Single-Phase Lithiation in Iron Hydroxy Fluorides with Pyrochlore Structure

Julian F Baumgärtner; Dragos C Stoian; Kenneth P Marshall; Mohammad Jafarpour; Matthias Klimpel; Huanyu Zhang; Faruk Okur; Wouter van Beek; Dmitry Chernyshov; Sina Abdolhosseinzadeh; Michael Wörle; Maksym V Kovalenko; Kostiantyn V Kravchyk

doi:10.1021/acsenergylett.5c00218

Single-Phase Lithiation in Iron Hydroxy Fluorides with Pyrochlore Structure

ACS Energy Lett. 2025 Feb 6;10(2):1082-1088. doi: 10.1021/acsenergylett.5c00218. eCollection 2025 Feb 14.

Authors

Julian F Baumgärtner^{1

2}, Dragos C Stoian³, Kenneth P Marshall³, Mohammad Jafarpour^{2

4}, Matthias Klimpel^{1

2}, Huanyu Zhang^{1

2}, Faruk Okur^{1

2}, Wouter van Beek³, Dmitry Chernyshov³, Sina Abdolhosseinzadeh^{2

4}, Michael Wörle¹, Maksym V Kovalenko^{1

2}, Kostiantyn V Kravchyk^{1

2}

Affiliations

¹ Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, CH-8093 Zürich, Switzerland.
² Laboratory for Thin Films and Photovoltaics, Laboratory for Functional Polymers, Empa - Swiss Federal Laboratories for Materials Science & Technology, CH-8600 Dübendorf, Switzerland.
³ Swiss-Norwegian Beam Lines at the European Synchrotron Radiation Facility, Grenoble 38000, France.
⁴ Institute of Materials Science and Engineering, Swiss Federal Institute of Technology Lausanne, CH-1015 Lausanne, Switzerland.

Abstract

3D transition metal fluorides have long been recognized as appealing low-cost, high-energy-density cathode materials for Li-ion batteries, but their conversion-type lithiation mechanism induces structural and morphological changes, limiting their cycling stability. Our findings now suggest that metal fluorides may undergo single-phase lithiation when crystallized in a pyrochlore structure, enabled by the presence of Li-ion storage sites within interconnected hexagonal channels. By conducting a detailed analysis of pyrochlore iron(III) hydroxy fluorides during lithiation using operando X-ray absorption spectroscopy, X-ray total scattering, and electron microscopy, we provide evidence for a possible single-phase lithiation mechanism and robust structural stability. These results challenge the traditional view of conversion-type lithiation in metal fluorides and highlight their potential for achieving high cycling stability and eventual commercialization in Li-ion batteries.