Separator substrate for electrochemical device, separator including said substrate, and method for forming separator for battery cell

Abstract

The present invention relates to a separator substrate for an electrochemical device. The separator substrate according to the present invention has small and uniform pore sizes, and therefore has excellent physical strength and durability, and can ensure a high dielectric breakdown voltage, resulting in a low incidence of short circuits.

Description

[Technical field]

[0001] The present invention claims priority to Korean Patent Application No. 10-2022-0023883, filed on February 23, 2022.

[0002] The present invention relates to a separator substrate for electrochemical devices having excellent voltage resistance characteristics, and a separator including the same. [Background technology]

[0003] Polyolefin microporous membranes are widely used for battery separators, including lithium batteries, diaphragms for electrolytic capacitors, moisture-permeable waterproof clothing, various filtration membranes, etc. When such polyolefin microporous membranes are used as battery separators, their performance is closely related to the characteristics, productivity, and safety of the battery. Therefore, in addition to excellent mechanical properties and permeability, lithium-ion battery separators are particularly required to have the following properties: shutdown property (shutdown property) that closes pores when heated abnormally to prevent battery heat generation, fire, explosion, etc. caused by short circuiting of an external circuit, overcharging, etc.; and heat shrinkage resistance (heat shrinkage property) that maintains its shape even at high temperatures to prevent dangerous situations in which the positive electrode material and the negative electrode material directly react with each other.

[0004] Generally, a microporous membrane made of only polyethylene has a low meltdown temperature, and a microporous membrane made of only polypropylene has a high shutdown temperature, so battery separators made of microporous membranes mainly composed of polyethylene and polypropylene have been proposed.

[0005] For example, Japanese Patent No. 3235669 discloses a battery separator having excellent heat shrinkage resistance and shutdown characteristics, the battery separator having at least one first layer formed of a polymer selected from low-density polyethylene, an ethylene-butene copolymer, or an ethylene-hexene copolymer, and at least one second layer formed of a polymer selected from high-density polyethylene, ultra-high molecular weight polyethylene, or polypropylene.

[0006] Japanese Patent No. 3422496 discloses a battery separator having excellent shutdown characteristics, the battery separator having at least one first layer formed of a polymer selected from an ethylene-butene copolymer, an ethylene-hexene copolymer, an ethylene-methacrylate copolymer, or polyethylene, and at least one second layer formed of a polymer selected from polyethylene or polypropylene.

[0007] Japanese Patent No. 2883726 discloses a battery separator having excellent shutdown and meltdown properties, which is formed by simultaneously extruding polypropylene with a melting point of 150°C or higher and polyethylene with a melting point of 100°C to 140°C, uniaxially stretching the resulting laminated film at a temperature of -20°C to [melting point of polyethylene (Tm0)-30]°C, and further stretching in the same direction at a temperature of (Tm0-30)°C to (Tm0-2)°C to make it porous.

[0008] Japanese Patent Application Laid-Open No. 11-329390 proposes a battery separator with excellent shutdown characteristics and strength, which is formed from two microporous strength layers made of polypropylene material and a blocking layer made of filler-containing polyethylene material interposed between them, with the blocking layer made of filler-containing polyethylene material being formed from a microporous membrane manufactured by a particle stretching method.

[0009] The separator is bonded to the electrode to manufacture the battery, and the bonding is performed by a lamination process in which the electrode and the separator are laminated and then heat and / or pressure are applied. The higher the heat and pressure conditions applied in the lamination process, the higher the adhesive strength with the electrode. In recent years, in order to improve productivity, the process speed has been increased to shorten the time that heat is applied to the separator, so that adhesive strength is ensured by increasing the pressure, but there is a problem that the separator is deformed by high pressure. In particular, when a separator substrate that is weak against pressure is used, the thickness is significantly reduced and the pores are significantly damaged, which reduces not only the performance of the battery but also the dielectric breakdown voltage of the separator, which may result in Hi-pot failure and low voltage failure. Therefore, there is a need to develop a porous polymer film substrate for separators that is less deformed even under high pressure lamination conditions. Summary of the Invention [Problem to be solved by the invention]

[0010] The present invention provides a separator substrate for electrochemical devices having a low thickness deformation rate and a high dielectric breakdown voltage, and a separator including the same. Another object of the present invention is to provide a method for selecting a separator substrate having excellent voltage resistance characteristics. It will be easily understood that the objects and advantages of the present invention can be achieved by the means or methods described in the claims and combinations thereof. [Means for solving the problem]

[0011] According to an embodiment of the present invention to achieve the above object, there is provided a separation membrane substrate for an electrochemical element, the separation membrane substrate being a sheet-shaped porous membrane including a plurality of pores and including a polymer material, the polymer material including a polyolefin-containing resin, and the separation membrane substrate having a difference between maximum and minimum pressures of 100 psi or less when measuring a wet curve of a wet sample.

[0012] The separation membrane substrate includes fibrils having a diameter of 50 nm to 250 nm, and spaces between the fibrils form pores, and the separation membrane substrate may have a porosity of 30 vol % to 65 vol %.

[0013] The thickness of the separation membrane substrate may be 5 μm to 50 μm.

[0014] The polyolefin-containing resin may be contained in an amount of 90 wt % or more relative to 100 wt % of the polymer material.

[0015] The polyolefin-containing resin may include one or more selected from the group consisting of polyethylene, polypropylene, polybutylene, and polypentene.

[0016] Meanwhile, the polymer material may have a polydispersity index (PDI) value of 1.0 to 12.0.

[0017] In this case, the polymer material may contain 90 wt % or more of polyethylene having a polydispersity index (PDI) of 1.0 to 12.0 with respect to 100 wt % of the polymer material.

[0018] The polymer material may have a polydispersity index (PDI) value of 5.0 to 10.0.

[0019] In this case, the polymer material may contain 90 wt % or more of polyethylene having a polydispersity index (PDI) of 5.0 to 10.0 relative to 100 wt % of the polymer material.

[0020] Meanwhile, the polymer material may have a weight average molecular weight (Mw) of 200,000 g / mol to 1,800,000 g / mol.

[0021] According to another embodiment of the present invention, there is provided a method for forming a battery cell separator, including a first step of preparing a separator substrate; a second step of measuring a wet curve of a wet sample of the separator substrate and determining a minimum pressure and a maximum pressure from the measurement result of the wet sample's air permeability curve; a third step of calculating a difference between the maximum pressure and the minimum pressure determined in the second step; a fourth step of comparing the calculated value with a standard of 100 psi or less; and a fifth step of using the prepared separator substrate as a battery cell separator if the calculated value is 100 psi or less.

[0022] Before carrying out the second step, a dry curve of a dry sample is measured, and if the pressure on the separation membrane substrate is 30 psi to 500 psi as a result of measuring the dry curve of the dry sample, the separation membrane substrate can be selected and the third step can be carried out.

[0023] The separation membrane substrate can be produced by a wet production method or a dry production method.

[0024] Meanwhile, according to another embodiment of the present invention, there is provided a separator for an electrochemical device, comprising: a separator substrate according to the present invention; and a heat-resistant layer formed on one or both sides of the separator substrate; wherein the heat-resistant layer comprises a binder resin and inorganic particles.

[0025] In this case, the binder resin may include a particulate binder polymer.

[0026] Meanwhile, according to another embodiment of the present invention, there is provided an electrochemical device including a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, wherein the separator is the separator according to the present invention described above. Effect of the Invention

[0027] The separator substrate according to the present invention has a small pore size and a uniform pore distribution. When used in a separator, the separator and an electrode are laminated together and subjected to a lamination process, and the substrate exhibits little thickness deformation and high breakdown voltage. [Brief description of the drawings]

[0028] The drawings attached to this specification are illustrative of preferred embodiments of the present invention, and serve to facilitate a better understanding of the technical concept of the present invention together with the above-mentioned contents of the invention, and therefore the present invention should not be interpreted as being limited to only the matters shown in such drawings. Meanwhile, the shape, size, scale, ratio, etc. of elements in the drawings described in this specification may be exaggerated in order to emphasize a clearer description.

[0029] [Figure 1] 1 is a SEM (scanning electron microscope) image showing a separation membrane substrate according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing an example of an air permeability curve of a dry sample and an example of an air permeability curve of a wet sample. [Diagram 3] FIG. 2 is a schematic diagram showing a separation membrane substrate formed by laminating three-layer films according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] The present invention will be described in detail below. Prior to that, the terms or words used in the present specification and claims should not be interpreted limited to their ordinary or dictionary meanings, but should be interpreted with a meaning and concept that matches the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the concept of the term to best describe his / her invention. Therefore, it should be understood that the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred embodiments of the present invention and do not fully represent the technical idea of ​​the present invention, and therefore there may be various equivalents and modifications that can replace them at the time of this application.

