Positive electrode and secondary battery including same

Abstract

The present invention relates to a positive electrode comprising: a positive electrode current collector; a first positive electrode active material layer formed on the upper surface of the positive electrode current collector; and a second positive electrode active material layer formed on the lower surface of the positive electrode current collector, wherein the first positive electrode active material layer and the second positive electrode active material layer contain a first conductive material and a second conductive material, respectively, the average length of the second conductive material is greater than the average length of the first conductive material, and K represented by Equation 1 is 0.82-1.16. A detailed description of Equation 1 is given in the present specification.

Description

Anode and secondary battery containing the same Cross-citation with related applications This application claims the benefit of priority from Korean Patent Application No. 10-2023-0193706, filed December 27, 2023, the entire contents of which are incorporated herein by reference. Technical field The present invention relates to a cathode and a secondary battery including the cathode, and more specifically, to a cathode having improved cycle characteristics and a high-capacity secondary battery including the cathode. With the advancement of technology in electric vehicles and portable devices, the demand for secondary batteries as an energy source is rapidly increasing. In particular, as the scope of application of secondary batteries expands to electric vehicles (EVs) and hybrid electric vehicles (HEVs), the demand for high-capacity secondary batteries with excellent capacity retention is increasing. The above capacity retention ratio can be determined by the components of the secondary battery. Specifically, the secondary battery includes a battery case and an electrode assembly accommodated in the battery case, and the electrode assembly can have a structure in which a positive electrode having a positive electrode active material layer formed on both sides of a positive electrode current collector, a negative electrode having a negative electrode active material layer formed on both sides of a negative electrode current collector, and a separator interposed between the positive electrode and the negative electrode are wound. At this time, when the positive electrode is wound, the properties of the positive electrode active material layer formed on both sides of the positive electrode current collector may be changed, and this may cause a problem in that charge and discharge may be uneven, thereby lowering the capacity retention ratio of the secondary battery. Therefore, research is needed on technologies that can improve the capacity retention rate of coil-type electrode assemblies. In order to solve the above-described problems, the problem to be solved by the present invention is to provide a positive electrode having improved cycle characteristics and capacity and a secondary battery including the same by reducing the difference in conductivity between the first positive electrode active material layer and the second positive electrode active material layer caused by winding of the positive electrode by applying a first positive electrode active material layer and a second positive electrode active material layer having different compositions to the positive electrode. [1] The present invention provides a positive electrode including a positive electrode current collector and a first positive electrode active material layer formed on an upper surface of the positive electrode current collector, and a second positive electrode active material layer formed on a lower surface of the positive electrode current collector, wherein each of the first positive electrode active material layer and the second positive electrode active material layer includes a first conductive material and a second conductive material, and an average length of the second conductive material is longer than an average length of the first conductive material, and K, represented by the following [Formula 1], is 0.82 to 1.16: [Formula 1] In the above equation 1, W T1 The content (weight%) of the first conductive material included in the first positive electrode active material layer is W T2 W is the content (weight%) of the second conductive material included in the first positive electrode active material layer. B1 The content (weight%) of the first conductive material included in the second positive electrode active material layer is W B2 is the content (weight%) of the second conductive material included in the second positive electrode active material layer, and BET1 is the specific surface area (m) of the first conductive material. 2 / g), BET2 is the specific surface area of ​​the second challenge material (m 2 / g), L1 is the average length (㎛) of the first conductive material, L2 is the average length (㎛) of the second conductive material, η1 is the true density (g / cc) of the first conductive material, η2 is the true density (g / cc) of the second conductive material, P T is the porosity (%) of the first positive electrode active material layer, P B refers to the porosity (%) of the second positive electrode active material layer. [2] The present invention can provide an anode in which the average length (L2) of the second conductive material in the above [1] is 2 to 6 times the average length (L1) of the first conductive material. [3] The present invention can provide an anode in which the average length (L2) of the second conductive material is 4 µm to 8 µm in the above [1] and / or [2]. [4] The present invention, in at least one of the above [1] to [3], comprises a second conductive material content (W) of the first positive electrode active material layer. T2 ) is the second conductive material content (W) of the second positive electrode active material layer. B2 ) can provide a larger anode. [5] The present invention, in at least one of the above [1] to [4], comprises a weight ratio (W) of the first conductive material and the second conductive material included in the first positive electrode active material layer. T1 : W T2 ) can provide a positive electrode of 15:1 to 45:1. [6] The present invention, in at least one of the above [1] to [5], comprises a weight ratio (W) of the first conductive material and the second conductive material included in the second positive electrode active material layer. B1 : W B2 ) can provide a cathode having a ratio of 35:1 to 45:1. [7] The present invention, in at least one of the above [1] to [6], comprises the content (W) of the second conductive material of the first positive electrode active material layer. T2 ) can provide a positive electrode having 0.015 wt% to 0.025 wt% of the total weight of the first positive electrode active material layer. [8] In the present invention, in at least one of the above [1] to [7], the specific surface area (BET1) of the first conductive material may be 0.15 to 0.5 times the specific surface area (BET2) of the second conductive material. Preferably, the specific surface area (BET1) of the first conductive material is 200 m 2 / g to 420m 2 / g, and the specific surface area (BET2) of the second challenge material is 850 m 2 / g to 1250m 2 / g can provide a positive electrode. [9] The present invention can provide an anode in which, in at least one of the above [1] to [8], the first conductive material can be a multi-walled carbon nanotube, and the second conductive material is a single-walled carbon nanotube or a double-walled carbon nanotube.

