KR100623608B1 - Polyethylene minute porosity film for battery separator and manufacturing method thereof - Google Patents

Abstract

The film prepared using the ultra high molecular weight polyethylene resin is poured into a solvent lower than the melting temperature of the ultra high molecular weight polyethylene resin to swell to induce a porous structure, and the solvent is removed from the swelled film to fix the porous structure. A method for producing a polyethylene microporous membrane for a battery separator having high porosity and high mechanical strength by axial stretching is disclosed.

Lithium Battery, Separator, Polyolefin, Ultra High Molecular Weight Polyethylene, Swelling, Boiling Point, Tecarin, Hexane

Description

Polyethylene minute porosity film for battery separator and manufacturing method thereof             

1 is a schematic diagram illustrating a manufacturing process of a polyethylene microporous membrane for a battery separator according to the present invention.

2 is a flowchart illustrating a method for producing a polyethylene microporous membrane for a battery separator according to the present invention.

3 is a view showing a porous film prepared according to the method for producing a polyethylene microporous membrane for a battery separator according to the present invention.

The present invention relates to a polyethylene microporous membrane for a battery separator and a method for manufacturing the same, and more particularly, to a battery separator for providing a porous film for a separator manufactured by using ultra-high molecular weight polyethylene having excellent mechanical properties alone. It relates to a polyethylene microporous membrane and a method of manufacturing the same.

Looking at the structure of every cell, there are six components, that is, a positive electrode, a negative electrode, a housing, a separator, an electrolyte, and a current collector. The separator is important because it allows the reversible ions to pass during charge and discharge but prevents electrons from flowing directly from the plate to the plate. The direct flow of electrons from the pole plate to the pole plate causes a short circuit of current and prevents electrons from flowing from the cathode to the anode through the circuit.

In rechargeable lithium ion batteries, which are usually formed of ultra-thin film components, it is very important that the distance between electrodes should be as short as possible to allow efficient ion transfer, but not so short that electron flow can occur.

In general, polyolefin microporous membranes have excellent safety against electrolytes and electrode active materials, and thus are used in separators of batteries, in particular, separators of lithium ion batteries and lithium ion polymer batteries, separators for large batteries of electric vehicles, and also widely used as various separators. It is applied.

Conventional polyolefin microporous membrane manufacturing method is mainly used to produce a microporous membrane by mixing the organic material and the inorganic powder, such as powdered silica and melt-molded, and then extract the organic medium and the inorganic powder. This method requires the extraction process of the inorganic material, there is a problem that it is difficult to control the permeability of the prepared membrane largely depends on the particle size of the inorganic powder.

Accordingly, a number of methods for producing high strength microporous membranes using ultra high molecular weight polyolefins as a part of components have been proposed. As an example, there is a method of forming a microporous membrane by forming a gel-like sheet from a solution in which a polyolefin composition containing ultra-high molecular weight polyolefin is heated and dissolved in a solvent, heating and stretching the gel-like sheet, and extracting and removing the solvent. .

However, in recent years, there is a demand for improving battery capacity characteristics, battery safety, and battery productivity of lithium batteries. Therefore, lowering the thickness of the ion transport resistance within the range where the safety is maintained can reduce the battery capacity characteristics. In order to secure safety in special situations such as deformation of the battery and to improve the working characteristics in the battery production process, high tensile strength and protrusion strength are required.

For this reason, it is advantageous to use a porous film made of ultra high molecular weight polyethylene having excellent mechanical properties as a battery separator. It is a reality.

Accordingly, the present invention has been made to solve such a conventional problem, and an object of the present invention is to provide an ultra-high molecular weight polyethylene microporous membrane for a battery having high mechanical properties and suitable pore characteristics and a method of manufacturing the same.                         

Another object of the present invention is to increase the volumetric energy density of the battery according to the thinning and to improve the assembly processability to prevent the short-circuit caused by contact between the positive electrode and the negative electrode of the battery, and the flammable materials used as materials of the battery It is to provide a microporous membrane and a method of manufacturing the same that can prevent accidents such as fire caused by use.

Other objects and features of the present invention will be more clearly understood through the preferred embodiments described below.

