KR20170014223A - Separator Manufacturing Method Comprising Thermal Resistant Coating Layer Having an Improved Ion Conductivity and Separator Manufactured thereby - Google Patents

Abstract

Provided are a separator manufacturing method for a secondary battery, and a separator manufactured thereby. The separator manufacturing method for a secondary battery comprises the steps of: (1) preparing a porous thin film consisting of a polyolefin-based polymer; (2) preparing a coating liquid including a heat resistant polymer having a melting point which is relatively higher than the porous thin film consisting of a polyolefin-based polymer, a porogen inducing to form a pore in a coating layer, and a solvent; (3) forming a heat resistant coating layer in at least one side of the porous thin film by drying after coating the porous thin film with the coating liquid; and (4) forming a pore on the heat resistant coating layer.

Description

TECHNICAL FIELD [0001] The present invention relates to a process for producing a separator comprising a heat-resistant coating layer having improved ionic conductivity and a separator prepared therefrom,

The present invention relates to a separation membrane production method, and more particularly, to a separation membrane production method comprising coating a porous membrane containing a porogen to induce pore formation in a coating layer to form a porous membrane, To a separator comprising a heat-resistant coating layer having improved ionic conductivity.

As technology development and demand for mobile devices are increasing, the demand for batteries as energy sources is rapidly increasing, and accordingly, a lot of researches on batteries capable of meeting various demands have been conducted.

Particularly in terms of the shape of the battery, there is a high demand for a prismatic secondary battery and a pouch type secondary battery which can be applied to products such as mobile phones with a thin thickness, and has advantages such as high energy density, discharge voltage, There is a high demand for lithium secondary batteries such as lithium ion batteries and lithium ion polymer batteries.

Such a lithium secondary battery uses a metal oxide such as LiCoO ₂ as a positive electrode active material, a carbon material and a lithium metal as a negative electrode active material, and a separator between the negative electrode and the positive electrode, and a nonaqueous electrolyte solution containing a lithium salt such as LiPF ₆ .

Since the separation membrane is required to serve as a lithium ion passage while insulating the positive electrode and the negative electrode from each other, it is necessary to secure mechanical safety necessary for mass production including a predetermined gap. In recent years, As heat accumulates in the cell, high thermal stability is required.

However, the polyolefin-based separator generally used has a melting point of 80 to 200 DEG C. When the internal temperature of the battery cell rises sharply due to a metal foreign substance or the like, the separator shrinks (thermal shrinkage) There was a risk of additional heat generation and ignition due to an internal short circuit.

In the case of coating an organic / inorganic compound on the separator to secure the thermal and mechanical stability of the separator, since the coated oil / inorganic compound blocks the pores of the separator substrate, electrolyte impregnability of the separator is lowered, This is not smooth.

Therefore, there is a high need for a process for producing a separator having a high thermal and mechanical safety while securing the porosity of the separator substrate and having sufficient ion conductivity.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art and the technical problems required from the past.

Specifically, the object of the present invention is to provide a polyolefin-based porous thin film having a heat resistant coating layer having a predetermined porosity by coating a coating liquid containing a porogen and a heat-resistant polymer, forming pores using the porogen, , Thermal and mechanical stability as well as ion conductivity of lithium ion and a method for producing the separator.

According to an aspect of the present invention,

(i) preparing a porous thin film made of a polyolefin-based polymer;

(ii) preparing a heat resistant polymer having a relatively higher melting point than the polyolefin-based polymer of the porous thin film, a porogen for inducing formation of pores in the coating layer, and a coating solution;

(iii) coating the coating liquid on the porous thin film and drying the coating liquid to form a heat resistant coating layer on at least one surface of the porous thin film; And

(iv) forming pores in the heat resistant coating layer;

And a control unit.

Therefore, the separation membrane manufacturing method according to the present invention ensures mechanical stability for maintaining the process through the porous thin film made of a polyolefin-based polymer, secures thermal stability through the heat resistant coating layer, It is possible to produce a separator having improved ionic conductivity.

When the coating layer is formed on the porous thin film as described above, conventionally, the particles contained in the coating liquid in the coating process clog the pores of the porous thin film, or the coating liquid having poor porosity on the surface of the porous thin film as the coating base forms a coating film , The electrolyte impregnability of the separator is lowered and the migration of lithium ions is not smooth.

