Large enhancement of the thermopower in Na$_x$CoO$_2$ at high Na doping

TL;DR

This study reveals a significant enhancement of thermopower and figure of merit in Na$_x$CoO$_2$ at high sodium doping levels, especially around $x$=0.85, indicating potential for improved thermoelectric applications.

Contribution

The paper demonstrates a large increase in thermopower and figure of merit in Na$_x$CoO$_2$ at high doping levels, expanding understanding of its thermoelectric properties.

Findings

Thermopower $S$ sharply increases for $x > 0.75$.

At $x \,\sim\,$ 0.85, $Z$ reaches 40 times higher than in less doped samples.

Enhanced thermoelectric performance at low temperatures.

Abstract

Research on the oxide perovskites has uncovered electronic properties that are strikingly enhanced compared with those in conventional metals. Examples are the high critical temperatures of the cuprate superconductors and the colossal magnetoresistance in the manganites. The conducting layered cobaltate $\rm Na_xCoO_2$ displays several interesting electronic phases as $x$ is varied including water-induced superconductivity and an insulating state that is destroyed by field. Initial measurements showed that, in the as-grown composition, $\rm Na_xCoO_2$ displays moderately large thermopower $S$ and conductivity $\sigma$. However, the prospects for thermoelectric cooling applications faded when the figure of merit $Z$ was found to be small at this composition (0.6$<x<$0.7). Here we report that, in the poorly-explored high-doping region $x>$0.75, $S$ undergoes an even steeper enhancement. At the critical doping $x_p\sim$ 0.85, $Z$ (at 80 K) reaches values $\sim$40 times larger than in the as-grown crystals. We discuss prospects for low-temperature thermoelectric applications.