Method for manufacturing a gel polymer electrolyte secondary battery and a gel polymer electrolyte secondary battery manufactured by the same method

Abstract

The secondary battery according to the present invention has a structure in which an electrolyte having a relatively low degree of cross-linking is contained in an internal core portion, and the core portion is surrounded by an outer shell portion containing an electrolyte having a relatively high degree of cross-linking. This structural feature provides the effect of improving both ionic conductivity and mechanical properties. Furthermore, the low-cross-linked electrolyte portion is enclosed by the highly cross-linked electrolyte portion, thereby preventing electrolyte leakage. Furthermore, the secondary battery according to the present invention can be manufactured using a simple method in which only the outer shell portion is cross-linked in an environment preheated above the cross-linking temperature, and the core portion reaches the cross-linking temperature and is cross-linked. This eliminates the need for a separate device or equipment line for the cross-linking step, thereby not adversely affecting process efficiency.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a lithium secondary battery including a gel polymer electrolyte and a gel polymer electrolyte secondary battery manufactured by the method.

[0002] This application claims priority based on Korean Patent Application No. 10-2021-0069524 filed on May 28, 2021, and all the contents disclosed in the specification and drawings of the application are incorporated herein.

Background Art

[0003] Recently, interest in energy storage technology has been increasing. In particular, the application fields are expanding to the energy of mobile phones, camcorders, notebook PCs, and even electric vehicles, and efforts for research and development of electrochemical devices are gradually materializing. Electrochemical devices are the most spotlighted fields in such energy storage technology fields, and among them, interest in secondary batteries capable of charging and discharging is emerging.

[0004] Among the currently applied secondary batteries, lithium secondary batteries developed in the early 1990s are attracting attention due to their advantages of high operating voltage and much higher energy density compared to conventional batteries such as Ni-MH, Ni-Cd, and lead-sulfate batteries using water-soluble electrolytes.

[0005] Such lithium secondary batteries can be divided into lithium-ion batteries using a liquid electrolyte and lithium polymer batteries using a polymer electrolyte depending on the electrolyte used.

[0006] Lithium-ion batteries have the advantage of high capacity, but since they use a liquid electrolyte containing a lithium salt, there is a risk of leakage and explosion, and thus there is a disadvantage that the battery design becomes complicated to prepare for this.

[0007] In contrast, lithium polymer batteries use solid polymer electrolytes or gel polymer electrolytes containing electrolyte solutions as the electrolyte, thus improving safety and potentially offering flexibility. This allows for the development of various forms, such as small or thin-film batteries. Gel polymer electrolytes can be classified into coated and liquid-injected types depending on the manufacturing method. Liquid-injected gel polymer electrolytes can be manufactured by injecting a liquid electrolyte containing crosslinking monomers into a cell, wetting the electrode assembly evenly with the liquid electrolyte, and performing a crosslinking process. During the crosslinking process, the electrolyte forms a matrix and transforms into a non-flowing gel electrolyte.

[0008] Such gel electrolytes have advantages such as being non-fluid, thus avoiding thermal safety and leakage problems, improving cell strength, making them resistant to external impacts, and offering high physical safety. However, their ionic conductivity is lower and their resistance is higher compared to liquid electrolytes. As a result, their lifespan tends to be shorter compared to using only liquid electrolytes. Therefore, there is a need for development aimed at improving the ionic conductivity of gel polymer electrolytes. [Overview of the project] [Problems that the invention aims to solve]

[0009] The present invention aims to provide a secondary battery containing a gel polymer electrolyte and having high ionic conductivity.

[0010] Furthermore, another objective of the present invention is to provide a method for improving the ionic conductivity of a gel polymer electrolyte produced in a method for producing a secondary battery containing a liquid-injection gel polymer electrolyte using a radical thermal initiation reaction.

[0011] Other objects and advantages of the present invention can be understood from the following description and will be more clearly understood from the embodiments of the present invention. Furthermore, the objects and advantages of the present invention can be realized by the means and combinations thereof set forth in the claims. [Means for solving the problem]

[0012] According to one aspect of the present invention, a method for manufacturing a secondary battery containing a gel polymer electrolyte, the method comprising: (S1) the step of loading an electrode assembly and a composition for forming a gel polymer electrolyte into a battery case to manufacture a spare battery; (S2) the step of carrying out a crosslinking reaction of the composition for forming a gel polymer electrolyte; and (S3) the step of cooling the result of step (S2), wherein step (S2) is carried out by a heating device, the heating device is preheated to a predetermined temperature before step (S2) is carried out, and the secondary battery contains a gel polymer electrolyte in which a portion of the electrolyte is crosslinked to a predetermined degree of crosslinking or higher, and the degree of crosslinking increases as you move from the center outwards.

[0013] According to a second aspect of the present invention, in the first aspect, the secondary battery includes a central part in which the degree of crosslinking of the electrolyte is low, and an outer part surrounding the central part, which contains a gel polymer electrolyte exhibiting a higher degree of crosslinking than the central part.

[0014] According to a third aspect of the present invention, in the first or second aspect, step (S1) includes the step of sealing the battery case under normal pressure.

[0015] According to the fourth aspect of the present invention, in any one of the first to third aspects, the composition for forming the gel polymer electrolyte comprises a lithium salt, a non-aqueous organic solvent, a polymerization initiator, and at least one polymerizable compound selected from the group consisting of polymerizable monomers, oligomers, and copolymers.

[0016] According to the fifth aspect of the present invention, step (S2) is performed at a temperature of 60°C or higher on any one of the first to fourth surfaces.

[0017] According to the sixth aspect of the present invention, in any one of the first to fifth surfaces, an aging step at room temperature is further performed before step (S2) is performed.

[0018] According to the seventh aspect of the present invention, after performing the room temperature aging step on any one of the first to sixth surfaces, a vacuum treatment step is further performed.

[0019] According to the eighth aspect of the present invention, in any one of the first to seventh aspects, the cooling in step (S3) is performed in a cooling chamber controlled to a temperature below room temperature, and the temperature of the battery reaches the ambient temperature of the cooling chamber within a time of 10 minutes or less.

[0020] According to the ninth aspect of the present invention, a secondary battery comprising a central part containing a gel polymer electrolyte with a low degree of crosslinking, wherein the degree of crosslinking of the gel polymer electrolyte increases stepwise or progressively from the inside to the outside of the battery, and an outer part surrounding the central part containing a gel polymer electrolyte having a higher degree of crosslinking than the central part.

