Graphene-encapsulated SnO2 hollow spheres as high-performance anode materials for lithium ion batteries

Graphene-encapsulated SnO2 hollow spheres as high-performance anode materials for lithium ion batteries

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CERAMICS INTERNATIONAL

Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint

Graphene-encapsulated SnO2 hollow spheres as high-performance anode materials for lithium ion batteries Hang Yanga,b, Zhaohui Houb,n, Ningbo Zhoub, Binhong Heb, Jianguo Caoa,b, Yafei Kuanga,n b

a College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China School of Chemistry and Chemical Engineering, Hunan Institute of Science and Technology, Yueyang 414006, China

Received 24 March 2014; received in revised form 23 May 2014; accepted 23 May 2014

Abstract Graphene-encapsulated SnO2 hollow spheres (GE-SnO2 HS) are synthesized by a simple electrostatic self-assembly process. The prepared composite consists of SnO2 hollow spheres (SnO2 HS) in the interior, which are encapsulated by flexible ultra-thin graphene shells at the exterior. This novel architecture is expected to buffer large volume changes and effectively prevent the detachment and agglomeration of SnO2 during the lithiation and delithiation processes. The GE-SnO2 HS composite exhibits much better electrochemical performance than bare SnO2 HS. In addition, the composite delivers a high reversible capacity of 422 mA h g  1 after 100 cycles at a current density of 158 mA g  1 and a capacity of 237 mA h g  1 after 30 cycles at a higher current density of 1580 mA g  1. These results suggest that the GE-SnO2 HS composite can provide new opportunities to enhance the properties of tin-based materials for use in high-capacity lithium ion batteries. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: B. Nanocomposites; C. Electrical properties; E. Batteries

1. Introduction Lithium ion batteries (LIBs) are gaining considerable attention as a primary power source for electric vehicles and portable electronic devices, primarily due to their high energy density, high electromotive force, and low self-discharge rate [1,2]. To date, graphite-based electrodes are the most widely used commercial anode materials for LIBs due to their high coulombic efficiency and excellent cycle performance [3,4]. Nevertheless, the limited theoretical specific capacity of graphite-based anodes (372 mA h g  1) is not sufficient to meet the increasing demand of LIBs with higher energy rate density. To this end, metal oxides such as SnO2 [5–7], Fe3O4 [8], Co3O4 [9], and Mn3O4 [10] have been actively investigated as potential anode materials, as they are expected to offer high specific lithium storage capacity. Among these metal n

Corresponding authors. Tel.: þ86 730 8648502; fax: þ 86 730 8640122. E-mail addresses: [email protected] (Z. Hou), [email protected] (Y. Kuang).

oxides, SnO2 is an abundant, low-cost, nontoxic material that is considered as a promising candidate anode material [11]. In principle, when SnO2 is used as an anode material in the LIBs, the electrochemical reaction at the anode can be simplified into the following two principal reactions [12]: SnO2 þ 4Li þ þ 4e  -2Li2 O þ Sn

ð1Þ

Sn þ xLi þ þ xe  2Lix Sn ð0 r x r 4:4Þ

ð2Þ

Based on reaction (2), the theoretical lithium storage capacity of SnO2 is estimated to be approximately 790 mA h g  1 [13], twice than that of the graphite-based anode. Nevertheless, the practical application of SnO2 anodes is hindered by its poor cycling stability arising from the large volume change ( 300%) and severe particle aggregation during charge– discharge cycles. This, in turn, leads to electrode pulverization and loss of the electrical contact of the anode [14–16]. To date, several methodologies have been proposed to circumvent the abovementioned problem. One of the commonly adopted strategies is the nanostructuring of SnO2, especially in the

http://dx.doi.org/10.1016/j.ceramint.2014.05.109 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: H. Yang, et al., Graphene-encapsulated SnO2 hollow spheres as high-performance anode materials for lithium ion batteries, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.05.109

