Performance of Li-Alloy/Ag2CrO4 Couples in Molten LiNO3-KNO3 Eutectic Electrolyte

There is interest in the development of a hightemperature power source that can be used to power data logging instrumentation while drilling in geothermal boreholes. Temperature can range from 250C to well over 400C in such an environment. To avoid the use of expensive dewar systems, the batteries must be able to function by using the ambient heat in the borehole. Typical electrolytes used in high-temperature (thermal) batteries are halide based with melting points between 313 and 436C. There is a need to develop electrolytes with much lower melting points for this application. The melting points need to be even lower to function in oil and gas boreholes, where temperatures reach only 250C or so. As part of this effort, we have performed a number of compatibility studies with some of the more-promising electrolytes—both inorganic as well as organic. In earlier work, we presented the results of characterization efforts with various electrochemical couples and, more specifically, with the Li(Si)/FeS2 couple in a low-melting CsBrbased eutectic. The latter systems works fine above 250C but experiences severe polarization due to higher internal impedance at lower temperatures. In an attempt to extend the liquid region to lower temperatures, the use of lower-melting electrolytes was explored. One electrolyte that shows promise is the LiNO3-KNO3 eutectic that melts at 124.5C. The use of high-activity anodes with this electrolyte is possible only due to the formation of a protective passive film if Li2O on the anode, much in the same way that a film of LiCl prevents continued reaction of the anode in Li/SOCl2 cells. A considerable amount of work has been done by Miles in studying the basic electrochemistry of various materials in LiNO3-KNO3 eutectic. 5 Giwa has also examined the Li(Al)/Ag2CrO4 couple with this electrolyte but over a narrow range of discharge conditions. In this work, we have extended the range of discharge conditions for this couple and have included the Li(Si)/Ag2CrO4 and Li(Si)/Ag2CrO4 couples as well. These materials were tested in single cells heated between platens at temperatures of 150 to 300C at current densities of 8 – 32 mA/cm. This paper will report on the performance of these cells and wil l present preliminary data for 5-cell stacks tested in a simulated borehole environment.