Separation membrane for electrochemical elements and electrochemical elements containing the same

Abstract

To provide a separation membrane for an electrochemical device having improved compression resistance, and an electrochemical device including the same.SOLUTION: The present invention relates to a separation membrane for an electrochemical device, and an electrochemical device including the same. The separation membrane according to an embodiment of the present invention is a separation membrane for an electrochemical device. The separation membrane includes a porous polymer substrate and one porous coating layer formed on at least one surface of the substrate. The porous coating layer includes inorganic hydroxide particles, cellulose nanofibers, and an aqueous binder polymer. The content of the cellulose nanofibers is 25 wt.% or more and 70 wt.% or less based on the total weight of the porous coating layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention claims the benefit as of the filing date of Korean Patent Application No. 10-2021-0184392, filed with the Korean Intellectual Property Office on December 21, 2021, and all its contents are included in this invention. This invention relates to a separation membrane for an electrochemical element having improved compression resistance and an electrochemical element containing the same. [Background technology]

[0002] In recent years, interest in energy storage technologies has been steadily increasing. As applications expand to mobile phones, camcorders, laptops, and even electric vehicles, efforts toward the research and development of electrochemical elements are becoming increasingly concrete. From this perspective, electrochemical elements are a field attracting the most attention, with particular interest in the development of rechargeable secondary batteries and lithium-ion batteries with high energy density. Recently, ensuring safety has become a major focus in the development of such secondary batteries.

[0003] Currently produced lithium-ion secondary batteries use a porous substrate made of polyolefin polymer resin as the separator membrane substrate to prevent short circuits between the positive and negative electrodes. However, this porous substrate has the problem of low heat resistance, as it shrinks or melts at high temperatures. Therefore, when the battery becomes hot due to internal or external stimuli, there is a high possibility that the positive and negative electrodes will come into contact with each other and short-circuit due to the shrinkage or melting of the separator membrane, causing a rapid release of electrical energy and potentially leading to the battery exploding or catching fire.

[0004] Therefore, in order to solve the aforementioned problems, a method is widely used in which a porous coating layer, in which inorganic particles and a binder polymer are mixed, is formed on at least one surface of a porous substrate to improve its heat resistance.

[0005] On the other hand, during the battery assembly process, a lamination process is carried out in which heat and pressure are applied to impart adhesion between the electrodes and the separation membrane. At this time, the pressure applied to the separation membrane causes inorganic particles within the porous coating layer to press against the porous substrate. In particular, since most of the inorganic particles used in the porous coating layer of the separation membrane are spherical, the inorganic particles within the porous coating layer come into contact with the porous substrate at points (dots), which applies localized pressure within the porous substrate, causing damage / deformation to the pore structure. Batteries employing such separation membranes suffer from reduced performance because their resistance and lifespan characteristics are affected. [Overview of the project] [Problems that the invention aims to solve]

[0006] The present invention was devised to solve the problems of the prior art described above, and aims to provide an electrochemical element separation membrane with improved compression resistance and an electrochemical element containing the same.

[0007] Furthermore, the present invention aims to provide a method for manufacturing a separation membrane for electrochemical elements having improved compression resistance.

[0008] It will be readily apparent that other objectives 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]

[0009] The present inventors have discovered that the above-mentioned problems can be solved by the following separation membrane for electrochemical elements, an electrochemical element containing the same, and a method for manufacturing the same.

[0010] According to one embodiment of the present invention, a separation membrane for an electrochemical element is provided, wherein the separation membrane comprises a porous polymer substrate and one porous coating layer formed on at least one surface of the porous polymer substrate, the porous coating layer contains hydroxide inorganic particles, cellulose nanofibers, and an aqueous binder polymer, and the content of the cellulose nanofibers is 25% by weight or more and 70% by weight or less based on the total weight of the porous coating layer.

[0011] According to one embodiment of the present invention, the content of the cellulose nanofibers may be 30% by weight or more and 60% by weight or less based on the total weight of the porous coating layer.

[0012] According to one embodiment of the present invention, the hydroxylated inorganic particles may form hydrogen bonds with at least one of the cellulose nanofibers and aqueous binder polymers.

[0013] According to one embodiment of the present invention, the content of the hydroxide inorganic particles may be 10% by weight or more and 60% by weight or less based on 100% by weight of the total weight of the porous coating layer.

