- Email: [email protected]
Synthesis and electrochemical characterization of 2D nanostructured Li4Ti5O12 with lithium electrode functionality
Journal of Physics and Chemistry of Solids 73 (2012) 1444–1447
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Synthesis and electrochemical characterization of 2D nanostructured Li4Ti5O12 with lithium electrode functionality Song-Yi Han, In Young Kim, Seong-Ju Hwang n Center for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano Sciences, Ewha Womans University, Seoul 120-750, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Available online 6 November 2011
Two-dimensional (2D) Li4Ti5O12 nanosheets with cubic spinel structure are synthesized via a lithiation process of exfoliated titanate 2D nanosheets and the subsequent heat-treatment at elevated temperatures. According to powder X-ray diffraction and field emission-scanning electron microscopy analysis, the obtained lithium titanium oxides show well-developed Bragg reflections of cubic spinel-structured Li4Ti5O12 phase and highly anisotropic 2D plate-like morphology. Ti K-edge X-ray absorption near-edge structure analysis reveals the stabilization of tetravalent titanium ions in the cubic spinel lattice of Li4Ti5O12. The 2D nanostructured Li4Ti5O12 materials display the electrochemical functionality as negative electrode for lithium secondary batteries. The most promising electrode performance is achieved by the heat-treatment of the lithiated titanate at 600 1C, highlighting the importance of heating temperature in optimizing the electrochemical property of the resulting materials. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides B. Chemical synthesis D. Electrochemical properties D. Phase transitions
1. Introduction Diverse metal oxides and metal alloys are intensively investigated as new anode materials in lithium ion batteries for replacing the currently commercialized graphite compounds [1]. Among many metal oxide-based anode materials ever developed, spinel-structured Li4Ti5O12 material receives prime attention because of its high electrochemical and structural stability, high lithium diffusion rate, and capability to prevent the formation of SEI layers on the electrode surface [2–4]. One of the most effective ways to improve the electrode functionality of electrode materials is the formation of nanostructures [5–7]. The reduction in the size of the electrode material to nanometer-scale is highly advantageous in facilitating Li intercalation at high current density. Based on this expectation, there are several reports about the formation of low dimensional nanostructured Li4Ti5O12 materials [8–11]. Typically the 1D nanostructures of lithium titanate are synthesized by lithiation process of 1D nanostructured titanium oxide prepared by hydrothermal treatment of bulk titania and the following heat-treatment at elevated temperatures [12,13]. Similarly 0D nanoparticle of titanium oxide is used as precursor for synthesizing the 0D nanocrystalline lithium titanate [14,15]. In addition, 3D mesoporous Li4Ti5O12 phase can be obtained by lithiation reaction of mesoporous titanium oxide [16]. However, 2D nanostructured Li4Ti5O12 material has never been reported.
n
Corresponding author. Tel.: þ82 2 3277 4370; fax þ 82 2 3277 3419. E-mail address: [email protected] (S.-J. Hwang).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.10.044
The soft-chemical exfoliation reaction of layered titanate produces the highly anisotropic 2D nanosheets of titanium oxide with subnanometer-thickness [17]. Taking into account the very thin thickness of exfoliated titanium oxide 2D nanosheet, this material is expected to be promising precursors for 2D nanostructured Li4Ti5O12 electrode material. Till date, however, there is no report on the synthesis of 2D nanostructured lithium titanate with the precursor titanate 2D nanosheets. In the present study, the 2D nanostructured Li4Ti5O12 materials are synthesized by n-BuLi treatment of exfoliated titanate nanosheets and the following calcination process at elevated temperatures. The crystal structure, crystal morphology, and the Ti oxidation state of the Li4Ti5O12 2D nanosheets are investigated with X-ray diffraction (XRD), electron microscopy, and X-ray absorption near-edge structure spectroscopy (XANES), respectively. The electrochemical properties of the obtained lithium titanate materials are examined to probe their functionality as the negative electrode material in lithium secondary batteries.
2. Experimental 2.1. Synthesis The precursor titanate 2D nanosheets were prepared in the form of aqueous colloidal suspension by exfoliation of lepidocrocite-type layered titanate. The exfoliation of layered titanate was achieved by the intercalation of tetrabutylammonium (TBA) cations into the protonated titanate as reported previously [17].