Accordingly, the inventors of the present application have conducted intensive research and various experiments, and have found that a coating solution contains a porogen, that is, a porogen inducing material, coating a coating liquid on the porous thin film, It was confirmed that the electrolyte membrane impregnation property of the separator prepared from this was remarkably improved and the lithium ion conductivity was improved, and the present invention was accomplished.

In addition, the separation membrane produced by the production method according to the present invention comprises a heat-resistant polymer having a relatively higher melting point than the polyolefin-based polymer in a porous thin film made of a polyolefin-based polymer having a relatively low mechanical stability but a somewhat lower thermal stability There is an effect that thermal and mechanical stability is improved by forming a heat resistant coating layer with a coating liquid.

Hereinafter, a method for producing a separation membrane according to the present invention will be described in more detail.

First, in step (i), a porous thin film made of a polyolefin-based polymer is prepared. As described above, the porous thin film has mechanical properties required for the assembly process and sufficient porosity to allow lithium ions to migrate.

At this time, the porous thin film may be made of a polyolefin-based polymer having excellent chemical resistance and hydrophobicity, such as polyethylene and polypropylene. The porous thin film made of such a polyolefin-based polymer is excellent in stretchability and can easily control the porosity, and the pores are formed through stretching or stretching processes.

The polyolefin-based porous thin film having such voids is advantageous in that it has less ionic conductivity and less deformation due to external chemical reaction, but its thermal stability is somewhat deteriorated. Thus, the production method according to the present invention is advantageous in that the porous thin- A separator having both a mechanical stability and a thermal stability can be manufactured by forming an excellent heat resistant coating layer.

The melting point of the polyolefin-based polymer may be from 80 to 200 ° C, and the melting point of the heat-resistant polymer may be from 230 to 500 ° C under a condition higher than the melting point of the polyolefin-based polymer.

When the melting point of the polyolefin-based polymer is less than 80 ° C., the pore of the separating membrane is closed at a low temperature, so that the migration of lithium ions can be blocked even though the battery cell is not sufficiently heated. On the contrary, when the melting point of the polyolefin- ° C., the above-described closing function does not work at a high temperature, which makes it difficult to prevent excessive heat generation of the battery cell, which is not preferable. For the same reason, the melting point of the polyolefin-based polymer may be in the range of 90 占 폚 to 180 占 폚, more specifically, 100 占 폚 to 160 占 폚.

On the other hand, the melting point of the heat-resistant polymer is preferably higher than the melting point of the polyolefin-based polymer so that heat shrinkage of the separator does not occur even when the internal temperature of the battery cell instantaneously rises due to a metal foreign substance or the like, Specifically, it may be 250 ° C to 450 ° C, more specifically 270 ° C to 400 ° C.

Accordingly, the separation membrane manufacturing method according to the present invention provides a separation membrane for a secondary battery, which has sufficient lithium ion mobility, mechanical rigidity, and thermal stability.

Next, in the processes (ii) and (iii), a coating liquid containing the heat-resistant polymer, the porogen and the solvent is prepared, the coating liquid is coated on the porous thin film prepared in the step (i) To form a heat-resistant coating layer.

Specifically, in the step (ii), a coating liquid containing a porogen is prepared. The coating solution is prepared by dissolving or dispersing a heat-resistant polymer having a relatively higher melting point and a porogen in a solvent than the polyolefin-based polymer constituting the porous thin film.

At this time, the heat-resistant polymer contained in the coating liquid may be contained in an amount of 0.5% by weight to 50% by weight based on the total weight of the coating liquid.

If the content of the heat resistant polymer exceeds 50 wt%, the coating liquid may be difficult to uniformly coat, and dispersion in the solvent may become difficult. When the content of the heat resistant polymer is less than 0.5 wt%, the desired heat resistance It is not preferable because it is hard to secure. For this reason, the content of the heat-resistant polymer may be 0.5 wt% to 40 wt%, more specifically 1 wt% to 30 wt%, based on the total weight of the coating solution.

The heat-resistant polymer is not particularly limited as long as it is a material that satisfies the above-mentioned range of melting point under a condition higher than the melting point of the polyolefin-based polymer, but may be a polyamide, a polycarbonate, a polyaniline, , Polystyrene, and polytetrafluoroethylene, and specifically may be a polyaniline having a melting point exceeding 300 ° C.

On the other hand, the solvent is not particularly limited as long as it can dissolve the heat-resistant polymer easily, but may be formic acid, dimethyl formamide, tetrahydrofuran, dichlromethane, And may be one or a mixture of two or more selected from the group consisting of chloroform, toluene, and methylethyl ketone.

