A facile route to carbon-coated SnO2 nanoparticles combined with a new binder for enhanced cyclabili

Electrochimica Acta 54 (2009) 7519–7524Contents lists available at ScienceDirectElectrochimicaActaj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c taA facile route to carbon-coated SnO 2nanoparticles combined with a new binder for enhanced cyclability of Li-ion rechargeable batteriesShu-Lei Chou a ,b ,∗,Jia-Zhao Wang a ,b ,Chao Zhong a ,b ,M.M.Rahman a ,b ,Hua-Kun Liu a ,b ,Shi-Xue Dou aa Institute for Superconducting and Electronic Materials,University of Wollongong,NSW 2522,Australia bARC Center of Excellence for Electromaterials Science,University of Wollongong,NSW 2522,Australiaa r t i c l e i n f o Article history:Received 14July 2009Accepted 2August 2009Available online 11 August 2009Keywords:SnO 2Carbon coatingLithium-ion battery BinderNanoparticlesa b s t r a c tCarbon-coated SnO 2nanoparticles were prepared by a novel facile route using commercial SnO 2nanopar-ticles treated with concentrated sulfuric acid in the presence of sucrose at room temperature and ambient pressure.The key features of this method are the simple procedure,low energy consumption,and inexpensive and non-toxic source materials.As-prepared core/shell nanoparticles were characterized by X-ray powder diffraction (XRD),thermogravimetric analysis (TGA),scanning electron microscopy (SEM),energy-dispersive X-ray spectrometry (EDX),transmission electron microscopy (TEM),and high-resolution transmission electron microscopy (HRTEM).The electrochemical measurements showed that the carbon-coated SnO 2nanoparticles with 10%carbon and using carboxymethyl cellulose (CMC)as a binder displayed the best electrochemical performance with the highest specific capacity of 502mAh g −1after 50cycles at a current density of 100mA g −1.In addition,owing to the water solvability of CMC,the usage of CMC as binder makes the whole electrode fabrication process cheaper and more environmental friendly.© 2009 Elsevier Ltd. All rights reserved.1.IntroductionRechargeable lithium-ion batteries are currently the technol-ogy of choice for portable electronic devices [1,2].In order to meet the requirements of the rapidly increasing portable electron-ics market,the performance of rechargeable lithium-ion batteries needs to be improved with higher energy density and better rate capability.Currently,the commercial anode material of choice is graphite,which has a relatively low theoretical capacity around 370mAh g −1[3].Materials based on tin oxide have been pro-posed as promising alternative anode materials for rechargeable lithium batteries owing to their high theoretical capacity (SnO 2:790mAh g −1)[4–9].The severe volume expansion and contraction during the alloying–dealloying cycles with Li +ions is the main hin-drance to the commercial use of SnO 2[10].In order to alleviate the volume changes,two main methods have been investigated,fabricating SnO 2into nanostructures [11–18]and adding/coating with buffer materials (such as carbon or other conductive materi-als),inside or outside the particles [19–26].Various types of SnO 2nanostructures,such as nanotubes [11–14],nanowires [15,16],and nanorods [17,18],have been investigated and were found to∗Corresponding author at:Institute for Superconducting and Electronic Mate-rials,University of Wollongong,Northfields Avenue,Wollongong,NSW 2522,Australia.E-mail address:sc478@.au (S.-L.Chou).have enhanced electrochemical performance.They provide more reaction sites,short Li +diffusion lengths,and considerable kinetic enhancement.However,the high cost for synthesizing the nanos-tructured materials and the relatively low capacity retention still need to be addressed for future commercial application.On the other hand,surface-coated/composite SnO 2showed better elec-trochemical performance than the bare material.Among the buffer materials,carbon has to be considered as the best choice because of its cheap,light,and conductive nature.To date,carbon coated onto or added into SnO 2has been prepared by spray pyrolysis,polymer coating and then carbonization,and hydrothermal/solvothermal methods [19–25].However,it is still important to explore