A separator membrane for lithium secondary batteries, a lithium secondary battery containing the same, and a method for manufacturing the separator membrane for lithium secondary batteries.

Abstract

A separator for a lithium secondary battery, a lithium secondary battery including the same, and a method for manufacturing the separator for the lithium secondary battery are disclosed. The separator for the lithium secondary battery includes a substrate, a first layer on one side of the substrate that is composed of inorganic particles, and a second layer on the first layer that is composed of polymer binder particles, the inorganic particles having an average particle size of less than 300 nm, and the polymer binder having an average particle size of 200 nm to 500 nm. The separator has a first layer including inorganic particles that has reduced surface roughness and improved packing density, allowing for uniform coating. A lithium secondary battery including the separator can have improved binding strength between the separator and an electrode, and improved bending strength.

Description

[Technical Field]

[0001] The present invention relates to a separator for lithium secondary batteries, a lithium secondary battery including the same, and a method for producing the separator for lithium secondary batteries. [Background Art]

[0002] A separator for electrochemical cells not only functions as an intermediate membrane in a battery that isolates the positive electrode and the negative electrode from each other, continuously maintains ionic conductivity, and enables charging and discharging of the battery, but also functions to prevent short circuits.

[0003] Substrates such as commonly used polyolefin-based base fabrics undergo significant thermal shrinkage at high temperatures and have low physical durability. As a result, when an abnormality occurs in a battery and the internal temperature rises, the separator is likely to deform, and in severe cases, an explosion may occur. In order to improve the heat resistance and safety of such separators, development is underway on coated separators obtained by coating one or two layers of inorganic particles and a binder on a substrate.

[0004] However, when two layers are coated by simultaneously and continuously applying inorganic particles and a binder onto a substrate, it is difficult to uniformly coat an aqueous binder slurry onto a wet inorganic particle coating layer.

[0005] Accordingly, there remains a need for a separator for lithium secondary batteries with improved adhesion between the separator and an electrode and improved bending strength by realizing uniform coating between a first layer containing inorganic particles and a second layer containing polymer binder particles, a lithium secondary battery including the same, and a method for producing the separator for lithium secondary batteries. [Summary of the Invention] [Problem to be Solved by the Invention]

[0006] In one aspect, the present invention provides a separator for a lithium secondary battery, wherein a first layer containing inorganic particles has reduced surface roughness and improved packing density, and enables uniform coating.

[0007] In another aspect, the present invention provides a lithium secondary battery including the separator, wherein binding force between the separator and an electrode is improved and flexural strength is enhanced.

[0008] In still another aspect, the present invention provides a method for producing the separator for a lithium secondary battery. [Means for Solving the Problem]

[0009] According to one aspect, there is provided a separator for a lithium secondary battery comprising: a substrate; a first layer containing inorganic particles on at least one surface of the substrate; and a second layer containing polymer binder particles on the first layer, wherein the average particle diameter (D50) of the inorganic particles is less than 300 nm, and the average particle diameter (D50) of the polymer binder particles is 200 nm to 500 nm.

[0010] The thickness of the first layer may be 1.5 µm to 4.0 µm.

[0011] The thickness of the second layer may be 1.0 µm to 3.0 µm.

[0012] The surface roughness (Ra) of the first layer may be 70 nm or less.

[0013] The polymer binder particles of the second layer that have penetrated into the inorganic particles of the first layer can penetrate from the interface between the first layer and the second layer toward the substrate to a depth of 0.5 µm or less.

[0014] The inorganic particles may be one or more selected from the group consisting of alumina (Al2O3), boehmite, BaSO4, MgO, Mg(OH)2, clay, silica (SiO2), TiO2, SnO2, CeO2, NiO, CaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3 and MgF2.

[0015] The polymer binder particles may also include acrylate-based binders, fluorine-based binders, rubber-based binders, cellulose-based binders, or combinations thereof.

[0016] In another embodiment, a lithium secondary battery is provided, comprising a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and the aforementioned separation membrane disposed between the positive electrode and the negative electrode.

[0017] The bending strength of the aforementioned lithium secondary battery is also 400N or more.

[0018] In yet another embodiment, a method for manufacturing a separation membrane for a lithium secondary battery is provided, comprising the steps of: preparing a substrate; preparing a first composition containing inorganic particles having an average particle size (D50) of less than 300 nm; preparing a second composition containing polymer binder particles having an average particle size (D50) of 200 nm to 500 nm; applying the first composition to at least one surface of the substrate to form a first layer containing the inorganic particles; and applying the second composition to one surface of the first layer and drying it to form a second layer containing the polymer binder particles, thereby manufacturing the aforementioned separation membrane for a lithium secondary battery.

[0019] The solid content of the first composition is 40% to 50% by weight, based on 100% by weight of the total.

[0020] The surface roughness (Ra) of the first layer is also 70 nm or less.

[0021] The thickness of the first layer is 1.5 μm to 4.0 μm, and the thickness of the second layer is 1.0 μm to 3.0 μm. [Effects of the Invention]

[0022] A separation membrane for a lithium secondary battery according to one embodiment includes a first layer containing inorganic particles on at least one surface of a substrate, and a second layer containing polymer binder particles on the first layer, wherein the average particle size (D50) of the inorganic particles is less than 300 nm, and the average particle size (D50) of the polymer binder particles is 200 nm to 500 nm. In the separation membrane for a lithium secondary battery, the first layer containing the inorganic particles has reduced surface roughness and improved packing density, and uniform coating is possible. In a lithium secondary battery including the separation membrane, the bonding force between the separation membrane and the electrode is improved, and the bending strength may be improved. [Brief explanation of the drawing]

[0023] [Figure 1] This is a schematic diagram of a lithium secondary battery based on one actual example. [Figure 2] This is a field emission scanning electron microscope (FE-SEM) image taken by cutting a cross-section of the separation membrane manufactured according to Example 1. [Figure 3] This is a schematic diagram showing the standard battery sample size measured using a three-point bending analyzer during the bending strength measurement according to Evaluation Example 2. [Modes for carrying out the invention]

[0024] Hereinafter, a separation membrane for lithium secondary batteries, a lithium secondary battery containing the same, and a method for manufacturing the separation membrane for lithium secondary batteries will be described in detail with reference to the embodiments and drawings of the present invention. It will be obvious to those of ordinary skill in the art that these embodiments are presented only as illustrative examples to further illustrate the present invention, and that the scope of the present invention is not limited by these embodiments.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this invention pertains. In case of any conflict, this specification, including its definitions, shall prevail.

