Separator for lithium secondary battery, method for manufacturing the same, and electrochemical device comprising the same

Abstract

The present invention provides a separator for a lithium secondary battery, which includes a porous substrate and a coating layer containing a binder and inorganic particles, the coating layer being formed on one surface or both surfaces of the porous substrate, wherein the binder is a binder including the following polymerized units: (a) a (meth)acrylamide-based monomer polymerized unit, (b) a (meth)acryl-based monomer polymerized unit containing a hydroxyl group, and (c) a multifunctional (meth)acrylamide-based monomer polymerized unit. The separator according to the present invention provides a separator for a lithium secondary battery, which has improved adhesion strength between inorganic particles and the separator, shows reduced interfacial resistance characteristics, and shows improved air permeability.

Description

Technical Field

[0001] This invention relates to a separator for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the separator. More specifically, this invention relates to a separator comprising a coating containing inorganic particles and a binder formed on the surface of a porous substrate, and an electrochemical device including the separator. Background Technology

[0002] A battery consists of a positive electrode, a negative electrode, a separator, and an electrolyte. Currently, most batteries are lithium-ion batteries, and due to their long recharge and discharge lifespan and excellent portability, they are used in portable electronic devices such as smartphones.

[0003] In recent years, due to advancements in the performance of mobile devices and laptops, the energy density of lithium-ion batteries has further increased, leading to various safety issues.

[0004] Specifically, due to their material properties and manufacturing process characteristics, the separators commonly used in lithium secondary batteries often pose safety issues because the shrinkage of the separator at high temperatures can lead to internal short circuits.

[0005] Ensuring the safety of lithium-ion batteries in manufacturing and use is a critical issue that urgently needs to be addressed. Recently, an organic-inorganic composite porous membrane has been proposed, in which a slurry composition of inorganic particles and binder polymers is coated onto a porous substrate to form a porous inorganic coating, used to ensure the safety of lithium-ion batteries. However, when electrode assemblies are formed by laminating electrodes and separators, insufficient interlayer adhesion strength results in a high risk of desorbing the inorganic materials and the separator. Furthermore, during this process, the detached inorganic particles may act as localized defects in the device.

[0006] To address this issue, Korean Patent Publication No. 10-2016-0033692 (March 28, 2016) discloses a method for forming a coating on a porous polymer substrate using an aqueous slurry comprising a poly(meth)acrylamide dispersion containing inorganic and organic particles. It mentions that the resulting membrane possesses the heat resistance and excellent electrochemical performance of a porous substrate; however, this membrane has not yet solved the problems of reduced porosity and relatively severe degradation of shrinkage performance at high temperatures, and the battery resistance and stability do not meet requirements. Therefore, improvements are urgently needed.

[0007] Therefore, since coatings containing binders and inorganic particles formed on the surface of porous polymer substrates can help improve the thermal stability of electrochemical devices, there is a need to develop diaphragms to further improve the heat resistance of electrochemical devices. Summary of the Invention

[0008] One embodiment of the present invention aims to provide a diaphragm with excellent heat resistance, which imparts high-temperature safety to the electrochemical device by reducing its shrinkage rate at high temperatures, and a method thereof. Another embodiment of the present invention aims to provide a novel diaphragm that prevents reduction in permeability caused by adhesives and has reduced electrical resistance.

[0009] Another embodiment of the present invention aims to provide a separator for lithium secondary batteries comprising a coating containing inorganic particles and a binder, wherein the adhesive strength of the inorganic particles is improved to prevent desorption of the inorganic particles, exhibiting reduced surface resistivity characteristics and improved air permeability.

[0010] In one general aspect, a separator for a lithium secondary battery comprises: a porous substrate and a coating comprising a binder and inorganic particles formed on one or both surfaces of the porous substrate.

[0011] The adhesive is an adhesive comprising the following polymer units: (a) (meth)acrylamide-based monomer polymerization unit, (b) (meth)acrylamide-based monomer polymerization unit containing hydroxyl groups, and (c) polyfunctional (meth)acrylamide-based monomer polymerization unit.

[0012] In a separator for a lithium secondary battery according to an exemplary embodiment of the present invention, the adhesive may be an adhesive made by comprising: (a) 65 mol% to 96 mol% of (meth)acrylamide monomer, (b) 3 mol% to 34 mol% of hydroxyl-containing (meth)acryloyl monomer and (c) 0.001 mol% to 1 mol% of polyfunctional (meth)acrylamide monomer.

