Pouch for secondary battery and lithium secondary battery including the same

Abstract

The present invention relates to a pouch for a secondary battery, and a lithium secondary battery including the pouch, which includes a barrier layer, a base layer disposed on one side of the barrier layer, and a sealant layer disposed on the other side of the barrier layer, wherein the sealant layer includes a first sealant layer disposed so that one side thereof contacts the other side of the barrier layer, and a second sealant layer disposed on the other side of the first sealant layer, and the sealant layer has a melt flow rate (MFR) of 14.0 g / 10 min or less measured under conditions of 230°C and a load of 2.16 kg.

Description

[Technical Field]

[0001] The present invention relates to a pouch for a secondary battery and a lithium secondary battery including the same, and more particularly to a pouch for a secondary battery that has excellent high-temperature reliability due to a high cell vent pressure resistance and little occurrence of venting in high-temperature environments, and a lithium secondary battery including the same. [Background technology]

[0002] Lithium secondary batteries are generally manufactured by coating a positive electrode active material slurry on a positive electrode current collector and a negative electrode current collector to form a positive electrode and a negative electrode, laminating these on both sides of a separator to form an electrode assembly of a predetermined shape, and then placing the electrode assembly in a pouch and injecting an electrolyte solution.

[0003] Secondary batteries are divided into pouch types and can types depending on the material of the case that houses the electrode assembly. Pouch-type secondary batteries are manufactured by pressing a flexible pouch film laminate to form a cup, placing an electrode assembly in the cup, injecting electrolyte, and then sealing the seal. Can-type secondary batteries are manufactured by placing an electrode assembly in a metal can, injecting electrolyte, and then assembling a top cap on the top of the can to seal it.

[0004] Pouch-type secondary batteries have the advantages of being lightweight, having excellent space utilization, and being able to achieve high energy density using a stacked electrode assembly, and are therefore widely used in high-capacity batteries such as electric vehicle batteries. However, compared to can-type secondary batteries, pouch-type secondary batteries have the disadvantage of being more vulnerable to fire, explosion, electrolyte leakage, etc. when subjected to external impact or when the internal temperature or pressure is high, and have lower durability. When used as a power source for electric vehicles, secondary batteries require excellent safety features to protect passengers.

[0005] Furthermore, when pouch-type secondary batteries are stored at high temperatures, gas is generated through a side reaction with the electrolyte, which increases the internal pressure of the battery and causes the pouch seal to burst, resulting in a vent phenomenon. If venting occurs early, the safety and reliability of the secondary battery will be significantly reduced. Therefore, there is a need for the development of a pouch-type secondary battery that has high cell vent resistance, is less likely to vent in high-temperature environments, and has excellent high-temperature reliability. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention is intended to solve the above problems, and provides a pouch having a specific melt flow rate, including a sealant layer formed in a two-layer structure, high cell vent pressure resistance, and excellent high-temperature reliability, and a lithium secondary battery including the pouch. [Means for solving the problem]

[0007] According to one embodiment, the present invention provides a pouch for a secondary battery, comprising: a barrier layer; a substrate layer disposed on one side of the barrier layer; and a sealant layer disposed on the other side of the barrier layer, the sealant layer comprising a first sealant layer disposed so as to be in direct contact with the other side of the barrier layer; and a second sealant layer disposed on the first sealant layer, wherein the sealant layer has a melt flow rate (MFR) of 14.0 g / 10 min or less, preferably 8.5 g / 10 min to 14.0 g / 10 min, and more preferably 9.0 g / 10 min to 13.5 g / min, measured under conditions of 230°C and a load of 2.16 kg.

[0008] Here, the first sealant layer and the second sealant layer may have a co-extruded structure.

[0009] The first sealant layer may include an acid-modified polyolefin resin, and the second sealant layer may include a polyolefin resin.

[0010] The ratio of the thickness of the second sealant layer to the thickness of the first sealant layer may be 0.8 to 1.2, preferably 0.9 to 1.1. For example, the thickness of the first sealant layer may be 25 μm to 80 μm, preferably 30 μm to 70 μm, and more preferably 30 μm to 60 μm, and the thickness of the second sealant layer may be 20 μm to 80 μm, preferably 25 μm to 70 μm, and more preferably 30 μm to 60 μm.

[0011] The total thickness of the sealant layer, which is the sum of the first sealant layer and the second sealant layer, can be 45 μm to 100 μm, preferably 50 μm to 100 μm, more preferably 60 μm to 100 μm, and even more preferably 70 μm to 90 μm.

[0012] Meanwhile, the barrier layer may include an aluminum alloy layer.

[0013] The substrate layer may contain one or more materials selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, and Teflon (registered trademark).

[0014] According to another embodiment, the present invention provides a lithium secondary battery including an electrode assembly formed by stacking a positive electrode, a separator, and a negative electrode, an electrolyte, and a pouch-type battery case that houses the electrode assembly and the electrolyte, wherein the battery case is the pouch according to the present invention.

[0015] The lithium secondary battery may have a cell vent pressure resistance at 60°C of 7.7 bar or more, preferably 7.7 bar to 15 bar, and more preferably 8 bar to 15 bar.

