KR101692412B1 - Polyolefin film with improved physical properties and electrochemical device comprising the same as separator substrate - Google Patents

Abstract

The present invention relates to a polyolefin-based film and a separator-based film, wherein the fibrils constituting the surface layer of the polyolefin-based film have a larger particle diameter than the fibrils constituting the inner layer, Striking strength, tensile strength and shrinkage ratio.

Description

TECHNICAL FIELD The present invention relates to a polyolefin film having improved physical properties and an electrochemical device comprising the same as a separator substrate,

The present invention relates to a polyolefin-based film having improved physical properties and an electrochemical device including the same as a separator substrate. More specifically, the present invention relates to an electrochemical device comprising a polyolefin-based film having a larger particle diameter than the fibrils constituting the inner layer And to an electrochemical device comprising the polyolefin film as a separator substrate.

Recently, interest in energy storage technology is increasing. As the application fields of cell phones, camcorders, notebook PCs and even electric vehicles are expanding, efforts for research and development of electrochemical devices are becoming more and more specified.

Electrochemical devices have attracted the greatest attention in this respect, among which the development of rechargeable secondary batteries has become a focus of attention. In recent years, in order to improve the capacity density and specific energy in developing such batteries, And research and development on the design of batteries are underway.

The polyolefin resin has been widely used as a material for a separator for electrochemical devices and has a disadvantage in that the mechanical strength is weak.

In order to solve this problem, an organic-inorganic composite porous separator has been proposed in which a porous coating layer is formed by coating a mixture of inorganic particles and a binder polymer on at least one surface of a porous polyolefin-based film. There is a high possibility of separation, lithium ions are not effectively transferred, and inorganic particles are desorbed.

Disclosure of Invention Technical Problem [8] The present invention provides a porous polyolefin film having improved physical properties such as mechanical strength by differentiating conditions of a fibril constituting a surface layer and an inner layer of a polyolefin resin, and a process for producing the same . In addition, the present invention provides a separator for an electrochemical device based on such a polyolefin-based film and an electrochemical device comprising the same.

According to one aspect of the present invention, there is provided a polyolefin-based film in which fibrils are mutually connected to each other to form a three-dimensional network structure, wherein the fibrils constituting the surface layer are a polyolefin-based film having a particle diameter larger than that of fibrils constituting the inner layer / RTI >

The ratio of (the particle diameter of the fibrils constituting the surface layer) / (the diameter of the fibrils constituting the inner layer) may be in the range of about 1.5 to 2.5.

The fibrils constituting the surface layer may have a particle diameter of 30 to 70 nm.

The fibrils constituting the inner layer may have a particle diameter of 16 to 30 nm.

The polyolefin-based film may be obtained by polymerizing one or more selected from the group consisting of ethylene, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene and 1-hexene.

According to another aspect of the present invention, there is provided a separation membrane for an electrochemical device based on the above-mentioned polyolefin-based film.

According to still another aspect of the present invention, there is provided an electrochemical device comprising an anode, a cathode, a separator interposed between the anode and the cathode, and an electrolyte, wherein the separator is the separator for the electrochemical device.

The electrochemical device may be a lithium secondary battery.

According to still another aspect of the present invention, there is provided a method of producing a polyolefin-based film by melt-extruding a polyolefin-based resin and then thermally fixing the same, wherein the heat-setting is performed at a temperature of 125 to 135 ° C for 15 seconds to 10 minutes Based on the total weight of the polyolefin-based film.

The polyolefin-based film produced in one embodiment of the present invention has excellent air permeability, stiffness, tensile strength and shrinkage ratio because the fibrils constituting the surface layer have a larger particle diameter than the fibrils constituting the inner layer, It is more suitable as a separator for chemical devices.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments of the invention and, together with the description of the invention, It should not be construed as limited.
Each of Figs. 1A and 1B is a photograph in which the surface layer and the inner layer of the separation membrane composed of fibrils having a large particle diameter are enlarged 50,000 times.
Each of Figs. 2A and 2B is a photograph in which the surface layer and the inner layer of the separation membrane composed of fibrils having a small particle diameter are enlarged 50,000 times.
Fig. 3 is a photograph of the separation membrane surface layer 50,000 times magnified after only extraction without heat fixing.
4 is a graph showing the puncture strength of the polyolefin-based films obtained in Examples 1 to 7 and Comparative Example 1. Fig.
5 is a graph showing the penetration strength of the polyolefin-based films obtained in Examples 1 to 7 and Comparative Example 1. Fig.

