Silicon

Directing Silicon?Graphene Self-Assembly as a Core/Shell Anode for High-Performance Lithium-Ion Batteries

Yuanhua Zhu,?,§Wen Liu,∥,§Xinyue Zhang,?Jinchao He,?Jitao Chen,*,∥Yapei Wang,*,?

and Tingbing Cao?

?Department of Chemistry,Renmin University of China,Beijing100872,P.R.China

∥College of Chemistry and Molecular Engineering,Peking University,Beijing100871,P.R.China

large volume change associated with the conversion reaction and

as-prepared GNS@Si NWs delivered a reversible capacity of1648

Moreover,capacity remained1335mAh·g?1after80cycles at a

electrochemical performance in terms of rate capability and cycling

INTRODUCTION

A critical demand for portable electronic devices has been rising sharply in recent years,in part because of the increasing global dependence on mobile information,entertainment,and communication.All of these devices are critically dependent on portable power sources.Lithium-ion batteries(LIBs)are one of the most popular types of rechargeable power sources in electronic devices.E?orts have been devoted to improving energy density,prolonging cycle life,stabilizing working voltage,and impairing memory e?ect for high-performance LIBs.1?7New anode materials,such as silicon(Si),sulfur(S8), titanium dioxide(TiO2),and tin oxide(SnO),8?11have been suggested.In terms of abundant amount in earth and high association ability with lithium ions,Si is attracting enormous attention for high-capacity anode materials.Its theoretical capacity of4200mAh·g?1(ca.Li4.4Si)is10times higher than the graphite(372mAh·g?1)which is currently used as an anode material.12?14However,the increase of lithium insertion leads a signi?cant volume expansion of Si,which causes pulverization of anodes and a rapid decrease in cycling stability.15,16Creating nanovoids or hollow structures to Si routinely alleviates the anode pulverization resulting from volume expansion,while they are technically challenging to be scaled up.17?25Another e?ective strategy is integrating carbon materials with nano-structural Si that addition of carbon materials can resist the volume expansion to some extent.26?28There are examples of hybrid Si/carbon composites,such as physical blends of Si nanowires(Si NWs)with graphene nanosheets(GNS),29 chemical vapor deposition(CVD)of Si onto carbon surface,30 calcination of carbon-rich polymers on Si nanowires.31The improvements of anode stability and energy density using hybrid structures encourage us to develop analogous structures for anode materials while using an easy process.

From a brief survey,graphene may serve as an excellent carbon source for preparing carbon/Si hybrid anodes with respect to its high mechanical strength,high chemical and thermal stability,and high electrical conductivity.32?37Direct blending of graphene with Si cannot form a distinct association between them.As such,the giant interstitial volume between carbon and Si still leaves too much space to con?ne fatal expansion of anodes.The promise that graphene can tailor Si expansion cannot be ful?lled without their intimate association. In this contribution,we present a versatile strategy for decorating graphite layers on Si nanowires.Graphene oxide (GO),having plenty of hydroxyl,epoxy,and carboxyl moieties,38was uniformly wrapped on positively charged Si nanowires by electrostatic self-assembling.The preassembled GO?Si NWs composite can be straightforward converted to reduced graphene nanosheets(GNS)@Si NWs composite by subsequent heat treatment.35,39The as-prepared GNS@Si NWs hybrid material was characterized and demonstrated improving electrochemical performance as anode for high-performance LIBs.In addition,this system integrates nanoscale in diameter and microscale in length.It renders a possibility to bridge the research among nano,micro,and macro,fundamentally leading to a roadmap for balancing high performance and high throughput of anode materials,Scheme1.

Received:November4,2012

Revised:December12,2012

Published:December26,2012

■

EXPERIMENTAL SECTION

Materials.Natural graphite powder (325mesh)was purchased from Qingdao HuaTai Lubricant Sealling S&T Co.Ltd.Silicon wafer was purchased from Zhejiang Kaihua Lijing Ltd.(3-Aminopropyl)-triethoxysilane (APTES)was bought from Alfa Aesar.Other reagents were purchased from TCI.All reagents described above were used as received without further puri ?cation.

