KR20210081297A - Manufacturing method of crosslinked polyolefin separator - Google Patents

Abstract

Provided is a method of preparing a crosslinked polyolefin separator, which comprises the steps of: (S1) preparing a silane-grafted polyolefin composition by injecting and mixing a raw material including polyolefin, a diluent, an initiator, and a carbon-carbon double anchoring group-containing alkoxysilane compound into an extruder, followed by extruding; (S2) molding and stretching the extruded silane-grafted polyolefin composition into a sheet form; (S3) preparing a porous membrane by extracting a diluent from the stretched sheet; (S4) forming a porous coating layer by applying and drying a slurry for forming a porous coating layer including inorganic particles, a binder polymer, a crosslinking catalyst, and a dispersion medium on at least one surface of the porous membrane; (S5) placing and winding a porous release substrate having a water vapor transmission rate of 2,000 g/m^2/day or more on the porous coating layer of the porous membrane; and (S6) water cross-linking the resulting product of step (S5).

Description

Manufacturing method of crosslinked polyolefin separation membrane {MANUFACTURING METHOD OF CROSSLINKED POLYOLEFIN SEPARATOR}

The present invention relates to a method for manufacturing a crosslinked polyolefin membrane, and more particularly, to a method for manufacturing a crosslinked polyolefin membrane having improved crosslinking reactivity.

Recently, interest in energy storage technology is increasing. Efforts for research and development of electrochemical devices are becoming more concrete as the field of application expands to energy of mobile phones, camcorders and notebook PCs, and even electric vehicles. Electrochemical devices are receiving the most attention in this aspect, and among them, the development of rechargeable batteries that can be charged and discharged has become the focus of interest. Recently, in developing such batteries, new electrodes have been developed to improve capacity density and specific energy. and battery design research and development.

Among the currently applied secondary batteries, lithium secondary batteries developed in the early 1990s have a higher operating voltage and significantly higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and lead sulfate batteries that use aqueous electrolyte solutions. is gaining popularity as

This lithium secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator, among which the separator has high ionic conductivity to increase lithium ion permeability based on insulation and high porosity to separate and electrically insulate the positive and negative electrodes. is required

In addition, in order to secure the safety of the lithium secondary battery, such a separator should have a wide interval between the shutdown temperature (shut down) and the melt down temperature (melt down). In order to widen the gap between the shutdown temperature and the melt-down temperature, it is necessary to control the shutdown temperature in a decreasing direction and the melt-down temperature in an increasing direction.

In order to control the meltdown temperature of a wet polyethylene (PE) separator, a method of increasing the meltdown temperature by composing polyolefin, which is the main component of the separator, with two or more polyolefins having different thermal properties has been attempted, but there is a limit that the increase in the meltdown temperature is not large. there was.

In the separator, the safety of the lithium secondary battery including the separator may be secured only when the interval between the shutdown temperature and the melt down temperature is wide. In order to widen the gap between the two, it is necessary to control the shutdown temperature in a decreasing direction and the melt-down temperature in an increasing direction.

Attempts have been made to increase the meltdown temperature using the cross-linked separator concept. As one of the techniques for increasing the meltdown temperature, an attempt has been made to prepare a cross-linked polyolefin separator by mixing polyethylene, a diluent, an initiator, a cross-linking agent, and a cross-linking catalyst at a time in a separator used for a lithium secondary battery. However, as described above, when a crosslinking agent, a crosslinking catalyst, etc. are added to the extruder at once, die drool occurs in the extruder or gel is generated in the separation membrane, thereby reducing productivity.

Therefore, the problem to be solved by the present invention is to provide a method for producing a cross-linked polyolefin separator in which cross-linking reaction is progressed to the inside of the separator by improving cross-linking reactivity while post-addition of a cross-linking catalyst.

One aspect of the present invention provides a method for producing a cross-linked polyolefin separator according to the following embodiments.

According to a first embodiment,

(S1) preparing a silane-grafted polyolefin composition by injecting and mixing a polyolefin, a diluent, an initiator, and a raw material including a carbon-carbon double bond-containing alkoxysilane compound into an extruder and extruding;

(S2) molding and stretching the extruded silane-grafted polyolefin composition into a sheet form;

(S3) preparing a porous membrane by extracting a diluent from the stretched sheet;

(S4) forming a porous coating layer by applying and drying a slurry for forming a porous coating layer comprising inorganic particles, a binder polymer, a crosslinking catalyst, and a dispersion medium on at least one surface of the porous membrane;

(S5) winding a porous release substrate having a water vapor transmission rate ^{of 2,000 g/m 2} /day or more on the porous coating layer of the porous membrane; and

(S6) a step of hydrolyzing the resultant of the step (S5); is provided a method for producing a cross-linked polyolefin membrane comprising a.

According to a second embodiment, according to the first embodiment,

The porous release substrate may have a moisture permeability of 2,000 to 12,000 g/m ^{2 /day.}

According to a third embodiment, according to the first or second embodiment,

The porous release substrate may have an average pore size of 1 μm to 1,000 μm.

According to a fourth embodiment, according to any one of the first to third embodiments,

The porous release substrate may have a strength of ^{250 kgf/cm 2 or more.}

According to a fifth embodiment, according to any one of the first to fourth embodiments,

The porous release substrate may include a nonwoven substrate.

According to a sixth embodiment, according to any one of the first to fifth embodiments,

The method may further include removing the porous release substrate after the step (S6).

According to a seventh embodiment, according to any one of the first to sixth embodiments,

The method may further include removing the porous release substrate after the step (S6).

According to an eighth embodiment, according to any one of the first to seventh embodiments,

The cross-linked polyolefin separator may include a siloxane (-Si-O-Si-) bonding structure, which is a silane-derived cross-linked structure derived from a silane compound grafted to the main chain of the polyolefin.

According to the ninth embodiment, there is provided a cross-linked polyolefin separator prepared by the manufacturing method of any one of the first to eighth embodiments.

According to a tenth embodiment,

There is provided an electrochemical device comprising an anode, a cathode, and a separator interposed between the anode and the cathode, wherein the separator is the crosslinked polyolefin separator of the ninth embodiment.

According to the eleventh embodiment, according to the tenth embodiment,

The electrochemical device may be a lithium secondary battery.

The method for producing a cross-linked polyolefin separator according to an embodiment of the present invention includes adding a cross-linking catalyst to a slurry for forming a porous coating layer during the production of a cross-linked polyolefin separator having an organic-inorganic composite porous coating layer formed on a porous polyolefin substrate. By introducing the catalyst post-addition method, compared to the method of adding a conventional cross-linking catalyst together with a polyolefin, a diluent, an initiator, and an alkoxy silane compound containing a carbon-carbon double bond group to an extruder, the problem of lowering productivity due to gel generation is solved.

In addition, after the step of forming the porous coating layer on the porous membrane, a release substrate with high moisture permeability is interposed on the porous coating layer, so that the moisture is smoothly transferred to the inside of the wound porous membrane in the hydro-crosslinking process after winding the porous membrane on which the porous coating layer is formed. cross-linking reactivity can be greatly improved.

