SnO2-graphene composite as highly reversible anode materials for lithium ion batteries

Short communication

SnO2/graphene composite as highly reversible anode materials for lithium ion batteries

Qi Guo,Zhe Zheng,Hailing Gao,Jia Ma,Xue Qin*

School of Science,Tianjin University,No.92Weijin Road,Tianjin300072,China

h i g h l i g h t s

We explored a facile method for the preparation of SnO2/graphene composite.

The SnO2nanoparticles are uniformly coated on the graphene nanosheets.

The graphene is of high degree of purity.

The initial discharge capacity is as high as1995.8mAh gà1at the current density of1A gà1.

After40cycles the reversible discharge capacity is still remained at1545.7mAh gà1.

a r t i c l e i n f o

Article history:

Received15January2013 Received in revised form

19March2013

Accepted25March2013 Available online1April2013

Keywords:

Tin oxide

Graphene

Composite

Lithium-ion batteries a b s t r a c t

Tin oxide(SnO2)/graphene composite is synthesized via a simple wet chemical method using graphene oxide and SnCl2$2H2O as raw materials.Graphene of high reduction degree in the composite can provide high conductivity and large-current discharge capacity.SnO2nanoparticles with dimension around5nm are uniformly distributed on the graphene matrix.

The SnO2/graphene composite exhibits outstanding electrochemical performance such as high reversible capacities,good cycling stability and excellent high-rate discharge performance.The initial discharge and charge capacities are1995.8mAh gà1and1923.5mAh gà1,respectively.After40cycles, the reversible discharge capacity is still maintained at1545.7mAh gà1at the current density of1A gà1, indicating that the composite is a promising alternative anode material used for high-storage lithium ion batteries.

Crown Copyrightó2013Published by Elsevier B.V.All rights reserved.

1.Introduction

Lithium-ion batteries are regarded as the promising power source for portable electrical devices and electrical/hybrid vehicles due to their high electromotive force and high energy density[1,2]. Graphite,as anode material,has been widely used owing to its extraordinary electronic transport properties,large surface area, and high electrocatalytic activities although its limited speci?c capacity(372mAh gà1)cannot ful?l the increasing demand for lithium-ion batteries with higher energy density[3e5].To settle this problem,many studies have been taken into consideration to investigate new electrode materials and metal oxide/graphene composites are selected as a kind of promising material for lithium-ion batteries as their speci?c capacities are much higher than graphene[6,7].

Among them,SnO2,an n-type and wide band gap semi-conductor,has attracted much attention as an anode material for the new-generation lithium-ion batteries with its high theoretical capacity(790mAh gà1)[8,9].However,it suffers from large volume changes and agglomeration associated with the Li-ion insertion and extraction processes,which brings about failure and loss of electrical contact of the anode[10,11].In addition,there is also a huge irreversible capacity during the?rst cycle due to the forma-tion of amorphous Li2O matrix.

In order to address the problems discussed above,SnO2/gra-phene composite which can take both advantages of SnO2and graphene has been prepared and its synthesis has drawn tremen-dous research interest for further application in the anode of lithium-ion batteries[12e14].

Here,we develop a simple wet chemical method to obtain SnO2/ graphene composite.The as-prepared composite can possess both of the advantages of tin oxide and graphene and exhibits superior electrochemical performance with large reversible capacity, excellent cycling performance and good rate https://www.sodocs.net/doc/b717357784.html,pared

*Corresponding author.Tel.:t8602227403670. E-mail address:qinxue@https://www.sodocs.net/doc/b717357784.html,(X.

Qin).Contents lists available at SciVerse ScienceDirect Journal of Power Sources

journal h omepage:www.elsevier.co m/lo cate/jp owsou

r

0378-7753/$e see front matter Crown Copyrightó2013Published by Elsevier B.V.All rights reserved. https://www.sodocs.net/doc/b717357784.html,/10.1016/j.jpowsour.2013.03.116

Journal of Power Sources240(2013)149e154

to other similar composites,this SnO2/graphene composite could be obtained via an easier method but exhibits much higher speci?c capacities at large current density.

2.Experimental

2.1.Sample preparation

The SnO2/graphene composite was synthesized using a wet chemical method.All chemicals were of analytical grade.The gra-phene oxide employed here was synthesized from natural graphite by a modi?ed Hummers’method[15,16].

