the total time spent on performing bead milling several times.

[0095] The bead milling conditions are for appropriately dispersing the bundled single-walled carbon nanotubes, and in particular, the conditions for completely dispersing the bundled single-walled carbon nanotubes into single-chain single-walled carbon nanotubes are excluded. That is, the bead milling conditions are for appropriately dispersing the bundled single-walled carbon nanotubes to form a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded side by side to each other in the prepared conductive agent dispersion. This can be achieved only when strictly regulating the composition of the mixed solution, the conditions of the dispersion process (such as the bead milling process), etc.

[0096] Through the above method, a carbon nanotube structure dispersion can be formed.

[0097] (2) Step (S2) for forming an electrode paste including a graphene dispersion, a carbon black dispersion, a carbon nanotube structure dispersion, and an electrode active material

[0098] Through the above method, when preparing the graphene dispersion, the carbon black dispersion, and the carbon nanotube structure dispersion, an electrode paste including the dispersion and the electrode active material is formed. In this case, the above electrode active material can be used as the electrode active material.

[0099] In addition, if necessary, the electrode paste may further include a binder and a solvent. In this case, the binder of the above-described embodiment may be used as the binder. The solvent may include, for example, dimethylformamide (DMF), diethylformamide, dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP); alcohols such as methanol, ethanol, 1-propanol, 2-propanol (isopropanol), 1-butanol (n-butanol), 2-methyl-1-propanol (isobutanol), 2-butanol (sec-butanol), 1-methyl-2-propanol (tert-butanol), pentanol, hexanol, heptanol, or octanol; diols such as ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, 1,3-propanediol, 1,3-butanediol, 1,5-pentanediol, or hexanediol; polyhydric alcohols such as glycerol, trimethylolpropane, pentaerythritol, or sorbitol; ethylene glycol ethers such as ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, triethylene glycol monomethyl ether, tetraethylene glycol monomethyl ether, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, triethylene glycol monobutyl ether, or tetraethylene glycol monobutyl ether; ketones such as acetone, methyl ethyl ketone, methyl propyl ketone, or cyclopentanone; esters such as ethyl acetate, γ-butyrolactone, and ε-propiolactone, and any one or a mixture of two or more thereof may be used, but the present invention is not limited thereto. The solvent may be the same as or different from the dispersion medium used in the predispersion, and the solvent may preferably be N-methylpyrrolidone (NMP, N-methylpyrrolidone).

[0100] Next, the electrode paste prepared as described above is dried to form an electrode active material layer. Specifically, the electrode active material layer may be formed by a method of coating the electrode paste on an electrode current collector and then drying the coated current collector, or may be formed by a method of casting the electrode paste on a separate carrier and then laminating the film separated from the carrier on the current collector. If necessary, after forming the electrode active material layer by the above method, a rolling process may be further performed. In this case, drying and rolling may be performed under appropriate conditions in consideration of the physical properties of the finally prepared electrode, and there is no particular limitation.

[0101] Secondary Battery

[0102] A secondary battery according to another embodiment of the present invention may include a negative electrode, a positive electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte, and at least one of the positive electrode and the negative electrode may be the electrode of the above-described another embodiment. More specifically, the electrode of the above-described another embodiment may be the positive electrode of the present embodiment.

[0103] The separator separates the negative electrode and the positive electrode and provides a path for the movement of lithium ions. Any separator can be used as the separator without particular limitation as long as it is commonly used in secondary batteries. In particular, a separator having a high moisture retention ability for the electrolyte and a low resistance to the transport of electrolyte ions can be used. Specifically, a porous polymer membrane can be used, for example, a porous polymer membrane made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure having two or more layers thereof can be used. In addition, a conventional porous non-woven fabric can be used, for example, a non-woven fabric formed of high melting point glass fiber or polyethylene terephthalate fiber. In addition, a coated separator including a ceramic component or a polymer component can be used to ensure heat resistance or mechanical strength, and a separator having a single-layer or multi-layer structure can be selectively used.

[0104] The electrolyte may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten-type inorganic electrolyte that can be used in the preparation of a lithium secondary battery, but is not limited thereto.