[0031] Throughout the specification of this application, when a part is said to "comprise" certain elements, this means that it can further include other elements, rather than excluding other elements, unless specifically stated to the contrary.

[0032] In addition, the terms "about," "substantially," and the like used throughout the specification of this application are used to mean a numerical value or a approximation thereof when the manufacturing and material tolerances inherent in the referred meaning are given, and are used to prevent unscrupulous infringers from unfairly exploiting the disclosure in which precise or absolute numerical values ​​are recited to aid in the understanding of this application.

[0033] Throughout the specification of this application, the phrase "A and / or B" means "A or B, or both."

[0034] The present invention relates to a separator substrate for an electrochemical device. In the present invention, the electrochemical device is a device that converts chemical energy into electrical energy by an electrochemical reaction, and is a concept that includes a primary battery and a secondary battery. The secondary battery is capable of being charged and discharged, and is a concept that includes a lithium ion battery, a nickel-cadmium battery, a nickel-hydrogen battery, etc.

[0035] Separation membrane In the present specification, the separator serves as a porous ion-conducting barrier that allows ions to pass while blocking electrical contact between the negative and positive electrodes in an electrochemical device. It is preferable that a plurality of pores are formed therein and the pores are interconnected so that gas or liquid can pass from one side of the separator to the other side.

[0036] The separation membrane according to the present invention includes a porous separation membrane substrate including a plurality of pores and a polymer material, and the separation membrane substrate may be a polymer film. In addition, the separation membrane may additionally have another layer disposed on at least one surface of the separation membrane substrate, as necessary, in terms of material and function. In one embodiment of the present invention, the separation membrane may include the separation membrane substrate and a heat-resistant layer, and for example, the heat-resistant layer may be formed on at least one surface of the separation membrane substrate. The heat-resistant layer may include inorganic particles and / or a binder resin.

[0037] In one embodiment of the present invention, the inorganic particles in the heat-resistant layer may have a layered structure bound by a binder resin, and may exhibit a porous structure resulting from spaces (interstitial volumes) formed between the inorganic particles. Such a porous structure has the effect of improving the electrolyte retention of the separator. Meanwhile, in one embodiment of the present invention, when the separator includes a heat-resistant layer, the heat-resistant layer may be 3 vol% to 40 vol% relative to 100 vol% of the total volume of the separator, and simultaneously or independently, the heat-resistant layer may be 5 vol% to 50 vol% relative to 100% of the total thickness of the separator.

[0038] Separation membrane base material FIG. 1 is a SEM (scanning electron microscope) image showing a separation membrane substrate according to one embodiment of the present invention.

[0039] In one embodiment of the present invention, the separation membrane substrate is a sheet-like porous membrane containing a polymer material and having a plurality of pores. For example, the separation membrane substrate may be a porous polymer film. The pores include open pores, and the open pores are connected to each other, so that gas or liquid can pass from one side of the separation membrane substrate to the other side. In one embodiment of the present invention, the separation membrane substrate preferably has an air permeability of 2000 sec / 100 cc or less and a porosity of 30 vol% to 65 vol%, more specifically 35 vol% to 55 vol%, from the viewpoint of the output and cycle characteristics of the battery. Meanwhile, in the present invention, the separation membrane substrate preferably has an average pore diameter of 10 nm to 100 nm, specifically 15 nm to 70 nm, more specifically 20 nm to 40 nm, and even more specifically 20 nm to 30 nm or 30 nm to 40 nm. If the average pore diameter is less than 10 nm, there is a problem that shutdown may occur even at low temperatures, causing the separation membrane to lose its function, and if the average pore diameter is more than 100 nm, there is a possibility that the separation membrane may lose its voltage resistance.

[0040] Meanwhile, in the present invention, the separation membrane substrate is stretched during the preparation of the separation membrane substrate, which causes cleavage between polyolefin lamellae, resulting in finer polymers and the formation of a large number of fibrils. The fibrils form a very fine network structure that is irregularly connected three-dimensionally, and the spaces between the fibrils serve as pores.

[0041] In the present invention, the fibrils may have a diameter of 50 nm to 250 nm, specifically 100 nm to 200 nm, more specifically 120 nm to 180 nm, and even more specifically 140 nm to 160 nm. If the diameter of the fibrils is thicker than 250 nm, the size of the pores formed around the fibrils may become large, making it difficult to ensure insulation. On the other hand, if the diameter of the fibrils is less than 50 nm, the fibril structure may easily deform during the lamination process in the manufacture of the battery, which may cause a problem of blocking the pores of the separator substrate.

[0042] Furthermore, the average diameter of the fibrils may be in the range of 100 nm to 200 nm, specifically 140 nm to 160 nm. If the average diameter of the fibrils is greater than 200 nm, a problem of increased overall resistance of the separator substrate may occur. On the other hand, if the average diameter of the fibrils is less than 100 nm, the fibril structure may easily deform during the lamination process in the manufacture of the battery, and the pores of the separator substrate may easily become clogged.

[0043] In one embodiment of the present invention, the diameter of the fibrils may be measured through an image of a scanning electron microscope (SEM) secured to a separation membrane substrate, but is not limited thereto.

[0044] In the present invention, the air permeability means the time (seconds) required for 100 ml of air to pass through a separation membrane substrate or separation membrane of 1 square inch size under a constant air pressure of 4.8 inches H2O. The air permeability can be measured, for example, using an EG01-55-1MR device manufactured by Asahi Seiko Co., Ltd.

[0045] The porosity means the ratio of the volume of pores to the total volume, and is expressed in vol%. It can be used in the same sense as terms such as porosity and porosity. In the present invention, the measurement of the porosity is not particularly limited, and can be measured, for example, by BET (Brunauer-Emmett-Teller) measurement using nitrogen gas or mercury penetration method (Hg porosimeter) according to one embodiment of the present invention. Alternatively, in one embodiment of the present invention, the true density of the electrode active material layer is calculated from the density (apparent density) of the obtained electrode (electrode active material layer), the composition ratio of the materials contained in the electrode (electrode active material layer), and the density of each component, and the porosity of the electrode active material layer can be calculated from the difference between the apparent density and the true density (net density).

[0046] In the present invention, the separator substrate has small and uniform pore size, and when applied to a separator for an electrochemical device, it can exhibit excellent dimensional stability and voltage resistance. Meanwhile, the pore distribution of the separator substrate can be confirmed through a sample air permeability curve test. The separator substrate according to the present invention preferably has a difference between maximum and minimum pressure of 100 psi or less when measuring the "wet curve".

[0047] If the pressure difference exceeds 100 psi, it means that there is a large difference in size between the largest and smallest pores in the separation membrane substrate, i.e., the pore sizes vary widely and are not uniform or consistent, which may result in a decrease in the insulating properties of the separation membrane after compression.

[0048] FIG. 2 is a schematic diagram showing an example of an air permeability curve of a dry sample and an air permeability curve of a wet sample. Referring to FIG. 2, in the present invention, the maximum pressure of the wetting curve means the pressure at the point where the air permeability curve of the wet sample intersects with the air permeability curve of the dry sample. Also, the minimum pressure of the wetting curve means the pressure at the point where the bubble point occurs. For the measurement of the bubble point, the following description may be referred to.

[0049] The wet curve is obtained by wetting a porous structure with a solution (Galwick solution) having a low surface tension of about 15.9 dynes / cm, pressurizing the structure with gas, and determining the pressure required to push out the solution filling the pores of the structure. In one embodiment of the present invention, the wet curve can be obtained by preparing a dry separation membrane substrate and a test solution, filling the pores of the separation membrane substrate with the test solution, pushing the test solution out of the pores, and measuring the relationship between air pressure and air flow rate using a porosimeter when the test solution is pushed out of the pores. The term "dry state" means that the state after the preparation of a separation membrane substrate by a conventional method for preparing the substrate is maintained and that the substrate is not in contact with liquids such as water or organic solvents. In one embodiment of the present invention, the air permeability curve of the wet sample can be measured by the following method. The wet sample in which the pores are filled with the test solution exhibits the same characteristics as a capillary filled with a liquid. When a wet sample is attached to a porosimeter and the air pressure is gradually increased, the air pressure becomes higher than the surface tension of the liquid in the pores in the order from the larger diameter pores to the smaller diameter pores, and the liquid is forced out of the pores. As a result, the air flow rate gradually increases, and finally the sample becomes dry. Therefore, by measuring the pressure when the liquid is forced out of the pores, the minimum and maximum pressure values ​​can be confirmed, and the diameter of the pores can be calculated. Here, assuming that the shape of the pores is approximately cylindrical, the condition for air at pressure P to enter a pore with diameter D is expressed by the Washburn equation shown in the following equation 1, where the surface tension of the liquid is γ and the contact angle of the liquid is θ. PD=4γcosθ……(Equation 1)

[0050] In particular, the measurement point where the generation of bubbles is first detected (the measurement point showing the maximum pore size) is called the bubble point. A standard method for measuring the bubble point is, for example, the method described in ASTM F316-86.