[0013] In the separator for a lithium secondary battery according to an exemplary embodiment of the present invention, (c) the multifunctional (meth)acrylamide monomer polymerization unit can be prepared by polymerizing a multifunctional monomer represented by the following chemical formula 3:

[0014] [Chemical Formula 3]

[0015]

[0016] R1 and R2 are independently hydrogen or C1 to C6 alkyl groups, and R is a straight-chain C1 to C6 alkyl group. 10 C1 to C2 of hydrocarbon group or branched chain 10 Hydrocarbon group, where a is an integer from 2 to 6.

[0017] In a separator for a lithium secondary battery according to an exemplary embodiment of the present invention, the weight-average molecular weight of the adhesive can be from 100,000 g / mol to 2,000,000 g / mol.

[0018] In a separator for a lithium secondary battery according to an exemplary embodiment of the present invention, the separator for a lithium secondary battery may comprise 50% to 99.9% by weight of inorganic particles and 0.1% to 50% by weight of adhesive, which constitute 50% to 99.9% by weight of the total weight of the coating.

[0019] According to an exemplary embodiment of the present invention, the separator for a lithium secondary battery, after being left to stand at 150°C for 60 minutes, exhibits a shrinkage rate of less than 5% in both the machine direction (MD) and transverse direction at high temperature.

[0020] According to an exemplary embodiment of the present invention, the separator for a lithium secondary battery, after being left to stand at 150°C for 60 minutes, exhibits a shrinkage rate of less than 3% in both the mechanical and lateral directions at high temperature.

[0021] The change in air permeability (ΔG) of the separator for a lithium secondary battery according to an exemplary embodiment of the present invention can satisfy the following equation 1:

[0022] [Formula 1]

[0023] ΔG=G1–G2≤70

[0024] Where G1 is the Gurley permeability of the membrane including the coating, and G2 is the Gurley permeability of the porous substrate itself. The Gurley permeability is measured according to ASTM D726 and the unit is seconds per 100 cubic centimeters (sec / 100cc).

[0025] In a separator for a lithium secondary battery according to an exemplary embodiment of the present invention, the thickness of the coating can be from 0.3 μm to 10 μm.

[0026] In another general aspect, lithium secondary batteries include the separator described above for lithium secondary batteries.

[0027] In another general aspect, a method for manufacturing a separator for lithium secondary batteries includes: (s1) preparing a slurry composition comprising a binder and inorganic particles; and

[0028] (s2) Apply the slurry composition to one or both surfaces of a porous substrate to form a coating.

[0029] The adhesive is an adhesive comprising the following polymeric units: (a) a (meth)acrylamide monomer polymeric unit, (b) a (meth)acryloyl monomer polymeric unit containing hydroxyl groups, and (c) a multifunctional (meth)acrylamide monomer polymeric unit.

[0030] Other features and aspects will become apparent from the following detailed description and claims. Detailed Implementation

[0031] The invention will be described in more detail below. However, the specific examples or exemplary embodiments described below are merely for reference in describing the invention in detail, and the invention is not limited thereto and can be implemented in various forms.

[0032] Furthermore, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein is merely for the purpose of effectively describing a particular instance and is not intended to limit the invention.

[0033] Furthermore, unless the context otherwise indicates, the singular form used in the specification and appended claims may also include the plural form.

[0034] Furthermore, unless specifically stated otherwise, “include” any element will be understood as implying the further inclusion of other elements, rather than excluding any other elements.

[0035] In this specification, "(meth)acryl" refers to acryl and / or methacryl.

[0036] The present invention is intended to solve the above-mentioned problems, and the object of the present invention is to provide a diaphragm with excellent heat resistance and a method for manufacturing the same, which can provide safety at high temperatures by reducing the shrinkage rate of the device at high temperatures.

[0037] A separator for a lithium secondary battery according to an exemplary embodiment of the present invention comprises a porous substrate and a coating comprising an adhesive and inorganic particles, the coating being formed on one or both surfaces of the porous substrate.

[0038] The adhesive includes: (a) a (meth)acrylamide monomer polymerization unit, (b) a (meth)acryloyl monomer polymerization unit containing hydroxyl groups, and (c) a multifunctional (meth)acrylamide monomer polymerization unit.

[0039] When using an adhesive, a separator for lithium-ion batteries can be provided that achieves the following effects compared to using conventional water-based adhesives: reduced shrinkage of the manufactured separator at high temperatures, improved detachment and separation of inorganic particles in the coating, reduced interfacial resistance, and improved permeability. Furthermore, lithium-ion batteries manufactured using a separator incorporating an adhesive can reduce resistance, resulting in improved performance in terms of capacity and output.

[0040] The above structure will be described in detail below.

[0041] A separator for a lithium secondary battery according to an exemplary embodiment of the present invention comprises a porous substrate.

