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Free-standing SnO2 nanoparticles@graphene hybrid paper for advanced lithium-ion batteries
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 6891–6897 www.elsevier.com/locate/ceramint
Free-standing SnO2 nanoparticles@graphene hybrid paper for advanced lithium-ion batteries Tian Gao, Kai Huangn, Xing Qi, Hongxing Li, Liwen Yang, Jianxin Zhongn Hunan Key Laboratory for Micro-Nano Energy Materials and Devices, Laboratory for Quantum Engineering and Micro-Nano Energy Technology, Faculty of Materials and Optoelectronic Physics, Xiangtan University, Hunan 411105, PR China Received 5 November 2013; received in revised form 3 December 2013; accepted 3 December 2013 Available online 14 December 2013
Abstract A free-standing SnO2/reduced graphene oxide (re-G) hybrid paper has been successfully fabricated by a facile vacuum filtration approach through a uniformly dispersed SnO2 nanoparticle and graphene oxide mixed solution. As potential anode material for high power and energy applications, the hybrid nanostructures were directly evaluated as anode for rechargeable lithium ion batteries without adding any polymer binder, conductive additives or current collectors. And the composite exhibits a greatly enhanced synergic effect with extremely high energy storage stability of 700 mAh g 1 after 100 cycles benefiting from the advanced structural features. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Lithium ion batteries; Tin oxide nanoparticles; Reduced graphene oxide
1. Introduction The development of high energy density and long-lasting lithium ion batteries (LIBs) is of great technological challenge for next-generation energy storage systems such as portable electronic devices, hybrid electrical vehicles and electrical vehicles, etc [1–3]. However, commercial graphite anode used currently has already reached its theoretical limit (372 mAh g 1), and exploring alternative anode materials with higher charge/discharge rate, reversible capacity as well as long cycle life and low cost has become an urgent task nowadays. From the material point of view, SnO2 could be a potential candidate for commercial anode materials due to its remarkably high theoretical charge capacity (782 mAh g 1) [4,5], nontoxicity, natural abundance and eco-friendliness, which has been extensively exploited for high-performance LIBs. The capacity of lithium storage is mainly achieved through the reversible conversion reaction mostly resulting from reversible alloying/ de-alloying processes of Sn with Li [6,7]. Despite those intriguing features, the main obstacle in developing SnO2based anodes lies in the severe volume change during lithium n
Corresponding authors. Tel./fax: þ 86 0731 58292468. E-mail addresses: [email protected] (K. Huang), [email protected] (J. Zhong).
ion insertion/extraction, which can result in pulverization of the initial particle morphology and cause the breakdown of electrical connection between the anode materials and current collectors, thereby leading to poor cycling performance [7,8]. In addition, the low electrical conductivity of pristine SnO2 challenges the achievement of high capacity at high charge/discharge rates. To overcome these above issues, nanostructures SnO2/C with various morphologies, such as nanofibers [9-12], nanoparticles [13,14], nanotubes [15] and nanopores [16,17] have been extensively studied as promising alternative anode materials in LIBs applications. Among them, the SnO2/ graphene presents a more superior performance than common carbon composite, and attracts unmatched attention and also triggered tremendous experimental activities [5,8,18–22]. For this composite, graphene sheets can substantially relieve mechanical strain, which otherwise causes an electrical disconnection resulting from the crumbling and pulverization of Sn domains during repeated charge/discharge cycles [23–29]. Simultaneously, graphene endowed with high conductivity and Young's modulus can also mitigate the aggregation of SnO2 nanoparticles, has a moderate theoretical capacity (744 mAh g 1) [30], which further contributes to a stable capacity response during extended charge/discharge cycles [31].
