Separation membrane for electrochemical device and electrochemical device including 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] The present invention claims the benefit of the filing date of Korean Patent Application No. 10-2021-0184392, filed with the Korean Intellectual Property Office on December 21, 2021, the entire contents of which are incorporated herein by reference. The present invention relates to a separator for an electrochemical device having improved compression resistance and an electrochemical device including the same. [Background technology]

[0002] In recent years, interest in energy storage technology has been growing. As the range of applications expands from mobile phones, camcorders, and laptops to the energy sources of electric vehicles, efforts toward the research and development of electrochemical devices have gradually taken shape. From this perspective, electrochemical devices are the field that has attracted the most attention, and among them, the development of rechargeable secondary batteries and lithium secondary batteries with high energy density has been the focus of particular attention. Recently, ensuring safety has been a major focus of attention in the development of such secondary batteries.

[0003] Currently produced lithium secondary batteries use porous substrates made of polyolefin polymer resins as separator substrates to prevent short circuits between the positive and negative electrodes. However, these porous substrates have the problem of shrinking or melting at high temperatures, resulting in poor heat resistance. Therefore, when the battery becomes hot due to internal or external stimuli, the separator shrinks or melts, increasing the likelihood of the positive and negative electrodes coming into contact with each other and shorting, which can lead to a sudden release of electrical energy and the resulting explosion or fire of the battery.

[0004] Therefore, in order to solve the above problems, a method of improving heat resistance by forming a porous coating layer containing a mixture of inorganic particles and a binder polymer on at least one surface of a porous substrate has been widely used.

[0005] Meanwhile, during the battery assembly process, a lamination process is performed in which heat and pressure are applied to impart adhesion between the electrodes and separator. During this process, the pressure applied to the separator causes the inorganic particles in the porous coating layer to press against the porous substrate. In particular, because most inorganic particles used in the separator's porous coating layer are spherical, the inorganic particles in the porous coating layer come into contact with the porous substrate at points, which applies localized pressure to the porous substrate, damaging or deforming the pore structure. Batteries using such separators suffer from performance issues due to impacts on resistance and lifespan characteristics. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made to solve the above-mentioned problems of the prior art, and an object of the present invention is to provide a separator for an electrochemical device having improved compression resistance and an electrochemical device including the same.

[0007] Another object of the present invention is to provide a method for manufacturing a separator for an electrochemical device having improved compression resistance.

[0008] It will be readily apparent that other objects and advantages of the present invention can be realized by the instrumentalities or methods, and combinations thereof, as recited in the appended claims. [Means for solving the problem]

[0009] The present inventors have discovered that the above problems can be solved by the following separator for an electrochemical device, an electrochemical device including the separator, and a method for manufacturing the electrochemical device.

[0010] According to one embodiment of the present invention, there is provided a separator for an electrochemical device, the separator comprising a porous polymer substrate and a porous coating layer formed on at least one surface of the porous polymer substrate, the porous coating layer comprising inorganic hydroxide particles, cellulose nanofibers, and a water-based binder polymer, and the content of the cellulose nanofibers is 25 wt % to 70 wt % 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 inorganic hydroxide particles may form hydrogen bonds with at least one of the cellulose nanofibers and the water-based binder polymer.

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

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

[0015] According to one embodiment of the present invention, the water-based 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 thereof.

[0016] According to one embodiment of the present invention, the cellulose nanofibers may have a length of 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 at a temperature range of 60°C to 70°C for 1 second to 60 seconds, the thickness of the porous polymer substrate may change by 5% or less before and after the pressure is applied.

[0018] One embodiment of the present invention provides an electrochemical device comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is the separator 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 provides a method for manufacturing a separator for an electrochemical device, the method comprising the steps of: preparing a porous polymer substrate; and coating at least one surface of the porous polymer substrate with a slurry containing inorganic hydroxide particles, cellulose nanofibers, an aqueous binder polymer, and an aqueous solvent to form at least one porous coating layer. [Effects of the Invention]

[0021] The separation membrane according to the present invention exhibits improved compression resistance, which can prevent deformation of the separation membrane even when pressure is applied. In particular, when pressure is applied to the separation membrane according to the present invention, at least one porous coating layer formed on at least one surface of the porous substrate acts as a buffer, thereby reducing the pressure on the porous substrate and preventing deformation of the separation membrane.

