NMR and X-ray Absorption Study of Lithium Intercalation in Micro- and Nanocrystalline Anatase

The intercalation of Li into micro- and nanocrystalline anatase xerogel powders using n-butyllithium and various preparation conditions has been studied by 6Li and 7Li solid-state NMR and X-ray absorption spectroscopy. The orthorhombic lithium titanate phase which forms for Li/Ti < 0.56 undergoes further structural modification as the Li/Ti ratio increases to 0.70. NMR shows the existence of one type of lithium environment in microcrystalline samples when Li/Ti ≤ 0.30 and two lithium environments when Li/Ti > 0.30. These two lithium environments are assigned to Li+ ions in similar structural sites that are uncoupled (species A) and weakly coupled (species B) to conduction band electrons. Nanocrystalline anatase samples produced by sol−gel methods differ from microcrystalline anatase in that the proportion of species A is greater. NMR further suggests that the coordination number of species A cannot be greater than five while no definite conclusions could be reached from NMR concerning species B. Titanium K-edge X-ray absorption near edge structure (XANES) provides a method of monitoring changes in electronic structure as Li is intercalated. Extended X-ray absorption fine structure (EXAFS) analysis provides information on the Ti local environment which changes considerably with the degree of intercalation in microcrystalline samples. For nanocrystalline anatase samples greater amounts of intercalation can be achieved which dramatically affects both the XANES and the EXAFS.