Lithium insertion and mobility in the TiO 2 -anatase/titanate structure: A periodic DFT study F. Tielens a,b,1 , M. Calatayud b , A. Beltra ´n a , C. Minot b, * , J. Andre ´s a, * a Dept. Cie `ncies Experimentals, Universitat Jaume I, P.O. Box 224, E-12080 Castello ´ , Spain b Laboratoire de Chimie The ´orique, Universite ´ Pierre et Marie Curie, Paris VI, Case 137, 4 Place Jussieu, F-75252 Paris Cedex 05, France Received 22 February 2005; received in revised form 4 April 2005; accepted 11 April 2005 Available online 16 June 2005 Abstract The Li atom positions are calculated in Li x TiO 2 anatase, by means of periodic density functional theory calculations, followed by the predictions of the energy barriers for the mobility of Li atoms, done for the first time for a range of Li concentrations (x < 0.5). A fivefold-coordinated environment was confirmed for the Li atoms in the empty octahedral sites in titanate (orthorhombic) Li x TiO 2 structures. For the low Li concentration structures (anatase) a fourfold-coordination was found for the Li atoms. The energy barrier to move a Li atom from one octahedral site to a neighboring one is found to converge for x value higher than 0.5–0.6 eV. The barrier used to calculate the hopping rate was found to decrease with increasing Li concentration. Ó 2005 Elsevier B.V. All rights reserved. Keywords: TiO 2 anatase; Titanate; Li-diffusion; Density functional theory 1. Introduction Lithium ion secondary batteries are widely applied to electronic devices used in our everyday life, because of their high energy density and rechargeability [1–4]. Due to the fact that TiO 2 is a lightweight, inexpensive and environmentally friendly material, and moreover, having favorable sites for lithium insertion, it can be considered as a potential material for such devices. Such batteries have indeed shown their efficiency in photo- electrochemical solar cells [5] and are expected to be a power source in electric vehicles. Another property which is frequently studied as well is its electrochromic- ity [6,7], which can be used in display applications. The electrochemical intercalation reaction can be written as xLi + + TiO 2 + xe  M Li x TiO 2 , where x is the mole fraction of lithium in titanium dioxide. The reversibility of the above mentioned reaction is of ut- most importance for its utilization in technical devices. Insertion of lithium ions into the anatase framework re- sults in a phase transition from the original tetragonal anatase structure toward the orthorhombic titanate phase for concentrations (x > 0.3–0.5) [8–10]. This tran- sition represents a loss of symmetry referring to TiO 2 anatase as central crystal structure. Indeed, LiTiO 2 crys- tals are often mixtures between both structures due to inhomogeneous Li distribution [11]. Li insertion in ana- tase could be seen as a two-phase equilibrium beginning with the intercalation of the first ions. All subsequent processes could be described by a superposition of these two phases in different amounts depending on the Li concentration. Another point of interest is the nature of the intersti- tial sites for the insertion of Li. The topology of the ana- tase-TiO 2 structure has been investigated theoretically [12–16] and experimentally [11,17–20], however not for a wide range of Li concentrations. It is generally 0022-0728/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2005.04.009 * Corresponding authors. E-mail addresses: tielens@lct.jussieu.fr (F. Tielens), minot@lct. jussieu.fr (C. Minot), andres@exp.uji.es (J. Andre ´s). 1 Tel.: +33 144 279 660. www.elsevier.com/locate/jelechem Journal of Electroanalytical Chemistry 581 (2005) 216–223 Journal of Electroanalytical Chemistry