Abstract:
An electrochemical cell having a can for containing electrochemically active materials including positive and negative electrodes and electrolyte. The can has a bottom end and an open top end, and positive and negative electrodes disposed in the can. An inner metal cover is disposed in the open end of the can and has a first peripheral flange. An outer metal cover is disposed over the inner cover in the open end of the container and has a second peripheral flange juxtaposed to the first peripheral flange. A seal is disposed between the can and both the first and second peripheral flanges. The low profile seal assembly advantageously provides for reduced volume consumption thereby allowing for a greater amount of electrochemically active materials in the can.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to electrochemical cells, i.e., batteries, and more particularly to a low profile closure assembly for closing and sealing the open end of a battery container. 
     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. In a bobbin-type cell construction, the cathode is typically 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 steel can to seal the active electrochemical materials in the sealed volume of the can. 
     Cylindrical alkaline cells are commonly closed by inserting a preassembled collector and seal assembly in the open end of the steel can. The collector and seal assembly typically includes the collector nail, an annular nylon seal, and an inner metal cover for radially supporting the nylon seal. The can typically has a taper or an inwardly extending bead at its open end which serves to support the collector and seal assembly in the desired orientation prior to securing it in place. After the collector and seal assembly has been inserted, an outer metal cover is placed over the assembly and the assembly and cover are secured in place by radially squeezing the can against the collector and seal assembly and outer cover and crimping the edge of the can over the peripheral lip of the collector and seal assembly and outer cover to secure the outer cover and collector and seal assembly within the open end of the can. The conventional rollback outer cover has a peripheral edge that is folded back, which results in a non-uniform compression of the seal. Additionally, electrochemical cells commonly employ electrochemically active materials, such as zinc, which generate hydrogen gas during storage and sometimes during or following service use. When the battery can is closed, excessive build-up of high pressure gases within the sealed can may cause damage to the cell and/or the device in which cell is employed. 
     In order to handle the potentially excessive build-up of pressure in the electrochemical cell, conventional batteries have employed voluminous seals which provide pressure release venting. One pressure relief approach has employed a resealable valve system that periodically releases excessive gas pressure from within the active volume. Another approach employs the use of a vent formed in the annular nylon seal which is intended to rupture upon experiencing excessive pressure build-up in the cell. According to yet another approach, the cell employs a circular thinned region formed in the annular nylon seal. However, the amount of space occupied by the conventional seal, the inner metal cover, and the outer metal cover, can be significant. 
     The greater the space occupied by the collector and seal assembly, the less space that there is available within the cell for the electrochemically active materials. Consequently, a reduction in the amount of electrochemically active materials that may be provided within the cell results in shorter service life for the cell. It is therefore desirable to maximize the internal volume within an electrochemical cell that is available for the electrochemically active components. 
     SUMMARY OF THE INVENTION 
     The present invention minimizes the closure assembly volume for sealing the open end of a cell container. 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 an electrochemical cell employing a container having a bottom end and an open top end, and positive and negative electrodes disposed in the container. An inner cover, such as a metal cover, having a first peripheral flange is disposed in the open end of the container. An outer cover is disposed over the inner cover in the open end of the container. The outer cover has a second peripheral flange juxtaposed to the first peripheral flange. A seal is disposed between the container and at least one of the first and second peripheral flanges. The resultant low profile seal assembly advantageously provides for reduced volume consumption thereby allowing for a greater amount of electrochemically active materials in the container. 
    
    
     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 low profile seal assembly for closing the open end of the can according to the present invention; 
     FIG. 2 is an enlarged sectional view of section III prior to can crimping; 
     FIG. 3 is an enlarged sectional view of section III of FIG. 1 after can crimping; and 
     FIG. 4 is an exploded elevational view of the low profile seal assembly for closing the open end of the cell can. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a cylindrical alkaline electrochemical cell  10  is shown therein having a low profile seal assembly for closing the open end of the cell container according to the present invention. The electrochemical cell  10  includes a cylindrical steel can  12  having a closed bottom end  14 , an open top end  16  and side walls extending between the top and bottom ends. The closed bottom end of can  12  includes a protruding nub  18  formed at its center region for providing the positive contact terminal of cell  10 . While the positive contact terminal  14  is shown integrally formed in the bottom end  14  of the can  12 , it should be appreciated that a separate positive contact terminal could be welded or otherwise attached to a flat bottom end of the can  12 . 
