Structure defects engineering in Prussian blue cathode materials for high-performance sodium-ion batteries

Prussian blue analogues (PBAs) have been considered as promising cathode materials for sodium-ion batteries (SIBs) due to high theoretical energy density and stable three-dimensional framework structure, while the existence of anionic vacancies and coordinated water have become the key scientific issues affecting electrochemical performances. Herein, PBA materials with different structural defects, constructed via regulating electronic state of Fe-ion, are regarded as cathode materials for SIBs. It is confirmed that anionic vacancies and coordinated water molecules gradually decrease with the increase of Fe^Ⅲ-ion with the weaker metal activity than Fe^Ⅱ. As a result, Fe^Ⅲ[Fe^Ⅲ(CN)₆] electrode with the lowest structural defects displays the highest reversible specific capacity of 121.4 mA h g⁻¹ with an energy density of 356.9 Wh kg⁻¹ at 10 mA g⁻¹ and the best cyclic stability compared with NaFe^Ⅲ[Fe^Ⅱ(CN)₆] and Na₂Fe^Ⅱ[Fe^Ⅱ(CN)₆]. Na-ion intercalation/deintercalation proceeds by a reversible phase transition mechanism (cubic↔cubic/rhombohedral↔rhombohedral) relied on 2-electron transfer per formula using Fe^Ⅱ/Fe^Ⅲ as redox sites, which is verified by ex situ XRD and XPS tests. Besides, high anionic vacancies and specific surface area boost the NaFe^Ⅲ[Fe^Ⅱ(CN)₆] electrode to exhibit fast Na-ion transfer ability, thus contributing superior rate property. This work elucidates a new technique strategy to regulate the structural defects and improve Na-ion storage properties of PBAs.