New Silver Containing Cathode for Primary Lithium Batteries for Medical Devices
INVENTION: Northwestern researchers have synthesized a new cathode, Ag4V2O6F2, (SVOF) with significantly higher accessible silver content than current medical battery cathodes employing silver vanadium oxide (SVO). Greater capacity and potential are produced compared to SVO. In addition the chemical nature of SVOF affords higher open circuit potential than that in SVO devices. The SVOF cathode maintains a greater potential than SVO electrodes for extended periods of time, allowing optimal device performance essential for medical battery applications, such as defibrillators.
The SVOF cathode affords 50% greater capacity above 3 V potential than the commercial SVO cathode. SVOF also generates a 3.52 V open circuit potential versus 3.25 V for SVO batteries. Improved battery performance and life are anticipated with the new cathode.
SUMMARY: The current standard cathode material in medical batteries is silver vanadium oxide (SVO, Ag2V4O11). SVO has a theoretical capacity of 450 mAh/g; however, not all of this capacity is accessible at practical voltages. The open circuit potential of SVO versus lithium is 3.25 V. The cathode inserts seven lithium ions during discharge, until it reaches a cut-off voltage of 1.5 V, resulting in a practical capacity of 315 mAh/g. Initially, lithium reduces Ag+ to Ag0 at ~ 3.25 V, the potential at which defibrillators work most efficiently. However lithium continues to react and reduces the V5+ to V4+ at 2.5 V. The potential does not plateau for the final of V4+ to V3+ reduction, but drops precipitously. Owing to the low reduction potential the full theoretical capacity of SVO is not utilized in practical applications. Voltage below 1.5 V is inadequate for defibrillator operation.
The new SVOF cathode has more accessible silver content above 3 V than SVO. Accordingly, SVOF has an available maximum capacity of 160 mAh/g from silver reduction versus 90 mAh/g with SVO (Figure). Silver reduction of SVOF achieves 50% greater capacity than maximum SVO performance, under non-optimized conditions. SVOF offers significantly higher voltage than present primary cathodes owing to the presence of fluoride in the material. The approximately 300 mV higher voltage yields more power to offset any future device shrinkage. SVOF based medical batteries promise improved performance and life versus current systems. The SVOF cathode utilizes a new composition Ag4V2O6F2,and synthesis, characterized by X–ray diffraction and analysis. See Sorensen et al. J. Am. Chem. Soc. 2005, 127, 6347-6352p>
STATUS: A patent application has been filed.
Kenneth Poeppelmeir, Heather Izumi, Erin Sorensen ,John Vaughey
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