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Thermodynamics and kinetics of Na-ion Insertion into Hollandite-TiO2 and O3-Layered NaTiO2: an Unexpected Link between two Promising Anode Materials for Na-ion Batteries Alexandros Vasileiadis, and Marnix Wagemaker Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03928 • Publication Date (Web): 06 Dec 2016 Downloaded from http://pubs.acs.org on December 9, 2016
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Chemistry of Materials
Thermodynamics and kinetics of Na-ion Insertion into Hollandite-TiO2 and O3-Layered NaTiO2: an Unexpected Link between two Promising Anode Materials for Na-ion Batteries Alexandros Vasileiadis and Marnix Wagemaker Department of Radiation Science and Technology, Delft University of Technology, Mekelweg 15, 2629JB, Delft, The Netherlands ABSTRACT: First principle DFT calculations are used to study the thermodynamic and kinetic properties of Na-ion insertion in TiO2 hollandite, a potential anode for Na-ion batteries. The experimentally observed phase transformation from tetragonal TiO2 (I4/m) to monoclinic Na0.25TiO2 (I2/m) is confirmed. At high Naion concentrations the calculated formation energies predict a first order phase transition towards the layered O’3-Na0.68TiO2 structure. Further sodiation initiates a solid solution reaction towards the layered O3-NaTiO2 phase, which was recently brought forward as a promising anode for Na-ion batteries. This transformation irreversibly transforms the one dimensional Hollandite tunnel structure into the layered structure, and potentially brings forward an alternative route towards the preparation of the hard to prepare O3-NaTiO2 material. Energy barrier calculations reveal fast Na-ion diffusion at low concentrations and sluggish diffusion upon reaching the N0.25TiO2 phase, rationalizing why experimentally the Na0.5TiO2 phase is not achieved. Detailed analysis of the kinetic behaviour in the hollandite structure via Molecular Dynamics simulations reveal the importance of correlated atomic motions and dynamic lattice deformations for the Na-ion diffusion. In addition, exceptional Na-ion kinetics were predicted for the layered O’3-Na0.75TiO2 phase through a dominating divacancy hopping mechanism.
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The high gravimetric and volumetric energy density of Li-ion batteries revolutionized modern society by enabling mobile applications such as laptops, tablets and mobile phones. However, the increasing demand for Li-ion batteries by the development of electrical cars and static storage for instance in houses, challenges the practical abundance of lithium world-wide1, 2. A highly promising alternative charge carrier, already considered by the scientific community decades ago, is Sodium being much more abundant and displaying comparable redox potentials and insertion behaviour3, 4 compared to Lithium. Sodium, however, has a larger ionic radius compared to Lithium imposing larger volume changes on the host structure lattice which is detrimental for the cycle life5. Although several materials have been reported as candidates for Na-ion positive electrodes, Na-ion anode materials are scarcely reported on. Because Na-ion intercalation into graphite layers appears not possible6, research focuses mainly on other carbonaceous materials such as hard carbons7, 8 and petroleum cokes9 while non-carbonaceous alloys10 and vanadium oxides11, 12 are investigated as well. TiO2 polymorphs are well studied as negative electrode materials for Li-ion batteries, primarily because of the success of spinel Li4Ti5O1213-16 and anatase17-20 and secondarily because of rutile21, layered TiO222, brookite and ramsdellite23. Recently those polymorphs were also investigated as potential negative electrode materials for Na-ion insertion24-28. Amongst them, the layered O3-NaTiO2 was identified as one of the most promising Na-ion anode candidates due to its exceptional electrochemical performance and the ability to intercalate Na-ions without large volume changes (