Hereinafter, the present invention will be described in detail. The terms or words used in the present specification and claims should not be construed as being limited to their ordinary or dictionary meanings, and the inventor may properly define the concept of the term in order to best describe his invention. It should be interpreted as meaning and concept consistent with the technical idea of the present invention based on the principle that there is.

According to one aspect of the present invention,

(S1) preparing a silane-grafted polyolefin composition by injecting and mixing a polyolefin, a diluent, an initiator, and a raw material including a carbon-carbon double bond-containing alkoxysilane compound into an extruder and extruding;

(S2) molding and stretching the extruded silane-grafted polyolefin composition into a sheet form;

(S3) preparing a porous membrane by extracting a diluent from the stretched sheet;

(S4) forming a porous coating layer by applying and drying a slurry for forming a porous coating layer comprising inorganic particles, a binder polymer, a crosslinking catalyst, and a dispersion medium on at least one surface of the porous membrane;

(S5) winding a porous release substrate having a water vapor transmission rate ^{of 2,000 g/m 2} /day or more on the porous coating layer of the porous membrane; and

(S6) a step of hydrolyzing the resultant of the step (S5); is provided a method for producing a cross-linked polyolefin membrane comprising a.

Hereinafter, a method for manufacturing a separator according to the present invention will be described in detail.

First, a polyolefin, a diluent, an initiator, and a raw material including a carbon-carbon double bond-containing alkoxysilane compound are added to and mixed in an extruder, and then extruded to prepare a silane-grafted polyolefin composition (S1).

In a specific embodiment of the present invention, the polyolefin is polyethylene; polypropylene; polybutylene; polypentene; polyhexene; polyoctene; copolymers of two or more of ethylene, propylene, butene, pentene, 4-methylpentene, hexene, and octene; or mixtures thereof.

In particular, the polyethylene includes low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), and the like, and among them, high-density polyethylene with high crystallinity and high melting point of the resin is most preferable.

In a specific embodiment of the present invention, the polyolefin may have a weight average molecular weight of 200,000 to 1,000,000, or 220,000 to 700,000, or 250,000 to 500,000.

In a specific embodiment of the present invention, the diluent may be liquid or solid paraffin oil, wax, soybean oil, etc. generally used in the manufacture of wet separation membranes.

In a specific embodiment of the present invention, as the diluent, a diluent capable of liquid-liquid phase separation from polyolefin may be used, for example, dibutyl phthalate, dihexyl phthalate, phthalic acid esters such as dioctyl phthalate; aromatic ethers such as diphenyl ether and benzyl ether; fatty acids having 10 to 20 carbon atoms, such as palmitic acid, stearic acid, oleic acid, linoleic acid, and linolenic acid; C10-20 fatty acid alcohols, such as palmitic acid alcohol, stearic acid alcohol, and oleic acid alcohol; Palmitic acid mono-, di-, or triesters, stearic acid mono-, di-, or triesters. Saturated and unsaturated fatty acids having 4 to 26 carbon atoms in the fatty acid group such as oleic acid mono-, di-, or triesters, linoleic acid mono-, di-, or triesters, or one in which the double bond of the unsaturated fatty acid is substituted with an epoxy Or two or more fatty acids, 1 to 8 hydroxyl groups, and ester-bonded fatty acid esters with alcohols having 1 to 10 carbon atoms; may include.

The diluent may be used alone or as a mixture including at least two or more of the above-mentioned components.

In a specific embodiment of the present invention, the total content of the diluent may be 100 to 350 parts by weight, or 125 to 300 parts by weight, or 150 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, as the polyolefin content is too large, the porosity decreases, the pore size becomes small, the interconnection between the pores is small, so the permeability is greatly reduced, and the viscosity of the polyolefin composition rises to reduce the extrusion load. The problem that processing may be difficult due to the rise can be prevented, and as the polyolefin content is too small, the kneading property of the polyolefin and the diluent is lowered, so that the polyolefin is not thermodynamically kneaded with the diluent but is extruded in the form of a gel, resulting in breakage and Problems such as thickness non-uniformity can be prevented.

In a specific embodiment of the present invention, the carbon-carbon double bond group-containing alkoxy silane compound is a crosslinking agent that causes a silane crosslinking reaction, and is grafted to polyolefin by a carbon-carbon double bond group, and the crosslinking reaction proceeds by means of an alkoxy group It serves to crosslink the polyolefin. Accordingly, it is possible to increase the melt-down temperature of the separator.

The carbon-carbon double bond group is a reactive group that can be grafted to polyolefin as described above, and is a substituent having a double bond between two carbons, for example, a vinyl group, acryloxy group, or methacrylic group. There may be an oxy group and the like.

In a specific embodiment of the present invention, it may include a compound represented by the following Chemical Formula 1:

[Formula 1]

In Formula 1, R _1, R _2, and R ₃ is each independently an alkoxy group having 1 to 10 carbon atoms, an alkylcarbonyloxy group having 1 to 10 carbon atoms, an alkoxyalkoxy group having 1 to 10 carbon atoms, or an alkyl group having 1 to 10 carbon atoms, wherein R _{1 and} R _{2 ,} and at least one of R ₃ is an alkoxy group or an alkoxyalkoxy group;

R is a vinyl group, an acryloxy group, a methacryloxy group, or an alkyl group having 1 to 20 carbon atoms, wherein at least one hydrogen of the alkyl group is substituted with a vinyl group, an acryloxy group, or a methacryloxy group.

According to one embodiment of the present invention, the R _1, R _2, and R ₃ is each independently an alkoxy group having 1 to 10, or 1 to 5, or 1 to 3 carbon atoms, 1 to 10, or 1 to 5, or 1 to 3 alkylcarbonyloxy group, 1 to 10 carbon atoms, or an alkyl group of 1 to 5, or 1 to 3, or an alkoxyalkoxy group having 1 to 10, or 1 to 5, or 1 to 3 carbon atoms, among the alkoxy group, alkylcarbonyloxy group, alkyl group, or alkoxyalkoxy group At least one hydrogen may be substituted with an alkoxy group having 1 to 10 carbon atoms or an alkylcarbonyloxy group having 1 to 10 carbon atoms. In this case, the alkylcarbonyloxy group is a substituent defined as "-O-(CO)-alkyl", and the alkoxyalkoxy group is a substituent defined as "-O-alkyl-O-alkyl".

In a specific embodiment of the present invention, the carbon-carbon double bond group-containing alkoxy silane compound is vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilane, (3-methacryloxypropyl)trimethoxy silane, (3-methacryloxypropyl)triethoxysilane, vinylmethyldimethoxysilane, vinyl-tris(2-methoxyethoxy)silane, vinylmethyldiethoxysilane, or a mixture of at least two or more thereof. can

In a specific embodiment of the present invention, the content of the carbon-carbon double bond group-containing alkoxy silane compound is 0.1 to 3.0 parts by weight, or 0.15 to 2.0 parts by weight, or 0.2 to 3.0 parts by weight based on 100 parts by weight of the total of the polyolefin and the diluent. It may be 1.5 parts by weight. When the content of the carbon-carbon double bond group-containing alkoxy silane compound satisfies the above numerical range, the content of the carbon-carbon double bond group-containing alkoxy silane compound is small and the graft rate is reduced, so that sufficient crosslinking is not achieved or carbon-carbon double bond groups It is possible to prevent the problem of poor appearance of the extruded sheet due to the presence of unreacted silane due to the high content of the contained alkoxysilane compound, and the shutdown temperature can be lowered by reducing the crystallinity of the polyolefin due to proper grafting with the polyolefin. .

