Abstract:
A process for assembling an electrochemical cell that utilizes an electrode containment shield to prevent internal electrical short circuits that are caused by fragments of a frangible electrode coming into contact with the battery&#39;s opposing electrode. The shield, which may be secured to the current collector, is particularly useful with low volume seals that do not support the separator.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention generally relates to a process for assembling electrochemical cells having a frangible electrode and, more particularly, to cells having a current collector assembly with a shield secured thereto which limits movement of the frangible electrode.  
           [0002]    Cylindrically shaped batteries are suitable for use by consumers in a wide variety of devices such as flashlights, radios and cameras. Batteries used in these devices typically employ a cylindrical metal container to house two electrodes, a separator, a quantity of electrolyte and a closure assembly that includes a current collector. Typical electrode materials include manganese dioxide as the cathode and zinc as the anode. The zinc is commonly employed in particulate form suspended in a gel. An aqueous solution of potassium hydroxide is a common electrolyte. A separator, conventionally formed from one or more strips of paper, is positioned between the electrodes. The electrolyte is readily absorbed by the separator and gelling medium.  
           [0003]    One of the issues that battery manufacturers must address is the requirement that direct contact between the anode and cathode within the battery be prevented. If the anode and cathode are allowed to physically contact one another, a chemical reaction takes place and the useful electrochemical capacity of the battery is reduced. The function of the separator is to prevent direct contact between the anode and cathode while allowing for ionic conductivity therebetween.  
           [0004]    Small cylindrical batteries must be manufactured to withstand the physical rigors associated with the manufacturing and distribution processes as well as the handling of batteries by consumers. In particular, batteries must be able to withstand being accidentally dropped by consumers. In batteries with a frangible electrode, such as the gelled anode used in many cylindrical alkaline batteries, dropping the battery may cause a small portion of the anode to fragment and thereby break free from the rest of the anode. The fragmented anode must be prevented from coming into direct contact with the cathode. Similarly, if the cathode is hard and susceptible to fragmentation when the cell is dropped, small fragments of the cathode may become separated from the body of the cathode and need to be contained. As shown in FIG. 1, many conventional cell constructions have addressed this problem by using an clastomeric seal  78  that includes a V-shaped leg  80  that projects toward the interior of the cell and contacts the top of the coiled separator  20  thereby forming a barrier that prevents anode fragments  64  from contacting cathode  54 . However, the V-shaped projections on the conventional seal designs occupy a portion of the cell&#39;s internal volume which could be better used to hold an additional quantity of the cell&#39;s electrochemically active materials. Consequently, many cell designs have been proposed that utilize low volume seals which do not cooperate with the separator to form a barrier that isolates the anode from the cathode. Unfortunately, eliminating the portion of the seal that helps to contain the anode has led to an increased level of internal electrical shorting between the anode and cathode when cells are dropped by consumers. The internal shorting problem is due to the freestanding portion of the separator, located above the anode/cathode interface, losing its stiffness when it absorbs some of the cell&#39;s electrolyte and then collapsing away from the low volume seal so that an unobstructed path is created between the anode and the cathode. As shown in FIG. 2, the collapsed portion  46  of the separator  20  has allowed a fragment  64  of anode  66  to contact cathode  54 .  
           [0005]    One solution to the problem of preventing internal electrical short circuits caused by fragmented electrodes in cylindrical alkaline batteries is disclosed in Japanese Kokai Patent Application No. 7 [1995]-134977. In one embodiment, this reference discloses applying an adhesive to a portion of the separator where the separator and sealing gasket contact one another. The adhesive secures the separator to the gasket so that small portions of the electrode that break free when the battery is dropped will not be able to contact the opposing electrode and cause an internal short circuit. One disadvantage with this approach is that the application of the adhesive to the edge of the separator in a large scale commercial manufacturing operation would slow down the production process thereby increasing the cost of the battery.  
           [0006]    Disclosed in U.S. Pat. No. 3,056,849 is a cell construction with a washer placed on top of an out-turned edge of the separator that overlays the depolarizer and thus prevents the anode slurry from contacting the depolarizer cylinder. One disadvantage of this cell construction is that the washer effectively limits the height of the depolarizer and thus reduces the discharge capacity of the cell.  
