Fuel gauge for an alkali metal electrochemical cell

Abstract: A "fuel gauge" for a pulse dischargeable alkali metal/solid cathode cell is described. The rate of voltage recovery is used to determine the state of charge of the cell. Voltage recovery includes recovery from one load to a second, lighter load, or a loaded condition to OCV. The present invention is particularly useful as an end-of-life indicator for a Li/CF x cell powering an implantable medical device.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to the conversion of chemical energy to electrical energy. More particularly, the present invention relates to the use of data collected from a discharging alkali metal/solid cathode cell to provide a "fuel gauge" for determining the state of charge of the cell. According to the present invention, the use of voltage recovery data from one load to a second, lighter load in a pulse discharging cell, particularly a Li/CF x cell, is used to estimate the depth-of-discharge (DOD) for the cell. The depth-of-discharge is directly related to the remaining discharge capacity. The use of the pulse discharge data as a fuel gauge according to the present invention is, therefore, particularly useful in an electrochemical cell powering an implantable medical device where the cell may discharge under a light load for extended periods of time interrupted by pulse discharge. 2. Prior Art In a discharging electrochemical cell, especially an implantable cell, it is desirable to know the amount or quantity of available capacity that remains. This affords the physician an opportunity to schedule surgery for device replacement in a timely and orderly manner without causing undue or unnecessary harm to the patient. Up to now, attempts to determine the charge condition or consumed battery capacity in a pulse dischargeable cell have generally relied on a counter to tabulate the number of pulses delivered by the battery, and hence, the remaining capacity. Representative of these types of devices are U.S. Patent Nos. 4,556,061 to Barreras et al. and 5,144,218 to Bosscha. The present invention is not dependent on a raw or cumulative pulse count, but rather, the recovery rate from one load to a second, lighter load or to open circuit voltage (OCV) to determine the depth of discharge for the cell. SUMMARY OF THE INVENTION According to the present invention, the voltage recovery data from one load to a second, lighter load in a pulse discharging cell, particularly a Li/CF x cell, are used to calculate the depth-of-discharge (DOD) for the cell. In that respect, voltage recovery according to the present invention is determined from one load to a second, lighter load or from a loaded condition to open circuit voltage. Such load variations can occur in an implantable medical device wherein the cell may discharge for extended periods under a light load interrupted by pulse discharge. The foregoing and additional advantages and characterizing features of the present invention will become clearly apparent upon a reading of the following detailed description together with the included drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic of an implantable medical device useful with the fuel gauge of the present invention. Fig. 2 is a graph of the voltage change/time change (ΔOCV/ΔHrs) ratio versus depth-of-discharge for a Li/CF x cell. Fig. 3 is a graph of the overlay of the predicted fuel gauge curve computed from the data graphed in Fig. 2, with the observed values. DETAILED DESCRIPTION OF THE INVENTION As used herein, the term "pulse" means a short burst of electrical current of a greater amplitude than that of a current immediately prior to the pulse. A pulse train consists of at least two pulses of electrical current delivered in relatively short succession with or without open circuit rest between the pulses. Referring now to the drawings, Fig. 1 shows a schematic embodiment of an electrochemical cell 10 according to the present invention provided as the power source for an implantable medical device 12. Representative medical devices include drug pumps, pacemakers, arterial defibrillators and neurostimulators, and the like. The cell 10 is connected to electronic circuitry 14 for the medical device 12. The electronics 14 enable the cell 10 to power the medical device 12 both at a relatively constant load 16 in a device monitoring mode, for example in a cardiac pacemaker for monitoring the heartbeat, and at a pulse load 18 during a device operating mode for charging a capacitor (not shown) or for delivering therapy. An electrochemical cell that possesses sufficient energy density and discharge capacity required of implantable medical devices comprises an anode of anode active materials selected from Groups IA, IIA and IIIA of the Periodic