KR102510290B1 - Method for preparing lithium negative electrode coated with lithium nitride, lithium negative electrode prepared therefrom and lithium-sulfur battery including the same - Google Patents

Abstract

Disclosed is a method for manufacturing a lithium nitride surface-treated lithium anode, in which a protective layer is formed on the surface of a lithium anode to improve lithium ion conductivity and lifespan characteristics of a battery, a lithium anode manufactured thereby, and a lithium-sulfur battery including the same . The manufacturing method of the lithium nitride surface-treated lithium anode includes the steps of a) dissolving an amine-based compound in an organic solvent; and b) forming a lithium nitride (Li ₃ N) protective layer on the surface of the lithium metal by supporting the lithium metal from which the surface oxide film is removed in the solution in which the amine compound is dissolved.

Description

Method for preparing lithium negative electrode coated with lithium nitride, lithium negative electrode prepared therefrom and lithium-sulfur battery including the same}

The present invention relates to a metal negative electrode for a lithium-sulfur battery, and more particularly, to a method for manufacturing a lithium nitride surface-treated lithium negative electrode, which improves lithium ion conductivity and lifespan characteristics of a battery by forming a protective layer on the surface of the lithium negative electrode , It relates to a lithium negative electrode prepared thereby and a lithium-sulfur battery including the same.

As interest in energy storage technology continues to grow, the field of application expands to the energy of mobile phones, tablets, laptops and camcorders, and even electric vehicles (EVs) and hybrid electric vehicles (HEVs). Research and development of chemical elements is gradually increasing. The electrochemical device is the field that has received the most attention in this respect, and among them, the development of secondary batteries such as lithium-sulfur batteries capable of charging / discharging has become a focus of attention, and recently, in the development of such batteries, the capacity density And in order to improve the specific energy, research and development on the design of new electrodes and batteries are being conducted.

Such an electrochemical device, among which a lithium-sulfur battery, has a high energy density and is in the limelight as a next-generation secondary battery that can replace a lithium ion battery, and accordingly, the importance of a lithium negative electrode is also increasing. In the case of conventional lithium ion battery (LIB) anode materials, when lithium ions are reduced, an initial irreversible reaction that forms a solid electrolyte interface (SEI) layer on the surface of the anode material proceeds, and the SEI layer formed in this way is very stable. Subsequently, the direct reaction between the electrolyte and the negative electrode is suppressed during subsequent cycles, resulting in improved lithium efficiency.

However, when lithium metal is used as an anode, it is difficult to keep the interface constant because the redox reaction of the battery continuously occurs on the metal surface even if such an irreversible reaction initially occurs. In general, since the SEI layer on the surface of the lithium metal formed by the redox reaction between the electrolyte and the lithium metal has low mechanical strength, the structure collapses while the cycle continues, revealing a new lithium surface, which causes a decrease in lithium efficiency. It is known. The non-uniformly formed surface of lithium in this way preferentially grows lithium ions on the protruded parts again, and forms dendrites. In order to solve the dendrite problem, which is the biggest problem preventing the commercialization of lithium metal anodes, various research groups have been trying to reform separators, introduce interlayers, and protective layers, but there is no way to fundamentally prevent dendrite growth of lithium. A solution has not yet been presented.

Therefore, an object of the present invention is to form a lithium nitride protective layer on the surface of the lithium negative electrode to inhibit dendrite growth, thereby improving the lithium ion conductivity and lifespan characteristics of a battery, a method for producing a lithium nitride surface-treated lithium negative electrode , To provide a lithium negative electrode prepared thereby and a lithium-sulfur battery including the same.

In order to achieve the above object, the present invention, a) dissolving an amine-based compound in an organic solvent; And b) forming a lithium nitride (Li ₃ N) protective layer on the surface of the lithium metal by supporting the lithium metal from which the surface oxide film is removed in the solution in which the amine-based compound is dissolved. A method for manufacturing a surface-treated lithium anode is provided.

In addition, the present invention, the surface oxide film is removed lithium metal; and a lithium nitride (Li ₃ N) protective layer formed on the surface of the lithium metal.

