KR101269203B1 - Microporous polyolefin film with a thermally stable layer at high temperature - Google Patents

Abstract

The present invention relates to a polyolefin composite microporous membrane which can be used as a battery separator. The polyolefin composite microporous membrane according to the present invention is a composite microporous membrane having a polymer coating layer formed of one or more layers selected from one side, both sides, or inside the membrane of the polyolefin microporous membrane, wherein the polymer coating layer is formed of a polyolefin microporous membrane. It is coated along the surface shape and the pore structure of the surface of the polyolefin microporous membrane is kept open, the permeability is 1.5x10 ^-5 Darcy or more, the melt breaking temperature is 160 ° C or more, and the lateral / longitudinal shrinkage is 150 ° C, It is 40% or less for 60 minutes, and the thickness of the said polymer coating layer is 0.5 micrometer or less in each surface. The polyolefin composite microporous membrane according to the present invention has excellent quality stability and excellent thermal stability at high temperature, and is suitable as a separator for high capacity / high power batteries.

Polyolefin, Microporous Membrane, Battery, Separator

Description

Microporous polyolefin film with a thermally stable layer at high temperature

The present invention relates to a polyolefin composite microporous membrane having excellent quality stability and excellent thermal stability at high temperature, and more particularly, to a polyolefin composite microporous membrane excellent as a separator for a high capacity / high output lithium secondary battery.

Polyolefin microporous films are widely used as battery separators, separation filters, and microfiltration membranes due to their chemical stability and excellent physical properties. Among these, secondary battery separators are required to have the highest level of quality along with demands for battery safety. In recent years, there is a growing demand for improving the characteristics of the separator for the thermal stability of the separator and the electrical safety of the secondary battery during charging and discharging according to the trend of high capacity and high output of the secondary battery. In the case of a lithium secondary battery, when the thermal stability of the separator decreases, damage or deformation of the separator caused by the temperature increase in the battery and a short circuit between the electrodes may occur, resulting in a risk of overheating or ignition of the battery.

The thermal stability of the battery is largely determined by the thermal stability and quality uniformity of the separator at high temperatures, and is affected by the closing temperature of the separator, the melt fracture temperature, the high temperature melt shrinkage rate, and the quality uniformity.

The separator having excellent thermal stability at high temperature prevents separator damage at high temperature and prevents short circuit between the electrodes. If a short circuit between electrodes occurs due to dendrites generated in the electrodes during the charge / discharge process of the battery, heat generation of the battery occurs. In this case, a separator having excellent thermal stability at high temperature prevents damage of the separator to suppress the occurrence of ignition / explosion. can do.

Methods for improving the thermal stability of the separator include a method of crosslinking the separator, a method of adding an inorganic substance, and a method of using a heat resistant resin.

Among these, a method of crosslinking the membrane is shown in US Pat. No. 6,127,438 and US Pat. No. 6,562,519. These methods are methods for electron beam crosslinking or chemical crosslinking of a film. However, among these methods, electron beam cross-linking requires the installation of an electron beam cross-linking device using radiation, and there are disadvantages such as limitations in production speed and quality variation due to non-uniform crosslinking. In addition, in the case of chemical crosslinking, the extrusion kneading process is complicated and there is a high possibility that gel is generated on the film due to irregular crosslinking, and there is a disadvantage that long time high temperature fixing is required.

U. S. Patent No. 6,949, 315 discloses a method for improving the thermal safety of a separator by kneading an ultrahigh molecular weight polyethylene with 5-15% by weight of an inorganic material such as titanium oxide. However, this method not only raises the extrusion load due to the use of ultra high molecular weight polyethylene, decreases the extrusion kneading property and decreases the productivity due to unstretching, but also causes problems such as poor kneading due to the input of inorganic materials, resulting in uneven quality and pinholes. It is easy to do so, and the lack of affinity between the inorganic material and the polymer resin interface causes a decrease in film properties.

A method of kneading and using a resin having excellent heat resistance is disclosed in U.S. Patent No. 5,641,565. This technology requires ultra-high molecular weight molecules with a molecular weight of 1 million or more to prevent the deterioration of the properties caused by the addition of polyethylene, heterogeneous polypropylene and inorganic materials. In addition, there is a disadvantage in that the process is complicated by adding a process for extracting and removing the used inorganic material.

