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TiO2-B nanofibers with high thermal stability as improved anodes for lithium ion batteries
Electrochemistry Communications 27 (2013) 124–127
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
TiO2-B nanofibers with high thermal stability as improved anodes for lithium ion batteries Wei Zhuang, Linghong Lu ⁎, Xinbing Wu, Wei Jin, Meng Meng, Yudan Zhu, Xiaohua Lu ⁎ State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 7 November 2012 Accepted 12 November 2012 Available online 27 November 2012 Keywords: Lithium ion batteries TiO2-B Nanofibers High thermal stability Titania
a b s t r a c t TiO2-B nanofibers with large surface area, high thermal stability and high crystallinity were synthesized by steam thermal method from K2Ti2O5. Compared to the bulk TiO2-B (TB-bulk) prepared from K2Ti4O9, these nanofibers exhibited much higher reversible capacity, cycling stability and rate capability. Such excellent electrochemical performances were derived from the facile charge transport due to the specific framework of a large specific surface area and one dimensional structure. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Batteries with long cycle life and high energy and power densities are important approaches to address the challenges of energy shortage. Next generation of lithium ion batteries is essential for applications in electric vehicles, stationary energy storage systems for solar and wind energy as well as smart grids [1]. However, the performance of current lithium ion batteries cannot meet the requirements in such application fields because of the safety issue and poor rate performance [2]. Titanates are being intensively investigated as anodes for lithium-ion batteries due to their superior safety and rate capability compared with graphite [3]. Li4Ti5O12 spinel is already used in commercial lithium-ion batteries [4]. Titanium dioxide (TiO2) is one of the most focused metal oxide materials for its unique properties and great importance in many areas such as anode materials of lithium ion battery. TiO2 has twice the theoretical specific capacity (335 mAh g−1) as compared to Li4Ti5O12 (175 mAh g−1) does [5]. Furthermore, as a novel member of TiO2 materials, TiO2-B shows a favorable channel structure for lithium mobility, so as to enhance the charge–discharge capabilities of a lithium cell. It has been identified that the lithium intercalation in TiO2-B features a pseudocapacitive process, rather than the solid-state diffusion process observed for anatase and rutile [6,7]. Theoretical studies have uncovered that this pseudocapacitive behavior originates from the unique sites and energetics of lithium absorption and diffusion in TiO2-B crystalline structure [8]. Moreover, one dimensional structure allows for the accommodation of the strain of lithium insertion/removal along a specific orientation, which avoids the irregular expansion [9]. Meanwhile, this structure ⁎ Corresponding authors. Tel./fax: +86 25 83588063. E-mail addresses: [email protected] (L. Lu), [email protected] (X. Lu).
facilitates the diffusion of electrolyte into the inner region of the electrode and therefore accelerates charge transfer in the interior [10]. Hence, it draws our attention to synthesize the TiO2-B nanofibers. However, TiO2-B is a metastable phase, which can gradually lead to degradation of both structure stability and lithium intercalation cycling stability of TiO2-B [11]. Actually, TiO2-B fibers (specific surface areab 20 m2 g−1) can be prepared from K2Ti4O9 via the soft chemistry method and exhibit considerably lithium ion intercalation performance [12]. However, the TiO2-B with large surface area and high thermal stability cannot be easily prepared directly from potassium titanate. Moreover, high purity TiO2-B materials are difficult to be synthesized through hydrothermal method. In our previous work, mesoporous anatase TiO2 with minor TiO2-B has been synthesized from K2Ti2O5 and shows high thermal stability [13]. In this work, we report a facile steam thermal method to transform K2Ti2O5 into TiO2-B nanofibers with a specific surface area of 112 m 2 g −1, high crystallinity and high thermal stability. The TiO2-B electrodes show high storage capacity, excellent rate capability and reliable stability. 2. Experimental 2.1. Preparation of samples K2Ti2O5 or K2Ti4O9 was prepared by adding K2CO3 (reagent grade) to TiO2 nH2O with a proper TiO2/K2O molar ratio and then sintered [14]. After that, 2 g K2Ti2O5 was mounted in a crucible and then with 20 ml deionized water out the crucible was sealed in a Teflon-lined autoclave. It was placed in an oven at the temperature of 180 °C for a set duration of 12 h, and then cooled to room temperature. Then, the intermediate was obtained after ion exchange in vigorously stirred
1388-2481/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.11.012
W. Zhuang et al. / Electrochemistry Communications 27 (2013) 124–127
