Synthesis and Characterization of a Novel KFeSO4 F Polymorph: Structural, Electrochemical and Magnetic Properties
Authors: Lander Laura , Rousse Gwenealle , Abakumov Artem M. , Sougrati Moulay-Tahar , Van Tendeloo Gustaaf , Tarascon Jean-Marie 
Published: February 2020

Since the discovery of the promising electrode material LiFePO4, recent research has been focusing on the development of new iron-based polyanionic materials for next generations Li- and Na-ion batteries displaying better performances while still preserving cost and sustainability benefits.1,2 Towards this  quest, our group explored the wide family of sulfate-based compounds, with  the most prominent members being monoclinic Li2Fe(SO4)2 and triplite LiFeSO4F having redox potentials of 3.83 V and 3.9 V vs. Li+/Li0, respectively.3,4

In order to further investigate the rich crystal chemistry offered by 3d-metal-based fluorosulfates, we explored the feasibility of using other alkali metals such as Na and K instead of Li. Through this approach  we  discovered a KFeSO4F  phase, which adopts a KTiOPO4 structure and releases K+ ions via  a complex electrochemical process.5

Knowing that sulfate-based compounds are prone to polymorphism, we recently unveiled a new low-temperature KFeSO4F polymorph.6 Using combined synchrotron and neutron powder diffraction as well as electron diffraction, it was shown that the compound adopts a complex layered-like structure that crystallizes in a large monoclinic unit cell. Impedance measurements together with the Bond Valence Energy Landscape approach show that the K+ ions, which are located between the layers, are mobile within the structure and can be electrochemically removed at an average potential of 3.7 V vs. Li+/Li0. Lastly, neutron diffraction experiments coupled with SQUID measurements reveal a long range antiferromagnetic ordering of the Fe2+ magnetic moments. These results confirm once again the richness of polymorphisms in sulfate-based materials, which, besides unusual electrochemical properties, show interesting physical properties.

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