Abstract:
Vulnerable surfaces of feedthroughs employed in electrochemical cells or batteries, particularly miniaturized, high energy density primary batteries for implantable medical devices (IMDs), are provided with protective coatings to protect from degradation by the cell electrolyte or deposition of conductive materials bridging the feedthrough pin and ferrule. A liquid polyimide coating is applied to the vulnerable surfaces and cured into a substantially uniformly thick polyimide coating that tenaciously adheres to the vulnerable surfaces during subsequent welding and molding assembly steps. A further insulator is preferably molded in situ of a polymer adhere wells to the polyimide coating during molding. Gaps between the insulator and the polyimide coating are advantageously minimized.

Description:
FIELD OF THE INVENTION  
         [0001]    This application relates to feedthroughs employed in electrochemical cells or batteries, particularly miniaturized, high energy density primary batteries for implantable medical devices (IMDs), and more particularly to protective coatings for such feedthroughs to protect feedthrough insulators from degradation by the cell electrolyte.  
         BACKGROUND OF THE INVENTION  
         [0002]    Electrical feedthroughs serve the purpose of providing an electrical circuit path extending from the interior of a hermetically sealed container to an external point that is electrically insulated from the container itself, such as in an electrochemical cell. Many such feedthroughs that provide such an electrical path are known in the art, but they generally comprise a ferrule adapted to be attached, by welding or adhesive, to a hermetically sealed enclosure, a feedthrough pin or “lead” for conducting battery current, and simple single-element or more complex multi-element insulator structures supporting the feedthrough pin within the ferrule and electrically insulated from the ferrule. One of the simplest single-element insulator structure is formed of glass that is deposited in its molten state or melted in situ with the feedthrough pin supported centered in the ferrule. The non-conductive glass supports the feedthrough pin centered in the ferrule upon cooling and solidification into a solid ring adhered to the feedthrough pin and the ferrule wall. A wide range of non-conductive glass compositions have been used or proposed for such use over the years.  
           [0003]    Electrochemical cells having such a hermetically sealed enclosure and requiring use of a feedthrough or feedthroughs to make an electrical connection with an anode or cathode or both have been developed to power a variety of equipment and devices. A great deal of effort has been expended over many years to miniaturize, reduce cost, increase features, and prolong useful life of such battery powered equipment and devices. These efforts make it important to be able to design and fabricate highly miniaturized, long lasting, electrochemical cells and their components, including the electrical feedthroughs, that are correspondingly smaller in dimension. Even as feedthrough dimensions are miniaturized, it is of paramount importance that the feedthroughs are not attacked or degraded by the electrolyte, the anode or the cathode materials of the cell.  
           [0004]    For example, electrochemical cells used in IMDs, e.g., implantable monitors and therapy delivery devices, to power circuitry or delivery a therapy have been highly miniaturized over time, yet have retained high energy density through the use of reactive anodes, cathodes, and electrolytes and advanced manufacturing techniques. Such electrochemical cells provide power for stimulation therapies delivered by implantable pulse generators (IPGs) of cardiac pacemakers, brain stimulators, gastric stimulators, nerve and muscle stimulators, and implantable cardioverter/defibrillators (ICDs) or to operate an implantable drug pump to dispense a drug bolus. A wide variety of electrical feedthroughs have been proposed or employed in commercialized electrochemical cells to provide the electrical paths from the anodes and/or cathodes within the cell enclosures to the electronic circuitry within the housing of the IPG or monitor or drug dispenser.  
           [0005]    Lithium/iodine electrochemical cells as described in U.S. Pat. Nos. 4,166,158, 4,460,664, and 5,306,581, and in commonly assigned U.S. Pat. No. 5,643,694, for example, have been widely used to power circuitry of cardiac pacemaker IPGs. The anode is formed of lithium metal usually supported on a conductive perforated current collector electrically attached to an anode feedthrough pin. The cathode comprises a charge transfer complex of an organic electron donor component material and iodine that is in direct contact with the conductive cell enclosure that the feedthrough ferrule is welded to. The electron donor can be any organic compound having a double bond or an amine group. The electron donor functions to give iodine sufficient conductivity. One preferred form of the organic donor is poly-2-vinylpyridine (P2VP). A solid lithium iodide electrolyte forms over the exposed surface of the lithium anode.  
