Method for manufacturing a separator and separator obtained thereby

Abstract

The present disclosure relates to a method of manufacturing a separator for a lithium secondary battery. In particular, the method of manufacturing a separator for a lithium secondary battery includes the steps of preparing a slurry for forming a porous coating layer including inorganic particles dispersed in an asymmetric linear ketone solvent and a binder polymer dissolved therein, the binder polymer including a first binder polymer and a second binder polymer; and applying the slurry for forming a porous coating layer to a porous polymer substrate having a plurality of pores, followed by drying. According to embodiments of the present disclosure, the slurry for forming a porous coating layer is prepared by dissolving a first binder polymer and a second binder polymer having predetermined properties in an asymmetric ketone solvent, and a separator can be obtained by using the slurry for forming a porous coating layer. In this way, adhesion to an electrode (lamination strength) can be improved and compression resistance of the separator can be increased.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method of manufacturing a separator applicable to an electrochemical device such as a lithium secondary battery, and a separator obtained therefrom.

[0002] This application claims priority to Korean Patent Application No. 10-2020-0074358 filed in Korea on June 18, 2020, the disclosure of which is incorporated herein by reference. BACKGROUND

[0003] Recently, energy storage technology has received growing attention. Efforts to research and develop electrochemical devices have been increasingly realized as the application of energy storage technology has been expanded to energy for mobile phones, camcorders, and notebook PCs, and even to energy for electric vehicles. In this context, electrochemical devices have been most highlighted. Among these electrochemical devices, development of rechargeable secondary batteries has been focused. Recently, active research has been conducted on designing new electrodes and batteries in order to improve the capacity density and specific energy in developing these batteries.

[0004] Among commercially available secondary batteries, lithium secondary batteries developed in the early 1990s have been highlighted because they have a higher operating voltage and a significantly higher energy density compared to conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries using aqueous electrolytes.

[0005] Although electrochemical devices such as lithium secondary batteries have been produced from many production companies, their safety characteristics have shown different signs. It is very important to evaluate and ensure the safety of these electrochemical devices. For example, a separator prevents short-circuiting between a positive electrode and a negative electrode and provides a passage for transporting lithium ions. Therefore, the separator is an important factor that affects the safety and output characteristics of a battery.

[0006] Meanwhile, such a separator is interposed between electrodes and is laminated under the application of pressure in order to obtain an electrode assembly including a positive electrode and a negative electrode. Here, the pore structure in the separator can be deformed by the pressure applied to the separator. In addition to this, when a silicon-based negative electrode is used to achieve high energy density, the negative electrode active material can undergo volume expansion, causing deformation of the pore structure in the separator.

[0007] Therefore, there is a need to provide a separator having improved compression resistance and showing improved adhesion to electrodes. SUMMARY

[0008] TECHNICAL PROBLEM

[0009] The present disclosure is designed to solve problems of the related art, and thus the present disclosure is directed to providing a method of manufacturing a separator showing improved adhesion to an electrode and having improved compression resistance, and a separator obtained therefrom.

[0010] The present disclosure is also directed to providing an electrochemical device provided with the separator.

[0011] Technical Solution

[0012] In one aspect of the present disclosure, there is provided a method of manufacturing a separator according to any one of the following embodiments.

[0013] According to a first embodiment, there is provided a method of manufacturing a separator for a lithium secondary battery, comprising the steps of:

[0014] preparing a slurry for forming a porous coating layer including inorganic particles dispersed in an asymmetric linear ketone solvent and a binder polymer dissolved therein, the binder polymer including a first binder polymer and a second binder polymer; and

[0015] applying the slurry for forming a porous coating layer to a porous polymer substrate having a plurality of pores, followed by drying,

[0016] wherein the first binder polymer is a thermoplastic polyurethane including a soft segment having a polyol-derived repeating unit and a hard segment having a urethane bonding structure,

[0017] the second binder polymer is an acrylate-based polymer having a glass transition temperature (Tg) of 25℃ to 125℃, and

[0018] the first binder polymer is introduced in an amount of more than 10 parts by weight based on a total content of 100 parts by weight of the binder polymer.

[0019] According to a second embodiment, there is provided a method of manufacturing a separator for a lithium secondary battery as defined in the first embodiment,

[0020] wherein the first binder polymer has a melting point of 30℃ to 150℃.

[0021] According to a third embodiment, there is provided a method of manufacturing a separator for a lithium secondary battery as defined in the first or second embodiment,

[0022] wherein the first binder polymer includes the hard segment and the soft segment in a molar ratio of 10:90 to 90:10.

[0023] According to a fourth embodiment, there is provided a method of manufacturing a separator for a lithium secondary battery as defined in any one of the first to third embodiments,

[0024] wherein the first binder polymer has a weight average molecular weight of 10,000 to 1,000,000.

[0025] According to a fifth embodiment, there is provided a method of manufacturing a separator for a lithium secondary battery as defined in any one of the first to fourth embodiments,

[0026] wherein the second binder polymer has a weight average molecular weight of 10,000 to 1,000,000.

[0027] According to a sixth embodiment, there is provided a method of manufacturing a separator for a lithium secondary battery as defined in any one of the first to fifth embodiments,

[0028] wherein the second binder polymer includes a repeating unit derived from at least one monomer selected from the group consisting of a methyl acrylate monomer, an ethyl acrylate monomer, a butyl acrylate monomer, a 2-ethylhexyl acrylate monomer, an acrylic acid monomer, and a methyl methacrylate monomer.

