Topochemically De-Intercalated Phases of V₂O₅ as Cathode Materials for Multivalent Intercalation Batteries: A First-Principles Evaluation

The scarce inventory of compounds that allow for diffusion of multivalent cations at reasonable rates poses a major impediment to the development of multivalent intercalation batteries. Here, we contrast the thermodynamics and kinetics of the insertion of Li, Na, Mg, and Al ions in two synthetically accessible metastable phases of V₂O₅, ζ- and ϵ-V₂O₅, with the relevant parameters for the thermodynamically stable α-phase of V₂O₅ using density functional theory calculations. The metastability of the frameworks results in a higher open circuit voltage for multivalent ions, exceeding 3 V for Mg-ion intercalation. Multivalent ions inserted within these structures encounter suboptimal coordination environments and expanded transition states, which facilitate easier ion diffusion. Specifically, a nudged elastic band examination of ion diffusion pathways suggests that migration barriers are substantially diminished for Na- and Mg-ion diffusion in the metastable polymorphs: the predicted migration barriers for Mg ions in ζ-V₂O₅ and ϵ-V₂O₅ are 0.62-0.86 and 0.21-0.24 eV, respectively. More generally, the results indicate that topochemically derived metastable polymorphs represent an interesting class of compounds for realizing multivalent cation diffusion because many such compounds place cations in "frustrated" coordination environments that are known to be useful for realizing low diffusion barriers.