Method for manufacturing gel polymer electrolyte secondary battery and gel polymer electrolyte secondary battery manufactured thereby

Abstract

To provide a secondary battery containing a gel polymer electrolyte and having high ionic conductivity.SOLUTION: There is provided a secondary battery including: a central portion in which a crosslinking degree of a gel polymer electrolyte increases step by step or gradually from the inside of a battery toward the outside and which contains a gel polymer electrolyte with a low crosslinking degree; and an outer shell portion which surrounds the central portion and contains a gel polymer electrolyte having a crosslinking degree higher than the crosslinking degree of the central portion. The outer shell portion has a crosslinking degree of 80 wt.% or more and 100 wt.% or less, and the central portion has a crosslinking degree of 75 wt.% or more and less than 80 wt.%. Preferably, the secondary battery further includes, between the central portion and the outer shell portion, a transition portion which has a crosslinking degree higher than the crosslinking degree of the central portion and a crosslinking degree lower than the crosslinking degree of the outer shell portion.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application claims priority from Korean Patent Application No. 10-2021-0148315, filed November 1, 2021.

[0002] The present invention relates to a method for manufacturing a lithium secondary battery containing a gel polymer electrolyte and a gel polymer electrolyte secondary battery manufactured thereby. [Background technology]

[0003] Interest in energy storage technology has been growing in recent years. In particular, as its application fields expand to include mobile phones, camcorders, notebook PCs, and even the energy of electric vehicles, efforts in the research and development of electrochemical devices are becoming more and more concrete. Electrochemical devices are the field that has attracted the most attention in this field of energy storage technology, and among them, interest is beginning to grow in secondary batteries that can be charged and discharged.

[0004] Among the secondary batteries currently in use, lithium secondary batteries, which were developed in the early 1990s, are attracting attention due to their advantages of higher operating voltage and significantly higher energy density compared to conventional batteries that use aqueous electrolytes, such as Ni-MH, Ni-Cd, and sulfuric acid-lead batteries.

[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 have the disadvantage of using a liquid electrolyte containing lithium salt, which can cause leakage and explosion, making battery design complex to prevent such risks.

[0007] Meanwhile, lithium polymer batteries use solid polymer electrolytes or gel polymer electrolytes containing an electrolyte solution as electrolytes, which improves safety and flexibility. This allows them to be developed in various forms, such as small or thin-film batteries. Gel polymer electrolytes can be divided into coating and injection types depending on the manufacturing method. Injection-type gel polymer electrolytes can be manufactured by injecting a liquid electrolyte containing a crosslinking monomer into a cell, wetting the electrode assembly uniformly with the liquid electrolyte, and then performing a crosslinking process. During the crosslinking process, the electrolyte forms a matrix and transforms into a non-fluid gel-state electrolyte.

[0008] Such gel electrolytes have the advantages of high physical safety due to the lack of electrolyte fluidity, which eliminates the problems of heat resistance and leakage, and improving cell strength and resistance to external impacts. However, they have lower ionic conductivity and higher resistance than liquid electrolytes. Therefore, they tend to have shorter lifespans than liquid electrolytes alone. Therefore, development aimed at improving the ionic conductivity of gel polymer electrolytes is needed. Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention provides a secondary battery containing a gel polymer electrolyte and having high ionic conductivity. Another object 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-injected gel polymer electrolyte using a radical thermal initiation reaction. It will be readily apparent that other objects and advantages of the present invention can be achieved by the means or methods recited in the claims, and combinations thereof. [Means for solving the problem]

[0010] A first aspect of the present invention is a method for manufacturing a secondary battery including a gel polymer electrolyte, the method including the steps of: placing an electrode assembly in a battery case (S10); injecting a gel polymer electrolyte-forming composition into the battery case (S20); primarily sealing the battery case in an atmospheric environment and aging the battery case (S30); releasing the primary sealing and degassing the battery case (S40); and crosslinking the result of step (S40) (S50). The secondary battery includes a gel polymer electrolyte in which a portion of the electrolyte is crosslinked to a predetermined degree or more, and the degree of crosslinking increases from the center of the battery to the outside.

[0011] A second aspect of the present invention is that in the first aspect, step (S40) is performed in a gas atmosphere having an oxygen concentration of less than 20 vol%, in a nitrogen (N2) atmosphere having an oxygen concentration of 90 vol% or more, or in an inert gas atmosphere having an oxygen concentration of 90 vol% or more.

[0012] In a third aspect of the present invention, in the first or second aspect, the secondary battery includes a central portion having a low degree of cross-linking of the electrolyte, and an outer shell portion surrounding the central portion and containing a gel polymer electrolyte exhibiting a higher degree of cross-linking than the central portion.

[0013] In a fourth aspect of the present invention, in any one of the first to third aspects, the gel polymer electrolyte composition 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.

[0014] A fifth aspect of the present invention is any one of the first to fourth aspects, further comprising the step of injecting oxygen into the battery case before performing the step (S30).

[0015] A sixth aspect of the present invention is any one of the first to fifth aspects, wherein the aging process of the step (S30) is carried out at room temperature to 45°C.

[0016] A seventh aspect of the present invention is any one of the first to sixth aspects, wherein the degassing in step (S40) is carried out within a range that maintains a volume of 75 vol% to 90 vol% relative to 100 vol% of the volume of the battery before unsealing.

[0017] An eighth aspect of the present invention is any one of the first to seventh aspects, wherein the crosslinking reaction in step (S50) is carried out at 60° C. or higher.

[0018] A ninth aspect of the present invention is any one of the first to eighth aspects, wherein an activation step is further carried out after the step (S50) is performed.

[0019] A tenth aspect of the present invention relates to a secondary battery, the secondary battery being manufactured by the method according to any one of the first to ninth aspects, the secondary battery comprising a core containing a gel polymer electrolyte with a low degree of cross-linking, the cross-linking degree of the gel polymer electrolyte increasing stepwise or gradually from the inside to the outside of the battery, and an outer shell surrounding the core and containing a gel polymer electrolyte with a higher degree of cross-linking than the core.

[0020] An eleventh aspect of the present invention is the tenth aspect, wherein the degree of cross-linking of the outer shell is 80% by weight or more and the degree of cross-linking of the central portion is less than 80% by weight.

[0021] A twelfth aspect of the present invention is the tenth or eleventh aspect, wherein the degree of crosslinking of the central portion is less than 40% by weight. [Effects of the Invention]

[0022] The secondary battery according to the present invention has a structure in which an inner core contains a liquid electrolyte that remains liquid without gelling due to crosslinking, and the core is surrounded by an outer shell that is crosslinked to a predetermined degree or more and gelled. This structural feature improves both ionic conductivity and mechanical properties, thereby improving both the rigidity and safety of the battery. Furthermore, since the liquid electrolyte is confined by the gel polymer electrolyte, leakage of the liquid electrolyte can be prevented. Furthermore, the secondary battery according to the present invention can be manufactured using a simple method in which only the outer shell of the electrode assembly is gelled by controlling the gelation degree of the electrolyte according to the oxygen concentration gradient within the electrode assembly. Therefore, there is no need to install a separate device or equipment line for the crosslinking step, which does not adversely affect process efficiency.