Hereinafter, the present invention will be described in detail. The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

The separation membrane according to an embodiment of the present invention is characterized in that the fibrils constituting the surface layer are polyolefin films having a larger particle diameter than the fibrils constituting the inner layer.

In the present specification, a surface layer is referred to as a 'surface layer' in the thickness direction from the surface of the polyolefin film, and a portion between both surface layers of the polyolefin film is referred to as an 'inner layer'.

The polyolefin-based film has a three-dimensional network structure or a three-dimensional mesh structure in which fibrils formed of a polyolefin-based resin are mutually connected successively. Due to the three-dimensional network structure of the fibrils, the polyolefin-based film can maintain the pores, thereby preventing deterioration of the cell characteristics (cycle characteristics) without impairing the ionic conductivity and also imparting flexibility.

Conventionally, the polyolefin-based film was formed so that the thickness of the fibrils was generally uniform throughout the surface layer and the inner layer.

The inventors of the present invention have found that the fibril thickness of the surface can be formed differently depending on the heat setting conditions as shown in FIGS. 1A and 1B, and FIGS. 2A and 2B, and the thickness of the internal fibril is maintained similarly even when the heat fixing conditions change And the polyolefin-based film according to the present invention is produced such that the fibrils constituting the surface layer are significantly larger than the fibril particles constituting the inner layer.

In the present invention, the fibrils constituting the surface layer of the polyolefin-based film have a particle diameter of about 30 to about 70 nm. For this purpose, when the heat fixing conditions are adjusted to apply heat energy intensively to the surface layer, the crystallization degree of the surface layer is higher than that of the inner layer. The pore size of the surface layer is lower than that of the inner layer due to the heat energy concentrated on the surface, while the thick fibrils are formed on the surface and the pore size is formed to be large. The coarse fibril structure of the surface thus formed is very advantageous for enhancing the mechanical strength of the separation membrane by making a strong bond between fibrils in longitudinal / transverse and thickness directions. Further, the surface roughness is increased by the thick fibrils formed on the surface, which is advantageous for improving the hygroscopicity of the electrolyte solution.

In the present invention, the fibrils constituting the inner layer of the polyolefin-based film have a particle diameter of about 16 to about 30 nm. The inner layer has a lower degree of crystallinity than that of the surface layer, and is formed of thin fibrils having a thickness of about 16 nm to 30 nm formed at the time of extraction, and has high porosity because there is no thickness shrinkage due to heat fixation occurring in the surface layer. Such a high air gap in the inner layer is advantageous for ensuring the liquid electrolyte of the separator.

Table 1 below shows the change in crystallinity of the layer and the inner layer after and after heat setting.

After heat fixation After extraction
Crystallinity Whole layer 62.4% 58.1% Skin layer 63.5% 58.6% The inner layer 61.4% 58.2%

The ratio of (the particle diameter of the fibrils constituting the surface layer) / (the diameter of the fibrils constituting the inner layer) ranges from about 1.5 to about 2.5. (The particle diameter of the fibrils constituting the surface layer) / (the diameter of the fibrils constituting the inner layer) is less than the above lower limit, the effect of improving the physical properties by the structure formed by the heat setting becomes insignificant. The pore structure collapses and the air permeability decreases.

The structure of such a polyolefin film is obtained by designing heat fixing conditions specifically during the manufacturing process, and the heat setting conditions will be described later.

The polyolefin-based film may be produced from a crystalline homopolymer or copolymer obtained by polymerizing ethylene, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene or 1-hexene. The weight average molecular weight of the polyolefin-based resin may be 300,000 or more and 1,000,000 or less in consideration of the formation of the three-dimensional network structure and the mechanical strength of the finally obtained film.

Various additives such as an antioxidant, an ultraviolet absorber, a lubricant, an anti-blocking agent, a pigment, a dye, and an inorganic filler may be included in the polyolefin-based film as needed without departing from the purpose of the present invention.

