Copolymer for separator and secondary battery using thereof

Abstract

The present invention relates to a copolymer comprising a monomer unit containing a sulfur atom, an acrylonitrile-based monomer unit, an acrylate-based monomer unit comprising a linear alkyl group having 1 to 20 carbon atoms, and a vinyl acetate-based monomer unit, a slurry composition containing the same, a separator, and a secondary battery.

Description

Technology Field

[0001] The present invention relates to a copolymer and a slurry composition containing the same, a separator, and a secondary battery. Background Technology

[0002] Due to their high energy density, lithium-ion batteries are widely used in the electrical, electronic, telecommunications, and computer industries. Following small lithium-ion batteries for portable electronic devices, their applications are expanding to include high-capacity batteries for hybrid and electric vehicles.

[0003] Although lithium-ion batteries are insulated by a separator, ensuring the thermal and chemical safety of the separator is crucial, as internal or external battery malfunctions or impacts can cause a short circuit between the positive and negative electrodes, potentially leading to overheating and explosion.

[0004] Currently, polyolefin-based films are widely used as separators, but polyolefins have the disadvantage of severe thermal shrinkage at high temperatures and poor mechanical properties.

[0005] To improve the stability of such polyolefin-based separation membranes, a porous separation membrane has been developed in which a mixture of inorganic particles and a binder is coated onto a polyolefin porous substrate film.

[0006] That is, in order to suppress thermal shrinkage caused by high temperature of polyolefin-based separators and battery instability caused by dendrites, inorganic particles are coated together with a binder on one or both sides of a porous separator substrate, thereby providing the inorganic particles with the function of suppressing the shrinkage rate of the substrate, and at the same time, a safer separator can be manufactured through the coating layer.

[0007] To ensure excellent battery characteristics, the coating layer must be uniformly coated while simultaneously requiring strong adhesion to the substrate. In particular, improvements in the performance and stability of secondary batteries are required through this. Prior art literature

[0008] Republic of Korea Registered Patent No. 10-1430975 Republic of Korea Published Patent Application No. 10-2006-0072065 The problem to be solved

[0009] Accordingly, the present invention aims to provide a slurry composition with excellent inorganic adhesion and electrode adhesion to a porous substrate using a copolymer.

[0010] In addition, the present invention aims to provide a separator with excellent adhesion and electrical properties to which the above slurry composition is applied, and a battery with excellent performance using said separator.

[0012] However, the problems that this invention seeks to solve are not limited to those mentioned above, and other unmentioned problems will be clearly understood by those skilled in the art from the description below. means of solving the problem

[0013] One aspect of the present invention comprises a monomer unit containing a sulfur atom, an acrylonitrile-based monomer unit, an acrylate-based monomer unit containing a linear alkyl group having 1 to 20 carbon atoms, and a vinyl acetate-based monomer unit.

[0014] Provides a copolymer.

[0015] Another aspect of the present invention is the copolymer; and

[0016] Inorganic particles; including,

[0017] A slurry composition is provided.

[0018] Another aspect of the present invention is a slurry composition comprising,

[0019] Provides a separation membrane.

[0020] Another aspect of the present invention is that the separation membrane comprises,

[0021] Provides a secondary battery. Effects of the invention

[0022] The copolymer of the present invention can increase the adhesion of inorganic materials and electrodes to the separator substrate, and can improve the heat resistance of the separator by increasing the inorganic material content due to the high adhesion.

[0023] In addition, by improving adhesion, the defect rate and void formation that may occur during battery assembly can be minimized.

[0024] In addition, excellent air permeability and electrical resistance characteristics can be imparted to improve ion conductivity, reaction stability, and lifespan characteristics within the battery electrolyte. Specific details for implementing the invention

[0025] The operation and effects of the invention will be described in more detail below through specific embodiments. However, these embodiments are merely examples of the invention and do not define the scope of the invention.

[0026] Prior to this, terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0027] Therefore, it should be understood that the configuration of the embodiments described in this specification is merely one of the most preferred embodiments of the present invention and does not represent all of the technical ideas of the present invention, and that various equivalents and modifications that can replace them may exist at the time of filing this application.

[0028] In this specification, singular expressions include plural expressions unless the context clearly indicates otherwise. In this specification, terms such as “comprising,” “comprising,” or “having” are intended to specify the existence of the implemented features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the existence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0029] In the present specification, "a to b" and "a~b" indicating numerical ranges, "to" and "~" are defined as ≥ a and ≤ b.

[0031] A copolymer of one aspect of the present invention may include a monomer unit containing a sulfur atom, an acrylonitrile-based monomer unit, an acrylate-based monomer unit containing a linear alkyl group having 1 to 20 carbon atoms, and a vinyl acetate-based monomer unit.

[0032] The above-mentioned monomer unit containing a sulfur atom can exhibit reactive emulsifier behavior. Typically, reactive emulsifiers can perform functions and roles such as controlling the chemical structure of the polymer, controlling particle size, stabilization, and dispersion during emulsion polymerization.

[0033] The above-mentioned sulfur atom-containing monomer unit, which exhibits reactive emulsifier behavior, can surround the edges of the copolymer (water-soluble resin) of the present invention and can control the stable generation and dispersion of particles with large particle sizes compared to conventional water-soluble resins.