Preparation of Silicon Nanowires.Si nanowires were obtained via an electroless etching process.40?42Commercial Si wafers (p-type,?100?oriented,0.3?10Ωcm)were cut to 4×4cm pieces.Si wafer was ultrasonicated in acetone and ethanol at room temperature for 10min to remove contamination from organic grease.Degreased Si substrate was heated in boiling Piranha solution (H 2SO 4/H 2O 23:1v/v)for 1h.Then,the wafer was dipped into HF (5%)solution to remove the native oxide layer.Subsequently,the Si substrate was rinsed several times with deionized water.The treated Si wafers were immediately dipped into an Ag coating solution containing 10%HF and 0.04M AgNO 3.The solution was slowly stirred for 1min under ambient air.After a thin uniform ?lm of Ag was deposited,wafers were washed with deionized water to remove extra Ag +and then immersed in an etchant composed of 4.6M HF and 0.5M H 2O 2.In this manner,Si NWs were formed on the Si wafer.After 240min of etching in the dark at room temperature,the wafer was washed by a HNO 3solution (30%)to remove excess Ag.Finally,the black wafer was washed 3times with deionized water and dried in the N 2atmosphere.

Preparation of NH 2-Terminated Silicon Nanowires.Pretreated Si wafers were cleaned in boiling Piranha solution (H 2SO 4/H 2O 23:1v/v)for 1h,rinsed thoroughly with deionized water,and dried.Then the OH-terminated Si wafers were immersed in a 1wt %toluene solution of APTES for 24h at room temperature,subsequently washed by THF and acetone three times,and dried in vacuum overnight.To remove the Si nanowires from the substrate,functionalized Si wafers were ultrasonicated in deionized water and free Si nanowires were collected from the solution.

Preparation of Silicon Nanowires Core/Graphene Shell Structure.GO nanosheets were prepared from natural graphite (325mesh,Qingdao)by a modi ?ed Hummers ’method.46,47The solution of APTES-functionalized Si NWs (1mg mL ?1)was stepwise dropped into a diluted (GO)dispersion (1mg mL ?1)with slight ultrasonication for 0.5h.The precipitate was freeze dried,followed by a thermal reduction process at 950°C in a furnace for 10h with a reductive atmosphere (95%N 2and 5%H 2).35Afterward,the graphene-wrapped Si NWs were soaked in 20%HF solution for 0.5h and dried in vacuum for 24h,which was thereafter called GNS@Si NWs.

Characterization.The morphology and nanostructure were examined on a JEOL JSM-7401F ?eld-emission scanning electron microscope (FESEM)and Hitachi T20transmission electron microscope (TEM).X-ray photoelectron spectroscopy (XPS)was carried out by a Kratos Axis Ultra spectrometer with Al K αmonochromatized X-ray source.

Galvanostatic charge and discharge cycling (LAND CT2001A,Wuhan Kinguo Electronics Co.,Ltd.)were performed in the potential window from 0.0005to 2.0V vs Li/Li +with a two-electrode 2032coin-type half cell,where Li metal foil was used as the counter electrode.The working electrode was prepared by mixing the GNS@Si NWs (80wt %),carbon black (10wt %),and PVDF (10wt %)with solvent (NMP).After coating the slurry onto Cu foils and drying for 1

h at 120°C,the electrode was punched into disks with a diameter of 10mm and pressed at 10MPa thereafter.The electrolyte was comprised of 1.0M LiPF 6in a mixture of ethylene carbonate (EC),dimethyl carbonate (DEC),and ethyl methyl carbonate (1:1:1,v/v)(LB315,Huarong Co.Ltd.).Celgard 2400was used as a separator.Cells were assembled in an Ar-?lled glovebox (Master 100Lab,Braun,Germany)with less than 1ppm of both oxygen and moisture.The cyclic voltammogram for GNS@Si NWs was measured from 2.0to 0V versus Li/Li +at a 1mV s ?1scan rate at an electrochemistry workstation (CHI660D,CH Instruments).