In a specific embodiment of the present invention, the initiator can be used without limitation as long as it is an initiator capable of generating radicals. Non-limiting examples of the initiator include 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane (2,5-dimethyl-2,5-di(tert-butylperoxy)hexane , (DHBP)), benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di-ter-butyl peroxide, dicumyl peroxide, cumyl peroxide, hydrogen peroxide, potassium persulfate, and the like.

In a specific embodiment of the present invention, the content of the initiator is 0.1 to 20 parts by weight, or 0.5 to 10 parts by weight, or 1 to 5 parts by weight, based on 100 parts by weight of the carbon-carbon double bond group-containing alkoxy silane compound. can be wealth When the content of the initiator satisfies the above numerical range, the problem of crosslinking between polyolefins in the extruder can be prevented due to a decrease in the silane graft rate due to a low content of the initiator or a high content of the initiator.

In a specific embodiment of the present invention, the raw material may further include an antioxidant. Examples of the antioxidant may include a phenol-based antioxidant, a phosphorus-based antioxidant, an amine-based antioxidant, a sulfur-based antioxidant, a polyurethane-based antioxidant, or a mixture of at least two or more thereof.

In a specific embodiment of the present invention, non-limiting examples of the phenolic antioxidant include 2,6-di-t-butyl-4-methylphenol, 4,4'-thiobis(2-t-butyl-5). -methylphenol), 2,2'-thio diethyl bis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)-propionate], pentaerythritol-tetrakis-[3 -(3,5-di-t-butyl-4-hydroxyphenyl)-propionate] (Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), 4,4 '-thiobis(2-methyl-6-t-butylphenol), 2,2'-thiobis(6-t-butyl-4-methylphenol), octadecyl-[3-(3,5-di- t-Butyl-4-hydroxyphenyl)-propionate], triethylene glycol-bis-[3-(3-t-butyl-4-hydroxy-5-methylphenol)propionate], thiodiethylene Bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 6,6'-di-t-butyl-2,2'-thiodi-p-cresol, 1 ,3,5-tris(4-t-butyl-3-hydroxy-2,6-xylyl)methyl-1,3,5-triazine-2,4,6-(1H,3H,5H)- trione, dioctadecyl 3,3'-thiodipropionate, or a mixture of at least two or more thereof.

The phenolic antioxidant may react with peroxide (ROOH) derived from the initiator to prevent auto-oxidation of polyolefin, and remove the initiator with good reactivity.

In a specific embodiment of the present invention, the phosphorus-based antioxidant can increase the reactivity with ROOH to prevent deterioration during polyolefin processing to obtain high-quality polyolefin. Specifically, it is possible to prevent in advance the problem that the melt viscosity changes according to the cutting of the polyolefin main chain by mechanical shearing force and oxidation, resulting in unstable processing conditions or deterioration of physical properties.

In a specific embodiment of the present invention, the phosphorus-based antioxidant may be a trivalent phosphorus compound, and may be a pentavalent phosphorus compound by ionically decomposing ROOH into harmless alcohol as shown in Scheme 1 below.

[Scheme 1]

In a specific embodiment of the present invention, the phosphorus-based antioxidant is, as a non-limiting example, 3,9-bis(2,6-di-t-4-methylphenoxy)-2,4,8,10- Tetraoxa-3,9-diphosphaspiro[5,5]undecane (3,9-Bis(2,6-di-tert-butyl-4-methylphenoxy)-2,4,8,10-tetraoxa- 3,9-diphosphaspiro[5.5]undecane), bis(2,6-dicumylphenyl)pentaerythritol diphosphite (Bis(2,4-dicumylphenyl)pentaerythritol diphosphate), 2,2'-methylenebis(4,6- di-t-butylphenyl) 2-ethylhexyl phosphite (2,2'-Methylenebis(4,6-di-tert-butylphenyl) 2-ethylhexyl phosphite), bis(2,4-di-t-butyl-6 -Methylphenyl)-ethyl-phosphite (bis(2,4-di-tert-butyl-6-methylphenyl)-ethyl-phosphite), bis(2,6-dit-butyl-4-methylphenyl) pentaerythritol diphosphite , bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite (bis(2,4-di-t-butylphenyl)Pentaerythritol Diphosphite), bis(2,4-dicumylphenyl)pentaerythritol diphosphite, dis tearyl pentaerythritol diphosphite, or a mixture of at least two or more thereof.

In a specific embodiment of the present invention, the sulfur-based antioxidant may decompose the first two ROOHs by oxidizing divalent sulfur atoms to hexavalent. In addition, when oxidation proceeds, acid can be generated, thereby promoting ionic decomposition of ROOH.

In a specific embodiment of the present invention, as a non-limiting example of the sulfur-based antioxidant, 3,3'-thiobis-1,1'-didodecyl ester (3,3'-thiobis-1,1'-didodecyl ester) ), dimethyl 3,3'-thiodipropionate (Dimethyl 3,3'-Thiodipropionate), dioctadecyl 3,3'-thiodipropionate (Dioctadecyl 3,3'-thiodipropionate), 2,2- Bis{[3-(dodecylthio)-1-oxopropoxy]methyl}propane-1,3-diyl-bis[3-(dodecylthio)propionate](2,2-Bis{[ 3-(dodecylthio)-1-oxopropoxy]methyl}propane-1,3-diyl bis[3-(dodecylthio)propionate]), or a mixture of at least two or more thereof.

In a specific embodiment of the present invention, the content of the antioxidant may be 0.01 to 1.0 parts by weight, or 0.03 to 0.8 parts by weight, or 0.05 to 0.7 parts by weight, based on 100 parts by weight of the total amount of the polyolefin and the diluent.

In a specific embodiment of the present invention, the silane-grafted polyolefin composition may further include general additives for improving specific functions, such as surfactants, UV stabilizers, antistatic agents, nucleating agents, etc., if necessary. .

In a specific embodiment of the present invention, a single screw extruder or a twin screw extruder may be used in the reaction extrusion step.

In step (S1), the extruder includes a feed and mixing zone for raw materials, a silane grafting reaction zone, and a termination zone. In these three zones, in the extruder, raw materials such as polyolefin, diluent, initiator, and carbon-carbon double bond-containing alkoxy silane compound are introduced into the hopper of the extruder, mixing and silane grafting reaction are performed, and the reaction is completed Then, through the die of the extruder, the silane-grafted polyolefin composition may be disposed in the order in which it is extruded to the outside.

In this case, a reverse screw element rotating in the stopping direction or in the reverse direction may be installed at the end of each section. The reverse screw element is a screw element designed to return the raw material fed into the extruder in the direction of the hopper (back), mix while stopping in place, or return it in the reverse direction while stopping at the same time, which is a single piece It may be configured, or it may be configured by two or more.

