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

Nanostructured nickel cobalt sulfide (Ni4.5Co4.5S8) 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 Ni4.5Co4.5S8 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 Ni4.5Co4.5S8 and g-Fe3C/Fe as the positive and negative electrodes, respectively. The (Ni4.5Co4.5S8//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.