Sodium storage in Na-rich Na_xFeFe(CN)₆ nanocubes

Na-rich prussian blue analogues (PBs) with high coulombic efficiency are of fundamental and technological importance for sodium-ion batteries (SIBs). Here we report high-quality Na_xFeFe(CN)₆ nanocubes as cathode materials for SIBs and investigate their sodium storage mechanism. Among them, Na-rich Na_1.70FeFe(CN)₆ shows highly reversible electrochemical reactions, delivering a capacity as high as 120.7mAhg^-1 at a current density of 200mAg^-1; even at 1200mAg^-1, the capacity still retains up to 73.6mAhg^-1. The first-cycle coulombic efficiency strongly depends on the sodium content in Na_xFeFe(CN)₆. Experimental results and first-principle calculations demonstrate that sodium cations in the large cavities of PBs have a priority to occupy the 8c site, while in the Na-rich samples, Na⁺ ions can be pushed into other 24d site. We believe that our findings can provide new insights into sodium storage mechanism for the PBs.