[0029] According to a seventh embodiment, there is provided a method of manufacturing a separator for a lithium secondary battery as defined in any one of the first to sixth embodiments,

[0030] wherein the asymmetric linear ketone solvent is an asymmetric linear ketone having a number of carbon atoms of 4 to 10.

[0031] According to an eighth embodiment, there is provided a method of manufacturing a separator for a lithium secondary battery as defined in any one of the first to seventh embodiments,

[0032] wherein the asymmetric linear ketone solvent includes methyl ethyl ketone, methyl ethyl ketone, methyl isobutyl ketone, ethyl propyl ketone, ethyl isobutyl ketone, or two or more thereof.

[0033] According to a ninth embodiment, there is provided a method of manufacturing a separator for a lithium secondary battery as defined in any one of the first to eighth embodiments,

[0034] wherein the drying step is performed at a relative humidity of 30% to 80%.

[0035] According to a tenth embodiment, there is provided a method of manufacturing a separator for a lithium secondary battery as defined in any one of the first to ninth embodiments,

[0036] wherein the weight ratio of the inorganic particles to the total of the binder polymers is 95:5 to 5:95.

[0037] According to an eleventh embodiment, there is provided a method of manufacturing a separator for a lithium secondary battery as defined in any one of the first to tenth embodiments,

[0038] wherein the asymmetric linear ketone solvent is methyl ethyl ketone, the first binder polymer is a thermoplastic polyurethane having a molar ratio of the hard segment to the soft segment of 10:90 to 90:10, and the second binder polymer is polymethyl methacrylate.

[0039] According to a twelfth embodiment, there is provided a method of manufacturing a separator for a lithium secondary battery as defined in any one of the first to eleventh embodiments,

[0040] wherein the separator has an adhesion to a counter electrode (laminating strength, Lami Strength) of 60 gf / 25 mm to 300 gf / 25 mm, and shows a compression ratio of the porous polymer substrate of 0% to 7% after lamination.

[0041] In another aspect of the present disclosure, there is provided a separator according to the following embodiments.

[0042] According to a thirteenth embodiment,

[0043] There is provided a separator for a lithium secondary battery obtained by the method as defined in any one of the first to twelfth embodiments, comprising:

[0044] a porous polymer substrate; and

[0045] a porous coating layer formed on at least one surface of the porous polymer substrate and including inorganic particles, a first binder polymer, and a second binder polymer.

[0046] According to a fourteenth embodiment,

[0047] There is provided a lithium secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is the same as defined in the thirteenth embodiment.

[0048] Advantageous effects

[0049] According to embodiments of the present disclosure, a slurry for forming a porous coating is prepared by dissolving a first binder polymer and a second binder polymer having predetermined properties in an asymmetric ketone solvent, and a separator can be obtained by using the slurry for forming a porous coating. In this way, adhesion to electrodes (lami strength) can be improved and compression resistance of the separator can be increased. BRIEF DESCRIPTION OF DRAWINGS

[0050] Figure 1 is a schematic view illustrating the structure of a thermoplastic polyurethane. DETAILED DESCRIPTION

[0051] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it is to be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings and should be interpreted based on the meanings and concepts corresponding to technical aspects of the present disclosure on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation of the invention.

[0052] In an electrochemical device such as a lithium secondary battery, a separator generally uses a porous polymer substrate, and thus has a problem in that it shows thermal shrinkage behavior. Accordingly, a porous coating has been introduced in order to reduce thermal shrinkage of the separator.

[0053] Meanwhile, the separator is interposed between electrodes and is laminated under pressure in order to obtain an electrode assembly including a positive electrode and a negative electrode. Here, a pore structure in the separator can be deformed by pressure applied to the separator. In addition to this, when a silicon-based negative electrode is used to achieve high energy density, the negative electrode active material can undergo volume expansion, causing deformation of the pore structure in the separator.

[0054] Accordingly, there is a need to provide a separator having improved compression resistance and showing improved adhesion to electrodes.

[0055] To solve the above-mentioned problems, the inventors of the present disclosure have conducted intensive research to provide a method of manufacturing a separator having improved compression resistance and improved adhesion to electrodes, and a separator obtained therefrom.

[0056] In one aspect of the present disclosure, a method of manufacturing a separator for a lithium secondary battery is provided, including the steps of:

[0057] a slurry for forming a porous coating is prepared, the slurry for forming a porous coating including inorganic particles dispersed in an asymmetric linear ketone solvent and a binder polymer dissolved therein, the binder polymer including a first binder polymer and a second binder polymer; and

[0058] applying the slurry for forming a porous coating to a porous polymer substrate having a plurality of pores, followed by drying,

[0059] wherein the first binder polymer is a thermoplastic polyurethane including soft segments having polyol-derived repeating units and hard segments having urethane bonding structures,

[0060] the second binder polymer is an acrylate-based polymer having a glass transition temperature (Tg) of 25°C to 125°C, and

[0061] The first binder polymer is introduced in an amount of more than 10 parts by weight based on 100 parts by weight of the total content of the binder polymer.

