Electrode assembly and secondary battery including the same

Abstract

The present invention relates to an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the dry adhesive strength of the separator to the negative electrode is greater than the dry adhesive strength of the separator to the positive electrode.

Description

[Technical Field]

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2023-0140906, filed with the Korean Intellectual Property Office on October 20, 2023, the entire contents of which are incorporated herein by reference.

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

[0003] Secondary batteries are classified into coin-type batteries, cylindrical batteries, prismatic batteries, and pouch-type batteries depending on the shape of the battery case. Unlike primary batteries, secondary batteries are rechargeable and have been the subject of much research and development in recent years due to the potential for miniaturization and increased capacity.

[0004] Recently, as the popularity of electric vehicles has increased, technological development and demand for high-capacity secondary batteries for use in electric vehicles has increased. To manufacture high-capacity secondary batteries, the size of electrode assemblies mounted inside battery cases has also increased.

[0005] The electrode assembly is a chargeable and dischargeable power generating element consisting of a laminated structure of electrodes and a separator. The separator includes an organic / inorganic composite porous coating layer, which provides adhesion between the electrodes and the separator.

[0006] The electrode assembly is mounted inside a battery case and then undergoes a packaging process and / or an activation process to manufacture a secondary battery. However, after the activation process, the expansion rate of the negative electrode is greater than that of the positive electrode, which causes uneven adhesion between the separator and the electrode, resulting in a bending phenomenon in which the electrode assembly bends or warps, which can cause deformation of the shape of the secondary battery itself.

[0007] Therefore, there is a need to develop a technology to prevent bending of the electrode assembly or the secondary battery including the electrode assembly. Summary of the Invention [Problem to be solved by the invention]

[0008] In view of the above-mentioned problems of the prior art, the present invention provides an electrode assembly and a secondary battery that can prevent bending of the electrode assembly or a secondary battery including the electrode assembly. [Means for solving the problem]

[0009] One embodiment of the present invention provides an electrode assembly including a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, wherein the dry adhesive strength of the separator to the negative electrode is greater than the dry adhesive strength of the separator to the positive electrode.

[0010] One embodiment of the present invention provides a secondary battery including a sealed battery case and an electrode assembly according to the present invention inside the battery case. [Effects of the Invention]

[0011] The electrode assembly according to the embodiment of the present invention has an effect of preventing bending that may occur after activation of a secondary battery including the electrode assembly.

[0012] The electrode assembly according to the embodiment of the present invention prevents bending, so that the electrodes can be aligned and fixed without being displaced, thereby improving energy density. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a graph showing the results of measuring the dry adhesive strength of the separator to the positive electrode, the dry adhesive strength of the separator to the negative electrode, the wet adhesive strength of the separator to the positive electrode, and the wet adhesive strength of the separator to the negative electrode according to Example 1 and Comparative Example 1. [Figure 2]3 is a photograph showing the interfaces between the negative electrode, positive electrode, and separator of each of the secondary batteries of Example 1 and Comparative Example 1 after activation. DETAILED DESCRIPTION OF THE INVENTION

[0014] Although the present invention may be embodied in many different forms, it is not intended to be limited to the embodiments set forth herein, and the present invention will be described in detail below so as to enable those skilled in the art to easily practice the present invention.

[0015] In this specification, when a part is said to "comprise" a certain component, this does not mean that it may further include other components, unless specifically stated to the contrary, but rather that it does not exclude other components.

[0016] As used herein, the term "dry adhesive strength" refers to the adhesive strength of a separator measured when the electrode assembly including the separator is not immersed in an electrolyte. That is, it refers to the adhesive strength of a separator measured before the activation step of the electrode assembly. As used herein, the term "dry adhesive strength" also refers to the adhesive strength of a separator in a dry state.

[0017] As used herein, the term "wet adhesive strength" refers to the adhesive strength of a separator measured when an electrode assembly including the separator is immersed in an electrolyte. That is, it refers to the adhesive strength of a separator measured after activating the electrode assembly. As used herein, the term "wet adhesive strength" also refers to the adhesive strength of a separator when immersed in an electrolyte.