[0022] The drawings attached to this specification illustrate preferred embodiments of the present invention and serve to facilitate a better understanding of the technical concept of the present invention together with the contents of the invention described above, and therefore the present invention should not be interpreted as being limited to only the matters depicted in such drawings. Meanwhile, the shape, size, scale, or ratio of elements in the drawings described in this specification may be exaggerated to emphasize a clearer description. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a SEM (scanning electron microscope) image of a cross section of the separation membrane of Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention will be described in detail below. The terms and phrases used in the specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical concept of the present invention, based on the principle that the inventor can appropriately define the concept of the term in order to best describe his or her invention.

[0025] Throughout this specification, when a part is said to "comprise" or "include" certain elements, this means that it can further include or comprise other elements, rather than excluding other elements, unless specifically stated to the contrary.

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

[0027] A separator for an electrochemical device 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, the porous coating layer comprising inorganic hydroxide particles, cellulose nanofibers, and a water-based binder polymer, and the content of the cellulose nanofibers is 25 wt % to 70 wt % based on the total weight of the porous coating layer.

[0028] Typically, separators contain inorganic particles in a porous coating layer to improve heat resistance, and spherical inorganic particles are commonly used. However, when pressure is applied to the separator, the inorganic particles in the porous coating layer come into contact with the porous polymer substrate at points, causing localized pressure within the porous polymer substrate, which can damage and / or deform the pore structure within the porous polymer substrate. Therefore, batteries using separators with damaged pore structures within the porous polymer substrate suffer from reduced resistance and lifespan, resulting in performance problems.

[0029] To solve this problem, the inventors of the present invention adjust the composition of the porous coating layer so that the porous coating layer acts as a buffer even when pressure is applied to the separation membrane, thereby reducing the impact on the porous substrate.

[0030] First, by including a material that can make line contact in the porous coating layer, localized pressure within the porous polymer substrate, i.e., the application of localized pressure due to point contact, can be reduced. Specifically, in the present invention, the porous coating layer contains a predetermined amount of linear cellulose nanofibers, thereby reducing the pressure applied to the porous polymer substrate.

[0031] Second, the components contained in the porous coating layer can bond together through interactions, dispersing the pressure exerted by the inorganic particles on the porous polymer substrate. In the present 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 capable of hydrogen bonding between each component, allowing for organic bonding between each component. 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. Furthermore, for example, the cellulose nanofibers can form hydrogen bonds with the aqueous binder polymer. Therefore, the bonding strength between the cellulose nanofibers and the aqueous binder polymer is strong, preventing the porous coating layer from detaching. This improves the physical properties, such as the increased peel strength of the porous coating layer to the porous polymer substrate, which can help maintain the shape of the porous coating layer. Furthermore, when pressure is applied to the separator, the pressure transmitted to the cellulose nanofibers is also distributed to the aqueous binder polymer, allowing the entire porous coating layer to function as a buffer. In particular, the inorganic hydroxide particles and cellulose nanofibers of the present invention receive pressure over a larger area than spherical inorganic particles, thereby reducing damage to the porous polymer substrate. Furthermore, the materials present in the porous coating layer have a high tensile modulus due to the formation of hydrogen bonds between numerous functional groups, such as OH groups, allowing the porous coating layer to function as a buffer for the porous polymer substrate. Therefore, even when pressure is applied to the separator, deformation of the separator is reduced, thereby preventing the problem of reduced battery performance.

[0032] The porous polymer substrate refers to a substrate having multiple pores formed therein, which act as a porous ion-conducting barrier that allows ions to pass through while blocking electrical contact between the negative and positive electrodes. The pores are interconnected, allowing gas or liquid to pass from one side of the substrate to the other. Such a substrate can be a porous polymer film containing a thermoplastic resin to provide a shutdown function. The shutdown function refers to the ability of the thermoplastic resin to dissolve and block the pores of the porous substrate when the battery temperature rises, thereby blocking ion migration 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 from 1 μm to 100 μm, or from 5 μm to 50 μm. The size and porosity of the pores present in the porous polymer substrate are also not particularly limited, but may be from 0.01 μm to 50 μm and from 10% to 95%, respectively.

