Preparation of yolk-shell and filled Co9S8 microspheres and comparison of their electrochemical properties.

ABSTRACT

In this study, we report the first preparation of phase-pure Co9S8 yolk­shell microspheres in a facile two-step process and their improved electrochemical properties. Yolk­shell Co3O4 precursor microspheres are initially obtained by spray pyrolysis and are subsequently transformed into Co9S8 yolk­shell microspheres by simple sulfidation in the presence of thiourea as a sulfur source at 350 °C under a reducing atmosphere. For comparison, filled Co9S8 microspheres were also prepared using the same procedure but in the absence of sucrose during the spray pyrolysis. The prepared yolk­shell Co9S8 microspheres exhibited a Brunauer­Emmett­Teller (BET) specific surface area of 18 m(2) g(−1) with a mean pore size of 16 nm. The yolk­shell Co9S8 microspheres have initial discharge and charge capacities of 1008 and 767 mA h g(−1) at a current density of 1000 mA g(−1), respectively, while the filled Co9S8 microspheres have initial discharge and charge capacities of 838 and 638 mA h g(−1), respectively. After 100 cycles, the discharge capacities of the yolk­shell and filled microspheres are 634 and 434 mA h g(−1), respectively, and the corresponding capacity retentions after the first cycle are 82% and 66%.