Structural, Electrochemical and Magnetic Properties of a Novel KFeSO4f Polymorph
Authors: Lander Laura , Rousse Gwenaelle , 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 On this quest, our group explored the wide family of fluorosulfate-based compounds, where the most prominent member is LiFeSO4F crystallizing either in a tavorite or triplite crystal structure with the latter presenting a potential of 3.9 V vs Li+/Li0.3

In order to further investigate the rich crystal chemistry offered by 3d-metal-based fluorosulfates, we studied the effect of the replacement of Li by other alkali metals such as Na and K. One of the so discovered phases was KFeSO4F, which adopts an orthorhombic structure analogue to KTiOPO4 (KTP) and from which K+ ions can be extracted in a complex electrochemical process.4

During the synthesis of the KTP-like KFeSO4F, a secondary phase was observed at lower temperature, which turned out to be a new KFeSO4F polymorph.5 Using combined synchrotron and neutron powder diffraction as well as electron diffraction, it is shown that the new 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 suggest that the K+ ions, which are located between the layers, are mobile within the structure and indeed, they 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 more the richness of polymorphisms in sulfate-based materials and we hereby want to encourage the further exploration of their interesting electrochemical and physical properties.
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