Kröhnkite Na2M(SO4)2.2H2O (M = 3d metals) Family of Sulphate Cathodes for Sodium-Ion Batteries

Banking upon Sodium, the fifth most-abundant element, materials chemists have recently given renewed effort to develop large-scale and economic Na-ion batteries. Similar to Li-ion batteries, the battery community has explored many possible oxides and oxyanionic insertion systems with great success. In the quest to unravel new positive insertion system, our group has discovered Na₂MP₂O₇ pyrophosphate cathodes with redox potentials ranging from 2.5 to 3.8 V (Adv. Energy Mater., 2, 841, 2012). In an effort to further increase the redox potential, one possible way is to replace the PO₄ units by more electronegative SO₄ units, where the role of inductive effect triggers potential up shift.

Using the sulphate chemistry, we have discovered a new Fe-based cathode material, namely Na₂Fe^II(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 Na₂Fe^II(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.