Hierarchical NiCo₂O₄ nanosheets/nitrogen doped graphene/carbon nanotube film with ultrahigh capacitance and long cycle stability as a flexible binder-free electrode for supercapacitors

Developing flexible and lightweight energy storage systems for miniaturized electronic equipment and high volumetric performance is arousing increasing interest. For practical applications, the parameters of gravimetric and volumetric performance should both be taken into account and given the same importance. Here, a novel hierarchical structure of free-standing, binder-free electrodes of corrugated NiCo₂O₄ nanosheets on nitrogen doped graphene/carbon nanotubes (NGN/CNTs) film has been designed for use in high-performance supercapacitors. This unique lightweight electrode structure achieved extremely high electrochemical performance, with a volumetric capacitance of 482.7 F cm⁻³, a gravimetric capacitance of 2292.7 F g⁻¹ at 5 A g⁻¹, and very long-term cycle stability (125% capacitance retention after 10 000 circles at 30 A g⁻¹). Optimal supercapacitive performance was also achieved in our fabricated asymmetric supercapacitor (ASC) by using NiCo₂O₄/NGN/CNTs as the cathode and NGN/CNTs as the anode. The device exhibited high gravimetric energy densities of 42.71 W h kg⁻¹ at 775 W kg⁻¹ and of 24.69 W h kg⁻¹ at a high gravimetric power density of 15 485 W kg⁻¹; it even demonstrated a high volumetric energy density of 25.90 W h L⁻¹ and a high volumetric power density of 9389 W L⁻¹. This strategy provides a new method to design flexible high-performance electrodes for a new generation of energy storage applications.