Ionic and Electronic Conduction in TiNb2O7.
Seymour, Ieuan D
Lamontagne, Leo K
Journal of the American Chemical Society
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Griffith, K., Seymour, I. D., Hope, M., Butala, M. M., Lamontagne, L. K., Preefer, M. B., Koçer, C. P., et al. (2019). Ionic and Electronic Conduction in TiNb2O7.. Journal of the American Chemical Society, 141 (42), 16706-16725. https://doi.org/10.1021/jacs.9b06669
TiNb2O7 is a Wadsley-Roth phase with the crystallographic shear structure and a promising candidate for high-rate lithium-ion energy storage. Fundamental aspects of the lithium insertion mechanism and conduction in TiNb2O7, however, are not well-characterized. Herein, experimental and computational insights are combined to understand inherent properties of bulk TiNb2O7. The results show an increase in electronic conductivity of seven orders-of-magnitude upon lithiation and indicate that electrons exhibit both localized and delocalized character, with a maximum Curie constant and Li NMR paramagnetic shift near Li0.60TiNb2O7. Square planar or distorted five-coordinate lithium sites are calculated to invert between thermodynamic minima or transition states. Lithium diffusion in the single-redox region (i.e. x ≤ 3 in Li x TiNb2O7) is rapid with low activation barriers from NMR and DLi = 10-11 m2·s-1 at the temperature of the observed T1 minima of 525-650 K for x ≥ 0.75. DFT calculations predict that ionic diffusion, like electronic conduction, is anisotropic with activation barriers for lithium hopping of 100-200 meV down the tunnels but ca. 700-1000 meV across the blocks. Lithium mobility is hindered in the multiredox region (i.e. x > 3 in Li x TiNb2O7), related to a transition from interstitial-mediated to vacancy-mediated diffusion. Overall, lithium insertion leads to effective n-type self-doping of TiNb2O7 and high-rate conduction while ionic motion is eventually hindered at high lithiation. Transition-state searching with beyond Li chemistries (Na+, K+, Mg2+) in TiNb2O7 revealed high diffusion barriers of 1-3 eV, indicating that this structure is specifically suited to Li+ mobility.
EPSRC (via University of Oxford) (EP/M009521/1)
External DOI: https://doi.org/10.1021/jacs.9b06669
This record's URL: https://www.repository.cam.ac.uk/handle/1810/297016
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