Understanding the Structural Evolution of the New Iron-Based Mixed-Phosphate Na4-XFe3(PO4)2(P2O7) in a Na Rechargeable Battery

Storing the energy from renewable energy sources has become a critical issue in recent years. In order to develop a large-scale energy storage system (ESS) connected to green and sustainable energy beyond the conventional size of batteries, cost effectiveness must be the foremost requirement. For a large-scale ESS, battery electrode materials that are based on earth-abundant, readily available, and low-cost elements are highly desired. In this regard, using electrochemistry that utilizes an iron-based redox reaction combined with Na guest ions would be an optimal choice for such batteries because of the unlimited availability of Na from seawater and the ready availability of iron. However, only a limited number of such materials have been reported to date (NaFePO₄, Na₂FePO₄F, NaFeSO₄F, and Na₂FeP₂O₇). ^1–6

 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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