Thermodynamics of spinel LixTiO2 from first principles

The thermodynamic and structural properties of LixTiO2 spinel are investigated by means of a cluster expansion based on pseudopotential ground state energy calculations in the Generalized Gradient approximation (GGA). The cluster expansion enables a Monte Carlo simulation of configurational thermodynamics, giving the Li configurations, chemical potential and the insertion potential as function of Li composition at 300 K. For 1/2 < x < 1LixTiO2 we find a two-phase region, consistent with what is found experimentally. The two coexisting phases differ in the sites occupied by Li: in Li1/2TiO2 and LiTiO2 Li occupies the crystallographic 8a and 16c sites, respectively. This site occupation and the changes in the unit cell dimensions compare well with X-ray and neutron diffraction experiments. For x < 1/2 in LixTiO2 solid solution behavior is found and Li extraction can only occur at higher potentials. The potential step at Li1/2TiO2 is calculated to be 1.4 V, in good agreement with experiment, but considerably higher than in the comparable Co and Mn-spinel.