[0062] First, a slurry for forming a porous coating is prepared, the slurry for forming a porous coating including inorganic particles dispersed in an asymmetric linear ketone solvent and a binder polymer dissolved therein, the binder polymer including a first binder polymer and a second binder polymer.

[0063] Herein, the first binder polymer is a thermoplastic polyurethane including soft segments having polyol-derived repeating units and hard segments having urethane bonding structures. Since the first binder polymer has a property of being melted by heat and solidified again, it can increase adhesion between a separator and an electrode (lami strength) and adhesion between a porous polymer substrate and a porous coating (peel strength).

[0064] The first binder polymer is a block copolymer, in which a molar ratio of the hard segments to the soft segments can be 10:90 or more, 20:80 or more, or 25:75 or more, and 90:10 or less, 85:15 or less, or 80:20 or less. When linear hard segments and curved soft segments form an association region by phase separation, within the above-defined range, the first binder polymer can act as a physical crosslinking point. In particular, when a proportion of the number of moles of the hard segments based on the total number of moles of the hard segments and the soft segments is 0.25 or more (i.e., when a molar ratio of the hard segments to the soft segments is 25:75 or more), the first binder polymer can have enhanced properties as a thermoplastic elastomer. Meanwhile, such an association region dissociates at a temperature higher than a predetermined temperature, and thus the first binder polymer can act as an elastomer.

[0065] The first binder polymer can be a thermoplastic polyurethane having a weight average molecular weight of 10,000 or more, 20,000 or more, or 50,000 or more, and 1,000,000 or less, 300,000 or less, or 200,000 or less. When the first binder polymer has a weight average molecular weight within the above defined range, it can be mixed with the second binder polymer to form the surface adhesive layer. In particular, when the first binder polymer has a weight average molecular weight of 100,000 to 200,000, the desired purpose of the present disclosure can be effectively achieved.

[0066] For example, the first binder polymer can be a thermoplastic polyurethane having a melting point of 30°C or more, 40°C or more, 50°C or more, or 60°C or more, and 150°C or less, 145°C or less, 140°C or less, or 135°C or less.

[0067] For example, the thermoplastic polyurethane can have a structure represented by the following Chemical Formula 1. Since the thermoplastic polyurethane has a hard segment and a soft segment as represented by Chemical Formula 1, it provides a different phase separation effect.

[0068] [Chemical Formula 1]

[0069]

[0070] More particularly, the hard segment and the soft segment of the linear polyurethane form an association region as shown in Figure 1 Chemical Formula 2, so that the polyurethane can act as a physical crosslinking point. Such a crosslinking point is dissociated at a temperature higher than a predetermined temperature, so that the thermoplastic polyurethane can act as an elastomer.

[0071] When a thermosetting polyurethane is used instead of a thermoplastic polyurethane as the first binder polymer, the crosslinking point cannot be dissociated, so that the polyurethane can not achieve adhesion to the electrode and can not act as an elastomer. First, the polyurethane that has been thermally cured cannot be dissolved in the above-mentioned solvent, thereby making it difficult to achieve a coating layer. In addition to this, since the solution-type polyurethane cannot maintain its shape at room temperature, it is not suitable as a binder capable of forming a separator surface adhesive layer.

[0072] Herein, the first binder polymer is introduced in an amount of more than 10 parts by weight based on the total content of the binder polymer of 100 parts by weight. If the first binder polymer is used in an amount of 10 parts by weight or less, the effects of improving compression resistance and improving adhesion to the electrode are reduced. This is because the amount of the first binder is not sufficient to improve the elasticity of the coating layer and to strengthen the binding of the coating layer.

[0073] According to embodiments of the present disclosure, the first binder polymer can be used in an amount of 15 to 95 parts by weight, 20 to 90 parts by weight, or 30 to 90 parts by weight, based on the total content of 100 parts by weight of the binder polymer.

[0074] Meanwhile, when only the first binder polymer is used, it can be impossible to form pores and a binder layer in the porous coating layer through a wet phase separation process. In addition to this, when the first binder polymer is used in combination with a conventional fluorine-based binder polymer at the same time, the first binder polymer is not miscible with the fluorine-based binder polymer, such that phase separation can not occur and surface adhesion can hardly be achieved. Furthermore, since intermediate layer separation occurs in the presence of an asymmetric linear ketone solvent, it can be impossible to form the porous coating layer itself.

[0075] That is, according to the present disclosure, a second binder polymer having desired properties as described below is used as an essential constituent element.

[0076] The second binder polymer is an acrylate-based polymer having a glass transition temperature (Tg) of 25℃ to 125℃. In particular, when an acrylate-based polymer is used as the second binder polymer, the acrylate-based polymer can be mixed with the first binder polymer to form a surface adhesive layer, thereby providing improved adhesion of the electrode and a reduced compression ratio. In particular, the acrylate-based polymer can have a glass transition temperature of 25℃ or more, 30℃ or more, 35℃ or more, 40℃ or more, 45℃ or more, or 50℃ or more, and 125℃ or less, 120℃ or less, 115℃ or less, 110℃ or less, 105℃ or less, or 100℃ or less.

