Polyolefin separator for electrochemical devices and electrochemical device comprising the same

Abstract

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

Description

TECHNICAL FIELD

[0001] This application claims priority to Korean Patent Application No. 10-2022-0072072, filed on June 14, 2022, in the Korean Intellectual Property Office, the content of which is incorporated herein in its entirety. The present invention relates to a polyolefin resin-based separator for an electrochemical device and an electrochemical device comprising the same. BACKGROUND

[0002] A film substrate based on a polymer resin such as polyolefin having a plurality of pores is used as a separator for an electrochemical device such as a secondary battery. Generally, an electrode assembly is manufactured through a lamination process in which a separator and an electrode are bonded by heat and pressure. The higher the heat and pressure applied in this process, the higher the bonding force between the electrode and the separator. Recently, to improve productivity by increasing the processing speed, the time to apply heat to the separator is shortened, and thus, to secure the bonding force, the bonding force is secured by increasing the pressure, but such an increased pressure can deform the separator. In addition, in the lamination process, the thickness of the polymer film substrate is greatly reduced, and the damage to the pores is increased, thereby not only reducing the performance of the battery, but also reducing the dielectric breakdown voltage of the separator, resulting in hi-pot defects and low voltage defects. Therefore, it is necessary to develop a porous polymer film substrate for a separator that is less deformed even under high pressure lamination conditions. SUMMARY

[0003] TECHNICAL PROBLEM

[0004] An object of the present invention is to provide a polyolefin separator for an electrochemical device having a low thickness change rate or strain and a high dielectric breakdown voltage in a lamination process of manufacturing an electrode assembly by improving compression resistance.

[0005] Another object of the present invention is to provide an electrochemical device comprising a separator having the above characteristics.

[0006] It is apparent that the objects and advantages of the present invention can be achieved by the means or combinations thereof described in the claims.

[0007] TECHNICAL SOLUTION

[0008] A first aspect of the present invention provides a separator for an electrochemical device comprising a polyolefin resin, the separator having a plurality of pores, wherein the polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.2, the average pore size of the pores is in the range of 20 nm to 40 nm, and the maximum pore size is 50 nm or less,

[0009] The polyolefin separator for an electrochemical device has a strain rate of 25% or less when a tensile stress of 15 MPa is applied for 60 seconds at 60°C, and a recovery time of 200 seconds or less is required for a recovery rate to reach 70% when a tensile stress of 2 MPa is applied for 180 seconds at 70°C and the tensile stress is removed.

[0010] According to a second aspect of the present application, there is provided the polyolefin separator for an electrochemical device according to the first aspect, wherein the polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.0, the average pore size of the pores is in the range of 20 nm to 39 nm, and the maximum pore size is 48 nm or less, and

[0011] The polyolefin separator for an electrochemical device has a strain rate of 23% or less when a tensile stress of 15 MPa is applied for 60 seconds at 60°C, and a recovery time of 190 seconds or less is required for a recovery rate to reach 70% when a tensile stress of 2 MPa is applied for 180 seconds at 70°C and the tensile stress is removed.

[0012] According to a third aspect of the present application, there is provided the polyolefin separator for an electrochemical device according to the first aspect or the second aspect, wherein the polyolefin resin has a polydispersity index (PDI) of 2.6 to 3.9, and the average pore size of the pores is in the range of 21 nm to 38 nm, and the maximum pore size is 46 nm or less, and

[0013] The polyolefin separator for an electrochemical device has a strain rate of 21% or less when a tensile stress of 15 MPa is applied for 60 seconds at 60°C, and a recovery time of 180 seconds or less is required for a recovery rate to reach 70% when a tensile stress of 2 MPa is applied for 180 seconds at 70°C and the tensile stress is removed.

[0014] According to a fourth aspect of the present application, there is provided the polyolefin separator for an electrochemical device according to the third aspect, wherein the average pore size of the pores is in the range of 22.2 nm to 36.1 nm, and

[0015] The polyolefin separator for an electrochemical device has a strain rate of 20.1% or less when a tensile stress of 15 MPa is applied for 60 seconds at 60°C, and a recovery time of 178 seconds or less is required for a recovery rate to reach 70% when a tensile stress of 2 MPa is applied for 180 seconds at 70°C and the tensile stress is removed.

[0016] According to a fifth aspect of the present application, there is provided the polyolefin separator for an electrochemical device according to any one of the first aspect to the fourth aspect, wherein the polyolefin resin has a weight average molecular weight of 500,000 to 1,500,000.

