Abstract:
A feedthrough assembly and method for a battery includes a corrosion-prone pin coupled to a corrosion-resistant current collector and protected from the battery electrolyte by a protective covering. The current collector is connected to the battery electrode without danger of exposing the pin to the electrolyte.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to battery components, and more particularly, a battery feedthrough assembly and method for making it.  
         BACKGROUND  
         [0002]    In many applications, particularly those in medical and aerospace fields, minimizing 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 to provide various battery functions is decreasing.  
           [0003]    Electronic device seals that bond glass and other ceramics 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. It is desirable for the glass and metal to have similar coefficient of thermal expansions (CTE) to avoid stress breaks during the heating and cooling processes. Preferably, a compression seal is created, for example, where an outer body (typically a case) has a CTE that is greater than an insulating component (typically glass), and the insulating component has a CTE that is greater for 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. Nonglass ceramics may be sealed to the metal using a braze, for example, as described in pending application U.S. Ser. No. 09/774,450 filed Jan. 30, 2001, which is assigned to the assignee of the present invention and is hereby incorporated herein by reference in its entirety.  
           [0004]    To manufacture a battery, typically, an electrode assembly is placed in a case having a cover with an opening that exposes the battery terminal. To keep weight at a minimum, it is desirable to use strong, yet lightweight materials for the battery case and cover. These materials ideally 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.  
           [0005]    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, there exists a tendency for these materials to expand and contract at differing rates. The CTE of the insulating member, which may be a glass or nonglass ceramic, may be different from 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.  
           [0006]    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 CTE are larger than that for titanium and the Ti-6Al-4V alloy, essentially eliminating this combination of materials for forming a seal.  
                                                                 TABLE 1                           shows the CTE of various materials.                CTE           [10 −6 /° C.]                        Conductors                Aluminum               1000 series (1004)   23.5           Gold           Au 100   14           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            Insulators                Nonglass Ceramics               Al 2 O 3     7.6           Glass           CaBAl 12   6.7                      
 
         SUMMARY  
         [0007]    The present invention is an efficient and economical feedthrough assembly and method used in a battery. The battery is preferably made of strong, lightweight material such as titanium. A pin, substantially housed by the battery cover, is insulated from the cover by an insulative member. The assembly is at least partially enveloped by a protective covering to protect it from contact with corrosive electrolyte. In some preferred embodiments of the invention, the pin is sealed to the insulative member after the protective coating is applied. In other embodiments of the invention, the protective coating may be applied after sealing the pin to the insulative member. The present invention allows for use of numerous embodiments and materials as may be desirable in varying applications. While the invention herein is illustrated using a positive terminal, the same technology may be applied to make a negative terminal.  
           [0008]    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. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    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.  
         [0010]    [0010]FIG. 1 is a diagram of a preferred embodiment of the present invention illustrating its principal components.  
         [0011]    [0011]FIG. 2 is a diagram illustrating an electrode connected to the feedthrough of FIG. 1.  
         [0012]    [0012]FIG. 3 is a diagram of an alternative preferred embodiment of the present invention illustrating its principal components.  
         [0013]    [0013]FIG. 4 is a diagram of another alternative preferred embodiment of the present invention illustrating its principal components.  
         [0014]    [0014]FIG. 5 is a diagram of yet another preferred embodiment of the present invention.  
         [0015]    [0015]FIG. 6 is a diagram of an alternative preferred embodiment of the present invention illustrating a pin with protruding base.  
         [0016]    [0016]FIG. 7 is a diagram of an alternative embodiment of the present invention wherein the current collector forms a core of the terminal.  
         [0017]    [0017]FIG. 8 is a diagram of an alternative embodiment of the present invention wherein the coating is applied to the pin after scaling the pin to the insulative member.  
         [0018]    [0018]FIG. 9 is a diagram of an alternative embodiment of the present invention wherein the current collector extends to cover a portion of the pin.  
         [0019]    [0019]FIG. 10 is a diagram of an alternative embodiment of the present invention wherein the current collector extends to cover a portion of the pin. 
     
    
     DETAILED DESCRIPTION  
       [0020]    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. While the invention herein has been illustrated using a positive terminal, which derives appreciable benefit from the invention, the same technology may be applied to make a negative terminal.  
