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Hollow nanotubular SiOx templated by cellulose fibers for lithium ion batteries
Electrochimica Acta 74 (2012) 271–274
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Hollow nanotubular SiOx templated by cellulose fibers for lithium ion batteries Hong Guo a,b,∗ , Rui Mao a , Xiangjun Yang a , Jing Chen a a b
School of Chemistry Science and Engineering, Yunnan University, Kunming 650091, Yunnan, China School of Chemistry and Chemical Engineering, Qujing Normal University, Qujing 655000, Yunan, China
a r t i c l e
i n f o
Article history: Received 8 January 2012 Received in revised form 21 April 2012 Accepted 22 April 2012 Available online 27 April 2012 Keywords: Silicon oxide Nanotube Natural cellulose Li-ion batteries
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-TEM analysis, 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 reversible capacity of 940 mAh g−1 at constant current density of 100 mA g−1 , and capacity retention keeps over 91.5% after 50 cycles. The nano-scale characteristics of SiOx particle embed in the nanotube ensures the fast Li-ion diffusion in the electrode. The nanotube came from cellulose fiber template provides the electrode 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 mechanical stress caused by volume change. All these factors are responsible for the stable electrochemical performance of SiOx electrode. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction The constant strong demand for lithium rechargeable batteries as a power supply for portable electronic devices and electric vehicles has accelerated the research and development of new electrode materials with higher energy density, better stability, longer cycle life and improved safety [1,2]. Si-based materials with a high theoretical specific capacity (4200 mAh g−1 ) have been projected as one of the most promising candidates in substitute of the already-commercialized graphitic carbon anode. However, increasing concerns on huge volume change (over 300%) concomitant with Li ion insertion/extraction entails the problem of structural instability, and thus results in the rapid fading of capacity [3,4]. Improvements are made by decreasing the particle size [5], using thin-films [6], or selecting an optimized binder [7]. For example, Besenhard and coworkers [8] and Dahn and coworkers [7,9] have demonstrated that improvements in the cycle life of silicon electrodes can be obtained by preparing them in the form of nanoparticles. Our previous work, Si thin film on rough Cu foil fabricated by rf magnetron sputtering also showed a high reversible specific capacity [10]. However, the cycle stability and the high cost of fabrication are still insufficient for practical use [6,10,11]. Therefore, intensive attentions are paid to prepare nano-sized SiOx composite, because inactive Li2 O and uniformly distributed
Si (Si/Li2 O composite) will be formed in situ during the initial discharge process. The formed Li2 O can act as a buffer component to improve the cycling performance of electrode and the nano-sized Si may provide the electrode with a high specific capacity [12,13]. In the present work, we propose a new strategy to synthesize hollow SiOx nanotubular materials in large quantities. It is based on the surface sol–gel process as applied to covering of cellulose fibers (filter paper) with a SiOx gel layer with nanometer precision. Subsequent calcinations of the as-prepared SiOx gel/filter paper composite resulted in SiOx nanotubes. The prepared SiOx nanotubes possess higher surface and stable hollow configuration. The former can render much contact area between Si and Li ion in the process of electrochemical reaction, and the latter can help the active silicon to accommodate large volume change without pulverizing. Meanwhile, the little remanent carbon produced in the process of calcinations can also prevent the aggregation of active particles. All the factors will contribute greatly to a high specific capacity and a good cyclic performance of SiOx nanotube anode. To our best knowledge, in contrast to the conventional methods that produce nanostructured SiOx , reports on hollow nanotubular SiOx anode are quite rare. This approach provides an effective route to develop nanotubular materials for lithium ion batteries. The electrochemical performance of synthesized hollow SiOx nanotubular materials was investigated primarily. 2. Experimental
∗ Corresponding author at: School of Chemistry Science and Engineering, Yunnan University, Kunming 650091, Yunnan, China. Tel.: +86 871 5032180. E-mail address: [email protected] (H. Guo). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.04.086
Tetraethylorthosilicate (TEOS), 2-propanol and ammonia were acted as raw reagents without further purification. Filter paper
272
H. Guo et al. / Electrochimica Acta 74 (2012) 271–274
