Positive electrode for lithium secondary battery with improved structural stability, method for manufacturing the same, and lithium secondary battery comprising the same

Abstract

This invention relates to a positive electrode for lithium secondary batteries with improved structural stability, a method for preparing the same, and a lithium secondary battery including the same. When manufacturing a positive electrode containing positive electrode additives, the conditions of the first and second rolling processes are controlled, thus offering the advantage of improving the structure and electrical properties of the first and second additive layers.

Description

Technical Field

[0001] This invention relates to a positive electrode for lithium secondary batteries with improved structural stability, a method for preparing the same, and a lithium secondary battery comprising the same.

[0002] This application claims priority to Korean Patent Application No. 10-2021-0071223, filed on June 2, 2021, the entire contents of which are incorporated herein by reference. Background Technology

[0003] Recently, the demand for secondary batteries as an energy source has been growing rapidly. Among these secondary batteries, lithium secondary batteries, which have high energy density, high voltage, long cycle life, and low self-discharge rate, have been commercialized and are widely used.

[0004] Graphite is primarily used as the anode material in lithium-ion batteries. However, due to the relatively low capacity per unit mass of graphite (372 mAh / g), it is difficult to achieve high capacity in lithium-ion batteries. Therefore, to achieve high capacity in lithium-ion batteries, anode materials that form intermetallic compounds with lithium, such as silicon, tin, and their oxides, have been developed and used as non-carbon-based anode materials with higher energy densities than graphite. However, the problem with these non-carbon-based anode materials is that, despite their high capacity, they have low initial efficiency, resulting in significant lithium consumption during the initial charging and discharging phases and substantial irreversible capacity loss.

[0005] In this regard, a method has been proposed that provides a lithium-ion source or reservoir for the positive electrode material and uses a material that is electrochemically active after the first cycle to overcome the irreversible capacity loss of the negative electrode without degrading the overall performance of the battery. Specifically, methods are known to apply an oxide containing excess lithium (e.g., Li6CoO4) as a sacrificial positive electrode material or an irreversible additive (or over-discharge inhibitor) to the positive electrode.

[0006] Meanwhile, existing irreversible additives such as Li6CoO4 are typically prepared by reacting cobalt oxide with an excess of lithium oxide. The irreversible additives prepared as described above are structurally unstable and generate a large amount of oxygen (O2) during charging. When the secondary battery is first charged, i.e., when all the irreversible additives have not reacted and remain after battery activation, reactions may occur during subsequent charge / discharge processes, causing side reactions or a large amount of oxygen inside the battery. The generated oxygen causes the electrode components to expand in volume and becomes one of the main factors leading to a decline in battery performance.

[0007]

[0008] Furthermore, due to the two-dimensional (2D) permeation network, conventionally used irreversible additives exhibit approximately 10-11 The powder conductivity is extremely low, approaching that of an insulator. This low powder conductivity increases the resistance of the positive electrode. Under these conditions, a large capacity of over 200 mAh / g is exhibited at low C-rates, but as the C-rate increases, the performance rapidly decreases with charging and discharging due to the high resistance. Therefore, there is a limitation on the reduction of battery charge / discharge capacity, and high-speed charging and discharging are difficult.

[0009] Therefore, there is a need to develop lithium secondary batteries with excellent electrical performance and improved battery safety.

[0010] Existing technical documents

[0011] Patent documents

[0012] Japanese Patent Application Publication No. 2019-061751 Summary of the Invention

[0013] Technical issues

[0014] The purpose of this invention is to provide a positive electrode for lithium secondary batteries with improved electrical performance and improved structural stability, a method for preparing the same, and a lithium secondary battery comprising the same.

[0015] Technical solution

[0016] The present invention aims to provide a positive electrode for lithium secondary batteries, the positive electrode comprising a current collector and an additive layer formed on one or both surfaces of the current collector, wherein the additive layer is a bilayer structure formed by stacking a first additive layer and a second additive layer, the first additive layer comprising a positive electrode active material, a positive electrode additive represented by the following chemical formula 1, a conductive material and a binder, the porosity of the first additive layer being 15% to 40%, and the porosity of the second additive layer being 40% to 70%.

[0017] [Chemical Formula 1]

[0018] Li p Co (1-q) M 1 q O4

[0019] In chemical formula 1, M 1 It means that at least one element is selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B and Mo, and p and q are 5≤p≤7 and 0≤q≤0.5, respectively.

[0020] The average thickness of the first compound layer can be from 0.1 μm to 20 μm, and the average thickness of the second compound layer can be from 50 μm to 300 μm.

[0021] The ratio (D1:D2) of the average thickness (D1) of the first binder layer to the average thickness (D2) of the second binder layer can be from 4:6 to 1:10.

[0022] Based on the total weight of the first binder layer, the content of the positive electrode additive can be from 0.1% by weight to 5% by weight.

[0023] The positive electrode active material can be a lithium nickel composite oxide represented by the following Chemical Formula 2.

[0024] [Chemical Formula 2]

[0025] Li x [Ni y Co z Mn w M 2 v O u

[0026] In Chemical Formula 2, M 2 represents at least one element selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and x, y, z, w, v, and u are respectively 1.0 ≤ x ≤ 1.30, 0.1 ≤ y < 0.95, 0.01 < z ≤ 0.5, 0.01 < w ≤ 0.5, 0 ≤ v ≤ 0.2, and 1.5 ≤ u ≤ 4.5.

[0027] The present invention also aims to provide a method for manufacturing a positive electrode for a lithium secondary battery of the present invention, the method comprising: a first binder layer forming step of forming a first binder layer by applying a first slurry containing a positive electrode active material and a positive electrode additive represented by Chemical Formula 1 on one or both sides of a current collector; a first rolling step of rolling the formed first binder layer; a second binder layer forming step of forming a second binder layer by applying a second slurry containing a positive electrode active material on the rolled first binder layer, and a second rolling step of rolling the formed second binder layer, wherein the porosity of the first binder layer is from 15% to 40%, and the porosity of the second binder layer is from 40% to 70%.

[0028] [Chemical Formula 1]

[0029] Li p Co (1-q) M 1 q O4

[0030] In Chemical Formula 1, M 1It means that at least one element is selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B and Mo, and p and q are 5≤p≤7 and 0≤q≤0.5, respectively.

