Local structural modulation via sulfate substitution for high-performance NFPP-based sodium-ion battery cathode

Sodium-ion batteries (SIBs) are compelling low-cost candidates for large-scale energy storage. Nevertheless, NASICON-type Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) polyanionic cathodes suffer from intrinsically inferior electronic conductivity and sluggish Na + diffusion, severely restricting their practical deployment. Herein, a sulfate anion (SO 4 2− ) lattice modulation strategy is implemented to construct modified NFPP electrodes. The incorporated SO 4 2− induces structural rearrangement and electronic redistribution, synergistically accelerating Na + migration, stabilizing electrode interface, and structural reversibility upon repeated sodiation/desodiation cycling. The optimized NFPPS-2 electrode delivers a reversible capacity of 104.2 mAh g −1 at 1C, coupled with remarkable high-rate capability (50C) and long-term cycling durability at 10C, as well as mitigated voltage polarization and enhanced energy efficiency. Systematical mechanistic validation via in-situ XRD, GITT, EIS, and DRT techniques corroborate that SO4 2− incorporation effectively promotes reaction kinetics and structural stability of NFPP. Moreover, the fabricated NFPPS-2//hard carbon full cell exhibits excellent electrochemical behavior, achieving a higher energy density than most documented polyanionic cathode-based SIB full cells. This work provides a reliable and versatile anion-substitution engineering methodology to enhance the overall electrochemical performance of low-cost polyanionic cathodes, which affords a viable route for the further development and industrial application of high-performance SIBs.