Large-Scale Synthesis of SnO2 Nanotube Arrays as High-Performance Anode Materials

ARTICLE

https://www.360docs.net/doc/171861479.html,/JPCC Large-Scale Synthesis of SnO2Nanotube Arrays as High-Performance Anode Materials of Li-Ion Batteries

Jiazheng Wang,Ning Du,*Hui Zhang,Jingxue Yu,and Deren Yang

State Key Lab of Silicon Materials and Department of Materials Science and Engineering,Zhejiang University,Hangzhou310027, People’s Republic of China

’INTRODUCTION

SnO2has been regarded as one of the most promising candidates for alternative anodes of Li-ion batteries due to the high theoretical capacity(781mAh/g)and low potential of lithium ion intercalation.1The main hindrance against commer-cial use of SnO2-based anode materials in lithium-ion batteries is the large volume change and the formation of electrochemical inactive Li2O during the charging and discharging processes, resulting in the poor cyclability.2The hybridizing of carbon can enhance the cyclability;however,this method sacri?ces the capacity itself due to the introduction of carbon with low theoretical capacity(372mAh/g).3Of pure SnO2,as anode materials of Li-ion batteries,it is suggested that nanosized SnO2with hollow interior structures could shorten the pathway lengths of lithium ions and accommodate the large volume change,thus showing improvement in cyclic performance.4The performance of SnO2 nanotubes and hollow spheres as anode materials of Li-ion batteries has con?rmed these views.4However,the volume change of SnO2still exists,which results in pulverization and thus a loss of electrical contact that limits the cycling life of electrodes.4Recently,ordered active nanostructures grown di-rectly on current collectors have shown the signi?cant improve-ments in Li-ion battery performance.5There are many advantages envisioned such as good contact of the active materials and adhesion with the current collector as well as good strain accommodation.Therefore,direct growth of SnO2nanostruc-tures on current collectors is expected to show good performance in Li-ion batteries.Synthesis of SnO2nanotube arrays on metal substrates still remains a challenge.More recently,Kuang et al. reported the synthesis of SnO2nanotube arrays on a silicon substrate via a chemical vapor deposition(CVD)process by using ZnO nanowire arrays as sacri?cial templates.7d The e?ec-tive two-step templates-assisted method for fabricating aligned SnO2nanotube arrays is of great interest.However,the CVD process needs the high reaction temperature and usually results in the inhomogenous deposition of SnO2layer along the ZnO nanowires.Moreover,it should be noted that silicon substrate is not suitable for a current collector of Li-ion batteries.Herein,we report a solution strategy to synthesize SnO2nanotube arrays on a titanium substrate by using ZnO nanowire arrays as sacri?cial templates.

’EXPERIMENT SECTION

Synthesis of SnO2Nanotube Arrays on Ti Substrate.ZnO nanowire arrays on Ti substrate were synthesized by a seed-assisted chemical reaction as described in our previous report.6 The synthesis of ZnO/SnO2coreàshell nanowire arrays was achieved by a hydrothermal shell-by-shell templating strategy using the prefabricated ZnO nanowire arrays as templates. Briefly,0.2g of Na2SnO334H2O and1.8g of urea were added into62mL of ethanol/water(60vol%ethanol)mixed solvents. After magnetic stirring for about10min,the suspension was transferred to a100-mL Teflon-lined stainless steel autoclave.Then the Ti substrates with as-prepared ZnO nanowire arrays were hung vertically onto the bottom of the autoclave,which was heated in an air flow electric oven at170°C https://www.360docs.net/doc/171861479.html,stly,the samples were removed from the autoclave and washed with clean water.

The as-synthesized ZnO/SnO2coreàshell nanowire arrays on Ti substrates were annealed at600°C under Ar for1h and then immersed in0.2M HCl solution for2h.After that,the samples were cleaned with deionized water,and dried in the air.

Characterization and Electrochemical Measurement.The obtained samples were characterized by X-ray powder diffraction (XRD)with a Rigaku D/max-ga X-ray diffractometer with graphite monochromatized Cu K radiation(λ=1.54178?). Received:February17,2011

Revised:May4,2011

ABSTRACT:This paper presents the large-scale synthesis of SnO2nanotube arrays on titanium substrate via ZnO nanowire arrays as sacri?cial templates.The SnO2nanotube arrays on titanium substrate feature the large surface area,good electronic conductivity,and adhesion with the current collector,leading to the enhanced performance in lithium-ion

batteries.

Published:May18,2011

The morphology and structure of the samples were examined by scanning electron microscopy (SEM HITACH S4800)with energy-dispersive X-ray spectrometry (EDX),transmission elec-tron microscopy (TEM,PHILIPS CM200),and high-resolution transmission electron microscopy (HRTEM,JEOL JEM-2010).Electrochemical measurements were performed by coin-type cells (CR2025),which were assembled in a glovebox (Mbraun,labstar,Germany)under an argon atmosphere by directly using the as-synthesized SnO 2nanotube arrays on Ti substrates as the anode.The counter and reference electrodes were lithium metal foils (15mm diameter),and the electrolyte solution was 1M solution of LiPF 6in ethylene carbonate (EC)and Dimethyl carbonate (DMC)(1:1by volume).Finally,the cells were then aged for 12h before measurements.

