NaFe₃(HPO₃)₂((H,F)PO₂OH)₆: A Potential Cathode Material and a Novel Ferrimagnet

A novel iron fluorophosphite, NaFe₃(HPO₃)₂­((H,F)­PO₂OH)₆, was synthesized by a dry low-temperature synthesis route. The phase was shown to be electrochemically active for reversible insertion of Na⁺ ions, with an average discharge voltage of 2.5 V and an experimental capacity at low rates of up to 90 mAhg⁻¹. Simple synthesis, low-cost materials, excellent capacity retention, and efficiency suggest this class of material is competitive with similar oxyanion-based compounds as a cathode material for Na batteries. The characterization of physical properties by means of magnetization, specific heat, and electron spin resonance measurements confirms the presence of two magnetically nonequivalent Fe³⁺ sites. The compound orders magnetically at T_C ≈ 9.4 K into a state with spontaneous magnetization.