Density-functional simulations of lithium intercalation in rutile

Density-functional simulations of lithium intercalation into rutile structured titanium dioxide are presented. Full relaxation of structures for a wide range of insertion concentrations is used to identify the thermodynamically most stable configurations and Li-ion site preferences. The host lattice is found to undergo large deformations upon Li insertion, which can be related to the excitation of soft vibrational modes. The dominant screening interaction is found to be due to these elastic distortions of the lattice rather than to dielectric screening. This leads to highly anisotropic and concentration-dependent effective Li-Li interactions, which are not easily amenable to empirical parametrization. The anisotropic volume expansion is found to be largely due to the increase in the radii of reduced Ti ions as they accommodate charge donated to the lattice. The computed open circuit voltage (OCV) reproduces the characteristic features of experimental discharge curves at elevated temperature. The computed Li-ion energy surfaces reveal highly anisotropic diffusion. A model of Li intercalation is proposed, which takes account of both the thermodynamic and kinetic properties computed here. This model is used to resolve apparent contradictions in the current interpretation of the measured OCV and its dependence on temperature. Predicted changes in the electronic structure and their relationship to the interaction between structural, charge, and spin degrees of freedom are discussed in detail.