ES3030487T3 - Method of preparing pouch type secondary battery - Google Patents

Abstract

The present invention relates to a method for producing a pouch-type secondary battery. More specifically, the present invention relates to a method for producing a pouch-type secondary battery. The method comprises: placing an electrode array inside a pouch-type case to produce a preliminary pouch-type electrode; placing an ultrasonic element made of metallic material against both sides of the preliminary pouch-type secondary battery; injecting a gel-polymer electrolyte composition into the preliminary pouch-type secondary battery; pressing the preliminary pouch-type secondary battery by means of the ultrasonic element and applying ultrasonic vibrations thereto; forming the preliminary pouch-type secondary battery; curing the gel-polymer electrolyte composition; and a degassing step, wherein the ultrasonic member is maintained at a temperature of 30-80°C and the step of pressing the preliminary pouch-type secondary battery is carried out while applying a pressure of 0.1-3,000 kgf/cm2 per unit area of the preliminary pouch-type secondary battery. (Automatic translation with Google Translate, without legal value)

Description

DESCRIPTION

Bag-type secondary battery preparation method

Technical field

The present invention relates to a method of preparing a pouch-type secondary battery having improved electrolyte wetting.

Background of the technique

The demand for secondary batteries as a power source has increased significantly as the development and demand of mobile device technology has increased, and among these secondary batteries, lithium secondary batteries, which have high energy density, high operating potential, long cycle life, and low self-discharge rate, have been widely commercialized and used.

A lithium secondary battery can be divided into a prismatic type lithium secondary battery and a pouch type lithium secondary battery according to its shape.

Regarding the prismatic lithium secondary battery, since its shape is fixed, its design is limited, and the venting for discharging gas or liquid presents safety problems, the prismatic lithium secondary battery is disadvantageous because there is a high risk of explosion due to the accumulation of heat and gas inside the battery.

On the contrary, compared to the pouch-type lithium secondary battery, since it has no limitations in shape and size, is easy to assemble by thermal fusion, and is easy to vent to discharge gas or liquid when abnormal behavior occurs, the pouch-type lithium secondary battery is advantageous in that it has high safety.

The pouch-type lithium secondary battery is prepared such that, after an electrode assembly, in which a porous separator is arranged between a positive electrode and a negative electrode, is inserted into a pouch-type housing having a predetermined size and shape, the electrode assembly is impregnated by injecting an electrolyte including a lithium salt.

The electrode array is impregnated as the electrolyte permeates between the positive electrode, the negative electrode, and the separator due to a capillary force, whereby battery performance can be optimized only when the electrodes constituting the electrode array are quickly and completely impregnated with the electrolyte.

However, due to the nature of the materials, the positive electrode, the negative electrode, and the separator are highly hydrophobic, but, since the electrolyte is a hydrophilic material, a significant amount of time and particular process conditions are required for the electrolyte to sufficiently impregnate the electrode assembly.

Furthermore, since the volume through which the electrolyte can permeate decreases, but the permeation area increases as a device such as a secondary battery gradually becomes larger, the interior of the electrode array may not be permeated with electrolyte, and there is a high possibility that the electrolyte will only exist locally on the exterior. As a result, a significant reduction in secondary battery capacity and performance due to uneven electrolyte distribution within the secondary battery has emerged as a problem.

In addition, a gel polymer electrolyte composition with a relatively high viscosity has been increasingly used, while lithium secondary batteries that include a gel polymer electrolyte to ensure safety have recently been commercialized.

Therefore, a method to improve the wetting of the composition for a gel polymer electrolyte has been required.

Typically, to improve wetting, a method of injecting a gel polymer electrolyte composition at a high temperature or a method of injecting a gel polymer electrolyte composition at a reduced pressure has been proposed. However, in this case, another problem arises, such as an internal short circuit, while pregelation occurs before impregnating a conventional electrode assembly with the gel polymer electrolyte composition, or the injected gel polymer electrolyte composition is modified by heat or pressure.

Therefore, there is a need for a preparation method of a pouch-type secondary battery that can improve electrolyte wetting without composition modification for a gel polymer electrolyte.

Prior art document: Korean Patent Application Laid-Open Publication No. 2014-0059746. Other relevant prior art documents disclosing a method for preparing secondary batteries are US 2015/079480 A1, KR2017 0118452 A, EP 2 648 259 A2, and US 2017/324115 A1.

Disclosure of the invention

Technical problem

An aspect of the present invention provides a method of preparing a pouch-type secondary battery having improved wetting of a composition for a gel polymer electrolyte with respect to an electrode assembly.

Technical solution

According to the present invention, there is provided a method for preparing a pouch-type secondary battery comprising:

housing a set of electrodes in an interior space of a pouch-type housing to form a preliminary pouch-type secondary battery;

providing a metallic ultrasonic element maintained at a temperature of 30°C to 60°C and formed of a metallic material to facilitate temperature and pressure control by tightly bonding to both surfaces of the bag-type preliminary secondary battery, the metallic ultrasonic element being formed in a structure surrounding the entire surface of the bag-type preliminary secondary battery;

injecting a gel polymer electrolyte composition into the pouch-type preliminary secondary battery; applying ultrasonic vibration having a frequency of 20 kHz to 200 MHz to the pouch-type preliminary secondary battery while pressurizing the pouch-type preliminary secondary battery using the metallic ultrasonic element that applies a pressure of 0.1 kgf/cm2 to 500 kgf/cm2 per area of the pouch-type preliminary secondary battery; forming the pouch-type preliminary secondary battery;

curing the composition to a gel polymer electrolyte; and

degas.

Preferred embodiments of the invention are described in the appended dependent claims.

The metallic ultrasonic element used in the method of the present invention may be formed of a metallic material selected from the group consisting of stainless steel, iron, aluminum, copper, nickel, and alloys of two or more thereof, and may be specifically formed of stainless steel or aluminum.

In addition, the metallic ultrasonic element can be formed in a structure surrounding the entire surface of the preliminary pouch-type secondary battery.

Furthermore, in the present invention, the pressurization of the bag-type preliminary secondary battery can be performed by applying a pressure of, for example, 0.1 kgf/cm2 to 100 kgf/cm2 per area of the bag-type preliminary secondary battery.

The application of ultrasonic vibration can be carried out simultaneously with the injection of the composition for a gel polymer electrolyte.

Furthermore, the method of the present invention may further include applying ultrasonic vibration after formation and before curing of the composition for a gel polymer electrolyte.