[0016] The lithium secondary battery may have an accelerated high-temperature storage period of 15 days or more, preferably 15 to 30 days, and more preferably 15 to 25 days, as measured at a temperature of 70°C while charging to 100% SOC at daily intervals.

[0017] According to yet another embodiment, the present invention provides a method for manufacturing a pouch for a secondary battery, the method comprising the steps of: laminating a substrate layer on a first surface of a barrier layer; and co-extruding a sealant layer, the sealant layer including a first sealant layer and a second sealant layer, on a second surface of the barrier layer, wherein the sealant layer has a melt flow rate (MFR) of 14.0 g / 10 min or less, measured under conditions of 230°C and a load of 2.16 kg.

[0018] After the co-extrusion step, the first sealant layer can be in direct contact with the barrier layer, and the second sealant layer can be laminated onto the surface of the first sealant layer.

[0019] The first sealant layer may include an acid-modified polyolefin resin, and the second sealant layer may include a polyolefin resin.

[0020] During the co-extrusion step, the resin pressure can be adjusted using a co-extrusion device.

[0021] The co-extrusion step may include the step of adjusting the resin pressure by replacing a filter of the co-extrusion device when the resin pressure exceeds a set value.

[0022] The sealant layers may be co-extruded to have a thickness ranging from about 45 μm to about 100 μm, or the first sealant layer may be co-extruded to have a thickness ranging from about 25 μm to about 80 μm, and the second sealant layer may be co-extruded to have a thickness ranging from about 20 μm to about 80 μm. [Effects of the Invention]

[0023] The pouch for a secondary battery according to the present invention, which is composed of two sealant layers and designed so that the melt flow rate (MFR) of the entire sealant layer is 14 g / 10 min or less, has higher sealing strength than conventional pouches. Therefore, when the pouch of the present invention is applied to a lithium secondary battery, the sealant layer does not easily rupture even when the internal pressure of the secondary battery increases. Specifically, the internal pressure of the cell at which venting occurs (referred to as "cell vent pressure") is high at 7.7 bar or more, and the number of days that can be stored without venting during an accelerated high-temperature storage test (accelerated high-temperature storage period) is 15 days or more, demonstrating excellent high-temperature reliability. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a diagram illustrating the cross-sectional structure of a pouch according to one embodiment of the present invention. [Figure 2] FIG. 2 is an exploded perspective view of a secondary battery according to another embodiment of the present invention. [Figure 3] 1 is a graph showing the measurement results of the cell vent pressure resistance and the number of accelerated high-temperature storage days of the lithium secondary batteries of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0025] The terms and words used in this specification and claims should not be interpreted in a limited way to their ordinary or dictionary meanings, but should be interpreted in a way that is 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.

[0026] The present invention will be described in more detail below.

[0027] As a result of extensive research into the development of a pouch-type secondary battery with excellent high-temperature reliability, the inventors discovered that when the melt flow index of the pouch sealant layer satisfies a specific range, the resistance of the sealant layer to the internal pressure of the cell (cell vent pressure resistance) increases significantly, thereby suppressing the occurrence of venting during high-temperature storage and significantly improving high-temperature reliability, leading to the completion of the present invention.

[0028] Pouch The pouch according to the present invention comprises a barrier layer, a substrate layer disposed on the outer surface of the barrier layer, and a sealant layer disposed on the inner surface of the barrier layer, the sealant layer including a first sealant layer disposed on the inner surface of the barrier layer and a second sealant layer disposed on the first sealant layer. The entire sealant layer (i.e., the combination of the first and second sealant layers) has a melt flow rate (MFR) measured at 230°C under a load of 2.16 kg of 14.0 g / 10 min or less, preferably 8.5 to 14.0 g / 10 min, and more preferably 9.0 to 13.5 g / 10 min.

[0029] Fig. 1 shows a cross-sectional structure of a pouch constituting a pouch according to the present invention, and Fig. 2 shows the configuration of a lithium secondary battery according to the present invention. Hereinafter, the pouch according to the present invention will be described with reference to Figs. 1 and 2.

[0030] The pouch 100 is a battery case for accommodating an electrode assembly and an electrolyte, and as shown in FIG. 1, includes a barrier layer 20, a substrate layer 10 disposed on the outer surface of the barrier layer, and a sealant layer 30 disposed on the inner surface of the barrier layer.

[0031] The pouch 100 includes a first case 101 and a second case 102, and may be manufactured by molding a pouch film laminate including a barrier layer 20, a base layer 10 formed on one side of the barrier layer, and a sealant layer 30 formed on the other side of the barrier layer. For example, the pouch 100 may be manufactured by a method of inserting the pouch film laminate into a press molding device and applying pressure to the pouch film laminate to stretch it, thereby forming a recessed cup portion in at least one of the first case 101 and / or the second case 102.

[0032] (1) Sealant layer The sealant layer 30 is located as the innermost layer of the pouch 100 and serves to seal the pouch when the sealant layers of the first case and the second case are bonded to each other by heat and pressure.