[0018] In this specification, the dry adhesive strength and wet adhesive strength are measured by cutting the electrode assembly into a size of 55 mm wide and 20 mm long to prepare a test specimen in which the positive electrode, separator, and negative electrode are stacked.The separator of the test specimen is then peeled from one side of the separator in a 90° peel mode using a tensile tester (UTM equipment) at a rate of 100 mm / min. That is, the measurement methods for the dry adhesive strength and wet adhesive strength are the same except for whether the electrode assembly is impregnated with an electrolyte.The absolute value of the difference between the measured adhesive strengths can be defined as the variation in adhesive strength.

[0019] In this specification, "activation" or "activation process" of a secondary battery refers to a process (or step) of activating the secondary battery and removing gas through a charging process of the secondary battery.

[0020] <Electrode assembly> After the activation process, the negative electrode and the positive electrode each expand. The expansion rates of the negative electrode and the positive electrode differ after the activation process. This difference in expansion rates between the negative electrode and the positive electrode can cause uneven adhesion between the separator and the electrode, resulting in bending, or bowing of the electrode assembly.

[0021] Therefore, one embodiment of the present invention provides an electrode assembly in which the dry adhesive strength of the separator to the negative electrode is greater than the dry adhesive strength of the separator to the positive electrode, thereby preventing bending after an activation process of a secondary battery including the electrode assembly and also preventing a dogbone shape from appearing at the interface between the separator and the electrode after an activation process of the secondary battery.

[0022] As a result, it is possible to provide a secondary battery including an electrode assembly with excellent performance.

[0023] In one embodiment of the present invention, the dry adhesive strength of the separator to the negative electrode may be 0.5 gf / 20 mm to 30 gf / 20 mm, preferably 3 gf / 20 mm or more and 25 gf / 20 mm or less, and more preferably 5 gf / 20 mm or more and 15 gf / 20 mm or less.

[0024] In one embodiment of the present invention, the dry adhesive strength of the separator to the positive electrode may be 0.2 gf / 20 mm to 20 gf / 20 mm, preferably 1 gf / 20 mm or more and 15 gf / 20 mm or less, and more preferably 1.5 gf / 20 mm or more and 10 gf / 20 mm or less.

[0025] In one embodiment of the present invention, the dry adhesive strength of the separator to the negative electrode may be 1.5 to 5 times, preferably 1.5 to 2 times or 2.5 to 5 times, the magnitude of the dry adhesive strength of the separator to the positive electrode.

[0026] In one embodiment of the present invention, the variation between the dry adhesive strength of the separator to the negative electrode and the dry adhesive strength of the separator to the positive electrode may be 10 gf / 20 mm or less, preferably 8 gf / 20 mm or less, more preferably 5 gf / 20 mm or less, where the variation refers to the absolute value of the difference in adhesive strength.

[0027] When the dry adhesive strength of the separator to the positive and negative electrodes satisfies the above range, bending after an activation process can be prevented, and the stacked structure of the electrode assembly manufactured in the stacking process can be maintained and transported more easily. That is, deterioration in performance of the electrode assembly due to misalignment of the electrodes during transportation can be prevented, resulting in superior performance of the manufactured electrode assembly.

[0028] In one embodiment of the present invention, the wet adhesive strength of the separator to the positive electrode may be greater than the wet adhesive strength of the separator to the negative electrode, i.e., the adhesive strength of the separator to the electrode in a state immersed in the electrolyte may be greater in the case of the positive electrode than in the case of the negative electrode.

[0029] In one embodiment of the present invention, the wet adhesive strength of the separator to the positive electrode may be 10 gf / 20 mm or more and 30 gf / 20 mm or less, preferably 10 gf / 20 mm or more and 20 gf / 20 mm or less.

[0030] In one embodiment of the present invention, the wet adhesive strength of the separator to the negative electrode may be 3 gf / 20 mm or more and 15 gf / 20 mm or less, preferably 5 gf / 20 mm or more and 10 gf / 20 mm or less.

[0031] In one embodiment of the present invention, the wet adhesive strength of the separator to the negative electrode may be 2 gf / 20 mm or more and 15 gf / 20 mm or less, more preferably 2.5 gf / 20 mm or more and 8 gf / 20 mm or less.

[0032] In one embodiment of the present invention, the wet adhesive strength of the separator to the positive electrode may be 1.5 to 3 times, preferably 1.5 to 2.5 times, greater than the wet adhesive strength of the separator to the negative electrode.

[0033] In one embodiment of the present invention, the variation between the wet adhesive strength of the separator to the positive electrode and the wet adhesive strength of the separator to the negative electrode may be 8 gf / 20 mm or less, preferably 5 gf / 20 mm or less, where the variation refers to the absolute value of the difference in adhesive strength.

