Separation membranes for electrochemical devices

Abstract

The present invention relates to a separator and an electrochemical device including the same. The separator of the present invention includes a porous polyolefin substrate having improved wettability and a porous coating layer having excellent adhesion to the porous polyolefin substrate. Specifically, the separator includes a porous polyolefin substrate and a porous coating layer formed on at least one surface of the porous polyolefin substrate, the porous polyolefin substrate including a cross-linked polyolefin resin, the cross-linked polyolefin resin including silicon-containing organic groups grafted to polyolefin chains, and the porous coating layer including inorganic particles and a binder polymer, the inorganic particles including bohemite (AlO(OH)).

Description

[Technical Field]

[0001] The present invention relates to a separator for an electrochemical device and an electrochemical device including the same.

[0002] This application claims priority based on Korean Patent Application No. 2023-0030218, filed on March 7, 2023, the entire contents of which are incorporated herein by reference in their entirety in the specification and drawings thereof. [Background technology]

[0003] Secondary batteries, such as lithium-ion secondary batteries, are widely used as power sources for portable electronic devices such as laptops, mobile phones, digital cameras, and video cameras. Recently, these batteries have been applied to various fields, such as automobiles, due to their high energy density.

[0004] Lithium secondary batteries have been gaining attention due to their advantages of higher operating voltage and significantly greater energy density compared to conventional batteries that use aqueous electrolytes, such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries. However, lithium-ion batteries have drawbacks, such as safety issues such as fire and explosion due to the use of organic electrolytes, and they are difficult to manufacture. Recent lithium-ion polymer batteries have overcome these weaknesses and are attracting attention as one of the next-generation batteries. However, their battery capacity is still relatively low compared to lithium-ion batteries, and their discharge capacity, especially at low temperatures, is insufficient, and improvements are urgently needed.

[0005] Evaluating the stability and ensuring the safety of such electrochemical devices is extremely important. Regarding the safety characteristics of electrochemical devices, overheating can lead to thermal runaway or penetration of the separator, which can lead to an explosion. In particular, polyolefin-based separator substrates commonly used as separators for electrochemical devices exhibit severe thermal shrinkage at temperatures above 100°C due to material properties and manufacturing process characteristics, including stretching, which can lead to short circuits between the positive and negative electrodes.

[0006] To address these safety issues in electrochemical devices, separators have been proposed that include a porous inorganic coating layer formed by coating at least one surface of a separator substrate having a plurality of pores with a mixture of an excess of inorganic particles and a binder polymer. However, there is a continuing need for further enhancement of stability. For example, the need for strengthening the adhesion strength at the interface between the separator substrate and a porous inorganic coating layer containing an excess of inorganic particles has been discussed. Summary of the Invention [Problem to be solved by the invention]

[0007] Therefore, the problem to be solved by the present invention is: The object of the present invention is to provide a separation membrane having enhanced wettability of a separation membrane substrate and improved adhesive strength between the separation membrane substrate and a porous inorganic coating layer containing an excess amount of inorganic particles.

[0008] Thus, the present invention aims to provide an electrochemical device with improved stability and excellent resistance characteristics. [Means for solving the problem]

[0009] In order to solve the above problems, According to one aspect of the present invention, there is provided a separation membrane substrate of the following embodiment.

[0010] The separation membrane substrate according to the first embodiment is A porous polyolefin substrate and a porous coating layer formed on at least one surface of the porous polyolefin substrate, The porous polyolefin substrate is Contains a crosslinked polyolefin resin, The crosslinked polyolefin resin comprises silicon-containing organic groups grafted onto polyolefin chains; The porous coating layer comprises: Contains inorganic particles and a binder polymer, The inorganic particles include bohemite (AlO(OH)).

[0011] According to the second embodiment, in the first embodiment, The porous polyolefin substrate is The gel fraction is 3% to 80%; The standard deviation (Δd) of the thickness measured at any of at least 100 locations may be 0.5 μm or less.

[0012] According to the third embodiment, in the first or second embodiment, The adhesive strength between the porous polyolefin substrate and the porous coating layer may be 50 gf or more.

[0013] According to the fourth embodiment, in any one of the first to third embodiments, The adhesive strength between the porous polyolefin substrate and the porous coating layer may be 70 gf to 95 gf.

[0014] According to the fifth embodiment, in any one of the first to fourth embodiments, The content of the bohemite is The inorganic particles may account for 80 wt % or more of the total weight of the inorganic particles.

[0015] According to the sixth embodiment, in the fifth embodiment, the silicon-containing organic group is a residue derived from a vinyl group-containing silane compound, The vinyl group-containing silane compound may include vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilane, (3-methacryloxypropyl)trimethoxysilane, (3-methacryloxypropyl)triethoxysilane, vinylmethyldimethoxysilane, vinyl-tris(2-methoxyethoxy)silane, vinylmethyldiethoxysilane, or a mixture of two or more thereof.

[0016] According to the seventh embodiment, in any one of the first to sixth embodiments, The porous polyolefin substrate may further include chromium (Cr).

[0017] According to the eighth embodiment, in any one of the first to seventh embodiments, The chromium content may be 0.1 ppm to 20 ppm.

[0018] According to the ninth embodiment, in any one of the first to eighth embodiments, The crosslinked structure in the crosslinked polyolefin resin may include a structure resulting from a radical polymerization reaction between vinyl groups via a thermal initiator.

[0019] According to the tenth embodiment, in any one of the first to ninth embodiments, The thermal initiator may include a peroxide-based compound, a persulfate-based compound, an azo-based compound, or a mixture thereof.

[0020] According to another aspect of the present invention, there is provided an electrochemical device having the following embodiments.

[0021] The electrochemical device according to the eleventh embodiment is The battery includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and the separator is according to any one of the first to tenth embodiments. [Effects of the Invention]

[0022] The separator according to one embodiment of the present invention may have excellent adhesive strength between the porous polyolefin substrate and the porous coating layer, and excellent wettability of the separator substrate.

[0023] In particular, the porous polyolefin substrate as the separator substrate exhibits improved thickness uniformity and heat resistance due to the inclusion of many crosslinked structures in the polyolefin chains, improved wettability with the electrolyte due to the inclusion of silicon-containing organic groups grafted to the polyolefin chains, and improved adhesion between the porous coating layer and the separator substrate. Furthermore, the porous coating layer exhibits improved adhesion to the porous polyolefin substrate due to the inclusion of bohemite as inorganic particles.

[0024] This can effectively improve the operational stability, such as heat resistance, of an electrochemical device including a separator using such a separator substrate, but the mechanism of the present invention is not limited to this. DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention will be described in detail below.

[0026] The present invention relates to a separator for an electrochemical device and an electrochemical device including the separator. In the present invention, the electrochemical device is a device that converts chemical energy into electrical energy through an electrochemical reaction, and includes both primary and secondary batteries. The secondary battery is capable of charging and discharging, and includes lithium-ion batteries, nickel-cadmium batteries, nickel-metal hydride batteries, etc.

[0027] In one embodiment of the present invention, the separator for an electrochemical device may be a safety reinforced separator (SRS), a ceramic coated separator (CCS), or a separator formed by another known manufacturing method.

[0028] In one embodiment of the present invention, the SRS is a manufacturing method that typically uses a solution-based binder resin, i.e., a binder resin-containing solution, to form the porous coating layer. This solution-based binder resin penetrates into the porous separation membrane substrate, thereby improving the adhesive strength between the separation membrane substrate and the porous coating layer. However, there are still issues that need to be addressed, such as weak bonding strength with the solution-based binder resin due to the hydrophobic properties of typical separation membrane substrates, and weak adhesive strength inherent to the porous coating layer due to the large amount of inorganic particles that make up the porous coating layer.

[0029] In another embodiment of the present invention, the CCS is generally known to have a weaker adhesive strength between the separator substrate and the porous coating layer than the SRS because the absolute amount of binder resin used in the CCS production method is generally smaller than that of the SRS production method and a particulate binder resin is used in forming the porous coating layer.

[0030] According to one aspect of the present invention, a separation membrane having excellent adhesion between a separation membrane substrate and a porous coating layer is provided, without being limited to a manufacturing method such as an SRS manufacturing method, a CCS manufacturing method, or other known manufacturing methods.