         [0021]    [0021]FIG. 1 is a diagram of a preferred embodiment of the feedthrough of the present invention illustrating its principal components. The battery case  10  in the present invention can be made of strong, durable, and lightweight material such as titanium. A feedthrough pin  50  is insulated from case  10  by an insulative member  20 , typically made of glass such as CaBAl 12 or Fusite 435, and extends through the battery case  10  for connection to an electrode within the battery case via current collector  70 . Because of the design flexibility achieved by the present invention, the feedthrough may be built directly into the side or cover of a battery case without the need for a ferrule; alternatively, the feedthrough may include a separate metal ferrule that surrounds the insulative member and that is welded or otherwise sealed to a battery case wall or cover. Therefore, as used herein, the term “case” and the reference numeral “10” may refer to either a wall or cover of the battery case or to a ferrule welded thereto.  
         [0022]    Thermal expansion is particularly problematic where the coefficient of thermal expansion (CTE) of the battery case material differs substantially from that of the pin or insulative member material. For feedthrough constructions using a glass as the insulative member, it is generally desirable that the CTE of the battery case be greater than that of the glass, and that the CTE of the glass be greater than that of the pin. In these typical constructions, it is also important that the pin material be inert to the electrolyte, which is typically very corrosive. However, the protection afforded by the present invention allows for the use of multiple and varying pin  50  materials and construction. For example, the pin  50  may effectively be constructed of steels, such as stainless steels, and nickel alloys, such as KOVAR® alloy, and 42 alloy, that have CTEs compatible with a titanium case, even though they may not be suitable for direct contact with the electrolyte.  
         [0023]    In order to couple the battery electrode to the feedthrough pin, a current collector  70  comprising a material selected to be compatible with the electrolyte is mechanically and electrically connected to the feedthrough pin  50  at an attachment point  67 . Such compatible materials include aluminum, platinum, gold, niobium, tantalum, molybdenum, and stainless steel. 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 be applied to connecting a current collector to a feedthrough pin. Connection may be accomplished by a number of means including the use of a resistance weld  100  as is illustrated. 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.  
         [0024]    A protective covering  65  is provided to protect the pin  50  from exposure to the electrolyte, and may be in the form of a coating dispersed over at least a portion of both the pin  50  and current collector  70 , covering at least the pin attachment point  67  and the portion of the pin that will lie within the interior of the battery case. The protective covering  65  comprises a material that can withstand exposure to electrolyte, and may comprise ceramic or a noble metal such as gold, with gold being a preferred material because of its high conductivity for improved rate capability.  
         [0025]    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, preferably 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 , and mixtures thereof. Solvents include esters, linear and cyclic ethers and 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. A preferred electrolyte for a cell of the present invention comprises LiPF 6  in a mixture of cyclic and linear carbonates, preferably 30:70 EC:DEC.  
         [0026]    The protective covering  65  protects the pin  50  from contact with electrolyte. The protective covering  65  may comprise a coating applied by plating such as electroplating, barrel plating, etc., chemical or physical vapor deposition, sputtering, ion implantation, or the like. The coating is preferably of a thickness less than 0.002 inches, and more preferably between 1 and 80 microns, while the most preferable thickness is approximately 5 microns. These thin coatings provide the needed barrier to the electrolyte without unduly diminishing the sealing ability between the pin and the insulative member  10 . In the instance where gold is used for the coating, especially on stainless steel, a nickel strike, preferably between 10 and 300 angstroms can be done to improve the adhesiveness of the coating to the pin  50 .  
         [0027]    [0027]FIG. 2 is a diagram of a preferred embodiment of the present invention illustrating the current collector  70  of the feedthrough of FIG. 1 joined to an element  80 , which comprises an electrode or a tab that is or will be joined to an electrode; element  80  is hereinafter referred to as electrode  80 . The material of electrode  80  and that of current collector  70  are compatible with the electrolyte within battery case  10 . Therefore, even though protective covering  65  may be penetrated during the connection of electrode  80  to current collector  70  at electrode attachment point  77 , the materials that are exposed will not corrode upon exposure to the electrolyte. The connection of electrode  80  to current collector  70  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. When the coating is applied to form the protective covering  65 , certain portions of the current collector  70  may be masked to prevent it from being deposited on those portions. When ceramic is used for the protective covering  65 , it is especially useful to mask the portion of the current collector  70  that will be joined to electrode  80  at electrode attachment point  77  to keep ceramic from depositing on it. This obviates the need to actually break through a ceramic coating to make the electrical connection to the current collector.  
