Global softening to manipulate sound velocity for reliable high-performance MgAgSb thermoelectrics

High-performance thermoelectric materials at room temperature are eagerly pursued due to their promising applications in the Internet of Things for sustainable power supply. Reducing sound velocity by softening chemical bonds is considered an effective approach to lowering thermal conductivity and enhancing thermoelectric performance. Here, different from softening chemical bonds at the atomic scale, we introduce a global softening strategy, which macroscopically softens the overall material to manipulate its sound velocity. This is demonstrated in MgAgSb, one of the most promising p-type thermoelectric materials at room temperature to replace (Bi,Sb)₂Te₃, that the addition of inherently soft organic compounds can easily lower its sound velocity, leading to an obvious reduction in lattice thermal conductivity. Despite a simultaneous reduction of the power factor, the overall thermoelectric quality factor B is enhanced, enabling softened MgAgSb by C₁₈H₃₆O₂ addition to achieve a figure of merit zT value of ∼0.88 at 300 K and a peak zT value of ∼1.30. Consequently, an impressive average zT of ∼1.17 over a wide temperature range has been realized. Moreover, this high-performance MgAgSb is verified to be highly repeatable and stable. With this MgAgSb, a decent conversion efficiency of 8.6% for a single thermoelectric leg and ∼7% for a two-pair module have been achieved under a temperature difference of ∼276 K, indicating its great potential for low-grade heat harvesting. This work will not only advance MgAgSb for low-grade power generation, but also inspire the development of high-performance thermoelectrics with global softening in the future.