[0031] To this end, a separator for an electrochemical device according to one aspect of the present invention includes a separator substrate having improved heat resistance and wettability, and a porous coating layer having excellent adhesion to the separator substrate, as described below.

[0032] Specifically, the separator includes a porous polyolefin substrate and a porous coating layer formed on at least one surface of the porous polyolefin substrate, the porous polyolefin substrate comprises a crosslinked polyolefin resin, the crosslinked polyolefin resin comprising silicon-containing organic groups grafted onto polyolefin chains; The porous coating layer includes inorganic particles and a binder polymer, and the inorganic particles include bohemite (AlO(OH)).

[0033] According to one embodiment of the present invention, the porous polyolefin substrate may contain a crosslinked polyolefin resin and chromium (Cr).

[0034] First, a porous polyolefin substrate will be described in detail as a separation membrane substrate having improved heat resistance and wettability.

[0035] Porous polyolefin substrate The porous polyolefin substrate is a separator substrate for electrochemical devices that contains a crosslinked polyolefin resin, and the crosslinked polyolefin resin contains silicon-containing organic groups grafted to polyolefin chains.

[0036] In one embodiment of the present invention, the crosslinked polyolefin resin contains chromium (Cr) derived from a chromium (Cr)-containing catalyst used in the production of polyolefin, but the present invention is not limited thereto.

[0037] In one embodiment of the present invention, the porous polyolefin substrate may have a gel fraction of 3% to 80%.

[0038] In one embodiment of the present invention, the porous polyolefin substrate may have a standard deviation (Δd) of thickness measured at any one of at least 100 points of 0.5 μm or less.

[0039] In another embodiment of the present invention, the porous polyolefin substrate may have a gel fraction of 3% to 80%, and a standard deviation (Δd) of thickness measured at at least 100 arbitrary points may be 0.5 μm or less.

[0040] In one embodiment of the present invention, the porous polyolefin substrate is 1 m 2 The number of spots each having a long side length of 50 μm or more may be 10 or less.

[0041] In another embodiment of the present invention, the porous polyolefin substrate has a gel fraction of 3% to 80%, a standard deviation (Δd) of thickness measured at at least 100 arbitrary points is 0.5 or less, and 2 The number of spots each having a long side length of 50 μm or more may be 10 or less.

[0042] According to an embodiment of the present invention, the polyolefin resin is not particularly limited in its monomer as long as it is used as a porous separation membrane substrate. For example, the polyolefin resin may be, but is not limited to, a homopolymer of a monomer selected from polyethylene, polypropylene, polybutylene, polypentene, polyhexene, polyoctene, ethylene, propylene, butene, pentene, 4-methylpentene, hexene, and octene, a copolymer of two or more thereof, or a mixture thereof.

[0043] As will be described later in the description of the method for producing the porous polyolefin substrate, the polyolefin resin may contain chromium (Cr) by being produced using an olefin polymerization catalyst containing chromium (Cr). Specifically, the porous polyolefin substrate contains chromium as a residue of the chromium catalyst used in the polymerization of the polyolefin resin.

[0044] In one embodiment of the present invention, the chromium-containing olefin polymerization catalyst comprises, for example, chromium oxide and a support supporting the chromium oxide. The support may comprise, for example, at least one component selected from the group consisting of silica, titania, alumina, zirconia, and aluminum phosphate, but the present invention is not limited thereto.

[0045] In one embodiment of the present invention, the chromium content in the porous polyolefin substrate may be, for example, but not limited to, 0.1 ppm to 20 ppm, 1 ppm to 10 ppm, or 5 ppm to 10 ppm. The chromium content in the porous polyolefin substrate may be, for example, a value measured using an inductively coupled plasma mass spectrometer (ICP-MS). When the chromium content in the porous polyolefin substrate is within the above range, advantageous effects can be obtained in terms of the number of vinyl groups in the polyolefin resin before crosslinking and the degree of crosslinking of the crosslinked polyolefin resin produced using the same, but the present invention is not limited thereto.

[0046] According to one embodiment of the present invention, the polyolefin resin produced using the chromium-containing olefin polymerization catalyst is characterized by containing a large number of active terminal vinyl groups that can be crosslinked by a thermal initiator contained in a coating solution in a subsequent process. As a result, the porous polyolefin substrate includes a polyolefin resin in which multiple crosslinked structures are formed between polyolefin chains through a crosslinking reaction caused by a thermal initiator.

[0047] According to one embodiment of the present invention, the polyolefin resin produced using the chromium-containing olefin polymerization catalyst is characterized by containing many active terminal vinyl groups that provide sites for grafting of a vinyl-containing silane compound contained in a coating solution in a subsequent process. As a result, the porous polyolefin substrate contains a silicon-containing organic group due to the grafting of a plurality of vinyl-containing silane compounds through the terminal vinyl groups.

[0048] In one embodiment of the present invention, the silicon-containing organic groups grafted to the polyolefin chains are present on the surface of the porous polyolefin substrate, thereby imparting polarity to the surface of the porous polyolefin substrate and improving chemical affinity with inorganic particles in a porous coating layer formed on the porous polyolefin substrate.

[0049] In this specification, the term "crosslinked polyolefin resin" refers to a polyolefin resin in which vinyl groups present in the chains of a polyolefin resin used as a raw material for a porous polyolefin substrate are activated by an initiation reaction, thereby forming a crosslinked structure within and / or between the chains of the polyolefin resin.

[0050] Specifically, in the crosslinked polyolefin resin, a vinyl group present at one end of the polyolefin chain is activated by a thermal initiator to form a radical, and the formed radical reacts with a radical formed in the polyolefin chain of another molecule and / or a radical formed at another end of the polyolefin chain of the same molecule to form C(Sp 2 )-C(Sp 2 ) linked cross-linked structures.

[0051] In one embodiment of the present invention, the "silicon-containing organic group grafted onto a polyolefin chain" refers to a residue derived from the vinyl-group-containing silane compound, which is formed by activating a vinyl group present in the chain of a polyolefin resin used as a raw material for a porous polyolefin substrate with a vinyl group present in a vinyl-group-containing silane compound through an initiation reaction, and forming a new covalent bond at the activated position.

[0052] In one embodiment of the present invention, the silicon-containing organic group refers to an organic residue derived from the vinyl group-containing silane compound. The vinyl group-containing silane compound may include, for example, vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilane, (3-methacryloxypropyl)trimethoxysilane, (3-methacryloxypropyl)triethoxysilane, vinylmethyldimethoxysilane, vinyl-tris(2-methoxyethoxy)silane, vinylmethyldiethoxysilane, or a mixture of two or more thereof, but the present invention is not limited thereto.

[0053] In one embodiment of the present invention, the crosslinked polyolefin resin may contain no terminal vinyl groups or may contain a number of terminal vinyl groups that is less than the number of terminal vinyl groups present in the polyolefin resin before crosslinking.

[0054] In a similar manner, the crosslinked polyolefin resin has a C(Sp 2 )-C(Sp 2 ) bonds. 2 )-C(Sp 2 ) bonds.

[0055] In one embodiment of the present invention, the polyolefin resin produced using the chromium-containing olefin polymerization catalyst may contain 100 or more, 200 or more, 300 or more, 400 or more, 500 or more, 600 or more, 700 or more, 800 or more, 900 or more, or 950 or more terminal vinyl groups per million carbon atoms, as determined from the number of functional groups in a H-NMR spectrum using a nuclear magnetic resonance spectrometer (Bruker 500 NMR, 14.1 telsa). The upper limit of the number of terminal vinyl groups is, but is not limited to, 1,500 or 1,000 within the above range.

[0056] Thus, in one embodiment of the present invention, the number of terminal vinyl groups of the polyolefin resin before crosslinking may be, for example, 100 or more, 200 or more, 300 or more, 400 or more, 500 or more, 600 or more, 700 or more, 800 or more, 900 or more, or 950 or more per million carbon atoms.

