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 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 cathode, and a lithium secondary battery including the cathode.

Background Art

[0003] With the rapid increase in the use of fossil fuels, the need for alternative and clean energy has been increasing, and as part of this, the fields of power generation and power storage using electrochemical reactions are the most actively studied.

[0004] Currently, a typical example of an electrochemical device using such electrochemical energy is a secondary battery, and its usage area is 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 includes an anode active material that inserts and desorbs lithium ions coming out of 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 a low price. 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 battery cycles proceed.

[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 silicon-based active material particles. For this purpose, a so-called prelithiation process of pre-inserting lithium ions into the negative electrode containing silicon-based active material particles 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 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. Also, in the process of performing notching and punching on the negative electrode, the possibility of ignition can be further increased due to an 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 pre-insert lithium ions into the negative electrode before battery operation to improve 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 that the present invention aims to solve is to provide a positive electrode and a method for manufacturing the same, which can improve safety during the battery manufacturing process by controlling the usable area of ​​the negative electrode through overlithiation, thereby having a crack rate of a certain amount or more within the surface of the positive electrode.

[0010] Another problem that the present invention aims to solve is to provide a secondary battery that includes the positive electrode, improves lifespan characteristics by controlling the usable region of the negative electrode, and realizes the inherent properties of the negative electrode active material without drawbacks. [Means for solving the problem]

[0011] To solve the above problems, in one aspect of the present invention, a positive electrode is provided which includes 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 refers to the region from the surface of the layer to a depth of 20 μm.

[0012] To solve the above problems, in another aspect of the present invention, a method for manufacturing a positive electrode is provided, comprising: step P1 of arranging a transfer laminate including a base film and a lithium metal layer located on the base film on a pre-positive electrode active material layer to form a positive electrode structure such that the lithium metal layer and the pre-positive electrode active material layer are in contact; step P2 of rolling the positive electrode structure; and step P3 of removing the base film from the transfer laminate after 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 mathematical formula 1, is 5.0% to 14.2%.

[0013] To solve the above problems, in yet another aspect of the present invention, a secondary battery is provided comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode comprises 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%. [Effects of the Invention]

[0014] The positive electrode according to the present invention has the advantage of having a certain amount of cracking on its surface by being overlithified by a specific method of lithium metal transfer. This reduces the usable area of ​​the negative electrode by allowing lithium ions to move to the negative electrode during the activation process, thereby increasing the battery life and reducing process risks that occur when prelithifying the negative electrode.

[0015] Furthermore, the secondary battery according to the present invention, by including the positive electrode, can increase the usable area of ​​the negative electrode, thereby increasing its capacity. In particular, when a silicon-based negative electrode active material is used, only a portion of the usable area can be used by overlithified lithium without lithium loss in the positive electrode, which has the advantage of minimizing volume changes and improving lifespan characteristics. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic diagram showing step P1 in a method for manufacturing a positive electrode according to one embodiment of the present invention. [Figure 2] This is a schematic diagram showing step P1 in a method for manufacturing a cathode according to one embodiment of the present invention, in which a transfer laminate containing a polymer layer is used. [Figure 3] This is a schematic diagram showing step P2 in a method for manufacturing a positive electrode according to one embodiment of the present invention. [Figure 4] This is a schematic diagram showing step P3 in a method for manufacturing a positive electrode according to one embodiment of the present invention. [Modes for carrying out the invention]

[0017] The present invention will be described in more detail below to facilitate understanding of it.

[0018] The terms and words used herein and in the claims should not be interpreted in a manner limited to their ordinary or dictionary meanings, but rather in a manner consistent with the technical idea of ​​the present invention, in accordance with the principle that inventors may define the concepts of terms as appropriate to best describe their invention.

[0019] The terms used herein are for illustrative purposes only and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0020] In this specification, terms such as “includes,” “equip,” or “have” indicate the presence of implemented features, figures, steps, components, or combinations thereof, but should be understood not to preclude the existence or possibility of adding one or more different features, figures, steps, components, or combinations thereof.

[0021] In this specification, D 50 This can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 This can be measured, for example, using the laser diffraction method. The laser diffraction method can generally measure particle sizes from the submicron region to several millimeters in size, and can obtain highly reproducible and high-resolution 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 ratio (%), 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 an 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 an 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 bodies, etc.

