Mechanochemical synthesis and electrochemical behavior of Na ₃FeF ₆ in sodium and lithium batteries

We report Na ₃FeF ₆ as a novel cathode material for sodium and lithium batteries. The material is synthesized through a mechanochemical process by reacting precursors (NaF and FeF ₃) in a ball mill under argon flow. These syntheses are environmentally friendly and safe because the entire processes do not require any hazardous gasses or high temperatures. XRD spectra confirm that the synthesized material has a well-defined crystal structure and can be indexed to a monoclinic crystal structure under the P2 ₁/C space group. Thermal analyses up to 500°C confirm the stability of the crystal structure and safety of this material. Electrochemical results indicate that Na ₃FeF ₆ is electrochemically active in both Na and Li cells while undergoing conversion reactions and exhibiting decent reversible capacities of larger than 100 and 200 mAh g ^{- 1} at room temperature, respectively.