GB1580230A

GB1580230A - Electro chemical cells

Info

Publication number: GB1580230A
Application number: GB53614/77A
Authority: GB
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1977-01-03
Filing date: 1977-12-22
Publication date: 1980-11-26
Also published as: FR2376524A1; JPS5385347A; SE7800043L; DE2759142A1

Description

(54) IMPROVEMENTS IN ELECTRO CHEMICAL CELLS (71) We, GENERAL ELECTRIC COMPANY, a corporation organized and existing under the laws of the State of New York, United States of America, of I River Road, Schenectady 12345, State of New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to electrochemical cells.

There have been numerous attempts to design containers for small electrochemical cells and closures therefor which reliably provide a fluid and gas tight seal. Typically, such designs comprise cylindrical metallic can with an enlarged or bell mouthed open end into which a closure assembly is disposed. The assembly is seated on an annular shelf or abutment formed in the can wall at the inner end of the bell mouthed portion.

The closure assembly typically includes a central metal disc and an annular gasket circumferentially mounted around the disc. The outer diameter of the gasket is sized such that the closure assembly loosely fits in the open end of the container and may be assembled by dropping the closure in place or by pushing the closure into a slight friction fit between the outer wall of the gasket and the inner wall of the container.

As shown in U.S. Patent Nos. 3,069,489 and 3,068,313, one method for sealing such a design is to radially compress and reduce the diameter of the bell mouthed open end of the can around the closure assembly which results in the radial compression of the gasket between the can wall and the edge of the cover disc.

Another prior art sealing method for such a design is to radially and axially compress the gasket between the can wall including the seating shelf for the closure and the edge of the cover disc by rolling or crimping the open end of the can around and over the side and top surfaces of the gasket.

Another container/closure design differing slightly from the foregoing designs is shown in U.S. Patent No. 3,554,813. In this design, the closure gasket has an outer diameter which is slightly greater than the inner diameter of the can wall. To insert the closure into the can, the assembly is pushed by the plunger through a reducing die and into the can end. As in the foregoing designs, the closure assembly is seated on an annular shelf formed in the can wall. To complete the seal, the can end is crimped over and around the closure assembly to axially and radially compress the gasket between the can wall and cover disc.

All the foregoing designs require the formation of a shelf or abutment in the can wall and, therefore, requires the can end to be non-uniformly shaped relative to the remainder of the can wall. In addition to special tooling, such special shaping usually requires the can to be annealed to increase the ductility prior to shaping.

Further, it has also been found that, even after annealing, the shaping causes the can wall to be stressed and also may cause the can wall to be weakened by thinning and cracking of the shaped region of the can wall. This may cause "stress corrosion"; i.e., cause the can wall to become perforated and leak after extended exposure to the corrosive cell environment.

Another problem with seals of these types is that they all depend upon compressing the can wall around the closure assembly thereby to compress the gasket between the can wall and the edge of the central cover disc. The can wall, after compression, has been found to have a tendency to elastically spring back to a limited extent and diminish the compression of the gasket and the sealing force exerted by it on its sealing interfaces with the can wall and cover disc. Thus, if an adequate seal is to be formed, the gasket must be sufficiently resilient to expand outwardly and tightly conform to the expanded inner diameter of the can wall. For this reason such prior art gaskets must be constructed of high resiliency material.

Additionally, to insure that seal integrity is maintained over the life of the cell, other characteristics such as the resistance to creep and deterioration in the cell environment must be considered in choosing the gasket material.

In seal applications requiring extended exposure to high temperatures (e.g. between 55"C to 100 C) high integrity, long life seals have been difficult to obtain because the commonly used gasket material such as nylon, polytetrafluoroethylene (PTFE), and elastomers as ethylene propylene rubber with good resiliency and creep resistance properties generally exhibits poor performance under extended exposure to high temperature. Certain plastic materials such as polysulfone which have good high temperature properties have been found either to be difficult to use or unuseable in these prior art seals because of their brittleness and lack of resilience.

