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

Abstract 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 2 FeP 2 O 7 , the first example ever reported in the pyrophosphate family for SIBs, are investigated. Na 2 FeP 2 O 7 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 Na 2 FeP 2 O 7 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. Na 2 FeP 2 O 7 shows excellent thermal stability up to 500 °C, even in the partially desodiated state (NaFeP 2 O 7 ), which suggests its safe character, a property that is very critical for large‐scale battery applications.