KR102869816B1 - Crosslinked polyolefin separator and manufacturing method thereof - Google Patents

Abstract

(S1) a step of introducing a polyolefin, a diluent, an initiator, a silane compound containing an alkoxy group and a carbon-carbon double bond group, and a crosslinking catalyst into an extruder and mixing them in a mixing section, and then extruding the mixture through a gear pump and a die outlet to produce a silane-grafted polyolefin composition; (S2) a step of forming and stretching the extruded silane-grafted polyolefin composition into a sheet shape; (S3) a step of extracting a diluent from the stretched sheet to produce a porous membrane; (S4) a step of heat-setting the porous membrane; and (S4) a step of crosslinking the heat-set porous membrane; A method for producing a crosslinked polyolefin membrane and a crosslinked polyolefin membrane are provided, wherein the residence time of the polyolefin after being introduced into the extruder in step (S1) and then mixed and transferred to the rear end of the gear pump is 3 minutes 30 seconds to 6 minutes 30 seconds.

Description

Crosslinked polyolefin separator and manufacturing method thereof {CROSSLINKED POLYOLEFIN SEPARATOR AND MANUFACTURING METHOD THEREOF}

The present invention relates to a crosslinked polyolefin separator and a method for manufacturing the same.

Interest in energy storage technology has been growing steadily. As its applications expand to include energy storage for mobile phones, camcorders, laptops, and even electric vehicles, research and development efforts in electrochemical devices are becoming increasingly concrete. Electrochemical devices are receiving the most attention in this regard, and the development of rechargeable secondary batteries is a particular focus. Recent research and development efforts in these batteries are focused on novel electrode and battery designs to improve capacity density and specific energy.

Among the secondary batteries currently in use, lithium secondary batteries developed in the early 1990s are attracting attention due to their advantages of higher operating voltage and significantly higher energy density than conventional batteries such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries that use aqueous electrolytes.

These lithium secondary batteries are composed of a positive electrode, a negative electrode, an electrolyte, and a separator. Among these, the separator is required to have high ionic conductivity to increase the permeability of lithium ions based on its insulating properties and high porosity to electrically insulate the positive and negative electrodes.

Furthermore, to ensure the safety of lithium secondary batteries, these separators must have a wide gap between the shutdown temperature and the meltdown temperature. To widen the gap between the shutdown and meltdown temperatures, the shutdown temperature must be controlled to decrease, while the meltdown temperature must be controlled to increase.

To control the meltdown temperature of wet polyethylene (PE) membranes, a method has been attempted to increase the meltdown temperature by mixing some polyolefin into the main component of the membrane. However, this method has been limited to a small increase in meltdown temperature. While the concept of cross-linked membranes can significantly increase meltdown temperature, controlling the cross-linking reaction is difficult without controlling the reaction within the extruder.

Therefore, the problem to be solved by the present invention is to provide a method for manufacturing a crosslinked polyolefin separator having a good appearance and small air permeability deviation, and a separator manufactured thereby.

One aspect of the present invention provides a method for manufacturing a crosslinked polyolefin separator according to the following embodiments.

According to the first implementation example,

(S1) A step of producing a silane-grafted polyolefin composition by introducing polyolefin, a diluent, an initiator, a silane compound containing an alkoxy group and a carbon-carbon double bond group, and a crosslinking catalyst into an extruder and mixing them in a mixing section, and then extruding them through a gear pump and through a die outlet;

(S2) A step of forming and stretching the extruded silane grafted polyolefin composition into a sheet form;

(S3) A step of manufacturing a porous membrane by extracting a diluent from the above-mentioned extended sheet;

(S4) a step of opening and fixing the porous membrane; and

(S4) A step of cross-linking the above-mentioned porous membrane;

A method for manufacturing a crosslinked polyolefin separation membrane is provided, wherein the residence time from the time the polyolefin is fed into the extruder in the step (S1) until it is mixed and transferred to the rear end of the gear pump is 3 minutes 30 seconds to 6 minutes 30 seconds.

According to the second embodiment, in the first embodiment,

The extruder is a twin-screw extruder having a hopper for feeding polyolefin, a port for feeding a diluent, an initiator, an alkoxy group and a carbon-carbon double bond group-containing silane compound, and a crosslinking catalyst, a kneading section for feeding and mixing the materials fed from the hopper and the port, a gear pump for feeding the materials fed through the kneading section to a die outlet, and a die outlet for extruding the materials fed through the gear pump to the outside.

The above residence time may be the time required from the time the polyolefin is fed into the hopper of the extruder to the time it is mixed with the diluent, initiator, silane compound containing an alkoxy group and a carbon-carbon double bond group, and a crosslinking catalyst in the kneading section and transferred to the rear end of the gear pump.

According to the third embodiment, in the first embodiment or the second embodiment,

The above residence time can be controlled by the arrangement of a plurality of screw elements provided within the kneading section.

According to the fourth embodiment, in the second embodiment or the third embodiment,

The above mixing unit has at least two zones among a forward zone that sends the input material into the extruder toward the die exit (forward), a stationary zone that mixes the input material into the extruder in place, and a backward zone that sends the input material into the extruder toward the hopper (backward), and each of the zones can independently include at least one screw element.

