Synthesis of Na2FePO4F/C and its electrochemical performance

Using NaF as the only Na precursor, fluorophosphate Na2FePO4F/C materials have been synthesized via the solid-state reaction. Na2FePO4F starts to form at 300 °C, becomes the sole crystalline form between 400 and 600 °C, and decomposes at 650 °C. Comparing with the theoretical capacity of 124 mAh g−1, the sample prepared at 600 °C delivered a discharge capacity of 118 mAh g−1. The outstanding electrochemical performance is believed to result from the good crystallization and high purity of the synthesized materials. The capacity retentions at 0.5 C and 2 C are 95.8% and 89.8%, respectively, of that at 0.05 C. Furthermore, a discharge capacity of 122 mAh g−1 is maintained under the cycling between 2.0 and 5.2 V vs. Li/Li+, indicating that the second Na+ is not extracted from the Na2FePO4F lattice.