Low-strain spherical Na_2.5Fe_1.75(SO₄)₃ cathode enabled by morphology control for long-cycle sodium-ion batteries

Alluaudite-type Na_2+2δFe_2-δ(SO₄)₃, characterized by its cost-effectiveness and high operating voltage, has emerged as a prospective cathode material for sodium-ion batteries (SIBs). However, poor electronic conductivity and sluggish reaction kinetics hinder its practical application. In this study, we developed a low-strain spherical Na_2.5Fe_1.75(SO₄)₃@C@KB (NFS@C@KB) cathode, enclosed within highly conductive Ketjen Black (KB) using a scalable spray-drying method. In-situ X-ray diffraction analysis revealed that the nearly zero-volume strain originates from the synergistic effect of the dense spherical configuration and pearl-like KB branched chains, effectively enhancing the structural stability and prolonging the cycle life. Moreover, the interconnected KB network establishes a continuous electron transport pathway, thereby improving the electronic conductivity. The optimized NFS@C@KB cathode delivered a capacity of 75 mAh g^-1 at 5C and exhibited remarkable cycling stability over 4000 cycles. Even under the extreme condition of -20 °C, it retained a discharge capacity of 63 mAh g^-1 at a rate of 1C. Consequently, the combination of a low-strain structure and outstanding electrochemical performance is expected to stimulate further exploration of iron-based sulfate cathodes and promote the development of rechargeable SIBs for energy storage applications.