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

The content of the silane compound containing the alkoxy group and carbon-carbon double bond group may be 0.1 to 3 parts by weight based on 100 parts by weight of the total of the polyolefin and the diluent.

The sixth embodiment is, in any one of the first to fifth embodiments,

The above alkoxy group and carbon-carbon double bond group-containing silane compound may include a compound represented by the following chemical formula 1:

[Chemical Formula 1]

In the above chemical formula 1, R _1, R _2, and R ₃ is each independently an alkoxy group having 1 to 10 carbon atoms, an alkoxyalkoxy group having 2 to 10 carbon atoms, an alkylcarbonyloxy group having 1 to 10 carbon atoms, or an alkyl group having 1 to 10 carbon atoms, wherein R _1, 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 replaced by a vinyl group, an acryloxy group, or a methacryloxy group.

The seventh embodiment is, in any one of the first to sixth embodiments,

The above alkoxy group and carbon-carbon double bond 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 at least two or more thereof.

According to one aspect of the present invention, a crosslinked polyolefin separator of the following embodiment is provided.

According to the 8th implementation example,

A crosslinked polyolefin separator is provided, which includes a -Si-O-Si- bonding group between the main chains of the polyolefin and has a gel number of 0.15 gels/m ² or less and a permeability deviation of 10 sec/100 cc or less.

According to the ninth embodiment, in the eighth embodiment,

The number of the above gels may be 0.05 to 0.13/ ^m2 , and the air permeability deviation may be 6.4 to 8.2 sec/100cc.

According to the 10th implementation example,

A lithium secondary battery is provided, comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the separator is a cross-linked polyolefin separator according to the eighth or ninth embodiment.

A method for manufacturing a crosslinked polyolefin separator according to one embodiment of the present invention comprises, in the step of manufacturing a silane-grafted polyolefin composition, controlling the residence time from when the polyolefin is fed into an extruder until it is mixed and transferred to the rear end of a gear pump to 3 minutes 30 seconds to 6 minutes 30 seconds, thereby providing a crosslinked polyolefin separator suitable as a separator for secondary batteries, with a small number of gels, a good appearance of the separator, a small deviation in air permeability, and excellent uniformity of the porosity of the separator.

Hereinafter, the present invention will be described in detail. Terms and words used in this specification and claims should not be construed as limited to their conventional or dictionary meanings. Rather, they should be interpreted with meanings and concepts consistent with the technical spirit of the present invention, based on the principle that the inventor can appropriately define the concept of a term to best explain his or her invention.

The present invention relates to a method for manufacturing a crosslinked polyolefin separator and a separator manufactured thereby.

Electrochemical devices, such as lithium secondary batteries, that contain separators with a large difference between their shutdown and meltdown temperatures possess superior safety. Consequently, separators with low shutdown and high meltdown temperatures are required.

The inventors of the present invention have produced a crosslinked polyolefin porous membrane using a crosslinking reaction to produce a separator having a high meltdown temperature, but the membrane still has insufficient capacity to lower the shutdown temperature.

The present inventors have taken note of these points and have sought to provide a crosslinked polyolefin separator and a method for manufacturing the same, which has improved heat resistance by having a high meltdown temperature and a low shutdown temperature at the same time, and which improves heat resistance by reducing the shutdown temperature and increasing the meltdown temperature by controlling the residence time until the polyolefin is mixed after being fed into the extruder and transferred to the rear end of the gear pump, and which has a good membrane appearance and a small deviation in air permeability, thereby providing excellent uniformity of the porosity of the membrane.

A method for manufacturing a cross-linked polyolefin separator according to one aspect of the present invention is as follows:

(S1) A step of producing a silane-grafted polyolefin composition by introducing polyolefin, a diluent, an initiator, a silane compound containing an alkoxy group and a carbon-carbon double bond group, and a crosslinking catalyst into an extruder and mixing them in a mixing section, and then extruding them through a gear pump and through a die outlet;

(S2) A step of forming and stretching the extruded silane grafted polyolefin composition into a sheet form;

(S3) A step of manufacturing a porous membrane by extracting a diluent from the above-mentioned extended sheet;

(S4) a step of opening and fixing the porous membrane; and

(S4) A step of cross-linking the above-mentioned porous membrane;

In the above step (S1), the residence time from when the polyolefin is fed into the extruder to when it is mixed and transferred to the rear end of the gear pump is 3 minutes 30 seconds to 6 minutes 30 seconds.

Hereinafter, the method for manufacturing a separation membrane according to the present invention will be examined in detail.

First, polyolefin, diluent, initiator, silane compound containing an alkoxy group and a carbon-carbon double bond group, and a crosslinking catalyst are fed into an extruder and mixed in a mixing section, and then extruded through a die outlet via a gear pump to manufacture a polyolefin composition in which a silane compound is grafted onto the main chain of the polyolefin (S1).

