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

A new mixed anion compound, Na2Fe(C2O4)F2, has been prepared by hydrothermal synthesis. The crystal structure exhibits infinite chains of corner-linked FeII-centered octahedra, with coordination composed of both oxalate and fluoride ligands. This compound exhibits promising reversible lithium and sodium insertion. On extended cycling, Na2Fe(C2O4)F2 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.