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Electrochimica Acta 116 (2014) 103– 110

Contents lists available at ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

orous SnO2@C/graphene nanocomposite with 3D carbon conductiveetwork as a superior anode material for lithium-ion batteries

eichao Liana,∗, Jingyi Wanga, Dandan Caib, Liangxin Dingb,ingming Jiaa, Haihui Wangb,∗∗

Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, ChinaSchool of Chemistry & Chemical Engineering, South China University of Technology, Guangzhou 510640, China

r t i c l e i n f o

rticle history:eceived 7 October 2013eceived in revised form 1 November 2013ccepted 1 November 2013vailable online 15 November 2013

eywords:raphene sheetsnO2

arbon shellanocomposite

a b s t r a c t

Porous nano-sized SnO2@C/graphene electrode material with three-dimensional carbon conductive net-work was designed and prepared. The carbon shell was introduced to suppress the aggregation ofnanoparticles and undesired reactions. The excellent electronic conductivity can be guaranteed by a 3Dcarbon conductive network consisted of graphene sheets and carbon shell. The porous structure can facil-itate liquid electrolyte diffusion into the bulk materials. As a result, the as-prepared SnO2@C/graphenenanocomposite as an anode material for lithium-ion batteries exhibits high reversible specific capacity,outstanding cyclability and good rate capability. The first reversible specific capacity is as high as 1115mAh g−1 at a specific current of 100 mA g−1. After 100 cycles at different specific currents from 100 to1000 mA g−1, the reversible specific capacity was still maintained at 1015 mAh g−1 at the specific currentof 100 mA g−1. Even at the high specific current of 1000 mA g−1, the reversible specific capacity is still

−1

ithium-ion batteries as high as 499 mAh g , higher than the theoretical specific capacity of the commonly used graphiteanode material (372 mAh g−1). The results give the clear evidence that the electrochemical performanceof graphene-based electrode materials can be improved by designing proper structure. The preparationapproach of porous SnO2@C/graphene nanocomposite reported in this paper may also be applied tofabricate other porous metal oxide@C/graphene electrode materials for high performance lithium-ion batteries.

. Introduction

Nano-sized electrode materials for lithium-ion batteries havettracted much attention because they have high reactivity, shortath lengths for electronic and Li+ transport, which is beneficialor achieving high reversible capacity and good rate capability1–3]. However, the large electrolyte/electrode interface arisingrom their nano-size leads to more undesired reactions involv-ng electrolyte decomposition, which result in a high level ofrreversibility (i.e., low columbic efficiency) and poor cycling per-ormance [1–3]. Moreover, their application is also hindered byhe poor electrochemical stability arising from the aggregation ofannoparticles during Li insertion/extraction process (Fig. 1a) [3].

o circumvent these problems, nano-composite electrode materi-ls were developed [4–11]. Among the reported nanocomposites,arbon-based nanocomposites are the most promising electrode

∗ Corresponding author: Tel.: +8687165920242.∗∗ Corresponding author. Tel.: +86 20 87110131.

E-mail addresses: lianpeichao@126.com (P. Lian), hhwang@scut.edu.cnH. Wang).

013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.11.007

© 2013 Elsevier Ltd. All rights reserved.

materials because the carbon can not only prevent the nanopar-ticles from aggregating to large particles but also increase theelectronic conductivity of the composites due to its good electronicconductivity [4,5,7,10]. Graphene, a single-atom-thick sheet ofhoneycomb carbon lattice, exhibits a number of intriguing uniqueproperties, such as superior electronic conductivity, high surfacearea (theoretical value 2620 m2 g−1), large surface-to-volume ratio,good mechanical properties [12,13]. Therefore, graphene is an idealcarbon nanostructure which can be used to design high perfor-mance electrode materials. So far, important progress has beenmade in the design and synthesis of graphene-based electrodematerials. A variety of metal oxide/graphene nanocomposites havebeen explored as anode materials for lithium-ion batteries, such asSnO2/graphene [14–23], Co3O4/graphene [24–26], Fe3O4/graphene[27–30], TiO2/graphene [31,32], Fe3O4/SnO2/graphene [33], etc.Although the graphene sheets in these reported nanocompositescan act as a barrier to prevent the aggregation of nanoparticles toa certain extent [16,30,33], the nanoparticles on the same piece

of graphene still tend to aggregate into large particles (Fig. 1b). Inaddition, side reactions involving electrolyte decomposition can becaused by the catalytic activity of nanoparticles due to the directcontact of the active material with the electrolyte [3]. Therefore,

104 P. Lian et al. / Electrochimica Acta 116 (2014) 103– 110

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ig. 1. Schematic illustration of (a) bare nannoparticles, (b) aggregated nanopartanocomposite with 3D carbon conductive network.

t’s very possible to improve the electrochemical performance ofraphene-based electrode materials by designing proper structure34–37].

