Patent Publication Number: US-2010112430-A1

Title: Electrochemical Cells Employing Expandable Separators

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. Ser. No. 11/380,789, filed Apr. 28, 2006, now allowed, the disclosure of which is incorporated by reference in its entirety herein. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to an implantable medical device (IMD) and, more particularly, to an electrochemical cell for use in an IMD wherein separation between the electrodes is maintained by means of a separator having an expandable joint. 
     BACKGROUND OF THE INVENTION 
     Electrochemical cells, such as batteries and capacitors, are important components in IMDs (e.g., implantable defibrillators) because they store and deliver the energy necessary to correct cardiac arrhythmias (e.g., tachycardia, bradycardia, atrial fibrillation, and/or ventricular fibrillation). Ideally, such batteries should have a high rate capability to provide the required charge, possess low self-discharge to increase useful life, and be highly reliable. Further, because these medical devices are being surgically implanted within a patient&#39;s body, the battery should be as compact as possible. Lithium batteries are now commonly used in IMDs and generally include a lithium anode and a cathode that may contain carbon monofluoride and/or silver vanadium oxide. The anode and cathode are enveloped in an electrolyte, or electrolytic solution, containing a solute (typically a lithium salt such as LiCF 3 ) and a solvent (e.g., dimethoxyethane). 
     It is known that the electrodes (e.g., anode and cathode) are separated to prevent arcing and to allow charge to accumulate without short-circuiting the electrochemical cell. Such separators should be resistant to degradation, have sufficient thickness to maintain inter-electrode separation without interfering with cell performance, and exhibit sufficient surface energy to augment electrolyte wettability and absorption. In addition, the separator should have an electrical resistivity sufficiently high to prohibit short circuit current from flowing directly between the electrodes through the separator. These requirements are balanced by the need for a porosity sufficient to freely permit ionic communication between the electrodes. 
     Separators may be made from a roll or sheet of separator material, and a variety of separator materials have been used. Paper (e.g., Kraft paper) is a cellulose-based separator material that is sometimes used and may be manufactured with high chemical purity. An alternative to paper separators are polymeric separators that may be made of microporous films (e.g., polytetrafluoroethylene) or polymeric fabrics (e.g., a woven synthetic halogenated polymer). Hybrid separators employing polymers (e.g., polypropylene or polyester) and paper are also known. 
     Separators having strong tensile properties are less likely to tear or break during fabrication and are better able to withstand internal stresses due to changes in the electrode volumes during discharge and re-charging cycles. Cathode material may swell as a battery is discharged. Thus, the space made available for batteries in medical devices may be somewhat larger than the non-swollen size of the battery thereby increasing the overall size of the medical device. When a separator is sealed around and envelopes a cathode, the volumetric expansion places stresses on the separator, perhaps causing tearing or rupturing of the separator that, in turn, may cause short circuits. This problem is exacerbated when thicker cathodes, which experience greater expansion (e.g., 100 percent), are employed. 
     According, it would be desirable to provide an electrochemical cell separator assembly that accommodates greater electrode expansion without requiring increased separator margin (i.e., the distance between the edge of a cathode and the edge of the separator seal). In addition, it would further be desirable to provide a separator assembly including an expandable separator joint for accommodating electrode expansion while reducing the possibility of separator rupture. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following drawings are illustrative of particular embodiments of the invention and therefore do not limit the scope of the invention, but are presented to assist in providing a proper understanding. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed descriptions. The present invention will hereinafter be described in conjunction with the appended drawings, wherein like reference numerals denote like elements, and: 
         FIG. 1  is an exploded view an implantable medical device; 
         FIG. 2  is an isometric cutaway view of the pulse generator employed in the implantable medical device shown in  FIG. 1 ; 
         FIGS. 3-5  illustrate steps in the manufacture of a separator in accordance with the prior art; 
         FIG. 6  is an isometric view of the separator shown in  FIGS. 3-5 ; 
         FIG. 7  is cross-sectional view of the separator shown in  FIGS. 3-6  in an expanded state; 
         FIG. 8  is a cross-sectional view of a second known separator enveloping a relatively thick electrode; 
         FIG. 9  is a cross-sectional view of a separator prior to trimming in accordance with a first embodiment of the present invention; 
         FIG. 10  is an isometric view of the separator shown in  FIG. 9  after trimming; 
         FIGS. 11 ,  12 , and  13  are isometric views of the separator shown in  FIG. 9  illustrating first, second, and third alternative ways, respectively, in which the separator may be made; 
         FIG. 14  is a cross-sectional view of the separator shown in  FIG. 9  in an expanded state; 
         FIGS. 15 ,  16 , and  17  are cross-sectional views of separators in accordance with second, third, and fourth embodiments of the present invention, respectively; and; 
         FIGS. 18 and 19  are side and top cutaway views, respectively, of an electrochemical cell employing the inventive separator. 
