Electronic transport properties of Li$_2$Sn$X_3$($X$=S, Se) from first principles calculation

Lithium-based materials usually exhibit very attractive properties such as fast ionic conduction and high thermal stability. Here, the calculated electronic transport properties of fast ion conductors using the first-principles method will be presented. All the calculations have been performed by using Tran-Blaha-modified Becke-Johnson (TB-mBJ) potential. The obtained indirect bandgap of Li$_{\mathrm{2}}$SnS$_{\mathrm{3}}$ is 2.14 eV, which fairly agrees with the measured value of 2.38 eV. But Li$_{\mathrm{2}}$SnSe$_{\mathrm{3\thinspace }}$shows direct bandgap (1.95 eV). The energy bands of both materials are flat that lead to high Seebeck coefficient approaching \textasciitilde 326 \textmu V K$^{\mathrm{-1\thinspace }}$at 700 K. The combination of high Seebeck coefficient and conductivity leads to high power factor (reaching \textasciitilde 6 mW m$^{\mathrm{-1}}$ K$^{\mathrm{-1\thinspace }}$at 700 K). Because of their high thermal stability and high power factor, both materials are the potential candidate for thermoelectric device applications. The electronic structure and physical mechanism behind high power factor will be explained in details.