Abstract:
An electrochemical cell is presented having a first component that provides a seal to prevent the escape of internal cell ingredients from the battery and into the ambient environment during normal operation of the cell. A second component, independent of the first component, provides a vent and allows built-up pressure to dissipate when the pressure reaches a predetermined maximum threshold.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims priority to provisional patent application number 60/273,804, filed Mar. 7, 2001, and entitled “Seal and Vent Combination for an Electrochemical Cell” the disclosure of which is hereby incorporated by reference as if set forth in its entirety herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to electrochemical cells, and in particular, relates to an apparatus for sealing and venting an electrochemical cell.  
           [0004]    2. Description of the Related Art  
           [0005]    Conventional electrochemical cells, such as alkaline cells, comprise a cathode, typically graphite, compressed into annular rings that is often wetted with an alkaline electrolyte. The cathode is then placed into a metal container which then serves as the positive current collector. A separator is typically disposed radially inwardly of the cathode and separates the cathode from the anode, which generally comprises a powdered zinc disposed in a gel, such as carboxymethylcellulose. The negative current collector, usually a brass pin or nail, is placed in electrical contact with the anode.  
           [0006]    Such cells typically include a seal that prevents the materials disposed within the cell from escaping at the interface between the negative endplate and the container. During normal use of the cell, the pressure within the cavity is sufficiently low, thereby presenting substantially no threat to the integrity of the cell structure. However, when the battery is misused, substantial pressure may build up within the cell. For example, if a user attempts to charge a non-chargeable cell, or exposes the cell to extreme heat, significant pressure may accumulate within the cell. If no means exists to dissipate the pressure, the battery could fail in an unpredictable manner. To prevent this occurrence, a vent is installed in the cell that remains closed until the pressure exceeds a threshold limit, at which time the vent will open, thereby permitting the pressure to dissipate from the cell and into the ambient environment. The vents of conventional cells have traditionally been constructed as a unitary member with the seal, and thus possess inherent drawbacks that will be now be described with reference to FIG. 1.  
           [0007]    In particular, FIG. 1 illustrates a conventional cell including a unitary seal/vent member in the form of a gasket  10 . Gasket  10  comprises an annular outer flange  14  that is seated in a ridge  11  formed within the inner surface of the container  13 . The negative end of the cell is sealed by crimping the container  13  (and outer flange  14 ) over the endplate  16 . Outer flange  14  is connected to an elongated neck  12  that is slid axially along the negative current collector and into place such that the outer flange  14  rests against the upper surface of  11 . The neck  12  is connected to the outer flange via a radially extending disk having a breakaway section  19  of reduced thickness compared to the rest of the gasket  10 . A washer  18  rests against the neck  12  and flange  14  to ensure that the outer flange  14  of gasket  10  remains in contact with the container, thereby maintaining the structural integrity of the cell. A first aperture  15  extends through the endplate  16 , and a second aperture  17  extends through washer  18 . When the internal cell pressure reaches a predetermined threshold, the breakaway section  19  will rupture, thereby allowing pressure to dissipate into the ambient environment via apertures  15  and  17 .  
           [0008]    Thus, the gasket  10  provides both a seal and a vent for the cell. Several drawbacks are associated with this conventional design. For example, the washer  18  must be manufactured within tight tolerance such that is has a large enough diameter to retain the gasket  10  in its proper position, but not so large so as to create additional stresses on the breakaway section  19  that could cause the gasket  10  to fail prematurely. Accordingly, because no significant force exists to bias the neck  12  against the current collector, the neck must extend along a substantial amount of the nail&#39;s length to produce an adequate seal that prevents the internal cell ingredients from escaping through the interface between the current collector and neck  12 , and ultimately out aperture  15 . The elongated neck  12  thus consumes valuable space within the cell that could otherwise be occupied by the anode mixture, thereby reducing the active volume of the cell and correspondingly reducing the life of the cell. Moreover, gasket  10  is required to have a high density and creep resistance in order to provide the necessary seal, however the breakaway section  19  must be adequately ductile so as to remain intact until the pressure buildup exceeds a predetermined amount. Furthermore, the thickness of breakaway section  19  must be manufactured within tight tolerance to ensure that it fails under predetermined conditions. Accordingly, conventional cells of this type are expensive and difficult to manufacture. Additionally, the internal volume located upstream of breakaway section  19  is vacant to allow adequate space for the rupture of section  19  such that the broken section  19  will not block aperture  17  during venting. This further reduces the active internal volume of the cell.  