(quantitative ashless) was purchased from Advantec Toyo (Tokyo, Japan). Surface sol–gel deposition of SiOx gel layers was carried out at about 50 ◦ C. The preparation procedure of SiOx nanotube is described as follows: a piece of filter paper was placed in a suction filter funnel, and was washed by suction filtration of ethanol, then followed by drying with air flow. A certain amount of 2-C3 H7 OH:TEOS = 6:1 (v/v) was then added to the filter funnel. The solution was slowly suction-filtered through the filter paper standing for 30 min to ensure adsorption of TEOS. Immediately 2-propanol was filtered to move the excessively adsorbed TEOS. NH3 ·H2 O:H2 O = 0.1:8 (v/v) was then added to the funnel and allowed to pass through the filter paper slowly within 20 min to promote hydrolysis of TEOS and condensation of the resultant SiOx gel layer. Individual cellulose fibers in the filter paper were thus coated with a nanometer-thick SiOx gel layer after dried in air. Finally, the gel heated in a furnace at 650 ◦ C for 6 h in air to remove the original filter paper template, and then white sheets composed of SiOx nanotubes were obtained. X-ray diffraction (XRD) was carried out to identity the phase composition of synthesized samples over the 2 range from 20◦ to 100◦ using a Rigaku D/max-A diffractometer with Co K␣ radiation. A Fourier transform infrared spectroscope (FTIR, Themo Nicolet 670FT-IR) was used for recording the FTIR spectra of the sample ranged from 460 to 4000 cm−1 . Morphologies of the synthesized SiOx nanotubes were observed with a AMRAY 1000B scanning electron microscope (SEM), and the microstructural characteristics of SiOx nanotubes were observed by high-resolution transmission electron microscope (HR-TEM, JEOL JEM-2010) working at 200 kV accelerating voltage and the lattice structure was identified by selected area electron diffraction (SAED) technique. For electrochemical performance evaluation, half-cell studies were performed. In the experimental SiOx nanotubes electrode, acetylene black powder and polyvinylidene fluoride (PVDF) were used as conductive additive and binder, respectively. The synthesized SiOx nanotubes were mixed with acetylene black and PVDF dissolved in N-methyl-pyrrolidinone in the weight ratio of 75:15:10 to form slurry, which was painted on a copper foil used as current collector. After solvent evaporation, the electrode was pressed and dried at 120 ◦ C under vacuum for 24 h. The cells were assembled in argon filled glove-box. Metallic lithium foil was used as counter electrode. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in vol. ratio). Cycling tests were carried out at different current densities of 100, 200 and 500 mA g−1 with a cut-off of 0.01/1.5 V versus Li/Li+ by Land2100 tester.
Fig. 1. XRD pattern of SiOx nanotubular materials synthesized by combination of the surface sol–gel process and subsequent calcinations at 650 ◦ C.
disappeared in the synthesized SiOx nanotubes spectrum, indicating these groups have decomposed after calcinations. Fig. 3a and b are low-magnification SEM images of the initial filter paper and SiOx nanotubes yielded by calcinations at 650 ◦ C, respectively. The SiOx sheet possesses overall morphological characteristics of the initial filter paper except for a little shrinkage in size. From Fig. 3c, it can be clearly distinguished that the samples are hollow nanotubes with outer diameter ca. 200–250 nm, and the thickness of the tube wall is estimated to be 30–40 nm. Being replicas of the natural cellulose fiber, the SiOx nanotubes possess very high aspect ratios. The tubular morphology was also characterized by TEM, as illustrated in Fig. 3d the inset in Fig. 3b. The SiOx nanotubes are composed of fine particles with sizes smaller than 20 nm. The fine particle is most likely in a low crystallinity or an amorphous state, because no typical crystalline structure was observed in the electron diffraction measurement of the SiOx nanotube assembly as SAED analysis inserted in Fig. 3d, which is in agreement with the analysis of XRD. Fig. 3e demonstrates that SiOx particle is actually composed of two distinct parts, well-ordered and disordered parts. In the ordered part, lattice fringe is observed, and the lattice spacing (0.245 nm) agrees with SiO2 (1 1 0) plane spacing. Combined with EDS detected atomic ratio of Si/O is near 1:1, it is reasonable to deduce SiOx nanotubes should include low-crystallinity SiO2 , amorphous Si or SiO corresponding to the disordered part. This result is consistent with other research groups [15–17]. Additionally, 1.08 wt% carbon was remained in the prepared nanotubular composite according to the infrared carbon analysis, which may be
3. Results and discussion The XRD pattern of the synthesized SiOx nanotubes at 650 ◦ C as shown in Fig. 1, declares that the product is an amorphous or a low crystallinity variety. The relatively high background implies the coexistence of amorphous component. The broad peak located in the 2 range of 20–35◦ is assignable to SiO2 with low crystallinity. The FTIR spectrum images of the prepared SiOx nanotubes and that of the precursor are shown in Fig. 2. The broad absorption centered at ca. 3425 cm−1 is due to adsorbed water. The peaks about 1090, 800, and 480 cm−1 are assigned to Si O bond [14]. The peak intensity of Si O bond for SiOx nanotubes is different from that of precursor, implying the structure of prepared sample has a little discrepancy with its precursor. The peaks nearby 1610 and 1408 cm−1 are corresponding to C C and C C bond, respectively. For the precursor, the weak peak at 957 cm−1 implies the presence of asymmetric stretching vibration of Si OH groups and the peak at 1548 cm−1 is corresponding to the C N bond [14]. These peaks
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
Fig. 3. SEM (a) micrograph of the filter paper, SEM (b, c), TEM (d), and HR-TEM (e) micrograph of SiOx nanotubes. The inset in (d) is the selected area electron diffraction.
