Sodium‐Deficient NASICON Na 3+ x VFe(PO 4 ) 3 Cathode for High‐Performance Sodium‐Ion Batteries

Abstract NASICON‐type Na 3 V 2 (PO 4 ) 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 3.15 VFe 0.86 (PO 4 ) 3 (NVFP‐650) cathode. It is found that the regulation of sintering temperature for Na 3+ x VFe(PO 4 ) 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.