High voltage cathode materials for Na-ion batteries of general formula Na3V2O2x(PO4)2F3−2x

Abstract

Different samples of the sodium–vanadium fluorophosphate cathodic materials have been synthesized via the hydrothermal method, varying the type and content of carbon used in the synthesis. Structural characterization of the composites was performed by powder X-ray diffraction. Magnetic susceptibility measurements and EPR (Electron Paramagnetic Resonance) polycrystalline spectra indicate that some of the samples exhibit V³⁺/V⁴⁺ mixed valence, with the general formula Na₃V₂O_2x(PO₄)₂F_3−2x where 0 ≤ x < 1. The morphology of the materials was analyzed by Transmission Electron Microscopy (TEM). A correlation between the type and content of carbon with the electrochemical behavior of the different samples was established. Electrochemical measurements conducted using Swagelok-type cells showed two voltage plateaux at 3.6 and 4.1 V vs. Na/Na⁺. The best performing sample, which comprised a moderate percentage of electrochemical grade carbon as additive, exhibited specific capacity values of about 100 mA h g⁻¹ at 1C (≈80% of theoretical specific capacity). Cyclability tests at 1C proved good reversibility of the material that maintained 98% of initial specific capacity for 30 cycles.