favourable to the improvement of electronic conductivity of SiOx electrode. Lithium ions insertion into and extraction from SiOx nanotube is defined as discharge and charge processes in this work, respectively. The charge–discharge curves for the first two and 30th cycles are shown in Fig. 4a. In the initial discharge, the potential drops rapidly to a plateau of 0.45 V and then decreases gradually to 0.01 V. There is a little gradual slope ranged from 0.90 to 0.70 V corresponding to the formation of SEI film. During the discharge–charge, the plateau of potential shifted to be more obtuse, which suggests that the poor crystallinity of Si component in the electrode. This is similar to other reported works [18,19]. The potential profiles of batteries become almost overlapping except the 1st cycle evidently, illuminating the excellent durability of SiOx nanotube electrode. The cycling performance profiles of SiOx nanotube electrode at different current densities are shown in Fig. 4b. The stable discharge capacity of electrode is 940 mAh g−1 at the current density of 100 mA g−1 . The coulombic efficiencies are always over 98.7% except for the first cycle, and the capacity retention keeps over 91.5% after 50 cycles. The initial coulombic efficiency is 70.3%, which may be resulted from inactive Li2 O formed in the initial cycle [12,20], and the detailed reason needs to study further. The initial delithiation capacity is slightly lower than that of on the second cycle, probably due to the activation process of SiOx nanotube system, which was often observed in other nano anodes [10,11]. From the next cycle on, the electrode shows an excellent cyclic stability. To investigate electrochemistry performance under the high rate discharge, the current density of 200 and 500 mA g−1 were used as constant test. The synthesized electrode exhibits good stability with discharge capacities of ca. 810 and 740 mAh g−1 at the current density of 200 and 500 mA g−1 , respectively. Though the capacity decreases with the higher rate, the synthesized product exhibits a stable cycle capability at 500 mA g−1 . The nano-scale characteristics of SiOx particle embed in the nanotube ensures the fast Li-ion diffusion in the electrode, and the fractional carbon coating layer on the SiOx particle surface renders the electrode having a good electronic conductivity. The nanotube came from cellulose fiber template provides the electrode with firm framework, which avoids the electrode to pulverize in the charge–discharge process. Moreover, the hollow structure offers a sufficient void space, which sufficiently alleviates the mechanical stress caused by volume change. Therefore, it shows a good cyclic stability and a high rate performance.
Fig. 4. Electrochemical performance of prepared SiOx nanotubular materials: (a) charge–discharge curves for the 1st, 2nd, and 30th cycles at current density 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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4. Conclusions
References
Applied of natural cellulosic substances such as filter paper acted as superior templates, hollow SiOx nanotubular materials were synthesized by combination of the surface sol–gel process and subsequent calcination. The prepared nanotubular materials retained the morphological hierarchy of the filter paper, and each nanotube was composed of nanosized SiOx . The stable discharge capacities of ca. 940 mAh g−1 and the columbic efficiencies of 98.7–99.5% were obtained at the current density of 100 mA g−1 . The nanotube from natural cellulosic template possessing firm framework avoids the electrode to pulverize in the charge–discharge process. The hollow structure offers a sufficient void space, in which SiOx nanoparticles will experience a volume change without a collapse, and the nano-scale characteristics of SiOx particle ensures the fast Li-ion diffusion in the electrode. Additionally, the fractional carbon coating layer on the SiOx particle surface renders the electrode having a good electronic conductivity. All these factors contribute greatly to the excellent cycling stability of electrode. Our efforts are simple, low-cost and mass-productive, which should be viable to fabricate aligned bundles of other nanotubular materials of rechargeable lithium ion batteries.