[0017] According to a 6th aspect of the present application, there is provided the polyolefin separator for an electrochemical device according to any one of the 1st to 5th aspects, wherein the polyolefin separator comprises:

[0018] a core portion comprising a mixture of polyethylene and polypropylene, and

[0019] a polyethylene skin portion laminated on both sides of the core portion.

[0020] According to a 7th aspect of the present application, there is provided the polyolefin separator for an electrochemical device according to any one of the 1st to 6th aspects, wherein the polyolefin separator is manufactured by a wet manufacturing method in which a pore former is extracted to form pores.

[0021] An 8th aspect of the present application provides an electrochemical device having an electrode assembly, wherein the electrode assembly comprises a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is the polyolefin separator according to any one of the 1st to 7th aspects.

[0022] According to a 9th aspect of the present application, there is provided the electrochemical device according to the 8th aspect, wherein the electrochemical device is a lithium secondary battery.

[0023] Advantageous Effects

[0024] The polyolefin separator according to the present application can suppress the strain rate and the recovery rate of the separator under predetermined conditions to be below predetermined values by controlling the polydispersity index of the polyolefin resin and the average pore size and the maximum pore size of the pores formed in the separator within predetermined ranges, thereby improving the compression resistance of the separator.

[0025] Accordingly, the thickness reduction rate of the separator due to the pressure applied in the lamination process for manufacturing the electrode assembly can be low. Therefore, the separator of the present application can not reduce the dielectric breakdown voltage and can have high insulation performance. Furthermore, even if a high pressure is applied in the lamination process, the damage to the separator is small, and the processing speed can be improved, thereby improving the processability. BRIEF DESCRIPTION OF DRAWINGS

[0026] The accompanying drawings, which are included to provide a further understanding of the application and are incorporated in and constitute a part of this specification, illustrate embodiments of the application and together with the description serve to explain the principles of the application. In the drawings:

[0027] Figure 1 SEM images of the cross section of the separator of Example 1 are shown; and

[0028] Figure 2 SEM images of the cross section of the separator of Comparative Example 1 are shown. DETAILED DESCRIPTION

[0029] Hereinafter, the present application will be described in detail. Before proceeding with the description, the terms or words used in the specification and claims should not be construed as common or dictionary meanings, but should be interpreted as meanings and concepts in accordance with the technical idea of the present application on the basis of the principle that the inventor is able to properly define the concept of the terms so as to describe his application in the best way. Therefore, the embodiments described herein and the configurations described in the drawings are merely the most preferred embodiments of the present application, and do not represent the entire technical idea of the present application, so it should be understood that there can be a variety of equivalents and modifications that can replace these at the time of filing the present application.

[0030] Throughout the specification, unless otherwise specified, when a part "comprises" a certain constituent element, it means that other constituent elements can also be included, rather than excluding other constituent elements.

[0031] In the present specification, a feature having a hole means that a gaseous and / or liquid fluid can pass therethrough, and an object containing a structure in which the holes are connected to each other transmits the fluid from one side to the other side of the object.

[0032] The present application relates to a separator for an electrochemical device, which can be applied to a separator itself or one constituent element of a separator. Therefore, in the separator of the present application, other layers can exist on at least one surface of a separator substrate, as necessary, in consideration of materials or functions. In one embodiment of the present application, the separator can have an organic / inorganic composite coating layer containing inorganic particles and / or a binder resin on at least one side or both sides of a porous substrate.

[0033] Further, the present application relates to an electrochemical device containing the separator. In the present application, the electrochemical device is a device that converts chemical energy into electrical energy through an electrochemical reaction, and is a concept including primary batteries and secondary batteries. In the present specification, the secondary battery can be charged and discharged, and refers to a lithium secondary battery, a nickel-cadmium battery, a nickel-hydrogen battery, etc. The lithium secondary battery uses lithium ions as an ion conductor, and can include a non-aqueous electrolyte secondary battery containing a liquid electrolyte, a full solid-state battery containing a solid electrolyte, a lithium polymer battery containing a gel polymer electrolyte, and a lithium metal battery using lithium metal as a negative electrode, etc., but is not limited thereto.

[0034] In the present specification, the separator has a porous property containing a plurality of holes, and functions as an ion conduction barrier to transfer ions while blocking electrical contact between a positive electrode and a negative electrode in an electrochemical device.

[0035] Hereinafter, a polyolefin separator for an electrochemical device according to the present application will be described in detail.

[0036] The polyolefin separator for an electrochemical device of the present application is a separator for an electrochemical device having a plurality of pores and containing a polyolefin resin, wherein the polyolefin resin has a polydispersity index (PDI) of 2.5 to 4.2, and the average pore size of the pores is in the range of 20 nm to 40 nm, and the maximum pore size is 50 nm or less.