eco-nomical synthesis techniques with low energy consumption for the formation of carbon-coated SnO 2.Recently,the choice of binder has become a very important issue in finding a solution to the problem of the large capac-ity fade observed for anode materials after cycling [27–31].Buqa et al.reported that nano-Si electrode containing 1%sodium car-boxymethyl cellulose (CMC)as binder shows the same cycle stability as an identical electrode containing 10%conventional polyvinylidene fluoride (PVdF)binder [27].Lestriez et al.claimed that the extended conformation of CMC in solution facilitates an efficient networking process between the conductive agent and the Si particles [28].Hochgatterer et al.reported that the chemi-cal bonding between CMC binder and Si particles contributes to the enhanced capacity retention of Si/C composite electrodes [29].Li et al.reported that Fe 2O 3electrodes using CMC binder and two other0013-4686/$–see front matter © 2009 Elsevier Ltd. All rights reserved.doi:10.1016/j.electacta.2009.08.0067520S.-L.Chou et al./Electrochimica Acta54 (2009) 7519–7524new binders show better cycling performance(about800mAh g−1 for100cycles)compared to electrodes made from conventional PVdF binder[30].Another advantage of using CMC as the binder is that CMC can be dissolved and processed in water,which makes the whole electrode fabrication process cheaper and more environ-mental friendly.However,there are still no reports on using CMC binder with SnO2-based anode materials.Here,carbon-coated SnO2nanoparticles were prepared by a novel facile route using commercial SnO2nanoparticles treated with concentrated sulfuric acid in the presence of sucrose at room temperature and ambient pressure.The key features of this method are the simple procedure,low energy consumption,and inexpen-sive and non-toxic source materials.In addition,CMC was used as a binder to further investigate its effects on the electrochemical performance.2.Experimental2.1.Preparation of carbon-coated SnO2nanoparticlesThe method used here is adopted from the literature[32].The typical procedure to prepare Sample CS-1is as follows.A mix-ture of the SnO2nanoparticles(1.7193g,99.5%,Nanostructured& Amorphous Materials Inc.,61nm),sucrose(3.0159g,98%,Aldrich), and water(1mL)was ultrasonicated for30min at room temper-ature,and then H2SO4(5mL,98wt.%,Sigma–Aldrich)was slowly added with stirring.After reacting for about20min,thefinal prod-ucts were washed with N,N-dimethylformamide three times,with water three times,and with ethanol three times,and then dried under vacuum at80◦C for12h.Sample CS-2was prepared by using the similar method as Sample CS-1.The only difference is that sam-ple CS-2increased the mass of the sucrose to6.025g.The pure carbon-based sample was also prepared via the similar method as CS-1without using SnO2nanoparticles.2.2.CharacterizationThe morphology and microstructure of the as-prepared carbon-coated SnO2nanoparticles were characterized by X-ray diffraction (XRD;GBC MMA017),the Brunauer–Emmett–Teller technique (BET;Quanta Chrome Nova1000),scanning electron microscopy (SEM;JEOL JSM6460A,30kV)with JEOL energy-dispersive X-ray (EDX)spectroscopy and EDX mapping systems,and transmission electron microscopy(TEM;JEOL2011,200KV).The thermal proper-ties of the as-prepared carbon-coated SnO2were characterized by thermogravimetric analysis(TGA;TA Instruments2000)under air at aflow rate of40mL min−1over a temperature range of80–800◦C with a ramp rate of5◦C min−1.For infrared(IR)spectroscopy,the samples were mixed with KBr powder,placed in a sample cup,and measured using a Shimadazu IRPrestige-21Fourier transform IR (FT-IR)spectrometer.KBr was used as the backgroundfile.All spec-tra were measured from4000to500cm−1,and the number of scans was typically10,with a resolution of2cm−1.2.3.Electrochemical testingTo test the electrochemical performance,carbon-coated SnO2 nanoparticles were mixed with acetylene black(AB)and a binder, sodium carboxymethyl cellulose(CMC,average Mw:∼250,000, Aldrich)or poly(vinylidenefluoride)(PVdF,Mw:∼534,000, Aldrich),in a weight ratio of70:20:10in a solvent,consisting of either water or N-methyl-2-pyrrolidone(99.5%,Aldrich),respec-tively.The