As in the past, when polyolefin, an initiator, a silane compound containing an alkoxy group and a carbon-carbon double bond group, and a crosslinking catalyst were fed into the extruder at the same time, the meltdown temperature could be increased, but the effect of reducing the shutdown temperature was still minimal.

However, to increase the safety of the separator, the gap between the shutdown temperature and the meltdown temperature must be large.

In one embodiment of the present invention, the polyolefin may include polyethylene; polypropylene; polybutylene; polypentene; polyhexene; polyoctene; a copolymer of two or more of ethylene, propylene, butene, pentene, 4-methylpentene, hexene, and octene; or a mixture thereof.

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

In one embodiment of the present invention, the weight average molecular weight of the polyolefin may be 200,000 to 1,000,000, or 220,000 to 700,000, or 250,000 to 500,000. In the present invention, by using a high molecular weight polyolefin having a weight average molecular weight of 200,000 to 1,000,000 as a starting material for manufacturing a separator, uniformity and membrane forming processability of the separator can be secured, while ultimately obtaining a separator having excellent strength and heat resistance.

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

In one embodiment of the present invention, the diluent may be a diluent capable of liquid-liquid phase separation with polyolefin, and examples thereof include phthalic acid esters such as dibutyl phthalate, dihexyl phthalate, and 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; fatty acid alcohols having 10 to 20 carbon atoms such as palmitic alcohol, stearic alcohol, and oleic alcohol; palmitic acid mono-, di-, or triester, stearic acid mono-, di-, or triester. It may include fatty acid esters in which one or more fatty acids having saturated and unsaturated fatty acids having 4 to 26 carbon atoms in a fatty acid group such as oleic acid mono-, di-, or triester, linoleic acid mono-, di-, or triester, or unsaturated fatty acids in which the double bonds of the unsaturated fatty acids are substituted with epoxy are ester-bonded with an alcohol having 1 to 8 hydroxyl groups and 1 to 10 carbon atoms.

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

In one 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 increases, the porosity decreases, the pore size becomes smaller, and the interconnection between the pores decreases, resulting in a significant drop in permeability, and the viscosity of the polyolefin composition increases, making processing difficult due to an increase in the extrusion load. In addition, as the polyolefin content decreases, the miscibility of the polyolefin and the diluent decreases, so that the polyolefin is not thermodynamically mixed with the diluent but is extruded in a gel form, thereby reducing problems such as breakage and thickness unevenness during stretching.

In one embodiment of the present invention, a silane compound containing an alkoxy group and a carbon-carbon double bond group acts as a crosslinking agent that causes a silane crosslinking reaction, grafting the polyolefin via the carbon-carbon double bond group and crosslinking the polyolefin via a crosslinking reaction via the alkoxy group. Accordingly, the meltdown temperature of the separator can be increased.

The above carbon-carbon double bond group is a reactive group capable of grafting onto polyolefin as described above, and is a substituent having a double bond between two carbons. Examples thereof include a vinyl group, an acryloxy group, or a methacryloxy group.

In one embodiment of the present invention, the silane compound containing an alkoxy group and a carbon-carbon double bond group may include a compound represented by the following chemical formula 1:

[Chemical Formula 1]

In the above chemical formula 1, R _1, R _2, and R ₃ is each independently an alkoxy group having 1 to 10 carbon atoms, an alkoxyalkoxy group having 2 to 10 carbon atoms, an alkylcarbonyloxy group having 1 to 10 carbon atoms, or an alkyl group having 1 to 10 carbon atoms, wherein R _1, 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 replaced by 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 3 carbon atoms, an alkoxyalkoxy group having 2 to 5 carbon atoms, an alkylcarbonyloxy group having 1 to 3 carbon atoms, or an alkyl group having 1 to 3 carbon atoms, and at least one hydrogen atom of the alkoxy group, the alkylcarbonyloxy group, the alkoxyalkoxy group, or the alkyl group may be replaced with an alkoxy group having 1 to 10 carbon atoms, or an alkylcarbonyloxy group having 1 to 10 carbon atoms. At this time, the alkoxyalkoxy group is a substituent defined as "-O-alkylene-O-alkyl", and the alkylcarbonyloxy group is a substituent defined as "-O-(CO)-alkyl".

In one embodiment of the present invention, the silane compound containing an alkoxy group and a carbon-carbon double bond group may include vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilane, (3-methacryloxypropyl)trimethoxysilane, (3-methacryloxypropyl)triethoxysilane, vinylmethyldimethoxysilane, vinyl-tris(2-methoxyethoxy)silane, vinylmethyldiethoxysilane, or a mixture of at least two or more thereof.

In one embodiment of the present invention, the content of the silane compound containing an alkoxy group and a carbon-carbon double bond group may be 0.1 to 3.0 parts by weight, or 0.15 to 2.0 parts by weight, or 0.2 to 1.5 parts by weight based on 100 parts by weight of the total of the polyolefin and the diluent. When the content of the silane compound containing an alkoxy group and a carbon-carbon double bond group satisfies the above numerical range, the content of the silane compound containing an alkoxy group and a carbon-carbon double bond group is small, which lowers the grafting rate and lowers the crosslinking, or the content of the silane compound containing an alkoxy group and a carbon-carbon double bond group is large, which leaves unreacted silane behind, resulting in poor appearance of the extruded sheet, etc., can be prevented, and the shutdown temperature can be lowered by lowering the crystallinity of the polyolefin due to appropriate grafting with the polyolefin.

