Fluorine-Doped NaTi2(PO4)3 Via Electronic Orbital Modulation and Bandgap Engineering for Aqueous Li/Na/K-Ion Batteries

Sodium titanium phosphate (NaTi₂(PO₄)₃, NTP) has emerged as a promising electrode material due to its three-dimensional open framework. This study investigates the use of NTP in aqueous dilute Li⁺/Na⁺ electrolytes and extends its application to high-concentration K⁺ electrolytes. X-ray photoelectron spectroscopy, X-ray absorption near-edge structure analysis, and density functional theory calculations revealed that highly electronegative fluorine partially substitutes for oxygen in the NTP lattice, resulting in the formation of Ti-F bonds. The substitution effectively modulates the electronic structure of Ti⁴⁺, alters the local coordination environment, and influences the redox dynamics. Enhanced long-term cycling stability and rate performance were demonstrated across aqueous sodium-ion, lithium-ion, and potassium-ion half-cells. Among the investigated systems, the aqueous sodium-ion system exhibited the best electrochemical performance, characterized by a single, well-defined charge–discharge plateau, stable cycling behavior with 88.7% capacity retention after 500 cycles at 1 A g⁻¹, and an initial specific discharge capacity of 121.7 mAh g⁻¹ at 0.2 A g⁻¹. The results establish F-doped NTP as a promising candidate for advanced energy storage applications in aqueous alkali metal-ion batteries.