[0051] On the other hand, when the difference between the maximum and minimum pressures in the measurement of the "wet curve" is within the range of 100 psi or less, the separator substrate has small pore size and high pore size uniformity. A separator substrate having such characteristics can exhibit high dimensional stability and high dielectric breakdown voltage.

[0052] Meanwhile, in one embodiment of the present invention, the pressure range of the separation membrane substrate when measuring the "dry curve of a dry sample" is preferably 30 psi to 500 psi, specifically 140 psi to 400 psi. The dry curve is obtained by pressurizing a porous structure using a gas and determining the pressure required to push out the existing gas filling the pores of the structure. In one embodiment of the present invention, the dry curve can be obtained by preparing a separation membrane substrate in a dry state, pushing out the gas from the pores, and measuring the relationship between the air pressure and the air flow rate using a porosimeter when the gas is pushed out from the pores.

[0053] In the present invention, the separator substrate may have a thickness of 3 μm to 50 μm, specifically 5 μm to 40 μm, more specifically 5 μm to 30 μm, from the viewpoint of thinning the electrochemical device and increasing the energy density. If the thickness of the separator substrate is less than 3 μm, the conductive barrier function may be insufficient, while if the thickness exceeds 50 μm (i.e., if it is too thick), the resistance of the separator may increase excessively, and the content of the electrode active material in the electrochemical device may relatively decrease, resulting in a decrease in the capacity of the electrochemical device.

[0054] Meanwhile, in one embodiment of the present invention, the polymer material is preferably a thermoplastic resin having a melting point of 200° C. or less from the viewpoint of providing a shutdown function, and may include one or more polyolefin-containing resins. The shutdown function refers to a function in which, when the battery temperature becomes high, the polymer resin melts and blocks the pores of the separation membrane substrate, thereby blocking the movement of ions between the positive and negative electrodes, thereby preventing thermal runaway of the battery. Incidentally, the shutdown performance of the separation membrane is related to the melting point, molecular weight, and crystallinity due to stretching of the polymer material, and the melting point and crystallinity can be determined by the heat setting process in the manufacturing method of the separation membrane substrate described later.

[0055] On the other hand, in the present invention, the polyolefin-containing resin preferably accounts for 90 wt % or more, or 95 wt % or more of 100 wt % of the polymer material.

[0056] The polyolefin-containing resin may include, for example, one selected from the group consisting of polyethylene, polypropylene, polybutene, and polypentene, or a mixture of two or more of these. In particular, the polyolefin-containing resin may be polyethylene and / or polypropylene.

[0057] Meanwhile, in a specific embodiment of the present invention, the polymeric material has a polydispersity index (PDI) value in the range of 1.0 to 12.0, preferably 3.8 to 12.0, more preferably 5.0 to 10.0. In one embodiment, when two or more polymeric materials are mixed, the mixed polymeric materials can satisfy the polydispersity index (PDI).

[0058] In a more preferred embodiment, the polymeric material preferably contains a single-component polymer resin in an amount of more than 50 wt%, 70 wt% or more, or 90 wt% or more relative to 100 wt% of the polymeric material, within the range of the polydispersity index (PDI). For example, the polymeric material may be composed of only a single component. The term "single component" means that the polymeric material has the same chemical structure and the polydispersity index (PDI) is in the range of 1.0 to 12.0.

[0059] In one specific embodiment of the present invention, the polymer material may contain 90 wt % or more of polyethylene having a polydispersity index (PDI) of 1.0 to 12.0 relative to 100 wt % of the polymer material, or may be composed solely of such polyethylene.

[0060] As described above, when the polydispersity index (PDI) value is satisfied or the polymer resin content range is satisfied together with or independently, it can help improve the uniformity in terms of the size and distribution of pores in the polymer substrate. Here, when the polydispersity index (PDI) exceeds 12.0, the uniformity of pores and mechanical strength may be reduced, and problems may occur that may adversely affect the voltage resistance characteristics, which is not preferable.

[0061] Meanwhile, the polydispersity index (PDI) can be calculated from the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn).

[0062] Meanwhile, in the present invention, the weight average molecular weight (Mw) and number average molecular weight (Mn) can be measured by gel permeation chromatography (GPC, PL GPC220, Agilent Technologies), and the measurement conditions can be set as follows. -Column: PL Olexis (Polymer Laboratories) -Solvent: TCB (Trichlorobenzene) -Flow rate: 1.0ml / min -Sample concentration: 1.0mg / ml -Injection volume: 200μl -Column temperature: 160℃ -Detector: Agilent High Temperature RI detector -Standard: Polystyrene (corrected by a cubic function)

[0063] In another embodiment, the polymeric material may have a melting index (MI) in the range of 0.02 g / 10 min to 1.0 g / 10 min. The MI is based on the condition that a load of 21.6 kg is applied at 190° C.

[0064] Meanwhile, in one embodiment of the present invention, the polymer material may have a weight average molecular weight (Mw) of 200,000 g / mol to 1,800,000 g / mol, preferably 300,000 g / mol to 1,500,000 g / mol, and more preferably 500,000 g / mol to 1,000,000 g / mol. If the weight average molecular weight is less than 200,000 g / mol, the mechanical properties and heat resistance may be deteriorated, and if the weight average molecular weight is more than 1,800,000 g / mol, the viscosity of the polymer material may be excessively high, which may cause problems such as deterioration of extrusion performance when produced as a film, and difficulty in stretching due to excessive elasticity.

[0065] In one embodiment of the present invention, the polymer material may have a number average molecular weight (Mn) of 30,000 g / mol to 300,000 g / mol. If the number average molecular weight is less than 30,000 g / mol, the mechanical properties and heat resistance may be deteriorated, and if the number average molecular weight is more than 300,000 g / mol, the viscosity of the polymer material may be too high, which may cause problems such as poor extrusion performance when produced as a film, and difficulty in stretching due to excessive elasticity.

[0066] Meanwhile, in a specific embodiment of the present invention, the separation membrane substrate may contain polyethylene, and may contain polypropylene as necessary. In this case, the polyethylene may be 95 wt% or more of 100 wt% of the polymer material, and the remainder may be polypropylene. The separation membrane substrate may contain polypropylene, but the content of polypropylene in the separation membrane substrate is preferably controlled to 5 wt% or less, for example, less than 5 wt%. The higher the polypropylene content, the higher the heat resistance of the separation membrane can be. However, if the polypropylene content exceeds 5 wt% of the above range, it is chemically unstable and pores are not well formed when preparing the separation membrane substrate by the wet method described below, which is disadvantageous for the development of porous characteristics, so that the content is preferably appropriately controlled to less than 5 wt% of the above range.

[0067] In addition, when the polypropylene content is high, it is easier to produce a separation membrane substrate with well-developed pores by a dry production method rather than a wet production method, but when a dry production method is applied, it is more difficult to control the thickness of the separation membrane substrate to be thinner than when a wet production method is applied.

[0068] Meanwhile, in a specific embodiment of the present invention, the separation membrane substrate may further include at least one polymer resin, such as polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalene, etc., as necessary.

[0069] Meanwhile, in a specific embodiment of the present invention, the separation membrane substrate may be a porous polymer film prepared by the method described below, and may be a single monolayer film or a multilayer film formed by laminating two or more sheets.

[0070] More specifically, the separation membrane substrate of the present invention may be a multilayer film formed by laminating two or more layers or three or more layers.

[0071] FIG. 3 is a schematic diagram showing a separator substrate formed by laminating three-layer films according to an embodiment of the present invention. Referring to FIG. 3, the pores formed in the films 10 and 30 present at the surface portion of the separator substrate 100 may be formed to be smaller in size than the pores formed in the film 20 present at the middle portion. For example, the size of the pores formed in the films present at the surface portion of the separator substrate may be 20 nm to 40 nm, and the size of the pores formed in the film present at the middle portion may be 30 nm to 50 nm. When such conditions are met, the possibility that the binder resin present in the selective heat-resistant layer penetrates into the pores inside the separator substrate can be further reduced, and the safety of the electrochemical device can be improved by imparting shutdown performance.