[0077] The second binder polymer can be an acrylate-based polymer having a weight average molecular weight of 10,000 or more, 50,000 or more, or 100,000 or more, and 1,000,000 or less, 600,000 or less, or 300,000 or less. When the second binder polymer has a weight average molecular weight in this predetermined range, it can achieve adhesion of the electrode and can be mixed with the first binder polymer to form a porous coating layer.

[0078] For example, the second binder polymer can include a repeating unit derived from at least one monomer selected from the group consisting of a methyl acrylate monomer, an ethyl acrylate monomer, a butyl acrylate monomer, a 2-ethylhexyl acrylate monomer, an acrylic acid monomer, and a methyl methacrylate monomer.

[0079] In the case where the separator includes only a conventional fluorine-based copolymer as the binder polymer, an adhesive layer has been formed on the surface of the porous coating through so-called wet phase separation, or a separate adhesive layer has been coated to impart adhesion between the separator and the electrodes. In this case, as described above, when a silicon-based negative electrode is used as the negative electrode, there is a problem in that the negative electrode active material undergoes volume expansion to cause deformation of the pores in the separator, or the pore structure in the separator is deformed due to the pressure applied to the separator during lamination of the separator between the two electrodes.

[0080] In contrast, when the first binder polymer and the second binder polymer are simultaneously used according to the embodiments of the present disclosure, a separator having improved adhesion to the electrodes and showing a low porous polymer substrate compression ratio can be provided. It is believed that this is because the first binder polymer imparts elastomeric properties to the porous coating, and the first binder polymer and the second binder polymer are mixed with each other to form a surface adhesive layer.

[0081] Meanwhile, the solvent used according to the present disclosure is an asymmetric linear ketone. In particular, the solvent is an asymmetric linear ketone having a number of carbon atoms of 4 to 10. For example, the solvent can include methyl ethyl ketone, methyl propyl ketone, methyl isobutyl ketone, ethyl propyl ketone, ethyl isobutyl ketone, or two or more thereof. The solvent can dissolve both the first binder polymer and the second binder polymer. To this end, the slurry used to form the porous coating is stabilized, so that the porous coating can be uniformly formed. However, acetone can be further added to the asymmetric linear ketone solvent in order to better dissolve the first binder polymer or the second binder polymer.

[0082] According to the present disclosure, the inorganic particles are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles are not particularly limited as long as they do not cause oxidation and / or reduction in the operating voltage range of an applicable electrochemical device (e.g., 0-5V based on Li / Li + In particular, when inorganic particles having a high dielectric constant are used, they help to increase the degree of dissociation of an electrolyte salt, particularly a lithium salt, in a liquid electrolyte, and thus they can improve the ionic conductivity of the electrolyte.

[0083] For these reasons, the inorganic particles can include inorganic particles having a dielectric constant of 5 or more, inorganic particles capable of transporting lithium ions, and a mixture thereof.

[0084] The inorganic particles having a dielectric constant of 5 or more can be inorganic particles selected from the group consisting of Al2O3, SiO2, ZrO2, AlO(OH), TiO2, BaTiO3, Pb(Zr x Ti 1-x)O3(PZT, where 0 < x < 1), Pb 1-x La x Zr 1-y Ti y O3(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), hafnium oxide (HfO2), SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZO3, and SiC, or a mixture of two or more thereof.

[0085] The inorganic particles capable of transporting lithium ions can be at least one selected from the group consisting of lithium phosphate (Li3PO4), lithium titanium phosphate (Li x Ti y (PO4)3, 0 < x < 2, 0 < y < 3), lithium titanium aluminum phosphate (Li x Al y Ti z (PO4)3, 0 < x < 2, 0 < y < 1, 0 < z < 3), (LiAlTiP) x O y -based glass (1 < 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), or a mixture of two or more thereof.

[0086] Although the average particle diameter of the inorganic particles is not particularly limited, the inorganic particles can preferably have an average particle diameter of 0.001 to 10 μm, more preferably 100 nm to 2 μm, and most preferably 150 nm to 1 μm, so as to form a porous coating having a uniform thickness and to provide a suitable porosity.

[0087] According to embodiments of the present disclosure, the weight ratio of the inorganic particles to the total binder polymer can be 95:5 to 5:95. When the weight ratio of the inorganic particles to the total content of the binder polymer satisfies the above-defined range, a problem of a decrease in the pore size and porosity of the resulting coating layer due to an increase in the content of the binder can be prevented. A problem of a decrease in the peeling resistance of the resulting coating layer due to a decrease in the content of the binder polymer can also be solved.

[0088] The separator according to embodiments of the present disclosure can further include a dispersant and other additives as components of the porous coating layer in addition to the above-described inorganic particles and the binder polymer.

[0089] Herein, the slurry for forming the porous coating layer can be mixed by using a method generally known in the art. According to embodiments of the present disclosure, the slurry for forming the porous coating layer can be prepared by dispersing the inorganic particles in a polymer dispersion including the first binder polymer and the second binder polymer dispersed in a solvent.

[0090] Then, the prepared slurry for forming the porous coating layer can be applied to the porous polymer substrate, followed by drying, to obtain a separator provided with the porous coating layer.

[0091] In particular, in the separator according to embodiments of the present disclosure, the porous polymer substrate can be a porous polymer film substrate or a porous polymer nonwoven web substrate.