The present invention provides an electro chemical cell comprising (a) a metallic container comprising: (1) a tubular side wall having a substantially uniform inner diameter portion of diameter d,; (2) a retaining lip extending from one end of said wall inwardly of said side wall; and (3) an end wall extending from the second end of said wall and closing the second end of said container, and (b) a cover assembly comprising; (1) a cover member terminating in a circumferential edge; and (2) an annular gasket of a hard, cold flow resistant material circumferentially disposed around said edge, said gasket disposed inwardly of said lip and solely, radially, compressed between said edge and the inner surface of said side wall, said uniform diameter portion of said side wall extending from said lip through the interface with said gasket and through a region directly adjacent said interface; said gasket in an uncompressed state comprising an outer wall with a cylindrical section of outer diameter d2, d2 being greater than d1, and a tapered section terminating in an end of a diameter d3, d3 being less than d,.

The present invention also provides a method for sealing an electrochemical cell comprising the steps of (a) sealing a substantially uniform diameter cylindrical, cup-shaped container of inner diameter d, in a collet having an inner wall diameter slightly greater than the outer diameter of the container, said collet providing a rigid back-up for the mouth portion of said container.

(b) force fitting a cover assembly comprising (1) a metallic cover member, and (2) an annular gasket of a hard, cold flow resistant, material circumferentially disposed around said member, said gasket comprising an outer wall with a cylindrical section of outer diameter d2, d2 being greater than d1, and a tapered section terminating in an end of a diameter d3, d3 being less than d1, into the mouth of said container in said collet such that said gasket is disposed inwardly of said mouth and is solely radially compressed between said container and said cover member; and (c) reducing the diameter of said mouth portion to diameter less than the compressed outer diameter of said gasket.

Thus, after the disposition of the assembly in the container end, the end is rolled over or crimped in above the cover assembly to prevent expulsion of the assembly due to internal gas pressure which may build up in the cell during use.

The present invention will be further described by way of Example, with reference to the accompanying drawings in which: FIG. 1A is a fragmentary elevational view of an electrochemical cell sealed in accordance with the features of this invention.

FIG. I B is, an enlarged fragmentary view of a portion of the seal region of FIG.

IA.

FIGS. 2A, 2B and 2C are fragmentary views of an electrochemical cell showing the successive steps of being sealed in accordance with this invention and an exploded, fragmentary, cross-sectional view of a machine for performing the successive steps of assembling and sealing the closure in the cell container.

In FIGS. IA and 1B, a cell 10 is shown which is constructed in accordance with an exemplary embodiment of this invention. Cell 10 is comprised of (1) a can 11 including a tubular side wall 43, end wall 44 and lip 41, and (2) a cover assembly 13 which is frictionally mounted in the open end 15 of can 11. The cover assembly 13 is comprised of a metallic cover member 19 and an annular gasket 21 of a hard, creep-resistant, alkali-resistant plastic. The cover member 19 includes a conventional resealable vent assembly 23, which forms no part of this invention, and an outer disc-shaped rim 25, terminating in a circumferential edge.

The gasket 21 is coated on its external surfaces with a thin layer of tacky sealing material 18 for enhancing the seal at the interfaces 22, 24 between the gasket 21 and can 11 and between the gasket 21 and the circumferential edge of cover disc 25, respectively. The predominant leakage path is at interface 22 and therefore the material 18 may be omitted from interface 24. Sealing material 18 is applied to the gasket 21 and to the metal parts by spraying, dipping or other conventional means, and it need only be applied in an amount sufficient to fill the imperfections and crevices of the meal and/or gasket parts which are inherently produced in the parts during the manufacture thereof. Suitable sealing materials are asphalt and similar soft, tacky materials inert to the alkaline or acidic electrolyte of the cell.

