First-principles prediction of large thermoelectric efficiency in superionic Li2SnX3 (X=S, Se)
Thermoelectric materials create an electric potential when subject to a temperature gradient and vice versa hence they can be used to harvest waste heat into electricity and in thermal management applications. However, finding highly efficient thermoelectrics with high figures of merit, zT ≥ 1, is very challenging because the combination of high power factor and low thermal conductivity is rare in materials. Here, we use first-principles methods to analyze the thermoelectric properties of Li2SnX3 (X=S,Se), a recently synthesized class of lithium fast-ion conductors presenting high thermal stability. In p-type Li2SnX3 , we estimate highly flat electronic valence bands that render high Seebeck coefficients exceeding 400 μVK-1 at 700K. In n-type Li2SnX3, the electronic conduction bands are slightly dispersive however the accompanying weak electron-acoustic phonon scattering induces high electrical conductivity. The combination of high Seebeck coefficient and electrical conductivity gives rise to high power factors, reaching a maximum of ~4.5 mWm-1K-2 at 300 K in both n-type Li2SnS3 and Li2SnSe3. Likewise, the thermal conductivity in Li2SnX3 is low as compared to conventional thermoelectric materials, 1.25-4.65 Wm-1K-1 at room temperature. As a result, we estimate a maximum zT = 1.1 in n-type Li2SnS3 at 700 K and 2.2 (1.1) in n-type Li2SnSe3 at the same temperature (300 K). Our findings of large zT in Li2SnX3 suggest that lithium fast-ion conductors, typically employed as electrolytes in solid-state batteries, hold exceptional promise as thermoelectric materials.