In bipropulsion systems, hypergolic ionic liquids with B-H groups, as promising alternatives to toxic hydrazine-based fuels, can improve propulsion performance. A series of borohydride cluster-based salts with excellent hydrolytic and thermal stabilities (T-d > 200 degrees C), and the highest density (1.18 g cm(-3)) of any known borohydride hypergolic ionic liquids were prepared. Upon contact with white fuming nitric acid or N2O4, these new salts have ultra-fast ignition delay times (as low as 1 ms), which is a superior ignition performance. Theoretical studies were employed in order to understand their high stability and good ignition performance.