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

Abstract

Thermoelectric materials create an electric potential when subjected 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 a 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, and 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 produce high Seebeck coefficients exceeding 400 μV K⁻¹ at 700 K. In n-type Li₂SnX₃, the electronic conduction bands are slightly dispersive; however, the accompanying electron–acoustic phonon scattering is weak, which induces high electrical conductivity. The combination of a high Seebeck coefficient and electrical conductivity gives rise to high power factors, reaching a maximum of ∼4.5 mW m⁻¹ K⁻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, 1.35–4.65 W m⁻¹ K⁻¹ at room temperature. As a result, we estimate a maximum zT of 1.1 in n-type Li₂SnS₃ at 700 K and of 2.1 (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.