[0014] According to one embodiment of the present invention, the hydroxide inorganic particles may include aluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, nickel hydroxide, or a mixture of two or more of these.

[0015] According to one embodiment of the present invention, the aqueous binder polymer may include carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN), polyacrylamide (PAM), or a mixture of two or more of these.

[0016] According to one embodiment of the present invention, the length of the cellulose nanofiber may be 1 μm to 100 μm.

[0017] According to one embodiment of the present invention, when a pressure of 1 MPa to 10 MPa is applied to the separation membrane in the temperature range of 60°C to 70°C for 1 second to 60 seconds, the thickness change rate of the porous polymer substrate before and after the pressure application may be 5% or less.

[0018] One embodiment of the present invention provides an electrochemical device including a positive electrode, a negative electrode, and a separation membrane interposed between the positive electrode and the negative electrode, wherein the separation membrane is the separation membrane described above.

[0019] According to one embodiment of the present invention, the electrochemical device may be a lithium secondary battery.

[0020] One embodiment of the present invention is a method for manufacturing a separation membrane for an electrochemical device, including the steps of preparing a porous polymer substrate, and coating a slurry containing inorganic hydroxide particles, cellulose nanofibers, an aqueous binder polymer, and an aqueous solvent on at least one surface of the porous polymer substrate to form at least one porous coating layer.

Advantages of the Invention

[0021] Since the separation membrane according to the present invention exhibits improved compressibility, it can show the effect of suppressing deformation of the separation membrane even when pressure is applied. In particular, in the separation membrane of the present invention, when pressure is applied, at least one porous coating layer formed on at least one surface of the porous substrate serves as a buffer, thereby reducing the pressure on the porous substrate and suppressing deformation of the separation membrane.

[0022] The drawings attached to this specification illustrate preferred embodiments of the present invention and serve to better understand the technical idea of the present invention together with the content of the invention described above. Therefore, the present invention is not construed as being limited only to the matters described in such drawings. On the other hand, the shape, size, scale, or ratio of elements in the drawings described in this specification may be exaggerated for the purpose of emphasizing a clearer explanation.

Brief Description of the Drawings

[0023] [Figure 1] It is a SEM (scanning electron microscope) image of the cross-section of the separation membrane of Example 1.

Modes for Carrying Out the Invention

[0024] Hereinafter, the present invention will be described in detail. Terms and words used in this specification and the claims should not be construed as being limited to their ordinary meanings or dictionary meanings. Based on the principle that the inventor can appropriately define the concept of terms in order to explain his invention in the best way, they must be construed in meanings and concepts consistent with the technical idea of the present invention.

[0025] Throughout the specification of the present application, when a part says that a certain component "includes" or "comprises", this means that, unless otherwise stated, it does not exclude other components, but can further include or comprise other components.

[0026] Throughout the specification of this application, the phrase "A and / or B" means "A or B, or both."

[0027] A separation membrane for an electrochemical element according to one embodiment of the present invention comprises a porous polymer substrate and a porous coating layer formed on at least one surface of the substrate, wherein the porous coating layer contains hydroxide inorganic particles, cellulose nanofibers, and an aqueous binder polymer, and the content of the cellulose nanofibers is 25% by weight or more and 70% by weight or less based on the total weight of the porous coating layer.

[0028] Generally, separation membranes contain inorganic particles in a porous coating layer to improve heat resistance, and spherical particulate inorganic particles are mainly used as these inorganic particles. However, when pressure is applied to the separation membrane, the inorganic particles in the porous coating layer come into point (dot) contact with the porous polymer substrate, and localized pressure is applied to the porous polymer substrate, causing damage and / or deformation to the pore structure within the porous polymer substrate. Therefore, batteries using separation membranes with damaged pore structures within the porous polymer substrate will experience reduced performance due to the impact on resistance and lifespan characteristics.

[0029] To solve these problems, the inventors of the present invention attempt to reduce the impact on the porous substrate by adjusting the composition within the porous coating layer so that the porous coating layer acts as a buffer even when pressure is applied to the separation membrane.

[0030] Firstly, by including a material that can make linear contact within the porous coating layer, the aim is to reduce localized pressure within the porous polymer substrate, that is, localized pressure applied by point contact. Specifically, in this invention, the pressure applied to the porous polymer substrate can be reduced by including linear cellulose nanofibers in a predetermined amount within the porous coating layer.