           [0006]    The reactive iodine cathode is capable of attacking many insulator materials that are used in such feedthroughs, e.g., glass insulator compounds of the types disclosed in the above-referenced &#39;664 and &#39;581 patents. Severe degradation of the insulator can result in an electrical short of the lead to the ferrule and/or migration of the cathode along the lead or pin and through electrical conductors coupled therewith to the circuitry and cause circuit malfunctions. Therefore, the insulator and the feedthrough pin used in such lithium/iodine cells are insulated by materials that are not degraded by the iodine cathode.  
           [0007]    For example, in the above-referenced &#39;694 patent, the feedthrough ferrule is formed with an internal cavity surrounding the inwardly extending feedthrough pin. An extension of the anode current collector is welded to the feedthrough pin. A nonconductive insulating material that is nonreactive with iodine, i.e., does not exhibit electronic conduction when exposed to iodine, is injection molded over the anode current collector extension and feedthrough pin and into the cavity to provide a single piece seamless insulator. Fluoropolymer materials including ethylene-chlorotrifluoroethylene (E-CTFE) and ethylene-tetrafluoroethylene (E-TFE) that can be injection molded and that perform satisfactorily are suggested for use.  
           [0008]    In a further approach disclosed in commonly assigned U.S. Pat. No. 5,549,985, a lithium/iodine-P2VP “button” cell is disclosed wherein an insulating layer overlies the inner surface of the feedthrough insulator. The preformed insulator cup is formed of one of polypropylene, a modified polytetrafluoroethylene, a fluoropolymer, or a polyvinylidenefluoride. In this design, the cathode is isolated from the region of the insulator cup, and the insulator cup does not necessarily have to be in intimate contact with the glass insulator.  
           [0009]    More recently, higher energy, miniaturized, hermetically sealed, electrochemical cells have also been developed to supply energy to charge high voltage capacitors in ICD IPGs to deliver cardioversion/defibrillation shocks in the range of 30 joules to counter ventricular tachyarrhythmias. As disclosed in commonly assigned U.S. Pat. Nos. 5,716,729, 5,766,797, 5,811,206, 6,017,656, 6,132,896, and 6,232,012 typically such electrochemical cells are formed having a lithium anode, a silver vanadium oxide, i.e., Ag 2 V 4 O 11  (aka SVO) cathode or a hybrid CF x /SVO cathode formed of SVO and carbon monofluoride (CF x ), and a liquid organic type electrolyte that comprises a lithium salt in combination with an organic solvent. The SVO cathode may include a binder selected from among powdered PTFE, polyimide, graphite or carbon black pressed onto a metal current collector formed of Ni or Ti, for example, and enveloped by a separator of microporous materials, e.g., polyethylene, polypropylene, ethylene tetrafluoroethylene (ETFE), or the like. These lithium/SVO and hybrid lithium/CF x /SVO cells have also been used to power circuitry of DUAL CHANNEL ITREL® (DCI) implantable spinal nerve stimulator IPGs manufactured by MEDTRONIC, INC., that deliver electrical stimulation into the spinal column, and may be used in the future to power circuitry of other IPGs and implantable drug dispensers and monitors.  
