Understanding the Structural Evolution of the New Iron-Based Mixed-Phosphate Na4-XFe3(PO4)2(P2O7) in a Na Rechargeable Battery
Recently, we reported a new iron-based mixed-phosphate cathode, Na4Fe3(PO4)2(P2O7), which shows promising electrochemical properties in both Na- and Li-ion cells (H. Kim et al., J. Am. Chem. Soc. 2012, 134, 10369−103722). 7 However, a fundamental understanding of the electrochemical reaction mechanism and structural stability under various conditions, which is essential for further development of this promising cathode, had not yet been achieved. In this report, we clarify the electrochemical reaction of Na4Fe3(PO4)2(P2O7) in Na‑ion cells. We discovered that the electrode reaction is governed mainly by a one-phase reaction with a small volumetric change of less than 4%, despite significant distortion of pyrophosphate (P2O7) occurring upon electrochemical cycling. Furthermore, the partially desodiated phases are thermally stable at temperatures up to 530 °C at all states of charge. With its open framework, high voltage (~3.2 V vs. Na), low volumetric change (~4%) and safety characteristics, Na4Fe3(PO4)2(P2O7) should be an outstanding candidate for a Na rechargeable battery electrode.
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