The polyolefin-based film may have a thickness of 5 to 100 占 퐉 or 30 to 70 占 퐉. When the polyolefin-based film has a thickness in the above-mentioned range, the battery has an internal short-circuit function and mechanical strength required in the separator and does not excessively increase the electrical resistance of the battery.

One aspect of the polyolefin-based film according to the present invention can be obtained by a manufacturing process including a heat melting step, an extrusion step, a stretching step and a heat setting step of a polyolefin, and if necessary, the stretching step can be excluded .

The heating and dissolving step may be carried out by stirring in a solvent at a temperature at which the polyolefin-based resin is completely dissolved, or by homogeneously mixing in an extruder.

When the heating and dissolution is carried out in a solvent, the melting temperature differs depending on the polymer and the solvent to be used. For example, in the case of the polyethylene composition, it is in the range of 140 to 250 ° C. The solvent is not particularly limited as long as it can sufficiently dissolve the polyolefin-based resin. Examples thereof include aliphatic or cyclic hydrocarbons such as nonane, decane, undecane, dodecane and liquid paraffin, or mineral oil fractions corresponding to their boiling points. A composition having a state similar to that of a gel having a stable solvent content Non-volatile solvents such as liquid paraffin are preferred for this purpose.

In the case of dissolving in an extruder, the above-mentioned polyolefin-based resin is first fed into an extruder and melted. The melting temperature depends on the kind of the polyolefin used, but the melting point of the polyolefin + 30 to 100 DEG C is preferable. The melting point will be described later. For example, in the case of polyethylene, it is preferably 160 to 230 DEG C, particularly 170 to 200 DEG C, and in the case of polypropylene, it is preferably 190 to 270 DEG C, particularly 190 to 250 DEG C. Then, a liquid solvent is supplied to the molten polyolefin-based resin.

The blending ratio of the polyolefin resin and the solvent is 10 to 50% by weight or 10 to 30% by weight, the solvent is 90 to 50% by weight or 90 to 50% by weight based on 100% by weight of the polyolefin resin and the solvent, To 70% by weight. When the polyolefin-based resin is used in an amount of less than 10% by weight and molded into a sheet, the swelling or neck-in of the die outlet becomes large, so that the formability and self-supportability of the sheet become difficult, If the resin is used in an amount of more than 50% by weight, the shrinkage in the thickness direction becomes large, the porosity decreases, and a microporous film having a large pore size can not be obtained.

Subsequently, in the extrusion molding step, the heated solution of the melt-kneaded polyolefin resin is extruded directly or from a die or the like using a separate extruder so that the film thickness of the final product is 20 to 100 μm.

As the die, a sheet die having a rectangular orifice shape is generally used, but it is possible to use a double die type hollow die, an impression die or the like. The die gap in the case of using a sheet die is usually 0.1 to 5 mm, and in extrusion molding, it is heated to 140 to 250 ° C. At this time, the extrusion speed is usually 20 to 30 cm / min and 15 m / min.

The solution extruded from the die is formed into a gel-like molding by cooling. Cooling is performed by cooling the die or cooling the gel sheet. The cooling is carried out at a rate of at least 50 캜 / min up to 90 캜, preferably 80 to 30 캜. As the cooling method of the gel sheet, there can be used a method of bringing the sheet into direct contact with cold air, cooling water or other cooling medium, a method of contacting with a cooling roll, or the like.

If the cooling rate is low, the higher order structure of the obtained gel-like shaped product becomes rough, and if the cooling rate is high, a dense structure is obtained. When the cooling rate is less than 50 ° C / minute, the gel-like structure becomes close to the independent foil and the degree of crystallization increases, thus making it difficult to remove the solvent.

The temperature of the cooling roll is preferably 30 캜 to the crystallization temperature of the polyolefin-based resin, particularly 40 to 90 캜. When the cooling roll temperature is high, the gel sheet slowly cools and the wall constituting the polyolefin lamellar structure forming the gel-like structure becomes thick, and the micropores tend to become independent grains, so that the desolvability is lowered and the permeability is lowered. When the cooling roll temperature is low, the gel-like sheet is quenched and the gel-like structure becomes dense, which is excessive, so that the pore size is reduced and the permeability is lowered.