According to an aspect of the present invention, the film prepared using the ultra high molecular weight polyethylene resin is poured into a solvent lower than the melting temperature of the ultra high molecular weight polyethylene resin to swell to induce a porous structure, and remove the solvent from the swelling film to remove the porous structure After fixing, the polyethylene microporous membrane for battery separator having high porosity and high mechanical strength was prepared by biaxial stretching.

According to another aspect of the invention, the step of solid-state processing the ultra-high molecular weight polyethylene resin to produce a film having a predetermined draw ratio; Inducing a porous structure to the film by swelling the film in a first solvent which is lower than the melting temperature of the ultra high molecular weight polyethylene resin and capable of dissolving the ultra high molecular weight polyethylene resin; Fixing the porous structure by introducing the swollen film into a second solvent having a boiling point lower than that of the first solvent to replace the first solvent with a second solvent; Drying the film to remove the second solvent; And it is disclosed a method for producing a polyethylene microporous membrane for a battery separator comprising the step of stretching the film.

Preferably, the predetermined draw ratio may have a range between the maximum draw ratio such that the swellability is not lost in the swelling step and the minimum draw ratio that can sufficiently maintain the film form upon treatment with the first solvent.

In addition, the production of the film may be made through a solid state processing process at a temperature lower than the melting temperature of the ultra high molecular weight polyethylene resin.

Preferably, tekarin may be applied as the first solvent, and hexane, ethanol or water as the second solvent may be applied individually or continuously in the order of lower boiling point.

Also, preferably, the film may be tensioned such that the film does not shrink completely upon swelling with the first solvent.

Preferably, the dried film is 1.5 times or more biaxially stretched and thermally fixed in the machine direction and the transverse direction, respectively, to maintain a thickness of 25 μm or less.

Preferably, the ultrahigh molecular weight polyethylene resin has a molecular weight of 10 ⁶ or more, a melting temperature of 139 ° C to 142 ° C, a heat of fusion of 40 to 55 cal / g, a bulk density of 0.05 to 0.2 g / cc, and a pore volume of 1.8 to 5.1 cc / g.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a manufacturing process chart for manufacturing a polyethylene microporous membrane for a battery separator according to the present invention.

A raw film is manufactured through the solid state processing apparatus 10 using ultra high molecular weight polyethylene resin. Ultra-high molecular weight polyethylene (UHMWPE) applied to the manufacture of the raw film applied to the present invention refers to polyethylene having a molecular weight of 3 to 6 million.

The fabric film manufactured through the solid-state processing process the fabric film with a high boiling point solvent through a swelling & rolling process using the swelling treatment tank 20 and the washing tank 30, Subsequently replace with a low boiling solvent.

The porous film swelled through the porosity process is dried by the drying apparatus 40 to remove all solvents.

Thereafter, a biaxial stretching process using the stretching apparatus 50 is performed to increase the strength of the porous film, reduce the thickness, and obtain sufficient and uniform porosity.

2 is a flowchart for explaining a method for producing a polyethylene microporous membrane for a battery separator according to the present invention.

First, a fabric film is continuously manufactured through a solid state processing process at a temperature lower than the melting temperature of the ultra high molecular weight polyethylene resin using the ultra high molecular weight polyethylene resin (step S10).

The ultra high molecular weight polyethylene resin applied to the production of the film may be a powdered ultra high molecular weight polyethylene resin obtained directly from the reactor, and between the maximum draw ratio not to lose swelling and the minimum draw ratio to sufficiently maintain the film form when treated with an organic solvent Has a draw ratio, and the draw ratio is preferably 3 to 6.

Preferably, the ultrahigh molecular weight polyethylene resin applied to the present invention has a molecular weight (Mv) of 10 ⁶ or more, a melting temperature of 139 ° C to 142 ° C, a heat of fusion of 40 to 55 cal / g, and a bulk density of 0.05 to 0.2 g / cc, and the pore volume has a property of 1.8 to 5.1 cc / g.

The film produced through the solid state processing is put into the swelling treatment tank 20 in which the high boiling point first solvent is contained in the tensioned state to swell to form a porous structure in the film (step S20). The film is treated with an organic solvent at a temperature lower than the melting temperature of the ultrahigh molecular weight polyethylene resin, and it is preferable to apply tekarin having a high boiling point as the first solvent.

