KR101529210B1 - Separator for lithium secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

Disclosure of the Invention The present invention relates to a polymer electrolyte membrane which exhibits excellent uptake of an electrolyte composed of an electrolyte and a lithium salt and exhibits excellent cell stability because the electrolyte is absorbed and swollen inside the polymer chain, And a lithium secondary battery comprising the separator.
The separator for a lithium secondary battery includes a film of an acrylamide polymer containing one or more predetermined acrylamide repeating units and having a plurality of pores having a diameter of 2.0 to 10.0 nm.

Description

TECHNICAL FIELD [0001] The present invention relates to a separator for a lithium secondary battery, and a lithium secondary battery including the separator.

The present invention relates to a separator for a lithium secondary battery and a lithium secondary battery including the same. More specifically, the present invention relates to a separator for a lithium secondary battery, which exhibits excellent uptake performance for an electrolyte, exhibits excellent stability due to less leakage of an electrolyte, exhibits suitable mechanical properties and conductivity as a separator, To a lithium secondary battery.

Cells generate electricity by using materials that can electrochemically react to the positive and negative electrodes. A representative example of such a battery is a lithium secondary battery that generates electrical energy by a change in chemical potential when the lithium ions are intercalated / deintercalated in the positive electrode and the negative electrode.

The lithium secondary battery can be obtained by using a material capable of reversible intercalation / deintercalation of lithium ions as a positive electrode and a negative electrode active material, and an electrolyte and a separator interposed between the positive electrode and the negative electrode.

Among them, the separator has a great influence on the stability and performance of the lithium secondary battery, and the porous polyethylene film is most widely used as the separator. Such a porous polyethylene film not only exhibits excellent permeability to an electrolyte but also exhibits excellent ionic conductivity and mechanical properties, and thus can be used as a separator of a lithium secondary battery.

However, the polyethylene-based film needs to be produced in the form of a porous film so as to exhibit excellent permeation performance to an electrolyte. For this purpose, the polyethylene film is subjected to processes such as controlling the crystal structure of the polymer, incorporating and extracting the plasticizer, and then, through a uniaxial or biaxial stretching process or the like, micropores having a diameter of several hundred nm or less are formed Have been produced and used in the form of porous films.

However, due to the presence of micropores having a relatively large diameter, problems such as leakage of the electrolyte from the separation membrane of the porous polyethylene film during use of the lithium secondary battery have been caused. Particularly, lithium dendrite generated at the cathode due to incomplete charge / discharge behavior of the battery grows through the pores of the separator to reach the anode, and ultimately leads to an internal short-circuit It is known to cause. The internal short circuit caused by the lithium dendrite is considered to be the biggest problem that threatens the safety of the battery, and control of the pore structure of the separator is attracting attention as a very important matter.

In recent years, various battery manufacturers have actively developed a membrane having a composite membrane structure in which a porous ceramic layer is applied to a polyolefin-based membrane, in an effort to suppress such short-circuiting within the battery. Particularly, the present applicant has developed a safety-reinforcing separator (SRS), which is a product name, and mass-applied it to an electric vehicle battery.

Meanwhile, in order to fundamentally solve the problems of the porous polyolefin-based film, attempts have been made to use a gel polymer material or the like. However, unlike a porous polyolefin-based film, such a gel-like polymer material does not have a microporous structure and exhibits characteristics much lower than that of the porous polyethylene film in properties such as electrolyte uptake and ionic conductivity of the electrolyte It is true that we have shown. In addition, due to the liquid electrolyte impregnated in the gel polymer material for the purpose of improving the ionic conductivity and the like, the mechanical strength is remarkably lowered and can be used in a battery assembling process including a roll-to-roll process There is no limit.

Due to such a problem, development of a material which can be applied as a separation membrane of a lithium secondary battery in place of a porous polyethylene film and which can prevent the leakage of an electrolyte and inhibit the growth of lithium dendrites and improve the safety of the battery is continuously required .

Accordingly, it is an object of the present invention to provide a separation membrane for a lithium secondary battery, which exhibits excellent impregnation rate for an electrolyte, exhibits excellent safety due to less leakage of an electrolyte, and exhibits suitable mechanical properties and conductivity as a separation membrane.

The present invention also provides a lithium secondary battery including the separator.

The present invention provides a separator for a lithium secondary battery comprising a film of an acrylamide polymer comprising a plurality of pores each having a diameter of 2.0 to 10.0 nm and comprising at least one repeating unit represented by the following formula 1:

 [Chemical Formula 1]

Wherein n is an integer from 15 to 1800, R is hydrogen or methyl,

,

,

또는

이고, R 'is selected from X,

,

,

or

ego,

X is -ZR ", Y is alkylene having 1 to 10 carbon atoms, Z is arylene having 6 to 20 carbon atoms, and R "is a linear or branched hydrocarbon having 10 to 20 carbon atoms or a linear or branched hydrocarbon having 10 to 20 carbon atoms Or branched perfluorohydrocarbons.

The present invention also provides a positive electrode comprising a positive electrode active material of a metal oxide; A negative electrode comprising a negative electrode active material of graphite or lithium metal oxide; The separator interposed between the anode and the cathode; And a non-aqueous electrolyte interposed between the positive electrode and the negative electrode, the non-aqueous electrolyte including an ionizable lithium salt and an organic solvent.

Hereinafter, a separator for a lithium secondary battery and a separator for a lithium secondary battery including the same according to a specific embodiment of the present invention will be described in detail.

According to one embodiment of the present invention, there is provided a separator for a lithium secondary battery comprising a film of an acrylamide polymer comprising a plurality of pores each having at least one kind of repeating units represented by the following formula (1) and having a diameter of 2.0 to 10.0 nm do:

 [Chemical Formula 1]

Wherein n is an integer from 15 to 1800, R is hydrogen or methyl,

,

,

또는

이고, R 'is selected from X,

,

,

or

ego,

X is -ZR ", Y is alkylene having 1 to 10 carbon atoms, Z is arylene having 6 to 20 carbon atoms, and R "is a linear or branched hydrocarbon having 10 to 20 carbon atoms or a linear or branched hydrocarbon having 10 to 20 carbon atoms Or branched perfluorohydrocarbons.