[0027] According to an embodiment of the present invention, the positive electrode active material is a particulate substance 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), a lithium nickel cobalt manganese composite oxide represented by; LI[Ni 1-y M 2 y O2 (where 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), a lithium nickel-based oxide represented by; Li 1+z [M 3 1-q M 4 q PO 4-r X r (where 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 the above Chemical Formula 1 can be in a particulate form.

[0039] The compound of the above 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 the above 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 into and desorbed from the inside of the positive electrode active material.

[0040] D of the compound of the aforementioned chemical formula 1 50 The size can be 5 μm to 15 μm, more specifically 7 μm to 12 μm, and more specifically 9 μm to 10 μm. 50 D is a secondary particle. 50 This can be the case. When the above range is satisfied, the dispersion of the positive electrode slurry becomes easier, and a uniform coating of the positive electrode active material layer becomes possible.

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

[0042] According to one embodiment of the present invention, the positive electrode active material layer may further include a positive electrode binder. The positive electrode binder plays a role in improving adhesion between positive electrode active material particles and adhesion 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, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, of which one alone or a mixture of two or more may be used.

[0043] The positive electrode binder can be contained in the positive electrode active material layer in an amount of 0.5% to 5.0% by weight, more specifically 1.0% to 2.5% by weight, and more specifically 1.0% 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 limitations as long as it does not cause a chemical change and has electronic conductivity in the battery in which it is constructed. Specific examples include graphite such as natural graphite or 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, and one of these alone or a mixture of two or more 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% to 30.0% by weight, more specifically 0.5% to 10.0% by weight, and more specifically 1.0% to 4.0% by weight.

[0046] According to one embodiment of the present invention, the positive electrode is characterized in that the crack rate of the surface portion of the positive electrode active material layer, which is 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 region, PA is the particle region, and the surface portion of the positive electrode active material layer refers to the region from the surface of the layer to a depth of 20 μm.

[0049] The crack rate is determined by analyzing the cross-section of the positive electrode, setting a reference region of 20 μm in the depth direction and 200 μm in the surface direction from the surface of the active material layer, and dividing the area of ​​the crack region and particle region within this region. The crack region is the part where the particles are fractured, and the particle region is the area where the particles are not fractured. The electrode is cut by ion milling, and the cut cross-section is photographed with a scanning electron microscope (SEM). Based on digital transformation, the crack region and particle region are quantified from the SEM cross-sectional image, and in this process, the ratio of the region in which the specific surface area increases within the positive electrode by an additional step is calculated and quantified. Furthermore, in order to improve accuracy, more than 50 reference regions are specified for the cross-section of the positive electrode, the crack rate of each region is determined, and the average value is calculated.

[0050] According to one embodiment of the present invention, the crack rate 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 14.1% or less, or 14.0% or less. A crack rate of less than 5.0% means that the lithium ions in the positive electrode active material layer are not sufficiently overlithified, and the amount of lithium that can move to the negative electrode by activation is insufficient, which may make it difficult to increase the usable area of ​​the negative electrode, and thus problems may arise that can reduce life characteristics, such as volume expansion of the negative electrode.

[0051] In other words, during the manufacturing process of 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 containing the lithium metal layer is rolled, and the lithium ions from the lithium metal layer are inserted into the interior of the positive electrode active material layer. During this process, lithium by-products are generated by the lithium ions. Therefore, the fact that the crack rate is 5.0% or more is considered to mean that the lithium metal layer was transferred to and rolled onto the positive electrode active material layer during the manufacturing of the positive electrode of the present invention.

[0052] On the other hand, if the crack rate exceeds 14.2%, the capacity of the positive electrode active material decreases, and side reactions caused by the broken particles can increase battery resistance or gas generation, potentially leading to performance degradation. Therefore, when overlithiation is performed to control the usable area of ​​the negative electrode, it is necessary to ensure that the crack rate meets a specific range through appropriate control.

