KR20170044499A - Porous film, method for manufacturing the porous film, and electro-chemical battery comprising the porous film - Google Patents

Abstract

The present invention relates to a porous film where a first microporous layer containing a polyethylene-based polymer and a first porous particle, a polypropylene-based polymer and a second microporous layer containing the second pore-forming particles are stacked. The average particle diameter of the first and second pore-forming particles is 300 nm or less. The present invention relates to a porous film which includes 5 to 25% by volume of the first pore-forming particles based on the total volume of the first microporous layer, and includes 5% by volume to 25% by volume of the second pore-forming particles based on the total volume of the second microporous layer, a method for manufacturing the porous film, a membrane comprising the porous film, or an electrochemical battery. So, battery stability and long-term reliability can be improved.

Description

TECHNICAL FIELD [0001] The present invention relates to a porous film, a method for producing the porous film, and an electrochemical cell including the same. BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to a porous film and a method for producing the porous film. The present invention also relates to an electrochemical cell comprising the porous film.

A separator for an electrochemical cell is an interlayer which separates an anode and a cathode from each other in a battery while maintaining the ionic conductivity to enable charging and discharging of the battery. The separator is made of a porous film.

Examples of methods for producing such a porous film include a dry process and a wet process. The dry process is a process in which a precursor film is extruded and then the orientation of the lamellar is controlled through heat treatment such as annealing and stretched to form pores. Since the dry process (Korean Patent Publication No. 2008-0085922) does not use an extraction solvent as compared with a wet process, it is eco-friendly and cost-competitive, but since it forms pores by stretching between crystals and amorphous of a single substance, There is a problem that the tensile strength in the direction is lowered. In addition, when pores are formed between crystalline and amorphous materials, the pore size is determined by the difference in the strength of crystal and amorphous material, and thus the pore size is determined by the type of material. Therefore, in the case of polyethylene, the pore size is excessively large, making it difficult to use it as a separator for a secondary battery. The wet process refers to a method of mixing a polymer material with a plasticizer, extruding the polymer material to form a sheet, and removing the plasticizer from the sheet to form pores. In the wet process, the size of the pores is determined by controlling the compatibility with the plasticizer. However, it is difficult to apply the same process to other materials except polyethylene.

Korean Patent Publication No. 2008-0085922

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The present invention is to provide a porous film which is simpler in manufacturing process and has better production efficiency as well as superior uniformity of porosity, porosity, air permeability and ion permeability as compared with the above-mentioned prior arts. Further, by controlling the porosity and air permeability of the porous film, it is desired to improve the cell stability and long-term reliability in the electrochemical cell including the same.

In one embodiment of the present invention, a porous film in which a first microporous layer containing a polyethylene-based polymer and a first pore-forming particle, and a second microporous layer containing a polypropylene-based polymer and a second pore- Wherein the first and second pore-forming particles have an average particle diameter of 300 nm or less, the first pore-forming particles are contained in an amount of 5 vol% to 25 vol% based on the total volume of the first microporous layer, And a second pore-forming particle is contained in an amount of 5% by volume to 25% by volume based on the total volume of the porous layer.

In one embodiment of the present invention, a first composition comprising a polyethylene-based polymer and first pore-forming particles is prepared, a second composition comprising a polypropylene-based polymer and second pore-forming particles is prepared, Layer precursor film is formed by extrusion molding the composition and the second composition, annealing the multi-layered precursor film at a temperature of from 80 캜 to 150 캜, annealing the annealed multilayered film at a temperature of from 50 캜 to 50 캜 % To 400% of the first stretched film, and stretching the stretched multilayer film at a temperature of 80 to 150 ° C for 40 to 400% second stretching.

In one embodiment of the present invention, the first composition and the second composition are respectively extrusion-molded to form respective precursor films, and the respective precursor films are heated at a temperature of 100 ° C to 150 ° C, a pressure of 0.1 MPa to 5 MPa And then stretching the laminated film by 50 to 400% at a low temperature of 0 占 폚 to 50 占 폚 and second stretching by 40 to 400% at a temperature of 80 to 150 占 폚. Is provided.

In another embodiment of the present invention, there is provided an electrochemical cell comprising a porous film, a cathode, a cathode, and an electrolyte.

The porous film according to one embodiment of the present invention is characterized by having uniform pores, excellent porosity, air permeability and ion permeability, and having a multi-layered structure, thereby improving shutdown characteristics and tensile strength. Furthermore, by controlling the porosity and air permeability, the cell stability and long-term reliability of the electrochemical cell including the porous film are improved.

In addition, the manufacturing method of the porous film according to another embodiment of the present invention can simplify the manufacturing process and produce the production efficiency, thus remarkably lowering the manufacturing cost.

1 is a scanning electron microscope (SEM) photograph of a polyethylene (PE) layer of a porous film prepared according to one embodiment of the present invention.
2 is a scanning electron microscope (SEM) photograph of a polypropylene (PP) layer of a porous film prepared according to one embodiment of the present invention.
3 is an exploded perspective view of an electrochemical cell according to an embodiment of the present invention.

Hereinafter, the present invention will be described in more detail. Those skilled in the art will appreciate that various modifications, additions, and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

An embodiment of the present invention is a porous film comprising a first microporous layer containing a polyethylene-based polymer and first pore-forming particles, and a second microporous layer comprising a polypropylene-based polymer and a second pore- Wherein the first and second pore-forming particles have an average particle diameter of 300 nm or less, the first pore-forming particles are contained in an amount of 5 vol% to 25 vol% based on the total volume of the first microporous layer, Wherein the second pore-forming particles are contained in an amount of 5% by volume to 25% by volume based on the total volume of the porous layer.

The porous film according to the above embodiments may be advantageous in terms of pore uniformity, air permeability and wettability with respect to an electrolyte by including pore-forming particles in each layer. Further, by having a multilayer structure, it can be advantageous in terms of mechanical properties, shutdown characteristics, and heat resistance.

