LOW PROFILE VENTABLE SEAL FOR AN ELECTROCHEMICAL CELL

Abstract: An electrochemical cell having a low profile seal assembly for sealing the open end of the cell container. The seal assembly includes a support member having a surface area that covers substantially the open end of the cell container and has a plurality of openings therein. A seal member is disposed against the support member to provide a sealed closure to the open end of the cell container. The seal member has a membrane with a groove formed in a surface thereof which acts as a stress concentrator. The groove is located against the support member such that said groove is located adjacent of the openings formed in the support member and is further supported by the support member at other locations.

Description: Low Profile Veritable Seal for an Electrochemical Cell The present invention generally relates to electrochemical cells and, more particularly, to a low profile seal assembly for sealing the open end of a cell container such that the seal vents when exposed to excessive pressure. Conventional alkaline electrochemical cells generally include a cylindrical can, normally made of steel, having a positive electrode, referred to as the cathode, which commonly comprises manganese dioxide as the active material. The electrochemical cell also includes a negative electrode, referred to as the anode, which commonly comprises zinc powder as the active material. The cathode is generally formed against the interior surface of the steel can, while the anode is generally centrally disposed in the can. Alternatively, in jellyroll cells, the anode and cathode are spirally wound. A separator is located between the anode and the cathode, and an alkaline electrolyte solution simultaneously contacts the anode, the cathode, and the separator. A conductive current collector is commonly inserted into the anode active material, and a seal assembly, which includes a seal member, provides closure to the open end of the cell's steel can to seal the active electrochemical materials in the sealed volume of the can. Cylindrical alkaline cells are commonly sealed closed by placing an annular nylon seal above a bead formed near the open end of the cell can and then crimping the upper end of the can inwardly and over the outer periphery of the seal to compress the seal against the bead. However, electrochemical cells employ electrochemically active materials, such as zinc, which generate hydrogen gas during storage under abusive conditions and sometimes during or following service use. When the container can is sealed, the build-up of high pressure gases within the sealed container may cause damage to the cell and/or to the device in which the cell is employed. One approach to avoiding a potentially excessive build-up of pressure in the cell container has been to employ a resealable valve system that periodically releases excessive gas pressure from within the active cell volume. However, the periodic and continuous release of gas pressure may. in some situations, permit electrolyte leakage containing salt and other particulate which may foul the resealable valve, and generally requires additional costly components Another approach to avoiding excessive build-up of pressure involves employing a sealed membrane that is intended to blowout when exposed to excessive pressure, either by puncture or by rupture of the membrane itself. Puncture mechanisms, such as a spiked member, may be employed to punch a hole in the thin membrane once the pressure reaches a predetermined amount. Alternately, a rupture mechanism may be employed in the form of a thin membrane which ruptures when the internal pressure of the cell becomes too great. One example of a thermoformed film membrane employed as a vent mechanism is disclosed in U.S. Patent No. 4,581,304, entitled "THERMOFORMED FILM MEMBER VENT FOR GALVANIC CELLS". The aforementioned patent discloses the use of a thermoformed film member retained across a vent aperture located in the inner cover of the electrochemical cell such that the thermoformed film member is intended to rupture at high pressure to provide a vent passage from the sealed internal volume to the surrounding atmosphere. Other approaches to venting excessive cell pressure have included the use of a vent formed in the seal of the battery which is intended to rupture upon experiencing an excessive pressure build-up in the cell. For example, U.S. Patent No. 5,080,985 discloses a groove formed in both the top and bottom surfaces of a plastic grommet seal such that the groove is designed to shear open at very high pressure. While the pπor approaches for venting high pressure gas from the cell have resulted in the ability to vent excessive pressure, many of the pπor approaches having not optimised the volume consumed by the seal member, while other approaches lack an accurate rupture pressure mechanism. According to the present invention we now provide an electrochemical