[0057] In still another embodiment of the present invention, when the porous polyolefin substrate also contains a polyolefin resin produced using a catalyst other than the chromium-containing olefin polymerization catalyst, the number of terminal vinyl groups of the polyolefin resin before crosslinking and the content of terminal vinyl groups of the polyolefin resin before crosslinking are preferably measured based on the number of terminal vinyl groups and the content of terminal vinyl groups of the entire polyolefin resin.

[0058] As described above, the crosslinked structure in the crosslinked polyolefin resin includes a structure resulting from a radical polymerization reaction between vinyl groups via a thermal initiator.

[0059] In one embodiment of the present invention, the thermal initiator can be any initiator that can activate a vinyl group present in the polyolefin chain to form a radical. Specifically, any initiator that can activate a terminal vinyl group present in the polyolefin chain to form a radical can be used. The thermal initiator can include, for example, a peroxide-based compound, a persulfate-based compound, an azo-based compound, or a mixture thereof.

[0060] Examples of the peroxide compound include, but are not limited to, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DHBP), benzoin peroxide, acetyl peroxide, dilauryl peroxide, di-tert-butyl peroxide, dicumyl peroxide, cumyl peroxide, hydrogen peroxide, or a mixture of two or more thereof.

[0061] The persulfate compound has a peroxymonosulfate ion (SO5 2- ) and peroxydisulfate (peroxydisulfate: S2O8 2- The persulfate compound is not particularly limited as long as it contains at least one of the following: sodium peroxymonosulfate (NaSO), potassium peroxymonosulfate (KHSO), sodium peroxydisulfate (NaSO), ammonium peroxydisulfate ((NH)SO), potassium peroxydisulfate (KSO), or a mixture of two or more of these compounds, but is not limited to these.

[0062] The azo-based compound may include, but is not limited to, 2,2'-azobis(2-methylpropionitrile) (AIBN).

[0063] In one embodiment of the present invention, the porous polyolefin substrate may further include a polyolefin resin produced from another type of olefin polymerization catalyst in addition to the polyolefin resin produced from the chromium-containing olefin polymerization catalyst as described above.

[0064] The other type of olefin polymerization catalyst may be, for example, an olefin polymerization catalyst containing at least one of titanium (Ti), aluminum (Al), magnesium (Mg), zirconium (Zr), and vanadium (V). Accordingly, the separation membrane substrate may further contain, for example, at least one of titanium (Ti), aluminum (Al), magnesium (Mg), zirconium (Zr), and vanadium (V).

[0065] In one embodiment of the present invention, the other types of olefin polymerization catalysts can be, for example, metallocene catalysts, Ziegler catalysts, or mixtures thereof, but the present invention is not limited thereto.

[0066] In this specification, polyolefin resins produced using the chromium-containing olefin polymerization catalyst may be referred to as "Cr-type polyolefins," and polyolefin resins produced using other types of the olefin polymerization catalyst may be referred to as "ZT-type polyolefins."

[0067] In one embodiment of the present invention, when the porous polyolefin substrate contains a polyolefin resin produced using an olefin polymerization catalyst other than the polyolefin resin produced using a chromium-containing olefin polymerization catalyst, the weight ratio of the Cr-type polyolefin to the ZT-type polyolefin is, for example, but not limited to, 1:9 to 9:1, specifically 2:8 to 8:2, 3:7 to 7:3, or 5:5. When the separation membrane substrate contains both the Cr-type polyolefin and the ZT-type polyolefin, the high molecular weight ZT-type polyolefin can advantageously increase the molecular weight of the separation membrane substrate, but the present invention is not limited thereto.

[0068] In one embodiment of the present invention, the separation membrane may further include, in addition to a polyolefin material, at least one polymer resin such as polyethylene terephthalate, polybutylene terephthalate, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfide, polyethylene naphthalene, etc. The porous polyolefin substrate may be, but is not limited to, a nonwoven fabric, a porous polymer film, or a laminate of two or more of these.

[0069] In one embodiment of the present invention, as described below, the porous polyolefin substrate comprises a polyolefin resin produced using a chromium-containing olefin polymerization catalyst, and when a polyolefin sheet derived from the polyolefin resin is heat-set during the process of manufacturing a separator substrate, a radical polymerization reaction mediated by the thermal initiator forms crosslinked structures within and / or between chains in the polyolefin resin, thereby improving the thickness uniformity and heat resistance of the porous polyolefin substrate and also improving the appearance characteristics of the porous polyolefin substrate. Furthermore, when a polyolefin sheet derived from the polyolefin resin is heat-set, the vinyl group-containing silane compound is grafted, thereby improving the wettability of the porous polyolefin substrate, although the mechanism of the present invention is not limited thereto.

[0070] According to one embodiment of the present invention, the porous polyolefin substrate exhibits a gel fraction of 3% to 80%. This may be a characteristic exhibited by the porous polyolefin substrate containing a polyolefin resin with a high degree of crosslinking, but the characteristics of the present invention are not limited thereto. The gel fraction of the porous polyolefin substrate may be, for example, 3% to 70%, 3% to 60%, 3% to 50%, 3% to 45%, 3% to 40%, 3% to 30%, 3% to 20%, or 3% to 10%.

[0071] As described above, one of the characteristics of the polyolefin resin is that it exhibits a high degree of crosslinking due to the crosslinked structures formed within and / or between polyolefin chains. As a result, while the polyolefin resin before crosslinking dissolves in a benzene-based solvent, the crosslinked polyolefin resin does not dissolve in a benzene-based solvent, allowing the gel fraction to be measured.

[0072] In this specification, the gel fraction can be measured by the following method. First, 0.2 g of a test piece of a separation membrane substrate sample to be measured is placed in a 120 mesh stainless steel net, extracted in trichlorobenzene at 100°C for 12 hours, and then dried in a vacuum oven at 100°C for 12 hours. Thereafter, the weight of the test piece remaining on the stainless steel net is measured, and the gel fraction is measured using the following formula. The gel fraction can be expressed as the average value of the measurements of three test pieces to improve the accuracy of the measurement: Gel fraction (%) = {(weight of remaining test piece in g) / 0.2 g} × 100.

[0073] In one embodiment of the present invention, the porous polyolefin substrate has a standard deviation (Δd) of thickness measured at at least 100 random locations of 0.5 μm or less, thereby enabling the separator substrate to exhibit excellent thickness uniformity.

[0074] In one embodiment of the present invention, the standard deviation (Δd) of the thickness measured at any 100 points of the porous polyolefin substrate is, for example, 0.5 μm or less, 0.45 μm or less, 0.40 μm or less, 0.35 μm or less, 0.3 μm or less, or 0.25 μm or less. The closer the standard deviation is to 0, the greater the thickness uniformity, so the lower limit of the standard deviation of the thickness may be 0.

[0075] The thickness of the separation membrane substrate is measured by a method for measuring the thickness of a separation membrane substrate, and may be expressed as a value measured by, for example, SEM image analysis of the prepared separation membrane substrate or by using a known thickness measuring device, such as, but not limited to, a VL-50S-B (Mitutoyo).

[0076] Generally, when a separator substrate is produced by adding raw materials such as a polymer resin, a diluent, a crosslinking agent, and other additives into an extruder at once and reacting them, problems have been observed in that side reactions occur in the extruder or the raw materials are not mixed well with each other, resulting in spots on the surface of the separator substrate that are different in brightness from the surrounding area due to unmelted raw materials.

[0077] In this specification, the term "spots" refers to white spot-like regions on the surface of a separation membrane substrate that are brighter and less transparent than the surrounding area.

[0078] In one embodiment of the present invention, the number of spots can be evaluated by visual observation or microscopic observation such as SEM.

[0079] According to one embodiment of the present invention, the porous polyolefin substrate is prepared by adding a polyolefin resin as a raw material, forming it into a polyolefin sheet, and then adding a thermal initiator and a vinyl group-containing silane compound thereto. This reduces the occurrence of spots and improves appearance defects. For example, the separator substrate can be prepared in a 1m 2 The number of spots with a long side length of 50 μm or more per unit area may be 10 or less. Specifically, within the above conditions, the number of spots may be 0 to 7, 0 to 5, 0 to 3, or 0. By forming the number of spots within the above range, it may be possible to have an advantageous effect in preventing uncoated areas due to appearance defects during subsequent ceramic coating and the occurrence of short circuits within the battery.