The present inventors have found that, when a film is provided with a specific acrylamide polymer, it exhibits an excellent impregnation rate with respect to the electrolyte due to its mesoporosity, but also because the relatively fine mesopores are formed due to the properties of the polymer itself And the electrolyte is less likely to leak when used as a separator of a lithium secondary battery, and thus the present invention has been completed.

The electrolytic impregnation rate and safety of the film of the acrylamide polymer and the separator containing the same are basically due to the characteristics such as the mesoporosity of the acrylamide polymer. The acrylamide-based polymer is prepared by radical polymerization (for example, RAFT polymerization) of a predetermined acrylamide monomer (monomer of the following formula 2; the same applies hereinafter) by a specific method. Meso-porosity including many mesopores without chemical treatment of the acrylamide-based polymer. Particularly, the film of the acrylamide-based polymer may have a large number of particles having a diameter of about 2.0 to 10.0 nm Of mesopores of mesopores.

The acrylamide-based monomer used in the production of the acrylamide-based polymer is prepared by copolymerizing a non-polar aliphatic hydrocarbon long-chain branch (having a carbon number of 10 or more) capable of self-assembly with an arylene group causing an interaction of? Or an amide group capable of causing intermolecular hydrogen bonding. Through the self-assembling behavior of these long chain aliphatic hydrocarbon branches, π-π interaction of arylene groups, intramolecular hydrogen bonding of amide groups, etc., the monomers can form a regular stereostructure in the solid state have.

Therefore, when specific radical polymerization or the like proceeds to such a monomer, the polymerization reaction occurs in a state in which the monomer molecules are well aligned, so that the respective monomer molecules can be regularly arranged in the polymer chain. More specifically, well-oriented monomer molecules can be combined through the polymerization reaction to form one polymer chain (for example, one polymer building block), and these polymer building blocks are gathered to form a regularly arranged polymer . Therefore, due to the ordered arrangement of the polymer building blocks in such a polymer, the acrylamide based polymer may comprise a plurality of mesopores having a uniform size without any additional treatment after polymerization. For the same reason, the acrylamide-based polymer may have crystallinity.

Due to such properties, the film of the acrylamide polymer may contain a plurality of fine mesopores, which may exhibit an excellent impregnation rate with respect to the electrolyte of the lithium secondary battery. Further, unlike the case where the porous polyethylene film previously used as a separator includes pores having a relatively large diameter of about several hundred nm or less through biaxial stretching or the like, the film of the acrylamide polymer can be produced by the above- Due to the nature of the polymer itself, it can comprise a number of mesopores which themselves have a fine diameter of about 2.0 to 10.0 nm. Therefore, the acrylamide polymer film maintains an electrolyte well in fine mesopores, and when used as a separation membrane of a lithium secondary battery, there is less risk of leakage of the electrolyte and can exhibit excellent safety.

In addition, since the film of the acrylamide polymer can be produced from a polymer having a relatively large molecular weight, excellent mechanical properties can be exhibited even after the electrolyte is impregnated and swollen, and suitable ion conductivity and the like, which can be used as a separator of a lithium secondary battery, It was confirmed that it can be shown.

Therefore, according to one embodiment, the film of the acrylamide polymer can be provided as a separator for a lithium secondary battery that replaces a porous polyethylene film used before, and the separator of this embodiment can exhibit excellent properties .

Hereinafter, the acrylamide polymer will be described in more detail, and a film and a separator including the acrylamide polymer will be described.

), 메타페닐렌(meta-phenylene,

), 파라페닐렌(para-phenylene,

), 나프탈렌(naphthalene,

), 아조벤젠(azobenzene,

), 안트라센(anthracene,

), 페난스렌(phenanthrene,

), 테트라센(tetracene,

), 파이렌(pyrene,

) 또는 벤조파이렌(benzopyrene,

) 등을 들 수 있으며, 기타 다양한 아릴렌으로 될 수 있다.In the acrylamide-based polymer as a main component of the separator, Z may be any arylene having 6 to 20 carbon atoms. More specifically, examples of such arylene include ortho-phenylene,

), Meta-phenylene,

), Para-phenylene,

), Naphthalene (naphthalene,

), Azobenzene (azobenzene,

), Anthracene,

), Phenanthrene (phenanthrene,

), Tetracene (tetracene,

), Pyrene (pyrene,

) Or benzopyrene,

), And the like, and may be various other arylene.

The R "may be a linear or branched aliphatic hydrocarbon substituted at the ortho, meta or para position of the aromatic ring included in Z, and the hydrocarbon may have 10 or more carbon atoms, more specifically 10 to 20 carbon atoms Lt; / RTI > chain length. Further, such hydrocarbons may be substituted or unsubstituted with fluorine, and may be, for example, linear or branched perfluorohydrocarbons having 10 to 20 carbon atoms.

As the repeating units of the above formula (1) and the monomers of the following formula (2) have such long chain hydrocarbons and arylene, properties such as mesoporosity due to the self-assembling behavior of the polymer may be more conspicuously exhibited, And the separator can exhibit improved electrolyte impregnation rate and excellent safety due to electrolyte leakage suppression.

The polymer may be a homopolymer composed of one kind of the repeating unit represented by the formula (1) or a copolymer composed of two or more kinds.

In addition, the acrylamide-based polymer may include a plurality of mesopores having a diameter of about 2.0 to 10.0 nm, or about 2.0 to 6.0 nm in a solid state. Here, the "diameter" of the pores may be defined as the length of the longest straight line connecting two different points in a circular, elliptical or polygonal shape forming a cross section of each pore. That is, since the polymer includes a plurality of uniform mesopores having a diameter within this range, the film and the separator containing the mesopores can be well impregnated with the electrolyte of the lithium secondary battery, It is possible to exhibit an excellent electrolyte impregnation rate and excellent safety as described above.

