Copolymer for separator and secondary battery including same

Abstract

The present invention relates to a copolymer containing 1 wt % to 40 wt % of monomer units containing a heterocyclic structure having 2 to 6 carbon atoms and at least one oxygen atom, based on 100 wt % of the total weight of the copolymer, and to a core-shell particle, a slurry composition, a separator, and a secondary battery containing the copolymer.

Description

[Technical Field]

[0001] The present invention relates to a copolymer, and a core-shell particle, a slurry composition, a separator, and a secondary battery containing the copolymer. [Background technology]

[0002] Due to their high energy density, lithium secondary batteries are widely used in the electrical, electronic, communications, and computer industries. Following their use as small lithium secondary batteries for portable electronic devices, their application is expanding to include high-capacity secondary batteries for hybrid and electric vehicles.

[0003] Lithium-ion secondary batteries are insulated by a separator, but internal or external abnormalities or impacts to the battery can cause a short circuit between the positive and negative electrodes, which can lead to heat generation and explosion, so ensuring the thermal and chemical safety of the separator is extremely important.

[0004] Currently, polyolefin-based films are widely used as separators, but polyolefins have drawbacks such as severe heat shrinkage at high temperatures and poor mechanical properties.

[0005] In order to improve the safety of such polyolefin-based separators, porous separators have been developed in which a polyolefin porous substrate film is coated with a mixture of inorganic particles and a binder.

[0006] That is, in order to suppress thermal shrinkage of polyolefin-based separators due to high temperatures and battery instability due to dendrites, inorganic particles are coated on one or both sides of a porous separator substrate together with a binder. This allows the inorganic particles to suppress the shrinkage rate of the substrate, and the coating layer also allows for the production of a safer separator.

[0007] Meanwhile, the electrode adhesive layer on the separator reduces the dead space between the electrodes, accelerating the transfer of lithium ions and acting as a migration path.

[0008] To achieve these electrode adhesive properties, various coating methods are used, including a single coating of a separator made by dispersing a resin in an inorganic material (Single Coated Separator / SCS), and a double-coated separator / DCS, in which a resin is coated on a separator that is already ceramic coated (Ceramic Coated Separator / CCS).

[0009] In order to reduce the volume of the coated separator and attached electrode and to protect the electrode, a lamination method using heat and pressure is used.

[0010] When multiple layers are stacked to manufacture a battery with high energy density, pressure is transferred to the inside of the battery, but heat is not transferred properly, which can reduce the adhesive strength of the coating separator and result in dead space.

[0011] To overcome this, there is a need to develop a pressure-sensitive binder (PSB) for the separation membrane.

[0012] Furthermore, in order to ensure excellent battery characteristics, the coating layer must be uniformly coated and must have strong adhesive strength to the substrate.

[0013] In particular, in high-capacity secondary batteries, electrodes and separators are stacked uniformly at high density (staking), which results in larger and more dead space inside the secondary battery during high-power charging / discharging.

[0014] The resulting dead space increases the internal resistance of the secondary battery and reduces its lifespan. By applying a resin with electrode adhesive properties to the separator, the performance degradation of the secondary battery caused by the dead space can be significantly reduced. [Prior art documents] [Patent documents]

[0015] [Patent Document 1] Korean Patent No. 10-1430975 [Patent Document 2] Korean Patent Publication No. 10-2006-0072065 Summary of the Invention [Problem to be solved by the invention]

[0016] Therefore, the present invention aims to provide a slurry composition using a copolymer that has excellent adhesive strength between inorganic materials and electrodes on a porous substrate.

[0017] In particular, the present invention provides a copolymer that is pressure-sensitive and has excellent pressure-sensitive adhesive properties under room temperature and heated (40°C to 70°C) conditions.

[0018] The present invention also provides a separator that is applied with the slurry composition and has excellent adhesive strength, breathability, and heat resistance, and a battery that uses the separator and has excellent performance.

[0019] However, the problems to be solved by the present invention are not limited to those mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0020] In one aspect of the present invention, the copolymer contains 1 wt % or more and 40 wt % or less of a monomer unit containing a heterocyclic structure having 2 to 6 carbon atoms and containing at least one oxygen atom, based on 100 wt % of the total weight of the copolymer. A copolymer is provided.

[0021] Another aspect of the invention is a core; and a shell surrounding the core; the shell comprises the copolymer; Core-shell particles are provided.

[0022] Another aspect of the present invention is a method for producing the copolymer; and Inorganic particles; A slurry composition is provided.

[0023] Yet another aspect of the present invention includes the slurry composition: A separation membrane is provided.

[0024] Another aspect of the present invention includes the separation membrane, A secondary battery is provided. [Effects of the Invention]

[0025] The copolymer of the present invention can increase the adhesive strength of inorganic materials and electrodes to the separation membrane substrate, and can improve the heat resistance and breathability of the separation membrane.

[0026] In particular, the copolymer of the present invention is sensitive to pressure and can improve the reduced pressure adhesive properties under normal temperature and heated (40°C to 70°C) conditions.

[0027] In addition, the copolymer of the present invention can reduce the defect rate and minimize the formation of dead space caused by stacking and fixing during battery assembly, and can improve ionic conductivity and air permeability in the electrolyte.