The polyethylene-based polymer contained in the first microporous layer is a crystallizable polyethylene-based polymer, and may be a polyethylene homopolymer or a polyethylene copolymer. In one example, a polyethylene polymer having a melting index of 10 or less, or 5 or less, such as 3 or less, may be used.

In one embodiment, high density polyethylene (HDPE), ultra high molecular weight polyethylene (UHMWPE), and the like can be used. For example, the high-density polyethylene may be used alone, or two or more kinds of high-density polyethylene may be used in combination, the ultra-high-molecular-weight polyethylene may be used alone, or two or more types of ultrahigh-molecular-weight polyethylene may be used in combination, And the ultrahigh molecular weight polyethylene may be used in combination. In one embodiment, the melt index of the high density polyethylene can be from 0.03 to 3, and can be, for example, from 0.1 to 3. In one embodiment, two types of high-density polyethylene having different melt indices may be mixed and used. Specifically, high-density polyethylene having a melt index of 0.5 or less and high-density polyethylene having a melt index of 0.5 to 1 may be mixed and used.

When the polyethylene-based polymer is used, a porous film having excellent air permeability and porosity can be obtained, and the separator manufactured using the polyethylene polymer has an advantage of improving long-term reliability.

The polypropylene-based polymer that can be used for the second microporous layer is a polypropylene-based polymer that can be crystallized, and may be a polypropylene homopolymer or a polypropylene copolymer. For example, a polypropylene polymer having an isotacticity of 85% to 99% as measured from the solubility in xylene can be used. In one example, the melt index of the polypropylene-based polymer may be 1 to 5 or more, specifically 2 to 4 or more. More specifically from 2 to 10, such as from 4 to 10.

In another example, the polyethylene-based polymer or the polypropylene-based polymer may be a compound having a glass transition temperature (Tg) or a melting temperature (Tm) of 100 or more. When a compound having a glass transition temperature or a melting temperature in the above range is used, not only the heat resistance of the porous film can be improved but also the pore size can be more advantageously controlled for the desired physical property.

 By using the above-mentioned polypropylene-based polymer, strength and thermal stability of the separator are improved and the oxidation resistance is better than that of the polyethylene-based polymer, thereby enabling the capacity of the battery to be increased.

In another example, the first microporous layer and the second microporous layer may comprise a polyethylene-based polymer or a polypropylene-based polymer, as well as other resins. Examples of other resins include poly (4-methylpentene), polyimide, polyester, polyamide, polyetherimide, polyamideimide, polyacetal, or a combination thereof. When other resins are included, the polyethylene-based polymer or polypropylene-based polymer and other resins may be blended in an appropriate solvent. In another example, the first microporous layer and the second microporous layer may further comprise a copolymer of an olefin and a non-olefin monomer.

The first and second pore-forming particles may be inorganic particles or organic particles or composite particles thereof.

Examples of the inorganic particles include alumina, silica, titania, zirconia, magnesia, ceria, zinc oxide, iron oxide, silicon nitride, titanium nitride, boron nitride, calcium carbonate, barium sulfate, aluminum sulfate, aluminum hydroxide, calcium titanate, , Calcium silicate, and magnesium silicate. In one example, alumina, calcium carbonate or aluminum hydroxide may be used.

Examples of the organic particles include a polymer of a monomer containing a double bond or a crosslinked polymer made by a method of emulsion polymerization or suspension polymerization, and a polymer precipitate prepared in a solution by controlling the compatibility. Specific examples include non-crosslinked or crosslinked polystyrene (PS), polyethylene (PE), polypropylene (PP), polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), polyurethane (PU) (PMP), polyethylene terephthalate (PET), polycarbonate (PC), polyester, polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polymethylene oxide (PMO) (PAI), polysulfone (PSF), polyethyl sulfone (PES), polyacrylamide (PAI), acrylonitrile butadiene rubber, methacrylate (PMMA), polyethylene oxide , Polyphenylene sulfide (PPS), polyarylate (PAR), polyimide (PI), polyaramid (PA), cellulose, cellulose modified product, melamine resin and phenol resin One or more organic particles can be used. More specifically, a crosslinked silicone rubber composition comprising a silicone rubber, an ethylene-methyl acrylate copolymer, a polystyrene (PS), a polyethylene (PE), a polypropylene (PP), a polysulfone (PSF), and a polyimide (Ethylene-methyl acrylate copolymer), polystyrene (PS), or polyethylene (PE) may be used.

The average particle diameter of the first and second pore-forming particles may be 300 nm or less, specifically 30 nm to 300 nm. Specifically from 30 nm to 250 nm, and more specifically from 30 nm to 100 nm. The dispersibility and processability of the pore-forming particles are not lowered within the above range, and the mechanical properties of the microporous membrane can be prevented from deteriorating. Using pore-forming particles having the above-described range may be advantageous in terms of controlling the size of pores, uniformity of pores, and air permeability. The pore-forming particles may be treated with a surfactant or the like to improve the dispersibility in the composition.

The first pore-forming particles and the second pore-forming particles may be contained in each of the first microporous layer and the second microporous layer in an amount of 5% by volume to 25% by volume based on the total volume of each layer. Specifically, it may be contained in an amount of 5% by volume to 20% by volume, more specifically, from 8% by volume to 20% by volume. In this range, sufficient pores are formed due to the pore-forming particles, and the air permeability can be improved.