[0034] When the wet adhesive strength satisfies the above range, it is easy to prevent bending of a secondary battery including the electrode assembly during an activation process of the secondary battery, thereby improving the performance of the secondary battery.

[0035] That is, the electrode assembly according to the present invention is characterized in that the separator has a constant adhesive strength to the negative electrode before and after immersion in the electrolyte, and the relative adhesive strength of the separator to the positive electrode and the negative electrode is different before and after immersion in the electrolyte, thereby preventing bending of the secondary battery after the activation process.

[0036] In one embodiment of the present invention, the separator may include a porous polymer substrate, a first organic / inorganic composite porous coating layer formed on one side of the polymer substrate, and a second organic / inorganic composite porous coating layer formed on the other side of the polymer substrate. That is, the separator may include different organic / inorganic composite porous coating layers on both sides.

[0037] In one embodiment of the present invention, the first organic / inorganic composite porous coating layer and the second organic / inorganic composite porous coating layer may each contain one or more types of particulate binder resin and one or more types of inorganic particles.

[0038] In one embodiment of the present invention, the first organic / inorganic composite porous coating layer refers to a layer in contact with a negative electrode, and the second organic / inorganic composite porous coating layer refers to a layer in contact with a positive electrode.

[0039] In one embodiment of the present invention, the first organic / inorganic composite porous coating layer may be a cured product of a first composition including a particulate binder resin and inorganic particles, and may include 50 to 80 parts by weight, preferably 60 to 70 parts by weight, of the particulate binder resin based on 100 parts by weight of the first composition.

[0040] In one embodiment of the present invention, the second organic / inorganic composite porous coating layer may be a cured product of a second composition including a particulate binder resin and inorganic particles, and may include 10 parts by weight to 40 parts by weight, preferably 15 parts by weight to 35 parts by weight of the particulate binder resin based on 100 parts by weight of the second composition.

[0041] In one embodiment of the present invention, the particulate binder resin may include one or more selected from the group consisting of acrylic polymer particles, fluorine-containing polymer particles, and hybrid polymer particles of fluorine-containing polymer and acrylic polymer. Specifically, the hybrid polymer particles of fluorine-containing polymer and acrylic polymer may contain the fluorine-containing polymer and the acrylic polymer in a weight ratio of 50:50 to 80:20.

[0042] The binder resin contains hybrid polymer particles of a fluorine-based polymer and an acrylic polymer, which allows the separator to maintain a certain level of adhesive strength before and after impregnation with an electrolyte, thereby improving the shape stability of the electrode assembly and making it easier to provide electrode assemblies with uniform performance.

[0043] In one embodiment of the present invention, the fluoropolymer may be a homopolymer of vinylidene fluoride (PVDF), a copolymer of vinylidene fluoride and another polymerizable monomer, or a mixture of two or more of these.

[0044] More specifically, the vinylidene fluoride and other polymerizable monomers may include, but are not limited to, one or more selected from the group consisting of tetrafluoroethylene, hexafluoropropylene, trifluoroethylene, chlorofluoroethylene, 1,2-difluoroethylene, perfluoro(methyl vinyl) ether, perfluoro(ethyl vinyl) ether, perfluoro(propyl vinyl) ether, difluoro(1,3-dioxole), perfluoro(2,2-dimethyl-1,3-dioxole), trichloroethylene, and vinyl fluoride. In particular, the fluorine-based polymer may be a copolymer of vinylidene fluoride and hexafluoropropylene (HFP). The content of vinylidene fluoride and other polymerizable monomers in the copolymer may be, but is not limited to, 1% by weight to 20% by weight, preferably 1% by weight to 5% by weight. The other polymerizable monomer is used to increase wet adhesion. If the content is less than 1 wt %, wet adhesion may be difficult to achieve. If the content exceeds the above range, the resistance of the separator may become too high, which may reduce the performance of the electrode assembly.

[0045] In the present invention, the comonomer content in the PVDF-based polymer can be measured by H-NMR using a Varian 500 MHz NMR. For details of the measurement method, see Journal of Materials Chemistry, 2012, 22, 341 or AMT-3412-0k. The NMR spectrum can be confirmed using appropriate equipment, such as a Bruker Avance III HD 700 MHz NMR or a Varian 500 MHz NMR.

