Patent Publication Number: US-6656629-B1

Title: High rate electrochemical cell

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 09/198,802, filed Nov. 24, 1998, entitled “HIGH RATE ELECTROCHEMICAL CELL AND ASSEMBLY METHOD” which issued Dec. 4, 2001 as U.S. Pat. No. 6,326,102 B1 entitled “HIGH RATE ELECTROCHEMICAL CELL WITH INCREASED ANODE-TO-CATHODE SURFACE AREA”. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to an electrochemical cell and, more particularly, relates to a high rate capable electrochemical cell having an increased anode-to-cathode interface surface area. 
     Electrochemical cells are commonly employed to supply voltage for electrically operated devices, and particularly for portable electrically operated devices. Currently, the popular alkaline cells of the generally cylindrical type are commercially available in industry standard sizes including D-, C-, AA-, AAA-, and AAAA-size cells, as well as other sizes and configurations. Electrochemical cells, such as the aforementioned type, commonly provide a predetermined open circuit voltage supply. 
     Conventional cylindrical alkaline cells generally have a cylindrical-shaped steel can provided with a positive cover at one end and a negative cover at the opposite end. The cylindrical cell has a positive electrode, commonly referred to as the cathode, which is often formed of a mixture of manganese dioxide, graphite, potassium hydroxide solution, deionized water, and a TEFLON® solution formed about the interior side surface of the cylindrical steel can. A cup-shaped separator is generally centrally disposed in an inner cylindrical volume of the can about the interior surface of the cathode. A negative electrode, commonly referred to as the anode, is typically formed of zinc powder, a gelling agent, and other additives, and is disposed within the separator. An electrolyte solution is also disposed in the can. One example of a conventional cylindrical cell is disclosed in U.S. Pat. No. 5,501,924, which is hereby incorporated by reference. 
     Conventional cells of the aforementioned cylindrical type commonly have a single anode and a single cathode contained within the steel can, with the separator interfaced between the two electrodes. With the bobbin type cell construction, the cathode is disposed adjacent the inner wall of the steel can, while the anode is disposed within a cylindrical volume centrally formed in the cathode. Accordingly, the separator has an anode-to-cathode interface surface area generally defined by the shape and size of the anode and the cathode. With the conventional bobbin type cell, the anode-to-cathode interface surface area is approximately equal to the surface defining the periphery of the cylindrical anode. 
     Another cell construction, commonly referred to as the jelly-roll cell construction, employs a sheet of anode and a sheet of cathode tightly wound together with a separator interdisposed between the two electrode sheets. While conventional jelly-roll wound cells offer high rate capability with a large anode-to-cathode interface area, such cells have inherent limitations. For instance, the process of forming jelly-roll cells is time consuming and relatively expensive. Further, the jelly-roll separator consumes a relatively large amount of available volume, thereby compromising the volume that remains for active cell materials. 
     A primary goal in designing alkaline cells is to increase the service performance which is the length of time for the cell to discharge under a given load to a specific voltage at which the cell is no longer useful for its intended purpose. Commercially available alkaline cells have an external size that is defined by industry standards, thereby limiting the ability to increase the amount of active materials that can be utilized. Conventional approaches for improving high rate performance have focused on increasing the efficiency of the internal cell materials. The need for high rate capable cells is becoming even more important with the increasing demand from consumers using high tech, high drain electronics devices. To meet this demand, the need to find ways to increase high rate service performance remains a primary goal of the cell designers. 
     SUMMARY OF THE INVENTION 
     The present invention improves the high rate performance of an electrochemical cell by providing an easy-to-manufacture cell construction having an enhanced anode-to-cathode interfacial surface area to realize improved high rate service performance. To achieve this and other advantages, and in accordance with the purpose of the invention as embodied and described herein, the present invention provides an electrochemical cell including a container, a first electrode, and a second electrode. In addition, the cell further has an outer electrochemically active layer formed around the first electrode and separated therefrom by a separator. The first electrode and the second electrode are disposed in the container and separated from each other via a separator. The outer electrochemically active layer is in electrical contact with the second electrode, and a current collector is disposed in contact with the second electrode. 