The thickness of the porous film may be from 3 탆 to 30 탆, specifically from 3 탆 to 25 탆, more specifically from 5 탆 to 25 탆. It is possible to produce a porous film having an appropriate thickness which is thick enough to prevent a short circuit between the positive electrode and negative electrode of the battery in the above range but not thick enough to increase the internal resistance of the battery. Specifically, the thickness ratio of the first microporous layer and the second microporous layer may be from 1: 9 to 9: 1, more specifically from 3: 7 to 7: 3, for example from 4: 6 to 6: 4 Lt; / RTI >

Hereinafter, a porous film according to another embodiment of the present invention will be described. The porous film according to this embodiment is characterized in that, in addition to the first microporous layer and the second microporous layer, at least one of a polyethylene-based polymer, a polypropylene-based polymer, and poly (4-methylpentene) A third microporous layer may be further included. Specifically, the third microporous layer may include a polyethylene-based polymer or a polypropylene-based polymer and a third pore-forming particle.

The polyethylene-based polymer or the polypropylene-based polymer may be the same as those usable for the first microporous layer and the second microporous layer.

The third pore-forming particles may be the same as the first pore-forming particles and the second pore-forming particles. The average particle size of the third pore-forming particles may be 300 nm or less, specifically 30 nm to 300 nm, specifically 30 nm to 250 nm, 30 nm to 200 nm, more specifically 30 nm to 100 nm .

The third pore-forming particles may be contained in an amount of 5% by volume to 25% by volume based on the total volume of the third microporous layer. Specifically, it may be contained in an amount of 5% by volume to 20% by volume, more specifically, from 8% by volume to 20% by volume.

In one embodiment, the third microporous layer may include a polypropylene-based polymer, and the first microporous layer and the second microporous layer may be laminated on the porous film to form a porous film having a three-layer structure. Specifically, a porous film having a three-layer structure in which the second microporous layer / the first microporous layer / the third microporous layer are stacked in this order can be formed. By having the above-described three-layer structure, there is an advantage that the mechanical strength, the shutdown characteristic, the oxidation resistance, and the heat resistance of the porous film can be improved.

In the three-layer porous film, the thickness ratio of the second microporous layer: first microporous layer: third microporous layer may be in the range of 1 to 10: 1 to 10: 1 to 10. Specifically, it may range from 2 to 8: 2 to 8: 2 to 8, for example from 3 to 7: 3 to 7: 3 to 7. [ The shutdown characteristics, tensile strength and oxidation resistance of the porous film can be further improved within the above-mentioned thickness range.

In another embodiment, each microporous layer of the porous film comprises a first pore formed by the pore-forming particles and a second pore formed between the lamella of each of the polyethylene layer and the polypropylene layer, wherein the volume of the first pore May be greater than the volume of the second pore.

The first pore has a / b of 1 to 7, for example, 1 to 6, while the second pore has a / b of 0.5 Or more. When the porous film is a polyethylene-based polymer film, the a / b of the first pore is 1 to 7, for example, 1 to 6, and the second pore 6 has a / b of more than 7, 10 < / RTI > When the porous film is a polypropylene-based polymer film, a / b of the first pore is 1 to 7, for example, 1 to 6, and the second pore 6 has a / b of more than 0.5, 0.5 to 5. 1, the porous film of the present embodiment includes a fibril 8 formed between a lamellar 7 and a lamellar 7, and includes second pores 6 between neighboring fibrils 8, Can be formed. In the porous film according to the present embodiment, the thickness of the lamella may vary depending on the kind of the crystalline resin used, but may be 200 nm or less, more specifically 100 nm or less, and more specifically 80 nm or less . For example, in the case of a crystalline polyethylene resin, it may be 100 nm or less, for example, 10 nm to 100 nm, for a polypropylene polymer crystalline resin, 100 nm or less, specifically 80 nm or less, more specifically 5 nm A lamellar having a thickness of from 60 nm to 60 nm can be formed. Since the volume of the first pores is larger than that of the second pores, it is advantageous in terms of air permeability, electrolyte wettability, and battery capacity. The second pore having a relatively small volume and a rectangular shape is advantageous in terms of heat shrinkage, strength and pore strain.

The porous film according to embodiments of the present invention may have a degree of crystallinity of from 35% to 70%, specifically from 40% to 60%, and more specifically from 40% to 50%. In the case of having the crystallinity in the above range, a thin lamellar structure can be formed at the time of stretching, thereby improving the air permeability and ion permeability of the porous film. A non-limiting example of measuring the crystallinity of a porous film is as follows: The crystallinity is calculated from the heat of fusion (H) of the film. The measurement of the heat of fusion is carried out using a TA Instrument Discovery as a differential scanning calorimeter. Specifically, put the measurement sample in approximately 10 mg Al pan, and place the Al pan containing the reference pan and the sample in the DSC cell. After stabilizing the equipment in a nitrogen atmosphere, the temperature was raised to 200 ° C at a rate of 10 ° C / min. And the melting heat value H is obtained from the second melting temperature profile at the time of raising the temperature to 200 ° C again. The theoretical heat of fusion of polyethylene and polypropylene resin is 293 J / g and 207 J / g, and the crystallinity is obtained by the following formula. Here, the content ratio of the polyethylene polymer a, the polypropylene polymer b, and the polyethylene polymer in the multilayered film is represented by the content ratio alpha and the content ratio beta of the polypropylene polymer.

Crystallinity (%) = 100 x {H _a / (293 x?) + H _b / (207 x?)}

The porosity of the porous film may be 40% to 70%, specifically 45% to 65%. The porous film is cut into a size of 10 cm x 10 cm, and its volume (cm < 3 >) and mass (g) are determined. The volume and mass of the porous film are measured, The porosity is calculated from the density (g / cm < 3 >

Porosity (%) = (volume-mass / density of porous film) / volume × 100

The air permeability of the porous film may be 500 sec / 100cc or less, specifically 450 sec / 100cc or less, more specifically 400 sec / 100cc or less. The porous film having air permeability in the above range has an advantage in that it can be easily moved between the positive electrode and the negative electrode. The method of measuring the air permeability of the porous film is not limited, and the following non-limiting examples are as follows: Ten specimens cut at 10 different points of the porous film are prepared, and then EG01-55-1 MR (manufactured by Asahi Seiko Co., ) Model is used to measure the average time taken for each 100 cm diameter air to pass through a 1 inch diameter circular membrane in each of the above specimens, measured five times each, and the average value is then measured to determine the air permeability.