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form of hollow nanostructures, such as nanotubes [17], nanoboxes [18], and nanospheres [19]. The local empty space in these hollow nanostructured materials is expected to partially accommodate the large volume change [20]. Another effective method to buffer the volume change is to fabricate nanocomposites of SnO2 with carbonaceous materials, such as carbon [21], carbon nanotubes [22], and graphene [23,24]. These carbonaceous materials not only buffer the volume change but also enhance the conductivity [25]. Graphene, a two-dimensional crystalline allotrope carbon characterized by a hexagonal arrangement of carbon atoms, has been widely studied as a promising material in energy storage applications due to its exceptional properties, including its large specific surface area (2600 m2 g  1), extraordinary electron transport properties, and structural flexibility [26]. Recently, several studies have reported the preparation of graphene-SnO2 nanocomposites for use as anode materials in LIBs. For example, Hwang reported the fabrication of a new type of SnO2/graphene nanocomposite that shows a reversible capacity of 500 mA h g  1 in the 50th cycle at a current density of 100 mA g  1 [27]. These SnO2/graphene nanocomposites exhibited improved electrochemical properties compared to those of pure SnO2 nanoparticles. Nevertheless, the high-rate capability and cycle performance is still insufficient for practical implementation. This may be attributed to the low weight fraction of SnO2 in the composite (40–60%) [7,23] and the aggregation of SnO2 nanoparticles during the cycle processes due to the relatively simple distribution of the SnO2 nanoparticles on the surface of graphene or between the graphene layers [28]. Therefore, it is highly desirable to develop novel architectures to overcome the abovementioned limitations. Electrostatic self-assembly is one of the most versatile techniques for the fabrication of hybrid structures. This technique is based on the principle of electrostatic attraction between two oppositely charged particles suspended in a solution. Recently, this technique has been reported for the preparation of graphene-based composites, utilizing the negatively charged functional groups, including epoxy, carboxyl, and hydroxyl groups, that are abundant on the basal planes and edges of the graphene oxide (GO) sheets. For example, Yu demonstrated the synthesis of graphene/carbon nanotube hybrid films [29] using electrostatic self-assembly. Similarly, this technique was used for the synthesis of graphene-wrapped polyaniline hollow spheres [30] and graphene-encapsulated hollow Fe3O4 nanoparticle aggregates for use as a highperformance anode material in LIBs [31]. Herein, we report the facile synthesis of grapheneencapsulated SnO2 hollow spheres (GE-SnO2 HS) using electrostatic self-assembly (Scheme 1). In a typical process, SnO2 hollow spheres (SnO2 HS) were initially modified to acquire a positive charge and then further self-assembled with the negatively charged GO under mild reaction conditions to form a composite with flexible and ultrathin graphene shells. This novel hybrid architecture combines the unique properties of graphene and the exceptional advantages of hollow nanostructures, leading to remarkable cycle performance (422 mA h