[0021] In the tenth surface of the present invention, the outer portion of the ninth surface has a degree of crosslinking of 80% by weight or more, while the central portion has a degree of crosslinking of less than 40% by weight. [Effects of the Invention]

[0022] The secondary battery according to the present invention has a structure in which the central part contains an electrolyte with a relatively low degree of crosslinking due to crosslinking in the central part, and the central part is surrounded by an outer part containing an electrolyte with a relatively high degree of crosslinking. Due to these structural features, both ionic conductivity and mechanical properties are improved. In addition, the lower degree of crosslinking of the electrolyte is contained by the higher degree of crosslinking of the electrolyte, which prevents electrolyte leakage. Furthermore, the secondary battery according to the present invention can be manufactured by a simple method by applying a method in which only the outer part is crosslinked before the central part reaches the crosslinking temperature and is crosslinked in an environment that has been preheated to above the crosslinking temperature. As a result, there is no need for separate equipment or equipment lines to perform the crosslinking step, and thus it does not negatively affect the efficiency of the process.

[0023] The following drawings accompanying this specification illustrate preferred embodiments of the invention and, together with the detailed description of the invention, serve to further illustrate the technical idea of ​​the invention. Therefore, the invention should not be construed as being limited solely to what is shown in the drawings. The shapes, sizes, scales, or proportions of elements in the drawings accompanying this specification may be exaggerated to emphasize a clearer description. [Brief explanation of the drawing]

[0024] [Figure 1] This is a cross-sectional view of a secondary battery according to one embodiment of the present invention. [Figure 2] This graph shows the temperature changes and temperature gradients both inside and outside the battery. [Modes for carrying out the invention]

[0025] Preferred embodiments of the present invention will now be described in detail with reference to the attached drawings. Prior to this, terms and words used in this specification and in the claims should not be interpreted in a manner limited to their usual or dictionary meanings, but rather in a manner corresponding to the technical idea of ​​the present invention, in accordance with the principle that the inventor himself can appropriately define the concepts of terms in order to best describe the invention. Accordingly, it should be understood that the embodiments and configurations shown in the drawings described herein are merely the most preferred embodiments of the present invention and do not represent the entirety of the technical idea of ​​the present invention, and that there may be a variety of equivalents and modifications that can be substituted for them at the time of this application.

[0026] Furthermore, throughout the specification, when a part of it "includes" a certain component, unless otherwise stated, this does not mean that other components are excluded, but rather that other components may be included.

[0027] Terms used throughout this specification, such as “approximately” and “substantially,” are used to mean, when specific manufacturing and material tolerances are presented, the numerical values ​​or values ​​close to those values, and are used to prevent unscrupulous infringers from unfairly using disclosures that refer to precise or absolute numerical values ​​to aid in understanding this application.

[0028] Throughout this specification, the phrase "A and / or B" means "A or B, or both."

[0029] The specific terms used in the following detailed descriptions are for convenience only and are not restrictive. The words “right,” “left,” “top,” and “bottom” indicate directions in the referenced drawings. The words “inward” and “outward” indicate directions toward or away from the geometric center of the specified device, system, and its components, respectively. “Forward,” “backward,” “upward,” “downward,” and related words and phrases indicate location and orientation in the referenced drawings and are not restrictive. Such terms include the words exemplified above, their derivatives, and words with similar meanings.

[0030] In this invention, unless otherwise specified, "*" means a link between identical or different atoms or end parts of a chemical formula.

[0031] Furthermore, in this specification, "substitution" means, unless otherwise defined, that at least one hydrogen atom bonded to a carbon atom is replaced by an element other than hydrogen. For example, substitution by an alkyl group having 1 to 5 carbon atoms or by a fluorine element.

[0032] Next, the present invention will be described in detail based on the attached drawings.

[0033] The secondary battery according to the present invention includes one or more independent negative electrodes, a separator membrane, and a positive electrode, and comprises an electrode assembly in which the negative electrode, separator membrane, and positive electrode are stacked in that order so as to be electrically insulated from the negative electrode by the separator membrane. The secondary battery also contains an electrolyte, and the electrode assembly is impregnated with the electrolyte. In one embodiment of the present invention, the battery has a configuration in which the degree of crosslinking of the electrolyte increases from the inside to the outside. That is, the central part of the electrode assembly has a relatively low degree of crosslinking of the electrolyte and is fluid, while the outer part of the electrode assembly has a higher degree of crosslinking of the electrolyte than the central part and is very low in fluidity or has no fluidity at all. The central part is surrounded by the outer part, and the low-degree-of-crosslinking electrolyte present in the central part is encapsulated by the high-degree-of-crosslinking electrolyte and exists in a state where it is not released to the outside of the electrode assembly. On the other hand, in one embodiment of the present invention, a transition region exists between the central part and the outer part, and the transition region means a portion in which the degree of crosslinking increases as you move from the central part to the outer part.

[0034] Figure 1 is a cross-sectional view of a secondary battery 10 according to one embodiment of the present invention. Referring to this, the battery includes an electrode assembly 100 in which a negative electrode, a separator membrane, and a positive electrode are stacked in that order, and a battery case 120 in which the electrode assembly is housed. The battery may have electrode tabs 110 that are drawn out from the electrode assembly to the outside. The battery also contains an electrolyte, and the electrode assembly is impregnated with the electrolyte. The central part C of the electrode assembly has a low degree of crosslinking of the electrolyte and is fluid. In one embodiment of the present invention, the electrolyte in the central part may have a viscosity in the range of 0 cP to 20,000 cP, preferably 15,000 cP or less. On the other hand, in one embodiment of the present invention, it is desirable that the degree of crosslinking in the central part is in the range of less than 40% by weight.

[0035] On the other hand, the central part is surrounded by an outer part P, and the outer part has a high degree of crosslinking of the electrolyte and preferably does not exhibit fluidity. In one embodiment of the present invention, the electrolyte of the outer part may exhibit a relatively high degree of crosslinking compared to the central part, for example, a degree of crosslinking of 40% by weight or more, preferably 80% to 100% by weight.

[0036] In one specific embodiment of the present invention, the battery has a central portion exhibiting a degree of crosslinking of less than 40% by weight, and an outer portion exhibiting a degree of crosslinking of 80% to 100% by weight, in which case the difference in degree of crosslinking between the outer portion and the central portion may be 50% by weight or more. In one specific embodiment, the inside of the battery has an outer portion and a central portion with a difference in degree of crosslinking of 50% by weight or more, and a transition portion may be arranged between the outer portion and the central portion.

[0037] In one embodiment of the present invention, the degree of crosslinking can be determined by calculating the proportion of C=C bonds in each electrolyte constituting the outer and central parts of the electrode assembly using nuclear magnetic resonance (NMR) analysis. However, the invention is not limited to this method.

[0038] On the other hand, in one embodiment of the present invention, the outer shell may include a transition portion T. The transition portion is located between the central part and the outermost surface of the outer shell, and the degree of crosslinking in the transition portion gradually increases as you move from the central part towards the outermost surface of the outer shell. That is, in terms of the degree of crosslinking, it increases in the order of central part → transition portion → outermost surface.

