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Journal of Power Sources 162 (2006) 837–840
Hybrid cathode lithium batteries for implantable medical applications
Kaimin Chen, Donald R. Merritt, William G. Howard, Craig L. Schmidt, Paul M. Skarstad ∗
Medtronic Energy and Components Center, 6700 Shingle Creek Parkway, Minneapolis, MN 55430, USA
Received 11 February 2005 Available online 22 August 2005
Lithium batteries with hybrid cathodes of Ag2V4O11 and CFx have been developed that combine the best features of both cathode components.
2005 Elsevier B.V. All rights reserved.
h a m s b v e c m d u t l t r c r r v
eywords: Lithium battery; Implantable medical device; Silver vanadium o
The earliest practical implantable cardiac pacemakers ppeared in the late 1950s, powered by Zn/HgO batteries 1]. Throughout the 1960s most implanted pacemakers con- inued to use Zn/HgO batteries. Efforts were made in the ate 1960s to introduce nuclear batteries based either on the eebeck effect or the betavoltaic effect . However, follow-
ng the introduction of lithium/iodine batteries for cardiac acemakers in 1972 [3,4], the usage swung rapidly toward lectrochemical power sources based on lithium. Several ithium-based battery chemistries were introduced besides ithium/iodine including Li/SOCl2, Li/Ag2CrO4, Li/CuS and i/MnO2. By the mid-1980s most of the industry had set-
led on lithium/iodine for implantable pulse generators (IPGs) equiring less than about 200 W, notably cardiac pacemak- rs. IPGs that required power in the range 200–500 W, such s implantable neurological stimulators, came to be pow- red mainly by Li/SOCl2 batteries. Implantable cardioverter- efibrillators (ICDs) were initially powered by Li/V2O5
∗ Corresponding author. Tel.: +1 763 514 1217; fax: +1 763 514 1163. E-mail address: firstname.lastname@example.org (P.M. Skarstad).
p f s l t
378-7753/$ – see front matter © 2005 Elsevier B.V. All rights reserved. oi:10.1016/j.jpowsour.2005.07.018
arbon monofluoride; Ag2V4O11; CFx; Hybrid cathode
The lithium/iodine system has provided small, simple, ighly reliable power sources with power characteristics lmost ideally suited to the requirements of cardiac pacing for ore than three decades. The system provided energy den-
ity of about 1 Wh/cm3 at drains of less than about 100 A in atteries of conventional construction. The system also pro- ides predictable increases in electrical resistance to signal nd-of-service . However, in the late 1990s implantable ardiac devices began to have increased peak power require- ents that challenged the power capability of conventionally
esigned lithium/iodine batteries. These include increased se of addressable memory to capture and store informa- ion about the electrical activity of the heart, faster and onger-range telemetry to transmit this information outside he body, and new sensors and therapies with higher power equirements. These new therapies include cardiac resyn- hronization and treatment of atrial fibrillation. Peak power equirements for these features can run into the milliwatt ange, well outside the range of efficient discharge for con- entionally designed lithium/iodine batteries.
Analysis of battery chemistries that had been used to ower implantable devices showed several that met the need
8 ower Sources 162 (2006) 837–840
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38 K. Chen et al. / Journal of P
ew therapies and features without compromise . This hemistry uses hybrid cathodes consisting of mixtures of ilver vanadium oxide, Ag2V4O11, and carbon monofluo- ide CFx. Ag2V4O11 is a cathode material with high power apability used most often to power implantable cardioverter- efibrillators. CFx is a commercial cathode material with imited power capability and fairly abrupt end-of-service haracteristics, but with very high capacity density. The blend f the two cathode materials gives a primary battery that has nergy density equal to that of a lithium/iodine battery of dentical form factor, with 40–50 times the power. The sys- em supports a voltage-based indicator of depth-of-discharge nd end-of-service and, in addition, reduces the mass of the attery almost by half, saving 5–10 g for a typical design.
Medtronic introduced the system in 1999 powering a evice for treating atrial fibrillation. This was followed in 000 with implant of a three-electrode IPG providing car- iac resynchronization therapy for treatment of congestive eart failure. The hybrid cathode battery system has since een introduced in other implantable devices including car- iac pacemakers, hemodynamic monitors and drug-delivery evices. The total number of implants powered by hybrid athode batteries stood at more than 100,000 as of January 005. In the future this chemistry is expected to find appli- ation in the full range of implantable device applications equiring primary batteries, including ICDs.