cell having a pressure release mechanism which occupies a minimum amount of cell volume. Such a pressure release mechanism may also effectively vent gas when exposed to an expected rupture pressure. The present invention improves the protective safeguards of an electrochemical cell with an enhanced low profile seal assembly for sealing the open end of the electrochemical cell's container to provide controlled pressure venting. To achieve this and other advantages, the present invention provides for a seal assembly for sealing the open end of an electrochemical cell container, an electrochemical cell having a low profile seal assembly, and a method of assembling an electrochemical cell having a low profile seal assembly. The seal assembly comprises a seal member disposed in the open end of the cell's container to provide a sealed closure to the open end of the container. The seal member has a stress concentrator including a groove formed in a surface of the seal member. In addition, the seal assembly has a support member disposed in the open end of the container and against the seal member. The support member has a surface area that covers substantially the open end of a cell container and has at least one opening formed therein. The groove formed in the seal member is located against the support member so that the groove is located adjacent to at least one opening in the support member and is located against the support member at other locations. The low profile seal assembly occupies a minimal amount of cell volume and serves as an accurate pressure relief mechanism. The electrochemical cell of the present invention may be of any shape or form, as is well known in the art. However, the majority of cells used today are of cylindrical shape, and this is the preferred shape of the cells of the present invention. Accordingly, the invention is described herein with reference to cylindrical cells. However, it will be appreciated that the invention can be applied, mutatis mutandis, to cells of other shapes and configurations. The can containing the active components of the cell is preferably made of steel. The cathode is preferably formed of a mixture of manganese dioxide, graphite, an aqueous solution of potassium hydroxide (whose concentration may vary as is well known in the art, but which is preferably about 45% w/v), and additives. This cathode is normally formed about the interior surface of the can. A separator, which may be of any material known in the art, but which is preferably formed of a non-woven fabric, is disposed about the interior surface of the cathode. The anode may be of any composition known in the art, and is preferably formed of zinc powder, a gelling agent and additives. The anode is disposed with an electrolyte inside the separator and is in contact with a current collector, for example a brass nail. The seal assembly preferably comprises an annular seal member for sealing the contents of the can against leakage, and a seal support for supporting the seal member. The seal support is preferably made of a rigid material, such as steel. The seal support includes one or more vent openings, which are preferably arranged annularly and are adjacent the groove in the surface of the seal member. The seal member preferably has a circular stress concentration groove formed in its top surface, which acts as a pressure release mechanism when exposed to an excessive pressure differential. The seal member directly abuts the seal support, and the stress concentration groove or grooves is or are located adjacent to vent openings in the seal support, as well, preferably, as adjacent non-vented portions of the seal support. The support member is preferably configured in the shape of a disk having one or more, for example two or three, vent openings, which, where there are more than one, are preferably equi -angularly spaced from each other. Each of the vent openings is preferably formed in the shape of a kidney bean having an effective arcuate opening length adjacent the stress concentration groove. The vent opening thus provides an open passageway adjacent to the stress concentration groove, such that excessive pressure build-up from within the cell will concentrate stress on the groove. When sufficient excessive pressure is present, the groove will rupture, i.e., shear, near the centre of the vent opening to release pressure to the outside atmosphere. The non-venting portions of the seal support provide structural support to the seal, such that the seal is supported both on the radially outward and radially inward edges of the vent openings, as well as at certain locations adjacent to the stress concentration groove. The stress concentration groove is preferably centrally located below the vent opening in the seal support, midway between the radially outward and inward edges of the vent opening. The thickness of the material of the seal member at the groove is preferably about one half