Separator for electrochemical device and method for producing same

Abstract

The present invention relates to a separator for an electrochemical device and a manufacturing method thereof, and in particular, is characterized in that the drying rate of a composition for forming a second porous coating layer is controlled to be faster than the drying rate of a composition for forming a first porous coating layer, thereby improving the interfacial adhesion between an electrode and a separator while ensuring the adhesive strength at the interface between the porous polymer substrate and the porous coating layer.

Description

[Technical field]

[0001] This application claims priority to Korean Patent Application No. 10-2020-0113902, filed on September 7, 2020, the entire contents of which are incorporated herein by reference in their entirety in the specification and drawings.

[0002] The present invention relates to a separator for an electrochemical device and a method for producing the same. [Background technology]

[0003] In recent years, interest in energy storage technology has been increasing. As the fields of application expand to include mobile phones, camcorders, and laptops, as well as the energy of electric vehicles, efforts in the research and development of electrochemical elements have been increasingly realized. Electrochemical elements are the field that has attracted the most attention from this perspective, and in particular, the development of rechargeable secondary batteries has attracted attention. In recent years, in the development of such batteries, research and development related to the design of new electrodes and batteries has been conducted in order to improve the capacity density and specific energy.

[0004] Among the secondary batteries currently in use, lithium secondary batteries, developed in the early 1990s, have been attracting attention due to their advantages of high operating voltage and significantly higher energy density compared to conventional batteries that use aqueous electrolytes, such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries.

[0005] Electrochemical devices such as lithium secondary batteries are produced by many manufacturers, but their safety characteristics vary from one another, making it extremely important to evaluate and ensure the safety of such electrochemical devices.

[0006] Meanwhile, when a cell is charged beyond the upper limit voltage (e.g. 4.2~4.3V) and the normal capacity of the cell is exceeded, this is called overcharging. At this time, side reactions occur in the electrodes and electrolyte inside the cell, which causes the internal temperature of the cell to rise, causing the separator to shrink and an internal short circuit to occur. At this time, the instantaneous short circuit causes a sudden increase in cell temperature, which may react with flammable gas inside the cell and cause an explosion. This sudden increase in cell temperature is further amplified by the cell's low thermal conductivity.

[0007] Previously, attempts were made to solve this problem by inserting a pad or foil with excellent thermal conductivity onto the cell surface, which improved heat dissipation capacity (the ability to release heat) and reduced the sudden rise in temperature. However, this method does not meet the high energy density currently required for automotive cells, because inserting a pad or foil reduces the energy density per unit volume.

[0008] Therefore, there is a demand for providing a cell that has improved heat dissipation characteristics while at the same time maintaining the same energy density as conventional cells. Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a separator for an electrochemical device that can improve overcharge safety without the need for an additional device.

[0010] Another goal is to improve safety by improving the heat dissipation characteristics of the cells while maintaining the conventional energy density. [Means for solving the problem]

[0011] An aspect of the present invention provides a separator for an electrochemical device according to the following embodiment.

[0012] The first embodiment is A porous polymer substrate; a porous coating layer located on at least one surface of the porous polymer substrate, the porous coating layer including inorganic particles and a binder polymer; The porous coating layer includes a first region in contact with the porous polymeric substrate and a second region opposite the first region and not in contact with the porous polymeric substrate; the binder polymer in the first region and the second region has a concentration gradient that increases from the porous polymer substrate side toward the outermost side of the porous coating layer, The present invention relates to a separator for electrochemical devices, wherein the gradient of the concentration of the binder polymer contained in the second region is greater than the gradient of the concentration of the binder polymer contained in the first region.

[0013] According to the second embodiment, in the first embodiment, The content of the binder polymer in the first region may be the same as the content of the binder polymer in the second region.

[0014] According to the third embodiment, in the first or second embodiment, The ratio of the thickness of the first region to the thickness of the second region may be 4:6 to 1:9.

[0015] According to the fourth embodiment, in any one of the first to third embodiments, The binder polymer may include polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, acrylonitrile-styrene-butadiene copolymer, polyimide, or two or more thereof.

[0016] Another aspect of the present invention provides an electrochemical device according to the following embodiment.

[0017] The fifth embodiment is An electrochemical device in which an electrode assembly including two electrodes having opposite polarities and a separator interposed between the two electrodes is housed in a battery case, The separator for an electrochemical device is the separator for an electrochemical device according to any one of the first to fourth embodiments.

[0018] According to the sixth embodiment, in the fifth embodiment, The porous coating layer may face the positive electrode.

[0019] According to the seventh embodiment, in the sixth embodiment, The adhesive strength at the interface between the separator and the positive electrode may be 30 gf / 25 mm or more when measured by applying a force at 180° at a speed of 100 mm / min after heating and pressurizing the separator and the positive electrode at 80° C. and a pressure of 1000 kgf for 1 second.

[0020] Yet another aspect of the present invention provides a method for manufacturing an electrochemical device according to the following embodiment.

[0021] The eighth embodiment is: (S1) preparing a composition for forming a first porous coating layer and a composition for forming a second porous coating layer; (S2) coating the composition for forming the first porous coating layer on at least one surface of the porous polymer substrate and drying it; (S3) coating the composition for forming the second porous coating layer on the resultant of (S2) and drying it; The present invention relates to a method for producing a separator for an electrochemical device, wherein the drying rate of the composition for forming the second porous coating layer is faster than the drying rate of the composition for forming the first porous coating layer.

[0022] According to the ninth embodiment, in the eighth embodiment, The drying rate of the composition for forming the first porous coating layer and the drying rate of the composition for forming the second porous coating layer can be controlled so that the amount of solvent dried per second is 1 mg / sec to 50 mg / sec, and the drying rate of the composition for forming the second porous coating layer can be made faster than the drying rate of the composition for forming the first porous coating layer.

[0023] According to the tenth embodiment, in the eighth or ninth embodiment, The composition for forming the second porous coating layer may be dried 1 to 20 mg / sec faster than the composition for forming the first porous coating layer.

[0024] According to the eleventh embodiment, in any one of the eighth to tenth embodiments, The drying rate of the composition for forming the first porous coating layer may be 10 to 50% of the drying rate of the composition for forming the second porous coating layer.

[0025] According to the twelfth embodiment, in any one of the eighth to eleventh embodiments, In the step (S1), a composition for forming a first porous coating layer in which the first inorganic particles are dispersed is prepared by adding first inorganic particles to a first binder solution in which a first binder polymer is dissolved in a first solvent, and stirring the mixture. and adding second inorganic particles to a second binder solution in which a second binder polymer is dissolved in a second solvent, and stirring the mixture to prepare a composition for forming a second porous coating layer in which the second inorganic particles are dispersed; In this case, the composition for forming the first porous coating layer and the composition for forming the second porous coating layer may have the same components and composition ratio. Effect of the Invention

[0026] According to one aspect of the present invention, the binder polymer contained in the second region has a concentration gradient that increases in the thickness direction of the separator toward the electrode, and has a larger concentration gradient than the binder polymer contained in the first region, thereby increasing the adhesive strength between the electrode and the separator.

