Na0.97KFe(SO4)(2): an iron-based sulfate cathode material with outstanding cyclability and power capability for Na-ion batteries

In this work, a novel cathode material for Na-ion batteries, Na0.97KFe(SO4)(2), was successfully prepared via slow evaporation and a low-temperature process, and its outstanding electrochemical performance was demonstrated. Based on the structural information of KFe(SO4)(2) obtained from Rietveld refinement of X-ray diffraction data, the possible atomic sites for Na ions entering the structure were verified using bond-valence sum energy maps. Electrochemical measurements and X-ray absorption near-edge structure analyses revealed that approximate to 0.97 mol Na ions can be reversibly (de)intercalated into the structure via the Fe3+/Fe2+ redox reaction. The average redox potential of Na0.97KFe(SO4)(2) was shown to be approximate to 3.27 V (vs. Na+/Na), which is higher than that of other Fe-based phosphates owing to the inductive effect of (SO4)(2-). It was verified that the specific capacity of Na0.97KFe(SO4)(2) at C/10 was approximate to 85 mA h g(-1). At 10C (full charge/discharge in 12 min), approximate to 53% of its capacity measured at C/10 was retained. In addition, up to approximate to 99% of its initial capacity was retained over 200 cycles at 1C with a high coulombic efficiency of 99.3097%, which was attributed to the negligible volume change (approximate to 1.8%) during charge/discharge.