Phase Formation in NaH2PO4–VOSO4–NaF–H2O System and Rapid Synthesis of Na3V2O2x(PO4)2F3-2x

Abstract

Renewable electricity products, for example, from wind and photovoltaic energy, need large-scale and economic energy storage systems to guarantee the requirements of our daily lives. Sodium-ion batteries are considered more economical than lithium-ion batteries in this area. Na3V2(PO4)2F3, NaVPO4F, and Na3(VO)2(PO4)2F are one type of material that may be used for Na-ion batteries. In order to better understand the synthesis of these materials, the phase formation in a NaH2PO4–VOSO4–NaF–H2O system under hydrothermal conditions was studied and is reported herein. This research focused on the influences of the sodium fluoride content and hydrothermal crystallization time on phase formation and phase purity. The phase transformation between Na(VO)2(PO4)2(H2O)4 and Na3V2O2x(PO4)2F3-2x was also studied. Na3V2O2x(PO4)2F3-2x with a high degree of crystallinity can be obtained in as short as 2 h via hydrothermal synthesis using a conventional oven at 170 °C without agitation. All compounds obtained in this research were studied mainly using powder X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometry, and Fourier-transform infrared spectroscopy.