Polyolefin separator for electrochemical device and electrochemical device including the same

Abstract

The present invention provides a separator for electrochemical devices having a large number of pores and comprising a polyolefin resin, the polyolefin resin having a polydispersity index (PDI) of 2.5 to 4.2, an average pore size of 20 nm to 40 nm, and a maximum pore size of 50 nm or less, the separator for electrochemical devices having a deformation rate of 25% or less when a tensile force of 15 MPa at 60°C is applied for 60 seconds, and a time required for a recovery rate of 70% is 200 seconds or less when a tensile force of 2 MPa at 70°C is applied for 180 seconds and then removed.

Description

[Technical Field]

[0001] The present invention claims the benefit of the filing date of Korean Patent Application No. 10-2022-0072072, filed with the Korean Intellectual Property Office on June 14, 2022, the entire contents of which are incorporated herein by reference. The present invention relates to a separator for an electrochemical device based on a polyolefin resin and an electrochemical device including the same. [Background technology]

[0002] Separators for electrochemical devices such as secondary batteries typically use porous polymer film substrates based on polymer resins, such as polyolefin. Electrode assemblies are typically manufactured through a lamination process, in which the separator and electrode are bonded together using heat and pressure. The higher the heat and pressure applied during this process, the stronger the adhesion between the electrode and separator. Recently, to improve productivity, the process speed has increased, shortening the time the separator is heated. This has led to increased pressure to ensure adhesion, but this has raised concerns about deformation due to high pressure. Furthermore, the lamination process significantly reduces the thickness of the polymer film substrate, significantly damaging the pores. This reduces not only battery performance but also the separator's breakdown voltage, ultimately resulting in high-pot and low-voltage failures. Therefore, there is a need to develop porous polymer film substrates for separators that exhibit minimal deformation even under high-pressure lamination conditions. Summary of the Invention [Problem to be solved by the invention]

[0003] An object of the present invention is to provide a polyolefin separator for electrochemical devices that has improved compression resistance, a low thickness change rate and deformation rate during a lamination process for manufacturing an electrode assembly, and a high breakdown voltage.

[0004] Another object of the present invention is to provide an electrochemical device including a separator having the above-mentioned properties.

[0005] It will be readily apparent that the objects and advantages of the present invention can be realized by means or methods, and combinations thereof, as recited in the appended claims. [Means for solving the problem]

[0006] A first aspect of the present invention is a separator for an electrochemical device having a large number of pores and comprising a polyolefin resin,

[0007] The polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.2;

[0008] The average size of the pores is 20 nm to 40 nm, and the maximum size of the pores is 50 nm or less;

[0009] The separator for an electrochemical element comprises:

[0010] The deformation rate is 25% or less when a tensile force of 15 MPa is applied at 60°C for 60 seconds,

[0011] Provided is a polyolefin separator for electrochemical devices, which takes 200 seconds or less to reach a recovery rate of 70% when a tensile force of 2 MPa at 70°C is applied for 180 seconds and then removed.

[0012] A second aspect of the present invention is the method for manufacturing the semiconductor device according to the first aspect,

[0013] The polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.0;

[0014] The average size of the pores is 20 nm to 39 nm, and the maximum size of the pores is 48 nm or less;

[0015] The separator for an electrochemical element comprises:

[0016] The deformation rate is 23% or less when a tensile force of 15 MPa is applied at 60°C for 60 seconds,

[0017] Provided is a polyolefin separator for electrochemical devices, which takes 190 seconds or less to reach a recovery rate of 70% when a tensile force of 2 MPa at 70°C is applied for 180 seconds and then removed.

[0018] A third aspect of the present invention is the method according to the first or second aspect,

[0019] The polyolefin resin has a polydispersity index (PDI) of 2.6 to 3.9;

[0020] The average size of the pores is 21 nm to 38 nm, and the maximum size of the pores is 46 nm or less;

[0021] The separator for an electrochemical element comprises:

[0022] The deformation rate is 21% or less when a tensile force of 15 MPa is applied at 60°C for 60 seconds,

[0023] Provided is a polyolefin separator for electrochemical devices, which takes 180 seconds or less to achieve a recovery rate of 70% when a tensile force of 2 MPa at 70°C is applied for 180 seconds and then removed.

