KR101692411B1 - Method for preparing porous separator using foaming agent - Google Patents

Abstract

The present invention relates to a method for producing a porous separator, and more particularly, to a method for producing a porous separator using a foaming agent. According to an aspect of the present invention, there is provided a process for preparing a polymer solution, which comprises dissolving a high-temperature-resistant polymer having a melting point (T _m ) of 200 ° C or higher and a foaming agent in a polar solvent to form a polymer solution, applying the polymer solution to a support, Forming a polymer film, and heating the polymer film to a temperature not lower than the decomposition temperature of the foaming agent to decompose and remove the foaming agent to form a porous separation membrane. According to the present invention, the pores of the separation membrane are evenly distributed from the center to the surface, particularly in the cross section, so that a porous separation membrane having excellent porosity and air permeability can be produced.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a porous separator using foaming agent,

The present invention relates to a method for producing a porous separator, and more particularly, to a method for producing a porous separator using a foaming agent.

Recently, interest in energy storage technology is increasing. As the application fields of cell phones, camcorders, laptops and even electric vehicles are expanded, efforts for research and development of electrochemical devices are becoming more and more specified. The electrochemical device is one of the most remarkable fields in this respect, and the development of a rechargeable secondary battery has become a focus of attention.

Among the currently applied secondary batteries, the lithium secondary batteries developed in the early 1990's are attracting attention because they have higher operating voltage and higher energy density than conventional batteries such as Ni-MH.

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.

However, unlike conventional polyolefin materials, a separator using a high heat-resistant polymer having a relatively high glass transition temperature or melting point as a substrate has problems in that when pores are formed by using a pore-forming agent as in the prior art, Poor formation of the pores and nonuniform distribution of the pores.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a porous separation membrane in which pores are uniformly formed on the surface of a high heat-resistant polymer separation membrane and a cross-sectional center thereof.

According to an aspect of the present invention, there is provided a method of preparing a polymer solution, comprising: forming a polymer solution by dissolving a high-temperature-resistant polymer having a melting point ( _Tm ) of at least 200 ° C and a foaming agent in a polar solvent; Drying the polar solvent to form a polymer film; and heating the polymer film to a temperature not lower than the decomposition temperature of the foaming agent to decompose and remove the foaming agent to form a porous separation membrane. / RTI >

According to the present invention, the pores of the high-heat-resistant polymer separation membrane are uniformly distributed from the center to the surface, particularly in the cross section thereof, and thus can have excellent porosity and air permeability.

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 FIG. 1 is a process flow chart for manufacturing a porous separation membrane according to one embodiment of the present invention. FIG.
2 is a SEM photograph of a pre-foaming film produced according to one embodiment of the present invention.
3 is a SEM photograph of a film after foaming manufactured according to one embodiment of the present invention.
Figs. 4 and 5 are SEM photographs of a surface contacting with the outside air and a portion contacting the glass plate in Comparative Example 1 of the present invention, respectively.

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.

The present invention is characterized in that a foaming agent is used in a method for producing a porous separator for a secondary battery, and then a plurality of pores are formed in the separator by its decomposition, thereby obtaining a porous separator.

In the physical properties of the porous separator of the present invention, the permeability used refers to the time (e.g., second) (Gurley value) through which 100 mL of air passes for a separation membrane having a thickness of 20 mu m, (Or discharge rate) of the cell performance, that is, the rate at which ions in a certain amount of electrolyte reach the both electrodes through the separator, that is, s / 100 mL. < / RTI >

The method for manufacturing a porous separation membrane according to one aspect of the present invention includes the steps of (S1) forming a polymer solution, (S2) forming a polymer film, and (S3) forming a porous separator.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process flow chart for manufacturing a porous separation membrane according to one embodiment of the present invention. FIG. Specifically, referring to FIG. 1, it is as follows.

In step S1, a high-temperature-resistant polymer and a foaming agent are dissolved in a polar solvent to form a polymer solution.

The high heat-resistant polymer is a raw material of a separator which is provided between an anode and a cathode of a secondary battery so as to maintain an insulation state to prevent a short circuit. The type of the polymer is selected according to a desired separation membrane. T _m ) is a high heat-resistant polymer of about 200 ° C or more.

