Electrode assembly and secondary battery including the same

Abstract

The present invention provides an electrode assembly in which positive and negative electrodes are alternately stacked with separators interposed therebetween, and the electrode assembly includes two or more separators having different temperatures at which their adhesive strength becomes maximum, in which the temperature at which the adhesive strength of the separator located at the center of the electrode assembly to the electrode becomes maximum is lower than the temperature at which the adhesive strength of the separator located in the remaining region to the electrode becomes maximum, and a secondary battery including the electrode assembly.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0145991, filed on October 28, 2021, and all contents disclosed in the documents of that Korean patent application are incorporated herein by reference.

[0002] The present invention relates to an electrode assembly including a plurality of separators and a secondary battery including the same. [Background technology]

[0003] In general, secondary batteries are formed in a structure in which an electrode assembly and an electrolyte are sealed inside a battery case, and are broadly classified into cylindrical batteries, prismatic batteries, pouch batteries, etc. depending on their external shape, and may also be classified into lithium ion batteries, lithium ion polymer batteries, lithium polymer batteries, etc. depending on the form of the electrolyte. With the recent trend toward miniaturization of mobile devices, there is an increasing demand for thin prismatic batteries and pouch batteries, and there is particularly growing interest in pouch batteries, which are easily deformed and lightweight.

[0004] Meanwhile, the electrode assembly housed in the battery case can be classified into a jelly roll type (wound type), a stack type (layered type), a stack and folding type (composite type), or a stack and lamination type structure depending on its shape.

[0005] Generally, secondary batteries are manufactured by assembling an electrode assembly together with an electrolyte into a battery case and then undergoing an activation process. The activation process stabilizes the battery structure and prepares it for use by charging, aging, and discharging the assembled battery. During the activation process, the negative electrode expands during charging, and defects where the electrode and separator are not sufficiently bonded may separate, resulting in bending, in which the electrode assembly bends in one direction or randomly. This bending problem is particularly prevalent in stacked electrode assemblies and stack and lamination electrode assemblies. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made to solve the above-mentioned problems, and aims to provide an electrode assembly and a secondary battery including the same that have been developed to suppress the occurrence of bending by disposing separators with different adhesive properties at the center and surface of the electrode assembly. [Means for solving the problem]

[0007] The present invention provides an electrode assembly in which positive and negative electrodes are alternately stacked with separators interposed therebetween, and the electrode assembly includes two or more separators having different temperatures at which their adhesive strength is maximized, wherein the temperature at which the adhesive strength of the separator located in the center of the electrode assembly to the electrode is maximized is lower than the temperature at which the adhesive strength of the separator located in the remaining region to the electrode is maximized, and a secondary battery including the electrode assembly. [Effects of the Invention]

[0008] The present invention suppresses the bending phenomenon of the electrode assembly that occurs due to temperature variations inside the electrode assembly during the activation process by disposing separators with different adhesive properties at the center and surface (outer edge) of the electrode assembly.

[0009] In the activation process, a hot press process, i.e., a pressurized preheating process, can be performed to apply a certain amount of pressure and heat to the secondary battery before or during initial charging to increase the adhesive strength between the electrodes and the separator. If the same separator is used throughout the electrode assembly, a temperature difference will occur between the surface and center due to the thickness of the electrode assembly.

[0010] If the heating and pressurizing conditions are set to suit the surface region of the electrode assembly, the separator located in the central region may not reach a temperature sufficient for adhesive strength, resulting in a higher adhesive strength to the electrode of the separator located in the surface region than the center region. When the attached electrode expands during initial charging, the separator located in the surface region expands with the higher adhesive strength, and the difference in the degree of expansion or elongation between the separator located in the central region and the separator located in the surface region causes bending of the electrode assembly.

[0011] Conversely, if the heating and pressurizing conditions are set to match the center of the electrode assembly, the separators located in the surface region are exposed to a lower temperature, resulting in weaker adhesive strength to the electrode than the separators located in the center. The uneven adhesive strength can cause differences in the degree of expansion or elongation of the separators between the center and the surface, resulting in bending of the electrode assembly. However, in the present invention, by disposing separators with the highest adhesive strength at relatively low temperatures in the center of the electrode assembly and separators with the highest adhesive strength at relatively high temperatures in the surface region, the difference in adhesive strength due to the temperature difference between the center and surface regions of the electrode assembly during the activation process can be alleviated, thereby preventing bending.

