Wednesday, 12 October 2016 15:29

Graphene Anchored with Nanocrystal Fe2O3 with Improved Electrochemical Li-Storage Properties

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Graphene Anchored with Nanocrystal Fe2O3 with Improved Electrochemical Li-Storage Properties


Graphene anchored with nanocrystal Fe2O3 was synthesized by a two-step solvothermal route. The nanocomposite was characterized by X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), scanning electron spectroscopy (SEM) and transmission electron microscopy (TEM). It was found that the Fe2O3 nano-particles were formed and homogeneously anchored on graphene sheets.

The electrochemical properties of the nanocomposite were investigated by galvanostatic cycling. The results showed that the Fe2O3/graphere nanocomposite exhibits obviously improved electrochemical properties compared to bare Fe2O3 because of the buffering, confining and conducting effects of the introduced grapheme.

Keywords: Fe2O3, graphene, nanocomposite, electrochemical properties, anode

1. INTRODUCTION In the early 1980s, some transition metal oxides, such as CuO [1], Fe2O3 [2], and CoO [3] have already been used as cathode in primary batteries. In recent years, these oxides have received a renewed interest since they showed reversible cycling as anodes for Li-ion batteries [47]. Among these oxides, Fe2O3 is an attractive anode material for lithium ion batteries due to the high theoretical capacity (1005mAh g−1 ), high abundance of Fe, low cost and low environmental impact [8–10]. However, Fe2O3 generally shows a rapid capacity fade because of the large volume changes and agglomerations during the conversion reactions. Great effort has been made to improve the electrochemical properties of Fe2O3. A stable cycling could be obtained for Fe2O3 by using nanostructured [11, 12], porous [13] and thin-film materials [14] with relieved volume changes.

Dispersing the particles on conducting carbon materials has been proved to be a useful strategy to improve cycling stability of the transition metal oxides due to the effective buffering and conducting effects of carbon materials. In addition, carbon materials also contribute to the overall capacity for the Int. J. Electrochem. Sci., Vol. 7, 2012 355 composite. Various forms of carbon, such as amorphous carbon [15], carbon fibers [16], and carbon nanotubes [17] have been suggested as the ideal matrices to disperse Fe2O3. Recently, great importance has been attached to another form of carbon, graphene, since reported by Novoselov et al. in 2004 [18]. The intensive researches on tin oxides [19-22], cobalt oxides [2329], and copper oxides [30, 31] have shown that the cycling stability of these oxides could be greatly improved by loading them onto the graphene.

Graphene, a flat monolayer of sp2 -bonded carbon atoms, is considered as an ideal matrix to disperse nanoparticles due to its advantages such as large specific surface area [32], high electronic conductivity [33], and high mechanical strength [34]. These merits make graphene or graphene-based materials very promising for use as anode materials for Li-ion batteries. In this work, we will investigate the effect of graphene on the electrochemical performance of Fe2O3. The Fe2O3/graphene (Fe2O3/G) nanocomposite was prepared by a two-step solvothermal route. The results showed that the nanocomposite exhibits an obviously improved cycling stability compared with bare Fe2O3.


2.1 Synthesis of Fe2O3/graphene nanocomposite Graphite oxide (GO, 20 mg), synthesized by a modified Hummer’s method [35], was ultrasonically dispersed in a N,N-dimethylformamide (DMF)/H2O (10:1 in volume) mixed solvent for 1 h to get graphene oxide followed by adding of 1 mmol FeSO4·7H2O. The above dispersion was then heated at 80 ºC for 3 h. Afterwards, the mixture was sealed into a Teflon-lined stainless steel autoclave and maintained at 180 ºC for 20 h. After it was cooled down to the room temperature, the resulting product was separated by centrifugation, washed with deionized water and ethanol for several times, and dried at 40 ºC under vacuum overnight. For comparison, bare Fe2O3 was prepared using the similar procedure without adding GO.

2.2 Materials Characterization X-ray diffraction (XRD) patterns of the products were collected on a Rigaku D/Max-2550pc powder diffractometer equipped with Cu Kα radiation (λ = 1.541 Å). X-ray photoelectron spectroscopy (XPS) analysis was performed on a KRATOS AXIS ULTRA-DLD spectrometer with a monochromatic Al K radiation (hv = 1486.6 eV). The microstructures were observed by field emission scanning electron microscopy (FE-SEM) on a FEI-sirion microscope and transmission electron microscopy (TEM) on a JEM 2100F microscope. Raman spectra were recorded on a JobinYvon Labor Raman HR-800 using 514.5 nm Ar-ion laser. Thermogravimetric analysis (TGA) was performed on a DSCQ1000 instrument from 80 to 800 ºC at a heating rate of 10 ºC min-1 in air.

2.3 Electrochemical measurements The electrochemical properties of the products (Fe2O3/G, Fe2O3) were evaluated using CR2025- type coin cells. The electrode slurry was prepared by mixing the active material, acetylene black and Int. J. Electrochem. Sci., Vol. 7, 2012 356 polyvinylidene fluoride in a weight ratio of 75:15:10 in N-methyl pyrrolidone (NMP) with stirring for 2 h. The working electrodes were made by coating the slurry onto Ni foam current collectors and dried at 100 °C under vacuum overnight. The working electrodes were assembled into half cells in an Arfilled glove box using Li foil as the counter electrode and polypropylene microporous sheet (Celgard 2300) as the separator. The electrolyte was 1 M LiPF6 dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1, in volume). The cells were galvanostatically charged and discharged between 0.005 and 3 V (vs. Li/Li+ ) on a Neware BTS-5V10mA battery cycler (Shenzhen, China). Cyclic voltammetry (CV) measurements were conducted on an Arbin BT2000 system in the voltage range 0.0053.0 V (vs. Li/Li+ ) at 0.1 mV s-1 . Electrochemical impedance spectroscopy (EIS) measurements were performed on a CHI660C electrochemistry workstation by applying an ac voltage of 5 mV amplitude in the frequency range from 10 mHz to 100 kHz at de-lithiated states. All of the electrochemical measurements were carried out at room temperature.

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