[0026] Methods and materials similar to or equivalent to those described herein may be used in carrying out or testing the present invention, but suitable methods and materials are described herein. Unless otherwise explicitly stated in the context, singular expressions include plural expressions.

[0027] In this specification, terms such as “includes” or “having” indicate the presence of any feature, number, stage, operation, component, part, ingredient, material, or combination thereof as described in the specification, and should be understood not to preemptively rule out the possibility of the presence or addition of one or more other features, number, stage, operation, component, part, ingredient, material, or combination thereof.

[0028] In this specification, the term “their combination” means a mixture or combination with one or more of the listed components.

[0029] In this specification, the term "and / or" means any combination of one or more items described in relation to, and all combinations thereof. In this specification, the term "or" means "and / or". In this specification, expressions such as "at least one," "one or more," or "one or more" preceding a component may complement the overall list of components, but not necessarily complement the individual components described above.

[0030] In the drawings, thicknesses are shown enlarged or reduced to clearly represent various layers and regions. Throughout the specification, similar parts are denoted by the same reference numerals. Throughout the specification, when a layer, film, region, plate, or other part is described as being "on top of" or "above" another part, this includes not only cases where it is directly above another part, but also cases where there are other parts in between. Throughout the specification, terms such as 1 and 2 may be used to describe various components, but such components are not limited by these terms. These terms are used solely for the purpose of distinguishing one component from another.

[0031] In this specification, the "average particle size (D50) value" refers to the particle size value corresponding to 50% of the smallest particles in a distribution curve accumulated from the smallest to the largest particle size, where the total number of particles is set to 100%. The D50 value can be measured by methods widely known to those skilled in the art, for example, by measuring with a particle size analyzer or by measuring from TEM or SEM images. Alternatively, it can be measured using a measuring device that utilizes dynamic light-scattering, and after performing data analysis and counting the number of particles for each particle size range, the D50 value can be easily obtained through calculation.

[0032] Generally, for lithium secondary batteries, coated separation membranes have been developed to improve heat resistance and safety by coating a substrate with inorganic particles and a binder. However, these coated separation membranes are almost always single-layer, making it difficult to achieve effective bonding between the separation membrane and the electrodes. Furthermore, when coating two layers of inorganic particles and a binder simultaneously and continuously on a substrate, it is difficult to uniformly coat the binder slurry in aqueous solution state onto the wet inorganic particle coating layer.

[0033] To solve such problems, the inventors of the present invention propose the following separation membrane for lithium secondary batteries, a lithium secondary battery containing the same, and a method for manufacturing the separation membrane for lithium secondary batteries.

[0034] A separation membrane for a lithium secondary battery according to one embodiment comprises a substrate, a first layer containing inorganic particles on at least one surface of the substrate, and a second layer containing polymer binder particles on the first layer, wherein the average particle size (D50) of the inorganic particles is less than 300 nm, and the average particle size (D50) of the polymer binder particles is 200 nm to 500 nm. The average particle size is also 200 nm to 450 nm, or 200 nm to 400 nm. By setting the average particle size (D50) of the inorganic particles and the average particle size (D50) of the polymer binder particles within the above range, the surface roughness of the first layer containing the inorganic particles can be reduced, the packing density can be improved, and the content of polymer binder particles from the second layer that have penetrated into the inorganic particles of the first layer can be reduced. Therefore, the separation membrane can embody a solid first layer containing the inorganic particles, while uniform coating is possible on at least one surface of the substrate, with the interface between the first layer and the second layer clearly separated.

[0035] The thickness of the first layer is 1.5 μm to 4.0 μm. The thickness of the second layer is 1.0 μm to 3.0 μm. Within the thickness range of the first and second layers, the bonding force between the separation membrane and the electrode is improved, and the bending strength can be enhanced. The thin film thickness of the first and second layers can be used to maximize the capacity per unit volume of the lithium secondary battery.

[0036] The surface roughness (Ra) of the first layer is also 70 nm or less. The surface roughness (Ra) of the first layer is also 65 nm or less, 60 nm or less, or 55 nm or less. By reducing the surface roughness (Ra) of the first layer to the above range, the content of binder particles penetrating between inorganic particles can be minimized. The polymer binder particles of the second layer that have penetrated into the inorganic particles of the first layer can penetrate to a depth of 0.5 μm or less from the interface between the first layer and the second layer in the direction of the substrate. By reducing the depth to which the polymer binder particles of the second layer have penetrated into the inorganic particles of the first layer to the above range, the packing density of the first layer containing the inorganic particles can be improved, and a robust inorganic particle layer can be realized.

[0037] The inorganic particles are one or more selected from alumina (Al2O3), boehmite, BaSO4, MgO, Mg(OH)2, clay, silica (SiO2), TiO2, SnO2, CeO2, NiO, CaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, and MgF2. The inorganic particles can be used alone or in mixtures of two or more. For example, the inorganic particles may be alumina (Al2O3), boehmite, or Mg(OH)2. The inorganic particles may be spherical, plate-shaped, or fibrous, but are not limited to these forms; any form usable in the art is acceptable. For example, if the inorganic particles are plate-shaped, the reduction in separation membrane area at high temperatures can be further suppressed, relatively high porosity can be ensured, and the characteristics may be improved when evaluating the penetration of lithium secondary batteries.

