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

Prussian blue analogs (Na2FeFe(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-pi 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 Fe2+/Fe3+ and low-spin Fe2+/Fe3+ couples during desodiation. Moreover, the facile three-dimensional diffusion channels for Na+ ions exhibit low diffusion barriers of 0.4 eV similar to 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.

Automated summary

Prussian blue analogs Na2FeFe(CN)(6) are considered as potential cathode materials for sodium-ion batteries due to low-cost iron resources and open framework. Detailed first-principles calculations reveal a phase transition and high rate performance during Na ion extraction process.