[0023] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the detailed description of the invention to be given later, serve to further understand the technical concept of the present invention, so the present invention should not be interpreted as being limited to only the matters depicted in such drawings. Meanwhile, the shape, size, scale, or ratio of elements in the drawings included in this specification may be exaggerated to emphasize a clearer description. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a cross-sectional view of a secondary battery according to an embodiment of the present invention; [Figure 2] 1 is a diagram illustrating a method for manufacturing a secondary battery according to an embodiment of the present invention, in which the oxygen concentration in the outer part of an electrode assembly is adjusted to be lower than the oxygen concentration in the center part by degassing before performing a gelation process. [Figure 3] 1 is a diagram illustrating a method for manufacturing a secondary battery according to an embodiment of the present invention, in which the oxygen concentration in the outer part of an electrode assembly is adjusted to be lower than the oxygen concentration in the center part by degassing before performing a gelation process. DETAILED DESCRIPTION OF THE INVENTION

[0025] Aspects of the present invention will be described in detail below. Prior to this, the terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts corresponding to the technical ideas 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. Therefore, it should be understood that the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred embodiments of the present invention and do not represent the entire technical ideas of the present invention, and therefore, various equivalents and modifications may be available as of the time of filing this application.

[0026] Throughout this specification, when a part is said to "include" or "comprise" a certain component, this does not mean that other components are excluded, and that other components may also be included, unless otherwise specified.

[0027] Furthermore, the terms "about," "substantially," and the like used throughout this specification are used to mean a numerical value or approximation of a numerical value when the manufacturing and material tolerances inherent in the stated meaning are given, and are used to prevent unscrupulous infringers from unfairly exploiting the contents of the disclosure in which precise or absolute numerical values ​​are stated to aid in the understanding of the present invention.

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

[0029] Certain terminology used in the Detailed Description of the Invention that follows is for ease of description only and is not limiting. The words "right," "left," "top," and "bottom" designate directions in the drawings to which reference is made. The words "inwardly" and "outwardly" designate directions toward or away from the geometric center of the designated device, system, or component thereof, respectively. "Front," "rear," "upward," "downward," and related words and phrases designate locations and orientations in the drawings to which reference is made and are not intended to be limiting. These terms encompass the above-listed words, derivatives thereof, and words of similar import.

[0030] The invention will now be described in more detail with reference to the accompanying drawings.

[0031] The secondary battery according to the present invention includes an electrode assembly including at least one anode, at least one separator, and at least one cathode, in which the anode, the separator, and the cathode are sequentially stacked so that the anode and the cathode are electrically insulated by the separator. The secondary battery also includes an electrolyte, and the electrode assembly is impregnated with the electrolyte. In one embodiment of the present invention, the battery has a shape in which the degree of cross-linking of the electrolyte increases from the inside to the outside.

[0032] In one embodiment of the present invention, the center portion of the electrode assembly has a relatively low degree of cross-linking of the electrolyte impregnated in the electrode assembly and thus has fluidity, while the outer portion has a higher degree of cross-linking of the electrolyte impregnated in the electrode assembly than the center portion and therefore has very low or no fluidity. Because the center portion is surrounded by the outer portion, the low-cross-linked electrolyte present in the center portion is encapsulated by the high-cross-linked electrolyte and does not escape to the outside of the electrode assembly.

[0033] Meanwhile, in one embodiment of the present invention, the electrode assembly may include a transition region in which the degree of cross-linking increases from the center to the outer periphery. The transition region may be located between the center and the outer periphery, and the degree of cross-linking of the electrolyte may be higher than that of the center and lower than that of the outer periphery. Alternatively, the transition region may not be a region separating the center and the outer periphery, but may refer to a region in which the degree of cross-linking changes from a given point in the center to a given point in the outer periphery. In other words, the electrode assembly of the present invention may include a center and an outer periphery surrounding the center, where the center has a relatively lower degree of cross-linking of the electrolyte than the outer periphery, and the outer periphery has a relatively higher degree of cross-linking of the electrolyte than the center, and may include a transition region in which the degree of cross-linking increases from the center to the outer periphery. FIG. 1 shows a cross-sectional view of a secondary battery 10 according to one embodiment of the present invention. The cross-sectional view exemplarily shows a cross-section parallel to the stacking direction of the electrode assembly. Referring to FIG. 1, the battery includes a battery case 120 incorporating an electrode assembly 100. The electrode assembly includes one or more negative electrodes and one or more positive electrodes stacked in sequence with a separator sandwiched therebetween, and includes an electrode tab 130 drawn out from the negative electrode and / or the positive electrode. The battery also includes an electrolyte, and the electrode assembly is impregnated with the electrolyte. The center portion C of the electrode assembly has a low degree of cross-linking of the electrolyte and is fluid. In one embodiment of the present invention, the electrolyte in the center portion may have a viscosity of 0 cP to 10 cP. Meanwhile, in one embodiment of the present invention, the center portion may have a degree of cross-linking of less than 80 wt %, preferably less than 40 wt %.

[0034] Meanwhile, the central portion is surrounded by an outer shell (P), and the outer shell has a relatively higher degree of cross-linking of the electrolyte than the central portion, and preferably has no fluidity. In one embodiment of the present invention, the electrolyte in the outer shell may have a cross-linking degree of 80% to 100% by weight. In a specific embodiment of the present invention, the outer shell may have a cross-linking degree of 80% to 100% by weight, and the central portion may have a cross-linking degree of less than 80% by weight, or less than 40% by weight.

[0035] Alternatively, in a specific embodiment, the electrode assembly includes a center portion, a transition portion, and an outer portion, and in this case, the outer portion and the center portion may have a difference in cross-linking degree of 10 wt % or more, 20 wt % or more, 30 wt % or more, 40 wt % or more, or 50 wt % or more. A transition portion having a higher cross-linking degree than the center portion and a lower cross-linking degree than the outer portion may be included between the center portion and the outer portion.

[0036] In one embodiment of the present invention, the degree of crosslinking can be determined by performing NMR analysis on each electrolyte sample collected from the outer and central parts of the electrode assembly, determining the amount of unreacted polymerizable compound remaining from a carbon-carbon double bond peak, and substituting the result into Equation 1 below to calculate the proportion of unreacted polymerizable compound in the collected electrolyte sample:

[0037] [Formula 1] Degree of crosslinking (wt%) = 100 - {amount of unreacted polymerizable compound / amount of collected electrolyte sample} x 100

[0038] Meanwhile, the NMR can be measured by, for example, 1D normal NMR (One Dimensional Normal Nuclear Magnetic Resonance Spectroscopy). Specifically, a Bruker 700 MHz NMR analyzer can be used, acetone-d6 can be used as the sample solvent, and the measurement conditions can be 1H NMR (zg30), ns = 32, d1 = 3 seconds.

