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Surfactant-assisted encapsulation of uniform SnO2 nanoparticles in graphene layers for high-

performance Li-storage

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2015 2D Mater. 2 014005

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2DMater. 2 (2015) 014005 doi:10.1088/2053-1583/2/1/014005

PAPER

Surfactant-assisted encapsulation of uniform SnO2 nanoparticles ingraphene layers for high-performance Li-storage

WeiAi1,2, Jianhui Zhu1, Jian Jiang1, DongliangChao1, YanlongWang1, Chin FanNg1, XiuliWang1,ChaoWu1,6, Changming Li6, Zexiang Shen1,WeiHuang2,4 andTingYu1,3,5

1 Division of Physics andApplied Physics, School of Physical andMathematical Sciences, NanyangTechnological University, 637371,Singapore

2 Key Laboratory of Flexible Electronics (KLOFE)& Institue of AdvancedMaterials (IAM),National Jiangsu Synergistic InnovationCenterfor AdvancedMaterials (SICAM),Nanjing TechUniversity (NanjingTech), 30 South PuzhuRoad,Nanjing 211816, People’s Republic ofChina

3 Graphene ResearchCentre, National University of Singapore, 117546, Singapore4 Key Laboratory forOrganic Electronics & InformationDisplays (KLOEID) and Institute of AdvancedMaterials (IAM),Nanjing

University of Posts &Telecommunications, Nanjing 210023, Jiangsu, People’s Republic of China5 Department of Physics, Faculty of Science, National University of Singapore, 117542, Singapore6 Institute for Clean Energy&AdvancedMaterials, Southwest University, Chongqing 400715, People’s Republic of China

E-mail: wei-huang@njtech.edu.cn and yuting@ntu.edu.sg

Keywords: SnO2 nanoparticles, graphene, Li-ion battery, surfactant

Supplementarymaterial for this article is available online

AbstractSnO2/graphene composite has been regarded as the alternative anodematerial for next generationhigh-performance lithium-ion batteries (LIBs). Herewe report an efficient and facile one-pot strategyfor the synthesis of SnO2/graphene composite through a surfactant-assisted redox process. The pre-sence of surfactant can provide homogeneous nucleation sites for SnO2 nanoparticles formation, thusensuring the generated SnO2 nanoparticles have a tiny size of∼5 nmand are uniformly distributed onthe graphene sheets. Simultaneously, the randomaggregation of graphene sheets leads to the in-situencapsulation of SnO2 nanoparticles into graphene layers, forming amechanically robust compositestructure. These unique structural features are not only favorable for fast electrons transport and Liions diffusion, but also capable of effectively buffering the volume changes of SnO2 nanoparticles. As aconsequence, the SnO2/graphene composite exhibits superior Li storage performance in terms oflarge reversible capacity, good cycling stability and excellent rate capability.

1. Introduction

Since their birth in the early 1990s, lithium-ionbatteries (LIBs) have been extensively used as theprimary power source for portable electronics rangingfrom digital cameras and mobile phones to laptopcomputers, due to their significant advantages overother traditional batteries [1]. Despite these notablesuccesses, a sustainable world still requires lighter,smaller, cheaper and more efficient LIBs with morepower that can compete with traditional fossil fuels forpowering electric vehicles. However, the currentelectrochemical performance (e.g. limited Li storagecapacity, poor rate capability, etc) of commerciallyutilized graphite anode is insufficient to meet these

requirements [2]. To this end, intense efforts havebeen focused on the research and development ofalternative anode materials for next generation LIBs.SnO2 is regarded as one of the most promisingcandidates due to its high Li storage capacity(782 mAh g−1), low discharge potential, non-toxicityand low cost [3]. Unfortunately, the practical applica-tion of SnO2 anode suffers from a severe pulveriza-tion-induced capacity fading, arising from its largevolume changes (∼300%) and agglomeration duringthe repeated Li insertion/extraction process [4]. Toaddress these issues, researchers have proposed severalapproaches to improve the electrochemical perfor-mance of SnO2. Owing to the high surface area tovolume ratio, nanostructured materials offer