[0072] At the same time or independently, the safety of the separation membrane substrate can be improved by adjusting the melting point of the film present in the middle portion of the separation membrane substrate to be higher than that of the film present in the surface portion. That is, when a problem of a sudden rise in temperature occurs during the operation of an electrochemical device, the surface portion of the separation membrane substrate can be melted by heat to block the pores, thereby providing shutdown performance. In this case, the melting point of the film present in the surface portion of the separation membrane substrate can be 125°C to 130°C, and the melting point of the film present in the middle portion can be 135°C to 140°C.

[0073] At the same time or independently, the weight average molecular weight of the film present in the middle part of the separation membrane substrate can be adjusted to be higher than that of the film present in the surface part of the separation membrane substrate, which can help improve the compression resistance of the separation membrane substrate. In this case, the weight average molecular weight of the film present in the surface part of the separation membrane substrate can be 200,000 g / mol to 400,000 g / mol, and the weight average molecular weight of the film present in the middle part can be 300,000 g / mol to 800,000 g / mol.

[0074] As described above, when the separator substrate is a multilayer film having two or more layers, the size of the pores formed in the film in the middle portion is larger than that of the film in the surface portion, and the melting point and weight average molecular weight of the film in the middle portion are adjusted to be higher than those of the film in the surface portion. However, the present invention is not limited thereto, and the separator substrate may satisfy the opposite conditions to those described above depending on the intended use of the electrochemical device, etc.

[0075] In the present invention, a separator that satisfies the above values ​​has improved voltage resistance characteristics of a battery, increasing the dielectric breakdown voltage, and reducing the rate of short circuit occurrence (Hi-pot failure rate) even under high voltage conditions.

[0076] Meanwhile, in the present invention, the dielectric breakdown voltage is the maximum voltage that an insulator can withstand, and dielectric breakdown means that when a voltage is applied to an insulator, if the voltage exceeds a certain value, the insulator breaks down and loses its insulating properties.

[0077] In one embodiment of the present invention, the withstand voltage characteristics can be confirmed by measuring the dielectric breakdown voltage of the separator, which can be confirmed by disposing a separator, which is an insulator, between two conductors, applying a voltage, and measuring the voltage at which dielectric breakdown occurs.

[0078] The dielectric breakdown voltage can be measured, for example, with an AC / DC / IR Hi-pot tester. For example, a stainless steel mesh and a separator substrate are hot-pressed together at 90°C, 4 MPa, and 1 sec, and then a DC current of 0.5 mA and a voltage rise of 100 V / s (voltage 3 kV, ramp-up time 3 s) are set. When the experiment is started, the measurement is completed when the voltage rises and a short circuit occurs, and the voltage at that time is defined as the dielectric breakdown voltage.

[0079] In one embodiment of the present invention, the short circuit occurrence rate (Hi-pot failure rate) can be evaluated by determining the voltage of the bottom 1% of test pieces that exhibit low breakdown voltage through a Weibull distribution analysis of the total number of test pieces tested.

[0080] The dielectric breakdown voltage of the separator substrate according to the present invention may be 400 V or more, and satisfying such a dielectric breakdown voltage is sufficient to achieve the voltage resistance characteristics of the separator substrate.

[0081] Method for producing separation membrane substrate In one embodiment of the present invention, the separation membrane substrate can be prepared by a method for preparing a polymer film, including a wet preparation method and a dry preparation method. In the dry preparation method, a separation membrane is prepared by forming an extrusion sheet, stretching the extrusion sheet, and heat fixing the extrusion sheet without using a pore-forming agent and an extraction solvent, as compared with the wet preparation method described below.

[0082] Among the above methods, the wet manufacturing method is preferred, which includes the steps of (S1) preparing a mixture, (S2) extruding the mixture and forming an extruded sheet, (S3) stretching the extruded sheet, (S4) removing the pore-forming agent, and (S5) heat fixing the extruded sheet.

[0083] In the step (S1), the type of polymer resin is appropriately selected according to the final properties of the separation membrane, and the selected polymer resin is mixed with a pore-forming agent. The polymer resin may refer to the above description of the polymer resin of the separation membrane substrate. For example, the polymer resin may be a polyolefin-containing polymer resin. Examples of the polyolefin-containing polymer resin include one selected from polyethylene, for example, high density polyethylene, linear low density polyethylene, low density polyethylene, or ultra-high molecular weight polyethylene, polypropylene, polybutylene, polypentene, etc., or a combination of two or more of them.

[0084] The pore former is a material dispersed in a polymer, which shows the heterogeneity of the substrate produced by extrusion, stretching, etc., and is subsequently removed from the substrate. Thus, the portion of the substrate where the pore former was located remains in the form of pores in the substrate. The pore former may be a material that maintains a liquid or solid state during the extrusion process. The pore former may be an aliphatic hydrocarbon-containing solvent such as liquid paraffin, paraffin oil, mineral oil, or paraffin wax; a vegetable oil such as soybean oil, sunflower oil, rapeseed oil, palm oil, coconut oil, corn oil, grapeseed oil, cottonseed oil, etc.; or a plasticizer such as dialkyl phthalate. In particular, the plasticizer may be di-2-ethylhexyl phthalate (DOP), di-butyl-phthalate (DBP), di-isononyl phthalate (DINP), di-isodecyl phthalate (DIDP), butyl benzyl phthalate (BBP), etc. Among these, liquid paraffin (LP, also called "liquid paraffin") is particularly preferred.

[0085] In addition, the content of the pore-forming agent during the preparation of the separator may be appropriately adjusted to realize a desired level of porosity. In consideration of improving the air permeability, it is preferable that the content of the pore-forming agent is high, but if it is contained in an excessive amount, it may have a negative effect on the strength of the substrate finally produced. Therefore, the content of the pore-forming agent may be 1 wt% to 80 wt% based on the total amount of the polymer resin and the pore-forming agent (100 wt%), and may be adjusted to 70 wt% or less, 60 wt% or less, or 50 wt% or less within the above range as necessary, or may be adjusted to 1 wt% or more, 20 wt% or more, or 40 wt% or more. Meanwhile, in a specific embodiment of the present invention, in order to realize a suitable porosity of the separator substrate, for example, to realize a porosity of about 45% or less, the pore-forming agent may be included in a range of 1 wt% to 60 wt% based on the total amount of the polymer resin and the pore-forming agent.

[0086] Next, the mixture prepared in the above step is extruded through an extruder to obtain an extruded sheet. The extruder is not particularly limited and may be an extruder commonly used in the art, for example, an extruder equipped with a T-die or a tubular die. The extrusion process may be performed at a normal extrusion temperature, and is preferably performed at a temperature 10°C to 100°C higher than the melting point of the polymer resin used. If the extrusion process is performed excessively beyond the above range, the polymer resin may be thermally degraded, which may make film formation difficult and may cause a deterioration in the physical properties of the substrate produced, which is not preferable. An extruded sheet can be obtained by such an extrusion process.

[0087] The extruded sheet is then subjected to a stretching process. The stretching process is carried out using a stretching machine commonly used in the art. The stretching machine may be, but is not limited to, a sequential biaxial stretching machine. The mechanical strength of the separator substrate can be increased by stretching the extruded sheet in this way. The stretching process is carried out in the machine direction (MD) and / or the transverse direction (TD). The stretching causes cleavage between the lamellae of the polyolefin to form a large number of fibrils, which are connected three-dimensionally irregularly to form a very fine mesh structure. In addition, the stretching process in all or one of these directions can increase the tensile strength in the corresponding stretching direction. If necessary, the separation membrane of the present invention can be stretched in the machine direction (MD) and / or the transverse direction (TD) alone (e.g., uniaxial stretching), simultaneously or sequentially (e.g., biaxial stretching) in the stretching process. Meanwhile, in one embodiment of the present invention, the temperature of the membrane during the stretching can be controlled to 100° C. to 130° C., preferably 110° C. to 125° C. For example, the temperature of the membrane during the stretching can be controlled in the range of 115° C. to 121° C. When the stretching step is performed within the above temperature range, a uniform membrane with small pores can be obtained.

