Abstract:
A current collector and seal assembly for use in an electrochemical cell is provided. An electrochemical cell comprises a container generally having a closed bottom end, an open top end, and side walls extending between the top and bottom ends. Electrochemically active materials are disposed in the container, and generally include a positive electrode and a negative electrode. A current collector and seal assembly is provided to close the open end of the container and includes a resilient and electrically non-conductive seal body having an upstanding wall defining an opening therethrough. A current collector is inserted through the opening and into one of the electrochemically active components. A polymeric compression bushing compresses the upstanding wall so that the collector and compression bushing cooperate to exert radial compressive stress and tangential compressive stress on the upstanding wall after insertion of the current collector through the opening.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to electrochemical cells and, more particularly, to a current collector and seal assembly for an electrochemical cell. 
     Alkaline electrochemical cells typically include a cylindrical steel can having a closed end, an open end, and side walls extending therebetween. The can contains electrochemically active materials which include a positive electrode, commonly referred to as the cathode, comprising manganese dioxide as the active material, and a negative electrode, commonly referred to as the anode, comprising zinc powder. Conventional cells often employ a bobbin-type construction in which the anode is centrally located and is surrounded by a tubularly shaped cathode which contacts the can walls. A separator is generally located between the anode and cathode, and an alkaline electrolyte solution simultaneously contacts, the cathode, the anode, and the separator. 
     Conventional electrochemical cells have a conductive current collector, which is typically in the shape of an elongated brass nail, inserted into the anode active material, and a seal assembly which provides closure to the open end of the steel can. The current collector generally extends through an opening, i.e., aperture, provided in the seal and provides an electrical connection between the anode and an outer conductive cover which serves as the negative terminal. The current collector, seal, and an inner metal cover are usually preassembled together to form what is commonly referred to as a current collector and seal assembly. The seal has a centrally located opening through which the current collector protrudes, and a sealant is typically disposed between the current collector and seal. The outer perimeter of the seal and the central portion of the seal which surrounds the centrally located opening are usually reinforced by a thickening of the seal&#39;s material. Between the outer perimeter and central portion, is a diaphragm which often has a thinned section for providing a stress concentration pressure release vent for allowing the seal to rupture when the cell&#39;s internal pressure exceeds a predetermined limit, to thereby vent high pressure gases from within the cell. 
     The reinforced central portion of the seal that surrounds and defines the centrally located opening is commonly referred to as the “hub.” The current collector is inserted through the opening in the hub so that an interference fit exists between the seal hub and the collector. In some commercially available cells, the diameter of the collector nail is usually greater than the inside diameter of the opening to create an interference fit so that electrolyte cannot escape from the cell along the surface of the collector. Often, the interference fit results in the creation of tangential tension which, if excessive, may exceed the seal hub&#39;s material strength and cause the seal to split and allow electrolyte solution to escape. On the other hand, if the interference fit is insufficient, electrolyte solution may escape between the collector and the seal. 
     A number of current collector and seal assemblies are known for maintaining a sealed closure between the seal and current collector. One approach uses a plastic coated metallic sleeve located on the interior surface of the seal for compressing the plastic against the collector to prevent leakage of electrolyte. Yet, another approach discloses the use of an inner metal cover, in which the collector is inserted through the seal&#39;s central opening so that the upstanding wall of the seal hub which surrounds the collector is forced outward against the inner metal cover. However, the current collector exerts tangential tension against the seal&#39;s upstanding wall forming the hub. Additionally, one commercially available battery has employed a flat metallic ring around the seal body so that the seal is compressed between the collector and ring as the collector is forced through the seal&#39;s central opening. Many of the above assemblies result in the creation of tangential tension in the seal which, when exposed to potassium hydroxide, may result in stress corrosion cracking which may allow for electrolyte leakage. In an attempt to minimize the likelihood of stress corrosion cracking, prior approaches often require coating the seal with a protectant such as asphalt. 
     A more recent approach is disclosed in U.S. Pat. No. 5,422,201, entitled “CURRENT COLLECTOR ASSEMBLY FOR AN ELECTROCHEMICAL CELL,” which is incorporated herein by reference. The aforementioned patent discloses the use of a compression means in the form of a tubularly shaped metallic component having one end flared radially outward. The metallic component is inserted around the upstanding wall forming the hub of the seal to compress a central opening formed in the hub. With the tubular metallic component in place, the current collector nail is driven upward through the compressed opening from the bottom side. In doing so, the compressed opening is forcibly increased in diameter by an amount that prevents the creation of tangential tension in the seal&#39;s hub, yet allows for compression of the hub against the current collector. The flared end of the tubular shaped metallic component creates a shear edge against the seal&#39;s vent; however, the metallic component is orientation sensitive and, therefore, may be difficult to handle and assemble with modern cell manufacturing equipment. 
     While known current collector assemblies have been used for many years, there exists a need for an improved current collector and seal assembly that is easy to assemble, imparts little or no tangential tension on the seal&#39;s hub to avoid hub splitting, and provides sufficient radial compressive stress and tangential compressive stress on the seal&#39;s hub. 