[0028] Meanwhile, when the copolymer of the present invention is coated on an inorganic coating binder (DCS) or coated alone for multipurpose use (SCS), the adhesive strength can be maximized, ultimately minimizing battery defects and improving battery performance and safety. DETAILED DESCRIPTION OF THE INVENTION

[0029] The functions and effects of the present invention will be described in more detail below through specific examples of the present invention, but these examples are presented only as examples of the present invention and do not define the scope of the invention.

[0030] Prior to this, the terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is consistent with the technical idea of ​​the present invention, in accordance with the principle that an inventor can appropriately define the concept of a term in order to best explain his or her invention.

[0031] Therefore, it should be understood that the configurations of the embodiments described in this specification are merely the most preferred embodiments of the present invention and do not represent the technical ideas of the present invention, and that at the time of filing this application, there may be various equivalents and modifications that can replace them.

[0032] In this specification, the singular expression includes the plural expression unless the context clearly dictates otherwise. In this specification, the terms "comprise," "comprise," "have," and the like are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the presence or additional possibility of one or more other features, numbers, steps, components, or combinations thereof.

[0033] In the present specification, when numerical ranges are indicated as "a to b" and "a to b," "to" and "to" are defined as ≧a and ≦b.

[0034] The copolymer of one embodiment of the present invention may contain 1 wt % to 40 wt % of monomer units containing a heterocyclic structure having 2 to 6 carbon atoms and at least one oxygen atom, based on 100 wt % of the total weight of the copolymer.

[0035] The monomer unit having a heterocyclic structure containing one or more oxygen atoms and having 2 to 6 carbon atoms can improve the adhesive strength of the copolymer and thereby improve the performance of the separator using the copolymer of the present invention.

[0036] That is, the cyclic ether structure increases the electron density of the copolymer, increasing the van der Waals force and forming hydrogen bonds with the new hydrophilic functional groups, improving adhesive strength through physical / chemical interactions with the electrode. As a result, the copolymer can exhibit reduced-pressure dry adhesive properties despite having a relatively high glass transition temperature (e.g., 4°C).

[0037] Generally, vacuum dry adhesive properties are exhibited in polymers having a glass transition temperature of -10°C or lower.

[0038] In addition, the monomer units containing a heterocyclic structure having 2 to 6 carbon atoms and one or more oxygen atoms can exhibit adhesive strength in response to pressure at room temperature and heated (40 to 70°C) conditions, even if the glass transition temperature of the copolymer is not 0°C or lower.

[0039] Meanwhile, the monomer unit having a heterocyclic structure containing one or more oxygen atoms and having 2 to 6 carbon atoms has a single bond and a ring structure, and therefore has amphiphilicity and can increase affinity with lithium ions.

[0040] In addition, the monomer unit containing a heterocyclic structure having 2 to 6 carbon atoms and one or more oxygen atoms may increase the rigidity of the copolymer due to the cyclic ether structure, thereby increasing the glass transition temperature of the copolymer.

[0041] If the content of the monomer unit having a heterocyclic structure containing one or more oxygen atoms and having 2 to 6 carbon atoms is below the range of the present invention, the air permeability of the separator may increase or the adhesive strength may decrease.

[0042] If the content of the monomer unit containing the heterocyclic structure having 2 to 6 carbon atoms and one or more oxygen atoms exceeds the range of the present invention, the polymerization reactivity may be reduced.

[0043] In one embodiment, the copolymer may further include an acrylonitrile-based monomer, an acrylate-based monomer unit, an acrylic acid-based monomer unit, a vinyl acetate-based monomer unit, and a styrene-based monomer unit.

[0044] The styrene-based monomer unit contains a benzene structure and has little interaction with the carbonate functional group constituting the electrolyte, which can contribute to reducing the swelling degree of the copolymer in the electrolyte.

[0045] Meanwhile, based on 100% by weight of the total weight of the copolymer, the copolymer may contain 3% to 15% by weight of the acrylonitrile-based monomer, 20% to 60% by weight of the acrylate-based monomer unit, 5% to 15% by weight of an acrylic acid-based monomer unit, 5% to 15% by weight of a vinyl acetate-based monomer unit, and 10% to 20% by weight of a styrene-based monomer unit.

[0046] If the content of the acrylonitrile-based monomer unit is above or below the range of the present invention, the dispersibility of the polymer particles and the dispersibility of the inorganic slurry may be reduced, or the adhesive strength may be reduced.

[0047] If the content of the acrylate-based monomer unit is above or below the range of the present invention, it may cause a decrease in adhesive strength and polymerization reactivity.

[0048] If the content of the acrylic acid-based monomer unit is above or below the range of the present invention, aggregation and precipitation of the polymer or a decrease in adhesive strength may occur.

[0049] If the content of the vinyl acetate-based monomer unit is above or below the range of the present invention, safety issues such as storage safety may occur.

[0050] If the content of the styrene-based monomer unit is above or below the range of the present invention, the adhesive strength may be reduced.