A galvanostatic cycling test of the assembled cells was carried out on a Land CT2001A system in the potential range of 5mV-2.0V at a discharge/charge current density of 200mA/g.Cyclic voltammetry (CV)were recorded on a MSTAT4(Arbin In-struments)system at a scan rate of 0.5mVs à1.

’RESULTS AND DISCUSSIONS

The schematic illustration of the typical synthetic strategy is displayed in Figure 1a.It can be seen that the whole process involves three steps:(1)Growth of ZnO nanowire arrays on titanium substrate by a seed-assisted method as previously reported by our group;6(2)coating of SnO 2layer onto the surface of ZnO nanowires by a hydrothermal process,which was based on a simple coating method developed by Archer et al.7a àc with some modi ?cations;(3)Removal of ZnO nanowires by HCl solution etching.7d From the photographs of the titanium substrate along with the synthetic processes,we can see that the surface seems to change from the metallic polish to black and ?nally to semitransparent with a slight

white,which primarily indicate the growth of ZnO nanowires and SnO 2nanotube arrays,respectively.Figure 2a is a typical SEM picture of ZnO nanowire arrays grown on titanium substrate.Figure 2b shows XRD pattern obtained from the as-prepared ZnO nanowire arrays.The ZnO (002)and (103)peaks can be found at 34.4and 62.8,respectively.Other peaks are relatively weak,con ?rming the synthesis of ZnO nanos-tructures which are well aligned and perpendicular to the substrate.To further clarify the synthetic processes,the morphological,structural,and compositional characterizations of the interim and ?nal products have been performed.Parts c and d of Figure 2show the morphological,structural,and compositional characterizations of the products by coating of SnO 2layer onto the surface of ZnO nanowire arrays.It can be seen that homogeneous core àshell nanowire arrays with a ZnO nanowire core and SnO 2outer layer have been obtained,which is a prerequisite for the synthesis of SnO 2nanotube arrays.Similar to the previously reported SiO 2nanostructures,7the as-synthesized ZnO nanowires are terminated with hydro-xyl (OH à)groups,which can facilitate the homogeneous deposition of the SnO 2layer.This hydrothermal method can avoid the surface modi ?cation before the coating process compared to our previously reported layer-by-layer coating method,8which may be more suitable for industrial fabrication.Parts a and b of Figure 3show the SEM images of ZnO-SnO 2core àshell nanowires after HCl solution etching.It can be seen that large-area nanotube arrays have been obtained,most of which exhibit the opening

ends.Parts c and d of Figure 3

Figure 2.Morphological and structural characterizations of ZnO nanowire arrays:(a)SEM image;(b)XRD pattern.Morphological and structural characterizations of ZnO àSnO 2core àshell nanowire arrays:(c)SEM image;(d)EDX

pattern;(e)TEM image;(f)HRT-EM image.

Figure 1.Schematic illustration for the synthetic strategy of SnO 2nanotube arrays (a);photographs of the surface of the titanium substrate (b);ZnO nanowire arrays on Ti substrate (c);SnO 2nanotube arrays on Ti substrate (d).

display the TEM and HRTEM images of the as-synthesized nanotubes which were scrapped from the titanium substrate. The nanotube exhibits a diameter of about100à300nm and a shell thickness of about10à20nm,which is composed of nanoparticles with diameters of2à5nm.The lattice spacing of 0.34nm corresponds to the{110}plane of SnO2,indicating the formation of SnO2layer.The as-prepared?lm on the surface of the titanium substrate was characterized by X-ray di?ractmeter glancing attachments.The XRD pattern(Figure3e)further con?rms the synthesis of pure SnO2nanotube arrays.The small TiO peaks(JCPDS card no.86à2352)indicate the slight oxidation on the surface of titanium substrate.Figure3f is the energy-dispersive X-ray(EDX)spectrum of the as-synthesized SnO2nanotube arrays after acid treatment.EDX analysis shows that the as-prepared sample contains elements Sn,O,and Ti, which come from the SnO2nanotube arrays and the substrate, respectively.After HCl solution etching,element Zn does not exist anymore indicating that ZnO has been dissolved and removed completely.This result is consistent with the XRD pattern in Figure3e.Yang et al.pioneered the synthesis of nanotube arrays via sacri?cial templates,such as GaN nanotube arrays obtained by using ZnO nanowire arrays as sacri?cial templates on silicon substrates.9ZnO nanowire arrays can be selectively eliminated by acid,alkali,and high-temperature reduction,which could also act as sacri?cial templates to synthesize nanotube arrays.More recently,Song et al.fabri-cated silicon nanotube arrays on steel substrate via CVD by using ZnO nanowire arrays as sacri?cial templates.10Herein, we apply the hydrothermal method to deposit SnO2layer onto the surface of ZnO nanowires,thus obtaining SnO2nanotube arrays on a titanium substrates.