■

RESULTS AND DISCUSSION

Generation of Si ?graphene hybrid with a core/shell structure was separated into three steps.First,Si-based nanowires were prepared and their surfaces modi ?ed with primary amines that endowed nanowires with positive charge.Second,negative-charged GO with carboxyl and hydroxyl groups was obtained via oxidation of natural graphite.Third,direct self-assembly based on electrostatic interaction was undergone between Si nanowires and GO nanosheets.

There are several ways to produce Si nanowires:top-down approach and bottom-up approach,such as CVD,RIE,and solution process,which need very expensive instruments and very strict conditions.43,44A simple top-down method of metal-assisted chemical etching (MACE)was utilized to produce Si NWs.40?42An array of Si NWs was created via an electroless process as described below.Typically,a p-type doped Si (1000.3?10Ωcm)wafer was soaked in a HF ?AgNO 3mixture,causing the adjacent Ag ions to be quickly reduced on the Si surface

++→++??

4Ag Si 6F 4Ag SiF 62The resulting Ag clusters uniformly distributing on Si surface started to be reoxidized into the ionic state in the presence of hydrogen peroxide (H 2O 2)

++→+++2Ag H O 2H 2Ag 2H O

222In case those Ag ions could be reduced by Si again,the Ag clusters acted as catalysts that accelerated the etching of Si in the HF solution.As such,Si underneath the Ag nanoparticles was indeed dissolved much faster than that without the coverage of Ag nanoparticles.The etching process of Si followed a total reaction as given below

++→++?↑

n n n Si /2H O 6HF

H O H SiF (4)/2H 222262The Ag-covering regions gradually went down after the etching reaction occurred for a while.The Si without Ag covering apparently su ?ered little from chemical etching;in turn,they stood as distinct nanowires.Ag-assisted etching could be rapidly quenched by removing Ag clusters from the Si surface.In this regard,desirable nanowires lengths are excellently controlled by the etching time,e.g.,the average length of Si nanowires in this contribution is 10μm with a diameter of about 200nm after the etching reaction performs for 4h,as shown in Figure 1a and 1b.

The high degree of uniformity a ?ords relative ease of surface functionalization for Si nanowires on the wafer.The interstitial areas among the Si nanowires could be easily fully ?lled with liquid chemical agents due to capillary forces.In an e ?ort to functionalize nanowire surfaces with primary amines,the as-synthesized Si NWs on a wafer were treated by a harsh Piranha

Scheme 1.Schematic Illustration of the Direct Self-Assembly Between GO Nanosheets and Si

Nanowires

solution (H 2SO 4/H 2O 23:1v/v),(3-aminopropyl)-triethoxysilane (APTES)dissolved in anhydrous toluene entirely wetted Si NWs,leading to a hydrolysis reaction where silane covalently bonds to hydroxyl groups.45Free Si nanowires functionalized with primary amines (NH 2?Si NWs)were collected via a vigorous sonication for 0.5h.As shown in Figure 1d,APTES-functionalized Si nanowires could be well dispersed in water as the surface hydrophilicity had been extremely enhanced.As a control,nonfunctionalized Si nanowires precipitate from aqueous solution after a few minutes.It should be noted that successful surface function-alization was veri ?ed by X-ray photoelectron spectroscopy (XPS)as indicated by the appearance of the N 1s peak at 398eV (Figure 1f).

Well-dispersed GO nanosheets were prepared by the modi ?ed Hummer ’s method.46,47A typical GO solution was presented in Figure 2a.Since GO has lots of functional groups,mainly hydroxyl and carboxylic acids (?OH,?COOH),GO exhibit unique a ?nity to water that the dispersed GO did not precipitate over months.Individual GO nanosheets were conceivably visualized by an atomic force microscope (AFM)and transmission electron microscope (TEM),as shown in Figure 2b and 2c,respectively.The thickness of a GO nanosheet with a single layer was evaluated as around 1nm,which is entirely consistent with previous results.48,49

Self-assembly between Si nanowires and GO nanosheets occurs based on electrostatic interaction.As shown in Figure 3a,insoluble solids separated from the GO aqueous solution

upon dropwise addition of NH 2?Si NWs solution.This precipitation was due to an electrostatic attraction that depressed the solubility of both GO nanosheets and Si

NWs.