           [0010]    Organic solvents known for use in such lithium/SVO cells in combination with lithium salts can be, for example, 3-methyl-2-oxazolidone, sulfolane, tetrahydrofuran, methyl-substituted tetrahydrofuran, 1,3-dioxolane, propylene carbonate (PC), ethylene carbonate, gamma-butyrolactone, ethylene glycol sulfite, dimethylsulfite, dimethyl sulfoxide or mixtures thereof and also, for example, low viscosity cosolvents such as tetrahydrofuran (THF), methyl-substituted tetrahydrofuran (Met-THF), dioxolane (DIOX), dimethoxyethane (DME), dimethyl isoxazole (DMI), diethyl carbonate(DEC), ethylene glycol sulfite (EGS), dioxane, dimethyl sulfite (DMS), dimethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, ethylene carbonate, gamma-butyrolactone, acetonitrile, formamide, dimethyl formamide, triglyme (tri(ethylene glycol)dimethyl ether), diglyme (diethylene glycol dimethyl ether), glyme (ethylene glycol dimethyl ether), nitromethane and mixtures thereof or the like. The ionizing solute for these cells can be a simple salt, as for example, LiCF 3  SO 3 , LiBF 4 , LiAsF 6 , LiPF 6  and LiClO 6 , or double salts or mixtures thereof, that produce an ionically conductive solution when dissolved in one or more solvents.  
           [0011]    The ether-based liquid organic electrolytes described above are highly reactive and can migrate within the cell enclosure and cause the anode and cathode to electrically short, ending the battery life. The anode, cathode, electrolyte and the separator between the anode and cathode are therefore typically enclosed within the conductive enclosure by case liners that are also formed of polyethylene, polypropylene, ETFE, or the like, to contain the electrolyte. In particular, the ether-based organic electrolytes described above can also damage many polymers and the lithium in the lithium salts can build up over time until an electrically conductive lithium layer extends across the exposed glass between the feedthrough pin and ferrule and electrically short-circuit the anode and cathode, causing the cell to fail.  
           [0012]    As described and shown in the figures of the above-referenced &#39;206 patent, a feedthrough pin insulator  90  is injection molded or overmolded over the interior portions or surfaces of the feedthrough pin  25 , the glass insulator  28 , and the ferrule  27 . The feedthrough pin insulator  90  is molded from polyethylene, polypropylene, ETFE, or the like, over these interior feedthrough components after the feedthrough ferrule  27  is welded into an aperture  20  through the enclosure cover  15  to form a cover sub-assembly. Although it is referred to as a “feedthrough pin insulator”, the feedthrough pin insulator  90  is intended to inhibit migration of the electrolyte along the feedthrough pin  25  into contact with the ferrule  27  and the annular glass insulator  28  and to present a very long path for an electrical short to develop by lithium deposition over its exterior surface. As described in the above-referenced &#39;206 patent, the protective layer afforded by the feedthrough pin insulator  90  may or may not make contact with the inner surface of the annular glass insulator  28 .  
           [0013]    In practice, it has been found that the feedthrough pin insulator  90  tends to shrink upon cooling, and gaps can form between the material of the feedthrough pin insulator  90  and the feedthrough pin  25 , the ferrule  27 , and the annular glass insulator  28 . In particular, it has been found that if such gaps occur, the electrolyte can migrate along the feedthrough pin  25  and into any gap between the feedthrough pin insulator  90  and the annular glass insulator  28  eventually resulting in an electrical short caused by lithium deposition, leading to premature failure of the battery. The gaps may not be readily apparent from inspection of the cover sub-assembly or may appear after the cover subassembly is welded to the cell enclosure.  
           [0014]    Therefore, a technique has been developed and employed to lessen the possibility of cell failure should such gaps occur. In particular, the manufacturing technique set forth in the above-referenced &#39;206 patent and illustrated in FIGS. 1-4 has been modified by applying a coating of ETFE over the surfaces of the feedthrough pin  25 , the ferrule  27 , and the annular glass insulator  28  before the feedthrough pin insulator  90  is molded over these surfaces. Like polyethylene and polypropylene, the ETFE coating is resistant to degradation by the ether-based solvents and lithium salts of the electrolyte. Unlike polyethylene and polypropylene, the thin ETFE coating also resists melting when heat is subsequently transmitted either in the molding step or along the feedthrough pin during the welding of the external end of the feedthrough pin to a substrate pad or the like to make a circuit connection or through the ferrule when the cover is welded to the case.  