[0037] The strain rate of the polyolefin separator for an electrochemical device at 60°C under a tensile stress of 15 MPa applied for 60 seconds is 25% or less, and the recovery time required for the recovery rate to reach 70% after the tensile stress is removed after a tensile stress of 2 MPa is applied at 70°C for 180 seconds is 200 seconds or less.

[0038] In the present application, the polyolefin separator is manufactured using a polyolefin resin as a base resin. Examples of the polyolefin resin can include polyethylene, polypropylene, polyamylene, etc., and can contain one or more of these resins. A porous separator, i.e., a separator having a plurality of pores, manufactured using such a polyolefin resin as a base resin is advantageous in providing a closed pore function at an appropriate temperature. In particular, when polyethylene and polypropylene are simultaneously contained as the polyolefin resin, physical properties such as closed pore characteristics and mechanical strength can be simultaneously improved.

[0039] Generally, the higher the molecular weight of the resin, the more advantageous it is in terms of compression resistance, so the weight average molecular weight of the polyolefin resin can be 500,000 to 1,500,000, which is higher than before. When a mixture of different kinds of polyolefin resins or a multi-layer structure containing different kinds of polyolefin resins is used to form the separator, the weight average molecular weight of the polyolefin resin is calculated by adding the weight average molecular weights according to the content ratio of each polyolefin resin.

[0040] In addition to containing the above-described polyolefin base resin, other resin components can be further mixed as needed, and in addition to containing the resin components, for example, filler particles can be contained. The filler particles can be introduced for the purpose of a pressure barrier, such that the thickness of the separator substrate, the size of the pores, and the porosity are not excessively reduced with respect to the high pressure applied in the lamination process described later. The filler particles can include organic fillers or inorganic fillers having a predetermined particle diameter, and are not limited to a specific component as long as having a strength greater than or equal to that of the polyolefin resin.

[0041] In the present application, the polyolefin resin has a polydispersity index (PDI) in the range of 2.5 to 4.2, an average pore size in the range of 20 nm to 40 nm, and a maximum pore size of 50 nm or less. That is, in the present application, the polyolefin resin has a low polydispersity index, and the average pore size and the maximum pore size are small. When these ranges are satisfied simultaneously, the compression resistance is improved. When the polydispersity index is less than 2.5, there can be a problem of a decrease in processability and deterioration in film uniformity, and when the polydispersity index exceeds 4.2, there can be a problem of a decrease in compression resistance. In addition, when the average pore size is less than 20 nm, the gas permeability can decrease, and there can be a problem of clogging of small pores by by-products at the time of charge and discharge of the battery. When the average pore size is greater than 40 nm, there can be a problem of uneven thickness of the separator, causing thickness deformation and a decrease in compression resistance due to local thickness deformation. In addition, when the maximum pore size exceeds 50 nm, there can also be a problem of a decrease in compression resistance.

[0042] The size of the pores can be calculated from the measured pore size distribution using a capillary flow porometry method. For example, first, after wetting the separator to be measured with a wetting agent such as a Galwick solution, the air pressure on the substrate side is gradually increased. At this time, when the applied air pressure is greater than the capillary force of the wetting agent present in the pores, the wetting agent clogging the pores is pushed out, and the size and distribution of the pores are measured by the pressure and flow rate at the moment of being pushed out. From this, the average pore size (size) and the maximum pore size can be determined.

[0043] In one aspect, the polydispersity index (PDI) of the polyolefin resin can be 2.5 to 4.0, more specifically 2.6 to 3.9. In addition, the average pore size can be 20 nm to 39 nm, more specifically 21 nm to 38 nm, most specifically 22.2 nm to 36.1 nm. In addition, the maximum pore size can be 48 nm or less, more specifically 46 nm or less.

[0044] On the other hand, in the present application, the polyolefin separator for electrochemical devices has a strain rate of 25% or less when a tensile stress of 15 MPa is applied for 60 seconds at 60°C, and at the same time, when a tensile stress of 2 MPa is applied for 180 seconds at 70°C and then the tensile stress is removed, the time taken until the recovery rate reaches 70% is 200 seconds or less.

[0045] When the strain rate under the above conditions exceeds 25% or the time taken until the recovery rate reaches 70% exceeds 200 seconds, the compression resistance after the lamination process with the electrode deteriorates.

[0046] In one aspect, the electrochemical device can have a strain rate of 23% or less when a tensile stress of 15 MPa is applied to the polyolefin separator at 60°C for 60 seconds, and a recovery time of 190 seconds or less can be required for the recovery rate to reach 70% when a tensile stress of 2 MPa is applied to the polyolefin separator at 70°C for 180 seconds and then the tensile stress is removed.

