18420
J. Phys. Chem. C 2009, 113, 18420–18423
Spinel Li4Ti5O12 Nanotubes for Energy Storage Materials Soon Chang Lee,† Sang Man Lee,‡ Jae Won Lee,§ Jin Bae Lee,† Sang Moon Lee,† Sang Sup Han,| Hee Cheon Lee,‡ and Hae Jin Kim*,† Nano Material Research Team, Korea Basic Science Institute, Daejon 305-333, Korea; Department of Chemistry, Pohang UniVersity of Science and Technology, Pohang 790-784, Korea; Energy & Applied Materials Laboratory, Korea Institute of Ceramic Engineering & Technology, Seoul 153-801, Korea; and Chemical Process R&D Center, Korea Institute of Energy Research, Daejon 305-343, Korea ReceiVed: May 15, 2009; ReVised Manuscript ReceiVed: August 31, 2009
We investigated new potential materials, such as hydrogen storage and lithium rechargeable batteries, for application in the field of advanced energy conversion and storage. We were able to synthesize Li4Ti5O12 nanotubes using heat treatment and an alkali-hydrothermal reaction through a simple structural rearrangement, and we examined their H2 storage characteristics and suitability as electrode materials for application in lithium-ion batteries. The Li4Ti5O12 nanotubes could store up to 0.7 wt % H2 at ambient temperature and their reversible capacity was approximately 156 mAh/g at a rate of 0.1C. Introduction Energy storage is indubitably one of the great challenges in the 21st century.1 Recently, nanostructured materials have become increasingly important in the areas of physical, chemical, and electrochemical energy storage, such as hydrogen storage and electrochemical performance for applications in Liion batteries.2-8 Nanotubes are one-dimensional nanostrucures with hollow interior channels, which have potentially large storage capacity, safety, and fast adsorption and desorption times compared to their bulk counterparts.9-11 Because of their large surface areas and surface-to-weight ratios, nanotubes are the most attractive candidates for the energy storage applications already mentioned.12 In actuality, good hydrogen storage capacities have been reported for some nanotubular shaped materials (e.g., BN, MoS2, and TiS2).13-16 In addition, alkali and transition-metal doping systems have been used in an attempt to overcome the limitations of these materials.17,18 A major mechanism for hydrogen storage is the physical or chemical adsorption of hydrogen molecules onto the surface of materials.19 Tubularstructured materials have more virtues than other structures due to their potentially fast surface adsorption kinetics. Subsequently, many transition-metal containing nanotubes have been synthesized, and their hydrogen storage characteristics examined.15,16,20-27 Recently, many successful results, beyond expectation, have been achieved for the hydrogen storage of titanium-containing nanotubes. However, H2 storage of spinel Li4Ti5O12 nanotubes has not been documented. Until now, spinel Li4Ti5O12 has attracted great attention as a superior electrode and supercapacitor material because of its high lithium mobility and its lack of structural changes during charge-discharge cycling.28,29 Nanotubes also draw great attention as electrode materials for lithium rechargeable batteries. Lithium and electron conductivity are the most importantly * To whom correspondence should be addressed. Phone: +82-42-8653953. Fax: +82-42-865-3419. E-mail:
[email protected]. † Nano Material Research Team, Korea Basic Science Institute. ‡ Department of Chemistry, Pohang University of Science and Technology. § Energy & Applied Materials Laboratory, Korea Institute of Ceramic Engineering & Technology. | Chemical Process R&D Center, Korea Institute of Energy Research.