And, the polymer may have a number average molecular weight of about 70000 to 500000, or a number average molecular weight of about 100000 to 300000. The polymer may also have a molecular weight distribution (PDI) of about 2.0 to 6.0, or a molecular weight distribution of about 3.0 to 5.0. As the polymer exhibits such molecular weight and molecular weight distribution characteristics and the like, the film and the separator containing the polymer can exhibit better mechanical properties even after swelling with the electrolyte. Therefore, the separator of one embodiment can more suitably be used as the separator of the lithium secondary battery.

The polymer may also be a crystalline polymer having a melting point ( _Tm ) of about 200 to 300 占 폚, or a melting point of about 220 to 280 占 폚. As the polymer has such a range of melting point, etc., it can exhibit excellent thermal stability. Therefore, the film and the separator containing the same can exhibit excellent heat resistance.

In addition, it has been found that the acrylamide-based polymer can be a crystalline polymer having a melting point within the above-mentioned range. As such, it can exhibit mesoporosity and crystallinity different from previously known polymers of similar structure. Due to such mesoporosity and the like, the polymer and the film and separator containing the polymer can exhibit excellent electrolyte impregnation rate and excellent safety as a separator. The melting point or crystallinity of these polymers can be determined by analyzing the structure of the solid polymer by means of small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) and by measuring the phase transition temperature of the polymer by differential scanning calorimetry And can be confirmed by thermal analysis for example.

When the polymer is heat-treated at a temperature of about 200 ° C or higher to a temperature lower than the melting point, for example, about 220 ° C to 240 ° C, it is confirmed that the pore diameter of the polymer phase may decrease as the heat treatment temperature is increased . For example, as the heat treatment temperature is increased, the pore diameter may be reduced by about 0.4 to 0.7 nm, more specifically by about 0.5 to 0.6 nm.

Also, the polymer corresponds to the chemical structure of R 'bonded to the amide (-CO-NH-) group contained in the repeating unit of the formula (1) or the length of the aliphatic hydrocarbon bonded at the terminal thereof, for example, R " It was confirmed that as the number of carbon atoms of the hydrocarbon increases, the pore diameter of the polymer increases. For example, as the carbon number increases from 12 to 16, the pore diameter may increase by about 0.1 to 1.0 nm, more specifically about 0.2 to 0.7 nm. Also, the pore diameter may increase as the chemical structure of Z contained in R 'is changed from phenylene to various aromatic structures such as naphthalene or anthracene.

The cause of the change in pore diameter is that the mesoporous steric structure (or crystal structure) of the polymer is changed according to the heat treatment, the chemical structure of R 'bonded to the amide group or the change of the carbon number of R " And this can be supported by DSC thermal analysis results of the embodiments described later.

As described above, in the acrylamide-based polymer, since the pore size can be easily controlled by a heat treatment, a change in the chemical structure introduced into the amide group of the repeating unit, or a control of the length of the hydrocarbon, The size of the mesopore can be easily controlled in the membrane. Therefore, the separator of this embodiment can easily control the mesoporosity, so that it becomes easier to express the desired electrolytic impregnation rate and safety as a separator due to electrolyte leakage suppression. Accordingly, the separator of one embodiment can be suitably used as a separator in various sizes or types of lithium secondary batteries.

On the other hand, the separator of one embodiment may have the form of a mesoporous film containing the acrylamide-based polymer described above.

Such a film may be obtained by solvent casting in which the acrylamide-based polymer is dissolved in a solvent and cast to a predetermined thickness and dried, or the polymer is melted and then pressure is applied Compression molding method for molding, and the like, and may be suitably obtained by the above-mentioned solvent casting method. At this time, since the film of the acrylamide polymer can exhibit a mesoporosity in which a large number of mesopores are formed due to the characteristics of the polymer itself, And can be used as a separator for a battery. Therefore, it is possible to simplify the production process of the separation membrane, but also to suppress the leakage of the electrolyte due to the existence of mesopores having a fine diameter, and to exhibit excellent safety as a separation membrane.

The film of the acrylamide polymer may be formed by a solvent casting method using a polar or nonpolar organic solvent such as chloroform, benzene, toluene, tetrahydrofuran (THF), ethyl acetate (EA) or dimethylformamide In addition, they can be formed to have better mechanical properties by a solvent casting method using a polar organic solvent such as chloroform, THF, or EA.

The acrylamide polymer film may have a thickness of about 20 to 100 占 퐉, or about 30 to 80 占 퐉. With such a thickness, the film can be suitably used as a separation membrane of a lithium secondary battery and can exhibit excellent mechanical properties even after swelling the electrolyte by impregnation.

On the other hand, the film of the acrylamide polymer described above may further include a trialkylammonium bromide-based additive such as dodecyltrimethylammonium bromide (DTAB) or didodecyldimethylammonium bromide (DDAB). As a result of further including such an additive, it has been confirmed that the film and the separator of one embodiment exhibit a better impregnation rate and ion conductivity with respect to an electrolyte, and thus can be more preferably used as a separator of a lithium secondary battery.

This is presumably because the trialkylammonium bromide-based additive can interact with the polymer chain of the acrylamide-based polymer to further improve the orientation of the polymer film. In order to optimize the effect of adding such an additive and to exhibit improved physical properties of the separator, the additive may be added in an amount of about 3 to 30 parts by weight, or about 5 to 20 parts by weight per 100 parts by weight of the acrylamide polymer contained in the film Parts by weight, or about 7 to 10 parts by weight.

In addition, the above-mentioned additives may be dissolved in a solvent and added to the acrylic amide polymer in forming a film of the acrylamide polymer by a solvent casting method or the like.

On the other hand, the above-mentioned acrylamide polymer can be produced by the following production method.

First, the acrylamide-based polymer is subjected to radical polymerization (for example, RAFT polymerization) in the presence of a radical initiator and, optionally, a reverse addition fragmentation transfer (RAFT) reagent, a reactant containing at least one monomer represented by the following formula ; And precipitating the polymerisation product in a non-solvent. ≪ RTI ID = 0.0 >

(2)

In the above formula (2), R and R 'are as defined for formula (1).