[0047] More specifically, the strain rate can be 21% or less when a tensile stress of 15 MPa is applied at 60°C for 60 seconds, and the recovery time taken until the recovery rate reaches 70% can be 180 seconds or less when a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then the tensile stress is removed.

[0048] Most specifically, the strain rate can be 20.1% or less when a tensile stress of 15 MPa is applied at 60°C for 60 seconds, and the recovery time taken until the recovery rate reaches 70% can be 178 seconds or less when a tensile stress of 2 MPa is applied at 70°C for 180 seconds and then the tensile stress is removed.

[0049] The polyolefin separator described above can be manufactured as follows, but is not limited thereto.

[0050] In one embodiment of the present application, the separator can be manufactured by a method (wet method) in which a polyolefin resin is kneaded with a diluent at a high temperature to form a single phase, the polymer material and the diluent are phase-separated during cooling, the diluent is extracted to form pores, and then stretching and heat fixation treatment are performed. In particular, the polyolefin separator can contain a core portion including a mixture of polyethylene and polypropylene, and polyethylene skin portions laminated on both sides of the core portion, but is not limited thereto.

[0051] By controlling the mixing ratio of the diluent, the stretching ratio, and the heat fixation treatment temperature, etc., the average pore size and the maximum pore size of the separator within the scope of the present application can be easily manufactured.

[0052] In the present application, the polyolefin separator substrate prepared by the above method can have a thickness in the range of 5 μm to 30 μm.

[0053] On the other hand, in one embodiment of the present application, the separator can further include an organic / inorganic composite coating layer formed on at least one surface of the polyolefin separator substrate.

[0054] The organic / inorganic composite coating layer can contain a binder resin and inorganic particles and have a porous property. In one embodiment of the present application, the binder resin and inorganic particles in the organic / inorganic composite coating layer can be present in a weight ratio of 1:99 to 30:70. The weight ratio can be appropriately adjusted within the above range, for example, when the sum of the binder resin and inorganic particles is 100% by weight, the binder resin can be 1% by weight or more, 5% by weight or more, or 10% by weight or more, and the inorganic particles can be 80% by weight or more, 85% by weight or more, 90% by weight or more, or 95% by weight or more.

[0055] The organic / inorganic composite coating layer can be formed by adhering inorganic particles with a binder resin and aggregating them within the layer. The pores inside the organic / inorganic composite coating layer can be formed by interstitial volumes, which are voids between the inorganic particles.

[0056] In one embodiment of the present application, the porosity of the organic / inorganic composite coating layer can be 30% by volume to 70% by volume. When the porosity is 70% by volume or less, it is possible to ensure the mechanical properties that can withstand the pressing process of bonding to the electrode, and because the surface opening ratio is not excessively high, it is suitable to ensure the bonding force. On the other hand, when the porosity is 30% by volume or more, it is advantageous in terms of ion permeability.

[0057] The thickness of the organic / inorganic composite coating layer can be formed to be 1 μm to 20 μm with respect to either side of the separator substrate, but is not particularly limited thereto. The thickness can be adjusted within an appropriate range by a person skilled in the art in terms of heat resistance or electrical resistance.

[0058] In the present application, non-limiting examples of the binder resin that can be used in the organic / inorganic composite coating layer can include any one of the polymer resins selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl methacrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxymethyl cellulose, or a mixture of two or more thereof. However, the present application is not particularly limited thereto.

[0059] In the detailed description of the present application, 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 application are not particularly limited as long as oxidation and / or reduction reactions do not occur within the operating voltage range of the electrochemical device to which they are applied (for example, based on Li / Li + from 0 to 5 V).

[0060] Non-limiting examples of the inorganic particles include BaTi03, Pb(Zr,Ti)03(PZT), Pb 1-x La x Zr 1-y Ti y O3(PLZT, 0 < x < 1, 0 < y < 1), Pb(Mg 1 / 3 Nb 2 / 3 )03-PbTi03(PMN-PT), hafnium dioxide (Hf02), SrTi03, Sn02, Ce02, MgO, Mg(OH)2, NiO, CaO, ZnO, Zr02, Si02, Y203, Al203, SiC, Al(OH)3, Ti02, aluminum peroxide, zinc tin hydroxide (ZnSn(OH)6), tin zinc oxide (Zn2Sn04, ZnSn03), antimony trioxide (Sb203), antimony tetroxide (Sb204), antimony pentoxide (Sb205), and the like, and the inorganic particles can include one or two or more thereof.

