Metal deforming – By use of closed-die and coacting work-forcer – Cup or shell drawing
Reexamination Certificate
2002-02-28
2003-03-04
Larson, Lowell A. (Department: 3725)
Metal deforming
By use of closed-die and coacting work-forcer
Cup or shell drawing
C072S379400
Reexamination Certificate
active
06526799
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to casings for electrochemical cells, typically cylindrical casings, and a method of forming such casings.
BACKGROUND
The casings for electrochemical cells are typically elongated and of cylindrical shape. The casings are typically in the shape of a cylinder having a closed end and open end. Conventional alkaline cells or lithium cells which are in common usage today are representative of cells employing such cylindrical casings.
Primary alkaline cell typically have a cylindrical casing which contains zinc anode active material, alkaline electrolyte, preferably aqueous potassium hydroxide, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically comprising cellulose. Such cells have a fresh voltage of about 1.5 Volt and are in widespread use. (Alkaline cells as referenced herein shall be understood to be conventional commercial alkaline cells having an anode comprising zinc, a cathode comprising manganese dioxide, and an electrolyte comprising potassium hydroxide.) Primary (non-rechargeable) lithium cells can have a cylindrical casing typically containing an electrode composite comprising an anode formed of a sheet of lithium, a cathode formed of a coating of cathode active material comprising manganese dioxide or lithiated manganese dioxide on a conductive metal substrate such as a stainless steel mesh. The lithium cells can have a sheet of electrolyte permeable separator material between the anode and cathode sheets. The separator sheet is typically placed on opposite sides of the lithium anode sheet and the cathode sheet is placed against one of the separator sheets, thereby separating the anode and the cathode sheets. The electrolyte used is typically comprises a lithium salt such as lithium trifluoromethane sulfonate (LiCF
3
SO
3
) dissolved in a nonaqueous solvent. The electrode composite is spirally wound and inserted into the cell casing, for example, as shown in U.S. Pat. No. 4,707,421. Lithium cells having a cylindrical casing can be of varying size and commonly are in the form of cylindrical cells having about ⅔ the height of a conventional AA size alkaline cell or even smaller heights. The lithium cells have a voltage of about 3.0 volts which is twice that of conventional Zn/MnO
2
alkaline cells and also have higher energy density (watt-hrs per cm
3
of cell volume) than that of alkaline cells. Primary lithium cells are in widespread use as a power source for many conventional photographic flash cameras, which require operation at higher voltage and at higher power output than supplied by individual alkaline cells.
The cylindrical casings for such alkaline and lithium cells have good mechanical strength and corrosion resistance and are typically of steel such as nickel plated cold rolled or nickel plated stainless steel. The cylindrical casings are formed typically from a flat sheet of metal. The metal sheet can be held in place over a block die having a cylindrical channel opening in its surface. The channel opening can run through a portion of the die's thickness. The flat metal sheet can be drawn in a single stage or in a plurality of stages by action of a punch on the sheet until the casing of desired shape and diameter is obtained. If multiple staging is used to fabricate the casing, a series of block dies can be used each having a progressively smaller diameter channel opening. Thus, a metal sheet can be punched in the first stage to a first cup shape having a first diameter which is smaller than the diameter of the starting metal sheet. The cup product from the first stage die can be placed in a second stage die having a cylindrical channel opening of diameter less than the diameter of the first die opening. The cylindrical cup formed in the final stage is of the desired shape, diameter and length.
The casing can then be filled with active anode material, electrolyte and cathode material. An end cap assembly comprising a terminal plate with attached electrical insulator such as an insulator plug can then be placed in the open end of the casing. One of the anode or cathode is in electrical contact with the casing and the other is in electrical contact with the terminal plate. The peripheral edge of the casing at the casing open end can be crimped over the edge of the end cap assembly thereby sealing the casing with a portion the insulator between the casing and terminal plate. During crimping a portion of the casing can also be radially compressed around the end cap assembly to provide a tight seal.
It is desirable to design the cylindrical cell so that the amount of internal volume available for active material is as great as possible for a cell of given overall size. This results in increased cell capacity and service life. In order to accomplish this objective various designs of the end cap assembly, for example by flattening the end cap assembly or by using thinner insulator plugs, have been tried. Such designs have their limitations since the end cap assemblies typically include an insulator plug, and must be strong enough to withstand the crimping force needed to provide a tight seal. Another approach is to reduce the wall thickness of the casing. When conventional methods are used to form the casing, for example, by punching a flat sheet as above described, the casing wall thickness is uniform from one end of the casing to the other. If the casing is fabricated to wall thickness which is below a threshold level, the peripheral edge of the casing at the casing open end cannot be crimped effectively around the edge of the end cap assembly. For example, when the wall thickness of the casing peripheral edge thereof becomes too thin, the peripheral edge does not hold its crimped position with time but rather tends to spring back radially from its original crimped position. The “spring back” effect is a result of the change in physical properties occurring if the metal becomes thinned during the punching process. A casing peripheral edge which has been thinned results in a relaxation of the crimp forces around the end cap assembly and a gradual loosening of the seal between the casing and end cap assembly. This of course is undesirable since it can result in leakage of electrolyte from the cell and could also allow ambient moisture to seep into the cell. Also, if the wall thickness of the casing peripheral edge is too thin it may crack as the crimping forces are applied to it.
SUMMARY OF THE INVENTION
An aspect of the invention is directed to a casings for cylindrical electrochemical cells, for example, alkaline cells having an anode comprising zinc and a cathode comprising manganese dioxide, or lithium cells, comprising lithium metal anode and cathode comprising manganese dioxide or lithiated manganese dioxide. The casing of the invention is characterized by having a non-uniform wall thickness. The casing of the invention is not intended to be restricted to any one cell size. Thus, the casing having non-uniform wall thickness can be made with varying overall length and diameter so that it can be used as a casing for any desired cylindrical cell size, for example, AAAA, AAA, AA, C or D, ⅔ A size (same diameter as AA cell but ⅔ its length) or CR2 size (15 mm×25 mm). Thus, the casing of the invention has particular application to cells having an outside diameter between about 7 and 35 mm and a length of between about 20 mm and 60 mm. The casing comprises a cylindrical body surface, an open end and an integrally formed closed end. The closed end forms the cell bottom which functions as a cell terminal. The bottom can be flat or can have an integrally formed pip protruding from the center thereof. A peripheral edge of the casing at the open end thereof extends from the casing body. The peripheral edge desirably has a length of between 3 and 5 mm. The peripheral edge is preferably stepped so that it has an outside diameter which is greater than the outside diameter of the remainder of the casing body. After the casing is filled with active mat
Buckle Keith
Ferraro Charles R.
Lucas Jay
Sargeant Sean A.
Douglas Paul I.
Josephs Barry D.
Krivulka Thomas G.
Larson Lowell A.
The Gillette Company
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