Theoretical Study of the Structural Evolution of a Na2FeMn(CN)6 Cathode upon Na Intercalation

The Prussian Blue analog, NaxFeMn(CN)6, is a potential new cathode material for Na-ion batteries. During Na intercalation, the dehydrated material exhibits a monoclinic to rhombohedral phase transition, while the hydrated material remains in the monoclinic phase. With density functional theory calculations, the phase transition is explained in terms of a competition between Coulomb attraction, Pauli repulsion, and d–π covalent bonding. The interstitial Na cations have a strong Coulomb attraction to the N anions in the host material, which tend to bend the Mn–N bonds and reduce the volume of the structure. The presence of lattice H2O enhances the Pauli repulsion so that the volume reduction is suppressed. The calculated volume change, as it depends upon the presence of lattice H2O, is consistent with experimental measurements. Furthermore, a new LiFeMn(CN)6 phase is predicted where MnN6 octahedra decompose into LiN4 and MnN4 edge-sharing tetrahedra.