The gel-like shaped article can then be stretched through an optional stretching step. The stretching is carried out at a predetermined magnification by heating the gel-like molded article and combining with a conventional turner method, a roll method, rolling or a combination of these methods. The stretching may be uniaxial stretching or biaxial stretching. In the case of biaxial stretching, longitudinal and transverse simultaneous stretching or sequential stretching can be performed.

The stretching temperature is preferably from the crystal dispersion temperature to the crystal melting temperature. When the stretching temperature is higher than the melting point + 10 캜, the orientation of the molecular chains due to the stretching can not be achieved by melting the resin. When the stretching temperature is lower than the crystal dispersion temperature, softening of the resin is insufficient, Can not be controlled. For example, in the case of a polyethylene composition, it may be in the range of 90 to 140 캜, more preferably 90 to 125 캜.

The drawing magnification is not particularly limited, but may be 2 to 100 times or 15 to 100 times as large as the surface magnification.

Then, optionally, the hot-fixated molding may be rinsed with a solvent to remove the remaining solvent. Examples of the cleaning solvent include hydrocarbon solvents such as hydrocarbons such as pentane, hexane and heptane, fluorinated hydrocarbons such as chlorinated hydrocarbons such as methylene chloride and carbon tetrachloride, fluorinated hydrocarbons such as trifluoroethane, ethers such as diethyl ether and dioxane, It is possible to use. These solvents are appropriately selected corresponding to the solvent used for dissolving the polyolefin composition, and they are used alone or in combination. The cleaning method may be performed by a method of dipping in a solvent to extract, a method of spraying a solvent, or a combination of these methods.

Then, through the heat fixing step, the gel-like molded product is heat-set at a temperature of 125 to 135 DEG C for 15 seconds to 10 minutes. When the heat setting temperature is lower than 125 캜, the effect of improving the shrinkage ratio due to heat setting can not be expected. When the heat setting temperature is higher than 135 캜, the surface of the resin is melted and the surface pores are clogged. When the heat setting time is shorter than 15 seconds, the residual stress removing effect by heat is lowered. When the heat setting time is longer than 10 minutes, not only the productivity is lowered but also the structural difference between the surface layer and the inner layer can not be secured.

The thus prepared polyolefin-based film according to one embodiment of the present invention can be used as a separator substrate. The separator is interposed between the anode and the cathode to be used in an electrochemical device. The electrochemical device includes all the devices that perform an electrochemical reaction, and specific examples include capacitors such as all kinds of primary, secondary cells, fuel cells, solar cells, or super-capacitor devices. Particularly, a lithium secondary battery including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery is preferable.

The electrochemical device may be manufactured according to a conventional method known in the art, and may be manufactured, for example, by assembling the positive electrode and the negative electrode with the separator interposed therebetween, and then injecting an electrolyte solution .

The electrode to be used together with the separator is not particularly limited and may be manufactured in a manner that an electrode active material is bound to an electrode current collector according to a conventional method known in the art.

Examples of the cathode active material include, but are not limited to, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a combination thereof It is preferable to use a 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 a negative electrode of an electrochemical device can be used. In particular, lithium metal or a lithium alloy, carbon, petroleum coke, activated carbon, Lithium-adsorbing materials such as graphite or other carbon-based materials and the like are preferable.

Non-limiting examples of the positive current collector include aluminum, nickel, or a combination thereof. Examples of the negative current collector include copper, gold, nickel, or a copper alloy or a combination thereof Foil to be manufactured, and the like.

The electrolyte solution that can be used in one embodiment of the present invention is a salt having a structure such as A ⁺ B ^- , where A ⁺ includes ions consisting of alkali metal cations such as Li ⁺ , Na ⁺ , K ^{+ -} it 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} ^- anion, or a salt containing an ion composed of a combination of propylene carbonate (PC) such as, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl (DMP), dimethylsulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (EMC), gamma-butyrolactone Or an organic solvent composed of a mixture thereof, but the present invention is not limited thereto.