As described above, the acrylamide-based monomer having a specific chemical structure represented by the general formula (2) is subjected to radical polymerization (for example, RAFT polymerization or the like) by a specific method under specific conditions and precipitated under a non- Acrylamide-based polymer having good properties can be easily produced. In addition, the reason why the polymer produced by this method has regularity and crystallinity has already been fully explained, so that further explanation will be omitted. As a result, even if only a relatively simple radical polymerization process is carried out without a separate chemical treatment process, the acrylamide-based polymer can be produced.

On the other hand, the manufacturing method may further include: before the polymerization step, forming a reaction solution including the radical initiator, the RAFT reagent, and the reactant; Placing the reaction solution in a polymerization ampoule and removing oxygen by a freeze-thaw method; And sealing the ampoule. As described above, the RAFT polymerization process, which is known as a kind of living radical polymerization, can be more suitably proceeded by carrying out the subsequent polymerization process after putting each reactant, initiator, etc. into the oxygenated polymerization ampoule, and the acrylamide polymer Can be obtained at a high conversion rate.

The production method may further comprise, after the precipitation step, dissolving the precipitated polymerization product in an organic solvent; And reprecipitating the polymerization product solution using a non-solvent. By further proceeding to such a re-precipitation step, an acrylamide polymer having crystallinity or the like can be more preferably obtained.

In the above production process, any of acrylamide monomers satisfying the formula (2) can be used as the monomer. Specific examples thereof include p-dodecyl phenyl acrylamide (DOPAM) , P-tetradecyl phenyl acrylamide (TEPAM), para-hexadecyl phenyl acrylamide (HEPAM), paradodecylnaphthyl acrylamide [N - (p-dodecyl) naphthyl acrylamide, DONAM, p-tetradecyl naphthyl acrylamide, TENAM, p-hexadecyl naphthyl acrylamide , HENAM, p-dodecyl azobenzenyl acrylamide, DOAZAM, p-tetradecyl azobenzenyl acrylamide, TEAZAM, para-hexadecyl azobenzene (P-hexadecyl) azobenzenyl acrylamide, HEAZAM], or N- [4- (3- (5- (4-dodecyl-phenylcarbamoyl) pentylcarbamoyl) -dodecyl-phenylcarbamoyl) pentyl-carbamoyl) -propyl) phenyl acrylamide, DOPPPAM), or monomers in which one or more fluorine atoms are substituted at the terminals thereof. Of course, two or more monomers selected from these monomers may be used .

As such a monomer, those described in Korean Patent Application No. 2011-0087290 (Korean Patent Registration No. 1163659) of the present inventors can be used. By using these monomers to produce a polymer through living radical polymerization such as RAFT polymerization, each monomer molecule is arranged more regularly in the polymer chain and well-oriented monomer molecules are combined to form a polymer that exhibits mesoporosity Can be more preferably obtained.

The monomer of Formula 2 and the method for producing the same are obviously disclosed to those skilled in the art such as Korean Patent Application No. 2011-0087290 (Korean Patent Registration No. 1163659) by the inventors of the present invention, and a detailed description thereof will be omitted.

The radical initiator, the RAFT reagent and the monomer may be prepared as a reaction solution dissolved in an organic solvent, and the RAFT polymerization process may proceed in the state of the reaction solution. The organic solvent may be at least one nonpolar solvent selected from the group consisting of normal hexane, cyclohexane, benzene, toluene, chlorobenzene, dichlorobenzene, methylene chloride and 1,2-dichloroethane, acetone, chloroform, (1) selected from the group consisting of tetrahydrofuran (THF), dioxane, monoglyme, diglyme, dimethylformamide (DMF), dimethylsulfoxide (DMSO) and dimethylacetamide More than one species of polar solvent may be used. A mixed solvent of the nonpolar solvent and the polar solvent may also be used. Also, in the above-mentioned re-precipitation step, the polymerization product can be dissolved by using such an organic solvent.

In the reaction solution, the monomer may be dissolved at a concentration of about 3.0 to 50% by weight, or about 5.0 to 40% by weight, based on the organic solvent. The subsequent polymerization process can proceed efficiently in the state of the reaction solution dissolved at such a concentration.

In addition, as the radical initiator used together with the monomer, any initiator known to be usable for radical polymerization can be used without any limitation. Examples of such radical initiators include azobisisobutyronitrile (AIBN), 2,2'-azobis- (2,4-dimethylvaleronitrile), 2,2'- Benzoyl peroxide (BPO), di-t-butyl peroxide (DTBP), and the like, and two or more selected from these may be used.

S-1-dodecyl-S '- (?,?' - (?,? '- dimethyl-? "- acetic acid) trithiocarbonate [ -dimethyl-α "-acetic acid trithiocarbonate], cyanoisopropyl dithiobenzoate, cumyl dithiobenzoate, cumyl phenylthioacetate, 1-phenylethyl-1 1-phenyldithioacetate, or 4-cyano-4- (thiobenzoylthio) -N-succinimide (4-cyano- valerate may be used, and a mixture of two or more selected from these may be used.

The radical initiator and the RAFT reagent may be used in a ratio of about 0.001 to 5.0% by weight based on the weight of the monomer.

And, in the above-mentioned production method, the radical polymerization process can be carried out at a reaction temperature of about 60 to 140 ° C. In addition, the radical polymerization process may be carried out for about 30 to 200 hours, more specifically about 50 to 170 hours.

In the precipitation or reprecipitation of the production method, the non-solvent may be a product of the polymerization process or a solvent that does not dissolve the acrylamide polymer. Examples of such non-solvent include methanol, ethanol, n-propanol, isopropanol, or Polar solvents such as ethylene glycol, and nonpolar solvents such as n-hexane, cyclohexane, and n-heptane. Of course, a mixture of two or more solvents selected from these may also be used. Through the precipitation and reprecipitation processes using such a non-solvent, the above-mentioned polymer having mesoporosity and crystallinity can be obtained with higher purity with ease.

