Abstract:
An electrical contact for connecting current collecting elements of a stack of electrochemical laminates. The electrical contact is formed of a current collecting terminal and a ductile electrically conductive material. The current collecting terminal has a pair of arms defining a space therebetween for receiving the ends of the current collecting elements as stacked. The ductile electrically conductive material is located within the space and is adapted to form an electrical bridge between the ends of the current collecting elements and the current collecting terminal.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to polymer electrolyte batteries. More particularly, the present invention relates to electrical contacts for current collectors consisting of a metal or metal oxide layer on a plastic substrate film, for use in polymer electrolyte batteries.  
       BACKGROUND OF THE INVENTION  
       [0002]     Rechargeable batteries manufactured from laminates of solid polymer electrolytes and sheet-like anodes and cathodes display many advantages over conventional liquid electrolyte batteries. These advantages include having a lower overall battery weight, a higher power density, a higher specific energy and a longer service life, as well as being environmentally friendly since the danger of spilling toxic liquid into the environment is eliminated. Solid polymer battery components include positive electrodes, negative electrodes and an electrolyte separator capable of permitting ionic conductivity, such as a solid polymer electrolyte mixed with an alkali salt sandwiched between the electrodes. The anode or negative electrode is usually made of alkali metal and alloys, typically Lithium metal, lithium alloys and the like or carbon, such as coke or graphite intercalated with lithium ion to form Li x C. The composite cathode or positive electrode is usually formed of a mixture of an active material (such as a transitional metal oxide), an electronically conductive filler (usually carbon or graphite particles), an ionically conductive polymer electrolyte material, an alkali salt and a current collector (usually a thin sheet of aluminum).  
         [0003]     Composite cathode thin films are usually obtained by coating or extruding directly onto a current collector. The current collector conducts the flow of electrons between the cathode active material and the battery terminals and also provides support for the cathode material, which has a paste-like structure. Current collectors such as metal foils have a tendency to corrode or to form an insulating film, which impairs the passage of electrons between the collector and the active material of the electrode when in direct contact with the cathode active material, thereby increasing the internal resistance of the electrochemical cell and reducing power density and cycle life of such rechargeable batteries. Corrosion of the metal current collector often occurs when very thin current collectors are used. This corrosion leads to loss of contact, electronic isolation and poor battery performance. It is known to use a protective coating between the electrode material and the metal current collector in order to enhance the contact and adhesion of the electrode material to the metal current collector. Such a protective coating also serves to protect the current collector from the corrosive effects of the electrolyte, the anodic material and the cathodic material.  
         [0004]     The current collector is considered as a passive component of the electrochemical cell because it does not generate energy but simply provides a means for conducting electrical current generated by the electrochemical cell. One exception is the use of a lithium or lithium alloy metal anode, which is an active component of the electrochemical cell and fully capable of conducting electrical current. It is therefore imperative to reduce the volume and weight of the current collector to a minimum for a given application.  
         [0005]     Thin metallic foil current collectors are fragile and have a tendency to break when subjected to tension through the various manufacturing processes of producing electrochemical cells. Every breakage of the metallic current collector effectively interrupts the production process, thereby increasing cost by reducing efficiency. To alleviate this problem, thin current collectors need to be less fragile and more flexible or malleable, while remaining good electric conductors.  
         [0006]     It is known to use metallized dielectric plastic films as electrodes in electrostatic condensers. The metals generally deposited on plastic films are in this case aluminum, zinc and their alloys. These metallizations are generally obtained, under vacuum, by thermal evaporation or by other assisted processes of evaporation: cathodic projection or electron beam. The thickness thus obtained is however very low, typically 100-500 Å and the surface resistance is consequently very high, approximately 1-100 Ω/square. In addition to the fact that the metals known and deposited are not chemically stable with the anode of polymer electrolyte generators, the surface conductivities obtained are insufficient to permit the draining of the range of currents provided for the average or large-size generators. The processes of metallization under vacuum are also known to be limited to a thickness lower than about 750 Å. These electrodes of electrostatic condensers are therefore not applicable as current collectors for most of the polymer electrolyte lithium generators, except possibly in the case of the metallization of aluminum applied to a positive electrode in small size batteries, where the mean current densities (I mean /cm 2 ) are low.  
