Specification Title of Invention: A positive electrode and a secondary battery including the positive electrode Technical field [One] Mutual citation with related applications [2] This application claims the benefit of priority based on Korean Patent Application No. 10-2018-0015313 filed on February 7, 2018, and all contents disclosed in the documents of the Korean patent application are included as part of this specification. [3] [4] Technical field [5] The present invention includes a current collector and a positive electrode active material layer disposed on the current collector, the positive electrode active material layer includes a positive electrode active material, a binder, and multi-walled carbon nanotubes, and the average length of the multi-walled carbon nanotubes is It is 1㎛ to 2㎛, the standard deviation of the length of the multi-walled carbon nanotubes relates to a positive electrode and a secondary battery of less than 0.5㎛. Background [6] Recently, as technology development and demand for mobile devices increase, the demand for batteries as an energy source is rapidly increasing, and accordingly, various studies on batteries that can meet various demands are being conducted. In particular, research on a lithium secondary battery having high energy density and excellent lifespan and cycle characteristics as a power source of such a device is actively being conducted. [7] A lithium secondary battery includes a positive electrode including a positive electrode active material capable of intercalating/detaching lithium ions, a negative electrode including a negative active material capable of intercalating/deintercalating lithium ions, and an electrode with a microporous separator interposed between the positive electrode and the negative electrode It means a battery in which the non-aqueous electrolyte containing lithium ions is contained in the assembly. [8] The positive electrode and/or the negative electrode may include a conductive material to improve conductivity. Conventionally, point-type conductive materials such as carbon black were mainly used. However, if the content of the conductive material is increased to improve conductivity, the amount of the positive electrode active material or the negative electrode active material is relatively reduced, making it difficult to achieve a high energy density of the battery. Therefore, there is a demand for satisfying the output and durability required for the required battery with only a small amount of conductive material. In particular, in the case of the positive electrode, the conductivity of the positive electrode active material itself is at a low level, so the above problem is more pronounced. [9] In order to solve this problem, a method of using a nano-sized conductive material, such as carbon nanotubes and carbon nanofibers, has a large specific surface area and thus allows many conductive contact with a small amount has been introduced. However, since such a nano-sized conductive material is difficult to be smoothly dispersed in the positive electrode slurry, it is difficult to obtain the desired conductivity unless the conductive material content in the positive electrode active material layer exceeds an appropriate level, for example, 1% by weight. [10] Accordingly, there is a need for development of a positive electrode capable of improving the dispersibility of the conductive material, ensuring conductivity despite a small content of the conductive material, and improving the output and life characteristics of the battery. Detailed description of the invention Technical challenge [11] An object of the present invention is to ensure the conductivity of the positive electrode even if the content of the conductive material is drastically reduced, so that the life characteristics of the battery can be improved, and the content of the positive active material can be increased, so that the output characteristics of the battery can be improved. It is to provide a secondary battery including this. Means of solving the task [12] The present invention includes a current collector and a positive electrode active material layer disposed on the current collector, the positive electrode active material layer includes a positive electrode active material, a binder, and a multi-walled carbon nanotube, and the average length of the multi-walled carbon nanotube is A positive electrode having a standard deviation of 1 μm to 2 μm and a length of the multi-walled carbon nanotube is 0.5 μm or less. [13] In addition, the present invention is the anode; cathode; A separator interposed between the anode and the cathode; And it provides a secondary battery containing an electrolyte. Effects of the Invention [14] According to the present invention, multi-walled carbon nanotubes are used as a conductive material, and the multi-walled carbon nanotubes have an appropriate level of average length and an appropriate level of standard deviation. Accordingly, while the multi-walled carbon nanotubes can be uniformly dispersed in the conductive material dispersion and the positive electrode slurry, the positive electrode active materials can be electrically connected smoothly by the multi-walled carbon nanotubes in the prepared positive electrode. Accordingly, the life characteristics of the battery can be improved. In addition, while the dispersibility of the multi-walled carbon nanotubes is improved, the conductivity of the positive electrode can be secured even when the multi-walled carbon nanotubes are used in a small amount, so that the content of the positive electrode active material can be relatively increased. Accordingly, the output characteristics of the manufactured secondary battery may be improved. Brief