In FIGS. 2A, 2B and 2C, fragmentary views are illustrated of a seal assembly machine 30 performing the successive steps for inserting and sealing a closure assembly 13 in the open end 15 of cell can 11 in accordance with this invention. The seal assembly machine 30 is comprised of a collet 31 for initially seating the can 11 in its unsealed condition. In this position, the closure assembly 13 is inserted into open end or mouth 15 of the cell 10 by being forced into the can mouth with an insert arbor 33. Arbor 33 has a cylindrical stepped portion 20 sized to apply the initial insertion force to rim 25 rather than to gasket 21. The collet 31 has a cylindrical mouth 35 with an inner diameter substantially equal to the outer can diameter and provides a rigid back-up for the cell wall as the cover assembly 13 is inserted into the can.

As shown in FIG. 2A, the gasket 21 in its uncompressed state has a tapered lower portion 29 and a larger, cylindrical upper portion 31. The portion 29 is sized such that the smallest diameter thereof is less than the inner diameter of can mouth 15 and progressively increases to diameter of the cylindrical portion 31. The cylindrical portion 31 is preferably 7 to 30% larger in diameter than the inner diameter of mouth 15.

As shown in FIG. 2B, the assembly 13 is force fitted into the can mouth until the outer surface 34 of gasket 21 is approximately coplanar with an upper surface 36 of collet 31. The extent of insertion of assembly 13 into the can 11 is controlled by a stop (not shown) operating in conjunction with the arbor 33 to limit the downward movement thereof. The insertion of assembly 13 flares can mouth 15 outwardly at the portion 37 which extends beyond the collet mouth 35. While the can 11 is in the collet 31, the portion of the can 11 which is circumscribed by the collet mouth 35 maintains its original diameter, and compression forces are stored in the gasket to provide a gas-tight seal. The sealing forces exerted by gasket 21 are solely in a radial direction due to the compression of the gasket between the parallel opposed edge of rim 25 and the wall of can 11. When inserted, the gasket 21 is compressed approximately 730% between the edge of rim 25 and the mutually opposed inner can wall.

In FIG. 2C, after insertion of the closure assembly 13 is completed, the plunger 33 is withdrawn and the seal is completed by spinning over the protruding and flared portion 37 of the can to form a retaining lip 41. Lip 41 functions to keep the closure assembly 13 from being displaced outwardly due to excess gas pressure which may build up during use of the cell 10. After portion 37 is spun over to form lip 41, the remaining cylindrical side wall 43 of can 11 has a uniform inner and outer diameter throughout.

The spinning tool is comprised of spinning wheels 39 and 40 and preferably a third spinning wheel which is behind the can wall and is not viewable in FIG. 2C.

The spinning wheels are preferably spaced 1200 apart around the can mouth. Each is rotatable about its own axis and the axis of the can, and is movable radially inwardly toward the can axis so as to be engageable with lip 41.

When the can 13 is removed from the collet 31, the gasket 21 forces the portion of the can wall adjacent interface 22 to spring elastically outward a slight amount. Prior to removal, this portion of the can wall is essentially unstressed.

Also, the region of the can wall below the gasket 21 is essentially unstressed because no shaping of the region is required to produce an effective seal. Because 7 this region is exposed to the corrosive cell environment, its unstressed condition is thought to provide enhanced cell wall integrity under extended life and high temperature conditions, i.e., enhanced resistance to "stress corrosion".

The portion of the can mouth 37 forming the lip 41 may be in a stressed condition due to the fact that it has been flared outwardly (FIG. 2B) and subsequently spun over (FIG. 2C). The potentially stressed condition. of lip 41 is of no consequence to seal integrity because it is above the sealing interface between the gasket 21 and can 11, and therefore does not contribute to the seal integrity.

The lip 41 retains the closure 13 inwardly of the can 11 in a plane perpendicular to the axis of can 11 and also tends to strengthen the can wall in the seal region prior to removal of the can 11 from collet 31 so that the compressed state of the gasket 21 is maintained. Thus, there is little, if any, outward bulging of the can wall upon removal of can 11 from collet 31 which would reduce the sealing force exerted by the gasket 21.