[0033] Meanwhile, in the pouch according to the present invention, the sealant layer 30 has a two-layer structure, specifically including a first sealant layer 32 and a second sealant layer 34. The first sealant layer 32 is disposed so that one side thereof is in direct contact with the barrier layer 20, and the second sealant layer 34 is disposed on the side of the first sealant layer 32 opposite the side that is in contact with the barrier layer 20.

[0034] The first sealant layer 32 and the second sealant layer 34 may be formed on the barrier layer 20 by co-extruding a resin constituting the first sealant layer and a resin constituting the second sealant layer.

[0035] Methods for forming a sealant layer in a pouch include a dry lamination method in which a sealant layer is attached to a barrier layer using a thermosetting adhesive, and a co-extrusion method in which a thermoplastic resin is co-extruded onto a barrier layer to form a sealant layer. Of these, when a sealant layer is formed using the co-extrusion method, it is possible to form a sealant layer with a higher melt flow rate than when a sealant layer is formed using the dry lamination method, which has the advantage of shortening the time required for the sealing process and facilitating production.

[0036] Furthermore, unlike the dry lamination method, the co-extrusion method does not use a thermosetting adhesive to bond the sealant layer 30 to the barrier layer 20, making it possible to form a pouch with excellent moisture resistance and high temperature durability.

[0037] Meanwhile, the first sealant layer 32 and the second sealant layer 34 may have different compositions. Specifically, the first sealant layer 32 may contain an acid-modified polyolefin resin, and the second sealant layer 34 may contain a polyolefin resin. When the first sealant layer 32 is formed of an acid-modified polyolefin resin and the second sealant layer 34 is formed of a polyolefin resin as described above, the adhesive strength with the barrier layer 20 and the high-temperature seal strength can be increased.

[0038] The sealant layer 30, which contacts the electrolyte and electrode assembly after the pouch is formed, must have insulating and corrosion-resistant properties. It must also have high sealing properties because it must completely seal the interior and prevent the transfer of substances between the inside and outside. Polyolefin resins are suitable as sealant layer materials because they have excellent mechanical properties, such as tensile strength, rigidity, surface hardness, abrasion resistance, and heat resistance, as well as chemical properties, such as corrosion resistance. However, because polyolefin resins do not have strong adhesive properties with the barrier layer, if a sealant layer is formed solely from polyolefin resin, interfacial delamination between the barrier layer and the sealant layer may occur when exposed to high temperatures or when the internal pressure of the battery increases. Therefore, in the present invention, an acid-modified polyolefin resin, which is a polyolefin resin that incorporates an acid component that can improve adhesive strength with the barrier layer, is used as the first sealant layer to improve adhesive strength with the barrier layer, thereby achieving excellent adhesive strength with the barrier layer, mechanical properties, and chemical resistance. When the sealant layer is formed as a single layer of an acid-modified polyolefin resin, rather than as two layers as in the present invention, the chemical resistance and mechanical properties may be reduced, and the seal strength may be reduced. When the sealant layer is formed as a single layer of a polyolefin resin, the adhesive strength with the barrier layer is reduced, and venting may occur at the interface with the barrier layer when exposed to high temperatures.

[0039] On the other hand, the acid-modified polyolefin resin is a polymer modified by block polymerization or graft polymerization of a polyolefin with an acid component, and may be, for example, a polymer obtained by polymerizing a polyolefin with a carboxylic acid or an anhydride thereof, such as acrylic acid, methacrylic acid, maleic acid, itaconic acid, crotonic acid, maleic anhydride, or itaconic anhydride.

[0040] Meanwhile, the polyolefin may be, for example, polyethylene such as low-density polyethylene, medium-density polyethylene, high-density polyethylene, linear low-density polyethylene, etc.; ethylene-α-olefin copolymer; polypropylene such as homopolypropylene, polypropylene block copolymer (e.g., propylene and ethylene block copolymer), polypropylene random copolymer (e.g., propylene and ethylene random copolymer); propylene-α-olefin copolymer; ethylene-butene-propylene trimer, etc., but is not limited thereto.

[0041] Meanwhile, in the pouch according to the present invention, the sealant layer has a melt flow rate (MFR) of 14.0 g / 10 min or less, preferably 8.5 g / 10 min to 14.0 g / 10 min, and more preferably 9.0 g / 10 min to 13.5 g / min, measured at 230°C and a load of 2.16 kg. If the MFR of the sealant layer exceeds 14.0 g / 10 min, the resistance to the internal pressure of the battery is significantly reduced, making venting more likely to occur when exposed to high temperatures. In other words, the cell vent pressure is significantly reduced. Here, the cell vent pressure refers to the maximum pressure at which venting does not occur when gas is injected into the cell. The higher the cell vent pressure, the less venting occurs in high-temperature environments and the better the high-temperature reliability.