[0053] According to one 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 plays a role in supplying lithium ions to the positive electrode active material layer. Specifically, the positive electrode active material layer may be located between the positive electrode current collector and the lithium metal layer. The lithium metal layer may be in contact with the positive electrode active material layer. The lithium metal layer includes solid-phase lithium metal, and specifically, the lithium metal layer may consist 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, and as a result, the lithium metal layer may not be present after activation, or it may exist in an extremely thin state.

[0054] According to one 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 serve to ensure that the lithium metal layer is effectively peeled from the transfer laminate and easily transferred to the positive electrode active material layer during the manufacturing of the positive electrode. That is, the polymer layer may be located on the positive electrode active material layer, apart from the transfer laminate, together with the lithium metal layer. The polymer layer may be in contact with the positive electrode active material layer, or, conversely, a lithium metal layer may be present between the polymer layer and the positive electrode active material layer.

[0055] The polymer layer can be at least one selected from the group consisting of polyethylene terephthalate (PET), polyimide (PI), poly(methylmethacrylate) (PMMA), polypropylene, polyethylene, and polycarbonate. This allows the polymer layer to dissolve in the electrolyte contained in the secondary battery, thereby preventing an increase in the battery's resistance. In particular, the polymer layer may contain PMMA, in which case the above-mentioned effects can be further improved.

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

[0057] In one embodiment of the present invention, the positive electrode is formed by a transfer method in which a lithium metal layer is placed on the positive electrode, and the lithium from the lithium metal layer is inserted into the positive electrode active material layer by solid-phase diffusion through rolling. This lithium can move to the negative electrode when the battery is running, thereby reducing the usable area of ​​the negative electrode.

[0058] Therefore, by using overlithified lithium in the positive electrode, the usable region of the negative electrode capacity can be reduced without lithium loss in the positive electrode active material. In particular, excessive volume changes in the silicon-based active material can be suppressed, improving the battery's lifespan. Furthermore, this has the advantage of allowing the entire capacity of the positive electrode active material to be utilized, increasing battery efficiency and maximizing energy density.

[0059] The positive electrode according to one embodiment of the present invention is characterized in that, instead of pre-lithification being performed by bringing a lithium metal layer into contact with the negative electrode as is normally done, a lithium metal layer is transferred and rolled onto the positive electrode, and then the lithium ions inserted into the positive electrode are moved to the negative electrode during the battery activation process.

[0060] In other words, since the negative electrode and the lithium metal layer do not come into contact, and lithium ions are not directly inserted into the negative electrode from the lithium metal layer, the phenomenon of excessive heat generation due to the alloy reaction between lithium and silicon at the negative electrode can be avoided, and the possibility of ignition due to the reaction between lithium and water can be significantly reduced. Furthermore, when notching and punching are performed on the negative electrode, lithium ions are not inserted into the negative electrode (because it is not in a pre-lithified state), so the possibility of ignition during the notching and punching processes can be significantly reduced.

[0061] Method for manufacturing a positive electrode The present invention provides a method for manufacturing a positive electrode, comprising: step P1 of arranging 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; step P2 of rolling the positive electrode structure; and step P3 of removing the base film from the transfer laminate after rolling to manufacture a positive electrode, wherein the positive electrode includes a positive electrode active material layer containing 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%.

[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 refers to the region from the surface of the layer to a depth of 20 μm.

[0064] According to one embodiment of the present invention, step P1 of the method for manufacturing the positive electrode is a step of arranging a transfer laminate including a base film and a lithium metal layer 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.

[0065] Referring to Figure 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 made of a material that can withstand the high temperature conditions that occur during the process of depositing the lithium metal layer 320 onto the base film 310. Specifically, the base film may include one or more materials selected from the group consisting of polyethylene terephthalate (PET), polyimide (PI), poly(methylmethacrylate) (PMMA), polypropylene, polyethylene, and polycarbonate.

[0066] According to one embodiment of the present invention, the lithium metal layer may be located on the substrate film. The lithium metal layer may serve to supply lithium ions to the pre-positive electrode active material layer. The lithium metal layer comprises solid-phase lithium metal, and more specifically, the lithium metal layer may consist 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 2.0 μm or more, 2.5 μm or more, or 3.0 μm or more, and can also be 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 met, the degree of cracking of the positive electrode active material particles on the surface of the positive electrode can be reduced to the extent that it does not affect performance, the application to usable lithium can be controlled to be small, i.e., lithium loss can be controlled to be small, the decrease in the initial capacity of the battery can be suppressed, and the lifespan can be improved by compensating for the irreversible capacity of the negative electrode.