           [0009]    What is therefore needed is an electrochemical cell having an improved seal and vent mechanism that increases the usable internal volume of the cell cavity.  
         BRIEF SUMMARY OF THE INVENTION  
         [0010]    The present invention provides an electrochemical cell having a cylindrical container having an axially extending sidewall. A cathode is disposed within the sidewall and has an inner surface that defines a centrally disposed void. The void is filled with internal cell ingredients. An endplate is connected to the container, and defines an aperture extending therethrough. An ionically permeable separator is interposed between the cell ingredients and the cathode. A negative current collector is at least partially disposed within the void and electrically connected to the anode and has a distal end that is connected to the endplate. A compressed seal member is disposed between the endplate and container, wherein the seal prevents the cell ingredients from flowing therebetween. A vent member, separate from the seal member, provides a blockage to the aperture, and opens when an internal cell pressure has exceeded a predetermined threshold.  
           [0011]    It is thus a general object of the invention to provide an electrochemical cell having a seal member separate from a vent member. The seal prevents leakage of internal cell ingredients into the ambient environment during normal operation, while the vent provides internal pressure dissipation once the internal cell pressure has exceeded a maximum permissible threshold. Advantageously, the seal and vent may be designed having properties that are compatible with their respective functions.  
           [0012]    This and other aspects of the invention are not intended to define the scope of the invention for which purpose claims are provided. In the following description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, and not limitation, preferred embodiments of the invention. Such embodiments do not define the scope of the invention and reference must be made therefore to the claims for this purpose. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Reference is hereby made to the following figures in which like reference numerals correspond to like elements throughout, and in which:  
         [0014]    [0014]FIG. 1 is a side sectional view of the negative terminal end of a prior art electrochemical cell having a unitary seal and vent member;  
         [0015]    [0015]FIG. 2 is a sectional side elevation view of the negative terminal end of an electrochemical cell in accordance with a preferred embodiment having independent vent and seal members; and  
         [0016]    [0016]FIG. 3 is a sectional side elevation view of the negative terminal end of an electrochemical cell in accordance with an alternate embodiment having independent seal and vent members. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    Referring now to FIG. 2, the negative terminal end  20  of an electrochemical cell constructed in accordance with a preferred embodiment of the invention includes an outer cylindrical steel container  22  that provides a positive current collector. A plurality, typically two to four, cylindrical annular cathode rings  24  (only one shown) is formed within the cell. Preferably, the outer diameter of rings  24  is greater than the inner diameter of the positive current collector  22  prior to installation, such that a pressure contact is formed between the outer walls of the rings  24  and the inner surface of container  22 . Cathode rings  24  present a radially inner surface that defines a centrally disposed void  26 .  
         [0018]    Alternatively, the cathode rings  24  could have a diameter smaller than the inner diameter of container  22 , such that the rings would be loosely placed inside the container and then forced to conform to the inner diameter of the container  22 . In particular, a downward force is applied at the top surface of the uppermost ring, and a rod having a fixed outer diameter is disposed within the central aperture of the ring while the downward force is being applied. It has been discovered, however, that this alternate process of inserting and forming cathode rings  24  may cause the container  22  to stretch and expand somewhat.  
         [0019]    A separator  30  is disposed radially inwardly of the inner surfaces of rings  24 , and thus circumscribes the void  26 . The void  26  is filled with an anode  28  that is mixed with electrolyte. Separator  30  comprises a non-woven, inert fabric that has sufficient porosity to enable permeability to gas and liquid such as an electrolyte, but substantially solid so as to prevent cathode  24  from electrically shorting with anode  28 . Alternatively, separator  30  could be conformal, as is understood by those having ordinary skill in the art. Separator  30  prevents the anode  28  and cathode  24  from coming into physical contact with each other while permitting the flow of electrolyte therebetween. Separator  30  further ensures that any pressure buildup within the cell is distributed substantially equally within the cell. Anode  28  is generally cylindrically shaped, has an outer peripheral surface which engages the inner surfaces of separator  30 , and comprises gelled zinc in accordance with the preferred embodiment.  