[0027] The increased adhesive strength between the electrode and the separator can improve overcharge safety, thereby improving overcharge safety using a conventional separator without the need for additional devices.

[0028] According to one embodiment of the present invention, the binder polymer contained in the first region has a concentration gradient smaller than the concentration gradient of the binder polymer contained in the second region, and the adhesive strength at the interface between the porous polymer substrate and the porous coating layer can be ensured.

[0029] According to one aspect of the present invention, the adhesive strength between the separator and the positive electrode is increased to improve the low thermal conductivity between the positive electrode and the separator. If the adhesive strength between the positive electrode and the separator is low, the thickness of the electrolyte interface layer impregnated between the positive electrode and the separator increases, resulting in a low thermal conductivity for dissipating heat generated in the cell to the outside. On the other hand, the battery cell according to one aspect of the present invention has an interface between the separator and the positive electrode with increased adhesive strength, resulting in improved heat dissipation characteristics.

[0030] According to one aspect of the present invention, a composition for forming a first porous coating layer and a composition for forming a second porous coating layer are sequentially coated, and the drying rate of the composition for forming the second porous coating layer is made faster than that of the composition for forming the first porous coating layer, thereby increasing the migration of the binder polymer generated in the composition for forming the second porous coating layer and controlling the binder distribution to be increased on the surface of the porous coating layer facing the electrode, thereby increasing the interfacial cohesion between the separator and the electrode.

[0031] In addition, according to one embodiment of the present invention, the interfacial adhesion between the porous polymer substrate and the porous coating layer can be ensured by sequentially coating a composition for forming a first porous coating layer and a composition for forming a second porous coating layer having the same composition. [Brief description of the drawings]

[0032] [Figure 1] 1 is a schematic diagram illustrating a separation membrane according to an embodiment of the present invention; [Diagram 2] FIG. 2 is a schematic diagram showing how the distribution of a binder polymer in a porous coating layer changes depending on the drying rate. [Diagram 3] FIG. 2 is a schematic diagram showing how the distribution of a binder polymer in a porous coating layer changes depending on the drying rate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Hereinafter, the embodiment of the present invention will be described in detail. The terms and words used in the present specification and claims should not be interpreted as being limited to their ordinary and dictionary meanings, but should be interpreted as being in accordance with the meaning and concept of the technical idea of ​​the present invention, in accordance with the principle that the inventor himself can appropriately define the concept of the term in order to best describe the invention. Therefore, it should be understood that the configurations shown in the examples and drawings described in this specification are merely the most preferred embodiment of the present invention, and do not represent the entire technical idea of ​​the present invention, and therefore there may be various equivalents and modifications that can be substituted for them at the time of this application.

[0034] Throughout this specification, when a part "comprises" another component, it means that the part may further include the other component, not excluding the other component, unless otherwise specified.

[0035] In addition, the terms "about," "substantially," and the like, used throughout this specification, when given the manufacturing and material tolerances inherent in the stated meaning, are used to mean a numerical value or close to that numerical value, and are used to prevent unscrupulous infringers from unfairly taking advantage of disclosures in which precise or absolute numerical values ​​are stated to aid in the understanding of this application.

[0036] Throughout this specification, the phrase "A and / or B" means "A, B, or all of these."

[0037] In the case of a cell catching fire due to overcharging, the internal temperature of the cell rises due to a side reaction of the components inside the cell caused by the overcharging current. This results in the contraction of the separator, causing an internal short circuit. The instantaneous short circuit that occurs at this time causes the cell temperature to rise rapidly, which can react with flammable gases inside the cell and cause an explosion. This rapid rise in cell temperature is further amplified by the low thermal conductivity of lithium-ion cells.

[0038] Previously, attempts were made to solve this problem by inserting a pad or foil with excellent thermal conductivity onto the cell surface, which improved heat dissipation capacity (the ability to release heat) and reduced the sudden rise in temperature. However, this method does not meet the high energy density currently required for automotive cells, because inserting a pad or foil reduces the energy density per unit volume.

[0039] The inventors conducted extensive research to solve the above problems and found that the part of the thermal resistance element of a cell that has the highest thermal resistance is the electrode / separator interface, particularly the positive electrode / separator interface. In view of the need to increase the adhesive strength at the electrode / separator interface, particularly the adhesive strength at the positive electrode / separator interface, in order to reduce the thermal resistance at that interface, the inventors focused on the present invention.

[0040] According to one aspect of the present invention, there is provided a separator for an electrochemical device, comprising: a porous polymer substrate; and a porous coating layer disposed on at least one surface of the porous polymer substrate, the porous coating layer including inorganic particles and a binder polymer, The porous coating layer includes a first region in contact with the porous polymeric substrate and a second region opposite the first region and not in contact with the porous polymeric substrate; the binder polymer in the first region and the second region has a concentration gradient that increases from the porous polymer substrate side toward the outermost side of the porous coating layer, The gradient of the concentration of the binder polymer contained in the second region is greater than the gradient of the concentration of the binder polymer contained in the first region.

[0041] In the present invention, the porous polymer substrate can be used without any particular limitation as long as it is a porous membrane that can electrically insulate the negative electrode and the positive electrode to prevent short circuiting while providing a path for lithium ion migration and can be used as a material for a separator in a general electrochemical device.

[0042] Specifically, the porous polymer substrate may be a porous polymer film substrate or a porous polymer nonwoven fabric substrate.

[0043] The porous polymer film substrate may be a porous polymer film made of a polyolefin such as polyethylene or polypropylene, and such a polyolefin porous polymer film substrate can exhibit a shutdown function at a temperature of, for example, 80 to 150°C.

[0044] In this case, the polyolefin porous polymer film may be formed from a polyolefin polymer such as polyethylene (such as high density polyethylene, linear low density polyethylene, low density polyethylene, and ultra-high molecular weight polyethylene), polypropylene, polybutylene, and polypentene, either alone or in combination of two or more of these.

[0045] In addition, the porous polymer film substrate may be manufactured by forming various polymers such as polyester in addition to polyolefin into a film shape. In addition, the porous polymer film substrate may be formed in a structure in which two or more film layers are laminated, and each film layer may be formed from the above-mentioned polymers such as polyolefin and polyester alone or a mixture of two or more of them.

[0046] In addition, the porous polymer film substrate and the porous nonwoven fabric substrate may be formed from a polymer such as polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalate, etc., either alone or in combination, in addition to the above-mentioned polyolefin-based polymers.

[0047] The thickness of the porous polymer substrate is not particularly limited, but may be specifically 1 to 100 μm, more specifically 5 to 50 μm. In accordance with the recent trend toward higher output / higher capacity batteries, it is advantageous to use a thin film as the porous polymer substrate.

[0048] The pore size of the porous polymer substrate may be 10 nm to 100 nm, 10 nm to 70 nm, 10 nm to 50 nm, or 10 nm to 35 nm, and the porosity may be 5% to 90%, preferably 20% to 80%, although in the present invention, such numerical ranges may be easily modified according to specific embodiments or needs.