Dried graphene oxide of0.1g was added into100mL distilled water.The obtained mixture was ultrasonicated for15min in the ultrasonic cell crusher.Then the suspension was centrifuged for 20min at4000r minà1and the supernatant was used for the next step reaction,the resultant graphene oxide solution was labelled as solution A.Solution B was prepared by dissolving0.1g of tin chloride dehydrate in100mL of distilled water with some appro-priate concentrated hydrochloric acid.The two solutions were mixed and transferred to a250mL three distillation?ask in which the above mixture was re?uxed at80 C in an oil bath for12h under stirring.After standing for another24h,the system was

cooled down to room temperature.The as-prepared products were rinsed with distilled water until the pH value of the solution reached to neutral and then dried at50 C for12h.The bare SnO2 nanoparticles were synthesized under the same condition except replacing the graphene oxide with hydrogen peroxide as oxidizing agent.

2.2.Sample characterization

The structure and morphology of the as-prepared SnO2/gra-phene composite were characterized by X-ray diffraction(XRD,D/ MAX-2500),Raman spectroscopy(In Via Re?ex),Transmission electron microscopy(TEM,Tecnai G2F20),X-ray photoelectron spectroscopy(XPS,PHI1600)and Thermogravimetric analysis(TGA, STA409PC Luxxò).

2.3.Electrochemical measurements

The working electrodes were prepared by mixing80wt%as-prepared active materials with10wt%acetylene black and10wt %polyvinylidene?uoride(PVdF)binder in N-methyl-2-pyrrolidi-none(NMP)to form homogeneous slurry through grinding in a mortal.Then the slurry was pasted onto a copper foil substrate and dried in vacuum oven at120 C for at least12h.The electro-chemical cells used this active materials as the working electrode,Li foil as counter electrode and reference electrode,and1mol Là1 LiPF6in a1:1(volume:volume)mixture of ethylene carbonate(EC) and dimethyl carbonate(DMC)as the electrolyte which was assembled in an argon-?lled glove box.The calculation of the ca-pacity of the composites was based on the SnO2/graphene com-posites.The cells were galvanostatically charge e discharge in the voltage range0.01e2.5V vs.Li/Litby a battery testing system (LAND CT-2001A system).Cyclic voltammetry(CV)curves were collected using Zahner Ennium electrochemical workstation.Elec-trochemical impedance spectra of the electrodes were recorded from100kHz to100mHz at5mV of the amplitude of the perturbation.

3.Results and discussion

Fig.1shows the XRD patterns of graphene and the as-prepared SnO2/graphene composites.The peaks at about2q?26 and 2q?43 can be assigned to the[002]and[100]plane of graphene nanosheets[17].The diffraction peaks for the crystalline SnO2 nanoparticles are clearly observed.The four intense diffraction peaks at2q?26.06 ,33.04 ,51.76 and65.04 can be indexed as the (110),(101),(211),(301)plane of the standard tetragonal SnO2 phase,respectively.The quite broadened diffraction peaks indicate the small sizes of the SnO2nanoparticles,which will contribute to the high electrochemical performance.

The as-product was also characterized by Raman spectroscopy. As shown in Fig.2,a weak peak at about621cmà1con?rms the presence of SnO2[18,19].The peaks at about1342cmà1,1592cmà1 are assigned to the D and G band of graphene,respectively,and the intensity ratio of the D to G band(I D/I G)is as high as1.8,revealing that most of the oxygen functional groups intercalated into the interlayer spacing of graphite have been removed during the GO reduction process.The peaks at about2676cmà1,2929cmà1are assigned to the2D and DtG band of graphene[20],respectively, which also suggests the substantial removal of oxygen containing groups during the GO reduction process.Graphene of

high Fig.1.XRD patterns of graphene and SnO2/graphene

composite.

Fig.2.Raman spectrum of SnO2/graphene composite.

Q.Guo et al./Journal of Power Sources240(2013)149e154 150

reduction degree can provide high conductivity and large-current discharge capacity.

The graphene nanosheets are partially overlapped as shown in the low-resolution TEM image of SnO2/graphene composite (Fig.3a).Fig.3b is a TEM image of the raw material graphene oxide as a contrast to Fig.3c where SnO2nanoparticles are uniformly coated on the surface of graphene.Fig.3d is a high-resolution TEM image of SnO2/graphene from which we can see layered structures of graphene decorated with homogeneously SnO2nanoparticles whose size is about5nm,and the small stripes of the prints indi-cate good crystal lattices of SnO2.

