Patent Description:
The present invention relates to a pouch type battery case and a pouch type secondary battery, and more particularly, to a pouch type battery case, in which formability is improved by improving tensile strength and elongation, and a pouch type secondary battery.

In general, types of secondary batteries include a nickel cadmium battery, a nickel hydride battery, a lithium ion battery, and a lithium ion polymer battery. These secondary batteries are not only applied and used in small products such as digital cameras, P-DVDs, MP3Ps, mobile phones, PDAs, portable game devices, power tools, and E-bikes, but are also applied and used in large products requiring high output, such as electric vehicles and hybrid vehicles, and a power storage device and a power storage device for backup which store surplus generated power or renewable energy.

In order to prepare such a secondary battery, first, a positive electrode collector and a negative electrode collector are respectively coated with electrode active material slurries to prepare a positive electrode and a negative electrode, and the positive electrode and the negative electrode are then stacked on both sides of a separator to form an electrode assembly having a predetermined shape. Then, after the electrode assembly is accommodated in a battery case and an electrolyte solution is injected, the battery case is sealed.

The secondary battery is classified into a pouch type, a can type, and the like, according to a material of a case for accommodating the electrode assembly. The pouch type accommodates the electrode assembly in a pouch formed of a flexible polymer material. In addition, the can type accommodates the electrode assembly in a case formed of a material such as metal or plastic.

The pouch, which is a case of the pouch type secondary battery, is prepared by press working of a flexible pouch film laminate to form a cup portion. Then, when the cup portion is formed, the electrode assembly is accommodated in an accommodating space of the cup portion and a sealing portion is sealed to prepare a secondary battery.

Drawing in the press working is performed by inserting the pouch film laminate into press equipment and applying a pressure to the pouch film laminate with a punch to stretch the pouch film laminate. The pouch film laminate is composed of a plurality of layers, and a gas barrier layer disposed therein is formed of metal. However, a conventional pouch film laminate had a limitation in forming a deeper cup portion, and also had a limitation in reducing a radius of filleting when edges of a bottom portion and edges of an open portion of the cup portion are filleted. Furthermore, the conventional pouch film laminate had a limitation in forming an outer wall of the cup portion close to vertical. Accordingly, there was a problem in that a dead space of the secondary battery was increased and a size of the electrode assembly was decreased to reduce energy efficiency to volume. <CIT> discloses a battery packaging material, a battery, a method for producing battery packaging material, and an aluminum alloy foil.

An aspect of the present invention provides a pouch type battery case, in which formability is improved by improving tensile strength and elongation, and a pouch type secondary battery.

According to the present invention, there is provided a pouch type battery case according to the appended claims. Also, the aluminum alloy film may include iron in an amount of <NUM> wt% to <NUM> wt%.

Furthermore, the aluminum alloy film may include iron in an amount of <NUM> wt% to <NUM> wt%.

Also, the aluminum alloy film may include silicon in an amount of <NUM> wt% or less.

Furthermore, the aluminum alloy film may have a grain size of <NUM> to <NUM>.

Also, the aluminum alloy film may have alloy number AA8021.

Furthermore, the gas barrier layer may have a thickness of <NUM> to <NUM>.

Also, a thickness of the sealant layer , for example, may be in a range of <NUM> to <NUM>.

Furthermore, the first polymer may include polypropylene (PP).

Also, the surface protection layer may have a thickness of <NUM> to <NUM>.

Furthermore, the second polymer may include polyethylene terephthalate (PET).

Also, the pouch type battery case may further include a drawing assistance layer which is formed of a third polymer and is laminated between the surface protection layer and the gas barrier layer.

Furthermore, the drawing assistance layer may have a thickness of <NUM> to <NUM>.

Also, the third polymer may include Nylon.

Furthermore, the pouch film laminate may have a total thickness of <NUM> or more, for example, <NUM> to <NUM>.

Also, the pouch film laminate may have a tensile strength, which is measured while the pouch film laminate is pulled at a tensile speed of <NUM>/min after being cut to a size of <NUM> × <NUM>, of <NUM> N/<NUM> to <NUM> N/<NUM>, and may have an elongation of <NUM>% to <NUM>%.

According to another aspect of the present invention, there is provided a pouch type secondary battery including an electrode assembly therein which is formed by stacking a positive electrode, a separator, and a negative electrode; and a pouch type battery case accommodating the electrode assembly, wherein the battery case is as defined above.

Other specific details of the present invention are included in the detailed description and drawings.

According to the embodiments of the present invention, at least the following effects may be achieved.

A pouch type battery case according to the present invention improves tensile strength, elongation, and toughness of a pouch film laminate by using the pouch film laminate including an aluminum alloy film having specific thickness and grain size as a gas barrier layer. If the pouch film laminate is used, since a forming depth may be increased without occurrence of pinholes or cracks during forming of a cup portion and a radius of curvature of an edge of the cup portion may be reduced, an accommodating space volume of a battery assembly may be increased.

Also, since the pouch film laminate according to the present invention has excellent puncture strength, it may more effectively protect an internal electrode assembly even if it is under great pressure from the outside or is damaged by being pierced by a sharp object.

