KR101522373B1 - Method of preparing separator, separator produced by the method, and electrochemical device comprising the separator - Google Patents

Abstract

The present invention relates to a method for producing a separator, a separator prepared by the method, and an electrochemical device having the separator. More particularly, the present invention relates to an electrochemical device comprising a pore- A separation membrane produced by the method, and an electrochemical device including the separation membrane. According to an aspect of the present invention, there is provided a method for producing a polymer electrolyte membrane, comprising the steps of preparing a mixture by mixing polymer particles with a pore-forming agent, extruding the mixture through an extruder to form an extruded sheet, Removing the pore-forming agent from the formed membrane using a solvent in the extraction vessel, maintaining the pore-forming agent concentration in the solvent at 0.3 wt% or less, and thermally securing the pore- And forming a separation membrane. INDUSTRIAL APPLICABILITY According to the present invention, a separation membrane having excellent porosity and air permeability can be produced without problems of heat resistance, pore closure and thickness reduction.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a separator, a separator manufactured by the method, and an electrochemical device including the separator,

The present invention relates to a method for producing a separator, a separator prepared by the method, and an electrochemical device having the separator. More particularly, the present invention relates to an electrochemical device comprising a pore- A separation membrane produced by the method, and an electrochemical device including the separation membrane.

A secondary battery is classified into a lead acid battery, a nickel-cadmium battery, a nickel-hydrogen battery, and a lithium secondary battery, which can be used semi-permanently by continuously repeating charging and discharging using an electrochemical reaction. Among them, lithium secondary batteries are superior to other batteries in terms of high voltage and energy density characteristics, leading the secondary battery market. Depending on the types of electrolytes, lithium ion secondary batteries using liquid electrolytes and solid electrolytes Lithium-ion polymer secondary battery.

The lithium secondary battery is composed of an anode, a cathode, an electrolyte, and a separator. Among them, separator is required to separate and electrically isolate an anode and a cathode, and has high permeability (permeability) of lithium ion based on high porosity. To increase the ionic conductivity. Polyolefin-based materials, such as polyethylene (PE) and polypropylene (PP), which are advantageous for pore formation and have excellent chemical resistance, mechanical properties, and thermal properties, are mainly used as the polymer substrate of a membrane which is generally used.

Desirable characteristics of the separator for a lithium secondary battery include excellent air permeability, low heat shrinkage, and high puncture strength. However, due to the development of a high capacity and high output cell, continuous air permeability is required. At present, in order to produce a porous separator from a polyolefin, a wet process is widely used in which a polyolefin and a pore-forming agent are mixed at a high temperature, extruded, stretched, and then a pore-forming agent is extracted to form a porous separator. A method of increasing the amount of pore-forming agent (for example, diluent, plasticizer, etc.) in order to improve air permeability in a separation membrane produced by such a wet method has been used, but there is a limit to lower the strength of the separation membrane.

The present invention aims to provide a method for improving porosity and air permeability of a separation membrane without the problem of closure and thickness reduction of pores through control of production conditions of the separation membrane.

According to an aspect of the present invention, there is provided a method of preparing a polymer composition, comprising the steps of preparing a mixture by mixing polymer particles with a pore-forming agent, extruding the mixture through an extruder to form an extrusion sheet, Removing the pore-former from the formed film using a solvent in one or more extraction tanks, maintaining the concentration of the pore-forming agent in the solvent at 0.3 wt% or less, and removing the pore- And thermally fixing the removed membrane to form a porous separation membrane.

According to another embodiment of the present invention, the polymer particles may be a polyolefin-based material.

According to another embodiment of the present invention, the pore-forming agent may be at least one selected from the group consisting of an aliphatic hydrocarbon-based solvent, vegetable oil, and a plasticizer.

According to another embodiment of the present invention, in the mixing step of the polymer particles and the pore-forming agent, based on 100% by weight of the final mixture, the mixture comprises 20 to 50% by weight of the polymer particles and 50 to 50% 80% by weight.