Specifically, when the heat-resistant polymer includes polyamide, a formic acid solvent can be used; When polycarbonate is included, dimethylformamide and tetrahydrofuran may be mixed in a ratio of 1: 1, or dichloroform or tetrahydrofuran may be used; When polyaniline is included, chloroform can be used; When polystyrene is contained, tetrahydrofuran, dimethylformamide, toluene, methyl ethyl ketone, chloroform, and tetrahydrofuran may be used, but the present invention is not limited thereto and may be appropriately selected depending on the physical properties of the heat resistant polymer and the physical properties of the porogen Of course.

The porogen contained in the coating solution may be contained in an amount of 0.01 wt% to 60 wt% with respect to the total weight of the coating solution. When the porogen content is more than 60% by weight, pores are formed excessively and thereafter, the removal process time is increased. When the porogen content is less than 0.01% by weight, it is difficult to secure the desired porosity. For the same reason, the content of the porogen may be 0.1 wt% to 55 wt%, more specifically 1 wt% to 50 wt%, based on the total weight of the coating liquid.

The porogen may be one or a mixture of two or more selected from the group consisting of cellosolves, esters, glycols, and ethers, although it is not particularly limited as long as it is a substance that induces pore formation in the heat resistant coating layer.

Specifically, the cellosolve may be methylcellosolve or ethylcellosolve; The esters are ethylene glycol monomethyl ether acetate or propylene glycol monomethyl ether acetate; The glycols are polyethylene glycol or polypropylene glycol; The ethers may be polyoxyethylene monomethyl ether or polyoxyethylene dimethyl ether. By using these porogens, a uniform three-dimensional mesh-like skeleton or uniform pores can be formed.

Next, in the step (iii), the coating liquid prepared in the step (ii) is coated on the porous thin film of the step (i) on the porous thin film and dried to form a porous thin film, To form a heat resistant coating layer. The heat-resistant coating layer may be formed on one side of the porous thin film or on both sides of the porous thin film.

The coating process for forming the coating layer is not particularly limited as long as it can form a heat resistant coating layer on the surface of the porous thin film, and commonly used spraying method, dip coating method, die coating method and the like can be suitably used.

In one specific example, step (iii) may be performed by dip coating. The dip coating method is a method in which a coating material is dipped in a coating solution or slurry and then a coating layer is formed on the surface of the coating material, and the process is simple and the utilization efficiency of the coating solution is high.

However, when such a dip coating method is applied to a porous membrane to introduce a coating layer into the porous membrane, the coating liquid may penetrate into the pores of the porous membrane during immersing the porous membrane in the coating liquid, thereby blocking the pores. Decrease the conductivity.

Accordingly, the separation membrane manufacturing method according to the present invention includes a porogen in the coating liquid so as to secure a predetermined porosity even when the separation membrane is coated by the dip coating method, and then the porogen is formed using the porogen And has the effect of manufacturing a separator having a high ionic conductivity while ensuring thermal stability and mechanical stability.

Therefore, a heat-resistant coating layer containing porogen is formed on the porous thin film by using a dip coating method or the like, and in the step (iv), pores are formed in the heat-resistant coating layer.

The process (iv) is carried out using porogens contained in the applied coating layer. The method is not particularly limited, but may be carried out by removing the porogen or by phase reversal of the solvent and the porogen. Describes the process of forming pores by using porogens.

First, the step (iv) may be performed by removing the porogen from the heat resistant coating layer.

Specifically, the step (iv) is performed by immersing a porous thin film having a heat resistant coating layer formed on a solvent dissolving the porogen.

The solvent is not particularly limited as far as the porogen is dissolved and the polyolefin-based polymer and the heat-resistant polymer are not dissolved and the porogen can be removed from the separation membrane. However, the solvent is not limited to water, alcohol, DMF, DMSO, and THF And may be a selected one or a mixture of two or more.

This removal process may be performed once or repeatedly, and in some cases, supercritical fluid may be used or aggressive cleaning may be performed. In one specific example, the removal process is performed in the state of applying ultrasonic waves, so that the porogen can be removed more quickly and efficiently, but the present invention is not limited thereto.

The step (iv) may be performed by phase inversion of the solvent and the porogen, which is performed by adding an electrolyte or by physical vibration. This phase reversal process may be performed singly or simultaneously with the above-described porogen removal process.

