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

Abstract

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 CoS_x (CoS and Co₉S₈) quantum dots embedded N/S-doped carbon nanosheets (CoS_x@NSC) is prepared by a sol-gel method. The CoS_x quantum dots are in situ formed within ultrafine carbon nanosheets without further sulfidation, thus resulting in ultrafine CoS_x 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 CoS_x@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 CoS_x@NSC//Na_0.44MnO₂ full cell is demonstrated, and it can deliver a specific capacity of 414 mAh g^-1 (based on the mass of CoS_x@NSC) at a current density of 0.2 A g^-1.