07) 4370–4373www.elsevier.com/locate/matlet

Materials Letters 61 (20

Synthesis and characterisation of SnO2 nano-single crystals as anodematerials for lithium-ion batteries

Ying Liang ⁎, Jing Fan, Xiaohong Xia, Zhijie Jia

Center of Nano-Science and Technology, Department of Physics, Central China Normal University, Wuhan 430079, P.R. China

Received 22 November 2006; accepted 1 February 2007Available online 8 February 2007

Abstract

Rutile structure SnO2 nano-single crystals have been synthesized using tin (IV) chloride as precursor by the modified hydrothermal method.Controllable morphology and size of SnO2 could be obtained by adjusting the concentration of the hydrochloric acid. The SnO2 nanoparticleswere characterised by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), scanning electronmicroscopy (SEM), X-ray diffraction (XRD) and electrochemical methods. The SnO2 nanoparticles as anode materials in lithium-ion batteriesexhibit high lithium storage capacities. The reversible capacities are more than 630 mA h g−1.© 2007 Elsevier B.V. All rights reserved.

Keywords: Characterisation methods; Nanomaterials; Lithium-ion batteries; Electrochemical properties

1. Introduction

Tin oxide with nanometer-scale dimension and morpholog-ical specificity as anode materials for lithium-ion batteriesexhibits high volumetric and gravimetric capacities in Li-ionbatteries [1–6]. Theoretically, a tin oxide anode can give amaximum charge-storage capacity of 781 mA h g−1, which isabout twice the theoretical capacity of carbon anode.

Some of the previous studies on Li+ storage in SnO2 aresubstantially lower than the theoretical values and have verypoor cycle-life. For instance, Courtney and Dahn [2] showedthat the capacity decreases from the first cycle to the secondcycle by 200 mA h g−1 for SnO2. Sivashanmugam [4] used aprecipitation technique to obtain sub-micron size SnO2 particlesas active materials, the first-cycle charge capacity for SnO2

heat-treated at 400 °C is 485 mA h g−1. Even if SnO2 is heat-treated at 800 °C, the initial capacity only increases to 556 mAh g−1. Above all, there is a substantial loss of charge and Li+ inthe first cycle due to the irreversible electrochemical reductionof oxide. Meanwhile, volume changes by as much as 259%

⁎ Corresponding author. Tel./fax: +86 276 786 1185.E-mail address: xfliangy@163.com (Y. Liang).

0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.02.008

occurred when Li+ was inserted and deinserted from Sn-basedmaterials, which led to mechanical disintegration of theelectrode. Prior work has shown that different sizes andmicrostructures of tin dioxides play important roles in theirelectrochemical performance. Therefore, controllable morphol-ogy and size of SnO2 could improve their electrochemicalproperty.

Various methods for preparing tin oxide thin film, nano-particles, and nanorods have been reported, such as electro-chemical synthesis [7], solid-state reactions [8], a templatemethod using a microporous polycarbonate filter and thermaloxidation of tin substrates in air [9] or preparation withcetyltrimethylammonium bromide (CTAB) surfactant [10]. Butthese techniques involve either more than one step or theexpensive starting materials. Moreover, these products need tobe sintered at high temperature, which led to a more definedgrain structure. In this work, SnO2 nano-single crystals withcontrollable morphology and high crystallinity were preparedby a modified hydrothermal method, which was developed inour laboratory to fabricate nanoparticles, for example SnO [11]and ZnO [12]. This method has advantages over other processesbecause of its simplicity, low equipment cost and the possibilityin making some nanometer oxides. The electrochemical

Fig. 1. X-ray diffraction patterns of (a) the as-prepared SnO2 and (b) the as-prepared SnO2 sintered at 900 °C for 4 h.

4371Y. Liang et al. / Materials Letters 61 (2007) 4370–4373

performance as anode materials in Li-ion batteries was alsoinvestigated. The as-prepared SnO2 with higher crystallinitydisplayed good electrochemical behavior.

2. Experimental

All chemicals used in this study were of analytical grade andused without further purification.

2.1. Preparation of tetragonal rutile structure nano-SnO2

Through a series of experiments, the preferable reactionconditions are as follows. The precursor was prepared bydissolving 3.50 g SnCl4·5H2O and HCl (38%) in distilled waterto form a solution ([HCl]=0.45 mol/L) under constant stirring.After pretreatment in ultrasonic water bath for about 30 min, themixed solution was transferred to an autoclave with about200 mL capacity. The temperature of the autoclave charged with1.0 MPa air was raised to 280 °C and held constant for 5 h. Afterthe autoclave was cooled to room temperature, the whitesuspension was washed with distilled water several times anddried in vacuum at 80 °C for 4 h. The as-prepared SnO2 and the

Fig. 2. Micrographs of SnO2 nanoparticles. (a) TEM micrograph of SnO2 nanopartic50 nm SnO2 nanoparticle.

product sintered at 900 °C for 4 h were used as anode materialsin Li-ion batteries, respectively.

