Separator for electrochemical device containing organic / inorganic composite porous coating layer and electrochemical device containing the same

Abstract

The separator for an electrochemical device according to the present invention includes a porous polymer substrate and an organic / inorganic composite porous coating layer formed on at least one side of the polymer substrate, the organic / inorganic composite porous coating layer including a particulate binder resin and inorganic particles, the particulate binder polymer including hybrid polymer particles of a fluorine-based polymer and an acrylic polymer and acrylic polymer particles, and the organic / inorganic composite porous coating layer has a heterogeneity of composition morphology in a thickness direction such that a content ratio of the hybrid polymer particles / acrylic polymer particles present on a surface portion opposite to a surface contacting the polymer substrate is greater than a content ratio of the hybrid polymer particles / acrylic polymer particles present inside the organic / inorganic composite porous coating layer.

Description

[Technical Field]

[0001] The present invention claims the benefit of the filing dates of Korean Patent Application No. 10-2022-0054734 filed with the Korean Intellectual Property Office on May 3, 2022, and Korean Patent Application No. 10-2022-0061568 filed with the Korean Intellectual Property Office on May 19, 2022, each of which is incorporated herein in its entirety.

[0002] The present invention relates to a separator for an electrochemical device having improved adhesive strength in a dry state (dry adhesive strength) and adhesive strength when immersed in an electrolyte (wet adhesive strength). [Background technology]

[0003] Electrochemical devices such as lithium secondary batteries are typically composed of a cathode, a separator, an anode, and an electrolyte. They are high-energy density energy storage devices that can be charged and discharged by reversibly converting chemical energy and electrical energy, and are widely used in small electronic devices such as mobile phones and laptops. In recent years, in response to environmental issues, high oil prices, and the need for energy efficiency and storage, their application to hybrid electric vehicles (HEVs), plug-in electric vehicles (PEVs), electric bicycles (e-bikes), and energy storage systems (ESSs) has rapidly expanded.

[0004] Ensuring the safety of lithium secondary batteries is an important issue to be resolved during their manufacture and use. In particular, separators commonly used in electrochemical devices exhibit extreme thermal shrinkage at high temperatures due to their material properties and manufacturing process characteristics, resulting in stability issues such as internal short circuits. Recently, to ensure the safety of lithium secondary batteries, organic / inorganic composite porous separators have been proposed. These separators are made by coating a porous polymer substrate with a mixture of inorganic particles and a binder resin to form an organic / inorganic composite porous coating layer. However, when an electrode and a separator are stacked to form an electrode assembly, there is a high risk of the electrode and the separator separating from each other due to insufficient interlayer adhesion. In this case, inorganic particles detached during the separation process may cause local defects within the device. Therefore, in order to improve the adhesion between the electrode and the separator, a separator in which an acrylic polymer binder is applied to an organic / inorganic composite porous coating layer has been proposed. However, while the use of an acrylic polymer binder improves dry adhesion, there is a problem in that after application to a battery, the acrylic polymer binder can swell or be eluted by the electrolyte, resulting in a decrease in wet adhesion.

[0005] As such, there is a need to develop a separator that maintains high adhesive strength from the time of manufacturing the separator including the organic / inorganic composite porous coating layer to its application in a battery. Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide an organic / inorganic composite porous separator that has improved adhesion to an electrode in both a dry state and a wet state.

[0007] Another object of the present invention is to provide a separator that maintains high durability and insulating properties without causing inorganic particles to fall off from the organic / inorganic composite porous coating layer.

[0008] Other objects and advantages of the present invention will be apparent from the following description, and it will be readily apparent that the objects and advantages of the present invention can be realized by the means or methods recited in the claims, and combinations thereof. [Means for solving the problem]

[0009] A first aspect of the present invention relates to a separator for an electrochemical device, comprising: a porous polymer substrate; and an organic / inorganic composite porous coating layer formed on at least one side of the polymer substrate; The organic / inorganic composite porous coating layer includes a particulate binder resin and inorganic particles, the particulate binder polymer includes hybrid polymer particles of a fluorine-based polymer and an acrylic polymer, and acrylic polymer particles; the acrylic polymer constituting the hybrid polymer particles does not contain a styrene-based repeating unit, and the acrylic polymer constituting the acrylic polymer particles contains a styrene-based repeating unit; The organic / inorganic composite porous coating layer has heterogeneity in composition morphology in the thickness direction, in which the content ratio of hybrid polymer particles / acrylic polymer particles present on the surface opposite to the surface in contact with the polymer substrate is greater than the content ratio of hybrid polymer particles / acrylic polymer particles present inside the organic / inorganic composite porous coating layer.

[0010] In a second aspect of the present invention, in the first aspect, the organic / inorganic composite porous coating layer has heterogeneity in composition morphology in a thickness direction, in which the content of hybrid polymer particles present on a surface portion opposite to the surface in contact with the polymer substrate is greater than the content of hybrid polymer particles present inside the organic / inorganic composite porous coating layer.

[0011] In a third aspect of the present invention, in the second aspect, the organic / inorganic composite porous coating layer has heterogeneity in composition morphology in a thickness direction, in which the content of acrylic polymer particles present on a surface portion opposite to the surface in contact with the polymer substrate is greater than the content of acrylic polymer particles present inside the organic / inorganic composite porous coating layer.

[0012] In a fourth aspect of the present invention, in any one of the first to third aspects, the average particle size (D50) of the hybrid polymer particles is smaller than the average particle size (D50) of the acrylic polymer particles.

