${\mathrm{CuBiSCl}}_2$ exhibits strongly contrasting anisotropies in charge and phonon transport, showing superior electron mobility (138 ${\mathrm{cm}}^2/\mathrm{V}\mathrm{s}$) yet ultralow lattice thermal conductivity (0.40 W/mK) along the $a$ axis at 300 K. This exceptional combination arises from the anisotropic bonding hierarchy where ${\left[{\mathrm{BiSCl}}_2\right]}_n$ ribbons facilitate efficient charge conduction through delocalized Bi-$6p$/S-$3p$ networks, while soft Cu-dominated vibrations strongly scatter heat-carrying phonons in the same direction. The resulting high-power factor (1.71 mW/${\mathrm{mK}}^2$ at 700 K) and peak $ZT$ of 1.57 establish ${\mathrm{CuBiSCl}}_2$ as a paradigm for realizing the “phonon glass-electron crystal” concept through crystallographic engineering.

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• Thermal conductivity
• Layered semiconductors
• Thermoelectrics
• Ab initio molecular dynamics
• Boltzmann theory
• Density functional calculations