Delocalized electronic engineering by ionic potential enabling fluorophosphates cathode towards ultrahigh rate capacity and stability for sodium-ion batteries

Polyanionic fluorophosphates (NFPF) are promising cathodes for sodium-ion batteries, yet their application is hindered by structural instability, low electronic conductivity, and sluggish Na⁺ diffusion. This work demonstrates an ionic potential tuning strategy by introducing Zn2+ (ionic potential: ∼2.7 Å−1) with stable delectronic configuration into the iron sites (ionic potential: ∼2.56 Å−1). DFT calculations reveal Zn2+ with higher ionic potential induces delocalized electronic around Fe, narrowing the band gap and attenuating Na+-O2- electrostatic interactions during the process of sodium ion removal and insertion, thereby reducing the Na⁺ diffusion barrier. Besides, the stable d10 configuration of Zn2+ renders its electron cloud symmetric and resistant to deformation, functioning as a "lattice pillar" during Fe2+/Fe3+ redox to suppress structural collapse. The optimized NFPF/C-Zn0.06 cathode exhibits exceptional cyclability, retaining 91% capacity after 500 cycles at 5 C. Full-cell tests with hard carbon anodes show a stable voltage (2.86 V) and minimal polarization, underscoring the practical promise of cationic potential engineering.