[0039] In addition, the analysis of the results 1This may be done by fixing the least volatile ethylene carbonate (EC) peak in the H NMR spectrum as 100% as the reference, determining the integral value of the acrylate double bond peak of each sample, and normalizing the acrylate double bond area of ​​the reference sample to 100 to calculate the relative area value.

[0040] However, the present invention is not limited to the above method.

[0041] Alternatively, the initial weight of the collected electrolyte sample can be measured, and then the sample can be immersed in a decalin solution at 135°C and boiled for 4 hours in accordance with ASTM D 2765, and the remaining weight can be measured, and the remaining dry weight can be calculated as a percentage of the initial weight.

[0042] The viscosity may be measured using a viscometer such as a Brookfield digital viscometer (RV DV2T). Specifically, the viscosity may be measured by contacting a spindle connected to the viscometer with a measurement target at a torque of 20% (±1%) rpm. In the present invention, the boundary between the outer portion and the center, or the boundary between the outer portion and the transition portion, or the boundary between the transition portion and the center is not determined by a specific position based on the dimensions or ratios of the length, width, or thickness of the electrode assembly, but is determined by the degree of cross-linking of the electrolyte.

[0043] Therefore, in one specific embodiment of the present invention, the portion of the electrode assembly that shows a cross-linking degree of 80% or more from the outside toward the center of the interior may be defined as the outer shell portion, and the portion that is surrounded by the outer shell portion but shows a cross-linking degree of less than 80% may be defined as the central portion.

[0044] Meanwhile, in one embodiment of the present invention, the empty space of the battery case outside the electrode assembly of the battery may be filled with an electrolyte. This will be referred to as a filling portion below. The electrolyte filled in the battery case may be in a gel form and may be formed in a shape that is inseparably connected to the outer shell portion. As will be described later, the secondary battery according to the present invention may be manufactured by placing the electrode assembly in the battery case, injecting a gel composition into the battery case, and then performing a gelling process, so that the outer shell portion of the electrode assembly may be connected to the filling portion and formed as an integrated unit.

[0045] Meanwhile, in one embodiment of the present invention, the transition portion may extend to the outside of the electrode assembly and occupy a portion of the filling portion. That is, in the present invention, the center portion may be disposed within the electrode assembly, the center portion may be surrounded by the transition portion, and the transition portion may be surrounded by the outer shell portion, so that the liquid electrolyte does not come into direct contact with the case. Figure 1 illustrates a shape in which the transition portion extends beyond the outer boundary of the electrode assembly and occupies a portion of the filling portion.

[0046] 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 surface or both surfaces of the positive electrode current collector. The positive electrode active material layer may include a positive electrode mixture, which may include a positive electrode active material, a binder, and a conductive material. The present specification states that the positive electrode mixture does not include an electrolyte impregnated in the positive electrode. In the present invention, the positive electrode active material layer has a porous characteristic including a plurality of pores, and the pores are filled with the electrolyte as described above. The electrolyte may be in a solid state that exhibits fluidity due to a low degree of cross-linking or does not exhibit fluidity due to a high degree of cross-linking, depending on the location of the pores in the electrode assembly.

[0047] The positive electrode current collector is not particularly limited as long as it has conductivity without inducing chemical changes in the battery. 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. may be used.

[0048] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium. Specifically, it may contain a lithium composite metal oxide having at least one metal such as cobalt, manganese, nickel, or aluminum and lithium. More specifically, the lithium composite metal oxide is a lithium-manganese-based oxide (e.g., LiMnO2, LiMn2O4, etc.), a lithium-cobalt-based oxide (e.g., LiCoO2, etc.), a lithium-nickel-based oxide (e.g., LiN i O2, etc.), a lithium-nickel-manganese-based oxide (e.g., LiNi1- Y Mn Y O2 (where 0 < Y < 1), LiMn 2-z Ni z O4 (where 0 < Z < 2), etc.), a lithium-nickel-cobalt-based oxide (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), a lithium-manganese-cobalt-based oxide (e.g., 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 (e.g., 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), or lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Nip2Co 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 atomic fractions of independent elements, 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, and p2 + q2 + q2 + r3 + s2 = 1)), etc. may be included, and any one or two or more of these compounds may be contained. Among these, from the viewpoint of enhancing the capacity characteristics and stability of the battery, the lithium composite metal oxide is LiCoO2, LiMnO2, LiNiO2, lithium nickel manganese cobalt oxide (e.g., 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. 5Mn 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 (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.) may be used.

[0049] The positive electrode active material may be contained in an amount of 50% to 99% by weight based on 100% by weight of the positive electrode binder.

[0050] The binder is a component that aids in bonding the positive electrode active material to the conductive material and the current collector, and may typically be included in an amount of 1 to 30 wt % relative to 100 wt % of the positive electrode mixture. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers.

[0051] The conductive material may be added in an amount of usually 0.5% by weight to 30% by weight, for example 1% by weight to 30% by weight, based on the total weight of the solid content in the positive electrode mixture.

[0052] Such a conductive material is not particularly limited as long as it is conductive without inducing a chemical change in the battery, and examples thereof include carbon powders such as carbon black, acetylene black (or denka black), ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite with highly developed crystal structures; conductive fibers such as carbon fiber and metal fiber; 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.

[0053] 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 surface or both surfaces of the negative electrode current collector. The negative electrode active material layer may include a negative electrode composite, which may include a negative electrode active material, a binder, and a conductive material. The present specification states that the negative electrode composite does not include an electrolyte impregnated in the negative electrode. In the present invention, the negative electrode active material layer has a porous characteristic including a plurality of pores, and the pores are filled with the electrolyte as described above. The electrolyte may exhibit a fluid liquid state or a non-fluid solid state due to a predetermined degree of cross-linking, depending on the location of the pores in the electrode assembly.

[0054] The negative electrode current collector may generally have a thickness of 3 μm to 500 μm. Such a negative electrode current collector is not particularly limited as long as it has high conductivity and does not induce chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, baked carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc. may be used. Furthermore, the surface may be provided with fine irregularities to enhance the binding strength of the negative electrode active material, and the negative electrode current collector may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0055] 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, a metal, or an alloy of such a metal with lithium, a metal composite oxide, a material capable of doping and dedoping lithium, and a transition metal oxide.

[0056] As the carbon material capable of reversibly intercalating / deintercalating the lithium ions, any carbonaceous negative electrode active material generally used in lithium ion secondary batteries may be used without particular limitation, and typical examples thereof include crystalline carbon, amorphous carbon, or a combination of these. 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 (low-temperature calcined carbon) or hard carbon, mesophase pitch carbide, calcined coke, and the like.

