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Electrochimica Acta 74 (2012) 271– 274

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

ollow nanotubular SiOx templated by cellulose fibers for lithium ion batteries

ong Guoa,b,∗, Rui Maoa, Xiangjun Yanga, Jing Chena

School of Chemistry Science and Engineering, Yunnan University, Kunming 650091, Yunnan, ChinaSchool of Chemistry and Chemical Engineering, Qujing Normal University, Qujing 655000, Yunan, China

r t i c l e i n f o

rticle history:eceived 8 January 2012eceived in revised form 21 April 2012ccepted 22 April 2012vailable online 27 April 2012

a b s t r a c t

An effective method of using cellulosic substances (filter paper) as template was employed to prepare SiOx

nanotubular materials examined as anode for Li-ion battery. According to XRD, SEM, TEM and HR-TEManalysis, the synthesized nanotubular materials retained the morphological hierarchy of the filter paper,and each nanotube was composed of nanosized SiOx ranged from 5 to 20 nm. It exhibits a stable reversiblecapacity of 940 mAh g−1 at constant current density of 100 mA g−1, and capacity retention keeps over

eywords:ilicon oxideanotubeatural cellulosei-ion batteries

91.5% after 50 cycles. The nano-scale characteristics of SiOx particle embed in the nanotube ensures thefast Li-ion diffusion in the electrode. The nanotube came from cellulose fiber template provides the elec-trode with firm framework, and thus avoids the electrode to pulverize in the charge–discharge process.Moreover, the hollow structure offers a sufficient void space, which sufficiently alleviates the mechan-ical stress caused by volume change. All these factors are responsible for the stable electrochemicalperformance of SiOx electrode.

. Introduction

The constant strong demand for lithium rechargeable batter-es as a power supply for portable electronic devices and electricehicles has accelerated the research and development of newlectrode materials with higher energy density, better stability,onger cycle life and improved safety [1,2]. Si-based materials with

high theoretical specific capacity (4200 mAh g−1) have been pro-ected as one of the most promising candidates in substitute of thelready-commercialized graphitic carbon anode. However, increas-ng concerns on huge volume change (over 300%) concomitant

ith Li ion insertion/extraction entails the problem of structuralnstability, and thus results in the rapid fading of capacity [3,4].mprovements are made by decreasing the particle size [5], usinghin-films [6], or selecting an optimized binder [7]. For exam-le, Besenhard and coworkers [8] and Dahn and coworkers [7,9]ave demonstrated that improvements in the cycle life of sili-on electrodes can be obtained by preparing them in the form ofanoparticles. Our previous work, Si thin film on rough Cu foil fab-icated by rf magnetron sputtering also showed a high reversiblepecific capacity [10]. However, the cycle stability and the high

ost of fabrication are still insufficient for practical use [6,10,11].herefore, intensive attentions are paid to prepare nano-sizediOx composite, because inactive Li2O and uniformly distributed

∗ Corresponding author at: School of Chemistry Science and Engineering, Yunnanniversity, Kunming 650091, Yunnan, China. Tel.: +86 871 5032180.

E-mail address: guohongcom@126.com (H. Guo).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.04.086

© 2012 Elsevier Ltd. All rights reserved.

Si (Si/Li2O composite) will be formed in situ during the initial dis-charge process. The formed Li2O can act as a buffer component toimprove the cycling performance of electrode and the nano-sizedSi may provide the electrode with a high specific capacity [12,13].

In the present work, we propose a new strategy to synthesizehollow SiOx nanotubular materials in large quantities. It is basedon the surface sol–gel process as applied to covering of cellulosefibers (filter paper) with a SiOx gel layer with nanometer precision.Subsequent calcinations of the as-prepared SiOx gel/filter papercomposite resulted in SiOx nanotubes. The prepared SiOx nanotubespossess higher surface and stable hollow configuration. The formercan render much contact area between Si and Li ion in the pro-cess of electrochemical reaction, and the latter can help the activesilicon to accommodate large volume change without pulverizing.Meanwhile, the little remanent carbon produced in the process ofcalcinations can also prevent the aggregation of active particles.All the factors will contribute greatly to a high specific capacityand a good cyclic performance of SiOx nanotube anode. To our bestknowledge, in contrast to the conventional methods that producenanostructured SiOx, reports on hollow nanotubular SiOx anode arequite rare. This approach provides an effective route to developnanotubular materials for lithium ion batteries. The electrochemi-cal performance of synthesized hollow SiOx nanotubular materialswas investigated primarily.