[0039] On the other hand, in one embodiment of the present invention, the empty space in the battery case outside the external boundary of the electrode assembly in the battery can be filled with an electrolyte. Hereinafter, this will be referred to as the filled portion. The electrolyte filling the empty space in the battery case in this manner may be formed in a form in which it is located closest to the battery case, has the highest degree of crosslinking, is not divided into the outer casing and / or transition portion, but is bonded as an inseparable whole. The secondary battery according to the present invention is manufactured by placing an electrode assembly in a battery case, then pouring a gel polymer electrolyte formation composition into the battery case, and then going through a crosslinking step, as described below, and as a result, the outer casing of the electrode assembly can be formed in a state in which it is connected to the filled portion and becomes an integrated unit.

[0040] On the other hand, in one embodiment of the present invention, the outer casing and / or transition portion may extend to the outside of the electrode assembly and occupy a part of the filling portion. That is, in the present invention, the central portion is located within the electrode assembly, and the central portion is directly surrounded by the outer casing or by the transition portion, the transition portion is surrounded by the outer casing, and the external boundary of the electrode assembly may belong to the outer casing or the transition portion. As a result, the liquid electrolyte can be arranged so as not to come into direct contact with the battery case.

[0041] In the present invention, the positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on at least one or both surfaces of the positive electrode current collector. The positive electrode active material layer includes a positive electrode mixture, which may include a positive electrode active material, a binder, and a conductive material. In this specification, the positive electrode mixture is described as not including an electrolyte impregnated into the positive electrode. In the present invention, the positive electrode active material layer has a porous property including a plurality of pores, the pores being filled with an electrolyte as described above, the electrolyte being in a solid state with a low degree of crosslinking and fluidity, or with a high degree of crosslinking and not fluidity, depending on the location of the pores in the electrode assembly.

[0042] The positive electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has conductivity. For example, stainless steel, aluminum, nickel, titanium, fired carbon, or those obtained by surface treatment of the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. can be used.

[0043] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium. Specifically, it may include a lithium composite metal oxide containing one or more metals such as cobalt, manganese, nickel, or aluminum and lithium. More specifically, the lithium composite metal oxide is a lithium manganese-based oxide (for example, LiMnO2, LiMn2O4, etc.), a lithium cobalt-based oxide (for example, LiCoO2, etc.), a lithium nickel-based oxide (for example, LiNiO2, etc.), a lithium nickel manganese-based oxide (for example, LiNi 2-z1 , 1-Y2 , q2 , Y2 , r3 , 1-Y1 , r2 , Y1 , p2 , p1 , q1 , q , r1 , z1 , p Mn Y O2 (where 0 < Y < 1), LiMn 2-z Ni z O4 (where 0 < Z < 2), etc.), a lithium nickel cobalt-based oxide (for example, LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), a lithium manganese cobalt-based oxide (for example, LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-z1 Co z1 O4 (where 0 < Z1 < 2), etc.), a lithium nickel manganese cobalt-based oxide (for example, Li(Ni p Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, p1 + q1 + r2 = 2), etc.), or a lithium nickel cobalt transition metal (M) oxide (for example, Li(Ni p2 Co q2 Mn r3 MS2 )O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r3, and s2 are, as the atomic fractions of independent elements, 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, and p2 + q2 + r3 + s2 = 1), and the like can be mentioned, and one or more of these compounds may be included.

[0044] Among these, the lithium composite metal oxide can enhance the capacity characteristics and stability of the battery, and is LiCoO2, LiMnO2, LiNiO2, lithium nickel manganese cobalt oxide (for example, Li(Ni 1 / 3 Mn 1 / 3 Co 1 / 3 )O2, Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, and Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc.), or lithium nickel cobalt aluminum oxide (for example, Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.).

[0045] The positive electrode active material may be contained at 50% to 99% by weight based on 100% by weight of the positive electrode binder.

[0046] The binder is a component that assists in the bonding of the positive electrode active material to a conductive material and to the current collector, and is usually included in an amount of 1 to 30% by weight relative to 100% by weight of the positive electrode mixture. Examples of such binders include polyvinyllidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene propylene dienterpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers.

[0047] The conductive material can typically be added to the positive electrode mixture in an amount of 1 to 30% by weight, based on the total weight of the solid content.

[0048] Such conductive materials are not particularly limited as long as they do not induce chemical changes in the battery and are conductive. For example, carbon powders such as carbon black, acetylene black (or Denka black), Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powders such as natural graphite, artificial graphite, or graphite with a well-developed crystalline structure; conductive fibers such as carbon fibers or 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; and conductive materials such as polyphenylene derivatives may be used.

[0049] In the present invention, the negative electrode may include a negative electrode current collector and a negative electrode active material layer formed on at least one or both surfaces of the negative electrode current collector. The negative electrode active material layer includes a negative electrode mixture, which may include a negative electrode active material, a binder, and a conductive material. In this specification, the negative electrode mixture is described as not including an electrolyte impregnated into the negative electrode. In the present invention, the negative electrode active material layer has porous properties including a plurality of pores, which are filled with an electrolyte as described above, and which may exhibit a fluid liquid state or a solid state with a predetermined degree of crosslinking that does not exhibit fluidity, depending on the location of the pores in the electrode assembly.

[0050] The negative electrode current collector may typically have a thickness of 3 to 500 μm. Such a negative electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface treatments with carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used. Also, similar to the positive electrode current collector, it is possible to form fine irregularities on the surface to strengthen the bonding force of the negative electrode active material, and it can be used in a variety of forms such as films, sheets, foils, nets, porous materials, foams, and nonwoven fabrics.

[0051] Furthermore, the negative electrode active material may include at least one selected from the group consisting of lithium metal, carbon material capable of reversibly intercalating / deintercalating lithium ions, metal or alloys of these metals with lithium, metal composite oxides, materials capable of doping and dedoping lithium, and transition metal oxides.

[0052] As the carbon material capable of reversibly intercalating / deintercalating the lithium ions, any carbonaceous negative electrode active material commonly used in lithium ion secondary batteries can be used without particular limitation. Representative examples thereof include crystalline carbon, amorphous carbon, or both of them can be used. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, fired coke and the like.

[0053] Examples of the metal composite oxide include PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5, Li x Fe2O3(0≦x≦1), Li x WO2(0≦x≦1) and Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0<x≦1; 1≦y≦3; 1≦z≦8) selected from the group consisting of can be used.

[0054] Examples of the substance capable of doping and undoping lithium include Si, SiO x(0 < x ≤ 2), Si-Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Sn, SnO2, Sn-Y (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Sn), etc. may be mentioned, and it is also possible to mix and use at least one of these with SiO2. As the element Y, it may be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0055] Examples of the transition metal oxide may include lithium-containing titanium composite oxide (LTO), vanadium oxide, lithium vanadium oxide, etc.

[0056] The negative electrode active material may be contained at 50% to 99% by weight based on 100% by weight of the negative electrode binder.

