Electrical Conductivities and Conduction Mechanism of Lithium-Doped High-Entropy Oxides at Different Temperature and Pressure Conditions

Li-doped high-entropy oxides (Li-HEO) are promising electrode materials for Li-ion batteries. However, their electrical conduction in a wide range of temperatures and/or at high pressure is unknown, hindering their applications under extreme conditions. Especially, a clear understanding of the conduction mechanism is needed. In this work, we determined the carrier type of several Li-doped (MgCoNiCuZn)O semiconductor compounds and measured their electrical conduction at temperatures 79-773 K and/or at pressures up to 50 GPa. Three optical band gaps were uncovered from the UV-vis-NIR absorption measurements, unveiling the existence of defect energy levels near the valence band of p-type semiconductors. The Arrhenius-like plot of the electrical conductivity data revealed the electronic conduction in three temperature regions, i.e., the ionization region from 79 to 170 K, the extrinsic region from ∼170 to 300 K, and the intrinsic region at ≥300 K. The closeness of the determined electronic band gap and the second optical band gap suggests that the conduction electrons in the intrinsic region originate from a thermal excitation from the defect energy levels to the conduction band, which determines the electronic conductivity. It was also found that at or above room temperature, ionic conduction coexists with electronic conduction with a comparable magnitude at ambient pressure and that the intrinsic conduction mechanism also operates at high pressures. These findings provide us a fundamental understanding of the band structure and conduction mechanism of Li-HEO, which would be indispensable to their applications in new technical areas.