Self-sacrificing strategy enables amorphous precursor mediated crystallization of phase-pure Na₄Fe₃(PO₄)₂P₂O₇ for advanced sodium-ion batteries

Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) has garnered significant attention as a promising cathode material for practical sodium-ion batteries, owing to its high structural stability and low cost. However, the susceptibility to form inactive maricite-NaFePO₄ during synthesis and poor intrinsic electronic conductivity limit the high Na-storage performance in NFPP. A self-sacrificing and clean-exit strategy was developed to synthesize phase-pure NFPP. The formation mechanism was elucidated through molecular dynamics simulations, UV–Vis, Raman and XRD: an amorphous precursor from complex coordination ensures homogeneous elemental mixing, creating a kinetically and thermodynamically favorable environment for crystallization. Crucially, oxalic acid serves only as a coordinating agent and is fully volatilized, mitigating the high carbon content issue associated with conventional synthesis methods using citric acid. Consequently, the phase-pure NFPP/OA composite exhibits a high reversible capacity of 110.8 mAh g⁻¹ at 0.1C, outstanding rate capability (101.5 mAh g⁻¹ at 10C), and exceptional cycling stability (99.75 % retention after 2000 cycles). Furthermore, The Na⁺ storage mechanism and electrochemical kinetics were systematically investigated via in-situ XRD and EIS-DRT analysis. These results provide new insights into the research on mixed polyanion-type cathode materials and offer valuable guidance for their practical application.