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

ABSTRACT Impurities remain the key challenge for Na _4‐x Fe _3‐x (PO ₄ ) _2‐x P ₂ O ₇ (NFPP, 0 ≤ x ≤ 1) cathode materials in sodium‐ion batteries (SIBs), addressed by modulating the Na ₂ FeP ₂ O ₇ : Na ₄ Fe ₃ (PO ₄ ) ₂ P ₂ O ₇ ratio in this study. The optimal Na _3.5 Fe _2.5 (PO ₄ ) _1.5 P ₂ O ₇ (Na _3.5 Fe _2.5 PP) 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 _3.5 Fe _2.5 PP 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 _3.5 Fe _2.5 PP//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.