[0024] A fourth aspect of the present invention is the method according to the third aspect,

[0025] The average size of the pores is 22.2 nm to 36.1 nm,

[0026] The separator for an electrochemical element comprises:

[0027] The deformation rate is 20.1% or less when a tensile force of 15 MPa is applied at 60°C for 60 seconds;

[0028] Provided is a polyolefin separator for electrochemical devices, which takes 178 seconds or less to reach a recovery rate of 70% when a tensile force of 2 MPa at 70°C is applied for 180 seconds and then removed.

[0029] A fifth aspect of the present invention is any one of the first to fourth aspects,

[0030] The polyolefin resin provides a polyolefin separator for electrochemical devices having a weight-average molecular weight of 500,000 to 1,500,000.

[0031] In a sixth aspect of the present invention, in any one of the first to fifth aspects,

[0032] The polyolefin separation membrane for electrochemical devices comprises a core made of a mixture of polyethylene and polypropylene, and polyethylene skins laminated on both sides of the core.

[0033] A seventh aspect of the present invention is any one of the first to sixth aspects,

[0034] The polyolefin separator for an electrochemical device is prepared by a wet preparation method in which pores are formed by extracting a pore-forming agent.

[0035] An eighth aspect of the present invention provides an electrochemical device including an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is a polyolefin separator according to any one of the first to seventh aspects.

[0036] A ninth aspect of the present invention is the method according to the eighth aspect,

[0037] The electrochemical device is a lithium secondary battery. [Effects of the Invention]

[0038] The polyolefin separation membrane according to the present invention has a polydispersity index of the polyolefin resin, an average pore size formed in the separation membrane, and a maximum pore size controlled within a predetermined range, and the deformation rate and recovery rate of the separation membrane are controlled to be below a certain value under a predetermined condition, thereby improving the compression resistance of the separation membrane.

[0039] As a result, the separator thickness is reduced due to the pressure applied during the lamination process for manufacturing the electrode assembly. As a result, the separator of the present invention has high insulation properties without a decrease in breakdown voltage. Furthermore, even when high pressure is applied during the lamination process, the separator is less damaged, and the process speed can be increased, improving processability.

[0040] 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, or ratio of elements in the drawings described in this specification may be exaggerated to emphasize a clearer description. [Brief explanation of the drawings]

[0041] [Figure 1] 1 shows an SEM image of a cross section of the separation membrane of Example 1;

[0042] [Figure 2] 1 shows an SEM image of a cross section of the separation membrane of Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0043] The present invention will be described in detail below. Prior to that, the terms and words used in the specification and claims should not be interpreted in a limited manner to their ordinary or dictionary meanings, but should be interpreted in a manner consistent with the technical concept of the present invention, based on the principle that the inventor can appropriately define the concept of the term 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 concept of the present invention, and that various equivalents and modifications may exist as of the time of filing this application.

[0044] Throughout this specification, when a part is said to "comprise" certain elements, this does not mean that it excludes other elements, but that it can further include other elements, unless specifically stated to the contrary.

[0045] In the present specification, the characteristic of having pores means that an object contains a plurality of pores, and the pores are interconnected to allow gas and / or liquid fluids to pass from one side of the object to the other side.

[0046] The present invention relates to a separator for an electrochemical device, which may be used as a separator itself or as a component of a separator. Therefore, the separator of the present invention may have other layers disposed on at least one surface of the separator substrate, depending on the material or function. In one embodiment of the present invention, the separator may have an organic / inorganic composite coating layer containing inorganic particles and / or a binder resin formed on at least one or both surfaces of the porous substrate.

[0047] The present invention also relates to an electrochemical device including the separator. In the present invention, the electrochemical device is a device that converts chemical energy into electrical energy through an electrochemical reaction, and is a concept that encompasses primary and secondary batteries. In this specification, the secondary battery is capable of being charged and discharged, and refers to a lithium secondary battery, a nickel-cadmium battery, a nickel-metal hydride battery, etc. The lithium secondary battery uses lithium ions as an ion conductor, and examples thereof include, but are not limited to, a nonaqueous electrolyte secondary battery including a liquid electrolyte, an all-solid-state battery including a solid electrolyte, a lithium polymer battery including a gel polymer electrolyte, and a lithium metal battery using lithium metal as an anode.

[0048] In the present specification, the separator has porous properties including a large number of pores, and serves as a porous ion-conducting barrier that allows ions to pass through while blocking electrical contact between the negative electrode and the positive electrode in the electrochemical device.

[0049] The polyolefin separator for electrochemical devices according to the present invention will be described in detail below.

