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

Abstract

The present invention relates to a binder for coating a separator of a secondary battery, which contains a (meth)acrylic copolymer consisting of a first structural unit derived from (meth)acrylonitrile, a second structural unit derived from (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate, and a third structural unit derived from an (acet)amide system, a separator for a secondary battery containing the same, and a secondary battery.

Description

[Technical Field]

[0001] The present invention relates to a binder for coating a separator of a secondary battery and a secondary battery including the same. [Background technology]

[0002] Separators for electrochemical batteries are interlayers that separate the positive and negative electrodes within a battery and maintain ionic conductivity, enabling the battery to be charged and discharged. Furthermore, if a battery is exposed to a high-temperature environment due to abnormal behavior, the separator may mechanically shrink or be damaged due to its melting properties at low temperatures. This can cause the positive and negative electrodes to come into contact with each other, potentially resulting in the battery catching fire. To overcome this issue, researchers are attempting to coat the separator with inorganic materials to suppress separator shrinkage and ensure battery stability.

[0003] For example, Korean Patent No. 10-2016-0061202 discloses a separator for a lithium secondary battery in which a coating layer containing an acrylic copolymer formed by polymerization of (meth)acrylate and (meth)acrylonitrile and a polyvinyl alcohol-based compound is disposed on one side of a substrate. However, this technology required the additional introduction of a polyvinyl alcohol-based compound due to the insufficient adhesive properties of the acrylic copolymer. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] KR10-2016-0061202A Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention has been made to provide a binder for coating a separator of a secondary battery, which can significantly improve the heat resistance of the separator by improving adhesion to a porous substrate and heat resistance. The present invention has also been made to provide a separator for a secondary battery including the binder for coating a separator of a secondary battery. The present invention still further has been made to provide a secondary battery including the separator of a secondary battery. [Means for solving the problem]

[0006] In order to achieve the above object, one embodiment of the present invention comprises a (meth)acrylic copolymer comprising: a first structural unit derived from (meth)acrylonitrile; a second structural unit derived from (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate; and a third structural unit derived from an (acet)amide; wherein the first structural unit derived from (meth)acrylonitrile is present in an amount of: 40 ~ 45 % by weight of the (meth)acrylic copolymer, and the second structural unit derived from the (meth)acrylic acid, the (meth)acrylic acid salt, or the (meth)acrylate is contained in an amount of 30 to 100 parts by weight of the (meth)acrylic copolymer. 40 % by weight of the third structural unit derived from an (acet)amide based copolymer, and the third structural unit is contained in an amount of 15 The present invention provides a binder for coating separators in secondary batteries, which contains up to 30% by weight of the binder.

[0007] Another embodiment of the present invention includes a porous substrate and a coating layer located on at least one surface of the porous substrate, the coating layer including a (meth)acrylic copolymer including a first structural unit derived from (meth)acrylonitrile; a second structural unit derived from (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate; and a third structural unit derived from an (acet)amide, wherein the first structural unit derived from (meth)acrylonitrile is present in an amount of: 40 ~ 45 % by weight of the (meth)acrylic copolymer, and the second structural unit derived from the (meth)acrylic acid, the (meth)acrylic acid salt, or the (meth)acrylate is contained in an amount of 30 to 100 parts by weight of the (meth)acrylic copolymer. 40 % by weight of the third structural unit derived from an (acet)amide based copolymer, and the third structural unit is contained in an amount of 15 The present invention provides a separator for a secondary battery containing up to 30% by weight of a polyaniline compound.

[0008] Yet another embodiment of the present invention provides a secondary battery including the separator for a secondary battery. [Effects of the Invention]

[0009] By applying a binder containing the (meth)acrylic copolymer according to the present invention to the separator coating of a secondary battery, the adhesive strength to the porous substrate and heat resistance can be improved, and the heat resistance properties of the separator can be significantly improved. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, specific examples of the present invention will be described in detail, but these are merely examples of the present invention, and the present invention is not limited to these examples, but is defined by the appended claims.