2. Experimental

Tetraethylorthosilicate (TEOS), 2-propanol and ammonia wereacted as raw reagents without further purification. Filter paper

2 mica Acta 74 (2012) 271– 274

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amorphous Si or SiO corresponding to the disordered part. Thisresult is consistent with other research groups [15–17]. Addition-ally, 1.08 wt% carbon was remained in the prepared nanotubularcomposite according to the infrared carbon analysis, which may be

72 H. Guo et al. / Electrochi

quantitative ashless) was purchased from Advantec Toyo (Tokyo,apan). Surface sol–gel deposition of SiOx gel layers was carriedut at about 50 ◦C. The preparation procedure of SiOx nanotubes described as follows: a piece of filter paper was placed in

suction filter funnel, and was washed by suction filtration ofthanol, then followed by drying with air flow. A certain amountf 2-C3H7OH:TEOS = 6:1 (v/v) was then added to the filter fun-el. The solution was slowly suction-filtered through the filteraper standing for 30 min to ensure adsorption of TEOS. Imme-iately 2-propanol was filtered to move the excessively adsorbedEOS. NH3·H2O:H2O = 0.1:8 (v/v) was then added to the funnel andllowed to pass through the filter paper slowly within 20 min toromote hydrolysis of TEOS and condensation of the resultant SiOx

el layer. Individual cellulose fibers in the filter paper were thusoated with a nanometer-thick SiOx gel layer after dried in air.inally, the gel heated in a furnace at 650 ◦C for 6 h in air to removehe original filter paper template, and then white sheets composedf SiOx nanotubes were obtained.

X-ray diffraction (XRD) was carried out to identity the phaseomposition of synthesized samples over the 2� range from 20◦ to00◦ using a Rigaku D/max-A diffractometer with Co K� radiation.

Fourier transform infrared spectroscope (FTIR, Themo Nicolet70FT-IR) was used for recording the FTIR spectra of the sampleanged from 460 to 4000 cm−1. Morphologies of the synthesizediOx nanotubes were observed with a AMRAY 1000B scanning elec-ron microscope (SEM), and the microstructural characteristics ofiOx nanotubes were observed by high-resolution transmissionlectron microscope (HR-TEM, JEOL JEM-2010) working at 200 kVccelerating voltage and the lattice structure was identified byelected area electron diffraction (SAED) technique.

For electrochemical performance evaluation, half-cell studiesere performed. In the experimental SiOx nanotubes electrode,

cetylene black powder and polyvinylidene fluoride (PVDF) weresed as conductive additive and binder, respectively. The syn-hesized SiOx nanotubes were mixed with acetylene black andVDF dissolved in N-methyl-pyrrolidinone in the weight ratio of5:15:10 to form slurry, which was painted on a copper foil useds current collector. After solvent evaporation, the electrode wasressed and dried at 120 ◦C under vacuum for 24 h. The cells weressembled in argon filled glove-box. Metallic lithium foil was useds counter electrode. The electrolyte was 1 M LiPF6 in a mixture ofthylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in vol.atio). Cycling tests were carried out at different current densitiesf 100, 200 and 500 mA g−1 with a cut-off of 0.01/1.5 V versus Li/Li+

y Land2100 tester.

. Results and discussion

The XRD pattern of the synthesized SiOx nanotubes at 650 ◦C ashown in Fig. 1, declares that the product is an amorphous or a lowrystallinity variety. The relatively high background implies the co-xistence of amorphous component. The broad peak located in the� range of 20–35◦ is assignable to SiO2 with low crystallinity.

The FTIR spectrum images of the prepared SiOx nanotubes andhat of the precursor are shown in Fig. 2. The broad absorptionentered at ca. 3425 cm−1 is due to adsorbed water. The peaksbout 1090, 800, and 480 cm−1 are assigned to Si O bond [14]. Theeak intensity of Si O bond for SiOx nanotubes is different fromhat of precursor, implying the structure of prepared sample has

little discrepancy with its precursor. The peaks nearby 1610 and

408 cm−1 are corresponding to C C and C C bond, respectively.or the precursor, the weak peak at 957 cm−1 implies the presencef asymmetric stretching vibration of Si OH groups and the peakt 1548 cm−1 is corresponding to the C N bond [14]. These peaks

Fig. 1. XRD pattern of SiOx nanotubular materials synthesized by combination ofthe surface sol–gel process and subsequent calcinations at 650 ◦C.

disappeared in the synthesized SiOx nanotubes spectrum, indicat-ing these groups have decomposed after calcinations.

Fig. 3a and b are low-magnification SEM images of the initialfilter paper and SiOx nanotubes yielded by calcinations at 650 ◦C,respectively. The SiOx sheet possesses overall morphological char-acteristics of the initial filter paper except for a little shrinkage insize. From Fig. 3c, it can be clearly distinguished that the samplesare hollow nanotubes with outer diameter ca. 200–250 nm, and thethickness of the tube wall is estimated to be 30–40 nm. Being repli-cas of the natural cellulose fiber, the SiOx nanotubes possess veryhigh aspect ratios. The tubular morphology was also characterizedby TEM, as illustrated in Fig. 3d the inset in Fig. 3b. The SiOx nano-tubes are composed of fine particles with sizes smaller than 20 nm.The fine particle is most likely in a low crystallinity or an amorphousstate, because no typical crystalline structure was observed in theelectron diffraction measurement of the SiOx nanotube assemblyas SAED analysis inserted in Fig. 3d, which is in agreement with theanalysis of XRD. Fig. 3e demonstrates that SiOx particle is actuallycomposed of two distinct parts, well-ordered and disordered parts.In the ordered part, lattice fringe is observed, and the lattice spac-ing (0.245 nm) agrees with SiO2 (1 1 0) plane spacing. Combinedwith EDS detected atomic ratio of Si/O is near 1:1, it is reasonableto deduce SiOx nanotubes should include low-crystallinity SiO2,

Fig. 2. FTIR spectra of the prepared SiOx nanotube (a) and its precursor (b).