[0092] The porous polymer film substrate can be a porous polymer film including a polyolefin such as polyethylene or polypropylene. Such a polyolefin porous polymer film substrate can achieve a shut-down function at a temperature of 80°C to 150°C.

[0093] Herein, the polyolefin-based porous polymer film can be formed of a polymer including a polyolefin polymer such as polyethylene including high-density polyethylene, linear low-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene, polypropylene, polybutylene, or polypentene, alone or in a combination of two or more thereof.

[0094] In addition thereto, the porous polymer film substrate can be obtained by molding various polymers other than polyolefins such as polyesters into a film shape. Further, the porous polymer film substrate can have a stacked structure of two or more film layers, each of which can be formed of a polymer including the above-mentioned polymers such as polyolefins or polyesters, alone or in a combination of two or more thereof.

[0095] In addition to the above-mentioned polyolefins, the porous polymer film substrate and the porous polymer nonwoven web substrate can be formed of polyethyleneterephthalate, polybutyleneterephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenyleneoxide, polyphenylenesulfide, or polyethylenenaphthalene, alone or in combination, in addition to the above-mentioned polyolefins.

[0096] The thickness of the porous polymer substrate is not particularly limited, and the porous polymer substrate can have a thickness of 1 μm to 100 μm, particularly 5 μm to 50 μm. Although the size of the pores and the porosity present in the porous polymer substrate are not particularly limited, the pore size and the porosity can be 0.01 μm to 50 μm and 20% to 75%, respectively.

[0097] The thickness of the porous coating is not particularly limited. Particularly, the porous coating can have a thickness of 1 μm to 15 μm, more particularly 1.5 μm to 10 μm, based on single-sided coating. In addition to this, the porosity of the porous coating is not particularly limited, but the porous coating can preferably have a porosity of 35% to 85%.

[0098] Although the process for coating the slurry used to form the porous coating onto the porous polymer substrate is not particularly limited, it is preferable to use a slot coating or dip coating process. The slot coating process includes coating the slurry supplied via a slot die onto the entire surface of the substrate, and is capable of controlling the thickness of the coating depending on the flow rate supplied from the metering pump. In addition to this, the dip coating process includes dipping the substrate into a tank containing the slurry to perform the coating, and is capable of controlling the thickness of the coating depending on the concentration of the slurry and the rate at which the substrate is removed from the tank. Further, in order to more precisely control the coating thickness, post-metering by a Mayer bar or the like can be performed after dipping.

[0099] Then, the porous polymer substrate coated with the slurry used to form the porous coating can be dried in a dryer such as an oven to form the porous coating on at least one surface of the porous polymer substrate.

[0100] In the porous coating, the inorganic particles are bound to each other by the binder polymer, while they are packed and in contact with each other, whereby interstitial volumes are formed between the inorganic particles, and the interstitial volumes between the inorganic particles become empty spaces to form pores.

[0101] That is, the binder polymer attaches the inorganic particles to each other so that they can maintain their bound state. For example, the binder polymer connects the inorganic particles to each other and fixes the inorganic particles. In addition thereto, the pores of the porous coating are those formed of the interstitial volumes between the inorganic particles that become empty spaces. These spaces can be defined by the inorganic particles substantially facing each other in a close-packed or densely packed structure of the inorganic particles.

[0102] The drying can be performed in a drying chamber, wherein conditions of the drying chamber are not particularly limited by the application of the non-solvent.

[0103] According to an embodiment of the present disclosure, the separator is dried under a humidified condition. For example, the drying can be performed at a relative humidity of 30% or more, 35% or more, or 40% or more, and 80% or less, 75% or less, or 70% or less.

[0104] Meanwhile, the separator according to the embodiment of the present disclosure can have an adhesion to a counter electrode (lami strength) of 60 gf / 25 mm to 300 gf / 25 mm, and can show a compression ratio of the porous polymer substrate of 0% to 7% after lamination.

[0105] In another aspect of the present disclosure, there is provided an electrochemical device including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is the above-described separator according to the embodiment of the present disclosure.

[0106] The electrochemical device includes any device that performs an electrochemical reaction, and specific examples thereof include primary batteries, secondary batteries, fuel cells, solar cells, or capacitors such as supercapacitor devices of all types. In particular, in the secondary battery, lithium secondary batteries including lithium metal secondary batteries, lithium ion secondary batteries, lithium polymer secondary batteries, or lithium ion polymer batteries are preferred.

[0107] The two electrodes, positive and negative electrodes, used in combination with the separator according to the present disclosure are not particularly limited, and can be obtained by binding an electrode active material to an electrode current collector by a method generally known in the art. Among the electrode active materials, non-limiting examples of positive electrode active materials include conventional positive electrode active materials that can be used for positive electrodes of conventional electrochemical devices. In particular, it is preferable to use lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a lithium composite oxide containing a combination thereof. Non-limiting examples of negative electrode active materials include conventional negative electrode active materials that can be used for negative electrodes of conventional electrochemical devices. In particular, it is preferable to use a lithium-intercalating material such as lithium metal or lithium alloy, carbon, petroleum coke, activated carbon, graphite, or other carbonaceous materials. Non-limiting examples of positive electrode current collectors include foils made of aluminum, nickel, or a combination thereof. Non-limiting examples of negative electrode current collectors include foils made of copper, gold, nickel, copper alloys, or a combination thereof.

