Binder for coating separator for secondary battery and secondary battery including the same

Abstract

To provide a binder for coating a separator for a secondary battery, which improves the thermal stability of the separator for the secondary battery.SOLUTION: The present invention relates to a binder for coating a separator for a secondary battery, and includes a mixture of a copolymer A derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylic acid monomer and including a carboxylic acid group, and a copolymer B derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylate-based monomer and including an alcohol group, and to a separator for a secondary battery and a secondary battery including the same.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a binder for coating a separator for a secondary battery, which improves the thermal stability of the separator for a secondary battery, and a secondary battery including the same.

Background Art

[0002] A separator for an electrochemical battery is a porous intermediate film that continuously maintains ionic conductivity while separating the positive electrode and the negative electrode in the battery, enabling charging and discharging of the battery. However, generally used polyolefin-based separators have melting characteristics at low temperatures. Therefore, when the battery is exposed to a high-temperature environment due to abnormal behavior or receives a physical impact from the outside, the separator may be mechanically shrunk or damaged, so that the positive electrode and the negative electrode in the battery may come into contact with each other and the battery may catch fire. In order to solve such problems, technologies have been tried in which the separator is coated with an inorganic substance to suppress the shrinkage of the separator and ensure the stability of the battery.

[0003] For example, Korean Patent Publication No. 10-2016-0061202 discloses a separator for a lithium secondary battery provided with a coating layer containing an acrylic copolymer formed by polymerization of (meth)acrylate and (meth)acrylonitrile, and a polyvinyl alcohol-based compound on one surface of a substrate. However, the disclosed separator has room for improvement in terms of heat resistance.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The present invention has been made in view of the above prior art, and an object of the present invention is to provide a binder for coating a secondary battery separator that improves the thermal stability of the secondary battery separator.

[0006] Another object of the present invention is to provide a secondary battery separator including the binder for coating the secondary battery separator.

[0007] Another object of the present invention is to provide a secondary battery including the secondary battery separator.

Means for Solving the Problems

[0008] To achieve the above object, one embodiment of the present invention provides a binder for coating a secondary battery separator, which includes a mixture of a copolymer A containing a carboxylic acid group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylic acid monomer, and a copolymer B containing an alcohol group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylate-based monomer.

[0009] Another embodiment of the present invention provides a secondary battery separator including a porous substrate and a coating layer located on at least one surface of the porous substrate. The coating layer has a heat-treated mixture of a copolymer A containing a carboxylic acid group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylic acid monomer, and a copolymer B containing an alcohol group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylate-based monomer.

[0010] Another embodiment of the present invention provides a secondary battery including the secondary battery separator.

Effects of the Invention

[0011] The binder according to the present invention can greatly improve the heat resistance characteristics of the separator and improve the stability of the secondary battery when applied to the separator by heat-treating a copolymer containing a carboxylic acid group and a copolymer mixture containing an alcohol group to form physical and chemical bonds between the functional groups.

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present invention will be described in detail. However, this is presented as an example and the present invention is not limited thereby, and the present invention is only defined by the scope of the claims described below.

[0013] The inventors of the present invention, when using a mixed water-soluble polymer mainly composed of a (meth)acrylamide-based monomer and vinylpyrrolidone and mixing a copolymer A containing a carboxylic acid group and a copolymer B containing an alcohol group as a binder, confirmed that the heat resistance of the separator is greatly improved by physical and chemical bonds between the functional groups compared to the case of using the copolymer A and the copolymer B alone without mixing. The present invention is based on this.

[0014] <Manufacture of Binder> One embodiment of the present invention provides a binder for coating a secondary battery separator containing a mixture of a copolymer A containing a carboxylic acid group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylic acid monomer, and a copolymer B containing an alcohol group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylate-based monomer.

[0015] In the binder of the present invention, the copolymer A and the copolymer B may be water-soluble polymer resins.

