graphene composite for high-capacity lithium storage

Applied Surface Science 258 (2012) 4917–4921

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Flower-like SnO2 /graphene composite for high-capacity lithium storage Hongdong Liu, Jiamu Huang ∗ , Xinlu Li ∗ , Jia Liu, Yuxin Zhang, Kun Du College of Materials Science and Engineering, Chongqing University, Chongqing 400045, PR China

a r t i c l e

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Article history: Received 12 October 2011 Received in revised form 4 January 2012 Accepted 20 January 2012 Available online 28 January 2012 Keywords: Flower-like SnO2 Graphene Lithium-ion batteries Anode

a b s t r a c t Flower-like SnO2 /graphene composite is synthesized by a simple hydrothermal method for high-capacity lithium storage. The as-prepared products are characterized by XRD, FTIR, FESEM, TGA and Nitrogen adsorption/desorption. The electrochemical performance of the flower-like SnO2 /graphene composite is measured by cyclic voltammetry and galvanostatic charge/discharge cycling. The results show that the flower-like SnO2 nanorod clusters are 800 nm in size and homogeneously adhere on graphene sheets. The flower-like SnO2 /graphene composite displays superior Li-battery performance with large reversible capacity, excellent cyclic performance and good rate capability. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Lithium-ion batteries, as power sources for mobile communication devices, portable electronic devices, and electrical/hybrid vehicles, have attracted special attention in the scientific and industrial fields due to their high electromotive force and high energy density [1]. Graphite has been employed as an anode material in lithium-ion batteries. However, graphite has an inherent limitation with a theoretical gravimetric capacity at 372 mAh g−1 . For the purpose of improving the energy density of batteries, scientists have made great efforts to explore alternative anode materials with higher capacity, such as Si [2–4], Sn [5], polymers [6], transition-metal oxides [7–12] and their composites [13–19]. Among these potential anode materials, SnO2 shows high capacity (782 mAh g−1 ), low cost, eco-friendliness, and natural abundance, thus has attracted considerable attention. However, its large volume expansion/contraction and severe aggregation associated with the Li-ion insertion and extraction process lead to electrode pulverization, consequently, result in a large irreversible capacity loss and poor cycling stability. To circumvent these problems, SnO2 nanorods/graphite [20], SnO2 /amorphous carbon [21] SnO2 /carbon nanotube hybrid [22] and SnO2 particles/graphene composite [23–26] have been successfully prepared and used as anode material for lithium ion batteries. According to previous studies [27–29], the electrochemical properties of SnO2 anode materials seem to be strongly correlated to the morphology of the nanocrystalline SnO2 . 1-D SnO2

∗ Corresponding authors. E-mail addresses: [email protected] (J. Huang), [email protected] (X. Li). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.01.119

nanorods or nanowires are well demonstrated as building blocks much more superior than nanopowders due to their high aspect ratio which may provide better Li-ion transfer and more reaction sites. Graphene, a 2-D macromolecular sheet of carbon atoms with a honeycomb structure, has excellent electronic conductivity and mechanical properties, and become an ideal conductive additive for hybrid nanostructured electrodes [30]. Thus, we use a simple hydrothermal synthesis to obtain the flower-like SnO2 /graphene composite, in which the flower-like SnO2 structures are composed of several 1-D SnO2 nanorods. This flower-like SnO2 /graphene composite displays superior lithium-ion batteries’ performance with large reversible capacity, excellent cyclic performance and good rate capability.

2. Experimental 2.1. Synthesis of the flower-like SnO2 /graphene composite A typical hydrothermal process was used to synthesize the flower-like SnO2 /graphene composite. All chemicals were of analytical grade and were used without further purification. Graphene oxide was synthesized from natural graphite by a modified Hummer’s method [31]. 0.1 g graphene oxide was dispersed in 40 mL of deionized water. Then, 1.05 g of SnCl4 ·5H2 O and 1.40 g of NaOH were added into the above suspension. After stirring for 20 min, the mixture was transferred into a PTFE-lined autoclave. The autoclave was sealed and kept at 200 ◦ C for 16 h. The system was then cooled naturally to room temperature. The as-prepared products were washed with deionized water and ethanol several times and dried at 70 ◦ C. The bare flower-like SnO2 was synthesized under

