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 ReO 3 -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 VTa 9 O 25 is reported, enabling a direct isostructural comparison with the better-known VNb 9 O 25 . 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 VNb 9 O 25 was an order of magnitude faster than that for VTa 9 O 25 . These experimental trends aligned well with density functional theory calculations combined with molecular dynamics that show a factor of 7 faster diffusion in VNb 9 O 25 compared to VTa 9 O 25 . Nudged elastic band calculations of the probable hopping pathways indicate that VNb 9 O 25 consistently exhibits a lower activation barrier for lithium diffusion than VTa 9 O 25, which can be attributed to the larger net charge transfer during Li hopping in VNb 9 O 25 . DFT calculations indicate that the structures show only a small overall volume change of about 6% across lithiation, with little structural difference between VNb 9 O 25 and VTa 9 O 25 . In contrast, the electronic structures differ, with VNb 9 O 25 undergoing an insulator–to–metal transition at a state of charge lower than that of VTa 9 O 25 . Overall, the results indicate that the choice of cation (Nb or Ta) influences the electronic and transport properties during lithiation.