     
    
    
     DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT 
     The following description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides a convenient illustration for implementing an exemplary embodiment of the invention. Various changes to the described embodiment may be made in the function and arrangement of the elements described herein without departing from the scope of the invention. 
       FIG. 1  is an exploded view of an implantable medical device  80 . Medical device  80  may be, for example, a cardioverter defibrillator capable of monitoring cardiac signals and delivering therapy pulses to pace a patient&#39;s heart and, if necessary, to treat ventricular fibrillation. Device  80  includes a pulse generator  82  comprising a canister  84  (e.g., a biocompatible metal, such as titanium, aluminum, steel, etc.) having a connector block  86  fixedly coupled thereto. Connector block  86  is coupled to a lead  88  by way of an extension  90 . The proximal portion of extension  90  comprises a connector  92  having two connection prongs  94  extending therefrom. Prongs  94  are configured to be received or plugged into two ports  96  provided within connector block  86 . One or more setscrew blocks may be provided within connector block  86  and tightened (e.g., via a torque wrench) upon insertion of prongs  94  into ports  96  to fixedly secure connector  92  to pulse generator  82 . 
     The distal end of extension  90  is provided with a connector portion  98  having a plurality of contacts  100  disposed thereon. The proximal end of lead  88  also includes a plurality of contacts  102  thereon, and the distal end of lead  88  includes a plurality of distal electrodes  104 . Connector portion  98  is configured to receive the proximal end of lead  88  so as to electrically couple contacts  100  to contacts  102 . Lead  88  and extension  90  each comprise an insulative tubing that carries a plurality of conductive filers. For example, lead  88  and extension  90  may each comprise a polyurethane or silicon tube having an insulative silicon core. Each filer passes through the silicon core, possibly within a narrower polyurethane tube to provide redundant insulation. The filers carried within lead  88  electrically couple proximal contacts  102  to distal electrodes  104 , and the filers carried within extension  90  electrically couple prongs  94  to contacts  100 . Thus, when connector  92  is plugged into connector block  86 , and when the proximal end of lead  88  is received by connector portion  98 , pulse generator  82  may send electrical signals to and receive electrical signals from distal electrodes  104 . 
       FIG. 2  is an isometric cutaway view of pulse generator  82  ( FIG. 1 ). Here, it may be seen that canister  84  houses a battery  108  and control circuitry  106 , which is mounted on a printed circuit board  110 . Battery  108  may be coupled to circuitry  106  via a lead  112 . A multipolar feedthrough assembly  114  guides an array of terminal pins through canister  84 . At their first ends, the terminal pins of assembly  114  are electrically coupled to circuitry  106  via a first plurality of connective wires  116  (e.g., gold). At their opposite ends, the terminal pins of assembly  114  may be coupled to a second plurality of wires (not shown) that extends through connector block  86  to contact prongs  94  ( FIG. 1 ). In this way, circuitry  106  may be electrically coupled to distal electrodes  104 . When lead  88  is positioned within or proximate a patient&#39;s heart, circuitry  106  may monitor cardiac signals detected by distal electrodes  104 . If determining that the heart is experiencing an arrhythmia (e.g., bradycardia, tachycardia, or fibrillation), circuitry  106  may cause battery  108  to deliver therapy (e.g., pacing or defibrillating electrical pulses) via electrodes  104  to return the heart to its normal rhythm. 
