KR20150030102A - A method of manufacturing separator for electrochemical device and separator for electrochemical device manufactured thereby - Google Patents

Abstract

The present invention relates to a manufacturing method of a separation membrane for an electrochemical device and the separation membrane for an electrochemical device manufactured by the method. The present invention has improved heat shrinkage and improved peel strength by: applying coating slurry after extracting a plasticizer from a drawn polyolefin film; and then performing a heat setting process in succession.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a separator for electrochemical devices and a separator for electrochemical devices manufactured therefrom,

The present invention relates to a process for producing a separator for an electrochemical device and a separator for an electrochemical device produced therefrom. More specifically, the process comprises applying a coating slurry after extracting a plasticizer from a stretched polyolefin film, And a separator for an electrochemical device produced from the separator. The present invention also relates to a separator for electrochemical devices.

Recently, interest in energy storage technology is increasing. As the application fields of cell phones, camcorders, notebook PCs and even electric vehicles are expanding, efforts for research and development of electrochemical devices are becoming more and more specified.

Electrochemical devices have attracted the greatest attention in this respect, among which the development of rechargeable secondary batteries has become a focus of attention. In recent years, in order to improve the capacity density and specific energy in developing such batteries, And research and development on the design of the battery.

It is very important to evaluate and secure safety in such an electrochemical device. The most important consideration is that the electrochemical device should not injure the user in case of malfunction. For this purpose, the safety standard strictly regulates the ignition and fuming in the electrochemical device. In the safety characteristics of the electrochemical device, there is a high possibility that the electrochemical device will be overheated to cause thermal runaway or explosion if the separator is penetrated. Particularly, a polyolefin porous substrate, which is typically used as a separator of an electrochemical device, exhibits extreme thermal shrinkage behavior at a temperature of 150 캜 or more due to characteristics of the manufacturing process including material properties and elongation, There is a problem.

In order to solve this problem, a composite membrane including a porous coating layer on which a slurry containing inorganic particles / organic particles and a binder polymer is coated on at least one surface of a polyolefin porous substrate having a plurality of pores has been proposed. In such a composite separator, the inorganic / organic particles in the coating layer coated on the polyolefin porous substrate serve as a support for maintaining the physical form of the coating layer, thereby suppressing the heat shrinkage of the polyolefin porous substrate when the lithium ion battery is overheated. In addition, micropores are formed between the inorganic particles or between the organic particles because of interstitial volume.

That is, in the conventional separation membrane production process, as shown in FIG. 1, the polyolefin resin composition is subjected to extrusion / casting, stretching, and extraction after heat setting to prepare a porous polyolefin film, and then a coating slurry is applied to the porous polyolefin film There is a limitation that the heat fixation process should be performed at a temperature at which the polyolefin film is not melted. After the coating slurry is coated on the porous substrate and dried, to further prevent destruction of the structural stabilization, There was a problem that was difficult to make.

The present invention provides a method for improving the heat shrinkage ratio of the composite separator having the porous coating layer and improving the peel strength of the porous coating layer.

In addition, the present invention provides a method of structurally stabilizing a composite separator having a porous coating layer.

According to one aspect of the present invention, there is provided a method for producing a polyolefin film, comprising the steps of extruding and stretching a resin composition containing a polyolefin and a plasticizer to obtain a sheet-shaped polyolefin film; extracting a plasticizer from the polyolefin film to obtain a porous polyolefin film A step of coating a slurry for forming a porous coating layer on at least one surface of the porous polyolefin film, and a step of heat setting.

The heat setting may be carried out at 128 to 135 占 폚.

The slurry may include inorganic particles and a binder polymer.

The slurry may include organic particles and a binder polymer.

According to another aspect of the present invention, a separation membrane for an electrochemical device is provided as described above.

According to still another aspect of the present invention, there is provided an electrochemical device including a positive electrode, a negative electrode, and a separation membrane interposed between an anode and a cathode, wherein the separation membrane is the separation membrane for an electrochemical device, / RTI >

The electrochemical device may be a lithium secondary battery.

According to the present invention, a composite slurry is prepared by applying a coating slurry after extracting a plasticizer in a polyolefin film, followed by heat setting to form a porous coating layer.

According to this, since the heat applied in the heat fixing process is transferred to the polyolefin film through the porous coating layer, heat treatment at a high temperature becomes possible, and the wettability of the coating slurry to the fibrilar structure of the polyolefin film is improved.

Further, since the heat applied in the heat fixing process is transferred to the polyolefin film through the porous coating layer, the polyolefin film has a fibril having a smaller diameter, thereby increasing the fibrilar number density per unit area, and the coating slurry Is increased.