[0010] The present invention can provide an anode in which the first conductive material has a density (η1) of 1.97 g / cc to 2.07 g / cc in at least one of the above [1] to [9].

[0011] The present invention can provide an anode in which the second conductive material has a density (η2) of 1.85 g / cc to 1.95 g / cc in at least one of the above [1] to

[0010] .

[0012] The present invention can provide a secondary battery including an electrode assembly in which a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode are wound in one direction according to at least one of the above [1] to

[0010] ; and a battery case in which the electrode assembly is accommodated, wherein when the positive electrode is wound, the first positive electrode active material layer is arranged to face the battery case, and the second positive electrode active material layer is arranged to face the winding center.

[0013] The present invention can provide a cylindrical secondary battery having a form factor ratio of 0.4 or more in the above

[0012] .

[0014] The present invention can provide a secondary battery, wherein the secondary battery is a 46110 cell, a 4875 cell, a 48112 cell, a 4880 cell or a 4680 cell, in the above

[0012] and / or

[0013] .

[0015] The present invention provides a secondary battery having a capacity retention rate of 80% according to the following [Formula 2] when charging the secondary battery at a constant current of 0.5 C in a temperature range of 20°C to 55°C until it reaches 4.2 V and then discharging the secondary battery at a constant current of 0.5 C until it reaches 2.5 V, which constitutes one charge / discharge cycle, in at least one of the above [1] to

