Dual-Phase Engineered Iron-Based Polyanion Cathodes for Fast and Durable Sodium-Ion Batteries

Iron-based polyanion material Na₂Fe₂(SO₄)₃ has gained attention for sodium-ion batteries due to its excellent electrochemical performance and low cost. However, Na₂Fe₂(SO₄)₃ suffers from residual FeSO₄ formation during synthesis, which limits its capacity and rate performance. Herein, we introduce a dual-phase engineering strategy by incorporating NaF during synthesis that effectively eliminates FeSO₄ residues and enables the formation of a dual-phase composite: Na_2.56Fe_1.72(SO₄)₃ (primary phase) and Na₃Fe₂(SO₄)₃F (secondary phase). This dual-phase structure not only eliminates excess FeSO₄ but also enhances Na⁺ diffusion by introducing abundant phase boundaries, leading to a superior electrochemical performance. The optimized Na_2.375Fe₂(SO₄)₃F_0.375 (NF-0.375) cathode achieves a high discharge capacity of 112 mAh g^-1 at 0.1 C, an exceptional rate capability of 82.9 mAh g^-1 at 30 C, and outstanding long-term stability, retaining 80% capacity after 10,000 cycles at 30 C. This dual-phase design provides a pathway for optimizing polyanion cathodes and accelerates the development of fast and durable SIBs for large-scale electric energy storage systems.