Figure 1.(a)Top view and (b)side view of the as-prepared Si NWs (inset shows a high-magni ?cation image).(c)Etched Si wafer after ultrasonication for 0.5h.(d)Free Si NWs functionalized with primary amines (inset shows a digital photograph of the solution having dispersed Si NWs).(e)Molecular structure of APTES.(f)XPS spectra of blank Si NWs and Si NWs functionalized with

APTES.

Figure 2.(a)Photograph image of dispersed GO solution;the concentration of GO nanosheets is 0.5mg mL ?1.(b)AFM and (c)TEM images of GO

nanosheets.

Figure 3.(a)Photographic images of rapid self-assembly between Si nanowires and GO nanosheets in aqueous solution.(b)FESEM image.(c)Low-and (d)high-magni ?cation TEM images of the core/shell Si ?graphene complex.(e)HRTEM image of NH 2?Si NWs core/graphene shell structure (inset shows electron di ?raction of single-crystal Si).

As a consequence,the self-assembled complex was solidi ?ed by a powerful means of lyophilization which can not only remove water but also enhance the electrostatic association.The dry solid was calcinated at a high temperature under a ?ow of N 2/H 2gas mixture (95:5),enabling conversion of GO to graphene.Noting that a native SiO 2layer may exist on the Si nanowire,the calcinated hybrid material was etched by a diluted HF solution to remove SiO 2.Subtitle structure of graphene on Si NWs was identi ?ed by ?eld-emission scanning electron microscopy (FESEM),transmission electron microscopy (TEM),and high-resolution TEM (HRTEM).As shown in Figure 3b,an intriguing thin layer apparently exists on a Si nanowire.Such a core/shell structure could be also observed by TEM (Figure 3c and 3d).In addition,the HRTEM image indicates that there is a clear boundary between Si NWs and the graphene layer.The thickness of the graphene layer is approximately 9?10nm (Figure 3e).

Both the rate of electron di ?usion and the conductivity of Si nanowires coated with graphene shells are assumed to be improved.In addition,the core/shell structure is also envisioned to accommodate the Si expansion during lithiation and delithiation cycling.To validate these hypotheses,the core/shell complex as a function of anode electrode was evaluated in a sealed 2032coin cell with Li foil as the counter electrode.Figure 4a shows the 1st,2nd,3rd,10th,25th,50th,and 80th charge ?discharge curves of GNS@Si NWs electrode at a current density of 200mA ·g ?1in a voltage range of 0.005?2.0V vs Li/Li +.The initial small plateaus at 0.8V and long slope pro ?les of electrode are similar to bare Si electrode that were reported previously.The GNS@Si NWs discharged a capacity of 2142mAh ·g ?1and a charge capacity of 1649mAh ·g ?1with

Coulombic e ?ciency at 80%.The irreversible capacity observed in the ?rst cycle can be attributed to formation of a solid electrolyte interface (SEI).It is noteworthy that the GNS@Si NWs still exhibit a ?at charging voltage plateau after 80cycles,showing excellent cyclability.