[0042] Meanwhile, the melt flow rate of the sealant layer refers to the melt flow rate of the entire sealant layer, including the first and second sealant layers, and is different from the melt flow rates of the first and second sealant layers individually. The melt flow rate of the sealant layer is a value measured after co-extrusion. The safety of lithium secondary batteries is affected by the physical properties of the sealant layer after co-extrusion, so the melt flow rate after co-extrusion is important. Because the melt flow rate of a thermoplastic resin can change depending on the extrusion conditions during the co-extrusion process, the melt flow rate after co-extrusion may be different from the value derived by simply arithmetically calculating the melt flow rates of the raw materials before extrusion.

[0043] In the present invention, the melt flow rate of the sealant layer can be measured by the following method.

[0044] First, the pouch was cut into 50mm x 300mm pieces and placed in 37% by weight hydrochloric acid for 3-48 hours to dissolve the aluminum and separate the sealant layer into a film. The sealant layer was then thoroughly rinsed with water and dried at 60°C for at least 2 hours. The film-like sealant layer was then rolled into a cylindrical shape to prepare a sample. Three of the prepared samples were placed in a measuring device (Gottfert MI-40) and melted at 230°C for 5 minutes. Then, a load of 2.16 kg was applied and the melt flow rate was measured using the volumetric flow rate (MVR) method. The melt density of the sealant layer was 0.728 g / cm. 3 The melt flow rate can be calculated by converting the volume into mass.

[0045] In the case of a two-layer sealant layer formed by coextrusion, the first and second sealant layers have different compositions, resulting in different physical properties such as melting point and melt flow rate. The melt flow rate of the entire sealant layer changes due to differences in the physical properties and thickness of the first and second sealant layers. If the melt flow rate of the entire sealant layer exceeds 14.0 g / 10 min, peeling may occur at the interface between the sealant layer and the barrier layer when the battery is exposed to high temperatures or the internal pressure of the battery increases, which may result in venting. As a result of extensive research into solving these problems, the inventors have found that the melt flow rate of the entire sealant layer, consisting of the first and second sealant layers, can be changed by adjusting the pressure conditions (hereinafter referred to as "resin pressure") applied to the filter of the co-extrusion device during co-extrusion. Furthermore, by adjusting the resin pressure, the melt flow rate of the entire sealant layer can be controlled to a specific range, i.e., 14.0 g / 10 min or less, thereby suppressing interfacial peeling between the sealant layer and the barrier layer, and significantly improving resistance to the internal pressure of the battery, i.e., cell vent pressure resistance characteristics.

[0046] The resin pressure increases with the number of resin co-extrusions. Therefore, the pressure on the filter of the co-extrusion device can be monitored over time, and if the resin pressure exceeds a set range, the resin pressure can be adjusted by replacing the filter. The resin pressure range required to form a sealant layer with a desired melt flow rate can vary depending on the types of resins constituting the first and second sealant layers, the thicknesses of the first and second sealant layers, the type of co-extrusion device, etc. A person of ordinary skill in the art can determine, through routine experimentation, the resin pressure range that results in a sealant layer with a melt flow rate of 14.0 g / 10 min or less.

[0047] Meanwhile, in the present invention, the ratio of the thickness of the second sealant layer to the thickness of the first sealant layer may be 0.8 to 1.2, preferably 0.9 to 1.1. When the ratio of the thickness of the second sealant layer to the thickness of the first sealant layer satisfies this range, a pouch can be realized that is excellent in all respects: adhesion to the barrier layer, insulation, and seal strength. If either the first sealant layer or the second sealant layer is too thick or too thin, problems may arise in that the resin constituting the sealant layer leaks out during sealing, or the adhesion and sealability between the sealant layer and the barrier layer decrease.

[0048] Preferably, the first sealant layer has a thickness of 25 μm to 80 μm, preferably 30 μm to 70 μm, and more preferably 30 μm to 60 μm. When the thickness of the first sealant layer satisfies the above range, the adhesive strength with the barrier layer is excellent.

[0049] Preferably, the second sealant layer has a thickness of 20 μm to 80 μm, preferably 25 μm to 70 μm, and more preferably 30 μm to 60 μm. When the thickness of the second sealant layer satisfies the above range, excellent sealing properties are achieved.

[0050] Meanwhile, the total thickness of the sealant layer, which is the sum of the first and second sealant layers, can be 45 μm to 100 μm, preferably 50 μm to 100 μm, more preferably 60 μm to 100 μm, and even more preferably 70 μm to 90 μm. When the total thickness of the sealant layer satisfies this range, defects such as resin leakage to the outside during sealing can be suppressed, and the amount of heat and time required for sealing can be appropriately adjusted.

[0051] (2) Barrier layer The barrier layer 20 is intended to ensure the mechanical strength of the pouch, to block the entry and exit of gas or moisture outside the secondary battery, and to prevent leakage of the electrolyte.

[0052] The barrier layer 20 may have a thickness of 40 μm to 100 μm, more preferably 40 μm to 90 μm, and even more preferably 50 μm to 80 μm. When the thickness of the barrier layer satisfies the above range, appropriate mechanical strength and barrier properties can be ensured.

[0053] The barrier layer 20 may be made of a metal material, specifically, an aluminum alloy thin film.