         [0007]     U.S. Pat. Nos. 5,423,110 and 5,521,028 both disclose a current collector and a process for making same in which one metal is deposited under vacuum on an insulating support film of synthetic resin, the metal for the metallization being selected so as to constitute a substrate promoting an electrochemical deposit and having its thickness adjusted between about 0.005 and 0.01 μm in order to give a metallized film having sufficient electric conductivity to initiate a uniform electrochemical deposit. Also disclosed is the step of electrochemically depositing at least one additional metallic layer, of a total thickness between 0.1 and 4 μm, on at least one part of the surface of the metallized film so as to constitute a metallized-plate conductor and to reduce the electrical surface resistance of the collector at a level sufficient to prevent significant voltage losses by resistive effect in the collector during operation of the generator. The metal of the additional metallic layer deposited is selected for its compatibility with the corresponding electrode of the generator.  
         [0008]     The applicant&#39;s co-pending U.S. application Ser. No. 10/329,364 discloses a current collector made of a polymer substrate support film having a thickness of between 1 and 15 μm; a conductive metallic layer having a thickness of less than 3 μm, which is coated by metal vapor deposition onto preferably both sides of the polymer substrate film which are able to conduct high current densities; and a protective metal or metal oxide layer deposited onto each conductive metallic layer, this protective layer being electrically conductive and having a thickness of between 5 and 500 nm for protecting the conductive metallic from the corrosive effects of the polymer electrolyte cells components.  
         [0009]     The advantages of a current collector as described in co-pending U.S. application Ser. No. 10/329,364 are numerous, and include being lightweight, providing a very thin film, having resilience and having high current density conductivity. However, one draw back of this configuration is the fact that the polymer substrate support film may act as electrical insulation between its two conductive metallic layers, making it difficult to electrically connect two or more such current collectors, especially in parallel.  
         [0010]     There is therefore a need for an electrical contact and method adapted to electrically connect two or more current collectors having conductive metallic layers over a polymer substrate support film.  
       STATEMENT OF THE INVENTION  
       [0011]     It is an object of the present invention to provide an electrical contact for current collectors having conductive metallic layers over a polymer substrate support film.  
         [0012]     It is another object of the present invention to provide a method for electrically connecting two or more current collectors having conductive metallic layers over a polymer substrate support film.  
         [0013]     It is a further object of the present invention to provide an electrical contact for current collectors having conductive metallic layers over a polymer substrate support film for use in electrochemical generators.  
         [0014]     As embodied and broadly described, the invention provides an electrical contact for connecting current collecting elements of a plurality of stacked electrochemical laminates, said electrical contact comprising: 
        a current collecting terminal having a pair of arms, said arms defining therebetween a space in which the ends of the current collecting elements are received; and     a ductile electrically conductive material located within said space, said ductile electrically conductive material adapted to form an electrical bridge between the ends of said current collecting elements and said current collecting terminal.        
 
         [0017]     As embodied and broadly described, the invention also provides an electrochemical generator comprising: 
        a plurality of stacked electrochemical laminates, each electrochemical laminate including: 
            a) at least one electrolyte separator disposed between an anode film and a cathode film;     b) a current collecting element associated with one of said anode film and said cathode film, said current collecting element comprising a polymer substrate support film coated on both sides with a conductive metallic layer;    
            a current collecting terminal having a pair of arms defining therebetween a space in which the ends of said current collecting elements are received, said current collecting terminal being crimped onto the ends of the current collecting elements;     a ductile electrically conductive material located within said space, said ductile electrically conductive material filling at least a portion of said space thereby forming an electrical bridge between the ends of said current collecting elements and said current collecting terminal.        