[0013] A fifth aspect of the present invention is the fourth aspect, wherein the hybrid polymer particles have an average particle size (D50) of 100 nm to 500 nm, and the acrylic polymer particles have an average particle size (D50) of 200 nm to 700 nm, more specifically, the hybrid polymer particles have an average particle size (D50) of 200 nm to 400 nm, and the acrylic polymer particles have an average particle size (D50) of 300 nm to 500 nm.

[0014] In a sixth aspect of the present invention, in any one of the first to fifth aspects, the mixing weight ratio of the hybrid polymer particles to the acrylic polymer particles is 8:2 to 2:8.

[0015] A seventh aspect of the present invention is any one of the first to sixth aspects, wherein the Tg of the acrylic polymer contained in the hybrid polymer particles is lower by 10°C or more than the Tg of the acrylic polymer contained in the acrylic polymer particles, more specifically, the Tg of the acrylic polymer contained in the hybrid polymer particles is 10°C to 30°C, and the Tg of the acrylic polymer contained in the acrylic polymer particles is 30°C to 50°C.

[0016] An eighth aspect of the present invention is any one of the first to seventh aspects, wherein the fluorine-based polymer is a homopolymer of vinylidene fluoride, a copolymer of vinylidene fluoride and another polymerizable monomer, or a mixture of two or more of these.

[0017] A ninth aspect of the present invention is any one of the first to eighth aspects, wherein the monomer is at least one selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, chlorofluoroethylene, 1,2-difluoroethylene, perfluoro(methyl vinyl) ether, perfluoro(ethyl vinyl) ether, perfluoro(propyl vinyl) ether, perfluoro(1,3 dioxole), perfluoro(2,2-dimethyl-1,3-dioxole), trichloroethylene, and vinyl fluoride.

[0018] A tenth aspect of the present invention is any one of the first to ninth aspects, wherein the fluorine-based polymer is a copolymer of vinylidene fluoride and hexafluoropropylene.

[0019] In an eleventh aspect of the present invention, in any one of the first to tenth aspects, the content of the monomer is 1% by weight to 20% by weight of the copolymer.

[0020] A twelfth aspect of the present invention is any one of the first to eleventh aspects, wherein the acrylic polymer constituting the hybrid polymer particles and the acrylic polymer constituting the acrylic polymer particles each independently contain an alkyl (meth)acrylate repeating unit having an alkyl group having 1 to 18 carbon atoms.

[0021] A thirteenth aspect of the present invention is any one of the first to twelfth aspects, wherein the organic / inorganic composite porous coating layer contains the particulate binder resin in a range of 1 wt % to 30 wt % of the organic / inorganic composite porous coating layer.

[0022] In a fourteenth aspect of the present invention, in any one of the first to thirteenth aspects, the inorganic particles have an average particle size (D50) in the range of 200 nm to 3 μm.

[0023] A fifteenth aspect of the present invention is any one of the first to fourteenth aspects, wherein the organic / inorganic composite porous coating layer is formed by coating a slurry in which the particulate binder resin and inorganic particles are dispersed in an aqueous dispersion medium onto at least one surface of a porous polymer substrate and drying the slurry.

[0024] A sixteenth aspect of the present invention relates to an electrochemical element, the electrochemical element comprising a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, the separator being related to any one of the first to fourteenth aspects.

[0025] In a seventeenth aspect of the present invention, in the sixteenth aspect, the electrochemical device is a lithium secondary battery. [Effects of the Invention]

[0026] The organic / inorganic composite porous coating layer of the separator according to the present invention simultaneously contains hybrid polymer particles of a fluorine-based polymer and an acrylic polymer, and acrylic polymer particles, and has heterogeneity in composition morphology in the thickness direction, in which the content ratio of the hybrid polymer particles / acrylic polymer particles present on the surface opposite to the surface in contact with the polymer substrate is greater than the content ratio of the hybrid polymer particles / acrylic polymer particles present inside the organic / inorganic composite porous coating layer.

[0027] According to the present invention, the content ratio of hybrid polymer particles / acrylic polymer particles present on the surface of the organic / inorganic composite porous coating layer opposite the surface in contact with the polymer substrate is higher than the content ratio of hybrid polymer particles / acrylic polymer particles present inside the organic / inorganic composite porous coating layer. Because the fluorine-based polymer contained in the hybrid polymer particles is insoluble in the electrolyte, even if the hybrid polymer particles also contain an acrylic polymer, they maintain their shape even when wet with the electrolyte. As a result, the separator of the present invention maintains its adhesion to the electrode without significantly reducing its adhesion to the electrode even in a wet state. Meanwhile, the acrylic polymer particles further contribute to maintaining the adhesion of the separator to the electrode in a dry state.

[0028] Therefore, in a roll-to-roll continuous process for manufacturing an electrode assembly by stacking electrodes and the separator of the present invention, the dimensional stability and processability of the electrode assembly are improved. Furthermore, when a battery is manufactured using an electrode assembly including the separator, a high binding strength can be maintained between the separator and the electrode even when the electrode is immersed in an electrolyte, preventing a decrease in interfacial resistance. Furthermore, because the binder resin particles maintain high adhesive strength in both dry and wet states, the inorganic particles contained in the organic / inorganic composite porous coating layer are well fixed without falling off, improving the dimensional stability of the separator. Therefore, the thermal safety and insulating properties of the battery are improved.

[0029] Furthermore, the acrylic polymer constituting the hybrid polymer particles does not contain a styrene-based repeating unit, while the acrylic polymer constituting the acrylic polymer particles contains a styrene-based repeating unit, thereby improving the adhesive strength between the porous polymer substrate and the organic / inorganic composite porous coating layer. [Brief explanation of the drawings]

[0030] The accompanying drawings illustrate preferred embodiments of the invention and, together with the detailed description, serve to explain the principles of the invention, but are not intended to limit the scope of the invention. Meanwhile, the shape, size, scale, and ratio of elements in the drawings described in this specification may be exaggerated to emphasize a clearer description.

