Anton van der Ven

Phase diagrams of lithium transition metal oxides: investigations from first principles

Abstract The thermodynamics and phase transitions in lithium intercalation oxides are discussed. Changes in the host structure can be driven by configurational Li-vacancy interactions, variations in electron count or by changes in the stability of the oxygen packing. The formalism to predict the lithium-vacancy ordered configurations and their free energy is presented and calculations of the phase diagram of Li x CoO 2 in the spinel and layered structure using this formalism are reviewed. Layered Li x CoO 2 has the richest phase diagram with ordering and staging transitions, and changes in host structure at low lithium contents. In general we find relatively low order–disorder transitions due to the strong screening of the lithium–lithium interaction by oxygen. From calculating the energy difference between the spinel and layered structure for several transition metal oxides it is found that a driving force for transition to spinel will always exist when a layered lithium transition metal oxide is delithiated. The limitations of current first principles methods in studying electronic transitions are discussed.

Lateral interactions between oxygen atoms adsorbed on platinum (111) by first principles

In this paper, first principles were used to obtain a set of lateral interactions of oxygen atoms adsorbed on Pt (111) which can be applied in future studies of the phase diagram of O–Pt (111). Density functional theory (DFT) total energy computations were performed for fifteen configurations of oxygen adsorbed on Pt (111), with oxygen coverages ranging from zero to one. A cluster expansion was then carried out to fit lateral interaction parameters between and among oxygen atoms. The interaction parameters extracted from first principles indicate that strong repulsions exist between first (237.1u2009meV) and second nearest neighbours (39.5u2009meV), and weak attractions exist between third nearest neighbours (−5.8u2009meV). These weak attractions may explain the formation of islands of oxygen with p(2 × 2) symmetry, observed to form on Pt (111).

Ab INITIO CALCULATION OF THE Li x CoO 2 PHASE DIAGRAM

The electrochemical properties of the layered intercalation compound LiCoO 2 used as a cathode in Li batteries have been investigated extensively in the past 15 years. Despite this research, little is known about the nature and thermodynamic driving forces for the phase transformations that occur as the Li concentration is varied. In this work, the phase diagram of Li x CoO 2 is calculated from first principles for x ranging from 0 to 1. Our calculations indicate that there is a tendency for Li ordering at x = 1/2 in agreement with experiment [1]. At low Li concentration, we find that a staged compound is stable in which the Li ions selectively segregate to every other Li plane leaving the remaining Li planes vacant. We find that the two phase region observed at high Li concentration is not due to Li ordering and speculate that it is driven by a metal-insulator transition which occurs at concentrations slightly below x

Understanding the Formation Energy of Transition Metal Hydrides

A detailed analysis of the formation energies of transition metal hydrides is presented. The hydriding energies are computed for various crystal structures using Density Functional Theory. The process of hydride formation is broken down into three consecutive, hypothetical reactions in order to analyse the different energy contributions, and explain the observed trends. We find that the stability of the host metal is very significant in determining the formation energy, thereby providing a more fundamental justification for Miedemas “law of inverse stability” [1] (the more stable the metal, the less stable the hydride). The conversion of the host metal to the structure formed by the metal ions in the hydride (fcc in most cases) is only significant for metals with a strong bcc preference such as V and Cr - this lowers the driving force for hydride formation. The final contribution is the chemical bonding between the hydrogen and the metal. This is the only contribution that is negative, and hence favourable to hydride formation. We find that it is dominated by the position of the Fermi level in the host metal.