[0057] The binder is a component that helps bind the conductive material, the active material, and the current collector, and is usually added at 1% to 30% by weight based on 100% by weight of the negative electrode binder. Examples of such binders may include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene polymer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluorine rubber, various copolymers thereof, etc.

[0058] The conductive material is a component for further improving the conductivity of the negative electrode active material and can be added in an amount of 1 to 20% by weight relative to 100% by weight of the negative electrode mixture. Such a conductive material may be the same as or different from the conductive material used in the manufacture of the positive electrode. For example, carbon powders such as carbon black, acetylene black (or Denka black), Ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powders such as natural graphite, artificial graphite, or graphite with a well-developed crystalline structure; conductive fibers such as carbon fibers or 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; and conductive materials such as polyphenylene derivatives may be used.

[0059] The separation membrane serves to block internal short circuits between the two electrodes and impregnate them with an electrolyte. It can be formed by mixing a polymer resin, a filler, and a solvent to produce a separation membrane composition, then directly coating the top of the electrodes with the separation membrane composition and drying it to form a separation membrane film, or by casting the separation membrane composition onto a support, drying it, and then laminating the separation membrane film, which has been peeled off the support, onto the top of the electrodes.

[0060] The separation membrane may be made from commonly used porous polymer films, such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, either alone or in a laminated configuration, or from ordinary porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, but is not limited to these.

[0061] In this case, the diameter of the pores in the porous separation membrane is typically 0.01 to 50 μm, and the porosity can be 5 to 95%. Furthermore, the thickness of the porous separation membrane can generally be in the range of 5 to 300 μm.

[0062] In the present invention, the separation membrane has porous properties including a plurality of pores, the pores being filled with an electrolyte as described above, and the electrolyte may be in a fluid liquid state or in a solid state having a predetermined degree of crosslinking and not being fluid, depending on the location of the pores in the electrode assembly.

[0063] On the other hand, the battery case is not particularly limited in terms of material or shape. For example, it may be cylindrical or rectangular using a metal can. Alternatively, it may be a pouch type using a pouch film. Or, a coin-shaped case may also be used.

[0064] Manufacturing method of secondary batteries Next, a method for manufacturing a secondary battery according to the present invention will be described.

[0065] In one embodiment of the present invention, the method for manufacturing the secondary battery is as follows: (S1) A step of manufacturing a spare battery by loading the electrode assembly and the composition for forming the gel polymer electrolyte into a battery case, (S2) The step in which the crosslinking reaction of the composition for forming the gel polymer electrolyte is carried out, (S3) The step of cooling the result of step (S2) is included.

[0066] The (S2) step is performed by a heating device, and the heating device can be preheated to a predetermined temperature before performing the (S2) step.

[0067] On the other hand, a secondary battery obtained by the above manufacturing method may have a central part containing an electrolyte with a low degree of crosslinking and fluidity, and the central part may be encapsulated by an outer part containing a gel polymer electrolyte that is crosslinked to a predetermined degree or higher.

[0068] In this specification, the term "reserve battery" is used to distinguish it from the final product and refers to an intermediate product in the manufacturing process.

[0069] First, the electrode assembly and the composition for forming the gel polymer electrolyte are manufactured, and then these are placed in a battery case (S1).

[0070] The electrode assembly is identical to that described for the secondary battery according to the present invention. Therefore, for the sake of convenience, it will not be described again. In one embodiment of the present invention, the electrode assembly may be provided in the form of a jelly roll wound up, or in a stacked or stacked / folded form, depending on the purpose and application of the battery.

[0071] Although not particularly limited, the process may be carried out in the following order: first, the electrode assembly is placed inside the battery case, and then the composition for forming the gel polymer electrolyte is poured in.

[0072] In one embodiment of the present invention, the composition for forming the gel polymer electrolyte may comprise (a) a lithium salt; (b) a non-aqueous organic solvent; (c) a polymerization initiator; and (d) at least one polymerizable compound selected from the group consisting of polymerizable monomers, oligomers, and copolymers.

[0073] Lithium salt The aforementioned lithium salt is used as an electrolyte salt in lithium secondary batteries and is used as a medium for transferring ions. Typically, lithium salts contain Li as a cation. + It contains, and as an anion, F - Cl - , Br - , I - NO3 - , N(CN)2 - BF4 - ClO4 - AlO4 - AlCl4 - PF6 - SbF6 - AsF6 - BF2C2O4 - BC4O8 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF- (CF3)6P - CF3SO3 - , C4F9SO3 - CF3CF2SO3 - (CF3SO2)2N - (F2SO2)2N - CF3CF2(CF3)2CO - (CF3SO2) 2CH - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - It may include at least one selected from the group consisting of the following.

[0074] The lithium salt may be used alone or, if necessary, in a mixture of two or more types. The lithium salt may be appropriately modified within a range of normal use, but may be included in the gel polymer electrolyte composition at a concentration of 0.5 M to 2 M, preferably 0.9 M to 1.5 M, in order to obtain an optimal film-forming effect for preventing corrosion of the electrode surface.

[0075] The gel polymer electrolyte formation composition of the present invention contains a lithium salt of 0.5 M or more, thereby reducing resistance due to lithium ion depletion during high-rate charge and discharge. Furthermore, in the gel polymer electrolyte formation composition of the present invention, when the concentration of the electrolyte salt satisfies the above range, the lithium cation present in the gel polymer electrolyte formation composition increases, resulting in high lithium cation (Li + This ensures the ion transfer characteristics (i.e., the cation transfer number), reduces the diffusion resistance of lithium ions, and improves the cycle capacity characteristics.

[0076] Non-aqueous organic solvents The non-aqueous organic solvent is not particularly limited as long as it minimizes decomposition due to oxidation reactions during the charging and discharging process of the secondary battery and can exhibit the desired properties together with the additive. For example, carbonate-based organic solvents, ether-based organic solvents, or ester-based organic solvents can be used individually or in combination of two or more.

[0077] Of the aforementioned organic solvents, the carbonate-based organic solvent may include at least one of cyclic carbonate-based organic solvents and linear carbonate-based organic solvents. Specifically, the cyclic carbonate-based organic solvent may include at least 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, vinylethylene carbonate, and fluoroethylene carbonate (FEC). Specifically, it may include a mixed solvent of ethylene carbonate having a high dielectric constant and propylene carbonate having a relatively lower melting point compared to ethylene carbonate.

[0078] Furthermore, the linear carbonate-based organic solvent contains at least 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, as a solvent having low viscosity and low dielectric constant, and more preferably contains dimethyl carbonate.

[0079] The ether-based organic solvent may be 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 mixture of two or more of these, but is not limited to these.

[0080] The ester-based organic solvent may be at least one selected from the group consisting of linear ester-based organic solvents and cyclic ester-based organic solvents.

[0081] The linear ester-based organic solvent may, but is not limited to, one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate, or a mixture of two or more of these.

[0082] The cyclic ester organic solvent may, but is not limited to, one selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone, or a mixture of two or more of these.