The separator for a lithium secondary battery including the film of the acrylamide polymer described above can exhibit an excellent impregnation rate with respect to the electrolyte due to the unique characteristics such as the mesoporosity of the acrylamide polymer itself and can suppress the leakage of the electrolyte, Safety can be shown. In addition, the separator of this embodiment exhibits excellent mechanical properties due to a large molecular weight and the like even after being swollen with an electrolyte, and exhibits a conductivity that can be used as a separator of a lithium secondary battery. Therefore, a separator of a porous polyethylene film It can be preferably applied as an alternative material.

According to another embodiment of the present invention, there is provided a lithium secondary battery comprising the separator of one embodiment described above. Such a lithium secondary battery includes a positive electrode comprising a positive electrode active material of a metal oxide; A negative electrode comprising a negative electrode active material of graphite or lithium metal oxide; A separator of the above-described embodiment interposed between the anode and the cathode; And a non-aqueous electrolyte interposed between the positive electrode and the negative electrode, the non-aqueous electrolyte including an ionizable lithium salt and an organic solvent.

Such a lithium secondary battery may have a conventional configuration of a lithium secondary battery, except that the lithium secondary battery includes the separator of one embodiment.

For example, the negative electrode may be prepared by preparing a negative electrode active material composition by mixing a graphite material or a lithium metal oxide negative electrode active material, a conductive material, a binder and a solvent, and then coating the resultant directly on the copper current collector and drying . Alternatively, the negative active material composition may be cast on a separate support, and then the film may be peeled off from the support and laminated on an aluminum current collector.

Examples of the conductive material include carbon black, graphite, and lithium metal oxide powder. Examples of the binder include vinylidene fluoride / hexafluoropropylene copolymer, polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate Polytetrafluoroethylene, and mixtures thereof may be used, but are not limited thereto. Examples of the solvent include, but are not limited to, N-methylpyrrolidone, acetone, tetrahydrofuran, decane, and the like. At this time, the content of the negative electrode active material, the conductive material, the binder, and the solvent may be used at a level commonly used in a lithium secondary battery.

The positive electrode may be prepared by preparing a positive electrode active material composition by mixing a positive electrode active material of a metal oxide including a lithium composite metal oxide, a binder and a solvent, as in the case of a negative electrode, and coating the same on an aluminum current collector, And then laminating the positive electrode active material film peeled from the support on the copper current collector. At this time, the cathode active material composition may further contain a conductive material if necessary.

As the cathode active material, a material capable of intercalating / deintercalating lithium may be used. As the cathode active material, a metal oxide or a lithium composite metal oxide may be used. However, the present invention is not limited thereto.

As the electrolyte to be charged into the lithium secondary battery, a non-aqueous electrolyte or the like may be used, and a lithium salt dissolved therein may be used.

Examples of the solvent for the non-aqueous electrolyte include cyclic carbonates (CC) such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC) Chain carbonates such as ethyl methyl carbonate (EMC) and diethyl carbonate (DEC), esters such as methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate and? -Butyrolactone, Ethers such as ethers such as dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane and 2-methyltetrahydrofuran, nitriles such as acetonitrile, amides such as dimethylformamide , Dimethylsulfoxide, and the like, but the present invention is not limited thereto. These can be used singly or in combination. Particularly, a mixed solvent of a cyclic carbonate such as ethylene carbonate and a chain carbonate such as diethyl carbonate can be preferably used.

The lithium salt may be LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlO ₂ , LiAlCl ₄ , LiCl, and LiI may be used, but the present invention is not limited thereto.

As described above, according to the present invention, there is provided a separator for a lithium secondary battery comprising a predetermined acrylamide polymer film. Such a separation membrane can exhibit an excellent impregnation rate with respect to the electrolyte due to its unique characteristics such as the mesoporosity of the acrylamide polymer itself, and can exhibit improved safety by suppressing leakage of the electrolyte.

In addition, the separator according to the present invention exhibits excellent mechanical properties due to large molecular weight and the like even after being swollen with an electrolyte, and exhibits a conductivity that can be used as a separator of a lithium secondary battery. Therefore, a porous polyethylene film Can be suitably applied as a substitute material.

1 shows the DSC thermal curve of the polymer prepared in Example 1. Fig.
Fig. 2 shows a TEM photograph of a thin film comprising the polymer of Example 1. Fig.
3 is a SEM photograph of a thick film (48 탆) film prepared by the method of Example 8 using the polymer synthesized in Example 4. Fig.
4 is a SEM photograph of a thick film (30 탆) film prepared by the method of Example 9 using the polymer synthesized in Example 5 and the additive of DTAB.
5 is an SEM photograph of a thick film (30 탆) film prepared by the method of Example 9 using the polymer synthesized in Example 6 and the additive of DDAB.
6A is a graph showing the oxidation stability measurement results of a thick film (48 mu m) film prepared by the method of Example 8 using the polymer synthesized in Example 4. Fig.
6B is a graph showing a resistance impedance of a thick film (48 mu m) film produced by the method of Example 8 using the polymer synthesized in Example 4. Fig. Using this graph, ion conductivity as a lithium secondary battery separation membrane can be obtained.

Best Mode for Carrying Out the Invention Hereinafter, the function and effect of the present invention will be described in more detail through specific examples of the present invention. It is to be understood, however, that these embodiments are merely illustrative of the invention and are not intended to limit the scope of the invention.

Example 1-7: Preparation of a novel acrylamide-based polymer

[Example 1] Poly (DOPAM) -1 manufacture

The acrylamide monomer (DOPAM; 1.0 g) synthesized in Example 1 of Korean Patent Registration No. 1163659 was dissolved in 6.3 mL of THF and then RAFT reagent cyanoisopropyldithiobenzoate (1.75 mg) and the radical initiator AIBN (0.87 mg ) In a 10 mL Schenk flask. The mixture was stirred in a nitrogen atmosphere for 30 minutes to remove the oxygen present in the solution, and the resulting mixture was placed in a silane cone oil vessel at 70 ° C. and subjected to RAFT polymerization for 72 hours. After the polymerization reaction, the reaction solution was precipitated in 200 mL of methanol and then filtered to obtain an orange solid. The resulting solid was redissolved in 8 mL of THF and reprecipitated in an excess of methanol. The resulting pale yellow solid was dried in a vacuum oven for 24 hours to obtain a pure homopolymer, Poly (DOPAM) -1, represented by the following formula (3). The polymerization conversion and the number average molecular weight were 48% and 14900, respectively. The molecular weight distribution of the homopolymer was very narrow as 1.25 and the melting temperature (T _m ) was 241 ^° C.