[0031] [Figure 1] FIG. 1 is a schematic diagram showing a cross section of a separation membrane according to a specific embodiment of the present invention. [Figure 2a] FIG. 2a shows an SEM image of the surface of the separation membrane of Example 1 before impregnation with the electrolyte. [Figure 2b] FIG. 2b is an enlarged view of a portion of FIG. 2a. [Figure 3a] FIG. 3a shows an SEM image of the surface of the separation membrane of Comparative Example 1. [Figure 3b] FIG. 3b is an enlarged view of a portion of FIG. 3a. [Figure 4] FIG. 4 is an SEM image of a cross section of the separation membrane according to Example 1. As shown in FIG. [Figure 5] FIG. 5 is an EDS image of a cross section of the separation membrane according to Example 1. As shown in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0032] The terms and words used in this specification and claims should not be construed as being limited to their ordinary or dictionary meanings, but should be construed as meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that the inventor can appropriately define the concept of terms to best describe his or her invention. Therefore, it should be understood that the embodiments described in this specification and the configurations illustrated in the drawings are merely the most preferred embodiments of the present invention and do not represent the entire technical idea of ​​the present invention, and that various equivalents and modifications that can replace them may exist at the time of filing this application.

[0033] FIG. 1 is a schematic cross-sectional view of a separator for an electrochemical device (10) according to one specific embodiment of the present invention.

[0034] In the present invention, the separation membrane 10 comprises a porous polymer substrate 1 and an organic / inorganic composite porous coating layer 3 formed on at least one surface of the polymer substrate 1. While Fig. 1 illustrates the organic / inorganic composite porous coating layer 3 formed on only one surface of the polymer substrate 1, it is understood that the organic / inorganic composite porous coating layer 3 may also be formed on the other surface of the polymer substrate 1.

[0035] The organic / inorganic composite porous coating layer (3) includes particulate binder resins (5, 7) and inorganic particles (9). The particulate binder polymers (5, 7) include hybrid polymer particles (5) of a fluorine-based polymer and an acrylic polymer, and acrylic polymer particles (7).

[0036] In the present invention, the acrylic polymer constituting the hybrid polymer particles does not contain a styrene-based repeating unit, while the acrylic polymer constituting the acrylic polymer particles contains a styrene-based repeating unit. Specifically, the absence of a styrene-based repeating unit means that a styrene-based compound, a copolymer containing a styrene-based repeating unit, or a polymer containing a styrene-based repeating unit is not contained in the process of polymerizing the acrylic polymer constituting the hybrid polymer particles. The presence of a styrene-based repeating unit means that a styrene-based compound, a copolymer containing a styrene-based repeating unit, or a polymer containing a styrene-based repeating unit is contained in the process of polymerizing the acrylic polymer constituting the acrylic polymer particles.

[0037] As shown in FIG. 1, the organic / inorganic composite porous coating layer (3) has heterogeneity in composition morphology in the thickness direction, in that the content ratio of hybrid polymer particles (5) / acrylic polymer particles (7) present on the surface opposite to the surface in contact with the polymer substrate (1) is greater than the content ratio of hybrid polymer particles (5) / acrylic polymer particles (7) present inside the organic / inorganic composite porous coating layer (3).

[0038] In the present specification, "heterogeneity of composition morphology in the thickness direction, in which the content ratio of hybrid polymer particles / acrylic polymer particles present in the surface portion of the organic / inorganic composite porous coating layer opposite to the surface in contact with the polymer substrate is greater than the content ratio of hybrid polymer particles / acrylic polymer particles present in the interior of the organic / inorganic composite porous coating layer" should be construed to include any form in which the content ratio of hybrid polymer particles / acrylic polymer particles present in the surface portion of the organic / inorganic composite porous coating layer opposite to the surface in contact with the polymer substrate is greater than the content ratio of hybrid polymer particles / acrylic polymer particles present below (inside) the surface portion of the porous coating layer. For example, the term should be construed to include a porous coating layer formed so that the content ratio of hybrid polymer particles / acrylic polymer particles decreases linearly from the surface of the porous coating layer toward the porous substrate, a porous coating layer formed so that the content ratio of hybrid polymer particles / acrylic polymer particles decreases nonlinearly from the surface of the porous coating layer toward the porous substrate, and a porous active layer formed so that the content ratio of hybrid polymer particles / acrylic polymer particles decreases discontinuously from the surface of the porous coating layer toward the porous substrate.

[0039] Thus, the content ratio of hybrid polymer particles / acrylic polymer particles present on the surface of the organic / inorganic composite porous coating layer opposite the surface in contact with the polymer substrate is greater than the content ratio of hybrid polymer particles / acrylic polymer particles present inside the organic / inorganic composite porous coating layer. Because the fluoropolymer contained in the hybrid polymer particles is insoluble in the electrolyte, even if the acrylic polymer is also contained, the hybrid polymer particles maintain their shape even when wet with the electrolyte. As a result, the separator of the present invention maintains its adhesion to the electrode without significantly reducing its adhesion to the electrode even in a wet state. Meanwhile, the acrylic polymer particles further contribute to maintaining the adhesion of the separator to the electrode in a dry state.