[0031] Secondly, the pressure exerted by the inorganic particles on the porous polymer substrate can be dispersed by enabling the components contained within the porous coating layer to bond through interactions. In this invention, one porous coating layer contains hydroxylated inorganic particles, cellulose nanofibers, and an aqueous binder polymer. The hydroxylated inorganic particles, cellulose nanofibers, and aqueous binder polymer may contain functional groups that can form hydrogen bonds between components, thus enabling organic bonding between components. For example, the hydroxylated inorganic particles can form hydrogen bonds with the cellulose nanofibers and / or the aqueous binder polymer, thereby dispersing the pressure exerted by the hydroxylated inorganic particles on the porous polymer substrate. Also, for example, cellulose nanofibers can form hydrogen bonds with the aqueous binder polymer. Therefore, the bonding force between the cellulose nanofibers and the aqueous binder polymer is strengthened, preventing the detachment of the porous coating layer itself and improving its physical properties, such as increasing the peel strength of the porous coating layer to the porous polymer substrate, which can help maintain the morphology of the porous coating layer. Furthermore, when pressure is applied to the separation membrane, the pressure transmitted to the cellulose nanofibers is also dispersed to the aqueous binder polymer, allowing the entire porous coating layer to act as a buffer. In particular, the hydroxide inorganic particles and cellulose nanofibers of the present invention are subjected to pressure over a larger area compared to spherical inorganic particles, thus reducing damage to the porous polymer substrate. In addition, the substances present in the porous coating layer have many functional groups such as OH groups that form hydrogen bonds and have a strong tensile modulus, enabling them to act as a buffer for the porous polymer substrate. Therefore, even when pressure is applied to the separation membrane, the deformation of the separation membrane is reduced, which can suppress the problem of reduced battery performance.

[0032] The porous polymer substrate refers to a porous ion-conducting barrier that allows ions to pass through while blocking electrical contact between the negative and positive electrodes, and is a substrate in which multiple pores are formed internally. Since the pores are interconnected, gas or liquid can pass from one side of the substrate to the other. As such a substrate, a porous polymer film containing a thermoplastic resin can be used from the viewpoint of providing a shutdown function. Here, the shutdown function refers to the function that, when the battery temperature rises, the thermoplastic resin melts and blocks the pores of the porous substrate, thereby blocking ion movement and preventing thermal runaway of the battery. The thermoplastic resin has a melting point of less than 200°C, and polyolefin resins such as polyethylene, polypropylene, polybutylene, and polypentene are preferred.

[0033] The thickness of the porous polymer substrate is not particularly limited, but may be 1 μm or more and 100 μm or 5 μm or more and 50 μm or less. The size and porosity of the pores present in the porous polymer substrate are also not particularly limited, but may be 0.01 μm or more and 50 μm or less and 10% or more and 95% or less, respectively.

[0034] The porous coating layer is formed on at least one surface of the porous polymer substrate. The porous coating layer contains hydroxide inorganic particles, cellulose nanofibers, and an aqueous binder polymer.

[0035] The hydroxylated inorganic particles contained in the porous coating layer may be bound together by cellulose nanofibers and / or aqueous binder polymers. In the present invention, hydrogen bonds can be formed between each of the hydroxylated inorganic particles, cellulose nanofibers, and aqueous binder polymers, or between each other, forming an interstitial volume, which is a space limited by the structure linked by hydrogen bonds, and the interstitial volume can form pores. For example, since cellulose nanofibers are not perfectly packed with each other, pores may exist between the fibers.

[0036] According to one embodiment of the present invention, the porous coating layer may be formed by bonds between two or more components selected from the hydroxylated inorganic particles, cellulose nanofibers, and aqueous binder polymer. For example, the bonds may be hydrogen bonds. In the present invention, the formation of hydrogen bonds between the components can be confirmed by X-ray diffraction analysis (XRD). Specifically, the bond length between molecules can be confirmed by X-ray diffraction analysis, allowing for the analysis of whether or not hydrogen bonds have been formed.

[0037] Specifically, since the hydroxylated inorganic particles can form hydrogen bonds with cellulose nanofibers and / or aqueous binder polymers, the pressure applied by the hydroxylated inorganic particles to the porous polymer substrate can be dispersed, thus suppressing deformation of the porous polymer substrate even when pressure is applied to the separation membrane.

