Abstract:
A feedthrough is insulated and hermetically sealed by brazing a ceramic disk to a case cover and by brazing the top surface of the ceramic disk to the bottom surface of a feedthrough pinhead. Using this technique instead of forming a compression seal, the surface area for bonding is increased, increasing bond strength. The ceramic disk electrically insulates the feedthrough pin from the cover, and provides a large surface area for mechanically sealing the cell with the braze. Considering the small size of many cells, this increased surface area is important for getting a good seal and increasing bond strength. This design also creates a longer fluid path, providing greater hermeticity. Furthermore, a greater range of component material combinations is available because CTE compatibility limitations of the feedthrough pin, cover, and insulator are minimized. This feedthrough is applicable to broad array of applications and numerous material combinations.

Description:
TECHNICAL FIELD 
     The present invention relates to battery components, and more particularly, a battery feedthrough assembly and method for making it. 
     BACKGROUND 
     In many applications, particularly in medical and aerospace fields, minimizing volume and weight is a major goal in battery design. As battery technology continues to make great strides, battery sizes have greatly decreased. Because of size and weight constraints, the number of available materials used for various battery components is decreasing. Furthermore, when providing batteries for a replacement market, the size of the battery is constrained by the available space in the existing device. For example, for replacement batteries for certain models of a hearing aid already in use, the battery thickness is limited to 3.6 mm, and the diameter is 5.8 mm. In order to not reduce the capacity of the cell, the space taken up by nonreactive components, such as the battery case and sealing components, must be minimized, thus reducing the amount of room available to seal the battery case. 
     Lithium-ion batteries provide high energy densities; however, a major problem associated with these cells is the highly corrosive nature of lithium battery chemistry. Hermetic seals are used to protect living tissue from corrosive battery components and to protect battery components from corrosive bodily fluids. Hermetic seals must be manufactured as ruggedly as possible for applications where hermeticity will be required for extended exposures to harsh environments. 
     Electronic device seals that bond glass to metal are generally known in the art. Molecular bonding is accomplished by oxidizing the surface of the metal component to facilitate bonding to the glass component. Heating the components causes the glass to soften and flow into the oxidized area of the metal component thereby creating a hermetic seal when the components are cooled. For typical feedthrough constructions using a glass as the insulator, a compression seal is created, for example, where an outer body (typically a metal case) has a coefficient of thermal expansion (CTE) that is greater than that of an insulating component (typically glass), and the insulating component has a CTE that is greater than that of a metal component (typically a pin). Once heated to 950° C. or greater, the differing CTE facilitates the glass flowing into the case to form a seal, and likewise, the glass to compress the pin to form yet another seal. It is desirable for the glass and metal to have similar CTE to avoid stress breaks during the heating and cooling processes. Thermal expansion is particularly problematic where the CTE of the battery case material differs substantially from that of the pin or insulator material. 
     Therefore, to form an acceptable glass-to-metal seal in a lithium or lithium-ion battery, the glass must have a high resistance to lithium corrosion; it must be able to make a hermetic seal between the metal header and the metal pin, which requires a thermal expansion match between the glass and the pin; and it must be an electrical insulator so that the case cover and the pin are electrically isolated. Also, where feedthroughs may come into contact with bodily fluids, it is necessary to choose biostable materials. 
     To manufacture a battery, typically, an electrode assembly is placed in a case having a cover. To keep weight at a minimum, it is desirable to use strong, yet lightweight materials for the battery case and cover. These materials may, as an example, include titanium and titanium alloys. However, titanium presents problems in most applications in that its CTE varies greatly from materials traditionally used for the feedthrough pin, resulting in seal failures. 
     The battery case is hermetically sealed to prevent corrosion and to avoid leakage of the internal electrolyte, which is typically very corrosive. Because of corrosion issues, only a limited number of materials can be used in contact with the electrolyte. For the positive feedthrough of a lithium ion battery, these materials include aluminum, platinum, gold, niobium, tantalum, molybdenum, and stainless steel. Because the CTE of the desirable battery cover material, e.g. titanium, is generally markedly different from the CTE of desirable pin material, e.g., stainless steels that can withstand electrolyte exposure, these materials tend to expand and contract at differing rates. The CTE of the insulating member may be different from that of one or both components as well. These differences in CTE make it difficult to form a good seal between the insulating body and the case or terminal pin during manufacturing, or may cause the seal to break during use. 
     To prevent these problems, the prior art has generally called for the requirement of materials that have compatible CTEs. As mentioned previously, a compression seal can be formed when the CTE for the pin material is less than that of the battery cover material. A quick look at stainless steel CTE reveals that these CTEs are larger than that for titanium and Ti-6A1-4V alloy, essentially eliminating this combination of materials for forming a glass compression seal. 
     