[0014] : [Formula 2] Capacity retention rate (%) = 100*{discharge capacity after 200 cycles} / {discharge capacity after 1 cycle}. According to one embodiment of the present invention, a positive electrode and a secondary battery including the same can improve the cycle characteristics of the secondary battery by applying first and second positive electrode active material layers having different content ratios of first and second conductive agents to the positive electrode, thereby reducing changes in physical properties between the first and second positive electrode active material layers caused by winding of the positive electrode. In addition, by applying the positive electrode to a high-capacity secondary battery, a secondary battery having improved capacity and capacity retention rate can be implemented. The effects obtainable from the exemplary embodiments of the present invention are not limited to the effects mentioned above, and other effects not mentioned can be clearly derived and understood by those skilled in the art to which the exemplary embodiments of the present disclosure belong from the following description. That is, unintended effects resulting from practicing the exemplary embodiments of the present disclosure can also be derived by those skilled in the art from the exemplary embodiments of the present disclosure. Figure 1 is a perspective view showing an anode according to exemplary embodiments. FIG. 2 is a perspective view showing a secondary battery according to exemplary embodiments. The terms or words used in this specification and claims should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted as having meanings and concepts that conform to the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of the term in order to explain his or her own invention in the best manner. It should be understood that the terms “comprise,” “include,” or “have,” as used herein, are intended to specify the presence of a feature, number, step, component, or combination thereof, but do not exclude in advance the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof. Each of the phrases "A or B", "at least one of A and B", "at least one of A or B", "A, B, or C", "at least one of A, B, and C", and "at least one of A, B, or C" can include any one of the items listed in that phrase, or all possible combinations of them. Terms such as "first" or "second" may be used merely to distinguish one component from another, and do not qualify the components in any other respect (e.g., importance or order). The "specific surface area" is measured by the BET method, and can be specifically calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mino II of BEL Japan. The “average length” of the first and second conductive materials can be measured using a field emission scanning electron microscope. For example, the average length of the first conductive material is measured by the following method. A solution in which the first conductive material and carboxymethyl cellulose are added to water in a weight ratio of 40:60 (solid content is 1 wt% based on the total weight of the solution) is diluted 1,000 times in water. Thereafter, 20 ml of the diluted solution is filtered through a filter, and the filter from which the first conductive material is filtered is dried. 100 images of the dried filter are taken using a scanning electron microscope (SEM), and the length of the first conductive material is measured using the imageJ program, and the average value of the lengths can be defined as the average length of the first conductive material. Hereinafter, the present invention will be described in detail. The cathode and / or secondary battery according to the present invention comprises at least one of the configurations disclosed below, and may comprise any combination between technically possible configurations among the configurations below. FIG. 1 is a perspective view showing an anode (210) according to exemplary embodiments. FIG. 2 is a perspective view showing a secondary battery (100) according to exemplary embodiments. Referring to FIGS. 1 and 2, a positive electrode (210) according to the present invention includes a positive electrode current collector (211) and a first positive electrode active material layer (212_1) formed on an upper surface (211_1) of the positive electrode current collector, and a second positive electrode active material layer (212_2) formed on a lower surface (211_2) of the positive electrode current collector, and each of the first positive electrode active material layer (212_1) and the second positive electrode active material layer (212_2) includes a first conductive material (not shown) and a second conductive material (not shown), and the average length of the second conductive material is longer than the average length of the first conductive material. In addition, a secondary battery (100) according to the present invention includes an electrode assembly (200) in which a cathode (210), an anode (220), and a separator (230) interposed between the cathode (210) and the anode (220) are wound in one direction; and a battery case (300) in which the electrode assembly (200) is accommodated, and when the electrode assembly (200) is wound, a first cathode active material layer (212_1) is arranged to face the battery case (300), and a second cathode active material layer (212_2) is arranged to face the center of the winding. The first positive electrode active material layer (212_1) arranged on the battery case (300) side has a larger radius of curvature than the second positive electrode active material layer (212_2) arranged on the winding center side, and accordingly, the distance between the positive electrode active material particles included in each of the first positive electrode active material layer (211_1) and the second positive electrode active material layer (211_2) and the distance between the positive electrode active material and the conductive material may be different. For example, when an electrode assembly is manufactured by winding positive electrodes in which the first positive electrode active material layer and the second positive electrode active material layer having the same composition are formed, the first positive electrode active material layer has a larger radius of curvature than the second positive electrode active material layer. Accordingly, the distance between the active material particles in the first positive electrode active material layer is larger than the distance between the active material particles in the second positive electrode active material layer, and the conductivity of the first positive electrode active material layer is lower than that of the second positive electrode active material layer, resulting in a difference in conductivity between the first and second positive electrode active material layers. Due to this, a problem of deterioration in the output characteristics and cycle characteristics of the secondary battery may occur. Accordingly, the present invention aims to provide a secondary battery (100) with improved cycle characteristics by preventing a decrease in conductivity of the first positive electrode active material layer (212_1) caused by a difference in the radius of curvature by applying a first positive electrode active material layer (212_1) and a second positive electrode active material layer (212_2) having different content ratios of the first and second conductive materials to the positive electrode (210). Hereinafter, the present invention will be described in detail. <Bipolar> The positive electrode (210) according to the present invention includes a positive electrode current collector (211) and a first positive electrode active material layer (212_1) and a second positive electrode active material layer (212_2) formed on each of both sides of the positive electrode current collector (211). Specifically, the positive electrode (210) includes a first positive electrode active material layer (212_1) formed on the positive electrode current collector (211) and the upper surface (211_1) of the positive electrode current collector, and a second positive electrode active material layer (212_2) formed on the lower surface (211_2) of the positive electrode current collector. The first positive electrode active material layer (212_1) and