Computer Simulation of the Phase Stabilities of Lithiated TiO2 Polymorphs

The structure and phase stability of a series of titillated titania polymorphs were determined using energy minimizations of the periodic bulk crystal structures and both density functional theory (DFT) and it potential shell model. The DFT calculations were performed spin unrestricted following the linear combination of atomic orbital approach with the B3LYP exchange-correlation potential. For the potential shell model, a new set of force field parameters was derived, independently of the DFT calculations, to describe the lithium-lattice interactions. The eight polymorphs considered in this study are the rutile, anatase, brookite, TiO2-B, ramsdellite, hollandite, spinel, and hexagonal structures. The lithium to titanium ratio, x, of each Initiated titania polymorph was varied from 0.0 to 1.0 with 0.25 increments. The potential model predictions were found to he in good agreement with the structure and energetics of the lithiated titania polymorphs determined from DFT calculations, at all lithium contents. Both computational approaches indicate the following relationships. The naturally occurring titania polymorphs (i.e., rutile, anatase, brookite, and TiO2-B) were found to be the most stable of die eight phases in the absence of lithium. Anatase, brookite, and ramsdellite become energetically favored over ruble upon lithium insertion. The hexagonal and spinel polymorphs have stabilities approaching that of rutile with increasing lithium content and showed essentially equivalent total energies for x = 1.0. This prediction is consistent with numerous experimental studies, which have reported that ruffle electrodes, in particular those that consist of nanomaterials, can undergo phase transformations to the hexagonal or spinel structure upon lithium insertion. The calculations indicate that the main factors controlling the relative stability of the lithiated titania polymorphs are the lithium bonding environment, the arrangement of LiOx and TiO6 polyhedra, and the extent of lattice deformation upon lithiation.