Through the pore formation process, pores are formed in the heat resistant coating layer formed by the dip coating method or the like, and a separation membrane having improved ionic conductivity can be obtained. After the step (iv), the drying process of the porous film having the heat resistant coating layer ) Can be further performed to remove the solvent remaining in the coating layer, the solvent used for removing the porogen, and the like from the separation membrane.

The drying process (v) may be performed before and / or after the pore formation process (iv) using the porogen. The drying process (v) may be performed at room temperature or may be performed at a constant temperature to control the curing rate and porosity of the heat resistant coating layer. have.

The present invention also provides a separation membrane produced by the above production method,

A porous thin film made of a polyolefin-based polymer; And

A heat-resistant coating layer which is in close contact with at least one surface of the porous thin film and is made of a heat-resistant polymer having a relatively higher melting point than the polyolefin-based polymer of the porous thin film;

And a separator for a secondary battery.

The porous thin film may be formed of a polyolefin-based polymer having excellent chemical resistance and hydrophobicity, such as polyethylene and polypropylene. As a representative example of a commercially available porous thin film made of a polyolefin-based polymer, a cell guard series (Celgard 占 2400, 2300 (manufactured by Hoechest Celanese Corp.), polypropylene membrane (manufactured by Ube Industries Ltd. or Pall RAI) (Tonen or Entek) separator may be used, but the present invention is not limited thereto.

The porous thin film made of such a polyolefin-based polymer is excellent in stretchability and can easily control the porosity. It is possible to improve the mobility of lithium ions by forming voids in the range of 0.1 탆 to 5 탆 in diameter, The porosity can be in the range of 20% to 60%.

If the porosity of the porous thin film is less than 20%, the electrolyte impregnability of the separator is lowered and the lithium ion can not move smoothly. If the porosity of the porous thin film is more than 60%, it is difficult to secure the desired mechanical rigidity. For the same reason, the porosity may be between 25% and 55%, in particular between 30% and 50%.

The thickness of the porous thin film is not particularly limited, but is preferably within a range of 1 탆 to 100 탆 in order to facilitate the electrode assembly process.

The heat-resistant polymer may be one or a mixture of two or more selected from the group consisting of polyamide, polycarbonate, polyaniline, polystyrene, and polytetrafluoroethylene , Specifically, polyaniline.

All of these polymers have a high melting point and heat resistance is imparted to the separator through the heat resistant coating layer. As a result, even when the internal temperature of the battery cell rises sharply, the polyolefin separator It prevents melting and prevents explosion and ignition by internal short circuit.

The porosity of the heat resistant coating layer may be 10% to 80%, more specifically 20% to 80%, and more specifically 40% to 80%. Porosity can be controlled by porogen content, drying conditions, and the like.

In general, the coating layer produced by dip coating or the like has a low porosity. However, the separator according to the present invention has a porosity formation process using a porogen in the course of production, and thus has a high porosity. Therefore, the ionic conductivity and the electrolyte impregnation The separator according to the present invention can provide a secondary battery having excellent battery characteristics such as rate characteristics, cycle characteristics and stability together with stability.

Further, the thickness of the heat-resistant coating layer may be 0.1 to 60 탆, specifically 1 to 55 탆, more specifically 3 to 50 탆. If the heat resistant coating layer is too thick, the process may be delayed, which is undesirable. When the heat resistant coating layer is too thin, it is difficult to secure the desired thermal stability.

The present invention also provides an electrode assembly including the positive electrode and the negative electrode, the separator for the secondary battery sandwiched between the positive electrode and the negative electrode, and a lithium secondary battery including the electrode assembly. The lithium secondary battery has a structure in which an electrode assembly is impregnated with a lithium-containing non-aqueous electrolyte.

The anode can be prepared, for example, by applying a slurry prepared by mixing a cathode mixture to a solvent such as NMP, onto a cathode current collector, followed by drying and rolling.

In addition to the positive electrode active material, the positive electrode mixture may optionally include a conductive material, a binder, a filler, and the like.