In one 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 (DHBP), benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di-tert-butyl peroxide, dicumyl peroxide, cumyl peroxide, hydrogen peroxide, potassium persulfate, etc.

In one embodiment of the present invention, the content of the initiator may be 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 silane compound containing the alkoxy group and the carbon-carbon double bond group. When the content of the initiator satisfies the numerical range, the problem of a decrease in the silane grafting rate due to a low initiator content or crosslinking between polyolefins in the extruder due to a high initiator content can be prevented.

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

In one embodiment of the present invention, the crosslinking catalyst may be a carboxylate of a metal such as tin, zinc, iron, lead, or cobalt, an organic base, an inorganic acid, or an organic acid. Non-limiting examples of the crosslinking catalyst include carboxylates of the metal such as dibutyltin dilaurate, dibutyltin diacetate, stannous acetate, stannous caprylate, zinc naphthenate, zinc caprylate, cobalt naphthenate, and the like; the organic base may include ethylamine, dibutylamine, hexylamine, pyridine, and the like; the inorganic acid may include sulfuric acid, hydrochloric acid, and the like; and the organic acid may include toluene sulfonic acid, acetic acid, stearic acid, or maleic acid, and the like. In addition, the crosslinking catalyst may be used alone or in a mixture of two or more thereof.

In one embodiment of the present invention, the content of the crosslinking catalyst may be 0.1 to 20 parts by weight, 0.5 to 10 parts by weight, or 1 to 5 parts by weight based on 100 parts by weight of the content of the silane compound containing an alkoxy group and a carbon-carbon double bond group. When the content of the crosslinking catalyst satisfies the above numerical range, a desired level of silane crosslinking reaction can occur, and undesirable side reactions do not occur within the lithium secondary battery. In addition, cost problems such as waste of the crosslinking catalyst do not occur.

In one embodiment of the present invention, the silane-grafted polyolefin composition may further include general additives for improving specific functions, such as surfactants, oxidation stabilizers, UV stabilizers, antistatic agents, and nucleating agents, as needed.

In one embodiment of the present invention, in the step of preparing the silane-grafted polyolefin composition, a single-screw extruder or a twin-screw extruder may be used as the extruder.

In the above step (S1), the residence time from when the polyolefin is fed into the extruder and then mixed until it is transferred to the rear end of the gear pump is 3 minutes 30 seconds to 6 minutes 30 seconds, and may also be 3 minutes 45 seconds to 6 minutes 15 seconds, or 3 minutes to 6 minutes. If the residence time is less than 3 minutes 30 seconds, the desired silane grafting reaction may not proceed sufficiently, and if it exceeds 6 minutes 30 seconds, there is a risk of gel formation due to the initiator reaction.

According to one embodiment of the present invention, the extruder may be a twin-screw extruder having a hopper for feeding polyolefin, a port for feeding a diluent, an initiator, a silane compound containing an alkoxy group and a carbon-carbon double bond group, and a crosslinking catalyst, a kneading section for feeding and mixing the fed materials, a gear pump for feeding the mixed materials to a die outlet, and a die outlet for extruding the materials fed by the gear pump to the outside.

Here, the residence time in the extruder is defined as the time required from the time the polyolefin is fed into the hopper of the extruder to the time it is mixed with the diluent, initiator, silane compound containing an alkoxy group and a carbon-carbon double bond group, and a crosslinking catalyst in the kneading section and transported to the rear end of the gear pump of the extruder.

The residence time within the extruder can be controlled by the arrangement of multiple screw elements provided within the mixing section.

According to one embodiment of the present invention, the extruder may have one or more kneading sections, each kneading section being configured with three different zones, namely a forward zone, a stationary zone, and a backward zone in the direction from the hopper to the die exit. Each of the zones may independently include one or more screw elements.

Specifically, the forward zone may include one or more screw elements designed to act to direct the input material within the extruder toward the die (forward); the stationary zone may include one or more screw elements designed to rotate the input material within the extruder in place (parallel); and the reverse zone may include one or more screw elements designed to return the input material within the extruder toward the hopper (backward).

For example, according to one embodiment of the present invention, the forward section may have two to five forward screw elements, the stationary section may have two to five neutral screw elements, and the backward section may have one to five reverse screw elements.

According to one embodiment of the present invention, the mixing section of the extruder may be composed of one or more mixing sections depending on the purpose, and further, a conveying section including one or more forward screw elements at the end in the die direction may be further included to configure the screw structure of the entire extruder.

Next, the extruded silane grafted polyolefin composition is formed and stretched into a sheet shape (S2).

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

In one embodiment of the present invention, a separation membrane having improved mechanical strength and puncture strength can be provided by performing the stretching step as described above.