     According to the assembly method of the present invention, a first electrode is formed having a conductive grid current collector integrally embedded therein. An outer separator covers the outer surface of the first electrode. An outer electrochemically active layer is disposed on top of the outer separator, and therefore wraps around the outside of the first electrode. The first electrode, outer electrochemically active layer and separator are disposed as a bobbin assembly into a container having a closed bottom end and an open top end. An inner separator is disposed in an inner cylindrical volume in the first electrode. A second electrode is disposed in the inner cylindrical volume of the first electrode and against the inner separator. A current collector is disposed in contact with the second electrode and the outer electrochemically active layer. A cover assembly is assembled to the open end of 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 cross-sectional view of a high rate capable electrochemical cell constructed in accordance with the present invention; 
     FIG. 2 is a flow diagram illustrating a methodology of assembling the electrochemical cell according to the present invention; 
     FIGS. 3A-3C are cross-sectional views of a die assembly illustrating the assembly of a cathode bobbin using full impact molding according to one embodiment; 
     FIGS. 4A-4E are cross-sectional views of a die assembly illustrating the assembly of the cathode bobbin using a ring molding technique according to a second embodiment; 
     FIGS. 5A-5E are cross-sectional views of a die assembly illustrating the assembly of the cathode bobbin using a combination of impact and ring molding according to a third embodiment; 
     FIG. 6 is a cross-sectional view of a partially assembled cathode bobbin illustrating the assembly of a cup-shaped separator according to a first embodiment; 
     FIG. 7 is an exemplary view of the partially assembled cathode bobbin illustrating application of a liquid separator according to a second embodiment; 
     FIG. 8 is a perspective view of the cathode bobbin illustrating application of an outer separator, an electrochemically active zinc strip and a shrink-wrap coating to the outer surface; 
     FIG. 9 is a cross-sectional assembly view of a partially assembled electrochemical cell illustrating insertion of the cathode bobbin into the steel can; and 
     FIG. 10 is a cross-sectional assembly view of the electrochemical cell illustrating insertion of the collector and seal assembly into the steel can. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, a cross-sectional view of a cylindrical alkaline electrochemical cell  10  is shown therein constructed according to the present invention. The electrochemical cell  10  is constructed with enhanced interfacial surface area between the positive and negative electrodes to increase current carrying capacity to thereby achieve high rate performance capability with capacity efficiency. While electrochemical cell  10  is described in connection with a cylindrical alkaline zinc/MnO 2  cell, it should be appreciated that the cell construction of the present invention is applicable to various types of cells including lithium, alkaline air, and air-assisted cells, and may be configured in different cell sizes and configurations. 
     Electrochemical cell  10  includes a steel can  12  having a cylindrical shape with a closed bottom end and an open top end. Steel can  12  has a protruding nub  14  integrally formed in the bottom closed end which serves as the cell&#39;s positive contact terminal. Alternately, steel can  10  may be formed with a flat bottom end, in which the protruding nub  14  can be welded or otherwise attached to the exterior to form the positive contact terminal. Additionally, a metalized, plastic film label (not shown) may be formed about the exterior surface of steel can  12 , except for the ends thereof. 
     The electrochemical cell  10  of the present invention employs a bobbin-type cell construction having a positive electrode, referred to herein as the cathode  16 , and a negative electrode, referred to herein as the anode  20 . The cathode  16  is molded into a bobbin-type construction and is disposed into steel can  12 , and the anode  20  is dispensed in an inner cylindrical volume of the cathode  16 . According to the present invention, an outer layer of electrochemically active material, referred to as a zinc strip  24 , is also provided in steel can  12 . Zinc strip  24  is a thin sheet of metal foil that contains an electrochemically active material, and more particularly contains zinc to provide a negative electrode. Zinc strip  24  is preferably thin, with a thickness on the order of approximately 1 mil; however, different thicknesses of zinc strip could be employed. Alternatively, the outer anode  24  can be made of other anode materials which may include lithium, cadmium, metal hydride, or other anode materials. The forms of the outer anode  24  may include foam, bonded powder, perforated or expanded metals, or others. The inner anode  20  and zinc strip  24  cover both sides of cathode  16  to provide for an increase in the anode-to-cathode surface interface area, which provides for a reduction in current density and provides for enhanced high rate service performance. 
     The cathode is formed into a bobbin  16  made up of an outer cathode layer  16 A and an inner cathode layer  16 B. The cathode  16 A and  16 B is preferably formed of a mixture of manganese dioxide, graphite, 45% potassium hydroxide solution, deionized water, and aqueous TEFLON® solution, and any additives. The cathode bobbin  16  is molded such that a conductive nickel grid  18  is embedded between inner and outer cathode layers  16 A and  16 B. The conductive grid  18 , which is preferably basket-shaped, is therefore integrally formed between inner and outer cylindrical cathode layers  16 A and  16 B. The conductive grid  18  may be made of woven wire or metal foil coated with a carbon conductive coating and provides a conductive current path having a large contact surface area contacting cathode  16 . 
