Structural, Electronic, Mechanical, and Thermodynamic Properties of Na Deintercalation from Olivine NaMnPO(4): First-Principles Study

The impact of Na atom deintercalation on olivine NaMnPO(4) was investigated in a first-principle study for prospective use as cathode materials in Na-ion batteries. Within the generalized gradient approximation functional with Hubbard (U) correction, we used the plane-wave pseudopotential approach. The calculated equilibrium lattice constants are within 5% of the experimental data. The difference in equilibrium cell volumes for all deintercalated phases was only 6%, showing that NaMPO(4) is structurally more stable. The predicted voltage window was found to be between 3.997 and 3.848 V. The Na(1)MnPO(4) and MnPO(4) structures are likely to be semiconductors, but the Na(0).(75)MnPO(4), Na(0).(5)MnPO(4), and Na(0).(25)MnPO(4) structures are likely to be metallic. Furthermore, all independent elastic constants for Na(x)MPO(4) structures were shown to meet the mechanical stability requirement of the orthorhombic lattice system.