the second positive electrode active material layer (212_2) of the positive electrode (210) according to the present invention may each include two types of conductive materials having different physical properties. When the two types of conductive materials are mixed and used, the mixing ratio of the conductive materials can be adjusted to complement each other's shortcomings and exert their advantages, thereby improving the performance of the positive electrode. In addition, by adjusting the mixing ratio of the two types of conductive materials, the performance of the positive electrode can be adjusted more finely. Each of the first positive electrode active material layer (212_1) and the second positive electrode active material layer (212_2) includes a first conductive material (not shown) and a second conductive material (not shown), and the compositions of the first positive electrode active material layer (212_1) and the second positive electrode active material layer (212_2) are different from each other. Specifically, the contents of the first and second conductive materials in the first positive electrode active material layer (212_1) and the second positive electrode active material layer (212_2) may be different from each other. At this time, the anode according to the present invention may have K of 0.82 to 1.16 as expressed by the following [Formula 1]. [Formula 1] In the above equation 1, W T1 The content (weight%) of the first conductive material included in the first positive electrode active material layer (212_1) is W T2 The content (weight%) of the second conductive material included in the first positive electrode active material layer (212_1) is W B1The content (weight%) of the first conductive material included in the second positive electrode active material layer (212_2) is W B2 is the content (weight%) of the second conductive material included in the second positive electrode active material layer (212_2), and BET1 is the specific surface area (m) of the first conductive material. 2 / g), BET2 is the specific surface area of ​​the second challenge material (m 2 / g), L1 is the average length (㎛) of the first conductive material, L2 is the average length (㎛) of the second conductive material, η1 is the true density (g / cc) of the first conductive material, η2 is the true density (g / cc) of the second conductive material, P T is the porosity (%) of the first positive electrode active material layer (212_1), P B refers to the porosity (%) of the second positive electrode active material layer (212_2). The above [Formula 1] reflects the porosity of the first and second positive electrode active material layers (212_1, 212_2), which can be an indicator of the difference in the conductive path according to the properties of the first and second conductive materials, which are the main factors affecting the conductivity, and the radius of curvature. Specifically, the K is characterized by having as factors the content, specific surface area, average length, and true density of the first and second conductive materials included in each of the first and second positive electrode active material layers (212_1, 212_2), and the porosity of the first and second positive electrode active material layers. The conductivity of the first and second positive electrode active material layers (212_1, 212_2) can vary depending on the content, specific surface area, average length, and true density of the first and second conductive materials. In addition, since the conductivity of the first and second positive electrode active material layers (212_1, 212_2) can increase as the porosity decreases, the porosity can also affect the conductivity of the first and second positive electrode active material layers (212_1, 212_2). The above K may be an index representing the degree of identity of conductivity between the first and second positive electrode active material layers (212_1, 212_2). The above K may be 0.82 to 1.16, preferably 0.85 to 1.15, and more preferably 0.9 to 1.1. Specifically, the above K may be 0.82 or more, 0.85 or more, 0.88 or more, 0.90 or more, or 0.92 or more, and the above K may be 1.16 or less, 1.15 or less, 1.1 or less, 1.08 or less, or 1.05 or less. The closer the above K is to 1, the smaller the difference in conductivity between the first positive electrode active material layer (212_1) and the second positive electrode active material layer (212_2). The conductivity of the first positive electrode active material layer (212_1) and the second positive electrode active material layer (212_2) must be similar so that charging and discharging can proceed uniformly, thereby realizing a secondary battery (100) with excellent cycle characteristics. On the other hand, the further the value of the above K is from 1, the greater the difference in conductivity between the first positive electrode active material layer (212_1) and the second positive electrode active material layer (212_2), and the difference in conductivity may cause problems such as overcharging of a portion with low resistance, thereby causing uneven charging and discharging. The above K is a factor of the average length (L1) of the first conductive material and the average length (L2) of the second conductive material included in each of the first and second positive electrode active material layers (212_1, 212_2). The average length (L2) of the second conductive material is longer than the average length (L1) of the first conductive material, and thus, it is easy to form a conductive path between the positive electrode active materials, thereby improving conductivity. Specifically, the average length (L2) of the second conductive material may be 2 to 6 times, preferably 3 to 5 times, the average length (L1) of the first conductive material. In addition, the average length (L1) of the first conductive material may be 0.5 μm to 3.5 μm, and the average length (L2) of the second conductive material may be 4 μm to 8 μm. When the lengths of each of the first and second conductive materials satisfy the above ranges, the conductivity of the first and second positive electrode active material layers may be excellent and uniform. Specifically, the first conductive material has a small area that can come into contact with the active material due to its short length, but has relatively excellent dispersibility, so that it can have a uniform distribution within the active material layer, thereby exhibiting uniform conductivity. On the other hand, the second conductive material has a long length, so that it can come into contact with a large number of active material particles within the active material layer, and thus has excellent conductivity. That is, if the length of each of the first and second conductive materials is far from the above range, agglomeration between particles may occur, which may increase conductive unevenness or cause a problem in which the material properties deteriorate due to a decrease in length, thereby reducing the conductivity. Meanwhile, the specific surface area (BET1) of the first conductive material may be 0.15 to 0.5 times, preferably 0.15 to 0.45 times, and more preferably 0.25 to 0.4 times, of the specific surface area (BET2) of the second conductive material. Specifically, the specific surface area (BET1) of the first conductive material may be 200 m 2 / g to 420m 2 / g, and the specific surface area (BET2) of the second challenge material is 850 m 2 / g to 1250m 2 / g may be. The specific surface area (BET2) of the second conductive material is larger than the specific surface area (BET1) of the first conductive material, and therefore, the conductivity of the second conductive material is superior to the conductivity of the first conductive material. When the specific surface areas of the first and second conductive materials satisfy the above range, the conductivity of the active material layer is uniform, and the contact area between the active material and the conductive material can be secured, so that a secondary battery having superior conductivity can be implemented. That is, the second conductive material has higher conductivity than the first conductive material, and the conductivity of the first and second positive electrode active material layers (212_1, 212_2) can be controlled by controlling the contents of