[0016] The (meth)acrylamide-based monomer may be at least one selected from the group consisting of acrylamide, methacrylamide, N-substituted methacrylamide, and methacrylamide containing an aliphatic and / or aromatic ring.

[0017] The vinylpyrrolidone (N-Vinylpyrrolidone) can be used as a monomer of polyvinylpyrrolidone and is derived from 2-pyrrolidone.

[0018] The (meth)acrylic acid monomer may be acrylic acid or methacrylic acid.

[0019] The (meth)acrylate-based monomer may be at least one selected from the group consisting of 2-hydroxyethyl acrylate and 2-hydroxyethyl methacrylate.

[0020] The copolymer A may be a polymer of monomers containing 60 to 90% by weight of (meth)acrylamide-based monomer, 5 to 40% by weight of vinylpyrrolidone, and 1 to 10% by weight of (meth)acrylic acid based on the total weight of the monomers.

[0021] The copolymer B may be a polymer of monomers containing 60 to 90% by weight of (meth)acrylamide-based monomer, 5 to 40% by weight of vinylpyrrolidone, and 1 to 10% by weight of (meth)acrylate-based monomer based on the total weight of the monomers.

[0022] In one embodiment of the present invention, the copolymer A and the copolymer B can be synthesized by an aqueous solution polymerization method.

[0023] In the present invention, by producing a polymer resin crosslinked with a small amount of a water-soluble chemical crosslinking agent, the stability of the polymer in the electrolytic solution was improved.

[0024] The chemical structure of copolymer A may be the same as Chemical Formula 1 below.

[0025] [Chemical Formula 1] JPEG2025088673000001.jpg35166(R 1 is H or CH 3 and R 2 is H or CH 3 and a, b, c are 0 to 1, and a + b + c = 1.)

[0026] As shown in the above scientific formula, copolymer A may be a copolymer of a (meth)acrylamide-based monomer containing a carboxylic acid functional group and vinyl pyrrolidone.

[0027] The chemical structure of copolymer B may be the same as Chemical Formula 2 below.

[0028] [Chemical Formula 2] JPEG2025088673000002.jpg41166(R 3 is H or CH 3 and R 4 is H or CH 3 and d, e, f are 0 to 1, and d + e + f = 1)

[0029] As shown in the above scientific formula, copolymer B may be a copolymer of a (meth)acrylamide-based monomer containing an alcohol functional group and vinyl pyrrolidone.

[0030] In the present invention, two kinds of water-soluble polymer resins having a carboxylic acid group or an alcohol functional group, namely copolymer A and copolymer B, were mixed and used as a binder for separator coating having better heat resistance characteristics.

[0031] In the present invention, a simple mixture of copolymer A and copolymer B is used as a binder.

[0032] [Chemical Formula 3] JPEG2025088673000003.jpg38167(R 1is H or CH 3 wherein, R 2 is H or CH 3 wherein, a, b, c are 0 to 1, a + b + c = 1, and R 3 is H or CH 3 wherein, R 4 is H or CH 3 wherein, d, e, f are 0 to 1, d + e + f = 1)

[0033] On the other hand, when polymerizing a copolymer of a (meth)acrylamide monomer containing both a carboxylic acid functional group and an alcohol functional group and vinylpyrrolidone in one polymer chain instead of a mixture of copolymer A and copolymer B, physical and chemical bonds may occur between the carboxylic acid functional group and the alcohol functional group during the polymerization process, resulting in gelation.

[0034] Therefore, in the present invention, as shown in Chemical Formula 3, a simple mixture in which copolymer A and copolymer B are mixed is used as a binder.

[0035] The mixture may be one in which copolymer A and copolymer B are mixed at a weight ratio of 2:8 to 8:2.

[0036] Preferably, copolymer A and copolymer B may be mixed at a weight ratio of 3:7 to 7:3.