[0011] The present inventors have produced a (meth)acrylic copolymer containing a first structural unit derived from (meth)acrylonitrile, a second structural unit derived from (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate, and a third structural unit derived from an (acet)amide system, and have found that the adhesive strength and heat resistance of the separator are greatly improved as a result of coating the separator with the (meth)acrylic copolymer containing a porous membrane coating slurry, thereby completing the present invention.

[0012] <Binder manufacturing> One embodiment of the present invention provides a binder for coating a separator of a secondary battery, comprising a (meth)acrylic copolymer consisting of a first structural unit derived from (meth)acrylonitrile; a second structural unit derived from (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate; and a third structural unit derived from an (acet)amide system.

[0013] The (meth)acrylonitrile may include acrylonitrile, methacrylonitrile, or a combination thereof. The (meth)acrylic acid may be acrylic acid or methacrylic acid. The (meth)acrylate salt may include an alkali metal, alkaline earth metal, ammonium, or amine salt, or a combination thereof. Examples of the (meth)acrylate salt include, but are not limited to, lithium acrylate, lithium methacrylate, sodium acrylate, sodium methacrylate, magnesium acrylate, magnesium methacrylate, ammonium acrylate, and ammonium methacrylate.

[0014] The structural unit derived from the (meth)acrylate can be derived from a (meth)acrylic acid alkyl ester, a (meth)acrylic acid perfluoroalkyl ester, or a (meth)acrylate having a functional group on the side chain, for example, a (meth)acrylic acid alkyl ester. The (aceto)amide-based compound may be an acetamide or amide-based compound.

[0015] In the present invention, a copolymer contains structural units derived from (aceto)amides to enhance the adhesive strength and heat resistance of the binder. The reason why (aceto)amides improve adhesive strength and heat resistance is that they have a higher glass transition temperature than (meth)acrylonitrile and (meth)acrylic acid, and contain more highly electronegative elements than (meth)acrylonitrile that can exert cohesive strength and adhesive properties such as hydrogen bonds or ionic bonds.

[0016] However, depending on the reactivity of (acet)amide itself and the compatibility between raw materials, the greater the amount of (acet)amide used, the greater the effect on the polymerization stability and reactivity of the binder copolymer. Therefore, there is a problem that (acet)amide cannot be used as the main component of the binder copolymer.

[0017] In the present invention, the third structural unit derived from an (acet)amide is preferably contained in an amount of 1 to 30% by weight relative to 100 parts by weight of the (meth)acrylic copolymer. If the third structural unit is contained in an amount of less than 1% by weight, the effect of improving adhesive strength and heat resistance may be insufficient, and if it is contained in an amount of more than 30% by weight, a polymer with a low molecular weight may be obtained or the polymerization conversion rate may decrease.

[0018] In the present invention, in addition to the third structural unit derived from an (acet)amide system, the binder copolymer contains a first structural unit derived from (meth)acrylonitrile and a second structural unit derived from (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate at a controlled weight ratio.

[0019] Since (meth)acrylonitrile has a higher glass transition temperature and melting point than (meth)acrylic acid, when a structural unit derived from (meth)acrylonitrile is used, the heat resistance of the separator can be significantly improved. Furthermore, the carboxyl group of (meth)acrylic acid improves the affinity with the substrate and the hydrogen bond or ionic bond with inorganic substances, thereby enabling the separator to exhibit higher adhesive performance than (meth)acrylonitrile.

[0020] Thus, the present invention includes a binder copolymer containing a first structural unit derived from (meth)acrylonitrile and a second structural unit derived from (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate in a controlled weight ratio.

[0021] According to one embodiment of the present invention, the first structural unit derived from the (meth)acrylonitrile is preferably contained in an amount of 30 to 70% by weight relative to 100 parts by weight of the (meth)acrylic copolymer, and the second structural unit derived from the (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate is preferably contained in an amount of 30 to 70% by weight relative to 100 parts by weight of the (meth)acrylic copolymer.