[0050] The polyolefin separator for electrochemical devices of the present invention is a separator for electrochemical devices having a large number of pores and containing a polyolefin resin,

[0051] The polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.2;

[0052] The average size of the pores is 20 nm to 40 nm, and the maximum size of the pores is 50 nm or less;

[0053] The separator for an electrochemical element comprises:

[0054] The deformation rate is 25% or less when a tensile force of 15 MPa is applied at 60°C for 60 seconds,

[0055] When a tensile force of 2 MPa is applied at 70°C for 180 seconds and then removed, it takes 200 seconds or less for the recovery rate to reach 70%.

[0056] In the present invention, the polyolefin separation membrane is manufactured using a polyolefin resin as a base resin. Examples of polyolefin resins include polyethylene, polypropylene, and polypentene, and the membrane may contain one or more of these. A porous separation membrane manufactured using such a polyolefin resin as a base resin, i.e., having a large number of pores, is advantageous from the viewpoint of imparting a shutdown function at an appropriate temperature. In particular, when the polyolefin resin contains both polyethylene and polypropylene, the shutdown property and physical properties such as mechanical strength can be simultaneously improved.

[0057] Generally, since a higher molecular weight of a resin is advantageous for compression resistance, the weight-average molecular weight of the polyolefin resin may be higher than conventionally, ranging from 500,000 to 1,500,000. When a mixture of different polyolefin resins is used or a separator membrane having a multi-layer structure made of different polyolefin resins is formed, the weight-average molecular weight of the polyolefin resin is calculated by adding the weight-average molecular weights of the respective polyolefin resins according to their content ratio.

[0058] In addition to the polyolefin-based resin, other resin components may be further mixed as needed, and in addition to the resin components, for example, filler particles may be included. The filler particles may be introduced for the purpose of a pressure barrier to prevent excessive reduction in the thickness, pore size, and porosity of the separator substrate due to the high pressure applied in the lamination process described below. The filler particles may include organic fillers or inorganic fillers having a predetermined particle size, and are not limited to a specific component as long as they have strength equal to or greater than that of the polyolefin resin.

[0059] In the present invention, the polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.2, an average pore size of 20 nm to 40 nm, and a maximum pore size of 50 nm or less. That is, in the present invention, the polyolefin resin has a low polydispersity index, and the average pore size and maximum pore size are small. When these ranges are satisfied simultaneously, compression resistance is improved. If the polydispersity index is less than 2.5, there are problems with reduced processability and film uniformity. If it exceeds 4.2, there are problems with reduced compression resistance. Furthermore, if the average pore size is less than 20 nm, there are problems with reduced air permeability and by-products blocking small pores during battery charge and discharge. If it exceeds 40 nm, there are problems with the separator having an uneven thickness, resulting in thickness deformation and local thickness deformation, resulting in reduced compression resistance. Furthermore, if the maximum pore size exceeds 50 nm, there is also a problem with reduced compression resistance.

[0060] The pore size can be calculated from the pore size distribution measured using a capillary flow porometer. For example, the separation membrane to be measured is first wetted with a wetting agent such as Galwick solution, and then air pressure is gradually increased on one side of the substrate. When the applied air pressure exceeds the capillary attraction of the wetting agent present in the pores, the wetting agent blocking the pores is expelled. The pore size and distribution can be measured based on the pressure and flow rate at the moment of expulsion, and the average pore size and maximum size can be determined from the measured air pressure and flow rate.

[0061] In this regard, the polyolefin resin may have a polydispersity index (PDI) of 2.5 to 4.0, more specifically 2.6 to 3.9. The average pore size may be 20 nm to 39 nm, more specifically 21 nm to 38 nm, and most specifically 22.2 nm to 36.1 nm. The maximum pore size may be 48 nm or less, more specifically 46 nm or less.

[0062] Meanwhile, in the present invention, the separator for electrochemical devices has a deformation rate of 25% or less when a tensile force of 15 MPa at 60°C is applied for 60 seconds, and simultaneously, when a tensile force of 2 MPa at 70°C is applied for 180 seconds and then removed, it takes 200 seconds or less for the recovery rate to reach 70%.

[0063] If the deformation rate under the above conditions exceeds 25% or the time required for the recovery rate to reach 70% exceeds 200 seconds, the compression resistance decreases after the lamination process with the electrode.