In one embodiment of the present invention, the stretching may be performed by sequential or simultaneous stretching in a roll manner or a tender manner. The stretching ratio may be 3 times or more, or 4 to 10 times in the longitudinal and transverse directions, respectively, and the total stretching ratio may be 14 to 100 times. When the stretching ratio satisfies the above numerical range, the problem of insufficient orientation in one direction and simultaneously a breakdown in the balance of physical properties between the longitudinal and transverse directions, resulting in a decrease in tensile strength and puncture strength, can be prevented. In addition, when the total stretching ratio satisfies the above numerical range, the problem of non-stretching or pore formation can be prevented.

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

In one 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 may be 70 to 160°C, or 90 to 140°C, or 100 to 130°C for machine direction (MD), 90 to 180°C, or 110 to 160°C, or 120 to 150°C for traverse direction (TD), and 90 to 180°C, or 110 to 160°C, or 110 to 150°C when stretching in both directions is performed simultaneously.

When the above-mentioned stretching temperature satisfies the above-mentioned numerical range, the problem of fracture or non-stretching due to lack of softness caused by the low temperature range of the stretching temperature can be prevented, and partial over-stretching or differences in physical properties caused by high stretching temperature can be prevented.

After this, a diluent is extracted from the above-mentioned molded and stretched sheet to manufacture a porous membrane (S3).

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

In one 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., which have high extraction efficiency and quick drying, are suitable.

In one embodiment of the present invention, the extraction method may be any general solvent extraction method such as an immersion method, a solvent spray method, an ultrasonic method, etc., either individually or in combination. The content of the residual diluent after the extraction treatment should preferably be 1 wt% or less. If the content of the residual diluent exceeds 1 wt%, the physical properties deteriorate and the permeability of the porous membrane decreases. 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, a high extraction temperature is preferable, but considering the safety issue due to boiling of the organic solvent, it is preferable that it is 40°C or lower. If the extraction temperature is lower than the freezing point of the diluent, the extraction efficiency is greatly reduced, so it must be higher than the freezing point of the diluent.

Additionally, the extraction time varies depending on the thickness of the porous membrane being manufactured, but for a porous membrane with a thickness of 5 to 15 μm, 2 to 4 minutes is appropriate.

After this, the porous membrane is opened and fixed (S4).

The above heat fixation is to fix the porous membrane and apply heat to forcibly hold the porous membrane that is trying to shrink, thereby removing residual stress.

In one embodiment of the present invention, when the polyolefin is polyethylene, the heat-setting temperature may be 100 to 140°C, or 105 to 135°C, or 110 to 130°C. When the polyolefin is polyethylene and the heat-setting temperature satisfies the above numerical range, rearrangement of polyolefin molecules may occur, thereby removing residual stress of the porous membrane, and reducing the problem of pores of the porous membrane being clogged due to partial melting.

In one embodiment of the present invention, the time for the heat-setting temperature may be 10 to 120 seconds, 20 to 90 seconds, or 30 to 60 seconds. When heat-setting is performed for the above time, rearrangement of polyolefin molecules may occur, thereby removing residual stress in the porous membrane, and reducing the problem of pores of the porous membrane being clogged due to partial melting.

Next, the heat-fixed porous membrane includes a cross-linking step in the presence of moisture (S5).

In one 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 one embodiment of the present invention, the crosslinking can be performed at a humidity of 60 to 95% for 6 to 50 hours.

According to one aspect of the present invention, a crosslinked polyolefin separator is provided, which includes a -Si-O-Si- bonding group between the main chains of the polyolefin and has a gel number of 0.15 gels/m ² or less and a permeability deviation of 10 sec/100 cc or less.

A silane compound containing an alkoxy group and a carbon-carbon double bond group is grafted onto the main chain of the polyolefin via the carbon-carbon double bond group, and then the alkoxy group of the grafted silane undergoes a cross-linking reaction with an adjacent alkoxy group, thereby forming a -Si-O-Si- bond (siloxane bond) group between the main chains of the polyolefin.

The number of the gels is 0.15 pieces/m ² or less, and in one embodiment of the present invention, it may be 0.01 to 0.15 pieces/m ² , or 0.05 to 0.13 pieces/m ² , or 0.05 to 0.12 pieces/m ² , or 0.05 to 0.10 pieces/m ² . When the number of the gels satisfies this range, pore formation is uniform, and the thickness can be controlled to be constant without protrusions.

The above number of gels can be obtained by cutting the manufactured separation membrane into 1 m X 1 m to prepare a sample, and then measuring the number of gels (gels/m ² ) with a long axis of 100 ㎛ or more in the prepared sample.

The above air permeability deviation is 10 sec/100cc or less, and according to one embodiment of the present invention, it may be 5 to 10 sec/100cc, or 6.4 to 8.2 sec/100cc, or 6.5 to 8.0 sec/100cc, or 7.0 to 8.0 sec/100cc. When the air permeability deviation satisfies this range, the membrane resistance can be uniform in the surface direction.

The above air permeability deviation is a standard for evaluating the porosity uniformity of a cross-linked polyolefin separator, and can be evaluated by the air permeability deviation of the following formula.

