Method for pre-detecting defective porous polymer substrates for separators

Abstract

Disclosed is a method of pre-inspecting a defective porous polymer substrate for a separator, including the steps of: selecting a porous polymer substrate having a plurality of pores to inspect whether it is a good product or a defective product; observing the selected porous polymer substrate using a scanning electron microscope (SEM) to obtain an image of the porous polymer substrate; quantifying an average of a pore distribution index (PDI) by using the obtained image of the porous polymer substrate; correcting the quantified average of the pore distribution index to obtain a corrected average of the pore distribution index; determining whether the corrected average of the pore distribution index is 60 a.u. (arbitrary unit) or less; classifying the porous polymer substrate as a good product when the corrected average of the pore distribution index is determined to be 60 a.u. or less, and classifying the porous polymer substrate as a defective product when the corrected average of the pore distribution index is determined to be greater than 60 a.u.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method of pre-detecting a defective porous polymer substrate for a separator.

[0002] This application claims priority to Korean Patent Application No. 10-2021-0044855, filed on April 6, 2021 in Korea, the disclosure of which is incorporated herein by reference. BACKGROUND

[0003] In recent years, energy storage technology has received increasing attention. As the application of energy storage technology has been extended to the energy sources of cell phones, camcorders, and notebook PCs, and even to the energy sources of electric vehicles, research and development efforts for electrochemical devices have been increasingly realized. In this case, electrochemical devices have received the most attention. Among such electrochemical devices, the development of rechargeable secondary batteries has been focused. Recently, active research has been conducted on designing new electrodes and batteries in order to improve the capacity density and specific energy of developing such batteries.

[0004] Among the commercially available secondary batteries, lithium secondary batteries developed in the early 1990s have received attention because they have a higher operating voltage and a significantly higher energy density compared to conventional batteries such as Ni-MH, Ni-Cd, and lead sulfate batteries using aqueous electrolytes.

[0005] Such lithium secondary batteries include a positive electrode, a negative electrode, an electrolyte, and a separator. Specifically, the separator needs to have an insulating property for separating and electrically insulating the positive electrode and the negative electrode from each other and a high ionic conductivity for increasing lithium ion permeability based on high porosity.

[0006] Generally, such a separator can be applied in the form of a porous polymer substrate obtained by kneading a polymer such as a polyolefin with a diluent, performing extrusion and orientation to form a film, and extracting the diluent to form pores by using a solvent or the like. In addition thereto, the separator can be applied in the form of a composite separator including an organic-inorganic composite porous layer containing a binder polymer and inorganic particles and disposed on at least one surface of the porous polymer substrate.

[0007] Herein, the porous polymer substrate applied to the separator of the secondary battery can have differently controlled physical properties depending on the processing conditions during its manufacture. Specifically, the pore structure of the porous polymer substrate serving as a lithium ion passage can also be differently changed depending on the processing conditions.

[0008] Specifically, when excessive heat is applied during the orienting step, some problems can occur, for example, the completed porous polymer substrate can have a large pore size, the porous polymer substrate can exhibit an uneven surface morphology, or fibrils of the polymer contained in the porous polymer substrate can be formed to have a large thickness. The problems of these problems are that, when the porous polymer substrate is applied in the form of a composite separator, the separator exhibits an excessively increased air permeation time after an organic-inorganic composite porous layer is formed.

[0009] This cannot be observed by determining the physical properties of the separator unit, such as the air permeation time. Specifically, in the case of determining the pore size using a capillary flow meter, the measurement results vary at different sites, which is not good for quality control.

[0010] Therefore, for the purpose of quality control of the porous polymer substrate used as a substrate for a separator, there is a need for a method of easily detecting a defective product (uneven morphology) in terms of pore size. SUMMARY

[0011] Technical problem

[0012] The present disclosure is designed to solve the problems of the related art, and thus, the present disclosure aims to provide a method of pre-detecting a defective porous polymer substrate for a separator to facilitate determination of the appearance of the separator for the purpose of quality control.

[0013] Technical solution

[0014] In one aspect of the present disclosure, there is provided a method of pre-detecting a defective porous polymer substrate for a separator according to any one of the following embodiments.

[0015] According to a first embodiment, there is provided a method of pre-detecting a defective porous polymer substrate for a separator, comprising the steps of:

[0016] selecting a porous polymer substrate having a plurality of pores to detect whether it is a good product or a defective product;

[0017] observing the selected porous polymer substrate using a scanning electron microscope (SEM) to obtain an image of the porous polymer substrate;

[0018] quantifying an average value of a pore distribution index (PDI) by using the obtained image of the porous polymer substrate;

[0019] correcting the quantified average value of the pore distribution index to obtain a corrected average value of the pore distribution index;

[0020] determining whether the corrected average of the pore distribution index is 60 a.u. (arbitrary unit) or less; and

[0021] when the corrected average of the pore distribution index is determined to be 60 a.u. or less, classifying the porous polymer substrate as a good product, and when the corrected average of the pore distribution index is determined to be more than 60 a.u., classifying the porous polymer substrate as a defective product.

[0022] According to a second embodiment, there is provided a method of pre-detecting a defective porous polymer substrate for a separator as defined in the first embodiment,

[0023] wherein the step of quantifying the average of the pore distribution index is performed by mathematically processing (Euclidean distance measurement) and computer engineering algorithm processing an image of the porous polymer substrate taken by a scanning electron microscope (SEM).

[0024] According to a third embodiment, there is provided a method of pre-detecting a defective porous polymer substrate for a separator as defined in the second embodiment,

[0025] wherein the step of quantifying the average of the pore distribution index comprises calculating a distance between pores and a pore size from an image of the porous polymer substrate taken by a scanning electron microscope (SEM).