         [0020]    The cathode  24  may comprise a mixture of manganese dioxide, MnO 2 , and a carbonaceous material, such as graphite, and is typically wetted with an alkaline electrolyte before being compressed into annular rings. The negative end  20  of the cell is sealed by an endplate  36  further presents the negative connection to the cell. A nail that comprises, for example brass, extends downstream from the endplate  36  and is in electrical contact with the anode  28  to provide negative current collector  32 . Current collector  32  includes a head  34  that is in electrical contact with the endplate  36 , and a substantially cylindrical body  38  extending axially downstream therefrom.  
         [0021]    Alkaline cells of this type are described, for example, in U.S. Pat. No. 5,814,419 assigned to Rayovac Corporation, the disclosure of which is hereby incorporated by reference as is set forth in its entirety herein for the purposes of background information. It should be appreciated that the term “downstream” is used herein to indicate a direction from the negative end of the cell towards the positive end, while “upstream” is used to indicate a direction from the positive end of the cell towards the negative end.  
         [0022]    A ridge  42  is formed in the inner surface of the steel container  22  proximal the negative end  20  of the cell, and provides a seat for an annular gasket  40 . Gasket  40  is made of an elastomeric material or plastic in accordance with the preferred embodiment, and includes a hub  46  having an inner radial surface  44  that defines an inner diameter of the hub. The hub  46  thus enables the gasket  40  to seal against the outer diameter of current collector  32  upon installation, as will be described in more detail below. The hub  46  further presents a radially outer surface that defines a hub outer diameter. The gasket  40  further includes an annular sealing member  50  at its outer end that is disposed adjacent and radially inwardly of container  22 . Outer member  50  extends axially upstream of the endplate  36  prior to the completion of cell the fabrication process. The outer member  50  presents a radially inner surface that defines an inner diameter of member  50 .  
         [0023]    The outer member  50  and hub  46  are connected via a gasket disk  48  that extends generally radially outwardly from the hub  46 . In a direction radially outwardly from the hub  46 , the disk  48  initially extends axially upstream at section  45 , extends downstream at section  47 , and then upstream again at section  59  prior to forming a flat radial surface  61  that joins disk  48  to outer member  50 . The axial components of disk  48  provides flexibility to the disk in the radial direction, thereby enabling a spring constant to bias the hub  46  against the negative current collector  32  to form a tight seal, as will be described in more detail below.  
         [0024]    The negative end  20  of the cell further includes a washer  52  that is disposed axially upstream of and adjacent the gasket  40 . Washer  52  generally follows the contour of gasket  40 , and includes an axially extending inner member  54  whose inner surface defines an inner diameter of the washer, and an annular axially extending outer member  56  whose outer surface defines an outer diameter of the washer. A washer disk  53  joins the inner and outer members  54  and  56 , and slopes along with gasket disk  48  such that it comprises both radially and axially extending components to provide a spring force against the gasket  40 . A flat radial surface  63  joins outer member  56  to washer disk  63 , and sits on radial surface  61  of gasket  40 . The radially inner surface of inner member  54  rests against the radially outer surface of hub  46 . The radially outer surface of outer member  56  rests against the radially inner surface of outer member  50 .  
         [0025]    The outer diameter of the washer  52  is greater than the inner diameter of member  50 , and the inner diameter of the washer  52  is less than the outer diameter of hub  46  prior to installation of the washer. Accordingly, the sloped disk  53  of washer further biases inner surface  54  against hub  46 , thereby further sealing the hub against current collector  32 . The outer member  56  of washer  52  also biases outer member  50  of gasket  40 , thereby further sealing the outer member  50  against the container  22 .  
         [0026]    The endplate  36  has an annular outer radially extending flange  58  that sits on surface  63  of washer  52 . Prior to installation of the current collector  32 , endplate  36  is welded to flat surface  39  of current collector, using the weld projection  41  disposed thereon, as is well known in the art. The washer  52  is then fitted over the gasket  40 , and the current collector  32  is driven through hub  46 , creating a single assembly comprised of washer  52 , gasket  40 , current collector  32 , and endplate  36 . Because the hub  46  presents a smaller inner diameter with respect to the outer diameter of the current collector  32 , a tight seal is formed therebetween that prevents the electrolyte and other fluids disposed in void  26  from escaping during normal operation of the cell.  