       
         
               
             
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 shows the CTE of various materials. 
               
             
          
           
               
                   
                 CTE 
               
               
                   
                 [10 −6 /° C.] 
               
               
                   
                   
               
             
          
           
               
                   
                 Conductors 
                   
               
               
                   
                 Aluminum 
                 23.5 
               
               
                   
                 1000 series (1004) 
               
               
                   
                 Gold 
                 14 
               
               
                   
                 Au 100 
               
               
                   
                 Nickel 
               
               
                   
                 42 Alloy 
                 4.7 
               
               
                   
                 Kovar (Co17, Ni29) 
                 6 
               
               
                   
                 Platinum 
               
               
                   
                 Pt 100 
                 9 
               
               
                   
                 PtIr 
                 9.2 
               
               
                   
                 Stainless Steel 
               
               
                   
                 304 
                 17.2 
               
               
                   
                 304L 
                 17.2 
               
               
                   
                 305 
                 17.2 
               
               
                   
                 316 
                 15.9 
               
               
                   
                 316L 
                 15.9 
               
               
                   
                 410 
                 9.9 
               
               
                   
                 420 
                 10.3 
               
               
                   
                 446 
                 10.4 
               
               
                   
                 Titanium 
               
               
                   
                 Titanium CP 
                 8.4 
               
               
                   
                 Ti 6AL-4V 
                 8.8 
               
               
                   
                 Insulators 
               
               
                   
                 Nonglass Ceramics 
                 7.6 
               
               
                   
                 Al 2 O 3   
               
               
                   
                 Glass 
                 6.7 
               
               
                   
                 CaBAl 12 
               
               
                   
                   
               
             
          
         
       