As described above, the high-temperature-resistant polymer having a high melting point is exposed to an abnormally high temperature condition due to external impact, internal failure, or the like, and the separation membrane containing the high-heat-resistant polymer as a separation membrane substrate and the secondary battery having the separation membrane, Since the substrate has a relatively high ability to withstand such high temperatures (e.g., glass transition temperature, melting point), it will not easily recede or melt, which will be greatly advantageous in terms of the structural stability of the separator and thus the stability of the secondary battery.

The high heat-resistant polymer that can be used in the present invention is not particularly limited as long as it is a polymer having a melting point of about 200 DEG C or higher. Typical examples thereof include engineering plastic (EP) such as polyester resin, polyamide ) Based resin, a polyimide (PI) based resin, a fluorine resin, and the like.

Preferable examples of the high heat-resistant polymer include polyimide (PI) having a glass transition temperature of about 400 ° C. or higher, polyetheretherketone (PEEK) having a glass transition temperature of about 143 ° C., melting point of about 343 ° C., polyetherimide polyetherimide (PAI) at about 274 ° C, polysulfone (PSF) at about 190 ° C, polyarylsulfone, polyetherimide, polyetherimide (PEI) (Glass transition temperature: about 230 ° C), polyethersulfone (PES) having a glass transition temperature of about 225 ° C, polyphenylene oxide (PPO) having a glass transition temperature of about 215 ° C, polytetrafluoroethylene polytetrafluoroethylene and PTFE having a glass transition temperature of about -73 DEG C and a melting point of about 327 to 335 DEG C, perfluoroalkoxy (PFA) having a melting point of about 300 DEG C, fluorinated ethylene propylene (FEP) having a melting point of about 250 DEG C, , Ethylene tetraphenyl (Glass transition temperature: about 147 to 150 DEG C, melting point: about 155 to 230 DEG C), polyglycolic acid (PGA), glass transition temperature about 35 to 40 ℃, melting point (T _m) of about 225 to about 230 ℃), polyethylene terephthalate (polyethylene terephthalate, PET; glass transition temperature of about 70 ℃, melting point about 265 ℃), polybutylene terephthalate (polybuthylene terephthalate, PBT having a glass transition temperature of about 50 캜 and a melting point of about 245 캜), polyphenylenesulfide (PPS) having a glass transition temperature of about 90 캜 and a melting point of about 280 캜, polyethylene naphthalene (PEN), and polyacetal ), And mixtures of two or more thereof.

More preferably, the high heat-resistant polymer is one or a mixture of two or more selected from the group consisting of polyetherimide (PEI) and polyamideimide (PAI).

A foaming agent or blowing agent refers to a material that forms bubbles. The blowing agent used in the present invention is dispersed in the high-temperature-resistant polymer, dissolved in a polar solvent, and applied on a support in the form of a polymer solution. This blowing agent then exhibits a heterogeneity of the finally formed film together with the polar solvent which is decomposed per se, for example by drying, and is subsequently removed from such a film. Therefore, according to one embodiment of the present invention, a portion where the foaming agent is located in the high-temperature-resistant polymer of the membrane (separation membrane) remains in the form of a plurality of pores in the membrane together with a site where the solvent is located.

Conventionally, pore-formers such as aliphatic hydrocarbon solvents, vegetable oils, plasticizers and the like which have been conventionally used in the art are incorporated into the high-temperature-resistant polymer at the initial stage of the process and then removed through solvent extraction or the like to remove the pore- Pores are formed on the surface of the membrane, which is formed by drying the high heat-resistant polymer solution, that is, the pore formation near the surface of the separation membrane is not satisfactory.

Unlike the pore-forming agent used in the prior art, the foaming agent used in the production of the high-heat-resistant polymer separator of the present invention is excellent in the inside and the outside of the separator, particularly in the thickness direction, Forming pattern.

