In situ chemical synthesis of SnO 2–graphene nanocomposite as anode materials for lithium-ion batteries
Jane Yao,Xiaoping Shen,Bei Wang,Huakun Liu,Guoxiu Wang *
Institute for Superconducting and Electronic Materials,School of Mechanic,Materials and Mechatronic Engineering,University of Wollongong,NSW 2522,Australia
a r t i c l e i n f o Article history:
Received 15July 2009
Received in revid form 30July 2009Accepted 31July 2009
Available online 6August 2009Keywords:Graphene
SnO 2nanoparticles Nanocomposite Anode material
Lithium-ion batteries
a b s t r a c t
An in situ chemical synthesis approach has been developed to prepare SnO 2–graphene nanocomposite.Field emission scanning electron microscopy and transmission electron microscopy obrvation revealed the homogeneous distribution of SnO 2nanoparticles (4–6nm in size)on graphene matrix.The electro-chemical reactivities of the SnO 2–graphene nanocomposite as anode material were measured by cyclic voltammetry and galvanostatic charge/discharge cycling.The as-synthesized SnO 2–graphene nanocom-posite exhibited a reversible lithium storage capacity of 765mAh/g in the first cycle and an enhanced cyclability,which can be ascribed to 3D architecture of the SnO 2–graphene nanocomposite.
Ó2009Elvier B.V.All rights rerved.
1.Introduction
Graphene,a single-atom-thick sheet of honeycomb carbon lat-tice,exhibits many unique chemical and physical properties [1–3].Since its discovery in 2004,extensive efforts have been devoted to investigating the technological applications of graphene materi-als,such as graphene-bad electronics,high strength composite materials,liquid crystal displays,and energy storage and conver-sion devices [4–7].
Graphite (the 3D form of graphene)is currently ud as the an-ode material in commercial rechargeable lithium batteries.It has a maximum theoretical lithium storage capacity of 372mAh/g,while a single layer graphene has a theoretical lithium storage capacity of 744mAh/g if lithium is attached to both sides of the graphene sheets.The enhanced lithium storage capacity of graphene in lith-ium ion cells has been experimentally verified [8,9].However,graphene sheets always naturally stack into multilayers and there-fore lo their high surface area and intrinsic chemical and physical properties.Recently,a general strategy –jamming graphene sheets with nanoparticles,has been developed to minimize the aggrega-tion of graphene.Both metal nanoparticles (Au and Pt)[10,11]and metal oxide nanoparticles (TiO 2and SnO 2)[12,13]have been ud for this purpo.However,tho composites were prepared by mechanically mixing nanoparticles with graphene dispersions,which limits the homogeneous dispersion of nanoparticles and the paration of graphene sheets.
Transition metal oxides have been studied as alternative anode materials for lithium-ion batteries becau of their high specific capacity.Among them,tin oxide (SnO 2)is particularly attractive.The reaction between lithium ions and SnO 2can be expresd as:
8:4Li þ
þSnO 2$Li 4:4Sn þ2Li 2O
ð1Þ
The above reaction is reversible with a theoretical reversible capac-ity of 782mAh/g bad on the mass of SnO 2[14].Therefore,the for-mation of SnO 2–graphene nanocomposite should not only reduce the degree of stacking of graphene sheets,but also boost the lithium storage capacity.
Herein,we report the in situ chemical synthesis of a SnO 2–graphene nanocomposite and its enhanced electrochemical perfor-mance as anode materials in lithium-ion batteries.2.Experimental
Graphene oxide nanosheets (GONS)were synthesid from nat-ural graphite powders by a modified Hummer’s method [15].In a typical synthesis,40mg GONS was disperd in de-ionid (DI)water by ultrasonication.The dispersion was then mixed with 40ml aqueous solution of SnCl 2Á2H 2O (20mg)and citric acid (20mg).The mixture was transferred into a 250ml round-bot-tomed flask and was heated in an oil bath to 120°C under stirring.20ml NaBH 4(200mg)aqueous solution was gradually added,and the resulting mixture was refluxed at 120°C for 5h.During this process,GONS were reduced to graphene nanosheets (GNS)and Sn 2+to Sn nanoparticles.Simultaneously,the Sn nanoparticles
1388-2481/$-e front matter Ó2009Elvier B.V.All rights rerved.doi:10.1016/j.elecom.2009.07.035
*Corresponding author.Fax:+61242215731.E-mail address:gwang@uow.edu.au (G.Wang).