2.2. Materials characterisations

XRD patterns were obtained at room temperature withCu Kα radiation for crystal phase identification of SnO2

nanoparticles.The morphology and microstructure of the products were

characterised by HRTEM, SAED, SEM and TEM.

2.3. Electrochemical tests

The tin oxides were made into slurry containing 70 wt.%SnO2, 20 wt.% carbon black, and 10 wt.% polyvinylidenefluoride (PVDF) in N-methyl-2-pyrrolidon (NMP). The slurrywas used to coat 0.2-mm thick copper foils to a mass loading ofabout 4–5 mg/cm2 after drying in vacuum overnight at 120 °CThey were cut into circular discs of 14 mm diameter and pressedbetween stainless-steel plates at 20 MPa.

Each coated electrode was assembled into a test cell using aLi counter electrode, a microporous polypropylene separator,and an electrolyte of 1 M LiPF6 solution in a 1:1 (v:v) mixtureof ethylene carbonate (EC) and dimethyl carbonate (DMC). Theassembly was carried out in an Ar-filled glove box (Mbraun,Unilab, Germany) with less than 1 ppm each of oxygen andmoisture. The cells were charged and discharged between0.01 Vand 2.0 Vat a constant current density of 0.4 mA/cm2 ona multichannel battery cycling unit (PCBT138-8D, China).

3. Results and discussions

3.1. Microstructure and morphology of the nanoparticles

The X-ray diffraction patterns (Fig. 1) of the SnO2 revealed mainlypeaks corresponding to the tetragonal rutile structure of SnO2, agreeingwith the JCPDS file No. 41-1445. From the X-ray diffraction data, it isconcluded that the material is pure phase SnO2 and has a completecrystallization. After sintering at 900 °C, the intensity of the diffractionpeaks is consistent with as-prepared SnO2.

les, (b) SEM image of as-prepared SnO2 sintered at 900 °C and (c) SAED of a

Fig. 4. Discharge capacity of different SnO2 nanoparticle electrodes vs. cyclenumber (• as-prepared SnO2; △ sample sintered at 900 °C).

4372 Y. Liang et al. / Materials Letters 61 (2007) 4370–4373

The atomic structure details of the nanoparticles are revealed usingHRTEM and TEM (Fig. 2a). The HRTEM image shows that the SnO2

nanoparticles are structurally uniform, and the clear lattice fringesillustrate that the nanoparticles are single crystals. The selected-areaelectron diffraction (SAED) patterns (Fig. 2c) can be indexed to thereflection of rutile tetragonal SnO2 structure, agreeing with the aboveXRD results. The well-defined SAED pattern shows that the obtainedSnO2 grows along the (001) direction. After sintering at 900 °C, theSnO2 nanoparticle sizes slightly increase (Fig. 2b). Their diameters andlengths are up to 50–100 nm and 200–400 nm, respectively. Some ofthem are like short nanorods.

3.2. Reaction mechanism

SnCl4ðsÞ þ 2H2OðlÞ ¼ SnO2ðsÞ þ 4HClðlÞ ð1Þ

In conventional precipitation, tin oxide is easily formed by addingOH− to a Sn4+ solution [13]. This method is not suitable to producenanoparticles because SnCl4 as a precursor is easy to hydrolyze. Itrapidly changes the solution environment, making it difficult to controlthe nucleation and growth of the particles. In our experiments,hydrochloric acid was used as a means to dose OH− ions slowly anduniformly throughout the reaction. We found that the concentration ofHCl was enhanced and the hydrolysis temperature was also increased.Thus the nucleation and growth process would be complete, leading tonanoparticles with high crystal quality. At a certain temperature,accompanied by emission of HCl vapor, the concentration of HCldecreased, so it was propitious to hydrolyze SnCl4. According toreaction Eq. (1), chemical reactions occurred and the tin ions in thesolution became supersaturated then precipitated as hydrous SnO2

particles. As a crystallizing process, the crystallized degree wasdecided by the temperature of the precursor's hydrolysis. Thehydrolysis temperature improved with the concentration of hydro-chloric acid increasing, which resulted in the formation of stable nucleiand subsequent particle growth. The crystallized degree was increasedso that the crystals of SnO2 grew well.

3.3. Electrochemical performance

The charge–discharge curve of as-prepared tin oxides is shown inFig. 3. The SnO2 nanoparticle electrode exhibited an initial lithiumstorage capacity of 1166 mA h g−1 and a stable reversible specific

Fig. 3. First-cycle and second-cycle charge and discharge curves of the as-prepared SnO2.

capacity of about 631 mA h g−1. A substantial difference between thefirst and all subsequent cycles is observed. There is a plateau in the firstdischarging curves around 0.9 V, which corresponds well with thereported formation of Li2O and Sn metal [14]. When the electrode iscompletely activated, the plateau of 0.9 V disappeared in subsequentcycles replaced by a plateau of 0.75 V, which corresponds well with theformation of the Li–Sn alloy. SnO2 has a theoretical capacity of780 mA h g−1 when forming Li4.4Sn alloys [2–4].