In this study, porous nano-sized SnO2@C/graphene electrodeaterial with three-dimensional (3D) electronic conductive net-ork was designed and prepared (Fig. 1c). The carbon shell was

ntroduced to suppress the aggregation of nanoparticles and unde-ired reactions [4,10]. The excellent electronic conductivity cane guaranteed by a 3D carbon conductive network consisted ofraphene sheets and carbon shell. The porous structure can facili-ate liquid electrolyte diffusion into the bulk materials. Therefore,t could be expected that the porous nano-sized SnO2@C/graphenelectrode material possess perfect cycling performance and goodate capability.

. Experimental

All chemicals were of analytical grade and used as received,ithout further purification.

n graphene sheets, and (c) the synthesis route for the porous SnO2@C/graphene

2.1. Preparation of SnO2 nanoparticles

SnO2 nanoparticles were prepared by using a facile gas-liquidinterfacial synthesis approach, as reported in our previous paper[16]. Briefly, in a 15-mL beaker, 0.3647 g of SnCl4·5H2O (TianjinKermel Chemical Reagent Co., Ltd., China) was dissolved in 10 mLof ethylene glycol (EG) (Beijing Chemical Reagent Co., Ltd., China).Then the beaker was placed into a 100-mL Teflon-lined autoclavethat contained 14 mL of ammonia solution (Guangzhou ChemicalReagent Factory, China). Then the autoclave was sealed and placedin a drying oven preheated to 180 ◦C and held at this temperaturefor 12 h. After cooling and centrifugation, washing with ethanol(Beijing Chemical Reagent Co., Ltd., China) for three times, thecollected solid product (SnO2 nanoparticles) was dispersed in25 mL of ethanol by ultrasonication. The obtained suspension wasnamed SnO2-EG.

2.2. Synthesis of graphene oxide

Graphene oxide (GO) was prepared by a modified Hummersmethod [37]. In a typical reaction, 2 g of graphite powder with

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he average particle size 30 �m, 2 g of sodium nitrate (Tianjinu Chen Chemical Reagent Factory, China) and 100 mL of con-entrated sulfuric acid (Beijing Chemical Reagent Co., Ltd., China)ere mixed and strongly stirred for 15 min in a 1000 mL reactionask immersed in an ice bath. Then 12 g of potassium perman-anate (Guangzhou Chemical Reagent Factory, China) was slowlydded to the above solution and cooled for 30 min in the ice bath.fter, the ice bath was removed and the suspension was continu-usly stirred for 48 h, and then 184 mL of pure water was slowlydded to the suspension in 15 min. Subsequently, the suspensionas further treated with 560 mL of warm water and 40 mL of2O2 (30%, Guangzhou Chemical Reagent Factory, China) to reduce

esidual permanganate and manganese dioxide to colorless solubleanganese sulfate. The mixture was centrifuged at 8000 rpm andashed with a mixed aqueous solution of 6wt% H2SO4/1wt% H2O2,

hen washed with water. Finally, the residual impurity ions wereemoved through dialysis. The prepared graphite oxide was dis-ersed in deionized water by ultrasonic treatment for 1 h to obtain000 mL GO dispersion.

.3. Synthesis of SnO2@C/graphene nanocomposite

The fabrication process of the porous SnO2@C/grapheneanocomposite with 3D carbon conductive network is demon-trated in Fig. 1c. First, SnO2 nanoparticles were coated withlucose-derived carbon-rich polysaccharide (GCP) by a simpleydrothermal approach [39,40]. Briefly, 1 g of glucose was dis-olved in 40 mL deionized water, and 20 mL of SnO2-EG were addedo the glucose aqueous solution under gentle stir, then the result-ng suspension was transferred to a 100-mL Teflon-lined autoclave.he autoclave was sealed and placed into a drying oven preheatedo 180 ◦C and held at this temperature for 12 h. After cooling andentrifugation, washing with deionized water for three times, theollected product (carbon precursor coated SnO2 nanoparticles)as added to 100 mL GO dispersion and sonicated for 6 h to yield

homogeneous suspension. Subsequently, the suspension wasransferred to a 100-mL Teflon-lined autoclave. Then the autoclaveas sealed and placed into a drying oven preheated to 185 ◦C andeld at this temperature for 12 h. Finally, the as-obtained prod-ct was freeze-dried overnight, followed by thermal treatmentt 500 ◦C for 4 h under argon atmosphere to obtain the porousano-sized SnO2@C/graphene electrode material with 3D carbononductive network.

