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Journal of Alloys and Compounds 654 (2016) 357e362

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Journal of Alloys and Compounds

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SnS2@Graphene nanosheet arrays grown on carbon cloth asfreestanding binder-free flexible anodes for advanced sodiumbatteries

Wangwang Xu a, Kangning Zhao b, Lei Zhang b, Zhiqiang Xie a, Zhengyang Cai b,Ying Wang a, *

a Department of Mechanical & Industrial Engineering, Louisiana State University, Baton Rouge, LA 70803, USAb State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China

a r t i c l e i n f o

Article history:Received 26 June 2015Received in revised form16 August 2015Accepted 8 September 2015Available online 11 September 2015

Keywords:Tin sulfide-graphene nanosheet arraysFreestanding binder-free flexible electrodeHigh-rate anode materialSodium-ion battery

* Corresponding author.E-mail address: ywang@lsu.edu (Y. Wang).

http://dx.doi.org/10.1016/j.jallcom.2015.09.0500925-8388/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

Flexible electrodes with light weight, favorable mechanical strength, low cost and high energy/highpower density have attracted tremendous interest for next-generation sodium ion rechargeable batte-ries. This work reports the first effort to achieve novel flexible free-standing SnS2@Graphene nanosheetarrays as next-generation sodium ion battery anode materials by using a facile two-step solvothermalroute. The as-prepared nanosheet arrays serve as an integrated electrode without metal current col-lectors, binders or any conductive additives, demonstrating excellent cycleability with a capacity of378 mAh/g after 200 cycles at a specific current of 1200 mA/g and improved rate capability withdischarge capacities of 633, 523, 458 and 348 mAh/g at specific currents of 200, 1000, 2000, and3000 mA/g, respectively. In comparison with most of the widely reported Sn or SnO2-based anodematerials, our integrated SnS2@Graphene nanosheet arrays exhibit superior cycling performance andrate capability as well as novel flexible feature for application as anodes in high-energy high-powersodium ion batteries.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Sodium-ion batteries (SIBs) have been deemed as the substitutefor lithium-ion batteries for a long time owing to their advantagessuch as lowcost and abundant resources of sodium [1e5]. However,insufficient electrochemical performances such as short cycling lifeand low rate capability have limited their practical applications,due to the larger size of a Naþ ion (1.02 Å in radius) than a Liþ ion(0.76 Å in radius) [6e8]. Hence, it is critical to develop new high-performance electrode materials for sodium-ion batteries to alle-viate or solve the above issues [9,10].

At present, tin and tin-based compounds are promising anodematerials in sodium ion batteries due to their high theoreticaldischarge capacity of 847 mAh/g (based on the theoretical stoi-chiometry of Na15Sn4) [11,12]. SnS2 has recently attracted increasingattention owing to the large interlayer spacing in its layeredstructure, which facilitates the insertion and extraction of sodium

ions [13e15]. However, because of severe pulverization due to greatvolume expansion (~420%) and poor kinetics caused by slow elec-tron transport, SnS2 cracks and gradually loses contact with thecurrent collector, resulting in poor cycleability and low rate capa-bility [16,17,18]. A general approach to enhancing its electro-chemical performance is to fabricate SnS2 based nano-compositearchitectures. For example, Wang et al. have investigated theexfoliated-SnS2-graphene composite materials for application asanodes in sodium ion batteries [19], which exhibit a remarkablecapacity as high as 670 mAhg�1 at a specific current of 20 mAg�1.When cycled at 640 mAg�1, its capacity reaches 463 mAhg�1 muchhigher than that of bare SnS2 (152mAhg�1), showing improved ratecapability [20]. Though these results have shown that SnS2 basednanocomposites are promising anode materials for applications insodium ion batteries, the above nanocomposites still suffer fromagglomeration of nanomaterials and stress between differentcomponents in the composites as well as slow charge transport.Therefore, it is crucial to develop novel tin disulfide electrodes withboth high rate capability and long cycle life for next-generation SIBs[14,20,21].

W. Xu et al. / Journal of Alloys and Compounds 654 (2016) 357e362358

On the other hand, polyvinylidene fluoride (PVDF), the mostpopular polymer binder used in battery electrodes, can decreaseelectronic conductivity of electrodes [22]. Addition of carbon blackmay improve conductivity of electrodes, but the tap density of theelectrodemay be seriously affected. In addition, with the increasingdemand in portable electronic devices and electric automobiles, itremains a challenge to achieve flexible electrodeswith light weight,favorable mechanical strength, low cost and high energy/high po-wer density, for next-generation high-performance SIBs [23]. Afreestanding binder-free electrode with excellent mechanicalstrength and large capacity is a vital component of a flexiblerechargeable battery.