Examples.

To illustrate the practice of this invention with nickel-cadmium alkaline "Cs" cells with a potassium hydroxide electrolyte, the following experiment was conducted using the procedure described in connection with FIGS. 2A, 2B and 2C: 1. After the insertion of a standard battery roll in a steel can, .043 cm thick, the can was placed in a collet (FIG. 2A).

2. A cover assembly comprised of a cover plate (O.D.-1.96 cm) and an annular gasket (O.D.-2.4 cm, I.D.-19.6 cm) of polysulfone (Union Carbide product No. P-1700) was force fitted into the open can mouth (I.D.--2.11 cm.) until the outer disc-shaped rim of the cover plate was approximately .22 cm. below the outer edge of the can wall defining the can mouth (FIG. 2B). This resulted in the reduction of the gasket wall from .218 cm to .074 cm or 19% compression of the cylindrical gasket wall between the outer diameter of the cover plate and the inner diameter of the can mouth.

3. An outer portion approximately .165 cm was the spun over the outer edge of the closure assembly (FIG. 2C).

Following the foregoing procedure, six groups of 15 cells each were sealed with following design variations relating to whether the can was annealed prior to sealing and as to whether only one or both of the gasket and inner wall in the sealing region was coated with an asphalt coating prior to sealing. The six groups are identified in Table 1.

TABLE1.

Gasket Can Wall Group No. Annealed Can Asphalt Coated Asphalt Coated 1 No Yes No 2 Yes Yes No 3 No No Yes 4 Yes No Yes 5 No Yes Yes 6 Yes Yes Yes All cells of groups 1 to 6 were charged and then discharged.

Five cells from each group were than allowed to stand at room temperature (RT stand) for various periods of time and inspected for electrolyte leakage.

Leakage was determined by visual inspection for the presence of potassium carbonate.

The remaining 10 cells were subjected to a high temperature and seal integrity (HTSI test).

In this test each of the cells were: (1) Placed in 60"C oven for 24 hours; (2) Charged 72 hours at c/10 rate in 60"C oven; (3) Removed from oven and discharged at "C" rate to .6 volts; (4) Shorted 16 to 20 hours; (5) Charged at C/3 rate for 48 hours; (6) Discharged at 2C rate to 1.0 volt; and (7) Shorted 16 to 20 hours.

The cells were then visually inspected after the indicated time periods for the presence of potassium carbonate as a indication of electrolyte leakage.

The cells from Group 3 to 5 which passed the foregoing tests were then subjected to an additional high temperature exposure test (HTE) which consisted of (1) placing the cells in an oven, (2) subjecting the cells to a temperature of 120 F for 30 days, and (3) visually inspecting the cells for electrolyte leakage as before.

The cells were in an unchanged condition during this test. The purpose of this test was to further evaluate the (1) susceptibility of the can to stress corrosion cracking, and (2) the ability of the seal to remain leak-free after extended periods of high temperature exposure to the corrosive cell environment.

The results of the foregoing test are given in Table II.

TABLE II (Results are failures in each group)

RT Stand HTST HTE Group Formation 10 Days 77 Days Total % 0 Days 28 Days 66 Days Total % 30 Days 1 0 of 15 5 of 5 - 100 8 of 9*** 8 of 9 9 of 9 100 2 0 of 14* 3 of 4 4 of 4 100 7 of 10 7 of 10 10 of 10 100 3 0 of 15 0 of 5 0 of 5 0 2 of 10 2 of 10 2 of 10 20 0 of 5 4 0 of 15 0 of 5 0 of 5 0 0 of 10 0 of 10 0 of 10 0 0 of 4 5 0 of 15 0 of 5 0 of 5 0 0 of 10 0 of 10 0 of 10 0 0 of 5 6 0 of 15 0 of 5 0 of 5 0 0 of 4** 0 of 10 0 of 4 0 * 1 cell lost at closing ** 6 cells in this group failed to receive electrolyte during assembly *** 1 cell lost in formation From Table II, it can be seen that the practice of the invention as evidenced by Groups 4 to 6 provide a seal of high integrity. It can further be concluded that the conventional step of annealing the can wall at least in the area of the seal which is used in many prior art sealing approaches to increase the mallability of the can wall can be omitted without apparently any deleterious effects. This is thought to be due to the elimination of the need to form an annular shelf or bead in the can wall for supporting the closure assembly in the can mouth.

