Na2Fe(C2O4)(HPO4): a promising new oxalate-phosphate based mixed polyanionic cathode for Li/Na ion batteries

The new iron-based mixed polyanionic material Na₂Fe(C₂O₄)(HPO₄) has been synthesized by a hydrothermal technique. The compound contains four Na sites with a three-dimensional crystal structure. This compound shows promising reversible Li and Na insertion properties as a cathode material. The redox potentials observed were ∼3.2 V (vs Li⁺/Li) for the Li-ion cell and ∼3.1 V (vs Na⁺/Na) for the Na-ion cell with Fe^2+/3+ redox reactions. The reversible electrode operation was found to deliver 71 and 104 mAh g^-1 specific capacities in Li and Na half cells, respectively. This present study reveals promising performance from a mixed oxalate-phosphate based polyanionic material and opens up further possibilities for materials discovery.