Patent Application
20080007216
The term percent depth-of-discharge (% DoD) is defined as the ratio of delivered capacity to theoretical capacity, times 100.
The term “pulse” means a short burst of electrical current of significantly greater amplitude than that of a pre-pulse current or open-circuit voltage immediately prior to the pulse. A pulse train consists of at least one pulse of electrical current. The pulse is designed to deliver energy, power or current. If the pulse train consists of more than one pulse, they are delivered in relatively short succession with or without open circuit rest between the pulses. An exemplary pulse train may consist of one to four 5 to 20-second pulses (23.2 mA/cm2) with about a 10 to 30 second rest, preferably about 15 second rest, between each pulse. A typically used range of current densities for cells powering implantable medical devices is from about 15 mA/cm2 to about 50 mA/cm2, and more preferably from about 18 mA/cm2 to about 35 mA/cm2. Typically, a 10 second pulse is suitable for medical implantable applications. However, it could be significantly shorter or longer depending on the specific cell design and chemistry and the associated device energy requirements. Current densities are based on square centimeters of the cathode electrode.
Rdc is calculated as follows: Rdc=(Vcellbkd−Vcellloaded)/I, where Vcellbkd is the cell voltage under pacing or sensing load (low drain in the microampere range), Vcellload is the cell voltage under the pulse applied load (typically in amps for defibrillation), and I is the current of the defibrillation pulse load.
An electrochemical cell that possesses sufficient energy density and discharge capacity required to meet the vigorous requirements of implantable medical devices comprises an anode of lithium and its alloys and intermetallic compounds including, for example, Li—Si, Li—Al, Li—B and Li—Si—B alloys. The greater the amounts of alloy material present by weight, however, the lower the energy density of the cell.
The form of the anode may vary, but preferably the anode is a thin metal sheet or foil of lithium, pressed or rolled on a metallic anode current collector, i.e., preferably comprising titanium, titanium alloy or nickel. Copper, tungsten and tantalum are also suitable materials for the anode current collector. The anode has an extended tab or lead of the same material as the current collector, i.e., preferably nickel or titanium, contacted by a weld to a cell case of conductive metal in a case-negative electrical configuration. Alternatively, the anode may be formed in some other geometry, such as a bobbin shape, cylinder or pellet to provide a low surface cell design.
The electrochemical cell further comprises a cathode of electrically conductive material that serves as the counter electrode. The cathode is preferably of solid materials having the general formula SMxV2Oy where SM is a metal selected from Groups IB to VIIB and VIII of the Periodic Table of Elements, and wherein x is about 0.30 to 2.0 and y is about 4.5 to 6.0 in the general formula. By way of illustration, and in no way intended to be limiting, one exemplary cathode active material comprises silver vanadium oxide having the general formula AgxV2Oy in either its β-phase having x=0.35 and y=5.8, γ-phase having x=0.74 and y=5.37, or ∈-phase having x=1.0 and y=5.5, and combinations of phases thereof.
Other suitable cathode active materials include copper silver vanadium oxide (CSVO), manganese dioxide, carbon, fluorinated carbon, titanium disulfide, cobalt oxide, nickel oxide, copper vanadium oxide, LiNO2, LiMn2O4, LiCoO2, LiCu0.92Sn0.08O2, LiCO1xNixO2, and mixtures thereof.
Before fabrication into an electrode for incorporation into an electrochemical cell according to the present invention, the cathode active material is preferably mixed with a binder material such as a powdered fluoro-polymer, more preferably powdered polytetrafluoroethylene or powdered polyvinylidene fluoride present at about 1 to about 5 weight percent of the cathode mixture. Further, up to about 10 weight percent of a conductive diluent is preferably added to the cathode mixture to improve conductivity. Suitable materials for this purpose include acetylene black, carbon black and/or graphite or a metallic powder such as powdered nickel, aluminum, titanium, stainless steel, and mixtures thereof. The preferred cathode active mixture thus includes a powdered fluoro-polymer binder present at a quantity of at least about 3 weight percent, a conductive diluent present at a quantity of at least about 3 weight percent and from about 80 to about 99 weight percent of the cathode active material.
Cathode components for incorporation into the cell may be prepared by rolling, spreading or pressing the cathode active mixture onto a suitable current collector selected from the group consisting of stainless steel, titanium, tantalum, platinum, nickel, and gold. Cathodes prepared as described above may be in the form of one or more plates operatively associated with at least one or more plates of anode material, or in the form of a strip wound with a corresponding strip of anode material in a structure similar to a “jellyroll”.
