An open-system synthesis approach to achieve high-rate Na3(VO)2(PO4)2F/C microcubes cathode for sodium-ion batteries

Sodium vanadium fluorophosphate (Na3(VO)2(PO4)2F) is a promising cathode material for high-energy sodium-ion batteries (SIBs) due to high working potential, large specific capacity and robust structural framework, however being confronted with sluggish reaction kinetics and low electrochemical reversibility. In this study, a new open-system synthesis approach is proposed to achieve carbon-encapsulated Na3(VO)2(PO4)2F microcubes (demoted as NVPF/C) with enhanced reaction kinetics and electrochemical reversibility, which involves in-situ growth of NVPF crystals on graphene oxides (NVPF/GO) in aqueous reaction systems and subsequent conversion of NVPF/GO into conductive NVPF/C. The as-synthesized NVPF/C material is structurally and electrochemically characterized with X-ray diffraction, X-ray photoelectron spectroscopy, FTIR spectroscopy, scanning electron microscope, transmission electron microscope, and electrochemical measurements. Experimental results suggest that the material is composed of carbon-coated NVPF crystals with an average particle size of ∼ 2 μm and a carbon-layer thickness of ∼ 5 nm. It exhibits outstanding rate capability with impressive reversible capacities of 118.9 mAh g−1 at 0.1C and 87.9 mAh g−1 at 10C, and excellent cycling performance with 79.4 % retention after 500 cycles at 2C. The finding in this work indicates the open-system synthesis approach not only is suitable for large-scale production but also can achieve advanced NVPF/C cathode material for SIBs.