Positive electrode, method for manufacturing the same, and lithium secondary battery including the positive electrode

Abstract

The present invention relates to a positive electrode. The positive electrode includes a positive electrode active material layer containing a positive electrode active material, and the crack rate of the surface portion of the positive electrode active material layer derived from the following Mathematical Formula 1 is 5.0% to 14.2%. [Mathematical Formula 1] CR = (CA) / [(PA)+(CA)] Here, CR is the crack rate (%), CA is the crack region, PA is the particle region, and the surface portion of the positive electrode active material layer means a region from the surface of the layer to a depth of 20 μm in the depth direction.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0101636 filed on August 12, 2022 and Korean Patent Application No. 10-2022-0183744 filed on December 23, 2022, and all of the contents disclosed in the documents of the Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to an over-lithiated cathode, a method for manufacturing the same, and a lithium secondary battery including the same.

Background Art

[0003] With the rapid increase in the use of fossil fuels, the need for alternative and clean energy is increasing, and one of the most actively studied fields is power generation and power storage using electrochemical reactions.

[0004] Currently, a typical example of an electrochemical device using such electrochemical energy is a secondary battery, and its usage areas are gradually expanding. Recently, as technology development and demand for portable devices such as portable computers, mobile phones, and cameras have increased, the demand for secondary batteries as an energy source has increased rapidly. Among such secondary batteries, various studies have been conducted on lithium secondary batteries with high energy density, that is, high capacity, and they have also been commercialized and widely used.

[0005] Generally, a secondary battery is composed of a cathode, an anode, an electrolyte, and a separator. The anode contains an anode active material that inserts and desorbs lithium ions emitted from the cathode, and as the anode active material, silicon-based active material particles with a large discharge capacity can be used. The silicon-based active material particles are Si or SiO XIt can correspond to, for example, (0 < X < 2). Silicon-based active material particles have the advantages of a large theoretical capacity and low cost. However, silicon-based active material particles have the drawback that during battery operation, the volume change is excessively large, so the battery life rapidly decreases as the number of battery cycles progresses.

[0006] Therefore, in order to minimize the volume change of silicon-based active material particles, there is a method of using only a part of the total capacity of the silicon-based active material particles. For this purpose, a so-called prelithiation process of inserting lithium ions into the negative electrode containing silicon-based active material particles in advance is used. Specifically, when lithium ions are inserted into the negative electrode by a method such as transferring lithium metal to the negative electrode, the lithium ions react with the irreversible sites of the negative electrode, and the total capacity of the negative electrode can be reduced to the level of the reversible capacity. Therefore, during battery operation, the amount of lithium ions inserted can be suitably reduced to the level required for battery operation, and the volume change of the silicon-based active material particles can be minimized.

[0007] However, in the process of performing prelithiation by arranging lithium metal on the surface of the negative electrode, excessive heat is generated by the alloy reaction of lithium and silicon, and the possibility of ignition due to the reaction of lithium and moisture also increases. In addition, in the process of notching and punching the negative electrode, the possibility of ignition can be further increased due to the increase in the reaction area of lithium and the silicon-based active material, and there is a serious safety problem that there is also a possibility of ignition by the silicon-based active material particles subjected to prelithiation.

[0008] Therefore, there is a need for a new technology that can insert lithium ions into the negative electrode in advance before battery operation to improve the battery life and suppress the possibility of excessive heat generation and ignition. Summary of the Invention Problems to be Solved by the Invention

[0009] One problem to be solved by the present invention is to control the available area of the negative electrode by over-lithiation to have a crack rate of a certain amount or more in the surface portion of the positive electrode, and to provide a positive electrode capable of improving safety in the battery manufacturing process and a method for manufacturing the same.

[0010] Another problem to be solved by the present invention is to provide a secondary battery including the positive electrode, improving the life characteristics by controlling the available area of the negative electrode, and realizing the inherent characteristics of the negative electrode active material without drawbacks.

Means for Solving the Problems

[0011] In order to solve the above problems, in one aspect of the present invention, a positive electrode including a positive electrode active material layer containing a positive electrode active material is provided, wherein the crack rate of the surface portion of the positive electrode active material layer derived from the following Mathematical Formula 1 is 5.0% to 14.2%. [Mathematical Formula 1] CR=(CA) / [(PA)+(CA)] Here, CR is the crack rate (%), CA is the crack region, PA is the particle region, and the surface portion of the positive electrode active material layer means a region from the surface of the layer to a depth of 20 μm in the depth direction.

[0012] In order to solve the above problems, in another aspect of the present invention, a transfer laminate including a base film and a lithium metal layer located on the base film is disposed on a preliminary positive electrode active material layer to form a positive electrode structure such that the lithium metal layer is in contact with the preliminary positive electrode active material layer, a P2 step of rolling the positive electrode structure, and a P3 step of removing the base film from the transfer laminate after the rolling to manufacture a positive electrode, wherein the positive electrode includes a positive electrode active material layer containing a positive electrode active material, and the crack rate of the surface portion of the positive electrode active material layer derived from the Mathematical Formula 1 is 5.0% to 14.2%. A method for manufacturing a positive electrode is provided.

[0013] In order to solve the above problems, in still another aspect of the present invention, there is provided a secondary battery including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode includes a positive electrode active material layer containing a positive electrode active material, and the crack ratio of the surface portion of the positive electrode active material layer derived from the above Mathematical Formula 1 is 5.0% to 14.2%.

Advantages of the Invention

[0014] The positive electrode according to the present invention is over-lithiated by a specific method of lithium metal transfer, so that a certain amount of crack ratio is formed on the surface portion. As a result, when lithium ions move to the negative electrode in the activation process, the available area of the negative electrode can be reduced, thereby increasing the battery life, and the process risk generated by pre-lithiation of the negative electrode can be reduced.

