Effect of fluorine doping on the performance of manganese-titanium NASICON cathodes for sodium-ion batteries

An Na-excess NASICOM type cathode material, Na3.6Mn1.3Ti0.7(PO3.6F0.4)3 (NMTP@F), was synthesized using a sol-gel method. Electrochemical evaluations revealed that the NMTP@F half-cell exhibits a high discharge capacity of 189.8 mAh g−1 at 0.1 C, excellent high-rate performance with a capacity of 76.7 mAh g−1 at 10 C, and outstanding cycling stability with 90.6% capacity retention after 500 cycles at 1 C. Cyclic voltammetry measurements revealed rapid Na+ transport during the charge–discharge process. Electrochemical impedance spectroscopy further provided quantitative evidence of reduced charge transfer resistance after F doping. Density functional theory calculations revealed that F doping lowers the crystal formation energy of NMTP, enhancing its structural stability. It also reduces the Na extraction energy and narrows the energy bandgap, facilitating rapid Na+ ion transport for high-rate performance. Projected density of states analysis shows F doping induces new spin-up states in Ti and F below the Fermi level, reducing the bandgap from 1.16 to 0.75 eV, improving electronic conductivity.