description of the drawing [15] 1 is a graph showing the measurement of lengths of multi-walled carbon nanotubes included in an anode used in Example 1 of the present invention. [16] 2 is a graph showing the measurement of lengths of multi-walled carbon nanotubes included in the anode used in Example 2 of the present invention. [17] 3 is a graph showing the measurement of lengths of multi-walled carbon nanotubes included in the anode used in Comparative Example 1 of the present invention. [18] 4 is a graph showing the measurement of lengths of multi-walled carbon nanotubes included in the anode used in Comparative Example 2 of the present invention. [19] 5 is a graph showing an increase in discharge capacity and battery resistance according to cycles for the batteries according to Examples 1 and 2 and Comparative Examples 1 and 2; Mode for carrying out the invention [20] Hereinafter, the present invention will be described in more detail to aid understanding of the present invention. At this time, terms or words used in the present specification and claims should not be construed as being limited to a conventional or dictionary meaning, and the inventor appropriately defines the concept of terms in order to describe his own invention in the best way. It should be interpreted as a meaning and concept consistent with the technical idea of ​​the present invention based on the principle that it can be done. [21] [22] [23] [24] A positive electrode according to an embodiment of the present invention includes a current collector and a positive electrode active material layer disposed on the current collector, the positive electrode active material layer includes a positive electrode active material, a binder, and a multi-walled carbon nanotube, and the multiple The average length of the wall carbon nanotubes may be 1 μm to 2 μm, and the standard deviation of the length of the multi-walled carbon nanotubes may be 0.5 μm or less. [25] [26] The current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, as the current collector, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, or the like may be used. Specifically, a transition metal such as copper and nickel that adsorbs carbon well can be used as the current collector. [27] [28] The positive active material layer may be disposed on one or both surfaces of the current collector. The positive electrode active material layer may include a positive electrode active material, a binder, and a multi-walled carbon nanotube. [29] [30] The positive active material may be the same as the positive active material included in the positive electrode slurry of the above-described embodiment. Specifically, the positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may include a layered compound such as lithium cobalt oxide (LiCoO 2 ) or lithium nickel oxide (LiNiO 2 ), or a compound substituted with one or more transition metals; Lithium manganese oxides such as formula Li 1+y1 Mn 2-y1 O 4 (0≦ y1 ≦0.33), LiMnO 3 , LiMn 2 O 3 , and LiMnO 2 ; Lithium copper oxide (Li 2 CuO 2 ); LiV 3 O 8 , V 2 O 5 , Cu 2 V 2Vanadium oxides such as O 7 ; Ni site-type lithium nickel oxide represented by the formula LiNi 1-y2 M1 y2 O 2 (where M1 is Co, Mn, Al, Cu, Fe, Mg, B or Ga, and satisfies 0.01≦y2≦0.3); Formula LiMn 2-y3 M2 y3 O 2 (where M2 is Co, Ni, Fe, Cr, Zn or Ta, and satisfies 0.01≦y3≦0.1) or Li 2 Mn 3 M3O 8 (where M3 is Fe, A lithium manganese composite oxide represented by Co, Ni, Cu, or Zn); Li in the formula may include LiMn 2 O 4 in which a part of Li is substituted with an alkaline earth metal ion, but is not limited thereto. Specifically, the positive electrode active material is Li[Ni 0.6 Mn 0.2 Co 0.2]O 2 can be. [31] [32] The positive active material may be included in an amount of 96% to 99% by weight based on the total weight of the positive active material layer, and specifically, may be included in an amount of 97% to 98.5% by weight. When the above range is satisfied, the output of the battery can be improved, but the contents of the multi-walled carbon nanotube and the binder are not excessively reduced, so that the life characteristics of the battery can be maintained. [33] [34] The binder may be the same as the binder included in the positive electrode slurry of the above-described embodiment. Specifically, the binder is polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate. And at least one selected from the group consisting of materials in which hydrogen is substituted with Li, Na, or Ca, etc. For example, the binder may include various copolymers thereof. , It may be a polyvinylidene fluoride containing a functional group such as an ether group. [35] [36] The multi-walled carbon nanotube may refer to a carbon nanotube having two or more graphene layers arranged side by side with respect to the axis of the carbon nanotube. That is, it may mean a carbon nanotube having two or more walls. In the present invention, carbon nanotubes having a preferred length and standard deviation of the length may be formed in the dispersion process of the conductive material dispersion. [37] [38] The average length of the multi-walled carbon nanotubes may be 1 μm to 2 μm, specifically 1.1 μm to 1.4 μm, and more specifically 1.1 μm to 1.2 μm. When the average length of the multi-walled carbon nanotubes is less than 1 µm, a large number of multi-walled carbon nanotubes having an excessively short length may exist, so that the electrical connection between the positive electrode active materials cannot be made smoothly, and the output of the battery is degraded. On the other hand, when the average length of the multi-walled carbon nanotubes exceeds 2 μm, a number of excessively long multi-walled carbon nanotubes exist, so that the