Before and after the swelling treatment tank 20, a feed roll 22 and a take-off roll 24 are installed to adjust the rotation speed. That is, when the film is put into the swelling treatment tank 20, the shrinkage occurs in the longitudinal direction while being swelled and can be shrunk up to about 60%. Therefore, in order to prevent shrinkage completely, a method of applying tension in the longitudinal direction to the film to be introduced may be considered.

To this end, by setting the rotational speed of the take-up roll 24 to be greater than 60% of the rotational speed of the feed roll 22, it is possible to apply a certain amount of tension to the film passing through the swelling treatment tank 20.

The amount of tension applied to the film is set by adjusting the shrinkage to be smaller than or equal to this in consideration of the extent to which the film is completely contracted in the swelling treatment tank 20.

In the swelling treatment tank 20, the first solvent penetrates into the inside of the film made of ultra high molecular weight polyethylene resin to swell the film.

In this way, the film is continuously passed through the swelling treatment tank 20 in which the first solvent is contained, and the film swollen by the first solvent is moved to a cooling cylinder (not shown) and cooled to facilitate winding (step S30). .

The cooled film is wound on the winding device or immediately put into the washing tank 30, and the washing tank 30 is loaded with a second solvent having a lower boiling point than the first solvent.

Like the swelling treatment tank 20, a feed roll 32 and a take-off roll 34 are provided before and after the washing tank 30, respectively, and are driven at the same rotational speed.

When the film is put into the washing tank 30, the first solvent is replaced with the second solvent to substantially remove the first solvent (step S40).

As the second solvent, a solvent having a low boiling point compatible with the first solvent is used. For example, any one of hexane, ethanol, or water may be applied or continuously applied in the order of low boiling point.

In this way, the removal of the solvent from the film is facilitated by removing the first solvent capable of dissolving the film and replacing the second solvent with a low boiling point.

The film treated with the second solvent to remove the first solvent in the washing tank 30 is dried while passing through the drying apparatus 40 to remove the second solvent (step S50). The winding roll (not shown) installed at the rear end of the drying apparatus 40 may maintain the same speed as the take-up roll 34 to prevent shrinkage of the swelling film due to drying.

Then, to increase the strength of the porous film, to reduce the thickness, and to obtain sufficient and uniform porosity, the biaxial stretching is performed by the simultaneous stretching or the successive stretching using the stretching apparatus 50 (step S60). At this time, stretching in the mechanical direction and in the transverse direction can be easily performed by using a zone heating block locally heated at 125 ° C. Moreover, it is preferable to extend | stretch and heat-fix 1.5 times or more in each direction so that thickness may be 25 micrometers or less.

In addition, by applying biaxial stretching, it is possible to reduce the stress in the other direction during the stretching of the film, thereby reducing the phenomenon that the width of the film decreases during the stretching process.

By this process it is possible to produce a high-strength porous film.

3 is a view showing a porous film prepared according to the method for producing a polyethylene microporous membrane for a battery separator according to the present invention.

When the high-strength porous film is manufactured by the process described with reference to FIG. 2, the porous film shown in FIG. 3 may be obtained.

Thickness, tensile strength, elongation, puncture strength, average pore size, porosity, Tm, and crystallinity of the ultra-high molecular weight polyethylene porous membrane The result measured over is shown in [Table 1].

division One 2 3 Thickness (㎛) 24.5 21.0 18.3 Tensile Strength (Kg / ㎠) MD 2,145 2,054 2,021 TD 550 560 530 % Elongation MD 15 16 11 TD 430 450 470 Piercing strength (g) 837 796 812 Average Pore Size (nm) 46 39 43 Porosity (%) 42.3 43.5 38.6 Tm (℃) 144.5 144.3 144.6 Crystallinity (%) 74.4 78.1 79.7

As described above, a film prepared using ultra high molecular weight polyethylene is treated with a high boiling point first solvent and a low boiling second solvent, and stretched and heat-set, thereby forming an ultra high molecular weight polyethylene porous membrane having a thickness of 25 microns or less. It can manufacture.

