Positive electrode for lithium secondary battery, and lithium secondary battery including the same

Abstract

To provide a positive electrode for preventing the deterioration of a lithium secondary battery and realizing the long lifetime.SOLUTION: A positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material. The first positive electrode active material contains lithium nickel composite oxide and is in a secondary particulate mode including a plurality of primary particles. An average particle diameter (D50) of the secondary particles is 9 μm to 25 μm. The second positive electrode active material contains lithium nickel composite oxide and is in a single particulate mode. An average particle diameter (D50) of the single particles is 0.5 μm to 5 μm. A lower layer, which is a region corresponding to 10 length% in a thickness direction from a surface of the positive electrode active material layer that is in contact with a current collector, includes 5 wt.% to 30 wt.% of the second positive electrode active material per 100 wt.% of the first positive electrode active material and the second positive electrode active material. An upper layer, which is a region corresponding to 10 length% in the thickness direction from a surface of the positive electrode active material layer on a side opposite to a surface thereof that is in contact with the current collector, includes 31 wt.% to 60 wt.% of the second positive electrode active material per 100 wt.% of the first positive electrode active material and the second positive electrode active material.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery including the same. [Background technology]

[0002] Lithium secondary batteries, which have high energy density and are easy to carry, are mainly used as the driving power source for mobile information terminals such as mobile phones, laptops, and smartphones. Recently, active research has been conducted on using high-energy-density lithium secondary batteries as driving power sources or power storage sources for hybrid and electric vehicles.

[0003] Such lithium secondary batteries require high electrode density, but the rolling process used to achieve high electrode density can cause cracks in the active material, which can impair electrochemical properties. Therefore, research into how to control the cracks induced during the rolling process is still needed. Summary of the Invention [Problem to be solved by the invention]

[0004] A positive electrode for a lithium secondary battery is provided to prevent deterioration of the lithium secondary battery and to achieve a long life. [Means for solving the problem]

[0005] In one embodiment, a positive electrode for a lithium secondary battery includes a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector, the positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material, the first positive electrode active material includes a lithium nickel-based composite oxide and is in the form of secondary particles consisting of a plurality of primary particles, and the average particle size (D 50 ) is 9 μm to 25 μm, the second positive electrode active material contains a lithium nickel-based composite oxide and is in the form of a single particle, and the average particle diameter (D 50the thickness of the positive electrode active material layer from a surface in contact with a current collector to a thickness 10% region of the positive electrode active material layer contains 5 wt % to 30 wt % of the second positive electrode active material relative to a total of 100 wt % of the first positive electrode active material and the second positive electrode active material, and the thickness of the positive electrode active material layer is 10% to 30 wt % of the total of 100 wt % of the first positive electrode active material and the second positive electrode active material, and the thickness of the positive electrode active material layer is 10% to 30 wt % of the total of 100 wt % of the first positive electrode active material and the second positive electrode active material.

[0006] In one embodiment, a method for manufacturing a positive electrode for a lithium secondary battery includes the steps of: preparing a first layer composition, in which the content of the second positive electrode active material is 5 to 30 wt % relative to a total of 100 wt % of the first positive electrode active material and the second positive electrode active material; preparing a second layer composition, in which the content of the second positive electrode active material is 31 to 60 wt % relative to a total of 100 wt % of the first positive electrode active material and the second positive electrode active material; loading the first layer composition and the second layer composition into a dual slot die coater, respectively, and then simultaneously coating the first layer on a current collector such that the first layer is located on the current collector and the second layer is located on the first layer; and drying and rolling the resulting mixture. In this method, the first positive electrode active material includes a lithium nickel-based composite oxide and is in the form of secondary particles composed of a plurality of primary particles, and the average particle diameter (D 50 ) is 9 μm to 25 μm, the second positive electrode active material contains a lithium nickel-based composite oxide and is in the form of a single particle, and the average particle diameter (D 50 ) is 0.5 μm to 5 μm.

[0007] In one embodiment, a lithium secondary battery is provided, comprising: the positive electrode; a negative electrode; and an electrolyte. Effect of the Invention

[0008] According to an embodiment of the present invention, a positive electrode for a lithium secondary battery can prevent cracks from occurring during a rolling process and delay degradation during the life of the battery, thereby realizing a long life. [Brief description of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view illustrating a lithium secondary battery according to an embodiment. [Diagram 2] 1 is a cross-sectional view illustrating a lithium secondary battery according to an embodiment. [Diagram 3] 1 is a cross-sectional view illustrating a lithium secondary battery according to an embodiment. [Figure 4] 1 is a cross-sectional view illustrating a lithium secondary battery according to an embodiment. [Diagram 5] 1 is an image of a cross section of a positive electrode of Comparative Example 1. [Figure 6] The left side is an enlarged image of the upper layer part of the cross section of the positive electrode of Comparative Example 1, and the right side is an enlarged image of the upper layer part of the cross section of the positive electrode of Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is not limited to the embodiments described herein, but may be embodied in various different forms, and should not be construed as limited to the embodiments described herein.

[0011] The terms used herein are merely used to describe exemplary embodiments and are not intended to limit the present invention. A singular expression includes a plural expression unless it has a clearly different meaning in the context.

[0012] "Combinations thereof" means mixtures, laminates, composites, copolymers, alloys, blends, reaction products, and the like of compositions.

[0013] It is to be understood that the terms "including," "comprising," or "having" are intended to specify the presence of embodied features, numerals, steps, components, or combinations thereof, but do not preclude the presence or additional possibility of one or more other features, numerals, steps, components, or combinations thereof.

[0014] In the drawings, the thickness of the various layers and regions is exaggerated for clarity, and similar parts are designated by the same reference numerals throughout the specification. When a part, such as a layer, film, region, plate, etc., is said to be "on" or "on" another part, this includes not only the case where it is "directly on" the other part, but also the case where there is another part between them. Conversely, when a part is said to be "directly on" another part, it means that there is no other part between them.