According to another embodiment of the present invention, in the step of forming the film, uniaxial or biaxial stretching can be carried out.

According to another embodiment of the present invention, after the step of removing the pore-forming agent, the pore-forming agent may be further extracted and removed with a pure solvent.

According to another embodiment of the present invention, in the step of removing the pore-forming agent, the concentration of the pore-forming agent in the solvent may be maintained to be 0.06% by weight or less. According to another embodiment of the present invention, after the step of removing the pore-forming agent, the pore-forming agent may be further extracted and removed with a pure solvent.

INDUSTRIAL APPLICABILITY According to the present invention, a separation membrane having excellent porosity and air permeability can be produced without problems of heat resistance, pore closure and thickness reduction.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention and, together with the description of the invention given above, serve to better understand the spirit of the invention. And should not be construed as limiting.
BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a process flow diagram for the preparation of a separation membrane according to one embodiment of the present invention.
2 is a graph showing changes in porosity and air permeability according to changes in liquid paraffin (LP) concentration in methylene dichloride (MC).
3 is a SEM photograph of the sample of Example 1. Fig.
4 is an SEM photograph of the sample of Comparative Example 1. Fig.

The terms used in the specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may properly define the concept of a term to describe its invention in its best possible way And should be construed in accordance with the principles and meanings and concepts consistent with the technical idea of the present invention. Therefore, the constitution shown in the embodiments described herein is the most preferable embodiment of the present invention and does not represent all the technical ideas of the present invention, so that various equivalents It should be understood that water and variations may be present.

According to one aspect of the present invention, the concentration of the pore-forming agent in the solvent is maintained within a specified range in the extraction step in the process for preparing the separation membrane. The method for producing a separator according to an aspect of the present invention comprises the steps of (S1) preparing a mixture, (S2) forming an extruded sheet, (S3) forming a film, (S4) And forming a porous separation membrane. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a process flow diagram for the preparation of a separation membrane according to one embodiment of the present invention. Specifically, referring to FIG. 1, it is as follows.

In step S1, the kind of the polymer is selected according to a desired separation membrane, and the polymer particles thus selected are mixed with a pore-forming agent to prepare a mixture.

The polymer particles used in the present invention may be polyolefin-based polymer particles, for example, as raw material particles of a separation membrane that is provided between an anode and a cathode of a secondary battery to maintain an insulation state to prevent short-circuiting. Examples of the polyolefin-based polymer include polyethylene, such as high density polyethylene, linear low density polyethylene, low density polyethylene, ultrahigh molecular weight polyethylene, polypropylene, polybutylene and polypentene, or a combination of two or more thereof But is not limited thereto.

The pore-forming agent used in the present invention is a material which is dispersed in polymer particles and exhibits a heterogeneity of a film produced through extrusion, drawing and the like, and is subsequently removed from such a film. Therefore, the site where the pore-forming agent is located in the polymer of the membrane remains in the form of pores. The pore former is preferably a liquid material in the extrusion process, but a material that maintains a solid state may be used.

The pore-forming agent may be an aliphatic hydrocarbon-based solvent such as liquid paraffin, paraffin oil, mineral oil or paraffin wax; Vegetable oils such as soybean oil, sunflower oil, rapeseed oil, palm oil, palm oil, coconut oil, corn oil, grape seed oil, cotton seed oil and the like; Or a plasticizer such as a dialkyl phthalate. In particular, the plasticizer is selected from the group consisting of di-2-ethylhexyl phthalate (DOP), di-butyl phthalate (DBP), di- isononyl phthalate Di-isodecyl phthalate (DIDP), butyl benzyl phthalate (BBP), and the like. Of these, liquid paraffin (LP, also referred to as " liquid paraffin ") is particularly preferable.

