FastCAP proposes a novel solid state ultracapacitor ("ultracap") capable of reliable operation from 0°C to 300°C, with survival at a temperature range of -55°C to 350°C. When complete, the device will exceed FastCAP's current world record as the highest temperature ultracap in the world. The proposed ultracap will enable significant strides forward in design paradigms utilized in deep space high temperature environments- enabling reduction in the weight, volume and complexity of energy storage systems, while relaxing design constraints on scarce candidate high temperature battery technologies. Importantly, the device will not exhibit the typical volatility and toxicity associated with traditional electrolytes present in current aerospace battery technologies. During Phase I, we will demonstrate an ultracap comprising a high temperature ionic liquid-doped polymer electrolyte paired with binder-free carbon nanotube electrodes capable of operating at 0°C-300°C. At the end of Phase II, this device will exhibit a high power density (1kW/kg and 1.4kW/L), 10-15X the power density of Sodium Sulfur batteries, and a high energy density (0.7Wh/kg and 1Wh/L), 10,000X higher than ceramic capacitors. The prototype will have a cycle life of more than 1,000,000 cycles at room temperature and below, and more than 100,000 cycles at 300°C. We will leverage our experience in the development of extreme environment ultracaps for oil & gas industry applications to engineer a device able to withstand high shock (up to a 1000Gpeak) and vibration (up to 60Grms) conditions, as well as high pressure differentials (up to 100atm).