First principles Hartree-Fock description of lithium insertion in oxides: I The end members TiO₂ and LiTiO₂ of the system Li_xTiO₂

First principles periodic Hartree-Fock calculations are reported for the P4(2)/mnm (rutile), I4(1)/amd (anatase), Pbca (brookite), Pnma (ramsdellite), Pcbn (colombite), Fd (3) over bar m (spinel), and Imma (orthorhombic) polymorphs of TiO2, from which the predicted order of stability is

P4(2)/mnm > I4(1)/amd > Pbca approximate to Pcbn > Pnma > Imma > Fd (3) over bar M.

The calculated difference in energy between the rutile and anatase structures is 0.02-0.06 eV, in good agreement with a recent local density approximation (LDA) estimate of 0.033 eV and an experiment enthalpy difference of 0.05 eV, The corresponding Hartree-Fock and LDA differences for the brookite structure are 0.06 and 0.058 eV, respectively. The calculated volumes, which are based on isotropic volume-optimized Hartree-Fock energies, are also in good agreement with recent LDA calculations and with experiment. Spin-unrestricted calculations are reported for the Fm (3) over bar m, Imma, Pnma, and P4(2)/mmm of LiTiO2, where the stability is in the order

Imma > Fm (3) over bar m > Pnma > P4(2)/mnm.

The only reported phase for LiTiO2 is Fm (3) over bar m, for which the calculated volume is in good agreement with experiment. From the relative stabilities of TiO2 and LiTiO2, the relative lithium insertion potentials corresponding to TiO2 -->(Li) LiTiO2 are deduced, with a maximum variation of 1.6 eV for the different polymorphic routes. The maximum voltage predicted is that for the Imma route,which is similar to 1 eV larger than that for Pnma. Direct comparisons with the calculated energy for C2/m Li0.5MnO2 -->(Li) LiMnO2 lead to an estimate of the voltage for Imma TiO2 -->(Li) LiTiO2 of similar to 1.3 eV, which is similar to 2.5 eV anodic to the Mn system, The corresponding values for the Pnma polymorphic route are similar to 3 and similar to 3.5 eV, respectively. Mulliken population analyses indicate that lithium is completely ionized in LiTiO2 and that the charge transfer is predominantly to the oxygen sublattice. There is a rehybridization of the titanium valence orbitals leading to a slight increase in the 3d population and strong localization of spin density at the titanium sites with local moments of similar to 1 mu(B). (C) 1999 Academic Press.