In addition, the content of the pore-former may be 50 to 80% by weight based on the total amount of the polymer particles and the pore-forming agent. When the content of the pore-forming agent falls within this limited range, pores are produced in a desired range, for example, a pore size of about 38% or more and / or a pore size of about 20 m or less, about 400 sec / 100 cc or less. In step S2, the mixture formed in step S1 is extruded through an extruder. The extruder may be a twin-screw extruder and the temperature of the extrusion process may be approximately 160 to 240 ° C. Through this extrusion process, an extruded sheet is formed.

In step S3, the extruded sheet formed in step S2 is subjected to the stretching step. This stretching process results in the formation of a stretched film having improved mechanical strength.

The stretching process is carried out in machine direction (MD), machine direction, longitudinal direction) and / or transverse direction (TD), vertical direction. The tensile strength in the stretching direction is increased by the stretching process in all of these, or one of them. If desired, the separator of the present invention can be subjected to longitudinal (MD) stretching and / or transverse (TD) stretching alone (e.g., uniaxially stretching), simultaneously or sequentially (e.g., biaxially stretching) .

In step S4, the pore forming agent is removed from the stretched film in step S3. This removal can be accomplished using a solvent. Specifically, the pore-former present in the stretched film is removed, for example, by extraction and drying with a solvent in one or more extraction tanks used. In addition, through this removal, the space occupied by the pore former is formed as pores.

In the extraction process, a number of baths (i.e., extraction baths) are ordinarily or batchwise positioned in the art and the solvent is filled in the extraction baths. The solvent is then extracted by dissolving the pore-forming agent by immersing the previously prepared membrane in various ways. However, if multiple extraction tanks are typically used, the solvent used in the extraction tanks may be recovered for reuse. In this case, the concentration of the pore-former dissolved in the re-used solvent present in the extraction vessel can greatly affect the removal efficiency of the pore-former in the pore-forming agent removal process. Generally, the concentration of the pore-former in the solvent in the re-used extraction vessel is present in the range of about 2 to 3% by weight.

Thus, it has been found that the concentration of pore-forming agent in the solvent is advantageously kept below a certain value, and in accordance with this finding, this embodiment maintains this concentration below 0.3% by weight. If the residual pore-forming agent exceeds 0.3% by weight, physical properties of the resultant film are lowered and the permeability of the film is decreased. Further, it is more preferable that such a concentration is maintained at 0.06 wt% or less.

2 is a graph showing changes in porosity and air permeability according to changes in liquid paraffin (LP) concentration in methylene dichloride (MC). Referring to Figure 2, the porosity is about 50% for a liquid paraffin (LP) concentration in methylene dichloride (MC) of about 0.0 wt%, about 40% for about 0.2 wt%, and about 0.50 wt% , And 30%, respectively. The concentration is reduced to approximately 20% in the range exceeding 0.6% by weight. The value of the N permeability is about 200 s / 100 cc when the liquid paraffin (LP) concentration in methylene dichloride (MC) is about 0.0 wt%, about 0.2 wt% 100 cc, and in the case of about 0.50 weight%, it increased to about 500 s / 100 cc. In addition, when the content exceeds 0.6% by weight, the content thereof is rapidly increasing.

The permeability used herein refers to the time (e.g., seconds) through which 100 cc of air passes through the separation membrane with a thickness of 20 mu m and is a value representing the velocity of the electrolyte passing through the separation membrane, (Or discharge rate) in the cell performance, that is, the rate at which ions in the separator reach the both electrodes through the separation membrane, that is, expressed as a unit of s / 100cc. In the meantime, the N permeability used herein refers to the air permeability of the separation membrane corrected to a thickness of 20 탆, and further N is represented by the following formula:

N. permeability = permeability x 20 μm / thickness

The solvent usable in the present invention is not particularly limited and any solvent capable of extracting the pore forming agent used for extrusion can be used. Preferably, however, methyl ethyl ketone, methylene chloride ), Hexane and the like are suitable.

The extraction method can be used in combination with all common solvent extraction methods such as immersion method, solvent spray method, ultrasonic method and the like. Preferably, the solvent may be methylene chloride, such as methylene dichloride (MC).