(3)

[Example 2] Poly (DOPAM) -2 manufacture

(DOPAM; 1.5 g), benzene 7.8 mL, RAFT reagent cyanoisopropyldithiobenzoate (2.63 mg) and AIBN (1.3 mg) synthesized in Example 1 of Korean Patent Registration No. 1163659 Pure Poly (DOPAM) -2 was obtained in the same manner as in Example 1 except that The polymerization conversion and the number average molecular weight were 66% and 35000, respectively. The molecular weight distribution of this polymer was 1.39, which was relatively narrow, and the melting temperature (T _m ) was 242 ^° C.

[Example 3] Poly (DOPAM) -3 manufacture

(DOPAM; 1.5 g) synthesized in Example 1 of Korean Patent Registration No. 1163659, 7.8 mL of benzene, 1.75 mg of RAFT reagent cyanoisopropyldithiobenzoate, 1.3 mg of AIBN, (DOPAM) -3 was obtained in the same manner as in Example 1, except that 96 hours was used. The polymerization conversion and number average molecular weight were 86% and 87600, respectively. The molecular weight distribution of this polymer was 1.74, which was relatively narrow, and the melting temperature (T _m ) was 242 ^° C.

[Example 4] Poly (DOPAM) -4 production

(DOPAM; 1 g) synthesized in Example 1 of Korean Patent Registration No. 1163659, 6.5 mL of benzene mixture and 7.5 mg of initiator BPO were placed in a 20 mL ampoule, The ampoules were sealed and the radical polymerization was allowed to proceed in an oven at 80 ° C for 72 hours. After the polymerization reaction, the reaction solution was precipitated in 300 mL of methanol and then filtered to obtain a pale yellow solid. The resulting solid was redissolved in 10 mL of THF and reprecipitated in an excess of methanol. The obtained solid was dried in a vacuum oven for one day or more to obtain a pale yellow homopolymer, Poly (DOPAM) -4. The polymerization conversion and the number average molecular weight were 96% and 153000, respectively. The molecular weight distribution of this polymer was 4.37 and the melting temperature (T _m ) was 242 ^° C.

[Example 5] Poly (DOPAM) -5 manufacture

Except that 6.5 mL of the acrylamide monomer (DOPAM; 1 g) synthesized in Example 1 of Korean Patent Registration No. 1163659 and THF / benzene (30/70 by volume) was used, pure Poly (DOPAM) -5. The polymerization conversion and the number average molecular weight were 95% and 112100, respectively. The molecular weight distribution of this polymer was 2.54 and the melting temperature (T _m ) was 242 ^° C.

[Example 6] Poly (DOPAM) -6 manufacture

Except that 6.5 mL of the acrylamide monomer (DOPAM; 1 g) synthesized in Example 1 of Korean Patent Registration No. 1163659 and THF / benzene (20/80 by volume) was used, pure Poly (DOPAM) -6. The polymerization conversion and the number average molecular weight were 94% and 125,300, respectively. The molecular weight distribution of this polymer was 2.1 and the melting temperature (T _m ) was 242 ^° C.

[Example 7] Poly (DOPAM) -7 manufacture

(DOPAM; 1 g) synthesized in Example 1 of Korean Patent Registration No. 1163659, 6.5 mL of benzene, 5.0 mg of an initiator BPO and a polymerization temperature of 85 ° C were used in the same manner as in Example 4 To obtain pure Poly (DOPAM) -6. The polymerization conversion and the number average molecular weight were 98% and 17,400, respectively. The molecular weight distribution of this polymer was 2.91 and the melting temperature (T _m ) was 242 ^° C.

 [Example 8] Preparation of poly (DOPAM) film

Poly (DOPAM) -2 to Poly (DOPAM) -7 synthesized in Examples 2 to 7 was dissolved in a CHCl ₃ solvent to prepare a 5.0 wt% polymer solution. The polymer solution was placed in a Teflon mold (3 x 7 cm), dried in air for 24 hours, and then dried again in a vacuum oven at room temperature for 24 hours to prepare a uniform film having a thickness of about 40 μm. The thickness of the film (20-100 탆) was proportional to the concentration of solvent (2-10 wt%) in the polymer and the number average molecular weight of the polymer.

[Example 9] Preparation of Poly (DOPAM) Porous Film

Poly (DOPAM) -2 to poly (DOPAM) -7 synthesized in Examples 2 to 7 were dissolved in a CHCl ₃ solvent to prepare a polymer solution having a concentration of 4.0 wt%. Then, DTAB (or DDAB) (2 to 20 wt% based on the amount of the polymer), and the polymer solution was mixed well. The polymer solution was placed in a Teflon mold (3 x 7 cm) and dried in the air for 24 hours. Thereafter, it was dried again in a vacuum oven at room temperature for 24 hours to prepare a uniform film having a thickness of about 30 탆. The prepared uniform film (3 x 7 cm) was immersed in a petri dish (10 x 10 cm) containing 100 mL of methanol and then allowed to stand at room temperature for 48 hours to remove the additives contained in the polymer film, For 24 hours to prepare a porous film containing pores.

The prepared film was immersed in an electrolytic solution for 24 hours, and the oxidation stability and ionic conductivity of the swollen film were measured to evaluate the characteristics as a separator for a lithium secondary battery.

[Test Example 1]: Thermal properties, molecular weight and molecular weight distribution and mesoporosity of polymer

(1) Analysis of thermal properties of polymer by DSC

The phase transition behavior of the poly (DOPAM) -1 polymer prepared in Example 1 was investigated using a DSC thermal analyzer. As a result, it was confirmed that the crystalline polymer had a melting temperature (T _m ) of 241 ± 1 ^° C.

