Mehmet Kadri Aydınol

A study of the average voltage to intercalate lithium in various metal oxides is presented. By combining the ab initio pseudopotential method with basic thermodynamics the average intercalation voltage can be predicted without the need for experimental data. This procedure is used to systematically study the effect of metal chemistry, anion chemistry, and structure. It is found that Li is fully ionized in the intercalated compounds with its charge transferred to the anion and to the metal. The substantial charge transfer to the anion is responsible for the large voltage difference between oxides, sulfides, and selenides. Ionic relaxation, as a result of Li intercalation, causes nonrigid-band effects in the density of states of these materials. Suggestions for compounds that may have a substantially larger voltage than currently used materials are also presented.

In this work, the phase diagram of ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ is calculated from first principles for x ranging from 0 to 1. Our calculations indicate that there is a tendency for Li ordering at $x=\frac{1}{2}$ in agreement with experiment [J. N. Reimers and J. R. Dahn, J. Electrochem. Soc. 139, 2091 (1992)]. 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 do not find the two-phase region observed at high Li concentration and speculate that this two-phase region is caused by the metal-insulator transition that occurs at concentrations slightly below $x=1.$