Na₂Fe(C₂O₄)F₂: A New Iron-Based Polyoxyanion Cathode for Li/Na Ion Batteries

A new mixed anion compound, Na 2 Fe(C 2 O 4 )F 2, has been prepared by hydrothermal synthesis. The crystal structure exhibits infinite chains of corner-linked Fe II -centered octahedra, with coordination composed of both oxalate and fluoride ligands. This compound exhibits promising reversible lithium and sodium insertion. On extended cycling, Na 2 Fe(C 2 O 4 )F 2 is capable of reversibly inserting 0.67 Li + or 0.56 Na + per formula unit up to 50 cycles at the average discharge voltages of 3.3 and 3.0 V, respectively. This represents arguably the best performance as a prospective cathode material so far observed among oxalates and is comparable to many known iron phosphate-based cathode materials.