1. Field of Invention
This invention relates to the conversion of chemical energy to electrical energy. In particular, the present invention relates to preparation of an improved cathode for lithium electrochemical cells. The cathode contains a first metal oxide or metal sulfide, preferably silver vanadium oxide (SVO) or copper silver vanadium oxide (CSVO), coated with a second metal oxide or metal sulfide different than the first. One preferred composition has a core of ε-phase SVO (Ag2V4O11) provided with a coating of γ-phase SVO (Ag1.2V3O1.8). The core ε-phase SVO provides the cell with relatively high capacity and rate capability while the γ-phase SVO protective coating improves long-term cathode stability by reducing particle reactivity with the electrolyte. Improved long-term stability translates into increased discharge life in a lithium electrochemical cell. A sol-gel process preferably performs the coating. An exemplary application is having the cell power an implantable cardiac defibrillator, where the cell may run under a light load for extended periods of time interrupted by high rate pulse
2. Prior Art
As is well known by those skilled in the art, an implantable cardiac defibrillator is a device that 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. 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 to the heart to treat tachyarrhythmias, the irregular, rapid heartbeats that can be fatal if left uncorrected.
It is generally recognized that lithium cells containing silver vanadium oxide and, in particular, ε-phase silver vanadium oxide are preferred for powering cardiac defibrillators. U.S. Pat. Nos. 4,310,609 and 4,391,729, both to Liang et al., disclose the preparation of ε-phase SVO as a cathode material for use in a nonaqueous electrolyte electrochemical cell. These patents describe the preparation of silver vanadium oxide by a thermal decomposition reaction of silver nitrate with vanadium oxide (V2O5) at a maximum temperature of about 360° C. The Liang et al. patents are assigned to the assignee of the present invention and incorporated herein by reference.
The reason silver vanadium oxide is preferred for cardiac defibrillators is because of its relatively high rate capability. For example, U.S. Pat. No. 4,830,940 to Keister et al. discloses a primary cell containing silver vanadium oxide for delivering high current pulses with rapid recovery, high capacity and low self-discharge. The Keister et al. patent is assigned to the assignee of the present invention and incorporated herein by reference.