Real-Time Visualizing Nucleation and Growth of Electrodes for Post-Lithium-Ion Batteries.

ConspectusUntil recently, most studies on nucleation and growth mechanisms have employed electrochemical transient measurements, and numerous models have been established on various metal electrode elements. Contrary to the conventional tip-induced nucleation and growth model, a base-induced nucleation and growth mode was discovered not so long ago, which highlighted the importance of direct real-time observations such as visualization. As analysis techniques developed, diverse in situ/operando imaging methods have spurred the fundamental understanding of complex and dynamic battery electrochemistry. Experimental observations of alkali Li and Na metals are limited and difficult because their high reactivity makes not only the fabrication but also the analysis itself challenging. Na metal has high reactivity to electrolytes. Accordingly, it is difficult to visualize the Na deposition in real-time due to gas evolution and resolution limitation. Only a few studies have examined the Na deposition and dissolution reactions in operando. It is generally believed that the Mg anode is free from the dendrite growth of Mg metal, and Mg deposition preferentially occurs along the surface direction. However, whether the Mg anode always follows the dendrite-free growth has currently become a controversial topic and is being discussed and redefined based on real-time imaging analyses. In addition, a variety of morphological evolutions in the metal anodes are required to be systematically distinguished by key parameters. Real-time imaging analysis can directly confirm the solid-liquid-solid multiphase conversion reactions of S and Se cathodes. S and Se elements belong to the same chalcogen group, but their crystal structures and morphological changes significantly differ in each electrode during deposition and dissolution reactions. Therefore, it is necessitated to discuss the nucleation and growth behaviors by examining intrinsic properties of each element in chalcogen cathodes. Considering that a mechanistic understanding of the Se cathode is in its infancy, its nucleation and growth behaviors must be further explored through fundamental studies. In this Account, we aim to discuss the nucleation and growth behaviors of metal (Li, Na, and Mg) anodes and chalcogen (S and Se) cathodes. To elucidate their nucleation and growth mechanisms, we overview the morphological evolutions on the electrode surface and interface by in situ/operando visualizations. Our recent studies covering Li, Na, Mg, S, and Se electrodes verified by operando X-ray imaging are used as critical resources in understanding their nucleation and growth behaviors. Overall, with validation of the complex and dynamic nucleation and growth behaviors of metal and chalcogen electrodes by in situ/operando visualization methods, we hope that this Account can contribute to supporting the fundamental knowledge for the development of high-energy-density metal and chalcogen electrodes.