In another embodiment of the present invention, after the step of removing the pore-forming agent, the remaining pore-forming agent may be further removed by further extraction with a pure solvent. For this additional removal step, the concentration of pore-forming agent in the solvent is maintained at 0.06 wt% or less. Due to the addition of the removal step, the physical properties, porosity and air permeability of the finally produced separation membrane can be further improved.

In step S5, the pore former-removed film in step S4 is heat-set by heat treatment. This heat fixation results in the formation of a porous separation membrane.

The pore former-removed membrane is finally heat set to obtain a membrane having the desired porosity and air permeability. In particular, the membrane from which the pore former has been extracted in one or more extraction tanks, e. G., The dried and pore-former removed film, is heat set to reduce the final film shrinkage by removing residual stresses.

The heat fixation is to fix the membrane for a certain period of time in the state of being subjected to the tension at the temperature below the melting point of the polymer to be used and to force the membrane to be shrunk by applying heat to remove the residual stress. Heat setting is advantageous for lowering the shrinkage rate at a high temperature, but if it is too high, the membrane is partially melted. Such a heat setting temperature may be, for example, about 110 to about 135 캜.

The separator according to one aspect of the present invention thus manufactured can be usefully used as a separation membrane of an electrochemical device, that is, a separator interposed between an anode and a cathode.

The electrochemical device according to one aspect of the present invention includes all devices that perform an electrochemical reaction, and specific examples thereof include capacitors such as all types of primary cells, secondary cells, fuel cells, solar cells, or super capacitor devices ). In particular, a lithium secondary battery including a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, or a lithium ion polymer secondary battery may be included in the secondary battery.

The electrode to be applied together with the separator according to an aspect of the present invention is not particularly limited, and the electrode active material may be bound to an electrode current collector according to a conventional method known in the art. Examples of the cathode active material include, but are not limited to, lithium manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium iron oxide, or a combination thereof A lithium complex oxide may be used. As a non-limiting example of the negative electrode active material, a conventional negative electrode active material that can be used for a negative electrode of an electrochemical device can be used. In particular, lithium metal or a lithium alloy, carbon, petroleum coke, activated carbon, Lithium adsorbing materials such as graphite or other carbon-based materials and the like can be used. Non-limiting examples of the positive current collector include aluminum, nickel, or a combination thereof. Examples of the negative current collector include copper, gold, nickel, or a copper alloy or a combination thereof Foil to be manufactured, and the like.

The electrolytic solution which can be used in the electrochemical device according to one aspect of the present invention is a salt having a structure such as A ⁺ B ^- , wherein A ⁺ is an alkali metal cation such as Li ⁺ , Na ⁺ , K ⁺ And B ^- is a metal ion such as PF ₆ ^- , BF ₄ ^- , Cl ^- , Br ^- , I ^- , ClO ₄ ^- , AsF ₆ ^- , CH ₃ CO ₂ ^- , CF ₃ SO ₃ ^- , N (CF ₃ SO ₂ ) _{^{2 -, C (CF 2 SO}} 2) 3 - anion, or a salt containing an ion composed of a combination of propylene carbonate (PC) such as, ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC ), Dimethyl carbonate (DPC), dimethylsulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (G-butyrolactone), or a mixture thereof. However, it is not limited thereto. It is not.

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

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

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the above-described embodiments. The embodiments of the present invention are provided so that those skilled in the art can explain the present invention more clearly and completely.

[Example 1]

The polymer particle starting material used is high-density polyethylene having a weight average molecular weight of 500,000 and a melting temperature of 135 占 폚. The pore former used is liquid paraffin (LP) having a kinematic viscosity at 40 DEG C of 40 cSt. The content ratio of the polyethylene and the pore-forming agent is 30 wt% and 70 wt%, respectively.