XPS analysis was conducted in the range of0e1100eV to investigate the surface composition and chemical states in the SnO2/graphene composites.Fig.4a shows the wide-survey XPS spectrum of the SnO2/graphene,which attributes to the existence of C,O,Sn.In addition,the intense peaks of Sn and O suggest the presence of a great amount of SnO2nanoparticles.As depicted in Fig.4b,SnO2/graphene shows three different C1s building energies, 284.6eV for non-oxygenated C,286.3eV for carbon in C e O and 288.5eV for carbonyl carbon(C]O),respectively.The peak of C e O species is quite weak,which indicates the oxygen containing groups have been removed by the Sn2tions as well as certi?es that the Sn2tions lead to the reduction process of GO.Moreover,the presence of SnO2can be proved by the Sn3d spectrum(Fig.4c)in which two outstanding peaks are attributed to Sn3d5/2and Sn3d3/2.

The thermal properties and the compositions of the as-prepared products were characterized by thermogravimetric analysis(TGA) in air.TGA curves of the SnO2/graphene are shown in Fig.5.An abrupt weight loss occurs from350 C to650 C,indicating the oxidization of graphene[21].After650 C,there is no further mass loss.The stability of the trace indicates the complete removal of graphene.According to the TGA curves,58wt%of SnO2are coated on the surface of graphene nanosheets.

The electrochemical reactivity of the SnO2/graphene composite as anode in lithium-ion batteries was evaluated by cyclic voltam-metry(CV)in the potential range of0.01V e2.5V with scanning rates of0.2mV sà1,0.5mV sà1,1mV sà1,2mV sà1,respectively.As seen in Fig.6,there is a small cathodic peak in each curve,which can be ascribed to the formation of the solid electrolyte(SEI)layers at the surface of active materials[22]as well as the reduction of SnO2to Sn with the synchronous formation of Li2O and the Li insertion in graphene nanosheets to form Li x C(Eqs.(1)and(3)).In the anodic curve,the oxidation peak around0.5e0.85V can be assigned to Li extraction from graphene nanosheets,which is most likely due to the Li de-alloying from Li x Sn(Eq.(2)).The other oxidation peak around1.25e1.75V can be attributed to a conver-sion reaction occurring between Li2O and metallic Sn(Eq.(4))[23] as well as partly reversible reaction of the Eq.(1)[24].Even though the scanning rate changes,the positions of the oxidation peaks do not move much,which indicates the SnO2/graphene composite

has Fig.3.Transmission electron microscopy images of the SnO2/graphene composite(a,c,d)and graphene oxide(b).

Q.Guo et al./Journal of Power Sources240(2013)149e154151

an excellent cycling performance and good large-current discharge capacity due to the high conductivity of graphene.

4Li ttSnO 2t4e à

/Sn t2Li 2O

(1)

x Li ttSn tx e à4Li x Sn

(2)x Li ttC egraphene Ttxe à4Li x C (3)Li 2O tSn 4SnO t2Li tt2e à

(4)

Fig.4.XPS spectrum of the SnO 2/graphene (a),C1s (b),Sn3d

(c).

Fig.5.TGA curve of the SnO 2/graphene in

air.Fig.6.CV curves of the SnO 2/graphene composite.

Q.Guo et al./Journal of Power Sources 240(2013)149e 154

152

The electrochemical performance of the SnO2/graphene com-posite was evaluated via galvanostatic charge/discharge cycling at the current density of1A gà1with the voltage range from0.01to 2.5V.For comparison,the SnO2nanoparticles were also tested. Fig.7a and b show the typical charge/discharge pro?les of SnO2/ graphene and SnO2nanoparticles in the?rst two cycles.The initial discharge speci?c capacity of the SnO2/graphene composite is 1995.8mAh gà1with a reversible speci?c capacity of 1923.5mAh gà1while the SnO2nanoparticles are859.1mAh gà1 and689mAh gà1,respectively.Obviously,the initial coulombic ef?ciency of the SnO2/graphene composite is as high as96.4%, which is higher than that of the bare SnO2nanoparticles(80.2%). The large-current discharge capacity of the composites mainly re-sults from the formation of the Li x Sn alloy during the charging and discharging process[Eq.(2)]as well as the conversion reaction of Eq.(4).And it also can be attributed to the high reduction degree of graphene and the uniform distribution of SnO2nanoparticles on graphene nanosheets.This characteristic has never been explored and is highly desirable and important.The interaction between the two composites results to a much higher electrochemical capaci-tance than either of graphene or SnO2,which suggest that the SnO2 nanoparticles decorated graphene improves several properties of the composite such as the electronic and ionic conductivity,de-creases of the polarization of the charge/discharge processes[23].