[1] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W. Van-Schalkwijk, Nature Materials 4 (2005) 366. [2] W.J. Zhang, Journal of Power Sources 196 (2011) 13. [3] H. Li, Z.X. Wang, L.Q. Chen, X.J. Huang, Advanced Materials 21 (2009) 4593. [4] J. Yin, M. Wada, K. Yamamoto, Y. Kitano, S. Tanase, T. Sakai, Journal of the Electrochemical Society 153 (2006) A472. [5] A. Magasinski, P. Dixon, B. Hertzberg, A. Kvit, J. Ayala, G. Yushin, Nature Materials 9 (2010) 353. [6] R. Lv, J. Yang, J.L. Wang, Y. NuLi, Journal of Power Sources 196 (2011) 3868. [7] J. Li, R.B. Lewis, J.R. Dahn, Electrochemical Solid-State Letters 10 (2007) A17. [8] M. Wachtler, M. Winter, J.O. Besenhard, Journal of Power Sources 105 (2002) 151. [9] A. Timmons, A.D.W. Todd, S.D. Mead, G.H. Carey, R.J. Sanderson, R.E. Mar, J.R. Dahn, Journal of the Electrochemical Society 154 (2007) A865. [10] H. Guo, H. Zhao, C. Yin, W. Qiu, Materials Science Engineering B, Solid-State Materials for Advanced Engineering 131 (2006) 173. [11] S. Ohara, J. Suzuki, K. Sekine, T. Takamura, Journal of Power Sources 136 (2004) 303. [12] C.H. Doh, H.M. Shin, D.H. Kim, Y.C. Ha, B.S. Jin, H.S. Kim, S.I. Moon, A. Veluchamy, Electrochemistry Communications 10 (2008) 233. [13] A. Veluchamy, C.H. Doh, D.H. Kim, J.H. Lee, D.J. Lee, K.H. Ha, H.M. Shin, B.S. Jin, H.S. Kim, S.I. Moon, C.W. Park, Journal of Power Sources 188 (2009) 574. [14] P.R. Griffiths, J.A.D. Haseth, Fourier Transform Infrared Spectrometry, John Wiley & Sons, New York, 1986. [15] J. Wang, H. Zhao, J. He, C. Wang, J. Wang, Journal of Power Sources 196 (2011) 4811. [16] L.F. Cui, R. Ruffo, C.K. Chan, H.L. Peng, Y. Cui, Nano Letters 9 (2009) 491. [17] S. Zhang, Z. Du, R. Lin, T. Jiang, G. Liu, X. Wu, D. Weng, Advanced Materials 22 (2010) 5378. [18] H.Y. Lee, S.W. Jang, S.M. Lee, S.J. Lee, H.K. Baik, Journal of Power Sources 112 (2002) 8. [19] T. Momma, S. Aoki, H. Nara, T. Yokoshima, T. Osaka, Electrochemistry Communications 13 (2011) 969. [20] J.H. Kim, H.J. Sohn, H. Kim, G. Jeong, W. Choi, Journal of Power Sources 170 (2007) 456.
Acknowledgments The authors would like to acknowledge financial support provided by Department Science Foundation of China (No. 210204), Social Development Plan of Yunnan Province China (No. 2008cd148), National Natural Science Foundation of China (No. U0937601) and 863 Program of National High Technology Research Development Project of China (No. 2011AA03A405).