Table of the Elements, including lithium, sodium, potassium, calcium, magnesium or their alloys, or any alkali metal or alkali-earth metal capable of functioning as an anode. Lithium is preferred and in that case the alloys and intermetallic compounds include, for example, Li-Si, Li-Al, Li-Mg, Li-Al-Mg, Li-B and Li-Si-B alloys and intermetallic compounds. The form of the anode may vary, but typically, the anode comprises a thin sheet or foil of the anode metal or alloy thereof, and a current collector contacted to the anode material. The current collector includes an extended tab or lead for connection to the negative terminal. The cathode electrode comprises solid active materials such as are typically used in alkali metal/solid cathode electrochemical cells. Particularly preferred cathode active materials for use with the present invention are prepared from fluorine and carbon including graphitic and nongraphitic forms of carbon, such as coke, charcoal or activated carbon. The fluorinated carbon is represented by the formula (CF x ) n wherein x varies between about 0.1 to 1.9 and preferably between about 0.5 and 1.2, and (C 2 F) n wherein the n refers to the number of monomer units which can vary widely. Other electrode active materials suitable for use with the present invention include a metal, a metal oxide, a metal sulfide and carbonaceous materials, and mixtures thereof. Such electrode active materials include, but are not limited to, manganese dioxide, copper silver vanadium oxide, silver vanadium oxide, copper vanadium oxide, titanium disulfide, copper oxide, copper sulfide, iron sulfide, iron disulfide and carbon, and mixtures thereof. No matter what active material is used, the cathode preferably comprises about 80 to about 99 weight percent of the electrode active material. According to the present invention, the preferred cathode active mixture comprises CF x combined with a discharge promoter component such as acetylene black, carbon black and/or graphite. Metallic conductive diluents such as nickel, aluminum, titanium and stainless steel in powder form are also useful when mixed with the cathode active mixture of the present invention. Up to about 10 weight percent of the discharge promoter component/conductive diluent is added to the mixture to improve conductivity. Solid cathode active components for incorporation into a cell according to the present invention may be prepared by rolling, spreading or pressing a mixture of one or more of the above listed electrode active materials, a discharge promoter component and/or one or more of the enumerated conductive diluents onto a cathode current collector with the aid of a binder material. Preferred binder materials include a powdered fluoro-resin such as powdered polytetrafluoroethylene (PTFE) or powdered polyvinylidene fluoride present at about 1 to about 5 weight percent of the electrode active material. The cathode current collector includes a lead for connection to the positive cell terminal, and is preferably in the form of a thin sheet or metal screen, for example, a titanium, stainless steel, aluminum or nickel screen, preferably titanium, having the lead extending therefrom. Alternatively, prior to contact with the current collector, the cathode active mixture including the binder and the discharge promoter component/conductive diluent is formed into a free-standing sheet in a manner similar to that described in U.S. Patent No. 5,543,249 to Takeuchi et al., which is assigned to the assignee of the present invention and incorporated herein by reference. Cathodes prepared as described above may be in the form of a strip wound with a corresponding strip of anode material in a structure similar to a "jellyroll", or in the form of one or more plates operatively associated with at least one or more plates of anode material as in a prismatic configuration. Electrode assemblies having a bobbin shape, a button configuration and the like are also useful with the present invention. The electrochemical cell of the present invention further includes a separator disposed intermediate the Group IA, IIA and IIIA anode and the cathode to provide physical separation therebetween. 