High thermoelectric performance induced by strong anharmonicity in ternary Li-based chalcogenides

Thermoelectric materials enable heat-to-electricity conversion, while its efficiency has so far been restricted by the dimensionless figure of merit ($\mathit{ZT}$). In this study, we employ first-principles calculations combined with Boltzmann transport equation along with the thermal transport unified theory to investigate the electrical and thermal transport properties of a family of Li-based chalcogenides ($\mathrm{L}{\mathrm{i}}_{3}B{X}_{4}, B=\mathrm{V}$, Nb, Ta and $X=\mathrm{S}$, Se, Te). It is found that $\mathrm{L}{\mathrm{i}}_{3}\mathrm{BT}{\mathrm{e}}_{4}$ exhibit ultralow lattice thermal conductivity (${\ensuremath{\kappa}}_{L}$) at room temperature, as lows as \ensuremath{\sim}0.1 W/mK. This is mainly attributed to the relatively weak interaction between the B and Te atom as confirmed by the small number of bonding states below the Fermi level, which introduces strong anharmonicity and significantly enhances the phonon scattering rate. Moreover, the observed power factors of $\mathrm{L}{\mathrm{i}}_{3}\mathrm{BT}{\mathrm{e}}_{4}$ are more than 10 $\textmu{}\mathrm{W}\phantom{\rule{4pt}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}{\mathrm{K}}^{\ensuremath{-}2}$ at 300 K, giving rise to giant $\mathit{ZT}$ at intermediate and high temperatures in both p-type and n-type, for instance, $\mathit{ZT}$ reaches 2.25 \ensuremath{\sim} 2.46 in $\mathrm{L}{\mathrm{i}}_{3}\mathrm{NbT}{\mathrm{e}}_{4}$ even after considering both particlelike propagation and wavelike phonon tunneling transport mechanism. In addition, we find that $\mathit{ZT}$ of $\mathrm{L}{\mathrm{i}}_{3}\mathrm{B}{\mathrm{X}}_{4}$ can further be enhanced through moderate strain engineering and these Li-based chalcogenides exhibit high ductility which is favorable for the post machining process. The present work unravels the importance of strong anharmonicity induced by weak-bonded heavy element on the thermoelectric performance in $\mathrm{L}{\mathrm{i}}_{3}\mathrm{B}{\mathrm{X}}_{4}$ family which provides important insights for the rational design of flexible thermoelectric devices.