.4. Materials Characterization

The structure and morphology of the as-prepared SnO2anoparticles, carbon precursor coated SnO2 nanoparticles,nO2@C/graphene precursor, and SnO2@C/graphene nanocom-osite was characterized by X-ray diffraction (XRD, Bruker D8dvance), Fourier transform infrared spectroscopy (FTIR, Brukerector 33) and transmission electron microscopy (TEM, FEI, Tec-ai G2 F30 S-Twin). N2 adsorption/desorption isotherms wereeasured using a Micromeritics ASAP 2010 (USA) analyzer at liq-

id nitrogen temperature. Elemental analysis was carried out onario EL III elementar (Germany) by burning the SnO2@C/grapheneanocomposite to form carbon dioxide. The content of carbon innO2@C/graphene nanocomposite was calculated according to theass of carbon dioxide. The elemental analysis indicates that the

arbon content of the SnO2@C/graphene nanocomposite is about8.6%.

.5. Electrochemical Measurements

The electrochemical experiments were carried out usinghe coin-type cells. The working electrode was prepared by

Fig. 2. XRD patterns of the as-synthesized (a) SnO2 nanoparticles, (b) GCP coatedSnO2 nanoparticles, and (c) SnO2@C/graphene nanocomposite.

mixing active material (SnO2@C/graphene nanocomposite) withpoly(vinylidene fluoride) (PVDF) and Super P carbon at a weightratio of 75:10:15 in N-methyl-2-pyrrolidone (NMP) to form slurry.Then, the resultant slurry was uniformly pasted on Cu foil with ablade, dried at 120 ◦C in a vacuum oven and pressed under a pres-sure of 20 MPa. The active material loading density of the electrodeis ca. 1.0 mg cm−2. The Celgard 2325 microporous membrane wasused as separator. The electrolyte was 1 mol L−1 LiPF6 dissolvedin a mixture of ethylene carbonate (EC) and dimethyl carbonate(DMC) (1:1 by volume). The lithium sheet was used as both counterand reference electrode. CR2025-type coin cells were assembledin an argon-filled glove box and galvanostatically discharged andcharged in a voltage range from 3.0 to 0.01 V using a Battery Test-ing System (Neware Electronic Co., China). Cyclic voltammetry (CV)test was carried out on an electrochemical workstation (ZahnerIM6ex) over the potential range 0.01-3.0 V versus Li/Li+ at a scan-ning rate of 0.2 mV s−1.

3. Results and discussion

3.1. Microstructural characterization

Fig. 1c shows the schematic illustration of the synthesis routefor the porous SnO2@C/graphene nanocomposite. First, SnO2nanoparticles were prepared by using a facile gas-liquid inter-facial synthesis approach. The XRD pattern of the as-preparedSnO2 nanoparticles is shown in Fig. 2a. The diffraction peaks ofSnO2 nanoparticles are clearly distinguishable. All strong diffrac-tion peaks can be indexed to the standard tetragonal SnO2 phase,indicating crystalline SnO2 nanoparticles were formed during thegas-liquid interfacial reaction process. The broad diffraction peakssuggest that the SnO2 nanoparticles are very small in size, whichcan be further confirmed by the TEM image. As shown in Fig. 3a, theTEM image clearly revealed that the particle size of SnO2 nanopar-ticles is less than 6 nm. Both the structure and morphology are inagreement with those reported in the literature [16].

Secondly, the as-prepared SnO2 nanoparticles were coated withglucose-derived carbon-rich polysaccharide (GCP) by a simplehydrothermal approach. Fig. 2b shows the XRD pattern of the GCPcoated SnO2 nanoparticles. The diffraction peaks of GCP coated

SnO2 nanoparticles are clearly distinguishable. All strong diffrac-tion peaks can be indexed to the standard tetragonal SnO2 phase,and no characteristic peaks of impurities, such as elemental Snor other tin oxides, were observed. The XRD result indicates that