 
         [0023]     As embodied and broadly described, the invention also provides a method of connecting in parallel the current collecting elements of a plurality of electrochemical laminates, said method comprising: 
        a) stacking the current collecting elements;     b) applying a layer of ductile electrically conductive material on at least a portion of the inside surface of a current collecting terminal, the current collecting terminal having a pair of arms defining a space therebetween;     c) positioning the ends of the current collecting elements as stacked within the space defined by the pair of arms of the current collecting terminal; and     d) crimping said current collecting terminal onto the ends of the current collecting elements.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]     The invention will be better understood and other advantages will appear by means of the following description and the following drawings in which:  
         [0029]      FIG. 1  is an enlarged schematic cross-sectional view of an example of a metallized current collector of an electrochemical cell laminate;  
         [0030]      FIG. 2  is an enlarged schematic side elevational view of an example of a series of metallized current collectors connected together in parallel;  
         [0031]      FIG. 3  is an enlarged schematic side elevational view of an electrochemical cell comprising a series of laminates, wherein the current collectors are connected together in accordance with an embodiment of the present invention;  
         [0032]      FIG. 3A  is a enlarged schematic side view of the current collecting terminal of the electrochemical cell shown in  FIG. 3 ;  
         [0033]      FIG. 4  is a schematic perspective view of a current collecting terminal for an electrochemical cell prior to assembly in accordance with an embodiment of the present invention;  
         [0034]      FIG. 5  is an enlarged schematic cross-sectional view of a series of current collectors connected together in accordance with a second embodiment of the present invention;  
         [0035]      FIG. 6  is a schematic top plan view of a metallized current collector sheet in accordance with an embodiment of the present invention;  
         [0036]      FIG. 7  is an enlarged schematic side perspective view of a series of laminates comprising metallized current collectors as shown in  FIG. 5 , stacked together to form an electrochemical cell in accordance with another embodiment of the present invention;  
         [0037]      FIG. 8  is an enlarged schematic side perspective view of a series of laminates comprising another embodiment of a metallized current collector, stacked together to form an electrochemical cell;  
         [0038]      FIG. 8A  is an enlarged cross-sectional view of the stacked metallized current collectors shown in  FIG. 8 ;  
         [0039]      FIG. 8B  is an enlarged cross-sectional view of a variant of the stacked metallized current collectors shown in  FIG. 8 ;  
         [0040]      FIG. 8C  is an schematic cross-sectional view of the stacked metallized current collectors shown in  FIGS. 8 and 8 A showing the electrical contact paths;  
         [0041]      FIG. 9  is an enlarged schematic side perspective view of a metallized current collector in accordance with another embodiment of the present invention;  
         [0042]      FIG. 9A  is an enlarged schematic cross-sectional view of the metallized current collector shown in  FIG. 9 ;  
         [0043]      FIG. 10  is an enlarged schematic side perspective view of a metallized current collector in accordance with another embodiment of the present invention; and  
         [0044]      FIG. 10A  is an enlarged schematic cross-sectional view of the metallized current collector shown in  FIG. 10 . 
     
    
     DETAILED DESCRIPTION  
       [0045]     Current collectors in electrochemical cells are necessary passive components, responsible for transporting electrical current generated by the electrochemical reaction between the anode and the cathode. Current collectors are also necessary as mechanical supports for paste-like anodes or cathodes and as such should be as strong and as thin as practicable, in order to reduce the mass and volume penalty of the current collector to the overall weight and volume of the electrochemical cell.  FIG. 1  illustrates schematically a cross-section of an example of an electrochemical cell laminate  20  comprising a metallized current collector  22 , where this current collector  22  consists of a polymer substrate support film  24  having a metallic conductive layer  26  on each side thereof. The illustrated cell laminate  20  is a bi-face configuration and therefore comprises two layers of cathode material  28  as well as a pair of anode films  32 . Each layer of cathode material  28  is coated or directly extruded onto a respective side of the current collector  22 . Each anode file  32  is separated from a respective cathode layer  28  by an electrolyte separator  30 . The anode films  32  are laterally offset relative to the cathode current collector  22 , such that the anodes  32  extend from one end of the laminate  20  and the cathode current collector  22  extends at the other end of the laminate  20 . As a result, when a plurality of cell laminates  20  are stacked together, all cathode current collectors  22  may be connected together in parallel at one end of the cell stack and all anode films  32  may be connected together in parallel at the other end of the cell stack.  