[0012] Furthermore, in the case of a secondary battery including an electrode assembly with less bending, durability and stability are improved. [Brief explanation of the drawings]

[0013] [Figure 1]FIG. 1 is a schematic diagram of an electrode assembly produced in Example 1. [Figure 2] 1 shows the results of bending evaluation of the secondary battery produced in Example 1. [Figure 3] 1 shows the results of bending evaluation of the secondary battery produced in Comparative Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0014] 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 having meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0015] In this specification, the terms "comprise," "include," "comprise," or "have" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, but should be understood as not precluding the possible presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0016] The present invention will now be described in more detail.

[0017] The applicant discovered that during the activation process, particularly the pressurized pre-heating process, a temperature difference occurs between the surface and center of the electrode assembly due to the thickness of the secondary battery, resulting in bending of the electrode assembly. Specifically, the applicant discovered that during the pressurized pre-heating process, a temperature difference due to differences in heat transfer within the electrode assembly causes a difference in adhesive strength between the separators located on the surface and center of the electrode assembly, resulting in bending. The applicant therefore seeks to provide an electrode assembly that can prevent such bending, and a secondary battery including the electrode assembly.

[0018] electrode assembly The electrode assembly according to the present invention has a configuration in which positive electrodes and negative electrodes are alternately stacked with separators interposed therebetween, and the total number of positive electrodes, negative electrodes, and separators may be at least 20 to 50, preferably 20 to 45, and more preferably 30 to 45. If the total number of positive electrodes, negative electrodes, and separators included in the electrode assembly is less than the above range, it is difficult to achieve high capacity, whereas if it is more than the above range, it is difficult to suppress the bending phenomenon due to a complicated process.

[0019] Here, the materials and types of the positive electrode, negative electrode, and separator are not particularly limited, and various positive electrodes, negative electrodes, and separators known in the art may be used.

[0020] Meanwhile, the electrode assembly of the present invention is characterized in that separators having different adhesive properties, specifically, temperatures at which maximum adhesive strength is achieved, are disposed at the center and surface of the electrode assembly.

[0021] Specifically, the electrode assembly of the present invention is an electrode assembly in which positive and negative electrodes are alternately stacked with separators interposed therebetween, and includes two or more separators having different temperatures at which their adhesive strength is maximized. The temperature at which the adhesive strength of the separator located in the center of the electrode assembly to the electrode is maximized may be lower than the temperature at which the adhesive strength of the separator located in the remaining region to the electrode is maximized.

[0022] Here, the center of the electrode assembly refers to the region that is a distance from the outermost surface of the electrode assembly of D / 2-kD to D / 2+kD, and the remaining region refers to the surface of the electrode assembly. The k may be a constant that satisfies the range of 0.1≦k≦0.3, preferably 0.1≦k≦0.25, and most preferably 0.1≦k≦0.2. When k satisfies this range, the difference in adhesive strength between the separator located at the surface and the separator located at the center during the activation process is reduced, thereby suppressing bending.

[0023] Specifically, in the electrode assembly of the present invention, the difference between the temperature at which the adhesive strength of the separator located in the center to the electrode is maximized and the temperature at which the adhesive strength of the separator located in the remaining region (surface region) to the electrode is maximized may be 1°C to 15°C, preferably 1°C to 10°C, and more preferably 3°C to 8°C. The center of the electrode assembly exhibits a lower temperature than the surface region because less heat is transferred to the center during the heating and pressurizing process than the surface region. The temperature difference between the center and the surface region during the activation process varies depending on the thickness and material of the electrode assembly, but is generally on the order of 1°C to 15°C. Therefore, when the temperature difference at which the separator in the center and the separator in the surface region reach their maximum adhesive strength is controlled to the above level, the variation in adhesive strength between the separator in the center and the separator in the surface region is reduced, thereby suppressing the occurrence of bending due to the variation in adhesive strength.

[0024] More specifically, the temperature at which the adhesive strength of the separator located in the central region is maximized may be less than 70° C., preferably 50° C. or higher and lower than 70° C., and more preferably 55° C. or higher and 68° C. or lower. The temperature at which the adhesive strength of the separator located in the surface region is maximized may be 70° C. or higher and 80° C. or lower, preferably 70° C. or higher and 75° C. or lower, and more preferably 70° C. or higher and 72° C. or lower.