[0015] The term "layer" includes not only a shape formed on the entire surface when observed in a plan view, but also a shape formed on a portion of the surface.

[0016] The average particle size can be measured by a method well known to those skilled in the art, for example, by using a particle size analyzer, or by using a transmission electron microscope image or a scanning electron microscope image. Alternatively, the average particle size can be calculated by measuring using a dynamic light scattering method, counting the number of particles for each particle size range, and then calculating the average particle size. Unless otherwise defined, the average particle size is the diameter (D 50 In addition, unless otherwise defined, the average particle size is the diameter (D) of the particle that occupies 50% of the cumulative volume in the particle size distribution obtained by measuring the size (diameter or major axis length) of 20 or more randomly selected particles in a scanning electron microscope image. 50 ) may be taken as the average particle size.

[0017] "Or" is not to be construed as exclusive, for example "A or B" is to be construed as including A, B, A+B, etc.

[0018] The term "metal" is understood to include general metals, transition metals, and metalloids.

[0019] positive electrode In one embodiment, a positive electrode for a lithium secondary battery includes a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector, the positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material, the first positive electrode active material includes a lithium nickel-based composite oxide and is in the form of secondary particles consisting of a plurality of primary particles, and the average particle size (D 50 ) is 9 μm to 25 μm, the second positive electrode active material contains a lithium nickel-based composite oxide and is in the form of a single particle, and the average particle diameter (D 50 the thickness of the positive electrode active material layer from a surface in contact with a current collector to a thickness 10% region of the positive electrode active material layer contains 5 wt % to 30 wt % of the second positive electrode active material relative to a total of 100 wt % of the first positive electrode active material and the second positive electrode active material, and the thickness of the positive electrode active material layer is 10% to 30 wt % of the total of 100 wt % of the first positive electrode active material and the second positive electrode active material, and the thickness of the positive electrode active material layer is 10% to 30 wt % of the total of 100 wt % of the first positive electrode active material and the second positive electrode active material.

[0020] In a positive electrode containing a lithium nickel-based positive electrode active material, a design has been proposed to maximize capacity and energy density by mixing large particles in the form of secondary particles and small particles in the form of single particles, thereby realizing long life characteristics. A positive electrode is generally manufactured by applying a positive electrode composition containing a positive electrode active material onto a current collector, drying it, and then rolling it. In a positive electrode containing large particles in the form of secondary particles and small particles in the form of single particles, a problem may occur in that the large particles in the form of secondary particles break, crack, or have a damaged surface in the upper layer where the rolling roll or rolling plate comes into contact during the rolling process. This may lead to a problem that the deterioration of the large particles in the upper layer is accelerated as the battery is repeatedly charged and discharged, resulting in a decrease in life characteristics. Therefore, in one embodiment, a design is proposed that can effectively suppress the problem of large particle breakage in the upper layer that occurs during the rolling process by increasing the content of small particles in the form of single particles at a certain rate and decreasing the content of large particles in the form of secondary particles at a certain rate in the upper layer, thereby improving life characteristics and maintaining high capacity and energy density.

[0021] Furthermore, in general, the thicker the positive electrode active material layer, the higher the capacity, but there is a problem that the degree of impregnation of the electrolyte is reduced or the movement of lithium ions is insufficient in the lower layer of the positive electrode active material layer near the current collector side. On the other hand, the positive electrode for a lithium secondary battery according to an embodiment achieves high capacity, but contains a large amount of single particles having an average particle size of 0.5 μm to 5 μm in the upper layer closer to the surface side of the positive electrode active material layer than in the lower layer closer to the current collector side of the positive electrode active material layer, thereby increasing the porosity of the lower layer, i.e., the lower part of the electrode, and increasing the degree of impregnation of the electrolyte, thereby allowing smooth movement of lithium ions. In other words, the uniform reaction between the upper and lower parts of the electrode plate is ensured, thereby improving the life characteristics of a lithium secondary battery including the positive electrode. In addition, the lithium ions can be smoothly moved to the lower part of the electrode plate, and the electrochemical reaction occurs more actively, thereby improving the rate characteristics of the lithium secondary battery.

[0022] Cathode active material layer The positive electrode active material layer includes a first positive electrode active material containing a lithium nickel-based composite oxide and a second positive electrode active material containing a lithium nickel-based composite oxide. The positive electrode active material layer may further include another type of positive electrode active material, and may optionally further include a binder and / or a conductive material. The first positive electrode active material is in the form of secondary particles consisting of a plurality of primary particles, and the average particle size (D 50 ) is 9 μm to 25 μm, and may be, for example, 9 μm to 23 μm, 9 μm to 22 μm, 10 μm to 20 μm, or 12 μm to 18 μm. The first positive electrode active material may be expressed as large grains or large particles. When the particle size of the secondary particles satisfies the above range, high capacity and high energy density can be realized, and the electrolyte is well impregnated up to the lower layer, which is the lower part of the electrode plate, and the movement of lithium ions can be actively maintained, thereby eliminating reaction non-uniformity between the upper and lower parts of the electrode plate and improving the life characteristics and rate characteristics of the battery. Here, the average particle size (D 50 ) may be obtained by measuring the sizes (diameter or major axis length) of more than 20 randomly selected particles in a scanning electron microscope image to obtain a particle size distribution, and taking the diameter of the particle that makes up 50% of the cumulative volume in the particle size distribution as the average particle size.

[0023] The second positive electrode active material is in the form of a single particle, and the average particle diameter (D 50 ) is 0.5 μm to 5 μm, and may be, for example, 0.7 μm to 5 μm, 0.8 μm to 5 μm, 1 μm to 5 μm, or 1.5 μm to 4 μm. The second positive electrode active material may be expressed as small grains or small particles. When the particle size of the single particles satisfies the above range, the energy density of the positive electrode can be maximized, and the electrolyte can easily penetrate to the bottom of the electrode plate, which can help lithium ions to move smoothly, thereby achieving high capacity and simultaneously improving the life characteristics and rate characteristics of the battery. Here, the average particle size (D 50 ) may be obtained by measuring the sizes (diameter or major axis length) of more than 20 randomly selected particles in a scanning electron microscope image to obtain a particle size distribution, and taking the diameter of the particle that makes up 50% of the cumulative volume in the particle size distribution as the average particle size.

