Sodium ZincHexacyanoferrate: A Prussian Blue Analogwith Long Life

Abstract

As one of the cathode materials for sodium-ion batteries, Prussian blue analogs (PBAs) have the advantages of low cost, easy synthesis, and high theoretical capacity. However, the current dual electron reactions of Fe-, Mn-, and Co-based PBAs all have the disadvantage of poor cycling stability, and crystal water and defects are considered the main limiting factors. However, the stability of the crystal structure is crucial. PBAs formed by replacing Fe with some metals without redox activity can improve cycling stability, but they are mostly cubic and monoclinic phase structures and not the most stable. In this study, the Zn-substituted sodium zinc hexacyanoferrate (ZnHCF) was found to have a structure similar to the sodium superionic conductor (NASION) with large sodium-ion diffusion channels and excellent cyclic stability, with a retention rate of 95.3% for 8000 cycles at 850 mA/g. The energy storage mechanism of ZnHCF was analyzed through ex situ and in situ X-ray diffraction (XRD) and operando electrochemical impedance spectroscopy (EIS) as an insertion–extraction-type mechanism with low strain and excellent structural stability, which will provide a theoretical basis for subsequent modification studies on other PBAs.