Monoclinic α‐NaVOPO 4 as cathode materials for sodium‐ions batteries: Experimental and DFT investigation

Sodium vanadyl orthophosphate NaVOPO4 is an attractive High-Potential cathode material for Na-ions batteries with three polymorphs (α, β, and α1). In this work, pure phase of monoclinic α-NaVOPO4 was synthesized by sol-gel method and the origin of its poor electrochemical activity was investigated. Sample characterization using X-ray diffraction, Raman spectroscopy, magnetic measurement and Fourier transform infrared (FT-IR) spectroscopy confirm the formation of a single phase of monoclinic NaVOPO4 without any impurities. Electrochemical properties were also characterized using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. The results indicate that α-NaVOPO4 exhibit a good ionic conductivity but the charge transfer resistance is large. Optical analysis and DFT calculations both showed that the α-NaVOPO4 is a semi-conductor with a band-gap energy of 2.24 eV which explain the origin of the high charge transfer resistance measured using EIS technique. From all these experimental and theoretical results, the poor electrochemical activity of α-NaVOPO4 can be associated to its low intrinsic electronic conductivity. Electronic and ionic properties of the NaVOPO4 polymorphs are not yet fully characterized. Here, the monoclinic NaVOPO4 was investigated and the material exhibits a high voltage, a good ionic conductivity and good stability, but it exhibits a poor performance. The EIS, optical, and DFT result explain the origin of such behavior. A detailed magnetic study was also reported in this paper.