Japanese Laid-Open Patent Publication No. 2004-161899 discloses a microporous membrane containing a non-polyethylene-based thermoplastic resin which is excellent in heat resistance to polyethylene and does not completely dissolve during melt kneading. However, the microporous membrane manufactured by this method has a disadvantage in that the uniformity of the thickness is greatly reduced due to the heat-resistant resin on the particle. If the thickness uniformity of the microporous membrane is lowered, the defective rate increases during assembly of the battery, which deteriorates the productivity.

All of the above methods were developed with the focus on improving the melt rupture temperature of the separator to improve the heat resistance and do not consider any quality stability problems that may occur in actual battery applications. There are also disadvantages.

Accordingly, the present inventors have conducted extensive research to solve the above-described problems of the prior art, and found that a polyolefin microporous membrane having the following characteristics is excellent as a separator for a lithium secondary battery.

(1) A composite microporous membrane having a polymer coating layer formed of one or more layers selected on one side, both sides, or inside a membrane of a polyolefin microporous membrane, wherein the polymer coating layer is coated along the surface shape of the polyolefin microporous membrane to be polyolefin. The pore structure of the surface of the microporous membrane is kept open, the permeability is 1.5x10 ^-5 Darcy or more, the melt breaking temperature is 160 ° C or more, the lateral / longitudinal shrinkage is 150 ° C, 40% or less for 60 minutes, The thickness of the polymer coating layer is a polyolefin-based composite microporous membrane, characterized in that 0.5㎛ or less on each side

(2) The polyolefin-based composite microporous membrane according to (1), wherein the coating layer polymer has a melting temperature or glass transition temperature of 170 ° C or higher.

An object of the present invention is to provide a polyolefin composite microporous membrane having excellent quality stability and excellent thermal stability at high temperature, and furthermore, to provide a separator that is compatible with high output / high capacity of a battery.

The microporous membrane of the present invention and its manufacturing method capable of achieving the above object are a composite microporous membrane having a polymer coating layer formed of at least one layer selected from one side, both sides, or inside the membrane of the polyolefin microporous membrane. The polymer coating layer was coated along the surface shape of the polyolefin microporous membrane to keep the pore structure of the surface of the polyolefin microporous membrane open, so that the permeability was 1.5x10 ^-5 Darcy or more and the melt fracture temperature was 160 ° C or more, and the transverse direction It is to provide a polyolefin-based composite microporous membrane having a shrinkage ratio in the longitudinal direction of 150 DEG C for 40 minutes or less for 60 minutes and a thickness of the polymer coating layer of 0.5 mu m or less on each side.

Looking at the present invention in more detail as follows.

The polyolefin composite microporous membrane of the present invention maintains the pore structure of the polyolefin microporous membrane before coating as shown in FIG. 1. Maintaining the pore structure of the polyolefin microporous membrane before coating means that the polymer having excellent heat resistance does not block the pore structure of the polyolefin microporous membrane before coating in the process of forming the coating layer according to the surface shape of the polyolefin microporous membrane. it means.

When the polymer of the coating layer blocks the pore structure during the coating process, the gas permeability is lowered and the ion permeation performance is lowered, so that the electrical properties of the battery, such as high rate characteristics, charge and discharge characteristics, low temperature characteristics, and lifespan, are reduced. The gas permeability is at least 1.5 × 10 ⁻⁵ Darcy and preferably at least 2.0 × 10 ⁻⁵ Darcy. If the permeability is lower than 1.5 × 10 ⁻⁵ Darcy, the ion permeability is poor and the electrical properties are deteriorated.

In the composite microporous membrane, the thickness of the polymer coating layer is preferably 0.5 µm or less, more preferably 0.2 µm or less. In addition, the increase of the total film thickness is 1.0 micrometer or less. If the thickness of the polymer coating layer exceeds 0.5㎛ prevents the pore structure of the polyolefin before coating, the permeability is sharply reduced, the thickness uniformity of the entire polyolefin composite microporous membrane is inferior due to the non-uniform coating layer. When the film thickness uniformity is deteriorated, productivity decreases because the defective rate increases during battery assembly, and short circuit occurs due to nonuniformity inside the battery after battery assembly, which increases safety factor.

Melt fracture temperature is 160 ° C or higher. In order to increase the thermal safety of the battery, the higher the melt fracture temperature of the separator is, the better, and to increase the melt fracture temperature of the polyolefin microporous membrane, the use of ultra high molecular weight resin is used. It is impossible to raise more than ℃. When the polyolefin microporous membrane is coated with a polymer having a melting temperature or glass transition temperature of 170 ° C. or higher, the melt breaking temperature may be increased to 160 ° C. or higher.