[0025] Meanwhile, in the electrode assembly of the present invention, the ratio of the number of separators located in an area that is a distance of D / 2-kD to D / 2+kD from the outermost surface of the electrode assembly to the number of separators included in the entire electrode assembly may be 1:1 to 1:4, preferably 1:1 to 1:3, and more preferably 1:1.25 to 1:1.50. When the ratio of the number of separators located in an area that is a distance of D / 2-kD to D / 2+kD from the outermost surface of the electrode assembly to the number of separators included in the entire electrode assembly satisfies the above range, a sufficient number of positive and negative electrodes can be stacked, which has the advantages of ensuring the energy density of the cell and minimizing an increase in cell thickness due to temperature variations.

[0026] Each component of the electrode assembly will now be described in detail.

[0027] Separator In the electrode assembly, the separator serves to prevent internal short circuits between the electrodes and to impregnate the electrodes with an electrolyte.

[0028] In the present invention, the temperature at which the adhesive strength of the separator is maximized can be measured using a Universal Testing Machine (UTM). Specifically, the temperature can be calculated by: (1) sampling the positive electrode and separator, each measuring 2 cm x 6 cm; (2) stacking the sampled positive electrode and separator; (3) placing the stacked sample in a pouch sealed on three sides, and then injecting an electrolyte; (4) completely sealing the pouch, and applying heat and pressure for 5 minutes using jig formation; and (5) opening the pouch, connecting the separator portion of the positive electrode and separator laminate with UTM pliers, and then performing a peel-off test at a 90° angle.

[0029] The separator according to the present invention may be made of a commonly used porous polymer resin, for example, a porous polymer resin made of 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 combination. Alternatively, a commonly used porous nonwoven fabric, for example, a nonwoven fabric made of a high-melting point glass fiber, a polyethylene terephthalate fiber, or the like, may be used, but is not limited thereto.

[0030] Here, the pore diameter of the porous separator is generally 0.01 μm to 50 μm, and the porosity may be 5% to 95%.

[0031] The separator included in the electrode assembly of the present invention may include a coating layer or a binder coating layer containing inorganic particles or a polymer material.

[0032] Specifically, the separator included in the electrode assembly of the present invention may be a safety reinforced separator (SRS) separator formed with a coating layer containing inorganic particles or polymeric materials made of ceramic components to ensure heat resistance or mechanical strength.

[0033] Specifically, the separator included in the electrode assembly of the present invention may include a porous separator substrate and a porous coating layer that is coated on one or both sides of the separator substrate. The coating layer may include a mixture of inorganic particles selected from metal oxides, semi-metal oxides, metal fluorides, metal hydroxides, and combinations thereof, and a binder polymer that binds and fixes the inorganic particles to each other.

[0034] The coating layer may contain at least one inorganic particle selected from Al2O3 and AlOOH. The inorganic particles can improve the thermal stability of the separator. That is, the inorganic particles can prevent the separator from shrinking at high temperatures. The binder polymer can also fix the inorganic particles to improve the mechanical stability of the separator.

[0035] The coating layer may also contain one or more binder polymers selected from polyethylene, polyvinylidene fluoride, polyvinylidene fluoride-tetrafluoroethylene, polyvinylidene fluoride-chlorotrifluoroethylene, and polyvinylidene fluoride-hexafluoropropylene. The binder polymer can bond the electrodes to the separator. Because the binder polymer is distributed throughout the coating layer, it can achieve uniform adhesion across the entire bonding surface, unlike the adhesives described above. Therefore, using such a separator can more stably fix the electrodes to the separator.

[0036] In the present invention, the temperature at which the adhesive strength of the separator is maximized may be adjusted by, but is not limited to, changing the type of binder contained in the coating layer or adjusting the humidification conditions during coating.

[0037] The thickness of the separator according to the present invention can generally be in the range of 7 μm to 22 μm, preferably 9 μm to 15 μm. When the thickness of the separator is in this range, the number of layers of the positive electrode, separator, and negative electrode can be increased, which has the advantage of increasing the energy density.

[0038] positive electrode The positive electrode included in the electrode assembly of the present invention may be prepared by coating a positive electrode current collector with a positive electrode mixture slurry including a positive electrode active material, a binder, a conductive material, and a solvent.

[0039] The positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like may be used.