         [0027]    The assembly is then installed into the container  22 , already filled with active materials. The negative end of container  22 , along with outer member  50  of gasket  40 , is crimped over the outer flange  58  in the direction of arrow “A” so to retain the endplate  36  in position. FIG. 2 illustrates the container  22  and gasket  52  both before crimping  35 , and after crimping  37 . The crimping further biases the container against outer member  50 , thereby improving the seal therebetween. The radial forces of the crimping are further transferred to the washer  52 , whose inner member  54  is further biased against hub  46 , thereby increasing the biasing spring force of hub against current collector  32  and tightening the resulting seal.  
         [0028]    Advantageously, the forces provided by washer  52  provide a sufficiently strong seal between the hub  46  and current collector  32  so as to negate the need for an elongated hub, as in the prior art embodiment illustrated in FIG. 1. Accordingly, the space that was previously occupied by the inner hub of the gasket and prior art designs may now be occupied by active cell ingredients  28 .  
         [0029]    The cell further includes a vent  60  that enables pressurized cell ingredients (e.g., electrolyte and gas) to dissipate from the void  26  once a predetermined maximum pressure threshold has been reached. Advantageously, the vent  60  is separate from the sealing gasket  40 . A pair of apertures  49  and  55  extends through the gasket and washer disks  48  and  53 , respectively, and a third aperture  64  extends through the endplate  36 . Apertures  49 ,  55 , and  64  are aligned such that gas entering aperture  49  is able to travel through aperture  55  and exit the cell via aperture  64  during venting. Apertures  49 ,  55 , and  64  thus permit the dissipation of pressure from within the cell once the internal cell pressure has exceeded a predetermined threshold, as will now be described.  
         [0030]    In particular, a layer of film  62  is disposed between gasket  40  and washer  52 , and is positioned such that it overlaps apertures  49  and  55 . Film layer  62  is adhesively bonded to the inner surface of the gasket  40  at locations adjacent aperture  55  in accordance with the preferred embodiment to provide a direct blockage with respect thereto, but could alternatively be bonded to the outer surface of gasket  40 . Alternatively, film layer  62  could be bonded to either the inner or outer surface of the washer  52  to provide a direct blockage with respect to aperture  49 . The film  62  may comprise a  6 - 6  or  6 - 12  nylon in accordance with the preferred embodiment, or any suitable alternative material that is impervious to electrolyte attack (such as polypropylene, polyethylene, or polyolefin), and that is capable of stretching to providing a vent as described herein.  
         [0031]    During normal operation, the film  62  prevents electrolyte from travelling through aperture  55  and out the cell. If internal cell pressure rises, however, the film  62  will begin to bow outwardly. Once the pressure exceeds the breaking point of the film  62 , the film will rupture thus allowing the gasses and electrolyte that has traveled through the separator  30  and aperture  49  to further pass through aperture  55  and exit the cell via aperture  64 . Vent  60  thus permits the safe dissipation of pressure within the cell once the pressure has exceeded a predetermined threshold. The threshold may be determined by the thickness of the film  62 , may be further dependent upon whether the film has been pre-stretched prior to attachment. Advantageously, because no risk exists that the ruptured film  62  will block aperture  55 , the void between endplate  36  and washer  52  is reduced with respect to the prior art, thereby further increasing the internal active cell volume.  
         [0032]    The vent  60  may further include a nub  66  that extends downstream from the endplate  36  at a position aligned with the axis of extension of film layer  62 . The nub  66  has a sufficiently sharp distal end that, when brought into contact with the film  62 , will rupture the film thereby permitting the pressure to escape from the cell as described above. Alternatively, the nub may extend outwardly from washer  52 , or any other suitable surface that would permit the film  62  to contact the nub after a significant amount of expansion only once the pressure within the cell has exceeded the maximum permissible threshold.  