As a result, the maintenance of the physical form of the polyolefin film becomes easier, the heat shrinkage ratio of the composite separator is improved, and the peel strength of the coating layer is improved.

1 schematically shows a conventional separation membrane production method.
2 schematically shows a separation membrane manufacturing method according to the present invention.
FIG. 3 is an enlarged photomicrograph of the surface of a conventional separation membrane for an electrochemical device in which a porous coating layer containing inorganic particles is formed on a porous substrate.
4 is an enlarged photograph of the surface of the separation membrane for an electrochemical device according to the present invention in which a porous coating layer containing inorganic particles is formed on a porous substrate.
FIG. 5 is an enlarged photomicrograph of the surface of a conventional separation membrane for an electrochemical device in which a porous coating layer containing organic particles is formed on a porous substrate.
6 is an enlarged photograph of the surface of a separation membrane for an electrochemical device according to the present invention in which a porous coating layer containing organic particles is formed on a porous substrate.

Hereinafter, the present invention will be described in detail. The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

As shown schematically in Fig. 2, the method for producing an isolation membrane for an electrochemical device of the present invention comprises the steps of extruding / casting a polyolefin and a plasticizer to obtain a sheet-like material, a step of stretching the obtained sheet, a step of stretching the stretched polyolefin film , A step of coating a slurry for forming a porous coating layer on at least one side of the polyolefin film from which the plasticizer has been extracted, and a step of heat setting the polyolefin film coated with the slurry.

The polyolefin in the present invention is not particularly limited as long as it is commonly used in the art. Specific examples of the polyolefin include high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE), ultra high molecular weight polyethylene (UHMWPE), polypropylene, polyethylene terephthalate, polybutylene terephthalate polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyetheretherketone, polyethersulfone, polyphenylsulfone, But are not limited to, polyphenylene oxide, polyphenylenesulfroid, polyethylenenaphthalene, and mixtures and copolymers thereof.

The plasticizer in the present invention is not particularly limited as long as it is commonly used in the art. Non-limiting examples of plasticizers include decalin, xylenes, dioctyl phthalate, dibutyl phthalate, stearyl alcohol, oleyl alcohol, decyl alcohol, nonyl alcohol, diphenyl ether, n-decane, n-dodecane, .

The plasticizer content is not particularly limited, but may be 20 mass% or more in terms of the porosity of the intended porous film, and 90 mass% or less in terms of viscosity. And preferably not less than 50 mass% and not more than 70 mass%.

In order to produce the composite membrane in the present invention, first, a part or the whole of the material is mixed using a Henschel mixer, a ribbon blender, a tumbler blender, or the like. Then, it is melted and kneaded by a screw extruder such as a single screw extruder, a twin screw extruder, a kneader or a mixer, and extruded from a T-die or a ring-shaped die. The kneaded / extruded melt can be solidified by compression cooling. Examples of the cooling method include direct contact with a cooling medium such as cold wind or cooling water, contact with a roll or a press cooled with a coolant, and the like.

The extruded melt is then stretched in film form. The stretching method may be carried out by a conventional method known in the art. Non-limiting examples include MD uniaxial stretching by a roll stretcher, TD uniaxial stretching by a tenter, roll stretching by a tenter, or a combination of a tenter and a tenter Simultaneous biaxial stretching by simultaneous biaxial stretching, simultaneous biaxial stretching by inflation molding, and the like. The stretching magnification is a total area magnification, and can be set to, for example, 7 times or more in consideration of the desired tensile strength and tensile elongation.

Then, the plasticizer is extracted from the stretched film. The extraction method is not particularly limited. For example, the plasticizer can be extracted by immersing or showering the polyolefin film in an extraction solvent. The extraction temperature may be any temperature at which the water-soluble polymer can be extracted, for example, from room temperature to about 100 캜. When the extraction temperature is lower than room temperature, the extraction speed is not fast. When the extraction temperature is higher than 100 ° C, the purified water starts to vaporize. As the extraction solvent used for the extraction of the plasticizer, it is preferable that the extraction solvent is a poor solvent for the polyolefin and is a good solvent for the plasticizer and has a boiling point lower than the melting point of the polyolefin. Nonlimiting examples of such an extraction solvent include hydrocarbons such as n-hexane and cyclohexane, halogenated hydrocarbons such as methylene chloride, 1,1,1-trichloroethane, and fluorocarbon, alcohols such as ethanol and isopropanol, And ketones such as acetone and 2-butanone.

