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

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 degrees 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.