[0051] In one embodiment, the acrylate-based monomer unit is methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, butyl acrylate, butyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, ethyl hexyl acrylate, ethyl hexyl methacrylate, lauryl acrylate, lauryl methacrylate. The copolymer may be formed by polymerizing one or more selected from the group consisting of stearyl acrylate, stearyl acrylate, and stearyl methacrylate.

[0052] The acrylic acid-based monomer unit may be formed by polymerizing at least one selected from the group consisting of acrylic acid and methacrylic acid.

[0053] Meanwhile, the acrylonitrile-based monomer may be formed by polymerizing at least one selected from the group consisting of acrylonitrile and methacrylonitrile.

[0054] In addition, the vinyl acetate-based monomer unit may be formed by polymerizing a vinyl acetate monomer, and the styrene-based monomer unit may be formed by polymerizing a styrene monomer.

[0055] In one embodiment, the heterocyclic structure containing at least one oxygen atom and having 2 to 6 carbon atoms may be ethylene oxide, trimethylene oxide, furan, tetrahydrofuran, tetrahydropyran, pyran, dioxane, morpholine, or a combination thereof.

[0056] In one embodiment, the monomer having a heterocyclic structure containing an oxygen atom and having 2 to 6 carbon atoms may be formed by polymerizing tetrahydrofurfuryl acrylate, tetrahydrofurfuryl methacrylate, or a combination thereof.

[0057] In one embodiment, the acrylic acid-based monomer units may be combined with an alkali metal.

[0058] That is, the carboxylate group of the acrylic acid-based monomer unit may be bonded to an alkali metal, a hydroxide containing an alkali metal, or a combination thereof.

[0059] On the other hand, the weight ratio of the alkali metal to the copolymer (weight of the alkali metal:weight of the copolymer) may be 1 to 10:100.

[0060] If the weight ratio of the alkali metal to the copolymer is higher or lower than the weight ratio of the present invention, the adhesive properties of the separator may be reduced, particularly the adhesive strength of the separator to the electrode.

[0061] The adhesive strength of the separator containing the alkali metal may be determined by the strength of the cohesive force and repulsive force between elements.

[0062] The copolymer of the present invention may have a change in adhesive strength to inorganic materials or electrode due to changes in cohesive force, adhesive force, and repulsive force between elements depending on the content of the alkali metal.

[0063] When the alkali metal is used to prepare a binder copolymer, it improves the coating stability and overall adhesive strength by increasing the cohesive force between elements and the dispersive force within the slurry. However, if it is added in an excessive amount, it may increase the viscosity and reduce the overall adhesive strength.

[0064] Meanwhile, improvement in adhesive strength of the inorganic material means an increase in adhesive strength between the inorganic material and the substrate, which may mean that the heat resistance of the separator can be improved by increasing the amount of inorganic material.

[0065] The electrode adhesive strength of the separator can reduce the defect rate caused by stacking electrodes during battery assembly, minimize the formation of dead space, and improve ionic conductivity in the electrolyte.

[0066] In addition, when a binder layer is coated separately on an inorganic coating layer, the electrode adhesion can be maximized, which ultimately minimizes the battery defect rate and further improves battery performance.

[0067] In one embodiment, the copolymer may include a repeating monomer unit represented by the following Formula 1:

[0068] [ka] In the above formula 1, R1 to R4 are each independently hydrogen; a linear or branched hydrocarbon having 1 to 4 carbon atoms; or a combination thereof; R5 is a linear or branched hydrocarbon having 1 to 20 carbon atoms; and R6 is hydrogen, an alkali metal, or a combination thereof, and x+y+z+m+n+o=1.

[0069] In the above chemical formula 1, x, y, z, m, n, and o correspond to the weight fraction of each monomer unit, and the sum of the weight fractions of each monomer unit is 1.

[0070] In one embodiment, R1 to R4 in Formula 1 may each independently include at least one selected from the group consisting of hydrogen, methyl, and ethyl.

[0071] In addition, R5 in the above Chemical Formula 1 can be methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl, n-pentyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, 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, n-docosyl The alkyl group may include any one or more selected from the group consisting of butyl, iso-pentyl, iso-heptyl, iso-octyl, iso-nonyl, iso-decyl, iso-undecyl, iso-dodecyl, iso-tridecyl, iso-tetradecyl, iso-pentadecyl, iso-cetyl, iso-hexadecyl, iso-heptadecyl, iso-stearyl, iso-octadecyl, iso-nonadecyl, iso-icosyl, iso-heneicosyl, and iso-docosyl.

[0072] Meanwhile, improvement in adhesive strength of the inorganic material means an increase in adhesive strength between the inorganic material and the substrate, which may mean that the heat resistance of the separator can be improved by increasing the amount of inorganic material.

[0073] The electrode adhesive strength of the separator can reduce the defect rate caused by stacking electrodes during battery assembly, minimize the formation of dead space, and improve ionic conductivity in the electrolyte.

[0074] In addition, when a binder layer is coated separately on an inorganic coating layer, the electrode adhesion can be maximized, which ultimately minimizes the battery defect rate and further improves battery performance.

[0075] In one embodiment, the copolymer may be a random or block copolymer depending on the synthesis process.