[0038] In particular, the present invention is characterized by simultaneously containing hydroxide inorganic particles, cellulose nanofibers, and an aqueous binder polymer in a single porous coating layer. Since hydroxide inorganic particles are present between the cellulose nanofibers and hydrogen bonds are formed between the components, it can exhibit superior effects in terms of compressive resistance. Therefore, if hydroxide inorganic particles and cellulose nanofibers are each contained in multiple porous coating layers, the present invention is excluded.

[0039] The specific type of the inorganic hydroxide particles is not limited, but examples include aluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, nickel hydroxide, or mixtures thereof. Preferably, aluminum hydroxide can be used.

[0040] The content of the inorganic hydroxide particles may be 10% or more, 20% or more, 25% or more, or 30% or more by weight, and 60% or less, or 50% or less by weight, based on 100% by weight of the total weight of the porous coating layer. When the content of inorganic hydroxide particles relative to the total weight of the porous coating layer satisfies the above range, there are advantageous effects in terms of heat resistance and dispersion stability. In particular, if the inorganic hydroxide particles are present in amounts higher than the above range, dispersion stability may decrease, and it may become difficult to control the slurry viscosity for forming the porous coating layer, making it difficult to adjust the thickness of the porous coating layer. In addition, when pressure is applied to the separation membrane, high internal pressure acts on it due to the inorganic hydroxide particles, which may be disadvantageous in terms of compressive resistance.

[0041] The average particle size of the hydroxylated inorganic particles is not particularly limited, but for the formation of a porous coating layer of uniform thickness and appropriate porosity, it is preferably in the range of 0.001 μm to 10 μm, more preferably 100 nm to 2 μm, and even more preferably 150 nm to 1 μm.

[0042] The aforementioned cellulose nanofiber is lightweight yet possesses high strength and does not expand when heated, making it potentially advantageous for use in porous coating layers of separation membranes to improve strength and heat resistance.

[0043] In one embodiment of the present invention, the content of cellulose nanofibers may be 25% by weight or more and 70% by weight or less, based on 100% by weight of the total weight of the porous coating layer. According to one embodiment of the present invention, the content of cellulose nanofibers may be 30% by weight or more and 60% by weight or less, based on 100% by weight of the total weight of the porous coating layer. If the content of cellulose nanofibers is higher than the presented content range, it may exhibit an inferior effect in terms of heat resistance, and if the content of cellulose nanofibers is lower than the presented content range, it may exhibit an inferior effect in terms of compression resistance.

[0044] The cellulose nanofiber may contain functional groups capable of hydrogen bonding with hydroxylated inorganic particles and / or aqueous polymers. For example, the cellulose nanofiber may contain -OH, -COO-, -COOH, -NH2 groups, etc., as functional groups capable of hydrogen bonding. Therefore, the cellulose nanofiber can form hydrogen bonds with hydroxylated inorganic particles and / or aqueous polymers within the porous coating layer, thereby dispersing the pressure applied by the hydroxylated inorganic particles to the porous polymer substrate. As a result, the porous coating layer can perform its role as a buffer more effectively, providing a separation membrane with further improved compression resistance.

[0045] Furthermore, the diameter of the cellulose nanofiber may be 1 nm or more and 1 μm or less, preferably 50 nm or more and 500 nm or less, and more preferably 100 nm or more and 200 nm or less. When the diameter of the cellulose nanofiber is within the above range, more pores can be formed in the porous coating layer, and more hydrogen bonds can be formed with hydroxylated inorganic particles and / or aqueous binder polymers, thereby further improving the wetting properties after electrolyte injection.

[0046] Furthermore, the length of the cellulose nanofibers may be 1 μm or more and 100 μm or less, preferably 30 μm or more and 100 μm or less, and more preferably 50 μm or more and 100 μm or less. When the length of the cellulose nanofibers is within the above range, the pressure that the hydroxide inorganic particles will experience when pressure is applied to the separation membrane can be easily absorbed, and the pressure can be dispersed within the porous coating layer, which may be even more advantageous in terms of compressive resistance.