In order to prevent internal short circuit conditions, the cathode is separated from the anode material by a suitable separator material. The separator is of electrically insulative material, and the separator material also is chemically unreactive with the anode and cathode active materials and both chemically unreactive with and insoluble in the electrolyte. In addition, the separator material has a degree of porosity sufficient to allow flow therethrough of the electrolyte during the electrochemical reaction of the cell. Illustrative separator materials include fabrics woven from fluoropolymeric fibers including polyvinylidine fluoride, polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethylene used either alone or laminated with a fluoropolymeric microporous film, non-woven glass, polypropylene, polyethylene, glass fiber materials, ceramics, polytetrafluoroethylene membrane commercially available under the designation ZITEX (Chemplast Inc.), polypropylene membrane commercially available under the designation CELGARD (Celanese Plastic Company, Inc.), a membrane commercially available under the designation DEXIGLAS (C. H. Dexter, Div., Dexter Corp.), and a membrane commercially available under the designation TONEN®.
The electrochemical cell further includes a nonaqueous, ionically conductive electrolyte serving as a medium for migration of ions between the anode and the cathode during electrochemical reactions of the cell. The electrochemical reaction at the electrodes involves conversion of ions in atomic or molecular forms that migrate from the anode to the cathode. Thus, suitable nonaqueous electrolytes are substantially inert to the anode and cathode materials, and they exhibit those physical properties necessary for ionic transport, namely, low viscosity, low surface tension and wettability.
A suitable electrolyte has an inorganic, ionically conductive lithium salt dissolved in a mixture of aprotic organic solvents comprising a low viscosity solvent and a high permittivity solvent. Preferred lithium salts include LiPF6, LiBF4, LiAsF6, LiSbF6, LiClO4, LiO2, LiAlCl4, LiGaCl4, LiC(SO2CF3)3, LiN(SO2CF3)2, LiSCN, LiO3SCF3, LiC6F5SO3, LiO2CCF3, LiSO6F, LiB(C6H)4, LiCF3SO3, and mixtures thereof.
Low viscosity solvents useful with the present invention include esters, linear and cyclic ethers and dialkyl carbonates such as tetrahydrofuran (THF), methyl acetate (MA), diglyme, trigylme, tetragylme, dimethyl carbonate (DMC), 1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), 1-ethoxy,2-methoxyethane (EME), ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, diethyl carbonate, dipropyl carbonate, and mixtures thereof. High permittivity solvents include 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-2-pyrrolidone (NMP), and mixtures thereof. In the present invention, the preferred electrolyte is 0.8M to 1.5M LiAsF6 or LiPF6 dissolved in a 50:50 mixture, by volume, of propylene carbonate and 1,2-dimethoxyethane.
The preferred form of the electrochemical cell is a case-negative design wherein the anode/cathode couple is inserted into a conductive metal casing connected to the anode current collector, as is well known to those skilled in the art. A preferred material for the casing is stainless steel, although titanium, mild steel, nickel, nickel-plated mild steel and aluminum are also suitable. The casing header comprises a metallic lid having a sufficient number of openings to accommodate the glass-to-metal seal/terminal pin feedthrough for the cathode electrode. The anode is preferably connected to the case or the lid. An additional opening is provided for electrolyte filling. The casing header comprises elements having compatibility with the other components of the electrochemical cell and is resistant to corrosion. The cell is thereafter filled with the electrolyte described hereinabove and hermetically sealed such as by close-welding a stainless steel plug over the fill hole, but not limited thereto. The cell of the present invention can also be constructed in a case-positive design.
An exemplary implantable medical device powered by a Li/SVO cell is a cardiac defibrillator, which requires a power source for a generally medium rate, constant resistance load component provided by circuits performing such functions as, for example, the heart sensing and pacing functions. This requires electrical current of about 1 microampere to about 100 milliamperes. From time-to-time, the cardiac defibrillator may require a generally high rate, pulse discharge load component that occurs, for example, during charging of a capacitor in the defibrillator for the purpose of delivering an electrical shock therapy to the heart to treat tachyarrhythmias, the irregular, rapid heartbeats that can be fatal if left uncorrected. This requires electrical current of about 1 ampere to about 5 amperes.