           [0015]    To make the ETFE coating, an ETFE liquid suspension is prepared from ETFE powder mixed with ethanol, and the ETFE liquid suspension is painted or otherwise deposited on the surfaces. Then, the coating is cured to evaporate the ethanol and melt the ETFE powder into a continuous layer that bonds securely to the surfaces. After inspection, the feedthrough pin insulator  90  is then molded over the ETFE coated surfaces of the feedthrough pin  25 , the ferrule  27 , and the annular glass insulator  28 . The heat of the molding step does not affect the patency of the ETFE coating, and any molding gaps that may occur, occur between the polypropylene or polyethylene feedthrough pin insulator  90  and the ETFE coated surfaces of the feedthrough pin  25 , the ferrule  27 , and the annular glass insulator  28 . In this way, the ETFE coating inhibits short-circuiting due to migration of the organic electrolyte into the gaps as described above.  
           [0016]    However, an even, continuous ETFE coating must be obtained for the ETFE coating to reliably protect and electrically insulate the coated surfaces. The ETFE liquid suspension is applied after the feedthrough ferrule  27  is welded to the cover  15  to form a cover sub-assembly. Difficulties have been experienced in both the application and the curing of the ETFE suspension. The cured ETFE coatings frequently exhibit bubbles, voids and fissures, and the ETFE powder can be overheated and burned or charred. Minute defects in the ETFE coatings are not readily apparent. Those defects that can be observed in inspection cause the feedthrough and cover sub-assembly to be scrapped, increasing cost of manufacture.  
           [0017]    It has been found to be particularly difficult to obtain a substantially uniform ETFE coating over the annular surface of the glass insulator  28 . It is difficult to maintain the ETFE powder in uniform distribution in the ETFE liquid suspension, and so it is not possible to precisely determine the amount of ETFE that may be in the coating that is applied. It is difficult to deliver an appropriate amount of the ETFE liquid suspension into the annular space between the conductive feedthrough pin  25  and the ferrule  27  due to the wetting characteristics and surface tension of the ETFE liquid suspension and the minute ferrule dimensions. It is also difficult to see any gaps or bubbles that might be present after the ETFE liquid suspension is applied or to see other defects arising after curing.  
           [0018]    Consequently, there is a need for an improved method of protecting such electrochemical cells from premature cell failure due to electrical short-circuiting between the feedthrough pin and/or damage to the feedthrough insulator due to reactive electrolyte.  
         BRIEF SUMMARY OF THE INVENTION  
         [0019]    In accordance with the present invention, the vulnerable surfaces of a feedthrough employed in electrochemical cells having a reactive electrolyte prone to attack the feedthrough materials and/or form an electrically conductive deposit bridging a feedthrough insulator are coated with a substantially uniform, electrically insulating, coating of a thermoplastic polymer that can be applied in a thin, uniform layer, that has a high melting point, and that is resistant to degradation by the electrolyte to form an electrically insulating coating that strongly adheres to the feedthrough surfaces. The polymer is preferably an aliphatic or aromatic polyimide.  
           [0020]    In accordance with one aspect of the present invention, the polyimide is applied in a liquid form over the feedthrough component surfaces of interest by painting, dabbing, swabbing or spraying and then dried or cured.  
           [0021]    Preferably, the polyimide coating extends in a substantially uniform layer over the entire surface of the annular feedthrough insulator and for a first predetermined distance away from the insulator along the feedthrough pin and for a second predetermined distance away from the insulator over the ferrule surface.  
           [0022]    Additionally, a feedthrough pin insulator is preferably molded over and against the polyimide coating and over any uncoated surfaces of the feedthrough pin and the ferrule, whereby the feedthrough pin insulator tends to inhibit migration of the electrolyte along the feedthrough pin or the ferrule and the polyimide coating protects the annular glass insulator.  