Air permeability deviation (sec/100cc) = Maximum air permeability - Minimum air permeability

Here, the air permeability (Gurley) can be measured by the ASTM D726-94 method, and the Gurley used here, as resistance to air flow, can be measured by a Gurley densometer. The air permeability value described here is expressed as the time (in seconds) it takes for 100 cc of air to pass through the cross-section of 1 in ² of the membrane under a pressure of 12.2 inH ₂ O, i.e., the air permeability time.

In one embodiment of the present invention, the shutdown temperature of the separator may be 140°C or lower, and the meltdown temperature of the separator may be 175°C or higher.

In one embodiment of the present invention, the difference between the shutdown temperature and the meltdown temperature of the separator may be 30°C or more, or 35°C or more, for example, 37.2 to 61.5°C.

A lithium secondary battery according to one aspect of the present invention includes a cathode, an anode, and a separator interposed between the cathode and the anode, wherein the separator is a cross-linked polyolefin separator according to one embodiment of the present invention described above.

Such lithium secondary batteries may include lithium metal secondary batteries, lithium ion secondary batteries, lithium polymer secondary batteries, or lithium ion polymer secondary batteries.

The positive and negative electrodes to be applied together with the separator of the present invention are not particularly limited, and the electrode active material can be manufactured in a form in which it is bound to the electrode current collector according to a conventional method known in the art. Non-limiting examples of the positive electrode active material among the electrode active materials include conventional positive electrode active materials that can be used in the positive electrode of a conventional electrochemical device, and in particular, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a lithium composite oxide that combines these are preferably used. Non-limiting examples of the negative electrode active material include conventional negative electrode active materials that can be used in the negative electrode of a conventional electrochemical device, and in particular, lithium metal or lithium alloy, carbon, petroleum coke, activated carbon, graphite, or other carbons, and lithium adsorbents are preferable. Non-limiting examples of positive current collectors include foils made of aluminum, nickel, or combinations thereof, and non-limiting examples of negative current collectors include foils made of copper, gold, nickel, or copper alloys, or combinations thereof.

The electrolyte that can be used in the electrochemical device of the present invention is a salt having a structure such as A ⁺ B ^- , wherein A ⁺ contains an ion composed of an alkali metal cation such as Li ⁺ , Na ⁺ , K ⁺ or a combination thereof, and B ^- contains an anion such as 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 ^- or a salt composed of an ion composed of a combination thereof, and propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, Dissolved or dissociated in organic solvents including, but not limited to, N-methyl-2-pyrrolidone (NMP), ethyl methyl carbonate (EMC), gamma butyrolactone (g-butyrolactone), or mixtures thereof.

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

Hereinafter, the present invention will be described in detail with examples to specifically illustrate it. However, the examples according to the present invention may be modified in various ways, and the scope of the present invention should not be construed as being limited to the examples described below. These examples are provided to more fully explain the present invention to those of average skill in the art.

Example 1

Into the extruder, 10.5 kg/hr of high-density polyethylene (Daehan Yuhwa, VH035) with a weight average molecular weight of 350,000 as polyolefin, 13.65 kg/hr of liquid paraffin oil (Kukdong Yuhwa, LP 350F, 68 cSt) as a diluent, 450 g/hr of vinyltrimethoxysilane as a carbon-carbon double bond-containing alkoxysilane, and 6 g/hr of dibutyltin dilaurate as a crosslinking catalyst were fed and mixed. At this time, the carbon-carbon double bond-containing alkoxysilane was fed in a state mixed with the diluent.

Next, 1 g/hr of 2,5-dimethyl-2,5-di-(tert-butylperoxy)hexane (DHBP) was added and mixed as an initiator into the extruder.

At this time, the extruder was a twin-screw extruder having a hopper for feeding polyolefin, a port for feeding a diluent, an initiator, a silane compound containing an alkoxy group and a carbon-carbon double bond group, and a crosslinking catalyst, a kneading section for transporting and mixing the materials fed from the hopper and the port, a gear pump for transporting the materials passed through the kneading section to a die outlet, and a die outlet for extruding the materials transported through the gear pump to the outside.

At this time, the time required from the time the polyolefin is fed into the hopper of the extruder to the time it is mixed with the diluent, initiator, alkoxy group and carbon-carbon double bond group-containing silane compound, and crosslinking catalyst in the kneading section and transferred to the rear end of the gear pump is defined as the residence time.

The above residence time was controlled by the arrangement of multiple screw elements within the kneading section, and the extruder was configured with four kneading sections in series and an extruder screw configuration for mixing. Specifically, each kneading section was configured with a forward section, a stop section, and a reverse section arranged sequentially. The forward section served to send the input material within the extruder toward the die exit (forward) and included five forward screw elements, the stop section served to mix the input material within the extruder in place and included three neutral screw elements, and the reverse section served to send the input material within the extruder toward the hopper (backward) and included one reverse screw element. Following the four kneading sections, the extruder was provided with a conveying section having nine forward screw elements to convey the mixed material toward the T-die. At this time, the residence time was 5 minutes and 30 seconds.

