<p>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_3.4Fe_2.4(PO₄)_1.4P₂O₇ (NFPP)/C. A series of MoO₄²⁻-doped NFPP/C cathodes is synthesized via a high-shear mixer-assisted sol–gel method. The optimized Na_3.4Fe_2.4(PO₄)_1.3(MoO₄)_0.1P₂O₇/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 MoO₄²⁻ 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.</p>