[0022] If the content of the first structural unit derived from (meth)acrylonitrile is less than 30% by weight, the adhesive strength is good, but the effect of improving heat resistance due to the use of (meth)acrylonitrile may not be sufficiently obtained.

[0023] The copolymer used as the binder in the present invention may contain, in addition to the above-mentioned monomers, other monomers copolymerizable therewith, such as styrene-based monomers, olefins, diene-based monomers, halogen atom-containing monomers, vinyl acetate, vinyl esters, vinyl ethers, vinyl ketones, heterocycle-containing vinyl compounds, and amide-based monomers.

[0024] The (meth)acrylic copolymer may be in various forms, such as an alternating polymer in which the above-mentioned units are alternately distributed, a random polymer in which the units are randomly distributed, or a graft polymer in which some of the structural units are grafted.

[0025] The weight-average molecular weight of the (meth)acrylic copolymer may be 150,000 to 950,000, and preferably 200,000 to 600,000. When the weight-average molecular weight of the (meth)acrylic copolymer satisfies the above range, the (meth)acrylic copolymer and a separator containing the same can exhibit excellent adhesive strength, heat resistance, and air permeability. The weight-average molecular weight may be an average molecular weight in terms of PEO measured using gel filtration chromatography.

[0026] The (meth)acrylic copolymer can be produced by various known methods such as emulsion polymerization, suspension polymerization, bulk polymerization, solution polymerization, or bulk polymerization.

[0027] Examples of the polymerization initiator used in the polymerization include organic peroxides such as lauroyl peroxide, diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate, t-butyl peroxydicarbonate, and 3,3,5-trimethylhexanoyl peroxide, azo compounds such as α,α'-azobisisobutyronitrile, ammonium persulfate, and potassium persulfate. The polymerization reaction temperature can be set to 50 to 90°C.

[0028] The (meth)acrylic copolymer can be formed by polymerizing 30 to 70% by weight of a first structural unit derived from (meth)acrylonitrile, 30 to 70% by weight of a second structural unit derived from (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate, and 1 to 30% by weight of a third structural unit derived from an (aceto)amide. The acrylic copolymer formed by polymerizing the above three monomers in the above content ratio has improved adhesion to porous substrates and heat resistance, and can be used in separators to significantly improve heat resistance.

[0029] A metal hydroxide can be used to adjust the pH of the polymer, and preferably NaOH or LiOH can be used as the metal hydroxide.

[0030] Another embodiment of the present invention provides a slurry for coating a porous membrane, which includes a binder containing the (meth)acrylic copolymer, inorganic particles, and a solvent. When the inorganic particles are contained in the coating layer, they can further prevent the separator from shrinking due to heat, thereby suppressing short circuits between the positive and negative electrodes, and can also minimize the resistance of lithium ions, thereby improving battery performance. The inorganic particles can include SiO2, alumina (Al2O3), Al(OH3), AlO(OH), TiO2, BaTiO3, Mg(OH)2, MgO, Ti(OH)4, clay, glass powder, or a combination thereof.

[0031] The inorganic particles are preferably mixed with the (meth)acrylic copolymer in a ratio (solid content ratio) of 15:35 to 65 to 85.

[0032] The solvent may be either water or an organic solvent. The amount of the solvent is adjusted so that the solid content is preferably 5 to 90% by weight, more preferably 10 to 50% by weight.

[0033] The mixing device for the slurry is not particularly limited as long as it is a device that can uniformly mix the above components, and a ball mill, sand mill, pigment disperser, crusher, ultrasonic disperser, homogenizer, planetary mixer, etc. can be used, but it is particularly preferable to use a high-dispersion device that can impart a high dispersion shear, such as a bead mill, roll mill, or Filmix.

