First Principles HartreeFock Description of Lithium Insertion in Oxides. I. The End Members TiO _{2}and LiTiO _{2}of the System Li _{x}TiO _{2}
Abstract
First principles periodic HartreeFock calculations are reported for the P4 _{2}/ mnm(rutile), I4 _{1}/ amd(anatase), Pbca(brookite), Pnma(ramsdellite), Pcbn(colombite), Fdoverline3m(spinel), and Imma(orthorhombic) polymorphs of TiO _{2}, from which the predicted order of stability is The calculated difference in energy between the rutile and anatase structures is 0.020.06 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 HartreeFock and LDA differences for the brookite structure are 0.06 and 0.058 eV, respectively. The calculated volumes, which are based on isotropic volumeoptimized HartreeFock energies, are also in good agreement with recent LDA calculations and with experiment. Spinunrestricted calculations are reported for the Fmoverline3m, Imma, Pnma, and P4 _{2}/ mmmof LiTiO _{2}, where the stability is in the order The only reported phase for LiTiO _{2}is Fmoverline3m, for which the calculated volume is in good agreement with experiment. From the relative stabilities of TiO _{2}and LiTiO _{2}, the relative lithium insertion potentials corresponding to TiO _{2} → LiTiO _{2}are deduced, with a maximum variation of 1.6 eV for the different polymorphic routes. The maximum voltage predicted is that for the Immaroute which is ∼1 eV larger than that for Pnma. Direct comparisons with the calculated energy for C2/ mLi _{0.5}MnO _{2} → LiMnO _{2}lead to an estimate of the voltage for ImmaTiO _{2} → LiTiO _{2}of ∼1.3 eV, which is ∼2.5 eV anodicto the Mn system. The corresponding values for the Pnmapolymorphic route are ∼3 and ∼3.5 eV, respectively. Mulliken population analyses indicate that lithium is completely ionized in LiTiO _{2}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 3 dpopulation and strong localization of spin density at the titanium sites with local moments of ∼1 μ_{B}.
 Publication:

Journal of Solid State Chemistry France
 Pub Date:
 February 1999
 DOI:
 10.1006/jssc.1998.8058
 Bibcode:
 1999JSSCh.142..428M