[0046] In the present invention, the acrylic polymer may preferably be a (meth)acrylic acid ester or an acrylic-styrene copolymer. Specific examples of such (meth)acrylic acid esters include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, n-amyl (meth)acrylate, i-amyl (meth)acrylate, hexyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, nonyl (meth)acrylate, and (meth)acrylate. Examples of suitable acrylates include decyl acrylate, hydroxymethyl (meth)acrylate, hydroxyethyl (meth)acrylate, ethylene glycol (meth)acrylate, ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate, allyl (meth)acrylate, and ethylene di(meth)acrylate, and one or more selected from these may be used. Among these, one or more selected from methyl (meth)acrylate, ethyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate are preferred, with methyl (meth)acrylate being particularly preferred.

[0047] More specifically, the acrylic-styrene copolymer may include an acrylic binder, and the acrylic binder may be a polyacrylate. For example, the binder may be one or more selected from the group consisting of styrene-butadiene rubber, nitrile-butadiene rubber, acrylonitrile-butadiene rubber, acrylonitrile-butadiene-styrene rubber, and acrylate-based polymers, and more specifically, may be a copolymer containing acrylate.

[0048] In one embodiment of the present invention, the first organic / inorganic composite porous coating layer and the second organic / inorganic composite porous coating layer may each contain a first binder resin containing the acrylic polymer particles and a second binder resin containing mixed polymer particles of a fluorine-based polymer and an acrylic polymer.

[0049] In one embodiment of the present invention, the glass transition temperature (Tg) of the acrylic polymer particles may be 40°C to 90°C, preferably 40°C to 60°C.

[0050] In one embodiment of the present invention, the inorganic particles may have a non-rectangular structure. When inorganic particles having this structure are used, the separator has an appropriate air permeability without interfering with the adhesive strength of the separator.

[0051] Specifically, in one embodiment of the present invention, the inorganic particles may be Al2O3.

[0052] In one embodiment of the present invention, the separator may be stacked in a zigzag folded state, that is, the electrode assembly according to one embodiment of the present invention may be zigzag stacked.

[0053] In this specification, stacking the first electrodes and the second electrodes alternately between the folded separator is referred to as zigzag stacking.

[0054] In this regard, the configuration in which the first electrodes and the second electrodes are alternately disposed between the folded separators may be described in more detail as a separator in which the separators are stacked in a zigzag pattern. More specifically, the separators are folded alternately back and forth between the left and right sides of the stacking axis, and stacked in a zigzag pattern, with the first electrodes and the second electrodes alternately disposed between the stacked separators. Here, the stacking axis refers to an imaginary axis that is parallel to the direction in which the first electrodes, separators, and second electrodes are stacked and passes through the center of the stack of electrodes and separators.

[0055] In other words, the fact that the first electrodes and the second electrodes are alternately arranged between the separation membranes means that the separation membranes are stacked in a zigzag pattern in the direction of the stacking axis, and one first electrode and one second electrode are alternately inserted into the space (between the separation membranes) created by the overlapping separation membranes.

[0056] For zig zag stacking, techniques or devices commonly used in the art may be used.

[0057] In one embodiment of the present invention, the negative electrode may include at least one of graphite and a silicon-based compound. The silicon-based compound may be called a silicon-based active material, and may include, but is not limited to, SiO2, and any silicon-based compound commonly used in the art may be used.

[0058] More specifically, in one embodiment of the present invention, the negative electrode includes a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector and containing a negative electrode active material, a binder polymer, and a conductive material, the negative electrode active material layer including a lower layer region in surface contact with the negative electrode current collector and an upper layer region that is in surface contact with the lower layer region and extends to the surface of the negative electrode active material layer, and the lower layer region and the upper layer region may each independently contain at least one of graphite and a silicon-based compound as the negative electrode active material.

[0059] In one embodiment of the present invention, the silicon-based compound may have a morphology in which the content thereof increases with increasing distance from the surface of the negative electrode current collector in the negative electrode active material layer.

[0060] <Secondary battery> According to one embodiment of the present invention, there is provided a secondary battery including a sealed battery case and an electrode assembly according to the present invention disposed inside the battery case. The secondary battery according to the present invention is characterized in that bending does not occur after an activation process.

[0061] Specifically, a flat ruler (hereinafter, "ruler a") is placed from one end of the secondary battery to the other end, and another flat ruler (hereinafter, "ruler b") is used to measure the maximum distance from the surface of the secondary battery to the flat surface on which the secondary battery is placed. In this case, if the distance between the concave part of the secondary battery measured using ruler b and one surface of ruler a is less than 3 mm, it is defined as no bending.

