Cu2Se exhibits a high figure-of-merit (ZT) >1 due to the scattering of phonons by mobile Cu+ ions, but migration of the Cu+ ions leads to chemical instability at high temperature. It is demonstrated that the incorporation of W in the Cu2Se matrix results in peak ZT of 2.1 (ZT(avg) = 0.93) at 878 K. After the first thermal cycle, the peak ZT of the composite decreases to stable value of approximate to 1.5 (ZT(avg) = 0.97). The enhanced stability of the nanocomposite along with high ZT(avg) is attributed to the covering of Cu2Se grain boundaries with W that inhibits Cu+ migration, reduces Se loss, and improves charge carrier mobility.
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