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The exfoliated titanate nanosheets were restored from as-synthesized colloidal suspension via freeze-drying process. The lithiation of titanate nanosheets was carried out by n-BuLi treatment for the restored powdery titanate sample for 4 days at room temperature. The resultant lithiated titanate sample was washed thoroughly with hexane and dried in an oven at 50 1C. To obtain 2D nanostructured Li4Ti5O12 materials, the lithiated titanate was subjected to heat-treatment at elevated temperatures of 400–800 1C in N2 atmosphere for 2 h. 2.2. Characterization The crystal structures of the precursor titanate, lithiated titanate, and its calcined derivatives were examined with a powder X-ray diffractometer (Rigaku, l ¼0.15418 nm, 298 K). The thermal behavior of the as-prepared lithiated titanate was studied with thermogravimetric analysis (TGA) under N2 flow. The effect of calcination on the crystal morphology of the lithiated titanate was investigated using field emission-scanning electron microscopy (FE-SEM, Jeol JSM-6700F). Ti K-edge XANES spectra were collected with extended X-ray absorption fine structure (EXAFS) facility installed at the beam line 7C in the Pohang Accelerator Laboratory (PAL), Korea. The XANES data were obtained in a transmission mode using gasionization detectors at room temperature. The energy of the collected spectra was calibrated by the simultaneous measurement of the spectrum of Ti metal foil.
Fig. 1. Powder XRD patterns of (a) pristine cesium titanate, (b) protonated titanate, (c) TBA-intercalated titanate, (d) lithiated titanate, and its derivatives calcined at (e) 400, (f) 500, (g) 600, (h) 700, and (i) 800 1C.
2.3. Electrode performance analysis The electrode functionality of the obtained Li4Ti5O12 materials was tested using the 2016 coin-type cell of Li/1 M LiPF6 in ethylene carbonate (EC):diethyl carbonate (DEC) (50:50 v/v)/ composite cathode. The composite electrodes were fabricated by coating of N-methylpyrrolidone slurry containing active electrode material, conductive binder (Super P), and PVDF in the ratio of 8:1:1 on the copper foil. The electrochemical measurements were carried out in a galvanostatic mode with WonA Tech multichannel galvanostat/potentiostat. The potential profiles of the present materials were measured with a voltage range of 1.3–2.0 V and a constant current density 0.1 mA cm 2.
3. Results and discussion 3.1. Powder XRD and thermal analyses The structural variations of layered cesium titanate upon protonation, exfoliation, and lithiation processes are examined with powder XRD analysis. As plotted in Fig. 1, the protonation of cesium titanate leads to the increase of basal spacing, confirming the replacement of interlayer Cs cations with hydronium ions. The powder XRD pattern of the exfoliated titanate restored from colloidal suspension indicates the formation of TBA-intercalated ˚ Upon the titanate phase with a large basal spacing of 15 A. reaction with n-BuLi, the basal spacing of the exfoliated titanate becomes larger, suggesting the intercalation of lithium ions into the interlayer space of layered titanate. To determine the heat-treatment condition of the lithiated titanate, the thermal behavior of this material is examined with TGA. As illustrated in Fig. 2, the lithiated titanate displays a significant mass loss of 13% in the temperature region of 25–250 1C, which is attributed to the dehydration and dehydroxylation of the material. The second mass decrease corresponding to the removal of organic species appears in the higher temperature range of 250–550 1C. On the basis of the present TGA results, the as-prepared lithiated titanate is heated at elevated temperatures of 400–800 1C. The effect of heat-treatment on the
Fig. 2. TGA curve of the as-prepared lithiated titanate.
crystal structure of lithiated titanate is also probed with powder XRD analysis, (see Fig. 1). All the calcined derivatives of the lithiated titanate show well-developed Bragg reflections corresponding to cubic spinel Li4Ti5O12 structure as well as small peaks corresponding to impurity Li2TiO3 phase. In case of lithium titanate prepared by calcination at 700–800 1C, most XRD peaks display separated features, suggesting the formation of two kinds of spinel compounds with slightly different lattice parameters. 3.2. FE-SEM analysis The FE-SEM images of the lithiated titanate and its calcined derivatives are illustrated in Fig. 3, together with that of the
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Fig. 3. FE-SEM images of (a) TBA-intercalated titanate, (b) lithiated titanate, and its derivatives calcined at (c) 400, (d) 500, (e) 600, and (f) 700 1C.