the first and second conductive materials having different conductivity. The content of the second conductive material (W) in the first positive electrode active material layer (212_1) T2 ) is the second conductive material content (W) of the second positive electrode active material layer (212_2). B2 ) may be greater than that. Specifically, the weight ratio (W) of the first conductive material and the second conductive material included in the first positive electrode active material layer (212_1) T1 : W T2 ) may be 15:1 to 45:1, preferably 16:1 to 32:1, more preferably 16:1 to 30:1. In addition, the weight ratio (W) of the first conductive material and the second conductive material included in the second positive electrode active material layer (212_2) B1 : W B2) may be 35:1 to 45:1, preferably 38:1 to 42:1. In an electrode assembly (200) completed by winding the positive electrode (210), the negative electrode (220), and the separator (230), the first positive electrode active material layer (212_1) is located close to the battery case (300), unlike the second positive electrode active material layer (212_2) located close to the winding center. Due to this, the first positive electrode active material layer (212_1) of the positive electrode (210) has a larger radius of curvature than the second positive electrode active material layer (212_2), and thus a difference in the distance between the positive electrode active materials may occur. The present invention can prevent a difference in conductivity from occurring between the first positive electrode active material layer (212_1) and the second positive electrode active material layer (212_2) by controlling the contents of first and second conductive materials with different conductivities in the first positive electrode active material layer (212_1) to the above range. The content (W) of the second conductive material included in the first positive electrode active material layer (212_1) T2 ) is too small, it is impossible to prevent the conductivity of the first positive electrode active material layer (212_1) from decreasing as the radius of curvature increases. On the other hand, the content (W) of the second conductive material included in the first positive electrode active material layer (212_1) T2 ) is excessive, the difference in conductivity with respect to the second positive electrode active material layer (212_2) may increase. In addition, the content (W) of the second conductive material of the first positive electrode active material layer (212_1) T2 ) may be 0.015 wt% to 0.025 wt% based on the total weight of the first positive electrode active material layer (212_1). That is, in order to prevent the conductivity of the first positive electrode active material layer (212_1) from decreasing, the positive electrode (210) includes a second conductive material having a higher conductivity than the first conductive material in the first positive electrode active material layer (212_1) within the above range. As a result, the phenomenon of uneven charging and discharging occurring can be prevented, and a secondary battery (100) having excellent cycle characteristics can be implemented. In addition, the first conductive material may be a multi-walled carbon nanotube (MWCNT), and the second conductive material may be a single-walled carbon nanotube (SWCNT) or a double-walled carbon nanotube (DWCNT). At this time, the number of walls of the multi-walled carbon nanotube may be 7 to 12. The single-walled carbon nanotube and the double-walled carbon nanotube have a larger average length and a larger specific surface area than the multi-walled carbon nanotube, and thus have better conductivity. Therefore, by increasing the content of the second conductive material included in the first positive electrode active material layer (212_1), a decrease in the conductivity of the first positive electrode active material layer (212_1) can be prevented. The true density (η1) of the first conductive material may be 1.97 g / cc to 2.07 g / cc, and the true density (η2) of the second conductive material may be 1.85 g / cc to 1.95 g / cc. By including the true densities of the first and second conductive materials in the above formula, the k value can be obtained as a constant. As a device for measuring the true density, an example of a dry automatic density meter (Accupick II 1340 series, Shimadzu Corporation) can be mentioned. When the above device is used, a small amount of the first conductive material or the second conductive material can be put into the density meter to automatically measure the true density. The true density can also be obtained using a measuring method and / or device other than the above-described measuring method and / or device. In addition, the porosity of the first positive electrode active material layer (212_1) may be smaller than the porosity of the second positive electrode active material layer (212_2). Specifically, when the positive electrode (210) is wound, a compressive stress is more applied to the second positive electrode active material layer (212_2) located near the center of the winding, and a tensile stress is more applied to the first positive electrode active material layer (212_1) located near the battery case, thereby causing a change in the porosity between the first and second positive electrode active material layers (212_1, 212_2). At this time, the porosity of the first positive electrode active material layer (212_1) is greater than that of the second positive electrode active material layer (212_2), but the porosity of the first positive electrode active material layer (212_1) can be 21% to 27%, and the porosity (212_2) of the second positive electrode active material layer can be 20% to 26%. Preferably, the porosity of the first positive electrode active material layer (212_1) can be 22% to 26%, and the porosity (212_2) of the second positive electrode active material layer can be 21% to 25%. When the porosities of the first and second positive electrode active material layers (212_1, 212_2) satisfy the above range, the conductivity difference due to the porosity difference can be reduced without adding an excessive amount of the second conductive material to the first positive electrode active material layer (212_1). <Secondary battery> A secondary battery (100) according to the present invention includes an electrode assembly (200) in which a cathode (210), an anode (220), and a separator (230) interposed between the cathode (210) and the anode (220) are wound in one direction; and a battery case (300) in which the electrode assembly (200) is accommodated. The external shape of the secondary battery (100) is not particularly limited, and may be a cylindrical shape, a square shape, etc. using a can. For example, the secondary battery may be a cylindrical secondary battery, and the battery case may be a cylindrical can. The battery case may be a container made of a metal material, and may include, for example, aluminum or an aluminum alloy. In an exemplary embodiment, the secondary battery (100) according to the present invention may be a high-capacity cylindrical battery having a form factor ratio (defined as the ratio of the diameter (Ф) to the height (H) of the cylindrical battery, that is, the value obtained by dividing the diameter by the height) of 0.4 or more. Here, the form factor means a value representing the diameter and height of the cylindrical battery. In an exemplary embodiment, the secondary battery (100) according to the present invention may be, for example, a 46110 cell (diameter 46 mm, height 110 mm, form factor ratio 0.418), a 4875 cell (diameter 48 mm, height 75 mm, form factor ratio 0.640), a 48110 cell (diameter 48 mm, height 110 mm, form factor ratio 0.436), a 4880 cell (diameter 48 mm, height 80 mm, form factor ratio 0.600), a 4680 cell (diameter 46 mm, height 80 mm, form factor ratio 0.575). In the numerical value representing the form factor, the first two numbers represent the diameter of the cell, and the next two or three numbers represent the height of the cell. In addition, when the secondary battery (100) according to the present invention is charged at a constant current of 0.5 C in a temperature range of 20° C. to 55° C. until it reaches 4.2 V and then discharged at a constant current of 0.5 C until