Isostructural Synthesis of Iron-Based Prussian Blue Analogs for Sodium-Ion Batteries

Rechargeable sodium ion batteries (SIBs) have promising applications in large-scale energy storage systems. Iron-based Prussian blue analogs (PBAs) are considered as potential cathodes owing to their rigid open framework, low-cost, and simple synthesis. However, it is still a challenge to increase the sodium content in the structure of PBAs and thus suppress the generation of defects in the structure. Herein, a series of isostructural PBAs samples are synthesized and the isostructural evolution of PBAs from cubic to monoclinic after modifying the conditions is witnessed. Accompanied by, the increased sodium content and crystallinity are discovered in PBAs structure. The as-obtained sodium iron hexacyanoferrate (Na_1.75 Fe[Fe(CN)₆ ]_0.9743 ·2.76H₂ O) exhibits high charge capacity of 150 mAh g^-1 at 0.1 C (17 mA g^-1 ) and excellent rate performance (74 mAh g^-1 at 50 C (8500 mA g^-1 )). Moreover, their highly reversible Na⁺ ions intercalation/de-intercalation mechanism is verified by in situ Raman and Powder X-ray diffraction (PXRD) techniques. More importantly, the Na_1.75 Fe[Fe(CN)₆ ]_0.9743 ·2.76H₂ O sample can be directly assembled in a full cell with hard carbon (HC) anode and shows excellent electrochemical performances. Finally, the relationship between PBAs structure and electrochemical performance is summarized and prospected.