required features. Carbon coating or metal doping is generally used to enhance the electron conductivity, and poor lithium conductivity can be improved by enlarging the surface area contacting the electrolyte.30 Nanoparticle, nanotube, nanowire, micro/mesoporous electrode materials are being studied for this purpose.8,31,32 LiFePO4, one of the insulating materials, is a typical example which overcomes poor electron and lithium conductivity through carbon coating and fabrication of nanoparticles.33 However, there are several problems with nanoparticle electrode materials, such as handling difficult in the electrode preparation and smaller surface areas compared with other nanostructured materials. For this reason, 1-D nanostructured materials, like nanotubes or nanowires, and mesoporous electrode materials are considered promising candidates for lithium rechargeable batteries, especially for high-powered applications such as hybrid electric vehicles or powertools. Generally, spinel Li4Ti5O12 has been synthesized by high temperature solid-state reactions of TiO2 and lithium sources, but Li4Ti5O12 with a nanotubular structure could be obtained only by wet chemical processes such as lithium ion exchange reactions of hydrogen titanate nanotube precursor.34,35 However, our synthetic method is divided into two steps: the synthesis of 2-dimensional plate-like Li4Ti5O12 and the formation of Li4Ti5O12 nanotubes through simple alkali hydrothermal reactions via a rolling-up process. In this work, we report our synthesis of Li4Ti5O12 nanotubes with a spinel structure and examine the characteristics of hydrogen storage and electrochemical performance for application to Li-ion batteries. Experimental Procedures Titanium n-butoxide (97%, Aldrich), cetyltrimethyl ammonium chloride (CTACl, 25 wt % in water, Aldrich), lithium hydroxide (99%, Aldrich), and sodium hydroxide (99.5%, Aldrich) were used as received without further purification. Li4Ti5O12 nanotubes were obtained as white precipitates using titanium n-butoxide (TNBT), which is widely used as a titanium source to prepare uniform TiO2, cetyltrimethyl ammonium chloride 25 wt.% aqueous solution (CTACl), and lithium hydroxide. In a typical synthesis, 8.2 g (0.0241 mol) of TNBT and 8.2 g (0.0256 mol) of CTACl and 1.86 g (0.0776 mol) of
10.1021/jp905114c CCC: $40.75 2009 American Chemical Society Published on Web 09/28/2009
Spinel Li4Ti5O12 Nanotubes for Energy Storage
J. Phys. Chem. C, Vol. 113, No. 42, 2009 18421
Figure 1. High-resolution TEM images of Li4Ti5O12 nanotubes. (a) Uniform open-ended Li4Ti5O12 nanotubes with outer and inner diameters of 11 and 6 nm, respectively. (b) TEM images showing the interlayer spacing of the nanotubes (0.51 nm) and a fine fringe perpendicular to the nanotube orientation (0.28 nm).
lithium hydroxide were homogeneously mixed. Then, this mixture was placed in a reaction oven at 120 °C for 24 h. Filtered, washed, and dried white powder was put into a tubular furnace and heated at 500 °C for 6 h before hydrothermal reactions at 120 °C in 10 M NaOH aqueous solution. The resultant product was filtered, washed with deionized water and ethanol, and finally dried under vacuum at 40 °C. The X-ray powder diffraction (XRD) patterns were obtained on a BRUKER D8 ADVANCE diffractometer using Cu KR radiation. The transmission electron microscopy (TEM) images were obtained on a JEOL 2100F field-emission TEM operated at 200 kV. The samples were prepared by dispersing the particles in absolute ethyl alcohol, dipping a 400-mesh carbon coated copper grid into the suspension, and drying the grids immediately by evaporating the solvent. N2 adsorption/desorption isotherms were measured at liquid nitrogen temperature using a gas sorption analyzer (Micromeritics, ASAP 2020). The samples were degassed at 473 K and a vacuum below 10-3 torr for 12 h prior to the measurement. The specific surface area was calculated by the Brunauer-Emmet-Teller (BET) equation. Li4Ti5O12 nanoubes were finely ground to an average particle size of 5 µm and then mixed with 10 wt.% acetylene black, N-methyl pyrrolidone (NMP), and 10 wt.% polyvinylidenedifluoride (PVDF) binder. The mixture paste was coated on an aluminum sheet using a coater. The coated sheet was dried under air at 100 °C for 30 min and pressed. The electrode was vacuumdried at 120 °C for 24 h. Electrochemical cells were constructed in a nitrogen-filled glovebox using lithium metal as the negative electrode. Cells were constructed using a Rotec P01 separator and 1 M LiPF6 in a 1:1 wt.% mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) electrolyte. Capacity, rate capability, and cycling data were measured at room temperature using a charge-discharge analyzer (Wonatec, WBCS 5000). Results and Discussion On the basis of TEM and XRD results, we might tentatively propose a mechanism of spinel Li4Ti5O12 nanotube formation. The evidence for nanotubes formed by rolling 2-dimensional plate-like shapes was obtained on a field-emission transmission electron microscope (TEM). The simple structural rearrangement by alkali hydrothermal reaction is shown in Figure 1. The TEM image shows the uniform open-ended tubular structures with outer and inner diameters of ∼11 and ∼6 nm, respectively. The nanotube interlayer spacing was ∼0.51 nm and the fine fringe perpendicular to the nanotube orientation was 0.28 nm (see the Supporting Information).
Figure 2. Powder XRD patterns of spinel Li4Ti5O12 nanotubes.
Figure 3. N2 adsorption-desorption isotherms of Li4Ti5O12 nanotubes measured at liquid nitrogen temperature.