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

As a process for applying the separator according to an embodiment of the present invention to a battery, a lamination, stacking and folding process of a separator and an electrode can be performed in addition to a general winding process.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to examples. However, the embodiments according to the present invention may be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Example  One

High density polyethylene having a weight average molecular weight of 500,000 and liquid paraffin having a kinematic viscosity of 68.00 cSt at a weight ratio of 35:65 were blended by a compounding method and vacuum dried at 80 DEG C for 12 hours. Then, the blended chips were extruded using a twin-screw extruder according to a conventional film production method, and sheets having a thickness of 1000 to 1200 μm were prepared using a T-die and a casting roll. This sheet was longitudinally stretched 6.5 times at 120 DEG C and 6.5 times in the transverse direction. The solvent was then extracted with methylene chloride at room temperature to obtain a porous membrane.

Then, as a heat setting step, heat setting was performed at a temperature of 128 DEG C for 5 minutes to obtain a high-density polyethylene film having a thickness of 18 mu m.

The fibrils forming the three dimensional network structure in the surface layer of the high density polyethylene film had a particle diameter of about 49.0 nm and the fibrils forming the three dimensional network structure within the range of 4 to 6 nm from the surface layer had a thinner particle diameter of about 28.9 nm. The ratio of (the particle diameter of the surface layer fibrils) / (the particle diameter of the inner layer fibrils) was 1.69.

Example  2

A high-density polyethylene film having a thickness of 18 占 퐉 was obtained in the same manner as in Example 1, except that the heat setting time was changed to 129 占 폚 for 4 minutes.

The fibrils forming the three dimensional network structure in the surface layer of the high density polyethylene film had a particle diameter of about 51.6 nm and the fibrils forming the three dimensional network structure within the range of 4 to 6 nm from the surface layer had a thinner particle diameter of about 28.9 nm. The ratio of (the particle diameter of the surface layer fibrils) / (the particle diameter of the inner layer fibrils) was 1.78.

Example  3

A high-density polyethylene film having a thickness of 18 占 퐉 was obtained in the same manner as in Example 1, except that the heat setting time was changed to 131 占 폚 for 3 minutes.

The fibrils forming the three - dimensional network structure in the surface layer of the high - density polyethylene film had a particle size of about 54.5 nm and the fibrils forming the three - dimensional network structure within 4 to 6 nm from the surface layer had a thinner particle diameter of about 28.9 nm. The ratio of (the particle diameter of the surface layer fibrils) / (the particle diameter of the inner layer fibrils) was 1.88.

Example  4

A high-density polyethylene film having a thickness of 18 占 퐉 was obtained in the same manner as in Example 1, except that the heat setting time was changed to 133 占 폚 for 2 minutes.

The fibrils forming the three dimensional network structure in the surface layer of the high density polyethylene film had a particle diameter of about 54.0 nm and the fibrils forming the three dimensional network structure within the range of 4 to 6 nm from the surface layer had a thinner particle diameter of about 29.0 nm. The ratio of (the particle diameter of the surface layer fibrils) / (the particle diameter of the inner layer fibrils) was 1.86.

Example  5

A high-density polyethylene film having a thickness of 18 占 퐉 was obtained in the same manner as in Example 1, except that the heat setting time was changed to 134 占 폚 and 90 seconds.

The fibrils forming the three dimensional network structure in the surface layer of the high density polyethylene film had a particle diameter of about 55.2 nm and the fibrils forming the three dimensional network structure inside the surface layer in the range of 4 to 6 nm had a thinner particle diameter of about 29.0 nm. The ratio of (the particle diameter of the surface layer fibrils) / (the particle diameter of the inner layer fibrils) was 1.90.

Example  6

A high-density polyethylene film having a thickness of 18 占 퐉 was obtained in the same manner as in Example 1, except that the heat setting time was changed to 135 占 폚 for 1 minute.

The fibrils forming the three dimensional network structure in the surface layer of the high density polyethylene film had a particle size of about 59.6 nm and the fibrils forming the three dimensional network structure within the range of 4 to 6 nm from the surface layer had a thinner particle diameter of about 28.9 nm. The ratio of (the particle diameter of the surface layer fibrils) / (the particle diameter of the inner layer fibrils) was 2.06.

