A New Rutile-Type NaFe₂F₆ Cathode for Sodium-Ion Batteries

The development of low-cost and sustainable cathode materials is crucial for advancing sodium-ion battery technology. This work proposes a new rutile-type NaFe₂F₆ cathode synthesized through an energy-efficient mechanochemical method without high-temperature processing. The rutile-type NaFe₂F₆ possess a unique crystal structure of edge-sharing MF₆ octahedra, forming a 1D Na⁺ diffusion channels with an ultralow migration barrier of 0.299 eV. In situ X-ray diffraction analysis demonstrates its exceptional structural stability during cycling, with only a 1.94% volume variation and a single-phase reaction mechanism without phase transitions. The NaFe₂F₆ cathode can deliver a high reversible capacity of 181 mAh g^-1 via a Fe²⁺/Fe³⁺ redox couple. Comprehensive kinetic analyses confirm superior Na⁺ transport properties compared to conventional FeF₃. Given the earth's crust abundance of Na and Fe elements, this work establishes NaFe₂F₆ as an environmentally sustainable and economically viable cathode material, paving the way for large-scale energy storage applications.