Sodium iron sulfate cathodes with ultra-long cycle-life and high safety for sodium-ion batteries

The development of low-cost and high-safety cathode materials is critically important to sodium-ion battery (Na-ion) research. Here we report a carbon nanotube (CNT)-percolating Na₂Fe(SO₄)₂ cathode (NFS-CNT) prepared via a rationally designed mechano-chemical method. The material synthesis mechanism is elucidated for the first time by in situ X-ray diffraction and thermogravimetric analysis. It is discovered that Na₂Fe(SO₄)·4H₂O is formed as an intermediate phase during the mechano-chemical process, which is dehydrated to produce the Na₂Fe(SO₄)₂ cathode material upon a mild thermal treatment. The NFS-CNT composite cathode achieves an ultra-long cycle-life of over 13,000 cycles at 10 C at room temperature and over 6000 cycles at 55 °C, demonstrating its exceptional durability. The superior cycling performance is attributed to the small lattice change during Na-ion extraction/insertion and the percolating CNT network. Furthermore, the NFS/CNT cathode exhibits stable cycle performance in Na-ion full cells (93.4 % retention after 700 cycles) and a significantly lower heat release (∼ 229.2 J g⁻¹) at the fully charged state compared to a wide range of Na-ion and Li-ion cathodes. materials. demonstrating its high thermal stability and safety. This work provides a promising path towards developing low-cost, high-performance Na-ion batteries.