the thickness of the material of the seal member immediately about the groove. The width of the groove is preferably from 0.2 to 1.2 times the thickness of the material of the seal member at the groove The groove will be designed to rupture at a pressure that will vary, as is well known in the art, according to the type and size of electrochemical cell. If desired, the seal member, or a part of the seal member, may be coated with a mateπal, such as asphalt, which will resist chemical attack of the cell contents on the seal member. The seal member preferably includes a membrane located adjacent said groove. The membrane preferably has a ratio of thickness to width of the opening in said support member in the range of from 0.1 to 0.2 The present invention is further lllustiated by the accompanying drawings, in which: FIG 1 is a longitudinal cross-sectional view of an electrochemical cell having a seal assembly including a stress concentration groove integrally formed in a seal member according to the present invention; FIG. 2 is a top view of the seal assembly according to the present invention; FIG. 3 is an exploded elevational view of the seal assembly of FIG. 2; and FIG. 4 is an enlarged sectional view of the seal assembly taken from section IV in FIG. 1 which further illustrates the groove formed in the seal member. Referπng to FIG. 1, a cylindrical alkaline electrochemical cell 10 is shown therein. Electrochemical cell 10 includes a cyhndπcal steel can 12 having a closed bottom end 14 and an open top end 16. The closed bottom end of can 12 further includes a positive cover welded or otherwise attached thereto and formed of plated steel with a protruding nub 18 at its centre region which forms the positive contact terminal of cell 10. Assembled to the open end 16 of steel can 12 is a cover and seal assembly with an outer cover 30 which forms the negative contact terminal of cell 10. A metallized, plastic film label 20 is formed about the exteπor surface of steel can 12, except for the ends of steel can 12. Film label 20 is formed over the peπpheral edge of the positive cover. A cathode, preferably formed of a mixture of manganese dioxide, graphite, forty- five percent potassium hydroxide solution and additives, is formed about the interior surface of steel can 12. A separator 24, preferably formed of a non-woven fabric that prevents migration of any solid particles in the cell, is disposed about the interior surface of cathode 22. An anode 26, preferably formed of zinc powder, a gelling agent and additives, is disposed with an electrolyte inside the separator 24 and is in contact with a current collector 28, which may include a brass nail. Accordingly, the cathode 22 is configured as the cell's positive electrode, and the anode 26 is configured as the cell's negative electrode. The current collector 28 contacts the outer negative cover 30 which forms the negative contact terminal of cell 10. The outer negative cover 30 is preferably formed of plated steel, and may be held in contact with current collector 28 via pressure contact or a weld. An annular seal member 32 is disposed in the open end of the steel can 12 to prevent leakage of the electrochemically active cell materials contained in steel can 12. Seal member 32 directly contacts a support member 34 which forms an inner cell cover and is preferably formed of steel. Together, the seal member 32 and support member 34 form a seal assembly for sealing closed the active electrochemical materials inside of can 12. The assembling of the seal assembly includes disposing the seal member 32 on top of a bead 15 formed radially inward on the inner wall of the can 10 and reducing the diameter of the can 12 above the bead 15 such that the seal member 32 is compressed against the support member34. The upper end of the can 10 is crimped inwardly and over the outer periphery of the seal 32 to compress it against the bead 15. It should also be appreciated that the outer negative cover 30 is electrically insulated from the steel can 12 by way of seal member 32. The support member 34 serves as an inner cover to rigidly support seal member 32, and includes one or more vent openings, i.e., apertures, 36 formed through the surface thereof. The outer negative cover 30 also includes one or more vent openings 38 that serve to expose the cell's non-active volume, which is defined herein as the volume between seal member 32 and outer cover 30, to the outside atmosphere. Vent openings 36 and 38 serve to vent pressure build-up from within the cell 10 to the outside atmosphere. Together, the outer cover 30, the current collector 28, seal member 32, and inner support member 34 form the cover and seal assembly that is inserted into the open end 16 of steel can 12 to seal the active ingredients within an active cell volume. According to the present invention, the seal member 32 has a circular stress concentration groove 40 formed in the top surface thereof which acts as a