Voltage delay is a phenomenon typically exhibited in a lithium/silver vanadium oxide cell that has been depleted of about 25% to 70% of its capacity and is subjected to high current pulse discharge applications. The voltage response of a cell that does not exhibit voltage delay during the application of a short duration pulse or pulse train has a distinct signature. In particular, the cell potential decreases throughout the application of the pulse until it reaches a minimum at the end of the pulse.
On the other hand, the voltage response of a cell that exhibits voltage delay during the application of a short duration pulse or during a pulse train can take one or both of two forms. One form is that the leading edge potential of the first pulse is lower than the end edge potential of the first pulse. In other words, the voltage of the cell at the instant the first pulse is applied is lower than the voltage of the cell immediately before the first pulse is removed. The second form of voltage delay is that the minimum potential of the first pulse is lower than the minimum potential of the last pulse when a series of pulses have been applied.
In that respect, the initial drop in cell potential during the application of a short duration pulse reflects the resistance of the cell, i.e., the resistance due to the cathode, the cathode-electrolyte interphase, the anode, and the anode-electrolyte interphase. The formation of a passivating surface film is unavoidable for lithium metal anodes due to their relatively low potential and high reactivity towards organic electrolytes. In the absence of voltage delay, the resistance due to passivated films on the anode and/or cathode is negligible. Thus, the ideal anode surface film should be electrically insulating and ionically conducting. While most lithium electrochemical systems meet the first requirement, the second requirement is difficult to achieve. In the event of voltage delay, the resistance of these films is not negligible, and as a result, impedance builds up inside the cell due to this surface layer formation that often results in reduced discharge voltage and reduced cell capacity. In other words, the drop in potential between the background voltage and the lowest voltage under pulse discharge conditions, excluding voltage delay, is an indication of the conductivity of the cell, i.e., the conductivity of the cathode, anode, electrolyte, and surface films, while the gradual decrease in cell potential during the application of the pulse train is due to polarization of the electrodes and electrolyte.
An automatic implantable cardioverter defibrillator essentially consists of an electrochemical cell as a power source for charging at least one electrolytic capacitor to deliver the electrical shock therapy to the patient's heart. Microprocessors powered by the cell perform the heart sensing and pacing functions and initiate capacitor charging to deliver the electrical shock therapy. Not only do Li/SVO cells experience irreversible Rdc growth and voltage delay problems beginning at about 25% DoD as previously explained in the Prior Art section, but electrolytic capacitors can experience degradation in their charging efficiency after long periods of inactivity. It is believed that the anodes of electrolytic capacitors, which are typically of aluminum or tantalum, develop microfractures after extended periods of non-use. These microfractures consequently result in extended charge times and reduced breakdown voltages. Degraded charging efficiency ultimately requires the Li/SVO cell to progressively expend more and more energy to charge the capacitors for providing therapy.
To repair this degradation, microprocessors controlling the implantable medical device are programmed to regularly charge the electrolytic capacitors to or near a maximum-energy breakdown voltage (the voltage corresponding to maximum energy) before discharging them internally through a non-therapeutic load. The capacitors can be immediately discharged once the maximum-energy voltage is reached or they can be held at maximum-energy voltage for a period of time, which can be rather short, before being discharged. These periodic charge-discharge or charge-hold-discharge cycles for capacitor maintenance are called “reforms.” Reforming implantable defibrillator capacitors at least partially restores and preserves their charging efficiency. An industry-recognized standard is to reform implantable capacitors by pulse discharging the connected electrochemical cell about once every three months throughout the useful life of the medical device, which is typically dictated by the life of the cell.
As previously discussed, while the onset point is somewhat dictated by whether the cathode is of a pressed powder design as described in U.S. Pat. Nos. 4,830,940 and 4,964,877, both to Keister et al. or of a freestanding sheet of SVO as described in U.S. Pat. Nos. 5,435,874 and 5,571,640, both to Takeuchi et al., it is generally recognized that a typical Li/SVO cell experiences irreversible Rdc growth and voltage delay in the about 25% to about 70% DoD region. In any event, the discharge life of a Li/SVO cell can be divided into three regions. For a pressed powder cathode, the first region ranges from beginning of life (BOL) to about 35% DoD where voltage delay and irreversible Rdc growth are not significant. The second region is termed middle-of-life (MOL) and ranges from about 35% DoD to about 70% DoD. The third region ranges from about 70% DoD to end of life (EOL) and is where voltage delay and irreversible Rdc growth are significantly reduced, if not entirely absent again. On the other hand, for a freestanding sheet cathode, the first region ranges from beginning of life to about 25% DoD, the second region ranges from about 25% DoD to about 45% DoD and the third region ranges from about 45% DoD to end of life.
