Enhanced Thermoelectric Performance from Li₂SrSiS₄ to Li₂PbSiS₄ Driven by Band Degeneracy and Antibonding-Induced Ultralow Thermal Conductivity

Thermoelectric (TE) materials with effective energy conversion properties are essential for tackling energy crises and combating environmental challenges. Quaternary materials have recently emerged as candidates for TE applications due to their high performance. Here, we study the electronic and thermal transport properties of two tetragonal quaternary thiosilicates, Li2PbSiS4 (LPSS) and Li2SrSiS4 (LSSS), using first-principles calculations and Boltzmann transport theory. LPSS, a wide band gap semiconductor, is predicted to exhibit a figure of merit (ZT) of (n-type) 3.06 at 900 K, exceeding that of LSSS (n-type, 0.51) by over 6-fold. Notably, LPSS also achieves a comparable ZT for p-type (2.51) carriers, underscoring its potential for TE device applications requiring balanced n- and p-type performance. This high-performance behavior originates from the synergistic effect of multiple band degeneracy and band dispersion, resulting in an improved power factor. At 300 K, LPSS exhibits a substantially reduced lattice thermal conductivity (kl) of 0.82 W/mK, which is 2.5 times lower than that in LSSS (2.09 W/mK). The ultralow kl of LPSS is strongly influenced by the presence of a valence band antibonding effect and Pb-dominated flat vibrational modes in the 0–2 THz frequency range, substantially enhancing three-phonon scattering channels, as evidenced by the distinct features in the weighted phase space. Additionally, the “double rattler” involving the LiS4 and PbS8 polyhedra in LPSS exhibits weak interactions, leading to significant anharmonicity and low kl. The variation in the local crystal structures is manifested in the electronic band structures and phonon dispersion of these materials. These observations found in this study further enhance the considerable potential for investigating multielement systems and device applications.