[0057] Examples of the metal composite oxide include PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5, LixFe2O3 (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, elements of Group 1, Group 2, Group 3 of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8) selected from the group consisting of may be used.

[0058] 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 an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and combinations thereof, and is not Si), Sn, SnO₂, Sn - Y (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and combinations thereof, and is not Sn), etc. may be mentioned, and at least one of these may be mixed with SiO₂ and used. As the element Y, 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 may be selected.

[0059] Examples of the transition metal oxide include lithium - containing titanium composite oxide (LTO), vanadium oxide, lithium vanadium oxide, etc.

[0060] The negative electrode active material may be contained in an amount of 50% to 99% by weight based on 100% by weight of the negative electrode composite material.

[0061] The binder is a component that helps bond the conductive material, the active material, and the current collector, and is usually added in an amount of 1% to 30% by weight based on 100% by weight of the negative electrode composite material. Examples of such binders include polyvinylidene fluoride (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, and various copolymers thereof.

[0062] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 0.5% by weight to 30% by weight, for example, 1% by weight to 20% by weight, relative to 100% by weight of the negative electrode composite. Such a conductive material may be the same as or different from the conductive material used in preparing the positive electrode, and examples thereof include carbon powders such as carbon black, acetylene black (or denka black), ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite, each having a highly developed crystal structure; conductive fibers such as carbon fiber and metal fiber; 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.

[0063] The separator serves to prevent internal short circuits between the electrodes and to be impregnated with an electrolyte, and may include an insulating porous sheet such as a polymer film or a polymer nonwoven fabric. For example, a separator composition may be prepared by mixing a polymer resin, a filler, and a solvent, and then the separator composition may be directly coated on the top of an electrode and dried to form a separator film, or the separator composition may be cast on a support and dried, and the separator film may be peeled from the support and laminated on the top of an electrode to form the polymer film.

[0064] The separation membrane may be a commonly used porous polymer film, for example, a porous polymer film made from a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, either alone or in a laminate thereof, or a common porous nonwoven fabric, for example, a polymer nonwoven fabric made from a high-melting point glass fiber, polyethylene terephthalate fiber, or the like, but is not limited thereto.

[0065] The porous separator may have a pore diameter of generally 0.01 μm to 50 μm, a porosity of 5 vol% to 95 vol%, and a thickness of generally 5 μm to 300 μm.

[0066] In the present invention, the separator has a porous property including a plurality of pores, and the pores are filled with the electrolyte as described above. The electrolyte may be in a fluid liquid state or in a non-fluid solid state due to a predetermined degree of cross-linking depending on where the pores are located in the electrode assembly.

[0067] Meanwhile, in one embodiment of the present invention, the separator may include an organic / inorganic composite porous coating layer on its surface, the inorganic particles including a binder resin material. The inorganic particles may have a particle size of 0.01 μm to 2 μm, and are not particularly limited as long as they are electrochemically stable without undergoing oxidation and / or reduction reactions at the operating potential of the battery. For example, the inorganic particles may be BaTiO3, Pb(Zr,Ti)O3 (PZT), Pb 1-x La x Zr 1-y Ti y O3(PLZT, where 0 <x<1、0<y<1である)、Pb(Mg 1 / 3 Nb 2 / 3 The battery case may contain at least one selected from the group consisting of )O3-PbTiO3 (PMN-PT), hafnia (HfO2), SrTiO3, SnO2, CeO2, MgO, NiO, CaO, ZnO, ZrO2, Y2O3, Al2O3, SiC, TiO2, boehmite, aluminum oxide, and aluminum hydroxide. Meanwhile, the battery case is not limited to a particular material or shape, and may be, for example, a cylindrical or rectangular shape using a metal can. Alternatively, it may be a pouch-type case using a pouch film. Alternatively, a coin-shaped case may be used.

[0068] Secondary battery manufacturing method Next, a method for manufacturing a secondary battery according to the present invention will be described.

[0069] In one embodiment of the present invention, the method for manufacturing the secondary battery includes: A step (S10) of housing the electrode assembly in a battery case; A step (S20) of injecting a composition for forming a gel polymer electrolyte into the battery case; a step (S30) of primarily sealing the battery case under normal pressure conditions (atmospheric atmosphere); A step (S40) of releasing the primary sealing and degassing; a step (S50) of crosslinking the resultant of the step (S40); Includes:

[0070] First, an electrode assembly is manufactured and then housed in a battery case (S10).

[0071] The electrode assembly is the same as that described for the secondary battery according to the present invention. Therefore, for the sake of convenience, a redundant description will not be provided. In one embodiment of the present invention, the electrode assembly may be wound into a jelly roll type, a stack type, or a stack-fold type depending on the intended use and application of the battery.

[0072] Next, a gel polymer electrolyte composition is injected into the battery case containing the electrode assembly (S20).

[0073] In one embodiment of the present invention, the gel polymer electrolyte composition may contain at least one polymerizable compound selected from the group consisting of: (a) a lithium salt; (b) a non-aqueous organic solvent; (c) a polymerization initiator; and (d) a polymerizable monomer, oligomer, and copolymer.

[0074] lithium salts The lithium salt is used as an electrolyte salt in a lithium secondary battery and is used as a medium for transferring ions. Generally, lithium salts contain Li as a cation.+ 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 contain at least one selected from the group consisting of:

[0075] The lithium salt may be used alone or in combination of two or more as needed. The lithium salt may be varied as appropriate within a range that is normally usable, but may be contained in the gel polymer electrolyte composition at a concentration of 0.5 M to 2 M, specifically 0.9 M to 1.5 M, to obtain an optimal effect of forming a corrosion-preventing coating on the electrode surface.

[0076] The gel polymer electrolyte composition of the present invention contains 0.5M or more of a lithium salt, thereby reducing the resistance due to the depletion of lithium ions during high-rate charge and discharge. Furthermore, in the gel polymer electrolyte composition of the present invention, when the concentration of the electrolyte salt satisfies the above range, the lithium cations present in the gel polymer electrolyte composition increase, thereby reducing the amount of lithium cations (Li + This ensures high ion transport properties (i.e., cation transfer number) of the lithium ion, reduces the diffusion resistance of lithium ions, and improves cycle capacity characteristics.

[0077] Non-aqueous organic solvents The non-aqueous organic solvent is not particularly limited as long as it minimizes decomposition due to oxidation reactions or the like during the charge / discharge process of the secondary battery and exhibits desired properties together with the additives. For example, carbonate-based organic solvents, ether-based organic solvents, and ester-based organic solvents may be used alone or in combination.

[0078] The carbonate organic solvent among the organic solvents may include at least one of a cyclic carbonate organic solvent and a linear carbonate organic solvent. Specifically, the cyclic carbonate 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, the cyclic carbonate organic solvent may include a mixed solvent of ethylene carbonate, which has a high dielectric constant, and propylene carbonate, which has a melting point relatively lower than that of ethylene carbonate.