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the same condition without the addition of graphene oxide for comparison. 2.2. Characterization X-ray diffraction (XRD) patterns were obtained from DMAX˚ Fourier transform 2500PC using Cu/K␣ radiation ( = 1.5406 A). infrared spectroscopy (FTIR) spectra were recorded on a GX spectrometer. Thermogravimetric analyses (TGA) were performed on a TGA/DSC1/1100LF thermogravimetric analyzer from 30 to 1000 ◦ C at a heating rate of 15 ◦ C min−1 in airflow. Brunauer–Emmet–Teller (BET) specific surface area was determined from nitrogen adsorption/desorption using automatic specific surface area measuring equipment (ASAP 2020M). The structure and morphology of the flower-like SnO2 /graphene composite and the bare flower-like SnO2 were observed by field emission scanning electron microscope (FESEM, NOVA 400). 2.3. Electrochemical measurement The electrochemical properties of the as-obtained products were carried out in coin cells with a Li foil as the counter electrode, active material (flower-like SnO2 /graphene composite, flower-like SnO2 or graphene sheets), carbon black and polyvinylidone difluoride (PVDF) binder in the weight ratio of 80:10:10 as the working electrode, 1 M LiPF6 in the volume ratio of 1:1 ethyl methyl carbon (EMC)/dimethyl carbonate (DMC) as the electrolyte. The cells were galvanostatically charge–discharged in the voltage range 0–3.0 V vs. Li/Li+ at the current densities of 50, 100 and 500 mA g−1 via a Battery Testing System (Ningbo baite testing equipment Co., China). Cyclic voltammetry (CV) curves were collected at 0.2 mV s−1 within the range of 0–3.0 V using an electrochemistry working station (Solartron 1260 8 W). 3. Results and discussion XRD patterns of the graphene oxide, the flower-like SnO2 and the flower-like SnO2 /graphene composite are presented in Fig. 1. There is a broad (0 0 2) diffraction peak in the XRD pattern of the graphene oxide. This result indicates that the graphene oxide sheets stack into multilayers, leading to loss of their high surface area and intrinsic chemical and physical properties. Fig. 1(b) and (c) shows all the reflection peaks of the as-prepared products can be indexed as a tetragonal rutile-like SnO2 phase (JCPDS card no. 41-1445). No obvious diffraction peaks attributed to graphite in the XRD pattern of the SnO2 /graphene composite are observed. This result indicates that the flower-like SnO2 nanorod clusters adhered on graphene sheets can prevent stacking into multilayers [24]. Fig. 2 shows the FTIR spectra of the graphene oxide and the SnO2 /graphene composite. For the graphene oxide, the peaks at 3671 and 1214 cm−1 are attributed to stretching of the O H band of CO H [32,33]. The peak at 1732 cm−1 is associated with stretching of the C O bond of carboxyl groups and the peak at 1569 cm−1 can be attributed to C C bending vibrations [32,33]. The FTIR spectrum of the SnO2 /graphene composite differs from that of the graphene oxide as evidenced by the disappearing or weakening of the peaks at 3672, 1732 and 1214 cm−1 . It suggests that hydrothermal treatment removed the partial oxygen-bearing groups so that the starting graphene oxide evolved into reduced graphene due to the hydrothermal treatment, the result is in good agreement with previous work [34]. In addition, the peak at 724 cm−1 is observed, which can be ascribed to lattice absorption of iron oxide [35]. The structure and morphology of the as-prepared products are observed by SEM. Fig. 3a shows that the large-scale flower-like microspheres structures are formed. These microspheres agglomerate with each other. Fig. 3b presents the flower-like SnO2

Fig. 1. XRD patterns of (a) graphene oxide, (b) flower-like SnO2 , (c) flower-like SnO2 /graphene composite.