[0083] Among the ester solvents mentioned above, cyclic carbonate compounds are preferably used because they are high-viscosity organic solvents with high dielectric constants that effectively dissociate lithium salts in electrolytes. Furthermore, when such cyclic carbonate compounds are mixed in appropriate proportions with low-viscosity, low-dielectric-constant linear carbonate compounds and linear ester compounds such as dimethyl carbonate and diethyl carbonate, a gel polymer electrolyte with high electrical conductivity can be produced, and therefore may be preferably used.

[0084] Polymerization initiator The polymerization initiator may be a conventional thermal or photopolymerization initiator known in the art. For example, the polymerization initiator may be decomposed by heat to form radicals, which may react with the aforementioned crosslinking agent by free radical polymerization to form a gel polymer electrolyte.

[0085] More specifically, non-limiting examples of the polymerization initiator include benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di-tert-butyl peroxide, t-butyl peroxy-2-ethyl-hexanoate, cumyl hydroperoxide, and hydrogen peroxide. Examples include, but are not limited to, organic peroxides such as peroxides, hydroperoxides, and one or more azo compounds selected from the group consisting of 2,2'-azobis(2-cyanobutane), 2,2'-azobis(methylbutyronitrile), 2,2'-azobis(isobutyronitrile) (AIBN), and 2,2'-azobisdimethyl-valeronitrile (AMVN).

[0086] The polymerization initiator may decompose in the battery by heat, in non-limiting examples, at temperatures of 30°C to 100°C, or at room temperature (5°C to 30°C) to form radicals, and the polymerizable oligomer may react with an acrylate compound by free radical polymerization to form a gel polymer electrolyte.

[0087] The polymerization initiator may be present in an amount of 0.01 to 20 parts by weight, preferably 0.1 to 10 parts by weight, based on 100 parts by weight of the polymerizable compound.

[0088] When the polymerization initiator is in the range of 0.01 to 20 parts by weight, the gel polymer conversion rate can be increased, ensuring the gel polymer electrolyte properties, preventing pregel reactions, and improving the wettability of the electrolyte to the electrode.

[0089] polymerizable compound The polymerizable monomer, oligomer, or copolymer described above is not particularly limited as long as it has a polymerizable functional group selected from the group consisting of vinyl groups, epoxy groups, allyl groups, and (meth)acrylic groups, which can undergo polymerization reactions within its structure, and is a compound that can be transformed into a gel by polymerization or crosslinking, and is commonly used as a synthetic monomer, oligomer, or copolymer in the production of gel polymer electrolytes.

[0090] Among these, the polymerizable monomers include, as non-limiting examples, tetraethylene glycol diacrylate, polyethylene glycol diacrylate (molecular weight 50-20,000), 1,4-butanediol diacrylate, 1,6-hexandiold diacrylate, trimethylolpropane triacrylate, trimethylolpropane ethoxylate triacrylate, trimethylolpropane propoxylate triacrylate, ditrimethylolpropane tetraacrylate, pentaerythritol tetraacrylate, and pentaerythritol ethoxylate tetraacrylate. tetraacrylate), dipentaerythritol pentaacrylate, dipentaerythritol hexaacrylate, polyethylene glycol diglycidyl ether, 1,5-hexadiene diepoxide, glycerol propoxylate triglycidyl ether, vinylcyclohexene dioxide, 1,2,7,8-diepoxyoctane (1,2,7,Examples include, but are not limited to, 8-diepoxyoctane, 4-vinylcyclohexene dioxide, butyl glycidyl ether, diglycidyl 1,2-cyclohexanedicarboxylate, ethylene glycol diglycidyl ether, glycerol triglycidyl ether, and glycidyl methacrylate. These compounds can be used individually or in combination of two or more.

[0091] Furthermore, the copolymer may include, as a typical example, at least one selected from the group consisting of allyl 1,1,2,2-tetrafluoroethyl ether (TFE)-(2,2,2-trifluoroethyl acrylate) copolymer, TFE-vinyl acetate copolymer, TFE-(2-vinyl-1,3-dioxolane) copolymer, TFE-vinyl methacrylate copolymer, TFE-acrylonitrile copolymer, TFE-vinyl acrylate copolymer, TFE-methyl acrylate copolymer, TFE-methyl methacrylate (MMA) copolymer, and TFE-2,2,2-trifluoroethyl acrylate (FA) copolymer.

[0092] The polymerizable compound may be included in an amount of 0.01% to 10% by weight based on the total weight of the composition for forming the gel polymer electrolyte. If the content of the polymerizable compound exceeds 10% by weight, crosslinking may occur too quickly or become too dense when the composition for forming the gel polymer electrolyte is injected into the battery, resulting in the disadvantage of obtaining a gel with high resistance. Conversely, if the content of the polymerizable compound is less than 0.01% by weight, crosslinking may not occur easily.

[0093] additives Furthermore, the gel polymer electrolyte formation composition of the present invention may further include additional additives that can form a more stable ion-conductive film on the electrode surface, as needed, in order to prevent decomposition and induction of negative electrode collapse in a high-power environment during the production of the gel polymer electrolyte, or to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and swelling improvement effects during high-temperature storage.

[0094] Specifically, the additional additives may include, as typical examples, one or more first additives selected from the group consisting of sultone compounds, sulfite compounds, sulfone compounds, sulfate compounds, halogen-substituted carbonate compounds, nitrile compounds, cyclic carbonate compounds, phosphate compounds, borate compounds, and lithium salt compounds.

[0095] The sultone compound mentioned above includes at least one compound selected from the group consisting of 1,3-propanesultone (PS), 1,4-butanesultone, ethensultone, 1,3-propensultone (PRS), 1,4-butensultone, and 1-methyl-1,3-propensultone, and may be included in an amount of 0.3% to 5% by weight, preferably 1% to 5% by weight, based on the total weight of the composition for forming the gel polymer electrolyte. If the content of the sultone compound in the composition for forming the gel polymer electrolyte exceeds 5% by weight, an excessively thick film may be formed on the electrode surface, resulting in increased resistance and output degradation. Furthermore, an excess amount of additives in the composition for forming the gel polymer electrolyte may increase resistance and degrade the output characteristics.

[0096] Examples of the sulfite compounds include one or more compounds selected from the group consisting of ethylene sulfite, methyl ethylene sulfite, ethyl ethylene sulfite, 4,5-dimethyl ethylene sulfite, 4,5-diethyl ethylene sulfite, propylene sulfite, 4,5-dimethylpropylene sulfite, 4,5-diethylpropylene sulfite, 4,6-dimethylpropylene sulfite, 4,6-diethylpropylene sulfite, and 1,3-butylene glycol sulfite, and may be included in an amount of 3% by weight or less based on the total weight of the composition for forming the gel polymer electrolyte.