[0015] In addition, the secondary battery according to the present invention includes the positive electrode, so that the available area of the negative electrode can be increased, and thus the capacity can be increased. In particular, when a silicon-based negative electrode active material is used, only a part of the available area can be used without lithium loss of the positive electrode due to over-lithiated lithium, and thus the life characteristics can be improved by minimizing the volume change.

Brief Description of the Drawings

[0016]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0017] Hereinafter, to facilitate the understanding of the present invention, the present invention will be described in more detail.

[0018] The terms and words used in this specification and the claims should not be construed as being limited to their ordinary or dictionary meanings. The inventors should interpret them in accordance with the meaning and concept consistent with the technical idea of the present invention, in accordance with the principle that they can appropriately define the concept of the terms in order to explain their invention in the best way.

[0019] The terms used in this specification are merely used to illustrate exemplary embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless they have a clearly different meaning in the context.

[0020] In this specification, terms such as "comprising", "including" or "having" are used to specify the presence of implemented features, numbers, steps, components, or combinations thereof, and should be understood not to preclude the presence or addition possibility of one or more different features, numbers, steps, components, or combinations thereof.

[0021] In this specification, D 50 can be defined as the particle size corresponding to 50% of the volume cumulative amount in the particle size distribution curve of the particles. The above D 50 can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes in the range from the submicron region to about several millimeters, and can obtain highly reproducible and highly resolved results.

[0022] Positive electrode The positive electrode according to the present invention includes a positive electrode active material layer containing a positive electrode active material, and the crack rate of the surface portion of the positive electrode active material layer derived from the following Mathematical Formula 1 is 5.0% to 14.2%.

[0023] [Mathematical formula 1] CR = (CA) / [(PA)+(CA)]

[0024] Here, CR is the crack rate (%), CA is the crack area, PA is the particle area, and the surface part of the positive electrode active material layer means the region from the surface of the layer to a depth of 20 μm in the depth direction.

[0025] According to one embodiment of the present invention, the positive electrode includes a positive electrode active material layer. The positive electrode active material layer can itself constitute the positive electrode, but the positive electrode active material layer can be located on the positive electrode current collector, and the positive electrode active material layer can be located on one or both surfaces of the positive electrode current collector.

[0026] According to one embodiment of the present invention, the positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or those obtained by surface treatment of the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. can be used. Further, the positive electrode current collector can usually have a thickness of 3 μm to 500 μm, and fine irregularities can be formed on the surface of the current collector to increase the adhesive force of the positive electrode active material. For example, it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabric bodies, etc.

[0027] According to one embodiment of the present invention, the positive electrode active material is a particulate material capable of undergoing an electrochemical reaction and can be a lithium transition metal oxide. For example, the positive electrode active material is a layered compound such as lithium cobalt oxide or lithium nickel oxide substituted with one or more transition metals; lithium manganese oxide substituted with one or more transition metals; Li 1+x [Ni a Co b Mn c M 1 (1-a-b-c) O (2-d) Ad (Here, M 1 is at least one selected from the group consisting of Al, Mg, Cr, Ti, Si, and Y, A is at least one selected from the group consisting of F, P, and Cl, -0.5 ≦ x ≦ 0.5, 0.1 ≦ a ≦ 1, 0.05 ≦ b ≦ 0.5, 0.05 ≦ c ≦ 0.5, 0 ≦ d ≦ 0.2, 0 < a + b + c ≦ 1), lithium nickel cobalt manganese composite oxide represented by; LI[Ni 1-y M 2 y O2(Here, M 2 can be at least one selected from Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn, and Ga, and 0.01 ≦ y ≦ 0.7), lithium nickel-based oxide represented by; Li 1+z [M 3 1-q M 4 q PO 4-r X r (Here, M 3 is at least one selected from the group consisting of Fe, Mn, Co, and Ni, M 4 is at least one selected from the group consisting of Al, Mg, and Ti, X is at least one selected from the group consisting of F, S, and N, -0.5 ≦ z ≦ 0.5, 0 ≦ q ≦ 0.5, 0 ≦ r ≦ 0.1), and can include at least one selected from the group consisting of olivine-type lithium metal phosphates represented by.

[0028] Specifically, the positive electrode active material can include a lithium nickel-based transition metal composite oxide having a layered structure, and the lithium nickel-based transition metal composite oxide can include a compound of Chemical Formula 1 below, and more specifically, can be a compound of Chemical Formula 1 below.

[0029] [Chemical Formula 1] Li 1+x [Ni a Co b Mn c M 1(1-a-b-c) O (2-d) A d

[0030] In the above Chemical Formula 1, M 1 can be at least any one selected from the group consisting of Al, Mg, Cr, Ti, Si, and Y, and specifically can be Al.

[0031] A is at least any one selected from the group consisting of F, P, and Cl, and specifically can be F.

[0032] The above x can satisfy -0.5 ≤ x ≤ 0.5, specifically -0.3 ≤ x ≤ 0.3.

[0033] The above a can satisfy 0.6 ≤ a < 1, specifically 0.7 ≤ a ≤ 0.9.

[0034] The above b can satisfy 0.03 ≤ b ≤ 0.1, specifically 0.05 ≤ b ≤ 0.1.

[0035] The above c can satisfy 0.03 ≤ c ≤ 0.1, specifically 0.05 ≤ c ≤ 0.1.

[0036] The above d can satisfy 0 ≤ d ≤ 0.1, specifically 0 ≤ d ≤ 0.05.

[0037] The above a, b, and c satisfy 0 < a + b + c ≤ 1, specifically a + b + c = 1.

[0038] The compound of Chemical Formula 1 can be in a particulate form.

