Blocking Ion Migration Stabilizes the High Thermoelectric Performance in Cu 2 Se Composites

The applications of mixed ionic–electronic conductors are limited due to phase instability under a high direct current and large temperature difference. Here, it is shown that Cu 2 Se is stabilized through regulating the behaviors of Cu + ions and electrons in a Schottky heterojunction between the Cu 2 Se host matrix and in‐situ‐formed BiCuSeO nanoparticles. The accumulation of Cu + ions via an ionic capacitive effect at the Schottky junction under the direct current modifies the space‐charge distribution in the electric double layer, which blocks the long‐range migration of Cu + and produces a drastic reduction of Cu + ion migration by nearly two orders of magnitude. Moreover, this heterojunction impedes electrons transferring from BiCuSeO to Cu 2 Se, obstructing the reduction reaction of Cu + into Cu metal at the interface and hence stabilizes the β‐Cu 2 Se phase. Furthermore, incorporation of BiCuSeO in Cu 2 Se optimizes the carrier concentration and intensifies phonon scattering, contributing to the peak figure of merit ZT value of ≈ 2.7 at 973 K and high average ZT value of ≈ 1.5 between 400 and 973 K for the Cu 2 Se/BiCuSeO composites. This discovery provides a new avenue for stabilizing mixed ionic–electronic conduction thermoelectrics, and gives fresh insights into controlling ion migration in these ionic‐transport‐dominated materials.