Sodium-Deficient NASICON Na_{3+ x}VFe(PO₄)₃ Cathode for High-Performance Sodium-Ion Batteries

NASICON-type Na₃V₂(PO₄)₃(NVP) material possesses robust 3D structure and high sodium diffusivity, thus showcasing its immense potential in sodium-ion batteries (SIBs). However, considering the perspective of environmental conservation, it is imperative to substitute vanadium with elements that are both cost-effective and non-toxic in order to further enhance its application in SIBs. Herein, Fe is utilized to replace the V site in the sodium vanadium phosphate structure and successfully prepared a pure phase sodium-deficient NASICON (sodium superionic conductor) Na_3.15VFe_0.86(PO₄)₃ (NVFP-650) cathode. It is found that the regulation of sintering temperature for Na_{3+ x}VFe(PO₄)₃(NVFP) material can effectively mitigate the formation of secondary phases and enhance the electrochemical properties of the resulting product. The sodium-deficient cathode shows enhanced electrochemical performance and sodium ion diffusion kinetics. It exhibits a high capacity of 102.8 mAh g^-1 at 0.1 C, and exhibits a high-capacity retention of 95.7% after 2000 cycles at 20 C. The energy storage mechanism and structural evolution are further investigated through SEM, TEM, XPS, and in situ XRD characterizations. The compositional modulation of sodium-deficient NVFP and the elucidation of its cycling mechanisms in this work would provide valuable insights for enhancing the performance of sodium energy storage systems.