Abstract:
The present invention, a cell layer edge support, provides longer battery cell life while minimizing the possibility of shorts within the cell. More specifically, an edge support layer is positioned around at least a portion of the smaller electrode, either the anode or the cathode, in the cell to define a supporting perimeter comparable to the perimeter of the larger electrode. The support layer generally comprises a polymeric material which helps to absorb pressures exerted on the cell layers during packaging. The anode, cathode, support layer and a separator layer placed between the anode and the cathode may be joined to form a battery. Preferably, a plurality of cells having the support layer may be joined to form a higher energy, longer life battery.

Description:
TECHNICAL FIELD 
     This invention relates to a method of preparation of battery cells, and in particular to battery cells having a support layer to support the edges and corners of electrodes during packaging of the cells. 
     BACKGROUND OF THE INVENTION 
     Cells and batteries are energy storage devices well known in the art. Cells typically comprise electrodes and an ion conducting electrolyte therebetween. For example, the rechargeable lithium ion cell, known as a rocking chair type lithium ion battery, typically comprises essentially two electrodes, an anode and a cathode, and a non-aqueous lithium ion conducting electrolyte therebetween. The anode (negative electrode) is a carbonaceous electrode that is capable of intercalating lithium ions. The cathode (positive electrode), a lithium retentive electrode, is also capable of intercalating lithium ions. The carbon anode comprises any of the various types of carbon (e.g., graphite, coke, carbon fiber, etc.) which are capable of reversibly storing lithium species, and which are bonded to an electrochemically conductive current collector (e.g., copper foil) by means of a suitable organic binder (e.g., polyvinylidine fluoride, PVdF). 
     The cathode comprises such materials as transition metals and chalcogenides that are bonded to an electrochemically conducted current collector (e.g., aluminum foil) by a suitable organic binder. Chalcogenide compounds include oxides, sulfides, selenides, and tellurides of such metals as vanadium, titanium, chromium, copper, molybdenum, niobium, iron, nickel, cobalt, and manganese. Lithiated transition metal oxides are, at present, the preferred positive electrode intercalation compounds. Examples of suitable cathode materials include LiMnO 2 , LiCoO 2 , LiNiO 2 , and LiFePO 4 , their solid solutions and/or their combination with other metal oxides and dopant elements, e.g., titanium, magnesium, aluminum, boron, etc. 
     The electrolyte in such lithium ion cells comprises a lithium salt dissolved in a non-aqueous solvent which may be (1) completely liquid, (2) an immobilized liquid (e.g., gelled or entrapped in a polymer matrix), or (3) a pure polymer. Known polymer matrices for entrapping the electrolyte include polyacrylates, polyurethanes, polydialkylsiloxanes, polymethacrylates, polyphosphazenes, polyethers, polyvinylidine fluorides, polyolefins such as polypropylene and polyethylene, and polycarbonates, and may be polymerized in situ in the presence of the electrolyte to trap the electrolyte therein as the polymerization occurs. Known polymers for pure polymer electrolyte systems include polyethylene oxide (PEO), polymethylene-polyethylene oxide (MPEO), or polyphosphazenes (PPE). Known lithium salts for this purpose include, for example, LiPF 6 , LiClO 4 , LiSCN, LiAlCl 4 , LiBF 4 , LiN(CF 3 SO 2 ) 2 , LiCF 3 SO 3 , LiC(SO 2 CF 3 ) 3 , LiO 3 SCF 2 CF 3 , LiC 6 F 5 SO 3 , LiCF 3 CO 2 , LiAsF 6 , and LiSbF 6 . Known organic solvents for the lithium salts include, for example, alkyl carbonates (e.g., propylene carbonate and ethylene carbonate), dialkyl carbonates, cyclic ethers, cyclic esters, glymes, lactones, formates, esters, sulfones, nitrates, and oxazoladinones. The electrolyte is incorporated into pores in a separator layer between the anode and the cathode. The separator layer may be either a microporous polyolefin membrane or a polymeric material containing a suitable ceramic or ceramic/polymer material. Silica is a typical main component of this latter type of separator layer. 