Advantages and features of the present invention, and implementation methods thereof will be clarified through following embodiments described with reference to the accompanying drawings. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Further, the present invention is only defined by scopes of claims.

Unless defined otherwise, all terms (including technical and scientific terms) used herein may be intended to have meanings understood by those skilled in the art. In addition, terms defined in general dictionaries should not be interpreted abnormally or exaggeratedly, unless clearly specifically defined.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the present invention. In the specification, the terms of a singular form may include plural forms unless referred to the contrary. It will be further understood that the terms "comprises" and/or "comprising" when used in this specification, specify the presence of stated components, but do not preclude the presence or addition of one or more other components.

<FIG> is an assembly view of a secondary battery <NUM> according to an embodiment of the present invention.

According to an embodiment of the present invention, since toughness is increased by improving tensile strength and elongation of a pouch film laminate <NUM>, formability may be improved when the pouch film laminate <NUM> is formed to prepare a pouch type battery case <NUM>. Also, since the pouch film laminate has excellent puncture strength, it may more effectively protect an internal electrode assembly even if it is under great pressure from the outside or is damaged by being pierced by a sharp object.

For this purpose, the pouch type battery case <NUM> according to an embodiment of the present invention is the pouch type battery case <NUM> accommodating an electrode assembly <NUM> which is formed by stacking a positive electrode, a separator, and a negative electrode, and includes the pouch film laminate <NUM> including a sealant layer <NUM> formed of a first polymer as an innermost layer; a surface protection layer <NUM> formed of a second polymer as an outermost layer; and a gas barrier layer <NUM> laminated between the surface protection layer <NUM> and the sealant layer <NUM> and formed of an aluminum alloy film having a thickness of <NUM> to <NUM> and a grain size of <NUM> to <NUM>.

The secondary battery <NUM> according to the embodiment of the present invention includes the electrode assembly <NUM> which is formed by stacking a positive electrode, a separator, and a negative electrode; and the pouch type battery case <NUM> accommodating the electrode assembly <NUM>, wherein the battery case <NUM> includes the pouch film laminate <NUM> including: the sealant layer <NUM> formed of a first polymer as an innermost layer; the surface protection layer <NUM> formed of a second polymer as an outermost layer; and the gas barrier layer <NUM> laminated between the surface protection layer <NUM> and the sealant layer <NUM> and formed of an aluminum alloy film having a thickness of <NUM> to <NUM> and a grain size of <NUM> to <NUM>.

The electrode assembly <NUM> is formed by sequentially stacking a positive electrode, a separator, and a negative electrode. First, a slurry, in which an electrode active material, a binder, and a conductive material are mixed, is applied to a positive electrode collector and a negative electrode collector to prepare the positive electrode and the negative electrode, and, after the electrode assembly <NUM> having a predetermined shape is formed by stacking the positive electrode and the negative electrode on both sides of the separator, the electrode assembly <NUM> is inserted into the battery case <NUM>, and the battery case <NUM> is sealed after injecting an electrolyte solution.

Specifically, the electrode assembly <NUM> includes two types of electrodes, such as the positive electrode and the negative electrode, and the separator disposed between the electrodes to insulate the electrodes from each other. The electrode assembly <NUM> includes a stacked type, a jelly-roll-type, and a stack and folding type. The two types of electrodes, that is, the positive electrode and the negative electrode, have a structure in which active material slurries are applied to electrode collectors in the form of a metal foil or metal mesh including aluminum and copper, respectively. The active material slurry may be typically formed by stirring a granular active material, a conductive material, and a binder in a state in which a solvent is added. The solvent is removed in a subsequent process.

As illustrated in <FIG>, the electrode assembly <NUM> includes an electrode tab <NUM>. The electrode tab <NUM> is connected to each of the positive electrode and the negative electrode of the electrode assembly <NUM> and protrudes from one side of the electrode assembly <NUM> to the outside so that it becomes a path through which electrons may move between the inside and the outside of the electrode assembly <NUM>. The current collector of the electrode assembly <NUM> is composed of a portion to which the electrode active material is applied and an end portion to which the electrode active material is not applied, that is, an uncoated portion. The electrode tab <NUM> may be formed by cutting the uncoated portion or may be formed by connecting a separate conductive member to the uncoated portion by ultrasonic welding or the like. As illustrated in <FIG>, the electrode tabs <NUM> may protrude side by side from one side of the electrode assembly <NUM> in the same direction, but are not limited thereto and may protrude in different directions, respectively.

An electrode lead <NUM> is connected to the electrode tab <NUM> of the electrode assembly <NUM> by spot welding or the like. In addition, a portion of the electrode lead <NUM> is surrounded by an insulating portion <NUM>. The insulating portion <NUM> is limitedly located at a sealing portion <NUM>, to which a first case <NUM> and a second case <NUM> of the battery case <NUM> are thermally fused, to adhere the electrode lead <NUM> to the battery case <NUM>. In addition, the insulating portion <NUM> prevents flow of electricity generated from the electrode assembly <NUM> to the battery case <NUM> through the electrode lead <NUM>, and maintains sealing of the battery case <NUM>. Thus, the insulating portion <NUM> is formed of an insulator having non-conductivity which does not conduct electricity well. In general, as the insulating portion <NUM>, an insulating tape, which is easy to be attached to the electrode lead <NUM> and is relatively thin, is widely used, but the present invention is not limited thereto and various members may be used as long as they may insulate the electrode lead <NUM>.

