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TiNb 2 O 7 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 TiNb 2 O 7 , however, are not well-characterized. Herein, experimental and computational insights are combined to understand the inherent properties of bulk TiNb 2 O 7 . 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 0.60 TiNb 2 O 7 . 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   TiNb 2 O 7 ) is rapid with low activation barriers from NMR and D  Li = 10 -11 m 2 s -1 at the temperature of the observed T  1 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 &gt; 3 in Li   x   TiNb 2 O 7 ), related to a transition from interstitial-mediated to vacancy-mediated diffusion. Overall, lithium insertion leads to effective n-type self-doping of TiNb 2 O 7 and high-rate conduction, while ionic motion is eventually hindered at high lithiation. Transition-state searching with beyond Li chemistries (Na + , K + , Mg 2+ ) in TiNb 2 O 7 reveals high diffusion barriers of 1-3 eV, indicating that this structure is specifically suited to Li + mobility.

Ionic and Electronic Conduction in TiNb2O7

Ionic and Electronic Conduction in TiNb₂O₇