Advantageous effects

According to a method of the present invention, since a composition for a gel polymer electrolyte is efficiently dispersed by applying ultrasonic vibration as well as pressurizing at a temperature of 30° C. to 60° C. using a metallic ultrasonic element during or after injection of the composition for a gel polymer electrolyte, not only the wetting of the composition for a gel polymer electrolyte can be improved, but also the wetting time can be reduced. Accordingly, a pouch type secondary battery can be prepared having an improved initial capacity and a low initial resistance value.

Brief description of the drawings

The following drawings attached to the specification illustrate preferred examples of the present invention by way of example, and serve to enable the technical concepts of the present invention to be further understood in conjunction with the detailed description of the invention provided below, and therefore the present invention should not be construed solely on the basis of the content of such drawings.

Figure 1 is a cross-sectional view illustrating a preliminary pouch-type secondary battery, on which an ultrasonic element is arranged, in a method of preparing a pouch-type secondary battery according to an embodiment of the present invention; and

Figure 2 is a graph illustrating the endurance evaluation of a pouch-type secondary battery according to Experimental Example 1 of the present invention.

Description of the symbols

100: Preliminary secondary battery of bag type

110-1: First ultrasonic element

110-2: Second ultrasonic element

Mode of carrying out the invention

It is to be understood that the expressions or terms used in the specification and claims are not to be construed as having the meaning defined in commonly used dictionaries. It is further to be understood that the expressions or terms should be construed as having a meaning consistent with their meaning in the context of the relevant art and the technical idea of the invention, based on the principle that an inventor may appropriately define the meaning of the expressions or terms to better explain the invention. Hereinafter, a method of preparing a pouch-type secondary battery according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Specifically, the method of preparing a pouch-type secondary battery according to the present invention comprises:

housing a set of electrodes in an interior space of a pouch-type housing to form a preliminary pouch-type secondary battery;

providing a metallic ultrasonic element maintained at a temperature of 30°C to 60°C and formed of a metallic material to facilitate temperature and pressure control by tightly bonding to both surfaces of the bag-type preliminary secondary battery, the metallic ultrasonic element being formed in a structure surrounding the entire surface of the bag-type preliminary secondary battery;

injecting a gel polymer electrolyte composition into the pouch-type preliminary secondary battery; applying ultrasonic vibration having a frequency of 20 kHz to 200 MHz to the pouch-type preliminary secondary battery while pressurizing the pouch-type preliminary secondary battery using the metallic ultrasonic element that applies a pressure of 0.1 kgf/cm2 to 500 kgf/cm2 per area of the pouch-type preliminary secondary battery; forming the pouch-type preliminary secondary battery;

curing the composition to a gel polymer electrolyte; and

degas.

Preparation of the preliminary secondary battery of the bag type

According to the present specification, the preliminary pouch-type secondary battery of the present invention includes an electrode assembly and a pouch-type casing in which the electrode assembly is housed, and can be prepared according to a conventional method known in the art.

For example, after forming an electrode array by sequentially stacking a positive electrode, a separator, and a negative electrode, the preliminary pouch-type secondary battery of the present invention can be prepared by housing the electrode array in a pouch-type casing (battery).

In this case, since the pouch-type casing has a sealing portion at each edge, the preliminary pouch-type secondary battery can be prepared by first sealing three edges excluding an edge where a composition for a gel polymer electrolyte is injected, after accommodating the electrode assembly in an interior space.

The electrode array may be formed by sequentially stacking the positive electrode and the negative electrode in a state where the separator is arranged between the positive electrode and the negative electrode to insulate them from each other, and may be formed in various structures such as a spiral wound type, a stacked type, or a stacking and folding type according to one embodiment.

In this case, the positive electrode can be prepared by forming a positive electrode material blend layer on a positive electrode collector. The positive electrode material blend layer can be formed by coating the positive electrode collector with a positive electrode suspension comprising a positive electrode active material, a binder, a conductive agent, and a solvent, and then drying and laminating the coated positive electrode collector.

The positive electrode collector is not particularly limited as long as it has conductivity without causing adverse chemical changes in the battery, and, for example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface is treated with one of carbon, nickel, titanium, silver, or the like can be used.

Furthermore, the positive electrode active material is a compound capable of reversibly intercalating and deintercalating lithium, wherein the positive electrode active material may specifically include a lithium metal composite oxide that includes lithium and at least one metal such as cobalt, manganese, nickel, or aluminum. Specifically, the lithium metal composite oxide may include lithium-manganese based oxide (e.g., LiMnO<2>, LiMn<2>O<4>, etc.), lithium-cobalt based oxide (e.g., LiCoO<2>, etc.), lithium-nickel based oxide (e.g., LiNiO<2>, etc.), lithium-nickel-manganese based oxide (e.g., LiNh_YMnYO<2>(where 0<Y<1), LiMn<2>-ZNizO<4>(where 0<Z<2), etc.), lithium-nickel-cobalt based oxide (e.g., LiNi-mCoY-^ (where 0<Y1<1), lithium-manganesecobalt based oxide (e.g., LiCO-i_Y<2>MnY<2>O<2>(where 0<Y2<1), LiMn<2>-Z<1>CoZ<1>O<4>(where 0<Z1<2), etc.), lithium nickel manganese cobalt based oxide (e.g. Li(NipCoqMnri)O<2>(where 0<p<1, 0<q<1, 0<r1<1, and p+q+r1=1) or Li(Nip<1>Coq<1>Mnr<2>)O<4>(where 0<p1<2, 0<q1<2, 0<r2<2, and p1+q1+r2=2), etc.), or lithium nickel cobalt transition metal (M) oxide (e.g. Li(Nip<2>Coq<2>Mnr<3>MS<2>)O<2>(where M is selected from the group consisting of aluminium (Al), iron (Fe), vanadium (V), chromium (Cr), titanium (Ti), tantalum (Ta), magnesium (Mg), and molybdenum (Mo), and p2, q2, r3, and s2 are atomic fractions of each independent element, where 0<p2<1, 0<q2<1, 0<r3<1, 0<s2<1, and p2+q2+r3+s2=1), etc.), and may include any one of them or a compound of two or more of them.

The binder is a component that aids in the bonding between the active material and the conductive agent and in the bonding with the current collector, wherein the binder may be usually added in an amount of 1% by weight to 30% by weight based on the total weight of the solids content in the positive electrode suspension. Examples of the binder may be polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, an ethylene-propylene-diene thermonomer, a sulfonated ethylene-propylene-diene thermonomer, a styrene-butadiene rubber, a fluorinated rubber, various copolymers, and the like.