[0039] The compound of Chemical Formula 1 can be in the form of secondary particles in which a plurality of primary particles are bonded to each other. Specifically, the compound of Chemical Formula 1 can be in the form of secondary particles in which 10 or more primary particles are bonded to each other. Thereby, there is an effect that lithium can be uniformly inserted and desorbed inside the positive electrode active material.

[0040] D of the compound of Chemical Formula 1 50 can be 5 μm to 15 μm, specifically can be 7 μm to 12 μm, and more specifically can be 9 μm to 10 μm. The said D 50 is the D of the secondary particles 50 and can be. When the said range is satisfied, the dispersion of the positive electrode slurry becomes easy, and uniform coating of the positive electrode active material layer becomes possible.

[0041] The said positive electrode active material can be contained in the positive electrode active material layer at 90% by weight to 99% by weight, specifically can be contained at 92% by weight to 98% by weight, and more specifically can be contained at 95% by weight to 98% by weight.

[0042] According to one embodiment of the present invention, the positive electrode active material layer can further contain a positive electrode binder. The said positive electrode binder plays a role in improving the adhesion between the positive electrode active material particles and the adhesive force between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene - propylene - diene polymer (EPDM), sulfonated - EPDM, styrene - butadiene rubber (SBR), fluororubber, or various copolymers thereof, etc. Among these, one kind alone or a mixture of two or more kinds can be used.

[0043] The said positive electrode binder can be contained in the positive electrode active material layer at 0.5% by weight to 5.0% by weight, specifically can be contained at 1.0% by weight to 2.5% by weight, and more specifically can be contained at 1.0% by weight to 2.0% by weight.

[0044] According to one embodiment of the present invention, the positive electrode active material layer may further include a positive electrode conductive material. The positive electrode conductive material is used to impart conductivity to the electrode and can be used without particular limitation as long as it has electron conductivity without causing a chemical change in the configured battery. Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. Among these, one kind alone or a mixture of two or more kinds can be used.

[0045] The positive electrode conductive material can be contained in the positive electrode active material layer in an amount of 0.5% by weight to 30.0% by weight, specifically 0.5% by weight to 10.0% by weight, and more specifically 1.0% by weight to 4.0% by weight.

[0046] According to one embodiment of the present invention, the positive electrode has a feature that the crack rate of the surface portion of the positive electrode active material layer derived from the following Mathematical Formula 1 is 5.0% to 14.2%.

[0047] [Mathematical Formula 1] CR = (CA) / [(PA)+(CA)]

[0048] Here, CR is the crack rate (%), CA is the crack area, PA is the particle area, and the surface portion of the positive electrode active material layer means the region from the surface of the layer to a depth of 20 μm in the depth direction.

[0049] The crack ratio is obtained by analyzing the cross-section of the positive electrode. A reference region is set with a depth of 20 μm and a planar dimension of 200 μm in the depth direction from the surface of the active material layer. The crack region and the particle region within this region are distinguished, and the areas are determined. The crack region is the part where the particles are cracked, and the particle region is the region where the particles are not cracked. The electrode is cut by ion milling, and the cut cross-section is photographed with a scanning electron microscope (SEM). For the SEM cross-sectional image, based on digital transformation, the crack region and the particle region are quantified. At this time, the ratio of the region where the specific surface area increases inside the positive electrode is calculated and quantified by an additional process. Also, to improve accuracy, 50 or more reference regions are specified for the positive electrode cross-section, the crack ratio of each region is obtained, and the average value is obtained.

[0050] According to one embodiment of the present invention, the crack ratio can be 5.0% to 14.2%, preferably 6.0% or more, 7.0% or more, 8.0% or more, or 9.0% or more, and can be 14.1% or less, or 14.0% or less. A crack ratio of less than 5.0% means that the lithium ions in the positive electrode active material layer are not sufficiently over-lithiated, and the amount of lithium that can move to the negative electrode by activation is not sufficient, so it may be difficult to increase the available region of the negative electrode, and therefore, problems such as volume expansion of the negative electrode that can degrade the life characteristics may occur.

[0051] That is, in the process of manufacturing the positive electrode of the present invention, a lithium metal layer is transferred onto the positive electrode active material layer, and then the positive electrode including the lithium metal layer is rolled, and the lithium ions of the lithium metal layer are inserted into the positive electrode active material layer. In this process, lithium by-products are generated by lithium ions. Therefore, it is considered that the crack ratio of 5.0% or more means that the lithium metal layer has been transferred and rolled onto the positive electrode active material layer during the manufacture of the positive electrode of the present invention.

[0052] On the one hand, when the crack rate exceeds 14.2%, the capacity of the positive electrode active material decreases, and the occurrence of side reactions due to cracked particles increases the battery resistance or the gas generation amount, which may cause performance degradation. Therefore, when performing over-lithiation for controlling the available region of the negative electrode, it is necessary to ensure that the crack rate satisfies a specific range through appropriate control.

[0053] According to an embodiment of the present invention, the positive electrode may further include a lithium metal layer located on the positive electrode active material layer. The lithium metal layer serves to supply lithium ions to the positive electrode active material layer. Specifically, the positive electrode active material layer can be located between the positive electrode current collector and the lithium metal layer. The lithium metal layer can be in contact with the positive electrode active material layer. The lithium metal layer contains solid-phase lithium metal. Specifically, the lithium metal layer can be composed of solid-phase lithium metal. The lithium metal layer may undergo solid-phase diffusion of lithium metal into the positive electrode active material layer by transfer and rolling. As a result, there may be no lithium metal layer after activation, and it may exist in a state where its thickness is extremely thin.