what about nowElectrochemistry Communications 11(2009)
1849–1852
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were oxidized to SnO2due to heating at120°C in atmosphere.The resultant black solid products were parated byfiltration,washed with DI-water,and dried in vacuum at80°C.To improve the crys-tallinity of SnO2in GNS,the product was annealed at300°C for 10h in Ar atmosphere.The weight content of SnO2in Sn/graphene nanocomposite was quantitatively determined to be40wt%by thermogravimetric analysis(TGA)and chemical analysis,in which SnO2nanoparticles were dissolved by diluted HCl.The structure and morphology of the SnO2/graphene nanocomposite were ana-lyd by X-ray diffraction(XRD,Philips1730X-ray diffractometer),field emission scanning electron microscopy(FESEM),and trans-mission electron microscopy(TEM,JEOL2011TEM facility).
esperanza spalding
SnO2–graphene powders were mixed with a binder,poly(vinyl-idenefluoride)(PVdF),at the weight ratio of90:10in N-methyl-2-pyrrolidone(NMP)solvent to form a slurry.Then,the resultant slurry was uniformly pasted on Cu foil substrates with a blade. The prepared electrode sheets were dried at1
00°C in a vacuum oven for12h and presd under a pressure of approximately 200kg/cm2.CR2032-type coin cells were asmbled in a glove box for electrochemical characterization.The electrolyte was1M LiPF6in a1:1mixture of ethylene carbonate and dimethyl carbon-ate.Li metal foil was ud as the counter electrode.The cells were galvanostatically charged and discharged at a current density of 55mAh/g within the range of0.01–3.0V.Cyclic voltammetry (CV)curves were collected at0.1mV/s within the range of0.01–3.0V using an electrochemistry workingstation(CHI660C).
3.Results and discussion
Fig.1a shows the X-ray diffraction pattern of the SnO2–graph-ene nanocomposite.The major diffraction lines can be indexed to the tetragonal rutile SnO2pha.The graphene nanosheets only show a weak(100)diffraction line.The broad diffraction peaks of the SnO2indicate small crystal size.The general morphology of the SnO2–graphene nanocomposite was obrved by FESEM. Fig.1b shows a FESEM image of the SnO2–graphene nanocompos-ite,in which the graphene nanosheets appear corrugated into a
wavy shape.SnO2nanoparticles are embedded in the curly graph-ene nanosheets.The crystalline structure of the SnO2–graphene nanocomposite was analyd by TEM and high resolution TEM. Fig.
德语在线翻译器2a shows a low magnification TEM image of the SnO2–graph-ene nanocomposite.SnO2nanoparticles are uniformly distributed on2D graphene nanosheets.The int in Fig.2a is the lected area electron diffraction(SAED)pattern.All diffraction rings can be in-dexed to tetragonal SnO2pha.A high magnification TEM image of the SnO2–graphene nanocomposite is shown in Fig.2b,from which the average particle size of SnO2can be determined to be about4–6nm(more than200counts).Fig.2c shows HRTEM image of cross-ctional view of SnO2–graphene nanocomposite.SnO2 nanoparticles(black)are surrounded byflexible graphene nano-sheets,which can be distinguished as linear strips.The interplanar distance of the(002)crystal planes of the stacked graphene sheets was determined to be0.38nm,which is much larger than that in the pristine graphite(0.34nm).The stacking of graphene nano-sheets amounts of3–6layers,which can be counted from the num-ber of strips as marked with circles in Fig.2c.Fig.2d prents a lattice resolved HRTEM image of the SnO2–graphene nanocompos-ite,from which the lattices of SnO2nanoparticles and graphene nanosheets are clearly visible.The int shows the atomic resolu-tion HRTEM image of a SnO2nanoparticle,in which the(110) and(200)crystal planes of the SnO2tetragonal structure can be clearly identified from the interplanar distances of0.24and 0.34nm,respectively.In the chemical synthesis process,graphene oxide nanosheets werefirstly disperd in water.Graphene oxide nanosheets can be considered as macromolecules containing epoxyl and hydroxyl moieties on the b
asal plane and carboxylic acid group on the edge sites[16,17].When they were mixed with Sn2+cations,the Sn2+cations were attracted and anchored to tho functional groups.On chemical reduction by NaBH4,the graphene oxide nanosheets were reduced to graphene nanosheets,and the Sn2+cations were reduced to Sn and aggregated to Sn nanoparti-cles.Since the reduction process was carried out at120°C,the Sn nanoparticles were immediately oxidized to SnO2nanoparticles. The in situ formed SnO2nanoparticles are able to effectively p-arate the stacking of graphene nanosheets.
The electrochemical reactivity of SnO2–graphene nanocompos-ite as anode in lithium ion cells wasfirst evaluated by cyclic vol-tammetry(CV).Fig.3shows the CV curves of SnO2–graphene nanocomposite electrode in thefirst,cond,10th,and50th scan-ning cycles.In thefirst cycle,there is a small cathodic peak at0.7V, which can be attributed to the formation of the solid electrolyte interpha(SEI)layer.This peak disappears from the cond cycle. The other obvious reduction peaks are located around0.12and 0.01V,and can be ascribed to the lithium reaction with SnO2nano-particles and inrtion in graphene nanosheets,respectively.