[0105] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0106] As the non-aqueous organic solvent, for example, the following can be used: aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran (franc), 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate ester, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl propionate, and ethyl propionate.

[0107] In particular, as the cyclic carbonate in the carbonate-based organic solvent, ethylene carbonate and propylene carbonate are high-viscosity organic solvents and dissociate the lithium salt in the electrolyte well due to their high dielectric constants, so cyclic carbonates can be preferably used. Since an electrolyte having high conductivity can be prepared when the cyclic carbonate is mixed with a linear carbonate such as dimethyl carbonate and diethyl carbonate having low viscosity and low dielectric constant in an appropriate ratio, cyclic carbonates can be more preferably used.

[0108] A lithium salt can be used as the metal salt, and the lithium salt is a material that is easily soluble in the non-aqueous electrolyte. Among them, for example, as the anion of the lithium salt, one 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 - .

[0109] In order to improve the life characteristics of the battery, suppress the reduction of the battery capacity, and improve the discharge capacity of the battery, in addition to the above electrolyte components, at least one additive may be further included in the electrolyte, for example, halogenated alkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, diglyme, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0110] According to 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 battery module. Since the battery module and the battery pack include secondary batteries having high capacity, high rate characteristics, and high cycle characteristics, the battery module and the battery pack can be used as power sources for medium and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0111] Hereinafter, the present invention will be described in more detail with reference to specific examples and comparative examples.

[0112] Preparation Example 1: Preparation of graphene dispersion

[0113] Chemically exfoliated graphene (in powder form), hydrogenated nitrile rubber (H-NBR) as a dispersant, and N-methylpyrrolidone (NMP) as a dispersion medium were mixed at a weight ratio of 3.6:1.2:95.2 to form a mixture. The mixture was added to a bead mill in which 80% was filled with beads having a diameter of 0.65 mm, dispersed, and discharged at a discharge rate of 2 kg / min. By performing this process twice, the chemically exfoliated graphene was completely dispersed to prepare a graphene dispersion (see Figure 3 ).

[0114] Preparation Example 2: Preparation of carbon nanotube structure dispersion

[0115] A bundle-type single-walled carbon nanotube composed of single-walled carbon nanotube units having an average diameter of 1.5 nm and an average length of 5 μm or more (specific surface area 650 m 2 / g) and polyvinylidene fluoride (PVdF, KF9700, weight-average molecular weight: 880,000 g / mol) were mixed in N-methylpyrrolidone (N-Methylpyrrolidone: NMP) as a solvent to prepare a mixture having a solid content of 2.4% by weight.

[0116] The mixture was stirred by a bead-mill method, whereby the bundle-type single-walled carbon nanotubes were dispersed in the solvent, thereby preparing a carbon nanotube structure dispersion. In this case, the diameter of the beads was 1 mm, the rotation speed of the stirring container containing the beads was 3,000 RPM, and stirring was performed for 60 minutes. The carbon nanotube structure dispersion includes a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are arranged side by side (see Figure 2 (A)).

[0117] In the carbon nanotube structure dispersion, the amount of the carbon nanotube structure is 0.4% by weight, and the amount of polyvinylidene fluoride is 2.0% by weight.

[0118] Preparation Example 3: Preparation of carbon black dispersion

[0119] Carbon black having a specific surface area of 240 m 2Carbon black (in the form of secondary particles composed of primary particles with an average diameter of 25 nm) and hydrogenated nitrile rubber (weight-average molecular weight: 260,000 g / mol) were mixed in N-methylpyrrolidone (NMP) as a solvent to prepare a mixture with a solid content of 16.5 wt%.

[0120] The mixture was stirred by bead-mill method to disperse the carbon black in the solvent to prepare a carbon black dispersion. In this case, the diameter of the beads was 1 mm, the rotation speed of the stirring container containing the beads was 3,000 RPM, and stirring was carried out for 60 minutes.

[0121] In the carbon black dispersion, the amount of carbon black was 15 wt%, and the amount of hydrogenated nitrile rubber was 1.5 wt%.