[0042] In an exemplary embodiment of the present invention, a porous substrate is commonly used in the art and can be a woven fabric, a nonwoven fabric, a porous membrane, etc., but is not limited thereto.

[0043] The materials for the porous substrate are not limited, but specifically, for example, polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyether ether ketone, polyarylether ketone, polyetherimide, polyamide imide, polybenzimidazole, polyethersulfone, polyphenylene ether, cyclic olefin copolymers, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon, polytetrafluoroethylene, etc., and the material can be formed from any one or more resins selected from the group consisting of them.

[0044] More specifically, as a porous substrate, it is more preferable to adjust it to a polyolefin-based porous substrate with finer pores, but not limited thereto.

[0045] Polyolefin-based porous substrates are typically manufactured in the form of membranes and are not limited to any particular type, as long as they are commonly used as separators for lithium secondary batteries. Examples include polyethylene, polypropylene, and their copolymers, but are not necessarily limited to these.

[0046] There is no particular limitation on the thickness of the porous substrate; for example, it can be from 1 μm to 100 μm, specifically from 5 μm to 50 μm, more specifically from 5 μm to 30 μm, but it is not limited thereto.

[0047] A separator for a lithium secondary battery according to an exemplary embodiment of the present invention includes a coating.

[0048] The coating is formed on one or both surfaces of the porous substrate, and can be applied to the entire surface of one side.

[0049] There is no particular limitation on the thickness, and the coating can be applied with a thickness of, for example, 0.01 μm to 10 μm, specifically, 0.1 μm to 10 μm, more specifically, 0.3 μm to 5 μm, but the thickness is not limited thereto.

[0050] The weight of the coating per unit area can be 2 g / m². 2 Up to 100g / m 2 However, it is not necessary to limit oneself to this.

[0051] The weight ratio of inorganic particles to binder in the coating can be 50:50 to 99.9:0.1, specifically 80:20 to 99.9:0.1, more specifically 90:10 to 99:1, and more specifically 95:5 to 99:1.

[0052] In an exemplary embodiment of the present invention, a coating can be formed by applying a slurry composition comprising inorganic particles and a binder.

[0053] More specifically, for example, the slurry composition may include inorganic particles and a binder; more specifically, it may include inorganic particles, a binder, and a solvent. Furthermore, the solid content may be from 5% to 40% by weight, but the invention is not limited thereto.

[0054] Using a specific adhesive as the binder in this invention reduces shrinkage at high temperatures and forms a coating with excellent heat resistance. This coating can provide a separator for lithium secondary batteries, which has improved adhesion strength of inorganic particles, exhibits reduced interfacial resistance characteristics, and shows improved permeability. Lithium secondary batteries manufactured using separators containing the adhesive can reduce resistance, thereby resulting in improved performance in terms of capacity and output.

[0055] The adhesive may include: (a) a (meth)acrylamide monomer polymerization unit, (b) a hydroxyl-containing (meth)acryloyl monomer polymerization unit, and (c) a multifunctional (meth)acrylamide monomer polymerization unit.

[0056] More specifically, in the separator for a lithium secondary battery according to an exemplary embodiment of the present invention, the adhesive may be an adhesive made from the following components: (a) 65 mol% to 96 mol% of (meth)acrylamide monomer, (b) 3 mol% to 34 mol% of hydroxyl-containing (meth)acrylamide monomer, and (c) 0.001 mol% to 1 mol% of polyfunctional (meth)acrylamide monomer.

[0057] (a) The (meth)acrylamide monomer polymerization unit of the adhesive can be represented by the following chemical formula 1:

[0058] [Chemical Formula 1]

[0059]

[0060] R1 is hydrogen or a C1 to C6 alkyl group.

[0061] (b) The hydroxyl-containing (meth)acryloyl monomer polymer unit of the adhesive can be represented by the following chemical formula 2:

[0062] [Chemical Formula 2]

[0063]

[0064] R2 is hydrogen or a C1 to C6 alkyl group. In addition, L1 is a straight-chain C1 to C6 alkylene group or a branched C1 to C6 alkylene group.

[0065] (c) The polyfunctional (meth)acrylamide monomer polymerization unit can be manufactured by polymerizing a polyfunctional monomer represented by the following chemical formula 3:

[0066] [Chemical Formula 3]

[0067]

[0068] Wherein R1 and R2 are independently hydrogen or C1 to C6 alkyl groups, and R is a straight-chain C1 to C6 alkyl group. 10 C1 to C2 of hydrocarbon group or branched chain 10 Hydrocarbon group, and a is an integer from 2 to 6.

