Sodium iron pyrophosphate: A novel 3.0 V iron-based cathode for sodium-ion batteries

Extending the pyrophosphate chemistry for rechargeable Na-ion batteries, here we report the synthesis and electrochemical characterization of Na2FeP2O7, a novel Fe-based cathode material for sodium batteries. Prepared by conventional solid-state as well as solution-combustion synthesis (at 600 °C), the Na2FeP2O7 adopts a triclinic structure (space group: P-1) with three-dimensional channels running along [100], [−110] and [01-1] directions. With no further optimization, the as-synthesized Na2FeP2O7 cathode was found to be electrochemically active, delivering a reversible capacity of 82 mAh·g−1 with the Fe3+/Fe2+ redox potential centered around 3 V (vs. Na/Na+). With its theoretical capacity of ~100 mAh·g−1 and potential high rate-capability, Na2FeP2O7 forms a promising novel cathode material, with the distinction of being the first ever pyrophosphate-class of cathode for sodium-ion batteries. Keywords: Na-ion batteries, Cathode, Pyrophosphate, Na2FeP2O7