[0068] According to one embodiment of the present invention, the loading amount of the lithium metal layer (unit: mAh / cm²) 2 ) is the loading amount of the pre-positive electrode active material layer (unit: mAh / cm²). 2 This can be 4% to 40%, specifically 12% to 35%, and more specifically 20% to 30%. When this range is satisfied, the generation of by-products is small, lithium can be easily inserted into the positive electrode active material, and the target lithium insertion capacity can be easily achieved.

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

[0070] Here, the positive electrode current collector 110 is the same as the positive electrode current collector described in the above-described embodiment relating to the positive electrode.

[0071] On the other hand, referring to Figure 2, the transfer laminate 300 may further include a polymer layer 330. The polymer layer 330 may be located between the base film 310 and the lithium metal layer 320. The polymer layer can play a role in ensuring that the lithium metal layer is effectively peeled from the transfer laminate and easily transferred to the positive electrode active material layer during the manufacturing of the positive electrode. That is, the polymer layer may be located on the positive electrode active material layer, separate from the transfer laminate together with the lithium metal layer. The polymer layer may be in contact with the positive electrode active material layer, or, conversely, the lithium metal layer may be present between the polymer layer and the positive electrode active material layer.

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

[0073] The thickness of the polymer layer can be 0.1 μm to 10 μm, more specifically 0.5 μm to 5 μm, and more specifically 1 μm to 2.5 μm. When this range is met, 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 one embodiment of the present invention, step P2 of the method for manufacturing the positive electrode is a step of rolling the positive electrode structure.

[0075] Referring to Figure 3, in step P2, the manufactured positive electrode structure 400 can be rolled. The rolling can be performed using a roll press. Specifically, pressure can be applied vertically to the positive electrode structure 400 passing through a space between two rolls R spaced apart with a predetermined space between them in the vertical direction, and this pressure can be linear pressure. Through this rolling process, at least some of the lithium ions from the lithium metal layer contained in the transfer laminate can be inserted into the interior of the pre-positive electrode active material layer. In this process, the pre-positive electrode active material layer can become the positive electrode active material layer. Figures 3 and 4 illustrate the lithium metal layer being included in the positive electrode, but if the entire lithium metal layer is inserted into the pre-positive electrode active material layer during the rolling process, the lithium metal layer may not exist as a separate layer.

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

[0077] When the rolling pressure is applied within the aforementioned 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 be met at an appropriate level. This makes it possible to achieve the effect of simultaneously improving lifespan and capacity characteristics.

[0078] According to one embodiment of the present invention, step P3 of the method for manufacturing the positive electrode is a step of manufacturing the positive electrode by removing the base film from the transfer laminate after rolling.

[0079] Referring to Figure 4, in step P3, after the rolling performed in step P2, 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 polymer layer 330 makes it easier to remove the base film 310.

[0080] Furthermore, according to one embodiment of the present invention, the method for manufacturing the positive electrode may further include a P4 step in which the pre-positive electrode is left to rest (stand) for 1 to 600 minutes, specifically 1 to 30 minutes. The P4 step may be performed after the P2 step. Specifically, the P4 step may be performed in at least one of the following steps: "between the P2 step and the P3 step" and "immediately after the P3 step". The P4 step effectively releases the reaction heat generated by the reaction between the lithium metal layer and the pre-positive electrode active material layer in the P2 step, resulting in uniform insertion of lithium into the positive electrode and a reduction in the generation of by-products.

[0081] Since the description of other manufactured positive electrodes has been given above, it will be omitted here.

[0082] secondary battery The present invention provides a secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode comprises 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%.

[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 refers to the region from the surface of the layer to a depth of 20 μm.

[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) 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 (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 anode active material may further include a carbon-based anode active material. The carbon-based anode active material may include at least 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 contain a negative electrode binder. The negative electrode binder may contain at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which the hydrogen atoms of these are substituted with Li, Na, or Ca, and may also contain various copolymers thereof.

