Lattice expansion and pillaring effect via Nb doping enabling high-rate and ultra-stable sodium storage for Na4Fe3(PO4)2P2O7

Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) is considered a promising cathode material for sodium-ion batteries (SIBs) due to its abundant resources, low cost, and moderate operating voltage. However, its inherent low electronic conductivity and sluggish sodium ion (Na + ) diffusion kinetics severely limit its high-rate performance and practical application. Herein, carbon-coated Na 4 Fe 3-x Nb x (PO 4 ) 2 P 2 O 7 @C composites were synthesized through a ball-milling-assisted solid-state method. Significantly, Nb 5+ doping expands the lattice spacing, lowers Na + migration barrier to accelerate reaction kinetics and narrows the band gap to enhance the intrinsic electronic conductivity of the material. Meanwhile, strong Nb–O interactions are proposed to contribute to the stabilization of the polyanionic framework, thereby improving structural reversibility and long-term cycling stability under high-rate conditions. The optimal modified material, Na 4 Fe 2.95 Nb 0.05 (PO 4 ) 2 P 2 O 7 @C, exhibits a discharge capacity of 123.88 mAh g −1 at 0.1C (1 C = 129 mAh g −1 ), while also demonstrating excellent stability and rate performance (capacity retention rate of 83.83% after 10,000 cycles at 20 C). This work provides a promising doping strategy for developing high-performance polyanionic cathodes for sodium-ion batteries.