Understanding the lattice-dynamical and electronic transport mechanisms of thermoelectric materials is essential for identifying systems that combine intrinsically low lattice thermal conductivity with favorable charge-transport characteristics. We present a comprehensive first-principles investigation of the thermoelectric properties of Li-based chalcopyrite compounds ${\text{LiXTe}}_2(\mathrm{X}=\text{Al},\text{Ga},\text{In})$. By combining the Boltzmann transport equation with self-consistent phonon theory, including bubble-term corrections and both three-phonon and four-phonon scattering, we reveal the microscopic origin of their intrinsically ultralow lattice thermal conductivity. Te-dominated isolated optical soft modes in the low-to-mid-frequency region, together with strong higher-order anharmonicity, substantially enhance phonon scattering. In particular, the inclusion of four-phonon processes reduces the lattice thermal conductivity of ${\text{LiInTe}}_2$ by 45% (41%) along the $c$-axis ($a/b$-axis) at 300 K, demonstrating pronounced anharmonic behavior. Electronic transport is evaluated using the AMSET framework, explicitly incorporating key scattering mechanisms and spin-orbit coupling (SOC) effects. ${\text{LiGaTe}}_2$ exhibits a highly dispersive, multivalley-degenerate band structure that significantly decouples $S\text{–}\sigma \text{–}{\kappa }_e$, leading to outstanding performance with an $n$-type $ZT$ of 2.8 at 800 K (2.2 with SOC). The experimental synthesis of ${\text{LiXTe}}_2$ would confirm the structural stability and practical viability of this family. These results establish ${\text{LiXTe}}_2$ as a promising high-performance thermoelectric system and provide essential microscopic insights for tuning strongly anharmonic semiconductors.

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• Anharmonic lattice dynamics
• Electrical conductivity
• Energy conversion technologies
• Lattice thermal conductivity
• Thermoelectric transport
• Energy materials
• Semiconductor compounds
• Thermoelectrics