Mg‐Doped Na₄Fe₃(PO₄)₂(P₂O₇)/C Composite with Enhanced Intercalation Pseudocapacitance for Ultra‐Stable and High‐Rate Sodium‐Ion Storage

Abstract Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) (NFPP) is considered as a promising cathode material for sodium‐ion batteries (SIBs) due to its low cost, non‐toxicity, and high structural stability, but its electrochemical performance is limited by the poor electronic conductivity. In this study, Mg‐doped NFPP/C composites are presented as cathode materials for SIBs. Benefiting from the enhanced electrochemical kinetics and intercalation pseudocapacitance resulted from the Mg doping, the optimal Mg‐doped NFPP/C composite (NFPP‐Mg5%) delivers high rate performance (capacity of ≈40 mAh g −1 at 20 A g −1 ) and ultra‐long cycling life (14 000 cycles at 5 A g −1 with capacity retention of 80.8%). Moreover, the in situ X‐ray diffraction and other characterizations reveal that the sodium storage process of NFPP‐Mg5% is dominated by the intercalation pseudocapacitive mechanism. In addition, the full SIB based on NFPP‐Mg5% cathode and hard carbon anode exhibits the discharge capacity of ≈50 mAh g −1 after 200 cycles at 500 mA g −1 . This study demonstrates the feasibility of improving the electrochemical performance of NFPP by doping strategy and presents a low‐cost, ultra‐stable, and high‐rate cathode material for SIBs.