exfoliated titanate. A highly anisotropic sheet-like morphology of the exfoliated titanate is well-maintained after the lithiation process. The overall sheet-like morphology of the lithiated titanate remains nearly unchanged after the heat-treatment at elevated temperatures, clearly demonstrating the 2D nanosheets shape of the obtained Li4Ti5O12 materials. Even though there is no significant change in the highly anisotropic morphology of the lithiated titanate up to 700 1C, the increase of heating temperature induces cracks in the sheet-shape crystallites. The heattreatment at 800 1C induces remarkable morphological change to bulk polyhedral crystals. 3.3. Ti K-edge XANES analyses The oxidation state and local crystal structure of the titanium ions in the Li4Ti5O12 2D nanosheets are investigated through the Ti K-edge XANES spectroscopy. The Ti K-edge XANES spectra of the 2D nanostructured Li4Ti5O12 materials are presented in Fig. 4, compared with the reference spectra of anatase TiO2, protonated layered titanate, exfoliated titanate, and bulk Li4Ti5O12. The 2D nanostructured Li4Ti5O12 materials display three pre-edge peaks (denoted as P1, P2, P3) that are assigned as quadruple-allowed 1s-3d transitions [18,19]. The 2D nanostructured Li4Ti5O12 materials show typical pre-edge features of cubic spinel-structured Li4Ti5O12 phase, highlighting the same local crystal structure of titanium ion in these materials. In the main-edge region, all the materials under investigation exhibit several spectral features (denoted as A, B, and C) corresponding to the dipole-allowed 1s-4p transitions [19]. Like the pre-edge features, the overall spectral features in the main-edge region are nearly identical for the 2D nanosheets and bulk crystals of Li4Ti5O12, confirming the formation of cubic spinel phase after the heat-treatment of the lithiated titanate. 3.4. Electrochemical measurements The electrochemical activity of the 2D nanostructured Li4Ti5O12 materials is examined by measuring the negative electrode performance of these materials. The potential profiles of the Li4Ti5O12 2D nanosheets are presented in the left panel of Fig. 5, in comparison with that of the lithiated titanate. Because Li/electrolyte/lithium titanate cell is used for the electrochemical test, the discharging is lithiation of lithium titanate and the charging is delithiation. Like the lithiated titanate, the Li4Ti5O12 2D nanosheets prepared at 400 and
Fig. 4. Ti K-edge XANES spectra for (a) anatase TiO2, (b) protonated titanate, (c) TBA-intercalated titanate, (d) lithiated titanate, and its derivatives calcined at (e) 500 and (f) 700 1C and (g) bulk Li4Ti5O12.
500 1C show smooth variation of operating potential, which is a characteristic property of the nanocrystalline electrode materials [20]. Conversely, a distinct plateau is observable for the other lithium titanate nanosheets, which is typical of well-ordered Li4Ti5O12 phase. The observed behavior suggests that the electrode activity of these materials occurs by two phase mechanism between lithiated and delithiated titanate spinel lattices. The right panel of Fig. 5 illustrates the discharge capacity plot of the 2D nanostructured Li4Ti5O12 materials as a function of the cycle number. The Li4Ti5O12 nanosheets prepared at 400 1C display a large initial discharge capacity of 162 mAh g 1, which is followed by a rapid decay in the capacity up to less than 60 mAh g 1 after the tenth cycle. Such a remarkable capacity fading is also observed for the homolog synthesized at 500 1C. In the case of the Li4Ti5O12 materials synthesized at higher than
S.-Y. Han et al. / Journal of Physics and Chemistry of Solids 73 (2012) 1444–1447
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Fig. 5. (Left) Potential profiles and (right) discharge capacity plots of 2D nanostructured Li4Ti5O12 prepared by calcination at (a) 400 (circles), (b) 500 (triangles), (c) 600 (squares), (d) 700 (diamonds), and (e) 800 1C (hexagons) and (f) the lithiated titanate. The profiles were measured at a rate of 10 mAh g 1 and the potential range of 1.3–2.0 V.
500 1C, much better cyclability can be obtained. Among the present materials, the 2D nanosheets prepared at 600 1C show promising discharge capacity with excellent cyclability, which are much better than the starting lithiated titanate. This finding highlights that the present lithiation-calcination process of exfoliated titanate nanosheets is effective in synthesizing the 2D nanostructured Li4Ti5O12 phase with electrode functionality. Taking into account the highly anisotropic 2D morphology of the present lithium titanates suitable for hybridization with foreign species, their electrode functionality is expected to be further improved by the hybridization with other electrochemically active or highly conductive materials like graphene.