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15October 2014

ACCEPTED FOR PUBLICATION

11December 2014

PUBLISHED

13 January 2015

© 2015 IOPPublishing Ltd

significance advantages over their bulk counterparts,since smaller particles can decrease the deformation ofthe electrodematerial and endow better electrochemi-cal performance [5]. In this regard, the morphologi-cally engineering of SnO2 into various nanostructureshave been performed to achieve improved perfor-mances including nanocubes [6], nanotubes [7],nanowires [8], porous [9] and hollow nanoparticles[10]. Nevertheless, the complicated fabrication proce-dures and relatively high costs hinder the large-scaleproduction and commercialization of thesenanostructures.

Another effective strategy is to fabricate SnO2-based composites by coating other stable metal oxides[11, 12] or conductive carbon-basedmatrices [13, 14].In particular, the introduction of carbon-based mate-rials (e.g. carbon nanofiber, mesoporous carbon, car-bon nanotube and graphene) can not only improvethe conductivity of SnO2 nanoparticles but also pre-vent their agglomeration, as well as buffer the asso-ciated volume changes [15, 16]. Among these carbon-based materials, graphene has received enormousattention due to its special two-dimensional geometricstructure and fascinating physical and chemical prop-erties [17, 18]. So far, various techniques have beendeveloped to fabricate SnO2/graphene composites,such as the hydrothermal method [19, 20], physicalmixingmethod [21], solution-based synthesismethod[22, 23] and ball-milling method [24]. However, theelectrochemical performance of SnO2/graphene com-posites is still not satisfying due to: (i) the SnO2 nano-particles are only simply decorated on the surface ofgraphene in most cases rather than confined in gra-phene layers, making them easily peeled off from thegraphene sheets during long-term cycling [25, 26]. (ii)The inhomogeneous size distribution of SnO2 nano-particles makes it difficult for the graphene sheets toeffectively prevent their agglomeration within electro-des because of the high surface energy [27]. Until now,SnO2/graphene composites with uniform structureand thus superior electrochemical performance, espe-cially high rate capability have rarely beenachieved [28].

In this work, we develop a facile one-pot approachfor the synthesis of SnO2/graphene composite via asurfactant-assisted redox process. The SnO2 nano-particles are formed via Sn2+-induced reduction ofgraphene oxide (GO), and simultaneously encapsu-lated into graphene layers due to the random aggrega-tion of graphene sheets. Furthermore, the presence ofsurfactant is indispensable for guaranteeing the uni-formity of SnO2 nanoparticles with a tiny size of∼5 nm and homogeneous distribution on graphenesheets. As expected, the resultant SnO2/graphenecomposite exhibits superior Li storage properties withlarge reversible capacity, good cyclic lifespan andexcellent rate capability, highlighting its great poten-tial applications in future LIBs.

2. Experimental section

2.1. Preparation ofGOGO was obtained via the oxidation of natural graphite(China Jixi Jinyu Graphite Co., Ltd) using a modifiedHummers’ method, and purified by dialysis for oneweek [29].

2.2. Preparation of surfactant-assistedencapsulation of SnO2nanoparticles in graphenelayers (SAE SnO2/G)1 g hexadecyltrimethylammonium bromide(HTMAB) was dispersed in 150 mL GO aqueoussolution (∼0.5 mgmL−1) under stirring at 40 °C.Subsequently, 14 mL ethanol solution of SnCl2 (500,250 and 125 mg) was dropwised into the solution, andthen further stirred at 40 °C for 2 h. Finally, theobtained products were collected by centrifugationand repeat washingwith deionizedwater several times,and dried in an oven at 100 °C for 12 h. The resultantpowders were denoted as SAE SnO2/G-6.7, SAE SnO2/G-3.3 and SAE SnO2/G-1.7, respectively, according tothemass ratio of SnCl2 andGO.