[0034] The porous coating layer is formed on at least one surface of the porous polymer substrate and includes inorganic hydroxide particles, cellulose nanofibers, and a water-based binder polymer.

[0035] The inorganic hydroxide particles contained in the porous coating layer may be bound by cellulose nanofibers and / or an aqueous binder polymer. In the present invention, hydrogen bonds can be formed between the inorganic hydroxide particles, cellulose nanofibers, and the aqueous binder polymer, respectively, or between each other, forming interstitial volumes, which are spaces defined by the structure connected by hydrogen bonds. The interstitial volumes can form pores. For example, because cellulose nanofibers are not perfectly packed together, pores may exist between the fibers.

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

[0037] Specifically, because the inorganic hydroxide particles can form hydrogen bonds with the cellulose nanofibers and / or the aqueous binder polymer, the pressure applied by the inorganic hydroxide particles to the porous polymer substrate can be dispersed, thereby 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 inorganic hydroxide particles, cellulose nanofibers, and an aqueous binder polymer in one porous coating layer. The inorganic hydroxide particles are present between the cellulose nanofibers, forming hydrogen bonds between the components, which can provide superior compression resistance. Therefore, cases in which multiple porous coating layers each contain inorganic hydroxide particles and cellulose nanofibers are excluded from the present invention.

[0039] The inorganic hydroxide particles are not limited to a specific type, and may be, for example, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, nickel hydroxide, or a mixture thereof. Preferably, aluminum hydroxide is used.

[0040] The content of the inorganic hydroxide particles may be 10 wt% or more, 20 wt% or more, 25 wt% or more, or 30 wt% or more, or 60 wt% or less, or 50 wt% or less, based on 100 wt% of the total weight of the porous coating layer. When the content of the inorganic hydroxide particles relative to the total weight of the porous coating layer satisfies the above range, advantageous effects are obtained in terms of heat resistance and dispersion stability. In particular, when the inorganic hydroxide particles are contained in an amount greater than the above range, dispersion stability may be reduced, or it may be difficult to control the slurry viscosity for forming the porous coating layer, making it difficult to control the thickness of the porous coating layer. Furthermore, when pressure is applied to the separator, high internal pressure acts on the inorganic hydroxide particles, which may result in poor compression resistance.

[0041] The average particle size of the inorganic hydroxide particles is not particularly limited, but in order to form a porous coating layer of uniform thickness and with an 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 cellulose nanofiber is lightweight, yet has high strength and does not expand even when heated, so it may be useful for improving the strength and heat resistance of the porous coating layer of the separator.

[0043] In one embodiment of the present invention, the content of the 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 the 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 cellulose nanofibers are contained in an amount greater than the recommended range, poor heat resistance may be exhibited, and if the cellulose nanofibers are contained in an amount less than the recommended range, poor compression resistance may be exhibited.

[0044] The cellulose nanofibers may contain functional groups capable of hydrogen bonding with the inorganic hydroxide particles and / or aqueous polymer. For example, the cellulose nanofibers may contain -OH, -COO-, -COOH, -NH groups, etc. as functional groups capable of hydrogen bonding. Therefore, the cellulose nanofibers form hydrogen bonds with the inorganic hydroxide particles and / or aqueous polymer within the porous coating layer, dispersing the pressure exerted by the inorganic hydroxide particles on the porous polymer substrate. This allows the porous coating layer to more effectively function as a buffer, thereby providing a separation membrane with improved compression resistance.

[0045] The diameter of the cellulose nanofibers may be 1 nm to 1 μm, preferably 50 nm to 500 nm, and more preferably 100 nm to 200 nm. When the diameter of the cellulose nanofibers is within this range, more pores can be formed in the porous coating layer, and more hydrogen bonds can be formed with the inorganic hydroxide particles and / or the aqueous binder polymer, thereby further improving wetting after injection of the electrolyte.

[0046] Furthermore, the length of the cellulose nanofibers may be from 1 μm to 100 μm, preferably from 30 μm to 100 μm, and more preferably from 50 μm to 100 μm. When the length of the cellulose nanofibers is within this range, the inorganic hydroxide particles can easily absorb the pressure that is applied to the separation membrane and can disperse the pressure within the porous coating layer, which may be more advantageous in terms of compression resistance.