[0026] According to a fourth embodiment, there is provided a method of pre-detecting a defective porous polymer substrate for a separator as defined in the second or third embodiment,

[0027] wherein the step of quantifying the average of the pore distribution index comprises:

[0028] obtaining a two-dimensional (2-D) binary image in which pixels of an original image of the porous polymer substrate taken by a scanning electron microscope (SEM) are divided into pixels representing a plurality of polymer fibril regions adjacent to each other or striding over each other and pixels representing a pore region formed among the plurality of polymer fibrils;

[0029] performing distance transformation on the 2-D binary image by using a computational Euclidean distance transform method; and

[0030] applying local maximum value calculation to the distance-transformed image to extract distance-transformed values of all pixels in the image and to perform statistical processing.

[0031] According to a fifth embodiment, there is provided a method of pre-detecting a defective porous polymer substrate for a separator as defined in any one of the first to fourth embodiments,

[0032] wherein the step of correcting the quantified average value of the pore distribution index (average PDI before correction) to obtain a corrected average value of the pore distribution index (average PDI after correction) is performed by calculation according to the following equation:

[0033] Corrected average PDI = Average PDI before correction x (1 / η),

[0034] wherein η represents a correction factor and is defined by the equation of η = (number of polymer fibril pixels) / (number of total image pixels).

[0035] According to a sixth embodiment, there is provided a method of pre-detecting a defective porous polymer substrate for a separator as defined in any one of the first to fifth embodiments,

[0036] wherein the separator obtained by coating a slurry containing inorganic particles, a binder polymer, and a dispersion medium on at least one surface of the porous substrate classified as a good product and drying to form an organic-inorganic composite porous layer has an air permeability of 600 sec / 100cc or less.

[0037] According to a seventh embodiment, there is provided a method of pre-detecting a defective porous polymer substrate for a separator as defined in any one of the first to sixth embodiments,

[0038] wherein the porous polymer substrate is classified as a good product when the corrected average value of the pore distribution index is determined to be 40 a.u. or less, and is classified as a defective product when the corrected average value of the pore distribution index is determined to be greater than 40 a.u.

[0039] Advantageous effects

[0040] Unlike the conventional method of determining a defective product in terms of air permeability or the like after completing a separator including an organic-inorganic composite porous layer on at least one surface of a porous polymer substrate, the method of pre-detecting a defective porous polymer substrate for a separator according to the embodiments of the present disclosure can clearly detect a defective product by using the image difference of the porous polymer substrate in the state of the porous polymer substrate as a separator substrate before forming an organic-inorganic composite porous layer thereon. In other words, the image of the porous polymer substrate obtained by observation with a scanning electron microscope (SEM) is used to quantify the average value of the pore distribution index (PDI), and then the value obtained by correcting the quantified average value is compared with a predetermined reference value to determine whether the corrected value is greater than or less than the reference value, thereby detecting whether the porous polymer substrate is a good product or a defective product. In this way, compared to the conventional method of determining a defective product in the state of a separator, it is possible to significantly reduce the time and cost. BRIEF DESCRIPTION OF DRAWINGS

[0041] The accompanying drawings illustrate preferred embodiments of the present disclosure and, together with the foregoing disclosure, serve to provide further understanding of the technical features of the present disclosure, and therefore, the present disclosure should not be construed as being limited to the drawings.

[0042] Figure 1a 、 Figure 2a 、 Figure 3a 、 Figure 4a and Figure 5a respectively illustrate scanning electron microscope (SEM) images of the porous polyolefin substrates according to Preparation Examples 1-5.

[0043] Figure 1b 、 Figure 2b 、 Figure 3b 、 Figure 4b and Figure 5b respectively illustrate results of distance transformation of the porous polyolefin substrates according to Preparation Examples 1-5.

[0044] Figure 1c 、 Figure 2c 、 Figure 3c 、 Figure 4c and Figure 5c respectively illustrate graphs of corrected mean values of pore distribution index (corrected mean PDI) and corrected pore distribution index according to Preparation Examples 1-5.

[0045] Figures 6a to 6d illustrates a raw image of the porous polyolefin substrate according to Preparation Example 1, a two-dimensional (2-D) binary image thereof, a quantized mean value of pore distribution index (mean PDI before correction) thereof, and a graph of corrected mean values of pore distribution index (corrected mean PDI) thereof.

[0046] Figures 7a to 7d illustrates a raw image of the porous polyolefin substrate according to Preparation Example 4, a two-dimensional (2-D) binary image thereof, a quantized mean value of pore distribution index (mean PDI before correction) thereof, and a graph of corrected mean values of pore distribution index (corrected mean PDI) thereof.

[0047] Figure 8 illustrates a series of steps of quantifying a mean value of pore distribution index from a raw image. DETAILED DESCRIPTION

[0048] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Before the description, it should be understood that the terms used in the specification and the appended claims should not be construed as limited to general and dictionary meanings but interpreted based on the meanings and concepts corresponding to technical aspects of the present disclosure on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation of the patent to be granted.

