Method for manufacturing positive electrode for lithium secondary battery and positive electrode

Abstract

This document discloses a method for manufacturing a positive electrode for a lithium secondary battery, comprising: (S1) forming a positive electrode active material layer by applying a positive electrode slurry composition comprising lithium iron phosphate and a binder to a current collector and drying the positive electrode slurry composition; (S2) rolling the positive electrode active material layer N times (N is an integer greater than or equal to 2), wherein during the rolling steps, during the first rolling, the thickness change rate of the positive electrode active material layer is 5% to 15% according to Equation 1 below, and during rolling after the first rolling, the thickness change rate of the positive electrode active material layer is 3.5% or less according to Equation 1 below. [Equation 1] Thickness change rate (%) = {(K-1) times the thickness of the positive electrode active material layer rolled - K times the thickness of the positive electrode active material layer rolled} × 100 / the thickness of the positive electrode active material layer before the rolling step. In Equation 1, K is an integer greater than or equal to 1 and less than or equal to N.

Description

Technical Field

[0001] This application claims priority to Korean Patent Application No. 10-2021-0187036, filed on December 24, 2021.

[0002] The present invention relates to a method for manufacturing a positive electrode for a lithium secondary battery and a positive electrode, and more specifically, to a method for manufacturing a positive electrode for a lithium secondary battery by multiple rolling processes and a positive electrode manufactured by the method. Background Technology

[0003] With the increasing demand for electric vehicles and energy storage systems (ESS), and the development of related technologies, the demand for batteries as energy sources is rapidly increasing. Therefore, various studies have been conducted on batteries capable of meeting diverse needs. In particular, active research has been carried out on lithium-ion secondary batteries with high energy density, excellent lifespan, and superior cycle characteristics as power sources for such devices.

[0004] Lithium cobalt oxide (LCO), lithium nickel cobalt manganese oxide (LNCMO), lithium iron phosphate (LFP), and similar materials are used as positive electrode active materials for lithium secondary batteries.

[0005] Lithium iron phosphate (LFP) is inexpensive because it contains iron, which is an abundant and inexpensive resource. Furthermore, its low toxicity reduces environmental pollution. Additionally, due to its olivine structure, LFP maintains its active material structure stably at high temperatures compared to lithium transition metal oxides with layered structures. Therefore, the advantages lie in improved high-temperature stability and battery life.

[0006] However, due to the non-spherical shape of lithium iron phosphate (LFP), it is difficult to manufacture high-density cathodes due to its low tap density. To address this issue, a high rolling rate is required during the rolling of the cathode slurry composition onto the cathode current collector. However, when rolling the cathode slurry composition at a high rolling rate, a problem arises: the cathode active material layer can detach from the cathode current collector due to differences in mechanical properties (e.g., elongation, Poisson's ratio, etc.) between the cathode current collector and the cathode active material layer.

[0007] Therefore, traditionally, in order to obtain a high-density cathode while preventing the deintercalation of the cathode active material layer, an increased amount of binder has been used in the cathode slurry composition. However, when the binder content in the cathode slurry composition increases, the resistance of the lithium secondary battery increases, and the content of the cathode active material relatively decreases, thus leading to a reduction in the battery capacity of the cathode.

[0008] Therefore, when manufacturing cathodes including lithium iron phosphate, it is necessary to develop a technology for manufacturing high-density cathodes while preventing the deintercalation of the cathode active material layer. Summary of the Invention

[0009] [Technical Issues]

[0010] With regard to the method for manufacturing a positive electrode for lithium secondary batteries including lithium iron phosphate, the present invention aims to provide a method for manufacturing a high-density positive electrode while preventing the positive electrode active material layer from being deintercalated from the positive electrode current collector.

[0011] [Technical Solution]

[0012] According to an exemplary embodiment of the present invention, a method for manufacturing a positive electrode for a lithium secondary battery and a positive electrode manufactured according to the manufacturing method are provided, the method comprising: (S1) forming a positive electrode active material layer by applying a positive electrode slurry composition comprising lithium iron phosphate and a binder to a current collector and drying the positive electrode slurry composition; (S2) rolling the positive electrode active material layer N times (N is an integer greater than or equal to 2), wherein in the rolling steps, during the first rolling, the thickness change rate of the positive electrode active material layer according to Equation 1 below is 5% to 15%, and during the rolling after the first rolling, the thickness change rate of the positive electrode active material layer according to Equation 1 below is 3.5% or less.

[0013] [Equation 1]

[0014] Thickness change rate (%) = {thickness of the positive electrode active material layer after (K-1) rolling steps - thickness of the positive electrode active material layer after K rolling steps} × 100 / thickness of the positive electrode active material layer before rolling steps

[0015] In Equation 1, K is an integer greater than or equal to 1 and less than or equal to N.

[0016] In an exemplary embodiment of the present invention, the rolling step can be performed by multi-stage rolling of 3 to 6 times.

[0017] In an exemplary embodiment of the present invention, after the rolling step, the porosity of the positive electrode active material layer can be 28% to 36%.

[0018] In an exemplary embodiment of the present invention, after the rolling step, the thickness of the positive electrode active material layer can be 85 μm to 95 μm.

[0019] In an exemplary embodiment of the present invention, after the rolling step, the rolling rate of the positive electrode active material layer can be 20% to 26%.

[0020] In an exemplary embodiment of the present invention, the porosity of the dried positive electrode slurry composition may be 55% or less.