[0037] After coating the mixture of copolymer A and copolymer B on a separator and then performing heat treatment, the water-soluble polymer resins of copolymer A and copolymer B are used alone, or the heat resistance is improved compared to a separator that has not been heat-treated.

[0038] This is due to the formation of hydrogen bonds (physical bonds) as a result of the interaction between the carboxylic acid groups (-COOH) and alcohol groups (-OH) contained in the polymer, and the formation of ester (-COO-) bonds (chemical bonds) by the condensation reaction between the alcohol groups (-OH) and carboxylic acid groups (-COOH) that occurs while water is removed.

[0039] In the present invention, the mixture of copolymer A and copolymer B is heat-treated at 50 to 100°C for 30 minutes to 24 hours so that physical and chemical bonds between functional groups are formed.

[0040] In one embodiment of the present invention, the mixture of copolymer A and copolymer B is heat-treated after being coated on a separator.

[0041] The copolymer used as a binder in the present invention may contain, in addition to the above monomers, monomers copolymerizable with other substances. Examples of other copolymerizable monomers include styrene-based monomers, olefins, diene-based monomers, halogen atom-containing monomers, vinyl acetate, vinyl esters, vinyl ethers, vinyl ketones, heterocyclic ring-containing vinyl compounds, and amide-based monomers.

[0042] The copolymer can be in various forms such as an alternating polymer in which the above units are alternately distributed, a random polymer in which they are arbitrarily distributed, or a graft polymer in which some structural units are grafted.

[0043] Examples of polymerization initiators used for polymerization include organic peroxides such as lauroyl peroxide, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, t-butyl peroxypivalate, 3,3,5-trimethylhexanoyl peroxide, azo compounds such as α,α'-azobisisobutyronitrile, or ammonium persulfate, potassium persulfate, and the like.

[0044] The polymerization reaction temperature can be 50 to 90°C.

[0045] Metal hydroxides can be used for adjusting the pH of the polymer. Preferably, NaOH or LiOH can be used as the metal hydroxide.

[0046] Other embodiments of the present invention provide a separator coating slurry containing the binder, inorganic particles, and a solvent.

[0047] When the inorganic particles are included in the coating layer, they can suppress a short circuit between the positive electrode and the negative electrode by further preventing shrinkage of the separator due to heat, and can minimize the resistance of lithium ions to improve the performance of the battery.

[0048] The inorganic particles may include SiO 2 , alumina (Al 2 O 3 ), Al(OH) 3 , AlO(OH), TiO 2 , BaTiO 3 , Mg(OH) 2 , MgO, Ti(OH) 4 , clay, glass powder, or a combination thereof.

[0049] The inorganic particles are preferably mixed with the mixture of the copolymer A and the copolymer B at a ratio of 15 to 35:65 to 85 (solid content ratio). However, the composition of the solid content ratio is not limited, and the thickness, pore size, and degree of porosity of the separator coated by the composition are determined.

[0050] Both water and an organic solvent can be used as the solvent, but it is better that the copolymer dispersion and the solubility parameter are similar. The amount of the solvent is adjusted so that the concentration of the solid content is preferably 5 to 90% by weight, more preferably 10 to 50% by weight.

[0051] The mixing device for the slurry is not particularly limited as long as it can uniformly mix the components, and a ball mill, a sand mill, a pigment disperser, a meat mill, an ultrasonic disperser, a homogenizer, a planetary mixer, etc. can be used. However, it is particularly preferable to use a high-dispersion device such as a bead mill, a roll mill, or a film mixer that can apply a high dispersion share.

[0052] <Manufacture of Inorganic Coated Separator> Another embodiment of the present invention provides a separator for a secondary battery, comprising a porous substrate and a coating layer provided on at least one surface of the porous substrate. The coating layer has a copolymer A containing a carboxylic acid group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylic acid monomer, and a copolymer B containing an alcohol group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylate-based monomer.

[0053] A separator for a secondary battery separates a negative electrode and a positive electrode and provides a migration path for lithium ions, and may include a porous substrate and a coating layer provided on at least one surface of the porous substrate.