[0047] The aqueous binder polymer is soluble in an aqueous solvent such as water. The aqueous binder polymer may also contain hydroxylated inorganic particles and / or functional groups capable of hydrogen bonding with the aqueous polymer. The specific type of aqueous binder polymer is not limited, but may include, for example, carboxymethylcellulose (CMC), styrene butadiene rubber (SBR), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN), polyacrylamide (PAM), or mixtures of two or more of these. Preferably, carboxymethylcellulose may be included as the aqueous binder polymer to ensure uniform coating properties.

[0048] The content of the aqueous binder polymer may be 5% to 60% by weight, 5% to 30% by weight, or 10% to 20% by weight, based on 100% by weight of the total weight of the porous coating layer. When the content of the aqueous binder polymer relative to the total weight of the porous coating layer satisfies the above range, the dispersibility of the hydroxide inorganic particles and cellulose nanofibers can be improved during the formation of the porous coating layer, and sufficient hydrogen bonds can be formed with the hydroxide inorganic particles and / or cellulose nanofibers, resulting in advantageous effects in terms of compressive resistance.

[0049] According to one embodiment of the present invention, when a pressure of 1 MPa to 10 MPa is applied to the separation membrane of the present invention for a period of 1 second to 60 seconds at a temperature range of 60°C to 70°C, the thickness change rate of the porous polymer substrate before and after the pressure is applied may be 5% or less, 4.5% or less, or 3% to 4.5% or less. For example, at a temperature of 70°C, a pressure of 5.2 MPa can be applied for 10 seconds, and the thickness change rate of the porous polymer substrate before and after the pressure is applied can be measured. At this time, pressure can be applied to the separation membrane using a hot-press device, and the thickness of the porous polymer substrate can be measured using a thickness measuring instrument (Mitutoyo, VL-50S-B).

[0050] According to one embodiment of the present invention, a method for producing a separation membrane for an electrochemical element includes the steps of: preparing a porous polymer substrate; and coating at least one surface of the porous polymer substrate with a slurry containing hydroxide inorganic particles, cellulose nanofibers, an aqueous binder polymer, and an aqueous solvent to form at least one porous coating layer.

[0051] In a method for producing a separation membrane according to one embodiment of the present invention, a slurry can be produced by dispersing hydroxylated inorganic particles and cellulose nanofibers in an aqueous solvent which is a dispersion medium, and then adding an aqueous binder polymer. The aqueous solvent used as the dispersion medium at this time is a polar solvent and may be water, methanol, ethanol, ethylene glycol, diethylene glycol, glycerol, or a mixture of two or more of these.

[0052] Subsequently, by applying and drying the prepared slurry onto at least one surface of the porous polymer substrate, at least one porous coating layer can be formed on at least one surface of the porous polymer substrate.

[0053] An electrochemical element according to one embodiment of the present invention includes a positive electrode, a negative electrode, and a separation membrane interposed between the positive electrode and the negative electrode, wherein the separation membrane is the separation membrane according to the above-described embodiment of the present invention.

[0054] In one embodiment of the present invention, the electrochemical element is a device that converts chemical energy into electrical energy by an electrochemical reaction, and includes all elements that perform electrochemical reactions. Specific examples include all types of primary batteries, secondary batteries, fuel cells, solar cells, or capacitors such as supercapacitor elements. In particular, among the secondary batteries, lithium secondary batteries, including lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries, are preferred.

[0055] The electrodes to be applied with the separation membrane of the present invention are not particularly limited and can be manufactured in a form in which the electrode active material is bonded to an electrode current collector according to a conventional method known in the industry.

[0056] Among the electrode active materials, as non-limiting examples of the positive electrode active material, ordinary positive electrode active materials that can be used for the positive electrode of a conventional lithium secondary battery can be used. In particular, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a lithium composite oxide obtained by combining these is preferably used.

[0057] As non-limiting examples of the negative electrode active material, ordinary negative electrode active materials that can be used for the negative electrode of a conventional lithium secondary battery can be used. In particular, lithium metal or a lithium alloy, carbon, petroleum coke, activated carbon, graphite, or other carbonaceous materials such as lithium adsorbents are preferred.

[0058] Non-limiting examples of the positive electrode current collector include foils made of aluminum, nickel, or combinations thereof. Non-limiting examples of the negative electrode current collector include foils made of copper, gold, nickel or copper alloys, or combinations thereof.