[0080] In another embodiment of the present invention, the thickness of the porous polyolefin substrate may be, for example, 4 μm to 20 μm. When the thickness of the porous polyolefin substrate is in the above range, advantageous effects can be exhibited in terms of the conductive barrier function and the resistance of the separator, but the present invention is not limited thereto.

[0081] In one embodiment of the present invention, the weight-average molecular weight (Mw) of the polyolefin resin contained in the porous polyolefin substrate may be, for example, in the range of 100,000 to 5,000,000. When the weight-average molecular weight of the polyolefin resin is in the above range, advantageous effects can be achieved in terms of ensuring the mechanical properties and shutdown characteristics of the separation membrane substrate, but the present invention is not limited thereto.

[0082] In this specification, the weight average molecular weight (Mw) of the polyolefin resin can be measured using gel permeation chromatography (GPC, PL GPC220, Agilent Technologies) under the following conditions: - Column: PL Olexis (Polymer Laboratories) -Solvent: TCB (Trichlorobenzene) -Flow rate: 1.0ml / min - Sample concentration: 1.0 mg / ml -Injection volume: 200μl -Column temperature: 160°C -Detector: Agilent High Temperature RI detector - Standard: Polystyrene (corrected by a cubic function).

[0083] In one embodiment of the present invention, the porous polyolefin substrate has a structure including a large number of micropores, for example, an average pore diameter of 0.01 μm to 0.10 μm and a porosity of 30% to 70%. When the average pore diameter and porosity in the porous polyolefin substrate are within the above ranges, the separator using the porous polyolefin substrate can exhibit advantageous effects in terms of ion permeability and mechanical strength, but the present invention is not limited thereto.

[0084] In one embodiment of the present invention, the porous polyolefin substrate can exhibit excellent heat resistance. For example, the porous polyolefin substrate can exhibit a rupture temperature of 155°C or higher. For example, the rupture temperature of the porous polyolefin substrate can be 160°C or higher, 170°C or higher, 180°C or higher, 190°C or higher, 195°C or higher, or even 200°C or higher. The higher the rupture temperature of the porous polyolefin substrate, the better its heat resistance. Therefore, the upper limit of the rupture temperature of the porous polyolefin substrate is not particularly limited and can be, for example, 500°C or lower, 450°C or lower, 350°C or lower, or 300°C or lower.

[0085] In the present specification, the breaking temperature of the porous polyolefin substrate can be measured by applying a load of 0.01 N to a test piece of the porous polyolefin substrate to be measured using a thermal mechanical analysis (TMA) analyzer, and observing the degree of deformation while raising the temperature at a rate of 5°C / min. The breaking temperature can be measured as the temperature at which the porous polyolefin substrate shrinks with the rise in temperature, then expands again and breaks.

[0086] In one embodiment of the present invention, the porous polyolefin substrate may have the characteristic of improved wettability due to the inclusion of silicon-containing organic groups grafted to the polyolefin chains as described above.

[0087] In one embodiment of the present invention, the "wettability" of the porous polyolefin substrate refers to the property of permeating an electrolyte solution and can be evaluated by a conventional method for evaluating the degree of electrolyte solution permeation, for example, by measuring the area over which the electrolyte solution has spread after a certain period of time after a certain volume of electrolyte solution has been dropped onto the porous polyolefin substrate. The larger the area over which the dropped electrolyte solution has spread within the same period of time, the better the wettability of the porous polyolefin substrate.

[0088] The porous polyolefin substrate as described above may have the effect of improving appearance characteristics, thickness uniformity, heat resistance, and wettability by forming a crosslinked structure between polyolefin chains using a thermal initiator and a vinyl group-containing silane compound during heat setting of a polyolefin resin produced using a chromium-containing olefin polymerization catalyst and a polyolefin sheet stretched from the polyolefin resin, and by including a grafted silicon-containing organic group. However, the present invention is not limited thereto.

[0089] Next, the porous coating layer having excellent adhesive strength to the porous polyolefin substrate will be described in detail.

[0090] Porous coating layer The porous coating layer contains a binder resin and inorganic particles, has a plurality of micropores therein, and has a structure in which these micropores are interconnected, and has the structural characteristics of a porous layer that allows gas or liquid to pass from one side to the other.

[0091] In one embodiment of the present invention, the porous coating layer may have a porous structure resulting from pores formed by the interstitial volume between inorganic particles. The pore size and porosity (porosity ratio) can be adjusted depending on the particle size and size distribution. This structure enhances resistance to metallic foreign matter present in the electrode and suppresses shrinkage of the porous polyolefin substrate, thereby enhancing the safety of the electrochemical device.

[0092] In one embodiment of the present invention, the porous coating layer includes a plurality of nodes including the inorganic particles and a binder polymer covering at least a portion of the surface of the inorganic particles, and one or more filaments formed in a thread-like shape from the binder polymer at the nodes, the filaments having node-connecting portions extending from the nodes to connect other nodes, and the node-connecting portions may have a structure in which the plurality of filaments derived from the binder polymer cross each other to form a three-dimensional network structure.

[0093] In one embodiment of the present invention, as described above, the porous coating layer can be formed by a safety reinforced separator (SRS) manufacturing method, a ceramic coated separator (CCS) manufacturing method, or other known manufacturing methods.

[0094] As described above, according to one embodiment of the present invention, the porous polyolefin substrate may exhibit improved wettability due to the inclusion of silicon-containing organic groups grafted to polyolefin chains. In particular, the silicon-containing organic groups grafted to the polyolefin chains present on the surface of the porous polyolefin substrate may impart polarity to the surface of the porous polyolefin substrate, thereby improving chemical affinity with inorganic particles in a porous coating layer formed on the porous polyolefin substrate.

[0095] According to one aspect of the present invention, by providing a porous coating layer having particularly improved adhesive strength on a porous polyolefin substrate having improved wettability as described above, a separator can have excellent properties without being limited by its manufacturing method, and can also exhibit the effect of improving the properties of an electrochemical device using the separator.

[0096] For this purpose, the porous coating layer contains bohemite (AlO(OH)) as inorganic particles.

[0097] According to one embodiment of the present invention, the bohemite has an advantage of excellent adhesive strength with the porous polyolefin substrate of the present invention, even among aluminum-containing inorganic particles, due to the -OH functional group of the bohemite, but the present invention is not limited thereto.

[0098] According to one embodiment of the present invention, the separator may have an adhesive strength between the porous polyolefin substrate and the porous coating layer of, for example, 50 gf / 15 mm or more. Specifically, the adhesive strength between the porous polyolefin substrate and the porous coating layer may be 50 to 100 gf / 15 mm, 50 to 95 gf / 15 mm, 55 to 95 gf / 15 mm, 60 to 95 gf / 15 mm, 65 to 95 gf / 15 mm, 70 to 95 gf / 15 mm, 75 to 90 gf / 15 mm, or 80 to 85 gf / 15 mm. When the adhesive strength between the porous polyolefin substrate and the porous coating layer is within the above range, it can be advantageous in improving the performance of an electrochemical device using the separator, but the present invention is not limited thereto.

[0099] In this specification, the adhesive strength between the porous polyolefin substrate and the porous coating layer can be measured by a known method for measuring the peel strength between a porous polyolefin substrate and a porous coating layer, for example, as follows. Double-sided adhesive tape is attached to a glass plate, and a separator to be measured, cut to a size of 15 mm x 100 mm, is attached so that the surface of the porous coating layer is attached to the adhesive tape. The end of the attached separator is then placed in a Universal Testing Machine (UTM) and a force is applied at a measurement speed of 300 mm / min at an angle of 180° to measure the force required to peel the separator substrate from the porous coating layer. The UTM device used is not particularly limited, and for example, a LLOYD Instrument LF Plus can be used.

