Abstract:
A ventable seal is provided for closing the open end of an electrochemical cell. The seal incorporates indentations formed in the outer surface of the centrally located hub that abuts a flexible diaphragm at a ventable interface. If a cell&#39;s internal pressure reaches an unsafe level and the seal ruptures, the indentations prevent resealing of the ruptured seal thereby avoiding a second build up of pressure within the cell.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention generally relates to ventable seals for pressurized containers and, more particularly, to ventable seals for electrochemical cells.  
           [0002]    Electrochemical cells, such as cylindrical alkaline electrochemical cells, employ two electrochemically active materials and an aqueous electrolyte. The electrochemically active materials are typically manganese dioxide and zinc. These materials are conventionally housed in a cylindrical elongated container that is open on one end so that the electrochemically active materials and electrolyte can be inserted therein during the cell manufacturing process. A closure assembly that incorporates a disc shaped elastomeric seal body and an elongated metallic current collector that projects through the center of the seal body closes the open end of the container. The seal body usually includes a hub, which surrounds the collector, and a thin diaphragm integrally molded into the central region of the seal body. The function of the diaphragm is to rupture and release gas from within the cell when the internal pressure becomes too high. The collector provides a conductive path between the zinc and one of the cell&#39;s terminal covers which is located on the end of the cell.  
           [0003]    Manufacturers of electrochemical batteries constantly strive to improve the performance of their products in a wide variety of battery powered devices. While most batteries are used in a conventional manner, a small percentage of batteries are exposed to extreme or abuse conditions. One of the abuse conditions occurs when a battery experiences a direct electrical short. This condition occurs when a low resistance electrical path is established between the anode and cathode. In one scenario, a direct electrical short can occur when a contact spring in a device, such as a flashlight containing two D-size batteries, inadvertently bridges the gap between the edge of the battery&#39;s steel container which contacts the cathode and the negative terminal cover that electrically contacts the anode. The spring is made of a highly conductive material such as nickel plated steel and thus provides a low resistance electrical connection between the anode and cathode. As soon as the direct electrical short is established, the cell begins to discharge as quickly as possible. In D-size batteries, which measure approximately 61 mm high and 34 mm in diameter, currents in excess of 20 amps are possible. Due to the exothermic chemical reactions that take place within a cell during the rapid discharge, the entire battery may reach temperatures in excess of 70° C. The increase in temperature increases the pressure within the cell. In addition to increasing the temperature of the battery, the chemical reactions that take place during discharge rapidly generate quantities of hydrogen gas that substantially increase pressure within the cell. The simultaneous production of hydrogen gas and increase in temperature causes the elastomeric seal, which is typically made of nylon, to become soft and lose some of its structural rigidity. The thin ventable portion of the seal may become elongated due to both the heating of the nylon and the increase in internal pressure. Consequently, when the softened and distorted seal ruptures in response to the pressure buildup, an initial quantity of gas may escape from within the cell but the tear in the ruptured seal could be resealed when the softened ruptured seal contacts the smooth outer surface of the seal&#39;s hub and reseals against the hub.. If the ruptured seal does reseal against the hub and the cell continues to generate gas, the cell may eventually experience a crimp release wherein the crimped connection between the seal and container is broken and the closure assembly is forcefully ejected from the container.  
           [0004]    As disclosed in U.S. Pat. No. 6,270,919 B1, previous attempts to prevent resealing of a ruptured seal body have included modifying a seal&#39;s inner disc portion to include ribs. The ribs are designed to maintain the opening in a ruptured seal body thereby preventing resealing of the vent mechanism. However, while the inclusion of ribs in the seal&#39;s diaphragm is helpful in preventing resealing in most cells, some cells with the ribs incorporated therein may be deformed when exposed to the heat generated during a direct electrical short such that the ribs cannot maintain the opening in the seal after it has ruptured.  
           [0005]    In a seal design disclosed in U.S. Pat. No. 6,312,850 B1, vertical grooves were placed in the surface of a compression member that forms a part of the seal assembly. The grooves are designed to prevent resealing of a vented seal&#39;s diaphragm. The grooves create channels that allow the gas to vent and thereby prevent resealing of the vented diaphragm. While this embodiment does prevent resealing of a vented seal, the compression member is an extra part that must be manufactured and assembled onto the seal body. This increases the cost of the battery and complicates the cell manufacturing process. Furthermore, the compression member occupies volume within the cell that would be better used to house electrochemically active materials.  