[0069] For example, the content of (a) (meth)acrylamide monomers in the adhesive can be 65 mol% to 96 mol%, particularly 66 mol% to 95 mol%, particularly 67 mol% to 94 mol%, particularly 68 mol% to 93 mol%, particularly 70.5 mol% to 91.5 mol%. (b) The content of hydroxyl-containing (meth)acryloyl monomers can be 3 mol% to 34 mol%, particularly 4 mol% to 33 mol%, particularly 5 mol% to 32 mol%, particularly 6 mol% to 31 mol%, particularly 8 mol% to 29 mol%. (c) The content of polyfunctional (meth)acrylamide monomers can be 0.001 mol% to 1 mol%, particularly 0.005 mol% to 0.9 mol%, more particularly 0.01 mol% to 0.5 mol%, thereby preparing the adhesive. When the adhesive is prepared within the above content range, the bonding strength of the inorganic particles can be further improved, a more significant effect can be obtained in terms of shrinkage at high temperatures, better air permeability, and a separator with lower resistance for lithium secondary batteries can be provided.

[0070] The adhesive has a weight-average molecular weight of 100,000 or more, particularly 200,000 or more, and more particularly, 250,000 to 2,000,000. Within the weight-average molecular weight range that meets the above conditions, the adhesive strength can be further improved. The weight-average molecular weight is the average molecular weight based on polyethylene glycol, measured by gel permeation chromatography.

[0071] Adhesives can be prepared by various methods known in the art, such as emulsion polymerization, suspension polymerization, mass polymerization, solution polymerization, or bulk polymerization.

[0072] The viscosity of an aqueous solution containing a binder with a solid content of 10% by weight can be below 3000 cps, specifically below 2500 cps, more specifically below 2000 cps, and even more specifically below 1500 cps, but is not limited thereto. When the binder is mixed with inorganic particles within the above range to prepare a slurry, the viscosity of the slurry can be further reduced, and the coatability can be further improved.

[0073] In an exemplary embodiment of the invention, the solution of the slurry composition used to form the coating is typically prepared as an aqueous solution, and, if necessary, polar aprotic solvents or polar protic solvents, such as acetone, tetrahydrofuran, dimethylformamide, and N-methyl-2-pyrrolidone, may be used.

[0074] In an exemplary embodiment of the present invention, there are no particular limitations on the inorganic particles used to form the coating, as long as they are electrochemically stable.

[0075] Non-limiting examples of inorganic particles include, but are not limited to, boehmite, CeO2, MgO, CaO, ZnO, Al2O3, TiO2, BaTiO3, HfO2, SrTiO3, SnO2, NiO, ZrO2, Y2O3, SiC, and BaTiO3. Furthermore, these inorganic particles can be used alone or in combination of two or more.

[0076] In exemplary embodiments of the present invention, the size of the inorganic particles is not limited, but in order to impart a smooth coating and excellent electrical properties, the average particle size can be in the range of 10 nm to 10 μm. The dispersibility and coatability of the slurry composition are further improved, and the desorption of inorganic particles can be further prevented within the above range, but the present invention is not necessarily limited thereto.

[0077] Slurry compositions are typically prepared by adding inorganic particles to a binder solution and then using a ball mill to thoroughly pulverize the inorganic particles to break down the aggregates.

[0078] There are no particular limitations on the method of applying an adhesive solution containing dispersed inorganic particles onto a porous substrate. However, the solution can be applied to one or both surfaces of the porous substrate by various methods, such as roller coating, spin coating, dip coating, bar coating, stencil coating, slot coating, and inkjet printing.

[0079] Another embodiment of the present invention provides a separator for a lithium secondary battery made from a porous substrate including the above-described coating.

[0080] The separator used in lithium secondary batteries can exhibit a change in permeability (ΔG) as shown in Equation 1 below:

[0081] [Formula 1]

[0082] ΔG=G1–G2≤70

[0083] Where G1 is the Greilly permeability of the membrane including the coating, and G2 is the Greilly permeability of the porous substrate itself, and the Greilly permeability is measured according to ASTM D726 and the unit is seconds per 100 cubic centimeters.

[0084] In Equation 1, ΔG can be less than 70 seconds / 100 cubic centimeters, specifically, for example, from 1 second / 100 cubic centimeters to 65 seconds / 100 cubic centimeters, and more specifically, from 5 seconds / 100 cubic centimeters to 60 seconds / 100 cubic centimeters. The change in air permeability within the above range is significantly small, therefore, the adhesive has better air permeability.

[0085] Compared to conventional adhesives, the adhesive according to an exemplary embodiment of the present invention has a significantly smaller change in air permeability (i.e., lower ΔG), and when batteries are manufactured using this adhesive, the resistance is further reduced. Therefore, compared to other adhesives, the battery performance, i.e., the capacity, is relatively increased, and the shrinkage rate at high temperatures is also further reduced.