X-ray diffraction (XRD) patterns are shown in Figure 2. The XRD patterns revealed that the nanostructures were composed of spinel Li4Ti5O12 with a cubic structure. The unit cell parameters were a ) b ) c ) 8.358 Å, β ) 90°, in agreement with those of bulk Li4Ti5O12 (JCPDS Card No. 49-0207).34 Figure 3 shows the N2 adsorption-desorption isotherms of the Li4Ti5O12 nanotubes measured at liquid nitrogen temperature using a gas sorption analyzer. The samples were degassed at 473 K under vacuum below 10-3 Torr for 12 h prior to the measurement. The isotherm was type IV with slight hysteresis according to the IUPAC classification.35 A considerable increase of the adsorption of N2 occurs above P/P0 ) 0.8 due to a capillary condensation. The specific surface area of Li4Ti5O12 nanotubes was calculcated using the Brunauer-Emmett-Teller (BET) equation to be 53.7m2/g. 129 Xe nuclear magnetic resonance (NMR) spectroscopy is well-known as the most powerful tool available for the characterization of porous materials. Here, we extended the application of laser-polarized 129Xe NMR to the newly developed Li4Ti5O12 nanotube and investigated the physicochemical properties of the xenon adsorption sites in this material. Figure 4a shows the temperature dependence of 129Xe NMR spectra of laser-polarized Xe adsorbed on the Li4Ti5O12 nanotube. As can be seen clearly from the spectrum at 270 K, at least three peaks were observed at 20, 40, and 60 ppm from the adsorbed
18422
J. Phys. Chem. C, Vol. 113, No. 42, 2009
Lee et al.
Figure 5. Gravimetric hydrogen uptake at 296 K under a pressure range from 1 to 100 bar.
Figure 4. (a) Variable temperature 129Xe NMR spectra of laserpolarized Xe adsorbed on Li4Ti5O12 under continuous flow optical pumping conditions. Scans of 8 were accumulated with a relaxation delay of 1 s. (b) Two-dimensional exchange 129Xe NMR spectra with mixing times of 1 and 10 ms. Scans of 128 were accumulated with a relaxation delay of 1 s.
xenon. Since the chemical shift of 129Xe is induced by the interaction of xenon with the solid surface, it can act as a good probe of the pore size, where the chemical shift increases with decreasing pore size. As the temperature was lowered, all of the peaks moved increasingly downfield. This downfield shift of peaks with cooling is due to the longer residence time of xenon on the pore surface at lower temperatures. In addition, the relative intensities of the downfield peaks increased with cooling, which was attributed to the greater enthalpy of xenon adsorption in the smaller pores. These results suggest that xenon atoms occupy smaller pores at lower temperatures, but larger pores at high temperatures. Two-dimensional exchange 129Xe NMR was used to study the exchange dynamics between xenon in the different sites, and Figure 4b shows the 2D exchange spectra at 270 K with mixing times of 1 and 10 ms, respectively. Incidentally, the intensity of the peak at 60 ppm corresponding to the interlayer site diminished more rapidly than that of other peaks with increasing mixing time, presumably due to the close proximity of xenon to the interlayer surface and the resultant fast relaxation of the signal. The cross peaks in Figure 4 indicated a very fast exchange time scale between the intercrystalline and interlayer sites of 1 ms, while the exchange between the interior channel and the intercrystalline or interlayer site was highly restricted. At the longer mixing time of 10 ms, cross peaks between the resonances for the interior channel and
free gaseous xenon began to appear, while the cross peaks between the free xenon and the intercrystalline or interlayer site remained absent, indicating the severely restricted exchange between them. This observation is reasonable since the intercrystalline and interlayer sites were isolated from the free gaseous xenon due to aggregation or packing of the nanotubes, while the open end of the channel was relatively free from such isolation. In addition, the extremely fast exchange between the intercrystalline and interlayer sites can be explained by the close contact of the outmost interlayer with the intercrystalline porosity and the direct penetration of xenon from the intercrystalline to the outmost interlayer site, while the moderately slow exchange between the interior channel and free gaseous xenon might be attributed to the possible diffusion restrictions of xenon at the open nanotube tips.36 The high-pressure H2 uptake experiment was carried out under ambient temperature and with pressure ranging from 1 to 100 bar with a gravimetric apparatus. The amount of hydrogen adsorption on the Li4Ti5O12 nanotubes was measured after vacuum drying for 6 h at 70 °C and activation at 293 K, as shown in Figure 5. The rate of hydrogen adsorption increased rapidly at low pressure but decreased with increasing pressure. However, the equilibrium of hydrogen adsorption was not reached even at 100 bar. The Li4Ti5O12 nanotubes could store up to ∼0.7wt.% H2 at ambient temperature. The capacity of the Li4Ti5O12 nanotubes was better than that of general nanoporous materials with high surface area. The superior capacity indicated the greater per unit specific surface area. The Li4Ti5O12 nanotubes were also tested as an electrode material of use in lithium-ion batteries. The cell capacity at discharge and charge, shown in Figure 6, was 163 mAh/g and 156 mAh/g, respectively, indicating an irreversible capacity of less than 5%. The cell was discharged at constant current (0.2 C) and charged at various currents (0.1-2 C) to test the performance of the Li4Ti5O12 nanotubes as an anode material. About 93% of the charge capacity at 0.2 C was sustained at 2 C. The improved rate capability of lithium deintercalation was attributed to the enlarged surface area, which compensated for the slow lithium ion diffusivity in the solid. In spite of the large surface area of the Li4Ti5O12 nanotubes, the cell cyclability over 25 cycles clearly demonstrated the absence of any capacity fading in the Li4Ti5O12 nanotubes.