           [0007]    U.S. Pat. No. 3,756,859 discloses a process for assembling an annular disc on top of a depolarizer mass body. The body is covered at its upper end with the annular disk and then a carbon rod is inserted through the disk and into the depolarizer body. This process discloses the placement of the disk and carbon rod in two separate steps.  
           [0008]    U.S. Pat. No. 3,888,700 discloses a cell assembly process that includes a plastic or paper compression washer and a carbon pencil which serves as a current collector. The washer has a hole therein to receive the carbon pencil. The disclosed process inserts the cathode mix into the container and then the carbon pencil is driven into the cathode mix. The washer is then placed on top of the cathode mix so that the carbon pencil aligns with the hole in the washer. This process also discloses the placement of the washer and carbon pencil in two separate steps.  
           [0009]    There exists a need for a process that provides a cell with a volume efficient electrode containment shield that can be accurately and economically located within the cell and will prevent undesirable movement of a frangible electrode&#39;s fragments so that the cell does not experience an internal electrical short circuit when the cell is dropped. The shield should occupy a minimum amount of the cell&#39;s internal volume and should be compatible with low volume seal bodies that do not provide structural support to the cell&#39;s separator.  
         BRIEF SUMMARY OF THE INVENTION  
         [0010]    The present invention provides a process for assembling an electrochemical cell with an electrode containment shield that is preassembled onto another cell component before the shield is inserted into the cell. Consequently, the shield can be accurately and cost effectively positioned within the cell so that fragments of a frangible electrode that become dislodged when the cell is dropped are safely contained and do not cause an internal electrical short circuit.  
           [0011]    In one embodiment, the process of the present invention includes the following steps. In a step, providing a cylindrical open ended container having a first electrode that defines a tubularly shaped cavity. A separator lines the cavity. A second electrode, which contains a frangible composition including particulate zinc, is disposed within the separator lined cavity. In another step, producing a current collector assembly having a cover, an electrode containment shield with a width less than the inside diameter of the container&#39;s open end and an elongated electrically conductive member having a first end and a second end. The assembly is produced by contacting an end of the electrically conductive member to the cover and inserting an end of the electrically conductive member perpendicularly through a disc shaped electrode containment shield thereby forming a preassembled current collector assembly. The first end of the conductive member contacts the cover. The second end of the conductive member forms a freestanding end that extends from the shield. In another step, the preassembled current collector assembly is joined to the open end of the container by inserting the second end of the assembly&#39;s conductive member into the second electrode and securing the current collector assembly to the open end of the container whereby the electrode containment shield is positioned between the cover and the second electrode. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a cross section of a conventional alkaline cell;  
         [0013]    [0013]FIG. 2 is a cross section of an alkaline cell containing a low volume seal assembly;  
         [0014]    [0014]FIG. 3A is an exploded view of a current collector assembly;  
         [0015]    [0015]FIG. 3B is cross sectional view of a current collector assembly;  
         [0016]    [0016]FIG. 4A is an exploded view of a current collector assembly;  
         [0017]    [0017]FIG. 4B is cross sectional view of a current collector assembly;  
         [0018]    [0018]FIG. 5A is an exploded view of a current collector assembly;  
         [0019]    [0019]FIG. 5B is cross sectional view of a current collector assembly;  
         [0020]    [0020]FIG. 6A is an exploded view of a current collector assembly;  
         [0021]    [0021]FIG. 6B is cross sectional view of a current collector assembly;  
         [0022]    [0022]FIG. 7A is an exploded view of a current collector assembly;  
         [0023]    [0023]FIG. 7B is cross sectional view of a current collector assembly;  
         [0024]    [0024]FIG. 8 is an exploded view of the components used to assemble the cell shown in FIG. 9;  
         [0025]    [0025]FIG. 9 is a cross section of an electrochemical cell manufactured by a process of this invention;  
         [0026]    [0026]FIG. 10 is a chart showing the steps of this invention&#39;s cell manufacturing process;  
         [0027]    [0027]FIG. 11 is a top view of an electrode containment shield;  
         [0028]    [0028]FIG. 12 is a cross section of a current collector;  
         [0029]    [0029]FIGS. 13A, 13B and  13 C are cross sectional views of a cell as it is manufactured by a process of this invention;  
         [0030]    [0030]FIG. 14 is a cross sectional view of a conventional cell; and  
         [0031]    [0031]FIG. 15 is a cross sectional view of a cell manufactured by a process of this invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]    Referring now to the drawings and more particularly to FIG. 9, there is shown a cross-sectional view of an assembled electrochemical cell made by a process of this invention. Beginning with the exterior of the cell, the cell&#39;s components are the container  10 , first electrode  50  positioned adjacent the interior surface of container  10 , separator  20  which forms a lining against the interior surface  56  of first electrode  50 , second electrode  60  disposed within the cavity defined by separator  20  and current collector assembly  70  secured to container  10 . In this embodiment, collector assembly  70  includes current collector  76 , electrode containment shield  71 , seal body  78 , inner cover  86  and terminal cover  84 . Container  10  has an open end  12 , a closed end  14  and a sidewall  16  therebetween. The closed end  14 , sidewall  16  and current collector assembly  70  define an internal volume in which the cell&#39;s electrodes are housed.  