Furthermore, any conductive agent may be used without particular limitation as long as it has adequate conductivity without causing adverse chemical changes in the battery, and may be added in an amount of 1% by weight to 20% by weight based on the total weight of the solids content in the positive electrode slurry. For example, as the conductive agent, carbon powder such as carbon black, acetylene black (or Denka black), Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite with a well-developed crystal structure, artificial graphite, or graphite; conductive fibers such as carbon fibers or metal fibers; conductive powder such as fluorocarbon powder, aluminum powder, and nickel powder may be used.

The solvent may include an organic solvent, such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount such that a desirable viscosity is obtained when the positive electrode active material, as well as selectively the binder and the conductive agent, are included. For example, the solvent may be included in an amount such that the concentration of solids content in the suspension that includes the positive electrode active material, as well as selectively the binder and the conductive agent, is in a range of from 10% by weight to 60% by weight, for example, from 20% by weight to 50% by weight.

Furthermore, the positive electrode may include a positive electrode collector region where the positive electrode active material layer is not formed, that is, a positive electrode uncoated portion, and a positive electrode tab formed of a metal material, such as aluminum (Al), may be attached to a side end of the positive electrode uncoated portion of the positive electrode.

Furthermore, the negative electrode can be prepared by forming a negative electrode material blend layer on a negative electrode collector. The negative electrode material blend layer can be formed by coating the negative electrode collector with a negative electrode suspension comprising a negative electrode active material, a binder, a conductive agent, and a solvent, and then drying and laminating the coated negative electrode collector.

The negative electrode collector generally has a thickness of 3 pm to 500 pm. The negative electrode collector is not particularly limited as long as it has high conductivity without causing adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper, or stainless steel whose surface is treated with one of carbon, nickel, titanium, silver, or the like, an aluminum-cadmium alloy, or the like, can be used.

Furthermore, the negative electrode active material may include at least one selected from the group consisting of lithium metal, a carbon material capable of reversibly intercalating/deintercalating lithium ions, metal or an alloy of lithium and the metal, a material that may be doped and undoped with lithium, and a transition metal oxide.

As the carbon material capable of reversibly intercalating/deintercalating lithium ions, a carbon-based negative electrode active material generally used in a lithium-ion secondary battery can be used without particular limitation, and, as a typical example, crystalline carbon, amorphous carbon, or both thereof can be used.

As the metal or alloy of lithium and the metal, a metal selected from the group consisting of copper (Cu), nickel (Ni), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), silicon (Si), antimony (Sb), lead (Pb), indium (In), zinc (Zn), barium (Ba), radium (Ra), germanium (Ge), aluminum (Al), and tin (Sn), or an alloy of lithium and the metal may be used.

The material that may be doped and undoped with lithium may include Si, SiOx (O<x<2), Sn, or SnO<2>, and a mixture of SO<2> and at least one thereof may also be used.

The transition metal oxide may include lithium-containing titanium oxide (LTO), vanadium oxide, and lithium-vanadium oxide.

Materials, which are the same as or different from those of the binder and the conductive agent used in the positive electrode material blend layer as described above, may be used for the binder and the conductive agent, respectively.

Furthermore, the solvent may include water or an organic solvent, such as NMP and alcohol, and may be used in an amount such that a desirable viscosity is obtained when the negative electrode active material, as well as selectively the binder and the conductive agent, are included. For example, the solvent may be included in an amount such that the concentration of solids content in the suspension that includes the negative electrode active material, as well as selectively the binder and the conductive agent, is in a range of 50% by weight to 75% by weight, for example, 50% by weight to 65% by weight.

In addition, a negative electrode collector region where the negative electrode active material layer is not formed, i.e., an uncoated portion of the negative electrode, may be formed on both side portions of the negative electrode. A negative electrode tab formed of a metallic material, such as nickel (Ni), may be attached to one side end of the negative electrode.

The positive electrode tab and the negative electrode tab each extend from the electrode assembly, and portions or all of the positive electrode tab and the negative electrode tab may be connected to a positive electrode lead and a negative electrode lead, respectively, for electrical connection to an external device or terminal. The positive electrode lead and the negative electrode lead are electrically connected to the electrode tabs by welding or the like, and portions of the leads may be exposed to the outside of the battery housing. In the battery housing, the positive electrode lead and the negative electrode lead may be arranged in opposite directions to each other, or may be arranged side by side in the same direction.

The separator is arranged between the positive and negative electrodes to electrically isolate the positive and negative electrodes from each other. The separator may be formed in the form of a porous film so that lithium ions can pass between the positive and negative electrodes. The separator, for example, may be composed of a porous membrane using polyethylene (PE), polypropylene (PP), or a composite film thereof.

Arrangement of the ultrasonic element by means of a tight connection

Furthermore, according to the present specification, a metallic ultrasonic element surrounding the entire surface of the pouch-type preliminary secondary battery may be provided by close bonding to both surfaces of the pouch-type preliminary secondary battery prepared as described above.

Figure 1 is a cross-sectional view illustrating a preliminary pouch-type secondary battery 100 on which an ultrasonic element is closely attached and arranged in the method of preparing a pouch-type secondary battery according to the embodiment of the present invention.

Referring to Figure 1, in the method of the present invention, the ultrasonic element surrounding the entire surface of the pouch-type preliminary secondary battery may be arranged by close attachment to both surfaces of the pouch-type preliminary secondary battery 100 housing the electrode assembly.

In this case, the ultrasonic element may be a pair of ultrasonic elements composed of a first ultrasonic element 110-1 and a second ultrasonic element 110-2 that are independently separated as illustrated in Figure 1, or it may be an integrated ultrasonic element in which at least one surface of the first ultrasonic element and at least one surface of the second ultrasonic element are bonded.

Furthermore, instead of using an adhesive element, it is desirable for the ultrasonic element to be in contact with one side of the preliminary pouch-type secondary battery housing to facilitate transmission of the ultrasonic vibration.

The ultrasonic element is formed of a metallic material to facilitate temperature and pressure control. The metallic material may include at least one selected from the group consisting of stainless steel, iron, aluminum, copper, nickel, and alloys of two or more thereof. Specifically, the ultrasonic element may be formed of stainless steel or aluminum, for example, aluminum, taking into account cost and heat transfer efficiency.

In the method of the present invention, it is desirable that the ultrasonic element be maintained at a temperature of 30°C to 60°C using a temperature control device (not shown) that includes a listening wire that is coupled to a portion of the ultrasonic element.

