Ca2+-inspired Na2.76Fe1.62-xCax(SO4)3@FG cathode with high-rate capability and ultra-long cycle life for sodium-ion batteries.

Abstract

The development of sustainable energy storage technologies is critically dependent on cathode materials that combine high electrochemical performance with minimal environmental footprint. Na2+2xFe2-x(SO4)3 (NFS) has emerged as a promising candidate for sodium-ion batteries (SIBs) owing to its composition of earth-abundant, non-toxic elements and high operating voltage. However, its widespread application remains constrained by intrinsically poor electronic/ionic conductivity and inadequate cycling stability. In this work, a series of Ca2+ doped single-crystalline Na2.76Fe1.62-xCax(SO4)3@flash graphene (NFS@FG-xCa, x = 0, 0.1, 0.3) materials were prepared through a simple solvent-free solid-phase reaction. As confirmed by X-ray diffraction (XRD), synchrotron radiation analysis and density functional theory (DFT) calculations, the successful substitution of Fe2+ with Ca2+ within the NFS structure results in a slight expansion of the lattice volume and enhanced structural stability. With appropriate Ca2+ doping (NFS@FG-0.1Ca), the material exhibits facilitated electron transfer and smooth three-dimensional Na+ diffusion, achieving a remarkable rate capability of 80 mAh g-1 at 50C. UV-visible diffuse reflectance spectroscopy together with DFT and Ab initio molecular dynamics (AIMD) simulations reveals that the enhanced kinetics originate from promoted electron delocalization with a reduced band gap and a lower Na+ diffusion barrier along the conduction pathways. Furthermore, leveraging a Ca-induced pinning effect, NFS@FG-0.1Ca exhibited suppressed lattice volume variation of only 1.12% during cycling, which contributes to the exceptional long-term cycling stability with 82.5% capacity retention after 6500 cycles at 50C. Besides, the practical viability of NFS@FG-0.1Ca was further validated through its performance under high mass-loading conditions and in full-cell configurations. This study provides an effective and scalable strategy for designing high-performance, nontoxic and low-cost cathode materials of advanced SIBs.

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