[0076] In one embodiment, the copolymer may be cross-linked by a cross-linking monomer, and the cross-linking degree of the copolymer may be 80% or more.

[0077] The cross-linking monomer may be at least one selected from the group consisting of divinylbenzene, aliphatic difunctional methacrylate, and aromatic difunctional methacrylate.

[0078] The cross-linking monomer may be added in an amount of 0.01 to 5 parts by weight based on 100 parts by weight of the copolymer.

[0079] On the other hand, the degree of crosslinking may be 99% or less.

[0080] In one embodiment, the copolymer may have a water average molecular weight of 10,000 to 1,000,000.

[0081] If the water-average molecular weight of the copolymer is less than 10,000, the fluidity of the copolymer may increase, reducing dispersibility and the heat resistance of the separator.If the water-average molecular weight exceeds 1,000,000, the viscosity may be too high for use, blocking the pores of the separator and reducing the air permeability and resistance.

[0082] Core-shell particles according to another embodiment of the present invention comprise a core and a shell surrounding the core, and the shell may comprise the copolymer.

[0083] Preferably, the average particle size of the core may be between 50 and 250 nm, and the average particle size of the final core-shell particles may be between 300 and 1000 nm.

[0084] If a core is not used, the particle shape may be broken down due to the absence of a hard, durable structure, and film formation may proceed, blocking the pores of the separator, resulting in reduced air permeability and resistance.

[0085] This can also cause problems in coating the electrode adhesive layer, since if the core does not provide durability, film formation may progress and block the pores of the separator.

[0086] In one embodiment, the core may be a polymer containing, but is not limited to, an acrylate-based monomer unit, an acrylic acid-based monomer unit, or a combination thereof, and the core particle may be crosslinked.

[0087] For crosslinking the core particles, at least one crosslinking monomer selected from the group consisting of divinylbenzene, aliphatic difunctional methacrylate, and aromatic difunctional methacrylate may be used.

[0088] The degree of cross-linking of the core-shell particles may be 80% or more and 99% or less. The degree of cross-linking of the core-shell particles was measured using the same method as that for measuring the cross-linking degree of the copolymer used in the shell, except that the core-shell particles were used instead of the copolymer used in the shell.

[0089] A slurry composition according to another embodiment of the present invention may contain the copolymer and inorganic particles.

[0090] The inorganic particles may be any insulating particles without any limitation, and may preferably be high-dielectric insulating particles.

[0091] Specific examples of the 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.

[0092] The size of the inorganic particles is not particularly limited, and may be, for example, 0.01 μm to 30 μm, more preferably 0.1 μm to 10 μm, inclusive. If the average particle size of the inorganic particles is less than the preferred range, dispersibility may be reduced. If the average particle size of the inorganic particles is greater than the preferred range, the coating layer may become thick after coating, resulting in reduced mechanical properties.

[0093] The shape of the inorganic particles is not particularly limited, and may be, for example, spherical, plate-like, elliptical, or irregular.

[0094] A separation membrane according to another aspect of the present invention may include the slurry composition.

[0095] The slurry composition may be coated on at least one surface of a porous substrate film, or the slurry composition may be formed into a film and attached to a porous substrate film to prepare a separator.

[0096] Meanwhile, the separator may be used as a separator for a secondary battery, for example, a separator for a lithium secondary battery.

[0097] An example of a method for producing a separation membrane may include the steps of: (a) dissolving or dispersing the copolymer in a solvent to prepare a polymer solution; (b) adding and mixing inorganic particles into the polymer solution of step (a); and (c) coating 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) and drying.

[0098] First, 1) the copolymer is prepared in the form of a polymer solution by dissolving or dispersing it in a suitable solvent.

[0099] The solvent preferably has a solubility index similar to that of the copolymer used as the binder and a low boiling point. This facilitates uniform mixing and subsequent solvent removal. Non-limiting examples of solvents that can be used include dimethoxyethane, ethyl acetate, acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, water, or a mixture thereof. More preferably, the solvent can be dispersed in water.

[0100] 2) Inorganic particles are added to and dispersed in the prepared polymer solution to prepare a mixture of inorganic particles and polymer.

[0101] It is preferable to carry out a dispersion process of the polymer solution and inorganic particles. In this case, the appropriate dispersion time may be 10 minutes to 5 hours. As a dispersion method, a conventional method may be used, and a ball mill method is particularly preferable.

[0102] The composition of the mixture consisting of inorganic particles and polymer is not particularly limited, and the thickness, pore size, and porosity of the organic / inorganic composite porous separator of the present invention can be adjusted depending on the composition.

[0103] That is, as the ratio of inorganic particles (I) to polymer (P) (ratio = I / P) increases, the porosity of the separator increases, resulting in an increase in the thickness of the separator for the same solid content (inorganic particle weight + polymer weight). In addition, the possibility of pore formation between inorganic particles increases, increasing the pore size. However, as the size (particle size) of the inorganic particles increases, the interstitial distance between the inorganic particles also increases, resulting in an increase in pore size.

[0104] 3) The mixture of inorganic particles and polymer thus prepared is coated on a prepared polyolefin-based separation membrane substrate, and then dried to obtain the separation membrane of the present invention.