         [0046]     In a specific example of an electrochemical cell laminate  20  construction, the anode films  32  are thin sheets of lithium or lithium alloy, while the cathode films or layers  28  are composites formed of a mixture of an insertion material capable of occluding and releasing lithium ions, such as transitional metal oxide, and an electrically conductive filler, such as carbon or graphite particles. Furthermore, the electrolyte separators  30  consist of a polymer/alkali metal salt complex that is ionically conductive.  
         [0047]     The current collector  22  is formed of a very thin polymer support film  24  having a thickness of between 1 and 15 microns, preferably less than 10 microns, onto which are coated conductive metallic layers  26 . Each metallic layer  26  has a thickness of between 0.1 and 5 microns, preferably about 0.3 to 1 micron. The conductive metallic layers  26  may be further protected against corrosion by a second extremely thin layer having a thickness of between 5 and 500 nanometers, preferably less than 100 nanometers. Preferred methods of depositing the conductive metal layers  26  in thickness sufficient to permit the draining of current densities (I max /cm 2 ) generated by average or large-size electrochemical cells include vacuum metal vapor deposition and plasma activated vapor deposition.  
         [0048]     Typically, the substrate support film  24  is selected from the group consisting of: bi-axially oriented polystyrene (BO-PS), polyethylene terephthalate (BO-PET), polycarbonate (PC), polypropylene (PP), polypropylene sulphide (PPS) and polyethylene Naphthalate (PEN), amongst others. The conductive metallic layers  26  may be formed of any metal exhibiting good electrical and thermal conductivity, as well as low density and low cost. Suitable conductive metals are Aluminum (Al), Copper (Cu), Silver (Ag), Nickel (Ni) and Tin (Sn), or alloys based on these metals. However, Aluminum and Copper are preferred for their low cost and excellent conductivity and, in the case of Aluminum, for its lightness. Any of these metals may be vacuum vapor deposited or plasma activated deposited onto the polymer substrate film.  
         [0049]     The polymer support film  24  is generally not a good electric conductor. As such, when three or more metallized current collectors  22  are electrically connected in parallel by a metallic current collecting terminal  34  crimped onto the ends of the current collectors  22 , as shown in  FIG. 2 , only the surfaces of the current collectors  22 A and  22 D directly in contact with the current collecting terminal  34  are in electrical contact with the current collecting terminal  34 . Current collectors  22 B and  22 C, as well as the surfaces of the current collectors  22 A and  22 D not directly in contact with the current collecting terminal  34 , are electrically isolated and unable to conduct the electrochemical energy generated by their respective laminates. The polymer support film  24  of each metallized current collector  22 A,  22 B,  22 C and  22 D acts as an electrical insulator.  
         [0050]      FIG. 3  illustrates a first, non-limiting embodiment of the present invention, wherein a plurality of electrochemical cell laminates are stacked together, their respective metallized current collectors  22  being electrically connected together with a current collecting terminal  34  crimped thereto. Inside the collecting terminal  34 , between the inner surface of the collecting terminal  34  and the metallized current collectors  22 , there is provided a ductile electrically conductive material  36 . This ductile material  36  forms an electrical bridge between current collectors  22  and current collecting terminal  34 , and more specifically between the ends of the current collectors  22  not directly in contact with the inner surfaces of the arms  38  and  39  of current collecting terminal  34 .  
         [0051]     As illustrated in  FIG. 3A , the ends of the metallic conductive layers  26  of each metallized current collector  22  are in contact with the ductile electrically conductive material  36 , which is itself in contact with the inner surfaces of current collecting terminal  34 . As such, electrical current generated by each electrochemical cell laminate may circulate freely to current collecting terminal  34 .  
         [0052]      FIG. 4  illustrates a current collecting terminal  34  prior to being deformed and crimped onto the ends of the current collectors  22  of a stack of electrochemical cell laminates. The arms  38  and  39  of the current collecting terminal  34  are open wide enough to easily receive a stack of metallized current collectors  22 . A portion of the inner surface of the current collecting terminal  34  is covered with a layer of ductile electrically conductive material  36  prior to deformation or crimping. When the current collecting terminal  34  is deformed or crimped onto the stack of metallized current collectors  22 , the ductile electrically conductive material  36  saturates the volume created by the arms  38  and  39  of current collecting terminal  34 , and more specifically the void space  37  ( FIG. 3A ), thus forming an electrical bridge between the ends of metallized current collectors  22  and current collecting terminal  34 . The ductile electrically conductive material  36  may also partially penetrate between the metallized current collectors  22  when the arms  38  and  39  of current collecting terminal  34  are pressed and crimped onto the stack of metallized current collectors  22 , thereby providing more surface area through which electrical current may circulate.  