[0079] The linear carbonate organic solvent is a solvent having low viscosity and low dielectric constant, and may include 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, and more specifically, may include dimethyl carbonate.

[0080] The ether-based organic solvent may be 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 mixture of two or more thereof, but is not limited thereto.

[0081] 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.

[0082] As the linear ester-based organic solvent, there may be used, for example, 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 mixture of two or more of these, but the linear ester-based organic solvent is not limited thereto.

[0083] Specific examples of the cyclic ester organic solvent include, but are not limited to, any 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 thereof.

[0084] Among the ester-based solvents, cyclic carbonate-based compounds are preferably used because they are high-viscosity organic solvents with a high dielectric constant that easily dissociates the lithium salt in the electrolyte. When such cyclic carbonate-based compounds are mixed in an appropriate ratio with linear carbonate-based compounds and linear ester-based compounds that have low viscosity and low dielectric constant, such as dimethyl carbonate and diethyl carbonate, a gel polymer electrolyte with high electrical conductivity can be prepared, and therefore these compounds are more preferably used.

[0085] polymerization initiator The polymerization initiator may be a conventional thermal initiator or photoinitiator known in the art, for example, the polymerization initiator may be decomposed by heat to form radicals, which may react with the crosslinker via free radical polymerization to form a gel polymer electrolyte.

[0086] 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. peroxide), and at least one azo compound selected from the group consisting of 2,2'-azobis(2-cyanobutane), 2,2'-azobis(methylbutyronitrile), 2,2'-azobis(isobutyronitrile) (AIBN; 2,2'-azobis(iso-butyronitrile)), and 2,2'-azobisdimethyl-valeronitrile (AMVN; 2,2'-azobisdimethyl-valeronitrile), but are not limited thereto.

[0087] The polymerization initiator is decomposed in the battery by heat, for example, at 30°C to 100°C, or at room temperature (5°C to 30°C), to form radicals, and the polymerizable oligomer reacts with the acrylate compound through free radical polymerization to form a gel polymer electrolyte.

[0088] The polymerization initiator may be contained in an amount of 0.01 to 20 parts by weight, specifically 0.1 to 10 parts by weight, relative to 100 parts by weight of the polymerizable compound.

[0089] When the polymerization initiator is used in an amount ranging from 0.01 to 20 parts by weight, the conversion rate to a gel polymer can be increased to ensure the electrolytic properties of the gel polymer, and the pre-gel reaction can be prevented to improve the wettability of the electrolyte solution to the electrode.

[0090] polymerizable compound The polymerizable compound, i.e., polymerizable monomer, oligomer, or copolymer, is a compound that has a polymerizable functional group selected from the group consisting of a vinyl group, an epoxy group, an allyl group, and a (meth)acrylic group in its structure, in which a polymerization reaction can occur, and that can be gelled by polymerization or crosslinking, and is not particularly limited as long as it is a compound that is used as a synthetic monomer, oligomer, or copolymer in the production of a conventional gel polymer electrolyte.

[0091] Among them, examples of the polymerizable monomer include, but are not limited to, tetraethylene glycol diacrylate, polyethylene glycol diacrylate (molecular weight: 50 to 20,000), 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate, trimethylolpropane triacrylate, trimethylolpropane ethoxylate triacrylate, trimethylolpropane propoxylate triacrylate, ditrimethylolpropane tetraacrylate, pentaerythritol tetraacrylate, and pentaerythritol ethoxylate tetraacrylate. tetraacrylate), dipentaerythritol pentaacrylate, dipentaerythritol hexaacrylate, poly(ethylene glycol) diglycidylether, 1,5-hexadiene dioxide, glycerol propoxylate triglycidyl ether, vinylcyclohexene dioxide, 1,2,7,8-diepoxyoctaneExamples of suitable glycidyl ethers 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 may be used alone or in combination of two or more.

[0092] Representative examples of the copolymer include 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.

[0093] The polymerizable compound may be included in an amount of 0.01 wt % to 10 wt % based on the total weight of the gel polymer electrolyte composition. If the content of the polymerizable compound exceeds 10 wt %, the gelation may occur too quickly or the composition may become dense during injection into a battery, resulting in a gel with high resistance. Conversely, if the content of the polymerizable compound is less than 0.01 wt %, the gelation may not proceed smoothly.

[0094] additives Furthermore, the gel polymer electrolyte composition of the present invention may further contain an additional additive capable of forming a more stable ion-conductive coating on the surface of the electrode, if necessary, in order to prevent the collapse of the negative electrode due to decomposition under 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.

[0095] Specifically, the additional additive may include at least one first additive selected from the group consisting of, as representative examples, sultone-based compounds, sulfide-based compounds, sulfone-based compounds, sulfate-based compounds, halogen-substituted carbonate-based compounds, nitrile-based compounds, cyclic carbonate-based compounds, phosphate-based compounds, borate-based compounds, and lithium salt-based compounds.

[0096] The sultone compound may be at least one compound selected from the group consisting of 1,3-propane sultone (PS), 1,4-butane sultone, ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone, and may be contained in an amount of 0.3 to 5 wt %, specifically 1 to 5 wt %, based on the total weight of the gel polymer electrolyte composition. If the content of the sultone compound in the gel polymer electrolyte composition exceeds 5 wt %, an excessively thick coating may form on the surface of the electrode, resulting in increased resistance and decreased output power. Furthermore, an excessive amount of additives in the gel polymer electrolyte composition may increase resistance and degrade output power characteristics.

[0097] The sulfite-based compound may be at least one compound selected from the group consisting of ethylene sulfite, methyl ethylene sulfite, ethyl ethylene sulfite, 4,5-dimethylethylene sulfite, 4,5-diethylethylene 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 contained in an amount of 3 wt % or less based on the total weight of the gel polymer electrolyte composition.

[0098] The sulfone-based compound may be at least one compound selected from the group consisting of divinyl sulfone, dimethyl sulfone, diethyl sulfone, methyl ethyl sulfone, and methyl vinyl sulfone, and may be contained in an amount of 3 wt % or less relative to the total weight of the gel polymer electrolyte composition.

[0099] The sulfate-based compound may be ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS), and may be contained in an amount of 3 wt % or less based on the total weight of the composition for the gel polymer electrolyte.

[0100] The halogen-substituted carbonate-based compound may be fluoroethylene carbonate (FEC), and may be contained in an amount of 5 wt% or less based on the total weight of the gel polymer electrolyte composition. If the content of the halogen-substituted carbonate-based compound in the gel polymer electrolyte composition exceeds 5 wt%, cell swelling performance may be deteriorated.