[0086] Furthermore, when the separator used in lithium secondary batteries is left to stand in a hot air drying oven at 150°C for 60 minutes, the measured content of the separator used in lithium secondary batteries can be less than 5%, particularly less than 4%, and more particularly less than 3%.

[0087] Furthermore, by employing a separator according to an exemplary embodiment of the present invention, the resulting lithium secondary battery can have a resistivity of less than 1.10, specifically, for example, less than 1.07, and more specifically, less than 1.05, compared to a separator without a coating. The resistivity can be expressed by the following Equation 2:

[0088] [Equation 2]

[0089] Resistance ratio = Resistance 2 / Resistance 1

[0090] The resistance ratio is measured as the ratio between resistance 2 and resistance 1, where resistance 1 is the resistance when a porous substrate without a coating is used as the separator, and resistance 2 is the resistance when a separator with a coating is formed on one surface of the porous substrate. The lithium secondary battery of the present invention, having this resistance ratio, can reduce resistance, thereby resulting in an improved lithium secondary battery in terms of capacity and output.

[0091] Another embodiment of the present invention includes a method for manufacturing a separator for a lithium secondary battery comprising a coating and a porous substrate. The method for manufacturing the separator for a lithium secondary battery comprises the following steps: (s1) preparing a slurry composition comprising a binder and inorganic particles; and (s2) coating the slurry composition onto one or both surfaces of a porous substrate to form a coating, wherein the binder is a binder comprising polymeric units including: (a) a (meth)acrylamide monomer polymeric unit, (b) a hydroxyl-containing (meth)acryloyl monomer polymeric unit, and (c) a multifunctional (meth)acrylamide monomer polymeric unit.

[0092] Step (s1) is the step of preparing the slurry composition by stirring the above-mentioned aqueous solution of the adhesive, inorganic particles and water to prepare the slurry composition.

[0093] Step (s2) is to apply the slurry composition to one or both surfaces of a porous substrate to form a coating, wherein the slurry composition is applied to one or both surfaces of the porous substrate by the above-described coating method and then dried to produce a diaphragm.

[0094] The separator manufactured by the method described above according to an exemplary embodiment of the present invention can be applied to electrochemical devices, examples of which may include lithium secondary batteries. Since electrochemical devices are well known and their construction is also known, they will not be described in detail here.

[0095] The lithium secondary battery according to an exemplary embodiment of the present invention may include the above-described separator between the positive and negative electrodes. In this case, the positive and negative electrodes can be used without limitation, as long as they are commonly used in lithium secondary batteries.

[0096] The present invention will be described in more detail below with reference to embodiments and comparative examples. However, the following embodiments and comparative examples are merely examples for describing the present invention in more detail and are not intended to limit the present invention in any way.

[0097] Methods for measuring physical properties

[0098] 1. Weight-average molecular weight

[0099] Measurements were performed using a GPC (EcoSEC HLC-8320GPC differential refractive index detector purchased from Tosoh) and a TSKgel guardPWx column. Two columns, TSKgel GMPWxl and TSKgel G2500PWxl (7.8×300mm), were used as GPC columns. A 0.1M NaNO3 aqueous solution was used as the solvent, and polyethylene glycol was used as the standard. The analysis was performed at 40°C and a flow rate of 1 mL / min.

[0100] 2. Viscosity

[0101] Viscosity was measured at 25°C using a Brookfield viscometer (model RVDV2) with a CPA-52Z rotor, with the rotation speed (rpm) set to 60-70% of the torque.

[0102] 3. Adhesion strength

[0103] Cut the diaphragm into 50mm x 50mm pieces and place it on top, coating up. Place a sheet of black drawing paper (20mm x 150mm x 0.25mm thickness) on top and apply a constant pressure (10g / cm²) using a press. 2 Pull the black drawing paper firmly to one side to determine the degree of adhesion of the inorganic material to the surface. Based on the degree of adhesion, assign it to A / B / C / D / F, as follows:

[0104] A: No adhesion

[0105] B: Small amount of inorganic matter adhered

[0106] In CF, the adhesive and inorganic matter adhere together, and the degree of adhesion increases as it approaches F.

