Combined Experimental and Computational Analysis of Lithium Diffusion in Isostructural Pair VNb9O25 and VTa9O25

TL;DR

This study combines experimental and computational methods to compare lithium diffusion in isostructural VNb9O25 and VTa9O25, revealing that VNb9O25 exhibits significantly faster lithium diffusivity and lower activation barriers, informing battery material design.

Contribution

It provides the first direct comparison of lithium diffusion in VNb9O25 and VTa9O25, combining experimental data with density functional theory and molecular dynamics to elucidate structure-property relationships.

Findings

VNb9O25 has an order of magnitude faster lithium diffusivity than VTa9O25.

Lower activation barriers for lithium diffusion are observed in VNb9O25 due to stronger Coulombic interactions.

VNb9O25 undergoes an insulator to metal transition at a lower lithiation level.

Abstract

Wadsley-Roth crystal structures are an attractive class of materials for batteries because lithium diffusion is facilitated by the ReO3-like block structure with electron transport enabled by edge-sharing along shear planes. However, clear structure-property relationships remain limited, making it challenging to develop improved materials. Here, the first lithiation of VTa9O25 is reported, enabling a direct isostructural comparison with the better-known VNb9O25. These materials have similar unit cell volumes and atomic radii yet exhibit different voltage windows, C-rate dependent capacities, and transport metrics. Time-dependent overpotential analysis reveals ionic diffusion as the primary bottleneck to high rate-performance in both cases, however, the lithium diffusivity for VNb9O25 was an order of magnitude faster than that for VTa9O25. These experimental trends aligned well with density functional theory calculations combined with molecular dynamics that show a factor of six faster diffusion in VNb9O25. Nudged elastic band calculations of the probable hopping pathways indicate that VNb9O25 consistently exhibits a lower activation barrier for lithium diffusion. Bader charge analysis reveals a larger net charge on Li in VNb9O25 due to the higher electronegativity of Nb which stabilizes the transition state and lowers the barrier. This stabilization arises from the stronger Coulombic interaction between Li and its coordinated O-environment. These materials behave similarly upon lithiation wherein the lattice vectors (corresponding to the block plane) increase until about 50% lithiation and then decrease. However, the electronic structure differs, indicating that VNb9O25 undergoes a insulator to metal transition at a lower state of charge compared with VTa9O25. Overall, this work establishes the role of the cation (Nb or Ta) on the electronic and transport properties during lithiation.