4:4Li þ Sn↔Li4:4Sn ð2Þ

This alloying–dealloying process is reversible, but accompanied bysignificant volume change, which may induce damage to the anodes.The use of nanosized particles can decrease volume expansion in thelocal domain when forming Li4.4Sn alloys. The cyclability of differentSnO2 samples is shown in Fig. 4. We investigated the charge–dischargeproperties of as-prepared SnO2 and the product sintered at 900 °C for4 h (as shown in Fig. 2b).The cyclability of SnO2 sintered at 900 °Cwas better than that of as-prepared SnO2, but the differences betweenthe samples were very small, which were due to their better and similarcrystallinity. Their capacities decreased significantly in the first cycle.However, from the second cycle, the capacity became more stable. Atcurrent densities of 0.4 mA/cm2 the reversible capacity remainedgreater than 530 mA h g−1. A coulombic efficiency (the ratio ofcoulombs in discharge to coulombs in charge) was more than 89%except for the first cycle. This is better than with the SnO2 powdersprepared by conventional hydrolysis, where it exhibited a substantialcapacity fading from 400 to 300 mA h g−1 in 30 cycles [15]. Thisexcellent performance is partly due to the initial establishment of a highSn dispersion. Moreover, the single crystalline structure of tin oxide ismore favorable for a stable capacity of the electrochemical reaction oflithium. In the modified hydrothermal method, nanosized tin oxideparticles form in a controlled manner which prevents extensive growthand aggregation of precipitated particles, with control of Sn particlesize and prevention of the Sn particles from agglomerating into largeclusters. Therefore, good cycle-life would be obtained.

4. Conclusions

A modified hydrothermal method was successfully devel-oped to prepare tetragonal rutile tin oxide single crystals withdiameters of 20–100 nm. The results showed that the rutile

4373Y. Liang et al. / Materials Letters 61 (2007) 4370–4373

SnO2 nano-single crystals were obtained. The electrochemicalperformance of SnO2 nanoparticles as anodes in Li-ion batterieswas measured. At current densities of 0.4 mA/cm2 thereversible capacity still remained greater than 530 mA h g−1

after 20 cycles. The results showed that the as-prepared SnO2

with higher crystallinity displayed a good electrochemicalbehavior.

Acknowledgements

This work was supported by the Important Program for Nano-material Science of Hubei Province, China (No. 20041003068-09).

References

[1] Y. Idota, T. Kubota, A. Matsufuji, Y. Maekawa, T. Miyasaka, Science 276(1997) 1395.

[2] I.A. Courtney, J.R. Dahn, J. Electrochem. Soc. 144 (1997) 2943.[3] G.X. Wang, Y. Chen, L. Yang, J. Yao, S. Needham, H.K. Liu, J.H. Ahn,

J. Power Sources 146 (2005) 487.[4] A. Sivashanmugam, T. Premkumar, S. Gopukumar, N.G. Renganathan,

M. Wohlfahrt-Mehrens, J. Garche, J. Appl. Electrochem. 35 (2005) 1045.[5] J.Y. Kim, D.E. King, P.N. Kumta, G.E. Blomgren, J. Electrochem. Soc.

147 (2000) 4411.[6] A. Ulus, Y. Rosenberg, L. Burstein, E. Peled, J. Electrochem. Soc. 149

(2002) 635.[7] A. Dierstein, H. Natter, F. Meyer, H.-O. Stephan, Ch. Kropf, R.

Hempelmann, Scr. Mater. 44 (2001) 2209.[8] F. Li, X. Yu, H. Pan, M. Wang, X. Xin, Solid State Sci. 2 (2000) 767.[9] N.C. Li, C.R. Martin, B. Scrosati, Electrochem. Solid-State Lett. 3 (2000)

316.[10] Y. Wang, C. Ma, X. Sun, H. Li, Inorg. Chem. Commun. 5 (2002) 751.[11] L. Zhu, Z. Jia, Y. Tang, Chin. J. Chem. Phys. 18 (2005) 237.[12] F. Bai, P. He, Z. Jia, X. Huang, Y. He, Mater. Lett. 59 (2005) 1687.[13] Y. Wang, J.Y. Lee, Electrochem. Commun. 5 (2003) 292.[14] I.A. Courtney, J.R. Dahh, J. Electrochem. Soc. 144 (1997) 2045.[15] R. Zhang, J.Y. Lee, Z.L. Liu, J. Power Sources 112 (2002) 596.