[0038] The polymer binder particles may include acrylate-based binders, fluorine-based binders, rubber-based binders, cellulose-based binders, or combinations thereof. The polymer binder particles can enhance the binding properties with the inorganic particles. Examples of acrylate-based binders include polymethyl methacrylate and polybutyl acrylate. Examples of fluorine-based binders include polyvinylidene fluoride, polyvinylidene fluoride-co-hexafluoropropylene copolymer and polyvinylidene fluoride-co-trichloroethylene copolymer. Examples of rubber-based binders include styrene-butadiene copolymer and acrylonitrile-styrene-butadiene copolymer. Examples of the aforementioned cellulose-based binders include cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethylcellulose, and carboxyl methyl cellulose. For example, the polymer binder particles can use polyvinylidene fluoride or polyvinylidene fluoride-co-hexafluoropropylene copolymer as fluorine-based binders. The fluorine-based binder also has a weight-average molecular weight (Mw) in the range of 300,000 to 1,700,000 g / mol.By using a fluorine-based binder within the aforementioned molecular weight range, the adhesion between the first and second layers and the substrate is strengthened, effectively suppressing heat-sensitive substrate shrinkage due to heat, and enabling the production of a separation membrane with sufficiently improved electrolyte impregnation properties. This can then be used to produce a battery that generates electrical output efficiently.

[0039] The substrate is also a porous substrate. The porous substrate is also a porous film containing polyolefin. The polyolefin has an excellent short-circuit prevention effect and can improve battery stability through its shutdown effect. For example, the porous substrate is a film made of a resin such as polyethylene, polypropylene, polybutene, polyvinyl chloride, or mixtures or copolymers thereof, but is not necessarily limited to these, and any porous film that can be used in the art is possible. For example, a porous film made of polyolefin resin, a porous film woven from polyolefin fibers, a nonwoven fabric containing polyolefin, or an aggregate of insulating material particles can be used. For example, a porous film containing polyolefin has excellent applicability of binder solutions for producing a coating layer formed on the substrate, can reduce the film thickness of the separation film, increase the ratio of active material in the battery, and increase the capacity per unit volume.

[0040] For example, polyolefins used as materials for porous substrates can be homopolymers, copolymers, or mixtures thereof, such as polyethylene and polypropylene. Polyethylene can be low-density, medium-density, or high-density polyethylene, and high-density polyethylene may be used from the viewpoint of mechanical strength. Also, two or more types of polyethylene can be mixed to impart flexibility. The polymerization catalyst used in the preparation of polyethylene is not particularly limited, and Ziegler-Natta catalysts, Phillips catalysts, metallocene catalysts, etc., can be used. From the viewpoint of achieving both mechanical strength and high permeability, the weight-average molecular weight (Mw) of polyethylene can be 100,000 to 12,000,000 g / mol, or for example, 200,000 to 3,000,000 g / mol. Polypropylene can be a homopolymer, a random copolymer, or a block copolymer, and can be used alone or in mixtures of two or more. Also, the polymerization catalyst is not particularly limited, and Ziegler-Natta catalysts, metallocene catalysts, etc., can be used. Furthermore, the stereoregularity is not particularly limited, and isotactic propylene, syndiotactic propylene, or atactic polypropylene can be used. In addition, within the limits that do not impair the effects of the present invention, polyolefins other than polyethylene or polypropylene, and additives such as antioxidants can be added to the polyolefin.

[0041] For example, porous substrates may include polyolefins such as polyethylene and polypropylene, and multilayer films of two or more layers may be used. Mixed multilayer films such as polyethylene / polypropylene two-layer separation films, polyethylene / polypropylene / polyethylene three-layer separation films, and polypropylene / polyethylene / polypropylene three-layer separation films may be used, but are not limited to these. Any material and configuration that can be used as a porous substrate in the relevant art is acceptable.

[0042] For example, a porous substrate may also contain a diene polymer produced by polymerizing a monomer composition containing a diene monomer. The diene monomer may be a conjugated diene monomer or a non-conjugated diene monomer. For example, the diene monomer may include one or more selected from 1,3-butadiene, isoprene, 2-chloro-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 2-ethyl-1,3-butadiene, 1,3-pentadiene, chloroprene, vinylpyridine, vinylnorbornene, dicyclopentadiene, and 1,4-hexadiene, but is not necessarily limited to these; any monomer that can be used as a diene monomer in the art is acceptable.

[0043] The thickness of the porous substrate in the separation membrane can be 1 μm to 100 μm. For example, the thickness of the porous substrate can be 1 μm to 30 μm. For example, the thickness of the porous substrate can be 5 μm to 20 μm. For example, the thickness of the porous substrate can be 5 μm to 15 μm. For example, the thickness of the porous substrate can be 5 μm to 10 μm. If the thickness of the porous substrate is less than 1 μm, it is difficult to maintain the mechanical properties of the separation membrane, and if the thickness of the porous substrate exceeds 100 μm, the internal resistance of the lithium secondary battery will increase.

[0044] The porosity of the porous substrate in the separation membrane ranges from 5% to 95%. If the porosity is less than 5%, the internal resistance of the lithium battery increases, and if the porosity exceeds 95%, it becomes difficult to maintain the mechanical properties of the porous substrate.

[0045] The pore size of the porous substrate in the separation membrane can be between 0.01 μm and 10 μm. For example, the pore size of the porous substrate in the separation membrane can be between 0.01 μm and 5 μm. For example, the pore size of the porous substrate in the separation membrane can be between 0.01 μm and 1 μm. If the pore size of the porous substrate is less than 0.01 μm, the internal resistance of the lithium secondary battery will increase, and if the pore size of the porous substrate exceeds 10 μm, it will be difficult to maintain the mechanical properties of the porous substrate.