2.3. Preparation of SnO2 nanoparticles decoratedgraphene (SnO2/G)For comparison, SnO2/G was prepared under thesame condition as SAE SnO2/G (3.3) except for theabsence ofHTMAB, and denoted as SnO2/G-3.3.

2.4. CharacterizationField-emission scanning electron microscopy(FESEM) images were obtained using a JEOL JSM-6700 F electron microscope. Transmission electronmicroscopy (TEM) observations were carried out on aJEOL JEM-2010 high resolution transmission electronmicroscope (operated at 200 kV). X-ray photoelectronspectroscopy (XPS) analysis was conducted on aPerkin-Elmermodel PHI 5600 XPS system using an AlKα (1486.6 eV) x-ray source. The crystal phase of theproducts was characterized by powder x-ray diffrac-tion (XRD) with Cu Kα radiation (λ= 1.54184 Å).Thermogravimetric analysis (TGA) was carried out ona Shimadzu DTG-60 H instrument at a heating rate of10 °Cmin−1 in air. Raman spectra were recorded on aWITEC CRM200 Raman system with 457 nm excita-tion laser.

2.5. Electrochemical characterizationsElectrochemical tests were measured using coin-typecells (CR 2032) assembled in an argon-filled glove boxwith Li foil as the counter and reference electrode, 1 MLiPF6 in a 50:50 (w/w) mixture of ethylene carbonateanddimethyl carbonate as the electrolyte. Theworkingelectrodes were prepared by a slurry coating proce-dure. Typically, active material, acetylene black andpolyvinylidene fluoride, were mixed at a weight ratioof 80:10:10 in N-methyl-2-pyrrolidinone solvent to

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form slurry. The slurry was then uniformly spreadonto a copper foil current collector and dried in avacuum oven at 100 °C for 12 h. Circular electrodeswith a diameter of 1.2 cm were punched out using anelectrode punch. The mass of the active material oneach electrode was 1.2–1.5 mg. Galvanostatic charge-discharge tests were carried out on a NEWARE batterytesting system. Cyclic voltammogram (CV) and elec-trochemical impedance spectroscopy (EIS) measure-ments were performed on CHI 760D electrochemicalworkstation.

3. Results and discussion

The fabrication of SnO2/G and SAE SnO2/G compo-sites is schematically illustrated in scheme 1. Briefly,the composites were prepared via redox reactionbetween GO and Sn2+, which results in the reductionof GO into graphene and the oxidation of Sn2+ toSnO2. Normally, the preparation of SnO2/G compo-site by Sn2+-induced reduction is conducted bymixingGO dispersion with the Sn2+ ions [30]. Due to theelectrostatic forces, Sn2+ ions are captured by theoxygen-containing functional groups onGO, and thenthe redox reaction occurs. However, the randomlydistributed oxygen-containing functional groups leadsto the generation of multiple nucleation sites and thusinhomogeneous distribution of SnO2 nanoparticles,yielding unsatisfied electrochemical performance ofSnO2/G [31]. The improvedmethodweproposed hereis to introduce a cationic surfactant HTMAB. In thisprocess, the positively charged hexadecyltrimethylam-monium ions of HTMABwould be firstly absorbed onthe basal plane of GO, forming lamellarmicelles with asandwich-like structure. The close-packed micellesnot only serve as homogeneous nucleation sites forSnO2 nanoparticles formation, but also prevent theirfurther growth or agglomeration, resulting in a

uniform distribution of SnO2 nanoparticles loaded ongraphene sheets. In addition, the random aggregationof graphene sheets ensures the formation of SAESnO2/G and hence excellent Li-storage properties. Inaddition, this method is more efficient and effectivethan conventional solution-based synthesis method[22, 23], as high-temperature annealing treatment forphase-purity/crystallinity is not required.