Since the temperature of the ultrasonic element is maintained within the above temperature range, the impregnation effect can be improved during subsequent injection of the composition for a gel polymer electrolyte and it is possible to prevent the internal temperature of the pouch-type secondary battery from excessively rising when ultrasonic vibration is applied or prevent occurrences of exfoliation and damage of an internal material, such as the electrode active material, due to the application of ultrasonic waves. In a case where the temperature of the ultrasonic element is lower than 30°C, since the viscosity of the composition for a gel polymer electrolyte is increased during subsequent injection of the composition for a gel polymer electrolyte, wetting may be reduced, and if the temperature of the ultrasonic element is higher than 80°C, since an increase in the internal temperature of the pouch-type secondary battery causes damage due to chemical and physical side reactions between the composition for a gel polymer electrolyte and the electrode or separator during subsequent injection of the composition for a gel polymer electrolyte, an internal short circuit of a cell may occur.

Injection of the composition for a gel polymer electrolyte

Furthermore, according to the present specification, after arranging the ultrasonic element surrounding the entire surface of the preliminary pouch-type secondary battery by closely bonding to both surfaces of the preliminary pouch-type secondary battery, a composition for a gel polymer electrolyte may be injected into a pouch-type casing accommodating the electrode array through an edge of the pouch-type casing in an open state.

In this case, a surface on which an inlet of the composition for a gel polymer electrolyte is formed is adjacent to a surface of the sealing portion on which the negative electrode tab and the positive electrode tab are arranged, but it is desirable that these surfaces be different from each other.

As a composition for a gel polymer electrolyte used in the preparation method of a secondary battery of the present invention, one including (a) a lithium salt, (b) an organic solvent, (c) a polymerizable monomer, (d) a polymerization initiator, and selectively (e) an additive may be used.

In this case, as (a) the lithium salt, any lithium salt normally used in a gel polymer electrolyte may be used without limitation, and, for example, the lithium salt may include Li+ as a cation, and may include at least one selected from the group consisting of F-, Cl-, Br-, I-, NO<3>-, N(CN)<2>-, CO<4>-, BF<4>', B<10>Cl-i<0>', PF6-, CF<3>SO<3>-, CH<3>CO<2>-, CF<3>CO<2>-, AsFa-, SbFa-, AlCk, AO<4>-, CH<3>SO<3>-, BF<2>C<2>O<4>-, BC<4>O<8>-, PF<4>C<2>O<4>-, PF<2>C<4>O<8>-, (CF3)2PF4-, (CF3)3PF3-, (CF3)4PF2-, (CF3)aPF-, C<4>F<9>SO<3>-, CF<3>CF<2>SO<3>-, (FSO<2>)<2>N-, (CF3SO2)2N-, CF3CF2(CF3)2CO-, (CF3SO2)2CH-, (SF<5>)<3>C-, (CFaSO<2>)aC-, CF<3>(CF<2>)<7>SO<3>-, SCN-, and (CFaCF<2>SO<2>)<2>N- as anion.

Specifically, the lithium salt may include a single material selected from the group consisting of LiCl, LiBr, Li, LiCl<4>, LiBF4, LiB<10>C10>, LiPFa, LiCF3SO3, LiCH3CO2, LiCF3CO2, LiAsFa, LiSbFa, LiAlCl4, LiAlO4, LiCH3SO3, lithium fluorosulfonylimide (LiFSI, LiN(SO<2>F)<2>), lithium (bis)trifluoromethanesulfonimide (LiTFSI, LiN(SO<2>CF<3>)<2>), and lithium bisperfluoroethanesulfonimide (LiBETi, LiN(SO<2>C<2>F<5>)<2>), or a mixture of two or more thereof. More specifically, the lithium salt may include a single material selected from the group consisting of LiBF<4>, LiPF6, UCH<3>CO<2>, LiCF<3>CO<2>, LiCH<3>SO<3>, LiFSI, LiTFSI, and LiBETI, or a mixture of two or more thereof. In addition to these materials, lithium salts commonly used in an electrolyte for a lithium secondary battery may be used without limitation.

The lithium salt may be appropriately changed in a range that can be normally used, but may be included in a concentration of 0.8 M to 4.0 M, for example, 1.0 M to 3.0 M, in the composition for a gel polymer electrolyte to obtain an optimal effect of forming a film to prevent corrosion of an electrode surface.

In a case where the concentration of the lithium salt is less than 0.8 M, the effect of improving the low temperature output and the cycle characteristics during high temperature storage of the lithium secondary battery is negligible, and, in a case where the concentration of the lithium salt is greater than 4.0 M, the wetting of the composition for a gel polymer electrolyte may be reduced due to an increase in the viscosity of the composition for a gel polymer electrolyte.

Furthermore, (b) the organic solvent is not limited as long as it is a non-aqueous organic solvent that can minimize decomposition due to an oxidation reaction during charging and discharging of the secondary battery and can exhibit the desired characteristics with the additive. For example, a carbonate-based solvent, an ether-based solvent, or an ester-based solvent may be used alone or in a mixture of two or more thereof.

The carbonate-based solvent among the organic solvents may include a cyclic carbonate-based solvent or a linear carbonate-based solvent.

Specific examples of the cyclic carbonate-based solvent may be any one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, vinylene carbonate, and fluoroethylene carbonate (FEC), or a solvent of two or more thereof.

Furthermore, the linear carbonate-based solvent may include any one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, or a solvent of two or more thereof. The ether-based solvent may include any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, and ethyl propyl ether, or a solvent of two or more thereof. Furthermore, the ester-based solvent may include a linear ester-based solvent and a cyclic ester-based solvent, and specific examples of the linear ester-based solvent among these ester-based solvents may be any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate, or a solvent of two or more thereof.

Specific examples of the cyclic ester-based solvent may be Y-butyrolactone, Y-valerolactone, <y>-caprolactone, α-valerolactone, or Σ-caprolactone.

Furthermore, (c) the polymerizable monomer may include a multifunctional acrylate-based compound containing at least one acrylate group in the molecule, a multifunctional methacrylate-based compound containing at least one methacrylate group, or compounds having a polymerizable functional group selected from the group consisting of a vinyl group, an epoxy group, an ether group, an allyl group, an oxyalkylene group, and a (meth)acrylic group, so that the polymerizable monomers can be polymerized with each other to form a polymer.

Specifically, the polymerizable monomer may include a compound represented by formula 1 below.

[Formula 1]

A-B-A'

In Formula 1,

A and A' are each independently an acrylate group, a methacrylate group, an alkylene group having 1 to 10 carbon atoms containing at least one acrylate group or methacrylate group, or -O-R<1>, wherein R<1> is an alkyl group having 1 to 10 carbon atoms containing at least one acrylate group or methacrylate group, or -O-R2-O-R3, wherein R2 is an alkylene group having 1 to 10 carbon atoms containing at least one acrylate group or methacrylate group, and R<3> is an alkyl group having <1> to <10> carbon atoms containing at least one acrylate group or methacrylate group; and

B is an oxyalkylene group.