     Lithium ion battery cells, as are most cells, are often made by adhering, e.g., by laminating, thin films of the anode, cathode, and the electrolyte/separator layers together wherein the electrolyte/separator layer is sandwiched between the anode and cathode layers to form an individual cell. A plurality of such cells are generally bundled together, in what is typically known as a cell stack or winding, and packaged to form a higher energy/voltage battery. Packaging of the cell or cell stack generally involves a vacuum seal lamination process requiring complex packaging equipment. During packaging, the pressures and forces are exerted upon the individual cell layers, which may cause vulnerable edges and corners of the electrode layers in each cell to be bent, crushed or otherwise damaged. This damage often decreases the overall life and power of the cell. Specifically, damage to the electrode films results in non-uniform utilization of the active materials, which in turn, can lead to lithium plating and loss of life. In addition, the pressure exerted on the electrode layers may cause the separator to split thereby posing possible risks of shorting within the battery. 
     Thus, there is a need to develop a cell construct and method of assembly to produce a more robust battery cell having longer life and increased activity, and which is less prone to developing shorts. 
     SUMMARY OF THE INVENTION 
     The present invention provides a robust battery cell, and in particular, a robust lithium ion battery cell, having long cell life and high activity with minimal risk of shorting. The battery cell comprises a support layer surrounding at least a portion of the outer edge, referred to as edge perimeter, of the smaller of the anode and cathode electrode layers and adjacent to the electrolyte/separator layer. The support layer provides support and strength primarily to the larger of the anode and cathode electrode layers as well as to the electrolyte/separator layer therebetween during vacuum sealing and exposure to other associated cell-packaging pressures. The support layer thus provides added rigidity and resistance to crushing during manufacture and packaging of the battery cell. 
     In an exemplary embodiment, the support layer may generally be in the shape of a frame having an edge perimeter and an open central portion. The support layer frame is positioned around the smaller electrode such that the smaller electrode lies within the open central portion allowing the edge perimeter of the support layer to provide support to the larger electrode, in particular the edges of the larger electrode, when the two electrodes are assembled and packaged together. The support layer may extend outwardly in the horizontal plane of the smaller electrode and beyond the edge perimeter of the larger electrode, but optimally the extension should be substantially equal to the edge perimeter of the larger electrode. The support layer of the present invention advantageously comprises polymers, including homopolymers, copolymers, or mixtures thereof. 
     The present invention also provides a method of preparing the battery cell including the support layer described above. The method comprises joining at least one anode, at least one cathode, at least one support layer positioned in surrounding relation to at least a portion of the smaller of the anode or cathode, and at least one electrode/separator layer sandwiched between the anode and cathode to form at least a single cell. Similarly, a plurality of single cells may be joined to form a battery. Multiple cells may be arranged as a cell stack or winding with support layers placed around each of the smaller of the anode or cathode in each cell. The anodes, cathodes, electrolyte/separator layers, and support layers in the cell stack may be sealed together to form the battery. 
     In constructing or assembling the cell, the support layer may be first placed adjacent to the surface of the electrolyte/separator layer followed by placement of the smaller electrode inside the open central portion of the support layer. Alternatively, the support layer may be placed around the smaller electrode prior to placement adjacent the separator layer. The smaller electrode, the support layer, the larger electrode and the electrolyte/separator layer are then joined to form a battery cell. Joining is typically accomplished by conventional packaging equipment, such as vacuum sealing or laminating equipment, to seal or enclose the cell or cell stack to form a battery. By virtue of the support layer, the edges of the larger electrode resist bending and crushing during the joining process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is an exploded view of a battery cell. 
         FIG. 2  is a cross-sectional view of a packaged battery cell. 
         FIG. 3  is an exploded view of a battery cell stack having two cells. 
     
    
    
     DETAILED DESCRIPTION 
     The invention is hereinafter described with general reference to a lithium ion battery cell, however, it may also be applied to other non-lithium ion conducting battery cells. As shown in  FIG. 1 , an embodiment of a battery cell  10  comprises a first electrode  14  and a second electrode  22  separated by an electrolyte/separator layer  30 , referred to simply as separator layer  30 . Either of the first electrode  14  or the second electrode  22  may be an anode, with the other being a cathode in the cell  10 . The first electrode  14  has an outer edge or an edge perimeter  16  defining a surface area  15 . Similarly, the second electrode  22  has an edge perimeter  24  defining a surface area  23 . Surface area  23  is larger than surface area  15 . Thus, first electrode  14  is a smaller electrode than second electrode  22 . A support layer  18  having an edge perimeter  20  and an open central portion  9  is positioned around at least a portion of the first electrode  14 . Advantageously, the support layer  18  is positioned around the entire first electrode  14 . The open central portion  19  defines area which is generally equal to or larger than the surface area  15  of the first electrode  14 . Preferably, the area defined by central portion  19  can be substantially equal to surface area  15  to maximize support for the electrodes in the cell. In constructing the cell, edge perimeter  20  in positioned to surround the first electrode  14  generally in substantially the same plane in which first electrode  14  lies, whereby first electrode  14  occupies central portion  19  of support layer  18 . As shown in  FIGS. 1 and 2 , first electrode  14  and second electrode  22  are assembled together with separator layer  30  therebetween to form cell  10 , which is di packaged with laminate material  11   a  using laminating equipment to form seal  11 . Vacuum sealing is preferably used to form seal  11 . Support layer  18  provides beneficial support to the larger second electrode  22  during packaging of the cell  10  by adding strength and rigidity to the edges and corners of second electrode  22 , for example, to reduce or prevent the edges and corners from being bent or crushed under packaging pressures. 