In addition, the conveying and mixing zone may include a hopper for supplying polyolefin and a first supply port for supplying a diluent; and a forward screw element for sending the supplied polyolefin and diluent in the die exit direction (forward), and finally, a reverse screw element rotating in a stop direction or in a reverse direction may be disposed at the end of the section to distinguish it.

The stationary or counter-rotating counter screw element may create a stagnant or partial counter-current in terms of total flow, but in particular continuously fed materials, e.g. polyolefins and diluents, into the void space between the screw element and the barrel. will flow in the forward direction.

The conveying and mixing zone may be controlled to a temperature of 120 to 230°C, or 140 to 220°C. When the conveying and mixing zone is controlled in such a temperature range, it is advantageous in that the introduced raw material is melted and mixed with the diluent uniformly at the same time.

According to one embodiment of the present invention, the feed and mixing zone of the raw material may be composed of three different zones, namely, a forward zone, a stop zone, and a reverse zone in the die exit direction from the hopper. The zones may each independently comprise one or more screw elements.

Specifically, the advancing zone may comprise one or more screw elements designed to serve to direct (forward) the input material within the extruder to the die; the stop zone may comprise one or more screw elements designed to rotate the input material in the extruder in place (parallel); The retracting section may comprise one or more screw elements designed to return the feed material in the extruder in the direction of the hopper (back).

For example, according to one embodiment of the invention, the forward zone comprises 2 to 5 forward screw elements, the stop zone comprises 2 to 5 neutral screw elements, and the reverse zone comprises 1 to 5 reverse screw elements.

The residence time in the conveying and mixing zone may be from 30 seconds to 5 minutes or from 1 minute to 3 minutes,

The residence time in the conveying and mixing zone means from the point at which the polyolefin is supplied to the hopper, the diluent, the initiator, and the carbon-carbon double bond-containing alkoxy silane compound are supplied to the second supply port of the silane grafting reaction zone, which is the next zone. It can be defined as the time required before the

The residence time can be adjusted according to the number and speed of the forward screw element and the stationary or counter-rotating reverse screw element installed at the end of the conveying and mixing zone.

According to an embodiment of the present invention, the silane grafting reaction zone may include a second supply port for supplying the diluent, the initiator, and an alkoxy silane compound containing a carbon-carbon double bond group; and mixing the supplied diluent, initiator, carbon-carbon double bond group-containing alkoxysilane compound, and crosslinking catalyst with the previously supplied polyolefin, and induce a grafting reaction between the carbon-carbon double bond group-containing alkoxysilane compound and polyolefin A plurality of reverse screws rotating in the stopping direction or in the reverse direction; may be provided.

The silane grafting reaction zone may be controlled at a temperature of 160 to 240 °C, or 170 to 230 °C. When the silane grafting reaction zone is controlled in such a temperature range, it is advantageous in that the introduction of silane is performed smoothly and the problem of over-crosslinking does not occur.

The residence time in the silane grafting reaction zone may be adjusted in consideration of the reactivity of the carbon-carbon double bond group-containing alkoxy silane compound, for example, the screw configuration and the length of the reaction zone may be adjusted.

The residence time in the silane grafting reaction zone means from the point at which the diluent, the initiator, and the alkoxy silane compound containing a carbon-carbon double bond group are supplied to the second supply port, the diluent and It can be defined as the time required before the time the antioxidant is supplied.

The residence time may be adjusted according to the number and speed of the plurality of kneaders and the number and speed of stationary or counter-rotating reverse screw elements installed at the end of the silane grafting reaction zone.

According to one embodiment of the present invention, the termination zone may have a third supply port for supplying the diluent and the antioxidant. At this time, the supplied antioxidant is previously supplied through the second supply port and does not participate in the silane grafting reaction, so it reacts with the initiator that remains reactive and terminates all reactions so that no further silane grafting reaction proceeds. do

Meanwhile, in the conveying and mixing zone, the antioxidant may be further added in such a way as to be incorporated into the polyolefin or dissolved or dispersed in a diluent.

The antioxidant input in the transfer and mixing zone serves to prevent hydrolysis of silane by using an antioxidant that is easily hydrolyzed while not having a competitive reaction with the initiator compared to the antioxidant input in the termination zone. can do. For example, a phosphite-based antioxidant may be applied as the antioxidant that is easily hydrolyzed, and a hindered phenol-based antioxidant may be used at the end of the reaction to be described later.

In the conveying and mixing zone, when the antioxidant is further added in such a way as to be incorporated into the polyolefin or dissolved or dispersed in a diluent, the content (a) of the antioxidant added to the end zone and the amount of antioxidant added to the conveying and mixing zone The weight ratio of the antioxidant content (b) may be 90:10 to 50:50, or 80:20 to 70:30. When the weight ratio satisfies this range, it may be advantageous in that overcrosslinking can be controlled without interfering with the crosslinking reaction.

Next, the extruded silane-grafted polyolefin composition is molded and stretched in a sheet form (S2).

For example, the reaction-extruded silane-grafted polyolefin composition is extruded using an extruder equipped with a T-die, etc., and then cooled by using a general casting or calendaring method using water cooling or air cooling. can form.

In a specific embodiment of the present invention, it is possible to provide a separator having improved mechanical strength and puncture strength by performing the stretching step as described above.

In a specific embodiment of the present invention, the stretching may be performed by a roll method or a tender method sequentially or simultaneously stretching. The draw ratio may be 3 times or more, or 4 times to 10 times, respectively, in the longitudinal and transverse directions, and the total draw ratio may be 6 to 100 times. When the draw ratio satisfies the above numerical range, it is possible to prevent the problem that the orientation in one direction is not sufficient and the balance of physical properties between the longitudinal and transverse directions is broken at the same time, so that the tensile strength and the puncture strength are lowered, and the total draw ratio is within the numerical range By satisfying , it is possible to prevent the problem that unstretched or pore formation does not occur.

In a specific embodiment of the present invention, the stretching temperature may vary depending on the melting point of the polyolefin used, the concentration and type of the diluent.

In a specific embodiment of the present invention, for example, when the polyolefin used is polyethylene, the diluent is liquid paraffin, and the kinematic viscosity of the liquid paraffin is 50 to 150 cSt at 40° C., the stretching temperature is longitudinal stretching (MD). , Machine Direction) may be 70 to 160 ℃, or 90 to 140 ℃, or 100 to 130 ℃, in the case of transverse stretching (TD, Traverse Direction) 90 to 180 ℃, or 110 to 160 ℃ or 120 to It may be 150°C, and in the case of simultaneously performing stretching in both directions, it may be 90 to 180°C, or 110 to 160°C, or 110 to 150°C.

When the stretching temperature satisfies the numerical range, as the stretching temperature has a low temperature range, it is possible to prevent the problem of fracture or non-stretching due to lack of softness, and occurs as the stretching temperature is high Partial overstretching or differences in physical properties can be prevented.

Thereafter, a porous membrane is prepared by extracting a diluent from the molded and stretched sheet (S3).

In a specific embodiment of the present invention, a diluent may be extracted from the porous membrane using an organic solvent and the porous membrane may be dried.

In a specific embodiment of the present invention, the organic solvent is not particularly limited as long as it can extract the diluent, but methyl ethyl ketone, methylene chloride, hexane, etc. with high extraction efficiency and fast drying are suitable.