0.1 M HCl solution until K+ ion was completely exchanged. The intermediate was filtered and washed with distilled water and then dried in a desiccator at 80 °C, denoted as TBN-80. Calcinations of the dried samples were performed in a muffle oven at 400, 500, 600 °C in air for 2 h, denoted as TBN-400, TBN-500 and TBN-600, respectively. Bulk TiO2-B prepared from K2Ti4O9 was possessed after ion exchange, washing and drying, denoted as TB-bulk. 2.2. Characterization The crystalline structure of the materials was determined by powder X-ray diffraction (XRD, Bruker D8, Cu-Kα radiation) and Raman spectra (Super LabRam). The textural properties were studied by N2 adsorption–desorption measurements (ASAP 2020 M) at liquid nitrogen temperature of 77 K. The sample morphology and microstructure were observed by scanning electron microscopy (SEM, Hitachi S4800) and transmission electron microscopy (TEM, Philips Tecnai G2 20) at 200 kV. 2.3. Electrochemical measurement All electrochemical measurements were conducted in Swagelok type cells with pure lithium foil as the counter and reference electrodes and Whatman glass fiber as separator. For preparation of working electrode, a slurry was prepared by mixing the active material, Super P carbon (Alfa Aesar) and PVDF (Kynarflex) in a weight ratio of 85:8:7 in N-methyl-pyrrolidone (NMP). The slurry was cast on a copper foil (Alfa Aesar, thickness: 0.05 mm) and was dried in vacuum at 100 °C for 24 h. The electrolyte used for the above cell configuration was 1 M LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate (1:1 w/w). Cyclic voltammetry was performed between the voltage ranges of 1.0 and 3.0 V using an advanced electrochemical system of Autolab 302 N at the scanning rate of 0.1 mV s −1. The electrochemical impedance measurements were recorded potentiostatically by applying an AC voltage of 5 mV amplitude in the 100 kHz to 10 mHz frequency range. The galvanostatic charge–discharge characteristics of the cells were recorded over the voltage range between 1.0 and 3.0 V using a NEWARE BTS-5V50mA computer-controlled battery test station for different rates at room temperature (25 °C). 1 °C is equivalent to 330 mA g−1 for TiO2-B.
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3. Results and discussion Fig. 1(a) shows the XRD patterns of the prepared TiO2 materials, in which the diffraction peaks can be indexed to monoclinic TiO2-B phase. The intensity distribution of nanofibers is similar to that of the bulk material. There are no peaks identified as tetragonal anatase phase (JCPDS no. 21–1272) in the nanofibres until heat treated at 600 °C, indicates the high thermal stability of crystalline structure [15]. Fig. 1(b) shows the N2 adsorption–desorption isotherm curves of TBN-500, which is a typical Type IV isotherm representing mesoporous structure with a BET specific surface area of 112 m 2 g−1. Meanwhile, TiO2-B bulk shows much lower surface area of 14.5 m 2 g −1. This phenomenon of large surface area may bring the excellent capability to accommodate strain and structure changes during battery cycling. The morphology of TiO2-B after washing and heat-treatment processes is displayed in Fig. 1(c) with a magnified image in the inset. The sample consisted of regular shaped nanofibers with uniform widths of 100–200 nm and variable lengths around 10 μm. The highly crystalline structure of the nanofibers is confirmed by both HR-TEM in Fig. 1(d) and selected area electron diffraction (SAED, inset of Fig. 1(d)). The 0.62 nm lattice spacing is consistent with the (001) plane of the monoclinic TiO2-B structure. All these results reveal the formation of highly crystalline mesoporous TiO2-B nanofibers. Fig. 2(a) shows Raman spectra of the samples thermally treated at different temperatures and TB-bulk. The band positions and relative intensities match well with the spectrum of TiO2-B reported in previous literature [16]. TBN-80 corresponding to the H-titanate is characterized by a strong peak at 150 cm−1. These results are in accordance with the results reported in the literature [17]. After calcination, peaks corresponding to the monoclinic phase are more evident and dominate the spectrum, in part because of the substantial loss of intensity of the 120 cm−1 peak of TiO2-B. Also, the nanofibers can keep the TiO2-B structure until the temperature reaches 600 °C and exhibit higher thermal stability of structure than that reported in the literature [7]. TiO2-B nanofibers were mixed with Super P carbon and PVDF binder to form composite electrode. The increased ability to store lithium ions in the TiO2-B nanofibers compared to the bulk one, as shown in Fig. 2(b), is very obvious. On reducing the dimensions of crystalline materials to the nanoscale, a greater proportion of the material will be in the near surface region where it is subjected to different forces than in
Fig. 1. (a) XRD pattern of the materials, with JCPDS (no. 46–1237) data for TiO2-B; (b) N2 adsorption–desorption isotherms of TBN-500 and TB-bulk; (c) FE-SEM image of TBN-500 and the highly magnified image (inset); (d) HR-TEM image indicating the presence of TiO2-B(001), the selected area electron diffraction (SAED) of TBN-500 and scheme of TiO2-B (inset).
