Graphene-supported SnO2 nanoparticles prepared by a solvothermal approach for an enhanced electrochemical performance in lithium-ion batteries
© Wang et al.; licensee Springer. 2012
Received: 13 December 2011
Accepted: 9 March 2012
Published: 13 April 2012
SnO2 nanoparticles were dispersed on graphene nanosheets through a solvothermal approach using ethylene glycol as the solvent. The uniform distribution of SnO2 nanoparticles on graphene nanosheets has been confirmed by scanning electron microscopy and transmission electron microscopy. The particle size of SnO2 was determined to be around 5 nm. The as-synthesized SnO2/graphene nanocomposite exhibited an enhanced electrochemical performance in lithium-ion batteries, compared with bare graphene nanosheets and bare SnO2 nanoparticles. The SnO2/graphene nanocomposite electrode delivered a reversible lithium storage capacity of 830 mAh g−1 and a stable cyclability up to 100 cycles. The excellent electrochemical properties of this graphene-supported nanocomposite could be attributed to the insertion of nanoparticles between graphene nanolayers and the optimized nanoparticles distribution on graphene nanosheets.
KeywordsSnO2 Graphene nanosheets Nanocomposite Lithium-ion batteries
Graphene has been emerged as a rising star in materials science and as an excellent candidate for many applications due to its unique two dimensional (2D) nanostructure , outstanding electrical properties , and ultrahigh specific surface area . The applications of graphene include gas molecule adsorption , quantum dots , transistors , lithium-ion batteries , supercapacitors , lithium air batteries , and drug delivery . In particular, graphene has attracted worldwide attention for energy storage and conversion. With the formation of sandwich-like three dimensional (3D) nanostructured composite materials, the restacking of graphene nanosheets (GNS) can be effectively prevented and therefore the electrochemical properties of the nanocomposite electrodes could be significantly improved by using nanocrystallines to insert between layers of graphene nanosheets [3, 11].
SnO2 has been examined as an anode material for lithium-ion batteries with a high theoretical capacity of 782 mAh g−1. SnO2 forms metal alloys when reacting with lithium, leading to reversible transformations between lithium tin alloys (LixSn) and tin metal when the lithiation and delithiation proceed. However, the capacity of bulk SnO2 electrode fades quickly during prolonged cycling . To further improve the electrochemical performance and the cycle life of SnO2 electrodes for long-term cycling, one approach is to synthesize nanosized SnO2 crystals with different morphologies, such as nanowires , nanotubes , and mesoporous structure . These nanostructured SnO2 materials were reported to deliver greatly enhanced specific capacities with durable cycling stabilities. In order to mechanically buffer the volume expansion associated with the charge/discharge processes in the lithium-ion cells, the formation of SnO2/graphene nanocomposites has also been proved to be feasible. Many methods have been implemented to distribute SnO2 nanocrystals on graphene nanosheets, including in situ chemical preparation [13, 17], reassembling process , gas–liquid interfacial synthesis , as well as hydrothermal and solvothermal methods [20, 21].
In this paper, we employ a facile solvothermal technique to disperse SnO2 nanoparticles with a controlled size on graphene nanosheets. The as-prepared SnO2/GNS nanocomposite showed uniform SnO2 nanoparticles distribution and significantly improved electrochemical properties, compared with bare graphene nanosheets and SnO2 nanoparticles. The solvothermal approach developed in this investigation could be used for the synthesis of other metal oxide/graphene nanocomposites.
Graphene oxide (GO) powders were prepared via a chemical approach derived from Hummers' method , according to the previously reported procedure . In a typical synthesis process, 40 mg GO was firstly dispersed in 40 ml ethylene glycol by ultrasonification for 1 h, followed by the addition of 0.1 mmol SnCl2·2H2O powders. The mixture was vigorously stirred for half an hour, and then transferred to a 50 ml Teflon lined autoclave, which was sealed and maintained in an oven at 160°C for 6 h. Afterwards, the black precipitates (SnO2/GNS) were collected, washed with deionized water and ethanol to remove the impurities, and isolated by vacuum filtration. The product was then dried in a vacuum oven at 60°C, and further sintered at 300°C for 4 h in argon to increase the crystallinity. For the comparison, bare SnO2 nanoparticles were also synthesized by the same experimental procedure without the presence of GO in the mixture solution.