[0040] The organic / inorganic composite porous coating layer (3) may have a heterogeneity in composition morphology through its thickness such that the content of hybrid polymer particles (5) present on the surface opposite to the surface in contact with the polymer substrate (1) is greater than the content of hybrid polymer particles (5) present inside the organic / inorganic composite porous coating layer (3). Also, the organic / inorganic composite porous coating layer (3) may have a heterogeneity in composition morphology through its thickness such that the content of acrylic polymer particles (7) present on the surface opposite to the surface in contact with the polymer substrate (1) is greater than the content of acrylic polymer particles (7) present inside the organic / inorganic composite porous coating layer (3).

[0041] When the hybrid polymer particles (5) and / or acrylic polymer particles (7) have the above-described heterogeneous composition morphology in the thickness direction, the polymer particles (5, 7) are more abundant on the surface opposite the surface that contacts the polymer substrate (1) than in the interior of the organic / inorganic composite porous coating layer (3). Therefore, the adhesive properties of the polymer particles present in large amounts on the surface enhance dry / wet adhesion to the electrode. Furthermore, resistance to external stimuli such as peeling and scratching is enhanced, improving lamination characteristics to the electrode. This results in excellent performance during battery assembly processes such as winding and lamination. Furthermore, the increased number of inorganic particles toward the interior enhances porosity, resulting in excellent ionic conductivity and contributing to improved battery performance.

[0042] According to a specific embodiment of the present invention, the porous polymer substrate (1) can provide a path for lithium ion migration while electrically insulating the anode and cathode to prevent short circuits and can be any material that can be used as a separator polymer substrate for an electrochemical device without any particular limitation. Examples of such separator substrates include porous polymer films or nonwoven fabrics containing one or more polymer resins, such as polyolefins such as polyethylene, polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, and polyethylene naphthalene.

[0043] In the present invention, the thickness of the polymer substrate may be 3 μm to 50 μm. The thickness of the separator substrate is not particularly limited to the above range, but if the thickness is thinner than the above lower limit, the mechanical properties may be reduced, making the separator more susceptible to damage during battery use. Meanwhile, the pore size and porosity of the separator substrate are also not particularly limited, but may be 0.01 μm to 50 μm and 10 vol% to 95 vol%, respectively.

[0044] The organic / inorganic composite porous coating layer (3) is formed by mixing a plurality of inorganic particles (9) and particulate binder resins (5, 7). By coating the polymer substrate (1) with the porous coating layer (3) containing the inorganic particles (9), the heat resistance and mechanical properties of the separator (10) can be further improved. According to a preferred embodiment of the present invention, the organic / inorganic composite porous coating layer (3) is disposed on both sides of the polymer substrate (1). By forming the porous coating layer (3) on both sides of the porous substrate (1), the wet adhesion and dry adhesion between the positive electrode and the separator and between the negative electrode and the separator can be improved.

[0045] The organic / inorganic composite porous coating layer (3) may have a microporous structure due to the interstitial volume between the constituent inorganic particles (9) and the particulate binder polymers (5, 7). The inorganic particles (9) also function as a kind of spacer that can maintain the physical shape of the porous coating layer (3). The interstitial volume refers to the space defined by the substantial surface contact between the inorganic particles (9) and the particulate binder polymers (5, 7). In addition, the inorganic particles (9) generally have the property of not changing their physical properties even at high temperatures of 200°C or higher, so the organic / inorganic composite porous coating layer (3) provides the separator (10) with excellent heat resistance. In the present invention, the organic / inorganic composite porous coating layer (3) may have a thickness in the range of 1 μm to 50 μm, or 2 μm to 30 μm, or 2 μm to 20 μm, based on the thickness formed on any one surface of the porous substrate (1).

[0046] In the present invention, the particulate binder polymers (5, 7) are binder polymers that are added in particulate form to a dispersion medium during the formation of the porous coating layer (3), and are coated and dried, thereby maintaining the particle shape of the added binder polymers. These are distinguished from non-particulate binder polymers that are coated and dried in a form dissolved in a solvent.

[0047] In one embodiment of the present invention, the particulate binder polymers (5, 7) may account for 90% by weight or more, or 99% by weight or more, of the binder components present in the porous coating layer (3). In this specification, the particulate binder polymers (5, 7) may be referred to as polymer particles, resin particles, binder particles, etc. The particulate binder polymers (5, 7) form a layered porous coating layer (3) by bonding the inorganic particles (9) to each other and to the polymer substrate (1).

[0048] In the present invention, the average particle size (D50) of the hybrid polymer particles may be smaller than the average particle size (D50) of the acrylic polymer particles. By making the average particle size (D50) of the hybrid polymer particles smaller than the average particle size (D50) of the acrylic polymer particles, it is possible to more easily form an organic / inorganic composite porous coating layer having heterogeneity in composition morphology in the thickness direction.

[0049] In one embodiment of the present invention, the hybrid polymer particles have an average particle size (D50) of 100 to 500 nm, the acrylic polymer particles have an average particle size (D50) of 200 to 700 nm, and more specifically, the hybrid polymer particles have an average particle size (D50) of 200 to 400 nm, and the acrylic polymer particles have an average particle size (D50) of 300 to 500 nm. The hybrid polymer particles and the acrylic polymer particles may be mixed in a weight ratio of 8:2 to 2:8.

[0050] Hybrid polymeric particles can be produced, for example, by reference to WO 2020 / 263936, which is incorporated herein by reference.

[0051] The fluorine-based polymer contained in the hybrid polymer particles is insoluble in the electrolyte solution and may be a homopolymer of vinylidene fluoride, a copolymer of vinylidene fluoride and another polymerizable monomer, or a mixture of two or more of these.