multi-walled carbon nanotubes can be easily entangled with each other in the conductive dispersion and the anode slurry. have. Accordingly, since the multi-walled carbon nanotubes are not evenly distributed in the positive electrode active material layer, the conductivity of the positive electrode active material layer is lowered. Accordingly, the output and life characteristics of the battery may be deteriorated. [39] On the other hand, when the average length exceeds 2 μm, it means that the lengths of the multi-walled carbon nanotubes are not sufficiently shortened, and this may be due to insufficient particle size distribution control processes such as milling. Therefore, when the average length exceeds 2 μm, the standard deviation of the length of the multi-walled carbon nanotubes is also large, and specifically, it must exceed 0.5 μm. [40] [41] The standard deviation of the length of the multi-walled carbon nanotube may be 0.5 μm or less, and specifically 0.3 μm to 0.5 μm. When the standard deviation of the length of the multi-walled carbon nanotubes exceeds 0.5 μm, the difference in length between the multi-walled carbon nanotubes is large, and thus the conductivity in the positive electrode active material layer is not formed to a uniform degree. Accordingly, the output and life characteristics of the battery may be deteriorated. Further, the excessively large standard deviation may be due to insufficient multi-walled carbon nanotube particle size improvement processes such as milling during the preparation of the conductive material dispersion. Since the acidity is lowered, the output and life characteristics of the battery are further deteriorated. In addition, when the standard deviation is too large, the viscosity of the conductive material dispersion liquid and the positive electrode slurry is excessively increased. Accordingly, when the positive electrode active material layer is formed, since it is difficult to smoothly apply the positive electrode slurry, the output and life characteristics of the battery may further deteriorate. [42] [43] The length of the multi-walled carbon nanotube may be 0.5 μm to 3.0 μm, and specifically 0.7 μm to 2.5 μm. Here, the length is not the average length described above, but refers to the length of each observed multi-walled carbon nanotube. When the above range is satisfied, the electrical connection between the positive electrode active materials is maintained at an appropriate level, while the multi-walled carbon nanotubes may be uniformly dispersed in the conductive material dispersion and the positive electrode slurry. Even when a small amount of multi-walled carbon nanotubes exceeding 3.0 μm in length is included in the positive electrode active material layer, agglomeration between the multi-walled carbon nanotubes occurs due to the long multi-walled carbon nanotubes. Multi-walled carbon nanotubes are uniformly dispersed and difficult to exist. Accordingly, the output and life characteristics of the battery may be deteriorated. In addition, when a long multi-walled carbon nanotube is present, the aggregation occurs and the viscosity of the conductive material dispersion increases, so processability decreases when manufacturing the positive electrode active material layer. The dispersibility of wall carbon nanotubes may be further lowered. [44] [45] With respect to the multi-walled carbon nanotubes included in the positive electrode active material layer, the average length, standard deviation of the length, and length of the multi-walled carbon nanotubes described above may be measured in the following manner. First, a certain amount of the positive electrode active material layer is diluted in an NMP solution weighing several tens of times, and then materials constituting the positive electrode active material layer are separated through ultrasonic waves. After that, a part of the upper layer portion of the solution is extracted, and again diluted with an NMP solution having a weight of several tens of times the amount of extraction. Thereafter, the lengths of multiple, for example, 30 or 25 multi-walled carbon nanotubes are measured by observing through a scanning electron microscope (SEM), and the average and standard deviation thereof are calculated to obtain the multi-walled carbon nanotubes. The average length, standard deviation of the length, and length can be derived. [46] [47] The multi-walled carbon nanotubes may be included in an amount of 0.1% to 1% by weight based on the total weight of the positive electrode active material layer, specifically 0.2% to 0.9% by weight, and more specifically 0.2% to 0.7% by weight. It may be included in weight percent. When the above range is satisfied, conductivity of the positive active material layer may be ensured. [48] The content of the multi-walled carbon nanotubes in an amount of 1% by weight or less, particularly 0.7% by weight or less is difficult to achieve only with carbon nanotubes having general physical properties. Specifically, in order to increase the content of the positive electrode active material, if the content of relatively general carbon nanotubes is reduced to a level of 1% by weight or less, the electrical connection between the positive electrode active materials can never be made smoothly, so the output characteristics of the manufactured secondary battery This inevitably has to be greatly degraded. In addition, since the positive electrode active materials cannot be smoothly supported by the carbon nanotubes, the positive electrode active materials are easily separated from the positive electrode active material layer or the structure of the positive electrode active material layer is gradually collapsed, and thus the mechanical stability