In situ chemical synthesis of SnO2–graphene nanocomposite as anode materials for lithium-ion batteries

In situ chemical synthesis of SnO2–graphene nanocomposite as anode materials for lithium-ion batteries

Electrochemistry Communications 11 (2009) 1849–1852 Contents lists available at ScienceDirect Electrochemistry Communications journal homepage: www...

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Electrochemistry Communications 11 (2009) 1849–1852

Contents lists available at ScienceDirect

Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom

In situ chemical synthesis of SnO2–graphene nanocomposite as anode materials for lithium-ion batteries Jane Yao, Xiaoping Shen, Bei Wang, Huakun Liu, Guoxiu Wang * Institute for Superconducting and Electronic Materials, School of Mechanic, Materials and Mechatronic Engineering, University of Wollongong, NSW 2522, Australia

a r t i c l e

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Article history: Received 15 July 2009 Received in revised form 30 July 2009 Accepted 31 July 2009 Available online 6 August 2009 Keywords: Graphene SnO2 nanoparticles Nanocomposite Anode material Lithium-ion batteries

a b s t r a c t An in situ chemical synthesis approach has been developed to prepare SnO2–graphene nanocomposite. Field emission scanning electron microscopy and transmission electron microscopy observation revealed the homogeneous distribution of SnO2 nanoparticles (4–6 nm in size) on graphene matrix. The electrochemical reactivities of the SnO2–graphene nanocomposite as anode material were measured by cyclic voltammetry and galvanostatic charge/discharge cycling. The as-synthesized SnO2–graphene nanocomposite exhibited a reversible lithium storage capacity of 765 mAh/g in the first cycle and an enhanced cyclability, which can be ascribed to 3D architecture of the SnO2–graphene nanocomposite. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Graphene, a single-atom-thick sheet of honeycomb carbon lattice, exhibits many unique chemical and physical properties [1– 3]. Since its discovery in 2004, extensive efforts have been devoted to investigating the technological applications of graphene materials, such as graphene-based electronics, high strength composite materials, liquid crystal displays, and energy storage and conversion devices [4–7]. Graphite (the 3D form of graphene) is currently used as the anode material in commercial rechargeable lithium batteries. It has a maximum theoretical lithium storage capacity of 372 mAh/g, while a single layer graphene has a theoretical lithium storage capacity of 744 mAh/g if lithium is attached to both sides of the graphene sheets. The enhanced lithium storage capacity of graphene in lithium ion cells has been experimentally verified [8,9]. However, graphene sheets always naturally stack into multilayers and therefore lose their high surface area and intrinsic chemical and physical properties. Recently, a general strategy – jamming graphene sheets with nanoparticles, has been developed to minimize the aggregation of graphene. Both metal nanoparticles (Au and Pt) [10,11] and metal oxide nanoparticles (TiO2 and SnO2) [12,13] have been used for this purpose. However, those composites were prepared by mechanically mixing nanoparticles with graphene dispersions, which limits the homogeneous dispersion of nanoparticles and the separation of graphene sheets. * Corresponding author. Fax: +61 2 42215731. E-mail address: [email protected] (G. Wang). 1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2009.07.035

Transition metal oxides have been studied as alternative anode materials for lithium-ion batteries because of their high specific capacity. Among them, tin oxide (SnO2) is particularly attractive. The reaction between lithium ions and SnO2 can be expressed as: þ

8:4Li þ SnO2 $ Li4:4 Sn þ 2Li2 O

ð1Þ

The above reaction is reversible with a theoretical reversible capacity of 782 mAh/g based on the mass of SnO2 [14]. Therefore, the formation of SnO2–graphene nanocomposite should not only reduce the degree of stacking of graphene sheets, but also boost the lithium storage capacity. Herein, we report the in situ chemical synthesis of a SnO2– graphene nanocomposite and its enhanced electrochemical performance as anode materials in lithium-ion batteries. 2. Experimental Graphene oxide nanosheets (GONS) were synthesised from natural graphite powders by a modified Hummer’s method [15]. In a typical synthesis, 40 mg GONS was dispersed in de-ionised (DI) water by ultrasonication. The dispersion was then mixed with 40 ml aqueous solution of SnCl22H2O (20 mg) and citric acid (20 mg). The mixture was transferred into a 250 ml round-bottomed flask and was heated in an oil bath to 120 °C under stirring. 20 ml NaBH4 (200 mg) aqueous solution was gradually added, and the resulting mixture was refluxed at 120 °C for 5 h. During this process, GONS were reduced to graphene nanosheets (GNS) and Sn2+ to Sn nanoparticles. Simultaneously, the Sn nanoparticles

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(a)

GNS

SnO2 (112) SnO2 (301)

SnO2 (211)

SnO2 (101)