pressure release mechanism when exposed to an excessive pressure differential. The seal member 32 directly abuts the support member 34 such that the stress concentration groove 40 is located adjacent to vent openings 36 as well as non-vented portions of support member 34. Referring particularly to FIGS. 2 and 3, the support member 34 is configured in the shape of a disk having three vent openings 36 which are equi-angularly spaced from each other. Each of vent openings 36 in support member 34 is formed in the shape of a kidney bean having an effective arcuate opening length adjacent to groove 40 as indicated by reference letter L. According to the configuration shown, the vent opening 36 provides an open passageway adjacent to groove 40 such that excessive pressure build-up from within the cell 10 will concentrate stress on groove 40. When sufficient excessive pressure is present, groove 40 will rupture, i.e., shear, near the centre of the vent opening 36 to release pressure to the outside atmosphere. The non-venting portions of support member 34 provide structural support to seal member 32 such that seal member 32 is supported both on the radially outward and radially inward edges of opening 36, as well as at certain locations adjacent to groove 40. The groove 40 is preferably centrally located below opening 36 midway between the radially outward and inward edges of opening 36. With particular reference to FIG. 3, the seal member 32 is shown having a boss 46 formed at the centre thereof and vertical extending walls 42 formed at the outer peripheral edge. Seal member 32 has a disk-like membrane 44 formed between boss 46 and outer peripheral walls 42. Groove 40 is located in membrane 44. The support member 34 sits directly against the top surface of the disk-like membrane 44 of seal member 32 and includes a central opening 48 formed therein for receiving boss 46. The current collector 28 extends through opening 48 and boss 46 which provides a sealing engagement between boss 46 and current collector 28. Referπng to FIG 4, an enlarged view of stress concentration groove 40 is shown directly abutting the vent passage 36 in support member 34 Groove 40 preferably includes either one or two abrupt corners formed between adjacent walls According to the embodiment shown, groove 40 is configured as an annular channel that is rectangular cross section with πght angles formed at the base comers thereof by vertical walls joining a flat hoπzontal bottom wall By providing a shaφ, e.g., ninety degree, comer, the groove 40 effectively acts as a stress concentrator to provide a clean shear at the comer of groove 40 upon expeπencmg a predetermined pressure differential Groove 40 has a width identified by W g , while the vent opening 36 formed in support member 34 has a width identified as W c The groove 40 has a thickness T g that is preferably approximately one-half the thickness T s of the immediately surrounding seal mateπal. The groove 40 preferably has a width W g to thickness T g ratio (W g /T g ) in the range of 0.2-1.2. The \ent opening 36 preferably has an arcuate length L to width W c ratio (LAV C ) equal to or greater than 1.0. In addition, the ratio of thickness T s of the disk-like membrane 44 to the width W c of vent opening 36 is preferably in the range of 0.1-0.2 The groove 40 is suitable to rupture when subjected to a pressure differential of approximately 1500 (psi) pounds-per- square inch for an AAA-size cell and up to approximately 1000 psi for an AA-size cell According to one embodiment, seal member 32 is made of a nylon, such as ZYTEL® 101F which is commercially available from E.I. duPont de Nemours and Co., Inc. Seal member 32 can be integrally formed to include groove 40 by using a conventional injection moulding process. It should be appreciated that while nylon is a preferred mateπal, other polymers or other seal mateπals could be used In addition, the bottom surfaces of seal member 32 may be coated with a layer of asphalt 35 to prevent chemical degradation of the seal member 32 due to the presence of electrolyte. Accordingly, the present invention provides for a seal assembly having a seal member 32 directly abutting a support member 34, with the seal member 32 having a stress concentration groove 40 formed therein and positioned in relation to openings 36 in said support member 34 to provide an accurate ruptuπng pressure release mechanism that will rupture when subjected to a predetermined pressure differential addition, the seal assembly consumes less volume, and thereby allows for the employment of a greater quantity of active electrochemical cell mateπals It will be understood by those who practice the invention and those skilled in the art, that vaπous modifications and improvements may be made to the invention without departing from the spiπt of the disclosed concept The scope of protection afforded is to be determined by the claims and by the breadth of inteφretation allowed by law.