如何提升团队执行力4LiþþSnO2þ4eÀ!Snþ2Li2Oð2Þx LiþþSnþxeÀ$Li x Snð3Þx LiþþCðgrapheneÞþx eÀ$Li x Cð4Þ
Three oxidation peaks appear around0.13,0.51,and1.23V,respec-tively.The0.13V anodic peak corresponds to lithium extraction
(b)
1850J.Yao et al./Electrochemistry Communications11(2009)1849–1852
from graphene nanosheet (Eq.(4));the 0.51V oxidation peak can be assigned to the de-alloying of Li x Sn (Eq.(3)),while the weak oxida-tion at 1.23V could be the partly reversible reaction of the Eq.(2)[18,19].The CV measurements clearly elucidated the reversible electrochemical reactions between the lithium ions and the SnO 2–graphene nanocomposite in lithium ion cells.
The lithium storage capacity and cyclability of SnO 2–graphene nanocomposite as anode in lithium ion cells were determined via galvanostatic charge/discharge cycling.Fig.4a shows the charge/discharge profiles of SnO 2–graphene electrode in the first,cond,and 50th cycles,respectively.In the first cycle,the SnO 2–graphene nanocomposite delivered a lithium inrtion capacity of 1420mAh/g and a reversible charging capacity of 765mAh/g.From the cond cycle,the reversibility of the electrode was improved禁止吸烟的英文
falling out
significantly.The SnO 2–graphene nanocomposite consists of 40wt%SnO 2and 60wt%graphene.Therefore,the theoretical capacity,C,of the SnO 2–graphene nanocomposite should be 777.2mAh/g bad on C SnO 2of 782mAh/g and C graphene of 744mAh/g.The initial reversible capacity of the SnO 2–graphene nanocomposite is very clo to the theoretical capacity.The revers-ible lithium storage le number is shown in Fig.4b.The SnO 2–graphene nanocomposite electrode maintained a capac-ity of 520mAh/g after 100cycles.On the other hand,the SnO 2nanoparticle electrode exhibited a poor cyclability,retaining only 50mAh/g in the 20th cycle and then failing completely.Graphene electrode prepared in the same condition as SnO 2–graphene nano-composite also exhibited much wor performance than that of SnO 2–graphene nanocomposite electrode.Therefore,the SnO 2–graphene nanocomposite electrode demonstrated much better electrochemical performance than that of the bare SnO 2and graph-ene electrodes.When SnO 2reacts with lithium,there is a dramatic volume increa,inducing cracking and pulverization.By embed-ding SnO 2nanoparticles in graphene nanosheet matrix,the volume expansion and contraction of the SnO 2nanoparticles can be effec-tively buffered by the flexible graphene nanosheets.In addition,graphene nanosheets also provide a highly conductive medium for electron transfer during the lithiation and de-lithiation process.So,good electrochemical performance can be maintained.4.Conclusion
In summary,SnO 2–graphene nanocomposites with 3D architec-ture were synthesized by an in situ chemical reduction process.The mixing of graphene nanosheets and SnO 2nanoparticles on the molecular level can ensure homogeneous distribution of SnO 2nanoparticles on graphene nanosheets and effective paration of tho graphene nanosheets.HRTEM analysis confirmed the uni-form attachment of SnO 2nanoparticles (4–6nm in size)on the graphene nanosheet matrix.Cyclic voltammetry
measurements
Fig.2.(a)Low magnification TEM image of SnO 2–graphene nanocomposite,showing the uniform distribution of SnO 2nanoparticles on graphene matrix.The int is the corresponding SAED pattern.(b)High magnification TEM image of SnO 2/graphene nanocomposite,from which the average particle size of SnO 2was measured to be 4–6nm.(c)HRTEM image of SnO 2–graphene nanocomposite,showing SnO 2nanoparticles surrounded by graphene nanosheets.(d)Lattice resolved HRTEM image of SnO 2–graphene nanocomposite,in which the lattices of SnO 2nanoparticles and graphene nanosheets are clearly visible.The int is an atomically resolved lattice image of a SnO 2nanoparticle,from which two perpendicular crystal planes,(110)and (200),can be distinguished.
J.Yao et al./Electrochemistry Communications 11(2009)1849–18521851
show the highly reactive nature of SnO 2–graphene towards lithium storage in lithium ion cells.Our SnO 2–graphene nanocomposite demonstrated a reversible specific capacity of 765mAh/g in the first cycle and enhanced cyclability.Acknowledgement良医妙药
We are grateful for financial support from the Australian Rearch Council (ARC)through the ARC Discovery Project (DP0772999).References
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