GASKET FOR MINIATURE GALVANIC CELLS

Abstract: An insulating gasket (1) is provided for a galvanic cell, adapted for disposal against the peripheral area of a flat electrode within the cell, the gasket comprising a ribbed portion (11) that extends between opposing segments of the gasket. The ribbed portion (11) functions as a restraining means to prevent or reduce bowing of the central area of the flat electrode during or after assembly of the cell. Also provided is a galvanic cell, for example an air depolarised cell, that comprises the gasket, and a method of sealing galvanic cells using the gasket.

Description: GASKET FOR MINIATURE GALVANIC CELLS This invention relates to a gasket for miniature-type galvanic cells that contain a flat electrode, to a galvanic cell comprising the gasket, and to a method of sealing a galvanic cell using the gasket. More particularly, the present invention relates to a gasket having a ribbed portion that functions as a restraining means for the central area of the flat electrode during or after assembly of the cell, and thereby reduces any bowing at the central area of the electrode. The miniaturisation of electronic devices has created a demand for small but powerful electrochemical cells. Cells that utilise an alkaline electrolyte are known to provide high energy density per unit volume and are therefore well suited for applications in miniature electronic devices such as cameras, hearing aids, watches and calculators. However, alkaline electrolytes, such as aqueous potassium hydroxide and sodium hydroxide solutions, have an affinity for wetting metal surfaces and are known to creep through the sealed metal interface of an electrochemical cell. Leakage in this manner can deplete the electrolyte solution from the cell and can also cause a corrosive deposit on the surface of the cell that detracts from the cell's appearance and marketability. These corrosive salts may also damage the device in which the cell is housed. Typical cell systems where this problem is encountered include silver oxide-zinc cells, nickel- cadmium cells, air depolarised cells, and alkaline manganese dioxide cells. In the prior art it has been a conventional practice to incorporate insulating gaskets between the cell cup and can so as to provide a seal for the cell. Generally, the gasket must be made of a material that is electrically insulating and that is inert to the electrolyte contained in the cell and the cell environment. In addition, it has to be flexible and resistant to cold flow under pressure of the seal and must maintain these characteristics so as to ensure a proper seal for a long period of storage. Materials such as nylon, polypropylene, ethylene-tetrafluoroethylene copolymer and high density polyethylene have been found to be suitable as gasket materials for most applications. Typically, the insulating gasket is annular and, in cross-section, in the form of a 'T'-shaped configuration including a "U"-shaped portion into which the extended wall of the cup may be inserted, so that, upon radially squeezing the edge of the can, the bottom portion of the gasket forms a seal with the bottom portion of the wall of the cup. The gasket generally extends the entire height of the internal wall of the cell. To better ensure a good seal, a sealant is generally applied to the gasket, including the internal " P'-shaped portion of the gasket, so that, upon insertion of the cup into the gasket, the edge of the extended wall of the cup seats in the sealant and thus, upon the application of a radial squeeze, forms a good seal between the cup and the can. In conventional zinc-air cells of the button-type, the positive air electrode is flat. The flat positive electrode can be prepared by mixing a catalyst consisting of manganese dioxide, activated carbon and electroconducting acetylene black with further addition of polytetrafluoroethylene (PTFE). The mix is then applied to a metallic screen and dried. In assembling the air electrode into the housing of the cell using, as seal, a conventional "J"-shaped gasket that has a horizontal base, the base of the gasket is generally compressed onto the flat, disk-shaped positive electrode at its peripheral area. The positive electrode of a zinc-air cell is typically a thin, flexible air electrode containing a screen that is stiffer than the positive electrode mix. Since the screen of the air electrode is stiffer than the positive electrode mix, when the electrode is compressed at its peripheral area by the edge of the outer wall of the cup via the gasket, the central area of the air electrode tends to bow inwards, instead of yielding, with the result that the electrode mix is compressed and extruded inwards. Electrode bowing has a significant impact on air cell performance. It has been observed that the discharging zinc negative electrode is generally unable to reverse the bow. As the negative electrode discharges, it discharges first against the air electrode. This creates a hard layer of zinc oxide that will not move with the discharge of the remaining negative electrode. Consequently, a portion of the void volume of the cell is not available, causing the cell to shut down prematurely and resulting in a