The silane-grafted polyethylene composition extruded by the above extruder was formed into a sheet form through a T-die and a cooling casting roll, and then biaxially stretched using a tenter-type sequential stretching machine for MD stretching (longitudinal stretching) and TD stretching (transverse stretching). The MD stretching ratio and the TD stretching ratio were both 7.0 times. The stretching temperatures were 110°C for MD and 125°C for TD.

The above-mentioned stretched sheet was diluted with methylene chloride and heat-set at 126°C to produce a porous membrane. The porous membrane was crosslinked at 85°C and 60% relative humidity for 24 hours to produce a crosslinked polyethylene separator. The thickness of the obtained crosslinked polyethylene separator was 9.0 μm.

The cross-linked polyethylene separator manufactured above has a silane compound grafted onto the main chain of polyolefin, includes a silane-derived cross-linking structure, and the silane-derived cross-linking structure includes a -Si-O-Si- bonding group.

Example 2

A cross-linked polyethylene membrane was manufactured in the same manner as in Example 1, except that the extruder was configured with three continuous mixing sections, followed by a conveying section having nine forward screw elements, and the residence time was 3 minutes and 30 seconds. That is, the configuration of each mixing section among the three mixing sections was the same as in Example 1.

Example 3

A cross-linked polyethylene membrane was manufactured in the same manner as in Example 1, except that the number of forward screw elements in the mixing section of the extruder was reduced to four, the number of reverse screw elements was increased to two, and the residence time was 6 minutes and 30 seconds.

Comparative Example 1

A cross-linked polyethylene membrane was manufactured in the same manner as in Example 1, except that the extruder did not have a mixing section, but only a conveying section having nine forward screw elements, and the residence time was 2 minutes.

The manufactured membrane did not form normal pores because the high-density polyethylene and liquid paraffin oil were not efficiently mixed due to insufficient mixing.

Comparative Example 2

A cross-linked polyethylene membrane was manufactured in the same manner as in Example 1, except that the number of forward screw elements in the mixing section of the extruder was reduced to one, the number of reverse screw elements was increased to four, and the residence time was 11 minutes and 45 seconds.

The manufactured extruded sheet formed a gel and could not be used as a separator.

Comparative Example 3

A cross-linked polyethylene membrane was manufactured in the same manner as in Example 1, except that the number of forward screw elements in the mixing section of the extruder was increased to 5, the number of reverse screw elements was increased to 2, and the residence time was 2 minutes and 50 seconds.

The manufactured separator had a difference in porosity depending on the location of the extruded sheet due to insufficient mixing, making it impossible to obtain a uniform separator.

Comparative Example 4

A cross-linked polyethylene membrane was manufactured in the same manner as in Example 1, except that the number of forward screw elements in the mixing section of the extruder was reduced to three, the number of neutral screw elements was increased to four, the number of reverse screw elements was increased to two, and the residence time was 7 minutes and 10 seconds.

The manufactured extruded sheet had a partial gel formation and could not be used as a separator.

The shutdown temperature, meltdown temperature, number of gels, and porosity uniformity (air permeability deviation) of the cross-linked polyethylene separators manufactured in Examples 1 to 3 and Comparative Examples 1 to 4 were measured, and the results are shown in Table 1.

(1) Shutdown temperature

The shutdown temperature is determined by exposing the porous membrane to increasing temperature conditions (starting at 30°C and increasing at 5°C/min) while measuring the air permeability of the porous membrane. The shutdown temperature of the porous membrane is defined as the temperature at which the air permeability (Gurley value) of the microporous membrane first exceeds 100,000 sec/100 cc. At this time, the air permeability of the porous membrane can be measured using an air permeability measuring device (Asahi Seiko, EGO-IT) according to JIS P8117.

(2) Meltdown temperature

The meltdown temperature is measured by thermomechanical analysis (TMA) after collecting samples in the machine direction (MD) and transverse direction (TD) of the porous membrane manufacturing direction. Specifically, a 10 mm long sample is placed in a TMA device (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 increases, the length of the sample changes, and the temperature at which the length increases rapidly and the sample breaks is measured. MD and TD are measured separately, and the higher temperature is defined as the meltdown temperature of the corresponding sample.

(3) Evaluation of the number of membrane gels

The manufactured membrane was cut into 1 m X 1 m to prepare a sample. Afterwards, the number of gels (units/m ² ) with a long axis of 100 ㎛ or more in the prepared sample was measured.

(4) Uniformity of porosity (deviation of air permeability)

The uniformity of the air permeability was evaluated by the air permeability deviation as shown below.

Air permeability deviation (sec/100cc) = Maximum air permeability - Minimum air permeability

At this time, the air permeability (Gurley) was measured by the ASTM D726-94 method. The Gurley used here is the resistance to air flow, which is measured by a Gurley densometer. The air permeability value described here is expressed as the time (in seconds) required for 100 cc of air to pass through the cross-section of 1 in ² of the membranes manufactured in Examples 1 to 3 and Comparative Examples 1 to 4 under a pressure of 12.2 inH ₂ O, i.e., the air permeability time.