[0040] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and specifically, may include a lithium metal oxide containing lithium and one or more metals such as cobalt, manganese, nickel, or aluminum. More specifically, the lithium metal oxide may include lithium-manganese-based oxides (e.g., LiMnO2, LiMn2O4, etc.), lithium-cobalt-based oxides (e.g., LiCoO2, etc.), lithium-nickel-based oxides (e.g., LiNiO2, etc.), lithium-nickel-manganese-based oxides (e.g., LiNi 1-Y Mn Y O2 (where 0 <Y<1)、LiMn 2-Z Ni ZO4 (where 0 < Z < 2), such as), lithium-nickel-cobalt-based oxides (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), such as), lithium-manganese-cobalt-based oxides (e.g., LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-z1 Co Z1 O4 (where 0 < Z1 < 2), such as), lithium-nickel-manganese-cobalt-based oxides (e.g., Li(Ni p Co q Mn r )O2 (where 0 < p < 1, 0 < q < 1, 0 < r < 1, p + q + r = 1) or Li(Ni p1 Co q1 Mn r1 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r1 < 2, p1 + q1 + r1 = 2), etc.), or lithium-nickel-cobalt-transition metal (M) oxides (e.g., Li(Ni p2 Co q2 Mn r2 M s2 )О2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and p2, q2, r2 and s2 are, as atomic fractions of independent elements, 0 < p2 < 1, 0 < q2 < 1, 0 < r2 < 1, 0 < s2 < 1, p2 + q2 + r2 + s2 = 1), etc.), etc. may be mentioned, and one or two or more of these compounds can be included.

[0041] Among them, in terms of being able to enhance the capacity characteristics and safety of the battery, the lithium 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 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co0.15 )O2 and Li(Ni 0.8 Mn 0.1 Co 0.1 )O2), or lithium nickel cobalt aluminum oxide (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.), and in consideration of the remarkable improvement effect by controlling the types and content ratios of constituent elements forming the lithium composite metal oxide, the lithium composite metal oxide may be 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., and any one or a mixture of two or more of these may be used.

[0042] The positive electrode active material may be contained in an amount of 60 to 99 wt %, preferably 70 to 99 wt %, and more preferably 80 to 98 wt %, based on the total weight of the solid content in the positive electrode mixture slurry.

[0043] The binder is a component that facilitates binding of the active material and the conductive material and binding of the active material to the current collector.

[0044] Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene (PE), polypropylene, ethylene-propylene-diene, sulfonated ethylene-propylene-diene, styrene-butadiene rubber, fluororubber, various copolymers, and the like.

[0045] Generally, the binder can be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, based on the total weight of the solid content in the positive electrode mixture slurry.

[0046] The conductive material is a component for further improving the conductivity of the positive electrode active material.

[0047] 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 1 to 20 wt % based on the total weight of the solid content of the negative electrode slurry. The conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity. Examples of the conductive material include carbon powders such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite; conductive fibers such as carbon fiber, carbon nanotubes, and metal fiber; fluorinated carbon powders; conductive powders such as aluminum powder 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.

[0048] Generally, the conductive material can be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, based on the total weight of the solid content of the positive electrode mixture slurry.

[0049] The solvent may include an organic solvent such as NMP (N-methyl-2-pyrrolidone) and may be used in an amount that provides a desired viscosity when the positive electrode active material and, optionally, a binder and a conductive material are included. For example, the solvent may be included so that the concentration of the solids including the positive electrode active material and, optionally, a binder and a conductive material is 50 to 95 wt %, preferably 70 to 95 wt %, and more preferably 70 to 90 wt %.

[0050] The thickness of the positive electrode according to the present invention can generally be in the range of 120 μm to 180 μm, preferably 140 μm to 160 μm. When the thickness of the positive electrode is in this range, it is possible to stack a large number of positive electrodes in a laminate of the same thickness, which has the advantage of increasing the energy density.

[0051] negative electrode The negative electrode included in the electrode assembly of the present invention may be prepared by, for example, coating a negative electrode current collector with a negative electrode mixture slurry containing a negative electrode active material, a binder, a conductive material, and a solvent, or a graphite electrode made of carbon (C) or a metal itself may be used as the negative electrode.

[0052] For example, when the negative electrode is manufactured by coating the negative electrode mixture slurry on the negative electrode current collector, the negative electrode current collector generally has a thickness of 3 to 500 μm. Such a negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, aluminum-cadmium alloy, etc., can be used. Furthermore, like the positive electrode current collector, the surface can be formed with fine irregularities to strengthen the binding strength of the negative electrode active material, and the negative electrode current collector can be used in various forms such as a film, sheet, foil, mesh, porous material, foam, nonwoven fabric, etc.