[0107] 4: Shrinkage rate at high temperatures

[0108] Cut the diaphragm into squares with sides of 10cm and mark the transverse (TD) and mechanical (MD) directions. Place a sample in the center, with five sheets of paper on top and bottom, and wrap the four sides of the paper with tape. Place the paper-wrapped sample in a 150°C hot air oven for 60 minutes. Afterward, remove the sample from the oven, measure the diaphragm with a camera, and calculate the mechanical (MD) shrinkage rate using the following mathematical formula 1 and the transverse (TD) shrinkage rate using the following mathematical formula 2:

[0109] [Mathematical Formula 1]

[0110] Mechanical shrinkage rate (%) at high temperature = (Mechanical length before heating - Mechanical length after heating) × 100 / Mechanical length before heating

[0111] [Mathematical Formula 2]

[0112] Transverse shrinkage rate at high temperature (%) = (Transverse length before heating - Transverse length after heating) × 100 / Transverse length before heating

[0113] 5. Change in air permeability (increase in Grylloy permeability) (ΔG)

[0114] The change in air permeability is determined by measuring the Gare permeability using Equation 1-1. It is measured according to ASTM D726 using an air permeability meter purchased from Toyoseiki Co., Ltd. The time required for 100 cubic centimeters of air to pass through a 1 square inch diaphragm is recorded and compared in seconds.

[0115] [Equation 1-1]

[0116] ΔG=G1–G2

[0117] Where G1 is the Greilly permeability of the membrane including the coating, and G2 is the Greilly permeability of the porous substrate itself.

[0118] 6. Resistance characteristics of secondary batteries

[0119] Using a charge / discharge cycle instrument, each battery manufactured according to the assembly process of the examples and comparative examples was charged with a constant current (CC) - 4.2V and a constant voltage (CV), and then discharged. The DC internal resistance (DC-IR) at 60% state of charge (SOC) was then measured using the J-pulse method. At this time, the resistance was measured separately when a porous substrate without coating was used as the separator (resistance 1) and when a separator with a coating formed on one surface of the porous substrate was used (resistance 2), and the resistance ratio between resistance 2 and resistance 1 was calculated. The resistance ratio can be expressed by Equation 2 below:

[0120] [Equation 2]

[0121] Resistance ratio = Resistance 2 / Resistance 1

[0122] 7. High-temperature storage

[0123] Each battery (2Ah battery) manufactured using the assembly process according to the examples and comparative examples was stored for 24 days in an oven at 60°C. The DC internal resistance (DC-IR) was measured using the J-pulse method described above, and the rate of increase in resistance was calculated. The rate of increase in resistance can be expressed by Equation 3 below:

[0124] [Equation 3]

[0125] ΔR(%)=(R2-R1) / R1×100

[0126] Where R1 is the resistance of each battery before the experiment, and R2 is the resistance of the battery after being stored at 60°C for 24 days. The resistance increase rate (change, ΔR) is calculated according to Equation 3 and is shown in Table 2.

[0127] [Example 1]

[0128] [Preparation of Adhesives]

[0129] A 1.0 L flask was replaced with nitrogen gas. 64.7 g of acrylamide, 10.4 g of 2-hydroxyethyl acrylate, 0.008 g of N,N-methylenebisacrylamide, and 676 g of distilled water were added to the flask, and the mixture was heated to 70 °C. Subsequently, 0.200 g of potassium persulfate was added as a polymerization initiator, and the polymerization reaction was initiated. After 15 hours of reaction, the flask was exposed to air to stop the polymerization reaction. The temperature was lowered to room temperature, and 1 M sodium hydroxide solution was added to adjust the pH to 7 to prepare a 10% by weight aqueous solution. The viscosity was then measured. The results are shown in Table 1.

[0130] [Preparation of Coating Solution]

[0131] 97% by weight of boehmite particles (γ-AlO(OH), available from Nabaltec, model Apyral AOH60) with an average particle size of 700 nm as non-conductive particles and 3% by weight of the binder prepared above (based on solid content) were added to water as a solvent and stirred to prepare a slurry composition for coating with a solid content of 30% by weight.

[0132] [Preparation of the diaphragm]

[0133] A membrane substrate (ENPASS, SK ie technology Co.) with a width of 150 mm, a length of 100 mm, and a thickness of 9 μm was used. The prepared coating solution was applied to one surface of the substrate at a speed of 3 m / min to form an active inorganic coating. After coating, the substrate was dried using a hot air dryer at 40°C and then wound into a roll shape. After winding, the thickness of the coated membrane was measured, totaling 12 μm, with a coating thickness of 3 μm. The manufactured membrane was used to evaluate adhesive strength, shrinkage at high temperatures, storage at high temperatures, increase in Grylloy permeability, and electrical resistance characteristics.

[0134] [Battery manufacturing]

[0135] 95 wt% of artificial graphite as the negative electrode active material, 3 wt% of acrylic latex with a glass transition temperature (Tg) of -52°C (solid content of 20 wt%), and 2 wt% of carboxymethyl cellulose (CMC) as a thickener were added to water and stirred to prepare a uniform negative electrode slurry. The slurry was coated onto a copper foil with a thickness of 20 μm, dried, and pressed to form a negative electrode plate with a thickness of 150 μm.