 The shrinkage in the lateral / vertical direction is 40% or less for each of 150 ° C. and 60 minutes, but preferably 30% or less for each. Similar to the melt breaking temperature, the shrinkage in the transverse and longitudinal directions at 150 ° C. indicates thermal stability at high temperatures of the separator. If the shrinkage at high temperature is bad, shrinkage occurs as the temperature inside the battery increases, and the two electrodes are exposed to cause a short circuit between the electrodes, thereby causing additional overheating and ignition.

 Method for producing a polyolefin composite microporous membrane of the present invention for achieving the above object may include the following steps.

Preparing a polymer solution for forming a coating layer having a melting temperature or a glass transition temperature of 170 ° C. or higher;

Preparing a microporous membrane having pores made of polyolefin;

Forming a coating layer by coating the polymer solution on one or both surfaces of the microporous membrane;

As a method for producing a composite microporous membrane prepared, including

The coating layer of the composite microporous membrane is coated along the surface shape of the microporous membrane so that the pore structure of the surface of the microporous membrane remains open, and the permeability of the composite microporous membrane is 1.5x10 ^-5 Darcy or more, and the melt fracture temperature is increased. Provided is a method for producing a polyolefin composite microporous membrane having a temperature of 160 ° C. or more, a shrinkage ratio in a lateral / vertical direction of 150 ° C., 40% or less for 60 minutes, and a thickness of the polymer coating layer of 0.5 μm or less on each side.

Looking at this in more detail:

(a) melting / kneading / extruding a mixture containing 20-50% by weight of polyolefin and 80-50% by weight of diluent above the phase separation temperature to produce a thermodynamic single phase in an extruder;

(b) molding the melt of the single phase into a sheet form by performing phase separation;

(c) stretching the sheet prepared in step (b) by a lateral biaxial stretching method or a sequential biaxial stretching method with a transverse and longitudinal stretching ratio of 3.0 times or more, respectively;

(d) extracting and drying the diluent under constant tension in the stretched film;

(e) a heat setting step of reducing the shrinkage of the film by removing residual stresses and the like of the dried film;

(f) applying a solution of a heat resistant material dissolved on one or both surfaces of the surface of the separator prepared in step (e) or inside the membrane;

(g) removing the solvent from the solution applied in step (f) to form a coating layer

Looking at each step in more detail, as follows.

A low molecular weight organic material (hereinafter diluent) capable of forming a single phase with the polyolefin at high temperatures with the polyolefin is extruded / kneaded at a high temperature at which the polyolefin is melted to form a thermodynamic single phase. When these thermodynamic single phase polyolefin and diluent solutions are cooled to room temperature, phase separation of the polyolefin and diluent occurs during the cooling process. In this case, each phase separated from each other is a polyolefin rich phase in which polyolefin comprises most of the content, and a diluents rich phase composed of a small amount of polyolefin and diluent dissolved in the diluent. Is done. After the phase separation proceeds as desired, the melt is completely cooled to solidify the polyolefin-containing phase, and the polyolefin microporous membrane is formed by extracting the diluent multi-containing phase with an organic solvent.

The basic physical properties of the microporous membrane are determined by the concentration of polyolefin in the polyolefin-containing phase during the phase separation process. When the phase separation is sufficiently performed and the polyolefin concentration of the polyolefin-containing phase is sufficiently high, the fluidity of the polyolefin chain decreases during the stretching after cooling, resulting in an increase in the forced orientation effect, thereby increasing the mechanical strength after the stretching. do. In other words, assuming sufficient phase separation with diluent using a resin of the same molecular weight, the mechanical strength is much better than that of a composition that is not.

In addition, the basic pore structure of the microporous membrane is determined during the phase separation process. That is, the size and structure of the dilutive multi-containing phase formed after phase separation determine the pore size and structure of the final microporous membrane. Therefore, it is possible to control the pore structure according to the thermodynamic phase separation temperature of the composition, the rate and time of phase separation during processing, and the depth of the phase separation induction temperature.