In addition, the present invention provides a lithium-sulfur battery including the negative electrode, the positive electrode, and the electrolyte.

According to the method for manufacturing a lithium anode surface-treated with lithium nitride according to the present invention, a lithium anode produced thereby, and a lithium-sulfur battery including the same, a lithium nitride protective layer is formed on the surface of the lithium anode to prevent dendrite growth By suppressing the lithium-sulfur battery, the lithium ion conductivity and lifespan characteristics can be improved.

1 is an image showing a state (A) of a lithium anode surface-treated with lithium nitride according to the present invention and a state (B) of a conventional lithium anode.
Figure 2 is a graph (A) showing the resistance value applied to the lithium negative electrode according to the present invention and a graph (B) showing the resistance value applied to a conventional lithium negative electrode.
3 is a graph comparing and contrasting life characteristics of lithium-sulfur batteries according to an embodiment of the present invention and a comparative example.

Hereinafter, the present invention will be described in detail.

A method for producing a lithium anode with surface treatment of lithium nitride according to the present invention includes the steps of a) dissolving (diluting) an amine-based compound in an organic solvent, and b) removing a surface oxide film from a solution in which the amine-based compound is dissolved. and forming a lithium nitride (Li ₃ N) protective layer on the surface of the lithium metal by supporting the metal.

The amine-based compound includes at least one amine group, and may be largely exemplified by an aromatic diamine compound and an aliphatic diamine compound. The aromatic diamine compound is an amine-based carbon compound having 6 to 20 carbon atoms, preferably 8 to 12 carbon atoms, and may further include a sulfur atom or an oxygen atom, if necessary. Specific examples of the aromatic diamine compound include m-xylene diamine, o-xylene diamine, p-xylene diamine, p-phenylenediamine, m-phenylenediamine, and 4,4-diaminodiamine. phenylsulfone and 4,4-diaminodiphenyl ether.

In addition, the aliphatic diamine compound is an amine-based carbon compound having 2 to 20 carbon atoms, preferably 6 to 12 carbon atoms, and may further include a sulfur atom or an oxygen atom, if necessary. Specific examples of the aliphatic diamine compound include ethylenediamine, propanediamine, 1,4-butanediamine, 1,6-hexanediamine, 1,7-heptanediamine, 1,8-octanediamine, 1,9-nonanediamine, 1,10-decanediamine, 1,11-undecanediamine, 1,12-dodecanediamine, 2-methyl-1,5-pentanediamine, 3-methyl-1,5-pentanediamine, 2,2,4 -Trimethyl-1,6-hexanediamine, 2,4,4-trimethyl-1,6-hexanediamine, 2-methyl-1,8-octanediamine and 5-methyl-1,9-nonanediamine. .

The organic solvent is used to dilute the amine-based compound, and common organic solvents known in the art may be used without limitation, and alcohol-based solvents such as methanol, ethanol, isopropyl alcohol, benzene, dichlorobenzene, and toluene , benzene-based solvents such as xylene, tetrahydrofuran (THF), and at least one of N-methyl pyrrolidone (NMP).

The amount of the amine-based compound added to the organic solvent is 0.5 to 30 parts by weight, preferably 1 to 15 parts by weight, more preferably 5 to 10 parts by weight, based on 100 parts by weight of the organic solvent. If the content of is less than 0.5 parts by weight based on 100 parts by weight of the organic solvent, there is a concern that a reaction that is effective in forming a protective layer does not occur, and if it exceeds 30 parts by weight, the protective layer is excessively formed, resulting in lithium ion conductivity This degradation may cause problems.

In step a), the process of dissolving the amine-based compound in an organic solvent may be performed at a temperature of 0 to 40 ° C, preferably 20 to 25 ° C for 1 to 48 hours, preferably 4 to 24 hours, , In the step b), the step of supporting (or reacting) the lithium metal in the solution in which the amine compound is dissolved is 10 to 30 ° C., preferably 20 to 25 ° C. for 10 seconds to 30 minutes, preferably Preferably it may be performed for 30 to 60 seconds. In addition, the thickness of the lithium nitride protective layer is 0.1 to 100 μm, preferably 0.3 to 20 μm, more preferably 0.5 to 3 μm, and if the thickness of the lithium nitride protective layer is less than 0.1 μm, it is used as a protective layer There is a concern that it may not perform its function, and if it exceeds 100 μm, a problem may occur in cell operation due to a decrease in lithium ion conductivity.