[0054] The aluminum alloy thin film may contain aluminum and one or more metal elements other than aluminum, such as iron (Fe), copper (Cu), chromium (Cr), manganese (Mn), nickel (Ni), magnesium (Mg), silicon (Si), and zinc (Zn).

[0055] Preferably, the aluminum alloy thin film has an iron (Fe) content of 1.2 wt% to 1.7 wt%, preferably 1.3 wt% to 1.7 wt%, more preferably 1.3 wt% to 1.45 wt%. When the iron (Fe) content in the aluminum alloy thin film satisfies this range, it is possible to minimize the occurrence of cracks and pinholes even when the cup portion is formed deep.

[0056] (3) Base material layer The base layer 10 is disposed on the outermost layer of the pouch to protect the electrode assembly from external impact and to provide electrical insulation.

[0057] The substrate layer 10 may be made of a polymer material, and may include, for example, one or more polymers selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, and Teflon (registered trademark).

[0058] The substrate layer 10 may have a single layer structure, or may have a multi-layer structure in which different polymer films 12 and 14 are laminated, as shown in Fig. 2. When the substrate layer 10 has a multi-layer structure, an adhesive layer 16a may be interposed between the polymer films. Meanwhile, the substrate layer 10 having a single layer structure or a multi-layer structure may be attached to the barrier layer 20 via an adhesive layer 16b.

[0059] Meanwhile, the substrate layer 10 may have a total thickness of 10 μm to 60 μm, preferably 20 μm to 50 μm, and more preferably 30 μm to 50 μm. When the substrate layer has a multilayer structure, the thickness includes the thickness of the adhesive layer. When the substrate layer 10 satisfies the above range, it exhibits excellent durability, insulating properties, and moldability. If the thickness of the substrate layer is too thin, durability may decrease and the substrate layer may be damaged during the molding process. If the thickness of the substrate layer is too thick, moldability may decrease, the total thickness of the pouch may increase, the battery storage space may decrease, and the energy density may decrease.

[0060] According to one embodiment, the base layer 10 may have a laminated structure of a polyethylene terephthalate (PET) film and a nylon film. Preferably, the nylon film is disposed on the barrier layer 20 side, i.e., the inner side, and the polyethylene terephthalate film is disposed on the outer surface side of the pouch.

[0061] Polyethylene terephthalate (PET) has excellent durability and electrical insulation properties, and when a PET film is placed on the surface side, it also has excellent durability and insulation properties. However, PET film has weak adhesion to the aluminum alloy thin film that constitutes the barrier layer 20 and its stretching behavior is different. Therefore, when a PET film is placed on the barrier layer side, peeling between the substrate layer and the barrier layer can occur during the molding process, resulting in an uneven stretching of the barrier layer and reduced formability. In contrast, nylon film has a similar stretching behavior to the aluminum alloy thin film that constitutes the barrier layer 20, so when a nylon film is placed between the polyethylene terephthalate and the barrier layer, it can improve formability.

[0062] The polyethylene terephthalate film may have a thickness of 5 μm to 20 μm, preferably 5 μm to 15 μm, and more preferably 7 μm to 15 μm, and the nylon film may have a thickness of 10 μm to 40 μm, preferably 10 μm to 35 μm, and more preferably 15 μm to 25 μm. When the thicknesses of the polyethylene terephthalate film and the nylon film satisfy the above ranges, excellent formability and rigidity after forming are achieved.

[0063] Lithium secondary battery Next, the lithium secondary battery according to the present invention will be described.

[0064] The lithium secondary battery according to the present invention includes an electrode assembly formed by stacking a positive electrode, a separator, and a negative electrode, an electrolyte, and a pouch-type battery case that houses the electrode assembly and the electrolyte, and the battery case is the above-described pouch according to the present invention.

[0065] The pouch according to the present invention (i.e., a pouch in which the sealant layer includes a first sealant layer and a second sealant layer and the melt flow rate of the entire sealant layer, including the first and second sealant layers, is designed to be 14.0 g / 10 min or less) has higher sealing strength than conventional pouches. Therefore, when the pouch according to the present invention is applied to a lithium secondary battery, the sealant layer does not easily rupture even when the internal pressure of the secondary battery increases, and high-temperature reliability can be achieved.

[0066] Specifically, a lithium secondary battery using the pouch of the present invention as a battery case can have a cell vent pressure resistance at 60°C of 7.7 bar or more, preferably 7.7 bar to 15 bar, and more preferably 8 bar to 15 bar.

[0067] Furthermore, a lithium secondary battery using the pouch of the present invention as a battery case can have an accelerated high-temperature storage period of 15 days or more, preferably 15 to 30 days, and more preferably 15 to 25 days, measured at a temperature of 70°C while charging to 100% SOC at daily intervals.

[0068] The specific details regarding the pouch are as described above, and the remaining configuration other than the pouch will be described below.

[0069] An embodiment of the lithium secondary battery according to the present invention is shown in Fig. 2. Hereinafter, each component of the lithium secondary battery according to the present invention will be described in more detail with reference to Fig. 2.

[0070] electrode assembly The electrode assembly 200 may include a plurality of electrodes and a plurality of separators stacked on top of each other. The plurality of electrodes may include positive and negative electrodes having opposite polarities, stacked alternately with separators sandwiched between them.