         [0033]    Referring to the flow chart shown in FIG. 10, the process of this invention comprises the following steps. In step  200 , providing a cylindrical open ended container housing a first electrode which defines a tubularly shaped cavity, a separator lining the cavity and a second electrode comprising a frangible composition including particulate zinc disposed within the cavity. In step  220 , a current collector assembly comprising an electrode containment shield is produced. The assembly comprises a cover for the electrochemical cell, an electrode containment shield having a width less than the inside diameter of the container&#39;s open end and an elongated electrically conductive member having a first end and a second end. The assembly is produced by contacting an end of the electrically conductive member to the cover and inserting an end of the electrically conductive member through a disc shaped electrode containment shield thereby forming a preassembled current collector assembly. The first end of the electrically conductive member contacts the cover and the second end of the electrically conductive member forms a freestanding end that extends from the shield. In step  240 , the preassembled current collector assembly is joined to the open end of the container. This step includes inserting the second end of the assembly&#39;s conductive member into the second electrode and securing the current collector assembly to the open end of the container such that the electrode containment shield is positioned between the cover and the second electrode.  
         [0034]    Disclosed in FIG. 8 is a preferred embodiment of an open ended container useful in a process of this invention. First electrode  50  is a mixture of manganese dioxide, graphite and an aqueous solution containing potassium hydroxide. Electrode  50  is formed by disposing a quantity of the mixture into the open ended container and then using a ram to mold the mixture into a solid tubular shape that defines a cavity which is concentric with the sidewall of the container. First electrode  50  has an interior surface  56 . Alternatively, the cathode may be formed by preforming a plurality of rings from the mixture comprising manganese dioxide and then inserting the rings into the container to form the tubularly shaped first electrode.  
         [0035]    Separator  20  is a film that is typically made from nonwoven fibers. One of the separator&#39;s functions is to provide a barrier at the interface of the first and second electrodes. The barrier must be electrically insulating and ionically permeable. To form a tubularly shaped compartment, a single sheet of separator may be coiled about an axis. The coiled tube is inserted into the cavity defined by the first electrode so that the tube lines the cavity. The dimensions of the separator are selected so that an edge of the coiled tube extends beyond the first electrode toward the open end of the container.  
         [0036]    In a preferred embodiment, second electrode  60  is a frangible homogenous mixture of an aqueous alkaline electrolyte, zinc powder, and a gelling agent such as crosslinked polyacrylic acid. The aqueous alkaline electrolyte comprises an alkaline metal hydroxide such as potassium hydroxide, sodium hydroxide, or mixtures thereof. Potassium hydroxide is preferred. The gelling agent suitable for use in a cell of this invention can be a crosslinked polyacrylic acid, such as Carbopol 940®, which is available from B. F. Goodrich, Performance Materials Division, Cleveland, Ohio, USA. Carboxymethyylcellulose, polyacrylamide and sodium polyacrylate are examples of other gelling agents that are suitable for use in an alkaline electrolyte solution. The zinc powder may be pure zinc or an alloy comprising an appropriate amount of one or more of the metals selected from the group consisting of indium, lead, bismuth, lithium, calcium and aluminum. A suitable anode mixture contains 67 weight percent zinc powder, 0.50 weight percent gelling agent and 32.5 weight percent alkaline electrolyte having 40 weight percent potassium hydroxide. The quantity of zinc can range from 63 percent by weight to 70 percent by weight of the anode. Other components such as gassing inhibitors, organic or inorganic anticorrosive agents, binders or surfactants may be optionally added to the ingredients listed above. Examples of gassing inhibitors or anticorrosive agents can include indium salts (such as indium hydroxide), perfluoroalkyl ammonium salts, alkali metal sulfides, etc. Examples of surfactants can include polyethylene oxide, polyethylene alkylethers, perfluoroalkyl compounds, and the like. The second electrode may be manufactured by combining the ingredients described above into a ribbon blender or drum mixer and then working the mixture into a wet slurry.  