     
     Furthermore, the compression seal described above requires a minimum thickness for the various components. For applications in which the overall thickness of the battery is limited, such as in the hearing aid replacement battery market, there is simply not enough room allotted to the feedthrough to provide the thickness of material necessary to form a strong glass compression seal. 
     SUMMARY 
     The present invention provides a feedthrough that is insulated and hermetically sealed by brazing a ceramic disk to a case cover and by brazing the top surface of the ceramic disk to the bottom surface of a feedthrough pinhead. By brazing these components in this manner instead of forming a traditional compression seal, the surface area for bonding is increased, increasing bond strength. The ceramic disk electrically insulates the feedthrough pin from the cover, and provides a large surface area for mechanically sealing the cell with the braze. Considering the small size of many cells, this increased surface area is important for getting a good seal and increasing bond strength. This low profile design also maximizes internal volume available for the electrode assembly and electrolyte. This design also creates a longer fluid path, providing greater hermeticity. Furthermore, a greater range of component material combinations is available because CTE compatibility limitations of the feedthrough pin, cover, and insulator are minimized. This feedthrough is applicable to a broad array of applications and numerous material combinations. 
     Nonglass ceramics may be sealed to metal using a braze, for example, as described in U.S. Pat. No. 6,607,843 and pending U.S. application Ser. No. 10/430,036, both of which are assigned to the assignee of the present invention and hereby incorporated herein by reference. Brazed ceramic bonds can have greater mechanical strength than glass seals. Brazing allows material combinations that are not available using glass to metal sealing technology. 
     The battery case can be made of strong, lightweight material such as titanium. The invention herein may be used to make a positive or negative feedthrough terminal. Several embodiments of the present invention are disclosed that provide a new and improved feedthrough assembly and method that may be easily and efficiently manufactured at low cost with regard to both materials and labor. These feedthrough assemblies are of durable and reliable construction and are useful in a myriad of applications and situations. These feedthrough assemblies are not complicated, and are very small, so they can be made without reducing capacity. This feedthrough can be used for miniature batteries that require a hermetic seal and improved seal strength, such as implantable cells. Miniature batteries having a diameter greater than the height include coin cells and button cells, so-called due to their shapes. A typical miniature cell that could benefit from this seal is 5.8 mm diameter×3.6 mm thick. These cells typically have a crimped plastic seal that is generally inadequate for implantable applications. Cells having this type of seal typically have a leak rate of about 10 − 10-10 −6  atm-cc/sec He, while implantable cells have a maximum leak rate of about 10 −7 -10 −9  atm-cc/sec He, depending on the life expectance of the device being powered, with 10 −8 -10 −9  atm-cc/sec He being typical for a 10 year life. Because of the space constraints, and in particular, the limited height dimension, a compression seal may not be possible. The feedthrough of the present invention can be designed to be low profile, as is usually desired for such cells. Although this feedthrough provides significant improvements over the prior art, the shape of the cell can be maintained so as to preserve the form, fit, and function with existing battery-powered devices. 
     This feedthrough can be made with a double seal, for example, using an outer brazing material to provide good adhesion and an inner brazing material to provide good chemical stability in contact with the electrolyte. Alternatively or additionally, a polymer may be used as an inner sealing material to provide an electrically insulating seal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the present invention, and together with the preceding general description and the following Detailed Description, explain the principles of the present invention. 
         FIG. 1  is a diagram of a preferred embodiment of a cell of the present invention. 
         FIG. 2  is a cross sectional view of the cell shown in  FIG. 1 . 
         FIG. 3  is an exploded view of the feedthrough of  FIG. 2 . 
         FIG. 4  is a cross sectional view of a cell showing an alternative preferred embodiment of the feedthrough. 
         FIG. 5  is an exploded view of the feedthrough of  FIG. 4 . 
         FIGS. 6 and 7  are exploded views of alternative embodiments of the pin of  FIG. 4 . 
         FIG. 8  is a cross sectional view of a cell having an alternative embodiment of the feedthrough. 
         FIG. 9  is an exploded view of the cell of  FIG. 8 . 