Example  7

A high-density polyethylene film having a thickness of 18 占 퐉 was obtained in the same manner as in Example 1, except that the heat setting time was changed to 136 占 폚 for 30 seconds.

The fibrils forming the three dimensional network structure in the surface layer of the high density polyethylene film had a particle size of about 61.5 nm and the fibrils forming the three dimensional network structure inside the surface layer at 4 to 6 nm had a thinner particle size of about 29.0 nm. The ratio of (the particle diameter of the surface layer fibrils) / (the particle diameter of the inner layer fibrils) was 2.12.

Comparative Example  One

A high-density polyethylene film having a thickness of 18 占 퐉 was obtained in the same manner as in Example 1, except that the heat fixing conditions were changed to 120 占 폚 for 1 minute.

The fibrils forming the three dimensional network structure in the surface layer of the high density polyethylene film had a particle diameter of about 33.5 nm and the fibrils forming the three dimensional network structure within the range of 4 to 6 nm from the surface layer had a thinner particle diameter of about 28.9 nm. The ratio of (the particle diameter of the surface layer fibrils) / (the particle diameter of the inner layer fibrils) was 1.16.

evaluation

The physical properties of the films obtained in Examples 1 to 7 and Comparative Example 1 were measured by the following methods.

Film thickness: Measured using a Mitutoyo contact thickness meter.

Air permeability: The air permeability was measured in terms of the time required for 100 ml of air to pass through the use of a royal soft breathability measuring instrument.

Tensile strength: Measured according to ASTM D882.

Shrinkage ratio: The ratio of the reduced area to the initial area after leaving the film at 120 캜 for one hour after cutting the film to 50 x 50 mm was expressed in%.

The films obtained in Examples 1 to 7 and Comparative Example 1 were measured to have physical properties shown in the following Table 2. The air permeability and the stiffness of the films were shown in the graphs of FIGS. 4 and 5, Black dots are experimental values obtained in Comparative Example 1, and remaining values are the experimental values obtained in Examples 1 to 7.

Ventilation
(N.Permeability)
(sec / 100cc) Sting intensity
(N.Penccture strength)
(Gf 18um) Machine direction
The tensile strength
(kgf / cm2) Machine transverse direction
The tensile strength
(kgf / cm2) Shrinkage (%) Example 1 173 525.5 1529.9 1386.8 34.4 Example 2 186 548.0 1634.4 1393.9 26.3 Example 3 197 650.4 1517.0 1584.6 20.4 Example 4 198 600.2 1516.5 1346.1 22.8 Example 5 243 627.2 1827.7 1630.9 18.6 Example 6 220 619.7 1636.0 1573.0 19.9 Example 7 288 648.4 1809.7 1611.8 14.0 Comparative Example 1 163 476.1 1167.0 1079.9 -

As is apparent from the above, the polyolefin films obtained in Examples 1 to 7 had a large value in the range of (the particle diameter of the surface layer fibrils) / (the particle diameter of the inner layer fibrils) Directional tensile strength, transverse direction tensile strength and shrinkage ratio.

Claims (9)

In a polyethylene film in which fibrils are mutually connected to form a three-dimensional network structure,
(The particle diameter of the fibrils constituting the surface layer) / (the particle diameter of the fibrils constituting the inner layer) is in the range of 1.5 to 2.5.
delete The method according to claim 1,
Wherein the fibrils constituting the surface layer have a particle diameter of 30 to 70 nm.
The method according to claim 1,
Wherein the fibrils constituting the inner layer have a particle diameter of 16 to 30 nm.
delete A separator for an electrochemical device based on a polyethylene film according to any one of claims 1, 3 and 4.
1. An electrochemical device comprising an anode, a cathode, a separator interposed between the anode and the cathode, and an electrolyte,
The electrochemical device according to claim 6, wherein the separator is the separator for an electrochemical device according to claim 6.
8. The method of claim 7,
Wherein the electrochemical device is a lithium secondary battery.
Characterized in that the polyethylene-based resin is melt-extruded and then heat-set, and the heat-setting is carried out at a temperature of 125 to 135 DEG C for 15 seconds to 10 minutes
A method for producing a polyethylene film according to claim 1.

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