lower capacity. Moreover, the bowing of the air electrode is detrimental to air cell performance because, not only does it reduce the internal volume available for the active components, but also it contributes to delamination of the air electrode. It would therefore be desirable to be able to reduce or eliminate the problems associated with electrode bowing experienced in miniature-type galvanic cells, such as in zinc-air cells. It would also be desirable to provide a galvanic cell that does not suffer from the disadvantages until now associated with miniature-type galvanic cells due to electrode bowing. Moreover, it would be desirable to provide a method of sealing a galvanic cell to form a cell that does not suffer from these disadvantages. What we have found is that these problems until now associated with miniature- type galvanic cells can be reduced or solved by the use of a modified gasket for sealing the cells. Accordingly, in a first aspect, the present invention provides an insulating gasket for a galvanic cell, adapted for disposal against the peripheral area of a flat electrode within the cell, the gasket comprising a ribbed portion extending between opposing segments of the gasket, wherein the ribbed portion is effective to prevent or reduce bowing of the central area of the flat electrode during or after assembly of the cell. In a second aspect, the present invention provides a galvanic cell comprising a flat electrode and an insulating gasket disposed against the peripheral area of the flat electrode, wherein the gasket comprises a ribbed portion extending between opposing segments of the gasket, the ribbed portion being effective to prevent or reduce bowing of the central area of the flat electrode. In a third aspect, the present invention provides a method of sealing a galvanic cell having a flat electrode and a two-part housing, comprising disposing an insulating gasket against the peripheral area of the flat electrode and compressing the gasket between the two parts of the housing to form an insulating seal, wherein the gasket comprises a ribbed portion extending between opposing segments of the gasket, the ribbed portion being effective to prevent or reduce bowing of the central area of the flat electrode during or after assembly of the cell. An advantage of the ribbed gasket in accordance with the present invention is that any bowing encountered in the flat positive electrode component of miniature-type galvanic cells during assembly is decreased. Another advantage is that the ribbed gasket in accordance with the present invention is easy to make, cost effective to produce and can easily be assembled into miniature-type galvanic cells. Yet another advantage is that the ribbed gasket in accordance with the present enables the cell capacity to be increased, by decreasing the extent of inward bowing of the positive electrode member during assembly and thus increasing the space available for the active negative electrode component. Furthermore, it is an advantage of the ribbed gasket in accordance with the present invention that electrode bowing, and the problems associated with such bowing, can be reduced without the need to modify the other cell components, thereby allowing the gasket to be assembled into a cell using otherwise conventional components. In an embodiment of the second aspect of the present invention, there is provided a galvanic cell having a first electrode with a peripheral area and a central area, a second electrode of opposite polarity, a separator between the electrodes and an electrolyte, all contained within a two-part conductive housing, one part of which is a can that is electrically connected to the first electrode and the other part of which is a cup that is electrically connected to the second electrode. Between the can and the cup is an insulating gasket having a base member with an outer wall spaced apart from an inner wall, thereby defining a "U'-shaped groove. The gasket also has a ribbed portion comprising at least one rigid rib extending from one segment of the base at the inner wall of the gasket across to an opposing segment. In the assembled cell, the edge of the open end of the cup is disposed within, and compressed into, the gasket groove. The base of the gasket is compressed against the peripheral area of the first electrode and the gasket rib supports the central area of the first electrode to reduce any bowing at the central area. In an embodiment of the third aspect of the present invention, there is provided a method of sealing a galvanic cell, so as to assemble a first electrode, a second electrode and an electrolyte of a cell into a two-part conductive housing in which one part is a cup and the other part is a can, including the steps of: a) preparing a conductive cup having a peripheral wall; b) preparing a conductive can having a peripheral wall and placing a first electrode within the can; c) preparing an insulating gasket with a base member having an inner upstanding wall spaced-apart from an outer upstanding wall thereby defining a "U"- shaped groove, the base member also having a first segment at the inner wall, a second segment at the inner wall opposing the first segment and