One end of the electrode lead <NUM> is connected to the electrode tab <NUM>, and the other end thereof protrudes to the outside of the battery case <NUM>. That is, the electrode lead <NUM> includes a positive electrode lead <NUM>, which has one end connected to a positive electrode tab <NUM> and extends in a protruding direction of the positive electrode tab <NUM>, and a negative electrode lead <NUM> which has one end connected to a negative electrode tab <NUM> and extends in a protruding direction of the negative electrode tab <NUM>. As illustrated in <FIG>, the other ends of both of the positive electrode lead <NUM> and the negative electrode lead <NUM> protrude to the outside of the battery case <NUM>. Accordingly, the electricity generated inside the electrode assembly <NUM> may be supplied to the outside. Also, since the positive electrode tab <NUM> and the negative electrode tab <NUM> are formed to respectively protrude in various directions, the positive electrode lead <NUM> and the negative electrode lead <NUM> may also respectively extend in various directions.

Materials of the positive electrode lead <NUM> and the negative electrode lead <NUM> may be different from each other. That is, the positive electrode lead <NUM> may be formed of an aluminum (Al) material that is the same as the positive electrode collector, and the negative electrode lead <NUM> may be formed of a copper (Cu) material or nickel (Ni)-coated copper material that is the same as the negative electrode collector. Since a portion of the electrode lead <NUM> protruding to the outside of the battery case <NUM> becomes a terminal portion, it is electrically connected to an external terminal.

The battery case <NUM> is formed by forming a pouch film laminate which is formed of a flexible material. Hereinafter, the battery case <NUM> will be described as a pouch. The battery case <NUM> accommodates the electrode assembly <NUM> so that the portion of the electrode lead <NUM>, that is, the terminal portion is exposed and is sealed. As illustrated in <FIG>, the battery case <NUM> includes the first case <NUM> and the second case <NUM>. A cup portion <NUM> is formed in the second case <NUM> to provide an accommodation space <NUM> capable of accommodating the electrode assembly <NUM>, and the first case <NUM> covers the accommodation space <NUM> from the top so that the electrode assembly <NUM> is not separated to the outside of the battery case <NUM>. In this case, as illustrated in <FIG>, the cup portion <NUM> provided with the accommodation space <NUM> may also be formed in the first case <NUM> to accommodate the electrode assembly <NUM> from the top. The first case <NUM> and the second case <NUM> may be prepared by connecting one sides thereof to each other as illustrated in <FIG>, but the present invention is not limited thereto and the first case <NUM> and the second case <NUM> may be prepared in various ways, for example, the first case <NUM> and the second case <NUM> are separated from each other and prepared separately.

When the electrode lead <NUM> is connected to the electrode tab <NUM> of the electrode assembly <NUM> and the insulating portion <NUM> is formed on the portion of the electrode lead <NUM>, the electrode assembly <NUM> is accommodated in the accommodation space <NUM> provided in the cup portion <NUM> of the second case <NUM>, and the first case <NUM> covers the space from the top. Then, an electrolyte solution is injected therein and the sealing portion <NUM> formed on edges of the first case <NUM> and the second case <NUM> is sealed. The electrolyte solution is to move lithium ions which are generated by an electrochemical reaction of the electrode during charge and discharge of the secondary battery <NUM>, wherein it may include a non-aqueous organic electrolyte solution, which is a mixture of a lithium salt and high purity organic solvents, or a polymer using a polymer electrolyte. Through the above method, the pouch type secondary battery <NUM> may be prepared.

<FIG> is a cross-sectional view of the pouch film laminate <NUM> according to an embodiment of the present invention.

The battery case <NUM> of the pouch type secondary battery <NUM> according to the embodiment of the present invention is prepared by drawing the pouch film laminate <NUM>. That is, the battery case is prepared by stretching the pouch film laminate <NUM> with a punch or the like to form the cup portion <NUM>. According to an embodiment of the present invention, the pouch film laminate <NUM>, as illustrated in <FIG>, includes the sealant layer <NUM>, the gas barrier layer <NUM>, and the surface protection layer <NUM>, and, if necessary, may further include a drawing assistance layer <NUM>.