[0088] In one embodiment of the present invention, during the stretching step, the stretch ratio in the MD / TD direction is more than 3 times and less than 12 times, specifically more than 4 times and less than 10 times, more specifically more than 5 times and less than 7 times, and even more specifically about 6 to 7 times. The total stretch ratio is preferably about 20 to 80 times.

[0089] If the stretch ratio is less than the above range, the orientation in one direction may be insufficient, and at the same time, the balance of physical properties between the machine direction and the transverse direction may be lost, resulting in a decrease in tensile strength, pin puncture strength, etc. In addition, if the total stretch ratio is less than 20 times, there is a possibility that the film may not be fully stretched and pores may not be formed, and if it exceeds 80 times, there may be a disadvantage that breakage may occur during stretching and the shrinkage rate of the final film may increase.

[0090] In one embodiment of the present invention, the stretching amounts in MD and TD are approximately the same. If the stretching amounts in both MD and TD are not approximately the same, it may be unfavorable from the viewpoint of the non-uniformity of the film.

[0091] Next, the pore-forming agent is removed from the extruded sheet obtained above. The pore-forming agent is removed by extracting it using a solvent and drying it. Also, as a result of this removal, the spaces occupied by the pore-forming agent are formed as pores. Any solvent capable of extracting the pore-forming agent can be used as the solvent for extracting the pore-forming agent, but preferably, methyl ethyl ketone, methylene chloride, hexane, etc., which have high extraction efficiency and quick drying, are suitable. Preferably, the solvent is methylene chloride, for example, methylene dichloride (MC). As the extraction method, any commonly used solvent extraction method such as an immersion method, a solvent spray method, an ultrasonic method, etc., can be used individually or in combination.

[0092] After the extraction of the pore-forming agent, the substrate is heat-set, thereby finally obtaining a separation membrane having the desired physical properties, porosity, and air permeability. The heat-set step can be performed using a heating device, such as an oven, capable of applying an appropriate temperature required for heat-set. In particular, the first dried membrane is heat-set to reduce the shrinkage rate of the final membrane by removing the residual stress. Heat-set is a method of fixing a membrane and forcibly holding the membrane that is about to shrink by applying heat to remove the residual stress. A higher heat-set temperature is advantageous for reducing the shrinkage rate, but if the temperature is too high, the membrane may partially melt, so that the formed pores are blocked and the permeability is reduced. The preferred heat-set temperature is selected within a temperature range in which about 10wt% to 30wt% of the crystalline portion of the membrane melts. If the heat setting temperature is selected to be lower than the temperature at which about 10 wt% of the crystalline portion of the membrane melts, the reorientation of the polyethylene molecules in the membrane may be insufficient, and the effect of removing residual stress in the membrane may be insufficient, and if the heat setting temperature is selected to be higher than the temperature at which about 30 wt% of the crystalline portion of the membrane melts, the pores may be blocked due to partial melting, resulting in a decrease in permeability. More specifically, when the heat setting temperature is 130°C or lower, preferably 120°C to 130°C, the pores are formed small and appropriately sized at about 10 nm to 100 nm, thereby improving the shutdown performance and compression resistance of the separator. If the heat setting temperature exceeds 130°C, this may be detrimental to the shutdown performance of the separation membrane. However, this can be compensated for by using a pore-forming agent (diluent) with a relatively high kinetic viscosity of about 50 cSt or an ultra-high molecular weight polymer resin with a weight average molecular weight of 1,000,000 g / mol to 2,000,000 g / mol, specifically about 1,500,000 g / mol.

[0093] Meanwhile, in one embodiment of the present invention, after preparing the separation membrane substrate, a step of evaluating a wet curve of the separation membrane substrate, measuring a minimum pressure and a maximum pressure according to the result, calculating the difference between the minimum pressure and the maximum pressure, and comparing the difference with a preset standard may be further performed. As described above, it is preferable that the difference between the minimum pressure and the maximum pressure in the wet curve measurement is 100 psi or less.

[0094] Meanwhile, in one embodiment of the present invention, a dry curve may be measured before the wet curve is measured, and a separation membrane substrate for which the pressure when the dry curve is measured is in the range of 30 psi to 500 psi may be preliminarily selected, and a wet curve may be measured using the selected substrate.

[0095] In one embodiment of the present invention, the separation membrane substrate may be a single layer, or may be a laminated film in which two or more layers of films are laminated together, at least one of the films included in the laminated film may be formed by the above-mentioned method.

[0096] heat resistant layer In one embodiment of the present invention, the separation membrane may include a heat-resistant layer formed on at least one surface of the separation membrane substrate. The heat-resistant layer may include a viscous binder resin and inorganic particles, and may have a structure in which a large number of micropores are formed inside and these micropores are connected to each other, and may have a structural feature of a porous layer that allows gas or liquid to pass from one side to the other side. In one embodiment of the present invention, the binder resin and the inorganic particles in the heat-resistant layer may be included in a weight ratio of 1:99 to 30:70, preferably 5:95, more preferably 10:90, even more preferably 15:85, and even more preferably 20:80. The ratio may be appropriately adjusted within the above range, and for example, the binder resin may be 1 wt% or more, 5 wt% or more, or 10 wt% or more, and the inorganic particles may be 80 wt% or more, 85 wt% or more, 90 wt% or more, or 95 wt% or more, based on a total of 100 wt% of the binder resin and the inorganic particles. In the present invention, the heat-resistant layer preferably has a porous structure from the viewpoint of ion permeability.

[0097] The heat-resistant layer may be formed by binding inorganic particles with a binder resin as a medium, and pores may be formed by interstitial volumes between the inorganic particles. The interstitial volumes are spaces defined by inorganic particles that are in substantial surface contact with each other in a packed structure (closed packed or densely packed) of inorganic particles.

[0098] In one embodiment of the present invention, the porosity of the heat-resistant layer is 30vol% to 70vol%, and within the range, the porosity may be 35vol% or more, or 40vol% or more, and simultaneously or independently, 65vol% or less, or 60vol%. For example, the porosity may be 40vol% to 60vol%. When the porosity is 70vol% or less, it is possible to ensure mechanical properties that can withstand the pressing process for bonding with the electrode, and the surface opening ratio is not too high, so it is suitable for ensuring adhesive strength. On the other hand, when the porosity is 30vol% or more, it is advantageous in terms of ion permeability.

[0099] Meanwhile, in the present invention, the porosity can be measured using an adsorption gas such as nitrogen with a BELSORP (BET apparatus) manufactured by BEL JAPAN Co., Ltd., or by a method such as mercury intrusion porosimetry. Alternatively, in one embodiment of the present invention, the true density of the electrode active material layer can be calculated from the density (apparent density) of the obtained electrode (electrode active material layer), the composition ratio of the materials contained in the electrode (electrode active material layer), and the density of each component, and the porosity of the electrode active material layer can be calculated from the difference between the apparent density and the true density (net density).

[0100] The thickness of the heat-resistant layer may be 1 μm to 6 μm on one side of the separator substrate. Within the above range, the thickness of the heat-resistant layer may be 2 μm or more, or 3 μm or more, as necessary. Within the above numerical range, the adhesive strength with the electrode is excellent, and as a result, the cell strength of the battery is increased. On the other hand, if the thickness is 6 μm or less, it is advantageous in terms of the cycle characteristics and resistance characteristics of the battery.

[0101] In the present invention, non-limiting examples of binder resins that can be used in the heat-resistant layer include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, ethylene-vinyl acetate copolymer, polyethylene oxide, and the like. The binder resin may be any one selected from the group consisting of polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, cyanoethyl oxide, polyarylate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxyl methyl cellulose, or a mixture of two or more of these. However, the binder resin is not particularly limited thereto. More specifically, a particulate binder polymer, not a non-particulate binder polymer, may be used as the binder resin, and such a particulate binder polymer may have a D50 diameter of 500 nm or less. By using a binder polymer having a particle shape, not a non-particulate shape, the porosity in the heat-resistant layer can be secured. In addition, by using a particulate binder polymer having a predetermined numerical range, there is an advantage that the particulate binder polymer does not penetrate into the pores in the porous polymer substrate, and therefore the resistance does not increase.Furthermore, the particulate binder polymer has a shell-type gel structure, which ensures the elasticity of the heat-resistant layer.

[0102] For this purpose, the particulate binder polymer may have a D50 diameter larger than the pores of the porous polymer substrate, for example, a D50 diameter of 80 nm or more, 90 nm or more, or 100 nm or more.