[0031] The first rolling process can be carried out at a speed of 0.5 m / s to 6 m / s, the second rolling process can be carried out at a speed of 2 m / s to 7 m / s, and the second rolling process can be carried out at a faster speed than the first rolling process.

[0032] The first rolling process can be carried out in a temperature range of 10℃ to 40℃, and the second rolling process can be carried out in a temperature range of 40℃ to 100℃.

[0033] The first compound layer can satisfy the thickness change rate condition of the following mathematical formula 1.

[0034] [Mathematical Expression 1]

[0035] D 2 min / D 1 max ×100≥70%

[0036] In mathematical formula 1, D 1 max D can represent the maximum thickness of the first compound layer after the first rolling process. 2 min This can represent the minimum thickness of the first compound layer after the second rolling process.

[0037] The present invention also aims to provide a lithium secondary battery comprising the above-described positive electrode, negative electrode, and separator inserted between the positive and negative electrodes.

[0038] The negative electrode may include a negative electrode current collector and a negative electrode additive layer located on the negative electrode current collector and containing a negative electrode active material, and the negative electrode active material may include carbon material and silicon material.

[0039] Beneficial effects

[0040] The advantages of the positive electrode for lithium secondary batteries, the manufacturing method thereof, and the lithium secondary battery including the present invention are that, when manufacturing the positive electrode containing the positive electrode additive, the conditions of the first and second rolling processes are controlled, thereby improving the structure and electrical properties of the first and second additive layers. Detailed Implementation

[0041] This invention can be modified in many forms and can have many implementation methods; therefore, specific implementation methods will be described in detail.

[0042] However, these embodiments should not be construed as limiting the invention to that particular embodiment, but should be understood as including modifications, equivalents or substitutions within the spirit and technical scope of the invention.

[0043] In this invention, the terms “comprising”, “having”, etc., are used to specify the presence of the features, quantities, steps, operations, components, elements or combinations thereof described herein, and they do not exclude the presence or addition of one or more other features, quantities, steps, operations, components, elements or combinations thereof.

[0044] Furthermore, in this invention, when a portion of a layer, film, region, plate, etc., is described as being "on" another portion, this includes not only the case where the portion is "directly" "on" the other portion, but also the case where another portion exists between the portion and the other portion. Conversely, when a portion of a layer, film, region, plate, etc., is described as being "below" another portion, this includes not only the case where the portion is "directly" "below" the other portion, but also the case where another portion exists between the portion and the other portion. Moreover, in this application, "deposited on" can include not only the case where it is disposed at the upper part, but also the case where it is disposed at the lower part.

[0045] The invention will be described in more detail below.

[0046] Positive electrode for lithium secondary batteries

[0047] In one embodiment of the present invention, a positive electrode for a lithium secondary battery is provided, comprising a current collector and an additive layer formed on one or both surfaces of the current collector, wherein the additive layer is a bilayer structure formed by stacking a first additive layer and a second additive layer, the first additive layer comprising a positive electrode active material, a positive electrode additive represented by the following chemical formula 1, a conductive material and a binder, the porosity of the first additive layer being 15% to 40%, and the porosity of the second additive layer being 40% to 70%.

[0048] [Chemical Formula 1]

[0049] Li p Co (1-q) M 1 q O4

[0050] In chemical formula 1, M 1 It means that at least one element is selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B and Mo, and p and q are 5≤p≤7 and 0≤q≤0.5, respectively.

[0051] Specifically, in chemical formula 1, M 1 It can represent the Zn element, and q can be 0.2≤q≤0.4.

[0052] The positive electrode for lithium secondary batteries of the present invention has a double-layer structure, wherein a first agent layer and a second agent layer are sequentially stacked on a current collector, and the second agent layer is located at the outermost portion. In particular, the positive electrode for lithium secondary batteries of the present invention can improve the structure and electrical properties of the first and second agent layers by controlling the conditions of the first and second rolling processes during the manufacture of the positive electrode.

[0053] The positive electrode additive can contain excess lithium (Li) and can provide Li for the lithium consumption caused by irreversible chemical and physical reactions at the negative electrode during the initial charging period. Therefore, the charging capacity of the battery increases, the irreversible capacity decreases, and the life characteristics can be improved.

[0054] According to the present invention, lithium cobalt oxide represented by Chemical Formula 1 can be used as a positive electrode additive. In this case, Li6CoO4 or Li6Co can be used alone as the lithium cobalt oxide represented by Chemical Formula 1. 0.5 Zn 0.5 O4 and Li6Co 0.7 Zn 0.3 O4, or combinations thereof. The advantage of the lithium cobalt oxide represented by Formula 1, compared to nickel-containing oxides commonly used in the art, is that, due to its high lithium-ion content and low voltage range required for delithiation, lithium ions can be desorbed without affecting the reaction of the positive electrode active material when the battery is activated.

[0055] Furthermore, the lithium cobalt metal oxide represented by Formula 1 can have a tetragonal crystal structure and possess the space group P42 / nmc within that structure. While the lithium cobalt metal oxide represented by Formula 1 exhibits excellent charge / discharge and lifetime characteristics in lithium-ion batteries, it suffers from low thermal stability. Specifically, when the lithium cobalt metal oxide represented by Formula 1 is used as a cathode additive, structural deformation can occur due to the temperature (e.g., slurry temperature) during cathode additive layer preparation, potentially leading to a decrease in battery performance.

[0056] Therefore, when manufacturing a positive electrode containing positive electrode additives, the positive electrode for lithium secondary batteries of the present invention prevents structural deformation of the positive electrode additives and improves the structure and electrical properties of the first and second additive layers by controlling the conditions of the first and second rolling processes.

[0057] As an example, in the X-ray diffraction (XRD) analysis of the first compound layer, the positive electrode for lithium secondary batteries of the present invention can satisfy the following equation 1.

[0058] [Equation 1]

[0059] A / B≤1

[0060] A represents the intensity of the strongest peak displayed in the range of 2θ = 38.5 ± 0.1°, and B represents the intensity of the strongest peak displayed in the range of 2θ = 47.9 ± 0.1°.