[0034] <Manufacturing inorganic coated separators> Another embodiment of the present invention provides a separator for a secondary battery, comprising: a porous substrate; and a coating layer located on at least one surface of the porous substrate, the coating layer comprising a (meth)acrylic copolymer consisting of a first structural unit derived from (meth)acrylonitrile; a second structural unit derived from (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate; and a third structural unit derived from an (acet)amide.

[0035] The separator for a secondary battery separates the negative electrode and the positive electrode and provides a path for lithium ions to move, and may include a porous substrate and a coating layer located on at least one surface of the porous substrate.

[0036] The porous substrate has a large number of pores and may be a substrate typically used in electrochemical devices. The porous substrate may be, but is not limited to, 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, polyetherimide, polyamide imide, polybenzimidazole, polyethersulfone, polyphenylene oxide, cyclic olefin copolymers, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon®, and polytetrafluoroethylene, or a polymer membrane formed from a copolymer or mixture of two or more of these polymers.

[0037] The porous substrate can have a thickness of about 1 μm to 40 μm, for example, 1 μm to 30 μm, 1 μm to 20 μm, 5 μm to 15 μm, or 10 μm to 15 μm.

[0038] The coating layer may contain, as a binder, a (meth)acrylic copolymer consisting of a first structural unit derived from (meth)acrylonitrile, a second structural unit derived from (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate, and a third structural unit derived from an (aceto)amide. When this (meth)acrylic copolymer is used as a binder in forming a coating layer on a substrate, it can improve the adhesive strength and heat resistance to the porous substrate, thereby improving the heat resistance of the separator.

[0039] The coating layer can be formed on one side of the substrate by applying the porous membrane coating slurry onto the porous substrate. The porous membrane coating slurry is as described above.

[0040] The method for applying the porous membrane coating slurry to the porous substrate is not particularly limited, and for example, a dip coating method, a die coating method, a gravure coating method, a comma coating method, etc. can be used.

[0041] Drying methods include, for example, drying with warm air, hot air, or low-humidity air, vacuum drying, and drying by irradiation with (far) infrared rays or electron beams. The drying temperature varies depending on the type of solvent used. Water or an aqueous alcohol solution can be used as the solvent. The drying temperature range is preferably 60 to 120°C.

[0042] The thickness of the coating layer is preferably 1 to 6 μm. If it is less than 1 μm, the heat resistance of the separator will be significantly reduced, while if it is more than 6 μm, the separator will be too thick, which may reduce the energy density of the battery and increase the resistance.

[0043] The coating layer may further include a dispersant, which may include an acrylic compound different from the (meth)acrylic copolymer.

[0044] Another embodiment of the present invention provides a secondary battery including the separator for a secondary battery. The secondary battery may include a positive electrode, a negative electrode, an electrolyte, and the separator for a secondary battery. The secondary battery may be a lithium-ion secondary battery.

[0045] The present invention will be described below with reference to examples, but the present invention is not limited to these examples. In the examples, parts and percentages are by weight unless otherwise specified.

[0046] <Production example> Binder manufacturing To a reaction vessel, 600 parts by weight of distilled water and 5 to 20 parts by weight of a metal hydroxide, NaOH or LiOH, were added per 100 parts by weight of the monomer mixture, and the mixture was stirred. While injecting high-purity nitrogen gas, the temperature was raised to 60°C. After adding 0.3 parts by weight of ammonium persulfate, a decomposition initiator, per 100 parts by weight of the monomer mixture, the solution polymerization reaction was allowed to proceed at 60°C for 8 hours. The mixture was then cooled to room temperature to produce an acrylic copolymer (Polymer A).

[0047] Manufacturing of slurry for porous membrane coating Alumina (average particle size 0.5 μm) as inorganic particles and polymer A were mixed at a solid content ratio of 85:15, and distilled water was added and mixed to a solid content concentration of 24%. This mixture was thoroughly dispersed using a ball mill method to produce a slurry.

