Hollow and yolk-shell iron oxide nanostructures on few-layer graphene in Li-ion batteries.

We report a simple and template-free strategy for the synthesis of hollow and yolk-shell iron oxide (FeOx) nanostructures sandwiched between few-layer graphene (FLG) sheets. The morphology and microstructure of this material are characterized in detail by X-ray diffraction, X-ray absorption near-edge structure, X-ray photoelectron spectroscopy, Raman spectroscopy, scanning and transmission electron microscopy. Its properties are evaluated as negative electrode material for Li-ion batteries and compared with those of solid FeOx/FLG and two commercial iron oxides. In all cases, the content of carbon in the electrode has a great influence on the performance. The use of pristine FLG improves the capacity retention and further enhancement is achieved with the hollow structure. For a low carbon loading of 18 wt. %, the presence of metallic iron in the hollow and yolk-shell FeOx/FLG composite significantly enhances the capacity retention, albeit with a relatively lower initial reversible capacity, retaining above 97% after 120 cycles at 1000 mA g(-1) in the voltage range of 0.1-3.0 V.