Visualization of Anti-Pulverization of Nickel Sulfide in Hierarchical Carbons Shells

Abstract

Nickel sulfide (NiS) has attracted attention as an anode material for lithium ion batteries due to its high theoretical capacity (590 mAh g-1), high thermal stability and low resistance compared to traditional metal oxides. However, the main problem of this NiS was a pulverization of the electrode after some cycles, and hence it was electrochemically isolated. In this study, the mixture of NiS and CaCO3 nanoparticles were coated with carbon by toluene CVD. As a result, the carbon shell layer with a hierarchical structure was formed around NiS, and the resultant material had a superior capacity of 575 mAh g-1 at 0.1 A g-1 for the anode material of lithium ion battery. In long-term cycle test, nickel sulfide was pulverized, but resultant pieces were preserved by the coated carbon shell during 100th cycle. However, after 100th cycles, the pulverization occurred continuously, and NiS was separated into metallic nickel and lithium sulfide. This lithium sulfide converted into polysulfides that have high solubility and mobility in the electrolyte, and the polysulfides eventually escaped out of the carbon shell. As a result, the capacity increased up to 606 mAh g-1 until 97th cycle, and then it decreased to 483 mAh g-1 at 200th cycle. This phenomenon was observed by the visualization of cycle-by-cycle TEM images and confirmed using additional ex situ analyses.