Abstract:
A flexible laminate for packaging a polymer electrolytic cell or battery, comprising: 
     a first metallic foil layer; 
     a second metallic foil layer; 
     an intermediate separation layer disposed between the first and second metallic foil layers, the intermediate separation layer establishing a predetermined spacing between the first layer metallic foil and the second metallic foil layer; 
     a first layer formed of a first self-sealing polymer on the first metallic foil layer.

Description:
This application claims benefit to application Ser. No. 60/078,418, filed Mar. 18, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to packaging material, and more particularly, to a flexible laminate for packaging and hermetically sealing an electrochemical device. The invention is particularly applicable for packaging electrolytic cells and batteries, and will be described with particular reference thereto. Although, it will be appreciated that the present invention may find advantageous application in packaging electrochemical devices that utilize polymer electrolytes or that require hermetic packaging to isolate the device from the environment. 
     BACKGROUND OF THE INVENTION 
     Improvements in microelectronics have increased the demand for electrolytic cells that can be directly incorporated into electronic devices so as to produce a portable, finished package. As the improvements in microelectronics reduce the size of the electronic device, the space and weight allotted to a power supply within such device have likewise decreased. It is therefore important to maximize the power-per-unit space that a battery can provide, but at the same time, minimize weight and fully utilize the space allotted to a power supply within an electronic device. The use of flexible packaging to enclose an electrolytic battery or cell is one way to reduce the weight of a battery or cell, and such packaging lends itself to unusual shapes. Flexible packaging can still provide hermetic containment of a battery, and in addition, is lighter than traditional packaging, i.e., metal cans or molded plastic cases. Flexible packaging also conforms more easily to the shape of the battery structure. 
     The flexible packaging may be formed of a single layer of a polymer film or single layer of a metal foil, but is typically comprised of a multi-layer material having a polymer layer for toughness, a metal foil layer to form a hermetic barrier and a third layer of an adhesive or sealant material. This multi-layer material is dimensioned to be tightly wrapped, e.g., to be folded, around a battery or cell. Heat and pressure are typically applied to overlapping edges of the material to form a seal enclosing the cell or battery. Operations such as folding of the material and the tight wrapping necessary for efficient packaging of the battery or cell place considerable stress on the various layers forming the material. Such stresses may lead to cracks forming in the metal foil layer. Such cracks, in addition to “pinhole imperfections” that may exist in the foil, can destroy the hermeticity of the packaging and allow solvents to escape from the battery or water vapor and oxygen to penetrate into the battery. Increasing the thickness of the foil layer sufficiently to avoid hermeticity issues can require undesirable increases in weight, thickness, stiffness and cost. 
     The present invention overcomes these and other problems and provides a multi-layer, flexible material for forming a package for an electrochemical device such as an electrolytic battery or cell. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a flexible, multi-layer material for packaging a polymer electrolytic cell or battery, comprising: 
     a first metallic foil layer; 
     a second metallic foil layer; 
     an intermediate separation layer disposed between said first and second metallic foil layer, said intermediate separation layer establishing a predetermined spacing between said first metallic foil layer and said second metallic foil layer; 
     a first self-sealing layer on said first metallic foil layer, the first self-sealing layer formed of a material having adhesive-like properties and having a thickness of less than about 76 μm; and 
     a second self-sealing layer on the second metallic foil layer, the second self-sealing layer formed of a material having adhesive-like properties and having a thickness of less than about 76 μm. 
     In accordance with another aspect of the present invention, there is provided a flexible, multi-layer material for packaging a polymer electrolytic cell, comprising: 
     a first layer and a second layer of an aluminum foil, the aluminum foil layers having a thickness of about 2 μm to about 100 μm; and 
     a layer of a thermoplastic material having a thickness of about 2 μm to about 100 μm disposed between the aluminum foil layers; 
     layers of a sealant-adhesive material applied to each of the aluminum foil layers, the adhesive layers formed of a thermoplastic polymer. 
     It is an object of the present invention to provide a laminate material for packaging an electrochemical device. 
     Another object of the present invention is to provide a laminate as described above for packaging an electrolytic cell or battery. 
     Another object of the present invention is to provide a laminate material as described above that is self-sealing upon the application of heat and pressure. 
     A still further object of the present invention is to provide a laminate material as described above having multiple layers of metal foil to insure hermetic packaging. 
     These and other objects will become apparent from the following description of a preferred embodiment taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take physical form in certain parts and arrangement of parts, a preferred embodiment of which will be described in detail in the specification and illustrated in the accompanying drawings which form a part hereof, and wherein: 
     FIG. 1 is a plan view showing conventional battery package formed from a flexible laminate material; 
     FIG. 2 is a sheet of laminate material used in forming the battery package shown in FIG. 1; and 
     FIG. 3A is an enlarged cross-sectional view showing a flexible laminate illustrating a preferred embodiment of the present invention. 
     FIG. 3B is an enlarged cross-sectional view showing a flexible laminate illustrating a second embodiment of the present invention. 
     FIG. 3C is an enlarged cross-sectional view showing a flexible laminate illustrating a third embodiment of the present invention. 