[0122] Example 1: Fabrication of the positive electrode

[0123] The graphene dispersion of Preparation Example 1, the carbon nanotube structure dispersion of Preparation Example 2, the carbon black dispersion of Preparation Example 3, LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622) as the positive electrode active material and a binder (PVDF, KF9700) were mixed with N-methylpyrrolidone (NMP, N-methylpyrrolidone) to prepare a positive electrode slurry with a solid content of 70.4%. The positive electrode slurry was coated on a 20-μm-thick aluminum (Al) foil current collector, dried at 130 °C, and then roll-pressed to prepare a positive electrode including a positive electrode active material layer.

[0124] In the positive electrode active material layer, the content of LiNi 0.6 Co 0.2 Mn 0.2 O2 (NCM622) was 97.5 wt%, the content of the binder was 1.04 wt%, the content of hydrogenated nitrile rubber was 0.06 wt%, the content of polyvinylidene fluoride was 0.4 wt%, the content of graphene was 0.3 wt%, the content of carbon nanotube structure was 0.1 wt%, and the content of carbon black was 0.6 wt%.

[0125] Comparative Example 1: Fabrication of the positive electrode

[0126] A positive electrode was fabricated in the same manner as in Example 1, except that the graphene dispersion of Preparation Example 1 was not used.

[0127] Comparative Example 2: Fabrication of the positive electrode

[0128] The positive electrode was fabricated in the same manner as in Example 1, except that the carbon nanotube structure dispersion of Preparation Example 2 was not used.

[0129] Comparative Example 3: Fabrication of Positive Electrode

[0130] The positive electrode was fabricated in the same manner as in Example 1, except that the carbon black dispersion of Preparation Example 3 was not used.

[0131] Test Example

[0132] For the positive electrodes of Example 1 and Comparative Examples 1 to 3, after measuring the active material resistance and the interface resistance using a 4-probe measurement method with an MP tester (manufacturer: Hioki), the results are shown in Table 1.

[0133] [Table 1]

[0134]

[0135] Each content is based on 100% by weight of the total amount of the positive electrode active material layer.

Claims

1. An electrode including an electrode active material layer, wherein the electrode active material layer includes an electrode active material and a conductive agent, the conductive agent includes: graphene; a carbon nanotube structure in which 2 to 5,000 single-walled carbon nanotube units are bonded to each other; and carbon black, wherein the average diameter of the single-walled carbon nanotube units is 0.5 nm to 5 nm, wherein the carbon nanotube structure is included in the electrode active material layer in an amount of 0.01 wt% to 0.5 wt%, wherein the average diameter of the carbon nanotube structure is 2 nm to 200 nm, and The BET specific surface area of the graphene is 100 m 2 / g to 500 m 2 / g.

2. The electrode according to claim 1, wherein the average length of the graphene is 0.1 μm to 100 μm.

3. The electrode according to claim 1, wherein the average thickness of the graphene is 0.3 nm to 300 nm.

4. The electrode according to claim 1, wherein the graphene is included in the electrode active material layer in an amount of 0.01 wt% to 1.0 wt%.

5. The electrode according to claim 1, wherein the carbon nanotube structures are connected to each other to form a network structure in the electrode.

6. The electrode according to claim 1, wherein, In the carbon nanotube structure, the single-walled carbon nanotube units are arranged side by side and bonded.

7. The electrode according to claim 1, wherein the average length of the carbon nanotube structure is 1 μm to 500 μm.

8. The electrode according to claim 1, wherein the average length of the carbon nanotube structure is 10 μm to 70 μm.

9. The electrode according to claim 1, wherein the average diameter of the carbon nanotube structure is 50 nm to 120 nm.

10. The electrode according to claim 1, wherein the average particle size (D 50 ) of the carbon black is from 1 nm to 500 nm.

11. The electrode according to claim 1, wherein the carbon black is included in the electrode active material layer in an amount of 0.01 wt% to 1 wt%.

12. The electrode according to claim 1, wherein the weight ratio of the graphene, the carbon nanotube structure, and the carbon black is 0.01 to 3:0.01 to 0.5:0.1 to 10.

13. The electrode according to claim 1, wherein the carbon nanotube structure is a carbon nanotube structure in which 50 to 4,000 single-walled carbon nanotube units are bonded to each other.

14. A secondary battery including the electrode according to claim 1.

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