structures with the diameter of around 1 ␮m are built from several compact nanorods. The mechanism for the formation of the flower-like SnO2 structures is that the SnO2 particles agglomeration and anisotropic growth of the crystal along the [0 0 1] direction [36]. When the reaction system is added to the graphene oxide, the flower-like SnO2 nanorod clusters adhere to both sides of the graphene nanosheets homogeneously (Fig. 3c). Further high resolution SEM observation (Fig. 3d) reveals that the size of the flower-like SnO2 nanorod clusters in composite is approximately 800 nm in size, which is smaller than the diameter of the bare flower-like SnO2 structures. The growth of the flower-like SnO2 nanorod clusters in composite is hindered by the graphene. The N2 adsorption/desorption isotherms of the as-prepared products are shown in Fig. 4. The flower-like SnO2 /graphene composite possesses 65.8 m2 g−1 of special surface area much larger than 1.3 m2 g−1 of the bare flower-like SnO2 , which could lead to its increased electrochemical reactive activity. It is obvious that a distinct hysteresis loop at the relative pressure P/P0 ranging from 0.4 to 1.0, indicating that the flower-like SnO2 /graphene composite is porous material. The total pore volumes of the flower-like SnO2 /graphene composite and the bare flower-like SnO2 are about

Fig. 2. FTIR spectra of (a) graphene oxide, (b) flower-like SnO2 /graphene composite.

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Fig. 3. SEM image of (a) low-magnification of flower-like SnO2 , (b) high-magnification of flower-like SnO2 , (c) low-magnification of flower-like SnO2 /graphene composite, (d) high-magnification of flower-like SnO2 /graphene.

0.309 and 0.005 cm3 g−1 , respectively. Obviously, the flower-like SnO2 /graphene composite possesses a much better porosity than the flower-like SnO2 . The electrochemical reactivity of the flower-like SnO2 /graphene composite, the bare flower-like SnO2 and the bare graphene as anode in lithium-ion batteries is first evaluated by CV. As can be seen from Fig. 5a, in the first cycle, there is a strong cathodic peak at 0.95 V, which can be ascribed to the formation of solid electrolyte interface (SEI) layers on the surface of active materials, the reduction of SnO2 to Sn and the synchronous formation of Li2 O [23], as described in Eq. (1). Moreover, relatively weak

peaks are observed between about 0.5 and 0.25 V, which are related to the formation of Lix Sn (Li2 Sn5 , LiSn, Li7 Sn3 , Li5 Sn2 , Li22 Sn5 ) [20,27] given by Eq. (2). The peaks near 0.01 V are ascribed to Li intercalation into graphene to form LiC6 [26] according to Eq. (3). In the anodic curve, the peaks at 0.2, 0.5 and 1.26 V can be attributed to Li deintercalation from LiC6 [23], Li dealloying from Lix Sn [37] and partly reversible reaction [24] of the Eq. (1), respectively. Inset shows the content (SnO2 : 72% and graphene: 26%) in the flower-like SnO2 /graphene composite. For the bare flower-like SnO2 in Fig. 5b, the peak intensity and integral area of the CV curves of the flower-like SnO2 are obviously decreased

Fig. 4. Nitrogen adsorption/desorption isotherms of (a) flower-like SnO2 /graphene composite, (b) flower-like SnO2 , inset shows the porosity distribution by original density functional theory model.

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Fig. 5. (a) CV curves of flower-like SnO2 /graphene composite at a scanning rate of 0.2 mV s−1 , inset shows TGA curve of flower-like SnO2 /graphene composite at a heating rate of 15 ◦ C min−1 in air, (b) CV curves of flower-like SnO2 at a scanning rate of 0.2 mV s−1 , inset shows CV curves of graphene at a scanning rate of 0.2 mV s−1 , (c) charge/discharge profiles of flower-like SnO2 /graphene composite, flower-like SnO2 and graphene at current density of 50 mA g−1 , (d) rate capability of flower-like SnO2 /graphene composite, flower-like SnO2 and graphene at various current densities.

as the cycles increase, implying its poor capacity retention, inset in Fig. 5b shows the CV curves of the bare graphene at a scanning rate of 0.2 mV s−1 . SnO2 + 4Li+ + 4e− ↔ 2Li2 O + Sn

