Mechanistic understanding and strategies to design interfaces of solid electrolytes: insights gained from transmission electron microscopy

Solid electrolytes (SEs) have gained increased attention for their promise to enable higher volumetric energy density and enhanced safety required for future battery systems. SEs are not only a key constituent in all-solid-state batteries, but also important “protectors” of Li metal anodes in next-generation battery configurations, such as Li–air, Li–S, and redox flow batteries. The impedance at interfaces associated with SEs, e.g., internal grain/phase boundaries and their interfacial stability with electrodes, represents two key factors limiting the performance of SEs, yet analyzing these interfaces experimentally at the nano/atomic scale is generally challenging. A mechanistic understanding of the possible instability at interfaces and propagation of interfacial resistance will pave the way to the design of high-performance SE-based batteries. In this review, we briefly introduce the fundamentals of SEs and challenges associated with their interfaces. Next, we discuss experimental techniques that allow for atomic-to-microscale understanding of ion transport and stability in SEs and at their interfaces, specifically highlighting the applications of state-of-the-art and emerging ex situ and in situ transmission electron microscopy (TEM) and scanning TEM (STEM). Representative examples from the current literature that exemplify recent fundamental insights gained from these S/TEM techniques are highlighted. Applicable strategies to improve ion conduction and interfaces in SE-based batteries are also discussed. This review concludes by highlighting opportunities for future research that will significantly promote the fundamental understanding of SEs, specifically further developments in S/TEM techniques that will bring new insights into the design of high-performance interfaces for future electrical energy storage.