it reaches 2.5 V, which constitutes one charge / discharge cycle, the capacity retention rate according to the following [Formula 2] may be 80% or more: [Formula 2] Capacity retention rate (%) = 100 * {discharge capacity after 200 cycles} / {discharge capacity after 1 cycle}. By reducing the difference in the properties of the first and second positive electrode active material layers that may occur due to winding of the positive electrode, uniform charging and discharging of the secondary battery is possible, and thus a secondary battery with improved cycle characteristics can be provided. That is, the present invention can implement a secondary battery having a large capacity and excellent capacity retention rate by applying a cathode having controlled contents of first and second conductive materials having different conductivity to a high-capacity secondary battery. Electrode assembly The electrode assembly (200) according to the present invention may have a structure in which a positive electrode (210), a negative electrode (220), and a separator (230) interposed between the positive electrode (210) and the negative electrode (220) are wound. When the positive electrode (210), the negative electrode (220), and the separator (230) are wound, one side of the positive electrode (210) is arranged toward the center of the winding, and the other side of the positive electrode (210) is arranged toward the battery case (300). (1) Bipolar The positive electrode (210) according to the present invention includes a positive electrode current collector (211), a first positive electrode active material layer (212_1) applied to an upper surface (211_1) of the positive electrode current collector, and a second positive electrode active material layer (212_2) applied to a lower surface (211_2) of the positive electrode current collector. The positive electrode current collector (211) is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. The first and second positive electrode active material layers (212_1, 212_2) further include a positive electrode active material and a positive electrode binder. The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; and a chemical formula of Li 1+a1 Mn 2-a1Lithium manganese oxides such as O4 (0≤a1≤0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; Ni-site type lithium nickel oxide expressed by the chemical formula LiNi1-a2Mc2O2 (wherein, M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01≤a2≤0.3); chemical formula LiMn 2-a3 M a3 Lithium manganese composite oxide represented by O2 (wherein, M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 0.01≤a3≤0.1) or Li2Mn3MO8 (wherein, M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); LiMn2O in which a part of Li in the chemical formula is replaced with an alkaline earth metal ion. 4　 These may include, but are not limited to, the anode. The anode may be Li-metal. Next, the positive electrode binder serves to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples thereof include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof may be used. The positive electrode (210) can be manufactured according to a conventional electrode manufacturing method. For example, a positive electrode slurry can be manufactured by mixing a positive electrode active material, a positive electrode binder, and / or a positive electrode conductive material in a solvent, and the positive electrode slurry can be applied onto a positive electrode current collector, followed by drying and rolling to manufacture a positive electrode current collector having a positive electrode active material layer formed thereon. Meanwhile, solvents commonly used in the relevant technical field can be used as solvents for the positive electrode slurry, and for example, dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, water, etc. can be used alone or in a mixture of two or more. (2) Cathode The negative electrode (220) according to the present invention includes a negative electrode current collector and a negative electrode active material layer formed on both sides of the negative electrode current collector. The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. As the negative electrode active material, compounds capable of reversible intercalation and deintercalation of lithium, which are generally used in the relevant technical field, can be used, and the type is not particularly limited. Specific examples of the negative electrode active material include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; Si, Si-Me alloy (wherein, Me is at least one selected from the group consisting of Al, Sn, Mg, Cu, Fe, Pb, Zn, Mn, Cr, Ti, and Ni), SiOy (wherein, 0 <y<2), Si-C 복합체 등과 같은 실리콘계 물질; 리튬 금속 박막; Sn, Al 등과 같이 리튬과 합금화가 가능한 금속 물질; 등을 들 수 있으며, 이들 중 어느 하나 또는 둘 이상의 혼합물이 사용될 수 있다. The above negative electrode conductive material is used to provide conductivity to the negative electrode, and can be used without special restrictions in the battery being composed, as long as it does not cause a chemical change and has electronic conductivity. The negative electrode conductive material may be the same as the positive electrode conductive material. The above negative electrode binder serves to improve the adhesion between negative electrode active material particles and the adhesive strength between the negative electrode active material and the negative electrode current collector. The above negative electrode binder may be the same as the above positive electrode binder. Meanwhile, the negative electrode (220) can be manufactured according to a conventional electrode manufacturing method. For example, a negative electrode slurry can be manufactured by mixing a negative electrode active material, a negative electrode binder, and / or a negative electrode conductive material in a solvent, and the negative electrode slurry can be applied onto a negative electrode current collector, followed by drying and rolling to manufacture a negative electrode current collector having a negative electrode active material layer formed thereon. As the solvent of the above cathode slurry, solvents generally used in the relevant technical field can be used, and for example, dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, water, etc. can be used alone or in a mixture of two or more. (3) Membrane The separator (230) according to the present invention separates the positive electrode (210) and the negative electrode (220) and provides a passage for lithium ions to move. Any separator commonly used in secondary batteries can be used without special restrictions. Specifically, the separator may be a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc. may be used. In addition, a coated separator containing a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength. Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily implement the present invention. However, the present invention may be implemented in various different forms and is not limited to the embodiments described herein. Example 1 Positive electrode active material: First conductive material: Second conductive material PVDF binder was mixed in a weight ratio of 97.585: 0.4: 0.015: 2 in N-methyl-2-pyrrolidone (NMP) to prepare a first positive electrode active material slurry. At this time, Li(Ni) was used as the positive electrode active material. 