In this note, we report free-standing SnS2 nanosheet-arraybased negative electrodes synthesized by growing SnS2 nano-sheet arrays directly on carbon fiber cloth, followed with wrappingthe SnS2 nanosheets with graphene nanosheets [24e26]. Theproduct is an integrated electrode [27e30]. The design of theSnS2@graphene nanosheet arrays is based on following principles:(1) The large interlayer spacing in the SnS2 structure not onlybenefits the intercalation and diffusion of sodium ions but also caneffectively buffer the stress which brought about by volumechanges during cycling [13,14,31]. (2) Nanosheet-array structurecan effectively avoid the agglomeration of powder structures, at thesame time facile strain relaxation in the nanosheet arrays allowthem to increase in thickness and area without breaking [32e34].(3) Each SnS2 nanosheet homogeneously grows on conductivecarbon cloth working as current collector so that all SnS2 nano-sheets can effectively contribute to the capacity. (4) In this welldesigned construction, both carbon fiber cloth and graphene act asconducting material to improve overall electronic conductivity ofSnS2@graphene nanosheet arrays, leading to fast reaction kineticswith fast switching speed. More important, two-dimensionalstructural graphene, which is well coated on the surface of SnS2nanosheets, can effectively relieve the stress of volume expansionduring discharge, thereby reducing the serious crush andimproving the cycling stability (Fig. 1) [32]. (5) Integrated negativesodium battery electrode has been prepared by direct growth ontocarbon fiber cloth without any binders and conductive additives,which usually add extra weight to the whole electrode [34,35,36].To our knowledge, it is the first time that a SnS2@graphene nano-array architecture has been fabricated directly on carbon fiber cloth.

Fig. 1. Schematic illustration of the SnS2@graphene nanosheet arrays with efficient electronfiber, two-dimensional grapheme and carbon fiber can not only provide faster electron pextraction. For the whole carbon fiber cloth, combination of carbon fibers form an integrat

2. Material and methods

Materials synthesis and characterization: the graphite powder,carbon fiber cloth, tin chloride pentahydrate (SnCl4$5H2O), thio-acetamide (TAA). graphene was prepared from the graphite bymodifying the Hummers' method [34,35].

Preparation of SnS2 nanosheet arrays: 0.54 g SnCl4 5H2O and0.46 g thioacetamide were dissolved in 30 ml isopropyl alcohol.Aftermagnetic stirring for half an hour, the solutionwas transferredinto a Teflon-lined autoclave (50 mL) in which a piece of cleanedcarbon fiber cloth was placed standing against thewall, followed byheating at 180 �C for 24 h. After cooling to room temperature, therecovered carbon fiber cloth was washed with deionized water andabsolute alcohol twice respectively, and dried at 70 �C overnight.Themass loading of SnS2 nanosheet on carbon fiber clothwas about1.93 mg/cm2.

Preparation of SnS2@graphene nanosheet arrays: 0.5 ml gra-phene suspension with a concentration of 3 mg/ml was dispersedinto 29 ml isopropyl alcohol. Then the solution was transformedinto a Teflon-lined autoclave after it was dispersed uniformly. BareSnS2 nanosheet arrays, obtained using the above method, wereplaced in a Teflon-lined autoclave and heated at 180 �C for 12 h.After cooling to room temperature, the products were washed bydeionized water and absolute alcohol twice respectively, and driedat 70 �C overnight.

3. Characterization

Crystallographic information of the obtained products was ob-tained with a X-ray diffraction (XRD) measurement using Cu Karadiation in a 2q range from 10� to 80� at room temperature.Morphology of the samples was studied using a FEI Quanta 3D FEGfield emission scanning electron microscopy (FESEM), transmissionelectron microscopy (TEM) and high-resolution transmission elec-tron microscopy (HRTEM) by JEOL HRTEM (JEM-1400 electron mi-croscope) with an acceleration voltage of 120 kV. Electrochemicalperformances of the samples were characterized with coin cellsof CR2016 type assembled in a glove-box filled with pure argongas. Sodium foil was used as the anode; a 1 M solution of NaClO4 inethylene carbon (EC)-dimethyl carbonate (DMC) (1:2 w/w) wasused as the electrolyte; the thickness of the as-prepared electrode is

transport and effective stress relief in charge/discharge process. For every single carbonathway on radial direction, but also relieve the stress during sodium insertion anded three-dimensional conductive network, contributing to the high rate capability.

