Evaluation of Multivalent Cation Insertion in Single- and Double-Layered Polymorphs of V₂O₅

Multivalent intercalation batteries have the potential to circumvent several fundamental limitations of reigning Li-ion technologies. Such batteries will potentially deliver high volumetric energy densities, be safer to operate, and rely on materials that are much more abundant than Li in the Earth's crust. The design of intercalation cathodes for such batteries requires consideration of thermodynamic aspects such as structural distortions and energetics as well as kinetic aspects such as barriers to the diffusion of cations. The layered α-V₂O₅ system is a canonical intercalation host for Li-ions but does not perform nearly as well for multivalent cation insertion. However, the rich V-O phase diagram provides access to numerous metastable polymorphs that hold much greater promise for multivalent cation intercalation. In this article, we explore multivalent cation insertion in three metastable polymorphs, γ', δ', and ρ' phases of V₂O₅, using density functional theory calculations. The calculations allow for evaluation of the influence of distinctive structural motifs in mediating multivalent cation insertion. In particular, we contrast the influence of single versus condensed double layers, planar versus puckered single layers, and the specific stacking sequence of the double layers. We demonstrate that metastable phases offer some specific advantages with respect to thermodynamically stable polymorphs in terms of a higher chemical potential difference (giving rise to a larger open-circuit voltage) and in providing access to diffusion pathways that are highly dependent on the specific structural motif. The three polymorphs are found to be especially promising for Ca-ion intercalation, which is particularly significant given the exceedingly sparse number of viable cathode materials for this chemistry. The findings here demonstrate the ability to define cation diffusion pathways within layered metastable polymorphs by alteration of the stacking sequence or the thickness of the layers.