(1)

Sn + xLi+ + xe− ↔ Lix Sn

(2)

C6 (graphene) + Li+ + e− ↔ LiC6

(3)

The electrochemical performance of the flower-like SnO2 /graphene composite and the flower-like SnO2 is evaluated by galvanostatic charge/discharge cycling at the current density of 50 mA g−1 in the voltage range from 0 to 3.0 V. The typical charge/discharge profiles of the as-prepared products at the first and tenth cycles can be clearly seen from Fig. 5c. In the first discharge step, both of the flower-like SnO2 /graphene composite and the bare flower-like SnO2 present an obvious plane at 0.95 V (vs. Li/Li+ ) corresponds to the irreversible reduction of SnO2 to form Li2 O and Sn [27]. The first discharge specific capacity of the flower-like SnO2 /graphene composite is about 1588 mAh g−1 , which is lower than the values of the bare flower-like SnO2 and the bare graphene. The first charge capacities are 1240, 1012 and 1266 mAh g−1 for the flower-like SnO2 /graphene composite, the bare flower-like SnO2 and the bare graphene, respectively. The initial capacity loss may result from the incomplete conversion reaction and irreversible lithium loss due to the formation of a SEI layer [26]. After 10 cycles, the flower-like SnO2 /graphene composite still retains a reversible capacity of 980 mAh g−1 much higher than the values of the bare flower-like SnO2 (278 mAh g−1 ) and the bare graphene (525 mAh g−1 ).

In addition, the flower-like SnO2 /graphene composite exhibits much better rate capability compared to the bare flower-like SnO2 and the bare graphene electrodes operated at various rates of 50, 100, and 500 mA g−1 (Fig. 5d). For example, before 10 cycles at a current density of 50 mA g−1 , the reversible capacity fading of the flower-like SnO2 /graphene composite is average 2% per cycle, much lower than 7% of the bare flower-like SnO2 electrode and 6.6% of the bare graphene electrode. After 20 cycles at a current density of 100 mA g−1 , the flower-like SnO2 /graphene composite keeps a reversible discharge capacity of 794 mAh g−1 , whereas the reversible discharge capacities of the bare flower-like SnO2 electrode and the bare graphene electrode rapidly drop to 107 and 355 mAh g−1 , respectively. Even at a higher current density of 500 mA g−1 , the flower-like SnO2 /graphene composite still maintains good cycling stability. After 40 cycles, it is found that the reversible discharge capacity of the flower-like SnO2 /graphene composite is still maintained at 730 mAh g−1 .

4. Conclusions In summary, we have reported a novel flower-like SnO2 /graphene composite by a simple hydrothermal method in which the flower-like SnO2 nanorod clusters are uniformly distributed on graphene sheets. The size of the flower-like SnO2 structures is approximately 800 nm. The electrochemical performance testing shows that the first discharge and charge capacity of the flower-like/graphene composite are 1588 and 1240 mAh g−1 at the current density of 50 mA g−1 , respectively. After 40 cycles at different current densities of 50, 100, and 500 mA g−1 , the reversible discharge capacity is still maintained at 730 mAh g−1 ,

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which indicates that the prepared flower-like/graphene composite possesses a good cycle performance for the lithium storage. The multifunctional features of the SnO2 /graphene composite are considered as follows: (1) The flower-like SnO2 nanorod clusters between the graphene sheets effectively keep the neighboring graphene sheets separated. In turn, the graphene sheets effectively hinder the growth of the flower-like SnO2 nanorod clusters. (2) The flower-like SnO2 structures are composed of several SnO2 nanorods which can further shorten pathways lengths of the ion diffusion and reduce interface impedance. (3) The graphene sheets provide a large contact surface for individual dispersion of well-adhered flower-like SnO2 nanorod clusters and act as an excellent conductive agent to provide a highway for electron transport, improving the accessible capacity. Acknowledgment This work was supported by Fundamental Research Funds for the Central Universities (no. CDJXS10 13 11 58). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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