The high heat-resistant polymer selected in consideration of the stability of the secondary battery in the method of solution casting or the like commonly used in the production process of the porous separator is particularly soluble in a polar solvent having a high boiling point, It is extremely difficult to form a large number of pores which are uniformly distributed evenly to the surface of the separation membrane in addition to the central region of the finally formed separation membrane.

The present inventors have recognized this problem and have succeeded in producing a separator having excellent porosity by uniformly distributing the pores of the final separation membrane by using a foaming agent as a high-temperature-resistant polymer together with a polar solvent instead of the conventional pore-forming agent.

Examples of the foaming agent include azo compounds, urea compounds, carbonate salts, hydrazide compounds, carbazide compounds, nitrile compounds and amine compounds. , Or a mixture of two or more thereof.

Among them, azodicarbonamide (ADCA; decomposition temperature: about 200 to about 210 DEG C, gas evolution amount of about 190 to about 210 mL / g), sodium hydrogencarbonate, potassium hydrogencarbonate, ammonium carbonate, benzenesulfonyl Hydrazide, toluene sulfonyl hydrazide (TSH, decomposition temperature: about 103 to about 107 DEG C; gas generation amount about 108 to about 124 mL / g), 4,4'-oxydibenzenesulfonyl hydrazide , 4'-oxydibenzene sulfonyl hydrazide, OBSH, decomposition temperature of about 155 to about 160 DEG C, gas evolution of about 106 to about 116 mL / g), semicarbazide, carbazide, azobisformamide, azobisisobutyronitrile azobis isobutyro nitrile, AIBN having a decomposition temperature of about 90 to about 120 DEG C, a gas generation amount of about 130 to about 145 mL / g), dinitrosopenta methylene tetramine (DPT; G., From about 232 to about 252 mL / g), and Diazoaminobenzene, and mixtures of two or more thereof.

The content of the blowing agent may be about 5 to about 50 parts by weight, or about 10 to about 30 parts by weight based on 100 parts by weight of the high heat-resistant polymer. If the content of the foaming agent is within this limited range, the desired porosity pattern as described above can be achieved in the separation membrane, and the desired degree of porosity and permeability, such as porosity of about 30% or more and / The ventilation rate is about 5,000 sec / 100cc or less.

The solvent usable in the present invention is not particularly limited and is not particularly limited as long as it is a solvent capable of dissolving the high heat-resistant polymer and the foaming agent. This solvent can be used every day for polarity. Non-limiting examples of such polar solvents include acetone, chloroform, tetrahydrofuran (THF), dioxane, monoglyme, diglyme, N-methyl-2-pyrrolidone (NMP ), Dimethylformamide (DMF), dimethylsulfoxide (DMSO) and dimethylacetamide (DMAC), and the like.

In step S2, the polymer solution formed in step S1 is applied on a support. The support serves as a template or support capable of forming a film form thereon. The support is not particularly limited as long as such a mold or supporting function is possible and its physical or chemical transformation is not effected at the decomposition temperature of the foaming agent and at the temperature at which the polar solvent is applied at the time of drying. For example, such supports may be made of, but not limited to, glass, metal (e.g., iron, steel), high melting point plastics, etc. and may be used in the form of plates or frames.

The polymer solution is applied to such a support and dried to form a polymer film. This drying step is carried out through a drying device such as an oven, and the temperature and time of the drying used here depend on the high heat-resistant polymer, foaming agent and solvent used, but the solvent is removed and the film is finally formed Temperature and time are not limited. Typically, the drying temperature is from about 50 to about 150 캜, or from about 80 to about 100 캜, and the drying time can be from about 10 to about 120 minutes, or from about 20 to about 60 minutes. Further, the polymer film dried and formed may have a thickness of usually about 15 to about 25 mu m.

Further, in the step S2, the stretching process may be selectively carried out, and the stretching process may be performed in a machine direction (MD), a machine direction, a longitudinal direction) and / or a transverse direction (TD) . 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 S3, the foaming agent is removed from the polymer film formed in step S2. The foaming agent present in the polymer film is removed, for example, by heating and drying. By this removal step, the space occupied by the foaming agent is formed as pores to form a porous separation membrane having a plurality of pores.