[0064] In this aspect, the separator for an electrochemical device is

[0065] When a tensile force of 15 MPa at 60°C is applied for 60 seconds, the deformation rate is 23% or less, and when a tensile force of 2 MPa at 70°C is applied for 180 seconds and then removed, the time required for the recovery rate to reach 70% may be 190 seconds or less;

[0066] More specifically, when a tensile force of 15 MPa is applied at 60°C for 60 seconds, the deformation rate may be 21% or less, and when a tensile force of 2 MPa is applied at 70°C for 180 seconds and then removed, the time required for the recovery rate to reach 70% may be 180 seconds or less.

[0067] More specifically, when a tensile force of 15 MPa is applied at 60°C for 60 seconds, the deformation rate may be 20.1% or less, and when a tensile force of 2 MPa is applied at 70°C for 180 seconds and then removed, the time required for the recovery rate to reach 70% may be 178 seconds or less.

[0068] The polyolefin separator may be prepared as follows, but is not limited thereto.

[0069] In one embodiment of the present invention, the separator may be manufactured by a method (wet process) in which a polyolefin resin is mixed with a plasticizer (diluent) at a high temperature to form a single phase, the polymer material and the plasticizer are phase-separated during cooling, the plasticizer is extracted to form pores, and then the mixture is stretched and heat-set. In particular, the polyolefin separator may include, but is not limited to, a core made of a mixture of polyethylene and polypropylene, and polyethylene skins laminated on both sides of the core.

[0070] The average pore size and maximum pore size of the separator can be easily prepared by a person skilled in the art to meet the scope of the present invention by adjusting the mixing ratio of the plasticizer, the stretching ratio, the heat setting temperature, etc.

[0071] In the present invention, the polyolefin separation membrane substrate prepared by the above-mentioned method may have a thickness of 5 μm to 30 μm.

[0072] Meanwhile, in one embodiment of the present invention, the separator may further include an organic / inorganic composite coating layer formed on at least one surface of the polyolefin separator substrate.

[0073] The organic / inorganic composite coating layer includes a binder resin and inorganic particles and has porous properties. In one embodiment of the present invention, the binder resin and inorganic particles in the organic / inorganic composite coating layer may be contained in a weight ratio of 1:99 to 30:70. This ratio may be appropriately adjusted within this range. For example, out of a total of 100 wt% of the binder resin and inorganic particles, the binder resin may be 1 wt% or more, 5 wt% or more, or 10 wt% or more, and the inorganic particles may be 80 wt% or more, 85 wt% or more, 90 wt% or more, or 95 wt% or more.

[0074] The organic / inorganic composite coating layer may be formed by inorganic particles bound together by a binder resin and accumulated inside the layer, and the pores inside the organic / inorganic composite coating layer may be due to interstitial volume, which is the empty space between the inorganic particles.

[0075] In one embodiment of the present invention, the porosity of the organic / inorganic composite coating layer may be 30 vol% to 70 vol%. A porosity of 70 vol% or less ensures mechanical properties that can withstand the pressing process for bonding to the electrode, and is suitable for ensuring adhesive strength because the surface opening ratio is not too high. On the other hand, a porosity of 30 vol% or more is advantageous from the viewpoint of ion permeability.

[0076] The thickness of the organic / inorganic composite coating layer may be, but is not limited to, 1 μm to 20 μm on either side of the separator substrate, and can be adjusted by those skilled in the art to an appropriate range based on heat resistance or electrical resistance.

[0077] Non-limiting examples of binder resins that can be used in the organic / inorganic composite coating layer of the present invention include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, ethylene-co-vinyl acetate, polyethylene oxide, and the like. The polymer resin may be any one selected from the group consisting of polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxyl methyl cellulose, or a mixture of two or more of these polymer resins, but is not limited thereto.

[0078] In a specific embodiment of the present invention, the inorganic particles that can be used in the organic / inorganic composite coating layer are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles that can be used in the present invention are those that can be used in the operating voltage range (e.g., Li / Li) of the applied electrochemical device. + There are no particular limitations on the material, as long as it does not undergo oxidation and / or reduction reactions at a voltage (0 V to 5 V relative to the reference voltage).

