The electronic structure and ionic diffusion of nanoscale LiTiO2 anatase

Abstract

Upon lithium insertion in the pristine TiO₂ anatase phase the theoretical maximum of LiTiO₂ can be reached in crystallite sizes less than ∼10 nm, whereas bulk compositions appear limited to Li_x≈0.6TiO₂ at room temperature. Both X-ray absorption spectroscopy (XAS) and ab initio calculations have been applied to probe the electronic structure of the newly formed LiTiO₂ phase. These results indicate that a large majority of the Li-2s electrons reside at the Ti-3d(t_2g)/4s hybridized site. About 10% of these electrons are transferred to non-localized states which makes this compound a good electronic conductor. Ionic conductivity is probed by nuclear magnetic resonance (NMR) relaxation experiments indicating relatively small hopping rates between the Li-ion sites in LiTiO₂. Formation of the poor ionic-conducting LiTiO₂ at the surface of the particles explains why micro-anatase Li_xTiO₂ is not able to reach the theoretical maximum capacity at room temperature, and why this theoretical maximum capacity reached in nano-sized materials cannot be (dis)charged at high rates.