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.12.009
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So far, the SnO2/graphene composited electrodes were prepared by a multi-stage process to control hydrolysis of SnCl4 and reduced graphene oxide nanosheets [18–22,32–34]. Furthermore, the N-doped graphene–SnO2 sandwich paper was fabricated by dispersing tetracyanoquinodimethane anion acetonitrile and SnCl2 in reduced graphene oxide suspension solution followed by treating in a tube furnace [5]. Nevertheless, their complex material processes limit the development of innovative and facile techniques for next generation advanced LIBs. Therefore, it is still a big challenge to find a convenient and facile method for constructing SnO2/graphene composited electrodes to further improve or optimize their electrochemical performance. Herein, we develop a facile and scalable strategy to construct free-standing SnO2/graphene hybrid electrodes by the simple vacuum filtration method, with super-high rate performance and extremely excellent cycling stability at high rates. By entirely circumscribing SnO2 nanoparticles between the graphene sheets, which served as structural cushion to accommodate the huge mechanical stress induced during cycling and to prevent the aggregation of SnO2 nanoparticles, the excellent structural and electrical integrity of the electrodes are perfectly preserved. What is more important, is that the fabricated free-standing and paper-like composites can be straightforwardly used as the electrode, in association with the needles of the current collector, conducting agent or polymer binder, that leads to excellent electrochemical performance as the anode for lithium-ion batteries. 2. Experimental SnO2 (with average diameters 50 nm) was purchased from Sigma-Aldrich Chemical Company Inc (United States), and Graphite (-325 mesh) from Sinopharm Chemical Reagent Co., Ltd (Shanghai China). The graphene oxide (GO) was synthesized from graphite powder by the modified Hummers method [33]. The dried GO and SnO2 nanoparticles were scaled and then dispersed into de-ionized (DI) water to form a uniform mixed solution by ultrasonication. To attain the SnO2/ re-G hybrid nanostructures, the above suspension with GO (40 wt%) and SnO2 (60 wt%) was vacuum filtered with a Whatman membrane filter. After peeling the composite from the membrane, it is immersed into HBr acid and kept at 120 1C for 2 h to efficiently reduce GO [35], finally obtaining a SnO2/
re-G paper-like hybrid electrode. In this presentation, the synthesizing processes are facile and easy to control, and the schematic is illustrated in Fig. 1. The morphology and structure of the samples were characterized by a field emission scanning electron microscope (FESEM, Hitachi S-4800), micro-Raman spectroscope (WITec excited by λ¼ 633 nm laser, and X-Ray power diffraction (XRD, D/MAX 2500). Thermogravimetric analysis (NETZSCH, TG209F3) was employed to calculate the mass content of SnO2 in the composite. For the electrochemical characterization, standard CR2032 coin cells were assembled in an Ar-filled glove box (Mikrouna, China). The SnO2/re-G composited paper was directly used as a working electrode without any conducting material and binding agent, and the Li foil as the counter electrode. The electrolyte was 1.0 M LiPF6 in a solution of 1:1 w/w ethylene carbonate:diethyl carbonate (Novolyte Technologies). All the discharge/charge capacity tests of the fabricated electrodes are determined by galvanostatic cycling over potential range 0.01–3 V in a battery tester (NEWARE BTS-5 V 5 mA, Neware Technology Co., Ltd., China). Cyclic voltammetry curves were scanned using an electrochemistry system (CHI 660D, Chen Hua Shanghai corp., China). All measurements were performed at room temperature. 3. Results and discussion Fig. 2 shows optical photographs of the SnO2/GO hybrid paper before and after peeling off from the membrane filter. In Fig. 2(a), a brownish composite, which was a random stack of GO sheets and SnO2 nanoparticles by vacuum filtration, can be clearly observed on the membrane. After air-dried, the composite can be exfoliated carefully from the membrane just like a paper. It can be discerned that the paper-like SnO2/GO composite changes from brownish to black after reduction in HBr solution, forming a SnO2/re-G composited nanostructure [16], as shown in Fig. 2(b). Simultaneously, the composite shows very good flexibility and retains the integrity, without any damage found in Fig. 2(c), even bent to some extent. Furthermore, the composite can be readily washed by soaking in DI water followed by vacuum drying, and then cut with a razor-blade into the free-standing platelet we need. Raman system was used to analyze the composition of the SnO2/re-G composite. The characteristic peaks at 1352 cm 1 and 1600 cm 1 can be clearly discerned, in Fig. 3(a), which
Fig. 1. Schematic illustration of the synthesizing processes for the SnO2/re-G hybrid paper.
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Fig. 2. (a) The SnO2/GO hybrid composite on filter membrane, inset is the SnO2/GO mixed solution; (b) the SnO2/re-G hybrid paper peeled from the filter membrane and turned black after reduction; and (c) the flexible SnO2/re-G hybrid paper.
Fig. 3. (a) Raman spectra of the final fabricated SnO2/re-G composite; and (b) XRD patterns of the treated and untreated SnO2 nanoparticle.