The sealant layer <NUM> is formed of a first polymer, and is formed as an innermost layer to be directly in contact with the electrode assembly <NUM>. Herein, the innermost layer refers to a layer positioned at the end of the gas barrier layer <NUM> in a direction in which the electrode assembly <NUM> is positioned. When the pouch film laminate <NUM> having the above-described laminate structure is drawn using a punch or the like, a portion is stretched to form the cup portion <NUM> including the pocket-shaped accommodation space <NUM>. Then, the electrolyte solution is injected when the electrode assembly <NUM> is accommodated in the accommodation space <NUM>. Thereafter, if the upper pouch <NUM> and the lower pouch <NUM> are in contact with each other and the sealing portion <NUM> is thermally compressed, the sealant layers <NUM> are adhered to each other to seal the pouch. In this case, the sealant layer <NUM> must have insulation properties because it is directly in contact with the electrode assembly <NUM>, and must have corrosion resistance because it is also in contact with the electrolyte solution. Also, since it is necessary to completely seal the inside to block material movement between the inside and the outside, it must have high sealing properties. That is, the sealing portion <NUM>, in which the sealant layers <NUM> are adhered to each other, must have excellent thermal adhesive strength. In general, the first polymer for preparing the sealant layer <NUM> may be formed of at least one material selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymer, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, Nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, Teflon, and glass fibers. Particularly, a polyolefin-based resin, such as polypropylene (PP) or polyethylene (PE), is mainly used. Since polypropylene (PP) has excellent mechanical properties, such as tensile strength, stiffness, surface hardness, wear resistance, and heat resistance, and chemical properties such as corrosion resistance, it is mainly used for preparing the sealant layer <NUM>. Furthermore, the sealant layer may also be composed of casted polypropylene, acid modified polypropylene, or a polypropylene-butylene-ethylene terpolymer. Herein, the acid modified polypropylene may be MAH PP (maleic anhydride polypropylene). Also, the sealant layer <NUM> may have a single-layer structure formed of any one material or may have a composite layer structure which is formed by layering two or more materials, respectively.

A thickness of the sealant layer <NUM> is <NUM> times to <NUM> times, preferably may be <NUM> times to <NUM> times, and more preferably may be <NUM> times to <NUM> times a thickness of the gas barrier layer to be described later. In a case in which the thickness of the sealant layer is less than <NUM> times the thickness of the gas barrier layer, sealing durability may be reduced, and, in a case in which the thickness of the sealant layer is greater than <NUM> times, since a total thickness of the pouch may be excessively increased, formability may be reduced. Also, in order to ensure sufficient insulation properties, it is more desirable that the thickness of the sealant layer <NUM> is <NUM> times or more the thickness of the gas barrier layer.

Specifically, the thickness of the sealant layer <NUM> may be in a range of <NUM> to <NUM>, preferably <NUM> to <NUM>, and more preferably <NUM> to <NUM>. In a case in which the thickness of the sealant layer <NUM> is less than <NUM>, the sealing durability may be reduced, for example, internal destruction occurs during sealing, and, in a case in which the thickness of the sealant layer is greater than <NUM>, since the thickness of the pouch may be excessively increased, the formability may be reduced and battery energy density (energy per volume) may be reduced. Also, in order to ensure sufficient insulation properties, it is more desirable that the thickness of the sealant layer <NUM> is <NUM> or more. The reason for this is that, when the thickness of the sealant layer is small, a dielectric breakdown voltage of the pouch film laminate may be decreased to deteriorate the insulation properties, and, in a case in which a battery is prepared by using the pouch film laminate with poor insulation properties, a failure rate may be increased.

The gas barrier layer <NUM> is laminated between the surface protection layer <NUM> and the sealant layer <NUM> to secure mechanical strength of the pouch, block incoming and outgoing of gas or moisture from the outside of the secondary battery <NUM>, and prevent leakage of the electrolyte solution.

The gas barrier layer <NUM> is formed of an aluminum alloy film, and the gas barrier layer <NUM> according to an embodiment of the present invention may particularly be formed of an aluminum alloy film having a thickness of <NUM> to <NUM> and a grain size of <NUM> to <NUM>. The aluminum alloy film is light-weighted while securing more than a predetermined level of mechanical strength, may compensate for electrochemical properties by the electrode assembly <NUM> and the electrolyte solution, and may secure a heat dissipation property.

Specifically, the aluminum alloy film according to the present invention has a thickness of <NUM> to <NUM>, and a grain size of <NUM> to <NUM>. When the thickness and grain size of the aluminum alloy film satisfy the above ranges, a forming depth is increased without occurrence of pinholes or cracks during cup forming.

Conventionally, it is common to form the gas barrier layer <NUM> to a thickness of <NUM> to <NUM>. However, in a case in which the thickness of the gas barrier layer is <NUM> to <NUM>, there was a limitation in increasing the depth of the cup portion <NUM> or forming an outer wall of the cup portion <NUM> close to vertical even if the pouch film laminate <NUM> is drawn, and there was also a limitation in reducing a filleting radius of curvature of an edge of the cup portion <NUM>. Also, since puncture strength was low, there was a problem in that the internal electrode assembly <NUM> was easily damaged when the battery case <NUM> received an impact from the outside.