[0049] The pores of the porous polymer substrate may have various types of pore structures, and any one of the average sizes of pores measured using a porosimeter or observed under FE-SEM is included in the present invention as long as it satisfies the pore diameter of the porous polymer substrate described above.

[0050] Here, in the case of a commonly known uniaxially stretched dry separation membrane, the central pore size in the transverse direction (TD) pore size on the FE-SEM can be used as the standard, rather than the pore size in the machine direction (MD).

[0051] Alternatively, for a porous polymer substrate having a network structure (eg, a wet PE separation membrane), the pore size can be determined based on the pore size measured by a porosimeter.

[0052] The porosity of the porous polymer substrate can be measured by a scanning electron microscope (SEM) image, a mercury porosimeter, a capillary flow porometer, or a porosimetry analyzer (Bell Japan, Belsorp-II mini) using nitrogen gas adsorption flow in a BET 6-point method.

[0053] The porous coating layer may be coated on at least one surface of the porous polymer substrate and on one or more regions of the pores of the porous polymer substrate, and may include inorganic particles and a binder polymer. The porous coating layer may be located on both surfaces of the porous polymer substrate, or may be selectively located on only one surface.

[0054] In the present invention, the porous coating layer includes a first region in contact with the porous polymer substrate, and a second region located on the opposite side of the first region and not in contact with the porous polymer substrate.

[0055] The binder polymer contained in the first region has a concentration gradient that increases from the porous polymer substrate side toward the outermost side of the porous coating layer.

[0056] The gradient of the concentration of the binder polymer in the first region is smaller than the gradient of the concentration of the binder polymer in the second region, which will be described later, and a relatively large amount of the binder polymer is present at the interface between the porous polymer substrate and the porous coating layer, thereby ensuring sufficient adhesive strength at the interface between the porous polymer substrate and the porous coating layer. For example, the adhesive strength at the interface between the porous coating layer and the porous polymer substrate may be 200 kgf / cm or more.

[0057] The adhesive strength of the interface between the porous coating layer and the porous polymer substrate can be measured by fixing the separator to a glass plate using double-sided tape, attaching tape to the porous coating layer, and measuring the force required to separate the tape at 90° at 100 mm / min using a tensile strength measuring device. More specifically, the adhesive strength of the interface between the porous coating layer and the porous polymer substrate can be measured by fixing the separator to a glass plate using double-sided tape, attaching tape (transparent tape manufactured by 3M) firmly to the porous coating layer, and measuring the force required to separate the tape at 90° at 100 mm / min using a tensile strength measuring device.

[0058] The binder polymer in the second region has a concentration gradient that increases from the porous polymer substrate toward the outermost side of the porous coating layer, that is, in the thickness direction of the separator, toward the electrode.

[0059] In addition, the gradient of the concentration of the binder polymer in the second region is greater than the gradient of the concentration of the binder polymer in the first region, thereby increasing the content of the binder polymer in the surface portion of the porous coating layer facing the electrode, thereby increasing the adhesive strength at the interface between the electrode and the porous coating layer.

[0060] In one embodiment of the present invention, the second region may be thicker than the first region to optimize the desired binder polymer properties. For example, the ratio of the thickness (height) of the first region to the thickness (height) of the second region may be 4:6 to 1:9, 4:6 to 2:8, or 4:6 to 3:7. For example, the thickness of the first region may be 0.5 μm to 10 μm, and the thickness of the second region may be 1 μm to 10 μm.

[0061] The binder polymer can connect and fix the inorganic particles to each other by adhering the inorganic particles to each other so that the inorganic particles can maintain a bonded state.

[0062] In one embodiment of the present invention, the binder polymer may include polyvinylidene fluoride (PVdF), polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, acrylonitrile-styrene-butadiene copolymer, polyimide, or two or more thereof.

[0063] In a specific embodiment of the present invention, the weight average molecular weight of the binder polymer may be independently 300,000 or more, 350,000 or more, 400,000 or more, 450,000 or more, or 500,000 or more, and 1,500,000 or less, 1,300,000 or less, or 1,200,000 or less. For example, in order to ensure heat resistance and adhesiveness while also ensuring processability, the weight average molecular weight may be 450,000 to 700,000.

[0064] In this case, the weight average molecular weight can be measured by gel permeation chromatography (GPC, PLGPC220, manufactured by Agilent Technologies).

[0065] Specifically, the measurement can be performed under the following analytical conditions. -Column: PL MiniMixed Bx2 -Solvent: THF -Flow rate: 0.3ml / min -Sample concentration: 2.0mg / ml -Injection volume: 10μl -Column temperature: 40℃ -Detector: Agilent RI detector -Standard: Polystyrene (corrected by a cubic function) -Data processing: ChemStation

[0066] In the present invention, the inorganic particles are not particularly limited as long as they are electrochemically stable. In other words, the inorganic particles usable in the present invention are those that are within the operating voltage range (e.g., 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. In particular, when inorganic particles having a high dielectric constant are used as the inorganic particles, the degree of dissociation of an electrolyte salt, for example, a lithium salt, in the liquid electrolyte can be increased, thereby improving the ionic conductivity of the electrolyte.

[0067] For the reasons mentioned above, the inorganic particles may be inorganic particles having a dielectric constant of 5 or more, inorganic particles having lithium ion transport ability, or a mixture thereof.

[0068] The inorganic particles having a dielectric constant of 5 or more are Al2O3, SiO2, ZrO2, AlO(OH), Al(OH)3, TiO2, BaTiO3, Pb(Zr x Ti 1-x )O3(PZT, where 0 <x<1)、Pb 1-x La x Zr 1-y Ti yO3 (PLZT, where 0 < x < 1, 0 < y < 1), (1 - x)Pb(Mg 1 / 3 Nb 2 / 3 )O3 - xPbTiO3 (PMN - PT, where 0 < x < 1), and can be one or more mixtures selected from the group consisting of hafnia (HfO2), SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, and SiC.

[0069] The inorganic particles having lithium ion transport ability are lithium phosphate (Li3PO4), lithium titanium phosphate (Li x Ti y (PO4)3, 0 < x < 2, 0 < y < 3), lithium aluminum titanium phosphate (Li x Al y Ti z (PO4)3, 0 < x < 2, 0 < y < 1, 0 < z < 3), (LiAlTiP) x O y - based glass (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li x La y TiO3, 0 < x < 2, 0 < y < 3), lithium germanium thiophosphate (Li x Ge y P z S w , 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), lithium nitride (Li x N y , 0 < x < 4, 0 < y < 2), SiS2 - based glass (Li x Si y S z , 0 < x < 3, 0 < y < 2, 0 < z < 4) and P2S5 - based glass (Li x P y S z , 0 < x < 3, 0 < y < 3, 0 < z < 7) and can be one or more mixtures selected from the group consisting of these.