[0046] Furthermore, as can be selected by those skilled in the art, the porous substrate may be given common additives for improving specific functions, such as plasticizers, oxidation stabilizers, UV stabilizers, and antistatic agents.

[0047] Another embodiment of a lithium secondary battery also includes a positive electrode containing a positive electrode active material, a negative electrode containing a negative electrode active material, and the aforementioned separation membrane disposed between the positive electrode and the negative electrode. The lithium secondary battery includes the separation membrane, which can improve the bonding force between the separation membrane and the electrodes and enhance the bending strength.

[0048] The bending strength of the lithium secondary battery is also 400N or more. For example, the bending strength of the lithium secondary battery is also 405N or more, or 410N or more.

[0049] The thickness of the separation membrane is 5 μm to 15 μm.

[0050] The positive electrode may be manufactured, for example, by the exemplary methods described below, but is not necessarily limited to such methods and can be adjusted according to the required conditions.

[0051] First, the positive electrode active material, conductive agent, binder, and solvent are mixed to prepare the positive electrode active material composition. The prepared positive electrode active material composition is directly coated onto an aluminum current collector and dried to produce a positive electrode plate with a positive electrode active material layer formed on it. Alternatively, the positive electrode active material composition is cast onto a separate support, and the resulting film is peeled off the support and laminated onto the aluminum current collector to produce a positive electrode plate with a positive electrode active material layer formed on it.

[0052] Conductive agents include, but are not limited to, carbon black, graphite nanoparticles, natural graphite, artificial graphite, acetylene black, Ketjenblack, carbon fibers, carbon nanotubes, metal powders, metal fibers, or metal tubes such as copper, nickel, aluminum, and silver, and conductive polymers such as polyphenylene derivatives. Any conductive agent used in the relevant art is acceptable.

[0053] As binders, vinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polytetrafluoroethylene (PTFE), mixtures of the aforementioned polymers, and styrene-butadiene rubber polymers are used, and as solvents, N-methylpyrrolidone (NMP), acetone, and water are used, but are not necessarily limited to these; any solvent used in the relevant art is acceptable.

[0054] It is also possible to further add a plasticizer or pore-forming agent to the positive electrode active material composition to form pores inside the electrode plate.

[0055] The content of the positive electrode active material, conductive agent, binder, and solvent used in the positive electrode is at levels commonly used in lithium batteries. Depending on the application and configuration of the lithium battery, one or more of the aforementioned conductive agent, binder, and solvent may be omitted.

[0056] The positive electrode active material is a lithium-containing metal oxide, and any of those commonly used in the industry may be used without limitation. For example, one or more composite oxides of lithium with metals selected from cobalt, manganese, nickel, and combinations thereof may be used, and a specific example of this is Li a A 1-b B' b D'2 (In the above formula, 0.90 ≤ a ≤ 1 and 0 ≤ b ≤ 0.5); Li a E 1-b B' b O2-c D' c (in the above formula, 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05); LiE 2-b B' b O 4-c D' c (in the above formula, 0≤b≤0.5, 0≤c≤0.05); Li a Ni 1-b-c Co b B' c D' α (in the above formula, 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05, 0<α≤2); Li a Ni 1-b-c Co b B' c O 2-α F' α (in the above formula, 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li a Ni 1-b-c Co b B' c O 2-α F'2(in the above formula, 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li a Ni 1-b-c Mn b B' c D' α (in the above formula, 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05, 0<α≤2); Li a Ni 1-b-c Mn b B' c O 2-α F' α (in the above formula, 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li a Ni 1-b-c Mn b B' c O 2-α F'2(in the above formula, 0.90≤a≤1, 0≤b≤0.5, 0≤c≤0.05, 0<α<2); Li a Ni b E c G dO2(In the above formula, 0.90≦a≦1, 0≦b≦0.9, 0≦c≦0.5, 0.001≦d≦0.1);Li a Ni b Co c Mn d G e O2(In the above formula, 0.90≦a≦1, 0≦b≦0.9, 0≦c≦0.5, 0≦d≦0.5, 0.001≦e≦0.1);Li a NiG b O2 (In the above formula, 0.90 ≤ a ≤ 1 and 0.001 ≤ b ≤ 0.1); Li a CoG b O2 (In the above formula, 0.90 ≤ a ≤ 1 and 0.001 ≤ b ≤ 0.1); Li a MnG b O2 (In the above formula, 0.90 ≤ a ≤ 1 and 0.001 ≤ b ≤ 0.1); Li a Mn2G b O4 (In the above formula, 0.90 ≤ a ≤ 1 and 0.001 ≤ b ≤ 0.1); QO2; QS2; LiQS2; V2O5; LiV2O5; LiI'O2; LiNiVO4; Li (3-f) J2(PO4)3(0≦f≦2);Li (3-f) Compounds represented by any one of the following chemical formulas can be used: Fe2(PO4)3(0≦f≦2);LiFePO4 In the chemical formulas representing the aforementioned compounds, A is Ni, Co, Mn, or a combination thereof; B' is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements, or a combination thereof; D' is O, F, S, P, or a combination thereof; E is Co, Mn, or a combination thereof; F' is F, S, P, or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; Q is Ti, Mo, Mn, or a combination thereof; I' is Cr, V, Fe, Sc, Y, or a combination thereof; and J is V, Cr, Mn, Co, Ni, Cu, or a combination thereof.