W. Xu et al. / Journal of Alloys and Compounds 654 (2016) 357e362 359

about 0.8 mm, including the carbon cloth. Galvanostatic charge/discharge measurements were performed with a multichannelbattery testing system (LAND CT2001A). Electrochemical imped-ance spectroscopy (EIS) was measured by an Autolab PotentiostatGalvanostat at room temperature.

4. Results and discussion

Crystal structure of freestanding SnS2@graphene nanosheet ar-rays and SnS2 nanosheet arrays are determined by XRD (Fig. 2a). Itcan be seen there is no significant difference in XRD patterns ofSnS2@graphene nanosheet arrays and SnS2 nanosheet arrays.Meanwhile, both of these patterns can be identified as hexagonaltin sulfide with the lattice parameters a ¼ 3.638 Å, b ¼ 3.638 Å,c ¼ 5.88 Å, P-3m1 space group, which are in good agreement withstandard XRD patterns of hexagonal berndtite SnS2 (JCPDF cardno.00-022-0951).

In comparisonwith electrodes used in batteries, the as-preparedSnS2@Graphene nanosheet arrays form a completely binder-free,free-standing and flexible electrode. It can be easily bent withoutany damage as shown in Fig. 3. As shown in the SEM images(Fig. 3c), SnS2 nanosheets grow on the carbon fiber cloth homo-geneously. They are about 10 nm in thickness and 1 mm in width.Additionally, the lattice fringeswith different d-spacings of 3.16 and5.89 are clearly observed in HRTEM image (Fig. 3d), correspondingto the (100) and (001) planes of hexagonal SnS2 crystals. As for theSnS2@graphene nanosheet arrays, graphene nanosheets directlywrapped around the each carbon fiber with SnS2 nanosheetsgrowing on it, forming a continuous network structure (Fig. 3e).Further characterizationwith HRTEM also reveals that (Fig. 3f). Thethickness of graphene nanosheets wrapping around SnS2 nano-sheets arrays is about 2 nm. Meanwhile, the SAED pattern (the insetin Fig. 3f) also indicates monocrystalline nature of SnS2 nanosheets.

Furthermore, in order to compare electrochemical perfor-mances of SnS2@graphene nanosheet arrays and bare SnS2 nano-sheet arrays for sodium ion batteries, they are assembled into coincells and cycled in a voltage range of 0.01e1 V (versus Naþ/Na).Similar to the reaction of SnO2, the electrochemical process of SnS2with sodium can be expressed as follows [19]:

SnS2 þ 4Naþ þ 4e� ¼ 2Na2S þ Sn (1)

Fig. 2. (a) XRD patterns of SnS2@graphene nanosheet arrays (noted as SnS2@graphenenanosheet arrays) and SnS2 nanosheet arrays (noted as SnS2 nanosheet arrays).

Sn þ xNaþ þ xe� ¼ NaxSn (2)

Reaction (1) is the conversion reaction, generally regardedpartially reversible [19,37,38]. Reaction (2) is the reversible alloyingreaction which determines the theoretical capacity of SnS2 to be847 mA h/g. The conversion reaction is considered to occur beforealloying reaction even though there is no exactly clear boundarybetween the conversion and alloying reactions for SnS2 as anodematerials in sodium batteries. Conversion reaction usually occursfirst since its product Sn is the precursor for subsequent alloyingreactions [15,17,20]. Cyclic voltammetry (CV) profile of SnS2@gra-phene nanosheet arrays are performed at a scan rate of 0.1 mV/s ina potential region of 0.01e1 V (Fig. 4a). It can be seen that twocathodic peaks appear in the first cycle. The first peak at 0.6 V maybe due to the conversion process (reaction 1) and the formation ofan irreversible solid electrolyte interphase (SEI). The broad cathodicpeak ranging from 0.3 to 0.01 V may be ascribed to the reversiblealloying reaction (reaction 2), which is consistent with previousreports [17,20]. During the anodic scan, the intense oxidation peakaround 0.32 V stands for the dealloying process of NaxSn. Thewell-defined peaks after the first cycle indicate the high revers-ibility of the reaction. The charge and discharge characteristics ofSnS2@graphene nanosheet electrode between 0.01 V and 1 V isthen displayed (Fig. 4b). It shows the initial charge and dischargeprofiles at various current rates from 200,1000, 2000 to 3000mA/g.It can be observed that the first discharge and charge capacity ofSnS2@graphene are about 1121 and 643 mA h/g respectively whencycled at 200 mA/g. The polarization in the first two cycles at aspecific current of 200 mA/g is attributed to the partially irrevers-ible conversion process and the formation of SEI film. Disappear-ance of potential platform at about 0.3 V can bemainly attributed toirreversibility of the conversion reaction (reaction 1). The electrodeexhibits excellent rate capability and cycle stability, deliveringdischarge capacities of 652, 558, 463, 370 mA h/g at a specificcurrent of 200 of 2nd cycle, 1000, 2000, 3000 mA/g respectively.When the specific current returns to 200 mA/g, it still shows ca-pacity retention as high as 95% by delivering the discharge capacityof 620 mA h/g, indicating high rate capability and outstandingelectrochemical reversibility of SnS2@graphene nanosheet arrays.