106 P. Lian et al. / Electrochimica Acta 116 (2014) 103– 110

SnO2

StncSbc

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Fig. 3. TEM images of (a) SnO2 nanoparticles, (b) GCP coated

nO2 can not be reduced by glucose under hydrothermal condi-ion. Fig. 3b shows typical TEM image of the GCP coated SnO2anoparticles, which clearly indicates that the SnO2 nanoparti-les were coated successfully. The FTIR spectra of the GCP coated

nO2 nanoparticles is shown in Fig. 4a. A broad absorption bandetween 3000 and 3700 cm−1 corresponds to O-H (hydroxyl orarboxyl) stretching vibration [41,42]. The band at approximately

5001000150020002500300035004000

(c)

(b)

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Wavenumber / cm-1

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ig. 4. FTIR spectra of (a) GCP coated SnO2 nanoparticles, (b) SnO2@C/graphenerecursor, (c) graphene oxide, (d) graphene sheets derived from splitting graphenexide, and (e) SnO2@C/graphene nanocomposite.

nanoparticles, and (c, d) SnO2@C/graphene nanocomposite.

1624 cm−1 can be attributed to C = C stretching vibration [41,42],whereas the weak bands in the 1000-1450 cm−1 region corre-spond to C-O (hydroxyl, ester, or ether) stretching and O-H bendingvibrations [42]. The weak band at 893 cm−1 is attributed to aro-matic C-H out-of-plane bending vibration [41,42], and the bandat 571 cm−1 is associated with the vibration of the Sn-OH terminalbonds [14]. Obviously, the GCP coated SnO2 nanoparticles should behydrophilic due to the carbon precursor coating layer with oxygen-containing groups including hydroxyl. Therefore, the GCP coatedSnO2 nanoparticles could be easily dispersed in GO dispersion toform homogeneous suspension. This is very important to realizethe hydrothermal self-assembly of GCP coated SnO2 and GO sheetsin the following step.

Thirdly, the GCP coated SnO2 nanoparticles were dispersedin GO dispersion by sonication to yield a homogeneous suspen-sion. Then GCP coated SnO2 and GO sheets were hydrothermallyassembled at 185 ◦C for 12 h to form 3D carbon precursor network[43,44]. It should be pointed out that both GO sheets and the car-bon precursor coating layer of the GCP coated SnO2 nanoparticleshave oxygen-containing functional groups [38], which can reactwith each other to form a 3D carbon precursor network underhydrothermal condition. After hydrothermal self-assembly andfreeze-drying, the obtained product was named SnO2@C/grapheneprecursor. Fig. 4a, b and c show the FTIR spectra of GCP coated SnO2,SnO2@C/graphene precursor and graphene oxide, respectively. Theabsorption band between 3000 and 3700 cm−1 corresponds to O-

H (hydroxyl or carboxyl) stretching vibration in the FTIR spectraof SnO2@C/graphene precursor is weaker than those of grapheneoxide and GCP coated SnO2, which indicates some O-H disappearedafter hydrothermal self-assembly. The FTIR result gives the direct

ica Acta 116 (2014) 103– 110 107

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vidence that the oxygen-containing functional groups on the GOheets and carbon precursor coating layer of the GCP coated SnO2anoparticles can react with each other during the hydrothermalelf-assembly process.

Finally, the SnO2@C/graphene precursor was heat-treated at00 ◦C for 4 h under argon atmosphere to carbonize the 3D car-on precursor network and obtain the SnO2@C/graphene electrodeaterial with 3D carbon conductive network. Fig. 2c shows theRD pattern of the as-obtained SnO2@C/graphene nanocomposite.he XRD pattern of the SnO2@C/graphene nanocomposite matchesell with that of tetragonal SnO2 (JCPDS File Card No.41-1445),

nd no characteristic peaks of impurities, such as elemental Sn orther tin oxides, were observed, which indicates that SnO2 can note reduced by carbon precursor at 500 ◦C. The broad diffractioneaks suggest that the SnO2 nanoparticles in the SnO2@C/grapheneanocomposite are very small in size. Fig. 4d and e show the FTIRpectra of the graphene sheets derived from splitting graphenexide and SnO2@C/graphene nanocomposite [38]. The absorptionands in the 900-3700 cm−1 region are consistent with those ofraphene sheets derived from splitting graphene oxide, which indi-ates the GO sheets in the 3D carbon precursor network wereeduced to graphene sheets during the heat-treatment process. Theeak bands at 537 and 650 cm−1 can be attributed to the vibra-