[0103] The particulate binder polymer may include an acrylic particulate binder (e.g., a copolymer of butyl acrylate and ethylhexyl acrylate, a copolymer of methyl methacrylate and ethylhexyl acrylate, polyacrylonitrile, polycyanoacrylate, a copolymer of butyl acrylate and styrene, etc.), acrylonitrile-butadiene-styrene rubber, acrylonitrile-butadiene rubber, polyvinyl chloride, polyvinylidene fluoride, polyvinyl alcohol, polystyrene, or a mixture of two or more of these.

[0104] In a specific embodiment of the present invention, the inorganic particles usable in the heat-resistant layer are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles usable in the present invention are within the range of the operating voltage of the applied electrochemical element (for example, Li / Li + There are no particular limitations as long as the oxidation and / or reduction reaction does not occur at a voltage of 0 to 5 V relative to the reference voltage.

[0105] Non-limiting examples of the inorganic particles include BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb 1-x Lax Zr 1-y Ti y O3(PLZT, 0 <x<1、0<y<1)、Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3 (PMN-PT), hafnia (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH)2, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al2O3, SiC, Al(OH)3, TiO2, aluminum peroxide, zinc tin hydroxide (ZnSn(OH)6), tin-zinc oxide (Zn2SnO4, ZnSnO3), antimony trioxide (Sb2O3), antimony tetroxide (Sb2O4), antimony pentoxide (Sb2O5), etc., and one or more of these may be included.

[0106] Independently or in combination with the above-listed components, the inorganic particles may include inorganic particles having lithium ion transport capability. Non-limiting examples of inorganic particles having lithium ion transport capability include lithium phosphate (Li3PO4), lithium titanium phosphate (Li x Ti y (PO4)3, 0 <x<2、0<y<3)、リチウムアルミニウムチタンホスフェート(Li x Al y Ti z (PO4)3, 0 <x<2、0<y<1、0<z<3)、14Li2O-9Al2O3-38TiO2-39P2O5などといった(LiAlTiP) x O y Glass (0 <x<4、0<y<13)、リチウムランタンチタネート(Li x La y TiO3, 0 <x<2、0<y<3)、Li 3.25 Ge 0.25 P 0.75 Lithium germanium thiophosphate (Li S4) x Ge y P z S w , 0 <x<4、0<y<1、0<z<1、0<w<5)、Li3Nなどといったリチウムナイトライド(Li x Ny ., 0 < x < 4, 0 < y < 2), glass containing SiS2 such as Li3PO4-Li2S-SiS2 (Li x Si y S z ., 0 < x < 3, 0 < y < 2, 0 < z < 4), glass containing P2S5 such as LiI-Li2S-P2S5 (Li x P y S z ., 0 < x < 3, 0 < y < 3, 0 < z < 7) or mixtures thereof, etc.

[0107] Also, the average diameter (D 50 ) of the inorganic particles is not particularly limited, but is preferably in the range of 0.3 μm to 1 μm for forming a coating layer with a uniform thickness and an appropriate porosity. If it is less than 0.3 μm, the dispersibility of the inorganic particles in the slurry prepared for manufacturing the heat-resistant layer may decrease, and if it exceeds 1 μm, the thickness of the formed coating layer may increase.

[0108] In one embodiment of the present invention, the method for forming the heat-resistant layer is as follows, for example. First, a binder resin is dissolved in an appropriate organic solvent to produce a polymer solution. As the solvent, it is preferably similar in solubility index to the binder polymer to be used and has a low boiling point. This is to facilitate uniform mixing and subsequent solvent removal. Non-limiting examples of solvents that can be used include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, water, or mixtures thereof, etc.

[0109] Next, inorganic particles are added and dispersed in the prepared polymer solution. In the present invention, the content ratio of the inorganic particles to the binder is as described above, and is appropriately adjusted in consideration of the thickness, pore size, and porosity of the heat-resistant layer of the present invention to be finally prepared.

[0110] Next, the inorganic particle slurry prepared above is applied to at least one side of a separator and dried. The method of coating the slurry on the separator substrate is not limited to any particular method, and any common coating method known in the art can be used. For example, various methods such as dip coating, die coating, roll coating, comma coating, or a combination thereof can be used.

[0111] The drying process is performed by appropriately setting temperature and time conditions to minimize the occurrence of surface defects on the surface of the composite porous layer. The drying process can be performed using auxiliary drying devices such as a drying oven or hot air within an appropriate range.

[0112] The separation membrane of the present invention can also be produced by a method in which the heat-resistant layer and the separation membrane substrate are separately prepared, these sheets are superimposed, and composited by thermocompression bonding or adhesive, etc. As a method for obtaining the heat-resistant layer as an independent sheet, there can be mentioned a method in which the slurry is applied onto a release sheet, the heat-resistant layer is formed by the method described in detail, and only the heat-resistant layer is peeled off, etc.

[0113] Meanwhile, the present invention provides a secondary battery including the separator, the battery including a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, the separator being a low resistance separator having the above-mentioned characteristics.

[0114] In the present invention, the positive electrode includes a positive electrode current collector and a positive electrode active material layer containing a positive electrode active material, a conductive material, and a binder resin on at least one surface of the current collector. The positive electrode active material includes layered compounds such as lithium manganese composite oxides (LiMn2O4, LiMnO2, etc.), lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and compounds substituted with one or more transition metals; chemical formula Li 1+x Mn 2-x O4 (where x is 0 to 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, Cu2V2O7; chemical formula LiNi 1-x M x O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x = 0.01 to 0.3), Ni-site type lithium nickel oxides represented by; chemical formula LiMn 1-x M x O2 (where M = Co, Ni, Fe, Cr, Zn or Ta, and x = 0.01 to 0.1) or lithium manganese composite oxides represented by Li2Mn3MO8 (where M = Fe, Co, Ni, Cu or Zn); LiMn2O4 in which part of the Li in the chemical formula is substituted with alkaline earth metal ions; disulfide compounds; one or more mixtures of Fe2(MoO4)3 can be included.

[0115] In the present invention, the negative electrode includes a negative electrode current collector and a negative electrode active material layer containing a negative electrode active material, a conductive material, and a binder resin on at least one surface of the current collector. The negative electrode uses, as the negative electrode active material, carbon such as lithium metal oxide, non-graphitizable carbon, graphite-based carbon; Li x Fe2O3 (0 ≦ x ≦ 1), Li x WO2 (0 ≦ x ≦ 1), Si, SiO x (0 < x < 2), silicon-based materials such as SiC, Si alloy; Sn x Me 1-x Me’ y O z(Me: Mn, Fe, Pb, Ge; Me': Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8), etc. metal composite oxides; lithium metal; lithium alloy; silicon-containing alloy; tin-containing alloy; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5; conductive polymers such as polyacetylene; Li-Co-Ni-containing materials; one or more mixtures selected from titanium oxides can be included.

[0116] In a specific embodiment of the present invention, the conductive material can be, for example, any one selected from the group consisting of graphite, carbon black, carbon fiber or metal fiber, metal powder, conductive whisker, conductive metal oxide, carbon nanotube, activated carbon, and polyphenylene derivative, or a mixture of two or more of these conductive materials. The carbon nanotube has a cylindrical shape in which a graphite sheet has a nano-sized diameter and sp 2The graphite sheet has a bond structure and exhibits conductive or semiconductive properties depending on the angle and structure at which the graphite sheet is wound. Carbon nanotubes can be classified into single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT) according to the number of bonds forming the wall, and these carbon nanotubes can be appropriately selected depending on the use of the dispersion. More specifically, the carbon nanotube may be one selected from the group consisting of natural graphite, artificial graphite, super-p, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, denka black, SWCNT, DWCNT, MWCNT, aluminum powder, nickel powder, zinc oxide, potassium titanate, and titanium oxide, or a mixture of two or more conductive materials selected from the group consisting of these.

[0117] The current collector is not particularly limited as long as it has high conductivity without inducing a chemical change in the battery. For example, stainless steel, copper, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like can be used.

[0118] The binder resin may be a polymer commonly used in electrodes in the art. Non-limiting examples of such binder resins include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polyethylhexyl acrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, ethylene-vinyl acetate copolymer, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate. Examples of the binder resin include, but are not limited to, cyanoethyl acetatepropionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxyl methyl cellulose. More specifically, the binder resin may be a particulate binder polymer rather than a non-particulate binder polymer, and such a particulate binder polymer may have a D50 diameter of about 500 nm or less.The use of a particulate binder polymer in this manner has the advantage that the particulate binder polymer does not penetrate into the pores in the electrode active material layer, so that resistance does not increase, and the particulate binder polymer also has a shell-type gel structure, which ensures the elasticity of the electrode active material layer.

