Scalable fabrication of NiCo2O4/reduced graphene oxide composites by ultrasonic spray as binder-free electrodes for supercapacitors with ultralong lifetime

•NiCo2O4/rGO nanocomposites with hierarchical and laminated structure was fabricated by one-step ultrasonic spray.•In-situ grown NiCo2O4 nanoparticles act as spacers to prevent re-stacking of graphene.•Tight adhesion between active materials and Ni foam guarantees efficient electron transfer and alleviate peeling of active materials.•NiCo2O4/rGO composite exhibits a high specific capacitance and excellent cycling stability.•The assembled asymmetric supercapacitor manifests an ultralong cycling lifetime up to 30,000 cycles.