Facile synthesis and electrochemical performances of hollow graphene spheres as anode material for lithium-ion batteries
© Yao et al.; licensee Springer. 2014
Received: 2 June 2014
Accepted: 12 July 2014
Published: 28 July 2014
The hollow graphene oxide spheres have been successfully fabricated from graphene oxide nanosheets utilizing a water-in-oil emulsion technique, which were prepared from natural flake graphite by oxidation and ultrasonic treatment. The hollow graphene oxide spheres were reduced to hollow graphene spheres at 500°C for 3 h under an atmosphere of Ar(95%)/H2(5%). The first reversible specific capacity of the hollow graphene spheres was as high as 903 mAh g-1 at a current density of 50 mAh g-1. Even at a high current density of 500 mAh g-1, the reversible specific capacity remained at 502 mAh g-1. After 60 cycles, the reversible capacity was still kept at 652 mAh g-1 at the current density of 50 mAh g-1. These results indicate that the prepared hollow graphene spheres possess excellent electrochemical performances for lithium storage. The high rate performance of hollow graphene spheres thanks to the hollow structure, thin and porous shells consisting of graphene sheets.
81.05.ue; 61.48.Gh; 72.80.Vp
KeywordsLithium-ion batteries Hollow graphene spheres Electrochemical performance Cycle performance
Since the paper on freestanding graphene was published by Novoselov et al. , the preparation, structure, and property of graphene have attracted great attention owing to its particular quantum Hall effect, sensitivity, mechanical hardness, electrical conductivity, and so on [2–7]. Graphene is a two-dimensional one-atom-thick planar sheet of sp2 bonded carbon atoms, which is a basic building block for graphitic materials of all other dimensionalities. It is regarded as the ‘thinnest material in the universe’ with tremendous application potential. These attractive properties of graphene generate huge interest from different scientific communities in the possible implementation of graphene in different application fields such as biomedicine, reinforced composites, sensors, catalysis, energy conversion and storage device, electronics, and transparent electrodes for displays and solar cells .
Nowadays, lithium-ion batteries are widely used in various electronic devices, such as notebook computers, cellular phones, camcorders, electric vehicles, and electric tools due to their superior properties such as long cycle life, high energy density, no memory effect, and environmental friendliness. To meet the increasing demand for lithium-ion batteries with high reversible capacity and energy density, much effort has been made to develop new electrode materials or design novel structures of electrode materials [9–14]. Recently, graphene sheets as anode materials were investigated and exhibited large reversible capacity [15–19]; it has been demonstrated that the graphene sheets of ca. 0.7 nm thickness could provide the highest storage density (with a Li4C6 stoichiometry) by density of states calculations .
In this work, the hollow graphene oxide spheres (HGOSs) were fabricated directly from graphene oxide (GO) utilizing a water-in-oil emulsion technique, which were prepared from natural flake graphite by oxidation and ultrasonic treatment. The hollow graphene oxide spheres were reduced to hollow graphene spheres (HGSs) by heat treatment under a hydrogen atmosphere. Compared with the graphene sheets , the prepared HGSs possess better cycle and high rate performances for the lithium storage, which thanks to the hollow structure, thin and porous shells consisting of graphene sheets.
GO nanosheets were prepared in two steps: the oxidation of flake natural graphite powder via a modified Hummers' method and ultrasonication. KMnO4 was employed as the oxidant to obtain graphite oxide. Firstly, 1 g of flake natural graphite powder with the mean diameter of 15 μm (provided by Dong Xin Electrical Carbon Co., Ltd., Chongqing, China) was added to 23 mL of cooled (0°C) concentrated H2SO4. Then, 3 g of KMnO4 was added gradually with stirring and cooling, so that the temperature of the mixture was maintained below 10°C. The mixture was then stirred at 35°C for 30 min. After this, 46 mL of distilled water was slowly added to cause an increase in temperature to 98°C, and the mixture was maintained at that temperature for 15 min. The reaction was terminated by adding 140 mL of distilled water followed by 10 ml of 30% H2O2 solution. The suspension was then repeatedly centrifuged and washed twice with 5% HCl solution and then repeatedly with water until sulfate could not be tested with barium chloride. The collected precipitate was dispersed in 450 mL water and sonicated for 2 h. Then, the suspension was separated into the supernatant liquor and a golden colored residue by centrifugation at 5,000 rpm for 10 min. The supernatant was centrifuged again at 15,000 rpm for 5 min to remove the suspended substance. The precipitate was ultrasonicated, collected, and dried in a vacuum oven at 60°C; thus, GO nanosheets were obtained.