[0100] In one embodiment of the present invention, the porous coating layer includes bohemite (AlO(OH)) as inorganic particles to exhibit excellent adhesion to the separator substrate. Specifically, the content of the bohemite may be, for example, 80 wt % or more based on the total weight of the inorganic particles included in the porous coating layer. More specifically, the content of the bohemite may be, for example, 80 wt % to 100 wt %, 85 wt % to 99 wt %, 90 wt % to 95 wt %, 95 wt % to 99 wt %, or 95 wt % to 100 wt %, based on the total weight of the inorganic particles included in the porous coating layer, but the present invention is not limited thereto.

[0101] In another embodiment of the present invention, the porous coating layer may further include, as inorganic particles, conventional inorganic particles used in porous coating layers other than bohemite, within a range that does not impair the object of the present invention.

[0102] The inorganic particles that can be further included are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles that can be further included in the present invention are not particularly limited as long as they do not undergo oxidation and / or reduction reactions within the operating voltage range of the applied electrochemical device (e.g., 0 to 5 V based on Li / Li+). In particular, when inorganic particles with a high dielectric constant are further used as inorganic particles, they can contribute to increasing the degree of dissociation of electrolyte salts, such as lithium salts, in the liquid electrolyte, thereby improving the ionic conductivity of the electrolyte solution.

[0103] In one embodiment of the present invention, the inorganic particles that can be further included are specifically alumina (Al2O3), aluminum hydroxide (Al(OH)3), silica (SiO2), titanium dioxide (TiO2), magnesium oxide (MgO), magnesium hydroxide (Mg(OH)2), zinc oxide (ZnO), barium titanate (BaTiO), aluminum nitride (AIN), boron nitride (BN), silicon carbide (SiC), beryllium oxide (BeO), potassium nitrate (KNO3), monoammonium phosphate (NH4H2PO4), BaTiO3, Pb(Zr,Ti)O3 (PZT), P1-x La x Zr 1-y Ti y O3(PLZT, 0 <x<1、0<y<1)、Pb(Mg 1 / 3 Nb 2 / 3 )O3-PbTiO3 (PMN-PT), hafnia (HfO2), SrTiO3, SnO2, CeO2, NiO, CaO, ZrO2, SiC and TiO2, or a mixture of two or more thereof, but the present invention is not limited thereto.

[0104] In one embodiment of the present invention, when the inorganic particles further include inorganic particles other than bohemite, the content of the additional inorganic particles is, for example, 20% by weight or more, specifically 15% by weight or less, 10% by weight or less, 5% by weight or less, or 1% by weight or less, but the present invention is not limited thereto.

[0105] In one embodiment of the present invention, the average particle size (D 50 ) is not particularly limited, but is preferably in the range of 0.01 μm to 2 μm, specifically 0.5 μm to 1 μm, in order to form a porous coating layer with appropriate porosity and uniform thickness.

[0106] In one embodiment of the present invention, the binder material may include an acrylic polymer and a PVDF polymer, or a mixture thereof.

[0107] The acrylic polymer may include, for example, a (meth)acrylic polymer. The (meth)acrylic polymer contains a (meth)acrylic acid ester as a monomer, and examples of such a monomer include butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, ethyl (meth)acrylate, methyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, t-butyl (meth)acrylate, pentyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, isononyl (meth)acrylate, lauryl (meth)acrylate, and tetradecyl (meth)acrylate, and the polymer may include one or more of these monomers.

[0108] In one embodiment of the present invention, the acrylic polymer may include styrene-butyl acrylate.

[0109] According to one embodiment of the present invention, the acrylic polymer may include one having a glass transition temperature (Tg) of 0° C. to 80° C., specifically 20° C. to 60° C., 30° C. to 50° C., 35° C. to 45° C., or 40° C. to 45° C. When the glass transition temperature of the binder material is within the above range, excellent effects can be exhibited in terms of the breathability and adhesive strength of the porous coating layer, but the present invention is not limited thereto.

[0110] The PVdF-based polymer may include one or more of a homopolymer of vinylidene fluoride (i.e., polyvinylidene fluoride), a copolymer of vinylidene fluoride and a copolymerizable monomer, and a mixture thereof. In one embodiment of the present invention, the monomer may be, for example, a fluorinated monomer and / or a chlorinated monomer. Non-limiting examples of the fluorinated monomer include vinyl fluoride, trifluoroethylene (TrFE), chlorofluoroethylene (CTFE), 1,2-difluoroethylene, tetrafluoroethylene (TFE), hexafluoropropylene (HFP), perfluoro(alkyl vinyl) ethers such as perfluoro(methyl vinyl) ether (PMVE), perfluoro(ethyl vinyl) ether (PEVE), and perfluoro(propyl vinyl) ether (PPVE), perfluoro(1,3-dioxole), and perfluoro(2,2-dimethyl-1,3-dioxole) (PDD), among others.

[0111] In one embodiment of the present invention, the PVDF-based polymer may include at least one selected from polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), polyvinylidene fluoride-chlorofluoroethylene (PVDF-CTFE), polyvinylidene fluoride-tetrafluoroethylene (PVdF-TFE), and polyvinylidene fluoride-trifluoroethylene (PVdF-TrFE).

[0112] According to one embodiment of the present invention, the PVDF-based polymer may include, for example, one having a glass transition temperature (Tg) of -80°C to 0°C, specifically, -60°C to -20°C, -50°C to -30°C, -45°C to -35°C, or -45°C to -40°C.

[0113] When the glass transition temperature of the binder material is within the above range, excellent effects can be exhibited in terms of the breathability and adhesive strength of the porous coating layer, but the present invention is not limited thereto.

[0114] In this specification, the glass transition temperature can be measured by a conventional method for measuring the glass transition temperature of a polymeric material, for example, by differential scanning calorimetry (DSC) in accordance with ISO 1135762. Specifically, a graph showing the relationship between the sample temperature and the supplied heat flow is obtained while varying the temperature of a DSC sample. At a point where the slope of the graph changes suddenly, extension lines are drawn to the graph before and after the change, and the glass transition temperature can be measured at the point where the two extension lines intersect with the graph. The temperature can be increased and decreased once or repeatedly at a constant rate of 0.01°C / min to 100°C / min, for example, 10°C / min, in the range of -80°C to 500°C.

[0115] According to another aspect of the present invention, there is provided a method for producing a separation membrane having the above-described configuration.

[0116] Method for manufacturing a separator for electrochemical devices First, according to one embodiment of the present invention, the porous polyolefin substrate used in the separation membrane can be prepared by the following method.

[0117] The method for manufacturing the porous polyolefin substrate may include, for example, melt-extruding a raw material containing a polyolefin resin to obtain a polymer melt extrudate, molding and stretching the obtained polymer melt extrudate to obtain a polymer sheet, applying a coating liquid containing a thermal initiator and a vinyl group-containing silane compound to the polymer sheet, and drying and heat-setting the polymer sheet coated with the coating liquid. The polyolefin resin includes a polyolefin resin manufactured using an olefin polymerization catalyst containing chromium.

[0118] First, a raw material containing a polyolefin resin produced using the chromium-containing olefin polymerization catalyst is melt-extruded to obtain a polymer melt extrudate.

[0119] In one embodiment of the present invention, the raw materials for the melt extrusion preferably include a diluent as well as the polyolefin resin produced using the chromium-containing olefin polymerization catalyst.

[0120] The diluent may be, for example, liquid or solid paraffin oil, mineral oil, wax, soybean oil, or the like, which are commonly used in the manufacture of wet separation membranes.

[0121] In one embodiment of the present invention, the diluent may be a diluent capable of undergoing liquid-liquid phase separation with the polyolefin resin, and examples thereof include phthalic acid esters such as dibutyl phthalate, dihexyl phthalate, and dioctyl phthalate, diphenyl ether, and benzyl ether. Examples of the fatty acid esters that can be used include, but are not limited to, aromatic ethers such as palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, and other fatty acids having 10 to 20 carbon atoms; palmitic alcohol, stearic alcohol, oleic alcohol, and other fatty acid alcohols having 10 to 20 carbon atoms; saturated and unsaturated fatty acids having 4 to 26 carbon atoms in a fatty acid group such as mono-, di-, or triester palmitate, mono-, di-, or triester stearate, mono-, di-, or triester oleate, and mono-, di-, or triester linoleate; and fatty acid esters in which one or more fatty acids in which the double bond of an unsaturated fatty acid is substituted with epoxy are ester-bonded to an alcohol having 1 to 10 carbon atoms and having 1 to 8 hydroxy groups.

