Stabilization of Superionic Copper Selenide Based Thermoelectric Materials

Thermoelectric (TE) technology enables the direct conversion between heat and electricity, thereby contributing to the alleviation of the prevailing energy crisis and the mitigation of environmental concerns. Bismuth telluride-based TE materials have been commercially utilized, while they are mainly applied to solid-state cooling rather than energy harvesting. Superionic conductors including copper chalcogenides and silver chalcogenides are rendered as promising TE candidates for power generation due to their superior thermoelectric figure of merit (ZT) at middle or high temperatures; the liquid-like behavior of ions aligns these systems with the concept of phonon-liquid electron-crystal (PLEC). Although the mobile ions are beneficial to enhancing the electrical transport and phonon scattering, their directional migration driven by an electric field or temperature gradient can result in the unintended deposition of metals, which can impair both the TE properties and the service stability of the materials. Therefore, it is imperative to identify stabilized liquid-like TE materials that can withstand current strike and high temperature. In this Account, we select copper selenide as a model superionic TE material to elucidate the origins of excellent TE performance, liquid-like behaviors, and instability at high temperatures. First, the unique electrical and transport properties are analyzed based on the electronic band structure and spectral lattice thermal conductivity calculated using the Density Functional Theory (DFT). Second, the microstructures are evaluated based on the data collected from advanced electron microscopes to understand the characteristics of highly active Cu ions. Third, we summarize the reasons for the Cu instability under different circumstances, as well as several effective strategies toward enhanced stability, including defect engineering, interface engineering, and chemical bond engineering. In particular, these strategies are well implemented by applicable routes, such as manipulation of cation vacancies, establishment of internal electric field, voltage division, interfacial trapping, ion confinement, and phase separation; the advantages and disadvantages of each strategy are clearly presented. Most importantly, we reveal the underlying modulation mechanisms for each route and highlight their core concept, i.e., control of potential barriers during the activation or motion process of ions, which helps us to master and popularize these strategies for practical use. Finally, we raise additional concerns regarding the trade-off between stability enhancement and liquid-like feature as well as the loss of volatile elements, and then point out the meaningful directions for future studies. We hope that our work will help to draw attention to the potential for enhancing the stability of superionic conductors and anticipate that the proposed approaches will facilitate the development of stabilized liquid-like TE materials, leading to further breakthroughs in this field.