[0070] The average particle size of the inorganic particles is not particularly limited, but may be in the range of 0.001 to 10 μm, more preferably 10 nm to 2 μm, and more preferably 50 nm to 150 nm, in order to form a porous coating layer with a uniform thickness and to have a suitable porosity.

[0071] The average particle size of the inorganic particles means the D50 particle size, and "D50 particle size" means the particle size at 50% of the cumulative particle number distribution according to the particle size. The particle size can be measured using a laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium and then introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac S3500), and when the particles pass through a laser beam, the difference in the diffraction pattern according to the particle size is measured to calculate the particle size distribution. The D50 particle size can be measured by calculating the particle diameter at the 50% point of the cumulative particle number distribution according to the particle size in the measuring device.

[0072] In one embodiment of the present invention, the weight ratio of the inorganic particles to the binder polymer may be 90:10 to 50:50. When the weight ratio of the inorganic particles to the binder polymer content satisfies the above range, it is possible to prevent a problem of reduced pore size and porosity of the porous coating layer formed due to an increased binder polymer content, and also to solve a problem of reduced peel resistance of the porous coating layer formed due to a low binder polymer content.

[0073] In an embodiment of the present invention, the content of the binder polymer in the first region may be the same as the content of the binder polymer in the second region, and the weight ratio of the inorganic particles to the binder polymer in the first region may be the same as the weight ratio of the inorganic particles to the binder polymer in the second region.

[0074] In this case, the porous coating layer may further include other additives in addition to the inorganic particles and binder polymer described above.

[0075] In one embodiment of the present invention, in the porous coating layer, the inorganic particles and the binder polymer are filled and in contact with each other and are bound to each other by the binder polymer, thereby forming interstitial volumes between the inorganic particles, and the interstitial volumes between the inorganic particles may become empty spaces to form pores.

[0076] In one embodiment of the present invention, the average pore size of the porous coating layer may be 0.001 μm to 10 μm. The average pore size of the porous coating layer may be measured by capillary flow porometry. Capillary flow porometry is a method for measuring the diameter of the smallest pore in the thickness direction. Therefore, in order to measure the average pore size of only the porous coating layer by capillary flow porometry, the porous coating layer must be separated from the porous polymer substrate and the separated porous coating layer must be surrounded by a supportable nonwoven fabric, and the pore size of the nonwoven fabric must be much larger than the pore size of the porous coating layer.

[0077] In one embodiment of the present invention, the porosity of the porous coating layer is not particularly limited, but may be 35 to 65%. The porosity of the porous coating layer corresponds to a value obtained by subtracting a volume calculated from the weight and density of each component of the porous coating layer from a volume calculated from the thickness, horizontal length, and vertical length of the porous coating layer.

[0078] The porosity of the porous coating layer can be measured by a BET 6-point method using a scanning electron microscope (SEM) image, a mercury porosimeter, a capillary flow porometer, or a porosimetry analyzer (Bell Japan, Belsorp-II mini) with nitrogen gas adsorption flow.

[0079] FIG. 1 is a schematic diagram of a separation membrane according to an embodiment of the present invention.

[0080] As shown in FIG. 1, a separation membrane 100 according to an embodiment of the present invention includes a porous coating layer 40 including a first region 20 and a second region 30 on at least one surface of a porous polymer substrate 10 .

[0081] The separator for an electrochemical device according to one aspect of the present invention comprises: a first region in contact with the porous polymeric substrate and a second region located on the opposite side of the first region and not in contact with the porous polymeric substrate; the binder polymer in the first region and the second region has a concentration gradient that increases from the porous polymer substrate toward the outermost side of the porous coating layer, The porous coating layer has a gradient of the concentration of the binder polymer in the second region that is greater than the gradient of the concentration of the binder polymer in the first region, thereby increasing the adhesive strength between the electrode and the separator while ensuring the adhesive strength at the interface between the porous polymer substrate and the porous coating layer.

[0082] The separator for an electrochemical device described above is interposed between two electrodes having opposite polarities, and the resulting mixture is housed in a battery case to provide an electrochemical device.

[0083] In one embodiment of the present invention, the porous coating layer may face the positive electrode.

[0084] The active material used in the positive electrode has a lower thermal conductivity than the active material used in the negative electrode, so the thermal conductivity between the positive electrode and the separator may be reduced. Specifically, if the adhesive strength between the positive electrode and the separator is low, the thickness of the electrolyte interfacial layer impregnated between the positive electrode and the separator increases, resulting in a lower thermal conductivity for dissipating heat generated in the cell to the outside. As a result, the thermal conductivity between the positive electrode and the separator is the lowest, resulting in a so-called bottleneck phenomenon.

[0085] Therefore, the effect of the present invention is more pronounced when the porous coating layer faces the positive electrode.

[0086] In one embodiment of the present invention, in order to solve this problem, the adhesive strength between the positive electrode and the separator is increased to increase the thermal conductivity between the positive electrode and the separator.

[0087] Specifically, in one embodiment of the present invention, a porous coating layer having a large amount of a binder polymer distributed on a surface facing an electrode faces a positive electrode, thereby increasing the interfacial adhesion between the porous coating layer and the positive electrode, thereby providing an electrochemical device with significantly improved overcharge safety.

[0088] In one embodiment of the present invention, the interfacial adhesive strength between the separator and the positive electrode may be 30 gf / 25 mm or more when measured by applying a force at 180° at a speed of 100 mm / min after heating and pressurizing the separator and the positive electrode at 80° C. and a pressure of 1000 kgf for 1 second.

[0089] For example, the separator and the positive electrode are stacked, sandwiched between 100 μm PET films, and heated and pressed at 80° C. and 1000 kgf pressure for 1 second using a flat press. The bonded separator and positive electrode are attached to a slide glass using double-sided tape, and the end of the separator adhesive surface (10 mm or less from the end of the adhesive surface) is pulled away and attached to the PET film and cross-section adhesive tape so that the longitudinal direction is connected. Then, a slide glass is attached to the lower holder of the UTM device, and the separator and the attached PET film are attached to the upper holder of the UTM device, and a force is applied at a measurement speed of 100 mm / min and 180° to measure and determine the force required to peel the positive electrode and the porous coating layer facing the positive electrode. Here, the UTM device may be LFPlus manufactured by LLOYD Instrument.

[0090] Such an electrochemical device includes any device that performs an electrochemical reaction, and specifically includes all kinds of primary batteries, secondary batteries, fuel cells, solar cells, and capacitors such as supercapacitor devices, etc. In particular, the secondary batteries may be lithium secondary batteries, including lithium metal secondary batteries, lithium ion secondary batteries, lithium polymer secondary batteries, and lithium ion polymer secondary batteries.

[0091] The positive and negative electrodes to be used with the separator of the present invention are not particularly limited, and may be prepared by bonding an electrode active material to an electrode current collector by a conventional method known in the art. Among the electrode active materials, non-limiting examples of the positive electrode active material include conventional positive electrode active materials used in the positive electrode of conventional electrochemical devices, and in particular, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a lithium composite oxide obtained by combining these materials.