[0053] In addition, 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 and lithium, a metal composite oxide, a material capable of doping and dedoping lithium, and a transition metal oxide.

[0054] As the carbon material capable of reversibly intercalating / deintercalating the lithium ions, any carbonaceous negative electrode active material generally used in a lithium ion secondary battery 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. Examples of the amorphous carbon include soft carbon (low-temperature fired carbon) or hard carbon, mesophase pitch carbide, fired coke, and the like.

[0055] As the metal or an alloy of these metals with lithium, a metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn or an alloy of these metals with lithium can be used.

[0056] 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, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8) can be used.

[0057] As the substance capable of doping and undoping lithium, 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 at least one of these may be mixed with SiO2 and used. As the element Y, it can 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.

[0058] Examples of the transition metal oxide include lithium-containing titanium composite oxide (LTO), vanadium oxide, lithium vanadium oxide, etc.

[0059] The negative electrode active material can be contained in an amount of 60 to 99% by weight, preferably 70 to 99% by weight, more preferably 80 to 98% by weight, based on the total weight of the solid content in the negative electrode binder slurry.

[0060] The binder is a component that facilitates the bonding between the conductive material, the active material, and the current collector. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene, sulfonated ethylene-propylene-diene, styrene-butadiene rubber, fluorine rubber, various copolymers thereof, etc.

[0061] Generally, the binder can be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, based on the total weight of the solid content in the negative electrode mixture slurry.

[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 1 to 20 wt % based on the total weight of the solid content in the negative electrode slurry. The conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity. Examples of the conductive material include carbon powders such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, and graphite; conductive fibers such as carbon nanotubes, carbon fibers, and metal fibers; carbon fluoride powders; conductive powders such as aluminum powder 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 conductive material may be contained in an amount of 1 to 20% by weight, preferably 1 to 15% by weight, and more preferably 1 to 10% by weight, based on the total weight of the solid content in the negative electrode mixture slurry.

[0064] The solvent may include water or an organic solvent such as NMP (N-methyl-2-pyrrolidone), and may be used in an amount that provides a desired viscosity when the negative electrode active material, and optionally a binder and a conductive material, are included. For example, the solvent may be included so that the concentration of the solids, including the negative electrode active material, and optionally a binder and a conductive material, is 50 wt % to 95 wt %, preferably 70 wt % to 90 wt %.

[0065] When a metal is used as the anode, the anode can be fabricated by physically bonding, rolling, or depositing a metal thin film on the anode current collector or the metal itself. The deposition method can be an electrolytic deposition method or a chemical vapor deposition method.

[0066] For example, the metal thin film itself or the metal bonded / rolled / deposited on the negative electrode current collector may include one metal or an alloy of two metals selected from the group consisting of lithium (Li), nickel (Ni), tin (Sn), copper (Cu), and indium (In).

[0067] The thickness of the negative electrode according to the present invention can generally be in the range of 180 μm to 240 μm, preferably 200 μm to 220 μm. When the thickness of the negative electrode is in this range, it is possible to stack a large number of negative electrodes in a laminate of the same thickness, which has the advantage of increasing the energy density.

[0068] Manufacturing method of electrode assembly The electrode assembly according to the present invention exhibits the most remarkable effects, particularly in the case of a stacked or stack and lamination type structure.

[0069] The electrode assembly of the present invention can be manufactured by conventional methods well known in the art.

[0070] For example, the stack-type electrode assembly may be manufactured by sequentially stacking a positive electrode and a negative electrode with a separator interposed therebetween, and the stack-and-lamination type electrode assembly may be manufactured by sequentially stacking a positive electrode, a separator, a negative electrode, and a separator to manufacture a basic unit, repeatedly stacking the basic unit, and then laminating the stacked units by applying heat and pressure.

[0071] secondary battery Next, the secondary battery according to the present invention will be described. The secondary battery according to the present invention may include an electrode assembly including the positive electrode, the negative electrode, and a separator, and an electrolyte.

[0072] Since the electrode assembly has been described above, a detailed description will be omitted and only the remaining components will be described.

[0073] Examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in producing secondary batteries, but are not limited to these.

[0074] Specifically, the electrolyte may include an organic solvent and a lithium salt.