           [0023]    This summary of the invention has been presented here simply to point out some of the ways that the invention overcomes difficulties presented in the prior art and to distinguish the invention from the prior art and is not intended to operate in any manner as a limitation on the interpretation of claims that are presented initially in the patent application and that are ultimately granted. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    These and other advantages and features of the present invention will be more readily understood from the following detailed description of the preferred embodiments thereof, when considered in conjunction with the drawings, in which like reference numerals indicate identical structures throughout the several views, and wherein:  
         [0025]    [0025]FIG. 1 is a side view in cross-section of an electrochemical cell having a polyimide coating applied over the annular insulator of the feedthrough and adjacent surfaces of the feedthrough pin and ferrule in accordance with one preferred embodiment of the present invention;  
         [0026]    [0026]FIG. 2 is a cross-section view taken along the line A-A of the electrochemical cell of FIG. 1;  
         [0027]    [0027]FIG. 3 is an expanded cross-section view of a feedthrough welded to a cell cover depicting a polyimide coating over the feedthrough insulator surface and between the ferrule and the feedthrough pin;  
         [0028]    [0028]FIG. 4 is an expanded cross-section view of a feedthrough welded to a cell cover depicting a polyimide coating extending over the feedthrough insulator surface and for a first predetermined distance away from the insulator along the feedthrough pin and for a second predetermined distance away from the insulator over the ferrule surface;  
         [0029]    [0029]FIG. 5( a ) is an end view of the completed sub-assembly of the coated feedthrough and cover with the feedthrough pin insulator molded over and against the polyimide coating and over any uncoated surfaces of the feedthrough pin and the ferrule; and  
         [0030]    [0030]FIG. 5( b ) is a side view of the completed sub-assembly of the coated feedthrough and cover with the feedthrough pin insulator molded over and against the polyimide coating and over any uncoated surfaces of the feedthrough pin and the ferrule; and 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    In the following detailed description, references are made to illustrative embodiments of methods and apparatus for carrying out the invention. It is understood that other embodiments can be utilized without departing from the scope of the invention.  
         [0032]    [0032]FIG. 1 shows a lithium/SVO or lithium/CF x /SVO electrochemical cell  1  having a polyimide coating applied over the annular insulator of the feedthrough  5  and adjacent surfaces of the feedthrough pin  25  and ferrule  27  in accordance with one preferred embodiment of the present invention. The electrochemical cell  1  includes an enclosure or housing  10 , a housing cover  15 , and a feedthrough  5 . Cover  15  has an opening for receiving the feedthrough ferrule  27  and an electrolyte fill port  30  which permits a liquid electrolyte to be poured inside housing  10  after assembly of cell  1  has been substantially completed. A disc  35  is welded into the fill port  30  to provide a hermetic seal after the liquid electrolyte is poured into the interior of housing  10 . The housing  10 , cover  15 , and disc  35  are formed of a metals, e.g., stainless steel or titanium, that can be laser welded together to hermetically enclose the anode, cathode, electrolyte and other interior cell components described below.  
         [0033]    An electrically conductive feedthrough pin is supported within the annulus of the feedthrough ferrule  27  by an annular pin insulator  28  that electrically insulates pin  25  from housing  10  and also hermetically seals opening  20 . The feedthrough insulator  28  is preferably formed of TA- 23  or CABAL- 12  glass, but may also be formed of alumina or aluminum oxide, or any other suitable electrically insulating, ceramic-containing material comprising, for example, sapphire or zirconium oxide, all referred to for convenience herein as “glass”. Ferrule  27  is most preferably formed of MP35N alloy, grade 3 titanium or 316 stainless steel, and less preferably from niobium, titanium, titanium alloys such as titanium-6Al-4V or titanium-vanadium, platinum, molybdenum, zirconium, tantalum, vanadium, tungsten, iridium, rhodium, rhenium, osmium, ruthenium, palladium, silver, aluminum, and alloys, mixtures and combinations thereof, depending on the chemical system selected for cell  1 . Ferrule  27  may be welded by other means to the opening of housing  10 , or soldered or glued thereto so long as the seal so formed is hermetic. Pin  25  is most preferably formed of niobium, titanium, titanium alloys such as titanium-6Al-4V or titanium-vanadium, platinum, molybdenum, zirconium, tantalum, vanadium, tungsten, iridium, rhodium, rhenium, osmium, ruthenium, palladium, silver, aluminum, and alloys, mixtures and combinations thereof, depending on the chemical system selected for cell  1 .  