         [0053]     The ductile electrically conductive material  36  may be a metal that is very ductile at room temperature, such as lithium, tin, lead, alloys thereof or combinations thereof, among other possibilities. The ductile material  36  may also be a metal-based epoxy paste, such as silver or aluminium epoxy-based paste, or any other suitable conductive paste.  
         [0054]      FIG. 5  illustrates a second embodiment of the invention wherein, within the stack of electrochemical cell laminates, the metallized current collectors  22  are stacked in a stair-like or offset pattern. This stacking pattern leaves a portion of the conductive metal layers  26  of each metallized current collector  22  exposed, thereby providing an increased surface area through which electrical current may circulate.  
         [0055]     According to yet another embodiment of the present invention,  FIG. 6  is a top plan view of a metallized current collector sheet  45  onto which is coated a layer of cathode material  40 . The edges  43  and  44  of the metallized current collector sheet  45  are provided with a series of indentations  42  made prior to coating of the current collector sheet  45  with the cathode material  40 . Since only one edge ( 43  or  44 ) of the metallized current collector sheet  45  will be connected to another metallized current collector sheet  45 , it is sufficient to have indentations  42  made on one of the two edges  43  or  44 . Furthermore, the indentations  42  may be cut out after the cathode material  40  has been coated onto the metallized current collector sheet  45 .  
         [0056]      FIG. 7  illustrates the positive side of a stack of electrochemical cell laminates comprising a plurality of cathodes having metallized current collector sheets  45  as illustrated in  FIG. 6 . The series of indentations  42  have the effect of increasing the surface area of the ends of the metallized current collector sheets  45  in contact with the ductile electrically conductive material  36 , when these same ends of the metallized current collector sheets  45  are crimped together. More specifically, the overall length of the exposed ends of the metallic conductive layers  26  of all metallized current collector sheets  45  is increased, thereby increasing the total surface area in contact with the ductile electrically conductive material  36 . Furthermore, the indentations  42  expose portions of the sides of adjacent metallized current collector sheets  45 , thereby further increasing the total surface area of the metallic conductive layers  26  in contact with the ductile electrically conductive material  36 . The indentations  42  provide more surface area through which electrical current may circulate.  
         [0057]      FIG. 8  illustrates a further embodiment of the present invention, wherein cathode layers  40  are coated onto metallized current collector sheets  48  that are provided at one edge  49  with a series of perforations  50 . Perforations  50  allow ductile electrically conductive material  36  to infiltrate the various layers of metallized current collector sheets  48 . Perforations  50  also provide for direct contact between a first metallic conductive layer  26  of a first metallized current collector sheet  48  and a third metallic conductive layer  26  of a third metallized current collector sheet  48 , through the perforations  50  of a second metallized current collector sheet  48  located between the first and third metallized current collector sheets  48 .  
         [0058]      FIG. 8A  is a cross-sectional view taken at line  8 A- 8 A of  FIG. 8  and illustrates ductile electrically conductive material  36  infiltrating all of the perforations  50 . If the distance  51  between two adjacent perforations  50  is smaller than the diameter of the perforations  50 , the ductile electrically conductive material  36  will infiltrate the perforations  50  of the subsequent metallized current collector sheets  48  even with a random alignment of the perforations  50  as shown in  FIG. 8A .  
         [0059]      FIG. 8B  is also a cross-sectional view taken at line  8 A- 8 A of  FIG. 8  and illustrates a situation in which the ductile electrically conductive material  36  is unable to infiltrate all of the perforations  50  because the distance  51  between two adjacent perforations  50  is greater than the diameter of the perforations  50  themselves. In this case, a random alignment of the perforations  50  may prevent the ductile electrically conductive material  36  from infiltrating some of the subsequent metallized current collector sheets  48 .  