slurry was uniformly pasted onto pieces of Cu foil with an area of1cm2.Such prepared electrode sheets were dried at90◦C in a vacuum oven for12h.The electrochemical cells(CR2032coin-type cell)contained carbon-coated SnO2nanoparticles on a Cu foil as the working electrode,Li foil as the counter electrode and ref-erence electrode,a porous polypropylenefilm as separator,and 1M LiPF6in a1:2(v/v)mixture of ethylene carbonate(EC)and diethyl carbonate(DEC)as the electrolyte.The cells were assem-bled in an Ar-filled glove box.The cells were cycled at a current density of50mA g−1for thefirstfive cycles and then cycled at 100mA g−1for the following cycles between0.01and1.5V using a computer-controlled charger system manufactured by Neware Bat-tery Testers.The specific capacity is calculated based on the mass of CS-1,CS-2,and pristine SnO2in the electrode,respectively.3.Results and discussionSEM and TEM images and EDX mapping of carbon-coated SnO2 nanoparticles(CS-1sample)are shown in Fig.1.Fig.1(a)shows that the nanoparticles of the CS-1sample are agglomerated into big particles several micrometers in diameter.The EDX spectrum in Fig.1(d)shows that only the elements Sn,O,and C are present in the sample,indicating its high purity.Fig.1(b)and(c)contains the EDX mapping images for C and Sn,showing the good distribution of car-bon in the composite.That is to say,the carbon is uniformly coated onto the surface of the SnO2nanoparticles.The transmission elec-tron microscope(TEM)image of CS-1in Fig.1(e)demonstrates that the nanoparticles(dark particle)have a size of about60nm,which is in a good agreement with that of the pristine SnO2nanoparticles, and are trapped inside the carbon materials(bright,film-like mate-rial).The high-resolution TEM(HRTEM)image in Fig.1(f)displays lattice fringes with a spacing of0.36nm,which is in good agree-ment with the spacing of the(110)planes of SnO2.The amorphous layer outside of the SnO2crystal marking with red arrows is the amorphous carbon coating.For comparison,samples with different carbon contents,named CS-1and CS-2,were also prepared by increasing the ratio of sucrose to SnO2nanoparticles.XRD,FT-IR,and TGA results are shown in Fig.2.Fig.2(a)demonstrates that the SnO2and carbon-coated SnO2 materials have the same crystal structure,which can be indexed to rutile-type SnO2(Joint Committee on Powder Diffraction Standards (JCPDS)Card No.41-1445).The pure carbon obtained from sucrose treated with H2SO4only shows a broad peak centered at around 22◦,indicating the amorphous carbon structure.The carbon-coated SnO2samples show small broad peaks at around22◦,showing the presence of carbon-based materials in the CS-1and CS-2.FT-IR transmission spectra of SnO2nanoparticles and carbon-coated SnO2nanoparticles are shown in Fig.2b.It can be seen that all the samples show absorption peaks at637cm−1,which were assigned to the vibration of Sn–O–Sn[33].For pristine SnO2nanoparticles, the absorption peaks at around3500,1647,and1368cm−1were attributed to the hydroxyl vibrations due to absorbed water on the surface of the nanoparticles.For carbon-coated SnO2nanoparti-cles,the broad absorption peak at around3500cm−1and two other peaks at1717and1278cm−1were attributed to the carboxyl func-tional group,indicating that there are still organic functional groups in the carbon coating[34].The weight percentage of carbon-based materials was measured by means of thermogravimetric analysis (TGA).Due to the thermal stability of SnO2,the main weight loss results from the decomposition of carbon-based materials from 80◦C to500◦C.The carbon contents of pristine SnO2,CS-1,and CS-2are0%,10%,and27%,respectively.Typical discharge–charge curves of the pristine SnO2nanopar-ticle(a and c)electrodes and the CS-1(b and d)electrodes,using PVdF(a and b)and CMC(c and d)as binders in Li-ion coin cells, are shown in Fig.3.The curves are similar to what has been pre-viously reported[21–25].The main reason for the existence of the initial irreversible capacity for anode materials is the formation of aS.