[0075] The organic solvent may be any solvent that can act as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of solvents that can be used include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (where R is a linear, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, which may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with low viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) that have high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of batteries, are more preferred.

[0076] The lithium salt can be any compound that can provide lithium ions used in secondary batteries without any particular limitations. Specifically, the anion of the lithium salt can be F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - The lithium salt may be at least one selected from the group consisting of: LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The concentration of the lithium salt is preferably within the range of 0.1 to 2.0 M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance and allowing lithium ions to migrate effectively.

[0077] In addition to the electrolyte components, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethyl alcohol amine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethyl alcohol, or aluminum trichloride, for the purposes of improving battery life characteristics, suppressing battery capacity reduction, and improving battery discharge capacity. Here, the additives may be contained in an amount of 0.1 to 5 wt % based on the total weight of the electrolyte.

[0078] Meanwhile, the secondary battery may be manufactured by assembling the secondary battery by housing the electrode assembly in a battery case, injecting an electrolyte solution, and then performing an activation process.

[0079] The activation process may include a process of heating the assembled secondary battery while applying pressure, i.e., a pressurized pre-heating process, prior to charging, aging, and discharging the battery. The heat and pressure process may also be performed together with initial charging.

[0080] The binder contained in the active material layer and the binder contained in the coating layer formed on the surface of the separator soften and become flexible when the secondary battery is heated. For example, when the secondary battery is pressed and the coating layer formed on the surface of the separator comes into contact with the positive or negative electrode, the surface shape of the binder-containing coating layer formed on the surface of the separator can be appropriately deformed to match the surface shape of the positive or negative electrode. When the surface shape of the coating layer deforms to match the surface shape of the positive or negative electrode, the interfacial space between the coating layer and the positive and / or negative electrode is effectively eliminated, allowing them to come into contact with each other. Furthermore, when the coating layer comes into close contact with the positive and / or negative electrode, it can exert adhesive strength to the positive and / or negative electrode.

[0081] The heating temperature for the secondary battery can be 45° C. to 80° C., specifically 55° C. to 75° C., and more specifically 65° C. to 72° C. Here, when the temperature is within the above range, there is an effect of increasing the adhesive strength to the positive electrode and / or negative electrode.

[0082] By applying pressure to the secondary battery, the gap between the separator and the electrode is reduced, i.e., gas generated by the reaction between the electrode and the electrolyte during the activation process and present between the separator and the electrode can be released and removed outside the interface.

[0083] Here, the pressure applied to the secondary battery may be a pressure applied to the secondary battery using a separate pressurizing means from outside the secondary battery. That is, according to an example of a method for manufacturing a secondary battery according to the present invention, by applying pressure to the secondary battery using a separate pressurizing means from outside the secondary battery during an activation process of the secondary battery, gas generated during the activation process can be more efficiently removed from the battery cells of the secondary battery and the separator and the electrodes can be closely attached to each other.

[0084] The pressure applied to the secondary battery during the activation process is 0.2 to 20 kgf / cm 2, specifically 0.2 to 10 kgf / cm 2 , more specifically 0.2 to 5 kgf / cm 2 Here, the pressure can be 20 kgf / cm 2 If the pressure exceeds this limit, excessive pressure may damage the battery.

[0085] A secondary battery including an electrode assembly according to the present invention has little bending of the electrode assembly after an activation process. Specifically, when the degree of bending of a fully charged electrode assembly after an activation process of a secondary battery according to the present invention is measured, when the total thickness of the electrode assembly is D, the difference in height between the center and both ends in the total length direction of the electrode assembly may be 0.03D or less, preferably 0.001D to 0.02D or less.

[0086] Although the present invention may be embodied in various different forms, it is not intended to be limited to the embodiments set forth herein, and the present invention may be practiced in various different forms, without departing from the spirit or scope of the present invention.

[0087] Example Example 1 A positive electrode with a thickness of 158 μm, a negative electrode with a thickness of 211 μm, and separators A and B each having a coating layer formed on a substrate were prepared.

[0088] The separator A is made of polyvinylidene fluoride-tetrafluoroethylene (PVdF-TFE, TFE substitution rate 20%, weight average molecular weight 300,000 g / mol), polyvinylidene fluoride-chlorotrifluoroethylene (PVDF-CTFE, CTFE substitution rate 20%), and an average particle size (D 50 ) 500 nm alumina (Al2O3) was added to N-methylpyrrolidone (NMP) solvent in a weight ratio of 15:5:80 to form a slurry, which was then coated onto a polyolefin substrate using a dipping phase separation method.

