Abstract:
An electrochemical cell having a low profile seal assembly for sealing the open end of the cell container. The seal assembly includes a support member having a surface area that covers substantially the open end of the cell container and has a plurality of openings therein. A seal member is disposed against the support member to provide a sealed closure to the open end of the cell container. The seal member has a membrane with a groove formed in a surface thereof which acts as a stress concentrator. The groove is located against the support member such that said groove is located adjacent to the openings formed in the support member and is further supported by the support member at other locations.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to electrochemical cells and, more particularly, to a low profile seal assembly for sealing the open end of a cell container such that the seal vents when exposed to excessive pressure. 
     Conventional alkaline electrochemical cells generally include a steel cylindrical can having a positive electrode, referred to as the cathode, which comprises manganese dioxide as the active material. The electrochemical cell also includes a negative electrode, referred to as the anode, which comprises zinc powder as the active material. The cathode is generally formed against the interior surface of the steel can, while the anode is generally centrally disposed in the can. Alternately, in jelly-roll cells, the anode and cathode are spirally wound. A separator is located between the anode and the cathode, and an alkaline electrolyte solution simultaneously contacts the anode, the cathode, and the separator. A conductive current collector is commonly inserted into the anode active material, and a seal assembly, which includes a seal member, provides closure to the open end of the cell&#39;s steel can to seal the active electrochemical materials in the sealed volume of the can. 
     Cylindrical alkaline cells are commonly sealed closed by placing an annular nylon seal above a bead formed near the open end of the cell can and then crimping the upper end of the can inwardly and over the outer periphery of the seal to compress the seal against the bead. However, electrochemical cells employ electrochemically active materials such as zinc which generate hydrogen gas during storage, under abusive conditions and sometimes during or following service use. When the container can is sealed, the build-up of high pressure gases within the sealed container may cause damage to the cell and/or the device in which the cell is employed. 
     One approach to avoiding a potentially excessive build-up of pressure in the cell container has been to employ a resealable valve system that periodically releases excessive gas pressure from within the active cell volume. However, the periodic and continuous release of gas pressure may, in some situations, permit electrolyte leakage containing salt and other particulate which may foul the resealable valve, and generally requires additional costly components. Another approach to avoiding excessive build-up of pressure involves employing a sealed membrane that is intended to blowout when exposed to excessive pressure either by puncture or rupture of the membrane itself. Puncture mechanisms such as a spiked member may be employed to punch a hole in the thin membrane once the pressure reaches a predetermined amount. Alternately, a rupture mechanism may be employed in the form of a thin membrane which ruptures when the internal pressure of the cell becomes too great. One example of a thermoformed film membrane employed as a vent mechanism is disclosed in U.S. Pat. No. 4,581,304, entitled “THERMOFORMED FILM MEMBER VENT FOR GALVANIC CELLS,” the disclosure of which is incorporated herein by reference. The aforementioned patent discloses the use of a thermoformed film member retained across a vent aperture located in the inner cover of the electrochemical cell such that the thermoformed film member is intended to rupture at high pressure to provide a vent passage from the sealed internal volume to the surrounding atmosphere. 
     Other approaches to venting excessive cell pressure have included the use of a vent formed in the seal of the battery which is intended to rupture upon experiencing an excessive pressure build-up in the cell. For example, U.S. Pat. No. 5,080,985 discloses a groove formed in both the top and bottom surfaces of a plastic grommet seal such that the groove is designed to shear open at very high pressure. While the prior approaches for venting high pressure gas from the cell have resulted in the ability to vent excessive pressure, many of the prior approaches having not optimized the volume consumed by the seal member, while other approaches lack an accurate rupture pressure mechanism. 
     Accordingly, it is therefore an object of the present invention to provide for an electrochemical cell having a pressure release mechanism which occupies a minimum amount of cell volume. It is also an object of the present invention to provide for such a pressure release mechanism that effectively vents gas when exposed to an expected rupture pressure. 