The cathode active material is a material capable of causing an electrochemical reaction. The lithium transition metal oxide includes lithium cobalt oxide (LiCoO ₂ ), lithium cobalt oxide (LiCoO ₂ ) containing two or more transition metals and substituted with one or more transition metals, lithium Layered compounds such as nickel oxide (LiNiO ₂ ); Lithium manganese oxides substituted with one or more transition metals; Formula _{LiNi 1-y M y O 2} ( where, M = Co, Mn, Al , Cu, Fe, Mg, B, Cr, Zn or Ga and Lim, 0.01≤y≤0.7 include one or more elements of the element) A lithium nickel-based oxide represented by the following formula: _{Li 1 + z Ni 1/3 Co 1/3} Mn 1/3 O 2, Li 1 + z Ni 0.4 Mn 0.4 Co 0.2 O 2 , such as _{Li 1 + z Ni b Mn c} Co 1- (b + c + d _{) M d O (2-e} ) A e ( where, -0.5≤z≤0.5, 0.1≤b≤0.8, 0.1≤c≤0.8, 0≤d≤0.2 , 0≤e≤0.2, b + c + d <1, M = Al, Mg, Cr, Ti, Si or Y, and A = F, P or Cl; lithium nickel cobalt manganese composite oxide; And the formula _{Li 1 + x M 1-y} M 'y PO 4-z X z ( wherein, M = a transition metal, preferably Fe, Mn, Co or Ni, M' = Al, Mg or Ti, X = F, S or N, and -0.5? X? +0.5, 0? Y? 0.5, and 0? Z? 0.1), but the present invention is not limited thereto.

The conductive material is usually added in an amount of 1 wt% to 30 wt% based on the total weight of the mixture including the cathode active material. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing a chemical change in the battery, and includes, for example, graphite; Carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used. Concrete examples of commercially available conductive materials include acetylene black series such as Chevron Chemical Company, Denka Singapore Private Limited, Gulf Oil Company, etc.), Ketjenblack, EC (Armak Company), Vulcan XC-72 (Cabot Company), and Super P (Timcal).

The binder is a component that assists in bonding of the active material and the conductive material and bonding to the current collector, and is usually added in an amount of 1 to 30 wt% based on the total weight of the mixture containing the cathode active material. Such a binder may or may not be the same as the binder polymer component on the separator described above. Non-limiting examples include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, and various copolymers.

The filler is not particularly limited as long as it is a fibrous material which is selectively used as a component for suppressing the expansion of the electrode and does not cause chemical change in the battery. Examples of the filler include olefin-based polymerizers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

The cathode current collector is generally made to have a thickness of 3 to 500 mu m. Such a positive electrode collector is not particularly limited as long as it has conductivity without causing chemical change to the battery, and may be formed on the surface of stainless steel, aluminum, nickel, titanium, sintered carbon or aluminum or stainless steel Carbon, nickel, titanium, silver, or the like may be used. The current collector may have fine irregularities on the surface thereof to increase the adhesive force of the cathode active material, and various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric are possible.

The negative electrode is prepared, for example, by applying a negative electrode mixture containing a negative electrode active material on a negative electrode collector and then drying the same. The negative electrode mixture may contain a conductive material, a binder, a filler, May be included.

Examples of the negative electrode active material include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and Ti which can be alloyed with lithium and compounds containing these elements; Complexes of metals and their compounds and carbon and graphite materials; Lithium-containing nitrides, and the like. Among them, a carbon-based active material, a silicon-based active material, a tin-based active material, or a silicon-carbon based active material is more preferable, and these may be used singly or in combination of two or more.

The negative electrode current collector generally has a thickness of 3 to 500 mu m. The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery. Examples of the negative electrode current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

The lithium salt-containing nonaqueous electrolyte solution is composed of an electrolyte solution and a lithium salt. As the electrolyte solution, a nonaqueous organic solvent, an organic solid electrolyte, and an inorganic solid electrolyte may be used.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma -Butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran, 2-methyltetrahydrofuran, dimethylsulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane , Acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate Nonionic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyrophosphate, ethyl propionate and the like can be used.

Examples of the organic solid electrolyte include a polymer electrolyte such as a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, an agitation lysine, a polyester sulfide, a polyvinyl alcohol, a polyvinylidene fluoride, A polymer containing an ionic dissociation group and the like may be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides and sulfates of Li such as Li ₄ SiO ₄ -LiI-LiOH and Li ₃ PO ₄ -Li ₂ S-SiS ₂ can be used.

The lithium salt is a material that is readily soluble in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO 4, LiBF 4, LiB 10 Cl 10, LiPF 6, LiCF 3 SO 3, LiCF 3 CO 2, LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

For the purpose of improving the charge / discharge characteristics and the flame retardancy, the electrolytic solution is preferably mixed with an organic solvent such as pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylenediamine, glyme, Benzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, . In some cases, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further added to impart nonflammability. In order to improve the high-temperature storage characteristics, carbon dioxide gas may be further added. FEC (Fluoro-Ethylene Carbonate, PRS (Propene sultone), and the like.

