Cobalt Sulfide Quantum Dot Embedded N/S-Doped Carbon Nanosheets with Superior Reversibility and Rate Capability for Sodium-Ion Batteries.

Metal sulfides are promising anode materials for sodium-ion batteries due to their large specific capacities. The practical applications of metal sulfides in sodium-ion batteries, however, are still limited due to their large volume expansion, poor cycling stability, and sluggish electrode kinetics. In this work, a two-dimensional heterostructure of CoSx (CoS and Co9S8) quantum dots embedded N/S-doped carbon nanosheets (CoSx@NSC) is prepared by a sol-gel method. The CoSx quantum dots are in situ formed within ultrafine carbon nanosheets without further sulfidation, thus resulting in ultrafine CoSx particle size and embedded heterostructure. Meanwhile, enriched N and S codoping in the carbon nanosheets greatly enhances the electrical conductivity for the conductive matrix and creates more active sites for sodium storage. As a result, the hybrid CoSx@NSC electrode shows excellent rate capability (600 mAh g-1 at 0.2 A g-1 and 500 mAh g-1 at 10 A g-1) and outstanding cycling stability (87% capacity retention after 200 cycles at 1 A g-1), making it promising as an anode material for high-performance sodium-ion batteries. A CoSx@NSC//Na0.44MnO2 full cell is demonstrated, and it can deliver a specific capacity of 414 mAh g-1 (based on the mass of CoSx@NSC) at a current density of 0.2 A g-1.