4. Conclusions In the present study, the 2D nanostructured Li4Ti5O12 materials are successfully synthesized by a lithiation process of exfoliated titanate 2D nanosheets and the subsequent heat-treatment at elevated temperatures. The formation of cubic spinel-structured Li4Ti5O12 2D nanosheets is confirmed by conducting powder XRD and FE-SEM analyses. The tetravalent oxidation state and octahedral symmetry of titanium ions are evidenced by the XANES analysis. The electrochemical measurements clearly demonstrate that the obtained 2D nanostructured lithium titanates can be used as negative electrode for lithium secondary batteries and the material calcined at 600 1C shows a better performance than the homologs obtained by heat-treatment at different temperatures. In conclusion, the exfoliated 2D nanosheets of layered titanate can be used as an efficient precursor for 2D nanostructured lithium titanate phase.
Acknowledgments This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2010-C1AAA001-2010-0029065) and by National Research
Foundation of Korea Grant funded by the Korean Government (2010-0001485). The experiments at Pohang Accelerator Laboratory (PAL) were supported in part by MOST and POSTECH. The authors are grateful to Prof. J. Cho (UNIST) for helping us to test the electrode performance.
References [1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496–499. [2] T. Ohzuku, A. Ueda, N. Yamamoto, J. Electrochem. Soc. 142 (1995) 1431–1435. [3] S. Takai, M. Kamata, S. Fujine, K. Yoneda, K. Kanda, T. Esata, Solid State Ion. 123 (1999) 165–169. [4] C.Y. Ouyang, Z.Y. Zhong, M.S. Lei, Electrochem. Commun. 9 (2007) 1107–1112. [5] Y.J. Hao, O.Y. Lai, D.Q. Liu, Z.U. Xu, X.Y. Ji, Mater. Chem. Phys. 94 (2005) 382–387. [6] M.K. Jo, Y.-S. Hong, J.B. Choo, J.P. Cho, J. Electrochem. Soc. 156 (2009) A430–A434. [7] M. Okubo, E. Hosono, J. Kim, M. Enomoto, N. Kojima, T. Kubo, H. Zhou, I. Honma, J. Am. Chem. Soc. 129 (2007) 7444–7452. [8] D.K. Lee, H.-W. Shim, J.S. An, C.M. Cho, I.-S. Cho, K.S. Hong, D.-W. Kim, Nanoscale Res. Lett. 5 (2010) 1585–1589. [9] J. Chen, L. Yang, S. Fang, Y. Tang, Electrochim. Acta 55 (2010) 6596–6600. [10] M. Noh, Y. Kwon, H. Lee, J. Cho, Y. Kim, M.G. Kim, Chem. Mater. 17 (2005) 1926–1929. [11] J. Li, Z. Tang, Z. Zhang, Electrochem. Commun. 7 (2005) 894–899. [12] S.C. Lee, S.M. Lee, J.W. Lee, J.B. Lee, S.M. Lee, S.S. Han, H.C. Lee, H.J. Kim, J. Phys. Chem. C 113 (2009) 18420–18423. [13] J.Y. Kim, J.P. Cho, Electrochem. Solid-State Lett 10 (2007) A81–84. [14] A. Guerfi, S. Se´vigny, M. Lagace´, P. Hovington, K. Kinoshita, K. Zaghib, J. Power Sources 88 (2003) 119–121. [15] S.S. Lee, K.-T. Byun, J.P. Park, S.K. Kim, H.-Y. Kwak, I.-W. Shim, Dalton Trans. 47 (2007) 4182–4184. [16] Y. Tang, L. Yang, Zheng Qiu, J. Huang, J. Mater. Chem. 19 (2009) 5980–5984. [17] T. Sasaki, M. Watanabe, H. Hashizume, H. Yamada, H. Nakazawa, J. Am. Chem. Soc. 118 (1996) 8329–8335. [18] N.J. Hess, M.L. Balmer, B.C. Bunker, S.D. Conradson, J. Solid State Chem. 129 (1997) 206–213. [19] S.G. Hur, D.H. Park, T.W. Kim, S.-J. Hwang, Appl. Phys. Lett. 85 (2004) 4130–4132. [20] S.-J. Hwang, D.H. Park, C.W. Kwon, G. Campet, J.-H. Choy, J. Power Sources 125 (2004) 119–123.