[0059] The electrolyte that can be used in the electrochemical device of the present invention is A + B - is a salt having a structure such as A + is Li + 、Na + 、K + such as alkali metal cations, or ions composed of combinations thereof, and B - is PF6 - 、BF4 - 、Cl - 、Br - 、I - 、ClO4 - 、AsF6 - 、CH3CO2 - 、CF3SO3 - 、N(CF3SO2)2 - 、C(CF2SO2)3 -Salts containing anions such as these, or ions consisting of combinations thereof, may be dissolved or dissociated in organic solvents consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma-butyrolactone (γ-butyrolactone), or mixtures thereof, but are not limited to these.

[0060] The injection of the electrolyte can be carried out at an appropriate stage in the battery manufacturing process, depending on the manufacturing process and required physical properties of the final product. That is, it can be applied before battery assembly or at the final stage of battery assembly.

[0061] Furthermore, the present invention provides a battery module including a battery with an electrode assembly as a unit battery, 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-powered motor; electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs); electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters; electric golf carts; and power storage systems.

[0062] The present invention will be described in detail below with reference to examples. However, the examples of the present invention can be modified into 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 give a more complete explanation of the present invention to a person of average knowledge in the industry.

[0063] [Examples] Examples 1-4 and Comparative Examples 1-5 Separation membranes for Examples 1 to 4 and Comparative Examples 1 to 5 were manufactured according to the method described below. The ratios of components contained in the porous coating layer of the manufactured separation membranes are shown in Table 1.

[0064] [Manufacturing of Cellulose Nanofibers] 1. Cellulose powder was added to a 2% by weight NaOH solution and vigorously stirred for 1 hour.

[0065] 2. After washing the treated cellulose powder with distilled water, it was added to a mixed solvent of IPA (isopropyl alcohol) and distilled water, and then stirred using a homogenizer for 12 hours.

[0066] 3. The cellulose nanofibers (CNF) produced in the cellulose solution were filtered.

[0067] 4. After drying at 80°C for two days, the cellulose nanofibers were recovered.

[0068] The recovered cellulose nanofibers were a mixture of cellulose nanofibers with diameters ranging from 50 nm to 500 nm and lengths ranging from 30 μm to 100 μm.

[0069] [Manufacturing of separation membranes] 1. A porous polyethylene film (thickness 9 μm, porosity 45%) was prepared as a porous polymer substrate.

[0070] 2. Distilled water was prepared, and the cellulose nanofibers (CNF) produced above, aluminum hydroxide (inorganic hydroxide particles) with an average particle size of 400 nm, and carboxymethylcellulose (aqueous binder polymer) were added and stirred to produce a slurry for forming a porous coating layer. The solid content of the produced slurry was 30%.

[0071] 3. After applying the slurry to one surface of a porous polyethylene film using a bar coater, the film was dried to produce a separation membrane containing the porous coating layer shown in Table 1 below.

[0072] [Comparative Example 6] 1. A porous polyethylene film (thickness 9 μm, porosity 45%) was prepared as a porous polymer substrate.

[0073] 2. Distilled water was prepared, and the cellulose nanofibers (CNF) and carboxymethylcellulose (aqueous binder polymer) produced above were added and stirred to produce a slurry for forming the first porous coating layer. The solid content of the produced slurry was 30%.

[0074] 3. Distilled water was prepared, and aluminum hydroxide (inorganic hydroxide particles) with an average particle size of 400 nm and carboxymethylcellulose (aqueous binder polymer) were added. The mixture was then stirred to produce a slurry for forming the second porous coating layer. The solid content of the produced slurry was 30%.

[0075] 4. Using a bar coater, the slurry for forming the first porous coating layer was applied to one surface of a polyethylene porous film, and then dried to form the first porous coating layer (first layer, thickness 3 μm) as described in Table 1 below. Then, the slurry for forming the second porous coating layer was applied to one surface on the first porous coating layer, and then dried to form the second porous coating layer (second layer, thickness 3 μm) as described in Table 1 below, thereby producing a separation membrane.

[0076] Evaluation results For the separation membranes of Examples 1 to 4 and Comparative Examples 1 to 6, the physical properties were measured before applying pressure and after applying a pressure of 5.2 MPa for 10 seconds at a temperature of 70°C, and the rate of change is shown in Table 1.

[0077] The specific methods for measuring the physical properties to evaluate compressive resistance are as follows:

[0078] (1) Thickness of porous polymer substrate and rate of change in thickness of porous polymer substrate The thickness of the porous polymer substrate was measured using a thickness measuring instrument (Mitutoyo VL-50S-B).

