Sub-nanometer, Ultrafine α-Fe O Sheets Realized by Controlled Crystallization Kinetics for Stable, High-Performance Energy Storage.

The development of energy devices based on iron oxides/hydroxides is largely hindered by their poor conductivity and large volume changes, especially with regard to specific capacitance and cycle stability. Herein, superior capacitance (1575 F g at 1.25 A g ) and high rate performance (955 F g at 25 A g ) were realized by synthesizing sub-nanometer, ultrafine α-Fe O sheets loaded on graphene (SU-Fe O -rGO). An assembled asymmetric supercapacitor showed outstanding cycle stability (106 % retention after 30 000 cycles). This excellent performance arises from the unique structural characteristics of the α-Fe O sheets, which not only enrich electrochemically reactive sites, but also largely eliminate the volume changes after long-term charge/discharge cycling. The synthesis of SU-Fe O -rGO critically depends on control of the crystallization kinetics during growth. A controlled heterogeneous nucleation mechanism results in the formation of atomically thin α-Fe O sheets on graphene rather than large particles in solvent, as clarified by theoretical calculations. This strategy paves a new way to synthesizing atomically thin transition metal oxide sheets and low-cost, eco-friendly iron-based energy storage.