     FIG. 3D is an enlarged cross-sectional view showing a flexible laminate illustrating a fourth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein the showings are for the purpose of illustrating preferred embodiments of the invention only, and not for the purpose of limiting same, FIG. 1 illustrates a battery  10  having a package or enclosure  14  formed from a flexible, multi-layered material  20 . Package  14  is adapted to enclose an electrochemical device. In the embodiment shown, a planar, solid electrolytic cell  12  having leads  16 ,  18  is shown. Package  14  is generally formed from a sheet of multi-layered material  20  by placing the rectangular cell(s)  12  onto one side of a sheet of multi-layered material  20 . Cell(s)  12  is positioned such that a portion of each of leads  16 ,  18  is positioned on material  20 , and a portion of leads  16 ,  18  extends beyond the edge of material  20 . The other half of material  20  is then folded over onto cell  12  to overlay onto the other side of material  20 . Material  20  is dimensioned such that when folded over, it extends beyond three peripheral edges of cell  12  such that the adhesive and sealant layer engages itself. Heat or pressure or both are applied to these three edges to form a seal about the periphery of cell  12 . The method of forming a package  14  for battery  10  as heretofore described, as well as the shape of package  14 , are generally conventionally known. In this respect, cell  12  and package  14 , in and of themselves, form no part of the present invention. The present invention relates specifically to multi-layered material  20  for forming outer package  14 . 
     In accordance with the present invention, material  20  is preferably a multi-layered structure and includes at least two layers of metallic foil, a separation layer between the two layers of metal foil and at least one layer of a pressure-sensitive or heat-sensitive, self-sealing material. 
     The two metal foil layers are provided as barrier layers to form a hermetic barrier within package  14 . The metal foil layers are provided to prevent migration of air into the battery or cell, and/or solvents from the cell. The intermediate separation layer is provided to separate and “decouple” the metallic foil layers. Specifically, the separation between the metallic foil layers is provided to prevent alignment or registry of defects in the respective foil layers, such as pinholes or cracks, from destroying the hermeticity of the multi-layer material. In addition, a space between the metallic foils prevents cracks or punctures in one metallic foil layer from penetrating or propagating into the other. As shall be described in greater detail below, the separation layer may be comprised of a wide range of materials, and is primarily provided to create a space or separation between the two metallic foil layers. Preferably, in addition to creating a space between the two metallic layers the separation layer has mechanical properties to impart physical strength to package  14 , namely tensile strength, penetration resistance and stiffness. The self-sealing layer is provided as an adhesive-like material that when heated and cooled, or under pressure or both will bond or join onto itself or onto one of the other layers of multi-layer material  20  and form a seal with any surface it may contact. 
     Referring now to the drawings, preferred embodiments of material  20  are illustrated in FIGS. 3A-3D. Each of the laminates shown in FIGS. 3A-3D is basically formed of layers of like materials that are merely arranged in a different order or grouping. In accordance with the preferred embodiment, the multi-layer materials shown in FIGS. 3A-3D each include at least two metallic foil layers. The metallic foil layers may be formed of like or dissimilar metal foils that are chosen to provide specific mechanical properties to multi-layer material  20 . For example, one layer may be formed of aluminum foil, and the other layer formed of copper foil. Preferable metallic foils for use in material  20  are selected from a group consisting of aluminum, copper, nickel and stainless steel. The metallic foil layers preferably have a thickness of less than 100 μm (microns), more preferably, having a thickness of between 5 to 50 microns, and most preferably, having a thickness of between 5 to 12 microns. In preferred embodiments, both metal layers are formed of an aluminum foil. 
     Referring now to the intermediate separation layer, this layer may be formed of fabric, scrim, paper, a polymer film, an adhesive, another metallic foil or combinations thereof. In this respect, the intermediate separation layer may itself be formed of multiple layers. For example, the intermediate separation layer may be formed of a woven cloth having a layer of adhesive on both sides thereof to secure the cloth to each of the metallic layers. As indicated above, a primary function of the separation layer is to separate the metallic foil layers. In addition, the intermediate separation layer also provides structural strength to material  20 . This is preferably accomplished by forming the intermediate separation layer from a material that possesses desired mechanical properties and securing such material to the metallic foil layers, for example, with additional layers of thin adhesive. In accordance with the present invention, the intermediate separation layer is preferably formed of a polymeric material. The polymeric material may be a film or a resinous material, and such film or resinous material may have adhesive-like properties, wherein the film itself or the resinous material itself will adhere to the metallic foil layers. A layer of adhesive material alone may also form the intermediate separation layer between the metallic foil layers. 
     The total thickness of the intermediate separation layer is preferably about 2 μm to about 100 μm, and more preferably, about 3 μm to about 18 μm. In preferred embodiments, the intermediate separation layer of material  20  is formed of a polymeric material, including but not limited to, thermoplastics, engineering thermoplastics, blends of the same, thermosets and blends of thermoplastics and thermosets. The polymeric material is bonded to the metallic layer by thin layers of adhesive. The polymeric layer is provided to separate one metal foil layer from the other metal foil layer or another material, and to provide strength to material  20 . In a preferred embodiment, the intermediate separation layer is comprised of Mylar® bonded to the metallic foil layer by a thin layer of adhesive. 