         [0034]    [0034]FIG. 2 shows a cross-sectional view of cell  1  of FIG. 1 taken along the line A-A. In FIG. 2, various layered elements are disposed within housing  1 . Anode  40  is generally pressed onto anode current collector  45  comprising an electrically conductive metal such as stainless steel, nickel or titanium. Anode current collector  45  preferably has a plurality of holes to promote adhesion of the material forming anode  40  thereon, but may assume any of a number of different geometric and structural configurations. The end of feedthrough pin  25  extending into the cell housing  10  is preferably spot welded or otherwise attached to anode current collector  45 .  
         [0035]    Separator  50  is formed of a microporous material such as polypropylene, polyethylene or ETFE, and permits the transfer of a liquid ionic electrolyte (not shown) therethrough. In one embodiment of the present invention, the liquid electrolyte comprises a solvent and a lithium salt that is in contact with anode  40  and cathode  55 . Separator  50  completely surrounds and seals anode  40  and anode current collector  45 .  
         [0036]    In an alternative embodiment of separator  50 , a non-woven absorbent layer (not shown) may be provided in addition to the microporous layer forming separator  50 . Such a non-woven absorbent layer is preferably disposed between separator  50  and anode  40  and between adjoining surfaces of anode  40  and cathode  55 . In addition to acting as an electrolyte reservoir, such a non-woven absorbent layer may compress or expand in response to any changes in cathode or anode volume during cell discharge.  
         [0037]    Case liner  67  may be formed of materials such as polyethylene, polypropylene or ETFE, and electrically insulates anode  40  from the internal surface of housing  10 . Case liner  67  further separates cathode  55  from the internal surface of housing  10 , even though cathode  55  is electrically connected to housing  10  by cathode current collector  60 .  
         [0038]    A cathode assembly comprising cathode  55  and cathode current collector  60  is permeated by electrolyte. Cathode current collector  60  is placed in a closely fitting die fixture such that the die maintains the shape of cathode current collector  60  as the cathode assembly is formed. A measured volume of cathode mixture comprising, for example, a mixture of powdered manganese dioxide, an inert powdered binding material such as PTFE and conductivity enhancer such as graphite or carbon black is placed into the die inside the current collector. Other suitable cathode materials may be employed such as silver vanadium oxide (Ag 2 V 4 O 11 ) or mercuric oxide. The cathode mixture is compressed in a press (e.g., at ambient temperature and at 20-40 tons of gauge pressure for about 1-20 seconds) within cathode current collector  60  to form a self-supporting cathode body  55  having opposite, exposed, flat surfaces.  
         [0039]    The liquid electrolyte that is poured into the cell  1  before the disc  35  is welded in place can include an organic solvent in combination with an ionizing solute. Organic solvents known for use in such lithium/SVO cells in combination with lithium salts can be, for example, 3-methyl-2-oxazolidone, sulfolane, tetrahydrofuran, methyl-substituted tetrahydrofuran, 1,3-dioxolane, propylene carbonate (PC), ethylene carbonate, gamma-butyrolactone, ethylene glycol sulfite, dimethylsulfite, dimethyl sulfoxide or mixtures thereof and also, for example, low viscosity cosolvents such as tetrahydrofuran (THF), methyl-substituted tetrahydrofuran (Met-THF), dioxolane (DIOX), 1, 2 dimethoxyethane (DME), dimethyl isoxazole (DMI), diethyl carbonate(DEC), ethylene glycol sulfite (EGS), dioxane, dimethyl sulfite (DMS) dimethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, ethylene carbonate, gamma-butyrolactone, acetonitrile, formamide, dimethyl formamide, triglyme (tri(ethylene glycol)dimethyl ether), diglyme (diethylene glycol dimethyl ether), glyme (ethylene glycol dimethyl ether), nitromethane and mixtures thereof or the like. The ionizing solute for these cells can be a simple salt or double salts or mixtures thereof, as for example, LiBF 4 , LiAsF 6 , LiPF 6  and LiClO 4 , LiCF 3 SO 3 , LiN(SOCL 2 ) 3 , Li(SO 3 )(CF 3 ) 3 , or LiC(SO 2 CF 3 ) 2 , that produce an ionically conductive solution when dissolved in one or more solvents.  
