Revealing the Kinetic Limitation of Na₇Fe₇(PO₄)₆F₃ Polyanion Cathode with High Theoretical Capacity for Sodium-Ion Batteries

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., Na₇Fe₇(PO₄)₆F₃ 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 Na₇Fe₇(PO₄)₆F₃ is closely related to the Na⁺ extraction kinetics limitation. Specifically, the crystal structure of Na₇Fe₇(PO₄)₆F₃ 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.