Reducing lattice thermal conductivity via phonon engineering: Strategies for high-performance thermoelectrics

Thermoelectric materials convert heat directly into electricity and are therefore promising for energy harvesting and environmental applications. Ideal high-performance thermoelectrics combine ultralow lattice thermal conductivity, κ_L, with high carrier mobility, a paradigm commonly termed phonon-glass electron-crystal. However, strong coupling between electronic and phononic transport complicates simultaneous optimization of these properties. Because κ_L is largely independent of electronic transport, targeted suppression of κ_L is an effective route to partially decouple heat and charge transport. This review summarizes recent advances in reducing κ_L via two complementary approaches: phonon engineering of bulk nanostructured systems and phonon engineering of low-dimensional materials. In bulk systems, κ_L may be minimized while retaining high electrical conductivity and maximizing the thermoelectric figure of merit ZT by controlling three fundamental phonon parameters: the volumetric specific heat c_v, the phonon group velocity v_g, and the phonon relaxation time τ. Low-dimensional architectures, including superlattices, nanowires, and nanocomposites, supply additional levers to suppress lattice heat transport and to tailor the electronic structure. Integrating multiscale and multimodal phonon-control strategies enables significant reductions in κ_L without sacrificing electronic performance, thereby advancing the phonon-glass electron-crystal paradigm.