An electrochemical device, such as a battery or power source, provides improved performance under stringent or extreme conditions. Such an electrochemical device for use in high temperature conditions may include at least a cathode, a lithium-based anode, a separator, and an ionic liquid electrolyte. This device also may include a current collector and housing that are electrochemically inert with respect to other components of the device. This electrochemical device may operate at temperatures ranging from 0 to 180, 200, 220, 240, and 260° C.