[0089] The separator B is a mixture of polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP, HFP substitution rate 8%, weight average molecular weight 350,000 g / mol), polyvinylidene fluoride-chlorotrifluoroethylene (PVDF-CTFE, CTFE substitution rate 20%), and an average particle size (D 50 ) 500 nm alumina (Al2O3) was added to acetone solvent in a weight ratio of 15:5:80 to form a slurry, which was then coated onto a polyolefin substrate using a wet phase separation method.

[0090] The temperature at which the adhesive strength of the separator A to the electrode was maximized was measured to be 70° C., and the temperature at which the adhesive strength of the separator B to the electrode was maximized was measured to be 65° C. The thickness of the separator A was 15 μm, and the thickness of the separator B was 15 μm.

[0091] Next, the positive electrode, the separator, the negative electrode, and the separator were alternately stacked to prepare an electrode assembly as shown in FIG.

[0092] First, 12 separators A 20a were stacked so as to be positioned between the positive electrode 10 and the negative electrode 30, and then 12 separators B 20b were stacked on top of that so as to be positioned between the positive electrode 10 and the negative electrode 30, and then 12 separators A 20a were stacked on top of that so as to be positioned between the positive electrode 10 and the negative electrode 30. In other words, the separator B was positioned at the center of the electrode assembly, and the separator A was positioned on the surface of the electrode assembly.

[0093] The prepared electrode assembly was placed in a pouch outer casing, and an electrolyte solution was injected to complete the assembly of a secondary battery.

[0094] Example 2 A positive electrode having a thickness of 158 μm, a negative electrode having a thickness of 211 μm, and separators A and C each having a coating layer formed on a substrate were prepared.

[0095] The separator A is made of polyvinylidene fluoride-tetrafluoroethylene (PVdF-TFE, TFE substitution rate 20%, weight average molecular weight 300,000 g / mol), polyvinylidene fluoride-chlorotrifluoroethylene (PVDF-CTFE, CTFE substitution rate 20%), and an average particle size (D 50 ) 500 nm alumina (Al2O3) was added to N-methylpyrrolidone (NMP) solvent in a weight ratio of 15:5:80 to form a slurry, which was then coated onto a polyolefin substrate using a dipping phase separation method.

[0096] The separator C is a mixture of polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP, HFP substitution rate 15%, weight average molecular weight 300,000 g / mol), polyvinylidene fluoride-chlorotrifluoroethylene (PVDF-CTFE, CTFE substitution rate 20%), and an average particle size (D 50 ) 500 nm alumina (Al2O3) was added to acetone solvent in a weight ratio of 15:5:80 to form a slurry, which was then coated onto a polyolefin substrate using a wet phase separation method.

[0097] The temperature at which the adhesive strength of the separator A to the electrode was maximized was measured to be 70° C., and the temperature at which the adhesive strength of the separator C to the electrode was maximized was measured to be 60° C. The thickness of the separator A was 15 μm, and the thickness of the separator C was 15 μm.

[0098] Twelve separators A were stacked between the positive and negative electrodes, and then 12 separators C were stacked on top of them between the positive and negative electrodes, and then 12 separators A were stacked on top of them between the positive and negative electrodes. That is, separator C was placed in the center and separator A was placed on the surface. A secondary battery was manufactured in the same manner as in Example 1, except that the type of separator was changed.

[0099] Example 3 A secondary battery was manufactured in the same manner as in Example 1, except that when manufacturing an electrode assembly by alternately stacking the positive electrode, the separator, the negative electrode, and the separator, 6 separators A, 12 separators B, and 6 separators A were used in that order.

[0100] Example 4 A secondary battery was manufactured in the same manner as in Example 1, except that when manufacturing an electrode assembly by alternately stacking the positive electrode, the separator, the negative electrode, and the separator, 12 separators A, 6 separators B, and 12 separators A were used in that order.

[0101] Comparative Example 1 A secondary battery was manufactured in the same manner as in Example 1, except that 36 separators A were used when manufacturing an electrode assembly by alternately stacking the positive electrode, the separator, the negative electrode, and the separator, and the same separator was used for the entire electrode assembly.

