796
Chem. Mater. 2001, 13, 796-801
Lithium Intercalation in AnatasesStructural and Magnetic Considerations Vittorio Luca,*,† Brett Hunter,‡ Boujemaa Moubaraki,§ and Keith S. Murray§ School of Chemistry, University of New South Wales, Sydney, NSW 2053, Australia, and Physics Division, Australian Nuclear Science and Technology Organization, PMB 1, Menai, NSW 2234, Australia, and Department of Chemistry, Monash University, Clayton, Victoria 3168, Australia Received June 20, 2000. Revised Manuscript Received December 5, 2000
The intercalation of Li in anatase to give orthorhombic LixTiO2 compounds with x in the range 0-0.7 has been studied. Neutron diffraction patterns have been measured and refined as a function of x to shed light on the mechanism of the intercalation reaction. It appears that transformation of tetragonal anatase to the orthorhombic structure is preceded by an initial expansion of the anatase unit cell. As x increases, most of the added electrons fill the anatase conduction band and susceptibility measurements indicate that an insulator-tometal transition occurs for x > 0.3. Electron paramagnetic resonance and susceptibility measurements indicate that only very few of the injected electrons are localized on Ti to give Ti3+.
Introduction The reversible topotactic redox intercalation of lithium in oxide hosts is an important process in rechargeable lithium ion (rocking-chair) batteries and electrochromic devices1 and a possible soft chemical (chimie douce) route to novel oxide materials. The advantages of this light oxide in battery applications are that (1) the energy density is potentially very high, (2) the Ti cations appear to be difficult to reduce to the metal, suggesting good long-term stability,2 and (3) titania is an environmentally benign oxide. The intercalation of Li in anatase using n-butyllithium occurs stoichiometrically up to a composition of Li0.5TiO2. Cava et al.3 initially solved the structure of this phase through refinement of the neutron powder pattern in the orthorhombic space group Imma. The Li was located in five-coordinate interstitial sites (2× LiO1 1.9658 Å; 2× Li-O2 2.0389 Å; Li-O1 2.1726 Å). In contrast, a recent theoretical study located the Li in four-coordinate sites.4 Recently, we addressed the chemical intercalation of lithium ions into anatase using n-butyllithium through XRD, 6Li and 7Li solid-state NMR, and X-ray absorption spectroscopy.5 The solid-state MAS NMR spectra of the * To whom correspondence should be addressed. Present address: Materials Division, Australian Nuclear Science and Technology Organization, PMB 1, Menai, NSW 2234, Australia. Tel.: 61-2-9717 3087. Fax: 61-2-9543 7179. E-mail:
[email protected]. † University of New South Wales. ‡ Physics Division, Australian Nuclear Science and Technology Organization. § Monash University. (1) Owen, J. R. Chem. Soc. Rev. 1997, 26, 256. (2) Ceder, G.; Aydinol, M. K. Solid State Ionics 1998, 109, 151. (3) Cava, R. J.; Murphy, D. W.; Zahurak, S.; Santoro, A.; Roth, R. S. J. Solid State Chem. 1984, 53, 64. (4) Stashans, A.; Lunell, S.; Bergstro¨m, R. Phys. Rev. B 1996, 53, 159. (5) Luca, V.; Hanley, T. L.; Roberts, N. K.; Howe, R. F. Chem. Mater. 1999, 11, 2089.
Li-intercalated samples are reproduced in Figure 1. In LixTiO2 samples with x E 0.3 only one Li species with δ ) 0 ppm was observed by NMR while for x > 0.3 an additional, more abundant species with δ ∼ 4 ppm was observed. It was hypothesized that this second species is due to a weak Knight shift arising from the interaction of some Li ion sites with electrons in the anatase conduction band. Electronic correlation within the t2g and eg conduction band states, which become increasingly filled as x increases, was evinced through the X-ray absorption pre-edge structure (XANES), which reflects the unoccupied density of states. The observation of two Li species by NMR spectroscopy suggests that previous refinements of neutron powder data, which located all of the lithium ions in five-coordinate interstitial sites, may be in doubt. To explain this discrepancy, it was hypothesized that Li ions occupying distinct crystallographic sites observable by NMR could not be distinguished on the basis of refinement of the neutron powder pattern due to disorder. In any event, it is considered that a revisitation of the neutron diffraction pattern of the stoichiometric phase is warranted. In this study we examine in greater detail the effects of increasing Li intercalation on the crystallographic structure of anatase through Rietveld refinement of neutron powder data. At the same time changes in the magnetic and electronic structure are monitored through magnetic susceptibility measurements and electron paramagnetic resonance (EPR) spectroscopy. Experimental Section The microcrystalline anatase used in this work was from Aldrich Chemical Co. and XRD showed it to be pure anatase. Previous studies6 have shown by SEM that the particle size (6) Cabrera, M. I.; Alfano, O. M.; Cassano, A. E. J. Phys. Chem. 1996, 100, 20043.
10.1021/cm000504w CCC: $20.00 © 2001 American Chemical Society Published on Web 03/02/2001
Lithium Intercalation in Anatase
Chem. Mater., Vol. 13, No. 3, 2001 797
Figure 1. Room-temperature solid-state 6Li MAS NMR spectra of Li-intercalated anatase samples reproduced from Luca et al.3 range of this commercial anatase material ranges from 1000 to 2000 Å and light scattering has shown that when dispersed in water aggregates with an average diameter of about 3000 Å are present. Our own TEM, XRD, and light scattering measurements confirm these size ranges. Prior to treatment with n-butyllithium, the bulk microcrystalline anatase powders were conditioned by calcination in air at 400 °C for 24 h. Three 6.0-g samples of the conditioned microcrystalline anatase were transferred to Quickfit Pyrex vials where they were outgassed for about 1 h at 100 °C. To three separate samples of anatase were added 51.6, 25.8, and 12.9 mL, respectively, of the 1.44 M n-butyllithium/hexane solution. This was transferred to the sample vials under oxygen-free, dry argon gas on a Schlenk line and allowed to react for a prescribed interval of time at 60 °C. The amount of free hydroxide in the n-butyllithium solution was typically 0.3.5 To conclude, although refinement of the neutron powder pattern in the Imma space group with a single type of Li environment is preferred to Imm2 on goodness-of-fit grounds, the presence of two distinct Li environments in the present titanate compounds cannot be disputed. The inability for these two environments to be expressed in the neutron refinements is a result of static disorder, which blurs this distinction. CM000504W
