Monoclinic Fe2(SO4)3: A new Fe-based cathode material with superior electrochemical performances for Na-ion batteries

We report monoclinic Fe-2(SO4)(3) as a novel high-voltage cathode material of Na-ion batteries. The detailed structural information of monoclinic Fe-2(SO4)(3) are verified through Rietveld refinement and bond-valance sum energy mapping with X-ray diffraction analyses. Monoclinic Fe-2(SO4)(3) delivers a large specific capacity of similar to 120 mAh g(-1) corresponding to similar to 1.8 mol Na intercalation into the structure and it exhibits the average operation voltage of similar to 3.25 V (vs. Na+/Na). Even at 5C, monoclinic Fe-2(SO4)(3) exhibits retention of similar to 76% of the capacity measured at C/10, indicating its outstanding power capability. Moreover, monoclinic Fe-2(SO4)(3) maintains up to similar to 78% of the initial capacity at 1C over 1000 cycles, with a high Coulombic efficiency of similar to 99%. This finding demonstrates the excellent cyclability of monoclinic Fe-2(SO4)(3), which results from the small volume change (similar to 1%) during the Na de/intercalation process.