     The edge perimeter  20  of support layer  18  generally determines the level of support provided to second electrode  22 . Edge perimeter  20  defines an area  21  which includes the area defined by open central portion  19 . Area  21  may be larger than the surface area  23  of second electrode  22 . In other words, the edge perimeter  20  of the support layer  18  may extend outwardly beyond the edges and corners of edge perimeter  24  of second electrode layer  22 . Alternatively, support layer  18  may have an area  21  substantially equal to the surface area  23  of larger second electrode  22  thereby having an edge perimeter  20  substantially equal to the edge perimeter  24  of the second electrode  22  to provide adequate support. Such a support layer  18  would absorb pressures, for example, including air pressure and vacuum pressure, exerted on cell  10  during packaging thereby relieving second electrode  22  from exposure to the total pressure of packaging. Such a relief in pressure coupled with the support provided to second electrode  22  to withstand packaging pressures significantly reduces or may even prevent bending or crushing the otherwise vulnerable corners and edges of second electrode  22 . 
     While support layer  18  is illustrated and described as a frame-like structure surrounding the entire first electrode  14 , the present invention is not so limited. Persons of ordinary skill in the art will readily understand that the benefits of the support layer  18  may be available in instances where the support layer  18  only surrounds a portion of the first electrode  14 . For example, the support layer  18  may be provided to support only two opposing edges of first electrode  14  leaving the other edges free. Advantageously, the support layer  18  will be provided for and surround at least those portions of the first electrode  14  which are vulnerable to pressures during packaging of the battery cell  10 . 
     The support layer  18  may be any material compatible for use in a battery cell  10 . Ideally, the support layer  18  may comprise a polymeric material. This polymeric material may be the same material used in the binder of either the first electrode  14  or the second electrode  22 , or it may be different. Examples of polymeric materials suitable for the present invention include but are not limited to, homopolymers, copolymers, or mixtures of polymers such as vinylidine fluoride, vinylidine chloride fluoride, vinylidine chloride, vinylchloride, acrylonitrile fluoroethylene, fluoropropylene, chlorofluoroethylene, chlorofluoropropylene, chloroethylene, chloropropylene, ethylene, propylene, vinylalcohol, glycol, acetate, ester, acrylate, carbonate, ethylene oxide, propylene oxide, acrylic acid modified olefins, maleic acid modified olefins, cellulose, nylon, urethane, terephthalate, and styrene. 
     As shown in  FIGS. 1 and 2 , a separator layer  30 , which includes an electrolyte, is placed between the first electrode  14  and the second electrode  22  to provide a medium for cell activity and ultimate conduction of electricity. Separator layer  30  may be any size necessary for optimal cell activity. For example, the separator layer  30  shown in  FIGS. 1 and 2  generally has a surface area  31  larger than surface areas  15  and  23  of first and second electrodes  14  and  22 , respectively. Alternatively, surface area  31  may be substantially equal to area  21  defined by edge perimeter  20  of support layer  18  or substantially equal to surface area  23  of second electrode  22 . 
     The first electrode  14 , second electrode  22 , and separator layer  30 , typically comprise polymeric materials. The first electrode  14  and the second electrode  22  generally include an organic binder containing the polymeric material. By way of example only, and not limitation, suitable polymeric materials include homopolymers, copolymers, or mixtures of polymers such as polyvinylidine fluoride, polyvinylidine chloride fluoride, polyvinylidine chloride, polyvinylchloride, polyvinylchloride acetates, polyacrylonitriles, polyfluoroethylenes, and polyolefins such as polypropylene and polyethylene, acrylic or maleic acid modified polyethylene or polypropylene, polyvinylalcohols, polyglycols, and the like. These materials may be obtained from commercial sources as is known to one skilled in the art. 