Also, the penetration strength of the porous film may be in the range of 150 gf to 400 gf, and specifically in the range of 180 gf to 350 gf. The sting intensity can be measured according to a method conventionally used in the art as one measure of the degree of rigidity of the porous film. As a non-limiting example of the method of measuring the sting intensity, ten specimens were prepared by cutting the porous film at 10 different points with a width of 50 mm (MD) and a length of 50 mm (MD) , A test piece is placed on a 10-cm hole, and the force to be pierced by a 1-mm probe is measured three times, and the average value is calculated.

Hereinafter, a method of manufacturing a porous film according to an embodiment of the present invention will be described.

A method for producing a porous film according to an embodiment of the present invention comprises the steps of: preparing a first composition comprising a polyethylene-based polymer and first pore-forming particles, and preparing a second composition comprising polypropylene and second pore- Layer precursor film, annealing the multi-layered precursor film at a temperature of from 80 DEG C to 150 DEG C, and extruding the annealed multilayered film at a temperature of from 0 DEG C to 50 DEG C, Deg.] C and 50% to 400% first stretching the stretched multi-layer film at a temperature of 80 [deg.] C to 150 [deg.] C.

Since the polyethylene-based polymer, the polypropylene-based polymer, and the first and second pore-forming particles are the same as those described in the above-mentioned embodiments, the method for manufacturing the porous film will be described below.

First, the first composition is prepared by mixing the polyethylene-based polymer and the first pore-forming particles. The mixing method is not particularly limited, but in one example, the polyethylene-based polymer and the first pore-forming particles are melted in a temperature range of 150 ° C to 220 ° C using a twin-screw extruder, and the first composition is prepared through pelletization .

Further, the second composition is prepared by mixing the polypropylene-based polymer and the second pore-forming particles. The mixing method is also not particularly limited, and the second composition may be prepared in the same manner as the first composition.

Thereafter, the first composition and the second composition are extrusion-molded to form a multi-layered precursor film. The method for forming the multilayer precursor film is not particularly limited, and a commonly used method can be used. In one example, each of the first composition and the second composition is extruded to prepare polyethylene-based polymer pellets and polypropylene-based polymer pellets, respectively. The polyethylene-based polymer pellets thus prepared are heated at 200 ° C to 300 ° C and the polypropylene- In a single screw or twin screw extruder at a temperature ranging from 200 ° C to 280 ° C and then co-extruded using a T-die or a ring-shaped die to form a multi-layered precursor film. In one embodiment, the pellets compounded with the first composition and the second composition are placed in a hopper of an extruder equipped with a T die, and extruded at an extruder temperature set at 170 캜 to 280 캜. A precursor film can be formed by adjusting the extrusion amount of the extrudate to a casting roll set at 30 ° C to 120 ° C such that the draw ratio is 30 to 120, for example, 40 to 100. When the stretching ratio is within the above range, uniform pores can be formed, and fracture can not easily occur during the stretching process. As another precursor production method, the extruder temperature is set at 170 ° C to 220 ° C by a blowing method. The thickness of the precursor film may range from 1 [mu] m to 50 [mu] m, for example from 3 [mu] m to 30 [mu] m, in embodiments from 5 [mu] m to 25 [

The prepared precursor film can be annealed at a temperature of 80 to 150 ° C. In one example, it can be annealed at a temperature of (Tm-80) DEG C to (Tm-3) DEG C. Here, Tm means the melting temperature of the polyethylene-based resin. Annealing is a heating process for improving the crystal structure and orientation structure by heat treatment to promote the formation of micropores upon stretching. Through the annealing process, the elasticity recovery rate of the precursor film can be adjusted to 5% to 80%, specifically 10% to 70%, more specifically, 20% to 60%. When the elastic recovery rate is within the above range, pore formation and pore size control can be easily performed in a subsequent drawing step, and it is easy to realize a morphology including the first pore and the second pore. For example, annealing is performed by placing a substrate roll in an oven where a tropical stream occurs, contacting the heated roll or a heated metal plate, or applying heat to the precursor film through a hot air or IR heater in a tenter or the like . The annealing may be performed at a temperature range of 100 占 폚 to 150 占 폚, specifically 120 占 폚 to 140 占 폚, for 10 minutes to 60 minutes, specifically 20 minutes to 50 minutes, and for example 30 minutes.

Thereafter, a step of first stretching the annealed film at a low temperature of 50% to 400% may be performed. The low-temperature stretching process is a process for forming pores between the particles and the polymer and inducing crazing over the entire area of the film to form uniform pores. For example, a roll-type or a roll- Tensile stretching is possible. The low temperature stretching temperature may range from 0 占 폚 to 50 占 폚, in embodiments from 0 占 폚 to 40 占 폚, from 10 占 폚 to 35 占 폚. Further, the stretching ratio can be stretched once in the longitudinal (MD) direction at 50% to 400%, or 50% to 300%, specifically 50% to 200%, more specifically 80% to 100%. When the stretching ratio is within the above range, cracks are sufficiently formed in the low temperature stretching step, and the desired air permeability and porosity can be achieved. The second stretching process may be performed at a temperature of 80 to 150 ° C in a range of 40% to 400%. The second stretching is a step of stretching each longitudinally or transversely at 80 DEG C to 150 DEG C in the longitudinal direction and / or transverse direction using a roll or tenter system. The magnification is 40% or more in the longitudinal direction and / To 400%, for example from 50% to 200%. When the stretch ratio is in the above range, the pores of the lamellar-fibril structure produced in the first stretching are enlarged to improve the air permeability, but the film may not be broken. The high-temperature stretching can be carried out at a temperature in the range of 80 DEG C to 150 DEG C, specifically in the range of 90 DEG C to 150 DEG C, for example, 110 DEG C to 135 DEG C.

