Pouch for secondary battery and lithium secondary battery comprising same

Abstract

Provided are a pouch for a secondary battery and a lithium secondary battery including the same. The pouch for a secondary battery includes a barrier layer, a substrate layer provided on one surface of the barrier layer, and a sealing layer provided on the other surface of the barrier layer. The sealing layer includes a first sealing layer disposed in direct contact with the other surface of the barrier layer and a second sealing layer disposed on the first sealing layer. The sealing layer has a melt flow rate (MFR) of 14.0 g / 10 min or less as measured at a temperature of 230 DEG C under a load condition of 2.16 kg.

Description

Technical Field

[0001] The present invention generally relates to a pouch for a secondary battery and a lithium secondary battery including the pouch. More specifically, the present invention relates to a pouch for a secondary battery having high cell exhaust pressure resistance, low exhaust generation, and exhibiting excellent high-temperature reliability, and a lithium secondary battery including the pouch. Background Art

[0002] A lithium secondary battery is generally manufactured by coating an 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. Then, the positive electrode and the negative electrode are stacked on both sides of a separator to form an electrode assembly. Next, the electrode assembly is housed in a case, and an electrolyte is injected into the case.

[0003] Conventionally, secondary batteries are classified according to the shape of the case housing the electrode assembly. Exemplary secondary batteries include a pouch-type secondary battery, a can-type secondary battery, and a prismatic secondary battery. For example, a pouch-type secondary battery is manufactured by pressing a flexible pouch film to form a cup-shaped portion for housing the electrode assembly, then injecting an electrolyte and sealing a sealing portion of the case. On the other hand, a can-type secondary battery is manufactured by housing the electrode assembly in a can made of a metal material, injecting the electrolyte into the can, and fastening a top lid to the upper portion of the can to seal the electrode assembly and the electrolyte.

[0004] Although the pouch-type secondary battery is light in weight, exhibits excellent space utilization, and has a high energy density, the pouch-type secondary battery is more likely to catch fire, explode, and leak electrolyte when subjected to external impact and / or higher internal temperature or pressure compared to the can-type secondary battery due to its stacked electrode assembly.

[0005] Secondary batteries, particularly pouch-type secondary batteries, are applied to various products including electric vehicles to reduce and / or prevent greenhouse gas emissions. When using a secondary battery to power an electric vehicle, the battery needs to have excellent safety to protect vehicle passengers.

[0006] In addition, when the pouch-type secondary battery is stored at a high temperature, gas is generated due to a side reaction with the electrolyte. As a result, the internal pressure of the battery increases, leading to an exhaust phenomenon in which the sealing portion of the pouch bursts. If the exhaust phenomenon occurs early, the safety and reliability of the secondary battery are significantly deteriorated. Therefore, it is desirable to develop a pouch-type secondary battery having high cell exhaust pressure resistance and low exhaust generation in a high-temperature environment to make the secondary battery reliable at high temperatures. Summary of the Invention

[0007] Technical problem

[0008] To address the above limitations, one aspect of the present invention provides a pouch having a specific melt flow rate, including a sealing layer with a bilayer structure, having a high cell exhaust pressure resistance, and having excellent high-temperature reliability, and a lithium secondary battery including the pouch.

[0009] Technical solution

[0010] According to one aspect of the present invention, a pouch for a secondary battery includes: a barrier layer; a substrate layer provided on one surface of the barrier layer; and a sealing layer provided on the other surface of the barrier layer. The sealing layer includes: a first sealing layer provided in direct contact with the other surface of the barrier layer; and a second sealing layer provided on the first sealing layer, and the melt flow rate (MFR) of the sealing layer measured under a load condition of 2.16 kg at a temperature of about 230 °C is 14.0 g / 10 min or less. In some cases, the MFR can be 8.5 g / 10 min to 14.0 g / 10 min, preferably 9.0 g / 10 min to 13.5 g / 10 min.

[0011] The first sealing layer and the second sealing layer may have a co-extrusion structure.

[0012] The first sealing layer may include an acid-modified polyolefin resin, and the second sealing layer may include a polyolefin resin.

[0013] The ratio of the thickness of the second sealing layer to the thickness of the first sealing layer may be 0.8 to 1.2, preferably 0.9 to 1.1. The thickness of the first sealing layer may be 25 μm to 80 μm, preferably 30 μm to 70 μm, more preferably 30 μm to 60 μm, and the thickness of the second sealing layer may be 20 μm to 80 μm, preferably 25 μm to 70 μm, more preferably 30 μm to 60 μm.

[0014] The total thickness of the sealing layer (e.g., the combination of the first sealing layer and the second sealing layer) may 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.

[0015] The barrier layer may include an aluminum alloy layer.

[0016] The substrate layer may include polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymer, polyacrylonitrile, polyimide, polyamide, cellulose, aromatic polyamide, nylon, polyester, poly(p-phenylene benzobisoxazole), polyarylate, or Teflon.

[0017] According to another aspect of the present invention, a lithium secondary battery includes: an electrode assembly formed by stacking a positive electrode, a separator, and a negative electrode; an electrolyte; and the above-described pouch.

[0018] In the lithium secondary battery, at a temperature of 60 °C, the cell vent pressure resistance can be 7.7 bar or more, preferably 7.7 bar to 15 bar, more preferably 8 bar to 15 bar.