[0021] In an exemplary embodiment of the present invention, after the first rolling step, the rolling rate of the positive electrode active material layer can be 10% or more.

[0022] In an exemplary embodiment of the present invention, the rolling step can be performed by a roll pressing method.

[0023] In an exemplary embodiment of the present invention, lithium iron phosphate may be a compound represented by Formula 1.

[0024] [Formula 1]

[0025] Li 1+a Fe 1-x M x (PO 4-b )X b

[0026] (In Equation 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are -0.5≤a≤0.5, 0≤b≤0.1, and 0≤x≤0.5, respectively.)

[0027] In an exemplary embodiment of the present invention, lithium iron phosphate may be LiFePO4 having an olivine crystal structure.

[0028] In an exemplary embodiment of the present invention, lithium iron phosphate may be included in the solid components of the cathode slurry composition in an amount of 94.90% to 97.96% by weight.

[0029] In an exemplary embodiment of the present invention, the adhesive may be included in the solid components of the positive electrode slurry composition in an amount of 0.5% to 3.5% by weight.

[0030] [Beneficial Effects]

[0031] According to the present invention, when a cathode slurry containing lithium iron phosphate is rolled multiple times, during the rolling period after the first rolling, the thickness change rate of the cathode active material layer is 3.5% or less, effectively suppressing the deintercalation of the cathode active material layer from the cathode current collector. As a result, a high-density cathode with a significantly reduced electrode defect rate can be manufactured.

[0032] Furthermore, using the above method, even with a low binder content in the positive electrode slurry composition, a high-density positive electrode with improved adhesion (hereinafter referred to as positive electrode adhesion) between the positive electrode current collector and the positive electrode active material layer can be manufactured. Moreover, due to the low binder content in the positive electrode active material layer, the resistance of the lithium secondary battery can be reduced, and the content of the positive electrode active material in the positive electrode active material layer can be relatively increased, thereby improving the battery capacity of the positive electrode. Attached Figure Description

[0033] Figure 1 This is a photograph of the positive electrode prepared in Example 1.

[0034] Figure 2 This is a photograph of the positive electrode prepared in Comparative Example 1.

[0035] Figure 3 This is a photograph of the positive electrode prepared in Comparative Example 2.

[0036] Figure 4 This is a photograph of the positive electrode prepared in Comparative Example 3. Detailed Implementation

[0037] The advantages and features of the present invention, as well as methods for implementing them, will become clear from the following detailed description of embodiments in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below and can be implemented in various different forms. These embodiments are provided merely to complete the disclosure of the invention and to fully inform those skilled in the art of the scope of the invention, which is defined only by the scope of the claims. Throughout this specification, similar reference numerals refer to similar elements.

[0038] Unless otherwise defined, all terms used in this specification (including technical and scientific terms) are to be used in the sense commonly understood by one of ordinary skill in the art to which this invention pertains. Furthermore, unless explicitly and specifically defined, terms as defined in common dictionaries are not to be interpreted ideally or excessively.

[0039] The terminology used in this specification is for describing various embodiments and is not intended to limit the invention. In this specification, singular forms also include plural forms unless otherwise specified in the phrase. The terms "comprises" and / or "comprising" as used herein do not exclude the presence or addition of one or more other elements besides those described.

[0040] In this specification, when a component includes a certain element, unless otherwise stated, this means that it may further include other elements without excluding other components.

[0041] In this specification, the description of "A and / or B" means A, or B, or A and B.

[0042] In this specification, unless otherwise expressly stated, “%” refers to weight%.

[0043] In this specification, D 50 This refers to the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. For example, D can be measured using the laser diffraction method. 50 Laser diffraction can typically measure particle sizes from the submicron level to several millimeters, and can obtain results with high reproducibility and high resolution.

[0044] In this specification, multi-stage rolling refers to multiple rolling processes compared to single rolling, and the first rolling process refers to the first rolling process performed on the positive electrode active material layer formed by drying the positive electrode slurry composition.

[0045] In this specification, the rate of change of thickness (%) can be calculated according to Equation 1 below.

[0046] [Equation 1]

[0047] Thickness change rate (%) = {thickness of the positive electrode active material layer after (K-1) rolling steps - thickness of the positive electrode active material layer after K rolling steps} × 100 / thickness of the positive electrode active material layer before rolling steps

[0048] In Equation 1, K is an integer greater than or equal to 1 and less than or equal to N, and N is an integer greater than or equal to 2.

[0049] In this specification, the rolling rate (%) can be calculated according to Equation 2 below.

[0050] [Equation 2]

[0051] Rolling yield (%) = {(thickness of the positive electrode active material layer before rolling - thickness of the positive electrode active material layer after K rolling passes) / thickness of the positive electrode active material layer before rolling} × 100

[0052] In this specification, porosity can be calculated according to Equation 3 below.

[0053] [Equation 3]

[0054] Porosity = {1 - (density of the cathode active material layer after K rolling cycles / density of the ideal cathode active material layer)} × 100

[0055] Here, "the density of the ideal positive electrode active material layer" refers to the density of a virtual positive electrode active material layer filled with positive electrode active material, conductive material, binder, etc., in a positive electrode where lithium iron phosphate, as the positive electrode active material, is 95% to 100%, without any voids, and is 3.39 g / cc in the embodiments of the present invention. Furthermore, the porosity can vary depending on the mixing ratio between the positive electrode active material, conductive material, binder, etc., but its value is known to be in the range of 3.3 g / cc to 3.5 g / cc.