g  1 after 100 cycles at a current density of 158 mA g  1) and high-rate performance (over 200 mA h g  1 after 30 cycles at a current density of 1580 mA g  1). 2. Experimental 2.1. Materials and methods All reagents were of analytical grade and used without further treatment. Graphite oxide was synthesized from graphite flakes (Alfa Aesar, 99.8%) via a modified Hummers' method [32]. For the preparation of GO suspension, graphite oxide was dispersed in deionized water under ultrasonication for 4 h. Next, SnO2 HS was prepared by an unusual inside-out Ostwald ripening mechanism, as previously reported in the literature [33]. In a typical procedure, 0.13 g of K2SnO3  3H2O was dissolved in 30 mL of ethanol–H2O mixed solvent (40% ethanol by volume). After gentle stirring for 5 min, 0.18 g of urea was added to the solution under vigorous stirring. Next, the resulting solution was transferred to a 50-mL Teflon-lined stainless-steel autoclave and heated in an electric oven at 150 1C for 24 h. After the hydrothermal treatment, the autoclave was cooled to room temperature naturally. The resulting white precipitate was separated by centrifugation and washed with deionized water and absolute ethanol. The washed precipitates were finally dried overnight at 50 1C. The SnO2 HS thus obtained was surface-modified with 3-aminopropyltrimethoxysilane ((CH3O)3Si(CH2)3NH2, APS) [28] as follows. First, 0.5 g of SnO2 HS was dispersed in 50 mL of ethanol by sonication for 30 min. Subsequently, 1 mL of APS was added to the above solution and stirred for 12 h. The resultant APS-modified SnO2 HS was separated and collected by centrifugation. GE-SnO2 HS was then synthesized via the hydrothermal treatment. In this process, both GO and APS-modified SnO2 HS dispersions were adjusted to pH 3 by adding hydrochloric acid (1 mol L  1). Subsequently, 2 mL of GO suspension (1 mg mL  1) was added to 20 mL of APS-modified SnO2 HS dispersion (0.5 mg mL  1) under mild stirring for 30 min. Next, 0.3 mL of hydrazine (35 wt%) was added to the above suspension to reduce GO to graphene. The solution was then diluted with 30 mL of deionized water and transferred into a 100-mL Teflon-lined stainless-steel autoclave and heated in a conventional electric oven at 120 1C for 3 h. After the hydrothermal treatment, the autoclave was cooled to room temperature naturally. The resulting product was collected by centrifugation and vacuum dried at 60 1C. 2.2. Characterization The morphology and structure of the as-prepared samples were investigated using scanning electron microscopy (SEM, Hitachi S-4800), high-resolution transmission electron microscopy (HR-TEM, JEOL-3010), X-ray diffraction (XRD, D/maxUltima IV), and X-ray photoelectron spectroscopy (XPS, K-Alpha 1063). The specific surface area and pore size

Please cite this article as: H. Yang, et al., Graphene-encapsulated SnO2 hollow spheres as high-performance anode materials for lithium ion batteries, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.05.109

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Scheme 1. Schematic illustration of the fabrication process and structure of the resulting GE-SnO2 HS composite.

Fig. 1. XRD patterns of SnO2 HS (a) and of graphene and GE-SnO2 HS (b).

distribution of the samples were determined from the N2 adsorption/desorption isotherms (Micromeritics ASAP2020 HT88). Furthermore, thermogravimetric analysis (TGA, DTA7300) was performed from room temperature to 700 1C at a heating rate of 10 1C min  1 under flowing air. 2.3. Electrochemical measurements The electrochemical performance of the as-prepared samples was determined by assembling a two-electrode CR2025 coin cell in an argon-filled glove box. The active material (70 wt%), polyvinylidene fluoride (PVDF, 20 wt%), and conductive carbon black (10 wt%) were mixed in N-methyl-2-pyrrolidinone (NMP) to form the working electrode. Lithium foil was used as the counter and reference electrode. The electrolyte consisted of 1 M LiPF6 in 1:1 (volume) ethylene carbonate (EC):dimethyl carbonate (DMC). Cyclic voltammograms (CV) were recorded on an electrochemical workstation (CHI 660B). The cells were galvanostatically charged and discharged over a voltage range from 5 mV to 3 V on a LAND battery tester. 3. Results and discussion Fig. 1 shows the XRD patterns of the as-prepared samples. All of the diffraction peaks of the SnO2 HS shown in Fig. 1a could be unambiguously indexed to tetragonal SnO2 (JCPDS card No. 41-1445). However, the XRD pattern of the GE-SnO2 HS shown in Fig. 1b was quite similar to that of SnO2 HS, as

