Novel binary cobalt nickel oxide hollowed-out spheres for electromagnetic absorption applications

[Display omitted] •Co1.29Ni1.71O4 hollowed-out spheres have been successfully synthesized.•The ionic mobility in spinel structure will increase, leading to oxygen vacancy.•The effective bandwidth is up to 4.84 GHz with a matched thickness of 1.60 mm. Novel binary cobalt nickel oxide Co1.29Ni1.71O4 hollowed-out spheres have been successfully synthesized via a facile hydrothermal method with a further calcination process. The phase, crystal structure, morphology, and electromagnetic (EM) and EM wave absorption properties were characterized in detail. We find that as the temperature increases, the ionic mobility in the spinel structure will increase, leading to lattice disorder and an increase in the concentration of oxygen vacancy, which will improve the dielectric loss and EM wave absorption capacity of the material. The minimum reflection loss (RLmin) of the sample calcined at 500 °C reached up to −50.7 dB with a matching thickness (d) of 1.6 mm, and the corresponding effective absorption bandwidth (fE, RL