Specifically, A and A’ may each independently include at least one selected from the units represented by formulas A-1 through A-5 below.

[Formula A-1]

[Formula A-4]

The acrylate or methacrylate group located at one end of the polymerizable monomer can form a polymer network through a polymerization reaction with an organic binder containing an ethylenically unsaturated group. These compounds can be derived from monomers containing a monofunctional or multifunctional acrylate or methacrylate group.

Additionally, B may include a unit represented by the formula B-1 below.

[Formula B-1]

In formula B-1,

R is an alkylene group having 1 to 10 carbon atoms,

R3 is O or an alkylene group having 1 to 5 carbon atoms,

k1 is an integer from 1 to 30, and

m is an integer from 0 to 3.

For example, in formula B-1, R can each independently be -CH2CH2- or -CH2CH2CH2-.

More specifically, the compound represented by formula 1 can be represented by formula 1a below.

[Formula 1a]

The polymerizable monomer may be included in an amount of 0.5% by weight to 20% by weight, particularly 0.7% by weight to 15% by weight, and more particularly 1.0% by weight to 10% by weight, based on the total weight of the composition for a gel polymer electrolyte.

In a case where the amount of the polymerizable monomer is 0.5% by weight or more, the gel-forming reaction effect can be improved to ensure sufficient mechanical strength of the gel polymer electrolyte, and, in a case where the amount of the polymerizable monomer is 20% by weight or less, it is possible to prevent disadvantages such as an increase in resistance due to the excessive amount of oligomer and limitations in the movement of lithium ions (decrease in ionic conductivity).

Furthermore, as (d) the polymerization initiator, a conventional polymerization initiator known in the art may be used. For example, at least one selected from the group consisting of an ultraviolet (UV) polymerization initiator, a photopolymerization initiator, and a thermal polymerization initiator may be used as the polymerization initiator.

Specifically, as a representative example, the UV polymerization initiator may include at least one selected from the group consisting of 2-hydroxy-2-methylpropiophenone, 1-hydroxycyclohexylphenyl ketone, benzophenone, 2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propanone, oxyphenylacetic acid 2-[2-oxo-2-phenylacetoxyethoxy]ethyl ester, oxyphenylacetic acid 2-[2-hydroxyethoxy]ethyl ester, alpha-dimethoxy-alpha-phenylacetophenone, 2-benzyl-2-(dimethylamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone, 2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone, 2-[2-oxo-2-phenylacetoxy ... diphenyl(2,4,6-trimethylbenzoyl)-phosphine, bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide, bis(eta-5-2,4-cyclopentadien-1-yl), bis[2,6-difluoro-3-(1H-pyrrol-1-yl)phenyl]titanium, 4-isobutylphenyl-4’-methylphenyliodonium hexafluorophosphate, and methyl benzoylformate.

Furthermore, as a representative example, the photopolymerization initiator or the thermal polymerization initiator may include at least one selected from the group consisting of benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di-tert-butyl peroxide, t-butyl peroxy-2-ethylhexanoate, cumyl hydroperoxide, hydrogen peroxide, 2,2'-azobis(2-cyanobutane), 2,2'-azobis(methylbutyronitrile), 2,2'-azobis(iso-butyronitrile) (AIBN), and 2,2'-azobisdimethyl-valeronitrile (AMVN).

The polymerization initiator is a compound capable of forming a radical upon dissociation by UV or heat at 30°C to 100°C in the battery or upon dissociation by light at room temperature (5°C to 30°C), wherein the polymerization initiator can be used in an amount of 0.1 parts by weight to 10 parts by weight based on 100 parts by weight of the polymer A represented by formula 1. In a case where the polymerization initiator is included in an amount of 10 parts by weight or less, since the polymerization rate in the polymer electrolyte can be controlled, a disadvantage that the unreacted polymerization initiator remains and adversely affects the performance of the battery later can be prevented. Furthermore, since the polymerization reaction between the polymers A represented by formula 1 can be carried out smoothly only when the polymerization initiator is included in an amount of 0.1 part by weight or more, a polymer electrolyte having a uniform thickness can be prepared.

Furthermore, as (e) the additive, additives commonly used in a gel polymer electrolyte may be used without limitation, and, as a representative example, at least one selected from vinylene carbonate (VC), propanesultone (PS), succinonitrile (SN), adiponitrile, ethylene sulfate (ESa), 1,3-propenesultone (PRS), fluoroethylene carbonate (FEC), LiPO2F2, lithium difluorooxalatoborate (LiODFB), lithium bis(oxalato)borate (LiBOB), tris(trimethylsilyl)phosphate (TMSPa), tris(trimethylsilyl)phosphite (TMSPi), lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, lithium oxalyldifluoroborate, tris(2,2,2-trifluoroethyl)phosphate (TFEPa), or tris(trifluoroethyl)phosphite (TFEPi) may be included.

In the method of the present invention, after injecting the composition for a gel polymer electrolyte, the open sealing portions are adhered to each other and sealed, and then a subsequent process can be performed.

That is, since the inlet of the gel polymer electrolyte composition is sealed before the ultrasonic vibration is applied, it is possible to prevent volatilization of the gel polymer electrolyte composition or internal contamination of the battery that may occur when the pouch-type case is opened.

Sealing can be carried out at 130°C to 160°C for 1 second to 10 seconds, for example at 140°C to 150°C for 2 seconds to 3 seconds, so that the polymer layers of the bag-type housing can be bonded together.

Application of ultrasonic vibration

Furthermore, in the method of the present invention, after injecting the composition for a gel polymer electrolyte, ultrasonic vibration is applied while pressurizing the preliminary pouch-type secondary battery using the ultrasonic element arranged by a close junction on both sides of the case of the preliminary pouch-type secondary battery.

The application of ultrasonic vibration is carried out by applying vibration having a frequency of 20 kHz to 200 MHz, for example, 31 kHz to 200 MHz. In a case where the ultrasonic vibration is carried out at a frequency of less than 20 kHz, since it is difficult for the composition for a gel polymer electrolyte injected into the bag-type housing to decompose into fine particles, the wetting improvement effect of the composition for a gel polymer electrolyte may be negligible. In a case where the ultrasonic vibration is carried out at a frequency of more than 200 MHz, power consumption may be excessively increased to reduce productivity.

