Superionic conductors are prime candidates for the electrolytes of all-solid-state batteries. Our understanding of the mechanism and performance of superionic conductors is largely based on their idealized lattice structures. But how do defects in the lattice affect ionic structure and transport in these materials? This is a question answered here by in situ transmission electron microscopy of copper selenide, a classic superionic conductor. Nanowires of copper selenide exhibit antiphase boundaries which are a form of a planar defect. We examine the lattice structure around an antiphase boundary and monitor with atomic resolution how this structure evolves in an ordered-to-superionic phase transition. Antiphase boundaries are found to act as barriers to the propagation of the superionic phase. Antiphase boundaries also undergo spatial diffusion and shape changes resulting from thermally activated fluctuations of the neighboring ionic structure. These spatiotemporal insights highlight the importance of collective ionic transport and the role of defects in superionic conduction.
Keywords: fast-ion transport; in situ transmission electron microscopy; nanostructure; solid electrolytes.