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W. Zhuang et al. / Electrochemistry Communications 27 (2013) 124–127
Fig. 2. (a) Raman spectra of the samples thermally treated at different temperatures and TB-bulk; (b) the first charge–discharge curves for different samples at 100 mA g−1; (c) cyclic voltammetry (CV) curves for different samples at a scan rate of 0.1 mV s−1. (d) CVs of TBN-500 at different scan rates and the plot of peak currents against the scan rates (inset). (e) Rate performances of TiO2-B nanofibers derived from K2Ti2O5 and TB-bulk at different current rates and typical electrochemical impedance spectra for TBN-500 and TB-bulk (inset); (f) cycling performances of TBN-500 and TB-bulk electrodes at the rate of 1 °C.
the bulk, resulting in structural distortions. Fig. 2(c) shows the cyclic voltammetry (CV) curves of TiO2-B electrode at the scan rate of 0.1 mV s−1. They all show a pair of plateaus in the load curves in the range of 1.45–1.65 V, where peaks correspond to plateaus in the load curves. In the cases of the bulk and nanofiber TiO2-B materials, the plateaus can be obtained and have been used previously to assign the 2-phase processes to intercalation into the A1 and A2 sites of the TiO2-B crystal structure [10]. This is consistent with previous observations of intercalation compounds that exhibit a 2-phase intercalation process for large (bulk) fibers but increasingly sloping load curves on reducing the particle dimensions. Fig. 2(d) shows the CV curves of TBN-500 at the scan rates of 0.1, 0.2, 0.5, 1 mV s −1. The plot of peak currents against the scan rate is shown in the inset of Fig. 2(d). To analyze this dependence in a broader interval of scan rate, we selected here the anodic and cathodic S-peaks, which are better resolved at faster charging. This revealed that lithium ion intercalation in TiO2-B is a pseudocapacitive process. Titanate anodes are employed because of their superior safety and ability to sustain high charge/discharge rates compared with graphite. These unique features are critically important for high power
applications such as electric vehicles. Therefore, we have investigated the rate capability of TiO2-B nanofibers. The variation of gravimetric capacity (based on the active mass alone) with different charge/discharge rates is shown in Fig. 2(e). The data of TiO2-B nanofibers calcinated at different temperatures are compared with bulk TiO2-B, which is similar to the best previously reported rate capability of any titanate [12]. As shown in the inset of Fig. 2(e), the semicircle of TBN-500 becomes smaller than that of TB-bulk, indicating that the charge-transfer resistance of TBN-500 is lower. These results further confirm the rate capability performance. The relatively high gravimetric capacity of the TiO2-B nanofibers is in part due to the large specific surface area, which helps to ensure a higher diffusion rate of lithium ions. The significance of this result is that maximizing gravimetric capacity for future applications in lithium-ion cells. The other appealing feature of the nanofibers is its excellent cycling performance. The cycling performance of the samples is shown in Fig. 2(f). Both half cells were cycled at 1 °C for 100 cycles. In the case of the TiO2-B nanofibers, the initial capacity was 193.8 mAh g−1 and decreased to 189.5 mAh g−1 after 100 cycles, with very good capacity retention of 97.8%. However, for the electrodes of TB-bulk, the initial
W. Zhuang et al. / Electrochemistry Communications 27 (2013) 124–127
capacity was 171.1 mAh g−1 and decreased to 160 mAh g−1 after 100 cycles with moderate capacity retention of 93.5%. It's the best of TiO2-B nanofibers derived from K-titanate although a little worse than that fabricated by hydrothermal method [12]. These excellent rate and cycling performances could be attributed to: a) the large specific surface area; b) the high stability of crystalline structure; and c) one dimensional structure. 4. Conclusions In summary, nanofibrous TiO2-B was successfully obtained by steam thermal method using K2Ti2O5 as the precursor, which, to our knowledge, has never been reported before. The material exhibits a large specific surface area of 112 m2 g−1, high crystallinity and high thermal stability. The electrodes of TBN-500 show high first capacity of 277 mAh g−1, excellent rate capability and high cycling stability. Therefore, this material can be considered as an alternative anode material for lithium ion batteries. Acknowledgments This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT 0732), the National Natural Science Foundation of China Grants (Nos. 21136004, 20736002, 21176113, and 20876073), NSFC-RGC (No. 20731160614), China Postdoctoral Science Foundation (No. 20110491407) and the National Basic