[0052] Examples of the vinylidene fluoride and other polymerizable monomers include, but are not limited to, one or more selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, chlorofluoroethylene, 1,2-difluoroethylene, perfluoro(methyl vinyl) ether, perfluoro(ethyl vinyl) ether, perfluoro(propyl vinyl) ether, perfluoro(1,3 dioxole), perfluoro(2,2-dimethyl-1,3-dioxole), trichloroethylene, and vinyl fluoride. In particular, the fluoropolymer may be a copolymer of vinylidene fluoride and hexafluoropropylene. The content of vinylidene fluoride and other polymerizable monomers may be, but is not limited to, 1 to 20 wt % of the copolymer.

[0053] In the present invention, the comonomer content in the PVDF-based polymer can be measured by H-NMR using a Varian 500 MHz NMR. For detailed measurement methods, see Journal of Materials Chemistry, 2012, 22, 341, or AMT-3412-0k. The NMR spectrum can be confirmed using an appropriate device, such as a Bruker Avance III HD 700 MHz NMR or a Varian 500 MHz NMR.

[0054] The acrylic polymer constituting the hybrid polymer particles and the acrylic polymer constituting the acrylic polymer particles may each independently contain an alkyl (meth)acrylate repeating unit having an alkyl group having 1 to 18 carbon atoms, but are not limited thereto. However, the acrylic polymer constituting the hybrid polymer particles does not contain a styrene-based repeating unit, and the acrylic polymer constituting the acrylic polymer particles contains a styrene-based repeating unit.

[0055] In one embodiment of the present invention, the Tg of the acrylic polymer contained in the hybrid polymer particles may be lower by 10° C. or more than the Tg of the acrylic polymer contained in the acrylic polymer particles. Since a lower Tg of the acrylic polymer is advantageous for improving adhesive strength, selecting an acrylic polymer with a low Tg contained in the hybrid polymer particles can further improve the dry / wet adhesive strength of the hybrid polymer particles to the electrodes.

[0056] More specifically, the Tg of the acrylic polymer contained in the hybrid polymer particles may be 10°C to 30°C, and the Tg of the acrylic polymer contained in the acrylic polymer particles may be 30°C to 50°C. Even if the acrylic polymer contained in the hybrid polymer particles has a low Tg, it can maintain its particulate shape at room temperature due to the hybridized fluorine-based polymer. Furthermore, when an acrylic polymer contained in the acrylic polymer particles is selected that has a Tg equal to or higher than room temperature, it can maintain its particulate shape at room temperature and exhibit electrode adhesive strength during lamination with an electrode.

[0057] The acrylic polymer may have a glass transition temperature (Tg) of 40°C or less.

[0058] More specifically, the acrylic polymer may be a polymer containing a carboxylic acid ester as a repeating unit, and preferably a (meth)acrylic acid ester. Specific examples of such a (meth)acrylic acid ester include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, n-amyl (meth)acrylate, i-amyl (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, nonyl (meth)acrylate, and (meth)acrylic acid. Examples of the acrylate include decyl (meth)acrylate, hydroxymethyl (meth)acrylate, hydroxyethyl (meth)acrylate, ethylene glycol (meth)acrylate, ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, allyl (meth)acrylate, and ethylene di(meth)acrylate, and the acrylate may be one or more selected from these. Among these, the acrylate may be one or more selected from methyl (meth)acrylate, ethyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate, and particularly preferably methyl (meth)acrylate.

[0059] The organic / inorganic composite porous coating layer may contain the particulate binder resin in a range of 1 wt % to 30 wt % of the organic / inorganic composite porous coating layer, but is not limited thereto.

[0060] The inorganic particles contained in the organic / inorganic composite porous coating layer are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles are suitable for use within the operating voltage range (e.g., Li / Li) of the electrochemical device to which they are applied. +There are no particular limitations on the inorganic particles, as long as they do not undergo oxidation and / or reduction reactions at a potential of 0 to 5 V relative to the reference voltage. In particular, when inorganic particles with ion-transfer ability are used, the ionic conductivity in the electrochemical device can be increased, thereby improving performance. Furthermore, when inorganic particles with a high dielectric constant are used as the inorganic particles, they can contribute to an increase in the degree of dissociation of the electrolyte salt, for example, lithium salt, in the liquid electrolyte, thereby improving the ionic conductivity of the electrolyte solution.

[0061] For the reasons mentioned above, the inorganic particles may include high dielectric constant inorganic particles having a dielectric constant of 5 or more, or 10 or more, inorganic particles having lithium ion transfer ability, or a mixture thereof. Non-limiting examples of inorganic particles having a dielectric constant of 5 or more include BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb 1-x La x Zr 1-y Ti y O3(PLZT, where 0 <x<1、0<y<1である)、Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3 (PMN-PT), hafnia (HfO2), SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, Y2O3, Al2O3, SiC, TiO2, etc. can be used alone or in combination of two or more. In addition, when the above-mentioned high dielectric constant inorganic particles are used in combination with inorganic particles having lithium ion transport ability, the synergistic effect can be doubled.

[0062] Non-limiting examples of the inorganic particles having lithium ion transfer ability include lithium phosphate (Li3PO4), lithium titanium phosphate (Li x Ti y (PO4)3,0 <x<2、0<y<3)、リチウムアルミニウムチタンホスフェート(Li x Al y Ti z (PO4)3,0 <x<2、0<y<1、0<z<3)、14Li2O-9Al2O3-38TiO2-39P2O5などといった(LiAlTiP) x O ySystem glass (0 < x < 4, 0 < y < 13), lithium lanthanum titanate (Li x La y TiO3, 0 < x < 2, 0 < y < 3), Li 3.25 Ge 0.25 P 0.75 S4 and the like, such as lithium germanium thiophosphate (Li x Ge y P z S w , 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), lithium nitride such as Li3N (Li x N y , 0 < x < 4, 0 < y < 2), SiS2-based glass such as Li3PO4-Li2S-SiS2 (Li x Si y S z , 0 < x < 3, 0 < y < 2, 0 < z < 4), P2S5-based glass such as LiI-Li2S-P2S5 (Li x P y S z , 0 < x < 3, 0 < y < 3, 0 < z < 7) or mixtures thereof and the like.