Motivated by the unique structure of SnO2nanotube arrays on titanium substrate,we perform the lithium storage test as anode. The electrochemical reaction of Li with SnO2can be described in eqs1and2

SnO2t4Lit4eàf Snt2Li2Oe1T

x LittxeàtSn T Li x Sne2TOn the basis of the reversible reactions1and2,the theoretical total capacity of the SnO2in the?rst and second cycle is1494mAh/g(8.4Li)and790mAh/g(4.4Li), respectively.Cyclic voltammetry(CV)measurement was car-ried out to understand the process of alloying and dealloying process.Figure4shows the?rst three cyclic voltammogram (CV)curves of the SnO2nanotube arrays electrodes at a scan rate of0.5mVsà1and a temperature of20°C.The cathodic peaks located around0.9V can be attributed to the formation of Li2O as well as the solid state electrolyte interface(SEI)?lm. The peak between0and0.6V corresponds to the alloying reaction of Sn and Li.An anodic peaks k(0.6V)indicates the dealloying of Li x Sn alloy.A galvanostatic cycling test of the assembled cells was carried out in the potential range of5mV to2.0V at a dischargeàcharge current density of200mA/g. The plateaus were found at around0.9V in the?rst discharge curve,which may correspond to the formation of the solid state electrolyte interface(SEI)?lm and Li2O(Figure5a).4It can be seen from Figure5b that the SnO2nanotube arrays show a high discharge capacity of790mAh/g after20cycles with the retention of about73%.The discharge capacity is higher than the theoretical capacity of SnO2anode.The partial reversibility of the reduction process described by the?rst reaction may be responsible for the extra reversible capacities SnO2nanotube arrays anode.11aàc The Coulombic e?ciency for the?rst cycle is about61%,while the other cycles keep steadily more than 95%.Recently,Liu et al.reported the synthesis of SnO2 nanorod arrays on FeàCoàNi alloy substrate via a hydro-thermal process,which exhibit a discharge capacity of about 750à800mAh/g after20cycles at a current of0.1C(78.1mA/g).5e The capacity is comparable with our results.However,consider-ing our current(200vs78.1mA/g)is larger and?rst Coulombic e?ciency(61vs59%)is higher,the SnO2nanotube arrays reported here is very attractive.More recently,Wang et al. obtained the extremely thin SnO2nanosheet via a typical

hydrothermal process,which exhibit a discharge capacity of Figure4.The?rst three CV curves of SnO2nanotube arrays anode in the potential range of0.0à2.0V at a scan

rate of0.5mV sà1.

Figure 3.Morphological and structural characterization of SnO2

nanotube arrays on titanium substrate:(a and b)SEM image;(c)TEM

image;(d)HRTEM image;(e)XRD pattern;(f)EDX pattern.

559mAh/g after 20cycles with the retention of 57%at a constant current of 78.1mA/g.12Also,we can claim that the SnO 2nanotube arrays show better performance than SnO 2nanosheet as anode materials of Li-ion batteries.Figure 5c shows the SEM image of SnO 2nanotube arrays after 20cycles as anode materials of Li-ion batteries.It can be seen that the nanotube arrays were basically maintained after 20charge àdischarge processes,in-dicating the good adhesion with the current collector.However,most of the opening ends of the SnO 2nanotubes were closed,which may be due to the large volume change during the charge àdischarge process.The TEM image (Figure 5d)shows that the shell thickness of the SnO 2nanotubes increase slightly without the collapse of the nanotubular shape,indicating the good strain accommodation of SnO 2nanotube arrays.It is believed that the good contact and adhesion with the current collector as well as good strain accommodation of SnO 2nano-tube arrays on titanium substrate may be responsible for the good performance.

In summary,SnO 2nanotube arrays on titanium substrate were synthesized by a prefabricated ZnO nanowire array as sacri ?cial templates.The as-synthesized SnO 2nanotube arrays were applied as anode materials of Li-ion battery,which exhibit high capacity and improved cycling performance.The good contact and adhesion with the current collector,as well as good strain accommodation of SnO 2nanotube arrays on titanium substrates,may be responsible for the good performance.More-over,such strategy can be extended to synthesize other metal oxide nanotube arrays on metallic substrate.

’AUTHOR INFORMATION

Corresponding Author

*E-mail:dna1122@https://www.360docs.net/doc/171861479.html,.Phone:86-571-87953190.Fax:86-571-87952322.

’ACKNOWLEDGMENT

Theauthorswouldliketoacknowledgethe ?nancialsupportfrom973Project (No.2007CB613403),863Project (No.2007AA02Z476),NSFC(No.50802086),ChinaPostdoctoralScienceFoundation funded project (No.20090461350),NSFC (No.51002133).’REFERENCES

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Figure 5.(a)First,second,and twentieth discharge curves of the SnO 2nanotube arrays-based anode materials at a current density of 200mA/g and a temperature of 20°C.(b)Discharge capacity vs cycle number for the SnO 2nanotube arrays-based anode materials at a current density of 200mA/g and a temperature of 20°C.(c)SEM image of the SnO 2nanotube arrays after 20cycles.(d)TEM image of the SnO 2nanotube arrays after 20cycles.