are in good agreement with the typical D and G band. The D band is associated with disordered samples carbon, while the G band is the result of the first-order scattering of the E2g mode of sp2 carbon domains [17]. A broad signal appears in the range of 350–800 cm 1, which originates from SnO2 (inset of Fig. 3(a)). The Raman characteristic peaks (469 and 630 cm 1) in our test shifts slightly more than the typical bulk SnO2 Raman peaks which are at about 476 and 635 cm 1. This decrease in crystal size leads to blue shift and broadening of Raman peaks [36,37]. As it is shown in Fig. 3(b), the XRD spectrum of the treated and untreated SnO2 nanoparticles is made to exclude the possibility that the SnO2 nanoparticles are reduced by the HBr acid in the constructing process. When comparing these two curves, no difference can be detected, and all the peaks are in good agreement with the standard profiles of the tetragonal structure of SnO2 [JCPDS 41-1445]. Thus, we can explicitly conclude that we have effectively reduced the graphene oxide to graphene, whilst the SnO2 active material is preserved. Scanning electron microscopy (SEM) is employed to further confirm that we have successfully fabricated the hybrid SnO2
nanoparticles and graphene sheets electrode. From the crosssectional SEM image in Fig. 4(a) and (b), a paper-like SnO2/reG composite about 20 mm thickness is obviously and clearly observed, where the SnO2 nanoparticles (diameter with 20–50 nm) is ideally anchored or tightly restricted between the adjacent re-G sheets and the re-G sheets are firmly stacked into continuous films without apparent stacking order owing to the existing SnO2 nanoparticles. Under a filtration-induced directional flow, the dispersive SnO2 nanoparticles and the interlocking of individual re-G sheets eventually construct a paper-like SnO2/re-G composite, as shown in Fig. 2(b), just like the description of the process in Fig. 1. Furthermore, it is worth noting that only small SnO2 nanoparticles are scattered out from the surface of the composite as shown in Fig. 4(c) and (d), and suggesting the perfect integrity of the obtained SnO2/re-G composite architecture. Moreover, we can conclude that this architecture can effectively block the SnO2 nanoparticles agglomeration during the charge/discharge cycles. Thermogravimetric analysis (TGA) has been used to identify the mass content of SnO2 nanoparticles in the composite, and the result shows in Fig. 5. The initial weight loss about 3% range from 30 1C to 200 1C can be attributed to the loss of the
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Fig. 4. (a) and (b) The SEM image of cross section of the SnO2/re-G composite, (c) and (d) SEM images of the top view from the surface of the composite, the blue dotted line area is the exposed SnO2 nanoparticles.
Fig. 5. TGA curve of SnO2/re-G composite measured in air atmosphere with a heating rate of 20 1C/min.
moisture in the composite. Then from 250 1C to 650 1C, the weight loss increases dramatically with the rise of temperature, giving the final 52% mass of the composite after reaching 900 1C. From the results, we can approximately calculate the mass content of SnO2 is about 53%. To understand the Li-ion insertion and extraction process in the composite during discharge/charge cycling, cyclic voltammogram (CV) was measured between 0.01 V and 3 V vs Li/Li þ at the scan rate of 0.2 mV s 1. Fig. 6(a) presents typical CV characteristics related to the lithiation and delithiation processes of the SnO2/re-G composite. A cathodic peak appears at 0.78 V in the first cycle which can be attributed to the formation of the solid electrolyte interphase layer. Another small reduction peak below 0.28 V could be due to the reactions between lithium and SnO2 nanoparticles to form LixSn alloys, while the insertion of lithium in graphene nanosheets could be identified as the reduction peak
at 0.01 V. There are three oxidation peaks located around 0.15, 0.62, and 1.21 V. They correspond to different oxidation reactions during the charge process. The first anodic peak at 0.15 V represents the lithium extraction from graphene nanosheet. The 0.62 V oxidation peak can be assigned to the dealloying of LixSn, showing a reversible process. The third weak oxidation at 1.21 V could result from the partial transformation of Sn metal to SnO2. The high reversibility of the CV curves further confirmed the reversible redox reactions occurring in the lithium-ion cell between lithium and SnO2/re-G composite. Half-cells with a metallic Li counter electrode were carried out to evaluate the electrochemical performance of the SnO2/re-G composite using galvanostatic charge/discharge from 0.01 V to 3.0 V, without adding the conducting agent, polymer binder or current collector. Fig. 6(b) shows the initial three discharge/ charge curves of the composite and at a current density of 200 mA g 1. It is worth noting that the first discharge capacity value is about 1260 mAh g 1, and the second discharge capacity declines to 870 mAh g 1. The irreversible capacity can be assigned to the re-G material [38]. The reason is that since the GO is not completely reduced, plenty of functional groups are still residual and the reactions of Li with O and H-containing groups have trapped lots of in-extractable Li ions. As a result, this leads to poor electrochemical performance [18]. Another cause can be attributed to the formation of solid electrolyte interphase (SEI) in the composite [19]. However, an unanticipated substantially superior capacity retention about 700 mAh g 1 is found after 100 cycles, as shown by the red curve in Fig 6.(c). No obvious capacity fading or tendency of capacity decay can be found. A comparison of the charge/discharge cyclic performances of SnO2 is also shown in Fig. 6(c). After 50 cycles, the capacity of the SnO2 nanoparticles is fading rapidly from 1200 to 210 mAh g 1. The high cycling stability and rate capability of
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Fig. 6. (a) CV curves of the first three cycles of the SnO2/re-G composite at a scan rate of 0.2 mV s 1; (b) the first three voltage-capacity profiles at 200 mA g 1; (c) plot of charge capacities versus cycle number of the SnO2/re-G, SnO2 nanoparticles at 200 mA g 1, respectively; (d) rate capability of the SnO2/re-G composite electrodes with different currents, all the units are in mA g 1; and (e) schematic representation showing that the pathway for electrons and lithium-ion in the SnO2/ re-G hybrid paper during cycling.