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 electrochemical cell. Illustrative separator materials include woven and non-woven fabrics of polyolefinic fibers including polyvinylidene fluoride, polyethylenetetrafluoroethylene, and polyethylenechlorotrifluoroethylene laminated or superposed with a polyolefinic or fluoropolymeric microporous film, non-woven glass, glass fiber materials and ceramic materials. Suitable microporous films include a polytetrafluoroethylene membrane commercially available under the designation ZITEX (Chemplast Inc.), a polypropylene membrane commercially available under the designation CELGARD (Celanese Plastic Company, Inc.) and a membrane commercially available under the designation DEXIGLAS (C.H. Dexter, Div., Dexter Corp.). The electrochemical cell of the present invention further includes a nonaqueous, ionically conductive electrolyte which serves as a medium for migration of ions between the anode and the cathode electrodes during the electrochemical reactions of the cell. The electrochemical reaction at the electrodes involves conversion of ions in atomic or molecular forms which migrate from the anode to the cathode. Thus, nonaqueous electrolytes suitable for the present invention 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 or organic, ionically conductive salt dissolved in a nonaqueous solvent, and more preferably, the electrolyte includes an ionizable alkali metal salt dissolved in a mixture of aprotic organic solvents comprising a low viscosity solvent and a high permittivity solvent or, a single solvent. The ionically conductive salt serves as the vehicle for migration of the anode ions to intercalate or react with the cathode active material. In a solid cathode/electrolyte system, the preferred ion-forming alkali metal salt is similar to the alkali metal comprising the anode. Examples of salts useful with the present invention include LiPF 6 , LiAsF 6 , LiSbF 6 , LiBF 4 , LiAlCl 4 , LiO 2 , LiGaCl 4 , LiSO 3 F, LiB(C 6 H 5 ) 4 , LiClO 4 , LiC(SO 2 CF 3 ) 3 , LiSCN, LiO 3 SCF 2 CF 3 , LiC 6 F 5 SO 3 , LiO 2 CCF 3 , LiN(SO 2 CF 3 ) 2 and LiCF 3 SO 3 , and mixtures thereof. Low viscosity solvents include tetrahydrofuran (TFH), methyl acetate (MA), diglyme, trigylme, tetragylme, dimethyl carbonate (DMC), 1,2-dimethoxyethane (DME), diethyl carbonate, diisopropylether, 1,2-diethoxyethane (DEE), 1-ethoxy, 2-methoxyethane (EME), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC) and ethyl propyl carbonate (EPC), and mixtures thereof, and high permittivity solvents include cyclic carbonates, cyclic esters and cyclic amides such as propylene carbonate (PC), butylene carbonate (BC), ethylene carbonate (EC), acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, γ-valerolactone, γ-butyrolactone (GBL) and N-methyl-pyrrolidinone (NMP) and mixtures thereof. In the preferred electrochemical cell comprising the Li/CF x couple, the preferred electrolyte is 1.0M to 1.4M LiBF 4 in γ-butyrolactone (GBL). The preferred form of the electrochemical cell of the present invention is a case-negative design wherein the anode/cathode couple is provided in a prismatic configuration inserted into a conductive metal casing such that the casing, a header thereof, or both are connected to the anode current collector and serve as the negative cell terminal, as is well known to those skilled in the art. A preferred material for the casing is titanium although stainless steel, nickel and aluminum are suitable. The casing header has a sufficient number of openings to accommodate a glass-to-metal seal terminal pin feedthrough for the cathode electrode. An additional opening is provided for electrolyte filling. After the prismatic electrode assembly is inserted into the casing and the casing header is secured thereto, the cell is filled with the electrolyte solution 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. The following example describes the manner and process of manufacturing an electrochemical cell according to the present invention, and it sets forth the best mode contemplated by the inventors of carrying out the invention, but it is not to be construed as limiting. EXAMPLE I Twenty lithium/carbon monofluoride (Li/CF x ) cells commercially available under model no. 9424 from Wilson Greatbatch Ltd., Clarence, New York, the assignee of the present invention, were selected from production inventory for pulse discharge testing. These cells have a half-round profile with external dimensions of 45mm x 22mm x 5mm and a theoretical capacity of 1.32 Ah. The selected Li/CF x cells are fabricated for use in implantable medical applications and they are capable of providing currents in the microamp to milliamp range while offering high energy density, long shelf life, and low impedance throughout cell life. The Li/CF x couple is characterized by a relatively flat voltage discharge profile. First, the cells were discharged at 37°C for 9 hours under a 1.5 kohm constant resistance load during an initial predischarge period. The predischarge period depleted the cells of approximately 1% of their theoretical capacity. Following this constant resistance portion of the regime, the cells were placed on open circuit storage for one week at 37°C, after which time the cells were subjected to a 10 mA pulse train. This pulse train consisted of four, 10 second pulses, with 15 second rest intervals between pulses. All twenty cells were then pulse discharged at 37°C according to the following regime: 1) Pulse train 1 was applied to the cells (one cell at a time) immediately. Subsequent pulse trains were applied every 0.06 Ah. 2) A 2 kohm background load was applied between pulse trains, starting after pulse train 1. However, this load was removed from the cells for each pulse train application. 