     Disposed on both the inside and outside surface of the cathode is a separator material. The separator material includes an inner separator  22 B which may be formed of a non-woven fabric that is formed in a cup-shaped configuration to cover the inner cylindrical walls of the cathode  16 B. Also provided is an outer separator  22 A which may be made up of a sheet of nonwoven material covering the outer walls of cathode  16 A. While the separator material is shown having a first cup-shaped separator  22 B to cover the inner walls of the cathode and outer separator  22 A wrapped around the outside walls of the cathode, a single separator piece may be formed to provide the same overall configuration. Alternately, the separator material may be applied as a liquid separator  22 ′ as explained herein. In any event, the separator covers the inside surface of the cathode, as well as the outside surface. 
     The anode  20  is disposed within the inner cylindrical volume of cathode  16  and separated therefrom via the inner separator  22 B. Anode  20  is preferably formed of zinc powder, a gelling agent, and other additives, according to one embodiment. However, anode  20  may include a powdered or gel matrix anode, e.g., zinc powder/carbopol gel, or could alternately include a lithium powder coated with TEFLON®. 
     To increase the anode-to-cathode surface interface area, a layer of electrochemically active material is provided as zinc strip  24 , and is disposed around the outside surface of the cathode  16 A, on the outside surface of the separator  22 A. The outer layer  24  of zinc strip provides an electrochemically active anode material that acts as another negative electrode. The zinc strip  24  is assembled to be in conductive contact with the anode  20  by way of a conductive tab  28 . Zinc strip  24  may be in electrical contact with the anode  20 , the negative collector  36 , or the cell&#39;s negative terminal. In addition, a shrink-wrap dielectric layer  26  is formed on the outside surface of zinc strip layer  24  to electrically insulate the zinc strip layer  24  from the steel can  12 . According to one example, the outer separator  22 A has a preferred thickness of less than or equal to 2 mil (0.0502 mm), zinc strip layer  24  has a preferred thickness of less than or equal to 3 mil (0.0753 mm), and the shrink-wrap dielectric layer  26  has a preferred thickness in the range of 2 to 3 mil 0.0502-0.0753 mm). 
     A negative current collector  36  is disposed in the open end of steel can  12  in contact with anode  20 . Negative current collector  36  may be integrally formed with seal assembly  30  which seals the open end of steel can  12 . Seal assembly  30  further includes a nylon seal  32 , and a negative cover  34  which is preferably welded to the negative current collector  36 . Nylon seal  32  may contact a metal washer  38 , and provides a sealing closure to the steel can  12 . The negative cover  34  is electrically insulated from steel can  12  by way of nylon seal  32  and serves as the cell&#39;s negative contact terminal. 
     Referring to FIG. 2, a methodology  40  of assembling the high rate capable electrochemical cell  10  according to the present invention is illustrated therein. Cell assembly method  40  includes the step  42  of inserting the conductive grid  18  into a cathode molding die. The conductive grid  18  is made up of a basket-shaped carbon coated grid made of nickel or other suitable conductive material and is accurately located in the die so that it will be centrally formed in the cathode. Proceeding to step  44 , the cathode is formed in the shape of a cylindrical bobbin with the cup-shaped conductive grid  18  embedded centrally therein. This is accomplished by placing cathode mix on both sides of the conductive grid  18 , and forming the cylindrical cathode bobbin according to one of three cathode molding techniques explained herein. 
     Once the cathode is molded with the embedded conductive grid  18 , separator material is applied to cover both the outer and inner walls of the cylindrical cathode bobbin as provided in step  46 . The separator may be applied as a non-woven material or a liquid separator as explained herein. In step  48 , an electrochemically active zinc strip  24  is wrapped around the outside surface of the cylindrical cathode bobbin, directly over the outer separator. The zinc strip  24  preferably includes an extended zinc tab  28 . The zinc tab  28  is folded over the open end of the cathode bobbin and into the inner cylindrical cavity formed in the cathode bobbin. On the outer surface of the zinc strip  24  is formed the dielectric shrink tube to complete formation of the cathode bobbin as provided in step  50 . Once the cathode bobbin assembly is complete, cell assembly methodology  40  proceeds to step  52  in which the cathode bobbin is disposed into the steel can  12 . This includes disposing the cathode bobbin into the open end of steel can  12  so that the bottom cup-shaped portion of the bobbin conforms to the closed bottom end of the can. Once the bobbin is fully disposed in steel can  12 , the conductive grid  18  is preferably welded to the bottom end of the can in step  54  to ensure adequate conductive contact therewith. 
