Role of Lithium Ordering in the LixTiO2 Anatase → Titanate Phase Transition

The mechanism of the tetragonal <-> orthorhombic phase separation of Li-intercalated anatase TiO2 has previously been proposed to be a cooperative Jahn-Teller distortion due to occupation of low-lying Ti 3d(xz,yz) orbitals. Using density functional calculations, we show that the orthorhombic distortion of Li0.5TiO2 is not a purely electronic phenomenon and that intercalated Li plays a critical role. For a 2 x 1 x 1 expanded supercell for 0 <= x(Li) <= 1, the intercalation voltage is minimized for x(Li) = 0.5. The low-energy structures display a common structural motif of edge-sharing pairs of LiO6 octahedra, which allows all Li to adopt favorable oxygen coordination. Long-ranged disorder of these subunits explains the apparent random Li distribution seen in experimental diffraction data.