Article pubs.acs.org/cm
Lithium Insertion into Titanium Dioxide (Anatase): A Raman Study with 16/18O and 6/7Li Isotope Labeling Barbora Laskova,†,‡ Otakar Frank,† Marketa Zukalova,† Milan Bousa,†,‡ Martin Dracinsky,§ and Ladislav Kavan*,†,‡ †
J. Heyrovský Institute of Physical Chemistry, v.v.i. Academy of Sciences of the Czech Republic, Dolejskova 3, 18223 Prague 8, Czech Republic ‡ Department of Inorganic Chemistry, Faculty of Science, Charles University, Hlavova 2030/8, 12843 Prague 2, Czech Republic § Institute of Organic Chemistry and Biochemistry, v.v.i. Academy of Sciences of the Czech Republic, Flemingovo nam. 2, 16610 Prague 6, Czech Republic S Supporting Information *
ABSTRACT: Electrochemical and chemical (with n-butyllithium) insertion of Li into TiO2 (anatase) is studied by Raman spectroscopy and by in situ Raman spectroelectrochemistry. Four isotopologue combinations in the system, namely 6/7LixTi16/18O2 (with x being the insertion coefficient), are prepared and studied. Spectral assignment is supported by numerical simulations using DFT calculations. The combination of experimental and theoretical Raman frequencies with the corresponding isotopic shifts brings new inputs for several still open questions about the Li-insertion into TiO2 (anatase). KEYWORDS: titanium dioxide, lithium insertion, Raman spectroscopy, spectroelectrochemistry
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studies,3−6,14−18,20 and there was also hexagonal LiTiO2 found in yet another study of rutile.21 With a pure nano-anatase host, the tetragonal structure seems to be the only Li-rich (up to x ≈ 1) phase in the system.6 It can be obtained both by chemical lithiation using n-butyllithium6 and electrochemically.20 Its detection is not always straightforward. For instance, the mesoporous anatase (∼6 nm crystallite size) exhibited solely the orthorhombic Li-titanate up to x = 0.96.16 Analogously, the Li1.02TiO2 made from anatase nanotubes showed only the weak Raman lines of orthorhombic Li-titanate; hence, the evolution of tetragonal LiTiO2 is hard to detect in this way.17 Nevertheless, we have quite surprisingly found that almost pure cubic LiTiO2 was formed via the n-butyllithium reaction with special titania nanomaterials composed of anatase, TiO2(B) and a large proportion (91%) of an “amorphous” component under conditions, where pure orthorhombic LixTiO2 grew from an “ordinary” nanoanatase.22 Structural data about the Li/TiO2 system were previously acquired using X-ray3,23 and neutron diffraction6,14,24 (the latter is preferred for its greater sensitivity to Li), 7Li NMR3−5,24 and EXAFS (XANES).19,20 Raman spectroscopy is another popular technique of choice for structural studies.15−18,25,26 Because of the sensitive probing of short-range structures, Raman spectroscopy is preferable over diffraction methods for small
INTRODUCTION Lithium insertion into TiO2 (anatase) attracted considerable attention in the past. It is interesting both for addressing the fundamental questions1−8 and for its prospective application in Li-ion batteries.7−12 The reaction is described by the formal equation: TiO2 + x(Li+ + e−) → LixTiO2
(1)
At a certain level of lithiation (x ≈ 0.5), the tetragonal anatase TiO2 (space group I41/amd, No. 141, lattice constants: a = 3.792 Å, c = 9.497 Å) is converted reversibly to orthorhombic titanate, Li0.5TiO2 (space group Imma, No. 74, lattice constants: a = 3.819 Å, b = 4.084 Å c = 9.066 Å). This conversion was first reported by Ohzuku et al.13 and Cava et al.14 and confirmed by others.3−6,15−18 More precisely, the initial stages of Li-insertion were characterized by a coexistence of two phases: a Li-poor tetragonal phase (x ≈ 0.01−0.03) with an anatase structure, and orthorhombic Li-titanate.3−5 Ren et al.16 reported that the growth of orthorhombic titanate started already at x = 0.05, but it was difficult to detect the onset of the nucleation of the orthorhombic phase by X-ray diffraction due to the small size of the nuclei. Wagemaker et al.6,19 found another Li-titanate at a deeper lithiation, LiTiO2, but this phase was accessible solely with nanocrystalline (