[0092] Non-limiting examples of the negative electrode active material include conventional negative electrode active materials used in the negative electrodes of conventional electrochemical devices, and in particular, lithium metal or lithium alloys, carbon, petroleum coke, activated carbon, graphite, or other carbonaceous materials such as lithium adsorbent materials can be used.

[0093] Non-limiting examples of positive current collectors include foils made of aluminum, nickel, or combinations thereof, and non-limiting examples of negative current collectors include foils made of copper, gold, nickel, or copper alloys, or combinations thereof.

[0094] The electrolyte used in the electrochemical element of the present invention is A + B - A salt with the structure: + Li + , Na + , K + or a combination thereof, such as B - 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), γ-butyrolactone, or a mixture thereof.

[0095] The electrolyte may be injected at an appropriate stage in the battery manufacturing process depending on the manufacturing process and desired physical properties of the final product, i.e., before battery assembly or at the final stage of battery assembly.

[0096] The separator for an electrochemical device is prepared by the following method.

[0097] A method for producing a separator for an electrochemical device according to one aspect of the present invention includes the steps of: (S1) preparing a composition for forming a first porous coating layer and a composition for forming a second porous coating layer; (S2) coating the composition for forming the first porous coating layer on at least one surface of the porous polymer substrate and drying it; (S3) coating the composition for forming the second porous coating layer on the resultant of (S2) and drying it; The drying speed of the composition for forming the second porous coating layer is faster than the drying speed of the composition for forming the first porous coating layer.

[0098] First, in step (S1), a composition for forming a first porous coating layer and a composition for forming a second porous coating layer are prepared.

[0099] The composition for forming the first porous coating layer is for forming the first region of the above-mentioned porous coating layer, and the composition for forming the second porous coating layer is for forming the second region of the above-mentioned porous coating layer.

[0100] The composition for forming the first and second porous coating layers includes inorganic particles, a binder polymer, and a solvent, and may further include additives commonly used in the art, if necessary.

[0101] For the inorganic particles and the binder polymer, see the above.

[0102] The solvent is not particularly limited as long as it can disperse the inorganic particles while dissolving or dispersing the binder polymer, and disperse and dissolve the binder polymer. A solvent having a low boiling point may be more advantageous in order to facilitate subsequent solvent removal. Non-limiting examples of usable solvents include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, water, or mixtures thereof. The amount of the solvent used is sufficient to dissolve or disperse the inorganic particles and the binder, and to provide a viscosity that allows excellent thickness uniformity during subsequent application of the composition for forming a porous coating layer, taking into consideration the coating thickness and production yield of the composition for forming a porous coating layer.

[0103] In the composition for forming the first porous coating layer or the composition for forming the second porous coating layer, the inorganic particles may be added in a state in which they have been crushed in advance to have a predetermined diameter, or the inorganic particles may be added to a binder solution, and then crushed and dispersed while controlling the size of the particles to have a predetermined diameter using a ball mill method or the like.

[0104] In the composition for forming the first or second porous coating layer, the inorganic particles may be dispersed by a method known in the art, for example, an ultrasonic disperser, a ball mill, a bead mill, a disperser, a mixer, etc., and a ball mill or a bead mill is particularly preferable. In this case, the treatment time may vary depending on the capacity, but is preferably 1 to 20 hours, and the particle size of the crushed inorganic particles may be controlled by the size of the beads used in the ball mill or bead mill and the ball mill (or bead mill) time.

[0105] In the composition for forming the first porous coating layer or the composition for forming the second porous coating layer, the weight ratio of the inorganic particles to the binder polymer may be 90:10 to 50:50. When the weight ratio of the inorganic particles to the binder polymer content satisfies the above range, it is possible to prevent a problem of a decrease in the pore size and porosity of the formed porous coating layer caused by an increase in the binder polymer content, and also to solve a problem of a decrease in peel resistance of the formed porous coating layer caused by an increase in the binder polymer content.

[0106] In one embodiment of the present invention, the step (S1) may be a step of preparing a composition for forming a first porous coating layer in which the first inorganic particles are dispersed by adding first inorganic particles to a first binder solution in which a first binder polymer is dissolved in a first solvent and stirring the mixture, and preparing a composition for forming a second porous coating layer in which the second inorganic particles are dispersed by adding second inorganic particles to a second binder solution in which a second binder polymer is dissolved in a second solvent and stirring the mixture.

[0107] In one embodiment of the present invention, the first binder polymer and the second binder polymer may be the same type.

[0108] In an embodiment of the present invention, the first inorganic particles and the second inorganic particles may be of the same type.

[0109] In one embodiment of the present invention, the first and second solvents may be of the same type.

[0110] In one embodiment of the present invention, the first porous coating layer composition and the second porous coating layer composition may have the same components and composition. When the first porous coating layer composition and the second porous coating layer composition are prepared with the same components and composition, i.e., when prepared in one vessel, it is advantageous because it eliminates the troublesome process of preparing the first porous coating layer composition and the second porous coating layer composition separately.

[0111] When the first and second porous coating layer compositions are prepared with the same components and composition, the adhesive strength between the separator and the electrode can be improved depending on the process even if one type of composition for forming a porous coating layer is used. This has the advantage that it is not necessary to change the binder content or prepare two or more types of compositions for forming a porous coating layer in order to increase the adhesive strength. That is, according to one aspect of the present invention, a porous coating layer having regions with different physical properties can be prepared using a composition for forming a porous coating layer having the same composition.

[0112] In steps (S2) and (S3), a composition for forming a first porous coating layer is coated on a porous polymer substrate and dried, and then a composition for forming a second porous coating layer is coated and dried, and the second porous coating layer composition is dried at a faster rate or in a shorter time than the first porous coating layer composition.

[0113] The method of coating the composition for forming the first porous coating layer and the composition for forming the second porous coating layer may be selected from known methods or a new appropriate method depending on the properties of the composition for forming a porous coating layer, etc. For example, a method of uniformly dispersing the composition using a doctor blade, etc., or a method such as die casting or comma coating may be selected.

[0114] According to one aspect of the present invention, a first coating device connected to a first porous coating layer forming composition supply unit is located in order to apply a porous coating layer forming composition onto a porous polymer substrate, and a first drying oven is located downstream of the first coating device. Also, a second coating device connected to a second porous coating layer forming composition supply unit is located downstream of the first drying oven, and a second drying oven is located downstream of the second coating device. The porous polymer substrate is transferred on a conveyor, and the first porous coating layer forming composition is coated on the porous polymer substrate through the first coating device, and then the first porous coating layer forming composition is dried in the first drying oven to form a first porous coating layer, and then the second porous coating layer forming composition is coated on the first porous coating layer through the second coating device, and then the second porous coating layer forming composition is dried in the second drying oven to form a second porous coating layer.