Spinel Li4Ti5O12 Nanotubes for Energy Storage
J. Phys. Chem. C, Vol. 113, No. 42, 2009 18423 References and Notes
Figure 6. (a) Charge and discharge rate capabilities of Li4Ti5O12 nanotubes at room temperature. (b) Cyclability of the Li4Ti5O12 cell with no capacity fading despite of the large surface area of the Li4Ti5O12 nanotubes.
Conclusions In summary, we focused our interest on the synthesis of transition metals for one-dimensional nanostructured materials and their energy storage application to hydrogen storage and lithium-ion batteries. Therefore, we synthesized spinel Li4Ti5O12 nanotubes via heat treatment and alkali-hydrothermal reactions through simple structural rearrangement, and we examined their H2 storage characteristics and suitability as electrode materials for application in lithium-ion batteries. The obtained Li4Ti5O12 nanotubes had a relatively large surface area of ∼53.7 m2/g, compared with their bulk counterparts. The Li4Ti5O12 nanotubes could store up to 0.7 wt.% H2 at ambient temperature and their reversible capacity is ∼156 mAh/g at a rate of 0.1C. Acknowledgment. This work was performed for the Hydrogen Energy R&D Center, a 21st Century Frontier R&D Program, funded by the Ministry of Education, Science, and Technology of Korea. H.J.K. is also grateful to the Korea Basic Science Institute (N28021). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.
(1) Arico, A. S.; Bruce, P.; Scrosati, B.; Trascon, J.-M.; Schalkwijk, W. V. Nat. Mater. 2005, 4, 366. (2) Nazar, L. F. Int. J. Inorg. Mater. 2001, 3, 191. Hirshes, M. Mater. Sci. Eng., B 2004, 108, 1. (3) Zu¨ttel, A.; Sudan, P.; Mauron, Ph.; Kiyobayashi, T.; Emmenegger, Ch.; Schlapbach, L. Int. J. Hydrogen Energy 2002, 27, 203. (4) Stro¨bel, R.; Jorissen, L.; Schliermann, T.; Trapp, V.; Schutz, W.; Bohmhammel, K.; Wolf, G.; Garche, J. J. Power Sources 1999, 84, 221. (5) Nijkamp, M. G.; Raaymakers, J. E. M. J.; van Dillen, A. J.; de Jong, K. P. Appl. Phys. A: Mater. Sci. Process. 2001, 72, 619. (6) Hirscher, M. Mater. Sci. Eng., B 2004, 108, 1. (7) Meng, S.; Kaxiras, E.; Zhang, Z. Nano Lett. 2007, 7, 663. (8) Lee, Y. J.; Kim, M. G.; Cho, J. P. Nano Lett. 2008, 8, 957. (9) Ding, Y.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 12280. (10) Lee, H. K.; Choi, W. I.; Ihm, J. S. Phys. ReV. Lett. 2006, 97, 056104. (11) Antonelli, D. M.; Seayad, A. M. AdV. Mater. 2004, 16, 765. (12) Zhao, N. H.; Wang, G. J.; Huang, Y.; Wang, B.; Yao, B. D.; Wu, Y. P. Chem. Mater. 