Thermodynamics of atom motion and metal deposition in mixed ionic electronic conductors

The possibility of decomposition in superionic mixed ionic-electronic conductors (MIEC) has limited their engineering applications. Specifically, high efficiency MIEC thermoelectric materials have not been utilized due to decomposition under large electronic currents and large temperature gradients. Herein, we derive the critical condition for decomposition, which corresponds to a critical chemical potential difference defined from linear non-equilibrium thermodynamics. This analysis leads to the conclusion that voltage, not current density, is the relevant design parameter. Consequently, the decomposition condition is independent of the geometry of the device; whereby, a strategy is presented for improving stability in devices subjected to electrical and temperature gradients. By using a series of electronically conducting, but ion-blocking barriers to reset the chemical potential it is possible to keep the material below the threshold for decomposition. Experimentally, the thermodynamic theory is validated in the Cu_2-dSe MIEC system.