[0019] The accelerated high-temperature storage period measured by charging the lithium secondary battery to SOC 100% at intervals of 1 day at a temperature of 70 °C can be 15 days or more, preferably 15 days to 30 days, more preferably 15 days to 25 days.

[0020] According to still another aspect of the present invention, there is provided a method for forming a pouch for a secondary battery, which includes the steps of: stacking a base material layer on a first surface of a barrier layer; and co-extruding a sealing layer including a first sealing layer and a second sealing layer on a second surface of the barrier layer, wherein the melt flow rate (MFR) of the sealing layer measured at a temperature of 230 °C under a load condition of 2.16 kg is 14.0 g / 10 min or less.

[0021] After the co-extrusion step, the first sealing layer may be in direct contact with the barrier layer, and the second sealing layer may be stacked on the surface of the first sealing layer.

[0022] The first sealing layer may include an acid-modified polyolefin resin, and the second sealing layer may include a polyolefin resin.

[0023] During the co-extrusion step, the resin pressure may be controlled using a co-extrusion device.

[0024] The co-extrusion step may include replacing a filter of the co-extrusion device to control the resin pressure when the resin pressure exceeds a predetermined value.

[0025] The sealing layer may be co-extruded to have a thickness of 45 μm to 100 μm, such that the thickness of the first sealing layer may be 25 μm to 80 μm, and the thickness of the second sealing layer may be 20 μm to 80 μm.

[0026] Advantageous effect

[0027] The sealing layer of the pouch for a secondary battery of the present invention is designed such that the sealing layer is composed of two layers, and the melt flow rate (MFR) of the entire sealing layer is 14 g / 10 min or less, having a higher sealing strength compared to conventional pouches. Therefore, when the pouch of the present invention is applied to a lithium secondary battery, even if the internal pressure of the secondary battery increases, the sealing layer is not easily broken. Specifically, the secondary battery of the present invention exhibits an internal cell pressure (“cell vent pressure resistance”) of 7.7 bar or more, and during the accelerated high-temperature storage test, it can be stored for 15 days or more without venting, thus having excellent high-temperature reliability. Description of the Drawings

[0028] Aspects, features, and other advantages of the present invention will be more clearly understood from the following detailed description in conjunction with the accompanying drawings, where:

[0029] Figure 1 is a cross-sectional view of a pouch according to an embodiment of the present invention.

[0030] Figure 2 is an exploded perspective view of a secondary battery according to another embodiment of the present invention.

[0031] Figure 3 is a graph showing the results obtained by measuring the cell exhaust pressure resistance and the accelerated high-temperature storage period for the examples and comparative examples of the present invention. Detailed Embodiments

[0032] The terms or words used in this specification and the claims should not be construed as being limited to their dictionary meanings, but should be understood as having meanings consistent with the technical idea of the present invention based on the principle that the inventor can appropriately define the terms so as to best illustrate their invention.

[0033] Hereinafter, the present invention will be described in more detail.

[0034] As a result of repeated studies for developing a pouch-type secondary battery having excellent high-temperature reliability, the present inventors have found that when the melt flow index of the sealing layer of the pouch satisfies a specific range, even if the pressure inside the cell increases, the sealing strength (cell exhaust pressure resistance) of the sealing layer is significantly improved. As a result, the sealing layer suppresses the occurrence of exhaust even at high temperatures, thereby significantly improving the sealing and reliability of the secondary battery.

[0035] Pouch

[0036] The pouch of the present invention includes a barrier layer, a substrate layer provided on the outer surface of the barrier layer, and a sealing layer provided on the inner surface of the barrier layer. The sealing layer may include a first sealing layer directly provided on the barrier layer and a second sealing layer provided on the first sealing layer. When measured at a temperature of 230 °C under a load condition of 2.16 kg, the melt flow rate (MFR) of the entire sealing layer (e.g., the combination of the first and second sealing layers) is 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 / 10 min.

[0037] Figure 1 is a cross-sectional view of a pouch film stack according to an embodiment of the present invention. Figure 2 is an exploded perspective view of a secondary battery according to an embodiment of the present invention. Hereinafter, reference will be made to Figure 1 and Figure 2Describe the bag of the present invention.

[0038] The bag 100 can be a battery case for accommodating an electrode assembly and an electrolyte. As Figure 1 shown, it may include a barrier layer 20, a substrate layer 10 provided on the outer surface of the barrier layer, and a sealing layer 30 provided on the inner surface of the barrier layer.

[0039] The bag 100 includes a first case 101 and a second case 102, and can be manufactured by stacking and molding bag films. For example, the bag 100 can be manufactured by inserting a stack of bag films into a compression molding device and applying pressure to the stack of bag films to stretch the stack and form a concave cup portion in at least one of the first case 101 and / or the second case 102.

[0040] Sealing layer

[0041] The sealing layer 30 of the first case and the sealing layer 30 of the second case can be configured to seal the bag when the sealing layers of the first case and the second case are joined together by thermal compression and are provided on the innermost layer of the bag 100.

[0042] In the bag of the present invention, the sealing layer 30 has a double-layer structure, specifically including a first sealing layer 32 and a second sealing layer 34. The first sealing layer 32 is arranged such that one of its surfaces is in direct contact with the barrier layer 20, while the second sealing layer 34 is provided on the surface of the first sealing layer 32 opposite to the surface in contact with the barrier layer 20.