-L.Chou et al./Electrochimica Acta54 (2009) 7519–75247521Fig.1.SEM image(a),EDS mapping of carbon(b),Sn(c),EDX spectrum(d),TEM(e)and HRTEM(f)image of carbon-coated SnO2nanoparticles(CS-1).solid electrolyte interphase(SEI)layer,which forms on the surface of the anode material during thefirst discharge at around0.8V(vs. Li+/Li)[35],and the irreversible reaction from SnO2to Sn.In the initial cycle,SnO2was reduced to Sn(Eq.(1)),and then the Sn can reversibly react with Li+through alloying and dealloying processes in the following cycles(Eq.(2))[36]:SnO2+4Li++4e−→Sn+2Li2O(1) Sn+x Li++x e−↔Li x Sn(0≤x≤4.4)(2) The initial discharge capacities of the electrodes using CMC as a binder are much higher than those of the electrodes using PVdF as a binder.The initial discharge capacity for SnO2and CS-1with CMC is1637and1651mAh g−1,while the initial discharge capacity with PVdF as a binder is1589and1458mAh g−1,respectively.The reversible capacities of electrodes using CMC as the binder are also higher than for those using PVdF as the binder.The carbon-coated electrode(CS-1)shows lower specific capacity compared with the pristine SnO2electrode but relatively good capacity retention,no matter which kind of binder was used.It can be seen in Fig.3(d)that the charge and discharge curves of the CS-1sample remain almost the same,indicating the good capacity retention.The repeated cycling induces enormous volume changes in Sn particles,which tend to expand and coalesce with the nearby Sn atoms,leading to large agglomerates and thus reducing the available surface area for the Li-ion storage capacity of the pure SnO2.The carbon coating can prevent the agglomeration of the Sn particles and improve the cycle stability.The differential charge and discharge profiles are shown in Fig.4. In the initial discharge,a major reduction peak at around0.90V is observed in all the curves,which is derived from Li2O formation when SnO2reacts with Li+,while the shoulder peaks at around0.8V are due to the formation of the SEI layer.These peaks disappear in the following cycles and leave a large initial irreversible capacity, which is relevant to the galvanostatic voltage profile presented in Fig.3.The small reduction peaks around0.6and0.3V correspond to the formation of Li x Sn alloys[36].Oxidation peaks can be found at0.45,0.6,0.8,and1.2V,indicating the multi-step transporta-tion of electrons from Li x Sn(0≤x≤4.4)to Sn.The intensities of the peaks for the electrodes using PVdF as the binder decreased much more quickly than those of electrodes using CMC as the binder. The CS-1using CMC as binder shows the best stability in terms of the peak positions and intensities,indicating the best reversibility of the reaction between Li+ions and Sn.Therefore,by combining CMC as a binder with the application of a carbon coating,enhanced cycle stability can be achieved.Fig.5displays the cycle stability of the pristine SnO2,CS-1, and CS-2samples at a current density of50mA g−1for thefirst five cycles and then cycled at100mA g−1for the following cycles between0.01and1.5V.It can be seen in Fig.5(a)that the capacity of the samples using CMC as binder is much higher than for samples using PVdF,which shows that the binder plays an important role in the cycle stability of electrode[27–31].After50cycles,the pristine SnO2and CS-1using CMC as binders show capacities of382andFig.2.XRD patterns(a),FT-IR transmission spectra(b),and TGA curves(c)of commercial SnO2nanoparticles,pure carbon,and carbon-coated SnO2nanoparticle samples CS-1and CS-2.7522S.-L.Chou et al./Electrochimica Acta54 (2009) 7519–7524Fig.3.Typical discharge–charge curves of the pristine SnO 2nanoparticle (a and c),CS-1(b and d)electrode using PVdF (a and b),and CMC (c and d)as binder.507mAh g −1,respectively,while those using PVdF show capacities of only 221and 248mAh g −1,respectively.It can also be found that the carbon-coated sample CS-1shows better cycle stability than pristine SnO 2no matter which kind of binder is used.In order to compare the electrochemical performance with the references,the summary of the specific capacities for different SnO 2-based sam-ples was listed in Table 1.It can be seen that the commercial SnO 2nanoparticles show almost the lowest specific capacity comparing with other previously reported materials,while after coating withcarbon sample CS-1shows