the SnO2/re-G composite is further demonstrated in Fig. 6(d). Reversibility of storage capacity was observed when the rate was first stepwise increased from 200 to 2000 mA g 1. The capacities at each rate are also quite stable without notable fading. Even at the extremely high current density at 2000 mA g 1, we can also see the perfected cycling stability of our designed composites, the capacity of which is about 400 mAh g 1. When the current density switches back to the initial value at 200 mA g 1, the capacity can be restored to about 600 mAh g 1. When the SnO2 nanoparticles are combined with re-G by the simple vacuum filtration method, the composite has both the advantages of SnO2 and graphene, exhibiting a larger reversible capacity as well as excellent rate capability. As for the high capacity and remarkably superior capacity retention of the free-standing flexible robust composite of SnO2 and graphene, we strongly believe that these result from its unique architecture cachet. For one thing, the SnO2 nanoparticles are restrainedly anchored between the re-G nanosheets. Hence, we are confirmed that all the SnO2
nanoparticles can be lithiated/delithiaed during the charge/ discharge cycles, that is to say, the whole SnO2 material are serviceable, as shown in Fig. 6(e). For another, the overlapping re-G nanosheets act not only as a compromiser facilitating electron percolation against the low intrinsic electric conductivity of SnO2 but also serve as a structure cushion to alleviate the dramatic volume expansion. In addition, the overlapping re-G nanosheets can also be accepted as capable of accommodating the mechanical stresses/strains experienced by the SnO2 phase and maintain the structural and electrical integrity during the intercalation and deintercalation. This is also the case when considering that the fracturing and a loss of contact among the active materials would lead to the destruction of the electrode failure, which results in the ultimate capacity decay. Moreover, the overlapping re-G nanosheets can effectively prevent the SnO2 nanoparticles from agglomerating and thus further preserve the morphology integrity, avoiding the general pronounced capacity fading. In the last, owing to the mechanically strong graphene, which is formed by stacking and
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interlocking of individual sheets under a filtration-induced directional flow [20], the free-standing composite of SnO2 nanoparticles and graphene can sustain significant deformation during bending. In short, the more than simple processing method and architecture of the free-standing flexible composite, led to an extraordinarily improved overall mechanical and electrochemical performance. From the aforementioned, we are convincingly assured that, this unique SnO2/re-G composite architecture, combining the advantages of SnO2 nanoparticles and graphene nanosheets as well as its cost-effective, simple and easy scalability fabrication method, opens up an efficient new route to investigate transition metal oxide/graphene hybrid electrodes for next-generation rechargeable lithium ion batteries. 4. Conclusions In summary, a paper-like SnO2/re-G composite was successfully prepared by a facile vacuum filtration approach. Benefiting from the advanced hybrid nanoarchitecture, the composite presents remarkable cycling stability (about 700 mAh g 1 over 100 cycles), rate capability (about 400 mAh g 1 at 2000 mA g 1) and no obvious capacity fading or tendency of capacity decay. For this composited architecture, graphene not only serves as a conductive channel, but also can effectively alleviate the expansion of SnO2 nanoparticles and prevent aggregation on cycling. Without binder, the liquid electrolyte can access SnO2 more easily, which facilitates faster Li ion transfer among SnO2 and significantly reduces polarization. We believe that the synthesis concept could easily be extended to other electrode materials. Our system presented in this work offers significant implications on structural design for improving performances of electrodes in LIBs. Acknowledgments The authors would like to acknowledge financial supports provided by the National Natural Science Foundation of China (Nos. 51172191 and 51002129) and the Natural Science Foundation of Hunan (No. 13JJ3064). References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [2] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845–854. [3] J. Jiang, Y.Y. Li, J.P. Liu, X.T. Huang, C.Z. Yuan, X.W. Lou, Recent advances in metal oxide-based electrode architecture design for electrochemical energy storage, Adv. Mater. 24 (2012) 5166–5180. [4] X.W. Lou, C.M. Li, L.A. Archer, Designed synthesis of coaxial SnO2@carbon hollow nanospheres for highly reversible lithium storage, Adv. Mater. 21 (2009) 2536–2539. [5] X. Wang, X.Q. Cao, L. Bourgeois, H. Guan, S.M. Chen, Y.T. Zhong, D.M. Tang, H.Q. Li, T.Y. Zhai, L. Li, Y. Bando, D. Golberg, N-doped graphene–SnO2 sandwich paper for high-performance lithium-ion batteries, Adv. Funct. Mater. 22 (2012) 2682–2690. [6] J.Y. Huang, L. Zhong, C.M. Wang, J.P. Sullivan, W. Xu, L.Q. Zhang, S.X. Mao, N.S. Hudak, X.H. Liu, A. Subramanian, H.Y. Fan, L. Qi, A. Kushima, J. Li, In situ observation of the electrochemical lithiation of a single SnO2, Nanowire Electrode Sci. 330 (2010) 1515–1520.
