Electrochemical cell having a casing of mating portions

[0002] The present application claims priority based on provisional application Ser. No. 60/195,007, filed Apr. 6, 2000.

[0003] 1. Field of the Invention

[0004] The present invention relates to an electrochemical cell designed for improving energy density by forming a prismatic case of at least two stamped metal components mated to reduce cell size. More particularly, the present invention relates to an electrochemical cell housed in a casing of mated stamped metal halves that may have a recess or indentation provided in the body of at least one of the casing components.

[0005] 2. Prior Art

[0006] Lithium anode based electrochemical cells, such as silver vanadium oxide cells used in implantable medical device applications, are typically housed in cylindrical and prismatic casings. Both of these housing designs comprise two main components, a lid and a deep drawn case.

[0007] For example, the prior art reveals U.S. Pat. No. 6,004,692 to Muffoletto et al., assigned to the assignee of the present invention and incorporated herein by cell housed in a prismatic casing enclosure for lithium electrochemical cell. The cell includes a deep drawn conductive casing having an open end closed by a connected lid. The lid is provided with a terminal pin serving as a lead for making electrical connection to a load. The casing itself provides the other terminal for the cell. The generally prismatic or rectangular shape of the Muffoletto et al. casing makes it extremely difficult, if not impossible, to provide indentations or recesses in the casing side wall while simultaneously inserting a cell stack with the case in an efficient manner. Such recesses are useful for nesting components of the device being powered by the cell.

[0008] The use of casing halves constructed of resin materials is also known in the prior art. For example, U.S. Pat. No. 4,659,636 to Suzuki et al. relates to a sealed storage battery housed inside casing halves molded from a synthetic resin. A suitable synthetic material is a composition containing blended ABS and polycarbonate resins.

[0009] Also, dipping battery elements in liquid thermoplastic encapsulating materials has been described. For example, U.S. Pat. No. 5,270,133 to Baumann III describes a case-less battery wherein the electrode elements are housed in an encapsulating material. The electrode elements are encapsulated by dipping them into air or heat curable thermoplastic materials.

[0010] Also, the use of a sealing bag in a non-aqueous electrolyte battery is known. For example, U.S. Pat. No. 6,404,693 to Fukuda et al. describes a non-aqueous electrolyte cell having electrode elements house in a sealing bag whose peripheral parts are heat fused together. The sealing bag is formed from a film having a multilayered configuration including a metal layer (metal foil) and an electrolytic solution insulating barrier layer.

[0011] The problem with the prior art constructions is that synthetic materials are not suitable for providing a hermetic enclosure, especially for an electrochemical cell intended to power an implantable medical device. Also, a prismatic or cylindrically-shaped casing may not be acceptable in all instances, for example in batteries powering implantable medical devices where it is desirable to have an indentation in the body of the casing for nesting components of the medical device. What is needed is a new casing construction of stamped metal components which allows for greater flexibility in package shape than is offered by traditional designs. These battery cases permit the cell designs to be much more flexible in nature while allowing for extremely energy efficient designs.

[0012] Accordingly, the present invention is directed to an electrochemical cell having plate electrodes housed inside mating “clam shell” casing components. Also, the present casing design allows for indentation structures in the casing components by virtue of them being stamped metal parts. Depending on the design, some of these indentations are not possible with traditional cylindrical and prismatic casing designs. When mated together, the casing components are form-fitting with respect to the internal battery structure so as to reduce the overall size of the electrochemical package. This provides the cell with the greatest amount of energy per unit size.

[0013] The foregoing and additional advantages and characterizing features of the present invention will become clearly apparent upon reading of the ensuing description together with the included drawings wherein:

[0014] FIG. 1 is a perspective view showing an embodiment depicting two halves of a electrochemical cell casing.

[0015] FIG. 2 depicts an embodiment of the cell casing, the lower portion containing the electrode assembly.

[0016] FIGS. 3 and 4cdepict assembled embodiments of the electrochemical cell.

[0017] FIGS. 4aand 4bshow sectional elevations of methods of assembly of embodiments of the two stamped metal halves.

[0018] FIG. 5 depicts the welding detail of an embodiment of the clam shell halves peripheral mating seam.

[0019] FIGS. 6 through 10 depicts details of various embodiments of continuous peripheral mating seams for welding.

