Kröhnkite Na2M(SO4)2.2H2O (M = 3d metals) Family of Sulphate Cathodes for Sodium-Ion Batteries
Using the sulphate chemistry, we have discovered a new Fe-based cathode material, namely Na2FeII(SO4)2.2H2O (Chem. Mater., 26, 1297, 2014). It can be easily synthesized by simple dissolution and precipitation mechanism involving heat treatment below 80 °C. Unknown till date, this new compound assumes the kröhnkite mineral structure containing Fe Octahedra linked with two structural H2O units. It offers nice one dimensional tortuous channel for Na-ion migration (see the Figure). Upon electrochemical cycling at C/20 rate (at 25 °C), the as-synthesized compound delivers capacity approaching 80 mAh/g with the average Fe3+/Fe2+ redox potential located at 3.25 V. It is a relatively high voltage operation for Fe-based cathodes in sodium batteries. We will describe various aspects of synthetic, structural and electrochemical properties of novel Na2FeII(SO4)2.2H2O cathode. Further, exploring this system, we have discovered yet another Fe-based sulphate polyanionic system, which offers high rate performance with high operating potential above 3.5 V. We will update the current state-of-the-art on SO4-based polyanionic cathode systems for sodium-ion batteries.