[0024] In the lower layer, which is a region of the positive electrode active material layer from the surface in contact with the current collector to 10% in the thickness direction, the second positive electrode active material may be included in an amount of 5 wt% to 30 wt% with respect to a total of 100 wt% of the first positive electrode active material and the second positive electrode active material, for example, 10 wt% to 30 wt%, 15 wt% to 30 wt%, or 20 wt% to 30 wt%. In the lower layer, the first positive electrode active material may be included in an amount of 70 wt% to 95 wt%, for example, 70 wt% to 90 wt%, 70 wt% to 85 wt%, or 70 wt% to 80 wt%, with respect to a total of 100 wt% of the first positive electrode active material and the second positive electrode active material.

[0025] In the upper layer, which is a region of the positive electrode active material layer from the surface opposite to the surface in contact with the current collector to 10% in the thickness direction, the second positive electrode active material may be included in an amount of 31 wt% to 60 wt% with respect to a total of 100 wt% of the first positive electrode active material and the second positive electrode active material, for example, 32 wt% to 50 wt% or 35 wt% to 45 wt%. In the upper layer, the first positive electrode active material may be included in an amount of 40 wt% to 69 wt%, for example, 50 wt% to 68 wt% or 55 wt% to 65 wt%, with respect to a total of 100 wt% of the first positive electrode active material and the second positive electrode active material.

[0026] Therefore, the upper layer may further include 1 to 55 wt% of the second positive electrode active material relative to the lower layer, for example, 2 wt% to 50 wt%, 3 wt% to 40 wt%, 5 wt% to 30 wt%, 5 wt% to 20 wt%, or 5 wt% to 10 wt%, based on the total weight of the first and second positive electrode active materials, i.e., the content of the second positive electrode active material in the upper layer may be 1 wt% to 55 wt% more than the second positive electrode active material in the lower layer, for example, 2 wt% to 50 wt%, 3 wt% to 40 wt%, 5 wt% to 30 wt%, 5 wt% to 20 wt%, or 5 wt% to 10 wt% more.

[0027] When the content of the second positive active material in the upper layer and the content of the second positive active material in the lower layer are within the above ranges, and the content of the second positive active material in the upper layer is higher than that of the second positive active material in the lower layer by a certain ratio, the problem of the first positive active material, which is a large particle in the form of a secondary particle, being broken or damaged in the upper layer where the rolling roll or rolling plate contacts during the rolling process in manufacturing the positive electrode can be effectively improved, and the problem of accelerated deterioration of the large particles during charging and discharging can be suppressed, improving the life characteristics and maintaining high capacity and high energy density.In addition, the impregnation of the electrolyte can be improved or the movement of lithium ions can be made smoother, improving the overall performance of the battery.

[0028] Between the lower layer and the upper layer, only the lower layer composition may be present, or only the upper layer composition may be present, or the lower layer composition may be present on the side closer to the lower layer and the upper layer composition may be present on the side closer to the upper layer.

[0029] For example, the content of the second positive electrode active material having a smaller average particle size may increase from the lower layer to the upper layer, and the content of the first positive electrode active material having a larger average particle size may increase from the upper layer to the lower layer, so that the porosity of the lower part of the electrode where the content of the first positive electrode active material having a larger average particle size is high increases, the degree of impregnation of the electrolyte increases, and lithium ions move smoothly. In other words, by ensuring uniform reactions at the top and bottom of the electrode plate, the life characteristics of a lithium secondary battery including the positive electrode may be improved. In addition, the lithium ions may move smoothly to the bottom of the electrode plate, so that the electrochemical reaction occurs more actively, and the rate characteristics of the lithium secondary battery may be improved.

[0030] The density of the positive electrode active material layer may be 3.4 g / cc to 3.9 g / cc, for example, 3.5 g / cc to 3.8 g / cc, or 3.6 g / cc to 3.7 g / cc. The density of the positive electrode active material layer means a density measured on a positive electrode in a rolled state. When the density of the positive electrode active material layer satisfies the above range, a very high energy density and a high capacity can be realized. However, in order to realize such a high density positive electrode, a process of rolling the positive electrode with high strength is required, and in the process, a problem may occur that large particles in the form of secondary particles in the upper layer are broken or damaged. However, according to the positive electrode design of one embodiment, damage to the large particles in the upper layer can be effectively suppressed while achieving a very high density. For example, the first positive electrode active material in the upper layer in which cracks have occurred may be 0 to 30% by number, for example, 1% by number to 20% by number, or 3% by number to 10% by number. This may refer to the percentage of first positive active materials in which at least some cracks have occurred, relative to 100% by number of the first positive active materials in the upper layer. The presence or absence of cracks and the crack occurrence ratio may be measured using SEM or TEM images of a cross section of the positive electrode. This may also be a value measured on the positive electrode after rolling or after a chemical formation process. When the first positive active materials in the upper layer in which cracks have occurred satisfy the above range, the problem of deterioration of large particles due to repeated charging and discharging can be effectively prevented, and the life characteristics of the battery can be improved.

[0031] If the large grains in the upper layer are damaged during the rolling process, nickel ions are reduced, for example, Ni 4+ Ion is Ni 3+ In one embodiment, the reduction phenomenon of nickel is reduced while suppressing damage to large particles in the upper layer. For example, the reduction phenomenon of nickel is reduced to 100% of the total area of ​​the cross section of the upper layer. 3+ The area occupied by Ni may be about 1 to 30 area %, for example, 3 to 20 area %, or 5 to 10 area %. This can be measured by TXM (Transmission X-ray Microscopy) analysis of the cross section of the positive electrode after rolling or after the chemical conversion process. 3+ When the ratio satisfies the above range, the problem of the large particles deteriorating due to repeated charging and discharging can be effectively prevented, and the life characteristics of a lithium secondary battery including the positive electrode can be improved.