The morphologies of the as-prepared SnO2/G andSAE SnO2/G samples were investigated by FESEM andTEM. Figures 1(a), 2(a) and (b) reveal that the SnO2/G-3.3 composite presents a large number of SnO2

nanoparticles on graphene layers with the size varyingfrom several nanometers to dozens of nanometers. Asdiscussed above, the wide particle-size distributionrange is mainly due to the multiple nucleation sites forSnO2 nanoparticles formation. After the introductionof surfactant, the aggregation of SnO2 nanoparticlesdoes not appear in the SAE SnO2/G-1.7 and SAESnO2/G-3.3 composites (figures 1(b) and (c)), thusonly randomly aggregated graphene sheets areobserved. However, a large excess of Sn2+ precursor inthe composite (SAE SnO2/G-6.7) will induce furthergrowth or agglomeration of SnO2 nanoparticles(figure 1(d)). TEM and HRTEM images (figures 2(c)and (d)) disclose the homogenous distribution ofSnO2 nanoparticles on the graphene layers with a uni-form size of∼5 nm in the SAE SnO2/G-3.3 composite,demonstrating the importance of the introduced sur-factant for the formation of SnO2 nanoparticles.

The SnO2/G and SAE SnO2/G composites werefurther analyzed by XPS, XRD and Raman spectro-scopy to investigate their structural characteristics.From the XPS survey spectra (figure 3(a)), C, O and Snare clearly detected in the composites. Meanwhile, theSn 3d spectra of the composites (figure 3(b)) show twosymmetric peaks at about 487.6 and 496.0 eV, corre-sponding to the Sn 3d5/2 and Sn 3d3/2 peaks, respec-tively [32]. The energy splitting between the two peaks

Scheme 1. Schematic illustration of the fabrication of SnO2/G and SAE SnO2/G composites.

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Figure 1. SEM images of SnO2/G-3.3 (a), SAE SnO2/G-1.7 (b), SAE SnO2/G-3.3 (c) and SAE SnO2/G-6.7 (d) composites.

Figure 2.TEM images of SnO2/G-3.3 composite (a) and (b). TEM image (c) andHRTEM image (d) of SAE SnO2/G-3.3 composite.

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is 8.4 eV, indicating the formation of SnO2 nano-particles [30, 33]. Figure 3(c) shows the XRD patternsof samples. GO exhibits a characteristic diffractionpeak at 2θ= 11° due to the intercalation of oxygen-containing functional groups between graphenesheets. The diffraction peaks of the composites at2θ= 26.3°, 33.4°, 51.9° and 65.5° can be successivelyassigned to the (110), (101), (211) and (301) planes ofthe SnO2 phase (JCPDS No. 41–1445) [34], furtherconfirming the formation of SnO2 nanoparticles. Thesignificant structural changes of GO after the redoxprocess were detected by Raman spectroscopy. Asshown in figure S1 (available in the supplementarydata), the Raman spectrum of GO contains both a Dband at 1356 cm−1 (k-point phonons of A1g sym-metry) and a G band at 1584 cm−1 (E2g phonon of sp2

atoms), with a ID/IG ratio of 0.84 [2, 18]. Notably, thefrequencies of the D and G bands in the composites donot show any relative shifts in comparison with thatobserved in GO. However, an increased ID/IG ratio isobserved in the composites, which is attributed to theformation of numerous small sp2 domains in gra-phene sheets after Sn2+ induced reduction [35, 36].

To determine the content of graphene in the com-posites, TGA was carried out in air. As shown in the

TGA curves (figure 3(d)), the initial mass loss of thesamples below 100 °C is due to the loss of adsorbedwater. A rapid mass loss can be observed with theincrease of the temperature from 100 to 700 °C, whichis assigned to the combustion of graphene in air (yield-ing CO2) [19, 37]. The content of graphene in thesecomposites is calculated to be 28%, 51%, 24% and18% for SnO2/G-3.3, SAE SnO2/G-1.7, SAE SnO2/G-3.3 and SAE SnO2/G-6.7, respectively.