The thickness of the porous polyolefin film obtained above is not particularly limited, but is preferably in the range of 5 to 50 占 퐉, and the pore size and porosity present in the porous substrate are also not particularly limited, but 0.001 to 50 占 퐉 and 10 to 99% desirable.

Then, at least one surface of the porous polyolefin film is coated with a slurry for forming a porous coating layer. To this end, a slurry for forming a porous coating layer must first be prepared. The slurry is prepared by dispersing a binder polymer together with inorganic particles or organic particles in a solvent.

The inorganic particles are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles usable in the present invention are not particularly limited as long as oxidation and / or reduction reaction does not occur in the operating voltage range of the applied electrochemical device (for example, 0 to 5 V based on Li / Li ⁺ ). Particularly, when inorganic particles having ion transfer ability are used, the ion conductivity in the electrochemical device can be increased to improve the performance. When inorganic particles having a high dielectric constant are used as the inorganic particles, dissociation of an electrolyte salt, for example, a lithium salt, in the liquid electrolyte is also increased, and ion conductivity of the electrolyte can be improved.

Non-limiting examples of the inorganic particles include high-permittivity inorganic particles having a dielectric constant of 5 or more, preferably 10 or more, inorganic particles having lithium ion-transporting ability, or a mixture thereof.

Non-limiting examples of inorganic particles greater than a dielectric constant of 5 is _{BaTiO 3, Pb (Zr, Ti} ) O 3 (PZT), Pb 1 - x La x Zr 1 - y Ti y O 3 (PLZT), PB (Mg 3 _{Nb 2/3) O 3 -PbTiO} 3 (PMN-PT), hafnia _{(HfO 2), SrTiO 3,} SnO 2, CeO 2, MgO, NiO, CaO, ZnO, ZrO 2, Y 2 O 3, Al 2 O ₃ , TiO _2, SiC Or a mixture thereof.

The term "inorganic particle having lithium ion-transferring ability" as used herein refers to an inorganic particle containing a lithium element but not having lithium stored therein and having a function of moving lithium ions. The inorganic particle has a lithium ion- examples include lithium phosphate (Li ₃ PO _4), lithium titanium phosphate _{(Li x Ti y (PO 4} ) 3, 0 <x <2, 0 <y <3), lithium aluminum titanium phosphate (Li _x Al _y Ti _z (LiAlTiP) _x O _y series such as (PO ₄ ) ₃ , 0 <x <2, 0 <y <1, 0 <z <3), 14Li ₂ O-9Al ₂ O ₃ -38TiO ₂ -39P ₂ O ₅ (0 <x <4, 0 <y <13), lithium lanthanum titanate (Li _x La _y TiO ₃ , 0 <x <2, 0 <y <3), Li _{3 .25} Ge _{0 . 25} P _{0 .75} S ₄ Lithium, such as germanium Mani help thiophosphate lithium nitro, such as _{(Li x Ge y P z S} w, 0 <x <4, 0 <y <1, 0 <z <1, 0 <w <5), Li 3 N (Li _x N _y , 0 <x <4, 0 <y <2), Li ₃ PO ₄ -Li ₂ S-SiS ₂ SiS ₂ family, such as _{glass (Li x Si y S z} , 0 <x <3, 0 <y <2, 0 <z <4), LiI-Li 2 SP 2 S 5 There is P ₂ S ₅ based _{glass (Li x P y S z} , 0 <x <3, 0 <y <3, 0 <z <7) or a mixture thereof as such.

The coating slurry may contain organic particles in place of or in combination with the inorganic particles. Organic particles are advantageous in terms of air permeability, heat shrinkability and peel strength, and are excellent in binding property with a binder polymer.

Nonlimiting examples of organic particles that can be used in the slurry for forming the porous coating layer include polystyrene, polyethylene, polyimide, melamine resin, phenol resin, cellulose, modified cellulose (such as carboxymethylcellulose), polypropylene, polyester (Polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate and the like), polyphenylene sulfide, polyaramid, polyamideimide, and polyimide. The organic particles may be composed of two or more kinds of polymers.

The size of the inorganic particles or organic particles is not limited, but may be in the range of 0.001 to 10 탆 in terms of forming a coating layer of uniform thickness and having an appropriate porosity.

The binder polymer used in the slurry for forming the porous coating layer is not particularly limited as long as it can function as a link between inorganic particles or organic particles and stably fix them. Non-limiting examples of the binder polymer include polyvinylidene fluoride-hexafluoro Polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichlorethylene, polymethylmethacrylate, polyacrylonitrile, polyvinylpyrrolidone polyvinylpyrrolidone, polyvinylacetate, polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate, cellulose acetate butyrate, cellulose acetate But are not limited to, cellulose acetate propionate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, Carboxyl methyl cellulose, acrylonitrile-styrene-butadiene copolymer, and polyimide. These resins may be used alone or in combination of two or more thereof. Can be used.