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Hollow nanotubular SiOx templated by cellulose fibers for lithium ion batteries Hong Guo a,b,∗ , Rui Mao a , Xiangjun Yang a , Jing Chen a a b
School of Chemistry Science and Engineering, Yunnan University, Kunming 650091, Yunnan, China School of Chemistry and Chemical Engineering, Qujing Normal University, Qujing 655000, Yunan, China
a r t i c l e
i n f o
Article history: Received 8 January 2012 Received in revised form 21 April 2012 Accepted 22 April 2012 Available online 27 April 2012 Keywords: Silicon oxide Nanotube Natural cellulose Li-ion batteries
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-TEM analysis, 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 reversible capacity of 940 mAh g−1 at constant current density of 100 mA g−1 , and capacity retention keeps over 91.5% after 50 cycles. The nano-scale characteristics of SiOx particle embed in the nanotube ensures the fast Li-ion diffusion in the electrode. The nanotube came from cellulose fiber template provides the electrode 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 mechanical stress caused by volume change. All these factors are responsible for the stable electrochemical performance of SiOx electrode. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction The constant strong demand for lithium rechargeable batteries as a power supply for portable electronic devices and electric vehicles has accelerated the research and development of new electrode materials with higher energy density, better stability, longer cycle life and improved safety [1,2]. Si-based materials with a high theoretical specific capacity (4200 mAh g−1 ) have been projected as one of the most promising candidates in substitute of the already-commercialized graphitic carbon anode. However, increasing concerns on huge volume change (over 300%) concomitant with Li ion insertion/extraction entails the problem of structural instability, and thus results in the rapid fading of capacity [3,4]. Improvements are made by decreasing the particle size [5], using thin-films [6], or selecting an optimized binder [7]. For example, Besenhard and coworkers [8] and Dahn and coworkers [7,9] have demonstrated that improvements in the cycle life of silicon electrodes can be obtained by preparing them in the form of nanoparticles. Our previous work, Si thin film on rough Cu foil fabricated by rf magnetron sputtering also showed a high reversible specific capacity [10]. However, the cycle stability and the high cost of fabrication are still insufficient for practical use [6,10,11]. Therefore, intensive attentions are paid to prepare nano-sized SiOx composite, because inactive Li2 O and uniformly distributed
Si (Si/Li2 O composite) will be formed in situ during the initial discharge process. The formed Li2 O can act as a buffer component to improve the cycling performance of electrode and the nano-sized Si may provide the electrode with a high specific capacity [12,13]. In the present work, we propose a new strategy to synthesize hollow SiOx nanotubular materials in large quantities. It is based on the surface sol–gel process as applied to covering of cellulose fibers (filter paper) with a SiOx gel layer with nanometer precision. Subsequent calcinations of the as-prepared SiOx gel/filter paper composite resulted in SiOx nanotubes. The prepared SiOx nanotubes possess higher surface and stable hollow configuration. The former can render much contact area between Si and Li ion in the process of electrochemical reaction, and the latter can help the active silicon to accommodate large volume change without pulverizing. Meanwhile, the little remanent carbon produced in the process of calcinations can also prevent the aggregation of active particles. All the factors will contribute greatly to a high specific capacity and a good cyclic performance of SiOx nanotube anode. To our best knowledge, in contrast to the conventional methods that produce nanostructured SiOx , reports on hollow nanotubular SiOx anode are quite rare. This approach provides an effective route to develop nanotubular materials for lithium ion batteries. The electrochemical performance of synthesized hollow SiOx nanotubular materials was investigated primarily. 2. Experimental
∗ Corresponding author at: School of Chemistry Science and Engineering, Yunnan University, Kunming 650091, Yunnan, China. Tel.: +86 871 5032180. E-mail address: [email protected] (H. Guo). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.04.086
Tetraethylorthosilicate (TEOS), 2-propanol and ammonia were acted as raw reagents without further purification. Filter paper
272
H. Guo et al. / Electrochimica Acta 74 (2012) 271–274