In addition, the lithium metal surface resistance (in the electrolyte solution) on which the lithium nitride protective layer is formed is 10 to 500 ohms, preferably 50 to 300 ohms, more preferably 100 to 250 ohms, and 1 to 240 hours, preferably Preferably, there is no change in resistance for 24 to 120 hours, more preferably 36 to 48 hours. This means that, unlike the prior art, continuous redox reactions of the battery are prevented on the surface of the lithium metal, and thus the formed lithium nitride protective layer is maintained in a stable state with the electrolyte.

On the other hand, lithium metal from which the surface oxide film is removed means lithium from which the surface oxide layer is physically and chemically removed. The binding force between the metal and the protective layer is very good. In addition, the lithium ion conductivity of the lithium nitride protective layer formed on the surface of the lithium metal is 10 ⁻⁴ to 5×10 ⁻³ Scm ⁻¹ , preferably about 10 ⁻³ Scm ^{−1 ,} which is very excellent. Therefore, according to the present invention, the growth or formation of dendrites can be suppressed by binding such lithium nitride and lithium metal and applying the same as a negative electrode, and furthermore, the lifespan characteristics of the battery can also be improved. In addition, the manufacturing method of the lithium anode has the advantage that the process is very simple and easy.

1 is an image showing a state (A) of a lithium anode surface-treated with lithium nitride according to the present invention and a state (B) of a conventional lithium anode. Next, the lithium anode prepared by the method for manufacturing a lithium anode surface-treated with lithium nitride according to the present invention, as shown in FIG. 1A, is described. It includes a metal and a lithium nitride (Li ₃ N) protective layer formed on the surface of the lithium metal. A detailed description of the elements constituting the lithium anode, such as the lithium nitride protective layer, applies mutatis mutandis as described in the manufacturing method of the lithium anode.

Finally, the present invention provides a lithium-sulfur battery including a lithium negative electrode, a positive electrode, and an electrolyte coated with lithium nitride. The description of the lithium negative electrode included in the lithium-sulfur battery is replaced with the above (the description of the binder, etc., which is not described in the negative electrode, applies mutatis mutandis), and other positive electrodes and electrolytes applied to the lithium-sulfur battery It may be a conventional one used in the art, and a description thereof will be described later.

Meanwhile, in the present invention, it is also possible to provide a battery module including the lithium-sulfur battery as a unit cell and a battery pack including the same. The battery module or battery pack may include a power tool; electric vehicles, including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or systems for power storage; It can be used as a power supply for any one or more medium or large sized devices.

Hereinafter, a description of a positive electrode, an electrolyte, and a separator applied to a lithium-sulfur battery including a lithium negative electrode coated with lithium nitride according to the present invention will be added.

anode

Regarding the positive electrode used in the present invention, after preparing a positive electrode composition including a positive electrode active material, a conductive material, and a binder, the slurry prepared by diluting it in a predetermined solvent (dispersion medium) is directly coated on the positive electrode current collector and An anode layer can be formed by drying. Alternatively, the positive electrode layer may be prepared by casting the slurry on a separate support and then laminating a film obtained by peeling from the support on a positive electrode current collector. In addition, the positive electrode may be prepared in various ways using a method widely known to those skilled in the art.