         [0037]    Electrolyte suitable for use in a cell made by a process of this invention is a thirty-seven percent by weight aqueous solution of potassium hydroxide. The electrolyte may be incorporated into the cell by disposing a quantity of the fluid electrolyte into the cavity defined by the first electrode. The electrolyte may also be introduced into the cell by allowing the gelling medium to absorb an aqueous solution of potassium hydroxide during the process used to manufacture the second electrode. The method used to incorporate electrolyte into the cell is not critical provided the electrolyte is in contact with the first electrode  50 , second electrode  60  and separator  20 .  
         [0038]    Shown in FIGS. 3A and 3B is current collector assembly  110  which includes terminal cover  84 , current collector  76  and electrode containment shield  71 . Current collector  76  is an elongated electrically conductive member made from brass. Collector  76  has a first end  79 , referred to herein as the terminal contact end, and a second end  77 , referred to herein as the freestanding end. Freestanding end  77  of collector  76  is tapered to facilitate inserting the collector through other cell components such as electrode containment shield  71 . Shield  71  is a thin, disc shaped, flexible component made from polypropylene. Alternate materials from which the disc may be made include polyvinyl chloride, nylon, zinc and copper. An interference fit is established between collector  76  and shield  71 . As shown in FIG. 11, the center of the shield may be cut out to allow collector  76  to easily pass through the shield during the assembly process and to insure that collector  76  is coaxial with the circumference of shield  71 .  
         [0039]    Specific processes useful in producing current collector assemblies shown in FIGS. 3B and 4B will now be described. Referring to FIG. 3A, current collector  110  can be produced using the following process steps. First, contacting the terminal contact end  79  of collector  76  to terminal cover  84 . Then, inserting freestanding end  77  of collector  76  through electrode containment shield  71 . In this embodiment, the step of contacting first end  79  of collector  76  to cover  84  precedes inserting freestanding end  77  of collector  76  through electrode containment shield  71 . A similar process may be used to produce collector assembly  90  shown in FIG. 4B. In this embodiment, the current collector assembly includes seal body  78  and inner cover  86  as well as collector  76 , terminal cover  84  and electrode containment shield  71 . When seal body  78  and inner cover  86  are present, the assembly process includes contacting the terminal contact end  79  of collector  76  to terminal cover  84  and bringing seal body  78  into contact with inner cover  86  to form closure member  72 . Then, inserting freestanding end  77  of electrode  76  through an opening in seal body  78  and then inserting freestanding end  77  through electrode containment shield  71 .  
         [0040]    Referring to FIG. 5B, current collector assembly  100  includes current collector  76 , terminal cover  84  and electrode containment shield  71 . Terminal cover  84  contacts the terminal contact end  79  of elongated current collector  76 . Electrode containment shield  71  is positioned between terminal cover  84  and the freestanding end  77  of collector  76 .  
         [0041]    Collector assembly  100 , shown in FIG. 5, can be manufactured using the following process steps. Inserting terminal contact end  79  of collector  76  through the center of electrode containment shield  71 . Then securing the terminal contact end  79  of collector  76  to the central portion of terminal cover  84 . In this embodiment, the terminal contact end  79  of collector  76  is inserted through the electrode containment shield prior to contacting the terminal contact end  79  of the collector  76  to terminal cover  84 . In a similar process, collector assembly  70 , shown in FIG. 6B, can be assembled using the following process steps. Forming closure member  72  by bringing inner cover  86  and seal body  78  into contact with one another. Placing terminal cover  84  into contact with inner cover  86 . Inserting terminal contact end  79  of collector  76  through central opening  88  in seal body  78  until the terminal cover and terminal contact end of collector  76  abut one another. Then inserting freestanding end  77  of collector  76  through electrode containment shield  71 . The terminal cover and collector may be secured to one another by soldering, welding, or with a suitable adhesive.  