When cycled at a specific current of 200 mA/g (Fig. 5a), theinitial discharge capacity of SnS2@graphene nanosheet arrays is ashigh as 1172 mA h/g. After 100 cycles, SnS2@graphene nanosheetarrays can still provide a high discharge capacity of 548 mA h/g,revealing capacity fading of only 0.187% per cycle from the 2ndcycle to the 100th cycle and demonstrating excellent cycling sta-bility. The carbon cloth contributes a specific capacity of ~12 mA h/g, which is negligible compared with the high capacity of SnS2(Figure S1a and b). The first Coulombic efficiency is about 49.1%,attributing to irreversible conversion reaction (reaction 1) [16], theformation of solid electrolyte interphase (SEI) layers. When thespecific current is increased to 1200 mA/g (Fig. 5b), the initialCoulombic efficiencies of SnS2@graphene nanosheet arrays andSnS2 nanosheet arrays are about 72.2% and 56.4% respectively.SnS2@graphene nanosheet arrays exhibit a prominent improve-ment in cycling performance in comparison with bare SnS2 nano-sheet arrays. SnS2@graphene nanosheet arrays show a dischargecapacity of 378 mA h/g after 200 cycles, showing a very low ca-pacity fading rate of 0.208%, much lower than that of SnS2 nano-sheet arrays (0.790%), again demonstrating better cycling stabilitycompared to SnS2 nanosheet arrays that exhibit rapid capacitydecaying. The phenomenon of capacity fluctuation from SnS2anode materials in sodium batteries has been widely reported inliterature [13,20] and the fluctuation is more distinct when theanode is cycled at a higher specific current. For example, Yuan et al.[20] observed the capacity fluctuationwhen the SnS2 electrode was

Fig. 3. (a) Front view and (b) side view of flexible and binder free SnS2@graphene nanosheet arrays. (c) SEM images (lower-magnification SEM image in the inset showing widerview of the sample) and (d) HRTEM image of SnS2 nanosheet arrays (TEM image in the inset). (e) SEM images (lower-magnification SEM image in the inset showing wider view ofthe sample) and (d) HRTEM image of SnS2@graphene nanosheet arrays. (TEM and SAED images in the inset).

Fig. 4. (a)The first three cycle voltammograms of SnS2@graphene nanosheet arrays at a scan rate of 1 mV/s. (b) The charge/discharge curves of SnS2@graphene nanosheet arrays at acurrent rate of at 200, 1000, 2000, 3000 and 200 mA/g, corresponding to the rate performance testing.

W. Xu et al. / Journal of Alloys and Compounds 654 (2016) 357e362360

Fig. 5. (a) Cycling performances of SnS2@graphene nanosheet arrays at 200 mA/g for 100 cycles. (b) Cycling performance of SnS2@graphene and SnS2 nanosheet arrays at 1200 mA/g for 200 cycles, respectively. (c) Rate performances of SnS2@graphene and SnS2 nanosheet arrays at the specific current ranging from 200, 1000, 2000, to 3000, and back to 200 mA/g. (d) Nyquist plots of SnS2@graphene and SnS2 nanosheet arrays at 100% depth of discharge in the 3rd cycle.