Furthermore, the bag-type preliminary secondary battery is pressurized to a pressure of 0.1 kgf/cm2 to 500 kgf/cm2, and more particularly of 0.1 kgf/cm2 to 100 kgf/cm2, for example, of 0.1 kgf/cm2 to 50 kgf/cm2, per area of the bag-type preliminary secondary battery using a hydraulic press while applying ultrasonic vibration.

In a case where the pressure applied is within the above range, the dispersion effect of the composition for a gel polymer electrolyte can be further improved. In a case where the applied pressure is less than 0.1 kgf/cm2 per area of the preliminary pouch-type secondary battery, it may be difficult to transmit ultrasonic vibration to the interior of the cell, and, in a case where the applied pressure is greater than 3,000 kgf/cm2 per area of the preliminary pouch-type secondary battery, excessive pressure may be applied to the pouch-type secondary battery to cause damage to the cell.

In the application of ultrasonic vibration, a cycle having a rest period of 5 seconds to 10 seconds after the application of ultrasonic vibration for 5 seconds to 10 seconds is set as a cycle, and the cycle may be repeated from about 50 times to about 150 times, for example, about 100 times.

In this case, in the present invention, since the ultrasonic element is formed in the form of completely surrounding the entire surface of the pouch-type preliminary secondary battery, the ultrasonic element can not only prevent heat loss from the interior of the pouch-type preliminary secondary battery to more stably maintain the internal temperature of the cell, but also can apply uniform ultrasonic pressure and vibration to the entire surface of the pouch-type preliminary secondary battery and improve the working environment by reducing the noise generated when the ultrasonic vibration is applied.

In this case, the application of ultrasonic vibration in the method of the present invention may be carried out simultaneously with the injection of the composition for a gel polymer electrolyte, or may be carried out after the injection of the composition for the gel polymer electrolyte is completed and the bag-type casing is sealed.

As in the method of the present invention, in a case where ultrasonic vibration is applied during or after injection of the gel polymer electrolyte composition, since the viscosity of the gel polymer electrolyte composition is reduced and its molecular mobility is improved to facilitate impregnation of the electrode plates and the separator constituting the electrode assembly, the impregnation (wetting) ability of the gel polymer electrolyte composition can be significantly improved. Therefore, the wetting time of the pouch type secondary battery after injection of the gel polymer electrolyte composition and the resulting preparation time can be reduced.

Particularly, in the method of the present invention, the pre-gelling reactivity of the composition for a gel polymer electrolyte can be suppressed by applying ultrasonic vibration after injecting the composition for a gel polymer electrolyte and sealing the bag-type casing.

In the method of preparing a pouch-type secondary battery of the present invention, when ultrasonic vibration is applied, the exothermic temperature range of the interior of the preliminary pouch-type secondary battery caused by the ultrasonic vibration can be controlled using the ultrasonic element.

Conducting training

In the present specification, the formation of the preliminary pouch-type secondary battery can be carried out after the application of ultrasonic vibration.

Training can be performed by charging and discharging at 100 mA to 200 mA down to 1.7 V to 4.4 V at 0.05 C to 0.1 C, for example, at 0.05 C to 1.7 V at 70 °C or less.

Furthermore, in the method of the present invention, aging may be additionally performed after forming.

Aging may be carried out in a temperature range of 25°C to 60°C, for example, 30°C to 40°C, for 1 hour to 72 hours (3 days) or less so that pregelation is prevented and the electrodes are sufficiently wetted with the composition for a gel polymer electrolyte.

In the method of the present invention, a sufficient solid-electrolyte interface (SEI) can be formed by performing the forming process and the aging process at 70°C or lower, and a side reaction can be prevented by controlling the wetting enhancement effect and the electrolyte salt decomposition reaction. Furthermore, in the method of the present invention, vacuum wetting can be additionally performed to improve wetting after aging.

It is desirable that the vacuum wetting be carried out by exposure for a short period of time to a vacuum pressure lower than that produced under conventionally carried out wetting conditions, where, specifically, the vacuum wetting can be repeated three times while reducing the pressure for 10 seconds to 20 seconds in a vacuum chamber, and more specifically, the vacuum wetting can be repeated three times while reducing the pressure for 15 seconds to 20 seconds.

When forming in the method of the present invention, large amounts of gas generated from the positive electrode active material and gas generated due to a side reaction between the positive electrode active material and the composition for a gel polymer electrolyte may be generated. If the gases generated in the battery cell are not efficiently removed in performing the forming, the gases may occupy a certain space in the battery cell to prevent uniform formation and adversely affect the battery life and battery performance such as capacity and output. In addition, the battery capacity may be rapidly reduced or the battery cell may swell due to the gas remaining in the battery cell as the number of charge and discharge cycles increases.

Therefore, the method of the present invention may further include degassing the gas generated in performing the forming by opening some areas of the casing, after forming.

Furthermore, the method of the present invention may further include applying ultrasonic vibration after degassing.

The application of ultrasonic vibration can be carried out under the same conditions as those of the application of ultrasonic vibration described above.

As described above, in the method of the present invention, since the dispersibility of the composition for a gel polymer electrolyte is improved by performing the various steps of ultrasonic vibration before a curing process for preparing the gel polymer electrolyte to increase wetting, the gel polymer electrolyte can be cured more uniformly and stably.

Curing the composition for a gel polymer electrolyte

In the present specification, curing (gelling) of the composition for a gel polymer electrolyte may be performed after formation.

Curing (gelation) can be carried out by a conventional photocuring procedure using irradiation with heat, electron beams, or gamma rays.

Specifically, curing (gelation) can be carried out by thermal curing in a temperature range of 30°C to 70°C, for example, 40°C to 65°C, for 5 hours to 24 hours under inert conditions.

As described above, if the curing is carried out in an inert atmosphere, since the reaction of radicals with oxygen, as a radical scavenger, in the atmosphere is essentially blocked, the degree of reaction can be increased to such an extent that unreacted oligomer is practically absent. Therefore, it is possible to improve the gel conversion rate and prevent the degradation of secondary battery performance caused by a large amount of unreacted oligomer remaining in the battery. A gas having low reactivity known in the art can be used for the inert atmosphere, and, in particular, at least one inert gas selected from the group consisting of nitrogen, argon, helium, and xenon can be used.

The polymerizable monomers can be crosslinked with each other by curing (gelation) to form a polymer network in the form of a gel, and the electrolyte salt dissociated from the composition for a gel polymer electrolyte can be uniformly impregnated into the polymer network.

Furthermore, the method of the present invention may further include aging at 25°C to 70°C, for example, 30°C to 60°C, for 1 hour to 24 hours, after curing. In the aging process, additional SEI may be formed by allowing the battery to stand at room temperature for a certain period of time after the charging and discharging and curing process are completed, and as a result, additional gas generation may be induced.