         FIGS. 10-12  are cross sectional views of alternative preferred embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments consistent with the present invention address the need for an efficient and reliable feedthrough assembly and method. The device and method described herein may be implemented in a variety of manners. Accordingly, the description of a particular embodiment herein is intended only for the purposes of example, and not as a limitation. Features described with respect to an embodiment described herein are not limited to that embodiment and may be applied to other embodiments described herein. For example, several case configurations are shown herein, and each case configuration may be combined with any feedthrough configuration herein. Furthermore, the case configuration is not limited to those described herein. 
       FIG. 1  is an isometric view of a preferred embodiment of the cell  10  of the present invention illustrating its principal components. The battery case  11  in the present invention can be made of strong, durable, and lightweight material such as titanium. Preferred battery case materials for applications wherein weight is less critical include 304, 304L, 316, and 316L stainless steels. A preferred battery case material for applications not requiring hermeticity is nickel plated iron. A case cover may be part of case  11  as case cover  66  in  FIGS. 8 and 9 , or may be a separate component as case cover  26  in  FIGS. 1-5 . Case cover  26  may comprise the same material as case  11  or may be a different material. A feedthrough pin  22  is insulated from case cover  26  by an insulator  24 , made of a nonglass ceramic such as alumina, and extends through the battery case cover  26  for connection to an electrode within the battery case. 
     The present invention allows for the use of multiple and varying materials for pin  22 . For example, pin  22  may effectively be constructed of steels, such as stainless steels, and nickel alloys, such as KOVAR®&amp; alloy, and 42 alloy. Pin diameters can be about 0.1 to about 3 mm. 
     The type of electrode assembly housed within case  11  is not limited and may comprise a pellet, a stack, a jellyroll, or any other type known in the art. As shown in  FIG. 2 , electrode  18  is mechanically and electrically connected to the feedthrough pin  22 , preferably directly, or via a current collector comprising a material selected to be compatible with the electrolyte. Such compatible materials include aluminum, platinum, gold, niobium, tantalum, molybdenum, and stainless steel. The material of electrode  18  and that of the current collector are chosen to be compatible with the electrolyte. In one material combination, when using a titanium case, the pin  22  can be KOVAR® alloy and the current collector can be a corrosion resistant stainless steel. The connection of electrode  18  to the current collector, or tab, or to pin  22  may be made by any means known in the art and may comprise resistance welding, laser welding, and other forms of welding, or mechanical fasteners, such as crimps, clamps, rivets, screws, pressure fits, and adhesives, including conductive adhesives. Alternatively, the mechanical and electrical connections can be separated, using the principles taught in U.S. Pat. Nos. 6,063,523 and 6,458,171, each of which is assigned to the assignee of the present invention and incorporated herein by reference in its entirety. These two patents teach a method for connecting a tab to an electrode, but the principle of separating the electrical and mechanical connections can also be applied to connecting a current collector to a feedthrough pin. Another alternative that can be used to connect the feedthrough pin to the electrode is the Feedthrough Assembly and Method taught in U.S. patent application Ser. No. 10/307,560, filed Nov. 27, 2002, and assigned to the assignee of the present invention and incorporated herein by reference in its entirety. Connection may be accomplished by a number of means including the use of a resistance weld, and may be facilitated using a feature formed on the pin such as pin slot  28 . Other connection methods include other forms of welding, such as laser welding, and mechanical fasteners, such as crimps, clamps, rivets, screws, pressure fits, adhesives including conductive adhesives, and combinations thereof. Pin  22  may be used as a winding arbor as described in U.S. patent application Ser. No. 10/167,688, filed Jun. 12, 2002, and assigned to the assignee of the present invention and incorporated herein by reference in its entirety. To facilitate assembly, pin slot  28  may be used to engage an electrode (or tab for connection to an electrode), via crimping, welding, friction, or the like. When used as a winding arbor, the shaft of pin  22  preferably extends almost to the bottom of the battery case. When pin  18  is used as a winding arbor, insulator  24  is preferably brazed to cover  26  prior to winding the electrode assembly so that the brazing process does not disturb the wound electrode assembly. Furthermore, the fixture used to hold to brazed feedthrough during winding may be constructed to apply forces only to the pin and not to the brazed joints, or to constrain the components such that the brazed joints of insulator  24  to pin and cover  26  are mainly in compression and not in shear. 