In order to improve this problem in the present invention, the gas barrier layer <NUM> was formed to a thickness of <NUM> to <NUM>, particularly <NUM> to <NUM>. In a case in which the thickness of the gas barrier layer satisfies the above range, since formability of the gas barrier layer <NUM> is improved, the cup portion <NUM> may be formed deep when the pouch film laminate <NUM> is drawn, the outer wall of the cup portion <NUM> is close to vertical, and the radius of curvature of the edge of the cup portion <NUM> may also be reduced. Then, since a volume of the accommodation space <NUM> is increased, more electrodes and separators may be stacked in the electrode assembly <NUM> accommodated therein and energy efficiency to volume may be increased. However, if the thickness of the gas barrier layer <NUM> is greater than <NUM>, since the total thickness of the pouch may be excessively increased, the energy density to volume of the secondary battery <NUM> may be rather reduced.

Also, in the case that the thickness of the gas barrier layer satisfies the above range, since the puncture strength of the pouch film laminate <NUM> is improved, it may more effectively protect the internal electrode assembly <NUM> even if it is under great pressure from the outside or is damaged by being pierced by a sharp object. Herein, the expression "excellent puncture strength" means that strength when puncturing a hole in the pouch film laminate <NUM> is high.

However, in a case in which only the thickness of the aluminum alloy film is increased, the forming depth may be increased, but, since pinholes or cracks occur in the aluminum alloy film after forming, a problem in the sealing durability occurs. Thus, as a result of a significant amount of research, the present inventors have found that, in a case in which an aluminum alloy film having a grain size of <NUM> to <NUM> was used, the occurrence of pinholes or cracks may be suppressed even if the forming depth was increased, thereby leading to the completion of the present invention. According to the study of the present inventors, in a case in which the grain size of the aluminum alloy film was greater than <NUM>, the strength of the aluminum alloy film was reduced to increase the occurrence of cracks or pinholes during forming, and, in a case in which the grain size was less than <NUM>, since flexibility of the aluminum alloy film was reduced, there was a limitation in improving the formability.

The grain size varies depending on a composition of the aluminum alloy film and a processing method of the aluminum alloy film, and may be measured by observing a cross section in a thickness direction of the aluminum alloy film with a scanning electron microscope (SEM). Specifically, in the present invention, a cross-sectional SEM image in the thickness direction of the aluminum alloy film was obtained using a scanning electron microscope, maximum diameters of <NUM> random grains among grains observed in the SEM image were measured, and an average value thereof was then evaluated as the grain size.

In the aluminum alloy film according to the present invention, a metallic element other than aluminum, for example, at least one selected from the group consisting of iron (Fe), copper (Cu), chromium (Cr), manganese (Mn), nickel (Ni), magnesium (Mg), silicon (Si), and zinc (Zn) may be included.

An iron (Fe) content of the aluminum alloy film may be in a range of <NUM> wt% to <NUM> wt%, preferably <NUM> wt% to <NUM> wt%, and more preferably <NUM> wt% to <NUM> wt%. In a case in which the iron (Fe) content in the aluminum alloy film is less than <NUM> wt%, the strength of the aluminum alloy film may be reduced to cause cracks and pinholes during forming, and, in a case in which the iron content is greater than <NUM> wt%, since the flexibility of the aluminum alloy film is reduced, there is a limitation in improving the formability.

Also, a silicon (Si) content of the aluminum alloy film may be in a range of <NUM> wt% or less, preferably <NUM> wt% to <NUM> wt%, and more preferably <NUM> wt% to <NUM> wt%, for example, <NUM> wt% to <NUM> wt%. In a case in which the silicon content is greater than <NUM> wt%, the formability may be reduced.

Specifically, the aluminum alloy film according to the present invention may be an aluminum alloy with alloy number AA8021.

<FIG> is a graph illustrating iron and silicon contents of an aluminum alloy with alloy number AA8079, which has mainly been used in a conventional pouch for a battery, and the aluminum alloy with alloy number AA8021 used in the present invention.

As illustrated in <FIG>, alloy number AA8079 contains <NUM> wt% to <NUM> wt% of iron and <NUM> wt% to <NUM> wt% of silicon. In general, mechanical strength is improved when a large amount of iron is contained in an aluminum alloy, and flexibility is improved when a small amount of iron is contained. With respect to the aluminum alloy of alloy number AA8079, relatively little iron is contained, and, in a case in which the gas barrier layer <NUM> is prepared by using the same, flexibility may be improved, but, since strength may be reduced, there may be a limitation in formability.

In contrast, as illustrated in <FIG>, alloy number AA8021 contains <NUM> wt% to <NUM> wt%, particularly <NUM> wt% to <NUM> wt% of iron, and <NUM> to <NUM> wt%, particularly <NUM> wt% to <NUM> wt% of silicon. In a case in which the gas barrier layer <NUM> is prepared from the aluminum alloy of alloy number AA8021, since iron is contained in a relatively large amount, tensile strength, elongation, and puncture strength may be improved.

Tensile strength (Rm), elongation (A), and grain size of the aluminum alloy with alloy number AA8079 and the aluminum alloy with alloy number AA8021 are illustrated in <FIG>.

As illustrated in <FIG>, AA8079 has low tensile strength and elongation so that there is a limitation in increasing the formability, and, since the grain size is relatively large at <NUM> to <NUM>, internal stress is less dispersed during stretching, and thus, there is a problem in that the number of pinholes is increased.