[0101] Furthermore, examples of the nitrile compound include at least one compound selected from the group consisting of succinonitrile, adiponitrile (Adn), acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0102] The cyclic carbonate-based compound may be vinylene carbonate (VC) or vinylethylene carbonate, and may be contained in an amount of 3 wt% or less based on the total weight of the gel polymer electrolyte composition. If the content of the cyclic carbonate-based compound in the gel polymer electrolyte composition exceeds 3 wt%, the cell swelling suppression performance may be deteriorated.

[0103] The phosphate-based compound may be at least one compound selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tetramethyltrimethylsilylphosphate, trimethylsilylphosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite, and may be contained in an amount of 3 wt % or less based on the total weight of the gel polymer electrolyte composition.

[0104] The borate-based compound may be lithium oxalyl difluoroborate, which may be contained in an amount of 3 wt % or less based on the total weight of the composition for the gel polymer electrolyte.

[0105] The lithium salt-based compound is a compound different from the lithium salt contained in the gel polymer electrolyte composition, and may be at least one compound selected from the group consisting of LiPO2F2, LiODFB, LiBOB (lithium bis(oxalatoborate) (LiB(C2O4)2), and LiBF4), and may be contained in an amount of 3 wt% or less relative to the total weight of the gel polymer electrolyte composition.

[0106] Two or more of the additional additives may be mixed and included in an amount of 20 wt % or less, specifically 0.1 wt % to 10 wt %, based on the total weight of the gel polymer electrolyte composition. If the content of the additional additive is less than 0.01 wt %, the effect of improving the low-temperature output of the battery and the high-temperature storage and life characteristics may be minimal. If the content of the additional additive is more than 20 wt %, excessive side reactions may occur in the gel polymer electrolyte composition during battery charge and discharge. In particular, the additive may not be fully decomposed at high temperatures and may remain unreacted or precipitated in the gel polymer electrolyte composition at room temperature. This may result in side reactions that reduce the life and resistance characteristics of the secondary battery.

[0107] Next, the battery case is primarily sealed and aged in an atmospheric environment (S30). This sealing is sufficient as long as the electrolyte does not leak out of the battery case during the aging process described below. Since the battery case is sealed in an atmospheric environment, the battery is sealed with air remaining inside, and the air may contain approximately 21 vol% oxygen. In one embodiment of the present invention, prior to the primary sealing, a process of injecting oxygen into the battery case may be performed to increase the oxygen concentration inside the battery case to above atmospheric levels, i.e., 21 vol% or more. The pressure range for the primary sealing is not particularly limited, and the primary sealing may be performed under atmospheric pressure, for example.

[0108] After the primary sealing, an aging step may be performed. The aging step allows the electrode assembly to be sufficiently impregnated with the composition, and the composition may be uniformly impregnated throughout the electrode assembly. Furthermore, since the battery is stably maintained in a fixed position during the aging step, the oxygen concentration may be uniform throughout the electrode assembly. The aging step is not particularly limited, but may be performed for a period ranging from several hours to several days. For example, it may be performed within 72 hours. While typical liquid electrolytes are often subjected to high temperatures (60°C or higher) to enhance the aging effect, the aging step is preferably performed at a temperature ranging from room temperature to 45°C due to the risk of pre-gel formation in gel polymers.

[0109] Meanwhile, oxygen (O2) can suppress the chain reaction of polymer monomers through radical quenching when radicals are generated. Therefore, since the aging process is performed in the presence of oxygen, polymerization caused by radical reactions can be suppressed during the aging process. As a result, the composition remains in a liquid state without gelation, achieving uniform wetting of the composition throughout the electrode assembly. Figure 2 is a schematic diagram of the uniform distribution of oxygen 300 within a primary sealed battery.

[0110] Thereafter, an oxygen concentration gradient is created between the outer and central portions of the electrode assembly (S40). In step S40, the oxygen concentration in the central portion of the electrode assembly is made higher than the oxygen concentration in the outer portion. In one embodiment of the present invention, step S40 may be performed by re-opening the secondary battery, which is the result of step S30, and degassing it. For example, the opening may be performed by cutting out the portion indicated by the dotted line A in FIG. 2. FIG. 3 shows the shape of an oxygen concentration gradient created by degassing.

[0111] In one embodiment of the present invention, the degassing may be performed within a range of maintaining a volume of 75% to 90% of the 100% volume of the battery before unsealing. Generally, degassing is performed so that the volume is 70% or less of the 100% volume of the battery before unsealing in order to remove as much gas as possible generated during the aging process. Meanwhile, in the present invention, degassing is performed under conditions that satisfy the above range in order to maintain a predetermined oxygen concentration within the battery. Meanwhile, in one embodiment of the present invention, the volume of the battery refers to the total volume of the space surrounded by the exterior material, and does not exclude the empty space inside the battery surrounded by the exterior material.

[0112] In one embodiment of the present invention, the volume of the battery after degassing may be larger than the volume of the battery immediately after primary sealing (before aging, ie, before gas generation).

[0113] In one embodiment of the present invention, the volume of the battery before unsealing may vary from battery to battery depending on the amount of gas generated. For example, the amount of gas generated may vary depending on the type of electrolyte, the type of additives in the electrolyte, the type of battery materials such as the electrode active material, the aging time, and the temperature. However, rather than calculating and applying a quantitative gas generation amount and gas removal amount, degassing is performed within the above range, considering that oxygen may remain in the battery by retaining a predetermined amount of gas within the battery without removing all of the gas generated during the aging process. This degassing process partially retains oxygen in the center, and the outer shell, which has a relatively short path connecting to the outside and is therefore more likely to release gas, has a higher degassing efficiency. This allows the subsequent crosslinking process to maintain a low degree of crosslinking in the center and a higher degree of crosslinking in the outer shell than in the center.

[0114] In one embodiment of the present invention, in the case of mass production of batteries, the amount of gas generated during aging of a battery designed before mass production and the amount of volume change due to gas release may be measured in advance, and the degassing process may be controlled so that the degassing is performed within the range of the volume change rate.

[0115] Meanwhile, in one embodiment of the present invention, at least the degassing process or the unsealing and degassing process may be performed in a gas atmosphere with an oxygen concentration lower than that of atmospheric air. For example, the process may be performed in a gas atmosphere with an oxygen concentration of less than 20 vol%, a nitrogen (N2) atmosphere with an oxygen concentration of 90 vol% or more, or an inert gas atmosphere such as helium, neon, argon, krypton, or xenon with an oxygen concentration of 90 vol% or more. In a specific embodiment, when the unsealing and degassing process or at least the degassing process is performed in such a gas atmosphere, the cross-linking reaction of the outer shell is not hindered by oxygen released from the inside to the outside of the electrode assembly. As a result, a battery with a relatively low degree of cross-linking in the center and a relatively high degree of cross-linking in the outer shell can be manufactured.

[0116] In one embodiment of the present invention, after the degassing process, the battery case may be resealed (secondary sealing) before the next crosslinking reaction proceeds. As described above, the secondary sealing process may be performed in a gas atmosphere with an oxygen concentration of less than 20 vol%, in a nitrogen (N2) atmosphere of 90 vol% or more, or in an inert gas atmosphere such as helium, neon, argon, krypton, or xenon of 90 vol% or more.

