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@SnSx@C) by a facile method and applied as negative electrode material in sodium ion batteries. The C@SnSx@C2 (at optimized carbon ratio) negative electrode can deliver an initial reversible capacity of 636.5 mAh center dot g(-1) at 0.1 A center dot g(-1), superior rate capability (265.1 mAh center dot g(-1) at rate of 10.0 A center dot g(-1)) and long cycle life (capacity retention of 96.3 % at 1.0 A center dot 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 SnSx 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.