[0020] Turning now to the drawings, FIGS. 1 to 4aillustrate an electrochemical cell 10 according to the present invention. The cell 10 includes a conductive casing of metal, such as stainless steel, having first and second clam shell portions 12 and 14. As shown in FIG. 4, the clam shell portions 12 and 14 are mated together and sealed about their periphery to provide a hermetic enclosure for an electrode assembly 16 (FIGS. 2 and 4a).

[0021] In particular, the first clam shell 12 comprises spaced apart side walls 18 and 20 extending to and meeting with spaced apart end walls 22 and 24. The side walls 18, 20 and the end walls 22, 24 meet each other at rounded corners and extend to a front wall 26. Opposite the front wall 26 is an continuous edge 28 of side walls 18, 20 and end walls 22, 24.

[0022] End wall 24 has a greater length than end wall 22. The difference in length between these walls accommodates a plurality of step portions 30 and 32. In this manner, side wall 20 include a first portion 34 extending from end wall 22 and forming into an angled side wall portion 36 which meets a second portion 38 extending to end wall 24. Recessed below the edge 28 and partially bordered by the angled side wall 36 and the second portion 38 of the side wall 20 is the major surface of the first step 30 which in turn leads to the second step 32. The major surface of the second step 32 is spaced further from the edge 28 than the major surface of the first step 30. The second step 32 ends at the front wall 26. The stepped portion of the first clam shell 12 is completed by an extension portion 40 of the first portion 34 of side wall 20. The extension portion 40 has a shape similar to the steps 30 and 32 and ends at the front wall.

[0023] The second clam shell 14 comprises spaced apart side walls 49 and 50 extending to and connecting with spaced apart end walls 42 and 44. The side walls 49 and 50 and end walls 42 and 44 meet at rounded corners and extend to front wall 47. Opposite the front wall is a continuous edge 51 of the side walls 49 and 50 and end walls 42 and 44. End wall 44 has a greater length than end wall 42. In this manner, side wall 49 includes a first portion 55 extending from end wall 42 and forming into an angled side wall portion 54 which meets a second portion 50 of side wall 49 extending to the end wall 44.

[0024] In one preferred embodiment of the present invention shown in FIGS. 3, 4a, and 5, the first clam shell 12 is sized to fit inside the periphery of the second clam shell 14 in a closely spaced relationship. This means that side wall 18 is somewhat shorter than side wall 50, end wall 22 is somewhat shorter than side wall 42, side wall 20 is somewhat shorter than side wall 49 and end wall 24 is somewhat shorter than end wall 44. Also, the first and second portions 34 and 38 of side wall 20 are sized to be received inside the first and second portions 55 and 50 of side wall 49 with the intermediate angled side wall portion 36 of side wall 20 aligned with and received inside of the intermediate angled side wall portion 54 of side wall 49.

[0025] In the embodiment of the present invention shown in FIG. 3, the first clam shell 12 is received and nested inside the second clam shell 14 in a closely spaced relationship. This figure clearly shows the first step 30 and the second step 32 recessed into the front wall 26 of the first clam shell 12. In particular, the first step 30 is surrounded and defined by the angled side wall 36, the second portion 38 of side wall 20, an offset 43 leading to the second step 32 and the extension portion 40. Similarly, the second step 32 is defined by the second portion 38 of side wall 20, a back wall 37 extending from portion 38 and leading to the extension 40.

[0026] In the embodiment shown, the first and second steps 30 and 32 of the recesses are generally parallel to the major surface of front wall 26. However, this is not necessary to practice the present invention. Instead, the recess may have virtually any shape as long as it is confined to the body of the clam shell 12 including walls 18, 20, 22 and 24. In that respect, the walls 36, 38, 43 and 40 surrounding the first step 30, and the walls 38, 37, 38 and 40 surrounding the second step 32 need not be at a perpendicular angle with respect to the respective steps and the front wall 26 of the clam shell 12. Instead, they may be at an obtuse angle or a combination of perpendicular and obtuse angles.

[0027] Thus, the steps 30 and 32 are recessed into the front wall 26 of the first clam shell 12, bordered by walls on all sides. The steps 30 and 32 are provided in the front wall 26 of the first clam shell 12 for the purposed of received a component (not shown) of the device intended to be powered by the electrochemical cell housed inside the casing. The casing constructions known in the prior art do not provide for such recessed structure. Both the conventionally known cylindrical case and the prismatic case are deep drawn structures that do not allow for a recessed structure being provided therein without compromising the integrity of the deep drawn container. As those skilled in the art can also see other embodiments of the current invention can be practiced in the absence of recesses or indentations in one or both of the metal stamped mated stamped metal halves.