In contrast, since AA8021 has excellent formability due to its high tensile strength and elongation and has a relatively small grain size of <NUM> to <NUM>, the internal stress may be more dispersed during stretching, and thus, the occurrence of pinholes may be effectively suppressed.

When a tensile force is applied to a certain material, a relationship between tensile strength and elongation may be expressed as a graph. In this case, if a vertical axis of the graph is the tensile strength and a horizontal axis is the elongation, an area under the graph is toughness of the corresponding material. The toughness refers to a degree of toughness against destruction of the material, wherein the higher the toughness is, the more the material may be elongated until it does not break. In a case in which the gas barrier layer <NUM> is prepared by using the aluminum alloy of alloy number AA8021, since the tensile strength and elongation are improved, the toughness may be increased and the formability may be improved.

The surface protection layer <NUM> is formed of a second polymer, and is formed as an outermost layer to electrically insulate the electrode assembly <NUM> from the outside while protecting the secondary battery <NUM> from friction and collision with the outside. Herein, the outermost layer refers to a layer positioned at the end of the gas barrier layer <NUM> in a direction opposite to the direction in which the electrode assembly <NUM> is positioned. The second polymer for preparing the surface protection layer <NUM> may be at least one material selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymer, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, Nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, Teflon, and glass fibers. Particularly, it is desirable that a polymer, such as polyethylene terephthalate (PET) having wear resistance and heat resistance, is mainly used. The surface protection layer <NUM> may have a single-layer structure formed of any one material or may have a composite layer structure which is formed by layering two or more materials, respectively.

According to an embodiment of the present invention, a thickness of the surface protection layer <NUM> may be in a range of <NUM> to <NUM>. If the thickness of the surface protection layer <NUM> is less than <NUM>, there is a problem in that external insulation is reduced. In contrast, if the thickness of the surface protection layer <NUM> is greater than <NUM>, since the total thickness of the pouch may be increased, the energy density to volume of the secondary battery <NUM> may be rather reduced.

PET is inexpensive, has excellent durability, and has excellent electrical insulation properties, but has weak adhesion to aluminum which is frequently used as the gas barrier layer <NUM>, and the PET and the aluminum have different behaviors from each other when they are stretched under stress. Thus, if the surface protection layer <NUM> and the gas barrier layer <NUM> are directly adhered, the surface protection layer <NUM> and the gas barrier layer <NUM> may be delaminated during drawing. As a result, since the gas barrier layer <NUM> may not be stretched uniformly, the formability may be reduced.

According to an embodiment of the present invention, the battery case <NUM> may further include the drawing assistance layer <NUM> which is formed of a third polymer and is laminated between the surface protection layer <NUM> and the gas barrier layer <NUM>. The drawing assistance layer <NUM> is laminated between the surface protection layer <NUM> and the gas barrier layer <NUM> to prevent the surface protection layer <NUM> and the gas barrier layer <NUM> from being delaminated during stretching. The third polymer for preparing the drawing assistance layer <NUM> may be at least one material selected from the group consisting of polyethylene, polypropylene, polycarbonate, polyethylene terephthalate, polyvinyl chloride, acrylic polymer, polyacrylonitrile, polyimide, polyamide, cellulose, aramid, Nylon, polyester, polyparaphenylene benzobisoxazole, polyarylate, Teflon, and glass fibers. Particularly, since a Nylon resin is easy to adhere to the polyethylene terephthalate (PET) of the surface protection layer <NUM> and has a similar behavior to the aluminum alloy of the gas barrier layer <NUM> when stretched, it is desirable that the Nylon resin is mainly used as the third polymer. The drawing assistance layer <NUM> may have a single-layer structure formed of any one material or may have a composite layer structure which is formed by layering two or more materials, respectively.

As described above, according to an embodiment of the present invention, since the gas barrier layer <NUM> has a thickness of about <NUM> to about <NUM>, the formability of the gas barrier layer <NUM> is improved. In this case, in order to also improve formability of the drawing assistance layer <NUM>, the drawing assistance layer <NUM> may have a thickness of <NUM> to <NUM>, and, particularly, it is desirable that the drawing assistance layer <NUM> has a thickness of <NUM> to <NUM>. If the thickness is less than <NUM>, since the drawing assistance layer <NUM> may not match the improved formability of the gas barrier layer <NUM>, it may be broken during stretching. In contrast, if the thickness is greater than <NUM>, since the total thickness of the pouch may be increased, the volume of the secondary battery <NUM> may be increased and the energy density may be reduced.

The pouch film laminate according to the present invention may have a total thickness of <NUM> or more, for example, <NUM> to <NUM>. In a case in which the thickness of the pouch film laminate is <NUM> or more, the cup forming depth may be further increased than that of a conventional case. In a case in which the total thickness of the pouch film laminate is excessively large, it is not desirable because a total volume of the secondary battery is increased.

