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

In this study, we report the first preparation of phase-pure Co ₉S₈ yolk-shell microspheres in a facile two-step process and their improved electrochemical properties. Yolk-shell Co₃O ₄ precursor microspheres are initially obtained by spray pyrolysis and are subsequently transformed into Co₉S₈ yolk-shell microspheres by simple sulfidation in the presence of thiourea as a sulfur source at 350 °C under a reducing atmosphere. For comparison, filled Co ₉S₈ microspheres were also prepared using the same procedure but in the absence of sucrose during the spray pyrolysis. The prepared yolk-shell Co₉S₈ microspheres exhibited a Brunauer-Emmett-Teller (BET) specific surface area of 18 m² g ^-1 with a mean pore size of 16 nm. The yolk-shell Co ₉S₈ 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 yolk-shell 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 %.