     Assembled to the open top end  16  of steel can  12  is an inner metal cover  30  and an outer metal cover  36 , which substantially closes the open end  16  of steel can  12 . Outer metal cover  36  serves as the negative contact terminal and preferably covers the top surface of inner metal cover  30 . According to the embodiment shown, outer metal cover  36  and inner metal cover  30  are electrically coupled to each other. However, it should be appreciated that inner metal cover  30  could be electrically insulated from outer metal cover  36 , such as to provide a neutral cover, for example. 
     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 bottom end  14  of steel can  12  and may extend partially onto the negative cover  36  as shown. 
     A cathode  22  is formed about the interior surface of steel can  12 . Cathode  22  may be formed of a mixture of manganese dioxide, graphite, potassium hydroxide solution, and additives, according to one example. The separator  24 , which may be 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  is disposed with an electrolyte into the separator  24  and in contact with a current collector  28  which may include a brass nail having an elongated body and an enlarged head at one end. The anode  26  may be formed of zinc powder, a gelling agent, and additives, according to one example. Accordingly, the cathode  22  is configured as the positive electrode, and the anode  26  is configured as the negative electrode. While a bobbin-type cell construction is shown, it should be appreciated that the electrochemical cell  10  may be otherwise constructed, such as a jelly-roll construction. 
     The inner metal cover  30  is preferably formed of plated steel, and preferably has a nickel coating, an asphalt coating, and/or other corrosion resistant material on the bottom surface thereof. The current collector  28  contacts the inner metal cover  30  and cover  30  is shown welded on its bottom surface to the current collector  28  via weld  34 . A ring-shaped nylon seal  32  having a J-shaped cross section, or alternately an L-shaped cross-section, is disposed in the open end  16  of steel can  12  to provide a seal between the steel can  12  and the inner and outer metal covers  30  and  36  to prevent leakage of electrochemically active cell materials contained in the steel can  12 . The assembly of the seal  32  may include disposing the seal  32  in the open end  16  of steel can  12  on top of a bead  15  that is formed radially inward on the inner walls of steel can  12 , disposing the inner metal cover  30  and outer metal cover  36  on top of seal  32 , and crimping the upper end of can  12  radially inward and over the outer periphery of the seal  32  to compress it against bead  15 . Therefore, the nylon seal  32  is compressed between the peripheral flanges of inner and outer metal covers  30  and  36  and the upper end walls of steel can  12 . It should be appreciated that the inner metal cover  30  and outer metal cover  36  are electrically insulated from the steel can  12  by way of seal  32 . Additionally, the inner metal cover  30 , current collector  28 , and J-shaped seal  32  may be preassembled as a unit to form a collector and seal assembly prior to disposal in the can  12 . Further, the outer metal cover  36  may also be preassembled onto the collector and seal assembly for easy disposal in the open end  16  of can  12 . 
     According to the present invention, the inner metal cover  30  has a peripheral flange  42  that extends substantially parallel to the side walls of the steel can  12 , which in turn are substantially parallel to the longitudinal axis of the cell  10 . In addition, the outer metal cover  36  likewise has a peripheral flange  44  that extends substantially parallel to the side walls of the steel can  12 . The peripheral flanges  42  and  44  extend in opposite directions with flange  42  extending upward, and flange  44  extending downward, and are arranged juxtaposed to each other so that they are parallel and contact each other. 
     The inner metal cover  30  is further shown having a pressure release mechanism  40  formed therein for venting to release high pressure gases from within the sealed volume of the can  12  upon reaching a predetermined pressure. The pressure release mechanism  40  has a reduced thickness section that is intended to fracture and bow upward to release high pressure gases. The outer metal cover  36  is spaced above the pressure release mechanism  40  to provide a space therebetween to allow for the pressure release mechanism  40  to bow outward a sufficient distance to provide adequate venting. 
     The seal assembly is further shown prior to the can crimping in FIG. 2, and again following can crimping in FIG.  3 . The ring-shaped seal  32  is disposed into the open end  16  of steel can  12  so that it rests on top of bead  15 . The inner metal cover  30  has an inverted V-section which receives the upper end of separator  24 . The vertically disposed peripheral flange  42  is provided as an extension of inner metal cover  30  located radially outward from the inverted V-section and conforms to the inside surface of seal  32 . The outer metal cover  36  has the peripheral flange  44  juxtaposed to the radially inward surface of the peripheral flange  42 . During the can crimping process, the open end  16  of steel can  12  is pressed radially inwardly so as to compress seal  32  and to crimp the upper end of can  12  over the upper edge of seal  32 . Accordingly, seal  32  is compressed between the bead  15  and outer crimped edge of steel can  12 . 
