Sandwich confinement engineering of tin sulfide in nitrogen/sulfur-co-doped hollow carbon spheres for efficient sodium storage

Due to the ultralow sodium-ion storage capacity (300 mAh g⁻¹) of carbon materials, incorporating high-capacity components into a carbon matrix has attracted wide attention. To maximize the advantages of high-sodium-storage-capacity composites and mitigate common drawbacks (volume expansion, poor electronic conductivity, and sluggish Na⁺ diffusion kinetics), the microstructural and morphological optimization of the carbon matrix is crucial. Herein, a sandwich confinement strategy was developed to confine SnS — a high-sodium-storage-capacity — within N/S-co-doped hollow double carbon layered spheres (SnS@h-NSCs). Robust Sn-C and C-S bonds promote structural stability and strong interfacial interactions between SnS and the N/S-co-doped carbon matrix. With increasing SnS loading, the optimized double-layer carbon matrix in SnS@h-NSCs efficiently alleviated volume expansion and enhanced electronic conductivity and facilitated Na⁺ migration kinetics. As a result, the anodic SnS@h-NSCs in sodium-ion batteries exhibited significant improvements in initial discharge capacities from 464 to 710 mAh g⁻¹ and the initial Coulombic efficiencies from 50.5 to 68.3 % compared to hollow N-doped carbon spheres (h-NCs). Low-SnS-loading SnS@h-NSCs showed excellent reversibility, while high SnS-loading SnS@h-NSCs exhibited a moderate capacity fading after 100 cycles, highlighting the importance of structural integrity through strong SnS‒carbon interactions (Sn‒C and C‒S bonds) in mitigating the volume expansion, and promoting stability, efficient electron transport, and Na⁺ migration, and underscoring the challenge of maintaining structural integrity at elevated active material content.