In a specific embodiment of the present invention, as the extraction method, all general solvent extraction methods such as an immersion method, a solvent spray method, and an ultrasonic method may be used individually or in combination. The content of the residual diluent after the extraction treatment may be preferably 1% by weight or less. When the content of the residual diluent satisfies this range, the problem of reduced physical properties and reduced permeability of the porous membrane can be prevented. The content of the residual diluent may be affected by the extraction temperature and extraction time. In order to increase the solubility of the diluent and the organic solvent, it is preferable that the extraction temperature is high, but in consideration of the safety problem due to the boiling of the organic solvent, it is preferable to be below 40℃. . If the extraction temperature is below the freezing point of the diluent, the extraction efficiency is greatly reduced, so it must be higher than the freezing point of the diluent.

In addition, the extraction time varies depending on the thickness of the porous membrane to be prepared, but in the case of a porous membrane having a thickness of 5 to 15 μm, 2 to 4 minutes is suitable.

Next, a porous coating layer is formed by coating and drying a slurry for forming a porous coating layer including inorganic particles, a binder polymer, a crosslinking catalyst, and a dispersion medium on at least one surface of the porous membrane (S4).

In the case of manufacturing a conventional cross-linked polyolefin membrane, a cross-linking catalyst was added to an extruder together with polyolefin, a silane compound, an initiator, and a diluent for reaction extrusion, but in this case, a cross-linking reaction proceeded in the extruder during reaction extrusion, resulting in gel and drool (Drool) occurred and the yield was lowered, and the filter pressure of the extruder was increased to shorten the filter replacement cycle and increase the non-operation time.

In the present invention, in order to prevent the problem of adding the cross-linking catalyst to the extruder first, a post-addition method is adopted in which the cross-linking catalyst is added to the slurry for forming the porous coating layer.

The inorganic particles are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles that can be used in the present invention are not particularly limited as long as oxidation and/or reduction reactions do not occur in ^{the operating voltage range of the applied electrochemical device (eg, 0-5 V based on Li/Li +).} In particular, when inorganic particles having a high dielectric constant are used as the inorganic particles, the ionic conductivity of the electrolyte can be improved by contributing to an increase in the degree of dissociation of an electrolyte salt, such as a lithium salt, in a liquid electrolyte.

For the reasons described above, the inorganic particles may include high dielectric constant inorganic particles having a dielectric constant of 5 or more, preferably 10 or more, inorganic particles having lithium ion transport ability, or a mixture thereof.

Non-limiting examples of inorganic particles having a dielectric constant of 5 or more include BaTiO ₃ , Pb(Zr _x Ti _1-x )O ₃ (PZT, 0<x<1), Pb _1-x La _x Zr _1-y Ti _y O ₃ (PLZT, 0<x<1, 0<y<1), (1-x)Pb(Mg _1/3 Nb _2/3 )O _3-x PbTiO ₃ (PMN-PT, 0<x<1) , hafnia (HfO ₂ ), SrTiO ₃ , SnO ₂ , CeO ₂ , MgO, NiO, CaO, ZnO, ZrO ₂ , SiO _{2 ,} Y ₂ O ₃ , Al ₂ O ₃ , SiC and TiO ₂ selected from the group consisting of Inorganic particles, such as any one or a mixture of two or more of them, not only exhibit high dielectric constant characteristics with a dielectric constant of 100 or more, but also have piezoelectricity ( By having piezoelectricity, it is possible to prevent the occurrence of an internal short circuit of the positive electrode due to an external impact, thereby improving the stability of the electrochemical device.

In addition, the inorganic particles having the lithium ion transport ability refers to inorganic particles containing a lithium element, but having a function of moving lithium ions without storing lithium. Non-limiting examples of inorganic particles having lithium ion transport ability include lithium phosphate (Li ₃ PO ₄ ), lithium titanium phosphate (Li _x Ti _y (PO ₄ ) ₃ ,0<x<2,0<y<3), Lithium aluminum titanium phosphate (Li _x Al _y Ti _z (PO ₄ ) ₃ ,0<x<2,0<y<1,0<z<3), (LiAlTiP) _x O _y series glass (0<x<4,0<y<13), lithium lanthanum titanate (Li _x La _y TiO ₃ ,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), lithium nitride (Li _x N _y , 0<x<4, 0<y<2), SiS ₂ (Li _x Si _y S _z ,0<x<3,0<y<2,0<z<4) series glass and P ₂ S ₅ (Li _x P _y S _z ,0<x<3,0< There are y<3,0<z<7) series glass or mixtures thereof, and when the above-described high-k inorganic particles and inorganic particles having lithium ion transport ability are mixed, their synergistic effect can be doubled.

The inorganic particle size is not limited, but is preferably in the range of 0.001 to 10 nm as possible for the formation of a coating layer having a uniform thickness and an appropriate porosity.

In addition, as the binder polymer applicable to the present invention, a binder that can be used to form the porous coating layer together with the inorganic particles may be used without limitation, but preferably polyvinylidene fluoride, polyvinylidene fluoride - Hexafluoropropylene (polyvinylidene fluoride-co-hexafluoropropylene), polyvinylidene fluoride-tetrafluoroethylene (polyvinylidene fluoride-co-tetrafluoroethylene), polyvinylidene fluoride-trichloroethylene (polyvinylidene fluoride-co-trichlorethylene) , polyvinylidene fluoride-trichlorofluoroethylene (polyvinylidene fluoride-co-trichlorofluoroethylene), polymethylmethacrylate (polymethylmethacrylate), polybutylacrylate (polybutylacrylate), polyacrylonitrile (polyacrylonitrile), polyvinylpyrroly Don (polyvinylpyrrolidone), polyvinylacetate (polyvinylacetate), ethylene-vinyl acetate copolymer (polyethylene-co-vinyl acetate), polyethylene oxide (polyethylene oxide), polyarylate (polyarylate), cellulose acetate (cellulose acetate), cellulose acetate Butyrate, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose (cyanoethylsucrose), pullulan, carboxymethyl cellulose (car boxyl methyl cellulose), or two or more of these.

In this case, the content ratio of the inorganic particles to the binder polymer may be, for example, 50:50 to 99:1, or 70:30 to 95:5. When the content ratio of inorganic particles to the binder polymer satisfies this range, the content of the polymer is appropriately controlled to improve the thermal stability of the separator and the peeling resistance of the porous coating layer, and the void space formed between the particles is secured to ensure the pore size. And the porosity may be increased to improve the performance of the final battery.

In addition, the dispersion medium is not limited as long as it is electrochemically stable, and specifically, acetone, methylethylketone, tetrahydrofuran, methylene chloride, chloroform, dimethyl It may be at least one selected from the group consisting of formamide (dimethylformamide), dimethylacetamide, N-methyl-2-pyrrolidone (NMP), and cyclohexane. .

In a specific embodiment of the present invention, the crosslinking catalyst is added to promote the silane crosslinking reaction.