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 6891–6897 www.elsevier.com/locate/ceramint
Free-standing SnO2 nanoparticles@graphene hybrid paper for advanced lithium-ion batteries Tian Gao, Kai Huangn, Xing Qi, Hongxing Li, Liwen Yang, Jianxin Zhongn Hunan Key Laboratory for Micro-Nano Energy Materials and Devices, Laboratory for Quantum Engineering and Micro-Nano Energy Technology, Faculty of Materials and Optoelectronic Physics, Xiangtan University, Hunan 411105, PR China Received 5 November 2013; received in revised form 3 December 2013; accepted 3 December 2013 Available online 14 December 2013
Abstract A free-standing SnO2/reduced graphene oxide (re-G) hybrid paper has been successfully fabricated by a facile vacuum filtration approach through a uniformly dispersed SnO2 nanoparticle and graphene oxide mixed solution. As potential anode material for high power and energy applications, the hybrid nanostructures were directly evaluated as anode for rechargeable lithium ion batteries without adding any polymer binder, conductive additives or current collectors. And the composite exhibits a greatly enhanced synergic effect with extremely high energy storage stability of 700 mAh g 1 after 100 cycles benefiting from the advanced structural features. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Lithium ion batteries; Tin oxide nanoparticles; Reduced graphene oxide
1. Introduction The development of high energy density and long-lasting lithium ion batteries (LIBs) is of great technological challenge for next-generation energy storage systems such as portable electronic devices, hybrid electrical vehicles and electrical vehicles, etc [1–3]. However, commercial graphite anode used currently has already reached its theoretical limit (372 mAh g 1), and exploring alternative anode materials with higher charge/discharge rate, reversible capacity as well as long cycle life and low cost has become an urgent task nowadays. From the material point of view, SnO2 could be a potential candidate for commercial anode materials due to its remarkably high theoretical charge capacity (782 mAh g 1) [4,5], nontoxicity, natural abundance and eco-friendliness, which has been extensively exploited for high-performance LIBs. The capacity of lithium storage is mainly achieved through the reversible conversion reaction mostly resulting from reversible alloying/ de-alloying processes of Sn with Li [6,7]. Despite those intriguing features, the main obstacle in developing SnO2based anodes lies in the severe volume change during lithium n
Corresponding authors. Tel./fax: þ 86 0731 58292468. E-mail addresses: [email protected] (K. Huang), [email protected] (J. Zhong).
ion insertion/extraction, which can result in pulverization of the initial particle morphology and cause the breakdown of electrical connection between the anode materials and current collectors, thereby leading to poor cycling performance [7,8]. In addition, the low electrical conductivity of pristine SnO2 challenges the achievement of high capacity at high charge/discharge rates. To overcome these above issues, nanostructures SnO2/C with various morphologies, such as nanofibers [9-12], nanoparticles [13,14], nanotubes [15] and nanopores [16,17] have been extensively studied as promising alternative anode materials in LIBs applications. Among them, the SnO2/ graphene presents a more superior performance than common carbon composite, and attracts unmatched attention and also triggered tremendous experimental activities [5,8,18–22]. For this composite, graphene sheets can substantially relieve mechanical strain, which otherwise causes an electrical disconnection resulting from the crumbling and pulverization of Sn domains during repeated charge/discharge cycles [23–29]. Simultaneously, graphene endowed with high conductivity and Young's modulus can also mitigate the aggregation of SnO2 nanoparticles, has a moderate theoretical capacity (744 mAh g 1) [30], which further contributes to a stable capacity response during extended charge/discharge cycles [31].