[0105] The method for coating the mixture of inorganic particles and polymer onto the polyolefin-based separation membrane substrate can be a conventional coating method known in the art, such as dip coating, die coating, roll coating, comma coating, or a combination thereof. In addition, when the mixture of inorganic particles and polymer is coated onto the polyolefin-based separation membrane substrate, it can be coated on both sides of the separation membrane substrate or selectively on only one side.

[0106] When the 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 the separator can exhibit the aforementioned improved safety in the event of an internal short circuit caused by an external impact.

[0107] The secondary battery may include a positive electrode, a negative electrode, the separator interposed between the positive electrode and the negative electrode, and an electrolyte.

[0108] The secondary battery may be manufactured by a conventional method known in the art. For example, the electrodes and a separator are assembled together, and then an electrolyte is injected into the assembly.

[0109] The electrode used with the separator is not particularly limited, and the positive electrode active material may be any of the common positive electrode active materials used in the positive electrode of a secondary battery, including, but not limited to, lithium intercalation materials such as lithiated magnesium oxide, lithiated cobalt oxide, lithiated nickel oxide, or composite oxides formed by combinations thereof. The negative electrode active material may be any of the common negative electrode active materials used in the negative electrode of a conventional electrochemical device, including, but not limited to, lithium metal or a lithium alloy and lithium intercalation materials such as carbon, petroleum coke, activated carbon, graphite, or other carbons. The above-mentioned electrode active materials are bound to a positive electrode current collector, i.e., a foil made of aluminum, nickel, or a combination thereof, and a negative electrode current collector, i.e., a foil made of copper, gold, nickel, copper alloy, or a combination thereof, to form two electrodes.

[0110] The electrolyte is a salt having a structure such as A+B-, where A+ includes an alkali metal cation such as Li+, Na+, or K+, or an ion consisting of a combination thereof, and B- includes an anion such as PF6-, BF4-, Cl-, Br-, I-, ClO4-, AsF6-, CH3CO2-, CF3SO3-, N(CF3SO2)2-, or C(CF2SO2)3-, or an ion consisting of a combination thereof. The salt may be propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide (dimethyl It is preferable that the compound is dissolved and dissociated in an organic solvent such as N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma butyrolactone (GBL), or a mixture thereof.

[0111] In applying the separator to a battery, in addition to the general winding process, processes such as stacking and folding of the separator and electrodes are possible. [Mode for carrying out the invention]

[0112] The present invention will be described in more detail below using examples, but the present invention is not limited thereto.

[0113] [Production Example 1] Production of copolymer Production of core particles 400 parts by weight of distilled water and 0.1 to 3 parts by weight of a sulfate-based emulsifier per 100 parts by weight of the monomer mixture (A) were placed in a reaction vessel and stirred, and the temperature was raised to 83°C while injecting high-purity nitrogen gas.

[0114] A decomposition initiator, ammonium persulfate, was added in an amount of 0.1 to 3 parts by weight per 100 parts by weight of the monomer mixture (A) to a reaction vessel prepared at 83°C, and a crosslinking monomer was added in an amount of 0.1 to 1 part by weight per 100 parts by weight of the monomer mixture (A), and a continuous emulsion polymerization reaction was carried out to produce core particles.

[0115] The monomer mixture (A) used was a crosslinking monomer, an acrylic acid monomer (AA) and methyl methacrylate (MMA) in a weight ratio of 1:1:98.

[0116] Fabrication of core-shell particles A reaction vessel was charged with 250 parts by weight of distilled water, 0.1-3 parts by weight of a sulfate-based emulsifier per 100 parts by weight of monomer mixture (B), and 0.1-15 parts by weight of the produced core particles, and the mixture was stirred. High-purity nitrogen gas was injected into the reaction vessel, which was then heated to 70°C. To the reaction vessel heated to 70°C, 0.1-3 parts by weight of ammonium persulfate, a decomposition initiator, per 100 parts by weight of monomer mixture (B), and 0.05 parts by weight of a crosslinking monomer per 100 parts by weight of monomer mixture (B), were added, and a continuous emulsion polymerization reaction was carried out. An aging process was carried out at 75°C for 5-12 hours to produce core-shell particles.

[0117] On the other hand, the crosslinking monomer may be added in an amount of 0.01 to 5 parts by weight per 100 parts by weight of the monomer mixture (B).

[0118] Thereafter, if necessary, the carboxylic acid moiety of the prepared copolymer was ionized by adding an aqueous solution of metal hydroxide (NaOH, LiOH, KOH).

[0119] The monomer mixture (B) was adjusted to have appropriate contents of tetrahydrofurfuryl methacrylate (THMFA), styrene (ST), acrylonitrile (AN), butyl acrylate (BA), acrylic acid (AA) and vinyl acetate (VAc).

[0120] The produced core-shell particles were used as a binder.