     As used herein, the term electrolyte refers to any solution or molten compound that conducts electricity. The electrolyte may be of various compositions, such as those formed from strong acids (HF, HCl, HBr, HI, HNO 3 , H 2 SO 4  and HClO 4 ), strong bases (all the Group IA and IIA hydroxides) and all soluble salts. Furthermore, the electrolyte may be formed by placing a liquid, such as a strong base, into a battery case containing battery components and allowing the liquid to physically or chemically react with the case and/or components to create the electrolyte for the battery. For a lithium ion battery, the electrolyte may comprise a nonaqueous, ionically conductive electrolyte comprising a salt, which can be an ionizable alkali metal salt, dissolved in a mixture of organic solvents chosen for their physical properties, such as viscosity, permittivity, and ability to dissolve the solute. Lithium salts known to be useful in lithium ion batteries include LiPF 6 , LiBF 4 , LiAsF 6 , LiSbF 6 , LiClO 4 , LiAlCl 4 , LiGaCl 4 , LiC(SO 2 CF 3 ) 3 , LiN(SO 2 CF 3 ) 2 , LiSCN, LiO 3 SCF 3 , LiC 6 F 5 SO 3 , LiO 2 CCF 3 , LiSO 6 F, LiB(C 6 H 5 ) 4 , LiCF 3 SO 3 , lithium bis(chelato)borates such as lithium bis(oxalato)borate (LiBOB), and mixtures thereof. Solvents include esters, linear and cyclic ethers, dialkyl carbonates such as tetrahydrofuran (THF), methyl acetate (MA), diglyme, triglyme, tetraglyme, dimethyl carbonate (DMC), 1,2-dimethoxyethane (DME),1,2-diethoxyethane (DEE),1-ethoxy,2-methoxyethane (EME), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, cyclic carbonates, cyclic esters and cyclic amides such as propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate, acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, γ-valerolactone, γ-butyrolactone (GBL), N-methyl-pyrrolidinone (NMP), and mixtures thereof. One such electrolyte for a cell of the present invention comprises LiPF 6  in a mixture of cyclic and linear carbonates, such as 30:70 EC:DEC. 
       FIG. 2  is a cross sectional view of the cell of  FIG. 1 , and  FIG. 3  is an exploded view of the feedthrough of  FIG. 1 . Pin  22  is mechanically coupled to insulator  24  using braze  23  to bond the underside  31  of pinhead  32  to the top surface  33  of insulator  24 . Braze  23  may optionally extend onto the surface of insulator  24  forming the hole through which pin  22  extends to provide a larger bonding area. Insulator  24  is mechanically coupled to cover  26  using braze  25  to bond the bottom surface  34  of insulator  24  to the top surface  35  of cover  26 . Exemplary dimensions for the various components are as follows: pin head thickness of 0.1 to 1.5 mm, insulator thickness of 0.05 to 0.5 mm, cover thickness of 0.05 to 0.6 mm, pin diameter of 0.1 to 3 mm, braze thickness of 0.1 mm max, and an overall sealed cover thickness of 1 mm or less. Preferably, the thicknesses of the pinhead  32 , insulator  24 , and cover  26  are each about 0.25 mm or less, and more preferably about 0.2 mm or less. This produces an overall sealed cover thickness of preferably about 0.8 mm or less, and more preferably about 0.7 mm or less. 
     Braze  23  and braze  25  may have the same or different material composition, and preferably are selected to be compatible with the chosen electrolyte and components being joined so that only one brazing material is needed for each braze  23  and  25 . Using only one brazing material for each braze simplifies assembly, especially in very small batteries that have very little surface area for brazing. Alternatively, as shown in  FIG. 4 , two different brazing materials  47  and  48  may be used for each braze  23  and  25 , with an outer brazing material  47  providing strong adhesion and an inner brazing material  48  providing chemical stability with the electrolyte. For example, Mn—Mo, Mo—W, or W can be used for bond strength, and precious metals such as Au, Pt, Au—Pt, and Ag can be used for preventing corrosion from the electrolyte. See, for example, JP56-086454. As an alternative to or in addition to inner brazing material  48 , epoxy or another polymer may be used as an inner sealing material to provide protection against corrosion of the outer brazing material  47 ; if the epoxy or other polymer is nonconductive, it may also provide an electrically insulating seal. 
     For a positive polarity feedthrough, the braze can be 90Au/10Cu or 90Au/10Pt. Other brazes include Au—Ti and Au—Ag. In general, gold alloy brazes are preferred, particularly those having higher gold percentages. The brazing temperature will depend on the materials used, and is typically above about 430° C. It is preferable that the melting point of the brazing material be less than the solidus of the pin and cover materials. For example, if a titanium cover is used, having a solidus of 1725° C., the brazing should be performed below 1725° C. However, if using a cover made of aluminum, having a solidus of about 650° C., or a low melting point aluminum alloy, which may have a solidus in the 500° C. range, the brazing material must be carefully considered because the temperature must be kept much lower. A preferred aluminum alloy for use with typical lithium and lithium ion chemistries and their associated electrolytes is Aluminum 3003, which is preferably brazed with brazing materials having a melting point of 571 to 621° C. By comparison, 300 and 400 series stainless steels utilize brazes having a melting point between 927 and 1204° C., and iron/nickel alloys utilize brazes having a melting point of 871 to 1232° C. 