         [0033]    It should therefore be appreciated that the embodiment illustrated in FIG. 2 employs two separate components to provide a seal and a vent for an alkaline cell. Advantageously, the two components may be designed having properties compatible with their respective functions. For example, the film is ductile and will rupture only under predetermined conditions, while the gasket  44  may accept a significant spring force to provide a tight seal between itself and current collector  32  and container  22  without jeopardizing the integrity of the vent. The seal  60  furthermore occupies less space within the cell when compared with current designs, thus increasing the active volume within the cell and resulting in a longer usable life.  
         [0034]    In particular, the cell illustrated in FIG. 2 in a AA size provides a total available internal volume of approximately 6,568 mm 3 , which is an increase in volume of approximately 7.6% over prior designs, thus significantly adding to the life of the battery. It should be easily appreciated to those having ordinary skill in the art that the present invention is equally applicable to other size cells, such as AAA, C, and D, which would present varying internal volumes and corresponding percentage increases in volume.  
         [0035]    Referring now to FIG. 3, the negative terminal  120  of an electrochemical cell constructed in accordance with an alternate embodiment of the invention is illustrated having reference numerals corresponding to like elements of FIG. 2 incremented by 100 for the purposes of clarity and convenience. In accordance with this embodiment, the annular separator  130  is captured in position not only by the radially inner surface of cathode rings  124 , but also by the inner surface of endplate  136 . The outer flange  158  of endplate  136  is seated within a sealing gasket  140  prior to crimping. The gasket is supported by ridge  142 . Once the gasket  140  and endplate are in place, the container  122  and outer member  150  are crimped as described above to capture the endplate  136  in position. The crimping additionally provides a tight seal at the interface between the gasket  140  and endplate  136 , and between the gasket and container  122 . Because the gasket  122  is used only to seal the endplate  136  and container  122 , it need not extend radially inwardly of separator  130  as in the embodiment illustrated in FIG. 2. Additionally, the current collector  132  is entirely captured within the void  126  and self-sealed at the endplate  136 . Accordingly, no seal is needed at the current collector with respect to the anode mixture  128 , and therefore no washer is present in accordance with this embodiment.  
         [0036]    The vent  166  is provided that, as described above, is separate from the sealing gasket  140 . In particular, a layer of film  162  is attached to the inner surface of endplate  136  and spans from a location radially inwardly of aperture  164  to a location radially outwardly of aperture  164 . Preferably, the layer  162  is disposed between the separator  130  and the endplate  136  such that the radially outer end of layer  162  is secured to flange  158 , while the radially inner end is attached to the radially extending surface of endplate  136 . While the film layer  162  is adhesively bonded to the endplate  136  in accordance with this embodiment, it should be appreciated that any suitable alternative fastening mechanism could be used. Because the film covers aperture  164 , gasses and liquids are prevented from escaping from the cell during normal use. If, however, that cell is misused such that a significant amount of pressure builds within the cell, the film  162  will expand outwardly, and will rupture when the pressure within the cell exceeds the predetermined threshold, thus allowing dissipation from the cell to the ambient environment through aperture  164 .  
         [0037]    Because the only seal that is necessary in this embodiment is disposed at adjacent the endplate  136 , the entire interior cavity  126  of the cell defined by the endplate  136  and separator  130  is sealed and may therefore be filled with active cell ingredients. As a result, the active volume within the cell is further increased to 6102 mm 3 , or approximately 12% greater than the conventional alkaline cells having a unitary seal/vent member.  
         [0038]    It should be appreciated that while film  162  directly provides a blockage with respect to aperture  164  in accordance with the embodiment illustrated in FIG. 3, film  62  also provides a blockage with respect to aperture  64  (in FIG. 2) by preventing anode material and gas from entering the cavity defined by endplate  36  and washer  52  and escaping from the cell via aperture  64 . Accordingly, the present invention provides a vent that is closed so as to block an aperture extending through the endplate only when the internal cell pressure is below a predetermined threshold, and that opens to allow pressurized internal cell ingredients to escape from the cell when the internal pressure has exceeded the predetermined threshold.  
         [0039]    The invention has been described in connection with what are presently considered to be the most practical and preferred embodiments. However, the present invention has been presented by way of illustration and is not intended to be limited to the disclosed embodiments. Accordingly, those skilled in the art will realize that the invention is intended to encompass all modifications and alternative arrangements included within the spirit and scope of the invention, as set forth by the appended claims.