ions of the SnO2 [14], which further indicates that SnO2 exists inhe obtained SnO2@C/graphene nanocomposite. Fig. 3c and d showhe typical TEM images of the SnO2@C/graphene nanocomposite,hich clearly indicate that the obtained product has a 3D carbon

onductive network consisted of graphene sheets and carbon shell.he particle size of SnO2 nanoparticles in the SnO2@C/grapheneanocomposite is similar to that of initial SnO2 nanoparticles,hich is also less than 6 nm. Moreover, it can be clearly observed

hat the SnO2 nanoparticles distributed in the 3D carbon con-uctive network homogeneously, there is almost no aggregatednO2 particles in the final SnO2@C/graphene nanocomposite. The2 adsorption-desorption isotherms and pore size distribution of

he final SnO2@C/graphene nanocomposite are shown in Fig. 5.he nitrogen adsorption-desorption isotherms present obviousressure hysteresis, which indicates that the SnO2@C/grapheneanocomposite is porous [4,14,41]. The Brunauer–Emmett–TellerBET) surface area of the SnO2@C/graphene nanocomposite is 196.4

2 g−1, and the total pore volume is 0.15 cm3 g−1. It should beoted that some nanopores with pore sizes smaller than 1 nmxist in the SnO2@C/graphene nanocomposite, which can be ofreat benefit to the reversible specific capacity because lithium cane stored in such nanopores [3,30]. The XRD, FTIR, TEM and BETesults clearly indicate that the designed porous SnO2@C/grapheneanocomposite with 3D carbon conductive network was prepareduccessfully.

.2. Electrochemical properties of the porous SnO2@C/grapheneanocomposite with 3D carbon conductive network

The electrochemical properties of the porous SnO2@C/grapheneanocomposite with 3D carbon conductive network were inves-igated using coin-type cells. The first 50 discharge/charge cyclesere tested at a specific current of 100 mA g−1, and the first

our discharge/charge curves are shown in Fig. 6a. The shape ofhe discharge/charge curves is generally consistent with thoseeported previously for SnO2/graphene nanocomposites [14,15,18],ndicating the same electrochemical reaction pathway. The firstischarge and charge specific capacities of the SnO2@C/graphene

anocomposite are as high as 2055 and 1115 mAh g−1, respec-ively. The initial coulombic efficiency calculated based on the firstischarge/charge specific capacities is 54%. The large irreversibleapacity during the first cycle can be attributed to the irreversible

Fig. 5. (a) Nitrogen adsorption-desorption isotherms of the SnO2@C/graphenenanocomposite, (b) Porosity distribution by Original Density Functional TheoryModel.

lithium loss due to the formation of thick solid-electrolyte inter-face (SEI) layer on the electrode surface [3,14], and the factthat some Sn(0) cannot be re-oxidized back to Sn(IV) [16]. Thehigh initial reversible specific capacity should be related to thehigh reactivity of the nano-sized SnO2@C/graphene composite. Itshould be pointed out that not only SnO2 nanoparticles in theSnO2@C/graphene nanocomposite can react with lithium but alsothe 3D carbon network consisted of graphene sheets and carbonshell is an active material for additional Li+ storage [30,32,38],which is of great benefit to the reversible specific capacity. In addi-tion, the nanopores in the SnO2@C/graphene nanocomposite canstore some lithium, which also contributes to the high reversiblespecific capacity [3,30]. The reversible specific capacity in the sec-ond cycle reduced to 1026 mAh g−1, but the discharge/chargecurves in the second, third, and fourth cycles almost overlap, whichindicates the SnO2@C/graphene nanocomposite possesses stablecyclic performance from the second cycle because of the formedstable SEI film during the first discharge process. After two cycles,the coulombic efficiency of the SnO2@C/graphene nanocompos-ite increases to above 98% in the subsequent cycles, revealing anexcellent reversibility.

Fig. 6b presents the cyclic voltammograms of the porousSnO2@C/graphene nanocomposite with 3D carbon conductive net-work. In the cathodic polarization process of the first cycle, two

peaks were observed. The weak peak between 0.45 and 0.85 Vcould be ascribed to the formation of solid electrolyte interface(SEI) layer on the surface of active material, reduction of SnO2to Sn and the synchronous formation of Li2O [15,16], whereas

108 P. Lian et al. / Electrochimica Acta 116 (2014) 103– 110

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he strong peak at 0.02 V corresponds to alloying of Sn with Lind the reversible reaction between lithium and the 3D carbonetwork [15,16]. Meanwhile, in the anodic polarization process,hree peaks were recorded at about 0.24, 0.56 and 1.23 V. The peakt 0.24 V corresponds to lithium extraction from the 3D carbon net-ork [15], and the peak at 0.56 V represents the de-alloying process

f Li+ ions [16], while the following weak peak at 1.23 V could bescribed to the reaction between Sn and Li2O [15,16]. After the firstycle, the CV curves of the SnO2@C/graphene nanocomposite inhe subsequent three cycles almost overlap completely, revealingn excellent cyclic performance. This result matches well with thatf the discharge/charge tests.