The polyolefin used in the present invention may be a polyolefin alone or a copolymer such as polyethylene, polypropylene, poly-4-methyl-1-pentene using ethylene, propylene, alpha olefin, 4-methyl-1-pentene, etc. as monomers and comonomers. And mixtures thereof. One example of a suitable polyolefin is a high density polyethylene having a comonomer content of less than 2% in terms of strength, extrusion kneading and elongation, with a weight average molecular weight of 2.0x10 ⁵ to 4.5x10 ⁵ . If the weight average molecular weight is lower than 2.0x10 ⁵ , the microporous membrane having excellent final properties cannot be obtained. If the weight average molecular weight is higher than 4.5x10 ⁵ , the extruder load is increased by the extruder due to the viscosity increase during the extrusion process, and the viscosity difference with the diluent is large. The kneading | mixing fall by this generate | occur | produces, and the surface shape of the sheet | seat extruded also becomes rough. The solution to this problem is to increase the extrusion temperature or make the shear configuration of the screw configuration of the twin screw compound higher, but in this case, deterioration of the resin occurs, thereby reducing physical properties.

The diluent used in the present invention may be any organic liquid that forms a single phase with the resin at the extrusion processing temperature. Examples thereof include aliphatic or cyclic hydrocarbons such as nonane, decane, decalin and paraffin oil; 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 acid alcohol, and oleate alcohol; Of fatty acid groups such as palmitic acid mono-, di- or triesters, stearic acid mono-, di- or triesters, oleic acid mono-, di- or triesters, linoleic acid mono-, di- or triesters One or two or more fatty acids of saturated and unsaturated fatty acids having 4 to 26 carbon atoms have 1 to 8 hydroxyl groups, and there are fatty acid esters ester-bonded with alcohols having 1 to 10 carbon atoms. If the conditions for phase separation are met, it is also possible to mix and use one or more of the above substances.

The composition of the polyolefin and diluent used in the present invention is preferably 20 to 50% by weight of polyolefin and 80 to 50% by weight of diluent. If the content of the diluent is less than 50% by weight, the porosity is reduced and the pore size is reduced, and the interconnection between the pores is small, so that the permeability is greatly reduced. On the other hand, when the diluent exceeds 80% by weight, the kneading property of the polyolefin and the diluent is reduced, so that the polyolefin is extruded in a gel form without thermodynamic kneading into the diluent, causing problems such as fracture and thickness unevenness in stretching. You can.

If necessary, general additives for improving specific functions, such as an oxidative stabilizer, a UV stabilizer, an antistatic agent, and the like may be added in a range where the properties of the separator are not significantly reduced.

As a method of making a sheet-shaped molding from the melt, both a general casting or calendering method may be used. A single phase melt, which has undergone melting / kneading / extrusion, is cooled to room temperature to produce a sheet having a constant thickness and width.

The sheet produced through the phase separation process is stretched at a total draw ratio of 24 to 70 times with a lateral biaxial stretching method or a sequential biaxial stretching method of 3.0 times or more in the transverse and longitudinal directions, respectively. If the draw ratio in one direction is less than 3.0 times, the orientation in one direction may not be sufficient, and the balance of properties between the longitudinal and transverse directions may be broken, resulting in a decrease in tensile strength and puncture strength, which may cause internal short circuits due to dendrites. . Thus, by increasing the orientation in the transverse direction and the longitudinal direction to increase the mechanical strength and also the high permeability can be compensated for the physical properties sacrificed by the thermal stability in the post-stage process. In addition, if the total draw ratio is less than 24 times, unstretched may occur, and sufficient permeability may not be obtained after coating the heat resistant polymer. If the total draw ratio exceeds 70 times, breakage during stretching is likely to occur, and shrinkage of the final film is increased.

The stretched film in the stretching step is to extract the internal diluent using an organic solvent and to dry. The organic solvent usable in the present invention is not particularly limited, and any organic solvent capable of extracting the diluent used in the resin extrusion may be used, but preferably methyl ethyl ketone and methylene chloride having high extraction efficiency and rapid drying. And hexane are suitable. As the extraction method, all general solvent extraction methods, such as an immersion method, a solvent spray method, and an ultrasonic method, may be used individually or in combination. The residual diluent content at the time of extraction should be 1% by weight or less. Exceeding 1% by weight of residual diluent deteriorates the physical properties and reduces the permeability of the film. The amount of residual diluent depends greatly on the extraction temperature and extraction time. The extraction temperature is good to increase the solubility of the diluent and the solvent, but it is better than 40 ℃ considering the safety problems due to the boiling (boiling) of the solvent. If the extraction temperature is below the freezing point of the diluent, the extraction efficiency is greatly reduced, so it must be higher than the freezing point of the diluent.

