Electronic structure, sodium diffusion and redox potentials of low-symmetry NaMFe(MoO₄ )₃ (M = Mg, Ni)

Sodium polyanionic compounds with transition metals are of considerable research interest for the search of new cathode materials for sodium-ion batteries. In this work, we employed ab initio calculations to evaluate three key battery properties: phase stability, diffusion barriers and redox voltage of low-symmetry molybdates NaMFe(MoO₄ )₃ (M = Mg, Ni) with α- and β-NaFe₂ (MoO₄ )₃ structures. First, we showed that the Hubbard Ucorrection within the GGA + Uapproach is necessary to correctly refer these molybdates to semiconductors. The sodium diffusion along various pathways was thoroughly examined, which allowed us to establish the most probable pathways with the lowest migration barriers. These compounds have different directions of sodium diffusion with twice different barriers that was associated to the peculiarities of their crystal structure. The high potentials and structural stability during sodium extraction, along with low-diffusion barriers predicted for NaMFe(MoO₄ )₃ (M = Mg, Ni) indicate that these molybdates may be promising high-voltage cathode materials for sodium-ion batteries.