Flexible all solid-state niobium nitride//activated carbon lithium-ion hybrid capacitor with high volumetric power and energy densities

•The NbN microspheres made of porous 2D nanoflakes of cubic crystal are synthesized.•The NbN shows high capacity of 347.73 mAh g−1 at 0.2 a g−1 with superb cycle life.•A highly flexible solid-state Li-ion hybrid capacitor NbN//AC is demonstrated.•The designed capacitor exhibits a high capacitance of 2597 mF cm−3 at 60 mA cm−3.•The device displays a high energy density of 3.25 mWh cm−3 at a high power density.