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Research Program of China (Nos. 2009CB623407, 2009CB219902 and 2009CB226103). References [1] B. Dunn, H. Kamath, J.M. Tarascon, Science 334 (2011) 928–935. [2] A.S. Arico, P. Bruce, B. Scrosati, J.M. Tarascon, W. Van Schalkwijk, Nature Materials 4 (2005) 366–377. [3] Z. Chen, I. Belharouak, Y.K. Sun, K. Amine, Advanced Functional Materials (2012), http://dx.doi.org/10.1002/adfm.201200698. [4] Z. Yang, D. Choi, S. Kerisit, K.M. Rosso, D. Wang, J. Zhang, G. Graff, J. Liu, Journal of Power Sources 192 (2009) 588–598. [5] S.T. Myung, N. Takahashi, S. Komaba, C.S. Yoon, Y.K. Sun, K. Amine, H. Yashiro, Advanced Functional Materials 21 (2011) 3231–3241. [6] J.M. Li, W. Wan, H.H. Zhou, J.J. Li, D.S. Xu, Chemical Communications 47 (2011) 3439–3441. [7] H. Jang, S. Suzuki, M. Miyayama, Journal of Power Sources 203 (2012) 97–102. [8] C. Arrouvel, S.C. Parker, M.S. Islam, Chemistry of Materials 21 (2009) 4778–4783. [9] A.I. Hochbaum, P. Yang, Chemical Reviews 110 (2010) 20. [10] M.V. Koudriachova, Surface and Interface Analysis 42 (2010) 1330–1332. [11] G.N. Zhu, Y.G. Wang, Y.Y. Xia, Energy & Environmental Science 5 (2012) 6652–6667. [12] M. Inaba, Y. Oba, F. Niina, Y. Murota, Y. Ogino, A. Tasaka, K. Hirota, Journal of Power Sources 189 (2009) 580–584. [13] W. Li, C. Liu, Y.X. Zhou, Y. Bai, X. Feng, Z.H. Yang, L.H. Lu, X.H. Lu, K.Y. Chan, Journal of Physical Chemistry C 112 (2008) 20539–20545. [14] N.Z. Bao, X. Feng, X.H. Lu, Z.H. Yang, Journal of Materials Science 37 (2002) 3035–3043. [15] Y. Ren, Z. Liu, F. Pourpoint, A.R. Armstrong, C.P. Grey, P.G. Bruce, Angewandte Chemie International Edition 51 (2012) 2164–2167. [16] T. Beuvier, M. Richard-Plouet, L. Brohan, Journal of Physical Chemistry C 113 (2009) 13703–13706. [17] K. Kiatkittipong, C. Ye, J. Scott, R. Amal, Crystal Growth and Design 10 (2010) 3618–3625.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
TiO2-B nanofibers with high thermal stability as improved anodes for lithium ion batteries Wei Zhuang, Linghong Lu ⁎, Xinbing Wu, Wei Jin, Meng Meng, Yudan Zhu, Xiaohua Lu ⁎ State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 7 November 2012 Accepted 12 November 2012 Available online 27 November 2012 Keywords: Lithium ion batteries TiO2-B Nanofibers High thermal stability Titania
a b s t r a c t TiO2-B nanofibers with large surface area, high thermal stability and high crystallinity were synthesized by steam thermal method from K2Ti2O5. Compared to the bulk TiO2-B (TB-bulk) prepared from K2Ti4O9, these nanofibers exhibited much higher reversible capacity, cycling stability and rate capability. Such excellent electrochemical performances were derived from the facile charge transport due to the specific framework of a large specific surface area and one dimensional structure. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Batteries with long cycle life and high energy and power densities are important approaches to address the challenges of energy shortage. Next generation of lithium ion batteries is essential for applications in electric vehicles, stationary energy storage systems for solar and wind energy as well as smart grids [1]. However, the performance of current lithium ion batteries cannot meet the requirements in such application fields because of the safety issue and poor rate performance [2]. Titanates are being intensively investigated as anodes for lithium-ion batteries due to their superior safety and rate capability compared with graphite [3]. Li4Ti5O12 spinel is already