[0061] In Equation 1, peak B, which appears in the range of 2θ = 47.9 ± 0.1°, is realized by the crystal of lithium cobalt metal oxide represented by Formula 1, and peak A, which appears in the range of 2θ = 38.5 ± 0.1°, is realized by impurities. This means that the proportion of lithium cobalt metal oxide represented by Formula 1 increases as the peak intensity ratio, i.e., "A / B", decreases. In the cathode of the present invention, the proportion of lithium cobalt metal oxide represented by Formula 1 in the cathode additive contained in the first additive layer is increased to more than 98%, such that A / B can be satisfied to be less than 1, less than 0.35, less than 0.25, or less than 0.1. In some cases, since the proportion of lithium cobalt metal oxide represented by Formula 1 in the cathode additive is 100%, A / B can be satisfied to be zero.

[0062] Furthermore, the porosity of the first mixture layer can range from 15% to 40%, specifically, from 20% to 40%, 20% to 35%, 25% to 40%, 25% to 35%, or 25% to 30%.

[0063] When the porosity of the first compound layer is too small, the electrolyte may have difficulty penetrating to the surface of the current collector. Conversely, when the porosity is too large, the electrode density decreases, thus reducing the amount of active material in contact with the current collector, potentially leading to a decrease in capacity and output. Therefore, the porosity of the first compound layer can be within the aforementioned range.

[0064] The measurement of porosity is not particularly limited, and in one embodiment of the invention, porosity can be measured by, for example, the Brunauer-Emmett-Teller (BET) measurement method or a mercury (Hg) porosimeter.

[0065] Furthermore, based on 100 parts by weight of the first compound layer, the content of the positive electrode additive can range from 0.1 parts by weight to 5 parts by weight, specifically from 0.1 parts by weight to 3 parts by weight, or from 1 part by weight to 3 parts by weight.

[0066] The first composite layer is a positive electrode active material capable of reversible insertion and extraction, and may contain a lithium-nickel composite oxide represented by the following chemical formula 2.

[0067] [Chemical Formula 2]

[0068] Li x [Ni y Coz Mn w M 2 v O u

[0069] In Chemical Formula 2, M 2 represents at least one element selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and x, y, z, w, v, and u are 1.0 ≤ x ≤ 1.30, 0.1 ≤ y < 0.95, 0.01 < z ≤ 0.5, 0.01 < w ≤ 0.5, 0 ≤ v ≤ 0.2, and 1.5 ≤ u ≤ 4.5, respectively.

[0070] The lithium nickel composite oxide represented by Chemical Formula 2 is a composite metal oxide containing Li, nickel, cobalt, and manganese, and in some cases, may have the form of doping with another transition metal (M 2 ). For example, the positive electrode active material may include one or more compounds selected from the group consisting of LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.8 Co 0.1 Mn 0.05 Al 0.05 O2, and LiNi 0.7 Co 0.1 Mn 0.1 Al 0.1 O2. As an example, in the positive electrode active material, LiNi 0.6 Co 0.2 Mn 0.2 O2 and LiNi 0.8 Co 0.1 Mn 0.1 O2 can be used alone or in combination as the lithium nickel composite metal oxide represented by Chemical Formula 2.

[0071] Furthermore, based on 100 parts by weight of the first compound layer, the content of the positive electrode active material can range from 85 parts by weight to 95 parts by weight, specifically, from 88 parts by weight to 95 parts by weight, 90 parts by weight to 95 parts by weight, 86 parts by weight to 90 parts by weight, or 92 parts by weight to 95 parts by weight.

[0072] In this case, conductive materials can be used to improve the properties of the positive electrode, such as its conductivity, and one or more materials selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, and carbon fiber can be used as the conductive material. For example, the conductive material may include acetylene black.

[0073] Furthermore, based on 100 parts by weight of the first compound layer, the content of conductive material can be from 1 part by weight to 10 parts by weight, specifically from 2 parts by weight to 8 parts by weight, or from 2 parts by weight to 6 parts by weight.

[0074] In addition, the adhesive may contain one or more resins selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, and copolymers thereof. For example, the adhesive may contain polyvinylidene fluoride.

[0075] Furthermore, based on 100 parts by weight of the first compound layer, the content of the adhesive can be from 1 part by weight to 10 parts by weight, specifically from 2 parts by weight to 8 parts by weight, or from 2 parts by weight to 6 parts by weight.

[0076] In addition, there is no particular limitation on the average thickness of the first compound layer, but specifically, it can range from 0.1 μm to 20 μm, more specifically, from 0.1 μm to 15 μm, 0.1 μm to 10 μm, 2 μm to 10 μm, 4 μm to 10 μm, or 5 μm to 9 μm.

[0077] In addition, the second compound layer is formed on the first compound layer formed on the current collector, and contains the positive electrode active material. In addition to the positive electrode active material, it may further contain conductive material and adhesive as needed.

[0078] Furthermore, the porosity of the second additive layer can range from 40% to 70%, specifically 40% to 65%, 40% to 60%, 45% to 60%, 45% to 55%, or 45% to 50%. In particular, the second additive layer located on the outermost side of the positive electrode can increase the impregnation performance of the electrolyte in the electrode by increasing its porosity. Therefore, the energy density increases, allowing for improvements in the capacity, output characteristics, and charge / discharge rate of the secondary battery including the aforementioned positive electrode.

[0079] On the other hand, the first and second compound layers, as described above, can be applied and rolled separately to have a porosity difference and form an interface without mixing with each other.

[0080] The positive electrode active material included in the second composite layer can be applied without particular restrictions, as long as it is a lithium metal composite oxide represented by chemical formula 2, and the positive electrode active material can contain materials selected from LiNi. 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, LiNi 0.9 Co 0.05 Mn 0.05 O2, LiNi 0.8 Co 0.1 Mn 0.05 Al 0.05 O2 and LiNi 0.7 Co 0.1 Mn 0.1 Al 0.1 One or more compounds in the group consisting of O2.

[0081] As an example, the positive electrode active material can be a lithium-nickel composite metal oxide represented by chemical formula 2, and LiNi 0.6 Co 0.2 Mn 0.2 O2 and LiNi 0.8 Co 0.1 Mn 0.1 O2 can be used alone or in combination as a positive electrode active material.

[0082] Furthermore, based on 100 parts by weight of the second compound layer, the content of the positive electrode active material can be from 80 parts by weight to 98 parts by weight, specifically from 84 parts by weight to 96 parts by weight, or from 88 parts by weight to 96 parts by weight.

[0083] In this case, a conductive material contained in the second composite layer can be used to improve the properties of the positive electrode, such as its conductivity. One or more materials selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, and carbon fiber can be used as the conductive material. For example, the conductive material may contain acetylene black.

