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

► Discovery of a new 3 V sodium-based cathode material Na2FeP2O7. ► Na2FeP2O7 is the first reported sodium-based pyrophosphate class of cathode. ► Na2FeP2O7 has excellent rate kinetics and reversible capacity. ► Na2FeP2O7 has the lowest volume change upon cycling among all Na-based cathodes.