[0032] On the other hand, the thickness of the entire positive electrode active material layer may be about 40 μm to 300 μm, for example, 40 μm to 250 μm, 50 μm to 200 μm, or 60 μm to 100 μm.

[0033] Meanwhile, a positive electrode according to another embodiment may include a positive electrode current collector; a first layer located on the current collector and including 5 wt % to 30 wt % of the second positive electrode active material relative to 100 wt % of the first positive electrode active material and the second positive electrode active material; and a second layer located on the first layer and including 31 wt % to 60 wt % of the second positive electrode active material relative to 100 wt % of the first positive electrode active material and the second positive electrode active material.

[0034] In the above embodiment, the first layer and the second layer may be in contact with each other, or a third layer may be present between the first layer and the second layer. For example, the third layer may have a composition of the first layer mixed with a composition of the second layer. According to the method for producing a positive electrode described below, the first layer may be simultaneously coated with the second layer on top of the second layer using a dual slot die coater, and then, the first layer and the second layer may be mixed during the rolling process to form a layer such as a third layer in which these compositions are mixed in between.

[0035] In the above embodiment, the first layer containing 5 to 30 wt % of the second positive electrode active material means that the first layer contains 5 to 30 wt % of the second positive electrode active material which is small particles relative to the total amount of the first and second positive electrode active materials contained in the first layer, and similarly, the second layer contains 31 to 60 wt % of the second positive electrode active material which is small particles relative to the total amount of the first and second positive electrode active materials contained in the second layer.

[0036] According to the embodiment, the lower layer, which is a region of the positive electrode active material layer from the surface in contact with the current collector to 10% in the thickness direction, contains 5% to 30% by weight of the second positive electrode active material with respect to a total of 100% by weight of the first positive electrode active material and the second positive electrode active material, and the upper layer, which is a region of the positive electrode active material layer from the surface opposite to the surface in contact with the current collector to 10% in the thickness direction, contains 31% to 60% by weight of the second positive electrode active material with respect to a total of 100% by weight of the first positive electrode active material and the second positive electrode active material. It can be said that the first layer contains the lower layer, and the second layer contains the upper layer.

[0037] The thickness of the first layer and the second layer may be the same or different, and may be independently 40 μm to 270 μm, for example 40 μm to 200 μm, 40 μm to 150 μm, 40 μm to 100 μm, or 40 μm to 80 μm.

[0038] The ratio of the thickness of the first layer to the thickness of the second layer is not particularly limited, and may be 50:50 to 90:10, for example, 50:50 to 85:15, 50:50 to 80:20, 50:50 to 75:25, or 55:45 to 70:30. By appropriately adjusting the thicknesses of the first layer and the second layer, it is possible to realize high capacity and high energy density while simultaneously improving the life characteristics.

[0039] The lithium nickel-based composite oxide of the first positive electrode active material and the lithium nickel-based composite oxide of the second positive electrode active material may be the same or different, and each may be independently represented by Chemical Formula 1. [Chemical formula 1] Li a1 Ni x1 M 1 y1 M 2 z1 O 2-b1 X b1

[0040] In the above chemical formula 1, 0.9≦a1≦1.2, 0.3≦x1<1, 0 <y1≦0.7、0≦z1≦0.7、0.9≦x1+y1+z1≦1.1、および0≦b1≦0.1であり、M 1 and M. 2 are each independently one or more elements selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr, and X is one or more elements selected from F, P, and S.

[0041] For example, in the above formula 1, 0.6≦x1<1, 0 <y1≦0.4、0≦z1≦0.4であってもよく、または0.7≦x1<1、0<y1≦0.3、0≦z1≦0.3であるか、0.8≦x1<1、0<y1≦0.2、0≦z1≦0.2であるか、または0.9≦x1<1、0<y1≦0.1、0≦z1≦0.1であってもよい。

[0042] As a specific example, the lithium nickel composite oxide of the first positive electrode active material and the lithium nickel composite oxide of the second positive electrode active material can each be independently represented by Chemical Formula 2. The compound represented by Chemical Formula 2 can be said to be a lithium nickel-cobalt-aluminum composite oxide or a lithium nickel-cobalt-manganese composite oxide. [Chemical Formula 2] Li a2 Ni x2 Co y2 M 3 z2 M 4 w2 O 2-b2 X b2

[0043] In the above chemical formula 2, 0.9≦a2≦1.2, 0.3≦x2≦0.98, 0.01≦y2≦0.69, 0.01≦z2≦0.69, 0≦w2≦0.69, 0.9≦x2+y2+z2+w2≦1.1, and 0≦b2≦0.1; M 3 is Al, Mn or a combination thereof, M 4 is one or more elements selected from B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr, and X is one or more elements selected from F, P, and S.

[0044] In the above formula 2, 0.4≦x2≦0.98, 0.01≦y2≦0.59, 0.01≦z2≦0.59, and 0≦w2≦0.59 may be satisfied, 0.5≦x2≦0.98, 0.01≦y2≦0.49, 0.01≦z2≦0.49, and 0≦w2≦0.49 may be satisfied, or 0.6≦x2≦0.98, 0.01≦y2≦0.39, 0.01≦z2≦0.39, and 0≦w2≦0.39 may be satisfied. , or 0.7≦x2≦0.98, 0.01≦y2≦0.29, 0.01≦z2≦0.29, and 0≦w2≦0.29, or 0.8≦x2≦0.98, 0.01≦y2≦0.19, 0.01≦z2≦0.19, and 0≦w2≦0.19, or 0.9≦x2≦0.98, 0.01≦y2≦0.09, 0.01≦z2≦0.09, and 0≦w2≦0.09.