In a specific embodiment of the present invention, as the crosslinking catalyst, carboxylate salts of metals such as tin, zinc, iron, lead, and cobalt, organic bases, inorganic acids and organic acids may be used. Non-limiting examples of the crosslinking catalyst include dibutyl tin dilaurate, dibutyl tin diacetate, stannous acetate, stannous caprylate, zinc naphthenate, zinc caprylate, and cobalt naphthenate, and the organic base includes ethylamine, dibutylamine, hexylamine, pyridine, and the like, and the inorganic acid includes sulfuric acid and hydrochloric acid, and the organic acid includes toluene sulfonic acid, acetic acid, and stearic acid. , maleic acid, and the like. In addition, the crosslinking catalyst may be used alone or a mixture of two or more of them.

In a specific embodiment of the present invention, the content of the crosslinking catalyst may be 0.01 to 0.05 parts by weight based on 100 parts by weight of the inorganic particles. When the content of the cross-linking catalyst satisfies the above numerical range, a desired level of silane cross-linking reaction may occur, unwanted side reactions in the lithium secondary battery may be prevented, and defects in the film forming process due to the cross-linking catalyst may be prevented. can be prevented

In addition, the slurry for forming the porous coating layer may be applied to at least one surface of the porous membrane to a thickness of 0.001 to 20 μm, or 2 to 7 μm. When the thickness of the coated slurry satisfies this range, the thermal stability effect is improved, and the problem of battery cycle performance degradation due to the resistance of the coating layer can be prevented.

A method of coating the slurry on the porous polymer substrate is not particularly limited, but a slot coating method or a dip coating method may be used. The slot coating is a method in which the slurry supplied through the slot die is applied to the entire surface of the substrate, and the thickness of the coating layer can be adjusted according to the flow rate supplied from the metering pump. In addition, the dip coating is a method of coating by immersing the substrate in a tank containing the composition, and the thickness of the coating layer can be adjusted according to the concentration of the slurry and the speed of taking the substrate out of the tank. Post-weighing can be done through bars, etc.

By drying the slurry-coated porous polymer substrate using a dryer such as an oven, a porous coating layer formed on at least one surface of the porous polymer substrate is formed. After the slurry is coated on the porous polymer substrate, the dispersion medium may be removed by drying at, for example, 90 to 180°C, or 100 to 150°C.

Thereafter, the porous membrane on which the porous coating layer is formed may be heat-set.

In the heat setting, the porous membrane is fixed by applying heat, and the residual stress is removed by forcibly holding the porous membrane to be contracted.

In a specific embodiment of the present invention, when the polyolefin is, for example, polyethylene, the heat setting temperature may be 100 to 140 ℃, or 105 to 135 ℃, or 110 to 130 ℃. When the polyolefin is polyethylene and the heat setting temperature satisfies the above numerical range, the polyolefin molecules are rearranged to remove the residual stress of the porous membrane, and the problem of clogging the pores of the porous membrane due to partial melting. can

In a specific embodiment of the present invention, the heat setting time may be 10 to 120 seconds, 20 to 90 seconds, 30 to 60 seconds. In the case of heat setting at this time, it is possible to remove the residual stress of the porous membrane due to rearrangement of the polyolefin molecules, and it is possible to reduce the problem of clogging the pores of the porous membrane due to partial melting.

Next, a porous release substrate having a moisture permeability ^{of 2,000 g/m 2} /day or more is interposed and wound on the porous coating layer of the porous membrane (S5).

Since the porous coating layer contains a binder polymer, the porous coating layer basically has surface adhesive force, so when the porous membrane is wound with the porous coating layer exposed to the outermost side, the facing porous membranes adhere to each other. In order to form an electrode assembly, the separator obtained by performing the hydrocrosslinking treatment of the porous membrane adhered to each other cannot be unwinded smoothly in the wound state.

In order to solve this problem, conventionally, a non-porous polyethylene terephthalate (PET) release film was interposed on the porous coating layer and wound up was performed. However, in the present invention, the porous membrane is essentially subjected to a water-crosslinking process after the porous membrane is wound. When the PET release film is interposed between the porous membranes, moisture cannot sufficiently penetrate to the inside of the wound porous membrane, and thus the wound porous membrane is wound up. A problem may arise in that the water bridge does not occur uniformly to the inside of the roll of the membrane.

Therefore, in the present invention, a porous release substrate having a water vapor transmission rate ^{of 2,000 g/m 2} /day or more is interposed and wound on the porous coating layer of the porous membrane.

According to an embodiment of the present invention, the moisture permeability of the porous release substrate is 2,000 to 12,000 g/m ² /day, or 2,000 to 10,000 g/m ² /day, or 2,000 to 3,080 g/m ² /day, or 3,080 to 10,000 g/m ² /day.

At this time, the moisture permeability of the porous release substrate can be measured using a moisture permeability tester (eg, model name: SJTM-014, manufacturer: Sejin Test Technology Co., Ltd.).

According to an embodiment of the present invention, the moisture permeability of the porous release substrate is mounted on the inside of a moisture permeability tester after cutting the porous release substrate to a size of 108 mm Ⅷ 108 mm. Then, dry nitrogen gas not containing water vapor is introduced into one side of the porous release substrate, and water vapor is introduced into the other side. At this time, the two spaces in which the gases are introduced are separated from each other so that the gases flowing into both sides of the porous release substrate are not mixed. Meanwhile, during the experiment, the temperature is set to 38°C, and the humidity is set to 100%RH and maintained. Then, for 24 hours, the amount of water vapor is measured from the one surface into which dry nitrogen gas is introduced using a humidity sensor. The amount of water vapor is divided by the area of the one surface to derive the amount of water vapor per unit area that has passed through the porous release substrate for 24 hours, and this is evaluated as a water vapor transmission rate (WVRT).

According to one embodiment of the present invention, the porous release substrate may have an average pore size of 1 to 1,000 μm, or 5 μm to 500 μm, or 5 μm to 15.4 μm, or 15.4 μm to 500 μm.

The average pore size of the porous release substrate may be measured using a scanning electron microscope (FE-SEM) (Hitachi S-4800 Scanning Electron Microscope). Specifically, the sample surface was magnified 2,500 times and measured using a scanning electron microscope (FE-SEM) (Hitachi S-4800 Scanning Electron Microscope), and then randomly sampled within the measured photo range (more than 10um in width and more than 15um in length) Measure the major axis length among the surface pores identified in , as the pore size, and set the number of measurements to at least 10, and the average value of the pore sizes obtained after measurement can be obtained.

According to an embodiment of the present invention, the porous release substrate is 250 kgf/cm ² or more, or 250 to 800 kgf/cm ² , or 300 to 700 kgf/cm ² , or 300 to 482 kgf/cm ² , or 482 to 700 kgf/cm ² may have a strength.

According to one embodiment of the present invention, the strength of the porous release substrate is cut to 15 mm × 100 mm to prepare a sample, and using the prepared sample, according to ASTM-D882, at a speed of 500 mm/min in the MD direction and the TD direction It can be measured as the strength at the time the sample is broken when each is pulled.

As the porous release substrate, any release substrate satisfying the moisture permeability range may be applied without limitation, and examples thereof may include a non-woven substrate and the like.

Next, the result of the step (S5) is cross-linked (S6).

In a specific embodiment of the present invention, the crosslinking may be performed at 60 to 100 °C, or 65 to 95 °C, or 70 to 90 °C.