SnO2 (110)

were oxidized to SnO2 due to heating at 120 °C in atmosphere. The resultant black solid products were separated by filtration, washed with DI-water, and dried in vacuum at 80 °C. To improve the crystallinity of SnO2 in GNS, the product was annealed at 300 °C for 10 h in Ar atmosphere. The weight content of SnO2 in Sn/graphene nanocomposite was quantitatively determined to be 40 wt% by thermogravimetric analysis (TGA) and chemical analysis, in which SnO2 nanoparticles were dissolved by diluted HCl. The structure and morphology of the SnO2/graphene nanocomposite were analysed by X-ray diffraction (XRD, Philips 1730 X-ray diffractometer), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM, JEOL 2011 TEM facility). SnO2–graphene powders were mixed with a binder, poly(vinylidene fluoride) (PVdF), at the weight ratio of 90:10 in N-methyl-2pyrrolidone (NMP) solvent to form a slurry. Then, the resultant slurry was uniformly pasted on Cu foil substrates with a blade. These prepared electrode sheets were dried at 100 °C in a vacuum oven for 12 h and pressed under a pressure of approximately 200 kg/cm2. CR2032-type coin cells were assembled in a glove box for electrochemical characterization. The electrolyte was 1 M LiPF6 in a 1:1 mixture of ethylene carbonate and dimethyl carbonate. Li metal foil was used as the counter electrode. The cells were galvanostatically charged and discharged at a current density of 55 mAh/g within the range of 0.01–3.0 V. Cyclic voltammetry (CV) curves were collected at 0.1 mV/s within the range of 0.01– 3.0 V using an electrochemistry workingstation (CHI660C).

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3. Results and discussion Fig. 1a shows the X-ray diffraction pattern of the SnO2–graphene nanocomposite. The major diffraction lines can be indexed to the tetragonal rutile SnO2 phase. The graphene nanosheets only show a weak (1 0 0) diffraction line. The broad diffraction peaks of the SnO2 indicate small crystal size. The general morphology of the SnO2–graphene nanocomposite was observed by FESEM. Fig. 1b shows a FESEM image of the SnO2–graphene nanocomposite, in which the graphene nanosheets appear corrugated into a wavy shape. SnO2 nanoparticles are embedded in the curly graphene nanosheets. The crystalline structure of the SnO2–graphene nanocomposite was analysed by TEM and high resolution TEM. Fig. 2a shows a low magnification TEM image of the SnO2–graphene nanocomposite. SnO2 nanoparticles are uniformly distributed on 2D graphene nanosheets. The inset in Fig. 2a is the selected area electron diffraction (SAED) pattern. All diffraction rings can be indexed to tetragonal SnO2 phase. A high magnification TEM image of the SnO2–graphene nanocomposite is shown in Fig. 2b, from which the average particle size of SnO2 can be determined to be about 4–6 nm (more than 200 counts). Fig. 2c shows HRTEM image of cross-sectional view of SnO2–graphene nanocomposite. SnO2 nanoparticles (black) are surrounded by flexible graphene nanosheets, which can be distinguished as linear strips. The interplanar distance of the (0 0 2) crystal planes of the stacked graphene sheets was determined to be 0.38 nm, which is much larger than that in the pristine graphite (0.34 nm). The stacking of graphene nanosheets amounts of 3–6 layers, which can be counted from the number of strips as marked with circles in Fig. 2c. Fig. 2d presents a lattice resolved HRTEM image of the SnO2–graphene nanocomposite, from which the lattices of SnO2 nanoparticles and graphene nanosheets are clearly visible. The inset shows the atomic resolution HRTEM image of a SnO2 nanoparticle, in which the (1 1 0) and (2 0 0) crystal planes of the SnO2 tetragonal structure can be clearly identified from the interplanar distances of 0.24 and 0.34 nm, respectively. In the chemical synthesis process, graphene oxide nanosheets were firstly dispersed in water. Graphene oxide nanosheets can be considered as macromolecules containing

Fig. 1. (a) X-ray diffraction pattern of SnO2–graphene nanocomposite. (b) FESEM image of SnO2–graphene nanocomposite.

epoxyl and hydroxyl moieties on the basal plane and carboxylic acid group on the edge sites [16,17]. When they were mixed with Sn2+ cations, the Sn2+ cations were attracted and anchored to those functional groups. On chemical reduction by NaBH4, the graphene oxide nanosheets were reduced to graphene nanosheets, and the Sn2+ cations were reduced to Sn and aggregated to Sn nanoparticles. Since the reduction process was carried out at 120 °C, the Sn nanoparticles were immediately oxidized to SnO2 nanoparticles. These in situ formed SnO2 nanoparticles are able to effectively separate the stacking of graphene nanosheets. The electrochemical reactivity of SnO2–graphene nanocomposite as anode in lithium ion cells was first evaluated by cyclic voltammetry (CV). Fig. 3 shows the CV curves of SnO2–graphene nanocomposite electrode in the first, second, 10th, and 50th scanning cycles. In the first cycle, there is a small cathodic peak at 0.7 V, which can be attributed to the formation of the solid electrolyte interphase (SEI) layer. This peak disappears from the second cycle. The other obvious reduction peaks are located around 0.12 and 0.01 V, and can be ascribed to the lithium reaction with SnO2 nanoparticles and insertion in graphene nanosheets, respectively. þ

