Two phase morphology limits lithium diffusion in TiO2 (anatase): A Li-7 MAS NMR study

Li-7 magic angle spinning solid-state nuclear magnetic resonance is applied to investigate the lithium local environment and lithium ion mobility in t etragonal anatase TiO2 and orthorhombic lithium titanate Li0.6TiO2 Upon lit hium insertion, an increasing fraction of the material changes its crystall ographic structure from anatase TiO2 to lithium titanate Li0.6TiO2. Phase s eparation occurs, and as a result, the Li-rich lithium titanate phase is co existing with the Li-poor TiO2 phase containing only small Li amounts appro ximate to 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 rat es at room temperature (D-micr = 4.7 x 10(-12) cm(2) s(-1) in anatase and D -micr = 1.3 x 10(-11) cm(2) s(-1) in lithium titanate). However, macroscopi c intercalation data show activation energies of similar to 0.5 eV and smal ler 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 anatas e is independent of temperature up to similar to 250 K but decreases at hig her temperatures, reflecting a change in the 3d conduction electron densiti es. The Li mobility becomes prominent from this same temperature showing th at such electronic effects possibly facilitate the mobility.