[0063] In the organic / inorganic composite porous coating layer, the content ratio of the inorganic particles is determined in consideration of the thickness, pore size, and porosity of the finally manufactured organic / inorganic composite porous coating layer, and the inorganic particles can be contained in the range of 70% to 99% by weight based on 100% by weight of the porous coating layer. When the content of the inorganic particles is less than 70% by weight, the heat resistance may decrease. On the other hand, when the content of the inorganic particles is too high, the amount of the binder is too small, so the adhesion of the porous coating layer may decrease.

[0064] According to a specific embodiment of the present invention, the inorganic particle size of the organic / inorganic composite porous coating layer is not limited, but may be in the range of 0.001 μm to 10 μm to form a coating layer of uniform thickness and with an appropriate porosity. For example, the inorganic particle size may be 200 nm to 3 μm, 200 nm to 2 μm, or 200 nm to 1 μm. When the inorganic particle size is within this range, dispersibility is maintained, making it easy to control the physical properties of the separator, and an increase in the thickness of the organic / inorganic composite porous coating layer can be prevented, resulting in improved mechanical properties. In addition, the likelihood of an internal short circuit occurring during battery charge and discharge due to excessively large pore size is reduced.

[0065] Meanwhile, in one embodiment of the present invention, the separator including the organic / inorganic composite porous coating layer may be prepared by mixing binder particles and inorganic particles with an aqueous dispersion medium to prepare a slurry for forming a coating layer, and then coating the slurry on at least one side of a polymer substrate.

[0066] The coating method may be dip coating, die coating, roll coating, comma coating, or a combination thereof.

[0067] In one embodiment of the present invention, the aqueous dispersion medium may include one or more of water and an alcohol having 1 to 5 carbon atoms. For example, the aqueous dispersion medium may include a mixture of water and isopropyl alcohol. By using an aqueous dispersion medium in the preparation method, the binder particles do not dissolve in the dispersion medium and are dispersed in the aqueous dispersion medium while maintaining their particle shape. For this reason, the binder particles can maintain their particle state in the prepared organic / inorganic composite porous coating layer and do not flow into the pores of the polymer substrate.

[0068] Meanwhile, in one embodiment of the present invention, the slurry for forming the coating layer is preferably controlled so that the solid content, excluding the dispersion medium, is in the range of 20 wt% to 50 wt%. Controlling the solid content, the average particle size and content ratio of the added binder particles, etc., within the above-mentioned ranges is advantageous for obtaining a separator having an organic / inorganic composite porous coating layer with heterogeneous composition morphology according to the present invention.

[0069] Meanwhile, in the present invention, the separator may be applied to an electrochemical device. The electrochemical device may include an anode and a cathode, with the separator interposed between the anode and the cathode. The electrochemical device may include any device that performs an electrochemical reaction, and specific examples include all types of primary batteries, secondary batteries, fuel cells, solar cells, and capacitors. In particular, among the secondary batteries, lithium ion secondary batteries, including lithium metal secondary batteries, lithium ion secondary batteries, lithium polymer secondary batteries, and lithium ion polymer secondary batteries, are preferred.

[0070] In one embodiment of the present invention, the electrochemical device may be manufactured according to a conventional method known in the art. According to one embodiment of the present invention, the electrochemical device may be constructed by interposing the separator between the positive electrode and the negative electrode. The electrochemical device may be manufactured by inserting an electrode assembly, in which the negative electrode, the separator, and the positive electrode are stacked, into a battery case and then injecting an electrolyte solution.

[0071] In one embodiment of the present invention, the electrode is not particularly limited, and an electrode active material can be prepared in a form bonded to an electrode current collector according to a conventional method known in the art. Among the electrode active materials, non-limiting examples of the positive electrode active material include conventional positive electrode active materials that can be used in the positive electrodes of conventional electrochemical devices, particularly lithium intercalation materials such as lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or composite oxides formed by combinations thereof. Non-limiting examples of the negative electrode active material include conventional negative electrode active materials that can be used in the negative electrodes of conventional electrochemical devices, particularly lithium intercalation materials such as lithium metal or lithium alloys, carbon, petroleum coke, activated carbon, graphite, or other carbons. Non-limiting examples of positive electrode current collectors include foils made of aluminum, nickel, or combinations thereof, and non-limiting examples of negative electrode current collectors include foils made of copper, gold, nickel, or copper alloys, or combinations thereof.

[0072] The electrolyte that can be used in the present invention is A + B - A salt having the structure: + Li + , Na + , K. + or a combination thereof, and B - is PF6 - , BF4 - , Cl - , Br - , I - , ClO4 - , AsF6 - , CH3CO2 - , CF3SO3 - , N(CF3SO2)2 - , C(CF2SO2)3 -or a combination thereof, dissolved or dissociated in an organic solvent such as, but not limited to, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma butyrolactone (γ-butyrolactone), or a mixture thereof.

[0073] The electrolyte injection may be performed at an appropriate stage during the battery manufacturing process depending on the manufacturing process and required properties of the final product. That is, it may be applied before battery assembly or at the final stage of battery assembly. In addition to the general winding process, the process of applying the electrode assembly of the present invention to a battery may include lamination (stack) and folding processes of the separator and electrodes.