[0056] In this specification, "after the rolling step" means after all rolling processes consisting of multiple steps have been completed.

[0057] The invention will be described in more detail below.

[0058] A method for manufacturing a positive electrode for lithium secondary batteries

[0059] The method of the present invention for manufacturing a positive electrode for a lithium secondary battery includes: (S1) forming a positive electrode active material layer by applying a positive electrode slurry composition comprising lithium iron phosphate and a binder to a current collector and drying the positive electrode slurry composition; (S2) rolling the positive electrode active material layer N times (N is an integer greater than or equal to 2).

[0060] In this case, during the first rolling, the thickness change rate of the positive electrode active material layer according to Equation 1 below is 5% to 15%, and during the rolling after the first rolling, the thickness change rate of the positive electrode active material layer according to Equation 1 below is 3.5% or less.

[0061] [Equation 1]

[0062] Thickness change rate (%) = {thickness of the positive electrode active material layer after (K-1) rolling steps - thickness of the positive electrode active material layer after K rolling steps} × 100 / thickness of the positive electrode active material layer before rolling steps

[0063] In Equation 1, K is an integer greater than or equal to 1 and less than or equal to N.

[0064] Because lithium iron phosphate has a non-spherical shape and therefore a low tap density, a high rolling rate is required during the cathode slurry rolling process to obtain a high-density cathode. However, when the cathode slurry is rolled at a high rolling rate, a problem arises: the cathode active material layer may deintercalate due to the mechanical property differences between the cathode current collector and the cathode active material layer.

[0065] As a result of repeated research to solve this problem, the inventors have discovered that when a cathode slurry containing lithium iron phosphate is rolled multiple times, a thickness change rate of 3.5% or less after two rolls can prevent the cathode active material layer from deintercalating from the cathode current collector, thus completing this invention. In this invention, when rolling is performed such that the thickness change rate between two rolls is 3.5% or less, a high-density cathode with excellent cathode adhesion can be manufactured even when the binder content in the cathode slurry composition is relatively reduced. This will be explained in detail below.

[0066] (1) Application and drying of the positive electrode slurry composition

[0067] The method for manufacturing a positive electrode for a lithium secondary battery according to the present invention may include applying a positive electrode slurry composition to a positive electrode current collector and drying the positive electrode slurry composition.

[0068] A cathode slurry composition can be prepared by mixing or dispersing the positive electrode active material and binder in a solvent. Furthermore, in addition to the positive electrode active material and binder, a conductive material and dispersant can be additionally mixed or dispersed in a solvent as needed to prepare the cathode slurry composition.

[0069] The positive electrode current collector can be any material that is conductive without causing chemical changes in the battery, and there are no particular limitations. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with surface treatments of carbon, nickel, titanium, silver, and the like can be used as current collectors.

[0070] The positive electrode current collector can have a thickness from 3 μm to 500 μm, and the adhesion to the positive electrode active material layer can be increased by forming micron / nano-level irregularities on the surface of the positive electrode current collector. For example, the positive electrode current collector can be used in various forms such as films, sheets, foils, meshes, porous materials, foams, nonwoven fabrics, and the like.

[0071] The positive electrode active material according to the present invention may include lithium iron phosphate. Because lithium iron phosphate has an olivine structure, it stably maintains its active material structure at high temperatures compared to lithium transition metal oxides with layered structures. As a result, when lithium iron phosphate is used as the positive electrode active material, the high-temperature stability and high-temperature lifetime characteristics of the positive electrode are significantly improved, thereby reducing the risk of fire in lithium secondary batteries including said positive electrode.

[0072] Lithium iron phosphate can be a compound represented by the following formula 1.

[0073] [Formula 1]

[0074] Li 1+a Fe 1-x M x (PO4-b )X xb

[0075] (In Equation 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn, and Y; X includes one or more elements selected from the group consisting of F, S, and N; and a, b, and x are -0.5 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.1, and 0 ≤ x ≤ 0.5, respectively.)

[0076] For example, lithium iron phosphate can be LiFePO4 with an olivine crystal structure.

[0077] Furthermore, lithium iron phosphate can have a monolithic structure composed of primary particles. In this invention, a "monolithic structure" refers to a structure in which the particles exist as independent phases, and the particles do not aggregate with each other in the morphological phase. In contrast to this monolithic structure, particle structures can be referred to as those in which small-sized particles ("primary particles") physically and / or chemically aggregate to form relatively large-sized particles ("secondary particles").

[0078] When lithium iron phosphate (LFP) has a monolithic structure composed of primary particles, the likelihood of LFP particle breakage during rolling is lower compared to when it is composed of secondary particles. This is preferable because the capacity reduction due to the deintercalation of broken particles is smaller. Furthermore, when LFP is composed of primary particles with a monolithic structure, binder migration can be reduced during the drying process of the cathode slurry, which is preferable in terms of interfacial adhesion between the cathode current collector and the cathode active material layer.

[0079] Lithium iron phosphate can have a carbon coating on its surface. When a carbon coating is formed on the surface of lithium iron phosphate, conductivity can be improved, thereby improving the resistive characteristics of the positive electrode.

[0080] The carbon coating can be formed using at least one raw material selected from the group consisting of glucose, sucrose, lactose, starch, oligosaccharides, polyoligosaccharides, fructose, cellulose, furfuryl alcohol, block copolymers of ethylene and ethylene oxide, vinyl resins, cellulose resins, phenolic resins, pitch-based resins, and tar-based resins. Specifically, the carbon coating can be formed by a heat treatment process after mixing the raw material with lithium iron phosphate.