[0054] According to an embodiment of the present invention, the positive electrode may further include a polymer layer located on the positive electrode active material layer. The polymer layer can play a role in effectively peeling the lithium metal layer from the transfer laminate during the manufacture of the positive electrode, so that the lithium metal layer can be easily transferred to the positive electrode active material layer. That is, the polymer layer can be located on the positive electrode active material layer away from the transfer laminate together with the lithium metal layer. The polymer layer can be in contact with the positive electrode active material layer. Different from this, a lithium metal layer can also exist between the polymer layer and the positive electrode active material layer.

[0055] The polymer layer can be at least any one selected from the group consisting of polyethylene terephthalate (PET), polyimide (PI), poly(methylmethacrylate) (PMMA), polypropylene, polyethylene, and polycarbonate. Thereby, in the secondary battery including the positive electrode, the polymer layer can be dissolved in the electrolyte solution contained in the secondary battery, and an increase in the resistance of the battery can be prevented. In particular, the polymer layer can include PMMA, and in that case, the above-described effects can be further improved.

[0056] According to an embodiment of the present invention, in the positive electrode, the porosity of the positive electrode active material layer can be 10% to 40%, specifically 15% to 35%, and more specifically 25% to 30%. In this case, no further change in thickness can occur during rolling.

[0057] According to an embodiment of the present invention, in the positive electrode, a lithium metal layer is disposed on the positive electrode by a transfer method, and lithium in the lithium metal layer is inserted into the positive electrode active material layer by solid-phase diffusion by rolling. This lithium can move to the negative electrode during the driving of the battery and play a role in reducing the available area of the negative electrode.

[0058] Therefore, it is possible to reduce the available area of the negative electrode capacity without lithium loss in the positive electrode active material due to over-lithiated lithium in the positive electrode. In particular, it is possible to suppress an excessive volume change of the silicon-based active material, and the life characteristics of the battery can be improved. Further, thereby, the capacity of the positive electrode active material can be used over the entire range, and there is an advantage that the efficiency of the battery can be increased and the energy density can also be maximized.

[0059] The positive electrode according to an embodiment of the present invention does not usually perform prelithiation by bringing a lithium metal layer into contact with the negative electrode, but after transferring and rolling the lithium metal layer onto the positive electrode, the lithium ions inserted into the positive electrode are moved to the negative electrode in the battery activation process.

[0060] That is, since the negative electrode and the lithium metal layer do not come into contact with each other and lithium ions are not directly inserted into the negative electrode from the lithium metal layer, it is possible to avoid the phenomenon of excessive heat generation due to the alloy reaction between lithium and silicon in the negative electrode, and the possibility of ignition due to the reaction between lithium and moisture can also be significantly reduced. Further, when notching and punching the negative electrode, since lithium ions are not inserted into the negative electrode (because prelithiation has not been performed), the possibility of ignition in the notching and punching processes can also be significantly reduced.

[0061] Method for manufacturing a positive electrode The method for manufacturing a positive electrode according to the present invention includes a P1 step of disposing a transfer laminate including a base film and a lithium metal layer located on the base film on a preliminary positive electrode active material layer to form a positive electrode structure such that the lithium metal layer and the preliminary positive electrode active material layer are in contact with each other, a P2 step of rolling the positive electrode structure, and a P3 step of removing the base film from the transfer laminate after the rolling to manufacture a positive electrode. The positive electrode includes a positive electrode active material layer including a positive electrode active material, and provides a method for manufacturing a positive electrode in which the crack rate of the surface portion of the positive electrode active material layer derived from the following Mathematical Formula 1 is 5.0% to 14.2%.

[0062] [Mathematical Formula 1] CR = (CA) / [(PA)+(CA)]

[0063] Here, CR is the crack rate (%), CA is the crack region, PA is the particle region, and the surface portion of the positive electrode active material layer means a region from the surface of the layer to a depth of 20 μm in the depth direction.

[0064] According to an embodiment of the present invention, in the method for manufacturing the positive electrode, the P1 step is a step of forming a positive electrode structure by disposing a transfer laminate including a base film and a lithium metal layer on a preliminary positive electrode active material layer such that the lithium metal layer is in contact with the preliminary positive electrode active material layer.

[0065] Referring to FIG. 1, the transfer laminate 300 may include a base film 310 and a lithium metal layer 320 located on the base film 310. The base film 310 can be used without limitation as long as it is a material that can withstand high temperature conditions generated during the process of depositing the lithium metal layer 320 on the base film 310. Specifically, the base film may include one or more selected from the group consisting of polyethylene terephthalate (PET), polyimide (PI), poly(methylmethacrylate) (PMMA), polypropylene, polyethylene, and polycarbonate.

[0066] According to an embodiment of the present invention, the lithium metal layer can be located on the base film. The lithium metal layer can serve to supply lithium ions to the preliminary positive electrode active material layer. The lithium metal layer includes solid-phase lithium metal, and specifically, the lithium metal layer can be composed of solid-phase lithium metal.

[0067] According to one embodiment of the present invention, in the transfer laminate, the thickness of the lithium metal layer can be 1.0 μm to 10.0 μm, specifically, it can be 2.0 μm or more, 2.5 μm or more, or 3.0 μm or more, and also 9.5 μm or less, 9.0 μm or less, 8.5 μm or less, 8.0 μm or less, 7.5 μm or less, 7.0 μm or less, or 6.5 μm or less. When the above range is satisfied, the degree of cracking of the cathode active material particles on the surface of the cathode can be reduced so as not to affect the performance, the application to available lithium, that is, the lithium loss amount can be controlled to be small, the decrease in the initial capacity of the battery can be suppressed, and the life can be improved by compensating for the irreversible capacity of the anode.

[0068] According to one embodiment of the present invention, the loading amount (unit: mAh / cm 2 ) of the lithium metal layer is 4% to 40% of the loading amount (unit: mAh / cm 2 ) of the preliminary cathode active material layer, specifically, it can be 12% to 35%, more specifically, it can be 20% to 30%. When the above range is satisfied, the generation of by-products is small, lithium can be easily inserted into the cathode active material, and the target lithium insertion capacity can be easily achieved.