the (002) peak of graphene almost overlapped with the main (110) peak of the SnO2 [12]. The absence of significant diffraction peaks corresponding to graphene sheets also suggests a low graphene content in the composite (further demonstrated by TGA) and a uniform distribution of graphene around the SnO2 HS. Furthermore, the size and morphology of the SnO2 HS before and after graphene encapsulation were examined using SEM and HR-TEM. The SEM image of the SnO2 HS shown in Fig. 2a indicates the formation of spherical particles with diameters of 100–200 nm. The rough surface of the particles suggests that the spheres are formed by an orderly assembly of nanoparticles. The hollow structure of the assembly was confirmed in the HR-TEM image (Fig. 2b), as evidenced from the strong contrast between the darker edges (shell, approximately 20 nm) and paler central region (interior). The wellresolved lattice fringes with an interplanar spacing of 0.335 nm correspond to the (110) plane of SnO2 (Fig. 2c). During the formation of SnO2 HS, solid SnO2 nanospheres, which are comprised of numerous smaller crystallites, are formed in the initial stages of the reaction. With prolonged reaction time, these solid nanospheres undergo inside-out Ostwald ripening due to the high surface energy of the crystallites located in the inner core relative to the crystallites in the outer surface [34]. Lastly, this evacuation process leads to the formation of hollow nanospheres. As shown in Fig. 2d and e, the graphene sheets encapsulate the SnO2 particles, thereby preventing the agglomeration of SnO2 and facilitating good dispersion of these oxide particles.

Please cite this article as: H. Yang, et al., Graphene-encapsulated SnO2 hollow spheres as high-performance anode materials for lithium ion batteries, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.05.109

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Fig. 2. SEM (a) and HR-TEM (b, c) images of SnO2 HS; SEM (d, e) and HR-TEM (f) images of GE-SnO2 HS.

In most cases, the edges of individual as well as overlapping graphene layers could be observed, particularly at the interface between aggregated particles, where the graphene layers appear to link the neighboring spheres. Furthermore, the HRTEM image of the GE-SnO2 HS composite shown in Fig. 2f confirms the encapsulation of SnO2 nanospheres. The HRTEM image also indicates that the SnO2 sphere consists of numerous SnO2 nanoparticles with voids between the nanoparticles. This accounts for the porosity of the composite. In addition, small voids are observed on the interface between the graphene sheets and SnO2 spheres, which are believed to have originated from the wrinkled texture of the graphene sheets and the spherical morphology of the SnO2 particles. The surface composition and chemical states of the APSmodified SnO2 HS and GE-SnO2 HS were further analyzed using XPS. The wide-scan survey XPS spectrum of the APSmodified SnO2 HS shown in Fig. 3a (inset) features peaks corresponding to tin (Sn 3p, 3d, 4d), which are attributed to SnO2 [7]. Sn (MNN) and O (KLL) are the poorly resolved peaks. The presence of C 1s, N 1s, and Si 2p peaks suggests that the APS components are successfully grafted onto the surface of SnO2 spheres during the modification process. In addition, the N 1s peak could be resolved into two different binding energies (Fig. 3a): the nitrogen peak with lower binding energy (NI, near 400 eV) represents free amines ( NH2), while the higher-energy peak (NII, approximately 402 eV) corresponds to hydrogen-bonded and protonated amines (NH3þ ) [35]. Fig. 3b shows the typical N 1s peak of GE-SnO2 HS and the wide-scan survey XPS spectrum (inset).

The presence of graphene sheets in the composite can be substantiated from the intense C 1s peak. Furthermore, the NII/ NI ration in Fig. 3b is greater than that in Fig. 3a, indicating that GE-SnO2 HS possesses a greater number of positively charged groups (NH3þ ). This could be attributed to the amount of free amines transforming to protonated amines when the APS-modified SnO2 HS was dispersed in acidic aqueous solution [36]. This triggers an electrostatic interaction between the negatively charged GO sheets and the positively charged SnO2 HS, resulting in the encapsulation of GO sheets onto the SnO2 HS. Subsequently, GE-SnO2 HS was obtained by chemical reduction. The total content of graphene in the composite was determined by TGA (Fig. 4). The TGA curve of the GESnO2 HS composite displays two distinct regions of weight loss. The initial weight loss (up to 150 1C) is due to the evaporation of physisorbed water, while the second weight loss between 150 and 650 1C could be attributed to the oxidization of graphene. At temperatures above 650 1C, the composite was rather stable, indicating the complete removal of graphene sheets. From the TGA curve, the SnO2 content in the composite was determined to be approximately 86% by weight. The porosity and Brunauer–Emmett–Teller (BET) specific surface area of bare SnO2 HS (Fig. 5a) and those of the GE-SnO2 HS composite (Fig. 5b) were determined from the nitrogen adsorption–desorption isotherms. The isotherms of both samples displayed typical IV features, characterized by an H1 hysteresis loop. The hysteresis loop of the GE-SnO2 HS composite exhibits a higher integral area than bare SnO2 HS,