         [0042]    Referring to FIG. 7, collector assembly  68  can also be assembled using the following steps. Forming closure member  72  by bringing inner cover  86  and seal body  78  into contact with one another. Inserting terminal contact end  79  of collector  76  through central opening  88  in seal body  78  until the terminal cover and terminal contact end  79  of collector  76  abut one another. Then, inserting freestanding end  77  of collector  76  through electrode containment shield  71 .  
         [0043]    To facilitate accurate and secure placement of shield  71  on collector  76 , the diameter of collector  76  can be changed in order to create a shoulder against which the shield can be located. As shown in FIG. 12, shoulder  75  on collector  76  is created by reducing the diameter of collector  76  for a portion of the distance between freestanding end  77  and terminal contact end  79 . Since the diameter of freestanding end  77  is less than the diameter of terminal contact end  79 , the electrode containment shield can be forced along the length of collector  76  until it abuts shoulder  75 .  
         [0044]    The outside diameter of electrode containment shield  71  must be selected to cooperate with the inside diameter of the cylinder defined by separator  20 . Preferably, the outside diameter of shield  71  is slightly less than the inside diameter of the open end  12  of container  10 . More preferably, the outside diameter of shield  71  is slightly less than the inside diameter of the tube defined by the separator so that shield  71  does not touch separator  20  during the cell assembly process. If the outside diameter of the shield is too small, small fragments of the frangible electrode could possibly escape past shield  71  and contact first electrode  50  if the top of separator  20  becomes bent or wrinkled thereby providing an unobstructed path between the first and second electrodes. However, if the outside diameter of the shield is too large, the shield could come into contact with the separator and cause it to tear thereby allowing a fragment of one electrode to contact the other electrode. In an alternate embodiment, the perimeter of the shield could be serrated to facilitate the edge of the shield bending upward if the shield should contact the separator during the cell assembly process.  
         [0045]    Referring to FIG. 8, after producing the current collector assembly and providing the open ended container, electrochemical cell  102  is assembled by joining current collector assembly  69  to the open end  12  of container  10 . The joining process involves inserting freestanding end  77  of collector  76  into second electrode  60  until the current collector assembly&#39;s terminal cover  84  is proximate the open end  12  of container  10  and shield  71  is positioned between second electrode  60  and terminal cover  84 . Preferably, shield  71  is positioned between edge  22  of separator  20  and second electrode  60  as shown in FIG. 9. Collector assembly  69  is secured to container  10  by crimping the open end of container  10  inwardly over the periphery of terminal cover  84 . Alternate means for securing the collector assembly to the container include gluing and welding.  
         [0046]    As shown in FIG. 13, electrode containment shield  71  can also be used to properly locate collector  76  within the central portion of the separator lined cavity defined by first electrode  50 . Using the shield to center collector  76  as it is inserted into second electrode  60  is useful in preventing an electrical short circuit within the cell which is caused by an off center collector piercing the separator and establishing an electrical path between the first and second electrodes as shown in FIG. 14. Preferably, the process of using shield  71  as a locator for collector  76  incorporates the following step. Prior to joining the preassembled current collector assembly to the open ended container, shield  71  is positioned closer to the second end  77  of collector  76  than to the first end  79  of collector  76 . Subsequently, during the insertion of the second end  77  of collector  76  into second electrode  60 , shield  71  is forced along the length of collector  76  by second electrode  60  until the collector assembly  70  is proximate the open end  12  of container  10  and shield  71  is closer to the first end  79  of collector  76  than to the second end  77  of collector  76 . To facilitate accurate centering of the collector, the diameter of shield  71  is preferably at least ninety percent of the diameter of the opening defined by coiled separator  20 . More preferably, the diameter of the shield is between ninety-five percent and one-hundred percent of the diameter of the opening defined by coiled separator  20 .  
         [0047]    As used herein, the term “seal body” refers to a three dimensional elastomeric component that electrically insulates the cell&#39;s positive and negative components from one another and cooperates with the cell&#39;s container to close the open end of the container. The seal body may have a ventable diaphragm formed therein. Alternatively, the seal body may be formed as a circular gasket  81  as shown in FIG. 15. Seal bodies are conventionally formed by injection molding materials such as nylon, polypropylene and polystyrene. The term “low volume seal body” refers to a seal body that does not provide support to the upstanding tubularly shaped separator  20  as shown in FIG. 15.  
         [0048]    The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and are not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the Doctrine of Equivalents.