     SUMMARY OF THE INVENTION 
     The present invention improves the reliability and process assembly of the collector and seal assembly for use in an electrochemical cell. To achieve this and other advantages, and in accordance with the purpose of the invention as embodied and described herein, a collector and seal assembly and an electrochemical cell including the collector and seal assembly are provided according to the present invention. The collector and seal assembly includes a resilient and electrically non-conductive seal body having an upstanding wall defining an opening therethrough. A current collector is inserted through the opening and is adapted to contact electrochemically active material in a cell. A polymeric compression bushing contacts and compresses the upstanding wall of the seal. Accordingly, the collector and compression bushing preferably cooperate to exert radial compressive stress and tangential compressive stress on the upstanding wall of the seal after insertion of the current collector through the opening. 
     The collector and seal assembly of the present invention is assembled for use in an electrochemical cell. The electrochemical cell has a container generally including a closed bottom end, an open top end, and side walls extending between the top and bottom ends. Electrochemically active materials are disposed in the container, and generally include a positive electrode and a negative electrode. The collector and seal assembly is disposed in the open end of the container to seal closed the open end. The collector and seal assembly is easy to assemble and minimizes tension in the seal to prevent stress corrosion cracking of the seal. 
     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 collector and seal assembly containing a compression bushing according to the present invention; 
     FIG. 2 is an elevated exploded view of the collector and seal assembly of FIG. 1; 
     FIG. 3 is a sectional top view of the seal prior to assembly; 
     FIG. 4 is a sectional top view of the seal partially assembled with the compression bushing; and 
     FIG. 5 is a sectional top view of the seal further illustrating assembly of the collector inserted through the central opening in the seal hub. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a cylindrical electrochemical cell  10 , such as an alkaline cell according to one example, 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  14  of can  12  includes a positive cover  18  welded or otherwise attached thereto and formed of plated steel, with a protruding nub at its center region, which forms the positive 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  may be formed over the peripheral edge of the positive cover and may extend partially onto the negative cover as shown. 
     A cathode  22  is formed about the interior surface of steel can  12 , and is generally tubular shaped. According to one example, the cathode  22  is formed of a mixture of manganese dioxide, graphite, potassium hydroxide (KOH) solution, and additives. A separator  24 , which may include a cup-shaped separator and is preferably formed of a non-woven fabric that prevents migration of any solid particles in the cell  10 , is disposed about the interior surface of cathode  22 . An anode  26  is disposed with alkaline electrolyte inside the separator  24 , generally in the center of the cell  10 . According to one example, the anode  26  is formed of zinc powder, a gelling agent, and additives. Disposed in contact with anode  26  is a current collector  28 , which may include a brass nail having an elongated body and an enlarged head at one end. 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 anode  26 , cathode  22 , and separator  24  may alternately be configured in a spiral wound configuration for a jelly-roll cell construction, or may be otherwise configured in primary or secondary cells, without departing from the teachings of the present invention. 
     The electrochemical cell  10  includes a collector and seal assembly which closes the open end  16  of steel can  12 . Included in the collector and seal assembly is current collector  28 , an annular nylon seal  30 , and a polymeric compression bushing  42 . The current collector  28 , nylon seal  30 , and polymeric compression bushing  42  are preferably pre-assembled and inserted into the open end  16  of steel can  12  as an assembled unit. In addition, an outer conductive cover  46 , which forms the negative contact terminal of cell  10 , is disposed over the collector and seal assembly. The outer negative cover  46  is preferably formed of plated steel, and is in contact with current collector  28 , preferably via pressure contact or a weld. The outer negative cover  46  includes one or more vent openings  48  that serve to expose the non-sealed volume of cell  10  to the surrounding outside atmosphere to allow for venting of gases during a cell venting condition. 
     The annular nylon seal  30  has an outer peripheral upstanding wall  32  formed at its outer perimeter, and an inner upstanding wall which forms a thickened hub  38  at the center of seal  30 . Formed between central hub  38  and outer upstanding wall  32  is an inwardly curved, i.e., concave, diaphragm  34  and an inverted V-section  36 . The inverted V-section provides a raised channel between the diaphragm  34  and outer upstanding wall  32  for receiving the top open end of separator  24 . Accordingly, the separator  24  fits into the inverted V-section and conforms thereto. 
     The concave diaphragm  34  is curved or bent inward towards the sealed inner volume of the can  12  so as to cause compressive force to be applied to the seal  30  when the pressure in the sealed volume of cell  10  is greater than the atmospheric pressure in the non-sealed volume. The concave diaphragm  34  may be in the shape of an elbow, or other configuration. When the seal  30  is disposed in can  12 , the sealed volume becomes slightly pressurized and, as the cell discharges, the pressure generally increases. Since the sealed volume of cell  10  generally always has a pressure greater than the atmospheric pressure, the seal  30  is generally always under compression. By maintaining the seal  30  in a compressed state, the seal  30  is less susceptible to damage when exposed to KOH. Since the adverse effects caused by KOH are reduced by maintaining the seal  30  under compression, little or no protective asphalt coating may be required. 