3) As soon as one of the cells delivered 0.06 Ah under the 2 kohm load since the end of the previous pulse train, the 2 kohm background load was removed from all of the cells simultaneously, and all of the cells were then pulse discharged to deliver one pulse train (applied to one cell at a time). After the last cell was pulse discharged, all of the cells were simultaneously placed back on the 2 kohm background load until the next train. 4) Each pulse train consisted of 9 pulses such that: pulses 1 to 3 were 0.5 mA for 0.02 seconds, pulses 4 to 6 were 5 mA for 0.2 seconds, and pulses 7 to 9 were 20 mA for 2 seconds. 5) The time between pulses within a train was 5 minutes (typical). 6) The end-of-life (EOL) cutoff was 1.0V under pulse. Thirteen of the twenty test cells received a total of 20 pulse trains before reaching the 1.0V end-of-life cutoff, while seven cells received 21 pulse trains. In order to develop a fuel gauge for a Li/CF x cell, voltage recovery as a function of depth-of-discharge (DOD) was examined. Because the 2 kohm background loads were removed prior to the application of each pulse train (starting prior to train 2), the cells in this example experienced a net voltage recovery during the pulse trains. The DOD analyses were conducted on the voltage recovery after removal of the 2 kohm background load but prior to the application of the fourth pulse in the pulse train. This was done in order to separate the voltage recovery data from one pulse train to the next pulse train. Voltage change (ΔOCV) was calculated for each pulse train by subtracting the voltage reading after the background load was removed (PB.OCV) from the open circuit voltage immediately prior to the application of the fourth pulse in the train (PP4.OCV). The results are presented in Table 1. Note that there are no pulse train 1 data in Table 1 because application of the 2 kohm background load was not initiated until after pulse train 1. Next, the voltage change experienced during the interval between removal of the 2 kohm background load but prior to the application of the fourth pulse was divided by the-corresponding change in time (ΔHrs). This ratio, ΔOCV/ΔHrs, is designated as the "Ratio" values in Table 1, and plotted as the ordinate coordinates in Fig. 2. As previously discussed, the model 9424 Li/CF x cells have 1.32 Amp-hours of theoretical or stoichiometric capacity. In order to obtain the corresponding depth-of-discharge (DOD) for each pulse train, the capacity delivered up to the beginning of the fourth pulse of each train (PP4.Ah) was divided by the theoretical or stoichiometric capacity of the tested cells. The resulting DODs listed in Table 1 were plotted as the abscissa in Fig. 2. The data plotted in Fig. 2 (curve 20) were then partitioned to allow analysis of points up to 70% DOD. The fit for the relationship of ΔOCV/ΔHrs to DOD was achieved using a natural logarithm transformation in both the independent variable (DOD) and the dependent variable (ΔOCV/ΔHrs). The equation determined for this curve fit is: ln(ΔOCV/ΔHrs) = -2.0539 - 0.41048 ln(DOD) The RSquare value, used to indicate goodness-of-fit, is 0.995986 out of a possible 1.00. The graph in Fig. 3 (curve 22) shows an overlay of the predicted curve fit with the observed data. The resulting equation allows for the estimation of DOD based on relieving the load from the cell and monitoring both the voltage change and the time change. Solving the equation, DOD is calculated as: DOD = e {-5.00 - 2.44 ln(ΔOCV/ΔHrs)} Thus, the present invention describes a method for determining the remaining discharge capacity in an alkali metal cell powering an electronic device such as an implantable medical device. While the example is directed to Li/CF x cells, the method for determining the remaining discharge capacity according to the present invention is believed to be applicable to all pulse dischargeable alkali metal/solid cathode cells. Exemplary electrochemical couples include Li/SVO, Li/CSVO and Li/MnO 2 as well as lithium and the other cathode active materials described hereinabove. Such a "fuel gauge" is useful for determining and planning elective surgery when the power source for the medical device is approaching its end-of-life and needs to be replaced. 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 scope of the present invention as defined by the appended claims.