[0084] Furthermore, based on 100 parts by weight of the second compound layer, the content of conductive material can be from 1 part by weight to 10 parts by weight, specifically from 2 parts by weight to 8 parts by weight, or from 2 parts by weight to 6 parts by weight.

[0085] Furthermore, the adhesive included in the second adhesive layer may comprise one or more resins selected from the group consisting of PVDF-co-HFP, polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, and copolymers thereof. For example, the adhesive may comprise polyvinylidene fluoride.

[0086] Furthermore, based on 100 parts by weight of the second compound layer, the content of the adhesive can be from 1 part by weight to 10 parts by weight, specifically from 2 parts by weight to 8 parts by weight, or from 2 parts by weight to 6 parts by weight.

[0087] In addition, the average thickness of the second compound layer can range from 50 μm to 300 μm, and in particular, from 100 μm to 200 μm, 80 μm to 150 μm, 120 μm to 170 μm, 150 μm to 300 μm, 200 μm to 300 μm, or 150 μm to 190 μm.

[0088] In certain instances, the thickness of the second compound layer may be greater than the thickness of the first compound layer.

[0089] Furthermore, the ratio (D1:D2) of the average thickness of the first additive layer to the average thickness (D2) of the second additive layer can be from 4:6 to 1:10. Specifically, the average thickness ratio (D1:D2) between the first and second positive electrode additive layers can be from 3:7 to 1:10, or from 2:8 to 1:9. In this invention, the second negative electrode additive layer is formed to be relatively thick, thereby increasing the bonding force with the current collector and increasing the stability of the electrode.

[0090] Meanwhile, in the positive electrode for lithium secondary batteries of the present invention, a material with high conductivity that will not cause chemical changes in the battery can be used as the current collector. For example, stainless steel, aluminum, nickel, titanium, and calcined carbon can be used, and aluminum or stainless steel with a surface treated with carbon, nickel, titanium, and silver can be used as the current collector. Furthermore, fine irregularities can be formed on the surface of the current collector to enhance the bonding force of the positive electrode active material, and various forms of positive electrode current collectors can be used, such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics. Moreover, considering the conductivity and total thickness of the positive electrode to be manufactured, the average thickness of the current collector can be appropriately applied in the range of 3 μm to 500 μm.

[0091] Method for manufacturing positive electrodes for lithium secondary batteries

[0092] An embodiment of the present invention provides a method for manufacturing a positive electrode for a lithium secondary battery, comprising: a first additive layer forming step, wherein a first additive layer is formed by applying a first slurry comprising a positive electrode active material and a positive electrode additive represented by Chemical Formula 1 to one or both sides of a current collector; a first rolling step, wherein the formed first additive layer is rolled; a second additive layer forming step, wherein a second additive layer is formed by applying a second slurry comprising a positive electrode active material onto the rolled first additive layer; and a second rolling step, wherein the formed second additive layer is rolled, wherein the porosity of the first additive layer is 15% to 40%, and the porosity of the second additive layer is 40% to 70%.

[0093] [Chemical Formula 1]

[0094] Li p Co (1-q) M 1 q O4

[0095] In chemical formula 1, M 1 It means that at least one element is selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B and Mo, and p and q are 5≤p≤7 and 0≤q≤0.5, respectively.

[0096] The method for manufacturing a positive electrode for a lithium secondary battery according to the present invention can improve the excellent charge / discharge characteristics and lifespan characteristics of the lithium secondary battery by adding a positive electrode additive to a first compound layer in contact with the current collector. Furthermore, by sequentially forming a second compound layer on the first compound layer, such that the second compound layer is located in the outermost portion, while controlling the conditions of the first and second rolling processes, it has the advantage of being able to improve the structure and electrical properties of the first and second compound layers.

[0097] Furthermore, in the method for manufacturing a positive electrode for a lithium secondary battery according to the present invention, the first additive layer is rolled using a roller press before the second additive layer is formed, and two rolling processes are performed to perform a second rolling after the second additive layer is formed, thereby improving the thickness uniformity of the first additive layer.

[0098] As an example, when the cross-section of the positive electrode for a lithium secondary battery manufactured by the above-described method for manufacturing a positive electrode for a lithium secondary battery is analyzed in the thickness direction using a scanning electron microscope (SEM), the following mathematical formula 1 can be satisfied.

[0099] [Mathematical Expression 1]

[0100] D 2 min / D 1 max×100≥70%

[0101] In mathematical formula 1, D 1 max D represents the maximum thickness of the first compound layer after the first rolling process. 2 min This indicates the minimum thickness of the first compound layer after the second rolling process.

[0102] Formula 1 represents the thickness change rate of the first compound layer during the two rolling processes, and implies that, when the first and second rolling processes are performed and the cross-section of the positive electrode is analyzed, the minimum thickness D of the first compound layer after the second rolling process is... 2 min The maximum thickness D of the first compound layer after the first rolling process 1 max The higher the ratio, the lower the thickness deviation of the first compound layer. Specifically, due to the very high uniformity of the first compound layer, the minimum thickness D... 2 min and maximum thickness D 1 max The deviation between them is very small, and the condition of 1 can be satisfied more than 75% when writing (e.g., D). 2 min / D 1 max ×100≥75%), 80% or more (e.g., D) 2 min / D 1 max ×100≥80%), 85% or more (e.g., D) 2 min / D 1 max ×100≥85%), 90% or more (e.g., D) 2 min / D 1 max ×100≥90%), or a range of 80% to 95% (e.g., 80%≤D). 2 min / D 1 max ×100≤95%.

[0103] Firstly, in this invention, the first additive layer formation process is performed within a temperature range of 10°C to 40°C. Specifically, the first additive layer formation process means that when the first slurry is applied to the current collector, the temperature of the first slurry or the temperature of the reactor storing the first slurry is controlled within a temperature range of 10°C to 40°C. More specifically, in the first additive layer formation process, the temperature of the reactor storing the first slurry can be controlled within the ranges of 10°C to 30°C, 10°C to 20°C, 15°C to 25°C, 22°C to 28°C, 20°C to 30°C, or 18°C ​​to 25°C. When the temperature of the slurry is controlled below 10°C in the first additive layer formation process, the slurry may not be uniformly applied to the current collector, and when the temperature is controlled above 40°C, the positive electrode additive contained in the first slurry may deteriorate. Therefore, in the first additive layer formation process, the temperature of the first slurry can be controlled within the above-mentioned ranges.