0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.12.009
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So far, the SnO2/graphene composited electrodes were prepared by a multi-stage process to control hydrolysis of SnCl4 and reduced graphene oxide nanosheets [18–22,32–34]. Furthermore, the N-doped graphene–SnO2 sandwich paper was fabricated by dispersing tetracyanoquinodimethane anion acetonitrile and SnCl2 in reduced graphene oxide suspension solution followed by treating in a tube furnace [5]. Nevertheless, their complex material processes limit the development of innovative and facile techniques for next generation advanced LIBs. Therefore, it is still a big challenge to find a convenient and facile method for constructing SnO2/graphene composited electrodes to further improve or optimize their electrochemical performance. Herein, we develop a facile and scalable strategy to construct free-standing SnO2/graphene hybrid electrodes by the simple vacuum filtration method, with super-high rate performance and extremely excellent cycling stability at high rates. By entirely circumscribing SnO2 nanoparticles between the graphene sheets, which served as structural cushion to accommodate the huge mechanical stress induced during cycling and to prevent the aggregation of SnO2 nanoparticles, the excellent structural and electrical integrity of the electrodes are perfectly preserved. What is more important, is that the fabricated free-standing and paper-like composites can be straightforwardly used as the electrode, in association with the needles of the current collector, conducting agent or polymer binder, that leads to excellent electrochemical performance as the anode for lithium-ion batteries. 2. Experimental SnO2 (with average diameters 50 nm) was purchased from Sigma-Aldrich Chemical Company Inc (United States), and Graphite (-325 mesh) from Sinopharm Chemical Reagent Co., Ltd (Shanghai China). The graphene oxide (GO) was synthesized from graphite powder by the modified Hummers method [33]. The dried GO and SnO2 nanoparticles were scaled and then dispersed into de-ionized (DI) water to form a uniform mixed solution by ultrasonication. To attain the SnO2/ re-G hybrid nanostructures, the above suspension with GO (40 wt%) and SnO2 (60 wt%) was vacuum filtered with a Whatman membrane filter. After peeling the composite from the membrane, it is immersed into HBr acid and kept at 120 1C for 2 h to efficiently reduce GO [35], finally obtaining a SnO2/
re-G paper-like hybrid electrode. In this presentation, the synthesizing processes are facile and easy to control, and the schematic is illustrated in Fig. 1. The morphology and structure of the samples were characterized by a field emission scanning electron microscope (FESEM, Hitachi S-4800), micro-Raman spectroscope (WITec excited by λ¼ 633 nm laser, and X-Ray power diffraction (XRD, D/MAX 2500). Thermogravimetric analysis (NETZSCH, TG209F3) was employed to calculate the mass content of SnO2 in the composite. For the electrochemical characterization, standard CR2032 coin cells were assembled in an Ar-filled glove box (Mikrouna, China). The SnO2/re-G composited paper was directly used as a working electrode without any conducting material and binding agent, and the Li foil as the counter electrode. The electrolyte was 1.0 M LiPF6 in a solution of 1:1 w/w ethylene carbonate:diethyl carbonate (Novolyte Technologies). All the discharge/charge capacity tests of the fabricated electrodes are determined by galvanostatic cycling over potential range 0.01–3 V in a battery tester (NEWARE BTS-5 V 5 mA, Neware Technology Co., Ltd., China). Cyclic voltammetry curves were scanned using an electrochemistry system (CHI 660D, Chen Hua Shanghai corp., China). All measurements were performed at room temperature. 3. Results and discussion Fig. 2 shows optical photographs of the SnO2/GO hybrid paper before and after peeling off from the membrane filter. In Fig. 2(a), a brownish composite, which was a random stack of GO sheets and SnO2 nanoparticles by vacuum filtration, can be clearly observed on the membrane. After air-dried, the composite can be exfoliated carefully from the membrane just like a paper. It can be discerned that the paper-like SnO2/GO composite changes from brownish to black after reduction in HBr solution, forming a SnO2/re-G composited nanostructure [16], as shown in Fig. 2(b). Simultaneously, the composite shows very good flexibility and retains the integrity, without any damage found in Fig. 2(c), even bent to some extent. Furthermore, the composite can be readily washed by soaking in DI water followed by vacuum drying, and then cut with a razor-blade into the free-standing platelet we need. Raman system was used to analyze the composition of the SnO2/re-G composite. The characteristic peaks at 1352 cm 1 and 1600 cm 1 can be clearly discerned, in Fig. 3(a), which
Fig. 1. Schematic illustration of the synthesizing processes for the SnO2/re-G hybrid paper.
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Fig. 2. (a) The SnO2/GO hybrid composite on filter membrane, inset is the SnO2/GO mixed solution; (b) the SnO2/re-G hybrid paper peeled from the filter membrane and turned black after reduction; and (c) the flexible SnO2/re-G hybrid paper.
Fig. 3. (a) Raman spectra of the final fabricated SnO2/re-G composite; and (b) XRD patterns of the treated and untreated SnO2 nanoparticle.