[0071] The positive electrode and the negative electrode may be manufactured by applying an active material layer-forming composition containing an electrode active material onto a current collector, and then drying the composition.

[0072] The positive electrode active material layer forming composition may include a positive electrode active material, a binder, and a conductive material, and the negative electrode active material layer forming composition may include a negative electrode active material, a binder, and a conductive material.

[0073] The current collector may be any material that does not cause chemical changes in the battery and has high conductivity, and examples of such materials include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surfaces that have been surface-treated with carbon, nickel, titanium, silver, or the like, and aluminum-cadmium alloys. The current collector typically has a thickness of 3 μm to 500 μm, and the current collector may have fine irregularities on its surface to enhance the bonding strength of the negative electrode active material. The current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0074] The positive electrode active material is a material capable of causing an electrochemical reaction, and various positive electrode active materials used in the art can be used. For example, layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2); lithium manganese oxide; 1-y M y Lithium nickel-based oxides represented by the formula: O2 (where M=Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn, or Ga, containing one or more of the above elements, and 0.01≦y≦0.7); Li 1+z Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2, Li 1+z Ni 0.4 Mn 0.4 Co 0.2 O2, etc., Li 1+z Ni b Mn c Co 1-(b+c+d) Md O (2-e) A e (wherein -0.5≦z≦0.5, 0.1≦b≦0.8, 0.1≦c≦0.8, 0≦d≦0.2, 0≦e≦0.2, b+c+d<1, M=Al, Mg, Cr, Ti, Si or Y, A=F, P or Cl) lithium nickel cobalt manganese composite oxide; 1+x M 1-y M' y PO 4-z X z (wherein M=transition metal, preferably Fe, Mn, Co, or Ni; M′=Al, Mg, or Ti; X=F, S, or n; −0.5≦x≦+0.5, 0≦y≦0.5, and 0≦z≦0.1), but is not limited thereto. The positive electrode active material may be included in an amount of 80 wt % to 99 wt % based on the total weight of the positive electrode active material layer.

[0075] The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and SiO. βExamples of the negative electrode active material include metal oxides capable of doping and dedoping lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide (0<β<2); and composites containing the metallic compounds and carbonaceous materials, such as Si-C composites and Sn-C composites. These may be used alone or in combination. A thin film of metallic lithium may also be used as the negative electrode active material. The carbon material may be either low-crystalline carbon or high-crystalline carbon. Representative examples of low-crystalline carbon include soft carbon and hard carbon, while representative examples of high-crystalline carbon include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-sintered carbon such as petroleum or coal tar pitch-derived cokes. The negative electrode active material may be included in an amount of 80 wt% to 99 wt% of the total weight of the negative electrode active material layer.

[0076] The binder is a component that helps bind the conductive material, active material, and current collector together, and is usually added in an amount of 0.1 to 10% by weight based on the total weight of the active material layer. 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 polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, nitrile-butadiene rubber, fluororubber, and various copolymers thereof.

[0077] The conductive material is a component for further improving the conductivity of the active material and may be added in an amount of 10 wt % or less, preferably 5 wt % or less, based on the total weight of the active material layer. Such a conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity. Examples of such a conductive material include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; carbon fluoride; metal powders such as 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.

[0078] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in lithium secondary batteries can be used without particular limitations. In particular, a separator that exhibits low resistance to electrolyte ion movement and excellent electrolyte humidification capacity is preferred. Specifically, a porous polymer film, such as a porous polymer film 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, or a laminate structure of two or more layers thereof, can be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, can also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material can be used, and it can be selectively used in a single-layer or multi-layer structure.

[0079] Meanwhile, the electrode assembly 200 may be provided with a plurality of electrode tabs 230 welded to each other. The plurality of electrode tabs 230 may be connected to a plurality of electrodes, protrude from the electrode assembly 200 to the outside, and act as a path through which electrons can move between the inside and outside of the electrode assembly 200. The plurality of electrode tabs 230 may be located inside the pouch 100.

[0080] The electrode tab 230 connected to the positive electrode and the electrode tab 230 connected to the negative electrode may protrude in different directions from each other with respect to the electrode assembly 200. However, without being limited thereto, the electrode tab 230 connected to the positive electrode and the electrode tab 230 connected to the negative electrode may protrude in the same direction in parallel to each other.

[0081] A lead 240 for supplying electricity to the outside of the secondary battery may be connected to the plurality of electrode tabs 230 by spot welding, etc. One end of the lead 240 may be connected to the plurality of electrode tabs 230, and the other end may protrude outside the pouch 100.

[0082] A portion of the lead 240 may be surrounded by an insulating portion 250. For example, the insulating portion 250 may include insulating tape. The insulating portion 250 may be located between the terrace 120 of the first case 101 and the second case 102, and in this state, the terrace 120 and the second case 102 may be heat-sealed to each other. In this case, portions of the terrace 120 and the second case 102 may be heat-sealed to the insulating portion 250. Therefore, the insulating portion 250 prevents electricity generated from the electrode assembly 200 from flowing to the pouch 100 via the lead 240, thereby maintaining the seal of the pouch 100.

