Hollow NiCo 2 S 4 Nanospheres Hybridized with 3D Hierarchical Porous rGO/Fe 2 O 3 Composites toward High‐Performance Energy Storage Device

Hierarchical hollow NiCo 2 S 4 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/Fe 2 O 3 composite (rGO/Fe 2 O 3 ) with precisely controlled particle size and morphology is successfully prepared through a scalable facile approach, with well‐dispersed Fe 2 O 3 nanoparticles decorating the surface of rGO sheets. The fixed Fe 2 O 3 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/Fe 2 O 3 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 NiCo 2 S 4 microspheres as the positive electrode, the 3D porous rGO/Fe 2 O 3 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 .