[0104] Furthermore, in the first rolling process, the thickness of the first additive layer is maintained uniformly, and the rolling density is increased, so that when the second additive layer is rolled, the influence of the positive electrode active material on the interface uniformity can be reduced. In addition, the rolling temperature is controlled in the first rolling process, thus minimizing the degradation of the positive electrode additive contained in the first additive layer. For this purpose, the first rolling process can be performed at a low temperature and high speed. Specifically, similar to the control of the slurry temperature when forming the aforementioned additive layer, the first rolling process can be performed within a temperature range of 10°C to 40°C, 10°C to 30°C, 10°C to 20°C, 15°C to 25°C, 22°C to 28°C, 20°C to 30°C, or 18°C ​​to 25°C. Furthermore, the rolling speed for the first rolling process can range from 0.5 m / s to 6 m / s, specifically from 0.5 m / s to 5.5 m / s, 0.5 m / s to 5 m / s, 0.5 m / s to 4.5 m / s, 0.5 m / s to 4 m / s, 0.5 m / s to 3.5 m / s, 0.7 m / s to 3 m / s, 0.7 m / s to 2.5 m / s, 0.7 m / s to 2 m / s, 0.9 m / s to 2 m / s, 1 m / s to 1.5 m / s, 1 m / s to 1.2 m / s, 2 m / s to 5 m / s, 2 m / s to 5.5 m / s, or 3 m / s to 5 m / s.

[0105] Next, similar to the first additive layer formation step, the second additive layer formation step can be carried out within a temperature range of 10°C to 40°C. As described above for the first additive layer formation step, the second additive layer formation step means that when the second slurry is applied to the current collector, the temperature of the second slurry or the temperature of the reactor storing the second slurry is controlled within the range of 10°C to 40°C. Since the second slurry does not contain positive electrode additives, the second additive layer formation step can be carried out at a temperature higher than the first additive layer formation temperature. However, when the temperature of the second slurry is high, the positive electrode additives contained in the first additive layer will be directly affected; therefore, the temperature of the reactor storing the second slurry can be controlled within the range of 10°C to 40°C.

[0106] Furthermore, compared to the first rolling process, the second rolling process can be performed at relatively higher temperatures and higher speeds. Specifically, the temperature range for the second rolling process can be 50°C to 100°C, more specifically, 60°C to 100°C, 75°C to 100°C, 85°C to 100°C, 50°C to 90°C, 60°C to 80°C, or 65°C to 90°C. Specifically, the rolling speed range for the second rolling process can be from 2 m / s to 7 m / s, more specifically, from 2 m / s to 6.5 m / s, 2 m / s to 6 m / s, 2 m / s to 5.5 m / s, 2 m / s to 5 m / s, 2 m / s to 4.5 m / s, 2 m / s to 4 m / s, 2.5 m / s to 4 m / s, 2.5 m / s to 3.5 m / s, 3.5 m / s to 5 m / s, 5 m / s to 7 m / s, 5.5 m / s to 6.5 m / s, or 6 m / s to 7 m / s.

[0107] Furthermore, the pressure conditions for performing the first and second rolling processes can each range from 50 MPa to 200 MPa, specifically 50 MPa to 150 MPa, 50 MPa to 100 MPa, 100 MPa to 200 MPa, 150 MPa to 200 MPa, or 80 MPa to 140 MPa. In this case, to optimize the rolling density of the first compound layer, the first rolling process can be performed under the same pressure conditions as the second rolling process, or under pressure conditions less than twice that of the second rolling process, for example, less than 1.5 times, less than 1.2 times, or 1.1 to 1.4 times that of the second rolling process. Thus, the ratio (D1:D2) of the average thickness (D1) of the prepared first compound layer to the average thickness (D2) of the prepared second compound layer can be from 4:6 to 1:10. Specifically, the average thickness ratio (D1:D2) between the first and second positive electrode binder layers can be 3:7 to 1:10, or 2:8 to 1:9. In this invention, the second positive electrode binder layer is formed to be relatively thick, thereby increasing the bonding force with the current collector and increasing the stability of the electrode.

[0108] According to the present invention, under the above-mentioned temperature conditions and the above-mentioned rolling conditions, a first additive layer and a second additive layer are sequentially formed on the current collector, thereby enabling the manufacture of a positive electrode containing positive electrode additives and with minimized degradation.

[0109] Lithium secondary batteries

[0110] In one embodiment of the present invention, a lithium secondary battery is provided, comprising the above-described positive electrode, negative electrode, and separator inserted between the positive electrode and the negative electrode.

[0111] The lithium secondary battery of the present invention may include the above-described positive electrode of the present invention, thereby reducing the amount of oxygen generated during charging and discharging and exhibiting excellent charging and discharging performance.

[0112] The lithium secondary battery of the present invention has a structure including a positive electrode, a negative electrode and a separator inserted between the positive electrode and the negative electrode.

[0113] Here, the negative electrode is manufactured by applying, drying, and pressing the negative electrode active material onto the negative electrode current collector, and, as needed, may be further selectively included with conductive materials, organic binder polymers, and additives, as in the positive electrode.

[0114] Furthermore, the negative electrode active material may include, for example, carbon materials and silicon materials. Carbon materials refer to carbon materials containing carbon atoms as their main component. Carbon materials may include one or more selected from the group consisting of: graphite with a fully layered crystal structure, such as natural graphite; soft carbon with a low-crystallinity layered crystal structure (graphene structure is a structure in which hexagonal honeycomb planes of carbon are layered); hard carbon formed by mixing these structures with amorphous portions; artificial graphite; expanded graphite; carbon fibers; non-graphitized carbon; carbon black; acetylene black; Ketjen black; carbon nanotubes; fullerenes; activated carbon; graphene; and carbon nanotubes. More preferably, the carbon material may include natural graphite and / or artificial graphite, and may include any one or more of graphene and carbon nanotubes, as well as natural graphite and / or artificial graphite. In this case, based on 100 parts by weight of all carbon material, the carbon material may contain 0.1 parts by weight to 10 parts by weight of graphene and / or carbon nanotubes. More specifically, based on 100 parts by weight of all carbon material, the carbon material may contain 0.1 parts by weight to 5 parts by weight, or 0.1 parts by weight to 2 parts by weight of graphene and / or carbon nanotubes.