[0062] The secondary battery according to an embodiment of the present invention may have an electrolyte injected into the battery case.

[0063] The secondary battery according to an embodiment of the present invention may be subjected to an activation process. The secondary battery according to the present invention may be free of bending after the activation process. In this case, the electrode assembly removed by opening the battery case after the activation process may also be free of bending.

[0064] In one embodiment of the present invention, the activation step of the secondary battery is performed under a temperature condition of 45°C to 55°C and a pressure of 0.5 kgf / cm 2 ~1.5kgf / cm 2 The secondary battery may be charged at 0.3 C to 0.8 C at least once under the above pressure condition.

[0065] Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and it will be obvious to those skilled in the art that various modifications and variations are possible within the scope that does not deviate from the technical idea of ​​the present invention described in the claims. [Example]

[0066] 1) Example 1 <Manufacturing of electrode assembly> To fabricate the negative electrode, a slurry for the first active material layer was prepared by mixing a mixture of artificial graphite as the carbon-based active material, a binder polymer (SBR (styrene-butadiene rubber) and CMC (carboxymethyl cellulose)), and carbon black as the conductive material in a weight ratio of 95.5:3:1.5 with water as the dispersant in a weight ratio of 1:2. As the silicon-based active material, SiO2 (silicon oxide), which has a Coulombic efficiency of 80% or higher when charged and discharged at 0.1 C, was prepared. The slurry for the second active material layer was identical to that for the first active material layer, except that the carbon-based active material and the silicon-based active material were mixed in a weight ratio of 9:1. That is, the weight ratio of the active material, binder polymer, carbon black, and carbon nanotubes (CNTs) was 95.5:3:1.0:0.5, and the weight ratio of the carbon-based active material to the silicon-based active material in the active material was 9:1.

[0067] The specific surface area of ​​the carbon nanotube (CNT) is 550m 2 The carbon nanotubes (CNTs) were multi-walled carbon nanotubes.

[0068] The slurry for the first active material layer was coated on one side of a copper (Cu) thin film, which was a 10 μm thick negative electrode current collector, using a double slot die. Subsequently, the slurry for the second active material layer was coated on the slurry for the first active material layer, and dried at 130° C. under vacuum for 1 hour to form a first active material layer and a second active material layer.

[0069] The first and second active material layers thus formed were simultaneously rolled using a roll pressing method to fabricate a negative electrode having a double-layered active material layer with a thickness of 80 μm. The thickness ratio of the first and second active material layers was 1:1. The loading amount of the negative electrode active material layer was 512 mg / 25 cm based on the dry weight. 2 (dry basis: 292 μm).

[0070] Next, to manufacture the positive electrode, Li(Ni 0.3 Mn 0.5 Co 0.2 A positive electrode active material slurry was prepared by adding 02 (NCM-352), carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder to N-methylpyrrolidone (NMP) as a solvent in a weight ratio of 96:2:2. The slurry was coated on one side of an aluminum current collector with a thickness of 15 μm, and dried and rolled under the same conditions as the negative electrode to prepare a positive electrode. In this case, the loading amount of the positive electrode active material layer based on the dry weight was 988 mg / cm. 2 (dry basis: 242 μm).

[0071] Finally, a polyethylene film (PE, thickness 9 μm) was prepared as a porous polymer substrate to fabricate a separation membrane.

[0072] Thereafter, a first composition was prepared by adding a first binder, inorganic particles, and a dispersant to distilled water at room temperature, and a second composition was prepared by adding a second binder, inorganic particles, and a dispersant to distilled water at room temperature.

[0073] In this case, the first binder was a copolymer of styrene and butyl acrylate, in which the weight ratio of the two monomers (styrene and butyl acrylate) was adjusted to 80:20, and the first binder had a glass transition temperature (Tg) of 45°C.

[0074] As the second binder, a PVDF-HFP binder containing 5% by weight of repeating units derived from HFP was used.

[0075] In addition, Al2O3 was used as the inorganic particles.

[0076] Then, 0.7 μm zirconia beads were added to the distilled water in the same amount as the inorganic particles, and the mixture was bead milled using a paint shaker for 2 hours to prepare an inorganic dispersion.

