Electrochemical properties of micron-sized Co₃O₄hollow powders consisting of size controlled hollow nanospheres

Micron-sized Co₃O₄hollow powders consisting of size controlled hollow nanospheres are prepared by applying Ostwald ripening and Kirkendall effect to the spray pyrolysis process. The Co-C composite powders uniformly dispersed with different sizes of metallic Co nanocrystals are formed by reduction of the cobalt oxide-carbon composite powders prepared using spray pyrolysis. Subsequent oxidation of the Co-C composite powders with filled structures forms the micron-sized Co₃O₄hollow powders consisting of size controlled hollow nanospheres. The mean sizes of the Co₃O₄hollow nanospheres oxidized from Co-C composite powders formed at reduction temperatures of 400, 600, and 800 °C are 37, 55, and 73 nm, respectively. The discharge capacities of the Co₃O₄powders formed from the Co-C composite powders reduced at temperatures of 400, 600, and 800 °C for the 300^thcycle are 644, 702, and 660 mA h g⁻¹, respectively, and their capacity retentions calculated from the second cycle are 81, 86, and 84%, respectively. The porous-structured Co₃O₄powders formed from the Co-C composite powders reduced at 800 °C show slightly better rate performance than those of the other two samples.