[0048] Manufacturing of inorganic coated separators The prepared slurry was applied to one side of a polyethylene porous substrate by a comma coating method to form a coating layer with a thickness of 6 μm, and then dried with hot air at a temperature of 60 to 85°C to prepare an inorganic coated separator.

[0049] Example 1 A binder and a separator for secondary batteries were produced in the same manner as in Production Example, except that a monomer mixture of 52 parts by weight of (meth)acrylonitrile, 43 parts by weight of (meth)acrylic acid, and 5 parts by weight of (acet)amide was used as the monomer mixture.

[0050] <Example 2> A binder and a separator for secondary batteries were produced in the same manner as in Production Example, except that a monomer mixture of 45 parts by weight of (meth)acrylonitrile, 40 parts by weight of (meth)acrylic acid, and 15 parts by weight of (acet)amide was used as the monomer mixture.

[0051] Example 3 A binder and a separator for secondary batteries were produced in the same manner as in Production Example, except that a monomer mixture of 40 parts by weight of (meth)acrylonitrile, 30 parts by weight of (meth)acrylic acid, and 30 parts by weight of (acet)amide was used as the monomer mixture.

[0052] <Comparative Example 1> A binder for a secondary battery and a separator were produced in the same manner as in Production Example, except that a monomer mixture of 60 parts by weight of (meth)acrylonitrile and 40 parts by weight of (meth)acrylic acid was used as the monomer mixture.

[0053] <Comparative Example 2> A binder and a separator for secondary batteries were produced in the same manner as in Production Example, except that a monomer mixture of 59 parts by weight of (meth)acrylonitrile, 39 parts by weight of (meth)acrylic acid, and 2 parts by weight of polyvinyl alcohol was used as the monomer mixture.

[0054] <Comparative Example 3> A binder for a secondary battery and a separator were produced in the same manner as in Production Example, except that a monomer mixture of 20 parts by weight of (meth)acrylonitrile and 80 parts by weight of (meth)acrylic acid was used as the monomer mixture.

[0055] <Comparative Example 4> A binder and a separator for secondary batteries were produced in the same manner as in Production Example, except that a monomer mixture of 10 parts by weight of (meth)acrylonitrile, 85 parts by weight of (meth)acrylic acid, and 5 parts by weight of (acet)amide was used as the monomer mixture.

[0056] [Table 1]

[0057] <Experimental Example 1>: Adhesion test The separators produced in Examples 1 to 3 and Comparative Examples 1 to 4 were cut to a size of 18 mm wide and 100 mm long to prepare test specimens. Double-sided tape with an area of ​​20 mm wide and 40 mm long was attached to an acrylic plate with an area of ​​40 mm wide and 100 mm long. The prepared separator was attached to the double-sided tape and then lightly pressed five times with a hand roller.

[0058] The test piece was attached to a UTM (20 kgf load cell), and one side of the separator was attached to the upper clip of the tensile strength tester, while a tape attached to one side of the separator was attached to the lower clip. The 180° peel strength was measured at a speed of 100 mm / min. Five or more test pieces were prepared per sample and measured, and the average values ​​were calculated. The results are shown in Table 2.

[0059] <Experimental Example 2>: Heat resistance test The separators produced in Examples 1 to 3 and Comparative Examples 1 to 4 were cut into test pieces measuring 3 cm x 5 cm in length and width. These test pieces were left in an oven at 150°C for 1 hour, and then the shrinkage rates were measured. The results are shown in Table 2.

[0060] [Table 2]

[0061] In Table 2, the separators of Examples 1 to 3 according to the present invention had higher average adhesive strengths, smaller standard deviations, and equivalent or higher thermal shrinkage rates than the separator of Comparative Example 1, which did not contain (acet)amide, and the separator of Comparative Example 2, which did not contain (acet)amide but contained polyvinyl alcohol.

