Article pubs.acs.org/cm
Lithium Insertion into Anatase Nanotubes V. Gentili,† S. Brutti,*,‡ L.J. Hardwick,§ A.R. Armstrong,† S. Panero,⊥ and P.G. Bruce† †
EaStCHEM, School of Chemistry, University of St Andrews, St. Andrews, Fife KY16 9ST, United Kingdom Dipartimento di Scienze, Università della Basilicata, Potenza, I-85100, IT § Stephenson Institute for Renewable Energy, Department of Chemistry, The University of Liverpool, Crown Street, L69 7ZD Liverpool, United Kingdom ⊥ Dipartimento di Chimica, Sapienza Università di Roma, Roma I-00185, Italy ‡
S Supporting Information *
ABSTRACT: Anatase nanotubes were synthesized by a hydrothermal route and characterized by FE-SEM, TEM, XRD, and N2 adsorption. The optimized synthesis route permits careful control of the crystal structure and morphology of the final product, thus giving the highest phase and morphological purities (>90%) of any anatase nanotubes reported to date. The anatase nanotubes were tested in lithium cells at various current rates and their performances compared with bulk and nanoparticulate anatase. The Li uptake of the nanotubes in lithium cells reaches 0.98 per formula unit. Moreover the nanotubes show better reversibility and cyclability compared to both bulk and nanoparticulate anatase. The excellent rate performance is comparable with the best literature values reported for mesostructured anatase nanopowders. By combining experimental data from neutron diffraction and Raman microscopy the lithium insertion mechanism into the anatase nanotubes was investigated. Ex situ neutron diffraction experiments were carried out on pristine, partially lithiated, and fully lithiated anatase nanotubes. In parallel, the structural changes associated with electrochemical lithium insertion were investigated by in situ Raman microscopy. This analysis suggests a Li-poor tetragonal/orthorhombic/Li-rich tetragonal double phase transition mechanism analogous to that previously observed by Wagemaker et al. for anatase nanoparticles smaller than 7 nm. KEYWORDS: titanium oxide, nanotubes, lithium insertion mechanism, nanostructured anodes for Li-ion cells
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a theoretical capacity of 335 mA h g−1. Bulk TiO2(B) can store 240 mA h g−1; in nanowire form, this increases to 305 mA h g−1, and for nanotubes, the capacity increases further to 330 mAh g−1.8−10 The same trend is also observed for anatase nanostructures.4,7,11 In this case, although the anatase lattice can accommodate only 0.5 Li per formula unit (167.5 mA h g−1) as a bulk material, the reduction of the particle size to the nanoscale allows lithium insertion beyond this limit. Intensive research on lithium insertion into nano TiO2 has been reported by many authors (as an example see refs 4, 7, 11, 12). In the case of 1D anatase nanomorphologies, although some authors reported encouraging performance of anatase nanotubes,13−22 there is a lack of information about which phase transitions occur, how the lattice changes on Li insertion, and where the lithium ions are located upon insertion. In this work, we report the synthesis of anatase nanotubes by a modified hydrothermal route. The synthesis route has been optimized in order to allow a careful control of the crystal structure and morphology of the final product thus giving the highest phase and morphological purities (>90%) ever reported
INTRODUCTION Nanostructured materials are being intensively studied as electrodes in rechargeable Li-ion batteries.1−3 Among such nanomaterials, TiO2-based nanoparticles and 1D morphologies (i.e., nanorods, nanowires, nanotubes) are attracting attention as possible negative electrode materials.4−7 These nanostructured titanates can deliver high specific capacities (≈200−300 mA h g−1), fast kinetics and superior safety compared to graphite due to the higher operating voltage (>1 V vs Li+/Li0).8−10 As a consequence, they are possible alternatives to the carbon-based negative electrodes in Li-ion cells. The formation of a solid-electrolyte interphase layer (SEI) due to electrolyte decomposition below 1 V vs Li+/Li0, although critical to the operation of carbonaceous anodes, also compromises high rate performance and leads to gas production and safety problems on cell overheating. Of course, a disadvantage of the higher potential of the anodes (∼1.5 V vs Li+/Li) is that it lowers the overall cell voltage, when coupled with the same cathode. Among the titanates, in recent years the spinel, Li4Ti5O12, has received most attention but can only store ∼175 mA h g−1 (compared to the theoretical capacity of graphite of 372 mA h g−1). The TiO2(B) polymorph of titanium dioxide can accommodate one lithium per formula unit, corresponding to © 2012 American Chemical Society