[0092] According to one embodiment of the present invention, the negative electrode active material layer may 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 is conductive, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives can be used.

[0093] According to one embodiment of the present invention, the secondary battery includes a separator. The separator can be any material commonly used as a separator in secondary batteries to separate the negative electrode and the positive electrode and provide a passage for lithium ions, and is not particularly limited. In particular, a material that has low resistance to ion movement of the electrolyte while having excellent electrolyte moisture absorption capacity is preferred. Specifically, porous polymer films, such as porous polymer films made from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or laminated structures of two or more layers thereof can be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. Furthermore, coated separators containing ceramic components or polymeric substances can be used to ensure heat resistance or mechanical strength, and can be selectively used in single-layer or multi-layer structures.

[0094] According to one embodiment of the present invention, the secondary battery may further include an electrolyte. The electrolyte may include, but is not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type 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 non-aqueous organic solvents that can be used include 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, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl pyropionate, and ethyl propionate.

[0096] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, can be preferably used because they are high-viscosity organic solvents with high dielectric constants and readily dissociate lithium salts. Furthermore, when such cyclic carbonates are mixed with low-viscosity, low-dielectric-constant linear carbonates such as dimethyl carbonate and diethyl carbonate in appropriate ratios, electrolytes with high electrical conductivity can be produced, and these can be used even more preferably.

[0097] The metal salt can be a lithium salt, which is a substance that dissolves easily in the non-aqueous electrolyte, for example, the anion of the lithium salt is 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 - You may use one or more selected from the group consisting of the following.

[0098] In addition to the components of the electrolyte, the electrolyte may also contain one or more additives for the purpose of improving the battery's lifespan, suppressing the decrease in battery capacity, and improving the battery's discharge capacity, such as haloalkylene carbonate compounds like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0099] As described above, the secondary battery containing the positive electrode according to the present invention exhibits excellent capacity and stable capacity retention, making it useful in portable devices such as mobile phones, notebook computers, and digital cameras, as well as 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 aforementioned battery module or battery pack can be used as a power source for one or more medium-to-large devices, including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0102] Hereinafter, preferred embodiments are presented to facilitate understanding of the present invention. However, these embodiments are merely illustrative examples, and it will be obvious to those skilled in the art that various changes and modifications are possible within the scope of the present description and the technical concept. It goes without saying that such variations and modifications fall within the scope of the appended claims.

[0103] electrode manufacturing Example 1-1: Manufacturing of a positive electrode A PET film (base film), a polymer layer made of PMMA (2.5 μm thick) disposed on the PET film, and a lithium metal layer made of solid-phase lithium metal (3.0 μm thick, 0.62 mAh / cm²) disposed on the polymer layer. 2 A transfer laminate containing the loading amount 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 positive electrode active material was Li[Ni 0.86 Co 0.05 Mn 0.08 Al 0.01 Multiple (10 or more) primary O2 particles combine with each other to form secondary particles, and the average particle size D of the secondary particles 50 The thickness was 9 μm. A positive electrode was prepared containing a preliminary positive electrode active material layer containing the positive electrode active material, PVdF as a positive electrode binder, and carbon nanotubes as a positive electrode conductive material in a weight ratio of 98:1:1, and aluminum foil (thickness: 12 μm) as a positive electrode current collector. The loading amount of the preliminary positive electrode active material layer was 4.5 mAh / cm². 2 The thickness was 140 μm.

[0105] The transfer laminate is placed in the pre-positive electrode active material layer so that the lithium metal layer and the pre-positive electrode active material layer are in contact.

[0106] Subsequently, the positive electrode on which the transfer laminate was placed was rolled using a roll press method, and then the positive electrode was left to stand for 24 hours. The pressure during the rolling was 20 kgf / cm. As a result, lithium ions from the lithium metal layer were inserted into the preliminary positive electrode active material layer, forming the positive electrode active material layer. Next, the base film was removed and left for 10 minutes to produce a positive electrode containing the positive electrode current collector, positive electrode active material layer, and polymer layer.