         [0060]      FIG. 8C  is further a cross-sectional view taken at line  8 A- 8 A of  FIG. 8 , which illustrates in more detail the various layers of the metallized current collector sheets  48  and the electrical contacts between them. When pressure is applied onto the stack of metallized current collector sheets  48  with the jaws of a crimping apparatus, the polymer substrate  24  may be deformed or compressed to such an extent that the conductive layer  26  of a first metallized current collector sheet  48  may physically reach through the perforations  50  of a second metallized current collector sheet  48  and contact the conductive layer  26  of a third metallized current collector sheet  48 . This phenomenon is illustrated in  FIG. 8C  by the electrical paths  52  and  54 . Electrical paths  52  show that the conductive layer  26  of metallized current collector sheet  48 A is in contact with the conductive layer  26  of metallized current collector sheet  48 C, which is in turn in direct contact with the conductive layer  26  of metallized current collector sheet  48 B. Furthermore, electrical paths  54  show that the conductive layer  26  of metallized current collector sheet  48 B is in contact with the conductive layer  26  of metallized current collector sheet  48 D, also through the deformation or compression of the polymer substrate  24  of metallized current collector sheet  48 C. The combination of the infiltration of ductile conductive material through the perforations  50  and the compression and deformation of the polymer substrate  24  of the various metallized current collector sheets  48  increases the electrical contacts between the plurality of crimped metallized current collector sheets  48  of an electrochemical cell.  
         [0061]      FIG. 9  illustrates another embodiment of the present invention, wherein a metallized current collector sheet  60  is provided with perforations  62  along its edge  63 . As shown in  FIG. 9A , the perforations  62  are made to the polymer substrate  24  prior to applying the metallic conductive layers  64 , such that the inner surfaces of the perforations  62  are also coated with a metallic conductive layer  64 . The metallic conductive layers  64  on both sides of the metallized current collector sheet  60  are therefore in electrical contact with each other through the metallic conductive layers  64  of the inner surfaces of the perforations  62 . The parallel electrical connections of a plurality of metallized current collector sheets  60  therefore offer less resistance, since there is an electrical path provided through the polymer substrate  24  between the metallic conductive layers  64  on opposite sides of each metallized current collector sheet  60 . The use of a ductile conductive material  36  with a crimped current collecting terminal to effect the parallel electrical connection, as well as the compression or deformation of the polymer substrate layer  24 , provide a further increase in the electrical conductivity of the various metallized current collector sheets  60  within the current collecting terminal.  
         [0062]      FIG. 10  illustrates a variant of the embodiment shown in  FIG. 9 , wherein a metallized current collector sheet  70  comprises oblong perforations  72  made along its edge  73 . As shown in  FIG. 10A , the inner surfaces of the perforations  72  are also coated with a metallic conductive layer  74 . The metallic conductive layers  74  on both sides of the metallized current collector sheet  70  are therefore in electrical contact with each other through the metallic conductive layers  74  of the inner surfaces of the perforations  72 . The oblong perforations  72  provide an increased contact area between the metallic conductive layers  74  of both sides of the metallized current collector sheet  70 .  
         [0063]     The embodiments of metallized current collector sheets  45 ,  48 ,  60  and  70  as illustrated in  FIGS. 6, 7 ,  8 ,  9  and  10  may also be stacked and crimped together in a stair-like or offset pattern as illustrated in  FIG. 5 , thereby leaving a greater portion of the conductive metal layers of each metallized current collector sheets exposed to the ductile electrically conductive material and providing increased total surface area through which electrical current may circulate.  
         [0064]     In a further embodiment (not shown), it is also possible to first stack the metallized current collector sheets as illustrated in  FIG. 3  and, prior to crimping the assembly, to punch a series of perforations as illustrated in  FIG. 8  such that the perforations will be aligned. As a result, the ductile electrically conductive material will penetrate and fill the perforations, thereby providing electrical contact within the perforations as well as outside of the perforations.  
         [0065]     Although the present invention has been described in relation to particular embodiments thereof, other variations and modifications are contemplated and are within the scope of the present invention. Therefore, the present invention is not to be limited by the above description but is defined by the appended claims.