Bifunctional Effects of Nitrogen-Doped Carbon Quantum Dots on Cos2/Mesoporous Carbon Composites for High-Performance Lithium-Ion Batteries

Abstract

Cobalt disulfide (CoS2) stands as a promising candidate for electrode materials in lithium-ion batteries due to its high theoretical capacity of 872 mAh g-1. Nevertheless, its electrode performance faces challenges associated with inadequate electronic/ionic conductivity, significant volume expansion, and the shuttle effect of lithium polysulfide during cycling. To address these issues, researchers have extensively explored the utilization of zeolitic imidazolate frameworks (ZIF)-derived porous carbon composites. These composites offer distinct advantages, including the formation of nanosized particles, heteroatom doping, and highly porous structures. However, ZIF-derived carbon supports primarily consist of ultra-micropores, which can impede lithium-ion diffusion. Herein, we aimed to enhance cycling stability by introducing a nitrogen-doped carbon quantum dot (NCQD) solution derived from N-methyl-2-pyrrolidone (NMP) into cobalt-based ZIF-67 to modify the porosity and to dope heteroatoms for CoS2 nanoparticles-embedded composites. The treatment of ZIF-67 with the mildly acidic NCQD solution resulted in partial etching of Co-N bonds within the structure, along with the deposition of NCQDs as a nitrogen source. Notably, the size of the pores could be adjusted by varying the concentration of the NCQD solution, while the nitrogen functional groups were retained during carbonization. Subsequent solvothermal sulfurization led to the conversion of metallic Co nanoparticles into CoS2 within a nitrogen-doped carbon matrix, yielding CoS2/nitrogen- and sulfur-co-doped carbon (NSC). The electrode using CoS2/NSC composite with the 2.8 mL NCQD pre-treatment as an anode of LIBs exhibited a higher capacity of 621 mAh g-1 and enhanced C-rate capability with the capacity of 392 mAh g-1 at 2.0 A g-1 than those of pristine CoS2/NSC. Moreover, the cycling stability was improved with a capacity retention of 77% after 100 cycles.

Keywords: Nitrogen-doped quantum dots, Cobalt-based ZIF-67, Mesoporous Carbon, CoS2/carbon composites, Anode material, Lithium-ion batteries