First principles Hartree-Fock description of lithium insertion in oxides I. The end members TiO2 and LiTiO2 of the system LixTiO2

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) poly morphs of TiO2, from which the predicted order of stability is P4(2)/mnm > I4(1)/amd > Pbca approximate to Pcbn > Pnma > Imma > Fd (3) ove r bar M. The calculated difference in energy between the rutile and anatase structur es is 0.02-0.06 eV, in good agreement with a recent local density approxima tion (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, whic h are based on isotropic volume-optimized Hartree-Fock energies, are also i n good agreement with recent LDA calculations and with experiment. Spin-unr estricted 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 calc ulated volume is in good agreement with experiment. From the relative stabi lities of TiO2 and LiTiO2, the relative lithium insertion potentials corres ponding 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) L iMnO2 lead to an estimate of the voltage for Imma TiO2 -->(Li) LiTiO2 of si milar to 1.3 eV, which is similar to 2.5 eV anodic to the Mn system, The co rresponding values for the Pnma polymorphic route are similar to 3 and simi lar to 3.5 eV, respectively. Mulliken population analyses indicate that lit hium is completely ionized in LiTiO2 and that the charge transfer is predom inantly to the oxygen sublattice. There is a rehybridization of the titaniu m valence orbitals leading to a slight increase in the 3d population and st rong localization of spin density at the titanium sites with local moments of similar to 1 mu(B). (C) 1999 Academic Press.