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

The Prussian Blue analog, Na_xFeMn(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.