Abstract:
A unitary lid for the casing of an electrochemical energy storage device is described. The lid has a terminal lead ferrule and a fillport formed from a single blank in a machining process. The lid does not require any welding except for securing it to the open end of a casing container. This helps the lid contribute to the cell&#39;s volumetric efficiency, which is especially important for cells powering implantable medical device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. application Ser. No. 09/837,778, filed Apr. 18, 2001, now abandoned, which claims priority on U.S. provisional application Ser. No. 60/198,175, filed Apr. 19, 2000. 
    
    
     FIELD OF INVENTION 
     The present invention relates to electrical energy storage devices, such as electrochemical cells and capacitors. More particularly, the present invention relates to lids or covers for casings housing electrical energy storage devices. 
     BACKGROUND OF THE INVENTION 
     Electrochemical cells and capacitors typically include a container with an opening that is closed by a lid or cover welded to the container to form a casing for the electrical energy storage device. Inside the container is an anode/cathode electrode assembly activated by an electrolyte. The container and the lid are of electrically conductive material and serve as a contact for either the anode electrode or the cathode electrode. In a case negative cell, the anode current collector is in contact with the casing while for a case positive design, the opposite is true. The other of the anode electrode and the cathode electrode not in contact with the casing is connected to a terminal lead electrically insulated from the casing by a glass-to-metal seal. When a load is connected to the casing and the terminal pin in an electrochemical cell, a chemical reaction in the cell results in a voltage differential that generates an electrical current to power the load, for example, a medical device. 
     The lid must provide access to the interior of the casing for at least two purposes. First, the terminal lead connected to the anode or the cathode current collector must pass through one of the lid openings to a position exterior of the casing. Second, the electrolyte must be filled into the housing through the other lid opening. Conventionally, two openings are defined in the lid for this purpose. The openings usually have structures connected to the lid to aid in sealing them. For example, a terminal lead ferrule is attached to the lid to accommodate the electrical lead and a fillport/closure assembly is used for sealing the fill opening. 
       FIG. 1  shows an exemplary prior art construction where the lid  10  is formed of a generally rectangular blank  12  stamped from a sheet of electrically conductive material. During stamping, two openings  14 ,  16  are provided through the blank  12 . A terminal lead ferrule  18  and a fillport  20  are sleeve-shaped members formed of discrete parts that are welded to the blank  12 , each in registration with one of the openings  14 ,  16 . 
     This prior art lid requires a number of manufacturing, inspection, and assembly steps due to the use of at least three discrete parts, i.e. the blank  12 , the terminal lead ferrule  18  and the fillport  20 . Specifically, the blank  12  is punched from a sheet of metal using a fine blanking or stamping operation. Simultaneously, the two openings  14 ,  16  are punched through the blank  12 . The lid  10  goes through an annealing process, a passivation process (e.g. removal of free iron from the surface of the part) and a cleaning process before it is inspected. The discrete terminal lead ferrule  18  and the discrete fillport  20  go through the same process steps prior to attachment to the blank  12 . The terminal lead ferrule  18  and the fillport  20  are then positioned in registration with the openings  14 ,  16  and welded thereto. These welds are vulnerable to variations in quality and each must be inspected. As those who are skilled in the art will readily recognize, these manufacturing, assembly, and inspection steps require time and labor. Also, inventory of the parts must be tracked and maintained, further adding to the cost of an electrical energy storage device. 
     Another problem with the prior art cover construction is that crevice corrosion can occur where the terminal lead ferrule  18  and the fillport  20  are secured to the openings  14 ,  16  in the lid  10 . Typically, the terminal lead ferrule  18  and the fillport  20  are inserted from the bottom or interior surface of the blank  12  before being welded. This welding may leave cracks or crevices between the mating surfaces leading to entrapment of materials such as cleaning solutions. As such, corrosion can occur around these crevices. 