[0061] Further, the average diameter (D 50 ) of the inorganic particles is not particularly limited, but is preferably in the range of 0.3 μm to 1 μm, to form a coating layer having a uniform thickness and an appropriate porosity. When the average diameter of the inorganic particles is less than 0.3 μm, the dispersibility of the inorganic particles in a slurry prepared for the preparation of an organic / inorganic composite coating layer can decrease, and when the average diameter of the inorganic particles is greater than 1 μm, the thickness of the formed coating layer can increase.

[0062] In one embodiment of the present application, a method of forming an organic / inorganic composite coating layer is, for example, as follows. First, a polymer solution is prepared by dissolving a binder resin in a suitable organic solvent. It is preferable that the solvent can have a similar solubility parameter and a low boiling point as the binder polymer to be used. This is to facilitate uniform mixing and subsequent solvent removal. Non-limiting examples of the solvent that can be used can include acetone, tetrahydrofuran, dichloromethane, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), cyclohexane, water, or a mixture thereof.

[0063] Thereafter, inorganic particles are added to the prepared polymer solution and dispersed. In the present application, the content ratio of the inorganic particles and the binder is as described above and is appropriately adjusted in consideration of the thickness, the size of the pores, and the porosity of the organic / inorganic composite coating layer of the present application finally manufactured.

[0064] Next, the inorganic particle slurry prepared above is coated onto at least one side of the prepared diaphragm substrate and dried. There are no particular limitations on the method of coating the slurry onto the surface of the diaphragm substrate, and conventional methods known in the art can be used. For example, various methods can be used, such as dip coating, die coating, roll coating, comma coating, or combinations thereof.

[0065] During the drying process, temperature and time conditions can be appropriately set to minimize surface defects in the organic / inorganic composite coating. The drying can be carried out within suitable limits using drying aids such as drying ovens or hot air.

[0066] When the diaphragm contains an organic / inorganic composite porous layer, damage caused during the lamination process by inorganic particles pressing against the surface of the diaphragm substrate against the organic / inorganic composite porous layer can be reduced.

[0067] The diaphragm prepared by the above method is inserted between the negative electrode and the positive electrode, and manufactured as an electrode assembly by a lamination process in which heat and / or pressure are applied to bond the diaphragm. In one embodiment of the invention, the lamination process can be performed by a roller press device comprising a pair of pressure rollers. That is, the negative electrode, the diaphragm, and the positive electrode are sequentially laminated and placed between the pressure rollers to achieve interlayer bonding. In this case, the lamination process can be performed by a hot pressing method.

[0068] On the other hand, the present invention provides an electrochemical device including the aforementioned separator, particularly a lithium secondary battery. The battery includes a negative electrode, a positive electrode, and a separator interposed between the negative and positive electrodes, and the separator includes a separator substrate having the aforementioned properties.

[0069] The positive electrode comprises a positive current collector and a positive active material layer on at least one surface of the current collector, the positive active material layer comprising a positive active material, a conductive material, and a binder resin. The positive active material may comprise one or a mixture of two or more of the following: layered compounds such as lithium manganese oxide (LiMn2O4, LiMnO2, etc.), lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or compounds replaced by one or more transition metals; lithium manganese oxide, such as Li... 1+x Mn 2-x O4 (where x is 0 to 0.33), LiMnO3, LiMn2O3, LiMnO2, etc.; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5 and Cu2V2O7; LiNi 1-x M x Ni-site type lithium nickel oxides represented by O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x = 0.01 to 0.3); LiMn 1-xM x O2(wherein M = Co, Ni, Fe, Cr, Zn or Ta and x = 0.01 to 0.1) or Li2Mn3MO8(wherein M = Fe, Co, Ni, Cu or Zn); LiMn2O4in which a part of Li in the formula is replaced by an alkaline earth metal ion; a disulfide compound; and Fe2(MoO4)3.

[0070] In the present application, the negative electrode comprises a negative electrode current collector and a negative electrode active material layer on at least one surface of the current collector, the negative electrode active material layer comprising a negative electrode active material, a conductive material and a binder resin. The negative electrode can comprise as the negative electrode active material one or a mixture of two or more selected from the group consisting of lithium metal oxides; carbon such as non-graphitizable carbon and graphite-based carbon; metal composite oxides such as 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, elements of Group 1, Group 2 and Group 3 of the periodic table, halogen; 0 < x < 1; 1 < y < 3; 1 < z < 8); 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, Bi2O4and Bi2O5; conductive polymers such as polyacetylene; Li-Co-Ni-based materials; and titanium oxides.