[0121] [Production Example 2] Production of slurry for porous membrane coating Inorganic particles (alumina (average particle size: 0.5 μm) or boehmite (average particle size: 0.7 μm)) and the core-shell particles (binder) prepared in Preparation Example 1 were mixed in a solid powder weight ratio of 80:20, and then additional distilled water was added and mixed to a solid powder concentration of 35 wt %. The mixture was thoroughly dispersed using a ball mill or a mechanical stirrer to prepare a slurry.

[0122] [Production Example 3] Production of separation membrane An inorganic coating layer is formed by applying the porous membrane coating slurry prepared in 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] After coating, the coating was dried using warm air, hot air, vacuum drying, infrared drying, or other methods, and the drying temperature range was 50 to 80°C.

[0124] The thickness of the inorganic coating layer is 1 to 6 μm on one or both sides. If the thickness is less than 1 μm, the heat resistance of the separator is significantly reduced. If the thickness is more than 6 μm, the separator is too thick, which reduces the energy density of the battery and increases the resistance.

[0125] [Examples 1 to 3 and Comparative Examples 1 to 3] Examples 1 to 3 and Comparative Examples 1 to 3 were prepared by adjusting the content of the monomers in the preparation of core-shell particles in Preparation Example 1 as shown in Table 1 below.

[0126] [Table 1] In Table 1, the monomers THMFA, ST, AN, acrylonitrile, BA, butyl acrylate, AA, and VAc represent tetrahydrofurfuryl methacrylate, acrylic acid, acrylic acid, and vinyl acetate, respectively.

[0127] THMFA is a monomer containing a heterocyclic structure with 2 to 6 carbon atoms and at least one oxygen atom.

[0128] Meanwhile, the degree of crosslinking of the copolymer used in the shell of the core-shell particles was measured by making a film using the copolymer used in the shell, measuring its weight, and then impregnating it with DEC (Diethyl carbonate) at 60°C for 24 hours, drying it, and then measuring its weight.

[0129] Hereinafter, the crosslinking rate was calculated by [(weight of the copolymer film used for the shell after impregnation and drying) / (weight of the copolymer film used for the shell before impregnation and drying)]×100.

[0130] The crosslinking degree of the copolymer used in the cells of the core-shell particles of Examples 1 to 3 was measured to be 80% or more and 99% or less.

[0131] The crosslinking rates of the core-shell particles of Examples 1 to 3 measured in the same manner using the core-shell particles of Examples 1 to 3 instead of the copolymer used in the cell were measured to be 80% or more and 99% or less.

[0132] [Evaluation Example 1] Average diameter, conversion rate and glass transition temperature of core-shell particles The average particle diameters of the core-shell particles of Examples 1 to 3 and Comparative Examples 1 to 3 were analyzed using a particle size analyzer (product name: Z3000, manufacturer: Nicomp).

[0133] The conversion rates of the core-shell particles of Examples 1 to 3 and Comparative Examples 1 to 3 were analyzed by drying 0.5 to 1 g of the core-shell particles (binder) at 180°C for 15 minutes, and then calculating the conversion rate as (actual dry weight / theoretical dry weight) x 100%.

[0134] 10 to 20 mg of the core-shell particles (binder) of Examples 1 to 3 and Comparative Examples 1 to 3 were placed in a 40 μL aluminum pan, and the glass transition temperature was analyzed by checking the thermal behavior using a DSC (Differential Scanning Calorimeter).

[0135] [Evaluation Example 2] Electrolyte swelling of core-shell particles The core-shell particles (binders) of Examples 1 to 3 and Comparative Examples 1 to 3 were dried in a 16π circular mold at 60°C for 24 hours. The prepared specimens were immersed in an electrolyte (EC:DEC=3:7+VC1%, 1.6M LiPF6) and left at 60°C for 24 hours, and the weight change rate of the specimens was measured.

[0136] [Evaluation Example 3] Dry electrode adhesion of separation membrane The separator prepared according to Preparation Example 3 using the core-shell particles (binder) of Examples 1 to 3 and Comparative Examples 1 to 3 was cut into a width of 20 mm and a length of 70 mm.

[0137] An electrode cut to a width of 25 mm and a length of 70 mm was placed on the prepared separator, and a test specimen was manufactured by applying a temperature of 65°C and a pressure of 500 kg for 10 seconds in a hot press.

[0138] The prepared specimen was attached to a UTM (1 kgf Load Cell), one side of the separator was attached to the upper clip of the tensile machine, and the tape attached to one side of the separator was attached to the lower clip, and the 180° peel strength was measured at a speed of 100 mm / min. At least five specimens were prepared per sample and measured, and the average value was calculated.

[0139] [Evaluation Example 4] Wet electrode adhesion of separation membrane The separator prepared in Preparation Example 3 using the core-shell particles (binder) of Examples 1 to 3 and Comparative Examples 1 to 3 was cut into a size of 20 mm in width and 70 mm in length.

[0140] An electrode cut to a width of 25 mm and a length of 70 mm was placed on the prepared separator, and then a temperature of 65°C and a pressure of 500 kg were applied for 10 seconds using a hot press.Then, the separator was sealed in an aluminum pouch and impregnated with the electrolyte at room temperature for 12 hours.

