Effect of Diffusion on Lithium Intercalation in Titanium Dioxide

Abstract

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...

Nanostructuring of β-MnO2: The Important Role of Surface to Bulk Ion Migration

“…This work highlighted anisotropic diffusion and the ability of local structural distortions to create Li ion traps that inhibit diffusion. 33 They also argued that short diffusion lengths and increased structural flexibility near the surface of nanostructures reduce these effects. 34 …”

Density-functional simulations of lithium intercalation in rutile

“…This work extends a preliminary report in which the importance of Li-ion diffusion on the performance of this system as a battery electrode was noted. 14 The preferred intercalation sites, and the most stable ordered configurations for inserted ions have been analyzed for a wide range of insertion concentrations. The diffusion pathways for Li ions have been explored in detail and a model of Li intercalation is presented.…”

Atomic resolution observation of conversion-type anode RuO2during the first electrochemical lithiation

“…According to the energetically calculation for rutile structure crystals including TiO2, SnO2 and RuO2, <001> crystallographic direction with the lowest energy cost is the most favorable lithium diffusion pathway. 27,[38][39][40][41] …”