       FIG. 4  is a cross sectional view of a cell and  FIG. 5  is an exploded view of an alternative embodiment of the feedthrough  40  illustrating a design wherein pin  42  has a pinhead  52  having a diameter larger than the diameter of the hole in cover  26 . This structure provides greater support both during manufacture and during use, wherein applying pressure to the feedthrough pinhead  52  transfers force through the insulator  24  to the cover  26 . Because ceramics are much better in compression than in tension, this structure provides an advantage by removing much of the tensile component of stress resulting from pressing on the feedthrough pin. 
       FIGS. 6 and 7  are exploded views showing that the feedthrough pin of the present invention may comprise two pieces joined by bonding, swaging, crimping, welding, screwing, brazing, or the like. As shown, pin  42  comprises a first piece forming the head  52  and a second piece forming the shaft  57  and having an optional connection feature, pin slot  58 . In an alternative embodiment (not shown) the pin may comprise a threaded washer forming head  52  screwed onto a threaded shaft  57 . 
       FIG. 8  is a cross sectional view and  FIG. 9  is an exploded view of a cell having another embodiment of the feedthrough  60 . Here, the insulator  64  has an outer diameter that is about the same as the inner diameter of the opening of cover  66 , allowing room for braze  25 , and the outer surface  77  of the insulator  64  is brazed to the inner surface  78  of the opening of the cover  66  using braze  25 , as illustrated. In this feedthrough embodiment, cover  66  is shown as integral with case  11  and sealed with bottom plate  12 ; however, this feedthrough embodiment may be used with any case design known in the art, including others shown herein. The underside  71  of the pinhead  72  is brazed to the top surface  73  of insulator  64  using braze  23 . The pin  62  extends through the insulator  64 . Braze  23  may also extend onto the shaft portion of pin  62  as shown to provide more bonding surface with insulator  64 . With this feedthrough configuration, the thickness can be kept very small. For example, if the pinhead  72  is 0.2 mm thick and the cover  66  and insulator  64  are each 0.25 mm thick, the overall thickness is only about 0.45 mm thick. 
       FIGS. 10-12  are cross sectional views of alternative preferred embodiments of the present invention. In these embodiments, the overall thickness of the sealed cover can be minimized while maximizing brazing area and utilizing a preferred case to cover weld configuration. In these embodiments, both the pin ( 102 ,  112 , or  122 ) and the cover  26  are brazed to the underside of the insulator  24 . Although the cover can be welded to the case from the side, top-down welding of a cover  26  to case  11  is preferred for ease of manufacturing; it also protects the feedthrough and electrode assembly from overheating during the welding process. With this feedthrough configuration, the thickness can be minimized. For example, if the pinhead, cover, and insulator are each 0.2 mm thick, the overall thickness is only about 0.4 mm thick. 
     The pin design will depend on a number of factors, including the type of external contact it is to be used with and the type of electrode assembly with which it will connect. When the pin is used as a winding arbor, a pin slot may be formed to engage an electrode for winding, and the pinhead may be designed with one or more features, such as a blind keyhole (not shown) for fixturing to an electrode winding machine. 
     The preferred pin  102  of  FIG. 10  has a pinhead that extends above the bottom of insulator  24 , and preferably slightly above the top of insulator  24 , as shown, to facilitate connection with a contact such as a spring contact. The shape of the pinhead provides maximum surface for brazing, including the underside of insulator  24  and the surface of the hole through insulator  24 . 
     The pin  112  of  FIG. 11  also has a pinhead that extends slightly above the top of insulator  24  to facilitate contact. However, the shape of the top portion of the pinhead extending through the insulator hole is narrower than the diameter of the hole in insulator  24 , so only the lower portion of the pinhead is brazed to the insulator  24 . 
     The pin  122  of  FIG. 12  has a pinhead that does not extend above the bottom of the insulator  24 ; although a small spring contact could be used with such a design, other types of contact known in the art would likely be more suitable for use with pin  122 . 
     In the embodiments of  FIGS. 10-12 , the outer edge of the pinhead may be spaced sufficiently far from the inner edge of cover  26  such that electrical insulation is not needed between them. Alternatively, the space between them may be filled with an insulative epoxy, polymer, or a ridge formed on insulator  24  (not shown) to electrically insulate the pin from the cover. 
     The specific implementations disclosed above are by way of example and for enabling persons skilled in the art to implement the invention only. We have made every effort to describe all the embodiments we have foreseen. There may be embodiments that are unforeseeable and which are insubstantially different. We have further made every effort to describe the invention, including the best mode of practicing it. Any omission of any variation of the invention disclosed is not intended to dedicate such variation to the public, and all unforeseen, insubstantial variations are intended to be covered by the claims appended hereto. Accordingly, the invention is not to be limited except by the appended claims and legal equivalents.