[0117] Next, heat is applied to the battery to carry out a crosslinking reaction (S50). The crosslinking reaction can be carried out by placing the battery in a heating device and heating it. Heat applied from the outside of the battery is gradually conducted into the battery, causing the electrolyte composition to gel continuously from the outside to the inside of the battery. However, since the battery has an oxygen concentration gradient between the inside and outside of the battery, the gelation reaction occurs more favorably outside the battery than inside. However, inside the battery, where the oxygen concentration is relatively high, the chain reaction of polymer monomers, i.e., the gelation reaction, is suppressed due to the radical quenching function of oxygen. That is, the center of the battery has a relatively low degree of crosslinking, meaning that the electrolyte remains liquid without gelling, while the outer periphery surrounding the center is crosslinked to a predetermined degree or higher to form a gel polymer electrolyte.

[0118] In one embodiment of the present invention, the crosslinking reaction may be carried out at a temperature of 60° C. or higher, and preferably within a range of 60° C. to 75° C. If the temperature is lower than 60° C., the crosslinking reaction may not be initiated, and if the temperature is higher than 75° C., decomposition of the electrolyte additive may occur.

[0119] Meanwhile, in one embodiment of the present invention, the heating device may be preheated to a predetermined temperature before the battery is inserted, in which case heat may be rapidly transferred to the outer shell of the electrode assembly, further accelerating gelation of the outer shell.

[0120] In this way, the method for manufacturing a secondary battery of the present invention creates an oxygen concentration gradient between the outside and inside of the secondary battery, thereby causing a sufficient cross-linking reaction in the outer shell portion where the oxygen concentration is low and suppressing cross-linking in the center portion where the oxygen concentration is high, thereby forming a center portion where the electrolyte inside the battery has a low degree of cross-linking and is maintained in a fluid state. In other words, the manufacturing method of the present invention uses the difference in oxygen concentration between the center portion and the outer shell portion of the battery to vary the degree of cross-linking of the gel composition, ultimately resulting in the center portion of the battery being rich in liquid electrolyte and the outer shell portion of the battery being rich in gel electrolyte, thereby simultaneously improving the durability and safety of the battery.

[0121] In the method for producing a lithium secondary battery of the present invention, the crosslinking reaction in step (S50) may be carried out for 5 to 24 hours at 60 to 75° C. For example, step (S50) may be carried out at about 70° C. for about 5 hours.

[0122] In one embodiment of the present invention, after step (S50), one or more activation processes may be further performed. Furthermore, after the activation process, a degassing process may be further performed to remove gas generated inside the battery. When the activation process is performed multiple times, a degassing process may be performed after the entire activation process or between activation processes.

[0123] Meanwhile, the activation step of the battery may be performed by a known method. For example, the battery may be subjected to one or more charge / discharge cycles. In a specific embodiment, the activation step may include a step of charging the battery at a 0.2 C rate to an SOC of 60% or higher. Meanwhile, the additional degassing step may be performed for several seconds to several minutes by pressurizing the battery case at a reduced pressure of -85 kPa to -95 kPa or -90 kPa to -95 kPa compared to atmospheric pressure to remove gas remaining in the battery case. For example, the additional degassing step may be performed for approximately 10 seconds.

[0124] As described above, the secondary battery obtained by the manufacturing method according to the present invention may have a shape in which the center contains an electrolyte with a low degree of cross-linking, and the center is encapsulated by an outer shell in the form of a gel electrolyte that is cross-linked to a predetermined degree or more.

[0125] In the present invention, the secondary battery is preferably a lithium secondary battery, and non-limiting examples of the lithium secondary battery include a lithium metal secondary battery, a lithium ion secondary battery, a lithium polymer secondary battery, and a lithium ion polymer secondary battery.

[0126] Hereinafter, the present invention will be described in detail with reference to examples. However, the examples according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the examples detailed below. The examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0127] Example 1 and Example 2 (Manufacturing of electrode assemblies) As a positive electrode active material (LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 A positive electrode active material slurry (solid content 50%) was prepared by adding 94 wt% O2 (NCM), 3 wt% carbon black as a conductive material, and 3 wt% polyvinylidene fluoride (PVDF) as a binder to N-methyl-2-pyrrolidone (NMP) as a solvent. The positive electrode active material slurry was applied to an approximately 20 μm-thick aluminum (Al) thin film as a positive electrode current collector, dried, and then roll-pressed to prepare a positive electrode.

[0128] Anode active material slurry (solid content 80%) was prepared by adding carbon powder (a mixture of 90% artificial graphite and 10% natural graphite by weight) as the anode active material, PVDF as the binder, and carbon black as the conductive material at 96 wt%, 3 wt%, and 1 wt%, respectively, to NMP as the solvent. The anode active material slurry was applied to a 10 μm-thick copper (Cu) thin film as the anode current collector, dried, and then roll-pressed to prepare the anode.

[0129] The positive electrode, negative electrode, and a three-layer separator made of polypropylene / polyethylene / polypropylene (PP / PE / PP) were stacked in order to fabricate a stacked electrode assembly, which consisted of 20 stacked positive electrodes.

[0130] (Production of Gel Polymer Electrolyte Composition) 96.96 g of a non-aqueous electrolyte solution 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. To the non-aqueous electrolyte solution, 5 wt% of trimethylolpropane triacrylate as a polymerizable compound and 0.02 wt% of AIBN as a polymerization initiator were added relative to 100 wt% of the gel polymer electrolyte composition to produce a gel polymer electrolyte composition.

[0131] (Lithium secondary battery manufacturing) The electrode assembly was inserted into a pouch-shaped battery case, and the gel polymer electrolyte composition was injected. The battery case was then sealed under normal pressure. The sealing was performed by applying pressure to the sealed portion of the pouch at 140°C for 2 seconds. After sealing the battery case, it was left at room temperature for 3 days. Next, the battery case was opened and degassed (primary), and then resealed. Finally, the battery was placed in a heating chamber and heated at 70°C for 5 hours to produce a lithium secondary battery containing a thermally polymerized gel polymer electrolyte. The primary degassing conditions (pressure and time) were as shown in Table 1 below.

[0132] On the other hand, after the primary degassing step, the battery was charged at 0.2 C to an SOC of 60%, and the battery case was opened and degassed (additional degassing) for 10 seconds under reduced pressure of -95 kPa.

[0133] Example 3 An electrode assembly and a composition for a gel polymer electrolyte were prepared in the same manner as in Examples 1 and 2.

