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 (VO 1− x PO 4 ) 2 F 1+2 x (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 (VO 1− x PO 4 ) 2 F 1+2 x (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 (VO 1− x PO 4 ) 2 F 1+2 x (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 (VO 1− x PO 4 ) 2 F 1+2 x (0 ≤ x ≤ 1) with special microarchitecture. Furthermore, it is found that NaH 2 PO 4 ·2H 2 O is the most appropriate phosphorus source, while VCl 3 , VOSO 4 · x H 2 O, and NH 4 VO 3 are the best choices of vanadium sources to prepare Na 3 (VO 1− x PO 4 ) 2 F 1+2 x (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 (VO 1− x PO 4 ) 2 F 1+2 x (0 ≤ x ≤ 1) with superior performance is provided.