[0049] In one aspect of the present disclosure, a method of pre-detecting a defective porous polymer substrate for a separator includes the steps of:

[0050] selecting a porous polymer substrate having a plurality of pores to detect whether it is a good product or a defective product;

[0051] observing the selected porous polymer substrate using a scanning electron microscope (SEM) to obtain an image of the porous polymer substrate;

[0052] quantifying an average value of a pore distribution index (PDI) using the image of the porous polymer substrate;

[0053] correcting the quantified average value of the pore distribution index to obtain a corrected average value of the pore distribution index;

[0054] determining whether the corrected average value of the pore distribution index is 60 a.u. (arbitrary unit) or less; and

[0055] classifying the porous polymer substrate as a good product when the corrected average value of the pore distribution index is determined to be 60 a.u. or less, and as a defective product when the corrected average value of the pore distribution index is determined to be greater than 60 a.u.

[0056] First, a porous polymer substrate having a plurality of pores is selected to detect whether it is a good product or a defective product.

[0057] Herein, the porous polymer substrate refers to a separator substrate before an organic-inorganic composite porous layer is formed thereon in a separator provided with the organic-inorganic composite porous layer. The porous polymer substrate includes a plurality of polymer fibers arranged in parallel to each other or across each other, and a plurality of pores formed among the plurality of polymer fibers, and thus can include a fiber region and a pore region.

[0058] Specifically, the porous polymer substrate can be a porous polymer film substrate or a porous polymer nonwoven web substrate.

[0059] The porous polymer film substrate can be a porous polymer film including a polyolefin such as polyethylene or polypropylene. Such a polyolefin porous polymer film substrate realizes a shut-off function at a temperature of 80-130℃.

[0060] Herein, the polyolefin porous polymer film can be formed of a polymer including a polyolefin polymer such as polyethylene including high-density polyethylene, linear low-density polyethylene, low-density polyethylene, or ultra-high molecular weight polyethylene, polypropylene, polybutylene, or polypentene, alone or in combination of two or more thereof.

[0061] In addition, the porous polymer film substrate can be obtained by molding various polymers other than polyolefins, such as polyesters, into a film shape. Further, the porous polymer substrate can have a stacked structure of two or more film layers, each of which can be formed of a polymer including the above-described polymers such as polyolefins or polyesters, alone or in combination of two or more thereof.

[0062] In addition, the porous polymer film substrate and the porous polymer nonwoven web substrate can be formed of, alone or in combination, polyolefins, polyesters (e.g., polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, etc.), polyacetals, polyamides, polycarbonates, polyimides, polyether ether ketones, polyether sulfones, polyphenylene ethers, polyphenylene sulfides, etc., in addition to the above-described polyolefins.

[0063] The thickness of the porous polymer substrate is not particularly limited, and the thickness of the porous polymer substrate is 1-100 μm, particularly 5-50 μm. In addition, although the size of the pores and the porosity present in the porous polymer substrate are not particularly limited, the pore size and the porosity can be 0.01-50 μm and 10-95%, respectively.

[0064] Specifically, the porous polymer substrate can be a porous polyolefin substrate, in which the polyolefin can include polyethylene; polypropylene; polybutylene; polyamylene; polyhexene; polyoctene; a copolymer of at least two of ethylene, propylene, butylene, amylene, 4-methylamylene, hexane, heptane, and octene; or a mixture thereof.

[0065] More specifically, the polyethylene can include low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), etc. Among them, high-density polyethylene having high crystallinity and a high resin melting point can be frequently used.

[0066] According to an embodiment of the present disclosure, the polyolefin can have a weight average molecular weight of 200,000-1,500,000, 220,000-100,000, 250,000-800,000, or 250,000-600,000. According to the present disclosure, a high-molecular-weight polyolefin having a weight average molecular weight of 200,000-1,000,000 can be used as a starting material for manufacturing the partition. In this way, a partition having excellent strength and heat resistance can be obtained while ensuring uniformity and film-forming processability of the partition.

[0067] Next, the selected porous polymer substrate is observed using a scanning electron microscope (SEM) to obtain an image of the porous polymer substrate.

[0068] According to embodiments of the present disclosure, images of the porous polymer substrate can be obtained using a scanning electron microscope at 20,000 times magnification under conditions of 5 kV and 10 μA.

[0069] The obtained images of the porous polymer substrate are used to quantify the average value of the pore distribution index (PDI).

[0070] The step of quantifying the average value of the pore distribution index can be performed by mathematically processing (Euclidean distance measurement) and computer engineering algorithmically processing the images of the porous polymer substrate taken using a scanning electron microscope (SEM).

[0071] The step of quantifying the average value of the pore distribution index can include calculating the distance between pores and the pore size (area ratio) from the images of the porous polymer substrate taken using a scanning electron microscope (SEM).

[0072] According to embodiments of the present disclosure, the step of quantifying the average value of the pore distribution index can include obtaining a two-dimensional (2-D) binary image in which the pixels of the original image of the porous polymer substrate taken using a scanning electron microscope (SEM) are divided into pixels representing a plurality of polymer fibril regions adjacent to each other or spanning each other, and pixels representing a pore region formed among the plurality of polymer fibrils; performing distance transformation on the 2-D binary image by using a computational Euclidean distance transformation method; and applying local maximum value calculation to the distance-transformed image to extract the distance-transformed values of all the pixels in the image and perform statistical processing.

[0073] Herein, the step of applying local maximum value calculation to the distance-transformed image to extract the distance-transformed values of all the pixels in the image and perform statistical processing can include separating the distance-transformed image into sections with local minimum values of the distance-transformed values as boundaries; filling local maximum values of the distance-transformed values of the separated sections with the distance-transformed values of all the pixels of the separated sections, and extracting the distance-transformed values of each pixel by the totality and performing statistical processing.

[0074] Figure 8 A series of steps for quantifying the average value of the pore distribution index from the original image are illustrated.