     With the cathode bobbin assembled to the can  12 , the remaining internal cell materials, including the anode and electrolyte solution are dispensed in the steel can  12  pursuant to step  56 . The anode  20  may include powdered or gel-type anode that is dispensed in the inner cylindrical volume provided in, the cathode bobbin. The anode  20  conforms to this shape of the inner cylindrical volume and abuts the inner separator  22 B, which in turn is disposed against the inner surface of cathode  16 . It should be appreciated that the anode  20  is inserted so that it contacts the zinc tab  28  such that the outer zinc strip layer  24  is in conductive contact with anode  20 . Finally, once all the internal components are assembled, cell assembly methodology  40  proceeds to step  58  to assemble the negative current collector and the seal assembly to close the open end of steel can  12 . The negative current collector is disposed in contact with the zinc powder or other active material in the anode  20 . The seal assembly  30  provides a sealing closure to the open end of can  12 , and serves to provide the cell&#39;s negative contact terminal. In addition, a metalized, plastic film label can be formed about the exterior surface of the steel can  12 , except for the ends thereof to complete the cell assembly. 
     Referring to FIGS. 3-5, sequences of steps for forming a molded cathode bobbin are shown therein for each of three embodiments. With particular reference to FIGS. 3A-3C, a cathode bobbin forming process is shown using full impact molding according to a first embodiment. To begin, the basket-shaped conductive grid  18  is located in a cylindrical cathode molding die  60  as shown in FIG.  3 A. The conductive grid  18  is accurately located and evenly spaced from the cylindrical walls of the die  60 . Next, as shown in FIG. 3B, cathode mix  15  is dispensed in die  60  both between die  60  and conductive grid  64  as well as to substantially fill the remaining internal volume of the die  60 . A ramrod  62  is then forcibly impacted into the central cylindrical volume of die  60  so as to compact the cathode mix  15  to form a rigid cathode bobbin structure with inner and outer cathode layers  16 B and  16 A and the conductive grid  18  integrally embedded therein. As shown in FIG. 3C, the cathode bobbin  16  is forcibly ejected from the die  60  by an ejector rod  64 . The cathode bobbin  16  is then ready to receive the separator material, zinc strip, and shrink tube, prior to its dispensing into steel can  12 . 
     Referring to FIGS. 4A-4E, a second embodiment of the cathode bobbin molding process is illustrated therein using a ring molding technique. Beginning in FIG. 4A, a series of four cathode rings are inserted into the die  60  and stacked one on top of another to form the outer cathode layer  16 A. The process of forming the cathode rings generally includes adding a measured charge of cathode mix to a ring shaped die set and, with the use of a die press, molding the cathode mix into the shape of a ring. The insertion of the cathode rings into the die  60  may be achieved by press fitting the cathode rings one on top of another. The process of forming single layer ring molded cathodes is widely known in the art. 
     Referring particularly to FIG. 4B, the conductive grid  18  is inserted in the inner cylindrical volume of the outer cathode layer  16 A within die  60 . Next, four smaller diameter cathode rings are disposed in the inner cylindrical volume of the conductive grid  18 , and are stacked one on top of another to form the inner cathode layer  16 B as shown in FIG.  4 C. Accordingly, the conductive grid  18  is embedded between the outer ring molded cathode layer  16 A and the inner ring molded cathode layer  16 B. As shown in FIG. 4D, a ramrod  62  is forcibly inserted centrally through the inner cylindrical volume of the cathode  16  to further compact the cathode rings together and forming a solid two-layer cathode ring with the conductive grid  18  embedded therein. Next, the cathode bobbin  16  is ejected by the ejector rod  64  as shown in FIG.  4 E. 
     Referring to FIGS. 5A-5E, the cathode bobbin is molded according to yet another embodiment which employs a combination of ring molding and impact molding. As shown in FIG. 5A, the outer cathode layer  16 A is formed by inserting four cathode rings stacked one on top of another into the die  60 . Next, the conductive grid  18  is inserted on the inner face of the outer cathode layer  16 A as shown in FIG.  5 B. In FIG. 5C, the remaining cathode mix  15  is disposed in the die  60  to substantially fill the remaining volume of die  60 , including the volume of the conductive grid  18 . In FIG. 5D, ramrod  62  is forcibly injected into the inner cylindrical volume of cathode mix  15  to compact and form the inner cathode layer  16 B. The ramrod  62  is removed from die  60 , and the ejector rod  64  ejects the molded cathode bobbin  16  from the die  60  as shown in FIG.  5 E. 