[0081] The average particle size D of lithium iron phosphate 50 The particle size can range from 0.5 μm to 20.0 μm, preferably from 0.5 μm to 10.0 μm, more preferably from 0.6 μm to 3 μm, and even more preferably from 0.6 μm to 2.5 μm. When the average particle size D of the positive electrode active material...s0 When the above range is met, the lithium mobility in lithium iron phosphate increases, thus improving the battery's charge / discharge characteristics.

[0082] The BET specific surface area of ​​lithium iron phosphate can be 5m². 2 / g to 20m 2 / g, specifically 7m 2 / g to 18m 2 / g, and more specifically 9m 2 / g to 16m 2 / g. This range corresponds to a lower value compared to conventional lithium iron phosphate. When the above range is met, the aggregation of lithium iron phosphate can be effectively suppressed even in cathode slurry compositions with relatively low dispersant content.

[0083] Lithium iron phosphate can be included in the solids composition of the cathode slurry composition in an amount of 94.90% to 97.96% by weight, specifically 95.40% to 97.68% by weight, and more specifically 95.96% to 97.55% by weight. When the content of lithium iron phosphate meets the above range, the battery capacity of the cathode can be improved by maintaining sufficient cathode energy density.

[0084] Adhesives serve to facilitate the bonding between the positive electrode active material and conductive materials, as well as the bonding with the current collector. Specific examples include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, or various copolymers thereof and the like, and mixtures of one, two, or more of these may be used.

[0085] In the process of preparing the cathode slurry, stirring involves a mixture of cathode active material, binder, conductive material, dispersant, and solvent, and shear force is applied to the cathode slurry during the stirring process. Here, when the carbon coating formed on the surface of lithium iron phosphate is coupled with the functional groups of the binder, gelation of the cathode slurry composition may occur during the cathode slurry manufacturing process. This gelation may be enhanced as the size of lithium iron phosphate particles becomes smaller.

[0086] To prevent the positive electrode slurry from gelling, polyvinylidene fluoride preferably satisfies the following equation 4.

[0087] [Equation 4]

[0088] 0 ≤ {(2A+B) / (C+D)} × 100 < 0.2

[0089] (A, B, C, and D represent the conditions for processing polyvinylidene fluoride.) 1 (Integrated area of ​​each peak appearing at 11.5 ppm to 12.8 ppm, 3.9 ppm to 4.2 ppm, 2.6 ppm to 3.2 ppm, and 2.1 ppm to 2.35 ppm during H-NMR measurements)

[0090] In this context, the range of 11.5 ppm to 12.8 ppm represents the concentration of COOH functional groups included in polyvinylidene fluoride (PVdF). 1 The H-NMR peak region, from 3.9 ppm to 4.2 ppm, represents the OCH2 functional groups included in polyvinylidene fluoride (PVdF). 1 The H-NMR peak region. Furthermore, the range of 2.6 ppm to 3.2 ppm represents the head-to-head coupling of polyvinylidene fluoride (PVdF) monomers. 1 The H-NMR peak region, ranging from 2.1 ppm to 2.35 ppm, represents the head-to-tail coupled polyvinylidene fluoride (PVdF) monomer. 1 H-NNMR peak region.

[0091] Meanwhile, the fact that polyvinylidene fluoride (PVdF) satisfies Equation 4 means that PVdF contains a relatively small number of polar functional groups (such as COOH and OCH2).

[0092] When the polyvinylidene fluoride in the binder contained in the cathode slurry does not satisfy Equation 4, many hydrogen bonds are formed between the functional groups (e.g., COOH, OCH2) in the binder and the hydrogen on the carbon coating, resulting in possible gelation of the cathode slurry composition.

[0093] In contrast, when the polyvinylidene fluoride in the binder contained in the cathode slurry satisfies Equation 4, the number of hydrogen bonds between the functional groups and the hydrogen on the carbon coating is reduced due to the presence of fewer functional groups in the binder. Therefore, gelation of the cathode slurry is prevented, the coating processability of the cathode slurry is improved, and the thickness and / or surface of the coated cathode active material layer can be uniformly formed.

[0094] In particular, the effect of preventing gelation of the cathode slurry is more significant when lithium iron phosphate is used as the positive electrode active material. Specifically, because lithium iron phosphate has a smaller average particle size and a larger specific surface area than traditional positive electrode active materials such as lithium nickel cobalt manganese oxide, the number of sites where hydrogen bonding can occur increases, making gelation more likely. Therefore, when the polyvinylidene fluoride in the binder contained in the positive electrode slurry satisfies Equation 4, the possibility of gelation in the positive electrode slurry using lithium iron phosphate as the positive electrode active material can be greatly reduced.

[0095] Furthermore, the polyvinylidene fluoride (PVDF) polymer binder tends to bond with lithium iron phosphate (LFP) more strongly than with the cathode current collector. Moreover, since the bonding strength between PVDF satisfying Equation 4 and LFP is lower than that between PVDF not satisfying Equation 4 and LFP, the interfacial adhesion between the cathode active material layer and the current collector can be improved.

[0096] Preferably, the binder can be a homopolymer of polyvinylidene fluoride (PVDF). For example, when the binder is a PVDF homopolymer, hydrogen bonds are not formed between the carbon coating and the binder because there are no polar functional groups in the binder, thus preventing gelation of the cathode slurry composition.