The step of removing the foaming agent is carried out through a heating / drying apparatus such as an oven in a similar manner to the step of removing the solvent to form a film form, and the temperature and time of heating / drying depend on the high heat- And is not particularly limited as long as it is a temperature and a time at which the foaming agent is decomposed to form a gas to have a porous membrane form. Typically, the heating / drying temperature may be from about 100 to about 300 占 폚, or from about 125 to about 175 占 폚, and the heating / drying time may be from about 10 to about 120 minutes, or from about 20 to about 60 minutes.

In another embodiment of the present invention, after the step of removing the foaming agent, the porous separator may be heat-set by further heat treatment. By this heat setting, the porous separator having the desired porosity and air permeability can be finally obtained as a stable structure. In particular, the porous separator obtained by extracting the blowing agent and the solvent by the foregoing process can largely or completely remove the residual residual stress by the heat fixation process, thereby greatly reducing the shrinkage ratio of the final porous separator.

This heat fixation fixes the porous separator for a certain period of time in the state of being subjected to the tension at a temperature not higher than the melting point of the high heat-resistant polymer to be used and forcibly retains the porous separator to be shrunk by removing heat to remove the residual stress. Heat fixation is advantageous for lowering the shrinkage rate at a high temperature, but if it is too high, the porous separator melts partially and thus a large number of pores formed are clogged, resulting in a decrease in air permeability. Such a heat setting temperature may be, for example, about 100 to about 200 캜.

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

The secondary battery according to one aspect of the present invention may include 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.

The electrode to be applied together with the porous separator according to one aspect of the present invention is not particularly limited, and electrode active materials 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 Lithium composite 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 a secondary battery can be used. In particular, lithium metal or a lithium alloy, carbon, petroleum coke, activated carbon, (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 electrolyte which can be used in the secondary battery 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 ⁺ It includes and B ^- is _{^{PF 6 -, BF 4 -,}} Cl -, Br -, I -, ClO 4 -, AsF 6 -, CH 3 CO 2 -, CF 3 SO 3 -, N (CF 3 SO 2) 2 _{^{-, C (CF 2 SO 2}} ) 3 - the salt containing ions consisting of the anionic, or a combination thereof, such as propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC) , Dipropyl carbonate (DPC), dimethylsulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate Lactone (g-butyrolactone), or an organic solvent composed of a mixture thereof, but is limited thereto No.

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

As a process for applying the porous 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 high heat-resistant polymer and foaming agent used are polyamideimide (PAI; trade name Torlon 4000T, T _g about 270 ° C., T _m about 400 ° C.) and azodicarbonamide (ADCA; decomposition temperature about 120 to about 126 C; CELLCOM-F from CELLCOM). About 100 parts by weight of the polyamideimide and about 20 parts by weight of the azodicarbonamide were mixed to form a mixture.

The mixture was dissolved in dimethyl formamide (DMF) as a polar solvent at a weight ratio of about 1:10 to form a polymer solution. The polymer solution was cast on a glass plate, placed in an oven at about 100 ° C, and dried for about 30 minutes to obtain a polymer film.

Again, it was placed in an oven at about 150 ° C, and azodicarbonamide was decomposed at a high temperature to generate a gas, thereby preparing a porous separation membrane from which the azodicarbonamide was removed. The physical properties of the prepared porous separator are shown in Table 1 below.

2 and 3 are SEM photographs of the pre-foaming and post-foaming films produced according to Example 1 of the present invention, respectively. In Fig. 2, the arrow indicates the foaming agent, and the circle in Fig. 3 indicates the portion where the foaming agent has escaped after foaming to form pores.

The Gurley values of the porous separator prepared according to Example 1 of the present invention were approximately infinite (∞) and about 300 s / 100 mL before and after foaming, respectively.

 [Comparative Example 1]

A separator was prepared under the same conditions as in Example 1 except that polyvinyl alcohol (PVA), which is a pore former, was used in place of the foaming agent.