The dried film is subjected to a heat setting step to remove residual stress and reduce shrinkage. The hot fix is to fix the film and apply heat to force the film to be shrunk to remove the residual stress. The high heat setting temperature is advantageous for lowering the shrinkage rate, but if it is too high, the film is partially melted and the micropores formed are clogged to lower the permeability. The preferred heat setting temperature is preferably selected in the temperature range in which 10 to 30% by weight of the crystalline portion of the film is melted. When the heat setting temperature is selected in the temperature range lower than the melting temperature of 10% by weight of the crystal part of the film, the reorientation of the polyolefin molecules in the film is insufficient and there is no effect of removing residual stress of the film, and the crystal part of the film If 30% by weight of is selected in the temperature range higher than the melting temperature, the microporous is blocked by partial melting, the permeability is reduced.

Here, the heat fixation time should be relatively short when the heat fixation temperature is high, and may be relatively long when the heat fixation temperature is low. Preferably about 15 second-2 minutes are suitable.

In order to improve the thermal stability of the polyolefin microporous membrane thus prepared, forming a coating layer of the heat resistant polymer while maintaining the pore structure of the polyolefin may be performed by melting the heat resistant polymer to form a solution, and applying the heat resistant polymer solution to the polyolefin microporous membrane. And removing the solvent to form a coating layer.

The polymer of the coating layer preferably has a melting temperature or glass transition temperature of 170 ° C or higher. If the melting temperature of the coating layer polymer is less than 170 ° C., sufficient thermal stability may not be ensured to withstand a sudden temperature increase due to a short circuit inside the battery. The polymer of the coating layer is not particularly limited as long as the melting temperature or glass transition temperature is 170 ° C or higher, and polyamide, polyimide, polyamideimide, polyethylene terephthalate, polycarbonate ( polycarbonate, polyarylate, polyetherimide, polyphenylene sulfone, polysulfone, and the like.

The coating layer polymer is dissolved in an organic solvent and applied in the form of a solution. The organic solvent is not particularly limited as long as it can dissolve the coating layer polymer, and may be N, N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), or N, N-dimethylacetamide (DMAc). , Dichloromethane (MC) and the like. In this case, the viscosity of the solution is preferably between 1 and 100 cps, more preferably 3 to 50 cps. If the viscosity of the solution exceeds 100cps, it is difficult for the coating layer polymer solution to be sufficiently applied into the polyolefin microporous membrane. If the viscosity of the solution is lower than 1cps, sufficient coating layer is not formed, and thus heat resistance characteristics cannot be expected.

The method of applying the coating layer polymer solution prepared in the above manner to one side or both sides of the surface of the polyolefin microporous membrane or the inside of the membrane is not particularly limited as long as it is known in the art, and the bar coating method and the die coating Method, comma coating method, Micro Gravure / Gravure method, Dip coating method, the spray (Spray) method or a mixture thereof may be used. Thereafter, the process may include removing a portion of the coating layer on the surface using a doctor blade or an air knife.

Polyolefin composite microporous membrane according to the present invention is excellent in thermal stability at high temperature with excellent permeability and excellent quality uniformity for the thickness can exhibit an excellent effect when used in high capacity / high output battery.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Hereinafter, the present invention will be described in more detail with reference to the following examples, but the scope of the present invention is not limited thereto.

[Example]

Various properties of the polyolefin composite microporous membrane of the present invention were evaluated by the following test method.

(1) film thickness

A contact type thickness measuring device with a thickness of 0.1 mu m was used.

(2) coating layer thickness

When the polymer coating layer is formed on the surface of the microporous membrane, the thickness of the coating layer is determined from the difference between the thickness of the microporous membrane before coating and the thickness of the microporous membrane after coating, using a thickness measuring instrument of a contact method having a thickness accuracy of 0.1 μm. Measured. In the case of a microporous membrane having a coating layer on both sides, 1/2 of the difference in thickness before and after coating was used as the thickness of the coating layer.

When the polymer coating layer was formed inside the microporous membrane, the thickness of the coating layer was measured from the size of the average pore size. The mean pore size was measured using UOP578-02 and measured using Mercury Porosimetry (PoreMaster from Quantachrome). One half of the size difference between the average pore size of the microporous membrane before coating and the microporous membrane after coating was used as the thickness of the coating layer.

(3) Viscosity

The viscosity of the polymer solution of the polymer coating layer was measured using an Ostwalt viscometer.

(4) space rate (%)

A rectangular sample of Acm x Bcm was cut out and calculated from Equation (1). A / B was cut and measured in the range of 5-20 cm each.