[0034] Therefore, the above-mentioned sulfur atom-containing monomer unit can help increase adhesion to the electrode and achieve low resistance within the secondary battery compared to particles with small particle sizes.

[0035] In addition, the monomer unit containing the sulfur atom can have a large dipole moment. Therefore, the adhesion to the inorganic material and the porous substrate or electrode can be improved through electrostatic attraction caused by the induction of polarization phenomena.

[0036] In a single covalent bond, electrons are attracted more towards the atom with higher electronegativity than the other atom. At this time, the atom with relatively higher electronegativity becomes negatively charged and the atom with lower electronegativity becomes positively charged. This is called a dipole, and its magnitude is called the dipole moment.

[0037] In one embodiment, the copolymer of the present invention may further comprise an acrylic acid-based monomer unit, an acrylate-based monomer unit comprising a branched alkyl group having 3 to 10 carbon atoms, or a combination thereof.

[0038] Meanwhile, the copolymer of the present invention may not include styrene-based monomer units formed by polymerizing styrene monomers.

[0039] For example, the copolymer of the present invention comprises a monomer unit containing a sulfur atom, an acrylonitrile-based monomer unit, an acrylate-based monomer unit containing a linear alkyl group having 1 to 20 carbon atoms, and a vinyl acetate-based monomer unit; or comprises a monomer unit containing a sulfur atom, an acrylonitrile-based monomer unit, an acrylate-based monomer unit containing a linear alkyl group having 1 to 20 carbon atoms, a vinyl acetate-based monomer unit, and an acrylic acid-based monomer unit; or comprises a monomer unit containing a sulfur atom, an acrylonitrile-based monomer unit, an acrylate-based monomer unit containing a linear alkyl group having 1 to 20 carbon atoms, a vinyl acetate-based monomer unit, and an acrylate-based monomer unit containing a branched alkyl group having 3 to 10 carbon atoms; or comprises a monomer unit containing a sulfur atom, an acrylonitrile-based monomer unit, and a linear alkyl group having 1 to 20 carbon atoms It may include acrylate-series monomer units containing an alkyl group, vinyl acetate-series monomer units, acrylic acid-series monomer units, and acrylate-series monomer units containing a branched alkyl group having 3 to 10 carbon atoms.

[0040] In one embodiment, the copolymer of the present invention may comprise, based on 100% by weight of the total weight of the copolymer, 0.1% by weight or more and 30% by weight or less of the sulfur atom-containing monomer unit, 1% by weight or more and 40% by weight or less of the acrylonitrile-based monomer unit, 1% by weight or more and 65% by weight or less of the acrylate-based monomer unit containing a linear alkyl group having 1 to 20 carbon atoms, and 1% by weight or more and 30% by weight or less of the vinyl acetate-based monomer unit.

[0041] For example, the copolymer of the present invention may comprise, based on 100% by weight of the total weight of the copolymer, 0.5% by weight or more and 30% by weight or less of the sulfur atom-containing monomer unit, 20% by weight or more and 35% by weight or less of the acrylonitrile-based monomer unit, 10% by weight or more and 65% by weight or less of the acrylate-based monomer unit containing a linear alkyl group having 1 to 20 carbon atoms, and 5% by weight or more and 15% by weight or less of the vinyl acetate-based monomer unit.

[0042] If the content of a monomer unit containing the sulfur atom having a large dipole moment, or a combination thereof, falls below the content range of the present invention, the adhesive strength is reduced, and the properties of the separation membrane may be degraded.

[0043] If the content of a monomer unit containing the sulfur atom having a large dipole moment, or a combination thereof, exceeds the content range of the present invention, reaction stability is reduced, and the characteristics of the separation membrane may be degraded.

[0044] If the above acrylonitrile-based monomer unit exceeds or falls below the content range of the present invention, it may cause a decrease in the dispersibility of polymer particles and inorganic slurry or a decrease in adhesion.

[0045] If the content of the acrylate-based monomer unit containing a linear alkyl group having 1 to 20 carbon atoms exceeds or falls below the content range of the present invention, it may cause a decrease in adhesive strength.

[0046] If the above vinyl acetate monomer unit exceeds or falls below the content range of the present invention, it may cause stability problems such as stability and storage stability.

[0047] In one embodiment, based on 100% by weight of the total weight of the copolymer of the present invention, the copolymer may further comprise 0.1% by weight or more and 20% by weight or less of the acrylic acid-based monomer unit, 0.1% by weight or more and 35% by weight or less of the acrylate-based monomer unit comprising a branched alkyl group having 3 to 10 carbon atoms, or a combination thereof.

[0048] For example, based on 100% by weight of the total weight of the copolymer of the present invention, it may further include 0.5% by weight or more and 10% by weight or less of acrylic acid-based monomer units, 10% by weight or more and 35% by weight or less of acrylate-based monomer units containing branched alkyl groups having 3 to 10 carbon atoms, or a combination thereof.

[0049] If the above acrylic acid-based monomer unit exceeds or falls below the content range of the present invention, it may cause polymer aggregation and precipitation or a decrease in adhesion.

[0050] If the content of the acrylate-based monomer unit containing a branched alkyl group having 3 to 10 carbon atoms exceeds or falls below the content range of the present invention, it may cause a decrease in adhesive strength.