[0079] Non-limiting examples of the inorganic particles include BaTiO3, Pb(Zr,Ti)O3 (PZT), 1-x La x Zr 1-y Ti y O3(PLZT, 0 <x<1、0<y<1)、Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3 (PMN-PT), hafnia (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH)2, NiO, CaO, ZnO, ZrO2, SiO2, Y2O3, Al2O3, SiC, Al(OH)3, TiO2, aluminum peroxide, zinc tin hydroxide (ZnSn(OH)6), tin-zinc oxide (Zn2SnO4, ZnSnO3), antimony trioxide (Sb2O3), antimony tetroxide (Sb2O4), antimony pentoxide (Sb2O5), etc., and one or more of these may be included.

[0080] In addition, the average diameter of inorganic particles (D 50 Although there are no particular limitations on the thickness of the inorganic particles, it is preferable that the thickness be in the range of 0.3 μm to 1 μm in order to form a coating layer of uniform thickness and to have an appropriate porosity. If the thickness is less than 0.3 μm, the dispersibility of the inorganic particles in the slurry prepared for producing the coating layer may decrease, and if the thickness is more than 1 μm, the thickness of the formed coating layer may increase.

[0081] In one embodiment of the present invention, the organic / inorganic composite coating layer may be formed as follows. First, a polymer solution is prepared by dissolving a binder resin in an appropriate organic solvent. The solvent preferably has a solubility index similar to that of the binder polymer to be used and a low boiling point. This facilitates uniform mixing and subsequent solvent removal. Non-limiting examples of solvents that can be used include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, water, or a mixture thereof.

[0082] Next, inorganic particles are added and dispersed in the prepared polymer solution. In the present invention, the content ratio of the inorganic particles to the binder is as described above and is appropriately adjusted taking into consideration the thickness, pore size, and porosity of the final coating layer of the present invention.

[0083] Next, the inorganic particle slurry prepared as described above is coated on at least one surface of the prepared separation membrane substrate and dried. The method for coating the slurry on the surface of the separation membrane substrate is not limited to any particular method, and any conventional method known in the art can be used. For example, various methods such as dip coating, die coating, roll coating, comma coating, or a combination thereof can be used.

[0084] The drying process is performed by appropriately setting temperature and time conditions to minimize the occurrence of surface defects in the organic / inorganic composite coating layer, and drying auxiliary devices such as a drying oven or hot air may be used within an appropriate range.

[0085] When the separator includes an organic / inorganic composite porous layer, damage caused by pressure of inorganic particles on the surface of the separator substrate facing the organic / inorganic composite porous layer during the lamination process can be reduced.

[0086] The separator prepared by the above method is interposed between the anode and cathode and fabricated into an electrode assembly by a lamination process in which heat and / or pressure are applied to bond them together. In one embodiment of the present invention, the lamination process may be performed using a roll press device including a pair of pressure rollers. That is, the anode, separator, and cathode may be sequentially stacked and then inserted between the pressure rollers to achieve interlayer bonding. In this case, the lamination process may be performed using a hot press method.

[0087] The present invention also provides an electrochemical device, particularly a lithium secondary battery, including the separator. The battery includes an anode, a cathode, and a separator interposed between the anode and the cathode, and the separator includes a separator substrate having the above-described properties.

[0088] The positive electrode comprises a positive electrode current collector and a positive electrode active material layer on at least one surface of the current collector, the positive electrode active material layer including a positive electrode active material, a conductive material, and a binder resin. The positive electrode active material can be a layered compound such as lithium manganese composite oxide (LiMn2O4, LiMnO2, etc.), lithium cobalt oxide (LiCoO2), or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; 1+x Mn 2-x Lithium manganese oxides such as LiMnO4 (where x is 0 to 0.33), LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, and Cu2V2O7; and the chemical formula LiNi 1-x M x Ni-site type lithium nickel oxide represented by the chemical formula LiMnO2 (where M=Co, Mn, Al, Cu, Fe, Mg, B, or Ga, and x=0.01 to 0.3) 1-x M xLithium manganese composite oxides represented by O2 (where M = Co, Ni, Fe, Cr, Zn or Ta and x = 0.01 to 0.1) or Li2Mn3MO8 (where M = Fe, Co, Ni, Cu or Zn); LiMn2O4 in which part of Li in the chemical formula is substituted with alkaline earth metal ions; disulfide compounds; it can contain one or a mixture of two or more of Fe2(MoO4)3.