[0069] In the P1 step, referring to FIG. 1, the preliminary cathode active material layer 120' is located on the cathode current collector 110, the transfer laminate 300 is disposed on the preliminary cathode active material layer 120', and the cathode structure 400 can be formed such that the lithium metal layer 320 and the preliminary cathode active material layer are in contact with each other.

[0070] Here, the cathode current collector 110 is the same as the cathode current collector described in the above embodiment regarding the cathode.

[0071] On the one hand, referring to FIG. 2, the transfer laminate 300 can further include a polymer layer 330. The polymer layer 330 can be located between the base film 310 and the lithium metal layer 320. The polymer layer can play a role in effectively peeling the lithium metal layer from the transfer laminate during the manufacture of the positive electrode, so that the lithium metal layer can be easily transferred to the positive electrode active material layer. That is, the polymer layer can be separated from the transfer laminate together with the lithium metal layer and located on the positive electrode active material layer. The polymer layer can be in contact with the positive electrode active material layer. Different from this, a lithium metal layer can also exist between the polymer layer and the positive electrode active material layer.

[0072] Here, since the polymer layer has been described above in the description of the positive electrode, its description is omitted.

[0073] The thickness of the polymer layer can be 0.1 μm to 10 μm, specifically 0.5 μm to 5 μm, and more specifically 1 μm to 2.5 μm. When the above range is satisfied, the lithium metal layer can be easily transferred to the positive electrode active material layer, and the reverse transfer phenomenon in which the positive electrode active material layer is transferred to the transfer laminate can be prevented.

[0074] According to an embodiment of the present invention, in the manufacturing method of the positive electrode, the P2 step is a step of rolling the positive electrode structure.

[0075] Referring to FIG. 3, in the P2 step, the manufactured positive electrode structure 400 can be rolled. The rolling can be performed by a roll press method. Specifically, pressure can be applied in the vertical direction to the positive electrode structure 400 passing through the space via two rolls R spaced apart with a predetermined space in the vertical direction, and the pressure can be a linear pressure. By the rolling process, at least a part of the lithium ions of the lithium metal layer contained in the transfer laminate can be inserted into the interior of the preliminary positive electrode active material layer. In this process, the preliminary positive electrode active material layer can become a positive electrode active material layer. FIGS. 3 and 4 illustrate the case where the lithium metal layer is included in the positive electrode, but in the rolling process, when the entire lithium metal layer is inserted into the interior of the preliminary positive electrode active material layer, the lithium metal layer may not exist as a separate layer.

[0076] During the rolling, the pressure applied to the positive electrode structure can be 10 kgf / cm to 90 kgf / cm, specifically 15 kgf / cm to 80 kgf / cm, and more specifically 20 kgf / cm to 40 kgf / cm.

[0077] When the pressure during the rolling is applied within the above range, the lithium metal layer can be effectively transferred, and lithium can be inserted into the positive electrode at a desired level. The crack rate can also meet an appropriate level. Thereby, the effect of simultaneously satisfying the improvement of the lifespan and the capacity characteristics can be realized.

[0078] According to an embodiment of the present invention, in the P3 step of the method for manufacturing the positive electrode, after rolling, the substrate film is removed from the transfer laminate to manufacture a positive electrode.

[0079] Referring to FIG. 4, in the P3 step, after the rolling performed in the P2 step, the base film 310 can be removed from the transfer laminate to produce the positive electrode 100. When the polymer layer 330 is located between the base film 310 and the lithium metal layer 320, the base film 310 can be more easily removed by the polymer layer 330.

[0080] Further, according to an embodiment of the present invention, the method for manufacturing the positive electrode may further include a P4 step of allowing the preliminary positive electrode to stand (leave) for 1 minute to 600 minutes, specifically 1 minute to 30 minutes. The P4 step can be performed after the P2 step. Specifically, the P4 step can be performed at least in one of the steps "between the P2 step and the P3 step" and "immediately after the P3 step". By the P4 step, the reaction heat generated by the reaction between the lithium metal layer and the preliminary positive electrode active material layer in the P2 step can be effectively released, and lithium can be uniformly inserted into the positive electrode, reducing the generation of by-products.

[0081] Since the description of the positive electrode manufactured otherwise has been described above, the description thereof is omitted.

[0082] Secondary battery The secondary battery according to the present invention includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. The positive electrode includes a positive electrode active material layer containing a positive electrode active material. The crack rate of the surface portion of the positive electrode active material layer derived from the following Mathematical Formula 1 is 5.0% to 14.2%.

[0083] [Mathematical Formula 1] CR = (CA) / [(PA)+(CA)]

[0084] Here, CR is the crack rate (%), CA is the crack region, PA is the particle region, and the surface portion of the positive electrode active material layer means the region from the surface of the layer to a depth of 20 μm in the depth direction.

[0085] Since the description of the positive electrode has been given above, its description is omitted.

[0086] According to an embodiment of the present invention, the negative electrode includes a negative electrode active material layer, the negative electrode active material layer can include a negative electrode active material, and the negative electrode active material can be applied with a silicon-based negative electrode active material, a carbon-based negative electrode active material, or a combination thereof.

[0087] According to an embodiment of the present invention, the negative electrode active material can include a silicon-based negative electrode active material. The silicon-based negative electrode active material can include Si and SiO x (0 < X < 2) and can include at least any one of them.

[0088] The Si is silicon particles and can be silicon particles (particles made of silicon) called so-called pure silicon. The silicon particles can effectively improve the capacity of the negative electrode. The SiO x (0 < X < 2) can be a form containing Si and SiO2, and the Si can also form a phase. That is, the X corresponds to the ratio of the number of O to Si contained in the SiO x (0 < X < 2). When the silicon-based composite particles contain the SiO x (0 < X < 2), the discharge capacity of the secondary battery can be improved.