[0051] In one embodiment, the monomer unit containing the sulfur atom may be a monomer unit containing a sulfide group, a monomer unit containing a sulfonate group, a monomer unit containing a sulfone group, a monomer unit containing a sulfonyl ester group, a monomer unit containing a sulfonamide group, a monomer unit containing a sulfonimide group, a monomer unit containing a sulfonyl azide group, a monomer unit containing a sulfonyl hydrazide group, a monomer unit containing a sulfonyl aziridine group, a monomer unit containing a sulfonyl azitidine group, a monomer unit containing a sulfonyl carbamate group, a monomer unit containing a sulfonylurea group, a monomer unit containing a sulfonyl halide group, or a combination thereof, but is not limited thereto.

[0052] In one embodiment, the acrylate-series monomer unit comprising a linear alkyl group having 1 to 20 carbon atoms may be formed by polymerizing one or more selected from the group consisting of methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, butyl methacrylate, lauryl acrylate, lauryl methacrylate, stearyl acrylate, and stearyl methacrylate.

[0053] Meanwhile, the above-mentioned acrylonitrile-based monomer unit may be formed by polymerizing one or more selected from the group consisting of acrylonitrile and methacrylonitrile.

[0054] In addition, the above vinyl acetate series monomer unit can be formed by polymerizing vinyl acetate.

[0055] In one embodiment, the monomer unit containing the sulfur atom may be formed by polymerizing sodium vinyl sulfonate.

[0056] In one embodiment, the acrylic acid-based monomer unit may be formed by polymerizing one or more selected from the group consisting of acrylic acid and methacrylic acid.

[0057] In addition, the above-mentioned acrylate-series monomer unit containing a branched alkyl group having 3 to 10 carbon atoms may be formed by polymerizing one or more selected from the group consisting of isopropyl acrylate, isopropyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, 2-ethylhexyl acrylate, and 2-ethylhexyl methacrylate.

[0058] In one embodiment, the acrylic acid-based monomer unit can be combined with an alkali metal.

[0059] That is, the carboxylate group of the above acrylic acid-based monomer unit can be bonded to an alkali metal.

[0060] Meanwhile, the weight ratio of the alkali metal to the copolymer (weight of one or more selected from the group consisting of the alkali metal and acetate salt compounds containing the alkali metal: weight of the copolymer) may be 0.1 to 15:100.

[0061] For example, the weight ratio of the alkali metal and the copolymer may be 1 to 10:100.

[0062] If the weight ratio of the alkali metal and the copolymer exceeds or falls below the weight ratio of the present invention, the adhesive properties and heat resistance properties of the separator may be reduced. In particular, the peel adhesion and electrode adhesion of the separator may be reduced.

[0063] The adhesive strength of the separator containing the above alkali metal can be determined by the strength of the cohesive and repulsive forces between the elements.

[0064] The copolymer of the present invention may have inorganic adhesion or electrode adhesion due to changes in the relative strength of cohesive force, adhesion force, and repulsive force between elements depending on the content of the alkali metal.

[0065] When the above alkali metal is used in the manufacture of a binder copolymer, the overall adhesive strength is improved due to the harmony between adhesion and cohesion caused by the increase in cohesion between elements; however, if added in excess, the cohesion may decrease, and the overall adhesive strength may be reduced.

[0066] Meanwhile, since the improvement in the adhesion of the copolymer to inorganic materials implies an increase in adhesion between the inorganic material and the substrate, this may mean that the heat resistance properties of the separator can be improved by increasing the amount of inorganic material.

[0067] The electrode adhesion of the separator can reduce the defect rate that occurs when loading electrodes during battery assembly, minimize void formation, and improve ion conductivity within the electrolyte.

[0068] In addition, when a binder layer is coated alone on an inorganic coating layer, electrode adhesion can be maximized, and ultimately, electrode adhesion can minimize the battery failure rate and further improve battery performance.

[0069] In one embodiment, the copolymer may include a monomer repeating unit represented by the following chemical formula 1.

[0071] [Chemical Formula 1]

[0072]

[0074] In the above formula 1, R1, R2, and R3 are each independently hydrogen, a linear or branched hydrocarbon having 1 to 4 carbon atoms, or a combination thereof; R4 is a linear alkyl group having 1 to 20 carbon atoms; R5 is a branched alkyl group having 3 to 10 carbon atoms; R6 is hydrogen or an alkali metal; R7 is a sulfide group, a sulfonate group, a sulfone group, a sulfonyl ester group, a sulfonamide group, a sulfonimide group, a sulfonyl azide group, a sulfonyl hydrazide group, a sulfonyliaziridine group, a sulfonyliazitidine group, a sulfonyl carbamate group, a sulfonylurea group, a sulfonyl halide group, or a combination thereof; and 0.01≤a≤0.65, 0≤b≤0.35, 0≤c≤0.2, 0.01≤d≤0.3, 0.001≤e≤0.3 and 0.01≤f≤0.4 may be possible.

[0075] However, a+b+c+d+e+f=1.

[0077] a, b, c, d, e, and f of the above chemical formula 1 correspond to the weight fraction of each monomer unit, and the sum of the weight fractions of each monomer unit is 1.