[0054] The porous substrate has a large number of pores and can be one normally used in an electrochemical element. The porous substrate is, without limitation, any one polymer selected from the group consisting of polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyether ether ketone, polyaryl ether ketone, polyether imide, polyamide imide, polybenzimidazole, polyether sulfone, polyphenylene oxide, cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon (registered trademark), and polytetrafluoroethylene, or a polymer film formed of a copolymer or mixture of two or more of these.

[0055] The porous substrate can have a thickness of about 1 μm to 40 μm, for example, a thickness of 1 μm to 30 μm, 1 μm to 20 μm, 5 μm to 15 μm, or 10 μm to 15 μm.

[0056] The coating layer may include a heat-treated mixture of a first copolymer containing a carboxylic acid group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylic acid monomer, and a second copolymer containing an alcohol group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylate-based monomer. When the mixture of the copolymers is used as a binder during the formation of the coating layer on the substrate, the heat resistance characteristics of the separator can be improved by enhancing the heat resistance of the porous substrate.

[0057] The coating layer can be provided on one side of the substrate by applying the separator coating slurry onto the porous substrate.

[0058] The separator coating slurry is as described above.

[0059] The method for applying the separator coating slurry onto the porous substrate is not particularly limited. For example, dip coating, die coating, gravure coating, comma coating, etc. can be used.

[0060] Examples of the drying method include drying by warm air, hot air, low humidity air, vacuum drying, and drying methods by irradiation with (far) infrared rays or electron beams. The drying temperature varies depending on the type of solvent used. Water or an organic solvent can be used as the solvent, but water or an organic solvent having a solubility parameter similar to that of the copolymer dispersion is preferred.

[0061] The drying temperature range is preferably 40 to 80°C.

[0062] The thickness of the coating layer is preferably 1 to 6 μm on one side or both sides. When it is less than 1 μm, there is a problem that the heat resistance characteristics of the separator are significantly reduced. When it exceeds 6 μm, since the thickness of the separator is too thick, it may cause a decrease in the energy density of the battery and an increase in resistance.

[0063] Another embodiment of the present invention provides a secondary battery including the separator for a secondary battery.

[0064] The secondary battery includes a positive electrode, a negative electrode, an electrolyte, and the separator for a secondary battery.

[0065] The secondary battery may be a lithium-ion secondary battery.

[0066] Hereinafter, the present invention will be described with reference to examples. However, the present invention is not limited thereto. In addition, parts and % in this example are based on weight unless otherwise clearly specified.

[0067] <Production Example> Production of Binder Copolymer Polymer While injecting high-purity nitrogen gas into the reaction vessel, 620 parts by weight of distilled water is heated to 70°C. 0.1 to 0.5 parts by weight of ammonium persulfate, which is a decomposition-type initiator, and 0.01 to 0.3 parts by weight of methylene-bis-acrylamide (MBA) are added to the reaction vessel maintained at 70°C, and 100 parts by weight of a monomer mixture is added respectively to proceed with a continuous solution polymerization reaction to produce copolymer A polymer and copolymer B polymer.

[0068] Production of Mixed Water-Soluble Polymer Resin Copolymer A polymer and copolymer B polymer each having a carboxylic acid functional group and an alcohol functional group are mixed at ratios of 3:7, 5:5, and 7:3 to produce a mixed water-soluble polymer resin (polymer C) with improved heat resistance.

[0069] Production of Slurry for Separator Coating Alumina (average particle size 0.5 μm) as inorganic particles and a polymer binder (A or mixed polymer C) were mixed so that the solid content ratio was 96:4, and additional distilled water was mixed so that the solid content concentration became 35%. This mixture was sufficiently dispersed through a ball mill or a mechanical stirrer (stirrer) to produce a slurry.