Thereafter, heat fixing can be additionally performed as necessary. The heat setting may be to stand for a certain time without stretching while applying heat. Alternatively, the hot fix may be stretched 110% to 150% in the longitudinal direction or in the transverse direction, respectively, in a state of applying heat using a roll or tenter system, and then relaxed to 80% to 100% of the stretched length or width Thereby reducing residual stress and shrinkage. When the heat is not fixed, the crystal tends to be restored to its original state, and it is preferable that the heat is fixed. In addition, the heat shrinkage of the porous film can be improved by heat setting.

In another example, the production method of the porous film includes extrusion molding together a third composition comprising at least one of a polyethylene-based polymer, a polypropylene-based polymer, and poly (4-methylpentene) and a third pore- . For the above materials, the same materials as described above can be used. In one embodiment, the method for producing the porous film may be a method for producing a porous film having a three-layer structure of polypropylene / polyethylene / polypropylene.

Another embodiment of the present invention relates to a method for preparing a first composition comprising a first composition comprising a polyethylene-based polymer and a first pore-forming particle, preparing a second composition comprising a polypropylene-based polymer and a second pore- The composition and the second composition are respectively extruded to form respective precursor films, and the respective precursor films are laminated at a temperature of 100 ° C to 150 ° C and a pressure of 0.1 MPa to 5 MPa, To 50% to 400% first elongation at a low temperature of 50 to 50 占 폚, and second stretching at 40 to 400% at a temperature of 80 to 150 占 폚.

The present embodiment is different from the first embodiment in that the first composition and the second composition are respectively extruded to form respective precursor films and laminated, and the first and second stretches are the same as those in the above-described embodiment. Therefore, the difference will be mainly described below.

Specifically, the polyethylene-based polymer pellets and the polypropylene-based polymer pellets were produced in the same manner as in the previous example, and the polyethylene-based polymer pellets were heated at 200 ° C to 300 ° C and the polypropylene- Screw or twin-screw extruder, and then a polyethylene-based polymer precursor film or a polypropylene-based polymer precursor film is formed using a T-die or an annular die, respectively. In one embodiment, each of the polyethylene-based polymer pellets and the polypropylene-based polymer pellets is placed in a hopper of an extruder equipped with a T-die, and the extruder is set at a temperature of 170 ° C to 330 ° C and extruded. The extruded product is extruded to a casting roll set at 30 ° C to 120 ° C so as to have a draw ratio of 30 to 120, for example , 40 to 100, to obtain a polyethylene polymer precursor film and a polypropylene polymer precursor A film can be formed. As another precursor production method, the extruder temperature is set at 170 ° C to 220 ° C by a blowing method.

Each of the precursor films prepared above may be laminated at a temperature of 100 ° C to 150 ° C and a pressure of 0.1 MPa to 5 MPa to prepare a multi-layered precursor film. The laminating condition may be specifically 110 ° C to 140 ° C, 0.1 MPa to 2 MPa.

Thereafter, if necessary, the precursor film may be annealed at a temperature of 80 ° C to 150 ° C. In one example, it can be annealed at a temperature of (Tm-80) DEG C to (Tm-3) DEG C. Annealing is a heating process for improving the crystal structure and orientation structure by heat treatment to promote the formation of micropores upon stretching. Through the annealing process, the elasticity recovery rate of the precursor film can be adjusted to 5% to 80%, specifically 10% to 70%, more specifically, 20% to 60%. When the elastic recovery rate is within the above range, pore formation and pore size control can be easily performed in a subsequent drawing step, and it is easy to realize a morphology including the first pore and the second pore. For example, annealing is performed by placing a substrate roll in an oven where a tropical stream occurs, contacting the heated roll or a heated metal plate, or applying heat to the precursor film through a hot air or IR heater in a tenter or the like . The annealing may be performed at a temperature range of 100 占 폚 to 150 占 폚, specifically 120 占 폚 to 140 占 폚, for 10 minutes to 60 minutes, specifically 20 minutes to 50 minutes, and for example 30 minutes.

Thereafter, a step of stretching each of the annealed films at a low temperature by 50% to 400% may be performed. The second stretching process may be performed at a temperature of 80 to 150 ° C in a range of 40% to 400%. After the second stretching, heat fixation may be additionally carried out if necessary.

In another embodiment, the method for producing a porous film according to the above embodiment is characterized in that, in forming each precursor film, in addition to the first composition and the second composition, a polyethylene-based polymer, a polypropylene- And further extruding a third composition comprising at least one third pore-forming particle. The materials included in the third composition may be the same as those described above. In one embodiment, the method for producing the porous film may be a method for producing a porous film having a three-layer structure of polypropylene / polyethylene / polypropylene.

The method for producing a porous film according to the above embodiments may contain an appropriate amount of an antioxidant, an antistatic agent, a neutralizing agent, a dispersant, an anti-blocking agent, and a slip agent, if necessary.

Referring to FIGS. 1 and 2, the porous film produced by the above-described manufacturing methods has a first pore 5 formed by pore-forming particles and a second pore 5 formed between the polyethylene-based polymer or the polypropylene- 6), and the volume of the first pore is larger than the volume of the second pore. In addition, the lamellar thickness 7 is very thin compared to the porous film produced by the conventional dry method, and the second pores 6 are formed in the fibril structure between the lamellar and the neighboring lamellar.

According to another embodiment of the present invention, there is provided a separation membrane comprising a porous film according to an embodiment of the present invention. The separation membrane may be formed of only a porous film, or may further include a porous adhesive layer formed on one or both surfaces of the porous film. The porous adhesive layer may comprise a binder resin and / or an inorganic particle.