[0043] The first sealing layer 32 and the second sealing layer 34 can be formed by co-extruding the resin constituting the first sealing layer and the resin constituting the second sealing layer onto the barrier layer 20.

[0044] Alternatively, a dry lamination method can be used to stack the sealing layer on the barrier layer, where a thermosetting adhesive is used to attach the sealing layer to the barrier layer. However, when the co-extrusion method is used to form the sealing layer, the sealing layer is formed to have a higher melt flow rate than when the dry lamination method is used to form the sealing layer, which in turn speeds up the subsequent sealing process and simplifies the manufacture of the secondary battery.

[0045] In addition, when the sealing layer 30 is not attached to the barrier layer 20 using a thermosetting adhesive (as in the case of the dry lamination method), the bag 100 is formed to have excellent moisture resistance and high-temperature durability.

[0046] The first sealing layer 32 and the second sealing layer 34 can have different compositions. Specifically, the first sealing layer 32 can contain an acid-modified polyolefin resin, and the second sealing layer 34 can contain a polyolefin resin. When the first sealing layer 32 is made of an acid-modified polyolefin resin and the second sealing layer 34 is made of a polyolefin resin, the adhesion to the barrier layer 20 and the high-temperature sealing strength can be improved.

[0047] Since the sealing layer 30 is the surface that contacts the electrolyte and the electrode assembly 200 after the bag is formed, the sealing layer 30 may have to be insulating and corrosion-resistant. In addition, since the inside of the sealing layer 30 must be completely sealed to prevent the movement of materials between the inside and the outside, the sealing layer 30 may have to have high sealing performance. Polyolefin resins 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, and thus are suitable as materials for the sealing layer. However, since polyolefin resins do not have high adhesiveness to the barrier layer, when the sealing layer is made only of polyolefin resin, interfacial peeling may occur between the barrier layer and the sealing layer when exposed to high temperatures or when the internal pressure of the battery increases. Therefore, in the present invention, an acid-modified polyolefin resin obtained by introducing an acid component capable of improving the adhesiveness to the barrier layer into the polyolefin resin can be used as the first sealing layer to improve the adhesiveness to the barrier layer, thereby excellently achieving all of the adhesiveness to the barrier layer, mechanical properties, and chemical properties. When the sealing layer is not formed into two layers as in the present invention but into a single layer made of an acid-modified polyolefin resin, the chemical resistance and mechanical properties may deteriorate, resulting in a decrease in the sealing strength, and when the sealing layer is formed into a single layer made of a polyolefin resin, the adhesiveness to the barrier layer may deteriorate, and thus gas evolution may occur at the interface with the barrier layer when exposed to high temperatures.

[0048] The acid-modified polyolefin resin may be a polymer modified by block polymerization or graft polymerization of a polyolefin with an acid component, for example, a polymer obtained by polymerizing a carboxylic acid or its anhydride such as acrylic acid, methacrylic acid, maleic acid, itaconic acid, crotonic acid, maleic anhydride, itaconic anhydride, etc. in a polyolefin.

[0049] 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, block copolymer of polypropylene (e.g., block copolymer of propylene and ethylene), random copolymer of polypropylene (e.g., random copolymer of propylene and ethylene); propylene-α-olefin copolymer; ethylene-butene-propylene terpolymer, etc., but not limited thereto.

[0050] The melt flow rate (MFR) of the sealing layer measured at a temperature of 230°C under a load condition of 2.16 kg is 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 / 10 min. When the melt flow rate of the sealing layer exceeds 14.0 g / 10 min, the resistance of the battery to internal pressure is significantly reduced. As a result, gas exhaust may occur when the battery is exposed to high temperatures. That is, when the melt flow rate of the sealing layer exceeds 14.0 g / 10 min, the gas exhaust pressure resistance of the battery cell is significantly reduced. Here, the gas exhaust pressure resistance of the battery cell refers to the maximum pressure at which no gas exhaust occurs when gas is injected into the battery cell. Therefore, the higher the gas exhaust pressure resistance of the battery cell, the better the high-temperature reliability.

[0051] The melt flow rate of the sealing layer refers to the melt flow rate of the entire sealing layer including the first sealing layer and the second sealing layer, and is a value different from the melt flow rates of the first sealing layer and the second sealing layer respectively. In addition, the melt flow rate of the sealing layer is a value measured after co-extrusion. Since the melt flow rate of a thermoplastic resin varies according to the conditions during co-extrusion, the melt flow rate after co-extrusion is usually a value different from the value obtained by simply arithmetically calculating the melt flow rates of the raw materials before extrusion.

[0052] The melt flow rate of the sealing layer can be measured by the following method.

[0053] First, cut the bag into a size of 50 mm × 300 mm, place it in hydrochloric acid with a concentration of 37 wt%, and leave it therein for 3 hours to 48 hours to melt the aluminum of the barrier layer 20 and separate the sealing layer in the form of a film. Then, after thoroughly washing the film with water, dry the sealing layer at a temperature of 60°C for 2 hours or more. Then, roll the separated sealing layer (in the form of a film) into a cylinder to prepare a sample. Put the three prepared samples into a measuring device (Gottfert Mi-40), melt them at a temperature of 230°C for 5 minutes, and then measure the melt flow rate value by the melt volume rate (MVR) method while applying a load of 2.16 kg. Assuming the melt density of the sealing layer is 0.728 g / cm 3 and convert the volume to mass to obtain the melt flow rate.