Claims: CLAIMS: 1. A seal assembly for sealing the open end of an electrochemical cell container, said seal assembly comprising: a seal member for locating in said open end of said container to provide a sealed closure to the open end of the container, said seal member having a stress concentrator comprising a groove formed in a surface of said seal member; and a support member for locating in said open end of said container and against the seal member, said support member having a surface area that covers a substantial portion of the open end of the container and further having at least one opening formed therein, wherein said groove is located against said support member such that said groove is located adjacent to said at least one opening. 2. A seal assembly according to claim 1, wherein said support member supports said seal on a radially inward edge of said opening and a radially outward edge of said opening. 3. A seal assembly according to claim 2, wherein said support member further provides support directly against portions of said groove. 4. A seal assembly according to any one of claims 1 to 3, wherein said groove is centrally located within said opening. 5. A seal assembly according to any proceeding claim, wherein said support member comprises a circular inner cover. 6. A seal assembly according to any proceeding claim, wherein said seal member provides a seal for a substantially circular opening, and said groove comprises a substantially circular groove. 7. A seal assembly according to any proceeding claim, wherein said seal member comprises nylon. 8. The seal assembly according to any proceeding claim, wherein said support member comprises metal. 9. A seal assembly according to any preceding claim, wherein said groove includes a comer formed between two adjacent walls. 10. A seal assembly according to any proceeding claim, wherein said groove has a ratio of width to thickness in the range of from 0.2 to 1.2. 11. A seal assembly according to any preceding claim, wherein said opening formed in the support member has a ratio of length to width equal to or greater than 1.0. 12. A seal assembly according to any preceding claim, wherein said seal member includes a membrane located adjacent said groove. 13. A seal assembly according to claim 12, wherein said membrane has a ratio of thickness to width of the opening in said support member in the range of from 0.1 to 0.2. 14. An electrochemical cell sealed by a seal member according to any one of the preceding claims. 15. An electrochemical cell comprising: a container having a closed bottom end and an open top end; a positive electrode disposed in said container; a negative electrode disposed in said container; a separator disposed between said positive electrode and said negative electrode; an electrolyte disposed in said container; a seal member disposed in the open top end of said container for closing said open top end of said container, said seal member having a stress concentrator including a groove formed in a surface thereof; and a support member disposed in said open end of said container and against a surface of said seal member for providing structural support thereagainst, said support member having a surface area that covers substantially the open end of the container and further having at least one opening formed therein at a location adjacent to said groove formed in said seal member. 16. An electrochemical cell according to claim 15, wherein said support member supports said seal member on a radially inward edge of said opening and a radially outward edge of said opening. 17. An electrochemical cell according to claim 15 or claim 16, wherein said support member further provides radial support directly against portions of said groove. 18. An electrochemical cell according to any one of claims 15 to 17, wherein said groove is centrally located within said opening. 19. An electrochemical cell according to any one of claims 15 to 18, wherein said seal member provides a seal for a substantially circular opening, and said groove comprises a substantially circular groove. 20. An electrochemical cell according to any one of claims 15 to 19, wherein said seal member comprises nylon. 21. An electrochemical cell according to any one of claims 15 to 20, wherein said support member comprises metal. 22. An electrochemical cell according to any one of claims 15 to 21, wherein said groove includes a co er formed between two adjacent walls. 23. An electrochemical cell according to any one of claims 15 to 22, wherein said groove has a ratio of width to thickness in the range of from 0.2 to 1.2. 24. An electrochemical cell according to any one of claims 15 to 23, wherein said opening formed in the support member has a ratio of length to width equal to or greater than 1.0. 25. An electrochemical cell according to any one of claims 15 to 24, wherein said seal member includes a membrane located adjacent said groove. 26. An electrochemical cell according to any one of claims 15 to 25, wherein said membrane has a ratio of thickness to width of said opening in said support member in the range of from 0.1 to 0.2. 27. A method of assembling an electrochemical cell comprising the steps of: providing a container having a closed bottom end and an open top end; disposing a positive electrode in said container; disposing a negative electrode in said container; disposing a separator between said positive electrode and said negative electrode; disposing an electrolyte in said container; providing a seal member having a membrane with a stress concentrator including a groove formed in a surface thereof; disposing said seal member in the open top end of said container for closing said open top end of said container; and disposing a support member in said open top end of said container and against a surface of said seal member for providing structural support thereagainst, said support member having a surface area that covers substantially the open end of the container and further having at least one opening formed therein at a location adjacent to said groove formed in said seal member. 28. A method according to claim 27 further comprising the step of crimping a peripheral edge of said container at said open top end onto an outer cover located above said support member. 29. A method according to claim 27 further comprising the step of disposing a collector through an aperture formed in said seal member, said seal member forming a seal against said collector.

Inventors: WU XIXIAN, WU, XIXIAN
Applicants: EVEREADY BATTERY INC, EVEREADY BATTERY COMPANY, INC.
Application year: 2000
Main classification: H01M2
IPC codes: H01M2, H01M6