[0089] According to an embodiment of the present invention, the silicon-based negative electrode active material has a problem that the volume change during the driving of the battery is severe and the life characteristics are very poor. However, when combined with the positive electrode according to the present invention, instead of lithium in the positive electrode active material that determines the capacity with the silicon-based negative electrode active material during activation from the over-lithiated positive electrode, the remaining lithium ions move and react with the silicon-based negative electrode active material in advance to form an irreversible phase, reducing the available area. As a result, the volume change during the driving of the battery does not increase, the life can be significantly improved, and the high-capacity characteristic, which is the inherent characteristic of the silicon-based negative electrode active material, can be fully exhibited.

[0090] According to one embodiment of the present invention, the negative electrode active material may further include a carbon-based negative electrode active material. The carbon-based negative electrode active material may include at least any one selected from the group consisting of artificial graphite, natural graphite, and graphitized mesocarbon microbeads.

[0091] According to one embodiment of the present invention, the negative electrode active material layer may further include a negative electrode binder. The negative electrode binder may include at least any one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, poly acrylic acid, and substances obtained by substituting hydrogen thereof with Li, Na, Ca, etc., and may also include various copolymers thereof.

[0092] According to an embodiment of the present invention, the negative electrode active material layer can further include a negative electrode conductive material. The negative electrode conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbons, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives can be used.

[0093] According to an embodiment of the present invention, the secondary battery includes a separator. As the separator, as long as it separates the negative electrode and the positive electrode and provides a migration path for lithium ions, it can be used without particular limitation as long as it is usually used as a separator in a secondary battery. In particular, those having low resistance to ion migration of the electrolyte and excellent electrolyte moisture retention ability are preferred. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin-based polymer such as a polyethylene homopolymer, a polypropylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof can be used. Also, a normal porous non-woven fabric, for example, a non-woven fabric made of high-melting-point glass fibers, polyethylene terephthalate fibers, etc. can also be used. Further, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer substance can also be used, and optionally, it can be used in a single-layer or multi-layer structure.

[0094] According to an embodiment of the present invention, the secondary battery may further include an electrolyte. Examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0095] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt. Examples of the non-aqueous organic solvent that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triethyl phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl pyruvate, and ethyl propionate.

[0096] In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, have a high dielectric constant as high-viscosity organic solvents and can preferably be used because they can dissociate lithium salts well. When such cyclic carbonates are mixed with low-viscosity and low-dielectric-constant linear carbonates such as dimethyl carbonate and diethyl carbonate in an appropriate ratio, an electrolyte having high electrical conductivity can be produced and can be more preferably used.

[0097] The metal salt may be a lithium salt, and the lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte solution. For example, as the anion of the lithium salt, F - , Cl - , I - , NO3- , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - One or more selected from the group consisting of can be used.

[0098] In addition to the constituent components of the electrolyte, the electrolyte may further contain one or more additives such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n - glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N - substituted oxazolidinone, N,N - substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2 - methoxyethanol or aluminum trichloride for the purpose of improving the life characteristics of the battery, suppressing the capacity reduction of the battery, improving the discharge capacity of the battery, etc.

[0099] As described above, the secondary battery including the positive electrode according to the present invention stably exhibits excellent capacity and capacity retention rate, and thus is useful in portable devices such as mobile phones, notebook computers, digital cameras, and in the field of electric vehicles such as hybrid electric vehicles (HEVs).

[0100] Accordingly, according to another embodiment of the present invention, a battery module including the secondary battery as a unit cell and a battery pack including the same are provided.

[0101] The battery module or battery pack can be used as a power source for one or more medium to large-sized devices such as a power tool; electric vehicles including an electric vehicle (EV), a hybrid electric vehicle, and a plug-in hybrid electric vehicle (PHEV); or a power storage system.

[0102] Hereinafter, preferred examples are presented to facilitate understanding of the present invention. However, it is obvious to those skilled in the art that the examples are only for illustrating the description, and various changes and modifications are possible within the scope of the description and the scope of the technical idea. Needless to say, such variations and modifications belong to the scope of the appended claims.

[0103] Electrode manufacturing Example 1-1: Manufacturing of a positive electrode A transfer laminate including a PET film (base film), a polymer layer (2.5 μm thick) made of PMMA disposed on the PET film, and a lithium metal layer (3.0 μm thick, 0.62 mAh / cm 2 loading amount) made of solid lithium metal disposed on the polymer layer was prepared.

[0104] On the other hand, as the positive electrode active material, Li[Ni 0.86 Co 0.05 Mn 0.08 Al0.01 O2 was used. The cathode active material is Li[Ni 0.86 Co 0.05 Mn 0.08 Al 0.01 O2, and a plurality (10 or more) of primary particles are combined with each other to form secondary particles, and the average particle diameter D of the secondary particles 50 was 9 μm. A cathode was prepared that included a preliminary cathode active material layer containing the cathode active material, PVdF as the cathode binder, and carbon nanotubes as the cathode conductive material in a weight ratio of 98:1:1, and an aluminum foil (thickness: 12 μm) as the cathode current collector. The loading amount of the preliminary cathode active material layer was 4.5 mAh / cm 2 and the thickness was 140 μm.

[0105] The transfer laminate was placed on the preliminary cathode active material layer so that the lithium metal layer was in contact with the preliminary cathode active material layer.

[0106] Thereafter, the cathode on which the transfer laminate was placed was rolled by a roll press method and then left for 24 hours. The pressure during rolling was 20 kgf / cm. As a result, lithium ions in the lithium metal layer were inserted into the preliminary cathode active material layer to form a cathode active material layer. Next, the base film was removed and left for 10 minutes to produce a cathode including a cathode current collector, a cathode active material layer, and a polymer layer.