     One prior art lid or cover for an electrochemical cell is described in U.S. Pat. No. 6,010,803 to Heller, Jr. et al. This lid is formed by a metal injection molding process which requires that the intersections between the terminal lead ferrule and the main body of the lid and between the fillport structure and the lid be slightly curved or “radiused.” Heller, Jr. et al. believes that radiused junctions facilitate the flow of material during the metal injection molding process. This eliminates areas of stress concentration that can cause the molded material to crack. 
     There are several problems with the Heller, Jr. et al. metal injection-molded lid. First, the radiused areas detract from the internal volume available to active and other no-active cell components. Electrical energy storage devices of the present invention are used in implantable medical devices such as cochlear implants. These are extremely small devices that require extremely compact power sources where maximizing internal volume is important. 
     Secondly, a machined lid according to the present invention has a higher density and, consequently, less porosity than the metal injection molded lid. Metal injection molded materials require a binder, and even though technology advances have reduced the amount of binder required, metal density is still about 98% to about 98.5%, after curing. In contrast, a one piece lid according to the present invention machined from bar or rod stock has a density of about 99.99%, and maintains acceptable mechanical properties required for glassing the terminal lead in the glass-to-metal seal. 
     Lastly, design structures can be repositioned or changed to accommodate a particular electrical energy storage device. Typically, this requires a program change to offset features, change tolerances or add new features. In contrast, metal injection molded components require a whole new set of tooling which is capitol intensive. 
     U.S. Pat. No. 5,173,375 to Cretzmeyer et al. related to a unitary lid for closing a casing for an electrochemical cell. This prior art lid is stamped from a metal blank. The problem is that the stamping process introduces minute stress fracture into the product lid. These are sights of potential seal failure and corrosion. 
     Accordingly, what is needed is a unitary lid having a terminal ferrule and fillport structure that reduces the manufacturing, assembly, and inspection steps described above and is as compact in size as possible. 
     SUMMARY OF THE INVENTION 
     The present invention meets the above-described needs by providing a unitary lid including a terminal ferrule and a fillport structure formed from a single blank of conductive material. A starting blank is provided with a thickness sufficient to meet the design features for a particular electrical energy storage device. The terminal ferrule and fillport are then created in the blank via a machining process such that the junctions where both the terminal lead ferrule and the fillport structure meet with the under side of the lid are at right angles. The process of the present invention eliminates the need for welding of the sleeve-shaped terminal lead ferrule and fillport, and requires fewer handling operations while optimizing the cell&#39;s internal volume. 
     These and other aspects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following description and the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is illustrated in the drawings in which like reference characters designate the same or similar parts throughout the figures of which: 
         FIG. 1  is cross-sectional side elevation view of a conventional lid with a welded terminal lead ferrule and fillport structure. 
         FIG. 2  is a cross-sectional side elevation view of the unitary lid of the present invention. 
         FIG. 2A  is a cross-sectional view taken along line  2 — 2  of  FIG. 2 . 
         FIG. 3  is a cross-sectional side elevation view of an exemplary electrochemical cell with the unitary lid of the present invention attached to a container to provide a casing for the electrode assembly. 
         FIG. 4  is a schematic of a patient P provided with an implantable medical device  100 . 
         FIG. 5  is an enlarged schematic of the indicated area in  FIG. 4  particularly showing the control circuitry  106 , the electrochemical cell  109  and capacitor for the medical device  100  connected to the patient&#39;s heart H. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Throughout this description the terms “lid” and “cover” are used interchangeably to refer to the member shown in  FIG. 2  that is attached to the open end of a container or housing to form a casing for an electrical energy storage device. 
     Electrochemical cells or batteries generate electrical current from chemical energy. Capacitors are used to store relatively large quantities of electrical energy for subsequent discharge. Often, these types of electrical energy storage devices are used in medical devices such as heart pacemakers, cardiac defibrillators, neurostimulators, cochlear implants, and the like. In that light, the lid of the present invention is a compact unitary member with space saving right angle surfaces at the junction of the lid body and both the terminal ferrule and the fillport. As will be described hereinafter, this makes the present lid particularly applicable for electrical energy storage devices intended for incorporation into implantable medical devices. 