Alternatives to this mixed hybrid cathode system used ince 1999 are reported to be under development. For exam- le, a cathode system in which the silver vanadium oxide and Fx are deposited in distinct layers and laminated to form
he cathode has been described [12–14]. First implant of this ystem is expected in 2005 .
. Description and characteristics of hybrid cathode atteries electrolyte
Most commercial Li/CFx cells use electrolyte consisting f LiBF4 dissolved in -butyrolactone to minimize secondary ntercalation reactions known to cause swelling when CFx is sed with many other commonly used lithium battery elec- rolytes. Ag2V4O11 cells for ICDs use a high-conductivity lectrolyte containing 1 M LiAsF6 dissolved in equal vol- mes of propylene carbonate and dimethoxyethane. Elec- rolyte composition can affect solubility of Li-Ag2V4O11 ischarge products, leading to undesirable side reactions, articularly for high-rate ICD batteries. However, stability f impedance is important for medium-rate applications as ell. Thus, a key element of hybrid-cathode battery technol- gy is selection of an electrolyte composition that gives stable erformance over the long service life.
Proprietary electrolyte compositions have been developed
ulses requiring 5–10 W. Different formulations have been
t w e p
ig. 1. Discharge voltage of Li/Ag2V4O11 cell and Li/CFx cell at A/cm2.
eveloped for medium-rate and for ICD applications. These ormulations have been tested extensively under application onditions and at elevated temperature. Voltage, resistance nd deliverable capacity have been shown to be stable and redictable in long-term discharge experiments. In the case of edium rate electrolyte, 100% of theoretical cathode capac-
ty was delivered over periods as long as 6 years, indicating o detectable loss of cathode capacity to parasitic reactions.
. Variation of voltage with degree of lithiation
The voltage curves for Ag2V4O11 and CFx versus Li re shown in Fig. 1. The voltage curve of hybrid cathode ehaves like a superposition of the two in proportion to he starting composition of the mixture. CFx by itself reacts ith one equivalent of lithium per formula unit at 2.9–3.0 V
n a single heterogeneous reaction. Ag2V4O11 reacts with p to six chemical equivalents of Li per formula unit at otentials between 3.1 and 2.2 V. The open-circuit voltage urve of Ag2V4O11  consists of two voltage plateaus, ach followed by a voltage ramp. Each plateau contributes wo electrochemical equivalents, and each ramp contributes ne electrochemical equivalent to the overall reaction. The lateaus lie at 3.1 and 2.6 V and bracket the CFx potential of bout 2.9 V. The voltage under load shown in Fig. 1 shows gradual decline on the first plateau; this is typical of dis-
harge of Ag2V4O11, even at low background discharge rates 1 A cm−2. As a hybrid-cathode battery discharges at very
ow rates, the Ag2V4O11 will first discharge some of its capac- ty, followed by the CFx and then the remaining Ag2V4O11 apacity, as shown in Fig. 2.
At significantly higher rates, as in a defibrillation pulse, he more rapid discharge kinetics of Ag2V4O11 may cause t to discharge in preference to CFx, shifting and blurring
pisode, the CFx recharges the Ag2V4O11 back to the com- osition at which the two materials have equal potentials.
K. Chen et al. / Journal of Power Sources 162 (2006) 837–840 839
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ig. 2. Discharge voltage of a Li/CFx-Ag2V4O11 medium-power hybrid athode cell at 2 A/cm2.
The composition of the hybrid mixture can be chosen ased on the application, enhancing the capacity with a reater proportion of CFx or enhancing the power capability nd end-of-service characteristics with more Ag2V4O11. For ow- and medium-rate applications the composition can be hosen such that 85–90% of the battery capacity comes from Fx. Discharge curves in Fig. 3 show the variation in oper- ting voltage with current density over the range of current ensity 3.8–30.1 A cm−2. In this composition range effi- iently designed medium-rate hybrid cathode batteries can atch lithium/iodine in energy density at about 1 Wh cm−3. ischarge data for hybrid cathode and iodine cells with iden-
ical form factor are shown in Fig. 4. For ICD applications, a higher proportion of Ag2V4O11 is
equired to give adequate power. The composition is still cho- en with CFx as the major component, contributing 65–75% f the total capacity. High-power ICD batteries designed
ig. 3. Comparison of discharge characteristics of medium-power hybrid athode cell and lithium/iodine cell of same form factor. Note falloff in elivered capacity of iodine cell at 1 mA.
