Solid-State Synthesis of Na₄Fe₃(PO₄)₂P₂O₇/C by Ti-Doping with Promoted Structural Reversibility for Long-Cycling Sodium-Ion Batteries

The cathode material Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) has shown great potential for sodium-ion batteries (SIBs) due to its cost-effectiveness, prolonged cycle life, and high theoretical capacity. However, the practical large-scale production of NFPP is hindered by its poor intrinsic electron conductivity and the presence of a NaFePO 4 impurity. In this study, we propose a mutually reinforcing approach involving Ti doping, mechanical nano treatment, and in situ carbon coating to produce Ti-NFPP via the solid-state methods of synthesis. Ti doping strengthens the covalent Fe–O interaction, hence accelerating the electron transfer and the redox reactions Fe 2+ /Fe 3+ . In situ carbon coating improves electrical conductivity and allows for accommodating the volumetric variation. Nanosized treatment promotes the uniform progression of solid-state reactions. The synthesized Na 4 Fe 2.98 Ti 0.01 (PO 4 ) 2 P 2 O 7 material (Ti-NFPP) exhibits promising electrochemical properties with an initial discharge specific capacity of 112.5 mA h g –1 at 0.1 C. A volumetric change of only 2.98% was observed during the de/sodiation process, indicating an enhanced reversibility of the crystal lattice. Moreover, it demonstrates exceptional cycling stability with a capacity retention rate of 97.2 mA h g –1 at 10 C over 5000 cycles. These findings offer a promising pathway for the large-scale production of Ti-NFPP in SIBs.