Received: September 11, 2012 Revised: October 29, 2012 Published: November 1, 2012 4468
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Table 1. TiO2-Based Nanotube Precursor List: Study of the Effect of the Ion-Exchange Conditions on the Na/Ti Ratio and XRD Phase Identification after Calcination at 380 C for 2 h in Air sample
ion exchange procedure
Na/Ti ratio (EDS)
estimated stoichiometry
phase identification after calcination
A B C Da E F
333 mL of H2O per g of TiO2 333 mL of H2O per g of TiO2 + 33.3 mL per g of TiO2 HCl(aq) 0.1 M 333 mL of H2O per g of TiO2 + 66.7 mL per g of TiO2 HCl(aq) 0.1 M 333 mL of H2O per g of TiO2 + 83.3 mL per g of TiO2 HCl(aq) 0.1 M 333 mL of H2O per g of TiO2 + 91.7 mL per g of TiO2 HCl(aq) 0.1 M 333 mL of H2O per g of TiO2 + 108 mL per g of TiO2 HCl(aq) 0.1 M
0.43 0.24 0.10 0.03−0.04 0.7. Going beyond the orthorhombic lithium titanate the formation of a lithium-rich tetragonal lithium titanate (LiTiO2) is also observed in the case of the anatase nanotubes. These results confirm the ability of nanostructured anatase to host larger amounts of lithium per formula unit compared to bulk materials. Apparently the anatase nanotubes do not follow the so-called interfacial lithium storage mechanism but exhibit the Li-poor tetragonal/ orthorhombic/Li-rich tetragonal double phase transition suggested by Wagemaker and co-workers for nanoparticles smaller than 7 nm.
As expected, an enhancement of the capacity was observed with a decrease in the particle size, together with a reduction in the fade rate. An analogous comparison can be made for the volumetric performance which is presented in Figure 8B. In this case, the anatase nanotubes outperform all the other anatase morphologies under investigation. The nanotubes exhibit a capacity of ∼300 mA h cm−3, greater than that of the 11 nm nanoparticles, which may be attributed to the higher density of the nanotube electrodes. Moreover the anatase nanotubes exhibit superior capacity retention on cycling, exceeding 70% at the 100th cycle with respect to the first discharge, compared with only 51% for the 11 nm nanoparticles. Between the 10th and 100th cycles, capacity retention was greater than 88% for the nanotubes and 75% for the 11 nm nanoparticles. It is to be noted that post mortem TEM image investigation of the nanotube electrodes showed retention of the 1D morphology of the nanomaterial upon repeated lithium insertion/extraction, as shown in Figure S6 in the Supporting Information. The performance of the anatase nanotubes at different cycling rates was also investigated. Figure 9 reports extended
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
S Supporting Information *
Additional figures and tables (PDF). This material is available free of charge via the Internet at http://pubs.acs.org/. Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS P.G.B. is indebted to the EPSRC for financial support and to the ALISTORE-ERI consortium for financial support of S.B. S.P. and S.B. from University of Rome Sapienza acknowledge the financial support from “Ministero dello Sviluppo Economico − Accordo di Programma MSE-CNR per la ricerca di Sistema elettrico Nazionale”. S.B. and S.P. thank Prof. Bruno Scrosati for the constant support and helpful discussions. Thanks are due to Jesus Canales for the HRTEM micrographies.
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Figure 9. Variation in charge (open circles) and discharge (closed circles) capacities vs cycle number for anatase nanotubes cycled at different rates between voltage limits of 1 and 2.5 V.
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CONCLUSIONS Anatase nanotubes were synthesized by a hydrothermal route and characterized using a range of techniques including XRD, FE-SEM, TEM and N2-adsorption. The optimized synthesis route allows a careful control of the crystal structure and morphology of the final product thus giving the highest phase and morphological purities (>90%) ever reported. These revealed tubes with a phase and morphological purity of >90% with a wall thickness between 2 and 3 nm, an external diameter of 8−10 nm and lengths of 30−100 nm. Electrochemical testing revealed a Li+ uptake of 0.98 per formula unit, greater than any other tested anatase morphology. In addition, the nanotubes showed better reversibility and cyclability compared to both bulk and nanoparticulate anatase along with superior rate capability. A combination of in situ Raman micoscopy and ex situ powder neutron diffraction measurements were used to investigate the lithium insertion mechanism. These revealed behavior that parallels the results 4475
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