better performance than porous SnO 2nanopowder [37],SnO 2microspheres [38],SnO 2/carbon composite [19]and SnO 2/carbon hollow spheres [23].CS-1even shows as good electrochemical performance as SnO 2nanospheres [39].Although the performance of present materials is not as good as SnO 2nan-otubes [11]or meso-scale tubes [14],the simple and low-cost method and the relatively high electrochemical performance make the present materials a very promising anode materials for the future industrial usage.In addition,because CMC can bedissolvedFig.4.Differential charge–discharge capacity versus potential from the first to the 50th cycle between 0.01and 1.50V for pristine SnO 2(a and b)and CS-1(c and d)samples using PVdF (a and c)and CMC (b and d)as binders.S.-L.Chou et al./Electrochimica Acta54 (2009) 7519–75247523Fig.5.The cycle life(a),coulombic efficiency(b),and capacity retention(c)of as-prepared carbon-coated SnO2(CS-1and CS-2)and pristine SnO2nanoparticles using CMC and PVdF as binder.Table1The comparison of the specific capacities for different SnO2samples.Materials Capacity(mAh g−1)Cycle number Current density(mA g−1)ReferenceCS-150750100This work Commercial SnO2nanoparticles38750100This work Porous SnO2nanopowder41010079[37]SnO2microspheres48040100[38]SnO2nanospheres50040158[39]SnO2meso-scale tubes4621501580[14]SnO2nanotubes720503160[11]SnO2/carbon composite35050–[19]SnO2/carbon hollow spheres41060158[23]and processed in water,it makes the whole electrode fabrication process cheaper and more environmentally friendly.The coulombic efficiency shown in Fig.5(b)illustrates that the initial coulombic efficiency is in the order of pristine SnO2>CS-1>CS-2,which is the reverse order of the carbon contents.A possible reason is that the carbon coating contains organic func-tional groups(such as–COOH)which can irreversibly react with lithium ions and decrease the initial coulombic efficiency.The more carbon in the composite,the lower the initial coulombic efficiency will be.The average coulombic efficiency for the following cycles are in the order of CS-1>CS-2>pristine SnO2,indicating that the certain amount(10%)of carbon coating could prevent the agglom-eration of Sn particles and improve coulombic efficiency.Since the coated carbon contains a lot organic functional groups,the further increased amount(27%)of carbon will bring a lot of side reason which will decrease the coulombic efficiency again.The capacity retentions shown in Fig.5(c)are in the same order as the coulom-bic efficiency.It can be found that the capacity retention curves of pristine–SnO2,CS-1and CS-2show similar slopes for thefirst 20cycles.After20cycles,the slopes of the capacity retention are in the order of CS-1<CS-2<pristine SnO2.Therefore,the CS-1sample shows the best capacity retention.The possible reason for this phenomenon is that the produced Li2O from the reaction between lithium and SnO2nanoparticles can somehow prevent the agglomeration of Sn for a few cycles(20cycles)[40].Without carbon coating,after20cycles the agglomeration of Sn becomes quicker and the capacity drops more quickly.On the contrary,the carbon coating can effectively prevent the agglomeration of Sn par-ticles and improve the capacity retention for the following cycles [19,23–25].4.ConclusionsCarbon-coated SnO2nanoparticles were prepared by a novel facile route using commercial SnO2nanoparticles treated with concentrated sulfuric acid in the presence of sucrose at room tem-perature and ambient pressure.The key features of this method are the simple procedure,low energy consumption,and inexpensive and non-toxic source materials.The electrochemical measure-ments showed that both the carbon coating and the binder selection affected the electrochemical performance.The carbon-coated SnO2 nanoparticles(CS-1)with10%carbon using CMC as a binder dis-played the best electrochemical performance,with the highest specific capacity being502mAh g−1up to50cycles at a current den-sity of100mA g−1.Owing to the water solvability of CMC,the usage of CMC as binder makes the whole electrode fabrication process cheaper and more environmental friendly.7524S.-L.Chou et 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