Ionic and Electronic Conduction in TiNb2O7.

Kent J Griffith,Leo K Lamontagne,Ieuan D Seymour,Molleigh B Preefer,Andrew J Morris,Megan M Butala,Can P Koçer,Clare P Grey,Matthew J Cliffe,Graeme Henkelman,Siân E Dutton,Michael A Hope

doi:10.1021/jacs.9b06669

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 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.

Highlights

Next-generation energy storage materials with fast recharging and high power capability are of immediate interest to accelerate widespread electric vehicle adoption.[1]
For negative electrodes in rechargeable lithium ion batteries, graphite and other materials that store a large quantity of lithium in a potential range close to the Li+/Li redox couple are favored for high-energy density applications
The electrochemical properties of TiNb2O7 have been reported for a variety of particle sizes/morphologies and electrode preparations.[79,24,28,37,80]

Summary

Introduction

Next-generation energy storage materials with fast recharging and high power capability are of immediate interest to accelerate widespread electric vehicle adoption.[1]. For negative electrodes (anodes) in rechargeable lithium ion batteries, graphite and other materials (e.g., silicon) that store a large quantity of lithium in a potential range close to the Li+/Li redox couple are favored for high-energy density applications. Large overpotentials and spatial overpotential inhomogeneities at high current densities can lead to lithium plating on the surface of low-voltage electrodes.[2−6] When lithium deposits as mossy or dendritic structures[7−10] rather than plating smoothly onto a Li anode the cell can short-circuit and undergo rapid heating that may lead to a battery fire/explosion.[11−14]. In order to overcome the inherent challenges associated with the use of low-voltage electrode materials in high-rate applications, a series of higher-voltage anode material candidates are emerging.

Results

Discussion

Conclusion