Excessive iodine addition leads to room-temperature superionic Cu2S with enhanced thermoelectric properties and improved thermal stability

Abstract

Highly disordered superionic phases lead to high thermoelectric performance (ZT~1.5) for Cu2M (M = S, Se, Te) alloys but bring challenges concerning thermal instability. Here we demonstrate that enhanced thermoelectric properties and improved thermal stability could be simultaneously realized in Cu2S through excessive I addition, mechanical alloying and radio-frequency (RF) hot pressing. The addition of excessive I beyond stoichiometric ratio not only induces higher Cu deficiency and thus better electrical properties for the prepared Cu2SIx (x = 0–0.15) alloys, but also leads to ultra-low thermal conductivity ascribed to multiple superionic phases and abundant nano-crystals. As a result, a ZT of 1.8 at 973 K is achieved for sample x = 0.10 (Cu2SI0.1), which is nearly twice the value of undoped Cu2S. Moreover, excessive I addition eliminates the ultrafast phase transformation that results in abrupt volume changes in pristine Cu2S and suppresses the rest transformations to be very sluggish. The high ZT value together with significantly suppressed phase transitions and impeded copper segregation enable the excessively I-added samples more beneficial for practical thermoelectric power generation.