Deciphering the Basis of Enhanced Na⁺ Storage Induced by Halogen Doping in Na₂VTi(PO₄)₃

Elemental doping is a widely adopted strategy to enhance the electrochemical performance of electrode materials, yet limited studies have explored the resulting variations in the bonding environments and the interactions between dopant atoms and their neighboring atoms. In this study, halogen-doped (fluorine, chlorine, and bromine) polyanionic phosphates are synthesized to investigate the effects of halogen doping on the fine crystal structure, chemical micro-environment, and electronic structure of Na₂VTi(PO₄)₃ for Na ion storage. Density functional theory analysis reveals that halogen doping strengthens interactions at the Na sites while disrupting their symmetry, thereby promoting Na⁺ conductivity. Simultaneously, the increased electron density and expanded electron cloud potentially bridge the electron clouds previously isolated by [PO₄] tetrahedra, facilitating electron transport. As a result, the doped samples demonstrate improved performance in rate capability, capacity, and cycling stability. This study provides deeper insights into the influence of elemental doping on electrode materials properties.