H. Guo et al. / Electrochimica Acta 74 (2012) 271– 274 273

F micrograph of SiOx nanotubes. The inset in (d) is the selected area electron diffraction.

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Fig. 4. Electrochemical performance of prepared SiOx nanotubular materials: (a)charge–discharge curves for the 1st, 2nd, and 30th cycles at current density

ig. 3. SEM (a) micrograph of the filter paper, SEM (b, c), TEM (d), and HR-TEM (e)

avourable to the improvement of electronic conductivity of SiOx

lectrode.Lithium ions insertion into and extraction from SiOx nanotube

s defined as discharge and charge processes in this work, respec-ively. The charge–discharge curves for the first two and 30th cyclesre shown in Fig. 4a. In the initial discharge, the potential dropsapidly to a plateau of 0.45 V and then decreases gradually to 0.01 V.here is a little gradual slope ranged from 0.90 to 0.70 V corre-ponding to the formation of SEI film. During the discharge–charge,he plateau of potential shifted to be more obtuse, which suggestshat the poor crystallinity of Si component in the electrode. Thiss similar to other reported works [18,19]. The potential profiles ofatteries become almost overlapping except the 1st cycle evidently,

lluminating the excellent durability of SiOx nanotube electrode.The cycling performance profiles of SiOx nanotube electrode at

ifferent current densities are shown in Fig. 4b. The stable dis-harge capacity of electrode is 940 mAh g−1 at the current densityf 100 mA g−1. The coulombic efficiencies are always over 98.7%xcept for the first cycle, and the capacity retention keeps over1.5% after 50 cycles. The initial coulombic efficiency is 70.3%,hich may be resulted from inactive Li2O formed in the initial cycle

12,20], and the detailed reason needs to study further. The initialelithiation capacity is slightly lower than that of on the secondycle, probably due to the activation process of SiOx nanotube sys-em, which was often observed in other nano anodes [10,11]. Fromhe next cycle on, the electrode shows an excellent cyclic stability.

To investigate electrochemistry performance under the highate discharge, the current density of 200 and 500 mA g−1 were useds constant test. The synthesized electrode exhibits good stabilityith discharge capacities of ca. 810 and 740 mAh g−1 at the currentensity of 200 and 500 mA g−1, respectively. Though the capacityecreases with the higher rate, the synthesized product exhibits atable cycle capability at 500 mA g−1. The nano-scale characteristicsf SiOx particle embed in the nanotube ensures the fast Li-ion diffu-ion in the electrode, and the fractional carbon coating layer on theiOx particle surface renders the electrode having a good electroniconductivity. The nanotube came from cellulose fiber templaterovides the electrode with firm framework, which avoids the elec-

rode to pulverize in the charge–discharge process. Moreover, theollow structure offers a sufficient void space, which sufficientlylleviates the mechanical stress caused by volume change. There-ore, it shows a good cyclic stability and a high rate performance.

100 mA g−1; (b) cycling performance of electrode at constant current density of 100,200 and 500 mA g−1, respectively. Electrode potential range of 0.01–1.5 V vs. Li/Li+.

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[19] T. Momma, S. Aoki, H. Nara, T. Yokoshima, T. Osaka, Electrochemistry Commu-nications 13 (2011) 969.

74 H. Guo et al. / Electrochi

. Conclusions

Applied of natural cellulosic substances such as filter paper acteds superior templates, hollow SiOx nanotubular materials wereynthesized by combination of the surface sol–gel process and sub-equent calcination. The prepared nanotubular materials retainedhe morphological hierarchy of the filter paper, and each nanotubeas composed of nanosized SiOx. The stable discharge capacities

f ca. 940 mAh g−1 and the columbic efficiencies of 98.7–99.5%ere obtained at the current density of 100 mA g−1. The nanotube

rom natural cellulosic template possessing firm framework avoidshe electrode to pulverize in the charge–discharge process. Theollow structure offers a sufficient void space, in which SiOx nano-articles will experience a volume change without a collapse, andhe nano-scale characteristics of SiOx particle ensures the fast Li-ioniffusion in the electrode. Additionally, the fractional carbon coat-

ng layer on the SiOx particle surface renders the electrode having aood electronic conductivity. All these factors contribute greatly tohe excellent cycling stability of electrode. Our efforts are simple,ow-cost and mass-productive, which should be viable to fabri-ate aligned bundles of other nanotubular materials of rechargeableithium ion batteries.

cknowledgments

The authors would like to acknowledge financial supportrovided by Department Science Foundation of China (No.

10204), Social Development Plan of Yunnan Province China (No.008cd148), National Natural Science Foundation of China (No.0937601) and 863 Program of National High Technology Researchevelopment Project of China (No. 2011AA03A405).

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