Two Phase Morphology Limits Lithium Diffusion in TiO2 (Anatase): A 7Li MAS NMR Study

Abstract

⁷Li magic angle spinning solid-state nuclear magnetic resonance is applied to investigate the lithium local environment and lithium ion mobility in tetragonal anatase TiO₂ and orthorhombic lithium titanate Li_0.6TiO₂. Upon lithium insertion, an increasing fraction of the material changes its crystallographic structure from anatase TiO₂ to lithium titanate Li_0.6TiO₂. Phase separation occurs, and as a result, the Li-rich lithium titanate phase is coexisting with the Li-poor TiO₂ phase containing only small Li amounts ≈ 0.01. In both the anatase and the lithium titanate lattice, Li is found to be hopping over the available sites with activation energies of 0.2 and 0.09 eV, respectively. This leads to rapid microscopic diffusion rates at room temperature (D_micr = 4.7 × 10^-12 cm² s^-1 in anatase and D_micr = 1.3 × 10^-11 cm² s^-1 in lithium titanate). However, macroscopic intercalation data show activation energies of ∼ 0.5 eV and smaller diffusion coefficients. We suggest that the diffusion through the phase boundary is determining the activation energy of the overall diffusion and the overall diffusion rate itself. The chemical shift of lithium in anatase is independent of temperature up to ∼250 K but decreases at higher temperatures, reflecting a change in the 3d conduction electron densities. The Li mobility becomes prominent from this same temperature showing that such electronic effects possibly facilitate the mobility.