           [0006]    Therefore, there exist a need for an inexpensive and simple to manufacture elastomeric seal body that occupies a minimum amount of volume within the cell and can reliably prevent resealing of a vented electrochemical cell.  
         BRIEF SUMMARY OF THE INVENTION  
         [0007]    The present invention provides a ventable seal body that prevents resealing of a ruptured seal in a pressurized container. The disc shaped seal body is manufactured as a single component including a first surface, a second surface, a flexible diaphragm formed between the surfaces and a protrusion that projects from the first surface. The protrusion includes a proximal section and a venting section. The proximal section, which abuts the flexible diaphragm at a ventable interface, includes an outer surface free of one or more indentations. The venting section, which is concentrically aligned with and abuts the proximal section, includes an outer surface with at least one indentation formed therein. The indentation in the venting section creates an unobstructed path along the outer surface of the venting section.  
           [0008]    The present invention also provides for an electrochemical cell having a container with an open end, a closed end and a sidewall therebetween. The container includes a separator and two electrochemically active materials arranged on opposite sides of the separator. A disc shaped seal body formed as a single component is secured to the open end of the container. The seal body has a top surface, a bottom surface and a perimeter that contacts the top and bottom surfaces. A flexible diaphragm is formed between the surfaces and positioned around a centrally located protrusion that projects perpendicularly from the center of the seal body&#39;s top surface. The protrusion defines an opening between the top and bottom surfaces. The protrusion has a proximal section abutting the flexible diaphragm at a ventable interface and a venting section concentrically aligned with and abutting the proximal section. The proximal section comprises an outer surface that is free of one or more indentations. The venting section has an outer surface with at least one indentation formed therein. The venting section&#39;s indentation creates an unobstructed pressure relief path along the outer surface of the venting section. A current collector extends through the opening in the protrusion and contacts an electrochemically active material in the cell. A cover is positioned between the flexible diaphragm and the cell&#39;s external environment. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a cross section of a conventional cylindrical alkaline electrochemical cell;  
         [0010]    [0010]FIG. 2 is a perspective view of a seal of the present invention;  
         [0011]    [0011]FIG. 3 is a perspective view of another seal of the present invention;  
         [0012]    [0012]FIG. 4 is a cross section of the seal shown in FIG. 2; and  
         [0013]    [0013]FIG. 5 is a partial cross section of an electrochemical cell of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    Referring now to the drawings and more particularly to FIG. 1, there is shown a cross section of a conventional alkaline electrochemical cell  90 .  
         [0015]    Beginning with the exterior of the cell, the cell components are the container  10 , first electrode  50  positioned adjacent the interior surface of container  10 , separator  20  contacting the interior surface  56  of first electrode  50 , second electrode  60  disposed within the cavity defined by separator  20  and closure assembly  70  secured to container  10 . Container  10  has an open end  12 , a closed end  14  and a sidewall  16  therebetween. The closed end  14 , sidewall  16  and closure assembly  70  define a cavity in which the cell&#39;s electrodes are housed.  
         [0016]    A seal body of this invention is shown in FIG. 2. The seal body may be manufactured by injection molding an electrically nonconductive material such as nylon, polystyrene, polypropylene or other plastic material. The mold is designed to provide the seal body with the desired features, such as a rupturable vent and a centrally located opening for accommodating a current collector, which make the seal body suitable for use in electrochemical cells. As shown in FIG. 2, seal body  100  is a generally disc shaped component with a first surface  102 , a second surface  104  and an edge  106  that defines the perimeter of the seal. First surface  102  may be referred to herein as the top surface. Second surface  104  may be referred to herein as the bottom surface. The edge  106  includes an upstanding wall  108 . Inwardly disposed from the wall is central region  110  of seal body  100 . The central region comprises a flexible diaphragm  112  that abuts ventable interface  126 . The ventable interface contacts protrusion  114  that projects perpendicularly from the center of the seal body&#39;s top surface. The protrusion is also known herein as a hub. Located between wall  108  and flexible diaphragm  112  is a nonventing portion  116  of central region  110 . The nonventing portion is thicker than flexible diaphragm  112  that surrounds hub  114 .  