The cycling performance of the porous SnO2@C/grapheneanocomposite with 3D carbon conductive network is shown

n Fig. 6c. The reversible specific capacity is almost constantrom the second cycle to the fiftieth cycle, exhibiting excellentycling stability. To the best of our knowledge, such excellentycling performance has rarely been reported for SnO2/grapheneanocomposite. The outstanding cycling performance of the porousnO2@C/graphene nanocomposite with 3D carbon conductiveetwork can be attributed to the following three factors. Firstly,he 3D carbon network consisted of graphene sheets and carbonhell can not only act as a barrier to prevent the aggre-

ation of SnO2 nanoparticles as shown in Fig. 3d, but alsouppress the aggregation of the in situ formed nanoparticlessuch as Sn and Li2O) during the discharge/charge process. Sec-ndly, the porous SnO2@C/graphene nanocomposite can provide

a scanning rate of 0.2 mV s−1, (c) Capacities versus cycle number between 0.01 andnanocomposite.

a void space, which help to accommodate the volume changesof SnO2 nanoparticles during Li+ insertion/extraction process.Finally, the carbon shell can avoid direct contact between SnO2nanoparticles and electrolyte to suppress the undesired reactions[3,4,10].

After 50 cycles, the same cell was further evaluated for ratecapability as shown in Fig. 6d. When the same cell was testedat various specific currents from 100 to 1000 mA g−1, the porousSnO2@C/graphene nanocomposite with 3D carbon conductive net-work exhibits good rate capability. At the high specific currentsof 300, 500 and 1000 mA g−1, the SnO2@C/graphene nanocompos-ite can still exhibit high reversible specific capacities of 801, 669and 499 mAh g−1, respectively. All of these values are higher thanthe theoretical specific capacity of the commonly used graphiteanode material (372 mAh g−1). The good rate capability of theporous SnO2@C/graphene nanocomposite with 3D carbon conduc-tive network can be related to its unique structure. The 3D carbonconductive network could increase the electronic conductivity ofthe nanocomposite and the porous structure can facilitate the liquidelectrolyte diffusion into the bulk materials. In addition, the nano-sized particles can shorten the path length for electronic and Li+

transport [1–3], which also favors the high rate capability. After 90cycles at various specific currents from 100 to 1000 mA g−1, when

−1

the specific current returns to initial 100 mA g , a high reversiblespecific capacity of 1015 mAh g−1 can be obtained in the 100thcycle, that is, about 99% retention of the reversible specific capacityin the second cycle.

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. Conclusions

In summary, porous nano-sized SnO2@C/graphene electrodeaterial with 3D carbon conductive network was designed and

repared. The as-prepared SnO2@C/graphene nanocomposite as annode material for lithium-ion batteries exhibits high reversiblepecific capacity, outstanding cyclability and good rate capability.he excellent electrochemical performances can be attributed tohe following reasons: (1) nanometer-sized materials have higheactivity, short path lengths for electronic and Li+ transport. (2)arbon shell can suppress the aggregation of nanoparticles andndesired reactions. (3) 3D carbon conductive network consistedf graphene sheets and carbon shell can guarantee the excel-ent electronic conductivity. (4) porous structure can facilitateiquid electrolyte diffusion into the bulk materials. The resultives the clear evidence that the electrochemical performance ofraphene-based electrode materials can be improved by design-ng proper structure. We believe that the strategy presentedn this work will pave the way for the practical application ofraphene-based electrode materials. The preparation approach oforous SnO2@C/graphene nanocomposite reported here may alsoe applied to fabricate other porous metal oxide@C/graphene elec-rode materials for high performance lithium-ion batteries.

cknowledgements

This work was financially supported by the National Scienceund for Distinguished Young Scholars of China (No. 21225625),cientific Research Start-up Fund of Kunming University of Sci-nce and Technology for Introducing Talents (KKSY201205133),earl River Scholar Program of Guangdong Province, and Educa-ion Department Science Foundation of Yunnan Province (Granto. 2012Y539).

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