Inside the extruder
Residence time Shut down
Temperature (℃) Meltdown
Temperature (℃) Number of gels
(pcs/m ² ) ventilation deviation
(sec/100cc) Membrane quality assessment Example 1 5 minutes and 30 seconds 139.1 196 0.11 7.1 Good (can be used as a separator for lithium secondary batteries) Example 2 3 minutes and 30 seconds 139.8 177 0.05 8.2 Good (can be used as a separator for lithium secondary batteries) Example 3 6 minutes and 30 seconds 139.5 201 0.13 6.4 Good (can be used as a separator for lithium secondary batteries) Comparative Example 1 2 minutes Not applicable Not applicable 0.01 48.9 Porosity is not uniform Comparative Example 2 11 minutes 45 seconds 138.1 209 3:30 5.5 Formation of multiple gels Comparative Example 3 2 minutes and 50 seconds Not applicable Not applicable 0.01 37.5 Porosity is not uniform Comparative Example 4 7 minutes and 10 seconds 138.3 205 2.63 6.2 Formation of multiple gels

Referring to Table 1, the separators of Examples 1 to 3 have a small number of gels, thus providing a good appearance of the separator, and a small deviation in air permeability, thus providing excellent uniformity of porosity of the separator, making them suitable as separators for secondary batteries. On the other hand, the separators of Comparative Examples 1 to 4 have poor uniformity of porosity or have a large number of gels formed, resulting in poor outer membrane quality, making them difficult to apply as separators for secondary batteries.

Claims (10)

(S1) A step of producing a silane-grafted polyolefin composition by introducing polyolefin, a diluent, an initiator, a silane compound containing an alkoxy group and a carbon-carbon double bond group, and a crosslinking catalyst into an extruder and mixing them in a mixing section, and then extruding them through a gear pump and through a die outlet;
(S2) A step of forming and stretching the extruded silane grafted polyolefin composition into a sheet form;
(S3) A step of manufacturing a porous membrane by extracting a diluent from the above-mentioned extended sheet;
(S4) a step of opening and fixing the porous membrane; and
(S4) A step of cross-linking the above-mentioned porous membrane;
In the above step (S1), the residence time from when the polyolefin is fed into the extruder until it is mixed and transferred to the rear end of the gear pump is 3 minutes 30 seconds to 6 minutes 30 seconds,
The above extruder is a twin-screw extruder having a hopper for feeding polyolefin, a port for feeding a diluent, an initiator, a silane compound containing an alkoxy group and a carbon-carbon double bond group, and a crosslinking catalyst, a kneading section for transporting and mixing the materials fed from the hopper and the port, a gear pump for transporting the materials that have passed through the kneading section to a die outlet, and a die outlet for extruding the materials transported through the gear pump to the outside.
The above mixing unit has at least two zones among a forward zone that sends the input material in the extruder toward the die exit direction (forward), a stationary zone that mixes the input material in the extruder in place, and a backward zone that sends the input material in the extruder toward the hopper direction (backward).
The above forward section comprises one or more forward screw elements,
The above-mentioned stopping zone comprises one or more neutral screw elements,
The above backward zone comprises one or more reverse screw elements,
Further comprising a conveying section including one or more forward screw elements at the end in the die direction,
A method for manufacturing a crosslinked polyolefin separation membrane, wherein the above residence time is the time required from the time the polyolefin is fed into the hopper of the extruder to the time it is mixed with the diluent, initiator, silane compound containing an alkoxy group and a carbon-carbon double bond group, and a crosslinking catalyst in the kneading section and transported to the rear end of the gear pump. delete In the first paragraph,
A method for manufacturing a crosslinked polyolefin separation membrane, characterized in that the above residence time is controlled by the arrangement of a plurality of screw elements provided in the above mixing unit. delete In the first paragraph,
A method for producing a crosslinked polyolefin separator, characterized in that the content of the silane compound containing the alkoxy group and carbon-carbon double bond group is 0.1 to 3 parts by weight based on 100 parts by weight of the total of the polyolefin and the diluent. In the first paragraph,
A method for manufacturing a crosslinked polyolefin separator, characterized in that the silane compound containing an alkoxy group and a carbon-carbon double bond group includes a compound represented by the following chemical formula 1:
[Chemical Formula 1]

In the above chemical formula 1, R _1, R _2, and R ₃ is each independently an alkoxy group having 1 to 10 carbon atoms, an alkoxyalkoxy group having 2 to 10 carbon atoms, an alkylcarbonyloxy group having 1 to 10 carbon atoms, or an alkyl group having 1 to 10 carbon atoms, wherein R _1, 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 replaced by a vinyl group, an acryloxy group, or a methacryloxy group. In the first paragraph,
A method for producing a crosslinked polyolefin separator, characterized in that the silane compound containing an alkoxy group and a carbon-carbon double bond group comprises vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilane, (3-methacryloxypropyl)trimethoxysilane, (3-methacryloxypropyl)triethoxysilane, vinylmethyldimethoxysilane, vinyl-tris(2-methoxyethoxy)silane, vinylmethyldiethoxysilane, or a mixture of at least two or more thereof. delete delete delete