Marina V. Koudriachova

Diffusion of Li-ions in rutile. An ab initio study

Ab initio calculations of lithium diffusion into titanium dioxide in the form of rutile are presented. An analysis of the site preference for intercalation and diffusion pathways of Li-ions is performed. The expansion of the host structure on Li-insertion is found to contribute to the enhanced diffusion of Li-ions along the tetragonal c direction. At the same time, a large distortion of the rutile framework makes Li diffusion in the ab planes prohibitively slow. Anisotropy of Li diffusion is used to explain the observed electrochemical behaviour of rutile. Computed diffusion coefficients were found in excellent agreement with the measured values.

Open circuit voltage profile for Li-intercalation in rutile and anatase from first principles

Ab initio calculations of lithium intercalation into rutile- and anatase structured titanium dioxide are presented. An analysis of self-ordering at different depths of discharge is performed. The open circuit voltage profile is calculated. It reproduces and explains the characteristic features of experimental discharge curves.

A new phase of lithiated titania predicted from first principles

A new phase of Li-intercalated rutile-structured titania, LixTiO2 for x=0.75, is predicted on the basis of first principles calculations. The existence of this phase has been noted in X-ray diffraction measurements but its structure has not previously been determined. The new phase has important consequences for the performance of LixTiO2 as a battery cathode material. On the basis of the computed energetics of phases at x=0.75, 0.5 and 0.25 phases, and a careful comparison of the computed and measured diffraction data, a two phase model of intercalation for 0.25<x<0.75 is proposed. It is suggested that nucleation of the new x=0.75 phases results from kinetic restrictions under operating conditions. The structural features of the new phase and their manifestation in X-ray diffraction are discussed. The transformation of the new phase to an hexagonal phase at x=0.8 is analysed and its role in cathode degradation discussed.

Li sites and phase stability in TiO2-anatase and Zr-doped TiO2-anatase

The phase stability of Zr-doped Li-anatase has been studied for a wide range of Li concentrations using density functional theory calculations of representative ordered structures. Thermodynamically stable structures have been established through the full relaxation of the internal coordinates and the size and shape of the periodic unit cell. The stability of the Li-poor tetragonal phase is found to correspond to the Li occupancy in the centre of the octahedral sites. Occupancy of the off-centre site results in the formation of an orthorhombic phase, which is similar to that observed in Li-anatase. It is demonstrated that doping of anatase with Zr-ions does not affect the two-phase nature of Li insertion. Hypothetical high doping with Zr ions results in a decrease of the Li intercalation potential for anatase but within the solubility limit the effect is negligible. It is confirmed that Ti4+ ions play a major role in the accommodation of the electron density donated by Li and that the intercalation potential in the two-phase region is correlated to the participation of Ti ions in accommodation of the electron density.

Structural deformations in lithium doped titanium dioxide

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. 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.

Lattice–gas model for intercalation compounds

Abstract A lattice–gas model of an intercalation compound is discussed in which the latter is described as an effective one-component medium of equally charged intercalated ions forming a lattice of interstitial sites with short-range nearest-neighbor repulsive interactions. For sufficiently strong interactions a second order phase transition has been found accompanied by the formation of a fine structure of intercalated ions at certain critical concentrations. As a result an open circuit voltage (OCV) shows a step-like behavior with nonanalytic features at the critical concentrations. The phase diagram and OCV are studied theoretically, within the framework of the mean field theory and the Bethe approximation, and by means of Monte Carlo simulations. Theoretical predictions are in perfect agreement with simulations for the OCV and in a fair agreement with simulations for the phase diagram.

Theory of anomalous voltage-discharge behavior for topotactic intercalation

Abstract Anomalous voltage–discharge behavior observed in topotactic intercalation materials is explained in terms of a lattice–gas model for interstitial sites with short-range nearest-neighbor repulsive interactions. For sufficiently strong interactions a second order phase transition results in the formation of fine structure of intercalated ions at certain critical concentrations. As a result a voltage–discharge curve shows a step-like behavior with nonanalytic features at the critical concentrations.

Phase Transformations of Anatase TiO 2 on Cation Intercalation from First Principles Simulation.

The phase transformations of anatase on Li-, H- and Na- intercalation are analyzed, for a variety of different insertion concentrations, using first principle calculations. Predicted structures are based on symmetry unconstrained optimization of all internal degrees of freedom and the unit cell shape and volume. The maximum insertion concentrations are determined, the phase stability of the predicted structures examined and the mechanism of the phase transformations discussed.

Comparison of ab initio and empirical approaches to the quartz surface

We investigate several possible reconstructions of the (001) α-quartz surface by using a combinatory approach of classical and ab initio molecular dynamics (MD). Configurational space is efficiently explored by fast classical MD simulations with a semi-empirical three-body potential, which has been shown to be accurate in simulations of the bulk. These runs generate initial structures for further refinement by the accurate quantum MD method of Car–Parrinello (P. Car, M. Parrinello, Phys. Rev. Lett. 55 (1985) 2471). Stable reconstructions of the quartz surface have been produced. They show the same pattern: formation of six- and three-membered rings. The obtained structures were found to be local minima in the classical potential, thereby demonstrating the usefulness of the empirical potential for surface calculations.

Influence of doping on the electrochemical properties of anatase

The effect of substitution on the intercalation properties of anatase-structured titania has been investigated in first principles calculations. Ti 4+ -ions were substituted by Zr 4+ , Al 3+ and Sc 3+ respectively and O 2- -ions by N 3- . For each compound the open circuit voltage profile (OCV) was calculated and compared to anatase. Lithium intercalation proceeds as in pure anatase through a phase separation into a Li-rich and a Li-poor phase in all cases examined here. The Li-content of the phases depends on the nature of the dopant and its concentration. Substitution by N 3- -ions does not lead to lower potentials, whereas doping with trivalent Sc 3+ - and Al 3+ - ions decreases the intercalation voltage. Substitution by tetravalent Zr 4+ -ions within the range of solubility does not significantly affect the OCV of anatase. A correlation is observed between the predicted equilibrium voltage and the participation of the Ti 4+ -ions in accommodating the donated electron density upon lithiation.