Balancing Electron Spin States of Na_3.12Fe_2.44(P₂O₇)₂ Through F‐Doping Realized High Performance on Sodium Ions Storage

Abstract Iron‐based pyrophosphate Na 3.12 Fe 2.44 (P 2 O 7 ) 2 (NFPO) has attracted significant attention due to its low cost, high safety, and open large framework structure, as a promising cathode material for sodium ion batteries. However, the structural instability arising from the uneven electron distribution in the e g orbitals of the 3d transition metal Fe limits the long‐cycle life of NFPO. Herein, a spin‐state regulation strategy is proposed by introducing the highly electronegative fluorine (F) element to modulate the local electronic structure of Fe, achieving a balanced electron distribution in the e g orbitals and enhancing structural stability. A range of analyses and density functional theory (DFT) calculations indicate that F doping can induce electron transitions from the t 2g to the e g orbitals of Fe, transforming it from an intermediate‐spin state to a high‐spin state, thereby stabilizing the crystal structure. Thus, an optimal Na 3.08 Fe 2.44 (P 2 O 6.98 ) 2 F 0.04 cathode synthesized via a simple sol‐gel method exhibits a high capacity of 116.4 mAh g −1 at 0.05 C (theoretical capacity of 117 mAh g −1 ), as well as superior rate capability (61.6 mAh g −1 at 80 C) and ultra‐long cycle life (86.3 % capacity retention rate over 20 000 cycles at 50 C). This work provides new insights into enhancing the structural stability of polyanion cathode materials by regulating the spin state of 3d transition metals.