The rapid microwave-assisted hydrothermal synthesis of NASICON-structured Na 3 V 2 O 2x (PO 4 ) 2 F 3−2x (0 < x ≤ 1) cathode materials for Na-ion batteries

NASICON-structured Na 3 V 2 O 2x (PO 4 ) 2 F 3−2x (0 < x ≤ 1) solid solutions have been prepared using a microwave-assisted hydrothermal (MW-HT) technique. Well-crystallized phases were obtained for x = 1 and 0.4 by reacting V 2 O 5 , NH 4 H 2 PO 4 , and NaF precursors at temperatures as low as 180–200 °C for less than 15 min. Various available and inexpensive reducing agents were used to control the vanadium oxidation state and final product morphology. The vanadium oxidation state and O/F ratios were assessed using electron energy loss spectroscopy and infrared spectroscopy. According to electron diffraction and powder X-ray diffraction, the Na 3 V 2 O 2x (PO 4 ) 2 F 3−2x solid solutions crystallized in a metastable disordered I 4/ mmm structure ( a = 6.38643(4) Å, c = 10.62375(8) Å for Na 3 V 2 O 2 (PO 4 ) 2 F and a = 6.39455(5) Å, c = 10.6988(2) Å for Na 3 V 2 O 0.8 (PO 4 ) 2 F 2.2 ). With respect to electrochemical Na + (de)insertion as positive electrodes (cathodes) for Na-ion batteries, the as-synthesized materials displayed two sloping plateaus upon charge and discharge, centered near 3.5–3.6 V and 4.0–4.1 V vs. Na + /Na, respectively, with a reversible capacity of ∼110 mA h g −1 . The application of a conducting carbon coating through the surface polymerization of dopamine with subsequent annealing at 500 °C improved both the rate capability (∼55 mA h g −1 at a discharge rate of 10C) and capacity retention (∼93% after 50 cycles at a discharge rate of C/2).