Sustainable synthesis of a high-performing off-stoichiometric sodium iron sulfate/N-rGO composite cathode for sodium-ion batteries

Sodium iron sulfate (Na_2+2xFe_2-x(SO₄)₃) is a polyanionic compound with a high operating potential (3.8 V vs Na/Na⁺) that is synthesised using abundant precursors. As a result, it is an attractive Na-ion cathode material, however, its poor electronic conductivity limits the capacity and stability during cycling. Herein, we report the synthesis of Na_2.5Fe_1.75(SO₄)₃/C45/N-doped reduced graphene oxide composite using solid-state and continuous hydrothermal flow synthesis methods. The coupling of both C45 and N-rGO creates a carbon matrix that surrounds the active material and offers increased surface contact with NFS and the conductive materials than observed with C45 alone. The NFS@C45/N-rGO cathode delivers discharge capacities of 98.9 mAh g⁻¹(at 10 mA g⁻¹) and 79.9 mAh g⁻¹(at 320 mA g⁻¹) respectively, with 85.3 % capacity retention at 10 mA g⁻¹over 250 cycles. Microstructural analysis confirms that the 2D N-rGO flakes form a continuous conductive scaffold around the active material, ensuring more uniform electronic pathways. This enhanced internal architecture leads directly to the superior capacity retention and lower impedance observed for the NFS@C45/N-rGO electrode during long-term cycling. This work demonstrates that high-performance NFS cathodes can be realised through fully sustainable synthesis routes, offering a viable pathway toward greener battery manufacturing.