The conducting material serves as a path for electrons to move from the positive electrode current collector to the positive electrode active material, thereby imparting electronic conductivity, and electrically connecting the electrolyte and the positive electrode active material so that lithium ions (Li+) in the electrolyte It moves to sulfur and acts as a pathway to react at the same time. Therefore, if the amount of the conductive material is not sufficient or it does not properly perform its role, the unreacted portion of the sulfur in the electrode increases, eventually causing a decrease in capacity. In addition, since it adversely affects high-rate discharge characteristics and charge/discharge cycle life, it is necessary to add an appropriate conductive material. The content of the conductive material is preferably added within the range of 0.01 to 30% by weight based on the total weight of the positive electrode composition.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery, and examples thereof include graphite; carbon blacks such as denka black, acetylene black, ketjen black, channel black, furnace black, lamp black and summer black; conductive fibers such as carbon fibers and metal fibers; metal powders such as carbon fluoride, aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used. Specific examples of commercially available conductive materials include Chevron Chemical Company, which is an acetylene black series, Denka Singapore Private Limited, Gulf Oil Company products, Ketjenblack, and EC series Armac Company (Armak Company) product, Vulcan (Vulcan) XC-72 Cabot Company (Cabot Company) product and Super-P (product of Timcal), etc. may be used.

The binder is for attaching the positive electrode active material to the current collector well, and must dissolve well in a solvent, form a conductive network between the positive electrode active material and the conductive material, and also have appropriate impregnation of the electrolyte. The binder may be any binder known in the art, specifically, a fluororesin-based binder including polyvinylidene fluoride (PVdF) or polytetrafluoroethylene (PTFE); rubber-based binders including styrene-butadiene rubber, acrylonitrile-butadiene rubber, and styrene-isoprene rubber; cellulosic binders including carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, and regenerated cellulose; polyalcohol-based binders; polyolefin binders including polyethylene and polypropylene; A polyimide-based binder, a polyester-based binder, or a silane-based binder; may be a mixture or copolymer of one or more selected from the group consisting of, but is not limited thereto.

The content of the binder may be 0.5 to 30% by weight based on the total weight of the positive electrode composition, but is not limited thereto. When the content of the binder resin is less than 0.5% by weight, the physical properties of the positive electrode are deteriorated and the positive electrode active material and the conductive material may be eliminated, and when the content exceeds 30% by weight, the ratio of the active material and the conductive material in the positive electrode is relatively reduced Battery capacity may be reduced, and efficiency may be reduced by acting as a resistance element.

The positive electrode composition including the positive electrode active material, the conductive material, and the binder may be diluted in a predetermined solvent and coated on the positive electrode current collector using a conventional method known in the art. First, a positive electrode current collector is prepared. The cathode current collector generally has a thickness of 3 to 500 μm. Such a positive electrode current collector is not particularly limited as long as it does not cause chemical change in the battery and has high conductivity. For example, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel. The steel surface treated with carbon, nickel, titanium, silver, etc. may be used. The current collector may form fine irregularities on its surface to increase the adhesion of the positive electrode active material, and various forms such as films, sheets, foils, nets, porous materials, foams, and non-woven fabrics are possible.

Next, a slurry obtained by diluting a positive electrode composition including a positive electrode active material, a conductive material, and a binder in a solvent is applied on the positive electrode current collector. A slurry may be prepared by mixing the positive electrode composition including the positive electrode active material, the conductive material, and the binder with a predetermined solvent. At this time, the solvent should be easy to dry and can dissolve the binder well, but it is most preferable that the positive electrode active material and the conductive material can be maintained in a dispersed state without dissolving. When the solvent dissolves the cathode active material, since the specific gravity of sulfur (D = 2.07) is high in the slurry, the sulfur sinks in the slurry, and the sulfur tends to gather in the current collector during coating, causing problems in the conductive network, resulting in problems in battery operation there is The solvent (dispersion medium) may be water or an organic solvent, and the organic solvent may be at least one selected from the group consisting of dimethylformamide, isopropyl alcohol, acetonitrile, methanol, ethanol, and tetrahydrofuran.

Subsequently, there is no particular limitation on the method of applying the positive electrode composition in the slurry state, for example, doctor blade coating, dip coating, gravure coating, slit die coating coating), spin coating, comma coating, bar coating, reverse roll coating, screen coating, cap coating, etc. It can be manufactured by The positive electrode composition that has undergone such a coating process is then dried to evaporate the solvent (dispersion medium), increase the density of the coating film, and achieve adhesion between the coating film and the current collector. At this time, drying is carried out according to a conventional method, which is not particularly limited.

electrolyte

The electrolyte constituting the lithium-sulfur battery of the present invention includes a solvent and a lithium salt, and may further include additives, if necessary. As the solvent, a common non-aqueous solvent serving as a medium for movement of ions involved in the electrochemical reaction of the battery may be used without particular limitation. Examples of the non-aqueous solvent include carbonate-based solvents, ester-based solvents, ether-based solvents, ketone-based solvents, alcohol-based solvents, and aprotic solvents.

