Encaging sodium iron sulfate into hollow carbon cube for high efficiency sodium storage

Alluaudite-type Na_2+2xFe_2-x(SO₄)₃ (NFS) is recognized as a cost-effective and high-voltage cathode material for sodium-ion batteries. However, the inherent drawback of poor intrinsic conductivity significantly degrades the sodiation reactivity, which becomes a formidable challenge to impede its commercial pace. In this work, we propose a refined design strategy to encage NFS particles into hollow carbon cubes (HCC) for advanced sodium energy storage. The HCC affords triple functions of an electrically conductive network for fast electron transfer, rich diffusion channels for smooth ion transportation, and hollow cavities for alleviating volume change during de/sodiation. These beneficially structural advantages impose a remarkable improvement in reaction kinetics and electrode stability. Consequently, the sodiation capacity of NFS@HCC composite is augmented to as large as 115 mAh g−1 along with excellent rate/cycling properties. Kinetic analyses further elucidate the underlying reason for the performance enhancement and the potential practicality is demonstrated by the full-cell configuration. This study offers an efficacious structure design insight for the development of low-cost and high-performance cathode materials for sodium-ion batteries.