Sodium storage in Na-rich NaxFeFe (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 NxFeFe(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. (C) 2015 Elsevier Ltd. All rights reserved.