[0054] When the sealing layer has a double-layer structure formed by co-extrusion, due to the different compositions of the first sealing layer and the second sealing layer, physical properties such as melting point and melt flow rate are different from those of the sealing layer with a single-layer structure. Due to the different physical properties and thicknesses of the materials of the first sealing layer and the second sealing layer, the melt flow rate of the entire sealing layer may vary. Moreover, when the melt flow rate of the entire sealing layer exceeds 14.0 g / 10 min, peeling may occur at the interface between the sealing layer and the barrier layer when the battery is exposed to high temperature or high internal pressure, which may lead to gas venting. As a result of repeated studies to address this limitation, the inventors of the present invention found that by controlling the pressure applied to the filter of the co-extrusion device during the co-extrusion process (hereinafter referred to as "resin pressure"), the melt flow rate of the entire sealing layer including the first sealing layer and the second sealing layer can be changed. When the melt flow rate of the entire sealing layer is controlled within a specific range of 14.0 g / 10 min or less by resin pressure control, peeling at the interface between the sealing layer and the barrier layer is suppressed, thereby significantly improving the resistance to the internal pressure of the battery (e.g., cell venting pressure resistance).

[0055] The resin pressure increases as the number of resin co-extrusion times increases. In this way, the pressure applied to the filter of the co-extrusion device can be monitored over time, and when the resin pressure exceeds the set range, the resin pressure can be controlled by replacing the filter. The resin pressure range for forming a sealing layer with a desired melt flow rate may vary depending on the types of resins constituting the first sealing layer and the second sealing layer, the thicknesses of the first sealing layer and the second sealing layer, and the type of co-extrusion device. With this knowledge, those skilled in the art can obtain the resin pressure range that makes the melt flow rate of the sealing layer 14.0 g / 10 min or less through routine experiments.

[0056] The ratio of the thickness of the second sealing layer to the thickness of the first sealing layer can be 0.8 to 1.2, preferably 0.9 to 1.1. When the ratio of the thickness of the second sealing layer to the thickness of the first sealing layer satisfies the above range, the sealing layer has excellent adhesion to the barrier layer, and the bag has excellent insulation and sealing strength. If one of the first sealing layer and the second sealing layer is too thick or too thin, the resin constituting the sealing layer may flow out during the sealing process, or the adhesion and sealing performance between the sealing layer and the barrier layer may deteriorate.

[0057] More specifically, the thickness of the first sealing layer can be 25 μm to 80 μm, preferably 30 μm to 70 μm, more preferably 30 μm to 60 μm. When the thickness of the first sealing layer satisfies the above range, the adhesion to the barrier layer is excellent.

[0058] The thickness of the second sealing layer can be from 20 μm to 80 μm, preferably from 25 μm to 70 μm, and more preferably from 30 μm to 60 μm. When the thickness of the second sealing layer meets the above range, the sealing performance is excellent.

[0059] The total thickness of the sealing layer (the combination of the first sealing layer and the second sealing layer) can be from 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 sealing layer meets the above range, the outflow of the resin during sealing is suppressed, and the heat and time required for sealing can be appropriately adjusted.

[0060] (2) Barrier layer

[0061] The barrier layer 20 can be configured to provide mechanical strength to the bag, block the introduction and discharge of gas or moisture from outside the secondary battery, and prevent electrolyte leakage.

[0062] The thickness of the barrier layer 20 can be from 40 μm to 100 μm, preferably from 40 μm to 90 μm, and more preferably from 50 μm to 80 μm. When the thickness of the barrier layer meets the above range, appropriate mechanical strength and barrier properties can be achieved.

[0063] The barrier layer 20 can be made of a metal material, specifically an aluminum alloy film.

[0064] The aluminum alloy film can contain aluminum and metal elements other than aluminum, for example, one or more metal elements selected from iron (Fe), copper (Cu), chromium (Cr), manganese (Mn), nickel (Ni), magnesium (Mg), or zinc (Zn).

[0065] The iron (Fe) content of the aluminum alloy film can be from 1.2 wt% to 1.7 wt%, preferably from 1.3 wt% to 1.7 wt%, and more preferably from 1.3 wt% to 1.45 wt%. When the iron (Fe) content in the aluminum alloy film meets the above range, even if the cup-shaped portion is deeply drawn, defects such as cracks or pinholes are not easily formed in the barrier layer 20.

[0066] (3) Substrate layer

[0067] The substrate layer 10 can be provided on the outermost layer of the bag and can be configured to protect the electrode assembly from external impacts and electrically insulate the electrode assembly.

[0068] The substrate layer 10 can be made of a polymer material, for example, one or more polymer materials including polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymer, polyacrylonitrile, polyimide, polyamide, cellulose, aromatic polyamide, nylon, polyester, poly(p-phenylene benzobisoxazole), polyarylate, or Teflon.

[0069] The base material layer 10 may have a single-layer structure, or as Figure 2 shown, it may have a multi-layer structure formed by stacking different polymer films 12 and 14. When the base material layer 10 has a multi-layer structure, an adhesive layer 16a may be provided between the polymer films. Whether it is a single-layer structure or a multi-layer structure, the base material layer 10 can be adhered to the barrier layer 20 via the adhesive layer 16b or any other mechanism known in the art.