[0108] The electrolyte that can be used in the electrochemical device according to the present disclosure is a salt having a structure of A + B - wherein A + includes an alkali metal cation such as Li + , Na + , K + , or a combination thereof, B - includes an anion such as PF6 - , BF4 - , CI - , Br - , I - , CIO4 - , AsF6 - , CH3CO2 - , CF3SO3 - , N(CF3SO2)2 - , C(CF2SO2)3 - , or a combination thereof, which dissolves or dissociates in an organic solvent including 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 combination thereof. However, the present disclosure is not limited thereto.

[0109] Depending on the manufacturing process of the final product and the properties required of the final product, the injection of electrolyte can be implemented in a suitable step during the process for manufacturing the battery. That is, the injection of electrolyte can be implemented before assembling the battery or in the final step of assembling the battery.

[0110] Embodiments will be described more fully hereinafter with reference to the accompanying drawings, in which some embodiments are shown. Embodiments may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0111] Comparative Example 1

[0112] First, polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) as a binder polymer was introduced into acetone as a solvent and dissolved therein at 50°C for about 4 hours to prepare a binder polymer solution. Next, aluminum hydroxide (Al(OH)3) (particle size: 800 nm) as an inorganic particle was introduced into the binder polymer solution. Then, isopropyl trioleyl titanate as a dispersant was introduced therein. Here, the weight ratio of inorganic particle: dispersant: binder polymer was controlled to be 73:2:25 to prepare a slurry for forming a porous coating.

[0113] The slurry for forming a porous coating was applied to both surfaces of a polyethylene porous film (porosity 45%) having a thickness of 9 μm by a dip coating process at 23°C under a relative humidity of 40%, so that the total loading amount can be 9 g / m 2 , followed by drying to obtain a separator having a porous coating. The test results are shown in Table 1 below.

[0114] Comparative Example 2

[0115] A separator was obtained in the same manner as in Comparative Example 1, except that only thermoplastic polyurethane (AM 160, SunYang Global Co.) was used as a binder polymer, instead of polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP). The test results are shown in Table 1 below.

[0116] Comparative Example 3

[0117] A separator was obtained in the same manner as Comparative Example 1, except that a thermoplastic polyurethane (AM160, Sun Yang Global Co.) was used as the binder polymer instead of polyvinylidene-co-hexafluoropropylene (PVDF-HFP), and the thermoplastic polyurethane was introduced into methyl ethyl ketone as the solvent instead of acetone. The test results are shown in Table 1 below.

[0118] Comparative Example 4

[0119] A separator was obtained in the same manner as Comparative Example 1, except that a thermoplastic polyurethane (AM100, Sun Yang Global Co.) was used as the binder polymer instead of polyvinylidene-co-hexafluoropropylene (PVDF-HFP), and the thermoplastic polyurethane was introduced into methyl ethyl ketone as the solvent instead of acetone. The test results are shown in Table 1 below.

[0120] Comparative Example 5

[0121] A separator was obtained in the same manner as Comparative Example 1, except that instead of using only polyvinylidene-co-hexafluoropropylene (PVDF-HFP) as the binder polymer, a thermoplastic polyurethane (AM160, Sun Yang Global Co.) and polyvinylidene-co-hexafluoropropylene (PVDF-HFP) were used in a weight ratio of 70:30 as shown in Table 1 below. The test results are shown in Table 1 below.

[0122] Comparative Example 6

[0123] A separator was obtained in the same manner as Comparative Example 1, except that instead of using only polyvinylidene-co-hexafluoropropylene (PVDF-HFP) as the binder polymer, a thermoplastic polyurethane (AM160, Sun Yang Global Co.) and polyvinylidene-co-hexafluoropropylene (PVDF-HFP) were used in a weight ratio of 70:30 as shown in Table 1 below and were introduced into methyl ethyl ketone as the solvent. The test results are shown in Table 1 below.

[0124] Comparative Example 7

[0125] A separator was obtained in the same manner as Comparative Example 1, except that instead of using only polyvinylidene-co-hexafluoropropylene (PVDF-HFP) as the binder polymer, a thermoplastic polyurethane (AM160, Sun Yang Global Co.) and polymethacrylate (weight average molecular weight 150,000, Tg 80°C) were used in a weight ratio of 70:30 as shown in Table 1 below and were introduced into acetone as the solvent. The test results are shown in Table 1 below.

[0126] Example 1

[0127] First, a thermoplastic polyurethane (AM160, Sun Yang Global Co.) as a first binder polymer and a polymethacrylate (weight average molecular weight 150,000, Tg 80℃) as a second binder polymer were introduced into methyl ethyl ketone as a solvent in a weight ratio of 90:10 and dissolved therein at 50℃ for about 4 hours to prepare a binder polymer solution. Next, aluminum hydroxide (Al(OH)3) as an inorganic particle (particle size: 800 nm) was introduced into the binder polymer solution. Then, isopropyl trioleyl titanate as a dispersant was introduced therein. Here, the weight ratio of the inorganic particle: the dispersant: the binder polymer was controlled to be 73:2:25 to prepare a slurry for forming a porous coating (the binder polymer in the weight ratio refers to the total content of the first binder polymer and the second binder polymer).