[0078] The above alkali metal may be Li, Na, or K, but is not limited thereto.

[0079] In one embodiment, R1, R2, and R3 are each independently one or more selected from the group consisting of hydrogen and methyl, and R4 may be one or more selected from the group consisting of methyl, ethyl, propyl, butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, lauryl, n-dodecyl, n-tridecyl, n-tetradecyl, n-pentadecyl, cetyl, n-hexadecyl, n-heptadecyl, stearyl, n-octadecyl, n-nonadecyl, n-icosyl, n-henicosyl, and n-docosyl.

[0080] In addition, the above R5 may be one or more selected from the group consisting of isopropyl, sec-butyl, tert-butyl, ethylhexyl, 2-ethylhexyl, iso-pentyl, iso-heptyl, and iso-octyl.

[0081] In one embodiment, the copolymer may be a random or block copolymer depending on the synthesis process.

[0082] In one embodiment, the number average molecular weight of the copolymer may be 10,000 to 1,000,000.

[0083] If the number average molecular weight of the copolymer is less than 10,000, the fluidity of the copolymer increases, which may reduce dispersibility and lower the heat resistance of the separator. If the number average molecular weight exceeds 1,000,000, the viscosity is too high for use and may block the pores of the separator, thereby reducing air permeability and resistance.

[0084] In one embodiment, the copolymer may be in the form of particles, and the average particle size (diameter) of the particles may be 500 nm or more and 1,500 nm or less.

[0085] For example, the average particle size (diameter) of the above particles may be 550 nm or more, 600 nm or more, or 650 nm or more, and may be 1,000 nm or less.

[0086] A slurry composition according to another aspect of the present invention may include the copolymer and inorganic particles.

[0087] The above inorganic particles can be used without restriction if they are insulating particles.

[0088] Specific examples of the above-mentioned inorganic particles include Al2O3, AlOOH, SiO2, TiO2, ZrO2, ZnO, NiO, CaO, SnO2, Y2O3, MgO, BaTiO3, CaTiO3, SrTiO3, SiC, Li3PO4, Pb(Zr,Ti)O3(PZT), (Pb,La)(Zr,Ti)O3(PLZT), and mixtures thereof.

[0089] The above inorganic particles are not subject to any special size limitations, but for example, the average particle size may be 0.01 μm to 30 μm, and more preferably 0.1 μm to 10 μm. If the average particle size of the inorganic particles is below the above preferred range, dispersibility may be reduced, and if it exceeds the above preferred range, the thickness of the coating layer after coating may increase, and mechanical properties may be degraded.

[0090] In addition, the above-mentioned inorganic particles have no particular restrictions on shape and may be, for example, spherical, plate-shaped, elliptical, or irregular.

[0091] A separation membrane according to another aspect of the present invention may include the slurry composition.

[0092] A separator can be manufactured by coating the above slurry composition on at least one surface of a porous substrate film, or by manufacturing the above slurry composition into a film form and laminating it onto a porous substrate film.

[0093] Meanwhile, the above separator can be used as a separator for a secondary battery, for example, as a separator for a lithium secondary battery.

[0094] As an example of manufacturing a separation membrane, the method may include: (a) a step of preparing a polymer solution by dissolving or dispersing the copolymer in a solvent; (b) a step of adding and mixing inorganic particles to the polymer solution of step a); and (c) a step of coating and drying one or more regions selected from the group consisting of the surface of a polyolefin-based separation membrane substrate and a portion of the pores in the substrate with the mixture of step b).

[0095] First, 1) the copolymer is prepared and prepared in the form of a polymer solution by dissolving or dispersing it in a suitable solvent.

[0096] As for the solvent, it is preferable to have a solubility index similar to that of the copolymer used as a binder and a low boiling point. This is to facilitate uniform mixing and subsequent solvent removal. Non-limiting examples of usable solvents include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, water, or mixtures thereof. More preferably, it can be used in a water-dispersed state.

[0097] 2) Inorganic particles are added and dispersed in the prepared polymer solution to prepare a mixture of inorganic particles and polymer.

[0098] It is desirable to carry out a dispersion process of the polymer solution and inorganic particles. At this time, a dispersion time of 0.1 to 24 hours may be appropriate. Conventional methods can be used as the dispersion method, and the ball mill method is particularly preferred.

[0099] The composition of the mixture consisting of inorganic particles and polymers is not significantly restricted, but accordingly, the thickness, pore size, and porosity of the organic / inorganic composite porous separation membrane of the present invention finally manufactured can be controlled.

[0100] In other words, as the ratio of inorganic particles (I) to polymers (P) (ratio = I / P) increases, the porosity of the membrane increases, which results in an increase in the thickness of the membrane at the same solid content (weight of inorganic particles + weight of polymers). Additionally, the possibility of pore formation between inorganic particles increases, leading to an increase in pore size. At this time, as the size (particle diameter) of the inorganic particles increases, the interstitial distance between the inorganic particles increases, thus increasing the pore size.

[0101] 3) A mixture of manufactured inorganic particles and polymers can be coated onto a prepared polyolefin-based separator substrate and subsequently dried to obtain the separator of the present invention.

