Biphasic Synergy Engineering in Iron-Based Polyanionic Cathodes for High-Performance Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are increasingly favored for large-scale energy storage owing to their cost-effectiveness and elemental abundance. Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) is a promising polyanionic cathode material due to its high operating voltage and environmental compatibility, but it suffers from poor electronic and ionic conductivity and structural degradation during deep cycling. To address these issues, a biphasic mixture and phase-ratio tuning strategy are employed to construct NFPP&NFPO composite cathodes, integrating NFPP with Na₂FeP₂O₇ (NFPO), a structurally compatible phase that offers superior stability. This architecture gives rise to interphase synergy, where NFPO alleviates lattice collapse and phase transition of NFPP, while NFPP enhances Na⁺ mobility and mitigates polarization in NFPO. We propose a sodium-ion bifurcation theory, wherein Na⁺ ions are dynamically redistributed between NFPP and NFPO domains during cycling, forming complementary transport channels that enhance the rate capability and cycling durability. A series of NFPP&NFPO composites with varied phase ratios was synthesized via spray-drying and calcination. Among them, the optimized NFPP&NFPO-2 composite achieves a high discharge capacity of 99.38 mAh g^-1 at 0.1 C, maintains 82.1 mAh g^-1 at 30 C, and achieves a capacity retention of 97.02% after 6000 cycles at 20 C. These enhancements are attributed to the synergistic sodium-storage mechanism and ion-bifurcation dynamics, which jointly optimize structural robustness, interfacial stability, and Na⁺ transport kinetics. This work provides insight into the design of biphasic electrode systems and presents a scalable route toward high-performance polyanionic cathodes for next-generation SIBs.