A stack / folding type electrode assembly having a structure in which a lithium secondary battery of the present invention is formed by winding a bi-cell or full cell stacked with a separator sheet in a state where a predetermined unit of positive and negative electrodes are stacked with a separator interposed therebetween, The separator sheet to which the unit cells are bonded may be the same as or different from the separator interposed between the anode and the cathode. In one specific example, the separator sheet may be a sheet having a heat-resistant coating layer formed on a polyolefin porous thin film like the separator, and a sheet including a heat-resistant coating layer having improved ionic conductivity using a porogen as described above.

Also, the present invention provides a battery module including the lithium secondary battery as a unit cell, a battery pack including the battery module, and a device including the battery pack.

Specific examples of the device include a power tool which is powered by an electric motor and moves; An electric vehicle including an electric vehicle (EV), a hybrid electric vehicle (HEV), a plug-in hybrid electric vehicle (PHEV), and the like; An electric motorcycle including an electric bike (E-bike) and an electric scooter (E-scooter); An electric golf cart; And a power storage system, but the present invention is not limited thereto.

As described above, the separation membrane manufacturing method according to the present invention is characterized in that a coating liquid containing porogen is coated on a polyolefin porous thin film, pores are formed with the porogen, and at least one surface of the porous thin film has a high porosity A separator having excellent electrolyte impregnability and ionic conductivity can be produced even when a dip coating method is used. Therefore, the manufacturing process is simple, the utilization efficiency of the material is high, the cost is reduced, and the thermal and mechanical stability And the ion conductivity is always maintained.

Hereinafter, the content of the present invention will be described in detail by way of examples, but the scope of the present invention is not limited thereto.

Preparation of Membrane

&Lt; Example 1 >

Was added to a chloroform solvent such that the content of polyaniline was 10% by weight and the content of polyethylene glycol (porogen) was 40% by weight, and the mixture was stirred to prepare a coating solution.

A polyolefin-based porous film (Celgard ^TM , polyolefin-based membrane) was repeatedly immersed in a coating liquid twice to form a heat-resistant coating layer containing porogen in the porous film, and after drying, the porogen was removed from the membrane using water Thus, a separation membrane having a porous heat-resistant coating layer was prepared.

The thickness of the polyolefin porous thin film was 20 탆, the porosity was 40%, the thickness of the heat resistant coating layer was 3 탆, and the porosity was 40%.

&Lt; Example 2 >

A separator was prepared in the same manner as in Example 1, except that the coating liquid was prepared by adding a polyethylene glycol (porogen) to a chloroform solvent so that the content of the polyethylene glycol (porogen) was 45% by weight.

&Lt; Example 3 >

A separator was prepared in the same manner as in Example 1, except that the coating solution was prepared by adding polyethylene glycol (porogen) to a chloroform solvent so that the content of polyethylene glycol (porogen) was 50% by weight.

<Example 4>

A separator was prepared in the same manner as in Example 1, except that the coating liquid was prepared by adding polyethylene glycol (porogen) to a chloroform solvent so that the content of the polyethylene glycol (porogen) was 55% by weight.

&Lt; Example 5 >

A separator was prepared in the same manner as in Example 1, except that polyoxyethylene dimethyl ether was used as a porogen to prepare a coating liquid.

&Lt; Example 6 >

A separator was prepared in the same manner as in Example 1, except that the porogen was removed by using methanol as a solvent.

&Lt; Comparative Example 1 &

A porous polyolefin thin film (Celgard ^TM , polyolefin separator) was used as a separator.

&Lt; Comparative Example 2 &

Was added to a chloroform solvent such that the content of polyaniline became 10% by weight and stirred to prepare a coating solution.

A polyolefin porous thin film (Celgard ^TM , polyolefin type separator) was repeatedly immersed in a coating liquid twice to form a heat resistant coating layer containing porogen in the porous thin film and dried to prepare a separator having a heat resistant coating layer.

Heat shrinkage measurement

<Experimental Example 1>

The separation membrane (5 cm 2 cm) prepared in the above Examples and Comparative Examples was sandwiched between the slide glasses, and both ends were fixed with a clip. The resultant was placed in an oven maintained at a set temperature of 150 ° C and 200 ° C and maintained for 30 minutes After the separator was taken out from the oven and cooled at room temperature, the area shrinkage (%) was measured and shown in Table 1 below.