4Li þ SnO2 þ 4e ! Sn þ 2Li2 O

ð2Þ

þ

ð3Þ

þ

ð4Þ

xLi þ Sn þ xe $ Lix Sn xLi þ CðgrapheneÞ þ xe $ Lix C

Three oxidation peaks appear around 0.13, 0.51, and 1.23 V, respectively. The 0.13 V anodic peak corresponds to lithium extraction

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Fig. 2. (a) Low magnification TEM image of SnO2–graphene nanocomposite, showing the uniform distribution of SnO2 nanoparticles on graphene matrix. The inset is the corresponding SAED pattern. (b) High magnification TEM image of SnO2/graphene nanocomposite, from which the average particle size of SnO2 was measured to be 4–6 nm. (c) HRTEM image of SnO2–graphene nanocomposite, showing SnO2 nanoparticles surrounded by graphene nanosheets. (d) Lattice resolved HRTEM image of SnO2–graphene nanocomposite, in which the lattices of SnO2 nanoparticles and graphene nanosheets are clearly visible. The inset is an atomically resolved lattice image of a SnO2 nanoparticle, from which two perpendicular crystal planes, (1 1 0) and (2 0 0), can be distinguished.

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from graphene nanosheet (Eq. (4)); the 0.51 V oxidation peak can be assigned to the de-alloying of LixSn (Eq. (3)), while the weak oxidation at 1.23 V could be the partly reversible reaction of the Eq. (2) [18,19]. The CV measurements clearly elucidated the reversible electrochemical reactions between the lithium ions and the SnO2– graphene nanocomposite in lithium ion cells. The lithium storage capacity and cyclability of SnO2–graphene nanocomposite as anode in lithium ion cells were determined via galvanostatic charge/discharge cycling. Fig. 4a shows the charge/ discharge profiles of SnO2–graphene electrode in the first, second, and 50th cycles, respectively. In the first cycle, the SnO2–graphene nanocomposite delivered a lithium insertion capacity of 1420 mAh/g and a reversible charging capacity of 765 mAh/g. From the second cycle, the reversibility of the electrode was improved

significantly. The SnO2–graphene nanocomposite consists of 40 wt% SnO2 and 60 wt% graphene. Therefore, the theoretical capacity, C, of the SnO2–graphene nanocomposite should be 777.2 mAh/g based on CSnO2 of 782 mAh/g and Cgraphene of 744 mAh/g. The initial reversible capacity of the SnO2–graphene nanocomposite is very close to the theoretical capacity. The reversible lithium storage capacity vs. cycle number is shown in Fig. 4b. The SnO2–graphene nanocomposite electrode maintained a capacity of 520 mAh/g after 100 cycles. On the other hand, the SnO2 nanoparticle electrode exhibited a poor cyclability, retaining only 50 mAh/g in the 20th cycle and then failing completely. Graphene electrode prepared in the same condition as SnO2–graphene nanocomposite also exhibited much worse performance than that of SnO2–graphene nanocomposite electrode. Therefore, the SnO2– graphene nanocomposite electrode demonstrated much better electrochemical performance than that of the bare SnO2 and graphene electrodes. When SnO2 reacts with lithium, there is a dramatic volume increase, inducing cracking and pulverization. By embedding SnO2 nanoparticles in graphene nanosheet matrix, the volume expansion and contraction of the SnO2 nanoparticles can be effectively buffered by the flexible graphene nanosheets. In addition, graphene nanosheets also provide a highly conductive medium for electron transfer during the lithiation and de-lithiation process. So, good electrochemical performance can be maintained. 4. Conclusion In summary, SnO2–graphene nanocomposites with 3D architecture were synthesized by an in situ chemical reduction process. The mixing of graphene nanosheets and SnO2 nanoparticles on the molecular level can ensure homogeneous distribution of SnO2 nanoparticles on graphene nanosheets and effective separation of those graphene nanosheets. HRTEM analysis confirmed the uniform attachment of SnO2 nanoparticles (4–6 nm in size) on the graphene nanosheet matrix. Cyclic voltammetry measurements

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show the highly reactive nature of SnO2–graphene towards lithium storage in lithium ion cells. Our SnO2–graphene nanocomposite demonstrated a reversible specific capacity of 765 mAh/g in the first cycle and enhanced cyclability.

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Acknowledgement 2.0

We are grateful for financial support from the Australian Research Council (ARC) through the ARC Discovery Project (DP0772999).

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