[0047] The aqueous binder polymer is soluble in an aqueous solvent such as water. The aqueous binder polymer may contain functional groups capable of hydrogen bonding with the inorganic hydroxide particles and / or the aqueous polymer. The aqueous binder polymer may be, but is not limited to, 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. To ensure uniform coating, the aqueous binder polymer preferably contains carboxymethylcellulose.

[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 the total weight of the porous coating layer (100% by weight). When the content of the aqueous binder polymer relative to the total weight of the porous coating layer satisfies this range, the dispersibility of the inorganic hydroxide particles and cellulose nanofibers can be improved during the formation of the porous coating layer, and sufficient hydrogen bonds can be formed with the inorganic hydroxide particles and / or cellulose nanofibers, resulting in advantageous effects in terms of compression resistance.

[0049] According to one embodiment of the present invention, when a pressure of 1 MPa to 10 MPa is applied for 1 to 60 seconds at a temperature of 60°C to 70°C, the thickness of the porous polymer substrate can be compared before and after application of pressure, with the thickness change rate of the porous polymer substrate being 5% or less, 4.5% or less, or 3% to 4.5% or less. For example, a pressure of 5.2 MPa can be applied for 10 seconds at 70°C, and the thickness change rate of the porous polymer substrate can be measured before and after application of pressure. Pressure can be applied to the separation membrane using a hot press, and the thickness of the porous polymer substrate can be measured using a thickness gauge (Mitutoyo VL-50S-B).

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

[0051] In one embodiment of the method for producing a separation membrane according to the present invention, inorganic hydroxide particles and cellulose nanofibers are dispersed in an aqueous solvent as a dispersion medium, and then an aqueous binder polymer is added to produce a slurry. The aqueous solvent as a dispersion medium used here 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] Thereafter, the prepared slurry is applied to at least one surface of a porous polymer substrate and dried to form at least one porous coating layer on at least one surface of the porous polymer substrate.

[0053] An electrochemical device according to an embodiment of the present invention includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and the separator is the separator according to the embodiment of the present invention described above.

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

[0055] The electrode to be used with the separator of the present invention is not particularly limited, and can be prepared in the form of an electrode active material bound to an electrode current collector according to a conventional method known in the art.

[0056] Among the electrode active materials, non-limiting examples of the positive electrode active material include conventional positive electrode active materials that can be used for the positive electrode of conventional lithium secondary batteries. In particular, it is preferable to use lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a lithium composite oxide that is a combination thereof.

[0057] Non-limiting examples of the negative electrode active material include common negative electrode active materials that can be used in the negative electrodes of conventional lithium secondary batteries, and particularly preferred are lithium metal or lithium alloys, and lithium-absorbing materials such as carbon, petroleum coke, activated carbon, graphite, or other carbons.

[0058] Non-limiting examples of positive electrode current collectors include foils made of aluminum, nickel, or a combination thereof, and non-limiting examples of negative electrode current collectors include foils made of copper, gold, nickel, or a copper alloy, or a combination thereof.

[0059] The electrolyte that can be used in the electrochemical element of the present invention is A + B - A salt having the structure: + Li + , Na + , K. + or a combination thereof, and B - is 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), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma butyrolactone (γ-butyrolactone), or a mixture thereof.

[0060] The electrolyte injection can be performed at an appropriate stage during the battery manufacturing process depending on the manufacturing process and required properties of the final product, i.e., before battery assembly or at the final stage of battery assembly.

[0061] The present invention also provides a battery module including a battery having an 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, power tools 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 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 skilled in the art.

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

[0064] [Production of cellulose nanofibers] 1. Cellulose powder was added to a 2 wt% NaOH solution and stirred vigorously for 1 hour.

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

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

[0067] 4. After drying at 80°C for 2 days, the cellulose nanofibers were collected.

[0068] The collected cellulose nanofibers had diameters of 50 nm to 500 nm and were mixed with cellulose nanofibers having lengths of 30 μm to 100 μm.