[0119] For this purpose, the particulate binder polymer may have a D50 diameter larger than the pores formed in the electrode active material layer, for example, a D50 diameter of 80 nm or more, 90 nm or more, or 100 nm or more.

[0120] The particulate binder polymer may include an acrylic-containing particulate binder (e.g., a copolymer of butyl acrylate and ethylhexyl acrylate, a copolymer of methyl methacrylate and ethylhexyl acrylate, polyacrylonitrile, polycyanoacrylate, etc.), acrylonitrile butadiene-styrene rubber, styrene butadiene rubber (SBR), acrylonitrile-butadiene rubber, polyvinyl chloride, polyvinylidene fluoride, polyvinyl alcohol, polystyrene, or a mixture of two or more thereof.

[0121] Meanwhile, more specifically, the binder used in the negative electrode may be styrene butadiene rubber (SBR), and the binder used in the positive electrode may be polyvinyl chloride.

[0122] The electrode assembly prepared as above can be placed in a suitable case and an electrolyte can be injected to manufacture a battery.

[0123] In the present invention, the electrolyte is A + B - A salt having the structure: + Li + , Na + , K + or a combination thereof, - PF6 - , BF4 - , Cl - , Br - , I - , ClO4 - , AsF6 - , CH3CO2 - , CF3SO3 - , N(CF3SO2)2 - , C(CF2SO2)3 - or a combination thereof, dissolved or dissociated in an organic solvent such as, but not limited to, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethylsulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma butyrolactone (g-butyrolactone), or a mixture thereof.

[0124] The present invention also provides a battery module including a battery having the electrode assembly as a unit cell, a battery pack including the battery module, and a device including the battery pack as a power source. Specific examples of the device include, but are not limited to, a power tool powered by a battery-type motor, electric vehicles including electric vehicles (EV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and the like, electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters (E-scooters), electric golf carts, and power storage systems.

[0125] The present invention will be described in detail below with reference to examples. However, the examples of the present invention can be modified in various different forms, and the scope of the present invention should not be construed as being limited to the examples detailed below. The examples of the present invention are provided to more completely explain the present invention to those having ordinary skill in the art.

[0126] 1. Manufacturing of separation membrane substrate [Example 1-1] 30 parts by weight of polyethylene resin (Mw 475,500 g / mol, PDI 4.5, MI 1.7 g / 10 min, Daehan Oil & Chemical Co., Ltd.) and 70 parts by weight of liquid paraffin oil (dynamic viscosity at 40°C: 40 cSt) were fed into a twin-screw extruder, kneaded, and then extruded. After extrusion, the mixture was formed into a sheet through a T-die and a cooling casting roll, and then biaxially stretched in a tenter-type sequential stretching machine with MD stretching followed by TD stretching. The liquid paraffin oil, which is a pore-forming agent (diluent), was extracted from the stretched sheet with methylene chloride and heat-set at approximately 128°C to obtain a separation membrane substrate.

[0127] [Example 1-2] A separation membrane substrate was obtained in the same manner as in Example 1-1, except that a polyethylene resin having Mw of 250,000 g / mol, PDI of 3.7, and MI of 2.1 g / 10 min was used.

[0128] [Examples 1-3] A separation membrane substrate was obtained in the same manner as in Example 1-1, except that a polyethylene resin having Mw 620,000 g / mol, PDI 4.5, and MI 0.5 g / 10 min was used.

[0129] [Examples 1-4] A separation membrane substrate was obtained in the same manner as in Example 1-1, except that a polyethylene resin having Mw 800,000 g / mol, PDI 5.4, and MI 0.2 g / 10 min was used.

[0130] [Examples 1-5] The polyethylene resin was a mixture of 70 parts by weight of a first polyethylene resin having Mw 250,000 g / mol, PDI 3.8, and MI 2.1 g / 10 min, and 30 parts by weight of a second polyethylene resin having Mw 620,000 g / mol, PDI 4.5, and MI 0.5 g / 10 min. A separation membrane substrate was obtained in the same manner as in Example 1-1, except that a polyethylene resin composition having a PDI of 10.0 and an MI of 1.2 g / 10 min was used.

[0131] [Examples 1-6] A separation membrane substrate was obtained in the same manner as in Example 1-1, except that the polyethylene resin used was a polyethylene resin composition having a PDI of 11.0 and a MI of 1.08 g / 10 min, which was a mixture of 60 parts by weight of a first polyethylene resin having a Mw of 250,000 g / mol, a PDI of 3.8, and a MI of 2.1 g / 10 min and 40 parts by weight of a second polyethylene resin having a Mw of 620,000 g / mol, a PDI of 4.5, and a MI of 0.5 g / 10 min.

[0132] [Examples 1-7] A separation membrane substrate was obtained in the same manner as in Example 1-1, except that the polyethylene resin used was a mixture of 50 parts by weight of a first polyethylene resin having Mw 250,000 g / mol, PDI 3.8, and MI 2.1 g / 10 min and 50 parts by weight of a second polyethylene resin having Mw 620,000 g / mol, PDI 4.5, and MI 0.5 g / 10 min, resulting in a polyethylene resin composition having a PDI of 12.0 and an MI of 0.9 g / 10 min.

[0133] [Examples 1-8] A first separation membrane substrate was obtained in the same manner as in Example 1-1, except that the thickness of the separation membrane substrate was 1 μm.

[0134] A second separation membrane substrate was obtained in the same manner as in Example 1-2, except that the thickness of the separation membrane substrate was 10 μm.

[0135] The first separation membrane substrate was laminated on both sides of the second separation membrane substrate, to obtain a multilayer separation membrane substrate with a final thickness of 12 μm.

[0136] [Comparative Example 1-1] A polyethylene resin composition was prepared by mixing 60 parts by weight of a first polyethylene resin (Mw 250,000 g / mol, PDI 3.8, MI 2.1 g / 10 min, Daehan Petrochemicals Co., Ltd.) and 40 parts by weight of a second polyethylene resin (Mw 800,000 g / mol, PDI 5.4, MI 0.2 g / 10 min, Daehan Petrochemicals Co., Ltd.). The composition had a Mw of 497,000 g / mol, a PDI of 12.8, and a MI of 0.32 g / 10 min. A separation membrane substrate was then obtained in the same manner as in Example 1-1.

[0137] [Comparative Example 1-2] A separation membrane substrate was obtained in the same manner as in Example 1-1, except that a polyethylene resin having Mw of 1,900,000 g / mol, PDI of 12.1, and MI of 0.01 g / 10 min was used.

[0138] [Comparative Example 1-3] A separation membrane substrate was obtained in the same manner as in Example 1-1, except that a polyethylene resin having Mw of 1,950,000 g / mol, PDI of 12.5, and MI of 0.01 g / 10 min was used.

[0139] The properties of the separation membrane substrates obtained in the above examples and comparative examples are shown in the following Tables 1 and 2. Incidentally, the separation membrane substrates according to Comparative Examples 1-2 and 1-3 were not smoothly formed, and the thickness was not uniform.

[0140] [Table 1]

[0141] [Table 2]

[0142] 2. Evaluation of the physical properties of separation membrane substrate (1) Molecular weight measurement The weight average molecular weight (Mw) and number average molecular weight (Mn) were measured by gel permeation chromatography (GPC, PL GPC220, Agilent Technologies) under the following conditions: -Column: PL Olexis (Polymer Laboratories) -Solvent: TCB (Trichlorobenzene) -Flow rate: 1.0ml / min -Sample concentration: 1.0mg / ml -Injection volume: 200μl -Column temperature: 160℃ -Detector: Agilent High Temperature RI detector -Standard: Polystyrene (corrected by a cubic function)

[0143] (2) Wet curve measurement method Galwick solution was prepared, and the separation membrane substrates obtained in the Examples and Comparative Examples were immersed in it and left for 1 minute. After that, the wet curves of the separation membrane substrates soaked in the solution were measured using a capillary flow porometer to measure the maximum and minimum pressures, and the difference between them was calculated. The measured pressure was in the range of 0 to 3500 MPa. The capillary flow porometer was measured using a PMI device (Porous Materials Inc.).

[0144] (3) Measurement of resistance (ER) change rate The resistance of the separation membrane substrate obtained in each Example and Comparative Example was measured by applying a current of 1 mA using a Hi-Tester manufactured by HIOKI. Meanwhile, each separation membrane substrate was hot pressed at 90° C., 4 MPa, and 1 sec, and the resistance was measured, and the resistance change rate (%) of each separation membrane substrate was calculated according to the following [Equation 2].