     In battery  108  and other such electrochemical cells, the electrodes (e.g., the anode and cathode) must remain physically separated to prevent shorting. As explained above, separators have been employed that physically partition the electrodes while permitting the flow of electrolytic fluid there between. Generally, such separators comprise one or more sheets of insulative material (e.g., paper, polymer, paper/polymer hybrids, etc.) that form a pouch or envelope, which receives a selected electrode (e.g., a cathode) therein. As explained more fully below, the enveloped electrode may swell during operation of the electrochemical cell. Therefore, the separator envelope must be capable of expanding without rupturing to accommodate the volumetric growth of the electrode. 
       FIGS. 3-6  illustrate steps in the manufacture of a separator  130  in accordance with the prior art. Referring to  FIG. 3 , an electrode  132  is disposed between first and second sheets  134  and  136  of separator material (e.g., polymeric film). Electrode  132  may be, for example, the cathode of a battery (e.g., battery  108  described above in conjunction with  FIG. 2 ). Separator sheets  134  and  136  are sealed together to enclose electrode  132 . In particular, the edges of separator sheet  134  may be folded toward sheet  136  and sealed thereto along seam  138  as shown in  FIG. 4 . The edges of separator sheet  134  are joined to sheet  136  using known heating techniques. The excess portions of sheet  136  (and/or sheet  134 ) extending beyond seam  138  may be trimmed away to leave electrode  132  sealed within an insulative envelope  142  formed by separator sheets  134  and  136  as is shown in  FIG. 5  (a cross-sectional view) and in  FIG. 6  (an isometric view). 
     As mentioned above, electrode  132  may swell during operation of an electrochemical cell employing electrode  132 . If electrode  132  is the cathode of a battery, for example, it may absorb electrolytic fluid and anode material (e.g., lithium) during discharge. To provide for the expansion of electrode  132  and to avoid rupturing of the separator, additional room is allotted within the separator envelope beyond that which is needed to accommodate electrode  132  in its normal, unexpanded state. Specifically, a lateral separator margin (designated X in  FIG. 5 ) is provided between the lower periphery of electrode  132  and seam  138 . As electrode  134  swells, the height (and, perhaps, the width and length) of electrode  134  increases. The outer surfaces of electrode  134  thus press outwardly on the inner surfaces of separator  130 , and separators sheets  134  and  136  diverge. As shown in  FIG. 7 , separator  130  expands and the additional volumetric space provided by separator margin X is consumed by electrode  132  in its swollen condition. 
     The need to provide separator margin results in an increase in the overall size of the separator envelope, which, in turn, increases the dimensions of an electrochemical cell employing the separator. The separator margin may be made even larger with the increased use of thicker cathodes, which may experience even greater expansion (e.g., 100%). To illustrate this point,  FIG. 8  is a cross-sectional view of a separator  147  disposed around a relatively thick electrode  146 . As can be seen, separator  147  is provided with a lateral margin Y, which is considerably larger than margin X of separator  130  ( FIG. 5 ). 
       FIGS. 9 and 10  are cross-sectional and isometric views, respectively, of a separator  149  in accordance with a first embodiment of the present invention. In a similar manner to separator  130  ( FIGS. 3-7 ), separator  149  comprises upper and lower separator sheets  150  and  152  between which an electrode  148  is disposed. However, unlike separator  130 , an expansion member or joint  154  (e.g., an accordion-shaped sleeve) is circumferentially disposed around electrode  148 . Expansion joint  154  is sealed to separator sheet  150  along seam  156  and to separator sheet  152  along seam  158 . Sealing may be accomplished with, for example, a soldering iron or other tool capable of applying heat while pressing portions of sheets  150  and  152  together. As was the case previously, separator sheets  150  and  152  are comprised of an insulative and porous material, such as an insulative paper, polymer, or paper/polymer hybrid. Though joint  154  may also comprise a similar or identical material, expansion joint  154  need not be porous. Thus, a wide variety of materials may be utilized for expansion joint  154 , providing that the selected material is insulative and may be bonded to separator sheets  150  and  152 . Preferably, a material having a high degree of strength and flexibility is employed, such as a polyolefin film. Expansion joint  154  may be formed by pre-setting (e.g., hand-creasing, heat-setting, etc.) one or more folds bends into a section of the selected insulative material. The folds or bends may each be generally V-shaped as shown in  FIGS. 9-14  or, instead, may each be generally curved as described below. After expansion joint  154  has been bonded to separators sheets  150  and  152 , any excess material (e.g., the portions of sheet  150  and/or sheet  152  extending beyond seams  156  and  158 ) is trimmed off and discarded ( FIG. 10 ). 