The composition ratio of the inorganic particles / organic particles and the binder polymer in the slurry for forming the porous coating layer may be, for example, in the range of 50:50 to 99: 1 by weight or 70:30 to 95: 5. If the content of the inorganic particles / organic particles relative to the binder polymer is too small, the improvement of the thermal stability of the separation membrane may be deteriorated and the void space formed between the inorganic particles / organic particles may not be sufficiently formed to decrease the pore size and porosity Resulting in deterioration of final cell performance. On the other hand, if the amount of the inorganic particles / organic particles relative to the binder polymer is too large, the peeling property of the porous coating layer may be weakened.

The solvent of the coating slurry is preferably such that the dispersion of the inorganic particles / organic particles and the binder polymer can be made uniform and can be easily removed thereafter. Non-limiting examples of usable solvents include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone ( N-methyl-2-pyrrolidone, NMP), cyclohexane, water or a mixture thereof.

The coating slurry is applied to at least one side of the porous substrate. The specific coating method may be a conventional coating method known in the art. For example, a dip coating, a die coating, a roll coating, , Comma coatings or mixtures thereof

And the like can be used. Further, the porous coating layer can be selectively formed on both or only one side of the porous substrate.

The slurry for forming the porous coating layer is coated on at least one surface of the porous substrate, and then the heat setting process is performed.

The hot fix is a process of holding the film and applying heat to force the film to be shrunk to remove the residual stress. If the heat fixing temperature is higher, the shrinkage rate is decreased. However, if the heat setting temperature is too high, the polyolefin film is partially melted, so that the formed micropores may be clogged and the permeability may be lowered.

In the present invention, since the plasticizer is extracted after the stretching with the polyolefin film, and then the slurry for coating the porous coating layer is coated and heat-set is performed, unlike the conventional process of extracting the plasticizer after stretching with the polyolefin film and heat- , The coating slurry which is not a polyolefin film is subjected to heat setting, so that heat is not directly applied to the polyolefin film. Therefore, even if heat fixing is performed at a higher temperature than in the prior art, melting of the polyolefin film can be suppressed. In addition, since the amount of heat directly applied to the polyolefin film is small, the fibril thickness of the polyethylene-based adjacent to the porous coating layer is made thinner than that of the conventional heat-treated polyolefin film. As a result, per unit area of the porous film surface adjacent to the porous coating fibrilar number density is increased, because the coating is increased interfacial contact area between the slurry, the high temperature region above the glass transition temperature of the coating slurry (T _g) or melting point (T _m) The wettability of the slurry with respect to the polyolefin porous fibrous structure can be improved.

According to the present invention, when a thermal process is performed at a temperature of from 128 to 135 占 폚, the bonding strength (peel strength) between the porous coating layer and the porous substrate is improved, and structural stability can also be secured.

The thickness of the porous coating layer may be in the range of 0.01 to 20 占 퐉, and the pore size and porosity may be in the range of 0.001 to 10 占 퐉, and the porosity may be in the range of 10 to 99% Lt; / RTI &gt; The pore size and porosity mainly depend on the size of the inorganic particles / organic particles. For example, when inorganic particles / organic particles having a particle size of 1 μm or less are used, pores formed also show about 1 μm or less. Such a pore structure is filled with an electrolyte solution to be injected at a later stage, and the filled electrolyte serves as an ion transfer function.

The electrochemical device may be manufactured according to a conventional method known in the art, and may be manufactured, for example, by assembling the cathode and the anode with the separator interposed therebetween, and then injecting an electrolyte solution .

The electrode to be used together with the separator 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 It is preferable to use a lithium composite oxide. As a non-limiting example of the anode active material, a conventional anode active material that can be used for an anode 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 are preferable. Non-limiting examples of the cathode current collector include aluminum, nickel, or a combination thereof, and examples of the anode current collector include copper, gold, nickel, or a copper alloy or a combination thereof Foil to be manufactured, and the like.

The electrolyte solution that can be used in one embodiment of the present invention is a salt having a structure such as A ⁺ B ^- , where A ⁺ includes ions consisting of alkali metal cations such as Li ⁺ , Na ⁺ , K ^{+ -} it 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} ^- 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), dipropyl (DMP), dimethylsulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP), ethylmethyl carbonate (EMC), gamma-butyrolactone Or an organic solvent composed of a mixture thereof, but the present invention is not limited thereto.