[0097] The sulfone compound mentioned above includes one or more compounds selected from the group consisting of divinyl sulfone, dimethyl sulfone, diethyl sulfone, methyl ethyl sulfone, and methyl vinyl sulfone, and may be included in an amount of 3% by weight or less based on the total weight of the composition for forming the gel polymer electrolyte.

[0098] Examples of the sulfate compounds include ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS), and may be included in an amount of 3% by weight or less based on the total weight of the composition for forming the gel polymer electrolyte.

[0099] Furthermore, examples of halogen-substituted carbonate compounds include fluoroethylene carbonate (FEC), which may be included in an amount of 5% by weight or less based on the total weight of the composition for forming the gel polymer electrolyte. If the content of the halogen-substituted carbonate compound in the composition for forming the gel polymer electrolyte exceeds 5% by weight, the cell swelling performance may deteriorate.

[0100] Furthermore, the nitrile compounds include at least one compound selected from the group consisting of succinate nitrile, adiponitrile (Adn), acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonile, cyclohexanecarbonile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0101] Examples of the cyclic carbonate compound include vinylene carbonate (VC) or vinylethylene carbonate, and may be included in an amount of 3% by weight or less based on the total weight of the composition for forming the gel polymer electrolyte. If the content of the cyclic carbonate compound in the composition for forming the gel polymer electrolyte exceeds 3% by weight, the cell swelling suppression performance may deteriorate.

[0102] Examples of the phosphate compound include one or more compounds selected from the group consisting of lithium difluto(bisoxalato) phosphate, lithium difluorophosphate, tetramethyltrimethylsilyl phosphate, trimethylsilyl phosphate, tris(2,2,2-trifluoroethyl) phosphate, and tris(trifluoroethyl) phosphate, and may be included in an amount of 3% by weight or less based on the total weight of the composition for forming the gel polymer electrolyte.

[0103] Examples of the borate-based compound include lithium oxalyl difluoroborate, which may be included in an amount of 3% by weight or less based on the total weight of the composition for forming the gel polymer electrolyte.

[0104] The lithium salt compound is a compound different from the lithium salt contained in the composition for forming the gel polymer electrolyte, and includes one or more compounds selected from the group consisting of LiPO2F2, LiODFB, LiBOB (lithium bisoxalate borate (LiB(C2O4)2)), and LiBF4, and may be included in an amount of 3% by weight or less based on the total weight of the composition for forming the gel polymer electrolyte.

[0105] The aforementioned additional additives may be mixed in a quantity of two or more types, and may be included in an amount of 20% by weight or less, preferably 0.1% to 10% by weight, based on the total weight of the composition for forming the gel polymer electrolyte. If the content of the additional additives is less than 0.01% by weight, the improvement effect on the low-temperature power characteristics, high-temperature storage characteristics, and high-temperature life characteristics of the battery will be minimal. If the content of the additional additives exceeds 20% by weight, excessive side reactions may occur in the composition for forming the gel polymer electrolyte during charging and discharging of the battery. In particular, unreacted substances or precipitated substances may remain in the composition for forming the gel polymer electrolyte at room temperature if they are not sufficiently decomposed at high temperatures. This may result in side reactions that reduce the life or resistance characteristics of the secondary battery.

[0106] On the other hand, in one embodiment of the present invention, a further step may be taken to control the oxygen concentration inside the battery case after the electrolyte solution has been injected.Oxygen (O2), when radicals are generated by a thermal initiator or the like, can suppress the chain reaction of polymer monomers through radical quenching. That is, the oxygen concentration can be controlled to suppress side reactions in which the polymerizable monomers are crosslinked during the electrolyte injection stage or the subsequent aging stage. In one embodiment of the present invention, the oxygen concentration can be controlled by injecting oxygen into the battery case after injecting the composition for forming the gel polymer electrolyte, or by sealing the battery case under atmospheric pressure. In this way, the oxygen concentration inside the battery can be maintained at or above the oxygen concentration in the air. In this case, by omitting the degassing step, the oxygen contained in the air can be left inside the battery case and the oxygen concentration can be controlled to a desired level. On the other hand, the oxygen can be removed from the battery case at an appropriate stage before the subsequent curing of the gel polymer composition. For example, the oxygen can be removed from the battery case by vacuum treatment, pressurization, or degassing to lower the oxygen concentration.

[0107] Furthermore, in one embodiment of the present invention, an aging step may be performed on the result after the injection of the gel polymer electrolyte formation composition (S1). The aging allows the electrode assembly to be sufficiently impregnated with the composition and to be uniformly impregnated throughout the electrode assembly. The aging is not particularly limited, but can be performed within a few hours to a few days. For example, it can be performed within 72 hours. In the present invention, it is desirable to perform the aging at room temperature below 30°C in order to prevent pre-gelation.

[0108] On the other hand, in one embodiment of the present invention, after the aging stage, a part of the battery case may be opened and one or more steps selected from vacuum treatment, pressurization, and degassing may be further performed. Since oxygen is removed from the electrode assembly by this process, it is advantageous for increasing the degree of crosslinking of the outer part in the subsequent crosslinking stage. In a specific embodiment of the present invention, The vacuum wear ッ The ting stage can be carried out under reduced pressure conditions of -85kPa to -99kPa.Furthermore, the vacuum wetting time is performed within a few minutes and can be repeated two or more times. In one specific embodiment of the present invention, a vacuum atmosphere can be formed under reduced pressure conditions of approximately -95 kPa, followed by eight cycles of 1 to 5 minutes each. In addition, the vacuum treatment ensures that the electrolyte is sufficiently transmitted to the fine voids in the electrodes and separation membrane, thereby improving the wettability of the electrode assembly.

[0109] Next, the crosslinking reaction of the gel polymer electrolyte formation composition is carried out (S2). In the present invention, the crosslinking reaction may be carried out by placing the spare battery in a predetermined heating device and maintaining it in the device for a predetermined time.

[0110] In one embodiment of the present invention, it is desirable that the heating device is preheated to a predetermined temperature before the spare battery is placed inside the heating device. This allows the outer casing of the battery to quickly reach the reaction start temperature, and crosslinking of the outer casing can be preferentially performed. Such a preheating step is advantageous for obtaining a secondary battery in which the electrolyte in the center of the battery is kept liquid, and the outer casing surrounding the center is crosslinked to a predetermined degree or higher, containing a gel polymer electrolyte.

[0111] In one embodiment of the present invention, the preheating temperature of the heating device can be controlled to be above the crosslinking start temperature. For example, the heating device can be preheated to a temperature of 50°C or higher or 60°C or higher. The upper limit of the preheating temperature is not particularly limited, but it is desirable that it be controlled to a range in which the battery and the materials contained in the battery, such as polymer materials and electrolyte materials, do not deteriorate. In one embodiment of the present invention, the preheating temperature can be controlled to be 75°C or lower, preferably 70°C or lower.

