The Rapid Microwave-assisted Hydrothermal Synthesis of NASICON-structured NaVO (PO)F (0 < ≤ 1) Cathode Materials for Na-ion Batteries

NASICON-structured NaVO (PO)F (0 < ≤ 1) solid solutions have been prepared using a microwave-assisted hydrothermal (MW-HT) technique. Well-crystallized phases were obtained for = 1 and 0.4 by reacting VO, NHHPO, 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 NaVO (PO)F solid solutions crystallized in a metastable disordered 4/ structure ( = 6.38643(4) Å, = 10.62375(8) Å for NaVO(PO)F and = 6.39455(5) Å, = 10.6988(2) Å for NaVO(PO)F). 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 Na/Na, respectively, with a reversible capacity of ∼110 mA h g. 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 at a discharge rate of 10C) and capacity retention (∼93% after 50 cycles at a discharge rate of C/2).