         [0040]    The embodiment of the present invention illustrated in the figures is representative of a SIGMA electrochemical cell manufactured by MEDTRONIC, INC., which is a medium-rate electrochemical cell having a lithium anode and a Combination Silver Vanadium Oxide (CSVO) cathode for electrically powering pacemakers and the like. The SIGMA battery comprises a CSVO pressed powder cathode, a pressed lithium metal anode and a liquid electrolyte containing 1 molar LiAsF 6 , in a mixture of 50% PC/50% glyme.  
         [0041]    Insulator  90  is most preferably molded from polypropylene, ETFE, polyethylene or any other suitable, preferably polymeric, material capable of withstanding exposure to the various constituents and components disposed inside cell  1 , such as the liquid electrolyte. Those skilled in the art will now understand that other compositions of matter than those set forth explicitly herein may also find application in the formation of insulator  90  of the present invention.  
         [0042]    In accordance with the present invention, the vulnerable surfaces of a feedthrough, e.g., feedthrough  5 , employed in electrochemical cells, e.g., cell  1 , are coated with a substantially uniform, electrically insulating, coating of a thermoplastic polymer that can be applied in a thin, uniform layer, that has a high melting point, and that is resistant to degradation by the electrolyte to form an electrically insulating coating that strongly adheres to the feedthrough surfaces. The applied thermoplastic polymer protects the vulnerable surfaces prone to attack by the reactive electrolyte and/or blocks the formation of an electrically conductive deposit bridging the surface of the feedthrough insulator  28 . The thermoplastic polymer used to form the coating is preferably a polyimide.  
         [0043]    In the process of fabricating the cell  1 , the feedthrough ferrule  27  is inserted into the opening  20  through the cell cover  15  and welded to the cell cover  15  forming a circular weld. Preferably, a leak test, e.g., a helium leak test, is conducted to determine if any cracks have formed in the glass insulator  28 . The interior vulnerable surfaces of the feedthrough  5  are then protected by applying a thermoplastic coating, particularly a polyimide coating, in liquid form and curing or drying the applied layer to form a thin film that is tightly bonded to the vulnerable surfaces.  
         [0044]    [0044]FIG. 3 is an expanded cross-section view of the feedthrough ferrule  27  welded into the opening  20  through the cell cover  15  depicting one version of the polyimide coating  100  applied over the surface of the feedthrough insulator  28  to extend between the ferrule  27  and the feedthrough pin  25 . The polyimide coating  100  extends substantially over the inner glass surface of the glass insulator  28 .  
         [0045]    [0045]FIG. 4 is an expanded cross-section view of the feedthrough ferrule  27  welded into the opening  20  through the cell cover  15  depicting a second version of the polyimide coating  100 ′ applied over the surface of the feedthrough insulator  28  to extend for a first predetermined distance D 1  away from the insulator  28  over the surface of ferrule  27  and for a second predetermined distance D 2  away from the insulator  28  along the feedthrough pin  25 .  
         [0046]    The liquid polyimide coating  100  or  100 ′ can be applied manually or automatically to the vulnerable surfaces as depicted in FIGS. 3 and 4 through a variety of application or deposition techniques. For example, the polyimide coating may be manually applied using a syringe and fine needle or pipette or using a brush or dabbing instrument. The wetting characteristics of liquid polyimides ensure wetting of the vulnerable surfaces. Techniques, e.g., centrifuging, may be employed to drive out any air bubbles if necessary. But, generally, the application is easier and the wetting is superior to those using suspended ETFE powder. The non-uniformity in the thickness of the applied ETFE powder suspension and the discoloration or charring of the ETFE during curing are avoided.  