[0107] Example 1-2: Manufacturing of a positive electrode A cathode was produced in the same manner as in Example 1-1 except that the thickness of the lithium metal layer was 6.2 μm and the loading amount was 1.28 mAh / cm 2 .

[0108] Example 1-3: Manufacturing of a positive electrode A cathode was produced in the same manner as in Example 1-2 except that the pressure during rolling was 30 kgf / cm.

[0109] Comparative Example 1-1: Manufacturing of a positive electrode A cathode was produced in the same manner as in Example 1-1 except that the transfer laminate was not applied.

[0110] Comparative Example 1-2: Manufacturing of a positive electrode The thickness of the lithium metal layer was 11.6 μm and the loading amount was 1.90 mAh / cm 2 The positive electrode was manufactured in the same manner as in Example 1-1 except for the above conditions.

[0111] Comparative Example 1-3: Manufacturing of a positive electrode The positive electrode was manufactured in the same manner as in Example 1-2 except that the pressure during rolling was 100 kgf / cm.

[0112] Experimental Example 1: Physical property evaluation of a positive electrode For the positive electrodes manufactured in Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-3, the crack rate (%) was measured by the following method.

[0113] One day after the completion of the production of each positive electrode, the cross-section of the positive electrode was analyzed. A reference region was set with a depth of 20 μm and a planar dimension of 200 μm from the surface of the active material layer in the depth direction. The crack regions and particle regions within this region were distinguished and the areas were determined. The crack regions were the parts where the particles were cracked, and the particle regions were distinguished as the regions where the particles were not cracked. The cross-section of the electrode cut by ion milling was photographed with a scanning electron microscope (SEM) to obtain an image.

[0114] For the SEM cross-sectional image, the crack regions and particle regions were quantified based on digital transformation, and the ratio of the regions where the specific surface area increased inside the positive electrode due to additional processes was calculated and quantified. Also, to improve the accuracy, 50 or more reference regions were specified for the cross-section of the positive electrode, the crack rate of each region was determined, and the average value was obtained.

[0115] [Table 1]

[0116] As shown in Table 1 above, in the cases of Examples 1-1 to 1-3, it can be confirmed by the crack rate that lithium was inserted into the interior of the positive electrode due to over-lithiation, and from Comparative Example 1-1, it can be confirmed that there is a difference in the crack rate between the case of over-lithiation and the case where it is not. On the other hand, in the case of Comparative Example 1-2, at the time of lithium transfer, the thickness of the lithium layer was excessively thick, and in the case of Comparative Example 1-3, as a result of slightly increasing the pressure during rolling, it can be confirmed that the degree of cracking of the positive electrode active material particles is excessively shown.

[0117] Manufacturing of a secondary battery Example 2-1: Manufacturing of a secondary battery As the negative electrode active material, silicon particles with an average particle diameter D 50 of 5 μm were used. A negative electrode including a preliminary negative electrode active material layer containing the negative electrode active material, CMC which is a negative electrode binder, and carbon nanotubes which are a negative electrode conductive material in a weight ratio of 80:10:10 was prepared. The loading amount of the preliminary negative electrode active material layer was 10 mAh / cm 2 and the thickness was 75 μm.

[0118] The positive electrode of Example 1-1, the negative electrode, and a porous polyethylene separator were assembled using a winding method, and an electrolyte (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 3 / 7 (volume ratio), 1 mol of lithium hexafluorophosphate (LiPF6)) was injected into the assembled battery to manufacture a preliminary lithium-ion secondary battery.

[0119] The preliminary lithium-ion secondary battery was charged at 4.2 V at a 0.1C C-rate and then discharged to 2.5 V to perform an activation process.

[0120] Examples 2-2 and 2-3: Manufacturing of a secondary battery A lithium-ion secondary battery was manufactured in the same manner as in Example 2-1, except that the positive electrodes of Examples 1-2 and 1-3 were used instead of the positive electrode of Example 1-1.

[0121] Comparative Examples 2-1 to 2-3: Manufacturing of a secondary battery A lithium-ion secondary battery was manufactured in the same manner as in Example 2-1, except that the positive electrodes of Comparative Examples 1-1 to 1-3 were used instead of the positive electrode of Example 1-1.

[0122] Experimental Example 2: Evaluation of initial efficiency Under the following conditions, the initial efficiencies of the lithium secondary batteries of Examples 2-1 to 2-3 and Comparative Examples 2-1 to 2-3 were evaluated.

[0123] Charging conditions: CC (constant current) / CV (constant voltage) (5 mV / 0.005C current cut-off) 4.2 V Discharging conditions: CC (constant current) condition 2.5 V

[0124] [Table 2]

[0125] Referring to Table 2 above, in the case of Comparative Example 2-1, although the crack rate of the positive electrode active material particles is low and the initial efficiency is excellent, it can be seen that the initial charge capacity is significantly low, which can be said to be a difference due to over-lithiation. Also, in the case of Comparative Example 2-2, over-lithiation is performed, but the degree is severe. Although a high capacity is shown in the initial charge capacity, it is an irreversible capacity that does not recover to the discharge capacity, and it can be confirmed that the effect can rather be reduced in that the lithium loss amount is large. Similarly to Comparative Example 2-2, in Comparative Example 2-3, although the initial capacity is shown to be high due to over-lithiation, it can be seen that the recoverable capacity is small and the lithium loss is large. On the other hand, in the case of Examples 2-1 to 2-3, lithium is replenished by over-lithiation to increase the initial capacity, and it can be confirmed that the lithium loss is small and the capacity can be used at a higher level compared to the comparative examples.

