Zinc-substituted Fe-based Prussian blue analogues induce a weak Jahn-Teller effect to enhance stability of sodium ion batteries

Prussian blue analogues (PBAs) have attracted wide attention as cathode materials for sodium-ion batteries. Nevertheless, the poor cycle performance caused by the [Fe(CN)6]4− defects and Jahn-Teller (J-T) effect hinders its practical application. Here, Zn-substituted PBAs were synthesized through co-precipitation method and studied as cathodes for Na-ion batteries. The synthesized Na1.14Zn0.39Fe0.61[Fe(CN)₆]·4.14H2O (NZFC) and Na1.18Zn0.42Fe0.58[Fe(CN)₆]·4.96H2O (NZFE) showed outstanding exceptional electrochemical properties, including high capacity (first capacity of 114.56 and 97.76 mAh·g−1 at 10 mA·g−1, respectively), excellent rate property (71.81 and 68.69 mAh·g−1 at 500 mA·g−1, respectively), and cycling stability (64.72 % and 80.27 %, respectively) after 350 cycles at 200 mA g−1, which is attribute to introduction of Zn element induced a weak Jahn-Teller effect and a low-spins and a high-spins FeII/FeIII redox sites coordinated by C and N atoms prompt reversible diffusion of sodium ions and without structural transformation in the Na-ion storage. Using combined experiment and theory via ex situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations, it is confirmed that the weak J-T distortion contributes significantly to fast-overall kinetics, structural stability, and high electronic conductivity of the electrode. These findings provide new insights into comprehending for the future development of high-performance and stable sodium ion batteries.