The pouch film laminate according to the present invention has excellent tensile strength and elongation by including the aluminum alloy film having specific thickness and grain size. Specifically, the pouch film laminate according to the present invention may have a tensile strength, which is measured while the pouch film laminate is pulled at a tensile speed of <NUM>/min after being cut to a size of <NUM> × <NUM>, of <NUM> N/<NUM> to <NUM> N/<NUM>, preferably <NUM> N/<NUM> to <NUM> N/<NUM>, and more preferably <NUM> N/<NUM> to <NUM> N/<NUM>, and may have an elongation of <NUM>% to <NUM>%, preferably <NUM>% to <NUM>%, and more preferably <NUM>% to <NUM>%. As described above, since the pouch film laminate according to the present invention has high tensile strength and elongation and, as a result, the toughness is increased, the occurrence of cracks is less likely even when the forming depth is large during cup forming.

Also, the pouch film laminate according to the present invention has excellent puncture strength by including the aluminum alloy film having specific thickness and grain size. Specifically, the pouch film laminate according to the present invention may have a puncture strength of <NUM> N or more, for example, <NUM> N to <NUM> N.

Hereinafter, the present invention will be described in more detail, according to specific examples.

A surface protection layer, a drawing assistance layer, and a gas barrier layer were formed by sequentially bonding a Nylon film having a width of <NUM>, a length of <NUM>, and a thickness of <NUM> and a polyethylene terephthalate (PET) film having a width of <NUM>, a length of <NUM>, and a thickness of <NUM> on one surface of an aluminum (Al) alloy thin film of alloy number AA8021 having a width of <NUM>, a length of <NUM>, and a thickness of <NUM> by a dry lamination method using a urethane adhesive.

Next, casted polypropylene (CPP) was melted at a high temperature and then co-extruded on the other surface of the aluminum (Al) alloy thin film to form a sealant layer having a thickness of <NUM> to prepare a pouch film laminate. A total thickness of the pouch film laminate was <NUM>.

A pouch film laminate was prepared in the same manner as in Example <NUM> except that a sealant layer was formed to a thickness of <NUM>. A total thickness of the pouch film laminate was <NUM>.

A pouch film laminate was prepared in the same manner as in Example <NUM> except that an aluminum alloy film of alloy number AA8021 having a thickness of <NUM> was used as a gas barrier layer, a Nylon film having a thickness of <NUM> was used as a drawing assistance layer, and a sealant layer was formed to a thickness of <NUM>. A total thickness of the pouch film laminate was <NUM>.

A pouch film laminate was prepared in the same manner as in Example <NUM> except that an aluminum alloy film of alloy number AA8021 having a thickness of <NUM> was used as a gas barrier layer. A total thickness of the pouch film laminate was <NUM>.

A pouch film laminate was prepared in the same manner as in Example <NUM> except that an aluminum alloy film of alloy number AA8079 having a thickness of <NUM> was used as a gas barrier layer. A total thickness of the pouch film laminate was <NUM>.

A pouch film laminate was prepared in the same manner as in Example <NUM> except that an aluminum alloy film of alloy number AA8079 having a thickness of <NUM> was used as a gas barrier layer, a Nylon film having a thickness of <NUM> was used as a drawing assistance layer, and a sealant layer was formed to a thickness of <NUM>. A total thickness of the pouch film laminate was <NUM>.

Cross sections in a thickness direction of the AA8021 and AA8079 aluminum alloy films respectively used as gas barrier layers in Example <NUM> and Comparative Example <NUM> were observed with a scanning electron microscope (SEM) to measure grain sizes. Specifically, after measuring maximum diameters of <NUM> grains observed in cross-se11ctional SEM images in the thickness direction of the aluminum alloy films which were obtained using a scanning electron microscope, the grain size was measured by a method of calculating an average value thereof.

The SEM image of the AA8021 aluminum alloy film and the SEM image of the AA8079 aluminum alloy film are illustrated in <FIG>. As a result of measuring grain sizes based on the illustrated SEM images, the grain size of AA8021 was <NUM> and the grain size of AA8079 was <NUM>.

After cutting each of the pouch film laminates prepared in Examples <NUM> and <NUM> and Comparative Examples <NUM> to <NUM> to the same size of <NUM> × <NUM>, forming was performed while changing a forming depth in a battery case forming device having a forming part with a size of <NUM> in width × <NUM> in length. The forming depth at which cracks occurred in each sample was recorded. Herein, corners and edges of a punch and the forming part of the battery case forming device were filleted, the corner of the punch had a curvature of <NUM>, the edge thereof had a curvature of <NUM>, the corner of the forming part had a curvature of <NUM>, and the edge thereof had a curvature of <NUM>. In addition, clearance of the punch and the forming part was <NUM>. Measurement results are presented in Table <NUM> below.

As listed in Table <NUM>, with respect to the pouch film laminates according to Examples <NUM> and <NUM> of the present invention, cracks did not occur and they may be formed to a depth of <NUM> or more, but, with respect to the pouch film laminates of Comparative Examples <NUM> to <NUM> in which gas barrier layer thicknesses and/or grain sizes were outside the ranges of the present invention, cracks occurred at a forming depth of less than <NUM>. Thus, it may be confirmed that the formability of the pouch film laminate was improved when the gas barrier layer thickness and grain size of the present invention were satisfied. If a battery case is prepared by using the pouch film laminate according to the preparation example of the present invention, since one cup portion is formed in each of a second case and a first case, an accommodation space having a depth of <NUM> or more may be obtained. Therefore, since a thicker electrode assembly may be accommodated, the energy efficiency to volume of the secondary battery may be increased.

