Heterogeneous-Interface-Induced Charge Redistribution Toward Fe-Based Polyanion Cathode for Advanced Sodium-Ion Batteries

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) is gradually developing into one of the most commercially prospective cathode materials for sodium-ion batteries. However, the inactive phase maricite-NaFePO₄ (m-NFP) normally tends to be formed during the synthesis process of NFPP, as well as the intrinsic poor electronic conductivity, which impacts the realization of high Na-storage performance. Herein, for the first time, we have constructed a heterostructure in Fe-based polyanionic cathode materials by fine-tuning the stoichiometric ratio of the Na site; the inactive phase m-NFP is fully transformed to the active Na₂FeP₂O₇ or NFPP. In NFPP-NFPO heterogeneous composites, density functional theory calculations reveal that the charge redistribution occurs at the heterogeneous interface, leading to stronger and more uniform interactions that can strengthen the structural stability and enhance the charge transport kinetics. Benefiting from the heterogeneous intergrowth structure and the formation of the electrochemically active phase, a high discharge specific capacity, ultralong cycle life (71.4% capacity retention after 10,000 cycles at 50 C), ultrafast rate capability (60.2 mAh g^-1 at 200 C), and impressive high-temperature tolerance have been achieved. This work achieves heterogeneous composites by manipulation of the phase composition, providing a new approach for designing high-performance polyanionic cathodes for sodium-ion batteries.