Cu2Se-based materials have good thermoelectric performance, but their poor stability precludes their practical application. Writing in Nature Materials, Jincheng Yu, Jing Zhu and Jing-Feng Li report the use of a co-doping strategy to produce a Cu2Se-based superionic material that has a figure of merit of 3 at 1,050 K, an efficiency of over 13% when integrated into a thermoelectric module and good operational stability.

A path to controlling ion migration consists in increasing the energy barrier that controls ion motion. Guided by theoretical calculations, the researchers adopted a cation–anion dual-doping strategy based on Ag and F co-doping, which increases the energy barrier. Ag ions occupy Cu sites, hindering the migration of Cu ions, whereas F atoms, with their high electronegativity, change the chemical bonding in the system. “This is the first time that highly electronegative F is successfully doped at the anion site in a superionic conductor for thermoelectric applications,” comments Li. “The mobile Cu ions are effectively ‘shackled’, suppressing Cu deposition and hence improving both stability and thermoelectric performance.”