High superionic conduction arising from aligned large lamellae and large figure of merit in bulk Cu_1.94Al_0.02Se

Good thermoelectric materials should have low thermal conductivity, high electrical conductivity, and Seebeck coefficient, which cannot be easily balanced in bulk materials. Exceptionally, the super-ionics in β-Cu₂Se can favorably contribute large ionic electrical conductivity and a liquid-like thermal conductivity by Cu⁺ ions. In the previous work, the superionic mechanism was found to be enhanced by small and randomly orientated lamellae with alternating ordered Se ion monolayer and disordered Cu ion bilayers. Here, we further enhance the superionic mechanism by increasing and better aligning lamellae in bulk Cu_1.94Al_0.02Se, resulting in a large thermoelectric figure of merit of 2.62 at 756°C.