P1-C7N4O8: improved stability and energy density via a covalent framework with uniform single bonds.

Energy can be stored in the chemical bonds of CNO energetic materials. However, non-uniform chemical bonds hinder the trade-off between structural stability and energy density: strong chemical bonds reduce energy density, whereas weak bonds lower the decomposition barrier. We propose a 3D close-packed structure of P1-C7N4O8 with uniform chemical single bonds using a first-principles crystal structure search method. The results show that P1-C7N4O8 formation becomes thermodynamically favorable through the reaction between C3N4 and dry ice (CO2) at 31 GPa. P1-C7N4O8 exhibits excellent structural stability under high pressure and can be retained at ambient conditions. C-N and C-O single bonds are elongated during decompression to atmospheric pressure, which can increase the energy density further. As a result, P1-C7N4O8 shows outstanding mass density (3.08 g cm-3) and volumetric energy density (18.12 kJ cm-3) at ambient conditions, outperforming those of CHON and CxNyOz energetic materials. This work indicates that a 3D covalent structure with uniform chemical single bonds is crucial for balancing the stability and performance of energetic materials.