Multiphase Transformation of NaFeF₃ During 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 Na_0.5FeF₃ 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.