Hollow Cobalt Selenide Microspheres: Synthesis and Application as Anode Materials for Na-Ion Batteries

The electrochemical properties of hollow cobalt oxide and cobalt selenide microspheres are studied for the first time as anode materials for Na-ion batteries. Hollow cobalt oxide microspheres prepared by one-pot spray pyrolysis are transformed into hollow cobalt selenide microspheres by a simple selenization process using hydrogen selenide gas. Ultrafine nanocrystals of Co₃O₄ microspheres are preserved in the cobalt selenide microspheres selenized at 300 °C. The initial discharge capacities for the Co₃O₄ and cobalt selenide microspheres selenized at 300 and 400 °C are 727, 595, and 586 mA h g^-1, respectively, at a current density of 500 mA g^-1. The discharge capacities after 40 cycles for the same samples are 348, 467, and 251 mA h g^-1, respectively, and their capacity retentions measured from the second cycle onward are 66, 91, and 50%, respectively. The hollow cobalt selenide microspheres have better rate performances than the hollow cobalt oxide microspheres.