A typical cyclic voltammetry (CV)characterization of the GNS@Si NWs composite is shown in Figure 4b.A discharge potential plateau around 0.79V is clearly observed in the ?rst discharge curves,corresponding mainly to formation of SEI ?lm and subsequent possible irreversible accumulation of Li +in the active Si particles.Below 0.3V,a sharp reduction peak for insertion of Li +into Si could be observed,and subsequently,the extraction process occurred at 0.39and 0.53V with a broad peak;these redox peaks were attributed the alloying/dealloying of Li with active Si NWs.In the following cycles,the cathodic peak of 0.6V disappears,indicating a stable SEI layer has maintained.The cathodic peak at 0.17V gradually evolved,corresponding to generation of Li ?Si alloy phases.The anodic part shows two peaks at 0.39and 0.53V,which could be assigned to dealloying of Li ?Si alloys,and become more distinct after the following cycles.Figure 4c compared the rate performance of GNS@Si NWs and bare Si NWs at various current densities.The cell was ?rst cycled at 200mA ·g ?1for 10cycles,in which a stable reversible capacity of about 1649mAh ·g ?1was observed.At the high current densities of 500,1000,5000,and 200mA ·g ?1,the GNS@Si NWs can still deliver high reversible speci ?c capacities of 1090,701,441,and 1251mAh ·g ?1,respectively.In contrast,the bare Si NWs showed sharply deteriorating capacity with the increasing charging current.When the charging current reached 1000mA ·g ?1,the bare Si NWs cannot deliver lithiation capacity

at

Figure 4.(a)Galvanostatic discharge ?charge pro ?les of the ?rst three cycles for graphene-wrapped Si NWs.(b)Cyclic voltammetry (CV)di ?erential capacity curves of GNS@Si NWs electrode.(c)Rate capability of the GNS@Si NWs composite compared with Si NWs.(d)Cycling performance of the GNS@Si composite compared with bare Si NWs.

all.Figure4d shows the cycling performance of various anodes at a current density of200mA·g?1.The bare Si NWs electrode shows dramatic capacity fading,with charge capacity at250 mAh·g?1after80cycles.With the graphene wrapping,GNS@Si NWs exhibit great enhanced stability,maintaining a capacity of 1335mAh·g?1after80cycles.We assume that the enhanced reversibility and rate capability of GNS@Si NWs is attributed mainly to the good mechanical?exibility of graphene that it can readily accommodate the large volume change associated with the conversion reaction and preventing cracking of the Si NWs. Use of graphene can also contribute to the enhanced electronic conductivity and act as a three-dimensional conductive network for the GNS@Si NWs electrode,which could promote electron

transfer during the lithiation and delithiation processes.■CONCLUSION

In summary,we demonstrate a facile,low-cost,and scalable approach to prepare GNS@Si NWs for high-performance LIBs. The approach has enabled preparation of a well-de?ned self-assembly structure of Si nanowires with spatially de?ned wrap of graphene.This unique structure enhances electron di?usion and conductivity of Si nanowires.More importantly,the core/ shell self-assembly structure can accommodate Si expansion during lithiation and delithiation cycling,thus signi?cantly improving the discharge capacity and prolonging the anode life in contrast to bare Si anodes materials.These Si nanowires/ graphene-based structures and the self-assembly method used to produce them represent a new paradigm for integrating nanoscale materials properties into microscale structures, opening opportunities in generating functional nanomaterials for the next generation of LIBs.It is envisioned that the capacity retention can be further improved by decreasing the diameter scale of the Si nanowires and introducing extra space

to reversibly tailor the volume expansion.

■AUTHOR INFORMATION

Corresponding Author

*E-mail:yapeiwang@https://www.sodocs.net/doc/127321385.html,;chenjitao@https://www.sodocs.net/doc/127321385.html,. Author Contributions

§These authors contributed equally.

Notes

The authors declare no competing?nancial interest.

?In memory of Prof.Cao.Deceased on March,16,2012■ACKNOWLEDGMENTS

This work was?nancially supported by the Fundamental Research Funds for the Central Universities and the Research Funds of Renmin University of China(20334010,20473045,

and20574040).

■REFERENCES

(1)Armand,M.;Tarascon,J.M.Building better batteries.Nature 2008,451,652?657.

(2)Bruce,P.G.;Scrosati,B.;Tarascon,J.M.Nanomaterials for rechargeable lithium batteries.Angew.Chem.,Int.Ed.2008,47,2930?2946.