[0071] In one embodiment of the present application, the conductive material can be, for example, any one or a mixture of two or more conductive materials selected from the group consisting of graphite, carbon black, carbon fibers or metal fibers, metal powders, conductive whiskers, conductive metal oxides, activated carbon and polyphenylene derivatives. More specifically, the conductive material can be one or a mixture of two or more conductive materials selected from the group consisting of natural graphite, artificial graphite, super-p, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal crack black, denka black, aluminum powder, nickel powder, zinc oxide, potassium titanate and titanium oxides.

[0072] The current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery, and materials such as stainless steel, copper, aluminum, nickel, titanium, calcined carbon; or materials obtained by surface treatment of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. can be used.

[0073] As the binder resin, a polymer generally used for electrodes in the art can be used. Non-limiting examples of such a binder resin include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethyl methacrylate, polyethylhexyl acrylate, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate, polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, cyanoethyl pullulan, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, pullulan, and carboxymethyl cellulose, etc., but are not limited thereto.

[0074] In the present application, the cathode slurry for manufacturing the cathode active material layer can include a dispersant, and the dispersant can be a pyrrolidone-based compound. Specifically, the dispersant can be N-methyl pyrrolidone (ADC-01, LG Chem Co.).

[0075] In the present application, the content of the dispersant in the cathode slurry for manufacturing the cathode active material layer can be greater than 0 parts and 0.5 parts or less, with respect to 100 parts by weight of the cathode slurry. Specifically, the content of the dispersant in the cathode slurry for manufacturing the cathode active material layer can be greater than 0.05 parts and 0.4 parts or less, with respect to 100 parts by weight of the cathode slurry.

[0076] In the present application, the anode slurry for manufacturing the anode active material layer can include a dispersant, and the dispersant can be a polyvinylpyrrolidone-based compound. Specifically, the dispersant can be polyvinylpyrrolidone (Junsei Co., Japan).

[0077] In the present application, the content of the dispersant in the anode slurry for manufacturing the anode active material layer can be greater than 0 parts or more and 0.5 parts or less, with respect to 100 parts by weight of the anode slurry. Specifically, the content of the dispersant in the anode slurry for manufacturing the anode active material layer can be greater than 0.05 parts and 0.4 parts or less, with respect to 100 parts by weight of the anode slurry.

[0078] The electrode assembly prepared as described above can be loaded into a suitable case, and an electrolyte can be injected to manufacture a battery.

[0079] In the present application, the electrolyte is a salt having the same structure as A + B - , and A + includes an ion consisting of an alkali metal cation such as Li + , Na + , K + , or a combination thereof, and B - includes PF6 - , BF4 - , Cl- Br - I - ClO4 - AsF6 - CH3CO2 - CF3SO3 - N(CF3SO2)2 - C(CF2SO2)3 - or a combination thereof. In the electrolyte, the salt can be dissolved or dissociated in an organic solvent or an organic solvent consisting of a mixture thereof, including 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), and gamma-butyrolactone (gamma-butyrolactone), but is not limited thereto.

[0080] Further, the present application provides a battery module containing a battery including the electrode assembly as a unit cell, a battery pack containing the battery module, and a device containing the battery pack as a power source. Specific examples of the device include: a power tool driven by a battery motor; an electric vehicle, including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and the like; an electric two-wheeled vehicle, including an electric bicycle and an electric scooter; an electric golf cart; and a power storage system, but are not limited thereto.

[0081] Modes for carrying out the application

[0082] Hereinafter, for the specific description of the present application, examples will be given to describe in detail. However, the embodiments according to the present application can be modified in various other forms, and the scope of the present application should not be construed as being limited to the following embodiments. The embodiments of the present application are provided in order to more completely explain the present application to those having ordinary skill in the art.

[0083] [Example 1]

[0084] A resin obtained by mixing polyethylene (weight average molecular weight: 900,000) and polypropylene (weight average molecular weight: 350,000) at a weight ratio of 93:7 as a core portion, and positioning polyethylene (weight average molecular weight: 900,000) resin on both sides of the core portion as a skin portion, was co-extruded, the stretching temperature was adjusted to 105°C and the heat fixation temperature was adjusted to 130°C, and a polyolefin separator substrate (total thickness: about 9 μm, core portion thickness: 7 μm, total thickness of two skin portions: 2 μm) was prepared by a wet method. The PDI of the resin used to prepare the separator was calculated according to the following Formula 1.

[0085] The following Table 1 shows the PDI, average pore size, maximum pore size, strain rate at 60°C under a tensile stress of 15 MPa for 60 seconds (referred to as recovery strain rate), and time required for the recovery rate to reach 70% upon removal of the tensile stress after applying a tensile stress of 2 MPa at 70°C for 180 seconds (referred to as recovery time) of the prepared polyolefin separators.