     Referring now to the self-sealing layer, this layer preferably comprises at least one of the outer layers of the laminate. The self-sealing layer (or layers) is formed of a material that is either heat-sensitive or pressure-sensitive or both so as to bond to another layer of like material so as to enable formation of the battery package by a heat and/or pressure process. The self-sealing layer(s) may be formed of any mono, co-extruded or laminated material that is electrically insulating, and chemically and physically compatible with any component or components of the electrochemical device with which it may come in contact (e.g., it must not dissolve in the electrolyte solution of a battery). Representative of such materials are acrylics, ethylene acrylic acid (EAA) copolymers, EEA copolymer, ethylene methacrylic acid (EMA) copolymers, ethylene vinyl acetate (EVA) copolymers, polymethyl acrylate (PMA) acrylonitrite, vinyl chloride polypropylene (VCPP) copolymers, acrylic nitrile-butadiene-styrene (ABS) copolymers, polyethylene (PE) and polypropylene (PP), and rubber modified compounds of the foregoing. In a preferred embodiment, the self-sealing layer is formed of SURLYN® or NUCREL® thermoplastic materials manufactured by DuPont. The thickness of the self-sealing layer is less than about 76 μm, and more preferably, is between about 38 μm to about 64 μm. 
     The overall thickness of material  20  is preferably kept as thin as possible so that material  20  maintains sufficient flexibility to be wrapped about cell  12 , as will hereinafter be described. At the same time, material  20  and its individual layers preferably have sufficient thicknesses to impart to material  20  the mechanical properties necessary to provide the toughness and hermeticity required of package  14 . In this respect, the overall thickness of material  20  is preferably less than 152.4 μm, preferably in the range of about 60 μm to about 127.0 μm, and more preferably, in the range of about 101.6 μm to about 127.0 μm. 
     Referring now to the drawings, specific embodiments of the present invention are shown. FIG. 3A shows a preferred material  20  wherein the intermediate separation layer is comprised of two layers of a polymer film. The two polymer layers are sandwiched between two layers of metallic foil. Thin layers of adhesive (not shown) are preferably used to bond the structural, polymer layers, together and to the metallic foil layers. A self-sealing layer or adhesive is provided over each metallic foil layer. 
     FIG. 3B shows a multi-layer material  20  wherein the inner layers of polymer material and metallic foil of FIG. 3A are rearranged to alternate. In this configuration, one of the polymer layers forms the intermediate separation layer between the two metallic foil layers and the other polymer layer is provided as an additional structural layer. As in the embodiment shown in FIG. 3A, adhesive (not shown) may be used to secure one or more layers to another layer. The two layers of metallic foil and two layers of polymer material are sandwiched between two layers of a self-sealing polymer. 
     FIG. 3C shows a material  20  that is generally similar to that shown in FIG. 3B, except that the material has only one self-sealing layer. The self-sealing layer preferably covers a metallic foil layer. 
     FIG. 3D shows a material comprised of two layers of metallic foil and a single layer of an adhesive forming the separation layer. A single layer of a self-sealing material is disposed over one metallic foil layer. In this embodiment, no reinforcing polymer is provided. In this embodiment, to bond the metallic foil layers to each other, an actual adhesive such as a pressure-sensitive adhesive is applied between the metallic foil layers. 
     The present invention thus provides a multi-layer material  20  having two layers of spaced-apart, metallic foil  24 ,  26  to improve the hermeticity of material  20 . With two layers of metallic foil  24 ,  26 , the possibility of a leak in package  14  is significantly reduced. Even if cracks or imperfections exist in both metallic foil layers  24 ,  26 , because of the separation of the two metallic layers, the likelihood that such cracks or defects would be in registry (i.e., alignment) with each other is small. Thus, if fluid penetrates through a void or crack in one metallic layer, it must travel laterally along the planes of the respective layers to reach another void or crack in the second metallic foil layer. Migration of fluid through the planes of material  20  significantly retards seepage and significantly reduces the likelihood that such seepage will leak through the second metallic foil layer. By providing two, spaced-apart layers of metallic foil, the hermeticity of the package is improved, together with the physical strength thereof. 
     The foregoing is a description of a preferred embodiment of the present invention. Numerous alterations and modifications will occur to those skilled in the art upon a reading of the present specification. It will be obvious to those skilled in the art that two or more layers of metal foil to provide durable hermeticity can be combined with numerous other combinations of structural and/or adhesive polymer layers to achieve desired degrees of strength, flexibility, durability, thickness, weight, adhesive strength, etc. For example, other types of adhesive besides thermoplastic can in principle be used, for example, contact adhesives. Sealing methods may include heat, pressure, ultrasonic, RF induction, etc., and combinations thereof. It is intended that all such modifications and alterations be included insofar as they come within the scope of the invention as claimed or the equivalents thereof.