(3)Kang,B.;Ceder,G.Battery materials for ultrafast charging and discharging.Nature2009,458,190?193.

(4)Dunn,B.;Kamath,H.;Tarascon,J.M.Electrical energy storage for the grid:A battery of choices.Science2011,334,928?935. (5)Li,H.;Wang,Z.;Chen,L.;Huang,X.Research on advanced materials for Li-ion batteries.Adv.Mater.2009,21,4593?4607.

(6)Zhang,D.Q.;Wen,M.C.;Zhang,P.;Zhu,J.;Li,G.S.;Li,H.X. Microwave-induced synthesis of porous single-crystal-like TiO2with excellent lithium storage https://www.sodocs.net/doc/127321385.html,ngmuir2012,28,4543?4547.

(7)Wu,X.L.;Jiang,L.Y.;Cao,F.F.;Guo,Y.G.;Wan,L.J.LiFePO4 nanoparticles embedded in a nanoporous carbon matrix:superior cathode material for electrochemical energy-storage devices.Adv. Mater.2009,21,2710?2714.

(8)Ning,J.J.;Dai,Q.Q.;Jiang,T.;Men,K.K.;Liu,D.H.;Xiao,N. R.;Li,C.Y.;Li,D.M.;Liu,B.B.;Zou,B.;Zou,G.T.;Yu,W.W.Facile synthesis of Tin oxide nanoflowers:A potential high-capacity lithium-ion-storage https://www.sodocs.net/doc/127321385.html,ngmuir2009,25,1818?1821.

(9)Wang,D.H.;Choi,D.;Li,J.;Yang,Z.G.;Nie,Z.M.;Kou,R.; Hu,D.H.;Wang,C.M.;Saraf,L.V.;Zhang,J.G.;Aksay,I.A.;Liu,J. Self-assembled TiO2?graphene hybrid nanostructures for enhanced Li-ion insertion.ACS Nano2009,3,907?914.

(10)Ji,X.L.;Lee,K.T.;Nazar,L. F.A highly ordered nanostructured carbon?sulphur cathode for lithium?sulphur batteries. Nat.Mater.2009,8,500?506.

(11)Bourderau,S.;Brousse,T.;Schleich,D.M.Amorphous silicon as a possible anode material for Li-ion batteries.J.Power Sources1999, 81?82,233?236.

(12)Kasavajjula,U.;Wang,C.S.;Appleby,A.J.Nano-and bulk-silicon-based insertion anodes for lithium-ion secondary cells.J.Power Sources2007,163,1003?1039.

(13)Chan,C.K.;Peng,H.L.;Liu,G.;McIlwrath,K.;Zhang,X.F.; Huggins,R.A.;Cui,Y.High-performance lithium battery anodes using silicon nanowires.Nat.Nano2008,3,31?35.

(14)Magasinski,A.;Dixon,P.;Hertzberg,B.;Kvit,A.;Ayala,J.; Yushin,G.High-performance lithium-ion anodes using a hierarchical bottom-up approach.Nat.Mater.2010,9,353?358.

(15)Boukamp,B.A.;Lesh,G.C.;Huggins,R.A.All-solid lithium electrodes with mixed-conductor matrix.J.Electrochem.Soc.1981,128, 725?729.

(16)Lee,J.K.;Kung,M.C.;Trahey,L.;Missaghi,M.N.;Kung,H.

H.Nanocomposites derived from phenol-functionalized Si nano-particles for high performance lithium ion battery anodes.Chem. Mater.2008,21,6?8.

(17)Lee,J.K.;Smith,K.B.;Hayner,C.M.;Kung,H.H.Silicon nanoparticles?graphene paper composites for Li ion battery anodes. https://www.sodocs.net/doc/127321385.html,mun2010,46,2025?2027.

(18)Park,M.H.;Kim,M.G.;Joo,J.;Kim,K.;Kim,J.;Ahn,S.;Cui, Y.;Cho,J.Silicon nanotube battery anodes.Nano Lett.2009,9,3844?3847.

