Enhancing the High-Temperature Thermoelectric Performance of Li(CoNi)O2 by Replacement of Ni with Earth-Abundant Mg

Abstract

Oxides are mainly high-temperature thermoelectric materials due to their inherent thermal stability and also because of their low figure of merit at low temperatures, < 500 K. The compound Li1.1Co0.85Ni0.15O2, which was recently reported to exhibit a high figure of merit, has been modified by replacing divalent Ni with Mg, an earth-abundant element, to synthesize Li1.1Co0.85Ni0.15−xMgxO2 compounds with x = 0–0.15. All the compounds have the rhombohedral single phase belonging to the R$$ \bar{3} $$mH space group. The lattice parameters are found to be larger compared with LiCoO2 due to the substitutions. The electrical conductivity of all the compounds increases with increasing temperature, which is a semiconducting behavior, and is found to increase by four orders of magnitude from 0.2 Ω−1 m−1 to 2000 Ω−1 m−1 in the range 300–1100 K. The Seebeck coefficient in this range is positive and is found to be nearly independent of temperature ∼ 300 μV K−1, indicating a saturation behavior. The thermal conductivity decreases with increasing temperature, reaching a value of ∼ 1.5 W m−1 K−1. The high Seebeck coefficient combined with enhanced electrical conductivity and low thermal conductivity results in an increase in the power factor to 0.2 mW m−1 K−2, with a simultaneous increase in the figure of merit zT to 0.16 at 1100 K. These results clearly show that partial replacement of Ni2+ with Mg2+ not only makes the oxide more environmentally friendly, but also increases zT. The enhanced properties are found to be due to oxidation of Co3+ to Co4+ and Ni2+ to Ni3+ due to the addition of Mg.