Visualized Pulverization via Ex Situ Analyses: Nickel Sulfide Anode Caged in a Hierarchical Carbon

Abstract

Nickel sulfide has been studied extensively as a next-generation anode material for lithium ion batteries due to its great theoretical capacity (590 mAh g−1), relatively high electrical conductivity and thermal stability compared to metal oxides. However, if it is used alone as an anode, performance is drastically reduced due to its pulverization during the cycling. To solve such problem, in this work, hierarchical carbon coated nickel sulfide is synthesized using both chemical vapor deposition of toluene and CaCO3 nano-template, and the resultant material is employed as an anode material of lithium ion battery. It shows a capacity of 575 mAh g−1 at a current density of 100 mA g−1, and maintains over 270 mAh g−1 at a high current density of 2000 mA g−1. In the long-term cycle test, the capacity gradually increases to 606 mAh g−1 around the 100th cycle and shows the high retaining ratio relative to the initial capacity over 84% after 200 cycles. During the cycles, the nickel sulfide pulverizes into nanoparticles, but these particles remain inside the hierarchical carbon structure. Finally, they separate into nickel and lithium sulfide. This mechanism is confirmed using the cycle-by-cycle transmission electron microscopy images and additional ex situ analyses.