[0083] electrolyte The electrolyte is used to transport lithium ions generated by electrochemical reactions at the electrodes when the secondary battery is charged or discharged, and can contain an organic solvent and a lithium salt.

[0084] 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 C2-C20 linear, branched, or cyclic hydrocarbon group that 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 high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred.

[0085] The lithium salt can be any compound capable of providing lithium ions used in lithium secondary batteries. Specifically, the lithium salt may be LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, or LiB(C, O) . The lithium salt concentration is preferably within a range of 0.1 to 5.0 M, and more preferably 0.1 to 3.0 M. When the lithium salt concentration is within this range, the electrolyte exhibits appropriate conductivity and viscosity, resulting in excellent electrolyte performance and efficient lithium ion migration.

[0086] In addition to the constituent components of the electrolyte, the electrolyte may further contain additives for the purposes of improving the life characteristics of the battery, suppressing a decrease in the capacity of the battery, and improving the discharge capacity of the battery.

[0087] The present invention will now be described in more detail with reference to specific examples.

[0088] Manufacturing example A laminated film was prepared by laminating a polyethylene terephthalate film (12 μm thick) / adhesive layer (3 μm thick) / nylon film (15 μm thick) in this order as the base layer. Next, a urethane adhesive was applied to a thickness of 3 μm onto the nylon film of the base layer, and an aluminum alloy thin film (barrier layer, thickness: 40 μm) was laminated thereon, followed by lamination using the dry lamination method.

[0089] Next, a maleic anhydride-modified polypropylene resin and a polypropylene resin were co-extruded onto the surface of the aluminum alloy thin film opposite to the surface on which the substrate layer was laminated, to form a sealant layer including a first sealant layer and a second sealant layer. During the co-extrusion, the maleic anhydride-modified polypropylene resin layer was in contact with the aluminum alloy thin film, and the thickness of the first sealant layer and the second sealant layer was adjusted to 40 μm each.

[0090] During the formation of the sealant layer, the resin pressure was varied while co-extrusion was carried out to produce pouch film laminates A to F.

[0091] The pouch film laminate prepared as described above was cut into a size of 50 mm x 300 mm and then placed in 37 wt% hydrochloric acid to dissolve the aluminum alloy thin film and separate the film-shaped sealant layer. The separated sealant layer was then thoroughly washed with water, dried at 60°C for 2 hours, and rolled into a cylindrical shape to prepare a sample. The sample was placed in a melt flow rate measurement device (Gottfert MI-40) and melted at 230°C for 5 minutes. A load of 2.16 kg was then applied to measure the melt flow rate (MFR) of the sealant layer. The measurement was performed using the volumetric measurement (MVR) method, and the melt density of the sealant layer was 0.728 g / cm. 3 It was assumed that: The measurement results are shown in Table 1 below.

[0092] [Table 1]

[0093] Examples and Comparative Examples Each of the pouch film laminates A to F produced according to the above manufacturing examples was drawn to produce a cup-shaped pouch for a secondary battery according to Examples 1 to 5 and Comparative Example 1. The sealant layer of each pouch produced according to Examples 1 to 5 and Comparative Example 1 was then heat-sealed, and the sealed portion was cut into a 15 mm width to prepare a sample. The sealed portion of each sample was then opened and placed in a peel strength tester so that the distance between the grips was 30 mm. A T-peel test was then performed at 25°C (room temperature) or 60°C (high temperature) at a speed of 5 mm / min to measure the seal strength. The seal strength was expressed as a percentage of the peel strength of Examples 2 to 5 and Comparative Example 1 relative to the reference value (100) of the peel strength of Example 1.

[0094] The measurement results are shown in Table 2 below.

[0095] [Table 2]

[0096] As shown in Table 2, the pouches of Examples 1 to 5, in which the melt flow rate of the sealant layer is 14 g / 10 min or less, exhibit significantly better sealing properties at room temperature and at high temperatures than the pouch of Comparative Example 1, in which the melt flow rate of the sealant layer is greater than 14 g / 10 min.

[0097] Experimental example: Secondary battery cell vent pressure resistance and high temperature accelerated test The stacked electrode assemblies manufactured in Examples 1 to 5 and Comparative Example 1 were housed in the pouches for secondary batteries, and electrolytes were injected into them. The sealant layer was then sealed to fabricate lithium secondary batteries. The cell vent pressure resistance and accelerated high-temperature storage period of each of the manufactured lithium secondary batteries were measured as follows. For accurate evaluation, the measurements were performed two or three times for each lithium secondary battery, and the measurement results are shown in Figure 3.

[0098] (1) Cell vent pressure measurement method A hole of approximately 1mm or less was drilled in the lithium secondary battery, and inert gas was injected at 25℃ and 60℃ while measuring the cell pressure over time. The maximum pressure reached until venting was evaluated as the cell vent pressure. Meanwhile, since the cell pressure drops to atmospheric pressure when venting occurs, the point at which the cell pressure drops to atmospheric pressure was evaluated as the time of venting.

