Structural, Electronic, Mechanical, and Thermodynamic Properties of Na Deintercalation from Olivine NaMnPO₄:...

The impact of Na atom deintercalation on olivine NaMnPO₄ 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 NaMPO₄ is structurally more stable. The predicted voltage window was found to be between 3.997 and 3.848 V. The Na₁MnPO₄ and MnPO₄ structures are likely to be semiconductors, but the Na₀.₇₅MnPO₄, Na₀.₅MnPO₄, and Na₀.₂₅MnPO₄ structures are likely to be metallic. Furthermore, all independent elastic constants for Na_xMPO₄ structures were shown to meet the mechanical stability requirement of the orthorhombic lattice system.