l m c a m
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ig. 4. Voltage of medium-power hybrid cathode cells at range of current ensities.
s with ICD batteries balanced for three-electron reduction f Ag2V4O11 . However, the energy density is enhanced ore than 50% relative to such “charge-time-optimized” ICD
. Comparison with other implantable battery ystems
Models have been developed, using techniques previ- usly reported in these Symposia, describing both design f practical hybrid cathode batteries and the expected ong-term discharge behavior. These have been developed or both the medium-power and high-power hybrid cathode attery systems. Corresponding models for the iodine and wo SVO systems have been described previously at these ymposia [9,18,19] and elsewhere . In all cases the odels incorporate data from cell discharge experiments
asting at least 5 years and as long as 7 years. These odels were used to explore the relationship between power
apability and energy density for these battery systems for region of design space accessible with current design and anufacturing technology. Fig. 5 shows the relationship between power density and
nergy density for cells of the two hybrid-cathode systems nd for corresponding cells of several other important mplantable lithium battery chemistries, including iodine nd two forms of silver vanadium oxide.
The two silver vanadium oxide systems differ in the form f silver vanadium oxide used. The two forms are crys- alline, single phase Ag2V4O11 , termed “CSVO”, and ilver vanadium oxide of the same composition made by ow-temperature decomposition of AgNO3 in the presence f V2O5 , termed “DSVO”.
CSVO and DSVO are similar in power capability for
840 K. Chen et al. / Journal of Power So
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Sources Committee, Crowborough, UK, 1993, pp. 167–176.
ig. 5. Ragone plots of medium-power and high-power hybrid cathode cells ystems compared with other implantable battery systems, silver vanadium xide (CSVO and DSVO) and lithium/iodine.
onger service life would increase the difference between SVO and DSVO on this plot. Both three-equivalent balance nd six-equivalent balance are presented for CSVO.
For non-iodine systems calculations were made assuming ases with external footprint dimensions 2.245 cm× 3.01 cm nd thicknesses of 0.75, 1.0, 1.25 and 1.50 cm. Prismatically oiled electrodes fit within these cases with 1, 3, 5, 8 and 0 turns. Two layers of 25 m separator were assumed. or each system an “elective replacement indicator” (ERI) oltage was chosen appropriate to the system such adequate apacity remained after this point to operate an ICD for t least three months. Background voltage and minimum ulse voltage for the fourth pulse in a train of four 60-J ulses were calculated as functions of discharged capacity. hese were used to calculate the cell impedance for pulses f this magnitude. From this the impedance-matched pulse ower density (power per unit external case volume) was alculated as a function of discharged capacity. Likewise nergy density to ERI was calculated from capacity and verage background voltage to ERI. For each cell design he minimum value of impedance-matched power density to RI versus the energy density to ERI is shown in Fig. 5.
For iodine cells an encasement with similar footprint imensions was chosen, with thicknesses of 0.3, 0.6 and .9 cm. Baseline anode area was calculated for a two-sided entral-anode design, and this area was enhanced by factors f 3, 5, 8 and 10. Lithium excess was taken to be proportional o anode area. None of these designs was capable of support- ng a defibrillation pulse, so ERI was taken to be at 90% of he capacity delivered to 2.0 V. Only the design with baseline lectrode area has ever been constructed, but the calculations o accurately reflect the performance expected of an iodine attery built with the enhanced areas as well. Impedance- atched power for the iodine system was based on 1 kHz
mpedance, representing a pulse of short duration. The com- arison between iodine and the other systems may not be xact in these calculations, but Fig. 5 does show the large dis- arity in power density between iodine and the other systems.
urces 162 (2006) 837–840
Fig. 5 also shows the advantageous energy density with ompetitive power density of high-power hybrid relative o other high-rate systems. It also shows the advantageous ower density and competitive energy density of medium- ower hybrid relative to iodine.
Lithium batteries with hybrid cathodes of Ag2V4O11 and Fx combine the best features of both cathode compo- ents. They can offer power density and energy density that re competitive with or superior to other developed battery hemistries, along with the stability and reliability needed for mplantable medical applications.
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