         [0028]    [0028]FIG. 3 is a diagram of another alternative preferred embodiment of the present invention illustrating its principal components. In this embodiment, the coating covers the bottom portion and optionally the top portion of the pin  50 , leaving the center portion of the pin  50  uncoated. The coating extends onto the current collector  70  to at least completely cover the junction between pin  50  and current collector  70 . Additionally, the coating  65  may partially or completely cover current collector  70 . The insulative member  20  flows and conforms to the coated and uncoated portions of the pin, forming a seal. This embodiment is particularly useful when adhering the insulative member  20  directly to the material of pin  50  provides a better seal than the seal obtained with the material of protective covering  65 . This may be the case either when forming a compression seal with a glass insulative member or when using a braze to seal a nonglass insulative member.  
         [0029]    [0029]FIG. 4 is a diagram of an alternative embodiment of the present invention illustrating a design wherein pin  50  and current collector  70  essentially form a two-material pin. In this embodiment, the pin  50  and current collector  70  are joined by a screw (shown), a butt weld, a crimp, or the like. The end of pin  50  is aligned with the end of the insulative member  20  or may extend into the interior of the battery case such that the current collector  70  is positioned completely beneath the insulative member  20 . A coating, forming protective covering  65 , is dispersed at least over the portion of the pin  50  that would be exposed to electrolyte, ensuring that the junction between pin  50  and current collector  70  is covered; alternatively, the coating may be dispersed over the entire subassembly of pin  50  and current collector  70 . By using the protective covering  65 , alignment of the end of pin  50  with the end of the insulative member  20  is not critical. In a preferred material combination, when using a titanium case, KOVAR® alloy is utilized for the pin  50  and a corrosion resistant stainless steel is utilized for the current collector  70 .  
         [0030]    [0030]FIG. 5 is a diagram of yet another preferred embodiment of the present invention. Here, the pin  50  is connected to the current collector  70  by a resistance weld  100 , as illustrated, or laser welding, mechanical fastener, crimp, clamps, rivets, screws, adhesives, or other method. The pin  50  and current collector  70  assembly receives a contiguous coating to form a protective covering  65  around the entire perimeter of the assembly. Alternatively, as in earlier-described embodiments, portions of the pin not exposed to the electrolyte and all except the pin attachment point  67  of the current collector may be left uncovered. The coated pin  50  extends through the insulative member  20  in battery case  10  as illustrated in previous embodiments.  
         [0031]    [0031]FIG. 6 is a diagram of an alternative preferred embodiment of the present invention illustrating a custom pin  52  having a flattened base  53 . The flattened base  53  of the custom pin  52  is designed to further support and strengthen the attachment between the pin  52  and the current collector  70 . Here again, both the pin  52  and the current collector  70  have a contiguous coating forming protective covering  65 . Also similar to the previous figure, the pin  50  can be connected to the current collector  70  by a mechanical fastener  103 , as illustrated schematically, or welding, crimps, clamps, rivets, screws, pressure fits, adhesives, other methods, and combinations thereof. In both configurations illustrated in FIGS. 5 and 6, KOVAR® alloy may be used for the pin  50  material, and stainless steel, preferably SS446, may be used for the current collector  70  material.  
         [0032]    [0032]FIG. 7 is a diagram of an alternative embodiment of the present invention illustrating an alternative pin construction design. In this configuration, the current collector  70  extends to form a core  55  through a sheath  56 , together forming a composite alternative pin  54 . Because the thickness of the sheath  56  is large with respect to the core  55 , the sheath  56  dictates the CTE of the composite pin. A coating is dispersed over the alternative pin  54  after assembly of the sheath  56  around the core  55  to form protective covering  65 . Then the coated pin  54  is sealed within the insulative member  20 . The battery case  10  and insulative member  20  are positioned similarly to previous embodiments. In this embodiment, SS316L is a preferred material for the core  55 , gold is used for the protective covering  65 , and KOVAR® alloy is used for the sheath  56 .  
