Eliminating Voltage Tailing Illusion: Purity Enhancement and Defining Effective Voltage Range for NFPP Cathodes in Sodium‐Ion Full‐Cells

Impurities remain the key challenge for Na_4-xFe_3-x(PO₄)_2-xP₂O₇ (NFPP, 0 ≤ x ≤ 1) cathode materials in sodium-ion batteries (SIBs), addressed by modulating the Na₂FeP₂O₇ : Na₄Fe₃(PO₄)₂P₂O₇ ratio in this study. The optimal Na_3.5Fe_2.5(PO₄)_1.5P₂O₇ (Na_3.5Fe_2.5PP) material at a 1:3 ratio with the least impurities is scalably synthesized via a dual-iron-source system. Crucially, this study reveals the illusion of seemingly high discharge capacities caused by the "voltage tailing" phenomenon in some cathode materials, defining the "effective voltage range" as 2.4-3.7 V in full-cells (N/P ratio = 1.1) through the triple-electrode measurements. Within this window, Na_3.5Fe_2.5PP delivers: (1) the highest initial charge/discharge capacities (116.05 / 102.33 mAh g^-1 at 0.1 C); (2) excellent rate capability (74.23 mAh g^-1 at 30 C), and (3) superior cycling performance (75.4% capacity retention ratio after 10 000 cycles at 20C). Finally, combined with hard carbon (HC) anode, Na_3.5Fe_2.5PP//HC pouch cells exhibit excellent safety, low-temperature performance (88.8% capacity retention ratio at -30°C), rate capability (84.1% capacity retention ratio at 30 C), and cycling stability (83.25% capacity retention ratio after 2300 cycles), enabling practical energy storage applications. This work refines NFPP purity enhancement strategies and reveals the capacity mismatch between half-cell and full-cell-a finding broadly applicable to SIBs using HC anodes, thereby facilitating practical SIBs applications in energy storage.