In a specific embodiment of the present invention, the hydro-crosslinking may be performed at a humidity of 60 to 95% for 6 to 50 hours.

According to another aspect of the present invention, there is provided a cross-linked polyolefin membrane comprising a cross-linked structure derived from a silane compound grafted to the main chain of the polyolefin.

The cross-linked polyolefin separator may be a silane cross-linked separator in which silane is grafted to the polyolefin main chain and polyolefin is cross-linked via these neighboring silanes, that is, silane-derived cross-linking derived by a silane compound grafted to the polyolefin main chain. structure may be included. In this case, the silane-derived cross-linked structure may include a siloxane (-Si-O-Si-) bonding structure.

According to one aspect of the present invention, there is provided an electrochemical device comprising an anode, a cathode, and a separator interposed between the anode and the cathode, wherein the separator is the aforementioned cross-linked polyolefin separator. The electrochemical device may be a lithium secondary battery.

Such an electrochemical device may be manufactured according to a conventional method known in the art, and an embodiment thereof may be manufactured by inserting an electrolyte solution after assembling by interposing the above-described separator between the positive electrode and the negative electrode. have.

The electrode to be applied together with the separator is not particularly limited, and the electrode active material may be prepared in a form in which the electrode active material is bound to the electrode current collector according to a conventional method known in the art.

Among the electrode active materials, non-limiting examples of the positive electrode active material include conventional positive electrode active materials that can be used in positive electrodes of conventional electrochemical devices, and in particular, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a combination thereof. It is preferable to use one lithium composite oxide. As a non-limiting example of the negative electrode active material, a conventional negative electrode active material that can be used for the negative electrode of a conventional electrochemical device can be used, and in particular, lithium metal or lithium alloy, carbon, petroleum coke, activated carbon, Lithium adsorption materials such as graphite or other carbons are preferred.

Non-limiting examples of the positive current collector include a foil made of aluminum, nickel, or a combination thereof, and non-limiting examples of the negative current collector include copper, gold, nickel or a copper alloy or a combination thereof. There are foils manufactured by

The electrolyte solution that can be used in one embodiment of the present invention is a salt having the same structure as ^{A +} B ^{- , where A +} contains an ion consisting of an alkali metal cation such as Li ⁺ , Na ⁺ , K ^{+ or a combination thereof, and B -} is PF ₆ ^-, BF ₄ ^- , Cl ^- , Br ^- , I ^- , ClO ₄ ^- , AsF ₆ ^- , CH ₃ CO ₂ ^- , CF ₃ SO ₃ ^- , N(CF ₃ SO ₂ ) ₂ ^- , C (CF ₂ SO ₂ ) ₃ ^- A salt containing an anion or a combination thereof, such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethylsulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (EMC), gamma-butyrolactone or Some are dissolved or dissociated in an organic solvent consisting of a mixture thereof, but the present invention is not limited thereto.

The electrolyte injection may be performed at an appropriate stage during the battery manufacturing process according to the manufacturing process and required physical properties of the final product. That is, it may be applied before battery assembly or at the final stage of battery assembly.

A lithium secondary battery according to an embodiment of the present invention forms an electrode assembly by disposing a separator between the positive electrode and the negative electrode, and puts the electrode assembly in, for example, a pouch, a cylindrical battery case or a prismatic battery case, and then the electrolyte When injected, the secondary battery can be completed. Alternatively, a lithium secondary battery may be completed by stacking the electrode assembly, impregnating it with an electrolyte, and sealing the obtained result in a battery case.

In this case, as a process of forming the electrode assembly, in addition to the winding process used in manufacturing the jelly roll, which is a general process, lamination, stack, and folding processes of the separator and the electrode are possible.

According to an embodiment of the present invention, the lithium secondary battery may be a stack type, a winding type, a stack and fold type, or a cable type.

The lithium secondary battery according to an embodiment of the present invention can be used not only in a battery cell used as a power source for a small device, but also can be preferably used as a unit cell in a medium or large battery module including a plurality of battery cells. Preferred examples of the medium and large devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems, and are particularly useful in areas requiring high output, such as hybrid electric vehicles and new and renewable energy storage batteries. can be used

Hereinafter, to help the understanding of the present invention, examples will be described in detail. However, the embodiments according to the present invention may be modified in various other forms, and the scope of the present invention should not be construed as being limited to the following examples. The embodiments of the present invention are provided to more completely explain the present invention to those of ordinary skill in the art.

Method for evaluating physical properties of a porous release substrate

(1) moisture permeability

The moisture permeability of the porous release substrate used in the following Examples and Comparative Examples was measured using a moisture permeability tester (model name: SJTM-014, manufacturer: Sejin Test Technology Co., Ltd.).

First, the porous release substrate was cut to a size of 108 mm × 108 mm, and then mounted inside the moisture permeability tester, respectively. Then, dry nitrogen gas not containing water vapor was introduced into one side of the porous release substrate, and water vapor was introduced into the other side. At this time, the two spaces in which the gases are introduced are isolated from each other so that the gases flowing into both sides of the porous release substrate are not mixed. Meanwhile, during the experiment, the temperature was set to 38°C, and the humidity was set to 100%RH and maintained. And for 24 hours, the amount of water vapor was measured from the one surface into which dry nitrogen gas was introduced using a humidity sensor. The amount of water vapor was divided by the area of the one surface to derive the amount of water vapor per unit area that passed through the porous release substrate for 24 hours, and this was evaluated as a water vapor transmission rate (WVRT).

(2) average pore size

The average pore size of the porous release substrate was measured using a scanning electron microscope (FE-SEM) (Hitachi S-4800 Scanning Electron Microscope).

Specifically, the sample surface was magnified 2,500 times and measured using a scanning electron microscope (FE-SEM) (Hitachi S-4800 Scanning Electron Microscope), and then randomly sampled within the measured photo range (more than 10um in width and more than 15um in length) The length of the major axis of the surface pores identified in Fig. 1 was measured as the pore size, the number of measurements was set to at least 10, and the average value of the pore sizes obtained after measurement was obtained.

(3) strength

When the strength of the porous release substrate is cut to 15 mm X 100 mm to prepare a sample, and the prepared sample is pulled in the MD direction and TD direction at a speed of 500 mm/min according to ASTM-D882, respectively, the time point at which the sample breaks was measured as the strength of

Evaluation of film breakage temperature of crosslinked polyolefin separator

The meltdown temperature of the separators prepared in Examples and Comparative Examples below is measured by thermomechanical analysis (TMA) after taking samples of the cross-linked polyolefin separator in the machine direction (Machine direction, MD), respectively. did.

Specifically, a 10 mm-long sample was placed in a TMA instrument (TA Instrument, Q400) and exposed to increasing temperature conditions (starting at 30° C. and 5° C./min) while applying a tension of 19.6 mN. As the temperature increased, the length of the sample was changed, and the temperature at which the sample was broken due to the rapid increase in length was measured, and this was defined as the film breaking temperature.

