We present a facile approach to synthesize amorphous iron oxyhydroxide nanosheet from the surfactant-assisted oxidation of iron sulfide nanosheet. The amorphous iron oxyhydroxide nanosheet is porous and has a high surface area of 223 m(2) g(-1). The lithium storage properties of the amorphous iron oxyhydroxide are characterized: it is a conversion-reaction electrode material, and it demonstrates superior rate capabilities (e.g., discharge capacities as high as 642 mAh g(-1) are delivered at a current density of 2 C). The impedance spectroscopy analysis identifies a RC series subcircuit originated by the conversion-reaction process. Investigation of the conversion-reaction kinetics through the RC subcircuit time constant reproduces the hysteresis in the discharge/charge voltage profile. Hysteresis is then connected to underlying thermodynamics of the conversion reaction rather than to a kinetic limitation.