[0075] Then, the quantified average value of the pore distribution index is corrected to obtain a corrected average value of the pore distribution index.

[0076] The porous polymer substrate includes an array of polymer fibrils forming the porous polymer substrate and pores formed among adjacent polymer fibrils, wherein the physical properties of the porous polymer substrate, such as air permeability, as required for the separator substrate, can be affected by the distribution and area of the pore regions.

[0077] In other words, even when the porous polymer substrates have the same quantified average value of the pore distribution index, they can exhibit different physical properties depending on the area of the pore region formed in each porous polymer substrate. Therefore, according to the present invention, the quantified average value of the pore distribution index is corrected.

[0078] Specifically, since the quantified pore distribution index is obtained by calculating the distance to the pore based on the pixel corresponding to the fibril, there is a limitation that the quantified pore distribution index cannot reflect the pore area characteristics. Therefore, the ratio of the number of polymer fibril pixels based on the total number of image pixels is defined as a correction factor (η), and the average value of the pore distribution index is divided by the correction factor, so that the average value of the pore distribution index can increase when the pores are large and abundant. In this way, the characteristics of the pore region can be reflected.

[0079] The quantified average value of the pore distribution index (average PDI before correction) is corrected to obtain a corrected average value of the pore distribution index (corrected average PDI) can be performed by calculation according to the following equation:

[0080] Corrected average PDI = Average PDI before correction x (1 / η),

[0081] where η denotes the correction factor and is defined by the equation η = (number of polymer fibril pixels) / (total number of image pixels).

[0082] Then, it is determined whether the corrected average value of the pore distribution index is 60 a.u. (arbitrary unit) or less. Herein, when the corrected average value of the pore distribution index is determined to be 60 a.u. or less, the porous polymer substrate is classified as a good product. On the other hand, when the corrected average value of the pore distribution index is determined to be greater than 60 a.u., the porous polymer substrate is classified as a defective product.

[0083] According to an embodiment of the present disclosure, when the corrected average value of the pore distribution index is determined to be 40 a.u. or less, the porous polymer substrate can be classified as a good product, and when the corrected average value of the pore distribution index is determined to be greater than 40 a.u., it can be classified as a defective product.

[0084] In the case where the corrected average of the pore distribution index of the porous polymer substrate exhibits 60 a.u. or less, the porous polymer substrate before the organic-inorganic composite porous layer is formed can exhibit a similar level of permeation time as compared to the porous polymer substrate exhibiting a corrected average of the pore distribution index of greater than 60 a.u. However, the separator obtained after the organic-inorganic composite porous layer is formed on the porous polymer substrate exhibiting a corrected average of the pore distribution index of 60 a.u. or less achieves significantly improved permeation characteristics as compared to the porous polymer substrate exhibiting a corrected average of the pore distribution index of greater than 60 a.u. In other words, in the porous polymer substrate exhibiting a corrected average of the pore distribution index of 60 a.u. or less, the pores in the porous polymer substrate are uniformly distributed in a more uniform area. Therefore, even when an organic-inorganic composite porous layer is introduced therein and a harsh condition is provided in terms of permeation retention, the porous polymer substrate does not experience significant deterioration of permeation characteristics, thus enabling performance suitable for a separator applied to a secondary battery.

[0085] According to the present disclosure, the porous polymer substrate to be detected whether it is defective or not can be used as a separator as it is. However, in a field requiring higher thermal stability, an organic-inorganic composite porous layer can be provided on at least one surface of the porous polymer substrate to provide a separator having enhanced stability.

[0086] Such a separator including an organic-inorganic composite porous layer on at least one surface thereof and having enhanced stability includes an organic-inorganic composite porous layer disposed on at least one surface of a porous polymer substrate, and comprising a plurality of inorganic particles, and a binder polymer partially or entirely disposed on the surface of the inorganic particles to interconnect and fix the inorganic particles.

[0087] The binder polymer can be one conventionally used in the art for forming an organic-inorganic composite porous layer. In particular, a polymer having a glass transition temperature (Tg) of -200 to 200°C can be used. This is because such a polymer is capable of improving the mechanical properties, such as flexibility and elasticity, of the organic-inorganic composite porous layer finally formed. Such a binder polymer serves as a binder to interconnect the inorganic particles with each other and stably fix them, and thus helps to prevent deterioration of the mechanical properties of the separator having the organic-inorganic composite porous layer.

[0088] In addition, the binder polymer does not essentially need to have ionic conductivity. However, when a polymer having ionic conductivity is used, it is possible to further improve the performance of the electrochemical device. Therefore, a binder polymer having as high a dielectric constant as possible can be used. In fact, since the degree of dissociation of a salt in an electrolyte depends on the dielectric constant of the electrolyte solvent, a binder polymer having a higher dielectric constant is able to increase the degree of dissociation of a salt in an electrolyte. The binder polymer can have a dielectric constant in the range from 1.0 to 100 (measured at a frequency of 1 kHz), particularly 10 or more.

[0089] In addition to the above functions, the binder polymer is characterized in that it gels when impregnated with a liquid electrolyte, and thus exhibits a high degree of swelling. Therefore, the binder polymer has a solubility parameter (i.e., Hildebrand solubility parameter) of 15-45 MPa 1 / 2 or 15-25 MPa 1 / 2 and 30-45 MPa 1 / 2 . Thus, compared to a hydrophobic polymer such as a polyolefin, a hydrophilic polymer having many polar groups can be used more frequently. When the solubility parameter is less than 15 MPa 1 / 2 and greater than 45 MPa 1 / 2 , the binder polymer is difficult to swell with a conventional liquid electrolyte for a battery.

