Revealing the Kinetic Limitation of Na ₇ Fe ₇ (PO ₄ ) ₆ F ₃ Polyanion Cathode with High Theoretical Capacity for Sodium‐Ion Batteries

Abstract Iron‐based sodium‐ion polyanionic materials have drawn extensive attention for developing advanced cathodes, primarily due to their abundant reserves and high safety. However, their typically low capacity impedes practical applications. Here, a novel candidate of this series, i.e., Na ₇ Fe ₇ (PO ₄ ) ₆ F ₃ is presented, which boasts a high theoretical capacity (159 mAh g ⁻¹ ) yet delivering a perplexingly low practical capacity (87 mAh g ⁻¹ ) within the typical voltage range (1.5–4.2 V). Combining theoretical calculations and experimental analysis, it is demonstrated that the capacity discrepancy in Na ₇ Fe ₇ (PO ₄ ) ₆ F ₃ is closely related to the Na ⁺ extraction kinetics limitation. Specifically, the crystal structure of Na ₇ Fe ₇ (PO ₄ ) ₆ F ₃ contains distinctive dam‐lake‐like regions (DLRs). The extraction of Na ⁺ ions located in DLRs (DLR‐Na) not only possesses a high energy barrier (1.53 eV) but also critically depends on the vacancies created by the migration of their neighboring Na ⁺ ions. Furthermore, a strategy of increasing the cutoff voltage is developed to unlock the DLR‐Na, which effectively increases the practical capacity (156 mAh g ⁻¹ ) to approach the theoretical value. This work is believed to open up new avenues for designing high‐capacity iron‐based sodium‐ion polyanionic cathodes.