The X-ray diffraction (XRD) pattern of the as-synthesized material was measured using a Siemens D5000 X-ray diffractometer (Siemens Company, Wittelsbacherplatz 2, Munich, Germany) from 10° to 80° under a scan rate of 1° min−1. The Raman measurement of the SnO2/GNS nanocomposite was conducted on a confocal Micro Raman Spectrometer with LabRAM HR system (HORIBA Korea Ltd., Pucheon, Kyunggido Korea) using a 632.8 nm He-Ne laser source. The spectra were recorded in the range of 200 to 2,000 cm−1 with accumulated scans for an enhanced resolution. Field emission scanning electron microscope (FESEM) observations were performed using a Zeiss Supra 55VP FESEM with an Oxford energy dispersive spectrometry system (Carl Zeiss Nanotechnology Center, Oberkochen, Germany). The transmission electron microscopy (TEM) analysis was carried out using a Jeol 2011 TEM facility (JEOL Ltd., Tokyo, Japan). The graphene (carbon) content in the composite material was determined by thermogravimetric analysis (TGA) on a TGA/DTA analyzer (TA Instruments, SDT 2960 module, New Castle, DE, USA) in air at 10°C min−1 ranging from room temperature to 1,000°C.
CR2032 coin cells were assembled in an argon-filled glove box (Unilab, M Braun Inertgas-Systeme GmbH, Garching, Germany) in which the levels of moisture and oxygen were controlled to be less than 0.1 ppm. The electrodes were made by mixing 80 wt% SnO2/GNS active materials, 10 wt% carbon black, and 10 wt% polyvinylidene fluoride binder in the N-methyl-2-pyrrolidone solvent to form a slurry. Then, the slurry was coated on copper foil substrate. Lithium foils were used as the negative electrodes. The electrolyte was 1 M LiPF6 in ethylene carbonate and dimethyl carbonate (1:1). Cyclic voltammetry (CV) tests were carried out on an electrochemistry workstation (CHI660D, CH Instrument, Inc., Austin, TX, USA) at a scan rate of 0.1 mV s−1 vs. Li/Li+ reference electrode in the voltage range of 0.01 to 3 V. Galvanostatic charge/discharge measurements were conducted on the Neware battery tester (Neware Co.,Ltd., Shenzhen, China) with a current rate of 0.1 C for 100 cycles. Electrochemical impedance spectroscopy was performed on the same electrochemistry workstation. The frequency was set in 0.01 Hz–100 kHz with the amplitude of 5 mV. The charge/discharge performance was also investigated for bare graphene nanosheets and SnO2 nanoparticles as a comparison.
Results and discussion
Simultaneously, GO nanosheets were also gradually reduced by EG to form graphene nanosheets. As EG is a mild reducing agent, the reduction processes take a long time (6 h) to complete at the high temperature (160°C). Then, the formed Sn nanoparticles were further oxidized by oxygen to become SnO2 nanoparticles during the cooling period. Consequently, the SnO2/GNS nanocomposite was obtained under the assistance of EG which acted not only as a dispersing agent but also as a reducing agent.
A facile solvothermal preparation method has been developed to synthesize the SnO2/GNS nanocomposite with a uniform nanoparticle distribution. The as-prepared SnO2/GNS nanocomposite exhibited an improved lithium storage capacity and cycling performance compared to bare GNS and bare SnO2 nanoparticles. The presence of GNS in the nanocomposite could increase the electrical conductivity and buffer the volume expansion associated with the lithiation and delithiation processes, leading to a significantly enhanced electrochemical performance. The solvothermal approach might be applicable for rapid and effective synthesis of other metal oxide/graphene nanocomposites.
, Graphene nanosheets
, Graphene oxide.
This work was financially supported by the Australian Research Council (ARC) through the ARC Discovery Project (DP1093855). We also acknowledge the support from the National Research Foundation of Korea through the World Class University program (R32-2008-000-20093-0).
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