For more specific examples, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC) as the carbonate-based solvent ), ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate (BC), etc., and the ester-based solvent includes methyl acetate, ethyl acetate, n-propyl acetate, 1,1-dimethylethyl acetate, methyl propionate, ethyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, and caprolactone. Ethyl ether, dipropyl ether, dibutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, diglyme, triglyme, tetraglyme, tetrahydrofuran, 2-methyltetrahydrofuran and polyethylene glycol dimethyl ether; and the like. In addition, the ketone-based solvent includes cyclohexanone, the alcohol-based solvent includes ethyl alcohol and isopropyl alcohol, and the aprotic solvent includes nitriles such as acetonitrile, amides such as dimethylformamide, and the like. Drew, dioxolanes such as 1,3-dioxolane (DOL) and sulfolane. The above non-aqueous solvents can be used alone or in combination of two or more, and the mixing ratio when two or more are mixed can be appropriately adjusted according to the performance of the target battery, and 1,3-dioxolane and dimethoxyethane A solvent mixed in a volume ratio of 1: 1 can be exemplified.

separator

A conventional separator may be interposed between the anode and the cathode. The separator is a physical separator having a function of physically separating electrodes, and can be used without particular limitation as long as it is used as a conventional separator, and in particular, it is preferable to have low resistance to ion movement of the electrolyte and excellent electrolyte moistening ability. In addition, the separator enables transport of lithium ions between the positive electrode and the negative electrode while separating or insulating the positive electrode and the negative electrode from each other. Such a separator may be porous and made of a non-conductive or insulating material. The separator may be an independent member such as a film, or a coating layer added to at least one of the positive electrode and the negative electrode. Specifically, a porous polymer film, for example, a porous polymer film made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate copolymer It may be used alone or by stacking them, or a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, etc. may be used, but is not limited thereto.

Hereinafter, preferred embodiments are presented to aid understanding of the present invention, but the following examples are merely illustrative of the present invention, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the present invention, It goes without saying that these changes and modifications fall within the scope of the appended claims.

[Example 1] Preparation of lithium anode with surface treatment of lithium nitride

First, after dissolving xylenediamine (amine-based compound) in toluene (organic solvent) in an amount of 1 part by weight relative to 100 parts by weight of toluene at 25 ° C. for 12 hours, lithium metal from which the surface oxide film was removed was heated at 25 ° C. by carrying it for 1 minute under the condition, to prepare a lithium negative electrode coated with a lithium nitride protective layer to a thickness of 1 μm on the surface of the lithium metal.

[Example 2] Preparation of lithium anode with surface treatment of lithium nitride

A lithium negative electrode coated with a lithium nitride protective layer to a thickness of 3 μm on the surface of the lithium metal in the same manner as in Example 1, except that the content of xylenediamine was changed to 10 parts by weight relative to 100 parts by weight of toluene. was manufactured.

[Example 3] Preparation of lithium anode with surface treatment of lithium nitride

A lithium negative electrode coated with a lithium nitride protective layer to a thickness of 5 μm on the surface of the lithium metal in the same manner as in Example 1, except that the content of xylenediamine was changed to 30 parts by weight relative to 100 parts by weight of toluene was manufactured.

[Comparative Example 1] Preparation of conventional lithium anode

A lithium anode was prepared by removing only the surface oxide layer without a separate protective layer coating.

[Example 4, Comparative Example 2] Preparation of lithium-sulfur battery

A lithium-sulfur battery including the lithium negative electrode prepared in Example 2, the remaining positive electrode, separator, and electrolyte was prepared. In addition, a lithium-sulfur battery including the lithium negative electrode prepared in Comparative Example 1, the remaining positive electrode, separator, and electrolyte was prepared.