[0141] The prepared specimen was attached to a UTM (1 kgf Load Cell), one side of the separator was attached to the upper clip of the tensile machine, and the tape attached to one side of the separator was attached to the lower clip, and the 180° peel strength was measured at a speed of 100 mm / min. At least five specimens were prepared per sample and measured, and the average value was calculated.

[0142] [Evaluation Example 5] Electrode adhesion strength of separator at room temperature and heated (40°C to 70°C) The separator prepared according to Preparation Example 3 using the core-shell particles (binder) of Examples 1 to 3 and Comparative Example 1 was cut into a width of 20 mm and a length of 70 mm.

[0143] An electrode cut to a width of 25 mm and a length of 70 mm was placed on the prepared separator, and then a test specimen was fabricated by applying pressures of 300 kg, 400 kg, and 500 kg at temperatures of 40°C, 50°C, 60°C, 70°C, and 80°C for 10 seconds in a hot press.

[0144] The prepared specimen was attached to a UTM (1 kgf Load Cell), one side of the separator was attached to the upper clip of the tensile machine, and the tape attached to one side of the separator was attached to the lower clip, and the 180° peel strength was measured at a speed of 100 mm / min. At least five specimens were prepared per sample and measured, and the average value was calculated.

[0145] [Evaluation Example 6] Change in air permeability of separation membrane The time (time) required for 100 cc of air to pass through the separator prepared in Preparation Example 3 using the core-shell particles (binders) of Examples 1 to 3 and Comparative Examples 1 to 3 was measured using an air permeability tester.

[0146] Meanwhile, the change in air permeability is the value obtained by subtracting the air permeability of an uncoated polyolefin separator from the air permeability of the separator prepared in Preparation Example 3 using the core-shell particles (binder) of Examples 1 to 3 and Comparative Examples 1 to 3.

[0147] The average diameter, glass transition temperature and electrolyte swelling degree of the core-shell particles (binder) of Examples 1 to 3 and Comparative Examples 1 to 3 evaluated by Evaluation Examples 1 and 2 are shown in Table 2 below.

[0148] [Table 2]

[0149] As shown in Table 2 above, the core-shell particles of Examples 1 to 3 had an average diameter of 280-500 nm, a conversion rate of 90-91%, and a glass transition temperature in the range of 3-40°C.

[0150] In addition, as the THMFA content increased and the BA content decreased, the average diameter of the core-shell particles decreased and the glass transition temperature tended to increase.

[0151] On the other hand, it was confirmed that the core-shell particles of Examples 1 to 3 prepared using THMFA had a lower degree of swelling in an electrolyte than the core-shell particles of Comparative Example 1 prepared without using THMFA.

[0152] It was also confirmed that the electrolyte swelling degree tended to decrease as the ST content increased.

[0153] The electrolyte swelling degree of the core-shell particles of Examples 1 to 3 was in the range of 600 to 900%.

[0154] The dry electrode adhesion and wet electrode adhesion of the separators prepared in Preparation Example 3 using the core-shell particles (binders) of Examples 1 to 3 and Comparative Examples 1 to 3, which were evaluated in Evaluation Examples 3 and 4, are shown in Table 3 below.

[0155] [Table 3] As shown in Table 3 above, it was confirmed that the separators employing the core-shell particles of Examples 1 to 3 prepared using THMFA had higher dry electrode adhesion and wet electrode adhesion than the separators employing the core-shell particles of Comparative Example 1 prepared without using THMFA or the core-shell particles of Comparative Examples 2 and 3 in which the ST content was adjusted.

[0156] In particular, it was confirmed that in the separators using the core-shell particles of Comparative Examples 1 to 3, the wet electrode adhesive strength was almost completely lost.

[0157] The room temperature and heated electrode adhesive strengths of the separators prepared in Preparation Example 3 using the core-shell particles (binders) of Examples 1 to 3 and Comparative Examples 1 to 3, which were evaluated in Evaluation Example 5, are shown in Table 4 below.

[0158] [Table 4] As shown in Table 4 above, the separators employing the core-shell particles of Examples 1 to 3 prepared using THMFA were found to have higher electrode adhesion strength under the same temperature and pressure conditions than the separator employing the core-shell particles of Comparative Example 1 prepared without using THMFA.

[0159] In particular, the core-shell particles of Example 1 exhibited room temperature and reduced pressure adhesive properties despite having a glass transition temperature of 4°C.

[0160] On the other hand, as the content of THMFA increases, the glass transition temperature increases and the pressure-reducing properties at room temperature disappear, but the pressure-reducing properties appear when heated, and it was confirmed that the adhesive strength improves.

[0161] The change in air permeability of the separators prepared in Preparation Example 3 using the core-shell particles (binders) of Examples 1 to 3 and Comparative Examples 1 to 3, evaluated in Evaluation Example 6, is shown in Table 5 below.

[0162] [Table 5] As shown in Table 5 above, the air permeability of the separators using the core-shell particles of Examples 1 to 3 prepared using THMFA showed a larger change in air permeability than the core-shell particles of Comparative Example 1 prepared without using THMFA.

[0163] It was also confirmed that the lower the THFMA content, the greater the change in air permeability.

[0164] That is, it can be seen that as the content of THFMA increases, the glass transition temperature increases but the change in air permeability decreases.