         [0033]    [0033]FIG. 8 is a diagram of an alternative embodiment of the present invention similar to that shown in FIG. 7, in which the coating is applied after assembly of the pin  54  to the insulative member  20  to form protective covering  65 . This order of steps can be applied to other embodiments described herein with similar results. When a conductive coating such as gold is used for protective covering  65 , the coating must not be allowed to bridge the case and the pin, forming a short circuit. Therefore, the case and the outer edge of the insulative member  20  could be masked to ensure electrical isolation. On the other hand, and as illustrated in FIG. 8, if an insulative coating such as a nonconductive ceramic or plastic is used, this masking is not necessary. In fact, by allowing the insulative coating to extend onto the case  10 , if good adhesion can be achieved between the insulative coating and the current collector  70 , pin  50 , and case  10 , then even if poor or no adhesion is achieved to the glass, the electrolyte will still be unable to reach the pin  50 . Furthermore, when using an insulative coating, the coating can be applied to the entire battery cover, thereby eliminating the need for a separate insulator component that is typically installed between the electrode assembly and the battery cover. When using a nonconductive protective covering  65 , it may be desirable to mask the portion of current collector  70  that will be attached to the electrode to eliminate the need to penetrate the ceramic coating when connecting the current collector  70  to the electrode.  
         [0034]    [0034]FIG. 9 is a diagram of an alternative embodiment of the present invention illustrating a current collector  70  that extends to cover pin  50  to form a protective covering  65  thereon. The material of current collector  70  is suitable for exposure to electrolyte, and the material of pin  50  has a CTE suitable for forming a seal with insulative member  20 . The protective covering  65  is sufficiently thin so that its effect on the CTE of the overall pin structure (pin  50  plus protective covering  65 ) is small enough to allow a reliable seal with the insulative member  20 . The maximum allowable thickness of protective covering  65  depends on the pin diameter, the materials, and the type of seal being formed. Ideally the thickness of the protective covering  65  does not exceed 0.020 inches (0.508 mm), and preferably is substantially thinner such as less than 0.002 inches, especially for smaller pin diameters. Preferable pin diameters are between 0.015-0.250 inches (0.381-6.350 mm), and most preferable pin diameters are between 0.015-0.120 inches (0.381-3.048 mm). As in all of the previous embodiments, the insulative member  20  may comprise a glass that forms a compression seal with the pin, or a nonglass ceramic that may be brazed to the pin. The protective covering  65  covers at least the bottom portion of pin  50  to prevent exposure of pin  50  to electrolyte. Ideally, KOVAR® alloy is utilized for pin  50  and stainless steel is utilized for the current collector  70 . The current collector  70  may include a feature  101 , such as a hole as shown, for mechanical fastening to an electrode.  
         [0035]    [0035]FIG. 10 is a diagram of an alternative embodiment of the present invention illustrating its principal components, including a custom pin  51  illustrating a different shape that could be applied to other embodiments described herein, and a current collector  70  in the form of a cup. In this embodiment, prior to coupling the pin  51  with the current collector  70 , a protective covering  65  is applied to protect at least the portion of the pin  51  that could otherwise be exposed to electrolyte. The protective covering  65  may comprise a braze that is used to couple pin  51  with current collector  70 . As another alternative, the protective covering  65  may comprise a coating that is thick and robust enough to withstand a press fit of the coated pin into the current collector  70 . As yet another alternative, the pin may be constructed of a material having a suitable CTE clad with a corrosion resistant material that forms the protective covering  65 . Current collector  70  is then press fit, glued, brazed, or otherwise adhered to pin  51  in such a way as to form a seal between the protective covering  65  and the current collector  70  so that electrolyte cannot leak in to attack the corrosion prone material of pin  51 . In this embodiment, the insulative member  20  may comprises a cutout  21  that allows for a portion of current collector  70  to nestle partially within the confines of insulative member  20 . Although the wall of the cup formed by the current collector  70  may be so thick as to not allow matching of the pin/cup composite CTE and therefore not form a good seal with insulative member  20 , a good seal is not required in this region, so long as a good seal is formed with the pin  51  in the region above the current collector  70  and the protective covering  65  keeps the electrolyte from contacting the pin  51 .  
         [0036]    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.