[0057] It is also possible to use compounds with a coating layer attached to their surface, and mixtures of the aforementioned compounds and compounds with a coating layer attached are also possible. The coating layer attached to the surface of the aforementioned compounds includes, for example, a coating element compound of oxide / hydroxyl, oxyhydroxyl, oxycarbonate, or hydroxycarbonate. The compounds forming such a coating layer are amorphous or crystalline. The coating elements included in the coating layer are Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or mixtures thereof. The method for forming the coating layer is selected within a range that does not adversely affect the physical properties of the positive electrode active material. The coating method is, for example, a spray coating method or an immersion method. The specific coating method is something that can be easily understood by those engaged in this field, so a detailed explanation is omitted.

[0058] Next, the negative electrode is manufactured as follows. The negative electrode is manufactured by substantially the same method as the positive electrode, except, for example, that the negative electrode active material is used instead of the positive electrode active material. Furthermore, in the negative electrode active material composition, the conductive agent, binder and solvent can be substantially the same as those used in the positive electrode.

[0059] For example, a negative electrode active material composition is manufactured by mixing a negative electrode active material, a conductive agent, a binder, and a solvent, and this is directly coated onto a copper current collector to manufacture a negative electrode plate. Alternatively, the manufactured negative electrode active material composition is cast onto a separate support, and the negative electrode active material film, peeled off from the support, is laminated onto a copper current collector to manufacture a negative electrode plate.

[0060] The negative electrode active material can be any material used as a negative electrode active material in lithium batteries in the art. For example, it may include one or more selected from lithium metal, metals alloyable with lithium, transition metal oxides, non-transition metal oxides, and carbon-based materials.

[0061] Examples of metals that can be alloyed with lithium include Si, Sn, Al, Ge, Pb, Bi, Sb, Si-Y' alloys (where Y' is an alkali metal, alkaline earth metal, group 13 element, group 14 element, transition metal, rare earth element, or a combination thereof, and not Si), and Sn-Y' alloys (where Y' is an alkali metal, alkaline earth metal, group 13 element, group 14 element, transition metal, rare earth element, or a combination thereof, and not Sn). Element Y' is, for example, Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ti, Ge, P, As, Sb, Bi, S, Se, Te, Po, or combinations thereof.

[0062] Examples of the transition metal oxides include lithium titanium oxide, vanadium oxide, and lithium vanadium oxide.

[0063] The aforementioned non-transition metal oxides are, for example, SnO2, SiO2 x (0 <x<2)などである。

[0064] The carbon-based material is, for example, crystalline carbon, amorphous carbon, or a mixture thereof. The crystalline carbon is, for example, graphite such as amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite. The amorphous carbon is, for example, soft carbon (low-temperature calcined carbon) or hard carbon, mesophase pitch carbide, calcined coke, etc.

[0065] The aforementioned concentrations of the negative electrode active material, conductive agent, binder, and solvent are at levels commonly used in lithium batteries. Depending on the application and configuration of the lithium battery, one or more of the aforementioned conductive agent, binder, and solvent may be omitted.

[0066] Next, a separator membrane is prepared to be inserted between the positive electrode and the negative electrode.

[0067] The separation membrane used is the one described above. The method for manufacturing the separation membrane will be described later.

[0068] Next, electrolytes are prepared.

[0069] The electrolyte is, for example, an organic electrolyte. This organic electrolyte is prepared, for example, by dissolving a lithium salt in an organic solvent.

[0070] Any organic solvent is acceptable as long as it is used as an organic solvent in the art. Examples of such organic solvents include propylene carbonate, ethylene carbonate, fluoroethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate, dipropyl carbonate, dibutyl carbonate, methyl propionate, ethyl propionate, propyl propionate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, γ-butyrolactone, dioxolane, 4-methyldioxolane, N,N-dimethylformamide, dimethylacetamide, dimethyl sulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, diethylene glycol, dimethyl ether, or mixtures thereof.

[0071] Any lithium salt is acceptable, as long as it is used as a lithium salt in the relevant art. Examples of such lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, LiAlO2, LiAlCl4, and LiN(C x F 2x+1 SO2)(C y F 2y+1SO2) (where x and y are natural numbers), LiCl, LiI, or mixtures thereof, etc.

[0072] Alternatively, the electrolyte may be a solid electrolyte. This solid electrolyte may be, for example, boron oxide or lithium oxynitride, but is not limited to these; any material used as a solid electrolyte in the relevant art is acceptable. The solid electrolyte may be formed on the negative electrode by a method such as sputtering, or a separate solid electrolyte sheet may be laminated on the negative electrode.

[0073] As can be seen from Figure 1, the lithium secondary battery 1 includes a positive electrode 3, a negative electrode 2, and a separator membrane 4. The positive electrode 3, negative electrode 2, and separator membrane 4 are wound or folded and housed in the battery case 5. An organic electrolyte is injected into the battery case 5 and sealed in a cap assembly 6 to complete the lithium secondary battery 1. The battery case 5 is cylindrical, but is not necessarily limited to this form; for example, it can be rectangular, thin film type, pouch type, etc.

[0074] A pouch-type lithium secondary battery comprises one or more battery structures. A separation membrane is placed between the positive and negative electrodes to form the battery structure. After the battery structures are stacked in a bicell structure, they are impregnated with an organic electrolyte, housed in a pouch, and sealed to complete the pouch-type lithium battery. Multiple such battery structures are stacked to form a battery pack, and such battery packs are used in all devices that require high capacity and high output. For example, they are used in notebook computers, smartphones, electric vehicles (EVs), etc.

[0075] Lithium-ion batteries have excellent lifespan and high efficiency characteristics, and are therefore used in electric vehicles (EVs), for example. They are also used in hybrid vehicles such as plug-in hybrid electric vehicles (PHEVs). Furthermore, they are used in fields where large amounts of power storage are required, such as electric bicycles and power tools.