     SUMMARY OF THE INVENTION 
     The present invention improves the protective safeguards of an electrochemical cell with an enhanced low profile seal assembly for sealing the open end of the electrochemical cell&#39;s container to provide controlled pressure venting. To achieve this and other advantages, and in accordance with the purpose of the invention as embodied and described herein, the present invention provides for a seal assembly for sealing the open end of an electrochemical cell container, an electrochemical cell having a low profile seal assembly, and a method of assembling an electrochemical cell having a low profile seal assembly. The seal assembly comprises a seal member disposed in the open end of the cell&#39;s container to provide a sealed closure to the open end of the container. The seal member has a stress concentrator including a groove formed in a surface of the seal member. In addition, the seal assembly has a support member disposed in the open end of the container and against the seal member. The support member has a surface area that covers substantially the open end of a cell container and has at least one opening formed therein. The groove formed in the seal member is located against the support member so that the groove is located adjacent to at least one opening in the support member and is located against the support member at other locations. The low profile seal assembly occupies a minimal amount of cell volume and serves as an accurate pressure relief mechanism. 
     These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a longitudinal cross-sectional view of an electrochemical cell having a seal assembly including a stress concentration groove integrally formed in a seal member according to the present invention; 
     FIG. 2 is a top view of the seal assembly according to the present invention; 
     FIG. 3 is an exploded elevational view of the seal assembly of FIG. 2; and 
     FIG. 4 is an enlarged sectional view of the seal assembly taken from section IV in FIG. 1 which further illustrates the groove formed in the seal member. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a cylindrical alkaline electrochemical cell  10  is shown therein. Electrochemical cell  10  includes a cylindrical steel can  12  having a closed bottom end  14  and an open top end  16 . The closed bottom end of can  12  further includes a positive cover welded or otherwise attached thereto and formed of plated steel with a protruding nub i 8  at its center region which forms the positive contact terminal of cell  10 . Assembled to the open end  16  of steel can  12  is a cover and seal assembly with an outer cover  30  which forms the negative contact terminal of cell  10 . A metalized, plastic film label  20  is formed about the exterior surface of steel can  12 , except for the ends of steel can  12 . Film label  20  is formed over the peripheral edge of the positive cover. 
     A cathode, preferably formed of a mixture of manganese dioxide, graphite, forty-five percent potassium hydroxide solution and additives, is formed about the interior surface of steel can  12 . A separator  24 , preferably formed of a non-woven fabric that prevents migration of any solid particles in the cell, is disposed about the interior surface of cathode  22 . An anode  26 , preferably formed of zinc powder, a gelling agent and additives, is disposed with an electrolyte inside the separator  24  and is in contact with a current collector  28 , which may include a brass nail. Accordingly, the cathode  22  is configured as the cell&#39;s positive electrode, and the anode  26  is configured as the cell&#39;s negative electrode. 
     The current collector  28  contacts the outer negative cover  30  which forms the negative contact terminal of cell  10 . The outer negative cover  30  is preferably formed of plated steel, and may be held in contact with current collector  28  via pressure contact or a weld. An annular seal member  32  is disposed in the open end of the steel can  12  to prevent leakage of the electrochemically active cell materials contained in steel can  12 . Seal member  32  directly contacts a support member  34  which forms an inner cell cover and is preferably formed of steel. Together, the seal member  32  and support member  34  form a seal assembly for sealing closed the active electrochemical materials inside of can  12 . The assembling of the seal assembly includes disposing the seal member  32  on top of a bead  15  formed radially inward on the inner wall of the can  10  and reducing the diameter of the can  12  above the bead  15  such that the seal member  32  is compressed against the support member  34 . The upper end of the can  10  is crimped inwardly and over the outer periphery of the seal  32  to compress it against the bead  15 . It should also be appreciated that the outer negative cover  30  is electrically insulated from the steel can  12  by way of seal member  32 . 
     The support member  34  serves as an inner cover to rigidly support seal member  32 , and includes one or more vent openings, i.e., apertures,  36  formed through the surface thereof. The outer negative cover  30  also includes one or more vent openings  38  that serve to expose the cell&#39;s non-active volume, which is defined herein as the volume between seal member  32  and outer cover  30 , to the outside atmosphere. Vent openings  36  and  38  serve to vent pressure build-up from within the cell  10  to the outside atmosphere. Together, the outer cover  30 , the current collector  28 , seal member  32 , and inner support member  34  form the cover and seal assembly that is inserted into the open end  16  of steel can  12  to seal the active ingredients within an active cell volume. 