are in good agreement with the typical D and G band. The D band is associated with disordered samples carbon, while the G band is the result of the first-order scattering of the E2g mode of sp2 carbon domains [17]. A broad signal appears in the range of 350–800 cm 1, which originates from SnO2 (inset of Fig. 3(a)). The Raman characteristic peaks (469 and 630 cm 1) in our test shifts slightly more than the typical bulk SnO2 Raman peaks which are at about 476 and 635 cm 1. This decrease in crystal size leads to blue shift and broadening of Raman peaks [36,37]. As it is shown in Fig. 3(b), the XRD spectrum of the treated and untreated SnO2 nanoparticles is made to exclude the possibility that the SnO2 nanoparticles are reduced by the HBr acid in the constructing process. When comparing these two curves, no difference can be detected, and all the peaks are in good agreement with the standard profiles of the tetragonal structure of SnO2 [JCPDS 41-1445]. Thus, we can explicitly conclude that we have effectively reduced the graphene oxide to graphene, whilst the SnO2 active material is preserved. Scanning electron microscopy (SEM) is employed to further confirm that we have successfully fabricated the hybrid SnO2
nanoparticles and graphene sheets electrode. From the crosssectional SEM image in Fig. 4(a) and (b), a paper-like SnO2/reG composite about 20 mm thickness is obviously and clearly observed, where the SnO2 nanoparticles (diameter with 20–50 nm) is ideally anchored or tightly restricted between the adjacent re-G sheets and the re-G sheets are firmly stacked into continuous films without apparent stacking order owing to the existing SnO2 nanoparticles. Under a filtration-induced directional flow, the dispersive SnO2 nanoparticles and the interlocking of individual re-G sheets eventually construct a paper-like SnO2/re-G composite, as shown in Fig. 2(b), just like the description of the process in Fig. 1. Furthermore, it is worth noting that only small SnO2 nanoparticles are scattered out from the surface of the composite as shown in Fig. 4(c) and (d), and suggesting the perfect integrity of the obtained SnO2/re-G composite architecture. Moreover, we can conclude that this architecture can effectively block the SnO2 nanoparticles agglomeration during the charge/discharge cycles. Thermogravimetric analysis (TGA) has been used to identify the mass content of SnO2 nanoparticles in the composite, and the result shows in Fig. 5. The initial weight loss about 3% range from 30 1C to 200 1C can be attributed to the loss of the
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Fig. 4. (a) and (b) The SEM image of cross section of the SnO2/re-G composite, (c) and (d) SEM images of the top view from the surface of the composite, the blue dotted line area is the exposed SnO2 nanoparticles.
Fig. 5. TGA curve of SnO2/re-G composite measured in air atmosphere with a heating rate of 20 1C/min.
moisture in the composite. Then from 250 1C to 650 1C, the weight loss increases dramatically with the rise of temperature, giving the final 52% mass of the composite after reaching 900 1C. From the results, we can approximately calculate the mass content of SnO2 is about 53%. To understand the Li-ion insertion and extraction process in the composite during discharge/charge cycling, cyclic voltammogram (CV) was measured between 0.01 V and 3 V vs Li/Li þ at the scan rate of 0.2 mV s 1. Fig. 6(a) presents typical CV characteristics related to the lithiation and delithiation processes of the SnO2/re-G composite. A cathodic peak appears at 0.78 V in the first cycle which can be attributed to the formation of the solid electrolyte interphase layer. Another small reduction peak below 0.28 V could be due to the reactions between lithium and SnO2 nanoparticles to form LixSn alloys, while the insertion of lithium in graphene nanosheets could be identified as the reduction peak
at 0.01 V. There are three oxidation peaks located around 0.15, 0.62, and 1.21 V. They correspond to different oxidation reactions during the charge process. The first anodic peak at 0.15 V represents the lithium extraction from graphene nanosheet. The 0.62 V oxidation peak can be assigned to the dealloying of LixSn, showing a reversible process. The third weak oxidation at 1.21 V could result from the partial transformation of Sn metal to SnO2. The high reversibility of the CV curves further confirmed the reversible redox reactions occurring in the lithium-ion cell between lithium and SnO2/re-G composite. Half-cells with a metallic Li counter electrode were carried out to evaluate the electrochemical performance of the SnO2/re-G composite using galvanostatic charge/discharge from 0.01 V to 3.0 V, without adding the conducting agent, polymer binder or current collector. Fig. 6(b) shows the initial three discharge/ charge curves of the composite and at a current density of 200 mA g 1. It is worth noting that the first discharge capacity value is about 1260 mAh g 1, and the second discharge capacity declines to 870 mAh g 1. The irreversible capacity can be assigned to the re-G material [38]. The reason is that since the GO is not completely reduced, plenty of functional groups are still residual and the reactions of Li with O and H-containing groups have trapped lots of in-extractable Li ions. As a result, this leads to poor electrochemical performance [18]. Another cause can be attributed to the formation of solid electrolyte interphase (SEI) in the composite [19]. However, an unanticipated substantially superior capacity retention about 700 mAh g 1 is found after 100 cycles, as shown by the red curve in Fig 6.(c). No obvious capacity fading or tendency of capacity decay can be found. A comparison of the charge/discharge cyclic performances of SnO2 is also shown in Fig. 6(c). After 50 cycles, the capacity of the SnO2 nanoparticles is fading rapidly from 1200 to 210 mAh g 1. The high cycling stability and rate capability of
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Fig. 6. (a) CV curves of the first three cycles of the SnO2/re-G composite at a scan rate of 0.2 mV s 1; (b) the first three voltage-capacity profiles at 200 mA g 1; (c) plot of charge capacities versus cycle number of the SnO2/re-G, SnO2 nanoparticles at 200 mA g 1, respectively; (d) rate capability of the SnO2/re-G composite electrodes with different currents, all the units are in mA g 1; and (e) schematic representation showing that the pathway for electrons and lithium-ion in the SnO2/ re-G hybrid paper during cycling.
