Orchestrated Spatial Confinement and Phase Engineering with SDS for Air-Stable and High-Rate Alluaudite Sodium-Ion Cathodes

Abstract

Alluaudite-type sodium iron sulfate (NFS) stands out as a promising cathode candidate for sodium-ion batteries due to its high operating voltage and low raw material cost; however, its practical viability is severely hampered by extreme environmental sensitivity and sluggish reaction kinetics. Addressing these challenges, this work proposes a spatial confinement and phase engineering strategy driven by the anionic surfactant sodium dodecyl sulfate (SDS), aiming to simultaneously enhance environmental stability and accelerate reaction kinetics. In the precursor stage, SDS acts as a ligand occupying the structural water reaction sites, effectively buffering the structural shrinking stress during dehydration. During thermal treatment, the steric hindrance from SDS long chains elevates the crystallization energy barrier, suppressing premature nucleation. Subsequently, SDS decomposition then triggers burst nucleation at elevated diffusion rates, yielding fine and uniform grains. Concurrently, the in situ formed carbon coating from SDS exerts robust spatial confinement, which not only effectively restricts grain growth to achieve nanosizing but also arrests structural relaxation, thereby stabilizing the metastable heterophase with abundant diffusion channels. Furthermore, rGO and the SDS-derived carbon jointly construct a robust three-dimensional dual-conductive network, significantly enhancing electronic conductivity. These combined effects fundamentally improve the reaction kinetics. Notably, the polar hydrophilic groups within the SDS/GO-derived carbon layer not only enhance electrolyte wettability but also establish robust interfacial adhesion via chemical anchoring to the NFS lattice. This anchoring effect shields the lattice surface, reducing the adsorption energy with external water molecules to as low as -0.06 eV, constructing a delicate "hydrophilic-yet-moisture-repellent" interface that facilitates ion conduction while protecting the lattice, thereby elevating the critical relative humidity (CRH). Consequently, the modified material exhibits substantially improved air stability (CRH increased from 40% to 55%) and superior rate performance (delivering 81.46 mAh g -1 at 30 C).