Degassing

Furthermore, in the specification of the present invention, degassing may be performed after curing of the composition for a gel polymer electrolyte.

Degassing can be performed while applying pressure to the battery casing. Degassing can be performed at a pressure of -85 kPa to -95 kPa.

The open area of the bag-type casing can then be sealed at a temperature of 120°C to 150°C for approximately 2 seconds to approximately 5 seconds.

As described above, in the present invention, since the ultrasonic element is arranged by close attachment to the preliminary pouch-type secondary battery during the preparation of the pouch-type secondary battery, and the ultrasonic vibration and pressure are applied simultaneously after the injection of the composition for a gel polymer electrolyte or simultaneously with the injection of the composition for a gel polymer electrolyte to effectively disperse the composition for a gel polymer electrolyte, the wetting of the composition for a gel polymer electrolyte can be improved. As a result, it is possible to reduce the wetting time during the preparation of the pouch-type secondary battery and the resulting preparation costs. Therefore, a pouch-type secondary battery can be prepared having improved stability and overall performance.

Hereinafter, the present invention will be described in detail by specific examples. However, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these exemplary embodiments are provided so that this description will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art.

Examples

Example 1.

(Preparation of composition for a gel polymer electrolyte)

A composition for a gel polymer electrolyte was prepared by adding 5 g of the compound represented by formula 1a and 0.01 g of dimethyl 2,2'-azobis(2-methylpropionate) (CAS No. 2589-57-3), as a polymerization initiator, to 94.99 g of an organic solvent (ethylene carbonate (EC):ethyl methyl carbonate (EMC) = volume ratio of 3:7) in which LiPF61 M was dissolved.

(Bag-type secondary battery preparation)

A positive electrode mixture slurry (solid content of 65% by weight) was prepared by adding LiCoO<2> as a positive electrode active material, carbon black as a conductive agent, and polyvinylidene fluoride (PVDF) as a binder in a weight ratio of 94:3:3 to N-methyl-2-pyrrolidone (NMP) as a solvent. An approximately 20 µm thick aluminum (Al) thin film, as a positive electrode collector, was coated with the positive electrode mixture slurry and dried, and then the coated Al thin film was roll-laminated to prepare a positive electrode.

Subsequently, a negative electrode mixture slurry (solid content of 75% by weight) was prepared by adding carbon powder as the negative electrode active material, PVDF as a binder, and carbon black as a conductive agent in a weight ratio of 96:3:1 to NMP as a solvent. A 10 µm thick copper (Cu) thin film, as a negative electrode collector, was coated with the negative electrode mixture slurry and dried, and then the coated Cu thin film was roll-laminated to prepare a negative electrode.

Subsequently, after a 20 µm thick polypropylene (PE) separator was arranged between the previously prepared electrodes, rolled up, and compressed to prepare an electrode array, the electrode array was inserted into a pouch-type battery case to prepare a preliminary pouch-type secondary battery.

Subsequently, an ultrasonic element formed by aluminum (Al) was arranged surrounding the entire surface of the preliminary pouch-type secondary battery by tightly bonding to both surfaces of the preliminary pouch-type secondary battery.

The gel polymer electrolyte composition prepared above was injected while maintaining the temperature of the ultrasonic element at 40 °C, and an inlet of the gel polymer electrolyte composition was sealed at 140 °C.

Subsequently, a pressure of 5 kgf/cm2 was applied per area of the preliminary bag-type secondary battery using the ultrasonic element, and simultaneously, a 20 kHz ultrasonic vibration was applied for 100 cycles (1 cycle: 5 seconds of ultrasonic vibration/5 seconds of rest).

Subsequently, after performing a forming procedure for 3 hours at 0.1 C and 200 mA, aging was performed at room temperature for 2 days, and a curing procedure was performed at 65 °C for 5 hours to prepare a gel polymer electrolyte.

Subsequently, degassing was performed while applying a pressure of -85 kPa to remove the generated gas from the interior of the preliminary bag-type secondary battery, and thus, a bag-type secondary battery of the present invention was prepared.

Example 2.

A gel polymer electrolyte composition was injected while maintaining the temperature of an ultrasonic element at 40 °C during the preparation of a pouch-type secondary battery, and an inlet of the gel polymer electrolyte composition was sealed at 140 °C.

Subsequently, the pouch-type secondary battery was prepared in the same manner as in Example 1, except that a pressure of 10 kgf/cm2 was applied per area of a preliminary pouch-type secondary battery using the ultrasonic element, and an ultrasonic vibration of 25 kHz was applied for 100 cycles (1 cycle: 5 seconds of ultrasonic vibration/5 seconds of rest).

Comparative example 1.

A pouch-type secondary battery was prepared in the same manner as in Example 1, except that, during the preparation of the pouch-type secondary battery, a composition for a gel polymer electrolyte was injected at room temperature (25 °C), an inlet of the composition for a gel polymer electrolyte was sealed at 140 °C, and then a forming process and a curing process were performed without performing an ultrasonic process and a pressurizing process.

Comparative example 2.

A pouch-type secondary battery was prepared in the same manner as in Example 1, except that, during the preparation of the pouch-type secondary battery, a composition for a gel polymer electrolyte was injected while maintaining the temperature of an ultrasonic element at 40 °C, an inlet of the composition for a gel polymer electrolyte was sealed at 140 °C, and then a forming process and a curing process were performed without performing an ultrasonic process and a pressurizing process.

Comparative example 3.

A gel polymer electrolyte composition was injected while maintaining the temperature of an ultrasonic element at 40 °C during the preparation of a pouch-type secondary battery, and an inlet of the gel polymer electrolyte composition was sealed at 140 °C.

Subsequently, the pouch-type secondary battery was prepared in the same manner as in Example 1, except that no ultrasonic procedure was performed, and a pressure of 5 kgf/cm2 was applied per area of a preliminary pouch-type secondary battery using the ultrasonic element.

Comparative example 4.

A pouch-type secondary battery was prepared in the same manner as in Example 1, except that, during the preparation of the pouch-type secondary battery, the temperature of an ultrasonic element was maintained at room temperature (25 °C).

Comparative example 5.

A pouch-type secondary battery was prepared in the same manner as in Example 2, except that, during the preparation of the pouch-type secondary battery, the temperature of an ultrasonic element was maintained at room temperature (25 °C).

Comparative example 6.

