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