(19)Chan,C.K.;Patel,R.N.;O’Connell,M.J.;Korgel,B.A.;Cui,Y. Solution-grown silicon nanowires for lithium-ion battery anodes.ACS Nano2010,4,1443?1450.

(20)Ma,H.;Cheng,F.Y.;Chen,J.;Zhao,J.Z.;Li,C.S.;Tao,Z.L.; Liang,J.Nest-like silicon nanospheres for high-capacity lithium storage.Adv.Mater.2007,19,4067?4070.

(21)Kim,H.;Han,B.;Choo,J.;Cho,J.Three-dimensional porous silicon particles for use in high-performance lithium secondary batteries.Angew.Chem.,Int.Ed.2008,47,10151?10154.

(22)Cao,F.F.;Deng,J.W.;Xin,S.;Ji,H.X.;Schmidt,O.G.;Wan, L.J.;Guo,Y.G.Cu-Si nanocable arrays as high-rate anode materials for lithium-ion batteries.Adv.Mater.2011,23,4415?4420.

(23)Kim,H.;Seo,M.;Park,M.H.;Cho,J.A critical size of silicon nano-anodes for lithium rechargeable batteries.Angew.Chem.,Int.Ed. 2010,49,2146?2149.

(24)Yao,Y.;McDowell,M.T.;Ryu,I.;Wu,H.;Liu,N.;Hu,L.B.; Nix,W.D.;Cui,Y.Interconnected silicon hollow nanospheres for lithium-ion battery anodes with long cycle life.Nano Lett.2011,11, 2949?2954.

(25)Hertzberg,B.;Alexeev,A.;Yushin,G.Deformations in Si-Li anodes upon electrochemical alloying in nano-confined space.J.Am. Chem.Soc.2010,132,8548?8549.

(26)Etacheri,V.;Haik,O.;Goffer,Y.;Roberts,G.A.;Stefan,I.C.; Fasching,R.;Aurbach,D.Effect of fluoroethylene carbonate(FEC)on the performance and surface chemistry of Si-nanowire Li-ion battery https://www.sodocs.net/doc/127321385.html,ngmuir2012,28,965?976.

(27)Wang,W.;Kumta,P.N.Nanostructured hybrid silicon/carbon nanotube heterostructures:reversible high-capacity lithium-ion anodes. ACS Nano2010,4,2233?2241.

(28)Xin,X.;Zhou,X.F.;Wang,F.;Yao,X.Y.;Xu,X.X.;Zhu,Y.M.; Liu,Z.P.A3D porous architecture of Si/graphene nanocomposite as high-performance anode materials for Li-ion batteries.J.Mater.Chem. 2012,22,7724?7730.

(29)Wang,X.L.;Han,W.Q.Graphene enhances Li storage capacity of porous single-crystalline silicon nanowires.ACS Appl.Mater. Interfaces2010,2,3709?3713.

(30)Cui,L.F.;Ruffo,R.;Chan,C.K.;Peng,H.L.;Cui,Y. Crystalline-amorphous core?shell silicon nanowires for high capacity and high current battery electrodes.Nano Lett.2008,9,491?495. (31)Guo,J.C.;Wang,C.S.A polymer scaffold binder structure for high capacity silicon anode of lithium-ion https://www.sodocs.net/doc/127321385.html,mun. 2010,46,1428?1430.

(32)Novoselov,K.S.;Geim,A.K.;Morozov,S.V.;Jiang,D.;Zhang, Y.;Dubonos,S.V.;Grigorieva,I.V.;Firsov,A.A.Electric field effect in atomically thin carbon films.Science2004,306,666?669.

(33)Geim,A.K.Graphene:status and prospects.Science2009,324, 1530?1534.

(34)Zhao,X.;Hayner,C.M.;Kung,M.C.;Kung,H.H.In-plane vacancy-enabled high-power Si?graphene composite electrode for lithium-ion batteries.Adv.Energy Mater.2011,1,1079?1084. (35)Zhou,X.S.;Yin,Y.X.;Wan,L.J.;Guo,Y.G.Self-assembled nanocomposite of silicon nanoparticles encapsulated in graphene through electrostatic attraction for lithium-ion batteries.Adv.Energy Mater.2012,2,1086?1090.

