Synthesis of spinel-type magnesium cobalt oxide and its electrical conductivity

This work devotes to investigate synthesis and electrical properties of spinel MgCo₂O₄ that is considered as a candidate of the cathode materials of magnesium-ion batteries in future. Samples were synthesized by two types of techniques: solid-state reaction and wet process. The crystal structures of the samples were analyzed by X-ray diffraction, and their electrical conductivities were obtained through the dc resistance and ac impedance measurements. By solid-state reaction, sample of a single MgCo ₂O₄ phase was not obtained; a sample synthesized at 800°C for 24 h after milling of Co₃O₄ and MgO powder was comprised of dual phases of spinel-type and rocksalt-type structures, and samples synthesized at temperatures higher than 880°C showed a single rocksalt-type phase without electrical conductance. The former sample showed p-type semiconducting behavior as well as spinel-type Co₃O ₄, but its electrical conductivity around room temperature was shown to exceed that of Co₃O₄. In contrast, a sample consisting of a single spinel-type phase was successfully synthesized by wet process, which exhibited an electrical conductivity of the order 10^-2Scm ^-1, being much higher than those of the samples synthesized by solid-state reaction. Thus, substitution of Co²⁺ in Co ₃O₄ with Mg²⁺ is found to enhance the electrical conductivity of the spinel-type phase.