     According to the present invention, the seal member  32  has a circular stress concentration groove  40  formed in the top surface thereof which acts as a pressure release mechanism when exposed to an excessive pressure differential. The seal member  32  directly abuts the support member  34  such that the stress concentration groove  40  is located adjacent to vent openings  36  as well as non-vented portions of support member  34 . Referring particularly to FIGS. 2 and 3, the support member  34  is configured in the shape of a disk having three vent openings  36  which are equi-angularly spaced from each other. Each of vent openings  36  in support member  34  is formed in the shape of a kidney bean having an effective arcuate opening length adjacent to groove  40  as indicated by reference numeral L. According to the configuration shown, the vent opening  36  provides an open passageway adjacent to groove  40  such that excessive pressure build-up from within the cell  10  will concentrate stress on groove  40 . When sufficient excessive pressure is present, groove  40  will rupture, i.e., shear, near the center of the vent opening  36  to release pressure to the outside atmosphere. The non-venting portions of support member  34  provide structural support to seal member  32  such that seal member  32  is supported both on the radially outward and radially inward edges of opening  36 , as well as at certain locations adjacent to groove  40 . The groove  40  is preferably centrally located below opening  36  midway between the radially outward and inward edges of opening  36  . 
     With particular reference to FIG. 3, the seal member  32  is shown having a boss  46  formed at the center thereof and vertical extending walls  42  formed at the outer peripheral edge. Seal member  32  has a disk-like membrane  44  formed between boss  46  and outer peripheral walls  42 . Groove  40  is located in membrane  44 . The support member  34  sits directly against the top surface of the disk-like membrane  44  of seal member  32  and includes a central opening  48  formed therein for receiving boss  46 . The current collector  28  extends through opening  48  and boss  46  which provides a sealing engagement between boss  46  and current collector  28 . 
     Referring to FIG. 4, an enlarged view of stress concentration groove  40  is shown directly abutting the vent passage  36  in support member  34 . Groove  40  preferably includes either one or two abrupt corners formed between adjacent walls. According to the embodiment shown, groove  40  is configured as an annular channel that is rectangular in cross section with right angles formed at the base corners thereof by vertical walls joining a flat horizontal bottom wall. By providing a sharp, e.g., ninety degree, corner, the groove  40  effectively acts as a stress concentrator to provide a clean shear at the corner of groove  40  upon experiencing a predetermined pressure differential. Groove  40  has a width identified by W g , while the vent opening  36  formed in support member  34  has a width identified as W c . The groove  40  has a thickness T g  that is preferably approximately one-half the thickness T s  of the immediately surrounding seal material. The groove  40  preferably has a width W g  to thickness T g  ratio (W g /T g ) in the range of 0.2-1.2. The vent opening  36  preferably has an arcuate length L to width W c  ratio (L/W c ) equal to or greater than 1.0. In addition, the ratio of thickness T s  of the disk-like membrane  44  to the width W c  of vent opening  36  is preferably in the range of 0.1-0.2. The groove  40  is suitable to rupture when subjected to a pressure differential of approximately 1500 (psi) pounds-per-square inch for an AAA-size cell and up to approximately 1000 psi for an AA-size cell. 
     According to one embodiment, seal member  32  is made of a nylon, such as ZYTEL®  101 F which is commercially available from E.I. duPont de Nemours and Co., Inc. Seal member  32  can be integrally formed to include groove  40  by using a conventional injection molding process. It should be appreciated that while nylon is a preferred material, other polymers or other seal materials could be used. In addition, the bottom surfaces of seal member  32  may be coated with a layer of asphalt  35  to prevent chemical degradation of the seal member  32  due to the presence of electrolyte. 
     Accordingly, the present invention provides for a seal assembly having a seal member  32  directly abutting a support member  34 , with the seal member  32  having a stress concentration groove  40  formed therein and positioned in relation to openings  36  in the support member  34  to provide an accurate rupturing pressure release mechanism that will rupture when subjected to a predetermined pressure differentially addition, the seal assembly consumes less volume, and thereby allows for the employment of a greater quantity of active electrochemical cell materials. 
     It will be understood by those who practice the invention and those skilled in the art, that various modifications and improvements may be made to the invention without departing from the spirit of the disclosed concept. The scope of protection afforded is to be determined by the claims and by the breadth of interpretation allowed by law.