[0112] In the battery placed in the heating device, heat applied from the outside of the battery is sequentially conducted into the inside of the battery, and as a result of this heat conduction, the injected electrolyte composition begins to crosslink from the outside to the inside of the battery.

[0113] In this case, if the battery is introduced into an environment preheated to a predetermined temperature or higher, a temperature gradient is formed inside / outside the battery before heat is conducted to the inside of the battery. That is, the outside of the battery reaches a temperature at which crosslinking can begin relatively quickly compared to the inside, but the temperature rise inside the battery is slower, and it takes a relatively longer time to reach the temperature at which crosslinking can begin.

[0114] Referring to the following examples and Figure 2, when a secondary battery injected with an electrolyte composition is reacted in a chamber preheated to 70°C, the outside of the battery reaches the crosslinking temperature relatively quickly, ensuring a sufficient crosslinking reaction time. However, the inside of the battery is slower to reach the crosslinking temperature, delaying the start of the crosslinking reaction, and thus ensuring a shorter crosslinking reaction time compared to the outside.

[0115] Thus, the method for manufacturing a secondary battery of the present invention creates a rapid temperature gradient between the outside and inside of the secondary battery, thereby allowing the crosslinking reaction to proceed sufficiently in the outer casing, while delaying the time it takes for the central part to reach a temperature at which crosslinking is possible, thereby creating a central part in the battery where the electrolyte is maintained in a state with a low degree of crosslinking.

[0116] If the reaction time required to crosslink the entire battery is approximately 5 hours, then by controlling the crosslinking reaction time to be shorter than this, the inside of the battery may not be completely crosslinked and may be maintained in a state with a low degree of crosslinking. In other words, the manufacturing method of the present invention uses the temperature difference between the center and outer part of the battery to make the degree of crosslinking of the gel composition different, so that the center of the battery has a high content of electrolyte components with a low degree of crosslinking, and the outer part has a high content of electrolyte components with a high degree of crosslinking, thereby improving both the durability and safety of the battery.

[0117] In the method for manufacturing a lithium secondary battery of the present invention, step (S2) may be carried out under conditions of 50°C to 75°C. In a specific embodiment of the present invention, step (S2) may be carried out under conditions of 60°C to 70°C. On the other hand, step (S2) may be carried out for 30 minutes to 24 hours. In a specific embodiment of the present invention, step (S2) may be carried out at 70°C for 3 hours or less. However, the above time and temperature conditions are not limited to the above ranges, and the reaction time and temperature can be appropriately controlled within a range in which the outer part exhibits a relatively high degree of crosslinking after crosslinking has begun, while the central part maintains a lower degree of crosslinking compared to the outer part.

[0118] Next, the product resulting from step (S2) is cooled (S3). The aforementioned cooling is performed so that the internal temperature of the battery is below the reaction temperature of the initiator, for example, This means that the temperature drops below room temperature. Preferably, the cooling can be carried out at a rate faster than natural cooling. According to one embodiment of the present invention, the cooling can be carried out by removing the spare battery from the heating device and placing it in a cooling chamber controlled to be below room temperature, and preferably within 10 minutes the temperature inside the battery reaches the same temperature as the ambient temperature of the chamber. This is to prevent unwanted crosslinking from occurring due to latent heat. In one specific embodiment of the present invention, the temperature of the cooling chamber can be controlled to 0°C to 20°C. On the other hand, in the present invention, it is desirable to start step (S3) as quickly as possible after step (S2) to suppress the progress of the crosslinking reaction due to latent heat after step (S2). The cooling step can be carried out for 30 minutes or more. That is, in terms of blocking the possibility of additional crosslinking reactions, it is desirable to maintain the cooling process for a predetermined time or longer even after the temperature of the battery reaches the ambient temperature of the cooling chamber.

[0119] As described above, a secondary battery obtained by the manufacturing method according to the present invention may have a central part containing an electrolyte with a low degree of crosslinking, and the central part may be encapsulated by an outer part in the form of a gel electrolyte that is crosslinked to a predetermined degree or higher.

[0120] In the present invention, the secondary battery is preferably a lithium secondary battery, and non-limiting examples of the lithium secondary battery include lithium metal secondary batteries, lithium-ion secondary batteries, lithium polymer secondary batteries, or lithium-ion polymer secondary batteries.

[0121] The present invention will be described in detail below with reference to examples. However, the examples of the present invention can be modified into a variety of other forms, and the scope of the present invention is not limited to the examples detailed below. The examples of the present invention are provided to give a more complete explanation of the present invention to a person of average knowledge in the industry.

[0122] (1) Example 1, Example 2 (Manufacturing of electrode assemblies) NCM (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 A cathode active material slurry (solid content 50%) was prepared by adding 94% by weight of O2, 3% by weight of carbon black as a conductive material, and 3% by weight of polyvinylidene fluoride (PVDF) as a binder to N-methyl-2-pyrrolidone (NMP) as a solvent. The cathode active material slurry was then applied to an aluminum (Al) thin film, which was a cathode current collector with a thickness of approximately 20 μm, and dried to produce a cathode. The cathode was then manufactured by roll pressing.

[0123] Carbon powder was used as the negative electrode active material, PVDF as the binder, and carbon black as the conductive material, at concentrations of 96% by weight, 3% by weight, and 1% by weight, respectively. These were added to NMP as a solvent to produce a negative electrode active material slurry (solid content 80%). The negative electrode active material slurry was then applied to a 10 μm thick copper (Cu) thin film, which served as the negative electrode current collector, and dried to produce the negative electrode. Finally, the negative electrode was manufactured by roll pressing.

[0124] A stack-type electrode assembly containing 20 positive electrodes was manufactured by alternately stacking the aforementioned positive electrode, negative electrode, and separation membrane consisting of three layers of polypropylene / polyethylene / polypropylene (PP / PE / PP) in sequence.

[0125] (Manufacturing of compositions for forming gel polymer electrolytes) A non-aqueous electrolyte was prepared by dissolving LiPF6 in a non-aqueous organic solvent having a composition of ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 30:70 (volume ratio) to a concentration of 1.0 M. A composition for forming a gel polymer electrolyte was prepared by adding trimethylolpropane triacrylate as a polymerizable compound at a concentration of 5% by weight relative to 100% by weight of the composition for forming the gel polymer electrolyte, and AIBN as a polymerization initiator at a concentration of 0.02% by weight relative to 100% by weight of the composition for forming the gel polymer electrolyte.

[0126] (Manufacturing of lithium-ion batteries) The electrode assembly was inserted into the battery case, and the gel polymer electrolyte formation composition was injected. Subsequently, the battery case was sealed at 140°C for 2 seconds under normal pressure, and then left to rest at room temperature for 3 days. After that, a portion of the battery case was opened, and the air was removed eight times for 5 minutes each under reduced pressure of -95kPa to remove oxygen from the inside.