[0089] In the present invention, the negative electrode includes a negative electrode current collector and a negative electrode active material layer containing a negative electrode active material, a conductive material, and a binder resin on at least one surface of the current collector. As the negative electrode active material, the negative electrode includes carbon such as lithium metal oxide, graphitizable carbon, and graphite-based carbon; Li x Fe2O3 (0 ≤ x ≤ 1), Li x WO2 (0 ≤ x ≤ 1), Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8) and other metal composite oxides; lithium metal; lithium alloy; silicon-based alloy; tin-based alloy; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, and Bi2O5; conductive polymers such as polyacetylene; Li-Co-Ni-based materials; it can contain one or a mixture of two or more selected from titanium oxides.

[0090] In one specific embodiment of the present invention, the conductive material may be, for example, any one selected from the group consisting of graphite, carbon black, carbon or metal fiber, metal powder, conductive whiskers, conductive metal oxide, activated carbon, and polyphenylene derivatives, or a mixture of two or more of these conductive materials. More specifically, the conductive material may be any one selected from the group consisting of natural graphite, artificial graphite, Super-P, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, Denka black, aluminum powder, nickel powder, zinc oxide, potassium titanate, and titanium oxide, or a mixture of two or more of these conductive materials.

[0091] The current collector is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery. For example, stainless steel, copper, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like may be used.

[0092] The binder resin may be a polymer commonly used in electrodes in the art. Non-limiting examples of such binder resins include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polyethylhexyl acrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, ethylene-vinyl acetate copolymer, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, and cellulose acetate propionate. Examples of suitable dispersants include, but are not limited to, cyanoethyl acetatepropionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxyl methyl cellulose. In the present invention, the positive electrode slurry for preparing the positive electrode active material layer may include a dispersant, which may be a pyrrolidone-based compound. Specifically, the dispersant may be N-methylpyrrolidone (ADC-01, LG Chemicals).

[0093] In the present invention, the content of the dispersant in the positive electrode slurry may be more than 0 parts by weight and not more than 0.5 parts by weight, relative to 100 parts by weight of the positive electrode slurry. Specifically, the content of the dispersant in the positive electrode slurry may be more than 0.05 parts by weight and not more than 0.4 parts by weight, relative to 100 parts by weight of the positive electrode slurry.

[0094] In the present invention, the negative electrode slurry for preparing the negative electrode active material layer may include a dispersant, which may be a polypyrrolidone-based compound. Specifically, the dispersant may be polyvinylpyrrolidone (manufactured by Junsei Corporation).

[0095] In the present invention, the content of the dispersant in the negative electrode slurry may be more than 0 parts by weight and not more than 0.5 parts by weight, relative to 100 parts by weight of the negative electrode slurry. Specifically, the content of the dispersant in the negative electrode slurry may be more than 0.05 parts by weight and not more than 0.4 parts by weight, relative to 100 parts by weight of the negative electrode slurry.

[0096] The electrode assembly prepared as described above can be placed in a suitable case and an electrolyte injected to manufacture a battery.

[0097] In the present invention, the electrolyte solution is + B - A salt having the structure: + Li + , Na + , K. + or a combination thereof, - is PF6 - , BF4 - , Cl - , Br - , I - , ClO4 - , AsF6 - , CH3CO2 - , CF3SO3 - , N(CF3SO2)2 - , C(CF2SO2)3- or a salt containing an anion such as, 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.

[0098] The present invention also provides a battery module including a battery having the electrode assembly as a unit cell, a battery pack including the battery module, and a device including the battery pack as a power source. Specific examples of the device include, but are not limited to, power tools powered by a battery-type motor, electric vehicles including electric vehicles (EVs), hybrid electric vehicles (HEVs), and plug-in hybrid electric vehicles (PHEVs), electric two-wheeled vehicles including electric bicycles (E-bikes) and electric scooters, electric golf carts, and power storage systems.

[0099] 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.

[0100] [Example 1]

[0101] A resin blend of polyethylene (weight average molecular weight 900,000) and polypropylene (weight average molecular weight 350,000) in a weight ratio of 93:7 was used as the core, and polyethylene (weight average molecular weight 900,000) resin was placed on both sides of the core and co-extruded to form the skins. The stretching temperature was adjusted to 105°C and the heat setting temperature to 130°C, and a polyolefin separator substrate (total thickness approximately 9 μm, core thickness: 7 μm, total thickness of both skins: 2 μm) was produced using a wet method. The PDI of the resin used to manufacture the separator was calculated using Equation 1 below.

