Battery case, cover, and feedthrough

[0002] This application is a Continuation-in-Part of U.S. application Ser. No. 09/421,171 filed on Oct. 19, 1999, now U.S. Pat. No. 6,554,178 which is a Divisional of U.S. application Ser. No. 09/290,012 filed on Apr. 8, 1999, now abandoned.

[0003] Not Applicable

[0004] Not Applicable

[0005] 1. Technical Field

[0006] This invention relates to a battery case, cover, and feedthrough and more particularly to a method and apparatus for providing a sealed battery having a lightweight case.

[0007] 2. Background Art

[0008] Battery cases typically require a conductive pin that feeds into the case through a dielectric material that insulates the pin from the conductive case wall. It is generally critical that the feedthrough be hermetically sealed.

[0009] Techniques are known for forming a hermetic seal between a metal pin of high melt point material and a ceramic cylinder surrounding the pin. Moreover, an annular metal collar of high melt point material can be hermetically sealed around the ceramic cylinder.

[0010] It is often desirable to be able to form a battery case of lightweight low cost material, such as aluminum. Aluminum can be deep drawn and has a density of 2.7 g/cm³as compared with the density of stainless steel of 7.8 g/cm³. Unfortunately, aluminum melts at a temperature lower than the temperatures normally required to fabricate and install the aforedescribed ceramic-metal feedthrough structure in the case. For example, the fabrication of the aforedescribed structure typically requires the application of a temperature of about 800° C. or more to form a hermetic seal between two high melt point metals and/or between a high melt point metal and a ceramic cylinder. Such an elevated temperature could destroy a case formed of aluminum, which melts just above 600° C.

[0011] The present invention is directed to an improved case/feedthrough construction enabling the use of a low melt point case material, e.g., aluminum, in conjunction with a ceramic-metal feedthrough subassembly that requires the application of a high temperature to form a hermetic seal.

[0012] In accordance with the invention, a case cover comprising a layer of high melt point material and a layer of low melt point material is used to respectively seal to a high melt point collar on the feedthrough subassembly and a low melt point case.

[0013] More particularly, in accordance with a preferred embodiment, a case cover is formed of clad metal comprised of a layer of high melt point material, e.g., stainless steel or titanium, and a layer of low melt point material, e.g., aluminum. A feedthrough hole is formed through the cover. The feedthrough subassembly annular collar is aligned with the hole and hermetically sealed to the clad metal high melt point layer using a high temperature welding process.

[0014] The clad metal low melt point layer, e.g., aluminum, can be laser welded to the aluminum case at a lower temperature to form a hermetic seal.

[0015] In another embodiment, the feedthrough does not have an annular metal collar, and both layers of cladding can withstand the temperatures required for sealing the dielectric material, e.g. glass, to the cover.

[0016] FIG. 1 is an exploded sectional view of a feedthrough subassembly, a battery case, and a case cover in accordance with the invention formed of clad metal;

[0017] FIG. 2 is a sectional view similar to FIG. 1 but showing the feedthrough subassembly, case, and case cover assembled together;

[0018] FIGS. 3A-C show the steps for making a case cover and sealing a battery case in accordance with one embodiment of the invention;

[0019] FIGS. 4A and 4B are sectional views of an alternative embodiment of the present invention;

[0020] FIGS. 5A and 5B are sectional views of an alternative embodiment of the present invention;

[0021] FIG. 6 is a sectional view of an alternative embodiment of the present invention; and

[0022] FIG. 7 is a sectional view of another alternative embodiment of the present invention.

[0023] The following text describes the preferred mode presently contemplated for carrying out the invention and is not intended to describe all possible modifications and variations consistent with the spirit and purpose of the invention. The scope of the invention should be determined with reference to the claims.

[0024] FIG. 1 depicts a feedthrough subassembly 10 comprising a feedthrough metal pin 1 penetrating a cylindrical ceramic glass core 2. A hermetic seal can be formed between the ceramic and metal by proper application of processes and materials.

