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

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.