[0086] [Example 2]

[0087] The separator was prepared in the same manner as in Example 1 except that the stretching temperature was adjusted to 120°C and the heat fixation temperature was adjusted to 115°C.

[0088] [Example 3]

[0089] The separator was prepared in the same manner as in Example 1 except that the heat fixation temperature was adjusted to 115°C.

[0090] [Comparative Example 1]

[0091] The separator was prepared in the same manner as in Example 1 except that a resin in which polyethylene (weight average molecular weight: 900,000) and polypropylene (weight average molecular weight: 350,000) were mixed at a weight ratio of 97:3 was used as the skin portion, and the stretching temperature was adjusted to 120°C.

[0092] [Comparative Example 2]

[0093] The separator was prepared in the same manner as in Example 1 except that a resin in which polyethylene (weight average molecular weight: 900,000) and polypropylene (weight average molecular weight: 350,000) were mixed at a weight ratio of 97:3 was used as the skin portion, and the heat fixation temperature was adjusted to 115°C.

[0094] [Comparative Example 3]

[0095] The separator was prepared in the same manner as in Example 1 except that a resin in which polyethylene (weight average molecular weight: 900,000) and polypropylene (weight average molecular weight: 350,000) were mixed at a weight ratio of 97:3 was used as the skin portion.

[0096] [Comparative Example 4]

[0097] The separator was prepared in the same manner as in Example 1 except that a resin in which polyethylene (weight average molecular weight: 900,000) and polypropylene (weight average molecular weight: 350,000) were mixed at a weight ratio of 97:3 was used as the skin portion, and the stretching temperature was adjusted to 120°C and the heat fixation temperature was adjusted to 115°C.

[0098] [PDI Measurement]

[0099] (Formula 1) PDI = (weight average molecular weight) / (number average molecular weight)

[0100] In this case, the values of the weight average molecular weight and the number average molecular weight were obtained by cutting the separator into a predetermined size and analyzing gel permeation chromatography (GPC).

[0101] [Measurement of average pore size and maximum pore size]

[0102] The determination was made according to the size distribution of the pores using a capillary flow porometry (CFP method).

[0103] [Measurement of strain at the time of applying a tensile stress of 15 MPa for 60 seconds at 60°C]

[0104] The strain rate was measured by dynamic mechanical analysis creep evaluation (DMA 850, TA Instruments) at 60°C by applying a stress of 15 MPa for 60 seconds.

[0105] [Measurement of time taken until the recovery rate reaches 70% at the time of applying a tensile stress of 2 MPa for 180 seconds at 70°C and then removing the tensile stress]

[0106] After applying a stress of 2 MPa at 70°C for 180 seconds using dynamic mechanical analysis (DMA 850, TA Instruments), the stress was removed, and the time taken until the recovery rate reached 70% was measured.

[0107] [Table 1]

[0108]

[0109] [Manufacture of electrode assembly]

[0110] 1) Preparation of positive electrode

[0111] A slurry for a positive active material layer having a concentration of 50% by weight excluding water was prepared by mixing a positive active material (LiNi 0.8 Mn 0.1 Co 0.1 O2), a conductive material (carbon black), a dispersant (N-methylpyrrolidone, ADC-01, LG Chem), and a binder resin (a mixture of PVDF-HFP and PVDF) with water. Next, the slurry was coated on the surface of an aluminum thin film (thickness: 10 µm) and dried, thereby preparing a positive electrode having a positive active material layer (thickness: 120 µm).

[0112] 2) Preparation of negative electrode

[0113] A slurry for a negative active material layer having a concentration of 50% by weight of the components except water was prepared by mixing graphite (a blend of natural graphite and artificial graphite), a conductive material (carbon black), a dispersing agent (polyvinylpyrrolidone, Junsei Corporation, Japan), and a binder resin (a mixture of PVDF-HFP and PVDF) in a weight ratio of 97.5:0.7:0.14:1.66. Next, the slurry was coated on the surface of a copper film (thickness: 10 μm) and dried, thereby preparing a negative electrode having a negative active material layer (thickness: 120 μm).

[0114] 3) Laminating Process

[0115] An electrode assembly was obtained by laminating the separator of the examples and comparative examples between the prepared negative electrode and positive electrode and performing a laminating process. The laminating process was performed at 70°C for 10 seconds at 5.2 MPa using a hot press.

[0116] [Reduction rate of pore size]

[0117] The reduction rate of pore size was calculated based on the following Formula 2.

[0118] (Formula 2) Reduction rate of pore size (%) = (C - D / C) x 100

[0119] In Formula 2, C is the average pore size of the separator substrate before the laminating process, and D is the average pore size of the separator substrate obtained after the laminating process.