[0025] Ceramic to stainless and ceramic to titanium are particular examples of materials that can be bonded to form hermetic seals. Braze bonding, for example, aluminum oxide and zirconium oxide ceramic to metals, including titanium, stainless steel, molybdenum, tantalum, and cobalt-chromium alloys, can be done using a braze, comprising 30% nickel and 70% titanium (U.S. Provisional Patent Application Serial No. 60/126,531; PCT Application WO00/56677). Another example of a braze bond includes the preferred method for joining zirconium oxide containing 3% yttrium to preferably a metal alloy, namely, titanium and niobium (55% Ti and 45% Nb), using the nickel-titanium braze (50% Ni and 50% Ti) (U.S. Provisional Patent Application Serial No. 60/126,514; PCT Application WO00/56395).

[0026] Other ceramics, such as CaBAl 12 glass, can be hermetically bonded to high melting temperature metals, such as PtIr or Ti-6Al-4V, by a baking process without the addition of a brazing material, as is well known in the art.

[0027] Thus, by selecting a pin 1 of stainless steel or titanium or one of the other aforementioned metals and a ceramic core 2 of aluminum oxide, zirconium oxide, CaBAl 12 glass, or other suitable ceramic, the pin 1 and core 2 can be hermetically bonded utilizing a high temperature brazing or baking process. Similarly, an annular collar 3 of stainless steel or titanium or one of the other aforementioned metals can be hermetically sealed around the core 2.

[0028] The feedthrough subassembly 10 is used in combination with a battery case 28 comprising a case body 30 defining an interior cavity 32 and a case cover 20 for sealing the cavity. The subassembly 10 is mounted in the cover 20 for providing an electrical connection into and out of the cavity 32. In accordance with the invention, the case cover 20 is formed of clad metal comprising a first layer 4 of a high melt point material, e.g., stainless steel or titanium, and a second layer 5 of a low melt point material, e.g., aluminum.

[0029] There are different methods for forming clad metal, such as a plate of stainless steel clad with aluminum. In one method (U.S. Pat. No. 4,213,558, Hirobe, et al.), sheet metal cladding is fed onto one or both faces of the hot solidified continuous cast strip being withdrawn from the casting means and the assembly is then passed to a hot roll bonding system for cladding. Another method (U.S. Pat. No. 4,966,748, Miyasaka et al.) produces a clad metal by forming a layer of the dissimilar metal powder on the surface of a base metal by cold fixing the powder to the surface under pressure, denting only the surface and a subsurface area of the layer of the dissimilar metal powder by melting and immediately solidifying in a vacuum, compressing the layer of the dissimilar metal powder together with the base metal at a temperature not higher than the solidus-line temperature of the two dissimilar metals under a pressure of not lower than 300 kgf/cm²using a hot isostatic press, and hot working the layer of the dissimilar metal powder together with the base metal. Explosion bonding is a method suited to cladding thick layers of metal such as might be used in 100-Ah cells. Roll cladding is preferred for making case covers up to 3 mm thick.

[0030] The case cover 20 defines a feedthrough mounting hole 6 of smaller diameter through layer 4 and larger diameter through layer 5. The feedthrough subassembly 10 is aligned with the hole 6 to place the annular collar 3 contiguous with the upper surface of case cover layer 4. Inasmuch as these contiguous materials are both formed of high melt point materials, e.g., stainless steel or titanium, they can be welded together using a high temperature process to form a hermetic seal. This high temperature is in excess of the melting point of the low melt point case material, e.g., aluminum. In accordance with the invention, the case cover lower layer 5 is formed of a low melt point material, e.g., aluminum, that can then be welded to the aluminum case body material 7 using a lower temperature to form a hermetic seal.

[0031] If the clad metal were not used, the high temperature required to hermetically bond the feedthrough pin subassembly to the case would require use of a high melt point case cover. However, in trying to bond the high melt point cover to the aluminum case, the welding or brazing temperature would melt the aluminum. Exemplary melting point temperatures of the various metals are aluminum 660.37° C., stainless steel-303, 1427° C., and titanium 1660° C. In FIG. 2, low temperature hermetic welding produces a low temperature weld shown at 11. High temperature welding produces a high temperature weld shown at 13.