[0115] The first drying oven and the second drying oven may each be composed of a preheating zone, a constant drying zone, and a falling drying zone. The preheating zone is a zone in which the porous coating layer in a room temperature environment is heated to the evaporation temperature of the solvent. The constant drying zone is a zone in which the solvent is evaporated on the surface of the porous coating layer, and the solvent content in the porous coating layer is reduced approximately linearly. The falling drying zone is usually understood to mean a zone in which the solvent is gently evaporated from the fine gaps between the particles constituting the porous coating layer.

[0116] In the present invention, the drying speed of the composition for forming the first porous coating layer and the composition for forming the second porous coating layer is made different in the constant rate drying region, because binder migration actively occurs in the constant rate drying region.

[0117] In the present invention, the drying of the composition for forming a first porous coating layer and the drying of the composition for forming a second porous coating layer are performed under the same temperature, humidity and atmospheric conditions, and it is understood that the solvent contained in the composition for forming a porous coating layer is substantially completely dried after the constant rate drying.

[0118] In one aspect of the present invention, the amount of solvent dried per unit time can be adjusted by varying the speed at which the composition for forming the first porous coating layer and the composition for forming the second porous coating layer pass through a drying oven of the same length, more specifically, a constant rate drying area.

[0119] For example, the time for which the first porous coating layer forming composition passes through a drying oven, more specifically, a constant rate drying region, can be longer than the time for which the second porous coating layer forming composition passes through a drying oven, more specifically, a constant rate drying region, to adjust the amount of solvent dried per unit time. For example, it can be 2 to 3 times longer. For example, when the length of the drying oven for drying the porous coating layer forming composition is 10 m, the first porous coating layer composition is dried by passing through the 10 m drying oven for 10 minutes, and the second porous coating layer forming composition is dried by increasing the heat amount and air volume of the drying oven and passing through the 10 m drying oven for 5 minutes to complete drying within 5 minutes, so that the amount of solvent dried per unit time can be different.

[0120] As a non-limiting example, the moving speed of the composition for forming a first porous coating layer and the moving speed of the composition for forming a second porous coating layer in the constant rate drying area may be set to 0.1 m / min to 10 m / min, and the moving speed of the composition for forming a second porous coating layer may be faster than the moving speed of the composition for forming the first porous coating layer.

[0121] In this case, the size and number of the drying ovens are not particularly limited and may be designed differently depending on the size of inorganic particles to be produced, the components of the composition for forming a porous coating layer, the application amount of the composition for forming a porous coating layer, etc. For example, a drying apparatus may be designed in which 2 to 10 drying ovens having a length of 10 to 15 m are used as a constant rate drying region and one drying oven having a length of 10 to 15 m is used as a falling rate drying region.

[0122] The drying temperatures of the first drying oven and the second drying oven can be independently controlled to 50°C or higher, or 80°C or higher, and set to 110°C or lower so that the porous coating layer does not peel off or collapse and productivity is improved.

[0123] The evaporation rate (drying rate) of the solvent can be adjusted by the atmospheric pressure (pressure inside the drying oven), drying temperature and relative humidity. In general, if other conditions are the same, the lower the atmospheric pressure and / or the higher the drying temperature, the faster the drying rate tends to be.

[0124] The drying speed of the falling-rate drying region is advantageously set to 1 mg / sec or more (usually 1.5 mg / sec or more, preferably 2 mg / sec or more) in order to complete the falling-rate drying in a short time, and is advantageously adjusted to 5 mg / sec or less (usually 3 mg / sec or less, preferably 2.8 mg / sec or less) in order to prevent oxidation of the surface of the porous polymer substrate.

[0125] According to one embodiment of the present invention, the drying rate of the composition for forming the first porous coating layer and the drying rate of the composition for forming the second porous coating layer can be controlled so that the amount of solvent dried per second is 1 mg / sec to 50 mg / sec, and the drying rate of the composition for forming the second porous coating layer can be controlled to be faster than the drying rate of the composition for forming the first porous coating layer.

[0126] For example, the composition for forming the second porous coating layer dries 1 mg / sec to 20 mg / sec faster than the composition for forming the first porous coating layer, or the drying rate of the composition for forming the first porous coating layer can be controlled to 10 to 50% of the drying rate of the composition for forming the second porous coating layer.

[0127] The drying rate of the composition for forming the first porous coating layer and the drying rate of the composition for forming the second porous coating layer are understood to mean drying rates in a constant rate drying region.

[0128] When the drying rate is set in this manner, the binder polymer contained in the composition for forming a second porous coating layer migrates to the upper layer as the composition for forming a second porous coating layer dries more actively than the binder polymer contained in the composition for forming a first porous coating layer dries, thereby increasing the distribution of the binder polymer in the porous coating layer portion on the surface side facing the electrode in the finally manufactured separator.

[0129] That is, a first region and a second region are formed having a binder concentration gradient that increases in a thickness direction of the separator toward the electrode, and the gradient of the binder polymer concentration gradient in the second region is greater than the gradient of the binder polymer concentration gradient in the first region.

[0130] The adhesive strength between the separator and the electrode is increased by increasing the distribution of the binder polymer in the porous coating layer on the surface facing the electrode. The increased adhesive strength between the separator and the electrode reduces the thermal resistance between the separator and the electrode, thereby providing a separator for an electrochemical device with improved overcharge safety and an electrochemical device including the same.

[0131] Furthermore, the presence of a sufficient amount of binder polymer on the porous polymer substrate side also ensures sufficient adhesive strength between the porous polymer substrate and the porous coating layer.

[0132] Meanwhile, the atmospheric pressure in the drying furnace in which the composition for forming the first porous coating layer is dried and the atmospheric pressure in the drying furnace in which the composition for forming the second porous coating layer is dried may be the same, for example, an atmospheric pressure of 1 atmosphere.

[0133] In addition, a drying oven in which the composition for forming the first porous coating layer and the composition for forming the second porous coating layer are dried may be maintained at a dew point of −10° C. or lower.

[0134] The drying in each of the first drying oven and the second drying oven can be performed by a method commonly used in the art, for example, by using hot air.

[0135] According to one aspect of the present invention, the composition for forming the first porous coating layer and the composition for forming the second porous coating layer have the same components and composition ratios, and are the same starting materials, but an electrochemical device can be provided in which the adhesion between an electrode, particularly a positive electrode, and a porous coating layer is improved. This is because the drying speed of the composition for forming the second porous coating layer is faster than the drying speed of the composition for forming the first porous coating layer, thereby increasing the migration of the binder polymer occurring in the second region of the porous coating layer and controlling the binder distribution on the surface of the porous coating layer facing the electrode. This can be seen in Figures 2 and 3.

[0136] 2 and 3 are schematic diagrams showing how the distribution of the binder polymer in the porous coating layer changes depending on the drying speed.

[0137] As shown in Fig. 2, the second binder polymer 32 is dissolved or dispersed in the second solvent 31. When this coating composition is dried, as shown in Fig. 3, the second solvent 31 volatilizes and the second binder polymer 32 migrates together with the second binder polymer 32 to the surface of the porous coating layer facing the electrode, thereby improving the adhesion between the porous coating layer and the positive electrode.