After cutting <NUM> samples of each of the pouch film laminates prepared in Examples <NUM> and <NUM> and Comparative Examples <NUM> to <NUM> to the same size of <NUM> × <NUM>, each of the samples was fixed to a lower jig of a tensile strength tester (manufacturer: Shimadzu, model: AGX-V). In addition, after fixing each sample with an upper jig to a point of <NUM> from a top end, the samples were elongated by moving the upper jig away from the lower jig at a speed of <NUM>/min. Then, tensile strength and elongation just before the pouch film laminate was fractured were measured. Measurement results are presented in Tables <NUM> and <NUM> below.

<FIG> is a graph of the results of testing tensile strength and elongation of the pouch film laminates according to Example <NUM>, Comparative Example <NUM>, and Comparative Example <NUM> of the present invention. Referring to <FIG> and Tables <NUM> and <NUM>, it may be confirmed that the pouch film laminates of Examples <NUM> and <NUM> had better tensile strength and elongation than the pouch film laminates of Comparative Examples <NUM> to <NUM>.

In a graph between tensile strength and elongation, an area under the graph is toughness of the corresponding material, wherein, as illustrated in <FIG>, it may be understood that a graph area of the pouch film laminate according to Example <NUM> of the present invention was larger than graph areas of the pouch film laminates according to Comparative Example <NUM> and Comparative Example <NUM>, and this showed that the pouch film laminate of Example <NUM> according to the present invention had excellent toughness.

After cutting <NUM> samples of each of the pouch film laminates prepared in Examples <NUM> and <NUM> and Comparative Examples <NUM> to <NUM> to the same size of <NUM> × <NUM>, each sample was horizontally fixed to a jig of a puncture strength tester (manufacturer: Shimadzu, model: AGX-V). A pin having a diameter of <NUM> and a tip curvature of <NUM> was installed vertically above the installed sample. Then, puncture strengths of the samples were measured by dropping the pin to the samples, respectively. Measurement results are presented in Table <NUM> below.

<FIG> is a graph of the results of testing puncture strength of the pouch film laminates according to Example <NUM>, Comparative Example <NUM>, and Comparative Example <NUM> of the present invention. Referring to <FIG> and Table <NUM>, it may be confirmed that the pouch film laminates of Examples <NUM> and <NUM> had better puncture strength than the pouch film laminates of Comparative Examples <NUM> to <NUM>. That is, the pouch film laminate according to the present invention may more effectively protect an internal electrode assembly even if it is under great pressure from the outside or is damaged by being pierced by a sharp object.

After each of the pouch film laminates prepared in Examples <NUM> and <NUM> was cut to the same size of <NUM> × <NUM> and then stored in a vacuum oven at <NUM> degrees for <NUM> hours, a dielectric breakdown voltage was measured in a dry room. Specifically, an aluminum thin film having a thickness of <NUM> t was disposed on both sides of the pouch film laminate, a (+) electrode of a measuring device was connected to the gas barrier layer of the pouch film laminate, and, after connecting a (-) electrode to the aluminum thin film in contact with the sealant layer, an applied voltage, when a leakage current measured while applying a voltage at a rate of <NUM> V/s was <NUM> Ma or more, was evaluated as the dielectric breakdown voltage. Measurement results are presented in Table <NUM> below.

Referring to Table <NUM>, it may be confirmed that a dielectric breakdown voltage of the pouch film laminate of Example <NUM> having a sealant layer thickness of <NUM> was higher than that of the pouch film laminate of Example <NUM> having a sealant layer thickness of <NUM>, wherein this showed that insulation properties were better when the thickness of the sealant layer was <NUM>.

Claim 1:
A pouch type battery case (<NUM>) adapted to accommodate an electrode assembly (<NUM>) therein which is formed by stacking a positive electrode, a separator, and a negative electrode, the pouch type battery case comprising a pouch film laminate (<NUM>) including:
a sealant layer (<NUM>) formed of a first polymer that is an innermost layer;
a surface protection layer (<NUM>) formed of a second polymer that is an outermost layer; and
a gas barrier layer (<NUM>) laminated between the surface protection layer (<NUM>) and the sealant layer (<NUM>) and formed of an aluminum alloy film having a thickness of <NUM> to <NUM> and a grain size of <NUM> to <NUM>,
wherein a thickness of the sealant layer (<NUM>) is <NUM> times to <NUM> times a thickness of the gas barrier layer (<NUM>), and wherein the grain size is evaluated as follows:
a cross-sectional SEM image in the thickness direction of the aluminum alloy film is obtained using a scanning electron microscope,
maximum diameters of <NUM> random grains among grains observed in the SEM image are measured, and
an average value thereof is then evaluated as the grain size.