[Equation 1]

Space rate = {(A x B x T) - (M / r) / (A x B x T)} x 100

Where T = thickness of the separation membrane (cm)

 M = sample weight (g)

ρ = resin density (g / cm ³ )

(5) Gas Permeability (Darcy)

Gas permeability was measured from a porometer (CFP-1500-AEL from PMI). In general, gas permeability is expressed by Gurley number, but the Gurley number is not corrected for the effect of film thickness, so it is difficult to know the relative permeability depending on the pore structure of the film itself. In order to solve this problem, Darcy's permeability constant was used in the present invention. Darcy's permeability constant is obtained from the following equation (2), and nitrogen is used in the present invention.

&Quot; (2) "

C = (8 FTV) / (πD ² (P ² -1))

Where C = Darcy permeability constant

       F = flow rate

       T = sample thickness

V = viscosity of the gas (0.185 for N ₂ )

       D = sample diameter

       P = pressure

In the present invention, the average value of Darcy's permeability constant was used in the range of 100 to 200 psi.

(6) Perforation strength (N / 占 퐉)

And measured with a UTM (Universal Test Machine) 3345 manufactured by INSTRON Inc. at a speed of 120 mm / min. The pin was a pin tip with a diameter of 1.0 mm and a radius of curvature of 0.5 mm.

&Quot; (3) "

Perforation strength (N / 占 퐉) = Measurement Load (N) 占 Isolation film thickness (占 퐉)

(7) Tensile strength was measured by ASTM D882.

(8) Shrinkage was measured in% in the longitudinal and transverse directions after leaving the polyolefin composite microporous membrane at 150 ℃ for 60 minutes.

(9) Closed temperature and melt fracture temperature

The closing temperature and melt breaking temperature of the polyolefin composite microporous membrane were measured in a simple cell capable of measuring impedance. The simple cell was assembled with a polyolefin composite microporous membrane positioned between two graphite electrodes and injected with an electrolyte therein, and the electrical resistance was measured by raising the temperature from 5 ° C./min to 25 ° C. to 200 ° C. using a 1 kHz alternating current. At this time, the temperature at the point where the electrical resistance rapidly increased by several hundreds to thousands of Ω was set as the closing temperature, and the temperature at the point where the electrical resistance decreased again and dropped below 100 Ω was used as the melting fracture temperature. As the electrolyte, lithium hexafluorophosphate (LiPF ₆ ) was dissolved in 1 mol of ethylene carbonate and propylene carbonate 1: 1 solution.

(10) Hot box test

The cell was assembled using the polyolefin composite microporous membrane as a separator. The positive electrode using lithium cobalt oxide (LiCoO ₂ ) as the active material and the negative electrode using graphite carbon as the active material were wound together with the prepared separator and put in an aluminum pack, followed by ethylene carbonate and diethylene. A battery was assembled by injecting and sealing an electrolyte solution in which lithium hexafluorophosphate (LiPF ₆ ) was dissolved in a 1 mol concentration in a carbonate 1: 1 solution.

The assembled battery was placed in an oven, and the temperature was raised at 5 ° C / min. After reaching 150 ° C, the battery was allowed to stand for 30 minutes to measure changes in the battery.

 Example 1

High density polyethylene having a weight average molecular weight of 3.8 × 10 ⁵ was used. As a diluent, dibutyl phthalate and a paraffin oil having a kinematic viscosity of 160 ° C. at 40 ° C. were mixed at a ratio of 1: 2. 30 wt% and 70 wt%.

The composition was extruded at 240 ° C. using a biaxial compounder equipped with a T-die and passed through a section set at 180 ° C. to induce phase separation, and a sheet was manufactured using a casting roll. Stretching was performed using a sequential biaxial stretching machine, and stretching temperatures, stretching ratios, heat setting temperatures and conditions were performed as shown in Table 1 below.

The polymer coating layer was coated using a bar coating method using a solution in which polyamideimide (PAI, polyamideimide) having a glass transition temperature of 272 ° C. was dissolved in NMP at a concentration of 9.1 wt%.

[Example 2]

High density polyethylene having a weight average molecular weight of 3.0 × 10 ⁵ was used, dibutyl phthalate was used as the diluent, and the content of polyethylene and the diluent was 30 wt% and 70 wt%, respectively.