[0074] The present invention will be described in detail below with reference to examples. However, the examples of the present invention may be modified in various different forms, and the scope of the present invention should not be construed as being limited to the examples detailed below. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0075] Example Example 1 A dispersion for forming a porous coating layer (solid concentration 35 wt%) was prepared by dispersing 8.2 parts by weight of hybrid polymer particles [D50: 300 nm, hybrid polymer particles composed of a copolymer of VDF and HFP polymerized in a molar ratio of 95:5 and a copolymer of ethyl acrylate and methyl methacrylate (Tg 20°C) mixed in a weight ratio of 7:3], 8.2 parts by weight of acrylic polymer particles [D50: 400 nm, copolymer of styrene and butyl acrylate (Tg 40°C)], and 80 parts by weight of inorganic particles (Al2O3, D50: 500 nm) in water.

[0076] Next, a polyethylene separator substrate (porosity 40%, thickness 9 μm) was prepared, and the dispersion was applied to both sides of the surface of the separator substrate by bar coating using a doctor blade. The substrate was then dried with hot air at 50°C using a heat gun to form a porous coating layer with a thickness of 12 μm on one side.

[0077] Figure 4 is an SEM image of a cross section of the separation membrane according to Example 1, and Figure 5 is an EDS image of a cross section of the separation membrane according to Example 1. Referring to Figure 5, the distribution of hybrid polymer particles contained in the organic / inorganic composite porous coating layer can be confirmed from an image related to F Kα1,2, which is obtained by analyzing the distribution of element F contained in the hybrid polymer particles. Meanwhile, the distribution of acrylic polymer particles can be confirmed from an image related to Ru Lα1, which is obtained by analyzing the distribution of element Ru after staining with RuO4.

[0078] Example 2 A porous coating layer was formed in the same manner as in Example 1, except that the content of the hybrid polymer particles was changed to 10.2 parts by weight, the content of the acrylic polymer particles was changed to 10.2 parts by weight, and the content of the inorganic particles was changed to 75 parts by weight to prepare a dispersion for forming a porous coating layer (solid concentration: 35% by weight).

[0079] Comparative Example 1 A porous coating layer was formed in the same manner as in Example 1, except that the hybrid polymer particles were not added and the content of the acrylic polymer particles was changed to 16.4 parts by weight to prepare a dispersion for forming a porous coating layer (solid concentration 35% by weight).

[0080] Comparative Example 2 A porous coating layer was formed in the same manner as in Example 1, except that the acrylic polymer particles were not added and the content of the hybrid polymer particles was changed to 16.4 parts by weight to prepare a dispersion for forming a porous coating layer (solid concentration 35% by weight).

[0081] Comparative Example 3 A porous coating layer was formed in the same manner as in Example 1, except that the ethyl acrylate-methyl methacrylate copolymer in the hybrid polymer particles was replaced with a styrene-butyl methacrylate copolymer to prepare a dispersion for forming a porous coating layer (solid concentration: 35 wt%).

[0082] Comparative Example 4 A porous coating layer was formed in the same manner as in Example 1, except that the acrylic polymer particles were changed from a copolymer of styrene and butyl acrylate to a copolymer of butyl acrylate to prepare a dispersion for forming a porous coating layer (solid concentration: 35 wt%).

[0083] Comparative Example 5 A porous coating layer was formed in the same manner as in Example 1, except that the ethyl acrylate and methyl methacrylate copolymer in the hybrid polymer particles was changed to a styrene and butyl methacrylate copolymer, and the styrene and butyl acrylate copolymer in the acrylic polymer particles was changed to a butyl acrylate copolymer to prepare a dispersion for forming a porous coating layer (solid concentration 35 wt%).

[0084] Measurement of average particle size D50 D50 may be defined as the particle size at 50% of the particle size distribution, measured using the laser diffraction method.

[0085] Measurement of Tg Using DSC, Tg was measured for a 25 mg sample in a nitrogen atmosphere at a temperature range of room temperature to 300°C at a heating rate of 10°C / min.

[0086] [Preparation of wet adhesion test pieces] The separator and positive electrode obtained in each example and comparative example were laminated, impregnated with 1.0 g of the electrolyte (ethylene carbonate: ethyl methyl carbonate = 7:3, content ratio, LiPF61M), and left at room temperature for 24 hours. Then, a test piece was prepared by lamination using a hot press. At this time, pressure was applied at 70 °C and 5 kgf for 5 minutes. The size of the test piece was 2 cm x 6 cm.

[0087] The positive electrode was prepared as follows: LiNi 0.8 Co 0.1 Mn 0.1 O2, PVDF, and carbon black were mixed in a weight ratio of 97.0:1.5:1.5 and dispersed in 2-methyl-2-pyrrolidone to prepare a positive electrode slurry. This was then coated on an aluminum current collector, dried, and rolled to prepare a positive electrode.

[0088] [Preparation of dry adhesive strength test pieces] The separator and negative electrode obtained in each example and comparative example were stacked and laminated using a hot press to prepare a test piece. The pressure was applied at 60°C and 6.5 MPa for 1 second. The size of the test piece was 2 cm x 6 cm.

[0089] The negative electrode was prepared as follows: graphite, SBR, and CMC were mixed in a weight ratio of 89.2:10:0.8 and dispersed in distilled water to prepare a negative electrode slurry, which was then coated on a copper current collector, dried, and rolled to prepare a negative electrode.

[0090] [Measurement of adhesive strength with electrode] The wet and dry adhesive strengths of the separators were evaluated using each test specimen prepared as described above, and the results are summarized in Table 1 below. Each test specimen was left at room temperature for 1 hour, and then the adhesive strength was measured. The adhesive strength was measured using a tensile tester (UTM device) by peeling at an angle of 180° for dry adhesive strength and 90° for wet adhesive strength.

