Precursor-Free Synthesis of Na₄Fe₃(PO₄)₂P₂O₇ Cathode Directly from Metallic Iron for Sodium-Ion Batteries

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) is an emerging sodium-ion battery cathode material due to its safety, environmental friendliness, and resource abundance. However, conventional synthesis suffers from high cost, long processing time, and potential environmental emissions, Here, we propose a sustainable one-step synthesis route is proposed for NFPP/C using metallic iron, avoiding ferric phosphate precursors. Thermodynamic analysis and reaction pathway simulations confirm that the formation of pure-phase NFPP can be achieved by precisely tuning the Fe content and sintering temperature, effectively suppressing the formation of impurities. The resulting NFPP/C exhibits a porous microstructure with uniform carbon coating, high tap density, and optimized ion transport characteristics. Electrochemical characterization of the NFPP/C material shows a reversible capacity of 108.5 mAh g^-1 at 0.1 C. Furthermore, the material exhibits excellent long-term cycling stability, retaining 81.7% of its capacity after 2000 cycles at 10 C. In situ XRD analysis confirms a highly reversible structural evolution accompanying Na⁺ intercalation and deintercalation. A techno-economic-environmental assessment shows that the metallic-iron route reduces synthesis costs by over 20% compared to conventional routes and eliminates wastewater and toxic emissions. This work provides an alternative approach for the industrial production of NFPP cathodes and offers valuable insights into the design of high-performance polyanionic materials.