Fabrication of S,N-doped carbon-coated SnS₂/SnS heterostructures supported by hollow carbon microspheres for sodium-ion storage

Developing novel anode materials containing electroactive heterostructures which boost ion and charge transfer kinetics in a carbon matrix is still a great challenge. Here we report on a new smartly designed material: SnS₂/SnS p-n heterostructures embedded in S,N-doped carbon layer supported by hollow carbon spheres (C@SnS_x@C) by a facile method and applied as negative electrode material in sodium ion batteries. The C@SnS_x@C2 (at optimized carbon ratio) negative electrode can deliver an initial reversible capacity of 636.5 mAh·g⁻¹ at 0.1 A·g⁻¹, superior rate capability (265.1 mAh·g⁻¹ at rate of 10.0 A·g⁻¹) and long cycle life (capacity retention of 96.3 % at 1.0 A·g⁻¹ after 150 cycles). The SnS₂/SnS p-n heterojunctions provide a lower sodium ion diffusion energy barrier (0.38 eV), higher Na⁺ adsorption energy (−4.66 eV) and higher electronic conductivity due to an internal electric field according to density functional theory calculations compared to plain SnS. Moreover, S,N-doped carbon facilitates electronic conductivity and buffers the volume changes during the conversion reaction-based SnS_x upon sodium insertion and extraction process. Porous hollow carbon spheres contribute to prevent the agglomeration of SnS₂/SnS nanosheets and keep the structural integrity. Our findings on this unique material might be extended to other ion battery technologies.