Na sub(2)FeP sub(2)O sub(7) 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 Na sub(2)FeP sub(2)O sub(7), the first example ever reported in the pyrophosphate family for SIBs, are investigated. Na sub(2)FeP sub(2)O sub(7) maintains well-defined channel structures (triclinic framework under the P1 space group) and exhibits a reversible capacity of approximately 90 mAh g super(-1) with good cycling performance. Both quasi-equilibrium measurements and first-principles calculations consistently indicate that Na sub(2)FeP sub(2)O sub(7) undergoes two kinds of reactions over the entire voltage range of 2.0-4.5 V (vs Na/Na super(+)): 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. Na sub(2)FeP sub(2)O sub(7) shows excellent thermal stability up to 500 degree C, even in the partially desodiated state (NaFeP sub(2)O sub(7)), which suggests its safe character, a property that is very critical for large-scale battery applications. Na sub(2)FeP sub(2)O sub(7) is reported as the first member in the pyrophosphate family for sodium battery cathodes. Utilizing the well-defined channel structure, Na sub(2)FeP sub(2)O sub(7) exhibits a reversible capacity of approximately 90 mAh g super(-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.