Iron Oxide Nanosheets and Pulse-Electrodeposited Ni-Co-S Nanoflake Arrays for High-Performance Charge Storage

Nanostructured nickel cobalt sulfide (Ni_4.5Co_4.5S₈) has been prepared through a single-step pulse-electrodeposition method. Iron oxide nanosheets at hollow graphite shells (Fe₃O₄@g-shells) were prepared from graphite-coated iron carbide/α-Fe (g-Fe₃C/Fe) in a two-step annealing/electrochemical cycling process. Electrochemical characterization of the Ni_4.5Co_4.5S₈ and g-Fe₃C/Fe materials showed that both have high specific capacities (206 mAh g^-1 and 147 mAh g^-1 at 1 A g^-1) and excellent rate capabilities (∼95% and ∼83% retention at 20 A g^-1, respectively). To demonstrate the advantageous pairing of these high rate materials, a full-cell battery with supercapacitor-like power behavior was assembled with Ni_4.5Co_4.5S₈ and g-Fe₃C/Fe as the positive and negative electrodes, respectively. The (Ni_4.5Co_4.5S₈//g-Fe₃C/Fe) device could be reversibly operated in a 0.0-1.6 V potential window, delivering an impressive specific energy of 89 Wh kg^-1 at 1.1 kW kg^-1 and a remarkable rate performance of 61 Wh kg^-1 at a very high specific power of 38.5 kW kg^-1. Additionally, long-term cycling demonstrated that the asymmetric full cell assembly retained 91% of its initial specific capacity after 2500 cycles at 40 A g^-1. The performance features of this device are among the best for iron oxide/hydroxide and bimetallic sulfide based energy storage devices to date, thereby giving insight into design principles for the next generation high-energy-density devices.