Please cite this article as: H. Yang, et al., Graphene-encapsulated SnO2 hollow spheres as high-performance anode materials for lithium ion batteries, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.05.109

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Fig. 3. Wide-scan survey XPS spectrum (inset) and the N 1s peak of APS-modified SnO2 HS (a) and GE-SnO2 HS (b).

corresponds to the alloying of Sn with Li þ and the Li þ insertion in graphene sheets to form LixC (reactions (2) and (3)) [12]. During the anodic sweep process, we could observe two corresponding anodic peaks. In the case of GE-SnO2 HS, the anodic peak at approximately 0.65–0.85 V could be attributed to the dealloying of the LixSn alloy and Li extraction from the graphene sheets [7]. The other anodic peak at approximately 1.25–1.65 V is likely due to the partial reversibility of reaction (1) [37] and the reaction occurring between Li2O and metallic Sn (reaction (4)), which is favorable for increasing the initial coulombic efficiency and specific capacity [7,38].

Fig. 4. TGA curve of GE-SnO2 HS.

indicating a larger average pore size for the composite. The BET specific surface area and pore volume of GE-SnO2 HS were estimated to be 130.7 m2 g  1 and 0.14 cm3 g  1, respectively, much higher than those of pure SnO2 HS (89.8 m2 g  1 and 0.09 cm3 g  1, respectively). The higher specific surface area and average pore size could be attributed to the addition of graphene and the voids observed at the interface between the graphene sheets and SnO2 spheres. The electrochemical performance of the as-prepared samples was evaluated from the cyclic voltammograms, which were recorded in the voltage range from 5 mV to 3.0 V (versus Li/ Li þ ) at a scan rate of 0.5 mV s  1 (Fig. 6). The cyclic voltammogram of GE-SnO2 HS (Fig. 6b) was found to be similar to that of the as-prepared SnO2 HS (Fig. 6a). The apparent cathodic peak at approximately 0.65–0.8 V could be ascribed to the formation of a solid electrolyte interface (SEI) layer as well as the reduction of SnO2 to Sn, as described by reaction (1) [12]. After the first cycle, this peak tends to diminish sharply, leaving the initial irreversible capacity. The peak undergoes a slight positive shift due to the change of the reaction components. The reduction process below 0.2 V

xLi þ þ C ðgrapheneÞþ xe  2Lix C

ð3Þ

Li2 O þ Sn2SnO þ 2Li þ þ 2e 

ð4Þ

Fig. 7a and b shows the typical discharge–charge voltage profiles of bare SnO2 HS and GE-SnO2 HS at a current density of 158 mA g  1 in the potential range from 5 mV to 3 V. As observed, bare SnO2 HS shows a capacity of 718 mA h g  1 in the first charging cycle and delivers a relatively low reversible capacity of 169 mA h g  1 after 100 cycles. This corresponds to a retention capacity of only 23%. The rapid decrease in capacity is related to the huge volume change associated with the alloying and dealloying process, which inflicts critical mechanical damage on the electrode [15,16]. On the other hand, the GE-SnO2 HS composite delivers a specific capacity of 1802 mA h g  1 in the first discharge and a reversible capacity of 909 mA h g  1. The capacity loss in the first cycle could be caused by the formation of a SEI layer, as predicted from the CV curve. Moreover, the GE-SnO2 HS composite shows a high reversible capacity of 682 mA h g  1 after 30 cycles, corresponding to a capacity retention of 75%. Furthermore, the reversible capacity was found to be as high as 422 mA h g  1 after the 100th cycle. The coulombic efficiency is 50% for the first cycle but is above 95% after several cycles, indicating good reversibility (Fig. 7c). Rate performance is an important parameter that determines the suitability of the anode material for practical applications in

