Significantly enhanced chemical stability in interface-controlled Cu_2+xSe-reduced graphene oxide composites and related thermoelectric performances

β-Cu₂Se has attracted much attention due to its high thermoelectric performances at high temperatures. However, the fast migration of Cu ions at elevated temperatures, which stems from its intrinsic liquid-like behavior, causes chemical instability in β-Cu₂Se, resulting in deterioration of its initial performance. Therefore, the improvement of its chemical stability is considered as the biggest challenge for practical thermoelectric application of β-Cu₂Se. Here we report significantly enhanced chemical stability in interface-controlled Cu_2+xSe-reduced graphene oxide (RGO) composites and related thermoelectric performances. The solid-state reacted Cu_2+xSe powder and RGO were consolidated into the bulk composites using spark plasma sintering at 823 K. By suppressing Cu migration using the impermeable RGO network, we successfully prepared the composites without Cu precipitates even in Cu-excess condition. Beneficial effects of the interface control on the thermoelectric properties of β-Cu₂Se were discussed, and the maximum ZT of 1.05 was obtained from the Cu_2.025Se-RGO composite at 823 K.