[0126] Experimental Example 3: Evaluation of the life characteristics of a secondary battery Charge and discharge were performed on each of the secondary batteries of Examples 2-1 to 2-3 and Comparative Examples 2-1 to 2-3, and the life characteristics of the secondary batteries were evaluated by the following method, and the results are shown in Table 3 below.

[0127] *Life characteristics (capacity retention rate): For the first and second cycles, charge and discharge were performed at 0.1C. From the third cycle to the 100th cycle, charge and discharge were performed at 0.5C. Charging conditions: CC (constant current) / CV (constant voltage) (5mV / 0.005C current cut-off) 4.2V Discharging conditions: CC (constant current) condition 3.27V The capacity retention rate was derived by the following calculations respectively. Capacity retention rate (%) = (Discharge capacity of the Nth cycle / Discharge capacity of the first cycle) × 100 (where N is 500 and 800)

[0128]

Table 3

[0129] Referring to Table 3 above, it can be confirmed that in the cases of Examples 2-1 to 2-3, the life characteristics are superior compared to Comparative Examples 2-1 to 2-3. In the case of Comparative Example 2-1, the crack rate of the positive electrode active material particles is low, and it shows excellent performance in the initial charge and discharge characteristics. However, the available area of the negative electrode cannot be limited, and it can be confirmed that the life characteristics are at a poor level due to the deterioration of the battery caused by reasons such as the volume expansion of the negative electrode. For Comparative Examples 2-2 and 2-3, it can be confirmed that the crack rate is high and the life is reduced due to side reactions with the electrolyte. Also, in the case of the examples, the capacity retention rate at 800 cycles maintains at about 75% of the level when it is 500 cycles, while in the case of the comparative examples, it can be seen that it does not even maintain at the 70% level. From this, it can be seen that the capacity retention rate itself is even lower and the life is shorter in the comparative examples, and it can also be seen that the rate of capacity deterioration is even faster in the case of the comparative examples.

Explanation of symbols

[0130] 110 Positive current collector 120, 120’ Preliminary positive electrode active material layer 300 Transfer laminate 310 Substrate film 320 Lithium metal layer 330 Polymer layer 400 Positive electrode structure R Roll

Claims

1. A positive electrode comprising a positive electrode active material layer containing a positive electrode active material, wherein the crack rate of the surface portion of the positive electrode active material layer derived from the following Mathematical Formula 1 is 5.0% to 14.2%. [Mathematical Formula 1] CR = (CA) / [(PA)+(CA)] Here, CR is the crack rate (%), CA is the crack region, PA is the particle region, and the surface portion of the positive electrode active material layer means a region from the surface of the layer to a depth of 20 μm in the depth direction.

2. The positive electrode according to Claim 1, further comprising a lithium metal layer located on the positive electrode active material layer.

3. The positive electrode according to Claim 1, further comprising a polymer layer located on the positive electrode active material layer.

4. The positive electrode according to Claim 1, wherein the crack rate of the surface portion of the positive electrode active material layer is 6.0% to 14.1%.

5. The positive electrode according to Claim 1, wherein the crack rate of the surface portion of the positive electrode active material layer is 7.0% to 14.0%.

6. The positive electrode active material is any one or more selected from the group consisting of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel-based composite oxide, lithium manganese-based composite oxide, and lithium transition metal phosphate. The positive electrode according to Claim 1.

7. A P1 step of forming a positive electrode structure by disposing a transfer laminate including a base film and a lithium metal layer located on the base film on a preliminary positive electrode active material layer so that the lithium metal layer is in contact with the preliminary positive electrode active material layer; A P2 step of rolling the positive electrode structure; After the rolling, a P3 step of removing the base film from the transfer laminate to manufacture a positive electrode, The positive electrode includes a positive electrode active material layer containing a positive electrode active material, A method for manufacturing a positive electrode, wherein the crack rate of the surface portion of the positive electrode active material layer derived from the following Mathematical Formula 1 is 5.0% to 14.2%. [Mathematical Formula 1] CR = (CA) / [(PA)+(CA)] Here, CR is the crack rate (%), CA is the crack region, PA is the particle region, and the surface portion of the positive electrode active material layer means a region from the surface of the layer to a depth of 20 μm in the depth direction.

8. The method for manufacturing a positive electrode according to Claim 7, wherein the rolling in the P2 step is performed by a roll-to-roll method.

9. The method for manufacturing a positive electrode according to Claim 7, wherein the thickness of the lithium metal layer is 1 μm to 10 μm.

10. The loading amount of the lithium metal layer is 4% to 40% of the loading amount of the preliminary cathode active material layer. The method for manufacturing a cathode according to claim 7.

11. The transfer laminate further includes a polymer layer, The polymer layer is located between the base film and the lithium metal layer. The method for manufacturing a cathode according to claim 7.

12. After the P2 step, the method for manufacturing a cathode according to claim 7 further includes a P4 step of leaving the preliminary cathode for 1 minute to 600 minutes.

13. During the rolling, the pressure applied to the cathode structure is 10 kgf / cm to 90 kgf / cm. The method for manufacturing a cathode according to claim 7.

14. A lithium secondary battery including a cathode, an anode, and a separator interposed between the cathode and the anode, The cathode includes a cathode active material layer containing a cathode active material, The crack rate of the surface portion of the cathode active material layer derived from the following Mathematical Formula 1 is 5.0% to 14.2%. 【Mathematical Formula 1】 CR = (CA) / [(PA)+(CA)] Here, CR is the crack rate (%), CA is the crack area, PA is the particle area, and the surface portion of the cathode active material layer means the area from the surface of the layer to a depth of 20 μm in the depth direction.

15. The anode includes an anode active material layer, and the anode active material layer contains an anode active material, The anode active material contains a silicon-based anode active material. The lithium secondary battery according to claim 14.

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