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'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 LiCF3) 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.