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

Abstract

The increasing demand for fast charging batteries has motivated the search for materials with improved transport characteristics. Wadsley–Roth crystal structures are an attractive class of materials for batteries because fast lithium diffusion is facilitated by the ReO₃-like block structure, with electron transport enabled by edge-sharing along the shear planes. However, a clear understanding of structure–property relationships remains limited, making it challenging to develop improved materials within this class of promising compounds. Here, the first lithiation of VTa₉O₂₅ is reported, enabling a direct isostructural comparison with the better-known VNb₉O₂₅. These materials have similar atomic radii and unit cell volumes yet exhibit different voltage windows, C-rate-dependent capacities, and transport metrics. Time-dependent overpotential analysis revealed ionic diffusion as the primary bottleneck to high-rate performance in both cases. However, the corresponding 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 7 faster diffusion in VNb₉O₂₅ compared to VTa₉O₂₅. Nudged elastic band calculations of the probable hopping pathways indicate that VNb₉O₂₅ consistently exhibits a lower activation barrier for lithium diffusion than VTa₉O₂₅, which can be attributed to the larger net charge transfer during Li hopping in VNb₉O₂₅. DFT calculations indicate that the structures show only a small overall volume change of about 6% across lithiation, with little structural difference between VNb₉O₂₅ and VTa₉O₂₅. In contrast, the electronic structures differ, with VNb₉O₂₅ undergoing an insulator–to–metal transition at a state of charge lower than that of VTa₉O₂₅. Overall, the results indicate that the choice of cation (Nb or Ta) influences the electronic and transport properties during lithiation.