At this time, it was judged as good when the measured membrane breaking temperature of the separator was equal to or higher than the temperature at which the cathode active material ran away due to self-heating. Specifically, since most of the commonly used nickel-rich cathode active materials have a self-heating temperature of 180° C., it was judged as good when the measured membrane breaking temperature of the separator was 180° C. or higher.

Example 1

10.5 kg of high-density polyethylene (Daehan Petrochemical, VH035) with a weight average molecular weight of 380,000 as polyolefin for extruder, 19.5 kg of liquid paraffin oil (Kukdong Petrochemical LP350F, kinematic viscosity 67.89 cSt at 40°C) as diluent 19.5 kg, alkoxy containing carbon-carbon double bond 450 g of vinyltrimethoxysilane as the silane and 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane (2,5-dimethyl-2,5-di(tert) as the initiator -Butylperoxy)hexane, (DHBP)) 6g was added and mixed at once to prepare a silane-grafted polyethylene composition under a temperature condition of 210 ° C., and the prepared composition was passed through a T-die of an extruder and a cooling casting roll to form a sheet. After molding, MD stretching (longitudinal stretching) followed by TD stretching (transverse stretching) was biaxially stretched using a tenter-type sequential stretching machine. Both the MD draw ratio and the TD draw ratio were set to 7 times. As for the extending|stretching temperature, MD was 113 degreeC, and TD was 121 degreeC.

The stretched sheet was prepared by extracting a diluent with methylene chloride and heat-setting at 129°C to prepare a porous membrane.

The thickness of the obtained porous membrane was 9.0 mu m. In addition, the average number of gels having a long side length of 100 μm or more per ^{1 m 2} in the porous membrane was 0.39.

Next, a slurry for forming a porous coating layer was applied to both surfaces of the porous membrane. Slurry for forming the porous coating layer is N-methyl-2-pyrrolidone (NMP) 400 g as a binder polymer as polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) (Solvay Solef 21510, Tm 132 ℃, weight average molecular weight) 300,000, HFP substitution degree 15 mol%) 30 g is dissolved, Al ₂ O ₃ (Japan Light Metals, LS235, D50 500 nm) 70 g is added and dispersed in a ball mill method, and then dibutyltin di as a crosslinking catalyst Preparation was completed by post-addition of 1 pt of laurate.

Thereafter, the slurry was sequentially coated on both sides of the porous membrane prepared previously by a micro-gravure method, and the slurry was sequentially immersed in a coagulation bath (NMP: water = 30: 70 weight ratio) and a washing bath (water), and then dried at 80 ° C. to porous A coating layer was formed. The prepared separation membrane into which the porous coating layer was introduced was simultaneously wound with a porous nonwoven fabric (thickness of 40 μm) made of PET as a porous release substrate to complete the preparation of the coated porous membrane. In this case, the porous nonwoven fabric made of PET as the porous release substrate had ^{a moisture permeability of 3,080 g/m 2} /day, an average pore size of 15.4 μm, and a strength of ^{482 kgf/cm 2 .}

Finally, the prepared porous coated membrane was cross-linked at 85° C. and 85% humidity for 48 hours to prepare a cross-linked polyolefin membrane. The thickness of the obtained crosslinked polyolefin separator was 15.0 μm, and the film breaking temperature by crosslinking was good at 193°C.

Example 2

A cross-linked polyolefin membrane was prepared in the same manner as in Example 1, except that the cross-linking catalyst was changed to the same amount of stearic acid. The thickness of the obtained crosslinked polyethylene separator was 15.0 μm, and the film breaking temperature by crosslinking was good at 188°C.

Comparative Example 1

A cross-linked polyolefin membrane was prepared in the same manner as in Example 1, except that a porous release substrate was not introduced. The thickness of the obtained crosslinked polyolefin separator was 15.0 μm, and the film breaking temperature by crosslinking was good at 191°C. However, due to the adhesion between the wound cross-linked polyolefin separators, the coating layer was detached, making it impossible to perform the normal assembly process.

Comparative Example 2

A cross-linked polyolefin membrane was prepared in the same manner as in Example 1, except that the cross-linking catalyst was introduced into the extruder. The number of gels with a long side length of 100 μm or more per ^{1 m 2} visually confirmed in the prepared separation membrane was 7.38 on average, and the number of gels greatly increased. Thereafter, the coating solution except for the crosslinking catalyst was coated in the same manner as in Example 1 to complete the preparation of the coated separator. The thickness of the obtained cross-linked polyethylene separator was 15.0 μm, and the film breaking temperature by cross-linking was 191° C., which was good.

Comparative Example 3

A cross-linked polyolefin separator was prepared in the same manner as in Example 1, except that a non-porous PET film (SKC, SG-31) was used instead of the porous release substrate. The thickness of the obtained crosslinked polyolefin separator was 15.0 μm, and the film breaking temperature was 155° C., so crosslinking did not proceed.

Claims (11)

(S1) preparing a silane-grafted polyolefin composition by injecting and mixing a polyolefin, a diluent, an initiator, and a raw material including a carbon-carbon double bond-containing alkoxysilane compound into an extruder and extruding;
(S2) molding and stretching the extruded silane-grafted polyolefin composition into a sheet form;
(S3) preparing a porous membrane by extracting a diluent from the stretched sheet;
(S4) forming a porous coating layer by applying and drying a slurry for forming a porous coating layer including inorganic particles, a binder polymer, a crosslinking catalyst, and a dispersion medium on at least one surface of the porous membrane;
(S5) winding a porous release substrate having a water vapor transmission rate ^{of 2,000 g/m 2} /day or more on the porous coating layer of the porous membrane; and
(S6) a step of hydrolyzing the resultant of step (S5); a method for producing a cross-linked polyolefin membrane comprising a. According to claim 1,
The method for producing a cross-linked polyolefin membrane, characterized in that the porous release substrate has a water vapor transmission rate of 2,000 to 12,000 g / m ^{2 /day.} According to claim 1,
The method for producing a cross-linked polyolefin separator, characterized in that the porous release substrate has an average pore size of 1 μm to 1,000 μm. According to claim 1,
The method for producing a cross-linked polyolefin separator, characterized in that the porous release substrate has a ^{strength of 250 kgf / cm 2 or more.} According to claim 1,
The method for producing a crosslinked polyolefin separator, characterized in that the porous release substrate comprises a nonwoven substrate. According to claim 1,
The method for producing a cross-linked polyolefin membrane, characterized in that the step of crosslinking in step (S6) is performed at 60 to 100° C. and 60 to 95% humidity for 6 to 50 hours. According to claim 1,
Method for producing a cross-linked polyolefin membrane, characterized in that it further comprises the step of removing the porous release substrate after the step (S6). According to claim 1,
The method for producing a cross-linked polyolefin separator, characterized in that the cross-linked polyolefin separator comprises a siloxane (-Si-O-Si-) bond structure, which is a silane-derived cross-linked structure derived by a silane compound grafted to the main chain of the polyolefin. A cross-linked polyolefin separation membrane prepared by the method of any one of claims 1 to 8. An electrochemical device comprising an anode, a cathode, and a separator interposed between the anode and the cathode, wherein the separator is the crosslinked polyolefin separator of claim 9. 11. The method of claim 10,
Electrochemical device, characterized in that the electrochemical device is a lithium secondary battery.