[0070] Production of Inorganic Coating Separator The slurry produced on one side of the polyethylene porous substrate was coated by the die coating method to form a coating layer with a thickness of 2 μm, and then dried with warm air at a temperature of 60 to 80 °C to produce an inorganic-coated separator.

[0071] <Example 1> Basically the same as the production example, 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylic acid monomer were polymerized in an aqueous system (copolymer A). After coating the copolymer A on the separator, the coated separator was heat-treated at a temperature of 100 °C for about 12 hours or more to synthesize a binder for a secondary battery, and a separator using this was produced.

[0072] <Example 2> Basically the same as the production example, 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylate-based monomer were polymerized in an aqueous system (copolymer B). After coating the copolymer B on the separator, the coated separator was heat-treated at a temperature of 100 °C for about 12 hours or more to synthesize a binder for a secondary battery, and a separator using this was produced.

[0073] <Example 3> Basically the same as the production example, 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylic acid monomer (copolymer A), and 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylate-based copolymer (copolymer B) were each subjected to aqueous polymerization. After producing a mixture C3-7 in which the polymerized copolymer A and copolymer B were mixed at a weight ratio of 3:7, it was coated on a separator. Then, the coated separator was heat-treated at a temperature of 100°C for about 12 hours or more to synthesize a binder for a secondary battery, and a separator with improved heat resistance using this was produced.

[0074] <Example 4> Basically the same as the production example, 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylic acid monomer (copolymer A), and 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylate-based copolymer (copolymer B) were each subjected to aqueous polymerization. After producing a mixture C5-5 in which the polymerized copolymer A and copolymer B were mixed at a weight ratio of 5:5, it was coated on a separator. Then, the coated separator was heat-treated at a temperature of 100°C for about 12 hours or more to synthesize a binder for a secondary battery, and a separator with improved heat resistance using this was produced.

[0075] <Example 5> Basically the same as the production example, 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylic acid monomer (copolymer A), and 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylate copolymer (copolymer B) were each subjected to aqueous polymerization. After producing a mixture C7-3 by mixing the polymerized copolymer A and copolymer B at a weight ratio of 7:3, it was coated on a separator. Then, the coated separator was heat-treated at a temperature of 100 °C for about 12 hours or more to synthesize a binder for a secondary battery, and a separator with improved heat resistance using this was produced.

[0076] <Comparative Example 1> Basically the same as the production example, after subjecting 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylic acid monomer (copolymer A) to aqueous polymerization, it was coated on a separator. Then, a binder for a secondary battery was synthesized by not performing a heat treatment step on the coated separator, and a separator using this was produced.

[0077] <Comparative Example 2> Basically the same as the production example, after subjecting 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylate monomer (copolymer B) to aqueous polymerization, it was coated on a separator. Then, a binder for a secondary battery was synthesized by not performing a heat treatment step on the coated separator, and a separator using this was produced.

[0078] <Comparative Example 3> Basically the same as the production example, 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylic acid monomer (copolymer A), and 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylate copolymer (copolymer B) were each subjected to aqueous polymerization. After producing a mixture C3-7 by mixing the polymerized copolymer A and copolymer B at a weight ratio of 3:7, it was coated on a separator. Then, by not performing a heat treatment step on the coated separator, a binder for a secondary battery was synthesized, and a separator using this was produced.

[0079] <Comparative Example 4> Basically the same as the production example, 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylic acid monomer (copolymer A), and 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylate copolymer (copolymer B) were each subjected to aqueous polymerization. After producing a mixture C5-5 by mixing the polymerized copolymer A and copolymer B at a weight ratio of 5:5, it was coated on a separator. Then, by not performing a heat treatment step on the coated separator, a binder for a secondary battery was synthesized, and a separator using this was produced.