[0145] [Formula 2] (Resistance (ER) of separation membrane substrate after pressing - Resistance (ER) of separation membrane substrate before pressing) / Resistance (ER) of separation membrane substrate before pressing * 100 (%)

[0146] (4) Average pore size The pore size distribution was obtained using a capillary flow porometer, from which the average pore size (nm) was calculated. The capillary flow porometer was measured using a PMI device (Porous Materials Inc.).

[0147] (5) Measurement of air permeability change rate The porosity and air permeability of the separation membrane substrates obtained in each of the Examples and Comparative Examples were measured using an EG01-55-1MR device manufactured by Asahi Seiko Co., Ltd.

[0148] On the other hand, each separation membrane substrate was hot pressed at 90° C., 4 MPa, and 1 sec, and then its air permeability was measured, and the air permeability change rate (%) of each separation membrane substrate was calculated according to the following [Equation 3].

[0149] [Formula 3] [(Initial air permeability - Air permeability after pressing) / Initial air permeability] x 100

[0150] (6) Thickness deformation rate The separation membrane substrate obtained in each Example and Comparative Example was measured for its thickness. Next, each separation membrane substrate was hot pressed at 90° C., 4 MPa, and 1 sec, and then its thickness was measured and the thickness deformation rate (%) of each separation membrane substrate was calculated by the following [Equation 4].

[0151] [Formula 4] [(initial thickness - thickness after pressing) / initial thickness] x 100

[0152] It was confirmed that the separation membrane substrates of the Examples had a lower thickness deformation rate than the Comparative Examples.

[0153] (7) Breakdown voltage Thirty test pieces were prepared for each of the examples and comparative examples, and their withstand voltage characteristics were evaluated. A stainless steel mesh and a separator substrate were hot-pressed at 70°C, 7.8 MPa, and 10 seconds, and then a DC current of 0.5 mA and a voltage rise of 100 V / s (voltage 3 kV, ramp-up time 3 seconds) were set. When the experiment started, the measurement was completed when the voltage rose to a point where each test piece was short-circuited, and the voltage at that time was measured as the breakdown voltage. The voltages of the bottom 1% of test pieces that showed low breakdown voltages were measured through a Weibull distribution analysis of the total number of test pieces tested, and are summarized in Table 1 above.

[0154] As confirmed by this experiment, the separator substrate according to the embodiment has small and uniform pore size, and therefore has superior voltage resistance and thickness deformation rate compared to the separator substrate according to the comparative example.

[0155] 3. Fabrication of Unit Cell [Example 2-1] First, the separation membrane substrate manufactured in Example 1-1 was prepared.

[0156] Next, aluminum hydroxide (average particle size: 800 nm, Huber) as inorganic particles and poly(ethylene glycol)-block-poly(acrylic acid) (BYK) as a dispersant were added to water at room temperature and stirred uniformly, and then an acrylic particulate binder (styrene-butyl acrylate, LGC, AD-S11, D50: 400 nm, copolymer of styrene and butyl acrylate (Tg 40°C)) as a particulate binder polymer was added sequentially to prepare a slurry for forming a heat-resistant layer. The weight ratio of the inorganic particles to the particulate binder polymer in the slurry was 85:15. The weight ratio of the inorganic particles to the dispersant was 99.5:0.5. The slurry was applied to one side of the separation membrane substrate prepared in Example 1-1 using a doctor blade and dried to prepare a separation membrane having a heat-resistant layer formed thereon.

[0157] Next, LiNi 0.8 Co 0.1 Mn 0.1 O2, PVDF and carbon black were mixed in a weight ratio of 97.0:1.5:1.5 and dispersed in 2-methyl-2-pyrrolidone to prepare a positive electrode slurry, which was then coated on an aluminum current collector, dried and rolled to prepare a positive electrode.

[0158] Next, graphite, SBR and CMC were mixed in a weight ratio of 89.2:10:0.8 and dispersed in distilled water to prepare a negative electrode slurry, which was then coated on a copper current collector, dried and rolled to prepare a negative electrode.

[0159] A separator having a heat-resistant layer was interposed between the positive and negative electrodes prepared as above, and the electrode assembly was laminated and packaged in a pouch together with an electrolyte to prepare a unit cell. The electrolyte was a 1.0M LiPF6 solution in a mixed solvent of ethylene carbonate (EC) / ethyl methyl carbonate (EMC) (volume ratio: 20 / 80).

[0160] [Comparative Example 2-1] A unit cell was manufactured in the same manner as in Example 2-1, except that the separator substrate manufactured in Comparative Example 1-1 was used. [Explanation of symbols]

[0161] 10, 30: Film present on the surface of the separation membrane substrate 20: Film present in the middle part of the separation membrane substrate 100: Separation membrane base material

Claims

1. A separator substrate for electrochemical elements, The separation membrane substrate is a sheet-like porous membrane including a plurality of pores and including a polymer material; the polymeric material comprises a polyolefin-containing resin; The separation membrane substrate has a difference between a maximum pressure and a minimum pressure of 100 psi or less when measuring a wet curve of a wet sample; The separation membrane substrate includes fibrils having a diameter of 50 nm to 250 nm, The separation membrane substrate has an average pore diameter of 10 nm to 100 nm; The polymer material has a weight average molecular weight (Mw) of 250,000 g / mol to 800,000 g / mol.

2. A separation membrane substrate for electrochemical elements as described in claim 1, wherein the spaces present between the fibrils form pores, and the separation membrane substrate has a porosity of 30 vol% to 65 vol%.

3. 2. The separator substrate for electrochemical devices according to claim 1, wherein the separator substrate has a thickness of 5 μm to 50 μm.

4. 2. The separator substrate for electrochemical devices according to claim 1, wherein the polyolefin-containing resin is contained in an amount of 90 wt % or more relative to 100 wt % of the polymer material.

5. 2. The separator substrate for electrochemical devices according to claim 1, wherein the polyolefin-containing resin comprises at least one selected from the group consisting of polyethylene, polypropylene, polybutylene, and polypentene.

6. 2. The separator substrate for electrochemical devices according to claim 1, wherein the polymer material has a polydispersity index (PDI) value of 1.0 to 12.

0.

7. 7. The separator substrate for electrochemical devices according to claim 6, wherein the polymer material contains 90 wt % or more of polyethylene having a polydispersity index (PDI) of 1.0 to 12.0 relative to 100 wt % of the polymer material.

8. The separator substrate for electrochemical devices according to claim 6, wherein the polymer material has a polydispersity index (PDI) value of 5.0 to 10.

0.

9. 9. The separator substrate for electrochemical devices according to claim 8, wherein the polymer material contains 90 wt % or more of polyethylene having a polydispersity index (PDI) of 5.0 to 10.0 relative to 100 wt % of the polymer material.

10. A first step of preparing a separation membrane substrate; A second step of measuring a wet curve of the separation membrane substrate and determining a minimum pressure and a maximum pressure from the wet curve; A third step of calculating a difference between the maximum pressure and the minimum pressure obtained in the second step; A fourth step of comparing the calculated value to a standard of 100 psi or less; and a fifth step of using the prepared separator substrate as a battery cell separator if the calculated value is less than or equal to 100 psi; The separation membrane substrate is a sheet-like porous membrane including a plurality of pores and including a polymer material; The separation membrane substrate includes fibrils having a diameter of 50 nm to 250 nm, The separation membrane substrate has an average pore diameter of 10 nm to 100 nm; The polymer material has a weight average molecular weight (Mw) of 250,000 g / mol to 800,000 g / mol.

11. 11. The method of claim 10, further comprising: measuring a dry curve of a dry sample before performing the second step; and selecting the separator substrate and performing the third step when a pressure on the separator substrate is 30 psi to 500 psi as a result of measuring the dry curve of the dry sample.

12. The method for forming a battery cell separator according to claim 10 , wherein the separator substrate is manufactured by a wet manufacturing method.

13. The method for forming a battery cell separator according to claim 10 , wherein the separator substrate is manufactured by a dry manufacturing method.

14. A separator for an electrochemical element, comprising: a separation membrane substrate according to any one of claims 1 to 9; and a heat-resistant layer formed on one or both surfaces of the separation membrane substrate, wherein the heat-resistant layer contains a binder resin and inorganic particles.

15. The separator for an electrochemical device according to claim 14 , wherein the binder resin comprises a particulate binder polymer.

16. An electrochemical device comprising a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, the separator being the separator for electrochemical devices according to claim 14 .

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