(quantitative ashless) was purchased from Advantec Toyo (Tokyo, Japan). Surface sol–gel deposition of SiOx gel layers was carried out at about 50 ◦ C. The preparation procedure of SiOx nanotube is described as follows: a piece of filter paper was placed in a suction filter funnel, and was washed by suction filtration of ethanol, then followed by drying with air flow. A certain amount of 2-C3 H7 OH:TEOS = 6:1 (v/v) was then added to the filter funnel. The solution was slowly suction-filtered through the filter paper standing for 30 min to ensure adsorption of TEOS. Immediately 2-propanol was filtered to move the excessively adsorbed TEOS. NH3 ·H2 O:H2 O = 0.1:8 (v/v) was then added to the funnel and allowed to pass through the filter paper slowly within 20 min to promote hydrolysis of TEOS and condensation of the resultant SiOx gel layer. Individual cellulose fibers in the filter paper were thus coated with a nanometer-thick SiOx gel layer after dried in air. Finally, the gel heated in a furnace at 650 ◦ C for 6 h in air to remove the original filter paper template, and then white sheets composed of SiOx nanotubes were obtained. X-ray diffraction (XRD) was carried out to identity the phase composition of synthesized samples over the 2 range from 20◦ to 100◦ using a Rigaku D/max-A diffractometer with Co K␣ radiation. A Fourier transform infrared spectroscope (FTIR, Themo Nicolet 670FT-IR) was used for recording the FTIR spectra of the sample ranged from 460 to 4000 cm−1 . Morphologies of the synthesized SiOx nanotubes were observed with a AMRAY 1000B scanning electron microscope (SEM), and the microstructural characteristics of SiOx nanotubes were observed by high-resolution transmission electron microscope (HR-TEM, JEOL JEM-2010) working at 200 kV accelerating voltage and the lattice structure was identified by selected area electron diffraction (SAED) technique. For electrochemical performance evaluation, half-cell studies were performed. In the experimental SiOx nanotubes electrode, acetylene black powder and polyvinylidene fluoride (PVDF) were used as conductive additive and binder, respectively. The synthesized SiOx nanotubes were mixed with acetylene black and PVDF dissolved in N-methyl-pyrrolidinone in the weight ratio of 75:15:10 to form slurry, which was painted on a copper foil used as current collector. After solvent evaporation, the electrode was pressed and dried at 120 ◦ C under vacuum for 24 h. The cells were assembled in argon filled glove-box. Metallic lithium foil was used as counter electrode. The electrolyte was 1 M LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 in vol. ratio). Cycling tests were carried out at different current densities of 100, 200 and 500 mA g−1 with a cut-off of 0.01/1.5 V versus Li/Li+ by Land2100 tester.
Fig. 1. XRD pattern of SiOx nanotubular materials synthesized by combination of the surface sol–gel process and subsequent calcinations at 650 ◦ C.
disappeared in the synthesized SiOx nanotubes spectrum, indicating these groups have decomposed after calcinations. Fig. 3a and b are low-magnification SEM images of the initial filter paper and SiOx nanotubes yielded by calcinations at 650 ◦ C, respectively. The SiOx sheet possesses overall morphological characteristics of the initial filter paper except for a little shrinkage in size. From Fig. 3c, it can be clearly distinguished that the samples are hollow nanotubes with outer diameter ca. 200–250 nm, and the thickness of the tube wall is estimated to be 30–40 nm. Being replicas of the natural cellulose fiber, the SiOx nanotubes possess very high aspect ratios. The tubular morphology was also characterized by TEM, as illustrated in Fig. 3d the inset in Fig. 3b. The SiOx nanotubes are composed of fine particles with sizes smaller than 20 nm. The fine particle is most likely in a low crystallinity or an amorphous state, because no typical crystalline structure was observed in the electron diffraction measurement of the SiOx nanotube assembly as SAED analysis inserted in Fig. 3d, which is in agreement with the analysis of XRD. Fig. 3e demonstrates that SiOx particle is actually composed of two distinct parts, well-ordered and disordered parts. In the ordered part, lattice fringe is observed, and the lattice spacing (0.245 nm) agrees with SiO2 (1 1 0) plane spacing. Combined with EDS detected atomic ratio of Si/O is near 1:1, it is reasonable to deduce SiOx nanotubes should include low-crystallinity SiO2 , amorphous Si or SiO corresponding to the disordered part. This result is consistent with other research groups [15–17]. Additionally, 1.08 wt% carbon was remained in the prepared nanotubular composite according to the infrared carbon analysis, which may be
3. Results and discussion The XRD pattern of the synthesized SiOx nanotubes at 650 ◦ C as shown in Fig. 1, declares that the product is an amorphous or a low crystallinity variety. The relatively high background implies the coexistence of amorphous component. The broad peak located in the 2 range of 20–35◦ is assignable to SiO2 with low crystallinity. The FTIR spectrum images of the prepared SiOx nanotubes and that of the precursor are shown in Fig. 2. The broad absorption centered at ca. 3425 cm−1 is due to adsorbed water. The peaks about 1090, 800, and 480 cm−1 are assigned to Si O bond [14]. The peak intensity of Si O bond for SiOx nanotubes is different from that of precursor, implying the structure of prepared sample has a little discrepancy with its precursor. The peaks nearby 1610 and 1408 cm−1 are corresponding to C C and C C bond, respectively. For the precursor, the weak peak at 957 cm−1 implies the presence of asymmetric stretching vibration of Si OH groups and the peak at 1548 cm−1 is corresponding to the C N bond [14]. These peaks
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
Fig. 3. SEM (a) micrograph of the filter paper, SEM (b, c), TEM (d), and HR-TEM (e) micrograph of SiOx nanotubes. The inset in (d) is the selected area electron diffraction.