     Accordingly, the molded cathode bobbin is formed according to one of the three techniques shown and explained in connection with FIGS. 3-5, respectively. The cathode bobbin assembly is subsequently completed as shown in FIGS. 6-8. With particular reference to FIG. 6, the cup-shaped inner separator  22 B is inserted in the inner cylindrical cavity that is formed in the cathode bobbin  16  so that separator  22 B abuts the inside walls of the inner cathode  16 B. It should be appreciated that the cup-shaped separator  22 B could, alternately, be inserted after the cathode bobbin is disposed in the steel can  12 , which may easily allow for the conductive grid  18  to be welded to the bottom of steel can  12  from within the inside of can  12 . 
     As an alternative to the non-woven separator, the cathode bobbin  16  can be coated with a liquid separator  22 ′ as shown in FIG.  7 . The liquid separator  22 ′ may include a polystyrene separator such as that disclosed in U.S. Pat. No. 4,315,062, which is hereby incorporated by reference. The liquid separator  22 ′ is applied by dipping the cathode bobbin  16  into a container of the liquid separator  22 ′ and subsequently removing the cathode bobbin  16  from the liquid separator and allowing the separator coating to dry. It should also be appreciated that the liquid separator  22 ′ could be sprayed on as an alternative coating technique. 
     Referring to FIG. 8, a sheet of outer separator material  22 A is wrapped around the outer surface of the outer cathode layer  16 A. It is preferred that the outer separator  22 A and inner cup-shaped separator  22 B together fully cover the outer cathode walls, the inner cathode walls, and the top open end of the cathode bobbin  16 . The zinc strip  24  is then wrapped around the outer surface of the outer separator  22 A. Zinc strip  24  provides an electrochemically active anode material with a conductive medium. Zinc strip  24  preferably includes the extended zinc tab  28  integrally formed or attached thereto. Wrapped around the outside of the zinc sheet  24  is the dielectric shrink tube  26  which electrically insulates the zinc strip  24  from steel can  12 . It should be appreciated that any combination of adjoining layers of the shrink tube  26 , zinc sheet  24 , and outer separator  22 A could be combined in a multi-layer material, which could be applied to the outer surface of the cathode  16  with a single wrap and adhered thereto. 
     Referring to FIG. 9, the completed cathode bobbin  16  is shown being inserted into steel can  12 . As shown, extended tab  28 , which extends from zinc strip  24 , is folded over and across the top open end of the cathode bobbin  16  and is further folded downward into the inner cylindrical volume thereof. The cathode bobbin  16  is press fit against the closed bottom end of steel can  12 , and the bottom surface of the conductive grid  18  and cup-shaped separator  22 B may reshape to conform to the shape of the integrated protruding nub  14 . The conductive grid  18  is then welded to the bottom end of the can. 
     Once the cathode bobbin  16  is fully assembled into the can  12 , the remaining internal materials are disposed in steel can  12 . This includes dispensing the anode  20  into the inner cylindrical cavity provided in the cathode bobbin  16 . In addition, electrolyte solution is dispensed in the can  12 . Once the internal materials have been disposed inside steel can  12 , the negative current collector  36  and seal assembly  30  are assembled to the open end of steel can  12 . The negative current collector  36  is disposed in contact with the anode  20  to provide contact with the zinc powder found therein. It should be appreciated that negative current collector  36  is also in contact with zinc strip  24  through extended tab  28 . The seal assembly  30  closes and seals the open end of steel can  12 , and may provide for the negative contact terminal of electrochemical cell  10 . In addition, it should be appreciated that a metalized label may be formed about the outer side walls of steel can  12 . 
     The electrochemical cell  10  constructed according to the present invention provides for a large interfacial surface area contact between the anode and cathode, in an easy-to-assemble cell construction. This provides for an increase in overall current carrying capacity of the electrodes, and offers reduced current density and decreased thickness of the electrodes to thereby result in better discharge efficiency and higher rate capability. In addition, the assembly construction allows for use of a thinner can, since the cathode molding formation is not performed in the can. 
     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.