[0062] Furthermore, it was found that the adhesive strength and heat resistance improved with an increase in the (aceto)amide content in Examples 1 to 3. This shows that when (aceto)amide is contained as a component of the acrylic copolymer as in the present invention, the adhesive strength and heat resistance improved without the need for mixing a polyvinyl alcohol polymer as in the prior art.

[0063] Furthermore, it was found that the separators of Examples 1 to 3 exhibited adhesive strength and heat resistance equivalent to or greater than the separator of Comparative Example 3, which had a (meth)acrylonitrile content of 20% by weight, and the separator of Comparative Example 4, which had a (meth)acrylonitrile content of 10% by weight. In particular, the separator of Comparative Example 4 had a low (meth)acrylonitrile content, and therefore, despite containing (aceto)amide, it had good adhesive strength but did not exhibit heat resistance. This shows that both adhesive strength and heat resistance can be exhibited only when (meth)acrylonitrile is contained in the copolymer at 30% by weight or more, as in the present invention.

Claims

1. The (meth)acrylic copolymer comprises a first structural unit derived from (meth)acrylonitrile, a second structural unit derived from (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate, and a third structural unit, which is (acet)amide; the first structural unit derived from the (meth)acrylonitrile is contained in an amount of 40 to 45% by weight relative to 100 parts by weight of the (meth)acrylic copolymer; the second structural unit derived from the (meth)acrylic acid, the (meth)acrylate salt, or the (meth)acrylate is contained in an amount of 30 to 40% by weight relative to 100 parts by weight of the (meth)acrylic copolymer; and the (acet)amide is contained in an amount of 15 to 30% by weight relative to 100 parts by weight of the (meth)acrylic copolymer.

2. The present invention comprises a porous substrate and a coating layer located on at least one surface of the porous substrate, the coating layer comprising a (meth)acrylic copolymer having a first structural unit derived from (meth)acrylonitrile, a second structural unit derived from (meth)acrylic acid, a (meth)acrylate salt, or a (meth)acrylate, and a third structural unit of (acet)amide; a separator for a secondary battery, wherein the first structural unit derived from the (meth)acrylonitrile is contained in an amount of 40 to 45% by weight relative to 100 parts by weight of the (meth)acrylic copolymer; the second structural unit derived from the (meth)acrylic acid, the (meth)acrylate salt, or the (meth)acrylate is contained in an amount of 30 to 40% by weight relative to 100 parts by weight of the (meth)acrylic copolymer; and the (acet)amide is contained in an amount of 15 to 30% by weight relative to 100 parts by weight of the (meth)acrylic copolymer.

3. 3. The separator for a secondary battery according to claim 2, wherein the porous substrate is a polymer membrane formed from any one polymer selected from the group consisting of polyolefin, polyester, polyacetal, polyamide, polyimide, polycarbonate, polyether ether ketone, polyaryl ether ketone, polyetherimide, polyamideimide, polybenzimidazole, polyethersulfone, polyphenylene oxide, cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon (registered trademark), and polytetrafluoroethylene, or a copolymer or mixture of two or more of these polymers.

4. The separator for a secondary battery according to claim 2 , wherein the coating layer is formed by applying a slurry for porous membrane coating onto the porous substrate.

5. The separator for a secondary battery according to claim 4, wherein the slurry for coating the porous membrane contains a binder containing the (meth)acrylic copolymer, inorganic particles, and a solvent.

6. 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 6. The secondary battery separator of claim 5, comprising a cellulose acylate, ...

7. 6. The secondary battery separator according to claim 5, wherein the inorganic particles are mixed with the (meth)acrylic copolymer in a ratio (solid content ratio) of 15 to 35:65 to 85.

8. 3. The secondary battery separator according to claim 2, wherein the coating layer has a thickness of 1 to 6 μm.

9. A secondary battery comprising the separator for secondary batteries according to any one of claims 2 to 8.

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