The intercalation phase diagram of Mg in V₂O₅ from first-principles

We have investigated Mg intercalation into orthorhombic V₂O₅, one of only three cathodes known to reversibly intercalate Mg ions. By calculating the ground-state Mg_xV₂O₅ configurations and by developing a cluster expansion for the configurational disorder in δ-V₂O₅, a full temperature-composition phase diagram is derived. Our calculations indicate an equilibrium phase-separating behavior between fully demagnesiated α-V₂O₅ and fully magnesiated δ-V₂O₅, but also motivate the existence of potentially metastable solid solution transformation paths in both phases. We find significantly better mobility for Mg in the δ polymorph, suggesting that better performance can be achieved by cycling Mg in the δ phase.