[0076] Furthermore, another embodiment of a method for manufacturing a separation membrane for lithium secondary batteries includes the steps of: preparing a substrate; preparing a first composition containing inorganic particles having an average particle size (D50) of less than 300 nm; preparing a second composition containing polymer binder particles having an average particle size (D50) of 200 nm to 500 nm; applying the first composition to at least one surface of the substrate to form a first layer containing the inorganic particles; and applying the second composition to one surface of the first layer and drying it to form a second layer containing the polymer binder particles, thereby manufacturing the aforementioned separation membrane for lithium secondary batteries.

[0077] The aforementioned substrate is also a porous film containing polyolefin. The porous film containing olefin is the same as described above, so its explanation will be omitted below.

[0078] The first composition containing the inorganic particles can be used as an inorganic dispersion liquid, in which inorganic particles having an average particle size (D50) of less than 300 nm are dispersed in a suitable solvent. The inorganic dispersion liquid can be dispersed using a ball mill, bead and / or screw mixer. The suitable solvent is not particularly limited, and solvents commonly used in the art can be used. The solvent used can also be an aqueous solvent. The aqueous solvent may contain 50% by weight or more of water as the main component, with other polar solvents added. The polar solvent is not particularly limited, as long as it is a solvent that can be well miscible with water and is usable in the art; for example, methanol, ethanol, polyvinyl alcohol, etc., can be used. If necessary, the first composition containing the inorganic particles may also contain an anionic polymer dispersant as an aqueous dispersant. The anionic polymer dispersant is included in the first composition in small amounts, and a low-viscosity slurry composition can be produced in a short time. Examples of the anionic polymer dispersant include ammonium polycarboxylate.

[0079] The solid content of the first composition is 40% to 50% by weight, based on 100% by weight of the total. Within the solid content range of the first composition, the surface roughness of the composition containing the inorganic particles can be reduced, and the content of polymer binder particles of the second layer that have penetrated into the inorganic particles of the first layer can be reduced. Therefore, the separation membrane can embody a solid first layer containing the inorganic particles, while uniform coating is possible on at least one surface of the substrate, with the interface between the first layer and the second layer clearly separated.

[0080] The surface roughness (Ra) of the first layer is also 70 nm or less.

[0081] The second composition containing the polymer binder particles is a polymer binder particle having an average particle size (D50) of 200 nm to 500 nm, and may include, for example, fluorine-based binder latex particles. For example, the fluorine-based binder latex particles may also be polyvinylidene fluoride-co-hexafluoropropylene copolymer latex particles. The fluorine-based binder latex particles are a semi-crystalline fluoropolymer and are produced by an emulsion polymerization process. The semi-crystalline fluorine-based binder latex particles produced by the emulsion polymerization process may be provided in the form of fine particles with a smaller average particle size (D50) than those produced by a suspension polymerization process. Such fine-particle fluorine-based binder latex particles can improve adhesion to inorganic particles. Furthermore, if the average particle size (D50) of the fluorine-based binder latex particles is within the aforementioned range, a composition with excellent water dispersibility and affinity to the first layer containing inorganic particles can be produced. Due to these properties, when a separation film is manufactured through the coating and drying stages, an effect of improved adhesion to the electrode may be observed. The weight-average molecular weight (Mw) of the fluorine-based binder latex particles can be 500,000 g / mol to 1,000,000 g / mol, for example, 400,000 g / mol to 600,000 g / mol. When the weight-average molecular weight (Mw) is within the above range, the fine particles produced by the emulsion polymerization process can have an appropriate average particle size so that the fine particles have excellent water dispersion properties. If necessary, the second composition containing the polymer binder particles may also contain additives such as leveling agents. An example of such leveling agent is a polysiloxane compound.

[0082] The first composition is applied to at least one surface of the substrate to form a first layer containing the inorganic particles. The second composition is applied to one surface of the first layer and dried to form a second layer containing the polymer binder particles, and the product is manufactured in the same manner as described above for manufacturing a lithium secondary battery separation film. The steps of forming the first layer and the second layer can be carried out simultaneously and continuously. The method of applying the first and second compositions onto the substrate is not limited, and methods commonly used in the art of the present invention can be used. Examples of the application method include, but are not limited to, dip coating, die coating, roll coating, comma coating, or gravure coating. These can be applied individually or in combination of two or more methods. The first and second layers may also be formed, for example, by gravure coating.

[0083] The thickness of the first layer is 1.5 μm to 4.0 μm, and the thickness of the second layer is 1.0 μm to 3.0 μm. Within the thickness range of the first and second layers, the bonding force between the separation membrane and the electrode is improved, and the bending strength can be enhanced. The thin film thickness of the first and second layers can be used to maximize the capacity per unit volume of the lithium secondary battery.

[0084] The first and second layers can be dried using methods such as drying with hot air, hot air, or low-humidity air, vacuum drying, or irradiation with far-infrared rays or electron beams. The drying temperature varies depending on the type of solvent, but generally, drying can be done at temperatures between 60°C and 120°C. The drying time also varies depending on the type of solvent, but generally, drying can be done for 1 minute to 1 hour.

[0085] The present invention will be described in more detail through the following examples and comparative examples. However, these examples are for illustrative purposes only and do not limit the scope of the present invention.

[0086] [Examples] (Manufacturing of separation membranes) Example 1 (First composition containing inorganic particles) Alumina (Al2O3) with an average particle size (D50) of 200 nm was added to ultrapure water at a concentration of 50% by weight. An anionic polycarboxylate ammonium (ELEXCEL-DS 540 (Sunopco, Korea)) additive was added at a concentration of 2% by weight, based on the solid content of the inorganic particle dispersion, and the mixture was stirred for 1 hour to produce a mixed solution. The mixed solution was then milled for approximately 2 hours using a bead mill with a 300 μm size bead to disperse the particles and produce an inorganic particle-containing dispersion. Based on the solid content of the inorganic particle-containing dispersion, 2% by weight of polyvinyl alcohol was added and the mixture was stirred for 1 hour to produce a first composition containing inorganic particles.

