Facile synthesis and cycling performance maintenance of iron hexacyanoferrate cathode for sodium-ion battery

Abstract

In this work, sodium-rich iron hexacyanoferrate Na1.74Fe[Fe(CN)6]0.90·2.07H2O (NaFeHCF) with a cubic structure is firstly prepared via a bottom-up approach. The electrochemical reaction mechanism and cycling performance maintenance of as-prepared NaFeHCF are carefully investigated. It is found that there exists some irreversible transformation of sodium positions in iron hexacyanoferrate during electrochemical cycling. Due to the highly itinerant electron state between high spin FeHS and low spin FeLS during redox reaction, the electron transfer in the FeLS3+/FeLS2+ at above 4 V gradually shift to that in the FeHS3+/FeHS2+ at about 3.35 V. However, such a dynamic transformation is not favorable for cycling stability, as more sodium content left in the 24d positions of cubic structure helps stabilize the framework and alleviate the capacity fading. The effects of the cut-off potential, the activated rate at 1st cycle, and the temperature are carefully investigated to find an operating window with insignificant transformation, where the sodium-rich iron hexacyanoferrate presents high cycling stability at 25 °C (with a capacity retention of 84.1% for 500 cycles at the current density of 600 mAg−1).