[0122] In one embodiment of the present invention, the content of the diluent is, for example, 100 parts by weight to 350 parts by weight, or 125 parts by weight to 300 parts by weight, or 150 parts by weight to 250 parts by weight, based on 100 parts by weight of the polyolefin. When the total content of the diluent satisfies the above numerical range, problems such as a decrease in porosity, a decrease in pore diameter, a decrease in interconnection between pores, a significant decrease in permeability, an increase in viscosity of the polyolefin composition, and an increase in extrusion load, which makes processing difficult, can be prevented. Problems such as breakage and thickness unevenness during stretching, which occur when the kneading property of the polyolefin and the diluent decreases as the polyolefin content decreases and the polyolefin is extruded in a gel state without being thermodynamically kneaded with the diluent, can be prevented, but the present invention is not limited thereto.

[0123] In one embodiment of the present invention, the raw materials may further include, in addition to the polyolefin resin and diluent produced using a chromium-containing olefin polymerization catalyst, a polyolefin resin produced using another type of olefin polymerization catalyst, for example, a Ziegler-Natta catalyst, or an olefin polymerization catalyst containing titanium (Ti), aluminum (Al), magnesium (Mg), zirconium (Zr), vanadium (V), or two or more thereof, or other polymer resins, and the above description also applies to these polyolefin resins and other polymer resins.

[0124] For example, in one embodiment of the present invention, the raw materials may further include a polyolefin resin produced by an olefin polymerization reaction using a Ziegler-Natta catalyst.

[0125] According to one embodiment of the present invention, the weight ratio of the polyolefin resin (Cr-type polyolefin) produced using a chromium-containing olefin polymerization catalyst to the polyolefin resin (ZT-type polyolefin) produced using another type of olefin polymerization catalyst among the raw materials is 1:9 to 9:1, specifically 2:8 to 8:2, 3:7 to 7:3, or 5:5, but the present invention is not limited thereto.

[0126] According to another embodiment of the present invention, when a ZT-type polyolefin is further included in addition to the Cr-type polyolefin, the content of the polyolefin resin produced using the chromium-containing olefin polymerization catalyst may be, for example, 10 wt% or more, based on 100 wt% of the total raw materials. Specifically, the content of the polyolefin resin produced using the chromium-containing olefin polymerization catalyst may be 15 wt% or more, 20 wt% or more, 25 wt% or less, 30 wt% or more, 35 wt% or more, 40 wt% or more, 45 wt% or more, 50 wt% or more, and / or 99.9 wt% or less, 99 wt% or less, 95 wt% or less, or 90 wt% or less, based on 100 wt% of the total raw materials, but the present invention is not limited thereto.

[0127] In one embodiment of the present invention, the step of obtaining the polymer melt extrudate can be carried out using, but is not limited to, a conventional single-screw extruder or a twin-screw extruder.

[0128] The resulting polymer melt extrudate is then shaped and stretched to obtain a polymer sheet.

[0129] In one embodiment of the present invention, after the polymer melt extrudate is extruded, it may be cooled by water cooling, air cooling, or the like, using a conventional casting or calendering method to form a cooled extrudate.

[0130] In one embodiment of the present invention, the molding and stretching steps can provide a separator substrate having improved mechanical strength and puncture strength.

[0131] In one embodiment of the present invention, the stretching can be performed by sequential or simultaneous stretching using a roll or a tender. The stretching ratio is, for example, 3 times or more, or 5 to 12 times in each of the longitudinal and transverse directions, and the total stretching ratio is 20 to 120 times. When the stretching ratio satisfies the above numerical range, advantageous effects can be obtained in terms of thickness uniformity of the separator substrate to be produced and balance of physical properties between the longitudinal and transverse directions, but the present invention is not limited thereto.

[0132] In one embodiment of the present invention, the stretching temperature varies depending on the melting point of the polyolefin resin used, and the concentration and type of the diluent, but the present invention is not limited thereto.

[0133] The diluent is then extracted from the stretched polymer sheet to obtain a porous polymer sheet.

[0134] In one embodiment of the present invention, the stretched sheet may be treated with an organic solvent having high solubility in the diluent to extract the diluent, followed by drying to form a porous sheet.

[0135] The organic solvent is not particularly limited as long as it can extract the diluent used, but methyl ethyl ketone, methylene chloride, hexane, etc. can be used in terms of extraction efficiency and drying speed.

[0136] In one embodiment of the present invention, the extraction method can be any common solvent extraction method, such as immersion, solvent spray, or ultrasonic, either individually or in combination. In one embodiment of the present invention, the residual diluent content after extraction may be preferably 1 wt% or less. When the residual diluent content is within the above range, advantageous effects can be observed in terms of the permeability and mechanical properties of the resulting separation membrane substrate and the efficiency of the manufacturing process, but the present invention is not limited thereto. The extraction time and extraction temperature vary depending on the thickness of the polymer sheet and the type of polymer, and the present invention is not limited thereto.

[0137] Next, a coating liquid containing a thermal initiator and a vinyl group-containing silane compound is applied to the polymer sheet.

[0138] According to one embodiment of the present invention, a coating liquid containing a thermal initiator and a vinyl group-containing silane compound is applied to a polymeric porous sheet with exposed pores. As described above, the thermal initiator is capable of activating terminal vinyl groups in the polyolefin resin chain to form radicals. This not only allows the thermal initiator to form numerous cross-linked structures in the polyolefin chain, but also allows the coating liquid to penetrate into fibrils present on the surfaces of the already formed pores, thereby demonstrating excellent effects in improving the heat resistance of the separator substrate, but the present invention is not limited thereto. Furthermore, the vinyl group-containing silane compound may be present by grafting silicon-containing organic groups to activated terminal vinyl groups in the polyolefin resin chain via covalent bonds. This may also demonstrate excellent effects in improving the wettability of the separator substrate, but the present invention is not limited thereto.

[0139] In one embodiment of the present invention, the thermal initiator and the vinyl group-containing silane compound in the coating liquid may be present in a weight ratio of, for example, 2:8 to 8:2, specifically, 3:7 to 7:3, or 4:6 to 6:4. When the weight ratio of the thermal initiator and the vinyl group-containing silane compound is within the above range, the vinyl groups in the chains of the polyolefin resin can be sufficiently activated to induce crosslinking and grafting reactions, which can advantageously improve the wettability of the separator substrate, but the present invention is not limited thereto.

[0140] In one embodiment of the present invention, the coating liquid may contain, as a solvent for the thermal initiator and the vinyl group-containing silane compound, for example, ethanol, propanol, acetone, NMP, DMAC, DMF, water, or a mixture of two or more of these. The total solid content of the coating liquid is preferably, for example, 5% by weight to 60% by weight, specifically 7% by weight to 40% by weight, from the viewpoint of improving radical activation of double bonds and wettability of the separation membrane substrate, but the present invention is not limited thereto.

[0141] In one embodiment of the present invention, the coating liquid may further contain common additives for improving specific functions, such as a surfactant, an oxidation stabilizer, a UV stabilizer, an antistatic agent, and a nucleating agent, if necessary, but the present invention is not limited thereto.

[0142] Thereafter, the polymer sheet coated with the coating solution is dried and heat-set to obtain a separator substrate.

[0143] The heat setting is performed to fix the porous membrane and apply heat to the membrane to forcibly fix the porous membrane, which tends to shrink, and remove residual stress.

[0144] According to the method for producing a separation membrane substrate, a coating liquid containing the thermal initiator and the vinyl group-containing silane compound is applied before heat setting. This allows a radical polymerization reaction mediated by the thermal initiator during the heat setting to form a large number of crosslinked structures through crosslinking reactions within and / or between polyolefin chains, thereby producing a polyolefin resin-based porous polyolefin substrate in which a large number of silicon-containing organic groups are grafted to the ends of the polyolefin chains.

[0145] In one embodiment of the present invention, the heat setting temperature and time vary depending on the vinyl group content in the polyolefin chain and the composition of the coating liquid, but the present invention is not limited thereto.