     The seal&#39;s central hub  38  has a cylindrical opening defined vertically therethrough for receiving the current collector  28  and providing an interference fit sealed closure between collector nail  28  and hub  38 . Formed about the outer perimeter of hub  38  is an outer cylindrical upstanding wall. The polymeric compression grommet  42  is force fitted around the outer upstanding wall of hub  38  so as to compress hub  38 , radially inward. Compression grommet  42  completely surrounds the outer upstanding wall of hub  38  and further extends above hub  38  and contacts the bottom surface of outer cover  46  to provide a support structure that enhances the vent shear edge and supports the collector and seal orientation. 
     Seal  30  further has a thinned-section  40  formed in the diaphragm  38  adjacent to the bottom outer peripheral edge of compression bushing  42 . Thinned-section  40  serves to provide a force concentration vent for venting pressure when the internal pressure in the sealed volume of the cell reaches a predetermined pressure limit. By placing the thinned-section  40  at a location adjacent to the bottom outer edge of polymeric compression bushing  42 , the bottom outer edge of compression bushing  42  serves to provide a shear edge to achieve a more consistent venting action. In addition, the polymeric compression bushing  42  has vertical grooves  44  formed therein for preventing sealing of the flexed seal diaphragm  34  against the upstanding walls of compression bushing  42  during a cell venting condition. 
     The assembly of the collector  28 , seal  30 , and compression bushing  42  to form the collector and seal assembly is illustrated in FIGS. 2-5. With particular reference to FIG. 2, the polymeric compression bushing  42  has a cylindrical opening  52  formed centrally therethrough with a diameter D B . The compression bushing  42  is made up of polymeric material, and more particularly, is made of polystyrene which is rigid and non-stretchable. Polymeric compression bushing  42  is therefore rigid and non-stretchable. Compression bushing  42  is formed in a generally cylindrical shape such that it is symmetric both with respect to the central longitudinal axis of the cell passing through opening  52  and an axis oriented perpendicular to the longitudinal axis. Compression bushing  42  has an outer peripheral wall  50  with a plurality of vertical channels  44  formed in its outer upstanding wall. Channels  44  are circumferentially spaced around the perimeter of the outer surface of compression bushing  42 , and have a preferred depth of approximately 5-7 mils. The compression bushing  42  is preferably an injected molded part that is orientation insensitive, since the top end is symmetric with the bottom end. Accordingly, compression bushing  42  can be inserted onto hub  38  from either its top or bottom end. This simplifies the cell assembly process, since some manufacturing equipment can easily handle and orient the compression bushing  42  into an upright position for insertion onto hub  38 . During manufacture, the polymeric compression bushing  42  is forcefully inserted onto hub  38 , and thereafter the current collector  28  is inserted downwardly from the top end into the central opening  54  formed in hub  38  of seal  30 . 
     With particular reference to FIG. 3, the nylon seal  30  is shown prior to assembly of the collector and seal assembly. The central opening  54  formed in the seal&#39;s hub  38  has an initial non-compressed inside diameter D O . The outer upstanding walls of hub  38  define an initial noncompressed diameter D H . The outer wall diameter D H  of hub  38  is larger than the inside diameter D B  of the central opening  52  formed in compression bushing  42 . In addition, the upstanding wall of hub  38  may have a slight taper with a smaller diameter at the top end to allow compression bushing  42  to more easily engage hub  38 . 
     As the compression bushing  42  is forcefully inserted onto the outer upstanding wall of hub  38 , the upstanding wall of hub  38  is compressed radially inward as is shown in FIG.  4 . When the polymeric compression bushing  42  is forcibly inserted onto the upstanding walls of hub  38 , the central opening  54 ′ is reduced to a reduced diameter D O ′ due to the radial compression caused by compression bushing  42  on hub  38 . Also, the outer wall diameter D H  of hub  38  is also reduced to a diameter of D H ′. 
     Once the compression bushing  42  has been force fitted onto hub  38  so as to reduce the size of opening  54 ′, the current collector nail  28  is then inserted into opening  54  as shown in FIG.  5 . With the collector nail  28  fully inserted, the size of opening  54 ′ is forced to expand to substantially its original size  54 . In doing so, compression bushing  42  provides a compressive force on hub  38 , so as to keep the hub  38  in a compressive state, instead of allowing the hub  38  to be in tension. 
     Accordingly, the seal hub  38  is compressed between polymeric compression bushing  42  and collector  28  so as to exert radial compressive stress and tangential compressive stress on the seal&#39;s hub  38 . It has been discovered that by maintaining the hub  38  in compression, rather than under tension, the hub  38  is less prone to stress corrosion cracking, which is particularly significant when the seal  30  is exposed to KOH. As a consequence, the amount of sealant employed between the collector and seal, and the amount of asphalt employed on the bottom of the seal can be reduced or even eliminated. It should also be appreciated that the collector and seal assembly does not employ an inner metal cover. 
     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.