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 of applying the separator according to an embodiment 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 general 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. Embodiments of the invention are provided to more fully describe the present invention to those skilled in the art.

Example  One

High-density polyethylene having a weight average molecular weight of 500,000 as polyolefin and liquid paraffin having a kinematic viscosity of 68.00 cSt were extruded at a temperature of 210 DEG C at a weight ratio of 35:65. The stretching temperature was set to 115 캜, and the stretching ratio was elongated seven times in the longitudinal direction. Then, after the plasticizer extraction, a porous polyolefin film was obtained.

Then, in order to prepare a slurry for forming the porous coating layer, a slurry having a weight composition ratio of Al ₂ O ₃ particles / cyano alcohol / PVDF-HFP / acetone 13.5 / 0.225 / 1.275 / 85 was prepared.

The coating slurry was coated on one side of the porous polyolefin film to the extraction step to a thickness of 3.5 탆 and then heat-set at 130 캜 at 5 m / min.

Example  2

A separator for an electrochemical device was prepared in the same manner as in Example 1, except that an organic particle slurry (acrylate / water = 20/80) was used as the coating slurry.

Comparative Example  One

High-density polyethylene having a weight average molecular weight of 500,000 and liquid paraffin having a kinematic viscosity of 68.00 cSt at a weight ratio of 35:65 were extruded at a temperature of 210 占 폚 in the same manner as in Example 1 except for the step of extracting the plasticizer from the polyolefin film Respectively. The stretching temperature was 115 캜, and the stretching ratio was elongated seven times in the longitudinal direction. Subsequently, the plasticizer was extracted and then heat-set at 130 ° C at 5 m / min to prepare a porous polyolefin film.

Comparative Example  2

A slurry for forming a porous coating layer having a weight composition ratio of Al ₂ O ₃ particles / cyano alcohol / PVDF-HFP / acetone 13.5 / 0.225 / 1.275 / 85 was prepared.

The coating slurry was coated on one side of the porous polyolefin film obtained in Comparative Example 1 and then heat-set at 130 占 폚 at 5 m / min.

Comparative Example  3

A separator for an electrochemical device was prepared in the same manner as in Comparative Example 2, except that an organic particle slurry (acrylate / water = 20/80) was used instead of Al ₂ O ₃ particles in the slurry for forming the porous coating layer.

Evaluation example

The aeration time, heat shrinkage and peel strength of each of the separators for electrochemical devices obtained in Examples 1 to 2 and Comparative Examples 1 to 3 were measured, and the results are shown in Table 1 below.

Example 1 Example 2 Comparative Example 1 Comparative Example 2 Comparative Example 3 Membrane Thickness (㎛) 14.0 14.0 10.5 14.0 14.0 Aeration time (s / 100 ml) 210 185 155 203 197 Porosity (%) - - 45.5 - - Peel strength (gf / 15 mm) 50 > 150 - 35 75 Heat shrinkage (%%, MD / TD) 5.5 / 7.5 3/4 8/17 6 / 9.8 6/7

As can be seen from the above, the separator of Comparative Example 1 in which the porous coating layer was not formed showed the lowest heat shrinkage rate. In Example 1 and Comparative Example 2 in which the heat fixing process was performed in a different order only and the same raw materials were used, Example 1 in which a slurry containing inorganic particles / organic particles was coated after extraction of a plasticizer from a polyolefin film was compared with Comparative Example 2 And exhibited better peel strength and heat shrinkage. Similarly, Example 2 exhibited superior peel strength and heat shrinkage than Comparative Example 3.

Claims (7)

Extruding and stretching a resin composition containing a polyolefin and a plasticizer to obtain a sheet-shaped polyolefin film,
Extracting a plasticizer from the polyolefin film to obtain a porous polyolefin film,
Coating at least one surface of the porous polyolefin film with a slurry for forming a porous coating layer, and
Wherein the step of forming the separator comprises the steps of:
The method according to claim 1,
Wherein the heat setting is performed at a temperature of from 128 to 135 占 폚.
The method according to claim 1,
Wherein the slurry comprises inorganic particles and a binder polymer.
The method according to claim 1,
Wherein the slurry comprises organic particles and a binder polymer.
A separator for an electrochemical device, which is produced by the method of any one of claims 1 to 4.
An electrochemical device comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode,
The electrochemical device according to claim 5, wherein the separation membrane is the separation membrane for an electrochemical device according to claim 5.
The method according to claim 6,
Wherein the electrochemical device is a lithium secondary battery.

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