[0102] At this time, the method of coating the mixture of inorganic particles and polymers onto a polyolefin-based separator substrate may use conventional coating methods known in the art, and various methods such as dip coating, die coating, roll coating, comma coating, or a combination thereof may be used. In addition, when coating the mixture of inorganic particles and polymers onto a polyolefin-based separator substrate, the coating may be applied to both sides of the separator substrate or selectively applied to only one side.

[0103] When the above separator is used in a secondary battery, lithium ions can be transferred not only through the separator substrate but also through the porous active layer, and in the event of an internal short circuit caused by external impact, the aforementioned safety enhancement effect can be exhibited.

[0104] In addition, the secondary battery may include a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0105] The above secondary battery can be manufactured according to conventional methods known in the industry, and as an example thereof, the electrode and the separator are assembled and then an electrolyte is injected into the assembly to manufacture it.

[0106] There are no significant limitations on the electrodes to be applied together with the above-mentioned separator; however, the cathode active material may be any conventional cathode active material that can be used for the cathode of a secondary battery. Non-limiting examples include lithium intercalation materials such as lithium manganese oxide (lithiated magnesium oxide), lithium cobalt oxide, lithium nickel oxide, or composite oxides formed by combinations thereof. Additionally, the anode active material may be any conventional anode active material that can be used for the anode of a conventional electrochemical device. Non-limiting examples include lithium metal or lithium alloys, as well as lithium intercalation materials such as carbon, petroleum coke, activated carbon, graphite, or other carbons. The two electrodes are configured by attaching the aforementioned positive electrode active materials to a positive current collector, i.e., a foil made of aluminum, nickel, or a combination thereof, and a negative current collector, i.e., a foil made of copper, gold, nickel, or a copper alloy, or a combination thereof, respectively.

[0107] The above electrolyte is a salt having a structure such as A+B-, where A+ comprises ions composed of alkali metal cations such as Li+, Na+, and K+ or combinations thereof, and B- comprises anions composed of anions such as PF6-, BF4-, Cl-, Br-, I-, ClO4-, AsF6-, CH3CO2-, CF3SO3-, N(CF3SO2)2-, and C(CF2SO2)3- or combinations thereof, and is a salt comprising propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, and tetrahydrofuran It is preferable that it be dissolved and dissociated in an organic solvent composed of tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma butyrolactone (GBL), or a mixture thereof.

[0108] In addition to the general winding process, the process of applying the above separator to a battery may include stacking and folding of the separator and electrode.

[0110] The present invention will be explained in more detail below using examples, but the present invention is not limited thereto.

[0112] [Preparation Example 1] Preparation of a copolymer

[0113] 245 parts by weight of distilled water and a portion by weight (A) of the monomer mixture, and 0.1 to 5 parts by weight of an emulsifier per 100 parts by weight of the monomer mixture were added to a reaction vessel, and the temperature was raised to 70°C and stirred while injecting high-purity nitrogen gas.

[0114] 0.15 parts by weight of ammonium persulfate, a decomposition initiator, was prepared per 100 parts by weight of the monomer mixture, and a portion by weight (B) of the remaining monomer mixture was prepared and each was continuously added to a reaction vessel heated to 70°C to start emulsion polymerization.

[0115] A copolymer was prepared by continuously adding the remaining weight part (C) of the remaining monomer mixture immediately after the addition of a portion of the remaining monomer mixture (B) was finished.

[0116] A copolymer subjected to a neutralization reaction was prepared by adding an aqueous solution of a metal hydroxide (NaOH, LiOH, KOH) to the copolymer prepared by the above polymerization reaction (emulsion polymerization reaction).

[0118] [Preparation Example 2] Preparation of a slurry for porous membrane coating

[0119] Inorganic particles [alumina (average particle size 0.5 μm) or boehmite (average particle size 0.7 μm)] and the binder copolymer prepared according to Preparation Example 1 were mixed in a solid weight ratio of 80:20, and then distilled water was added and mixed to achieve a solid concentration of 35%. This mixture was sufficiently dispersed using a ball mill or a mechanical stirrer to prepare a slurry.

[0121] [Preparation Example 3] Preparation of a separation membrane

[0122] An inorganic coating layer was formed by applying the porous membrane coating slurry prepared according to Preparation Example 2 to a polyolefin porous substrate (polyethylene (PE), polypropylene (PP), etc.). Various coating methods can be used, such as dip coating, die coating, gravure coating, and comma coating.

[0123] In addition, after coating, it was dried using methods such as hot air, hot air, vacuum drying, and infrared drying, and the drying temperature range was 50 to 80℃.

[0124] The thickness of the above inorganic coating layer was 1 to 6 μm on one side or both sides, and if the thickness was less than 1 μm, there was a problem that the heat resistance of the separator was significantly reduced, and if the thickness exceeded 6 μm, the thickness of the separator was too thick, which could reduce the energy density of the battery and increase the resistance.

[0126] [Examples and Comparative Examples]

[0127] Examples 1 to 10 and Comparative Examples 1 and 4 were prepared according to Preparation Example 1 by adjusting the monomer content as shown in Table 1 below, and the prepared copolymer was used to prepare a porous membrane coating slurry according to Preparation Example 2 and a separation membrane according to Preparation Example 3.