Temperature Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Comparative Example 1 Comparative Example 2 150 ℃ 10 13 17 20 11 11 72 5 200 ℃ 15 19 25 28 14 16 melt 12

Referring to Table 1, Comparative Example 1 in which the heat resistant coating layer was not formed was shrunk by 72% at a temperature of 150 ° C and significantly melted at a temperature of 200 ° C, whereby the separation membrane melted. That is, when the internal temperature of the battery instantaneously rises due to an internal short circuit or the like, the separation membrane made of a polyolefin-based polymer as in Comparative Example 1 is liable to ignite and explode.

On the other hand, the shrinkage percentage of the membranes of Examples 1 to 6 and Comparative Example 2 including the heat resistant coating layer was similar. Example 1 in which the content of porogen in the coating liquid is 40% shows higher thermal stability than that in Example 4 in which porogen content is 55%, and Comparative Example 2 in which porogen is not contained shows higher thermal stability It can be seen that the smaller the pores in the separator, the better the protective effect of the heat-resistant coating layer.

Therefore, it is preferable that the porogen is included in the range of forming the pores so that the protective effect of the heat resistant coating layer is maintained, and it is preferable that the porosity of the heat resistant coating layer is not more than 80% as described above.

Measurement of electrolyte absorption

<Experimental Example 2>

The separator samples (3 cm × 3 cm) prepared in the above Examples and Comparative Examples were immersed in a 1 M LiPF ₆ EC: DEC: EMC (4: 3: 3, volume ratio) electrolyte solution at room temperature for about 2 hours, Was removed with paper filter paper, and the weight was measured. The weight increase rate (%) was measured and shown in Table 2 below.

Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Comparative Example 1 Comparative Example 2 170 164 160 150 168 172 166 53

Referring to Table 2, the electrolyte uptake of Comparative Example 1 in which the heat resistant coating layer was not formed was similar to that of Examples 1 to 6, and Comparative Example 2 in which porogen was not included in the coating solution absorbed a significantly smaller amount of electrolyte.

This seems to be due to the fact that the heat-resistant coating layer of Comparative Example 2 formed a film to prevent the electrolyte from penetrating into the separator. On the other hand, Example 4, which is expected to have a high porosity, shows a lower amount of absorption than the other examples. It is not necessary that the porosity is necessarily high, Should be secured.

Manufacture of lithium secondary battery

LiCoO ₂ was used as the positive electrode active material and 2.5 wt% of LiCoO ₂ , 2.5 wt% of Super-P (conductive agent) and 2.5 wt% of PVDF (binder) were added to NMP (N-methyl-2-pyrrolidone) The mixture slurry was prepared and then coated on an aluminum current collector, dried and pressed to prepare a positive electrode.

Artificial graphite was used as an anode active material, and an anode mixture slurry was prepared by adding 95.5 wt% of artificial graphite, 2.5 wt% of Super-P (conductive agent) and 2 wt% of PVDF (binder) to NMP as a solvent, The whole phase was coated, dried and pressed to prepare a negative electrode.

The positive electrode and the negative electrode were interposed in the separator prepared in Examples and Comparative Examples, and an electrolyte solution was injected to prepare a lithium secondary battery. The electrolytic solution was prepared by dissolving LiPF ₆ electrolyte at a concentration of 1M using a mixed solvent of EC: DEC: EMC = 4: 3: 3 (volume ratio).

High Temperature Storage Characteristics

<Experimental Example 3>

The secondary batteries prepared using the separators according to Examples 1 to 4 and Comparative Example 1 were charged and discharged at 1 C in the interval of 2 V to 3.35 V to measure the capacity and discharge resistance, After storing SOC 100% for 2 weeks, the capacity and discharge resistance were measured again, and the results are shown in Table 3 below.

1st discharge 80 C, 2 wek, SOC 100% Capacity increase and decrease
(%) Increase / decrease of resistance (%) 1C Capacity (mAh) Discharge resistance
(10s, m) 1C Capacity (mAh) Discharge resistance
(10s, m) Example 1 32.8 3.5 29.3 3.9 -11 11 Example 2 33.5 3.7 29.5 4.2 -12 13.5 Example 3 33.1 3.9 30.1 4.3 -9.1 10.3 Example 4 33.2 4.0 29.8 4.5 -7.5 12.5 Comparative Example 1 33.4 3.4 29.1 3.8 -12.9 12.9 Comparative Example 2 - - - - - -

Referring to Table 3, the initial capacity and discharge resistance of the battery manufactured using the separator of Examples 1 to 4 and Comparative Example 1 are similar, but Comparative Example 1 in which the heat resistant coating layer is not formed has the following characteristics: Lt; RTI ID = 0.0 &gt; increased &lt; / RTI &gt; resistance.

