A new model of Li intercalation into rutile and anatase structured titania has been developed from first principles calculations. The model includes both thermodynamic and kinetic effects and explains the observed differences in intercalation behavior and their temperature dependence. The important role of strong local deformations of the lattice and elastic screening of interlithium interactions is demonstrated. In addition, a new phase of LiTiO 2 is reported. DOI: 10.1103/PhysRevLett.86.1275 Transition metal oxides are promising electrode materials in advanced high-energy density batteries [1]. The performance of an electrode depends on its ability to intercalate lithium reversibly into the host lattice. The transition metal oxides have open structures capable of accommodating guest ions and a flexible electronic structure which can accommodate donated electrons and provide sufficient ionic and electronic conductivity. These properties result in a number of low-energy sites for guest ions within the lattice and the potential for high capacity lithium ion intercalation.Battery performance is usually characterized by low current discharge curves which approximate the equilibrium voltage difference between the electrodes, often referred to as the open circuit voltage (OCV). The key factors in applications are high-energy density and reversible structural changes on Li intercalation for a large range of the insertion concentration.For a material in thermodynamic equilibrium the OCV is fully determined by the difference in the chemical potential of lithium between the anode and the cathode which is determined by the ordering of the lithium ions in the host structure. However, in practical applications the OCV is also often limited by lithium ion diffusion which prevents ions from reaching thermodynamically stable sites. The design of new battery materials requires a microscopic understanding of the stable sites and diffusion pathways which is very difficult to obtain from experiment. Therefore, first principles simulation, which provides accurate and reliable energy surfaces, has a key role to play in identifying and characterizing prospective electrode materials.Previous simulations, based on a combination of first principles energetics and a Monte Carlo treatment of the statistical mechanics, have focused on the thermodynamics of lithium insertion. Thus, phase diagrams and equilibrium OCV's have been computed for Li . This methodology requires the representation of configurational energies using effective interaction parameters which are assumed to be valid over a wide range of insertion concentrations.Another interesting electrode material is nanostructured TiO 2 which is currently used in solar cell applications [6]. It has a number of practical advantages as it is readily available, chemically stable, semiconducting, inexpensive, and nontoxic [7]. The most common natural forms of TiO 2 are rutile and anatase. Lithium intercalation into both forms has been studied extensively. In Li x TiO 2 the reported maximum...

Density-functional simulations of lithium intercalation into rutile structured titanium dioxide are presented. Full relaxation of structures for a wide range of insertion concentrations is used to identify the thermodynamically most stable configurations and Li-ion site preferences. The host lattice is found to undergo large deformations upon Li insertion, which can be related to the excitation of soft vibrational modes. The dominant screening interaction is found to be due to these elastic distortions of the lattice rather than to dielectric screening. This leads to highly anisotropic and concentration-dependent effective Li-Li interactions, which are not easily amenable to empirical parametrization. The anisotropic volume expansion is found to be largely due to the increase in the radii of reduced Ti ions as they accommodate charge donated to the lattice. The computed open circuit voltage ͑OCV͒ reproduces the characteristic features of experimental discharge curves at elevated temperature. The computed Li-ion energy surfaces reveal highly anisotropic diffusion. A model of Li intercalation is proposed, which takes account of both the thermodynamic and kinetic properties computed here. This model is used to resolve apparent contradictions in the current interpretation of the measured OCV and its dependence on temperature. Predicted changes in the electronic structure and their relationship to the interaction between structural, charge, and spin degrees of freedom are discussed in detail.

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The mechanism of the tetragonal-to-orthorhombic transformation upon Li-intercalation into anatase structured titania has been studied using first principle calculations. The primary mechanism for the formation of the orthorhombic phase is found to be the accommodation of donated charge in localized Ti-d yz orbitals leading to a cooperative Jahn-Teller-like distortion of the lattice. This model is examined further by considering electron addition states in pure anatase and the analogous structures of H 0.5 TiO 2 and Na 0.5 TiO 2 . It is shown that the rigid band model is not valid and population of the degenerate Ti-d xy,yz orbitals occurs beyond a critical concentration due to the repulsive interaction of the localized electrons. It is shown that the stability of the Li 0.5 TiO 2 structure is related to the similarity of the ionic radius of Li ϩ and Ti ϩ3 ions. Optimal configurations of H 0.5 TiO 2 and Na 0.5 TiO 2 are also predicted.

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