[0115] Furthermore, silicon materials are particles with silicon (Si) as the main component, containing metallic elements, and may also contain Si particles and silicon oxide (SiO). XAnd 1 ≤ X ≤ 2) particles. For example, silicon materials may comprise silicon particles, silicon monoxide (SiO) particles, silicon dioxide (SiO2) particles, or mixtures thereof.

[0116] Furthermore, the silicon material can be in a mixed form of crystalline and amorphous particles, and based on 100 parts by weight of total silicon material, the proportion of amorphous particles is 50 to 100 parts by weight, specifically 50 to 90 parts by weight, 60 to 80 parts by weight, or 85 to 100 parts by weight. In this invention, by controlling the proportion of amorphous particles contained in the silicon material within the above range, thermal stability and flexibility can be improved without reducing the electrical performance of the electrode.

[0117] In addition, the negative electrode active material includes carbon and silicon materials, and is based on 100 parts by weight of negative electrode compound layer, the content of which can be from 1 part by weight to 20 parts by weight, specifically, from 5 parts by weight to 20 parts by weight, from 3 parts by weight to 10 parts by weight, from 8 parts by weight to 15 parts by weight, from 13 parts by weight to 18 parts by weight, or from 2 parts by weight to 7 parts by weight.

[0118] In this invention, the content of carbon and silicon materials contained in the negative electrode active material is adjusted to the above-mentioned range, so as to increase the charging capacity per unit mass, while reducing Li consumption and irreversible capacity loss during the initial charging and discharging of the battery.

[0119] As an example, based on 100 parts by weight of the negative electrode mixture layer, the negative electrode active material may contain 95±2 parts by weight of graphite and 5±2 parts by weight of a mixture obtained by uniformly mixing SiO particles and SiO2 particles. In this invention, the content of carbon and silicon materials contained in the negative electrode active material is adjusted to the above range, thereby increasing the charging capacity per unit mass while reducing Li consumption and irreversible capacity loss during the initial charging and discharging of the battery.

[0120] In addition, the average thickness of the negative electrode mixture layer can be from 100 μm to 200 μm, specifically from 100 μm to 180 μm, 100 μm to 150 μm, 120 μm to 200 μm, 140 μm to 200 μm, or 140 μm to 160 μm.

[0121] Furthermore, there are no particular limitations on the negative electrode current collector, as long as it has high conductivity without causing chemical changes in the battery. For example, copper, stainless steel, nickel, titanium, and calcined carbon can be used as negative electrode current collectors, and copper or stainless steel with a surface treated with carbon, nickel, titanium, or silver can also be used. In addition, similar to the positive electrode current collector, minute irregularities can be formed on the surface of the negative electrode current collector to improve the adhesion to the negative electrode active material, and various forms of negative electrode current collectors can be used, such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics. Moreover, considering the conductivity and total thickness of the negative electrode to be manufactured, the average thickness of the negative electrode current collector can be appropriately applied in the range of 3 μm to 500 μm.

[0122] Furthermore, a separator is inserted between the positive and negative electrodes, and an insulating film with high ion permeability and high mechanical strength is used as the separator. There are no particular limitations on the separator, as long as it is commonly used in the art; specifically, chemically resistant and hydrophobic polypropylene, glass fiber, or sheets or nonwoven fabrics made of polyethylene can be used as separators. In some cases, composite separators can be used, in which a porous polymer substrate such as a sheet or nonwoven fabric is coated with inorganic / organic particles by an organic adhesive polymer. When a solid electrolyte, such as a polymer, is used as the electrolyte, this solid electrolyte can also be used as the separator. Furthermore, the separator can have an average pore size of 0.01 μm to 10 μm and an average thickness of 5 μm to 300 μm.

[0123] Meanwhile, the positive and negative electrodes can be wound into gel rolls and housed in cylindrical, prismatic, or pouch-shaped batteries, or they can be housed in pouch-shaped batteries in a folded or stacked-folded manner, but the present invention is not limited thereto.

[0124] Furthermore, the lithium salt-containing electrolyte of the present invention can be composed of an electrolyte and a lithium salt, and can use a non-aqueous organic solvent, an organic solid electrolyte, or an inorganic solid electrolyte as the electrolyte.

[0125] For example, as non-aqueous organic solvents, the following aprotic organic solvents can be used: N-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, butyl carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, or ethyl propionate.

[0126] For example, as organic solid electrolytes, polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate polymers, polylyzed lysine, polyester sulfides, polyvinyl alcohol (PVA), polyvinylidene fluoride, or polymers containing ion-dissociating groups can be used.

[0127] Lithium nitrides, halides, and sulfates, such as Li3N, LiI, Li5NI2, Li3N-LiI-LiOH, LiSiO4, LiSiO4-LiI-LiOH, Li2SiS3, Li4SiO4, Li4SiO4-LiI-LiOH, and Li3PO4-Li2S-SiS2, can be used as inorganic solid electrolytes.

[0128] Lithium salts are materials that are readily soluble in non-aqueous electrolytes. For example, LiCl, LiBr, LiI, LiClO4, LiBF4, and LiB can be used. 10 Cl 10 LiPF6, LiCF3SO3, LiCF3CO2, LiAsF6, LiSbF6, LiAlCl4, CH3SO3Li, (CF3SO2)2NLi, lithium chloroborane, lithium lower aliphatic carboxylic acids, lithium tetraphenylborate, or imides are used as lithium salts.

[0129] Furthermore, to improve charge / discharge characteristics and flame retardancy, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, triammonium hexaphosphate, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolides, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride can be added to the electrolyte. In some cases, to impart non-flammability, halogenated solvents such as carbon tetrachloride or trifluoroethylene can be further included, and to improve high-temperature storage characteristics, carbon dioxide gas can be further included, and fluoroethylene carbonate (FEC) or propylene sulpholone (PRS) can be further included.

[0130] Meanwhile, according to one embodiment of the present invention, a battery module including the above-mentioned secondary battery as a unit cell is provided, and a battery pack including the battery module is provided.

[0131] The battery pack can be used as a power source for medium to large-sized equipment requiring high-temperature stability, long cycle life, and high-speed characteristics. Specific examples of medium to large-sized equipment include electric tools driven by electric motors, electric vehicles (EVs) including EVs, hybrid EVs (HEVs), and plug-in HEVs (PHEVs), electric two-wheeled vehicles including e-bikes and e-scooters, electric golf carts, electric trucks, and systems for energy storage. More specifically, examples include HEVs, but the invention is not limited thereto.

