Structural TopologyEngineering via Heterovalent Dual-ElementSubstitution for High Thermoelectric Performance in AgBiS2 Monolayer

Abstract The development of high-performance thermoelectric materials is challenging due to the intrinsic coupling of electronic and thermal transport, demanding a synergistic optimization strategy. While the strategic introduction of weakly bonded structural units holds great promise for phonon engineering, its potential as a versatile design principle to govern both thermal and electronic transport is not yet fully established. Here, we demonstrate that a loosely bound [AgS] atomic layer in the AgBiS2 monolayer enables an exceptional combination of ultralow lattice thermal conductivity (κL) and high thermoelectric efficiency. Through heterovalent dual-element substitution (2IIIA ↔ IB + VA) of the TlS monolayer, the derived AgBiS2 maintains a layered architecture but features a critical structural topology innovation: a [AgS] sublayer featuring softened intralayer Ag–S bonds that are weakly coupled to the neighboring [BiS] layer. These bonding characteristics directly produce strong lattice anharmonicity and localized vibrations, leading to drastically suppressed κL. Furthermore, the localized vibrations induced by the loosely bound [AgS] layer enhance polar optical phonon scattering, which dominates carrier transport at moderate doping concentrations. Concurrently, the electronic structure, featuring a light electron effective mass (0.13 m0) due to the preserved covalent framework in the [BiS] sublayers, endows the material with superior electron mobility. This unique situation creates an ideal scenario where electronic transport remains reasonably good while thermal transport is strongly inhibited. Consequently, despite a moderate power factor, the AgBiS2 monolayer achieves a peak ZT of 1.93 at 900 K for p-type carriers, significantly outperforming the TlS monolayer (ZT = 0.90). This work establishes the engineering of loosely bound atomic layers via cation substitution as a potent strategy for designing high-efficiency two-dimensional thermoelectric materials. Our findings reveal that strategically weakened interatomic bonding within specific sublayers can create a favorable balance between electronic and thermal transport channels, providing a design paradigm for advanced thermoelectrics.