     Turning now to the drawings,  FIG. 2  shows a unitary lid or cover  30  according to the present invention formed by machining a rectangular blank (now shown) of an electrically conductive material such as stainless steel, titanium, nickel, aluminum, and the like. Lid  30  has generally opposing major planar upper and lower surfaces  36  and  39 . When in place closing the open end of the container  42  of a casing  45  ( FIG. 3 ), upper surface  36  is an exterior surface and inner surface  39  is an interior surface. Lid  30  is formed of generally three portions or regions: a main body portion  48  having the opposed upper and lower surfaces  36 ,  39 , a terminal ferrule portion  51 , and a fillport portion  54 . The terminal lead ferrule  51  and fillport portion  54  are completely integral or unitary with main body portion  48 . As used herein, completely integral means being of a single continuous body of material. In other words, by machining the lid  30  from a suitable blank, the terminal ferrule  51  and the fillport portion  54  are not separate or discrete parts, but rather are completely unitary with the main body portion  48  forming a single part. 
     Lid  30  terminates along a peripheral edge  57  that is generally perpendicular to the planar upper and lower lid surfaces  36 ,  39 . In the embodiment shown, main body portion  48  is generally rectangular in peripheral shape. The terminal ferrule  51  is a sleeve-shaped portion having a surrounding side wall  52  with a cylindrical outer surface  52 A. The cylindrically shaped inner surface  52 B of the surrounding side wall has a machined roughened texture. In other words, the inner surface  52 B is not threaded, but is roughened by the action of a rotating machining bit removing material from the blank to form the cylindrically shaped opening. The machined roughness enhances the integrity of the glass-to-metal seal by providing scoring marks that serve as attachment structures for the glass to fill and anchor into. With a prior art metal injection molded lid according to the previously discussed U.S. Pat. No. 6,010,803 to Heller, Jr. et al. or a metal stamped lid according to U.S. Pat. No. 5,173,375 to Cretzmeyer et al., there is no such surface roughness to which the glass can anchor. Also, the hoop strength of a lid produced by a metal injection molding process is reduced due to the extended curing time required. 
     The cylindrical outer surface  52 A of the ferrule side wall  52  meets the lower surface  39  of the lid main body portion  48  at a right angle or a normal orientation. Similarly, the fillport  54  is a sleeve-shaped portion having a cylindrically shaped opening provided by a surrounding side wall  55 . The cylindrical outer surface  55 A of the fillport side wall  55  meets the lower surface of the lid main body portion  48  at a right angle. 
     While the upper ends of the terminal ferrule  51  and the fillport  54  are shown co-planar with the upper surface  36  of the lid  30 , the present invention should not be so limited. In that respect, these structures can extend above the upper lid surface  36 . What is important is that they are unitary with the main body portion  48 . 
     It will be understood by those of ordinary skill in the art that the main body portion  48  of the lid  30  may be of any suitable shape to mate with and close an opening in a container  42  for a casing, which also may be of any suitable shape. Therefore, the present invention contemplates any configuration of two portions of an electrical energy storage device casing which when mated form a cavity therein. This includes casing of a cylindrical shape, prismatic shape, button shape and casing formed of mating portions, such as described in U.S. application Ser. No. 09/757,232, filed Jan. 9, 2001. This application is assigned to the assignee of the present invention and incorporated herein by reference. 
     Either one of such portions of the casing, i.e., the lid or the container, or either one of the mating portions, may include the terminal ferrule portion  51  and the fillport portion  54  and be formed as a single part. What is important is that the respective outer surfaces  52 A,  55 A of the terminal ferrule  52  and the fillport  55  meet the lower or inner surface  39  of the lid main body portion  48  at a normal orientation. This means that as little internal casing volume as possible is occupied by the unitary lid  30 . Such a construction benefits volumetric cell efficiency, which is especially important in electrical energy storage devices intended for incorporation in implantable medical devices. 