[0032] FIGS. 3A-3C show the steps for making a three-layer case cover and sealing a battery case. FIG. 3A shows a clad sheet 40 comprising a low melting point strip 42 having a top layer 44 and a bottom layer 46 of high melting point material. The sheet is preferably no more than 3 mm (0.12 inches) thick, and more preferably no more than 2 mm (0.08 inches) thick. The low melting point strip 42 preferably comprises aluminum and is preferably 0.5 to 2 mm (0.02 to 0.08 inches), and more preferably about 0.89 mm (0.035 inches) thick. The top and bottom layers of high melting point material preferably comprise stainless steel, titanium, or a titanium alloy, or a combination thereof. As used herein, “titanium” generally refers to titanium or an alloy thereof. Each layer is preferably 0.25 to 0.76 mm (0.01 to 0.03 inches), and more preferably about 0.5 mm (0.02 inches) thick. Using stainless, titanium, or titanium alloy on both faces of an aluminum sheet helps keep the clad material flat both during the cladding process and during welding. FIG. 3B is a cross sectional view of the clad case cover 50 comprising clad sheet 40 having two feedthrough holes 52, 54 and having the bottom edge 56 machined away to expose the material of the low melting point strip 42 for contacting the case and welding to it. The feedthrough holes are preferably stepped for ease of assembly. FIG. 3C is an exploded view of feedthrough subassemblies 62 and 64, a clad case cover 50, and a battery case body 30. As shown in FIG. 3C, a shoulder 66, 68 of each feedthrough assembly 62, 64 is conveniently supported by the step 53, 55 of each hole 52, 54, during welding of the feedthrough to the top layer 44 of high melting point material of the cover 50. The feedthrough subassemblies 62, 64 preferably comprise feedthrough pins 72, 74 made of PtIr or Ti-6Al-4V hermetically sealed in cylinders 76, 78 of ceramic such as CaBAl 12 glass, which are preferably hermetically sealed within annular rings 82, 84 of high melting point material. The high melting point material of the annular rings 82, 84 is welded to the high melting point material of the cover 50. The low melting point strip 42 is welded, preferably laser welded, to the battery case body 30, which comprises a low melting point material. Bottom edge 56 is machined such that top of case body 30 occupies the space of the material that was machined away, such that the outer lower edge of cover 50 is flush with the outer edge of case body 30, and the laser is directed at the junction of the cover at strip 42 and the case body 30. This configuration ensures that the metal at surface 57 of strip 42 or 46 will absorb any energy not going into forming the weld, preventing burning through to inner components such as the separator of the battery electrode assembly. The embodiment of FIGS. 3A-3C is advantageous for those applications in which high energy density is important, since the configuration is very space efficient.

[0033] FIGS. 4A and 4B are sectional views of an alternative embodiment of the present invention. Cover 90 comprises a layer of high melting point material 92 clad to a layer of low melting point material 94. The top layer of material at the edge 93 may optionally be machined away so that the high temperature material 92 does not act as a heat sink or otherwise affect the subsequent welding of the low melt temperature layer 94. Then, the cover 90 may be stamped, punched, or otherwise formed as in FIG. 4B so that the perimeter of the cover is bent 90° from the main body of the cover. This allows the low melting point material 94 to be joined to the high melting point case body 30 with a “top down” weld 96. Welding from the top of the case on the single planar surface formed by aligning the edge of low melting point material 94 with the top edge of battery case body 30 is easier and therefore can be less costly than welding from the side. Weld 96 is preferably a laser weld.

[0034] FIGS. 5A and 5B are sectional views of an alternative embodiment of the present invention. Cover 100 comprises a top layer of high melting point material 102 and a bottom layer of high melting point material 106 clad to a layer of low melting point material 104. Material at the edge 107 may be machined away to expose the low melt temperature layer 104. Then, the cover 100 may be stamped, punched, or otherwise formed as in FIG. 5B so that the low melting point material 104 can be joined to the low melting point case body 30 with a “top down” weld 108. Weld 108 is preferably a laser weld.