Another advantage realized by the invention and supported by the foregoing tests is the ability to form a tight seal using lower resiliency gasket materials such as polysulfone which have enhanced resistance to seal degradation in high temperature applications in contrast to materials such as rubber and nylon.

Claims

WHAT WE CLAIM IS:

1. An electrochemical cell comprising (a) a metallic container comprising: (1) a tubular side wall having a substantially uniform inner diameter portion of diameter dl; (2) a retaining lip extending from one end of said wall inwardly of said side wall; and (3) an end wall extending from the second end of said wall and closing the second end of said container, and (b) a cover assembly comprising: (1) a cover member terminating in a circumferential edge; and (2) an annular gasket of a hard, cold flow resistant material circumferentially disposed around said edge, said gasket disposed inwardly of said lip and solely radially compressed between said edge and the inner surface of said side wall, said uniform diameter portion of said side wall extending from said lip through the interface with said gasket and through a region directly adjacent said interface; said gasket in an uncompressed state comprising an outer wall with a cylindrical section of outer diameter d2, d2 being greater than d1, and a tapered section terminating in an end of a diameter d3, being less than d1.

2. A cell as claimed in Claim 1, wherein said material is polysulfone, nylon or ethylene propylene rubber.

3. A cell as claimed in Claim I or Claim 2, further comprising a soft material interposed between the gasket and said side wall.

4. A cell as claimed in Claim 3, wherein said material encapsulates said gasket.

5. A cell as claimed in Claim 4, wherein said material is asphalt.

6. A cell as claimed in any one of claims 1 to 5, wherein the can wall is substantially unstressed in the region of the can wall adjacent the sealing interface between the gasket and the wall.

7. A method for sealing an electrochemical cell comprising the steps of (a) seating a substantially uniform diameter cylindrical, cup-shaped container of inner diameter d, in a collet having an inner wall diameter slightly greater than the outer diameter of the container, said collet providing a rigid back-up for the mouth portion of said container, (b) force fitting a cover assembly comprising (1) a metallic cover member, and (2) an annular gasket of a hard, cold flow resistant material circumferentially disposed around said member, said gasket comprising an outer wall with a cylindrical section of outer diameter d2, d2 being greater than d1, and a tapered section terminating in an end of a diameter d3, d3 being less than dl, into the mouth of said container in said collet such that said gasket is disposed inwardly of said mouth and is solely radially compressed between said container and said cover member; and (c) reducing the diameter of said mouth portion to diameter less than the compressed outer diameter of said gasket.

8. A method as claimed in Claim 7, wherein said step of reducing is performed while said container is seated in said collet.

9. A method as claimed in Claim 7 or Claim 8, wherein said cover assembly is force fitted into said container by introducing first said tapered section and then said cylindrical section into said mouth.

10. A method as claimed in any one of Claims 7 to 9, wherein said gasket, during said force fitting step, is uncompressed prior to engagement with said container.

11. A method as claimed in any one of Claims 7 to 10, wherein said gasket is sized such that the gasket thickness between plate and container is compressed between 7% and 30% by said force fitting step.

12. An electrochemical cell substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

13. A method for sealing an electrochemical cell substantially as hereinbefore described with reference ot the accompanying drawings.

14. An electrochemical cell when sealed by a method as claimed in any one of claims 7 to Il and 13.