0.89 Co 0.06 Mn 0.04 Al 0.01)O2, the first challenge material has a surface area of ​​320 m 2 / g, an average length of 1.5 ㎛, and a true density of 2.02 g / cc, multi-walled carbon nanotubes (MWCNTs) are used as the second conductive material, and have a specific surface area of ​​1050 m 2 / g, an average length of 6 μm, and a true density of 1.9 g / cc were used. A second positive electrode active material slurry was prepared in the same manner as the first positive electrode active material slurry, except that the positive electrode active material: first conductive agent: second conductive agent: PVDF binder were mixed in a weight ratio of 97.59: 0.4: 0.01: 2. The first cathode active material slurry was applied to the upper surface of the cathode current collector (Al) at a density of 0.454 g / 25 cm. 2 The first positive electrode active material layer is manufactured by applying a loading amount of 0.430 g / 25 cm to the lower surface of the positive electrode current collector, and the second positive electrode active material slurry is applied at a loading amount of 0.430 g / 25 cm 2 A second positive electrode active material layer was manufactured by applying a loading amount of . Thereafter, the first and second positive electrode active material layers and the positive electrode current collector were dried and roll pressed to manufacture a positive electrode. Negative active material: Conductive agent: Binder: CMC was mixed in water at a weight ratio of 96.2:035:2.2:1.1 to prepare a negative electrode slurry. At this time, natural graphite and artificial graphite mixed at a weight ratio of 7:3 were used as the negative active material, BM-L203 from Zeon was used as the binder, and Super C65 from Imerys was used as the conductive agent. The above-mentioned negative electrode slurry was applied to a negative electrode current collector (Cu thin film), dried, and roll pressed to prepare a negative electrode. A jelly-roll type electrode assembly was manufactured by interposing a separator between the positive and negative electrodes manufactured as described above and winding the electrodes, in which the second positive electrode active material layer of the positive electrode was positioned on the inside to face the winding axis and the first positive electrode active material layer was positioned on the outside. After the electrode assembly manufactured as described above was placed in a cylindrical battery can, ethylene carbonate (EC): dimethyl carbonate (DMC) was mixed in a volume ratio of 25:75, and then an electrolyte in which LiPF6 was dissolved to a concentration of 1.0 M was injected to manufacture a secondary battery having 4680 cells. Example 2 A secondary battery was manufactured in the same manner as in Example 1, except that the first positive electrode active material slurry was manufactured by mixing the positive electrode active material: first conductive agent: second conductive agent: PVDF binder in a weight ratio of 97.58: 0.4: 0.02: 2. Example 3 A secondary battery was manufactured in the same manner as in Example 1, except that the first positive electrode active material slurry was manufactured by mixing the positive electrode active material: first conductive agent: second conductive agent: PVDF binder in a weight ratio of 97.575: 0.4: 0.025: 2. Comparative Example 1 A secondary battery was manufactured in the same manner as in Example 1, except that the first positive electrode active material slurry was manufactured by mixing the positive electrode active material: first conductive agent: second conductive agent: PVDF binder in a weight ratio of 97.59: 0.4: 0.01: 2. Comparative Example 2 A secondary battery was manufactured in the same manner as in Example 1, except that the first positive electrode active material slurry was manufactured by mixing the positive electrode active material: first conductive agent: second conductive agent: PVDF binder in a weight ratio of 97.57: 0.4: 0.03: 2. The contents of the first and second conductive materials included in each of the positive electrodes manufactured in Examples 1 to 2 and Comparative Examples 1 to 3 are summarized and shown in Table 1 below. First positive electrode active material layerSecond positive electrode active material layerFirst conductive material (WT1 )2nd Challenge (W T2 )1st Challenge (W B1 )2nd Challenge (W B2 )Example 10.4 wt% 0.015 wt% 0.4 wt% 0.01 wt%Example 20.4 wt% 0.02 wt% 0.4 wt% 0.01 wt%Example 30.4 wt% 0.025 wt% 0.4 wt% 0.01 wt%Comparative Example 10.4 wt% 0.01 wt% 0.4 wt% 0.01 wt%Comparative Example 20.4 wt% 0.03 wt% 0.4 wt% 0.01 wt% Experimental Example 1: Evaluation of Porosity and Calculation of K Value in [Equation 1] The porosity of each of the rolled first and second positive electrode active material layers of each secondary battery manufactured in Examples 1 to 3 and Comparative Examples 1 to 2 was evaluated and shown in Table 2 below. Specifically, the porosity was calculated by the following [Equation 3]. [Formula 3] Porosity of the positive electrode active material layer after winding (%) = {1-(electrode density of the positive electrode active material layer after winding / true density of the positive electrode active material layer)} × 100 The true density of the positive electrode active material layer is the density of the positive electrode active material layer measured when the positive electrode active material layer is collected to a certain size and pressed with a press device until the thickness of the positive electrode active material layer does not change, and the electrode density of the positive electrode active material layer after rolling is the density of the positive electrode active material layer calculated by dividing the electrode weight (g) per unit area (cm2) measured by collecting the positive electrode active material layer to a certain size after rolling by the electrode thickness (cm). In addition, the K values ​​of each of the secondary batteries manufactured in Examples 1 to 3 and Comparative Examples 1 to 2 were measured and calculated using the following [Formula 1], and are shown in Table 2 below. [Formula 1] In the above equation 1, W T1 The content (%) of the first conductive material included in the first positive electrode active material layer is W T2is the content (%) of the second conductive material included in the first cathode active material layer, W B1 The content (%) of the first conductive material included in the second positive electrode active material layer is W B2 is the content (%) of the second conductive material included in the second positive electrode active material layer, and BET1 is the specific surface area (m) of the first conductive material. 2 / g), BET2 is the specific surface area of ​​the second challenge material (m 2 / g), L1 is the average length (㎛) of the first conductive material, L2 is the average length (㎛) of the second conductive material, η1 is the true density (g / cc) of the first conductive material, η2 is the true density (g / cc) of the second conductive material, P T is the porosity (%) of the first positive electrode active material layer, P B refers to the porosity (%) of the second positive electrode active material layer. Porosity (%) K1st positive electrode active material layer (P T )Second positive electrode active material layer (P B )Example 124.48023.4000.908Example 224.27623.2051.004Example 324.58223.4981.100Comparative Example 124.17423.1080.813Comparative Example 224.37823.3031.196 Experimental Example 2: Capacity Retention Rate Evaluation Secondary batteries including each of the positive electrodes manufactured by Examples 1 to 2 and Comparative Examples 1 to 3 were manufactured, and the capacity retention rates of each were measured, which are shown in Table 3 below. Specifically, for each of the secondary batteries manufactured as described above, when charging to 4.2 V at a constant current of 0.5 C in a temperature range of 20°C to 55°C and then discharging to 2.5 V at a constant current of 0.5 C is considered one charge / discharge cycle (n=1), 200 cycles (n=200) of charge / discharge were performed, and then the capacity retention rate was measured using a cycler from PNE. K Capacity Maintenance Rate (%) Example 10.90883.2 Example 21.00485.1 Example 31.10081.5 Comparative Example 10.81379.5 Comparative Example 21.19674.7 Referring to Table 3 above, it can be confirmed that the closer K is to 1, the better the capacity retention rate. Specifically, Examples 1 to 3, where k is 0.9 to 1.1, have higher capacity retention rates than Comparative Example 1, where k is 0.813, and Comparative Example 2, where k is 1.196. It is believed that this is because Examples 1 to 3 adjust the content of the second conductive material in the first positive electrode active material layer, so that the conductivity between the first positive electrode active material layer and the second positive electrode active material layer is similar to each other. [Explanation of symbols] 100: Secondary battery 200: Electrode assembly 210: Bipolar 211: Bipolar collector 211_1: Top surface of the cathode collector 211_2: The lower surface of the positive electrode collector 212_1: First positive electrode active material layer 212_2: Second positive electrode active material layer 220: Negative 230: Membrane 300: Battery Case