[0045] In each of the lithium nickel-based composite oxide of the first positive electrode active material and the lithium nickel-based composite oxide of the second positive electrode active material, the nickel content relative to 100 mol% of metals excluding lithium may be, for example, 60 mol% or more, specifically 70 mol% or more, 80 mol% or more, or 90 mol% or more. The higher the nickel content, the higher the capacity can be achieved. However, the higher the nickel content, the more side reactions with the electrolyte may occur or the more cation mixing may increase, and the more problems may occur in which large particles are broken or deteriorated during the rolling process. However, according to the positive electrode design of one embodiment, even if the nickel content is increased, the problem of large particles being damaged in the upper layer during the rolling process is effectively suppressed, and the problem of accelerated deterioration of large particles during charging and discharging is prevented, thereby improving the life characteristics of the lithium secondary battery.

[0046] binder The binder according to an embodiment serves to adhere the positive electrode active material particles to each other and to adhere the positive electrode active material to the current collector. Representative examples of the binder include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, nylon, etc.

[0047] Conductive material The conductive material according to one embodiment is used to impart electrical conductivity to the electrode, and may be any material that does not cause a chemical change in the battery that is constructed and is electronically conductive. Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0048] The content of the positive electrode active material may be 90 wt% to 99.8 wt% or 95 wt% to 99 wt%, and the contents of the binder and the conductive material may be 0.1 wt% to 5 wt%, or 0.5 wt% to 2.5 wt%, respectively, relative to 100 wt% of the positive electrode active material layer.

[0049] Current collector The positive electrode current collector according to an embodiment is not particularly limited as long as it is conductive while not inducing a chemical change in the lithium secondary battery, and specific examples thereof include aluminum (Al), stainless steel (SUS), indium (In), magnesium (Mg), titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), germanium (Ge), lithium (Li), or a combination thereof, and one example thereof is aluminum (Al). In this case, the shape of the current collector may be a plate or foil.

[0050] Positive electrode manufacturing method In one embodiment, the present invention provides a method for manufacturing a positive electrode for a lithium secondary battery, the method including: preparing a first layer composition, in which the content of the second positive electrode active material is 5 to 30 wt % relative to 100 wt % of the total of the first positive electrode active material and the second positive electrode active material; preparing a second layer composition, in which the content of the second positive electrode active material is 31 to 60 wt % relative to 100 wt % of the total of the first positive electrode active material and the second positive electrode active material; loading the first layer composition and the second layer composition into a dual slot die coater, respectively, and then simultaneously coating the compositions such that the first layer is located on a current collector and the second layer is located on the first layer; and drying and then rolling.

[0051] The positive electrode can be manufactured through the above method. The first positive electrode active material and the second positive electrode active material are the same as those described above, so detailed description will be omitted.

[0052] The first layer composition includes a first positive electrode active material and a second positive electrode active material in a predetermined ratio, and may optionally include a binder and / or a conductive material. Similarly, the second layer composition includes a first positive electrode active material and a second positive electrode active material in a predetermined ratio, and may optionally include a binder and / or a conductive material.

[0053] The first layer composition is characterized by including 5 wt% to 30 wt% of the second positive electrode active material relative to 100 wt% of the first positive electrode active material and the second positive electrode active material, for example, 10 wt% to 30 wt%, 15 wt% to 30 wt%, or 20 wt% to 30 wt%. The second layer composition is characterized by including 31 wt% to 60 wt% of the second positive electrode active material relative to 100 wt% of the first positive electrode active material and the second positive electrode active material, for example, 32 wt% to 50 wt%, or 35 wt% to 45 wt%. When the ratio of small particles in the form of single particles in each of the first layer (lower layer) and the second layer (upper layer) is appropriately adjusted in this way, the problem of damage to large particles in the form of secondary particles in the upper layer contacting the rolling roll in the rolling step can be effectively improved.

[0054] The ratio of the thickness of the first layer to the thickness of the second layer can be designed as, for example, 50:50 to 90:10, specifically, 50:50 to 85:15, 50:50 to 80:20, 50:50 to 75:25, or 55:45 to 70:30. By appropriately adjusting the thicknesses of the first layer and the second layer, it is possible to realize high capacity and high energy density while simultaneously improving the life characteristics.

[0055] The rolling step may be performed so that the density of the final rolled positive electrode is in the range of 3.4 g / cc to 3.9 g / cc, for example, 3.5 g / cc to 3.8 g / cc, or 3.6 g / cc to 3.7 g / cc. By rolling at such a high strength, a battery with a high energy density can be realized. However, in this case, a problem of large particles in the form of secondary particles in the upper layer being broken may occur. However, according to an embodiment, by appropriately adjusting the composition of the first layer and the second layer, damage to the large particles in the upper layer can be effectively suppressed, thereby achieving a high energy density and improving the life characteristics at the same time.

[0056] Lithium secondary battery In one embodiment, there is provided a lithium secondary battery comprising the positive electrode; a negative electrode; and an electrolyte, wherein the electrolyte may be a liquid electrolyte or a solid electrolyte.

[0057] For example, in one embodiment, a lithium secondary battery may be provided that includes the above-described positive electrode, negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte solution. As another example, an all-solid-state secondary battery may be provided that includes the above-described positive electrode, negative electrode, and a solid electrolyte layer located between the positive electrode and the negative electrode.

[0058] As an example, a lithium secondary battery using an electrolyte will be described below.

[0059] Lithium secondary batteries can be classified into cylindrical, square, pouch, coin, and other types according to their shapes. FIGS. 1 to 4 are schematic diagrams showing a lithium secondary battery according to an embodiment, in which FIG. 1 shows a cylindrical battery, FIG. 2 shows a square battery, and FIGS. 3 and 4 show a pouch battery. Referring to FIGS. 1 to 4, the lithium secondary battery 100 may include an electrode assembly 40 having a separator 30 interposed between a positive electrode 10 and a negative electrode 20, and a case 50 in which the electrode assembly 40 is housed. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte (not shown). The lithium secondary battery 100 may include a sealing member 60 that seals the case 50 as shown in FIG. 1. Also, in FIG. 2, the lithium secondary battery 100 may include a positive electrode lead tab 11, a positive electrode terminal 12, a negative electrode lead tab 21, and a negative electrode terminal 22. As shown in FIGS. 3 and 4, the lithium secondary battery 100 may include electrode taps 70, i.e., a positive electrode tap 71 and a negative electrode tap 72, which serve as electrical paths for conducting the current generated in the electrode assembly 40 to the outside.