[0146] According to one embodiment of the present invention, the porous coating layer used in the separator as described above may be prepared by an SRS method, a CCS method, or other known methods.

[0147] In one embodiment of the present invention, the porous coating layer can be prepared by the following method.

[0148] First, the binder material is dispersed or dissolved in a solvent to prepare a binder dispersion or binder solution, and then inorganic particles are added to the binder dispersion or binder solution and dispersed to prepare a slurry for forming a porous coating layer.

[0149] In one embodiment of the present invention, the solvent can function as either a solvent that dissolves the binder polymer or a dispersion medium that disperses the binder polymer without dissolving it, depending on the type of binder polymer. The solvent can have a solubility index similar to that of the binder polymer to be used and a low boiling point. This facilitates uniform mixing and subsequent solvent removal. Non-limiting examples of such solvents include one or a mixture of two or more selected from the group consisting of water, acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP), and cyclohexane.

[0150] In one embodiment of the present invention, the inorganic particles can be added in a state where they have been crushed to a predetermined average particle size, or they can be added to a slurry in which the binder polymer is dissolved or dispersed, and then crushed and dispersed while controlling the crushing to a predetermined average particle size using a ball mill or the like. In this case, crushing can be carried out for 1 to 20 hours, and the average particle size of the crushed inorganic particles can be as described above. Conventional crushing methods can be used, such as a ball mill method.

[0151] In one embodiment of the present invention, the solid content of the porous coating layer forming slurry is 5 wt % to 60 wt %, or 30 wt % to 50 wt %. When the solid content of the porous coating layer forming slurry is within the above range, it is easy to ensure coating uniformity and to prevent the slurry from flowing and causing unevenness, or to prevent a large amount of energy from being required to dry the slurry.

[0152] The method for coating the porous coating layer-forming slurry onto the separation membrane substrate may be a conventional coating method known in the art, such as dip coating, die coating, roll coating, comma coating, microgravure coating, doctor blade coating, reverse roll coating, Mayer bar coating, direct roll coating, or a combination thereof. Furthermore, the drying may be any conventional drying method, such as natural drying or air drying, without any particular limitation.

[0153] Electrochemical elements An electrochemical device according to another aspect of the present invention includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and the separator is the same as the separator described above.

[0154] In this specification, the specific configuration of the positive electrode, negative electrode, and electrode assembly can be a conventional one, so a description thereof will be omitted.

[0155] According to another aspect of the present invention, a secondary battery can be provided by preparing an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and then inserting the prepared electrode assembly into a suitable case and injecting an electrolyte solution into the case.

[0156] The present invention will be further described below with reference to examples. However, the following examples are for the purpose of illustrating the present invention, and the scope of the present invention is not limited to these examples.

[0157] [Production of porous polyolefin substrate] Manufacturing example Cr-type polyolefin (DL Chemical, TR570) 9 kg / hr and diluent (Kyukdong Petrochemical, LP350F) 21 kg / hr were fed into an extruder (Korea EM, φ32 twin-screw extruder, L / D=56) at a melt extrusion rate of 200°C to obtain a polyolefin melt extrudate. The resulting polyolefin melt extrudate was passed through a T-die and formed into a sheet using a cooling caster. It was then biaxially stretched in a tenter-type sequential stretching machine, stretching in MD followed by TD, to obtain a polyolefin sheet. The MD and TD stretch ratios were 7x and 6x, respectively, and the stretching temperatures were 115°C in MD and 125°C in TD. The diluent was extracted from the stretched polyolefin sheet using methylene chloride to obtain a porous polyolefin sheet.

[0158] Next, a coating liquid containing 10 wt % of dicumyl peroxide as a thermal initiator and 20 wt % of vinyltrimethoxysilane as a vinyl group-containing silane compound (wetting agent) in an ethanol solvent was applied to one surface of the porous polyolefin sheet.

[0159] Thereafter, the polyolefin sheet coated with the coating liquid was dried and heat-set at 128° C. to prepare a separator substrate, which had an average thickness of 9.0 μm.

[0160] Comparative manufacturing example 9 kg / hr of ZT-type polyolefin (Daehan Yuka, VH035) as a raw material and 21 kg / hr of liquid paraffin oil (Kyukdong Yuka, LP350F) as a diluent were fed into an extruder (Korea EM, φ32 twin-screw extruder L / D=56), and melt-extruded at 200°C to obtain a polyolefin melt extrudate.

[0161] The resulting polyolefin melt extrudate was passed through a T-die and then cooled and cast into a sheet, which was then biaxially stretched in a tenter-type sequential stretching machine (MD stretching followed by TD stretching) to obtain a polyolefin sheet. The MD and TD stretch ratios were 7x and 6x, respectively, and the stretching temperatures were 115°C in MD and 125°C in TD.

[0162] The diluent was extracted from the stretched polyolefin sheet using methylene chloride to obtain a porous polyolefin sheet, which was then dried and heat-set at 128°C to prepare a separator substrate. The average thickness of the resulting separator substrate was 9.0 μm.

[0163] [Evaluation of the physical properties of porous polyolefin substrates] Gel fraction measurement First, 0.2 g of a separation membrane substrate sample was placed in a 120 mesh stainless steel net, extracted in trichlorobenzene at 100°C for 12 hours, and then dried in a vacuum oven at 100°C for 12 hours.

[0164] Thereafter, the weight of the test piece remaining on the stainless steel net was measured, and the gel fraction was calculated using the following formula. The gel fraction was expressed as the average value of the measured values ​​of three test pieces: Gel fraction (%) = {(weight of remaining test piece in g) / 0.2 g} × 100.

[0165] Measurement of chromium (Cr), silicon (Si) and aluminum (Al) content First, a test piece of the separation membrane substrate was reacted with sulfuric acid and carbonized on a hot plate, after which the sulfuric acid was removed. It was then incinerated in an electric furnace (temperature: 600°C) for 4 hours and decomposed with nitric acid and hydrogen peroxide. Once the test piece was clearly dissolved, it was diluted with tertiary ultrapure water to prepare an analytical sample.

[0166] The contents of chromium (Cr), silicon (Si), and aluminum (Al) in the separation membrane substrate were measured using an inductively coupled plasma mass spectrometer (ICP-MS) (Axiom MC model, Thermo Elemental Ltd, UK).

[0167] When a Ziegler-Natta catalyst is used, aluminum (Al) is present on the separation membrane substrate, so detecting the aluminum content is necessary to confirm the use of a Ziegler-Natta catalyst.

[0168] Breaking Temperature The rupture temperature of the prepared separation membrane substrate was analyzed using a thermal mechanical analysis (TMA) analyzer (TA Instruments, TMA Q400).

[0169] Specifically, a load of 0.01 N was applied to the separation membrane substrate, and the temperature was raised at a rate of 5°C / min while observing the degree of deformation. The temperature at which the separation membrane substrate contracted with the rise in temperature and then expanded again until it broke was measured as the "breaking temperature."

[0170] Measure the number of spots Separation membrane base material 1m 2 The number of spots with a long side length of 50 μm or more was visually observed and counted.

[0171] Thickness uniformity measurement The thickness of the separation membrane substrate was measured at 10 cm intervals across 1 m in the width direction and at 3 m intervals across 30 m in the length direction, and the standard deviation of the thickness was calculated using the thickness measurements at a total of 100 points.

[0172] Wettability Measurement After 4 μm of DMC (dimethyl carbonate) was dropped onto one surface of the separation membrane substrate, the area of ​​the spread was measured. Specifically, after a certain time had passed since the dropwise addition of DMC, a photograph of the surface of the separation membrane substrate was taken, and the area of ​​the spread of DMC on the photograph was measured.

[0173] [Table 1] [Separation membrane manufacturing] Example 1 As a separation membrane substrate, a porous polyolefin substrate according to a manufacturing example was prepared.

[0174] The porous coating layer was formed by the W-SRS (water-based SRS) manufacturing method as follows. Specifically, styrene-butyl acrylate (Tg 40°C, particle size (D 50 ) 500 nm) and PVDF-HFP copolymer (HFP 5 wt%, Tg -40°C, particle size (D 50 ) 500 nm), and bohemite (AlO(OH) as inorganic particles, particle size (D 50 ) 500 nm) were added in a weight ratio of 10:10:80 and mixed for about 2 hours using a bead mill mixer to prepare a slurry for forming a porous coating layer.