The electrochemical cycling performances of the SnO2/graphene composite and bare SnO2nanoparticles were investigated at the current density of1A gà1(Fig.8).The SnO2/graphene composite exhibits a high initial discharge speci?c capacity of1995.8mAh gà1 (Fig.8a).The?rst charge capacity is1923.5mAh gà1which could remain about1707.7mAh gà1up to10cycles.And even after40 cycles,the reversible discharge capacity is still remained at 1545.7mAh gà1.On the other hand,the SnO2nanoparticles present a poor cycling performance(Fig.8b).The severe pulverization leads to a rapid fading of capacity and the charge capacity reduces rapidly from689mAh gà1to237.2mAh gàhttps://www.sodocs.net/doc/b717357784.html,pared with the bare SnO2nanoparticles,the SnO2/graphene composite shows superior charge capacity and cycling performance.This can be ascribed to the uniform distribution of SnO2nanoparticles on gra-phene,which fully utilizes SnO2to release the stress caused by the drastic volume variation during the lithium intercalation/dein-tercalation process.With the increasing of cycle numbers,the curves of discharge and charge are getting closer and closer,which proves the excellent cycling stability of the SnO2/graphene

composite.

Fig.7.Charge/discharge pro?les of the SnO2/graphene composite(a)and bare SnO2

nanoparticles(b)for the1st and

2nd.

Fig.8.Capacities vs.cycle number for SnO2/graphene composite(a)and bare SnO2

nanoparticles(b)at the current density of1A gà1.

Q.Guo et al./Journal of Power Sources240(2013)149e154153

In order to verify the electrochemical performance of the com-posite in comparison with pure SnO 2nanoparticles,electro-chemical impedance spectroscopy measurements were carried out.Fig.9shows the electrochemical impedance spectra of SnO 2/gra-phene composite and the bare SnO 2nanoparticles electrodes after 3charge/discharge cycles as well as their simulated spectra respectively.As shown in Fig.9,the Nyquist plots of both SnO 2/graphene composite and the SnO 2nanoparticles consist of two semicircles and a slope.The equivalent circuit of the electrodes is shown in Fig.10where R1,R2,R3,W1are denoted as solution resistance,SEI ?lm resistance,reaction resistance and Warburg impedance,respectively.A constant phase element is expressed as CPEi (CPEi ?{Yi(jw )}à1).A nonlinear,least-square ?tting calcula-tion is performed using the equivalent circuit shown in Fig.10.For SnO 2/graphene composite electrode,the electrochemical imped-ance value (R3)and CPE2value (T )are 54.5U cm à2and 112m F cm 2,and SnO 2nanoparticles electrode are 101.4U cm à2and 74m F cm 2.The value of CPE2value (T )is suggested to be the value of the double layer capacitance of the electrode and related to the true reaction areas of the electrode.So the true reaction area of SnO 2/graphene composite electrode is larger than that of the SnO 2electrode.The electrochemical impedance of the SnO 2/graphene composite electrode is lower than that of the SnO 2nanoparticles electrodes,which leads to a higher electrochemical activity of the SnO 2/graphene composite electrode.This result shows that gra-phene in the composite can largely enhance the electrochemical activity of SnO 2nanoparticles during the cycle processes.

4.Conclusions

SnO 2/graphene composite with uniform SnO 2nanoparticles coating was prepared by a simple wet chemical method.The as-prepared products exhibited an excellent lithium intercalation/deintercalation performance as anode materials for lithium-ion batteries.Our composite displayed large speci ?c capacities espe-cially at high current density.All of the superior electrochemical properties were ascribed to the high purity of graphene,the ho-mogeneously dispersion and thin-layered SnO 2on graphene.Our results could facilitate the practical applications of SnO 2/graphene composite in lithium-ion battery anodes.Acknowledgements

The authors gratefully acknowledge the ?nancial support from the National Natural Science Foundation of China (20603024)and the Open Project of Key Lab Adv.Energy Mat.Chem.(Nankai Univ.)(KLAEMC-OP201201).References

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Fig.9.Electrochemical impedance spectra of bare SnO 2nanoparticles electrode (a)and SnO 2/graphene electrode

(b).

Fig.10.The equivalent circuit of the bare SnO 2electrode and SnO 2/graphene electrode.

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