[0097] The weight-average molecular weight of the binder can be from 20,000 g / mol to 1,200,000 g / mol, specifically from 100,000 g / mol to 1,000,000 g / mol, and more specifically from 400,000 g / mol to 980,000 g / mol. When the weight-average molecular weight of the binder meets the above range, the positive electrode slurry composition can have a viscosity suitable for the coating process. Therefore, it is desirable that the uniformity of the positive electrode active material layer formed by the composition is maintained and the positive electrode adhesion is improved.

[0098] According to an exemplary embodiment of the invention, the adhesive may be included in the solid components of the positive electrode slurry composition in an amount of 0.5% to 3.5% by weight, specifically 0.5% to 3.0% by weight, and more specifically 0.5% to 1.5% by weight.

[0099] According to the present invention, when a cathode slurry containing lithium iron phosphate is rolled multiple times, the thickness change rate in the second rolling process is 3.5% or less. Even if the binder content in the cathode slurry composition is at a low level, a high-density cathode with excellent cathode adhesion can be manufactured. Furthermore, since the binder content in the cathode active material layer is low, the resistance of the lithium secondary battery can be reduced, and the content of the cathode active material in the cathode active material layer can be relatively increased, thereby improving the battery capacity of the cathode.

[0100] The conductive material is not particularly limited, as long as it is conductive and does not cause chemical changes in the battery. Examples include: graphite; carbon black, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lampblack, summer black, and the like; conductive fibers, such as carbon fibers, metal fibers, and the like; metal powders, such as fluorinated carbon powder, aluminum powder, and nickel powder and the like; conductive whiskers, such as zinc oxide, potassium titanate, and the like; conductive metal oxides, such as titanium oxide and the like; and conductive materials, such as polyphenylene derivatives and the like. Among these, carbon nanotubes, carbon nanofibers, and carbon black are preferred as the conductive material of the present invention, with carbon nanotubes being the most preferred. Since the conductive network of carbon nanotubes can reduce the migration of the binder during the drying process of the positive electrode slurry composition, it is most preferred as the conductive material included in the positive electrode of the present invention.

[0101] Carbon nanotubes possess an sp2 coupled structure and consist of cylindrical graphite sheets with a diameter of nanometers. They exhibit conductive or semiconductor properties depending on the rolling angle and structure of the graphite sheets. Based on the number of couplings forming the walls, carbon nanotubes can be classified into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes, and these types of carbon nanotubes can be appropriately selected according to the intended use of the dispersion.

[0102] Furthermore, carbon nanotubes can have secondary shapes formed by the aggregation or arrangement of multiple carbon nanotubes. For example, it can be a bundle-type carbon nanotube in which multiple carbon nanotubes are arranged side by side or aligned in a specific direction, or an entangled-type carbon nanotube in which multiple carbon nanotubes are entangled in a spherical or potato-shaped manner without a specific orientation. In terms of dispersibility, bundle-type carbon nanotubes are preferred.

[0103] The BET specific surface area of ​​carbon nanotubes can be 100 m². 2 / g to 1000m 2 / g, 150m 2 / g to 800m 2 / g, 150m 2 / g to 500m 2 / g, 150m 2 / g to 300m 2 / g, or 150m 2 / g to 200m 2 / g.

[0104] When the conductive material is carbon nanotubes, the conductive material can be included in the solid component of the positive electrode slurry composition in an amount of 1.5% by weight or less, specifically from 0.5% to 1.0% by weight, and more specifically from 0.6% to 1.0% by weight. When the content of the conductive material in the solid component of the positive electrode slurry composition meets the above range, the conductivity of the positive electrode can be improved by maintaining the positive electrode conductive network.

[0105] The dispersant inhibits the excessive aggregation of lithium iron phosphate in the positive electrode slurry composition and enables lithium iron phosphate to be effectively dispersed and present in the prepared positive electrode active material layer.

[0106] The dispersant may include hydrogenated nitrile copolymers, and specifically, the dispersant may be a hydrogenated nitrile copolymer.

[0107] Specifically, the hydrogenated nitrile copolymer is a copolymer comprising structural units derived from α,β-unsaturated nitriles and structural units derived from hydrogenated conjugated dienes, or a copolymer comprising structural units derived from α,β-unsaturated nitriles, structural units derived from conjugated dienes, and structural units derived from hydrogenated conjugated dienes. As α,β-unsaturated nitrile monomers, for example, acrylonitrile or methacrylonitrile, and the like, and one or a mixture of two or more of these monomers may be used. As conjugated diene monomers, for example, conjugated diene monomers having 4 to 6 carbon atoms, such as 1,3-butadiene, isoprene, or 2,3-methylbutadiene, and the like, and one or a mixture of two or more of these monomers may be used.

[0108] More specifically, the hydrogenated nitrile copolymer can be hydrogenated nitrile butadiene rubber (H-NBR).

[0109] The weight-average molecular weight of the dispersant can be from 10,000 g / mol to 150,000 g / mol, preferably from 15,000 g / mol to 140,000 g / mol, and more preferably from 20,000 g / mol to 130,000 g / mol. This corresponds to a value lower than the weight-average molecular weight of the dispersant included in conventional cathode slurry compositions.

