Tension ring-functionalized bicyclic ammonium ionic liquids as hypergolic fuels with superior energy density

Hypergolic ionic liquids are a new type of liquid propellants, and the energy density is critical to propulsion performance. We propose to combine cage structures with tensile rings to design and synthesize new ionic liquid molecules. Cycloalkane-substituted 1-aza-bicyclo[2.2.2]octane and 1,4-diazabicyclo[2.2.2]octane-like ionic liquids were synthesized using dicyanamide root as an anion, and their structure and properties were determined. The obtained ionic liquids possess higher energy density up to 1.87 kJ·mL −1 . The three-membered ring substituent can increase the energy density of ionic liquids by 79.8 % and reduce the ignition delay time by 52.5 % than that of straight-chain alkanes. This work provides an important basis for the design and synthesis of the new type of hypergolic ionic liquids.