[0091] [Table 1]

[0092] As can be seen from Table 1, the separators according to the present invention exhibited excellent wet and dry adhesion to the electrodes. Comparative Example 1 exhibited high dry adhesion, but the wet adhesion was low, which could result in reduced battery performance when actually used in a battery. Comparative Example 2 exhibited low dry adhesion, which could result in reduced adhesion during the electrode assembly manufacturing process. Furthermore, Comparative Examples 3 to 5 exhibited low wet and dry adhesion, which could result in reduced adhesion during the electrode assembly manufacturing process and reduced battery performance when actually used in a battery. However, the separators according to the examples exhibited high wet and dry adhesion, which could result in excellent electrochemical effects during the electrode assembly manufacturing process and battery operation. [Explanation of symbols]

[0093] 1: Porous base material 3: Organic / inorganic composite porous coating layer 5: Hybrid polymer particles 7: Acrylic polymer particles 9: Inorganic particles 10: Separation membrane

Claims

1. The present invention includes a porous polymer substrate and an organic / inorganic composite porous coating layer formed on at least one side of the polymer substrate, The organic / inorganic composite porous coating layer includes a particulate binder resin and inorganic particles, the particulate binder polymer includes hybrid polymer particles of a fluorine-based polymer and an acrylic polymer, and acrylic polymer particles; the acrylic polymer constituting the hybrid polymer particles does not contain a styrene-based repeating unit, and the acrylic polymer constituting the acrylic polymer particles contains a styrene-based repeating unit; The organic / inorganic composite porous coating layer has heterogeneity in composition morphology in a thickness direction, in which a content ratio of hybrid polymer particles / acrylic polymer particles present on a surface opposite to a surface in contact with the polymer substrate is greater than a content ratio of hybrid polymer particles / acrylic polymer particles present inside the organic / inorganic composite porous coating layer, the Tg of the acrylic polymer contained in the hybrid polymer particles is lower by 10° C. or more than the Tg of the acrylic polymer contained in the acrylic polymer particles; The acrylic polymer contained in the hybrid polymer particles has a Tg of 10°C to 30°C, and the acrylic polymer contained in the acrylic polymer particles has a Tg of 30°C to 50°C.

2. 10. The separator for an electrochemical device according to claim 1, wherein the organic / inorganic composite porous coating layer has a heterogeneity in composition morphology in a thickness direction, in which a content of hybrid polymer particles present on a surface opposite to a surface in contact with the polymer substrate is greater than a content of hybrid polymer particles present inside the organic / inorganic composite porous coating layer.

3. 10. The separator for an electrochemical device according to claim 1, wherein the organic / inorganic composite porous coating layer has a heterogeneity in composition morphology in a thickness direction, in which a content of acrylic polymer particles present on a surface opposite to a surface in contact with the polymer substrate is greater than a content of acrylic polymer particles present inside the organic / inorganic composite porous coating layer.

4. 2. The separator for an electrochemical device according to claim 1, wherein the hybrid polymer particles have an average particle size (D50) smaller than the average particle size (D50) of the acrylic polymer particles.

5. 5. The separator for electrochemical devices according to claim 4, wherein the hybrid polymer particles have an average particle size (D50) of 100 nm to 500 nm, and the acrylic polymer particles have an average particle size (D50) of 200 nm to 700 nm.

6. 2. The separator for an electrochemical device according to claim 1, wherein the hybrid polymer particles and the acrylic polymer particles are mixed in a weight ratio of 8:2 to 2:

8.

7. 2. The separator for an electrochemical device according to claim 1, wherein the fluorine-based polymer is a homopolymer of vinylidene fluoride, a copolymer of vinylidene fluoride and another polymerizable monomer, or a mixture of two or more thereof.

8. 8. The separator for electrochemical devices according to claim 7, wherein the monomer is at least one selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, chlorofluoroethylene, 1,2-difluoroethylene, perfluoro(methyl vinyl) ether, perfluoro(ethyl vinyl) ether, perfluoro(propyl vinyl) ether, perfluoro(1,3 dioxole), perfluoro(2,2-dimethyl-1,3-dioxole), trichloroethylene, and vinyl fluoride.

9. 8. The separator for an electrochemical device according to claim 7, wherein the fluorine-based polymer is a copolymer of vinylidene fluoride and hexafluoropropylene.

10. 8. The separator for an electrochemical device according to claim 7, wherein the content of the monomer is 1% by weight to 20% by weight of the copolymer.

11. 2. The separator for electrochemical devices according to claim 1, wherein the acrylic polymer constituting the hybrid polymer particles and the acrylic polymer constituting the acrylic polymer particles each independently comprise an alkyl (meth)acrylate repeating unit having an alkyl group having 1 to 18 carbon atoms.

12. 10. The separator for an electrochemical device according to claim 1, wherein the organic / inorganic composite porous coating layer comprises the particulate binder resin in an amount ranging from 1 wt % to 30 wt % of the organic / inorganic composite porous coating layer.

13. 2. The separator for an electrochemical device according to claim 1, wherein the inorganic particles have an average particle size (D50) in the range of 200 nm to 3 μm.

14. 2. The separator for an electrochemical device according to claim 1, wherein the organic / inorganic composite porous coating layer is formed by coating a slurry in which the particulate binder resin and inorganic particles are dispersed in an aqueous dispersion medium on at least one surface of a porous polymer substrate and then drying the coating.

15. An electrochemical element comprising a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode, An electrochemical device, wherein the separation membrane is the separation membrane according to any one of claims 1 to 14.

16. 16. The electrochemical device according to claim 15, wherein the electrochemical device is a lithium secondary battery.

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