[0132] Example

[0133] The invention will be described in more detail below with reference to embodiments and experimental examples.

[0134] However, the following embodiments and experimental examples are merely illustrative of the present invention, and the content of the present invention is not limited to the following embodiments and experimental examples.

[0135] Examples 1 to 4 and Comparative Examples 1 to 7: Manufacturing of positive electrodes for lithium secondary batteries

[0136] Weigh 95 parts by weight of LiNi as the positive electrode active material 0.8 Co 0.1 Mn 0.1 O2, 0.9 parts by weight of Li6CoO4 as a positive electrode additive, 1.6 parts by weight of PVdF as a binder, and 2.5 parts by weight of carbon black as a conductive material are mixed with N-methylpyrrolidone (NMP) solvent to prepare a first slurry for the first binder layer.

[0137] In addition, 95 parts by weight of LiNi were weighed as the positive electrode active material. 0.8 Co 0.1 Mn 0.1 O2, 2 parts by weight of PVdF as a binder and 3 parts by weight of carbon black as a conductive material are mixed with NMP solvent to prepare a second slurry for the second binder layer.

[0138] A first slurry is applied to aluminum foil and dried, followed by a first rolling process to form a first composite layer (average thickness: 8 μm). Subsequently, a second slurry is applied to the first composite layer and dried, followed by a second rolling process to form a second composite layer (average thickness: 100 μm). In this case, the maximum thickness D of the first composite layer after the first rolling process is measured by analyzing the cross-section of the first composite layer immediately after the first and second rolling processes, respectively. 1 max Minimum thickness D of the first compound layer after the second rolling process 2 min And the ratio of the measured values ​​(D)2 min / D 1 max The conditions for performing the first and second rolling processes are shown in Table 1 below. Furthermore, the conditions for performing the first and second compound layer forming processes and the first and second rolling processes are shown in Table 1 below.

[0139] [Table 1]

[0140]

[0141] Experimental Example 1:

[0142] 1) Observe the XRD pattern of the first mixture layer.

[0143] To confirm the temperature-dependent changes in cathode additives during the manufacture of cathodes for lithium-ion batteries, X-ray diffraction measurements were performed on the first additive layers of the cathodes for lithium-ion batteries manufactured in Examples 1 to 4 and Comparative Examples 1 to 4, and the proportion of each cathode additive was calculated from the measured X-ray diffraction. In this case, X-ray diffraction was measured using an X-ray diffraction analyzer from Rigaku Corporation, and the X-ray diffraction was scanned. X-ray diffraction patterns in the range of 15° to 64° were obtained at wavelengths of Cu Ka radiation, 40 kV and 100 mA, and at an angle of 2θ and a scan rate of 5° / sec.

[0144] As a result, peaks were observed in the Li6CoO4 of the first compound layer in Examples 1 to 4. Specifically, peak A was observed in the range of 2θ = 38.5 ± 0.1°, and peak B was observed in the range of 2θ = 47.9 ± 0.1°. For reference, the A / B ratio of Examples 1 to 4 is less than or equal to 0.1.

[0145] On the other hand, in Comparative Examples 1 to 4, the peaks shown in Examples 1 to 4 were not observed. It is believed that during the manufacture of the positive electrode, the structure of the positive electrode additive is deformed due to the high temperatures in the additive layer formation process and the rolling process.

[0146] Experimental Example 2:

[0147] To evaluate the performance of the positive electrode for lithium secondary batteries and the lithium secondary batteries of the present invention, the following experiments were conducted.

[0148] 1) Evaluation of positive electrode porosity

[0149] The electrode porosity of the first and second agent layers of the positive electrode manufactured in the examples and comparative examples was measured. The results are shown in Table 2 below.

[0150] 2) Evaluation of initial resistance value

[0151] Lithium-ion batteries were manufactured using the various positive electrodes produced in the Examples and Comparative Examples. Specifically, natural graphite, carbon black conductive material, and PVdF binder, used as negative electrode active materials, were mixed with NMP solvent in a weight ratio of 85:10:5 to prepare a slurry for forming the negative electrode, and this slurry was applied to copper foil to manufacture the negative electrode. A separator made of porous polyethylene (PE) membrane (thickness: approximately 16 μm) was laminated and inserted between each positive electrode and the manufactured negative electrode produced in the Examples and Comparative Examples to manufacture an electrode assembly. The manufactured electrode assembly was placed in a battery case, and an electrolyte was injected into the case to manufacture a lithium-ion battery. In this case, the electrolyte was prepared by dissolving lithium hexafluorophosphate (LiPF6) at a concentration of 1.0 M in an organic solvent composed of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate (EC / DMC / EMC mixed volume ratio of 3 / 4 / 3).

[0152] Furthermore, each manufactured lithium-ion secondary battery was charged using a CC-CV (constant voltage) method at a constant current (CC) at a rate of 0.3C, causing the battery voltage to reach 4.2V at a current of 333mA. After the battery voltage reached 4.2V, it was charged once by cutting off the current at a rate of 0.05C while maintaining a CV of 4.2V. For a single-charge battery, a cycle of three CC discharges was repeated at 333mA and a rate of 0.3C until the battery voltage reached 3V. After recording the voltage drop that occurred when discharging at a current of 2A (2C) for ten seconds, the DC discharge resistance values ​​measured during the initial evaluation and calculated using R = V / 1 (Ohm's Law) are shown in Table 2 below. In this case, the DC resistance is closely related to the output characteristics of the secondary battery.

[0153] 3) Evaluation of cycle life performance

[0154] Lithium-ion batteries were manufactured using each positive electrode produced in the Examples and Comparative Examples, in the same manner as the evaluation of the initial resistance value. For each manufactured lithium-ion battery, capacity retention was measured after 100 charge / discharge cycles (n=100) and 200 charge / discharge cycles (n=200) at 25°C under conditions of a final charge voltage of 4.25V, a final discharge voltage of 2.5V, and 0.5C / 0.5C. In this case, capacity retention was calculated using the following mathematical formula 2, and the results are shown in Table 2 below.