[0175] The prepared slurry for forming a porous coating layer was applied to both sides of the prepared porous polyolefin substrate using a bar coater and dried at 60°C for 10 minutes to obtain a separator having a porous coating layer with a thickness of 3 μm on each side of the porous polyolefin substrate.

[0176] Example 2 As a separation membrane substrate, a porous polyolefin substrate according to a manufacturing example was prepared.

[0177] The porous coating layer was formed by the CCS manufacturing method as follows. Specifically, styrene-butyl acrylate (Tg 40°C, particle size (D 50 ) 500 nm), and bohemite (AlO(OH) as inorganic particles, particle size (D 50 ) 500 nm) were added in a weight ratio of 5:95, and mixed for about 2 hours using a bead mill mixer to prepare a slurry for forming a porous coating layer.

[0178] The prepared slurry for forming a porous coating layer was applied to both sides of the prepared porous polyolefin substrate using a bar coater and dried at 60°C for 10 minutes to obtain a separator having a porous coating layer with a thickness of 2 μm on each side of the porous polyolefin substrate.

[0179] Comparative Example 1 As a separation membrane substrate, a porous polyolefin substrate according to a comparative production example was prepared.

[0180] Alumina (Al2O3) as inorganic particles, particle size (D 50 A separation membrane having a porous coating layer with a thickness of 3 μm provided on each side of a porous polyolefin substrate was obtained in the same manner as in Example 1, except that a 500 nm thick porous polyolefin substrate was used.

[0181] Comparative Example 2 As a separation membrane substrate, a porous polyolefin substrate according to a comparative production example was prepared.

[0182] Alumina (Al2O3) as inorganic particles, particle size (D 50 A separation membrane having a porous coating layer with a thickness of 2 μm provided on each side of a porous polyolefin substrate was obtained in the same manner as in Example 2, except that a 500 nm thick porous polyolefin substrate was used.

[0183] Comparative Example 3 As a separation membrane substrate, a porous polyolefin substrate according to a manufacturing example was prepared.

[0184] Alumina (Al2O3) as inorganic particles, particle size (D 50 A separation membrane having a porous coating layer with a thickness of 2 μm provided on each side of a porous polyolefin substrate was obtained in the same manner as in Example 2, except that a 500 nm thick porous polyolefin substrate was used.

[0185] Comparative Example 4 As a separation membrane substrate, a porous polyolefin substrate according to a manufacturing example was prepared.

[0186] Aluminum hydroxide (Al(OH)3) was used as inorganic particles, and particle size (D 50 A separation membrane having a porous coating layer with a thickness of 2 μm provided on each side of a porous polyolefin substrate was obtained in the same manner as in Example 2, except that a 500 nm thick porous polyolefin substrate was used.

[0187] [Evaluation of separation membrane properties] The physical properties of the separation membrane were evaluated by the following methods, and the results are shown in Table 2.

[0188] Adhesion strength evaluation To evaluate the adhesive strength between the porous polyolefin substrate and the porous coating layer, the peel strength was measured by the following method.

[0189] First, the separator to be measured was cut into a size of 15 mm x 100 mm. Double-sided adhesive tape was attached to a glass plate, and the surface of the porous coating layer of the prepared separator was attached so that it adhered to the adhesive tape. Next, the end of the attached separator was attached to a UTM device (Universal Testing Machine, LLOYD Instrument LF Plus), and a force was applied at a 180° angle at a measurement speed of 300 mm / min to measure the force required to peel the porous coating layer from the porous polymer substrate.

[0190] Breathability The air permeability was measured according to the Gurley method in ASTM D726-94. The Gurley used here is the resistance to air flow and is measured using a Gurley densometer. The air permeability value is measured by passing 100 cc of air through 1 in of the separator under test at a pressure of 12.2 inH2O. 2 The time (seconds) required for the air to pass through the cross section was expressed as the ventilation time.

[0191] Heat shrinkage rate The separator to be measured was cut into a size of 50 mm x 50 mm, sandwiched between A4 paper sheets, and placed in a convection oven at 120°C for 1 hour, after which the thermal shrinkage in the machine direction (MD) and transverse direction (TD) was measured.

[0192] At this time, the heat shrinkage rate was calculated as [(initial length - length after heat treatment) / (initial length) x 100].

[0193] [Table 2] As can be seen from Table 2 above, the separators of Examples 1 and 2 according to one embodiment of the present invention had excellent adhesive strength between the porous polyolefin substrate and the porous coating layer, whereas the separators of Comparative Examples 1 to 4 had poor adhesive strength between the porous polyolefin substrate and the porous coating layer.

[0194] In particular, in the preparation examples, when a crosslinked polyolefin resin having silicon-containing organic groups grafted to polyolefin chains and a porous polyolefin substrate containing chromium are used, the porous coating layer contains bohemite (AlO(OH)) as inorganic particles, which eliminates significant limitations on the method for forming the porous coating layer and significantly improves the adhesion between the porous polyolefin substrate and the porous coating layer. More specifically, in Comparative Example 3, in which alumina was used as the inorganic particles, the adhesion between the porous polyolefin substrate and the porous coating layer was significantly poor. In Comparative Example 4, in which aluminum hydroxide was used as the inorganic particles, not only was the adhesion poor, but the heat resistance was also significantly poor due to the low density of aluminum hydroxide.

[0195] The present invention has been described above with reference to the embodiments and drawings. However, a person having ordinary knowledge in the field to which the present invention belongs will be able to make various applications and modifications within the scope of the present invention based on the above content.

Claims

1. A porous polyolefin substrate and a porous coating layer formed on at least one surface of the porous polyolefin substrate, The porous polyolefin substrate is Contains a crosslinked polyolefin resin, The crosslinked polyolefin resin comprises silicon-containing organic groups grafted onto polyolefin chains; The porous coating layer comprises: Contains inorganic particles and a binder polymer, The separator for an electrochemical device, wherein the inorganic particles contain bohemite (AlO(OH)).

2. The porous polyolefin substrate is The gel fraction is 3% to 80%; 2. The separator for an electrochemical element according to claim 1, wherein the standard deviation (Δd) of thickness measured at at least 100 arbitrary points is 0.5 μm or less.

3. 2. The separator for an electrochemical device according to claim 1, wherein the adhesive strength between the porous polyolefin substrate and the porous coating layer is 50 gf / 15 mm or more.

4. 4. The separator for electrochemical devices according to claim 3, wherein the adhesive strength between the porous polyolefin substrate and the porous coating layer is 70 gf / 15 mm to 95 gf / 15 mm.

5. The content of the bohemite is 2. The separator for an electrochemical device according to claim 1, wherein the inorganic particles are present in an amount of 80 wt % or more based on the total weight of the inorganic particles.

6. the silicon-containing organic group is a residue derived from a vinyl group-containing silane compound, 2. The separator for electrochemical elements according to claim 1, wherein the vinyl group-containing silane compound comprises vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilane, (3-methacryloxypropyl)trimethoxysilane, (3-methacryloxypropyl)triethoxysilane, vinylmethyldimethoxysilane, vinyl-tris(2-methoxyethoxy)silane, vinylmethyldiethoxysilane, or a mixture of two or more thereof.

7. The separator for an electrochemical device according to claim 1 , wherein the porous polyolefin substrate further comprises chromium (Cr).

8. 8. The separator for electrochemical devices according to claim 7, wherein the chromium content is 0.1 ppm to 20 ppm.

9. 2. The separator for electrochemical devices according to claim 1, wherein the crosslinked structure in the crosslinked polyolefin resin includes a structure resulting from a radical polymerization reaction between vinyl groups via a thermal initiator.

10. 10. The separator for an electrochemical device according to claim 9, wherein the thermal initiator comprises a peroxide-based compound, a persulfate-based compound, an azo-based compound, or a mixture thereof.

11. The battery includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, An electrochemical device, wherein the separator is the separator for electrochemical devices according to claim 1 .

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