     Though expansion joint  154  was described above as an independent sleeve that was sealed between separator sheets  150  and  152 , it should be appreciated that joint  154  may be formed in other ways. For example, as indicated in  FIG. 11 , expansion joint  154  may be formed (e.g., folded, heat-set, etc.) in a periphery of separator sheet  150  that is subsequently folded toward separator sheet  152  and sealed thereto along seam  162 . Similarly, as indicated in  FIG. 12 , expansion joint  154  may be formed in a periphery of separator sheet  152  and subsequently sealed to separator sheet  150  along seam  166 . Furthermore, a combination of these two approaches may be taken wherein a portion of expansion joint  154  is formed in the peripheries of each of separator sheets  150  and  152 , which are then folded toward one another and sealed together along seam  170  as shown in  FIG. 13 . 
       FIG. 14  is a cross-sectional view of separator  149  after electrode  148  has swelled. As can be seen, separator  149  has expanded along expansion joint  154  to accommodate the growth of electrode  148  and the corresponding divergence of separator sheets  150  and  152 . It should thus be appreciated that, like conventional separators (e.g., separator  130 ), inventive separator  149  is capable of accommodating a relative large degree of electrode swelling without rupture. However, in contrast to known separators, separator  149  has a smaller separator margin and, therefore, a more compact size. This may be appreciated by comparing the separator margin X of separator  130  ( FIG. 5 ) and margin Y of separator  147  ( FIG. 7 ) to the separator margin of separator  149  ( FIGS. 9 ,  11 - 13 ). Thus, a more compact electrochemical cell may be achieved by employing the inventive separator. 
     Although the exemplary expansion joint (i.e., joint  154 ) is generally accordion-shaped, it should be noted that expansion joints of a variety shapes are contemplated. To this end,  FIGS. 15-17  illustrate three alternative expansions joints. In particular,  FIG. 15  illustrates a second expansion joint  174  having the general shape of a strain relief spring or the Greek letter omega. Expansion joint  174  is disposed between separator sheets  150  and  152  and sealed thereto as described above.  FIG. 16  illustrates a V-shaped expansion joint  176  also disposed between and sealed to sheets  150  and  152 . Finally,  FIG. 17  illustrates an generally a scalloped or crenate expansion joint  178  that is formed by folding separator sheet  150  downward and sealing sheet  150  to sheet  152  in the manner previously described. Alternatively, expansion joint  178  (or other expansion joint) may be formed from a single, relatively large separator sheet by folding the sheet in half and sealing three overlapping portions of the sheet together proximate electrode  148 . 
       FIGS. 18 and 19  are side and top cutaway views of an electrode assembly  180  employing separator  149  ( FIGS. 9-14 ). A current collector  182  comprises a flattened metal plate (e.g., titanium) having a plurality (e.g., a grid) of apertures therethrough and a lead  184  extending therefrom. Active material  186  (e.g., silver vanadium oxide powder) is disposed on either side of collector  182  to form an electrode  188  (e.g., a cathode), which is encased by separator  149  as described above. A slot (not shown) may be provided through separator  149  to accommodate lead  184 . A second electrode  190  (e.g., an anode) having a second lead  192  extending therefrom is disposed adjacent electrode  188 . Electrode assembly  180  may be disposed within a battery casing, which is sealed and filled with an electrolytic fluid. When so disposed, separator  149  prevents physical contact between electrodes  188  and  190 , but permits the flow of electrolytic fluid there between. 
     Considering the foregoing, it should be appreciated that there has been provided an electrochemical cell separator assembly that accommodates greater electrode expansion without requiring increased separator margin (i.e., the distance between the edge of a cathode and the edge of the separator seal). It should further be appreciated that a separator assembly has also been provided including an expandable separator joint for accommodating electrode expansion while reducing the possibility of separator rupture. Although the invention has been described with reference to a specific embodiment in the foregoing specification, it should be appreciated that various modifications and changes can be made without departing from the scope of the invention as set forth in the appended claims. Accordingly, the specification and figures should be regarded as illustrative rather than restrictive, and all such modifications are intended to be included within the scope of the present invention.