used in commercial lithium-ion batteries [4]. Titanium dioxide (TiO2) is one of the most focused metal oxide materials for its unique properties and great importance in many areas such as anode materials of lithium ion battery. TiO2 has twice the theoretical specific capacity (335 mAh g−1) as compared to Li4Ti5O12 (175 mAh g−1) does [5]. Furthermore, as a novel member of TiO2 materials, TiO2-B shows a favorable channel structure for lithium mobility, so as to enhance the charge–discharge capabilities of a lithium cell. It has been identified that the lithium intercalation in TiO2-B features a pseudocapacitive process, rather than the solid-state diffusion process observed for anatase and rutile [6,7]. Theoretical studies have uncovered that this pseudocapacitive behavior originates from the unique sites and energetics of lithium absorption and diffusion in TiO2-B crystalline structure [8]. Moreover, one dimensional structure allows for the accommodation of the strain of lithium insertion/removal along a specific orientation, which avoids the irregular expansion [9]. Meanwhile, this structure ⁎ Corresponding authors. Tel./fax: +86 25 83588063. E-mail addresses: [email protected] (L. Lu), [email protected] (X. Lu).
facilitates the diffusion of electrolyte into the inner region of the electrode and therefore accelerates charge transfer in the interior [10]. Hence, it draws our attention to synthesize the TiO2-B nanofibers. However, TiO2-B is a metastable phase, which can gradually lead to degradation of both structure stability and lithium intercalation cycling stability of TiO2-B [11]. Actually, TiO2-B fibers (specific surface areab 20 m2 g−1) can be prepared from K2Ti4O9 via the soft chemistry method and exhibit considerably lithium ion intercalation performance [12]. However, the TiO2-B with large surface area and high thermal stability cannot be easily prepared directly from potassium titanate. Moreover, high purity TiO2-B materials are difficult to be synthesized through hydrothermal method. In our previous work, mesoporous anatase TiO2 with minor TiO2-B has been synthesized from K2Ti2O5 and shows high thermal stability [13]. In this work, we report a facile steam thermal method to transform K2Ti2O5 into TiO2-B nanofibers with a specific surface area of 112 m 2 g −1, high crystallinity and high thermal stability. The TiO2-B electrodes show high storage capacity, excellent rate capability and reliable stability. 2. Experimental 2.1. Preparation of samples K2Ti2O5 or K2Ti4O9 was prepared by adding K2CO3 (reagent grade) to TiO2 nH2O with a proper TiO2/K2O molar ratio and then sintered [14]. After that, 2 g K2Ti2O5 was mounted in a crucible and then with 20 ml deionized water out the crucible was sealed in a Teflon-lined autoclave. It was placed in an oven at the temperature of 180 °C for a set duration of 12 h, and then cooled to room temperature. Then, the intermediate was obtained after ion exchange in vigorously stirred
1388-2481/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.11.012
W. Zhuang et al. / Electrochemistry Communications 27 (2013) 124–127
0.1 M HCl solution until K+ ion was completely exchanged. The intermediate was filtered and washed with distilled water and then dried in a desiccator at 80 °C, denoted as TBN-80. Calcinations of the dried samples were performed in a muffle oven at 400, 500, 600 °C in air for 2 h, denoted as TBN-400, TBN-500 and TBN-600, respectively. Bulk TiO2-B prepared from K2Ti4O9 was possessed after ion exchange, washing and drying, denoted as TB-bulk. 2.2. Characterization The crystalline structure of the materials was determined by powder X-ray diffraction (XRD, Bruker D8, Cu-Kα radiation) and Raman spectra (Super LabRam). The textural properties were studied by N2 adsorption–desorption measurements (ASAP 2020 M) at liquid nitrogen temperature of 77 K. The sample morphology and microstructure were observed by scanning electron microscopy (SEM, Hitachi S4800) and transmission electron microscopy (TEM, Philips Tecnai G2 20) at 200 kV. 2.3. Electrochemical measurement All electrochemical measurements were conducted in Swagelok type cells with pure lithium foil as