[0080] <Comparative Example 5> Basically the same as the production example, 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylic acid monomer (copolymer A), and 65 parts by weight of a (meth)acrylamide-based monomer, 25 parts by weight of vinylpyrrolidone, and 10 parts by weight of a (meth)acrylate copolymer (copolymer B) were each subjected to aqueous polymerization. After producing a mixture C7-3 by mixing the polymerized copolymer A and copolymer B at a weight ratio of 7:3, it was coated on a separator. Then, by not performing a heat treatment step on the coated separator, a binder for a secondary battery was synthesized, and a separator using this was produced.

[0081] <Comparative Example 6> Basically the same as the production example, in order to produce a copolymer of a (meth)acrylamide-based monomer containing a carboxylic acid functional group and an alcohol functional group and vinyl pyrrolidone, 60 parts by weight of a (meth)acrylamide-based monomer, 20 parts by weight of vinyl pyrrolidone, 10 parts by weight of a (meth)acrylic acid monomer and 10 parts by weight of a (meth)acrylate were subjected to aqueous polymerization, but gelation occurred during the polymerization process.

[0082] [Table 1] Type of binder for secondary battery and presence or absence of heat treatment JPEG2025088673000004.jpg71160

[0083] Experimental Example 1: Heat shrinkage rate of coated separator (Dry heat resistance) Samples with a vertical and horizontal size of 5×5 cm were prepared using the separators manufactured using the binders of the above Examples and Comparative Examples. Each sample was heat-treated by leaving it in an oven at 100°C for 0 or 12 hours. Then, the same sample was left in an oven at 150°C for 1 hour, and the final shrinkage rate was measured and the results are shown in Table 2 below.

[0084] Experimental Example 2: Heat shrinkage rate of coated separator on electrolyte (Wet heat resistance) Samples with a vertical and horizontal size of 5×5 cm were prepared using the separators manufactured using the binders of the above Examples and Comparative Examples. Each sample was heat-treated by leaving it in an oven at 100°C for 0 or 12 hours. Then, the electrolyte and the sample were put into the pouch and sealed, and then impregnated for about 12 hours. After making a gas ejection port in the prepared pouch, it was left in an oven at 150°C for 1 hour, and then the sample was taken out and the shrinkage rate was measured, and the results are shown in Table 2 below.

[0085] [Table 2] Heat shrinkage rate of polymer-coated separator [%] JPEG2025088673000005.jpg74159

[0086] Examples in Table 2 show the shrinkage rate of the coated separator heat-treated according to the present invention. It can be seen that the mixture C-coated separator having both carboxylic acid functional groups and alcohol functional groups exhibits a lower Dry shrinkage rate compared to the coated separator of copolymer A or copolymer B having only one functional group. In the examples, the coated separator undergoes shrinkage of about 1 to 2% while being heat-treated at 100 °C for about 12 hours or more. When the heat-treated sample is left in an oven at 150 °C for 1 hour, the coated separator of copolymer A or copolymer B having only one functional group exhibits a final Dry heat shrinkage rate of 5% or more, while the coated separator of mixture C having both carboxylic acid functional groups and alcohol functional groups exhibits a final Dry heat shrinkage rate within 5%.

[0087] In particular, as in Example 5, when the carboxylic acid functional group is in excess over the alcohol functional group, a better Dry heat shrinkage rate is shown. This is the result of the carboxylic acid functional group interacting with the amide functional groups present in the polymer resin in addition to the alcohol functional group.

[0088] Comparative examples in Table 2 show the shrinkage rate of the non-heat-treated coated separator. All the coated separators in the comparative examples exhibited a final Dry heat shrinkage rate of 5% or more. This is a numerical value similar to that of Examples 1 and 2 where no interaction between functional groups occurred during the heat treatment process.

[0089] The improvement in heat resistance by heat treatment is the result of the bonding between the carboxylic acid (-COOH) functional groups and the alcohol (-OH) functional groups contained in the particulate polymer and the chain polymer. During the heat treatment process, the formation of new ester (-COO-) functional groups and hydrogen bonds due to the bonding between functional groups at a temperature of 100 °C improves the heat resistance performance of the polymer, and the heat resistance performance of the separator coated with the corresponding polymer mixture is improved due to the formation of these bonds.