favourable to the improvement of electronic conductivity of SiOx electrode. Lithium ions insertion into and extraction from SiOx nanotube is defined as discharge and charge processes in this work, respectively. The charge–discharge curves for the first two and 30th cycles are shown in Fig. 4a. In the initial discharge, the potential drops rapidly to a plateau of 0.45 V and then decreases gradually to 0.01 V. There is a little gradual slope ranged from 0.90 to 0.70 V corresponding to the formation of SEI film. During the discharge–charge, the plateau of potential shifted to be more obtuse, which suggests that the poor crystallinity of Si component in the electrode. This is similar to other reported works [18,19]. The potential profiles of batteries become almost overlapping except the 1st cycle evidently, illuminating the excellent durability of SiOx nanotube electrode. The cycling performance profiles of SiOx nanotube electrode at different current densities are shown in Fig. 4b. The stable discharge capacity of electrode is 940 mAh g−1 at the current density of 100 mA g−1 . The coulombic efficiencies are always over 98.7% except for the first cycle, and the capacity retention keeps over 91.5% after 50 cycles. The initial coulombic efficiency is 70.3%, which may be resulted from inactive Li2 O formed in the initial cycle [12,20], and the detailed reason needs to study further. The initial delithiation capacity is slightly lower than that of on the second cycle, probably due to the activation process of SiOx nanotube system, which was often observed in other nano anodes [10,11]. From the next cycle on, the electrode shows an excellent cyclic stability. To investigate electrochemistry performance under the high rate discharge, the current density of 200 and 500 mA g−1 were used as constant test. The synthesized electrode exhibits good stability with discharge capacities of ca. 810 and 740 mAh g−1 at the current density of 200 and 500 mA g−1 , respectively. Though the capacity decreases with the higher rate, the synthesized product exhibits a stable cycle capability at 500 mA g−1 . The nano-scale characteristics of SiOx particle embed in the nanotube ensures the fast Li-ion diffusion in the electrode, and the fractional carbon coating layer on the SiOx particle surface renders the electrode having a good electronic conductivity. The nanotube came from cellulose fiber template provides the electrode with firm framework, which avoids the electrode to pulverize in the charge–discharge process. Moreover, the hollow structure offers a sufficient void space, which sufficiently alleviates the mechanical stress caused by volume change. Therefore, it shows a good cyclic stability and a high rate performance.
Fig. 4. Electrochemical performance of prepared SiOx nanotubular materials: (a) charge–discharge curves for the 1st, 2nd, and 30th cycles at current density 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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4. Conclusions
References
Applied of natural cellulosic substances such as filter paper acted as superior templates, hollow SiOx nanotubular materials were synthesized by combination of the surface sol–gel process and subsequent calcination. The prepared nanotubular materials retained the morphological hierarchy of the filter paper, and each nanotube was composed of nanosized SiOx . The stable discharge capacities of ca. 940 mAh g−1 and the columbic efficiencies of 98.7–99.5% were obtained at the current density of 100 mA g−1 . The nanotube from natural cellulosic template possessing firm framework avoids the electrode to pulverize in the charge–discharge process. The hollow structure offers a sufficient void space, in which SiOx nanoparticles will experience a volume change without a collapse, and the nano-scale characteristics of SiOx particle ensures the fast Li-ion diffusion in the electrode. Additionally, the fractional carbon coating layer on the SiOx particle surface renders the electrode having a good electronic conductivity. All these factors contribute greatly to the excellent cycling stability of electrode. Our efforts are simple, low-cost and mass-productive, which should be viable to fabricate aligned bundles of other nanotubular materials of rechargeable lithium ion batteries.
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Acknowledgments The authors would like to acknowledge financial support provided by Department Science Foundation of China (No. 210204), Social Development Plan of Yunnan Province China (No. 2008cd148), National Natural Science Foundation of China (No. U0937601) and 863 Program of National High Technology Research Development Project of China (No. 2011AA03A405).