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. Additionally, a new LiFeMn(CN)6 phase is predicted where MnN6 octahedra decompose into LiN4 and MnN4 edge-sharing tetrahedra. ■ INTRODUCTION Cathode materials for Li and Na batteries have attracted a great deal of interest for renewable energy applications.1−3 They are also of interest scientifically, owing to the rich chemistry associated with ion intercalation processes. For example, intercalation of Li or Na ions can be coupled with displacive transformations of the host structures, resulting in complicated phase transitions and multiple voltage plateaus during electrochemical cycling.4−6 The Prussian blue analog (PBA), NaxFeMn(CN)6, has recently been discovered as a good cathode material for Na-ion batteries for its high gravimetric energy density and long cycle life. The actual structure of the active material, however, is under debate since different crystal structures have been reported due to small differences in the synthetic methodology. Song et al. found that the amount of lattice H2O plays an important role in the structural variation. By completely dehydrating the material, two distinct phases are confirmed at different Na concentrations, a Na-poor monoclinic phase and a Na-rich rhombohedral phase. However, when fully hydrated, the structure of NaxFeMn(CN)6·2H2O remains monoclinic over the entire range of Na concentration. These structural differences further affect the charge/discharge voltages. Understanding the atomic scale mechanism of these transformations is difficult solely from experiment. Even neutron diffraction, which is a powerful method for determining the structure of materials with light elements, has problems determining the Na occupation sites in the PBA and distinguishing atoms of Fe from Mn. In this paper, theoretical studies are conducted to determine the structural changes in PBA during Na charge/discharge with consideration of the presence of lattice H2O. Then, the underlying forces that influence the phase transitions are analyzed. In brief, during Na intercalation, the Coulomb attraction between Na and N anions tend to shrink the volume and stabilize the rhombohedral phase. The d−π covalent bonding between Mn and N has the opposite effect and favors the linear structures found in the monoclinic phase with the larger volume. The introduction of H2O into the lattice increases the Pauli repulsion, favoring the larger-volume monoclinic phase. The latter two forces together can eliminate the presence of the dense rhombohedral phase over the entire Na intercalation range. Finally, the correlation between the experimentally measured charging voltage and the host crystal structure is rationalized in this study with our computational model. ■ COMPUTATIONAL METHOD Calculations based on density functional theory (DFT) were performed using the Vienna Ab-initio Simulation Package (VASP) with a plane-wave basis set and the projector augmented wave method to account for core electrons.10−12 Two levels of theory were employed for the exchange correlation energy. First the general gradient approximation having the PW91 functional with a Hubbard on-site U term (GGA+U) on the transition metal d orbitals was used for structural optimization. The DFT+U method is necessary to avoid artificial delocalization of electronic states, such as 3d electrons on the metal centers, as a result of the self-interaction error that is present in pure DFT. Effective U values (Ueff) of 4.3 eV for Fe and 5.0 eV for Mn were taken from the literature, originally developed for olivine LiFePO4 and LiMnPO4. 15−17 Soft potentials for C and N were employed with an energy cutoff of 360 eV for the plane-wave basis set. No significant difference was found with tests using harder potentials Received: March 26, 2015 Revised: May 1, 2015 Published: May 12, 2015 Article