[0128] In addition, the average particle size (diameter) of the copolymers of Examples 1 to 10 and Comparative Examples 1 and 4, analyzed by Particle Sizer NICOMP 380, is shown in Table 1 below.

[0130] Copolymer monomer and content (weight%) Average particle size (nm) AN BA EHA MAA VAc Na-VS Example 1 24 30 30 5 10 1 650 Example 2 24 60 - 5 10 1 650 Example 3 24 28 28 5 10 5 750 Example 4 24 56 - 5 10 5 750 Example 5 24 61 - - 10 5 750 Example 6 25 25 25 5 10 10 750 Example 7 25 50 - 5 10 10 750 Example 8 25 55 - - 10 10 750 Example 9 25 15 15 5 10 30 1000 Example 10 25 30 - 5 10 30 1000 Comparative Example 1 25 30 30 5 10 0 450 Comparative Example 2 25 60 - 5 10 0 450 Comparative Example 3 61 12 12 5 10 0 450 Comparative Example 4 61 24 - 5 10 0 450

[0132] In Table 1 above, the monomers AN represents acrylonitrile, BA represents butyl acrylate, EHA represents 2-ethylhexyl acrylate, MAA represents methacrylic acid, VAc represents vinyl acetate, and Na-VS represents sodium vinyl sulfonate.

[0134] [Evaluation Example 1] Adhesion strength of slurry for porous membrane coating (Peel strength test)

[0135] A tape with a width of 18 mm and a length of 30 mm or more was attached to a separator prepared according to Preparation Example 3 using the binder copolymers of Examples 1 to 10 and Comparative Examples 1 and 4, and then lightly pressed 5 times with a hand roller to prepare a specimen.

[0136] The manufactured specimen was mounted on a UTM (1 kgf Load cell), one side of the membrane was fixed to the upper clip of the tensile strength tester, and the tape attached to one side of the membrane was fixed to the lower clip. The 180° peel strength was measured at a speed of 100 mm / min. At least 5 specimens were prepared and measured per sample, and the average value was calculated.

[0138] [Evaluation Example 2] Dry electrode adhesion of the separator

[0139] A separator prepared according to Preparation Example 3 using the binder copolymers of Examples 1 to 10 and Comparative Examples 1 to 4 was cut to a width of 20 mm and a length of 70 mm.

[0140] After placing an electrode cut to a width of 25 mm and a length of 70 mm on the prepared separator, a specimen was prepared by applying a temperature of 65°C and a pressure of 500 kg for 10 seconds using a hot press.

[0141] The manufactured specimens were mounted on a UTM (1 kgf Load cell), one side of the separator was fixed to the upper clip of the tensile strength tester, and the electrode was fixed to the lower clip. The 180° peel strength was measured at a speed of 100 mm / min. At least 5 specimens were prepared and measured per sample, and the average value was calculated.

[0143] [Evaluation Example 3] Wet electrode adhesion of the separator

[0144] A separator prepared according to Preparation Example 3 using the binder copolymers of Examples 1 to 10 and Comparative Examples 1 to 4 was cut to a size of 20 mm in width and 70 mm in length.

[0145] After placing an electrode cut to a width of 25 mm and a length of 70 mm on the prepared separator, seal it in an aluminum pouch and immerse it in the electrolyte at room temperature for 12 hours, then apply a hot press at 65°C and 65 kg / cm² 2 The temperature and pressure were applied for 30 seconds.

[0146] The prepared specimen was mounted on a UTM (1kgf Load cell), one side of the separator was fixed to the upper clip of the tensile strength tester, and the electrode was fixed to the lower clip. The 180° peel strength was measured at a speed of 100 mm / min. At least 5 specimens were prepared and measured per sample, and the average value was calculated.

[0148] [Evaluation Example 4] Air permeability of the separator

[0149] The time (in seconds) required for 100 cc of air to pass through the membrane prepared according to Preparation Example 3 using the binder copolymers of Examples 1 to 10 and Comparative Examples 1 to 4 was measured using an air permeability measuring device. The amount of change in air permeability was calculated based on the air permeability of the uncoated membrane.

[0151] [Evaluation Example 5] Electrical resistance of the separator

[0152] A separator prepared according to Preparation Example 3 using the binder copolymers of Examples 1 to 10 and Comparative Examples 1 to 4 was punched to a diameter of 18 mm, and a CR2032 coin cell consisting of a graphite cathode and an NCM622 anode was assembled.

[0153] After forming the assembled coin cell in a Biologics impedance device, the resistance was measured at SOC 50. The resistance values ​​of each coated separator were calculated as a percentage compared to the uncoated separator as a reference.

[0155] The peel adhesion, dry electrode adhesion, and wet electrode adhesion of the separator prepared according to Preparation Example 3 using the binder copolymers of Examples 1 to 10 and Comparative Examples 1 to 4, evaluated by Evaluation Examples 1 to 3, are shown in Table 2 below.