In yet another embodiment of the present invention, there is provided an electrochemical cell comprising a separator comprising a porous film as described herein, and a cathode and a cathode and filled with an electrolyte.

The type of the electrochemical cell is not particularly limited and may be a battery of a kind known in the technical field of the present invention.

The electrochemical cell according to an embodiment of the present invention may be a lithium secondary battery such as a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery.

The method of manufacturing the electrochemical cell according to an embodiment of the present invention is not particularly limited, and a method commonly used in the technical field of the present invention can be used.

3 is an exploded perspective view of an electrochemical cell according to one embodiment. The electrochemical cell according to one embodiment is explained as an example of square type, but the present invention is not limited thereto, and may be applied to various types of cells such as a lithium polymer battery and a cylindrical battery.

3, an electrochemical cell 100 according to an embodiment includes an electrode assembly 40 wound around a separator 30 between an anode 10 and a cathode 20, (Not shown). The anode 10, the cathode 20 and the separator 30 are impregnated with an electrolyte (not shown).

The separation membrane 30 is as described above.

The anode 10 may include a cathode current collector and a cathode active material layer formed on the cathode current collector. The cathode active material layer may include a cathode active material, a binder, and optionally a conductive material.

The cathode current collector may be aluminum (Al), nickel (Ni) or the like, but is not limited thereto.

As the cathode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specifically, it is possible to use at least one of cobalt, manganese, nickel, aluminum, iron or a composite oxide or composite phosphorus of metal and lithium in combination thereof. More specifically, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate or a combination thereof may be used.

The binder not only adheres the positive electrode active materials to each other well but also adheres the positive electrode active material to the positive electrode current collector. Specific examples thereof include polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, diacetylcellulose, polyvinyl chloride , Carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide containing polymer, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene-based polymer, polypropylene-based polymer, styrene Butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, and the like. These may be used alone or in combination of two or more.

The conductive material imparts conductivity to the electrode. Examples of the conductive material include, but are not limited to, natural graphite, artificial graphite, carbon black, carbon fiber, metal powder, and metal fiber. These may be used alone or in combination of two or more. The metal powder and the metal fiber may be made of metals such as copper, nickel, aluminum, and silver.

The cathode 20 may include a negative electrode collector and a negative electrode active material layer formed on the negative collector.

The negative electrode current collector may be copper (Cu), gold (Au), nickel (Ni), copper alloy, or the like, but is not limited thereto.

The negative electrode active material layer may include a negative electrode active material, a binder, and optionally a conductive material.

Examples of the negative electrode active material include a material capable of reversibly intercalating and deintercalating lithium ions, a lithium metal, an alloy of lithium metal, a material capable of doping and dedoping lithium, a transition metal oxide, Can be used.

Examples of the material capable of reversibly intercalating and deintercalating lithium ions include carbonaceous materials, and examples thereof include crystalline carbon, amorphous carbon, and combinations thereof. Examples of the crystalline carbon include amorphous, plate-like, flake, spherical or fibrous natural graphite or artificial graphite. Examples of the amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, fired coke, and the like. As the lithium metal alloy, a lithium-metal alloy may be selected from the group consisting of lithium, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, An alloy of a selected metal may be used. As the material capable of doping and dedoping lithium, Si, SiO _x (0 <x <2), Si-C composite, Si-Y alloy, Sn, SnO ₂ , Sn-C composite, Sn- And at least one of them may be mixed with SiO ₂ . The element Y may be at least one element selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Tl, Ge, P, As, Se, Te, Po, and combinations thereof. Examples of the transition metal oxide include vanadium oxide, lithium vanadium oxide, and the like.

The kinds of the binder and the conductive material used for the cathode are the same as those used for the anode and the conductive material.

The positive electrode and the negative electrode may be prepared by mixing each active material and a binder with a conductive material in a solvent to prepare each active material composition and applying the active material composition to each current collector. The solvent may be N-methyl pyrrolidone or the like, but is not limited thereto. The method of manufacturing the electrode is well known in the art, and therefore, a detailed description thereof will be omitted herein.

The electrolytic solution includes an organic solvent and a lithium salt.

The organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move. Specific examples thereof may be selected from a carbonate-based solvent, an ester-based solvent, an ether-based solvent, a ketone-based solvent, an alcohol-based solvent and an aprotic solvent.

Examples of the carbonate solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), ethyl methyl carbonate Carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). Specifically, when a mixture of a chain carbonate compound and a cyclic carbonate compound is used, it can be prepared from a solvent having a high viscosity and a high dielectric constant. Here, the cyclic carbonate compound and the chain carbonate compound may be mixed in a volume ratio of 1: 1 to 1: 9.

Examples of the ester solvent include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate,? -Butyrolactone, decanolide, valerolactone, Mevalonolactone, caprolactone, and the like. Examples of the ether solvent include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran and the like. Examples of the ketone-based solvent include cyclohexanone, and examples of the alcohol-based solvent include ethyl alcohol, isopropyl alcohol, and the like.

The organic solvents may be used alone or in combination of two or more. When two or more of them are used in combination, the mixing ratio may be appropriately adjusted according to the performance of the desired cell.

The lithium salt is dissolved in an organic solvent to act as a source of lithium ions in the battery to enable the operation of a basic electrochemical cell and to accelerate the movement of lithium ions between the positive electrode and the negative electrode.

For example the lithium salt _{is, LiPF 6, LiBF 4, LiSbF} 6, LiAsF 6, LiN (SO 3 C 2 F 5) 2, LiN (CF 3 SO 2) 2, LiC 4 F 9 SO 3, LiClO 4, LiAlO 2 , LiAlCl ₄ , LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ) where x and y are natural numbers, LiCl, LiI, LiB (C ₂ O ₄ ) _2, .