of the positive electrode may be deteriorated. Accordingly, the cycle characteristics of the manufactured secondary battery are inevitably deteriorated. [49] However, in the present invention, since the positive electrode active material layer contains multi-walled carbon nanotubes having physical properties such as an appropriate average length and standard deviation of the length in a uniformly distributed state, the content of the multi-walled carbon nanotubes is 1% by weight. Even if included below, electrical connection between the positive electrode active materials is maintained, and mechanical stability of the positive electrode active material layer may be ensured, and thus the output and life characteristics of the battery may be improved. In addition, since the content of the multi-walled carbon nanotube is maintained at a small level of 1% by weight or less, the positive electrode active material layer may contain a relatively higher amount of the positive electrode active material, so that the output of the manufactured battery will be further improved. I can. [50] [51] The loading amount of the positive active material layer may be 15 mg/cm 2 to 40 mg/cm 2 , and specifically 20 mg/cm 2 to 30 mg/cm 2 . When the above range is satisfied, the energy density of the positive electrode is secured, and the thickness of the positive electrode may not be excessively increased. In addition, when applying the positive electrode slurry, a problem may not occur in processability. [52] [53] [54] [55] A method of manufacturing a positive electrode according to another embodiment of the present invention includes preparing a conductive material dispersion; Forming a positive electrode slurry including a conductive material dispersion, a positive electrode active material, a binder, and a solvent; And coating and drying the positive electrode slurry on a current collector, wherein the conductive material dispersion comprises a multi-walled carbon nanotube, a dispersant, and a dispersion medium, and the average length of the multi-walled carbon nanotubes is from 1 μm to 2 μm, and the standard deviation of the length of the multi-walled carbon nanotube may be 0.5 μm or less. [56] [57] Preparing the conductive material dispersion may include forming a mixture by mixing a bundle-type multi-walled carbon nanotube, a dispersant, and a dispersion medium, and adjusting the particle size distribution of the bundled multi-walled carbon nanotubes. May include doing. [58] The dispersant may be at least one selected from the group consisting of hydrogenated nitrile butadiene rubber (H-NBR), polyvinylpyrrolidone (PVP), and carboxymethylcellulose (CMC). [59] The dispersion medium may be at least one of N-methyl-2-pyrrolidone (NMP) and water. [60] In the bundle-type multi-walled carbon nanotube, the bundle-type is a bundle or rope in which a plurality of carbon nanotube units are arranged in parallel or entangled in a substantially the same orientation in the longitudinal direction of the unit. Refers to the secondary shape of the shape. In the case of the bundled multi-walled carbon nanotubes, the carbon nanotubes have a partially agglomerated form, and the lengths are also excessively varied, so if the shape and length are not adjusted and used directly as a conductive material, the bundled multi-walled carbon nanotubes in the positive electrode active material layer It is difficult for the wall carbon nanotubes to be uniformly dispersed and to be present, and it is difficult to secure a conductive path. Accordingly, after mixing the bundled multi-walled carbon nanotubes, the dispersant, and the dispersion medium, there is a need for a process of adjusting the particle size distribution, that is, the shape and length of the bundled multi-walled carbon nanotubes. [61] Particularly, in the bundle-type multi-walled carbon nanotubes immediately after synthesis is completed, carbon nanotubes of 5 μm to 50 μm are randomly agglomerated so that the total size has a particle shape of several tens of μm. Typically, pellets are manufactured from the bundled multi-walled carbon nanotubes for ease of handling such as transport, storage, and input. Therefore, in order to use the anode as a conductive material, a milling operation of disassembling the pellets to separate the bundled carbon nanotubes from each other and making the length uniform is required. [62] Adjusting the particle size distribution may be performed by a method such as milling or ultrasonic treatment, and may preferably be performed by milling. The milling may be performed by a method such as a ball mill, a spike mill, a bead mill, a basket mill, and an attrition mill, and specifically This can be done by means of a spike mill. [63] The spike mill may be performed in the following manner. The spike mill is operated while introducing a mixture containing the bundled carbon nanotubes, a dispersant, and a dispersion medium into a spike mill device filled with beads. At this time, the rotor inside the device rotates, and this rotational force imparts kinetic energy to the beads, thereby dispersing the bundled carbon nanotubes in the mixture. After that, the mixture is discharged through the outlet at a specific discharge rate. By performing such a process under specific conditions, the multi-walled carbon nanotubes included in the anode of the present invention may be formed. In particular, the size of the beads, the filling amount of the beads, the discharge speed of the mixture, and the number of millings correspond to the main conditions, and the conductive material dispersion used in the present invention may be formed by an appropriate combination thereof. In other words, not only the range of each of the above conditions, but