[0107] Examples 1-2: Manufacturing of a positive electrode The lithium metal layer thickness is 6.2 μm, and the loading capacity is 1.28 mAh / cm². 2 Except for the above, the positive electrode was manufactured in the same manner as in Example 1-1.

[0108] Examples 1-3: Manufacturing of a positive electrode The positive electrode was manufactured using the same method as in Example 1-2, except that the rolling pressure was set to 30 kgf / cm.

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

[0110] Comparative Example 1-2: Manufacturing of the positive electrode The lithium metal layer thickness is 11.6 μm, and the loading capacity is 1.90 mAh / cm². 2 Except for the above, the positive electrode was manufactured in the same manner as in Example 1-1.

[0111] Comparative Example 1-3: Manufacturing of the positive electrode The positive electrode was manufactured using the same method as in Example 1-2, except that the rolling pressure was set to 100 kgf / cm.

[0112] Experimental Example 1: Evaluation of the physical properties of the positive electrode The crack rate (%) of the positive electrodes manufactured in Examples 1-1 to 1-3 and Comparative Examples 1-1 to 1-3 was measured using the following method.

[0113] One day after the completion of manufacturing each of the aforementioned positive electrodes, the cross-section of the positive electrode was analyzed. A reference region was set at 20 μm in the depth direction and 200 μm in the plane direction from the surface of the active material layer. The crack region and particle region within this region were separated and their areas were determined. The crack region was defined as the part where the particles were broken, and the particle region was defined as the part where the particles were not broken. Images were obtained by scanning electron microscopy (SEM) of the cross-section of the electrode, which was cut by ion milling.

[0114] Based on the aforementioned SEM cross-sectional image, crack regions and particle regions were quantified using digital transformation. In addition, the ratio of regions where the specific surface area increases within the cathode was calculated and quantified through an additional process. Furthermore, to improve accuracy, more than 50 reference regions were specified for the cross-section of the cathode, and the crack rate of each region was determined, and its average value was calculated.

[0115] [Table 1]

[0116] As shown in Table 1 above, in Examples 1-1 to 1-3, the insertion of lithium into the positive electrode due to overlithiation can be confirmed by the crack rate, and from Comparative Example 1-1, it can be confirmed that there is a difference in the crack rate between the case with overlithiation and the case without. On the other hand, in the case of Comparative Example 1-2, it can be confirmed that the thickness of the lithium layer was excessively thick during lithium transfer, and in the case of Comparative Example 1-3, it can be confirmed that the degree of cracking of the positive electrode active material particles was excessive as a result of slightly increasing the pressure during rolling.

[0117] Manufacturing of rechargeable batteries Example 2-1: Manufacturing of a secondary battery As the negative electrode active material, the average particle size D 50 Silicon particles with a diameter of 5 μm were used. A negative electrode was prepared containing a preliminary negative electrode active material layer containing the aforementioned negative electrode active material, CMC as a negative electrode binder, and carbon nanotubes as a negative electrode conductive material in a weight ratio of 80:10:10. The loading amount of the preliminary negative electrode active material layer was 10 mAh / cm². 2 The thickness was 75 μm.

[0118] The positive electrode, negative electrode, and porous polyethylene separator of Example 1-1 were assembled using a winding method, and an electrolyte (ethylene carbonate (EC) / ethyl methyl carbonate (EMC) = 3 / 7 (volume ratio) and lithium hexafluorophosphate (LiPF 61 moles)) was injected into the assembled battery to produce a preliminary lithium-ion secondary battery.

[0119] The aforementioned spare lithium-ion secondary battery was charged at 4.2V with a C-rate of 0.1C, and then discharged to 2.5V 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 Secondary Batteries 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 The initial efficiency of the lithium secondary batteries of Examples 2-1 to 2-3 and Comparative Examples 2-1 to 2-3 was evaluated under the following conditions.

