Synthesis and electrochemical properties of spherical and hollow-structured NiO aggregates created by combining the Kirkendall effect and Ostwald ripening

The Kirkendall effect and Ostwald ripening were successfully combined to prepare uniquely structured NiO aggregates. In particular, a NiO-C composite powder was first prepared using a one-pot spray pyrolysis, which was followed by a two-step post-treatment process. This resulted in the formation of micron-sized spherical and hollow-structured NiO aggregates through a synergetic effect that occurred between nanoscale Kirkendall diffusion and Ostwald ripening. The discharge capacity of the spherical and hollow-structured NiO aggregates at the 500^th cycle was 1118 mA h g^-1 and their capacity retention, which was measured from the second cycle, was nearly 100%. However, the discharge capacities of the solid NiO aggregates and hollow NiO shells were 631 and 150 mA h g^-1, respectively, at the 500^th cycle and their capacity retentions, which were measured from the second cycle, were 63 and 14%, respectively. As such, the spherical and hollow-structured NiO aggregates, which were formed through the synergetic effect of nanoscale Kirkendall diffusion and Ostwald ripening, have high structural stability during cycling and have excellent lithium storage properties.