2008, 20, 2612. (13) Tang, C.; Bando, Y.; Ding, X.; Qi, S.; Golberg, D. J. Am. Chem. Soc. 2002, 124, 14550. Ma, R.; Bando, Y.; Zhu, H.; Sato, T.; Xu, C.; Wu, D. J. Am. Chem. Soc. 2002, 124, 7672. (14) Wang, P.; Orimo, S.; Matsushima, T.; Fujii, H.; Major, G. Appl. Phys. Lett. 2002, 80, 318. (15) Chen, J.; Kuriyama, N.; Yuan, H.; Takeshita, H. T.; Sakai, T. J. Am. Chem. Soc. 2001, 123, 11813. Chen, J.; Li, S. L.; Tao, Z. L. J. Alloys Compd. 2003, 413, 356. (16) Chen, J.; Tao, Z. L.; Li, S. L. Angew. Chem., Int. Ed. 2003, 42, 2147. Chen, J.; Li, S. L.; Tao, Z. L.; Shen, Y. T.; Cui, C. X. J. Am. Chem. Soc. 2003, 125, 5284. (17) Yildirim, T.; Ciraci, S. Phys. ReV. Lett. 2005, 94, 175501. (18) Zhao, Y. F.; Kim, Y. H.; Dillon, A. C.; Heben, M. J.; Zhang, S. B. Phys. ReV. Lett. 2005, 94, 155504. (19) Shi, S. Z.; Hwang, J.-Y. Int. J. Hydrogen Energy 2007, 32, 224. (20) Feldman, Y.; Wasserman, E.; Srolovitz, D. J.; Tenne, R. Science 1995, 267, 222. (21) Zelinski, M.; Dorhout, P. K. J. Am. Chem. Soc. 1998, 120, 734. (22) Hsu, W. K.; Chang, B. H.; Zhu, Y. Q.; Han, W. Q.; Terrones, H.; Terrones, M.; Grobert, N.; Cheetham, A. K.; Kroto, H. W.; Walton, D. R. M. J. Am. Chem. Soc. 2000, 122, 10155. (23) Zhu, Y. Q.; Hsu, W. K.; Terrones, M.; Firth, S.; Grobert, N.; Clark, R. J. H.; Kroto, H. W.; Walton, D. R. M. Chem. Commun. 2001, 121. (24) Nath, M.; Rao, C. N. R. J. Am. Chem. Soc. 2001, 123, 4841. Nath, M.; Rao, C. N. R. Chem. Commun. 2001, 2236. (25) Whitby, R. L. D.; Hsu, W. K.; Boothroyd, C. B.; Fearon, P. K.; Kroto, H. W.; Walton, D. R. M. ChemPhysChem 2001, 2, 620. (26) Zhu, Y. Q.; Hsu, W. K.; Kroto, H. W.; Walton, D. R. M. Chem. Commun. 2001, 2184. (27) Chen, J.; Kuriyama, N.; Yuan, H.; Takeshita, H. T.; Sakai, T. J. Am. Chem. Soc. 2001, 123, 11813. (28) Guerfi, A.; Se´vigny, S.; Lagece´, M.; Hovington, P.; Kinoshita, K.; Zaghib, K. J. Power Sources 2003, 119-121, 88. (29) Sorensen, E. M.; Barry, S. J.; Jung, H.-K.; Rondinelli, J. R.; Vaughey, J. T.; Poeppelmeier, K. R. Chem. Mater. 2006, 18, 482. (30) Wang, G. J.; Gao, J.; Fu, L. J.; Zhao, L. J.; Wu, Y. P.; Takamura, T. J. Power Sources 2007, 174, 1109. (31) Wang, D.-W.; Fang, H.-T.; Li, F.; Chen, Z.-G.; Zhong, Q. -S.; Lu, G. Q.; Cheng, H.-M. AdV. Func. Mater. 2008, 18, 1. (32) Ohzuku, T.; Ueda, A.; Yamamoto, N. J. Electrochem. Soc. 1995, 142, 1431. (33) Zhao, B.; Jiang, Y.; Zhang, H. J.; Tao, H. H.; Zhong, M. G.; Jiao, Z. J. Power Sources 2009, 189, 462. (34) Li, J.-R.; Tang, Z.-L.; Zhang, Z.-T. Electrochem. Commum. 2005, 7, 894. (35) Sing, K. S. W.; Everrett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (36) Romanenko, K. V.; Fonseca, A.; Dumonteil, S.; Nagy, J. B.; d’Espinose de Lacaillerie, J. B.; Lapina, O. B.; Fraissard, J. Solid State NMR 2005, 28, 135.
JP905114C