[0138] The present invention will be described in detail below with reference to specific examples. However, the examples according to the present invention can be modified into many other forms, and the scope of the present invention should not be construed as being limited to the following examples. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0139] Example 1 Polyvinylidene fluoride (PVdF) having a weight average molecular weight of 330,000 was added as a first binder polymer to acetone as a first solvent, and dissolved at 50°C for about 4 hours to prepare a first binder solution. Al2O3 (particle size: 100 nm) was added as a first inorganic particle to the first binder solution. At this time, the weight ratio of the first inorganic particle: the first binder polymer was controlled to 80:20 to prepare a composition for forming a first porous coating layer. At this time, the content of the solid content (the first solvent removed from the composition for forming a first porous coating layer) was 18 parts by weight based on 100 parts by weight of the composition for forming a first porous coating layer.

[0140] Next, polyvinylidene fluoride (PVdF) having a weight average molecular weight of 330,000 was added as a second binder polymer to acetone as a second solvent, and dissolved at 50°C for about 4 hours to prepare a second binder solution. Al2O3 (particle size: 100 nm) was added as a second inorganic particle to the second binder solution. At this time, the weight ratio of the second inorganic particle: the second binder polymer was controlled to 80:20 to prepare a composition for forming a second porous coating layer. At this time, the content of the solid content (the second solvent removed from the composition for forming a second porous coating layer) was 18 parts by weight based on 100 parts by weight of the composition for forming a second porous coating layer.

[0141] Then, the composition for forming the first porous coating layer was applied to both sides of a 9 μm-thick polyethylene porous film (porosity: 45%) by dip coating under conditions of a relative humidity of 45% and a temperature of 110° C. Then, it was dried at a rate of 16 mg / sec to form a first region of a porous coating layer having a thickness of 0.9 μm on each side.

[0142] The composition for forming the second porous coating layer was then applied onto one side of the first region by dip coating at a speed of 5 m / min, a relative humidity of 45%, and a temperature of 110° C. The composition was then dried at a speed of 32 mg / sec to form a second region of a porous coating layer having a thickness of 2.1 μm on one side. At this time, the ratio of the thickness of the first region to the thickness of the second region on one side was 3:7. The results are shown in Table 1.

[0143] Comparative Example 1 Polyvinylidene fluoride (PVdF) with a weight average molecular weight of 330,000 was added as a binder polymer to acetone as a solvent and dissolved at 50°C for about 4 hours to prepare a binder solution. Al2O3 (particle size: 100 nm) was added as inorganic particles to the binder solution. At this time, the weight ratio of inorganic particles:binder polymer was controlled to 80:20 to prepare a composition for forming a porous coating layer. At this time, the content of solids (solvent removed from the composition for forming a porous coating layer) was 18 parts by weight based on 100 parts by weight of the composition for forming a porous coating layer.

[0144] The composition for forming a porous coating layer was then applied to both sides of a 9 μm-thick polyethylene porous film (porosity: 45%) by dip coating under conditions of 45% relative humidity and 110° C. Then, it was dried at a rate of 16 mg / sec to form a porous coating layer having a thickness of 3 μm on each side. The results are shown in Table 1.

[0145] Comparative Example 2 A separator for an electrochemical device was prepared in the same manner as in Comparative Example 1, except that the drying rate of the composition for forming a porous coating layer was controlled to 32 mg / sec. The results are shown in Table 1.

[0146] Comparative Example 3 A separator for an electrochemical device was prepared in the same manner as in Example 1, except that the drying rate of the composition for forming the first porous coating layer was controlled to 32 mg / sec and the drying rate of the composition for forming the second porous coating layer was controlled to 16 mg / sec. The results are shown in Table 1.

[0147] Evaluation example 1: Analysis of the physical properties of separation membranes In the separators for electrochemical devices prepared in Example 1 and Comparative Examples 1 to 3, the thickness ratio of the first region to the second region, the adhesive strength between the positive electrode and the separator, the overcharge safety evaluation, and the T max The results are shown in Table 1.

[0148] [Table 1]

[0149] As can be seen from Table 1, in Example 1, in which the drying rate of the composition for forming the second porous coating layer was faster than the drying rate of the composition for forming the first porous coating layer, 4 out of 5 samples passed the overcharge safety evaluation.

[0150] On the other hand, in the case of Comparative Example 1, the drying speed was slow, and the phenomenon of the binder polymer migrating to the surface facing the electrode was reduced. As a result, the adhesive strength between the positive electrode and the separator was low, and as a result, all five batteries failed the overcharge safety test.

[0151] In the case of Comparative Example 2, the composition was dried at the same drying speed as the composition for forming the second porous coating layer in Example 1. However, in the case of Comparative Example 2, unlike Example 1, the solvent was dried all at once, and the drying and coating speeds were too fast, resulting in the occurrence of undried areas.

[0152] Meanwhile, in Comparative Example 3, the drying speed of the composition for forming the first porous coating layer is faster than that of the composition for forming the second porous coating layer. In this case, the content of the binder polymer transferred to the surface of the porous coating layer is relatively less than that in Example 1, and the adhesive strength between the positive electrode and the porous coating layer is relatively not improved. It was also confirmed that all five overcharge safety evaluations failed.

[0153] The thickness of the separator, the adhesive strength between the positive electrode and the separator, the overcharge safety evaluation, and T max was measured by the following method.

[0154] 1) Thickness measurement The thicknesses of the first and second regions of the separation membrane were measured using a thickness measuring device (Mitutoyo Corporation, VL-50S-B).

[0155] 2) Measurement of adhesion strength between the positive electrode and the separator A positive electrode was prepared to measure the adhesive strength (Lami strength) between the electrode and the separator.

[0156] First, a positive electrode was produced in the following manner.

[0157] LiNi as a positive electrode active material 0.6 Co 0.2 Mn 0.2 O2, carbon black, and polyvinylidene fluoride (PVdF) were mixed in a weight ratio of 96:2:2 into N-methyl-2-pyrrolidone (NMP) to prepare a positive electrode slurry. The prepared positive electrode slurry was applied to an aluminum foil with a thickness of 20 μm as a positive electrode current collector at 3 mAh / cm 2 The positive electrode was prepared by coating with 100% water.

[0158] The prepared positive electrode was cut to a size of 20 mm x 50 mm. The separator prepared in the examples and comparative examples was cut to a size of 25 mm x 100 mm. The prepared separator and positive electrode were stacked and then sandwiched between 100 μm PET films and bonded using a flat press. The flat press was performed by heating and pressing at 80 ° C and a pressure of 1000 kgf for 1 second. The bonded separator and positive electrode were attached to a slide glass using double-sided tape. The end of the separator adhesive surface (10 mm or less from the end of the adhesive surface) was pulled away and attached to a 25 mm x 100 mm PET film and a cross-section adhesive tape so that the longitudinal direction was connected. Then, the slide glass was attached to the lower holder of a UTM device (LFPlus manufactured by LLOYD Instruments), and the separator and the attached PET film were attached to the upper holder of the UTM device. A force was applied at a measurement speed of 100 mm / min and 180° to measure the force required to peel the positive electrode from the porous coating layer facing the positive electrode.