         [0017]    Protrusion  114  in FIG. 2 can be generally described as comprising three distinct sections even though all sections of the hub, like the seal itself, are formed as a single component when the seal is manufactured. The portion of hub  114  that abuts ventable interface  126  is known herein as proximal section  115 . The outside diameter of the proximal section is constant throughout the height of the proximal section. The top of the proximal section appears to abut the hub&#39;s venting section  117 . The maximum outside diameter of venting section  117  is equal to or slightly smaller than the outside diameter of proximal section  115 . Venting section  117  includes one or more indentations  128 ,  130 ,  132  and  138  that function as pressure relief indentations for the seal body. In a preferred embodiment, the indentations are arcuate shaped. The ratio of the width of the indentation to the depth of indentation should be greater than 1:1 and less than 6:1. Preferably, the ratio of width to depth is greater than 2:1 and less than 4:1. More preferably, the ratio is about 3:1. If desired, the indentations could be shaped as rectangles, semicircles or ovals. In between the indentations are outwardly bowed extensions  140  of venting section  117 . Extensions  140  separate the indentations from one another. The top of the venting section forms a first shoulder  142  that abuts the bottom of the hub&#39;s third section that is known herein as the distal section  119 . The outside diameter of distal section  119  is less than the outside diameter of venting section  117  which is equal to or less than the outside diameter of proximal section  115 . The outer surface of distal section  119  is free of the indentations that characterize venting section  117 . The end of distal section  119  forms a second shoulder  120  at the end of hub  114 . The interior surfaces of distal section  119 , venting section  117  and proximal section  115  define an opening  122  through seal body  100 .  
         [0018]    The functionality of the protrusion&#39;s proximal, venting and distal sections will now be explained. Proximal section  115  forms ventable interface  126  with flexible diaphragm  112 . The interface is designed to rupture when the cell&#39;s internal pressure reaches a predetermined value. Preferably, ventable interface  126  will tear quickly when the seal body vents. In order to get predictable and rapid venting, proximal section  115  and flexible diaphragm  112  form an arc shaped ventable interface  126  having a uniform thickness around the proximal section. The arc is achieved by having a washer shaped proximal section abutting a thin ventable interface. Preferably, the arc is at least 180°, more preferably 270° and most preferably a circle. The height of the proximal section must be selected to insure that the one or more indentations in the venting section do not interfere with the tearing at interface  126 .  
         [0019]    The venting section&#39;s key function is to provide one or more indentations which serve as pressure relief paths for the entrapped gas as it is released from within the cell. While the length, width and height of the indentation can be altered to accommodate factors such as moldability of the seal body, indentations with a width to depth ratio of approximately 3:1 are preferred. An arcuate shaped indentation is particularly preferred because the arc is easy to incorporate into the mold used to form the seal body. Furthermore, an arcuate shaped indentation facilitates uniform manufacturing of the seal body. The height of the indentation must be sufficient to allow the gas to escape to the cell&#39;s environment.  
         [0020]    The principle function of distal section  119  is to provide a leakproof interface between current collector  276  and inner cover  150  (see FIG. 5). The interface between the collector and the inner cover must be able to stop electrolyte from leaving the cell by creeping along the surface of the collector. This is accomplished by using a collector with an outside diameter larger than the inside diameter of opening  122  in seal body  100  so that an interference fit is established between the collector and seal body. In order to achieve uniform compression of the distal section between the collector and cover the thickness of the distal section between these two parts must be consistent. Consequently, the one or more indentations that characterize the venting section must terminate below the distal section so that the indentation does not interfere with the compression of the seal body in distal section  119  of protrusion  114 .  
         [0021]    As shown in FIG. 2, one or more ribs  148  may be incorporated into the top surface of flexible diaphragm  112 . One end of each rib  148  abuts the outer surface of proximal section  115 . Each rib is integrally formed in top surface  102  of seal body  100  and is located along a line radiating from the center of seal body  100  toward the perimeter of the seal body. An end of each rib  148  that abuts the proximal section of the hub is approximately located between the indentations in venting section  117  of hub  114 . Ribs  148  are intended to prevent flexible diaphragm  112  from resealing against the interior surface of inner cover  150 . Since the ribs prevent a portion of the torn flexible diaphragm from moving upward and blocking the escape route of the gas that is trapped within the cell, the ruptured seal body does not reseal and allow the cell to become pressurized once again.  