[0090] In Table 2 above, for the final wet heat shrinkage rate as well as for the final dry heat shrinkage rate, the case where heat treatment was carried out while coating with Mixture C showed the best final wet shrinkage rate. On the other hand, in the case of the final wet heat shrinkage rate, better numerical values were shown when the alcohol functional group in Example 3 was in excess. Such a difference is due to the difference in the affinity between the functional group on the electrolytic solution and the functional group in the polymer resin.

Claims

1. A copolymer A containing a carboxylic acid group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylic acid monomer, and A mixture of a copolymer B containing an alcohol group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylate-based monomer, A binder for coating a secondary battery separator.

2. The copolymer A is a polymer of monomers containing 60 to 90% by weight of the (meth)acrylamide-based monomer, 5 to 40% by weight of the vinylpyrrolidone, and 1 to 10% by weight of the (meth)acrylic acid, based on the total weight of the monomers. The binder for coating a secondary battery separator according to Claim 1.

3. The copolymer B is a polymer of monomers containing 60 to 90% by weight of the (meth)acrylamide-based monomer, 5 to 40% by weight of the vinylpyrrolidone, and 1 to 10% by weight of the (meth)acrylate-based monomer, based on the total weight of the monomers. The binder for coating a secondary battery separator according to Claim 1.

4. In the mixture, the copolymer A and the copolymer B are mixed at a weight ratio of 2:8 to 8:

2. The binder for coating a secondary battery separator according to Claim 1.

5. The (meth)acrylic acid monomer is acrylic acid or methacrylic acid. The binder for coating a secondary battery separator according to Claim 1.

6. The (meth)acrylate-based monomer is at least one selected from the group consisting of 2-hydroxyethyl acrylate and 2-hydroxyethyl methacrylate. The binder for coating a secondary battery separator according to Claim 1.

7. The heat treatment is performed at 50 to 100°C for 30 minutes to 24 hours. The binder for coating a secondary battery separator according to Claim 1.

8. A separator for a secondary battery, comprising a porous substrate and a coating layer applied to at least one surface of the porous substrate, wherein the coating layer is a heat-treated mixture of a copolymer A containing a carboxylic acid group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylic acid monomer, and a copolymer B containing an alcohol group derived from a (meth)acrylamide-based monomer, vinylpyrrolidone, and a (meth)acrylate-based monomer. Separator for secondary battery.

9. The porous substrate is any one polymer selected from the group consisting of polyolefin, polyester, polyacetal, polyamide, polyimide, polycarbonate, polyetheretherketone, polyaryletherketone, polyetherimide, polyamideimide, polybenzimidazole, polyethersulfone, polyphenyleneoxide, cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon, and polytetrafluoroethylene, or a polymer film formed of two or more copolymers or mixtures thereof. The separator for a secondary battery according to claim 8.

10. The coating layer is formed by applying a slurry for separator coating onto the porous substrate. The separator for a secondary battery according to claim 8.

11. The slurry for separator coating contains a binder, inorganic particles, and a solvent according to any one of claims 1 to 7. The separator for a secondary battery according to claim 10.

12. The inorganic particles are SiO ２ , alumina (Al ２ O ３ ), Al(OH) ３ , AlO(OH), TiO ２ , BaTiO ３ , Mg(OH) ２ , MgO, Ti(OH) ４ , clay, glass powder, or a combination thereof. The separator for a secondary battery according to claim 11.

13. The inorganic particles are mixed with the mixture of the copolymer A and the copolymer B at a ratio (solid content ratio) of 15 to 35:65 to 85. The separator for a secondary battery according to claim 11.

14. The thickness of the coating layer is 1 to 6 μm. The separator for a secondary battery according to claim 8.

15. A secondary battery comprising the separator for a secondary battery according to any one of claims 8 to 14.

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