[0120] [Measurement of resistance]

[0121] For the resistance, various separator substrates were interposed between SUS, an electrolyte was injected to prepare a coin cell, and the resistance (ER) was measured by an EIS method. At this time, the frequency was in the range of 100000 Hz to 10000 Hz. The electrolyte was a mixture of LiPF6 having a concentration of 1 M in a non-aqueous solvent in which ethylene carbonate and ethylmethyl carbonate were mixed at a ratio of 3:7.

[0122] [Dielectric breakdown voltage]

[0123] After the SUS mesh and the separator substrate were thermally bonded at a temperature of 90°C, 4 MPa, and 1 second, the voltage at which the failure condition (>0.5 mA, 3 s) was reached was determined by increasing the voltage at a rate of 100 V / sec. For each of the examples and comparative examples, after the dielectric breakdown voltage of 30 samples was measured, the voltage of 1% lower was obtained by Weibull distribution analysis.

[0124] [Capacity retention rate]

[0125] For the prepared batteries, charge and discharge were each performed at a rate of 1C in the range of 2.5 to 4.25 V, charge and discharge were repeatedly performed, 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.

[0126] The results are shown in Table 2 below.

[0127] [Table 2]

[0128]

Claims

1. A polyolefin membrane for an electrochemical device, comprising: A polyolefin resin having a polydispersity index in the range of 2.5 to 4.

2. The diaphragm contains a plurality of pores, and The aperture has an average aperture size in the range of 26.7 nm to 36.1 nm and a maximum aperture size in the range of 40.5 nm to 46 nm. The polydispersity index refers to (weight-average molecular weight) / (number-average molecular weight), and The average pore size and the maximum pore size are calculated based on the pore size distribution using a capillary flow porosimeter method. The polyolefin resin described herein has a weight-average molecular weight of 500,000 to 1,500,000, and When a membrane is formed using a mixture of different types of polyolefin resins or a multilayer structure containing different types of polyolefin resins, the weight-average molecular weight of the polyolefin resins is calculated by summing the weight-average molecular weights based on the content ratio of each polyolefin resin.

2. The polyolefin membrane for an electrochemical device according to claim 1, wherein the strain rate of the polyolefin membrane is less than 25% when subjected to a tensile stress of 15 MPa for 60 seconds at 60°C. The recovery time required for the polyolefin membrane to achieve a 70% recovery rate after being subjected to tensile stress of 2 MPa at 70°C for 180 seconds and then having the tensile stress removed is less than 200 seconds.

3. The polyolefin membrane for electrochemical devices according to claim 2, wherein the polyolefin resin has a polydispersity index in the range of 2.5 to 4.

0. The strain rate of the polyolefin membrane is less than 23% when a tensile stress of 15 MPa is applied at 60°C for 60 seconds, and the recovery time required for the polyolefin membrane to achieve a recovery rate of 70% after the tensile stress is removed when a tensile stress of 2 MPa is applied at 70°C for 180 seconds is less than 190 seconds.

4. The polyolefin membrane for an electrochemical device according to claim 2, wherein the polyolefin resin has a polydispersity index in the range of 2.6 to 3.

9. The strain rate of the polyolefin membrane when subjected to tensile stress of 15 MPa at 60°C for 60 seconds is less than 21%, and the recovery time required for the polyolefin membrane to achieve a recovery rate of 70% after the tensile stress is removed when subjected to tensile stress of 2 MPa at 70°C for 180 seconds is less than 180 seconds.

5. The polyolefin diaphragm for electrochemical devices according to claim 4, The strain rate of the polyolefin membrane when subjected to tensile stress of 15 MPa at 60°C for 60 seconds is less than 20.1%, and the recovery time required for the polyolefin membrane to achieve a recovery rate of 70% after the tensile stress is removed when subjected to tensile stress of 2 MPa at 70°C for 180 seconds is less than 178 seconds.

6. The polyolefin membrane for an electrochemical device according to claim 1, wherein the polyolefin membrane comprises: The core comprises a mixture of polyethylene and polypropylene; and A polyethylene sheath is laminated on both sides of the core.

7. The polyolefin membrane for electrochemical devices according to claim 1, wherein the polyolefin membrane is manufactured by a wet manufacturing method in which a pore-forming agent is extracted to form pores.

8. An electrochemical device having an electrode assembly, The electrode assembly includes a positive electrode, a negative electrode, and a diaphragm inserted between the positive electrode and the negative electrode, wherein the diaphragm is the diaphragm according to any one of claims 1 to 7.

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

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