Decoupled anisotropic Charge-Phonon Transport Enables Exceptional n-Type Thermoelectric Performance in CuBiSCl${}_2$

First-principles calculations demonstrate an exceptional decoupling of charge and thermal transport along the $\mathit{\text{a}}$-axis in CuBiSCl${}_2$. The material achieves superior electron mobility (138 cm${}^2$/V$\cdot$s at 300 K) through delocalized Bi-6$\mathit{\text{p}}$/S-3$\mathit{\text{p}}$ networks while maintaining ultralow lattice thermal conductivity (0.40 W/mK at 300 K) via Cu-dominated anharmonic phonon scattering - both optimized along the same crystallographic direction. This simultaneous optimization originates from the anisotropic bonding hierarchy where [BiSCl${}_2$]${}_n$ ribbons enable efficient charge transport along $\mathit{\text{a}}$-axis, while the soft vibrational modes associated with Cu atoms strongly scatter heat-carrying phonons. The resulting high power factor (1.71 mW/mK${}^2$ at 700 K) and peak $\mathit{\text{ZT}}$ of 1.57 establish CuBiSCl${}_2$ as a model system that realizes the long-sought “phonon glass-electron crystal” paradigm through crystallographically engineered transport channels.