[0155] [Mathematical Expression 2]

[0156] Capacity retention rate (%) = (Discharge capacity during n charge-discharge cycles / Discharge capacity during 1 charge-discharge cycle) × 100

[0157] [Table 2]

[0158]

[0159] As shown in Table 2, in the case of the lithium secondary batteries manufactured in the examples, it was confirmed that the resistance values ​​were lower than those of the comparative examples. In particular, the lithium secondary batteries including the positive electrodes of Comparative Examples 1 to 4 had the highest resistance values. This is believed to be due to the structural deformation of the positive electrode additives caused by the high temperatures in the additive layer formation process and the rolling process when manufacturing the positive electrodes of the comparative examples. Furthermore, it was found that the batteries having the positive electrodes of the examples had improved electrical performance when subjected to 100 and 200 charge-discharge cycles, and had high capacity retention rates of 97% and 95% or higher, respectively.

[0160] On the other hand, in Comparative Example 5, it was confirmed that the porosity of the first additive layer was too small, making it difficult for the electrolyte to permeate to the surface of the current collector, thus reducing the battery's electrical performance. Furthermore, it was confirmed that in each battery having the positive electrode of Comparative Examples 6 and 7, the porosity of the first additive layer was significantly reduced, resulting in decreased battery electrical performance.

[0161] The results above demonstrate that the method for manufacturing a positive electrode for lithium-ion batteries according to the present invention can achieve excellent charge / discharge characteristics and improve the lifespan of lithium-ion batteries by adding a positive electrode additive to the first compound layer in contact with the current collector. Furthermore, by sequentially forming a second compound layer on the first compound layer, with the second compound layer located at the outermost portion, and by controlling the conditions of the first and second rolling processes, the degradation of the positive electrode additive contained in the first compound layer can be minimized, thus achieving high battery performance.

[0162] Although the invention has been described with reference to exemplary embodiments thereof, it should be understood that various changes and modifications may be made to the invention by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.

[0163] Therefore, the technical scope of the present invention should not be limited to the content described in the detailed description of this specification, but should be determined by the scope of the appended claims.

Claims

1. A positive electrode for a lithium secondary battery, comprising: current collector; and An agent layer formed on one or both surfaces of the current collector. The mixture layer is a double-layer structure formed by stacking a first mixture layer and a second mixture layer. The first compound layer comprises a positive electrode active material, a positive electrode additive represented by the following chemical formula 1, a conductive material, and a binder, and The porosity of the first compound layer is 15% to 40%, and the porosity of the second compound layer is 40% to 70%. [Chemical Formula 1] Li p Co (1-q) M 1 q O4 In chemical formula 1, M 1 The expression represents the selection of one or more elements from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, where p and q are 5 ≤ p ≤ 7 and 0 ≤ q ≤ 0.5, respectively. In the X-ray diffraction (XRD) analysis of the first compound layer, the positive electrode satisfies the following mathematical formula 1': [Mathematical Expression 1'] A / B≤1 In mathematical formula 1', A represents the intensity of the strongest peak among the peaks displayed in the range of 2θ = 38.5 ± 0.1°, and B represents the intensity of the strongest peak among the peaks displayed in the range of 2θ = 47.9 ± 0.1°.

2. The positive electrode according to claim 1, wherein, The average thickness of the first compound layer is 0.1 μm to 20 μm, and The average thickness of the second compound layer is 50 μm to 300 μm.

3. The positive electrode according to claim 1, wherein, The ratio of the average thickness D1 of the first compound layer to the average thickness D2 of the second compound layer, D1:D2, is 4:6 to 1:

10.

4. The positive electrode according to claim 1, wherein, Based on the total weight of the first compound layer, the content of the positive electrode additive is from 0.1% to 5% by weight.

5. The positive electrode according to claim 1, wherein, The positive electrode active material is a lithium metal composite oxide represented by the following chemical formula 2: [Chemical Formula 2] Li x [Ni y Co z Mr w M 2 v ]O u In Chemical Formula 2, M 2 represents one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, and x, y, z, w, v, and u are 1.0 ≤ x ≤ 1.30, 0.1 ≤ y < 0.95, 0.01 < z ≤ 0.5, 0.01 < w ≤ 0.5, 0 ≤ v ≤ 0.2, and 1.5 ≤ u ≤ 4.5, respectively.

6. A method for manufacturing a positive electrode for a lithium secondary battery, the method comprising: The first additive layer forming process involves applying a first slurry containing a positive electrode active material and a positive electrode additive represented by the following chemical formula 1 to one or both sides of the current collector to form the first additive layer. The first rolling process involves rolling the formed first mixture layer. The second compound layer forming process involves applying a second slurry containing a positive electrode active material onto the first compound layer after rolling to form the second compound layer. and The second rolling process involves rolling the formed second mixture layer. The porosity of the first compound layer is 15% to 40%, and the porosity of the second compound layer is 40% to 70%. [Chemical Formula 1] Li p Co (1-q) M 1 q O4 In chemical formula 1, M 1 The expression represents the selection of one or more elements from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, where p and q are 5 ≤ p ≤ 7 and 0 ≤ q ≤ 0.5, respectively. In the first mixture layer formation process, the temperature of the first slurry is between 10°C and 40°C; and in the second mixture layer formation process, the temperature of the second slurry is between 10°C and 40°C. The first rolling process is carried out in a temperature range of 10°C to 40°C, and the second rolling process is carried out in a temperature range of 40°C to 100°C.

7. The method according to claim 6, wherein, The first rolling process is carried out at a speed of 0.5 m / s to 6 m / s. The second rolling process is carried out at a speed of 2 m / s to 7 m / s, and The second rolling process is performed at a faster speed than the first rolling process.

8. The method according to claim 6, wherein, The first rolling process is carried out in a temperature range of 10°C to 30°C, and The second rolling process is carried out in a temperature range of 50°C to 90°C.

9. The method according to claim 6, wherein, The first compound layer satisfies the thickness change rate condition of the following mathematical formula 1: [Mathematical formula 1] D 2 min / D 1 max ×100≥70% In mathematical formula 1, D 1 max D represents the maximum thickness of the first compound layer after the first rolling process. 2 min This indicates the minimum thickness of the first compound layer after the second rolling process.

10. A lithium secondary battery, comprising: The positive electrode according to any one of claims 1 to 5; negative electrode; and A diaphragm is inserted between the positive electrode and the negative electrode.

11. The lithium secondary battery according to claim 10, wherein, The negative electrode includes a negative electrode current collector and a negative electrode mixture layer located on the negative electrode current collector and containing negative electrode active material. The negative electrode active material comprises carbon and silicon materials.

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