       FIG. 3  shows an illustrative exemplary electrochemical cell  60  incorporating a lid or cover according to the present invention. The exemplary cell  60  is described in U.S. Pat. No. 5,750,286 to Paulot et al., which is assigned to the assignee of the present invention and incorporated herein by reference. The cell  60  includes the casing  45  made of metal, such as stainless steel, titanium, nickel, aluminum, or other suitable electrically conductive material. Casing  45  is formed of two portions: the container shell  42  and the lid  30 . Container  42  has an interior surface  63  and an opposite exterior surface  65 . Further, container  45  terminates in a peripheral region  68  at a peripheral edge or rim  71 . Peripheral region  68  defines an open side or opening  74  leading into the container  42 . Accordingly, the container  42  forms generally all but one open side of casing  45 . Lid  30  closes opening  74  and is attached to the peripheral region  68 , such as by welding. 
     As shown in  FIG. 3 , the terminal ferrule  52  supports a glass-to-metal seal  77  for a terminal lead  80  connected to the current collector  83  of one of the electrodes, for example the cathode electrode  86 . The anode (not shown) is segregated from the cathode by a separator  89 . The anode/cathode electrode assembly is then activated by an electrolyte (not shown) filled in the casing, and sealed therein by a closure means, such as ball  92  sealed in the fillport portion  54 . Those skilled in the art will understand that the present invention is not limited to any particular closure structure. 
     In accordance with the previous description, it will be evident that the present invention is applicable to any type of electrical energy storage device in which a housing is used that has a container portion with an open side and a cover for closing the container, thereby forming a casing for the storage device. In that respect, the present invention is applicable to low rate, medium rate, high rate, case negative and case positive electrochemical cells of both primary and secondary chemistries. Examples of such cells include lithium iodine cells, lithium thionychloride cells, lithium silver vanadium oxide cells, lithium carbon monofluoride cells, lithium manganese dioxide cells, and secondary cells containing lithium cobalt oxide, and the like. 
     It will further be recognized that such cells may take one of various configurations. For example, depending on the type of cell, the configuration of the anodes, cathodes, terminal lead ferrule portions, fillports, etc. will vary. Also, for example, depending on the cell, the materials housed in the casing will vary. Such materials may take the form of a liquid or a solid depending on the type of cell. Therefore, it should be clear that the present invention is in no manner limited to the illustrative cell described herein and is applicable to all types of electrochemical chemistries. 
     The present invention is also applicable to capacitors, such as those described in U.S. Pat. Nos. 5,926,362 and 6,334,879, both to Muffoletto et al. These patents are assigned to the assignee of the present invention and incorporated herein by reference. 
       FIGS. 4 and 5  show a patient P having a medical device  100 , such as an implantable cardiac defibrillator, implanted inside the body. The enlarged schematic shows the medical device  100  comprising a housing  103  containing control circuitry  106  powered by an electrochemical cell  109 . The cell  109  is also connected to a capacitor  112 . The control circuitry  106  is connected to at least one conductor  115  by a hermetic feedthrough  118 , as is well known by those skilled in the art. The distal end of the conductor connects to the heart H for delivering a therapy thereto from the capacitor  112  charged by the cell  109 . The cell  109  and capacitor  112  are both built having a unitary lid  30  closing their casings according to the present invention. 
     Periodically, the patient will go to a medical facility, and the like, where the deliverable capacity determined by the control circuitry  106  is read to determine if the cell has discharged to the point that it is approaching its end-of-life, typically at an open circuit voltage of about 2.0 volts. If so, this indicates that it is time for the physician to schedule the patient for surgery to replace the medical device with a new one. 
     It is appreciated that various modifications to the inventive concepts described herein may be apparent to those of ordinary skill in the art without departing from the spirit and scope of the present invention as defined by the appended claims.