[Experimental Example 1] Evaluation of resistance change of lithium anode

The lithium anodes prepared in Examples 1 to 3 and Comparative Example 1 were made into a Li / Li symmetric cell, and the resistance change was measured in units of 1 hour for 48 hours through EIS measurement (measuring device name: VMP3 potentiostat, Bio-Logic Co.) By measuring, the change in resistance applied to each lithium negative electrode was evaluated. Figure 2 is a graph (A) showing the resistance value applied to the lithium negative electrode according to the present invention and a graph (B) showing the resistance value applied to a conventional lithium negative electrode. As shown in FIG. 2, in the case of Example 1 in which a lithium nitride protective layer was formed on the surface of lithium metal according to the present invention, there was little change in resistance value compared to Comparative Example 1 using a conventional lithium negative electrode, From this, it was confirmed that when the lithium nitride protective layer is formed on the surface of the lithium metal, the continuous redox reaction of the battery is prevented on the surface of the lithium metal, and the lithium nitride protective layer is maintained in a stable state with the electrolyte.

[Experimental Example 2] Evaluation of life characteristics of lithium-sulfur batteries

After setting the discharge current rate of the lithium-sulfur battery prepared in Example 4 and Comparative Example 2 to 0.5 C, life characteristics were observed. 3 is a graph comparing and contrasting life characteristics of lithium-sulfur batteries according to an embodiment of the present invention and a comparative example. As shown in FIG. 3, the lithium-sulfur battery of Example 4 in which a lithium negative electrode having a lithium nitride protective layer formed on the surface of lithium metal is applied, compared to the lithium-sulfur battery of Comparative Example 2 in which a conventional lithium negative electrode is applied. , Cycle life characteristics were also improved, and through this, it was confirmed that the lithium nitride (Li ₃ N) formed on the surface of the lithium metal according to the present invention has excellent lithium ion conductivity.

Claims (12)

a) dissolving an amine-based compound in an organic solvent; and
b) forming a lithium nitride (Li ₃ N) protective layer on the surface of the lithium metal by supporting the lithium metal from which the surface oxide film is removed in a solution in which the amine compound is dissolved; Method for manufacturing a treated lithium anode. The method of claim 1, wherein the amine compound is an aromatic diamine compound having 6 to 20 carbon atoms or an aliphatic diamine compound having 2 to 20 carbon atoms. The method according to claim 2, wherein the aromatic diamine compound is m-xylenediamine, o-xylenediamine, p-xylenediamine, p-phenylenediamine, m-phenylenediamine, 4,4-diaminodiphenylsulfone and A method for producing a lithium negative electrode surface-treated with lithium nitride, characterized in that selected from the group consisting of 4,4-diaminodiphenyl ether. The method according to claim 2, wherein the aliphatic diamine compound is ethylenediamine, propanediamine, 1,4-butanediamine, 1,6-hexanediamine, 1,7-heptanediamine, 1,8-octanediamine, 1,9-nonanediamine , 1,10-decanediamine, 1,11-undecanediamine, 1,12-dodecanediamine, 2-methyl-1,5-pentanediamine, 3-methyl-1,5-pentanediamine, 2,2, The group consisting of 4-trimethyl-1,6-hexanediamine, 2,4,4-trimethyl-1,6-hexanediamine, 2-methyl-1,8-octanediamine and 5-methyl-1,9-nonanediamine Method for producing a lithium negative electrode surface-treated with lithium nitride, characterized in that selected from. The method according to claim 1, wherein the organic solvent is an alcohol-based solvent selected from the group consisting of methanol, ethanol and isopropyl alcohol; a benzene-based solvent selected from the group consisting of benzene, dichlorobenzene, toluene and xylene; tetrahydrofuran; And N- methylpyrrolidone; characterized in that at least one selected from the group consisting of, a method for producing a lithium nitride surface-treated lithium negative electrode. The method of claim 1, wherein the amount of the amine-based compound added to the organic solvent is 0.5 to 30 parts by weight based on 100 parts by weight of the organic solvent. The method according to claim 1, characterized in that the supporting process of step b) is performed at a temperature of 10 to 30 ° C. for 10 seconds to 30 minutes. delete delete delete delete delete

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