[0110] When the weight-average molecular weight of the dispersant is less than 10,000 g / mol, there is a problem of reduced dispersibility of lithium iron phosphate and elution of the dispersant during electrode preparation. When the weight-average molecular weight of the dispersant exceeds 150,000 g / mol, the cathode slurry composition has high viscosity, which may reduce the stability and coating processability of the cathode slurry composition, and the conductive material aggregates in a linear manner, which is therefore undesirable in terms of the resistance of the lithium secondary battery.

[0111] The dispersant may be included in the solid components of the positive electrode slurry composition in an amount of 1.5% by weight or less, specifically 1.2% by weight or less, and more specifically from 0.1% by weight to 1.0% by weight. When the content of the dispersant meets the above range, the aggregation of conductive material in the solid components of the positive electrode slurry composition can be suppressed, thereby improving the positive electrode conductive network.

[0112] Meanwhile, in the method for manufacturing a positive electrode according to the present invention, the step of applying the positive electrode slurry composition to the positive electrode current collector can be carried out by methods known in the art, for example, the positive electrode slurry composition can be uniformly dispersed using a doctor blade or similar, or it can be carried out by methods such as die casting, comma coating, screen printing, etc.

[0113] Meanwhile, in the method for manufacturing the positive electrode according to the present invention, the drying process of the positive electrode slurry composition applied to the positive electrode current collector can be carried out by known methods, for example, by heat treatment methods such as vacuum heating or hot air injection within a constant temperature range.

[0114] Here, the temperature range for the drying process can be 60°C to 130°C, specifically 80°C to 130°C, and more specifically 100°C to 130°C. When the temperature meets the above range, the moisture content in the lithium iron phosphate can be minimized, and volatile components included in this process can be sufficiently removed, thereby preventing side reactions caused by these components and the degradation of battery characteristics during battery charging and discharging.

[0115] The drying process can take anywhere from 5 minutes to 3 hours, specifically from 5 minutes to 20 minutes, and more specifically from 5 minutes to 10 minutes.

[0116] The positive electrode slurry composition applied to the positive electrode current collector is dried through a drying process to form a positive electrode active material layer.

[0117] (2) Forming a positive electrode active material layer by multiple rolling N times.

[0118] The method for manufacturing a positive electrode for a lithium secondary battery according to the present invention includes a rolling step of forming a positive electrode active material layer by rolling a positive electrode active material layer formed by a dried positive electrode slurry composition N times (N is an integer of 2 or greater) in multiple stages. In this case, the first rolling step performed can be a step of pre-rolling the dried positive electrode slurry composition, so that subsequent rolling steps after the first rolling are easy to perform.

[0119] The first rolling process can be carried out by roll forming, but is not limited to this. For example, the first rolling process can be carried out by hot pressing.

[0120] Prior to the first rolling process, the porosity of the dried cathode slurry composition can be 55% or less, specifically 35% to 55%, and more specifically 40% to 55%. When the porosity of the dried cathode slurry composition meets the above range, it is preferable that the rolling process of the cathode slurry composition becomes easier.

[0121] The first rolling process can be performed until the thickness change rate of the positive electrode active material layer exceeds a certain value. For example, before / after the first rolling process, the thickness change rate of the positive electrode active material layer according to the following Equation 1 can be 5% to 15%, specifically 8% to 15%, and more specifically 10% to 15%.

[0122] [Equation 1]

[0123] Thickness change rate (%) = {thickness of the positive electrode active material layer after (K-1) rolling steps - thickness of the positive electrode active material layer after K rolling steps} × 100 / thickness of the positive electrode active material layer before rolling steps

[0124] In Equation 1, K is an integer greater than or equal to 1 and less than or equal to N.

[0125] During the first rolling process, when the thickness change rate of the positive electrode active material layer meets the above range, the positive electrode slurry composition is rolled sufficiently to the point that the positive electrode active material layer does not deintercalate, thereby minimizing the number of rolling operations in subsequent rolling processes.

[0126] When the thickness change rate of the positive electrode active material layer is within the above range, and the first rolling process is completed, the subsequent rolling process can be performed.

[0127] In the rolling process performed after the first rolling process, the method for manufacturing a positive electrode for a lithium secondary battery according to the present invention can perform rolling such that the thickness variation rate of the positive electrode active material layer according to Equation 1 above becomes 3.5% or less. Here, the thickness variation rate can be from 0.1% to 3.5%, more specifically from 0.5% to 3.5%.

[0128] Rolling processes following the first rolling process can be performed using a rolling mill method, but are not limited to this. For example, the second rolling process can be performed using a hot press method.

[0129] In the rolling process following the first rolling, when the thickness change rate of the positive electrode active material layer according to Equation 1 exceeds 3.5%, the low-strength portion of the initial positive electrode active material layer (e.g., the portion with a thickness less than the average thickness or the edge portion) breaks, causing the positive electrode active material layer to be de-intercalated from the positive electrode current collector.

[0130] The aforementioned thickness variation rate (%) corresponds to the rolling rate variation rate (%p). Here, the rolling rate (%) is calculated according to Equation 2 below, and the rolling rate variation rate (%p) can be calculated according to Equation 5 below.