[0035] FIG. 6 is a sectional view of an alternative embodiment of the present invention. Cover 110 comprises a layer of high melting point material 112 clad to a layer of low melting point material 114. Although not shown but similar to the embodiment shown in FIG. 4A, the top layer of material at the edge may be machined away so that the high temperature material 112 does not act as a heat sink or otherwise affect the subsequent welding of the low melt temperature layer 114. In this case, the cover 110 need not be formed and may remain flat. Case 116 may be stamped, drawn, or otherwise formed to create a ledge 118 so that the low melting point material 114 can be joined to the low melting point case 116 with a tungsten inert gas (TIG) weld, with the weld bead shown at 120. Again, this weld can be made “top down” and/or on a single planar surface, in this case, the ledge 118 of case 116, improving manufacturability.

[0036] FIG. 7 is a sectional view showing an assembled battery case body, cover, and feedthrough of another alternative embodiment of the present invention. In this embodiment, the case body 130, clad cover 140, and feedthrough pin 1 are made of metals that can withstand the temperatures required for hermetically sealing the ceramic cylinder 2 directly to the cover. The ceramic cylinder 2 is preferably a glass, such as CaBAl 12, which can be baked into the cover directly without requiring an annular metal collar. The cover comprises a first material 142 that has a coefficient of thermal expansion (CTE) suitably matched to the pin 1 and ceramic cylinder 2 for installing the feedthrough subassembly directly into the cover using a compression seal, and a second material 144 that is matched to the case body for forming a reliable hermetic weld 148, which is preferably a laser weld. Both materials 142 and 144 have sufficiently high melting points to withstand the baking-in process of the feedthrough directly to the cover 140. When using a glass and forming a compression seal, the materials 142 and 144 preferably have melting points that can withstand temperatures needed to flow the glass. Preferred materials for this embodiment include a first material 142 of stainless steel, and a second material 144 and a case body 130 of titanium or an alloy thereof. This configuration and material combination facilitates manufacturing of a battery having a strong yet lightweight battery case.

[0037] Titanium to stainless steel cladding can be made using any method known in the art that will produce a clad material capable of withstanding processing at the temperature necessary to seal ceramic cylinder 2 to cover 140. Examples can be found in WO 00/24542 entitled “Method of producing metal composites which can be processed at high temperatures,” which describes a cladding method that uses one or more interlayers of high melting point material between the titanium and stainless steel layers, and Japanese Patent Laid Open Application, Publication No. 06-234083 entitled “Titanium clad stainless steel plate and its production,” which describes a cladding method in which titanium material is directly clad to stainless steel.

[0038] The clad material used in FIG. 7 may comprise two layers, as shown therein, or may comprise three layers, as shown in FIG. 3A-C. In the three-layer version, the clad sheeting may comprise a titanium sheet having stainless steel clad onto both faces thereof, or a stainless steel sheet having titanium clad onto both faces thereof. For a titanium case body, where stainless steel is on the bottom layer of the cover, the edge of it is machined away to expose the titanium for welding to the case body, in a similar manner as shown in FIG. 3A-C. Likewise, in a cover comprising only one layer of titanium and one layer of stainless steel, either one may be comprise the bottom layer of the cover; if the stainless steel layer is on the bottom, the edge of it is machined away to expose the titanium for welding to a titanium case body. As shown in FIG. 7, even if the titanium is on the bottom layer 144 of cover 140, a step may be machined into it without breaking through to the stainless steel for seating the case body and to prevent burning through to inner battery components such as the separator during welding, as described with respect to FIG. 3A-C.

[0039] For compression sealing glass as the ceramic cylinder 2, the through hole in the clad sheet is preferably stepped, with the glass aligned with the cladding layer that has a CTE matched to that of the glass. Therefore, although FIG. 7 shows the ceramic cylinder 2 sealing with the top layer of the clad sheeting, if the bottom layer is the layer matching the CTE of the glass, the glass may be aligned with and form a seal with the bottom layer. Furthermore, for the three-layer clad material, a layer that has a CTE matched to that of the glass is aligned with the glass and sealed thereto; this may be the top, middle, or bottom layer.

[0040] Any of the configurations shown in FIGS. 1-6 may utilize a titanium case body, a stainless steel feedthrough collar, and a clad cover of titanium and stainless steel. In that case, the stainless steel annular collar is preferably welded to a stainless steel layer of the cover, and a titanium layer of the cover is welded to the titanium case body. Other material combinations are within the scope of the invention.

[0041] While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.