[0060] negative electrode The negative electrode for a lithium secondary battery includes a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector. The negative electrode active material layer includes a negative electrode active material, and may further include a binder and / or a conductive material.

[0061] The negative electrode active material includes a material capable of reversibly intercalating / deintercalating lithium ions, lithium metal, an alloy of lithium metal, a material capable of being doped and dedoped with lithium, or a transition metal oxide.

[0062] Examples of substances capable of reversibly intercalating / deintercalating lithium ions include carbon-based negative electrode active materials, which can include, for example, crystalline carbon, amorphous carbon, or combinations thereof. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flaky, spherical, or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, calcined coke, and the like.

[0063] As the alloy of the lithium metal, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn can be used.

[0064] As the substance capable of doping and undoping lithium, an Si-based negative electrode active material or an Sn-based negative electrode active material can be used. The Si-based negative electrode active material may be silicon, a silicon-carbon composite, SiOx (0 < x < 2), an Si-Q alloy (wherein Q is selected from alkali metals, alkaline earth metals, group 13 elements, group 14 elements (excluding Si), group 15 elements, group 16 elements, transition metals, rare earth elements, and combinations thereof), or a combination thereof. The Sn-based negative electrode active material may be Sn, SnO2, an Sn-based alloy, or a combination thereof.

[0065] The silicon-carbon composite may be a composite of silicon and amorphous carbon. According to one embodiment, the silicon-carbon composite may be in a form in which silicon particles are coated with amorphous carbon on the surface of the silicon particles. For example, it can include secondary particles (cores) formed by granulating primary silicon particles, and an amorphous carbon coating layer (shell) located on the surface of the secondary particles. The amorphous carbon may also be located between the primary silicon particles, and for example, the primary silicon particles may be coated with amorphous carbon. The secondary particles may be dispersed in an amorphous carbon matrix.

[0066] The silicon-carbon composite may further include crystalline carbon. For example, the silicon-carbon composite may include a core including crystalline carbon and silicon particles and an amorphous carbon coating layer located on the core.

[0067] The Si-based or Sn-based negative electrode active material may be used in combination with a carbon-based negative electrode active material.

[0068] For example, the negative electrode active material layer can contain 90% to 99% by weight of the negative electrode active material, 0.5% to 5% by weight of the binder, and 0% to 5% by weight of the conductive material.

[0069] The binder serves to adhere the negative active material particles to each other and to the current collector well. The binder may be a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof.

[0070] The non-aqueous binder may be polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

[0071] The aqueous binder may be selected from styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, poly(meth)acrylonitrile, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0072] When the negative electrode binder is an aqueous binder, it may further contain a cellulose-based compound capable of imparting viscosity. The cellulose-based compound may be a mixture of one or more of carboxymethylcellulose, hydroxypropylmethylcellulose, methylcellulose, or alkali metal salts thereof. The alkali metal may be Na, K, or Li.

[0073] The dry binder may be a polymeric material capable of being fiberized, such as polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

[0074] The conductive material is used to impart conductivity to the electrodes, and any material that does not cause a chemical change in the battery that is constructed and is electronically conductive can be used. Specific examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber, including copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0075] The negative electrode current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0076] electrolyte The electrolyte for the lithium secondary battery includes a non-aqueous organic solvent and a lithium salt.

[0077] The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0078] The non-aqueous organic solvent may be a carbonate, ester, ether, ketone, or alcohol solvent, an aprotic solvent, or a combination thereof.

[0079] Examples of the carbonate-based solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. Examples of the ester-based solvent include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanolide, mevalonolactone, valerolactone, caprolactone, etc. Examples of ether solvents that can be used include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and tetrahydrofuran. Examples of ketone solvents that can be used include cyclohexanone. Examples of alcohol solvents that can be used include ethyl alcohol and isopropyl alcohol. Examples of aprotic solvents that can be used include nitriles such as R-CN (R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, which may contain a double bond, an aromatic ring, or an ether group); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane and 1,4-dioxolane; and sulfolanes.

[0080] The non-aqueous organic solvents can be used alone or in combination of two or more kinds.

[0081] When a carbonate-based solvent is used, a cyclic carbonate and a chain carbonate can be mixed and used, and the cyclic carbonate and the chain carbonate can be mixed in a volume ratio of 1:1 to 1:9.

[0082] The lithium salt is a substance that is dissolved in an organic solvent and acts as a lithium ion source in a battery to enable basic lithium secondary battery operation and promote the movement of lithium ions between the positive electrode and the negative electrode. Representative examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide; LiFSI), LiC4F9SO3, LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2) (x and y are integers from 1 to 20), lithium trifluoromethanesulfonate, lithium tetrafluoroethanesulfonate, lithium difluorobis(oxalato)phosphate (LiDFOB), and lithium bis(oxalato)borate (LiBOB).

[0083] Separation membrane Depending on the type of lithium secondary battery, a separator may be present between the positive and negative electrodes. Such a separator may be made of polyethylene, polypropylene, polyvinylidene fluoride, or a multi-layered film of two or more of these materials, and of course, a mixed multi-layered film such as a polyethylene / polypropylene two-layered film, a polyethylene / polypropylene / polyethylene three-layered film, or a polypropylene / polyethylene / polypropylene three-layered film may be used.

[0084] The separator may include a porous substrate and a coating layer including an organic material, an inorganic material, or a combination thereof located on one or both sides of the porous substrate.

[0085] The porous substrate may be a polymer membrane formed of any one polymer selected from polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyether ketone, polyaryl ether ketone, polyetherimide, polyamide imide, polybenzimidazole, polyether sulfone, polyphenylene oxide, cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon (registered trademark), and polytetrafluoroethylene, or a copolymer or mixture of two or more of these.