[0090] Non-limiting examples of the binder polymer include, but are not limited to, 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 amylopectin, cyanoethyl polyvinyl alcohol, cyanoethyl cellulose, cyanoethyl sucrose, amylopectin, and carboxymethyl cellulose.

[0091] Non-limiting examples of the inorganic particles can include inorganic particles having a dielectric constant of 5 or more, particularly 10 or more, inorganic particles having lithium ion transportability, and mixtures thereof.

[0092] Non-limiting examples of the inorganic particles having a dielectric constant of 5 or more can include any one selected from the group consisting of BaTiO3, Pb(Zr,Ti)O3(PZT), Pb 1-x La x Zr 1-y Ti y O3(PLZT), Pb(Mg 1 / 3 Nb 2 / 3) O3-PbTiO3(PMN-PT), hafnium oxide (HfO2), SrTiO3, SnO2, CeO2, MgO, Mg(OH2), NiO, CaO, ZnO, ZrO2, Y2O3, Al2O3, TiO2, SiC, AlO(OH), Al2O3.H2O, or mixtures thereof.

[0093] As used herein, "inorganic particles having lithium ion transport capability" refers to inorganic particles that contain lithium element and are capable of transporting lithium ions but do not store lithium. Non-limiting examples of inorganic particles having lithium ion transport capability include: lithium phosphate (Li3PO4), lithium titanium phosphate (Li x Ti y (PO4)3, 0 < x < 2, 0 < y < 3), lithium aluminum titanium phosphate (Li x Al y Ti z (PO4)3, 0 < x < 2, 0 < y < 1, 0 < z < 3), (LiAlTiP) x O y based glass (1 < x < 4, 0 < y < 13), such as 14Li2O-9Al2O3-38TiO2-39P2O5, lithium lanthanum titanate (Li x La y TiO3, 0 < x < 2, 0 < y < 3), lithium germanium thiophosphate (Li x Ge y P z S w , 0 < x < 4, 0 < y < 1, 0 < z < 1, 0 < w < 5), such as Li 3.25 Ge 0.25 P 0.75 S4, lithium nitride (Li x N y , 0 < x < 4, 0 < y < 2), such as Li3N, SiS2-based glass (Li x Si y S z , 0 < x < 3, 0 < y < 2, 0 < z < 4), such as Li3PO4-Li2S-SiS2, and P2S5-based glass (Li x P y S z , 0 < x < 3, 0 < y < 3, 0 < z < 7), such as LiI-Li2S-P2S5, or mixtures thereof.

[0094] According to embodiments of the present disclosure, the organic-inorganic composite porous layer can be an organic coating using an organic slurry or an aqueous coating using an aqueous slurry. In particular, the aqueous coating is more useful because it facilitates thin film coating and reduces the electrical resistance of the separator.

[0095] A separator including an organic-inorganic composite porous layer on at least one surface of a porous polymer substrate and having enhanced stability can be obtained as follows.

[0096] First, to form the organic-inorganic composite porous layer, a slurry (synthetic material) for forming the organic-inorganic composite porous layer is prepared by dissolving or dispersing a binder polymer in a dispersion medium, adding inorganic particles thereto, and dispersing them therein. The inorganic particles can be added after being previously pulverized to a prescribed average particle diameter. Otherwise, the inorganic particles can be added to a mixture of the binder polymer and the dispersion medium, and then pulverized and dispersed while being controlled to have a predetermined average particle diameter by treatment using a ball milling method or the like.

[0097] The synthetic material for forming the organic-inorganic composite porous layer is coated on the porous polymer substrate. Although there is no particular limitation on the coating method, it is preferable to use a slot coating or dip coating process. The slot coating process includes coating the synthetic material supplied through a slot die onto the entire surface of the substrate, and the thickness of the coated layer can be controlled according to the flux supplied from a metering pump. In addition, the dip coating process includes immersing the substrate in a tank containing the synthetic material to perform coating, and the thickness of the coated layer can be controlled according to the concentration of the synthetic material and the rate at which the substrate is removed from the tank. Furthermore, to more precisely control the thickness of the coating, post-metering can be performed by a Mayer bar or the like after impregnation.

[0098] Then, the porous polymer substrate coated with the synthetic material for forming the organic-inorganic composite porous layer can be dried in a dryer such as an oven to form the organic-inorganic composite porous layer on at least one surface of the porous polymer substrate.

[0099] According to an embodiment of the present disclosure, the binder polymer adheres the inorganic particles to each other so that they can maintain their adhered state (i.e., the binder polymer interconnects and fixes the inorganic particles). In addition, the inorganic particles are bound to the porous polymer substrate by the binder polymer. In the organic-inorganic composite porous layer, the inorganic particles can form interstitial volumes while they are substantially in contact with each other. Herein, the interstitial volume refers to a space defined by the inorganic particles substantially in contact with each other in a close-packed or dense-packed structure of the inorganic particles. The interstitial volume among the inorganic particles becomes a void to form a pore of the organic-inorganic composite porous layer.

[0100] Non-limiting examples of the dispersion medium include any one selected from the group consisting of acetone, tetrahydrofuran, dichloromethane, chloroform, dimethylformamide, N-methyl-2-pyrrolidone, methyl ethyl ketone, cyclohexane, methanol, ethanol, isopropanol, propanol, and water, or a mixture of two or more thereof.