[0099] (2) Accelerated high-temperature storage period The lithium secondary battery was stored at a temperature of 70° C. and charged to 100% SOC at daily intervals, and the number of days that it could be stored without venting was measured.

[0100] 3, the lithium secondary batteries of Examples 1 to 5, which used pouches with a sealant layer having a melt flow rate of 14.0 g / 10 min or less, which is within the range of the present invention, exhibited high cell vent pressure resistance of 7.7 bar or more at both 60°C and 25°C, and also exhibited accelerated high-temperature storage days of 15 days or more. In contrast, the lithium secondary battery of Comparative Example 1, which used a pouch with a sealant layer having a melt flow rate of more than 14.0 g / 10 min, exhibited significantly reduced cell vent pressure resistance at 60°C and accelerated high-temperature storage days. [Explanation of symbols]

[0101] 10 Base material layer 12, 14 Polymer film 16a, 16b adhesive layer 20 Barrier Layer 30 Sealant Layer 32 First sealant layer 34 Second sealant layer 100 pouches 101 Case 1 102 Case 2 120 Terrace 200 electrode assembly 210 Electrode 230 Electrode Tab 240 leads 250 Insulation

Claims

1. a barrier layer, a substrate layer disposed on one surface of the barrier layer, and a sealant layer disposed on the other surface of the barrier layer; the sealant layer includes a first sealant layer disposed in direct contact with the other surface of the barrier layer, and a second sealant layer disposed on the first sealant layer; The pouch for a secondary battery, wherein the sealant layer has a melt flow rate (MFR) of 14.0 g / 10 min or less when measured under conditions of 230° C. and a load of 2.16 kg.

2. 2. The pouch for a secondary battery according to claim 1, wherein the sealant layer has a melt flow rate (MFR) of 8.5 g / 10 min to 14.0 g / 10 min measured under conditions of 230° C. and a load of 2.16 kg.

3. The pouch for a secondary battery according to claim 1 , wherein the first sealant layer and the second sealant layer are co-extruded.

4. The pouch for a secondary battery according to claim 1 , wherein the first sealant layer contains an acid-modified polyolefin resin.

5. The pouch for a secondary battery according to claim 1 , wherein the second sealant layer contains a polyolefin resin.

6. 2. The pouch for a secondary battery according to claim 1, wherein the ratio of the thickness of the second sealant layer to the thickness of the first sealant layer is 0.8 to 1.

2.

7. the first sealant layer has a thickness of 25 μm to 80 μm; 2. The pouch for a secondary battery according to claim 1, wherein the second sealant layer has a thickness of 20 μm to 80 μm.

8. 2. The pouch for a secondary battery according to claim 1, wherein the sealant layer has a total thickness of 45 μm to 100 μm.

9. The pouch for a secondary battery according to claim 1 , wherein the barrier layer includes an aluminum alloy layer.

10. 2. The pouch for a secondary battery according to claim 1, wherein the base material layer contains one or more materials selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymers, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, and Teflon (registered trademark).

11. an electrode assembly formed by stacking a positive electrode, a separator, and a negative electrode; Electrolytes, A lithium secondary battery comprising the pouch for secondary batteries according to claim 1.

12. The lithium secondary battery according to claim 11, wherein the lithium secondary battery has a cell vent pressure resistance of 7.7 bar or more at 60°C.

13. The lithium secondary battery according to claim 11, wherein the accelerated high-temperature storage period measured while charging the lithium secondary battery to 100% SOC at a temperature of 70°C at one-day intervals is 15 days or more.

14. laminating a substrate layer on the first surface of the barrier layer; co-extruding a sealant layer on the second surface of the barrier layer, the sealant layer comprising a first sealant layer and a second sealant layer; The method for manufacturing a pouch for a secondary battery, wherein the sealant layer has a melt flow rate (MFR) of 14.0 g / 10 min or less when measured under conditions of 230° C. and a load of 2.16 kg.

15. after the co-extruding step, the first sealant layer is in direct contact with the barrier layer; The method for manufacturing a pouch for a secondary battery according to claim 14 , wherein the second sealant layer is laminated on a surface of the first sealant layer.

16. The method for manufacturing a pouch for a secondary battery according to claim 14 , wherein the first sealant layer contains an acid-modified polyolefin resin, and the second sealant layer contains a polyolefin resin.

17. The method for manufacturing a pouch for a secondary battery according to claim 14 , wherein a resin pressure is adjusted using a co-extrusion device during the co-extrusion step.

18. The method for manufacturing a pouch for a secondary battery according to claim 17, further comprising the step of replacing a filter of the co-extrusion device when the resin pressure exceeds a preset value.

19. The method for manufacturing a pouch for a secondary battery according to claim 14, wherein the sealant layer is co-extruded to have a thickness of 45 μm to 100 μm.

20. 20. The method for manufacturing a pouch for a secondary battery according to claim 19, wherein the sealant layers are co-extruded so that the first sealant layer has a thickness of 25 μm to 80 μm and the second sealant layer has a thickness of 20 μm to 80 μm.

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