[0136] 94% by weight of LiCoO2 as the positive electrode active material, 2.5% by weight of polyvinylidene fluoride as the fusion agent, and 3.5% by weight of carbon black as the conductive agent were added to N-methyl-2-pyrrolidone (NMP) as the solvent, and the mixture was stirred to prepare a uniform positive electrode slurry.

[0137] The slurry was coated onto an aluminum foil with a thickness of 30 μm, dried, and pressed to form a positive electrode plate with a thickness of 150 μm.

[0138] A separator was placed between the positive and negative electrodes to manufacture a pouch cell. An electrolyte solution containing 1M lithium hexafluorophosphate (LiPF6) with a ethylene carbonate (EC) / ethyl methyl carbonate (EMC) / dimethyl carbonate (DMC) ratio of 25:45:20 (volume ratio) was injected into the battery pack. To seal the opening of the aluminum package, the opening on the aluminum exterior was sealed by heat sealing at 165°C to manufacture a 650mAh pouch lithium secondary battery. The resistance of the resulting lithium secondary battery was evaluated using the above method and is shown in Table 2.

[0139] [Examples 2 to 7]

[0140] The battery was manufactured in the same manner as in Example 1, except that the binder was prepared using the composition shown in Table 1 below. The physical properties are shown in Table 2 below.

[0141] [Examples 8 and 9]

[0142] Except that 2-hydroxyethyl methacrylate was used instead of 2-hydroxyethyl acrylate to prepare the binder having the composition shown in Table 1 below, the battery was manufactured in the same manner as in Example 1. The physical properties are shown in Table 2 below.

[0143] [Comparative Examples 1 to 11]

[0144] The battery was manufactured in the same manner as in Example 1, except that the binder was prepared using the composition shown in Table 1 below. The physical properties are shown in Table 2 below.

[0145] [Comparative Example 12]

[0146] A commercially available acrylic water-dispersible adhesive with a latex content of 20% by weight was used. The viscosity of the adhesive was measured and is shown in Table 1 below.

[0147] Furthermore, the coating solution was prepared in the same manner as in Example 1, a separator was fabricated, and then a battery was manufactured. The physical properties are shown in Table 2 below.

[0148] [Table 1]

[0149]

[0150] In Table 1, AM is acrylamide, 2-HEA is 2-hydroxyethyl acrylate, 2-HEMA is 2-hydroxyethyl methacrylate, and PAA (polyfunctional acrylamide) is N,N-methylenebisacrylamide.

[0151] [Table 2]

[0152]

[0153] *Resistivity increase rate (storage at high temperature): 2Ah battery, 60℃, stored for 24 hours.

[0154] As can be seen from Table 2, Examples 1 to 9 demonstrate superior adhesive strength and significantly lower shrinkage at high temperatures (below 5%, particularly below 3%) compared to Comparative Examples 1 to 12, when the weight-average molecular weight of the adhesive is the same. Furthermore, the increase in Gryllium permeability shows a significantly lower change in air permeability (ΔG), thus confirming the excellent air permeability of the adhesive.

[0155] Furthermore, it was confirmed that the resistance increase rate was significantly lower when the present invention was applied. Therefore, compared to batteries without the separator of the present invention, batteries including the separator for lithium secondary batteries prepared according to an exemplary embodiment of the present invention have a resistivity ratio of less than 1.1. This is considered an effect of lithium ions readily passing through the separator, because the adhesive of the present invention has low shrinkage and excellent permeability.

[0156] Furthermore, the separator for lithium secondary batteries according to exemplary embodiments of the present invention exhibits excellent high-temperature stability and excellent adhesive strength, thus confirming that the adhesive strength is improved.

[0157] That is, the separator for lithium secondary batteries of the exemplary embodiments of the present invention has a shrinkage rate of less than 5%, particularly less than 3%, at high temperatures that is significantly lower in both the MD and TD directions.

[0158] The separator for lithium secondary batteries according to the present invention has improved adhesion strength of inorganic particles, making it less likely for the inorganic particles to detach.

[0159] Furthermore, the separator for lithium secondary batteries according to the exemplary embodiment has a significantly reduced shrinkage rate at high temperatures, thus providing a separator for lithium secondary batteries with excellent heat resistance.

[0160] Furthermore, in battery manufacturing, the separator for lithium secondary batteries according to the present invention exhibits excellent air permeability and significantly reduced resistance. Therefore, lithium secondary batteries using the separator for lithium secondary batteries according to the present invention have improved capacity and significantly improved output and lifespan characteristics.