[0157] copolymer Peel adhesion (gf / mm) Dry electrode adhesion (gf / mm) Wet electrode adhesion (gf / mm) Example 1 4.98 3.31 7.01 Example 2 5.22 2.90 8.01 Example 3 6.89 4.81 12.01 Example 4 7.01 4.30 15.11 Example 5 7.13 4.44 15.41 Example 6 7.12 5.32 11.51 Example 7 7.22 4.77 13.56 Example 8 7.23 4.89 13.99 Example 9 4.85 3.95 6.99 Example 10 3.98 3.01 8.61 Comparative Example 1 3.54 2.86 4.10 Comparative Example 2 3.66 2.31 6.96 Comparative Example 3 3.22 1.86 4.11 Comparative Example 4 3.45 0.88 6.21

[0159] As shown in Table 2 above, the peel adhesion strength of the separator applied with the binder copolymer of Examples 1 to 10 is in the range of 3.5 gf / mm or more and 9.0 gf / mm or less, the dry electrode adhesion strength is in the range of 2.5 gf / mm or more and 7.0 gf / mm or less, and the wet electrode adhesion strength is in the range of 5.5 gf / mm or more and 20.0 gf / mm or less.

[0160] Specifically, the peel adhesion strength of the separator applied with the binder copolymer of Examples 1 to 10 is in the range of 3.90 gf / mm or more and 8.0 gf / mm or less, the dry electrode adhesion strength is in the range of 3.0 gf / mm or more and 6.0 gf / mm or less, and the wet electrode adhesion strength is in the range of 6.99 gf / mm or more and 16.0 gf / mm or less.

[0161] Meanwhile, it was confirmed that the separator membranes applied with the binder copolymers of Examples 1 to 10, which used sodium vinylsulfonate containing a high electronegativity atom as a monomer, had higher peel adhesion compared to the separator membranes applied with the binder copolymers of Comparative Examples 1 to 4, which did not use sodium vinylsulfonate as a monomer.

[0162] Meanwhile, it was confirmed that the separator membranes applied with the binder copolymer of Examples 1 to 10 using sodium vinylsulfonate as a monomer generally had higher dry electrode adhesion and wet electrode adhesion compared to the separator membranes applied with the binder copolymer of Comparative Examples 1 to 4.

[0163] In addition, based on 100% by weight of the total weight of the copolymer for the binder, it was confirmed that the peel adhesion, dry electrode adhesion, and wet electrode adhesion were best when the sodium vinylsulfonate content was 5 to 10% by weight.

[0164] In addition, when 2-ethylhexyl acrylate was included, dry adhesion tended to increase and wet adhesion tended to decrease.

[0166] The change in air permeability of the separation membrane prepared according to Preparation Example 3 using the binder copolymers of Examples 1 to 10 and Comparative Examples 1 to 4 evaluated by Evaluation Example 4 is shown in Table 3 below.

[0168] copolymer Air permeability (sec / 100cc) Example 1 △53 Example 2 △53 Example 3 △47 Example 4 △48 Example 5 △49 Example 6 △42 Example 7 △42 Example 8 △45 Example 9 △40 Example 10 △40 Comparative Example 1 △65 Comparative Example 2 △63 Comparative Example 3 △33 Comparative Example 4 △35

[0170] As shown in Table 3 above, the change in air permeability of the separator membrane to which the binder copolymer of Examples 1 to 10 was applied was in the range of △36 sec / 100cc or more and △60 sec / 100cc or less.

[0171] Specifically, the change in air permeability of the separator membrane to which the binder copolymer of Examples 1 to 10 was applied was in the range of △38 sec / 100cc or more and △55 sec / 100cc or less.

[0172] Meanwhile, it was confirmed that as the sodium vinylsulfonate content in the binder copolymer increased, the change in air permeability tended to decrease compared to the uncoated membrane.

[0173] This is because as the sodium vinylsulfonate content increases, the glass transition temperature of the copolymer rises, which reduces the film formation of the copolymer during membrane coating and prevents it from blocking the pores of the membrane.

[0175] The resistance of a cell with a separator prepared according to Preparation Example 3 using the binder copolymers of Examples 1 to 10 and Comparative Examples 1 to 4 evaluated by Evaluation Example 5 is shown in Table 4 below.

[0177] copolymer resistance(%) Example 1 -0.363 Example 2 -0.501 Example 3 -1.952 Example 4 -2.139 Example 5 -2.151 Example 6 -1.877 Example 7 -1.949 Example 8 -1.955 Example 9 -0.942 Example 10 +0.801 Comparative Example 1 +1.222 Comparative Example 2 +1.422 Comparative Example 3 +2.030 Comparative Example 4 +2.531

[0179] As shown in Table 4 above, the electrical resistance of the cell using the separator membrane to which the binder copolymer of Examples 1 to 10 was applied was in the range of -2.5% or more and -0.1% or less.

[0180] Specifically, the electrical resistance of the cell using the separator membrane to which the binder copolymer of Examples 1 to 10 was applied was in the range of -2.3% or more and -0.3% or less.

[0181] That is, compared to the cell with the separator with the binder copolymer of Comparative Examples 1 to 4, the electrical resistance of the cell with the separator with the binder copolymer of Examples 1 to 10 was lowered.

[0182] Meanwhile, it was confirmed that a cell with a separator membrane prepared using a binder copolymer containing sodium vinylsulfonate containing a highly electronegative atom of Examples 1 to 10 had a lower electrical resistance compared to a cell with a separator membrane prepared using a binder copolymer of Comparative Examples 1 to 4 in which sodium vinylsulfonate was not used.

