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.

(1)           Padhi, A. K.; Nanjundaswamy, K. S.; Masquelier, C.; Goodenough, J. B. Mapping of Transition Metal Redox Energies in Phosphates with NASICON Structure by Lithium Intercalation. J. Electrochem. Soc. 1997, 144 (8), 2581–2586.

(2)           Masquelier, C.; Croguennec, L. Polyanionic (Phosphates, Silicates, Sulfates) Frameworks as Electrode Materials for Rechargeable Li (or Na) Batteries. Chem. Rev. 2013, 113 (8), 6552–6591.

(3)           Reynaud, M.; Ati, M.; Melot, B. C.; Sougrati, M. T.; Rousse, G.; Chotard, J.-N.; Tarascon, J.-M. Li₂Fe(SO₄)₂ as a 3.83 V Positive Electrode Material. Electrochem. Commun. 2012, 21, 77–80.

(4)           Ati, M.; Melot, B. C.; Chotard, J.-N.; Rousse, G.; Reynaud, M.; Tarascon, J.-M. Synthesis and Electrochemical Properties of Pure LiFeSO₄F in the Triplite Structure. Electrochem. Commun. 2011, 13 (11), 1280–1283.

(5)           Recham, N.; Rousse, G.; Sougrati, M. T.; Chotard, J.-N.; Frayret, C.; Mariyappan, S.; Melot, B. C.; Jumas, J.-C.; Tarascon, J.-M. Preparation and Characterization of a Stable FeSO ₄ F-Based Framework for Alkali Ion Insertion Electrodes. Chem. Mater. 2012, 24 (22), 4363–4370.

(6)           Lander, L.; Rousse, G.; Abakumov, A. M.; Sougrati, M.; Tendeloo, G. van; Tarascon, J.-M. Structural, Electrochemical and Magnetic Properties of a Novel KFeSO4F Polymorph. J. Mater. Chem. A 2015, 3 (39), 19754–19764.