[0087] (Second composition containing polymer binder particles) A water-dispersed latex of a PVdF-HFP polymer (PVdF:HFP molar ratio = 98:2, glass transition temperature: -30°C, melting point: 150°C, average particle size (D50): 200 nm) was diluted in ultrapure water to a solid content of 5% by weight. Polyether-modified polysiloxane was added as a leveling agent at 2% by weight based on the solid content, and the mixture was stirred for 4 hours to produce a second composition containing polymer binder particles.

[0088] (separation membrane) A polyethylene substrate film (manufactured by SK Innovation) with a thickness of 7.0 μm was coated with a first composition containing the inorganic particles by gravure coating, and then a second composition containing the polymer binder particles was continuously coated through a slot die with a shim thickness of 75 μm. Subsequently, the surface coated with the first and second compositions was dried at a running speed of 20 mpm and a temperature of 75°C to produce a separation membrane in which a first layer containing inorganic particles and a second layer containing polymer binder particles were sequentially formed on one surface of the polyethylene substrate film. The thickness of the first layer containing inorganic particles was 1.5 μm, and the thickness of the second layer containing polymer binder particles was 1.0 μm.

[0089] Example 2 A separation membrane was manufactured in the same manner as in Example 1, except that alumina (Al2O3) with an average particle size (D50) of 200 nm was added to ultrapure water at a concentration of 40% by weight to prepare a first composition containing a mixture and inorganic particles.

[0090] Example 3 A separation membrane was manufactured in the same manner as in Example 1, except that a water-dispersible latex of PVdF-HFP polymer with an average particle size (D50) of 500 nm was used instead of a water-dispersible latex of PVdF-HFP polymer with an average particle size (D50) of 200 nm, and a second composition containing polymer binder particles was prepared.

[0091] Example 4 The separation membrane was manufactured using the same method as in Example 1, except that the thickness of the first layer containing the inorganic particles was 4.0 μm.

[0092] Example 5 A first composition containing a mixture and inorganic particles was prepared by adding 40% by weight of alumina (Al2O3) with an average particle size (D50) of 200 nm to ultrapure water, and a separation membrane was prepared in the same manner as in Example 1, except that the thickness of the second layer containing the polymer binder particles was 3.0 μm.

[0093] Comparative Example 1 A separation membrane was manufactured using the same method as in Example 1, except that alumina (Al2O3) with an average particle size (D50) of 450 nm was used to prepare the first composition containing a mixed solution and inorganic particles.

[0094] Comparative Example 2 A separation membrane was manufactured using the same method as in Example 1, except that alumina (Al2O3) with an average particle size (D50) of 650 nm was used to prepare the first composition containing a mixed solution and inorganic particles.

[0095] Comparative Example 3 A separation membrane was manufactured using the same method as in Example 1, except that alumina (Al2O3) with an average particle size (D50) of 310 nm was used to prepare the first composition containing a mixed solution and inorganic particles.

[0096] (Manufacturing of lithium-ion batteries) Example 6 (Manufacturing of negative electrodes) A slurry of 97% by weight of graphite particles with an average particle size of 25 μm, 1.5% by weight of styrene-butadiene rubber (SBR) and 1.5% by weight of carboxymethylcellulose (CMC) as binders was mixed and added to distilled water. The mixture was then stirred with a mechanical stirrer for 60 minutes to produce a negative electrode active material slurry. The slurry was applied to a 10 μm thick copper current collector using a doctor blade, dried in a hot air dryer at 100°C for 0.5 hours, and then dried again for 4 hours under vacuum conditions at 120°C before being rolled (roll pressed) to produce a negative electrode plate.

[0097] (Manufacturing of positive electrodes) A mixture of 97% by weight of LiCoO2, 1.5% by weight of carbon black powder and 1.5% by weight of polyvinylidene fluoride (PVdF) as conductive agents was added to an N-methyl-2-pyrrolidone solvent and stirred with a mechanical stirrer for 30 minutes to produce a positive electrode active material slurry. The positive electrode active material slurry was applied to a 20 μm thick aluminum current collector using a doctor blade, dried in a hot air dryer at 100°C for 0.5 hours, then dried again for 4 hours under vacuum and at 120°C, and rolled (roll pressed) to produce a positive electrode plate.

[0098] (Manufacturing of lithium-ion batteries) The separation membrane manufactured in Example 1 was inserted between the positive and negative electrode plates manufactured as described above into a pouch of jelly rolls facing each other, an electrolyte solution was injected, the pouch was vacuum-sealed, and a thermal press was applied at a temperature of 85°C for 3 minutes while applying a load of 450 kgf to manufacture a pouch cell.

[0099] The electrolyte used was 1.3M LiPF6 dissolved in a mixed solvent of ethylene carbonate (EC):propylene carbonate (PC):ethyl propionate (EP):propyl propionate (PP) (15:15:25:45 volume ratio).

[0100] Examples 7-10 A lithium secondary battery was manufactured in the same manner as in Example 6, except that the separation membranes manufactured in Examples 2 to 4 were used instead of the separation membrane manufactured in Example 1.

[0101] Comparative Examples 4-6 A lithium secondary battery was manufactured in the same manner as in Example 6, except that the separation membranes manufactured in Comparative Examples 1-3 were used instead of the separation membrane manufactured in Example 1.

[0102] Evaluation Example 1: Surface roughness (Ra) of the first layer and field emission scanning electron microscopy (FE-SEM) (1) Surface roughness (Ra) of the first layer The surface roughness (Ra) of the first layer containing inorganic particles of the separation membranes produced by Examples 1-5 and Comparative Examples 1-3 was measured using an atomic force microscope. The results are shown in Table 1 below.

