Covalent sulfur/CoS2-decorated honeycomb-like carbon hierarchies for high-performance sodium storage with ultra-long high-rate cycling stability

Transition metal sulfides (TMSs) with high theoretical sodium storage capacity have been regarded as one of the most competitive and promising anode materials for sodium-ion batteries. Nevertheless, great challenges still exist for TMS to achieve long-cycle life owing to the poor conductivity and volume expansion during the sodiation/desodiation process. The conventionally used pure carbon coating method can effectively solve the above problems. However, the low sodium ion storage capability of carbon will decrease the capacity of the anode. Thus the functionalization of well-designed carbon nanostructures with exotic elements (e.g., sulfur) or high-specific-capacity materials has been regarded as an efficient route to enhance sodium storage performance. Herein, we developed a facile strategy to incorporate covalent sulfur and CoS2 nanoparticles into honeycomb-like carbon matrices (denoted as S/CoS2@C), where sulfur is vaporized under vacuum and reacted with the Co-decorated carbon hierarchies, leading to the formation of CoS2 nanoparticles as well as the covalent C–Sx–C bonds. Consequently, the as-prepared S/CoS2@C composite demonstrates excellent sodium storage performance, with high initial coulomb efficiency (ICE) of 83 % and capacity retention of 95 % (478.5 mAh g−1) after 700 cycles at 0.2 A g−1, and an outstanding high-rate long-term cycling stability with a high reversible capacity of 396.1 mAh g−1 after 5000 cycles at 2 A g−1 (an average decay rate of only 0.0014 %). Furthermore, electrochemical analyses reveal that both covalent sulfur and CoS2 contribute to the sodium storage capacity via a predominantly surface-induced capacitive process, and the well-designed S/CoS2@C composite with encapsulation of covalent sulfur and CoS2 into the highly interconnected porous carbon matrices efficiently buffer the volume changes and well maintain the electrode integrity/conductivity upon sodiation/desodiation cycling.