In Comparative Example 2, the lithium ion had a remarkably low conductivity including a heat-resistant coating layer in which voids were not formed by the dip coating method, so that the capacity and resistance of the battery could not be measured.

In other words, unlike the comparative example, the separation membrane according to the present invention can form a heat resistant coating layer by a dip coating method on a porous thin film excellent in mechanical stability, thereby easily achieving thermal stability, A separation membrane for a secondary battery excellent in ion conductivity is provided.

Those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims (22)

(i) preparing a porous thin film made of a polyolefin-based polymer;
(ii) preparing a heat resistant polymer having a relatively higher melting point than the polyolefin-based polymer of the porous thin film, a porogen for inducing formation of pores in the coating layer, and a coating solution;
(iii) coating the coating liquid on the porous thin film and drying the coating liquid to form a heat resistant coating layer on at least one surface of the porous thin film; And
(iv) forming pores in the heat resistant coating layer;
Wherein the separator is formed of a metal. The polyolefin-based polymer according to claim 1, wherein the polyolefin-based polymer has a melting point of from 80 캜 to 200 캜;
Wherein the melting point of the heat-resistant polymer is 230 ° C to 500 ° C under a condition higher than the melting point of the polyolefin-based polymer. The method of claim 1, wherein the heat-resistant polymer contained in the coating liquid is contained in an amount of 0.5 to 50 wt% based on the total weight of the coating liquid. The method of claim 1, wherein the heat-resistant polymer is one or two selected from the group consisting of polyamide, polycarbonate, polyaniline, polystyrene, and polytetrafluoroethylene. Wherein the mixture is a mixture of two or more species. The method of claim 1, wherein the solvent is selected from the group consisting of formic acid, dimethyl formamide, tetrahydrofuran, dichlromethane, chloroform, toluene, methyl ethyl ketone methylethyleketone) or a mixture of two or more thereof. The method of claim 1, wherein the porogen contained in the coating liquid is contained in an amount of 0.01 to 60 wt% based on the total weight of the coating liquid. The process for producing a separation membrane according to claim 1, wherein the porogen is one or a mixture of two or more selected from the group consisting of cellosolves, esters, glycols, and ethers. 8. The method of claim 7, wherein the cellosolve is methylcellosolve or ethylcellosolve; The esters are ethylene glycol monomethyl ether acetate or propylene glycol monomethyl ether acetate; The glycols are polyethylene glycol or polypropylene glycol; Wherein the ether is polyoxyethylene monomethyl ether or polyoxyethylene dimethyl ether. The method of claim 1, wherein the step (iii) is performed by dip coating. The method of claim 1, wherein the step (iv) is performed by removing the porogen from the heat resistant coating layer. 11. The method of claim 10, wherein the removing process is performed by immersing a porous thin film having a heat resistant coating layer on a solvent that dissolves the porogen. The process for producing a separation membrane according to claim 11, wherein the solvent is one or a mixture of two or more selected from the group consisting of water, alcohol, DMF, DMSO, and THF. 11. The method of claim 10, wherein the removing process is performed in a state where ultrasonic waves are applied. The method of claim 1, wherein the step (iv) is performed by phase inversion of a solvent and a porogen. The method of claim 1, wherein the drying step (v) of the porous thin film having the heat resistant coating layer is further performed after the step (iv). 15. A separation membrane produced by the method according to any one of claims 1 to 15,
A porous thin film made of a polyolefin-based polymer; And
A heat-resistant coating layer which is in close contact with at least one surface of the porous thin film and is made of a heat-resistant polymer having a relatively higher melting point than the polyolefin-based polymer of the porous thin film;
Wherein the separator for a secondary battery comprises: The separator for a secondary battery according to claim 16, wherein porosity of the porous thin film is 20% to 60%. 17. The separation membrane for a secondary battery according to claim 16, wherein the heat-resistant coating layer has a porosity of 10% to 80%. 17. The separation membrane for a secondary battery according to claim 16, wherein the thickness of the heat-resistant coating layer is from 0.1 mu m to 60 mu m. The separator for a secondary battery according to claim 16, wherein the heat resistant coating layer comprises pores formed by a porogen for inducing pore formation. 16. The electrode assembly according to claim 16, comprising an anode and a cathode, and a separation membrane for a secondary battery interposed between the anode and the cathode. A lithium secondary battery comprising the electrode assembly according to claim 21.