Cation engineering via Ti4+-doping and carbon encapsulation unlocks superior rate and cycle performance in Na4Fe3(PO4)2P2O7 cathode

Despite its high theoretical capacity and robust framework, the NASICON cathode Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) suffers from two major drawbacks: low electronic conductivity and secondary-phase formation (electrochemically inactive NaFePO 4 impurity phase). Here, to address these intertwined issues, we propose a synergistic bulk-to-surface strategy via Ti 4+ -doping and carbon coating Na 4 Fe 3-x Ti x (PO 4 ) 2 P 2 O 7 (x = 0, 0.05, 0.1, 0.15) materials, aiming to enhance the Na + diffusion rate and structural stability from a structural-surface perspective. XRD and XPS characterizations confirm the successful incorporation of Ti 4+ into the NFPP framework. Ti 4+ -doping addresses both electronic and structural limitations; its electron-withdrawing capability enhances intrinsic conductivity by optimizing charge distribution, and the concomitant lattice contraction stabilizes the host framework against phase impurities. The partial substitution of Fe sites effectively suppresses the formation of NaFePO 4 impurities. In parallel, the in-situ carbon coating establishes a robust conductive network and acts as a resilient buffer, effectively accommodating volume variations during cycling. Cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT), and electrochemical impedance spectroscopy (EIS) reveal that the resulting Na 4 Fe 2.9 Ti 0.1 (PO 4 ) 2 P 2 O 7 (NFPP-Ti10%) possesses a higher Na + diffusion coefficient and lower interfacial impedance. It achieves a high capacity of 117.7 mAh g −1 and remarkable long-term cyclability, retaining 87.2% capacity after 5000 cycles at 20C. The superior electrochemical performance originates from the synergistic effect of Ti 4+ doping and carbon coating, which simultaneously enhances structural stability, electronic transport, and interfacial stability.