Lithium-ion batteries (LIBs) have been receiving increasing attention as attractive power sources, motivated especially by the rapid development of electric vehicles and portable devices. 2] As anode materials for LIBs, transition metal oxides utilize all of a metal’s redox potentials by the formation of metal through a chemical conversion mechanism, resulting in high theoretical capacities (500–1000 mAhg ) compared to commercially used graphite based on an intercalation mechanism (372 mAhg ). Among the oxides, Fe3O4 has gained considerable attention due to its low cost, abundance in nature, the fact that it is environmentally benign, and its high theoretical capacity of 926 mAhg 1 through the reaction Fe3O4+8Li + + 8e $3Fe+4Li2O. However, the application of Fe3O4 is hampered by a large volume expansion and structural rearrangements upon electrochemical cycling (problems that are common for conversion materials). To circumvent these problems, several hybrid nanostructures have been designed by mixing Fe3O4 with carbon. [4–6] Carbon plays a dual role in the electrodes: it can increase the electronic conductivity ; furthermore, it can work as a structural buffering material to accommodate the strain caused by the large volume change during the charge–discharge process. Various carbon materials, such as carbon nanotubes, carbon fibers, graphene, and others, have been successfully utilized to host Fe3O4 nanoparticles. Several approaches, such as hydrothermal, co-precipitation, as well as template syntheses, have been developed. Most strategies involve time-consuming procedures, usually requiring a separate step to, for example, deposit the carbon onto the surface of the presynthesized iron oxide or to fill or disperse iron oxide nanoparticles into pores or onto the carbon’s surface. Multistep processes or the use of several reagents might be required. It is thus desirable to develop a facile synthetic route to generate such functional composite materials. The present work describes a simple solvent-free process to obtain Fe3O4–C nanocomposites. In these composites Fe3O4 nanoparticles are anchored onto self-developed helical carbon fibers. Ferrocene, as a single precursor, acts as both iron and carbon source. As illustrated in Scheme 1, the synthesis is accomplished by a pyrolysis–oxidation route: pyrolysis of ferrocene in a closed stainless-steel reactor produces a fine black powder, referred to as “Fe–C” composite, consisting of two iron-rich phases: Fe and Fe3C; then, further mild oxidization of Fe–C under CO2, yields the final oxide product, referred to as “Fe3O4–C”. It is known that Fe and Fe3C can catalyze the formation of helical carbon. In the present procedure, Fe and Fe3C formed during pyrolysis of ferrocene and act as catalysts for the growth of helical carbon. Upon oxidation, both Fe and Fe3C are fully converted into Fe3O4, while the carbon morphology remains helical. Fe or Fe3C and later their transformation product (Fe3O4) are firmly embedded in the helical carbon structure. The thus-obtained Fe3O4–C composite shows a high reversible capacity, and good cycling and rate capability. Synergistic effects, through combining the redox reaction of metal oxide and carbon nanofibers, are discussed. X-ray diffraction (XRD) patterns of the material before and after oxidation (Figure 1a) show that upon pyrolysis of ferrocene, two distinct iron-rich phases of Fe3C (Joint Committee on Powder Diffraction Standards (JCPDS) card number 0350772) and a-Fe (JCPDS 006-0696) form in the Fe–C composite. The formation of Fe3C was due to the partial dissolution of carbon atoms into Fe. Upon further oxidation of the Fe–C composite, both Fe and Fe3C were oxidized to Fe3O4 (JCPDS 019-0629). In the Fe3O4–C composite, the narrow and sharp peaks suggest that the obtained Fe3O4 is of a highly crystalline nature. The mean crystallite size of Fe3O4 was calculated as 44 nm, according to the Scherrer equation. A strong peak due to graphitic carbon (2q=26.58) was observed in both Fe–C and Fe3O4–C composites. That the carbon morphology was well-maintained was confirmed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). N2 isotherms of the Fe3O4–C composite showed type IV curves with an H3 hysteresis loop (Figure 1b), indicating a mesoporous structure. The sample had a high Brunauer–Emmett– Teller (BET) surface area of 126 mg . The pore-size distribution (inset of Figure 1b) indicated the coexistence of both micropores (0.026 cmg ) and mesopores (0.08 cmg ). The relatively large specific surface area and high porosity offer a large material/electrolyte contact area and promote the diffusion of Li ions. Scheme 1. Fabrication of Fe–C and Fe3O4–C composites.
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Mar 29, 2017
Nilamadanthai Anbazhagan + 4
Open Access