Thermoelectric properties of LiCo_1-xM_xO₂ (M = Cu, Mg, Ni, Zn): Comparison with Li_yCoO₂ and Na_yCoO₂ systems

Li_yCoO₂ has a similar layered structure to Na_yCoO₂, which is a typical p-type oxide thermoelectric material, and the average Co valence of 3 + y is controlled by the Li content y. We investigated the thermoelectric properties of LiCo_1-xM_xO₂ samples (M = Cu, Mg, Ni, Zn) for the first time at high temperatures, in which Co³⁺ was substituted by the divalent M²⁺ ions, and the average Co valence of 3 + x can be controlled similarly to the Li content y in Li_yCoO₂. The substitution of the M²⁺ ions for the Co site was found to show thermoelectric properties similar to those of Li_yCoO₂ with the same average Co valence. The Mg-doped sample showed the highest thermoelectric performance at high temperatures in this study; the thermoelectric power factor P is 2.38 × 10^-4 W m^-1 K^-2 at 1173 K and the dimensionless figure of merit ZT is 0.024 at 876 K. The thermoelectric potential of LiCo_1-xM_xO₂ is discussed and compared with those of Li_yCoO₂ and Na_yCO₂ systems.