[0165] This is because the lower the glass transition temperature, the more difficult it is for the particle shape to be maintained at room temperature, and when heated to coat the separator, the particle melts easily and forms a film, which can clog the pores of the separator.

[0166] On the other hand, if the glass transition temperature is high, the particle shape is maintained even when heat is applied and the pores of the separation membrane are not blocked, so that the change in air permeability can be reduced.

[0167] That is, it was confirmed that a separator with excellent adhesive strength and breathability can be prepared by using a binder copolymer containing THMFA within the content range of the present invention.

[0168] In particular, the binder copolymer of the present invention can have pressure sensitive adhesive properties.

[0169] In addition, it was found that the separator having excellent heat resistance and adhesive properties, which uses a binder copolymer containing THMFA within the content range of the present invention, can improve the performance and safety of secondary batteries.

[0170] The scope of the present invention is indicated by the claims that follow rather than by the above detailed description, and all modifications and variations that fall within the meaning and scope of the claims and their equivalents should be interpreted as being included within the scope of the present invention. [Industrial Applicability]

[0171] The copolymer of the present invention can increase the adhesive strength of inorganic materials and electrodes to the separation membrane substrate, and can improve the heat resistance and breathability of the separation membrane.

[0172] In particular, the copolymer of the present invention is sensitive to pressure and can improve the reduced pressure adhesive properties under normal temperature and heated (40°C to 70°C) conditions.

[0173] In addition, the copolymer of the present invention can reduce the defect rate and minimize the formation of dead space caused by stacking and fixing during battery assembly, and can improve ionic conductivity and air permeability in the electrolyte.

[0174] Meanwhile, when the copolymer of the present invention is coated on an inorganic coating binder (DCS) or coated alone for multipurpose use (SCS), the adhesive strength can be maximized, ultimately minimizing battery defects and improving battery performance and safety.

Claims

1. The copolymer contains 1 wt% to 40 wt% of monomer units containing a heterocyclic structure having 2 to 6 carbon atoms and containing at least one oxygen atom, based on 100 wt% of the total weight of the copolymer. Copolymer.

2. Further containing acrylonitrile-based monomers, acrylate-based monomer units, acrylic acid-based monomer units, vinyl acetate-based monomer units and styrene-based monomer units, The copolymer of claim 1.

3. Based on 100% by weight of the total weight of the copolymer, the copolymer contains 3% by weight or more and 15% by weight or less of the acrylonitrile-based monomer, 20% by weight or more and 60% by weight or less of the acrylate-based monomer unit, 5% by weight or more and 15% by weight or less of the acrylic acid-based monomer unit, 5% by weight or more and 15% by weight or less of the vinyl acetate-based monomer unit, and 5% by weight or more and 20% by weight or less of the styrene-based monomer unit. The copolymer of claim 2.

4. The acrylate monomer units include methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl acrylate, propyl methacrylate, isopropyl acrylate, isopropyl methacrylate, butyl acrylate, butyl methacrylate, and sec-butyl acrylate. acrylate, sec-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, ethylhexyl acrylate, ethylhexyl methacrylate, lauryl acrylate, lauryl methacrylate, stearyl acrylate, and stearyl methacrylate. is formed by polymerizing one or more selected from the group consisting of The acrylic acid-based monomer unit is formed by polymerizing at least one selected from the group consisting of acrylic acid and methacrylic acid, The acrylonitrile-based monomer is formed by polymerizing at least one selected from the group consisting of acrylonitrile and methacrylonitrile. The copolymer of claim 2.

5. The heterocyclic structure containing at least one oxygen atom and having 2 to 6 carbon atoms is ethylene oxide, trimethylene oxide, furan, tetrahydrofuran, tetrahydropyran, pyran, dioxane, morpholine, or a combination thereof. The copolymer of claim 1.

6. The monomer unit containing a heterocyclic structure having 2 to 6 carbon atoms and an oxygen atom is formed by polymerizing tetrahydrofurfuryl acrylate, tetrahydrofurfuryl methacrylate, or a combination thereof. The copolymer of claim 1.

7. The acrylic acid-based monomer unit is bonded to an alkali metal. The copolymer of claim 2.

8. The monomer repeat unit is represented by the following formula 1: The copolymer of claim 1. 【Chemical 1】 In the above formula 1, R １ Or R ４ are each independently hydrogen; a linear or branched hydrocarbon having 1 to 4 carbon atoms; or a combination thereof; R ５ is a linear or branched hydrocarbon having 1 to 20 carbon atoms, R ６ is hydrogen, an alkali metal, or a combination thereof; x+y+z+m+n+o=1.

9. The copolymer is a random or block copolymer. The copolymer of claim 1.

10. The crosslinking degree of the copolymer is 80% or more. The copolymer of claim 1.

11. core; a shell surrounding the core; The shell comprises a copolymer according to any one of claims 1 to 10. Core-shell particles.

12. The copolymer according to any one of claims 1 to 10; and inorganic particles; Slurry composition.

13. The slurry composition of claim 12, Separation membrane.

14. The separation membrane according to claim 13, Secondary battery.

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