[0069] [Separation membrane manufacturing] 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 prepared cellulose nanofibers (CNF), aluminum hydroxide (hydroxide inorganic particles) with an average particle size of 400 nm, and carboxymethyl cellulose (aqueous binder polymer) were added and stirred to prepare a slurry for forming a porous coating layer. The solid content of the prepared slurry was 30%.

[0071] 3. The slurry was applied to one side of a polyethylene porous film using a bar coater and then dried to prepare a separator having a porous coating layer as 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 prepared cellulose nanofiber (CNF) and carboxymethyl cellulose (aqueous binder polymer) were added and stirred to prepare a slurry for forming a first porous coating layer. The solid content of the prepared slurry was 30%.

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

[0075] 4. The slurry for forming the first porous coating layer was applied to one side of a polyethylene porous film using a bar coater and then dried to form a first porous coating layer (first layer, thickness 3 μm) as shown in Table 1 below. The slurry for forming the second porous coating layer was applied to one side of the first porous coating layer and then dried to form a second porous coating layer (second layer, thickness 3 μm) as shown in Table 1 below, thereby manufacturing a separator.

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

[0077] Specific methods for measuring physical properties for evaluating compression resistance are as follows.

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

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

[0080] [Formula 1] Thickness change rate of porous polymer substrate (%) = [(thickness before pressure - thickness after pressure) / (thickness before pressure)] × 100 (2) Air permeability of the separation membrane and rate of change in air permeability of the separation membrane The air permeability of the separation membrane was measured using an Oken type air permeability measuring device manufactured by Asahi Seiko. 2 The time it took for 100cc of air to pass through was measured.

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

[0082] [Formula 2] Change in air permeability of 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 in resistance of the separation membrane The resistance value when the separator was immersed in the electrolyte was measured using the following electrolyte at 25°C by an AC method (frequency: 10,000 Hz to 100,000 Hz). The electrolyte was prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7 as a solvent, adding vinylene carbonate (VC) at 2 wt % relative to the solvent, and adding LiPF6 to a concentration of 1M.

[0083] The resistance change rate of the separator was calculated using the following Equation 3.

[0084] [Formula 3] Separation membrane resistance change rate (%) = [(resistance before pressure - resistance after pressure) / (resistance before pressure)] × 100

[0085] [Table 1]

Claims

1. A separator for an electrochemical element, The separation membrane comprises a porous polymer substrate and a porous coating layer formed on at least one surface of the porous polymer substrate; The porous coating layer includes inorganic hydroxide particles, cellulose nanofibers, and a water-based 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 separator for an electrochemical device, wherein the content of the aqueous binder polymer is 5 wt % to 30 wt % based on the total weight of the porous coating layer.

2. 2. The separator for an electrochemical device according to claim 1, wherein the content of the cellulose nanofibers is from 30 wt % to 60 wt % based on the total weight of the porous coating layer.

3. 2. The separator for an electrochemical device according to claim 1, wherein the inorganic hydroxide particles are hydrogen bonded to at least one of the cellulose nanofibers and the water-based binder polymer.

4. 2. The separator for an electrochemical device according to claim 1, wherein the content of the inorganic hydroxide particles is from 10 wt % to 60 wt % based on 100 wt % of the total weight of the porous coating layer.

5. 2. The separator for an electrochemical device according to claim 1, wherein the inorganic hydroxide particles comprise aluminum hydroxide, magnesium hydroxide, calcium hydroxide, chromium hydroxide, zirconium hydroxide, nickel hydroxide, or a mixture of two or more thereof.

6. 10. The separator for an electrochemical device according to claim 1, wherein 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 thereof.

7. 2. The separator for an electrochemical device according to claim 1, wherein the cellulose nanofibers have a length of 1 μm or more and 100 μm or less.

8. 2. The separator for electrochemical devices according to claim 1, wherein when a pressure of 1 MPa to 10 MPa is applied to the separator at a temperature of 60° C. to 70° C. for 10 seconds, the thickness of the porous polymer substrate changes by 5% or less before and after the pressure is applied.

9. The battery includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, An electrochemical device, wherein the separation membrane is the separation membrane according to any one of claims 1 to 8.

10. 10. The electrochemical device according to claim 9, wherein the electrochemical device is a lithium secondary battery.

11. A method for producing a separator for an electrochemical element according to any one of claims 1 to 8, providing a porous polymeric 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.

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