Comprehensive Studies on the Hydrothermal Strategy for the Synthesis of Na3(VO1-xPO4)2F1+2x (0 ≤ x ≤ 1) and their Na-Storage Performance

Sodium vanadium fluorophosphate, Na-3(VO1-xPO4)(2)F1+2x (0 <= x <= 1), is proved to be a series of very promising cathode materials for Na-ion batteries, owing to the high operating voltage and structural stability. However, the selective preparation of phase-pure Na-3(VO1-xPO4)(2)F1+2x (0 <= x <= 1) with outstanding electrochemical performance still remains a big challenge. Moreover, there are lots of discrepancies concerning the Na-storage properties, including a diversity of voltage profiles and capacities reported, based on the variety of synthetic methods. Herein, a comprehensive investigation is conducted about the fabrication of phase-pure Na-3(VO1-xPO4)(2)F1+2x (0 <= x <= 1) by employing different starting materials and corresponding synthetic conditions, based on a facile low-temperature hydrothermal strategy. Results show that the pH in the reaction system may be of vital importance for the successful preparation of Na-3(VO1-xPO4)(2)F1+2x (0 <= x <= 1) with special microarchitecture. Furthermore, it is found that NaH2PO4 center dot 2H(2)O is the most appropriate phosphorus source, while VCl3, VOSO4 center dot xH(2)O, and NH4VO3 are the best choices of vanadium sources to prepare Na-3(VO1-xPO4)(2)F1+2x (0 <= x <= 1) with superior Na-storage performance. This is the first time that the underlying correlations between the starting materials and the Na-storage performance, are investigated and further that guidance for the selective synthesis of phase-pure Na-3(VO1-xPO4)(2)F1+2x (0 <= x <= 1) with superior performance is provided.