[0102] The PDI, average pore size, maximum pore size, deformation rate when a tensile force of 15 MPa at 60°C is applied for 60 seconds (referred to as recovery deformation rate), and the time required for the recovery rate to reach 70% when a tensile force of 2 MPa at 70°C is applied for 180 seconds and then removed (referred to as recovery time) of the prepared polyolefin separator are shown in Table 1 below.

[0103] [Example 2]

[0104] A separator was produced in the same manner as in Example 1, except that the stretching temperature was 120°C and the heat setting temperature was 115°C.

[0105] [Example 3]

[0106] A separator was prepared in the same manner as in Example 1, except that the heat setting temperature was adjusted to 115°C.

[0107] [Comparative Example 1]

[0108] A separation membrane was manufactured in the same manner as in Example 1, except that a resin mixture of polyethylene (weight average molecular weight 900,000) and polypropylene (weight average molecular weight 350,000) in a weight ratio of 97:3 was used as the skin portion, and the stretching temperature was adjusted to 120°C.

[0109] Comparative Example 2

[0110] A separation membrane was manufactured in the same manner as in Example 1, except that a resin mixture of polyethylene (weight average molecular weight 900,000) and polypropylene (weight average molecular weight 350,000) in a weight ratio of 97:3 was used as the skin portion, and the heat setting temperature was adjusted to 115°C.

[0111] Comparative Example 3

[0112] A separation membrane was manufactured in the same manner as in Example 1, except that a resin mixture of polyethylene (weight average molecular weight 900,000) and polypropylene (weight average molecular weight 350,000) in a weight ratio of 97:3 was used as the skin portion.

[0113] Comparative Example 4

[0114] A separator was manufactured in the same manner as in Example 1, except that a resin mixture of polyethylene (weight average molecular weight 900,000) and polypropylene (weight average molecular weight 350,000) in a weight ratio of 97:3 was used as the skin portion, and the stretching temperature was set to 120°C and the heat setting temperature was set to 115°C.

[0115] [PDI measurement]

[0116] (Equation 1) PDI = (weight average molecular weight) / (number average molecular weight)

[0117] In this case, the weight average molecular weight and number average molecular weight were determined by cutting the separation membrane to a predetermined size and analyzing it through gel permeation chromatography (GPC).

[0118] [Measurement of average and maximum pore size]

[0119] The pore size distribution was measured using a capillary flow porometer (CFP method).

[0120] [Measurement of deformation rate when a tensile force of 15 MPa is applied for 60 seconds at 60°C]

[0121] Through Dynamic Mechanical Analysis creep evaluation (DMA850, TA Instruments), stress was applied at 60°C and 15 MPa for 60 seconds to measure the deformation rate.

[0122] [Measurement of the time it takes for the recovery rate to reach 70% after applying a tensile force of 2 MPa at 70°C for 180 seconds and then removing the force]

[0123] Using a dynamic mechanical analysis (DMA850, TA Instruments), stress was applied at 70°C and 2 MPa for 180 seconds, and the time required for the recovery rate to reach 70% was measured.

[0124] [Table 1]

[0125] [Manufacturing of electrode assemblies]

[0126] 1) Manufacturing of the positive electrode

[0127] Cathode active material (LiNi 0.8 Mn 0.1 Co 0.1 O2), conductive material (carbon black), dispersant (N-methylpyrrolidone, ADC-01, LG Chemicals), and binder resin (a mixture of PVDF-HFP and PVDF) were mixed with water in a weight ratio of 97.5:0.7:0.14:1.66 to prepare a slurry for the positive electrode active material layer, with the remaining components excluding water being 50 wt%. The slurry was then applied to the surface of an aluminum thin film (10 μm thick) and dried to prepare a positive electrode having a positive electrode active material layer (120 μm thick).

[0128] 2) Manufacturing of negative electrodes

[0129] Graphite (a blend of natural and artificial graphite), conductive material (carbon black), dispersant (Polyvinylpyrrolidone, Junsei), and binder resin (a blend of PVDF-HFP and PVDF) were mixed with water in a weight ratio of 97.5:0.7:0.14:1.66 to prepare a slurry for the negative electrode active material layer, with the remaining components (excluding water) at a concentration of 50 wt%. The slurry was then applied to the surface of a copper thin film (10 μm thick) and dried to prepare a negative electrode having a negative electrode active material layer (120 μm thick).

[0130] 3) Lamination process

[0131] The prepared negative and positive electrodes were stacked with the separators of the Examples and Comparative Examples interposed therebetween, and a lamination process was performed to obtain an electrode assembly using a hot press at 70°C and 5.2 MPa for 10 seconds.

