Superior electrochemical properties of SiO₂-doped Co₃O₄ hollow nanospheres obtained through nanoscale Kirkendall diffusion for lithium-ion batteries

Hollow SiO₂-doped Co₃O₄ (Si-Co₃O₄) nanospheres with excellent Li-ion storage properties were synthesized via flame spray pyrolysis by applying a nanoscale Kirkendall diffusion process. A solid SiO₂-doped CoO (filled Si-CoO) nanopowder was prepared through this process, and then it was transformed into hollow Si-Co₃O₄ nanopowder by way of a core-shell-structured Co-SiO₂ (filled Co@Si-CoO) composite nanopowder. In addition, the direct oxidation of the filled Si-CoO nanopowder at 300 °C under an air atmosphere resulted in the formation of a solid SiO₂-doped Co₃O₄ (filled Si-Co₃O₄) nanopowder. At a high current density of 2 A g^-1, the hollow Si-Co₃O₄ nanospheres exhibited a 150th-cycle discharge capacity of 971 mA h g^-1 and capacity retention of 99.5%, which was measured relative to the second cycle. However, the corresponding capacity retentions of the filled Si-CoO and Si-Co₃O₄ nanopowders were only 82.2% and 71.5%, respectively. The high structural stability during cycling and high Li-ion conductivity, which are caused by the hollow structure, are responsible for the excellent Li-ion storage properties of the hollow Si-Co₃O₄ nanospheres obtained through nanoscale Kirkendall diffusion.