The composition was extruded at 250 ° C. using a biaxial compounder equipped with a T-die and passed through a section set at 190 ° C. to induce phase separation, and a sheet was manufactured using a casting roll. Stretching was performed using a sequential biaxial stretching machine, and stretching temperatures, stretching ratios, heat setting temperatures and conditions were performed as shown in Table 1 below.

The polymer coating layer was coated using a bar coating method using a solution in which polyamideimide (PAI, polyamideimide) having a glass transition temperature of 272 ° C. was dissolved in DMF at a concentration of 16.6 wt%.

[Example 3]

High density polyethylene having a weight average molecular weight of 3.8 × 10 ⁵ was used. As a diluent, dibutyl phthalate and a paraffin oil having a kinematic viscosity of 160 ° C. at 40 ° C. were mixed at a ratio of 1: 2. 25 wt%, 75 wt%.

After the sheet was manufactured under the same conditions as in Example 1, the sheet was drawn using a sequential biaxial drawing machine, and drawing temperatures and draw ratios, heat setting temperatures and conditions were performed as shown in Table 1 below.

The polymer coating layer was coated using a bar coating method using a solution in which polyamideimide (PAI, polyamideimide) having a glass transition temperature of 272 ° C. was dissolved in DMF at a concentration of 16.7 wt%.

 Example 4

High density polyethylene having a weight average molecular weight of 3.0 × 10 ⁵ was used, dibutyl phthalate was used as the diluent, and the content of polyethylene and the diluent was 30 wt% and 70 wt%, respectively.

After the sheet was manufactured under the same conditions as in Example 2, the sheet was drawn using a sequential biaxial drawing machine, and drawing temperatures and draw ratios, heat setting temperatures and conditions were performed as shown in Table 1 below.

The polymer coating layer was coated using a bar coating method using a solution in which polyetherimide (PEI, polyetherimide) having a glass transition temperature of 217 ° C. was dissolved in NMP at a concentration of 4.3 wt%.

Comparative Example 1

High density polyethylene having a weight average molecular weight of 3.8 × 10 ⁵ was used. As a diluent, dibutyl phthalate and a paraffin oil having a kinematic viscosity of 160 ° C. at 40 ° C. were mixed at a ratio of 1: 2. 30 wt% and 70 wt%.

After the sheet was manufactured under the same conditions as in Example 1, the sheet was drawn using a sequential biaxial drawing machine, and drawing temperatures and draw ratios, heat setting temperatures and conditions were performed as shown in Table 2 below.

The polymer coating layer was not applied.

Comparative Example 2

High density polyethylene having a weight average molecular weight of 3.8 × 10 ⁵ was used, dibutyl phthalate was used as the diluent, and the content of polyethylene and diluent was 30% by weight and 70% by weight, respectively.

After the sheet was manufactured under the same conditions as in Example 2, the sheet was drawn using a sequential biaxial drawing machine, and drawing temperatures and draw ratios, heat setting temperatures and conditions were performed as shown in Table 2 below.

The polymer coating layer was coated using a bar coating method using a solution in which polyamideimide (PAI, polyamideimide) having a glass transition temperature of 272 ° C. was dissolved in NMP at a concentration of 8.0 wt%.

[Comparative Example 3]

High density polyethylene having a weight average molecular weight of 3.0 × 10 ⁵ was used, dibutyl phthalate was used as the diluent, and the content of polyethylene and the diluent was 25% by weight and 75% by weight, respectively.

After the sheet was manufactured under the same conditions as in Example 2, the sheet was drawn using a sequential biaxial drawing machine, and drawing temperatures and draw ratios, heat setting temperatures and conditions were performed as shown in Table 2 below.

The polymer coating layer was applied using a bar coating method using a solution in which polycarbonate (PC, polycarbonate) having a melting temperature of 231 ° C. was dissolved in dichloromethane (MC) at a concentration of 13.8 wt%.

[Comparative Example 4]

High density polyethylene having a weight average molecular weight of 3.8 × 10 ⁵ was used. As a diluent, dibutyl phthalate and a paraffin oil having a kinematic viscosity of 160 ° C. at 40 ° C. were mixed at a ratio of 1: 2. 30 wt% and 70 wt%.

After the sheet was manufactured under the same conditions as in Example 1, the sheet was drawn using a sequential biaxial drawing machine, and drawing temperatures and draw ratios, heat setting temperatures and conditions were performed as shown in Table 2 below.