the SnO2/re-G composite is further demonstrated in Fig. 6(d). Reversibility of storage capacity was observed when the rate was first stepwise increased from 200 to 2000 mA g 1. The capacities at each rate are also quite stable without notable fading. Even at the extremely high current density at 2000 mA g 1, we can also see the perfected cycling stability of our designed composites, the capacity of which is about 400 mAh g 1. When the current density switches back to the initial value at 200 mA g 1, the capacity can be restored to about 600 mAh g 1. When the SnO2 nanoparticles are combined with re-G by the simple vacuum filtration method, the composite has both the advantages of SnO2 and graphene, exhibiting a larger reversible capacity as well as excellent rate capability. As for the high capacity and remarkably superior capacity retention of the free-standing flexible robust composite of SnO2 and graphene, we strongly believe that these result from its unique architecture cachet. For one thing, the SnO2 nanoparticles are restrainedly anchored between the re-G nanosheets. Hence, we are confirmed that all the SnO2
nanoparticles can be lithiated/delithiaed during the charge/ discharge cycles, that is to say, the whole SnO2 material are serviceable, as shown in Fig. 6(e). For another, the overlapping re-G nanosheets act not only as a compromiser facilitating electron percolation against the low intrinsic electric conductivity of SnO2 but also serve as a structure cushion to alleviate the dramatic volume expansion. In addition, the overlapping re-G nanosheets can also be accepted as capable of accommodating the mechanical stresses/strains experienced by the SnO2 phase and maintain the structural and electrical integrity during the intercalation and deintercalation. This is also the case when considering that the fracturing and a loss of contact among the active materials would lead to the destruction of the electrode failure, which results in the ultimate capacity decay. Moreover, the overlapping re-G nanosheets can effectively prevent the SnO2 nanoparticles from agglomerating and thus further preserve the morphology integrity, avoiding the general pronounced capacity fading. In the last, owing to the mechanically strong graphene, which is formed by stacking and
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interlocking of individual sheets under a filtration-induced directional flow [20], the free-standing composite of SnO2 nanoparticles and graphene can sustain significant deformation during bending. In short, the more than simple processing method and architecture of the free-standing flexible composite, led to an extraordinarily improved overall mechanical and electrochemical performance. From the aforementioned, we are convincingly assured that, this unique SnO2/re-G composite architecture, combining the advantages of SnO2 nanoparticles and graphene nanosheets as well as its cost-effective, simple and easy scalability fabrication method, opens up an efficient new route to investigate transition metal oxide/graphene hybrid electrodes for next-generation rechargeable lithium ion batteries. 4. Conclusions In summary, a paper-like SnO2/re-G composite was successfully prepared by a facile vacuum filtration approach. Benefiting from the advanced hybrid nanoarchitecture, the composite presents remarkable cycling stability (about 700 mAh g 1 over 100 cycles), rate capability (about 400 mAh g 1 at 2000 mA g 1) and no obvious capacity fading or tendency of capacity decay. For this composited architecture, graphene not only serves as a conductive channel, but also can effectively alleviate the expansion of SnO2 nanoparticles and prevent aggregation on cycling. Without binder, the liquid electrolyte can access SnO2 more easily, which facilitates faster Li ion transfer among SnO2 and significantly reduces polarization. We believe that the synthesis concept could easily be extended to other electrode materials. Our system presented in this work offers significant implications on structural design for improving performances of electrodes in LIBs. Acknowledgments The authors would like to acknowledge financial supports provided by the National Natural Science Foundation of China (Nos. 51172191 and 51002129) and the Natural Science Foundation of Hunan (No. 13JJ3064). References [1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657. [2] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845–854. [3] J. Jiang, Y.Y. Li, J.P. Liu, X.T. Huang, C.Z. Yuan, X.W. Lou, Recent advances in metal oxide-based electrode architecture design for electrochemical energy storage, Adv. Mater. 24 (2012) 5166–5180. [4] X.W. Lou, C.M. Li, L.A. Archer, Designed synthesis of coaxial SnO2@carbon hollow nanospheres for highly reversible lithium storage, Adv. Mater. 21 (2009) 2536–2539. [5] X. Wang, X.Q. Cao, L. Bourgeois, H. Guan, S.M. Chen, Y.T. Zhong, D.M. Tang, H.Q. Li, T.Y. Zhai, L. Li, Y. Bando, D. Golberg, N-doped graphene–SnO2 sandwich paper for high-performance lithium-ion batteries, Adv. Funct. Mater. 22 (2012) 2682–2690. [6] J.Y. Huang, L. Zhong, C.M. Wang, J.P. Sullivan, W. Xu, L.Q. Zhang, S.X. Mao, N.S. Hudak, X.H. Liu, A. Subramanian, H.Y. Fan, L. Qi, A. Kushima, J. Li, In situ observation of the electrochemical lithiation of a single SnO2, Nanowire Electrode Sci. 330 (2010) 1515–1520.
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