TiNb2O7 is a Wadsley-Roth phase with a crystallographic shear structure and is a promising candidate for high-rate lithium ion energy storage. The 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 the 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 a composition of 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 LixTiNb2O7) 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 LixTiNb2O7), 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 reveals high diffusion barriers of 1-3 eV, indicating that this structure is specifically suited to Li+ mobility.
Bibliographical noteFunding Information:
K.J.G. thanks the Winston Churchill Foundation of the United States, the Herchel Smith Scholarship, and the EPSRC (EP/M009521/1) for funding. M.A.H. acknowledges support from the Oppenheimer Foundation. A.J.M. acknowledges support from the EPSRC (EP/P003532/1). Via our membership of the UK’s HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202, EP/R029431), this work used the ARCHER UK National Computing Service. M.J.C. thanks Sidney Sussex College, University of Cambridge, for financial support. K.J.G. thanks Dr. John Griffin, University of Lancaster, for discussions of homonuclear and heteronuclear dipolar coupling; Professor Ram Seshadri, University of California, Santa Barbara, for experimental advice; and Charles Creissen, University of Cambridge, for assistance with diffuse reflectance spectroscopy. I.D.S. thanks Professor David J. Wales and Dr. Roberta Pigliapochi, University of Cambridge, for discussions on transition-state energetics. S.E.D. and C.P.K. acknowledge funding from the Winton Programme for the Physics of Sustainability. Magnetic measurements were carried out on the Advanced Materials Characterisation Suite funded by EPSRC Strategic Equipment Grant EP/M000524/1.
Copyright © 2019 American Chemical Society.