Giant Enhancement in High-Temperature Thermoelectric Figure-of-Merit of Layered Cobalt Oxide, LiCoO2, Due to a Dual Strategy-Co-Substitution and Lithiation.

Abstract

The chemical composition of LiCoO2, a layered oxide commonly used as electrode in batteries, was changed to Li1+yCo1-xNixO2 by a combination of substitution and lithiation to enhance the thermoelectric figure-of-merit at high temperatures. Substitution of Ni as well as lithiation does not change the crystal structure, R3̅m. The lattice parameters c and a are found to increase slightly but maintain a nearly constant ratio, 4.99, indicating no lattice distortion. The trivalent Co was substituted with divalent Ni to synthesize LiCo1-xNixO2 series of p-type compounds with x varying up to 0.15. The high-temperature thermopower decreases drastically from ∼600 to 300 μV K-1, while the electrical resistivity drops by an order of magnitude from 1 × 10-2 to 1 × 10-3 Ω m due to substitution of 15 atom % Ni. The total thermal conductivity also decreases from ∼3 to 1.5 W m-1 K-1. Increasing the amount of Li in LiCo0.85Ni0.15O2 changes the thermophysical properties further and leads to enhancement of figure-of-merit. The power factor is found to change from 37.6 μW m-1 K-2 for the base compound to 120 μWm-1 K-2, a significant improvement for a p-type compound. The overall figure-of-merit as a result increases to 0.12 at ∼1100 K due to substitution and lithiation, a giant increase of ∼760% compared to 1 × 10-2 for the pure compound LiCoO2. These substituted and lithiated compounds are found to be extremely stable even after six months and exhibit totally reproducible thermophysical properties.