(36)Zhou,X.S.;Yin,Y.X.;Cao,A.M.;Wan,L.J.;Guo,Y.G. Efficient3D conducting networks built by graphene sheets and carbon nanoparticles for high-performance silicon anode.ACS Appl.Mater. Interfaces2012,4,2824?2828.

(37)Zhou,X.S.;Yin,Y.X.;Wan,L.J.;Guo,Y.G.Facile synthesis of silicon nanoparticles inserted into graphene sheets as improved anode materials for lithium-ion https://www.sodocs.net/doc/127321385.html,mun2012,48,2198?2200.

(38)Li,D.;Muller,M.B.;Gilje,S.;Kaner,R.B.;Wallace,G.G. Processable aqueous dispersions of graphene nanosheets.Nat.Nano 2008,3,101?105.

(39)Wang,Y.P.;Han,P.;Xu,H.P.;Wang,Z.Q.;Zhang,X.; Kabanov,A.V.Photocontrolled self-assembly and disassembly of block ionomer complex vesicles:A facile approach toward supra-molecular polymer https://www.sodocs.net/doc/127321385.html,ngmuir2010,26,709?715. (40)Huang,Z.P.;Fang,H.;Zhu,J.Fabrication of silicon nanowire arrays with controlled diameter,length,and density.Adv.Mater.2007, 19,744?748.

(41)Gao,Y.F.;Cheng,M.J.;Wang,B.L.;Feng,Z.G.;Shi,F. Diving?Surfacing Cycle Within a Stimulus-responsive Smart Device Towards Developing Functionally Cooperating Systems.Adv.Mater. 2010,22,5125?5128.

(42)Cheng,M.J.;Gao,Y.F.;Guo,X.P.;Shi,Z.Y.;Chen,J.F.;Shi,

F.A Functionally Integrated Device for Effective and Facile Oil Spill https://www.sodocs.net/doc/127321385.html,ngmuir2011,27,7371?7375.

(43)Wu,Y.Y.;Fan,R.;Yang,P.D.Block-by-block growth of single-Crystalline Si/SiGe superlattice nanowires.Nano Lett.2002,2,83?86.

(44)Heitsch,A.T.;Fanfair,D.D.;Tuan,H.Y.;Korgel,B.A. Solution-Liquid-Solid(SLS)growth of silicon nanowires.J.Am.Chem. Soc.2008,130,5436?5437.

(45)Wang,Y.P.;Han,P.;Wu,G.L.;Xu,H.P.;Wang,Z.Q.;Zhang, X.Selectively erasable multilayer thin film by photoinduced https://www.sodocs.net/doc/127321385.html,ngmuir2010,26,9736?9741.

(46)Hummers,W.S.;Offeman,R.E.Preparation of Graphitic Oxide.J.Am.Chem.Soc.1958,80,1339?1339.

(47)Kovtyukhova,N.I.;Ollivier,P.J.;Martin,B.R.;Mallouk,T.E.; Chizhik,S.A.;Buzaneva,E.V.;Gorchinskiy,https://www.sodocs.net/doc/127321385.html,yer-by-Layer assembly of ultrathin composite films from micron-sized graphite oxide sheets and polycations.Chem.Mater.1999,11,771?778.

(48)Xu,Y.X.;Bai,H.;Lu,G.W.;Li,C.;Shi,G.Q.Flexible graphene films via the filtration of water-soluble noncovalent functionalized graphene sheets.J.Am.Chem.Soc.2008,130,5856?5857.

(49)Wu,Q.;Xu,Y.X.;Yao,Z.Y.;Liu, A.R.;Shi,G.Q. Supercapacitors based on flexible graphene/polyaniline nanofiber composite films.ACS Nano2010,4,1963?1970.