Electrochemical and Structural Behavior of Trirutile-Derived FeF3 During Sodiation and Desodiation

Abstract

Despite significant advances in the Li-Fe-F systems for lithium-ion batteries, the investigation on the Na-Fe-F systems for sodium-ion batteries is still insufficient. Herein, trirutile-derived FeF₃ prepared through the electrochemical delithiation of trirutile Li₀.₅FeF₃ has been examined as the positive electrode of sodium-ion batteries at 90 °C with a thermally stable ionic liquid electrolyte to shed light on the structural evolutions occurring during sodiation–desodiation for the first time. Synchrotron X-ray diffraction revealed that the reversible topotactic extraction/insertion of 0.2 Na⁺ proceeds during cycling between 2.6 and 4.0 V, which triggers a two-phase reaction between tetragonal NaxFeF₃ and tetragonal FeF₃. A lower cutoff voltage of 2.3–4.0 V induces a partial structural transition from tetragonal FeF₃ into cubic FeF₃ along with cycling, where the topotactic Na⁺ extraction/insertion not only occurs in the tetragonal structure but also involves the reversible phase transformation between orthorhombic NaFeF₃ and cubic FeF₃ after several cycles.