[0070] The total thickness of the base material layer 10 can be 10 μm to 60 μm, preferably 20 μm to 50 μm, and more preferably 30 μm to 50 μm. When the base material layer has a multi-layer structure, this thickness can be the thickness including the adhesive layer. When the base material layer 10 meets the above range, the durability, insulation, and formability can be excellent. When the thickness of the base material layer is too thin, the durability may decrease, and the base material layer may be easily damaged during the forming process. When the thickness of the base material layer 10 is too thick, the formability of the bag film stack decreases. In addition, as the thickness of the base material layer 10 increases, the overall thickness of the bag may increase, thereby reducing the battery accommodation space and the size of the electrode assembly 200 that can be provided therein, resulting in a decrease in energy density.

[0071] According to one embodiment, the base material layer 10 may have a stacked structure of a polyethylene terephthalate (PET) film (e.g., the polymer film 12) and a nylon film (e.g., the polymer film 14). Here, the nylon film can be disposed at the position closest to the barrier layer 20, and the polyethylene terephthalate film can be stacked on the nylon film and disposed on the outer surface side of the bag.

[0072] The polyethylene terephthalate (PET) material has excellent durability and electrical insulation. Therefore, when the PET film is placed on the surface side, the durability and insulation can be excellent. However, the PET film may not adhere firmly to the aluminum alloy film constituting the barrier layer 20. Even if the PET film is fixed to the aluminum alloy film, the PET film may change the stretching behavior of the bag film stack. Therefore, during the forming process, the base material layer 10 and the barrier layer 20 may be peeled off from each other, and / or the barrier layer may be unevenly stretched, resulting in deteriorated formability of the bag film stack. In contrast, since the nylon film has a stretching behavior similar to that of the aluminum alloy film constituting the barrier layer 20, providing a nylon film between the polyethylene terephthalate and the barrier layer has the effect of improving the formability of the bag film stack.

[0073] The thickness of the polyethylene terephthalate film can be 5 μm to 20 μm, preferably 5 μm to 15 μm, and more preferably 7 μm to 15 μm. The thickness of the nylon film can be 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, the formability and the rigidity after forming can be excellent.

[0074] Lithium secondary battery

[0075] Next, the lithium secondary battery of the present invention will be described.

[0076] The lithium secondary battery may include: 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. Here, the battery case is the above-mentioned pouch.

[0077] More specifically, the pouch can be designed such that the sealing layer (for example, the first sealing layer and the second sealing layer) has a melt flow rate of 14.0 g / 10 min or less, thereby generating a higher sealing strength than the pouches of the prior art. Therefore, when forming a lithium secondary battery using the pouch of the present invention, even if the internal pressure of the secondary battery increases, the sealing layer is not easily broken. Therefore, the resulting secondary battery is reliable even at high temperatures.

[0078] Specifically, the cell vent pressure resistance of the lithium secondary battery at a temperature of 60 °C can be 7.7 bar or more, preferably 7.7 bar to 15 bar, and more preferably 8 bar to 15 bar.

[0079] In addition, the accelerated high-temperature storage period of the lithium secondary battery measured by charging to SOC 100% at one-day intervals under temperature conditions of 70 °C can be 15 days or more, preferably 15 days to 30 days, and more preferably 15 days to 25 days.

[0080] Since the specific details related to the pouch are the same as those described above, the remaining components other than the pouch will be described below.

[0081] Figure 2 It is a diagram of a lithium secondary battery according to an embodiment of the present invention. Hereinafter, each component of the lithium secondary battery of the present invention will be described in more detail with reference to Figure 2 Each component of the lithium secondary battery of the present invention will be described in more detail.

[0082] Electrode assembly

[0083] The electrode assembly 200 may include a plurality of electrodes and a plurality of separators alternately stacked. The plurality of electrodes may include a positive electrode and a negative electrode alternately stacked with a separator therebetween.

[0084] The positive and negative electrodes can be prepared by coating a collector with a composition for forming an active material layer containing an electrode active material and then drying the composition.

[0085] The composition for forming a positive electrode active material layer may include a positive electrode active material, a binder, and a conductive material. The composition for forming a negative electrode active material layer may include a negative electrode active material, a binder, and a conductive material.

[0086] The collector is not particularly limited as long as it does not cause a chemical change in the battery and has high conductivity. For example, the collector can be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel whose surface can be treated with carbon, nickel, titanium or silver, or an aluminum cadmium alloy. In addition, the thickness of the collector can be 3 μm to 500 μm, and fine irregularities can be formed on the surface of the collector to improve the binding force of the active material. For example, the negative electrode collector can have various shapes such as a film, sheet, foil, net, porous body, foam, or non-woven fabric.