[0128] The slurry for forming a porous coating was applied to both surfaces of a polyethylene porous film (porosity 45%) having a thickness of 9 μm by a dip coating process at 23℃ under a relative humidity of 40%, so that the total loading amount can be 9 g / m 2 , followed by drying to obtain a separator having a porous coating. The test results are shown in Table 1 and Table 2 below.

[0129] In this example, the weight average molecular weight of the binder polymer can be measured by using a gel permeation chromatography (GPC: gel permeation chromatography, PL GPC220, Agilent technologies).

[0130] Specifically, the weight average molecular weight can be measured under the following analysis conditions:

[0131] - Column: PL MiniMixed B x 2

[0132] - Solvent: THF

[0133] - Flow rate: 0.3 mL / min

[0134] - Sample concentration: 2.0 mg / mL

[0135] - Injection volume: 10 μL

[0136] - Column temperature: 40℃

[0137] - Detector: Agilent RI detector

[0138] - Standard: Polystyrene (corrected with tertiary function)

[0139] - Data processing: ChemStation

[0140] Example 2

[0141] The separator was obtained in the same manner as in Example 1, except that the weight ratio of the first binder polymer : the second binder polymer was controlled to be 70 : 30. The test results are shown in Table 2 below.

[0142] Example 3

[0143] The separator was obtained in the same manner as in Example 1, except that the weight ratio of the first binder polymer : the second binder polymer was controlled to be 50 : 50. The test results are shown in Table 2 below.

[0144] Example 4

[0145] The separator was obtained in the same manner as in Example 1, except that the weight ratio of the first binder polymer : the second binder polymer was controlled to be 30 : 70. The test results are shown in Table 2 below.

[0146] Comparative Example 8

[0147] The separator was obtained in the same manner as in Example 1, except that the weight ratio of the first binder polymer : the second binder polymer was controlled to be 10 : 90. The test results are shown in Table 2 below.

[0148] Example 5

[0149] The separator was obtained in the same manner as in Example 2, except that a thermoplastic polyurethane (AM100, Sun Yang Global Co.) was used as the first binder polymer, instead of a non-thermoplastic polyurethane (AM160, Sun Yang Global Co.). The test results are shown in Table 2 below.

[0150] [TEST EXAMPLE]

[0151] 1) Manufacture of a negative electrode

[0152] First, natural graphite, carbon black, carboxymethyl cellulose (CMC, Carboxy Methyl Cellulose), and styrene-butadiene rubber (SBR, Styrene-Butadiene Rubber) were mixed with water in a weight ratio of 96 : 1 : 2 : 2 to prepare a negative electrode slurry. Then, the negative electrode slurry was coated on a copper foil (Cu-foil) to a capacity of 3.55 mAh / g to form a thin electrode plate, and the electrode plate was dried at 135℃ for 3 hours or more and subjected to pressing to obtain a negative electrode.

[0153] 2) Manufacture of cathode

[0154] First, LiNi 0.6 Co 0.2 Mn 0.2 O2, carbon black, polyvinylidene fluoride (PVDF) were introduced into N-methyl-2-pyrrolidone (NMP) in a weight ratio of 96:2:2 and mixed therein to prepare a cathode slurry. The cathode slurry was coated on an aluminum foil having a thickness of 20 μm as a cathode current collector to a capacity of 3.28 mAh / g to obtain a cathode.

[0155] 3) Adhesion of separator and electrode

[0156] Then, the resulting separator was laminated with the electrode in a manner that the porous coating layer of the separator can face the negative active material layer of the electrode of 1), and then the resulting structure was pressed at 70°C under 600 kgf for 1 second to obtain an electrode assembly including a negative electrode laminated with a separator.

[0157]

[0158]

[0159] Test method

[0160] 1) Compression ratio of porous polymer substrate

[0161] First, each separator was pressurized at a temperature of 70°C under a pressure of 2000 kgf by using a hot-press. Next, the separator was sonicated for 2 minutes with a solvent introduced during preparation of a slurry for forming a porous coating layer to remove the porous coating layer formed on the porous polymer substrate. Then, the thickness of the porous polymer substrate without the porous coating layer was measured.

[0162] After that, the compression ratio was calculated according to the following equation:

[0163] Compression ratio = (initial thickness of porous polymer substrate - thickness of porous polymer substrate without porous coating layer) / initial thickness of porous polymer substrate) x 100%

[0164] 2) Method of measuring adhesion between electrode and separator (lami strength)

[0165] The negative electrode was obtained as described in Test Example 1) and cut into a size of 25 mm x 100 mm. Each separator according to the examples and comparative examples was cut into a size of 25 mm x 100 mm. The prepared separator was stacked with the negative electrode, and the resulting structure was inserted into a PET film having a thickness of 100 μm and adhered by using a flat press. Here, heating and pressurization were carried out at 70°C under a pressure of 600 kgf for 1 second by using a flat press. The adhered separator and negative electrode were attached to a glass slide by using double-sided tape. The end portion of the separator (10 mm or less from the end of the adhered surface) was peeled and attached to a PET film (25 mm x 100 mm) by using single-sided tape in the longitudinal direction. Then, the glass slide was mounted to the lower holder of a UTM instrument (LLOYD Instrument LFPlus), and the PET film to which the separator was attached was mounted to the upper holder of the UTM instrument. Thereafter, a force was applied thereto at an angle of 180° at a rate of 300 mm / min, and the force required to separate the negative electrode and the porous coating layer facing the negative electrode from each other was measured.

