Potential for superionic conductors in thermoelectric applications

Superionic conductors (SICs) have played a fascinating role in the development of thermoelectric (TE) materials, as exemplified by Cu2Se and Cu2S. Recently, materials engineering of Cu2Se and Cu2S has led to exceptional values of the thermoelectric figure of merit zT > 2, nearly double the efficiencies of traditional TE materials. However, the fluid-like motion of the ions in the SIC crystal structures results in strong electro-migration and consequent material degradation when the compounds are placed in TE power generation conditions. To make superionic conductors viable as TE materials, efforts to stabilize the ionic motion while maintaining excellent TE properties are essential. In this review, we discuss the potential for SICs in TE applications, and we specifically highlight two materials - Cu1.8Na0.01S and (AgCrSe2)(0.5)(CuCrSe2)(0.5) - where stability and high TE performance may have been achieved.