High Temperature Batteries for Venus Surface Missions

In-situ exploration of Venus is seriously hampered by its severe environment, which is benign (28 o C) at an altitude of 55 km, but rapidly becomes hostile, with increasing temperature and CO 2 pressure at lower altitudes, eventually reaching ~465°C and 90 bars at the surface. 1 These challenging conditions have limited the previous Venus surface missions, e.g., the Russian Venera series and Vega-2 Landers, 2 to barely two hours after deployment with lithium-primary batteries, despite the use of considerable insulation, phase-change materials, and similar heat sinks to isolate batteries and avionics from high surface temperatures. The recent decadal survey, ‘Vision and Voyages for Planetary Science in the Decade as well as the more recent Venus Exploration and Analysis Group (VEXAG) study 3 emphasized the need to gather basic information on the crust, mantle, core, atmosphere/exosphere, and bulk composition of Venus, to understand the evolutionary paths of Venus in relation to Earth and recommended long-duration landers and probes for future missions. In order to enable extended surface missions on Venus, e.g., landers, probes and seismometers, NASA has initiated the development of high temperature electronics and power technologies, under its ‘Hot Operating Temperature Technology’ (HOTTech) program. Under this program, we have been developing advanced primary batteries resilient to the hostile conditions on the Venus surface and operational for several days with high specific energy (>100 Wh/kg) and energy density (>150 Wh/l). Here, we will describe the development of high temperature batteries based on lithium alloy (e.g., Li-Al) anodes, molten salt electrolytes containing binary/ternary mixtures of alkali metal halides, cathodes consisting of transition metal sulfides, and designs similar to the aerospace thermal batteries. 4 With FeS cathode and appropriate changes in the electrolyte, binder and active material ratios, we have demonstrated the operation of the high temperature battery in prototype cells for 30 days in primary mode, and >150 days in rechargeable mode at 475 o C. Further, with suitable thin coatings of inorganic compounds, e.g., Al 2 O 3 , AlF 3 and AlBO 3 on the cathode particles, the utilization of the cathode, and hence the operational life of the cells have been improved by another 50%. Acknowledgements The work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with th