Ionic and Electronic Conduction in TiNb₂O₇

Abstract

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 Li_0.60TiNb₂O₇. 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., <i>x</i> ≤ 3 in Li<i>_<i>x</i></i>TiNb₂O₇) is rapid with low activation barriers from NMR and <i>D</i>_Li = 10^-11 m² s^-1 at the temperature of the observed <i>T</i>₁ minima of 525-650 K for <i>x</i> ≥ 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., <i>x</i> > 3 in Li<i>_<i>x</i></i>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 reveals high diffusion barriers of 1-3 eV, indicating that this structure is specifically suited to Li⁺ mobility.