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

[0124] [Table 2]

[0125] Referring to Table 2 above, in the case of Comparative Example 2-1, the crack rate of the positive electrode active material particles is low and the initial efficiency is excellent, but the initial charge capacity is remarkably low, which can be attributed to the difference in whether or not overlithiation is performed. In the case of Comparative Example 2-2, although overlithiation is performed, the degree is severe and although it shows a high initial charge capacity, it is an irreversible capacity that does not recover to discharge capacity, and it can be confirmed that the effect may actually be reduced in terms of the large amount of lithium loss. In Comparative Example 2-3, similar to Comparative Example 2-2, the initial capacity is shown to be high due to overlithiation, but it can be seen that the recovered capacity is small and lithium loss is large. On the other hand, in the cases of Examples 2-1 to 2-3, lithium is replenished by overlithiation to increase the initial capacity, and it can be confirmed that 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 The secondary batteries of Examples 2-1 to 2-3 and Comparative Examples 2-1 to 2-3 were each subjected to charging and discharging, and the life characteristics of the secondary batteries were evaluated using the method described below. The results are shown in Table 3 below.

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

[0128] [Table 3]

[0129] Referring to Table 3 above, it can be confirmed that Examples 2-1 to 2-3 exhibit superior lifespan characteristics 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, showing excellent initial charge-discharge characteristics. However, it is not possible to limit the usable area of ​​the negative electrode, and it can be confirmed that the battery deteriorates due to reasons such as volume expansion of the negative electrode, resulting in poor lifespan characteristics. In Comparative Examples 2-2 and 2-3, the crack rate is high, and it can be confirmed that the lifespan is reduced due to side reactions with the electrolyte. Furthermore, in the case of the examples, the capacity retention rate at 800 cycles is maintained at approximately 75% of the level at 500 cycles, while in the case of the comparative examples, it can be seen that it is not even maintained at 70%. This shows that the capacity retention rate itself is even lower and the lifespan is shorter in the comparative examples, and that the rate of capacity deterioration is also faster in the comparative examples. [Explanation of Symbols]

[0130] 110 Positive electrode current collector 120, 120' Pre-existing positive electrode active material layer 300 Transfer Layers 310 Base film 320 Lithium metal layer 330 Polymer layer 400 Positive Electrode Structure R Roll

Claims

1. It comprises a positive electrode active material layer containing a positive electrode active material, and a lithium metal layer located on the positive electrode active material layer. 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%. [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 refers to the region from the surface of the layer to a depth of 20 μm.

2. The positive electrode according to claim 1, wherein the positive electrode further comprises a polymer layer located on the positive electrode active material layer.

3. 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%.

4. 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%.

5. The positive electrode according to any one of claims 1 to 4, wherein the positive electrode active material is 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.

6. Step P1 involves arranging a transfer laminate including a base film and a lithium metal layer located on the base film on a pre-positive electrode active material layer to form a positive electrode structure such that the lithium metal layer and the pre-positive electrode active material layer are in contact; Step P2 involves rolling the positive electrode structure, The process includes step P3, in which the base film is removed from the transfer laminate after the rolling process to produce 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 refers to the region from the surface of the layer to a depth of 20 μm.

7. The method for manufacturing a positive electrode according to claim 6, wherein the rolling in step P2 is carried out by a roll-to-roll method.

8. The method for manufacturing a positive electrode according to claim 6, wherein the thickness of the lithium metal layer is 1 μm to 10 μm.

9. The method for manufacturing a positive electrode according to claim 6, wherein the loading amount of the lithium metal layer is 4% to 40% of the loading amount of the preliminary positive electrode active material layer.

10. The transfer laminate further comprises a polymer layer, The method for manufacturing a positive electrode according to claim 6, wherein the polymer layer is located between the base film and the lithium metal layer.

11. A method for manufacturing a positive electrode according to any one of claims 6 to 10, further comprising a P4 step of leaving the preliminary positive electrode active material layer for 1 minute to 600 minutes after the P2 step.

12. The method for manufacturing a positive electrode according to claim 6, wherein the pressure applied to the positive electrode structure during rolling is 10 kgf / cm to 90 kgf / cm.

13. It 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 and a lithium metal layer located on the positive electrode active material. A lithium secondary battery 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%. [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 refers to the region from the surface of the layer to a depth of 20 μm.

14. The aforementioned negative electrode includes a negative electrode active material layer, and the aforementioned negative electrode active material layer includes a negative electrode active material. The lithium secondary battery according to claim 13, wherein the negative electrode active material includes a silicon-based negative electrode active material.

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