Comparison of Lithium Diffusion in Isostructural Ta12MoO33 and Nb12MoO33: Experimental and Computational Insights from Single Crystals

The demand for fast charging requires high-performance battery materials with improved ionic transport. Wadsley–Roth (WR) structures have garnered attention, where the combination of blocks and shear planes addresses ionic and electronic conductivity, respectively. An improved understanding of structure–property relationships could lead to higher-performance materials. Herein, we report the first single-crystal structures of Nb₁₂MoO₃₃ and Ta₁₂MoO₃₃ that are consistent with other (3 × 4 × ∞) WR phases. The lithiation of Ta₁₂MoO₃₃ is reported to enable an isostructural comparison with Nb₁₂MoO₃₃. These two compounds have similar unit cell volumes and atomic radii, where the Ta₁₂MoO₃₃ unit cell is 0.2 vol % smaller. Despite the similarities in structure, the lithiation capacities, voltage windows, C rate-dependent capacities, and ionic diffusivities are distinctly different. These experimental trends align well with density functional theory calculations showing (1) a lower activation energy for Li transport within Ta₁₂MoO₃₃ consistent with its measured 1.5–4.9-fold higher diffusion coefficients (lithiation) and (2) an ∼25% greater measured lithiation stoichiometry for Nb₁₂MoO₃₃, which is attributed to the calculated smaller octahedral distortions (compared to Ta₁₂MoO₃₃). These findings reveal that smaller channels in Ta₁₂MoO₃₃ stabilize the transition state with 5-fold coordination, which both decreases the activation energy for diffusion and limits the extent of lithiation. Such structure–property trends help in the search for next-generation battery materials.