[0161] In the foregoing, although the present invention has been described with reference to specific content and limited exemplary embodiments, these are provided only to aid in the overall understanding of the present invention. The present invention is not limited to the exemplary embodiments, and those skilled in the art to which this invention pertains can make various modifications and changes based on the specification.

[0162] Therefore, the spirit of the present invention should not be limited to the above exemplary embodiments, and all modifications of the above claims and equivalent or equivalent to the claims are intended to fall within the scope and spirit of the present invention.

Claims

1. A separator for a lithium secondary battery, comprising: Porous substrate; and A coating comprising binder and inorganic particles, said coating being formed on one or both surfaces of the porous substrate. The adhesive described herein is an adhesive comprising the following polymeric units: (a) a (meth)acrylamide monomer polymeric unit, (b) a (meth)acryloyl monomer polymeric unit containing hydroxyl groups, and (c) a multifunctional (meth)acrylamide monomer polymeric unit. The polyfunctional (meth)acrylamide monomer polymerization unit described in (c) is obtained by polymerizing a polyfunctional monomer represented by the following chemical formula 3: [Chemical Formula 3] in R1 and R2 are independently hydrogen or C1 to C6 alkyl groups. R is a straight chain from C1 to C2. 10 C1 to C2 of hydrocarbon group or branched chain 10 hydrocarbon group, and a is an integer from 2 to 6; The adhesive described herein is an adhesive made by comprising: (a) 65 mol% to 96 mol% of the (meth)acrylamide monomer, (b) 3 mol% to 34 mol% of the hydroxyl-containing (meth)acryloyl monomer, and (c) 0.001 mol% to 1 mol% of the polyfunctional (meth)acrylamide monomer.

2. The separator for a lithium secondary battery according to claim 1, wherein the adhesive has a weight-average molecular weight of 100,000 g / mol to 2,000,000 g / mol.

3. The separator for a lithium secondary battery according to claim 1, wherein the coating comprises 50% to 99.9% by weight of the inorganic particles and 0.1% to 50% by weight of the binder, accounting for 50% to 99.9% by weight of the total weight of the coating.

4. The separator for lithium secondary batteries according to claim 1, wherein the separator for lithium secondary batteries, after being left to stand at 150°C for 60 minutes, has a shrinkage rate of less than 5% in both the mechanical direction (MD) and the transverse direction (TD) at high temperature.

5. The separator for a lithium secondary battery according to claim 4, wherein the separator for a lithium secondary battery, after being left to stand at 150°C for 60 minutes, has a shrinkage rate of less than 3% in both the mechanical direction (MD) and the transverse direction (TD) at high temperature.

6. The separator for a lithium secondary battery according to claim 1, wherein the change in air permeability ΔG of the separator for the lithium secondary battery satisfies the following formula: [Formula 1] ΔG = G1 – G2 ≤ ​​70 in G1 is the Grylloy permeability of the diaphragm including the coating. G2 is the Grylloy permeability of the porous substrate itself, and The Gurley permeability was measured according to ASTM D726 and is expressed in seconds per 100 cubic centimeters.

7. The separator for a lithium secondary battery according to claim 1, wherein the thickness of the coating is from 0.3 μm to 10 μm.

8. A lithium secondary battery comprising a separator for a lithium secondary battery as described in any one of claims 1 to 7.

9. A method for manufacturing a separator for a lithium secondary battery, the method comprising: (s1) Prepare a slurry composition containing binder and inorganic particles; and (s2) Apply the slurry composition to one or both surfaces of a porous substrate to form a coating. The adhesive described herein is an adhesive comprising the following polymeric units: (a) a (meth)acrylamide monomer polymeric unit, (b) a (meth)acryloyl monomer polymeric unit containing hydroxyl groups, and (c) a multifunctional (meth)acrylamide monomer polymeric unit. The polyfunctional (meth)acrylamide monomer polymerization unit described in (c) is obtained by polymerizing a polyfunctional monomer represented by the following chemical formula 3: [Chemical Formula 3] in R1 and R2 are independently hydrogen or C1 to C6 alkyl groups. R is a straight chain from C1 to C2. 10 C1 to C2 of hydrocarbon group or branched chain 10 hydrocarbon group, and a is an integer from 2 to 6; The adhesive described herein is an adhesive made by comprising: (a) 65 mol% to 96 mol% of the (meth)acrylamide monomer, (b) 3 mol% to 34 mol% of the hydroxyl-containing (meth)acryloyl monomer, and (c) 0.001 mol% to 1 mol% of the polyfunctional (meth)acrylamide monomer.

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