[0131] [Equation 2]

[0132] Rolling yield (%) = {(thickness of the positive electrode active material layer before rolling - thickness of the positive electrode active material layer after K rolling passes) / thickness of the positive electrode active material layer before rolling} × 100

[0133] [Equation 5]

[0134] The rate of change of rolling ratio (%p) = {rolling ratio of the positive electrode active material layer rolled K times - rolling ratio of the positive electrode active material layer rolled (K-1) times}

[0135] Accordingly, in the method for manufacturing the cathode according to the present invention, in the rolling process after the first rolling, the rate of change of the rolling ratio can be 3.5%p or less, specifically from 0.1%p to 3.5%p, more specifically from 0.5%p to 3.5%p.

[0136] In the rolling process, the total number of rolling operations can be less than 6, specifically 3 to 6, and more specifically 4 to 6.

[0137] If the total number of rolling passes exceeds 6, the thickness of the positive electrode active material layer becomes too thin. As a result, the weaker parts of the initial positive electrode active material layer (e.g., parts with a thickness less than the average thickness or edge parts) break, causing the positive electrode active material layer to be de-intercalated from the positive electrode current collector.

[0138] After the rolling step, the thickness of the positive electrode active material layer can be from 85 μm to 95 μm, specifically from 85 μm to 92 μm, and more specifically from 85 μm to 91 μm. When the thickness of the positive electrode active material layer after the rolling step meets the above range, a high-density positive electrode can be manufactured while preventing the deintercalation of the positive electrode active material layer.

[0139] After the rolling step, the porosity of the positive electrode active material layer can be 28% to 36%, specifically 28.5% to 36%, and more specifically 29% to 36%. Here, "after the rolling step" means after all rolling processes consisting of multiple steps have been completed, and the porosity can be calculated according to Equation 3 below.

[0140] [Equation 3]

[0141] Porosity = {1 - (density of the cathode active material layer after K rolling cycles / density of the ideal cathode active material layer)} × 100

[0142] Here, "the density of the ideal positive electrode active material layer" refers to the density of a virtual positive electrode active material layer filled with positive electrode active material, conductive material, binder, etc., in a positive electrode in which lithium iron phosphate, as the positive electrode active material, is present in a composition of 95% to 100% and no voids are present. Furthermore, the porosity can vary depending on the mixing ratio of the positive electrode active material, conductive material, binder, etc., but its value is known to be in the range of 3.3 g / cc to 3.5 g / cc.

[0143] The porosity of the cathode active material layer after the rolling step is related to the thickness of the cathode active material layer after the rolling step. Therefore, when the porosity of the cathode active material layer after the rolling step meets the above-mentioned range, a high-density cathode can be manufactured while preventing the cathode active material layer from being deintercalated.

[0144] After the rolling step, the rolling rate of the positive electrode active material layer can be 20% to 26%, specifically 22% to 26%, and more specifically 23% to 26%. When the rolling rate of the positive electrode active material layer after the rolling step meets the above range, a high-density positive electrode can be manufactured while preventing the positive electrode active material layer from being deintercalated.

[0145] The invention will be described in more detail below by way of examples. However, the following examples are for illustrative purposes only, and the scope of the invention is not limited thereto.

[0146] Example 1: Preparation of the positive electrode

[0147] (1) Preparation of positive electrode slurry composition

[0148] By average particle size D 50 A cathode slurry composition was prepared by adding 2.4 μm LiFePO4 positive electrode active material, carbon nanotube (CNT) conductive material, polyvinylidene fluoride (PVdF) binder, and hydrogenated nitrile butadiene rubber (H-NBR) dispersant to N-methylpyrrolidone (NMP) solvent and stirring them. In the cathode slurry composition, the positive electrode active material, conductive material, binder, and dispersant were present in a weight ratio of 97.4:0.8:1.0:0.8, and the solids content of the cathode slurry composition was 63% by weight.

[0149] (2) Preparation of the positive electrode

[0150] The positive electrode slurry composition was applied to a 20 μm thick aluminum thin film current collector to achieve a final positive electrode discharge specific capacity of 156.7 mAh / cm². 2 The positive electrode slurry composition was then vacuum dried at 130°C for 10 hours.

[0151] Then, a rolling step is performed on the dried positive electrode active material layer with an initial thickness of 115 μm. Specifically, the dried positive electrode active material layer is rolled by a roll forming method, and the rolling process is performed a total of 6 times.

[0152] During the first rolling process, the thickness of the positive electrode active material layer was rolled to a rate of 10.4%, and in subsequent rolling processes, the thickness of the positive electrode active material layer was rolled to a rate of 3.5%, 2.6%, 1.8%, 1.7%, and 3.5%, respectively, and the positive electrode was prepared.

[0153] Here, after each rolling process, the porosity of the positive electrode active material layer is calculated by replacing the "density of the ideal positive electrode active material layer" in Equation 3 below with 339 g / cc, and the results are shown in Table 1.

[0154] [Equation 3]

[0155] Porosity = {1 - (density of the cathode active material layer after K rolling cycles / density of the ideal cathode active material layer)} × 100

[0156] Comparative Example 1: Preparation of the positive electrode

[0157] In the rolling step, the positive electrode was prepared in the same manner as in Example 1, except that the positive electrode active material layer was rolled once in total, such that the thickness change rate of the positive electrode active material layer during rolling was 22.6%.

[0158] Comparative Example 2: Preparation of the positive electrode

[0159] In the rolling step, the positive electrode was prepared in the same manner as in Example 1, except that the positive electrode active material layer was rolled a total of 4 times, such that the thickness change rate of the positive electrode active material layer during the rolling period after the first rolling was 3.5%, 5.2%, and 3.5%, respectively.