also the combination of each condition must be appropriately satisfied. [64] The size of the bead may be 0.5mm to 2mm, specifically 0.6mm to 1mm, and more specifically 0.6mm to 0.75mm. When the size of the bead exceeds 2 mm, the shear force generated by the bead is insufficient, so that the dispersion and particle size distribution of the multi-walled carbon nanotube may not reach a desirable level. In addition, when the size of the bead is less than 0.5mm, not only the mixture but also the beads are discharged to the outlet, so that the dispersing ability of the spike mill is not kept constant, and a separate process of separating the beads from the discharged product is required. There is. [65] The filling amount of the bead may be 50% to 90%, and specifically 65% ​​to 80%. When the filling amount of the beads exceeds 90%, the pressure inside the spike mill device increases significantly, making it difficult to use the spike mill continuously. When the filling amount of the bead is less than 50%, it is difficult to form an appropriate kinetic energy required for dispersion. [66] The discharge rate of the mixture may be 1 kg/min to 5 kg/min, and specifically 2 kg/min to 4 kg/min. [67] The number of milling refers to the number of times the mixture is put into the container. The number of milling may be 2 to 3 times. [68] The viscosity of the conductive material dispersion may be 10,000 cps to 30,000 cps at 30°C to 50° C., and specifically 15,000 cps to 25,000 cps. If the above range is satisfied, it is easy to add the positive electrode slurry. In addition, satisfying the viscosity means that the multi-walled carbon nanotubes are smoothly dispersed and that the particle size distribution satisfies a desirable level. [69] [70] In forming the positive electrode slurry, the positive electrode slurry may include the conductive material dispersion, a positive electrode active material, a binder, and a solvent. [71] [72] The positive electrode active material, the binder, the multi-walled carbon nanotube, and the current collector are the same as the positive electrode active material, the binder, and the multi-walled carbon nanotube included in the positive electrode of the above-described embodiment, and thus a description thereof will be omitted. Meanwhile, the average length, standard deviation of the length, and the length of the multi-walled carbon nanotubes included in the conductive material dispersion liquid may be kept the same in the positive electrode active material layer. [73] The solvent is dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl-2-pyrrolidone (NMP), polyvinylidene fluoride-hexafluoro It may be at least one selected from the group consisting of a low propylene copolymer (PVDF-co-HFP) aqueous solution, a polyvinylidenefluoride aqueous solution, acetone, and water. For example, the solvent may be NMP. [74] The solid content of the positive electrode slurry may be 60% to 80% by weight, and specifically 65% ​​to 75% by weight, based on the total weight of the positive electrode slurry. When the above range is satisfied, the viscosity of the positive electrode slurry to be applied (coated) to the current collector is maintained, and the positive electrode slurry is easily dried. At this time, the viscosity of the positive electrode slurry is preferably 5,000 cps to 25,000 cps. [75] Since the cathode active material layer is prepared by drying the cathode slurry to remove the solvent, the content of each of the cathode active material, binder, and multi-walled carbon nanotubes contained in the solid content based on the total weight of the solid content of the cathode slurry described above is described above. It is equal to the value based on the total weight of the positive electrode active material layer included in the positive electrode of one embodiment. [76] In the coating and drying step, the coating and drying may be coating and drying the current collector to which the positive electrode slurry is applied at a temperature of 100° C. to 180° C. at a rate of 4 m/min to 80 m/min. In addition, after drying, roll rolling may be performed to adjust the thickness of the positive electrode, and additional drying may be performed to remove residual moisture from the positive electrode after rolling. [77] [78] [79] [80] [81] A secondary battery according to another embodiment of the present invention may include a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte. Here, since the anode is the same as the anode of the above-described embodiment, a description thereof will be omitted. [82] [83] The negative electrode may include a negative electrode current collector and a negative active material layer disposed on one or both surfaces of the negative electrode current collector. [84] [85] The negative electrode current collector is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, as the negative electrode current collector, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, etc. may be used. . Specifically, a transition metal such as copper and nickel that adsorbs carbon well can be used as the current collector. [86] [87] The negative active material layer may include a negative active material, a negative conductive material, and a negative binder. [88] [89] The negative active material may be graphite-based active material particles or silicon-based active material particles. The graphite-based active material particles may be one or more selected from the group consisting of artificial graphite, natural graphite, graphitized carbon fiber, and graphitized mesocarbon microbead, and particularly, when using artificial graphite, rate characteristics may be improved. . The silicon-based active material particles are Si, SiO x (0