Ab Initio Study of NaMFe(MoO4)3 (M = Mn, Fe, Co, Ni, Zn): Electron Structure, Sodium Diffusion and Potentials

The electronic structures of NaMFe(MoO4)(3) (M = Mn, Fe, Co, Ni, Zn) molybdates isostructural to alpha-NaFe2(MoO4)(3) or beta-NaFe2(MoO4)(3) is studied. Taking into account the electronic correlation in the Hubbard model, it is shown that these compounds are semiconductors with the band gap of 1.2-2.6 eV. The sodium diffusion pathways in the alpha-NaFe2(MoO4)(3) and beta-NaFe2(MoO4)(3) structures are determined, and it is shown that the energy barrier in the compounds with the beta-NaFe2(MoO4)(3) structure is almost twice lower. The simulation of sodium extraction shows that these molybdates must have a high potential from 3 to 5 V and insignificant changes in the volume and the formation enthalpy depend on the sodium content. According to the obtained data, low-symmetry NaMFe(MoO4)(3) (M = Mn, Fe, Co, Ni, Zn) molybdates exhibit diffusion properties and cathode voltage similar to those in the compounds of the NASICON structural type and also can be promising materials for sodium-ion batteries.