Claims

1. A positive electrode current collector and a first positive electrode active material layer formed on the upper surface of the positive electrode current collector, and a second positive electrode active material layer formed on the lower surface of the positive electrode current collector, Each of the first positive electrode active material layer and the second positive electrode active material layer includes a first conductive material and a second conductive material, The average length of the second challenge material is longer than the average length of the first challenge material, An anode having K of 0.82 to 1.16, represented by the following [Formula 1]: [Formula 1] In the above equation 1, W T1 The content (weight%) of the first conductive material included in the first positive electrode active material layer is W T2 W is the content (weight%) of the second conductive material included in the first positive electrode active material layer. B1 The content (weight%) of the first conductive material included in the second positive electrode active material layer is W B2 is the content (weight%) of the second conductive material included in the second positive electrode active material layer, and BET1 is the specific surface area (m) of the first conductive material. 2 / g), BET2 is the specific surface area of ​​the second challenge material (m 2 / g), L1 is the average length (㎛) of the first conductive material, L2 is the average length (㎛) of the second conductive material, η1 is the true density (g / cc) of the first conductive material, η2 is the true density (g / cc) of the second conductive material, P T is the porosity (%) of the first positive electrode active material layer, P B refers to the porosity (%) of the second positive electrode active material layer.

2. In paragraph 1, An anode in which the average length (L2) of the second conductive material is 2 to 6 times the average length (L1) of the first conductive material.

3. In paragraph 2, An anode having an average length (L2) of the second challenge material of 4 μm to 8 μm.

4. In paragraph 1, The second conductive material content (W) of the first positive electrode active material layer T2 ) is the second conductive material content (W) of the second positive electrode active material layer. B2 ) is larger than the anode.

5. In paragraph 1, The weight ratio (W) of the first conductive material and the second conductive material included in the first positive electrode active material layer T1 : W T2 ) is a positive electrode with a ratio of 15:1 to 45:

1.

6. In paragraph 1, The weight ratio (W) of the first conductive material and the second conductive material included in the second positive electrode active material layer B1 : W B2 ) is a positive electrode with a ratio of 35:1 to 45:

1.

7. In paragraph 1, The content of the second conductive material in the first positive electrode active material layer (W T2 ) is 0.015 wt% to 0.025 wt% based on the total weight of the first positive electrode active material layer.

8. In paragraph 1, An anode wherein the specific surface area (BET1) of the first conductive material is 0.15 to 0.5 times the specific surface area (BET2) of the second conductive material.

9. In paragraph 1, The specific surface area (BET1) of the first challenge material is 200 m 2 / g to 420m 2 / g and, The specific surface area (BET2) of the above second challenge material is 850 m 2 / g to 1250m 2 / g is positive.

10. In paragraph 1, The above first challenge material is a multi-walled carbon nanotube, The second challenge material is an anode which is a single-walled carbon nanotube or a double-walled carbon nanotube.

11. In paragraph 1, The true density (η1) of the first challenge material is 1.97 g / cc to 2.07 g / cc, An anode having a density (η2) of the second challenge material of 1.85 g / cc to 1.95 g / cc.

12. An electrode assembly in which the positive electrode, negative electrode, and separator interposed between the positive electrode and negative electrode according to Article 1 are wound in one direction; and a battery case in which the electrode assembly is accommodated; A secondary battery, wherein, when the positive electrode is wound, the first positive electrode active material layer is arranged to face the battery case, and the second positive electrode active material layer is arranged to face the winding center.

13. In paragraph 12, The above secondary battery is a cylindrical secondary battery having a form factor ratio of 0.4 or more.

14. In paragraph 12, The above secondary battery is a secondary battery having 46110 cells, 4875 cells, 48112 cells, 4880 cells or 4680 cells.

15. In paragraph 12, The secondary battery above is a secondary battery having a capacity retention rate of 80% or more according to the following [Formula 2] when charging to 4.2 V at a constant current of 0.5 C in a temperature range of 20°C to 55°C and then discharging to 2.5 V at a constant current of 0.5 C, which constitutes one charge / discharge cycle: [Formula 2] Capacity retention rate (%) = 100*{discharge capacity after 200 cycles} / {discharge capacity after 1 cycle}.

Images