The concentration of the lithium salt can be used within the range of 0.1M to 2.0M. When the concentration of the lithium salt is within the above range, the electrolytic solution has an appropriate conductivity and viscosity, and thus can exhibit excellent electrolytic solution performance, and lithium ions can effectively move.

Hereinafter, the present invention will be described in more detail with reference to Examples, Comparative Examples and Experimental Examples. It should be understood, however, that these examples, comparative examples and experimental examples are merely illustrative of the present invention, and thus the scope of the present invention is not limited thereto.

Example  And Comparative Example  : Preparation of Porous Film

Example  One

40% by volume of HDPE (HIVOREX 5200, Lotte Chemicals, hereinafter referred to as 'HDPE 1', Tm = 133 ° C) having a melting index of 0.3 and HDPE (HIVOREX 5000, Lotte Chemicals, hereinafter referred to as 'HDPE 2', Tm = 130 ° C) and 20% by volume of calcium carbonate (TC, octanoic calcium) having an average particle size of 60 nm were completely melted at a temperature equal to or higher than the melting point in a twin-screw extruder and kneaded to prepare a first composition Respectively. 80 volume% of a polypropylene-based polymer (PP) having a melt index of 2.0 (S802, Korean emulsification) and 20 volume% of calcium carbonate having an average particle size of 60 nm (equivalent) were melted and kneaded and extruded under the same conditions as above to obtain a polypropylene- . The prepared polyethylene polymer pellets were melted in a twin-screw extruder at 260 ° C and the polypropylene polymer pellets in a twin-screw extruder at 230 ° C, and then co-extruded using a multi-block type T-die to produce a two layer precursor film of PP / PE (Draw ratio: 40 to 45). The prepared two-layered precursor film of PP / PE was annealed in a hot air oven at 120 캜 for 30 minutes, and then dried at room temperature (25 캜) And then stretched 200% in the MD direction at a high temperature of 120 占 폚 and then heat-set at 130 占 폚 for 2 minutes to prepare a porous film of Example 1 having a two-layer structure of PP / PE having a thickness of 18 占 퐉.

Example  2

In Example 1, the polyethylene-based polymer pellets and the polypropylene-based polymer pellets were extrusion-molded without pneumatic extrusion to form a polyethylene-based polymer precursor film and a polypropylene-based polymer precursor film. Each of the precursor films was laminated at 135 DEG C under 0.3 MPa, and then subjected to first 100% stretching in the MD direction at room temperature, 200% second stretching once in the MD direction at 120 DEG C, Minute to prepare a porous film of Example 2 having a two-layer structure of PP / PE having a thickness of 18 mu m.

Example  3

A porous film of Example 3 having a two-layer structure of PP / PE was prepared in the same manner as in Example 1, except that organic particles having an average particle size of 200 nm were used instead of calcium carbonate in Example 1 .

Example  4

In the same manner as in Example 1, except that the polyethylene-based polymer substrate was an inner layer and the polypropylene-based polymer substrate was coextruded on both sides of the polyethylene-based polymer substrate to prepare a three-layered precursor film To prepare a porous film of Example 4 having a three-layer structure of PP / PE / PP.

Comparative Example  One

In Example 1, a porous film of Comparative Example 1 having a two-layer structure was produced in the same manner as in Example 1, except that calcium carbonate was not included in the production of the polyethylene-based polymer pellets.

Comparative Example  2

In Example 1, a porous film of Comparative Example 2 having a two-layer structure was produced in the same manner as in Example 1, except that calcium carbonate was not included in the production of the polypropylene polymer pellets.

Comparative Example  3

A porous film of Comparative Example 3 having a two-layer structure was produced in the same manner as in Example 1, except that calcium carbonate (equivalent calcium) having an average particle size of 500 nm was used in Example 1.

Comparative Example  4

In the same manner as in Example 1, except that the polyethylene-based polymer pellets and the polypropylene-based polymer pellets were prepared in the same manner as in Example 1, except that calcium carbonate having an average particle size of 60 nm was contained in each layer in an amount of 35 vol% To prepare a porous film of Comparative Example 4.

The composition and treatment conditions of the porous films of Examples 1 to 4 and Comparative Examples 1 to 4 are summarized in Table 1 below.

content
(volume%) Examples 1 and 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 PE layer HDPE1 40 40 40 100 40 40 32.5 HDPE2 40 40 40 - 40 40 32.5 Pore-forming particles 20 20 20 - 20 20 35 PP layer PP 80 80 80 80 100 80 65 Pore-forming particles 20 20 20 20 - 20 35 Pore forming particle size (nm) 60 200 60 60 60 500 60 The thickness ratio of each layer PP / PE
= 2: 3 PP / PE
= 2: 3 PP / PE / PP = 2: 3: 2 PP / PE
= 2: 3 PP / PE
= 2: 3 PP / PE
= 2: 3 PP / PE
= 2: 3 Total thickness of porous film (μm) 18 20 18 18 18 20 17

Experimental Example  One

Porosity Measurement of Porous Film

The volume (cm 3) and mass (g) of the porous film prepared in the above Examples and Comparative Examples were cut out to 10 cm × 10 cm, and the volume and mass and the density (g / cm 3) of the porous film The porosity was calculated using the following formula.

Porosity (%) = (volume-mass / density of porous film) / volume × 100

Experimental Example  2

Measurement of air permeability of porous film

In order to measure the air permeability of the porous film produced in the examples and comparative examples, each of the porous films was divided into 10 pieces, which were cut at 10 different points in a size capable of entering a circle having a diameter of 1 inch or more And then the time for passing 100 cc of air through each of the above specimens was measured using an air permeability measuring apparatus (Asahi Seiko Co., Ltd.). The time was measured five times each, and the average value was calculated to measure the air permeability.