[0183] In other words, it was confirmed that when a monomer unit formed by the polymerization of sodium vinylsulfonate into a binder copolymer is included, the electrical resistance of the cell tends to decrease.

[0184] This reduction in electrical resistance is due to the improved ion conductivity within the electrolyte caused by the high electronegativity of sodium vinylsulfonate and its larger particle size compared to inorganic materials.

[0186] In other words, it was confirmed that a separator with excellent substrate and electrode adhesion can be manufactured by using a copolymer containing monomer units containing sulfur atoms within the content range of the present invention as a binder.

[0187] In addition, it was found that the performance of a secondary battery can be improved by using a separator with excellent air permeability and electrical resistance characteristics, which utilizes a copolymer containing monomer units containing sulfur atoms of the present invention.

[0189] The scope of the present invention is defined by the claims set forth below rather than by the detailed description above, and all modifications or variations derived from the meaning and scope of the claims and equivalent concepts thereof should be interpreted as being included within the scope of the present invention.

Claims

Claim 1 A copolymer comprising a monomer unit containing a sulfur atom, an acrylonitrile-based monomer unit, an acrylate-based monomer unit containing a linear alkyl group having 1 to 20 carbon atoms, and a vinyl acetate-based monomer unit, wherein the monomer unit containing a sulfur atom is formed by polymerizing sodium vinylsulfonate, and the average particle size is 650 nm or more and 1,000 nm or less. Claim 2 A copolymer according to claim 1, further comprising an acrylic acid-based monomer unit, an acrylate-based monomer unit comprising a branched alkyl group having 3 to 10 carbon atoms, or a combination thereof. Claim 3 A copolymer according to claim 1, comprising, based on 100% by weight of the total weight of the copolymer, 0.1% by weight or more and 30% by weight or less of the sulfur atom-containing monomer unit; 1% by weight or more and 40% by weight or less of the acrylonitrile-based monomer unit; 1% by weight or more and 65% by weight or less of the acrylate-based monomer unit containing a linear alkyl group having 1 to 20 carbon atoms; and 1% by weight or more and 30% by weight or less of the vinyl acetate-based monomer unit. Claim 4 A copolymer according to claim 2, further comprising, based on 100% by weight of the total weight of the copolymer, 0.1% by weight or more and 20% by weight or less of the acrylic acid-based monomer unit, 0.1% by weight or more and 35% by weight or less of the acrylate-based monomer unit containing a branched alkyl group having 3 to 10 carbon atoms, or a combination thereof. Claim 5 A copolymer according to claim 1, wherein the monomer unit containing the sulfur atom comprises a monomer unit containing a sulfide group, a monomer unit containing a sulfonate group, a monomer unit containing a sulfone group, a monomer unit containing a sulfonyl ester group, a monomer unit containing a sulfonamide group, a monomer unit containing a sulfonimide group, a monomer unit containing a sulfonyl azide group, a monomer unit containing a sulfonyl hydrazide group, a monomer unit containing a sulfonyl aziridine group, a monomer unit containing a sulfonyl azitidine group, a monomer unit containing a sulfonyl carbamate group, a monomer unit containing a sulfonylurea group, a monomer unit containing a sulfonyl halide group, or a combination thereof. Claim 6 In claim 1, the acrylate-series monomer unit comprising a linear alkyl group having 1 to 20 carbon atoms is formed by polymerizing one or more selected from the group consisting of methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, butyl acrylate, butyl methacrylate, lauryl acrylate, lauryl methacrylate, stearyl acrylate, and stearyl methacrylate, and the acrylonitrile-series monomer is acrylonitrile and A copolymer formed by polymerizing one or more selected from the group consisting of methacrylonitrile. Claim 7 delete Claim 8 In paragraph 2, the above acrylic acid series monomer unit is formed by polymerizing one or more selected from the group consisting of acrylic acid and methacrylic acid, and the above acrylate series monomer unit containing a branched alkyl group having 3 to 10 carbon atoms is formed by polymerizing one or more selected from the group consisting of isopropyl acrylate, isopropyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, 2-ethylhexyl acrylate, and 2-ethylhexyl methacrylate. formed copolymer. Claim 9 The copolymer of claim 1, comprising a monomer repeating unit represented by the following chemical formula 1. [Chemical Formula 1] In the above chemical formula 1, R1, R2, and R3 are each independently hydrogen, a linear or branched hydrocarbon having 1 to 4 carbon atoms, or a combination thereof; R4 is a linear alkyl group having 1 to 20 carbon atoms; R5 is a branched alkyl group having 3 to 10 carbon atoms; R6 is hydrogen or an alkali metal; R7 is a sulfonate group; the monomer repeating unit containing R7 is formed by polymerizing sodium vinylsulfonate, 0.01≤a≤0.65, 0≤b≤0.35, 0≤c≤0.2, 0.01≤d≤0.3, 0.001≤e≤0.3, and 0.01≤f≤0.4, and a+b+c+d+e+f=1. Claim 10 A slurry composition comprising a copolymer of any one of claims 1 to 6, 8, and 9; and inorganic particles. Claim 11 A separation membrane comprising the slurry composition of claim 10. Claim 12 A secondary battery comprising the separator of Clause 11.

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