Please cite this article as: H. Yang, et al., Graphene-encapsulated SnO2 hollow spheres as high-performance anode materials for lithium ion batteries, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.05.109

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Fig. 5. Nitrogen adsorption/desorption isotherms of SnO2 HS (a) and GE-SnO2 HS (b).

Fig. 6. Cyclic voltammograms of SnO2 HS (a) and GE-SnO2 HS (b) at a scan rate of 0.5 mV s  1.

LIB. Therefore, the rate performance of the prepared GE-SnO2 HS was evaluated by galvanostatic charge–discharge cycling at the current densities of 395 mA g  1, 790 mA g  1, and 1580 mA g  1 for 30 cycles (Fig. 7d). The bare SnO2 HS exhibited poor rate performance; hence, the related test results are not shown here. On the other hand, the GE-SnO2 HS composite exhibited excellent rate performance despite the drop in specific capacity with increasing current density and cycling. At a current density of 395 mA g  1, the specific capacity remained at 458 mA h g  1 after 30 cycles, indicating a capacity retention of 57%. Even at a higher current density of 1580 mA g  1, the 30th reversible capacity was as high as 237 mA h g  1. Notably, the reversible capacity of the GESnO2 HS increases in the first several cycles and then slightly decreases at a higher current density of 1580 mA g  1. This could be attributed to the large particle size of the SnO2 spheres. From these results, it is apparent that the GE-SnO2 HS composite exhibits much better electrochemical performance than the SnO2 HS in terms of both lithium storage capability and capacity retention. The observed improvement in the

electrochemical performance could be attributed to both the presence of graphene, which provides a highly conductive medium for electron transfer, and the formation of the encapsulated composite structure. The voids in the interior of SnO2 HS provide additional space to accommodate the expansion of SnO2 during the lithiation and delithiation processes. In addition, the encapsulation of SnO2 HS by the flexible and continuous graphene sheets further buffers the volume change of SnO2 in the outer space and prevents the agglomeration of SnO2 particles. Furthermore, the diffusion paths of Li þ and electrons are greatly shortened by the higher surface area and porosity of the composite [31,39], which further enhances the electrochemical performance of the GE-SnO2 HS composite. 4. Conclusion In summary, graphene-encapsulated SnO2 hollow spheres were fabricated by adopting a simple process based on electrostatic interaction and hydrothermal treatment. The asprepared composite exhibited better lithiation and delithiation

Please cite this article as: H. Yang, et al., Graphene-encapsulated SnO2 hollow spheres as high-performance anode materials for lithium ion batteries, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.05.109

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Fig. 7. Charge–discharge profiles of SnO2 HS (a) and GE-SnO2 HS at a current density of 158 mA g  1 (b). Cycling performance and coulombic efficiency of GESnO2 HS at a current density of 158 mA g  1 (c). Rate capability of GE-SnO2 HS (d).

performance than bare SnO2 hollow spheres. The observed improvement in the electrochemical performance can be attributed to the unique features of the new SnO2–graphene composite with hollow-structured SnO2 spheres encapsulated by graphene shells.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Nos. 51272075 and 51238002).

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Please cite this article as: H. Yang, et al., Graphene-encapsulated SnO2 hollow spheres as high-performance anode materials for lithium ion batteries, Ceramics International (2014), http://dx.doi.org/10.1016/j.ceramint.2014.05.109