Experimental Example  3

Measurement of stiffness of porous film

Each of the porous films prepared in the above Examples and Comparative Examples was cut at 10 different points in MD 50 mm × TD 50 mm to prepare 10 specimens, and then, using KATO TECH G5 equipment The specimens were placed on a 10 cm hole, and the force was measured while pushing with a 1 mm probe. The stiffness of each of the above specimens was measured three times, and then the average value was calculated.

Experimental Example  4

Measurement of Heat Shrinkage of Porous Film

Each of the porous films prepared in the above-mentioned Examples and Comparative Examples was cut at 10 different points in MD 50 mm × TD 50 mm to prepare 10 specimens. Each of the specimens was allowed to stand in an oven at 105 ° C for 1 hour and then shrunk in the MD and TD directions and allowed to stand in an oven at 120 ° C for 1 hour and then shrunk longitudinally and transversely And the average heat shrinkage ratio in the MD and TD directions was calculated in consideration of the reduced size.

Experimental Example  5

Measurement of tensile strength of porous film

Each of the porous films prepared in the above Examples and Comparative Examples was cut at 10 different points in a rectangular shape of 10 mm in length × 50 mm in length (TD) to prepare 10 specimens, The specimens were mounted on a UTM (tensile tester) to measure the length to 20 mm, and the specimens were pulled to measure the average tensile strength in the MD and TD directions.

The results according to Experimental Examples 1 to 5 are shown in Table 2 below.

Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Comparative Example 4 Porosity (%) 48 45 34 49 25 30 38 56 Air permeability (sec / 100cc) 382 401 425 364 760 613 575 218 Stiff Strength (gf) 193 184 180 209 350 320 175 150 Heat Shrinkage (%) MD 9 11 15 5 17 13 11 15 TD 0 0 0 0 0 0 0 0 Tensile strength (kgf / cm ² ) MD 1100 1020 980 1360 1200 1180 890 840 TD 70 72 76 67 69 70 76 80

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that such detail is solved by the person skilled in the art without departing from the scope of the invention. will be. Accordingly, the actual scope of the present invention will be defined by the appended claims and their equivalents.

Claims (17)

A first microporous layer comprising a polyethylene-based polymer and first pore-forming particles,
A porous film laminated with a second microporous layer containing a polypropylene-based polymer and second pore-forming particles,
The average particle diameter of the first and second pore-forming particles is 300 nm or less,
Wherein the first pore-forming particles are contained in an amount of 5% by volume to 25% by volume based on the total volume of the first microporous layer,
Wherein the second pore-forming particles are contained in an amount of 5% by volume to 25% by volume based on the total volume of the second microporous layer. The method of claim 1, wherein the first and second pore-forming particles are each independently selected from the group consisting of polystyrene (PS), polyethylene (PE), polypropylene (PP), polyvinylidene fluoride ), Polytetrafluoroethylene (PTFE), polyurethane (PU), polymethylpentene (PMP), polyethylene terephthalate (PET), polycarbonate (PC), polyester, polyvinyl alcohol (PM), polyethylene oxide (PEO), polyamide (PA), polyamideimide (PA), polyacrylonitrile (PA), acrylonitrile (PAN), silicone acryl rubber, ethylene-methyl acrylate copolymer, polymethylene oxide (PA), polysulfone (PSF), polyethyl sulfone (PES), polyphenylene sulfide (PPS), polyarylate (PAR), polyimide (PI), polyaramid (PA), cellulose, cellulose A modified resin, a melamine resin and a phenol resin Wherein the organic particles are at least one selected from organic particles. The method of claim 1, wherein the first and second pore-forming particles are each independently selected from the group consisting of alumina, silica, titania, zirconia, magnesia, ceria, zinc oxide, barium sulfate, iron oxide, silicon nitride, titanium nitride, Wherein the inorganic particles are at least one selected from the group consisting of aluminum sulfate, aluminum hydroxide, calcium titanate, barium titanate, talc, calcium silicate, and magnesium silicate. The porous film according to claim 1, wherein the first and second pore-forming particles have an average particle diameter of 30 nm to 300 nm. The porous film according to claim 1, wherein the porous film further comprises a third microporous layer containing at least one of a polyethylene-based polymer, a polypropylene-based polymer, poly (4-methylpentene), and polyethylene terephthalate and a third pore- Containing, porous film. The porous film according to claim 5, wherein the average particle diameter of the third pore-forming particles is 30 nm to 300 nm. The porous film according to claim 5, wherein the third pore-forming particles are contained in an amount of 5% by volume to 25% by volume based on the total volume of the third microporous layer. The porous film according to claim 5, wherein the porous film has a three-layer structure of a second microporous layer / a first microporous layer / a third microporous layer. The porous film according to claim 1, wherein the porosity of the porous film is 40% to 70%. The porous film according to claim 1, wherein the air permeability of the porous film is 500 sec / 100cc or less. The porous film according to claim 1, wherein the porous film has a puncture strength of 150 gf to 400 gf. 12. The method of any one of claims 1 to 11, wherein the porous film is formed between a first pore formed by the first or second pore-forming particles and a lamella of each of the first microporous layer or the second microporous layer Wherein the volume of the first pore is larger than the volume of the second pore. [12] The method of claim 12, wherein the first pore has a / b of 1 to 7 and the second pore has a / b of at least 0.5, where the length of the major axis of the pore is a and the length of the minor axis is b. Porous film. 13. The porous film of claim 12, wherein the second pore is formed in a fibril structure between the lamella and the neighboring lamella. 13. The porous film of claim 12, wherein the lamellar thickness is 200 nm or less. An electrochemical cell comprising a porous film, a cathode, a cathode, and an electrolyte according to any one of claims 1 to 11. 17. The electrochemical cell of claim 16, wherein the electrochemical cell is a lithium secondary battery.

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