[0077] Next, a surfactant was further added to the inorganic dispersion, and the inorganic dispersion was stirred at 10 rpm for 10 minutes to prepare a first slurry and a second slurry in which the first binder particles and the second binder particles were dispersed, respectively.

[0078] In this case, the first binder is contained in an amount of 60 parts by weight based on 100 parts by weight of the first slurry, and the second binder is contained in an amount of 20 parts by weight based on 100 parts by weight of the second slurry.

[0079] Thereafter, the first slurry and the second slurry were coated on both sides of the polyethylene film (PE, thickness 9 μm) using a doctor blade, respectively, and dried using a hot air blower to form porous coating layers each having a thickness of 2.5 μm, thereby producing a separator of Example 1 having a total thickness of 14 μm.

[0080] The fabricated positive electrodes, negative electrodes, and separators were fed to a stacking table, and the separator was folded to stack the positive electrodes, negative electrodes, and separators. Specifically, the positive electrodes and negative electrodes were stacked on the stacking table so that they were alternately arranged between the folded separators, producing a stack using 39 electrodes. The stack was then heated and pressurized at a temperature of 60°C and a pressure of 2 MPa for 15 seconds (time conditions) to produce the electrode assembly of Example 1.

[0081] The dry adhesive strength to the negative electrode and the dry adhesive strength to the positive electrode of the electrode assembly of Example 1 were measured, and the results are shown in FIG.

[0082] Specifically, the dry adhesive strength of the electrode assembly of Example 1 to the negative electrode and the dry adhesive strength of the electrode assembly to the positive electrode were measured by cutting a portion of the electrode assembly to a size of 20 mm x 70 mm before immersing the electrode assembly in the electrolyte solution, and preparing a test specimen in which the positive electrode, separator, and negative electrode were stacked. The separator of the test specimen was then peeled from one side of the separator in a 90° peeling mode at a rate of 100 mm / min using a tensile tester (UTM equipment).

[0083] <Secondary battery manufacturing> The electrode assembly of Example 1 was placed in a pouch exterior material (battery case) of CPP / aluminum / nylon laminate sheet, an ethyl methyl carbonate electrolyte containing LiPF6 was injected, and the pouch exterior material was heat-sealed to complete the assembly of the secondary battery.

[0084] After that, the secondary battery was subjected to a temperature condition of 50°C and 1.0 kgf / cm 2 The secondary battery was activated by charging it at 0.5 C under the pressure condition.

[0085] Comparative Example 1 An electrode assembly and a secondary battery were manufactured in the same manner as in Example 1, except that the binder contained in the slurry coated on both sides of a polyethylene film (PE, thickness 9 μm) was a PVdF-based binder, and the content of the PVdF-based binder was 50 parts by weight based on 100 parts by weight of the slurry.

[0086] Experimental Example 1 - Evaluation of dry adhesive strength Each electrode assembly of Example 1 and Comparative Example 1 was cut into a size of 55 mm wide and 20 mm long to prepare a test specimen in which a positive electrode, separator, and negative electrode were stacked. Double-sided tape was attached to a glass slide, and the positive or negative electrode of the sample was attached to the double-sided tape. The sample was then pressed with a certain pressure to adhere to the double-sided tape. Then, a 90° peel test was performed to measure the dry adhesive strength of the separator to the positive electrode and the dry adhesive strength of the separator to the negative electrode.

[0087] Specifically, the dry adhesive strength of the separator to the positive electrode and the dry adhesive strength of the separator to the negative electrode were measured by pulling the separator at a speed of 100 mm / min using a tensile tester (UTM equipment) to measure the force at which the separator peeled from the positive electrode and the negative electrode, respectively.

[0088] Experimental Example 2: Evaluation of wet adhesive strength After activation in Example 1 and Comparative Example 1, the electrode assemblies were cut into pieces measuring 55 mm wide and 20 mm long to prepare test specimens in which a positive electrode, separator, and negative electrode were stacked. Double-sided tape was attached to a glass slide, and the positive or negative electrode of the sample was attached to the double-sided tape. The sample was then pressed with a certain pressure to adhere to the double-sided tape. Then, a 90° peel test was performed to measure the wet adhesion strength of the separator to the positive electrode and the wet adhesion strength of the separator to the negative electrode.

[0089] Specifically, the wet adhesive strength of the separator to the positive electrode and the wet adhesive strength of the separator to the negative electrode were measured by pulling the separator at a speed of 100 mm / min using a tensile tester (UTM equipment) to measure the force at which the separator peeled from the positive electrode and the negative electrode, respectively.