the counter and reference electrodes and Whatman glass fiber as separator. For preparation of working electrode, a slurry was prepared by mixing the active material, Super P carbon (Alfa Aesar) and PVDF (Kynarflex) in a weight ratio of 85:8:7 in N-methyl-pyrrolidone (NMP). The slurry was cast on a copper foil (Alfa Aesar, thickness: 0.05 mm) and was dried in vacuum at 100 °C for 24 h. The electrolyte used for the above cell configuration was 1 M LiPF6 in a mixture of ethylene carbonate and dimethyl carbonate (1:1 w/w). Cyclic voltammetry was performed between the voltage ranges of 1.0 and 3.0 V using an advanced electrochemical system of Autolab 302 N at the scanning rate of 0.1 mV s −1. The electrochemical impedance measurements were recorded potentiostatically by applying an AC voltage of 5 mV amplitude in the 100 kHz to 10 mHz frequency range. The galvanostatic charge–discharge characteristics of the cells were recorded over the voltage range between 1.0 and 3.0 V using a NEWARE BTS-5V50mA computer-controlled battery test station for different rates at room temperature (25 °C). 1 °C is equivalent to 330 mA g−1 for TiO2-B.
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3. Results and discussion Fig. 1(a) shows the XRD patterns of the prepared TiO2 materials, in which the diffraction peaks can be indexed to monoclinic TiO2-B phase. The intensity distribution of nanofibers is similar to that of the bulk material. There are no peaks identified as tetragonal anatase phase (JCPDS no. 21–1272) in the nanofibres until heat treated at 600 °C, indicates the high thermal stability of crystalline structure [15]. Fig. 1(b) shows the N2 adsorption–desorption isotherm curves of TBN-500, which is a typical Type IV isotherm representing mesoporous structure with a BET specific surface area of 112 m 2 g−1. Meanwhile, TiO2-B bulk shows much lower surface area of 14.5 m 2 g −1. This phenomenon of large surface area may bring the excellent capability to accommodate strain and structure changes during battery cycling. The morphology of TiO2-B after washing and heat-treatment processes is displayed in Fig. 1(c) with a magnified image in the inset. The sample consisted of regular shaped nanofibers with uniform widths of 100–200 nm and variable lengths around 10 μm. The highly crystalline structure of the nanofibers is confirmed by both HR-TEM in Fig. 1(d) and selected area electron diffraction (SAED, inset of Fig. 1(d)). The 0.62 nm lattice spacing is consistent with the (001) plane of the monoclinic TiO2-B structure. All these results reveal the formation of highly crystalline mesoporous TiO2-B nanofibers. Fig. 2(a) shows Raman spectra of the samples thermally treated at different temperatures and TB-bulk. The band positions and relative intensities match well with the spectrum of TiO2-B reported in previous literature [16]. TBN-80 corresponding to the H-titanate is characterized by a strong peak at 150 cm−1. These results are in accordance with the results reported in the literature [17]. After calcination, peaks corresponding to the monoclinic phase are more evident and dominate the spectrum, in part because of the substantial loss of intensity of the 120 cm−1 peak of TiO2-B. Also, the nanofibers can keep the TiO2-B structure until the temperature reaches 600 °C and exhibit higher thermal stability of structure than that reported in the literature [7]. TiO2-B nanofibers were mixed with Super P carbon and PVDF binder to form composite electrode. The increased ability to store lithium ions in the TiO2-B nanofibers compared to the bulk one, as shown in Fig. 2(b), is very obvious. On reducing the dimensions of crystalline materials to the nanoscale, a greater proportion of the material will be in the near surface region where it is subjected to different forces than in
Fig. 1. (a) XRD pattern of the materials, with JCPDS (no. 46–1237) data for TiO2-B; (b) N2 adsorption–desorption isotherms of TBN-500 and TB-bulk; (c) FE-SEM image of TBN-500 and the highly magnified image (inset); (d) HR-TEM image indicating the presence of TiO2-B(001), the selected area electron diffraction (SAED) of TBN-500 and scheme of TiO2-B (inset).