[0127] Next, the battery was placed in a chamber preheated to 70°C and heated for a predetermined time. After that, the battery was removed from the chamber and cooled. The cooling process was carried out in a cooling chamber set to 10°C, and it was confirmed that the internal temperature of the battery reached the ambient temperature inside the cooling chamber within 10 minutes. A lithium secondary battery containing a polymerized gel polymer electrolyte was manufactured in this manner. The crosslinking temperature and time are as shown in Table 1 below. Meanwhile, the internal temperature of the battery was confirmed by inserting a micro-probe type temperature measuring device into the center and outer casing of the battery, respectively.

[0128] (2) Comparative Example 1 A battery was manufactured in the same manner as in Example 1, except that the crosslinking reaction and cooling reaction were omitted.

[0129] (3) Comparative Examples 2-4 A battery was manufactured in the same manner as in Example 1, except that the crosslinking time for Comparative Example 2 was set to 6 hours, the crosslinking time for Comparative Example 3 was set to 12 hours, and the crosslinking time for Comparative Example 4 was set to 0.5 hours.

[0130] (4) Comparative Example 5 A battery was manufactured in the same manner as in Example 1, except that a cooling process was omitted.

[0131] [Table 1]

[0132] [Table 2] **Stiffness: This value is calculated by setting the cell stiffness of Comparative Example 4 to 100%, and the actual measurement unit is gf / mm.**

[0133] As can be seen from Table 1 above, in Examples 1 and 2, the degree of crosslinking in the central part was significantly lower than in the outer part, and it was confirmed that the outer part showed a degree of crosslinking of 80% or more. From this, it was confirmed that the resistance was low and the mechanical strength was high, and the life characteristics were also excellent.

[0134] In contrast, Comparative Example 1, which remained in an uncrosslinked state, exhibited good resistance characteristics, but its mechanical strength was found to be extremely weak.

[0135] On the other hand, in Comparative Example 5, no cooling process was performed, and the crosslinking reaction continued due to latent heat, resulting in a very high degree of crosslinking in the center, which increased the resistance.

[0136] In Comparative Example 2, the crosslinking time was short and the degree of crosslinking in both the central and outer regions was low. In Comparative Examples 3 and 4, the crosslinking time was long and a high degree of crosslinking was observed even in the central region, confirming that the resistance characteristics were weak.

[0137] (3) Experimental Examples 3-1) Experimental Example 1: Method for Measuring the Degree of Crosslinking The degree of crosslinking of the lithium secondary batteries produced in Examples 1 and 2 and Comparative Examples 1 to 5 was measured by the following method. After opening the battery case of each battery, the electrode assembly was removed and disassembled into the outer casing and the central part to obtain the sample. Each sample was placed in acetone (Acetone d-6), shaken at room temperature for 1 hour, and then filtered to remove solids and obtain a filtrate. The amount of unreacted oligomers remaining in the filtrate (based on C=C bonds) was measured by NMR analysis, and the degree of crosslinking was calculated by comparing this with the added oligomers using Equation 1 below. The NMR was measured using 1H-NMR with a Varian 500MHz. Specifically, in each experiment, 0.1g of the polymer solution was taken, dissolved in 1mL of the solvent for NMR described below, and 1H-NMR was measured using the analytical instrument described below according to the manufacturer's manual. For unreacted oligomers, a -H peak originating from the =CH2 at the double bond end was observed at approximately 5.7ppm to 6.4ppm. Analytical equipment: 500MHz NMR (Varian Unity Inova 500), 1H-NMR Concentration: 10~20mg / mL Solvent: CDCl3-D3 Temperature: 25℃

[0138] [Formula 1] Crosslinking degree (%) = 100 - {(Amount of unreacted oligomers remaining / Oligomers added) × 100}

[0139] 3-2) Experimental Example 2: Measurement and Calculation Method for Lithium Battery Stiffness The rigidity of the lithium secondary battery in Comparative Example 4 was measured against the center of the battery using a ball-type texture analyzer at a speed of 10 mm / min, a distance of 1.2 mm, and a trigger force of 50 g.

[0140] Subsequently, the stiffness of Comparative Example 4 was set to 100%, and the stiffness of the other experimental values ​​was calculated.

[0141] 3-3) Experimental Example 3: Safety Evaluation by Nail Penetration Test The lithium secondary batteries manufactured in Examples 1 and 2 and Comparative Examples 1 to 5 were fully charged to 4.4V at room temperature, and then subjected to a nail-penetration test under GB / T conditions (nail diameter 2.5 mm, nail penetration speed 6 m / min). The results are shown in Table 2.

Claims

1. A method for manufacturing a secondary battery containing a gel polymer electrolyte, The aforementioned method, (S1) A step of manufacturing a spare battery by loading the electrode assembly and the composition for forming the gel polymer electrolyte into a battery case, (S2) A step in which the crosslinking reaction of the composition for forming the gel polymer electrolyte is carried out, (S3) A step of cooling the result of step (S2), The (S2) step is performed by a heating device, and the heating device is preheated to a predetermined temperature before the (S2) step is performed. The secondary battery contains a gel polymer electrolyte in which a portion of the electrolyte is crosslinked to a predetermined degree or higher, and the degree of crosslinking increases from the center outwards. The secondary battery includes a central part with a low degree of crosslinking of the electrolyte, and an outer part surrounding the central part, which contains a gel polymer electrolyte exhibiting a higher degree of crosslinking than the central part. The outer portion has a degree of crosslinking of 40% by weight or more, and the central portion has a degree of crosslinking of less than 40% by weight. The electrolyte in the central part is fluid, while the electrolyte in the outer part is not fluid. The composition for forming the gel polymer electrolyte comprises a lithium salt, a non-aqueous organic solvent, a polymerization initiator, and at least one polymerizable compound selected from the group consisting of polymerizable monomers, oligomers, and copolymers. A method for producing a secondary battery, wherein the polymerizable monomer, oligomer, or copolymer is a compound having an acrylic group and capable of being transformed into a gel-like state by polymerization or crosslinking.

2. The method for manufacturing a secondary battery according to claim 1, wherein the step in (S1) includes sealing the battery case of the spare battery under normal pressure.

3. The method for manufacturing a secondary battery according to claim 1, wherein step (S2) is performed at a temperature of 60°C or higher.

4. The method for manufacturing a secondary battery according to claim 1, wherein, before performing step (S2), a further step is performed in which the electrode assembly is impregnated with the gel polymer electrolyte forming composition at room temperature.

5. The method for manufacturing a secondary battery according to claim 4, wherein after the step of impregnating the electrode assembly with the gel polymer electrolyte forming composition at room temperature, a vacuum treatment step is further performed.

6. The method for manufacturing a secondary battery according to claim 1, wherein the cooling in step (S3) is carried out in a cooling chamber controlled to a temperature below room temperature, and the temperature of the battery reaches the ambient temperature of the cooling chamber within a period of 10 minutes or less.

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