Specifically, polyamideimide (PAI) and polyvinyl alcohol (PVA) were mixed in a weight ratio of about 80:20, and the mixture was mixed with dimethyl formamide (DMF) in an amount of about 1 : ≪ / RTI > 10 parts by weight to form a polymer solution. The resulting polymer solution was cast on a glass plate, placed in an oven at about 100 캜, and dried for about 30 minutes to obtain a polymer film. Thereafter, the polymer film was immersed in water to extract polyvinyl alcohol (PVA) to obtain a porous separator. The physical properties of the porous separator are shown in Table 1 below.

Figs. 4 and 5 are SEM photographs of a surface contacting with the outside air and a portion contacting the glass plate in Comparative Example 1 of the present invention, respectively. In the case of the surface that meets the outside air, no traces of pores can be found, and it can be confirmed that the asymmetric structure with pores is formed due to the extraction of the pore forming agent at the part contacting with the glass plate.

Example 1 Comparative Example 1 Thickness (㎛) 20 20 Porosity (%) 20 20 Air permeability (Gurley, s / 100 mL) 300 ∞

The problem with the method of preparing the separator by extracting the pore-forming agent is that since the high-heat-resistant polymer is soluble only in a polar solvent having a high boiling point, the drying rate of the solvent is slow, and the pore- Due to this problem, it is not easy to achieve the desired permeability of the membrane due to the asymmetric structure formed in the method using the pore former. On the other hand, in the case of using the foaming agent as described above in the present invention, the asymmetric structure is alleviated due to the decomposition and foaming of the foaming agents distributed at the inside and the vicinity of the surface in contact with the air at high temperature.

Claims (10)

Dissolving a mixture of a high heat resistant polymer having a melting point (T _m ) of 200 ° C or higher and a foaming agent in a polar solvent to form a polymer solution,
Applying the polymer solution to a support, drying and removing the polar solvent to form a polymer film,
Heating the polymer film to a temperature not lower than the decomposition temperature of the foaming agent to decompose and remove the foaming agent to form a porous separation membrane, and
And separating the formed porous separation membrane from the support. The method according to claim 1,
Wherein the high heat resistant polymer is selected from the group consisting of polyimide (PI), polyetheretherketone (PEEK), polyetherimide (PEI), polyamideimide (PAI), polysulfone (PES), polyether sulfone (PES), polyphenylene oxide (PPO), polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA) Polypropylene, fluorinated ethylene propylene (FEP), ethylenetetrafluoroethylene (ETFE), polycarbonate (PC), polyglycolic acid (PGA), polyethylene terephthalate But are not limited to, polybutylene terephthalate (PBT), polyphenylenesulfide (PPS), polyethylene naphthalene, PEN), and polyacetal. ≪ RTI ID = 0.0 > 11. < / RTI > 3. The method of claim 2,
Wherein the high-temperature-resistant polymer is one or a mixture of two or more selected from the group consisting of polyetherimide (PEI) and polyamideimide (PAI). The method according to claim 1,
Wherein the foaming agent is selected from the group consisting of azo, urea, carbonate salt, hydrazide, carbazide, nitrile, and amine And a mixture of two or more selected from the above. 5. The method of claim 4,
Wherein the foaming agent is selected from the group consisting of azodicarbonamide (ADCA), sodium hydrogencarbonate, potassium hydrogencarbonate, lithium carbonate, ammonium carbonate, benzenesulfonyl hydrazide, toluene sulfonyl hydrazide (TSH) (4,4'-oxydibenzene sulfonyl hydrazide, OBSH), semicarbazide, carbazide, azobis formamide, azobis isobutyro nitrile (AIBN), dinitroso Wherein the porous separator is a mixture of at least one selected from the group consisting of dinitrosopenta methylene tetramine (DPT) and diazoaminobenzene. The method according to claim 1,
Wherein the foaming agent is contained in an amount of 5 to 50 parts by weight based on 100 parts by weight of the high-heat-resistant polymer. The method according to claim 1,
Wherein the polar solvent is selected from the group consisting of acetone, chloroform, tetrahydrofuran (THF), dioxane, monoglyme, diglyme, N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylsulfoxide (DMSO), and dimethylacetamide (DMAC). 2. The method for producing a porous separation membrane according to claim 1, delete delete delete

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