The mixture of the two components is extruded at 220 DEG C using a twin-screw extruder to obtain an extrudate. The resulting extrudate is passed through a T-die and then passed through a cooling roll to produce an extruded sheet. The melt temperature of the extruded sheet produced is 118 캜. The extruded sheet was stretched 6 times in the longitudinal direction and 7 times in the transverse direction using a biaxial stretching machine. The longitudinal drawing temperature is 115 캜, and the transverse drawing temperature is 123 캜. In the stretched sheet, liquid paraffin (LP) was removed using pure methylene dichloride (MC) as a solvent, and heat set at 128 DEG C for 30 seconds to obtain a final separator. The physical properties of the obtained membranes are shown in Table 1 below. An SEM photograph of the sample of Example 1 is shown in Fig.

[Example 2]

A separator was prepared under the same conditions as in Example 1 except that liquid paraffin (LP) was removed from the stretched sheet such that the concentration of liquid paraffin (LP) in methylene dichloride (MC) was 0.06 wt%. The physical properties of the obtained membranes are shown in Table 1 below.

[Example 3]

(LP) was removed from the stretched sheet such that the concentration of liquid paraffin (LP) in methylene dichloride (MC) was 0.06% by weight, and then the liquid paraffin (LP) LP) was extracted with liquid paraffin (LP) so that the concentration of liquid paraffin (LP) in methylene dichloride (MC) was 0.0 wt%. The physical properties of the obtained membranes are shown in Table 1 below.
[Example 4]
A separator was prepared under the same conditions as in Example 1 except that liquid paraffin (LP) was removed from the stretched sheet so that the concentration of liquid paraffin (LP) in methylene dichloride (MC) was 0.3 wt%. The physical properties of the obtained membranes are shown in Table 1 below.

[Comparative Example 1]

A separator was prepared under the same conditions as in Example 1 except that liquid paraffin (LP) was removed from the stretched sheet such that the concentration of liquid paraffin (LP) in methylene dichloride (MC) was 0.57% by weight. The physical properties of the obtained membranes are shown in Table 1 below. An SEM photograph of the sample of Comparative Example 1 is shown in Fig.

Concentration of LP in MC Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Thickness (㎛) 19.4 16.6 18.5 16.8 16.6 Porosity (%) 48.5 44.2 51.6 38 35.1 Ventilation
(permeability) 203.9 200.4 185.0 334.3 356.8 Ventilation
(N, permeability) 209.8 241.4 203.3 398 429.2

Referring to Table 1, when the LP concentration in the solvent MC exceeds 0.3 wt%, it can be seen that the pores formed by the extraction of the pore-forming agent are closed during the heat-setting process and the properties of the separator membrane and the air permeability are lowered there was.

Claims (9)

Mixing the polymer particles with a pore-forming agent to prepare a mixture,
Extruding the mixture through an extruder to form an extruded sheet,
Stretching the extruded sheet to form a film,
Removing the pore-former from the formed film using a solvent in one or more extraction tanks, maintaining the concentration of the pore-forming agent in the solvent at 0.3 wt% or less, and
And forming the porous separation membrane by thermally fixing the pore former-removed membrane. The method according to claim 1,
Wherein the polymer particles are polyolefin-based materials. The method according to claim 1,
Wherein the pore-forming agent is at least one selected from the group consisting of aliphatic hydrocarbon-based solvents, vegetable oils, and plasticizers. The method according to claim 1,
Characterized in that, in the mixing step of the polymer particles and the pore-forming agent, the mixture comprises 20 to 50% by weight of the polymer particles and 50 to 80% by weight of the pore-forming agent, based on 100% Gt; The method according to claim 1,
Wherein the film is uniaxially or biaxially stretched in the step of forming the film. The method according to claim 1,
Further comprising the step of further extracting and removing the pore-forming agent with a pure solvent after the step of removing the pore-forming agent and before the step of fixing the heat. The method according to claim 1,
Wherein the concentration of the pore-forming agent in the solvent is maintained at 0.06 wt% or less in the step of removing the pore-forming agent. A separator produced by the production method according to any one of claims 1 to 7. An electrochemical device comprising an anode, a cathode, and a separator interposed between the anode and the cathode, wherein the separator is the separator according to claim 8.

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