[0134] The electrode assembly was inserted into the battery case, and the gel polymer electrolyte composition was injected. The battery case was then sealed under normal pressure. The sealing was performed by applying pressure to the sealed portion of the pouch at 140°C for 2 seconds. After sealing the battery case, it was left at room temperature for 3 days. Next, the battery case was opened and degassed (primary) under a helium atmosphere. Then, it was resealed. Finally, the battery was placed in a heating chamber and heated at 70°C for 5 hours to produce a lithium secondary battery containing a thermally polymerized gel polymer electrolyte. The primary degassing conditions (pressure and time) were as shown in Table 1 below.

[0135] On the other hand, after the primary degassing step, the battery was charged at 0.2 C to an SOC of 60%, and the battery case was opened and degassed (additional degassing) for 10 seconds under reduced pressure of -95 kPa.

[0136] Comparative Example 1 A battery was produced in the same manner as in Example 1, except that the pressure applied in the primary degassing step was 95 kPa and the time was 30 minutes.

[0137] Comparative Example 2 A battery was produced in the same manner as in Example 1, except that the primary degassing was not carried out.

[0138] [Table 1]

[0139] [Table 2] **Stiffness: A numerical value converted by setting the cell stiffness of Comparative Example 1 as 100%, and the actual measurement unit is gf / mm.

[0140] Experimental example 5-1) Experimental Example 1: Method for measuring the degree of crosslinking by process The degree of crosslinking of the lithium secondary batteries manufactured in Examples 1 to 3 and Comparative Examples 1 and 2 was measured by the following method.

[0141] After opening the battery case of each battery, the electrode assembly was collected and disassembled into an outer shell and a center portion to collect electrolyte samples. Each collected sample was placed in acetone (Acetone d-6) and shaken at room temperature for approximately 1 hour. Then, it was filtered to remove impurities such as electrode active materials and conductive materials, obtaining a filtrate. NMR analysis of the filtrate was performed to measure the amount of unreacted polymerizable compound remaining from carbon-carbon double bond peak data. This was then compared with the amount of the collected electrolyte sample to calculate the degree of crosslinking using the following equation 1.

[0142] [Formula 1] Degree of crosslinking (%) = 100 - (amount of unreacted polymerizable compound / amount of collected electrolyte sample) x 100

[0143] The NMR analysis was performed by 1D Normal NMR (One Dimensional Normal Nuclear Magnetic Resonance Spectroscopy). The NMR measurement was performed using a Bruker 700 MHz NMR analyzer, and the sample was dissolved in acetone-d6 before use. The measurement conditions were 1H NMR (zg 30), ns = 32, d1 = 3 sec. 1The ethylene carbonate (EC) peak, the least volatile peak in the H NMR spectrum, was set at 100% as the reference, and the integral of the acrylate double bond peak of each sample was calculated. The acrylate double bond area of ​​the reference sample was normalized to 100 to calculate the relative area, and the results are summarized in the table above.

[0144] 5-2) Experimental Example 2: Measurement and calculation method of stiffness of lithium secondary battery The stiffness of the lithium secondary battery according to Comparative Example 1 was measured at the center of the battery using a texture analyzer ball type device at a speed of 10 mm / min, a distance of 1.2 mm, and a minimum trigger force of 50 g.

[0145] Thereafter, the stiffness of the other experimental values ​​was calculated, with the stiffness of Comparative Example 1 being set at 100%.

[0146] 5-3) Experimental Example 3: Safety evaluation by nail penetration test The lithium secondary batteries manufactured in Examples 1 and 2 and Comparative Examples 1 and 2 were fully charged to 4.4 V at room temperature, and then subjected to a nail penetration test under GB / T conditions (nail diameter 2.5 mm, penetration speed 6 m / min). The results are shown in Table 2.

[0147] As can be seen from Table 2, Example 1, which was degassed for a long time at a relatively low pressure, and Example 2, which was degassed for a short time at a relatively high pressure, were found to have superior resistance and lifespan characteristics compared to the other Examples and Comparative Examples. Furthermore, Examples 1 and 2 also had superior mechanical properties for the electrode assemblies compared to the other Experimental Examples. Furthermore, in Example 3, the opening, degassing, and resealing were performed under an inert gas atmosphere, which confirmed an increased degree of cross-linking in the outer shell. Therefore, it was confirmed that sufficient ionic conductivity and lifespan characteristics for battery operation were secured, while mechanical strength was further improved.

Claims

1. A method for manufacturing a secondary battery including a gel polymer electrolyte, comprising: The method comprises: A step (S10) of housing the electrode assembly in a battery case; Injecting a composition for forming a gel polymer electrolyte into the battery case (S20); a step (S30) of primarily sealing the battery case in an atmospheric environment and aging the battery case; A step (S40) of releasing the primary sealing and degassing; a step (S50) of crosslinking the resultant of the step (S40); Including, The secondary battery includes a gel polymer electrolyte in which a portion of the electrolyte is crosslinked to a predetermined degree or more, and the degree of crosslinking increases from the center of the battery to the outside.

2. The step (S40) is carried out in a gas atmosphere having an oxygen concentration of less than 20 vol % or a nitrogen (N ２ 2. The method for producing a secondary battery according to claim 1, wherein the heating is carried out in an atmosphere of an inert gas of 90 vol % or more.

3. 2. The method for manufacturing a secondary battery according to claim 1, wherein the secondary battery includes a central portion having a low degree of cross-linking of the electrolyte, and an outer shell portion surrounding the central portion and including a gel polymer electrolyte having a higher degree of cross-linking than the central portion.

4. 2. The method for producing a secondary battery according to claim 1, wherein the composition for 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 a polymerizable monomer, an oligomer, and a copolymer.

5. The method for manufacturing a secondary battery according to claim 1 , further comprising the step of injecting oxygen into the battery case before performing the step (S30).

6. The method for manufacturing a secondary battery according to claim 1, wherein the aging process in step (S30) is performed at room temperature to 45°C.

7. 2. The method for manufacturing a secondary battery according to claim 1, wherein the degassing in step (S40) is performed in a range of maintaining a volume of 75 vol % to 90 vol % relative to 100 vol % of the volume of the battery before unsealing.

8. The method for manufacturing a secondary battery according to claim 1 , wherein the crosslinking reaction in step (S50) is carried out at 60° C. or higher.

9. The method for manufacturing a secondary battery according to claim 1 , further comprising the step of activating the battery after the step (S50) is performed.

10. Produced by the method of any one of claims 1 to 9, A secondary battery comprising: a central portion containing a gel polymer electrolyte with a low degree of cross-linking, the degree of cross-linking of the gel polymer electrolyte increasing stepwise or gradually from the inside to the outside of the battery; and an outer shell portion surrounding the central portion and containing a gel polymer electrolyte with a higher degree of cross-linking than the central portion.

11. The secondary battery according to claim 10 , wherein the degree of cross-linking of the outer shell portion is 80% by weight or more and the degree of cross-linking of the center portion is less than 80% by weight.

12. The secondary battery according to claim 10 , wherein the degree of cross-linking of the central portion is less than 40% by weight.

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