Sodium storage in Na-rich Na x FeFe(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 NaxFeFe(CN)6 nanocubes as cathode materials for SIBs and investigate their sodium storage mechanism. Among them, Na-rich Na1.70FeFe(CN)6 shows highly reversible electrochemical reactions, delivering a capacity as high as 120.7 mA h g−1 at a current density of 200 mA g−1; even at 1200 mA g−1, the capacity still retains up to 73.6 mA h g−1. The first-cycle coulombic efficiency strongly depends on the sodium content in NaxFeFe(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.