[0160] Comparative Example 3: Preparation of the Positive Electrode

[0161] The positive electrode was prepared in the same manner as in Example 1, except that a total of 7 rolling processes were performed on the positive electrode active material layer with an initial thickness of 122 μm, such that the thickness change rate of the positive electrode active material layer during the first rolling was 13.9%, and the thickness change rates of the positive electrode active material layer during the subsequent rolling processes were 3.3%, 2.5%, 1.6%, 2.5%, 0.8%, and 1.6%, respectively.

[0162] Comparative Example 4: Preparation of the Positive Electrode

[0163] The cathode was prepared by rolling the cathode active material layer twice in the rolling step, such that the thickness change rate during the first rolling was 10.4% and the thickness change rate during the subsequent rolling was 12.1%.

[0164] [Table 1]

[0165]

[0166]

[0167] Experiment Example 1 - Visually confirm whether the positive electrode active material layer has been de-intercalated or de-intercalated.

[0168] Visually inspect the positive electrodes prepared in Example 1 and Comparative Examples 1 to 4 to determine whether the positive electrode active material layer has been deintercalated or deintercalated.

[0169] Specifically, the positive electrodes prepared in Example 1 and Comparative Examples 1 to 4 were visually observed to confirm whether the rolled positive electrode active material layer was deintercalated from the positive electrode current collector, as shown in Table 2 below.

[0170] O: Deintercalation / intercalation of the positive electrode active material layer occurs.

[0171] X: No deintercalation / intercalation of the positive electrode active material layer occurred.

[0172] In addition, Figures 1 to 4 The positive electrodes prepared in Example 1 and Comparative Examples 1 to 3 are shown. Specifically, Figure 1 This is a photograph of the positive electrode prepared in Example 1. Figure 2 This is a photograph of the positive electrode prepared in Comparative Example 1. Figure 3 This is a photograph of the positive electrode prepared in Comparative Example 2. Figure 4 This is a photograph of the positive electrode prepared in Comparative Example 3.

[0173] [Table 2]

[0174] Example 1 X Comparative Example 1 O Comparative Example 2 O Comparative Example 3 O Comparative Example 4 O

[0175] Through Table 2 and Figure 1 and Figure 2It can be confirmed that, unlike the cathode of Example 1, in the cathode of Comparative Example 1, which was rolled only once, deintercalation of the cathode active material layer occurred.

[0176] In addition, through Table 2 and Figure 1 and Figure 3 It can be confirmed that, unlike the positive electrode of Example 1, in the positive electrodes of Comparative Examples 2 and 4, which were rolled such that the thickness change rate during rolling after the first rolling exceeded 3.5%, deintercalation of the positive electrode active material layer occurred.

[0177] In addition, through Table 2 and Figure 1 and Figure 4 It can be confirmed that, unlike the cathode of Example 1, in the cathode of Comparative Example 3, in which the number of rolling cycles exceeds 5, deintercalation of the cathode active material layer occurs even when rolling is performed at a thickness change rate of 3.5% or less during rolling after the first rolling.

Claims

1. A method for manufacturing a positive electrode for a lithium secondary battery, comprising: (S1) A positive electrode active material layer is formed by applying a positive electrode slurry composition comprising lithium iron phosphate and a binder to a current collector and drying the positive electrode slurry composition; (S2) The positive electrode active material layer is rolled N times, where N is an integer greater than or equal to 2 and less than or equal to 6. In the rolling step, during the first rolling, the thickness change rate of the positive electrode active material layer is 5% to 15% according to the following Equation 1, and During the rolling process following the first rolling, the thickness variation rate of the positive electrode active material layer, according to Equation 1 below, is 3.5% or less. [Equation 1] Thickness change rate (%) = {thickness of the positive electrode active material layer after (K-1) rolling steps - thickness of the positive electrode active material layer after K rolling steps} × 100 / thickness of the positive electrode active material layer before rolling steps In Equation 1, K is an integer greater than or equal to 1 and less than or equal to N.

2. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein... The rolling process is carried out through multi-stage rolling of 3 to 6 times.

3. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein... After the rolling step, the porosity of the positive electrode active material layer is 28% to 36%.

4. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein... After the rolling step, the thickness of the positive electrode active material layer is 85 μm to 95 μm.

5. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein... After the rolling step, the rolling rate of the positive electrode active material layer is 20% to 26%.

6. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein... The porosity of the dried cathode slurry composition is 55% or less.

7. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein... After the first rolling step, the rolling rate of the positive electrode active material layer is 10% or more.

8. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein... The rolling step is performed by a roll pressing method.

9. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein... The lithium iron phosphate is a compound represented by the following formula: [Formula 1] Li 1+a Fe 1-x M x (PO 4-b )X b In Equation 1, M includes one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y, and X includes one or more elements selected from the group consisting of F, S and N, and a, b and x are -0.5≤a≤0.5, 0≤b≤0.1 and 0≤x≤0.5, respectively.

10. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein... The lithium iron phosphate is LiFePO4 with an olivine crystal structure.

11. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein... The lithium iron phosphate is included in the solid components of the cathode slurry composition in an amount of 94.90% to 97.96% by weight.

12. The method for manufacturing a positive electrode for a lithium secondary battery according to claim 1, wherein... The binder is included in the solid components of the positive electrode slurry composition in an amount of 0.5% to 3.5% by weight.

13. A positive electrode for a lithium secondary battery, said positive electrode being manufactured by the method according to claim 1.

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