a ribbed portion comprising at least one insulating rib extending inwardly from the first segment to the second segment; d) placing the edge of the wall of the cup within the groove of the gasket and placing the second electrode and electrolyte within the cup; e) assembling the cup of step d) within the can of step b), with the edge of the peripheral wall of the cup compressed against the gasket base and the gasket base compressed against the peripheral area of the first electrode, and with the rib portion of the gasket providing a restraining force on the central area of the first electrode, thereby effectively reducing any bowing of the first electrode at its central area; and f) compressing the edge of the can into the outer wall of the gasket to effectively seal the can to the cup via the gasket and thereby electrically insulate the cup from the can. By the term "opposing segment", as used herein, is meant any segment on the inner wall of the base member of the gasket that is different from the first segment, such as (1) a second segment directly or diametrically opposite to the first segment; or (2) a second segment disposed off-set from the central axis of the gasket. The ribbed portion of the gasket is intended to provide a restraining force against bowing of the flat positive electrode, so that bowing of the positive electrode during or following assembly of the cell is reduced. Thus, the term "flat electrode", as used herein, refers to electrodes that are generally planar and, as such, susceptible to, or capable of, bowing or flexing. The ribbed portion of the gasket should be sufficiently rigid that it restrains and reduces the bowing at the central area of the flat electrode, as compared with the bowing of an identical electrode in the absence of the ribbed portion. Therefore, the ribbed portion of the gasket can have, for example, a single rib disposed through the central axis or off-set from the central axis of the base member of the gasket, two or more ribs disposed through the central axis of the base member of the gasket and spaced apart at equal or unequal angle intervals, or two or more ribs disposed between segments that are off-set from the central axis of the base member of the gasket. Other rib arrangements are possible in accordance with the present invention, providing the ribbed portion is sufficiently rigid so as to be effective as a restraining means for the flat electrode, so as to reduce bowing of the central area thereof. The ribbed portion of the gasket can be made of any suitably stiff electrically insulating material, and preferably is moulded as a unitary structure with the remaining portion of the gasket. Alternatively, the ribbed portion of the gasket is formed as a separate component of the gasket and is disposed under the base of the gasket or secured to the bottom inward portion of the inner wall of the base member of the gasket. If formed as a separate component, the ribbed portion may be made of a material that is different from or the same as that of the remaining portion of the gasket. Since the ribbed portion of the gasket is an insulating member, the area of the rib covering the positive electrode should be sufficiently small so that it will not significantly interfere with the operation of the cell or decrease the operating capacity of the cell. The gasket in accordance with the present invention may be made of any suitable material, with consideration given to its stability in the presence of the electrolyte, its resiliency, and its resistance to cold flow. Suitable polymeric materials include, for example, nylon, polytetrafluoroethylene, fluorinated ethylene-propylene, chlorotrifluoroethylene, perfluoro-alkoxy polymer, polyvinyls, polyethylene, polypropylene and polystyrene. Other suitable materials will be apparent to those skilled in the art. For certain applications, additional precautions can be taken in conjunction with the ribbed gasket of the present invention to provide a more effective seal, for example by coating selected areas of the gasket surfaces with a sealing agent such as a fatty polyamide resin or asphalt. As mentioned, in order to reduce bowing of a positive electrode in a miniature galvanic cell, the ribbed gasket of the present invention is provided with a ribbed central portion that will restrain the central area of the flat positive electrode and thereby reduce any bowing at the central area of the positive electrode. Preferably, the ribbed portion has at least two ribs disposed at right angles. Also preferably, the ribbed portion is of a unitary construction with the remaining portion of the gasket, as can conveniently be produced using conventional moulding techniques. Although the present invention is herein described with reference to circular miniature cells, it will be appreciated that the cell can be of any appropriate shape in accordance with the present invention, including oval, ellipsoid or oblong, and that the gasket and its ribbed portion will be shaped accordingly. Typical cell systems in which the present invention can be used include alkaline manganese dioxide cells, air depolarised cells such as zinc-air cells, nickel-cadmium cells and silver oxide-zinc cells. The