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Synthesis and electrochemical characterization of 2D nanostructured Li4Ti5O12 with lithium electrode functionality Song-Yi Han, In Young Kim, Seong-Ju Hwang n Center for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano Sciences, Ewha Womans University, Seoul 120-750, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Available online 6 November 2011
Two-dimensional (2D) Li4Ti5O12 nanosheets with cubic spinel structure are synthesized via a lithiation process of exfoliated titanate 2D nanosheets and the subsequent heat-treatment at elevated temperatures. According to powder X-ray diffraction and field emission-scanning electron microscopy analysis, the obtained lithium titanium oxides show well-developed Bragg reflections of cubic spinel-structured Li4Ti5O12 phase and highly anisotropic 2D plate-like morphology. Ti K-edge X-ray absorption near-edge structure analysis reveals the stabilization of tetravalent titanium ions in the cubic spinel lattice of Li4Ti5O12. The 2D nanostructured Li4Ti5O12 materials display the electrochemical functionality as negative electrode for lithium secondary batteries. The most promising electrode performance is achieved by the heat-treatment of the lithiated titanate at 600 1C, highlighting the importance of heating temperature in optimizing the electrochemical property of the resulting materials. & 2011 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Oxides B. Chemical synthesis D. Electrochemical properties D. Phase transitions
1. Introduction Diverse metal oxides and metal alloys are intensively investigated as new anode materials in lithium ion batteries for replacing the currently commercialized graphite compounds [1]. Among many metal oxide-based anode materials ever developed, spinel-structured Li4Ti5O12 material receives prime attention because of its high electrochemical and structural stability, high lithium diffusion rate, and capability to prevent the formation of SEI layers on the electrode surface [2–4]. One of the most effective ways to improve the electrode functionality of electrode materials is the formation of nanostructures [5–7]. The reduction in the size of the electrode material to nanometer-scale is highly advantageous in facilitating Li intercalation at high current density. Based on this expectation, there are several reports about the formation of low dimensional nanostructured Li4Ti5O12 materials [8–11]. Typically the 1D nanostructures of lithium titanate are synthesized by lithiation process of 1D nanostructured titanium oxide prepared by hydrothermal treatment of bulk titania and the following heat-treatment at elevated temperatures [12,13]. Similarly 0D nanoparticle of titanium oxide is used as precursor for synthesizing the 0D nanocrystalline lithium titanate [14,15]. In addition, 3D mesoporous Li4Ti5O12 phase can be obtained by lithiation reaction of mesoporous titanium oxide [16]. However, 2D nanostructured Li4Ti5O12 material has never been reported.
n
Corresponding author. Tel.: þ82 2 3277 4370; fax þ 82 2 3277 3419. E-mail address: [email protected] (S.-J. Hwang).
0022-3697/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2011.10.044
The soft-chemical exfoliation reaction of layered titanate produces the highly anisotropic 2D nanosheets of titanium oxide with subnanometer-thickness [17]. Taking into account the very thin thickness of exfoliated titanium oxide 2D nanosheet, this material is expected to be promising precursors for 2D nanostructured Li4Ti5O12 electrode material. Till date, however, there is no report on the synthesis of 2D nanostructured lithium titanate with the precursor titanate 2D nanosheets. In the present study, the 2D nanostructured Li4Ti5O12 materials are synthesized by n-BuLi treatment of exfoliated titanate nanosheets and the following calcination process at elevated temperatures. The crystal structure, crystal morphology, and the Ti oxidation state of the Li4Ti5O12 2D nanosheets are investigated with X-ray diffraction (XRD), electron microscopy, and X-ray absorption near-edge structure spectroscopy (XANES), respectively. The electrochemical properties of the obtained lithium titanate materials are examined to probe their functionality as the negative electrode material in lithium secondary batteries.
2. Experimental 2.1. Synthesis The precursor titanate 2D nanosheets were prepared in the form of aqueous colloidal suspension by exfoliation of lepidocrocite-type layered titanate. The exfoliation of layered titanate was achieved by the intercalation of tetrabutylammonium (TBA) cations into the protonated titanate as reported previously [17].