[0028] In that respect, the mating clam shells 12 and 14 of the present invention are stamped or otherwise formed from sheet metal. This construction technique readily allows the provision of recessed structures into the major front wall 26 of the first clam shell. However, it should be readily apparent to those skilled in the art that the shape, size and exact positioning of the recessed steps 26 and 28 in the first clam shell is by way of example only. In a broader sense, either one of the first and second clam shells 12 and 14 can have a plurality of recesses formed into their respective major front walls 26 and 47 any where within their surface area. This provides for greater flexibility in adapting a cell casing to a particular application. For example, if the cell is intended to power an implantable medical device and the manufacturer requires that a component of the device be nested within the major face of one of the clam shells 12 and 14, the recess structure of the present invention provides for this.

[0029] FIG. 5 depicts welding of clam shell halves 12 and 14 at peripheral seam 62 formed by mating continuous edges 28 and 51. Weld 63 is created by laser 61 from welding source 60.

[0030] In FIG. 6, peripheral seam 62 is created by positioning continuous peripheral edge 51 of clam shell half 14 with continuous peripheral edge 28 of shell half 12 in a coplanar arrangement for weld 63.

[0031] In FIG. 7, clam shell half 14 contains an outwardly extending offset or stepped portion 45 at the clam shell periphery near continuous edge 51. The offset distance is equal to the thickness of the walls 18, 20, 22 and 24 forming the continuous edge 28. During construction of the casing, clam shell half 12 is partially disposed inside clam shell half 14. Weld 63 is then provided at the seam 62 created by contacting the continuous edge 51 with the upper surface 59 of clam shell 12 to provide a continuous hermetic seal there.

[0032] In FIG. 8, clam shell half 14 contains an outwardly extending continuous offset or stepped portion 45 on the periphery near continuous edge 51 and clam shell half 12 contains an inwardly extending continuous offset or stepped portion 46 on the periphery near continuous edge 28. The offset distances are equal to one-half the thickness of the various walls forming the continuous edges 28 and 51. When the clam shells 12 and 14 are mated together, the oppositely extending stepped portions 45 and 46 result in clam shell half 12 being partially disposed inside clam shell half 14 in a non-coplanar arrangement. Weld 63 is then provided at the seam 62 created by contacting the continuous edge 51 with the upper surface 59 of clam shell 12 to provide continuous hermetic seal.

[0033] In FIG. 9, clam shell half 12 contains an inwardly extending offset or stepped portion 46 at the clam shell periphery near continuous edge 28. The offset distance is equal to the thickness of the walls 42, 44, 49 and 50 forming the continuous edge 51. During construction of the casing, clam shell half 12 is partially disposed inside of clam shell half 14. With this construction, the continuous edge 51 of clam shell 14 butts against clam shell 12 with weld 63 providing a hermetic seal at edge 51 with the upper surface 59 of shell 12.

[0034] In FIG. 10, peripheral seam 62 is created by lapping continuous edge 51 of clam shell 14 over continuous edge 28 of clam shell 12. In this embodiment, clam shell 12 is partially disposed inside clam shell 14 in a non-coplanar arrangement. Again, weld 63 provides a hermetic seal at the continuous seam 62 formed at the mating of edge 51 with upper surface 59 of shell 12.

[0035] The casing of the present invention is readily adapted for housing various types of electrochemical chemistries such as alkali metal/solid cathode or alkali metal/oxyhalide electrochemical cells of both the solid cathode and liquid cathode types. The exemplary electrochemical cell 16 housed in the casing of the present invention is of the liquid electrolyte type comprised of a cathode electrode having a body 33 of solid cathode material in the form of plate 39 pressed together and bonded against a cathode current collector. The cathode active material is preferably comprised of a metal, a metal oxide, a mixed metal oxide or a metal sulfide, and the cathode current collector is fabricated from a thin sheet of metal. Suitable materials for the current collector include nickel, aluminum, stainless steel, mild steel and titanium.

[0036] The benefit of having a lap joint construction to the mating clam shells 12 and 14 of the present casing (FIGS. 7 to 10) is to prevent the laser beam 61 from compromising the cell components housed inside the casing. With the construction shown in FIG. 6, it is possible for the laser beam welding the clam shells 12 and 14 together to penetrate past the junction of peripheral edge 51 to the electrode assembly housed therein. This could lead to compromise of the electrode assembly. For that reason, a backing ring (not shown) is typically used in welding the casing configuration of FIG. 6.