A pouch-type secondary battery was prepared in the same manner as in Example 1, except that, during the preparation of the pouch-type secondary battery, a composition for a gel polymer electrolyte was injected while maintaining the temperature of an ultrasonic element at 40 °C, and then a 20 kHz ultrasonic vibration was applied for 100 cycles (1 cycle: 5 seconds of ultrasonic vibration/5 seconds of rest) using the ultrasonic element without pressurization.

Comparative example 7.

A pouch-type secondary battery was prepared in the same manner as in Example 1, except that, during the preparation of the pouch-type secondary battery, the temperature of an ultrasonic element was maintained at 100 °C.

Comparative example 8.

A pouch-type secondary battery was prepared in the same manner as in Example 1, except that, during the preparation of the pouch-type secondary battery, a pressure of 3,500 kgf/cm2 was applied per area of a preliminary pouch-type secondary battery using an ultrasonic element during pressurization while maintaining the temperature of the ultrasonic element at room temperature (25 °C).

[Table 1]

Experimental examples

Experimental example 1.

The wetting of each of the pouch-type secondary batteries including the gel polymer electrolyte, which were respectively prepared in Example 1 and Comparative Example 1, was measured at 25 °C using an alternating current (AC) impedance measurement method. In this case, the ionic conductivity was measured in a frequency range of 0.05 Hz to 100 MHz using a VMP3 measuring instrument and a 4294A analyzer. The results thereof are presented in Figure 2 below. In this case, in Figure 2, the horizontal axis represents an actual value (Zre) of impedance (Z) calculated by an impedance calculating unit, and the vertical axis represents an imaginary value (Zim) of the impedance.

Furthermore, the reference example in Figure 2 represents an AC impedance measured immediately after injection of the composition for a gel polymer electrolyte in Example 1.

In general, as the wetting of the gel polymer electrolyte composition increases, since fewer pores in the battery are not impregnated with the gel polymer electrolyte composition, the resistance decreases. Therefore, when measuring AC impedance, the bulk resistance is measured to be small, and the resistance value steadily converges as the impregnation progresses.

Referring to Figure 2, with respect to the pouch-type secondary battery of Example 1, in which ultrasonic vibration was applied, it can be understood that the mass resistance value was significantly reduced compared with that of the pouch-type secondary battery of Comparative Example 1, in which ultrasonic vibration was not applied, based on the value of the pouch-type secondary battery of Reference Example.

From these results, it can be understood that the wetting of the gel polymer electrolyte composition in the battery was improved when ultrasonic vibration was applied during the injection of the gel polymer electrolyte composition.

Experimental example 2. Evaluation of initial capacity

Each of the pouch-type secondary batteries prepared in Examples 1 and 2 and the pouch-type secondary batteries prepared in Comparative Examples 1 to 8 was charged at a constant current (Cco) of 333 mA at a rate of 0.3 C to a battery voltage of 4.2 V in a constant current-constant voltage (Cco-Tco) mode, and charged to cutoff at a rate of 0.05 C while maintaining a constant voltage (Tco) of 4.2 V after the battery voltage reached 4.2 V to perform the first charge. A cycle was repeated three times, in which the battery subjected to the first charge was discharged at a constant current (Cco) of 333 mA at a rate of 0.3 C to a battery voltage of 3 V, and the third discharge capacity was selected as the initial capacity. The results thereof are presented in Table 2 below.

[Table 2]

Referring to Table 2, with respect to the lithium secondary batteries of Examples 1 and 2, given the excellent electrolyte wetting, it can be understood that higher initial capacities were achieved than those of the pouch-type secondary batteries prepared in Comparative Examples 1 to 8.

Experimental example 3. Evaluation of the initial resistance value

Each of the pouch-type secondary batteries prepared in Examples 1 and 2 and the pouch-type secondary batteries prepared in Comparative Examples 1 to 8 was charged at a constant current (Cco) of 333 mA at a rate of 0.3 C to a battery voltage of 4.2 V in a constant current-constant voltage (Cco-Tco) mode, and charged to cut-off at a rate of 0.05 C while maintaining a constant voltage (Tco) of 4.2 V after the battery voltage reached 4.2 V to perform the first charge. A cycle was repeated three times, where the battery subjected to first charge was discharged at a constant current (Cco) of 333 mA at a rate of 0.3 C down to a battery voltage of 3 V, the voltage drop generated when a 10 second discharge was performed at a current of 2 A (2 C) was recorded, and the direct current (DC) discharge resistance values measured during the initial evaluation, which were calculated using R = V/I (Ohm's law), are then presented in table 3 below. In this case, the DC resistance had a close relationship with the output characteristics of the secondary battery.

[Table 3]

Referring to Table 3, with respect to the pouch-type secondary batteries prepared in Examples 1 and 2, since the electrolyte wetting was increased and, consequently, a uniform electrode-electrolyte reaction occurred, it can be understood that the internal resistance values were lower than those of the pouch-type secondary batteries prepared in Comparative Examples 1 to 8.

Claims (6)

1. Method for preparing a pouch-type secondary battery comprising: housing a set of electrodes in an interior space of a pouch-type housing to form a preliminary pouch-type secondary battery; providing a metallic ultrasonic element maintained at a temperature of 30°C to 60°C and formed of a metallic material to facilitate temperature and pressure control by tightly bonding to both surfaces of the bag-type preliminary secondary battery, the metallic ultrasonic element being formed in a structure surrounding the entire surface of the bag-type preliminary secondary battery; injecting a composition for a gel polymer electrolyte into the preliminary pouch-type secondary battery; applying ultrasonic vibration having a frequency of 20 kHz to 200 MHz to the bag-type preliminary secondary battery while pressurizing the bag-type preliminary secondary battery using the metallic ultrasonic element that applies a pressure of 0.1 kgf/cm2 to 500 kgf/cm2 per area of the bag-type preliminary secondary battery; carry out the formation of the preliminary bag-type secondary battery; curing the composition to a gel polymer electrolyte; and degas. 2. Method according to claim 1, wherein the metallic material is selected from stainless steel, iron, aluminum, copper and nickel, or an alloy of two or more thereof. 3. Method according to claim 1, wherein the metallic ultrasonic element comprises stainless steel or aluminum. 4. Method according to claim 1, wherein the pressurization of the bag-type preliminary secondary battery is performed by applying a pressure of 0.1 kgf/cm2 to 100 kgf/cm2 per area of the bag-type preliminary secondary battery. 5. Method according to claim 1, wherein the application of ultrasonic vibration is carried out simultaneously with the injection of the composition for a gel polymer electrolyte. 6. The method of claim 1, further comprising applying ultrasonic vibration after formation and before curing of the composition for a gel polymer electrolyte.