Structure Engineering of Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ via W-doping Enhanced Cycling Stability and Rate Performance for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are promising for large-scale energy storage, but their development is hindered by the lack of high-rate, long-life cathode materials. Na4Fe3(PO4)2P2O7 (NFPP) offers structural stability and cost advantages but suffers from low electronic conductivity and sluggish Na+ diffusion. Herein, a tungsten-doped NFPP (NFPP-Wx) is achieved via a scalable sol–gel method in which W substitution at Fe sites suppresses grain growth, increases surface area, and improves electrode–electrolyte contact. Combined density functional theory and systematic experimental analyses reveal that W-doping narrows the band gap from 2.639 to 0.696 eV, enhances electronic conductivity, and boosts the pseudocapacitive contribution. NFPP-W0.03 delivers 105.6 mAh g–1 at 0.1C and demonstrates exceptional cycling stability, retaining 93.7% after 970 cycles at 2C, 89.3% after 4000 cycles at 50C, and 73.8% after 6000 cycles. A full cell paired with hard carbon exhibits 97.8 mAh g–1 at 0.1C and long-term stability. This work demonstrates W-doping as an effective strategy to engineer NFPP for high-rate, durable SIB cathodes.