Polymer coating layer consists of polycarbonate (PC, polycarbonate) and 3-methacryloxypropyltrimethoxysilane (γ-MPS) with a melting temperature of 231 ° C and silica (SiO ₂ , average particle size 800 nm) in dichloromethane (MC). It was applied using a bar coating method using a solution dissolved in a concentration of 12.9wt%, 22.1wt%.

[Comparative Example 5]

High density polyethylene having a weight average molecular weight of 3.0 × 10 ⁵ was used, dibutyl phthalate was used as the diluent, and the contents of polyethylene and diluent were 35 wt% and 65 wt%, respectively.

After the sheet was manufactured under the same conditions as in Example 2, the sheet was drawn using a sequential biaxial drawing machine, and drawing temperatures and draw ratios, heat setting temperatures and conditions were performed as shown in Table 2 below.

The polymer coating layer is a solution in which vinylidene fluoride-nuclifluoropropylene copolymer (P (VdF-co-HFP) and vinyledene floride-co-hexafluoro propylene copolymer) having a melting temperature of 160 ° C. is dissolved in NMP at a concentration of 6.2 wt%. It was applied using a bar coating method using.

Experimental conditions of the Examples and Comparative Examples and the results obtained therefrom are summarized in Tables 1 and 2 below.

[Table 1]

[Table 2]

1 is an electron micrograph (20,000 times) of the microporous membrane surface of Example 1

2 is an electron micrograph (20,000 times) of the microporous membrane surface of Example 3

3 is an electron micrograph (20,000 times) of the microporous membrane surface of Comparative Example 1

4 is an electron micrograph (20,000 times) of the microporous membrane surface of Comparative Example 3

Claims (9)

A composite microporous membrane having at least one polymer coating layer selected from one side, both surfaces, or inside a membrane of a polyolefin microporous membrane, wherein the polymer coating layer is coated along the surface shape of the polyolefin microporous membrane to form a pore structure on the surface of the polyolefin microporous membrane. Is kept open, the permeability is 1.5 × 10 ⁻⁵ Darcy or more, the melt breaking temperature is 160 ° C. or more, and the transverse / longitudinal shrinkage is 150 ° C., 40% or less for 60 minutes, and the thickness of the polymer coating layer is A polyolefin-based composite microporous membrane having a thickness of 0.5 μm or less on each side. The method of claim 1, wherein The coating layer polymer is a polyolefin-based composite microporous membrane having a melting temperature or glass transition temperature of 170 ℃ or more. The method of claim 2 The coating layer polymer is a polyolefin-based composite microporous membrane formed by coating in a solution of 1 ~ 100cps. The lithium secondary battery separator which has a composite microporous film of any one of Claims 1-3. A lithium secondary battery having the separator of claim 4. Preparing a polymer solution for forming a coating layer having a melting temperature or a glass transition temperature of 170 ° C. or higher; Preparing a microporous membrane having pores made of polyolefin; Forming a coating layer by coating the polymer solution on one or both surfaces of the microporous membrane; As a method for producing a composite microporous membrane prepared, including The coating layer of the composite microporous membrane is coated along the surface shape of the microporous membrane so that the pore structure of the surface of the microporous membrane remains open, and the permeability of the composite microporous membrane is 1.5 × 10 ⁻⁵ Darcy or more, and the melt fracture temperature Is 160 ° C. or higher, the shrinkage in the transverse direction / longitudinal direction is 150 ° C., 40% or less for 60 minutes, and the thickness of the polymer coating layer is 0.5 μm or less on each surface. The method of claim 6 Forming the coating layer is a method for producing a composite microporous membrane, characterized in that to form a coating layer by coating a polymer solution in a state of maintaining the pore structure of the polyolefin microporous membrane. The method of claim 7, The solution is a method for producing a composite microporous membrane is a solution having a viscosity of 1 ~ 100cps. The method according to any one of claims 6 to 8, The manufacturing step of the microporous membrane is (a) melting / kneading / extruding a mixture containing 20 to 50% by weight of polyolefin and 80 to 50% by weight of diluent above the phase separation temperature to produce a thermodynamic single phase in the extruder step; (b) molding the melt of the single phase into a sheet form by performing phase separation; (c) stretching the sheet prepared in step (b) by a lateral biaxial stretching method or a sequential biaxial stretching method with a transverse and longitudinal stretching ratio of 3.0 times or more, respectively; (d) extracting and drying the diluent under constant tension in the stretched film; (e) a method of manufacturing a composite microporous membrane prepared by removing the residual stress of the dried film and the like to reduce the shrinkage of the film.

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