[0101] Further, according to one embodiment of the present disclosure, after the composition for forming the organic-inorganic composite porous layer is coated on the porous polymer substrate, it can be dried at a temperature of 20-70℃ or 23-60℃ and a relative humidity of 30-80% or 50-80% for 1 minute to 2 hours, 5 minutes to 1 hour, or 10 minutes to 1 hour to remove the solvent. In this way, it is possible to prepare a separator including the organic-inorganic composite porous layer on at least one surface thereof and having enhanced stability.

[0102] The separator obtained by coating a slurry including inorganic particles, a binder polymer, and a dispersion medium on at least one surface of the porous substrate classified as a good product and drying to form an organic-inorganic composite porous layer can have excellent air permeability characteristics of 600 sec / 100cc or less.

[0103] When the method for pre-detecting a defective porous polymer substrate for a separator according to the embodiment of the present disclosure is used, the result of pre-detecting a good product / defective product obtained by comparing the corrected average of the pore distribution index in the state of the porous polymer substrate with the reference value can completely correspond to the result of the air permeability characteristics of the finished separator having the organic-inorganic composite porous layer.

[0104] Examples will be described more fully below in order to enable easy understanding of the present disclosure. However, the following examples can be embodied in many different forms and should not be construed as being limited to the exemplary embodiments set forth therein. Rather, these exemplary embodiments are provided so that the disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0105] Preparation of Example 1

[0106] (1) Preparation of a porous polymer substrate

[0107] First, 9.0 kg / hr of high-density polyethylene (Korea Petrochemical Ind. Co. Ltd., VH035) having a weight average molecular weight of 350,000 as a polyolefin, and 21.0 kg / hr of liquid paraffin oil (Kukdong Oil & Chem. LP350F, 68 cSt) as a diluent were introduced into a press machine and mixed therein.

[0108] Herein, the press machine is a twin-screw press machine including a hopper configured to introduce the polyolefin and the diluent, a kneading unit configured to transport and mix the introduced components, and a die outlet configured to press the mixed components.

[0109] The polyethylene composition extruded by the extruder was molded into a sheet-like shape by a T-die and a cast roll. Then, biaxial orientation was performed by using a tenter-type sequential orientation machine to perform MD (machine direction) orientation and then TD (transverse direction) orientation. Herein, each of the MD orientation ratio and the TD orientation ratio was 7.0. The orientation temperature was 110°C in the MD and 125°C in the TD.

[0110] Then, the diluent was extracted from the oriented sheet by using dichloromethane, and the sheet from which the diluent was extracted was heat-fixed at a temperature of 126°C to obtain a polyolefin separator. Herein, in the heat-fixing step, the sheet introduced into the heat-fixing step was elongated to 40% of the width thereof in the width direction of the sheet, and then peeled to 15% to obtain a porous polyolefin base material (porous polymer base material). The thickness of the obtained porous polyolefin base material was 8.6 μm.

[0111] (2) Preparation of separator having organic-inorganic composite porous layer

[0112] First, polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) (weight average molecular weight 390,000, HFP content 8 wt%) and polyvinylidene fluoride-co-chlorotrifluoroethylene (PVDF-CTFE) (weight average molecular weight 450,000, CTFE content 20 wt%) as binder polymers and cyanoethylvinyl alcohol (also used as a binder polymer) as a dispersant were added to acetone and dissolved therein at 50°C for about 12 hours to prepare a binder polymer solution. Then, alumina Al203 (average particle diameter 500 nm) as an inorganic particle was added to the obtained binder polymer solution and dispersed therein to prepare a slurry for an organic-inorganic composite porous layer. Herein, the weight ratio of PVDF-HFP: PVDF-CTFE: dispersant: coupling agent: inorganic particle was 21:7:1:1:70.

[0113] The obtained slurry for an organic-inorganic composite porous layer was coated on both surfaces of the porous polyolefin base material prepared as described above, and dried at a temperature of 23°C and a relative humidity of 50% for 10 minutes to obtain a separator having an organic-inorganic composite porous layer on both surfaces thereof (total thickness of the organic-inorganic composite porous layer on both surfaces: about 6 μm).

[0114] Preparation of Example 2

[0115] A porous polyolefin substrate was obtained in the same manner as in Preparation Example 1, except that, in addition to 7.0 kg / hr of high-density polyethylene having a weight average molecular weight of 350,000 as a polyolefin (Korea Petrochemical Ind. Co. Ltd., VH035) and 2.0 kg / hr of high-density polyethylene having a weight average molecular weight of 600,000 (Korea Petrochemical Ind. Co. Ltd., VH105U) were introduced into the press, the orientation temperature at TD was controlled to 130°C. Herein, the resulting porous polyolefin substrate had a thickness of 9.1 μm. In addition, the temperature at TD orientation of 130°C simulated a slight increase in the processing temperature due to some malfunction of the air supply / exhaust motor, heater, filter, etc. during the manufacturing process at TD orientation.

[0116] In addition, a separator having organic-inorganic composite porous layers on both surfaces thereof (total thickness of the organic-inorganic composite porous layers on both surfaces: about 6 μm) was obtained in the same manner as in Preparation Example 1, except that the porous polyolefin substrate obtained as described above was used.

[0117] Preparation Example 3

[0118] A porous polyolefin substrate was obtained in the same manner as in Preparation Example 1, except that, in addition to 7.0 kg / hr of high-density polyethylene having a weight average molecular weight of 350,000 as a polyolefin (Korea Petrochemical Ind. Co. Ltd., VH035) and 2.0 kg / hr of high-density polyethylene having a weight average molecular weight of 600,000 (Korea Petrochemical Ind. Co. Ltd., VH105U) were introduced into the press, the orientation temperature at TD was controlled to 130°C. Herein, the resulting porous polyolefin substrate had a thickness of 9.1 μm. In addition, the temperature at TD orientation of 130°C simulated a slight increase in the processing temperature due to some malfunction of the air supply / exhaust motor, heater, filter, etc. during the manufacturing process at TD orientation.