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The exfoliated titanate nanosheets were restored from as-synthesized colloidal suspension via freeze-drying process. The lithiation of titanate nanosheets was carried out by n-BuLi treatment for the restored powdery titanate sample for 4 days at room temperature. The resultant lithiated titanate sample was washed thoroughly with hexane and dried in an oven at 50 1C. To obtain 2D nanostructured Li4Ti5O12 materials, the lithiated titanate was subjected to heat-treatment at elevated temperatures of 400–800 1C in N2 atmosphere for 2 h. 2.2. Characterization The crystal structures of the precursor titanate, lithiated titanate, and its calcined derivatives were examined with a powder X-ray diffractometer (Rigaku, l ¼0.15418 nm, 298 K). The thermal behavior of the as-prepared lithiated titanate was studied with thermogravimetric analysis (TGA) under N2 flow. The effect of calcination on the crystal morphology of the lithiated titanate was investigated using field emission-scanning electron microscopy (FE-SEM, Jeol JSM-6700F). Ti K-edge XANES spectra were collected with extended X-ray absorption fine structure (EXAFS) facility installed at the beam line 7C in the Pohang Accelerator Laboratory (PAL), Korea. The XANES data were obtained in a transmission mode using gasionization detectors at room temperature. The energy of the collected spectra was calibrated by the simultaneous measurement of the spectrum of Ti metal foil.
Fig. 1. Powder XRD patterns of (a) pristine cesium titanate, (b) protonated titanate, (c) TBA-intercalated titanate, (d) lithiated titanate, and its derivatives calcined at (e) 400, (f) 500, (g) 600, (h) 700, and (i) 800 1C.
2.3. Electrode performance analysis The electrode functionality of the obtained Li4Ti5O12 materials was tested using the 2016 coin-type cell of Li/1 M LiPF6 in ethylene carbonate (EC):diethyl carbonate (DEC) (50:50 v/v)/ composite cathode. The composite electrodes were fabricated by coating of N-methylpyrrolidone slurry containing active electrode material, conductive binder (Super P), and PVDF in the ratio of 8:1:1 on the copper foil. The electrochemical measurements were carried out in a galvanostatic mode with WonA Tech multichannel galvanostat/potentiostat. The potential profiles of the present materials were measured with a voltage range of 1.3–2.0 V and a constant current density 0.1 mA cm 2.
3. Results and discussion 3.1. Powder XRD and thermal analyses The structural variations of layered cesium titanate upon protonation, exfoliation, and lithiation processes are examined with powder XRD analysis. As plotted in Fig. 1, the protonation of cesium titanate leads to the increase of basal spacing, confirming the replacement of interlayer Cs cations with hydronium ions. The powder XRD pattern of the exfoliated titanate restored from colloidal suspension indicates the formation of TBA-intercalated ˚ Upon the titanate phase with a large basal spacing of 15 A. reaction with n-BuLi, the basal spacing of the exfoliated titanate becomes larger, suggesting the intercalation of lithium ions into the interlayer space of layered titanate. To determine the heat-treatment condition of the lithiated titanate, the thermal behavior of this material is examined with TGA. As illustrated in Fig. 2, the lithiated titanate displays a significant mass loss of 13% in the temperature region of 25–250 1C, which is attributed to the dehydration and dehydroxylation of the material. The second mass decrease corresponding to the removal of organic species appears in the higher temperature range of 250–550 1C. On the basis of the present TGA results, the as-prepared lithiated titanate is heated at elevated temperatures of 400–800 1C. The effect of heat-treatment on the
Fig. 2. TGA curve of the as-prepared lithiated titanate.
crystal structure of lithiated titanate is also probed with powder XRD analysis, (see Fig. 1). All the calcined derivatives of the lithiated titanate show well-developed Bragg reflections corresponding to cubic spinel Li4Ti5O12 structure as well as small peaks corresponding to impurity Li2TiO3 phase. In case of lithium titanate prepared by calcination at 700–800 1C, most XRD peaks display separated features, suggesting the formation of two kinds of spinel compounds with slightly different lattice parameters. 3.2. FE-SEM analysis The FE-SEM images of the lithiated titanate and its calcined derivatives are illustrated in Fig. 3, together with that of the
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Fig. 3. FE-SEM images of (a) TBA-intercalated titanate, (b) lithiated titanate, and its derivatives calcined at (c) 400, (d) 500, (e) 600, and (f) 700 1C.
