Structural Evolution, Redox Mechanism, and Ionic Diffusion in Rhombohedral Na2FeFe(CN)6 for Sodium-Ion Batteries: First-Principles Calculations

Prussian blue analogs (Na 2 FeFe(CN) 6 ) have been regarded as potential cathode materials for sodium-ion batteries (SIBs) due to their low-cost iron resources and open framework. Herein, the detailed first-principles calculations have been performed to investigate the electrochemical properties of Na x FeFe(CN) 6 during Na ion extraction. The material undergoes a phase transition from a dense rhombohedral to open cubic structure upon half-desodiation, which is resulted from competition of the Na−N Coulomb attraction and d− π covalent bonding of Fe−N. The analyses on the density of states, magnetic moments and Bader charges of Na x FeFe(CN) 6 reveal that there involve in the successive redox reactions of high-spin Fe 2+ /Fe 3+ and low-spin Fe 2+ /Fe 3+ couples during desodiation. Moreover, the facile three-dimensional diffusion channels for Na + ions exhibit low diffusion barriers of 0.4 eV ∼ 0.44 eV, which ensures a rapid Na + transport in the Na x FeFe(CN) 6 framework, contributing to high rate performance of the battery. This study gives a deeper understanding of the electrochemical mechanisms of Na x FeFe(CN) 6 during Na + extraction, which is beneficial for the rational design of superior PBA cathodes for SIBs.