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Hollow NiCo2S4 Nanospheres Hybridized with 3D Hierarchical Porous rGO/Fe2O3 Composites toward High‐Performance Energy Storage Device

Hierarchical hollow NiCo2S4 microspheres with a tunable interior architecture are synthesized by a facile and cost‐effective hydrothermal method, and used as a cathode material. A three‐dimensional (3D) porous reduced graphene oxide/Fe2O3 composite (rGO/Fe2O3) with precisely controlled particle size and morphology is successfully prepared through a scalable facile approach, with well‐dispersed Fe2O3 nanoparticles decorating the surface of rGO sheets. The fixed Fe2O3 nanoparticles in graphene efficiently prevent the intermediates during the redox reaction from dissolving into the electrolyte, resulting in long cycle life. KOH activation of the rGO/Fe2O3 composite is conducted for the preparation of an activated carbon material–based hybrid to transform into a 3D porous carbon material–based hybrid. An energy storage device consisting of hollow NiCo2S4 microspheres as the positive electrode, the 3D porous rGO/Fe2O3 composite as the negative electrode, and KOH solution as the electrolyte with a maximum energy density of 61.7 W h kg−1 is achieved owing to its wide operating voltage range of 0–1.75 V and the designed 3D structure. Moreover, the device exhibits a high power density of 22 kW kg−1 and a long cycle life with 90% retention after 1000 cycles at the current density of 1 A g−1.