[0159] 3) Evaluation of overcharging safety For the overcharging experiment, a battery was prepared that was fully charged to 0.3C / 4.2V. The overcharging experiment involved charging the battery to 8.4V at room temperature with a current of 1C to check for explosions.

[0160] 4) T max Measurement During the overcharge experiment, a thermocouple was attached to the center of the cell, and the highest temperature measured until the end of overcharge was defined as T max was selected as.

[0161] Evaluation example 2: Measurement of the peel strength in the thickness direction of the separation membrane The peel strength in the thickness direction of the separators prepared in Example 1, Comparative Example 1, and Comparative Example 3 was measured and is shown in Table 2 below.

[0162] The peel strength in the thickness direction of the separation membrane was measured by the following method.

[0163] The separation membranes prepared in the examples and comparative examples were cut to a size of 25 mm x 100 mm. The prepared separation membranes were attached to a slide glass using double-sided tape. The end of the adhesive surface of the separation membrane on the side not attached to the slide glass (10 mm or less from the end of the adhesive surface) was pulled away and attached to a 25 mm x 100 mm PET film using single-sided adhesive tape so that the longitudinal direction was connected. Then, the slide glass was attached to the lower holder of a UTM device (LFPlus manufactured by LLOYD Instruments), the separation membrane and the attached PET film were attached to the upper holder of the UTM device, and a force was applied at a measurement speed of 100 mm / min at 90° to measure the force required to peel off a part of the porous coating layer.

[0164] Then, a PET film was attached to the separator from which the porous coating layer had been partially peeled off, and a force was applied at a measurement speed of 100 mm / min at 90° to measure the force required to peel off a portion of the porous coating layer.

[0165] This was done two more times.

[0166] [Table 2]

[0167] As can be seen from Table 2, in the case of Example 1, the peel strength was the highest in the first measurement. This is believed to be because the drying speed of the composition for forming the second porous coating layer was faster than that of the composition for forming the first porous coating layer, and the binder polymer was distributed in greater amounts on the surface side facing the electrode. It can be seen that the peel strength was lower in the second to fourth measurements than in the first measurement, since the porous coating layer on the surface side facing the electrode had already been partially peeled off.

[0168] Meanwhile, in the case of Comparative Example 1, the peel strength of the separation membrane showed a similar trend from the first to fourth times. In the case of Comparative Example 1, the drying speed of the separation membrane was very slow and the distribution of the binder polymer along the thickness direction of the separation membrane did not change significantly, so it can be inferred that the peel strength of the separation membrane also showed a similar trend from the first to fourth times.

[0169] In the case of Comparative Example 3, the second peel strength was the highest. This is believed to be because the drying speed of the composition for forming the first porous coating layer was faster than the drying speed of the composition for forming the second porous coating layer, and the binder polymer was distributed most abundantly at the interface between the second region and the first region.

Claims

1. A porous polymer substrate; a porous coating layer located on at least one surface of the porous polymer substrate, the porous coating layer including inorganic particles and a binder polymer; The porous coating layer includes a first region in contact with the porous polymeric substrate and a second region opposite the first region and not in contact with the porous polymeric substrate; the binder polymer in the first region and the second region has a concentration gradient that increases from the porous polymer substrate side toward the outermost side of the porous coating layer, the gradient of the concentration of the binder polymer contained in the second region is greater than the gradient of the concentration of the binder polymer contained in the first region; A separator for an electrochemical device, wherein the first region has a thickness greater than a thickness of the second region.

2. 2. The separator for an electrochemical device according to claim 1, wherein a content of the binder polymer relative to a total amount of the inorganic particles and the binder polymer contained in the first region is the same as a content of the binder polymer relative to a total amount of the inorganic particles and the binder polymer contained in the second region.

3. 3. The separator for an electrochemical device according to claim 1, wherein a ratio of a thickness of the first region to a thickness of the second region is from 4:6 to 1:

9.

4. 4. The separator for electrochemical elements according to claim 1, wherein the binder polymer comprises polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene, polyvinylidene fluoride-trichloroethylene, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, carboxymethyl cellulose, acrylonitrile-styrene-butadiene copolymer, polyimide, or two or more thereof.

5. An electrochemical device in which an electrode assembly including two electrodes having opposite polarities and a separator interposed between the two electrodes is housed in a battery case, An electrochemical element, wherein the separator is the separator for electrochemical elements according to claim 1 .

6. 6. The electrochemical device according to claim 5, wherein the porous coating layer faces the positive electrode.

7. 7. The electrochemical device according to claim 6, wherein the adhesive strength at the interface between the separator and the positive electrode is 30 gf / 25 mm or more when measured by applying a force at 180° at a speed of 100 mm / min after heating and pressurizing the separator and the positive electrode at 80° C. and a pressure of 1000 kgf for 1 second.

8. A method for producing a separation membrane for an electrochemical element according to claim 1, comprising the steps of: (S1) preparing a composition for forming a first porous coating layer and a composition for forming a second porous coating layer; (S2) coating the composition for forming the first porous coating layer on at least one surface of the porous polymer substrate and drying it; (S3) coating the composition for forming the second porous coating layer on the resultant of (S2) and drying it; The method for manufacturing a separator for an electrochemical device, wherein the drying rate of the composition for forming a second porous coating layer is faster than the drying rate of the composition for forming a first porous coating layer.

9. 9. The method for producing a separator for an electrochemical element according to claim 8, wherein the drying rate of the composition for forming a first porous coating layer and the drying rate of the composition for forming a second porous coating layer are controlled so that the amount of solvent dried per second is 1 mg / sec to 50 mg / sec, and the drying rate of the composition for forming a second porous coating layer is faster than the drying rate of the composition for forming a first porous coating layer.

10. 10. The method for manufacturing a separator for an electrochemical device according to claim 8, wherein the composition for forming the second porous coating layer is dried 1 to 20 mg / sec faster than the composition for forming the first porous coating layer.

11. The method for producing a separator for electrochemical elements according to any one of claims 8 to 10, wherein a drying rate of the composition for forming the first porous coating layer is 10 to 50% of a drying rate of the composition for forming the second porous coating layer.

12. In the step (S1), a first inorganic particle is added to a first binder solution in which a first binder polymer is dissolved in a first solvent, and the mixture is stirred to prepare a composition for forming a first porous coating layer in which the first inorganic particle is dispersed; preparing a composition for forming a second porous coating layer in which the second inorganic particles are dispersed by adding second inorganic particles to a second binder solution in which a second binder polymer is dissolved in a second solvent and stirring the mixture; The method for manufacturing a separator for an electrochemical device according to claim 8 , wherein the composition for forming the first porous coating layer and the composition for forming the second porous coating layer have the same components and composition ratio.

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