Multiphase Transformation of NaFeF₃During Desodiation and Sodiation

The orthorhombic NaFeF3, which is envisioned to be an auspicious positive electrode for Na-ion batteries, has drawn considerable interest as an environmentally benign energy material with exceptionally high theoretical capacity. Despite these prospects, the reaction mechanism(s) during orthorhombic NaFeF3 operations are still not well understood. Thus, in a bid to expound on this research space, we report the reaction mechanism(s) of a carbon-coated orthorhombic NaFeF3 prepared through high-energy ball-milling and heat treatment processes. A thermally stable ionic liquid electrolyte at elevated temperatures is employed to maximize the utilization of NaFeF3. The orthorhombic NaFeF3 exhibits high electrochemical activity and long-term cycling stability of up to 400 cycles at 90 °C. Through a combination of galvanostatic intermittent titration technique and synchrotron X-ray powder diffraction measurements, we discover that the (de)sodiation processes are facilitated by a multiphase transformation mechanism. Further, we experimentally identify, for the first time, an orthorhombic Na0.5FeF3 compound as an intermediate phase of the transformations. The results discussed in this work are expected to provide invaluable insights for the future advancement of the Na–Fe–F system.