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Thermoelectric materials create an electric potential when subject to atemperature gradient and vice versa hence they can be used to harvest wasteheat into electricity and in thermal management applications. However, findinghighly efficient thermoelectrics with high figures of merit, zT$\geq$1, is verychallenging because the combination of high power factor and low thermalconductivity is rare in materials. Here, we use first-principles methods toanalyze the thermoelectric properties of Li$_2$Sn$X_3$ ($X$=S,Se), a recentlysynthesized class of lithium fast-ion conductors presenting high thermalstability. In p-type Li$_2$Sn$X_3$, we estimate highly flat electronic valencebands that render high Seebeck coefficients exceeding 400 ${\mu}$VK$^{-1}$ at700K. In n-type Li$_2$Sn$X_3$, the electronic conduction bands are slightlydispersive however the accompanying weak electron-acoustic phonon scatteringinduces high electrical conductivity. The combination of high Seebeckcoefficient and electrical conductivity gives rise to high power factors,reaching a maximum of 4 mWm$^{-1}$K$^{-2}$ in p-type Li$_2$SnS$_3$ and 8mWm$^{-1}$K$^{-2}$ in n-type Li$_2$SnSe$_3$ at 300 K. Likewise, the thermalconductivity in Li$_2$Sn$X_3$ is low as compared to conventional thermoelectricmaterials, 2-5 Wm$^{-1}$K$^{-1}$ at room temperature. As a result, we estimatea maximum zT = 1.05 in p-type Li$_2$SnS$_3$ at 700 K and an extraordinary 3.07(1.5) in n-type Li$_2$SnSe$_3$ at the same temperature (300 K). Our findings ofhuge zT in Li$_2$Sn$X_3$ suggest that lithium fast-ion conductors, typicallyemployed as electrolytes in solid-state batteries, hold exceptional promise asthermoelectric materials.

First-principles prediction of extraordinary thermoelectric efficiency in superionic Li2SnX3(X=S,Se)