[0102] Experimental example - Electrode assembly bending evaluation Each of the lithium secondary batteries manufactured in Examples 1 to 4 and Comparative Example 1 was subjected to a pressurized pre-heating process using a jig formation device. Specifically, the pressurized pre-heating process was carried out at 70°C and 0.2 kgf / cm. 2 pressure of 3kgf / cm for 2 minutes at 70℃ 2 The battery was then subjected to a heating and pressurizing process for 3 minutes at a pressure of 5 kgf / cm. After that, the primary activation process was carried out in a jig. Specifically, the battery was charged to a SOC (State of Charge) of 60% at a constant current of 1 C and charged at a pressure of 5 kgf / cm. 2 The primary activation process was performed by heating at 55°C under a pressure of 1000 kJ / cm. Then, the secondary activation process was performed. Specifically, the secondary activation process was performed by fully charging to 4.25V at a constant current of 0.33C in a chamber environment at 25°C without using a jig formation device. Then, the electrode assembly was visually observed for bending and evaluated as follows.

[0103] 〇: No bending occurs. ×: Bending occurs.

[0104] [Table 1]

[0105] Figure 2 is a photograph of the lithium secondary battery of Example 1 after completion of the activation process, and Figure 3 is a photograph of the lithium secondary battery of Comparative Example 1 after completion of the activation process. As shown in Figure 2, in Example 1, in which the separator arrangements at the center and outer periphery were adjusted, bending was not observed. On the other hand, as shown in Figure 3, bending of the electrode assembly was observed in Comparative Example 1. The bending of the electrode assembly of Comparative Example 1 was measured as a difference in height between the center and both ends in the full length direction, with D being 0.031D, relative to the thickness D of the electrode assembly. [Explanation of symbols]

[0106] 10: Positive electrode 20a: Separator A 20b: Separator B 30: Negative electrode D: Total thickness of the electrode assembly

Claims

1. An electrode assembly in which positive electrodes and negative electrodes are alternately stacked with separators interposed therebetween, and the electrode assembly includes two or more types of separators that have different temperatures at which their adhesive strength becomes maximum, the temperature at which the adhesive strength of the separator located at the center of the electrode assembly to the electrode is maximized is lower than the temperature at which the adhesive strength of the separator located in the remaining region to the electrode is maximized; The number of separators included in the electrode assembly is 20 to 50, The temperature at which the adhesive strength of the separator located at the center of the electrode assembly becomes maximum is 50°C or more and less than 70°C, The temperature at which the adhesive strength of the separator located in the remaining region becomes maximum is 70°C or more and 80°C or less.

2. 2. The electrode assembly according to claim 1, wherein the central portion is a region whose distance from the outermost surface of the electrode assembly is D / 2-kD to D / 2+kD (where 0.1≦k≦0.3), where D is the total thickness of the electrode assembly.

3. 2. The electrode assembly according to claim 1, wherein a difference between a temperature at which the adhesive strength of the separator located at the center of the electrode assembly to the electrode is maximized and a temperature at which the adhesive strength of the separator located in the remaining region to the electrode is maximized is greater than 0°C and not greater than 15°C.

4. 3. The electrode assembly of claim 2, wherein a ratio of the number of separators located in an area that is a distance from the outermost surface of the electrode assembly that is D / 2-kD to D / 2+kD to the number of separators included in the entire electrode assembly is 1:1 to 1:

4.

5. 2. The electrode assembly of claim 1, wherein the separator included in the electrode assembly comprises a porous separator substrate and a porous coating layer entirely coated on one or both sides of the separator substrate, the coating layer comprising a mixture of inorganic particles selected from metal oxides, semi-metal oxides, metal fluorides, metal hydroxides, and combinations thereof, and a binder polymer that binds and fixes the inorganic particles to each other.

6. The coating layer contains Al as inorganic particles. ２ O ３ 6. The electrode assembly of claim 5, comprising one or more selected from the group consisting of AlOOH and AlOOH.

7. 6. The electrode assembly of claim 5, wherein the coating layer includes at least one binder polymer selected from the group consisting of polyethylene, polyvinylidene fluoride, polyvinylidene fluoride-tetrafluoroethylene, polyvinylidene fluoride-chlorotrifluoroethylene, and polyvinylidene fluoride-hexafluoropropylene.

8. A secondary battery comprising the electrode assembly according to any one of claims 1 to 7.

9. The secondary battery of claim 8 , wherein when the electrode assembly has a total thickness D, the difference in height between the center and both ends in the total length direction is 0.03D or less.

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