Scalable Preparation of Low-Cost Na 2+ x Fe 2 (SO 4 ) 3− x (PO 4 ) x Cathode with Superior Air Stability and High-Temperature Performance

Alluaudite-type Na 2 Fe 2 (SO 4 ) 3 is a promising cathode material for low-cost sodium-ion batteries (SIBs). However, its practical application is severely hindered by the challenging pure-phase synthesis and poor air stability. Herein, PO 4 3− -doped Na 2 Fe 2 (SO 4 ) 3 (Na 2+ x Fe 2 (SO 4 ) 3− x (PO 4 ) x, NFSP- x ) was prepared via a scalable sand-milling/spray-drying strategy. Systematic characterizations confirm that homogeneous PO 4 3− doping is realized without disrupting the lattice framework of Na 2 Fe 2 (SO 4 ) 3 . The as-prepared NFSP-0.5 exhibits the highest phase purity and crystallinity among all samples. Electrochemical tests show that NFSP-0.5 delivers a reversible discharge capacity of 98.5 mA h g −1 at 0.1 C, excellent rate performance (87.5 mA h g −1 at 10 C), and long-term cycling stability (negligible capacity degradation over 1000 cycles at 10 C). Moreover, the NFSP-0.5 cathode achieves nearly no capacity decay over 50 cycles at 0.5 C in a sodium-ion full cell, demonstrating promising preliminary cycling stability for practical application. Meanwhile, NFSP-0.5 also shows superior air stability (no structural degradation after 7 days of air exposure) and outstanding high-temperature cycling performance (93.2% capacity retention after 800 cycles at 2 C and 60 °C). This work provides a scalable preparation strategy for high-performance polyanionic cathode materials for SIBs.