1 is a DSC thermal curve showing the thermal phase transition temperature behavior of Poly (DOPAM) -1 obtained in Example 1. Fig. 1, a mesoporous (mesoporous) a melting temperature (T _m) of the structure formed by the polymer chains of the polymer -1 Poly (DOPAM) is confirmed to 241 ^o C. In addition, the melting temperature of the microcrystals formed from the aliphatic hydrocarbon introduced at the end of the repeating unit is confirmed at about 5 ^° C. It is predicted that the mesoporous structure formed between the polymer chains of the poly (DOPAM) -1 polymer is relatively stably oriented because the phase transition melting temperatures of the poly (DOPAM) -1 polymer are similar in the heating curve and the cooling curve at the almost same temperature range. do. In addition, when the number average molecular weight of the synthesized poly (DOPAM) polymer was 8000 or more, the melting temperature did not show any significant difference.

(2) Molecular weight measurement of polymer

The number average molecular weight (Mn) and the molecular weight distribution (PDI) of the polymer were measured at room temperature using a gel permeation chromatography (GPC) instrument. Styragel ^R as a GPC filler and tetrahydrofuran (THF) as a solvent were used.

(3) Analysis of mesoporous structure of polymer by TEM

A thin film containing the poly (DOPAM) -1 polymer of Example 1 was prepared and its TEM (transmission electron microscope) photograph was taken and shown in Fig. At this time, the thin film was prepared by heat-treating a solid powder of Poly (DOPAM) -1 polymer at a melting temperature for 6 hours, followed by quenching with liquid nitrogen. FIG. 2 is a TEM photograph of this thin film cut to a thickness of about 50-120 nm and deposited on a film of RuO ₄ vapor. Referring to FIG. 2, the black part is formed by depositing RuO ₄ on a benzene group introduced into a polymer chain of poly (DOPAM) -1, which constitutes a skeleton of a cylindrical structure. The black part is a black structure having a pore size of about 3.5 nm Have been confirmed to be relatively uniformly distributed on the surface of the thin film by Korean Patent Registration No. 1163659 of the present inventors.

(4) Porosity analysis of polymer film by SEM

Pores of the surface of the pure Poly (DOPAM) polymer film and the porous film prepared in Examples 8 and 9 were measured using a scanning electron microscope (SEM).

3 is an SEM photograph of a uniform film (48 탆) prepared by the method of Example 8 using the polymer Poly (DOPAM) -4 synthesized in Example 4. Fig. As a result, it was confirmed that the surface of the film was a sponge structure containing a large number of very small fine pores.

4 is a view showing a uniform porous film (30 占 퐉) prepared by the method of Example 9, in which the polymer Poly (DOPAM) -5 synthesized in Example 5 and 10 wt% of the additive DTAB (ratio to the weight of the polymer) . Through this, it was confirmed that fine pores having a size of about 25-100 nm were formed on the surface of the film by the interaction between the additive and the long alkyl chain dodecyl groups introduced into the side chains of the polymer.

Fig. 5 is a graph showing the results of measurement of a uniform porous film (30 占 퐉) prepared by the method of Example 9, in which the polymer Poly (DOPAM) -6 synthesized in Example 6 and 10 wt% of the additive DDAB (ratio to the weight of the polymer) . Thus, it was confirmed that the pores having a size of about 120-220 nm were formed on the surface of the film by the interaction between the additive and the long alkyl chain dodecyl groups introduced into the side chains of the polymer.

[Test Example 2] Application of Poly (DOPAM) Film to Membrane for Lithium Secondary Battery

(1 ) Measurement of electrolyte uptake

The polymer films prepared in Examples 8 and 9 were cut into a circular film (1.8 cm in diameter and 2.54 cm ^{2 in} area) of a uniform size using a round frame, and then the weight was measured. A circular polymer film was placed in a liquid electrolyte mixture (EC / DEC = 1/1 or EC / PC = 1/1 volume ratio) containing 1.0 mole of lithium salt (LiPF ₆ ) dissolved therein. After 24 hours, The weight of the impregnated film was measured. All experiments were conducted under conditions of less than 1.0 ppm moisture in a glove box filled with argon gas.

The electrolyte impregnation rate (%) was calculated by dividing the weight of the film after impregnation by the weight of the entire film minus the weight of the film before impregnation, and then multiplying by 100 again. The measurement results of the electrolyte impregnation rate are summarized in Table 1 below.

(2) Measurement of oxidation stability

Linear sweep voltammetry was used to confirm the oxidation stability of the polymer films. First, an oxidation-stable cell (2032-type coin) was prepared using a polymer film impregnated with a liquid electrolyte mixture solution. Lithium metal and stainless steel were used as reference electrode (or counter electrode) and working electrode, respectively. In addition, a lithium metal electrode was placed in the bottom can of the cell, a polymer film prepared thereon was put up, and a stainless steel electrode was stacked. Finally, the can was closed to fabricate a closed oxidation stable cell. Applying a potential (1.0 mV s ^-1) at a constant rate to the polymer film of the fabricated cell, the density of the current suddenly increases above a certain voltage. At this time, the polymer film applied to the cell was regarded as being electrochemically oxidized, and this specific voltage was set as a reference value of electrochemical oxidation stability.

6A is a graph showing a representative oxidation stability of a uniform film (48 mu m) prepared by the method of Example 8, Polymer Poly (DOPAM) -4 synthesized in Example 4. Fig. According to FIG. 6A, the film produced from the result that the density of the current rapidly increases at 4.2 volt is electrochemically oxidized.

In addition, the oxidation stability of the polymer films prepared in the above-mentioned examples showed a value between about 4.0 and 6.0 voltage, and the measurement results of the oxidation stability are summarized in Table 1 below.