cup and can for the galvanic cells in accordance with the present invention are formed using conventional materials and techniques. For example, the cup may be of a triclad construction made of nickel, stainless steel and copper, in which a layer of copper has been laminated to the bare side of a nickel-clad steel strip. Other laminated materials from which the cup may be made include bilaminates of copper on a stainless steel substrate or laminates made from more than three layers. The can should be made of a conductive material that will not corrode or otherwise deteriorate when in contact with the materials of the cell. Suitable conductive materials include for example stainless steel, iron, nickel, or nickel-plated steel. The negative electrode mixture can comprise, for example, a mixture of zinc particles, electrolyte and organic binders. The present invention will be further understood by reference to the accompanying drawings, in which: Figure 1 is a cross-sectional view of a ribbed gasket in accordance with the present invention; Figure 2 is a top view of an embodiment of a ribbed gasket as shown in Figure 1; Figure 3 is a top view of another embodiment of a ribbed gasket in accordance with the present invention; Figure 4 is a top view of another embodiment of a ribbed gasket in accordance with the present invention; Figure 5 is a top view of another embodiment of a ribbed gasket in accordance with the present invention; Figure 6 is a cross-sectional view of an air electrode in a low profile can; Figure 7 is a cross-sectional view of a cup containing a negative electrode and a ribbed gasket in accordance with the present invention secured to the outer wall of the cup; Figure 8 is a cross-sectional view of the can of Figure 6 that has been inverted and then placed on and over the open end of the cup of Figure 7 containing the negative electrode and the ribbed gasket in accordance with the present invention; Figure 9 is a cross-sectional view of the cell of Figure 8 in which the wall of the can has been crimped to provide a sealed cell; Figure 10 is a cross-sectional view of a zinc-air cell comprising a ribbed gasket in accordance with the present invention; and Figure 11 is a cross-sectional view of a zinc-air cell of the prior art. Figures 1 and 2 show a ribbed gasket 1 having an outer wall 3, a base 5 and an inner wall 7. The outer wall 3 and inner wall 7 form a groove 9. A pair of spaced apart rigid ribs 1 1 is moulded as part of a "J"-shaped circular contour of the gasket. The rigid ribs are disposed at 90° right angles. Figure 3 shows a ribbed gasket 31 having an outer wall 33, a base 35 and an inner wall 37. Three rigid ribs 39 are shown moulded as a part of a "J"-shaped circular gasket 31. The ribs 39 are spaced apart at 60° intervals. Figure 4 shows a ribbed gasket 41 having an outer wall 43, a base 45 and an inner wall 47. Two rigid ribs 49 are shown moulded as a part of a "J"-shaped circular contour. The ribs 49 are disposed so that each rib 49 is aligned parallel to the other rib. Figure 5 shows a ribbed gasket 51 having an outer wall 53, a base 55, and an inner wall 57. Four rigid ribs 59 are shown moulded as a part of a ".T-shaped circular contour. The ribs 59 are disposed so that they do not intersect the central axis of the gasket 51. Figure 6 shows a can 2 that comprises a base 10 having a peripheral upstanding wall 12 and, disposed in base 10, an opening 14 to permit air entry. The can 2 has patterned internal embossed sections 6, to provide a defined gap for uniform air distribution across the surface of an electrode 8 disposed within can 2. An electrode assembly containing a membrane 4, polytetrafluoroethylene layer 16 and electrode 8 containing a metal screen is secured within the can 2. Figure 7 shows a cup 20 with a negative electrode mixture 22, placed in and making electronic contact with the cup 20. A ribbed gasket 1 of an electrically insulating material as shown in Figure 1 is disposed onto the outer surface of the peripheral wall 26 of cup 20 and extends over edge 28 of wall 26 within groove 18 of gasket 1. A ribbed portion 11 is shown moulded as a unitary unit for the gasket 1. Figure 8 shows a can 2 as depicted in Figure 6 but inverted, containing an inserted electrode assembly as depicted in Figure 7. The cup 20 is preassembled with gasket 1 and contain negative electrode 22. The edge 28 of the wall of cup 20 is disposed within groove 18 of gasket 1 and forces the base of gasket 1 against electrode 8. Figure 9 shows the assembly of Figure 8, in which the edge or rim 30 of the can 2 has been crimped inwardly to compress the electrically insulating gasket 1 between the cup 20 and the can 2, thereby forming a seal and an electrical barrier between the can 2 and the cup 20. Figure 10 shows a zinc-air cell in accordance with the present invention. The can 2 is in electrical contact with electrode 8 and the cup 20 is in electrical contact with electrode 22 and thus the terminals of the cell are at opposite ends. Figure 11 shows a zinc-air cell of the prior art (using the components as identified in Figure 10) that illustrates the bowing 40 of an air electrode 8 at its central area using a conventional gasket 42.