[0103] (2) Field emission scanning electron microscope (FE-SEM) Cross-sections of the separation membranes produced in Examples 1-5 and Comparative Examples 1-3 were cut and analyzed using a field emission scanning electron microscope (FE-SEM). The results are shown in Table 1 and Figure 2 below. The FE-SEM analysis was performed at a magnification of 10K, and the depth to which the polymer binder particles of the second layer penetrated into the inorganic particles of the first layer was measured from the interface between the first and second layers toward the substrate.

[0104] [Table 1]

[0105] Referring to Table 1 above, the separation membranes produced in Examples 1 to 5 had a surface roughness of 52.5 nm or less in the first layer containing inorganic particles, and the penetration depth of the polymer binder particles of the second layer into the inorganic particles of the first layer was 0.5 μm or less in all cases. Compared to the separation membranes produced in Comparative Examples 1 to 3, the separation membranes produced in Examples 1 to 5 showed reduced surface roughness in the first layer containing inorganic particles, and reduced penetration depth of the polymer binder particles of the second layer into the inorganic particles of the first layer.

[0106] Furthermore, referring to Figure 2, the separation membrane produced by Example 1 shows an interface between a first layer containing clearly separated inorganic particles and a second layer containing polymer binder particles on a substrate film. It can be seen that the polymer binder particles of the second layer, which have penetrated into the inorganic particles of the first layer, have infiltrated from the interface between the first and second layers toward the substrate to a depth of approximately 0.2 μm.

[0107] Therefore, it can be seen that the separation membranes produced by Examples 1 to 5, compared to the separation membranes produced by Comparative Examples 1 to 3, are able to achieve a uniform coating due to the formation of a robust first layer.

[0108] Evaluation Example 2: Bending Strength The bending strength of the lithium secondary batteries manufactured according to Examples 6-10 and Comparative Examples 4-6 was measured using a three-point bending analyzer. The results are shown in Table 2 below.

[0109] The instrument used for the analysis of the bending strength was a single-column analyzer (Instron-3344). A lithium secondary battery sample was prepared to the size of 60mm(L) x 60mm(W) x 5.0mm(T), based on the length (L), width (W), and thickness (T) shown in Figure 3. The lithium secondary battery sample was charged at 25°C with a constant current of 0.7C rate within a cutoff range of 4.40V and 0.05C, and discharged within a cutoff range of 3.0V and 0.5C. After repeating this charge-discharge cycle up to 10 times, the bending strength was evaluated in the discharged state.

[0110] The bending strength evaluation was performed as follows.

[0111] The midpoint of the length (L) of the lithium secondary battery sample was positioned in the middle of the span of the bending strength analyzer, and a jig equipped with a load cell with a maximum load of 1 kN was pressed vertically at a speed of 5 mm / min to measure the maximum strength of the lithium secondary battery sample when it was bent.

[0112] [Table 2]

[0113] As shown in Table 2 above, the lithium secondary batteries manufactured according to Examples 6 to 10 have a bending strength of 400 N or more compared to the lithium secondary batteries manufactured according to Comparative Examples 4 to 6. [Explanation of symbols]

[0114] 1. Lithium-ion rechargeable battery 2 negative electrode 3 Positive electrode 4 Separation membrane 5 Battery case 6 Assembly

Claims

1. Substrate and The substrate has a first layer containing inorganic particles on at least one surface, The first layer comprises a second layer containing polymer binder particles, The average particle size (D50) of the inorganic particles is less than 300 nm, and the average particle size (D50) of the polymer binder particles is between 200 nm and 500 nm. The thickness of the first layer is 1.5 μm to 4.0 μm. A separation film for lithium secondary batteries, wherein the surface roughness (Ra) of the first layer is 70 nm or less.

2. The separation membrane for lithium secondary batteries according to claim 1, wherein the thickness of the second layer is 1.0 μm to 3.0 μm.

3. The separation membrane for a lithium secondary battery according to claim 1, wherein the polymer binder particles of the second layer that have penetrated into the interior of the first layer penetrate to a depth of 0.5 μm or less in the direction of the substrate from the interface between the first layer and the second layer.

4. Said inorganic particles are alumina (Al ２ O ３ ), boehmite, BaSO ４ , MgO, Mg(OH) ２ , clay, silica (SiO ２ ), TiO ２ , SnO ２ , CeO ２ , NiO, CaO, ZnO, ZrO ２ , Y ２ O ３ , SrTiO ３ , BaTiO ３ and MgF ２ The separator for a lithium secondary battery according to claim 1, which is one or more selected from the group consisting of:

5. The separation membrane for lithium secondary batteries according to claim 1, wherein the polymer binder particles include an acrylate-based binder, a fluorine-based binder, a rubber-based binder, a cellulose-based binder, or a combination thereof.

6. A positive electrode containing a positive electrode active material, A negative electrode containing a negative electrode active material, A lithium secondary battery comprising a separation membrane according to any one of claims 1 to 5, disposed between the positive electrode and the negative electrode.

7. The lithium secondary battery according to claim 6, wherein the bending strength of the lithium secondary battery is 400 N or more.

8. The stage of preparing the base material, A step of preparing a first composition containing inorganic particles having an average particle size (D50) of less than 300 nm, A step of preparing a second composition containing polymer binder particles having an average particle size (D50) of 200 to 500 nm, The steps include applying the first composition to at least one surface of the substrate to form a first layer containing the inorganic particles, A method for producing a separation membrane for a lithium secondary battery, comprising the steps of: applying the second composition to one surface of the first layer and drying it to form a second layer containing the polymer binder particles, thereby producing the separation membrane for a lithium secondary battery according to claim 1.

9. The method for producing a separation membrane for a lithium secondary battery according to claim 8, wherein the solid content of the first composition is 40% to 50% by weight, based on 100% by weight of the total.

10. The method for manufacturing a separation membrane for a lithium secondary battery according to claim 8, wherein the thickness of the second layer is 1.0 μm to 3.0 μm.

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