[0086] The organic material may include a polyvinylidene fluoride-based polymer or a (meth)acrylic-based polymer.

[0087] The inorganic material may include, but is not limited to, inorganic particles selected from Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite, and combinations thereof.

[0088] The organic material and the inorganic material may be mixed in one coating layer, or may be in the form of a laminate of a coating layer containing an organic material and a coating layer containing an inorganic material.

[0089] Examples of the present invention and comparative examples are described below. The following examples are merely illustrative of the present invention, and the present invention is not limited to the following examples. EXAMPLES

[0090] Example 1 (1) Manufacturing of positive electrodes Li1Ni as positive electrode active material 0.916 Co 0.07 Al 0.014A positive electrode active material is prepared by mixing a first positive electrode active material having a secondary particle form and a particle size of about 17 μm with a second positive electrode active material having a single particle form and a particle size of about 3 μm in a weight ratio of 70:30 using O2. A first layer composition is prepared by mixing 97.7 wt% of the prepared positive electrode active material, 1.2 wt% of a binder PVDF, and 1.1 wt% of a conductive material CNT in an NMP solvent.

[0091] Li1Ni as positive electrode active material 0.916 Co 0.07 Al 0.014 A positive electrode active material is prepared by mixing a first positive electrode active material having a secondary particle form and a particle size of about 17 μm with a second positive electrode active material having a single particle form and a particle size of about 3 μm in a weight ratio of 60:40 using O2. A second layer composition is prepared by mixing 97.7 wt% of the prepared positive electrode active material, 1.2 wt% of a binder PVDF, and 1.1 wt% of a conductive material CNT in an NMP solvent.

[0092] The prepared first layer composition and second layer composition are loaded into a dual slot die coater, and then simultaneously coated so that the first layer is located on the aluminum current collector and the second layer is located on the first layer. After drying, the electrode is rolled to a plate density of 3.7 g / cc to produce a positive electrode in which the current collector, first layer, and second layer are laminated in that order. In the produced positive electrode, the thickness of the first layer is about 60% of the total, and the thickness of the second layer is about 40% of the total.

[0093] (2) Manufacturing of negative electrodes Anode active material slurry is prepared by mixing 97.3% by weight of graphite, 0.5% by weight of denka black, 0.9% by weight of carboxymethyl cellulose, and 1.3% by weight of styrene butadiene rubber in an aqueous solvent. The prepared anode active material slurry is applied to a copper foil, dried, and then rolled to prepare anode.

[0094] (3) Battery manufacturing The prepared positive electrode, a polyethylene / polypropylene multi-layered separator, and the prepared negative electrode are sequentially stacked to prepare a pouch-type cell, and then an electrolyte solution of 1.0M LiPF6 lithium salt added to a solvent of ethylene carbonate and diethyl carbonate mixed in a volume ratio of 50:50 is injected to prepare a lithium secondary battery.

[0095] Comparative Example 1 A positive electrode was prepared in which a first layer having a total thickness of about 100 μm was formed on a current collector using a general coater without using a second layer composition, and a lithium secondary battery was prepared in a manner substantially similar to that of Example 1.

[0096] Comparative Example 2 A lithium secondary battery was manufactured in a manner substantially similar to that of Example 1, except that a first layer composition including a positive electrode active material in which a first positive electrode active material and a second positive electrode active material were mixed in a weight ratio of 60:40 instead of 70:30 and a second layer composition including a positive electrode active material in which a first positive electrode active material and a second positive electrode active material were mixed in a weight ratio of 70:30 instead of 60:40 were used.

[0097] Comparative Example 3 A lithium secondary battery was manufactured in a manner substantially similar to that of Example 1, except that a first layer composition including a positive electrode active material in which the first positive electrode active material and the second positive electrode active material were mixed in a weight ratio of 60:40 instead of 70:30 was used.

[0098] Evaluation example 1: Analysis of deterioration of upper layer due to rolling A transmission X-ray microscopy (TXM) analysis was performed on the cross sections of the rolled positive electrodes manufactured in Example 1 and Comparative Examples 1 to 3. FIG. 5 is an image of the cross section of the positive electrode of Comparative Example 1, and Ni 3+6 shows a large area corresponding to Ni, which means that the reduction of nickel occurred and the positive electrode active material was broken or damaged. The left side of FIG. 6 is an enlarged image of the upper layer of the cross section of the positive electrode of Comparative Example 1, and the right side of FIG. 6 is an enlarged image of the upper layer of the cross section of the positive electrode of Example 1. Referring to FIG. 6, in Comparative Example 1, Ni 3+ On the other hand, in the case of Example 1, the large particles in the upper layer were Ni 3+ It can be seen that the area corresponding to the above decreased, and the phenomenon of breakage due to rolling was decelerated.

[0099] Evaluation example 2: Battery life characteristic evaluation The lithium secondary batteries manufactured in Example 1 and Comparative Examples 1 to 3 were charged at a constant current of 1.0 C rate at 45° C. until the voltage reached 4.25 V, and then cut off at a 0.05 C rate while maintaining 4.25 V in constant voltage mode. Then, the battery was discharged at a 1.0 C rate until the discharge voltage reached 2.8 V, and this cycle was repeated 1000 times. In all charge / discharge cycles, a 10-minute rest period was allowed to pass after each charge / discharge cycle, and the capacity change due to the cycle is shown in Table 1.

[0100] [Table 1]

[0101] Referring to Table 1, it can be seen that the life characteristics of the battery of Example 1 are significantly superior to those of Comparative Examples 1 to 3.

