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

Thursday, 9 October 2014: 11:40
Sunrise, 2nd Floor, Galactic Ballroom 1 (Moon Palace Resort)
P. Barpanda (Indian Institute of Science), G. Oyama (The University of Tokyo), C. D. Ling (The University of Sydney), and A. Yamada (The University of Tokyo)
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 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.