         [0022]    Shown in FIG. 3 is another embodiment of the present invention. Seal body  101  is disc shaped and formed as a single component having a first surface  102 , a second surface  104  and a perimeter  106 . Flexible diaphragm  112  is formed between the first surface and the second surface. Protrusion  114  projects from first surface  102  and abuts flexible diaphragm  112  at ventable interface  126 . Protrusion  114  comprises proximal section  115  and venting section  117 . The surface of proximal section  115  is free of any grooves, indentations or channels. In contrast, the surface of venting section  117  comprises at least one indentation. As shown in FIG. 3, the hub may have several indentations  128 ,  130 ,  132  and  138 . The number of indentations may be varied to accommodate differences in the seal&#39;s physical parameters such as: outer diameter of the seal&#39;s hub; elasticity of the seal material at elevated temperature; and pressure at which the seal is designed to vent. Preferably the hub has two, four or six indentations. Between any two adjacent indentations is an outwardly bowed portion  140 . The outwardly bowed portion separates the individual indentations, also referred to herein as relief paths, from one another.  
         [0023]    [0023]FIG. 4 shows a cross section of the seal of FIG. 2. Protrusion  114  comprises proximal section  115 , venting section  117  and distal section  119 . Proximal section  115  abuts flexible diaphragm  112  at ventable interface  126 . The surface of proximal section  115  is free of any grooves or indentations. Venting section  117  comprises indentation  130  and outwardly bowed portions  140  (not shown). Distal section  119 , which is concentrically aligned with proximal section  115  and venting section  117 , adjoins venting section  117 . The free end of protrusion  114  terminates at shoulder  120 . Opening  122 , located in the center of the seal body, defines a passage between top surface  102  and bottom surface  104 . Perimeter  106  is defined by wall  108 . Ledge  152  abuts wall  108 . Rib  148  extends from the surface of flexible diaphragm  112 .  
         [0024]    Shown in FIG. 5 is a partial cross sectional view of an electrochemical cell  200  of this invention. The cell includes container  210  having an open end  212 . Disposed within the container are first electrode  250 , second electrode  260 , separator  220  and a quantity of an aqueous alkaline electrolyte. Closure assembly  270  is secured to the open end of the container. The assembly comprises seal body  100  that includes proximal section  115  which adjoins flexible diaphragm  112  at ventable interface  126 , venting section  117  that abuts proximal section  115  and includes indentations  130  and  132 , and distal section  119  which is concentric with and adjoins venting section  117 . The closure assembly also includes a centrally located current collector  276  that protrudes through opening  122  in seal body  100  and inner cover  150 . The current collector is an elongated rod made of an electrically conductive material such as brass. One end of the collector contacts second electrode  260  and the opposite end of the collector protrudes through top surface  102  of seal body  100  and contacts one of the cell&#39;s terminal covers  158 . Inner cover  150  is located above top surface  102  of seal body  100 . The perimeter of cover  150  contacts seal ledge  152 . The center of cover  150  defines an opening  154 . The inside diameter of cover opening  154  is smaller than the outer diameter of the hub&#39;s venting section  117  and greater than the outer diameter of the hub&#39;s distal section  119 . The outside diameter of collector  276 , the inside diameter of opening  112  and the thickness of distal section  119  are selected so that after collector  276  has been inserted into opening  122  in seal body  100 , a portion of distal section  119  is forced outwardly against cover  150  by collector  276  thereby imparting tangential tension on the seal body&#39;s distal section and creating an interference fit between collector  276  and distal section  119  as well as distal section  119  and cover  150 . The purpose of creating the interference fit is to prevent the escape of electrolyte along the interface of collector  276  and hub  114 . Since distal section  119  must be uniformly compressed between collector  276  and inner cover  150 , the pressure relief indentations located in the hub&#39;s venting section  117  cannot extend into the distal section. A second opening  156  in cover  150  allows gases that vent through a rupture in the seal to escape from the space defined by the top surface  102  of the seal body and cover  150 . An opening  160  in terminal cover  158  allows gases that have passed through opening  156  to move beyond the terminal cover into the cell&#39;s external environment. Terminal cover  158 , positioned above closure assembly  270 , makes electrical contact with current collector  276 .  
         [0025]    First electrode  250 , also known herein as a cathode, contacts the inside surface of container  250  and defines a centrally located cavity. First electrode  250  is a mixture of manganese dioxide, graphite and an aqueous solution containing potassium hydroxide. The electrode 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  250  has a ledge  252  and an interior surface  256 . As an alternative to molding the cathode in the container, 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.  
         [0026]    Second electrode  260  is a 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.  
         [0027]    Electrolyte suitable for use in a cell 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  250 , second electrode  260  and separator  220 .  
         [0028]    Separator  220  is a coiled film of nonwoven fibers. The separator is disposed about the interior surface  256  of first electrode  250 . 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.  
         [0029]    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.