[0119] In addition, a separator having organic-inorganic composite porous layers on both surfaces thereof (total thickness of the organic-inorganic composite porous layers on both surfaces: about 6 μm) was obtained in the same manner as in Preparation Example 1, except that the porous polyolefin substrate obtained as described above was used.

[0120] Preparation Example 4

[0121] A separator for a filter, not a separator for a lithium secondary battery, was used to determine the corrected average PDI. This separator had an average pore diameter of 120 nm and a maximum pore size of 140 nm, and thus could not be used as a separator for a secondary battery.

[0122] A separator for a filter was obtained by the following process.

[0123] First, 100 parts by weight of a PTFE resin 650J TM(available from MCF Co.) was mixed with 22 parts by weight of a lubricating oil (Isopar H, available from Exxon Co.) to prepare a composition containing a fluorinated resin, and the composition was aged at 38°C for 24 hours. Next, a preform was produced by applying a pressure of 4 MPa, and the preform was pressed into the form of a sheet having a thickness of 1 mm using a paste press. Then, the sheet was calendered to a thickness of 300 μm to obtain a PTFE film. The resulting PTFE film was heat-treated by roll-to-roll processing in an oven heated to 200°C to completely remove the lubricant.

[0124] The heat-treated PTFE film was oriented 3 times in the machine direction by using a roll speed difference at 300°C, and the roll speed was lowered to allow relaxation, so that 10% shrinkage in the machine direction (MD) at 310°C can occur.

[0125] After that, the film was oriented 10 times in the transverse direction at 280°C by using a roll speed difference, and the film was heat-set at 370°C for 9 seconds by using a heated roll to obtain a PTFE porous film as a separator for a processed filter.

[0126] Preparation Example 5

[0127] A porous polyolefin substrate was obtained in the same manner as in Preparation Example 1, except that the heat-setting temperature was increased to 135°C to simulate problems related to temperature that occur at heat-setting. Here, the thickness of the resulting porous polyolefin substrate was 8.7 μm. This separator showed an excessively long air permeation time, and thus could not be used for a secondary battery.

[0128] Pre-detection of a defective porous polyolefin substrate

[0129] First, each of the porous polyolefin substrates according to Preparation Examples 1-5 was observed at a magnification of 20,000 using a scanning electron microscope (SEM) (S-4800, available from Hitachi) under conditions of 5 kV and 10 μA to obtain an image of each porous polymer substrate. The SEM images of the porous polyolefin substrates according to Preparation Examples 1-5 are shown in FIGS. 1-5, respectively. Figures 1a-5a

[0130] ​The image of the porous polymer substrate is used to quantify the average value of the pore distribution index (PDI). Herein, in the step of quantifying the average value of the PDI, a two-dimensional (2-D) binary image is obtained in which the pixels of the original image of the porous polymer substrate taken with a scanning electron microscope (SEM) are divided into pixels representing a plurality of polymer fibril regions adjacent to each other or striding over each other and pixels representing pore regions formed among the plurality of polymer fibril regions. Then, a distance transformation of the 2-D binary image is performed by using a computational Euclidean distance transformation method. The distance transformation results of the porous polyolefin substrates according to Preparation Examples 1-5 are respectively shown in Figures 1b-5b .

[0131] Then, a local maximum calculation is applied to the distance-transformed image to extract the distance-transformed values of all the pixels in the image and to perform a statistical processing, thereby providing a quantified average value of the pore distribution index (average PDI before correction).

[0132] Thereafter, the quantified average value of the pore distribution index (average PDI before correction) is corrected to obtain a corrected average value of the pore distribution index (average PDI after correction) according to the following formula:

[0133] Corrected average PDI = average PDI before correction x (1 / η),

[0134] wherein η represents a correction factor and is defined by the formula of η = (number of polymer fibril pixels) / (number of total image pixels).

[0135] The corrected average value of the pore distribution index (average PDI after correction) and the graph of the corrected pore distribution index of the porous polyolefin substrate according to each of Preparation Examples 1-5 are respectively shown in Figure 1c , Figure 2c , Figure 3c , Figure 4c and Figure 5c .

[0136] In particular, Figures 6a-6d the original image of the porous polyolefin substrate according to Preparation Example 1, its two-dimensional (2-D) binary image, the quantified average value of its pore distribution index (average PDI before correction), and the graph of the corrected average value of its pore distribution index (average PDI after correction) are respectively illustrated. Figures 7a-7d the original image of the porous polyolefin substrate according to Preparation Example 4, its two-dimensional (2-D) binary image, the quantified average value of its pore distribution index (average PDI before correction), and the graph of the corrected average value of its pore distribution index (average PDI after correction) are respectively illustrated.

[0137] Reference is made to Figures 6a-6d and Figures 7a-7dWhen comparing the raw images and 2-D binary images of the porous polyolefin substrate according to Preparation Example 1 and the porous polyolefin substrate according to Preparation Example 4, it can be seen that Preparation Example 1 has a uniformly dispersed and regularly sized pore region (black region in the 2-D binary image) compared to Preparation Example 4. However, both Preparation Example 1 and Preparation Example 4 showed an approximate average PDI of about 38 a.u. before correction. Then, with reference to the corrected average of the pore distribution index (corrected average PDI), the porous polyolefin substrate according to Preparation Example 1 showed a value of 39.46 a.u., while the porous polyolefin substrate according to Preparation Example 4 showed a significantly different value of 70.14 a.u.