Thus, the basis for the present invention is to precisely determine when the second discharge region constituting the onset of irreversible Rdc growth and voltage delay begins. By delineating the boundaries of the second discharge region, it is known when to end and again begin periodically pulse discharging a Li/SVO cell about once every 90 days, as deemed necessary for capacitor reform by current industry standards, so that the cell can be pulsed more frequently than every 90 days in this precisely defined second region. The problem is that in the second discharge region of a Li/SVO cell, more frequent pulse discharging never completely eliminates the voltage delay phenomenon. It merely lessens its severity to an acceptable amount. This is why the Rdc is termed irreversible. Nonetheless, it is beneficial to minimize the effects of irreversible Rdc growth and voltage delay as much as possible.
In regions 1 and 3 of the discharge curve for a Li/SVO cell all that is required to eliminate reversible Rdc anode passivation layer is to pulse discharge the cell. This serves to break up and dissipate the passivation layer, thereby eliminating the cause of reversible Rdc.
For that reason, the present invention does not require that a threshold value for Rdc and voltage delay be exceeded. Rather, it is based on the initiation of additional pulsing on the detection of an increase in Rdc or charge time. This difference allows for greater flexibility in initiating additional cell pulsing, and covers a situation where the cell is displaying a Rdc reading or charge time below a threshold value, but these parameters are increasing relative to previous measurements. In this situation, initiating additional cell pulsing early-on is beneficial in preventing the formation of irreversible resistance rise. The method of the present invention can be used when the cell is combined with a variety of capacitor types, such as wet-tantalum or aluminum electrolytic capacitors.
In any event, the amount of Rdc increase depends on the particular cell design and device design. Depending on a particular cell and device design, an increase in Rdc of from about 0.0005 ohms to about 0.008 ohms, more preferably from about 0.001 ohms to about 0.005 ohms, is sufficient to trigger increased pulsing. Additional pulsing may include more frequent pulsing, on the order of every few days or weeks. Preferably, the more frequent pulsing occurs from about every seven days to about every 60 days, more preferably about every 30 days. The pulsing consists of at least one pulse of electrical current or, if it is of a pulse train consisting of more than one pulse, they are delivered in relatively short succession with or without open circuit rest between the pulses.
According to another embodiment of the invention, an increase in the time it takes the cell to charge the ICD capacitors to a predetermined voltage, for example their maximum-energy breakdown voltage, is used to trigger increased cell pulsing. Again, both cell design and device design influence the magnitude of the change in charge time needed to trigger increased pulsing. Preferably, an increased charge time in the range of from about 2 seconds to about 5 seconds for a 50 J energy pulse is sufficient to trigger the application multiple cell pulses.
Additional pulsing may also include applying several pulses in a short time span (over several seconds). For example, if an increase in cell Rdc or device charge time is detected, then additional pulses can be applied immediately to the cell, with from about two to four pulses applied in a relatively short time span. In a typical protocol, multiple pulses are applied with from two to 15 seconds between each pulse. The application of multiple pulses over a short time span provides the benefit of efficiently reducing cell Rdc while simultaneously lowering the charge time of the device capacitors.
The following example describes the manner and process of a reducing the internal resistance of an electrochemical cell according to the present invention, and it sets forth the best mode contemplated by the inventors of carrying out the invention.
A plurality of Li/SVO cells of an identical design, materials, and construction were divided into Groups A and B. The cells were then discharged under conditions similar to that employed by ICDs. In particular, the Group A cells were discharged at 37° C., with a single 2.5 A pulse of 10 seconds duration applied every 90 days (curve 50). The Group B cells were discharged under the same conditions, except that they were switched to a 30 day pulse interval (curve 52). The initiation of the pulse interval switch was defined by an increase in Rdc observed in the cells.
As illustrated in
It is appreciated that various modifications to the inventive concepts described herein may be apparent to those of ordinary skill in the art without departing from the spirit and scope of the present invention as defined by the appended claims.
This application claims priority from U.S. Provisional Application Ser. No. 60/806,864, filed Jul. 10, 2006.
| Number | Date | Country | |
|---|---|---|---|
| 60806864 | Jul 2006 | US |