[0132] [Porosity reduction rate]

[0133] The reduction rate of the pore size was calculated based on the following formula 2.

[0134] (Equation 2) Pore size reduction rate (%) = (CD / C) * 100

[0135] In the above formula 2, C represents the average pore size of the separation membrane substrate before lamination, and D represents the average pore size of the separation membrane substrate obtained after lamination.

[0136] [Resistance measurement]

[0137] Resistance was measured by sandwiching each separator substrate between SUS plates, injecting the electrolyte into coin cells, and measuring the resistance (ER) using EIS. The frequency was set to 100,000 to 10,000 Hz. The electrolyte was a 1M LiPF6 solution mixed with a non-aqueous solvent of ethylene carbonate and ethyl methyl carbonate in a 3:7 ratio.

[0138] [Breakdown voltage]

[0139] The SUS mesh and separator substrate were hot-pressed together at 90°C, 4 MPa, and 1 s, and the voltage was increased at a rate of 100 V / s to determine the voltage at which the fail condition (>0.5 mA, 3 s) was reached. The breakdown voltage of 30 samples for each example and comparative example was measured, and the bottom 1% of voltages were determined through Weibull distribution analysis.

[0140] [Capacity maintenance rate]

[0141] The manufactured battery was repeatedly charged and discharged in the range of 2.5 V to 4.25 V at a rate of 1 C, and the ratio of the discharge capacity after each cycle to the initial discharge capacity was calculated. The capacity retention rate was evaluated at room temperature.

[0142] The results of the above measurements are shown in Table 2 below.

[0143] [Table 2]

Claims

1. A polyolefin separator for an electrochemical device having a large number of pores and containing a polyolefin resin, The polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.2; the average size of the pores is 20 nm to 40 nm, and the maximum size of the pores is 50 nm or less; The polyolefin separator for an electrochemical device comprises: The deformation rate is 25% or less when a tensile deformation force of 15 MPa is applied at 60°C for 60 seconds, When a tensile force of 2 MPa is applied at 70°C for 180 seconds and then removed, the time required for the recovery rate to reach 70% is 200 seconds or less; The polyolefin resin has a weight average molecular weight of 500,000 to 1,500,000. Polyolefin separation membrane for electrochemical elements.

2. The polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.0; the average pore size is 20 nm to 39 nm, and the maximum pore size is 48 nm or less; The polyolefin separator for an electrochemical device comprises: The deformation rate is 23% or less when a tensile deformation force of 15 MPa is applied at 60°C for 60 seconds, When a tensile force of 2 MPa is applied at 70°C for 180 seconds and then removed, the time required for the recovery rate to reach 70% is 190 seconds or less. The polyolefin separator for electrochemical devices according to claim 1 .

3. The polyolefin resin has a polydispersity index (PDI) of 2.6 to 3.9; the average pore size is 21 nm to 38 nm, and the maximum pore size is 46 nm or less; The polyolefin separator for an electrochemical device comprises: The deformation rate is 21% or less when a tensile deformation force of 15 MPa is applied at 60°C for 60 seconds, When a tensile force of 2 MPa is applied at 70°C for 180 seconds and then removed, the time required for the recovery rate to reach 70% is 180 seconds or less. The polyolefin separator for electrochemical devices according to claim 1 .

4. the average size of the pores is 22.2 nm to 36.1 nm; The polyolefin separator for an electrochemical device comprises: The deformation rate when a tensile force of 15 MPa is applied at 60°C for 60 seconds is 20.1% or less; When a tensile force of 2 MPa is applied at 70°C for 180 seconds and then removed, the time required for the recovery rate to reach 70% is 178 seconds or less. The polyolefin separator for electrochemical devices according to claim 3 .

5. 2. The polyolefin separation membrane for electrochemical devices according to claim 1, comprising a core portion made of a mixture of polyethylene and polypropylene, and polyethylene skin portions laminated on both sides of the core portion.

6. 10. The polyolefin separator for an electrochemical device according to claim 1, wherein the polyolefin separator for an electrochemical device is manufactured by a wet manufacturing method in which pores are formed by extracting a pore-forming agent.

7. An electrochemical device comprising an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, 7. An electrochemical device, wherein the separator is the polyolefin separator for electrochemical devices according to claim 1.

8. 8. The electrochemical device according to claim 7, wherein the electrochemical device is a lithium secondary battery.

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