     Lithium ion and other non-lithium ion batteries may comprise a plurality of the individual cells  10  illustrated in  FIG. 1 , each formed from individual small first and large second electrodes separated by a separator layer. The cells are generally arranged in a cell stack and packaged to from a battery. In such a battery, at least a portion of one or more of the first electrodes  14 , having a surface area  15  smaller than that of the counter second electrode  22 , may be surrounded by a support layer  18 . One embodiment of the present invention, shown in  FIG. 3 , is a cell stack  12  containing two cells sharing a single small first electrode  14 . The first electrode  14  has opposing surfaces  14   a ,  14   b  and a first edge perimeter  16  defining a first surface area  15 . A support layer  18  defining an open central portion  19  and an edge perimeter  20  is placed in surrounding relation to the first electrode  14 . Two separator layers  30  and  32  are placed adjacent to and in contact with opposing surfaces  14   a ,  14   b , respectively, of the first electrode  14 . The individual sizes and polymeric materials of the two separator layers  30  and  32  may be the same or different as determined by the user and generally depends upon the voltage requirements and cost of the battery. As shown in  FIG. 3 , the surface area  33  of separator layer  32  is substantially equal to the surface area  31  of separator layer  30 . Adjacent to and contacting the surfaces  30   a  and  32   a , respectively, of separator layers  30  and  32  opposing the first electrode  14  are second electrode  22  and third electrode  26 , respectively. Adjacent to and contacting the surface  30   b  of separator layer  30  is support layer  18 . Second electrode  22  and third electrode  26  have surface areas  23  and  27 , respectively, defined by edge perimeters  24  and  28 , which may be substantially equal to the area  21  defined by the edge perimeter  20  of the support layer  18 . The surface areas  23  and  27  of electrodes  22  and  26 , respectively, may be equal or different in size. Advantageously, the surface areas  23  and  27  are substantially equal to the area  21  of support layer  18 . In this fashion, the support layer  18  maximizes support to both larger second and third electrodes  22  and  26  by maintaining a uniform outer edge surface of cell stack  12  without allowing the edges and corners of electrodes  22  and  26  from becoming vulnerable to pressures exerted during packaging and internal pressures maintained during the lifetime of the vacuum sealed dual-cell battery. 
     The present invention also provides a method of constructing or assembling a battery cell, such as a lithium ion battery cell, having a support layer. Referring again to  FIG. 1 , the method comprises providing at least one anode and at least one cathode wherein either of the anode or the cathode is the first electrode  14  having a surface area  15  that is smaller relative to the larger surface area  23  of the other of the anode or the cathode, i.e., second electrode  22 . A support layer  18  is provided in surrounding contact with at least a portion of the first electrode  14  whereby the support layer  18  is adapted to provide reinforcing support to the larger second electrode  22 . Optimally, every first electrode  14  in a cell stack is surrounded by a support layer  18  during subsequent packaging in a multi-cell battery. Between each first electrode  14  and second electrode  22  in sequence, a separator layer  30  is placed, and first electrode  14 , support layer  18 , second electrode  22 , and separator layer  30  are joined to form a battery cell  10 . 
     Joining may involve vacuum sealing or other lamination methods to package the cell. As shown in  FIG. 2 , seal  11  of cell  10  encloses the cell  10  and its components, first electrode  14 , support layer  18 , second electrode  22 , and separator layer  30  under vacuum. Vacuum and air pressures exerted on the cell  10  by packaging equipment conventionally used to package batteries vary depending on equipment. Conventional techniques known in the art to join components of a battery cell are suitable in the method of the present invention. 
     Other aspects of the present method include placing support layer  18  adjacent to the separator layer  30  prior to adding the first electrode  14  and joining the layers together to provide a joined battery cell  10 . Upon joining cell  10  would have the support layer  18  in a surrounding relation with at least a portion of the edge perimeter  16  of the smaller surface area  15  of first electrode  14 , i.e., either the anode or the cathode. Alternatively, battery cell  10  may be assembled such that the support layer  18  is independently provided in a surrounding relation with the edge perimeter  16  of smaller first electrode  14  prior to joining the layers together to form the cell  10 . 
     Accordingly, the support layer of the present invention provides sufficient support for larger electrode layers during packaging to eliminate the need for more complex and expensive packaging equipment, materials and processes, such as pre-formed or rigid packaging processes, for the manufacture and packaging of battery cells. Consequently, a capital cost savings, including equipment, materials and fabrication process savings together with battery cells having longer lifetimes and fewer shorts, may be realized during production. 
     While the present invention has been illustrated by the description of one or more embodiments thereof, and while the embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of Applicants&#39; general inventive concept.