[0090] 1, in Example 1, before activation, the dry adhesion strength of the separator to the positive electrode was 0.5 gf / 20 mm and the dry adhesion strength of the separator to the negative electrode was 9.3 gf / 20 mm, indicating that the dry adhesion strength of the separator to the negative electrode was higher than the dry adhesion strength of the separator to the positive electrode. After the secondary battery was activated in a high-temperature and high-pressure environment, the wet adhesion strength of the separator to the positive electrode in Example 1 increased to 10.1 gf / 20 mm and the wet adhesion strength of the separator to the negative electrode decreased to 5.8 gf / 20 mm, indicating that the wet adhesion strength of the separator to the positive electrode and the wet adhesion strength of the separator to the negative electrode were balanced.

[0091] In contrast, in Comparative Example 1, the dry adhesion strength of the separator to the negative electrode before activation was 0.5 gf / 20 mm and the dry adhesion strength of the separator to the positive electrode was 11.3 gf / 20 mm, indicating a high dry adhesion strength of the separator to the positive electrode. After the secondary battery was activated in a high-temperature and high-pressure environment, the wet adhesion strength of the separator to the positive electrode in Comparative Example 1 decreased to 9.1 gf / 20 mm and the wet adhesion strength of the separator to the negative electrode increased to 0.9 gf / 20 mm, indicating a large variation and imbalance between the wet adhesion strength of the separator to the positive electrode and the wet adhesion strength of the separator to the negative electrode.

[0092] Referring to FIG. 2, it can be seen that the dog bone phenomenon and bending phenomenon did not occur in Example 1, which has uniform wet adhesive strength, whereas the dog bone phenomenon and bending phenomenon occurred in the electrode assembly of Comparative Example 1, which has non-uniform wet adhesive strength.

[0093] This indicates that when the dry adhesive strength of the separator to the negative electrode is higher than the dry adhesive strength of the separator to the positive electrode, the wetting of the electrolyte is excellent.

[0094] While the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various modifications and variations of the present invention may be made without departing from the spirit and scope of the present invention as set forth in the following claims.

Claims

1. a positive electrode, a negative electrode, and a separator provided between the positive electrode and the negative electrode; The electrode assembly, wherein the dry adhesive strength of the separator to the negative electrode is greater than the dry adhesive strength of the separator to the positive electrode.

2. 2. The electrode assembly of claim 1, wherein the separator has a dry adhesive strength to the negative electrode of 0.5 gf / 20 mm to 30 gf / 20 mm.

3. 2. The electrode assembly of claim 1, wherein the separator has a dry adhesive strength to the positive electrode of 0.2 gf / 20 mm to 20 gf / 20 mm.

4. 2. The electrode assembly of claim 1, wherein the dry adhesive strength of the separator to the negative electrode is 1.5 to 5 times greater than the dry adhesive strength of the separator to the positive electrode.

5. The separation membrane is a porous polymer substrate; a first organic / inorganic composite porous coating layer formed on one surface of the polymer substrate; and a second organic / inorganic composite porous coating layer formed on the other surface of the polymer substrate; The electrode assembly of claim 1 , comprising:

6. the first organic / inorganic composite porous coating layer and the second organic / inorganic composite porous coating layer each contain a particulate binder resin and inorganic particles; 6. The electrode assembly of claim 5, wherein the particulate binder resin comprises at least one selected from the group consisting of acrylic polymer particles, fluorine-based polymer particles, and hybrid polymer particles of fluorine-based polymer and acrylic polymer.

7. the first organic / inorganic composite porous coating layer is a cured product of a first composition including a particulate binder resin and inorganic particles; The electrode assembly of claim 5, wherein the particulate binder resin is contained in an amount of 50 to 80 parts by weight based on 100 parts by weight of the first composition.

8. the second organic / inorganic composite porous coating layer is a cured product of a second composition including a particulate binder resin and inorganic particles; The electrode assembly of claim 5, wherein the particulate binder resin is contained in an amount of 10 to 40 parts by weight based on 100 parts by weight of the second composition.

9. The electrode assembly of claim 1 , wherein the separator is stacked in a zigzag folded state.

10. a sealed battery case; and The electrode assembly according to any one of claims 1 to 9 is disposed inside the battery case. A secondary battery comprising:

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