Abstract
<h2>Summary</h2> Thermoelectric harvesting of low-temperature waste heat offers great opportunities for sustainable energy production. However, the investigations of related thermoelectric materials and modules remain sluggish. Here, we reported a great advance in the n-type Mg<sub>3</sub>Sb<sub>1.5</sub>Bi<sub>0.5</sub> system by minor Cu additions. Some Cu atoms preferentially occupy interstitial sites within the Mg<sub>3</sub>Sb<sub>2</sub> lattice and significantly modified phonon modes via filling in the phonon gap and increased anharmonic phonon scattering, thereby leading to the anomalously low thermal conductivity. Simultaneously, the detrimental behavior of thermally activated electrical conductivity was completely eliminated through grain-boundary complexion engineering. These two critical roles contributed to the remarkable improvement of <i>zT</i>. Based on this developed high-performance material coupled with p-type α-MgAgSb-based material, a fabricated thermoelectric module rivaling long-time champion Bi<sub>2</sub>Te<sub>3</sub>, demonstrated a record-high conversion efficiency ∼7.3% at the hot-side temperature of 593 K. These results pave the way for low-temperature thermoelectric harvesting.
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