Comparative Investigation of Na2FeP2O7 Sodium Insertion Material Synthesized by Using Different Sodium Sources

Na2FeP2O7 powders were synthesized by a solid-state method by using different sodium sources (Na2CO3, NaH2PO4, and NaOH), and their solid-state reaction mechanism and the electrochemical properties of the synthesized Na2FeP2O7 were studied in detail. The results indicate that NaH2PO4 is conducive to the growth of the crystal surface with a wider spacing, which contributes to sodium-ion intercalation and deintercalation. Na2FeP2O7 synthesized at 600 °C by using NaH2PO4 exhibits the best electrochemical performance among them, showing an initial discharge capacity of 82.8, 79.4, 77.9, 75.3, 68.8, and 47.8 mAh g–1 under the cycle rate of C/10, C/5, C/2, 1C, 2C, and 5C, respectively. Moreover, a reversible capacity of 81.0 mAh g–1 at C/10, about 83.5% of the theoretical capacity, can be achieved after different cycle rates. The capacity retention is about 93.3% after 140 cycles under C/2. The results indicate that it is very important to choose the right sodium sources to synthesize Na2FeP2O7 with a good perf...