Electrochemical Li storage properties of the com-posites were evaluated by using CR2032 type coincells. Figure 4 shows the first three CV curves for theSnO2/G and SAE SnO2/G electrodes at a scan rate of0.5 mV s−1. The cathodic peak at ∼0.9 V in the firstcycle is due to the formation of solid electrolyte inter-phase (SEI) and reduction of SnO2 to Sn [20]. Thispeak disappeared in the following cycles, and a newreversible peak at∼1 V is observed, indicating the con-version reaction of SnO2 [28]. While the peak at∼0.02 V can be ascribed to the alloying reaction of Snwith Li to form various LixSn alloys [38]. In the anodicsweep, two peaks at∼0.6 and∼1.3 V are observed. Thefirst peak corresponds to the dealloying reaction ofLixSn alloys, whereas the second peak represents thepartially reversible reaction of SnO2 with Li [39, 40].

Figure 3. (a) XPS survey spectra of SnO2/G and SAE SnO2/G composites. (b) XPS Sn 3d spectra of SnO2/G and SAE SnO2/Gcomposites. (c) XRDpatterns ofGO, SnO2/G and SAE SnO2/G composites. (d) TGA curves ofGO, SnO2/G and SAE SnO2/Gcomposites.

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After the first cycle, the CV curves of the compositesare almost overlapped, implying the good reversibilityof the electrochemical reactions on the electrodes.

Figure 5 shows the galvanostatic charge-dischargeprofiles of the SnO2/G and SAE SnO2/G electrodes at acurrent density of 100 mA g−1. The capacity valueswere calculated based on the mass of the composite.Notably, the first discharge capacity for the SnO2/G-3.3, SAE SnO2/G-1.7, SAE SnO2/G-3.3 and SAE SnO2/G-6.7 electrodes is 1779, 1755, 2033 and1821 mAh g−1, corresponding to the coulombic effi-ciency of 62%, 47%, 56% and 59%, respectively. Theinitial irreversible capacity loss is attributed to the SEIformation and irreversible reactions on the electrodes.However, after the first cycle, the electrochemicalreactions become more and more reversible uponcycling, leading to the significantly increased cou-lombic efficiency (figure S2). This is highly consistentwith theCV results.

The cycling performance of SnO2/G and SAESnO2/G electrodes was investigated at a current rate of100 mA g−1, as shown in figure 6(a). It can be seen thatthe SnO2/G-3.3 electrode delivers a high initial rever-sible capacity of 1110 mAh g−1. However, a rapidcapacity fading is observed from the 30th cyclesbecause of the severe pulverization of the electrodecaused by the significant volume change during the

charge and discharge process. The charge capacity ofSnO2/G-3.3 electrode is only 582 mAh g−1 after 120cycles, which is 52.4% retention of the reversible capa-city. The poor cycling stability of the SnO2/G-3.3 elec-trode might be attributed to the inhomogeneousdistribution of the SnO2 nanoparticles on graphenesheets, as is observed by previous SEM and TEM ima-ges. Thus, the graphene matrix cannot effectively buf-fer the associated volume changes. In particular, theSAE SnO2/G electrodes exhibit greatly improved cyclicstability over that of the SnO2/G-3.3. It is worth notingthat the electrochemical performances of SAE SnO2/Gcorrelate closely to the content of graphene in thecomposite. The SAE SnO2/G-3.3 electrode with a gra-phene loading content of 24% displays the best Li sto-rage performance in terms of high reversible capacityand good cycling stability. After 120 cycles, the SAESnO2/G-3.3 electrode can sustain a high reversiblecapacity of 998 mAh g−1, 87.5% retention of the initialreversible capacity, which suggests its good cycling sta-bility. The gradual increase of capacity from the 10thto 60th cycles could be attributed to the improvedaccess of Li ions into the electrode, leading to anincreased accommodation behavior for Li [41, 42].Further increasing the graphene content in the com-posite (SAE SnO2/G-1.7) will result in low Li storage

Figure 4.CV curves of the first three cycles of the SnO2/G-3.3 (a), SAE SnO2/G-1.7 (b), SAE SnO2/G-3.3 (c) and SAE SnO2/G-6.7 (d)electrodes at a scan rate of 0.5 mV s−1.