[0037] The embodiment depicted in FIGS. 2 and 4ashows an electrolyte fill opening 52 contained in side wall 42, a glass-to-metal seal 53, and a cathode terminal lead 70.

[0038] Cell 10 further includes an anode electrode, generally designated 71. The anode electrode comprises an anode active material, such as lithium pressed to the opposite sides of an anode current collector. The anode current collector is fabricated from a thin sheet of metal such as nickel.

[0039] The anode electrode 71 is in operative contact with the cathode plate 32 through a thin sheet of separator material 72. The separator surrounds and envelopes the cathode body 33 to prevent direct physical contact with the anode 71.

[0040] The terminal lead 70 connected to the cathode current collector extends through a header assembly comprising the glass-to-metal seal 53 fitted in the end wall 42 of clam shell 14. Lead 70 is the positive electrical terminal, being connected to the cathode electrode 33. The anode electrode 71 is in operative contact with the conductive casing through direct physical contact of the anode active material with the clam shells 12, 14. The cathode current collector is of titanium and terminal lead 70 is of molybdenum, separators 72 and 73 are of polypropylene. The activating electrolyte is a 1.0M to 1.4M solution of LiAsF₆or LiPF₆in a 50:50 mixture of, by volume, 1,2-dimethoxyethane and propylene carbonate. Glass seal 53 is of TA-23 Hermetic sealing glass, while the casing clam shells 12, 14 are of stainless steel.

[0041] This electrochemical system is of a primary cell type. However, those skilled in the art will readily recognize that the casing of the present invention is readily adopted to house both primary electrochemical systems of either a solid cathode or liquid catholyte type, or a secondary cell such as a lithium ion cell having a carbonaceous negative electrode and lithium cobalt oxide positive electrode.

[0042] It is not necessary that the casing have a rectangular cross section. For example, FIG. 4bshows an embodiment of a hexagonally shaped cross section. Although a hexagonal shape is shown, it can be appreciated by those skilled in the art, that a number of geometrical shapes can be used and a hexagon is shown for illustrative purposes only.

[0043] In this embodiment, clam shell 92 is coplanar with clam shell 94 about the horizontal axis so that shell 92 fits inside the periphery of shell 94. Thus, side walls 77 and 79 are somewhat shorter than side walls 76 and 78, and side walls 74 and 75 are somewhat shorter than side walls 82 and 83.

[0044] Furthermore, it will be readily apparent to those skilled in the art that the walls of the hexagonally shaped cross section shell halves terminate in equally spaced end walls defined by the junction of the side walls meeting at rounded edges. Specifically, clam shell half 94 is defined by walls 76 and 78, equal in length and emanating from an acute angle of 60°, each extending to and meeting walls 82 and 83 respectively, with rounded edges at an angle of 60°. Walls 82 and 83 are equal in length and extend from wall surfaces 85 and 84, respectively, up to continuous edge 51 formed by the junction of end walls 86 and 87 and side walls 82 and 83. Clam shell half 92 is defined by walls 77 and 79, equal in length and both emanating from an acute angle of 60°, each extending to and meeting with walls 74 and 75 respectively, with rounded edges at an angle of 60°. Walls 74 and 75 are equal in length and extend from wall surfaces 88 and 89 up to continuous edge 28 formed by the junction of side walls 74 and 75 and end walls 90 and 91 (FIG. 4c).

[0045] To manufacture the case it is necessary to appropriate the proper sheet thickness conductive metal and draw the individual halves by stamping or other such suitable means. The peripheral edges need to be trimmed, ensuring weld-ready continuous peripheral edges. The glass-metal seal with terminal lead 70 is installed in the clam shall 14 the lower drawn metal half. The electrode assembly is installed in the clam shell 14 which is then mated with the first clam shell 12. The edges of the mated clam shells 12, 14 form a continuous peripheral surface, which, as previously described with respect to FIGS. 6 to 10, is sealed by welding, creating a hermetically sealed electrochemical cell.

[0046] Use of the two stamped metal component configuration for the manufacture of the implantable grade lithium anode based electrochemical cells permits optimum utilization of available volume in an implantable medical device, yielding greater packaging efficiencies in smaller devices. The result is a highly efficient power source. Heretofore, the manufacturing process required many more steps to create a hermetically sealed cell capable of being implanted. The invention will enhance the art by decreasing manufacturing costs.

[0047] While preferred embodiments of the present invention have been disclosed, it will be appreciated that it is not limited thereto, but may be otherwise embodied with the scope of the following claims.