         [0047]    Moreover, the polyimide dries or cures into a substantially uniformly thick polyimide coating  100 ,  100 ′ that tenaciously adheres to the vulnerable surfaces. The melting temperature of the polyimide coating is advantageously higher than the molding temperature that is employed to later mold the insulator  90  in place. Consequently, the polyimide coating does not soften or loosen during molding of the insulator  90 . In this way, gaps between the insulator  90  and the polyimide coating  100 ,  100 ′, are advantageously minimized.  
         [0048]    In an initial screening test, various polyimides were found to be resistant to degradation upon exposure for 5 weeks at 70° C. to an electrolyte comprising 1 molar LiBF 6  in a 60% GBL/40% glyme solvent.  
         [0049]    A header assembly  19  comprising the feedthrough  5  welded with the cover  15  with the a pin insulator sub-assembly  97  formed after the molding of insulator  90  over the interior portion of the feedthrough pin  25 , the polyimide coating  100  (or  100 ′) and to or over the ferrule  27  is depicted in FIGS.  5 ( a ) and  5 ( b ). Various shapes and dimensions are illustrated and called out in FIGS.  5  ( a ) and  5 ( b ) related to the specific cell  1  disclosed in the above-referenced &#39;206 patent that are not material to the practice of the present invention but are included for completeness of description of a preferred embodiment of practicing the invention.  
         [0050]    In this respect, the lower portion of feedthrough pin  25  is most preferably bent at a 90° angle so that pin  25  extends laterally away from feedthrough vertical centerline  29  a sufficient distance to permit easy mechanical and electrical connection of pin  25  to anode current collector  45 . The lower end of feedthrough pin  25  is electrically connected to current collector  45 . First and second portions  98  and  99  of insulator  90  are disposed beneath an internal surface  16  of cover  15  and inside housing  10 . At least portions of feedthrough pin  25  are connected to or enclosed within feedthrough pin insulator  90 . First portion  98  is disposed at a location at or near ferrule  27 . Second portion  99  is disposed at a location at or near anode  40 . Feedthrough pin insulator  90  preferably extends between anode  40  and ferrule  27 , and at least portions of feedthrough pin  25  engage or are enclosed thereby. Feedthrough pin insulator  90  most preferably electrically insulates feedthrough pin  25  from ferrule  27  and other cell components having the same electrical potential as the cathode. Feedthrough pin insulator  90  has gate or dam  95  attached to or forming part of second portion  99 . Dam or gate  95  has sealing surface  91  forming a portion thereof for preventing or inhibiting anode material from being extruded or pushed therearound when sealing surface  91  is pushed against a sidewall of an anode formation cavity during an anode formation process as described in the above-referenced &#39;206 patent.  
         [0051]    Thus, an example of the coating of vulnerable surfaces of electrochemical cell feedthroughs with a polyimide is described above in relation to a known electrochemical cell  1  of the above-referenced &#39;206 patent. It will be understood that the present invention can be practiced in the protection of vulnerable surfaces of feedthroughs of any configuration affixed to a side wall of a housing of an electrochemical cell of any configuration or chemistry where such coating would be beneficial  
         [0052]    All patents and publications referenced herein are hereby incorporated by reference in their entireties.  
         [0053]    It will be understood that certain of the above-described structures, functions and operations of the above-described preferred embodiments are not necessary to practice the present invention and are included in the description simply for completeness of an exemplary embodiment or embodiments.  
         [0054]    In addition, it will be understood that specifically described structures, functions and operations set forth in the above-referenced patents can be practiced in conjunction with the present invention, but they are not essential to its practice.  
         [0055]    It is therefore to be understood, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described without actually departing from the spirit and scope of the present invention.