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W. Zhuang et al. / Electrochemistry Communications 27 (2013) 124–127
Fig. 2. (a) Raman spectra of the samples thermally treated at different temperatures and TB-bulk; (b) the first charge–discharge curves for different samples at 100 mA g−1; (c) cyclic voltammetry (CV) curves for different samples at a scan rate of 0.1 mV s−1. (d) CVs of TBN-500 at different scan rates and the plot of peak currents against the scan rates (inset). (e) Rate performances of TiO2-B nanofibers derived from K2Ti2O5 and TB-bulk at different current rates and typical electrochemical impedance spectra for TBN-500 and TB-bulk (inset); (f) cycling performances of TBN-500 and TB-bulk electrodes at the rate of 1 °C.
the bulk, resulting in structural distortions. Fig. 2(c) shows the cyclic voltammetry (CV) curves of TiO2-B electrode at the scan rate of 0.1 mV s−1. They all show a pair of plateaus in the load curves in the range of 1.45–1.65 V, where peaks correspond to plateaus in the load curves. In the cases of the bulk and nanofiber TiO2-B materials, the plateaus can be obtained and have been used previously to assign the 2-phase processes to intercalation into the A1 and A2 sites of the TiO2-B crystal structure [10]. This is consistent with previous observations of intercalation compounds that exhibit a 2-phase intercalation process for large (bulk) fibers but increasingly sloping load curves on reducing the particle dimensions. Fig. 2(d) shows the CV curves of TBN-500 at the scan rates of 0.1, 0.2, 0.5, 1 mV s −1. The plot of peak currents against the scan rate is shown in the inset of Fig. 2(d). To analyze this dependence in a broader interval of scan rate, we selected here the anodic and cathodic S-peaks, which are better resolved at faster charging. This revealed that lithium ion intercalation in TiO2-B is a pseudocapacitive process. Titanate anodes are employed because of their superior safety and ability to sustain high charge/discharge rates compared with graphite. These unique features are critically important for high power
applications such as electric vehicles. Therefore, we have investigated the rate capability of TiO2-B nanofibers. The variation of gravimetric capacity (based on the active mass alone) with different charge/discharge rates is shown in Fig. 2(e). The data of TiO2-B nanofibers calcinated at different temperatures are compared with bulk TiO2-B, which is similar to the best previously reported rate capability of any titanate [12]. As shown in the inset of Fig. 2(e), the semicircle of TBN-500 becomes smaller than that of TB-bulk, indicating that the charge-transfer resistance of TBN-500 is lower. These results further confirm the rate capability performance. The relatively high gravimetric capacity of the TiO2-B nanofibers is in part due to the large specific surface area, which helps to ensure a higher diffusion rate of lithium ions. The significance of this result is that maximizing gravimetric capacity for future applications in lithium-ion cells. The other appealing feature of the nanofibers is its excellent cycling performance. The cycling performance of the samples is shown in Fig. 2(f). Both half cells were cycled at 1 °C for 100 cycles. In the case of the TiO2-B nanofibers, the initial capacity was 193.8 mAh g−1 and decreased to 189.5 mAh g−1 after 100 cycles, with very good capacity retention of 97.8%. However, for the electrodes of TB-bulk, the initial
W. Zhuang et al. / Electrochemistry Communications 27 (2013) 124–127
capacity was 171.1 mAh g−1 and decreased to 160 mAh g−1 after 100 cycles with moderate capacity retention of 93.5%. It's the best of TiO2-B nanofibers derived from K-titanate although a little worse than that fabricated by hydrothermal method [12]. These excellent rate and cycling performances could be attributed to: a) the large specific surface area; b) the high stability of crystalline structure; and c) one dimensional structure. 4. Conclusions In summary, nanofibrous TiO2-B was successfully obtained by steam thermal method using K2Ti2O5 as the precursor, which, to our knowledge, has never been reported before. The material exhibits a large specific surface area of 112 m2 g−1, high crystallinity and high thermal stability. The electrodes of TBN-500 show high first capacity of 277 mAh g−1, excellent rate capability and high cycling stability. Therefore, this material can be considered as an alternative anode material for lithium ion batteries. Acknowledgments This work was supported by the Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT 0732), the National Natural Science Foundation of China Grants (Nos. 21136004, 20736002, 21176113, and 20876073), NSFC-RGC (No. 20731160614), China Postdoctoral Science Foundation (No. 20110491407) and the National Basic
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