Molybdate-Induced Rigid–Flexible Composite Framework for High-Rate and Durable Iron-Based Polyanionic Cathodes

Although iron-based polyanionic cathodes are promising for use in sodium-ion batteries because of their low cost and high structural stability, their practical application is hindered by low electronic conductivity and sluggish Na + transport kinetics at high rates. Herein, a molybdate-induced rigid–flexible composite framework strategy is proposed to regulate the polyanionic skeleton of Na 3.4 Fe 2.4 (PO 4 ) 1.4 P 2 O 7 (NFPP)/C. A series of MoO 4 2− -doped NFPP/C cathodes is synthesized via a high-shear mixer-assisted sol–gel method. The optimized Na 3.4 Fe 2.4 (PO 4 ) 1.3 (MoO 4 ) 0.1 P 2 O 7 /C cathode exhibits outstanding electrochemical performance, delivering 101.1 mAh/g after 2000 cycles at 5 C with 97.02% capacity retention and a decay rate of only 0.0015% per cycle. At 10 C, it retains 99.30% of its initial capacity after 2000 cycles. Mechanistic studies reveal that the incorporation of MoO 4 2− enhances the local flexibility of the framework while preserving the stability of the original three-dimensional structure, thereby accelerating Na + migration and improving electrochemical kinetics. At the same time, the composite framework features favorable pathways for electron migration, which enhance its electronic conductivity. In situ X-ray diffraction confirms the highly reversible Na + intercalation/deintercalation behavior of the modified framework. In addition, the assembled NFPP-0.1Mo||hard carbon full cell delivers 74.1 mAh/g after 2000 cycles at 10 C, with a capacity retention of 79.42%. This work provides an effective polyanion-group engineering strategy for the design of high-rate and durable iron-based cathodes for sodium-ion batteries.