[0138] From Figures 6a-6d and Figures 7a-7d It can be seen that this is because Preparation Example 1 shows a larger area ratio of the white region in the 2-D binary image compared to that of Preparation Example 4, and thus, has a larger correction factor η (0.914) (i.e., the ratio of the number of fibril (white region) pixels to the number of total image pixels) compared to the correction factor (0.667) according to Preparation Example 4.

[0139] Table 1 below shows the quantified average of the pore distribution index (average PDI before correction), the correction factor, and the corrected average of the pore distribution index (corrected average PDI) of the porous polyolefin substrate according to each of Preparation Examples 1-5.

[0140] Thereafter, it is determined whether the corrected average of the pore distribution index (corrected average PDI) is 60 a.u. or less. Then, when the corrected average of the pore distribution index is determined to be 60 a.u. or less, the porous polymeric substrate is classified as a good product, and when the corrected average of the pore distribution index is determined to be greater than 60 a.u., the porous polymeric substrate is classified as a defective product. The results are shown in Table 1.

[0141] Method for the evaluation of the air permeability (air permeation time)

[0142] The air permeability (Gurley) of the porous polymeric substrate according to each of Preparation Examples 1-5 and the air permeability of the separator including the organic-inorganic composite porous layer were determined by the method of ASTM D726-94. The results are shown in Table 1. Herein, the Gurley refers to the resistance to air flow and is determined by a Gurley densitometer. The air permeability value described herein is expressed in terms of the time (seconds) required for 100 cc of air to pass through a portion of the porous polymeric substrate or separator having an area of 1 in 2 of 12.2 in H2O of pressure, i.e., the air permeation time.

[0143] [Table 1]

[0144]

[0145] Referring to Table 1, in terms of the air permeation time of the porous polymer substrate before forming the organic-inorganic composite porous layer, it shows similar air permeation time to that of Preparation Example 3 which shows a corrected average of the pore distribution index of more than 60 a.u. in the case where the porous polymer substrate according to each of Preparation Examples 1 and 2 satisfies a corrected average of the pore distribution index of 60 a.u. or less. However, in terms of the air permeation time of the separator obtained after forming the organic-inorganic composite porous layer, the separator according to Preparation Example 1 shows a value of about 210-220 sec / 100 cc, which is about 30% lower than the air permeation time of the separator using the porous polymer substrate according to Preparation Example 3 after forming the organic-inorganic composite porous layer. From the above results, it can be seen that, in the case of using the pre-detection method for defects of the porous polymer substrate for the separator according to the present embodiment, the result of pre-detection of good products / defective products obtained by comparing the corrected average of the pore distribution index in the state of the porous polymer substrate with the reference value corresponds exactly to the result of the air permeation characteristics of the finished separator provided with the organic-inorganic composite porous layer. According to the prior art, the separator including the organic-inorganic composite porous layer is manufactured, and then evaluated in terms of air permeability, and defective products are classified and discarded. However, according to the pre-detection method of the present application, it is possible to pre-detect defective products in the state of the porous polymer substrate, and thus, it is possible to provide a significantly improved time and cost efficiency.

Claims

1. A method for pre-inspecting defective porous polymer substrates used as separators, comprising the following steps: Select a porous polymer substrate with multiple pores to test whether it is a good or defective product. The selected porous polymer substrate was observed using a scanning electron microscope (SEM) to obtain an image of the porous polymer substrate; The average value of the pore distribution index (PDI) was quantified using the obtained image of the porous polymer substrate; The quantized average value of the pore distribution index is corrected to obtain the corrected average value of the pore distribution index; Determine whether the corrected average value of the pore distribution index is 60 or less in any unit; and When the corrected average value of the pore distribution index is determined to be 60 or less in any unit, the porous polymer substrate is classified as a good product; and when the corrected average value of the pore distribution index is determined to be greater than 60 in any unit, the porous polymer substrate is classified as a defective product. The step of averaging the quantized aperture distribution index includes: A two-dimensional 2D binary image is obtained. In the two-dimensional 2D binary image, the pixels of the original image of the porous polymer substrate captured by scanning electron microscopy (SEM) are divided into pixels representing multiple polymer fibrillary regions that are adjacent to or cross each other, and pixels representing pore regions formed in the multiple polymer fibrils. The two-dimensional 2D binary image is subjected to a distance transformation using the Euclidean distance transformation method; and Local maxima calculation is applied to the distance-transformed image to extract the distance transform values ​​of all pixels in the image, and statistical processing is performed. The step involves correcting the quantized average PDI (Purity Distribution Index) before correction by calculating the pore distribution index (PDI) according to the following formula, thus obtaining the corrected average PDI: The corrected average PDI = the original average PDI × (1 / η), Where η represents the correction factor, and is defined by the formula η = (number of polymer fibril pixels) / (total number of image pixels).

2. The method for pre-detecting defective porous polymer substrates for separators according to claim 1, wherein, The separator obtained by coating a slurry containing inorganic particles, a binder polymer and a dispersion medium onto at least one surface of a porous substrate classified as good quality and drying it to form an organic-inorganic composite porous layer has an air permeability of 600 sec / 100cc or less.

3. The method for pre-detecting defective porous polymer substrates for separators according to claim 1, wherein, When the corrected average value of the pore distribution index is determined to be 40 or less in any unit, the porous polymer substrate is classified as good, and when the corrected average value of the pore distribution index is determined to be greater than 40 in any unit, the porous polymer substrate is classified as defective.

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