exfoliated titanate. A highly anisotropic sheet-like morphology of the exfoliated titanate is well-maintained after the lithiation process. The overall sheet-like morphology of the lithiated titanate remains nearly unchanged after the heat-treatment at elevated temperatures, clearly demonstrating the 2D nanosheets shape of the obtained Li4Ti5O12 materials. Even though there is no significant change in the highly anisotropic morphology of the lithiated titanate up to 700 1C, the increase of heating temperature induces cracks in the sheet-shape crystallites. The heattreatment at 800 1C induces remarkable morphological change to bulk polyhedral crystals. 3.3. Ti K-edge XANES analyses The oxidation state and local crystal structure of the titanium ions in the Li4Ti5O12 2D nanosheets are investigated through the Ti K-edge XANES spectroscopy. The Ti K-edge XANES spectra of the 2D nanostructured Li4Ti5O12 materials are presented in Fig. 4, compared with the reference spectra of anatase TiO2, protonated layered titanate, exfoliated titanate, and bulk Li4Ti5O12. The 2D nanostructured Li4Ti5O12 materials display three pre-edge peaks (denoted as P1, P2, P3) that are assigned as quadruple-allowed 1s-3d transitions [18,19]. The 2D nanostructured Li4Ti5O12 materials show typical pre-edge features of cubic spinel-structured Li4Ti5O12 phase, highlighting the same local crystal structure of titanium ion in these materials. In the main-edge region, all the materials under investigation exhibit several spectral features (denoted as A, B, and C) corresponding to the dipole-allowed 1s-4p transitions [19]. Like the pre-edge features, the overall spectral features in the main-edge region are nearly identical for the 2D nanosheets and bulk crystals of Li4Ti5O12, confirming the formation of cubic spinel phase after the heat-treatment of the lithiated titanate. 3.4. Electrochemical measurements The electrochemical activity of the 2D nanostructured Li4Ti5O12 materials is examined by measuring the negative electrode performance of these materials. The potential profiles of the Li4Ti5O12 2D nanosheets are presented in the left panel of Fig. 5, in comparison with that of the lithiated titanate. Because Li/electrolyte/lithium titanate cell is used for the electrochemical test, the discharging is lithiation of lithium titanate and the charging is delithiation. Like the lithiated titanate, the Li4Ti5O12 2D nanosheets prepared at 400 and
Fig. 4. Ti K-edge XANES spectra for (a) anatase TiO2, (b) protonated titanate, (c) TBA-intercalated titanate, (d) lithiated titanate, and its derivatives calcined at (e) 500 and (f) 700 1C and (g) bulk Li4Ti5O12.
500 1C show smooth variation of operating potential, which is a characteristic property of the nanocrystalline electrode materials [20]. Conversely, a distinct plateau is observable for the other lithium titanate nanosheets, which is typical of well-ordered Li4Ti5O12 phase. The observed behavior suggests that the electrode activity of these materials occurs by two phase mechanism between lithiated and delithiated titanate spinel lattices. The right panel of Fig. 5 illustrates the discharge capacity plot of the 2D nanostructured Li4Ti5O12 materials as a function of the cycle number. The Li4Ti5O12 nanosheets prepared at 400 1C display a large initial discharge capacity of 162 mAh g 1, which is followed by a rapid decay in the capacity up to less than 60 mAh g 1 after the tenth cycle. Such a remarkable capacity fading is also observed for the homolog synthesized at 500 1C. In the case of the Li4Ti5O12 materials synthesized at higher than
S.-Y. Han et al. / Journal of Physics and Chemistry of Solids 73 (2012) 1444–1447
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Fig. 5. (Left) Potential profiles and (right) discharge capacity plots of 2D nanostructured Li4Ti5O12 prepared by calcination at (a) 400 (circles), (b) 500 (triangles), (c) 600 (squares), (d) 700 (diamonds), and (e) 800 1C (hexagons) and (f) the lithiated titanate. The profiles were measured at a rate of 10 mAh g 1 and the potential range of 1.3–2.0 V.
500 1C, much better cyclability can be obtained. Among the present materials, the 2D nanosheets prepared at 600 1C show promising discharge capacity with excellent cyclability, which are much better than the starting lithiated titanate. This finding highlights that the present lithiation-calcination process of exfoliated titanate nanosheets is effective in synthesizing the 2D nanostructured Li4Ti5O12 phase with electrode functionality. Taking into account the highly anisotropic 2D morphology of the present lithium titanates suitable for hybridization with foreign species, their electrode functionality is expected to be further improved by the hybridization with other electrochemically active or highly conductive materials like graphene.
