Dynamics of Coupled Lithium/Electron Diffusion in TiO2 Polymorphs

Molecular dynamics simulations were performed with a potential shell model to investigate the diffusion of lithium ions and electron polarons in rutile and anatase. Simulations of an isolated lithium ion in rutile predict fast diffusion in the c channels with an activation energy of 0.05 eV, which corresponds to a jump rate of 4 x 10(11) s(-1) and a diffusion coefficient of 9 x 10(-5) cm(2).s(-1) at room temperature. In anatase, the activation energies for intra- and interoctahedron lithium hopping are 0.02 and 0.39 eV, respectively, and the lithium diffusion coefficient is 4-5 orders of magnitude slower than in rutile. When in the presence of an electron polaron, lithium hopping is predicted to be affected up to four hops away. The effects are more pronounced in rutile; whereby the first energy minimum along the c direction is absent due to the strong lithium-electron electrostatic interactions along the open c channels. Combining the lithium and electron polaron hopping rates, a Coupled diffusion mechanism emerges whereby the electron polarons hop rapidly back and forth around the lithium ions. This process can lead to the occurrence of an instantaneous driving force for lithium hopping. The lithium ion-electron polaron binding energies are found to be large, with a stronger binding in rutile than in as 0.45 and 0.28 eV, respectively, Suggesting that, at low lithium mole fractions, lithium ions and electron polarons will form strongly correlated pairs.