Claims: An electrochemical cell comprising an alkali metal anode and a solid cathode activated with a nonaqueous electrolyte a) which electrochemical cell has a determinable stoichiometric capacity; and b) which electrochemical cell is associated with means for discharging the electrochemical cell under a first load condition and at a second, lighter load or at an open circuit voltage condition; c) wherein said means provides for determining the remaining discharge capacity in the cell by having the first load condition removed from the cell at a first time so that the cell's discharge voltage relaxes from a first voltage at the first load to a second voltage at a second, lighter load or at an open circuit voltage condition and wherein the voltage change from the first voltage to the second voltage is measurable and wherein the time interval for the cell to relax from the first voltage at the first time to the second voltage at the second time is measurable and wherein the voltage change is divisible by the time interval to determine a ΔOCV/ΔHrs ratio and wherein the depth-of-discharge ratio for the cell is then determinable using the equation: DOD = e [-5.00-2.44 ln (ΔOCV/ΔHrs)] . An electrochemical cell according to claim 1 in combination with an implantable medical device requiring a substantially constant discharge rate during a medical device monitoring function and at least one pulse discharge for a medical device operating function wherein: said means for discharging the electrochemical cell is electronic circuitry powered by the electrochemical cell; said first load occurs during the medical device operating mode for charging a capacitor or delivering therapy; and said second load occurs during a medical device monitoring mode. An electrochemical cell according to claim 1 or claim 2 wherein the solid cathode comprises fluorinated carbon as an active material. An electrochemical cell according to any one of the preceding claims wherein the solid cathode is selected from the group consisting of silver vanadium oxide, copper silver vanadium oxide, manganese dioxide, cobalt oxide, nickel oxide, fluorinated carbon, copper oxide, copper sulfide, iron sulfide, iron disulfide, titanium disulfide and copper vanadium oxide, and mixtures thereof. An electrochemical cell according to any one of the preceding claims wherein the cathode comprises from 80 to 99 weight percent of a cathode active material. An electrochemical cell according to any one of the preceding claims wherein the cathode further comprises a binder material and a conductive additive. An electrochemical cell according to claim 6 wherein the binder material is a fluoro-resin powder. An electrochemical cell according to claim 6 or claim 7 wherein the conductive additive is selected from the group consisting of carbon, graphite powder, acetylene black and metallic powder selected from the group consisting of titanium, aluminum, nickel and stainless steel, and mixtures thereof. An electrochemical cell according to any one of the preceding claims wherein the cathode comprises from 0 to 3 weight percent carbon, 1 to 5 weight percent of a powder fluoro-resin and 94 weight percent of a cathode active material. An electrochemical cell according to any one of the preceding claims wherein the cell is activated with a nonaqueous electrolyte which comprises a nonaqeous solvent having an inorganic salt dissolved therein, wherein the alkali metal of the salt is the same as the alkali metal comprising the anode. An electrochemical cell according to claim 10 wherein the nonaqueous solvent is selected from the group consisting of tetrahydrofuran, methyl acetate, diglyme, triglyme, tetraglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy, 2-methoxyethane, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, propylene carbonate, ethylene carbonate, butylene carbonate, acetonitrile, dimethyl sulfoxide, dimethyl formamide, dimethyl acetamide, γ-valerolactone, γ-butyrolactone and N-methyl-pyrrolidinone, and mixtures thereof. An electrochemical cell according to claim 9 or claim 10 wherein the alkali metal salt is selected from the group consisting of 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 2 , LiO 3 SCF 2 CF 3 , LiC 6 F 5 SO 3 , LiO 2 CCF 3 , LiSO 3 F, LiB(C 6 H 5 ) 4 and LiCF 3 SO 3 , and mixtures thereof. An electrochemical cell according to any one of the preceding claims wherein the cell has a lithium anode active material in electrical contact with a nickel current collector and the cathode comprises fluorinated carbon active material in electrical contact with a titanium current collector and wherein the anode and the cathode are activated with the electrolyte solution comprising 1.0M LiBF 4 , in γ-butyrolactone. A method for determining the remaining discharge capacity in an electrochemical cell according to any one of the preceding claims, comprising the steps of: a) providing the electrochemical cell comprising a lithium anode and a fluorinated carbon cathode activated with a nonaqueous electrolyte; b) determining a stoichiometric capacity of the cell; c) discharging the cell under a first load condition; d) removing the first load from the cell at a first time so that the cell's discharge voltage relaxes from a first voltage at the first load to a second voltage at a second, lighter load or at an open circuit voltage condition and measuring the voltage change from the first voltage to the second voltage; e) measuring the time interval for the cell to relax from the first voltage at the first time to the second voltage at the second time; f) dividing the voltage change by the time interval to determine ΔOCV/ΔHrs ratio; and g) computing the depth of discharge equaling e [-5.00 - 2.44 ln (ΔOCV/ΔHrs)] . A method according to claim 14 including providing the cell powering an implantable medical device. A method according to claim 14 including providing an electronic device powered by the electrochemical cell.

Inventors: SYRACUSE KENNETH C, TAKEUCHI ESTHER S, WAITE NOELLE M, SYRACUSE, KENNETH C., TAKEUCHI, ESTHER S., WAITE, NOELLE M.
Applicants: GREATBATCH W LTD, WILSON GREATBATCH LIMITED
Application year: 2001
Main classification: H01M10
IPC codes: H01M6, H01M4, H01M10