Preparation of Yolk‐Shell and Filled Co 9 S 8 Microspheres and Comparison of their Electrochemical Properties

In this study, we report the first preparation of phase‐pure Co 9 S 8 yolk–shell microspheres in a facile two‐step process and their improved electrochemical properties. Yolk–shell Co 3 O 4 precursor microspheres are initially obtained by spray pyrolysis and are subsequently transformed into Co 9 S 8 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 9 S 8 microspheres were also prepared using the same procedure but in the absence of sucrose during the spray pyrolysis . The prepared yolk–shell Co 9 S 8 microspheres exhibited a Brunauer–Emmett–Teller (BET) specific surface area of 18 m 2  g −1 with a mean pore size of 16 nm. The yolk–shell Co 9 S 8 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 Co 9 S 8 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 %.