Background: Venus, despite being our closest neighboring planet, has not been explored extensively due to its hostile and extreme environment. Its surface temperature is as high as 740 K (467°C) and the pressure is 92 bar. Since the environment is relatively mild at high altitudes, several balloon missions were successfully deployed for moderate durations (up to 46h). Surface missions, landers and probes, e.g., Russian Veneras and Vega 2 Lander, however, continued to be of short duration, lasting <2 hours, due to limited survivability of the batteries (typically primary Li-SO2 batteries), payload and the electronics at these temperatures. Objective: We intend to develop and demonstrate, in two years, new high temperature primary batteries survivable and operational at Venus/Mercury surface temperatures (up to 500oC) for longer durations (30 days). These heat-resilient batteries will be based on similar chemistries and battery designs as the thermal batteries currently being used in planetary missions, e.g., Mars Exploration Rover, Phoenix Lander and Mars Science Laboratory, during Entry Descent and Landing (EDL), but will provide longer operational life (>30 days vs. 2h) and higher energy densities (3-4x over state-of-the-art). These systems will have inherent stability under Venus/Mercury environments, enabled by the use of a modified molten salt electrolyte and Li alloy anode, and will be operational over several days, due to reduced heat dissipation losses in the (hot) ambient conditions. Since battery designs and fabrication methods are identical to the thermal batteries widely used in Department of Defense applications, the proposed battery technology can be rapidly developed and infused into future Venus and Mercury surface missions. Approach: Our approach is based on adapting the proven and heritage chemistry and designs of lithium thermal batteries and modifying both cell designs and components for extended lifetimes in Venus environment. These batteries will be based on new Li alloy anodes, alkali halide molten salt electrolytes and high energy metal sulfide cathodes with high temperature stability. In the thermal batteries, the (solid) electrolyte is transformed into a molten and conducting state, with pyrotechnic materials and initiators. These batteries provide high power densities, but only for short durations (of <2h), depending on how long the cell remains hot. Prior to activation, however, these batteries have a long shelf life (>10 years). For the proposed batteries, the high ambient temperatures on Venus will be used to activate them and also enable them to retain heat and operate for longer durations than standard thermal batteries. Specific modifications will include minimizing/eliminating pyrotechnic activation in lieu of in-situ activation from the hot environment, reducing thermal insulation and reducing can thickness. These modifications will result in Gen-1 prototype high temperature batteries with a lifetime of 30 days, specific energy (>100 Wh/kg) and energy density (200 Wh/l). In parallel, we will develop advanced anode/cathode and electrolyte materials and use them in high-energy battery designs to further improve the energy densities to 150 Wh/kg and >200 Wh/l, and to increase the operational life to >30 days at 500oC (in Gen-2 batteries). Benefits: The proposed batteries will enable Venus/Mercury surface missions of >30 days (vs 2 h for current batteries). Their inherent thermal resilience will make the design of the lander/probe less complex and more flexible. Combining the heritage battery designs with our prior experience on the chemistry and manufacturing of high temperature batteries, we anticipate a rapid maturation of the proposed battery to a TRL 4 in two years. These batteries will also be beneficial to several terrestrial applications, e.g., guided missiles, sonobuoys and radar and electronics packages for nuclear weapons.