     The outer negative cover  36  also contacts the inner metal cover  30  above the inverted V-shaped section. Provided between inner metal cover  30  and outer negative cover  36  are gaps  50  which provide passages for vented gas to be released during a venting condition to the outside atmosphere via one or more openings  38  formed in outer metal cover  36 . Gaps  50  may include channels formed in the bottom surface of outer metal cover  36  or protruding ribs extending from the upper surface of inner metal cover  30  at a region above the inverted V-shaped section. 
     Additionally, inner metal cover  30  has a thickness T I  and the outer metal cover  36  has a thickness T O . It is preferred that the total combined thickness T I +T O  of the peripheral flanges  42  and  44  be in the range of 20 to 30 mils to provide sufficient constant hoop strength to seal the steel can  12  closed and maintain the structural integrity of the cell  10 . The thickness T I  may be substantially equal to the thickness T o . However, it should be appreciated that the thicknesses T I  and T O  relative to each other may be varied. For example, the thickness T I  of the inner metal cover  30  may be a reduced thickness as compared to the thickness T O  to allow for ease of forming the pressure release mechanism  40 . For example, inner metal cover  30  may have a thickness T I  of approximately 5 mils, as compared to the outer metal cover  36  having a thickness T O  of approximately 15 mils. 
     Referring to FIG. 4, the assembly of the electrochemical cell  10  is illustrated therein. The seal  32  receives the inner metal cover  30  with the collector nail  28  welded thereto. Once the electrochemically active materials including the cathode  22  and the anode  26 , as well as the separator  24  and electrolyte have been dispensed within the steel can  12 , the open end  16  of can  12  is ready for closure. The collector and seal assembly is then disposed in the open end  16  of steel can  12  so that it rests on top of a beaded or tapered wall. The outer metal cover  36  may be placed on top of the inner metal cover  30  prior to the seal insertion to form a preassembled collector and seal assembly. The steel can  12  is then crimped closed over the seal  32  to seal the can  12  closed. Thereafter, the label  20  may be applied to the outer wall of steel can  12 . 
     Accordingly, the inner metal cover  30 , outer metal cover  36  and seal  32  form a low volume closure to the open end  16  of steel can  12 . Peripheral flanges  42  and  44  matingly engage one another in a manner that allows the steel can  12  to be crimped closed so that the peripheral flanges  42  and  44  are uniformly compressed radially inwardly. Accordingly, as the steel can  12  is crimped closed, peripheral flanges  42  and  44  move radially inwardly together in concert and remain substantially parallel to one another, without creating void space therebetween. The inner metal cover  30  and outer metal cover  36  thereby serve to act in concert like a spring to absorb thermal expansion and contractions of the cell  10 . Preferably, the inner metal cover  30  and outer metal cover  36  are crimped to be under stress no greater than approximately the yield stress of the cover material. 
     Additionally, the stress concentration pressure release mechanism  40  formed in the inner metal cover  30  is intended to fracture to release high pressure gases upon reaching a predetermined pressure differential. Stress concentration pressure release mechanism  40  includes a reduced thickness arcuate groove formed about an approximate 270 degree rotation in the top surface and/or bottom surface of inner metal cover  30 . The pressure release mechanism  40  is intended to fracture along its groove and remain connected at the hinge. The stress concentration groove  40  may be formed by coining techniques or other thickness reduction techniques. It should be appreciated that by employing both inner and outer metal covers  30  and  36  in accordance with the present invention, a reduced thickness inner metal cover  30  having a thickness T I  such as 5 mils, for example, may be employed with an outer metal cover thickness T O  of approximately 15 mils, to provide an overall thickness at the peripheral flanges  42  and  44  equal to 20 mils. This allows a reduction in the thickness T I  of the inner metal cover  30  which enhances formation of the stress concentrator  40 . 
     The stress concentration pressure relief mechanism  40  of the present invention is positioned beneath the negative outer cover  36  so as to prevent the electrochemical materials from spraying directly outward from battery  10  upon rupture. To allow for release of venting gas to the outside atmosphere, the negative cover  36  has one or more openings  38  formed therein. Also, the provision of the negative cover  36  over pressure release mechanism  40  allows mechanism  40  to bow outwardly under the negative cover  36  and ultimately rupture. Accordingly, negative outer cover  36  prevents inadvertent damage to the pressure release mechanism  40  and shields mechanism  40  from the corrosive effects of the ambient environment and therefore reduces the possibility of premature venting and/or leakage. 
     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.