(3) Measurement of ion conductivity

The ion conductivity cell is composed of stainless steel for both the reference electrode and the working electrode. That is, a polymer film impregnated with a liquid electrolyte mixed solution was placed in a can at the bottom of a cell, a stainless steel electrode (area: 2 cm ² ) was placed thereon, and finally the can was closed to fabricate a closed cell. Ion conductivity measurements were made at room temperature using AC-impedance analysis. The frequency was measured from 1 to ¹⁰⁶ Hz.

That is, if an AC current is flowed into a sample in a specific frequency range, an impedance which is a resistance value due to an AC current is generated. Therefore, the ionic conductivity (C) of the applied polymer film can be obtained from the thickness (L), area (A) and resistance (R) of the film.

C (S cm ^-1 ) = L (cm) / [R (?) X A (cm ² )]

FIG. 6B is a graph showing impedance values for a representative AC current of the polymer film (48 μm) produced by the method of Example 8 using the polymer Poly (DOPAM) -4 synthesized in Example 4. The ionic conductivity of the polymer film was found to be 0.000105 S / cm. As a result, the ionic conductivity of the other polymeric film was measured in the same manner The results are summarized in Table 1 below.

In the following Table 1, examples of the polymer used in forming the film, examples relating to the method of forming the film, thickness of the film formed, contents of the additives used and the physical properties measured above are summarized.

Polymer film Electrolyte
(Solvent; volume ratio) thickness
(탆) Electrolyte
Impregnation rate
(volume%) resistance
(Ω) Ion conductivity
(X 10 ^-4 S / cm) Oxidation stability
(Voltage) Example 8 (Polymer: Example 2) EC / DMC
(1/1) 35 40 150.8 0.03 3.0 or less Example 8 (Polymer: Example 3 EC / DEC
(1/1) 45 56 40.7 0.38 4.3 Example 8 (Polymer: Example 4) EC / DEC
(1/1) 48 62 24.2 1.05 4.2 Example 8 (Polymer: Example 4) EC / PC
(1/1) 60 61 268.7 0.01 4.5 Example 8 (Polymer: Example 5) EC / DEC
(1/1) 40 60 26.4 0.47 4.2 Example 9 (polymer: Example 5; DTAB-10 wt%) [ EC / DEC
(1/1) 30 63 24.2 0.62 4.3 Example 9 (polymer: Example 5; DTAB-20 wt%) [ EC / DEC
(1/1) 30 61 50.5 0.30 4.3 Example 9 (Polymer: Example 6; DDTAB-10 wt%) [ EC / DEC
(1/1) 30 65 37.0 0.27 4.0 Example 8 (Polymer: Example 7) EC / DEC
(1/1) 30 53 32.0 0.46 6.0 or higher

Referring to Table 1, it was confirmed that the polymer films of the examples exhibited ion conductivity of about 10 ^-6 to 10 ^-4 S / cm, which is applicable to a separator for a lithium secondary battery. This is 10 to 100 times higher than the polymer gel type membranes reported so far.

In addition, the polymer films of the Examples were found to exhibit excellent oxidation stability up to 6.0 voltage, with an excellent electrolyte impregnation rate. This may mean that the oxidation stability value (4.2 voltage) of the polyethylene separator used as a separator for a lithium secondary battery is similar to or more stable than that of the present invention.

In addition, the polymer film of the embodiment absorbs / swells the electrolyte by an impregnation rate to form an elastic polymer gel, which is a single phase, and substantially does not cause leakage of the electrolyte solution, and has excellent safety as a separator for a lithium secondary battery . This may have a different meaning from the existing polyethylene separator membrane in which the electrolytic solution is impregnated with the electrolytic solution even when the electrolytic solution is separated from the solid separator membrane and the liquid phase.

Therefore, the polymer film of the embodiment can contribute to the provision of a separator membrane and a lithium secondary battery exhibiting more excellent stability.

Claims (10)

A separator for a lithium secondary battery comprising an unstretched film of an acrylamide polymer comprising a plurality of pores having at least one kind of repeating units represented by the following formula (1) and having a diameter of 2.0 to 10.0 nm:
[Chemical Formula 1]

In Formula 1,
n is an integer from 15 to 1800,
R is hydrogen or methyl,
R 'is selected from X,

,

,

or

ego,
X is < RTI ID = 0.0 > - ZR &
Y is alkylene having 1 to 10 carbon atoms,
Z is arylene having 6 to 20 carbon atoms,
R "is a linear or branched hydrocarbon having 10 to 20 carbon atoms, or a linear or branched perfluorohydrocarbon having 10 to 20 carbon atoms.
The separator for a lithium secondary battery according to claim 1, wherein the acrylamide polymer has a number average molecular weight of 70,000 to 500,000 and a molecular weight distribution (PDI) of 2.0 to 6.0.
The separator for a lithium secondary battery according to claim 1, wherein the unstretched film of the acrylamide polymer has a thickness of 20 to 100 탆.
The separator for a lithium secondary battery according to claim 1, wherein the unstretched film of the acrylamide polymer is an unstretched film produced by solvent casting.
The separator for a lithium secondary battery according to claim 4, wherein the solvent casting method is performed using one or more organic solvents such as chloroform, benzene, THF, or ethyl acetate.
The separator for a lithium secondary battery according to claim 1, wherein the unstretched film of the acrylamide-based polymer further comprises a trialkylammonium bromide-based additive.
The separator for a lithium secondary battery according to claim 6, wherein the additive is contained in an amount of 2 to 25 parts by weight based on 100 parts by weight of the acrylamide polymer.
A positive electrode comprising a positive electrode active material of a metal oxide;
A negative electrode comprising a negative electrode active material of graphite or lithium metal oxide;
A separation membrane according to claim 1 interposed between the anode and the cathode; And
And a non-aqueous electrolyte interposed between the positive electrode and the negative electrode, the non-aqueous electrolyte including an ionizable lithium salt and an organic solvent.
The lithium secondary battery according to claim 8, wherein the organic solvent comprises a mixed solvent of cyclic carbonate and chain carbonate.
The method of claim 8, wherein the ionizable lithium salt is _{LiPF 6, LiBF 4, LiSbF 6} , LiClO 4, or a lithium secondary battery comprising a LiCF ₃ SO _3.

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