Computational and Experimental Investigation of the Transformation of V₂O₅ Under Pressure

It has previously been reported that under high-pressure V₂O₅ (α-V₂O₅) transforms into a layered polymorph, β-V₂O₅, consisting of V⁵⁺O₆ octahedra instead of V⁵⁺O₅-square pyramids. Both polymorphs have a good performance as positive electrode for lithium batteries. In this work, we investigate the pressure-induced α → β transformation combining first principles and experimental methods. Density functional theory (DFT) predicts that α-V₂O₅ transforms to β-V₂O₅ at 3.3 GPa with a 11% volume contraction; experiments corroborate that at a pressure of 4 GPa, V₂O₅ (d = 3.36 g/cm³) transformed into a well-crystallized β-V₂O₅, with a much denser structure (d = 3.76 g/cm³). β-V₂O₅ can be also prepared at 3 GPa, although with a substantial degree of amorphization. The calculated bulk modulus of α-V₂O₅ (18 GPa) indicates that this is a very compressible structure; this being linked to the contraction along its b-axis (interlayer space) and to a significant decrease of a long V−O distance (V−O ≈ 2.9 Å). As a result, the vanadium coordination increases from five (square pyrmamid) in α-V₂O₅ to six (distorted octahedron), leading to the stabilization of the high-pressure (β) polymorph. This change of the coordination environment of vanadium ions also affects the electrical conductivity. The calculated density of states shows a narrowing of 0.5 V in the band gap for the β polymorph, in comparison to the ambient-pressure material; the measured resistivities at room temperature (10 000 Ω cm in α-polymorph and 400 Ω cm in β-polymorph) reveal that β-V₂O₅ is indeed a better electronic conductor than α-V₂O₅. In view of these results, similar transformations at moderate pressures are expected to occur in other V⁵⁺ frameworks, suggesting an interesting way to synthesize novel V⁵⁺ compounds with potential for electrochemical devices.