Density functional theory calculations applied to olivine-like NaMnPO4 with transition metal substitutions for energy storage applications

In this study, Density Functional Theory (DFT) calculations are applied to evaluate the structural and thermodynamic properties of MPO4 and NaMPO4 as cathode materials for sodium-ion batteries (SIBs). Using the modified Perdew-Burke-Ernzerhof (PBE) method and the projector augmented wave (PAW) method, the effect of metal substitution in MPO4 and NaMPO4 lattices (M = Mn, Fe, Co, Ni,), as well as Fe-Ni substituted NaMnPO4 was examined for its structural and electrochemical characteristics. As NaMnPO4 has less ionic and electronic conductor, the partial substitution of Mn by simultaneous Ni and Fe gives good physicochemical properties useful for good cathode materials in SIBs. For NaMn0.5Ni0.25Fe0.25PO4, its optimal values of gravimetric capacity (154 mAh.g(-1)), bandgap energy (0.45 eV) and intercalation potential (3.54 V) appeared very interesting to be an attractive cathode material for SIBs. Na+ diffusion required less energy in NaFePO4, NaMnPO4, and Fe-Ni co-doped NaMnPO4 systems, promoting a rapid recharge rate and good ionic conductivity thanks to the desodiation process in creating a mixed oxidation state particularly for Fe and Mn atoms.