4. Conclusions In the present study, the 2D nanostructured Li4Ti5O12 materials are successfully synthesized by a lithiation process of exfoliated titanate 2D nanosheets and the subsequent heat-treatment at elevated temperatures. The formation of cubic spinel-structured Li4Ti5O12 2D nanosheets is confirmed by conducting powder XRD and FE-SEM analyses. The tetravalent oxidation state and octahedral symmetry of titanium ions are evidenced by the XANES analysis. The electrochemical measurements clearly demonstrate that the obtained 2D nanostructured lithium titanates can be used as negative electrode for lithium secondary batteries and the material calcined at 600 1C shows a better performance than the homologs obtained by heat-treatment at different temperatures. In conclusion, the exfoliated 2D nanosheets of layered titanate can be used as an efficient precursor for 2D nanostructured lithium titanate phase.
Acknowledgments This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2010-C1AAA001-2010-0029065) and by National Research
Foundation of Korea Grant funded by the Korean Government (2010-0001485). The experiments at Pohang Accelerator Laboratory (PAL) were supported in part by MOST and POSTECH. The authors are grateful to Prof. J. Cho (UNIST) for helping us to test the electrode performance.
References [1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 407 (2000) 496–499. [2] T. Ohzuku, A. Ueda, N. Yamamoto, J. Electrochem. Soc. 142 (1995) 1431–1435. [3] S. Takai, M. Kamata, S. Fujine, K. Yoneda, K. Kanda, T. Esata, Solid State Ion. 123 (1999) 165–169. [4] C.Y. Ouyang, Z.Y. Zhong, M.S. Lei, Electrochem. Commun. 9 (2007) 1107–1112. [5] Y.J. Hao, O.Y. Lai, D.Q. Liu, Z.U. Xu, X.Y. Ji, Mater. Chem. Phys. 94 (2005) 382–387. [6] M.K. Jo, Y.-S. Hong, J.B. Choo, J.P. Cho, J. Electrochem. Soc. 156 (2009) A430–A434. [7] M. Okubo, E. Hosono, J. Kim, M. Enomoto, N. Kojima, T. Kubo, H. Zhou, I. Honma, J. Am. Chem. Soc. 129 (2007) 7444–7452. [8] D.K. Lee, H.-W. Shim, J.S. An, C.M. Cho, I.-S. Cho, K.S. Hong, D.-W. Kim, Nanoscale Res. Lett. 5 (2010) 1585–1589. [9] J. Chen, L. Yang, S. Fang, Y. Tang, Electrochim. Acta 55 (2010) 6596–6600. [10] M. Noh, Y. Kwon, H. Lee, J. Cho, Y. Kim, M.G. Kim, Chem. Mater. 17 (2005) 1926–1929. [11] J. Li, Z. Tang, Z. Zhang, Electrochem. Commun. 7 (2005) 894–899. [12] S.C. Lee, S.M. Lee, J.W. Lee, J.B. Lee, S.M. Lee, S.S. Han, H.C. Lee, H.J. Kim, J. Phys. Chem. C 113 (2009) 18420–18423. [13] J.Y. Kim, J.P. Cho, Electrochem. Solid-State Lett 10 (2007) A81–84. [14] A. Guerfi, S. Se´vigny, M. Lagace´, P. Hovington, K. Kinoshita, K. Zaghib, J. Power Sources 88 (2003) 119–121. [15] S.S. Lee, K.-T. Byun, J.P. Park, S.K. Kim, H.-Y. Kwak, I.-W. Shim, Dalton Trans. 47 (2007) 4182–4184. [16] Y. Tang, L. Yang, Zheng Qiu, J. Huang, J. Mater. Chem. 19 (2009) 5980–5984. [17] T. Sasaki, M. Watanabe, H. Hashizume, H. Yamada, H. Nakazawa, J. Am. Chem. Soc. 118 (1996) 8329–8335. [18] N.J. Hess, M.L. Balmer, B.C. Bunker, S.D. Conradson, J. Solid State Chem. 129 (1997) 206–213. [19] S.G. Hur, D.H. Park, T.W. Kim, S.-J. Hwang, Appl. Phys. Lett. 85 (2004) 4130–4132. [20] S.-J. Hwang, D.H. Park, C.W. Kwon, G. Campet, J.-H. Choy, J. Power Sources 125 (2004) 119–123.