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capacity, while decreasing the graphene content (SAESnO2/G-6.7) will lead to poor cycling stability.

The rate capability of the SnO2/G and SAE SnO2/Gelectrodes was evaluated at stepwise current densities(figure 6(b)). As the current densities increase from100 to 200, 400, 800, 1600 and 3200 mA g−1, the SAESnO2/G-3.3 electrode still exhibits stable capacities of1144, 1078, 1073, 1032, 910 and 682 mAh g−1, respec-tively. Moreover, with the current density returns to

100 mA g−1, the capacity of the electrode is able torecover to its initial value, indicating its excellent ratecapability. As far as we know, such superior Li storageperformance is rarely reported in the SnO2/G compo-sites, as summarized in table S1. Remarkably, the SAESnO2/G-3.3 electrode always delivers higher capacitiesthan the other electrodes at various current densities(also shown in figure S3). EISmeasurements (figure 7)illustrate that the charge transfer resistance of SAE

Figure 5.Galvanostatic charge-discharge profiles of SnO2/G-3.3 (a), SAE SnO2/G-1.7 (b), SAE SnO2/G-3.3 (c) and SAE SnO2/G-6.7(d) electrodes at a current density of 100 mA g−1.

Figure 6. (a) Cycling performance of the SnO2/G and SAE SnO2/G electrodes at a current density of 100 mA g−1. (b) Rate capability ofthe SnO2/G and SAE SnO2/G electrodes at various current densities.

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SnO2/G-3.3 electrode is 37Ω, which is much smallerthan the SnO2/G-3.3 (79Ω), SAE SnO2/G-1.7 (54Ω)and SAE SnO2/G-6.7 (111Ω) electrodes.

The excellent electrochemical performances of theSAE SnO2/G-3.3 composite can be attributed to itsunique structural features. First of all, the intrinsicactivity of the SnO2 nanoparticles component in thecomposite endows high Li storage capacity by provid-ing large interfacial areas for fast Li insertion/extrac-tion within the electrode, and shortening the solid-state diffusion length of Li ions. In addition, the uni-form distribution of SnO2 nanoparticles on graphenesheets can effectively prevent the agglomeration Snparticles formed during Li insertion. Secondly, thegraphene component in the composite can act as notonly continuous channels for electrons transport butalso Li storage material, which additionally con-tributes to its exceptional performance. More impor-tantly, the random aggregation of graphene sheetsresults in the encapsulation of SnO2 nanoparticles ingraphene layers, producing a mechanically robuststructure. Therefore, the graphene sheets can functionas a 3D scaffold for preventing the SnO2 nanoparticlesbeing peeled off and buffering the associated volumechanges.

4. Conclusions

In summary, we have developed a facile and effectiveapproach to synthesize SnO2/G composite through asurfactant-assisted redox method. This composite iscomposed of uniform SnO2 nanoparticles with a sizeof ∼5 nm, which are homogeneously encapsulated inthe graphene layers. Benefiting from the uniquestructural features, the composite exhibits high rever-sible capacity (1141 mAh g−1 at a current density of100 mA g−1) and long-term cycling stability (120cycles with 87.5% retention), as well as excellent rate

capability when applied as the anodematerial for LIBs.Considering the simple, low cost and high throughputfabrication process of this approach, we believe thecomposite may be a promising alternative anodematerial for next generation high-performance LIBs.Moreover, our strategy could also be helpful infabricating other graphene-based composites fordiverse applications.

Acknowledgments

This work is supported by the Singapore NationalResearch Foundation under NRF RF Award No.NRFRF2010-07, A*Star SERC PSF grant 1321202101and MOE Tier 2 MOE2012-T2-2-049. WH givesthanks for the support by the Natural Science Founda-tion of Jiangsu Province (BM2012010), Priority Aca-demic Program Development of Jiangsu HigherEducation Institutions (YX03001), Ministry of Educa-tion of China (IRT1148), Synergetic Innovation Cen-ter for Organic Electronics and Information Displays,and theNationalNatural Science Foundation of China(61136003, 51173081).

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