[0087] The positive electrode active material can utilize various positive electrode active materials known in the art that can cause an electrochemical reaction. Exemplary materials include: layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2); lithium manganese oxide; lithium nickel oxide represented by the formula Li 1-y M y O2 (where M = Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn or Ga, and contains one or more of these elements, 0.01 ≤ y ≤ 0.7); Li 1+z Ni b Mn c Co 1-(b+c+d) M d O (2-e) A e represented by lithium nickel cobalt manganese composite oxide, such as Li 1+z Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2 or Li 1+z Ni 0.4 Mn 0.4 Co 0.2 O2 (where -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); or the formula Li 1+x M 1-y M’ y PO 4-z X zOlivine lithium metal phosphate represented by (where M = transition metal, preferably Fe, Mn, Co or Ni, M' = Al, Mg or Ti, X = F, S or N, -0.55 ≤ x ≤ +0.5, 0 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.1). The above are only examples of materials and not an exhaustive list. Based on the total weight of the positive electrode active material layer, the content of the positive electrode active material can be 80% by weight to 99% by weight.

[0088] Compounds capable of reversibly inserting and extracting lithium can be used as the negative electrode active material. Specific examples can include: carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber and amorphous carbon; metal compounds capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys or Al alloys; metal oxides capable of doping and undoping lithium, such as SiO β (0 < β < 2), SnO2, vanadium oxides and lithium vanadium oxides; or composites containing the above metal compounds and carbonaceous materials, such as Si-C composites or Sn-C composites. Any one or a mixture of two or more of the above materials can be used. Additionally, a thin film of metallic lithium can be used as the negative electrode active material. Furthermore, both low-crystalline carbon and high-crystalline carbon can be used as carbon materials. Representative examples of low-crystalline carbon can include soft carbon and hard carbon, while high-crystalline carbon can include amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase carbon microspheres, mesophase pitch and high-temperature calcined coke such as coke derived from petroleum or coal tar pitch. Based on the total weight of the negative electrode active material layer, the content of the negative electrode active material can be 80% by weight to 99% by weight.

[0089] The binder is a component that aids in the binding between the conductive auxiliary material, the active material and the current collector. Based on the total weight of the active material layer, its addition amount is usually 0.1% by weight to 10% by weight. Examples of the binder can include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber, nitrile rubber, fluororubber and various copolymers thereof.

[0090] The conductive material can be a component for further improving the conductivity of the active material, and based on the total weight of the active material layer, its addition amount can be 10% by weight or less, preferably 5% by weight or less. The conductive material can be not particularly limited as long as the material does not cause chemical changes in the battery and has conductivity. For example, the conductive material can 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 cracking carbon black; conductive fibers, such as carbon fibers and metal fibers; fluorocarbons; metal powders, such as aluminum and nickel powders; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium dioxide; or polyphenylene derivatives.

[0091] The separator can separate the negative electrode from the positive electrode and provide a channel for the movement of lithium ions, and can be used without particular limitation as long as it is commonly used as a separator in lithium secondary batteries. That is, the separator preferably has a low resistance to the ion movement of the electrolyte and has an excellent ability to absorb the electrolyte. Specifically, the separator can include, for example, a porous polymer film made of polyolefin polymers (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, or ethylene / methacrylate copolymer), or a stacked structure of at least two layers thereof. In addition, the separator can be a conventional porous non-woven fabric, for example, a non-woven fabric made of glass fibers or polyethylene terephthalate fibers having a high melting point. Furthermore, a coated separator containing a ceramic component or a polymer material can be used to provide heat resistance or mechanical strength, and can be selectively used in a single-layer or multi-layer structure.

[0092] The electrode assembly 200 can include a plurality of electrode tabs 230 that are welded to each other. Each of the plurality of electrode tabs 230 can be connected to a corresponding one of the plurality of electrodes 210 and protrude from the electrode assembly 200 so as to serve as a channel through which electrons move between the inside and the outside of the electrode assembly 200. The plurality of electrode tabs 230 can be disposed inside the pouch 100.

[0093] As Figure 1 shown, the electrode tab 230 connected to the positive electrode and the electrode tab 230 connected to the negative electrode can protrude in opposite directions with respect to the electrode assembly 200. However, the present invention is not limited thereto. For example, the electrode tab 230 connected to the positive electrode and the electrode tab 230 connected to the negative electrode can protrude in the same direction from the same side of the electrode assembly, such that the electrode tab 230 connected to the positive electrode and the electrode tab 230 connected to the negative electrode are parallel to each other.

[0094] The lead 240 for supplying power to the outside of the secondary battery can be connected to the plurality of electrode tabs 230 by spot welding or the like. One end of the lead 240 can be connected to the plurality of electrode tabs 230, while the other end can protrude to the outside of the pouch 100.

[0095] A portion of the lead wire 240 may be surrounded by the insulating portion 250. For example, the insulating portion 250 may include insulating tape. The insulating portion 250 may be disposed between the stepped portion 120 of the second housing 102 and the stepped portion 120 of the first housing 101, and in this state, the stepped portions 120 of the first housing and the second housing may be heat-fused to each other. As a result, a portion of the stepped portion 120 of the first housing 101 and a portion of the stepped portion 120 of the second housing 102 may be heat-fused to the insulating portion 250. Thus, the insulating portion 250 may prevent the gas generated by the electrode assembly 200 from flowing through the bag 100 and may keep the bag 100 in a sealed state.

[0096] Electrolyte

[0097] The electrolyte may be configured to allow lithium ions generated by the electrochemical reaction of the electrodes during the charge and discharge process of the secondary battery to move, and may include an organic solvent and a lithium salt.

