Rsc_cc_c3cc47977c 3..5

In the past decades, lithium-ion batteries (LIBs), due to the high energy densities, have become a dominant power source in portable electronics and have found applications in transportation, such as in hybrid electrical vehicles (HEVs) and electrical vehicles (EVs). Particularly, they have become indispensable energy storage devices for intermittent energy conversions, like in solar cells and wind power. However, lithium resources are too limited to meet the increasing demand of the global market for LIBs. Large-scale applications of LIBs are challenging in terms of their availability and cost. The development of new types of batteries, such as sodium-ion and magnesium-ion batteries, is necessary. Among them, sodium-ion batteries (NIBs) possess electrochemical working principles that are similar to LIBs. In addition, sodium is both inexpensive and abundant. Sodium is the sixth richest element on earth. Therefore, NIBs could substitute LIBs in applications such as smart grids and large-scale energy storage for renewable solar power and wind power. NIBs have attracted tremendous attention because of their potential low cost. Numerous materials have been investigated as electrodes for NIBs, such as Na4Co3(PO4)2P2O7, 3 Na3V2(PO4)3, 4,5 red phosphorus, SnSb, Sb, TiO2 10 and MoO3. 11 Iron-based materials have promising sustainability and environmental safety for both LIBs and NIBs. Iron, the fourth richest element in the earth’s crust, is inexpensive. Therefore, these materials have the potential as costeffective future energy storage systems. Iron-based materials (i.e., NaxFe1/2Mn1/2O2, 12 Fe3O4, 13,14 and Fe2O3 ) have been used as cathodes and anodes of NIBs. Improving the performance of this series of iron-based materials is extremely important for the development of NIBs. Recently, transition metal oxides (e.g., SnO2 16,17 and MoO3 ) that were previously applied as high-capacity anodes in LIBs have been extended to NIBs. Transitionmetal oxides have exhibited high capacity and good cycling performance. In addition, they are also promising anode candidates inNIBs for large-scale energy storage. However, largevolume expansion/contraction associated with alkaline ion insertion– extraction is known to generate tremendousmicrostructural damage to electrodes, which leads to loss of electrical contact and subsequently rapid capacity fading. Anchoring Fe2O3 nanoparticles onto carbon matrices can effectively cushion the volume expansion/contraction. Graphene has been used successfully as a carbon matrix because of its superior conductivity, large surface area, and excellent mechanical flexibility. In this study, Fe2O3 nanocrystals uniformly anchored onto graphene nanosheets (Fe2O3@GNS) were obtained via the nanocasting technique and studied as the anode material of NIBs. Graphene functioned as both a supporter and a conductive additive. No additional conductive reagents were used in the Fe2O3@GNS electrode. The good contact between Fe2O3 nanocrystals and GNS and the flexibility of GNS were expected to improve the cycling performance and rate capability of NIBs. The corresponding electrochemical performance of the NIB with Fe2O3@GNS was evaluated. Graphite oxide (GO) was synthesized from natural graphite powders by a modified Hummers method. GNS was prepared through thermal exfoliation splitting of GO. GNS prepared by the thermal exfoliation method has numerous pores, which provide space for the nanoparticles. Therefore, nanocasting technology can be used to prepare Fe2O3@GNS. 19 The details of the experiment are separately described in the ESI.† After anchoring Fe2O3, a hump located at around 341 turns up, which is ascribed to the peaks of Fe2O3 and is visible in the XRD pattern of Fe2O3@GNS, as shown in Fig. S1 (ESI†). Fig. 1a shows a Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Umezono 1-1-1, Tsukuba, 305-8568, Japan. E-mail: [email protected]; Tel: +81-29-861-5795 b Nanjing National Laboratory of Microstructures, School of Electronic Science and Engineering & School of Modern Engineering and Applied Science, Nanjing University, Nanjing 210093, China. E-mail: [email protected]; Tel: +86-25-83621220 WPI Advanced Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan † Electronic supplementary information (ESI) available: Experimental details, characterization and measurements, XRD patterns of GNS and Fe2O3@GNS, a TG curve of Fe2O3@GNS, and a SEM image of the GNS. See DOI: 10.1039/ c3cc47977c Received 17th October 2013, Accepted 21st November 2013