[0102] Although the preferred embodiments have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements made by those skilled in the art using the basic concepts defined in the following claims also fall within the scope of the present invention. [Explanation of symbols]

[0103] 100: Lithium secondary battery 10: Positive electrode 11: Positive lead tap 12: Positive terminal 20: Negative electrode 21: Negative lead tap 22: Negative terminal 30: Separation membrane 40: Electrode assembly 50: Case 60: Sealing material 70: Electrode tap 71: Positive tap 72: Negative tap

Claims

1. A positive electrode for a lithium secondary battery comprising a positive electrode current collector and a positive electrode active material layer located on the positive electrode current collector, the positive electrode active material layer includes a first positive electrode active material and a second positive electrode active material, The first positive electrode active material includes a lithium nickel-based composite oxide and is in the form of secondary particles composed of a plurality of primary particles, and the average particle diameter (D 50 ) is 9 μm to 25 μm, The second positive electrode active material includes a lithium nickel-based composite oxide and is in the form of a single particle. The average particle diameter (D 50 ) is 0.5 μm to 5 μm, A lower layer, which is a region of the positive electrode active material layer that is in a thickness direction from a surface in contact with a current collector, contains a second positive electrode active material in an amount of 5 wt % to 30 wt % based on a total of 100 wt % of the first positive electrode active material and the second positive electrode active material, a positive electrode for a lithium secondary battery, wherein an upper layer, which is a region of the positive electrode active material layer up to 10% in a thickness direction from a surface opposite to a surface in contact with a current collector, contains 31 wt % to 60 wt % of a second positive electrode active material relative to a total of 100 wt % of a first positive electrode active material and a second positive electrode active material.

2. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the lower layer contains 20 to 30 wt % of the second positive electrode active material, based on a total of 100 wt % of the first and second positive electrode active materials.

3. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the upper layer contains 35 to 45% by weight of the second positive electrode active material, based on 100% by weight of the first and second positive electrode active materials combined.

4. 2. The positive electrode of claim 1, wherein the content of the second positive electrode active material in the upper layer is 1% by weight to 55% by weight higher than that of the second positive electrode active material in the lower layer.

5. 2. The positive electrode of claim 1, wherein the content of the second positive electrode active material increases from the lower layer to the upper layer.

6. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the density of the positive electrode active material layer is 3.4 g / cc to 3.9 g / cc.

7. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein cracks are generated in 0 to 30% of the first positive electrode active material particles in the upper layer out of 100% by number of the first positive electrode active material particles.

8. Ni with respect to 100% of the total cross-sectional area of ​​the upper layer 3+ 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the area occupied by said metal oxide is 1 to 30% by area.

9. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode active material layer has a thickness of 40 μm to 300 μm.

10. The positive electrode for the lithium secondary battery is Positive electrode current collector; a first layer located on the positive electrode current collector, the first layer including 5 wt % to 30 wt % of the second positive electrode active material relative to 100 wt % of the first positive electrode active material and the second positive electrode active material; and a second layer disposed on the first layer, the second layer including 31 wt % to 60 wt % of the second positive electrode active material relative to 100 wt % of the first positive electrode active material and the second positive electrode active material; 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the lower layer is included in a first layer and the upper layer is included in a second layer.

11. The first and second layers are in contact with each other, or The positive electrode for a lithium secondary battery according to claim 10 , further comprising a third layer between the first layer and the second layer, the third layer having a mixture of the composition of the first layer and the composition of the second layer.

12. the thickness of the first layer and the thickness of the second layer are the same or different and are each independently between 40 μm and 270 μm; 11. The positive electrode for a lithium secondary battery according to claim 10, wherein a ratio of a thickness of the first layer to a thickness of the second layer is 50:50 to 90:

10.

13. The lithium-nickel-based composite oxide of the first positive electrode active material and the lithium-nickel-based composite oxide of the second positive electrode active material are the same or different from each other, and each is independently represented by Chemical Formula 1: [Chemical formula 1] Li a1 Ni x1 M 1 y1 M 2 z1 O 2-b1 X b1 In the above formula 1, 0.9≦a1≦1.2, 0.3≦x1<1, 0<y1≦0.7, 0≦z1≦0.7, 0.9≦x1+y1+z1≦1.1, and 0≦b1≦0.1; M 1 and M. 2 are each independently one or more elements selected from Al, B, Ba, Ca, Ce, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr, and X is one or more elements selected from F, P, and S.

14. The positive electrode for a lithium secondary battery according to claim 13, wherein the lithium nickel-based composite oxide of the first positive electrode active material and the lithium nickel-based composite oxide of the second positive electrode active material are each independently represented by Chemical Formula 2: [Chemical formula 2] Li a2 Ni x2 Co y2 M 3 z2 M 4 w2 O 2-b2 X b2 In the above chemical formula 2, 0.9≦a2≦1.2, 0.3≦x2≦0.98, 0.01≦y2≦0.69, 0.01≦z2≦0.69, 0≦w2≦0.69, 0.9≦x2+y2+z2+w2≦1.1, and 0≦b2≦0.1; M 3 is Al, Mn or a combination thereof; M 4 is one or more elements selected from B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr, and X is one or more elements selected from F, P, and S.

15. preparing a first layer composition, the first layer composition having a second positive electrode active material content of 5 to 30 wt % based on a total of 100 wt % of the first and second positive electrode active materials; preparing a second layer composition, the second positive electrode active material being present in an amount of 31 to 60 wt % based on a total of 100 wt % of the first positive electrode active material and the second positive electrode active material; loading the first layer composition and the second layer composition into a dual slot die coater, respectively, and then simultaneously coating the first layer on a current collector such that the first layer is positioned on the current collector and the second layer is positioned on the first layer; and A method for producing a positive electrode for a lithium secondary battery, comprising the steps of: drying and then rolling the resulting mixture; The first positive electrode active material contains a lithium nickel-based composite oxide and is in the form of secondary particles composed of a plurality of primary particles, and the average particle diameter (D 50 The second positive electrode active material contains a lithium nickel-based composite oxide and is in the form of a single particle, and the average particle diameter (D 50 ) is 0.5 μm to 5 μm.

16. The positive electrode according to any one of claims 1 to 14; A negative electrode; and an electrolyte.

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