[0079] The rate of change in thickness of the porous polymer substrate was calculated using Equation 1 below.

[0080] [Formula 1] Percentage change in thickness of porous polymer substrate (%) = [(thickness before pressurization - thickness after pressurization) / (thickness before pressurization)] × 100 (2) Permeability of the separation membrane and rate of change of permeability of the separation membrane The air permeability of the separation membrane was measured using an Ogane-type air permeability measuring device manufactured by Asahi Seiko. The membrane had a diameter of 28.6 mm and an area of ​​645 mm². 2 The time it took for 100cc of air to pass through was measured.

[0081] The rate of change in the permeability of the separation membrane was calculated using the following formula 2.

[0082] [Formula 2] Percentage change in air permeability of the separation membrane (%) = [(Air permeability before pressurization - Air permeability after pressurization) / (Air permeability before pressurization)] × 100 (3) Resistance of the separation membrane and rate of change of resistance of the separation membrane The resistance value of the separation membrane when impregnated with the electrolyte was measured at 25°C using the AC method (frequency 10,000Hz to 100,000Hz) with the following electrolyte. The electrolyte was prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) as solvents in a 3:7 (volume ratio), adding 2% by weight of vinylene carbonate (VC) to the solvent, and adding LiPF6 to a concentration of 1M.

[0083] The resistance change rate of the separation membrane was calculated using the following formula 3.

[0084] [Formula 3] Resistance change rate of the separation membrane (%) = [(Resistance before pressurization - Resistance after pressurization) / (Resistance before pressurization)] × 100

[0085] [Table 1]

Claims

1. A separation membrane for electrochemical elements, The separation membrane comprises a porous polymer substrate and one porous coating layer formed on at least one surface of the porous polymer substrate. The porous coating layer comprises hydroxide inorganic particles, cellulose nanofibers, and an aqueous binder polymer. The content of the cellulose nanofibers is 25% by weight or more and 70% by weight or less based on the total weight of the porous coating layer. A separation membrane for an electrochemical element, characterized in that the content of the aqueous binder polymer is 5% by weight or more and 30% by weight or less based on the total weight of the porous coating layer.

2. The separation membrane for an electrochemical element according to claim 1, characterized in that the content of the cellulose nanofiber is 30% by weight or more and 60% by weight or less based on the total weight of the porous coating layer.

3. The separation membrane for an electrochemical element according to claim 1, characterized in that the hydroxylated inorganic particles form hydrogen bonds with at least one of the cellulose nanofibers and aqueous binder polymers.

4. The separation membrane for an electrochemical element according to claim 1, characterized in that the content of the hydroxylated inorganic particles is 10% by weight or more and 60% by weight or less based on 100% by weight of the total weight of the porous coating layer.

5. The separation membrane for an electrochemical element according to claim 1, characterized in that the hydroxide inorganic particles include aluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, nickel hydroxide, or a mixture of two or more of these.

6. The separation membrane for an electrochemical element according to claim 1, characterized in that the aqueous binder polymer comprises carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN), polyacrylamide (PAM), or a mixture of two or more of these.

7. The separation membrane for an electrochemical element according to claim 1, characterized in that the length of the cellulose nanofiber is 1 μm or more and 100 μm or less.

8. The separation membrane for an electrochemical element according to claim 1, characterized in that when a pressure of 1 MPa to 10 MPa is applied to the separation membrane for 10 seconds at a temperature range of 60°C to 70°C, the rate of change in the thickness of the porous polymer substrate before and after the pressure is applied is 5% or less.

9. The system includes a positive electrode, a negative electrode, and a separation membrane interposed between the positive electrode and the negative electrode. An electrochemical element characterized in that the separation membrane is the separation membrane described in any one of claims 1 to 8.

10. The electrochemical element according to claim 9, characterized in that the electrochemical element is a lithium secondary battery.

11. A method for manufacturing a separation membrane for an electrochemical element according to any one of claims 1 to 8, The steps include preparing a porous polymer substrate and A method for producing a separation membrane for an electrochemical element, comprising the step of coating at least one surface of a porous polymer substrate with a slurry containing hydroxide inorganic particles, cellulose nanofibers, an aqueous binder polymer, and an aqueous solvent to form at least one porous coating layer.

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