W. Xu et al. / Journal of Alloys and Compounds 654 (2016) 357e362 361

cycled at 200 mA/g. Zhao et al. [13] also reported a more obviousfluctuation occurring at 800 mA/g than at lower specific currents.The actual cause of such capacity fluctuation during cycling is notyet clear, though we deem it may be attributed to possible largestructural change of SnS2 during electrochemical cycling. In ourcase, the cycling performances of SnS2 electrodes are evaluated at aspecific current as high as 1200 mA/g, significantly higher thanthose used in previous reports [13,20], resulting in appreciablecapacity fluctuation. During the rate performance testing (Fig. 5c),the designed SnS2@graphene nanosheet arrays deliver dischargecapacities of 633, 523, 458 and 348 mA h/g at specific currents of200, 1000, 2000, and 3000 mA/g, respectively. The discharge ca-pacity recovers to 620 mA h/g when the specific current goes backto 200 mA/g, exhibiting much better rate performance than that ofbare SnS2 nanosheet arrays (482 mA h/g). It can be seen thatSnS2@graphene nanosheet arrays exhibit enhanced rate capabilitybecause graphene nanosheets help to relieve stress for volumechanges during sodium ion intercalation and deintercalation.Moreover, the continuous electron transport provided by both theconducting carbon fiber cloth and graphene nanosheets is anotherfactor contributing to the improved performance. As such, EIS re-sults confirmed the fast kinetics of electron transport (Fig. 5d). Inthis figure, Rs represents the Ohmic resistance of the electrodesystem, including the electrolyte and the cell components. Rct andRf represent the charge transfer resistance and the resistance of SEIfilm respectively. CPE and Zw represent double layer capacitanceand the Warburg impedance respectively. Both of the Nyquist plotsexhibit a depressed semicircle in the medium-frequency regionfollowed by a slash in the low-frequency region. SnS2@graphenenanosheet arrays show a much lower resistance of 125 U in com-parison with bare SnS2 nanosheet arrays (425 U), indicating thatgraphene nanosheets covering on the surface of SnS2 indeedimprove electron transport and facilitate kinetics of SnS2@gra-phene nanosheet arrays. Additionally, in comparison with SnS2nanosheet arrays, the slope of SnS2@graphene nanosheet arrays is

increased distinctly at low-frequency range, indicating that Rct ofSnS2@graphene nanosheet arrays is much lower than that of SnS2nanosheet arrays electrode. This phenomenon indicates thatwith the graphene layer wrapped around the SnS2 nanosheets,charge transfer at the electrode/electrolyte interface is more facileand the overall battery internal resistance can be reduced at thesame time.

To summarize, the significantly enhanced electrochemical per-formance of freestanding SnS2@graphene nanosheet arrays can beascribed to the following factors. First, the novel structure ofnanosheet arrays facilitates relaxation of strain during sodium ionintercalation/deintercalation processes. In this structure, graphenenanosheet wrapping around the surface of SnS2 nanosheets caneffectively relieve the stress of volume expansion/contraction andcontribute to the structural stability and mechanical integrityduring charge/discharge cycles. Second, the highly conductingcarbon fiber cloth and graphene nanosheets allow faster electrontransport in the electrode during electrochemical cycling, resultingin excellent rate capability of SnS2@graphene nanosheet arrays. Incomparison with most of the Sn or SnO2-based anode materialsreported in literature [21,25,39e44], our SnS2@graphene nano-sheet arrays show significantly improved rate capability, when atspecific current of 200 mA/g and even as high as 2000 mA/g(Table S1.). Most of the Sn or SnO2-based anode materials reportedin literature involve expensive Cu current collector, non-conductivepolymer binders and low tap-density carbon black or acetyleneblack as conductive additives for fabrication of electrodes, whichrequires long complicated procedure, thereby hindering its widecommercialization in SIBs or LIBs. Moreover, these electrodes arerigid and have the limitations in meeting the increasing demand inportable electronic devices and electric vehicles. Therefore, theintegrated freestanding binder-free flexible electrode achieved inthis work can effectively overcome the drawbacks above and findgreat potential applications in new-generation rechargeablebatteries.

W. Xu et al. / Journal of Alloys and Compounds 654 (2016) 357e362362

5. Conclusions

In conclusion, we successfully design and fabricate freestandingbinder-free flexible SnS2@graphene nanosheet arrays using a facile,scalable, and two-step solvothermal method. It is the first time torealize SnS2 directly grown on the current collector. Because of theirunique integrated structure that provides efficient electron trans-port pathway and faster kinetics, the as-prepared SnS2@graphenenanosheet arrays demonstrate facile and stable sodium ion storageperformance, leading to improved rate and cycling capability incomparison with bare SnS2 nanosheet arrays. At a specific currentof 1200 mA/g, the reversible specific discharge capacity after 200cycles is as high as 378 mA h/g. Such an ordered freestandingcomposite nanostructure demonstrates great potential for appli-cations in next-generation flexible energy conversion/storagedevices.

Acknowledgments

Thework is supported by the Chevron Innovative Research Fund(127996148) from LSU College of Engineering as well as theResearch Enhancement Award and Research Awards Programsponsored by LaSPACE.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jallcom.2015.09.050.

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