[0098] The organic solvent may be used without particular limitation as long as it can serve as a medium through which ions participating in the electrochemical reaction of the battery move. Examples of the organic solvent may include: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether or tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (R is a C2 to C20 linear, branched, or cyclic hydrocarbon group, which may contain a double bond, an aromatic ring, or an ether bond); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane; or sulfolane. Among the above examples, carbonate solvents are preferred, and more preferably a mixture of a cyclic carbonate (such as ethylene carbonate or propylene carbonate, etc.) having high ionic conductivity and high dielectric constant that can improve the charge and discharge performance of the battery and a low-viscosity linear carbonate compound (such as methyl ethyl carbonate, dimethyl carbonate, or diethyl carbonate).

[0099] The lithium salt can be used without particular limitation as long as it is a compound capable of providing lithium ions for a lithium secondary battery. Specifically, the lithium salt may include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2, etc. The concentration of the lithium salt is preferably in the range of 0.1 M to 5.0 M, more preferably 0.1 M to 3.0 M. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance and enabling effective movement of lithium ions.

[0100] To improve the life characteristics of the battery, suppress the decline in battery capacity, and improve the discharge capacity of the battery, the electrolyte may further contain an additive.

[0101] Example

[0102] Hereinafter, the present invention will be described in more detail through specific examples.

[0103] Manufacturing Example

[0104] In the manufacturing example, a stacked film was prepared. The stacked film includes a substrate layer in which a polyethylene terephthalate film layer with a thickness of 12 μm, an adhesive layer with a thickness of 3 μm, and a nylon film with a thickness of 15 μm are stacked in sequence. Subsequently, a polyurethane adhesive with a thickness of 3 μm was coated on the nylon film of the substrate layer, and an aluminum alloy thin film with a thickness of 40 μm (constituting a barrier layer) was stacked thereon by a dry lamination method.

[0105] Next, on the surface opposite to the surface of the stacked substrate layer of the aluminum alloy thin film, a sealing layer including a first sealing layer and a second sealing layer was formed by co-extruding a maleic anhydride-modified polypropylene resin and a polypropylene resin. During the co-extrusion process, the maleic anhydride-modified polypropylene resin layer is in direct contact with the aluminum alloy thin film, and the thickness of each of the first sealing layer and the second sealing layer is 40 μm.

[0106] During the formation of the sealing layer, bag film stacks A to F (as shown in Table 1 below) were manufactured by co-extrusion while controlling the resin pressure.

[0107] Then, stack the prepared bag films and cut them into a size of 50 mm × 300 mm. Put the stacked and cut bag films into hydrochloric acid with a concentration of 37 wt% to melt the aluminum alloy film, and then separate the film-like sealing layer. Next, thoroughly wash the separated sealing layer with water and dry it at a temperature of 60 °C for 2 hours, and then roll it into a cylinder to prepare a sample. Then, put the sample into a melt flow rate measuring device (Gottfert Mi-40) and melt it at a temperature of 230 °C for 5 minutes, and measure the melt flow rate (MFR) of the sealing layer while applying a load of 2.16 kg. The measurement is carried out by the melt volume rate (MVR) method, assuming that the melt density of the sealing layer is 0.728 g / cm 3 .

[0108] The measurement results are shown in Table 1 below.

[0109] [Table 1]

[0110] Sealing layer MFR (g / 10min) A 9.12 B 10.68 C 11.40 D 13.20 E 13.56 F 15.36

[0111] Examples and Comparative Examples

[0112] By stretching and forming each of the bag film stacks A to F prepared in the above manufacturing examples, pouches for secondary batteries of Examples 1 to 5 and Comparative Example 1 with cup-shaped portions were prepared. Then, the sealing layers of the respective pouches prepared according to Examples 1 to 5 and Comparative Example 1 were sealed by thermal bonding, and the sealed portions were cut into a width of 15 mm to prepare samples. Then, the sealed portions of the samples were opened, and the samples were mounted on a peel strength measuring device with a distance of 30 mm between the jigs. Then, a T-peel test was carried out at a speed of 5 mm / min at room temperature of 25 °C and high temperature of 60 °C to measure the sealing strength. Here, the sealing strength is expressed as the percentage of the peel strength of Examples 2 to 5 and Comparative Example 1 with respect to the reference value when the peel strength of Example 1 is defined as the reference value (100).

[0113] The measurement results are shown in Table 2 below.

[0114] [Table 2]

[0115] Film stack for bag Sealing strength at 25°C (%) Sealing strength at 60°C (%) Example 1 A 100 100 Example 2 B 94.7 100 Example 3 C 89.4 98.1 Example 4 D 75.2 93.3 Example 5 E 73.5 81.0 Comparative example 1 F 38.1 21.0

[0116] As shown in Table 2, the pouches of Examples 1 to 5 with a melt flow rate of the sealing layer of 14 g / 10 min or less have significantly better sealing performance at room temperature and high temperature than the pouch of Comparative Example 1 with a melt flow rate of the sealing layer exceeding 14 g / 10 min.

[0117] Experimental Example: Exhaust Pressure Resistance and High-Temperature Acceleration Test of the Core of a Secondary Battery

[0118] The following methods were also used to conduct the cell exhaust pressure resistance and high-temperature acceleration tests. The stacked electrode assemblies were housed in the secondary battery bags prepared in Examples 1 to 5 and Comparative Example 1, the electrolyte was injected, and then the sealing layer was sealed to manufacture lithium secondary batteries. The cell exhaust pressure resistance and accelerated high-temperature shelf life of each manufactured lithium secondary battery were measured as follows. For accurate evaluation, each lithium secondary battery was measured two or three times, and the measurement results are shown in Figure 3 .