As described above, the polyethylene microporous membrane for battery separator prepared by the method for producing a polyethylene microporous membrane for battery separator according to the present invention has a high porosity and high mechanical strength, thereby increasing the volumetric energy density of a battery due to thinning, It is possible to improve and prevent the short-circuit phenomenon caused by the contact between the positive electrode and the negative electrode of the battery, and can be used very effectively to prevent accidents such as ignition due to misuse of the combustible materials used as the material of the battery.

Although the above has been described based on the preferred embodiment of the present invention, it can be appropriately changed or modified by those skilled in the art without departing from the scope of the present invention. It is natural that such changes or modifications fall within the scope of the present invention without departing from the spirit of the present invention. Therefore, the technical scope of the present invention should not be limited to the contents described in the detailed description of the specification but should be defined by the claims of the patent described below.

As described above, the polyethylene microporous membrane for a battery separator according to the present invention and a method for producing the same are treated with a high boiling point first solvent and a low boiling point second solvent, respectively, of a fabric film prepared using ultra high molecular weight polyethylene. Thereby, the porous film for separators which used the ultrahigh molecular weight polyethylene which processing was not suitable independently can be provided.

In addition, there is an advantage that the production of ultra-high molecular weight polyethylene microporous membrane for lithium battery having high mechanical properties and appropriate porosity.

Furthermore, because of high porosity and high mechanical strength, it is possible to increase the volumetric energy density of a battery due to the thinning and to improve the assembly processability, to prevent the short and contact of the positive and negative electrodes of the battery and to use it as a material of the battery. It can be used very effectively to prevent accidents such as ignition due to misuse of the combustible materials.

Claims (11)

Solid-state processing the ultrahigh molecular weight polyethylene resin to produce a film having a predetermined draw ratio; Inducing a porous structure to the film by swelling the film in a first solvent which is lower than the melting temperature of the ultra high molecular weight polyethylene resin and capable of dissolving the ultra high molecular weight polyethylene resin; Fixing the porous structure by introducing the swollen film into a second solvent having a lower boiling point than the first solvent and replacing the first solvent with a second solvent; Drying the film to remove the second solvent; And Method for producing a polyethylene microporous membrane for a battery separator, comprising the step of stretching the film. The method of claim 1, wherein the predetermined draw ratio is characterized in that it has a range between the maximum draw ratio such that the swellability is not lost in the swelling step and the minimum draw ratio that can sufficiently maintain the film form when treated with the first solvent. Method for producing polyethylene microporous membrane for battery separator. The ultrahigh molecular weight polyethylene resin according to claim 1 or 2, the molecular weight is 10 ⁶ or more, the melting temperature is 139 to 142 ℃, the heat of melting is 40 to 55 cal / g, the bulk density is 0.05 to 0.2 g / cc and a pore volume of 1.8 to 5.1 cc / g. A method for producing a polyethylene microporous membrane for a battery separator. The method of manufacturing a polyethylene microporous membrane for a battery separator according to claim 1 or 2, wherein the film is produced through a solid state processing process at a temperature lower than a melting temperature of the ultrahigh molecular weight polyethylene resin. The method for producing polyethylene microporous membrane for battery separator according to claim 1 or 2, wherein tekarin is applied as the first solvent. The method of manufacturing a polyethylene microporous membrane for a battery separator according to claim 5, wherein tension is applied to the film so that the film does not completely shrink when swollen by the first solvent. The method for producing a polyethylene microporous membrane for a battery separator according to claim 1 or 2, wherein hexane, ethanol or water is continuously applied as the second solvent individually or in descending order of boiling point. The polyethylene microporous material of claim 1 or 2, wherein the dried film is 1.5 times or more biaxially stretched and thermally fixed in a machine direction and a transverse direction to maintain a thickness of 25 µm or less. Method of Making Membranes. delete As ultra-high molecular weight polyethylene resin for the production of polyethylene microporous membrane for battery separator, The molecular weight is 10 ⁶ or more, the melting temperature is 139 to 142 ℃, the heat of melting is 40 to 55 cal / g, the bulk density is 0.05 to 0.2 g / cc, the pore volume is 1.8 to 5.1 cc / g Ultra high molecular weight polyethylene resin. Polyethylene microporous membrane for a battery separator produced by the method of claim 1.

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