[0166] 3) Method of measuring thickness

[0167] The thickness of each separator was measured by using a thickness gauge (Mitutoyo Co., VL-50S-B).

[0168] 4) Method of measuring air permeability

[0169] The air permeability of each separator was measured by using a Gurley-type air permeability tester in accordance with JIS P-8117. Here, the air permeability was measured as the time required for 100 mL of air to pass through a 28.6 mm diameter and 645 mm 2 time required.

[0170] As can be seen from Tables 1 and 2, there was a problem in that the compression ratio was poor when only a conventional fluorine-based binder polymer was used. In addition to this, when only a thermoplastic polyurethane was used according to Comparative Examples 2 to 4, it can be difficult to form a pore and a surface adhesive layer under a humidification phase separation process. When a thermoplastic polyurethane was combined with a fluorine-based binder polymer according to Comparative Examples 5 and 6, the two polymers were not miscible, causing separation of the intermediate layer, and it was difficult to form an adhesive layer on the surface of the porous coating layer, thereby making it difficult to improve the adhesion to the electrode. In particular, when methyl ethyl ketone was used as a solvent according to Comparative Example 6, separation of the intermediate layer occurred, thereby making it difficult to form the porous coating layer itself.

[0171] In contrast, according to the embodiments of the present disclosure, as can be seen from Examples 1 to 5, the thermoplastic polyurethane and the acrylate-based adhesive polymer show high miscibility. In addition to this, when methyl ethyl ketone is used as the solvent, improved effects can be obtained in terms of compression, adhesion to the electrode, and air permeability.

Claims

1. A method for manufacturing a separator for a lithium secondary battery, comprising the following steps: A slurry for forming a porous coating is prepared, the slurry comprising inorganic particles dispersed in an asymmetric linear ketone solvent and a binder polymer dissolved therein, the binder polymer comprising a first binder polymer and a second binder polymer; and The slurry used to form the porous coating is applied to a porous polymer substrate having multiple pores, and then dried. The first adhesive polymer is a thermoplastic polyurethane comprising soft segments having repeating units derived from polyols and hard segments having urethane-bonded structures. The second adhesive polymer is an acrylate-based polymer having a glass transition temperature (Tg) of 25°C to 125°C, and The first adhesive polymer is introduced in an amount greater than 10 parts by weight based on the total content of the adhesive polymer of 100 parts by weight.

2. The method of manufacturing a separator for a lithium secondary battery according to claim 1, wherein the first adhesive polymer has a melting point of 30°C to 150°C.

3. The method of manufacturing a separator for a lithium secondary battery according to claim 1, wherein the first adhesive polymer comprises the hard segment and the soft segment in a molar ratio of 10:90 to 90:

10.

4. The method of manufacturing a separator for a lithium secondary battery according to claim 1, wherein the first adhesive polymer has a weight-average molecular weight of 10,000 to 1,000,000.

5. The method of manufacturing a separator for a lithium secondary battery according to claim 1, wherein the second adhesive polymer has a weight-average molecular weight of 10,000 to 1,000,000.

6. The method of manufacturing a separator for a lithium secondary battery according to claim 1, wherein the second adhesive polymer comprises repeating units derived from at least one monomer selected from methyl acrylate monomer, ethyl acrylate monomer, butyl acrylate monomer, 2-ethylhexyl acrylate monomer, acrylic acid monomer, and methyl methacrylate monomer.

7. The method of manufacturing a separator for a lithium secondary battery according to claim 1, wherein the asymmetric linear ketone solvent is an asymmetric linear ketone having 4 to 10 carbon atoms.

8. The method of manufacturing a separator for a lithium secondary battery according to claim 1, wherein the asymmetric linear ketone solvent comprises methyl ethyl ketone, methyl propyl ketone, methyl isobutyl ketone, ethyl propyl ketone, ethyl isobutyl ketone, or two or more thereof.

9. The method for manufacturing a separator for a lithium secondary battery according to claim 1, wherein the drying step is carried out at a relative humidity of 30% to 80%.

10. The method of manufacturing a separator for a lithium secondary battery according to claim 1, wherein the weight ratio of the inorganic particles to the total binder polymer is 95:5 to 5:

95.

11. The method of manufacturing a separator for a lithium secondary battery according to claim 1, wherein the asymmetric linear ketone solvent is methyl ethyl ketone, the first adhesive polymer is a thermoplastic polyurethane with a molar ratio of the rigid segment to the soft segment of 10:90 to 90:10, and the second adhesive polymer is polymethyl methacrylate.

12. The method of manufacturing a separator for a lithium secondary battery according to claim 1, wherein the separator has an adhesion to the counter electrode of 60 gf / 25 mm to 300 gf / 25 mm and exhibits a compression ratio of 0% to 7% of the porous polymer substrate after lamination.

13. A separator for a lithium secondary battery obtained by the method as defined in any one of claims 1 to 12, comprising: Porous polymer substrate; and A porous coating is formed on at least one surface of the porous polymer substrate and includes inorganic particles, a first binder polymer, and a second binder polymer.

14. An electrochemical device comprising a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the separator is the same as that defined in claim 13.

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