[0119] (1) Method for measuring cell exhaust pressure resistance:

[0120] A hole with a diameter of 1 mm or less was drilled in each lithium secondary battery, and while injecting an inert gas at temperatures of 25 °C and 60 °C, the internal pressure of the cell was measured over time, and the maximum internal pressure reached until exhaust occurred was evaluated as the cell exhaust pressure resistance.

[0121] Since the cell pressure drops to atmospheric pressure when there is exhaust, the point at which the cell pressure drops to atmospheric pressure can be expressed as the exhaust occurrence time point.

[0122] (2) Accelerated high-temperature shelf life:

[0123] The lithium secondary battery was stored at a temperature of 70 °C, then charged to SOC 100% at one-day intervals, and the shelf life that the secondary battery could be stored without exhaust was measured.

[0124] As Figure 3 shown, in the lithium secondary batteries of Examples 1 to 5 using bags with a melt flow rate of the sealing layer of 14 g / 10 min or less within the scope of the present invention, the cell exhaust pressure resistance at temperatures of 60 °C and 25 °C was higher than 7.7 bar, and the accelerated high-temperature shelf life was also higher than 15 days. On the contrary, in the lithium secondary battery of Comparative Example 1 using a bag with a melt flow rate of the sealing layer exceeding 14 g / 10 min, at a temperature of 60 °C, the cell exhaust pressure resistance and the accelerated high-temperature shelf life were significantly reduced.

Claims

1. A pouch for a secondary battery, comprising: Barrier layer; Substrate layer provided on one surface of the barrier layer; and Sealing layer provided on the other surface of the barrier layer, The sealing layer includes: A first sealing layer provided to be in direct contact with the other surface of the barrier layer; and A second sealing layer provided on the first sealing layer, and The melt flow rate (MFR) of the sealing layer measured at a temperature of 230°C under a load condition of 2.16 kg is 14.0 g / 10 min or less.

2. The pouch according to claim 1, wherein, The melt flow rate (MFR) of the sealing layer measured at a temperature of 230°C under a load condition of 2.16 kg is 8.5 g / 10 min to 14.0 g / 10 min.

3. The pouch according to claim 1, wherein, The first sealing layer and the second sealing layer have a co-extrusion structure.

4. The pouch according to claim 1, wherein, The first sealing layer contains an acid-modified polyolefin resin.

5. The pouch according to claim 1, wherein, The second sealing layer contains a polyolefin resin.

6. The pouch according to claim 1, wherein, The ratio of the thickness of the second sealing layer to the thickness of the first sealing layer is 0.8 to 1.

2.

7. The pouch according to claim 1, wherein, The thickness of the first sealing layer is 25 μm to 80 μm, and The thickness of the second sealing layer is 20 μm to 80 μm.

8. The pouch according to claim 1, wherein, The total thickness of the sealing layer is 45 μm to 100 μm.

9. The pouch according to claim 1, wherein, The barrier layer contains an aluminum alloy layer.

10. The pouch according to claim 1, wherein, The substrate layer contains polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymer, polyacrylonitrile, polyimide, polyamide, cellulose, aromatic polyamide, nylon, polyester, poly(p-phenylene benzobisoxazole), polyarylate, Teflon, or a combination thereof.

11. A lithium secondary battery, comprising: An electrode assembly formed by stacking a positive electrode, a separator, and a negative electrode; An electrolyte; and The pouch according to claim 1, wherein, The bag defines a cup-shaped portion for accommodating the electrode assembly and the electrolyte.

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

13. The lithium secondary battery according to claim 11, wherein, The accelerated high-temperature storage period measured by charging the lithium secondary battery to SOC 100% at intervals of 1 day at a temperature of 70°C is 15 days or more.

14. A method for forming a pouch for a secondary battery, the method comprising the following steps: Stack the substrate layer on the first surface of the barrier layer; and Co-extrude a sealing layer including a first sealing layer and a second sealing layer on the second surface of the barrier layer, wherein, after the co-extrusion step, the melt flow rate (MFR) of the sealing layer measured at a temperature of 230°C under a load condition of 2.16 kg is 14.0 g / 10 min or less.

15. The method according to claim 14, wherein, After the co-extrusion step, the first sealing layer is in direct contact with the barrier layer, and The second sealing layer is stacked on the surface of the first sealing layer.

16. The method according to claim 14, wherein, The first sealing layer contains an acid-modified polyolefin resin, and the second sealing layer contains a polyolefin resin.

17. The method according to claim 14, wherein, During the co-extrusion step, control the resin pressure using a co-extrusion device.

18. The method according to claim 17, further comprising: When the resin pressure exceeds a specified value, replace the filter of the co-extrusion device.

19. The method according to claim 14, wherein, The sealing layer is co-extruded to have a thickness of 45 μm to 100 μm.

20. The method according to claim 19, wherein, The sealing layer is co-extruded such that the thickness of the first sealing layer is 25 μm to 80 μm, and the thickness of the second sealing layer is 20 μm to 80 μm.

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