Claims: CLAIMS: 1. An insulating gasket for a galvanic cell, adapted for disposal against the peripheral area of a flat electrode within the cell, the gasket comprising a ribbed portion extending between opposing segments of the gasket, wherein the ribbed portion is effective to prevent or reduce bowing of the central area of the flat electrode during or after assembly of the cell. 2. A gasket according to claim 1 wherein the ribbed portion is an integral part of the gaskat. 3. A gasket according to claim 1 wherein the ribbed portion is a separate part of the gasket. 4. A gasket according to any preceding claim wherein the ribbed portion comprises two intersecting ribs disposed 90° apart; three intersecting ribs disposed 60° apart; or two or more non-intersecting ribs. 5. A gasket according to any preceding claim wherein the gasket is made of a material selected from nylon, polytetrafluoroethylene, fluorinated ethylene-propylene, chlorotrifluoroethylene, perfluoro-alkoxy polymer, polyvinyls, polyethylene, polypropylene and polystyrene, preferably nylon. 6. A galvanic cell comprising a flat electrode and an insulating gasket disposed against the peripheral area of the flat electrode, wherein the gasket comprises a ribbed portion extending between opposing segments of the gasket, the ribbed portion being effective to prevent or reduce bowing of the central area of the flat electrode. 7 A galvanic cell according to claim 6 comprising a) a flat first electrode having a peripheral area, a central area and a polarity, b) a second electrode of opposite polarity, c) a separator between the first electrode and the second electrode, d) an electrolyte, e) a two-part conductive housing containing the first electrode, second electrode, separator and electrolyte, the first part of the housing being a can having a wall with an edge defining an opening and electrically connected to the first electrode, and the second part of the housing being a cup having a wall with an edge defining an opening and being electrically connected to the second electrode, and f) an insulating gasket comprising a base member having an outer wall spaced apart from an inner wall thereby defining a "U"-shaped groove, the base member further having a first segment at the inner wall, a second segment at the inner wall opposing the first segment and a ribbed portion comprising at least one insulating rigid rib extending inwardly from the first segment to the second segment; wherein the edge of the open end of the cup is disposed within and compressed into the groove of the gasket, the base member is compressed against the peripheral area of the first electrode and the rib is a support for the central area of the first electrode and thereby effectively reduces any bowing at the central area 8 A galvanic cell according to claim 6 or claim 7 wherein the flat electrode is the positive electrode. 9 A galvanic cell according to claim 8 wherein the positive electrode is an air electrode 10 A galvanic cell according to claim 9 wherein the negative electrode comprises zinc 11. A method of sealing a galvanic cell having a flat electrode and a two-part housing, comprising disposing an insulating gasket against the peripheral area of the flat electrode and compressing the gasket between the two parts of the housing to form an insulating seal, wherein the gasket comprises a ribbed portion extending between opposing segments of the gasket, the ribbed portion being effective to prevent or reduce bowing of the central area of the flat electrode during or after assembly of the cell. 12. A method according to claim 11, comprising the steps of: a) preparing a conductive cup having a peripheral wall with an edge defining an opening; b) preparing a conductive can having a peripheral wall with an edge defining an opening, and placing a first electrode having a peripheral area and a central area within the can; c) preparing an insulating gasket comprising a base member having an inner upstanding wall spaced-apart from an outer upstanding wall thereby defining a "U"- shaped groove, the base member also having a first segment at the inner wall, a second segment at the inner wall opposing the first segment and a ribbed portion comprising at least one insulating rigid rib extending inwardly from the first segment to the second segment; d) placing the edge of the wall of the cup within the groove of the gasket and placing a second electrode and electrolyte within the cup; e) assembling the cup of step d) within the can of step b) with the edge of the peripheral wall of the cup compressed against the base member of the gasket and the base member compressed against the peripheral area of the first electrode, and with the ribbed portion of the gasket providing a restraining force on the central area of the first electrode, thereby effectively reducing the bowing of the first electrode at its central area; and f) compressing the edge of the can into the outer wall of the gasket to effectively seal the can to the cup via the gasket and thereby electrically insulate the cup from the can. 13. A method according to claim 12 wherein the first electrode is the positive electrode and the second electrode is the negative electrode. 14. A method according to claim 13 wherein the positive electrode is an air electrode.

Inventors: GETZ ROSE A, GETZ, ROSE, A.
Applicants: EVEREADY BATTERY INC, EVEREADY BATTERY COMPANY, INC.
Application year: 1998
Main classification: H01M2
IPC codes: H01M2, H01M12