Na2FeP2O7 as a Promising Iron-Based Pyrophosphate Cathode for Sodium Rechargeable Batteries: A Combined Experimental and Theoretical Study

Considering the promising electrochemical performance of the recently reported pyrophosphate family in lithium ion batteries as well as the increasing importance of sodium ion batteries (SIBs) for emerging large‐scale applications, here, the crystal structure, electrochemical properties, and thermal stability of Na2FeP2O7, the first example ever reported in the pyrophosphate family for SIBs, are investigated. Na2FeP2O7 maintains well‐defined channel structures (triclinic framework under the P1 space group) and exhibits a reversible capacity of ≈90 mAh g−1 with good cycling performance. Both quasi‐equilibrium measurements and first‐principles calculations consistently indicate that Na2FeP2O7 undergoes two kinds of reactions over the entire voltage range of 2.0–4.5 V (vs Na/Na+): a single‐phase reaction around 2.5 V and a series of two‐phase reactions in the voltage range of 3.0–3.25 V. Na2FeP2O7 shows excellent thermal stability up to 500 °C, even in the partially desodiated state (NaFeP2O7), which suggests its safe character, a property that is very critical for large‐scale battery applications. Na2FeP2O7 is reported as the first member in the pyrophosphate family for sodium battery cathodes. Utilizing the well‐defined channel structure, Na2FeP2O7 exhibits a reversible capacity of ≈90 mAh g−1 with several different plateaus corresponding to distinctive Na sites. The thermodynamic and kinetic behaviors of this compound during battery operations are explained well using first principles calculations.