Among the most promising strategies to enable Li metal anodes for next-generation rechargeable batteries is the use of solid-state electrolytes (SSEs). This new generation of batteries with higher gravimetric and volumetric energy density will be critical to the development of a range of applications, perhaps most importantly for electric vehicles (EVs). To date, however, stabilization of the Li metal/SSE interface at high rates of charge has proven challenging in many material systems. The nucleation and propagation of Li filaments has been observed to cause short-circuit in a range of promising SSE materials.¹ As fast-charge capability is a key requirement for many applications, but in particular for EVs, overcoming this challenge is critical to enabling the use of Li metal anodes. Limited understanding of the underlying mechanisms by which these Li structures nucleate and propagate within SSE materials has prevented rational design of solutions to the problem.² In this work, a new operando visualization platform is developed and used to study the propagation of Li filaments within garnet LLZO SSE materials at current densities relevant to fast charge capability. Time-synchronization of voltage traces with high resolution video microscopy reveal new insights into the nature of the dendritic structures, the mechanism by which they propagate, and their reversibility. Post-mortem electron microscopy analysis of specific sites in the operando experiments provides a high resolution, 3D perspective of the structures, and helps to identify pathways for improved performance. References 1. L. Porz et al., Adv. Energy Mater., 1701003, 1701003 (2017) 2. M. Wang, J. B. Wolfenstine, and J. Sakamoto, Electrochim. Acta, 296, 842–847 (2019).