GO nanosheets of 0.1 g were dispersed into aqueous ammonia (20 mL, pH = 12) through agitation and were stirred at 30°C for 1 h to obtain the GO nanosheet suspension. Then, the suspension was slowly poured into hot olive oil (provided by Asceites Del Sur-coosur, Seville, Spain; the acidity is <0.4%, and the saturated fat, polyunsaturated fat, and monounsaturated fat are 14, 9, and 77 wt%, respectively) preheated to 90°C and intensely stirred for 30 min at 90°C. Subsequently, with the formation of a water-in-oil emulsion, the viscosity of the emulsion rapidly increased with the appearance of a golden foam. Half an hour later, when the bath temperature was increased to 95°C, the viscosity decreased gradually. With the intensive stirring, water was gradually separated from the oil. In the meantime, emulsion turned clear as olive oil. Finally, the emulsion system was cooled to room temperature. The HGOSs were obtained by centrifugation, washing, and drying. The HGOSs were reduced to HGSs at 500°C for 3 h under an atmosphere of Ar(95%)/H2(5%).
The products were characterized by X-ray diffraction (XRD) on a Rigaku D/max-2500B2+/PCX system (Rigaku, Beijing, China) using Cu/K radiation (λ = 1.5406 Å) over the range of 5 to 90° (2θ) at room temperature. The morphologies of the samples were observed by scanning electron microscope (SEM, Hitachi S-4700, Hitachi, Ltd, Chiyoda-ku, Japan). The information of functional groups was measured by Fourier transform infrared spectroscopy instrument (FTIR, Nicolet Nexus 670, Thermo Fisher Scientific, Shanghai, China).
The electrochemical performances of the HGSs as anode materials for lithium-ion batteries were measured with the coin-type cells. The lithium sheets were used as both reference and counter electrodes, and composite electrodes comprising active mass (HGSs, 85 wt%), carbonaceous additive (acetylene black, 5 wt%), and poly(vinylidene difluoride) (PVDF, 10 wt%) binder were used as working electrodes. The thickness and density of electrode are 50 μm and 1.95 mg cm-2, respectively. One molar LiPF6 solution in a 1:1 (volume) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) from Merck & Co., Inc. (Whitehouse Station, NJ, USA) was used as electrolyte. The Celgard 2400 microporous polypropylene film provided by Jimitek Electronic (Shenzhen, China) Co. Ltd was used as separator. The coin-type cells were galvanostatically discharged (Li insertion) and charged (Li extraction) in the voltage range from 0.01 to 3.50 V vs. Li/Li+ at the different current densities. Electrochemical impedance spectroscopy measurements of the electrodes were carried out on an electrochemical workstation (Princeton VersaSTAT3-200, Princeton Applied Research, Oak Ridge, TN, USA) using the frequency response analysis. The impedance spectra were obtained by applying a sine wave with amplitude of 5.0 mV over the frequency range from 100 kHz to 0.01 Hz.
Results and discussion
After a thermal treatment in H2, these functional groups derived from the intensive oxidation were eliminated, which can be proved by the disappearance of the peaks at 1,724, 1,619, 1,224, and 1,053 cm-1 while an appearance of a new peak at 1,631 cm-1 (Figure 2b) reflecting the skeletal vibration of graphene sheets [15, 22]. These evidences indicate that HGOSs have been successfully reduced to HGSs by heat treatment.
The HGSs have been successfully fabricated from GO nanosheets utilizing a water-in-oil emulsion technique and thermal treatment. The electrochemical performance testing showed that the first reversible specific capacity of the HGSs was as high as high as 903 mAh g-1 at a current density of 50 mAh g-1. After 60 cycles at different current densities of 50 mA g-1, 100 mA g-1, 200 m mA g-1, 500 m mA g-1, and 1,000 mA g-1, the reversible specific capacity was still maintained at 652 mA g-1 at the current density of 50 mA g-1, which indicated that the prepared HGSs possess a good cycle performance for the lithium storage. The high rate performance of HGSs thanks to the hollow structure, thin and porous shells consisting of graphene sheets.
hollow graphene oxide spheres
hollow graphene spheres
scanning electron microscope
Fourier transform infrared
solid electrolyte interface
This work was supported by the National Natural Science Foundation of China (Grant No. 50672004), National High-Tech Research and Development Program (2008AA03Z513), and Doctoral Fund of Ministry of Education of China (20120010110001).
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